Using multiple metal-gill binding models and the toxic unit concept to help reconcile multiple-metal toxicity results

Metal-gill binding models and biotic ligand models (BLMs) in general are designed to predict metal toxicity to aquatic organisms. These models calculate the amount of a metal-binding to a sensitive biological membrane, such as a fish gill, which equates with metal toxicity. Cation competition at the metal-binding site and anionic complexation in the water decrease metal-binding to the membrane, decreasing metal toxicity. These models have, to date, been developed for individual metals. To assess how these models handle multiple-metal interactions, metal-gill binding models for two to six metals were created and their behavior tested against the toxic unit (TU) concept assuming strict additivity. The multiple-metal models yield greater than strict additivity at low aqueous metal concentrations (Sigma < 1 TU), strict additivity at intermediate metal concentrations (Sigma = 1 TU), and less than strict additivity at high metal concentrations (Sigma > 1 TU), independent of the combination of metals. Deviations from strict additivity are due to the non-linear nature of the models, where greater than linear filling of binding sites occurs at low metal concentrations, and where strong competition for binding sites occurs at high metal concentrations, with a point of strict additivity between, where the metals sum to one toxic unit. Simulations with natural organic matter (NOM) show similar trends but are complex. Mathematical modeling of multiple-metal interactions may help in the interpretation of toxicity results from mixed-metal exposures to aquatic organisms.