Study On The Stability Of Chlorine Dioxide And The Formation Of Disinfection Byproducts Under High Chloride Ionic Strength Conditions

Chlorine dioxide（ClO₂）is an environmentally friendly oxidant and disinfectant commonly used in water treatment because of its strong oxidizing property and no direct production of halogenated disinfection by-products（DBPs）.However,ClO₂is extremely unstable and prone to disproportionation and decomposition due to various environmental factors,resulting in free active chlorine（FAC,which can form halogenated DBPs with organic precursors）and inorganic DBPs—chlorite（ClO₂^-）and chlorate（ClO₃^-）.At present,the influence mechanism of high chloride ionic strength on the stability of ClO₂ and the formation/transformation of inorganic and organic DBPs is still unclear.On the basis of literature review,this paper studies the following three aspects:（1）The effect of high chloride ionic strength on the stability of ClO₂.（2）The effect of high chloride ionic strength on the formation/transformation of DBPs during ClO₂ oxidation of typical organic compounds（phenol,hydroquinone,and humic acid）.（3）The production of DBPs in ClO₂oxidation of actual wastewater（reverse osmosis concentrated water of steel enterprises）.The main conclusions are as follows:（1）The effects of high chloride ionic strength on the stability of ClO₂ were investigated at pH（4,7,8,10）and ClO₂ concentration（0-50mg/L）.The results showed:（1）Without high chloride ion strength,ClO₂ is the most unstable（decays by 6.5%）and generates the most FAC（about 50～70%of the decayed ClO₂ can be converted to FAC）at pH 7.On the contrary,ClO₂ is the most stable（decays by 2.8%）and generates the least FAC at pH 10.（2）With high chloride ion strength（4000mg/L Cl^-）,ClO₂ is more stable at pH 4（increases by 1.03%）and less stable at other pH values（decreases by 0.2%～0.5%）.In sum,the stability of ClO₂is not obviously affected by high chloride ion strength,but significantly affected by pH.（2）Based on the influence of pH on ClO₂ and the pH of actual high-salt wastewater,the effects of high chloride ion strength（4000mg/L Cl^-）on the oxidation of phenol by ClO₂were investigated at pH 7 and 8.The results showed:（1）In phenol,the existence of high chloride ion strength leads to a decrease in ClO₂^-（100%）,ClO₃^-（3.5%～8.4%）and the formation of FAC（10.2%～17.0%）,resulting in a decrease in DBPs formation（18.3%）.（2）In hydroquinone,the existence of high chloride ion strength leads to a decrease in ClO₂^-（6.9%～37.0%）,ClO₃^-（3.1%～10.1%）and the formation of FAC（8.0%～13.0%）,resulting in a decrease in DBPs formation（25.7%）.（3）In humic acid,the existence of high chloride ion strength leads to a decrease in ClO₂^-（4.0%～9.3%）,ClO₃^-（10.8%）and the formation of FAC（36.5%～38%）,resulting in a decrease in DBPs formation（29.7%）.However,when the concentration of ClO₂ is too high,the production of DBPs will increase by about 0.3%.In sum,high chloride ion strength can concomitantly reduce inorganic DBPs（ClO₂^-and ClO₃^-）and the production of organic DBPs（by reducing FAC production）.Therefore,ClO₂ has a good prospect for the treatment of organic matter in high-salt organic wastewater,but the dosage of ClO₂ should be reasonably controlled.（3）Finally,the production of inorganic and organic DBPs is studied in the treatment of actual reverse osmosis water with CIO₂in ClO₂ treatment of actual reverse osmosis water（COD:60～80mg/L,pH:7.55～8.06,Cl^-concentration:3951.8mg/L）.The results showed:When the dosage of ClO₂ was less than 60mg/L,ClO₂^-could not be detected,and FAC was rarely generated（only 5.8%）.When the dosage of ClO₂ was higher than 60 mg/L,the production of ClO₂^-was only 1%of ClO₃^-,and the production of FAC increased rapidly to59 mg/L,but the production of DBPs continued to decrease（up to about 14.4%）.To sum up,ClO₃^-is the main inorganic DBPs of ClO₂ treatment of high-salt organic wastewater.In addition,appropriate excess ClO₂ will increase the production of FAC and inorganic DBPs but benefit the control of organic DBPs.