Hierarchical porous biochar with Fe/Zn co-activation derived from orange waste: enhanced methylene blue adsorption and mechanistic insights

This study presents an innovative and eco-friendly method to convert citrus processing waste into advanced biochar adsorbents via a tailored ZnCl2/FeCl3 co-modification strategy. This dual-activation strategy simultaneously generates hierarchical porous architectures (a 16.1-fold increase in specific surface area and a 5.7-fold rise in total pore volume compared to the pristine biochar), and integrates active surface sites (through FeCl3 modification), thereby leading to substantial structural enhancements. Comprehensive analysis confirmed the formation of iron oxide species alongside preserved oxygen-containing functional groups, collectively generating multifunctional active sites. The optimized Fe/Zn-OPBC500, which balances tailored surface chemistry and hierarchical porous structure, exhibits exceptional methylene blue adsorption, with a capacity of 237.53 mg g−1, and a 96.8% removal rate within 60 min. The adsorption process predominantly adhered to the Langmuir isotherm model, and followed pseudo-second-order kinetic behavior, with thermodynamics confirming spontaneous chemisorption. This high efficiency is governed by multiple synergistic mechanisms, including surface complexation with iron sites, strong electrostatic and hydrogen-bonding interactions, π–π stacking, and pore confinement. Remarkably, the Fe/Zn-OPBC500 also demonstrates excellent practicality, retaining 113.24 mg g−1 over seven regeneration cycles, and maintaining performance across a broad pH range and in the presence of common ions. This study presents a practical approach for citrus waste valorization and provides fundamental insights into the multi-mechanistic adsorption processes, thereby guiding the rational design of advanced carbon-based adsorbents.