Mn^(2+)对草酸还原Cr(Ⅵ)的强化效果及机理研究

Strengthening Effect and Mechanism of Mn^(2+) on Reducing Cr(Ⅵ)by Oxalic Acid

地下环境中普遍存在的金属离子和小分子有机酸可以共同还原降解对环境和人体有害的Cr(Ⅵ)。通过探讨不同影响因素(体系组分、初始pH值、有机酸浓度、金属离子浓度)下,Cr(Ⅵ)的还原转化效果及其规律,确定了最佳反应组合,并探讨了Mn^(2+)对草酸还原Cr(Ⅵ)的强化效果及机理。结果表明:通过对比5种常见的小分子有机酸和5种常见的金属阳离子,确定了Mn^(2+)/草酸对Cr(Ⅵ)的还原效果最为显著;Mn^(2+)和草酸同时存在的反应体系中,Cr(Ⅵ)的还原反应分为缓慢诱导期和自动加速期,两个阶段均符合拟一级动力学;随着反应体系初始pH值的增大,Cr(Ⅵ)的还原反应速率逐渐减小;草酸和Mn^(2+)浓度分别上升时,明显加速了Cr(Ⅵ)的还原;在Mn^(2+)/草酸反应体系中引入乙二胺四乙酸(EDTA)对Mn^(2+)强化草酸还原Cr(Ⅵ)有较强的抑制作用;通过3次Cr(Ⅵ)循环还原试验,证实了Cr(Ⅵ)还原过程中Mn^(2+)/草酸/Cr(Ⅵ)和Mn^(3+)/草酸/Cr(Ⅵ)两种中间体络合物的重要作用,并且测定了Cr(Ⅵ)还原的最终产物为Cr(Ⅲ)。该研究结果对揭示地下环境中铬的形态变化及其迁移转化规律具有重要的环境意义。

Metal ions and low-molecular-weight organic acids(LMWOA),which are ubiquitous in subsurface environments,can reduce and degrade Cr(Ⅵ)which is harmful to environment and human body.Through the exploration of the performance and mechanism of reduction of Cr(Ⅵ)with different factors,this paper determines the optimal reaction conditions,and discusses the effect and mechanism of Mn^(2+)enhanced reduction of Cr(Ⅵ)by oxalic acid.The results show that by comparing five common LMWOAs and five common metal cations,it is found that Mn^(2+)/oxalic acid/Cr(Ⅵ)is the optimal reduction system with the highest rate.In the system of Mn^(2+)and oxalic acid,the reduction of Cr(Ⅵ)can be divided into induction period and automatic acceleration period,which all conform to the first-order kinetic model.The reduction rate of Cr(Ⅵ)decreases with the increase of initial pH.When the concentration of oxalic acid and Mn^(2+)increases respectively,the reduction of Cr(Ⅵ)is accelerated.The introduction of ethylenediaminete-traacetic acid(EDTA)into the Mn^(2+)/oxalic acid reaction system has a strong inhibitory effect on the reduction of Cr(Ⅵ).Through three cycles of experiments,the important role of Mn^(2+)/oxalic acid/Cr(Ⅵ)and Mn^(3+)/oxalic acid/Cr(Ⅵ)intermediate complexes in the reduction process is confirmed.And the final product of Cr(Ⅵ)reduction is determined to be Cr(Ⅲ).The research results are of great environmental significance to reveal the change of chromium speciation and its migration and transformation in subsurface environments.