Study On Efficiency And Mechanism Of Oxalic Acid And Ethanol Co-reduction Of Cr(Ⅵ) And Simulation Of In Situ Remediation Of Cr(Ⅵ) Contaminated Groundwater

China is a major producer of chromium(Cr)chemical,and the high concentration of Cr pollution seriously threatens the ecological environment and human health.Chromium exists mainly in two different valence states:hexavalent chromium(Cr(Ⅵ))and trivalent chromium(Cr(Ⅲ))in the natrral environment,and the degree of environmental impact and harm caused by the two differs greatly.Cr(Ⅵ)is recognized as a toxic carcinogen and is highly migratory.With the increase of environmental protection requirements,economical and effective environmental remediation technologies for chromium pollution are in urgent demand.Among many chromium remediation technologies,in situ chemical reduction remediation is considered to be a remediation technology with great potential because of its advantages such as economical and practical,and easy implementation.Based on the above background information,the thesis established an oxalic acid(OA)/ethanol(EtOH)system to reduce hexavalent chromium.The main studies and results are as follows:(1)An OA/EtOH in situ chemical reduction system was constructed with Cr(Ⅵ)as the target pollutant,and the effectiveness of OA concentration,EtOH concentration,pollutant concentration.solution p H and common components in water(anions,cations and humic acids)on the effectiveness of the OA/EtOH system for the reduction of Cr(Ⅵ)were investigated.The results showed that the reaction of the OA/EtOH system for the reduction of Cr(Ⅵ)is in good accordance with the pseudo-first-order reaction kinetics(R~2>99%),the OA concentration and EtOH concentration have a large effect on the reduction efficiency of Cr(Ⅵ),low p H is favorable for the reduction of Cr(Ⅵ)by the OA/EtOH system,and the common components have little effect on the reduction of Cr(Ⅵ)by the system.The reduction conditions of the OA/EtOH system were optimized by batch experiments and response surface design method(RSM),and a quadratic polynomial was fitted to the experimental results,and the derived model was found to be meaningful from the variance results.The correlation coefficient of the model(R~2=0.9964)showed a good agreement between the actual and predicted values.(2)The reduction products of Cr(Ⅵ)in the system were analyzed by UV-visible scanning.The Cr metal intermediate Cr(V)was detected by electron paramagnetic resonance(ESR),the absence of Cr(V)in the reaction process of Cr(Ⅵ)reduction in the OA/EtOH system was demonstrated.The reaction mechanism for the reduction of Cr(Ⅵ)by the OA/EtOH system was clarified by density functional theory(DFT)calculations.In the system of OA/EtOH reduction of hexavalent chromium,Cr and OA form the complex Cr-OA,which enhances the reactivity of Cr(Ⅵ)toward oxygen atom transfer or protonation reaction,reduces the reaction energy barrier for the conversion of Cr(Ⅵ)to Cr(Ⅲ)and facilitates the electron transfer between Cr species and EtOH.EtOH changes the configuration of Cr-OA,accelerates the electron transfer from OA to Cr,and at the same time directly transfers electrons to Cr by bonding with Cr-OA,and cooperates with OA multi-electron transfer to Cr and finally reduce Cr(Ⅵ)to Cr(Ⅲ).(3)The remediation of Cr(Ⅵ)contamination in groundwater using medium and fine sand as aquifer media respectively,was investigated using a three-dimensional simulation tank.The experimental results showed that the OA/EtOH system can form an effective reduction reaction zone,which has a good effect on the removal of Cr(Ⅵ)in the simulated aquifer.For the aquifer medium of fine sand,the removal rate of Cr(Ⅵ)reached 60.62%during the remediation process of 101 h in a small-scale simulated tank,and the plume of Cr(Ⅵ)contamination kept shrinking and the concentration kept decreasing during the remediation process.In the aquifer medium of medium sand,the removal rate of Cr(Ⅵ)reached 71.52%during 818 h of remediation in the larger-scale simulated tank.From the experimental results,it is clear that the aquifer medium of medium sand is more favorable than fine sand for the remediation of Cr(Ⅵ)reduction,and the remediation of Cr(Ⅵ)reduction can be accelerated by increasing the concentration of remediation reagent.The changing areas of p H and oxidation reduction potential(ORP)in the simulated tank aquifer are consistent with those of Cr(Ⅵ).