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 (Mg(2+)) 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 Mg(2+) linearly increased the EC(50) of Cu and Co, supporting the concept of competitive binding of Mg(2+) and metal ions in toxic action. The effects of pH on Cu and Co toxicity were related not only to free Cu(2+) and Co(2+) activity, respectively, but also to inorganic metal complexes. Stability constants for the binding of Cu(2+), CuHCO(3) (+), CuCO(3)(aq), CuOH(+), Mg(2+), Co(2+), CoHCO(3) (+) and Mg(2+) with biotic ligands were logK (CuBL) 5.87, [Formula: see text] 5.67, [Formula: see text] 5.44, logK (CuOHBL) 5.07, logK (MgBL) 2.93, logK (CoBL) 4.72, [Formula: see text] 5.81 and logK (MgBL) 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, R (2) = 0.84]), the BLM-based TU model fitted the observed effects better (RMSE = 6.70, R (2) = 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.