Cadmium uptake by earthworms as related to the availability in the soil and the intestine

The free metal concentration in the soil solution is often considered a key parameter for metal uptake by and toxicity to soft-bodied soil organisms. The equilibrium partitioning theory, which assumes a relationship between the contaminant concentration in pore water and the contaminant concentration in the body tissue, can be used to describe uptake by earthworms. This theory has proved useful for organic chemicals, but its applicability is less clear for metals. In this study, the Cd concentration in soil pore water (pw) was varied by increasing the soil pH by the addition of lime (Ca(OH)₂) and by adding manganese oxide (MnO₂), which has a high metal binding capacity. Both lime (0.135% w/w) and MnO₂ (1% w/w) decreased [Cd²⁺]_pw by a factor of 25, while Cd_worm was reduced only by a factor of 1.3 in lime-treated soils and 2.5 in MnO₂-treated soils. Cadmium uptake was weakly related to the free metal concentration (R²_adj = 0.66). Adding pH as an explanatory variable increased R²_adj to 0.89, indicating that Cd uptake from pore water is pH dependent, which might be attributed to competition of protons and Cd at the surface of the earthworm body. However, previous earthworm experiments in reconstituted groundwater showed a conspicuously smaller pH dependency of Cd uptake. The differences in metal uptake between earthworms in lime- and MnO₂-treated soils are therefore more likely to reflect the predominance of pH-independent intestinal uptake of Cd. Equilibrating the soil with a solution of 0.01 M CaCl₂ and 0.1 M triethanolamine (buffered at pH 7.2), simulating the conditions prevailing in the worm intestine, yielded free Cd concentrations that were closely (R²_adj = 0.83) and linearly related to the Cd concentration in the earthworm tissue.