Biosorption of selenium from aqueous solution by green algae (Cladophora hutchinsiae) biomass: Equilibrium, thermodynamic and kinetic studies

The equilibrium, thermodynamics and kinetics of selenium(IV) biosorption from aqueous solution by dead green algae (Cladophora hutchinsiae) biomass was investigated. Optimum biosorption conditions were determined with respect to pH, biomass concentration, contact time, and temperature. The equilibrium data were analyzed using the Langmuir, Freundlich and Dubinin-Radushkevich (D-R) isotherm models. The maximum biosorption capacity of C. hutchinsiae biomass for Se(IV) was found to be 74.9 mg/g at pH 5, biomass concentration 8 g/L, contact time 60 min, and temperature 20 °C. The biosorption percentage decreased from 96% to 60% as temperature was increased from 20 to 50 °C during the equilibrium time. From D-R model, the calculated mean biosorption energy (10.9 kJ/mol) indicated that the biosorption of Se(IV) onto C. hutchinsiae biomass was taken place by chemical ion-exchange. The highest recovery (95%) was achieved using 0.5 M HCI. The high stability of C. hutchinsiae permitted a slightly decrease about 20% in recovery of Se(IV) ions after ten times of adsorption-elution process. The calculated thermodynamic parameters, ΔG° (between -18.39 and -16.08 kJ/mol at 20-50 °C) and ΔH° (-45.96 kJ/mol) showed that the biosorption of Se(IV) onto C. hutchinsiae biomass was feasible, spontaneous and exothermic, respectively. The experimental data was also fitted to the Lagergren's first-order and pseudo second-order kinetic models. The results revealed that the pseudo second-order reaction model provided the best description these data with coefficients of determination in range of 0.992-0.999. The biosorption rate constant was calculated as 24.9 × 10^-2 g/(mg min).