Bioavailability of Cd in 110 polluted topsoils to recombinant bioluminescent sensor bacteria: effect of soil particulate matter

In this study, bioavailability and water extractability of Cd in a panel of 110 natural aged heavy metal-polluted soils from northern France containing up to 20.1 mg of Cd per kilogramme was evaluated. Particulate matter was removed by differential centrifugation of soil-water suspensions (liquid to solid ratio 10) resulting in soil-water extracts containing different size of particles. Chemical analysis of Cd and analysis of bioavailable Cd with recombinant bioluminescent Cd-sensing bacteria were applied in parallel to these fractionated soil solutions. Extractability of Cd from soil to the aqueous phase was low-only 0.13% of the soil total Cd as a mean; however, Cd-sensing recombinant luminescent bacteria Bacillus subtilis incubated in soil-water suspensions for 2 h showed that in the conditions of contact exposure, the bioavailable fraction of Cd increased about 30-fold being 3.74% of the soil total Cd as a mean value. The total Cd content of soils was not a good predictor of either bioavailable or water-extracted fraction of Cd, but these fractions were rather determined by the combination of soil total Cd and physico-chemical properties-texture and organic matter content. Analysis of two selected model soils with Cd sensor bacteria showed that about 90% of the bioavailable Cd was associated with larger soil particles that were removed from the soil suspensions by centrifugation at 4,500xg, and even settling of the soil suspensions for 2 h removed already 65% of bioavailable Cd. Thus, our results indicate a potential for remarkably higher environmental hazard for soil-associated heavy metals than just aqueous exposure.