University of California Los Angeles
Dr. Sanjay Mohanty is an assistant professor at the Department of Civil and Environmental Engineering at UCLA. His research aims to understand the links between weather conditions, subsurface contaminant removal processes, and water quality impairment during climate change and to develop engineered methods to increase the resiliency of stormwater infrastructure and remediation technologies for the protection of surface water and groundwater resources.
Particle-facilitated transport and release of PFAS in soil
Soils contaminated with perfluoroalkyl substances (PFAS) serve as a long-term source for contamination of groundwater. Thus, the processes that release and transport PFAS from soils have implications on the management of contaminated subsurface sites and groundwater. Among different processes that could release PFAS from soils, the particle-facilitated release of PFAS is rarely studied. This study reveals the importance of soil colloids—particles less than 2 µm—in releasing PFAS from subsurface soils subjected to natural weather conditions such as dry-wet and freeze-thaw cycles and their eventual transport to groundwater. To examine the transport, groundwater contaminated with 200 µg/L of perfluorobutanoic acid (PFBA), perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) was injected at 0.6 cm/ hr for 13 days through columns packed with two types of soil under saturated condition. To examine the processes that redistribute PFAS in pore water in the absence of advection, the flow was stopped for 6 days before injecting the contaminated groundwater for additional 13 days, following which PFAS-free groundwater was injected to flush PFAS from pores. To quantify the leaching of PFAS, the contaminated soil columns were subjected to dry-wet and freeze-thaw-wet cycles, where a control column was subjected to leaching without undergoing drying or freeze-thaw treatments. In both soils, PFOA was adsorbed and retarded, whereas the retardation of PFBA and PFOA was low, potentially because of the low organic carbon content of the soils. Due to the diffusion of PFAS into the clay-rich soil matrix, flow interruption was expected to decrease PFAS concentration in the pore water. However, the concentration of PFAS in porewater or effluent surprisingly increased after the flow interruption, indicating a release of PFAS in association with soil colloids during flow perturbation. Dry-wet cycles and freeze-thaw cycles increased the release of soil particulates and particle-associated PFAS. Presence of colloids and a difference in concentration of total and dissolved (centrifuged) PFAS in the samples confirmed the particle-facilitated release of PFAS. Potential of particle-facilitated release increased with the relative affinity of PFAS on soil: PFOS > PFOA > PFBA. Overall, these results suggest that particle-facilitated transport and release of PFAS, particularly long-chained PFAS, should be considered in the risk assessment models for both groundwater and subsurface soils.