Modeling acute toxicity of metal mixtures to wheat (Triticum aestivum L.) using the biotic ligand model-based toxic units method

The combined toxic effects of copper (Cu) and cobalt (Co) were predicted using the biotic ligand model (BLM) for different concentrations of magnesium (Mg2+) and pH levels, with parameters derived from Cu-only and Co-only toxicity data. The BLM-based toxic unit (TU) approach was used for prediction. Higher activities of Mg2+ linearly increased the EC50 of Cu and Co, supporting the concept of competitive binding of Mg2+ and metal ions in toxic action. The effects of pH on Cu and Co toxicity were related not only to free Cu2+ and Co2+ activity, respectively, but also to inorganic metal complexes. Stability constants for the binding of Cu2+, CuHCO3+, CuCO3(aq), CuOH+, Mg2+, Co2+, CoHCO3+ and Mg2+ with biotic ligands were logKCuBL 5.87, $$\mathrm{log}\,{K}_{{{\rm{CuHCO}}}_{3}{\rm{BL}}}$$ log K CuHCO 3 BL 5.67, $$\mathrm{log}\,{K}_{{{\rm{CuCO}}}_{3}{\rm{BL}}}$$ log K CuCO 3 BL 5.44, logKCuOHBL 5.07, logKMgBL 2.93, logKCoBL 4.72, $$\mathrm{log}\,{K}_{{{\rm{CoHCO}}}_{3}{\rm{BL}}}$$ log K CoHCO 3 BL 5.81 and logKMgBL 3.84, respectively. The combinations of Cu and Co showed additive effects under different conditions. When compared with the FIAM-based TU model (root mean square error [RMSE = 16.31, R2 = 0.84]), the BLM-based TU model fitted the observed effects better (RMSE = 6.70, R2 = 0.97). The present study supports the BLM principles, which indicate that metal speciation and major cations competition need to be accounted for when predicting toxicity of both single metals and mixtures of metals.