Study On The Removal Efficiency And Mechanism Of Roxarsone In Water By Chlorination And UV/chlorine

Organoarsenic could be transformed into highly toxic inorganic arsenic through biological or abiotic reaction into aquatic environments,which could harm to human health.Due to the frequent occurrence of organoarsenic in water,thoroughly removing organoarsenic from water became a hot topic in the arsenic pollution field.At present,various methods for removing organoarsenic have been studied,including adsorption,anaerobic biotransformation,photocatalysis,etc.However,most of them either failed to remove the inorganic arsenic produced during the degradation of organoarsenic or required a long time to achieve appreciable removal.The transformation of organoarsenic to arsenate（As（V））could improve the removal efficiency of total arsenic,because the adsorption capacity of aluminum and iron adsorbents to inorganic arsenic is stronger than that of organoarsenic.Therefore,this paper studied the degradation efficiency and mechanism of organoarsenic by chlorine and UV/chlorine as well as the removal efficiency of total arsenic.Roxarsone（ROX）was selected as the target,and the main research contents are as follows:（1）Chlorine could effectively degrade ROX,and the reaction conformed to the second order reaction kinetics.The degradation of ROX was p H-dependent,the degradation rate constant of ROX decreased from 5.12 M^-1·s^-1 to 0.05 M^-1·s^-1 when p H increased from 5 to 10.This was mainly because p H determined the forms of available chlorine and of ROX in the solution.The experimental rate constants at different p H values fitted well with the acid-base protonation model.The results showed that the degradation of ROX resulted from the reaction between chlorine and deprotonated ROX.In this study,humic acid（HA）had an inhibition for the decay of ROX.HA would not only compete with ROX for chlorine,but also reverse the ROX-phenoxy group back to the parent ROX via electron transfer,thus inhibiting the degradation of ROX.By analyzing the inhibition of HA on ROX based on the two-channel kinetic model,the results indicated that the electrophilic substitution pathway（65.2%）was the main pathway of ROX degradation.The change of arsenic speciation and products during ROX degradation were analyzed to predict the degradation pathway of ROX.Ti was found that 32.4%of ROX was oxidized to As（V）through electron transfer pathway,and the remaining ROX was converted to monochlorine-ROX through electrophilic substitution pathway.Compared with ROX,iron oxide has stronger adsorption capacity for As（V）and chlorinated organoarsenic,especially for chlorinated organoarsenic.Therefore,after the oxidation,the removal of total arsenic in the solution was improved.In addition,ROX could still be effectively removed in real water.（2）ROX could be efficiently decayed by the combination of UV and chlorine.At p H 7,5μM of ROX could be completely degraded by UV/chlorine（100μM Na Cl O）within 30 min with a degradation rate of 0.1198 min^-1.The degradation rate initially increased then dropped with the raise of p H,which was mainly related to the form of available chlorine,free radical capture and the generation of Cl₂^·-.The results of radical quenching experiments and competitive kinetics showed that hydroxyl radical（HO·）and reactive chlorine species（RCS）were major reactive species in the decay of ROX.The contribution of HO·and RCS to ROX degradation was 24.6%and 66.0%at p H 7,respectively;whereas RCS was the main oxidant for ROX decay at p H 10 with a contribution of 92.7%.The degradation pathway of ROX was deduced by the theoretical calculation of ROX and the analysis of transformation products.During the degradation of ROX by UV/chlorine,the As-C bond was broken by attack of radicals,and inorganic arsenic was released,81.8%of ROX was converted into As（V）.The conversion of organoarsenic to inorganic arsenic was beneficial to the removal of total arsenic,and the removal of total arsenic increased remarkably after UV/chlorine.In addition,cytotoxicity studies showed that cytotoxicity was enhanced after the degradation of ROX by UV/chlorine,owing to the production of highly toxic inorganic arsenic and chlorinated products in the degradation of ROX.However,the cytotoxicity was eliminated when the oxidized solution was adsorbed,since iron oxides adsorbed inorganic arsenic and chlorinated products.This study demonstrated that UV/chlorine process can be an effective method for the control of the potential environmental risks of organoarsenic...