Ziziphus spina-christi seed-based activated carbon for efficient methylene blue adsorption: Mechanistic studies

The discharge of dye-containing wastewater into aquatic environments poses serious environmental concerns, creating a need for efficient and sustainable treatment methods. In this study, activated carbon prepared from Ziziphus spina-christi seeds (ZSAC) was synthesized through chemical activation and evaluated for methylene blue (MB) adsorption from aqueous solutions. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) analyses confirmed the formation of a porous carbonaceous adsorbent with suitable surface characteristics for dye removal. Batch adsorption experiments showed that MB removal was influenced by solution pH, adsorbent dosage, contact time, initial dye concentration, agitation speed, and temperature. Maximum removal efficiency (98.7%) was achieved at pH 8, 15 mg adsorbent dosage, 20 min contact time, 140 rpm agitation speed, and 25 °C. Equilibrium data were best described by the Langmuir isotherm model (R² = 0.998), indicating monolayer adsorption with a maximum adsorption capacity of 81.3 mg g⁻¹ . Kinetic studies revealed that adsorption followed the pseudo-second-order model (R² = 0.998). Thermodynamic parameters confirmed that the adsorption process was spontaneous (ΔG° < 0) and exothermic (ΔH° = −43.52 kJ mol⁻¹). Furthermore, ZSAC retained 93.9% of its adsorption efficiency after four regeneration cycles, demonstrating its potential as a low-cost and sustainable adsorbent for wastewater treatment.