Modeling toxicity of binary metal mixtures (Cu(2+) -Ag(+) , Cu(2+) -Zn(2+) ) to lettuce, Lactuca sativa, with the biotic ligand model.

The biotic ligand model (BLM) was applied to predict metal toxicity to lettuce, Lactuca sativa. Cu(2+) had the lowest median effective activity (EA50(M) ), compared with Ag(+) and Zn(2+) (EA50(Cu)  = 2.60 × 10(-8) M, EA50(Ag) = 1.34 × 10(-7)  M, EA50(Zn) = 1.06 × 10(-4)  M). At the 50% response level, the fraction of the total number of biotic ligands occupied by ions (f50(M) ) was lowest for Ag(+) among the metals (f50(Ag) = 0.22, f50(Cu) = 0.36, f50(Zn) = 0.42). Cu(2+) had the highest affinity for biotic ligands compared with Ag(+) and Zn(2+) , as shown by stability constants of the cation-biotic ligand binding, expressed as log K(MBL) (log K(CuBL) = 7.40, log K(AgBL) = 6.39, log K(ZnBL) = 4.00). Furthermore, the BLM was combined with the toxic equivalency factor approach in predicting toxicity of mixtures of Cu(2+) -Zn(2+) and Cu(2+) -Ag(+) . The fraction of biotic ligands occupied by ions was used to determine the relative toxic potency of metals and the toxic equivalency quotient (TEQ) of mixtures. This approach allowed for including interactions in estimating mixture toxicity and showed good predictive power (r(2) = 0.64-0.84). The TEQ at the 50% response level (TEQ50, Cu(2+) equivalents) for Cu(2+) -Zn(2+) mixtures was significantly lower than the value for Cu(2+) -Ag(+) mixtures. Joint toxicity depended on both TEQ and specific composition of the mixture. The present study supports the use of the accumulation of metal ions at the biotic ligands as a predictor of toxicity of single metals and mixtures.