chapter 8 water pollution organic water...

Chapter 8 Water Pollution and the Hydrological Cycle Page 8 - 1

CHAPTER 8 WATER POLLUTION

Table of Contents CHAPTER 8 WATER POLLUTION .......................................................................... 2 Questions Addressed in This Chapter ........................................................................... 2 Pollutants and the Hydrologic Cycle ............................................................................ 3 Types of Water Pollutants ............................................................................................. 4

Metal Pollutants ........................................................................................................ 5 Metal toxicity ............................................................................................................ 6 Chemical form of metals in natural waters ............................................................... 7

Organic Water Pollutants .............................................................................................. 8 Benzene-containing pollutants .................................................................................. 8 Systemic Conditions that affect the fate of organic pollutants in water ................. 11

Interaction of pollutants with soil ............................................................................... 14 The structure of soil ................................................................................................ 14 The chemical structure of humus and humic acids ................................................. 17 Interactions of metal and organic water pollutants with soil .................................. 19 Migration of pollutants in soils ............................................................................... 21 Contamination and drawdown of aquifers .............................................................. 22

Water Purification Systems ......................................................................................... 25 Common water purification methods ...................................................................... 25 Reverse osmosis and desalination ........................................................................... 27

Bottled Water .............................................................................................................. 28 Water quality ........................................................................................................... 28 Regulation of water quality ..................................................................................... 29

Health effects of impure water .................................................................................... 30 Risk-benefit considerations in pollution mitigation ................................................ 30 Risk management .................................................................................................... 33 Purification at the water tap .................................................................................... 35

Sewage Treatment ....................................................................................................... 36 Stream pollution .......................................................................................................... 39 Lake Pollution ............................................................................................................. 41 Pollution in the bay regions ........................................................................................ 43 Ocean Pollution ........................................................................................................... 45 SUMMARY ................................................................................................................ 48 Review Questions ....................................................................................................... 50 Problems ..................................................................................................................... 52 Individual and Group Projects .................................................................................... 53 Readings and links (to be updated) ............................................................................. 53


CHAPTER 8

WATER POLLUTION

We have seen in previous chapters that precipitation deposited on Earth is already polluted with nitric and sulfuric acids as well as oxidized degradation products of organic air pollutants. Having reached the earth, this precipitation is now susceptible to the addition and subtraction of pollutants because of water’s solvent properties and contact with earth’s surface and atmosphere. Among the added pollutants are biological, organic, and inorganic contaminants, such as farm runoff, landfill leachate and industrial waste products. Soils filter some metal pollutants and biological contaminants by binding them to soil particles. However, this binding is not always permanent and metals can contaminate ground water supplies by slow, downward migration. The main concerns regarding water pollutants are their effect on human health and on the health of aquatic and land ecosystems. In this chapter, we explore the chemistry of selected water pollutants and the manner in which they are distributed and altered in the environment.

Questions Addressed in This Chapter

1. What is the chemical nature of metal water pollutants? 2. What is the chemical nature and fate of herbicide, pesticide, and other

organic water pollutants? 3. What is the fate of water pollutants in contaminated water percolating

through the soil? 4. How do some pollutants ultimately migrate down to groundwater? 5. What are the definitions of “water quality” and “threshold dose?” Of what

importance are these terms in water pollution? 6. How is water purified for drinking purposes and in sewage treatment? 7. What is the influence of equilibrium and solubility on lead pollution

hazards and how are these related to risk assessment and management? 8. What are the problems of stream and river water use and pollution in

rivers, bays, and oceans?

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Pollutants and the Hydrologic Cycle

Long before humans were on the planet Earth, water was repeatedly polluted and purified without sewage plants, water filters, bottled water, etc. We have much to learn about nature’s water purification systems. In fact, we are now mimicking many of these processes that evolved along with other Earth systems. We need to understand the advantages as well as the limitations of these systems. Let’s monitor a hypothetical drop of water as it works its way through the hydrologic cycle to illustrate its pollution status and what is done to purify it both by natural and human systems. We will examine the sources and sinks of water and air

pollutants. We include pollutants such as acid precipitation in this discussion because precipitation is the most important atmospheric sink for most air pollutants and their oxidation products. That is, through the precipitation process, air pollutants now become water pollutants. However, when precipitation falls on land, it can dissolve pollutants such as pesticides and fertilizers and other soluble chemicals. The fates of all of these water pollutants will vary, depending on whether they find their way into soil, groundwater, lakes, streams, rivers, estuaries or oceans. Figure 8-1 illustrates the above discussions in diagrammatic form and will serve as an organizing concept map for this chapter. At the beginning of the hydrologic cycle, nature provides an efficient water cleansing mechanism: the evaporation of water from liquid or solid surfaces. In order to accomplish this, energy must be provided by the sun or be taken from the surroundings to break the large number of hydrogen bonds between water molecules in liquid water, as well as to overcome the attractive forces between water molecules and dissolved substances, including pollutants. Energy must be provided to break bonds between adsorbed water and the substance to which the water is adsorbed. The

Figure 8-1 Overview of the sources and sinks of water pollutants. Airborne sources are introduced into water through precipitation or direct “dry” deposition. Ground-based pollutants can come either from point sources such as sewage disposal sites or industrial outfalls in streams or from broad-based sources such as fields containing fertilizers or pesticides. Underground aquifers, which provide drinking water, can be contaminated from water that works its way through the soil or from leaking underground storage tanks.

First step in the water

cycle: purification


water evaporation process is a major water purification step since nearly all of the chemical species with which the water was previously associated remain behind in the liquid or solid state. For example, water molecules evaporated from the surface of the ocean have to have enough energy to break away from the strong ion-dipole attractions of the positive sodium and negative chloride ions of dissolved ocean salt and other water dipoles. The water molecules that move into the gaseous state absorb vast amounts of energy from the sun or from the environment. This energy is given back to the environment when these same water molecules are condensed as water droplets or are attracted and adsorbed to some solid surface.

Types of Water Pollutants

Atmospheric water molecules ultimately are adsorbed to suspended solid particles or aerosols as microscopic droplets and become clouds that are ultimately contaminated with sulfuric and nitric acids and other air pollutants (Chapter 5). This is the first step in water pollution. A significant fraction of the water in clouds ultimately falls to the ground as precipitation. The probability of polluting the water from precipitation at ground level depends on the type of chemical that contacts the liquid water precipitation. The rule of thumb regarding solubility is "like tends to dissolve like." Thus, highly polar liquid water molecules are able to dissolve many different types of polar pollutants. However, water has been called the “universal solvent” because even the most “insoluble” compounds such as oil (remember the saying “oil and water don’t mix”) can still dissolve enough of the compound so that an "oily" taste can be detected easily in water that has been in contact with oil or gasoline. The solubility of various compounds varies widely depending on their polarity, but almost all compounds have at least a limited solubility in liquid water. This means that both hydrophobic (water hating, relatively insoluble) and hydrophilic (water loving, generally soluble) pollutants can be transported through the liquid water part of the hydrological cycle. Water pollutants can be classified into a number of different categories: (a) biological organisms; (b) metals and metal compounds; (c) organic compounds. Microorganisms in sewage and related pollutants, such as E. Coli and Cryptosporidium are, still are of great concern (see box next page). Two additional types of chemical pollutants, metal and chlorinated organic compounds have been of concern and will be the main focus of our chapter.

Types of water

pollutants


Metal Pollutants

There is a basic difference between metals and organic pollutants. Even though the process may sometimes be very slow, most organic compounds are generally biodegradable, that is, organisms assist in the transformation these compounds into

Mystery disease explodes into a Milwaukee epidemic In 1979, Randy Moffitt, outstanding relief pitcher for the San Francisco Giants, was suddenly stricken with a mysterious disease. Its symptoms were severe stomach cramps, nausea, diarrhea, and general malaise, and the illness nearly wrecked Randy's pitching career. After many physicians had diagnosed him with mental and psychological problems, the disease finally was diagnosed as Cryptosporidiosis. Only about one hundred people in the world had previously contracted this disease. Cryptosporidiosis had only been recognized as a human disease in 1976 and until 1982 was rarely encountered. It is caused by a parasitic protozoa contained in fecal matter from infected animals or human wastewater. In 1982, the increase in the disease began to rise with the rise in AIDS cases. Those with challenged immune systems, e.g., infants, pregnant women, elderly, and those with immune diseases such as AIDS, are especially prone to Cryptosporidiosis. Those with strong immune systems usually fight off the infection in one to two weeks and recover completely. Randy Moffitt's case was unusual. In early 1987, 13,000 people came down with the disease in Carrollton, Georgia in a town of 64,000 residents. The outbreak was traced to the municipal water supply. But health officials took special notice in April, 1993, when over 400,000 inhabitants of Milwaukee (population 800,000) became ill with diarrhea and about a hundred died. The illness was traced to drinking water contaminated with this protozoa. The mayor of Milwaukee, whose own son was ill from the epidemic, ordered one of the two city water filtration plants shut down, when it was noted that the water coming from that plant was unusually cloudy. Tests on this water confirmed the presence of Cryptosporidium. Investigators suggested that excessive rainfall washed Cryptosporidium from farmland waste into Lake Michigan, from which the city drew its water supply. Since the Milwaukee episode, researchers have discovered small amounts of Cryptosporidium in a number of municipal drinking water supplies of large and small cities. Sources of Cryptosporidium are any kind of natural water (lakes, rivers, streams, pools and Jacuzzis), stool, food, and objects contaminated with fecal matter from farm runoff (especially cattle farms), municipal sewage waste and wild animal feces. Unlike most germs and one-celled animals, Cryptosporidium is not killed by conventional water treatment employing chlorine. The Cryptosporidium species is transmitted by ingestion of oocytes (an immature egg) excreted in the feces of infected humans and animals. Thus infection can be transmitted from person-to-person, ingestion of fecally contaminated food, or contact with fecally contaminated surfaces. Swimming pools are potential sources of oocyte contamination. The Environmental Protection Agency has ruled that all municipal water suppliers of populations of over 10,000 must regularly monitor for Cryptosporidium. As few as 100 Cryptosporidium oocytes are sufficient to cause noticeable illness. It is recommended that anyone with AIDS or an otherwise compromised immune system not drink any tap water or enter any swimming pool. Smaller public water works find it too expensive to perform the type of monitoring required and are therefore often in non compliance with EPA regulations regarding Cryptosporidium contamination. UPDATE??? Recommended preventive measures are: washing of the hands before eating, avoiding contact with any fecal matter, directly or indirectly, and using water that is either boiled (rolling boil for a minimum of one minute) or filtered through a filter with holes one micron (0.000001 meter) or less.



simpler, non-toxic products, in some cases into water and carbon dioxide. Metals cannot be degraded in the same way. They can be chemically transformed into harmless compounds or oxidation states. However, metal atoms always retain their identity and, under the proper circumstances, can be transformed back into harmful forms. It is very difficult to reassemble toxic organic pollutants from carbon dioxide and water. If a metal is dangerous in one of its oxidation states, it is always capable of being dangerous, even if its other oxidation states are harmless. This is because both oxidizing and reducing environments occur in nature, especially in aqueous environments such as surface and deep waters.

Metal toxicity

Not all metals are toxic chemicals, chemicals that interfere with some human or natural chemical process that causes an illness or causes harm to a living organism. Small amounts of many metals, such as zinc (Zn) and iron (Fe), are essential in our diet. However, ingestion of relatively large quantities of the same metals can be toxic. There are other metallic elements, such as cadmium (Cd), that are apparently not needed by humans, but are highly toxic at low concentrations. Figure 8-2 illustrates some of the more prominent metallic elements that may be present in water. Not all of these metals are toxic to humans. These metals can be introduced into groundwater from a number of different sources. Drinking water containing these metals is often drawn from underground wells or other water sources and can cause exposure to toxic metal pollutants.

Figure 8-2 Incomplete Periodic Table showing important metals (underlined) often found in ground water

Metal pollutants



Chemical form of metals in natural waters

There is a difference between pure solid metallic elements and the chemical species found in water. The difference is the removal of one or more of the metal's valence electrons, which causes the metal atom to become a positive ion that may or may not be soluble in water, depending on the negative ion(s) with which it is found in solution. Recall from our discussions of electronegativity (Chapter 5) that metals are those elements that have relatively low electronegativities with respect to non-metal elements like oxygen and fluorine. Metals tend to lose electrons to non-metals or to covalently bond groups of non-metals, leading to ions such as sulfate and nitrate contained in acid rain. For example, solid cadmium sulfate (CdSO4), composed of Cd2+ and SO4

2– ions, is formed when the neutral cadmium atom (Group 2B) gives two of its valence electrons to a collection of sulfur and oxygen atoms to become the positively charged cadmium ion and the negatively charged sulfate ion. This compound could form in an acid rain cloud droplet that formed around a solid particle nucleus originating in a zinc smelting plant, a plant that extracts metallic zinc from zinc ores taken from the earth. Zinc and cadmium are in the same group of the periodic table (2B) and have very similar chemistry. Thus zinc ores have small amounts of cadmium, which can escape into the environment during the smelting process forming zinc oxide (ZnO) and cadmium oxide (CdO). A CdO particle could react with sulfuric acid in acid rain to form water and cadmium sulfate, a salt that is soluble in rainwater. CdO + H2SO4 –––> CdSO4 + H2O (8-1) Cadmium is a member of the group 2B transition metals. The elements in this group are metals in the (B) region of the periodic table (Figure 8-2), which include all the metals with atomic numbers from 21 (Sc-scandium) to 30 (Zn-zinc) and all elements below these metals. We indicated that the group numbers of the periodic table were indicative of the number of valence electrons of the respective elements in that group. As a Group 2B element, Cd should have two valence electrons. However, most transition elements can have a variable number of valence electrons and their chemistry can be highly complex because they can undergo oxidation-reduction reactions. That is, either their charge or their effective control of their valence electrons can increase or decrease, thus changing their oxidation state. The toxicity of certain transition metals, such as chromium (Cr), used in chrome plating, depends on the oxidation state of the metal, and this state depends upon the pH, the oxygen concentration, and the oxidation state of the various chemical species in the solution in which chromium finds itself. Cadmium, chromium, mercury, lead, and arsenic (a nontransition metal element) are toxic elements and are of great concern as water pollutants. Such valence electron complexity implies a more complicated electronic structure for transition metal atoms than the metals that we dealt with previously. Transition metals can lose or share a number of different valence electrons. For example, iron can lose either two or three valence electrons to for two different ions,

Different chemical forms of metals

Transition metals

Complex metal

chemistry


Chapter 8 Water Pollution and the Hydrological Cycle - 8

Fe2+ and Fe3+. Transition elements form many complex ions and coordination compounds. In these species, the central atom is attached to a number of atoms or groups of atoms through the donation and consequent sharing of a number of pairs of the metal atom’s electrons with the attached groups. For example, the hydrated ions of many transition metals are considered to be complex ions. The iron ion Fe2+ [also designated Fe (II)] can be represented as Fe(H2O)6

2+, with six water molecules contributing shared electron pairs of water oxygen atoms with the central iron atom. Many molecules present in soils are capable of forming complex compounds formed from metal ions and other ions or molecules.

A key feature of transition and some other metal pollutants is that the metal can change its oxidation state and its complex or coordination compound partners. The designation of the oxidation state of a metal M as M(VI) indicates that the metal is in an oxidation state of 6+. However, this does not necessarily mean that the metal atom has lost six of its valence electrons. It does mean that that the metal has lost control of six of its valence electrons to elements with greater electronegativities than the metal itself. As indicated previously, the metal forever retains its chemical characteristics as a metal. It may be recycled and changed in oxidation states, but if it is a toxic metal, it always retains the potential for toxic behavior. For example, certain parts of the Middle East are contaminated with copper and lead from metal smelters in operation 2000 years ago. Cr(III) is found naturally and is an essential nutrient. Cr(VI) is usually generated during industrial operations, is mobile in the environment, can penetrate the cell wall and act inside the cell to form cancerous cells. Cr(VI) causes skin and stomach irritation, damage to liver. It can be removed from contaminated waters by activated carbon, a form of pure carbon that is heated to high temperatures and has a very large surface area and efficiently adsorbs organic molecules as well as inorganic ions such as Cr(VI).

Organic Water Pollutants

Benzene-containing pollutants

Many organic compounds of environmental concern are chlorine-containing derivatives of the organic molecule benzene (C6H6) whose symmetrical structure is shown in Figure 8-3. Benzene forms a hexagon of six carbon atoms covalently bonded to each other, and each carbon is covalently bonded to six hydrogen atoms. Experimental measurements reveal that all of these hydrogen atoms lie in the same plane as the carbon atoms. Benzene is completely symmetric, flat molecule. The cyclic benzene molecule has an unusual electronic structure: the two Lewis structures for benzene shown in Figure 8-3a and 8-3b imply that the carbon-carbon bonds oscillate between single and double bonds. All of the carbon-carbon bonds are experimentally found to be of the same length and strength, implying that the Lewis structures shown are somehow averaged to give an intermediate structure. Chemists say that there is “resonance” between these two Lewis structures. Therefore, rather than showing the structures in Figure 8-3a or 8-3b, the structure with the circle in the

Organic water

pollutants




center of the hexagon at the bottom of Figure 8-3 is often used to depict the benzene molecule.

It has been found experimentally that a portion of the valence electrons in the double bonds between the carbon atoms are concentrated above and below the plane containing the carbon and hydrogen nuclei, and therefore can interact with similar types of electrons. For example, benzene is a known human carcinogen, a chemical that can cause cancer, because it disrupts the normal functioning of DNA by being inserted into regions of the DNA that can accept this flat molecule and others carcinogens related to it. Compounds derived from benzene are formed by replacing one or more of the six hydrogen atoms on the benzene with other atoms or groups of atoms. Compounds derived in this manner from benzene or related compounds, are called aromatic compounds. When a single hydrogen atom is removed from benzene and the resulting carbon atom is covalently joined to another atom or group of atoms, the benzene group is known as a phenyl group. Thus biphenyl and its many related chlorinated compounds, called PCBs (polychlorinated biphenyls), consist of two benzene rings covalently attached between two carbon atoms, one on each benzene ring, resulting from the elimination of two hydrogen atoms, with one or more remaining ring hydrogens replaced by chlorine atoms (Figure 8-4). DDT is a compound in which two benzene rings are joined to a common carbon atom that is itself attached to a hydrogen atom and another carbon atom that has three chlorine

Figure 8-3 Benzene, C6H6, can be depicted in Lewis structures (a) and (b), neither of which represents the observed chemical properties of benzene. [(c) and (d) are alternate ways of representing structure (a)]. The hexagon in the bottom row (left) with the circle is a “resonance hybrid” of structures (a) and (b), indicating that the carbon-carbon bonds are neither single nor double, but somewhere in between.

Benzene and

aromatic compounds


atoms attached to it. There are two additional chlorine atoms attached to ring carbons on the opposite sides of the two rings. Chemical compounds that include the benzene structure, especially those with many chlorine atoms attached (Figure 8-4), are very stable compounds, and are not particularly prone to biodegradation, the natural breakdown of the organic compound by microorganisms in the soil. For example, even though PCBs are not currently being manufactured or employed in industrial settings, they are still hazardous chemicals in a number of regions because of their previous accidental or deliberate release, their leakage from hazardous waste dump sites, their very long lifetimes in the environment, and their toxicity.

A number of toxic water pollutants, including pesticides and herbicides, are derived by substituting the hydrogens of benzene with other atoms, benzene molecules, or benzene derivatives. Among these compounds, whose structures are

Bishpenol A (BPA) Figure 8-4 Chemical formulas for a number of environmentally problematic derivatives of benzene.


illustrated in Figure 8-4, are dioxin, DDT (dichlorodiphenyl trichloroethane), and PCBs. Most of the benzene-containing compounds discussed above tend to have some limited water solubility. They also tend to be concentrated in the fatty tissues of mammals (bioaccumulation), leading to storage of increasing quantities of the pollutants until toxic concentrations are reached. The toxicity of a compound, its biodegradability, and its rate of introduction into water are important considerations in determining whether a particular water source will become polluted with that compound. Although not considered to be a general water pollutant, a related benzene derivative called bisphenol A (Fig. 8-4), BPA, has been found in manufactured plastics, especially in polycarbonate and epoxy plastics. Because of its chemical structure, BPA resembles a number of different naturally occurring estrogens and interferes with their functions. It has been banned in baby bottles in the European Union and Canada. In the US, the FDA raised concerns about its effect on fetuses, infants and young children. BPA has been found in the polymer linings of food cans and in sports water bottles.

There are many different types of synthetic and naturally occurring organic compounds that make their way into the natural water cycle on its ground segment. One pollutant of more recent concern has been the many pharmaceuticals and their metabolites, that is, small molecule compounds that result during the degradation and elimination of pharmaceuticals from the body. Many unused pharmaceuticals are simply deposited directly into the sewage system, where they can evade the sewage treatment system and work their way into streams and rivers, affecting wildlife. Pharmaceutical metabolites also work their way into the sewage system after being eliminated from the patient. Some waste products are affecting stream life. For example, black bass are being feminized in waters coming from farms and wastewater treatment plants. Male fish sex organs producing immature eggs. Many anti-inflammatory and analgesic medications are excreted relatively rapidly in urine. Agricultural and urban runoff (non point source pollution) often contains nutrients, pesticides, pathogens, sediment, salts, trace metals, and substances contributing to biological oxygen demand (BOD –consume oxygen in natural waters during their oxidation in biological degradation processes).

Systemic Conditions that affect the fate of organic pollutants in water

All natural waters (marshes, creeks, streams, rivers, lakes, bays, and oceans) are part of an ecosystem. A number of factors besides those discussed above are important in determining the fate of a pollutant dissolved in natural waters: presence of biological species such as bacteria and other living species, surfaces that can adsorb pollutants such as phase boundaries (see below), the rate of adsorption and chemical reaction on surfaces, oxygen concentration, oxidation or reduction capacity of the environment, and pH.



(a) Presence of biota. In every natural water system, many biological species are present that are characteristic of that particular ecosystem. The ecosystem has very slowly evolved to support these biota through chemical and biological feedbacks that may be subtle and not easily understood until one part of the ecosystem is altered. The addition of a chemical that is not normally present in a particular ecosystem may cause destruction of one or more species in that ecosystem or make less efficient one or more of the partners in the delicate balance that controls the viability of that ecosystem. When water pollutants are present, they often affect the biological systems. In some situations, the presence of the pollutant causes the environment to support as a major species one that was previously a minor species. For example, if the pollutant is a nutrient, the environment can be so much improved for a certain species that it undergoes luxuriant growth (bloom) to the detriment, possibly even causing the death, of other species. On the other hand, the pollutant can poison the environment and kill one or more of the biological species present. It is a rare case in water pollution studies when the presence of waterborne biota can be ignored in considering the fate of waterborne chemical pollutants. Much is yet to be learned about these complex chemical and biological systems. (b) Residence times The average residence times of a specific water molecule in various phases of the hydrological cycle are markedly different (atmosphere: ~11 days; land: ~ a year; ocean: ~ 3,500 years). The lifetime of a pollutant molecule dissolved in water in a particular environment depends on the efficiency of the natural “filters” present that either prevent the pollutant from entering the water source or remove the pollutant. The rate of removal of the pollutant by a natural filter system is critical if the rate of removal of the water itself from a particular environment is rapid. (c) Phase boundaries Three phases are present in most ground water environments: air-water, water-solid, and air-solid (Figure 8-5). Generally, in the water-solid phase, the solid surface has a coating of organisms and organic compounds interfacing with

the water phase. This coating can be thick enough that the water does not come in contact with the solid under the coating. Dissolved pollutants are attracted to the coating on the solid surface and partition (divide) themselves between the water phase and the organic coating on the solid phase. The fraction of the time that a pollutant spends in each of these two regions determines the water concentration of the pollutant.

Conditions affecting

the fate of organic

pollutants

Figure 8-5 Diagram indicating the three phase interfaces in any aerated soil: air-water; water-solid and solid-air


Some of the solids in contact with water in soils and sediments are extremely small particles called colloids, small particles that can remain suspended indefinitely in water without settling. Colloids are particles less than 0.001 mm in size. The clay fraction includes particles less than 0.002 mm in size. In soils these colloids are generally, but not always, aggregated into larger solids. There are two types of colloids, organic and inorganic. A very important property of inorganic colloids is their ability to adsorb and desorb (release) positively charged metal cations. Organic colloids contain both positive and negative charges, but always seem to have a net negative charge and therefore adsorb positively charged metal cations. Organic colloids have a high capacity to absorb water. These organic colloids are highly important in nearly all soils and especially critical in adsorbing or removing pollutants from natural waters for varying periods of time. Pollutants can be so strongly adsorbed to the surface coating of the colloids that their residence times on the colloid surface are extremely long. If these surface-contaminated colloids can migrate through the soil, however slowly, they will carry adsorbed pollutants with them. This type of migration is postulated as one of the mechanisms for the slow transport of pollutants, including metals, down to groundwater far below the topsoil, over periods as long as many decades. (d) Oxidation-reduction zones In the upper part of the soil, the oxygen concentration is essentially the same as that of the atmosphere in contact with the soil. Deeper in the soil, the oxygen concentration is reduced because oxygen is consumed by various biota at a rate that exceeds the diffusion rate of oxygen into that region from the atmosphere. Lakes and oceans are oxygenated to varying degrees. However, as one probes into the bottom sediment, one quickly finds that the oxygen concentration is essentially zero, i.e., it becomes anaerobic, devoid of oxygen, changing the chemical environment from an oxidizing to a reducing environment. Such conditions encourage the growth of anaerobic organisms, those that do not depend on oxygen to survive. Under anaerobic conditions, with little or no oxygen present, the oxidation states of various substances are generally found to be the lower, reduced states, i.e., states in which metals have more control over their valence electrons and in which other compounds have fewer oxygen atoms and central atoms have more “control” over their valence electrons. For example, the bubbles rising from the anaerobic "muck" of swamps, so-called “swamp-gas”, is methane, CH4, a compound containing no oxygen atoms and in which the carbon is in its most reduced state and more in control of its valence electrons with its C–H bonds. Part of the rotten egg smell of tidal flats is hydrogen sulfide (H2S), a result of the reduction of sulfate ions (SO4

2–) to H2S by anaerobic bacteria. The oxidation state, solubility, and, in some cases, toxicity of dissolved metals depends on the amount of oxygen present, the pH of the groundwater, the concentrations of other compounds that are capable of being oxidized or reduced, and the presence of compounds that can react with the metal pollutant in its different oxidation states.

Migration of colloids


Interaction of pollutants with soil

Topsoil has been called one of the most valuable and least appreciated systems of a nation. Yet, through erosion, it is being washed away downstream into the bays and oceans or swept away by wind at an alarming rate. Without soil, the growth of agricultural products becomes very difficult, if not impossible. Soils are open systems. That is, they undergo constant exchange of matter and energy with their environment, the hydrosphere, the atmosphere, and the biosphere (living species). There are many different types of soil in different parts of the world. We will consider below a typical dark-colored soil supporting vegetation such as trees, grasses, and agricultural products. The most important component of these soils, humic acids (discussed below), is a highly valuable chemical reagent, because its functions include providing: initiators of both oxidation and reduction reactions, materials that act as a pH buffer, a material for retaining water, a binder of metals and organic materials such as pollutants, a stimulator of plant growth, and, important for this chapter, a material that bio-transforms toxic pollutants through oxidation and reduction in various soil regions and adsorbs these pollutants and their degradation products.

The structure of soil

The topmost layer of soil, called the O horizon (Fig. 8-6) consists of decaying debris or litter from plant, animal, and microbial remains. It is the final product, humus, of a natural composting processes. This decay is caused by aerobic bacteria that, with the aid of oxygen from the air, oxidize some of the organic matter to carbon dioxide and water and to weak organic acids of the general form, RCOOH. Here R stands for a large, incompletely characterized, organic molecular complex attached to the weakly acidic -COOH groups. The acidic nature of this molecule gives rise

to the name of main constituent of brown to black soils – humic acids. Note the plural form, acids. There is no one humic acid. Humic acids are a complex mixture of compounds of unknown structure. Another important component of soils is fulvic acid, which is especially reactive with Fe3+(III), Al3+(III), and Cu3+ and forming strong complexes, leading to these ions being more soluble in natural waters. The difficulty in analyzing the chemical structure of humic and fulvic acids in soil is that it is difficult to analyze it in situ, that is, in its natural state in the ground. Instead, it is usually treated chemically and separated into fractions that can be studied with sophisticated chemical instrumentation. Even after this separation, only average characteristics can be determined. These humic acids, along with other organic decay products, such as proteins, amino acids, lignin, cellulose, etc., aggregate together to form the particulate matter that we observe as soil. However, the humic acids are water-soluble and can migrate through the soil. They can even be

Structure of

soil

Fig. 8-6 Structure of a typical soil



present even in drinking water, depending on the water treatment process. Soil gains its color because the many double bonds in humic acids. In addition, decaying plant and animal matter absorb light. The red clay color originates from the inorganic mineral content of the soil. HONO is formed when humic acid is exposed to both NO2 and light, according to a recent discovery. When exposed to light, HONO forms OH radicals, which are critical in helping oxidize air pollutants so their oxidation products are soluble in cloud water and eliminated in precipitation. Thus, there appears to be a direct link between soil and natural air pollution control. Since NO2 is more prevalent in urban environments, this may be a natural aid in controlling air pollutants in such locations. The carbon dioxide concentration in the O horizon is as high as eight times that in the atmosphere above the ground because this gas is a byproduct of decomposition of litter organic matter. CO2 mixed with the water and the weak organic acids in the O horizon makes the soil acidic. As water drains over the surface of the decaying vegetation, it picks up small amounts of humus, that part of the plant and animal material that bacteria are not able to digest. As water carrying this undigested brown, organic matter percolates down through the soil, the humus is adsorbed to rock grains in the A layer of soil, also designated as the A horizon. In many soils, the A horizon, also designated as topsoil, is located immediately below the O horizon. Both are well aerated because of the relatively open structure of the soil and of the living creatures such as earthworms that burrow through the soil. The specific organisms present in the soil are determined by the pH, the amount and type of mineral and organic matter, the nutrients present, and the

oxygen concentration. Critical to the dynamic functioning of soil are the billions of microorganisms that are contained in every gram of the average backyard soil. A pinch of soil from the back yard of the average home contains a billion organisms and approximately 10,000 distinct species of microbes, many of which have not yet been named or scientifically studied. Many

are probably direct descendants of the earliest life forms on Earth. Many of the carbon atoms in soil have been recycled many thousands of years as the biological soil recyclers die and are recycled themselves by their successors. Nearly all of the different size grains of rock in the A horizon, primarily minerals called silicates, have an organic humus coating. Pollutants in the soil moisture inventory are more intimately exposed to this humus surface than to the soil mineral, which is often completely covered with an organic film. In addition, the water is exposed to soil organisms that reside on and in the humus (organic) portion of the soil. This spatial arrangement provides an opportunity for randomly diffusing pollutant molecules and ions to come in contact with the soil surface humus film; the


outcome of this contact depends on the chemical nature of the pollutant, the humus, and the organisms present. Nitrogen is a critical element in agriculture and is often in short supply in the soil. Chemists have not come up with a more efficient way of reducing molecular

nitrogen (N2) to form critical nitrogen-containing plant nutrients than soil microorganisms. An enzyme in soil biota readily accomplishes this difficult feat in a symbiotic system involving root nodules and microorganisms (Fig. 8-7). The reason this is a difficult chemical reaction to carry out is because of the vast amounts of energy necessary to break the very strong triple bond in N2. The enzyme nitrogenase is the critical molecule that helps these organisms perform this chemical reduction of N2. Thus, water draining through the soil comes in contact with a variety of organic and biological entities, each of which may adsorb, react with, complex with, oxidize, or reduce, the pollutants

contained in the water. Again, we must treat soil, especially the A horizon part, as a system of inorganic and organic chemicals and organisms that have evolved over millions of years, along with the plants and animals that grow in and live on that soil. The chemical structure and reactions that take place in this soil are highly complex, as complex as the structure of the soil itself. Many of our natural antibiotics come from soil organisms, and many natural antibiotics from soil organisms protect plants from diseases. Thus, the message is clear that humans should protect and nourish their soil as much as it nourishes and protects them. The chemical structure of soil is complex and incompletely understood. Soil structure is studied by separating it into different fractions. These fractions are then investigated by heating or treating them with various chemicals. Different soils have certain features in common. Click here for more on these common chemical characteristics.

Fig. 8-7 Roots of a soybean plant showing round nodules containing enzymes that transform atmospheric N2 into reduced nitrogen compounds that the plant can utilize


––––––––––––––––––––––––––“Click here” material––––––––––––––––––––––––

The chemical structure of humus and humic acids

The humus in soil contains more carbon than in all living things. Humus is not a single type of molecule, but rather is a collection of complex molecules with common characteristics. Though the percentage of humus in soil is small, this material is responsible for most of the physical and chemical properties of the A horizon soil. Such properties as pH, metal binding capacity, adsorption capacity for organic pollutants, water holding capacity, and stability of soil aggregates are determined by the humus content. Because of topsoil erosion, humus is the prime organic matter in river sediments. Thus, humus is a critical component of the solid surfaces that first contact liquid water in the land segment of the hydrological cycle. The humic acids are highly complex, high molecular weight compounds with an empirical formula C187H186O89N9S1. The near correspondence between the average number of carbon and hydrogen atoms hints at the structure of humic acids. This means that there are not a large number of –CH2– groups, and there are a large number benzene (C6H6)-like groups. The relatively large number of oxygen atoms indicates the presence of both acids (RCOOH) and phenol (C6H5OH)-like (phenolic) compounds, the latter being derived from decayed leaves. Fulvic acid has an average atomic composition is C135H182O95N5S2, indicating a less aromatic (benzene-like) character than the humic acids The basic chemical structure of humus is derived from a substance called lignin. This material is part of the woody cell walls that holds plants together. We can imagine an oversimplified model dirt molecule that accounts for soil’s chemical properties with respect to water pollutants. One end of a very long, primarily hydrophobic molecule has been oxidized to form an organic acid (RCOOH), making water-soluble one end of a normally water insoluble molecule. Such types of molecules are called amphiphiles because a single molecule incorporates both water-soluble and water-insoluble chemical characteristics. (This is the same type of chemical structure present in soaps and detergents, whose function is to be both water soluble and attractive to grease and other hydrophobic “dirt.”) The structure of such an amphiphilic molecule is represented schematically below. XXXXXXXX-COOH The long XXXXXXXX... “tail” region represents the primarily water-insoluble region containing stretches of hydrophobic ...CH2CH2... groups and/or aromatic molecules containing benzene rings, and the COOH “head” represents a carboxylic acid region, which is hydrophilic because of its OH and CO groups of covalently bonded atoms.

Humus

and Humic Acids


The -COOH group is composed of a carboxyl group [ – C = O] covalently attached to a hydroxyl (–OH) group and to the ...XXX– group; this means that there are four bonds attached to the carbon atom in the -COOH group. There are several unshared pairs of electrons on the oxygen atoms of the carboxyl group. These electrons can be shared with certain types of transition metal atoms, forming complex compounds or ions that chemically bind these metal ions and remove them from the

rainwater. In addition, humus has so-called phenolic functional groups, collections of atoms with a similarity to phenol (C6HOH), a benzene ring with an OH group substituting for one H atom. Phenol behaves as a weak acid where the hydrogen proton on the phenol -OH group that is attached to a benzene ring is the donated acidic proton. Thus, humic acid has two different types of acidic groups attached to it. In addition, a few non-ionized polar groups may be attached in various regions of the predominantly hydrophobic tail.

In humic acid, these amphiphilic (containing amphiphile) molecules are joined with other degradation products called fats or lipids, proteins and amino acids, and cellulose (Chapter 14???). Lipids are structurally similar to the lignin derivatives in that they are quite water soluble at one end. The rest of the molecule, the “tail” region, is primarily water insoluble made up of long chains of –CH2– segments in a chain with a methyl group (–CH3) at the end of the chain. Together, the oxidized lignin and fat degradation products apparently spontaneously assemble to form structures that attach to the surface of minerals in soil. Since the ...COOH groups in the lignin are weak acids, a small fraction of these acidic (–COOH) groups dissociate into carboxylate (...COO–) and hydronium (H3O+) ions. These ions quickly react to re-form ...COOH groups because they are weak acids. Alternatively a base can neutralize the hydronium ion. The combination of the weak acids (...COOH) and the anions of that weak acid (...COO–) act as a buffering agent, i.e., one that resists change in pH with the addition of an acid or base to the soil. Thus, the pH of soil containing humus is fairly constant and is usually slightly acidic because of the soil’s humic and fulvic acid content. Amphiphilic molecules naturally form a bilayer structure such as that depicted in Figure 8-6. That is, the molecules line up with their hydrophobic “tails” pointed toward each other in two layers arranged tail-to-tail. The polar RCOO– and RCOOH groups are arranged in such a way that they occupy the top and bottom regions of the bilayer structure. (This general type of bilayer structure is abundant in nature. It is the same type of structure that makes up cell membranes.) Therefore these assemblies of molecules are able to anchor themselves through their polar groups to polar regions of the soil particles, forming the humus films that can adsorb pollutants. The grains that make up the inorganic (non-organic) part of soil, the mineral fragments, are ionic with a surface coated with positive and negative ions. The positive charges of this mineral ionic surface (Figure 8-8) attract the negatively charged carboxyl groups of the humus molecules and form an organic film on the inorganic mineral grain. If the entire grain or most of the grain is coated with such a film, the thin organic humus film will dominate the chemistry of the interaction

Chemical structure of humic

acid

Phenol


between soil and any pollutant molecules that come in contact with the soil grains.

The organic coating is apparently quite extensive since this model appears to explain the majority of the features of the top layers of soil. Figure 8-8 Bilayer structure of an organic film on a grain of soil. The bilayer is composed of two layers of amphiphilic molecules (see text) that are attracted to each other through their hydrophobic tails. Hydrophilic regions on one side of the bilayer are attracted to ionic regions on the soil particle and on the other side are exposed to dissolved pollutants, which they can adsorb. ––––––––––––––––––––––End of “click here” material–––––––––––––––––––––––

Interactions of metal and organic water pollutants with soil

We will focus on interactions of metal and chlorinated organic pollutants with soil particles. Metal ions can form covalent chemical complexes with humus carboxylic acid molecules (RCOOH) or can form salts with the ionized carboxyl ion (RCOO–), where R is a complex chemical structure attached to the –COOH group. Mineral matter in soil layers beneath the A horizon binds metals that are not attracted and bound by humic acids. Exposed mineral surface is found extensively in lower soil layers, where there is little or no humus coating. Most transition and heavy meals such as lead and mercury are removed from water by one of these lower soil layers. Chlorinated organic pollutants contain several chemical features that cause these pollutants to be strongly adsorbed to soil humus. The compounds that are of greatest concern generally contain one or more benzene rings and a number of chlorine atoms. Because of the rule "like tends to attract like," the benzene ring-

Interaction of pollutants

and soil


containing pollutants are attracted to the benzene rings in the soil humus and the highly polar carbon-chlorine bonds are attracted to the polar regions of the humus.

Figure 8-9 Chemical structures of chlorinated herbicides (chemicals that kill plants) 2,4-D and 2,4,5-T and a typical pesticide (chemicals that kill insects) carbaryl. Carbaryl, an insecticide (Sevin™) widely used on lawns and the herbicide 2,4-D, used for weed and brush control (Figure 8-9), have limited solubility in water because of their chemical structure. When these compounds are washed off plant material to which they are applied, both molecules are adsorbed on the attractive regions on the surface of amphiphilic humus molecules. The more hydrophobic parts such as the benzene ring regions of the adsorbed molecules can adsorb to the more hydrophobic interior of the humus bilayer. However, the humus mass is probably continually adding and subtracting amphiphilic molecules and, when this happens, molecules of Sevin and 2,4-D can make their way back out to the solution phase and migrate downward in the draining aqueous phase. Another possible fate for both the carbaryl and 2,4-D molecules during their soil entrapment is exposure to a microorganism that has enzymes (biological catalysts) that are capable of degrading these organic molecules. For 2,4-D molecules, one fate might be to liberate one or more chlorine atoms as Cl– ions, likely causing the molecule to be a less toxic molecule. The chemical reaction would be a reduction of the 2,4-D involving an electron transfer to a C–Cl bond, liberating a free Cl– ion. The carbaryl, with its fused benzene ring structure, may be biodegraded by a rare organism in the soil. Compounds with three or more fused benzene rings are called polyaromatic hydrocarbons (PAH). PAHs are usually carcinogenic and are generally difficult to biodegrade, although recent studies have identified soil and water organisms that are capable of performing this difficult task.

Pesticides and

herbicides


Migration of pollutants in soils

Many metallic and organic pollutants can be strongly adsorbed to the organic matter in humus and to the deeper clay particles in soil. Yet there is evidence that some of these pollutants migrate very slowly through the soil, ultimately contaminating deeper aquifers. Sometimes it takes decades for these contaminants to work their way through the soil. The reason for this migration was not immediately obvious nor was it expected because the water pollutants could bind so strongly to humus and to clay

particles. Humic acids are adsorbed in a number of different forms besides the coatings on large, immobile soil particles. Two of the most important forms are humus micelles and very small colloidal inorganic particles that have humus coatings. Both of these species are mobile in soil water and can slowly percolate down into the soil, sometimes taking many years to migrate from topsoil to groundwater regions. A micelle is an aggregate of molecules suspended in solution (Figure 8-10). Hydrophobic tails of humus molecules are tucked inside the micelle and polar head groups, usually carboxyl groups that are on the outside surface of a spherical assembly of amphiphilic molecules.

A colloid is a suspension of solid particles much smaller than the majority of soil particles. Suspensions of clay particles can form colloids. An excess of positive or negative charge on their surfaces stabilizes these colloids. These small particles, with either excess positive or negative charges on their surfaces repel each other (like charges repel) and thus avoid collisions that would otherwise cause aggregation of the particles into larger particles. If a small mineral colloid particle is coated with a bilayer of humus, the colloidal particle can be stabilized by the negative charges of the exterior carboxyl groups. Both micelles and humus-coated colloidal particles are able to move downward with the draining rainwater into deep soil layers. Any pollutant adsorbed to the surface of either of these types of particles or trapped in the interior of the micelle can be carried along with them if the pollutant is tightly bound. When these entities migrate to lower soil horizons, they may slow down to speeds considerably lower than those achieved in the A-horizon. Nevertheless, strongly adsorbed pollutants can still be mobile in the micelle and colloidal forms and, if not permanently entrapped on a large immobile soil grain, can ultimately work their way down into the ground water (Figure 8-11).

Role of colloids

and micelles in pollution migration

Figure 8-10 Cross section of polar amphiphile head groups on the surface of a micelle and the hydrophobic tails tucked inside the micelle. These are dynamic structures, with amphiphile molecules entering and leaving the structure often.





Contamination and drawdown of aquifers

The oxygen concentration begins to be depleted as one descends through the different soil layers where there is circulation of air, designated the vadose zone, until it becomes nearly zero when the water table is reached. At this point there is a zone of water saturation in the soil, the top layer of which is called the water table (Fig. 8-12). Below this point, nearly all the space between sand and gravel particles is filled with liquid water. Water is attracted above the water table because of capillary action due to the close packing of the soil particles and thus coats these particles with adsorbed water. However, because organisms consume oxygen faster than it can diffuse into the liquid water from above in the aerated vadose zone, the concentration of oxygen is zero or near zero below the water table. A subsurface region of loose rock particles containing large quantities of water is called an aquifer, which is used as a source of domestic and industrial water. This lack of oxygen results in important differences between chemical reactions above and below the water table. The water table and the aquifers below it are generally reducing zones, whereas the aerated zones above the water table are oxidizing zones. The organisms contained in the reducing zones below the water table are generally anaerobic organisms that use a chemical other than oxygen from which to obtain energy. Quite often this energy source is sulfate, which is reduced to hydrogen sulfide, H2S. In this region, the stable oxidation states of transition metals are usually different from those of the same metals under aerobic conditions and thus the location of the metal in the environment can affect the toxicity of the metal. For example, when present in a reducing environment, the iron (Fe) ion will usually be found in the iron (II) state (two valence electrons are lost). If one pumps well water containing iron (II) ion from an anaerobic zone up to the surface, the freshly introduced oxygen oxidizes iron (II) to iron (III). Iron (III) commonly precipitates as a rust-like precipitate. The extent of the reducing power of the environment depends upon the pH, the oxygen concentration, and the concentration of other oxidizing and reducing agents present. Minerals in contact with ground water contribute positive and negative ions through their dissolution and ionization. Through different tortuous paths in cracks

Water table and

aquifers

Hard water

Figure 8-11 Diagram showing the various forms in which a groundwater solute (S) may find itself: free in water or adsorbed to: an amphiphile, a rock (coated with bilayer), a colloid (coated with (bilayer) or a mobile micelle. The solute may desorb and move among all of these forms at different times and therefore work its way down deep into the soil, as far as the water table or possibly an aquifer.



and pores in bedrock, the dissolved ions, molecules, micelles, and colloids that originate in the upper layer of the soil can work their way down into groundwater and pollute aquifers (Figure 8-12). On the way through the soil, the water sample might pick up iron (III) calcium ions (Ca2+) carbonate ions (CO3

2–), magnesium ions (Mg2+), and sulfate ions (SO4

2–) from dissolved minerals, depending on the mineral content of the soil. If the ground water is high in calcium, magnesium, sulfate or hydrogen carbonate (bicarbonate) ion concentration, it is classified as “hard” water. These ions combine with soap, forming a solid and leaving a classic “ring” around the bathtub or washbasin. Water that joins an aquifer is joining one of the huge hidden bodies of fresh water, the groundwater pool that represents some of the most potable water sources available. A significant part of the US population depends upon groundwater in aquifers for drinking water. Because of the partial filtration and purification provided by the various layers of soil, the water in these aquifers is usually drinkable with minimum purification. That is, if it is not polluted because of the slow migration of pollutant-contaminated colloids or micelles mentioned previously.

Water from aquifers has become increasingly critical for agricultural use, especially for irrigation in arid regions or regions that are subject to extended periods of drought. In the Midwestern United States, the world’s largest aquifer, the Ogallala, is a huge eight-state (the high plains of Nebraska, South Dakota, Wyoming, Colorado, New Mexico, Kansas, Oklahoma and Texas) source of underground water that supplies a quarter of the water for US farmland irrigation. More water is being

Figure 8-12 Diagram of ground-water flow under the land surface, illustrating the effects on the water table of a water well, a stream, and the infiltration of a water pollutant into the ground water. Note that the water table is lowered near the pumping well and that the ground water from the stream flows away from rather than toward the stream as it does on the right.


pumped out than is being added to recharge this aquifer, leading to serious concerns about water shortages in this large region in the near future. The top of the Ogallala aquifer has gone in one generation in Texas from a depth of 95 feet to more than 335 feet, with only about 60 feet more until it runs dry. Drinking surface water in India and Bangladesh used to lead to many diseases, including cholera. In order to improve the quality of drinking water, many thousands of relatively inexpensive shallow tube wells have been drilled in Bangladesh and India. Unfortunately, this well-meaning project led to the world’s largest mass poisoning of a population. Many years following the introduction of the tube wells, they were found to be the source of new, mysterious skin rashes and cancers caused by arsenic (As) water contamination. The arsenic source was natural arsenic containing minerals in a geological feature common to both India and Bangladesh. Much research is now focusing on the geology, the hydrology and arsenic detection and filtration methods. Over 100 million inhabitants of India and Bangladesh are at risk from arsenic containing aquifers. Deeper wells are mostly free of arsenic, but if these aquifers are drawn down too fast, as they might be in regions with high population density, there is a danger of water from the upper arsenic-contaminated wells being drawn into the lower aquifers. Open shallow wells are not contaminated as much as closed tube wells because in the open wells, the water is exposed to air for days. Oxygen is able to diffuse into the water oxidizing the iron, forming solid iron oxides. The arsenic then adsorbs to the iron oxide surface and precipitates out of the water, drastically lowering the arsenic concentration. This does not happen in the closed tube well. Similar problems with arsenic contamination have been observed in China, Vietnam, Chile, and the United States. The problem is complicated by the difficulty in removing As contamination in common water purification processes. Runoff water from farms causes stream pollution problems from both animal waste and fertilizers containing nitrogen and phosphorous. A major cleanup of this runoff water can be provided by having vegetated stream buffers of about 60 meters width on either side of streams near farmland, with gentle buffer slopes. Planting trees, especially poplars, in these stream buffer zones is more effective than planting grass.

Tube well in Bangladesh


Water Purification Systems

There is a worldwide need for new water purification systems that are not technologically complex and highly centralized and that minimize residual chemicals and waste products. Water is purified rather effectively in the different parts of the natural hydrological cycle system. Scientists are beginning to imitate parts of this cycle in their attempts to provide both developed and developing nations with more “natural” systems that provide potable water and are less expensive and not environmentally damaging. The prime goal in water purification is to disinfect it by removing traditional and emerging pathogens (disease-causing organisms) without creating more problems due to the chemicals used in the disinfection process. Decontamination steps are also needed to detect and remove elements such as arsenic (As), heavy metals (Pb, Hg, etc.), and other toxic chemicals that are harmful to humans and to the environment. Some of these chemicals are toxic at very small concentrations. There are great difficulties in detecting and removing these contaminants when they are present at 3 to 9 orders of magnitude lower concentrations than other nontoxic dissolved substances. Chemically treating the total water volume to get rid of a trace contaminant is expensive. Low cost methods of detecting trace toxic contaminants and removing only these are urgently needed. Knowledge of oxidation states of elements is part of this challenge. For example, consider the elements arsenic (As) and chromium (Cr). As(III) is about 50 times as toxic as As(V). Cr(III) is an essential nutrient, yet can be toxic at high concentrations. Cr(VI) is highly toxic and workplace exposure to Cr(VI) is associated with increased risk of lung cancer.

Common water purification methods

In the US, there are 91 chemicals whose concentration is regulated in tap water. However, there are ~60,000 chemical compounds that are in commercial use. Hundreds of these that can cause cancer or other diseases are not similarly regulated. Arsenic is legal in drinking water at concentrations that can cause bladder cancer in 1 out of 600 people over a lifetime. There are risks of multiple contaminants, but nothing is being done about regulating for cumulative effects. One might think that highly complex methods must be used purify water to meet water purity standards.

Not necessarily. Water taken from aquifers, streams, and other freshwater supplies is first filtered through one to six feet of coarse sand, removing most of turbidity (suspended solids) and algae. Additionally, a chemical coagulant, usually a gelatinous Al(OH)3 precipitate, is added to further clarify stubborn turbidity. This chemical provides a gelatinous mass that provides a large surface area to adsorb

Water purification

Water purification system


colloids and other suspended materials. However, there is a disposal problem because of the chemical content and bulk of this sludge. The last step in most water treatment is to add chlorine (Cl2) to disinfect the water. This gaseous, diatomic molecule is a powerful oxidant and kills most pathogens in the water supply, notable exceptions being Cryptosporidium and C. parvum. Unfortunately, chlorine reacts with any remaining organic matter to produce chlorinated compounds. Some of these compounds have been cited as suspected carcinogens and some environmental groups are claiming that chlorinated compounds that mimic estrogen are being produced in the chlorination of drinking water. Natural estrogens that have been introduced during water treatment are reported to have caused reproductive problems in downstream aquatic life. The question being debated is whether the risk of consuming the chlorinated products present in the chlorine-treated water is greater than the risk of disease from inadequate treatment of the water by chlorine substitutes. Among the proposed substitutes for chlorine are ozone (O3) , hydrogen peroxide (HO2), ultraviolet light, and chloramine (NH2Cl). Some doubt has been raised as to whether any of these oxidizing agents are as effective as chlorine. Mixtures of these agents are also being considered, for example ozone treatment followed by chlorine. One problem with using ozone for disinfection is that if the water contains bromide ions (Br –), the ozone oxidizes this ion to form the potential human carcinogen bromate ion (BrO3

–). Sunlight is a natural disinfectant and new attention is being paid to artificial UV irradiation during the purification of water as an integral part of the disinfection process is being investigated. UV irradiation is usually used in conjunction with other treatments mentioned above. The decision of whether to utilize combination chemical treatments (e.g., ozone and chlorine) or to use filtration or adsorption to remove the offending chemical depends on cost, location, pollutant concentration, chemical nature and toxicology of the pollutant. There are usually problems with each treatment and no one universal water purification protocol has been adopted. The search for more effective water treatment protocols is a challenging, but urgent one, especially with the increasing worldwide water shortage. In developing countries, novel ideas are helping to reduce illnesses from contaminated water supplies. These range from cloth filtration (48% reduction in cholera) to chlorine bleach disinfection at the point of consumption to redesign of water containers. By simply making water storage containers with narrow mouths to prevent contaminated dippers from infecting stored water, illnesses traced to water consumption have been reduced. Boiling water is an obvious protective action, but heating fuel is often too scarce or expensive.

Chloramine


Reverse osmosis and desalination

One of the highest quality water purification systems is the reverse osmosis process. It is technologically complex and energy intensive and does have some other drawbacks, but will certainly be one of the purification systems called into increasing use with the pending water shortages around the world. In this process, water is forced under pressure through long, small, hollow fibers producing very high quality, highly purified water. This is because of the composition of the fiber, which has holes that are just slightly larger than the size of a water molecule. Thus only water molecules can move freely through the membrane. All ions, which are larger than water because they are all hydrated to some degree, are excluded from passage through the membrane. In addition, all biological molecules, including bacteria, viruses, proteins, etc., are excluded. However, all of the excluded ions, molecules, and biota become highly concentrated and must be discarded, which is not a serious problem for lightly contaminated water sources. However, when purifying ocean water or ground water with high salt content such as saline aquifers, there is a serious disposal problem with the highly concentrated brine that must be disposed of following the purification process. Desalinatization of water in combination with solar energy is becoming the source of water supplies for those in Arab countries where fresh water is in short supply. About half of the fresh water is multistage flash distillation in which seawater is heated with steam forming water vapor which is condensed by cold seawater, thereby heating it in a heat exchanger (Fig. 8-13). The other half is provided by reverse osmosis. An interesting modification of reverse osmosis is being called “forward osmosis.” In this technique, water molecules migrate by osmosis from a low salt impure water source through a water permeable membrane toward a high concentration ammonium salt solution and dilute this solution. Ammonia is then heated to remove it by distillation, with the pure water left behind. This technique uses significantly less energy than the reverse osmosis process.

Another unique process is that employing biomimetic membranes. These membranes have aquaporins embedded within them that preferentially draw water molecules through the membrane much like the kidney recovers water from urine.

Schematic of a multi-stage flash desalinator A - Steam in E - Steam out B - Seawater in F - Heat exchanger C - Potable water out G - Condensation collection D - Waste out H - Brine heater Fig. 8-13 The incoming cold seawater (B) is heated by the hot water vapor which condensed into pure water as it gives up its heat to the incoming seawater. Additional heat is added by the steam in the heater to make the seawater hot enough to vaporize some of the water. (H). Pure water is collected from each stage and exits the apparatus (C).


Aquaporins are proteins found in cellular membranes and act like the plumbing system for cells by moving water molecules in single file in and out of the cell.

Some molecules that are stubborn pollutants such as polyphenols, nitrites,

polyaromatic hydrocarbons, cyanides, and heavy metals are difficult to remove. In such cases, enzymes are used that either bind or degrade these compounds. Modern nanotechnology using very small embedded silver particles or nanowires or carbon nanotubes are used to inactivate bacteria and other unwanted biological entities.

Bottled Water

About one third of the American public drink bottled water regularly. With water shortages looming in certain parts of the US and emphasis on the health implications of consumption of large amounts of water, the search for convenient sources of drinking water has lead to large increases in the consumption of bottled water. The combination of the disposal problems with plastic bottles, leaching of compounds from the plastic into the water, the transporting of this water long distances (e.g., from Fiji), the question of purity when compared with tap water (which has more stringent purity standards) has lead to questions of both the economics and environmental damage from this trend. It takes 3 liters of source water to make 1 liter of bottled water. By some accounts, as much as 40% of this product is filtered tap water. The price of bottled water is anywhere from hundreds to many thousands of times the price of tap water.

Water quality

Water specialists prefer the term "water quality" to “water purity.” Scientists who have tried to purify water so that absolutely no contaminants are left have given up trying to achieve this goal. Modern measurement techniques are now so sensitive that they can always detect some impurities, no matter how highly purified the water. “Water quality” is defined in terms of the degree to which it is contaminated and how this contamination interferes with the desired use of the water. Water uses can be for drinking, aquatic life propagation, irrigation, recreation, aesthetics, or for an industrial water supply. Each of these water uses makes its own special demands on the purity of the water supply. What is an “allowable" contaminant for one use may not be acceptable for another. For example, a small amount of bacterial contamination, which causes the water to be unfit to drink, can be tolerated in many industrial processes. Some trace metals must be removed from the water used in certain industrial processes, while they are easily tolerated in drinking water.

Water quality


Regulation of water quality

According to an extensive study by the National Resources Defense Council (NRDC), the Federal Drug Administration (FDA) completely exempts 60-70% of the bottled water sold in the United States, water that is packaged and sold within the same state. Nearly 40 states say they do not regulate such waters. Even when the FDA regulates these bottled waters, the regulations are weaker in many ways than the US Environmental Protection Agency (EPA) regulations for tap water. City tap water can have no confirmed E. coli or fecal coliform bacteria (bacteria that are indicators of possible contamination by fecal matter). FDA bottled water rules contain no such prohibition (a certain amount of any type of coliform bacteria is allowed in bottled water). Most cities using surface waters have had to test for Cryptosporidium or Giardia, common pathogens that can cause diarrhea or more serious problems in certain vulnerable people. Bottle water companies are not required to do this. Tap water test results and notices of violations must be reported to state or federal officials. There is no mandatory reporting for water bottlers. City water system operators must be certified and trained to ensure that they know how to safely treat and deliver water – not so for bottlers. In fairness, EPA reported recently that 1 in 10 community tap water systems violated EPA’s tap water treatment or contamination standards. Therefore the NRDC concludes that “there is no assurance that bottled water is any safer than tap water. Other academic and government bottled water surveys generally are consistent with the testing NRDC commissioned (tests were done in independent laboratories)…these studies also found that most bottled water meets applicable enforceable standards, but that a minority of waters contain chemical or microbiological contaminants of potential concern.” The Environmental Working Group states that 18% of the water bottlers do not reveal the source of their water and that one third disclose nothing about the treatment or purity of their water inside their plastic bottles. This organization recommends that consumers use filtered tap water, which should be purer than tap water. There is disturbing report regarding studies of tap water contamination by anti-biotic resistant bacteria. Tests for these bacteria in water from several cities in Michigan and Ohio were performed on water taken directly from: the unpurified water source, the “finished” (purified) water at the water purification facility, water entering the home, and water taken from the tap. The highest concentration of these bacteria were found coming from the tap. These tests obviously need to be repeated under different circumstances and, if they are repeatable, further research will be needed. One of the serious problems with bottled water is the bottle. 2.4 million metric tons of PET (polyethylene terephthalate) derived from ~1.5 million barrels of oil are


used to make these bottles. Nearly 90% of these bottles wind up as litter or goes to the landfill. The problem has become so severe that San Francisco does not allow any purchase of bottled water in the city.

Health effects of impure water

Definitions of water quality generally involve a quantitative measure of the impurity concentration and whether or not the water will cause harm to living organisms or interference with an industrial process. To make regulatory decisions regarding potential health hazards of water pollutants, a dose-response relationship must be acquired for the pollutants. That is, some knowledge must be gained regarding the effects on human health as the concentration of each pollutant is increased. Regulatory agencies want to know at what concentration the pollutant affects individuals who are most sensitive to that pollutant, and then set an exposure limit at a concentration below that limit. Quite often, animal experiments are used to determine this level because of an assumed similarity in physiological responses to pollutants between certain animals and humans. However, surprisingly great variations in tolerances may exist from species to species for different pollutants, so it is important to find an appropriate animal model for human responses when these types of experiments are conducted, a desirable goal that is often not achieved. Some oppose the use of animals in such experiments, and scientists are searching for acceptable substitutes, such as human cell cultures.

Health effects of pollutants are usually measured with respect to either acute or chronic symptoms. Exposure to very large concentrations of pollutants, called acute exposure, will generally cause a dramatic response, ranging from death to temporary or permanent disability. Many environmental agencies enact legislation based upon these responses. However, it is very difficult to accurately measure chronic exposure symptoms that result from the effects of exposure to small concentrations of pollutants over an extended period of time. Animal experiments for such studies are expensive, and extrapolation to human beings is more suspect, since tolerances at these low levels may be different in humans and animals. It is also very difficult to separate out the effects from other pollutants usually present.

Risk-benefit considerations in pollution mitigation

In many cases, it is not possible to completely eliminate a pollutant at its source. Environmental regulations and regulators must have some criterion to make decisions regarding the allowable amount of pollutant that can be released into the aquatic environment. How much of the pollutant can be tolerated both from a health and a quality-of-life point of view, and what are the resultant costs to society of releasing different amounts of the pollutant into the environment? When does the cost of

Dose- response

relationship of

pollutants


reducing the concentration of a pollutant exceed the benefits obtained by lowering the pollutant’s concentration? Is it possible or desirable to reduce the concentration of a pollutant to zero? What is the dose-response relationship for the pollutant? These are all questions to be answered and decisions that must be made by government anything more than qualitative answers and there is, of necessity, much guesswork in estimates involved in risk-benefit calculations. For example, if there is some evidence that one person out of 1,000,000 will die by being exposed to a particular concentration of a pollutant, what cost ought to be associated with the loss of that person? We will use as an example of risk analysis the presence in drinking water of lead, one of the more dangerous heavy metal pollutants. A large number of children who ingested chips from peeling paint containing lead have suffered from mental retardation traced to this exposure. Earlier in the United States, extensive amounts of lead were released from exhausts of motor vehicles. Because of this, social and political pressures were brought to bear on the gasoline industry to remove tetraethyl lead [Pb(C2H5)4], an antiknock additive, from gasoline. As a result, the industry now offers “unleaded” gasoline. From 1976 to 1980 lead levels in the blood of the average person in the US. decreased from 146 to 92 micrograms per liter. The primary reason for this decrease was due to the introduction of unleaded gasoline formulations. However, the presence of lead in blood also originates from exposure to dust, lead paint, and contaminated drinking water.

In most cases of lead-contaminated drinking water, the lead does not come from the water source, but from household plumbing. In some older homes, lead pipes feed water into the house. In other cases, leaded solder is used in copper pipe joints. Some brass faucets have lead in them. When water comes in contact with the lead pipe, solder, or brass, corrosion can occur while the water is standing in the presence of a lead source for hours to days. Lead contained in plumbing is in the

form of the solid metal, represented in chemical equations as either Pb(metal) or Pb(s). All of the metal atoms in solid lead share valence electrons. One of the characteristics of metals is that they have a low electronegativity. That is, metals do not have as much affinity for their valence electrons as other, more electronegative elements. In addition, slightly acidic water that comes in contact with in the water will accept electrons from the metal and cause it to dissolve (corrode) to form the

Risk-Benefit analysis

regulatory agencies. Many of these questions do not have


aqueous lead ion, Pb(aq)2+. We can represent this process for lead in the plumbing in

the following equations:

Pb (s) → Pb(aq) 2+ + 2 e– (8-2)

2 e- + 2 H(aq) + → H2(gas) (8-3)

________________________________________

Pb (s) + 2 H(aq) + → Pb(aq)

2+ + H2(g) (8-4) = (8-2) + (8-3) Equation (8-4) is a reduction reaction showing the reduction of the hydronium ion, although there are many other potential chemicals that could be reduced by accepting electrons from the solid lead. In equation (8-4), which is the result of the addition of equations (8-2) and (8-3), “2 e–"” does not appear because when it appears on both sides of the equation, after the addition of the two equations, it can be canceled out on both sides. The above set of reactions represents reduction (of the hydrogen ion) and oxidation (of the lead) reactions, or in chemistry jargon, equation (8-4) is a redox reaction. This type of reaction generally represents the transfer of one or more electrons from one element or group of elements to another element or group of elements. Redox reactions represent one of the several different general classes of chemical reactions. Thus, lead ions dissolve in standing water in lead-containing pipes or plumbing fixtures containing lead. This water contains dissolved carbon dioxide and, therefore, also contains carbonate ions (CO3

2–) because of the following set of sequential chemical equilibrium reactions:

CO2 + 2 H2O H3O+(aq) + HCO3

–(aq)

(8-5)

HCO3–(aq) + H2O H3O+

(aq) + CO32–

(aq) (8-6)

Carbonate ions combine with lead ions to form insoluble lead carbonate solid:

Pb(aq)2+ + CO3

2–(aq) PbCO3 (s) (8-7)

Thus reaction (8-7) indicates that we can minimize soluble lead ion by converting it into the relatively insoluble solid PbCO3, which is found to coat the inside surface of water pipes. However, as is indicated by the two arrows, equation (8-7) as well as equations (8-5) and (8-6), are equilibrium reactions. Remember, equilibrium means that the reaction goes both forward and backward at all times with equal rates when at equilibrium. Thus, some lead remains in solution as ions at all times. The amount of lead in solution depends on the relative rates of the forward and backward reactions. However, these rates depend upon the concentrations of each of the reactants and

Lead chemistry

in plumbing and water

quality



products. The soluble lead ion [Pb(aq)2+] moves into the body with the ingested water,

and is able to move through the blood/brain barrier and, when accumulated in large enough quantities, can cause brain damage, especially in small children. Our chemical assessment of the risk of finding lead ion in drinking water reveals that there will always be lead in drinking water as long as the water has had some contact with metallic lead. Two questions arise: (1) can we get rid of the offending lead plumbing? (2) If this is too expensive, can we lower the lead ion concentration in drinking water to acceptable levels where little or no toxic effects are found?

Risk management

Using the above example of the risk analysis of lead in drinking water, we now illustrate how this risk can be managed. Note the use of the word “management” rather than “elimination,” since it is impossible to remove every single lead ion from a glass of drinking water. What steps involving chemistry can be taken to minimize the risk? The obvious solution to lead-contaminated water is to remove and replace the offending lead pipe, pipe joint, or faucet. However, the cost of this remedial action is often prohibitive. In certain parts of Scotland, for example, where much of the household plumbing contains lead, the costs of removal and replacement were prohibitive. Another way had to be found to manage this risk. One move, which was required by law, was to lower the lead content of solder, which is now banned in solder sold in the US. Another suggestion made to the general public, which immediately lowers the health hazard, was to run water a short time through a faucet before drinking it. By this technique, the standing water in contact with the lead in the plumbing is flushed out and replaced with fresh water. Still this was not enough. Another treatment employed, which greatly reduced the lead ion content, was based upon a fundamental principle pertaining to equilibrium systems, and uses the deliberate addition of chemicals containing carbonate ion to lead-contaminated water to reduce the lead ion concentration.

Risk management


Figure 8-14 Equilibrium between solid lead carbonate and aqueous lead and carbonate ions. Le Châtelier’s Principle states that when a chemical equilibrium is stressed (altered suddenly), the equilibrium shifts so as to relieve that stress (sudden change). A chemical equilibrium consists of two opposing chemical reactions, a “forward” and a “reverse” reaction, which have equal rates when at equilibrium. In the case of equilibrium reaction (8-8), the forward reaction represents the collision and chemical combination of the aqueous lead and carbonate ions to give insoluble lead carbonate (Figure 8-14), and the reverse reaction is the breaking apart of this lead carbonate solid into aqueous Pb2+ and CO3

2– ions. The speed of the forward reaction will depend on the solution concentrations of both the lead ion and the carbonate ion. Increasing the concentration of either of these aqueous ions will increase the probability of a collision between the oppositely charged ions and, therefore, will increase the rate of formation of the solid lead carbonate. Suppose the concentration of the carbonate ion is increased significantly by adding to the water supply a soluble salt containing carbonate ions. Free lead ions will collide more often with the carbonate ion and form solid lead carbonate, which adheres to the wall. When the reverse reaction yields a lead ion, that lead ion will quickly react with the excess quantities of carbonate ions present. The net result of both the forward and reverse reactions in the presence of excess carbonate ions is that the steady state concentration of the aqueous lead ion at the new equilibrium is noticeably smaller than in the absence of added carbonate ion, as symbolically illustrated in Figure 8-15.

Le

Chatelier’s Principle applied to the lead

problem


Figure 8-15 Effect of the addition of carbonate ions on the lead ion concentration in a lead pipe. Increasing carbonate ion concentration decreases lead ion concentration. Thus, to lower the lead ion concentration in drinking water, one can increase the steady state carbonate ion concentration. Water containing high carbonate ion concentrations is found in regions that obtain their water from sources near limestone (CaCO3) deposits or other reasonably soluble carbonate minerals. Therefore, the lead problem is most serious where water contains very little carbonate ion from natural sources. In such regions, carbonate can be added to the water supply to reduce lead concentrations in drinking water to acceptable levels. Carbonate ion is not harmful in the quantities needed to bring the lead ion concentration to more tolerable levels. The solution to the Scottish lead pipe problem was to add a soluble carbonate compound, such as sodium carbonate (Na2CO3) to the drinking water in regions that did not contain natural sources of carbonate ion in the water. Lead phosphate is also a relatively insoluble salt. Thus, lead ion concentrations can also be minimized by adding a salt containing soluble phosphate ions to the drinking water. These solutions to this problem are an example of putting principles of chemical equilibrium to work to protect the health of the consumer and are also good examples of risk management.

Purification at the water tap

Another alternative for water purification is to install a water filter at the tap for water purification. The chemicals contained in such a filter have binding sites containing positive ions that are displaced by more strongly bound metal ions in the drinking water. In the case of lead pollution, the positive lead ion displaces either the positive hydrogen or sodium ions contained in the ion exchange material (Fig. 8-16). Water filters usually also contain activated charcoal filters that have large surfaces that readily adsorb organic water pollutant molecules. The charcoal often originates from charred coconut waste and is sometimes impregnated with fine silver particles, which attack bacteria and prevent mold and algae. Combinations of ion exchange and charcoal filters remove most harmful pollutants. However, it is important to recognize that each of these filters has a limited capacity for pollutants. They must either be replaced or recharged on a regular basis and, if not carefully maintained, can contaminate water rather than purify it.

Deliberate shifting of equilibria


Purified water is used for many purposes in the home and in industry. In most of these uses, there are substances that dissolve in the water or are deliberately added that make it less pure, sometimes considerably so. This poses the problem of what to do to re-purify the contaminated water after discharging it back into the environment as sewage.

Sewage Treatment

Considering the relatively simple treatment given sewage, it is surprising that more health problems do not arise from sewage treatment effluent. The reason for the success of this treatment is the complexity of the responses of the biological systems that are employed to treat the sewage. Again, the treatment makes use of biological systems that have evolved naturally to treat animal and other wastes. In the United States, on the order of ~150 gallons of water per day per person are fed into a sewage treatment system, where the challenge is to rid this effluent of dangerous microorganisms and toxic chemicals. Treatment at most sewage plants consists of a series of steps that make use primarily of biological organisms. Sewage

Sewage treatment

Fig. 8-16 Illustration of chemical ion exchanges in an ion-exchange resin. “Hard” water containing Mg2+ and Ca2+ ions exchange these ions with Na+ ions attached by their positive charge to negatively charged resin beads. The Mg2+ and Ca2+ ions firmly attach to the resin beads liberating the Na+ ions into the water, making it now “soft.” When all resin beads are depleted of Na+ ions, the ion exchange resin must be again charged with a sodium salt such as NaCl.


treatment is essentially doing what nature would do, given less of a load of pollutants to purify, doing it in a smaller space, and in a short period of time. The keys to success are the availability of plenty of oxygen, the right organisms, and no interference with the natural system of water purification.

The first two of the three stages in sewage treatment (Fig. 8-17) are mostly biological. They involve providing hospitable surroundings for anaerobic and aerobic enzymatic digestion of the large variety of microorganisms and organic compounds delivered in the sewage. In the first stage, called the “primary treatment,” large objects, sand and grit are removed from the raw sewage and the remaining mixture is allowed to sediment (allow solids to settle out to the bottom layer). This sediment is recycled (Fig. 8-17). In the second stage, called “secondary treatment,” the liquid from the primary stage is fully aerated or bubbled with pure oxygen or air in the presence of either free floating or suspended bioorganisms (bacteria and protozoa) that digest and degrade the suspended organic matter. These bioorganisms adhere to a solid medium in trickling filter and rotating biological contactor methods or are suspended in the liquid in the activated sludge method. In all of these methods, there is agitation of the system to assure that the liquid comes into intimate contact with the bioorganisms, so that the liquid can be processed as quickly as possible.

Fig. 8-17 Diagram of a typical sewage treatment plant.


In the third, or “tertiary treatment” phase, the effluent from the secondary treatment phase is treated with one or more of the following processes: sand filtration or activated charcoal filtration, contact with reeds, filter feeding invertebrates. These latter natural processes remove nitrogen and phosphorous compounds. There may be some chemical “polishing” or “disinfection” at the end of the treatment such as chlorination and dechlorination, but microorganisms and higher order species do the bulk of the work of ridding sewage of unwanted organic and inorganic matter. One interesting combination of human-constructed and natural purification systems is where treated sewage plant effluent is discharged into slow moving natural wetlands before being released from these wetlands into streams. Such natural post-treatment significantly decreased estrogen activity of the effluent and removes nitrogen compounds. Once more, the incorporation of natural systems that have evolved over millions of years into human recycling efforts is shown to be effective. All organic compounds, whether they are estrogens, pharmaceuticals, or industrial chemicals are all high-energy compounds that potentially can be used as energy sources (food) for a microorganism. Given the richness of microbial life in soil, it is likely there is an organism somewhere that can use the waste organic compound as a food source. The challenge is to find the appropriate organisms and an economically viable home for it to aid in recycling of organic waste. A relatively recent development is the introduction into the secondary treatment phase of membrane bioreactors (MBRs). In this rapidly developing process, suspended biomass, similar to that found in the activated sludge method, is put in contact with immersed microfiltration or ultrafiltration membranes. These membranes contain very small holes of varying sizes that do not allow suspended solids and, in the case of ultrafiltration, microorganisms through the membrane. A serious problem with this system is the fouling (plugging) of the membrane, with the expensive need to clean or replace the membrane. Chemical modification of the membrane to render it last longer before fouling have improved the economics, but this method, though it has faster throughput, is more expensive than the older, more conventional methods. One problem with all of the sewage treatment processes is the difficulty in completely removing relatively small molecules and ions. For example, there is increasing concern regarding the effects on wildlife and humans downstream from the sewage outfall of pharmaceuticals and hormones that escape the treatment process. In particular, there is great concern regarding the chemicals released from sewage treatment plants that mimic estrogens. There is already strong evidence that these chemicals are causing changes in sexual behavior in wildlife, such as frogs and fish, in contact with waters contaminated with estrogen-like contaminants. Since much solid organic matter is a waste treatment byproduct, some thought has been given to using this energy rich material as an energy source. Much of the


organic solids that does not dissolve in the treatment process is currently used for fertilizer. The sewage treatment liquid waste of course is returned to the stream, sometimes containing unwanted dissolved chemical waste that is treated and purified by natural processes in the stream. However, certain sewage effluents that are rich in nitrogen and phosphorous compounds are affecting stream life by providing excessive amounts of nutrients for growth in a stream that does not naturally provide such high nutrient levels. This is a situation in which treatment effluent can be further treated by flowing it through marshland where the excess nutrients are taken up by natural vegetation.

Stream pollution

Streams can be so overloaded with excess organic material that aerobic organisms multiply rapidly and, in doing so, lower the oxygen concentration to undesirable levels. Biological oxygen demand (BOD) is a measure of this load of excess organic material in a stream. When BOD is too high, oxygen levels become low and stream quality suffers, sometimes even causing fish kills. When water rushes over rocks or

waterfalls, it fully aerates and can be purified through natural processes. However, the water in a rushing brook is generally not safe to drink because its source and contamination level is unknown. It does take time, and therefore distance, for streams to naturally clean up organic-based contamination. Some underground streams pose a different problem in that they may run through abandoned mines and are noted

for their high acid content, thus introducing a different type of contaminant that cannot be as easily purified by a biological system, and that sometimes is responsible for the demise of biological systems that would otherwise help purify the stream. Waterpower dams constructed during the past century have disrupted natural ecosystems, including preventing fish from moving upstream to spawn. Many of these dams are now being destroyed in an attempt to recover the natural ecosystems that took millions of years to evolve. Migrating fish are part of that ecosystem. Streams and rivers generally flow past urban areas with manufacturing plants that remove large quantities of water. Water is drawn into manufacturing plant water intakes during its passage down the river. This water is used for cooling and manufacturing purposes, often resulting in thermal and chemical pollution when much of the water is returned to the stream. Thermal pollution is the increase in the returned water temperature that causes lowering of the stream’s oxygen

Stream pollution


concentration, often converting the stream into an unfavorable environment for some aquatic species. In manufacturing plants, river water is used in industrial processes employing water as a solvent, e.g., in a copper electroplating process. Even though the water is normally treated before being released, there is an occasional release that is above the allowed Environmental Protection Agency (EPA) levels of toxic metals or organic compounds. Even though some of the pollutants, such as metal ions, are essential for aquatic life, excess concentrations can be toxic to these species. The key to metal ion toxicity is bioavailability. In other words, is the metal in a form in which it can be consumed by the organism and interact with the organism in a detrimental way? If the metal is in a complex chemical ion or compound that is not assimilated or digested by the organism, it may be non-toxic. Some of the metal ions are of such a form that, when they contact river sediments, they are irreversibly bound. Other metal ions adsorb to silt that washes off agricultural fields and that is suspended in the water stream. There is concern that antibacterial soaps and detergents that escape sewage treatment are building up to high enough concentrations in streams that they help to

produce drug resistant bacteria that are an integral part of water ecosystems and may harm these important parts of stream ecology. The same is true of pharmaceuticals that are discarded or that originate in human waste. Some of these compounds are not easily biodegraded or are consumed by stream organisms. The concentrations of these compounds, as well as illegal drugs such as marijuana,

heroin, and methamphetamine, as well as natural and synthetic estrogens, are increasing in rivers, streams, and bays. Analyzing concentrations of drugs or their metabolites in sewage treatment effluent from a particular urban area is being used to monitor trends in the use of various illegal drugs in urban areas. Caffeine is being used as a marker for determining when waste treatment plants exceed their capacity during storms. Particulate matter contained in rivers has a surface reactant capability that depends on the size of the particle. Scientists are only beginning to appreciate this difference as they study the reactions of mineral nanoparticles of different sizes. Reaction rates on these particles have been found to be strongly dependent on their size. Very little attention has been paid to these particles in past studies. With the introduction of manufactured nanoparticles in its infancy, the disposal of these nanoparticles in natural streams and elsewhere is beginning to be of concern. When a river flows through a large city, it receives runoff water from streets with salt and other road and urban debris. This runoff includes animal waste with all


of its pollution potential. Harbor and riverboat traffic introduce oil slicks and bilge that increases the chemical load in the river water.

Lake Pollution

One of the important pollution problems with lakes is the accumulation of the outputs of contaminated rivers and of the many factories that use the water from the lake. Silt from contaminated rivers accumulates at the mouths of these rivers and acts to adsorb pollutants carried down the rivers. The metals and organic compounds, carried from the industrial sites along the river, have accumulated in this silt over the periods when pollution control was lax or non-existent. During periods of heavy storms, some sewage treatment plants along the river are unable to accommodate the heavy influx of rainwater coming into their facilities and release raw sewage for short periods of time into the river and ultimately into the lake. Because of this, coliform bacteria counts from the sewage can become high enough to close lakefront beaches to swimming.

Environmentalists have pointed out the changing nature of the wildlife in the shore communities and lake islands on a number of the Great Lakes in the United States. Fishing is no longer advised in some areas along these lakes because of the high concentrations of metals, especially mercury, in the fish. In some areas, fish may not be sold from the lake with concentrations of mercury over 1 microgram per

Lake pollution

Fig. 8-18 Pollution sources in the Great Lakes. “AOCs” represent areas of pollution concern


gram of fish. Mercury, found in the lake in the chemical form methyl mercury (CH3Hg+), interferes with enzyme reactions in the brain and can lead to mental retardation. It is especially important to limit intake of this compound by children and those who are pregnant. Some environmental groups and several researchers claim to have identified the reason for the loss of species diversity in areas around the Great Lakes. The claim is made that the chlorinated compounds coming from many different types of pesticides, herbicides, dioxins, and PCBs, formerly used in electrical transformers, as well as from the chlorination of drinking water, are interfering with hormone

receptors in the cells of birds and animals by mimicking hormones and are causing the “feminization” of many species. The claim is that male hormone levels are lowered because of the chlorinated compounds, with a resulting loss of fertility. Claims also are being made that similar effects are being observed in terms of loss of sperm count in human males over the past decade. There are also claims that pesticides cause cancer among agricultural workers and base their claims on animal experiments in which laboratory animals are exposed to these pesticides.

Other scientists believe that the concentrations of manufactured pesticides present in the environment is insignificant in comparison with the natural pesticides produced by plants themselves. They maintain that cancer and hormone mimic risk claims are exaggerated because the concentrations utilized in animal experiments are much too high to be extrapolated to humans. They claim that millions of tons of chlorinated chemicals are being emitted by natural systems, such as algae, kelp, fungi, and vegetation. Other studies claim that there is no extensive evidence of sperm loss in males. The claims and counterclaims have been heated at times. Some groups have threatened to enact laws forbidding the use of any chlorine-containing compounds in the region. Chemical spokespersons counter that chorine-containing compounds are ubiquitous in the chemical industry and this type of law would have serious consequences in areas of real human need, such as the pharmaceutical industry, where chlorine-containing compounds are used extensively in the manufacture of drugs.


Pollution in the bay regions

The bay is another one of the places where nature has evolved a novel method for cleansing water running into it. Bay estuaries are very important in the biological

cycles of marine life and are generally more productive per acre than even rich agricultural lands. This is because these estuaries are the nurseries for ocean fish and other marine life. A series of interesting events occur when river water meets the ocean water in the bay. The flow rate slows down, the deep river channel

narrows, and the land area covered by water greatly expands. There are now tides to contend with and the river gives rise to wide tidal bay flats or salt marshes that are regularly covered and drained as tides ebb and flow. In the region where saltwater from the ocean meets fresh river water, the ability of the river water to hold all of the dissolved substances in it suddenly changes and some of the dissolved material comes out of solution as a suspended precipitate. Suddenly, because of complex chemical factors, the river water that has just mixed with salt water in the upper reaches of the estuary becomes turbid because of the formation of solid material from the dissolved compounds in the river water. Much of the solid material formed is deposited in the sediments at the bottom of the estuary and over the tidal flats. This sediment is in intimate contact with many dissolved chemicals as the tides come in and out. Salt marshes have developed as a filter for nutrients that help make the salt marsh very productive in nurturing aquatic life. At the same time, the salt marshes act as filters for certain toxic pollutants. As the estuary water moves down the bay (Figure 8-19), the water, increasing in its salt content (salinity), suddenly starts to become less turbid. A survey of the variety and number of living organisms shows very large numbers in comparison with either the river or the ocean. In this region, the bay has its highest productivity, from the smallest microorganism to the largest bay fish. That is, of course, if the nutrient load, e.g. nitrate ions, is not too high (as it has been in the Chesapeake Bay), and if the oxygen concentration is not too low. Since the bay is so productive, oxygen is consumed in massive amounts. The continual tidal flushing of the salt marshes replenishes the oxygen.

Bay pollution

The geochemical

filter


If we look at the input of metals at the top of the bay and output of metals into the ocean, we see a general trend in the chemical nature of the metals that are retained in the bay. Among the transition metals, the ones on the left part of the periodic table are more prone to be adsorbed in the sediments in the upper bay region and deposited there. These metals such as iron (Fe), cobalt (Co), and manganese (Mn) are removed by what is called the “geochemical filter.” Metals that escape the geochemical filter into the less turbid waters of the lower bay are then subject to the “biochemical filter” that consists of the large number and high density of organisms growing in the bay waters. Metals are critical to the functioning of many of these organisms and are incorporated in them from the bay water. However, as these organisms die, they release the metal back into circulation, unlike the case of the metals trapped in the geochemical filter. These metals, as a group, are those on the right-hand side of the transition metals in the periodic table. Such elements as copper (Cu) and zinc (Zn) are “conservative” elements. That is, approximately the same amount of these metal ions is put into the bay as comes out of the bay into the ocean during a given period of time. Recent research on the Delaware Bay has revealed that significant amounts of metal pollution in the bay is deposited from the air through snow and rainfall. The various metal pollutants are directly correlated with weather patterns. For example, when the wind comes from the sea, very little metal pollution (other than sodium from sea salt) is found in comparison with the extensive mix of metals deposited when the wind comes from heavily industrialized regions. If these metals from the industrial areas are falling into the Bay, it is certain that they are also being deposited on land.

Figure 8-19 Cross section of an estuary where fresh, river water meets salty ocean water.

Dissolved salts come out of solution and cause high turbidity in the upper region of the estuary. In the lower portion, the clearer water gives rise to very high productivity of marine life.

The biochemical

filter



Ocean Pollution

Oceans dominate seventy percent of the Earth’s surface. The oceans are the primary source of protein for a billion people. Microbes in the oceans make 50% of the oxygen we breathe. For most of human history, the oceans have been treated as infinite bodies of water, with little or no concern that they might become polluted. However, during the last several decades, it has become increasingly obvious that the combination of population increases and industrialization have created one more, very large area of serious pollution concern, the world’s oceans. Just as there are many different ecosystems on land, the oceans have highly developed ecosystems. There are at a minimum 250,000 known species, with many, perhaps in the millions, of other unidentified. Coral reefs, which have been called “rainforests of the sea,” are the most biologically diverse oceanic ecosystems. Pollution is beginning to affect these ecosystems in unanticipated ways, such as in the bleaching of ocean corals. As of 2004, 20 percent of the world’s reefs had been destroyed and 24 percent were under imminent danger of collapse. Two causes of this have been identified as increasing water temperatures from climate change and pollution originating from land sources. More recently, the acidification of the world’s oceans has been recognized as a potentially serious threat to ocean life. Ocean pollution consists of river-borne pollutants that are not removed by the estuaries, and the result of marine activity, such as ocean dumping, oil spills, floating debris originating from fishing and other ocean-going vessels, especially non-biodegradable plastics such as Styrofoam cups, etc. Ocean pollution also originates from contact with polluted air. Especially important is the role of uptake by the oceans of excess CO2. Oceans absorbed 30% of all CO2 produced by humans. Acidification of ocean waters (Chapter 6 link) affects ocean ecosystems. These ecosystems can possibly adapt to acidification, but the rate of change is faster then ever before in the geological history of the Earth. Acidity from excess CO2 will probably double by end of century.

The ocean is covered with a thin sheen of hydrophobic material coming from oil spills, the flushing of the holds of oil tankers while they are at sea, and petroleum product input from rivers and from natural hydrocarbon sources in the sea. Oil from undersea seepage was in the ocean long before oil drilling and oil tankers, and microorganisms have developed both in the marine and the land environments to take care of this type of natural pollutant. However, hydrocarbon pollution uptake by these organisms can be slow, and natural degradation of the hydrocarbon pollution can be easily overwhelmed by major oil spills. The two most prominent of these spills are the Exxon Valdez in Alaska’s 1989 and the more recent Deepwater Horizon oil platform blowout in the Gulf of Mexico in April, 2010, both of which caused massive economic hardships and affected sea life in significant ways – and probably still is doing so. The Exxon Valdez oil tanker spilled at least 11 million barrels of oil into Prince William Sound in Alaska and covered 1,300 miles of coastline. Greater than 100,000 seabirds, 2,500 sea otters, 300 harbor seals, 20 orcas, billions of salmon and


herring eggs were killed within a short time following the spill. However, the long-range effects were still being felt twenty years later and may last at least 10 more years. For example, the long-term effects on wildlife of a Valdez oil layer buried under beach sand are unknown. Oil from the Deepwater well blowout met with various fates. Lighter fractions evaporated and some surface oil was burned off. During the massive leak, large amounts of the oil dispersant Corexit were mixed with exiting crude oil at the ruptured wellhead. This dispersant is a mix of solvents that break up the crude oil into small globules that are supposed to remain suspended in the water. The dispersant is a surfactant that binds to the oil slick and, with wave action, lowers the energy necessary to break up the oil slick. The object is to increase the surface area so that organisms can react with and consume the oil faster. Although significant amounts of oil did make it to Gulf shore beaches and marshes, much oil sank to the ocean floor and its fate is being studied. As of this writing, relatively few independent studies of the remaining oil and the overall impact of the Deepwater Horizon oil spill have been reported. Of those that have been reported, there is ample evidence that there are significant amounts of oil on the seafloor near the spill site. However, the long-term effects of the spill, as in the Exxon Valdez spill, remain to be seen. The lowest molecular mass components of crude oil evaporate into the atmosphere, are attacked by the OH radicals in that region, and oxidized by atmospheric oxygen. The oxidized hydrocarbon products ultimately fall back to earth as a component of rain. Of the material left in the ocean, the lightest fraction is the most water-soluble and is the most toxic to marine wildlife. The heavier components are more viscous and coagulate into tar balls. Tar balls of greater density than water sink to the ocean floor while those lower in density than water float in the ocean until they are washed up on shore. The comparatively small surface area of these tar balls slow the degradation rate by microorganisms that are able to digest these hydrocarbons. Consequently their lifetime on the beaches is longer than bathers would like. During the Exxon Valdez spill, one of the methods employed in accelerating the natural oil cleanup during this episode was to fertilize the oil-contaminated beaches with nitrogen- and phosphorous-containing compounds. This provided vital nutrients, allowing resident microorganisms to rapidly multiply and break down the oil. However, the large loss of marine life in the aftermath of the spill has lead to the call for double-hulled tankers and other reform measures in oil transport.

Ocean pollution


There is much concern about the potential effects of oil spills on the organisms at the bottom of the ocean food chain since they are one of the primary consumers of ocean pollutants. Because there is a concentration increase of the pollutants (bioconcentration) as one moves up the food chain (e.g., small fish eating plankton, larger fish eating smaller fish, etc.), there is a danger that toxic pollutants can affect the entire food chain either because they harm organisms in the lower portion of the chain or because they increase in concentration until they harm species at the upper part of the food chain, which includes humans. The effect of bioaccumulated DDT on the thinning of peregrine falcon eggshells is a classic example of bioconcentration. The world’s oceans contain an increasing amount of plastic pollution that can be lethal or harmful to different participants in ocean ecosystems. Plastic objects degrade very slowly. These objects and their degradation products become attractive to various forms of wildlife. Seabirds have been observed to swallow plastic objects and then choke as they try to regurgitate it to feed their young. Organisms and toxic chemicals are attracted to small plastic objects and take free rides for long distances and invade and negatively affect foreign ocean ecosystems. In some sections of the Pacific Ocean, a surface density of about a million pieces of plastic per square mile has been measured. Discarded fishing lines and nets causes the deaths of many sea creatures. Research on biodegradable plastics with shorter lifetimes in aqueous environments is continuing, but is not helping to solve the immediate problem of plastic accumulation in the world’s oceans. Some ocean and bay fish have been found to be contaminated with water pollutants such as mercury, DDT, and PCBs and have been banned from human consumption. One particularly severe case was a case of mercury poisoning of a thousands of people who ate fish contaminated with mercury from a bay near Minamata, a small fishing village in Japan. Severe brain dysfunction is caused by excess quantities of dimethyl mercury, (CH3)2Hg, created from mercury waste contained in bottom sediments by methane-generating bacteria. A derivative of this mercury compound is soluble in water and can cross the blood-brain barrier, interfering with the brain’s normal chemical processes. Recent concern and research into the causes of red tides and pfiesteria outbreaks have led to searches for possible connections to processes on land such as nutrients from agricultural field runoff. Some ocean pollutants are toxic only because of their secondary effects. Plant nutrients containing excess nitrogen and phosphorous from agricultural runoff and sewage discharges into streams ultimately flow into the ocean. When they do, these nutrients encourage the growth of excess phytoplankton and zooplankton that ultimately die and fall to the ocean floor, where they cause further growth of microbes in the deeper water. The excess growth consumes oxygen faster than it can be replenished. This has led to coastal regions in which have oxygen concentrations so low that it cannot sustain normal marine life. Those fish, shrimp, and craps that can escape this zone do, but the marine life that is less mobile die off, leading to “dead zones.” The area and number of dead zones around the world is increasing. One of


the largest dead zones is in the Gulf of Mexico near the mouth of Mississippi River and covers over 8,000 square miles. This zone is seasonal, forming in the spring and breaking up in the fall. The Deepwater oil spill undoubtedly at least temporarily lowered the oxygen levels in those regions in which organisms were digesting the oil contaminant. However, no oxygen lowering has been discovered in the spill region. One year following the Deepwater spill, there is concern over the fate and effects of what is called “microbial spit” (slime) that has resulted from microbial digestion of the oil and was found on the seafloor. Potential genetic effects of oil spills and other pollutants may not be felt until one or more generations after exposure and may not easily be linked to the original exposure event. It is postulated by some scientists that life originated in the sea. Throughout the history of the Earth, the oceans have been relatively hospitable to human life. It is only in the last half century that there have been warning signs of threats to this complex ecosystem. The warning flags of the ocean pollution and overfishing threats are up, and the storm is beginning. Ocean acidification within a very short historical period is a unique and perhaps critically threatening geochemical event. There is a need for further research and action in this vast area of the Earth as well as in the other water-containing areas of the planet.

SUMMARY

1. What is the chemical nature of metal water pollutants?

Many toxic transition metals have complicated chemistry because of their ability to form complex compounds and to exhibit variable oxidation states. No metal can be destroyed by chemical reactions in the same way that organic compounds are destroyed, for example, by incineration. Positive metal ions are attracted to negatively charged ions in soil humus and in the mineral components of soil.

2. What is the chemical nature and fate of polychlorinated and pesticide

pollutants?


Some of these pollutants have structures consisting of an aromatic compound such as benzene with chlorine atoms substituting for hydrogen atoms. Such compounds are readily adsorbed by soil humus, which has similar aromatic ring structures.

3. What causes adsorption of pollutants in contaminated water percolating

through the soil? Upper layers of soil contain decayed organic matter called humus.

Humus is a collection of complex amphiphilic molecules with common characteristics that are responsible for most of the physical and chemical properties of the upper portions of soil. The basic structure of humus contains a number of different aromatic compounds. One end of a long, generally hydrophobic molecule has been oxidized to form an organic acid (RCOOH), converting that end into a hydrophilic region. This structure causes humic acid molecules to aggregate in bilayer structures that adhere strongly to and completely coat soil particles. Such bilayers alter the character of the soil particles in such a way that they attract and bind both polar and non-polar water pollutants.

4. How do some pollutants migrate down to groundwater, despite their

strong adsorption to soil matter?

Soil humus is contained in micelles and on the surface of colloids. Both of these structures are mobile, can adsorb pollutants, and slowly migrate through the soil, ultimately contaminating ground water.

5. What are the definitions of water quality and threshold dose? Of what importance are these in water pollution?

Water quality depends on the degree to which the water causes harm to

a living substance or interferes with an industrial process. Threshold dose is a level of pollutant concentration below which there is no observable detrimental effect. Both of these terms are important in generating regulations regarding water treatment and purity.

6. How is water purified for drinking purposes and in sewage treatment?

Water purification for drinking purposes as well as for sewage treatment relies heavily on filtration and the purifying action of microorganisms. Chlorine and other chemicals are used to kill through oxidation reactions most, but not all, harmful organisms in preparing drinking water and in the final treatment of sewage discharge.


7. What is the influence of equilibrium and solubility on lead pollution hazards? How might these characteristics influence risk assessment and management?

Lead ion contamination can be suppressed by the addition of carbonate

ion to lead ion-contaminated water, thus shifting the equilibrium by reducing the concentration of lead ions, increasing the amount of solid lead carbonate, and thereby reducing the danger of lead poisoning from drinking water.

8. What are the problems of stream and river water use and pollution in

rivers, bays, and oceans?

The challenge in using stream water for both domestic and industrial purposes is to return the stream water cleaner than at the intake from the stream. The bay estuary provides geochemical and biochemical filters for river water pollutants. The ocean receives the pollutants that escape these filters as well as marine pollutants, including significant oil and plastic pollution. Oil spills, “dead zones,” coral reef die-offs also are of concern throughout the world’s oceans.

Review Questions

1. Which type of compound will be found in greater abundance in natural

waters, hydrophilic or hydrophobic compounds? 2. What features characterize transition metals? What is a coordination

compound? 3. From the point of view of biodegradability, in what way does a toxic metal

differ fundamentally from a toxic organic pollutant? 4. In what way does the benzene molecule pose a problem for the Lewis octet

rule? How is this problem solved in terms of representing the benzene structure?

5. What are aromatic molecules? Give several specific examples. 6. Why is it important to include consideration of biological species when

considering the various components of the natural waters?


7. In which part of the hydrological cycle does the average water molecule spend the longest period of time: the air portion, the land portion (streams, rivers, lakes, bays), or the ocean portion?

8. Why is the oxygen concentration important in water pollution studies? 9. List several water pollutants picked up in the air portion of the hydrological

cycle. 10. Outline the structure of a typical soil profile as a function of depth. 11. What is the origin of humus? 12. What are some of the chemical characteristics of humus? 13. A pollutant molecule has the soap-like structure CH3CH2CH2CH2CH2COO–

Cu+. In what way or ways would you anticipate that this molecule would react with humus in a soil sample?

14. A water sample containing cadmium ions (Cd2+) is added to soil. Indicate the

type of processes that might occur in the soil to retard the sample on its way to becoming part of the groundwater below the site of contamination.

15. In what ways could micelles and colloids hasten the introduction of cadmium

ions into the groundwater in the above question? 16. What are the differences in the types of chemical processes taking place in the

A, B, and C soil horizons? 17. Outline the steps taken in the purification of drinking water. 18. Why do some object to the use of chlorine in the purification of drinking

water? 19. Distinguish between water purity and water quality. 20. Consider two pollutants that have different dose-response curves, one with a

linear response with no threshold dose and one with a threshold dose. Consider the effects of doubling a dose of pollutant in each of these two cases.

21. List all the ways you can think of to minimize the amount of lead contained in

drinking water that has been determined to contain lead ions. 22. Suppose lime, a base, is added to water before it enters a household. What

effect will this addition have on the amount of lead contamination in drinking water in that household?


23. What will the addition of sodium hydrogen carbonate (Na+HCO3–) do to the

free lead ion concentration in drinking water? 24. Explain the effects noted in the above question in terms of the Le Châtelier

principle. 25. What strictly chemical steps are taken to purify sewage in a sewage treatment

plant? 26. Can heat be considered a water pollutant? 27. What is the role of river and estuary sediments in the water purification

process? 28. What are thought to be the health problems with pollutants that are chlorinated

aromatic compounds? 29. Why is there a zone of high turbidity in the estuary? 30. In what two ways does an estuary filter compounds dissolved in river water. 31. Trace the fate of oil spills in the ocean.

Problems

32. You are working in a commercial firm that manufactures a very useful

product that benefits humans. Recently this firm has been accused of discharging into a nearby stream wastewater that contains a harmful chemical whose composition is unknown to you. As a responsible citizen and also as one who is concerned about keeping your job, what is the strategy you would follow in your own independent investigation into the alleged problem? What questions would you ask? What solutions would you suggest to municipal agencies? To your firm?

33. Following on with the situation in the above problem, suppose you read that

the toxic compound released into the stream contains a two benzene rings and six chlorine atoms. In what ways would this knowledge affect your strategy in helping solve the above problem?

34. Continuing with the above problem, suppose that you find out that your firm

is also releasing relatively large quantities of a metal compound into the same stream. In what ways would this knowledge affect your strategy?

35. Suppose the stream in the above problems is instead a bay. Would this make

any difference in your strategy?


36. Suppose you are the CEO of the above firm and are told that the risk from the

release of all these pollutant compounds into the stream is that at most, one person out of a million would run the risk of dying from the ingestion of these pollutants. What would be your action?

37. Suppose you are an employee of the EPA faced with the decision of what to

do regarding the above problem. What would be your course of action?

Individual and Group Projects

38. Search the Internet and library for information on the following subjects:

water pollution (be prepared for overload), heavy metals, pesticides and herbicides (link with "pollution"), eutrophication, BOD, humus, soils, water quality, water purification, sewage treatment. The US Environmental Protection Agency (EPA) home page is a good source for information.

39. Obtain information from one or more of the following agencies on water

pollution: EPA, your state water quality agency, and your local sewage disposal plant.

Readings and links (to be updated)

Water Fights, A. Markels, U.S. News & World Reports, May 19, 2003, p.58. Detecting Lead, J. Gorman, Science News, Vol. 163, May 24, 2003, pp 326-7. When Pollutants Take the Arctic Route, Science News, Vol. 163, May 24, 2003, p. 334. Countering Oil Spills, P. S. Zurer, Chemical & Engineering News, April 7, 2003, pp 32-3. Environmental Chemistry, S. E. Manahan, Lewis Publishers, Boca Raton, 1994.

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