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

Materials Preparation

Natural zeolite (clinoptilolite) sourced from PT. Paragon Perdana Mining served as the support for the synthesis of

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 ^\circ∘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 {C_0}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).

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

Similarly, in our result, the similarity in the XRD pattern between the pristine and

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 (

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 ^\circ∘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 ^\circ∘C to 800 ^\circ∘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 ^\circ∘C, 200 to 400 ^\circ∘C. Notably, the final step is markedly pronounced relative to the parent zeolite. As illustrated in Figure 1(d), the mass loss of NbX^\circ∘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 ^\circ∘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

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