Platinum enrichment in marine ferromanganese oxides: Constraints from surface adsorption behavior on synthetic feroxyhyte (δ-FeOOH)

Abstract

Several recent studies have reported a strong association between Pt and iron-bearing minerals in marine ferromanganese oxides. However, due to the low Pt abundance (hundreds of ppb) in these ferromanganese oxides, the mechanism and controlling factors of Pt enrichment in ferromanganese oxides are not clear. Feroxyhyte is an important constituent of marine ferromanganese oxides and is thought to play an important role in Pt enrichment. Here, we used feroxyhyte as the representative of Fe minerals in the marine ferromanganese oxides to study the adsorption behavior and molecular scale adsorption mechanisms of Pt at the water-feroxyhyte interface. The study employed a combination of Pt adsorption/desorption experiments with the participation of various ionic salts, and X-ray absorption fine structure (XAFS) analyses. The adsorption/desorption experimental results showed that the presence of phosphate, ammonium and low pH environment would inhibit Pt enrichment on feroxyhyte. Pt LIII-edge XAFS analysis shows that Pt is oxidized on the feroxyhyte surface, transforming Pt (II) to Pt (IV) (OH)62−. According to the experiment results, this study proposed a hypothesis of the mechanism of Pt enrichment by feroxyhyte. Pt (II) Cl42−is considered to be the main form of Pt in seawater. [PtCln(OH)4−n]2− produced by the hydrolysis of [PtCl4]2− is adsorbed to the feroxyhyte surface through dehydration condensation, and then oxidized to Pt (IV) (OH)62−. Our results clarify the molecular-scale mechanism of Pt adsorption and fixation on feroxyhyte, and provide insights on the geochemical behavior of Pt.