Hydrothermal synthesis of Nb₂O₅ - natural zeolite composite for enhanced adsorptive removal of anionic and cationic dye

2.1 Materials Preparation

Natural zeolite (clinoptilolite) sourced from PT. Paragon Perdana Mining served as the support for the synthesis of Nb₂O₅-containing adsorbent with varying niobium loading levels. Ammonium niobium oxalate (C₄H₄NNbO₉·xH₂O, Sigma-Aldrich, ANO) was employed as the niobium precursor. The materials were prepared by utilizing a facile hydrothermal method, with ANO loading of 5, 15, and 30 wt.%. Initially, 2 g of natural zeolite (NZ) was dispersed in 80 mL of distilled water and stirred for 15 min. This mixture was then subjected to ultrasonic dispersion for an additional 30 min to form a stable zeolite suspension. Following this, a specific amount of ANO was added to the zeolite suspension under vigorous stirring, and ultrasonic dispersion was maintained for the next 15 min. The resulting mixture was subsequently transferred to a Teflon-lined autoclave reactor and heated at 175 °C for 72 h. After the hydrothermal treatment, the obtained solid was collected using vacuum filtration and rinsed thrice with distilled water. The rinsed material was labeled as NbX_NZ, where X denotes the amount of ANO loaded, and NZ represents the natural zeolite.

2.2 Adsorption Study

The as-synthesized NbX_NZ was employed for the adsorption of both anionic (Congo red) and cationic (Methylene blue). The adsorption experiment was conducted in 250 mL conical flasks, each containing 100 mL of dye solution with a concentration of 50 mg/L and 0.05 g of the adsorbent. The flasks were agitated at 400 rpm at room temperature (25 – 30 °C). The effect of contact time was investigated by collecting samples at regular intervals (0 – 120 min) and analyzing the residual dye concentration using UV-Vis spectrophotometry. The adsorption capacity at equilibrium, qe (mg/g), was calculated using the following Eq. (1).

Where C₀ and Ce are the initial and equilibrium concentrations of dye (mg/L), respectively, V is the volume of the solution (L), and m is the mass of the adsorbent (g).

3.1 Materials Characterization

The X-ray diffraction (XRD) analysis was employed to scrutinize the crystalline structure and phase composition of both the NZ and NbX_NZ. Figure 1(a) shows the XRD pattern of these materials, where the characteristic peaks of clinoptilolite were identified and confirmed by JCPDS card number 47-1870. Interestingly, the XRD pattern of Nb₂O₅-supported zeolite demonstrated a resemblance to that of the pristine natural zeolite, which underscores the retention of the zeolite framework after niobium modification. This observation is in alignment with the findings of Ferreira et al., who reported analogous results when preparing Nb2O5-supported Y zeolite using the impregnation method 10. They disclose that the preservation of the faujasite structure after Nb impregnation was confirmed by the comparable XRD pattern of the parent NaY zeolite and the Nb-modified samples.

Similarly, in our result, the similarity in the XRD pattern between the pristine and Nb₂O₅-modified zeolite substantiates the successful incorporation of Nb₂O₅ within the zeolite framework without altering its intrinsic structure. Moreover, no additional peaks corresponding to Nb₂O₅ were discerned in the XRD pattern of Nb₂O₅-supported zeolite, which might suggest a well-dispersed state of Nb₂O₅ within the zeolite framework or potentially amorphous nature of Nb₂O₅ post-synthesis. Nb₂O₅ can exhibit several distinct phases based on various factors such as temperature and preparation methods. Based on our past studies on the preparation of Nb₂O₅ over a hydrothermal method using ANO precursor, it was reported that the obtained Nb₂O₅ exhibited an orthorhombic phase with a deformed structure16. However, the absence of crystalline structure of Nb₂O₅ in this study, as indicated by the XRD pattern, could be attributed to several factors. During the hydrothermal synthesis, the Nb₂O₅-support interaction can reduce the mobility of the niobium oxide species, making them more resistant to aggregation into crystalline phases. Moreover, this interaction can hinder the crystallization of Nb₂O₅, thereby preventing the formation of distinct crystalline phases 10.

The FTIR spectra of the prepared materials are presented in Figure 1(a). The acquired spectra both NZ and NbX_NZ, elucidate distinctive peaks at wavenumbers 3606, 3368, 1626, and 1017 cm^-1. The peaks observed at 3606 cm^-1 and 3368 cm^-1 are typically associated with the stretching vibration of hydroxyl (OH) groups 12. These peaks could signify the presence of hydroxyl groups either internally within the zeolite framework or on its surface, affirming the hydrophilic nature of zeolite material. The peak at 1626 cm^-1 is suggestive of bending vibration of molecular water (H₂O) entrapped within the zeolite pores or hydroxyl groups. While the most prominent peaks observed at 1017 cm^-1 is indicative of the asymmetric stretching vibration of Si-O or Al-O tetrahedral units within the zeolite framework 8 of the zeolite structure. Upon modification with niobium, a notable new peak emerged at 1444 cm^-1, which become more pronounced with increasing Nb loading. As inferred in our preceding work, the emergence of this peaks is postulated to signify the presence of ammonium ions within the pore structure of Nb₂O₅16. Given the preparation of Nb₂O₅ from ammonium niobium oxalate precursor, it’s plausible that the ammonium ions were retained within Nb₂O₅ post-synthesis.

The thermogravimetric (TGA) curve of the NbX_NZ and the zeolite support is presented in Figure 1(c). For NZ, it revealed a two-step decomposition pattern. A significant mass loss was observed from room temperature extending up to 400 °C, which could be attributed to the release of the physically adsorbed water, removal of hydroxyl groups, or the decomposition of any volatiles organic compound present 10. Following this phase, a slower and more gradual mass loss was noted from 400 °C to 800 °C, possibly indicating the dehydroxylation process. Interestingly, the TGA curve of NbX_NZ unveils a more complex decomposition behavior in contrast to the pristine zeolite, manifesting three principal decomposition steps. These steps across the temperature ranges of room temperature to 200 °C, 200 to 400 °C. Notably, the final step is markedly pronounced relative to the parent zeolite. As illustrated in Figure 1(d), the mass loss of NbX°C. Notably, the final step is markedly pronounced relative to the parent zeolite. As illustrated in Figure [FIGREF:1](d), the mass loss of NbX_NZ from 400 to 800 °C significantly surpass that of the parent natural zeolite. The calculated mass loss for NZ, Nb5_NZ, Nb15_NZ, and Nb30_NZ within this decomposition temperature regime amounts to 1.72%, 1.97%, 3.00%, and 2.91% respectively, showcasing the influential role of niobium modification on the thermal decomposition characteristics of the zeolite. These findings further substantiate the successful incorporation of Nb₂O₅ onto the zeolite structure, a conclusion that align with the observation made in other studies.

Figure 2(a) – (c) displays the FESEM images of the niobium-doped natural zeolite with different ANO weight percentages prior to the hydrothermal process. All niobium-doped natural zeolite samples exhibit characteristics of particle agglomeration, revealing an irregular and somewhat clumped morphology, showing a common natural zeolite morphology. This appearance is accentuated by the rough texture of the particle surfaces, dotted with smaller crystallites potentially resulting from the doping process. The FESEM images are presented alongside the EDX elemental mapping of niobium (Nb), aluminum (Al), and silicon (Si) atoms. A significant amount of Al and Si atoms were observed for all samples (i.e., Nb5_NZ, Nb15_NZ, Nb30_NZ). Interestingly, the Nb elemental mapping shows an increasing Nb atomic density, implying higher Nb percentages in the Nb30_NZ sample compared to the other samples (Nb5_NZ and Nb15_NZ). These results are corroborated by the weight percentage (wt.%) elemental analysis of the samples depicted in Figure 2(d). It demonstrates that the Nb weight percentage increased from 0.33 wt.% to 1.43 wt.% from the Nb5_NZ to Nb30_NZ samples, confirming the increase in Nb composition on the nanocomposites with increasing ANO weight percentage prior to the hydrothermal process.

The adsorption capacity of the NZ and NbX_NZ for CR dye adsorption is depicted in Figure 3(a), calculated using Eq. (1), based on the changes in UV-Vis absorption intensity at 498 nm after 150 minutes of reaction time. The Nb2O5 sample was used as a control and exhibited very little adsorption capacity (below 5 mg/g) for CR dye. The NZ sample displayed an adsorption capacity of up to 40 mg/g, which increased to 77 mg/g after adding 15wt.% of ANO (Nb15_NZ have higher CR adsorption capabilities compared to the NZ samples. Moreover, Figures 3(b) and 3(c) depict the adsorption capacity over time of the control _NZ have higher CR adsorption capabilities compared to the NZ samples. Moreover, Figures [FIGREF:3](b) and [FIGREF:3](c) depict the adsorption capacity over time of the control Nb₂O₅, unmodified natural zeolite, and niobium-doped natural zeolite against CR and methylene blue (MB) dyes, respectively. The addition of niobium significantly increased the adsorption capacity of the NZ sample for CR dyes, while no substantial increase in adsorption capacity for MB dyes was observed.