CHAPTER 7WATER, THE ULTIMATE GREEN SOLVENT: ITS USES

AND ENVIRONMENTAL CHEMISTRY

From Green Chemistry and the Ten Commandments of Sustainability, Stanley E. Manahan, ChemChar Research,

Inc., 2006manahans@missouri.edu

7.1. H2O: SIMPLE FORMULA, COMPLEX MOLECULE

Angled structure of the water molecule (next slide)

The water molecule is polar

• Positive ends toward anions

• Negative ends toward cations

Water molecule forms hydrogen bonds

The properties of water are due to the polar nature of the water molecule and its ability to form hydrogen bonds.

The Water Molecule

7.2. IMPORTANT PROPERTIES OF WATERExcellent solvent for salts, acids, bases, and substances that have H, O, and N atoms capable of forming hydrogen bonds

Solvent in biological fluids, such as blood or urine

Water weathers minerals and transports dissolved minerals in the geosphere

Transports nutrients to plant roots in soil

Many industrial uses

Very high surface tension—ducks float

Transparent to visible light enabling photosynthesis to occur in algae

Maximum density as a liquid at 4˚ C causing bodies of water to become stratified with colder, denser layers on the bottom.

Important Heat Characteristics of Water

High heat capacity of 4.184 joules per gram per ˚ C (J/g-˚C)

Very high heat of fusion of 334 joules per gram (J/g)

Very high heat of vaporization of water is 2,259 J/g, water vapor carries latent heat

7.3. WATER DISTRIBUTION AND SUPPLY

Numbers are for annual rainfall in centimeters.150-200100–15050–100<2525-5050–100>200

Areas with rapidpopulation growth

4.4. Water Utilization (in U.S.)

Trends in U.S, Water Use

Encouraging trends in water use in the U.S., the result of

• Water conservation efforts, especially in industry and agriculture

• Recycling water, including uses through several levels requiring progressively lower water quality

• Replacement of spray irrigators with direct application of water to soil including trickle irrigation

• Exact computer control of water usage

Where Earth’s Water is Found

About 97% of Earth’s water is in oceans

Most of the remaining water is in the form of solid snow and ice

Less than 1% of Earth’s water as water vapor and clouds in the atmosphere, as surface water in lakes, streams, and reservoirs, and as groundwater in underground aquifers

7.4. Bodies of Water and Life in Water

Stratification of a Body of Water Strongly Affects Chemical and Biological Processes

Living Organisms in WaterA normal body of water is an ecosystem

• Based upon a food supply consisting of the biomass produced photosynthetically by the algae and plants living in it:

6CO2 + 6H2O (sunlight) C6H12O6 + 6O2 (7.4.1)

• Glucose, C6H12O6, is converted to other forms of biomass

• Algae and plants are producers that generate biomass

• Heterotrophic organisms, usually bacteria in water, metabolize biomass:

C6H12O6 + 6O2 6CO2 + 6H2O (7.4.2)

Biologically Mediated Processes in Water

Specialized bacteria in water can utilize oxidized chemical species with high oxygen contents other than molecular O2 for oxygen sources.

Example: Nitrate ion, NO3-, acts as an oxidizing agent in the

bacterially-mediated biodegradation of biomass:

C6H12O6 + 3NO3- + 6H+ 6CO2 + 3H2O + 3NH4

+ (7.4.3)

By mediating chemical reactions, such as the one above, microorganisms, particularly bacteria, largely determine the chemistry that occurs in water.

Dissolved oxygen in water is very important.

Biodegradable organic pollutants cause biochemical oxygen demand, BOD.

7.5. CHEMICAL PROCESSES IN WATERBiochemical processes including photosynthesis

2HCO3- (sunlight energy) {CH2O} + O2 + CO3

2- (7.5.1)

• {CH2O} represents biomass

Acid-base reactions

CO32- + H2O HCO3

- + OH- (7.5.2)

Precipitation reactions

Ca2+ + CO32- CaCO3(s) (7.5.3)

Oxidation-reduction reactions, usually carried out by bacteria are generally ones in which chemical species gain or lose oxygen

Example: Oxidation of S in H2S

H2S + 2O2 SO42- + 2H+ (7.5.4)

7.6. FIZZY WATER FROM UNDERGROUND

Natural waters contain dissolved gases.

• Dissolved oxygen required by fish

• Dissolved carbon dioxide in some mineral waters• Carbon dioxide in Lake Nyos in the African country of Cameroon

which asphyxiated 1,700 people in 1986

Henry’s Law for gas solubilities states that the solubility of a gas in a liquid is proportional to the partial pressure of that gas in contact with the liquid.

• Gas solubility decreases with increasing temperature

Oxygen in Water

At 25˚ C the concentration of oxygen dissolved in water is only about 8 milligrams per liter of water (mg/L)

• Readily consumed by biodegradation of biomass (abbreviated {CH2O}) by oxygen-utilizing bacteria:

{CH2O} + O2 CO2 + H2O (7.6.1)

• Only about 8 mg of {CH2O} consumes 8 mg of O2

7.7. (WEAK) ACID FROM THE SKY

An acid is a substance that contains or produces H+ ion in water, whereas a base is a substance that accepts H+ ion in water or contains or produces hydroxide ion, OH-.

Whether water is acidic or basic is expressed by pH:

• pH = -log [H+] (7.7.1)• [H+] is the molar concentration of H+ in water, that is, the number

of moles of this ion per liter of water.

[H+], mol/L log[H+] pH

0.100 -1.00 1.00

1.00 10-3 -3.00 3.00

1.00 10-5 -5.00 5.00

1.00 10-9 -9.00 9.00

Acid in Water (Continued)

The value of [H+] in pure water at 25˚ C is 1.00 10-7 mol/L and the pH is 7.00.

• The concentration of dissolved carbon dioxide, [CO2(aq)], in water in equilibrium with 370 ppm atmospheric air at 25˚ C is 1.21 10-5

mol/L.

• Makes water slightly acidic because

CO2 + H2O H+ + HCO3- (7.7.2)

• [H+] = 2.3 10-6 mol/L corresponding to a slightly acidic pH of 5.6

• Such water is neutral, neither acidic nor basic.

• Water with a pH less than 7.00 is acidic, whereas water with a pH greater than 7.00 is basic.

The average global concentration of CO2 gas in air in the year 2001 was about 370 parts per million by volume, and going up by about 1 ppm per year.

7.8. WHY NATURAL WATERS CONTAIN ALKALINITY AND CALCIUM

Water alkalinity is the ability of water to react with and neutralize acid (H+).• Due to presence of bicarbonate ion, HCO3

-, which can react as follows with H+ ion:

HCO3- + H+ CO2(aq) + H2O (7.7.3)

Water hardness in the form of dissolved Ca2+ ionBoth water hardness and alkalinity are acquired when water containing dissolved CO2 reacts with limestone, CaCO3:

• CO2(aq) + CaCO3(s) + H2O Ca2+(aq) + 2HCO3- (7.7.4)

Atmospheric CO2 dissolved in water, and from biodegradation

HCO3- dissolved in water

Solid carbonates (CaCO3) in mineral formations in contact with water

Carbon Dioxide and Carbonate Species in Water

7.9. METALS IN WATERMetal ions in water are present as hydrated ions, such as Ca(H2O)6

2+.

Bound water molecules can be displaced reversibly by other species.

• Such species include chelating agents, which can bond to metal ions in 2 or more places to form a metal chelate.

• One such chelating agent is the nitrilotriacetate anion used in some cleaning formulations and capable of bonding to a metal ion on 4 separate sites

• Chelates tend to be particularly stable, and they are very important in natural water systems.

• Chelates are involved in life systems; for example, blood hemoglobin is a chelate that contains Fe2+ ion bonded simultaneously to 4 N atoms on the hemoglobin protein molecule

Humic Substances in Water

Water in nature may contain naturally-occurring chelating agents called humic substances that are complex molecules of variable composition left over from the biodegradation of plant material.

Humic substances bind with Fe2+ ion to produce gelbstoffe (German for “yellow stuff”) which is very difficult to remove by water treatment processes.

Humic substances produce trihalomethanes, such as chloroform, HCCl3 during disinfection of water by chlorine

Most important chemical and biochemical processes in water occur at interfaces between water and another phase (usually solid)

7.10. Water Interactions With Other Phases

Sediments are variable mixtures of minerals, clay, silt, sand, and organic matter

Formed by

• Erosion

• Sloughing of banks into water

• Washed in from watersheds

Chemical reactions, for example, as the result of photosynthesis:

• Ca2+ + 2HCO3 - + h{CH2O}(s) + CaCO3(s) + O2(g)

• Deposits solid CaCO3 (limestone)

• Deposits biomass, {CH2O}

Sediments

Very small particles suspended in water

Size ranging from very large molecules up to about 1m

Scatter light (Tyndall effect)

Unique characteristics

• High surface/volume • High interfacial energy

• High surface/charge

Colloids in Water

Behavior and stability of colloids are important in aquatic chemical phenomena

• Formation of sediments

• Dispersion and agglomeration of bacterial cells

• Dispersion and removal of pollutants

• Waste treatment processes

7.11. HEAVY METAL WATER POLLUTANTSThe heavy metals are those metals of relatively higher atomic numbers, many of which are toxic

Cadmium, Cd, widely used in metal plating and other industrial applications, is highly toxic.

Toxic lead, Pb (from its Latin name of plumbum), is the most common heavy metal pollutant because of its widespread use in industry, in the manufacture of lead storage batteries, formerly as a leaded additive to gasoline, as a pigment in white house paint, and as an anticorrosive primer applied prior to painting steel.

Mercury, Hg, in water caused poisoning in the Minamata Bay area of Japan.

Arsenic is a toxic metalloid

• Arsenic compounds used for pesticides were lead arsenate, Pb3(AsO4)2; sodium arsenite, Na3AsO3; and Paris Green, Cu3(AsO3)2.

• Arsenic-contaminated wells in Bangladesh

Organically Bound Metal Water PollutantsOrganically bound metals may be considerably more mobile and in some cases more toxic than hydrated metal ions in water.

Metals bound as chelates discussed in Section 7.9.

Metals bonded directly to carbon in hydrocarbon groups such as the methyl group (-CH3) to produce organometallic compounds.

Methylmercury compounds found in Lake Saint Clair in 1970 released from chloralkali process for making chlorine and sodium hydroxide

• HgCl2 (action of anoxic bacteria) CH3HgCl + Cl- (7.11.1)• The monomethylmercury ion in this compound, CH3Hg+, is soluble

and mobile in water and the dimethylmercury, (CH3)2Hg, also produced is volatile as well. These mobile species were released from the sediments and concentrated in fish tissue.

Organotin compounds used as industrial biocides and to prevent growth of organisms on boat and ship hulls have been common water pollutants

Heavy Metal Pollutants and Green Chemistry

Regulate releases

Better to find substitutes

7.12. INORGANIC WATER POLLUTANTS

Cyanide as HCN or CN-

• As little as 60 mg of cyanide can be fatal to a human.

Cyanide is sometimes released to water, especially from metal extraction operations (especially gold).

• Has caused some large fish kills

Green chemistry calls for use of substitutes for cyanide

Excessive levels of NH4+/NH3 cause water pollution.

Hydrogen sulfide, H2S, is a toxic gas with a foul odor that is produced by anaerobic bacteria acting upon inorganic sulfate (see Section 7.4), from geothermal sources (hot springs) and as a pollutant from chemical plants, paper mills, textile mills, and tanneries.

Inorganic Water Pollutants (continued)

Microbial degradation under ground may generate carbon dioxide, CO2, that exists as free carbon dioxide in water.

• Can be toxic to aquatic organisms

• Can make water corrosive because of its acidity and tendency to dissolve protective CaCO3 coatings on pipe.

Nitrite ion, NO2-, generated by bacteria or from industrial sources.

• Can cause methemoglobinemia by converting the hemoglobin in blood to methemoglobin, a form useless for transporting oxygen.

Nitrate ion, NO3-, is a more common water contaminant.

Acidity

Strong acid pollutants that cause water to have a low pH are very damaging to organisms living in water and can make water corrosive.

From bacterial oxidation of iron pyrite, FeS2, to produce sulfuric acid.

• 4FeS2(s) + 2H2O + 15O2 2H2SO4 + 2Fe2(SO4)3 (7.12.2)

Acid rain from dissolved HCl, H2SO4, and HNO3 can cause acidic water pollution.

• Especially damaging to lakes that are not in contact with basic rock

Alkalinity and Salinity

Alkalinity due to salts such as sodium carbonate, Na2CO3

• Human activities can cause alkaline salts to be leached from soil.

Water salinity is due to dissolved salts, such as sodium chloride and calcium chloride.

• From municipal water systems and irrigation

Water Pollutants That Are Just Too Nutritious

Growth of algae requires inorganic nitrogen (NH4+, NO3

-), phosphorus (H2PO4

-, HPO42-), and potassium (K+).

• Sources include fertilizers put on soil to enhance crop growth, some industrial pollutants, degradation of sewage in wastewater.

Excessive levels of algal nutrients cause algae to grow too well and generate too much biomass.• Biomass eventually dies and decays, which uses up all the oxygen

in the water and clogs a body of water with dead plant matter—eutrophication.

• Eutrophication is usually curtailed by lowering phosphate (limiting nutrient) input into bodies of water and streams.

• Lowering levels of phosphate-based detergent builders around 1970 has curtailed eutrophication.

7.13. ORGANIC WATER POLLUTANTS

Two extremes:

• Readily biodegradable organic compounds, such as biomass, {CH2O}, that consume dissolved O2

• Refractory organic compounds, such as PCBs, that tend to accumulate in sediments and in the lipid (fat) tissues of fish and birds that eat fish

Oxygen-Demanding Substances

Biodegradation of biomass, {CH2O}, consumes oxygen.

• {CH2O} + O2CO2 + H2O (7.13.1)

Biochemical oxygen demand, BOD, refers to the amount of oxygen consumed in biodegrading the organics in a liter of water.

SewageIn addition to BOD from fecal matter and food wastes, sewage contains oil, grease, grit, sand, salt, soap, detergents, degradation-resistant organic compounds, disease-causing microorganisms, and an incredible variety of objects that get flushed down the drain.

Detergents in Sewage

• Foam from poorly degradable surfactants in sewage

• The problem was branched-chain alkyl benzene sulfonate, ABS

• The problem was solved with linear alkyl sulfonate (LAS) surfactant that has a readily biodegraded straight chain.

LAS

C C C HH H H

H H H

CH

HHHHHHH

HHHHHHHHCCCCCCCCH

O- +Na

O OS

C C C C C C C C C C

H

H

H CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

S O- +Na

O

O

H

H

H

H

H

H

H

H

H

ABS

7.14. PESTICIDES IN WATER

Include insecticides, herbicides, bactericides to control bacteria, slimicides to control slime-causing organisms in water, and algicides used against algae.

Originally, highly persistent DDT was the greatest problem, but its use has been banned.

Now herbicides are the biggest pesticide problem because they are applied directly to soil and get into water as runoff.

Insecticides That Have Been Water PollutantsExample synthetic insecticides

P O NO2

SH3CO

H3COMethyl parathion

Carbamic acid

HN

HC

O

OH

Organochlorine compounds

Organophosphates

Carbamates

Carbofuran

C

OCH3

H3C

O C

O

N

H

CH3

HH

DDT

C

CCl Cl

Cl

Cl

H

Cl C C

CCl

Cl

Cl

Cl

C

C

CH

HH

H

HCl

Cl

ClCCl

Chlordane

H3C

C2H5O

CS

O

S

PO H

CC

CH3 OC2H5OC

HH O

MalathionHydrolizable bonds

CarbarylC

O

N

H

CH3O

Insecticides (continued)Example insecticides from natural sources

N

NC

CC

CH3

HH H

HH

H

Nicotine

Insecticides from plants

C

OC

H3CO

CC

O

O

O

OCH3

HHC

H CH3C

H

H

H

H

H

Rotenone

H3C

H H C O

CHH

C C

CC C

H

H

H H H

C

H3CC C C C

CH3

CH3H

CHCO O

C

H3C

H

C HH Pyrethrin I

Insecticides (continued)

Greatest water pollution problems from organochlorine insecticides.

• Dominant from 1940s to 1960s, especially DDT

• Not particularly toxic to humans and other animals

• Very persistent in the environment because they undergo biodegradation only slowly

• Tendency to undergo bioaccumulation in fish and other organisms, concentrating in fat tissue

• Undergo biomagnification through food chains

Insecticides (continued)

Organophosphate insecticides replaced organochlorine insecticides.

• Biodegradable with no tendency to undergo bioaccumulation.

• Act by inhibiting acetylcholinesterase enzyme involved with nerve function (same action as chemically-related military poison “nerve gases”)

• Can be very toxic; many people have been killed by parathion

• Malathion is only about 1/100 as toxic to mammals as is parathion because mammals and other animals can break down the molecule

Carbamates are biodegradable and relatively safe

• Carbaryl (Sevin) is used to kill insects on lawns or gardens and, because of its low toxicity to mammals, can be sprinkled on pets in (often futile) attempts to rid them of flea infestations.

• Carbofuran is a plant systemic insecticide that is taken up by roots and leaves and distributed through the plants

Naturally-Occurring Insecticides

Important insecticides derived from plants

• Rotenone extracted from the roots of certain kinds of legumes

• Nicotine from tobacco

• Pyrethrins extracted from dried chrysanthemum or pyrethrum flowers

• Pyrethroids, synthetic analogs of pyrethrins

• Bt insecticide from Bacillus thuringiensis bacteria transferred to plants by recombinant DNA techniques so that they can make their own insecticide

HerbicidesHerbicides to control weeds and grass on cropland

Susceptible to being washed off fields by rainfall with a high potential to become water contaminants

Commonly found in drinking water supplies, which may require removal measures

Typical herbicides:

Glyphosate is the active ingredient in Monsanto’s Roundup herbicide.

Some genetically modified soybeans and other crops are “Roundup ready”

GlyphosateHO C C N C P OH

NO H

H

H

H OH

O

N

CN

C

NCCl

N C

CH3

CH3

HNCCH

H

H H

HH

Atrazine

N

CN

C

NCCl

NNCCH

H

H H

HH H

C C H

H H

H HSimazine

The Infamous DioxinDioxin, known chemically as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD):

• Manufacturing byproduct (2,4,5-T herbicide) and generated in some combustion processes

• Extremely stable and poorly biodegradable

• Accumulates in sediments

• Times Beach

2,3,7,8-Tetrachlorodibenzo-p-dioxin (commonly called dioxin)

Cl

Cl

O

C

CC

C

CC O C

C

CC

C

CCl

Cl

H

H

H

H

7.15. POLYCHLORINATED BIPHENYLS (PCBs)Polychlorinated biphenyls (PCBs) consist of a class of 209 compounds made from substituting from 1 to 10 chlorine atoms for H atoms on biphenyl (see below):

PCBs are notable for their extreme chemical and thermal stability, resistance to biodegradation, low vapor pressure, and high dielectric constants.

Widely dispersed and persistent in the environment

Once widely used for electrical and other applications

Accumulate in sediments

Hudson River sediments due to pollution by General Electric plants

A typical polychlorinated biphenylBiphenyl

CC C

CCC

H

H H

CC C

CCC

H H H H

HH

H CC C

CCC

H Cl

H

Cl H

CC C

CCC

H Cl

H

Cl

H

7.16. RADIOACTIVE SUBSTANCES IN WATERRadioactive isotopes, or radionuclides can get into water from either natural sources or from the fission of uranium or plutonium in nuclear power reactors or (formerly) above-ground weapons testing.

Radionuclides have unstable nuclei that evolve ionizing radiation.

Radionuclides decay with specific half-lives that can range from fractions of a second to millions of years.

• Alpha particles, helium atom nuclei composed of two neutrons and two protons

• Beta radiation in the form of high-energy electrons

• Gamma rays, which are electromagnetic radiation that behaves like very short wavelength, high-energy X-rays.

Radionuclides in WaterGreatest concern with respect to radionuclides in water arises from natural geological sources• Alpha particle emitter radium-226, 226Ra, half-life 1620 years, is a

particular concern in drinking water.Radionuclides from above-ground testing of nuclear weapons and from reactor accidents (only one, 1986 Chernobyl accident)• Alpha particle emitter radium-226, 226Ra, half-life 1620 years, is a

particular concern in drinking water.

• Strontium-90 (90Sr), half-life 28 years, which is in the same chemical group as calcium and is incorporated into bone

• Cesium-137, (137Cs), an alkali metal that the body handles much like sodium and potassium ions

• Iodine-131 (131I), half-life 8 days, that is attracted to the thyroid and can impair its function and even cause thyroid cancer

Threat to water from nuclear weapons manufacturing and research operations

• Plume of radioactive water from the Hanford works, Washington

7.17. WATER TREATMENT

Municipal Water

Aeration of water

• 4Fe2+ + 4O2 + 4H2O 4Fe(OH)3(s) + 8H+ (7.17.1)• Gelatinous Fe(OH)3 product causes coagulation of

colloidal particles.

Municipal Water Treatment (Cont.)Excessive levels of dissolved calcium along with bicarbonate ion, HCO3

-. (temporary hardness) can be removed from water by adding lime, Ca(OH)2:

• Ca2+ + 2HCO3- + Ca(OH)2 2CaCO3(s) + 2H2O (7.17.2)

Disinfection of water by adding chlorine

• Cl2 + H2O H+ + Cl- + HOCl (7.17.3)

Green Ozone for Water Disinfection

Ozone, O3 disinfects water

• 3O2(g) (electrical discharge) 2O3(g) (7.17.5)

Ozone made from air using clean electrical energy as needed for water disinfection is a very good green chemistry practice.

• No storage of dangerous chemicals

FilteredairCompressorRefrigeratoranddesiccatorDriedairOzonegeneration,3O2 →2O320,000voltsO3 in air

WaterinWatercontactingozone

Disinfectedwater

Water for Industrial UseWide range of water quality and purification, depending upon use

Water recycling means recycling water back through a system for essentially the same use.

Sequential use recognizes that several applications may require water of successively lower quality.

Removal of salts by reverse osmosis

Waste brinePurified waterMembrane selectivelypermeable to waterPressurizedwater

Sewage TreatmentThe primary objective of sewage treatment is to remove oxygen-demanding substances from wastewater. • Substances of mostly biological origin, abbreviated {CH2O}, that

undergo biodegradation and consumption of dissolved oxygen, thus exerting a biochemical oxygen demand (BOD)

Three main categories of sewage treatment

(1) Primary treatment to remove grit, grease, and solids

(2) Secondary treatment to reduce BOD

(3) Tertiary treatment to further refine effluent water quality.

Removal of BOD in secondary wastewater treatment by

{CH2O} + O2 CO2 + H2O + biomass (7.17.6)

• Trickling filter in which sewage is sprayed over rocks coated with microorganisms

• Activated sludge process (next slide)

Activated Sludge Process for Wastewater Treatment

Air SludgesettlingPurified waterwith reducedBODWastewater containingBOD, {CH2O}Aeration tankDegradation of BODMineralization of C, N, P, and SSettled sludge with viable microorganismsExcess sludge toanaerobic digester

Anaerobic digesterByproduct methanefuel gas2{CH2O} →CH4+CO2