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Advancing the Diffusive Gradients in Thin-Films Passive Sampling Device for Monitoring PFAS in Drinking Water Systems
Citation:
Hodges, S., H. Pham, J. Fairey, D. Wahman, AND L. Haupert. Advancing the Diffusive Gradients in Thin-Films Passive Sampling Device for Monitoring PFAS in Drinking Water Systems. Presented at 2022 Water Quality & Technology Conference, Cincinnati, OH, November 14 - 17, 2022.
Impact/Purpose:
The environmental or health problem addressed by the study: Passive sampling of PFAS in drinking water A general description of the work and results: Experiments have been conducted to evaluate measurements of PFAS in drinking water using a diffusive gradients in thin-films (DGT) passive sampling device (PSD) The long-term importance or significance of the findings: Provides passive sampling of PFAS in drinking water Who would be interested in or could apply the results (e.g. program or regional partners, general public, local communities): Researchers, engineers, and drinking water utilities interested in time weighted average PFAS sampling.
Description:
Grab sampling is presently used to measure PFAS in waters but is labor intensive and captures a snapshot in time. The diffusive gradients in thin-films (DGT) passive sampling device (PSD), on the other hand, can be used to quantify time-weighted average (TWA) contaminant concentrations over several days to a few months, but several challenges need to be overcome to adapt the DGT technique for PFAS in drinking water systems. A DGT-PSD has a diameter similar to that of an American quarter and consists of a (1) binding layer containing a sorbent(s) with high affinity for the target analyte(s) and is overlain by a (2) gel layer of precise thickness that is in contact with the bulk water. The gel layer restricts PFAS mass transport into the binding layer to molecular diffusion and consequently requires accurate measurements of PFAS diffusion coefficients in the gel, D_PFAS:Gel. A diffusive boundary layer (DBL) exists between the gel layer and the bulk water, the thickness of which, ¿¿DBL, is dependent on the surrounding hydrodynamics and can approach 0.5 mm. Deploying multiple DGT passive samplers with different gel layer thicknesses (i.e., ¿¿gel = 0.8-, 1.2-, 1.6-, and 2.0 mm) for a given deployment period allows for in-situ determination of ¿¿DBL. A two-compartment diffusion cell (DC) can be used to measure D_PFAS:Gel. A DC consists of separate source and sink compartments bridged together by the gel layer. The target PFAS are spiked into the source compartment at time zero and are measured in the source and sink over the next few hours to few days. The rate of PFAS mass increase in the sink is used along with the source concentration, ¿¿gel, and the planar diffusive area to calculate the apparent diffusion coefficient, D_App. This approach assumes that ¿¿DBL is negligible compared to ¿¿gel. However, no standard methodology exists for the DC experiments, including the mixing rate and cell and gel-bridge configuration. To ensure D_PFAS:Gel values are accurate, ¿¿DBL on either side of the gel layer should be determined in the DC experiments and accounted for in DGT-PSD deployments. Triplicate DC experiments with the four different gel layer thicknesses were completed to determine ¿¿DBL on either side of the gel layer. Nitrate was used as a surrogate compound because it could be quantified in near real-time using ultraviolet (UV) spectroscopy. The linear regression of ¿¿gel/D_App vs. ¿¿gel was used to determine the actual diffusion coefficient and ¿¿DBL. This analysis indicated that ¿¿DBL was about 0.22 mm on either side of the diffusive gel. Accounting for this DBL increased D_PFAS:Gel by about 60%, which contradicts the common assumption used in DC experiments that ¿¿DBL is negligible. D_PFAS:Gel were measured for a suite of 24 PFAS, all of which were included in UCMR5. These values were applied to determine deployment times and method detection limits for DGT-PSDs in drinking water systems.