Efficiency And Mechanisms Of Copper And Zinc Contaminated Soil Remediation By Sodium Alginate Enhanced Electrokinetic Technology

Soil contaminated by heavy metals has been seriously increased in China.Compared with other remediation technologies,electrokinetic remediation（EKR）technology is widely used due to its easy applicability,lower cost,high efficiency.However,the metal desorption is one of restrict factors to development of EKR.In addition,the increase of p H in catholyte and the recovery of metal ions also limit its application.This study developed a highly efficient,environment-friendly,and easily practical coupling electrokinetic technology to remediate heavy metals contaminated soil.A biodegradable reagent was used to improve soil heavy metal desorption ability,coupling adsorption technology for controlling electrolyte p H and recovery of heavy metals,as well as buildinga modeling simulation prediction to discuss metal migration.Sodium alginate was used as biodegradable reagents to improve the current efficiency.Sodium alginate can significantly improve soil conductivity.For 0.5-2%dosage,soil conductivity increases with the increase of dosage.The introduction of reagent can change soil Zeta potential and change polarity of soil surface from positive to negative,change the direction of electroosmosis,and reduce the stability of soil colloidal dispersion.Sodium alginate has rich functional groups,such as carboxyl groups and hydroxyl groups which can desorb Cu²⁺and Zn²⁺from soil particles by ion exchange and complexation.The introduction of sodium alginate can change the soil metal fractions,and the exchangeable fraction can be transformed into reducible and oxidizable fractions.Chitosan could complex metal ions and migrate into thecatholyte under the electric field to form crosslinked chitosan gelations.Kinetic model of metal ions in electrolyte was pseudo-first order reaction.The highest concentration of Cu²⁺and Zn²⁺ions in electrolyte were 49.89mg kg^-1 and 38.13mg kg^-1 at 48 h.The introduction of sodium alginate improved the migration rate of Cu²⁺and Zn²⁺ions,and the highest concentration of Cu²⁺and Zn²⁺ions in electrolyte were 51.31 mg kg^-1 and 43.72mg kg^-1at 40 h.Alginate can desorb Cu²⁺and Zn²⁺ions from soil by forming unstable gels,which could be dissociated through electrolysis and then migrated out of the soil by electromigration,thus enhancing electrokinetic remediation.However,the effect decreased with the increase of sodium alginate dosage.Sodium alginate was selected as the best biodegradable reagent to remediate Cu and Zn contaminated soil,which of the best dosage ratio was 1%and Cu and Zn removal rate were 95.82%and 97.33%,respectively.Quantum chemistry theory was used to calculate and analyze the mechanism of sodium alginate desorption of heavy metals.GGMM unit in sodium alginate was used as the model molecule for calculation.The results showed that both of Cu-GGMM and Zn-GGMM complexes were bidentate coordination type.The Gibbs free energy of Cu-alginate gels was higher than Zn,which proves that Cu-alginate gels were difficult to dissociate by electrolysis.An electrokinetic remediation technology coupling with ion adsorption was constructed to remediate heavy metal contaminatedsoil.The coupled technology delayed the current rising and accelerated the current decreasing,and the introduction of ion exchange system reduced the energy consumption.The ion exchange system can relatively stabilize the cathode electrolyte p H at 2,and weaken the cathode polarization while absorbing metal ions,and effectively reduced the amount of OH^-produced by the cathode.The coupled technology can significantly improve the electrokinetic remediation removal efficiency.The coupled technology can improve the metal removal rate,promote metal ions migrating into the cathode electrolyte under the electric fieldand then removed from the electrolyte by ion resins absoption,which purify the cathode electrolyte and recycle the heavy metals.The recoveries of Cu and Zn in coupled technology were 95.47%and 97.01%,respectively.The ion exchange system can release H⁺into the electrolyte circulation system,neutralize OH^-generated by cathode polarization,inhibit the metal precipitation in soil nearby the cathode area,promoted the electromigration of metal ions,and improved the metal recovery.COMSOL software was used to simulate the electrokinetic remediation process.The soil metal desorption near the anode region preferentially occurs,and the soil residual metals firstlyincreased and then decreased from anode to cathode.Due to the soil position closed to the cathode,the metal accumulation phenomenon was more obvious.The OH^-produced by the cathode hindered the migration efficiency of metal ions,which also induced the metal desorption hysteresis except H⁺migration and the metal electromigration.The results of simulated electrokinetic remediation process by coupled technology showed that the introduction of ion system led to a stable cathode electrolyte p H,which inhibited the production of OH^-,strengthening the soil metal desorption,and avoided the metal precipitation,and consequentlyenhanced the metal removal efficiency.