Hybrid Chitosan Biosorbents: Tunable Adsorption at Surface and Micropore Domains

Abstract

Herein, we report a study that provides new insight on the knowledge gaps that relate to the role of biopolymer structure and adsorption properties for chitosan adsorbents that are cross-linked with glutaraldehyde. The systematic modification of chitosan cross-linked with glutaraldehyde (CG) and its quaternized forms (QCG) was studied in relation to the reaction conditions: mole ratios of reactants and pH conditions. Complementary adsorbent characterization employed ¹³C NMR/FTIR spectroscopy, TGA and DSC, point-zero-charge (PZC), solvent swelling, and sorption studies using selected dye probes. The spectral and thermal techniques provide complementary evidence that affirm the key role of cross-linker content and quaternization on variation of the physicochemical properties of chitosan. The PZC results reveal a neutral surface charge for the modified materials between pH 6.0 to 6.3 ± 0.3, as compared with pH 8.7 ± 0.4 for pristine chitosan. Solvent swelling in water decreased with greater cross-linking, while the QCG materials had greater swelling over CG materials due to enhanced hydration. The adsorption results reveal variable dye uptake properties according to the cross-linker content. Similarly, surface versus micropore adsorption was demonstrated, according to the nature and ionization state of the dye for the modified adsorbents, where the CG and QCG materials had tunable sorption properties that exceeded that of unmodified chitosan. A key step in tuning the structure and surface chemical properties of cross-linked chitosan involves pH control during synthesis. The facile tunability of the physicochemical properties of the modified biopolymers reported herein means that they possess features of biomimetics that are relevant to advanced drug delivery, antimicrobial materials for wound healing, biosensors, and biosorbents for biomedical applications.

Keywords: adsorption; biopolymer cross-linking; chitosan; glutaraldehyde; pore structure; surface quaternization.

Figure 1

FTIR spectra of (A) cross-linked chitosan (CG), (B) cross-linked and quaternized chitosan (QCG) and (C) cross-linked chitosan showing the 1660–1800 cm⁻¹ region (the highlighted section of the spectra shows new IR bands from the cross-linker). The acronyms for sample names are defined in Table 1.

Figure 2

(A–C). DTG of cross-linked chitosan (A) and cross-linked and quaternized chitosan (B), and DSC thermograms of CH and its modified forms (C). The acronyms for the sample names are defined in Table 1.

Figure 3

(A) ¹³C CP-MAS solids NMR spectra of chitosan, CG, and (B) QGC materials at variable levels of cross-linking and quaternization. The acronyms for sample names are defined in Table 1.

Figure 4

PZC of CG (A) and QCG (B). The acronyms for sample names are defined in Table 1.

Figure 5

Equilibrium water swelling properties of chitosan, CG, and QGC polymers at ambient pH (pH ca. 6.5). The acronyms for sample names are defined in Table 1.

Figure 6

(A) Removal efficiency of chitosan (CH), CG, and QGC polymers for methyl orange (MO), reactive black 5 (RB), phenolphthalein (Phth), and methylene blue (MB). (B) Effects of pH on the decolorization of MO by chitosan (CH), CG, and QGC hydrogels. The acronyms for sample names are defined in Table 1.

Figure 7

Effect of pH adjustment on (A) swelling properties, (B) thermal stability, and (C) adsorption properties of the CG polymers. The acronyms for the sample names are defined in Table 1.

Scheme 1

(A) Schematic description of the synthesis of CGx and QCGx polymer materials, and. (B) Reaction of chitosan with glutaraldehyde to yield micropore domains that contain solvent (not drawn to scale) that arise due to cross-linking between the biopolymer chains.