Ion exchange kinetics of magnetic alginate ferrogel beads produced by external gelation

This paper reports on a study of the influence of sodium alginate concentration and iron addition on the ion exchange kinetics of calcium alginate ferrogel beads produced by external gelation. The calcium absorption and sodium release of the beads were fitted to Fick's second law for unsteady state diffusion in order to obtain the effective diffusion coefficients of Na+ and Ca2+. The dried beads were characterized concerning their thermal stability, particle size distribution and morphology. The gelation kinetics showed that an increase in alginate concentration from 1% to 2% increased the Ca2+ equilibrium concentration, but presented no effect on Ca2+ effective diffusion coefficient. Alginate concentration higher than 2% promoted saturation of binding sites at the bead surfaces. The addition of iron promoted faster diffusion of Ca2+ inside the gel beads and reduced the Ca2+ equilibrium concentration. Also, iron particles entrapped in the alginate gel beads promoted greater absorption of water compared to pure alginate gel and lower thermal stability of the beads. The main diffusion of Ca2+ into and Na+ out from the bead took place during the first 60 min, during which almost 85-90% of the Ca2+ equilibrium concentration is achieved, indicating that this period is sufficient to produce a Ca-alginate bead with high crosslinking of the polymer network.