Adsorption Behavior of Pb2+ Ions on Alginate Beads and Capsules

The adsorption behavior of Pb2+ was compared between calcium alginate beads and capsules, which have different structures of alginate-gel core beads and liquid core alginate-membrane capsules, respectively. In terms of adsorption kinetics and isotherms, adsorption characteristics depending on pH and hardening time were compared for both adsorbents and also released calcium ion during the adsorption process was monitored. The adsorption of Pb2+ on both adsorbents was caused by surface complexation and ion exchange mechanisms, both of which have similar effects on adsorption process regardless of the amount of adsorbed Pb2+. The dependence of Pb2+ adsorption upon pH was also similar for both adsorbents indicating the existence of similar functional groups on the surface of adsorbents. However, a different Pb2+ adsorption behavior was observed considering the adsorption kinetics. The adsorption kinetic of Pb2+ on alginate beads was slower than on alginate capsules and the maximum adsorption loading (Q(max)) onto alginate beads was also less than onto alginate capsules by 49%. This drawback of alginate beads compared to capsules were ascribed to a diffusion limitation due to solid gel-core structure of alginate beads.