Equilibrium and Kinetic Studies on the Removal of a Textile Dye from Water by Adsorption onto a Waste Eggshell Biosorbent

This study investigates the adsorption of the textile dye Direct Blue BR 200 from aqueous solutions onto waste chicken eggshells, evaluated as a low-cost biosorbent, and the commercial activated carbon Sorbotech LGCO 100, used as a reference sorbent. Adsorption equilibrium was determined experimentally and interpreted using the Henry, Langmuir, Freundlich, Sips and modified BET isotherms. Among these models, the Sips isotherm provided the best fit for both adsorbents, indicating surface heterogeneity and a finite sorption capacity. The equilibrium data showed that eggshells exhibit higher dye uptake than activated carbon across the investigated concentration range. Kinetic studies were performed in a continuous-flow fixed-bed system at flow rates: 1.72, 2.31 and 2.90 cm3/s. Apparent effective diffusion coefficient (Ds) was determined using the analytical solution to Fick’s unsteady-state diffusion equation. The Ds values obtained for eggshells—2.55 × 10−11 m2/s, 4.59 × 10−11 m2/s, and 1.92 × 10−10 m2/s—consistently exceeded those for activated carbon. The combined equilibrium and kinetic analyses, supported by the calculated diffusion coefficients, demonstrate that waste eggshells outperform the activated carbon Sorbotech LGCO 100 in their ability to adsorb Direct Blue BR 200 from water. The results provide quantitative parameters for modeling adsorption processes and highlight the potential of eggshell-based biosorbents in water treatment applications.