Kinetics and Thermodynamics of Per - and Polyfluoroalkyl Substances (PFASs) Adsorption by Activated Carbon in Aqueous Systems

Per- and polyfluoroalkyl substances (PFASs) are persistent environmental contaminants posing significant challenges in water treatment due to their stability and resistance to conventional removal methods. To enhance removal efficiency in treatment facilities, this study investigates the adsorption mechanisms of PFAS on activated carbon, focusing on the influence of the C-F chain length and functional groups through kinetic and thermodynamic analyses. The adsorption kinetics followed a pseudo-second order model, with adsorption rates increasing with longer C-F chains. This trend is likely driven by increased hydrophobicity, which enhances interaction with the hydrophobic domains of activated carbon. In comparison, PFAS with SO₃⁻ functional groups showed faster adsorption rates than those with COO⁻ groups, attributed to their larger molecular size and higher hydrophobicity, leading to steric hindrance. In competitive adsorption scenarios, long-chain PFASs rapidly occupied adsorption sites, demonstrating a higher competitive potential compared to short-chain PFASs. The adsorption capacity decreased with increasing temperature, likely due to enhanced solubility of PFAS, which reduced their affinity for the activated carbon surface. Thermodynamic analyses indicated that the adsorption process was exothermic (ΔH<0) and more effective for shorter C-F chains, likely due to increased hydrophilic interactions. Positive changes in entropy (ΔS>0) suggested that adsorption disrupted the structure of surrounding water at the carbon interface, while Gibbs free energy values (ΔG, -16 to -11 kJ mol-1) confirmed that the mechanisms involved were primarily physical, characterized by hydrogen bonding and hydrophobic interactions. These insights contribute to understanding PFAS adsorption on activated carbon, offering critical data for improving PFAS removal in water treatment.