Biological Aging as an Overlooked Factor Controlling the Effectiveness of Colloidal Activated Carbon for PFAS Removal in Subsurface Environments

Abstract Colloidal activated carbon (CAC) is effective for removing per- and polyfluoroalkyl substances (PFAS) and has been used for in situ remediation. However, under subsurface conditions, biological aging may impact the performance of CAC sorption barriers. In this study, a column system was designed to accelerate interactions between CAC and microorganisms, enabling evaluation of biological aging effects on PFAS adsorption and CAC properties within a practical time frame. Aging was confirmed by indicators of microbial growth and activity (e.g., SEM, DNA, and ATP) relative to a control with NaN3. After four months of aging, CAC particles were recovered for PFAS isotherm and kinetic analyses. Results showed a significant reduction in adsorption capacity, particularly for shorter-chain and more hydrophilic perfluoroalkylcarboxylic acids (PFCAs). Characterization of fresh and biologically aged CAC revealed decreased surface area, micropore volume, and carbon-to-oxygen (C/O) ratio, along with increased particle diameter, point of zero charge (PZC), and anion exchange capacity (AEC). A washing procedure partially restored these properties and the adsorption capacity. Overall, biological aging is an often-overlooked factor that negatively affects PFAS removal by CAC through both reversible and irreversible changes associated with biofilm attachment and surface alteration. Incorporating biological aging into modeling frameworks will improve predictions of long-term CAC performance in the field.