Exploring Es-sifa clay as a high-performance adsorbent: experimental and density functional theory investigations

• Effective utilization of Es-Sifa clay as an adsorbent for removing methylene blue (MB). • Achieved an MB adsorption efficiency of 87%. • Comprehensive evaluation of kinetic and isotherm models through statistical error analysis. • Density Functional Theory (DFT) calculations revealed the chemical reactivity of MB dye. The present work explores the effectiveness of Es-Sifa clay, located in Morocco, especially in the Draa-Tafilalet region, as an adsorbent. The study focused on methylene blue (MB) as the model contaminant for evaluating adsorption efficiency. The clay was characterized using scanning electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, pH at the point of zero charge, and cation exchange capacity measurements. Batch adsorption experiments were conducted under varying conditions, including contact time (5-120 min), stirring speed (100- 400 rpm), initial dye concentration (10-100 mg/L), adsorbent dose (0.25-2.5 g/L), and temperature (25-60°C), to evaluate and optimize removal efficiency. Results showed that MB removal increased with contact time, adsorbent dose, and agitation speed, but decreased with rising temperature and dye concentration. The adsorption kinetics followed the pseudo-second-order model, and the isotherm data best fit the Langmuir model, with a maximum adsorption capacity of 49.41 mg/g. Thermodynamic parameters confirmed that the process is spontaneous and exothermic. DFT calculations were also performed to provide insight into the adsorption mechanism. MB was found to exhibit a narrow HOMO–LUMO energy gap, low chemical hardness, and high electrophilicity, reflecting strong reactivity and surface affinity. Mulliken charge analysis identified electron-rich heteroatoms and aromatic rings as key interaction sites, with adsorption driven by electrostatic forces, hydrogen bonding, and π-back-donation. These theoretical results are consistent with experimental findings, confirming the strong affinity of MB for Es-Sifa clay. This study underscores the potential of Es-Sifa clay as a sustainable, low-cost material for eco-friendly wastewater treatment, supporting Morocco’s efforts in waste valorization.