Influence of oxygen-containing functional groups of activated carbon and structural branching of per- and...

Advancing the fundamental knowledge of interfacial interactions between per- and polyfluoroalkyl substances (PFAS) and granular activated carbon (GAC) is at the forefront of water treatment industry due to the stringent and progressively developing regulatory framework. Understanding molecular level interactions can have a crosscutting impact on water treatment in terms of selecting the suitable adsorbent and implementing better management practices for storage, regeneration, and disposal of GACs. In this study, commercially available bituminous coal-based GACs were systematically oxidized with nitric acid to progressively alter graphitic basal plane surface oxygen chemistry while minimally changing pore structure. Increasing the bulk oxygen content to 16 wt.% led to a 30-fold decrease in perfluorooctane sulfonic acid (PFOS) adsorption at the studied concentration range, which could be attributed to: (i) weakened hydrophobic interactions from loss of aromatic structure, (ii) increased electrostatic repulsion associated with greater surface acidity, and (iii) enhanced pore water that reduced PFOS accessibility. These experimental observations were supported by density functional theory (DFT) calculations. Integrated experimental and DFT studies showed that linear PFOS exhibits higher adsorption affinity than branched isomers regardless of surface oxidation, though this isomer selectivity is concentration-dependent and becomes prominent primarily at higher equilibrium concentrations. This discrepancy was attributed to steric constraints that hindered the planar alignment of nonpolar PFOS tails along the graphitic basal planes, while simultaneously restricting the transport of bulky branched isomers into micropores. Together, the results establish a mechanistic framework linking carbon surface chemistry, PFAS molecular structure, and competitive pore water interactions.