Basic Studies Of Electrokinetic Treatment Of Copper Contaminated Soils

Soil heavy metal pollution is one of the serious environmental problems, which should be resolved as soon as possible. Electrokinetic soil remediation is an emerging technology that has attracted increasing interest of scientist and governmental officials. It applys a low intensity current between a cathode and an anode imbedded in the contaminated soil, and drived ions and small soil particles, in addition to water, to move between the electrodes by the means of electrokinetic, electroosmosis, electrophoresis and diffusion.Two soils, a yellow brown soil from Jiuhua (pH 7) with low metal mobility and a red soil from Guixi with low pH and high metal mobility, were used. Cu was the main pollutant in the two soils. We aim to evaluate the effects of EDDS and the polarity exchange on the electrokinetic treatments and the chang of soil organic matters and enzymatic activities after the treatments. The results are shown as following:The characteristics of the soils and chemical forms of soil metals greatly affected the metal removal from the soils. High acid/alkali buffering capacity and high pH of the yellow brown soil hindered the development of an acid front in the soil column during electrokinetic treatments, and the acid extractable Cu fractionation was only 9.34% of the total Cu. Because of the low mobility of Cu, little Cu was removed. The acid extractable Cu fractionation was 50% of the total in the red soil with a low pH. When catholyte pH was not controlled, the removal percent of Cu was 55.5%. It means that electrokinetict treatment is an effective technique for the acid soil.EDDS is a good extractant for soil Cu. When EDDS was used in the catholyte and the anolyte was controlled at pH=6 with NaOH, a high pH was developed in the soil column during the electrokinetic treatments. When EDDS was used as catholyte (5 mmol L^-1) or the soil was pre-saturated with 1.6 mmol EDDS kg^-1 soil, Cu removal was very low, which was ascribed that the mobility of EDDS/EDDS-Cu was low and their migration direction was opposed to the electroosmotic flow. Soil EDDS-Cu was transported to the catholyte. When the anolyte was controlled at pH=6 and high concentration of salts was contained in the catholyte, electroosmotic flow was non-uniform, and a positive pore pressure was developed, which resulted in a highter moisture in S1 than others after the treatments.When the contaminated yellow brown soil was treated with a polarity exchange, pH in the soil section close to the cathode (S5) did not fall into a low value during a short interval, which was advantageous for Cu removal to the cathode. The polarity exchange changed the electroosmotic flow, but this decreased the treatment efficiency if the electroosmotic flow was high. When HC1 was used to control catholyte pH, the performance was improved. However, the introduction of HC1 caused toxic chlorine produced on the anode, and the soil pH was very low after the treatment.The soil organic matter and urease activity did not significantly change after electrokinetic treatments, but invertase activity increased. The highest invertase activity was 170 times of initial one. The invertase activity and pH or Cu was negatively correlated, and the correlation between invertase activity and pH was significant at p< 0.01.