1 Introduction

Nowadays, water pollution is considered a serious issue for the whole world. Researchers developed various materials for treating this problem with different techniques. The textile and dye production industries discharge hazardous and non-biodegradable azo dyes into the environment, including methyl orange [1]. Industrial processing can result in the loss of up to 20% of total dye output, which pollutes the environment and fuels eutrophication, which affects aquatic life. Most of the present treatment methods for these effluents are physicochemical, which creates a disposal issue for the leftover sludge. Contaminants are not eliminated by processes such as chemical precipitation and separation of pollutants, electrocoagulation, and adsorption-based elimination; instead, they are transferred to solids, which are normally disposed of in landfills [2, 3]. Nanotechnology has a lot of promise for dye degradation since nanoparticles may chemically react with dyes to generate non-toxic compounds that don't need to be removed [4, 5]. Photocatalysis is a nanotechnology-based approach that is currently being researched [6,7,8,9,10,11,12].

An effective photocatalyst can function as a material during the photocatalytic process, but it has always been challenging for researchers to create the necessary form, size, and porosity while keeping prices low [13,14,15,16]. For more effective photocatalytic degradation, the researchers are using band gap materials that are visible light sensitive. In previous works lots of photocatalysts were employed such as titanium dioxide (TiO2) [17], zinc oxide (ZnO) [18, 19], copper oxide (CuO) [14], ferric oxide (Fe2O3) [20], and zinc sulfide (ZnS) [21,22,23,24].

In recent decades, gadolinium zirconate Gd2Zr2O7, as a model of pyrochlore (A2B2O7) structure is considered the comprehensive concern of scientists imputed to their unique properties including excellent mechanical properties, flexible space structure, good radiation resistance, strong ray absorption ability, high thermal expansion, phase stability in a wide range of temperature, Excellent chemical stability, high ionic conductivity of 10–2 S/cm at 1000 K, thermodynamic stability up to the melting point (> 2000 °C), and corrosion resistance are all characteristics of this material. It has a low thermal conductivity of 1.6 W/(mK) at 1000 °C [25,26,27,28]. Accordingly, Gd2Zr2O7 is a promising employ in immobilization matrix for high-level nuclear waste, especially actinides, solid oxide fuel cells, catalysts, sensors, oxygen separation membranes, thermal barrier coating, luminescent materials, optical fields, etc. [28,29,30]. The pyrochlore structure A2B2O7 belongs to the Fd-3m space group with a cubic structure where larger trivalent rare-earth atom cations, may occupy the A site in its unit cell, whereas smaller tetravalent transition atom cations, such as Ti, Zr, and Sn, may occupy the B site. The stability of the pyrochlore structure corresponds to the radius ratio of A and B site cations. When the ionic radius ratio of A and B sites is lower than 1.46, the disorder defect fluorite (Fm-3m) structure is favored when both cations, namely A3+ and B4+ randomly distributed on A and B positions [25, 31].

Therefore, this work focuses on tailoring gadolinium zirconate Gd2Zr2O7 nanopowders employing tartaric acid as fuel in a sol–gel auto-combustion technique. In this context, the synthesis of Gd2Zr2O7 nanopowders possesses fine particle size, uniform microstructure, good homogeneity, good surface area, and high chemical activities. The change in phase evolution, microstructure, and optical characteristics was explored based on X-ray diffraction profile (XRD), Fourier transforms infrared (FT-IR), transmission electron microscopy (TEM), and UV–VIS-Near IR spectrophotometer [32].

It is worth noting that nanomaterials have many applications that serve society and contribute effectively to solving environmental problems with great effectiveness and lower cost. There are lots of techniques that are used to prepare nanometer materials and are discussed widely in several types of research [33,34,35,36,37,38]. Environmental remediation is one of the most popular applications in which nanometric materials play an effective role.

The synthesized nanopowders were evaluated for the phytoremediation of crystal violet (CV) under UV light irradiation. For instance, crystal violet is a cationic triphenylmethane dye, toxic, carcinogenic, and may cause acute chronic effects on human and animal life. This dye is a prospective biohazard owing to its mitotic poisoning behavior with very intense color, therefore, a little quantity of CV in water can diminish the access to sunlight and hinder the photosynthesis process [39, 40]. Consequently, the complete removal of such dye from water is almost desirable.

2 Materials and Methods

2.1 Preparation of the Gd2Zr2O7 Nanoparticles

Materials include gadolinium nitrate hexahydrate Gd(NO3)3·6H2O supplied by Alfa Aesar Co. also involved zirconyl oxychloride octahydrate ZrOCl2·8H2O supplied by Alpha Chemika Co. and tartaric acid COOH(CHOH)2COOH acts as a fuel and a complexing agent supplied by Lanxess Co.

Sol–gel auto-combustion has been used for the synthesis of Gd2Zr2O7 nanoparticles with success. This procedure involved combining equal amounts of ZrOCl2·8H2O and Gd(NO3)3·6H2O, which were then dissolved in 40 ml of distilled water in a 1:1 molar ratio. Tartaric acid was added to the mixture while it was being heated on a hot plate with magnetic stirring until a viscous precursor was produced. The produced precursor was then completely dried in an oven for a whole night at 100 °C. To create Gd2Zr2O7 nanoparticles, the dried sample will then be fired in a furnace for 2 h at 1100 °C (Fig. 1).

Fig. 1
figure 1

Preparation of Gd2Zr2O7 pyrochlore nanopowder

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2.2 Sample Characterization

Thermo Scientific's Nicolet iS10 spectrophotometer is frequently used to measure the Fourier transform infrared (FTIR) absorption spectra for prepared samples at room temperature, which range from 4000 to 400 cm−1. The tested sample was created using the compression procedure, which entailed mixing 200 mg of KBr and 2 mg of powdered Gd2Zr2O7 sample to create a transparent disc. The measuring method started straight soon when the mixture was given at a weight of 5 tons/cm2. XRD is a useful tool for identifying the crystalline phases formed in our sample throughout the sintering phase, as well as ensuring the crystallinity of the final products. The diffraction pattern of the investigated material was obtained using an X-ray diffractometer (Philips PW 1390). UV/Vis. Spectroscopy is used to obtain the optical properties of the prepared pyrochlore and also its energy gap. Jasco V770, Japan was the UV–Vis spectrophotometer used in this investigation. Unless we use appropriate technology for that purpose, measuring the concentration of polluted water after treatment is a difficult task. Fortunately, the vis. Spectrometer (Jenway 7200) made this simple. Before and after the treatment procedure, the polluted water's absorption spectrum was examined, and the amounts were determined using a certain formula. To measure the size of the Gd2Zr2O7 particles, we needed a specialized instrument. A transmission electron microscope, The (Jeol JEM-1011, Japan), was employed.

2.3 Photoreactor and Photo-Catalytic Measurement

Crystal violet has a large absorption band in the visible portion of the electromagnetic spectrum (591 nm) so its solution can be analyzed by visible spectrophotometer. Although CV dye is used in the textile, culinary, and cosmetic industries, it has the potential to damage aquatic environments.

CV was chosen as the target organic pollutant to explore how it degraded utilizing Gd2Zr2O7 nanoparticles under the effect of UV irradiation. The photocatalytic oxidation studies of Gd2Zr2O7 nanoparticles were conducted in a 100 ml beaker mounted on a magnetic stirrer and exposed to two parallel, 36 Watt UV lamps above the beaker, which was contained in a lamina with silvered inside walls. The catalyst was added to a beaker containing 50 ml of an aqueous solution of CV to obtain even photocatalyst dispersion and to make it possible to create an adsorption–desorption equilibrium (10 ppm). The reaction vessel was filled with a 1 ml solution of 33% H2O2. The addition of H2O2 was considered to be the catalyst to undergo a Fenton-like reaction. At various times during the procedure, liquid aliquots were taken out of the vessel (15 min). Centrifuging was done on the liquid before analysis. Using a visible spectrophotometer, the liquid solutions were examined following reactions. Based on the organic dye's rate of degradation, kinetic investigations were carried out. The degradation processes of the dye molecules may be expressed as follows using the Langmuir–Hinshelwood model, which assumes that dye degradation follows pseudo-first-order kinetics:

$$ - {\text{dC}}/{\text{dt }} = {\text{ K}}_{{{\text{app}}}} \left[ {\text{C}} \right] $$
(1)

With the restriction that C = Co at t = 0, where Co is the initial concentration in the bulk solution following dark adsorption and t is the reaction time, the connection will be as follows.

$$ - {\text{ln}}\left[ {{\text{A}}/{\text{A}}_{{\text{o}}} } \right] \, = {\text{ K}}_{{{\text{app}}}} {\text{t}} $$
(2)

where A is the dye's absorbance at time t, and Ao is the dye's starting absorbance. t is the length of time the dye was exposed to light, and Kapp is the apparent reaction rate constant. The slope of the linear plots should equal the apparent first-order rate constant when graphing − ln[At/A0] versus time (Kapp). The (Kapp) values reveal the rate at which photocatalyst materials degrade CV molecules when UV light and H2O2 are present. This is how CV removal efficiency (η) was determined:

$$ \eta \left( \% \right) \, = {1}00[({\text{A}}_{{\text{o}}} - {\text{A}})/{\text{A}}_{{\text{o}}} ] $$
(3)

According to Beer-Lambert law, the dye's absorbance relates to the concentration of CV dye.

3 Results and Discussion

3.1 X-Ray Diffraction

Figure 2 depicts the XRD profile of gadolinium zirconate Gd2Zr2O7 powders synthesized using sol–gel auto-combustion route based on tartaric acid as a fuel annealed at temperature 1100 °C for 2 h. The broad diffraction peaks at 2θ = 29.41°, 34.12°, 48.90° 58.26°, and 61.18° linked to (111), (200), (220), (311), and (222) reflections of the defect-fluorite structured Gd2Zr2O7 with space group: Fm-3m (PDF#80-0471) were assigned. However, 111 (≈ 14°), 311 (≈ 28°), 331 (≈ 37°), and 511 (≈ 45°) corresponding to pyrochlore superstructure (space group: Fd-3m) were not detected [25, 29, 41, 42]. This conclusion is consistent with the earlier findings that defect fluorite will eventually form at temperatures between 700 and 1200 °C, and that pyrochlore will eventually form at temperatures between 1300 and 1400 °C. Due to the Gd2Zr2O7 structure's proximity to the pyrochlore–fluorite transition's geometric border (RGd/RZr = 1.46), 100% order is not recognized and is attributed to atom inversion. The movement of 10% of oxygen atoms from O1 sites to the initially unoccupied O3 sites is what causes the disordering in the oxygen sublattice.

Fig. 2
figure 2

XRD of Gd2Zr2O7 nanoparticles, Gd2Zr2O7 [JCPD01-080-0471(c)]

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The crystallite size of the formed Gd2Zr2O7 powder at an annealing temperature of 1100 °C was determined from the most intense peak (111) based on the Debye–Scherrer equation.

$${d_{RX}} = k\lambda /\beta {\text{cos}}\theta]$$
(4)

where dRX is the crystallite size, k = 0.9 is a correction factor to account for particle shapes, β is the full width at half maximum (FWHM), λ is the wavelength of Cu target = 1.5406 Å, and θ is the Bragg angle. It was found to be 33.12 nm on average. The lattice parameter (a) of the produced Gd2Zr2O7 powders was calculated using Bragg's equation a = dhkl \(\sqrt{{h}^{2}+{k}^{2}+{I}^{2}}\) where d represents the inter-planar spacing of the major diffraction peak of GZO and hkl represents the miller indices of the corresponding plane. The lattice parameter a of GZO was found to be 10.524 + 0.02 Å

3.2 Fourier Transform Infrared (FTIR)

Figure 3 displays the FT-IR spectra of the synthesized sample. It is noted that the absorption peak at a wavenumber of ~ 1640 cm−1 and 3425 cm−1 is associated with the bending and the bending vibrating of OH in water imputed to the adsorbed water from the atmosphere [25, 29]. Gd–O vibrations are responsible for the absorption bands at 547 and 1425 cm1. The Zr–O–Zr stretch-related vibrations correlate to the distinctive peak at 715 cm−1. A characteristic peak at 847 and 666 cm−1 can be attributed to the stretching vibration of the M–O bond (M = Zr–Gd) [25, 29, 41]. The peaks in 1505, 1398, and 1080 cm−1 are related to Zr–OH vibrations [41].

Fig. 3
figure 3

FTIR of Gd2Zr2O7 Nanoparticles

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3.3 TEM

Figure 4a illustrates the TEM image of the synthesized Gd2Zr2O7. The particle morphology with a uniform cubic-like structure was exhibited. The produced sample also has a small average size of ~ 17 nm and a narrow size dispersion. The particles also exhibit server aggregation. In contrast, the HRTEM picture in Fig. 3b. displays clear lattice fringes, demonstrating the sample's strong crystallinity. The lattice spacing matches the defect-fluorite structure of Gd2Zr2O7 (PDF#80-0471) rather well, further supporting the Gd2Zr2O7's formation.

Fig. 4
figure 4

a TEM micrographs of Gd2Zr2O7 nanoparticles, b electron diffraction pattern of Gd2Zr2O7 nanoparticles

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3.4 Optical Properties

The diffuse reflectance spectrum (DRS) of Gd2Zr2O7 in the wavelength range from 200 to 2500 nm is displayed in Fig. 5. There is a very weak absorption band at 276 cm−1 imputed to the 8S7/2–6I7/2 transition of Gd3+ ions. The absorption edge was found to be below 600 cm−1 due to the f–f transition of Gd ions. Above 700 cm−1, the reflectance was increased as a result of the weakening of the Gd ion absorption [43] (Fig. 6).

Fig. 5
figure 5

Diffuse reflectance spectrum of Gd2Zr2O7 nanoparticles

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Fig. 6
figure 6

Kubelka–Munk plot of pure Gd2Zr2O7

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The Kubelka–Munk (K–M) function is used to transform reflectance spectra into comparable absorption spectra.

The type of transition taken into account is noted in the summaries of the following equations taken from the bibliography for the Eg calculation.

$$ \alpha \left( {{\text{h}}\upsilon } \right) \approx {\text{B}}\left( {{\text{h}}\upsilon \, - {\text{ E}}_{{\text{g}}} } \right)^{{\text{n}}} $$

When plotted as \( \alpha \left( {{{h}}\upsilon } \right)^{1/2} \) vs E, an indirect allowed transition has n = 2; when plotted as \( \alpha \left( {{{h}}\upsilon } \right)^{1/3} \) versus E, an indirect forbidden transition has n = 3; In the case of a direct allowed transition, n = 1/2 (plotted as \( \alpha \left( {{{h}}\upsilon } \right)^{2} \) vs E), while in the case of a direct forbidden transition, n = 3/2 (plotted as \( \alpha \left( {{{h}}\upsilon } \right)^{2/3} \) versus E), where Eg stands for the band gap (eV), h for Planck's constant (J.s), B for absorption constant, (α) for the extinction coefficient, which is proportional to F. (R). and v for light frequency (s−1). The best linear fit in the absorption spectra can be used to experimentally calculate the n value for a particular transition using several formulas [44]. The band gap energy of the produced powder was 3.8 eV which was slightly higher than the previously published for pure Gd2Zr2O7 tailored by the hydrothermal route and then annealing at 1200 °C for 2 h (Eg = 3.63 eV). Such change may be related to the difference in the crystallite size [45].

3.5 Photoremedation of Crystal Violet

Figure 7 displays the completed calibration curve that establishes the concentration of crystal violet solutions during the purification process using the solutions' wavelength-dependent absorbance data (591 nm). The concluded equation:

Fig. 7
figure 7

Calibration curve of crystal violet solutions

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$$\mathrm{Conc}.=\frac{\mathrm{Absorbance}+0.0202}{0.20289}\qquad {\text{For Crystal Violet}}$$
(5)

According to measurements made with a visible spectrophotometer, purification times are correlated with a rise in the absorbance spectra of the contaminated water (crystal violet solutions).

Known concentrations were prepared from the crystal violet solution, specifically 1, 2, 3,…..and 10 ppm. The absorbance spectrum was measured for them employing the visible spectrophotometer, and a calibration curve was drawn for those concentrations then an equation was concluded to calculate the unknown concentrations resulting from the purification process by the means of their absorbance.

Crystal violet solution (10 ppm) was used to perform the photocatalyst test on it with a time ranging from 0 to 90 min with an interval of 15 min.

Hydrogen peroxide solution was used as a catalyst for gadolinium zirconium oxide nanoparticles to purify using Fenton Like reaction by exposing it to ultraviolet rays (32 watts).

To show the actual effect of gadolinium zirconium oxide, the purification process was carried out under the same conditions, using it alone, again using hydrogen peroxide alone, and a third by using them together with 1 ml of hydrogen peroxide with 0.025 g of pyrochlore on 50 ml of crystal violet solution.

It was noticed that when using pyrochlore alone, it proved a reasonable efficiency, and that efficiency was improved by using hydrogen peroxide with it. They demonstrated high efficiency in the degradation of crystal violet from a concentration of 10 ppm to a concentration of 0 ppm in 90 min or less.

As for the case of pyrochlore with hydrogen peroxide, the interpretation is slightly different, as it is done with a process called Fenton-Like Reaction, and in this case, hydrogen peroxide is indispensable. This case is characterized by its great effectiveness and a rate of removal that reaches zero in a time also close to 90 min, like the case of purification with hydrogen peroxide alone. It may seem superficial that pyrochlore has no work as long as we reach the same percentage with or without it, but the matter doesn’t work like that. The efficiency of the purification process in the case of pyrochlore with hydrogen peroxide is better than hydrogen peroxide alone, even if we get the same efficiency approximately at the end of the 90 min purification period. The evidence is the values ​​of relative concentrations as shown in Fig. 8. At the beginning of the process, we find that the efficiency of the Fenton-Like method, which contains pyrochlore and hydrogen peroxide, is superior to hydrogen peroxide alone, even if we reach the same result in the end. In addition, we used a very small percentage of pyrochlore, up to 0.025 g per 50 ml of the pollutant. If this percentage is increased, it will increase the purification process, and thus the efficiency of pyrochlore will be evident in the degradation process using a Fenton-like reaction (Fig. 9).

Fig. 8
figure 8

Absorbance of 10 ppm solution of crystal violet solution after treatment with a only 0.05 gm of Gd2Zr2O7 without H2O2 where c only H2O2. And b, d, and f are concentrations conversion C/C0 versus time for degradation process where g is the figure of combined data

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Fig. 9
figure 9

Graphical representation of photocatalysis prosses of CV using Gd2Zr2O7 nanoparticles

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The phtotocatlytic mechanism of Gd2Zr2O7 nanoparticles alone was dicussed by the following equations. Through a subsequent reaction with evaluated activity, the photogenerated VB (h+) nanoparticles mix with either H2O or OH− to create OH· and therefore indirectly degrade the dye molecule as shown in eqations from (6) to (9).

The hydroxyl groups, water, and oxygen present on the surface of Gd2Zr2O7 nanoparticles cause the electrons (e−) in the conduction band to react with the species absorbed on the surface of the material, free radicals, to produce O2·─, which then causes the production of OH· radicals as shown in eqations from (10) to (16).

$$ {\text{Gd}}_{{2}} {\text{Zr}}_{{2}} {\text{O}}_{{{7} }} + h\upsilon \to {\text{ h}}_{{{\text{VB}}}}^{ + } + {\text{e}}_{{{\text{CB}}}}^{ - } $$
(6)
$$ {\text{h}}^{ + } + {\text{e}}^{ - } \to {\text{heat}} $$
(7)
$$ {\text{H}}_{{2}} {\text{O}} + {\text{h}}_{{{\text{VB}}}}^{ + + } \to {\text{H}}^{ + } + {\text{OH}}^{ - } $$
(8)
$$ {\text{h}}_{{{\text{VB}}}}^{ + } + {\text{OH}}^{ - } \to {\text{OH}} $$
(9)
$$ {\text{e}}_{{{\text{CB}}}}^{ - } + {\text{O}}_{{2}} \to {\text{ O}}_{{2}}^{ - } $$
(10)
$$ {\text{O}}_{{2}}^{ - } + {\text{ HO}}_{{2}} + {\text{ H}}^{ + } \to {\text{ H}}_{{2}} {\text{O}}_{{2}} + {\text{ O}}_{{2}} $$
(11)
$$ {\text{H}}^{ + } + {\text{O}}_{2}^{{ \cdot { - }}} + {\text{ eCB}}^{{ - }} \to {\text{ HO}}^{ \cdot } $$
(12)
$$ {\text{O}}^{{ \cdot { - }}} + {\text{ OH}}_{2}^{ \cdot } \to {\text{ OHO}}_{2}^{ \cdot } + {\text{ O}}^{{ - }} $$
(13)
$$ {\text{HO}}_{{2}}^{ \bullet } \to {\text{H}}_{{2}} {\text{O}}_{{2}} + {\text{ O}}_{{2}} $$
(14)
$$ {\text{H}}_{{2}} {\text{O}}_{{2}} + {\text{ e}}_{{{\text{CB}}}}^{{ - }} \to {\text{ OH}}^{ \cdot } + {\text{ OH}}^{{ - }} $$
(15)
$$ {\text{H}}_{{2}} {\text{O}}_{{2}} + \, \to {\text{ 2OH}}^{ \bullet } $$
(16)

The dye pollution was afterwards degraded by the generated agents.

$$ {\text{CV }} + {\text{ O}}_{{2}}^{{ \cdot { - }}} \to {\text{ CV}}^{{ \cdot { - }}} \to {\text{ product}} $$
(17)
$$ {\text{CV }} + {\text{ OH}}^{ \cdot } \to {\text{ CV}}^{{ \cdot { - }}} \to {\text{ product}} $$
(18)
$$ {\text{CV }} + {\text{ OHO}}_{{2}}^{ \cdot } \to {\text{ CV}}^{ \cdot } \to {\text{ product}} $$
(19)

The photocatalytic degradation of CV dye under UV light in the presence of photocatalysts made of TiO2 and ZnO is described in several works [46]. The use of Fe-doped TiO2 for the removal of this dye is still understudied [47], even though the CV dye degradation under visible light has also been studied using various photocatalyst formulations (such as BiOxCly/BiOmIn composites) [48], BiVO4/FeVO4 [49], CdS-anchored porous WS2 [50], and -Cu2V2O7). Another literature survey about CV degradation was mentioned also in Table 1. Despite the efficiency of the aforementioned materials in degradation CV, Gd2Zr2O7 pyrochlore nanopowder has proven reasonable efficiency compared to them by conventional photodegradation or by Fenton Like reaction.

Table 1 Comparison of maximum degradation capacities of CV dye using different catalysts [51]
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4 Conclusion

This study aims to use a specific type of pyrochlore in the purification process using a special method. Gd2Zr2O7 pyrochlore alone demonstrated a moderate efficiency in the degradation process of crystal violet solution (50 ml–10 ppm) under UV light by photocatalysis. When 1 ml of hydrogen peroxide solution was added, the degradation process was improved to reach a concentration of nearly zero of the pollutant by Fenton-Like method. The effect of hydrogen peroxide alone on the pollutant, photocatalyst, as well as both of them, together were measured. A noticeable effect on the rate of degradation in the case of the catalyst was shown by adding hydrogen peroxide more than using them alone, and this was also observed from concentration calculations with time in a period not exceeding 90 min. It is found that in the case of the catalyst and hydrogen peroxide, the rate of degradation was faster, even if the final result or the result at the time of 90 min is almost the same for each of the hydrogen peroxide alone or when it is present with the catalyst. The functional groups of pyrochlores were measured using Fourier transform infrared analysis (FTIR). The crystalline nature of all samples of pyrochlore was also confirmed using X-ray diffraction analysis and both of the crystallite size and the lattic parmeter of the fabricated Gd2Zr2O7 were found to be 28.5 nm and 10.524 + 0.02 Å, respectively. The optical properties and energy-gap calculations were also measured using UV/Vis. spectrometer and the band gap energy of the synthesized powders was found to be 3.8 eV relating the impact of the crystallite size. As for monitoring the purification process, tracking the concentrations of the pollutant, and indicating the effect of the catalyst on it, all this was done using a visible light spectrophotometer. Because the prepared samples were supposed to be in the nano size, this was confirmed using the TEM. Electron diffraction was utilized as confirmation for the X-ray measurement that all prepared samples are crystalline. Finally, the synthesized nanoparticles was displayed a high photo Fenton catalytic activity with efficiency ~ 90% for the remedation of crystal violet dye after 60 min.