Fractionation and reactivity of platinum group elements during estuarine mixing

The fractionation of platinum group elements (PGE) rhodium(III), palladium(II), and platinum(IV), has been studied after their addition in aqueous form to unfiltered river water samples (Tugela river, South Africa) and to mixtures of river water and seawater. The particulate fraction of PGE averaged about 70 (Rh), 50 (Pd), and 25% (Pt) of total metal and was dependent on both particle concentration and salinity. The aqueous (<0.45 microm) pool of PGE was dominated by entities of less than 0.1 microm in diameter, and for Pd and Rh hydrophobic complexes of metal, operationally defined by their retention on a C-18 column, were significant. Distribution coefficients, based on the w/w concentration of metal on particles relative to the corresponding concentration in the aqueous pool, either increased (Rh and Pd) or declined (Pt) with increasing salinity. These observations are interpreted in terms of a number of general and metal-specific mechanisms. Thus, the behavior of Pd appears to be controlled by its association with relatively small (<0.1 microm) dissolved organic ligands, a significant fraction of which is hydrophobic and is subject to salting out upon estuarine mixing. Rhodium may also be partly subject to this mechanism of removal, but kinetic considerations and results of X-ray analysis of filter-retentates suggest that adsorption of cationic hydroxychlorides and the destabilization and precipitation of hydroxy-complexes induced by the rise in pH across the estuarine gradient are more important. Unlike Pd, the binding of Pt by organic ligands is kinetically hindered. Thus, in contrast to Pd, particle-water reactivity of Pt is controlled by electrostatic interactions between the particle surface and inorganic aqueous species. The results of this study improve our understanding of and ability to predict the transport and fate of PGE in estuaries where these metals are mobilized or discharged.