Adsorption kinetics of platinum group elements in river water

The uptake of platinum group elements (PGE) by different preparations of estuarine sediment suspended in filtered river water has been examined. For a given PGE, adsorption time courses to untreated sediment and to sediment whose hydrous metal oxides or organic matter had been removed by appropriate chemical treatments were similar. Adsorption of Rh(lll) and Pt(IV) proceeded via a first-order reversible reaction. For Rh, forward rate constants were 1 order of magnitude greater than reverse rate constants, but for Pt, forward and reverse constants were comparable. Respective system response times, required to attain 63% of the new equilibrium, ranged from about 10 to 30 h and 2 to 20 h. In contrast, rapid, initial uptake of Pd(ll) was succeeded, in most instances, by a protracted period of desorption, requiring a more complex mechanistic interpretation. In all cases, adsorption was reduced following a period of PGE equilibration with filtered river water, suggesting that complexation with natural organic ligands exerts a significant control on the adsorption process by, for example, stabilizing PGE in solution. Exchangeability of adsorbed PGE, evaluated by ammonium acetate extraction, decreased in the order Pd > Pt > Rh, in qualitative agreement with the proposed or modeled adsorption mechanisms. Experimental results, together with independent assessments of PGE mobility from secondary sources (e.g. road dust), indicate that Pd has the greatest potential for long-range transport and bioaccumulation in the aquatic environment.