Potential of protonated alginate beads for heavy metals uptake

Abstract

The potential for uptake of several heavy metal ions by alginate in the form of protonated beads was investigated at 25 °C. The morphological characteristics of the beads and their behavior in aqueous solutions were examined as well. The ability of protonated alginate beads to remove heavy metal ions from dilute aqueous solutions was demonstrated. The uptake of trivalent chromium, copper, zinc, nickel and cobalt was found to be about 75, 77, 46, 43 and 35 mg per g of beads, respectively. The uptake increased with solution pH and acid concentration for protonation, and with decreasing ionic strength. The cross-linking agents, Ba and Ca, were not released during uptake, and protons were liberated instead. Therefore, the uptake was associated with ion exchange between protons of the free carboxylic functional groups of the alginate and metal ions from the solutions. EPMA-EDX analyses showed that heavy metals were uniformly distributed throughout the surface (external and internal) of the beads, indicating that protonated dry alginate beads can be considered as porous ion exchangers. Observation of protonated dry alginate beads by scanning electron microscopy (SEM) showed a corrugated surface having a uniform distribution of regular nodules and cavities. A mean diameter of about 1.0 mm was obtained for these beads. They were found to have chemical and structural stability in acidic and slightly acidic environments. Bead forming reaction with Ba or Ca makes it possible to use alginate as sorbent for heavy metal ions in extremely dilute aqueous solutions.