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

Océane Hourtané , Scott Smith , Claude Fortin

Abstract

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 impacts on aquatic ecosystems. While speciation can be predicted for well-studied metals, the task is difficult when little information on thermodynamic constants is available, as it 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 hours 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 Pt (II and IV) 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 a 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.

EN25014  Accepted 22 May 2025

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