The present work explores the valorization of eggshell waste as a sustainable biosorbent (ESBS) for the removal of industrial azo dyes, with a focus on its structural features and adsorption performance. XRD analysis revealed that ESBS has a high level of crystallinity, with calcite ( \(\text{CaCO}_{3}\) ) being the most common mineral characterized by the sharp reflection at 29.67°. EDX analysis showed that Ca, C, and O account for more than 96% of the composition. ATR spectroscopy highlighted the characteristic carbonate vibrations, and SEM micrographs revealed fibrous, porous architecture, providing abundant active sites for adsorption. The point of zero charge (pHpzc = 7.6) favored the uptake of anionic dye species at the working pH (6.86) through electrostatic attraction. Comparable removal efficiencies were observed when ESBS were evaluated as both powdered and coarse fragments. This demonstrates the intrinsic reactivity of this material without any mechanical processing. The use of larger fragments is advantageous for practical applications, as it simplifies handling and minimizes hydraulic head losses in potential treatment systems. Adsorption and kinetic modeling revealed that the Sips isotherm and nth order provided the best fit, reflecting heterogeneous adsorption and a passive yet energy-efficient process suitable for large-scale wastewater storage basins. Overall, these findings establish eggshell waste as a low-cost, eco-friendly biosorbent with strong potential for industrial wastewater remediation within the circular economy perspective.
