Calcium alginate beads from Laminaria digitata for the removal of Cu+2 and Cd+2 from dilute aqueous metal solutions

Sodium alginate extracted from the brown algae Laminaria digitata was used to prepare metal sorbing beads. Their metal binding properties were investigated with respect to copper and cadmium ions in single and binary metal solutions. Their behaviour in batch kinetic experiments and in a flow through column for the remediation of dilute aqueous metal solutions was studied. Alginate was found to be a promising material for heavy metal sequestration while sorption was found to be following a competitive mechanism in multi-metal solutions. The obtained maximum adsorption capacities for Cu+2 and Cd+2 were 1.5 mmol/g and 2.09 mmol/g respectively. The calculated diffusion coefficients — 2.8–3.0×10!5 cm2/s for copper and 4.4–4.5×10!5 cm2/s for cadmium ions — were independent of the initial concentration. Finally, the obtained diffusion coefficients were used for the simulation of the breakthrough curves on column filtration experiments. Linear adsorption driving force model was used to provide a rough estimation of the operational and design parameters in flow column adsorption processes.

Fig. 7. Break-through curve and theoretical approximation. the whole concentration range. Nevertheless, the simplified expression can effectively be used for a rough estimation of a heavy metal concentration in fixed bed adsorption process. The complete study of a multicomponent (binary systems) column in which the mass transport of the one metal may be affected by the presence of the other metal will be part of a future study. 3.4. Desorption and reuse The desorption of the adsorbed Cu+2 and Cd+2 ions from the tested sorbents were studied in a flow system. The metal ions adsorbed onto calcium alginate beads were eluted with 0.1 M HCl. More than 97% of the adsorbed metal ions were desorbed. The volume of the acid used for the regeneration of the column was 450 ml (1.5 ml/min for 5 h) while the total volume of the metal solution used for the column experiment was .18 L (1.5ml/min for 12,140 min; final concentration of the regeneration solution: 29.1 mmol/l). In order to show the reusability of the beads, an adsorption–desorption cycle of metal ions was repeated three times using the same preparations. The adsorption capacities did not noticeably Alginate, extracted from Laminaria digitata, in the form of calcium cross-linked beads, exhibit high uptake capacity for the copper and cadmium cations compared to other low cost adsorbents due to a high M/G ratio. The Langmuir and the Sips adsorption models can adequately describe the relationship between solution equilibrium metal concentration and the amount adsorbed by the beads. Additionally, a competitive mechanism seems to be dominant during adsorption in binary systems. Batch adsorption kinetic model is in good agreement with the experimental results and prove that the diffusion coefficient is independent to the initial concentration. Finally, a linear adsorption driving force model can effectively be used for a rough estimation of operational and design parameters in flow bed experiments. The experimental data and the theoretical models used revealed that the prepared material can effectively be applied in a flow column process for the removal of heavy metal ions from dilute aqueous metal solutions.