Copper–zinc coergisms and metal toxicity at predefined ratio concentrations: Predictions based on synergistic ratio model

• We modelled exposure of multiple mixtures of Cu and Zn in aquatic ecosystem.
• Cu:Zn interaction against tilapia species were evaluated by synergistic ratio.
• Toxicity of metal mixtures differs from the single toxicity actions of metals.
• Synergism was realized in most of the ratio mixtures.
• Data from multiple exposures to contaminants is crucial in environmental management.

A significant number of studies have centred on the single actions of heavy metals against test animals in predicting aquatic toxicity. However, practical existence of environmental toxicants is in multiple mixtures and variable undefined ratio combinatorial concentrations. Pollution abatement approaches in setting representative safe boundaries for metal contaminants is crucial with factual data on predictively modelled exposures of organisms to multiple mixtures. In continuance of our approach to toxicity of individual heavy metals, we determined the toxicity of binary mixtures of copper and zinc at predetermined ratios against tilapia species and also evaluated the coergisms based on synergistic ratio model for effective formulations of safe limits. Orecohromis niloticus species were exposed to copper and zinc (Cu:Zn) at ratios of 1:1 and 1:2 on 96hLC50 index and mortality response analysed following the probit-log-dose regression with metal–metal interactions effectively modelled. The 96hLC50 values for Cu:Zn were calculated to be 68.898 and 51.197 mg/l for ratios 1:1 and 1:2, respectively. The joint action toxicity of the metal mixtures was observed to differ from the metals acting singly against the same animal species. Synergistic coergisms were realized in most of the ratio mixtures except the antagonistic effect displayed by the combination of Cu:Zn in the ratio 1:1 when compared to the single action of copper. Biological toxicity of heavy metals however still appears uncertain, and consideration of multiple mixtures and interactions of toxicants in natural milieu is very crucial in environmental management of the existing and emerging contaminating metals.