模拟环境条件下δ-MnO_2氧化As(Ⅲ)的搅拌流动动力学特征

Kinetic characteristics of As(Ⅲ) oxidation by δ-MnO_2 in the simulated environment: A stirred flow study

采用搅拌流动法研究了酸性水钠锰矿及水羟锰矿2种δ-MnO2矿物氧化As(Ⅲ)的动力学过程,构建了可用于多相体系的搅拌-流动氧化还原反应动力学模型.经过As吸附量的校正后,该模型对酸性水钠锰矿及水羟锰矿氧化As(Ⅲ)动力学数据拟合度分别为0.980和0.951,模型拟合得到pH 7时2种矿物单位比表面上氧化As(Ⅲ)的初始反应速率常数k分别为0.131,0.014min-1·m-2.相比而言,该速率常数明显高于批量法得到的表观速率常数kobs,0.021,0.001min-1?m-2更接近真实的化学动力学参数.搅拌流动法与批量法得到的不同矿物的速率常数大小趋势一致,即尽管酸性水钠锰矿对As(Ⅲ)的氧化率低于水羟锰矿,单位比表面上酸性水钠锰矿氧化As(Ⅲ)初始反应速率却远高于水羟锰矿.反应过程分析表明,反应初始阶段,As(Ⅲ)的吸附为主要限速步骤;而随着反应的进行,矿物表面反应位点逐渐钝化或减少,反应位点数量成为限速步骤.

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.