Enhanced Removal of Polyfluoroalkyl Substances by Simple Modified Biochar: Adsorption Performance and Theoretical Calculation

To cope with the widespread polyfluoroalkyl substance (PFAS) pollution in the global aquatic environment, this study prepared an acid-modified biochar adsorbent derived from sludge with satisfactory performance. The theoretical maximum adsorption capacities of the adsorbent for perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) were determined to be 72.17 and 45.88 mg·g–1, respectively. The theoretical calculation of the adsorption of biochar surface functional groups with PFOS and PFOA was first performed in this study, and the mechanism of the adsorption process was inferred with a combination of experimental results and redundancy analysis. The adsorption mechanism is composed mainly of the following three aspects: (I) electrostatic interaction between the positively charged biochar and the deprotonated PFAS; (II) hydrogen bonding between anionic species of PFAS and functional groups; and (III) hydrophobic interaction between long-chain hydrophobic PFAS and biochar. Electrostatic interaction plays a major role in the three main adsorption mechanisms under acidic media. Meanwhile, different functional groups have different adsorption energies for PFAS, and C═O has the highest adsorption energy. The findings in this work could provide great insights into the adsorption of PFAS on modified biochar as an environmentally friendly adsorbent to promote resource reuse, improve water quality, and ensure human health.