Chlorine dioxide, discovered in 1811 by Davy, was prepared from the reaction of potassium chlorate with hydrochloric acid. Early experimentation showed that chlorine dioxide exhibited strong oxidizing and bleaching properties. In the 1930s, the Mathieson Alkali Works developed the first commercial process for preparing chlorine dioxide from sodium chlorate. By 1939, sodium chlorite was established as a commercial product for the generation of chlorine dioxide. Chlorine dioxide uses expanded rapidly in the industrial sector. In 1944, chlorine dioxide was first applied for taste and odor control at a water treatment plant in Niagara Falls, New York. Other water plants recognized the uses and benefits of chlorine dioxide. In 1958, a national survey determined that 56 U.S. water utilities were using chlorine dioxide. The number of plants using chlorine dioxide has grown more slowly since that time.
At present, chlorine dioxide is primarily used as a bleaching chemical in the pulp and paper industry. It is also used in large amounts by the textile industry, as well as for the &aching of flour, fats, oils, and waxes. In treating drinking water, chlorine dioxide is used in this country for taste and odor control, decolorization, disinfection, provision of residual disinfectant in water distribution systems, and oxidation of iron, manganese, and organics. The principal use of chlorine dioxide in the United States is for the removal of taste and odor caused by phenolic compounds in raw water supplies.
Chlorine dioxide is a yellow-green gas and soluble in water at room temperature to about 2.9 g/1 chlorine dioxide (at 30 mm mercury partial pressure) or more than 10 g/1 in chilled water. The boiling point of liquid chlorine dioxide is 11° C; the melting point is - 59° C. Chlorine dioxide gas has a specific gravity of 2.4. The oxidant is used in a water solution and is five times more soluble in water than chlorine gas. In addition, chlorine dioxide does not react with water in the same manner that chlorine does. Chlorine dioxide is volatile; consequently, it can be stripped easily from a water solution by aeration.
Chlorine dioxide has a disagreeable odor, similar to that of chlorine gas, and is detectable at 17 ppm. It is distinctly irritating to the respiratory tract at a concentration of 45 ppm in air. Concentrations above 11 percent can be mildly explosive in air. As a gas or liquid, it readily decomposes upon exposure to ultraviolet light. It is also sensitive to temperature and pressure, two reasons why chlorine dioxide is generally not shipped in bulk concentrated quantities. Chlorine dioxide has a much greater oxidative capacity than chlorine and is therefore a more effective oxidant in lower concentrations. Chlorine dioxide also maintains an active residual in potable water longer than chlorine does. It does not react with ammonia or with trihalomethane precursors when prepared with no free residual chlorine. Chlorine dioxide is prepared from feedstock chemicals by several methods. The specific method depends on the quantity needed and the safety limitations in handling the various feedstock chemicals. The most common processes are: From sodium chlorite (NaCIOJ:
• Acid and sodium chlorite
• Gaseous chlorine and sodium chlorite
• Sodium hypochlorite, acid, and sodium chlorite.
From sodium chlorate (NaCI03):
• The sulfur dioxide process
• The methanol process.
The first group of processes is more commonly used. The second group of processes is frequently used by industry where the quantities produced are much greater than in water utilities.
Oxidation of phenols with chlorine dioxide or chlorine produces chlorinated aromatic intermediates before ring rupture. Oxidation of phenols with either chlorine dioxide or ozone produces oxidized aromatic compounds as intermediates which undergo ring rupture upon treatment with more oxidant and/or longer reaction times. In many cases, the same nonchlorinated, ringruptured aliphatic products are produced using ozone or chlorine dioxide. In oxidizing organic materials, chlorine dioxide can revert back to the chlorite ion. In the presence of excess chlorine (or other strong oxidant), chlorite can be preoxidized to chlorine dioxide. Using large excesses of chlorine dioxide over the organic materials appears to favor oxidation reactions (without chlorination), but slight excesses appear to favor chlorination. When excess free chlorine is present with the chlorine dioxide, chlorinated organics usually are produced, but in lower yields, depending on the concentration of chlorine and its reactivity with the particular organic(s) involved. Treatment of organic compounds with pure chlorine dioxide containing no excess free chlorine produces oxidation products containing no chlorine in some cases, but products containing chlorine in others.
Under drinking water plant treatment conditions, humic materials and/ or resorcinol do not produce trihalomethanes with chlorine dioxide even when a slight excess of chlorine (1 percent to 2 percent) is present. Also, saturated aliphatic compounds are not reactive with chlorine dioxide. Alcohols are oxidized to the corresponding acids.
The gaseous chlorine-sodium chlorite process for producing chlorine dioxide uses aqueous chlorine and aqueous sodium chlorite to produce a mixture of chlorine dioxide and chlorine (commonly as HOC1). Figure 4 shows such a system, consisting of a chlorine dioxide generator, a gas chlorinator, a storage reservoir for liquid sodium chlorite, and a chemical metering pump. (Sodium chlorite solution can be prepared from commercially available dry chemical by adding it to water.) The recommended feed ratio of chlorine to sodium chlorite is 1:1 by weight. Additional chlorine can be injected into the reactor vessel without changing the overall production of chlorine dioxide.
CI02 SOLUTION TO TREATMENT PROCESS
Figure 4. Components of a gaseous sodium chlorite-chlorine dioxide generation system.
A major disadvantage of this system is the limitation of the single-pass gas-chlorination phase. Unless increased pressure is used, this equipment is unable to achieve higher concentrations of chlorine as an aid to a more complete and controllable reaction with the chlorite ion. The French have developed a variation of this process using a multiple-pass enrichment loop on the chlorinator to achieve a much higher concentration of chlorine and thereby quickly attain the optimum pH for maximum conversion to chlorine dioxide. By using a multiple-pass recirculation system, the chlorine solution concentrates to a level of 5-6 g/1. At this concentration, the pH of the solution reduces to 3.0 and thereby provides the low pH level necessary for efficient chlorine dioxide production. A single pass results in a chlorine concentration in water of about 1 g/1, which produces a pH of 4 to 5. If sodium chlorite solution is added at this pH, only about 60 percent yield of chlorine dioxide is achieved. The remainder is unreacted chlorine (in solution) and chlorite ion. When upwards of 100 percent yield of chlorine dioxide is achieved, there is virtually no free chlorite or free chlorine carrying over into the product water. The French system can be designed for variable-feed rates with automatic control by an analytical monitor. This has the advantages of eliminating the chlorine dioxide storage reservoir. Production can be varied by 20 equal increments. A 10 kg/h (530 lb/day) reactor can be varied in 0.5 kg/h (26.5 lb/ day) steps over the range of 0-10 kg/h, and this can be accomplished by automatic control with the monitor located in the main plant control panel.
Another approach to chlorine dioxide production is the acid-sodium chlorite system. The combination of acid and sodium chlorite produces an aqueous solution of chlorine dioxide without production of significant amounts of free chlorine. The acid-based process avoids thé problem of differentiating between chlorine and chlorine dioxide for establishing an oxidant residual. This system uses liquid chemicals as the feedstock. Each tank has a level sensor to avoid overfilling. The tanks are installed below ground in concrete bunkers which are capable of withstanding an explosion. There are no floor drains in these bunkers. Any spillage must be pumped with corrosion-resistant pumps. Primary and backup sensors with alarms warn of any spillage. Because of the potential explosiveness, chemicals are diluted prior to the production of chlorine dioxide. The dilution is carried out on a batch basis controlled by level monitors. Proportionate quantities of softened dilution water along with the chemical reagents are pumped to mixing vessels by means of calibrated double-metering pumps. After the reactor is properly filled, an agitator within the container mixes the solution. Dilutions of 9 percent HC1 and 7.5 percent sodium chlorite are produced in the chemical preparation process. The chlorine dioxide is subsequently manufactured on a batch basis. The final strength of the solution is about 20 percent, 90 percent to 95 percent of this is chlorine dioxide and 4 percent to 7 percent is chlorine.
Chlorine is the most widely used disinfectant in water treatment. It appears, however, that it may not be the best disinfectant to use for drinking water where poor-quality raw water or completely recycled water is used. Other reasons for considering alternative disinfection techniques include the possibility that disinfection by chloramines will allow viruses to remain viable or that the inactivated virus particles have viable nucleic acids that may be released within humans, the reduction of germicidal efficiency with elevated pH, and the formation of persistent chlorinated organic compounds. Chlorine dioxide has proven to be a strong oxidizing agent. When free of chlorine, it does not form trihalomethane compounds in drinking water. It is less likely than chlorine to form chlorinated compounds with most organics commonly encountered in raw water supplies. Chlorine dioxide is effective in oxidizing organic complexes of iron and manganese, imparts no taste and odor to treated water, and provides a highly stable, long-lasting oxidant residual.
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