Effects of Physical and Chemical Aging of Colloidal Activated Carbon on the Adsorption of Per- and Polyfluoroalkyl Substances

Colloidal activated carbon (CAC) is an injectable adsorbent that sequesters per- and polyfluoroalkyl substances (PFAS) in the subsurface, serving as an in situ remediation technology for PFAS-impacted sites. However, the effectiveness of the CAC sorptive barrier could change over time due to alterations in its physicochemical properties induced by aging processes. In this study, the effects of CAC aging on surface properties of CAC and resulting impact on the adsorption behaviors of PFAS compounds were determined using four accelerated aging treatments, including wet–dry cycling (W/D), hydrogen peroxide (H), Fenton’s reagent (F), and mineral acid (A). Fenton’s reagent and mineral acid aging treatments showed a greater impact on the physical structure and chemical composition of CAC than either W/D-CAC and H-CAC. Aging the CAC lowered the CAC specific surface area and anion exchange capacity, increased surface oxygen content, and lowered the point of zero charge, suggesting negative impacts on the capacity of CAC for PFAS removal. This reduction in the sorption capacity was confirmed in batch sorption isotherm experiments. All aged CAC had lower PFAS adsorption in a multisolute system, including six PFAS compounds with different chain lengths (C4–C8) and functional groups (sulfonate or carboxylate), with the only exception being perfluorooctanesulfonic acid (PFOS). The results suggest that the aging process is an important, yet often overlooked, factor in determining the long-term effectiveness of the CAC sorptive barrier for PFAS removal, especially for shorter-chain hydrophilic PFAS. More research is needed to verify the influence of field-scale aging processes on the CAC performance and longevity.