Comparative Study of the Competitive Adsorption of Mg, Ca and Sr Ions onto Resins

Existing studies on the adsorption of alkaline earth metal ions have mainly focused on the adsorption of one alkaline earth metal ion. However, under natural conditions, alkaline earth metal ions usually co-exist. While separating them using an adsorbent, they compete with each other for adsorption onto the adsorbent. In this work, comparative and competitive adsorption of Mg, Ca and Sr ions onto four resins (D001, DJH003, LSC-100 and LSC-500) of sodium form were investigated in batch experiments at 25 degrees C. Our results showed that in all the experiments, the equilibrium point was attained in approximately 4 hours. In general, the pseudo-first-order model is more suitable to describe the adsorption kinetic data than the pseudo-second-order model. For both D001 and DJH003 resins, there are bigger differences in the adsorption amounts of Mg, Ca and Sr ions for the competitive adsorption than for the noncompetitive adsorption. Under competitive adsorption conditions, the separation factor follows the order of beta(Sr/Mg) > beta(Ca/Mg), and the order of preference for adsorption is Sr2+ > Ca2+ > Mg2+. Therefore, it can be seen that there is a clear inhibitory effect for the adsorption of Mg ions owing to the competition of other ions. However, for both LSC-100 and LSC-500 resins, the competitive adsorption of Mg, Ca and Sr ions does not show obvious differences. The competitive adsorption results for all the resins were explained by considering hydration of the metal ions, electrostatic attractions and complexation between the metal ions and the resins. The results obtained would be helpful for understanding the competitive adsorption processes and the recovery or removal of one or more alkaline earth metal ions from aqueous solutions.