Effect And Mechanism Of Manganese Enhanced Reduction Of Hexavalent Chromium By Oxalic Acid

Chromium is an important metal element in industrial production and people’s lives in modern society.However,due to unreasonable control of the three wastes（waste residue,waste water,exhaust gas）containing chromium and even random discharge into the environment,groundwater and soil in many areas of China have been seriously polluted by hexavalent chromium.One of the main ways to control hexavalent chromium pollution is to reduce and convert the highly toxic Cr（Ⅵ）into low toxicity Cr（Ⅲ）.Low-molecular-weight organic acids（LMWOAs）,metal ions and mineral particles,which are ubiquitous in subsurface environments,have an important effect on the migration and transformation of Cr（Ⅵ）and the change of toxicity.Oxalic acid is a common low-molecular-weight organic acid in subsurface environment.It has strong reducing properties and can complex,chelate or dissolve heavy metals.It is an important organic ligand in the soil;Mn²⁺is a ubiquitous metal ion in the environment and plays an important role in the geochemistry of subsurface environments.Another common substance in subsurface environment is birnessite,which is the most common manganese oxide mineral.It is widely involved in geochemical processes and has an important influence on the migration and transformation of heavy metal chromium.These substances are not only friendly and safe environmental elements,but also play an important role in the existence and transformation of hexvalent chromium.Therefore,this study chose oxalic acid,Mn²⁺and birnessite as the main research objects,to explore the effect and rule of manganese enhancing oxalic acid reduction of Cr（Ⅵ）,and explore its mechanism.This study is divided into two parts:In the first part,explore effect and mechanism of Mn²⁺enhanced reduction of hexavalent chromium by oxalic acid.Mn²⁺can significantly strengthen the reduction of Cr（Ⅵ）by oxalic acid.The strengthening process is divided into a slow induction period and an automatic acceleration period,both of which conform to the pseudo-first order kinetics.The reaction rate constant in the acceleration period is 6 times that of the induction period.The effects of environmental factors on Cr（Ⅵ）reduction are as follows:As the initial p H of the system increases,the reduction rate of Cr（Ⅵ）gradually decreases.When the concentration of oxalic acid and Mn²⁺increased respectively,the reduction of Cr（Ⅵ）was obviously accelerated.Among the common metal cations,Fe³⁺can promote the reduction of Cr（Ⅵ）;Al³⁺almost completely inhibits the reduction of Cr（Ⅵ）.The addition of several common anions hardly affected the reduction of Cr（Ⅵ）by Mn²⁺/oxalic acid.The introduction of ethylenediaminetetraacetic acid（EDTA）into the Mn²⁺/oxalic acid reaction system has a strong inhibitory effect on the Mn²⁺enhanced oxalic acid reduction of Cr（Ⅵ）.Analyze the content changes of Mn²⁺,Mn³⁺,and Mn⁴⁺during the reaction,and infer the different roles of Mn²⁺and Mn³⁺in different stages of the reaction.Through three cycles of experiments,the important role of the two intermediate complexes of Mn²⁺/oxalic acid/Cr（Ⅵ）and Mn³⁺/oxalic acid/Cr（Ⅵ）in the reduction process was confirmed,and the final product of Cr（Ⅵ）reduction was determined as Cr（Ⅲ）.The second part explores the effect law and mechanism of birnessite enhanced oxalic acid reduction of Cr（Ⅵ）.First,the laboratory prepares acid birnessite and conducts phase characterization to analyze its composition and properties.In the system where birnessite and oxalic acid exist,the reduction process of Cr（Ⅵ）conforms to the pseudo first-order kinetics.The effects of environmental factors on Cr（Ⅵ）reduction are as follows:With the increase of the initial p H of the system,the reduction rate of Cr（Ⅵ）gradually decreases;when the addition amount of birnessite increases,the reduction rate of Cr（Ⅵ）presents a trend of first increasing and then decreasing;the concentration of oxalic acid increases,The reduction rate of Cr（Ⅵ）increases accordingly;among the common anions and cations,Fe³⁺inhibits the reduction of Cr（Ⅵ）by about 60%,Al³⁺completely inhibits the reduction of Cr（Ⅵ）.The common anions have no obvious effect on the reduction of Cr（Ⅵ）.The reduction process of Cr（Ⅵ）in the birnessite/oxalic acid system can be divided into two aspects.On the one hand,the adsorption of birnessite makes Cr（Ⅵ）adsorb on the surface of the mineral,and the reduction reaction with oxalic acid occurs on the surface of the mineral.On the one hand,oxalic acid is used as a ligand to assist in dissolving of hydromanganite,so that manganese ions are continuously dissolved and Cr（Ⅵ）is reduced in the solution.Through the experiment of Cr（Ⅵ）reduction cycle,it can be concluded that oxalic acid is the main electron donor and complex ligand.The results showed that Cr（Ⅵ）was completely reduced to Cr（III）.In conclusion,this study explores the effect and mechanism of metal ion Mn²⁺and common manganese mineral birnessite enhanced reduction of Cr（Ⅵ）with oxalic acid,which has some enlightening significance for clarifying the existing form and migration and transformation rules of chromium in subsurface environment,and it also has certain enlightening significance for the remediation and treatment of hexavalent chromium pollution.