Chlorine (Cl) has three valences under normal environmental conditions, -1, 0 and +1. Environmental scientists often refer only to the chlorine forms having 0 and +1 valences as chlorine; they refer to the -1 form as chloride. Chlorine with a valence of 0 (Cl2) and chlorine with a valence of +1 (HOCl) both have the ability to oxidize materials, whereas chlorine at a -1 valence, chloride, is already at its lowest oxidation state and has no oxidizing power.
The functions of chlorination are to disinfect water or wastewater, decolorize waters or fabrics, sanitize and clean surfaces, remove iron and manganese, and reduce odors. The fundamental principle of each application is that due to its oxidizing potential, chlorine is able to effect many types of chemical reactions. Chlorine can cause alterations in DNA, cell-membrane porosity, enzyme configurations, and other biochemicals; the oxidative process can also lead to the death of a cell or virus. Chemical bonds, such as those in certain dyes, can be oxidized, causing a change in the color of a substance. Textile companies sometimes use chlorine to decolorize fabrics or process waters. In some cases, odors can be reduced or eliminated through oxidation. However, the odor of certain compounds, such as some phenolics, is aggravated through a reaction with chlorine. Certain soluble metals can be made insoluble through oxidation by chlorine (soluble Fe2+ is oxidized to insoluble Fe3+), making the metal easier to remove through sedimentation or filtration.
Chlorine is commercially available in three forms; it can also be generated on-site. For treating small quantities of water, calcium hypochlorite (Ca(OCl)2), commonly referred to as high test hypochlorite (HTH) because one mole of HTH provides two OCl- ions, is sometimes used. For large applications, chlorine gas (Cl2) is the most wide used source of chlorine. It reacts readily with water to form various chlorine species and is generally the least expensive source. There are, however, risks associated with the handling and transport of chlorine gas, and these have convinced some to use sodium hypochlorite (NaOCl) instead. Sodium hypochlorite is more expensive than chlorine gas, but less expensive than calcium hypochlorite. Some utilities and industries have generated chlorine on-site for many years, using electrolysis to oxidize chloride ions to chlorine. The process is practical in remote areas where brine, a source of chloride ions, is readily available.
Chlorine has been used in the United States since the early 1900s for disinfection. It is still commonly used to disinfect wastewater and drinking water, but the rules guiding its use are gradually changing. Until recently, chlorine was added to wastewater effluents from treatment plants without great concern over its effects on the environment. The environmental impact was thought to be insignificant since chlorine was being used in such low concentrations. However, evidence has accumulated showing serious environmental consequences from the discharge of even low levels of various forms of chlorine and chlorine compounds, and many plants now dechlorinate their wastewater after allowing the chlorine to react with the wastewater for 30–60 minutes.
Chloroform was the THM found in the highest concentrations during the surveys. The risks associated with drinking water containing high levels of chloroform are not clear. It is known that 0.2 quarts (200 ml) of chloroform is usually fatal to humans, but the highest concentrations in the drinking water surveyed fell far below (311 ug/l) this lethal dose. The potential carcinogenic effects of chloroform are more difficult to evaluate. It does not cause Salmonella typhimurium in the Ames test to mutate, but it does cause mutations in yeast and has been found to cause tumors in rats and mice. However, the ability of chloroform to cause cancer in humans is still questionable, and the EPA has classified it and other THMs as probable human carcinogens. Based on these data, the maximum contaminant level for THMs in drinking water is now 100 ug/l. This is an enforceable standard and requires the monitoring and reporting of THM concentrations in drinking water.
There are several ways to test for chlorine, but among the more common methods are iodometric, DPD (N,N- Diethyl-p-phenylenediamine) and amperometric. DPD and amperometric methods are generally used in the water and wastewater treatment industry. DPD is a dye which is oxidized by the presence of chlorine, creating a reddish color. The intensity of the color can then be measured and related to chlorine level; the DPD solution can be titrated with a reducing agent (ferrous ammonium sulfate) until the reddish color dissipates. In the amperometric titration method, an oxidant sets up a current in a solution which is measured by the amperometric titrator. A reducing agent (phenylarsine oxide) is then added slowly until no current can be measured by the titrator. The amount of titrant added is commonly related to the amount of chlorine present.
To minimize the problem of chlorinated byproducts, many cities in the United States, including Denver, Portland, St. Louis, Boston, Indianapolis, Minneapolis, and Dallas, use chloramination rather than simple chlorination. Chlorine is still required for chloramination, but ammonia is added before or at the same time to form chloramines. Chloramines do not react with organic precursors to form halogenated by-products including THMs. The problem in using chloramines is that they are not as effective as the free chlorine forms at killing pathogens.
Questions still remain about whether the levels of chlorine currently used are dangerous to human health. The level of chlorine in most water supplies is approximately 1 mg/l, and there is evidence that the chlorinated by- products formed are not hazardous to humans at these levels. There are some risks, nevertheless, and perhaps the most important question is whether these outweigh the benefits of using chlorine. The final issue concerns the short-term and long-term effects of discharging chlorine into the environment. Dechlorination would be yet another treatment step, requiring the commitment of additional resources. At the present time, the general consensus is that chlorine is more beneficial than harmful. However, it is important to note that a great deal of research is now underway to explore the benefits of using alternative disinfectants such as ozone, chlorine dioxide, and ultraviolet light. Each alternative poses some problems of its own, so despite the current availability of a great deal of research data, the selection of an alternative is difficult.
See also Drinking water supply ; Ozone .
Centers for Disease Control and Prevention (CDC). “Chlorine.” http://emergency.cdc.gov/agent/chlorine/index.asp (accessed November 9, 2010).
Gregory D. Boardman