Optimizing binary biosorption of cobalt and nickel ions on brown algae using a central composite design

One of the most valuable approaches to eliminate heavy metal ions from aqueous solutions is biosorption. Cobalt and nickel can be removed from aqueous solutions simultaneously using sodium chloride-treated Cystoseria indica, which is a kind of brown algae. Three parameters, including pH, initial concentration of heavy metals, and adsorbent mass, were designated to run batch adsorption experiments. The central composite design was employed to show the simultaneous change of all factors according to a pre-specified experimental matrix that creates a response model in which the interactions between the calculated responses and each variable as well as interaction influences, are revealed. R-2 values for the computed model were 0.96 and 0.95 for the response surface 1 (cobalt) and 2 (nickel), respectively. Central composite design calculated the optimum adsorption process as pH, 5.9; biomass dosage, 0.06 g; initial nickel concentration, 91.94 mg/l; and initial concentration of cobalt 89.36 mg/l. The kinetic and isotherm models were engaged to evaluate the equilibrium data at the optimum condition. The biosorption of both heavy metal ions obeyed the intra-particle diffusion kinetic model best, with the equilibrium adsorption time at 80 min. The extended Freundlich isotherm model is better fitted for the equilibrium biosorption data of nickel and cobalt ions. The maximum biosorption of nickel and cobalt ions simultaneously at optimized condition was 69.99 and 75.21 mg/g, respectively.