Waste-to-Resource Conversion of Cow Dung Ash for Sustainable Wastewater Treatment: Isotherm Modeling and MOORA Evaluation

Low-cost adsorbents derived from agricultural and livestock waste offer a sustainable approach to wastewater remediation. In this study, a material derived from cow dung ash (SMCDA) was prepared through a simple waste-valorization route without chemical activation and evaluated for the removal of methylene blue (MB) from aqueous solution. X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, Brunauer–Emmett–Teller analysis, and zeta-potential measurements were used to characterize the mineral composition, surface functional groups, morphology, pore structure, and surface charge of SMCDA. Batch adsorption experiments examined the effects of solution pH, adsorbent dosage, initial MB concentration, contact time, and temperature. The highest removal efficiency, 97.33%, was obtained at pH 8 with an SMCDA dosage of 1000 mg L−1. Equilibrium data were fitted using seven isotherm models and evaluated using multiple statistical criteria and Multi-Objective Optimization by Ratio Analysis (MOORA). The Freundlich model achieved the highest MOORA ranking and predicted an equilibrium adsorption capacity of 13.944 mg g−1 at the highest concentration investigated, which is close to the experimental value of 14.425 mg g−1. The results are consistent with heterogeneous adsorption involving electrostatic attraction, possible π–π interactions, hydrogen bonding, and pore filling. These findings demonstrate the potential of cow dung ash as a low-cost adsorbent prepared without hazardous chemical activating agents.