Chlorite removal with ferrous ions

Effective use of chlorine dioxide as an alternative disinfectant in water treatment may require removal of the by! product chlorite ion (ClO2 ). The goal of this research was to investigate the use of ferrous iron (Fe2+) for the chemical ! reduction of ClO2 from drinking water in order to define the operating conditions, process efficiency with different pH conditions and organic carbon concentration and the potential formation of chlorate during this process. The main results show that the reaction between the ferrous ion and chlorite is very fast (5–15 s) over a range of pH 6.5–8.0; in ! this condition a ferrous ion dose of 3.31 mg Fe/mg ClO2 completely reduced chlorite to chloride, producing minimal residual soluble iron. For pH higher than 8.0–8.5, chlorite removal is lower due to the natural transformation of the ferrou ions to ferric hydroxide. Within these pH values, chlorite can be removed completely with ferrous ion concentrations higher than the stoichiometric value. Moreover, the application of ferrous salts for chlorite removal during the coagulation process enhances the performance of the coagulation and flocculation treatment.

Complete removal of chlorite can be obtained with a ferrous ion stoichiometric dose at neutral pH. At higher pH conditions (pH >8) an additional 25% Fe2+ dose is required to reach complete chlorite removal. The influence of organic matter concentration on the process efficiency is negligible. Chlorite removal with ferrous ions can be successfully applied on new potabilisation plants or as an up-grading stage of existing plants (by adding ferrous ions in existing coagulation/ flocculation tanks or before sand filtration). This process is very interesting for its simplicity and economy.