Cow dung waste as an eco-friendly adsorbent for Alizarin Red S dye removal

Abstract

In this study, cow dung waste (CDW), acid-treated cow dung waste (ATCDW), and base-treated cow dung waste (BTCDW) were evaluated for the removal of Alizarin Red S (ARS) from aqueous solutions using UV-Vis spectroscopy. The adsorbents were characterized using Fourier Transform Infrared Spectroscopy (FT-IR), Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDX), and X-ray Diffraction (XRD) to understand their structural and surface properties. The adsorption optimal conditions for the adsorption of ARS were obtained as ARS concentration of 300 mg/L, pH of 2, temperature of 45 °C, and contact time of 100 min for CDW and ATCDW, 80 min for BTCDW respectively. The adsorption isotherms indicated that CDW and ATCDW followed the Freundlich model, suggesting a heterogeneous adsorption process, while the adsorption of ARS by BTCDW adhered to the Langmuir model, indicative of monolayer adsorption. The maximum adsorption capacities, calculated from the Langmuir model, were 58.24 mg/g for CDW, 87.54 mg/g for ATCDW, and 61.37 mg/g for BTCDW at 45 °C. Kinetic studies revealed that the adsorption of ARS onto CDW and ATCDW followed the pseudo-second-order model, while BTCDW adsorption was best described by the pseudo-first-order model. Thermodynamic analysis showed that the adsorption process was endothermic, with enthalpy values of 48.12 kJ/mol for CDW, 95.22 kJ/mol for ATCDW, and 7.55 kJ/mol for BTCDW. FT-IR analysis confirmed the presence of functional groups such as O-H, C=O, and C-O, which are associated with cellulose, hemicellulose, and lignin, facilitating dye interactions through electrostatic forces. These results suggest that cow dung waste, especially when treated, is a promising, cost-effective, and eco-friendly adsorbent for the removal of organic contaminants like ARS from wastewater.