The Transformation Rules Of Iodine Substances Oxidated By Ferrate (Ⅵ) During Water Treatment

The formation of iodinated disinfection byproducts（I-DBPs）was discovered during the oxidation of natural water containing iodide by traditional disinfectants（like liquid chlorine and chloramine）.I-DBPs have more genotoxicity and cytotoxicity than chlorinated disinfection by-products and brominated disinfection by-products.Iodide commonly exists in natural water.The raw water of drinking water plants in coastal areas has higher iodide concentration than in inland areas.Ferrate（Ⅵ）is an environment-friendly oxidizing agent which has a strong oxidation capacity.Ferrate（Ⅵ）can oxidize a variety of organic pollutants and kill various types of bacteria,virus,and so on.The research focuses on the formation potential of I-DBPs and the transformation rules of iodine substances during the oxidation of I-containing water by ferrate（Ⅵ）.First,the reaction rates of the reaction between ferrate（Ⅵ）and iodide/hypoiodous acid were investigated.The oxidation rates of the iodide and hypoiodous acid by ferrate（Ⅵ）decreased with the pH increased.As the pH increased from 5 to 10,the second-order rate constants of the reaction of ferrate（Ⅵ）with iodide decreased from2.16×10⁴ M^-1s^-1to 71 M^-1s^-1and the second-order rate constants of the reaction of ferrate（Ⅵ）with hypoiodous acid decreased from 1.6×105 M-1s-1 to 8.3×10² M^-1s^-1.The stoichiometry of the reaction of ferrate with iodide/hypoiodous acid was investigated.The stoichiometric coefficients of ferrate（Ⅵ）with iodide and hypoiodous acid were 2 and 1.3 respectively.The transformation rules of iodine substances during the oxidation of I-containing water by ferrate（Ⅵ）were investigated.Iodide was oxidized to hypoiodous acid quickly during the oxidation of iodide by ferrate（Ⅵ）and then the hypoiodous acid was oxidized to iodate.Hypoiodous acid was oxidized to iodate during the oxidation of hypoiodous acid by ferrate（Ⅵ）.An increase in ferrate（Ⅵ）concentration could promote the reaction rates of ferrate with iodide/hypoiodous acid.An increase in phosphate concentration can contribute the reaction of ferrate（Ⅵ）with iodide and inhibit the reaction of ferrate（Ⅵ）with hypoiodous acid.At pH 7,as the phosphate concentration increased from 0 to 100 mM,the second-order rate constants of the reaction of ferrate（Ⅵ）with iodide increased from 9×10³ M^-1s^-1to 1.8×10⁴ M^-1s^-1and the second-order rate constants of the reaction of ferrate（Ⅵ）with hypoiodous acid decreased from 1.1×10⁵ M^-1s^-11 to 5.4×10⁴ M^-1s^-1.The reaction kinetics of ferrate with iodide/hypoiodous acid in raw water were investigated.The second-order rate constants of ferrate with iodide and hypoiodous acid were 2.4×10² M^-1s^-1and 4.2×10³ M^-1s^-1,respectively.These two rate constants were less than the rate constants under the same condition in phosphate buffer,indicating the reductive substances in raw water consumed ferrate（Ⅵ）.Therefore,the reaction rates of ferrate with iodide/hypoiodous in raw water decreased.Choosing iodoform as a representative substance of I-DBPs.The generated laws of iodoform in ferrate（Ⅵ）/HA/iodide system were observed.At pH 7,the concentrations of iodide and HA were 20μM and 5mg/L respectively,no formation of iodoform was observed when the ferrate（Ⅵ）concentrations were 10、40 or 80μM.According to the above discussions,the risk of generating I-DBPs was very low in the oxidation system of ferrate（Ⅵ）.