The Control Effect And Mechanism Of Ferrate Pre-oxidation On Iodinated Aromatic Disinfection By-Products

Sea level rise caused by global warming has affected coastal areas by seawater intrusion.In some areas of China,the iodide content in the surface water and ground water has increased.Chlorine-containing disinfectant can oxidize the iodide in the source water into active iodine,and the active iodine will further react with the organic matter to generate iodinated disinfection by-products（I-DBPs）.The toxicity of iodinated disinfection by-products is much higher than that of chlorinated and brominated disinfection by-products.As a newly identified class of iodinated disinfection by-products,iodinated aromatic disinfection by-products are more toxic than iodinated aliphatic disinfection by-products（such as iodoform,iodoacetic acid,etc.）.Therefore,it is urgent to develop adv anced water treatment technologies to control the formation of iodinated aromatic disinfection by-products.The formation and transformation of iodinated aromatic compounds in the pre-chlorination of the source water were first explored.2-Iodophenol,4-iodophenol and 2,4,6-triiodophenol were selected as the representative compounds of iodinated aromatic disinfection by-products,and then the transformation mechanism of them by chlorine was investigated.The mechanism of ferrate pre-oxidation to control the formation of iodinated aromatic by-products was further explored,and the effect of ferrate pre-oxidation of iodine-containing water to control the formation of iodinated aromatic disinfection by-products was studied.Organic iodine at the level ofμg/L and iodophenols at the level of ng/L were detected in the source water.Iodophenols were selected as the representative compounds of iodinated aromatic organics in the water to explore the effect and mechanism of ferrate oxidation of them.In this study,it was found that a variety of iodinated aromatic compounds,including iodophenols,were generated in the pre-chlorination of the source water,and the concentrations of the iodinated aromatic compounds were at the level of ng/L.With increasing chlorination time,their concentrations first increased and then decreased,indicating that iodinated aromatic compounds could further react with chlorine to be converted.2-Iodophenol,4-iodophenol and 2,4,6-triiodophenol were selected as the representative compounds of iodinated aromatic disinfection by-products to investigate the reaction mechanism of chlorine with them.Under neutral conditions（p H=7.0）,the second order reaction rate constants of chlorine with 2-iodophenol,4-iodophenol and 2,4,6-triiodophenol were 1.86×10²M^-1s^-1,1.62×10² M^-1s^-1and 7.5×10¹ M^-1s^-1,respectively.Chlorine could finally convert iodophenols into chlorinated/iodinated aliphatic disinfection by-products and iodate.Although prolonging the chlorine disinfection time and increasing the chlorine dosage could reduce the generation of iodinated aromatic disinfection by-products,it would lead to an increase in the generation of chlorinated disinfection by-products.Therefore,it is necessary to develop effective and green water treatment technologies to control the formation of iodinated aromatic DBPs.Phenol,bisphenol A and p-hydroxybenzoic acid were selected as the representative compounds of the precursors of iodinated aromatic disinfection by-products to study the reaction kinetics of ferrate with them.Under neutral conditions（p H=7.0）,the second order reaction rate constants of ferrate with phenol,bisphenol A and p-hydroxybenzoic acid were 6.7×10¹ M^-1s^-1,7.3×10² M^-1s^-1and 2.6×10¹ M^-1s^-1,respectively.The reaction rate constants of ferrate with phenolic compounds decreased with the increase of p H.The connection of electron donor groups on the benzene ring could promote the oxidation of phenolic compound by ferrate,while the connection of electron withdrawing groups on the benzene ring could inhibit the oxidation of phenolic compound by ferrate.For the coexistence system of iodide and phenolic compounds,under acidic and neutral conditions（p H≤7.0）,iodide in water would inhibit the oxidation of phenolic compounds by ferrate,and promote the oxidation of phenolic compounds under alkaline conditions（p H≥8.0）.Under acidic conditions（p H=5.0）,no iodinated aromatic by-products were detected in the ferrate/iodide/phenolic compounds systems.Iodinated aromatic by-products were detected in the ferrate/iodide/phenolic compounds systems under neutral and alkaline conditions（p H≥7.0）.When ferrate oxidized iodine-containing water,iodide was first oxidized to hypoiodous acid by ferrate.Under acidic conditions（p H=5.0）,most of the iodide in the ferrate/iodide/phenolic compounds systems was oxidized to iodate in a very short time（within a few seconds）,making it difficult for hypoiodous acid to accumulate in water,thus avoiding the reaction of hypoiodous acid with phenolic compounds to produce iodinated aromatic disinfection by-products.Under alkaline conditions（p H≥8.0）,the formation rate of iodate was slowed down,and hypoiodous acid was easier to accumulate in the system,which was conducive to the reaction of hypoiodous acid with phenolic compounds to produce iodinated aromatic disinfection by-products.Although iodinated aromatic disinfection by-products were produced in the ferrate/iodide/phenolic compounds systems under neutral and alkaline conditions,ferrate could further oxidize the generated iodinated aromatic disinfection by-products in the systems.Therefore,ferrate pre-oxidation was effective to control the formation of iodinated aromatic disinfection by-products in water.In addition to iodide,organic iodine was also an important iodine source of iodinated disinfection by-products.2-Iodophenol,4-iodophenol and 2,4,6-triiodophenol were selected as the representative compounds of iodinated aromatic organic compounds,and the effect and mechanism of ferrate oxidation of them were investigated.The second order reaction rate constants of ferrate with iodophenols decreased with the increase of p H.Under neutral conditions（p H=7.0）,the second order reaction rate constant s of ferrate with 2-iodophenol,4-iodophenol and 2,4,6-triiodophenol were 1.39×10³M^-1s^-1,4.58×10²M^-1s^-1and2.89×10³M^-1s^-1,respectively.The formation of iodate was observed in the oxidation of iodophenols by ferrate,indicating that the electrophilic reaction between ferrate and iodophenol led that iodine atom on iodophenol was deprived from the benzene ring,and the deprived iodine could be further oxidized to stable and non-toxic iodate.Catechol and hydroquinone were the deiodination products of 2-iodophenol and 4-iodophenol,respectively.The results of quantum chemical calculation confirmed that the deiodination of iodophenols by ferrate was a spontaneous and energy reducing process.In addition to the deiodination products,iodinated products,oxidation products and coupling products were also detected in the ferrate oxidation of iodophenols.The acute toxicity of iodophenols was greatly reduced after ferrate oxidation.The deiodination of iodophenols by ferrate was also a detoxification process.A variety of iodinated aromatic disinfection by-products were detected in the source water after chlorine and chloramine disinfection.When 100μM ferrate was used to pre-oxidize the source water,no iodinated aromatic disinfection by-products were detected in the chlorine and chloramine disinfected source water.Ferrate pre-oxidation could convert iodide and organic iodine in water into iodate,thus avoiding iodide and organic iodine from participating in the formation of iodinated aromatic disinfection by-products in the subsequent disinfection process.Therefore,ferrate pre-oxidation could effectively control the generation of iodinated aromatic disinfection by-products in the disinfected source water.