Preparation and Characterization of Chromium Oxide Nanoparticle for Environmental Applications

INTRODUCTION
Water Contamination refers to the deterioration of water quality caused by harmful substances making it unsafe for consumption, agriculture, hygiene, and ecosystem health. According to the World Health Organization, contaminated water is that which has been altered to such an extent that it becomes unusable, often causing severe illnesses like cholera, dysentery, typhoid, and hepatitis, which claim hundreds of thousands of lives annually, Causes of water contamination stem from both human activity and natural processes[1- 3].Agricultural runoff: includes fertilizers, pesticides, and animal waste that enter rivers and groundwater, causing nutrient pollution and algal blooms. Industrial discharge: chemicals and heavy metals improperly released into water bodies contaminate them with toxic substances Sewage and wastewater: untreated or poorly treated sewage introduces pathogens and pollutants into freshwater systems. Urbanization and poor waste management: plastics, leachate, and household trash penetrate water supplies in rapidly growing cities. Natural events: storms, flooding, or mineral leaching can mobilize sediments and raw pollutants into water sources. 
Advanced Oxidation Processes (AOPs) denote a collection of chemical treatment methods aimed at eliminating organic and inorganic contaminants from water and air. These processes utilize highly reactive species, mainly hydroxyl radicals (•OH), to break down contaminants into harmless end products such as water and carbon dioxide. Various methods are employed for wastewater treatment, typically categorized into physical, chemical and biological techniques [4]. Regrettably, numerous procedures are expensive, produce environmentally detrimental by-products, and exhibit complexity. Because of their effectiveness, characteristics, and advantages, advanced oxidation processes (AOPs), especially photo catalysis, are among the most widely used technologies for treating these effluents [5]. Photo catalysis has proven to be incredibly simple, economical, and efficient. Because of its effectiveness in breaking down organic contaminants like industrial colors, it has attracted a lot of research interest [6,7]. The approach is advantageous and secure, as it effectively purifies water containing certain color pollutants without producing harmful intermediate products in the majority of instances [8]. These semiconductor nanoparticles can produce electron-hole pairs [9] when exposed to light at a particular wavelength. These pairs then take part in a sequence of oxidation and reduction reactions, producing hydroxyl groups as the main oxidizing agent and aiding in the breakdown of the dye’s organic components into products that are safe for the environment [10].
An organic substance called malachite green is employed in aquaculture as a color and, more controversially, as an antibacterial agent. Malachite is commonly used as a dye for materials such as silk, leather, and paper; however, despite its name, the dye is not derived from the mineral malachite; the name simply refers to the color similarity. In the dye industry, malachite green is categorized as a triarylmethane dye; it is also utilized in the pigment industry [11]. Although the name is commonly used more broadly to refer only to the colored cation, malachite green technically refers to the chloride salt [C6H5C(C6H4N(CH3)2)2] Cl. Cotton, silk, wool, leather, and paper are all colored with a synthetic dye called malachite green (di[4-dimethylamino-phenyl] phenyl cation). In the aquaculture industry, malachite green is a topical fungicide that works well. Malachite green (MG) is a green, metal-based triphenylmethane fertilizer that poses a serious risk to water quality [12]. The molecule is significantly more reactive when the benzene ring is present. It poses a serious risk to human life and health and can result in cancer and cell apoptosis [13].

MATERIALS AND METHODS
Chromium chloride hex hydrate salt (CrCl3•6H2O) supplied by Fluke, Ethanol (EtOH) supplied by Fluke, Ammonium hydroxide (NH4OH) supplied by Fluke, Every chemical was used without any additional purification.

Synthesis of Chromium oxide (Cr2O3) nanoparticles
Chromium oxide (Cr2O3) nanoparticles have been synthesized via the technique of sol-gel method. In 50 milliliters of ethanol (EtOH), around 2 grams (7,5 mmol) of chromium chloridhexahydrate (CrCl3.6H2O) was dissolved. After being shaken in a container on a magnetic stirrer for about half an hour, the resultant solution became green.
The resulting solution was then gradually supplemented with ammonium hydroxide solution (NH4OH) using a burette at a rate of around two drops per minute until the pH reached about 8. Ten minutes later, the solution was filtered through a centrifuge, the over stands were taken off, and the solution was washed twice with a little amount of distilled water. The precipitate was then dried for two hours at 90 °C. The chromium oxide precipitate powder, which was dark green, was obtained.

Malachite green dye is broken down by photo catalysis using chromium oxide nanoparticles
In degradation studies, chromium oxide (Cr2O3) nanoparticles were used as a photo catalyst to break down malachite green dye in aqueous solution when exposed to UV light as shown in Fig. 2. The suspension solution was chilled by circulating cooling water through the first apparatus. A 100 mL dye degradation suspension solution makes up the second component. Malachite green dye was prepared as a 100 ppm stock solution using distilled water. For every color concentration, a suspension solution was created by stirring. One hundred milliliters of each 0.17 g of chromium oxide nanoparticles were added to the liquid to amalgamate the hue, and it was then shaken. The matching suspension solution combination has been irradiated using an ultraviolet light source. A syringe was used to extract 2-3 mL of each sample every 10 minutes. After centrifuging the samples for ten minutes at 3000 rpm, a UV-Vis spectrophotometer was used to test each sample’s absorbance.

RESULTS AND DISCUSSION
X-ray diffraction of synthesized chromium oxide:
Fig. 3. shows the XRD pattern of synthesized chromium oxide nanoparticle, and explain the main bands which parallel to chromium oxide were observed having 2𝜃 values of 21.3826◦, 26.0844◦, 31.2530◦, 37.6206◦, 40.0649◦, 46.2634◦, 52.4677◦, 54.2889◦, 64.9477◦, 66.5431◦, and 69.3505◦, corresponds to the (012), (104), (110), (006), (113), (202), (024), (116), (122), (214), (300) and (101) planes of Cr2O3 (JCPDS Card No. 72-3533) respectively [14,15].
X-ray diffraction reveals that the produced Nano metal oxide is 100% Cr2O3 and has a hexagonal form. The oxide nanoparticles’ good crystal formation is indicated by the peaks’ sharpness. The average particle size (D) of the particles was calculated from the high intensity peak using the Debye-Scherer equation [16]:

𝐷 = 𝐾𝜆/ (𝛽𝑐𝑜𝑠𝜃) 

where D is the crystalline size of the Nano powders of chromium oxide.
 K is equal to 0.89, β is the line width at half-maximum height (FWHM), 𝜃 is the Bragg’s angle, and λ mean is the wavelength of the X-ray radiation, which is equal to (λ=0.15406 nm) for CuKα. Table 1 shows the crystalline size of chromium oxide.

Scanning Electron Microscopy (SEM) for synthesized Cr2O3 nanoparticle
The morphology of the created Cr2O3 nanoparticle was analyzed using scanning electron microscopy techniques (SEM, Zeiss, Germany) in order to obtain crucial information about its structure. SEM images. As illustrated in Fig. 4. The SEM micrograph shows a highly agglomerated morphology composed of numerous fine nanoparticles distributed densely across the surface [17]. The particles exhibit predominantly spherical to nearly spherical shapes, forming irregular clusters due to aggregation, which is common in nanoscale metal oxides because of their high surface energy. The image, taken at a magnification of 2500×, reveals that the material has a rough and porous surface texture, indicating a large surface area. This type of morphology is beneficial for applications such as photo catalysis, adsorption, and sensing, as the high surface area enhances active site availability. The uniform distribution of the nanoparticles also suggests that the synthesis method produced a relatively consistent particle formation without large micro-sized impurities [18,19].

Band Gap Energy of Cr2O3 nanoparticle
The plot shows the relationship between ((F(R).hν)2) and the photon energy (hν) Fig. 5, where (F(R)) is the Kubelka–Munk function derived from the UV–Vis diffuse reflectance data. In this analysis, the exponent n = 2 is used, indicating that Cr₂O₃ is treated as a material with a direct allowed electronic transition[20].The linear portion of the curve in the higher-energy region is extrapolated toward the energy axis (x-axis). The point where the extrapolated straight line intersects the x-axis corresponds to the optical band gap energy (E₉).From the extrapolation shown in the figure, the estimated band gap of Cr₂O₃ is: Eg = 2.49 Ev.
The energy needed for electrons to move from the valence band to the conduction band is represented by this value. A band gap of approximately 2.49 eV indicates that Cr₂O₃ can absorb visible light and is suitable for applications such as photocatalysis, sensors, pigments, and optoelectronic devices[21,22].

Effect of loaded mass of Chromium oxide nanoparticles on the Malachite green dye’s photocatalytic degradation
Using a dye concentration of 40 mg/L, an air flow rate of 10 mL/min, and at room temperature, the effect of chromium oxide nanoparticle mass on the photocatalytic degradation of malachite green dye was investigated. As the mass of chromium oxide nanoparticles increases to as seen in Figs. 6 and 7, the photocatalytic degradation of Malachite Green dye starts at 0.17 g/100 ml, climbs steadily, and then progressively diminishes. with a mass of 0.17 g/100 ml of chromium oxide nanoparticles. The semiconductor has the capacity to absorb the lightest possible. Only at concentrations of chromium oxide nanoparticles more than 0.17 g/100 ml will the primary layers of malachite green dye experience a reduction in photo degradation efficiency as a result of light absorption; the solution’s subsequent phases are not exposed to light photons. This effect was studied by several researchers 23]. The rate of photo degradation of malachite green dye also decreases when the loading mass of chromium oxide nanoparticles is below the optimal value of 0.17 g/100 ml. This is because the mass of the nanoparticles’ surface area decreases, which lowers the amount of light that the nanoparticles can absorb [24].

Impact of starting concentration of malachite green dye on photocatalytic degradation
The effects of the concentration solution of malachite green dye by maintaining constant conditions throughout, the experiment investigated the spectrum of photocatalytic degradation processes (40-90 mg/L). Fig. 8 presents a graphical representation of the findings. Our findings showed that when the initial dye concentration rose, the rate of photocatalytic degradation reduced. The number of photons that come into touch with the catalyst surface increases as the concentration of Malachite green dye drops because the photon’s journey lengthens as it enters the solution. This speeds up the production of superoxide ions and hydroxyl radicals, which in turn speeds up the rate at which materials degrade [25,26].
The high photo degradation efficiency (89.01%) was 40 mg/L when the quantity of malachite green dye was enough. The photocatalytic degradation efficiency (P.D.E.) at different concentrations of malachite green dye is shown in Fig. 9.
The Impact of Light Irradiation Type:
Fig. 10 illustrates how different forms of light irradiation affect the breakdown of malachite green dye. 88.89% of the dye destroyed under UV light in 60 minutes, whereas 56.25% and 45.41 percent of the dye decomposed under visible light in 60 minutes, respectively. This finding indicates that the degradation of malachite green dye was not significantly impacted by either visible or dark light. The experiment also examined the weak adsorption mechanism between chromium oxide nanoparticles and malachite green dye in a dark environment [27].

The effect of temperature on the photocatalytic degradation of malachite green dye
The effects of temperature on the photocatalytic degradation of malachite green dye were examined using a range of tests from 298 to 310 K. With all testing settings held constant, the created amount of chromium oxide nanoparticle catalyst was 0.17 g per 100 mL, whereas the initial concentration of malachite green dye was 40 ppm. 
The results in Fig. 11 demonstrate that the dye’s degradation accelerated with temperature. The existence of more reactive hydroxyl radicals might be the cause of this [28-32]. Plotting Fig. 12 illustrates The Arrhenius equation was used to calculate the activation energy related to dye photo degradation in k versus 1/T, and the result was 38.61kJ.mol-1.

CONCLUSION
Hexagonally structured chromium oxide nanoparticles have been synthesized utilizing the sol-gel method. The prepared sample was analyzed via the X-ray diffraction method. This article employed a co-precipitation approach to synthesize chromium oxide nanoparticles. The optimal concentration of chromium oxide nanoparticles was found to be 0.17 g per 100 mL. The catalyst dosage dictated the photocatalytic degradation of malachite green dye. The effects of malachite green dye on dye concentration have been studied at the ideal dosage of 40 mg/L. Because there is less OH⁻ adsorption on the catalyst surface, the rate of photocatalytic degradation decreases as the amount of Malachite green dye increases. The efficiency of the photocatalytic degradation of malachite green -farbstoff is 89.01 percent. The activation energy, according to calculations, is 38,61 kJ•mol⁻¹. 

ACKNOWLEDGMENTS
We would like to thank the University of Babylon and the Chemistry Department of the Faculty of Science for Women and College of science for their help in making this project successful.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.