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.

The mobility of groundwater and its reactivity with subsurface lithologies makes it an ideal medium for investigating both the mineralogy of the extensive volume of the rocks and soils that it comes into contact with, including the distribution of potential commodities, and the presence of contaminants. Groundwater grab sampling is potentially an effective tool for evaluating metal and metalloid concentrations but can suffer from poor replication and high detection limits. This study evaluates the diffusive gradients in thin films (DGT) technique to detect signatures of Au mineralization in groundwater, as well as associated pathfinder and potential contaminant elements (As and Sb). The DGT technique was modified for Au by evaluating a "gel-less" configuration, with diffusion onto an activated carbon binding layer being controlled by the 0.13 mm thick filter membrane (0.45 μm porosity) only, in order to increase sensitivity in quiescent solutions. Laboratory-based measurements indicated that the diffusive boundary layer (DBL) was ∼ 0.40 mm in thickness in quiescent solutions. The modified DGT samplers were then deployed alongside ferrihydrite DGT devices (fitted with 0.8 mm diffusive gels) to simultaneously measure Au, As and Sb in groundwaters surrounding a known arsenopyrite-hosted Au ore body. DGT-measured Au concentrations ranged from 2.0 ng/L to 38.5 ng/L, and were within a factor of 5 of grab sample concentrations. DGT-measured concentrations of As and Sb were above the detection limits, while grab sample concentrations of As and Sb were often close to or below detection. The DGT technique demonstrated methodological improvement over grab sampling of groundwater for the investigated elements with respect to sensitivity, replication, and portability, although DGT requires further evaluation in a wider range of groundwater environments and conditions.

No abstract

No abstract