La(III) and Ce(III) biosorption on sulfur functionalized marine brown algae Cystoseira indica by xanthation method: Response surface methodology, isotherm and kinetic study

In this study, chemically modified Cystoseira indica brown algae by xanthation method was utilized to study the biosorption of La(III) and Ce(III) ions from the aqueous solutions under batch mode. The effects of four variables including pH (2-6), temperature (25-55 degrees C), biosorbent dosage (0.5-2.5 g L-1) and initial metal concentration (50-250 mg L-1) were evaluated and optimized using Response Surface Methodology (RSM) based on the Central Composite Design (CCD). A low P-value and high coefficient of determination (R-2) implied the accuracy and validity of the proposed quadratic models. The results showed that the xanthation improved the performance of C. indica than the intact and Ca-pretreated ones. Performing experiments under optimal condition sets produced maximum biosorption capacities of 185.44 and 172.33 mg g(-1) for La(III) and Ce(III), respectively. In order to characterize the provided biosorbents, FTIR analyses were carried out. Results revealed that the dominant functional groups in the binding process were hydroxyl, carboxyl and amide in the intact algae while xanthation caused addition of sulfur groups that participated in binding process and increased the sorption capacities of both metal ions. The equilibrium and kinetic studies revealed that the Freundlich isotherm model and the double-exponential kinetic model were in the better fitness with the La(III) and Ce(III) experimental data. Moreover, the results showed that the presence of Th(IV) ions decreased the biosorption capacities of both metal ions. In conclusion, the present work showed that the xanthated C. indica could be a proper biosorbent for La(III) and Ce(III) biosorption from aqueous solutions.