Rccci3 oh rcoh cci3 h2o CHCI3 oh

This reaction will occur with other halogens and is the basis of a qualitative test for the CH3CO— group. Phenols are another class of organic compound that can be chlorinated by HOCl. Chlorinated phenols have very noticeable, antiseptic-like odors that are detectable at the parts-per-billion range.

These chlorination reactions are most expected in waters containing much industrial waste, where suitable organic molecules are present in significant amounts. Of course, such waters may already contain chlorinated hydrocarbon wastes, and the extent to which chlorination actually contributes to the total is not always clear in field situations. However, reactions with naturally occurring organic materials also are of concern. Humic substances (Section 9.5.7) in particular are suspected of being able to yield chlorinated products when they are present in large amounts in the water source. There are also concerns that harmful partially oxidized but not chlorinated by-products might be formed (e.g., epoxides).

In the presence of ammonia, HOCl will form chloramines (e.g., monochlor-amine) from the reaction

Under appropriate conditions, the reaction of the chlorine is fast and essentially complete, although at high chlorine/ammonia ratios or low pH values, di- and trichloramine can be formed. Analogous reactions take place with organic amines. Production of these materials may contribute to odor and taste problems in the treated water. On the other hand, combining ammonia with chlorine treatment can eliminate the formation of tastes or odors associated with the formation of chlorinated phenols, since the chloramine is a poor chlorinating agent and also gives a more persistent disinfectant action. This combined treatment (sometimes called chloramination) is used in a few public water supply treatment facilities in the United States. The chloramine slowly hydrolyzes to HOCl, prolonging the lifetime of the disinfectant, and producing it at a lower concentration so that chlorination reactions are reduced.

While bromine and iodine are not used to treat public water supplies, they have been used in disinfection of swimming pools. Iodine is used to treat water that is used for drinking in wilderness treks and so on, often as tetraglycine hydroperiodide: a complex of glycine, hydriodic acid and iodine having the composition [NH2CH2COOH)16(HI)4(I2)5]). The chemistry of bromine resembles that of chlorine closely, but in contrast to HOCl and HOBr, HOI is not stable and decomposes rapidly to I" and IO3 so that there is no residual disinfection action.

While in many U.S. localities the law allows no substitute for chlorine treatment of drinking waters, an alternate method is available and receives some use in Europe and Canada, namely, treatment of the water with ozone, O3. This method is as effective as treatment with chlorine, may have taste advantages, and avoids the risks of generating chlorinated products, although the question of whether it can produce partially oxidized toxic products has been raised. There is also some concern that ozone can react with inorganic bromide to produce substances that are effective in brominating organic compounds that have carcinogenic potentials as great as organochlorides. Ozonolysis is more expensive than chlorination, and because the ozone cannot be stored, it must be produced on site (by electric discharge in air). Ozone also does not provide a long-term disinfectant action in the distribution system, and so some chlorination remains necessary.

Another potential oxidizing agent for water purification is chlorine dioxide, also used in a few localities. This compound is unstable and cannot be stored; it must be prepared on site from sodium chlorite, usually by the reaction of chlorine, although an alternative preparation method is to treat the chlorite with acid. In both cases, side reactions produce chlorate, a substance whose health effects do not seem to be fully studied. A third, more economical but more complex, method used in some pulp and paper mills involves reduction, in a strong acid solution of sodium chlorate, NaClO3, with a reducing agent such as SO2.

Because it acts as an oxidizer, chlorine dioxide is reduced to the chlorite ion:

It is not as effective a chlorinating agent as HOCl, and it does not produce CHCl3 or chloramines, but there is some evidence that if organic material is in excess, ClO2 can produce partially oxidized materials such as epoxides, quin-ones, and chlorinated quinones. It has no residual disinfecting action.

Ultraviolet irradiation is still another purification technique that has received limited use (e.g., in Switzerland). It is rapid and independent of pH and temperature, but less effective if suspended matter is present.

Passage of water through beds of activated carbon will remove organic contaminants that are not oxidized readily by the chlorine treatment and will also remove chlorinated organic molecules. This process has been used on a limited scale in water treatment plants for many years. It can also be employed by the householder in an attachment to the water tap. Activated carbon is produced by charring a variety of organic materials, followed by partial oxidation. This material has a high surface area and adsorbs most organic molecules effectively. The activity can be regenerated by oxidation of the adsorbed materials by heating in an atmosphere of air and steam. This method is perhaps the most practical means now available for removing chlorinated hydrocarbons, aromatics, and so on, which are among the most dangerous of the organic pollutants likely to be present in a domestic water source.

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