Development of the diffusive gradients in thin films technique for the measurement of labile gold in natural waters.

Gold is a precious metal that exists in most soils, sediments, and natural waters at extremely low concentrations (<1 μg/kg). The diffusive gradients in thin films (DGT) technique, used extensively for measuring trace metal concentrations in soils, sediments, and waters, has potential for geochemical exploration for gold, but has not been developed for this metal. This work investigates the possibility of measuring labile gold using DGT by introducing a new binding layer based on activated carbon. The performance of this new technique was assessed using gold(III) chloride in solution by: (1) determining the diffusion coefficient of gold(III) in hydrogels; (2) determining the uptake of gold(III) chloride by the new activated carbon binding layer; (3) determining an elution methodology for the binding layer and evaluating its efficiency; (4) assessing the capacity of the activated carbon binding layer to adsorb gold; (5) determining the effect of pH and ionic strength (as NaCl) on performance, and (6) assessing the selectivity of the new binding layer for gold. It was found that the diffusion coefficient of gold(III) increased as solution pH decreased. The diffusion coefficient also increased at high ionic strength (≥0.1 M NaCl). Accounting for these phenomena, the DGT technique behaved predictably under all tested conditions. The technique can potentially be used as a geochemical exploration tool for gold in soils and in aqueous environments, with method detection limits as low as 0.9 ng/L for a 7-day deployment.