Enhanced Photocatalytic Degradation of Rhodamine-B under Led Light Using CuZnAl Hydrotalcite Synthesized by Co-Precipitation Technique

Abstract

The co-precipitation method was employed to synthesize a set of ZnAl hydrotalcite materials modified by Cu²⁺. The synthesized materials have a similar lamellar structure to that of hydrotalcite. The distance between layers is in the range of 7.73–8.56 Å. The network parameters a and c ranged from 3.058 to 3.074 Å and from 23.01 to 24.44, respectively. According to the IUPAC classification, the composites possess a mesoporous structure which belongs to class IV, type H 3. Particularly, the absorption edge shifts strongly to the visible light region when increasing the molar ratio of Cu²⁺ in the samples from 0 to 3.5. The photocatalytic activity of the synthetic materials was evaluated through the degradation efficiency of rhodamine-B (Rh-B) in the water and colorants in textile wastewater. The present study was the first to synthesize a material sample that contains a molar ratio of Cu²⁺ in the range of 2.5–3.5 and has high catalytic activities. They were able to degrade Rh-B at a high concentration (100 ppm) with a conversion rate of approximately 90% after 240 min of irradiation using a 30 W LED light. The catalytic activity of the composites depends on the molar ratio of modified Cu²⁺, the value of environmental pH, the H₂O₂ concentration and the irradiation time.

1. Introduction

ZnAl hydrotalcite materials have been of significant interest to researchers and scientists. The precursors used to synthesize these materials are also highly diverse, ranging from nitrate, chloride and sulfate salts with molar ratios of Zn²⁺/Al³⁺ of 2:1; 3:1 and 4:1, respectively [1,2,3,4,5]. The XRD analysis results show that the Zn²⁺/Al³⁺ molar ratios of approximately 2:1 and 1.5:1 were the most suitable for the structure of LDHs [1] and CuZnAl material [2]. The ZnAl materials were used as the modification substrate by various agents (e.g., Cu, Ti and Eu), as well as organic composes (e.g., EDTA and EDDS) [6,7]. Therefore, promising results of catalytic and adsorbent materials were obtained in environmental treatments.

Hydrotalcite materials MgAlCO₃ and ZnAlCO₃ modified by transition metals (for example, Cu²⁺, Co²⁺, Fe³⁺, Cr³⁺, and Ni²⁺) have been successfully synthesized, mainly by co-precipitation, yielding metal-supported catalysts, immobilized systems and forms of oxide film and nanooxides obtained by the calcination of hydrotalcite materials at a controlled temperature [7,8,9]. The obtained research results showed that the isomorphous substitution of transition metal ions in a brucite-like lattice leads to the appearance of oxidation–reduction centers in the sheets which may act as active sites for the catalytic oxidation reaction [8]. The synthesized hydrotalcite materials have a unique layered structure, high thermal stability, tunable components, acidic–basic properties and structure memory effects [9].

Hydrotalcite materials modified with Cu²⁺ ions have been successfully synthesized, characterized and applied as adsorbents, photocatalysts and catalysts in the various fields. For instance, MgCuAl hydrotalcites have been successfully synthesized according to the molar ratios of Cu²⁺ of 0; 0.07; 0.14. 0.21; 0.28; and 0.35 (total molars of Cu²⁺ and Mg²⁺ to Al³⁺ of 0.7:0.3) for the purpose of oxidizing styrene into styrene oxide and benzaldehyde [8]; whereas CuZnAl hydrotalcites were used for the adsorption of Cu²⁺ ions in electroplating wastewater [2].

However, the study of the systematic influence of the molar ratios of the Cu²⁺ ion on the ZnAlCO₃ hydrotalcite structure and the use of CuZnAlCO₃ materials as photocatalysts for the photocatalytic degradation of rhodamine-B at high concentrations using light 30 W LED visible light (λ > 400 nm, λ_max = 464 nm) has not been published. In the present study, we synthesized the ZnAlCO₃ substrate according to the molar ratio Zn²⁺/Al³⁺ = 7:3, followed by modification with the Cu²⁺ ion with the molar ratios Cu²⁺ of 0; 0.5; 1.0; 1.5; 2.0; 2.5; 3.0; and 3.5 using the co-precipitation method. The characterization analysis of the synthesized material was performed using XRD, EDX, SEM, TEM, BET, and UV-Vis DRS. The catalytic activity of the hydrotalcites was then evaluated by the degradation reaction of Rh-B using the 30 W LED light. This article will provide the results of the structural characterization of CuZnAlCO₃ materials, evaluate the photocatalytic activity of the synthesized materials for the rhodamine-B degradation reaction, select the optimal conditions for the degradation process of rhodamine-B under visible light, and evaluate the pigments decomposing performance in sedge textile dyeing wastewater.

2. Results

2.1. Structural Properties of Hydrotalcites

2.1.1. XRD

The results of the XRD patterns shown in Figure 1 reveal that all the material samples with typical peaks such as the crystal structures of hydrotalcite at diffraction angles of 11.6°, 23.4°, 34.8° and 60.5° corresponded to the lattice planes of (003), (006), (009) and (110), respectively [9,10]. Therefore, the H and CuH-n materials preserved the hydrotalcite structure. However, the peak height of the characteristic peaks of hydrotalcite structure at the planes of (003), (009) and (110) decreased with the increasing proportion of Cu²⁺ in the material [2,7]. Based on the distance values (d) between the layers of the hydrotalcites at the plane of (003) and (110), the a and c parameters were obtained, where the a parameter represents the cation–cation distance in the brucite layer and a = 2.d₁₁₀. The thickness of the brucite layer was characterized by parameter c and the inner layer spacing and was calculated according to the formula c = 3d₀₀₃ [3,11,12]. The calculation of parameters a and c shown in Table 1 revealed that they fluctuated in the range from 3.058 to 3.074 Å and from 23.18 to 24.15 Å, respectively. These a and c parameters are typical for hydrotalcites with the presence of CO₃^2– anions in the interlayers [1,5,13]. In addition, the characteristic diffraction peak for the CuO crystal at diffraction angle 2θ = 35.5° [9,14,15] appeared quite weakly in some samples with Cu²⁺ molar ratios from 1.5 to 3.5. This result was consistent with the composites’ color in Table 1, indicating that the Cu²⁺ ion was well dispersed in the brucite lattice structure of the hydrotalcite which is caused by the isomorphic substitution of Cu²⁺ with Zn²⁺ in the Brucite network. This could be explained by the fact that the Cu²⁺ and Zn²⁺ ions had a quite close ionic radius (0.73 and 0.74 Å, respectively) [1,9,11]. Moreover, the appearance of CuO particles in the material structure also contributes to promote the catalytic activity of materials [15,16].

Thus, the H and CuH-n materials synthesized with the molar ratio of Cu²⁺ = 0–3.5 retained a similar lamellar structure to that of the hydrotalcite and the layered structure was slightly degraded when increasing the Cu²⁺ molar ratio. Especially, the production cost of hydrotalcites was significantly reduced using the pure chemicals from Xilong Scientific (Shantou, Guangdong, China), which helps reduce the cost of environmental treatment.

2.1.2. SEM and TEM Images of the Materials

The SEM images of the H and CuH-3.0 hydrotalcites showed a lamellar structure (Figure 2a,b). The sample H contained hydrotalcite sheets with various sizes from small (30–50 nm) to large (>100 nm). In contrast, the CuH-3.0 material sample has more uniform slabs, and the plate size fluctuates in the range from 50 to 100 nm. However, the average grain size of the H and CuH-n materials ranges from 15 to 33 nm (Table 1). This result is also similar to Wu et al. (2020) [7]. Overall, the H and CuH-n materials were successfully synthesized with a hydrotalcite-like layered structure using the co-precipitation method and affordable chemicals with high purity.

The TEM images of the two materials samples H and CuH-3.0 clearly showed the lamellar structure of the hydrotalcites (Figure 2c,d). The plates of the H material had a lighter color than those of the CuH-3.0 material, while the colors of the CuH-3.0 sheets were bold and uniformly distributed. This result proves that Cu²⁺ ions are well-dispersed in the brucite network of hydrotalcite and benefit from the XRD characteristics of the composite samples.

2.1.3. Analysis Results of the Composition of Elements in the H and CuH-3.0 Hydrotalcites

The results of EDX spectral analysis of the H and CuH-3.0 hydrotalcites showed that there were the three elements O, Zn and Al presented in sample H and all four elements O, Al, Cu and Zn presented in the material sample CuH-3.0 (Figure 3). The percentage of atomic elements O, Al, Cu and Zn in the two samples of H and CuH-3.0 was shown in Table 1. The ratio of Zn and Al atoms in H was closed to a 2:1 ratio. It was possible to attach the H material with the composition Zn₄Al₂CO₃(OH)₁₂·3H₂O. The actual molar ratio of Cu²⁺ and Zn²⁺ ions in the CuH-3.0 sample is 5.32:6.86, which is quite similar to the theoretically calculated ratio of Cu²⁺ and Zn²⁺ to synthesize material.

2.1.4. The N₂ Adsorption/Desorption Isotherms (BET) of Hydrotalcites

The analysis results of N₂ adsorption/desorption isotherms (BET) in Figure 4 showed that the two hydrotalcites H and CuH-1.0 had the N₂ adsorption/desorption curves belonging to type IV and type H3, respectively, according to the IUPAC classification [2,11,17]. However, the two hydrotalcites CuH-2.0 and CuH-3.0 both had the N₂ adsorption/desorption curves belonging to type IV and type H4 characterized by the mesoporous materials. The hysteresis of the adsorption isotherms and the desorption curves of the four hydrotalcites gradually increased with the increase in the molar ratio of Cu²⁺ in the sample. In general, when increasing the molar ratio of Cu²⁺ in the sample from 0 to 3.0, the specific surface area increased while the pore diameter reduced from 37.59 nm to 24.51 nm (Table 2).

2.1.5. UV-Vis DRS Spectra of Material Samples

The photocatalytic activity of the hydrotalcites can be predicted based on the UV-Vis DRS spectrum (Figure 5a). The H sample displayed two absorption peaks in the range of 220–350 nm in the UV light region. The light absorption in this range was attributed to the charge transfer from O²⁻ to metal cations (Al³⁺ and Zn²⁺) [13]. The UV-Vis spectral shape of the CuH-n materials was completely different from that of the H sample. The first absorption band was extended to a higher wavelength region (approximately 260 nm) which was explained by the charge transfer from O²⁻ to Cu²⁺ ions. In particular, there was a strong shift in the second absorption region to the visible light spectrum that could be explained by the charge displacement d-d in the octahedral medium of Cu²⁺ ions. Therefore, the absorption edge of the material also shifted strongly to the visible light region when increasing the ratio of Cu²⁺ in the material samples from 0.5 to 3.5.

Figure 5b illustrated the conversion to the bandgap energy of the hydrotalcites according to the equation of the Kubelka–Munk model fitted with indirect transition [18,19,20]. The bandgap energy Eg of the H and CuH samples decreased from 3.02 eV (H) to 2.3 eV (CuH-3.5). This result could indicate that the samples modified by Cu²⁺ possess good photocatalytic activity under visible light.

2.2. Research on the Ability to Degrade Rh-B on Hydrotalcite Samples

2.2.1. Effect of the Dosage Cu²⁺ ion in the Sample on the Catalytic Activity of the Material

The results of the 30 ppm Rh-B degradation activity of the hydrotalcites are shown in Figure 6. After 30 min of stirring in the dark to reach adsorption equilibrium, the 30 ppm Rh-B adsorption efficiency of all tested samples was equal to less than 10%. During irradiation under the 30 W LED light, the Rh-B degradation efficiency increased in accordance with the irradiation time. The 30 ppm Rh-B degradation sharply increased only after the first 30 min of illumination under the 30 W LED light and slowed down during the subsequent irradiation periods. After 30 min of irradiation, the Rh-B conversion reached 90–93% on the three samples of CuH-2.5, CuH-3.0 and CuH-3.5 materials.

In contrast, the sample H exhibited the lowest ability to degrade Rh-B under the same tested conditions. The Rh-B conversion by this material was only 33% after 180 min of irradiation. The photocatalytic activity of CuH-n materials could be explained by the fact that the bandgap energy (Eg) decreased when increasing the dosage of Cu²⁺ in the sample. Furthermore, the 6500 K LED had three absorption peaks in the wavelength range from 446 to 477 nm and the strongest intensity was shown at the maximum wavelength of 464 nm [21], which was suitable for the light absorption region of CuH material samples to form photogenerated e⁻ and h⁺ pairs. In addition, Cu²⁺ ions reacted with H₂O₂ to generate the active intermediate radicals Cu^+•O₂H. Then, these active intermediate radicals continued to react with H₂O₂ to produce Cu²⁺, O₂, H₂O and ^•OH radicals. Afterwards, ^•OH radicals targeted the chromogenic groups to degrade the Rh-B [20,22,23]:

Cu²⁺ − ZnAl + hυ → Cu²⁺ − ZnAl (e⁻, h⁺) (1)

H₂O₂ + e⁻ → HO^• + OH⁻ (2)

Cu²⁺ZnAl + H₂O₂ → Cu^+•O₂H + H⁺(3)

Cu^+•O₂H + H₂O₂ → Cu²⁺ + O₂ + ^•OH(4)

OH^• + Rh − B → colorless reduction products(5)

h⁺ + Rh − B → colorless oxidation products(6)

Compared with the previous publications, all the H and CuH-n materials only insignificantly adsorbed Rh-B [19,24]. The first reason for this is that Rh-B, as a cationic dye, was unable to exchange anions with CO₃^2– ions in the interlayer. The second reason is the repulsion caused by metal ions in the plates for Rh-B molecules [11,25,26]. The activity of CuH materials mainly depends on the role of Cu²⁺ ions in the brucite network structure. The Cu²⁺ ions (1) reduced the activation energy of the materials; (2) enhanced the ability to absorb light in the visible light region; and (3) create catalytic centers that generate optical ^•OH radicals. The role of Cu²⁺ ions in the hydrotalcites also reduced the recombination rate of e⁻–h⁺ pairs, leading to an increasing rate of Rh-B degradation of these modified materials.

2.2.2. Effect of Rh-B Concentration on the Catalytic Activity of the Materials

The CuH-3.0 hydrotalcite sample was used to survey the Rh-B degradation ability in the selected concentration of Rh-B (Figure 7). The Rh-B ability degradation on the CuH-3.0 material sample gradually decreased with an increasing Rh-B concentration in the range of 30–100 ppm. The rate of Rh-B degradation rapidly increased after the first 30 min of irradiation at the concentrations of 30, 40 and 50 ppm of Rh-B. This rate slowly increased during the next 210 min. When the Rh-B concentration was 75 and 100 ppm, the Rh-B degradation efficiency gradually increased by 90% over approximately 240 min of the irradiation. Thus, it can be seen that the CuH-3.0 material was capable of degrading Rh-B at a high concentration of 100 ppm. It was proven that the H material modified by Cu²⁺ ions had obtained a new material with excellent photocatalytic activity towards the Rh-B degradation under the investigation conditions. This finding is the first to show that the synthesized hydrotalcites can degrade the Rh-B at high concentrations using only the 30 W LED light, resulting in lower disposal costs as compared to the 400 W halogen and the 500 W Xenon lights [18,24].

2.2.3. Effect of H₂O₂ Concentration on the Catalytic Activity of the CuH-3.0 Material

As shown in Figure 8a, the catalytic activity of (1) the CuH-3.0 sample; (2) the combination of CuH-3.0 material sample and H₂O₂ 30%; and (3) the blank confirmed the role of oxidizing precursor to the Rh-B degradation of the material. The Rh-B degradation efficiency at the concentration of 100 ppm increased more slowly after 240 min of the irradiation in two cases—the blank and CuH-3.0. In contrast, the Rh-B degradation efficiency rapidly increased when both the CuH-3.0 material and H₂O₂ were combined, indicating that the catalytic activity of CuH-n material samples can be improved by combining with a low level of H₂O₂. Equations (1)–(4) explained the catalytic activity of CuH-n in the presence of H₂O₂ to form the ^•OH photogenic radicals involved in Rh-B degradation.

The influence of the H₂O₂ concentration on the catalytic activity of CuH-3.0 hydrotalcite was shown in Figure 8b. The Rh-B degradation rate accelerated as the H₂O₂ concentration increased from 800 to 13,333 mg/L. In particular, the Rh-B degradation efficiency at a concentration of 100 ppm could reach approximately 70% after only 30 min of irradiation in combination with the 13,333 mg/L H₂O₂.

The survey results can be explained by the fact that a high concentration of H₂O₂ could promote the generation of more ^•OH photogenic radicals, thus increasing the rate of Rh-B degradation: H₂O₂ → 2 ^•OH. However, excessively high H₂O₂ concentration would hinder the Rh-B degradation rate and complicate the removal procedure. The reduced catalytic activity of the material at high H₂O₂ concentrations is explained by the fact that the generated ^•OH radicals react with H₂O₂ to produce ^•OOH radicals and O₂ molecules that are much weaker oxidizing agents than ^•OH radicals [27].

^•OH + H₂O₂ → ^•OOH(7)

^•OH + ^•OOH → O₂ + H₂O(8)

2.2.4. Effect of Medium pH on the Catalytic Activity of the Material

The medium pH values clearly influenced the catalytic activity of the CuH-3.0 material sample (Figure 9). The catalytic activity of the material decreased when changing the medium pH values of the 100 ppm Rh-B solution from the initial pH of 5.06 to the acidic region and the basic region. In a strongly acidic environment (pH = 2.0), the catalytic activity of the material decreased strongly. This result was due to the destruction of the structure of the material at a strong acidic pH media and the decreasing number of Cu²⁺ catalyst sites caused by leaching to the solution [28,29,30]. The catalytic activity of the material also decreased when increasing the medium pH value from 5.06 to the neutral, basic and strong base regions (pH = 10.0–12.0). This result was explained by the fact that the increasing solution viscosity at a high pH value reduced the diffusivity of Rh-B to the material surface and the light absorption by the material. In addition, the structure of the material could be destroyed by dissolving Zn(OH)₂ and Al(OH)₃ at a high pH value. Therefore, the optimal pH range for degrading the Rh-B of the material was the slightly acidic pH range of 5.06–6.0.

With the obtained survey results, it can be seen that the photocatalytic activity of CuH-n materials was significantly enhanced, depending on the main factors: the ratio molar of Cu²⁺ ion in the sample, lighting time, medium pH value and H₂O₂ concentration. It is possible to diagram the Rh-B decomposition reaction by the CuH-n photocatalyst in Figure 10 below.

2.3. Applications to Textile Wastewater Treatment

Due to the complex composition of wastewater and the addition of color stabilizers, 5.0 mL of 30% H₂O₂ solution was used in this experiment. The survey results showed that the intensity of the absorption peaks of the pigments at 500–553 nm gradually decreased with the irradiation time at pH 3.52 (Figure 11b). Such a decrease in the intensity became significant at pH 6.0 (Figure 11c). This result is in closed agreement with the investigation of the optimal conditions of the material for Rh-B. However, the degradation efficiency of pigments in wastewater was only approximately 88% after 360 min of lighting at pH 6.0 (Figure 11a). The obtained results concluded that the CuH-3.0 material can completely decompose the pigments in the textile wastewater composition, thus introducing the potential application of CuH-n material samples in textile wastewater treatment.

3. Materials and Methods

3.1. Materials

Zincat nitrate (Zn(NO₃)₂·6H₂O), aluminum nitrate (Al(NO₃)₃·9H₂O) and Rh-B were purchased from Xilong Scientific, China. Copper nitrate (Cu(NO₃)₂·3H₂O), sodium carbonate (Na₂CO₃), sodium hydroxide (NaOH), Rh-B and 30% hydrogen peroxide solution (H₂O₂) were purchased from Merck, Darmstadt, Germany. The chemicals have a purity of more than 99%.

3.2. Synthesis of the Materials

The hydrotalcite material ZnAl-CO₃ (H) and hydrotalcites modified by Cu²⁺ ions (denoted as CuH-n) were synthesized following the co-precipitation method described by Wu et al. (2020) with minor modifications [3,7,8,31]. Briefly, Al(NO₃)₃·9H₂O, Zn(NO₃)₂·6H₂O and Cu(NO₃)₂·3H₂O were simultaneously dissolved with 150 mL of deionized water in the 250 mL triangle vessel. The precursor salts were collected and adjusted to obtain the molar ratio (Cu²⁺ + Zn²⁺)/Al³⁺ = 7:3 (Table 3). The reaction vessel was placed on a heated magnetic stirrer to obtain a homogeneous solution. Then, 25 mL of 0.6 M Na₂CO₃ solution was added dropwise to the reaction flask and stirred for 60 min at room temperature. The resulting mixture was transferred to a 500 mL beaker and the pH of the mixture was adjusted to 9.5 by 2M NaOH solution, allowing gel formation. The gel was then stirred on a magnetic stirrer for 60 min and aged in a Teflon flask at 100 °C for 24 h. After aging, the synthesized gel was filtrated and washed with hot deionized water (70 °C) several times until pH = 7.0. The solids were dried at 80 °C for 24 h to obtain the samples of hydrotalcite ZnAlCO₃ (H) and hydrotalcites modified by Cu²⁺ ions (denoted as CuH-n where n is the molar ratio of Cu²⁺ in the sample). Finally, the samples were ground with an agate mortar and pestle and used for the subsequent experiments.

3.3. Structures and Catalytic Properties of Materials

The crystalline phase composition of the synthesized samples was characterized by X-ray diffraction (XRD) on the D8-ADVANCE 5005-Brucker instrument (Bruker AXS, Karlsruhe, Germany). The light shifting edge band gap energy was determined by the UV-Vis DRS spectra on a HITACHI U-4100 spectrometer (Tokyo, Japan). The specific surface area was determined by the N₂ adsorption–desorption isotherm (BET) on a MicroActive for TriStar II Plus Version 2.03 instrument (Micromeritics, Norcross, GA, USA). The SEM and TEM images of the material were taken by using the Hitachi S-4800 instrument, Jeol 3010 (Tokyo, Japan). The elemental composition was determined by EDX spectroscopy using the HORIBA instrument (MODEL 7593-H) (Kyoto, Japan).

3.4. Evaluation of the Catalytic Efficiency of Synthesized Materials through the rhodamine-B Decomposition Reaction

The Rh-B degradation efficiency of the synthesized material was evaluated through the following survey. A 30 W LED lamp which has a 6500 K color temperature (Philips, Amsterdam, The Netherlands) was used as the visible light source. The reaction mixture of 250 mL Rh-B solution and 0.2 g of the material was stirred in the dark for 30 min until the adsorption equilibrium was achieved. Then, 1.2 mL of 30% H₂O₂ was added to the reaction mixture and the sampling of Rh-B concentration was carried out every 30 min and centrifuged and the molecular absorbance was measured at the wavelength of 553 nm. The materials with good activity were selected to investigate the influence of Rh-B concentrations, H₂O₂ concentration and medium pH value and its application to treat sedge mat textile wastewater. The survey experiments were conducted 3 times. The results were then calculated to obtain the average values as well as the standard error values and used to evaluate the photocatalytic activity of the investigated materials.

3.5. Determination of Rh-B Concentration in Water

The standard curve for determining the Rh-B concentration in water was followed by equation y = 0.1674x + 0.0069 (R² = 0.9994). The standard curve for determining the Rh-B concentration is shown in Figure 12 below.

After the degradation progress, the concentration of Rh-B in the water was determined. For every 30 min of irradiation, a small amount (5–7 mL) of Rh-B sample was extracted from the reaction mixture and centrifuged to remove the sample material. The samples were diluted before measuring the absorbance of Rh-B at 553 nm on a Shimadzu UV−Vis 1700 instrument. The concentration of Rh-B remaining in the solution was calculated according to the calibration curve from Figure 12.

3.6. Research on the Ability to Treat Textile Wastewater

Dark red-colored sewage sludge mats containing a high concentration of pigments was collected from the wastewater tank of households located in Dong Bang village, Quynh Phu district, Thai Binh province, Vietnam. Prior to the experiment, the initial wastewater sample was diluted ten times with distilled water. Then, the diluted wastewater sample (250 mL) was investigated for the ability to degrade the colorants in wastewater according to the following parameters: the initial medium pH value of 3.52, and the adjusted pH value of 6.0 and 0.2 g of CuH-3.0 material sample. All reaction mixtures were contained in a 500 mL beaker and sealed with a black glossy bag. Afterwards, the reaction mixture was stirred in the dark for 30 min to reach adsorption equilibrium. The 5.0 mL of 30% H₂O₂ solution was added to the reaction mixture. The reaction mixtures were irradiated under a 30 W LED light at room temperature. The samples were collected to determine the pigments’ degradability of the composite after a period of every 60 min. After the sample was centrifuged to remove a small amount of catalyst, the wastewater was diluted 10 times for UV-Vis spectrum scanning on a UV 1700 SHIMADZU machine (Kyoto, Japan). The color degradation efficiency was calculated based on the absorbance of the pigments at wavelengths of 550–553 nm. The survey experiments were conducted 3 times. The results are then calculated to obtain average values, standard error values and used to evaluate the photodegradation ability for colorants in textile dyeing wastewater.

(9)$\mathrm{H}\% =\frac{{\mathit{Abs}}_{\mathit{o}}{ - \mathit{Abs}}_{\mathit{t}}}{{\mathit{Abs}}_{\mathit{o}}}\times 100 (\%)$

Here, ${\mathit{Abs}}_{\mathit{o}}$ and ${ \mathit{Abs}}_{\mathit{t} }$ is the absorbance of colorant at the initial time and sampling time, respectively.

4. Conclusions

The H and CuH-n-synthesized hydrotalcites possess a similar layered structure as hydrotalcite, despite a decrease in structure when increasing the molar ratio of Cu²⁺ ion in the brucite layers from 0 to 3.5. The results of XRD pattern characterization showed the presence of weak peaks of the crystalline phase of CuO in the samples with a molar ratio of Cu²⁺ from 1.5 to 3.5. The catalytic activity of H samples modified by Cu²⁺ ions increased significantly. The survey results indicated that the catalytic activity of the materials depends on various factors, such as the molar ratio of Cu²⁺ ions in the sample, the irradiation time, the concentration of rhodamine-B, the concentration of H₂O₂ and the medium pH value. The hydrotalcite samples with the molar ratio of Cu²⁺ ions in the range of 2.5–3.5 exhibit the highest photocatalytic activity for Rh-B degradation. The presence of a small amount of H₂O₂ significantly increased the ability to degrade Rh-B at a 100 ppm concentration on the CuH-3.0 sample. In addition, the medium pH value in the slightly acidic range (5.06–6.0) provided the most favorable environment for Rh-B degradation. In addition, the CuH-3.0 material sample can completely degrade the pigments in textile wastewater. The conversion efficiency of colorants in wastewater can reach up to 88%. Simultaneously, the color intensity of the mixture significantly decreased, and the solution became pale after 360 min of illumination. The results of the structural characterization and photocatalytic activity of the synthesized samples confirmed that the materials modified by the Cu²⁺ molar ratio of 2.5–3.5 are considered an excellent photocatalyst for the degradation of the high Rh-B concentration and the textile wastewater treatment.

Author Contributions

Conceptualization, M.C. and T.H.L.N.; methodology, M.C. and Q.D.N.; software, T.H.L.N. and Q.D.N.; formal analysis, T.K.N.T. and T.H.T.P.; data curation, V.N.V.; writing—original draft preparation, T.H.T.P., V.N.V. and T.H.D.; writing—review and editing, T.H.D. and T.K.N.T. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All the data are available within the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Tables

Figure 1. XRD pattern of the H and CuH-0.5–CuH-3.5 material samples.

Figure 2. SEM images of the H (a) and CuH-3.0 hydrotalcites (b), TEM images of H (c) and CuH-3.0 samples (d).

Figure 3. EDX spectrum of the H (a) and CuH-3.0 hydrotalcites (b).

Figure 4. The N2 adsorption and desorption isotherms of four hydrotalcites: H, CuH-1.0, CuH-2.0 and CuH-3.0.

Figure 5. UV-Vis DRS spectrum (a) and band gap energy of composite samples (b).

Figure 6. The 30 ppm Rh-B decolorization performance over irradiation time on hydrotalcites.

Figure 7. The comparison of the Rh-B degradation ability on the CuH-3.0 sample with the Rh-B concentrations are 30, 40, 50, 75 and 100 ppm.

Figure 8. The comparison of the ability to degrade Rh-B at a concentration of 100 ppm in three cases (a) and the ability to degrade the Rh-B of CuH-3.0 at different concentrations of H2O2 (b).

Figure 9. The comparison in the degradation ability of Rh-B at 100 ppm concentration of CuH-3.0 sample at different environmental pH values (a) and conversion Rh-B of 100 ppm at 180, 210, 240 min (b).

Figure 10. Schematic diagram of Rh-B photocatalytic degradation on CuH-n hydrotalcite.

Figure 11. Conversion of dyes in the textile dyeing wastewater composition at pH 3.52 and 6.0 (a), the UV-Vis spectrum of dyes in the survey progress after 360 min of the irradiation by 30 W LED light at pH = 3.52 (b), pH = 6.0 (c).

Figure 12. Standard curve for determination of Rh-B concentration in the water.

Table 1

Analysis results percentage of elements O, Al, Cu, Zn in the H, CuH-3.0 hydrotalcites.

Samples	% O Atom	% Al Atom	% Cu Atom	% Zn Atom
H	72.19	8.94	0	18.87
CuH-3.0	79.04	8.78	5.32	6.86

Table 2

Specific surface area (BET), pore diameter and pore volume of four hydrotalcites.

Sample	Surface Area BET (m2/g)	Porous Diameter (nm)	Porous Volume (cm3/g)
H	16.1	37.6	0.15
CuH-1.0	15.7	21.1	0.08
CuH-2.0	17.5	22.9	0.10
CuH-3.0	33.9	24.5	0.21

Table 3

The characterization of sample of ZnAl and Cu²⁺ modified hydrotalcites (n_Zn + n_Cu = 0.07, n_Al = 0.03, n_CO3 = 0.015 (mol))].

No.	Name	Molar RatioCu:Zn:Al:CO3	d003 (Å)	d006 (Å)	d110 (Å)	Average Grain Size in Scherrer (nm)	Lattice Parameter (Å)		Color
							A	c
1	H	0:7.0:3.0:1.5	8.025	3.929	1.534	33.28	3.068	24.075	White
2	CuH-0.5	0.5:6.5:3.0:1.5	8.027	3.913	1.537	18.64	3.074	24.081	Light blue
3	CuH-1.0	1.0:6.0:3.0:1.5	8.559	3.866	1.536	20.31	3.072	25.677	Light blue
4	CuH-1.5	1.5:5.5:3.0:1.5	8.051	3.854	1.535	17.96	3.07	24.153	Black blue
5	CuH-2.0	2.0:5.0:3.0:1.5	7.963	3.841	1.537	15.09	3.074	23.889	Black
6	CuH-2.5	2.5:4.5:3.0:1.5	7.806	3.812	1.536	21.20	3.072	23.418	Black
7	CuH-3.0	3.0:4.0:3.0:1.5	7.824	3.868	1.535	16.09	3.07	23.472	Black
8	CuH-3.5	3.5:3.5:3.0:1.5	7.726	3.808	1.529	19.02	3.058	23.178	Black

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