Synthesis and characterization of Zn and Fe doped magnetic biochar from Acacia falcata leaves for Cr(VI) adsorption

Abstract Hexavalent chromium (Cr(VI)), a toxic pollutant extensively utilized across multiple industries, necessitates effective treatment using low-cost and sustainable materials. In this research, magnetic biochar (MBC) was prepared using Acacia falcata leaves through chemical treatment with ZnCl 2 and incorporating Fe 3 O 4 nanoparticles using FeCl 3 ·6H 2 O as a precursor. BET analysis revealed a specific surface area of 248.11 m 2 /g and FESEM images showed a highly porous structure with uniformly embedded Fe 3 O 4 nanoparticles, which became smoother and more compact after Cr(VI) removal. XRD spectra confirmed the incorporation of Fe 3 O 4 through distinct (311) and (440) peaks, matching the cubic spinel structure, while VSM data revealed a magnetic saturation of 5.44 emu/g. XPS analysis indicated the participation of carboxyl, hydroxyl, carbonyl, and Fe 3 O 4 groups in Cr(VI) reduction and adsorption. Batch experiments identified an optimum pH of 2, a MBC dose of 0.4 g/L, and a contact time of 3 h. The adsorption data followed the Freundlich isotherm and pseudo-second-order kinetics. The maximum adsorption capacity was 25.62 mg/g at 303 K, and thermodynamic studies confirmed that Cr(VI) removal was spontaneous and endothermic. The enthalpy and entropy values for Cr(VI) adsorption were 17.71 kJ/mol and 63.22 J/mol·K, respectively. Reusability studies, conducted at the optimum pH of 2 and a MBC dose of 1.4 g/L, demonstrated that MBC could be reused for up to five cycles. MBC effectively removed over 97.80% of Cr(VI) from various water sources, highlighting its potential for Cr(VI) remediation.