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Modeling toxicity of binary metal mixtures (Cu2+–Ag+, Cu2+–Zn2+) to lettuce, Lactuca sativa, with the biotic ligand model

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The biotic ligand model (BLM) was applied to predict metal toxicity to lettuce, Lactuca sativa. The Cu2+ had the lowest median effective activity (EA50M), compared with Ag+ and Zn2+ (EA50Cu = 2.60 × 10−8 M, EA50Ag = 1.34 × 10−7 M, EA50Zn = 1.06 × 10−4 M). At the 50% response level, the fraction of the total number of biotic ligands occupied by ions (f50M) was lowest for Ag+ among the metals (f50Ag = 0.22, f50Cu = 0.36, f50Zn = 0.42). The Cu2+ had the highest affinity for biotic ligands compared with Ag+ and Zn2+, as shown by stability constants of the cation–biotic ligand binding, expressed as log KMBL (log KCuBL = 7.40, log KAgBL = 6.39, log KZnBL = 4.00). Furthermore, the BLM was combined with the toxic equivalency factor approach in predicting toxicity of mixtures of Cu2+–Zn2+ and Cu2+–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 the authors to include interactions in estimating mixture toxicity and showed good predictive power (r2 = 0.64–0.84). The TEQ at the 50% response level (TEQ50, Cu2+ equivalents) for Cu2+–Zn2+ mixtures was significantly lower than the value for Cu2+–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. Environ. Toxicol. Chem. © 2012 SETAC

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