Enhanced Electrokinetic Remediation of Cadmium-Contaminated Natural Clay by Phosphonates and EDTA

Abstract

Soil contamination by heavy metals is a worsening worldwide environmental problem. Electrokinetic extraction is a promising technology for in-situ remediation of contaminated soils of low hydraulic permeability. However, the extraction of heavy metals is usually hindered by the high acid/base buffer capacity of natural soils. EDTA is a strong metal chelate widely studied in the enhancement of electrokinetic remediation. In this study, batch desorption experiments and bench-scale electrokinetic extraction experiments were carried out to study the effect of two phosphonates, i.e. EDTMP and NTMP, and EDTA on the extraction of cadmium from a natural clay from Shanghai, China. The results of the batch experiments indicate that it is very difficult to extract the cadmium sorbed on soil particle surfaces when the soil pH is above 7.0 without any enhancement. However, with the addition of 0.1 M phosphonates or EDTA, more than 75% of the sorbed cadmium could be dissolved into solution in the pH range of 2.0-11.0. The results of the electrokinetic extraction experiment indicate that the cadmium spiked in the specimen can be transported towards the anode with the enhancement of EDTA, EDTMP, or NTMP. Although phosphonates are not as efficient as EDTA in solubilization of cadmium, accumulation of cadmium in the vicinity of the anode was not observed as in the case of EDTA. The average removal efficiencies of cadmium from the soil samples collected after approximately 5 days of electrokinetic extraction enhanced by 0.1 M EDTMP and NTMP were 22.4% and 22.8%, respectively, higher than the 15.1% by 0.1 M EDTA under similar conditions.