Efficient adsorptive removal of perfluorooctanoic acid by large surface area biochar modified with KHCO₃

Biochar is a promising adsorbent for remediating perfluorooctanoic acid (PFOA) in contaminated water. However, the performance of pristine biochar is limited. Considering pore filling is a crucial mechanism for PFOA adsorption on biochar, this study investigated the impact of KHCO3 modification on the pore structure of biochar and its adsorption capacity for PFOA. The characterization and experiment results revealed that both the specific surface area (SSA) and adsorption capacity of biochars were positively correlated with the pyrolysis temperature, and modified corn stalks biochar prepared at 800 °C (CBC-800) exhibited a noticeable SSA (1,471.6 m2/g) and adsorption capacity (514.8 mg/g), which were 3.6 and 37.1 times higher, respectively, compared to pristine corn stalk biochar (404.1 m2/g and 13.9 mg/g). The adsorption kinetics and the isotherm data followed pseudo-second-order kinetics and the Freundlich model, respectively, indicating chemisorption was the main factor limiting the adsorption rate. Thermodynamics demonstrated the adsorption process was physical, spontaneous, and exothermic. The porous biochars performed superior adsorption capacities under various environmental conditions (pH, inorganic salts, etc. ). The removal rate of CBC-800 for low concentrations of PFOA (10-1,000 μg/g) ranged from 87.4% to 99.6%, and this rate was positively correlated with the initial concentrations. Additionally, CBC-800 effectively removed PFOA (40 μg/L) through six consecutive adsorption cycles (93.4%-94.5%). Mechanism analysis indicated dominant pore filling was greatly enhanced, while hydrogen bonding, electrostatic, and hydrophobic interactions were also involved. Our study demonstrated that biochar derived from low-cost agricultural and forestry residues combining KHCO3 modification has great potential for the adsorptive removal of emerging PFAS in contaminated water.