Accumulation of platinum and palladium by a green alga and their toxicity: Influence of initial speciation
Résumé
In this project, we examined the accumulation and toxicity of Pd and Pt in the green alga Pseudokirchneriella subcapitata as a function of their initial forms and potential re-speciation during the exposure experiments. To do so, the microalgae were exposed for 96 h in a modified USEPA medium containing either chloride or ammonium Pt and Pd species. We hypothesized that, if these metals quickly reach thermodynamic equilibrium, they should mainly re-speciate into hydroxo-complexes and exert similar toxicity to the algae.
Similar growth inhibition thresholds (EC50s) and levels of metal accumulation were observed for chloro and amino Pd complexes. However, tests with Pt showed higher toxicity with chloro-complexes than with ammonia complexes. Surprisingly, similar accumulation levels of Pt were observed for both forms, indicating an intriguing disconnect between accumulation and effects. These results thus suggest that the changes in Pd speciation occur within a time frame short enough to provide similar toxicity to the green alga while Pt speciation remains slow enough to have toxicity effects that differ base on the initial speciation of the metal. Changes in metal speciation over time for all initial chemical species tested was further investigated by UV-visible absorbance to get insight on the time scale of these changes. Changes in UV-vis spectra were observed within hours for chloro-complexes of Pt but not for ammonium complexes, suggesting a higher stability of Pt ammonium complexes. Our work shows that the initial form of Pt used in toxicity tests needs to be considered and that thermodynamic equilibrium cannot be assumed to be reached over the exposure time.