δ-MnO_2对Cr(Ⅲ)氧化动力学特征

Kinetic Characteristics of Cr(Ⅲ) Oxidation by δ-MnO_2

采用分步离心法研究了δ-MnO2对Cr(Ⅲ)的氧化动力学.结果表明,δ-MnO2对Cr(Ⅲ)的氧化反应可以用一级动力学方程分段拟合.氧化反应是由2个速率不同的一级反应组成,Cr(Ⅲ)溶液在高浓度(400μmol/L)条件下,也可以用扩散方程和Elovich方程分段拟合.温度升高显著增加快反应阶段速率常数.随着反应的进行,MnO2表面吸附的Cr(Ⅵ)/Mn(Ⅱ)逐渐减小并远小于理论值(0.667),MnO2表面吸附的Cr(Ⅲ)、Cr(Ⅵ)和Mn(Ⅱ)只占总量的0.1%～3%.溶液中的Cr(Ⅵ)/Mn(Ⅱ)逐渐减小并接近理论值.所以Mn(Ⅱ)向溶液中的扩散滞后于Cr(Ⅵ).Cr(Ⅲ)的氧化反应的速率控制步骤,在低浓度条件下是Cr(Ⅲ)的扩散和吸附,在高浓度条件下是Mn(Ⅱ)向溶液中的扩散.

The kinetics of Cr（Ⅲ）oxidation by synthesized vernadite（δ-MnO2）was investigated through magnetic stirring and fractional centrifugation. The oxidation procedure of Cr（Ⅲ）by δ-MnO2could be divided into two first-order reactions,a fast reaction followed by a slow one. In high Cr（Ⅲ） concentration solution,the reaction was also well sub-simulated with diffusion equation and Elovich equation according to the reaction phases. The rate constants markedly increased with temperature rising. The ratio of Cr（Ⅵ） / Mn（Ⅱ）gradually decreased and eventually reached the theoretical value（0. 667） with the reaction. Cr（Ⅵ）,Cr（Ⅲ） and Mn（Ⅱ） adsorbed on δ-MnO2only accounted 0. 1 to 3% of the total amounts. The Cr（Ⅵ） /Mn（Ⅱ） ratio on the surface of MnO2 was much smaller than the theoretical value due to the release of Mn（Ⅱ） to the solution lagged behind Cr（Ⅵ）. When Cr（Ⅲ） was in low concentration,Cr（Ⅲ）oxidation was controlled by the diffusion and adsorption of Cr（Ⅲ）. While in high Cr（Ⅲ） concentration,the key step was the diffusion of Mn（Ⅱ） to solution.