Dissolution of a ZnO/CuO nanoparticle mixture, metal speciation, and microalgal responses in aqueous media of contrasting composition

Metal oxide nanoparticles, particularly ZnO and CuO, are widely used in photocatalytic water treatment. Their partial dissolution releases dissolved metal species that may affect downstream biological processes. This study examines the dissolution and speciation of a ZnO/CuO physical mixture (97/3%, w/w, 100 mg·L − 1 ) in four aqueous matrices: deionized water, Bold’s Basal Medium (BBM), BBM without EDTA, and OECD 209 synthetic wastewater. Dissolution experiments revealed a marked contrast between the two materials: while CuO dissolution remained negligible (maximum 0.11 mg·L − 1 ), ZnO released over 10 mg·L − 1 zinc within 7 days. Chemical speciation modelling (Visual MINTEQ 3.1) predicted free Zn 2+ to represent 17–48% of dissolved zinc, with the remainder forming inorganic complexes (ZnHPO 4 , ZnCO 3 ) or EDTA chelates. The experimentally measured < 3 kDa Zn fraction was consistently lower than the modelled free Zn 2+ fraction, particularly in OECD 209 (~ 50% lower), suggesting that additional processes may influence zinc distribution in the different matrices. Based on the concentrations observed during the dissolution experiments, a second set of experiments was performed with the microalga, Scenedesmus obliquus , in the three complex media (BBM, BBM without EDTA and OECD 209), using nominal Zn concentrations of 12.5, 25, and 50 mg·L − 1 added as ZnSO 4 . These experiments were used to evaluate how medium composition affects the biological response to added zinc, rather than to directly assess the toxicity of ZnO/CuO transformation products. Zinc addition did not prevent microalgal growth in any medium over the 7-day exposure, although growth trajectories differed from the control depending on the medium and the zinc concentration. For the photosynthetic indicators (Fv/Fm, pigments), no zinc concentration differed significantly from the control, whereas the inorganic fraction of the biomass increased with zinc concentration in all media, with medium-dependent amplitudes. Together, these results show that matrix chemistry influences zinc speciation and suggest that it may also contribute to medium-dependent biological responses.