Hierarchically porous Fe/Mg-modified biochar for simultaneous sequestration of PFAS and Cr(VI) in water: Experimental evaluation and DFT verification

Fe/Mg co-modified biochar enabled simultaneous PFAS adsorption and Cr(VI) reduction–immobilization. • Acid-washed Fe₄Mg₁-NBC showed enhanced removal, pore accessibility, and regenerability. • Experiments and DFT confirmed hierarchical pores, bimetallic sites, and interfacial synergy as key mechanisms. Simultaneous removal of per - and polyfluoroalkyl substances (PFAS) and hexavalent chromium (Cr(VI)) from water remains challenging because of their divergent adsorption, redox, and interfacial behaviors. Herein, Fe/Mg co-modified N-doped biochar (Fe₄Mg₁-NBC) was fabricated through a hydrothermal synthesis strategy followed by pyrolysis and acid washing for the synergistic remediation of PFAS/Cr(VI) co-contaminated water. The optimized Fe/Mg ratio regulated the pore architecture and surface chemistry, producing a hierarchically porous, partially graphitized carbon matrix enriched with oxygen-containing groups and highly dispersed Fe–O/Mg–O active sites. Fe₄Mg₁-NBC exhibited superior removal performance toward PFOA, PFOS, PFBA, PFBS, and Cr(VI) in both single-solute and binary systems, with favorable adsorption under weakly acidic conditions and moderate temperatures. Acid washing further improved performance by eliminating inactive residues, opening blocked pores, and exposing additional active sites, while more than 90% of the initial adsorption efficiency was retained after five regeneration cycles. XPS analysis revealed Fe 2+ /Fe 3+ transformation and Metal–F interactions, indicating that Cr(VI) removal proceeded through Fe-mediated reduction followed by Cr(III) immobilization, whereas PFAS uptake involved electrostatic attraction, coordination, hydrophobic interaction, and pore filling. DFT calculations further demonstrated that Fe/Mg co-doping markedly enhanced PFAS adsorption affinity, decreasing the adsorption energy from +0.43 eV on pristine biochar to −1.24 eV on Fe/Mg-NBC. Co-adsorption configurations also suggested that Fe and Mg sites provided complementary functions for Cr(VI) activation and PFAS stabilization in mixed systems. These findings establish Fe₄Mg₁-NBC as a reusable multifunctional adsorbent for efficient treatment of PFAS/Cr(VI)-contaminated water. Download: Download high-res image (167KB) Download: Download full-size image