Semi-unzipping of chitosan-sodium alginate polyelectrolyte gel for efficient capture of metallic mineral ions from tannery effluent

Recently, a rigorous discharge limit for metallic mineral ions containing effluent has been established to ensure the leather-making industry's long-term sustainability. Nevertheless, the existing intelligent biomass sodium alginate (SA) and chitosan (CS) adsorbents in remediating metal ions-contaminated effluent have been plagued by their compact structures as well as more carboxyl groups being occupied and masked. Herein, chitosan-sodium alginate polyelectrolyte gel with a semi-unzipping system (CS-SA(GTA0.3)) was rationally constructed with the help of unzipper glutaraldehyde (GTA) intruding. Half of the electrostatic interaction and half of the Schiff base reaction were the essence of the semi-unzipping effect. Such character endowed the CS-SA(GTA0.3) composite with a loose, porous, and amorphous structure, thus improving its swelling behavior and exposition of more functional carboxyl groups, which demonstrated an efficient capture ability of commonly metallic mineral ions (Cr3+, Fe3+, Al3+, and Zr4+) compared with the traditional CS-SA gel bearing a compact structure. Based on pseudo-second-order kinetic model fitting results, the equilibrium capture capacity of CS-SA(GTA0.3) for Cr3+, Fe3+, Al3+, and Zr4+ was 96.42, 95.95, 85.49, and 151.57 mg/g. Even in a large number of suspended solids, the semi-unzipping of chitosan-sodium alginate polyelectrolyte gel was still capable of capturing 149.69 mg/g of Cr3+ from tannery effluent, demonstrating the excellent potential for practical application.