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 4 h, and equilibrium was attained in 48 h. 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 0.13 mmol/g, respectively. The separation factor (α) values for αPb/Cu, αPb/Ni, and αCu/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.