Uptake of Zn²⁺ from dilute aqueous solutions using protonated dry alginate beads

The uptake of zinc ions from dilute aqueous solutions was studied at 25°C using protonated dry alginate beads (PDAB) of around 1 mm in diameter. The Zn²⁺ uptake increases with the pH of the Zn bearing solution, reaching a value of around 90 mg of Zn per gram of beads (dry weight) at pH 4·5. For an initial Zn concentration as low as 10 mg L^-1, the removal reached was complete. The mechanism of Zn uptake was found to be ion exchange between zinc ions and protons form the functional groups of the beads, which followed a pseudo-second order kinetic behaviour. In equilibrium condition the experimental data followed the Langmuir adsorption model. The maximum uptake reached were around 145 and 165 mg g21 at pH values of 3·5 and 4·5, respectively, which is higher than most of the sorbents used for zinc removal. EPMA-EDX analysis shows that the functional groups of the PDAB were homogeneously distributed during preparation of beads, and that zinc ions were able to reach functional groups in the entire structure of the beads without a concentration gradient across the beads.