Characterization

The X-ray diffraction (XRD) patterns of the samples were recorded using a Bruker AXS D8 Advance X-ray powder diffractometer (Karlsruhe, Germany) with CuKα radiation from 2 to 80°2θ at a scanning rate of 2°/min. Scanning electron micrographs (SEM) and energy-dispersive X-ray spectroscopy (EDS) were obtained using a Thermo Scientific Prisma E scanning electron microscope (Waltham, Massachusetts, USA). Samples were sputter-coated with either gold (Au) or carbon (C). Cr in the leaching test was measured by atomic absorption mass spectrometry (Analytik Jena GmbH—novAA 400 P, Jena, Germany) using a graphite furnace with a detection range of 10–50 ųg/L. The calibration solution was Cr/Mg/HNO₃. Fourier-transform infrared (FTIR) spectra were recorded between 4000 and 500 cm^–1 using a JASCO 4100 spectrometer (Jasco, Japan)(). Ultraviolet–Visible absorbance (UV–Vis) spectra were recorded on a Specord 210 Analytik Jena spectrometer with a holmium oxide filter (Jena, Germany).

Preparation of Catalyst

Prior to the intercalation process, the bentonite was first saturated with Na⁺ using a NaCl solution (Arbaoui & Boucherit, Reference Arbaoui and Boucherit2014; Korichi et al., Reference Korichi, Elias and Mefti2009), which was designed to promote the ion exchange of Cr(III). The saturation process was carried out as follows: 5 g of bentonite was dispersed in deionized water with a minimum resistivity of 18.2 megohm cm and stirred for 30 min at 30°C, then a previously prepared NaCl solution (1 M, 1000 mL) was added to the prepared mixture with vigorous stirring; the new resulting mixture was stirred for 48 h at 30°C, then filtered and washed with deionized water until free of unreacted NaCl. Finally, the Na-saturated solid obtained was dried at 70°C for 72 h.

The dispersion of Cr(III) cations in the interlayer spaces of the Mnt was carried out using an ion-exchange process, and the protocol was as follows: 1.4 g of bentonite was dispersed in deionized water, then the resulting mixture was stirred for 2 h at room temperature (solution A). Cr(III) nitrate solution (100 mL, 0.1 M) was prepared by adjusting the pH to avoid precipitation, and then the solution was added to solution A. After that, the Cr-bentonite material obtained was recovered by filtration, washed thoroughly with distilled water to remove excess unreacted Cr(III) cations, and dried at 70°C for 72 h. The amount of Cr-bentonite material was treated with NaBH₄ solution (100 mL, 4 M). The mixture was stirred for 2 h; the final Cr₂O₃NPs-bentonite product was filtered, washed, and dried overnight.

Catalytic Application

Bentonite, Cr-bentonite, and Cr₂O₃NPs-bentonite were evaluated first in order to choose the best catalyst from among them. The catalytic reduction of CR dye in the presence of NaBH₄ was carried out under the following conditions: 0.034 g of each catalyst, 0.05 M NaBH₄, and 0.06 mM of CR, no solution pH adjustment was made, and the reduction of CR dye was measured in situ by UV–Vis spectroscopy. Firstly, 2.5 mL of CR dye solution was added to 1.5 mL of freshly prepared NaBH₄. This solution was transferred to a quartz cuvette to which an amount of catalyst was also added. This system was then loaded into the UV–Vis spectrophotometer and the program was set to scan the sample every 30 s. Afterward, two parameters that affected the catalytic reduction, namely the dose of the catalyst (0.014–0.034 g) and the initial dye concentration (0.06–0.12 mM) were assessed. An adsorption test (dye + catalyst) and a test without catalyst (NaBH₄ + dye) were also realized. The kinetics of catalytic dye reduction was investigated by applying the pseudo-first order rate law (Eq. 1 and 2). The percentage degradation of CR dye was also established by using Eq. 3.

(1) $\frac{d{A}_t}{\mathrm{dt}}={-k_{\mathrm{app}}A}_t$

(2) $\ln \left(\frac{A_t}{A_0}\right)=-k_{\mathrm{app}}t$

where k_app (s⁻¹) is the rate constant, A _t is the absorbance at time t, and A ₀ is the initial absorbance of the CR dye.

(3) $\mathrm{Percentage}\mathrm{of}\mathrm{conversion}\mathrm{of}\mathrm{CR}(\%)=\frac{A_0-A_t}{A_0}\times 100$

Statistical Analysis

The residual sum of squares (RSS, Eq. 4) measures the accuracy of a regression model. The smaller the residual sum of squares, the better the model fits the data; the greater the residual sum of squares, the less well the model fits the data (Marquardt, Reference Marquardt1963).

(4) $\mathrm{RSS}=\sum _{i=1}^n{{(y}_i-{\mathrm{fx}}_i)}^2$

XRD

The structural properties of all prepared samples were examined by XRD analysis. The XRD patterns of bentonite, Cr-bentonite, and Cr₂O₃NPs-bentonite samples (Fig. 2) clearly showed a crystalline structure similar to that of pure Mnt, as described in previous studies, which was confirmed by the presence of reflections (001), (100), (110), and (210) corresponding to Mnt (Cheng et al., Reference Cheng, Zhang, Xie, Chen and Li2013). The XRD pattern exhibited the characteristics (001) peak of Mnt at 6.437°2θ, d ₀₀₁ = 1.372 nm. In addition, the diffraction peaks related to quartz, illite, calcite, and K-feldspar impurities were also observed. After the ion-exchange process, the (001) reflection shifted to a lower angle, giving a d-spacing of 1.423 nm, which was attributed to the species with a smaller ionic radius being replaced by the species with a larger ionic radius, indicating successful intercalation of Cr(III) cations in the interlayer spaces of montmorillonite. After the chemical treatment using NaBH₄ the d-spacing was then increased to 1.538 nm. This was probably due to the agglomeration of chromium oxide species on the surface of the bentonite. The characteristic peaks for chromium oxide (at 24.73, 32.62, 35.30, 39.74, 41.64, 45.35, 50.28, 54.89, 59.16, and 73.27°2θ, corresponding to the (012), (104), (110), (113), (202), (024), (116), (214), (300), and (101) planes (Almontasser & Parveen, Reference Almontasser and Parveen2020; Khan et al., Reference Khan, Shahid, Hanif, Almoallim, Alharbi and Sellami2021), respectively, were observed with low intensities, indicating that the Cr₂O₃NPs were highly dispersed on the bentonite support. This result implies the formation of the rhombohedral structure of chromium in the interlayer spaces of bentonite. By using the Scherrer equation (Eq. 5) (Patterson, Reference Patterson1939), the average particle diameter was calculated and found to be between 35 and 40 nm.

(5) $\mathrm{The}\mathrm{mean}\mathrm{size}\mathrm{of}\mathrm{particles}\left(\tau \right)=\frac{K\lambda }{\beta \cos \theta }$

where τ is the crystallite size (nm); K = 0.9, the shape factor; λ (= 0.154060 nm), the wavelength of Cu-Kα radiation; β, the integral breadth of the most intense peak (FWHM); and θ, the diffraction angle.

Fig. 2 XRD patterns of bentonite, Cr-bentonite, and Cr₂O₃NPs-bentonite samples

SEM and EDS

SEM–EDS analysis was used to study the surface morphology and the elemental constituents of the samples. The SEM images of Cr-bentonite and Cr₂O₃NPs-bentonite samples (Figs. 3a,b and 4a,b, respectively) confirmed the conservation of the structure of the Mnt even after the ion-exchange process and the chemical treatment with NaBH₄. All micrographs showed the same morphology, which corresponds to the presence of aggregates of ancillary minerals in the bentonite (supplementary information Fig. S1); these are heterogeneous in size and have irregularly shaped cavities, similar results to those found by Cheng et al. (Reference Cheng, Zhang, Xie, Chen and Li2013).

Fig. 3 SEM–EDS analysis of Cr-bentonite: a-b SEM images, c EDS spectrum, and d SEM–EDS element mapping images

Fig.4 SEM–EDS analysis of Cr₂O₃NPs-bentonite: a-b SEM images c EDS spectrum, and d SEM–EDS element mapping images

The EDS spectra of Cr-bentonite and Cr₂O₃NPs-bentonite (Figs. 3c and 4c, respectively) revealed brighter areas, indicating the presence of Cr (Ngah et al., Reference Ngah, Teong, Toh and Hanafiah2012). The brighter areas are because of the better electric conduction properties of chromium compared to the Mnt itself. O, Na, Al, and Si were found in both Cr-bentonite and Cr₂O₃NPs-bentonite, which is not surprising because these are the major components of bentonite. The weight percentages of these elements in Cr-bentonite were O (46.98%), Na (0.69%), Al (10.88%), Si (37.09%), and Cr (4.36%) (Fig. 3, embedded table). For the Cr₂O₃NPs-bentonite, the respective weight percentages were O (43.53%), Na (7.26%), Al (9.05%), Si (33.76%), and Cr (6.40%) (Fig. 4). The increase in Na weight was due to chemical treatment with NaBH₄, and the increase in Cr was due to the transformation of Cr(III) to chromium oxide nanoparticles which were dispersed homogeneously on the bentonite surfaces. In addition, the peaks situated at binding energies of 0.5, 5.4, and 5.9 eV indicated the existence of diverse chromium species. Following the element map scan (Figs. 3d and 4d), Cr was clearly intercalated homogeneously on the bentonite. EDS line scan analysis (Fig. 5) was carried out in order to examine the elemental distribution on the Cr-bentonite surfaces; it was distributed homogeneously. That sample thus provided a well dispersed Cr oxide system with a highly reactive catalytic surface. Cr in the Cr₂O₃NPs-bentonite (Fig. 6) was, therefore, uniformly and homogeneously distributed across the whole sample surface and produced a catalytic surface with uniform, active sites.

Fig. 5 SEM–EDS analysis of Cr-bentonite: a-b line scans of elements and c distribution of Cr

Fig. 6 SEM image and corresponding EDS line scan along the yellow line of the Cr₂O₃NPs-bentonite surface

UV–Vis DR

UV–Vis DR spectroscopy was used to identify the chemical environment and coordination states (Fig. 7) and a large characteristic band in the region of 200–365 nm was revealed and attributed to Al–O and Si–O bonding in the Mnt layers. UV–Vis spectra of the modified Cr-bentonite and Cr₂O₃NPs-bentonite samples indicated the presence of two absorption peaks at 413 and 596 nm, which can be attributed to the presence of several Cr species. In addition, the peak intensity at 314 nm of Cr₂O₃NPs-bentonite was much greater than for the Cr-bentonite. This was due to the transformation of Cr ions to chromium oxide nanoparticles (Ahmad et al., Reference Ahmad, Shamim, Mahmood, Mahmood and Khan2018). In addition, deconvolution of the UV–Vis spectra of the Cr₂O₃NPs-bentonite (Fig. S2) confirmed the presence of the ⁴A_2g → ⁴T_1g transition of the six-coordinate geometry at 413 nm and the ⁴A_2g → ⁴T_2g transition of chromium ions in an octahedral environment at 596 nm (Liang et al., Reference Liang, Zhang, Luo, Luo, Li, Xu and Zhang2014, Reference Liang, Zhang, Luo, Liu, Bai, Xu and Zhang2015).

Fig. 7 UV–Vis DR spectra of bentonite, Cr-bentonite, and Cr₂O₃NPs-bentonite samples

The CR dye also was characterized by UV–Vis spectroscopy and gave characteristic absorption bands of the conjugation throughout the structure and of the azo bond at 492 and 340 nm. Possible reduction products include amino derivatives which have absorption peaks around 284 nm.

FTIR

The FTIR spectrum of bentonite (Fig. 8) is very similar to that of Mnt, as would be expected from the large proportion of Mnt in bentonite (Hayati‐Ashtiani, Reference Hayati-Ashtiani2011; Ikhtiyarova et al., Reference Ikhtiyarova, Özcan, Gök and Özcan2012). The vibration band observed at 3622 cm^–1 was attributed to the Al–OH stretching of smectites with a large amount of Al in the octahedral sheet. The broad band observed at 3378 cm^–1 was attributed to intercalated water molecules linked through hydrogen bonds to the terminal silanol groups. The band at 1400 cm^–1 was attributed to the stretching and bending vibrations of physisorbed water (Hayati‐Ashtiani, Reference Hayati-Ashtiani2011).The band at 985 cm^–1 was attributed to the Si−O stretching vibrations (Madejová & Komadel, Reference Madejová and Komadel2001); the band at 610 cm⁻¹ can be attributed to the bending vibration of the Al−O−Si groups (Aroke et al., Reference Aroke, Abdulkarim and Ogubunka2013; Madejová & Komadel, Reference Madejová and Komadel2001). The modified materials, Cr-bentonite and Cr₂O₃NPs-bentonite, displayed FTIR spectra that differed in terms of the intensity of the main band as well as the presence of new bands in the latter. The FTIR spectrum of Cr-bentonite showed a decrease in the intensity of the band at 1600 cm^–1 attributed to the intercalated and physisorbed water molecules, which suggests that the intercalation takes place through an ion-exchange reaction between the Cr(III) ions and the interlayer hydrated sodium cations, thus causing the dehydration of the material obtained after intercalation. The FTIR spectrum of Cr₂O₃NPs-bentonite contained two new bands at 553 and 600 cm^–1 which were attributed to M−O (Cr−O) stretching (Madi et al., Reference Madi, Tabbal, Christidis, Isber, Nsouli and Zahraman2007).

Fig. 8 FTIR spectra of bentonite, Cr-bentonite, and Cr₂O₃NPs-bentonite samples