Regulating and intervening act of Cr chemical speciation effect on the electrokinetic removal in Cr contaminated soil in arid area

Heavy metals chemical speciation significantly effects the electrokinetic removal of heavy metals contaminated soil, but how to regulate and intervene the chemical speciation for improving the removal efficiency in the electrokinetic remediation of the contaminated soil is still an opened problem. In this study, we used increasing citric acid electrolyte concentration and soil pretreatment as regulating measures and the anolyte circulation and reduction wells as intervening act to explore the detailed transformation and migration of Cr speciation over time during electrokinetic remediation, the effect of the transformation and migration of Cr speciation in arid area contaminated soil on the removal efficiency. Based on the results of sequential extraction, the regulating measures can significantly improve the soil pH and the transformation of Cr speciation in arid areas, but the detailed transformation occurs in the early stage of remediation. The material flows showed that the transformation of other speciation to exchangeable (EXC) was higher than its migration to the electrode chamber and the EXC was not completely removed throughout the experiment. By adding HCl to the soil prior to the electrokinetic remediation, more Cr (III) can migrate to the cathode region. Regulating measure for pretreatment of real contaminated soil in arid area based on high concentration of citric acid and intervening act for the anolyte circulation could significantly reduce the bound to Fe/Mn oxides (FM), but the overall removal efficiency of the real contaminated soil (from 44.86% to 44.90%) was not improved, while the removal efficiency of artificially contaminated soil was higher. Furthermore, the multivariate linear regression analysis of the link between the five fractions and removal efficiency of Cr also shows that their correlation was much higher in artificially contaminated soils than in real contaminated soils. The scale effect would affect the transformation of the speciation, and the smaller the scale of the experiment is, the larger the influence on the migration and transformation of Cr speciation will be. The combination of regulating measures and intervening act provide a valuable reference for the specific transformation and migration of heavy metals speciation and the improvement of removal efficiency during EKR of contaminated soil in arid area.