Biogeochemistry of platinum group elements in aquatic ecosystems

Carlos Eduardo Salgueiro e Silva Monteiro

Key information

Authors:

Carlos Eduardo Salgueiro e Silva Monteiro (Carlos Eduardo Salgueiro e Silva Monteiro)

Supervisors:

Miguel José Martins Caetano; Margarida Maria Portela Correia dos Santos Romão (Margarida Maria Portela Correia dos Santos Romão); Antonio Cobelo-García

Published in

10/22/2020

Abstract

Platinum-group elements (PGE), in which platinum (Pt) and rhodium (Rh) are included, are technology-critical elements of environmental emerging concern with a large variety of applications and increasing global demand. Consequentially, Pt and Rh concentrations have increased in several environmental compartments, including aquatic ecosystems. This thesis presents data acquired recently in the Tagus River-estuary system. This system is adequate to study Pt and Rh due to active and historical anthropogenic pressures and its connection to the sea. Thus, the thesis provides new insights on the occurrence and transport from urban areas to the estuary, and geochemical implications as well in a highly hydrodynamic estuary. A voltammetric procedure was optimized for the simultaneous determination of Pt and Rh in a single scan, taking advantage of signal transformation using the second derivative. This procedure improves ill-defined peaks usually found at ultra-trace concentrations, such as those of Pt and Rh found in environmental matrices. The spatial distribution of Pt and Rh was assessed in surface sediments (n=72). Concentrations ranged between 0.18–5.1 ng Pt g-1 and 0.019–1.5 ng Rh g-1. Estimated reference (background) levels in the estuary were 0.55 ng Pt g-1 and 0.27 ng Rh g-1. Levels of Pt and Rh close to wastewater treatment plants (WWTP) outfalls and pluvial discharge sites were similar to the reference levels. The higher concentrations were found in the vicinity of industrial areas and the Vasco da Gama bridge, pointing out the source of Pt and Rh in industrial catalysts and automotive catalytic converters (ACC), respectively. Focusing on Pt and Rh transfer from the urban area into the estuary and their chemical alterations, a speciation analysis study of both elements was evaluated using a multi-method approach. Road dust collected (<63 μm) in a high traffic lane was thoroughly characterized and incubated during 7 days in synthetic rainwater and seawater. Truly dissolved species measured by adsorptive cathodic stripping voltammetry (AdCSV) corresponded to 0.01% and 0.1% of total Pt and Rh, respectively, in the road dust and concentrations were relatively constant over time. In the leachates (0.45 μm), truly dissolved concentrations of Pt and Rh were approximately 10 and 40 %, respectively, of total filter-passing species measured by inductively coupled plasma mass spectrometry (ICP-MS). This suggests the presence of colloidal- or insoluble (nano)particles. Furthermore, temporal variations were observed for total filter-passing Pt species, as opposed to Rh. The concentrations and partition of Pt and Rh were assessed in the water column of Tagus estuary. The simultaneous determination of particulate and dissolved Rh in the estuarine water column is reported for the first time. Samples were collected at the entrance (VFX – Vila Franca de Xira) and mouth (ALC – Alcântara) of the estuary during semi-diurnal tidal cycles over neap (NT) and spring tide (ST). Additionally, in VFX NT cycle, ICP-MS was used to determine Pt in the dissolved fraction (<0.45 μm). Both Pt and Rh followed the hydrodynamic regime, in general presenting higher concentrations during low tide. Concentrations of PtP (1.0–25.6 ng g˗1) and PtD (0.1–11.7 ng L˗1) were higher than RhP (0.1–5.1 ng g˗1) and RhD (0.03–0.12 ng L˗1), respectively, and both fractions of Pt were higher at ALC than in VFX, mirroring different anthropogenic inputs to the Tagus estuary. The main source of Pt and Rh was the ACC but, at ALC station, Pt also had origin in Pt-based compounds. The behavior of Pt appeared to be non-conservative due to the higher inputs than Rh. Distribution coefficients (KD) around 104 were calculated for Pt and Rh and were independent of the salinity gradient. In VFX NT survey, speciation analysis showed that truly dissolved forms represented 39±9 % of total Pt in the water column, while total filter-passing species were higher, 65±14 %. As also found for PtD in ALC, these results suggest that dissolved forms control Pt speciation in the water column. Contrarily, particulate forms represented the main proportion of bulk Rh in the water column since dissolved Rh was often below the LOD. Additionally, metals recirculation within the estuary and the potential exchange to the Atlantic Ocean were also assessed at the downstream station using a hydrodynamic model of Tagus River estuary (MOHID). Both Pt and Rh presented in general higher concentrations associated with the ebb opposing to the flood. Moreover, the Lagrangian modelling of the particles’ spread also pointed out that recirculation within the estuary takes place and the export towards the Atlantic Ocean can occur within 2 days after release. Ultimately, the obtained results show that estuaries are an important pathway to introduce PGE in the coastal region, transferring them towards the ocean with implications in regional and/or global circulation.