Diffusive gradient in thin film (DGT) samplers are being used to monitor for metals and predict their bioavailability in contaminated environments. The principle of the DGT samplers is based on the diffusion of metal species through a diffusion layer, followed by their uptake onto a resin layer beneath the diffusion layer. Due to the small pore size of the diffusion layer (i.e., d<sub>pore </sub>&lt; 100 nm), it was hypothesized that only dissolved species can pass through the diffusion layer. Thus, the mass of metal accumulated on the sampler is assumed to represent the dissolved metal concentration in the solution. In anoxic sediments, chalcogenic metals (e.g., Hg, Zn, Cu, and Cd) can persist as a mixture of dissolved and particulate species, including small metal sulfide nanoparticles (d &lt; 30 nm). When DGT samplers are deployed in these settings, it is unclear how the nanoparticles affect the sampler performance. We hypothesize that nanoparticles can migrate through the diffusion layer and accumulate on the resin layer of the sampler. Alternatively, the nanoparticles could deposit on the diffusion layer and clog its pore, thereby retarding the migration of the dissolved species. Both processes would result in a predicted metal concentrations that are different than the actual values. To test theses hypotheses, DGT samplers were exposed to solutions containing HgS and ZnS nanoparticles. The amount of Hg and Zn accumulated onto the DGT sampler was quantified and compared with the bulk solution dissolved Hg(II) and Zn(II) concentrations. X-ray absorption spectroscopy was also used to determine the speciation of the Hg accumulated on the resin layer and the possible presence of HgS particles. Our results indicated that in mixtures of dissolved and nanoparticulate Hg and Zn, the uptake of the metal by the DGT sampler was slower than in solutions with only dissolved metals. The nanoparticles did not appear to pass through the diffusion layer. The results suggested that the nanoparticles deposited on the surface of the DGT and retarded the migration of dissolved metal species into the samplers. Therefore, the metal concentration predicted by the DGT was smaller than the actual concentration in the solution. These findings suggest that DGT samplers cannot be used to predict dissolved metal concentration in sediment porewater. Future studies are needed to determine if metal uptake rates into DGT samplers could provide a proxy for metal bioavailability in sediments.

Diffusive gradient in thin film (DGT) passive samplers for monitoring metals in contaminated sediments: Contribution of metal sulfide nanoparticles

249th National Meeting (2015)

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