Mineral-specific ligand and redox dynamics of adsorbed platinum: Pathway-dependent mobility during terrestrial weathering

Emily G Wright , Fernanda M Loza, Elaine D. Flynn, Jeffrey G. Catalano 

Weathering zones developed above ultramafic rocks, such as oxidized ores, soils, and laterites, concentrate the critical mineral platinum and represent a promising future resource. Prior work has documented Pt accumulation in these zones, including its common association with iron oxides. However, the fundamental mechanisms that control Pt retention versus mobilization during weathering are not well understood, inhibiting resource prediction and our knowledge of Pt environmental behavior. Dissolved platinum is predominantly complexed by chloride and ammonia in surficial waters, but the influence of these environmental ligands on Pt mobilization versus retention by mineral surfaces is currently unclear. We studied how Pt(II) aqueous speciation controls adsorption to hematite and goethite, which typically dominate the reactive mineralogy of laterites, from pH 4 to 8 and for reaction times up to 113 days. Initial speciation is the primary control on the fate of Pt, with slow ligand exchange kinetics inhibiting chemical transformations in solution even when far from equilibrium. Platinum(II)-ammine species are kinetically inert and do not appreciably adsorb to either mineral, even after several months. In contrast, Pt(II)-chloro species readily interact with both mineral surfaces. These dissolved species likely slowly convert to Pt(II)-hydroxo species in solution, further enhancing adsorption. Thus, while chloride ligands mobilize a portion of the adsorbed platinum pool, Pt(II)-ammine complexes are highly mobile and fully unreactive with iron oxide surfaces. Weathering systems hosting active biogeochemical nitrogen cycling or organic matter mineralization may thus strongly mobilize Pt. X-ray absorption spectroscopy indicates that ternary Pt(II)-chloro surface complexes form on both hematite and goethite. Over time, these adsorbed species oxidize and undergo hydrolysis on the hematite surface, but resist redox transformations on goethite. These variations indicate that Pt surface speciation will differ substantially among weathering zones under different climate regimes, which dictate iron oxide mineralogy. Surface-mediated oxidation to Pt(IV), which occurs only in hematite-rich weathering zones, will further inhibit mineral-fluid reactions of platinum due to slower ligand exchange rates for Pt(IV) versus Pt(II) species. Such kinetic inhibition of platinum geochemical reactions will create persistent disequilibrium and result in spatially heterogeneous retention of this critical mineral at the pore scale that varies with fluid residence time.