Study On Kinetics And Mechanism Of Thallium(Ⅰ) Oxidation Catalyzed By Halogen Ions

Thallium（Tl）is a highly toxic heavy metal that exists widely in environmental water.Oxidation is a common and effective method for thallium removal,but it is easily affected by halogen ions in water.This study investigated the role of halogen ions commonly found in the environment in the treatment of common oxidants potassium permanganate（KMn O₄）and sodium hypochlorite（Na Cl O）in the oxidation of thallium（I）.In this experiment,ABTS spectrophotometry was used to detect the concentration change of reactants/products in the reaction process of potassium permanganate or sodium hypochlorite with Tl（Ⅰ）.The analysis of variance showed that the ABTS method at low buffer concentration would be affected by Tl（III）,which was corrected by the improved ABTS method.In the KMn O₄ experiment,the second-order reaction kinetics model was adopted,and the concentration of reactants was relatively close in the experimental process,while in the Na Cl O experiment,the pseudo-first-order reaction kinetics model was adopted,and the concentration of Na Cl O in the reaction was more than 166 times that of Tl（Ⅰ）.By combining p H-dependent oxidation experiments and kinetic modeling,the bromine ions（Br^-）at environmental levels exhibited a significant catalytic effect on Tl（Ⅰ）oxidation kinetics by permanganate and chlorine at a widely environmental p H range.Oxidation kinetics of chloride ion on Tl oxidation by KMn O₄ was mainly influenced by ionic strength,and iodine ions（I^-）in the Tl（Ⅰ）oxidation by sodium hypochlorite played a catalytic role.Under acidic conditions,the catalytic oxidation of Tl（Ⅰ）was driven by both the autocatalysis of Mn O₂ and the rapid oxidation kinetics of HOBr.Bromide ion mainly affected the reaction of Tl（Ⅰ）and KMn O₄ by participating in the homogeneous oxidation process,and the rate-limiting step was the stage of producing highly active Mn（V）and HOBr,while bromide ion had a weak influence on the reaction of heterogeneous phase.In the process of Tl（Ⅰ）oxidation by sodium hypochlorite,HOCl was the main active species,and the ionic strength had little effect on the oxidation.The Tl（Ⅰ）oxidation system of brominated HOCl can be described by the second-order reaction kinetics model,and the reaction rate is significantly increased with the addition of bromide ion.The results of quantification of the reactivity and contribution rate of each bromine species showed that,except HOBr,other bromine species,especially Br Cl and Br OCl,were also important active species in the Tl oxidation by HOCl.The addition of iodide ion increased the oxidation reaction rate to some extent,but the catalytic activity of iodide ion was relatively weak compared with bromide ion.The results highlight the key role of halogen ions in the oxidation kinetics of Tl（Ⅰ）,which is beneficial to the study of thallium REDOX kinetics and the evaluation of thallium removal strategies in water.