Evaluating the Performance of DGT Technique for Selective Measurement of Trace Metals and Assessment of Environmental Health in Coastal Waters

Abstract

The diffusive gradients in thin films (DGT) technique as a passive sampler for measurement of trace metals was validated and evaluated. A systematic determination of diffusion coefficients for a wide range of cationic (Al, Cd, Co, Cu, Mn, Ni, Pb, Zn) and oxyanionic (Al, As, Mo, Sb, V, W) metals in open (ODL) and restricted (RDL) diffusive layers used by the DGT technique was performed. The diffusion coefficients were determined at acidic and neutral pH, using two independent methods, diffusion cells and time-series DGT techniques. The calculated values for many oxyanions were the first reports in the RDL. The diffusion coefficients measured in the ODL were retarded compared to the values reported in water, and further retarded in the RDL for all elements with both methods. A DGT technique with mixed binding layer (MBL), containing both Chelex-100 and Metsorb, was validated for the measurement of Al at pH 4.01 and pH 8.30, where the dominant species shifts from cationic to anionic, respectively. The performance of this DGT- MBL was then evaluated in various coastal sites over a wide range of pH, for the simultaneous measurement of Al and other cationic and oxyanionic metals using both ODL and RDL. The results were compared to the 0.45 µm-filterable and also the measurements of individual binding layers to investigate the selectivity of each DGT type for trace metals. All measured concentrations with all measurement types were compared to the water quality guidelines defined by the Australian and New Zealand Environment and Conservation Council (ANZECC) to assess the environmental health of the studied field sites. The in-situ application of DGT-MBL confirmed the utility of this approach compared to the use of individual DGT-Chelex and DGT-Metsorb samplers, especially for metals like aluminium with complex speciation.