Developing Of Coupled Kinetic Models Of Adsorption,Desorption And Oxidation Of As(Ⅲ) In Ferrihydritemn(Ⅱ)/Manganese Oxide Systems

Soil arsenic pollution is becoming more and more serious with the rapid development of economy and industrialization.The environmental behavior of arsenic in soil is affected by many active components of soil minerals,such as ferrihydrite,hematite and manganese oxides,which can react with arsenite/arsenate by adsorption,desorption and oxidation reactions,thus changing the species and bioavailability of arsenite/arsenate.In the natural environment,the coexistence of ferrihydrite and Mn（II）/manganese oxide is very common,and the coupled kinetic reactions of adsorption,desorption and oxidation of arsenite/arsenate will happen at the same time.However,there is still a lack of quantitative tools for predicting the coupled kinetic processes of As reactions with Fe and Mn（oxyhydr）oxides.The three models developed in this study were based on the kinetics experiments published in the literature,which involved three systems:（i）ferrihydrite-Mn（II）-As（Ⅲ）-O₂ system（system-A）,（ii）ferrihydrite-manganese oxide-As（Ⅲ）system（system-B）,（iii）ferrihydrite-birnessite-As（Ⅲ）system（system-C）.The model in this study also considered the heterogeneity of different chemical conditions and mineral complexation sites.According to the results of the model,ferrihydrite mainly adsorbed As（Ⅲ）and As（V）,while manganese oxide mainly oxidized As（Ⅲ）when ferrihydrite and manganese oxide coexisted.Besides,the adsorbed As（V）is mainly distributed on the nonprotonated-bidentate sites among the three types of binding sites of ferrihydrite.It was also found that the oxidation rates of different reaction systems were different.At the beginning of the reaction,the order of the oxidation rates of three systems were:system-C>system-B>system-A.However,with the development of the reaction,the order of the oxidation rates was reversed:system-A>system-B>system-C.The oxidation rate of system-C was high only for a short period of time,while the oxidation rate of system-A was high for most time of the reation process.Thus,system-A was more stable and its oxidation rate for As（Ⅲ）was larger compared with other systems.System A explored the influence of solution p H and initial molar ratio of Mn（II）to As（Ⅲ）on arsenite oxidation rate.The results showed that the oxidation rate was positively related to the p H value and initial molar ratio of Mn（II）to As（Ⅲ）.This model has showed good performance for interpreting the experimental data,which would help us to understand the kinetic behavior between As and specific minerals,and also provides us a model framework for predicting the coupled kinetic behavior of As in the environment.