Evaluation of the effect of solution ageing on the DGT speciation of rhodium (Rh) and platinum (Pt)
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Synthetic solutions of Rh and Pt at near neutral pH, as well as spiked natural waters are subjected to an ageing effect, which means that over time the speciation of the PGEs will be changed from their original composition due to aquation and hydrolysis, forming various hydroxy and aqua complexes. Due to slow ligand exchange reactions equilibrium speciation may not be rapidly attained. Ageing may affect the reactivity of the PGE solutions towards chelating resins used in the analysis of PGEs by Diffusive Gradients in Thin films (DGT). The aim of the study was therefore to investigate the performance of the chelating resins (Purolite S914 and S920 with thiourea and isothiourea functional groups, and IONQUEST® MPX-317 with a phosphine oxide thiourea functional group) for their applicability as a binding resin for DGT applications for Pt and Rh in natural waters using 17-day aged spiked solutions. Uptake kinetics experiments revealed that the direct uptake of Pt on the resins from aged solutions was fast and quantitative, whereas only 30% of Rh was directly bound on the resin. Likewise, much lower apparent diffusion coefficients were obtained for Rh in aged spiked river water (70% decrease) which can be explained by the formation of Rh(OH)3(s) which is not taken up by the resin gel. The addition of dissolved organic matter results in an increased solubilisation of Rh(OH)3(s) and an increased DGT labile fraction. The formation of Rh(OH)3(s) is less pronounced in seawater. For Pt, no species are formed which are not DGT labile. Thus, DGT accurately measures labile metal species in solution, discriminating labile from inert metal species.