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The objective of this study was to valorise eggshell waste (ESW) by investigating its biosorption properties and evaluating its efficiency as a sustainable biosorbent for the removal of the synthetic dye Congo Red (CR) from model CR solutions and synthetic wastewater with the addition of CR. Batch biosorption experiments were conducted to investigate the influence of several factors on the biosorption process, including ESW concentration (1–15 g L−1), contact time (1–360 min), temperature (15, 25, 35, 45 °C) and initial CR concentration (10–100 mg L−1). Desorption experiments were performed using ultrapure water, 0.1 M NaCl, 50% ethanol, 0.1 M HCl, or 0.1 M NaOH as solvents. A higher ESW concentration improved CR removal, but the amount of CR adsorbed on ESW decreased. The dye uptake by ESW was increased with prolonged contact time and temperature increase. When the effect of CR initial concentration was investigated, the results indicated that the process is concentration-dependent and that overall, CR uptake by ESW was higher in synthetic wastewater than in the model dye solution. The biosorption process was better described by the Langmuir isotherm model than by the Freundlich model, indicating monolayer adsorption. Kinetic analysis showed that the pseudo-second-order model provided a better fit than the pseudo-first-order model. Desorption of CR from ESW under the applied experimental conditions was generally low (0.67–27.13%).
Velić et al. (Mon,) studied this question.