Redox Transformation of Platinum Mediated by Natural Organic Matter Revealed by X-ray Absorption Spectroscopy

Abstract Platinum (Pt) is an emerging environmental contaminant due to its increasing anthropogenic use, yet the processes controlling its speciation remain poorly understood. Here, we investigate Pt redox transformations in the presence of natural organic matter (NOM) using X-ray absorption spectroscopy (XAS). Under anoxic conditions, NOM promotes the progressive reduction of Pt(IV) to Pt(II), with Pt(II) detectable within 24 h and only ∼10% of Pt(IV) remaining after 2 months. In contrast, no reduction occurs under oxic conditions. The resulting Pt(II) is predominantly associated with reduced sulfur functional groups, highlighting the role of thiols in controlling Pt speciation. These results reveal a previously overlooked pathway for Pt transformation mediated by NOM. Such redox processes are likely to control Pt speciation in reducing environments including stratified freshwater water columns, hyporheic sediments, wetlands, and saturated soils. The formation of reduced Pt species may enhance Pt mobility, bioavailability, and potential toxicity, with important consequences for its environmental fate.