Utilization of Phosphogypsum as Sustainable Adsorbent for Removal of Crystal Violet Dye from Wastewater: Kinetics, Thermodynamics, and Applications in Textile Effluent Treatment

This study examines the potential of phosphogypsum—a by-product of the phosphoric acid production process—as a low-cost and sustainable adsorbent for the removal of crystal violet dye from aqueous solutions. Phosphogypsum was characterized using X-ray fluorescence, X-ray diffraction, particle size distribution, and zeta potential measurements, revealing that it is primarily composed of di-hydrate calcium sulfate, with a negatively charged surface in the pH range from 1.8 to 8.2 and a mean particle size of 12.2 microns. Experiments were conducted to evaluate the effects of pH, adsorbent dose, contact time, and temperature on its adsorption ability. The results indicated that the adsorption capacity increased with the pH up to a value of 5, while higher initial dye concentrations enhanced the uptake capacity but reduced the removal efficiency. The adsorption process was well described by the Langmuir isotherm, suggesting chemisorption as the dominant mechanism, while the pseudo-second-order kinetic model indicated that adsorption primarily occurred on the exterior surface. The thermodynamic analysis revealed that the process was exothermic and spontaneous at 20 °C and 30 °C, with a decrease in favorability at higher temperatures. The adsorbent demonstrated reusability, with a removal efficiency of 71% after five regeneration cycles. Furthermore, phosphogypsum was successfully applied to treat real textile effluent, achieving significant reductions in both biochemical oxygen demand (71%) and dye content (87%). These findings highlight the potential of phosphogypsum as an effective and eco-friendly adsorbent for wastewater treatment, contributing to waste valorization and environmental sustainability.