The Contrasting Roles of pH, Na+, and Ca2+ on Transport and Deposition of Colloidal Activated Carbon in Single and Mixed Na+-Ca2+ Systems in Saturated Porous Media

Abstract Colloidal activated carbon (CAC) barriers have been widely adopted for in situ groundwater remediation of legacy and emerging contaminants. The design and installation of CAC barriers require an understanding of CAC transport and deposition as a function of site geochemistry, particularly for coastal sites where spatial variations in salinities are high and CAC placement with or without in situ flocculation by CaCl2 may be complex. Our column study has explored this complexity over a wide range of salinities (0.1–0.7 M) and relevant CaCl2 concentrations (0.0125–0.23 M) by considering individual and combined effects of Na+ and Ca2+ at pH 3 and 7. For the first time, retention profiles (RPs) and SEM-EDX images of CAC retained on sand are presented alongside breakthrough curves (BTCs) to elucidate CAC transport and retention behaviors. The presence of Ca2+ increased CAC retention in sand, likely through cation bridging at pH 7 and electrostatic interactions at pH 3. For Na+, CAC retention increased significantly at pH 3 only, owing to Na+-driven double-layer compression. In Na+-Ca2+ systems, CAC retention decreased linearly with increasing Na+:Ca2+ ratio with an inflection point at 0.83, below which CAC retention was driven by Ca2+ interaction with pH and above it by Na+ interaction with pH. This highlighted the Na+:Ca2+ ratio as a viable design parameter to control CAC retention in saline aquifers. In single Ca2+ and mixed Na+-Ca2+ systems, increasing Ca2+ concentration also resulted in transitions of BTCs from blocking to ripening, indicating a change in CAC retention mechanisms. However, the corresponding transitions of RPs from linear to hyperexponential suggested that the change in retention mechanisms was primarily occurring close to the column inlet where the majority of CAC was retained. TEM and SEM-EDX confirmed that CAC retention was largely driven by CAC aggregation and sand-surface irregularities. These outcomes provide necessary foundational insights for CAC barrier design and placement in saline and hypersaline groundwater with or without in situ CaCl2 flocculation.