Environ. Sci. Technol. 2006, 40, 1524-1531
Adsorption Kinetics of Platinum Group Elements in River Water ANDREW TURNER,* MARK CRUSSELL, GEOFFREY E. MILLWARD, ANTONIO COBELO-GARCIA, AND ANDREW S. FISHER School of Earth, Ocean and Environmental Sciences, University of Plymouth, Drake Circus, Plymouth PL4 8AA, U.K.
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(III) and Pt(IV) proceeded via a firstorder 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(II) 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.
Introduction Platinum group elements (PGE), defined herein as Rh, Pt, and Pd, have a variety of industrial applications and are also used in dentistry, jewelry, and medicine. However, by far their most important use is in vehicle exhaust catalysts, and this has resulted in significant increases in both local and global environmental concentrations over the past few decades (1, 2). Abrasion and deterioration of the catalyst surface results in the emission of metallic particles, and a proportion of PGE is gradually mobilized from these particles on aging and weathering (3), ultimately allowing them to enter the aquatic environment (4). Here, the principal (or sole) oxidation states are Rh(III), Pt(II) and Pt(IV), and Pd(II), although information concerning their precise aqueous chemical speciation is lacking. * Corresponding author phone: +44 1752 233041; fax: +44 1752 233035; e-mail:
[email protected]. 1524
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 5, 2006
The subsequent behavior, transport, and fate of PGE are largely dependent on the nature and extent of their interaction with sediment particles. There is, however, very little information on the adsorptive characteristics of these metals, and data appear to be limited to selected PGE that have been studied as part of broad, multitracer experiments. For example, Chen et al. (5) reported sediment-water distribution coefficients for Rh and Pd in suspensions of a variety of minerals and sediments in artificial seawater and indicated that these metals are relatively soluble. In a later study, Takahashi et al. (6) examined the uptake of Rh and Pt (presumably as Pt(II)) by silica and kaolinite in the presence and absence of humic acid and concluded that the acid acted to modify the particle surface rather than form stable, soluble metal complexes. In this study, we examine the kinetics of and controls on the adsorption of these metals to estuarine sediment suspended in river water, using elevated concentrations of stable Rh, Pt, and Pd as tracers. A similar approach has recently been employed to investigate the uptake of PGE by the marine macroalga, Ulva lactuca L (7). The current study represents the first systematic investigation into the particle-water interactions of these metals in the environment and, as such, makes a valuable contribution to an improved understanding of their geochemical behavior as transition metals and their transport and fate as contaminants of growing concern.
Experimental Section Unless otherwise stated, all reagents used in this study were purchased from BDH and were AnalaR or Aristar grade. Prior to use, all polymer and glass containers and equipment were soaked in 10% HNO3 for 24 h and rinsed with Millipore Milli-Q water. Sample Collection, Preparation, and Characterization. Samples for this study were collected from the River Plym and its estuary, southwest England. The Plym drains about 80 km2 of protected granitic moorland in which there is little anthropogenic activity, and its estuary is partly urbanized along its 5 km length. River water was collected in a 2 L high-density polyethylene bottle and stored cool (
