Perfluoroalkyl chemical adsorption by granular activated carbon: Assessment of particle size impact on equilibrium parameters and associated rapid small-scale column test scaling assumptions.

Abstract

Single-solute batch kinetic and isotherm experiments were conducted in Type 1 (18.2 MΩ·cm resistivity) water supplemented with 10 mM carbonate buffer (pH 7.75, 25 °C) for nine drinking water relevant perfluoroalkyl chemicals and three bituminous-coal based granular activated carbons (GACs). Except for perfluorooctane sulfonic acid (PFOS), mass transfer was well represented by a film diffusion model, which estimated a film diffusion coefficient (k_L). For PFOS, a batch pore and surface diffusion model better represented the data and allowed estimation of both k_L and a surface diffusion coefficient (D_s). Adsorption was well described by the Freundlich isotherm, and for a given perfluoroalkyl chemical at equilibrium, the three GACs showed similar solid phase density (q_e) for a given liquid concentration (c_e). For each GAC and at c_e ≤ 1000 ng/L, log q_e increased linearly with perfluoroalkyl chemical carbon chain length for both perfluoroalkyl carboxylic acids (PFCAs) and perfluoroalkane sulfonic acids (PFSAs). GenX, a perfluoroalkyl ether carboxylic acid containing six carbon atoms, showed similar adsorption with the PFCA perfluorohexanoic acid. The impact of particle size (d_P) on kinetic and isotherm parameters was investigated: (i) for PFOS, D_s increased with increasing d_P, with some overlap in confidence intervals; (ii) for PFSAs, Freundlich K_f and 1/n showed decreasing and increasing trends, respectively, with increasing d_P. Potential implications of the d_P impact were explored with PFOS breakthrough simulations under different rapid small-scale column test conditions where carbon usage rate estimations varied by -49 % to 6 % from the baseline simulation, depending on the d_P and scaling approach.