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 chitosan (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 ZrO and ZrN 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.

Zr4+ cross-linked chitosan-thiourea composite for efficient detoxification of Cr(VI) ions in aqueous solution.

The effective strategy to solve the problems of low mechanical strength, low adsorption efficiency and poor stability of chitosan is to modify it. In this work, Zr4+ cross-linked chitosan-thiourea (CS-thiourea-Zr) composite was synthesized for the first time and used to remove Cr(VI). CS, ZrO, CN bonds in FTIR spectrum and Zr, S elements in XPS spectrum, SEM-mapping and EDS results verify the successful preparation of CS-thiourea-Zr. The main mechanism of Cr(VI) removal by CS-thiourea-Zr includes electrostatic action, ion exchange, reduction and complexation. The adsorption of Cr(VI) is enhanced by electrostatic adsorption and ion exchange, 80.6 % of the adsorbed Cr(VI) is reduced to Cr(III) by -OH group, and the adsorption of Cr(III) is enhanced by COO- group and CS bond. The maximum adsorption capacity of CS-thiourea-Zr is 246.72 mg g-1 at 298 K and pH = 2.0. CS-thiourea-Zr has excellent acid-base resistance (suitable pH 2-12) and reusability.