Environmental Research 91 (2003) 119–126 Mobility, bioavailability, and toxic effects of cadmium in soil samples Z. Prokop,* P. Cupr, V. Zlevorova-Zlamalikova, J. Komarek, L. Dusek, and I. Holoubek Research Centre for Environmental Chemistry and Ecotoxicology, Masaryk University, Kamenice 126/3, 625 00 Brno, Czech Republic Received 14 June 2001 Abstract Total concentration is not a reliable indicator of metal mobility or bioavailability in soils. The physicochemical form determines the behavior of metals in soils and hence the toxicity toward terrestrial biota. The main objectives of this study were the application and comparison of three approaches for the evaluation of cadmium behavior in soil samples. The mobility and bioavailability of cadmium in five selected soil samples were evaluated using equilibrium speciation (Windermere humic aqueous model (WHAM)), extraction procedures (Milli-Q water, DMSO, and DTPA), and a number of bioassays (Microtox, growth inhibition test, contact toxicity test, and respiration). The mobility, represented by the water-extractable fraction, corresponded well with the amount of cadmium in the soil solution, calculated using the WHAM (r 2 ¼ 0:96; Po0:001). The results of the ecotoxicological evaluation, which represent the bioavailable fraction of cadmium, correlated well with DTPA extractability and also with the concentration of free cadmium ion, which is recognized as the most bioavailable metal form. The results of the WHAM as well as the results of extraction experiments showed a strong binding of cadmium to organic matter and a weak sorption of cadmium to clay minerals. r 2003 Elsevier Science (USA). All rights reserved. Keywords: Bioavailability; Cadmium; Extractability; Mobility; Soil; Speciation 1. Introduction The behavior of metals in soils (e.g., mobility, bioavailability) cannot be reliably predicted on the basis of their total concentrations. The uptake and toxicity of many metals show marked dependence on speciation of the metals and these responses often correlate best with the activity of free metal ion (Laxen and Harrison, 1981; Knight and McGrath, 1995; Parker and Pedler, 1997). Exceptions to this generalization have been observed; however, the free metal form is considered the most bioavailable and the most active form (Janssen et al., 1997a). The bioavailability of metals in soils or sediments is often expressed in terms of concentration in a water phase. The metal distribution between a solid phase and pore water of a soil is commonly described by equilibrium partitioning (Janssen et al., 1997b). But the total dissolved metal concentration does not necessarily correspond to the amount available to biota. Ion pairs, complex ions, polymers, or microparticulates can reduce free ion species of heavy metals in solution (Green et al., 1993). The process of identifying and quantifying these different species of metals in a sample is referred to as speciation. Chemical equilibrium models, such as the Windermere humic aqueous model (WHAM) and free- ion activity model (FIAM), are available to calculate metal speciation in waters, sediments, or soils (Tipping, 1994; Parker and Pedler, 1997). Many studies refer to metal speciation in terms of extractable metals related to single or sequential extraction. For example DTPA, CaCl 2 , or other individual extractants are frequently used for prediction of availability of metals to plants (Liang and Karama- nos, 1993). Ahnstrom and Parker (2001) suggested that conventional sequential extraction procedures may be of limited utility for predicting bioavailability. The extrac- table fraction as well as the pore water fraction of metals does not necessarily correspond to the amount available to soil organisms. The concentrations of the bioavailable form can be related more closely to biological toxicity (Tokalioglu et al., 2000). Ecotoxicological evaluations of soils are usually associated with the measurement of aqueous or solvent extract toxicity. A bioassay in which the test organism directly interacts with untreated soil, with both the *Corresponding author. Research Centre for Environmental Chemistry and Ecotoxicology, Kamenice 126/3, 625 00 Brno, Czech Republic. Fax: +54-112-9506. E-mail address: zbynek@chemi.muni.cz (Z. Prokop). 0013-9351/03/$-see front matter r 2003 Elsevier Science (USA). All rights reserved. PII:S0013-9351(02)00012-9