The impact of biochar's physicochemical properties on sorption of perfluorooctanoic acid (PFOA)

To better characterize properties governing the sorption of per- and polyfluoroalkyl substances (PFAS) to biochar, twenty-three diverse biochars were characterized and evaluated as sorbents for perfluorooctanoic acid (PFOA). Biochars were produced at various temperatures, using two different technologies, and made from sewage sludge, food waste reject, wood wastes, and one reference substrate (wood pellets). The biochars were characterized in terms of surface area, pore volume and pore size distributions, elemental composition, leachable elements, ash content, pH, zeta potential, condensed aromatic carbon (ConAC) content (determined by benzenepolycarboxylic acid (BPCA) markers), and their -OH functional group content (infrared spectroscopy). PFOA sorption isotherms were determined using Polanyi-Dubinin-Manes (PDM) and Freundlich models. The sludge-based biochars [Freundlich coefficients (log K_F) between 2.56 ± 0.11 and 6.72 ± 0.22 (μg/kg)/(μg/L)^nF; fitted free energy of adsorption (E) and pore volume (V_o) from the PDM model between 13.27 and 17.26 kJ/mol, and 0.50 and 523.51 cm³/kg] outperformed wood biochars [log K_F between 1.02 and 4.56 ± 0.22 (μg/kg)/(μg/L)^nF; E between 9.87 and 17.44 kJ/mol; V_o between 0.21 and 7.16 cm³/kg] as PFOA sorbents. Multivariate statistical analysis revealed that the sorption capacity was mainly controlled by pore volume within the pore diameter region that could accommodate the molecular size of PFOA (3-6 nm). Hydrophobic interactions between PFOA and aromatic carbon rich regions controlled sorption affinity, especially in the wood biochars.