Effects of Calcium and Natural Organic Matter on the Transport and Remobilization of Colloidal Activated Carbon in Saturated Porous Media: Insights from Force Spectroscopy

Colloidal activated carbon (CAC) is a promising technology for the in situ remediation of groundwater impacted by perfluoroalkyl and polyfluoroalkyl substances (PFAS). The long-term performance of an engineered CAC barrier will depend, in part, on the emplacement and remobilization of CAC particles within aquifer media. We here explored the influence of calcium ions (Ca2+) and Suwanee River natural organic matter (SRNOM) on CAC deposition and remobilization within saturated sand columns. Our results showed that the presence of Ca2+ (e.g., >5 mM) under high ionic strength conditions (100 mM) enhanced CAC deposition and subsequently reduced its remobilization upon the introduction of a low ionic strength solution (i.e., DI water). A combination of cation bridging and electrostatic screening, driven by Ca2+, contributed to the increased retention of CAC in the sand column. In contrast, when SRNOM was present at concentrations above 5 mg/L, CAC exhibited reduced deposition under high ionic strength conditions (100 mM), followed by markedly enhanced remobilization upon flushing with a low ionic strength solution. This behavior is primarily driven by increased electrosteric repulsion at the CAC–sand interface when the sand surfaces are coated by NOM. Atomic force microscopy (AFM) force measurements showed that under the same ionic strength, Ca2+ increased the work of adhesion between CAC and silica surfaces, whereas NOM decreased it. Our work underscores the critical influence of both the presence and concentration of Ca2+ and NOM on the deposition and remobilization behaviors of CAC, providing valuable insights into the engineering design and practical implementation of in situ CAC sorptive barriers for effective PFAS remediation.