Non-monotonic breakthrough and competitive adsorption of perfluorooctanoic acid and perfluorooctane sulfonate across flow-rate-controlled empty bed contact times in Ca-modified carbon fixed-bed columns

초록

Empty bed contact time (EBCT) is widely used as a design parameter in fixed-bed adsorption systems, yet its influence on per- and polyfluoroalkyl substances (PFAS) breakthrough under competitive flow conditions remains insufficiently understood. This study investigated the breakthrough behavior of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) in columns packed with Ca-modified mandarin peel-derived carbon material under single- and binary-solute conditions. Four EBCT conditions (2.5, 5, 10, and 20 min) were tested by adjusting the influent flow rate while maintaining a constant bed configuration. The bed-volume (BV)-normalized breakthrough behavior was non-monotonic: increasing EBCT from 2.5 to 5 min increased BV0.5 and BV0.9 by 41.5–57.4% for PFOA and 50.8–59.7% for PFOS, whereas further increases to 10- and 20-min reduced BV-normalized performance. At 5-min EBCT, the Thomas model-derived capacities were 0.278 mg g−1 for PFOA and 0.320 mg g−1 for PFOS under single-solute conditions. Under binary conditions, these values decreased to 0.107 and 0.158 mg g−1, respectively, consistent with competitive adsorption and the preferential suppression of PFOA observed in the breakthrough curves. Mass transfer zone (MTZ) analysis and apparent parameters from the Yoon–Nelson, Thomas, and Bohart–Adams models enabled comparative evaluation of EBCT-dependent breakthrough responses. X-ray photoelectron spectroscopy and Brunauer–Emmett–Teller analyses of selected PFAS-loaded samples provided supporting evidence for Ca-associated surface interactions and internal pore occupation. Overall, flow-rate-controlled EBCT variation produced system-specific non-monotonic breakthrough behavior through combined effects of residence time, hydrodynamic conditions, mass transfer, and competitive adsorption.

키워드