Coconut-shell biochar fixed-bed for groundwater remediation: Co-removal of microplastics and perfluorooctanoic acid

Microplastics and PFOA are emerging contaminants that are widely present in groundwater, posing threats to human health and ecosystems. This study aimed to investigate the remediation of polystyrene microplastics (PS-MPs) and perfluorooctanoic acid (PFOA) in groundwater using waste coconut shell biochar in permeable reactive barriers (PRBs). Fixed-bed column experiments were conducted with different bed depths, flow rates, initial pollutant concentrations, and co-contamination conditions. Results demonstrated that coconut shell biochar exhibited an effective adsorption capacity, with an initial concentration of 1 mg/L, flow rate of 0.5 mL/min, and bed depth of 2.5 mm, removal efficiencies of 75.00 % for PFOA and 36.24 % for PS-MPs were achieved within 10 days. Increasing the depth of biochar from 1.5 mm to 5.0 mm enhanced the removal efficiencies (PFOA:96.43 %, PS-MPs:61.72 %) and extended breakthrough times (480 min for PS-MPs, 2410 min for PFOA). Lower flow rates (0.2 mL/min) extended hydraulic retention time, improving adsorption (both PFOA and PS-MPs were > 75 %), while higher pollutant concentrations accelerate PRB depletion and reduce removal efficiency. Additionally, the presence of PS-MPs exhibited dual effects on PFOA remediation: promoting adsorption at low concentrations but causing competitive inhibition at high concentrations. Meanwhile, increasing PFOA concentrations intensified electrostatic repulsion, which reduced the removal efficiency of PS-MPs from 36.24 % to 10.43 % as the PFOA concentration increased from 0 to 10 mg/L. Furthermore, among the tested models (Thomas, Yoon-Nelson and BDST), the Thomas model best fitted the fixed-bed adsorption data for PS-MPs, PFOA, and composite contamination.