Applicability of Gypsum for Enhanced Dye Removal: Structural Characterization, Equilibrium, Kinetics, and Adsorption Mechanism. Adsorption Mechanism.

Abstract

This study investigated the use of waste gypsum (GS) as a low-cost adsorbent for the removal of Crystal Violet (CV) dye from aqueous solutions. The adsorbent was characterized using XRD, FTIR, and SEM-EDX techniques. The characterization results confirmed the crystalline nature of gypsum and the successful adsorption of dye molecules onto its surface. Furthermore, a Box–Behnken Design (BBD) based on Response Surface Methodology (RSM) was employed to evaluate the influence of the main operating parameters on the adsorption performance. The results revealed that the maximum removal efficiency reached 95.30% under the optimum conditions of 2 g adsorbent dosage, pH 10, temperature of 45 °C, and contact time of 25 min. The adsorption kinetics were best described by the pseudo-second-order (PSO) model, while the Freundlich isotherm provided the best fit to the equilibrium data. In contrast, the Langmuir isotherm estimated a maximum monolayer adsorption capacity of 8.43 mg/g at 45 °C. Thermodynamic studies indicated that the adsorption process was spontaneous and endothermic. The proposed adsorption mechanism involved multiple interactions, including electrostatic interactions, n–π interactions, and hydrogen bonding. These findings highlight the potential of GS as an effective and eco-friendly adsorbent for wastewater treatment, contributing to waste valorization and environmental sustainability.