Efficient solar heterogeneous photocatalytic degradation of metronidazole using heterojunction semiconductors hybrid nanocomposite, layered double hydroxides

During the past few decades the consumption of pharmaceutical drugs was dramatically increased, which has increased their discharge into the environment. Several pharmaceuticals are detected in the environment originally from both human and veterinary use (Kanakaraju et al. 2018). The remediation of pharmaceuticals is not completely achieved; it has been reported that about 64% of emerging contaminants are only partially removed (50%), while up to 9% are not removed during their treatment (Pereira et al. 2019).

The application of a new process for removal of emerging contaminants from wastewaters has become necessary. In this context, the heterogeneous photocatalysis can be considered more promising for the removal of this kind of pollutants. The photocatalysis has attracted more attention nowadays, since it is considered low cost, environmentally friendly and efficient.

Recent reports have demonstrated the efficient methods as adsorption and photocatalytic degradation for the removal of pharmaceutical pollutions. Nguyen et al. have conducted a research involving the removal of acetaminophen using commercially activated carbon, their results demonstrated that the adsorption occurred rapidly, with around 52% of acetaminophen in solution (517 mg/L) being removed within 5 min of contact (Nguyen et al. 2020). Czech et al. studied the photocatalytic of metoprolol and triclosan onto elsmoreite/tungsten oxide@ZnS they observed degradation of 50% of metoprolol and 70% of triclosan after exposure time of about 60 min under visible light (Czech et al. 2020). Ahmadpour et al. demonstrate excellent photocatalytic efficiency of diclofenac onto TiO₂@ZnFe₂O₄/Pd; they obtained degradation efficiency of 86.1% within 120 min under solar light (Ahmadpour et al. 2020).

The activation of semiconductor by the solar light irradiation is considered as renewable and inexpensive energy sources (Fagan et al. 2016; Mecha et al. 2016). The semiconductors used as photocatalysts are ZnO (Zhang et al. 2017), TiO₂ (Pichat et al. 2005), Cds (He et al. 2012) and SnO₂ (Kim et al. 2016). This recent year's the use of layered double hydroxides (LDH) as photocatalysts attracted more attention due to their advantages such as high specific surface area, easily controllable interlayer anions, simple synthesis, ability to generate electron-hole carriers, low cost and stability. LDH, also known as hydrotalcite-like compounds, are a class of synthetic anionic layered clays containing brucite-like layers and positively-charged sheets. They can be represented by the general formula: ⁠; where M²⁺ and M³⁺ are the divalent and trivalent cations, respectively, Aⁿ⁻ represents the interlayer anion with a charge (n-) and x is equal to the ratio of M³⁺/(M²⁺ + M³⁺) with a value varying in the range of 0.17–0.50 (Zhao et al. 2012).

Photocatalysis using hybrids of semiconductors and LDH get great attention, and could be the most privileged catalysts (Prasad et al. 2019) as they have unique features because of the coupling of the individual characteristics of the parent sample.

In this work, we report the obtained results of high efficiency removal of pharmaceutical pollutant by efficient and environment friendly solar photocatalytic degradation using the heterojunction semiconductors hybrid nanocomposite, LDH. The photocatalysts were prepared via a coprecipitation method and theirs structural and physicochemical proprieties were analyzed by several techniques. The photocatalytic removal behavior of MNZ on the synthesized semiconductor/LDH using solar irradiations is investigated as a function of photocatalyst dose, initial pollutant concentration and nature of oxide incorporated. The results showed that the 5%MoO₃/MgAl-LDH-C can significantly improve remediation of pharmaceuticals.

The MgAl-LDH materials were prepared by co-precipitation method at constant pH (10) with cationic ratio (Mg²⁺/Al³⁺) equal to 2. 100 mL of nitrate salts solution containing Al(NO₃)₃·9H₂O and Mg(NO₃)₂·6H₂O was added dropwise to an alkaline solution containing Na₂CO₃ and NaOH under vigorous stirring. The suspension thus obtained was aged at 60 °C for 8 h, then filtered, washed with distilled water, was finally dried overnight at 80 °C, giving MgAl-LDH solids and the calcined solids were obtained by calcining MgAl-LDH samples at 600 °C for 5 h.

The CuO/MgAl-LDH and MoO₃/MgAl-LDH materials were synthesized by the same method as MgAl-LDH but by adding 5% of CuO or MoO₃ oxides in nitrate salts solution.

The samples were named as follows: MgAl-LDH, 5%CuO/MgAl-LDH, 5%MoO₃/MgAl-LDH and MgAl-LDH-C, 5%CuO/MgAl-LDH-C, 5%MoO₃/MgAl-LDH-C for calcined samples, where the letter C refers to calcined.

The chemical composition of each solid was established using atomic absorption spectroscopy (AAS) 240FS AA. X-ray diffraction (XRD) patterns were recorded with Siemens D500 diffrcatometer (CuKαλ = 1.54 Å) at a scanning speed of 2θ/ min from 5 to 80°. Fourier transform infrared (FTIR) spectra were recorded on a Bruker ALPHA spectrophotometer, at a resolution of 2 cm⁻¹ and averaging over 20 scans, in the range 400 cm⁻¹–4,000 cm⁻¹. The morphology of the samples were investigated using scanning electron microscopy (Quanta 250) with an accelerating voltage of 20 kV, combined with energy dispersive X-ray spectroscopy (Système EDX Bruker EDS Quantax 200) for the determination of materials composition.

The solar photocatalytic degradation process was conducted in a batch condition using a 500 mL beaker. The MNZ solution with photocatalyst was exposed to sunlight daily between 11:00 a.m. and 16:00 p.m. while magnetically stirred using a magnetic stirrer.

Prior to irradiation, the solution was magnetically stirred in the dark for 60 min to reach the adsorption-desorption equilibrium. Periodically 5 mL of sample was withdrawn from the beaker and centrifuged (4,000 rpm for 20 min) then the filtrate was analyzed by UV–vis spectrophotometer at 319 nm to determine the residual concentration of MNZ.

The chemical composition of 5%MoO₃/MgAl-LDH-C and 5%CuO/MgAl-LDH-C was determined by atomic absorption spectroscopy (AAS). Table 1 gives the chemical composition and formula of the synthesized materials.

X-ray diffraction (XRD) patterns of the LDH photocatalysts synthesized with addition of CuO and MoO₃ oxides are displayed in Figure 1.

The XRD pattern of both samples 5% CuO/MgAl-LDH and 5% MoO3/MgAl-LDH (Figure 1(A)–1(b) and 1(B)–1(e)) reveals the presence of LDH structure, which is related to the apparition of sharp and symmetric reflections at lower 2Ɵ value; 2Ɵ = 11.12 (d₀₀₃), 2Ɵ = 22.11 (d₀₀₆) and 2Ɵ = 61.22 (d₁₁₀) (Parida & Mohapatra 2012). At higher 2Ɵ value a broad and asymmetric peaks are observed, 2Ɵ = 39 (d₀₁₅), 2Ɵ = 47.1 (d₀₁₈) and 2Ɵ = 61 (d₁₁₀). Furthermore, in the XRD pattern of 5% CuO/MgAl-LDH (Figure 1(A)–1(b)) two peaks located at 2Ɵ = 36 and 39 corresponds to the reflection of CuO are observed (Rat'ko et al. 2012). XRD pattern of 5% MoO₃/MgAl-LDH (Figure 1(B)–1(e)) shows two peaks at 2Ɵ = 25 and 27 corresponds to MoO₃ reflections.

Upon calcination at 600 °C, we can observe the collapse of LDH structure and formation of oxides metal. The XRD spectra of 5%CuO/MgAl-LDH-C (Figure 1(A)–1(c)) shows peaks related to MgO and CuO, and the spectra of 5%MoO₃/MgAl-LDH-C (Figure 1(B)–1(f)) shows peaks related to MoO₃ and MgO; these results are similar to those previously published elsewhere (Valeikiene et al. 2020).

The FTIR pattern of synthesized samples is shown in Figure 2. The spectrum shows the typical absorption peaks of carbonate LDH. A broad peak observed at 3,381.72 cm⁻¹ for 5%MoO₃/MgAl/LDH sample (Figure 2(a)) and at 3,428.09 cm⁻¹ for 5%CuO/MgAl/LDH sample (Figure 2(b)) is related to the hydroxyl stretching vibrations ν(OH).

The sharp absorption peak at about 1,341 cm⁻¹ for 5%MoO3/MgAl/LDH sample and at 1,364.9 cm⁻¹ for 5%CuO/MgAl/LDH sample can be assigned to the asymmetric stretching mode of the carbonate anions (Jianfeng et al. 2015). Finally, the bands ranging from 400 to 800 cm⁻¹ can be attributed to the characteristic stretching bands of M-O and O-M-O vibration (He & Zhang 2007; Cheng et al. 2010).

As seen from the FTIR spectrum of the calcined samples (Figure 2(c) and 2(d)) all the absorption bands are weakened, compared to the as synthesized LDH, the peak intensity of ions become relatively weaker, indicating that more ions in the interlayer are removed.

The morphology and particle size of synthesized solids were analyzed by TEM and Figure 3 depicts the TEM images. The TEM images of 5%CuO/MgAl-LDH and 5%MoO₃/MgAl-LDH (Figure 3(c) and 3(b)) indicates that the solids show a well-dispersed LDH nanocrystal structure with average particles size in the range of 35–55 nm. While the TEM images of CuO and MoO₃ (Figure 3(a) and 3(d)) shows that the oxides metal CuO and MoO₃ exhibits a core-shell structure.

The LDH photocatalysts synthesized with addition of CuO and MoO₃ were tested for MNZ solar photocatalysis at the initial MNZ concentration of 10 mg/L for 160 min. As illustrated in Figure 4, the best photocatalysis efficiency was reached with 5%MoO₃/MgAl-LDH-C, the higher efficiency of 5%MoO₃/MgAl-LDH-C was related to the synergistic effects of MoO₃ semiconductor and MgAl-LDH, upon hybridation of these two materials a new semiconductor has been generated with new electric properties.

The photocatalytic performance of 5%MoO₃/MgAl-LDH-C as function of MNZ initial concentration was evaluated by varing the initial MNZ concentration from 10 m/L to 70 mg/L with photocatalyst dose of 1.5 g/L at neutral pH. As shown in Figure 5, the photocatalytic degradation efficiency decreased with increasing the initial MNZ concentration. The photocatalytic efficiency is related to the OH radicals formation on photocatalyst surface. In this case, the weight of photocatalyst is kept constant for each MNZ initial concentration, and the number of OH radicals generated remains constant, while MNZ concentration increases. The ration of OH radicals/MNZ molecules decreases with higher initial MNZ concentration and the probability of MNZ to react with OH radicals decreases, resulting in lower removal efficiency. On the other hand, the increasing in MNZ concentration can lead to the decreases of the photon path length that penetrates the pollution solution (Behzad et al. 2019).

Figure 6 shows the absorption intensity of MNZ solar photocatalysis using 1.5 g/L of 5%MoO₃/MgAl-LDH-C as photocatalyst, the important change in the peak intensity at 319 nm is observed. Increasing the solar exposure time the absorption intensity decreased.

In order to assess the effect of the photocatalyst dose on the photocatalytic efficiency, the photocatalytic experiments with different doses of photocatlyst were conducted. As shown in Figure 7 the increase of photocatalyst dose from 0.5 g/L to 2 g/L improves the photocatalytic degradation efficiency. With increasing the photocatalyst dose, the active sites on the photocatalyst surface will be increased, thus enhancing the photons absorbed by the photocatalyst causing the enhancement of photocatalytic efficiency. Therefore, the optimum photocatalyst dose was fixed as 1.5 g/L for the next studies.

By further increasing the photocatalyst dose until 2 g/L, high penetration may partly be impeded due to screening effects, thus reducing degradation efficiency (Behzad et al. 2019).

The reaction kinetic of MNZ removal by 5%MoO₃/MgAl-LDH-C was investigated using the Langmuir–Hinshelwood model.

The apparent rate constant K_app and R² were calculated from the linear plot ln (C₀/C) versus irradiation time (t) as shown in Figure 8 and the obtained results are illustrated in Table 2.

MNZ photocatalytic degradation results well-fitted the pseudo-first-order kinetic model, with coefficients (R²) more than 0.93. Increasing the MNZ initial concentration the apparent rate decreased, this result is related to the decreases in the hydroxyl radical generated, when increasing the MNZ initial concentration the solar light photons will be adsorbed by MNZ moleculs rather than the photocatalyst (Collazzo et al. 2012; Kalebaila & Fairbridge 2014).

Table 3 illustrates the photocatalytic MNZ degradation studies published in the literature. When comparing these photocatalytic MNZ degradation studies with our study, it can be seen that these works use artificial radiation, making the process expensive. In our work, we tested higher concentration of MNZ (10, 30, 50 and 70 mg/L), and used inexpensive sunlight radiation. The utilization of solar-light as a promoter in the field of molecular photodegradation can provide many advantages such as eliminating the use of energy-demanding UV-lamps, which are usually made of toxic mercury. Several studies have reported about MNZ photocatalytic degradation. However, few of them focus on the solar radiation.

The reusability of MoO₃/MgAl-LDH-C (1.5 g/L) catalyst in the photodegradation of MNZ (C_MNZ = 10 mg/L) at free pH was also studied under sunlight conditions over the course of 60 min. Figure 9 shows the results of the reusability test of MoO₃/MgAl-LDH-C photocatalyst for the photocatalytic degradation of MNZ within four successive cycles. The photocatalyst in any run was collected, washed with distilled water, dried and then used in a new experiment. It could be observed that the degradation efficiency of MNZ was decreased slightly after five repeated experimental runs, thereby indicating that the as-prepared MoO₃/MgAl-LDH-C could be used as a promising photocatalyst for the degradation of organic pollutants with a significant reusability potential.

The hybrid semiconductors MnO₃, CuO, uncalcined and calcined MgAl-LDH have been successfully synthesized by co-precipitation method and tested for MNZ removal by solar photocatalysis process. The 5%MnO₃/MgAl-LDH-C solid is very promising photocatalyst for the removal of organic pollutants for wastewater using photocatalysis method under solar irradiation, since the solar irradiation is an environmentally friendly light source, the MNZ solution is used at pH neutral without adjustment and the photocatalysis is realised without addition of scavengers.

All relevant data are included in the paper or its Supplementary Information.