Integrating voltammetry in ecotoxicology: Cu(II)-nitrocatechol complexes formation as a driver of Cu(II) and nitrocatechol toxicity in aquatic systems

Abstract

Nitrocatechols (NCs) are hazardous environmental pollutants that pose a significant risk to aquatic organisms. The toxicity of pollutants is determined by their speciation, which depends on interactions with coexisting inorganic and organic substances, emphasizing the need for detailed specific studies. Employing square wave and cyclic voltammetry, we investigated the complexation between NCs, specifically 4-nitrocatechol (4NC), 3-methyl-4-nitrocatechol (3M4NC), 3-methyl-5-nitrocatechol (3M5NC), and 4-methyl-5-nitrocatechol (4M5NC), and copper ions (Cu(II)), a micronutrient that is toxic at elevated concentrations. By confirming and following a quasi-reversible two-electron reduction of di-nitrocatecholate [Cu(NC)₂]^2- complexes in slightly alkaline solution (pH 8.2), we demonstrated fast complex formation in solution at different analyte concentrations and high kinetic stability. However, the pH lowering resulted in degradation of these complexes, yielding mono-nitrocatecholate [Cu(NC)] species and eventually uncomplexed Cu(II) ions. The chronic toxicity bioassays (AlgaeTox) with Scenedesmus subspicatus microalgae exposed to a mixture of Cu(II) and NCs at different concentrations showed decreased toxicity compared to the toxicity of individual Cu(II) and NCs. This decrease was linked to the formation of [Cu(NC)₂]²⁻ complexes, which was confirmed by the appearance of a corresponding voltammetric reduction peak. Conversely, increased toxicity was observed when the complexes were degraded, resulting in the presence of free/labile Cu(II) or NCs species in the solution. These results demonstrate that concentration-dependent complexation between pollutants significantly alters the toxicity profile of Cu(II)/NC systems. This study highlights the value of voltammetry with a mercury drop working electrode as a reliable method for studying pollutant complexation relevant to ecotoxicity in natural waters.