Colloidal properties and stability of colloidal activated carbon: effects of aqueous chemistry on sedimentation kinetics

Abstract

Recently, in situ injections of polymer-stabilized colloidal activated carbon (CAC) have shown successful immobilization of per/polyfluoroalkyl substances in groundwater. Performance of an in situ CAC barrier will depend on its subsurface distribution, governed partly by its colloidal properties/stability. Ours is the first study to provide key electrostatic properties of CAC and investigate the effects of aqueous chemistry on its ζ-potential, aggregation, and sedimentation kinetics. In this study, disparity between point of zero charge and isoelectric point of CAC suggests that protonation–deprotonation may not be its only surface charging mechanisms. The single and combined effects of pH, cations, and organic matter (bovine serum albumin (BSA), humic acid (HA)) observed in aggregation/sedimentation studies highlighted Ca²⁺ as a key factor in determining the CAC destabilization in aqueous environments. However, high Na⁺ concentrations reduced the effect of Ca²⁺, suggesting that high salinity environments might be favourable for CAC transport. Ca²⁺ showed ability to induce bridging of CAC particles through formation of chain-like CAC homo-aggregates and/or CAC-HA hetero-aggregates, seen in TEM images, indicating Ca²⁺-specific cation bridging as the main destabilizing mechanism for CAC. The sustained stability of CAC under aqueous conditions, where ζ-potential values (−30 to +30 mV) predicted aggregation/sedimentation, demonstrated that stabilization/destabilization mechanisms other than electrostatic forces were also present. Steric and/or Lewis acid base repulsion may be the main stabilizing forces, as indicated by reduction in CAC particle size prior to aggregation/sedimentation. Overall, this study highlights aqueous geochemical conditions that are critical for predicting subsurface CAC transport and spatial distribution.