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| Kinetic characteristics of As (III) oxidation by δ-MnO2 in the simulated environment: A stirred flow study |
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Abstract In this study, As (III) oxidation at the surfaces of two δ-MnO2 minerals, birnessite and vernadite, was investigated using stirred-flow technique, and the corresponding model was established to describe the kinetics of the redox reactions in the stirred-flow heterogeneous system. Following correction of As adsorption, the degree of fitting of the stirred-flow kinetic data of As (III) oxidation by birnessite and vernadite using the established model was 0.980and 0.951, respectively. The obtained initial rate constants (k) per unit specific area at pH 7were 0.131 and 0.014min-1m-2, respectively, which were much greater than apparent initial rate constant (kobs), 0.021min-1m-2 and 0.001min-1m-2, derived from the batch experiments. This indicated that rate constant (k) is much closer to the real chemical kintics that could be obtained using the stirred-flow technique. Both stirred-flow and the batch experiments showed that birnessite exhibited the greater reaction rate on a per surface area basis in As (III) oxidation than on vernadite, although birnessite had a relatively lower suface area and As (III) oxidation capacity. Analysis of the reaction process suggested that As (III) absorption was the rate determining step in the initial stage, and then the number of suface Mn reactive sites gradually became the rate determining step with the passivation and decrease of the sites.
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Received: 12 August 2013
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