Platinum speciation in the presence of natural organic matter in a simplified freshwater medium investigated using HPSEC-ICP-MS

Environmental context Platinum is a metal of emerging concern in ecotoxicology, mainly due to its dissemination in the environment through the abrasion of car catalysts. However, its aqueous speciation in natural waters remains poorly known, and its binding to organic matter may modify its effects on organisms. Experimental measurements revealed the complex chemistry of Pt, low complexation and very slow kinetics. The use of PtII would be recommended for chronic exposure experiments. Rationale It is a common consensus that the free ion concentration of a metal in an aqueous medium can be a good indicator of its toxicity. In this context, the ability to determine metal speciation is of paramount importance to evaluate possible effect on aquatic ecosystems. Although speciation can be predicted for well-studied metals, the task is difficult when little information on thermodynamic constants is available, as is the case for platinum. It is then necessary to turn to experimental methods. Methodology For this purpose, a high-performance size exclusion liquid chromatography method coupled with inductively coupled plasma–mass spectrometry for online metal detection was used. In a synthetic freshwater medium, we first explored the inorganic speciation of platinum (added as PtIV) at pH 5 and 6, and using three equilibration periods (4 min, 48 h and 1 week). We then tested two different conditions with freshwater natural organic matter (NOM) from four different origins, both expected to provide different levels of complexation: low (pH = 5; [NOM] = 3 mg C L−1) and high (pH = 6; [NOM] = 10 mg C L−1). Results Several inorganic forms of platinum (PtII and PtIV) were identified, with good separation and repeatability. In the absence of NOM, dissociation of [PtIVCl6]2− complexes was clearly more important at pH 5 than at pH 6. Also, [PtIVCl6]2−persisted over time, even after 1 week, even though this redox form is believed to be unstable in these conditions, suggesting that thermodynamic equilibrium was not reached. Discussion Weak complexation by NOM was observed, and the initial form of [PtIVCl6]2− persisted. However, the presence of NOM resulted in the formation of additional Pt inorganic species. These unidentified peaks, which were relatively more abundant at high NOM levels, were interpreted as intermediate species between [PtIVCl6]2− and Pt–MON complexes.