Soil Biology & Biochemistry 38 (2006) 1924–1932 Development and validation of an acute biotic ligand model (BLM) predicting cobalt toxicity in soil to the potworm Enchytraeus albidus K. Lock à , K.A.C. De Schamphelaere, S. Becaus, P. Criel, H. Van Eeckhout, C.R. Janssen Laboratory of Environmental Toxicology and Aquatic Ecology, Ghent University, J. Plateaustraat 22, 9000 Ghent, Belgium Received 19 October 2005; received in revised form 6 December 2005; accepted 9 December 2005 Available online 2 March 2006 Abstract An acute Biotic Ligand Model (BLM) was developed to predict the effect of cobalt on the survival of the potworm Enchytraeus albidus, exposed in nutrient solutions added to acid washed, precombusted sand. The extent to which Ca 2+ , Mg 2+ and Na + ions and pH independently mitigate cobalt toxicity to E. albidus was examined. Higher activities of Ca 2+ , Mg 2+ and H + linearly increased the 14 d LC50 Co 2þ (LC50 expressed as Co 2+ -activity) whereas Na + -activity did not. Stability constants for the binding of Co 2+ , Ca 2+ , Mg 2+ and H + to the biotic ligand (BL) were derived, i.e. log K CoBL ¼ 5.13, log K CaBL ¼ 3.83, log K MgBL ¼ 3.95 and log K HBL ¼ 6.53. It was calculated that at Co-concentrations corresponding to the 14d-LC50 value, 32% of the BL sites were occupied by cobalt. An initial validation of the applicability of this BLM in true soil exposure systems was performed by comparing observed and model-predicted 14 d LC50 s in a standard artificial soil and a standard field soil. By assuming pore water to be the only route of exposure and assuming equilibrium between pore water Co 2+ and solid phase Co, which is predicted by the geochemical WHAM-Model 6, LC50 s (as mg Co kg 1 dry wt of soil) were predicted within an error of less than a factor two. Further validation in true soil exposures, combined with more detailed knowledge of Co binding to soil solid phases is needed, if this model is to be used as a tool for risk assessment and derivation of soil quality criteria for Co. r 2006 Elsevier Ltd. All rights reserved. Keywords: Bioavailability; Metal toxicity; Speciation; Oligochaetes; Risk assessment; Ecotoxicology 1. Introduction Cobalt is a naturally occurring element and is mainly present in the earth’s crust as cobaltite [CoAsS], erythrite [Co 3 (AsO 4 ) 2 ] and smaltite [CoAs 2 ] (Barceloux, 1999). Cobalt is mainly used as a component of very hard, strong and heat-resistant alloys and in permanent magnets. It is also used as drying agent in paintings, as colour pigment in porcelain, as a catalyst in rubber manufacturing and as an additive in fertilizers and fodders (Barceloux, 1999). Elevated cobalt concentrations in the terrestrial environ- ment may, for example, result from deposition from burning of fossil fuels, wear of cobalt-containing alloys and spreading of sewage sludge and manure (Barceloux, 1999). Metals such as cobalt may present environmental risk when occurring at elevated concentrations and are being managed through the establishment of environmental quality criteria and standards. Recently it has been recognized by regulators, industry and academic scientists that standard procedures for deriving environmental quality criteria are inadequate to accurately assess the potential impact of metals on the ecological quality of ecosystems (Fairbrother et al., 1999; Janssen et al., 2000). This is because current environmental quality criteria and risk-assessment procedures of metals are predominantly based on total metal concentrations. However, there is extensive evidence that total metal concentrations in soils are not good predictors of metal bioavailability and toxicity. The 14 d LC50 of Zn, Cd, Cu and Pb to Enchytraeus albidus, for example, varied over more than two orders of magnitude depending on the composition of the soil (Lock et al., 2000; Lock and Janssen, 2001a). These differences in metal toxicity were mainly determined by pH ARTICLE IN PRESS www.elsevier.com/locate/soilbio 0038-0717/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2005.12.014 à Corresponding author. Tel.: +32 9 264 37 64; fax: +32 9 264 37 66. E-mail address: Koen_Lock@hotmail.com (K. Lock).