| Heavy metal contamination of soil has given rise to a severe problem in China. The selection of remediation technologies is a key factor among the projects for tackling this problem.The cleanup technologies include the electrokinetic remediation(EKR), flushing, and phytoremediation. The removal efficiency of the EKR exceeds those of the flushing and phytoremediation when processing heavy metal contaminated clay. Two sequential steps control the removal rate of contaminants in EKR. The first step is the desorption of metal ions from soil particles, while the second is the electromigration of charged pollutants under the electric field, separating contaminants from the contaminated soil.Prevalent EKR theories hypothesize that the electric field exerts no effect on the ionic desorption, on which the EKR numerical modeling is based. One, however, cannot obtain any validation research of this hypothesis from global mainstream academic databases. Therefore, this dissertation studies the effect of the electric field on the metal ionic desorption for the purpose of verifying the aforementioned assumption through the comparisons with various desorption experiments(i.e. desorption kinetics experiments, electrokinetic desorption kinetics experiments, and column desorption kinetics experiments) via using the Cr-contaminated soil from a factory site.First, it performed desorption kinetics experiments, using two kinds of Na NO3 desorption solutions(0.05 and 0.5 mol/L). The comparison between two kinds of experiments turns out that NO3- is capable of competing the absorption site with the Cr(VI). The concentration of NO3- is proportional to that of desorbed Cr(VI). To eliminate the interference of various NO3- concentration, this research used 0.05 mol/L Na NO3 desorption solution in the subsequent experiments.Then, it shows that the amount of desorbed Cr(VI)of column desorption kinetics experiments is less than that of desorption kinetics experiments and exists a regular fluctuation, which results from the porosity among soil particles. The extensive existence of capillary pores in clay could make Cr(VI) periodically adsorb and “explosively” desorb on these capillary pores, causing the concentration fluctuation. Meanwhile, the decrease of desorbed Cr(VI) amount attributes to the loss of the solid/water contact area for the tested soil of column desorption kinetics experiments is fixed in the apparatus rather than being mixed.Moreover, the electrical field could significantly affect the Cr(VI) desorption, which is demonstrated from the comparison of electrokinetic desorption kinetics experiments(whose voltage gradients upon the soil is 10 and 20 V/cm, individually) with the column desorption kinetics experiments. Three factors would influence the desorption process; i.e. the resultant force, provided by the applied electric field and the ion electrostatic field, which drives Cr(VI) in capillary pores, and the electro-enhanced ion exchange desorption would both promote the Cr(VI) desorption; whereas the “blind hole capture”(BHC) could have a negative effect. In this research, the positive effects overcame the negative. However, attributing to BHC, it cannot merely depend on increasing the voltage to raise the amount of desorbed Cr(VI).A 10V/cm bidirectional electrokinetic kinetics desorption experiment was conducted. When the direction of applied electric field was reversed, the amount of desorbed Cr(VI) remained stable due to a fact that some of desorbed Cr(VI) ions from BHCs in the upstream soil came into BHCs in the downstream soil again.In conclusion, it finds out that the electric field has double-edged effects on the ion desorption kinetics, denying the prevalent independent hypothesis. Further study should investigate the interaction between positive and negative effects, and factors which affect them. |
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