Optimization of batch process parameters for chromium (VI) removal from synthetic wastewater using eggshell–clay composite

In this study, a biodegradable spherical adsorbent bead was developed from eggshell (ES) and Bijoypur clay (BC) for the removal of hexavalent chromium from synthetic tannery wastewater. This Eggshell–Clay polymer composite (ES–BC) was made by incorporating them with PVA and alginate. To analyze the morphology and chemical composition of the adsorbent before and after the chromium adsorption, SEM, FTIR, XRD, and EDX were used. The impact of adsorption parameters such as adsorbent dosage (0.67–2.23 g/L), pH of the solution (1.5–9), contact time (10–240 min), initial Cr (VI) concentration (25–100 ppm), and solution temperature (25–65 °C), percentage clay in ES-BC adsorbent (0–40%) for Cr (VI) adsorption were investigated. The optimum value for adsorbent dosage, pH, contact time, initial Cr concentration, temperature, and clay to eggshell ratio was found 1.17 g/L, 1.5, 150 min, 58.2 ppm, 25 °C, and 80/20% respectively. The isotherm model was well-fitted to the Freundlich isotherm suggesting heterogeneous adsorption in multilayer on surface. The adsorption kinetics followed pseudo-second-order kinetics supporting the chemisorption process. Moreover, a decrease in enthalpy (H°) and negative Gibbs free energy (G°) with increased temperature indicated exothermic and spontaneous reaction. Therefore, this research investigated that the composite may serve as a viable option for treating Cr (VI) containing tannery effluent.