A novel biochar adsorbent for treatment of perfluorooctanoic acid (PFOA) contaminated water: Exploring batch and dynamic adsorption behavior

Perfluoroalkyl substances (PFAS), like perfluorooctanoic acid (PFOA), are of concern worldwide given they are ubiquitous in the environment. In this study, the treatment of PFOA-contaminated water was assessed using biochar adsorbents produced from raw canola straw (RCS) through chemical activation with H₃PO₄ and ZnCl₂ and microwave-assisted pyrolysis (MWP). MWP conditions were evaluated to create optimal H₃PO₄-treated (PBC) and ZnCl₂-treated (ZnBC) biochar adsorbents with treatments determined using a central composite design (CCD) based on the response surface methodology (RSM) considering activator concentration, and microwave heating time and power. The highest PFOA removal efficiency for PBC (3.0 mol/L) was achieved at 92 % (368 μg/g), while for ZnBC (0.55 mol/L) it was 84 % (336 μg/g). In contrast, untreated biochar and RCS had markedly lower PFOA removals of 5 % and 1 %, respectively. Activation of biochar under optimal pyrolysis conditions (6 min at 600 W) led to increased chemical functional groups, porosity, and surface area, as confirmed by FT-IR, XPS, and BET. The kinetic study indicated that chemisorption was the primary PFOA adsorption mechanism, while the Freundlich isotherm model suggested heterogeneous multilayer adsorption for PFOA removal. Further, background salts enhanced PFOA adsorption through divalent bridges and salting-out mechanisms. PBC and ZnBC adsorbents performed well over a broad pH range of 3 to 9. Lastly, Yan and Yoon-Nelson models were used to assess adsorption breakthrough for a model fixed-bed adsorption system. This study exhibits that PBC and ZnBC adsorbents, derived from accessible biomass, offer an environmentally friendly solution to remove PFOA from contaminated water.