Catalytic Reduction of Environmental Pollutants with Biopolymer Hydrogel Cross-Linked Gelatin Conjugated Tin-Doped Gadolinium Oxide Nanocomposites

Abstract

In the present study, a biopolymer nanocomposite hydrogel based on gelatin and tin-doped gadolinium oxide (Sn-Gd₂O₃@GH) was prepared for the efficient reduction of water pollutants. The method of Sn-Gd₂O₃@GH preparation consisted of two steps. A Sn-Gd₂O₃ nanomaterial was synthesized by a hydrothermal method and mixed with a hot aqueous solution (T > 60 °C) of gelatin polymer, followed by cross-linking. Due to the presence of abundant functional groups on the skeleton of gelatin, such as carboxylic acid (-COOH) and hydroxyl (-OH), it was easily cross-linked with formaldehyde. The structure, morphology, and composition of Sn-Gd₂O₃@GH were further characterized by the FESEM, XRD, EDX, and FTIR techniques. The FESEM images located the distribution of the Sn-Gd₂O₃ nanomaterial in a GH matrix of 30.06 nm. The XRD patterns confirmed the cubic crystalline structure of Gd₂O₃ in a nanocomposite hydrogel, while EDS elucidated the elemental composition of pure Sn-Gd₂O₃ powder and cross-linked the Sn-Gd₂O₃@GH samples. The synthesized Sn-Gd₂O₃@GH nanocomposite was used for the removal of different azo dyes and nitrophenols (NPs). It exhibited an efficient catalytic reduction of Congo red (CR) with a reaction rate of 9.15 × 10^-1 min^-1 with a strong NaBH₄-reducing agent. Moreover, the Sn-Gd₂O₃@GH could be easily recovered by discharging the reduced (colourless) dye, and it could be reused for a fresh cycle.

Keywords: Sn-Gd2O3@GH; azo dye; catalytic reduction; gelatin hydrogel; nanocomposite; nitrophenols.

Scheme 1

Molecular structures of 2–NP, 4–NP, 2,6–DNP, MB, MO, and CR.

Figure 1

Preparation of Sn-Gd₂O₃@GH nanocomposite.

Figure 2

FESEM images of pure Sn-Gd₂O₃ (a) and Sn-Gd₂O₃@GH (b) at low and their high magnification images (a’), (b’).

Figure 3

EDX spectra of Sn-Gd₂O₃ nanomaterial (a) and Sn-Gd₂O₃@GH nanocomposite (b).

Figure 4

XRD patterns of Sn-Gd₂O₃ nanomaterial (Black-line) and Sn-Gd₂O₃@GH nanocomposite (Red-line).

Figure 5

FTIR spectra of Sn-Gd₂O₃ nanomaterial and Sn-Gd₂O₃@GH nanocomposite.

Figure 6

Typical UV−visible absorbance spectra of 4−NP (a), 2,6−DNP (b), and 2−NP (c) and their (d) Ln(At/A0) vs. time plot for the reduction reactions where the amount of the Sn-Gd₂O₃@GH catalyst was 0.2 g.

Figure 7

Typical UV–visible absorbance spectra of 4-MO (a), CR (b), and MB (c) and their and Ln(At/A0) vs. time plot (d) for the reduction reactions with NaBH₄/Sn-Gd₂O₃@GH. The amount of the Sn-Gd₂O₃@GH catalyst was kept fixed (0.2 g).

Figure 8

UV–visible spectra of CR dye reduction with NaBH₄/Sn-Gd₂O₃@GH (a). Optimization of CR dye degradation. The plot of ln (Ct/C0) versus time for CR dye by changing amount of Sn-Gd₂O₃@GH catalyst (b). Different concentrations of CR dye (c) and changing the amount of NaBH4 (d).

Figure 9

Scheme for the reduction of the CR by NaBH4 using Sn-Gd₂O₃@GH hydrogel catalyst.

Figure 10

Recyclability of the Sn-Gd₂O₃@GH hydrogel catalyst. Degradation (red) and reaction rate (black) histograms are shown for the reusability of Sn-Gd₂O₃@GH catalyst during CR degradation in the presence of borohydrate.