Proportion Optimization Of Permeable Reactive Barrier For Removing Heavy Metals In Groundwater

As an in-situ remediation technology,permeable reaction barrier technology is more and more common used in groundwater heavy metal remediation.The reaction medium is the key link of PRB technology,and the repair efficiency is directly related to the reaction efficiency of the reaction medium.In this paper,through static experiments,central composite design（CCD）was used to optimize the mixing ratio of zero valent iron,corn stalk biochar and scoria.The optimum ratio was fitted by kinetic and isothermal adsorption.The results show that the optimal ratio is 2.163g of zero-valent iron,1.055g of biochar and 0.5g of scoria,and the mass ratio is about 4:2:1 of zero-valent iron:corn stalk biochar:scoria.The reaction of Cd（Ⅱ）and Cr（Ⅵ）is more consistent with Langmuir isothermal adsorption and quasi-second-order kinetics.And Freundlich isothermal adsorption and quasi-first-order kinetics are more representative of As（Ⅲ）reaction.The addition of anions Cl^-and SO₄^2-increases the reaction rates of Cr（Ⅵ）and As（Ⅲ）,while the addition of HCO₃^-decreases the removal rates of Cr（Ⅵ）and As（Ⅲ）,and the removal rate of Cd（Ⅱ）decreases after the addition of anions.The removal rate of Cr（Ⅵ）and As（Ⅲ）in the reaction medium is significantly increased with the addition of Fe³⁺,Mn²⁺,Na⁺and K⁺cations,while the removal efficiency of Cd（Ⅱ）is decreased with the addition of cations.The reaction materials with the best ratio were subjected to the dynamic column experiment for 45 days.The changes of pollutant concentration,pH and oxidation reduction potential in the column experiment were analyzed,and a series of barrier parameters were got according to the test data.After the dynamic experiment,the microscopic characterization of the reaction materials was carried out.The results show that the comprehensive exhaustion point of the reaction column is in 43d.The Results of the reaction column dynamic experiment were fitted by Thomas model,and the fitting results were good.The Thomas rate constants of Cr（Ⅵ）,Cd（Ⅱ）and As（III）of the reaction barrier were 0.213,0.239 and 1.39,respectively.The adsorption mass were1.114mg/g,0.552 mg/g and 0.109 mg/g.The pH of the reaction barrier fluctuates greatly.The existence of the three pollutants in the energy spectrum detection（EDS）proves that the reaction material has adsorption effect on the three pollutants.XPS analysis showed that there were four forms of Cr（III）,Cd（Ⅱ）,As（III）and As（V）on the surface of the material.It shows that Cr（Ⅵ）is reduced to Cr（III）in the reaction system,and the material has adsorption effect on Cr（III）.The reaction materials for Cd（Ⅱ）may be physical adsorption,chemical adsorption or surface complexation.The removal process of arsenic by medium involves both redox reaction and adsorption.According to the reaction barrier material parameters of the dynamic column experiment,the groundwater model are established by using GMS software.With the position and thickness of PRB barrier as dependent variables,the effective running time,the residence time of barrier and the removal rate of Cr（Ⅵ）as response quantities,response surface fitting was carried out.The optimal parameters of PRB reaction barrier are 60 meters long,5 meters thick and 188 meters away from pollution source.The pollutant concentration in front and behind of the barrier was compared,and the pollutant concentration after the reaction was lower than the groundwater quality standard III water quality standard,proving that the reaction barrier is feasible to repair and remove the three pollutants.