In situ remediation of per- and polyfluoroalkyl substances by colloidal activated carbon in groundwater and vadose zone

Per- and polyfluoroalkyl substances (PFAS) have reached global occurrence in the environment, including groundwater and vadose zone. Therefore, developing technologies to remediate PFAS in groundwater and vadose zone is urgent, especially for in situ remediation. Colloidal activated carbon (CAC) has emerged as a cost-effective adsorbent that shows potential to in situ immobilize PFAS in groundwater and vadose zone. This review provides state-of-the-art assessment of CAC-enabled technologies to in situ remediate PFAS-impacted groundwater and vadose zone, focusing on the technology feasibility, effectiveness, efficiency, longevity, and environmental factors affecting technology performance. The adsorption mechanisms of CAC for various types of long-chain and short-chain PFAS are discussed to evaluate technology effectiveness and efficiency. Significant efforts are devoted to assessing and predicting the longevity of CAC-based barriers for treating PFAS plumes in groundwater by mathematical modeling and field observations. Key factors on how nonuniform distribution, plume migration, and potential loss of CAC affect the barrier continuity and PFAS breakthrough are elucidated. The unique processes in the vadose zone, such as dynamic wetting-drying cycles, air-water interfacial partitioning, and oxic-anoxic transition zones are highlighted, since they may significantly affect CAC adsorption and migration capacities, underscoring the urgent needs for systematic studies and long-term monitoring efforts. Future research frontiers and challenges on CAC-enabled technologies for in situ remediation of PFAS-impacted groundwater and vadose zone are identified, contributing to the global efforts in addressing PFAS pollution.