Platinum-group elements in urban fluvial bed sediments—Hawaii

Abstract

Results from a detailed examination of the abundance, spatial variability and grain-size fractionation of platinum-group elements (PGEs; iridium, Ir; palladium, Pd; platinum, Pt; and rhodium, Rh) in bed sediments of an urban stream in Honolulu (Hawaii, USA) indicate significant contamination of Pd, Pt, and Rh. PGE concentrations in sediments located in close proximity to storm drains followed the sequence of Pt (10.3–24.5 ng g−1) > Pd (5.9–12.6 ng g−1) > Rh (0.82–2.85 ng g−1) > Ir (0.11–0.23 ng g−1). From a contamination perspective, enrichment ratios followed the sequence of Rh (25.3) ≫ Pd (6.9) = Pt (6.8) ≫ Ir (2.3). Iridium was primarily geogenic in origin, while the remaining PGEs indicated significant anthropogenic contamination. Attrition of the PGE-loaded three-way catalytic converters and their release to the road environment is the most likely source of PGEs in the stream sediments examined. PGE enrichment of bed sediments likely resulted from direct transport of sediment-associated road runoff via storm drains. Preliminary work on grain-size partitioning showed preferential enrichment and mass loading of Pd, Pt, and Rh in grain-size fractions ranging from 63 to 1,000 μm. Data from this study have direct implications for contaminant transport, and sediment source identification in urban catchments. Rhodium, in particular, emerged as an element potentially useful for sediment fingerprinting.