PHYSICO-CHEMICAL AND BIOLOGICAL EVALUATION OF THE EFFICACY OF IN SITU METAL INACTIVATION IN CONTAMINATED SOILS

Key words: phytoremediation, soil remediation, immobilization, heavy metals, risk assessment

In situ inactivation (immobilization) of metal contaminated soils is a developing technique which seems to be a valuable alternative for more expensive and complicated civil-engineering techniques. The main objective of in situ immobilization is to decrease the risks of the contamination for the environment and human health through reductions of relative mobility and bioavailability of the metals in the soil. Specific criteria for evaluation and monitoring of in situ inac- tivation of metals are needed. In this paper the combined use of sequential ex- traction procedures (according to Tessier et al. 1979), microbial heavy metal biosensors (BIOMET; Corbisier et al. 1996), phytotoxicity tests (Van Assche and Clijsters 1990; Vangronsveld and Clijsters 1992) and a zootoxicity test (using Eisenia fetida according to the OECD-guidelines) is presented as a test system for evaluation and monitoring of efficacy and durability of in situ inactivation of metal contaminated soils. A heavily contaminated sandy soil from the site of an old pyrometallurgical zinc smelter (Lommel, Maatheide, further mentioned as 'MH'; Vangronsveld et al. 1995, 1996) was 'diluted' with a compara- ble (pH, texture,...) uncontaminated soil resulting in a series of: 0 % MH, 20 % MH, 50 % MH and 100 % MH. The original MH soil contained 4800 mg Zn kg \ 2360 mg Pb kg"1, 830 mg Cu kg'1 and 20 mg Cd kg"1. Zn is the dominant metal for selection pressure (Diels and Mergeay 1990; Vangronsveld et al. 1996). These soil mixtures were treated with several soil additives in different proportions : steel shots 1 % (SG; Mench et al. 1994), steel powder (SC) 1 %, cyclonic ashes 5 % (formerly called beringite; Vangronsveld et al., 1995, 1996) (CA), bentonite 5 % (BE), and combinations (CA 5 % + SG 1 %; CA 5 % + BE 5 %). Choice of the additives used in this experiments was based on previous experience. The effect of the soil amendments on metal mobility and bioavail- ability were evaluated 12 months after their application.A good correlation was found between the different evaluation criteria (Fig.l). The soil additive cy- clonic ashes alone or in combination with steelshots or bentonite gave the best immobilization of the heavy metals, the lowest phyto- and zootoxicity indices as well as a strong reduction of heavy metal bioavailability for bacteria (not shown).