Zr4+and glutaraldehyde cross-linked polyethyleneimine functionalized chitosan composite: Synthesis, characterization, Cr(VI) adsorption performance, mechanism and regeneration

In order to improve the stability, electrostatic interaction and ion exchange ability of chitosan for Cr (VI) removal, it is an effective strategy to introduce polyvalent metal ions and polymers into chitosan molecular chain through crosslinking. In this paper, Zr4+ and glutaraldehyde crosslinked polyethyleneimine functionalized chi-tosan (CGPZ) composite was successfully synthesized and characterized by XRD, SEM, FTIR, BET, and XPS. The results showed that polyethyleneimine was successfully grafted onto chitosan by Schiff base reaction, while the appearance of Zr-O and Zr-N bonds verified the successful preparation of CGPZ. The monolayer maximum adsorption capacity of Cr(VI) by CGPZ was 593.72 mg g-1 at 298 K and t = 210 min. The removal efficiency of 100 mg L-1 Cr(VI) reached 95.7 %. The thermodynamic, isotherm and kinetic results show that the adsorption process of Cr (VI) by CGPZ is a spontaneous endothermic process controlled by entropy, which accords with Freundlich model and pseudo-second-order kinetic model. The regeneration experiments show that both HCl and NaOH can effectively desorb Cr(III) and Cr(VI) from the adsorbent surface, and the adsorbent has good acid-base resistance and regeneration performance. The removal of Cr(VI) mainly involves electrostatic attraction, ion exchange, reduction and complexation. CGPZ can synergistically adsorb Cr(VI) by electrostatic interaction of-NH2/-C=N and ion exchange of Cl- ion in the center of Zr, then reduce Cr(VI) to Cr(III) (45.4 % at pH = 2.0) by the-OH group on its surface, and chelate Cr(III) through COO-and -NH-groups.