Determining the selectivity of divalent metal cations for the carboxyl group of alginate hydrogel beads during competitive sorption

To investigate the competitive sorption of divalent metal ions such as Ca²⁺, Cu²⁺, Ni²⁺, and Pb²⁺ on alginate hydrogel beads, batch and column tests were conducted. The concentration of carboxyl group was found to be limited in the preparation of spherical hydrogel beads. From kinetic test results, 80% of sorption was observed within 4h, and equilibrium was attained in 48h. According to the comparison of the total uptake and release, divalent metal ions were found to stoichiometrically interact with the carboxyl group in the alginate polymer chain. From the Langmuir equation, the maximum capacities of Pb²⁺, Cu²⁺, and Ni²⁺ were calculated to be 1.1, 0.48, and 0.13mmol/g, respectively. The separation factor (α) values for α_Pb/Cu, α_Pb/Ni, and α_Cu/Ni were 14.0, 98.9, and 7.1, respectively. The sorption capacity of Pb²⁺ was not affected by the solution pH; however, the sorption capacities of Cu²⁺ and Ni²⁺ decreased with increasing solution pH, caused by competition with hydrogen. According to the result from the fixed column test, Pb²⁺ exhibited the highest affinity, followed by Cu²⁺ and Ni²⁺, which is in exact agreement with those of kinetic and isotherm tests. The sorbent could be regenerated using 4% HCl, and the regenerated sorbent exhibited 90% capacity upto 9 cycles.