Study On Mass Transfer Of Acid-base And Pollutants In Soil Electro-kinetic Remediation

Soil electro-kinetic remediation(EKR)is a technology for the separation and removal of heavy metals from soil,which is environmental friendly,easy to operate,low cost and no secondary pollution.The research on this technology has been extensive around the world,but the technology is still difficult to be applied in engineering.Based on the problems in engineering application,this work focuses on the process and mechanism of acid-base mass transfer and pollutant mass transfer in electro-kinetic remediation.In the EKR process,the p H of the soil system solution has an important influence on the state of pollutants and the mass transfer process.Therefore,based on the state and mass transfer process of pollutant controlled by acid-base,this paper studied the change of acid-base control area from three aspects:soil particle size,electro-kinetic remediation time and current density.The larger the soil particle size and soil porosity tortuosity are,the smaller the effective diffusion coefficient and the diffusion flux and electromigration flux of H~+and OH~-are,which will lead to a narrower acid-base control area.It was found that the longer the electro-kinetic remediation time,the acid control area and the base control area formed by the migration of H~+and OH~-to the cathode and anode will become wider,and finally the acid and base annihilation area disappears at a certain moment.Only the acid control area and the base control area exist in the soil system.It was found that the greater the current density,the greater the electric field strength,the greater the electromigration fluxes of H~+and OH~-,and the larger and wider the acid-base control area formed by H~+and OH~-mass transfer respectively.In the EKR process,if the mass transfer process of pollutants can be effectively controlled,it will be very helpful to solve the problem of electro-kinetic remediation efficiency.Therefore,K~+and F~-are selected as tracers for the study of cations and anions respectively,to study the mass transfer process of pollutants and try to find their mass transfer laws.Firstly,the processes of K~+and F~-contaminated soil systems are studied separately.Through the study of the diffusion flux of K~+and F~-,it was found that the when K~+and F~-are pulsed into the soil system,the concentration gradient and the diffusion flux are are the largest.With the increment of time,the concentration gradient decreases and when the final concentration gradient and diffusion flux are zero,K~+and F~-are evenly dispersed into the soil system.Secondly,EKR was used to remove K~+and F~-evenly dispersed in the soil system.The experiment found that the electromigration flux was much larger than the diffusion flux,and the mass transfer rate of K~+and F~-was mainly determined by electromigration.Finally,EKR was used to remove the K~+and F~-pulsed to the soil system again.The experiment found that when removing K~+,the cathode should be close to the K~+source,and when removing F~-,the anode should be close to the F~-source.Although the electromigration flux is greater than the diffusion flux,the mass transfer rates of K~+and F~-are determined by both diffusion and electromigration.