Effect of pH on metal speciation and resulting metal uptake and toxicity for earthworms

In the present study, relationships between changes in the solubility and speciation of metals in contaminated soils under different pH regimes and their toxicity to earthworms were investigated. Earthworms (Lumbricus rubellus) were exposed in a laboratory bioassay to metalliferous soils under three pH regimes: Unamended pH, pH lowered by one unit (pH −1), and pH increased by one unit (pH +1). In each soil, total (hot nitric acid–extractable) and 0.01 M CaCl2–extractable metal concentrations were measured and soil pore-water chemistry analyzed to allow metal speciation to be modeled using the Windermere Humic Aqueous Model. Earthworm metal accumulation was determined and toxicity assessed by measuring survival and reproduction and at the molecular level by recording expression of the gene encoding metallothionein-2 (MT-2) using quantitative reverse transcriptase– polymerase chain reaction. Both metal solubility and speciation were found to be highly pH dependent. Metal accumulation in earthworms was influenced by soil concentration and, in some cases (e.g., Cd), by pH. Reproduction was affected (reduced up to 90%) by soil metal level, pH, and their interaction. Relationships between analyzed and calculated Zn concentrations and toxicity and between analyzed and calculated Cd concentrations and tissue accumulation and MT-2 expression were compared by fitting dose–response models and assessing the fit of the data. This analysis indicated that values based on a pH-adjusted free ion concentration best explained toxicity (r2 = 0.82) and accumulation (r2 = 0.54). Expression of MT-2 was, however, poorly correlated (p > 0.05) with all analyzed and modeled soil metal concentrations.