ABSTRACT
Soil remediation or soil cleanup has become more common, especially in developed countries. The choice of extractant such as water, organic solvents, chelating agents, surfactants, acids, or bases depends on which types of contaminants are to be treated (Holden et al., 1989). Removal of metals by forming soluble complexes with the chelating agents is quite efficient, but the high cost of solvents has precluded their application in remediation of heavy metal-contaminated sites. Hydrochloric acid (HCl) is more effective in extracting metals than other acids, presumably because of the formation of soluble metal-chloride complexes (John et al., 1994). Ethylenediamine tetraacetic acid (EDTA) has been chosen as the representative of the group of chelating agents, since it can form very stable complexes in aqueous solution with Cd, Cr, Cu, Ni, Pb, and Zn ions (Tuin and Tels, 1990b). EDTA alone can remove over 90% of the lead and cadmium (Ellis et al., 1986). Two types of metal extraction agents, acids and chelating agents (Ehrenfeld and Bass, 1984; Rulkens and Assink, 1984), have been used for the removal of heavy metal from soil. Both of these extraction agents have shown a high efficiency in soil washing technology (Tuin and Tels, 1990a; Allen and Chen, 1993; Peters and Shem, 1992). However, after acid washing, the physical and chemical properties of the soil can drastically change from the natural condition. Farrah and Pickering (1978) found greater removal efficiencies for Cd and Pb with EDTA than with HCl in the case of artificially contaminated clay minerals. Norvell (1984) measured twice as large removal of Cu and Ni with 0.1 N HCl than with 0.005 M EDTA at pH=5.3; the amount of Cd extracted was about the same with HCl and with EDTA. Tuin and Tels (1990b) obtained a higher extraction percentage when 0.1 N HCl was applied before 0.005M EDTA, and the result could be further improved by using a higher EDTA concentration. Elliott et al. (1989) and Allen and Chen (1993) showed that with EDTA, pH does not influence the extraction efficiency. Peters and Shem (1992) found that over 64% of Pb could be removed by EDTA at the pH range of 4 to 12, with pH causing only a minor effect on the extraction efficiency.
