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Review

Photocatalytic Degradation of Environmental Contaminants: Transformation Products and Effects on Photocatalytic Performance

by
Ailton José Moreira
1,
Gleison Neres Marques
2,
Kelvin Costa de Araújo
2,
Alex Silva de Moraes
2,
Lucia Helena Mascaro
2 and
Ernesto Chaves Pereira
2,*
1
São Paulo State University (UNESP), Institute of Chemistry, Araraquara 14800-060, SP, Brazil
2
Department of Chemistry, Universidade Federal de São Carlos, São Carlos 13565-905, SP, Brazil
*
Author to whom correspondence should be addressed.
Catalysts 2025, 15(7), 643; https://doi.org/10.3390/catal15070643
Submission received: 15 May 2025 / Revised: 22 June 2025 / Accepted: 28 June 2025 / Published: 30 June 2025
(This article belongs to the Special Issue Recent Advances in Photocatalysis for Environmental Applications)

Abstract

Advanced oxidation processes are technologies currently being developed and applied to degrade many emerging contaminants that require special attention due to the risks associated with health and the environment. However, the transformation products (TPs) produced by the degradation of these contaminants have attracted little attention from the community regarding their effects on degradation processes, persistence, and environmental toxicity. To present this concern to the scientific community, this article provides data and information that allows us to identify that TPs cannot be pushed to the background or completely ignored in environmental decontamination studies. To this end, heterogeneous photocatalysis was chosen as the primary data collection mechanism due to its interdisciplinary nature. Computational simulation tools, such as Density Functional Theory (DFT), which are widely used to study the properties of materials and contaminants, are very useful and should be applied more frequently to understand the properties of TP. These compounds can interact with photocatalysts and impact the degradation performance of the primary contaminant. Monitoring TPs in degradation reactions is also a challenge due to the lack of analytical standards, the variability of the compounds formed, and the low concentrations produced. The results presented here allow us to conclude that these TPs can affect photocatalytic performance, induce questionable conclusions about their performance, be more toxic than the contaminant of origin, and, above all, contribute relevant information to conclude about the degradation mechanisms.

Graphical Abstract

1. Introduction

The scientific community has faced a worldwide problem of environmental contamination related to the presence of highly persistent organic pollutants (POPs) in different ecological systems [1,2]. These compounds, classified as emerging contaminants, encompass a range of substances, including pesticides, personal care products, sanitizers, and active ingredients in pharmaceutical products, among others (Figure 1). Their varied chemical composition confers these compounds chemical stability, limiting their degradation by physical, chemical, and biological processes that naturally occur in ecosystems [3,4]. The sources of these contaminants’ introduction into the environment are diverse, and their control has proven to be a significant global challenge [5]. The natural consequence of this contamination is the constant reports around the world of toxicity to living organisms, bioaccumulation along the food chain, mutation, alterations to the reproductive system, and evidence of a relationship with the emergence of new diseases affecting living beings [6]. One of the main aspects that raises concern about the bioaccumulation of these compounds in nature is their presence in foods such as meat, milk, and eggs, which humans widely consume. The impact of the bioaccumulation of these compounds in pregnant women has raised necessary evidence of the presence of severe neurodevelopmental disorders in fetuses [7]. On the other hand, a meta-analysis reported that exposure of pregnant women to per- and polyfluoroalkyl substances (PFASs), which are also classified as POPs, may be associated with gestational diabetes mellitus, hypertensive disorders of pregnancy, and gestational hypertension [8].
These health risks have led government agencies in developed countries to regulate the presence of new contaminants in the environment [9,10]. However, the global reality of contamination by this class of compounds highlights the urgent need for progress to minimize and eliminate these problems. Among the different classes of emerging contaminants found in the environment, the active ingredients of pharmaceuticals have emerged as one of the biggest concerns, given the number of molecules used to combat different diseases [11]. Found in environmental concentrations ranging from ng/L to mg/L in surface waters around the world, their transformations over time and their remaining in the ecosystem are still poorly understood, as is their toxicity to different classes of living organisms. Environmental monitoring studies have identified a high frequency for the presence of compounds such as atrazine, sodium diclofenac, ibuprofen, and naproxen, as well as concentrations of up to 2 µg/L, 836 µg/L, 1673 µg/L, and 464 µg/L, respectively [12]. The literature also shows that these contaminants are present in rainwater, aquatic sediments, drinking water, and other matrices [13,14,15] (Figure 2).
As a result of this environmental presence and the inability of conventional treatment methods to degrade these contaminants, advanced oxidation processes (AOPs) have been identified as an alternative for treating contaminated water and effluents. These methods involve the production of highly reactive species that promote the degradation of contaminants through redox reactions. The techniques that have always been investigated for this application are ozonation, Fenton, photo-Fenton, peroxidation, and others [16]. Among the oxidation methods mentioned above, heterogeneous photocatalysis has been widely investigated due to its particularities. In this process, the interaction of a semiconductor material with light radiation produces reactive species capable of degrading contaminants [17,18]. The efficiency of the process is directly related to the material’s ability to absorb electromagnetic radiation, promote an electron from the valence band (VB) to the conduction band (CB), maintain the separation of charge carriers, and thus use the oxidizing and reducing sites in the structure for degradation reactions. In addition, the interaction of contaminants with the semiconductor surface can increase the degradation kinetics of the compound by bringing it closer to the reaction sites, as well as influencing the degradation mechanisms and preferential formation of TP [19,20].
Concerning the TPs formed during degradation reactions, the literature has shown that there is great concern in identifying these species. However, there are few reports of studies aimed at investigating the effect of these compounds on the photocatalytic mechanism, toxicity, and environmental persistence [19,21,22]. As with any compound harmful to health and the environment, TPs must be given the same attention as the original contaminants once formed in the degradation process. This is because these compounds also interact with the semiconductor and can affect its performance, are discharged into the environment as a waste product from the treatment of the contaminant of origin, can remain in environmental matrices for a long time, and are potentially toxic, since they come from originally hazardous compounds [23,24].
Studies involving the origin contaminant (atrazine) and their TPs (hydroxyatrazine and desethylatrazine) have shown effects on some expression genes responsible for the metabolism of plant species [25]. Using computational toxicity evaluation, Fan et al. identified that, during the UV/Chlorine reaction of phenols degradation, halogenated coupling TPs were harmful due to human thyroperoxidase activity inhibition and aquatic toxicity [26]. Sandré et al. investigated the toxicity of furosemide (FUR) and two TPs [saluamine (SAL) and pyridinium (PYR)] in fish models. The authors identified that, in some situations, the toxicity of SAL and PYR to organisms is more evident than the toxicity of FUR itself [27]. Wang et al. also investigated the toxicity of methylformin and two TPs [(C4H6ClN3) e (C4H6ClN5)] that are not metabolized by the body and reach environmental ecosystems. Their studies confirmed that the two TPs evaluated showed hepatoxicity in mice. In addition, the TPs showed the potential to cause damage to the liver and brain, to alter the intestinal biota, as well as to disturb metabolic levels in the animals [28]. These are some of the studies that have shown the importance of paying attention to TPs originating from POPs degradation reactions. The presence of these TPs in the environment, the numerous reports of toxicity, and the little attention that the scientific community still directs to these compounds limits decision-making for their regulation. Considering the vast number of unregulated contaminants, it is unthinkable that the regulation of these TPs could happen in a short space of time. However, the problems caused by the class of Per- and polyfluoroalkyl substances have shown that TPs should be considered hazardous and require regulation [29]. Many PFAS are TPs of the original contaminant, and their toxicity has been reported worldwide and has drawn the attention of government authorities [10,30].
This review seeks to present a more comprehensive analysis of the TPs that originate in advanced oxidation studies, presenting evidence of their effect on photocatalytic performance, influence on the photodegradation mechanism of primary contaminants, selectivity potential for preferential mechanisms, analytical alternatives for their monitoring, and a future perspective for the scientific community to take a closer look at this stage of AOP applied to the treatment of water and effluents (Figure 3).
Recent advancements in analytical chemistry have significantly enhanced the identification of TPs. In many cases, compounds classified as persistent are already listed in existing databases, which include information such as exact mass, structural characteristics, and fragment ion data [31,32]. However, some compounds may be categorized as unknowns, meaning that limited or no information is available in current databases. In such cases, more robust and advanced analytical techniques are required to elucidate their structures and confirm their identity. In this sense, Wang et al. [33] developed an automated platform based on machine learning for the discovery of unknown compounds, achieving a highly accurate performance with a false positive rate of less than 0.7%, significantly outperforming existing algorithms. On the other hand, Mattoli et al. [31] conducted suspect screening and untargeted analysis using ultra-high-performance liquid chromatography coupled with quadrupole time-of-flight mass spectrometry (UHPLC-qTOF) to obtain a more comprehensive chemical profile of POPs. Among the identified compounds were omeprazole and its primary metabolite, omeprazole sulfide. Recently, Anagnostopoulou et al. [34]. employed high-resolution mass spectrometry (HRMS) to identify persistent organic pollutants (POPs) in wastewater from an urban area in Greece. Among the most frequently detected compounds were pharmaceuticals with hypertensive and beta-blocking activity, such as valsartan and irbesartan, which reached concentrations exceeding 5000 ng/L in influent samples and 1000 ng/L in effluent samples [34]. In another study, Nováková et al. [35] developed a robust extraction method applicable to both wet and dry soil samples, combined with a comprehensive analytical workflow for the identification and semi-quantification of transformation products (TPs). The approach integrated Liquid Chromatography/Mass Spectrometry/Mass Spectrometry (LC-MS/MS) and Liquid Chromatography/High Resolution Mass Spectrometry (LC-HRMS) using the Parallel Reaction Monitoring (PRM) method, which led to the creation of a suspect list used to screen targeted suspects with the LC-HRMS/PRM technique, which allowed for the sensitive and selective identification of transformation products in complex soil matrices. Additionally, extremely low concentrations of TPs can be detected using advanced analytical techniques, such as direct infusion Fourier transform ion cyclotron resonance mass spectrometry (DI-FT-ICR MS) [36,37]. This cutting-edge high-resolution mass spectrometry (HRMS) technique can resolve mass differences in the millidalton (mDa) range and achieve mass accuracy with errors of less than one part per million (ppm) [36]. These integrative strategies form a robust analytical framework to map the fate of POPs and TPs in environmental and biological matrices.

2. Photocatalysts, Synthesis, and Reaction Mechanisms

The growth of material science, especially nanostructured materials, has led to considerable technological advances in various fields of science [38]. Many nanostructures with different chemical compositions, architected morphologies, planned structures, and modulated physicochemical properties have been successfully achieved by the synthesis methods developed [39,40,41]. These materials have led to an exponential growth in new technologies, being applied as photocatalysts in different processes to produce renewable energy [42,43], environmental monitoring systems [44], and energy storage devices [45], in addition to environmental decontamination applications [17] (Figure 4). Therefore, knowing some primary methods for synthesizing these materials, their advantages, and opportunities for exploring new approaches contributes to advancing the field.
The physicochemical properties of interest for photocatalysts are, in general, their ability to interact with light energy, high surface area, easy synthesis, low cost, reusability, and the possibility of precise control of the reaction steps [46]. In this sense, synthesis methods, such as the sol-gel method, electrodeposition, and electrospinning, produce materials with high structural organization in the form of particles or nanometrically controlled structures [47,48]. Furthermore, controlling key parameters of these synthesis mechanisms, such as crystallization speed, promotes the formation of structures with high purity or the insertion of secondary elements as co-catalysts, thereby improving the photocatalytic properties [49]. On the other hand, hydrothermal synthesis methods are systems widely used to produce many nanomaterials under controlled temperature and pressure conditions, replacing multi-step reactions with direct reactions that consume fewer reagents [50]. The reaction media can be made up of various solvents, where their physicochemical properties promote preferential conditions for crystal growth in the x, y, and z directions, allow for the precise incorporation of co-catalysts, and give rise to easily controlled morphologies. However, one method that has stood out for its simplicity, speed in producing nanostructures, doping efficiency, precise control of polymorphs for polyphase semiconductors, and fine-tuning of energy parameters is the microwave hydrothermal method. In this method, the uniform heating of the reaction medium increases the reaction speed, and the nature of the solvent applied improves the control of heating due to the dielectric constant that influences the interaction with microwave radiation [51,52]. For each synthesis method chosen, specific properties can be induced in the nanomaterials (Figure 5), which are used for different technological applications.
A new version of the microwave-assisted hydrothermal method to create photocatalysts has been explored lately. This updated technique combines microwave, ultraviolet (UV), and visible light radiation, which work together to produce more effective photocatalysts than standard methods. First proposed by Moreira et al. (2022), the new synthesis proposal was called the microwave, ultraviolet, visible radiation-assisted hydrothermal method (MW-UV-Vis HM) [23]. Using a Microwave Discharge Electrodeless Lamp (MDEL) in the reaction vessel, nanostructures can be obtained under the effect of light in a hydrothermal condition, allowing secondary photochemical reactions to influence the physical and chemical properties of the nanostructures produced (Figure 6) [23,51]. Equations (1)–(3) represent the reactions that can occur from the photolysis of water in the reaction system during the crystallization stage of nanostructures.
H2O + hv → HO + H+ + e(aq)
HO + Mx+ → My+ + OH
e(aq) + Mx+ → Mz+
where x, y, and z represent the oxidation number of the precursor metal ion before the reaction, after oxidation by HO, and after reduction by e(aq).
These photochemical reactions during synthesis can produce materials with a mixture of cations/anions with varying NOX, which results in materials with a great defect number and optimized photocatalytic properties. The literature shows that materials containing a mixture of Mn2+,3+,4+, Fe2+,3+, or Mo3+,4+,5+ exhibit improved photoactivity due to their greater ability to trap photo-excited electrons between the different oxidation states of the metals, minimizing recombination [56,57]. Equations (4) and (5) represent these potential reactions that can occur during the synthesis of nanomaterials.
Mn2+ +HO → Mn3+ + HO
Mn3+ + e(aq) → Mn2+
For Ti4+ precursor solutions, they can react according to Equations (6) and (7):
Ti4+ + e(aq) → Ti3+
Ti3+ + HO → Ti4+ + OH
In the oxidizing peroxide synthesis method, nanomaterials containing more active surfaces are obtained, and the synergistic effect of light and temperature to accelerate the photochemical decomposition of the peroxide and form these surfaces with improved active sites has the potential to be achieved by applying this new method [58,59]. As the source for MDEL is microwave, the system allows its use with different chemical compositions. Under these conditions, nanomaterials are synthesized using an Hg-MDEL under the UV light effect characteristic of Hg, while using a Cd-MDEL, synthesis is conducted under the high-intensity visible light characteristic of Cd emission. When the MDEL comprises I2/Kr, high-energy emissions (λ = 178.3 nm) can be achieved, increasing secondary reactions in the reaction medium [60].
The use of MDEL has been widely investigated for the degradation of contaminants in aqueous media, achieving exceptional results [61,62,63]. However, their application to produce nanomaterials is still incipient, and many combinations can be explored to develop new photocatalysts. In addition to studying the effect of different radiation, using different MDEL reactors that enable precise adjustment of microwave power enables control of the irradiance emitted during synthesis [64]. These synthesis methods for producing photocatalysts can induce structural defects, surface modifications, and changes in composition, among other characteristics that can optimize the photocatalytic properties (Figure 7).
In a review of the few studies involving the application of MW-UV-Vis HM in the synthesis of nanomaterials, Moreira et al. [23] produced samarium-doped TiO2 photocatalysts containing 50% more of the metastable phase brookite compared to the same material produced by the conventional microwave-assisted hydrothermal method (MHM). The authors also identified an increase in deconvoluted peaks associated with possible oxygen vacancies, as determined by high-resolution XPS analysis of the O1s, which was attributed to an improvement in the photocatalytic efficiency of the material. During the application stage, the photocatalyst produced by this new synthetic approach demonstrated superior performance for the photocatalytic degradation of Atrazine and the RhB dye compared to the material produced by conventional MHM. Nobrega et al. [51] also applied the new method to produce 3D ZnO doped with Co and used Rietveld refinement to investigate its crystalline properties in depth. The results showed that the samples produced by MW-UV-Vis HM showed a reduction in micro deformation, indicating that this characteristic is the result of the reorganization of the network due to the formation of particles with defect densities, tensions, and distortions influenced by the light of the MDEL used in the synthesis [51]. The materials were applied to the electrocatalytic reduction of CO2 to CO, and the authors identified that the Co-doped ZnO produced by MW-UV-Vis HM achieved a 30% improvement in faradaic efficiency compared to the material of the same composition produced by conventional MHM. Regarding this is a new synthesis method, few studies have been found in the literature, limiting a more comprehensive discussion of this new approach to nanomaterial synthesis.
These new materials may have different active sites that interact with the molecules during the photocatalytic stage, allowing for optimum photoconversion results [65,66]. These active sites can be selective depending on the nature of the molecule in the reaction medium. In the context of photocatalysis of emerging contaminants, a TP can compete with the primary contaminant and thus affect the effectiveness of degradation. In this respect, computer simulation is an important tool for predicting the reaction nature of photocatalysts and their potential for degrading emerging contaminants and their TP.

3. Photocatalytic Degradation: Performance and Degradation Mechanisms

Various photocatalytic materials have been developed and applied in technological processes over the past few decades. In many of these studies, the search for materials with photoactivity under lower energy radiation, particularly visible light, has been prioritized, as this condition allows solar energy to serve as the driving source of the processes. In this context, the focus has been on finding materials with reduced bandgap energy (Eg). As an example, various synthesis strategies have been successfully employed, reducing the Eg = 3.2 eV of pure TiO2 to Eg values = 1.65 eV when doped with Mn [20], Eg = 1.9 eV when doped with V [67], and Eg = 2.37 eV when doped with Fe/Au [68]. However, other studies have shown that semiconductors with a high Eg value are active in visible light, as is the case with TiO2 P25 and its modification with Ti3+ (Eg = 3.14 eV) [69], and Sb-doped SnO2 (Eg = 3.71 eV) [70]. These results show that looking exclusively at the magnitude of a semiconductor’s Eg cannot be a decisive criterion for defining the light source for application studies. This is because other aspects, such as structural defects, vacancies, multiphase materials, surface properties, and other characteristics, influence the band structure’s light absorption and electronic transition processes. These properties, which can be further evaluated by photoluminescence (PL), X-ray photoelectron spectroscopy (XPS), and electrochemical measurements [24,40,71,72,73], have provided results more consistent with degradation results. Similarly, some computer simulation results confirm that TP can interact more effectively with a semiconductor’s surface, directly or indirectly degrading it. In addition, computational calculations also make it easier to identify the degradation of an organic compound from the gap values between the frontier orbitals (HOMO-LUMO) (Figure 8). In the case of fluoxetine and its TPs (MAEB, PPMA, and TFMP), the Eg values suggest a tendency towards degradation in the order of PPMA > FLX > TFMP > MAEB. Therefore, photocatalytic performance cannot be discussed exclusively based on the degradation results of the primary contaminant, and, in this respect, examining the effects of TP more closely is a relevant scientific discussion.
In addition to the TPs, the effect of the matrix, which can present different co-contaminants, is another aspect to consider in photodegradation studies. For example, dissolved organic matter (DOM) is a naturally occurring component present in environmental matrices, such as surface water and soil [74]. DOM can have a positive or negative impact on the efficiency of photocatalysis due to its ability to interact with light, its potential to adsorb different classes of contaminants, as well as its ability to undergo photochemical reactions and release oxidizing species consumed in the reaction of interest into the reaction medium. DOM can act as a pH regulator in the reaction medium, favoring or not favoring the degradation of some specific contaminants [75]. The effect of TPs on the photocatalytic process, being an even more complex aspect, is discussed in more detail at different points throughout this review.
In degradation processes, TPs can assume structures containing organic groups like the original contaminant, and these characteristics allow us to unravel the degradation mechanisms. Hydroxyl radicals are excellent electron acceptors and exhibit a strong tendency to hydroxylate aromatic rings in the reaction steps [76]. As shown in Figure 8, FLX can be degraded into MAEB, PPMA, and TFMP, which are distinct degradation mechanisms. Moreira et al. [77] showed that, in the degradation of FLX, when it occurs by photolysis, it preferentially forms PPMA and TFMP because of the breaking of the ether bond of the FLX structure. On the other hand, hydroxylation of the ether group gives rise to TFMP and MAEB. These two FLX degradation mechanisms were competitors in the reaction step and may be influenced by the nature of the degradation process, namely, UV photolysis, vacuum photolysis (VUV), and heterogeneous photocatalysis [22,64,77]. Diclofenac sodium is a contaminant containing two aromatic rings joined by an amine bond. During its degradation, cleavage of the secondary amine splits the molecule into a halogenated and carboxylate structure [78]. Hydroxylation, decarboxylation, and dehalogenation are also very common mechanisms in advanced oxidation processes. In heterogeneous photocatalysis, knowing, in depth, the nature of the photocatalyst and its physicochemical properties is a fundamental aspect to unravel the degradation mechanisms [79,80]. Materials with a high specific surface area can improve material–molecule interaction and act as adsorbents. In addition, this interaction can increase the concentration of contaminants near the surface of the photocatalyst and consequently cause degradation by the oxidizing species formed in the photoactivation mechanism [80].
Lima et al. [53] showed, in photocatalytic degradation studies of rhodamine B (RhB), ofloxacin (OFX), and diclofenac sodium (DCF), 100% removal in 120 min for RhB, 95% in 50 min for OFX, and 100% in 20 min for DCF. However, for DCF, three TPs were monitored, and degradation for two of these compounds was successfully achieved, while one of the TPs proved to be more persistent. These results show that the photocatalyst studied was not only efficient in the degradation of DCF but also in the degradation of its TP. In this context, computer simulation applied to suggested/known TP structures allows consistent conclusions to be reached about the persistence of these compounds throughout the reaction, as well as in environmental ecosystems. Studies conducted by Chatzimpaloglou et al. [81] showed that the photocatalytic degradation of the antineoplastic etoposide (ETO) by TiO2 P25 under UV light achieved >95% degradation within 5 min. However, the degradation rate was more pronounced up to 3 min, after which it decreased in speed. The authors monitored 29 TPs throughout the degradation process, and many of these TPs reached a formation peak at just 3 min. After this time, the TP degradation coincides with the ETO degradation [81]. Lincho et al. [82] also identified that, during the photocatalytic degradation of phenol, the TPs (p-benzoquinone, hydroquinone, resorcinol, and 1,2-dihydroxybenzene) reach a formation peak in 20 min, and then are degraded together with phenol up to 60 min. This degradation of TPs after 20 min confirms that, in the photocatalytic mechanism, these TPs act as competitors for the sites and reactive species initially consumed exclusively for phenol degradation [82]. Malafatti et al. [22] studied the photocatalytic degradation of Prozac® (Fluoxetine) in a high-efficiency system of hydroxyl radical production in the presence of a hydroxyapatite photocatalyst. A total of 10 TPs were identified during the degradation of Prozac, which was 100% degraded within 1 min by photocatalysis. By quantitatively analyzing Prozac and its TPs, the authors identified that the amount of TPs formed depended on the material used, and formation peaks were reached from 0.5 to 2 min, depending on the TP. After 0.5 min of processing, some of these TP were also degraded, again indicating that there is competition with the source contaminant during the photocatalytic process [22]. In another study conducted by Moreira et al. [64], the degradation of COU and its TPs (7-HC) under identical experimental conditions showed that 7-HC exhibited a value of k7HC >> kCOU, confirming that, in the photocatalytic mechanism, when a TP is produced, it can react with the oxidizing species with a greater velocity compared to the primary compound. This evidence reinforces the need to monitor the TP formation/degradation profile during a photodamage study, as the TP competes with its parent compound in the reaction medium [64].
The literature shows that the scientific community has consistently used qualitative monitoring of TPs through mass spectrometry analysis to accurately describe the degradation pathways of an emerging contaminant [83]. These degradation routes and structures proposed on numerous occasions show that TP competition with the contaminant of origin is a reality. Xu et al. [84] studied the photocatalytic degradation of 2,4,6-trichlorophenol and achieved a result of up to 88% removal in 90 min. Using mass spectrometry analysis, they identified the formation of TPs, which progressively formed through hydroxylation of the aromatic ring. The proposed degradation mechanism showed that four of the suggested TPs consume the hydroxyl radicals produced in the reaction medium, i.e., they compete with the contaminant of origin in the degradation mechanism. As the authors did not follow the TPs over time, a more in-depth analysis of when this competition begins is not possible, showing that the degradation, limited to 88% of phenol in 90 min, is related to the consumption of hydroxyl radicals by the TPs. The same approach is seen in a study conducted by Wang et al., which obtained photocatalytic degradation of chloramphenicol of up to 92% in 60 min. The authors identified 17 TPs. However, the lack of monitoring over time does not make it possible to define the peak of formation and the exact moment when these TPs start to compete with chloramphenicol in the consumption of reactive species [85]. Table 1 lists different published studies that present the efficiency of photocatalytic degradation, the TPs that were monitored over the degradation time of the primary contaminant, the amount of TP monitored, and the average times of maximum formation of these TPs. The availability of TP monitoring data over degradation time is still incipient in the literature. As discussed earlier, the qualitative approach focusing on identifying the TP structure at a defined degradation time has been preferred. However, the studies listed in Table 1 show that for all cases, a considerable amount of TP is formed in the early stages of degradation. Also considered in the survey was the time when TP reaches maximum formation and is subsequently degraded. In most of the studies reported, TP degradation still occurs at times when the concentration of the primary contaminant is considerable. This result reinforces the fact that TP plays a competing role in the degradation mechanism, consuming oxidizing species, interacting with the photocatalyst surface, and blocking active sites, thereby impairing photocatalytic performance when only the primary contaminant is analyzed. On the other hand, the degradation of TP also shows that the photocatalyst can follow a preferential route, and considering the monitoring of these compounds in chemical reactions is a relevant aspect for scientific discussion.
The dynamics of photocatalytic reactions are intrinsically related to the different forms of interaction between the catalyst, light energy, and the species that make up the reaction medium [94,95]. The surface properties (such as specific area and porosity) and electronic structure (valence and conduction bands) of the materials determine the preferred mechanisms for removing pollutants, which can be adsorption, direct photocatalysis in the valence band or conduction band, indirect photocatalysis by reaction with oxidants formed in the photocatalytic mechanism, and other processes [96,97]. Active sites can be related to the presence of a metal center, an oxygen vacancy, a metal vacancy, an acidic or basic surface group, a structural defect, and even the polarity of the surface [95,98,99]. Density Functional Theory (DFT) calculations have been widely employed to elucidate contaminant degradation mechanisms at the atomic level, often with experimental studies. For instance, Li et al. [96] used DFT to identify carbon defect sites in C3N5, revealing that defects increased the bandgap and formed terminal amino groups, which acted as electron traps to enhance charge separation. Similarly, Wang et al. (2025) [100] evaluated the electronic and optical properties of B-, P-, and B/O-doped graphitic carbon nitride (g-C3N4), showing that B/P-doping narrowed the band gap and enhanced photocatalytic activity.
The role of surface properties and charge distribution in degradation processes has also been explored using DFT. Li et al. [96] employed electrostatic surface potential (ESP) analysis to identify electron-deficient regions near defects that facilitate the generation of reactive oxygen species (ROS). Also, Zhang et al. [101] employed spin-polarized DFT to analyze the band structures of Bi2WO6 and MgFe2O4, demonstrating that Z-scheme heterojunctions enhance charge separation for tetracycline hydrochloride (TCH) degradation. Finally, Dai et al. [102] showed that Fe2O2@NiFe LDH heterojunctions modify interfacial band structures to suppress electron–hole recombination. Figure 9 shows the identification of active sites obtained by computer simulation for some photocatalysts. These results allow a preliminary analysis of the photoactive potential to degrade classes of contaminants with specific chemical structures [66,103].
Fukui indices and frontier molecular orbital (FMO) analyses have been essential for predicting degradation pathways and reactive sites. Essenni et al. (2024) [104] used Fukui indices to identify the most vulnerable methylene blue (MB) sites for hydroxyl radical attack. Yu et al. (2024) [105] applied similar methods to map doxycycline hydrochloride (DOC) degradation pathways, while de Paiva et al. (2024) [106] combined Fukui indices, ESP, and FMO analyses to predict ciprofloxacin (CIP) degradation mechanisms. Deng et al. (2023) [97] extended this approach by integrating natural population analysis (NPA) to characterize radical attack sites and propose degradation pathways for doxycycline hydrochloride (DH). DFT has also been used to study oxygen vacancies and their role in catalytic processes. Zhang et al. [99] employed Bader charge analysis and climbing-image nudged elastic band (CI-NEB) calculations to show that oxygen vacancies (VOs) in NiFe-LDH enhance peroxydisulfate (PDS) adsorption and activation. Through charge density distribution analyses, Yang et al. [98] further demonstrated that Cd vacancies in Co5:DCdS optimize the electronic structure and improve water adsorption, while Co atoms anchor active sites.
Besides DFT, Molecular Dynamics (MD) simulations have provided complementary insights into degradation mechanisms. Gomzi et al. [107] combined MD and DFT to model toluene adsorption on bimetallic oxide surfaces. At the same time, Wang et al. (2023) [108] used MD to study the interplay between hydrogen bonding and stacking in SubPC-3 molecules. The integration of DFT with experimental techniques has also been explored. Wan et al. (2024) [109] combined DFT with XPS to elucidate charge transfer mechanisms in CoS-based systems, and Yu et al. (2025) [110] correlated DFT-predicted Z-scheme band structures with experimental results to propose a degradation mechanism for CoAl-LDH/BiOBr. These combinations of computational and experimental approaches validate computational predictions and provide a more comprehensive understanding of degradation processes. However, the use of computer simulation to predict TP’s effect on the reaction system and the performance of photocatalysis has been underexplored. This approach is relevant in photocatalysis since, for some systems, the concentration of TP in the reaction medium is much higher than the concentration of the source contaminant after just a few minutes of reaction [111].
It is important to note the diversity of methodological approaches employed in the computational studies. For instance, regarding the software used for DFT calculations, most studies rely on VASP [97,99,100,102], Gaussian [104,105,106], or CP2K [108]. As for exchange-correlation functionals, the most frequently used are PBE [97,99,100,102,108] and B3LYP [104,105,106]. For basis sets, the common choices include plane waves [99,100,102,107] or GTOs [104,106]. Furthermore, implicit solvation models, such as CPCM and SMD, are also applied in some of these studies [104,106], highlighting the complexity and adaptability of computational tools for simulating realistic environments.

4. Higher-Order Calibration: An Alternative for Monitoring TP in Photodegradation Reactions

Chromatography is the standard technique and is established worldwide for analyzing organic contaminants in water and effluent samples. However, methods combining traditional techniques, such as UV-Vis spectroscopy and fluorescence spectroscopy, with chemometric approaches—particularly classification techniques and first- and second-order calibrations—have gained prominence for various applications [112,113,114,115].
Multi-way methods have become important in analytical chemistry, mainly due to the so-called second-order advantage. This advantage refers to the method’s ability to accurately predict the analyte even in the presence of unknown interferents, making it ideal for environmental, food, and other types of samples that are traditionally composed of many constituents. First-order multivariate methods, on the other hand, tend to produce false analyte predictions when interferent constituents not included in the calibration set are present. Another benefit of multilinear methods is that they require a smaller number of calibration standards compared to first-order multivariate methods, which typically need extensive calibration sets to model all possible variations in the sample matrix [116].
An interesting example of the application of UV-Vis spectroscopy coupled with chemometric tools was provided by Moreira et al. for the detection of atrazine (ATZ) and its TP after degradation by photolysis and photocatalysis [112]. Traditionally, a limitation of UV-Vis absorbance spectra lines is in the convolution of peaks from different components; in other words, the summation of peaks often results in a broad band. Using Multivariate Curve Resolution–Alternating Least Squares (MCR-ALS), the authors successfully deconvoluted this broad peak [112]. They identified two components: ATZ and one of its degradation products, as shown in Figure 10.
Although UV-Vis spectroscopy is an accessible technique with simple operation, one of its limitations is the need to work with high concentrations to obtain an adequate analytical signal. An alternative, therefore, is the use of fluorescence spectroscopy, which is characteristic of many organic contaminants found in environmental matrices. However, like UV-Vis absorption, fluorescence spectra obtained from environmental samples—or those containing multiple components—are complex. This complexity arises from the convolution of peaks in regions where fluorescent molecules have overlapping emission, making it challenging to identify and quantify each contaminant separately. In this context, the application of chemometric techniques can assist, as shown in UV-Vis examples, in deconvoluting overlapping peaks.
Excitation–emission matrix (EEM) fluorescence spectroscopy, when combined with Parallel Factor Analysis (PARAFAC), can yield even more robust results than UV-Vis spectroscopy. This is because EEM provides more detailed information from a single sample compared to UV-Vis spectroscopy. Several studies have demonstrated its robustness in detecting contaminants in aqueous matrices, even in the presence of unknown interferences. For instance, it has been successfully applied in the detection of petroleum contaminants, humic substances, and other aromatic hydrocarbons at levels as low as µg/L and ng/L [117,118,119,120,121,122,123,124,125,126,127].
The study by Zhou and collaborators is a strong example of the robustness of the EEM–PARAFAC approach. In their work, the authors investigated contamination resulting from the Deepwater Horizon oil spill, one of the most significant spill events in history [118,128]. Various contaminants, including phenanthrene, pyrene, anthracene, naphthalene, and fluorene, were identified and assessed in marine water. The authors performed several comparisons with gas chromatography-mass spectrometry (GC-MS), highlighting the ease of data acquisition via fluorescence, primarily due to the reduced number of sample preparation steps. This illustrates the advantages of the EEM–PARAFAC method in terms of simplicity and efficiency for analyzing oil spill samples.
The simplicity in sample preparation mentioned by the authors is mainly due to the absence (in most cases) of the pre-concentration step. Additionally, another advantage of the technique lies in its analysis time, which generally takes only a few minutes. In a study conducted by some of the authors of this review, the degradation of PAHs—anthracene, dibenzothiophene, and naphthalene—was monitored using EEM–PARAFAC [129]. Quantification was performed at low concentrations in the µg L−1 range, with a limit of quantification (LOQ) of 1.43 µg L−1 for anthracene. This is another advantage of the technique: its high sensitivity compared to liquid chromatography with UV detection. The deconvolution of the 3D spectrum, shown in Figure 11, enables the separation of individual PAH profiles, allowing their degradation to be tracked throughout the photocatalytic process.

5. Challenges and Future Perspectives

The natural or artificial degradation reactions of contaminants produce secondary compounds (TPs) that need attention due to their potential risks to health and the environment. In the development of new water and effluent treatment technologies, such as photocatalysis, these TP compounds influence the process and the conclusions about the effectiveness of the studied technologies. Therefore, monitoring the formation of these compounds cannot be pushed into the background due to the serious health and environmental consequences that this choice can have. However, the challenges of monitoring TPs in environmental ecosystems are exponentially increased due to the low concentrations of the primary contaminants. The variety of formation routes of these TPs is also an additional challenge, as it depends on environmental and anthropogenic factors, making it difficult to predict their presence in the environment even when their parent compound is present. Nevertheless, these challenges, which have already been identified for monitoring primary contaminants, can also be overcome by developing new analytical methods and techniques, efficient sample preparation processes, and computer simulation with a focus on TPs.
The increased availability of specialized libraries with suggested molecular structures for TPs observed in degradation studies should also be further developed. The combination of different analysis and characterization techniques, such as high-resolution mass spectrometry, nuclear magnetic resonance, and other spectroscopic methods used to characterize molecular structures, should be worked on in an interdisciplinary way. These specialized and robust libraries of TPs’ experimental data can be used with information sources for machine learning tools aiming to predict their potential environmental persistence, toxicity to different living organisms, and potential use in other applications. The same proposal applies to the physicochemical characteristics of different nanostructures produced by the new synthesis method presented here. These properties associated with data processing by machine learning can optimize the search for the applications of these materials, the most suitable synthesis conditions for a specific purpose, as well as the potential of these materials to produce TPs with less toxicity.
For the synthesis method proposed here, still little explored, the challenges are to produce new MDELs capable of operating at low microwave power. The diversity of elemental composition can also contribute to more in-depth studies of the influence of light on the properties of nanomaterials. Another challenge is the high cost of microwave equipment that allows precise control of energy parameters. The use of solid-state DFT to calculate surface energies can optimize the choice of structures that can benefit from the additional incidence of light energy during synthesis. Likewise, DFT can help ensure that the parameters calculated for the solid can be efficiently associated with the characteristics of the contaminants and their TPs. Using computational simulation to predict the photocatalysts with the potential to degrade specific contaminants optimizes the application stage.
These technologies, processes, and protocols applied to contaminants that are widely known today can be used to study TPs in more detail, helping the scientific community and environmental authorities to make important decisions. Identifying, quantifying, and proposing precise TP structures are tasks that require the collaboration of researchers from different fields of knowledge. This task requires highly sophisticated and expensive equipment to reliably and accurately obtain environmentally relevant information. It also requires a multidisciplinary team to overcome all the techno-scientific challenges. As an example, chromatography coupled with high-resolution mass spectrometry has been widely used for this purpose. However, it is a high-cost instrument that requires high qualifications and has limited availability in many regions of the world. The use of chemometric tools to overcome these challenges of monitoring TP at low concentrations using less sophisticated instruments has proved to be an important alternative.
The environmental toxicity associated with these TPs is also an urgent demand; however, the main challenges lie in the availability of these compounds in sufficient quantities to apply them in established protocols. As an example, ofloxacin is marketed for 0.74 USD/mg, however, its TP (10-Fluoro-2,3-dihydro-3-methyl-9-(4-methyl-1-piperazinyl)-7-oxo-7H-pyrido [1,2,3-de]-1,4-benzoxazine-6-carboxylic acid) is marketed for 8.35 USD/mg, while TP (Desmethyl Ofloxacin) is marketed for 17.84 USD/mg. Another example is sultamethoxazole, which is marketed for 0.26 USD/mg, while its TP1 (4-Amino-N-(3-methyl-5-isoxazolyl)benzenesulfonamide) costs 15.00 USD/mg and a TP2 (N4-Acetylsulfamethoxazole,N-{4-{[(5-Methyl-3-isoxazolyl)amino]sulfonyl}phenyl}acetamide, Sulfisomezole-N4-acetate) costs 45.00 USD/mg. This substantially higher cost for acquiring TP limits the development of research using TP as a primary contaminant in different experimental protocols. Degradation routes and TP formation potential, as well as the confirmation of proposed structures by mass spectrometry results, could be more reliably supported using computational methods. Computer simulation, with a focus on TP/nanostructure interaction, can also shed light on the effect on degradation efficiency and the potential for directing the reaction. There are many challenges associated with understanding the transformation of contaminants in the environment, their effects on health and biota, and environmental persistence. Therefore, interdisciplinary and collaborative work between different areas of science is the most appropriate way to overcome all the existing challenges.

Author Contributions

All the authors contributed equally to the writing, editing, and proofreading of this review manuscript. A.J.M.: experimental design, writing—original draft and methodology. G.N.M.: structural properties, writing, illustrations, and methodology. K.C.d.A. and A.S.d.M.: writing, editing, and methods and methodology. L.H.M.: writing, review and editing. E.C.P.: supervision, resources, writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Fundação de Amparo à Pesquisa do Estado de São Paulo (grants: 2013/07296–2, 2017/11986–5, 2021/06128-5, 2021/12394-0, 2022/06219-3, 2023/07525-3, 2024/07206-8), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPQ, Grants: 381789/2024-1), and Shell is gratefully acknowledged.

Data Availability Statement

All data are contained within the article.

Acknowledgments

GenAI was used for the grammatical organization and refinement of the language in the preparation of our manuscript. The tools used were ChatGPT, version GPT-4o, and Gemini (Google), version 2.5 Flash.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

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Figure 1. Some of the classes of emerging contaminants found in environmental ecosystems.
Figure 1. Some of the classes of emerging contaminants found in environmental ecosystems.
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Figure 2. Sources of contamination, presence in the environment, and health risks.
Figure 2. Sources of contamination, presence in the environment, and health risks.
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Figure 3. Presence of contaminants in the environment and some of their transformation products that have been observed.
Figure 3. Presence of contaminants in the environment and some of their transformation products that have been observed.
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Figure 4. Some technological applications of processes based on advanced oxidation.
Figure 4. Some technological applications of processes based on advanced oxidation.
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Figure 5. Morphology of nanomaterials produced by different synthesis methods. * [53], ** [54], *** [51], § [39], §§ [24], §§§ [55], §§§§ [40].
Figure 5. Morphology of nanomaterials produced by different synthesis methods. * [53], ** [54], *** [51], § [39], §§ [24], §§§ [55], §§§§ [40].
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Figure 6. MW-UV-Vis radiation-assisted hydrothermal method (MW-UV-Vis HM): A new approach for synthesizing nanostructures.
Figure 6. MW-UV-Vis radiation-assisted hydrothermal method (MW-UV-Vis HM): A new approach for synthesizing nanostructures.
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Figure 7. Possible chemical reactions triggered and nanostructures produced during the application of MW-UV-Vis HM.
Figure 7. Possible chemical reactions triggered and nanostructures produced during the application of MW-UV-Vis HM.
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Figure 8. Electronic properties of FLX and its TPs: molecular electrostatic potentials (MEPs), frontier orbitals (HOMO/LUMO), and energy gaps.
Figure 8. Electronic properties of FLX and its TPs: molecular electrostatic potentials (MEPs), frontier orbitals (HOMO/LUMO), and energy gaps.
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Figure 9. Interactions between contaminants and photocatalytic nanostructures as a function of surface potentials.
Figure 9. Interactions between contaminants and photocatalytic nanostructures as a function of surface potentials.
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Figure 10. MCR-ALS resolved pure UV–Vis spectra. Reproduced with permission from Moreira, A.J., et al., published by Springer Nature Link, 2022. License 6026530343348, 12 May 2025 [112].
Figure 10. MCR-ALS resolved pure UV–Vis spectra. Reproduced with permission from Moreira, A.J., et al., published by Springer Nature Link, 2022. License 6026530343348, 12 May 2025 [112].
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Figure 11. EEM spectra of an antracene, naphthalene, and dibenzothiophene mixture prepared in a surface water matrix. Reproduced with permission from Araújo, K.C.; et al., Published by Elsevier B.V, 2023. License 6026730602442, 12 May 2025 [129].
Figure 11. EEM spectra of an antracene, naphthalene, and dibenzothiophene mixture prepared in a surface water matrix. Reproduced with permission from Araújo, K.C.; et al., Published by Elsevier B.V, 2023. License 6026730602442, 12 May 2025 [129].
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Table 1. TP monitoring data in photochemical degradation studies of environmental contaminants.
Table 1. TP monitoring data in photochemical degradation studies of environmental contaminants.
Degradation ProcessContaminantDegradation (%)/Time (min)TP (Amount Found)Time of Maximum Formation
(min)
Ref.
g-C3N4 nanosheets photocatalysisDiclofenac96/120415 [n = 4] *[86]
Catalytic oxidation2-hydroxybenzophenone100/10121.7 [n = 12][87]
TiO2/BiPO4 photocatalysisCarbamazepine88/3601050 [n = 5]
110 [n = 3]
150 [n = 1]
240 [n = 1]
[88]
Catalytic oxidationtrichloroacetic acid100/60220 [n = 1]
90 [n = 1]
[89]
Co2O4/TiO2+PMSAtrazine100/50320 [n = 1]
35 [n = 2]
[90]
HAP:Nb2O5 photocatalysisFluoxetine100/161 [n = 3]
2 [n = 2]
5 [n = 1]
[22]
Sm-doped TiO2Atrazine45/3003300 [n = 3][23]
Co3O4/TiO2+(PMS)Atrazine100/20320 [n = 1]
35 [n = 2]
[90]
CuO/CuWO4 nanostructuresFluoxetine95/45340 [n = 3][91]
UV-H2O2chloramphenicol100/40410 [n = 2]
15 [n = 1]
25 [n = 1]
[92]
Plasma water treatmentPFAS100/543 [n = 1]
5 [n = 1]
10 [n = 2]
[93]
* represents the amount of TP with maximum formation at the respective time.
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MDPI and ACS Style

Moreira, A.J.; Marques, G.N.; Araújo, K.C.d.; Moraes, A.S.d.; Mascaro, L.H.; Pereira, E.C. Photocatalytic Degradation of Environmental Contaminants: Transformation Products and Effects on Photocatalytic Performance. Catalysts 2025, 15, 643. https://doi.org/10.3390/catal15070643

AMA Style

Moreira AJ, Marques GN, Araújo KCd, Moraes ASd, Mascaro LH, Pereira EC. Photocatalytic Degradation of Environmental Contaminants: Transformation Products and Effects on Photocatalytic Performance. Catalysts. 2025; 15(7):643. https://doi.org/10.3390/catal15070643

Chicago/Turabian Style

Moreira, Ailton José, Gleison Neres Marques, Kelvin Costa de Araújo, Alex Silva de Moraes, Lucia Helena Mascaro, and Ernesto Chaves Pereira. 2025. "Photocatalytic Degradation of Environmental Contaminants: Transformation Products and Effects on Photocatalytic Performance" Catalysts 15, no. 7: 643. https://doi.org/10.3390/catal15070643

APA Style

Moreira, A. J., Marques, G. N., Araújo, K. C. d., Moraes, A. S. d., Mascaro, L. H., & Pereira, E. C. (2025). Photocatalytic Degradation of Environmental Contaminants: Transformation Products and Effects on Photocatalytic Performance. Catalysts, 15(7), 643. https://doi.org/10.3390/catal15070643

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