Delineating the dynamic uptake and toxicity of Ni and Co mixtures in Enchytraeus crypticus using a WHAM-FTOX approach

• Metal uptake in enchytraeids linearly correlated with metal binding to humic acid.
• WHAM-FTOX well predicted Ni and Co toxicity at different exposure times.
• Ni toxicity increased with time until 14 d, while Co toxicity remained constant.
• Acute model parameters cannot be directly used to predict chronic metal toxicity.

Uptake and toxicity of Ni, Co and their mixtures in Enchytraeus crypticus after different exposure times (4, 7, 10 and 14 d) were predicted using the WHAM-FTOX model, which incorporates the effects of metal speciation, affinity and competition of metals for binding sites. The combined toxicity of metals was quantified by the toxicity function (FTOX), a linear combination of the amount of metal binding to non-specific ligand sites (vi) and a toxicity coefficient (αi). Observed body concentrations of Ni and Co in the animals correlated well with the WHAM-calculated amounts binding to humic acid, supporting the use of humic acid as a surrogate for metal binding sites of E. crypticus. The toxicity of metals at different exposure times was well predicted by the WHAM-FTOX model. The derived αNi increased with time and reached equilibrium after approximately 14 d, while αCo remained almost independent of time. This suggests for Ni more time is needed than for Co to reach equilibrium of body concentrations, so the toxicity of Ni is much more time-dependent. The WHAM-FTOX model provides a new tool for evaluating the potential mixture toxicity of metals to soil organisms in a dynamic environment. However, as αi varied with exposure time, caution is warranted when using the parameters estimated from acute toxicity experiments for predicting the chronic toxicity of metal mixtures.