Catalytic Oxidation Of As（Ⅲ） By Mn（HCO₃^-）_n-Activated H₂O₂/SO₃^2-

Arsenic（As）is a toxic contaminant widely distributed in groundwater,which can bring about serious damages to human organs.Given the toxicity and the mobilization ability of As（Ⅲ）are much higher than those of As（Ⅴ）,it’s necessary to oxidize As（Ⅲ）to As（Ⅴ）in the first step in the treatment of As（Ⅲ）.In this study,we will investigate the reactivity of the Mn^Ⅱ-H₂O₂-HCO₃^-(Mn^Ⅱ-BAP)and Mn^Ⅱ-SO₃^2--HCO₃^--[Dissolved oxygen（DO）systems in the oxidation of As（Ⅲ）,the catalytic mechanism of the Mn^Ⅱ/Ⅲ-（HCO₃^-）_n complexes,the oxidizing species and the corresponding oxidation mechanisms.First,kinetic results show that the Mn^Ⅱ-（HCO₃^-）_n complexes[including Mn^Ⅱ（HCO₃）⁺and Mn^Ⅱ（HCO₃）₂]play important roles in the oxidation of As（Ⅲ）in the study of Mn^Ⅱ-BAP system.Quenching experiments indicate that O₂^·-and the oxidizing species generated from O₂^·-may play predominant roles in the oxidation of As（Ⅲ）.We further reveal that the MnO₂⁺-（HCO₃^-）_n intermediate generated in the reaction between Mn^Ⅱ-（HCO₃^-）_n and O₂^·-,instead of O₂^·-,is the predominant oxidizing species in the Mn^Ⅱ-BAP system.Although CO₃^·-also contributes to As（Ⅲ）oxidation,the high reaction rate constant between CO₃^·-and O₂^·-indicates that CO₃^·-is not the predominant oxidizing species in the As（Ⅲ）-Mn^Ⅱ-BAP system.Meanwhile,the presence of Mn（Ⅲ）further indicates the importance of Mn（Ⅱ）-Mn（Ⅲ）cycling in the formation of MnO₂⁺-（HCO₃^-）_nin the Mn^Ⅱ-BAP system.We therefore suggest two important roles of Mn^Ⅱ-（HCO₃^-）_n in the Mn^Ⅱ-BAP system:（i）Mn^Ⅱ-（HCO₃^-）_nreacts with H₂O₂ to form the Mn^Ⅲ（HCO₃）₃ intermediate,followed by subsequent reaction between Mn^Ⅲ（HCO₃）₃and H₂O₂ to produce O₂^·-;（ii）Mn^Ⅱ-（HCO₃^-）_n can also stabilize O₂^·-with the formation of MnO₂⁺-（HCO₃^-）_n.MnO₂⁺-（HCO₃^-）_n plays the predominant role in the oxidation of As（Ⅲ）.Then,HCO₃^-was introduced to the Mn^Ⅱ-SO₃^2-system based on the conclusion in the above study that the HCO₃^-ligand lowered the oxidation potential of Mn（Ⅲ）/Mn（Ⅱ）and facilitated the transformation between Mn（Ⅲ）and Mn（Ⅱ）species.Kinetic results show that the concentration of HCO₃^-is a key factor in the oxidization of As（Ⅲ）in the Mn^Ⅱ-SO₃^2--HCO₃^--DO system.When the concentration of HCO₃^-is less than 125 m M,the complexation of HCO₃^-、Mn²⁺and SO₃^2-requires some time to reach equilibrium,resulting in the slow oxidation rate of As（Ⅲ）during the complexation.When the concentration of HCO₃^-is higher than 125 m M,excessive HCO₃^-competes with SO₃^2-for Mn（Ⅱ）,inhibiting the oxidation of As（Ⅲ）in the Mn^Ⅱ-SO₃^2--HCO₃^--DO system.Quenching experiments further indicate that both SO₄^·-and OH·contribute to As（Ⅲ）oxidation and SO₄^·-is the predominant oxidizing species.We therefore suggest that in the Mn^Ⅱ-SO₃^2--HCO₃^--DO system,complexation between Mn²⁺and HCO₃^-facilitates the transformation from Mn（Ⅱ）to Mn（Ⅲ）and the production of SO₄^·-,resulting in the efficient oxidation of As（Ⅲ）.The present study enriches our understanding of the oxidation reactivity and the oxidizing species in the Mn^Ⅱ-BAP and Mn^Ⅱ-SO₃^2--HCO₃^-systems and provides new insight in the efficient and cost-effective oxidation of As（Ⅲ）in the high-arsenic groundwater.