Valorization of eggshell waste into CaO adsorbents for efficient crystal violet dye removal: kinetics, isotherms, thermodynamic and activation energy studies

Abstract

Water pollution from persistent cationic dyes, such as Crystal Violet (CV), poses serious environmental challenges, highlighting the need for sustainable and cost-effective treatment methods. This study aimed to develop an eco-friendly adsorbent from waste eggshells by synthesizing high-purity calcium oxide (CaO) and evaluating its performance for CV removal. The material was characterized using XRF, XRD, and FTIR, confirming CaO as the dominant phase with surface hydration to Ca(OH)₂. Adsorption experiments were optimized using Response Surface Methodology (RSM) with a Central Composite Design, revealing that the initial dye concentration was the most influential factor. Under optimal conditions (pH 3, 6.1 g CaO, 90 min, 60 ppm CV), eggshell-derived CaO achieved 97.9% CV removal at. Adsorption followed the Dubinin–Radushkevich isotherm (R 2 = 0.9839) and the pseudo-second-order kinetic model (R 2 = 0.999), indicating physisorption-driven adsorption. Thermodynamic analysis showed the process to be spontaneous, exothermic (ΔH = −2.27 kJ/mol), and energetically favourable. Importantly, the activation energy (Ea = 14.85 kJ/mol) was determined, providing critical insight into the energy barrier and temperature dependence of adsorption, a parameter rarely reported in literature. This study reinforces and ascertains the potential of eggshell-derived CaO as a low-cost, sustainable adsorbent and fills a key knowledge gap in mechanistic understanding and industrial applicability. • High-purity CaO was sustainably synthesized from waste eggshells. • Optimized adsorption achieved 97.9% removal of Crystal Violet dye. • Adsorption followed the D–R isotherm and PSO kinetics, indicating physisorption. • Thermodynamic and activation energy (Ea = 14.85 kJ/mol) analyses confirmed an exothermic, and energetically favourable process.