Simultaneous Measurement of Trace Metal and Oxyanion Concentrations in Water using Diffusive Gradients in Thin Films with a Chelex-Metsorb Mixed Binding Layer

A new diffusive gradients in thin films (DGT) technique with a mixed binding layer (Chelex-100 and the titanium dioxide based adsorbent Metsorb) is described for the simultaneous measurement of labile trace metal (Mn, Co, Ni, Cu, Cd, and Pb) and oxyanion (V, As, Mo, Sb, W, and P) concentrations in freshwater and seawater. The mixed binding layer (MBL) DGT technique was evaluated against the Chelex-DGT and Metsorb-DGT techniques, and all elution efficiencies and diffusion coefficients have been remeasured for the above analytes. Diffusion coefficients (D) measured using MBL-DGT generally agreed well with those measured by Chelex-DGT (D-MBL/D-Chelex = 0.97-1.05), Metsorb-DGT (D-MBL/D-Metsorb = 0.97-1.01), and diffusion cell experiments. The measurement of trace metals and oxyanions by MBL-DGT was independent of pH (5.03-8.05) and ionic strength (I = 0.001-0.7 mol L-1). MBL-DGT accurately measured the concentration of trace metals and oxyanions in synthetic freshwater (C-MBL/C-Sol = 0.82-1.18) over the 4 day deployment and also agreed well with Metsorb-DGT (C-MBL/C-Metsorb = 0.84-0.94) and Chelex-DGT (C-MBL/C-Chelex = 0.88-1.11) measurements. In synthetic seawater, MBL-DGT accurately measured the concentration of metals and oxyanions (C-MBL/C-Sol = 0.85-1.12) over 4 days, with the exception of Mo-none of the DGT techniques were capable of measuring Mo in seawater. MBL-DGT measured the Mn concentration accurately over the entire 4 day period, whereas Chelex-DGT only measured Mn accurately up to 2 days. The MBL-DGT method described in this study offers significant advantages over the ferrihydrite-Chelex-DGT method reported previously. These advantages include the commercial availability of both Metsorb and Chelex-100, the higher accuracy of Metsorb for measuring some oxyanions in freshwater and seawater, and the possibility of measuring Fe, which would not be possible using the Chelex-ferrihydrite binding layer.