Drivers of Rh and Pt variability in the water column of a hydrodynamic estuary: Effects of contrasting environments

Rhodium and platinum are amongst the less studied elements in estuarine waters and the understanding of their speciation analysis and environmental fate remains limited. In this study, we address the occurrence and discrimination of soluble/insoluble Rh and Pt species in aquatic systems, as well as their potential transport. Particulate and dissolved (< 0.45 μm) rhodium (RhP and RhD) and platinum (PtP and PtD), respectively, were determined in the water column of contrasting environments during neap (NT) and spring (ST) tide semi-diurnal cycles: in the upper Tagus estuary (VFX) and near the mouth, close to a wastewater treatment plant outfall (ALC). Both elements were analyzed by AdCSV and ICP-MS, as well ancillary parameters were determined. Concentrations of Rh and Pt followed the tidal regime, presenting higher concentrations during low tide. Concentrations of RhP (0.1–5.1 ng g˗1) and RhD (0.03–0.12 ng L˗1) were lower than PtP (1.0–25.6 ng g˗1) and PtD (0.1–11.7 ng L˗1), respectively. Concentrations found in ALC were higher than VFX, except for RhD, mirroring anthropogenic inputs attributed to automotive catalytic converters and an additional Pt source originated in Pt-based compounds. Distribution coefficients (KD) of 104 were computed and were independent of the salinity gradient. The speciation analysis done at VFX during NT showed that truly dissolved forms measured by AdCSV represented 39 ± 9% of total Pt in the water column, while total filter-passing species measured by ICP-MS were higher, 65 ± 14%. Pt speciation was controlled by its dissolved forms, whereas particulate Rh forms represented the bulk value (> 65%). The potential transport evaluated at downstream station indicated recirculation within the estuary and export towards the Atlantic Ocean, with higher concentrations associated with the ebb opposing to the flood. These results show estuaries as important pathways to introduce PGE in coastal regions, transferring them towards the ocean.