Visible-Light-Induced Photocatalytic Degradation of Fast Green FCF and Orange II Dye by Yb₂O₃ Nanoparticles ^†

Abstract

The quest for efficient photocatalytic materials for eliminating synthetic dyes like Orange II Sodium Salt and Fast Green FCF (For Coloring Food) has been spurred by the mounting environmental problems associated with these dyes. Due to their excellent electrical characteristics, thermal stability, and potential to reduce electron–hole recombination, ytterbium oxide (Yb₂O₃) and other rare-earth metal oxides are gaining popularity. This study synthesized Yb₂O₃ nanoparticles and assessed their photocatalytic activity when exposed to visible light. Significant degradation efficiencies were revealed by the spectrophotometric analysis, suggesting that Yb₂O₃ is an effective nanocatalyst for dye remediation applications. The results demonstrate how rare-earth-based nanomaterials can improve environmentally friendly and sustainable wastewater treatment methods, supporting ongoing environmental cleanup initiatives.

1. Introduction

Water is an essential commodity for ecosystems and people equally. One of the most severe problems facing humanity worldwide now is the contamination of water [1]. Pollutants from industrial, agricultural, and human activities include pesticides, fertilizers, dyes, heavy metals, and surface-active compounds [2]. Dyes are commonly used as coloring agents in a range of industries to improve the appearance and value of products. Each year, more than 10,000 dyes totalling more than 7 × 10⁵ metric tons are produced, primarily for printing and dyeing [3]. Due to their capacity to impart vivid and long-lasting colors, dyes are a significant class of environmental pollutants that are widely utilized in sectors like textiles, paper, plastics, leather, and cosmetics [4]. There are dyes that are both natural and manufactured. Natural dyes are colorants or dyes derived from invertebrates, plants, or minerals. Most natural dyes are vegetable colors procured from plants, their different parts, and other biological sources like fungi. Chemical compounds that are deliberately created in laboratories, mostly from coal tar or petroleum-based chemicals, are known as synthetic dyes. Natural dyes are not as frequently utilized as synthetic dyes despite their historical relevance and beneficial effects on the environment because of their limited color range, unpredictable outcomes, high production costs, and poor fading resistance [5]. The chemical structure and application of dyes define them. The pigment’s color is derived from a class of molecules known as chromophores. Numerous functional groups, including azo, anthraquinone, methine, nitro, aryl methane, carbonyl, etc., form the basis for these chromophore-containing dyes. Electrons that are transferred from or provided to substituent groups to create or enhance the color of the chromophore are known as auxochromes. Amines, carboxyls, sulfonic acids, and hydroxyls are the four most common auxochromes [6]. For procedures including dying, mending, softening, and washing, the textile industry utilizes a lot of water. It has been found that 15% of the pigment is washed out in water during the dying process and becomes effluent [7]. Due to the chemicals’ significant potential to have detrimental impacts on natural water sources, the extensive industrial release of dyes into the environment raises serious ecological concerns [8]. When wastewater from different industries containing excessive concentrations of dyes enters water bodies. Plants and aquatic organisms have less access to light, which has a negative impact, and it makes self-purification impossible [9]. Aquatic plants and wildlife are impacted as a result of a decrease in photosynthesis and oxygen levels. Since algae are at the base of the food chain, light absorption predominantly reduces their photosynthetic activity, which has a significant impact on all organisms above them [10]. Long-term dye exposure can have harmful effects on the aquatic environment, including low aerobic biodegradability, accumulation in sediments, especially in aquatic life forms, and breakdown of pollutants into carcinogenic or mutagenic compounds. Numerous colors and the byproducts of their breakdown are harmful to life, carcinogenic, and mutagenic. Even minute concentrations in the water for days can have a significant negative impact on the transparency and quality of bodies of water like rivers, lakes, and others, harming the aquatic ecosystem. When they are consumed, gut microbes break them down, damaging DNA [11].

Dyes are extremely toxic and perhaps carcinogenic, generating several new health issues in both humans and animals in addition to wreaking havoc on the environment [12]. Textile dye toxicity to humans can be classified as either acute or chronic/genotoxic. Skin irritation and skin sensitization are the primary issues associated with acute poisoning with textile dyes, which can occur by oral intake and inhalation. The main long-term health risk associated with some textile dyes is chronic or genotoxicity. This became evident when, between 1930 and 1960, a significant prevalence of bladder cancer was noted among factory workers engaged in the production of specific dyes. Fuchsine, auramine, benzidine, and two naphthylamines were the specific substances involved [13]. Respiratory issues brought on by breathing in dye particles are the most frequent concern about colorants. A person’s immune system may occasionally be impacted by a dye; in severe circumstances, this could cause a violent reaction the next time the person breathes in the color. This condition is referred to as respiratory sensitization and manifests as sneezing, wet eyes, itching, wheezing, and coughing—all of which are symptoms of asthma [14]. The human body can absorb dyes through the gastrointestinal tract, respiratory system, or skin. Ingested dyes often pass through the intestinal epithelium and into the bloodstream after being absorbed by the stomach and small intestine. Particles of airborne dye can enter the bloodstream through the respiratory system after being inhaled and lodging in the lungs. Certain colors have been shown to penetrate the skin and enter the bloodstream, especially when they come into contact with damaged or broken skin. After being absorbed, dyes can circulate throughout the body and build up in different organs and tissues. Adipose tissue and organs such the liver, kidneys, lungs, spleen, and brain have all been found to contain dyes and their metabolites. Adipose tissue also has a tendency to accumulate dyes. Human dye bioaccumulation is a complicated process that is impacted by a number of factors, such as exposure dose, duration, frequency, individual metabolism, and physiological parameters. Over time, the body may gradually accumulate dyes due to prolonged exposure to low quantities from diet or environmental sources. Furthermore, specific metabolic pathways can convert dyes into metabolites with varying toxicity profiles and varying levels of bioavailability. For instance, gut bacteria have the ability to convert certain azo dyes into aromatic amines, which are recognized carcinogens and may increase the carcinogenicity of azo dyes [15]. Among these dyes, Orange II and Fast Green FCF dyes are well known for both their uses and adverse consequences. Fast Green FCF (C₃₇H₃₄N₂Na₂O₁₀S₃), a triphenylmethane dye with a molecular weight of 808.84 g/mol, is widely used as a food colorant and in textiles, while Orange II (C₁₆H₁₁N₂NaO₄S), an azo dye with a molecular weight of 248.71 g/mol, is utilized in the leather, paper, and cosmetics industries [16]. The environment and people are negatively impacted by both of these colors. Therefore, creating economical and effective ways to break down these dyes is crucial. Dyes have been eliminated from wastewater discharge using a variety of physicochemical techniques. However, many chemical or physical approaches have intrinsic flaws that render them unfeasible from an economic standpoint. The disadvantages include the necessity for labor-intensive operations, substantial sludge generation that could result in secondary contamination, higher energy and chemical requirements, and partial removal of organic metabolites. Enzymes, bacteria, algae, and other microorganisms have also been used to try dye breakdown, although each approach has disadvantages of its own [17]. The photocatalytic technique stands out as a cost-effective, sustainable, and environmentally safe way to remediate dye wastewater. This effective technique makes use of sunshine, which supplies energy higher than the photocatalysts’ band gap. As a result, electrons get excited and travel from the valence band to the conduction band, creating electron hole pairs that have the ability to break down wastewater contaminants. It is desirable because of its affordability, sustainability, and capacity to totally break down pollutants when exposed to sunshine. One practical solution to reduce the negative effects of industrialized dye wastewater on the environment and human health is photocatalysis [18]. The use of nanoparticles (NPs) or nanocomposites as a catalyst in photocatalytic dye degradation has attracted a lot of interest, and numerous studies have been published to support it [19].

Although noble metal-decorated systems (e.g., Ag/ZnO, Au/TiO₂) and transition metal oxide NPs like TiO₂ and ZnO have been thoroughly investigated for photocatalytic degradation of organic dyes, they frequently suffer from low visible-light activity and high recombination rates of photogenerated charge carriers. The pursuit for substitute photocatalysts has been spurred by these difficulties as well as the high price of noble metals [20]. Rare earth oxide NPs and materials doped with rare earth elements have recently become attractive options due to their better production of reactive species, reduced electron–hole recombination, and increased absorption of visible light. For example, rare earth dopants like Er³⁺ in TiO₂ and Yb-based composites greatly enhance photocatalytic dye degradation when exposed to visible light, underscoring the increasing interest in rare earth systems for environmentally friendly wastewater treatment [21]. Because of their special electrical and structural characteristics, rare earth oxide NPs have become extremely attractive photocatalysts for the visible-light degradation of organic dyes. Compared to undoped metal oxides, rare earth elements can introduce localized electronic states and altered band gaps, thereby improving visible light absorption, enhancing photogenerated charge carrier separation, and reducing electron–hole recombination [22]. For example, a study by Lakshamanan K. et al. described the synthesis of silver NP/yttrium oxide with titanium carbide MXene (Ag/Yb₂O₃@Ti₃C₂) using coprecipitation and hydrothermal methods to produce a catalytic material for photocatalysis and a stable and high-performance electrode material for supercapacitors. Methylene Blue (MB) dye was used to test its photocatalytic efficiency. Additionally, the Ag/Yb₂O₃@Ti₃C₂ nanocomposite demonstrated exceptional photocatalytic activity, degrading MB dye by 97% in 75 min [23]. Another study described a comparable green synthesis of Y₂O₃ NPs utilizing the sol–gel technique and aqueous leaf extracts of Lantana camara L (LC). The efficiency of these particles as a photocatalyst was investigated using LC Y₂O₃ NPs for the photocatalytic degradation of Rhodamine B. The findings showed that while the catalyst was present, 92% of the dye was broken down in 60 min, while when it was not, only 18% of the dye was broken down. This shows that the catalyst played a significant role in the dye’s photocatalytic breakdown [24]. As a result, rare earth oxide NPs have shown potential in eradicating organic dyes and preventing water contamination from wastewater. In the current study, Orange II and Fast Green FCF dyes were used to investigate Yb₂O₃ NPs’ dye degradation efficiency.

2. Materials and Methods

Analytical-grade reagents were used in the study as described in our previous manuscript [25].

3. Experimental

3.1. Synthesis of Yb₂O₃ NPs

Yb₂O₃ NPs were fabricated through a co-precipitation method. 0.3 M aqueous NaOH solution was added dropwise to a 0.1 M Yb(NO₃)₃·5H₂O solution, as described in our previous manuscript [25]. The schematic representation of the synthesis process has been illustrated in Figure 1, as given in detail in our previously published paper [25].

3.2. Photocatalytic Activity Evaluation

The photocatalytic efficacy of the NPs was measured under visible light against Fast Green FCF and Orange II dyes using the reported methodology [25]. The reaction was carried out in a wooden dark box with a visible light source fitted in it. 50 mL of 10 ppm dye solution was taken and 25 mg of NPs were directly added to it and stirred in dark for 30 min. After that the solution was exposed to the visible light source with continuous stirring and 3 mL of sample was extracted every 15 min up till 105 min. After completion of the reaction, the catalyst was separated by centrifugation prior to UV–Vis analysis. The % degradation of the dye was estimated using Equation (1).

%Degradation = (A_o − A_t)/A_o

(1)

where A_t is the dye absorbance at time t, and Ao is the dye absorbance at the start. The dynamics of Orange II Sodium Salt and Fast Green FCF degradation were also investigated. The degradation kinetics might be assessed by plotting ln (C_t) against time. The pseudo-first-order kinetic Equation (2) was used for the kinetic analysis.

ln(C_o/C_t) = kt

(2)

Here, C_o represents the initial concentration, C_t denotes the concentration at time t, and k is the rate constant. The slope of the plot of ln(C_o/C_t) against time gives a rate constant, k.

4. Results and Discussion

4.1. Characterization

The synthesized Yb₂O₃ NPs’ UV-Vis absorption spectra (Figure 2a) show a notable absorption peak at 368 nm, which is caused by intrinsic electronic transitions from the valence to the conduction band. The broad band gap shown by this absorption pattern is typical of rare-earth metal oxides. According to the tauc plot, the band gap was 4.91 eV (Figure 2b) [25]. The FT-IR spectra (Figure 2c) were obtained in the 4000–400 cm⁻¹ range. The strong peaks at 578.54 cm⁻¹ and 416.55 cm⁻¹ confirmed Yb–O stretching vibrations, characteristic of metal–oxygen bonding in rare earth metal oxides [26,27]. According to our earlier publication, the XRD pattern matched with the JCPDS Card No. 43-1037 [25]. As described in our earlier paper, the SEM micrograph of Yb₂O₃ NPs revealed the irregularly shaped agglomerates made up of fine NPs. Smaller NPs have grouped to form larger clusters, giving the appearance of a heterogeneous particle distribution. The detailed characterization was given previously [25].

4.2. Photocatalytic Potential

Because of their extensive industrial use and their chemical stability, Fast Green FCF and Orange II are synthetic dyes that are widely acknowledged as persistent environmental contaminants. Because of its vivid color and water solubility, Fast Green FCF is a triarylmethane dye that is frequently applied as a colorant in food, cosmetics, pharmaceuticals, and textile products. However, when released in industrial effluents, its complex aromatic structure makes it resistant to biodegradation and contributes to its persistence in aquatic environments [28]. Orange II, sometimes referred to as Acid Orange 7, is a water-soluble anionic azo dye that is often employed in the paper, leather, and textile industries. Like other azo dyes, it has strong coloration and structural durability, making it challenging to remove using traditional wastewater treatment methods [29]. By decreasing light penetration, raising biochemical oxygen demand, and posing toxicological concerns to aquatic life and human health, these dyes can degrade water quality. This has prompted the development of sophisticated remediation techniques including photocatalytic degradation [30]. In our study, the photocatalytic dye degradation efficacy of Yb₂O₃ NPs was tested against Fast Green FCF and Orange II (Figure 3a,b) under visible light irradiation in 105 min. The dye degradation of Fast Green FCF with Yb₂O₃NPs was found to be 85.47% and follows pseudo-first-order kinetics with the rate constant of 1.6 × 10⁻² min⁻¹ (R² = 0.94045) as given in Figure 4a, while photodegradation of Orange II with Yb₂O₃NPs was found 83.09%, also following pseudo-first-order kinetics, and the rate constant was found to be 0.6 × 10⁻² min⁻¹ (R² = 0.96556) as given in Figure 4b. Figure 5 illustrates the % degradation versus time plot for both dyes.

Due to its intrinsic semiconductor properties and electronic structure that are similar to well-known photocatalysts, Yb₂O₃, a lanthanide-based oxide semiconductor, has drawn research interest in photocatalytic applications. In particular, its wide band gap allows for the excitation of charge carriers under UV/visible irradiation, which is necessary for efficient photocatalysis. When exposed to the right light, Yb₂O₃’s band structure enables the production of photogenerated electrons and holes, making it a viable option for energy conversion research and environmental cleanup [27]. Furthermore, rare earth metal oxides generally have distinct 4f electronic configurations that can affect charge carrier dynamics and light absorption, which is known to improve charge separation and increase the range of photon absorption in metal oxides modified with lanthanide species. In comparison to undoped wide-band gap materials, the addition of rare earth oxides like Yb₂O₃ can improve photocatalytic performance by shifting optical absorption toward the visible range and lowering electron–hole recombination rates. These features demonstrate the promise of Yb₂O₃ and related rare-earth oxides as light-active photocatalytic materials with adjustable optical and electrical characteristics that can be used to degrade pollutants when exposed to solar or visible light [31]. In photocatalytic reactions, electron holes are generated when the photocatalyst absorbs sufficient light energy [16].

When photon energy is delivered to the surface of the NPs, the conduction band will excite electrons from the valence band, resulting in electron–hole pairs. Subsequently, the photogenerated electron holes convert hydroxide ions (OH) or water molecules (H₂O) into hydroxyl radicals (•OH). After the photoexcited electrons convert oxygen into superoxide radicals (•O₂), the H+ ions in water can then protonate these radicals to create hydroxyl radicals (HO₂•). These radicals fragment into the more reactive hydroxyl radical species OH• after being transformed into H₂O₂. The degradation of dyes depends on superoxide and hydroxyl radicals [32]. These reactive oxygen species attack the dye molecules adsorbed on the catalyst surface, leading to cleavage of the chromophoric groups responsible for color. Effective dye mineralization is achieved by further oxidizing the resultant intermediates, which yields inorganic ions, carbon dioxide, and water. The major inorganic ions formed during degradation include sulphate (SO₄²⁻), nitrate (NO₃⁻), and ammonium (NH₄⁺). Basic qualitative chemical analysis verified the existence of these ions in the treated solution, supporting the mineralization of Orange II and Fast Green FCF dyes.

The reactions of the photocatalytic degradation pathway are listed below as Equations (3)–(10). The proposed mechanism of dye degradation has been illustrated in Figure 6.

Activation: Catalyst + hν → e⁻ + h⁺

(3)

ROS Generation: h⁺ + H₂O → •OH + H

(4)

e⁻ + O₂ → •O₂⁻

(5)

•O₂⁻ + H⁺ → HO₂•

(6)

HO₂• + HO₂• → H₂O₂ + O₂

(7)

e⁻ + H₂O₂ → •OH + OH⁻ (or H₂O₂ + hν → 2•OH)

(8)

Dye Degradation: Dye (colored) + •OH/h⁺/•O₂⁻ → Organic Intermediates (Decolorized)

(9)

Mineralization: Organic Intermediates + •OH/h⁺/•O₂− → CO₂ + H₂O + SO₄²⁻ + NO³⁻ + NH⁴⁺

(10)

The photocatalytic potential of such NPs has also been shown in earlier studies. For instance, a recent work described the production of CeO₂ nanorods for the photocatalytic degradation of Congo Red (CR) dyes via hydrothermal synthesis and calcination. The results show that the CR solution degrades more quickly when exposed to UV light than when it is dark. Within 130 min, a high CR degradation efficiency of up to 97.7% was observed, demonstrating the rare-earth oxide’s efficacy in photodegradation [33]. Similarly, another work described the synthesis of ZnO NPs doped with rare earth elements (Sm, Dy, and Nd) and evaluated their photocatalytic dye degradation effectiveness against Crystal Violet (CV) and Malachite Green (MG). The highest degrading efficiency for Dy-doped ZnO NPs was discovered to be 97.18% for MG and 98% for C [34]. Furthermore, some examples of rare earth NPs and rare earth doped nanomaterials effective in photocatalytic dye degradation are mentioned in

Table 1. Some examples of rare earth NPs and rare earth-doped nanomaterials are effective in photocatalytic dye degradation.

S.No.	Nanomaterial	Dye	Time (Minutes)	Source	Degradation %	Reference
1	CeO2 NPs	MB	80	Sunlight	75	[35]
2	Sm-doped CeO2 NPs	MB	90	UV Light	80	[36]
3	Eu-doped CeO2 NPs	Rose Bengal	120	UV Light	90	[37]
4	La-doped ZnO	Rhodamine B	40	visible light	82	[38]
5	La-doped TiO2 NPs	MB	60	visible light	85	[39]
6	Gd-doped ZnO NPs	MB	120	UV Light	89	[40]
7	Nd-doped ZnO NPs	MB	180	UV Light	90	[41]
8	Eu-doped CeO2 NPs	Rhodamine B	100	UV Light	70	[42]
9	La2O3–CeO2	MB	100	UV Light	80	[43]
10	La2O3 NPs	MB	100	UV Light	60	[44]

.

4.3. Recyclability of Nanoparticles

The recyclability experiment for the NPs was performed for Fast Green FCF dye, up to third cycle to test the reusability of the photocatalyst. No significant loss in the degradation potential was observed, and after the third cycle, 83.11% degradation was observed as compared to 85.42%, of the fresh NPs (Figure 7). XRD analysis was also performed, after recovering the NPs via centrifugation from the treated dye solution to observe the changes in the crystalline structure of the NPs. The findings of the recyclability test show that the photocatalytic efficiency of the synthesized NPs does not significantly decrease even after the third cycle. Furthermore, the XRD spectrum (Figure 8) of the recovered NPs, after the second cycle also does not show significant change, confirming that the crystalline structure of the NPs remains intact even after being employed as photocatalyst. The findings are consistent with the previous literature [45,46,47].

5. Conclusions

Using a co-precipitation technique, Yb₂O₃ NPs were successfully produced and showed remarkable photocatalytic performance toward the degradation of Orange II and Fast Green FCF dyes in aqueous solutions. The catalyst’s significant photocatalytic activity was demonstrated by its 105 min degradation efficiencies of 85.47% for Fast Green FCF and 83.09% for Orange II. These results demonstrate the outstanding ability of Yb₂O₃ NPs in eliminating dyes from wastewater, providing a practical and sustainable method for advanced water treatment applications in accordance with international environmental sustainability goals.