water. treatment by oxidation processes

Redacción y Ejecución de Proyectos

Water. Treatment by Oxidation Processes.

Correa Álvarez, Adán 1


Oxidation processes are used in water treatment for disinfection and removal of

potentially toxic contaminants in the water, and for some industrial purposes such as pulp

bleaching and purification of wastewater before discharge into the environment.

The most familiar chemical oxidant is oxygen from air; other examples of oxidants

used in water treatment are chlorine, chloramines, ozone, chlorine dioxide, hydrogen

peroxide, and potassium permanganate. Sometimes these oxidants are used in

combination with each other (e.g., ozone with hydrogen peroxide) or with photons from

irradiation lamps.

1. Physical and Chemical Properties of Chemical Oxidants

The power of a chemical oxidant to cause a chemical oxidation reaction is

measured by two properties: (1) the thermodynamic driving force and (2) the rate constant

of oxidation.

All common chemical oxidants shown in Table 1 are relatively powerful and

should oxidize most organic compounds to carbon dioxide and water. The fact that this

does not always occur (e.g., organic compounds on our planet are stable to oxidation by

oxygen in most cases) indicates that thermodynamics is not the only important factor in

determining whether a chemical reaction will occur spontaneously.

The other factor that is important in rating chemical oxidants is how fast they

cause a chemical oxidation to proceed. This is the domain of chemical kinetics. In fact,

in very few cases do the oxidants listed in Table 1 oxidize environmental contaminants

as completely as would be expected from thermodynamic considerations.

Chemical reactions that should occur, as indicated by the thermodynamic driving

force, may do so very slowly if the rate constant for the process is low. For example, the

reaction with ozone is actually a very slow reaction, whereas that of another common

pollutant, phenol, is very fast.

-Chlorine.

The solubility of chlorine in water is high compared to that of oxygen because in

water, Cl2 hydrolyzes to form hypochlorous acid. Hypochlorous acid is a weak acid with




a pKa of 7.50 at 25 C, dissociating to a hydrogen ion and the hypochlorite ion. Chlorine

is a toxic gas, and worker exposure must be avoided.

-Chloramines.

Chloramines are formed by reaction of chlorine with ammonia. The reaction

occurs in three steps as shown below, leading to mono-, di-, and trichloroamines. These

have different oxidation potential and disinfection power, but in general, chloramines are

not as potent in both respects as chlorine. However, chloramines are more stable in water

than chlorine so they can maintain an active disinfection capacity for longer periods of

time.

-Ozone.

Ozone is always produced when oxygen is decomposed in air, e.g., in an electric

discharge or when shortwave UV radiation is absorbed by oxygen. This decomposition

occurs naturally in the atmosphere, but it may also be used to generate ozone for water

treatment by the reaction shown below using either air, oxigenen riched air, or pure

oxygen.

Ozone is more soluble in water than oxygen but less soluble than chlorine. In

water it decomposes slowly, with a half-life that depends on the other substances present

in water. Ozone is a toxic gas, and monitors are necessary where ozone is made and used,

to minimize worker exposure. Ozone is a powerful oxidant but reacts with different

chemicals at various rates that range over several orders of magnitude.

-Chlorine Dioxide.

Chlorine Oxides and Chlorine Oxygen Acids, is a clear, colorless gas that

dissolves in water without dissociation. It is a powerful disinfectant and oxidant (Table

1) and has been used extensively for water treatment. Chlorine dioxide is becoming an

increasingly important alternative to chlorine as a bleaching agent in pulp and paper

production. However, chlorine dioxide reacts rapidly with many substances in natural

water, such as natural organic matter, in a reduction – oxidation reaction that produces

chlorite ion. Chlorite has some documented health effects such as irreversible binding to

hemoglobin,

-Potassium Permanganate.

Has been used in water treatment for many years, primarily for oxidation of

manganese (II) and iron (II) to the corresponding insoluble oxides or hydroxides, for color

removal, and for taste and odor control. Potassium permanganate is usually not

recommended for drinking water disinfection. Also, care must be taken not to overdose

and produce a pink-colored water, which is objectionable to consumers.

-Hydrogen Peroxide.

Hydrogen Peroxide is a more oxidant than chlorine or ozone. It is used in many

industrial and medical applications.




2. Uses of Chemical Oxidants. Oxidants are used in water treatment for a variety of purposes, including

disinfection and oxidation, in the treatment of drinking water and wastewater, oxidation

in pulp and paper treatment, and disinfection and scale control in the treatment of cooling

tower water.

2.1. Drinking Water Treatment. Chemical oxidants are used for treatment of drinking water, mostly for

disinfection and removal of chemical compounds or potential toxicants. The most

common drinking water oxidant worldwide is chlorine (hypochlorous acid). In parts of

Europe, ozone has been used for as long as chlorine, and it is growing in use worldwide.

Disinfection Treatment with chemical oxidants has become the traditional method

for disinfection of water for distribution in municipal water systems. Ozone and chlorine

were first used for this purpose at about the beginning of this century and have been used

continuously since that time. Chlorine is the more preferred of the two because it is more

stable and maintains its disinfection capability for longer periods of time. Ozone

decomposes quickly but is a more powerful disinfectant.

2.2. Chemical Oxidation Oxidants are also used in drinking water treatment to remove substances that give

unwelcome color to the water, compounds that cause poor taste or odor, and

micropollutants that may have deleterious health effects.

In addition, oxidants are used to remove iron and manganese, which are not toxic

but may cause discoloration of household appliances. Color in natural water is caused by

natural organic matter (NOM) and sometimes by metallic elements complexed with

NOM. Oxidants destroy color by chemically oxidizing the color center in the

macromolecular NOM. Of the common chemical oxidants, ozone appears to be most

effective for removing color. However, since the chemical structures in NOM that cause

color vary from source to source, another oxidant may be more or less successful than

ozone in removing the color, depending on the water source.

2.3. Wastewater Treatment

The use of chemical oxidants is not as common in wastewater treatment as in

drinking water treatment because wastewater usually contains more impurities and,

therefore, would consume more oxidants. Since oxidants are rather costly commodities,

they are generally used for polishing (i.e., treating wastewater just before discharge) after

most impurities have been removed. Chlorine has been used traditionally for treatment of

domestic wastewater after biological treatment. The rationale was to minimize the

concentration of microorganisms and ammonia being discharged into receiving streams.




2.4. Advanced Oxidation Processes

The term advanced oxidation processes (AOPs) is used for water treatment

processes that involve the formation of highly reactive, short-lived chemical

intermediates. These intermediates (e.g., the hydroxyl radical) are powerful oxidants –

more powerful than the conventional oxidants discussed above – and are used to oxidize

water contaminants that resist other forms of treatment. Several AOPs have been

developed and commercialized including the following:

1. Ozone in combination with hydrogen peroxide (sometimes called Peroxone

process) with UV radiation, or with both hydrogen peroxide and UV.

2. Hydrogen peroxide in combination with iron (II) salts (Fenton process), with

UV radiation, or with UV radiation and other modifiers such as iodine ion,

iron (III) salts, etc.

3. Oxygen in combination with high-energy, high-frequency sound waves

(sonication), electron-beam irradiation, or gamma radiation.

Oxidation with AOPs is used worldwide in the treatment of drinking water for

oxidation of herbicides, and for oxidation of taste and odor compounds that are formed

in rivers, lakes, and reservoirs. AOPs are also used in groundwater treatment to oxidize

contaminants such as halogenated solvents (trichloroethylene and tetrachloroethylene).

In addition, AOPs are being investigated for treatment of industrial wastewater,

but they appear to be substantially more expensive than conventional biological

treatment. Thus, AOPs may be most useful for treating low-volume streams containing

compounds that are not easily biodegraded. In this case, oxidizing the target compounds

only partially (i.e., not completely to carbon dioxide) may be most economical.

Advanced oxidation processes that involve UV radiation utilize lamps such as

mercury arc lamps that produce radiation of various wavelengths, depending on the power

density of the current in the lamp.

3. Toxicology and Environmental Health

Chlorine and other chemical oxidants have had a very positive influence on human

health because they are the principal disinfectants used in drinking water treatment,

disinfection of sanitary facilities, and other applications. The incidence of waterborne

diseases such as cholera and typhoid fever decreased dramatically in the early decades of

the 20th century as water disinfection was practiced more extensively.

Chemical oxidants are generally toxic compounds themselves, so workers and the

general public should be protected from large doses. However, low levels of oxidants,

particularly chorine and chloramines, are consumed regularly in drinking water with no

apparent ill effects.

water. treatment by oxidation processes

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