Optimization Of Technology For Electrokinetic Remediation Of Chromium-contaminated Soil By Auxiliary Liquid Phase Chamber Coupled With Ion Exchange Membranes

With the acceleration of industrialization,the amount of chromium slag produced continues to increase,and the problem of soil chromium pollution caused by improper stacking and disposal methods has become more and more prominent.In particular,the soil of chromium(Cr)-contaminated sites in arid areas of northwest China is extremely difficult to remove due to its high proportion of Cr(III)content and poor migration,and it is possibly to be further oxidized to Cr(VI)with high toxicity,which will give rise to serious threat to the ecological environment and human health.The electrokinetic remediation(EKR)technology had been proven to be a highly promising remediation technology for Cr-contaminated soil,but the remediation process has general problems of limitation such as focusing phenomenon,polarization effects and secondary pollution.At present,the removal effect of Cr(VI)with strong solubility and good fluidity is better,while the removal of Cr(III)and total chromium needs to be improved.Based on that,this work designed a new type of modification of the EKR device,“an auxiliary liquid phase chamber coupled with anion and cation exchange membranes(AEM and CEM)in the middle of the soil chamber”,which separated the soil chamber into two parts,so that they were subject to the influence of acid front and base front from the electrode chamber,respectively.By using the chemical mechanisms that the acid front is more favorable for Cr(III)desorption and Cr(VI)is mobile in a wide p H range,while the base front is more favorable for Cr(VI)desorption and Cr(III)can be dissolved in a strong alkaline condition,we aimed to investigate the effectiveness of this scheme design in achieving simultaneous extraction of Cr(III)and Cr(VI)from the soil to further improve the total Cr removal efficiency.For further optimization,the enhancing reagents were added to the electrode chamber and the pulsed electric field was applied,in order to improve the total Cr removal efficiency while reducing the energy consumption of the remediation system and improving the energy utilization efficiency.The results showed as following:(1)The auxiliary liquid phase chamber coupled with AEM and CEM separated the traditional single soil chamber for the left and right chambers and was subject to the influence of acid front and base front respectively,and the soil p H of the two chambers showed a significant gradient difference,which improved its effectiveness on the soil environment and significantly avoided the antagonism caused by the encounter of traditional acid-base fronts in the soil chamber and the focusing phenomenon caused by the sudden changes of p H.(2)The auxiliary liquid phase chamber coupled with AEM and CEM uniformly regulated the soil moisture content and promoted electroosmotic flow,shortened the migration path of target metal ions and improved the current efficiency,further improved the effectiveness of the auxiliary liquid phase chamber for heavy metal collection,achieved the purpose of simultaneous extraction of Cr(III)and Cr(VI),and the removal rate of total chromium in all soil regions was increased and the horizontal difference was significantly reduced.(3)After the addition of enhancing reagents,the combination of oxalate-potassium oxalate mixed solution and D-gluconate sodium was beneficial to the conversion of strongly bound chromium to weakly bound state,especially the organic fractionation of left soil chamber and the iron-manganese oxides fractionation of right soil chamber,and the highest total chromium removal rate(54.29%)and energy use efficiency were obtained.(4)Under the action of acid front,the use of citric acid was conducive to the solubilization,chelation and migration of Cr(III)in the left chamber soil in the early stage of remediation,but it was easy to cause the adsorption of Cr(VI)in the later stage of remediation,and the combined pulse electric field was beneficial to the desorption of Cr(VI)and promoted the further migration of Cr(VI).Oxalic acid-potassium oxalate mixed solution was conducive to the reduction and removal of Cr(VI)in soil.Under the action of base front,D-gluconate sodium showed better solubility for Cr(III)than EDTA-2Na.(5)After the enhancing reagents combined with pulsed electric field with pulse hour intervals of "6OFF/18ON",the total chromium removal rate and energy utilization efficiency were improved in the recovery time of only 75% of the constant electric field.The combination of oxalate-potassium oxalate mixed solution and D-gluconate sodium was used to obtain relatively high chromium removal rate(58.12%)and the highest energy use efficiency.The design and optimization of this scheme aims to explore a EKR enhancement scheme of more efficient and energy-saving,which could provide a theoretical reference towards the wide application of EKR technology.