Evaluating the effect of DOC on PFAS sorption to soil and colloidal activated carbon using column tests

Remediation of sites contaminated with per- and polyfluoroalkyl substances (PFASs) is a significant step towards protecting drinking water sources and limiting human exposure. PFAS production and use is increasingly being restricted worldwide with a reduction of point sources; however, legacy plumes are still posing a threat due to the persistence of these chemicals against degradation. One of the most widely applied soil remediation techniques for PFASs is stabilisation, aiming to long-term entrapment of the contaminants with the addition of fixation agents in the subsurface, which prevent their leaching from soil to groundwater. In relation to this, the aim of this study was to identify the leaching behaviour of various PFASs in a treatment scenario using colloidal activated carbon (CAC). The effect of dissolved organic carbon (DOC), one of the major groundwater constituents in Sweden, on sorption was evaluated. Silt loam soil sampled from central Sweden was used, as well as a mixture of the soil with CAC at 0.1% w/w. Spiked artificial groundwater was prepared with a mixture of 21 PFASs, at a total concentration of 1.4 μg mL^-1. The sorption of PFASs to the solid phase was investigated using 15 cm long column experiments under saturated conditions, with and without DOC at 10 mg L^-1. Non-reactive tracer tests with NaCl were used to evaluate hydrological parameters. The desorption behaviour after treatment was also investigated, by switching the inflow from contaminated to clean water after steady state was achieved. Analysis of the compounds was conducted using ultra performance liquid chromatography coupled with tandem mass spectrometry (UPLC-MS/MS). Preliminary results showed retardation of PFASs with the addition of CAC, primarily for long-chain PFASs, exhibiting a correlation between sorption strength capacity and perfluorocarbon chain length. Some short-chain compounds, like perfluorobutanoate (PFBA) and perfluoropentanoate (PFPeA), exhibited immediate breakthrough. Slightly higher retardation of long-chain PFASs was noticed in the presence of DOC, for both treated and reference soil, indicating a potential increase of adsorption sites. Conversely, short-chain PFASs appeared to be outcompeted by DOC and showed faster breakthrough in its presence. These results indicate that humus content can have variable effect on PFAS sorption to soil and CAC, depending on perfluorocarbon chain length. Further experiments aim to the quantification of the sorption capacity.