Competitive biosorption of Ag(I) and Cu (II) by tripolyphosphate crosslinked chitosan beads

In this study, alkalization of chitosan before crosslinking was applied in to enhance the adsorption capacity of the modified chitosan. Competitive adsorption of Ag (I) and Cu (II) from bimetallic solutions was studied using the newly synthesized tripolyphosphate crosslinked chitosan beads. Results indicated that alkalization before crosslinking helps to protect free-amine groups from crosslinking and hence increases the uptake capacity and selectively of the synthesized beads towards Ag (I). The maximum uptakes of Ag (I) and Cu (II) were 82.9 and 15.5 mg/g respectively at room temperature with an initial concentration of each metal being 2.0 mmol/L and the sorbent dosage of 1.0 g L⁻¹. Langmuir isotherm and pseudo-second order kinetic model provide better descriptions of adsorption isotherm and kinetics of metal ions on sorbent surfaces. Analyses from FT-IR and XPS confirmed that free amine, hydroxyl and P₃O₁₀⁵⁻ groups are involved in metal binding with amine and hydroxyl groups more selective to Ag (I). Then, continuous adsorption with the newly synthesized chitosan beads was simulated using the lumped kinetic model. According to the parameters obtained in the batch adsorption, the overall mass-transfer coefficient (Kf), and axial dispersion coefficient (DL) were determined using the empirical correlations. The value of Kf for Ag (I) is in the range of 5.028×10⁻⁵ s⁻1 to 8.389×10⁻⁵ s⁻¹, the value of Kf for Cu (II) was in the range of 8.000×10⁻⁵ s⁻¹ to 1.283×10⁻⁴ s⁻¹; The range of axial dispersion coefficient for Ag (I) and Cu (II) were both varying from 1.806×10⁻⁴ cm²/s to 1.778×10⁻⁴ cm²/s. Results from the breakthrough and elution profiles indicated that decreasing the flow rate, sample concentration and injection time, or increasing the bed length could enhance the separation of the two metal ions. Besides, it was found that concentration overload by increasing the sample concentration is more effective to improve the separation of two metal ions in fixed-bed column than volume overload by increasing the injection time. In conclusion, the newly synthesized chitosan-based biosorbents showed great selective adsorption for Ag (I) in bimetallic solutions, and the simulation studies provided good potential in industrial applications to recover precious metal ions from water or wastewater