Hydrogel Beads of Amidoximated Starch and Chitosan as Efficient Sorbents for Inorganic and Organic Compounds

Abstract

The synthesis of hydrogel beads involving natural polymers is, nowadays, a leading research area. Among natural polymers, starch and chitosan represent two biomolecules with proof of efficiency and low economic impact in various utilization fields. Therefore, herein, the features of hydrogel beads obtained from chitosan and three sorts of starch (potato, wheat and rise starches), grafted with acrylonitrile and then amidoximated, were deeply investigated for their use as sorbents for heavy metal ions and dyes. The hydrogel beads were prepared by ionotropic gelation/covalent cross-linking of chitosan and functionalized starches. The chemical structure of the hydrogel beads was analyzed by FT-IR spectroscopy; their morphology was revealed by optical and scanning electron microscopies, while the influence of the starch functionalization strategies on the crystallinity changes was evaluated by X-ray diffraction. Molecular dynamics simulations were used to reveal the influence of the grafting reactions and grafted structure on the starch conformation in solution and their interactions with chitosan. The sorption capacity of the hydrogel beads was tested in batch experiments, as a function of the beads' features (synthesis protocol, starch sort) and simulated polluted water, which included heavy metal ions (Cu²⁺, Co²⁺, Ni²⁺ and Zn²⁺) and small organic molecules (Direct Blue 15 and Congo red).

Keywords: covalent cross-linking; grafted starch; ionotropic gelation; molecular dynamics simulation; sorption capacity.

Figure 1

SEM images (scale bar 20 µm) (A), circular equivalent (CE) diameter (B) and X-ray diffractograms (C) of initial (a) and grafted AN (b) and Ax (c) starches.

Figure 2

Snapshots depicting the starting and final structure of the 3 simulated systems: (A) amylose from starch, (B) SgAN and (C) SgAx. The starch molecules are colored in red and blue, the AN side chains are colored in green and the Ax side chains are colored in silver. Water has been omitted for clarity.

Figure 3

Optical (scale bar 5 mm) and SEM (scale bar 20 µm) images of composite beads.

Figure 4

Snapshots depicting the starting and final structure of the simulated systems containing CS and (A) SgAN and (B) SgAx. The starch molecules are colored in red and blue, the AN side chains are colored in green, the Ax side chains are colored in silver and the chitosan is colored in orange. Water has been omitted for clarity.

Figure 5

Deconvolution of 1800–1500 cm⁻¹ region of FTIR spectra of CS/amidoximated starch beads obtained by the two procedures.

Figure 6

X-ray diffractograms of CS/SgAx, SgAN and SgAN-AX composite beads as compared to bare CS.

Figure 7

Swelling of the composite hydrogel beads as a function of time at pH = 6 (a) and different pH values (b); inset in (b) potentiometric titration curves of SgAx and CS.

Figure 8

Sorption of Cu²⁺, Co²⁺, Ni²⁺ and Zn²⁺ onto composite beads based on PS (square), WS (circle) and RS (triangle) and using hydrogel beads CS/SgAx (close symbols) or CS/SgAN-Ax (open symbols).

Figure 9

(a) Schematical representation of the three active sites (colored) of amidoxime functional group and (b) sorption capacity of hydrogel beads CS/SgAN-Ax or CS/SgAx expressed as moles of metal (Me)/moles of active sites (AS) in hydrogel beads.

Figure 10

Sorption of dyes onto composite beads based on PS (square), WS (circle) and RS (triangle) and using CS/SgAx (close symbols) or CS/SgAN-Ax (open symbols) hydrogel beads.