Environmental issues have become raised much worse in recent decades due to economic and industrial growth . Large amounts of hazardous water containing harmful contaminants have been generated because of industrial and agricultural research. Organic dyes are amongst the most prominent kinds of pollutants found in the textile industry wastewater. Tint wastewater has a detrimental impact, including environmental contamination and genotoxicity from products . Textiles, cosmetics, paper mills, plastic, leather industries, pharmaceuticals industries, and even food recently use chemically synthesized hues extensively [3-6]. The majority of them are azo dyes, such as methyl orange, Congo red, Rhodamine 6G, methylene blue, and others, which already have chromophores in their microstructures and therefore are pigmented. Spilling these chemicals into lakes, rivers, and groundwater during manufacturing, on either hand, represents an important public health risk hence dyes mutate into noxious, unpredictable, and cytotoxic components . The discharge of large volumes of colorants into the ecosystem is a matter of social concern, legislative issues, and a significant challenge for scientific experts .
For the treatment of these toxins, many chemical, physical, and biological procedures have already been used such as comprising reverse osmosis, chemical coagulation/flocculation, biological treatments, membrane filtering, and adsorption processes [9-13]. Apart from these approaches, nanomaterials and their related photocatalysts have shown potential in the removal of organic wastes. By illuminating the target wastewater with UV light, a range of semiconductor metal oxide nanoparticles, nanotubes, and nanoflowers have recently been employed as photocatalysts to degrade a variety of organic dyes [14,15].
In the present decade, nanomaterials have received great attention in the literature due to their various properties and applications in drug delivery, electronic devices, microwave devices, biosensors, catalysts, and photocatalysts. Metal oxides may take on a wide variety of structural geometries with an electronic structure that can display metallic, semiconductor, or insulator properties, and so play a critical role in a wide range of chemistry, physics, and materials science applications [16-20]. As a result, dyes are the most studied photocatalytic degradation substrates under solar light or UV light, compared with other natural compounds, because they are heavy industry pollutants . TiO2 , Ag nanoparticles , ZnO , Graphene-Ag/ZnO , and are some of the nano photocatalysts. Heterogeneous photocatalysis utilizing metal oxides has garnered a lot of attention in recent years because of its potential uses in both environmental and organic synthesis [26-30]. Semiconducting metal oxide nanoparticles absorb photons and produce electron-hole pairs, which may be utilized to oxidize or reduce materials on the photocatalyst surface. [31,32]. This feature has sparked a lot of interest in photocatalysts and their potential uses in domains like environment purification [33,34] and sustainable energy like water electrolysis. According to the review, the characteristics of these kinds of materials for purifying water are their durability, relatively low cost, and enhanced degradability . Furthermore, the methodology eliminates the necessity for particle recovery after the treatment method .
In this study, we have synthesized the novel core-shell nanostructure MgZrO3@Fe2O3@ZnO photocatalyst for the degradation of Nigrosin dye [37-38]. The performance of MgZrO3, MgZrO3@Fe2O3, and MgZrO3@Fe2O3@ZnO nanostructure photocatalysts for mineralization of organic pollutants has been comprehensively proved in the current work, UV-DRS, XRD, FE-SEM, EDAX, and HR-TEM techniques were used to characterize the synthesized materials. The photocatalytic performance of MgZrO3@Fe2O3@ZnO was examined for the rapid decolorization of Nigrosin dye under UV-light exposure at room temperature. The catalyst was recycled five times and the structural stability of the recycled nanomaterial was confirmed. The degradation performance of Nigrosin dye was analyzed using UV-VIS and the mechanism of photodegradation was confirmed by liquid chromatography-mass spectrometry. The impact of a double Fe2O3 and ZnO coating on the MgZrO3 photocatalyst was investigated in detail.
Reagents and materials
Commercially available Magnesium chloride (MgCl2, Merck, 99.00%), Zirconium oxychloride (ZrOCl2, Merck, 99.90%), Ferric nitrate (Fe(NO3)3 Merck, 99.50%), Zinc chloride (ZnCl2, Merck, 99.90%), Triton-X-100 (Loba, 98.00%), Sodium chloride (NaCl, Loba, 99.00%), Sodium sulphate (Na2SO4, Loba, 98.00%), Sodium carbonate (Na2(CO3), Loba, 99.50%), Sodium nitrate (NaNO3, Loba, 99.90%), Ethylenediamine tetraacetic acid (EDTA, Sigma Aldrich, 99.00 %) Isopropyl alcohol (IPA, Merck, 99.00%) and Nigrosin dye (NI, Sigma Aldrich, 99.00 %), with an analytical grade were purchased and used without further purification. The stock (100 ml) solution of Nigrosin dye was prepared using deionized water. The experimental solution of the chosen concentration of Nigrosin dye (5, 10, 15 20, and 25 mg/L) was prepared by further dilution of the stock solution with deionized water.
Preparation of Magnesium (II) zirconate nanoparticles
The nanoparticle of MgZrO3 was synthesized by the sol-gel method. The Magnesium chloride [MgCl2] (1 M) and Zirconium oxychloride [ZrOCl2] (1 M) was taken in a Teflon autoclave and dissolved using 100 mL of double-distilled water, Triton X-100 was added in the above solution. To obtain a precipitate solution with a pH of 9-10, a 2 M NaOH solution was gently added under vigorous stirring. The steel-lined Teflon autoclave was kept in an oven at 120 oC for 24 h. The resulting gel was filtered and rinsed with distilled water multiple times. The washed precipitate was then dried at 110 oC for 6 h and then calcined at 800 oC for 6 h to remove the surfactant.
Preparation of MgZrO3@Fe2O3 core-shell nanoparticles
The Fe2O3 coating on magnesium (II) zirconate was carried out by mixing MgZrO3 (1 M) and Ferric nitrate Fe(NO3)3 (2 M) with the appropriate addition of Triton X-100 as a surfactant in 100 ml double distilled water. To achieve the precipitate, a further NaOH solution (1 M) was added dropwise under vigorous stirring for 1 h. The resulting slurry was kept in an oven at 120 oC for 24 h. To remove the organic materials, the precipitate was washed with distilled water and dried at 110 OC for 4 h before being calcined at 650 OC for 6 h. TEM studies validated the coating layer of Fe2O3.
Preparation of MgZrO3@Fe2O3@ZnO core-shell nanoparticles
The MgZrO3@Fe2O3@ZnO core-shell nanoparticles were prepared by mixing MgZrO3@Fe2O3 (1.1 M), Zinc chloride (ZnCl2) (1.2 M), and a known amount of Triton X-100 into 100 ml NaOH (2 M) solution under vigorous stirring for 2 h and kept into steel-lined Teflon autoclave in an oven at 120 oC for 24 h. When the reaction was completed, the precipitate obtained was filtered and cleaned with deionized water and dried at 120 oC for 4 h. The dried product was ground in mortar-pestle to prepare fine powder. The gained powder was smashed and afterward calcined at 700 oC for 6 h to obtain a polycrystalline powder. The two layers of coating on MgZrO3 are authenticated by TEM analysis.
A variety of analytical methods were used to characterize the materials. The optical and band gap energy was studied by using UV-DRS analysis which is confirmed on UV-DRS Shimadzu-2400 Instrument. An XRD analysis of the samples was carried out using a copper X-ray diffractometer operated at 40 kV and 40 mA with a scanning rate of 1o/minute. The morphology of the samples was analyzed using FE-SEM and TEM. A NOVA NANOSEM NPEP303 electron microscope equipped with a 15 kV accelerating voltage instrument was used for the FE-SEM study, and energy-dispersive X-ray analysis techniques (EDAX) were used in conjunction with the same instrument to determine elemental compositions. The crystallinity of magnesium (II) zirconate and core-shell nanostructures were determined using a transmission electron microscope (HRTEM- JEOL/JEM 2100, operating voltages 200 kV, LaB6 Electron gun, point resolution 0.23 nm, Lattice resolution 0.14 nm).
photocatalytic degradation experiments
The photocatalytic activity of the MgZrO3@Fe2O3@ZnO heterocatalyst was evaluated using the degradation of a Nigrosin dye aqueous solution exposed to visible light irradiation. A reaction mixture of dye (≈ 10−5 M) and 0.2 g photocatalyst was irradiated with a 20W UV lamp (Philips) in a photoreactor. The progress of the dye degradation was monitored by measuring the absorbance of the reaction mixture at regular time intervals using a UV-Vis spectrophotometer (Systronics, India) at 576 nm. It was observed that the absorbance of the dye solution decreases with the increasing time of exposure, which indicates that the concentration of Nigrosin dye decreases.
RESULTS AND DISCUSSION
Optical Studies of the MgZrO3@Fe2O3@ZnO Nanoparticles
The spectra of MgZrO3 core-shell nanocrystals for ultraviolet light are shown in Fig. 1. In nanocomposites, the band gap is determined by crystallite size, shape, and structural imperfections . Fig.1 gives the absorption peaks at 260 nm, 410 nm, and 432 nm for MgZrO3, MgZrO3@Fe2O3, and MgZrO3@Fe2O3@ZnO core-shell nanoparticles respectively. Both of the core-shell nanocrystals demonstrated high absorption spectra between 410 and 432 nm. As a result of this hypsochromic variation in absorption, the band gap energies of both materials were affected. The band gap energies were calculated by Eg (eV) = 1240/(wavelength in nm) using absorption peaks and found 4.96 eV for MgZrO3, 3.14 eV for MgZrO3@Fe2O3 core-shell nanoparticles, and 2.98 eV for MgZrO3@Fe2O3@ZnO core-shell nanoparticles.
X-ray diffraction patterns (XRD)
The X-ray diffraction pattern of magnesium zirconate is shown in Fig. 2(a). The pattern indicated a dominant cubic zirconia phase with a small contribution of cubic magnesium oxide, the structure is in good agreement with the standard JCPDS card No. 27-997 and 1-1235 respectively. No other impurity peaks were visualized in the XRD, this indicates good purity of the prepared material. In Fig.2 (b) the presence of Fe2O3 with MgZrO3 and the crystalline nature of hematite (α-Fe2O3) were confirmed by XRD. XRD patterns showed the different reflection peaks with respect to corresponding lattice planes which is matched with JCPDS no.89-1165. The diffraction planes such as (012), (104), (110), (113), (024), (116), (122), (214), and (300) were noticed in the irrespective diffraction angles 26.23o, 36.50o, 42.86o, 56.24o, 64.90o and 74.00o respectively, no other peak from impurities were noticed in the XRD spectra.
Figure 2(c) illustrates the existence of Fe2O3 and ZnO phases in MgZrO3@Fe2O3@ZnO core-shell nanoparticles, indicating that ZnO was coated on the Fe2O3 nanoparticles. The 2θ values with reflection planes at 28.25o (100), 34.39o (002), 36.23o (101), 47.44o (102), 54.34o (101) and 56.78o (200) corresponds to JCPDS Card No. 36-1451. So, all diffraction peaks fit well with the hexagonal wurtzite structure of ZnO. The particle size of core-shell nanoparticles was calculated by using Scherer’s formula , which gives 32.25, 41.74, and 34.25 nm for MgZrO3, MgZrO3@Fe2O3, MgZrO3@Fe2O3@ZnO nanocrystals respectively.
Scanning electron microscope (SEM) analysis
The structural morphology of nanoparticles was studied and analyzed using SEM, and it gives significant information regarding the size, shape, and growth mechanism as shown in Fig. 3(a-c). The SEM imaging of magnesium zirconate particles generated by the sol-gel method is shown in Fig. 3(a). The finding of spherical morphologies with the crystal habit minerals is supported by XRD data . Fig. 3(b) SEM analysis of the surface morphology and distribution of MgZrO3@Fe2O3 crystals reveals that multi-dimensional nanoparticles with spherical shapes were formed. Fig. 3(c) shows typical SEM images of MgZrO3@Fe2O3@ZnO, which reveal the sample’s anatomy, which is susceptible to aggregation due to the spherical and granular surface area to volume ratio at both resolutions.
The aggregation of particles occurs as a result of nanoparticles with an uneven spherical form grown to a bigger size with well-delineated limits. The surface-to-volume ratio of these spherical-shaped nanoparticles is high, giving for a large number of active sites. These sites promote the photocatalytic capacity of nanomaterials by absorbing UV light and forming an electron-hole pair and they are also dependent on grains inside interconnections.
EDAX spectroscopy was used to analyze the distribution and chemical composition of the MgZrO3@Fe2O3@ZnO core shell. The result of the EDAX analysis confirms the presence of Mg, Zr, O, Fe, and Zn elements in the prepared core-shell sample and is confirmed by the elemental peaks attributable to these elements in the inserted EDAX spectrum without other impure peaks.
In the EDAX spectrum, Fig. 3(a) shows strong peaks for magnesium at 1.2 keV and zirconate at 2.2, and 0.2 keV, while the iron is present at 6.5 keV in coated magnesium zirconate with Fe2O3, which is depicted in Fig. 3(b) on the same scale as magnesium and zirconate. In Fig. 3(c) the elemental analysis of MgZrO3@ Fe2O3 coated with zinc oxide shows a significant peak at 8.5 keV due to Zn.
Transmission electron microscopy (TEM) analysis
The surface shape and particle size of the obtained MgZrO3@Fe2O3@ZnO nanocrystals are shown in the TEM picture in Fig. 4(c). The image shows that MgZrO3@Fe2O3@ZnO powder is made up of nanometric particles with nanocrystal sizes of 20-40 nm. It was impossible to examine the particles separately since they overlapped. The morphology was validated by TEM which revealed a spherical shape and varied-sized agglomerates made up of smaller particles in the MgZrO3@Fe2O3@ZnO core-shell. This might be the result of tiny particles coalescing into agglomerates. Fig. 4 shows an analysis of periodic lattice fringes in a high-resolution transmission electron microscopy (HRTEM) picture.
The primary coating layer surrounds the core with Fe2O3 with ZnO seated on top of the Fe2O3 as the secondary coating layer. The homogeneous coating of Fe2O3 and ZnO on MgZrO3 is due to complete layer matching between Fe2O3 and ZnO. In Fig. 4(c), two types of coatings are visible on the MgZrO3 nanoparticles that are the subject of the dual-coated MgZrO3 sample.
Photocatalytic Degradation of Nigrosin dye
The obtained MgZrO3@Fe2O3@ZnO nanoparticles were exploited as an effective photocatalyst for degradation of the Nigrosin dye as a chosen pollutant. To choose the best condition for dye degradation, the different parameters were used such as the effect of dark and visible light irradiation, photocatalyst dosage, initial dye concentration, and irradiation time. A photocatalytic procedure was performed in a UV-transparent glass tube reactor with the absorbance being measured every 10 minutes using a spectrophotometer. Irradiating a dye solution (20 mg/L) with visible light has no effect on absorbance in the absence of a photocatalyst, as illustrated in Fig. 5. There were no changes in the dye degradation when 0.1 g MgZrO3@Fe2O3@ZnO was added to the Nigrosin dye solution (20 mg/L) in the absence of light sources for 80 minutes. This result indicates that there was no dye adsorption on the catalyst. As a result, MgZrO3@Fe2O3@ZnO alone has no deterioration.
Without light illumination, no Nigrosin dye degradation occurs in the presence of a catalyst, illustrating the necessity of light illumination . The effect of visible light on the photolysis degradation of Nigrosin dye was investigated, and the dye concentration was found to be reduced when the dye was exposed to an aqueous solution. This work shows that light irradiation with only 0.1 g of MgZrO3@ Fe2O3@ZnO strengthened catalytic performance for both discoloration and breakdown of Nigrosin dye.
Effect of concentration of dye
The dye degradation is the prime factor that can affect the dye degradation efficiency. The influence of dye concentration on the rate of photocatalytic degradation was investigated utilizing produced magnesium zirconate, MgZrO3@Fe2O3, and MgZrO3@Fe2O3@ZnO core-shell nanoparticles with varying preliminary concentrations of Nigrosin dye (5,10,15,20 and 25 mg/L).
MgZrO3@Fe2O3@ZnO core-shell nanoparticles exhibited improved 94 percent degradation of Nigrosin (20 mg/L) in 60 minutes, MgZrO3@ Fe2O3 degradation in 80 minutes, and MgZrO3 degradation in 80 minutes when the light was switched on for the appropriate period.
Under the light, MgZrO3@ Fe2O3@ZnO allows the color of Nigrosin to fade over time. This is attributed to the appropriate band location and increased surface area as a result of the unique shape. This is due to the success of consolidating Fe2O3 and ZnO onto the MgZrO3 spinel, which resulted in improved structural, morphological, and optical characteristics. The photocatalytic degradation adequacy of MgZrO3@Fe2O3@ZnO decreased as the starting concentration of Nigrosin dye grew.
Effect of amount of catalyst
The impact of varying the quantity of catalyst on the rate of dye degradation has been seen over a wide range (0.1-0.5 g) while maintaining all other parameters constant. The degradation rate rises as the concentration of Photocatalyst increases, with MgZrO3@ Fe2O3@ZnO core-shell reaching 0.2 g. With an increase in catalyst quantity above this limit, the rate of reaction becomes almost constant. The quantity of magnesium zirconate core-shell nanoparticles (0.1-0.5 g) in the presence of UV - visible light with Nigrosin dye (20 mM/L) solution is shown in Fig. 7.
As the catalyst concentration rises, the degradation rate lowered. The light scattering of the catalyst achieved this outcome. The photo-activated suspension volume was lowered when light penetrated the dye solution as the catalyst dosage was increased . Some areas of the catalyst surface were found to be unresponsive to photon and dye absorption in this condition .
To evaluate the optical time of photodegradation, a series of tests were performed at a variant time and the results are given in Fig. 8. The λmax of the chosen dye is located at 576 nm. It can be seen that 96 % of the dye degraded within 60 min. In this study, 0.2 g of MgZrO3@ Fe2O3@ZnO core-shell nanomaterials per 100 ml of 20 mM/L Nigrosin dye solution was said to be the least addition. The degradation of dye before and after exposure to visible light and photocatalyst is depicted in Fig. 8. As the irradiation duration lengthens, the chromophoric absorption peak at 576 nm completely reduces the dark purple colorization of the Nigrosin dye solution. The color of the solution (absorbance 576 nm) changed considerably, indicating that in the presence of MgZrO3@Fe2O3@ZnO core-shell nanoparticles, the absorbance falls to a minimum within 60 minutes.
Advanced oxidation processes (APO) include photocatalytic pollution clean-up, which has shown to be a difficult technique for organic compounds. Because semiconductors are inexpensive and can easily mineralize diverse organic contaminants, this approach is more suited and efficient than other APOs. The photocatalytic discoloration of Nigrosin dye occurs as a result of the following events.
This process prevents the recombination of electrons and holes. The radicals formed in the foregoing processes, .OH and .O2, can then react with the Nigrosin dye to make additional species, which is the direct cause of the dye’s discoloration.
LC-MS analysis of the Nigrosin dye breakdown pathway
To illustrate the degradation pathway and establish the degradation of colorless species, a qualitative evaluation was carried out using Liquid Chromatography-Mass Spectrometry (LC-MS). Under UV-visible irradiation, the mass spectrum of Nigrosin dye solution before photodegradation by MgZrO3@Fe2O3@ZnO core-shell nanoparticles is presented in Fig. 9(a). The decrease in m/z of Nigrosin dye revealed fragmentation products is depicted in Fig. 9(b). The structure of various products was postulated using HPLC-MS fragmentation. The m/z 512, 363, 159, 151, 143, 127, 123, 77, and 54 fragments showed excellent degradation of a large Nigrosin dye molecule with a molecular weight of 616. The possible mechanism of degradation of Nigrosin dye was given in Fig. 10.
Detection of active species by scavenger experiment
The use of scavengers to trap holes and free electrons is a beneficial method for elucidating the process of organic pollutant photocatalysis. Fig. 11 shows the photocatalytic activity of magnesium zirconate core-shell nanoparticles for Nigrosin dye.
The OH active species were captured using EDTA, whereas the O2 active species were trapped with isopropyl alcohol . During the scavenger experiment, it was discovered that a very little amount of EDTA hindered the breakdown process. As a deactivator of OH active species, EDTA shows that the most common active species involved in photocatalytic degradation were OH, which is produced by the absorption of UV-Visible light and forms a large number of holes.
Furthermore, EDTA molecules can interact with the surface of the catalyst to restrict dye molecule interaction, which is necessary for photodegradation, demonstrating the importance of surface interaction. Furthermore, adding IPA (Isopropyl alcohol) to magnesium zirconate and core-shell nanoparticles lowered their catalytic efficacy somewhat, supporting the little role of O2 radicals in the destruction of both organic molecules. As a consequence, the capturing experiment confirmed the OH’s predominance in Nigrosin dye degradation .
The effect of salinity
When salts are added, the salting-out effect may affect the removal efficiency. In the presence of 0.5 gm NaCl, Na2SO4, Na2(CO3), and NaNO3 aqueous solutions, the influence of salt on removal efficiency was investigated. In the addition of inorganic salts, the dye removal efficiency of MgZrO3@Fe2O3@ZnO rises, as seen in Fig.12. This demonstrates that dye adsorption on MgZrO3@Fe2O3@ZnO is aided by the presence of salt. The salting-out idea may be used to explain how salt can help with adsorption [47,48] The solubility of the dye in water is increasingly restricted as the number of salt increases. The salting-out effect reduces the dye’s solubility, causing more dye molecules to diffuse to the surface of MgZrO3@Fe2O3@ZnO and increasing adsorption efficiency. As a result, the highest Nigrosin dye elimination was obtained in the presence of NaNO3 salt.
In Fig. 13, when the absorbance data of Nigrosin dye are plotted in ln(Ao/A) versus time dependant normalized dye concentrations (which is the ratio between the initial concentration and the concentration upon reaction), a linear plot is obtained. This indicates that the decomposition of Nigrosin dye follows first-order kinetics. The rate constant of MgZrO3@Fe2O3@ZnO core-shell nanoparticles is 0.06329 min-1.
Chemical Oxygen Demand (COD)
The mineralization of nigrosin dye was confirmed by measuring the decrease in COD value. COD of nigrosin dye solution was estimated before and after the photocatalytic treatment and the photodegradation efficiency of the catalyst was calculated using the following equation:
%. η = x 100
% η = Photodegradation efficiency (%), COD before = COD of dye solution before exposure of light and CODafter = COD of dye solution after exposure of light COD of dye solution before and after exposure of light has been determined by the redox method. The photodegradation efficiency after 60 min of exposure to light on Nigrosin dye using MgZrO3@Fe2O3@ZnO nanoparticles was found to be 96.08. The COD of dye solution before dye degradation was found 71.34 mg/l whereas after dye degradation it was decreased up to 2.79 mg/l.
For each catalytic process, the catalyst’s stability and recyclability are more crucial. After the process, the MgZrO3@Fe2O3@ZnO core-shell nanostructure catalyst was recovered and its recyclability for dye degradation was investigated. For the recyclability investigation, the same degrading technique was used.
The produced MgZrO3@Fe2O3@ZnO core-shell nanostructure catalyst was photo catalytically reusable for three cycles, with a slight decline in efficacy until the fourth cycle, when the percent degradation reduces drastically, as shown in Fig. 14. This might be due to a loss of catalyst and a reduction in active sites on the catalytic surface throughout the recovery process.
Comparison of the results
Table 2 compares Nigrosin dye catalytic performance with other photocatalysts described in the literature. This shows that photocatalysts are mentioned in a modest number of papers. Moreover, the current work is the first of its sort to report the clearance of Nigrosin dye with novel core-shell nanoparticles. In comparison to Table 1, the photocatalytic degradation performance of the MgZrO3@Fe2O3@ZnO core-shell nanoparticles was considerably greater in a shorter time. In comparison to the mentioned papers, this shows that MgZrO3@Fe2O3@ZnO core-shell nanoparticles are a suitable choice and an alternative to traditional photocatalysts. The turnover number (TON) has been calculated by using this formula .
MgZrO3@ Fe2O3@ZnO core-shell nanoparticles were effectively synthesized using the sol-gel method and then employed for UV light-driven photocatalysis of Nigrosin dye. MgZrO3 was effectively stabilized in an aqueous solution by a double covering of Fe2O3 and ZnO, which also exhibited its oxidation. The developed catalyst showed enhanced photocatalytic efficiency at the lowest catalyst dosage. The catalyst was very efficient for several cycles, with the increased formation and low recombine frequencies of electron-hole pairs; consequently, it may be employed as an appropriate photocatalyst for dye pollutant treatment from polluted water. Furthermore, the MgZrO3@Fe2O3@ZnO core-shell nanoparticles can be used four times without losing their degrading activity, and the catalyst can be easily extracted via filtering. The photocatalytic material was also steady, according to the reusability tests. This study might lead to the development of a dye wastewater treatment technology.
DECLARATION OF CONFLICTS INTEREST
The authors state that they have no known competing financial interests or personal ties that might have influenced the research presented in this study.