Incorporating bioavailability into toxicity assessment of Cu-Ni, Cu-Cd, and Ni-Cd mixtures with the extended biotic ligand model and the WHAM-F _tox approach

There are only a limited number of studies that have developed appropriate models which incorporate bioavailability to estimate mixture toxicity. Here, we explored the applicability of the extended biotic ligand model (BLM) and the WHAM-F_tox approach for predicting and interpreting mixture toxicity, with the assumption that interactions between metal ions obey the BLM theory. Seedlings of lettuce Lactuca sativa were exposed to metal mixtures (Cu-Ni, Cu-Cd, and Ni-Cd) contained in hydroponic solutions for 4 days. Inhibition to root elongation was the endpoint used to quantify the toxic response. Assuming that metal ions compete with each other for binding at a single biotic ligand, the extended BLM succeeded in predicting toxicity of three mixtures to lettuce, with more than 82 % of toxicity variation explained. There were no significant differences in the values of f_mix50 (i.e., the overall amounts of metal ions bound to the biotic ligand inducing 50 % effect) for the three mixture combinations, showing the possibility of extrapolating these values to other binary metal combinations. The WHAM-F_tox approach showed a similar level of precision in estimating mixture toxicity while requiring fewer parameters than the BLM-f_mix model. External validation of the WHAM-F_tox approach using literature data showed its applicability for other species and other mixtures. The WHAM-F_tox model is suitable for delineating mixture effects where the extended BLM also applies. Therefore, in case of lower data availability, we recommend the lower parameterized WHAM-F_tox as an effective approach to incorporate bioavailability in quantifying mixture toxicity.