Reel silk from cocoons: Optimized fabrication of chitosan-sodium tripolyphosphate imprinted polymers for high-efficiency and selective capture of hexavalent chromium

The structure–function relationship of ion-imprinted polymers for toxic hexavalent chromium (Cr(VI)) is poorly understood. In this study, a series of Cr(VI)-imprinted chitosan (CS)-sodium tripolyphosphate (STPP) composites were prepared by sol–gel method using different alkali/acid combinations and drying modes. The elution of CS-STPP polymer with 1 M NaOH+5 mM HNO3 followed by freeze drying produced the optimal IIP-B+5A-fd material. The distinct structural stabilities and surface charges of imprinted CS-STPP composite determined their disparate Cr(VI) sorption abilities. The Cr(VI) sorption ratio on IIP-B+5A-fd kept higher than 97 % at pH<7, while that on non-imprinted NIP-B+5A-fd decreased from ∼ 73 % to ∼ 28 % as the solution pH rose from 4 to 7. The Cr(VI) sorption kinetics on IIP-B+5A-fd reached equilibrium after 12 h, faster than 24 h on NIP-B+5A-fd. The maximum Cr(VI) sorption capacity of IIP-B+5A-fd (41.29 mg/g) surpassed those of NIP-B+5A-fd (26.00 mg/g) and various previously reported adsorbents. IIP-B+5A-fd exhibited a much better Cr(VI) sorption efficiency (∼83 %) than NIP-B+5A-fd (∼20 %) in the presence of mixed aqueous components. The Cr(VI) sorption mechanism on IIP-B+5A-fd was electrostatic attraction–redox–inner-sphere complexation. Overall, the prepared IIP-B+5A-fd polymer holds a promising potential for highly efficient and selective removal of Cr(VI) from a polluted water environment.