Zn-Layered Double Hydroxide Intercalated with Graphene Oxide for Methylene Blue Photodegradation and Acid Red Adsorption Studies

Abstract

This study focuses on the synthesis of a novel layered double hydroxide and its application in two environmental remediation processes. Graphene oxide, a two-dimensional material, has potential applications in this field. However, its tendency to agglomerate restricts its usability. Our objective was to increase the morphology and performance of layered double hydroxide (LDH) by combining GO with hydrotalcite. The LDH/GO nanohybrids were utilized as photocatalysts for the degradation of methylene blue (MB) dye and were investigated as sorbents for acid red (A.R) dye in water. In order to achieve this objective, ZnAl-NO₃ LDH was synthesized using the co-precipitation method, with a Zn:Al ratio of ~3. Subsequently, the LDH was intercalated with varying ratios of as-received graphene oxide. An array of analytical techniques, including X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) measurements, N₂ physisorption, scanning electron microscopy–energy-dispersive X-ray analysis (SEM-EDX), and diffuse reflectance UV–vis spectra (DR UV-vis), were employed to examine the physicochemical properties of the synthesized LDH. These techniques confirmed that the obtained material is zinc-aluminum hydrotalcite intercalated with GO. The addition of graphene oxide (GO) to the layered double hydroxide (LDH) structure improved the performance of the hydrotalcite. As a result, the composite ZnAl-LDH-10 shows significant potential in the field of photocatalytic degradation of MB. Additionally, the incorporation of GO enhanced the absorption of light in the visible region of the spectra, leading to improved elimination of A.R compared to LDH without GO or other ratios of GO.

1. Introduction

Industrial growth has led to the persistent pollution of water bodies, which is a significant environmental issue. The presence of dye contamination in industrial wastewater, which originates from many sources such as textiles, paper, printing, plastics, and leather, has emerged as a significant issue due to its potential to cause harmful and even cancerous effects on both humans and the environment [1]. According to reports, the dye and textile sector produces 7 × 10⁵ m³ of wastewater every year [2]. Consequently, water filtration has become increasingly necessary in recent decades [3]. Methylene blue and acid red dyes are substances employed in the contemporary textile industry for the purpose of coloring [4]. The waste generated by textile companies poses a significant threat to human health and has a detrimental impact on water resources. These waste products cause dermatological, ocular, and renal ailments while also exerting harmful effects on the human physiological system. With the increasing stringency of international environmental standards, such as ISO 14001 [5] in October 1996, new technical solutions have been created to remove organic contaminants, including dyes. Layered materials are widely recognized for their ability to serve as excellent host matrices for incorporating a wide range of organic substances. They easily accommodate guest molecules through topotactic processes. Layered double hydroxides (LDHs), often referred to as hydrotalcite-like solids, have attracted significant attention due to their use as adsorbent and heterogeneous catalysts [6,7]. LDH nanomaterials are 2D ionic lamellar inorganic compounds classified as hydrotalcite-like materials or anionic clays. The structures consist of layers that are analogous to brucite, with an M²⁺ ion such as Cu²⁺, Ni²⁺, Zn²⁺, Ca²⁺, or Mn²⁺ coordinated with six OH groups. When a divalent metal cation is substituted by a trivalent metal cation such as Fe³⁺, Co³⁺, Cr³⁺, or Al³⁺, the resulting positive charge can be balanced by incorporating a negatively charged species such as NO₃⁻, CO₃²⁻, Cl⁻, or SO₄²⁻ into the inter-lamellar space. LDHs have a general formula of [M^(II)_1−x M^(III)_x (OH)₂]^x+[A^m−_x/m·nH₂O]^x−, where x represents the mole ratio of metal cations, specifically M^III/(M^III + M^II), and Aⁿ⁻ represents an inorganic or organic anion with a charge of n− [8]. An LDH is classified as a doped semiconductor. The semiconductor properties of an LDH can be modified by adjusting the laminate cations and interlaminar anions, thanks to the tunability of its laminate elements [9]. LDHs exhibit a stratified crystal structure and contain a significant capacity for ion exchange, rendering them valuable for the removal of pollutants from soil, water, and air. LDHs are commonly employed as adsorbents or catalysts in environmental cleanup. They have the ability to adsorb heavy metals, organic compounds, and other contaminants from water or soil that is contaminated, thereby decreasing their concentration and enhancing the overall quality of the environment. Moreover, LDHs have the ability to function as catalysts in chemical processes, facilitating the decomposition of noxious chemicals. This phenomenon facilitates the breakdown of harmful substances into compounds that are less harmful or not harmful at all, hence aiding in the process of remediation. Numerous two-component layered double hydroxides have been studied, including ZnCr, CoFe, ZnTi, MgAl, NiFe, and others. These materials exhibit excellent photocatalytic capabilities for breaking down organic pollutants, splitting water using light energy, and converting CO₂ into other compounds [10,11,12,13]. Nevertheless, the laminate contains many heavy metals, including Cr and Cu, which can have detrimental effects on the environment [14].

Graphene-based aerogels have garnered significant attention in recent years for their efficacy in eliminating harmful contaminants owing to their desirable characteristics, which encompass an ample number of anchoring sites, versatile morphologies, and modifiable functional groups and pore dimensions [15]. Graphene oxide (GO) possesses a distinctive two-dimensional (2D) conjugated chemical structure, which has garnered significant interest due to its advantageous characteristics, including a substantial theoretical surface area, exceptional chemical stability, and outstanding electrical capabilities. Therefore, graphene oxide has been widely utilized in various applications, including supercapacitors, solar cells, adsorption, and photocatalysts [16,17]. Recent research has shown that environmental cleanup via GO possesses four outstanding characteristics. Firstly, graphene oxide has the potential to enhance the conductivity of semiconductors by minimizing the recombination of electrons and holes. Secondly, the uniform dispersion of GO can significantly improve the performance of semiconductors [18]. Thirdly, GO has the ability to reveal several active sites on a semiconductor because of its substantial theoretical specific surface area [19]. Lastly, GO has the ability to broaden the spectrum of light absorption for semiconductors [20,21,22,23]. Both graphene oxide and layered double hydroxide sheets encounter the issue of aggregation when used as nanomaterials, posing a considerable hindrance to their application procedures. The amalgamation of these two substances can efficiently avert the occurrence of this issue. LDHs and GO possess analogous layered structures, and the majority of their features are mutually complementary. A possibility exists to mix GO and LDHs into hierarchical nanocomposites, allowing for the integration of their complementing features. GO exhibits both electrical conductivity and strength, whereas LDHs possess chemical reactivity. This facilitates the effortless creation of an electron pathway between GO and LDHs and the establishment of a stress transfer network between them, which is another significant component of advanced functional materials that should be taken into consideration [24,25].

The current investigation involved the preparation of a range of ZnAl-LDH materials with different amounts of GO using a modified co-precipitation technique [26]. The nanohybrids that were created were analyzed to investigate their physical form, chemical composition, and physical characteristics. This paper focuses on two water treatment technologies: photocatalytic removal of MB using ultraviolet irradiation, and extraction and removal of A.R dye. The photocatalytic properties were examined using UV light, taking into account the duration of illumination, pH of the solution, concentrations of the dye, dosage of the catalyst, and stability of the catalyst. Furthermore, the mechanism of photocatalytic degradation is discussed. The study also examines the adsorption qualities. The impact of solution pH on the adsorption of A.R dye, the influence of the mass of solid phases and the duration of shaking, the effect of temperature, and the behavior of the composite were examined using kinetic and thermodynamic experiments. The nanocomposite was also evaluated for its efficacy in removing colors from various raw water samples, including saltwater, wastewater, and tap water, to assess its practical use.

2. Results and Discussion

2.1. Physicochemical and Structural Characterization

The XRD patterns exhibit symmetric and crisp peaks with high intensity, while all examined solid samples display asymmetric and wide peaks at low 2θ angles (Figure 1) [27]. For the ZnAl-LDH sample, the characteristic reflections at 2θ = 11.6°, 23.4°, 34.5°, 39.1°, 46.6°, 60.1°, and 61.5° correspond to the (003), (006), (009), (015), (018), (110), and (113) planes of standard well-crystallized hydrotalcite-like LDH materials (JCPDS no. 38-0486) [28,29]. All diffraction peaks in the XRD pattern of the modified ZnAl-LDH nanocomposite correspond well with those of the pure ZnAl-LDH [30,31]. A comparison of the XRD patterns of unmodified and modified ZnAl-LDH reveals a diminished intensity of the (003) reflection peak for the modified ZnAl-LDH composite. This was ascribed to the disarray of the stacked configuration [32]. A pronounced and intense diffraction peak at 2θ = 9.7° is evident for pure GO, corresponding to the (002) plane reflection [33]. The intensities of the modified ZnAl-LDH diffraction peaks diminished considerably with the increasing GO content, attributable to the greater quantity of GO nanosheets intercalating between the ZnAl-LDH layers, resulting in a layered structure comprising inorganic (ZnAl-LDH) and organic (GO) layers. The layered structure may exhibit reduced crystallinity relative to the pristine ZnAl-LDH due to the incorporation of disorder and the amorphous characteristics of GO and the formation of a stratified structure of alternating organic (GO) and inorganic ZnAl-LDH layers. The introduction of graphene oxide nanosheets disrupts the orderly configuration of the ZnAl-LDH crystal lattice, leading to a decrease in the material’s crystallinity. The average crystallite sizes of pure ZnAl-LDH, ZnAl-LDH-5, ZnAl-LDH-10, ZnAl-LDH-15, and ZnAl-LDH-25, determined using the Scherrer equation, were around 12, 10, 9, 10, and 11 nm, respectively. The presence of a layered structure of alternating organic and inorganic layers, coupled with diminished crystallinity, may influence the material’s properties. It can affect its mechanical strength, thermal stability, and surface area, among other characteristics.

The FT-IR spectrum of the ZnAl-LDH/GO nanocomposite closely resembles that of the pure ZnAl-LDH sample, as seen from Figure S1 [34]. The band detected at 3455 cm⁻¹ corresponds to the stretching vibration mode of the O–H groups in the LDH. O–H was formed from the surface functional groups of GO sheets. For the LDH, O–H originated from the hydroxide layer. The modified ZnAl-LDH composites exhibited pronounced O–H stretching vibrations due to the integration of GO with LDH. The widening of this band results from the creation of hydrogen bonds [35]. The narrow band at around 1600 cm⁻¹ is attributed to the bending vibration of water. Furthermore, in ZnAl-LDH, the band at about 1380 cm⁻¹ is attributed to the vibration of interlayer nitrate anions. The peaks seen in the 400–800 cm⁻¹ range are attributed to the stretching and bending vibrations of M-${\mathrm{N}\mathrm{O}}_3^{-}$, M–O, and M–O–M bonds inside the ZnAl-LDH structure. The M–OH and M–O linkages in the modified ZnAl-LDH /GO nancomposites were more pronounced than in pure ZnAl-LDH due to the interaction between metals and the oxygen functional groups of GO [36,37].

XPS was employed to investigate the surface composition and chemical condition of the produced materials. Figure 2a illustrates that the survey spectra identified the presence of Zn, Al, C, and O components solely on the surface of the ZnAl-LDH composite. This indicates a lack of further contaminants, consistent with the previously reported XRD results. Subsequent high-resolution investigations concentrated on elements directly linked to the sample’s uses. Figure 2b illustrates that the high-resolution spectra of the Zn 2p state exhibit distinct doublet peaks at around 1022 and 1045 eV for the ZnAl-LDH samples. In the ZnAl-LDH-15 and -25 samples, the ZnO peaks begin to bifurcate into two separate peaks, maintaining the same oxidation state but differing in concentration. The peaks correspond to the binding energies of Zn 2p3/2 and Zn 2p1/2, respectively. These data strongly indicate that zinc was uniformly in a divalent oxidation state throughout all LDH samples [38]. The Al2p peak at around 75 eV can be attributed to Al(OH)₃ derived from the LDH structure (Figure 2c). In contrast to other samples, ZnAl-LDH-10 exhibited distinct behavior, with only Al (OH)₃ identified, indicating that this sample was more alkaline. This demonstrates the existence of Al³⁺ species on the surface of the samples. Furthermore, Al₂O₃ was observed on the surface region of the samples ZnAl-LDH-5, ZnAl-LDH-15, and ZnAl-LDH-25, correlating with the increased addition of GO [39,40,41]. The deconvoluted XPS examination of the O 1s spectra, illustrated in Figure 2d for the ZnAl-LDH samples, reveals unique peaks corresponding to different oxygen species. These species are classified as isolated oxygen (O²⁻), referred to as O_β, indicating a distinct type of oxygen within the lattice structure. A notable variant is lattice oxygen (OH⁻), referred to as O_γ, which demonstrates oxygen incorporated into the lattice structure. Furthermore, there is evidence of oxygen associated with adsorbed water, designated as O_α. The differences among oxygen species offer valuable insights into the composition and structure of the ZnAl-LDH samples [40,42]. Graphene oxide comprises many oxygen-containing functional groups, such as hydroxyl, epoxide, and carboxyl. Upon the addition of GO to a material, these groups can engage with the surface, modifying its chemical state. The injection of GO presumably enhances the overall oxygen concentration at the surface. The presence of functional groups from GO can result in the emergence of new peaks in the XPS spectrum, signifying isolated oxygen rather than oxygen in a bulk state or bonded in other compounds. The interaction between GO and the substrate may result in further oxidation or the generation of novel chemical species. This may be evident in the XPS data as alterations in binding energies or the emergence of new peaks associated with varying oxidation states of oxygen [43,44].

The C 1s core–shell analysis of modified ZnAl-LDH suggests that the peaks corresponding to C-C, C-O, and O-C=O relocated to a lower binding energy region, suggesting an interaction between LDH and GO, as illustrated in Figure 2e. The observed alterations in the C 1s core–shell peaks related to various bonds, with the emergence of a new peak indicative of C-O bonding, and the unidentified peaks collectively signify the efficient synthesis of ZnAl-LDH and effective modification by the intercalation of GO.

The N₂ gas adsorption/desorption isotherm, illustrated in Figure S2, reveals that the ZnAl-LDH exhibits a type IV isotherm according to the IUPAC classification, signifying its mesoporous characteristics. A distinct hysteresis loop is seen in the curve, commencing at roughly 0.1 (P/Po) and concluding near 1.0 (P/Po) with an H3 type hysteresis loop, indicating that the pores in the ZnAl-LDH result from the aggregation of plate-like particles. In other words, this indicates the presence of matrix-type inconsistent slit-like pores [45]. The NLDFT approach was employed to analyze the pore size distribution, as illustrated in Figure S2, revealing the existence of multimodal pores within the micro and meso size ranges, with an average pore size of approximately 2 nm.

Table 1. Textural properties of the synthesized nanocomposites obtained from N₂ physisorption.

Sample	SBET (m2/g) ±1	Smeso (m2/g) ±1	Vt CC/g	Vp CC/g	Vmeso CC/g	Vmic CC/g	Average Pore Size (nm)	HF
ZnAl-LDH	76	38	0.064	0.046	0.056	~0.004	2.94	0.033
ZnAl-LDH-5	83	12	0.045	0.008	0.008	0.038	1.05	0.122
ZnAl-LDH-10	115	17	0.071	0.017	0.012	0.052	1.05	0.108
ZnAl-LDH-15	89	13	0.055	0.012	0.005	0.040	1.05	0.108
ZnAl-LDH-25	71	55	0.094	0.066	0.088	0.006	2.44	0.059

delineates the alterations in surface area and average pore size distribution resulting from the incorporation of carbon into the ZnAl-LDH samples, as determined from the adsorption branch. The surface area of the unaltered ZnAl-LDH sample (without carbon addition) was first measured at 76 m²/g. Nevertheless, with the incorporation of carbon, the surface area demonstrated a notable enhancement in the ZnAl-LDH-5 (83 m²/g) and ZnAl-LDH-10 (115 m²/g) samples. Nevertheless, with the continued excessive addition of carbon, the samples began to aggregate once more, leading to a reduction in surface area for ZnAl-LDH-15 (89 m²/g) and ZnAl-LDH-25 (71 m²/g). The fluctuation in average pore size with the gradual incorporation of graphene oxide (GO) into a material, initially increasing and then decreasing, can be elucidated in relation to the interaction between graphene and the ZnAl-LDH porous structure as follows. A minuscule quantity of GO is injected, which can permeate the holes and augment the overall specific surface area owing to the inherent high surface area of graphene. Graphene oxide can create new active sites and modify the current pore structure, leading to a more complex and interconnected pore network that improves the specific surface area (the specific surface area increased by 34% for the ZnAl-LDH-10 sample). As GO grows, the presence of functional groups can enhance adsorption capacities and facilitate interactions with the surrounding material, optimizing pore volume (the ZnAl-LDH-10 nanocomposite had the biggest total pore volume, mesopore volume, and micropore volume and considerable hierarchical factor for increased surface area). At elevated concentrations, GO may initiate aggregation or stacking instead of distributing uniformly throughout the material. This may impede specific pores or reduce their functional dimensions [46].

The integration of graphene oxide into ZnAl-LDH enhanced the surface area owing to the huge surface area of the graphene oxide sheets. This also resulted in an increase in the interlayer spacing and an alteration of the composite material’s microstructure. These effects may lead to enhanced accessibility to active regions and greater efficacy in several applications, including catalysis and adsorption.

The SEM results depicted in Figure S3 elucidate the textural morphology of the materials. Uniform layers can be seen arranged in a feather-like cloud formation for the ZnAl-LDH and low GO-loaded samples (ZnAl-LDH-5 and -10). Specifically, the ZnAl-LDH-10 sample had a homogeneous distribution of nanosized particles, devoid of any discernible agglomerations, unlike the samples loaded with 15 and 25 GO. Figure S4 illustrates a histogram of the computed particle sizes obtained from the corresponding SEM images. The data unequivocally demonstrate that the sample ZnAl-LDH-10 exhibited the smallest particle sizes among all samples, resulting in the largest surface area per unit of material. The EDX examination of the processed samples verified that their elemental composition predominantly consisted of zinc and aluminum, as demonstrated by the data in Figure S5.

Figure 3a illustrates the band gap energy (E_g) measurements of the as-synthesized pure ZnAl-LDH and ZnAl-LDH-10, identified as the optimal catalyst based on the catalytic test section. Calculations of band gap energy in semiconductors are crucial for assessing their suitability for optoelectronic applications. E_g was established using the Tauc relations Equation (1).

$$\mathsf{\alpha }\mathrm{h}\mathsf{\upsilon }={\left(\mathrm{A}\mathrm{h}\mathsf{\upsilon }-{\mathrm{E}}_{\mathrm{g}}\right)}^{\mathrm{n}}$$

(1)

where h$\mathsf{\upsilon }$ represents the energy of a photon (1240/λ), and α denotes the linear absorption coefficient, with n equal to 0.5 for direct transitions and 2 for indirect transitions. The function of Kubelka–Munk (F(R)) is proportional to α; thus, Equation (2) is expressed as follows [47]:

$$\mathrm{F}\left(\mathrm{R}\right)\mathrm{h}\mathsf{\upsilon }={\left(\mathrm{A}\mathrm{h}\mathsf{\upsilon }-{\mathrm{E}}_{\mathrm{g}}\right)}^{\mathrm{n}}$$

(2)

The E_g values of the pure and modified ZnAl-LDH were determined by plotting (F(R). h$\mathsf{\upsilon }$)^0.5 vs. h$\mathsf{\upsilon }$. The ZnAl-LDH nanoparticles demonstrated a distinct photo response in the UV spectrum owing to their substantial energy band gap of 2.5 eV. The ZnAl-LDH composite, upon integration with GO, demonstrated a decreased band gap relative to ZnAl-LDH. This implies a change in the reference and demonstrates that the absorption intensity of GO-modified ZnAl-LDH was markedly increased in comparison to ZnAl-LDH. The production of heterojunction composites had a beneficial photosensitizing effect by the combination of ZnLDH and GO. Furthermore, the incorporation of a semiconductor material into the carbon matrix may induce the formation of vacancies, subsequently resulting in a decrease in the band gap width [48,49].

Figure 3b illustrates the UV-vis diffuse reflection spectra of both pure and modified ZnAl-LDH. ZnAl-LDH has significant absorption between 100 and 400 nm. The unmodified ZnAl-LDH material demonstrates considerable absorption in the near-ultraviolet spectrum. As the wavelength of the light increases, the absorption capacity of the unmodified ZnAl-LDH material progressively declines, ultimately stabilizing around zero. The altered ZnAl-LDH composite material shows an incremental increase in absorption capacity across the wavelength range of 400 to 800 nm, indicating a substantial and strong absorption capability. The LDH substance is removed by GO, modifying the band structure and markedly improving its photocatalytic efficiency. The LDH material has inadequate absorption in the visible light spectrum [50].

2.2. Photocatalytic Degradation of Methylene Blue

The photocatalysts underwent UV light exposure for 100 min, achieving MB removal rates of 39.6%, 65.5%, 92.2%, 78%, and 58.3% in wastewater for the ZnAl-LDH, ZnAl-LDH-5, ZnAl-LDH-10, ZnAl-LDH-15, and ZnAl-LDH-25 composites, respectively (Figure 4). This indicates that the composites displayed efficient photocatalytic activity. Nonetheless, merely 39.6% of the MB was eliminated by pure ZnAl-LDH under identical reaction conditions, indicating that the photocatalytic efficacy of the pure ZnAl-LDH composite is comparatively inadequate.

The data indicate that the incorporation of graphene oxide (GO) in ZnAl-LDH nanocomposites improved their photocatalytic color removal efficacy. The composite ZnAl-LDH-10 exhibited enhanced color removal efficacy relative to the other composites with differing GO loadings. The performance of the ZnAl-LDH/GO nanocomposite materials, which peaked at 10% GO addition and exhibited a notable alteration in degradation trend after 80 min, can be elucidated through various interconnected mechanisms. At 10% GO, the material presumably experiences optimal interactions between the ZnAl-LDH and GO. The functional groups on GO can augment the dispersion and stability of the LDH, resulting in enhanced catalytic or adsorption characteristics [51]. The inclusion of GO can enhance the specific surface area (increased by 34%), hence offering additional active sites for degrading events. This ideal concentration enhances the efficacy of the composite material [52]. In the initial 80 min, processes such as photocatalytic degradation or adsorption are expected to occur swiftly due to the abundant availability of active sites and the efficient synergy among the components [53]. As the reaction proceeds, the active sites on the surface of the nanocomposite may become saturated with the target pollutants, leading to a reduction in degradation efficiency. The kinetics of the degradation response may vary over time. The reaction may initially exhibit quick pseudo-first-order kinetics, but as saturation increases, the kinetics may transition to a slower rate, indicating the depletion of reactive sites [54,55]. Nevertheless, the efficacy of the ZnAl-LDH-5 composite, with a decreased quantity of GO, was relatively lower. The diminished charge transfer rates of the photocatalyst result from the reduced loading of GO in this composite. In contrast, the ZnAl-LDH-15 and ZnAl-LDH-25 composites with higher concentrations of GO exhibited diminished degradation efficiency. This occurrence is likely attributable to variables such as a surplus of electron and energy transfer or an increased rate of recombination between photoinduced electrons and holes. Furthermore, the presence of dark materials might reduce UV light absorption and hinder the generation and separation of electrons and holes on the composite surface.

In addition, it was observed that while GO demonstrated remarkable efficiency at low concentrations, it offered no further benefits at higher concentrations due to its constrained capacity to form a network [56]. Taking these variables into account, the ZnAl-LDH-10 catalyst was chosen for its capacity to attain an advantageous amalgamation of GO loading, uniform distribution of mixed metal oxide particles, and larger surface-supported GO sheets. This finally resulted in improved photocatalytic efficiency for dye elimination.

The effects of different parameters such as the solution pH, dye concentration, and mass of catalysts were extensively studied, and the results are illustrated in Figure 5. The research findings indicate that the photocatalytic effect, accompanied by an induction period, was most effective at a solution pH of 10 (Figure 5a). A pH of 10 provides the most favorable conditions for the effective execution of the degradation process. The reaction rate was markedly reduced at both elevated and diminished pH levels. Under these conditions, the degradation of MB exceeded 40% at pH~8, perhaps due to the development of an intermediate that also absorbs light at the detection wavelength. The observations indicate that the mechanism of photodegradation of MB in the ZnAl-LDH-10 composite is significantly pH-dependent [50]. The initial concentration of MB illustrated in Figure 5b shows that the extent of deterioration fluctuated between 92% and 99%. A substrate concentration of C_o = 5 mg/L was employed in the subsequent assays as it exhibited optimal degradation across the whole time frame. The degradation rate of MB was evaluated using various quantities of catalyst. Figure 5c illustrates that the gradient of the degradation curve escalates with an increase in the quantity of photocatalyst. In theory, an increase in the quantity of photocatalysts will result in greater production of photogenerated electrons and holes, hence enhancing photocatalytic efficacy. Consequently, in this experiment, the ideal photocatalyst quantity was 100 mg. To quantitatively quantify the kinetic behavior of the photodegradation of MB dye in aqueous solution, the experimental data relevant to the Langmuir–Hinshelwood model can be analyzed using the following equation [57]:

$$\ln {\displaystyle \frac{C}{C_0}}=k_{a}t$$

(3)

$${\mathrm{t}}_{1/2}={\displaystyle \frac{ln2}{k_a}}$$

(4)

where C₀ denotes the initial concentration of MB (mg/L), C signifies the MB concentration (mg/L) at a specific time t, and k_a represents the apparent pseudo-first-order rate constant (min⁻¹).

The findings are disclosed in Figure 5d, in which the elevated values of the linear regression coefficients (R²) suggest that the photocatalytic reaction aligned well with pseudo-first-order reaction kinetics, notwithstanding the variation in the starting MB concentration.

Stability is a critical feature for the practical application of photocatalysts in environmental remediation. Consequently, the reusability and stability of ZnAl-LDH were assessed utilizing the same methodology previously described [58]. Following every 100 min of the photodegradation process, the catalysts were subjected to centrifugation, rinsed with 100% ethanol and distilled water, and subsequently reused in the following cycle. Figure 6 illustrates that the photocatalytic activity of the ZnAl-LDH-10 composite retains 88% of its initial efficacy after four recycling iterations, demonstrating the catalyst’s exceptional reusability for prospective practical applications in dye degradation.

The UV-vis DRS study in the previous section indicated that the integration of GO with the ZnAl-LDH composite resulted in a reduction in the band gap to 1.9 eV (Scheme 1b), in contrast to 2.5 eV for pure ZnAl-LDH (Scheme 1a). This section will clarify the tentative mechanism by which the photocatalytic activity of the ZnAl-LDH composites is augmented, as demonstrated in Scheme 1c. A composite photocatalyst of layered double hydroxide (LDH) supported by graphene oxide (GO) is subjected to UV radiation, resulting in the formation of electron–hole pairs within the semiconductor. The research conducted by Sherryna et al. indicates that the excitation of an LDH under UV light facilitates the transition of electrons from the valence band (VB) to the conduction band (CB), hence generating a hole in the valence band ($h_{VB}^{+}$) [59]. The electrons produced in the conduction band are subsequently transported to the surface of the graphene oxide (GO). Wang et al. emphasized that GO serves as an efficient electron collector and transporter, promoting the separation of electron–hole pairs and markedly diminishing the probability of recombination [60]. The transport of electrons improves the total photocatalytic efficiency of the composite. The photogenerated hole may engage with OH⁻ or H₂O on the semiconductor surface, oxidizing them to the ${OH}^{•}$ radical. Simultaneously, the electrons in the conduction band interact with dissolved oxygen molecules to generate the O₂^•− radical, as seen by Lan et al. [13]. The augmented surface area of the ZnAl-LDH/GO composite, as illustrated in the preceding portion of this study, is pivotal to this mechanism. An increased surface area offers additional active sites for the adsorption of reactants, such as OH− and H₂O, which can engage with the photogenerated holes. This promotes the oxidation process, resulting in the generation of the hydroxyl ${OH}^{•}$ radical. Moreover, the augmented surface area facilitates a higher quantity of electrons to interact with dissolved oxygen molecules, resulting in the formation of superoxide radicals O₂^•− [61,62,63]. Ultimately, the produced radicals can degrade the organic dye (MB) into harmless byproducts such as CO₂ and H₂O, as established in the research by Pan. et al. [64], resulting in ZnAl-LDH/GO composites demonstrating better photocatalytic effectiveness compared to pure ZnAl-LDH.

2.3. Adsorption Study

The adsorption and removal efficiency of A.R dye utilizing ZnAl-LDH adsorbents was markedly influenced by pH, adsorbent mass, contact duration, and temperature. The examination of solution pH, conducted within a range of 2 to 10, demonstrated that acidic conditions, particularly at pH 2, resulted in the greatest adsorption effectiveness, as the percentage of dye removal was maximized at this level but decreased markedly at elevated pH values. The ideal pH of 2 was well maintained with HCl, as seen by the results in Figure 7A. The effect of adsorbent mass was examined at a fixed dye concentration of 20 mg/L. Increasing the ZnAl-LDH dosage from 5 to 17.5 mg enhanced the dye removal efficiency from 50.3% to 97.5% with pure ZnAl-LDH and from 62.6% to 99.5% with ZnAl-LDH-10, as illustrated in Figure 7B. The enhancement in efficiency is ascribed to the increased availability of active sites due to the augmented adsorbent mass, with 7.5 mg used for subsequent tests to facilitate the investigation of additional variables affecting adsorption. Moreover, the duration of contact between the dye and the adsorbent was essential for efficient removal, since the adsorption efficiency increased with extended contact time. The majority of dye adsorption transpired within the first 75 min, achieving equilibrium after 120 min, as illustrated in Figure 7C. This indicates a two-stage adsorption process: an initial rapid phase where dye molecules attach to the external surface, followed by a gradual diffusion into the adsorbent’s internal structure. Moreover, temperature was identified as a significant element influencing the adsorption process, with elevated temperatures (283 K to 323 K) improving dye removal effectiveness, as illustrated in Figure 7D. This rise substantiates that the adsorption of A.R dye onto ZnAl-LDH is an endothermic process, enhanced by increasing thermal energy that presumably facilitates increased contact between the dye molecules and the adsorbent surface.

2.4. Kinetic Study

The kinetic behavior of contaminants, such as dye species from aquatic solutions, utilizing pure and ZnAl-LDH/GO nano-sorbents, is critically important as it offers useful insights into chemical routes and adsorption mechanisms. The rates of gross transport, intraparticle diffusion, and film diffusion all affect the amount of dye retained on the adsorbing surface of solid phases, with the fastest process governing the overall transport rate, thereby influencing the quantity of dye retained on the solid phases’ adsorbing surface. The data derived from the experiment examining the impact of shaking duration and the calculation of the kinetic model for the removal of A.R dye by nano-sorbents were analyzed using the Weber–Morris model [65]:

q_t = R_d(t)^1/2

(5)

where R_d is the intraparticle transport rate constant, and q_t is the concentration of adsorbed dyes at any time (t). The numerical values of the constant R_d and qt were determined from the slope of the Weber–Morris curve, and the data are listed in

Table 2. The parameters for various kinetic models for the removal of A.R dye onto synthesized pure ZnAl-LDH and ZnAl-LDH/GO nanocomposites at 20 °C.

Weber–Morris Model
	Rd	R2
Pure	3.85	0.997
5%	4.67	0.994
10%	4.796	0.989
15%	4.53	0.997
Fractional Power Model
	A	B	Ab	R2
Pure	5.349	0.436	2.332	0.991
5%	4.178	0.525	2.194	0.997
10%	5.529	0.479	2.648	0.995
15%	3.603	0.539	1.942	0.997
Pseudo-First-Order Model
	qe, exp, (mg·g−1)	qe, calc, (mg·g−1)	k1	R2
Pure	43.82	47.64	0.027	0.951
5%	48.48	59.43	0.032	0.962
10%	50.87	62.66	0.037	0.967
15%	45.18	56.62	0.032	0.961
Pseudo-Second-Order Model
	qe, exp, (mg·g−1)	qe, calc, (mg·g−1)	k2	R2
Pure	43.82	52.91	5.7 × 10−4	0.982
5%	48.48	62.89	4.1 × 10−4	0.991
10%	50.87	64.94	4.6 × 10−4	0.994
15%	45.18	60.61	3.7 × 10−4	0.984
Elovich Model
	α, [g·(mg·min)−1]	β, [mg·(g·min)−1]		R2
Pure	0.049	10.59		0.938
5%	0.023	13.09		0.967
10%	0.027	13.39		0.974
15%	0.022	12.34		0.957

.

The kinetic model of the fractional power function can be expressed using the following equation [66]:

ln q_t = ln a + b ln t

(6)

where q_t is the quantity of adsorbed dye species per mass unit of pure and modified ZnAl-LDH at any time t, a and b are mathematical coefficients, and the value of b is less than one. The application of the fractional power function equation to the data from the experimental adsorption process, as illustrated in Figure 8B, yielded results that are consistent with the R² values for A.R dye, with the numerical values of a and b displayed in Table 2. These data may indicate the unsuitability of the kinetic model of the fractional power function for representing the adsorption process of A.R dye onto pure and modified ZnAl-LDH sorbents. The Lagergren equation is another important equation that examines the adsorption rate. The variation in adsorption of A.R dye species from an aqueous solution onto pure and modified ZnAl-LDH adsorbents was examined using the Lagergren first-order equation [67]:

$$\log ({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}})=\log {\mathrm{q}}_{\mathrm{e}}-{\displaystyle \frac{{\mathrm{K}}_{\mathrm{L}\mathrm{a}\mathrm{g}}}{2.303}}\mathrm{t}$$

(7)

where q_e represents the equilibrium quantity of adsorbed A.R dye per unit weight of solid phases, q_t denotes the quantity of adsorbed dye per unit mass of solid phases at any time t, and K_Lag is the first-order rate constant. The log (q_e − q_t) vs. time graph (Figure 8C) exhibits a linear correlation. The computed values of K_Lag, q_e, and R² for the elimination of A.R dye using pure and modified ZnAl-LDH are presented in Table 2. The data do not support the first-order kinetics hypothesis for the sorption of A.R dye species onto solid materials [68].

The linearized kinetic pseudo-second-order model equation can be illustrated using the equation below [65], predicated on the following assumptions: (1) the total number of binding sites is dictated by the quantity of adsorbate at equilibrium, and (2) the concentration of adsorbate remains constant over time. The pseudo-second-order model is represented by the following equation [69]:

$${\displaystyle \frac{t}{q_t}}={\displaystyle \frac{1}{{\mathrm{k}}_2{\mathrm{q}}_{\mathrm{e}}^2}}+\left({\displaystyle \frac{1}{{\mathrm{q}}_{\mathrm{e}}}}\right)t$$

(8)

where q_e represents the quantity of adsorbed dyes per unit weight of solid phases at equilibrium, q_t denotes the quantity of adsorbed dyes per unit weight of solid phases at any time t, and k₂ is the coefficient of the pseudo-second-order reaction. The plots of t/q_t vs. t exhibit linearity under these conditions, as illustrated in Figure 8D. The intercept and slope from the A.R dye curve were utilized to calculate the second-order coefficient (k₂) and the equilibrium capacity (q_e) for the dye species, with the resulting data presented in Table 2. The data demonstrate that the kinetic pseudo-second-order model is suitable for characterizing the adsorption process for the removal of A.R dye using both pure and modified ZnAl-LDH, with k₂ being significantly influenced by experimental variables such as contact time, solution pH, concentration, and temperature [70].

The rate equation is derived from the Elovich model [71], commonly utilized to represent adsorption capacity. This kinetic model is mostly applicable to chemisorption and is often beneficial for systems with a heterogeneous adsorption surface. This model can be expressed using the following equation:

$${\mathrm{q}}_{\mathrm{t}}=\mathsf{\beta }\ln \left(\mathsf{\alpha }\mathsf{\beta }\right)+\mathrm{}\mathsf{\beta }\ln \mathrm{t}$$

(9)

where α denotes the initial adsorption rate and β represents the desorption coefficient. The plots of qt vs. ln t are linear, as shown in Figure 8E, and the numerical values of coefficients α and β of the Elovich model were derived from the slopes and intercepts in Figure 8E for A.R dye. The findings acquired are presented in Table 2.

According to the correlation coefficients presented in Table 2 and the experimental data derived from the fractional power function, Lagergren pseudo-first-order, pseudo-second-order, and Elovich models applied to the adsorption process for the removal of A.R dye using pure and modified ZnAl-LDH, the pseudo-second-order kinetic model was determined to be the most suitable for characterizing the adsorption of A.R dye species onto both pure and modified ZnAl-LDH.

2.5. Thermodynamic Characteristics for A.R Dye Uptake by Pure and Modified ZnAl-LDH

Examination of the thermodynamic parameters was performed to elucidate the removal process of A.R dye on both pure and modified ZnAl-LDH solid phases within a temperature range of 283–323 K. The plot of ln K_C against 1000/T for dye adsorption on both pure and modified ZnAl-LDH exhibits a linear correlation across the temperature range of 283–323 K (see Figure 9). As the temperature rises, the equilibrium constant for A.R dye increases, signifying an endothermic nature of its adsorption onto both pure and modified ZnAl-LDH, as demonstrated by the positive enthalpy value (ΔH). The computed values of ΔH, ΔS, and ΔG at 295 K are displayed in

Table 3. The values of enthalpy (ΔH), entropy (ΔS), and Gibbs free energy (ΔG) at 295 K for the adsorption of A.R dye on pure and modified ZnAl-LDH solid phases.

Thermodynamic Equations	Solid Phase		Parameters
${\mathrm{l}\mathrm{n}\mathrm{K}}_{\mathrm{c}}=\frac{-\mathsf{\Delta }\mathrm{H}}{\mathrm{R}\mathrm{T}}+\frac{\mathsf{\Delta }\mathrm{S}}{\mathrm{R}}$ ΔG = ΔH − TΔS ${\mathrm{k}}_{\mathrm{c}}=\frac{{\mathrm{q}}_{\mathrm{e}}}{{\mathrm{C}}_{\mathrm{e}}}$		ΔH	ΔS	ΔG at 295K
	Pure	20.20 ± 1.97	74.35 ± 2.11	−1.58 ± 0.08
	5%	27.63 ± 2.12	102.5 ± 2.43	−2.39 ± 0.09
	10%	37.19 ± 2.04	137.5 ± 2.82	−3.11 ± 0.11
	15%	21.85 ± 1.89	80.93 ± 1.93	−1.86 ± 0.08

ΔH = enthalpy change (kJ/mol); ΔS = entropy (J/K mol); ΔG = Gibbs free energy (kJmol⁻¹). R = gas constant; T = temperature (K); K_C = equilibrium constant; C_e = equilibrium concentration of dyes remaining in solution (mg/L); q_e = the quantity of dyes adsorbed per unit mass of adsorbent after equilibrium (mg/g).

, derived from the linear regression in Figure 9. A positive ΔS value for A.R dye indicates a more flexible solid–liquid interaction, suggesting that the dye species are cohesively linked. The existence of negative (ΔG) values at 295 K for A.R dye signifies that these dyes are predisposed to spontaneously adsorbing onto both pure and modified ZnAl-LDH solid phases. Moreover, the ΔH results indicate that the adsorption of the dye onto both pure and modified ZnAl-LDH occurred through a physical adsorption mechanism for A.R dye.

2.6. Applications Study

To validate the efficacy of pure and modified ZnAl-LDH solid phases for the adsorption and elimination of dye species from aqueous solutions, it is essential to investigate their application to real samples. Three actual water samples—seawater, wastewater, and tap water—were examined; these samples were collected and processed as previously outlined. In the three samples, the concentrations of A.R dye were analyzed and determined to be beneath the UV-vis detection threshold. The three samples were infused with a concentration of 20 mg L⁻¹ of the dye, and 15 mg of pure and modified ZnAl-LDH adsorbents were added. The solution’s pH was adjusted to 2 and agitated for 120 min at 308 K. The removal percentages of A.R dye from the seawater, wastewater, and tap water samples were determined to be 92.0%, 93.1%, and 94.3% for the ZnAl-LDH solid phase and 94.5%, 95.3%, and 97.0% for the ZnAl-LDH-10 solid phase, respectively, as illustrated in Figure 10. The ZnAl-LDH and ZnAl-LDH-10 solid phases were subsequently collected, washed with acetone to eliminate the dyes, desiccated, and recycled for the dye removal procedure. Over four cycles, a virtually identical percentage of adsorption was attained. This indicates that ZnAl-LDH and ZnAl-LDH-10 can be recycled and reprocessed for several adsorption cycles without diminishing adsorption efficacy.

3. Materials and Methods

3.1. Materials

Graphene oxide (GO), Zn(NO₃)₂·6H₂O (99%), acid red (A.R) dye, HCl (37%), and NaOH were purchased from Sigma-Aldrich, Saint Louise, MO, USA. Al(NO₃)₃·9H₂O (98%) was purchased from Fluka, Newport News, VA, USA, and Na₂CO₃ was procured from Merck, Darmstadt, Germany. A sequence buffer solution from Britton–Robinson with a pH range of 2–10 and hydrochloric acid (0.1 M) were utilized as the aquatic adsorption medium in this method. All the chemicals used in this study were of analytical reagent grade and were used as received without further purification. The deionized water used in this study was obtained using a Wellix Plus water purification system (Suwon-si, Republic of Korea).

3.2. Zinc Aluminum–Layered Double Hydroxide/Graphene Oxide (ZnAl-LDH/GO) Synthesis

The ZnAl-LDH/GO samples were manufactured using a modified co-precipitation technique, with a molar ratio of Al:Zn = 1:3 [32]. A negatively charged colloidal dispersion of graphene oxide (GO) nanosheets was achieved by dispersing the desired quantity of GO in an aqueous solution containing 1 M ammonia. Afterwards, two solutions, labeled A and B, were added at the same time. Solution A was made by combining a certain quantity of Zn(NO₃)₂·6H₂O and Al(NO₃)₃·9H₂O, which were dissolved in distilled water. Solution B consisted of a combination of 1 M of NaOH and 0.5 M of Na₂NO₃. The pH of the solution was approximately 10, and it was vigorously stirred at room temperature. The pH was regulated to a value of 10 by manipulating the flow rate of solutions A and B. The entire preparation procedure was conducted using ultrasonic irradiation at a frequency of 50/60 Hz and a power of 400 W. Following the completion of the precipitation procedure, the liquid was thoroughly rinsed until the pH level reached 7. Subsequently, the cake underwent filtration and was subsequently subjected to a drying process at 80 °C for a duration of 18 h. Four distinct ZnAl-LDH/GO hybrids were synthesized, each with a different weight percentage of GO: 5%, 10%, 15%, and 20%. These hybrids were labeled as ZnAl-LDH-5, ZnAl-LDH-10, ZnAl-LDH-15, and ZnAl-LDH-25, respectively, based on their corresponding GO weight percentages. The bare ZnAl-LDH was synthesized using the same procedure, excluding the addition of GO.

3.3. Characterization Techniques

Diverse tools were used to analyze the fabricated bare ZnAl-LDH and modified samples, as described in detail in Text S1 (Page S-1, Supplementary Materials).

3.4. Photocatalytic Degradation of Methylene Blue (MB) Dye

The photocatalytic efficacy of the synthesized nanocomposites was assessed for the breakdown of methylene blue (MB) in aqueous solutions under UV irradiation, serving as a model reaction at ambient temperature and pH 10. The pH of the reaction mixtures was modified utilizing a 0.1 M NaOH and a 0.1 M HNO₃ solution. In a standard experiment, suitable quantities of the catalysts were suspended in MB solutions and agitated in darkness for 30 min to achieve an adsorption–desorption equilibrium between the MB and the photocatalyst surfaces. The photocatalytic system setup is detailed in Text S1 (Page S-2, Supplementary Materials).

3.5. Removal of Acid Red Dye by Adsorption Experiment

The detailed study of the efficacy of the pure and modified ZnAl-LDH solid phases in the extraction and removal of acid red (A.R) dye is given in Text S1 (Page S-3).

4. Conclusions

This study involved the synthesis of ZnAl-LDH and ZnAl-LDH/GO nanocomposites using an eco-friendly and cost-effective technique. The physicochemical characterization utilized many methods to identify the functional groups (FT-IR), crystalline structure (XRD), oxidation states (XPS), and BET surface area of the material. The morphologies of the synthesized materials were analyzed using SEM and TEM. The band gap energy, obtained from the UV-vis DSR data, demonstrated that the ZnAl-LDH/GO heterojunction displayed a reduced bandgap relative to the unaltered ZnAl–layered double hydroxide (2.5 eV). The ZnAl-LDH-10 heterojunction exhibited markedly improved photocatalytic activity relative to the unmodified ZnAl-LDH, promoting the photodegradation of organic pollutants under UV light exposure.

Subsequent adsorption studies utilizing acid red dye were performed, revealing that ZnAl-LDH-10 was the most effective adsorbent among the compounds analyzed. This unique material had the smallest crystallite size and a notably large surface area, as evidenced by XRD, SEM-EDAX, and N₂ physisorption investigations. This innovative material exhibits considerable potential as a solar-powered catalyst and adsorbent for the treatment of wastewater containing textile dyes.