Modification of biochar by iron containing adsorption centers as a method to enhance the remediation of perfluorooctanoic (PFOA) and (PFOS) acids from water and soil: a density functional theory study

A doubled crystallographic unit of hematite (Fe₂₄O₃₆) was constructed and fully optimized using density functional theory (DFT) with the M06-2X functional. Geometry optimization used the 6-31G(d,p) basis set, while single-point energies were calculated with 6-311 + + G(d,p). Antiferromagnetic conditions were ensured by setting the total spin to zero (Sz = 0), and triplet instability analysis was performed to evaluate ferromagnetic potential. To simulate bulk water effects on adsorption, the CPCM solvation model (ε = 78.3) was applied. Harmonic frequency analysis confirmed structural minima, and Gibbs free energies were calculated using Gaussian 16. PFOA and PFOS, with highly negative pKa values (~ -0.1 and <). Quadratic SCF convergence (scf = qc) addressed numerical challenges, and interaction energies were corrected for basis set superposition error using the counterpoise method. Calculated IR spectra and molecular visualizations were generated with Chemcraft, without applying scaling factors.