A Technology For Controlling The Rebound Of Cr(?) In Remediated Groundwater By Preparing Calcium Sulfite In Soil

Heavy metal pollution of groundwater has become a serious problem with the rapid development of urbanization and industrialization in China.Chromium???,one of the most prominent pollutants,has aroused extensive concern as it can easily migrate and transfer between soil and groundwater.As a result,not only groundwater itself but also the source of pollution should be treated during the process of remediation.However,the rebound of Cr concentration in groundwater occurs frequently due to the incomplete Cr???reduction.Thus,sustainable-release materials?SRM?are often used for solving this problem.SRM can be classified as solid SRM?SSRM?land liquid SRM?LSRM?in terms of their forms in soil.SSRM has good retention capacity with low mobility,while LSRM can be readily in situ injected into soil at low cost and distributed along groundwater path to achieve higher efficiency,but subject to being flushed away.As high mobility and retention capacity are not compatible for SSRM and LSRM,this study develop an innovative technology,called liquid-injection&solid-stay?LISS?which can produce SSRM in soil from an injected solution,to control the Cr???rebound in groundwater.In this paper,there are three experiments to validate the feasibility of this technology from the aspects of reduction efficiency,liquid-solid conversion characteristics of CaCl₂-Na₂S₂O₅ and practical application,which includes S?IV?-Cr???reduction experiments,experiment for verification of liquid-solid conversion characteristics of CaCl2-Na2S2O5 and column experiment.S???-Cr???reduction experiments show that the reaction of NaSO₃ with-Cr???belongs to pseudo-first order reaction in homogenous phase when pH ranges from 6 to 8,and reaction rate significantly decreases with the increasing pH.In heterogeneous phase,Cr???removal rate is 97.3%by CaSO₃·0.5H₂O?s?when pH is 6,but it decreases to38.8%and 9.3%at pH 7 and 8 respectively.In addition,Fe?III?has an impressive effect on catalyzing the reaction of S?IV?-Cr???and Cr???removal rate rise to 50.6%and42.4%at pH 7 and 8 with Fe?III?.In experiment of preparing CaSO₃·0.5H₂O?s?by CaCl₂-Na₂S₂O₅ solution,the CaCl₂-Na₂S₂O₅ solution is stable under room temperature.But it can be significantly facilitated by heating to produce a large amount of SO₃^2-which can form precipitateCaSO₃·0.5H₂O?s?combing with Ca²⁺.And 83.1%of S?IV?in CaCl₂-Na₂S₂O₅solution is pericipitated after heating.The liquid-solid transfer characteristic of CaCl₂-Na₂S₂O₅ solution by heating meets the requirements of LISS technology.The column experiment is mainly to simulate the groundwater process,and verify the effect of CaSO₃·0.5H₂O?s?on controlling the rebound of Cr???in remediated groundwater.During the flushing process by DI water,the concentrations of Cr???in the effluents of controlling experiments are higher than 0.1 mg·L^-1,and keep increasing in the whole flushing column experiments.While the concentrations of Cr???in the effluents of columns which added CaSO₃·0.5H₂O?s?or treated by LISS are always lower than 0.1 mg·L^-1,and no obvious increasement was observed.However,when it comes to NaAC-HAC flushing solution,its relatively low pH will promote the reduction rate and dissolution of soil resulting in the fluctuating of concentrations of Cr???(around 0.1 mg·L^-1)in the effluents.The results of pH and conductivity in effluents indicate that though the production of H⁺,Na⁺,and Cl^-by LISS cause a slightly disturbance,it can recover after a few days by itself.Therefore,LISS has potential ability for the application of controlling the rebound of Cr???in groundwater remediation.