Kinetics of As(III) and Cr(III) Oxidation by OH-birnessites with Various Average Oxidation States (AOSs)

摘要

A series of alkaline birnessites (OH-birnessites), termed as Bir-OH1, Bir-OH2 and Bir-OH3, with average oxidation states (AOSs) of 4.02, 3.85 and 3.70 were synthesized. The PZCs of the OH-bimessites, around 1～2, decreased with the increase of AOS. Oxidation of Cr(Ⅲ) and As(Ⅲ) by OH-birnessites in the initial reaction stage complied with the pseudo first order kinetics. The apparent reaction rate constants (Kobs) in the initial reaction stage increased with the increase of AOS. The Kobs of Cr(Ⅲ) oxidation by the tested samples in order were: Bir-OH1 (0.0342 min-1)>Bir-OH2 (0.0178 min-1)>Bir-OH3 (0.0148 min-1). Those of As(Ⅲ) oxidation were: Bir-OH1 (0.0951 min-1)>Bit-OH2 (0.0396 min-1)>Bir-OH3 (0.0071 min-1). The Kobs also increased with increase of AOS. It was indicated that Mn(IV) and Mn(Ⅲ) in birnessite had different reactivities, the AOS of Ohbirnessites which was determined by the ratio of Mn (Ⅳ) and Mn(Ⅲ) played an important role in affecting oxidation kinetics. In the initial stage of Cr(Ⅲ) oxidation, Mn(Ⅱ) release lagged behind Cr(VI) release. More and more Mn(Ⅱ) were released to the solution as the reaction went on, and the ratios of released Mn(Ⅱ) to Cr(Ⅳ) gradually approached the theoretical value of the reaction stoichiometry. While for As(Ⅲ) oxidation, nearly no Mn(Ⅱ) was detected in the whole oxidation process, the produced Mn(Ⅱ) adsorbed to the surface of OH-birnessites and then formed precipitate, preventing the reaction from going on. It was implied that Cr(Ⅲ) and as(Ⅲ) oxidation had different kinetic behaviors on the surface of OH-bimessites.