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.