Congo red dye adsorption using ZnAl layered double hydroxide fabricated using hydrothermal methods

Materials

All precursor materials and chemicals including Zinc Nitrate Hexahydrate (

Preparation of ZnAl Layered Double Hydroxide

The ZnAl layered double hydroxides were synthesized via a hydrothermal method similar to previous literature 17. A precursor solution was prepared by dissolving 5.95 grams of

Materials characterizations

The synthesized materials were first characterized by Scanning Electron Microscopy equipped with Energy Dispersive X-ray Spectroscopy (SEM-EDS JEOL JSM-6510) to assess surface morphology and elemental composition. X-ray Diffractometry (XRD, Rigaku, SmartLab SE basic) was used to evaluate crystallinity, and Fourier Transform Infrared Spectroscopy (FTIR, Shimadzu IRSpirit-X Compact) was employed to identify chemical bonding and functional groups.

Adsorption investigations

The adsorption properties of the synthesized materials were investigated through a comprehensive study. Initially, the adsorption capacity of each catalyst, ZnAl-HMTA and ZnAl-Urea, was determined using a 10 ppm Congo red dye solution. For this, 0.05 grams of each catalyst was added to 100 ml of the dye solution, which was then stirred at 300 rpm on a hot plate stirrer. Samples of 5 ml were collected at intervals of 10 minutes for a duration of 60 minutes. These samples were analyzed using UV-Vis spectroscopy to assess the dye’s adsorption. Subsequently, adsorption isotherm experiments were conducted using varying concentrations of Congo red dye, specifically at 5, 10, 15, 20, and 25 ppm. This series aimed to understand the adsorption behavior under different dye concentrations, following the same sampling and analytical procedures as the initial tests.

ZnAl LDH characteristics

The SEM images presented in Figure 1(a) and (b) reveal distinct morphological differences between the u- and h-ZnAl LDH samples. Figure 1(a) shows the u-ZnAl LDH exhibited an irregular and agglomerated morphology, characterized by uneven and less defined structures, suggesting that the urea precursor leads to a less uniform nucleation and growth process under hydrothermal conditions 18. In contrast, Figure 1(b) depicts the h-ZnAl LDH samples, in which exhibits a more uniform, “plate-like” morphology with better-defined particles 19,20. The consistent structure of the HMTA-based ZnAl LDH could be attributed to the more controlled decomposition of HMTA during synthesis, which promotes uniform nucleation and growth of the LDH crystals.

The accompanying EDS (Energy-Dispersive X-ray Spectroscopy) analysis provides insights into the elemental composition of the samples. The u-ZnAl LDH (Figure 1(a), right side) shows a Zn/Al atomic ratio of approximately 0.68:1, with zinc and aluminum atomic percentages of 10.78% and 15.94%, respectively. This ratio indicates a significant deviation from the targeted 2:1 ratio, with a lower zinc content and higher aluminum content than expected. This could suggest incomplete incorporation of zinc or over-incorporation of aluminum during the synthesis process. In contrast, the h-ZnAl LDH (Figure 1(b), right side) exhibits a Zn/Al atomic ratio of approximately 2.01:1, with zinc and aluminum atomic percentages of 18.13% and 9.03%, respectively. This ratio closely matches the desired 2:1 stoichiometry, indicating a more successful incorporation of zinc relative to aluminum in the LDH structure when using HMTA as a precursor 21.

The XRD patterns of the u-ZnAl LDH and h-ZnAl LDH samples are shown in Figure 2(a). The diffraction peaks observed in the h-ZnAl LDH sample correspond to the characteristic reflections of a well-formed layered double hydroxide structure, with sharp and intense peaks at 2\thetaθ values around 11.6 ^\circ∘, 23.3 ^\circ∘, 34.8 ^\circ∘, 39.3 ^\circ∘, and 60.8 ^\circ∘. These peaks can be indexed to the (003), (006), (012), (015), and (110) planes, respectively, indicating a high degree of crystallinity and a well-ordered layered structure with the

The FTIR spectra of the u-ZnAl LDH and h-ZnAl LDH are presented in Figure 2(b). Both spectra display characteristic absorption bands that correspond to the functional groups present in the ZnAl LDH structure 22. For the h-ZnAl LDH, the broad absorption band around 3400 cm^-1 is attributed to the OH stretching vibrations of the hydroxyl groups in the LDH layers and interlayer water molecules 24. The band around 1638 cm^-1 is due to the bending vibrations of water molecules in the interlayer 24. The u-ZnAl LDH also exhibits similar band around 3400 cm^-1 and 1638 cm^-1, but they are broader and less intense, reflecting the lower crystallinity and potentially less structured interlayer water in this sample. This aligns with the XRD results indicating poorer crystallinity in u-ZnAl LDH. The bands observed around 1500 cm^-1 and 1380 cm^-1 in both spectra correspond to the stretching vibrations of the interlayer carbonate anions, which are commonly present in LDHs due to their affinity for

ZnAl LDH adsorption kinetics

In the analysis of Congo red dye adsorption, UV-Vis absorption spectra reveal key insights into the effectiveness of the catalysts over a 60-minute period. Figure 3(a) and (b) displays the absorption spectra for u-ZnAl LDH and h-ZnAl LDH, respectively. Initially, both catalysts show strong absorption peaks around 498 nm, indicative of the presence of Congo red dye. As time progresses, these peaks diminish, with h-ZnAl LDH demonstrating a more pronounced decrease in intensity, suggesting more effective dye adsorption compared to u-ZnAl LDH. This observation is visually supported by Figure 3(c), where the upper row showing u-ZnAl LDH and the lower row h-ZnAl LDH both depict a reduction in dye colour intensity. However, the colour fading is notably more significant in the h-ZnAl LDH samples, aligning with the quantitative data from the UV-Vis absorption spectra.

Figure 3(d) quantifies the adsorption capacity, calculate using Equation that available in literatures 28,29. The result illustrating a rapid increase in dye removal within the first 20 minutes for both catalysts, which then plateaus towards the 40-minute mark. Here, h-ZnAl LDH reaches a higher equilibrium capacity (Qe) of 17.8 mg/g, compared to 12.3 mg/g for u-ZnAl LDH. The adsorption kinetics, analysed through pseudo-first-order (PFO) and pseudo-second-order (PSO) models, further elucidate the adsorption dynamics 30,31,32. The PFO model shows a poorer fit to the data, especially for u-ZnAl LDH as seen in Figure 3(e). Sample shows a low R² value higher than 0.995, as depicted in Figure 3(f). Overall, the analyses clearly demonstrate that h-ZnAl LDH is more effective than u-ZnAl LDH in adsorbing Congo red dye from aqueous solutions. The superior performance of h-ZnAl LDH is evident in both the visual and quantitative results and is further supported by kinetic modeling that confirms chemisorption as the predominant mechanism of dye removal. These findings highlight the potential of h-ZnAl LDH as a highly effective catalyst for environmental remediation applications, particularly in the treatment of wastewater containing dye pollutants.² value higher than 0.995, as depicted in Figure [FIGREF:3](f). Overall, the analyses clearly demonstrate that h-ZnAl LDH is more effective than u-ZnAl LDH in adsorbing Congo red dye from aqueous solutions. The superior performance of h-ZnAl LDH is evident in both the visual and quantitative results and is further supported by kinetic modeling that confirms chemisorption as the predominant mechanism of dye removal. These findings highlight the potential of h-ZnAl LDH as a highly effective catalyst for environmental remediation applications, particularly in the treatment of wastewater containing dye pollutants.

ZnAl LDH adsorption isotherm

The adsorption behavior of Congo red dye onto ZnAl LDH was investigated using UV-Vis absorption spectra and modeled through adsorption isotherms. The UV-Vis spectra displayed in Figure 4(a) for different concentrations (5, 10, 15, 20, and 25 ppm) show a marked decrease in intensity over 60 minutes, signaling effective dye adsorption. The corresponding photographs within the panel visually confirm this trend, where a noticeable fading of the dye’s color intensity is evident, especially prominent at higher concentrations. This indicates robust adsorption capabilities of ZnAl LDH, consistent across the tested dye concentration range.

Further quantification of the adsorption process is explored in Figure 4(b), (c), and (d) through Langmuir and Freundlich isotherm models 33,34. The experimental data plotted alongside the isotherm curves in Figure 4(b) fits both models well, suggesting simultaneous monolayer and potentially multilayer adsorption on heterogeneous sites. The Langmuir isotherm, depicted in Figure 4(c), shows a high correlation (R² = 0.98909) supporting a monolayer adsorption mechanism where each adsorbent site on the ZnAl LDH binds a single molecule of dye. This linear relationship emphasizes that ZnAl LDH provides specific, energetically uniform adsorption sites that become saturated without overlap once occupied.

Conversely, the Freundlich isotherm model presented in Figure 4(d) plots, yielding an R² value of 0.94382, indicative of multilayer adsorption on a heterogeneous surface. This model’s fit suggests that ZnAl LDH also possesses a variety of adsorption sites with different binding energies, likely accommodating multiple layers of dye molecules. The effectiveness of both isotherm models implies complex adsorptive interactions on ZnAl LDH’s surface, characterized by a combination of physical and chemical adsorption processes. Overall, the data illustrates that ZnAl LDH is a potent adsorbent for removing Congo red dye from aqueous solutions, validated by both visual and quantitative analyses. The congruence of the experimental results with Langmuir and Freundlich isotherms provides valuable insights into the nature of the adsorption mechanism, proposing that ZnAl LDH could be particularly effective in water treatment applications where comprehensive dye removal is essential.