Mesoporous phosphorus-doped activated carbon from Acacia falcata: Mechanistic insights into Cr (VI) removal, regeneration, and spiking studies

The escalating threat of hexavalent chromium (Cr(VI)) contamination in wastewater has rendered the development of high-performance carbonaceous materials from biomass a critical research imperative. The need for sustainable, efficient, and cost-effective solutions to mitigate Cr(VI) pollution has spurred interest in exploring biomass-derived carbonaceous materials as potential adsorbents. This study presents the preparation of phosphorus-doped activated carbon from Acacia falcata leaves via low-temperature activation with orthophosphoric acid. The resulting activated carbon exhibits a mesoporous structure, with a specific surface area of 406.65 m²/g. Characterization via FESEM, EDS, BET and XPS analyses revealed a mesoporous structure with an irregular surface, micropores, micro-cracks, and micro-pits, with an average pore size of 4.03 nm. Post-Cr(VI) adsorption analysis indicated a smoother surface, likely due to pore filling and chromium oxide formation, with EDS confirming chromium presence (peaks at 0.57 keV and 5.41 keV). XPS deconvolution identified Cr(III) (63.18 %) and Cr(VI) (36.82 %) species, highlighting the material's dual function in Cr(VI) adsorption and reduction. Batch experiments followed pseudo-second-order kinetics and the Freundlich model, achieving a maximum adsorption capacity of 60.14 mg/g at optimum conditions of pH 2, 150 rpm, 0.2 g/L adsorbent dose and 303 K. Thermodynamic analysis indicated a positive ΔH° (33.69 kJ/mol) and ΔS° (132.61 J/mol K), alongside a negative ΔG°, confirming the adsorption's spontaneous, endothermic nature with increased randomness. The presence of cations had a negligible effect on removing Cr(VI), but anions, especially nitrate and phosphate, slightly reduced adsorption efficiency. However, the adsorbent proved highly resilient, maintaining over 77.05 % Cr(VI) removal efficiency after four regeneration cycles. Moreover, it demonstrated exceptional performance in treating water from diverse sources, including river, lake, and well, highlighting its promise as a cost-effective and eco-friendly wastewater treatment solution.