Catalytic Oxidation Of As(Ⅲ)by Fe(Ⅱ) And Dissolved Oxygen

As（Ⅲ）is a toxic contaminant in aqueous environments,and oxidative flocculation of Fe（Ⅱ）is an important method to remove As（Ⅲ）contaminant.In ground water,ferrihydrite with a high surface area not only accelerates the oxidation of the adsorbed Fe（Ⅱ）,but also shows high adsorption capacities toward As（Ⅲ,V）contaminants.However,the concentrations of dissolved oxygen（DO）,Fe（Ⅱ）and bicarbonate on the oxidation and removal efficiency of As（Ⅲ）are not clear,and the speciation of the produced Fe（oxyhydr）oxides as well as their effects on the mobilization of As（Ⅲ,V）contaminants also needs to be investigated.In oxic aqueous environments,the presence of ligands such as HCO₃^-,Na₂C₂O₄and Na₂C₆H₅O₇on the oxidative flocculation of Fe（Ⅱ）as well as the oxidation and removal of As（Ⅲ）also needs to be explored.In this study,kinetic experiments and solid characterizations such as XPS,FT-IR and XRD will be used to investigate the oxidation kinetics of Fe（Ⅱ）and As（Ⅲ）,and study the concentrations of DO,Fe（Ⅱ）and bicarbonate on the oxidation efficiency of As（Ⅲ）,and on the speciation of the produced secondary minerals.The corresponding mechanisms involved in those processes will also be discussed.The results show that the oxidation efficiencies of As（Ⅲ）increase with increasing DO.As to the system containing 100 mg L^-1As（Ⅲ）,1 g L^-1ferrihydrite and 5 m M Fe（Ⅱ）,18.3%As（Ⅲ）can be oxidized with 93.42%total As being removed.Although the presence of HCO₃^-apparently desorbs As adsorbed on ferrihydrite,it can also increases the oxidation efficiency of As（Ⅲ）to 30.5%.The high removal percentage of total As can be attributed to the adsorption of As on ferrihydrite or the produce secondary minerals.Solid characterizations further indicate that lepidocrocite,magnetite and ferric arsenate are primary minerals in the reaction system,and siderite is also present in the bicarbonate system.The results also show that oxidative flocculation of Fe（Ⅱ）can effectively remove As（Ⅲ）in oxic aqueous environments.In the absence of ligand,45.8%of 10mg L^-1As（Ⅲ）can be catalytic oxidized by DO and 3 m M Fe（Ⅱ）,whereas in the presence of 1 m M Na HCO₃,Na₂C₂O₄or Na₂C₆H₅O₇,the respective 95.4%,77.5%or 71.4%As（Ⅲ）can be oxidized.Quenching experiments in the presence of SOD and CAT show that the oxidation efficiencies of As（Ⅲ）decrease by 50%,which further indicates that the produced H₂O₂and O₂^·-oxidative species play important roles in As（Ⅲ）oxidation.This study provides new insight into oxidation by As（Ⅲ）during oxidative flocculation of Fe（Ⅱ）.