Green conversion of artemisia residues into activated carbon for efficient adsorptive removal of toxic dyes from wastewater: Experimental and DFT insights

This study presents a sustainable strategy for converting post-essential-oil extraction Artemisia atlantica residues into phosphoric acid-activated carbon (ACAA) for the removal of cationic dyes from wastewater. This underutilized agro-industrial residue is a low-cost, renewable, lignocellulosic, and carbon-rich precursor whose valorization supports waste minimization and circular bioeconomy principles. The study also integrates Box–Behnken Design (BBD) for process optimization with Density Functional Theory (DFT) calculations to clarify the adsorption mechanism. Chemical activation increased the BET surface area from 25.87 to 98.45 m² g⁻¹ and produced a porous structure rich in oxygen-containing functional groups, as confirmed by FTIR, SEM, and TGA analyses. BBD identified pH, adsorbent dosage, and contact time as the most influential factors. The quadratic model accurately predicted optimum conditions of pH 9, 0.20 g adsorbent, and 200 min, achieving removal efficiencies above 95%. The maximum Langmuir adsorption capacities were 170.36 mg g⁻¹ for crystal violet (CV) and 153.14 mg g⁻¹ for methylene blue (MB), compared with 32.85 and 61.88 mg g⁻¹, respectively, for the raw precursor. Adsorption followed the pseudo-second-order model (R² > 0.96) and the Langmuir isotherm (R² > 0.99). Thermodynamic analysis confirmed spontaneous and exothermic adsorption, with ΔH° values of −23.38 kJ mol⁻¹ for CV and −9.91 kJ mol⁻¹ for MB. DFT calculations showed that the ACAA@MB complex had the lowest HOMO–LUMO energy gap (1.12 eV), indicating the strongest electronic interaction and adsorption affinity. Electrostatic interactions and charge transfer were identified as the dominant mechanisms. ACAA also maintained high efficiency over repeated adsorption–desorption cycles, resisted common competing inorganic ions, and successfully treated real textile wastewater, substantially reducing dye concentration, COD, BOD₅, turbidity, and electrical conductivity. These results demonstrate the strong potential of ACAA as a sustainable and robust adsorbent for advanced wastewater treatment.