Particle–water interactions of platinum group elements under estuarine conditions

Particle–water interactions of selected platinum group elements (PGE) have been studied as a function of pH (between 5 and 10), salinity, and concentrations of Cl− and Ca2+. Rhodium(III), Pd(II) and Pt(IV) were added to water samples and sediment suspensions and, following a period of equilibration and subsequent phase separation, filtrates and HCl-digested filters were analysed by ICP-MS. PGE removal from the aqueous phase to the particulate phase appeared to proceed via both coagulation of organic/colloidal associations and adsorption to estuarine sediment particles. In river water, removal of Rh increased and Pt decreased with increasing pH, whereas Pd was not detected in the particulate phase throughout much of the pH range studied. In end-member estuarine water (salinity = 28), removal of Rh and Pt increased with increasing pH, but removal of Pd was relatively insensitive to pH. Conditional particulate-aqueous distribution coefficients, KDs (mL g− 1), were on the order of a few hundred (Pd and Pt) to several thousand (Rh) along the estuarine gradient. Increasing salinity and Cl− concentration were accompanied by an increase in KD for Rh and Pd, and a reduction in KD for Pt, while increasing Ca2+ concentration resulted in an increase in KD for all PGE. Experimental observations were qualitatively interpreted in terms of what is known about the aqueous speciation of PGE. Thus, Rh data were consistent with the adsorption of cationic species of the form: [RhCl6−x(H2O)x]x−3, where x = 0 to 6; although in river water complexation by additional (e.g. organic) ligands may have also occurred. Speciation of Pd was predicted to be dominated by organic complexes under all experimental conditions, consistent with its stabilisation in solution in freshwater and apparent salting out and/or coagulation in the presence of dissolved ions. The presence of hydroxychloride complexes of Pt(IV), whose ligand number (and negative charge) increases with increasing chlorinity, could explain most of the experimental observations for this element.