Adsorptive removal of anionic dye using calcined oyster shells: isotherms, kinetics, and thermodynamics

Oyster shells are a type of biogenic materials with excellent characteristics in surface area, porosity, sorption capacity, and high concentration of CaCO3 (up to 90 wt%), and can be easily converted into a calcium-based alkali adsorbent. In this research, oyster shells calcined at 900 °C were applied as an adsorbent for acid green 25 (AG25) removal from aqueous solutions. The adsorption performances were evaluated, and the FTIR, SEM, and BET techniques were employed to characterize this material. Results showed that AG25 removal performance depended on adsorbent dosage, pH, adsorption temperature, contact time, and initial concentration. Adsorption capacity was maximized at 34.1 mg g−1 at pH of 11.0, an adsorbent dosage of 2.0 g L−1, an AG25 concentration of 70 mg L−1, and adsorption temperature of 40 °C. Both the Ho-McKay model and the pseudo-second-order model correlated with the adsorption kinetics well with the values of R2 > 0.99 (closer to unity). The Langmuir isotherm showed an excellent correlation coefficient of R2 > 0.99 with the equilibrium data. The thermodynamics study indicates that the adsorption was spontaneous and endothermic. These results demonstrate that the calcined oyster shells has the potential to be used as an eco-friendly and low-cost effective adsorbent for anionic dye removal from water.