不同氧化度碱性水钠锰矿氧化As(Ⅲ)和Cr(Ⅲ)的动力学特性

Kinetics of As(Ⅲ) and Cr(Ⅲ) oxidation by alkaline birnessites with various average Mn oxidation states

用批量法研究了Mn平均氧化度分别为4.02、3.85和3.70的碱性水钠锰矿对As(Ⅲ)和Cr(Ⅲ)氧化的动力学特性。结果表明,碱性水钠锰矿氧化As(Ⅲ)和Cr(Ⅲ)先经历准一级动力学反应阶段,随后表观速率常数(kobs)逐渐减小至趋近零,达到反应平衡,kobs逐渐减小除了逆反应影响外,还与产物在矿物表面不断积累,钝化反应位点有关。初始反应阶段kobs随氧化度增加而增大,氧化As(Ⅲ)的kobs分别为:0.095 1、0.039 6和0.007 1 min-1;氧化Cr(Ⅲ)的kobs分别为0.034 2、0.017 8和0.014 8 min-1。氧化As(Ⅲ)的初始反应阶段生成的Mn(Ⅱ)基本保留在矿物表面,对反应位点钝化大,使kobs减小速度快。而Cr(Ⅲ)氧化初始阶段,Mn(Ⅱ)释放明显滞后于Cr(Ⅵ)释放,但随反应进行逐渐增加,与Cr(Ⅵ)释放量比值接近于反应计量比。故Cr(Ⅲ)氧化生成Mn(Ⅱ)对位点的钝化远小于As(Ⅲ)氧化,其初始反应阶段后kobs减小速度较慢,且氧化度越高,保持一级动力学的初始反应阶段越长,kobs减小越慢。因此,水钠锰矿的Mn氧化度越高,氧化As(Ⅲ)和Cr(Ⅲ)的反应速率越快,As(Ⅲ)和Cr(Ⅲ)在碱性水钠锰矿表面氧化产物的释放行为是影响其反应动力学特性的重要因素。

Synthetic alkaline birnessites with the average Mn oxidation states of 4.02、3.85 and 3.70 are applied to investigate the kinetic characteristics of As（Ⅲ） and Cr（Ⅲ） oxidation.The results show that initial stage of As（Ⅲ） and Cr（Ⅲ） oxidation by Bir-OHs comply with the first order kinetics and then the apparent rate constant （kobs） decrease with the reaction down to zero due to both the effects of the reverse reaction and the reaction site passivation resulting from the product accumulating on the mineral surface.Nearly no Mn（Ⅱ） release was detected in the initial reaction stage of As（Ⅲ） oxidation,Mn（Ⅱ） were adsorbed to the surface of mineral and then shielded the reaction sites,thus the kobs decreased sharply.In contrast,for Cr（Ⅲ） oxidation,although Mn（Ⅱ） release lagged behind of Cr （Ⅵ） release in the initial reaction stage,then more and more Mn （Ⅱ） were released to the solution as the reaction went on,the ratios of released Mn （Ⅱ） to Cr （Ⅵ） gradually increased up to the theoretical reaction stoichiometry.The effect of Mn（Ⅱ） shielding the surface reaction sites in the Cr（Ⅲ） oxidation is much less than that in the Cr（Ⅲ） oxidaton,thekobs decreases much slowly after the initial reaction stage.The higher the Mn AOS,the longer it maintained the first order kinetics in the initial reaction stage.Therefore,alkaline birnessite with higher Mn AOS exhibits the more rapid oxidation reaction towards As（Ⅲ） and Cr（Ⅲ）,and the behavior of product release to the solution is a significant factor determining the kinetics of the reaction.