The Generation, Oxidation and Disinfection of
chlorine Dioxide

Cao rui yu Gu guo wei Huang zhi ming Ye hui
（School of Environmental Scientific and Engineering Tongji University）

Tongji Quality Supervision and Examination Center of Environment Protective product NEPA
Abstract:　This paper introduce you the generation and manufacture of chlorine dioxide, the oxidation of nonorganics and organics by chlorine dioxide. The practicing has been well established that chlorine dioxide in reaction with both humic and fulvic acid does not form trihalomethane. At a time the toxicity of ClO₂^- and ClO₃^- must be considered in evaluating the safety of chlorine dioxide as a disinfectant.
Keywords:　chlorine dioxide oxidation of nonorganics oxidation of organics trihalomethane

I. Introduction

　The chlorine had been used for the disnfectant in drinking water treatment, for long which the disinfection-effect had been known by consumer. But, the hazards by using chlorine in drinking water treatment, was being known by consumer. In 1974, H Florin (Rook) and U.S (Beller) examined the trihalomethane and chloroform. Up to now, about 2221 organic to health effects of man had been examined. And in drinking water have 765 type, among them 20 type are Carcinogenic, and 117 type are the mutagenic. Those organics not only can be caused the Carcinogenic but also can be caused the hepatotoxic, neurotoxic, and metabolic derangements. (Referenced in table 1).
　Because the man have studied the change of the disinfectant by chlorine; in which the chlorine dioxide is increased interest disinfectant using in drinking water treatment.
The diverse toxicological properites of these chlorination by-products are referenced in Table 1.

II. The history and the properties of Chlorine Dioxide

Eelier on, the chlorine dioxide was being called the green-yellow gas by Sir Humphrey Davy, and it was discovered in 1881. The green-yellow gas was produced by Davy with acidifying potassium chlorate with sulfuric acid.
In 1843, the first reference in the literature to chlorite was that of Millon, who obtained the green-yellow gas by acidifying potassium chlorate with hydrochloric acid. During that time, Millon did mot identify as contaimng chlorine dioxide, until 1881. Garzarolli-Thurnlackh identified the gas as a mixture of chlorine dioxide and chlorine. In 1944, the first report of using chlorine dioxide in drinking water treatment occurred, at the Niagara Falls, water treatment plant. The chlorine dioxide is green gas, its density is 2.4 time to chlorine, and its molecular weight is 67.44(g/mol). The solubility of chlorine dioxide is 5 times to the chlorine. The chlorine dioxide is easily explosive gas, which it is can be explosived when its concentration is over than 10 percent in the air and 30 percent in the water, and its concentration below about 10 g/L will not be produced sufficiently high vapor pressure to present an explosive hazard. Because of its explosive haxard, chlorine dioxide must be manufactured at the point of use.
In water treatment plant, the solution concentrations of the chlorine dioxide rarely exceed 4g/L, and its generally treatment levels range is from 0.1 to1.3 mg/L. Aqueous solution of chlorine dioxide are quite stable if kept cool, well sealed, and protected from light. Slight acidification of chlorine dioxide solutions (PH: 6) enhances stability by inhibiting its dosproportonation.

III. Generation and manufacture of chlorine dioxide

1. Generation of chlorine dioxide by sodium chlorate.
The chlorine dioxide can be generated by two ways.
1). Generation of chlorine dioxide is the reaction of sodium chlorate, sodium chlorine with sulphuric acid.

The health Effects Associated with Chlorination by-products (Table 1)

Chemical Class By-product Toxicological Effects Trihalo mthanes Chloroform
Dichlorobromomethan
Dibromochbromethane
Bromoform ①Carcinogenic ②hepatotoxic ③renal toxic ①Hepatotoxic ②renal toxic;
①Hepatotoxic ②renal toxic;
①Hepatotoxic ②renal toxic;
Haloacetonitriles Chloroacetonitrile
Dichloroacetonitrile
Trichloroacetonitrile
Bromochloroacetonitrile
Dibromoacetonitrile ①Genotoxic ②developmental;①②③
①Mulagenic ②genotoxic ③developmental;
①Genotoxic ②developmental;
①Mulagenic ②genotoxic ③developmental;
①Genotoxic ②developmental; Haloacid derivatives Dichloroacetic acid
Trichloroacetic acid
①Metabolic derangemental ②nerotoxic
①Increased hepatic peroxisomes
②③Oculorlesions ④aspermatogenesis Chlorophenols 2-chlorophenol
2,4-Dichlorophenol
2,4,6-Trichlorophenol ①Fetotoxic ②tumor promoter;
①Fetotoxic ②tumor promoter;
①Carcinogenic; Chlorinated Ketones 1,1-Dichloropropanone
1,1,1-Trichloropropanone
1,1,3,3-Tetrachloropropanone ①Mutagenic;
①Mutagenic;
①Mutagenic;

2NaClO₃+2NaCl+2H₂SO₄=2ClO₂+Cl₂+ 2Na₂SO₄+2H₂O------(1)

2). Generation of chlorine dioxide is reaction of sodium chlorite with chlorine.
　This reaction contains two stage: in first stage, the generation of the hypochlorous acid is by chlorine solute into the water; in second stage, the generation of chlorine dioxide is the reaction of hypochlorous acid with sodium chlorite.
Cl₂+H₂O=HOCl+HCl----------------2…(1)
HOCl+HCl+2NaClO₂=2ClO₂+2NaCl+H₂O------2…(2)
Can write be written as the follow equation
Cl₂ +2NaClO₂ =2ClO₂ +2NaCl------------2
Can see figure (1) and (2)
2. Generation of chlorine dioxide is mixed by the sodium chlorite with acid
(hydrochloric acid or sulfuric acid )
5NaClO₂ +4HCl=4ClO₂ 5NaCl+2H₂O---3
10NaClO₂+5H₂SO₄=8ClO₂+5Na₂SO₄+2HCl+4H₂O-------4

IV. The oxidation of the chlorine dioxide

1.Oxidation of manganese by chlorine dioxide.
As mentioned, chlorine dioxide is commonly used in drinking water treatment for manganese oxidation, and is sometimes used for sulphide oxidation. Chlorine dioxide is especially attractive for oxidation of manganese, because of the rapid reaction between chlorine and reduced manganese. The chlorine dioxide, as very strong oxidant, will oxidize manganese (2+) to manganese (4+), MnO2. This is typical of chlorine dioxide oxidation-reduction reactions at normal drinking water PH values.
And the chlorite produced from the reduction of chlorine dioxide also rapidly reacts with reduced manganese, so that the overall reaction is
2ClO₂+5Mn⁺²+6H₂O=5MnO₂+12H+2Cl-------------(5)
2.The oxidation of iron by chlorine dioxide.
The chlorine dioxide rapidly oxidizes iron (2+) to iron (3+), which precipitates as iron hydroxides like in the case of manganese oxidation, chlorite also reacts with reduced iron so that the overall reaction is:
ClO₂+5Fe(HCO₃)₂+3H₂O=5Fe(OH)₃+10CO₂+Cl^-+H⁺----(6)
The report has been presented about the chlorine dioxide to oxidize organically bound iron. Chlorine dioxide has been used in situations in which iron removal was not the primary concern, but where iron-bearing waters had promoted the growth of iron bacteria in the distribution system. Chlorine dioxide has also been used effectively to control the accumulation of biofilms, and may have been removing the attached bacteria and exposing them to the disinfectant in conjunction with oxidizing the iron present. The ability of chlorine dioxide to control biolfilms is thought to be a result of its reaction with the ‘polysaccharide‘ matrix that some organisms produce exocellularly and use to attach to surfaces.
3.The oxidation of sulphides by chlorine dioxide.
The chlorine dioxide can rapidly oxidize hydrogen sulfide. The end product of the oxidation has been reported to be exclusively the sulfate ion in the PH
Range of 5~9. The relative proportions of sulfur and sulfate are PH and temperature-dependent, if with higher PHs and temperatures favoring the formation of sulfate.
ClO₂+ S²-+H₂O—>SO₄²-+Cl-+4H⁺ ---------(7)
4.The oxidation of cyanide by chlorine dioxide.
The cyanide can be oxidized by chlorine dioxide; and its products are the carbon dioxide and nitrogen, and the reaction is:
ClO₂ + 2CN- + 2H₂O—> 2CO₂ + N₂ + Cl- + 4H⁺ ---------(8)

　

　

　

　

　

V. The oxidation of organics in drinking water by chlorine dioxide
1. The oxidation of phenols by chlorine dioxide.
In general, chlorine dioxide reacts primarily by oxidation reactions-resulting in few organic compounds, both volatile and nonvolatile, and in which chlorine atoms have been incorporated. In contrast, the chlorine reacts with organics only by electrophilic substitution, resulting a variety of volatile and nonvolatile chlorinated organic products, among them the Trihalomethanes. The follow equation can be clearly explained this problem.
The reaction of chlorine dioxide with phenolate is following equation.

The reaction of chlorin with phenolate is following equation.

Generally speaking, the sources of phenolic compounds in water supplies are reported to be industrial wastes, especially petroleum and wood-processing wasters, decaying vegetation and algae. The treatment for these waters containing phenolic compounds with chlorine dioxide does not produce the typical chlorophenolic medicinal taste and odor and it can be effectively removed existing tastes and odors. The organic products identified from the reaction between chlorine dioxide and phenol include the chlorophenals, p-benzoquinone and maleic and oxilic acids.
The shown by studying, the reaction between chlorine dioxide and phenol at PH 7 with it in excess is complete, and that the phenol is completely consumed, eliminating of chlorinated products.
2. Oxidation of humic substances by chlorine dioxide.
The practicing has been well established that chlorine dioxide in reaction with both humic and fulvic acid does not form THMs. These observations have been translated into effective alterations in drinking water treatment practices that serve to minimize THM formation. The most common mode of using chlorine dioxide for THM control is a replacement for prechlorination. The most way, chlorine dioxide is added to the raw water supply for prinary disinfection and oxidation. And the free chlorine and ocmbined chlorine, added postfiltration, is used as a residual disinfectant. Used this mode of operation, THM precursors are oxidized by the chlorine dioxide, and the other unit proceses of the water treatment plant, as coagulation, settling, and filtration, affect removal of THM precursors before final chlorination. The prechlorine-dioxide dosages are generally 30 to 50 percent of the required prechlorination dosage of the water treatment plant. For maintaining an adequate residual chlorine in the distribution system of the water supply are, however, generally slightly higher than before the chlorine dioxide. This process can be resulted in a 50 to 70 percent decrease in Trihalomathanes.
3. Chlorine dioxide for control of taste and odor compounds.
There are several reports on the successful application of chlorine dioxide for control of musty, or fishy tastes and odors in drinking water. The chlorine dioxide, as ozone, can be controled of specific taste and odor compounds from drinking water.
Colour removal by chlorine dioxide.
The result by practicing indicated that it is effective in colour removal by chlorine dioxide, because it is effective oxidant. For example, we had studied the colour removal by chlorine dioxide experiment in water treatment plant of city‘s river from the Ti Hang lake. The colour of raw water decreased from 17°to 9°when the chlorine dioxide dosages is from 1 to1.5 mg/l. In contrast, the colour of raw water decreased from 17°to 14°when the chlorine dosages is from 4 to 6 mg/l, and the water treatment processes contains the coagulation, setting, filtration and disinfection. Can see the figure 3.

　

　

　

　

VI. Toxicological problems by chlorine dioxide

The chlorine dioxide is a so strong oxidant that reacts with organic material to produce a variety of oxidized by-products. Numerous studies have been conducted with chlorine dioxide and its inorganic by-products-chlorite (Cl2-) and chlorate (ClO3-). So that the toxicity of ClO2- and ClO3- must be considered in evaluating the safety of chlorine dioxide as a disinfectant. Because chlorine dioxide and ClO3- can be rapidly converted to ClO2- in humans, some aspects of their toxicity are probably similar. The toxicological effects from exposure to chlorine dioxide, ClO2- and ClO3- were first associated with the hemopoietic system. Practicing indicated that ClO2- produced hemolytic anemia at lower exposure levels than those required to produce significant increases in methemoglobin. The hemolytic anemia to the red blood cell membrane. Additional studies extended these findings to chlorine dioxide and ClO3-. Chlorite remained the most potent of the three chemical species for causing signs of hemolytic oxidative stress in animals. These effects were see in varying degrees of sensitivity in several species.

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