Influence of initial speciation of platinum and palladium on their accumulation and toxicity towards phytoplankton

Environmental context The increasing global demand for platinum and palladium for the development of new technologies will likely enhance their biogeochemical mobility. However, our understanding of the environmental risks of these metals remains incomplete. To help fill this knowledge gap, we determined the toxicity of various forms of platinum and palladium on a green alga. We found that palladium was more toxic than platinum and that platinum toxicity depended on the form tested. Rationale Chemical equilibrium-based models such as the biotic ligand model (BLM) can be used to predict metal toxicity towards aquatic organisms. However, some metals such as platinum (Pt) and to a lesser extent palladium (Pd) are known to be kinetically hindered, i.e. reactions between these elements and ligands in solution might be slow relative to the biota exposure time. The BLM applicability may thus be limited for these metals and their ecological risk assessment may require consideration of their initial form of entry in aquatic ecosystems. Methodology This study investigates the relationship between inorganic Pt and Pd initial speciation and their bioavailability and toxicity toward the unicellular green alga Raphidocelis subcapitata. Chloro- or amino-coordinated Pt and Pd complexes such as PtIICl42−, PtIVCl62−, PtII(NH3)42+, PdIICl42− and PdII(NH3)42+ were examined in 96-h algae exposure experiments to determine growth inhibition and metal accumulation. Investigation of the complex stability over time of the tested complexes was carried out using spectrophotometry measurements and kinetic calculations. Results Similar accumulation and no toxicity (half maximal effective concentration, EC50 > 150 µg L−1) were observed for both in our tested conditions. Nevertheless, evidence of PtIICl42− dissociation was observed whereas PtII(NH3)42+ was stable. Exposure to PtIVCl62− showed stronger algal growth inhibition (EC50 = 142 µg L−1) than exposure to PtIICl42− but similar accumulation. This phenomenon might be partly explained by the generation of free chloride radicals through PtIV reduction in the presence of light. Similar accumulation and toxicity were observed for both PdIICl42− (EC50 = 4.69 µg L−1) and PdII(NH3)42+ (EC50 = 2.40 µg L−1). Discussion These results were coherent with the relatively rapid speciation changes of the PtII complexes as observed and modelled. We therefore conclude that the initial speciation of the studied chloro- and amino-coordinated PtII and PdII complexes is not a determining parameter of their accumulation and toxicity towards R. subcapitata.