Applications of Quantum Dots in Photo-Based Advanced Oxidation Processes for the Degradation of Contaminants of Emerging Concern—A Review

Abstract

Nanomaterials are interesting due to their unexpected and unique properties arising from phenomena occurring at the so-called mesoscale (that is, between single atoms and bulk solids). Among nanomaterials, one may distinguish quantum dots, which are highly crystalline nanocrystals with sizes up to c.a. 10 nm. Due to the quantum confinement effect, quantum dots exhibit extraordinary electronic and optical properties and may be utilized in photocatalysis. Semiconducting quantum dots may absorb photons, which results in the excitation of electrons from valence to conducting bands. Excited electrons in the conducting band and positive holes in the valence band may interact with chemical molecules (e.g., with water molecules), forming highly reactive radicals. Consequently, quantum dots may be utilized in advanced oxidation processes based on the action of light (i.e., photo-based advanced oxidation processes). Furthermore, quantum dots have advantages, such as having a tunable energy band gap and relative cost-effectiveness. Advanced oxidation processes are very important in the context of the constantly increasing pollution of the natural environment. Contaminants of emerging concern, such as pesticides, endocrine-disrupting compounds, and flame retardants, are still being detected in naturally present water. Such compounds may be degraded using advanced oxidation processes, utilizing quantum dots as photocatalysts. However, many operational parameters (such as quantum dots’ properties, including the means of their preparation) influence the efficiency of such processes; thus, detailed studies are being conducted.

1. Introduction

The last few decades have seen rapid developments in the field of nanotechnology. Nanomaterials produced by nanoscientists have been found to be useful for many wide-scale applications. This fact has contributed to the rapid development of nanotechnology. The current definition of a nanoparticle, provided by the International Union of Pure and Applied Chemistry, states that a nanoparticle is a particle of any shape but with a size lying in a range from 10⁻⁹ to 10⁻⁷ m [1]. Nanomaterials lie somewhere between the quantum and macroscopic scales; some of their properties are similar to those of molecules and, simultaneously, some of their other properties are rather similar to those of bulk solids. This fact contributes to the observed extraordinary behavior of nanomaterials. For example, their relatively large surface/volume ratio allows for increased adsorption of chemical molecules on their surface. This feature is especially demanded in catalysis [2]. Nanoparticles also exhibit a relatively high surface energy, which contributes to the ability of nanostructures to participate in various chemical reactions [2]. Quantum dots have especially found applications in the catalytic degradation of various organic compounds. For example, Cd_0.5Zn_0.5S and carbon dots (CDs) were combined in S-scheme heterojunction photocatalysts (Cd_0.5Zn_0.5S/Bi₂MoO₆/CDs) with improved stability and activity regarding the degradation of various antibiotics [3]. Another example of the application of quantum-dot-based photocatalytic degradation is the removal of levofloxacin (a third-generation quinolone antibiotic) by a multicomponent and enhanced fibrous photocatalyst consisting of carbon QDs, CdS, and Ta₃N₅; the combined heterostructure (CQDs/CdS/Ta₃N₅) exhibited a kinetic rate constant exceeding that of a CdS/Ta₃N₅ heterostructure by 2.1 fold and that of standalone Ta₃N₅ by 39.4 fold [4]. As may be seen from the above examples, the preparation of heterostructured photocatalysts consisting, at least partially, of quantum dots is a relatively new concept that may lead to increased photocatalytic activity [5,6,7].

The main advantage of the application of quantum dots in photocatalysis is their relatively high photocatalytic activity, which is related to their excellent optical and electrical properties [7,8]. On the other hand, the disadvantages of quantum dots include the following: they often require a problematic synthesis process; complicated operation, separation, and dosage; susceptibility to degradation (including chemical and photo-degradation); and they have relatively complicated and expensive characterization techniques. Moreover, one of the main advantages of quantum dots—the ability to generate reactive oxygen species (ROS)—is, at the same time, responsible for their unwanted and hazardous biological activity, which may include, for example, DNA damage [9,10]. Quantum dots may also release toxic elements into their surroundings, which brings additional concerns.

Contaminants of emerging concern (CECs) are a new group of different chemical compounds ranging from metals, polymers, and semiconductors to complex chemical compounds that are being constantly released into aquatic systems due to the immersive and rapid industrial development of various branches. CECs may be exemplified as pesticides, endocrine-disrupting compounds, and flame retardants. Such compounds may be degraded by advanced oxidation processes utilizing quantum dots as photocatalysts. However, many operational parameters (such as quantum dots’ properties, including the means of their preparation) influence the efficiency of such processes; thus, detailed studies are being conducted.

The aim of this review is to collect and discuss the most recent investigations on the synthesis and application of quantum dots in the photo-based removal of contaminants of emerging concern.

2. Quantum Dots—Preparation, Characterization, and Properties

Quantum dots are crystalline nanoparticles with sizes lying typically in a range from 1 to 10 nm and which exhibit semiconducting properties [2]. However, the most important criterion for a nanoparticle to be a quantum dot is for its size to be comparable to the exciton Bohr radius, which, in turn, depends on the kind of material that constitutes the nanoparticle (e.g., for CdSe, the exciton Bohr radius is c.a. 5 nm) [2]. Such a criterion ensures the exhibition of the quantum confinement effect, which leads to the quantization of energy levels in a quantum dot (similarly to the energy levels in a single atom) [2]. This means that even a metallic bulk solid may reveal semiconducting properties at the nanoscale [2].

There are many methods for the preparation of quantum dots, which may be, in general, divided into two subgroups: bottom-up and top-down [2,11]. The latter comprises mainly physical methods and includes mechanical methods (such as high-energy ball milling) and laser ablation, among others [11]. Bottom-up methods, on the other hand, depend on building the nanoparticle from the bottom and are mainly chemical in nature [2,11]. They include various colloidal syntheses (e.g., the precipitation of sulfides and reduction in noble metal cations), the sol–gel method, hydrothermal methods, microwave-assisted synthesis, sonochemical synthesis (i.e., ultrasound assisted), and electrochemical methods, among others [11,12,13].

Characterization techniques of quantum dots are numerous. Microscopic techniques include transmission electron microscopy (TEM), scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), high-resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM), and scanning tunneling microscopy (STM). They enable the investigation of the morphology, topology, size, and phase boundaries of samples [2]. Among spectroscopic techniques, one may list energy dispersive X-ray spectroscopy (EDX), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure (EXAFS), Mössbauer spectroscopy, inductively coupled plasma atomic emission spectroscopy (ICP-MS), Fourier-transform infrared spectroscopy (FTIR), and ultraviolet-visible spectroscopy (UV-Vis), among others. These spectroscopic techniques provide information on the chemical and elemental composition of quantum dots, their size and phase composition, optical properties (optical band gap), and the local arrangement of atoms within them [2]. Diffraction techniques, such as X-ray powder diffraction (PXRD), give information on the phase composition, crystalline structure, lattice deformations and crystallite sizes [2]. Lastly, absorption techniques such as gas adsorption (e.g., BET measurements) allow for the estimation of the specific surface area of a nanomaterial—a parameter that is especially important in catalytic processes [2].

The semiconducting properties of quantum dots make them suitable for use in photocatalysis. Due to quantum confinement and an increase in the energy band gap, quantum dots absorb photons of specific energy, and that energy is higher than for a bulk material. After photon absorption, electrons from the valence band are promoted to the conduction band. The ‘space’ that remains after the excitation of the electron is called a positive hole. Electrons and holes may recombine and emit energy in the form of heat or light. However, if they do not recombine, they may take part in various chemical reactions that are responsible for their photocatalytic applications. First of all, positive holes may oxidize the surrounding molecules of a solvent (typically: water), forming corresponding radicals. In the case of oxidizing water molecules, hydroxyl radicals are formed which, in chemistry, are known as the second strongest oxidizing agent, after fluorine. These radicals may further react and oxidize organic compounds, causing their degradation. Positive holes may also directly oxidize organic molecules that are adsorbed at the surface of the quantum dot. On the other hand, photogenerated electrons act as chemical reductors and react with oxygen adsorbed on the surface of the quantum dot. Such a reaction produces peroxide radicals, which also have a significant oxidizing capability and may destroy organic molecules.

From the point of view of enhancing photocatalytic activity, the following properties of quantum dots are important:

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size, which determines the optical band gap of the quantum dot (the higher the band gap, the higher the energy of the photoexcited electrons), surface (a larger surface means more likely absorption of organic molecules), the surface-to-volume ratio (smaller quantum dots have relatively more atoms on the surface that are reactive in contrast to the atoms present inside the quantum dot), and the path of photogenerated carriers to the reactive surface (smaller size means shorter path and thus smaller probability of electron and hole recombination),

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optical properties, which determine which photons are absorbed and how likely they are to be absorbed (i.e., the absorption coefficient should be high),

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electric properties, which influences the probability of electron and positive hole recombination.

The properties of quantum dots may be tuned to some extent by doping with various elements, forming heterostructures, and modifying the surface with passive layers or absorbed organic ligands. For example, in a heterostructure, an additional component may increase the specific surface area, improve the separation of photogenerated carriers, modify the optical energy band gap, enhance the absorption of photons, and improve the adsorption of target organic molecules by specific physicochemical interactions.

3. Quantum Dots for Photo-Based Removal of Pesticides

Pesticides are widely used in agriculture, forestry, and horticulture. However, they are hazardous to the environment and considered as contaminants of emerging concern. Photo-based degradation of pesticides is a promising sustainable way for pollutant removal; however, there are various challenges in both the design of materials as well as the selection of optimal operational parameters. Factors such as the pollutant and catalyst concentration, temperature, pH, and the presence of oxidizing agents significantly influence the removal efficiency. Quantum dots and their heterojunctions have great potential in generating reactive oxygen species due to their light absorption, charge carrier dynamics, and adsorption properties.

3.1. Influence of Operational Parameters on the Efficiency and Kinetics of Photo-Based Processes for Removal of Pesticides

3.1.1. Influence of Pollutant Concentration

Photocatalytic pesticide degradation is typically described by the Langmuir-Hinshelwood model; therefore, it follows pseudo-first order kinetics. However, experiments show that removal efficiency decreases as the pollutant concentration rises. This phenomenon is explained by the fact that with an increase in pollutant concentration, more pesticide molecules adsorb on the surface of the catalyst, blocking the active sites, so less reactive oxygen species (ROS) are generated, which causes a decrease in the degradation rate.

3.1.2. Influence of Catalyst Concentration

With the rise of catalyst amount, more reactive oxygen species are generated per unit of time, increasing the degradation rate. However, after some critical point, the addition of the catalyst results in only minor changes [14]; therefore, it is not optimal in terms of cost. Moreover, the excessive concentration of nanoparticles causes a reduction in light penetration and may lead to a decrease in the efficiency of pesticide removal [14]. The phenomenon is depicted in Figure 1.

3.1.3. Influence of Temperature

According to the Arrhenius equation, with an increase in temperature, the reaction rate of the elementary reaction increases. Additionally, temperature rise enhances the charge carrier dynamics, promoting the generation of ROS. On the other hand, an increase in temperature leads to enhanced electron hole recombination, reducing the efficiency of photocatalysis. In addition, the adsorption properties decrease after reaching a certain critical point. Experiments show that the removal rate of certain quantum dots (QDs) enhances with an increase in temperature [15,16,17], while others reach a maximum at a specific temperature and then decrease [18].

3.1.4. Influence of the pH of the Reaction Medium

The influence of pH on the photo-based removal of pesticides is complex. At first, it impacts the charge of the catalyst surface and, therefore, adsorption equilibrium. The second factor is the hydrolysis of pesticides under a certain pH. To optimize adsorption, the pH at the zero point of charge and pK_a of pollutants should be considered. Targhan et al. [14] reported the hydrolysis of imidacloprid in alkaline (pH = 9) conditions. The optimal pH is different for each system: in the case of the degradation of carbaryl with CQDs modified with ZnO and TiO₂, the optimal pH was found to be 7 [19], for CdS QDs on a molecularly imprinted polymer used in the degradation of buprofezin, it was 10 [17], while for the case of imidacloprid removal with the same catalyst, the optimal pH was found to be 9. In the case of ZnS/Ag₂S/thiol groups containing graphene QDs ternary heterojunctions, used in diazinon and fenitrothion degradation, the optimal pH was 3 for both pesticides. It has been reported that the optimal pH for the heterogeneous photo-Fenton process is acidic, because Fe²⁺ and Fe³⁺ ions are present in the form of active aqua complexes instead of less reactive hydroxide complexes [20,21].

3.1.5. Presence of Oxidizing Agents

The presence of H₂O₂ causes enhanced generation of ROS. Targhan et al. [15] reported that in the case of imidacloprid removal by a binary composite of CQDs containing thiol groups and CdS QDs, the addition of H₂O₂ at a concentration of 20 ppm increased the removal efficiency from 75.60% to 88.06%. However, a further increase in H₂O₂ concentration led to only a minor rise in removal efficiency. This result is consistent with research by Pervez et al. [21], in which GQDs/α-FeOOH used QDs for the photo-Fenton degradation of ciprofloxacin (CIP), and where the influence of the H₂O₂ concentration was investigated in the range of 0.125–0.75 mM. The difference between 0.50 mM and 0.75 mM was not significant compared to the increase from 0.25 mM to 0.50 mM. Therefore, 0.50 mM concentration was chosen as optimal.

3.2. Examples of the Application of Quantum Dots in Photo-Based Degradation of Pesticides

Figure 2 presents the molecular structures of pesticides that were degraded using various quantum dots.

3.2.1. Photocatalytic Degradation

Targhan et al. [15] studied the photocatalytic degradation of imidacloprid by CdS QDs passivated with CQDs containing thiol groups for stabilization. Under simulated visible light, at pH 9 and after 90 min of irradiation, the removal efficiency was 90.13%. The influence of components was investigated—photocatalytic performance decreased in the seria CQDs-SH/CdS QDs > CQDs-SH = CdS QDs, i.e., the formation of heterojunctions increased the observed efficiency. Photocatalytic experiments using a UV-C lamp as the light source demonstrated that it is more effective than simulated visible light. The optimal pH was basic—experiments were conducted in a pH range of 5–9; the highest degradation rate was observed at pH 9. This phenomenon was caused by higher adsorption, as a photocatalyst surface has a negative charge (pH at zero point of charge 5.2), while imidacloprid at pH 9 is positively charged (pK_a = 11.12). The influence of the photocatalyst concentration led to the conclusion that an increased catalyst dose provides the generation of more ROS per unit of time; however, an excessive concentration of catalyst causes reduced light penetration of the system, and the removal rate decreases. The effect of H₂O₂ addition was studied, and it was found that an increase in its concentration from 0 to 20 ppm led to a strong increase in photodegradation efficiency, i.e., from 75.60% to 88.06%. A further increase in the H₂O₂ concentration led to a minor enhancement in imidacloprid degradation. Those authors connected this observation with the fact that at relatively high concentrations, H₂O₂ can potentially act as an ·OH radical scavenger. With an increase in temperature from 25 to 40 °C, the degradation rate increased due to an enhancement in charge carrier mobility. Reusability tests showed a slight decrease in degradation over three cycles. The products of degradation were studied with the LC–MS technique (Figure 3); 6-chloronicotinaldehyde (Figure 3-(3)) was considered as the main product of degradation. Imidacloprid may be hydrolyzed to imidacloprid-urea (Figure 3-(2)), then degraded to 6-chloronicotinaldehyde, and oxidized to 6-chloronicotinic acid (Figure 3-(4)).

Targhan et al. [14] used terpyridine (Tpy)-coated CdS QD loaded on a metal–organic framework, i.e., ZIF-67 (Zeolitic imidazolate framework-67), nanocomposite for the photocatalytic degradation of imidacloprid. To investigate the partial influence of these components on the composite degradation efficiency of ZIF-67, CdS/Tpy QDs and ZIF-CdS/Tpy were compared. The removal efficiency was found to be 56.3%, 72.2%, and 91.0%, respectively. The effect of pH was investigated in the range of 5–9; it turned out that better degradation was obtained under alkaline conditions, with a removal efficiency at pH 9 of 80.9%, while at pH 5, it was only 59.2%. Under alkaline conditions, the hydrolysis of imidacloprid was also observed. Without a catalyst, the removal efficiency was 12.50%. Experiments with scavenger addition revealed that hydroxyl radicals are responsible for degradation. Removal rates under dark conditions, which characterize the adsorption equilibrium, increased as follows: CdS/Tpy QDs 15.4%, ZIF-67 43.6%, and ZIF-CdS/Tpy 47.4%. The BET surface area and the total pore volume of ZIF-CdS/Tpy were 1611.1 m²/g and 0.6805 cm³/g, respectively. These results suggest that the large specific surface area of ZIF-67 enhanced the adsorption of pollutants, while CdS/Tpy QDs ensured the generation of hydroxyl radicals and the degradation of imidacloprid.

Muangmora et al. [19] obtained carbon quantum dots using the hydrothermal method from spent coffee and modified them with ZnO and TiO₂. CQD modification increased the BET surface area: the values were 10.64 m²/g, 11.44 m²/g, and 24.148 m²/g for ZnO, TiO₂, and TiO₂/ZnO/CQD respectively. The combination of oxides and CQDs was studied in terms of carbaryl photodegradation. The removal efficiency growth was as follows: TiO₂ < ZnO < TiO₂/ZnO < TiO₂/CQD < ZnO/CQD < TiO₂/ZnO/CQD. The latter showed an almost six times higher degradation rate than TiO₂ (0.0570 vs. 0.0098 min⁻¹). With the increase in the carbaryl concentration, the rate of degradation decreased. Experiments with scavenger addition revealed that hydroxyl radicals make a major contribution to the process, while ·O²⁻ radicals play a minor role. The effect of pH was investigated in the range of 3–11, and the highest degradation rate was observed at pH 7. Reusability tests showed a decrease from 99.01% to 84.09% of removal efficiency after 5 cycles. Those authors connected this decrease with the loss of catalyst during the separating and residual adsorption of carbaryl molecules.

Malik et al. [17] modified CdS QDs with molecularly imprinted polymer (MIP) using gingerol as a template [19] to obtain CdS/MIP and for the comparison of synthesized CdS QDs modified with non-imprinted polymer CdS/NIP. They then used them for the photocatalytic removal of Imidacloprid and Buprofezin. The best removal was observed at pH 10 for Buprofezin and pH 9 for Imidacloprid. Under a temperature of 20 °C and an optimized pH, after 2 h removal, the efficiencies for Imidacloprid were 81% in the case of CdS/MIP and 76% for CdS/NIP. Under the same conditions, the degradation of Buprofezin reached 72% and 68%, respectively. With the increase in catalyst concentration, the degradation rate increased. The rise of temperature from 20 to 60 °C enhanced removal efficiency. The increase in pesticide concentration led to a lower degradation rate, as the pollutant adsorbed onto the catalyst surface and blocked reactive sites, preventing the formation of ROS.

Idrees et al. [18] synthesized carbon and phosphorus co-doped boron nitride quantum dots (CP-BNQDs) with a hydrothermal method and investigated the photocatalytic degradation of a fungicide, i.e., chlorothalonil, and nine other molecules. In the case of chlorothalonil, the observed kinetics were of the pseudo-first order type, with a rate parameter of 0.0053 min⁻¹. An additional investigation conducted on the degradation of the antibiotic tetracycline showed that the optimal temperature was 35 °C. Photocatalytic experiments with the addition of scavengers revealed that the active oxygen species causing degradation were both ·OH and ·O^2–.

Nekooei et al. [16] obtained ZnS QD heterojunctions with graphene quantum dots containing thiol groups (S-G GQDs) and modified with Ag₂S. The obtained ternary ZnS/SG QDs/Ag₂S was utilized in the photocatalytic degradation of diazinon and fenitrothion. According to UV-VIS and COD (chemical oxygen demand) analyses, photocatalytic degradation consisted of two main steps: 1. the degradation of the initial molecule, and 2. the degradation of intermediates. UV-VIS absorbance decreased significantly in the first 35–40 min of the process, while the COD remained almost constant; thus, a notably longer time is needed for the decomposition of intermediates. The influence of temperature was well described by the Arrhenius equation. The activation energies of photocatalytic degradation of diazinon and fenitrothion were 4.8 and 5.1 kJ/mol, respectively. The pH effect was investigated in the wide range of 1–11, and the best efficiency was at pH 3 for both pesticides. The influence of individual components of ternary ZnS/SG QDs/Ag₂S and their combination was also investigated. The degradation rate presented the same pattern for both investigated pesticides and increased as follows: ZnS QDs < ZnS/SG QDs < ZnS QDs/Ag₂S < ZnS/SG QDs/Ag₂S < Ag₂S. Although Ag₂S by itself showed better performance, the price of silver is high, so ZnS/SG QDs/Ag₂S is more sustainable for commercial reasons. ICP analysis showed that the proportion of silver in the final catalyst was only 0.9%. The reusability test showed no significant changes over four cycles compared to the first cycle.

John et al. [22] modified Bi₂O₃ nanoparticles with 5 wt. % of nitrogen and sulfur co-doped carbon quantum dots (N,S-CQDs) and the studied photocatalytic degradation of 2,4-dichloropenoxyacetic acid (2,4-D) and diuron. After 2 h of sunlight radiation, the TOC (total organic carbon) removal rate of 2,4-D was 92% and 97% for diuron. High-resolution mass spectrometry showed that the main product of 2,4-D degradation was the less toxic 2,4-dichloroanisole, while the degradation of diuron led to the creation of 3,4-dichloroaniline.

Abbasi et al. [23] synthesized InP QDs and InP QDs covered with a ZnS QD core–shell structure by applying a solvothermal method. The degradation of dyes, hydrocarbons, and the pesticide deltamethrin was studied under a 300 W Xe lamp. Interestingly, InP/ZnS QDs showed better performance in the degradation of dyes and hydrocarbons while, in the case of deltamethrin, the result was 90.29% after 90 min versus 93.36% in the case of InP QDs. Reusability tests conducted on dyes proved good stability of the catalyst over three cycles.

3.2.2. Photo-Fenton Degradation

Swedha et al. [20] used a Ni₂CuS₄ Qds @ Fe₃O₄ nanocomposite with different amounts of Fe₃O₄, from 10% to 30%, in the photo-Fenton process for the degradation of the pesticide bromoxynil (BRX) and the antibiotic cefixime (CEF). Tests were conducted under a 1000 W halogen lamp with 20 mg/L concentration of catalyst and pollutant and 100 µL of H₂O₂. Photocatalytic tests were conducted on the following systems: only H₂O₂ (without catalyst), Ni₂CuS₄ QDs, Fe₃O₄, Ni₂CuS₄ QDs @ 10%Fe₃O₄, Ni₂CuS₄ QDs @ 20%Fe₃O₄, and Ni₂CuS₄ QDs @ 30%Fe₃O₄. The highest degradation rate was in the case of Ni₂CuS₄ QDs @ 30% Fe₃O₄ for both BRX (0.0070 min⁻¹) and CEF (0.0038 min⁻¹). The lowest rate of degradation was in the case of the reaction with only H₂O₂, which proved that the photo-Fenton process was dominant. The effect of the pollutant and catalyst dosage showed that a higher catalyst dosage at the beginning caused higher degradation, but further increase led to a reaction between Fe³⁺ and ·OH radicals. An increase in pollutant concentration led to a higher percentage of degradation, but further increase caused decline due to the limitation of the generated reactive oxygen states. The influence of pH was investigated in a range of 4–9, and the highest degradation rate was obtained with acidic pH. The presence of anions that are common in real wastewater samples, e.g., Cl⁻, CO₃²⁻, NO₃⁻, PO₄³⁻, and SO₄²⁻, caused a minor decrease in degradation. Reusability tests proved high stability and reusability over six cycles. Experiments with scavengers revealed that ·OH and ·O²⁻ radicals were responsible for the degradation, while the addition of hole and electron scavengers did not significantly influence the process.

Pervez et al. [21] obtained nanocomposites from graphene quantum dots with α-FeOOH and utilized them in the photo-Fenton degradation of ciprofloxacin (CIP). Figure 4 illustrates the influence of operational parameters on removal efficiency. To investigate the impact of the GQD amount in the composite, different portions of GQDs, from 9 to 15 mg, were added during synthesis to 14.456 g of FeSO₄ · 7H₂O. The degradation efficiency was more than 90% in each case, and the highest value was in the case of 13.5 mg GQDs. For comparison, pure α-FeOOH resulted in only 52.6% photo-Fenton degradation. After 60 min, 93.73% of the pesticide was degraded; for comparison, during the Fenton process in dark conditions, the removal efficiency was only 52.50%, proving the significant effect of light in this process. With an increase in H₂O₂ concentration, i.e., in the range of 0.125–0.75 mM, the degradation rate increased; however, the difference between 0.50 mM and 0.75 mM was not significant compared to the increase from 0.25 mM to 0.50 mM. Therefore, 0.50 mM concentration was chosen as optimal. The influence of pH was investigated in the range of 3.01–10.40. CIP has two dissociation constants (pK_a1 = 6.05 and pK_a2 = 8.37), and the point of zero charge of GQDs/α-FeOOH is between 8.5 and 9.0. Generally, higher rates were obtained in acidic conditions: at pH 3.01 and 4.95, removal was the same (93.7%), while increases of pH to 7.00, 9.04, and 10.40 resulted in a decrease in the degradation rate to 79.78%, 58.7%, and 22.0% respectively. A series of experiments with the addition of radical inhibitors revealed that ·OH, ·O²⁻, and ¹O₂ play important roles in the process.

The applications of quantum dots in photo-based removal processes of pesticides are summarized in

Table 1. Summary of discussed examples of the usage of quantum dots in photo-based pesticide degradation.

Material	Pesticide	Removal Efficiency/Constant of Degradation Rate	Conditions	Ref.
CQDs-SH/CdS QDs	Imidacloprid	75.60% under simulated visible light 92.20% under UV-C	catalyst dose 1 g/L pesticide dose 10 ppm pH 7 t = 25 °C Degradation time 90 min	[15]
ZnS/SG QDs/Ag2S 5 mg/25 mL	Diazinon Fenitrothion	0.053 min−1 0.056 min−1	catalyst dose (0.2 g/L) pesticide dose (10 g/L) t = 25 °C 60 W LED light	[16]
CdS QDs/MIP	Imidacloprid (pH = 9) Buprofezin (pH = 10)	81% 72%	catalyst dose 10 ppm pesticide dose 10 ppm Degradation time 120 min UV light	[17]
CdS QDs/NIP	Imidacloprid (pH = 9) Buprofezin (pH = 10)	76% 68%	catalyst dose 10 ppm pesticide dose 10 ppm Degradation time 120 min UV light	[17]
Ag@CP-BNQDs	Chlorothalonil	0.0053 min−1	catalyst dose (1 g/L) pesticide dose (25 ppm) t = 25 °C 5 W of blue LED	[18]
ZIF-CdS/Tpy QDs (0.7 g/L)	Imidacloprid	90.95% 93.12%	catalyst dose 0.7 g/L pesticide dose 10 ppm 35 W LED lamp Degradation time 90 min pH 7	[14]
InP QDs InP/ZnS QDs	Deltamethrin	93.36% 90.29%	catalyst dose (10 mg/kg) 300 W Degradation time 90 min Xe lamp	[23]
TiO2/ZnO/CQD	Carbaryl	99.01%, 0.0570 min−1	catalyst dose 1 g/L pesticide dose 5 mg/L Degradation time 60 min	[19]
N,S-CQD-Bi2O3	2,4-D Diuron	92% 97%	catalyst dose 25 mg/30 mL pesticide dose 20 mg/L degradation time 2 h	[22]
Ni2CuS4 QDs@30% Fe3O4	Bromoxynil	0.0070 min−1	catalyst dose 20 mg/L pesticide dose 20 mg/L 1000 W halogen lamp degradation time 200 min amount of H2O2 100 µL	[20]
GQD/α-FeOOH	Ciprofloxacin	93.73%	catalyst dose 0.25 mg/L pesticide dose 10 mg/L 350 W Xe lamp degradation time 60 min H2O2 dose 0.50 mM	[21]

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4. Quantum Dots for Photo-Based Removal of Flame Retardants

Flame retardants are substances that reduce or lower the overall fire risk of materials. These substances are widely used in composites, textiles, clothes, plastics, building supplies, bedding, furniture, etc. [24]. Flame retardants may be categorized as, for example, inorganic, halogenides (based on bromine and chlorine), and organo-phosphorus.

Brominated flame retardants contaminate the environment, mainly water, dust, air, and soil, from which exposure to humans occurs, potentially affecting human health. Brominated flame retardants may act as endocrine disruptors—they can interfere with hormonal function [25].

Brominated aromatic compounds (including brominated flame retardants) demonstrate the ability to absorb the electromagnetic waves from the UV region. A sufficient amount of captured electromagnetic energy causes the facile homolytic breaking of the C–Br bond and provokes further structural rearrangements, including the breaking of the ether bond, which leads to the photodegradation process [26].

Quantum dots can boost the process of photodecomposition—they generate an electric field that enhances the catalytic activity on the conduction band on the conductor’s surface [27].

4.1. Examples of the Application of Quantum Dots in the Photo-Based Degradation of Flame Retardants

4.1.1. Photodegradation of 4-Bromophenol by Graphitic Carbon Nitride with Graphene Quantum Dots

Halophenols, such as 4-bromophenol are obstinate organic substances, which are used in the production of many substances, such as brominated flame retardants (BFRs), polymer intermediates, drilling fluid, pharmaceuticals, and wood preservatives.

Brominated phenols (BPs) exhibit toxicity. The degradation processes of these substances are slow, and they contaminate water and soil in the long term. The environmental pollution connected with these flame retardants is hard to overcome via natural processes. Notably, 4-BP is considered a priority toxic halogenated phenol by the USEPA (the United States Environmental Protection Agency) based on its mutagenic, genotoxic, and carcinogenic nature [25,27]. Diverse decomposition procedures (chemical, physical, and biological treatment procedures) have been utilized for the treatment of halophenols in wastewater.

Graphitic carbon nitride functionalized with graphene quantum dots (CN@GQN; Figure 5) exhibits an effective photodegradation of 4-bromophenol. CN@GQN is obtained by the in-plain induction of GQDs into exfoliated graphitic carbon nitride via visible light-driven photodegradation.

CN-Ex (carbon nitride) can convert electrons from the valence band, because it has exceptional conductivity. GQDs are responsible for generating an electric field to improve the catalytic activity on the conduction band. A metal-free CN@GQD composite showed a relatively high photodegradation rate of 4-BP under visible light, which could be debrominated and oxidized separately on the conduction and valence bands, respectively [27].

Research has shown that the inclusion of GQD and CN layers might be responsible for the modification of the band structure and the enhancement of hydrophilicity and photocatalytic effectiveness [25].

Graphene-based and various transition-metals catalysts have been considered as photocatalysts for the degradation of bromophenols due to their low cost, strong resistance to chemical reactions, and enhanced charge carrier separation.

CN@GQN, as a non-metallic composite, is more advantageous than metallic catalysts with regard to metal leaching, incomplete pollutant degradation, and the generation of toxic sludge, which are present in the case of metallic photocatalysts [27].

Visible light represents about 43% of the total solar spectrum, while UV light comprises only 5% (Figure 6). For this reason, decomposition under visible light could be more beneficial in practical terms. During a chemical analysis, a composite of GQD was made via the hydrothermal process, built by the induction into the layers and edges of the CN. The application of GQD@CN for the mineralization of 4-BP under LED visible light (λ = 420 nm) irradiation at room temperature led the researchers to the conclusions that the synergistic effect of GQD and CN had caused the complete mineralization of 4-BP, and CN@GQD markedly facilitated the red shift of the band gap and extensive solar light absorption [28].

The recycling ability of the catalyst demonstrated great potential as a metal-free catalyst for the treatment of micropollutants in the aquatic environment. It also indicated that the above-mentioned method would be effective and beneficial for the degradation of 4-bromophenol [25].

4.1.2. Double Heterojunction Carbon Quantum Dots (CQDs)/CeO₂/BaFe₁₂O₁₉ for the Degradation of Tetrabromobisphenol A (TBBPA)

Tetrabromobisphenol A (TBBPA) is often applied as a flame retardant in insulated wires, printed circuit boards, and polycarbonate plastics. TBBPA has been detected in soil, sediment samples, and living organisms [29,30]. As a brominated flame retardant, TBBPA may seriously affect metabolism, cause hormone secretion disorders, and even damage the endocrine system, bones, and brain [30]. Efficient techniques for the elimination of TBBPA from the environment are crucial due to the reasons outlined above [29,30].

Double heterojunction carbon quantum dot/CeO₂/BaFe₁₂O₁₉ magnetic separation photocatalysts (CCBFOMSPs) were prepared via a polyacrylamide gel method and low-temperature sintering combined with a hydrothermal method. The photocatalytic activity of this catalyst for the degradation of TBBPA was improved by regulation of the electromagnetic interaction between the BaFe₁₂O₁₉ contents and the photo-generated electrons.

The construction of a special heterojunction turned out to be an effective method to enhance the photocatalytic activity of the BaFe₁₂O₁₉ photocatalyst. Cerium dioxide (CeO₂) is a wide-gap semiconductor of n-type conductivity, which may be used to construct, together with BaFe₁₂O₁₉, a composite photocatalyst with n-n type heterojunctions. Carbon quantum dots (CQDs) may be additionally incorporated to serve as charge transport carriers, improving the transfer and separation of charge carriers in n-n type BaFe₁₂O₁₉/CeO₂ photocatalysts. In this method, CQDs could be made using glucose as a raw material by the hydrothermal method.

Figure 7a shows the structure of Tetrabromobisphenol A. Figure 7b shows the time-dependent photocatalytic degradation of TBBPA in the presence of Samples S1, S2, S3, and S4 under simulated sunlight radiation (S1—ultrafine CeO₂ nanoparticles, S2—15% CQDs and 5 wt. % BFO/CeO₂, S3—15% CQDs and 10 wt. % BFO/CeO₂, 15% CQDs and 10 wt. % BFO/CeO₂). Figure 7c,d shows plots of ln(C_t/C₀) vs. irradiation time and the first-order kinetic constant (k) of TBBPA in the presence of samples S1, S2, S3, and S4 under simulated sunlight radiation, respectively. The degradation rate of sample S3 was 3.29 times that of sample S1. The results showed that the constructed heterojunction could effectively degrade the Tetrabromobisphenol A [30].

The photocatalytic activity of the used photocatalysts could be linked to the oxidation and reduction capacity of positive holes (h_VB⁺), hydroxyl radicals (•OH), and superoxide radicals (•O₂⁻). It was observed that the saturation magnetization of CCBFOMSPs increased with an increasing BaFe₁₂O₁₉ content. However, the photocatalytic activity of CCBFOMSPs first increased and then decreased with an increase of BaFe₁₂O₁₉ content. This trend was consistent with the trend of the visible light absorption coefficient among CCBFOMSP samples. The results showed an internal correlation between magnetic and optical properties and photocatalytic activities [30]. Experiments demonstrated that CCBFOMSP is an effective photocatalyst in the photodecomposition of Tetrabromobisphenol A.

4.1.3. TiO₂ Nanomaterials with Various Types of QD to Remove Tetrabromobisphenol-A

A F-TiO₂/g-C₃N₄ heterojunction was successfully applied to degrade widely used flame retardants. Nanostructured TiO₂, due to its unique photochemical and electrochemical properties, has been widely applied in photocatalysis. TiO₂ can soak up only a few solar photons and scarcely responds to visible light because of its large bandgap of 3.27 eV. There are special mechanisms that are used to synthesize high-performance TiO₂-based materials to improve the absorption competence for solar light and the potential for charge separation and transportation. Those special mechanisms involve doping, controlling heterojunctions, and compositing with quantum dots (C, Ge, ZnS, CdS) [31].

A synthetic procedure for the preparation of a F-TiO₂/g-C₃N₄ heterojunction photocatalyst with a three-dimensional structure included one-step calcination, which improved the photocatalytic activity of the F-TiO₂/g-C₃N₄ photocatalysts. Remarkably, compared with pure g-C₃N₄, F-TiO₂, and TiO₂/g-C₃N₄, the photodegradation performance (under visible light) of F-TiO₂/g-C₃N₄ toward TBBPA was greatly enhanced [31]. This was due to the enhancement of the separation efficiency of photogenerated electron-hole pairs [31].

4.1.4. Detection of Melamine by Digital Image Colorimetry Using Carbon Nitride Quantum Dots in a Cellulose Matrix with a Smartphone-Based Portable Device

Melamine derivatives are salts of melamine (an organic base) with organic or inorganic acids such as cyanuric, boric, and phosphoric or polyphosphoric acids. For example, melamine cyanurate (MC) is used in the production of halogen-free flame-retarded unfilled and mineral-filled Pas (Polyamides) for electronic and electrical applications [32].

To integrate carbon nitride quantum dots into a cellulose matrix, carbon nitride quantum dots (CNQDs) were embedded in a sodium carboxymethyl cellulose (CMC) matrix to form a CNQD-CMC film, which was then used to explore the room temperature phosphorescence (RTP) of CNQDs. Due to the internal hydrogen bonding interactions between CMC and CNQDs, the non-radiative relaxation process was suppressed. During the experiments, a simple, background-free miniature device integrated with a CNQD-CMC film was made. In the presence of MEL, the yellow RTP color of the CNQD-CMC film was photographed using a smartphone. The Color Recognizer APP was used to recognize the red (R) value for the quantitative detection of MEL. As a result, a digital image colorimetry (DIC) determination of MEL was achieved due to the visible RTP color change of the CNQD-CMC film [33].

5. Quantum Dots for Photo-Based Removal of Dyes

Industrial effluents containing dyes make water unfit for drinking. Water pollution is nowadays a concern to the global community, since wastewater from the leather, textile, food, and chemical industries may contain hazardous dyes which can cause serious environmental issues. Various organic dyes are commonly used as coloring agents in the cosmetics, leather, textile, printing, paper, plastic, pharmaceutical, and rubber industries. Heterogeneous photocatalysis using QDs is one of many technologies for the advanced degradation of water contaminated with organic dyes.

5.1. Examples of the Application of Quantum Dots for the Photo-Based Degradation of Dyes

5.1.1. Carbon Quantum Dots with TiO₂ Nanocomposite for the Photodegradation of Rhodamine B

Rhodamine B (RhB) (Figure 8) is a toxic, non-biodegradable dye. It exhibits carcinogenic and neurotoxic effects, as well as the ability to cause several diseases in humans [34,35]. Rhodamine B is a synthetic dye utilized in the manufacturing of dye lasers, fireworks, stamp pad inks, ballpoint pens, and carbon sheets [35].

Pure TiO₂ and a carbon quantum dot (CQD)-doped TiO₂ nanocomposite (CQDs/TiO₂ nanocomposite) were prepared by a sol–gel method for the photocatalytic removal of toxic Rhodamine B [36].

TiO₂ is a photocatalyst that needs another compound for the modification of its nanoparticle structure to enhance its photocatalytic activity. This might be achieved by coupling with materials such as semiconductors, metals, dyes, and nonmetal elements. These additives may form the energies of the intermediate states in the band gap. Carbon nanomaterials have been demonstrated to efficiently enhance photocatalytic reactions due to their superior conductivity, capacitance, absorption of visible light, and chemical stability. For example, carbon quantum dots (CQDs), as a kind of novel and environmental carbon nanomaterial, have drawn much attention due to their multiple synthesis methods, favorable biocompatibility, and unique photo-induced electron transfer. It was demonstrated that the absorption wavelength of the composite material underwent a red shift to the visible region in comparison to pure TiO₂. When the content of CQDs in the composite was 5% (based on the mass of TiO₂), the obtained CQDs/TiO₂ exhibited excellent catalytic activity for the degradation of RhB. The degradation degree of CQDs/TiO₂ was 85.47%, which was 5.5 times higher than that of standalone TiO₂. The degree of catalytic degradation of RhB remained above 80% even after four cycles. Investigations of the photocatalytic mechanism of RhB by CQDs/TiO₂ revealed that positive holes (h⁺), superoxide anion radicals (•O₂^–), hydrogen peroxide (H₂O₂), and hydroxyl radicals (•OH) were the main active species in this process [37].

To obtain a composite photocatalyst, carbon quantum dots (CQDs) were embedded onto the external surface of the TiO₂ nanotubes (TNTs). CQDs were produced via the hydrothermal method. The results indicated that the CQDs played an important role in the visible-light photocatalytic process [37].

The TiO₂-doped CQD heterostructure possessed higher photocatalytic activity under UV-light irradiation compared to TiO₂. The photocatalytic degradation of Rhodamine B was improved when TiO₂-doped CQDs were used instead of TiO₂ [37].

5.1.2. Carbon Quantum Dots for the Photodegradation of Methylene Blue

Methylene blue (methylthioninium chloride) (MB) is a cationic organic dye from the phenothiazine family. It is a tricyclic phenothiazine compound that is soluble in water and some organic solvents. Over the years, it has been used for the treatment of malaria, methemoglobinemia, and other pathologies [38].

Carbon Quantum Dots (CQDs), as a typical functional carbonaceous material, have some excellent characteristics, such as luminescence, low toxicity, good up-converting, and relatively low cost of synthesis. The as-prepared samples of CQDs and TNTs exhibited significant photocatalytic activity in the case of the high initial concentration of the methylene blue solution (30 mg/L) within 50 min under visible-light irradiation. This excellent photocatalytic activity was mainly due to the up-conversion photoluminescence property of the CQDs, which can convert longer wavelength light (>600 nm) absorbed from visible light into light of shorter wavelengths (<600 nm) and activate the titania nanotubes to produce electron-hole pairs. Additionally, the stacking interaction of the π–π bonds between the CQDs and the benzene rings in molecules of dyes would improve the absorption ability of the CQD/TNT photocatalysts.

Carbon quantum dots (CQDs) were embedded onto the external surface of TiO₂ nanotubes (TNTs) through an improved hydrothermal method. The results indicated that the CQDs played an important role in the visible-light photocatalytic process—the electrons photogenerated from the TNTs could be trapped by the CQDs and retard the recombination of photoexcited [39].

5.1.3. TiO₂ and GQD Compounds for the Photodegradation of Methylene Blue

TiO₂/GQD compounds absorb electromagnetic waves from the visible spectrum of sunlight and thus may be utilized for photocatalytic organic matter degradation. GQDs show relatively good physicochemical properties, such as stable photoluminescence, thermal conductivity, and improved surface grafting [40]. GQDs could be additionally modified with polyethylenimine (PEI) and polyethylene glycol (PEG).

Polymer-modified GQDs showed enhanced photocatalytic organic matter decomposition [40]. The degradation ability of these modified photocatalysts was demonstrated on the basis of the decomposition of methylene blue. The photocatalytic rates of degradation decreased in the following order: GQDs, GQDs-PEIs, GQDs-PEGs. Thus, polymer-modified GQDs could qualitatively control the degradation rate of MB [40].

GQDs were synthesized via a hydrothermal method using citric acid as a precursor. The highly fluorescent GQDs were highly soluble and stable in water [40].

5.1.4. Phosphorus-Doped Graphene Quantum Dots Loaded on TiO₂ for Enhanced Photodegradation of the Methyl Orange

Methyl orange (MO) dye is carcinogenic, mutagenic, and toxic to aquatic organisms. Methyl orange (MO) is an anionic azo dye, having high chemical stability; it belongs to the sulfonated azo group (Figure 9) [41].

Photocatalytic efficiency may be enhanced by reducing the recombination of photogenerated electron-hole pairs. Transferring the photogenerated carriers to adsorbates is a typical way to diminish the direct recombination probability. It is possible to reduce this probability in TiO₂ by forming a p-n heterojunction with p-type phosphorus-doped graphene quantum dots (P-GQDs). In such a heterojunction, the photogenerated holes are transferred from TiO₂ to P-GQDs, which reduces the direct recombination of carriers. This results in the improvement of the Methyl Orange (MO) photodegradation efficiency to 95.5% after 14 min [42].

Graphene quantum dots (GQDs) are 0D carbon nanomaterials with a size of less than 10 nm. They have the following advantages over traditional semiconductor quantum dots: simple synthesis, unique boundary effects, and biocompatibility [42].

A P-GQD/TiO₂ photocatalyst was synthesized via facile hydrothermal method. It exhibited efficient photocatalytic activity in the MO degradation process under UV light irradiation. In the hybrid photocatalyst, photogenerated carriers were transferred by the electric fields generated by light excitation and formed by the p-n junction, which, in general, led to the high effectivity of the photodegradation of MO [42].

5.1.5. Methyl Orange Piezodegradation via GQD/ZnO S-Scheme—A Different Method of Degradation with QD Usage

Alternatively, there are other methods for the degradation using QDs which do not depend on the usage of light. Piezoelectric materials are a class of materials with non-central symmetries. Under the action of mechanical forces, they exhibit the captivating phenomenon of piezoelectric polarization, which causes the reorganization of surface charges and starts subsequent redox reactions. Consequently, part of the charge screened by the polarized charge is released and may participate in the catalytic reaction [43].

Piezocatalysis is a newly emerging catalysis technology that relies on the piezopotential and piezoelectric properties of the catalysts [44]. The piezocatalytic technique offers a unique advantage, as it can convert mechanical energy into chemical energy without the need for light or electricity [45].

The GQD/ZnO heterojunction exhibited relatively good piezocatalytic activity, such as a relatively high reaction kinetic rate constant (0.051 min⁻¹) and degradation efficiency (96.1% after 60 min) [45].

A GQD/ZnO S-scheme heterojunction has been applied as a piezocatalyst [45]. The S-scheme charge transfer pathway and piezocatalytic mechanism of GQD/ZnO were revealed, based on both experimental and theoretical analyses [45]. The process involves facilitating charge transfer and improving the redox ability [45].

6. Quantum Dots for Photo-Based Removal of Endocrine Disrupting Compounds

Endocrine disrupting compounds (EDCs) are chemical compounds that can interfere with the hormonal system and may produce harmful effects in both humans and wildlife.

Quantum dots have been applied in the photo-based removal of the following EDCs (see

Table 2. A summary of endocrine disrupting compounds degraded in photo-based processes using quantum dots.

No.	Chemical Structure	Name
1		Ethylparaben
2		Bisphenol A
3		Sulfamethoxazole
4		Norfloxacin
5		Diclofenac
6		Dimethyl Phthalate
7		Para-nitrophenol
8		Sulfadiazine

): Ethylparaben, Bisphenol A, Sulfamethoxazole, Norfloxacin, Diclofenac, Dimethyl Phthalate, Para-nitrophenol, and Sulfadiazine.

6.1. Examples of the Application of Quantum Dots for the Photo-Based Degradation of Endocrine Disrupting Compounds

6.1.1. Degradation of Sulfamethoxazole (SMX) Using a Hybrid CuOx–BiVO₄/SPS/Solar System

BiVO₄ catalysts (0.75, 3.0, and 10 wt. %) were synthesized using polyol-reduction with ethylene glycol as the reductant [46,47]. As may be seen in Figure 10, all obtained catalysts are able to remove all SMX with only the help of solar power and sodium persulfate activation (SPS). If only either SPS or solar was used with a catalyst, the efficiency could not exceed 60%, but with both, the contaminant was removed fairly quickly [48]. Among all of the tested Cu_xBVO₄ catalysts, the best performing one was Cu_3.0BVO₄, which was further proven by another test where all catalysts were used to remove SMX. Cu_3.0BVO₄ was able to remove the entirety of the contaminant in 25 min, while the others did it in 30 min. Next, the effect of reactive species scavengers on SMX degradation was examined within the same environment as in Figure 10. The results showed that ultrapure water and 10 mM of t-BuOH did not make affect SMX degradation, but 10 mM of MeOH caused degradation of around 90% after 60 min, and a sample containing 10 mM of EDTA could remove more than 30% of SMX after 60 min. Lastly, SPS consumption was studied. For normal SPS/solar, there was a small change in SPS concentration after 60 min, while for Cu_3.0BVO₄/SPS, 10% was gone after 60 min, and for Cu_3.0BVO₄/SPS/Solar around 40% was gone by 60 min. This occurred under solar light irradiation, whereby pairs of electron-holes were formed in copper-promoted BiVO₄ and SPS reacted with electrons in the semiconductor’s band forming SO₄^o− radicals that could participate in SMX degradation.

6.1.2. Hybrid Quantum Dots of Cadmium-Doped in Chitosan Forming Cd/CdIn₂S₄ for the Photodegradation of Ofloxacin (OFL) and Para-Nitrophenol (PNP)

Mishra et al. synthesized inorganic-organic hybrid quantum dots of cadmium-doped in chitosan, forming Cd/CdIn₂S₄ for the photodegradation of ofloxacin (OFL) and para-nitrophenol (PNP) [49].

The main reason why this catalyst was made was because CdIn₂S₄ has shown an ideal gap for a catalyst under visible light radiation, i.e., 2.18 eV. The catalyst was produced via a hydrothermal technique for electrocatalytic application, and Cd/CdIn₂S₄@Ch quantum dots were synthesized via the solvothermal method [50], but at a slightly higher temperature of 160 °C. Subsequently, the following research was done.

The first experiment was to determine the optimal amount of H₂O₂ for the best catalyst performance. This was done because H₂O₂ plays a critical part in the photo-Fenton -like oxidation process which was used here [49]. After testing in a range of 0–0.5 mL with a space of 0.1 mL between, the optimal amount of H₂O₂ was shown to be 0.2/0.3 mL for OFL/PNP (conditions: OFL/PNP—20/15 ppm, catalyst dosage 8 mg, pH = 7). The pH of the solution was greatly influenced by the surface charge of the synthesized Cd/CdIn₂S₄@Ch quantum dots; thus, to maximize degradation, the optimum pH must be calculated, including with the types of pollutants present in wastewater. The calculated values of optimal pH for OFL and PNP were 7 and 3, respectively, and these were used in later tests. The next experiment was to determine the influence of contaminant concentration on the degradation performance of our CDs; the best results were yielded for 20 ppm of OFL, but 15 and 10 ppm were very similar. Meanwhile, the best results for PNP were with 15 ppm. The decrease in catalyst performance with higher contaminant concentration may have been caused by the narrowing of photon path lengths. Also, higher amounts of contaminant would require a heavier catalyst load, which could enhance solution’s opacity and, in the end, inhibit photodegradation.

6.1.3. Carbon Dots Modified with g-C₃N₄ for Photo Oxidation of Bisphenol-A (BPA)

Iqbal et al. obtained carbon dots modified with g-C₃N₄ for the photo-oxidation of Bisphenol-A (BPA) under visible light irradiation [51]. The first obtained was g-C₃N₄ by using simple thermal condensation of urea. In this process, 10 g of urea was placed in a crucible and dried at 80 °C overnight, followed by heat treatment at 550 °C with a heating rate of 5 °C/min for 3 h. After heating, the obtained light-yellow powder was ground into a fine powder. Then, carbon quantum dots (CDs) were synthesized with a one-step microwave-assisted approach. The precursor for this synthesis was glucose, of which 70 mg was dissolved in 20 mL of ultrapure water in a beaker. After that, the beaker was placed into a microwave oven and heated at 500 W for 30 min. The obtained light to dark brown solid with some clusters was further dissolved with 10 mL of ultrapure water to produce a CD solution, which was then kept in the fridge. With both C₃N₄ and CDs having been obtained, doped CDs were synthesized by mixing 10 g of urea with 40 mL of ultrapure water, followed by the addition of 0.5, 1.0, and 1.5 mL CD solution for different doped catalysts. Next, the solution was mixed under a magnetic stirrer and heated at 80 °C until it was completely dry. Next, the mixture was cooled to form a solid, ground to a powder, and calcined at 550 °C for 2 h at a heating rate of 5 °C/min. With all catalysts obtained, the degradation of Bisphenol A was researched and the following results were obtained.

High-Resolution TEM microstructure was analyzed, and the size of CDs ranged from 2.2 nm to 3.9 nm, with an average of 3.75 nm. Besides that, the temperature used in the synthesis was not high enough, so the CDs may still have been multi-crystalline or amorphous and still containing significant amount of sp² clusters.

The band gap was measured; higher doping showed a decreased band gap: 1.5 CDs/g-C₃N₄ (2.11 eV), 1.0 CDs/g-C₃N₄ (2.14 eV), 0.5 CDs/g-C₃N₄ (2.26 eV), g-C₃N₄ (2.46 eV).

As shown in Figure 11, a lower pH strongly favored the degradation of BPA on this catalyst [51]. Higher pH caused BPA to exist as an anion or even a dianion, which caused BPA to be repelled from the catalyst, which was also negatively charged, thus lowering its oxidation. This was why the adsorption of BPA on the surface of 1.5 CDs/gC₃N₄ was also high, at pH 3–7. Thus, pH 10 was chosen as the best for tests, because of a good balance between the BPA adsorbed on the surface of the catalyst and the amount of catalyst that generated e⁻/h⁺ pairs.

The next test was the investigation of the influence of the initial concentration of BPA on BPA removal. The test was done with 5, 10, 20, and 30 mg/L (catalyst dosage 30 mg/L, solution volume 100 mL, and pH = 10), with 5 and 10 mg/L being optimal. In the case of higher dosages, the solution was probably less transparent, causing less light to be transported to the surface of the catalyst and making it work with less efficiency. Lastly, research were done on the effect of catalyst dosage and the effect of CD loading on BPA removal. All tests showed similar results, but the best performance was observed with 30 mg/L of 1.5 CD/g-C₃N₄ catalysts.

6.1.4. Photocatalytic Degradation of Fluoroquinolone and Norfloxacin (NOFX) by Mn:ZnS Quantum Dots

The synthesis of Mn:ZnS QDs was as follows [52]. First, solutions of sodium sulfide, zinc acetate, and manganese carbonate in water were prepared. Precursors for the obtaining process were 0.5 M solutions of Zn(CH₃COO)₂*H₂O and Na₂S. Then, 29.75 mL of 0.01 M MnCO₃ solution was added to 49.50 mL of Zn(CH₃COO)₂*H₂O. Subsequently, cetylpyridinium chloride (1 At. wt. %) was added as the capping agent. Next, the Na₂S solution was added dropwise. Soon, a white precipitate appeared, and stirring continued for 15 min. The reaction was then refluxed at 60 °C and centrifuged at 5000 rpm for 5 min. Lastly, the precipitate was filtered through Whattman filter paper and washed to eliminate impurities [52].

After the reaction, an XRD diffractogram showed that Mn:ZnS QDs comprised 83.92 wt. % Zn, 13.32 wt. % S and 2.77 wt. % Mn. The band gap was also measured Via Tauc’s relation; for pure ZnS, it was 3.88 eV, and for Mn-doped ZnS QDs, it was found to be 4.12, 4.39, 4.6, and 4.5 eV for 0.5%, 1%, 3%, and 5% Mn doping, respectively [52].

The first test done was the investigation of the efficiency of ZnS QDs for NOFX removal (15 mg/L). The best results were obtained with Mn:ZnS+UV, achieving degradation above 80%, followed by pure ZnS+UV, with removal of around 80% of NOFX. The next test conducted was the investigation of the effect of the initial pH of the NOFX solution on its removal (Figure 12). pH plays a significant role in photocatalytic aqueous oxidation because it influences the charge of the catalyst, the size of catalyst particles, and the positions of valence and conduction bands.

As may be seen from Figure 12, the optimal pH for the degradation of those drugs was found to be 10. The decrease in drug degradation at lower and higher pH was due to surface changes in the catalyst. The ZnS point of zero charge is between 7 and 7.5, so at lower pH values, too much of the contaminant was absorbed, whereas at higher pH, there was not much NOFX adsorbed on the surface of the catalyst.

Next, the amount of catalyst for this pH was researched. As shown in Figure 13, the optimal amount of catalyst was 60 mg for 25 mL of the drug with 60 min of irradiation. Higher amounts of the catalyst also worked well, achieving results of around 75% degraded NOFX. This was because higher amounts of the catalyst caused reduced light penetration, while the initial increase in degradation for lower values was possibly due to the augmentation in the active site accessibility with increased loading of the catalyst.

Lastly, the effect of the initial concentration of the drug was studied; the optimal amount was found to be 15 ppm, with a degradation efficiency of around 86%, while for 10 and 20 ppm, degradation lowered to 60 and around 45%, respectively. The decline in efficiency with rising drug concentration was because a higher proportion of drug molecules were adsorbed on the surface of QDs, leading to a decreased numbers of active sites, thus creating a lesser amount of OH radicals. Also, because of the presence of more molecules, fewer photons could reach the catalyst surface.

6.1.5. Photocatalytic Degradation of Diclofenac (DCF) by Nitrogen-Doped Carbon Quantum Dot-Graphitic Carbon Nitride (CNQD)

The prepared compounds were bulk g-C₃N₄ and g-C₃N₄ nanosheets [53]. The bulk was obtained via the thermal polymerization of urea. Firstly, 10 g of urea was placed in a 50 mL porcelain crucible with a lid and heated to 550 °C, applying a heating rate of 300 K/h. This temperature was sustained for 3 h. After cooling to room temperature, a bright yellow solid was obtained in the amount of 0.65 g. It was then crushed into powder (denoted as bCN). Afterward, bCN was subjected to a second polymerization via heating in a partially covered crucible. The yield was 0.53 g of a light-yellow substance which was crushed to powder, making g-C₃N₄ nanosheets (CNSs). An elemental analysis of bCN and CNS yielded the following: C 29.4, H 2.0, and N 55.9 (wt. %), and C 31.1, H 3.8, and N 59.3 (wt. %).

After that Nitrogen-Doped Carbon Quantum Dots (NCQDs) were synthesized via hydrothermal treatment [53]. In 15 mL of deionized water, 3 g of citric acid and 1 g of urea were combined and dissolved. After stirring at 500 rpm for 30 min, the mixture was heated at 150 °C for 4 h in a PTFE-lined autoclave. The obtained dark-brown solution was centrifuged at 12,000 rpm for 20 min. After that, the solution was filtered with an ultrafiltration membrane (M_wCO = 2000) to remove impurities. The final solution was freeze-dried to obtain solid NCQD samples. An analysis of the powder showed that it contained C 36.0, H 6.3, and N 14.7 in wt. %.

Lastly, CNQDs were prepared with different concentrations of NCQD (2, 4, 6, and 8 wt. %) via the hydrothermal method. Firstly, 1 g of CNS was ultrasonically treated in 90 mL of deionized water for 30 min to exfoliate and delaminate the sheet’s structure. Then, ultrasonication was applied for 15 min to disperse NCQDs in 10 mL of distilled water. After that, two solutions were mixed and stirred at room temperature for 30 min. Then, the resulting solution was put into a PTFE-lined autoclave and heated to 120 °C for 4 h. The precipitate was collected, washed three times with distilled water, centrifuged at 6000 rpm for 6 min, and then dried at 70 °C overnight. Four types of CNQDs were created based on NCQD concentration (2, 4, 6, and 8 wt. %).

Finally, a three-step process was carried out in which bCN was made via thermal polymerization with urea as the precursor. bCN underwent a second round of thermal polymerization to modify its morphology. NCQDs then were synthesized using citric acid and urea in the second step. The collected NCQD powder was added to the surface of CNS and the co-catalyst via a hydrothermal method at 120 °C for 4 h. After this process, the degradation of DCF via this catalyst was examined.

The band gap values of the obtained bCN and CNQD were between 2.65 eV to 2.76 eV, while the values of the band gaps of CNS and CNQDs were 2.80 eV and less than 1.00 eV, respectively.

Photolysis and the kinetic rate constant were both measured and graphed (Figure 14).

The initial pH and DCF concentrations were 3.17 and 10 mg/L, respectively. As may be seen in the graph (Figure 14), the best performing catalyst was CNQD-6, with DCF removal of 62%. More or less doped catalysts performed worse in the photolysis tests. This was because while the doping of catalysts helps with visible light absorption and improves charge separation of positive holes (h⁺) and electrons (e⁻), after a certain point, it can also cause a shield effect and inhibit the process.

Next, photolysis of DCF with different scavengers such as AO (ammonium oxalate), IPA (isopropanol), and p-BQ was done, because they may scavenge both •O₂⁻ radicals and h⁺. The experiment showed that IPA did not influence degradation, but while AO caused a small decrease in degradation, the sample with p-BQ stopped the degradation of DCF after removing 40% of it, probably due to some equilibrium established in the solution.

Lastly, TOC (total organic carbon content) removal during the photocatalysis of DCF over CNQD-6 and the removal of DCF with CNQD-6 were studied. TOC was removed by 34% in 3 h and up to 75% after 10 h. This indicated that an increase in reaction time enhanced DCS mineralization. The removal of DCF by CNQD-6 was measured over three cycles using HPLC. With each cycle, the was slightly lower, i.e., from 62% to 51%, which showed the great stability of the catalyst. During each cycle, C-N=C sp² hybridized aromatic N atoms were reported as critical components for this oxidation, and during the last cycle, only a slight loss in this band was observed, which indicated excellent stability of g-C₃N₄.

6.1.6. Porous Fe₂O₃ Nanoparticles Coupled with CdS Quantum Dots for the Degradation of Bisphenol A

In a study conducted by Liang and colleagues, porous Fe₂O₃ nanoparticles were coupled with CdS quantum dots for the degradation of Bisphenol A (BPA) [54]. MIL-100(Fe) was synthesized by a direct hydrothermal synthesis. A-Fe₂O₃ was prepared by heating MIL-100(Fe) at 300 °C for 2 h with a heating rate of 5 °C/min in air; afterwards, the temperature was increased to 450 °C at a rate of 1 °C/min. After reaching 450 °C, the ceramic crucible was removed and a reddish brown powder was observed (F450). Next was the synthesis of X-CdS/450. The first step was to immerse 100 mg of F450 in 20 mL of 0.1 M Cd(NO₃)₂ aqueous solution for 30 s and then centrifuge the mixture with distilled water at 4000 rpm for 5 min. Then, the collected sample was immersed in 20 mL of 0.1 M Na₂S aqueous solution for 30 s and again centrifuged with distilled water. The cycle was repeated 1, 3, 5, and 7 times for different catalyst samples. Next, the cycle samples were dried in a N₂ stream. The obtained powder was X-CdS/F450 (X = 1, 3, 5, 7, where x is the number of times of immersion). Lastly, pure CdS was prepared by dispersing 0.1 M Cd(NO₃)₂ in 200 mL distilled water containing 0.1 M Na₂S, which was then stirred overnight. After that, the resulting precipitate was centrifuged with distilled water and dried to obtain a bright-yellow solid. Next, research was conducted, and the results were as follows.

Porous F450 had a surface area of 201 m²/g and a pore volume of 0.26 cm³/g, while modified CdS QDs had a surface area of 79 m³/g. The next experiments were to assess the degradation of Bisphenol A via the obtained catalysts, the effect of H₂O₂ dosage on BPA degradation, the effect of pH, and how each catalyst worked under different conditions. The best performing catalyst in different environments was 5-CdS/Fe₂O₃+H₂O₂, which means that BPA degradation probably occurred through a Fenton-like pathway. The next test further confirmed that among all X-CdS/F450 catalysts, 5-CdS/F450 performed the best, with a degradation time of BPA 30 min. This was because a lower content of CdS led to insufficient visible light absorption, while too much CdS caused the agglomeration of CdS QD nanoclusters which overlapped on the surface and decreased its photoactivity. A pH of 4 was found to be optimal. Finally, the effect of the H₂O₂ dosage on the degradation of BPA was studied; the best value was 50 µL. This amount of H₂O₂ produced the most •OH radicals. Lower values of H₂O₂ produced fewer radicals, while higher concentrations caused the radicals to scavenge to form HOO• radicals, with lower oxidation capacity. The last test was to evaluate the reusability of 5-CdS/F450. After each test, the catalyst was recovered by centrifuge, washed with ethanol and water, centrifuged at 4000 rpm for 5 min, and dried in a vacuum at 100 °C for 4 h. After four cycles, the photocatalytic efficiency decreased from 100% to around 90%, which showed the great stability of the obtained catalyst.

6.1.7. Marimo-like Bi₂WO₆ and Mammillaria-like ZnO for the Photodegradation of Dimethyl Phthalate (DMP)

Chin et al. formed a composite catalyst using marimo-like Bi₂WO₆ and mammillaria-like ZnO for application in the photodegradation of dimethyl phthalate (DMP) [55]. Marimo-like Bi₂WO₆ was synthesized with the hydrothermal method. Firstly, 6 mmol of Bi(NO₃)₃*5H₂O was dissolved in 0.4 M HNO₃ and sonicated for 10 min. Subsequently, 3 mmol Na₂WO₄ solution was added dropwise into the Bi(NO₃)₃ solution under stirring. The mixture was stirred for 60 min and placed in a Teflon-lined autoclave for 16 h at 175 °C. The product was then washed with ethanol and deionized water and then dried at 65 °C for 24 h. Next, the BWZ composite was made by ultrasonicating 4 mmol of Zn(NO₃)₂*6H₂O and 0.0462 g of marimo-like Bi₂WO₆ in 80 mL deionized water for 30 min. Next, 0.024 mol of NaOH was added slowly, and the mixture was stirred for 17 h. The collected products were washed several times with ethanol and deionized water and dried at 65 °C in an oven. The dried products were then calcinated at 450 °C for 2 h to obtain 15-BWZ. With this method, other catalysts were also made with different contents of Bi₂WO₆ (15, 20, and 25), denoted as 15-BWZ, 20-BWZ, and 25-BWZ (Figure 15).

After obtaining the catalysts, research was conducted. The band gaps of each catalyst were measured; the band gap of pure ZnO was estimated to be 3.20 eV and that of Bi₂WO₆ at 2.94 eV.

First, the photodegradation of DMP under different conditions was tested. The best catalyst was proven to be 20-BWZ (20 wt. % Bi₂WO₆/ZnO), achieving degradation of 86.6%, whereas pure ZnO and Bi₂WO₆ only degraded 59.5% and 27.1%, respectively. This effect could be attributed to the fact that a low amount of Bi₂WO₆ can lead to fewer heterojunctions, while an excessive amount can become the recombination center of charge carriers. The next test was catalyst recycling. During this test, the 20-BWZ composite lost around 14% of its photoactivity (decrease from 86.6% to 72%) after four cycles, which could be connected to the unavoidable loss of catalyst during the photodegradation of DMP. Also, phototoxicity removal by the catalyst was assessed because of possible environmental risks associated with photocatalytically treated DMP. During the test, the 20-BWZ composite showed great phytotoxicity removal, being able to remove a high level of phytotoxicity (82.8%), which shows that the 20-BWZ composite is not only stable in terms of degrading DMP, but also can efficiently detoxify a treated DMP solution.

6.1.8. Hydrothermally Synthesized rGO-TiO₂ Composite for Ethylparaben Degradation

Firstly, graphene oxide (GO) was synthesized. This was done by adding 120 mL of concentrated sulfuric acid, 2.5 g of graphite, and 2.5 g of NaNO₃ into a beaker under agitation in a cold bath [56]. Subsequently, 15 g of KMnO₄ was added slowly in small doses at T < 20 °C. The suspension was continuously agitated for 2 h at 35 °C. Next, 325 mL of water was added to the cold mixture, raising the temperature to 90 °C. After that, 8.83 mL of 33% H₂O₂ was added to reduce KMnO₄, and then agitation was maintained for 30 min. The oxidized material was washed with 10% HCl, and the suspension was centrifuged and washed several times with water until a neutral pH was achieved. The product was then dried in an oven at 60 °C for 24 h to obtain graphene oxide, which was dispersed in 250 mL of water and sonicated for 1 h. After sonication, the dispersion was centrifuged for 30 min at 8000 rpm.

The next prepared compound was a rGO-TiO₂ composite. This was done with the hydrothermal method, by adding 3.7 mL of titanium isopropoxide to 3.3 mL of triethanolamine to obtain a 0.5 M Ti(IV) solution. rGO-TiO₂ composites were obtained by adding different amounts of GO dispersion (1 mg/mL) to 42.9 mL of a mixture of water with ethanol (proportions 1:14) under continuous agitation. Subsequently, 8.6 mL of 0.5 M Ti(IV) solution was added, followed by agitating for 24 h to obtain a homogeneous solution which was then heated in a stainless steel reactor for 24 h at 180 °C. The resulting solid was washed three times with ethanol, centrifuged at 13,000 rpm for 10 min, and oven-dried at 60 °C. The obtained composites were designated as xrGO-TiO₂, with x being the GO content (4, 7, 10, or 30%) [56].

Lastly, rGO-P25 composites were prepared by the same method. Different amounts of GO were added to 90 mL of a water:ethanol (2:1) mixture, which was then sonicated for 30 min. Next, 300 mg of P25 was added and then agitated for 2 h. Next, the mixture was placed into a stainless steel reactor and heated at 120 °C for 3 h. The resulting composite was recovered by filtration, washed several times with deionized water, and dried at 70 °C for 12 h.

Thermogravimetric analysis curves were made for TiO₂ and the obtained catalysts. The experiment showed curves in which we could distinguish four mass loss regions [56]. The first one was the slight initial mass loss, corresponding to dehydration and the elimination of water molecules adsorbed on the surface (20–200 °C). The second one (200–325 °C) corresponded to the decomposition of labile oxygenated groups to GO sheets. The third one (325–600 °C) corresponded to the combustion of carbon to more stable groups. Finally, the fourth one (600–800 °C) corresponded to the dihydroxylation process; thus, a higher % of GO in the catalyst caused it to be worse in operation at higher temperatures. The photocatalytic degradation of EtP was measured; the results are shown in Figure 16.

As shown in Figure 16, the worst-performing method of photodegradation of EtP was direct photolysis, while the best was photodegradation with a catalyst containing 7% rGO-TiO₂, which was able to photodegrade 98.6% of EtP in 40 min. The presence of rGO sheets in TiO₂ favored photocatalytic activity, because they improved the adsorption capacity by increasing the surface area. Graphene is an electron acceptor which suppresses the recombination of photo-generated charge carriers and enhances EtP degradation. rGO acted as a sensitizer by donating electrons to TiO₂. Lastly, the presence of C-O-Ti bonds reduced the band gap value to 2.55 eV, increasing the concentration of the generated radicals for the reaction. It was also found that rGO of 7% was the optimal because, for lower values, there were not many particles, so the surface still grew, while for higher values of rGO, performance decreased, because excess particles could cover active sites on the TiO₂ surface or act as recombination centers [56].

A comparison of various studies on the photo-based degradation of endocrine disrupting compounds using quantum dots is presented in

Table 3. Comparison of the described catalysts, including information on degraded contaminant, efficiency, time, and conditions.

Catalyst	CEC	Efficiency [%]	Time [min]	Conditions	Ref.
3.0 Cu.BVO	sulfamethoxazole	100	22	Solar/SPS	[48]
Cd/CdIn2S4@Ch	p-Nitrophenol	96.7	30	H2O2, pH = 3	[49]
	ofloxacin	85.5	90	H2O2, pH = 7
1.5CDs/g-C3N4	BPA	Over 90	180	pH = 10	[51]
Mn:ZnS	Norfloxacin	86	60	pH = 10	[52]
CNQD-6	Diclofenac	62	180	Light irradiation	[53]
5-CdS/F450	BPA	100	30	pH = 4, H2O2	[54]
20-BWZ	Dimethyl Phthalate	86.6	90	Light irradiation	[55]
7% rGO-TiO2	Ethylparaben	98.6	40	Light irradiation	[56]
PA66/TiO2/CQDs	AMX	Over 90	120	pH = 8	[57]
	SDZ	88	240	pH = 8
GCN-CQDs/BVO	Benzyl Paraben	85.7	150	pH = 4, light	[58]
CNQDs-2.0/TiO2	BPA	100	80	UV light	[59]
N20BiOBr	E2	100	240	pH = 6.5	[60]
	EE2	100	240
	4TOP	100	240
	BPA	100	240
NCQD(2.10)/Al2O3-700	BPF	100	120	pH = 6.4, 25 °C	[61]
5-N,S:CQD/MIL-101(Fe)	BPA	100	60	PS/VIS	[62]

.

7. Conclusions

The utilization of quantum dots in photo-based wastewater treatment is a promising and developing area of research. In this study, the analyzed QDs and their heterojunctions showed potential for the photocatalytic and photo-Fenton removal of organic pesticides. For flame retardants and dyes, the main similarity was that, among these substances, the most commonly used are GQD, CQD, and CNQD. Carbon/graphitic quantum dots improved the ability for photodegradation under the visible light. The application of quantum dots on the surface of the different compounds to achieve photodecomposition is a novel, environmentally friendly, and low-cost kind of photodegradation. The reactive oxygen species produced in the process degrade pollutants into organic intermediates and mineralization products; however, total oxidation requires a long time, and only few studies to date have considered TOC, while most research has been focused only on the initial pollutant. Moreover, operational parameters significantly influence the process and therefore should be optimized for each system individually.