Recyclable Xanthan/TiO₂ Composite Cryogels towards the Photodegradation of Cr(VI) Ions and Methylene Blue Dye

Abstract

Composite cryogels were prepared from xanthan gum (XG) precursor gels at 20 g L-1 containing TiO2 load at 5, 10, and 20 wt% and citric acid, as crosslinker. The effect of the pH over precursor gel on the properties of the resulting cryogels was evaluated. The characterization of the XG/TiO2 cryogels comprised compression tests, swelling degree (SD) determination, Fourier transform infrared vibrational spectroscopy in the attenuated total reflectance mode (FTIR-ATR), scanning electron microscopy (SEM), and X-ray microtomography (CT) analyses. The largest compressive modulus (E) was observed for XG/TiO2 10% cryogels prepared at pH 4.0, which amounted to 100±7 kPa, whereas the E value determined for bare XG cryogels was 29±3 kPa. XG/TiO2 10% cryogels presented larger pores and thicker walls than bare XG cryogels, as evidenced by SEM and CT analyses. FTIR-ATR spectra evidenced the ester bonds stemming from the esterification among carboxylic acid groups and/or XG hydroxyl groups. XG/TiO2 10% cryogels presented SD of (61±2) gwater/gcryogel, long-term stability in water, and outstanding photocatalytic properties in the presence of Cr(VI) ions and methylene blue (MB). The photocatalytic processes for the reduction of Cr(VI) to Cr(III) and for the photobleaching of MB fitted the first-order kinetic model, yielding rate constants of 0.019 varying min-1 and 0.0096 min-1, respectively. For both processes, the XG/TiO2 10% cryogels could be recycled five times without losing shape or efficiency.

Designing Highly Photoactive Hybrid Aerogels for In-Flow Photocatalytic Contaminant Removal Using Silica-Coated Bacterial Nanocellulose Supports

“…The FTIR spectrum of BC aerogel (Figure a) depicts bands assigned to different vibrational modes of nanocellulose network functionalities, including hydrogen-bonded hydroxyl groups (O–H stretching centered at 3346 cm –1 ), methyl and methylene groups (C–H stretching around 2800–3000 cm –1 ), as well as vibrational modes related to characteristic C–C and C–O–C linkages of polysaccharide rings (800–1200 cm –1 region, marked with * in Figure b). These include vibrational features of β-glucosidic linkage, such as δ C–O–C (1161 cm –1 ), υ C–C (1109 cm –1 ), υ C–O (1057 cm –1 ), and υ C–O at C 6 of glucopyranosic ring units (1034 cm –1 ). , The FTIR spectra of BC@SiO 2 hybrid aerogels reveal that the prominent vibration bands (3346 cm –1 ), corresponding to −OH stretching groups, greatly diminish in intensity and become broader as SiO 2 loading increases (from BCS1 to BCS5) (Figure a). Such spectral changes may be related to the disruption of the inter-fiber hydrogen-bond network of cellulose and/or condensation of the surface hydroxyl groups with the partially hydrolyzed silicon ethoxide species (Si­(OH) x (OC 2 H 5 ) 4– x ), thus leading to SiO 2 anchoring and nucleation selectively on the BC scaffold surface.…”

“…These include vibrational features of βglucosidic linkage, such as δ C−O−C (1161 cm −1 ), υ C−C (1109 cm −1 ), υ C−O (1057 cm −1 ), and υ C−O at C 6 of glucopyranosic ring units (1034 cm −1 ). 61,62 The FTIR spectra of BC@SiO 2 hybrid aerogels reveal that the prominent vibration bands (3346 cm −1 ), corresponding to −OH stretching groups, greatly diminish in intensity and become broader as SiO 2 loading increases (from BCS1 to BCS5) (Figure 6a). Such spectral changes may be related to the disruption of the inter-fiber hydrogen-bond network of cellulose and/or condensation of the surface hydroxyl groups with the partially hydrolyzed silicon ethoxide species (Si-(OH) x (OC 2 H 5 ) 4−x ), thus leading to SiO 2 anchoring and nucleation selectively on the BC scaffold surface.…”