Sorption of hexavalent chromium from aqueous solution using marine green algae Halimeda gracilis: Optimization, equilibrium, kinetic, thermodynamic and desorption studies

• Sorption of Cr(VI) onto Halimeda gracilis was studied at various operating conditions.• RSM was successfully employed to optimize the process variables.• Equilibrium, kinetics and thermodynamic studies were carried out for the sorption process.• Desorption studies were carried out to find the efficiency of the sorbent.

In this work, effect of operating variables on Cr(VI) uptake capacity of marine green algae Halimeda gracilis was studied. Response surface methodology (RSM) was applied to optimize the operating variables. A maximum percentage removal of Cr(VI) by H. gracilis occurs when, pH ∧ 4.9, sorbent dosage ∧ 2.2 g/L, agitation speed ∧ 136 rpm and contact time ∧ 47 min. The sorbent was characterized by using Fourier transform infrared spectroscopy (FTIR) and scanning electron microscope (SEM) analysis. Experimental data were analyzed in terms of pseudo first order, pseudo second order intra particle diffusion, power function and Elovich kinetic models. The results showed that the sorption process of Cr(VI) ions followed pseudo second order and power function kinetics. The sorption data of Cr(VI) ions are fitted to Langmuir, Freundlich, Dubinin•Radushkevich Temkin, Sips and Toth isotherms. Sorption of Cr(VI) onto H. gracilis biomass followed the Langmuir isotherm model (R2 = 0.997) with the maximum sorption capacity of 55.55 mg/g. The calculated thermodynamic parameters such as ΢G°, ΢H and ΢S showed that the sorption of Cr(VI) ions onto H. gracilis biomass was feasible, spontaneous and endothermic. Desorption study shows that the sorbent could be regenerated using 0.2 M HCl solution, with up to 80% recovery.