INTRODUCTION
THE mining and smelting of lead (Pb) and zinc (Zn) ore often results in contamination of soil with cadmium (Cd), Pb, and Zn. Metal-contaminated soil often presents an unacceptable risk to human and ecological health and must be remediated (Adriano et al., 1997; Pierzynski, 1997). Commonly used cleanup methods involve excavation and landfilling of smelter-contaminated soil; however, more permanent and less expensive in situ solutions have been favored during the last decade (Iskandar and Adriano, 1997). Chemical immobilization is a relatively inexpensive in situ remediation method in which soil is treated with chemical amendments that reduce metal solubility. Many studies report the use of chemical amendments including organic matter (Pierzynski and Schwab, 1993), alkaline materials (Mench et al., 1994), and phosphates (Boisson et al., 1999; Ma et al., 1994, 1995) for the chemical remediation of metal-contaminated soil. A recent study comparing biosolids, alkaline cement kiln dust, and rock phosphate (carbonated apatite) found that many of these amendments reduced phytoavailability and extractability of heavy metals (Basta and Gradwohl, 2000; Gradwohl, 1998). However, the effect of these amendments on metal bioavailability and toxicity to soil invertebrates was not evaluated.
Toxicity testing using earthworms is a well-developed means of studying the bioavailability and acute toxicity of soil contaminants (Edwards and Bohlen, 1992) and contaminated field soils (Marinussen et al., 1997a). Total heavy metal concentrations may not be directly related to soil organism toxicity due to a number of modifying factors such as pH, organic matter content, and clay content (Beyer et al., 1987; Conder and Lanno, 2000; Hopkin, 1989; Lanno and McCarty, 1997; Ma, 1982; Marinussen et al., 1997b; McLean and Bledsoe, 1992; Morgan and Morgan, 1988; Spurgeon et al., 1997). Cleanup guidelines are often based on total heavy metal content of soil and do not consider what proportion of the total may be biologically available (bioavailable) to organisms, even though environmental risk is related to the bioavailability of heavy metals in soil. The bioavailability of metals cannot be measured directly using chemical analyses; only living organisms can actually determine bioavailability (Lanno and McCarty, 1997). A method of measuring metal availability not involving organisms (reducing expense and variability) that is well related to bioavailability would be an extremely useful screening tool for evaluating metal-contaminated soils.
Two surrogate methods of measuring metal bioavailability in soils that are quick to perform and relatively inexpensive include (i) weak-electrolyte soil extractions and (ii) ion-exchange membrane metal uptake. Extractions using weak (M) CaCl2 or Ca(NO3)2 solutions have been used successfully as toxicity-related measures of metal availability in soils (Basta and Gradwohl, 2000; Conder and Lanno, 2000; Gradwohl, 1998; Marinussen et al., 1997b; Peijnenburg et al., 1997, 1999; Posthuma et al., 1997; Weljte, 1998). These solutions are hypothesized to extract exchangeable or weakly bound "available" metals in soil (Sloan et al., 1997), which are believed to be available for uptake by soil organisms (Peijnenburg et al., 1999; Posthuma et al., 1997). Conder and Lanno (2000) demonstrated that metal levels in weak Ca(NO3)2 extractions relate well to lethal Cd, Pb, and Zn toxicity in the earthworm E. fetida exposed to metal-spiked artificial soil. In contrast, ion-exchange membranes have a definite advantage over weak electrolyte extractions because they can be deployed in soils with a minimum of soil physicochemical alteration, even under in situ conditions (Liang and Schoenau, 1995, 1996; Qian and Schoenau, 1997). Cation-exchange membranes complex divalent metal ions such as Cd, Pb, and Zn directly from the soil solution, but suffer interference from Ca cations, which compete with heavy metals for membrane uptake (Liang, 1994). Anion-exchange membranes coated with a metal chelator, such as disodium-diethylenetriaminepentaacetic acid (DTPA), can chelate available metals while avoiding Ca saturation due to preferential binding of cationic transition metals (Evangelou, 1998; Liang and Schoenau, 1995). While Liang and Schoenau (1995) found a strong correlation between Cd, Cr, Pb, Ni, and Zn uptake in chelating membranes and uptake for lettuce (Lactuca sativa L.) in soil, Conder and Lanno (2000) found only a weak relationship between ion-exchange membrane uptake and toxicity in the earthworm E. fetida exposed to Cd, Pb, and Zn in spiked artificial soils.
The objectives of this research were to (i) assess the ability of chemical immobilization amendments (municipal sewage sludge biosolids and rock phosphate) to reduce metal bioavailability and ecotoxicity in a toxic, metal-contaminated smelter soil, and (ii) evaluate soil extraction methods using Ca(NO3)2 solution or ion-exchange membranes coated with DTPA as surrogate measures of metal bioavailability and ecotoxicity.
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