Magnetic activated date-pit biochar for efficient crystal violet removal from water: adsorption performance and economic analysis

Magnetic activated biochar (MAB) was synthesized from waste Deglet Nour date pits, an abundant palm byproduct from Algeria, through ZnCl2 chemical activation followed by Fe3O4 nanoparticle loading via co-precipitation, and applied for crystal violet (CV) removal from water. Comprehensive characterization by SEM–EDX, AFM, TEM, FTIR, TGA, XRD, VSM, and N2 physisorption verified the successful activation and magnetization of biochar. TEM images of MAB confirmed that Fe3O4/ZnFe2O4 nanoparticles (11–47 ± 0.1 nm) are well-dispersed on the micron-sized biochar support. MAB exhibited an enhanced BET surface area of 870.6 (± 0.01) m2.g−1 and a large pore volume of 0.514 (± 0.001) cm3.g−1, exceeding those of the pristine date-pit biochar (DPB; 143.2 ± 0.01 m2.g−1 and 0.0662 ± 0.001 cm3.g−1) by more than sixfold. Adsorption kinetics indicated a rapid uptake with 57% removal within 5 min under single-component conditions and up to 86% in the binary system with methylene blue (cooperative removal). The Langmuir maximum adsorption capacity of MAB reached 537.8 ± 10.18 mg.g−1, whereas DPB showed 63.0 ± 4.094 mg.g−1 (at 45 °C ± 1 °C). Thermodynamic studies indicated a spontaneous, exothermic, and entropy-driven behavior. Ethanol CV-desorption confirmed the reversibility of the process and the recyclability of MAB. Adsorption was controlled by π–π interactions, pore filling, hydrogen bonding, n–π interactions, surface complexation, and van der Waals forces. The production cost of MAB was estimated at 2.16–4.86 US$.kg−1, while the operational cost of CV removal was only 0.004–0.014 US$.g−1 of dye, demonstrating technical efficiency, economic feasibility, and sustainable valorization of date pit waste.