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

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.


Introduction
The presence of a low concentration of synthetic dyes is a serious threat to the reuse of industrial wastewater. Natural water sources are contaminated by heavy metals and hazardous dyes, such as nitrophenols, azo dyes, phthalates, and herbicides, with most contributions coming from oil refining and industrial discharges [1][2][3]. Dyes are mainly used in the textile, painting, leather, printing, and photography industries, among others. These dyes, after completing the dyeing process, are released as effluent to nearby water streams. The presence of these dyes creates environmental problems [4]. In particular, azo dyes contain a nitrogen-nitrogen double bond (-N=N-) and are usually applied in various industries. Various methods have been tried to remove these dyes, such as physical, chemical, and biological methods, electrochemical methods, catalysis, and adsorption [5]. Adsorption is well reported to aid in dye reduction.
Metals and their oxide nanoparticles have played the vital role of catalyst in numerous studies [6][7][8][9]. While noble metal nanoparticles have been recognized as the best catalysts for various reactions and sensing applications [10][11][12][13], their high cost limits their applications on a large scale. In contrast to noble metals, other transition metal nanoparticles, such as tin, copper, nickel, iron, and cobalt, have been reported [6][7][8][10][11][12][14][15][16][17][18][19][20][21][22] (23.1%). Gelatin composition is almost similar to collagen. Changes in its structure emerge during its manufacturing process. On heating, the collagen are transferred into helixes which, upon cooling, partly regain structure, are converted into gelatin containing water, and form gel. However, due to its brittle and fragile network, it is limited in application. To control these limitations, MNPs have been introduced to form hydrogel nanocomposites [55]. The entrapped NPs either cross-link the chains of the hydrogel or absorb on the polymer chain, forming a three-dimensional network [56]. These networks are resulted due to covalent and noncovalent bonds through physical or chemical interactions. The MNPs and polymer work together to produce unique properties, such as thermal, mechanical, and optical, which leads the hydrogel nanocomposites to be used in sensors, catalysts, electronics, optics, and in separation devices [57]. Gelatin hydrogel can also be used as biodegradable material in the pharmaceutical and medical industries [56,58,59]. Moreover, it can be cross-linked easily due to the presence of a large number of functional groups [60].
Therefore, herein, the gelatin hydrogel nanocomposite was prepared by the incorporation of gadolinium doped tin dioxide nanomaterials and cross-linked with formaldehyde. The prepared hydrogel nanocomposite was studied as a catalyst for the reduction of different environmental pollutants. Additionally, the prepared hydrogel exhibits low cost, highly efficiency for the removal of dyes, potential recyclability, and easy separation.

Synthesis of Sn-Gd 2 O 3 Nanomaterial
Gadolinium is a vital substance for doping due to its chemical stability. Here, a onestep way of fabrication is followed for Sn-Gd 2 O 3 nanomaterial. In the first step, 0.1 M solution of gadolinium salt [Gd(NO 3 ) 3 ·6H 2 O] was prepared, dissolving 2.26 gm in 50 mL of distilled water. Similarly, 0.01 M solution of SnCl 4 salt (0.1303 gm) was freshly prepared in 50 mL of ethanol. The SnCl 4 solution was set on vigorous stirring for 1hr, until a colourless solution was obtained. In the second step, both solution [Gd(NO 3 ) 3 ·6H 2 O] and SnCl 4 was mixed up in 100 mL of round bottom flask. The pH of the final solution was adjusted above 10 by adding drop wise NH 4 OH solution. Again, the solution was kept for stirring, forming colloidal suspension, after 1 h. Finally, the solution was transferred to a Teflonlined autoclave at 150 • C for 3 h, which gives white precipitate. The final product was collected after centrifugation. The resulting white product was washed with water and ethanol and dried at 80 • C overnight.

Preparation of Gelatin Hydrogel Solution
Gelatin hydrogel was prepared by dissolving of 2.4 gm of gelatin in 30 mL of distilled water, at room temperature. The beaker containing hydrogel was heated mildly at 60 • C, under constant stirring, to prepare a homogenous viscous solution of gelatin polymer.

Fabrication of Sn-Gd 2 O 3 @GH Nanocomposite
During adjustments, the 8 wt% concentration of gelatin was found to be good. Thus, 20 mL of gelatin solution was used to prepare hydrogel nanocomposite. The hydrogel was further treated with 4% by weight of Sn doped gadolinium oxide (Sn-Gd 2 O 3 ) in a beaker

Synthesis of Sn-Gd2O3 Nanomaterial
Gadolinium is a vital substance for doping due to its chemical stability. Here, a onestep way of fabrication is followed for Sn-Gd2O3 nanomaterial. In the first step, 0.1 M solution of gadolinium salt [Gd(NO3)3.6H2O] was prepared, dissolving 2.26 gm in 50 mL of distilled water. Similarly, 0.01 M solution of SnCl4 salt (0.1303 gm) was freshly prepared in 50 mL of ethanol. The SnCl4 solution was set on vigorous stirring for 1hr, until a colourless solution was obtained. In the second step, both solution [Gd(NO3)3.6H2O] and SnCl4 was mixed up in 100 mL of round bottom flask. The pH of the final solution was adjusted above 10 by adding drop wise NH4OH solution. Again, the solution was kept for stirring, forming colloidal suspension, after 1 h. Finally, the solution was transferred to a Teflonlined autoclave at 150 °C for 3 h, which gives white precipitate. The final product was collected after centrifugation. The resulting white product was washed with water and ethanol and dried at 80 °C overnight.

Preparation of Gelatin Hydrogel Solution
Gelatin hydrogel was prepared by dissolving of 2.4 gm of gelatin in 30 mL of distilled water, at room temperature. The beaker containing hydrogel was heated mildly at 60 °C, under constant stirring, to prepare a homogenous viscous solution of gelatin polymer.

Fabrication of Sn-Gd2O3@GH Nanocomposite
During adjustments, the 8 wt% concentration of gelatin was found to be good. Thus, 20 mL of gelatin solution was used to prepare hydrogel nanocomposite. The hydrogel was further treated with 4% by weight of Sn doped gadolinium oxide (Sn-Gd2O3) in a beaker under constant stirring for 40 min, to completely homogenize the Sn-Gd2O3 nanomaterial. The hydrogel nanocomposite was cross-linked by drained off slowly 5 mL of formaldehyde and left for 24 h to completely cross-link the hydrogel. The prepared solid hydrogel

Preparation of Dyes Solution
All of the dyes, 4-NP, 2-NP, 2,6-DNP, ArO, MO, and CR, were used. Standard solutions were prepared with desired concentrations of 0.07 mM. Ultra-pure distilled water was used throughout all the experiments.

Preparation of Dyes Solution
All of the dyes, 4-NP, 2-NP, 2,6-DNP, ArO, MO, and CR, were used. Standard solutions were prepared with desired concentrations of 0.07 mM. Ultra-pure distilled water was used throughout all the experiments.

Characterizations
In this approach, scanning electron microscopy, (FESEM) of JEOL, JSM-7600F, Tokyo, Japan, was used for the morphological investigation of Sn-Gd 2 O 3 @GH nanocomposite, at high accelerating voltage. The prepared sample was coated with platinum before analysis, and attached on 1.5 cm diameter of sticky black tape. The surface elemental composition of the sample was analysed using Oxford energy dispersive X-ray spectroscopy (EDS; Tokyo, Japan) equipment, attached with FESEM (Tokyo, Japan). X-ray diffraction (XRD) experiments were conducted on PANalytical diffractometer using Kα radiation (λ = 0.154 nm) source to investigate the crystallinity of the samples. The voltage of 40 KV and current of 50 mA was set for analysis. The analysis data was recorded in the range of 10 • and 80 • at a scan rate of 2 • 2θ min −1 . Fourier transform infrared (FTIR; Berlin, Germany) was used to characterize different functional groups in the Sn-Gd 2 O 3 @GH nanocomposite hydrogel. The spectra were recorded in the range of 40,000 to 500 cm −1 by averaging the 64 scans, using FTIR (Berlin, Germany).

FESEM Analysis
Field Emission Scanning Electron Microscope (FESEM; Tokyo, Japan) is an electron microscope which is suitable for the analysis of material surface. It is used for observing structures as small as 1 nm. For sample analysis, the primary electrons from a field source are focused and deflected by the lenses which produce a narrow scan beam that investigates the surface morphology of the sample. The secondary electrons are reflected from the material surface to the detectors, which transform it to an electronic signal.
FESEM is used to analyse the surface morphology of the samples, usually for the exterior and cross-sectional area. Figure 2a Figure 2b,b shows the as synthesized Sn-Gd 2 O 3 @GH nano-composite present in the white aggregated form. The average particle size of 30.06 nm was calculated. The incorporation of Sn-Gd 2 O 3 nanomaterial (red circle on SEM image) changed the morphology of the gelatin polymer; it appeared in grooves and wide roughness, as shown in Figure 2b .

EDX Analysis
Energy dispersive-ray spectroscopy is a surface microanalysis characterization to analyse the elemental composition of the sample. Figure 3 elucidates the EDX spectra of Sn-Gd 2 O 3 and Sn-Gd 2 O 3 @GH nanocomposite. EDX spectra showed the existence of Sn, Gd, and O elements into the Sn-Gd 2 O 3 doped nano-catalyst ( Figure 3a). In another EDX spectrum, it is found the Sn, Gd, O, and C elements into the gelatin polymer coated Sn-Gd 2 O 3 @GH nanocomposite ( Figure 3b). The platinum peak showed due to the platinum coating on the sample. The former EDX spectra for Sn and Gd revealed 26.25 % and 46.86 % by weight, while the second spectra showed 9.46 % and 20.54 % by weight amount of Sn and Gd, respectively. Figure 3b showed the elemental composition of Sn-Gd 2 O 3 nanomaterial in the hydrogel matrix. This homogenous distribution confirms the Sn-Gd 2 O 3 in the hydrogel nanocomposite. FESEM is used to analyse the surface morphology of the samples, usually for the exterior and cross-sectional area. Figure 2a,b shows the FESEM images of powder Sn-Gd2O3 and Sn-Gd2O3@GH nano-composite, respectively. The Sn-Gd2O3 particles are clearly seen in irregular form as shown in Figure 2a, while, Figure 2b confirms the dispersed nature of Sn-Gd2O3 nanomaterials (red circle on SEM image) in the hydrogel matrix. Figure 2b,b′ shows the as synthesized Sn-Gd2O3@GH nano-composite present in the white aggregated form. The average particle size of 30.06 nm was calculated. The incorporation of Sn-Gd2O3 nanomaterial (red circle on SEM image) changed the morphology of the gelatin polymer; it appeared in grooves and wide roughness, as shown in Figure 2b′.

EDX Analysis
Energy dispersive-ray spectroscopy is a surface microanalysis characterization to analyse the elemental composition of the sample. Figure 3 elucidates the EDX spectra of Sn-  Gd2O3 and Sn-Gd2O3@GH nanocomposite. EDX spectra showed the existence of Sn, Gd, and O elements into the Sn-Gd2O3 doped nano-catalyst ( Figure 3a). In another EDX spectrum, it is found the Sn, Gd, O, and C elements into the gelatin polymer coated Sn-Gd2O3@GH nanocomposite (Figure 3b). The platinum peak showed due to the platinum coating on the sample. The former EDX spectra for Sn and Gd revealed 26.25 % and 46.86 % by weight, while the second spectra showed 9.46 % and 20.54 % by weight amount of Sn and Gd, respectively. Figure 3b showed the elemental composition of Sn-Gd2O3 nanomaterial in the hydrogel matrix. This homogenous distribution confirms the Sn-Gd2O3 in the hydrogel nanocomposite.

XRD Analysis
The XRD profiles of powder Sn-Gd 2 O 3 nanomaterial and Sn-Gd 2 O 3 @GH nanocomposite are presented in Figure 4. Figure 4 (Black-line) shows the XRD profile of the Sn-Gd 2 O 3 nanomaterial having four sharp peaks. The pattern of Gd 2 O 3 powder is exactly coincident with cubic phase Gd 2 O 3 (JCPDS card no.12-0797), as published earlier in the related literature [61]. The prominent peaks appeared at 2-theta of 29.39 • , 34.12 • , 49.12 • , and 58.33 • , and could be assigned to (222), (400), (440), and (622) diffraction of Sn-Gd 2 O 3 nanomaterials, respectively. Figure 4 (Red-line) showed corresponding peaks for Sn-Gd 2 O 3 @GH nanocomposite and calculated no substantial difference in the XRD pattern of the Sn-Gd 2 O 3 nanomaterial sample, except a decrease in the intensity of the peaks. No significant peaks were observed for the dopant due to its low concentration.

FTIR Analysis
FTIR is used to investigate the chemical composition of the sample surface, examine the different functional groups residing in the molecular structure of the compound, and identify peaks change in the mixed substances. Herein, the spectra were systematically recorded in the range of 500-4000 cm −1 for the sample analysis. Figure 5 shows the ATR-FTIR spectra of Sn-Gd2O3, and dried Sn-Gd2O3@GH nanocomposite film. In the FTIR spectrum of powder Sn-Gd2O3, the characteristic peaks at 1643, 1513, and 1379 are attributed to the -OH vibration of water and asymmetric stretching vibrations of the carbonate group which may be due to the presence of the air [62], the two prominent peaks exhibited at 3322 and 618 cm −1 that are related to the symmetric stretching vibration of -OH, and the Gd = O vibrational band of Sn-Gd2O3. Similarly, many peaks appeared in the FTIR spectra of Sn-Gd2O3@GH nanocomposite. The peaks observed at 3284 and 2930 cm −1 were attributed to the stretching of N-H functional and C-H stretching vibration of gelatin structure. The peak at 1636 cm −1 is present amid I; the peak at 1409 cm −1 is attributed to the C-H bending vibration; while peaks appearing at 1438 and 1537 cm −1 suggest C-H aliphatic vibration and asymmetric stretching vibration of carboxyl group, respectively. Similarly, the peak which appeared at 1337 is attributed to the N-H stretching vibration. Moreover, the broader peak was appeared for Gd=O, at 553 cm −1 , which is similar to published report

FTIR Analysis
FTIR is used to investigate the chemical composition of the sample surface, examine the different functional groups residing in the molecular structure of the compound, and identify peaks change in the mixed substances. Herein, the spectra were systematically recorded in the range of 500-4000 cm −1 for the sample analysis. Figure 5 shows the ATR-FTIR spectra of Sn-Gd 2 O 3 , and dried Sn-Gd 2 O 3 @GH nanocomposite film. In the FTIR spectrum of powder Sn-Gd 2 O 3 , the characteristic peaks at 1643, 1513, and 1379 are attributed to the -OH vibration of water and asymmetric stretching vibrations of the carbonate group which may be due to the presence of the air [62], the two prominent peaks exhibited at 3322 and 618 cm −1 that are related to the symmetric stretching vibration of -OH, and the Gd=O vibrational band of Sn-Gd 2 O 3 . Similarly, many peaks appeared in the FTIR spectra of Sn-Gd 2 O 3 @GH nanocomposite. The peaks observed at 3284 and 2930 cm −1 were attributed to the stretching of N-H functional and C-H stretching vibration of gelatin structure. The peak at 1636 cm −1 is present amid I; the peak at 1409 cm −1 is attributed to the C-H bending vibration; while peaks appearing at 1438 and 1537 cm −1 suggest C-H aliphatic vibration and asymmetric stretching vibration of carboxyl group, respectively. Similarly, the peak which appeared at 1337 is attributed to the N-H stretching vibration. Moreover, the broader peak was appeared for Gd=O, at 553 cm −1 , which is similar to published report [63], and confirm the presence of a Sn-Gd 2 O 3 nanomaterial in the hydrogel nanocomposite. In FTIR spectrum of powder Sn-Gd 2 O 3 , the characteristics peaks at 1643, 1513, and 1379 are attributed the stretching vibration of hydroxyl and asymmetric stretching vibrations of carbonate group, which may be due to the presence of the air.

Dyes Reduction
The catalytic activity of Sn-Gd2O3 NPs was analysed against two series of different dyes, such as azo dyes, and different NPs. For each dye, 25 mL of (0.07 mM) of each MB, MO, and CR were added into a 50 mL beaker with strong reducing agent NaBH4 under constant stirring. The reduction of the dyes was investigated by introducing 0.4 g of NaBH4, which suddenly brought a small change in the colour of the dyes. A catalyst of 0.2 gm (Sn-Gd2O3@GH) was added to the beaker promptly after adding the NaBH4. The catalytic reduction was recorded through UV-visible spectrometer. For the recording of every spectral line the dye was introduced to quartz cuvette cell of 3.5 mL. For the reduction investigation, tiny amount of the aliquot was taken out in the beaker in quartz cuvette for spectral analysis. The cuvette solution is drained off into the beaker again and continued the process until completion of the reaction. The hydrogel nanocomposite reduced the MO and CR in 16 and 4 min, respectively. No change in the spectra of MB was observed for 20 min. Similarly, different nitro phenols, such as 4-NP, 2,6-DNP, and 2-NP, were investigated following the same process as discussed above. Hence, the 4-NP, 2,6-DNP, and 2-NP nitrophenols were reduced in 16, 12, and 8 min, respectively. Therefore, the catalytic efficiency of Sn-Gd2O3@GH catalyst reduced the MO, CR, 4-

Dyes Reduction
The catalytic activity of Sn-Gd 2 O 3 NPs was analysed against two series of different dyes, such as azo dyes, and different NPs. For each dye, 25 mL of (0.07 mM) of each MB, MO, and CR were added into a 50 mL beaker with strong reducing agent NaBH 4 under constant stirring. The reduction of the dyes was investigated by introducing 0.4 g of NaBH 4 , which suddenly brought a small change in the colour of the dyes. A catalyst of 0.2 gm (Sn-Gd 2 O 3 @GH) was added to the beaker promptly after adding the NaBH 4 . The catalytic reduction was recorded through UV-visible spectrometer. For the recording of every spectral line the dye was introduced to quartz cuvette cell of 3.5 mL. For the reduction investigation, tiny amount of the aliquot was taken out in the beaker in quartz cuvette for spectral analysis. The cuvette solution is drained off into the beaker again and continued the process until completion of the reaction. The hydrogel nanocomposite reduced the MO and CR in 16 and 4 min, respectively. No change in the spectra of MB was observed for 20 min. Similarly, different nitro phenols, such as 4-NP, 2,6-DNP, and 2-NP, were investigated following the same process as discussed above. Hence, the 4-NP, 2,6-DNP, and 2-NP nitrophenols were reduced in 16, 12, and 8 min, respectively. Therefore, the catalytic efficiency of Sn-Gd 2 O 3 @GH catalyst reduced the MO, CR, 4-NP, 2,6-DNP, and 2-NP, among which the CR was reduced efficiently in 4 min. The concentration of all of the dyes was 0.07 mM, using 0.4 gm of NaBH 4, and 0.2 gm of the catalyst throughout the experiment.

Catalytic Reduction of NPs
The catalytic activity of Sn-Gd 2 O 3 @GH nanocomposite was investigated against 4-NP, 2,6-DNP, and 2-NP with of strong reducing agent (NaBH 4 ), as model reactions. UV-visible absorption spectra were recorded verses irradiation time to investigate the colour changes in the reaction phase. It was observed that the addition of 0.4 g of strong reducing agent to different nitrophenol solutions caused an immediate change in their colour. However, no decrease in the intensity at main peaks was observed. Upon 0.2g of Sn-Gd 2 O 3 @GH nanocomposite addition as a catalyst to the solutions, the NPs were reduced. For each reaction, 25 mL of dye solution (0.07 mM) was added to a 50 mL beaker, proceeding with the addition of 0.4 gm NaBH 4 and 0.2 gm catalyst under constant stirring. Figure 6a shows the catalytic reduction of 4-NP to (4-aminophenol) 4-AP with reducing agent NaBH 4 . The aqueous solution of 4-NP showed a maximum absorption peak (λ max ) at 317 nm, which red shifted to 398 nm, with the addition of NaBH 4 . This shift confirms the formation of 4-nitropnolate ions as described in some previous reports [64,65]. Due to the addition of 0.2g hydrogel catalyst, the intensity of the peak at 398 nm starts decreasing with the concomitant increase in 4-AP peak at 300 nm. Therefore, the light yellow solution of 4-NP had disappeared after 16 min. A similar procedure was followed for the 2,6-DNP and 2-NP (0.07 mM), as shown in (Figure 6b,c). The catalytic activity of Sn-Gd2O3@GH nanocomposite was investigated against 4-NP, 2,6-DNP, and 2-NP with of strong reducing agent (NaBH4), as model reactions. UVvisible absorption spectra were recorded verses irradiation time to investigate the colour changes in the reaction phase. It was observed that the addition of 0.4 g of strong reducing agent to different nitrophenol solutions caused an immediate change in their colour. However, no decrease in the intensity at main peaks was observed. Upon 0.2g of Sn-Gd2O3@GH nanocomposite addition as a catalyst to the solutions, the NPs were reduced. For each reaction, 25 mL of dye solution (0.07 mM) was added to a 50 mL beaker, proceeding with the addition of 0.4 gm NaBH4 and 0.2 gm catalyst under constant stirring. Figure 6a shows the catalytic reduction of 4-NP to (4-aminophenol) 4-AP with reducing agent NaBH4. The aqueous solution of 4-NP showed a maximum absorption peak (λ max) at 317 nm, which red shifted to 398 nm, with the addition of NaBH4. This shift confirms the formation of 4-nitropnolate ions as described in some previous reports [64,65]. Due to the addition of 0.2g hydrogel catalyst, the intensity of the peak at 398 nm starts decreasing with the concomitant increase in 4-AP peak at 300 nm. Therefore, the light yellow solution of 4-NP had disappeared after 16 min. A similar procedure was followed for the 2,6-DNP and 2-NP (0.07 mM), as shown in (Figure 6b,c). Their peaks appeared at 428 nm and 413 nm were reduced in 12 and 8 min, respectively. In these catalytic reactions, the concentration of NaBH4 was much higher and can Their peaks appeared at 428 nm and 413 nm were reduced in 12 and 8 min, respectively. In these catalytic reactions, the concentration of NaBH 4 was much higher and can be considered constant. Kinetic study was proposed to investigate the mechanism of adsorption process of the dye on the contact surface of the catalyst. To quantitatively measure the rate of these redox reactions, the UV-vis spectral data was calculated through the following pseudo-first-order kinetics.
where C 0 is the initial concentration of the dye and C t is the concentration of the dye at any given interval of time. A 0 and A t are the absorbance value take before and after introducing NaBH 4 , respectively, and catalysts to the reaction media. K app represent the apparent rate constant, while t represents the rate of the reaction. Therefore, the catalytic efficiency of Sn-Gd 2 O 3 @GH catalyst on the reduction of 4-NP, 2,6-DNP, and 2-NP can be described as pseudo first-order. The reduction rates of 4-NP, 2,6-DNP and 2-NP were reported as 1.6 × 10 −1 , 1.4 × 10 −1 , and 1.8 × 10 −1 min −1 , respectively.

Catalytic Reduction of Azo Dyes
In this approach, the reduction of different dyes, such as CR, 4-MO, and MB was also examined using NaBH 4 in the presence of Sn-Gd 2 O 3 @GH catalyst. All dyes of the same concentration of (0.07 mM), were investigated following the same process as above. Figure 7 showed that CR and MO dyes were reduced within 4 and 16 min by NaBH 4 (in presence of Sn-Gd 2 O 3 @GH) in one min interval at room temperature, except MB which was investigated for 18 min showing no progress in reduction. The corresponding rate of reduction of the CR and MO was reported as 0.842 and 0.197 min −1 , respectively. The reduction rate was linearly associated with Equation (1). Therefore, the catalytic reduction with NaBH 4 (In presence of Sn-Gd 2 O 3 @GH catalyst) was reported efficient for CR dye. be considered constant. Kinetic study was proposed to investigate the mechanism of adsorption process of the dye on the contact surface of the catalyst. To quantitatively measure the rate of these redox reactions, the UV-vis spectral data was calculated through the following pseudo-first-order kinetics.
where C0 is the initial concentration of the dye and Ct is the concentration of the dye at any given interval of time. A0 and At are the absorbance value take before and after introducing NaBH4, respectively, and catalysts to the reaction media. Kapp represent the apparent rate constant, while t represents the rate of the reaction. Therefore, the catalytic efficiency of Sn-Gd2O3@GH catalyst on the reduction of 4-NP, 2,6-DNP, and 2-NP can be described as pseudo first-order. The reduction rates of 4-NP, 2,6-DNP and 2-NP were reported as 1.6 × 10 −1 , 1.4 × 10 −1 , and 1.8 × 10 −1 min −1 , respectively.

Catalytic Reduction of Azo Dyes
In this approach, the reduction of different dyes, such as CR, 4-MO, and MB was also examined using NaBH4 in the presence of Sn-Gd2O3@GH catalyst. All dyes of the same concentration of (0.07 mM), were investigated following the same process as above. Figure  7 showed that CR and MO dyes were reduced within 4 and 16 min by NaBH4 (in presence of Sn-Gd2O3@GH) in one min interval at room temperature, except MB which was investigated for 18 min showing no progress in reduction. The corresponding rate of reduction of the CR and MO was reported as 0.842 and 0.197 min −1 , respectively. The reduction rate was linearly associated with Equation (1). Therefore, the catalytic reduction with NaBH4 (In presence of Sn-Gd2O3@GH catalyst) was reported efficient for CR dye.

Catalytic Reduction Changing Experimental Parameter
The industrial development causes a serious threat to human beings owing to the release of huge number of pollutants, especially dyes, to direct stream flow. These dyes' molecules contain double bond azo group (-N=N-) in their skeleton of chemical structure [66,67]. Synthetic dyes are used for various purposes, mainly in the cosmetic, food, pulp, paper, and textile industries. In particular, CR and MO are both anionic azo dyes, which are used as indicators as well as in the food, textile, paper and pulp, cotton, silk, plastic, cosmetic, pharmaceutical, rubber, printing, and wool industries [68]. Despite the useful applications of CR, it also causes eye and skin irritation, gastro-intestinal irritation, and vomiting, while its long-term subjection to the body causes liver tumours in women [69]. CR dye is also used as a biological stain in medicine for diagnosis of amyloidosis and an indicator in acidic media [70,71].
Therefore, the catalytic reduction of CR dye was carried out with strong reducing agent (NaBH 4 ) and Sn-Gd 2 O 3 @GH, with Sn-Gd 2 O 3 @GH nanocomposite as the catalyst. For the spectral reduction of CR dye in the presence of NaBH 4 /Sn-Gd 2 O 3 @GH, the UV-visible spectrometer was found and presented in Figure 8a.
The UV-Vis spectrum of reduction of CR was recorded for 4 min in the presence of NaBH 4 , using 0.2 gm of Sn-Gd 2 O 3 @GH catalyst (Figure 8a). With the introduction of NaBH 4 to the aqueous medium of CR, the absorption peak at 495 nm was reduced to absorption band (λ max ) at 475 nm.

Catalytic Reduction Changing Experimental Parameter
The industrial development causes a serious threat to human beings owing to the release of huge number of pollutants, especially dyes, to direct stream flow. These dyes' molecules contain double bond azo group (-N = N-) in their skeleton of chemical structure [66,67]. Synthetic dyes are used for various purposes, mainly in the cosmetic, food, pulp, paper, and textile industries. In particular, CR and MO are both anionic azo dyes, which are used as indicators as well as in the food, textile, paper and pulp, cotton, silk, plastic, cosmetic, pharmaceutical, rubber, printing, and wool industries [68]. Despite the useful applications of CR, it also causes eye and skin irritation, gastro-intestinal irritation, and vomiting, while its long-term subjection to the body causes liver tumours in women [69]. CR dye is also used as a biological stain in medicine for diagnosis of amyloidosis and an indicator in acidic media [70,71].
Therefore, the catalytic reduction of CR dye was carried out with strong reducing agent (NaBH4) and Sn-Gd2O3@GH, with Sn-Gd2O3@GH nanocomposite as the catalyst. For the spectral reduction of CR dye in the presence of NaBH4/Sn-Gd2O3@GH, the UV-visible spectrometer was found and presented in Figure 8a. In the reaction mixture of CR+NaBH 4 , the concentration of NaBH 4 was kept higher, which resulted in the increase in the pH of the reaction mixture. The addition of hydrogel nanocomposite (Sn-Gd 2 O 3 @GH catalyst) helps in catalytic reduction by transferring electrons from electron donor species BH 4 − to electron acceptor species CR. The catalyst decreases the activation energy and stabilize the reaction mixture [72]. As the reaction mixture is independent on the concentration of the NaBH 4 , and hence, for the catalytic reduction of the present mixture a pseudo first order kinetics could be used. Therefore, the linear relationship between ln(C t /C 0 ) (where A 0 is absorbance of the initial concentration, A t is the absorbance at the specific reaction time) vs. time confirmed that the reaction is of the pseudo-first order. Further, the efficiency of Sn-Gd 2 O 3 @GH was further optimized and analysed against different catalyst amounts (Sn-Gd 2 O 3 @GH), NaBH 4 amounts, and CR concentrations.
The change in reduction rate was further analysed, changing 0.1, 0.2, and 0.4 gm of Sn-Gd 2 O 3 @GH catalyst against CR, in the presence of 0.4 gm NaBH 4 . The UV-vis absorbance was recorded in 1 min intervals. For each run, 25.0 mL (0.07 mM) CR was introduced into the beaker, followed by the addition of 0.4 gm NaBH 4 , adding 0.1, 0.2, or 0.4 gm catalyst, respectively. That widely records the decrease in reduction time of the dye by increasing catalyst amount. The reduction of the CR was complete in 4, 3, and 2 min with the reduction rate of 0.915, 0.999, and 1.96 min −1 , respectively. This clearly demonstrates that the reduction time decreases with the increase in the catalyst amount, as 0.4g of Sn-Gd 2 O 3 @GH reduced the CR in 2 min with reduction rate of 1.96 min −1 , as shown in Figure 8b. To determine the catalyst efficiency against different concentrations of the dye, three different concentrations, 0.07, 0.05, and 0.03 mM of the CR dye were prepared. Each concentration of 25.0 mL was consecutively introduced into a 50.0 mL beaker using 0.4 gm of NaBH 4 and 0.2g of the prepared catalyst. The CR was reduced in 3, 3, and 2 min with a corresponding rate of 0.875, 0.656, and 1.14 min −1 , respectively. The 0.03 mM was reduced with high reduction rate of 1.14 min −1 as shown in Figure 8c. Similarly, Figure 8d represents the effect of NaBH 4 for the reduction of 0.07 mM CR. Herein, three different weights of 0.2, 0.4, and 0.8 gm of NaBH 4 were added to 25 mL of the (0.07 mM) dye using 0.2 gm of the catalyst. By the transfer of the above NaBH 4 amount to the beaker in the presence of catalyst, the dye was reduced in 4, 3, and 3 min with reduction rate of 0.455, 0.875, and 1.42 min −1 , respectively. Again, the higher reduction rate of 1.42 min −1 confirmed the influence of NaBH 4 on the catalytic efficiency of Sn-Gd 2 O 3 @GH nanocomposite. The apparent rate constant (K app ) of the pseudo order kinetic for all changing parameters is shown in (Figure 8b-d) between ln(C t /C 0 ) and time, which were fitted with the above linear Equation (1). Similarly, the % reduction in the dyes was calculated using the given Equation (2).
where A 0 and A t are corresponding to the initial absorbance of the dye and absorbance at time 't', respectively.

Mechanism of Reduction
CR is a di-azo dye widely used in different industries. Besides its importance, it is also hazardous to nature. Therefore, from an environmental point of view, its reduction is absolutely paramount. Its chemical structure is composed of two phenyl groups which are connected by two diazo binding to the two naphthalene on the both side of the biphenyl group [73]. Reduction of CR molecules cannot be carried out by NaBH 4 because this reaction is not feasible kinetically. Therefore, here, the reduction of CR is carried out by Sn-Gd 2 O 3 @GH nanocomposite with NaBH 4 . Figure 9 shows the reduction of CR, which is converted into its corresponding constituents. Before introducing the catalyst to the reaction media, a high concentration of NaBH 4 was employed to the aqueous solution of CR, which raised the pH of the solution and changed its colour from dark red to lighter red. The NaBH 4 gives BH 4 and acts as an electron donor. By introducing the Sn-Gd 2 O 3 @GH catalyst, the BH 4 and dye molecules adsorb on the surface of the catalyst. The surface of the nanomaterial acts as mediator for transferring electrons from borohydride ions (BH 4 -) to CR molecules. The catalyst provides an efficient pathway to transfer electrons to CR molecules and reduced it in a small amount of time. Finally, the CR molecule was broken down into 1,1'-bipheny, sodium(4-amino-naphthalene)-1-sulfonate, and nitrogen.
by Sn-Gd2O3@GH nanocomposite with NaBH4. Figure 9 shows the reduction of CR, which is converted into its corresponding constituents. Before introducing the catalyst to the reaction media, a high concentration of NaBH4 was employed to the aqueous solution of CR, which raised the pH of the solution and changed its colour from dark red to lighter red. The NaBH4 gives BH4 -and acts as an electron donor. By introducing the Sn-Gd2O3@GH catalyst, the BH4 -and dye molecules adsorb on the surface of the catalyst. The surface of the nanomaterial acts as mediator for transferring electrons from borohydride ions (BH4 -) to CR molecules. The catalyst provides an efficient pathway to transfer electrons to CR molecules and reduced it in a small amount of time. Finally, the CR molecule was broken down into 1,1'-bipheny, sodium(4-amino-naphthalene)-1-sulfonate, and nitrogen. Figure 9. Scheme for the reduction of the CR by NaBH4 using Sn-Gd2O3@GH hydrogel catalyst.

Reusability of Catalyst
Here, the important factor of the efficient catalyst is its recyclability property [74]. The recyclability determines the durability and performance of the catalyst. Here, the Sn-Gd2O3@GH catalyst was checked for three cycles after successful completion of the reaction. Figure 10 exhibits the C/C0 (%) and kapp of three catalytic reduction reactions of CR dye. The catalyst was easily recovered from the reaction mixture simply by filtration and then washed with distilled water and reused again. Figure 9. Scheme for the reduction of the CR by NaBH4 using Sn-Gd 2 O 3 @GH hydrogel catalyst.

Reusability of Catalyst
Here, the important factor of the efficient catalyst is its recyclability property [74]. The recyclability determines the durability and performance of the catalyst. Here, the Sn-Gd 2 O 3 @GH catalyst was checked for three cycles after successful completion of the reaction. Figure 10 exhibits the C/C 0 (%) and k app of three catalytic reduction reactions of CR dye. The catalyst was easily recovered from the reaction mixture simply by filtration and then washed with distilled water and reused again.
It is clearly observed form Figure 10 that CR was reduced by Sn-Gd 2 O 3 @GH with sodium borohydrate in 4, 5, and 5 min, respectively, up to 97.6, 89.7, and 85.8 %, with catalytic rate of 9.15 × 10 −1 , 5.67 × 10 −1 , and 3.98 × 10 −1 min −1 , respectively. The concentration of NaBH 4 was kept constant throughout the recyclability. For each cycle, the recovered amount of Sn-Gd 2 O 3 @GH hydrogel catalyst was used against 0.7 mM solution of CR. A slight decrease was exhibited in the activity of the catalyst with reusability. It is clearly observed form Figure 10 that CR was reduced by Sn-Gd2O3@GH with sodium borohydrate in 4, 5, and 5 min, respectively, up to 97.6, 89.7, and 85.8 %, with catalytic rate of 9.15 × 10 −1 , 5.67 × 10 −1 , and 3.98 × 10 −1 min −1 , respectively. The concentration of NaBH4 was kept constant throughout the recyclability. For each cycle, the recovered amount of Sn-Gd2O3@GH hydrogel catalyst was used against 0.7 mM solution of CR. A slight decrease was exhibited in the activity of the catalyst with reusability.

Conclusions
Sn doped Gd2O3 nanomaterial was successfully prepared. The nanomaterial was simply prepared by mixing of 0.1 M solution of Gd[NO3]3.6H2O and SnCl4 at PH > 10. The solution was heated in a Teflon-lined autoclave at 150 °C to obtain the white precipitate of the sample. Then, 0.4 % by weight of the white precipitate of Sn-Gd2O3 was treated with 20 mL of gelatin solution and cross-linked with formaldehyde, and finally Sn-Gd2O3@GH nanocomposite was synthesized. The hydrogel nanocomposite was further used as a catalyst against different dyes, such as 4-NP, 2,6-DNP, 2-NP, MB, MO, and CR, in the presence of strong reducing agent NaBH4. The Sn-Gd2O3@GH catalyst reduced the CR efficiently with respect to other dyes, with a 9.15 × 10 −1 min −1 rate of reaction. The XRD pattern confirms the tetragonal rutile phase of the crystal. The SEM micrographs evaluate the average crystalline size of Sn-Gd2O3nanomaterial, which is calculated as 30.06 nm, while EDS peaks confirm the incorporated material in the hydrogel nanocomposite. Therefore, the present approach leads to the fabrication of a new hydrogel nanocomposite for the removal of water pollutants, which is efficient for the reduction activity of dyes and can be easily recycled and reused.

Conclusions
Sn doped Gd 2 O 3 nanomaterial was successfully prepared. The nanomaterial was simply prepared by mixing of 0.1 M solution of Gd[NO 3 ] 3 ·6H 2 O and SnCl 4 at PH > 10. The solution was heated in a Teflon-lined autoclave at 150 • C to obtain the white precipitate of the sample. Then, 0.4 % by weight of the white precipitate of Sn-Gd 2 O 3 was treated with 20 mL of gelatin solution and cross-linked with formaldehyde, and finally Sn-Gd 2 O 3 @GH nanocomposite was synthesized. The hydrogel nanocomposite was further used as a catalyst against different dyes, such as 4-NP, 2,6-DNP, 2-NP, MB, MO, and CR, in the presence of strong reducing agent NaBH 4 . The Sn-Gd 2 O 3 @GH catalyst reduced the CR efficiently with respect to other dyes, with a 9.15 × 10 −1 min −1 rate of reaction. The XRD pattern confirms the tetragonal rutile phase of the crystal. The SEM micrographs evaluate the average crystalline size of Sn-Gd 2 O 3 nanomaterial, which is calculated as 30.06 nm, while EDS peaks confirm the incorporated material in the hydrogel nanocomposite. Therefore, the present approach leads to the fabrication of a new hydrogel nanocomposite for the removal of water pollutants, which is efficient for the reduction activity of dyes and can be easily recycled and reused.  Data Availability Statement: Data will be available upon reasonable request.