Removal of platinum group elements in an estuarine turbidity maximum

Abstract

The particle–water interactions of Rh(III), Pd(II) and Pt(IV) (or platinum group elements, PGE) have been studied in an estuarine turbidity maximum (Tamar, south west England) by spiking suspensions with mixed standards to concentrations of 10 μg L − 1 and analysing acidified filtrates and digested particles by ICP–MS. Adsorption of PGE to estuarine particles was defined by the Frendlich equation, with Freundlich constants on the order of a few thousand mL g − 1 and the degree of convexity between about 0.7 and 0.9. Along an axial transect of the turbidity maximum, all metals exhibited increasing removal from the aqueous phase with increasing particle concentration. However, for a given metal, removal could not be accounted for by a single sediment–water distribution coefficient, K D, since the magnitude of this coefficient decreased with increasing particle concentration. This reduction in K D could not be accounted for by the non-linearity of the adsorption isotherms. However, it was partly replicated in experiments in which different quantities of the same, fractionated and filtered sediment were added to filtered river water, and was greatly accentuated when the experiment was repeated after sediment suspensions had been allowed to settle for different time periods, resulting in a reduction in net particle size with decreasing particle concentration. Thus, variations in both particle size (and presumably sorptive character) and particle concentration (through some undefined mechanism or artefact) appear to be responsible for controlling the partitioning, hence removal of PGE from the estuarine water column. The results of this study significantly improve our understanding of and ability to predict the likely transport and fate of PGE in an estuarine turbidity maximum, once these elements have been mobilised from secondary sources such as road dust.