Accounting for Non-Additive Effects in Multicomponent Aquatic Systems: Implications for Environmental Risk Assessment of Chemical Mixtures

Introduction: Environmental risk assessment of chemical mixtures in aquatic systems is often based on additive assumptions. However, trace metals and coexisting ligands, mineral particles, dissolved organic matter, and anthropogenic surfaces may interact through coupled equilibria, producing responses that differ from the sum of individual effects. Special attention is given to environmentally relevant metal ions such as Pb(II), Cu(II), Cd(II), and Zn(II), together with common complexing or competing components, including humic substances, phosphate, carbonate, chloride, iron and aluminum oxides, and microplastic-associated surfaces.

Methods: This study introduces a thermodynamic interpretation of non-additive effects in multicomponent aquatic systems. A coupled or observed equilibrium response is compared with an additive reference state defined under comparable conditions. Deviations are expressed using dimensionless ratios of equilibrium, solubility, adsorption, or distribution parameters and converted into apparent Gibbs free-energy terms. This provides a common energetic criterion for classifying additive, synergistic, and antagonistic behavior.

Results: The proposed approach shows that synergistic effects correspond to stabilization of the coupled system relative to the additive reference state, whereas antagonistic effects indicate destabilization or reduced retention. The same logic can be applied to metal complexation, precipitation-boundary displacement, adsorption on mineral or organic surfaces, and retention by microplastic-associated phases. These processes may alter dissolved metal concentrations, apparent bioavailability, and toxicity under realistic mixture conditions.

Conclusions: Non-additive interactions should be considered in aquatic mixture assessment when coupled chemical equilibria control contaminant speciation and mobility. The proposed criterion does not replace toxicological testing but provides a quantitative chemical basis for identifying mixture conditions that may increase or decrease environmental risk.