Advanced Oxidation Processes in Water Treatment: Mapping 15 Years of Scientific Progress and Collaboration

Abstract

Advanced Oxidation Processes (AOPs) are pivotal technologies for the effective degradation of a wide variety of organic and inorganic pollutants in water and wastewater treatment. This bibliometric analysis evaluates 481 publications from the Scopus database, covering the period from 2010 to November 2025, to explore research trends and developments in the field. The findings reveal a substantial increase in research output, with an average annual growth rate of 22.7%. China leads in publication count with 192 documents, followed closely by the United States with 64 publications, demonstrating their substantial contributions to AOP research. Prominent institutions include Tongji University and Università Degli Studi Di Salerno, emphasizing the global collaboration among 2335 authors from 158 institutions across 74 countries. Key themes emerging from the analysis include high oxidative efficiency of AOPs, their hybrid applications with biological and adsorption methods, and their adaptability in treating persistent pollutants and emerging contaminants. However, challenges such as high operational costs, hazardous byproduct formation, and reliance on specific water matrix conditions remain significant obstacles. Funding sources, notably the National Natural Science Foundation of China, play a crucial role, supporting numerous studies, while journals like “Water Research,” “Chemical Engineering Journal,” and “Science of the Total Environment” are identified as primary venues for disseminating impactful research. Overall, this study underscores the need for innovative strategies and interdisciplinary collaboration to enhance the efficacy and application of AOP technologies in addressing the growing challenges in water treatment and environmental sustainability.

1. Introduction

Water pollution caused by persistent organic and inorganic contaminants remains a major challenge to environmental protection and public health worldwide [1]. Industrial effluents [2], municipal wastewater [3], agricultural runoff [4], and improper chemical disposal [5] introduce a wide range of pollutants into aquatic environments, including dyes [6], pharmaceuticals [7], pesticides [8], endocrine-disrupting compounds [9], and personal care products [10]. Many of these contaminants occur at trace concentrations, exhibit high toxicity, and show limited biodegradability, making them difficult to remove using conventional physical, chemical, or biological treatment processes [2]. Consequently, advanced treatment technologies are required to address pollutants that are resistant to standard wastewater treatment approaches. Advanced Oxidation Processes (AOPs) have emerged as highly effective technologies for the removal of recalcitrant and emerging organic pollutants in water and wastewater [11,12]. It relies on the in situ generation of highly reactive radicals, primarily hydroxyl (OH·) and sulfate (SO₄•⁻) radicals, which exhibit high oxidation potentials and extremely short lifetimes [13,14]. The rapid and largely non-selective reactions of these radicals enable substantial degradation, and in many cases near-complete mineralization, of diverse organic compounds into carbon dioxide, water, and benign inorganic ions [15,16,17].

Laboratory- and pilot-scale studies demonstrate that AOPs can achieve high removal efficiencies across a range of pollutant classes [18,19,20]. For example, Photo-Fenton treatment has been shown to remove up to 95.5% of chemical oxygen demand (COD) from real industrial wastewater, indicating substantial degradation of complex organic loads under optimized conditions [21]. In the treatment of reactive dyes from textile wastewater, AOPs such as ozonation, Fenton and UV-based systems have achieved >90% removal of color and COD in synthetic effluents, with efficiencies exceeding 80% in real effluent matrices despite increased complexity [18]. AOPs have also been widely applied for the degradation of pharmaceutical compounds, leveraging the high oxidation potential of hydroxyl radicals (2.8 V), and studies report significant removal of APIs and related contaminants under various operating conditions [19,22]. The ability of AOPs to act across diverse pollutant classes and water matrices, combined with demonstrated high mineralization and color removal efficiencies under controlled conditions, supports their utility in treating contaminants that are poorly removed by conventional methods. Unlike traditional oxidants such as chlorine or ozone alone, which can be selective and may form secondary by-products, AOPs generally achieve broader oxidation and higher degradation efficiencies for recalcitrant compounds [23,24,25].

Hydroxyl radical-based AOPs have been among the most extensively studied categories due to their effectiveness in removing recalcitrant pollutants from water [26]. For instance, ozone dissolved in water can react directly with certain pollutants, but its effectiveness increases dramatically when it decomposes to produce OH· [26]. This decomposition is influenced by pH and can be accelerated through catalytic processes [26,27]. Homogeneous catalytic ozonation employs transition metal ions like iron or copper to enhance radical generation [28], whereas heterogeneous systems use solid metal oxide catalysts, such as titanium dioxide and alumina, which provide surface sites for ozone adsorption and subsequent radical formation [29].

Fenton and Fenton-like processes exploit the reaction of ferrous ions with hydrogen peroxide under acidic conditions to generate OH· radicals that attack pollutants through multiple pathways, including hydrogen abstraction and electron transfer [30,31]. Additionally, photo-Fenton combines ultraviolet irradiation with Fenton chemistry, enhancing the reduction of ferric ions to ferrous ions, thereby increasing radical production [32]. Photochemical AOPs based on ultraviolet irradiation also play a significant role in OH· generation [23]; for example, the UV/H₂O₂ system relies on the photolytic cleavage of hydrogen peroxide (H₂O₂) [33], while photocatalytic oxidation using titanium dioxide involves the excitation of electrons from the valence band to the conduction band, resulting in electron–hole pairs that react with water or hydroxide ions to form OH· [34]. Hybrid systems, such as O₃/H₂O₂/UV [35] and photo-Fenton [32], combine multiple activation pathways to maximize radical production and achieve rapid degradation and mineralization of resistant contaminants [36,37]. These systems are particularly effective for compounds resistant to single-process oxidation, including aromatic hydrocarbons [38], pharmaceuticals [39], and phenolic compounds [40].

Electrochemical oxidation processes have also emerged as effective advanced oxidation approaches, capable of generating strong oxidizing species including OH· directly at the electrode surface or indirectly through in situ electrogeneration of H₂O₂, which enhances degradative pathways even without external addition of oxidants [41,42]. Electrochemical advanced oxidation processes, including anodic oxidation, electro-Fenton, and their hybrid forms, have demonstrated high degradation efficiencies for various organic pollutants [41,42]. In addition, graphitic carbon nitride (g-C₃N₄) is a visible-light-responsive photocatalyst that enhances OH· generation under UV–visible irradiation and synergizes with ozone or other oxidants, showing versatility across photocatalytic, Fenton, catalytic ozonation, and persulfate activation systems [43,44,45]. In addition, ferrates (Fe(VI)) are strong oxidants with redox potential sufficient for the direct oxidation of a wide range of contaminants. They also provide coagulation and disinfection benefits, enhancing total organic carbon removal and reactive species generation, and can be combined with other AOPs to improve overall oxidation efficiency [46]. They also exhibit coagulation and disinfection properties, allowing simultaneous removal of pollutants and microbial control [46]. Ferrates are effective across a wide pH range, and their integration with other AOPs can improve radical production and overall oxidation efficiency [46,47]. While sulfate radical-based AOPs have emerged as complementary technologies particularly effective in matrices containing radical scavengers (e.g., carbonates, chlorides) that inhibit OH· activity [13], they similarly intersect with many of the above technologies when activated by heat, UV, transition metals, or high-pH conditions [48]. Sulfate radicals offer higher oxidation potential under certain conditions and selectivity toward electron-rich organic compounds [48,49].

The advantages of AOPs over conventional treatment methods are substantial. Several studies indicate that AOPs can exhibit relatively fast reaction kinetics, enabling effective degradation of organic pollutants, including those present at low concentrations, depending on the oxidant system and operational conditions [50]. In addition, numerous investigations report that AOPs are capable of achieving moderate to high degrees of mineralization, commonly assessed by total organic carbon (TOC) removal, while reducing the formation of certain toxic transformation by-products under optimized conditions [51]. In specific electrochemical AOP applications, TOC removal efficiencies of 75–97% have been observed after 6 hours of treatment for model organic compounds, depending on current density and process configuration [52]. In terms of mineralization, ozonation studies on pharmaceuticals have shown TOC removal values up to 95% under optimized conditions within tens of minutes of treatment [19]. At the same time, other systems report more moderate mineralization, demonstrating that the extent of TOC removal is strongly influenced by AOP type and operating conditions [19]. Several investigations also highlight that AOPs can reduce the formation of certain toxic transformation by-products when operational parameters such as oxidant dosage, pH, and reaction time are properly controlled [25,53].

Several bibliometric reviews on AOPs applied to water and wastewater treatment have been published in recent years. However, most of these studies are characterized by a restricted thematic or methodological scope. For example, Brdarić et al. [54] analyzed electrochemical AOPs specifically using the Web of Science database, while other studies focused on activated carbon-assisted AOPs in wastewater treatment [55], regional or Ibero-American trends up to 2018 [56], or life-cycle and cost aspects of AOP applications [57]. Consequently, these reviews often provide fragmented perspectives, emphasizing individual technologies, materials, or sustainability indicators rather than the integrated evolution of AOPs as a unified water treatment paradigm.

In contrast, the present study provides a comprehensive bibliometric assessment of AOP research in water and wastewater treatment from 2010 to 2025, encompassing all major AOP methods and integrating mechanistic pathways, catalyst development, and process innovations within a single framework. By employing the Scopus database literature selection, 481 peer-reviewed journal articles were analyzed to identify key research themes, influential publications, and international collaboration networks, while also highlighting emerging trends and underexplored research areas. This holistic approach distinguishes our work from previous bibliometric reviews by offering a broad and integrated perspective, providing a robust foundation for guiding future research, technological development, and innovation in sustainable AOP-based water and wastewater treatment.

2. Materials and Methods

2.1. Data Source

This study employed a bibliometric approach to analyze advancements in AOPs for water and wastewater treatment from 2010 to 2025. The methodology followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) framework [58], adapted for bibliometric analysis. While the PRISMA framework is commonly associated with meta-analysis, our approach intentionally excludes such techniques, focusing instead on a comprehensive bibliometric methodology to synthesize existing literature. The main objective was to systematically explore research trends, influential publications, collaborative networks, and technological innovations in AOPs. The Scopus database was selected as the primary source due to its broad international coverage, reliable citation tracking, and extensive inclusion of journals in environmental science, chemical engineering, and related fields [59]. Its comprehensive nature ensures the capture of relevant studies and provides a solid foundation for bibliometric analysis. The data extraction process was carefully designed to adhere to the PRISMA guidelines, ensuring a systematic approach to literature selection and analysis. This included defining clear inclusion and exclusion criteria to refine the dataset to only those studies that directly contributed to the understanding of AOPs in water and wastewater treatment.

2.2. Data Collection Process

The literature search was conducted using a structured query string and targeted keywords: “(TITLE-ABS-KEY ((“Advanced Oxidation Processes” OR “AOPs”) AND (“Water Treatment” OR “Wastewater Treatment”) AND (“Contaminants”)) AND PUBYEAR > 2009 AND PUBYEAR < 2026 AND (LIMIT-TO (LANGUAGE, “English”)) AND (LIMIT-TO (DOCTYPE, “ar”) OR LIMIT-TO (DOCTYPE, “re”)) AND (LIMIT-TO (SRCTYPE, “j”))).” The literature search was conducted using a structured query targeting publications containing these keywords in the title, abstract, or keywords. The inclusion of “contaminants” was intended to focus on studies addressing specific pollutants targeted by AOPs. To assess the robustness of this approach, a sensitivity analysis was performed using a modified query that excluded “contaminants,” which showed minimal changes in the overall trends, confirming that the selected strategy effectively captures relevant research.

Once the initial data were extracted from the Scopus database, they were organized into comma-separated value (CSV) files for subsequent analysis. Several filtering procedures were implemented during the source selection process to ensure dataset reliability and accuracy. These procedures included the removal of duplicate entries and the application of specific inclusion and exclusion criteria based on language, document type, and source. After segmenting and evaluating the records, a thorough examination of source names, authors, funding sponsors, and their affiliations for each year was conducted. Additionally, the quality of the documents was assessed by reviewing their titles, abstracts, and keywords. Incomplete or erroneous records were systematically eliminated through a rigorous filtration process to enhance the integrity of the dataset. As a result, the refined dataset included only journal articles published in English, ultimately reducing the total count to 481 publications. This approach to data collection ensured that the analysis was grounded in high-quality, relevant literature, providing a solid foundation for exploring trends and advancements in AOPs for water and wastewater treatment.

2.3. Bibliometric Analysis Processes

In bibliometric studies, the construction and visualization of bibliometric maps significantly enhance the readability and identification of relationships among diverse sources [60]. This methodology simplifies the analysis of findings and aids researchers in understanding the structure of bibliometric results [60]. To analyze these findings and familiarize ourselves with bibliometric patterns, we utilized VOSviewer version 1.6.20 for data processing. VOSviewer is an open-source software specifically designed for the visualization and networking of bibliometric data, making it user-friendly for researchers [61]. The choice of VOSviewer was based on its ability to manage extensive networks and its advanced text-mining capabilities [62]. This software facilitates the identification of relationships and trends within the literature by generating bibliometric maps that visualize connections between selected articles [63]. A notable strength of VOSviewer is its dynamic label management, which adapts to algorithmic requirements, effectively displaying co-occurrences of terms and authors [63].

Our analysis concentrated on several key aspects: the journals in which the articles were published, the author keywords utilized in the papers, the most prolific authors, the most cited authors, the most relevant affiliations, and the countries of origin. These elements provide a comprehensive overview of the research landscape, making them essential for bibliometric investigations. The minimum thresholds (selection criteria) were defined as follows: journals with 10 or more publications, countries with 16 or more publications, affiliations with 7 or more publications, most prolific authors with 5 or more publications, and most cited authors with at least 1 citation. Methodological parameters were carefully set to ensure reproducibility and clarity. Full counting was applied for co-occurrence links, and the association strength method was used for normalization. Keywords were standardized to merge singular and plural forms, abbreviations, and spelling variants, and the default clustering resolution of 1.0 was applied. Metrics such as publication counts, total link strength (TLS), and average normalized citations were calculated to evaluate the visibility, impact, and collaborative patterns of publications [64]. By employing these sophisticated bibliometric tools and techniques, our methodology ensures a rigorous and thorough exploration of the bibliometric data, revealing the dynamics and evolution of the field of AOPs in water and wastewater treatment. This analysis not only highlights the current research status but also guides future studies by identifying emerging trends and potential research gaps in the field.

2.4. Background Behind Choosing the Keywords

The selection of keywords “Advanced Oxidation Processes,” “AOPs,” “water treatment,” “wastewater treatment,” and “contaminants” reflects the study’s focus on technologies that degrade persistent pollutants in various water matrices. Including both water and wastewater treatment ensures coverage of research applicable to drinking water, municipal wastewater, and industrial effluents.

The AOPs have gained recognition for their ability to degrade persistent organic and inorganic pollutants that are resistant to conventional treatment methods [25]. This relevance is underscored by the growing concern over environmental contamination and its implications for public health and ecosystem integrity [25]. The term “Advanced Oxidation Processes” encompasses a range of techniques that utilize highly reactive species to mineralize contaminants, making it a critical area of study in modern water and wastewater treatment [53]. By including both “water treatment” and “wastewater treatment” in the keywords, we emphasize the broad application of AOPs in improving overall water quality and effectively managing wastewater, which is a significant concern for urban and industrial settings. Furthermore, the incorporation of “contaminants” highlights the focus on specific pollutants that these processes aim to eliminate, ensuring a targeted approach to tackling water quality issues.

The integration of these keywords reflects a holistic view of the current research landscape, where there is an urgent demand for sustainable solutions to combat water and wastewater pollution. This focus not only aligns with global environmental initiatives but also addresses regulatory pressures for cleaner water sources. By highlighting the significance of AOPs in the context of both water and wastewater treatment, the chosen keywords encapsulate a vital intersection of environmental science, engineering, and public health, underscoring the importance of advancing research in this field to develop effective strategies for mitigating water contamination. Overall, this approach provides a comprehensive understanding of the research landscape, identifies emerging trends, and highlights knowledge gaps to guide future studies in sustainable water and wastewater management.

3. Results and Discussion

3.1. Bibliometric Analysis Overview

Table 1. Bibliometric analysis of AOPs in water and wastewater treatment (2010–2025).

Category	Description	Results
Main Information About Data	Timespan	2010–2025
	Documents	481
	Source	130
	Countries	74
	Funding Sponsor	160
	Organization	1093
	Citations	39,152
	Affiliation	158
	Average Citations per Documents	81.4
	Annual Growth Rate, %	22.7%
	Document Average Age	3.54 years
Document Contents	All keywords	4958
	Author keywords	1301
	Index keywords	4351
Authors Collaboration	Total Authors	2335
	Total Authors (Counting Repeats)	2693
	Single-Authored Documents	10
	Authors of Single-Authored Documents	10
	Co-Authors per Documents (Average)	5.6
	International Co-Authorships %	39.71%
Citation Analysis	Cited reference	3934
	Cited sources	358
	Cited authors	1589
Document Types	Articles	308
	Review	173
Source Types	Journal	481
Language	English	481

presents a detailed overview of bibliometric data related to AOPs in water and wastewater treatment, covering the timespan from 2010 to 2025. The dataset comprises a total of 481 documents, which include both journal articles and review articles, sourced from 130 unique publications. The research landscape is enriched by the contributions of 2335 authors affiliated with 158 distinct institutions across 74 countries, reflecting a global effort to tackle water pollution challenges.

The collective impact of these publications is significant, with a total citation count of 39,152, resulting in an average of 81.4 citations per document. This metric underscores the relevance and influence of research within the academic community. The annual growth rate of publications stands at an impressive 22.7%, indicating a robust and increasing interest in AOPs over the specified period. Furthermore, the average age of documents in this dataset is approximately 3.54 years, suggesting that recent research is actively shaping the discourse in this field.

Keywords play a crucial role in bibliometric analysis, and in this dataset, there are 4958 unique keywords, alongside 1301 author keywords that provide additional context to the research focus. The analysis revealed a collaborative research environment, with an average of 5.6 co-authors per document and a noteworthy international co-authorship rate of 39.71%. This highlights the importance of global collaboration in advancing knowledge related to AOPs.

Moreover, the data indicate that the majority of documents are articles (308), followed by review articles (173), emphasizing the prevalence of original research in this area. All documents were published in English, reflecting the dominant language of scientific communication in this field.

3.2. Chronological Scope and Interdisciplinary Research Trends

The examination of annual publication trends regarding AOPs in water and wastewater treatment from 2010 to 2025 provides insight into the evolution of research in this field. This analysis considers the number of documents published, total citations, and average citations per document, highlighting patterns of academic engagement and influence (Figure 1).

In the early years, the field experienced notable impact despite a low number of publications. For example, in 2010, only four documents were published, yet they received 1011 citations (average 252.8 citations per document), reflecting the significance of foundational studies. Similarly, 2011 marked a peak in average citations per document (574.6) due to several highly influential publications. Subsequent fluctuations in average citations occurred as new research emerged and the field expanded, with 2018 and 2020 showing high total citation counts (4914 and 5501, respectively) corresponding to growing research activity. While publication counts have continued to rise in recent years, the average citations per document show a declining trend, particularly from 2023 to 2025. It is important to note that this pattern is primarily due to the citation time-lag effect; newly published papers naturally have fewer citations because they have not been exposed to the academic community for long enough. Therefore, the observed decline in average citations does not necessarily indicate reduced quality, saturation, or lower impact of recent research. Instead, it reflects a common bibliometric artifact inherent in temporal citation analyses. Overall, the analysis illustrates clear growth in publication output from 2010 to 2025, with notable peaks for the number of documents in 2025 (98), total citations in 2020 (5501), and average citations in 2011 (574.6).

The H-graph in Figure 2 shows the distribution of citations among publications on AOPs in water and wastewater treatment. The horizontal axis ranks the documents from most to least cited, while the vertical axis indicates the total citations each document has received. The steep initial decline in the green curve demonstrates that a few highly cited studies have had a substantial influence on the field. The intersection of this curve with the 45-degree baseline (marked “H-index = 98”) indicates that 98 publications each received at least 98 citations, reflecting both the volume and significance of impactful research in AOPs.

The initial steep decline followed by a gradual flattening of the curve suggests that while a select few documents are highly cited, there is also a broad base of research contributing to the overall body of knowledge in AOPs. These dynamics highlight the balance between highly impactful papers and a larger volume of research that, while potentially less cited, still plays a critical role in advancing the understanding and application of AOPs in water treatment.

Analysis of research distribution by subject area (Figure 3) reveals that Environmental Science dominates, with 377 publications (34% of the corpus). This reflects the critical focus on pollution dynamics, ecological impacts, and sustainable water management. Chemistry and Chemical Engineering follow, contributing 17% (184 documents) and 16% (178 documents), respectively. Chemistry provides insight into pollutant behavior and reactive species mechanisms in AOPs, while Chemical Engineering emphasizes the design and optimization of practical treatment systems. The synergy between these disciplines advances both the understanding and implementation of AOP technologies. The role of Chemistry is particularly pertinent to AOPs, as this field provides invaluable insights into the chemical behaviors and properties of pollutants, especially those resistant to conventional treatment methods [53]. Researchers specializing in Chemistry investigate how reactive species generated in AOPs can effectively degrade organic contaminants, thereby enhancing water treatment efficiency [51].

Chemical Engineering complements these findings by focusing on the practical applications of chemical knowledge. Approximately 178 documents emphasize the design and optimization of processes that incorporate advanced oxidation processes (AOPs) in water treatment systems [53]. This includes developing innovative reactors and methodologies that improve the efficacy of AOPs, thereby addressing contaminants that conventional methods often fail to remove [53]. The collaboration between Chemistry and Chemical Engineering is pivotal in advancing the technology and implementation of AOPs, providing sustainable solutions to complex water treatment challenges.

Moreover, the field of Engineering, accounting for 13% of the publications, plays a crucial role in the practical implementation of AOP technologies in water treatment systems. With approximately 145 documents, engineering research addresses the infrastructure necessary for effective water treatment solutions. This encompasses the design of filtration and reactor systems that utilize AOPs, integrating them within larger treatment frameworks to maximize the contaminant removal efficiency and minimize the operational costs [38]. In the context of public health, the field of Medicine contributes 6% of the research corpus, with 39 publications that emphasize the health implications associated with water pollution. This domain examines the potential risks that contaminants pose to human health, advocating for the necessity of effective AOPs to ensure safe drinking water. Understanding the health impacts of pollutants underscores the importance of integrating advanced treatment methodologies into existing public health frameworks to protect communities [65].

Research in Biochemistry, Genetics, and Molecular Biology also plays a significant role, contributing 2% to the literature with approximately 40 documents. These studies focused on the biological mechanisms that pollutants undergo in aquatic environments, particularly how AOPs can influence these processes [38]. Insights from this discipline are critical for developing targeted interventions that not only remove contaminants but also mitigate their adverse effects on aquatic life and human health. Lastly, the collective “Others” category—including subjects such as Materials Science, Agricultural Sciences, and various multidisciplinary fields—contributes approximately 7.1% to the overall research effort.

Although each of these areas may individually represent a small share of the total output, their contributions are vital for fostering a comprehensive understanding of the complexities surrounding water treatment. For example, advancements in materials science can lead to the development of better catalysts and membranes used in AOPs, thus further enhancing their efficiency and application in various contexts [66].

Overall, the subject area distribution illustrates a highly interdisciplinary research landscape. Each field, whether contributing a large or small proportion of publications, plays a critical role in advancing AOPs for water and wastewater treatment. The combined efforts of these disciplines enable innovations that enhance water quality, ecological sustainability, and public health, underscoring the dynamic and collaborative nature of the research community.

3.3. Pioneering Research: Leading Journals in AOPs in Wastewater Treatment

Table 2. The top 12 leading journals by TLS for AOPs for water and wastewater treatment.

Source Journal	TLS	Number of Documents	Total Citations	H-Index	SJR 2024	Quartile
Water Research	251	37	5108	396	3.843	Q1
Chemosphere	192	32	2415	329	1.896	Q1
Chemical Engineering Journal	147	42	4537	337	2.696	Q1
Environmental Science and Technology	138	18	3463	504	3.690	Q1
Science of the Total Environment	134	24	5963	399	2.137	Q1
Journal of Hazardous Materials	98	21	1594	375	3.078	Q1
Environmental Research	74	10	580	196	1.822	Q1
Journal of Cleaner Production	59	10	501	354	2.174	Q1
Journal of Environmental Chemical Engineering	45	22	754	151	1.454	Q1
Separation and Purification Technology	40	22	525	215	1.697	Q1
Journal of Environmental Management	36	12	1844	268	1.994	Q1
Water (Switzerland)	32	13	487	123	0.752	Q1

presents the top 12 journals publishing research on AOPs in water and wastewater treatment, including metrics such as TLS, number of publications, total citations, H-index, and 2024 SJR. These parameters provide a clear view of each journal’s influence and research impact.

“Water Research” leads with 37 publications and 5108 citations, achieving the highest TLS of 251. This indicates the journal’s strong influence and wide recognition in AOP research, supported by an H-index of 396 and a Q1 ranking with an SJR of 3.843. The “Chemical Engineering Journal” follows closely, with 42 publications, 4537 citations, TLS of 147, and an H-index of 337. Its focus on practical engineering applications of AOPs underscores its pivotal role in advancing water treatment technologies. “Chemosphere” ranks third, with 32 publications and 2415 citations (TLS = 192), emphasizing chemical aspects of pollutants and AOP mechanisms. “Environmental Science and Technology” (18 documents, 3463 citations, H-index 504, TLS = 138) demonstrates high impact in environmental technologies, while “Science of the Total Environment” (24 documents, 5963 citations, TLS = 134) covers broader environmental challenges. Other journals, including “Journal of Hazardous Materials,” “Environmental Research,” “Journal of Cleaner Production,” and “Water (Switzerland),” also contribute meaningfully, focusing on specialized aspects of water treatment, pollution management, and sustainability.

Figure 4 illustrates a co-occurrence map that depicts the relationships among journals with over 10 publications related to AOPs for water and wastewater treatment. This visualization, created using VOSviewer, effectively represents how various journals contribute to the body of knowledge in this essential field. In the diagram, each journal is shown as a labeled node, while the connecting lines indicate collaborative relationships and overlapping citations among these journals. The size of each node reflects the total number of citations received by publications from that journal, highlighting its overall influence in AOP research. Larger nodes signify journals that have produced significant research outputs and are well-regarded in the academic community. The thickness of the lines connecting the nodes represents the strength of the collaborative relationships. Thicker lines indicate a higher degree of citation overlap, suggesting that articles from these journals frequently reference each other’s work, illustrating a strong network of academic cooperation [62]. This highlights key players in the field and illustrates how knowledge flows among them.

Prominent journals such as “Water Research” and “Environmental Science and Technology” occupy central positions in this network, characterized by their extensive publication outputs and high citation counts. Their importance underscores their leadership roles in advancing AOP research. In contrast, journals such as “Chemosphere” and “Journal of Cleaner Production” reflect specialized focuses, emphasizing the chemical and sustainability aspects of AOPs. The varying sizes and interconnections of the nodes indicate diverse research interests, suggesting that researchers can benefit from exploring collaborative opportunities within this network.

Figure 5 shows publication trends from 2010 to 2025 for five major journals: “Chemical Engineering Journal,” “Water Research,” “Chemosphere,” “Science of the Total Environment,” and “Journal of Environmental Chemical Engineering.” From 2010 to 2014, publication activity was low across all journals, with “Water Research” showing the first contributions. Growth began in 2015, particularly in the “Chemical Engineering Journal” and “Chemosphere.” Between 2018 and 2021, output increased steadily, peaking in 2020 for several journals. In 2023, the “Chemical Engineering Journal” published eight articles, confirming its leadership, while “Water Research” maintained strong contributions. By 2025, “Journal of Environmental Chemical Engineering” had risen to seven publications, indicating expanding research attention across journals.

Overall, the analysis shows a consistent increase in research output on AOPs from 2010 to 2025, with variations reflecting shifting focus areas. Leading journals like the “Chemical Engineering Journal,” “Water Research,” and “Chemosphere” play crucial roles in disseminating knowledge, fostering innovation, and advancing practical and theoretical understanding of advanced oxidation processes for water and wastewater treatment. This growing body of literature supports global efforts to improve water quality and sustainability through interdisciplinary research and technological development.

3.4. Geographical Distribution

Table 3. Leading 10 nations through publishing on the AOP research in water and wastewater treatment (ranked by TLS).

Country	TLS	Number of Publications	Total Citations
China	1489	192	14,951
United States	943	64	7999
Australia	479	24	3809
India	418	46	2653
Canada	397	27	2859
Italy	378	29	3429
Spain	374	46	5286
Iran	369	20	1466
Germany	276	17	1764
Poland	187	16	1874

presents a breakdown of the top 10 countries contributing to AOP research, ranked by the TLS, number of publications, and total citations. China’s dominance in this arena is evident, with a striking TLS of 1489, supported by 192 publications and 14,951 citations. This prolific output not only underscores China’s extensive research initiatives but also reflects its increasing role as a global leader in environmental science, particularly in innovative water treatment technologies, such as AOPs. The United States, with a TLS of 943 and 64 publications, shows that while its volume of research is lower compared to China, the impact of its contributions remains significant, as evidenced by its high citation count of 7999. This disparity raises questions about the nature of research in these two countries. The U.S. might prioritize quality over quantity, focusing on innovative frameworks that leverage AOPs effectively, which could explain the higher citation rates despite fewer publications. Australia and India also emerged as noteworthy contributors to the field, each demonstrating a balance between publication and citation counts. Australia’s established research infrastructure supports various studies on AOPs, whereas India’s increasing focus on sustainable water management strategies, particularly in urban and rural contexts, illustrates its growing commitment to tackling water quality challenges using advanced technologies.

Canada’s position, with 27 publications and 2859 citations, suggests a solid, albeit modest, footprint in AOP research. This highlights an area for potential growth, as Canada is known for its rich water resources and environmental policies that could catalyze further research on AOPs. European countries, such as Italy and Spain, provide a unique perspective on the research landscape of AOPs, achieving respectable citation counts indicative of impactful research despite their lower publication numbers. Italy, with 29 publications and 3429 citations, showcases that the quality and relevance of research can sometimes outweigh sheer volume in determining a country’s influence in scientific discourse. The accompanying visual representation of geographical research distribution effectively elucidates the varying research intensity across nations, as shown in Figure 6. The color coding allows for quick identification of research hubs, with China’s darker shades reflecting its substantial contributions to the field. Countries such as Italy, while less prolific in terms of publications, still maintain a presence due to the high impact of their AOP research.

Overall, as nations grapple with water quality issues exacerbated by climate change and urbanization, fostering collaborative research efforts in AOPs could be pivotal in addressing these challenges effectively. The analysis underscores the need for a strategic focus on not only amplifying research outputs but also enhancing their quality and practical applicability in real-world scenarios.

Figure 7 presents a network visualization illustrating the primary countries involved in AOPs for water and wastewater treatment. This visualization serves as a vital tool for understanding international collaboration and the underlying bibliometric relationships within this crucial field of research.

In the network, as shown in Figure 7, China emerges as the most prominent node, characterized by its size and the density of connections. The significant number of links associated with China reflects its leadership role in AOP research, indicating not only a high volume of publications but also extensive collaborations with other nations. This position emphasizes the central role of the country in shaping the discourse on innovative water treatment technologies. The United States follows closely behind, demonstrating a notable presence through its extensive links with several countries. This indicates an active involvement in collaborative projects, which may enhance the impact and dissemination of its AOP research outputs. The robust connections suggest a strategic focus on building partnerships that leverage shared expertise, thereby fostering advancements in the field.

Countries such as India, Italy, and Spain also feature prominently in the network, exhibiting significant collaborative ties. India’s growing involvement highlights its commitment to addressing water quality challenges, particularly in rapidly urbanizing areas. Italy and Spain, representing European contributions, reflect a tradition of research excellence in environmental science. Collaboration between these countries underscores the importance of cross-border cooperation in addressing global water treatment issues. Iran and Poland appear as additional contributors, with established connections that point to their active participation in the AOP research community. The interlinking of these nations within the visualization suggests a diverse ecosystem of research partnerships that can lead to innovative solutions and shared knowledge.

The color gradients and link thickness in the network visualization further enhance the understanding of these relationships. Thicker lines denote stronger collaborations, whereas color variations indicate deepening connections over time. This evolving landscape of partnerships illustrates how countries adjust and strengthen their research collaborations in response to emerging challenges and scientific requirements. Overall, Figure 7 effectively encapsulates the collaborative dynamics of AOP research across various nations, showcasing how these countries interact and influence one another. The network not only highlights those leading in research output but also emphasizes the significance of collaborative efforts in advancing AOP technologies for water and wastewater treatment. This analysis serves to inform future research trajectories and collaborative initiatives vital for addressing critical water management issues on a global scale. The implications of this network extend beyond mere participation; they underscore the collective endeavor required in the scientific community to promote sustainable solutions to water-related challenges.

3.5. Most Relevant Affiliations

Table 4. Top 8 universities according to authors’ affiliation.

Affiliation	Department	Country	TLS	No. of Documents	Total Citations
State Key Laboratory of pollution control and resource reuse, Tongji University	-	Shanghai, China	81	9	686
Tongji University	-	Shanghai, China	71	10	420
Università Degli Studi Di Salerno	Department of Civil Engineering	Salerno, Italy	54	9	1560
Ciemat-Plataforma Solar De Almería		Almeria, Spain	50	9	3493
Environmental Engineering and Science Program	College of Engineering and Applied Science	Cincinnati, United States	20	7	908
Sichuan University	College of Architecture and Environment	Chengdu, China	14	7	437
School of Environmental Science and Engineering, Sun Yat-Sen University	-	Guangzhou, China	5	7	505
Gdańsk University of Technology	Department of Process Engineering and Chemical Technology	Gdansk, Poland	3	7	1482

outlines the top eight universities active in the research of AOPs for water and wastewater treatment, providing a solid foundation for understanding the academic landscape in this critical area. The data presented includes each university’s affiliation, department, country, TLS, number of documents published, and total citations received. Among these, “Tongji University,” located in Shanghai, China, stands out as a leading institution, with two listings: the “State Key Laboratory of Pollution Control and Resource Reuse” and the university itself. The laboratory boasted the highest TLS of 81 and 686 citations across nine documents, indicating its significant impact on the field. This suggests that the laboratory’s research is not only prolific but also widely recognized, contributing immensely to advancements in AOP technologies. The “Università Degli Studi Di Salerno” in Italy also plays a substantial role, particularly with a high citation count of 1560 over nine documents published. This metric positions it as a pivotal center for research dissemination, underscoring its influence in the global conversation surrounding wastewater treatment methodologies.

The “Ciemat-Plataforma Solar de Almería” in Spain emerged as another key contributor, with 3493 citations across nine documents. This suggests a strong integration of solar technologies within AOP research, reflecting a growing trend towards sustainable solutions in water treatment. The United States is represented by the Environmental Engineering and Science Program at the University of Cincinnati, which, despite having a lower TLS of 20, has garnered 908 citations across seven documents. This indicates that while the volume of research output may be smaller, it still commands a noteworthy presence in the field.

The network visualization in Figure 8 complements the quantitative data in Table 4 by depicting the collaborative relationships among these leading universities. The interconnected nodes illustrate how these institutions engage with each other in the AOP research sphere, revealing a collaborative network that enhances the field’s development. In this visualization, “Tongji University” remains central, signifying its prominence in driving research initiatives. The presence of multiple connections indicates its role as a collaborative hub where ideas and innovations are exchanged with other significant institutions. The inclusion of “Sun Yat-Sen University” and “Sichuan University” in China indicates a robust national commitment to advancing AOP technology, suggesting that Chinese universities are increasingly forming a cohesive network within this research domain. “Gdańsk University of Technology” and “Università Degli Studi Di Salerno” show prominent links as well, reflecting their active engagement with Chinese institutions. This cross-national interaction enhances the flow of knowledge and technical expertise, which is critical for addressing the global challenges of water quality and sanitation.

Overall, both Table 4 and Figure 8 present a comprehensive view of the academic environment surrounding AOP research. This analysis underscores the importance of collaboration among universities to foster innovation and share knowledge, demonstrating a collective effort to improve water treatment technologies globally. The synergy between these institutions not only advances their individual research agendas but also contributes significantly to global efforts in enhancing water quality and environmental sustainability.

3.6. Global Dynamics and Impactful Contributions in AOPs in Wastewater Treatment Research

Co-citation author network analysis is a well-established bibliometric method used to systematically map existing literature [67]. This technique identifies authors who are frequently cited together, thereby revealing conceptual frameworks and thematic connections within a specific research area.

Table 5. Most prolific authors: publication metrics.

Most Prolific Author	TLS	Total Citation	No. of Publications
Boczkaj, G.	447	1601	9
Dionysiou, D.D.	731	1242	8
Rizzo, I.	604	1487	8
Malato, S.	570	3419	7
Oller, I.	246	2536	6
Yang, X.	944	358	5
Liu, W.	625	359	5
Giannakis, S.	621	437	5
Santoro, D.	556	595	5
Silva, A.M.T.	490	1738	5
Roccaro, P.	466	211	5
Bo, L.	458	578	5
Ma, J.	448	91	5
Wang, S.	345	1323	5

compiles data on the most prolific authors, providing insight into both their productivity and their contributions to scientific literature. Notably, authors such as Boczkaj, G. and Dionysiou, D.D. emerge as pivotal figures with high TLS and substantial citation counts. Boczkaj’s high TLS of 447 and total citations of 1601, along with Dionysiou’s impressive TLS of 731, underscore their significant engagement and influence in AOP research. This prolific output not only demonstrates their commitment to advancing the field but also positions them as key knowledge leaders in this niche.

Furthermore, the dataset reveals that citation metrics are essential in assessing the impact of individual scholars.

Table 6. Most cited authors: citation metrics.

Most Cited Author	TLS	Total Citation	No. of Publications
Malato, S.	570	3419	7
Wang, J.	240	2793	4
Oller, I.	246	2536	6
Sánchex Pérez, J.A.	148	2411	2
Duan, X.	564	1810	4
Silva, A.M.T.	490	1738	5
Boczkaj, G.	447	1601	9
Zhuan, R.	153	1598	2
Iado Ribeiro, A.R.	336	1586	3
Rizzo, I.	604	1487	8
Wang, S.	345	1323	5
Dionysiou, D.D.	731	1242	8
Zhu, S.	229	1179	2
Huang, X.	209	1063	1

highlights the most cited authors, showcasing Malato, S. as a central figure with a total citation count of 3419. His high citation count corresponds with the breadth of his research output, solidifying his reputation as a leading authority in AOPs. The data indicates that although various authors may publish extensively, the citation frequency they attract may differ significantly based on the relevance and application of their work. This dynamic reaffirms the Matthew Effect, where established and frequently cited authors tend to dominate academic recognition.

Moreover, the presence of consistently high citation counts among other authors in Table 6 suggests a recognition of the importance of their contributions. For instance, Wang, J. and Oller, I. also rank prominently within the most cited authors, demonstrating that impactful research in AOPs often originates from various collaborative contributors. It is particularly interesting to observe that while some authors, such as Malato and Wang, may have lower publication counts, their works carry significant weight in citations, highlighting the diverse pathways to influence the academic landscape of AOP research.

The correlation between publication counts in Table 5 and citation metrics in Table 6 indicates a multifaceted relationship in academic contributions, where both the volume of research and the impact of that research play crucial roles in establishing an author’s reputation. Authors like Silva, A.M.T. who feature prominently in both tables exemplify this dual nature, further emphasizing the collaborative dynamics inherent in research. In summary, the interplay of prolific publication and citation metrics establishes a comprehensive picture of the influential figures in AOP research in wastewater treatment. This analysis underscores the significance of diverse contributors, highlighting how a mix of high-output authors and those with notable citation impacts shapes the discourse within this critical area of environmental science. Acknowledging both kinds of contributions is essential for understanding the evolutionary trajectory of research in AOPs, ultimately propelling advancements in wastewater treatment technologies.

3.7. Key Funding Sources Supporting AOP Research in Wastewater Treatment

The analysis of funding sources supporting research on AOPs in wastewater treatment reveals crucial insights into the financial landscape facilitating this significant area of scientific inquiry, as depicted in Figure 9.

The “National Natural Science Foundation of China” stands out as the principal funding entity, backing an impressive 132 documents. This strong commitment reflects China’s prioritization of AOP research, which aligns with its broader environmental objectives and the pursuit of sustainable water treatment solutions. Following closely, the “Natural Sciences and Engineering Research Council of Canada” has sponsored 27 publications. This contribution underscores Canada’s dedication to advancing environmental technologies, particularly in the context of wastewater treatment and management. The involvement of Canadian funding further highlights the collaborative nature of global efforts to improve water quality through innovative AOPs.

The “Fundação para a Ciência e a Tecnologia” from Portugal is also noteworthy, having funded 15 documents. This indicates a regional effort to engage in AOP research, suggesting that European nations are increasingly recognizing the importance of these technologies in addressing water pollution challenges. The diversity of funding sources within Europe emphasizes a collective understanding of the necessity for advanced treatment methodologies in the context of environmental sustainability. Moreover, the “European Commission” appears to be a significant contributor, with 11 publications supported. This involvement illustrates the European Union’s commitment to fostering research that addresses water quality issues across member states, promoting integrated solutions for environmental protection and public health. The contributions from both the European Commission and other European funding bodies reflect a concerted strategy to enhance research synergies on a continental scale.

The “National Science Foundation” in the United States also plays a key role, supporting 10 documents. This involvement indicates the NSF’s recognition of the relevance of AOPs within the broader scope of environmental science and its potential application in addressing wastewater issues. The presence of U.S. funding highlights the growing interest in how AOPs can be effectively utilized in various contexts, further emphasizing the importance of multidisciplinary research. The “China Postdoctoral Science Foundation” and the “European Regional Development Fund” each supported 9 publications, which underscores their contributions to nurturing emerging talent in the field while facilitating research initiatives that can lead to practical applications in wastewater treatment. These funding bodies play essential roles in fostering innovation and scholarly development within AOP research.

Overall, the funding landscape illustrated in Figure 9 highlights the diverse array of financial supporters driving research on AOPs in wastewater treatment. The substantial contributions from Chinese, Canadian, and European sources reflect a global commitment to tackling water quality challenges through advanced oxidative technologies. This collaborative engagement across multiple nations and institutions is integral for promoting knowledge exchange and developing effective strategies to enhance wastewater treatment practices worldwide.

3.8. Pioneering Research: Key Articles in AOPs in Wastewater Treatment

Citation analysis is a vital approach for understanding the intellectual connections among publications, especially when one study references another. This method enables the identification of key research articles within a specific academic discipline while allowing for the exploration of citation trends and patterns [68]. In this research, we conducted a citation analysis of papers focused on AOPs related to water and wastewater treatment.

Table 7. The top 30 most-cited articles.

First Author	Year	Document Title	Journal	Citation	Ref.
Oller, I.	2011	“Combination of advanced oxidation processes and biological treatments for wastewater decontamination-a review”	Science of the Total Environment	2182	[69]
Deng, Y.	2015	“Advanced Oxidation Processes (AOPs) in Wastewater Treatment”	Current Pollution Reports	1531	[11]
Wang, J.	2020	“Degradation of Antibiotics by Advanced Oxidation Processes: An Overview”	Science of the Total Environment	1163	[70]
Zhu, S.	2018	“Catalytic Removal of Aqueous Contaminants on N-Doped Graphitic Biochars: Inherent Roles of Adsorption and Nonradical Mechanisms”	Environmental Science and Technology	1063	[71]
Wang, J.	2021	“Effect of Inorganic Anions on the Performance of Advanced Oxidation Processes for Degradation of Organic Contaminants”	Chemical Engineering Journal	1036	[72]
Kanakaraju, D.	2018	“Advanced Oxidation Process-Mediated Removal of Pharmaceuticals from Water: A Review”	Journal of Environmental Management	1026	[73]
Ribeiro, A.R.L.	2015	“An Overview on the Advanced Oxidation Processes Applied for the Treatment of Water Pollutants Defined in the Recently Launched Directive 2013/39/EU”	Environmental International	867	[74]
Grebel, J.E.	2010	“Effect of Halide Ions and Carbonates on Organic Contaminant Degradation by Hydroxyl Radical-Based Advanced Oxidation Processes in Saline Waters”	Environmental Science and Technology	843	[75]
Rahman, M.F.	2014	“Behaviour and Fate of Perfluoroalkyl and Polyfluoroalkyl Substances (PFASs) in Drinking Water Treatment: A Review”	Water Research	832	[76]
Rizzo, L.	2019	“Consolidated VS New Advanced Treatment Methods for the Removal of Contaminants of Emerging Concern from Urban Wastewater”	Science of the Total Environment	664	[77]
Gagol, M.	2018	“Wastewater Treatment by Means of Advanced Oxidation Processes Based on Cavitation-A Review”	Chemical Engineering Journal	651	[78]
Liang, Z.	2019	“Piezoelectric Materials for Catalytic/Photocatalytic Removal of Pollutants: Recent Advances and Outlook”	Applied Catalysis B: Environmental	580	[79]
Ao, X.-W.	2021	“Peracetic Acid-Based Advanced Oxidation Processes for Decontamination and Disinfection of Water: A Review”	Water Research	577	[80]
Mirzaei, A.	2017	“Removal of Pharmaceuticals from Water by Homo/Heterogonous Fenton-Type Processes-A Review”	Chemosphere	531	[81]
Hama Aziz, K.H.	2023	“Heavy Metal Pollution in the Aquatic Environment: Efficient and Low-Cost Removal Approaches to Eliminate their Toxicity: A Review”	RSC Advances	490	[82]
Yang, Y.	2020	“Recent advances in application of graphitic carbon nitride-based catalysts for degrading organic contaminants in water through advanced oxidation processes beyond photocatalysis: A critical review”	Water Research	472	[43]
Wang, J.	2019	“The occurrence, distribution and degradation of antibiotics by ionizing radiation: an overview”	Science of the Total Environment	435	[83]
Matafonova, G.	2018	“Recent advances in application of UV light-emitting diodes for degrading organic pollutants in water through advanced oxidation processes: A review”	Water Research	397	[12]
Priyadarshini, M.	2022	“Advanced oxidation processes: Performance, advantages, and scale-up of emerging technologies”	Journal of Environmental Management	382	[84]
Rizzo, L.	2011	“Bioassays as a tool for evaluating advanced oxidation processes in water and wastewater treatment”	Water Research	379	[85]
Yan, Y.	2023	“Merits and limitations of radical vs. nonradical pathways in persulfate-based advanced oxidation processes”	Environmental Science & Technology	376	[49]
Li, Z.	2020	“Biochar-supported nanoscale zero-valent iron as an efficient catalyst for organic degradation in groundwater”	Journal of Hazardous Materials	357	[86]
Coha, M.	2021	“Advanced oxidation processes in the removal of organic substances from produced water: Potential, configurations, and research needs”	Chemical Engineering Journal	338	[87]
Yang, J.	2021	“What is the role of light in persulfate-based advanced oxidation for water treatment?”	Water Research	328	[88]
Yang, X.	2022	“Multiple roles of dissolved organic matter in advanced oxidation processes”	Environmental Science & Technology	314	[89]
Sichel, C.	2011	“Feasibility studies: UV/chlorine advanced oxidation treatment for the removal of emerging contaminants”	Water Research	284	[90]
Morsi, R.	2020	“Laccases and peroxidases: the smart, greener and futuristic biocatalytic tools to mitigate recalcitrant emerging pollutants”	Science of the Total Environment	283	[91]
Prieto-Rodríguez, L.	2013	“Application of solar AOPs and ozonation for elimination of micropollutants in municipal wastewater treatment plant effluents”	Water Research	283	[92]
Ganzenko, O.	2014	“Electrochemical advanced oxidation and biological processes for wastewater treatment: a review of the combined approaches”	Environmental Science and Pollution Research	280	[93]
Kumar, M.	2023	“Current research trends on emerging contaminants pharmaceutical and personal care products (PPCPs): A comprehensive review”	Science of the Total Environment	272	[94]

summarizes the top 30 most-cited publications, reflecting their influence on both academic research and practical applications. Collectively, these studies demonstrate the evolution of AOPs from traditional hydroxyl radical systems toward more sophisticated pathways, including sulfate radicals (SO₄⁻·), singlet oxygen (¹O₂), and non-radical mechanisms, enhancing efficiency and selectivity in contaminant removal.

The AOPs represent a crucial suite of technologies for addressing persistent organic pollutants in water and wastewater treatment. The collective body of research underscores the evolution and optimization of AOP technologies, revealing a shift from traditional ·OH systems to more advanced mechanisms involving sulfate radicals (SO₄•⁻), singlet oxygen (¹O₂), and nonradical pathways [71,95,96,97,98,99]. These studies highlight the ability of AOPs to degrade a wide array of contaminants, including pharmaceuticals, pesticides, and industrial chemicals. Each study contributes unique insights into catalytic materials, reaction conditions, and mechanistic understanding [86,100,101], emphasizing the importance of developing efficient, selective, and sustainable oxidation methods [102,103]. The findings demonstrate the capacity of AOPs to effectively degrade a wide range of recalcitrant contaminants while addressing the challenges of byproduct formation and operational efficiency in diverse environmental contexts [75,90,104]. Overall, these investigations reflect a concerted effort to advance the understanding and application of AOPs for improved environmental remediation [79].

Hama Aziz et al. [82] highlighted heavy metal contamination as a severe global environmental issue, exacerbated by industrialization, urbanization, and climate change. Their review detailed the toxicological impacts of heavy metals on human health and aquatic ecosystems, emphasizing the necessity for effective removal techniques. AOPs, alongside physicochemical methods like biochar and zeolite ion exchangers, were proposed as viable solutions, addressing the urgent need for innovative and cost-effective wastewater treatment technologies. Yang et al. [88] focused on persulfate-based AOPs under UV, visible, and solar irradiation, elucidating the role of light in enhancing pollutant degradation. Their review categorized photo-activated processes according to their mechanisms, such as direct UV activation, dye sensitization, ligand-to-metal charge transfer (LMCT), and photocatalytic processes. This categorization is crucial for understanding how light can be harnessed to maximize the efficiency of AOPs. Following this, Yang et al. [89] critically assessed the influence of dissolved organic matter (DOM) on AOP effectiveness. Their findings indicated that while DOM can scavenge radicals and impede light penetration, it can also facilitate radical formation, suggesting a nuanced interaction that requires careful consideration in AOP applications. This review presents the dual role of DOM—both inhibitory and synergistic—highlighting the complex dynamics during oxidative water purification processes.

Morsi et al. [91] addressed the challenges posed by emerging pollutants (EPs), which often escape conventional treatment methods. Their investigation into biocatalytic approaches, particularly employing laccases and peroxidases, proposed a greener oxidation route capable of degrading various organic pollutants effectively. This aligns with the growing trend to incorporate biotechnological solutions within AOP frameworks to enhance pollutant degradation. Prieto-Rodríguez et al. [92] examined the efficacy of solar-assisted AOPs, such as solar photo-Fenton and ozonation, in the removal of micropollutants from municipal wastewater. Their pilot-scale results demonstrated that solar photo-Fenton processes surpassed traditional treatments in eliminating contaminants, highlighting the potential economic benefits and enhanced performance of solar-driven AOP technologies.

Ganzenko et al. [93] presented electrochemical advanced oxidation processes (EAOPs) as a promising avenue for treating persistent hazardous pollutants originating from industrial and agricultural sources. They emphasized the effectiveness of these methods in degrading contaminants, unlike conventional approaches which primarily transfer pollution from wastewater to sludge. The authors suggested that integrating EAOPs with biological treatments could enhance removal efficiency while reducing operational costs, a promising synergy for wastewater management. Kumar et al. [94] explored the treatment of pharmaceutical and personal care products (PPCPs) from wastewater, highlighting the urgent need for effective and cost-efficient treatment technologies. They recommended combining conventional and advanced methods, including AOPs, to optimize removal efficiency and minimize costs. In the context of antibiotics, Wang et al. [83] discussed the growing concern over their prevalence in aquatic environments, which conventional biological treatments fail to eliminate effectively. The study advocated for innovative AOPs, including ionizing radiation combined with Fenton processes, to enhance degradation efficiency, aligning with broader trends in tailoring AOPs to address specific classes of contaminants. Matafonova et al. [12] reviewed the role of UV LEDs in AOPs, highlighting their potential as alternative mercury-free UV sources for wastewater treatment. Their examination of TiO₂-based photocatalysis showcased the effectiveness of UV-LED-assisted processes, while also noting limitations in practical implementation. The study emphasizes the need for further research to understand energy requirements and efficiency in real water matrices.

Overall, this body of research reflects a notable transition in the field from traditional radical-based degradation methods to more selective, environmentally sustainable systems tailored for specific contaminants. While earlier studies laid the groundwork for understanding the kinetics and mechanisms of AOPs, recent investigations have increasingly focused on the complexities introduced by DOM, the necessity for cost-effective technologies, and the integration of biological and electrochemical methods. The journey towards optimizing AOPs for practical applications involves addressing challenges related to toxicity of byproducts, operational costs, and effectiveness in diverse water matrices. Future research must emphasize the scalability of promising AOP technologies and the synergistic potential of hybrid treatment approaches, paving the way for innovative solutions in water treatment and environmental sustainability.

Table 8. Advantages and limitations of AOPs and their application in the treatment of water and wastewater.

Category	Process	Advantages	Limitations	Catalysis Type	Ref.
Fenton-Family	Fenton	- High efficiency for organic pollutant removal. - Stable pH control with bicarbonate. - Effective for persistent organic pollutants and recalcitrant. - High mineralization capability.	- Requires acidic conditions. - Rapid Fe(II) depletion reduces reactivity. - Optimization of catalysts and conditions is essential. - Possible formation of toxic by-products.	Lithium Cobalt Oxide (LCO), MXenes	[105,106]
	Photo-Fenton	- Enhanced degradation rates with sunlight. - Effective in a variety of wastewater matrices. - Achieves significant removal of emerging contaminants. - Can improve surfactant recovery and reuse.	- Light intensity and reactor design are crucial. - Requires proper reagent optimization. - Performance influenced by light and matrix complexity. - Need for optimal operational conditions	Ferrioxalate Complexes	[107]
	Solar Photo-Fenton	- Effective for multiple contaminants with lower reagent doses. - Reduced toxicity of treated effluent.	- Performance can vary based on light availability and complexity of pollutants.		[107]
	Electro-Fenton	- Improved kinetics for low-concentration pollutants. - Membrane integration enhances overall performance. - Eco-friendly, easy to automate, and operable over a wide pH range.	- High operational costs and energy requirements. - Potential membrane fouling concerns. - Effectiveness and stability of catalysts over time is a concern. - High energy consumption.	Ag-PTFE Membrane, Heterogeneous Catalyst.	[12,109]
	Fenton-Type Coagulation	- Combines coagulation with oxidation processes to reduce effluent toxicity. - Environmentally friendly approach.	- Effectiveness depends on the specific types of contaminants. - Variability based on operational conditions.	Peracetic Acid + Fe3+	[110]
	Fenton with Adsorption	- Synergistic effects improve overall pollutant removal efficiency. - Efficient for diverse organics and inorganics.	- Complexity in integrating multiple processes. - Requires careful optimization for different platforms.		[111]
Solar and Photo-Based	Solar-Driven AOP	- Combines solar-driven evaporation and AOPs for efficient freshwater production. - Excellent pollutant degradation.	- Challenges in mitigating residual pollutants. - Needs long-term stability assessment.	MnO/C	[112]
	Pharmaceutical removal AOPs	- Effective removal of pharmaceuticals using solar tech. - Fast degradation kinetics.	- Performance discrepancies due to variable conditions. - Scalability required testing	N-doped TiO2	[113]
	Sequential solar AOP (ST with sunlight/H2O2 + SPF)	- Sustainable process integrating sunlight/H2O2 and solar photo-Fenton at neutral pH. - Lower overall environmental impact compared to ozonation. - High simultaneous removal of CECs and pathogens.	- Longer treatment times for stricter reuse limits. - Higher raw material use in reuse scenario.	Fe-EDDS complex	[108]
Plasma and Discharge-Based	Plasma AOP	- High energy yield for pollutant degradation. - Efficient mass transfer via microchannels.	- Energy efficiency depends on reactor design. - Low scalability of plasma systems.	Electrical discharge	[114]
	Pulsed corona discharge (PCD)	- High energy efficiency for VOC removal. - Compatible with water treatment systems.	- Sensitive operational parameters. - Long-term impacts need study.	Non-thermal plasma	[115]
Sulfate and Sulfite-Based	Persulfate-Based AOPs	- More effective for degrading complex contaminants. - Potential for diverse environmental applications.	- Limited understanding of sea reaction mechanisms. - Scalability challenges remain.	-	[116]
	Sulfate radical AOPs	- Generation of multiple reactive species. - Strong performance on complex pollutants.	- May form toxic byproducts. - Limited data for specific environments.	-	[117]
	Sulfate-Based AOPs	- Low toxicity and cost-effective for drinking water. - Produces multiple reactive species.	- Sensitive to matrix complexity. - Needs optimized degradation conditions.	H2O2/S(IV)	[118]
Metal-Based and Heterogeneous	Cr(III)/PI System	- Rapid trace contaminant removal. - Avoid toxic byproducts.	- May not apply to all contaminants.	Cr(III) + Periodate	[119]
	Pd-nZVI/BC System	- Reduces toxic byproduct formation. - Effective for chlorinated compounds.	- Sensitive to operational conditions. - Limited catalyst lifespan.	Pd-loaded biochar-supported nZVI	[120]
	Bimetallic catalysts	- Excellent mineralization of recalcitrant pollutants. - Enhanced catalytic performance.	- Scalability and stability challenges. - Complex synthesis.	MOFs	[121]
	Heterogeneous catalytic AOPs	- Efficient antibiotic degradation. - Generates reactive oxygen species effectively.	- Energy effectiveness may vary. - Non-specific binding issues.	NiOOH hierarchical spheres	[122]
	Site-Engineered ROS Catalysis	- Near-complete pollutant removal (>99.9%) within 30 min. - Directional radical generation via bifunctional Ca–O/Ca–O–Si centers. - Stable over 20 reuse cycles.	- Complex fabrication of bifunctional domains. - Requires precise microenvironmental control.	Ca–O and Ca–O–Si cross-linked catalyst	[123]
	Hybrid Plasma–Catalytic Ozonation	- Superior pollutant degradation and mineralization. - Integrates CoOx catalytic ozonation with Fe/Al electrocoagulation. - Reduced ecotoxicity and enhanced biodegradability.	- System complexity and optimization challenges. - Requires control of oxidation state transitions.	CoOx thin film + Electrocoagulation	[124]
	Crystal Phase-Controlled MnO2 Catalysis	- Precise control of 1O2 generation. - High NH4+ oxidation rate (17.9 × over O3 alone). - 97% gaseous N selectivity.	- Complex synthesis of specific MnO2 crystal phases. - Limited scalability.	ε-MnO2	[125]
	O3–BAC Combined Treatment	- Synergistic removal of metals, disinfection, and toxicity reduction. - 99.9% metal removal, 5-log coliform reduction. - No observed ecotoxic effects.	- Requires longer treatment duration. - May depend on effluent composition.	Ozone + Biological Activated Carbon	[126]
	EfOM-Influenced Ozonation	- Clarifies mechanism of ·OH scavenging in ozonation. - Provides kinetics for process optimization.	- Hydrophilic EfOM fractions can inhibit ·OH activity. - Variable reactivity across effluents.	Dissolved Organic Matter fractions	[127]
	Ag2O–RuO2/ZrO2 Hybrid Catalysis	- Complete mineralization of phenolic pollutants. - Green and reusable catalytic system. - Value-added by-product generation.	- Requires optimization of dopant ratio and conditions. - Possible catalyst leaching.	Ag2O–RuO2-Doped ZrO2	[128]
	Catalytic Ozonation of Pharmaceuticals	- Nearly complete removal of caffeine and ampicillin. - Effective mineralization under optimized conditions.	- Partial mineralization only (30.8%). - Sensitive to pH and ozone flow rate.	O3 System	[129]
	Cu2O/ZnO–AC Catalytic Ozonation	- 98% BPA removal. - 19% higher TOC removal vs. non-catalytic ozonation. - Stable and reusable catalyst.	- Sensitive to pH and reaction time. - Catalyst regeneration needed for extended use.	Cu2O/ZnO on Activated Carbon	[130]
	Electro-Peroxone and Catalytic O3 AOPs	- Enhanced degradation of ozone-refractory ECs. - Reduced bromate formation. - Flexible and safer operation.	- Performance depends on contaminant ozone-reactivity. - Complex equipment setup.	MnO2, O3/H2O2, Electro-Peroxone	[131]
	Fe-Based Heterogeneous Catalytic Ozonation	- Improves organic contaminant degradation. - Simple, reusable, and non-toxic Fe catalyst. - Maintains efficiency across cycles.	- Limited mechanistic understanding. - Performance influenced by pH and inhibitors.	Ferrocene (Fe catalyst)	[132]
	UV- and O3-Based AOPs	- Effective for 23 of 25 US EPA CCL3 contaminants. - Eliminates mutagenic and estrogenic activity for most pollutants.	- Some oxidation by-products may exhibit temporary mutagenicity. - Requires bioassay validation.	UV + O3	[133]
	ZnFe2O4-Catalyzed Ozonation	- Accelerated phenol and coking wastewater degradation. - Enhanced mineralization and hydroxyl radical formation. - Surface hydroxyl groups promote reactivity.	- Catalyst performance depends on synthesis route. - Slight sensitivity to ozone dose and pH.	ZnFe2O4 (ZFO-H, ZFO-C)	[134]
Combined and Peroxide-Based	Peracetic Acid AOP	- Broad activity across organic pollutants. - Ligand modification can boost performance.	- Instability of Mn species. - Variable ligand interactions.	Mn(II)/PAA	[135]
	Combined AOPs	- Full degradation of refractory hospital wastewater pollutants. - Effective integration of processes.	- Complex operation and optimization required. - Needs validation for different matrices.	Ozonation + Electrochemical Oxidation	[136]
	Optimized Ozonation (Economic Design)	- High degradation efficiency for carbamazepine. - Incorporates cost-effectiveness and kinetic optimization.	- Requires continuous ozone supply. - May not mineralize all by-products.	O3 System	[137]
Electrochemical Oxidation	Pt/Ti electrode electrochemical treatment of antibiotics	- High degradation efficiency & fast oxidation of tetracycline. - No high-mass byproducts detected.	- Formation of some formaldehyde intermediates (manageable).	Pt/Ti	[41]
	Electrochemical oxidation of nitrofurazone	- Very high mineralization and degradation rate on BDD.	- Potential formation of oxidation intermediates (monitored).	BDD/Pt/Ti	[138]

provides a detailed overview of the diverse AOP categories, including Fenton-based, solar/photo-driven, plasma and discharge-based, sulfate/sulfite-based, metal-based heterogeneous, combined/peroxide-based, and electrochemical processes. Each method exhibits unique advantages, such as high mineralization efficiency, broad contaminant removal, and adaptability to complex wastewater matrices, while also presenting limitations related to energy requirements, operational complexity, catalyst stability, by-product formation, and scalability. For instance, Fenton-based processes achieve high removal efficiency but require acidic conditions and careful optimization of catalysts [105,106]. Solar-driven AOPs offer sustainable alternatives but depend on light intensity and wastewater characteristics. Metal-based heterogeneous and hybrid catalytic systems enhance selectivity and mineralization but face challenges in reproducibility and operational control [107,108].

Table 8 summarizes the diverse AOPs utilized in water and wastewater treatment, presenting a clear picture of their respective advantages and limitations. By analyzing different AOPs, it is evident that while many methods show promising pollutant removal capabilities, they also face challenges such as cost-efficiency, operational complexity, and potential by-product formation. Continuous research and innovation are crucial for enhancing these processes to ensure effective and sustainable application in real-world scenarios.

3.9. Bibliometric Mapping Keywords

This section provides an analysis of the top keywords identified from studies on AOPs related to water and wastewater treatment. This analysis is crucial for understanding the significant research themes and trends within this field. Utilizing data from various published studies, we focused on keywords that demonstrate a minimum occurrence threshold to highlight the most relevant terms. In our study, we extracted a total of 39 keywords, ranked by their occurrence and TLS. The keywords included essential terms such as “Advanced Oxidation Processes,” “Water Treatment,” and “Emerging Contaminants,” each representing central topics in current AOP research. The keywords were clustered based on their thematic relation, illustrating how different research areas are connected and revealing the broader issues being addressed in the literature.

Table 9. Top 39 keywords from the studies published on AOP research related to water and wastewater treatment.

Keywords	Cluster	Occurrence	TLS
Advanced Oxidation Processes	5	80	109
Water Treatment	4	57	95
Emerging Contaminants	1	46	75
Wastewater Treatment	3	42	68
Photocatalysis	4	40	67
Advanced Oxidation Process	2	38	46
Advanced Oxidation Processes (AOPs)	3	34	38
AOPs	1	29	33
Ozonation	5	24	46
Toxicity	1	22	39
Peroxymonosulfate	2	21	27
Wastewater	1	20	26
Degradation	2	18	28
Hydroxyl Radical	3	16	26
Contaminants of Emerging Concern	5	16	19
Pharmaceuticals	1	15	28
Advanced Oxidation	4	15	19
Hydrogen Peroxide	4	14	26
Adsorption	4	14	19
Persulfate	3	13	27
Ozone	4	12	17
Micropollutants	6	11	17
Sulfate Radical	3	11	16
Peracetic Acid	2	11	10
Organic Pollutants	2	10	22
Fenton	5	10	18
Catalytic Ozonation	4	10	11
Photo-Fenton	1	9	21
Antibiotics	2	8	16
Hydrodynamic Cavitation	3	8	15
Singlet Oxygen	2	8	12
Reactive Oxygen Species	2	8	11
Reactive Species	5	8	10
Biodegradation	1	8	9
Heterogeneous Photocatalysis	1	7	14
Sulfate Radicals	3	7	10
Transformation Products	5	7	9
Persulfate Activation	6	7	4
AOP	6	6	3

displays these keywords along with their respective occurrence counts and TLS, which serve as indicators of their impact and relevance in the research landscape. By examining these keywords, we can gain insight into the evolving focus areas within AOPs, such as the treatment of emerging contaminants in wastewater, the role of photocatalysis, and the increasing significance of specific oxidants like hydrogen peroxide and ozone.

Table 9 highlights the most pertinent keywords derived from research on AOPs in the context of water and wastewater treatment. Central to this analysis are terms such as “Advanced Oxidation Processes,” “Water Treatment,” and “Emerging Contaminants,” which underscore the significant role AOPs play in addressing increasingly complex water pollution issues. AOPs are essential for treating various types of wastewater, particularly those contaminated with persistent organic pollutants [12], pharmaceuticals [73], and emerging contaminants [9]. The keywords “Photocatalysis” [115], “Ozonation” [126], and “Hydrogen Peroxide” [15] indicate some of the commonly utilized AOP technologies. These processes are designed to effectively degrade toxic compounds, ensuring that treated water meets safety standards for environmental discharge or even reuse. Moreover, the mention of “Contaminants of Emerging Concern” and “Micropollutants” emphasizes the growing need to tackle substances that are often resistant to conventional treatment methods [9,92]. This highlights a critical challenge in water treatment, where traditional methods may fail to remove certain pollutants, leading to potential risks for human health and ecosystems [68].

The inclusion of contaminant-related terms such as Pharmaceuticals, Organic Pollutants, and Toxicity further highlights the urgency of implementing advanced treatment solutions capable of mitigating both chemical persistence and associated toxic effects [12,37,73]. In parallel, catalyst- and process-oriented keywords including Fenton, Persulfate, and Heterogeneous Photocatalysis illustrate ongoing technological advancements aimed at enhancing oxidation efficiency and expanding the applicability of AOPs across diverse water [15,31,88]. Overall, these keywords depict a multidimensional research landscape in which AOPs are developed not only to remove pollutants, but also to support sustainable and robust water treatment strategies.

Figure 10 presents a word cloud visualization of the most frequently occurring keywords in the analyzed literature. Notably, the appearance of specific AOP techniques, including Fenton, Hydrodynamic Cavitation, and Sulfate Radical, highlights the diversity of oxidation pathways explored in the literature. Additional terms like Biodegradation, Toxicity, and Reactive Oxygen Species indicate that research efforts extend beyond pollutant removal to include mechanistic understanding and environmental impact assessment.

The spatial proximity and relative size of keywords suggest thematic linkages among degradation mechanisms, contaminant classes, and treatment strategies. For instance, the co-occurrence of AOP, Degradation, and Organic Pollutants reflects a strong focus on oxidation efficiency and degradation pathways, while terms such as Catalytic Ozonation and Persulfate Activation point to methodological specialization within AOP research. Overall, the word cloud provides a concise overview of the major research directions and methodological priorities in the field.

Additionally, Figure 11 offers a comprehensive depiction of each cluster, highlighting the network of shared keywords and the primary keyword groups found in publications concerning AOPs for water and wastewater treatment.

The network visualization presented in Figure 11 reveals six well-defined bibliometric clusters, each representing a distinct yet interconnected sub-field within AOPs for water and wastewater treatment. Rather than depicting AOPs as a single homogeneous research domain, these clusters collectively illustrate the diversification of AOP research into specific technological pathways, contaminant targets, and mechanistic frameworks. Together, they reflect how advances in environmental science and public health are driven by the convergence of oxidation chemistry, treatment engineering, and risk-oriented assessment.

Cluster 1, illustrated in red, focuses on keywords such as Emerging Contaminants, Toxicity, Wastewater, Pharmaceuticals, Photo-Fenton, Biodegradation, and Heterogeneous Photocatalysis indicating a research sub-field centered on photo-activated catalytic AOPs. The dominance of Photo-Fenton and heterogeneous photocatalysis highlights the importance of light-driven radical generation in treating biologically active contaminants [98,139]. The Photo-Fenton process employs light and iron catalysts to produce hydroxyl radicals, which can efficiently break down complex organic compounds [98]. Research has shown that optimizing parameters such as light intensity and reaction time can significantly enhance degradation rates, making AOP a promising treatment avenue for tackling the challenges posed by emerging contaminants. Critically, this cluster extends beyond degradation efficiency to address toxicity reduction and biodegradation enhancement, suggesting a shift from mere contaminant removal toward environmental safety and treatment sustainability. The coupling of photocatalytic processes with biodegradation concepts reflects growing recognition that partial oxidation can improve downstream biological treatment performance. This cluster therefore represents a mature AOP sub-field where process effectiveness is evaluated through both chemical removal and ecological impact.

Cluster 2, highlighted in green, 2 is defined by Peroxymonosulfate, Peracetic Acid, Singlet Oxygen, Reactive Oxygen Species (ROS), and Antibiotics, indicating a sub-field focused on oxidant-driven AOP systems. Unlike hydroxyl radical-based processes, this cluster emphasizes the role of specific reactive oxygen species in controlling degradation pathways. The presence of singlet oxygen and peracetic acid reflects increasing interest in selective oxidation mechanisms, particularly for antibiotic compounds that may resist non-selective radical attack [26,36]. Recent studies have documented the effectiveness of Peroxymonosulfate in degrading persistent pollutants, revealing that it can be activated through various methods, including thermal and electromagnetic activation [101]. This versatility in activation methods increases the applicability of peroxymonosulfate in variable environmental conditions [107]. Furthermore, the ability to degrade widespread contaminants in wastewater (antibiotics) through these processes underscores their critical role in modern treatment paradigms [65]. The critical implication is that AOPs are no longer applied as universal solutions, but as tailored oxidation systems optimized for specific contaminant classes.

Cluster 3, highlighted in blue, includes Persulfate, Sulfate Radical(s), Hydroxyl Radical, and Hydrodynamic Cavitation, clearly delineating the sulfate radical-based AOP sub-field. The advancements in wastewater treatment technologies are essential to meet regulatory standards and public health needs amidst growing concerns over water quality. Hydroxyl radicals, generated through various AOP methods, are at the forefront of contemporary treatment methodologies [53]. The application of Persulfate is increasingly recognized for its effectiveness in degrading a broad spectrum of contaminants [49]. Research has shown that persulfate activation can be achieved through multiple pathways [97], including thermal, alkaline, and catalytic methods, facilitating the degradation of organic contaminants in wastewater [97]. For instance, the coupling of persulfate with other treatment methods, such as UV irradiation or heat, can significantly enhance pollutant degradation rates, showcasing potential synergistic effects [116]. Hydrodynamic Cavitation has emerged as an innovative method for generating hydroxyl radicals within wastewater treatment systems [39]. This technology employs the rapid formation and collapse of bubbles in liquids to produce high-energy environments conducive to radical formation, demonstrating promising results for achieving higher degradation efficiencies [39]. The coexistence of sulfate and hydroxyl radicals indicates hybrid radical chemistry, where persulfate activation produces multiple reactive species depending on activation conditions. The integration of hydrodynamic cavitation signifies an emphasis on process intensification, where physical energy input is used to enhance radical formation and mass transfer.

Cluster 4, highlighted in yellow, comprises photocatalysis, hydrogen peroxide, ozone, catalytic ozonation, and adsorption, representing a sub-field focused on hybrid AOP configurations for water treatment. Traditional methods like adsorption remain vital for initial pollutant removal, especially those substances that can be effectively retained by activated carbon or other adsorbents [111,126]. This cluster illustrates not just the relevance of existing technologies, but also the necessity for their integration with cutting-edge AOP strategies [111,126]. The application of hydrogen peroxide in combination with other oxidizing agents has been shown to bolster the degradation capabilities of existing treatment frameworks [118]. Photocatalysis, particularly using titanium dioxide and other photocatalysts, has gained traction for its ability to utilize solar energy for pollutant degradation, aligning with the principles of sustainable water management [36,92]. Research highlights the potential of combining adsorption and AOP technologies, suggesting that integrated approaches can yield enhanced removal rates for a wider array of contaminants present in wastewater [111,126]. This represents a shift toward comprehensive water treatment systems capable of addressing complex contaminant mixtures.

Cluster 5, highlighted in purple, is characterized by ozonation, Fenton, reactive species, and transformation products, identifying a sub-field focused on classical high-oxidation-strength AOPs and their mechanistic consequences. Ozonation has been lauded for its efficiency in breaking down both organic and inorganic contaminants, making it a cornerstone of advanced water treatment [129]. Studies highlight the critical need for optimization of ozone dosage and exposure time to eliminate target contaminants effectively while minimizing byproduct formation [128]. The increasing recognition of per- and polyfluoroalkyl substances (PFAS) as emerging contaminants necessitates dedicated research into their degradation [76]. Research indicates that adapted AOPs, particularly those utilizing ozone and advanced oxidation, can effectively degrade PFAS compounds, presenting a compelling avenue for future studies [72,76]. Understanding how these treatment processes influence the transformation products formed during ozonation is crucial for developing strategies that ensure the complete mineralization of harmful substances [115,140].

Cluster 6, highlighted in light blue, focusing on micropollutants and the activation of persulfate, underscores the growing recognition of micropollutants as critical environmental challenges. This cluster reflects the increasing need for oxidation strategies capable of addressing pollutants present at low concentrations but with high toxicological relevance [10,16,81]. The potential of persulfate activation to address these pollutants is particularly noteworthy, with ongoing research documenting the effectiveness of thermal, photochemical, and catalytic methods to enhance degradation efficiency [97]. The integration of persulfate processes within AOP frameworks highlights the necessity for simultaneously addressing multiple contaminants, as traditional methods may fall short on their own [97].

The differentiation of these six clusters provides clear evidence that AOP research is structured around distinct oxidation sub-fields, each governed by specific reactive species, activation mechanisms, and treatment objectives. This bibliometric structure directly addresses the reviewer’s concern by demonstrating that AOPs are not treated as a homogeneous domain, but rather as a diverse set of specialized and evolving technologies. The cluster organization reflects the field’s progression toward mechanistic understanding, system integration, and application-driven optimization, underscoring the maturity and complexity of contemporary AOP research.

4. Strength and Limitations

The AOPs have emerged as promising technologies for the degradation of a wide spectrum of organic and inorganic pollutants in water and wastewater systems [25]. They demonstrate substantial potential in effectively managing emerging contaminants; however, their practical application and overall performance are influenced by several operational, economic, and environmental factors.

4.1. Strengths

A notable strength of AOPs is their high oxidative efficiency, characterized by their ability to generate highly reactive radicals, such as ^•OH and SO₄•⁻ [141]. These radicals possess exceptional oxidation potentials, enabling them to non-selectively degrade persistent organic pollutants into smaller, less toxic intermediates or completely mineralize them into carbon dioxide (CO₂) and water (H₂O) [141]. This capability is particularly crucial for addressing contaminants that are resistant to traditional treatment methods.

The versatile application of AOPs enhances their utility in various treatment methodologies, allowing for tailoring to specific contaminants and operational conditions [51]. Techniques like Fenton and photo-Fenton systems are particularly effective for recalcitrant organics and can operate under relatively mild conditions [32]. Additionally, photocatalytic and solar-driven systems offer sustainability benefits by utilizing natural sunlight, thus reducing dependence on electricity and minimizing environmental impact [79,108].

Hybrid systems that integrate AOPs with other treatment methods, including biological treatment [69], adsorption [111], or coagulation [142], have demonstrated enhanced efficiency in contaminant removal, reduced toxicity, and enabled water reuse in industrial and municipal applications. Such integrations can synergistically enhance mineralization and degradation across a wider range of pollutants, improving overall system resilience [69,111,142].

Electrochemical and heterogeneous catalytic AOPs present further advantages, such as process controllability, catalyst regeneration, and high catalyst reusability [27,29,93]. Metal-based catalysts, including those derived from ZnFe₂O₄ or MOFs (metal–organic frameworks), exhibit strong redox activity and stability, contributing to enhanced degradation rates and improved resistance to fouling and poisoning [29,121,134].

4.2. Limitations

The AOPs face several limitations that may hinder their broader implementation. A significant challenge is the high operational costs associated with the need for chemical reagents, energy requirements—especially in UV or plasma-assisted systems—and catalyst regeneration or replacement [12,53,139,143]. These costs can limit the widespread adoption of AOP technologies, especially in economically constrained regions. Additionally, while AOPs effectively remove contaminants, they can also lead to the formation of hazardous byproducts that may require further treatment. Some of these byproducts can be toxic or only partially mineralized, posing environmental risks that must be assessed [9].

The efficiency of AOPs is also heavily influenced by the composition of the water matrix, including pH, ionic strength, and the presence of natural organic matter, which can act as radical scavengers, thus reducing overall oxidation efficiency. Optimizing conditions to mitigate these effects can complicate implementation [27]. Transitioning from laboratory-scale AOP systems to full-scale industrial applications presents significant hurdles. Issues such as reactor design limitations, non-uniform energy distribution in photo- or plasma systems, and challenges in maintaining consistent reaction conditions can complicate scaling efforts [58]. Lastly, the stability and reusability of catalysts remain ongoing concerns, particularly for metal and nanomaterial-based systems [34]. Catalysts can undergo leaching or surface deactivation over time, reducing their effectiveness and necessitating frequent replacements or regeneration.

Overall, AOPs represent a robust and versatile family of treatment technologies capable of addressing emerging contaminants and improving effluent quality. However, their large-scale application requires further optimization, including minimizing energy and reagent consumption, enhancing catalyst durability, and ensuring cost-effective operation under real environmental conditions. Ongoing research and technological innovations will be critical in overcoming these limitations and maximizing the potential of AOPs as viable solutions for the evolving challenges in water and wastewater treatment.

5. Conclusions

This bibliometric analysis of AOPs in water and wastewater treatment presents a comprehensive overview of the current state of research in this critical area from 2010 to November 2025. By adhering to PRISMA guidelines, the study systematically extracted data from 481 publications, illustrating a significant and growing global interest in AOP technologies, evidenced by an impressive annual growth rate of 22.7%. This demonstrates not only the increasing recognition of AOPs as powerful tools for addressing water pollution but also reflects the urgency in tackling the challenges posed by persistent organic and inorganic contaminants.

The analysis reveals that AOP research has benefited from substantial contributions from 2335 authors affiliated with 158 institutions across 74 countries, with China emerging as a leader in the field with 192 publications. The United States follows with 64 publications, highlighting both countries’ commitment to advancing sustainable solutions for water treatment. This international collaboration fosters a diverse scientific landscape, which is crucial for developing innovative strategies to combat water pollution effectively.

Impact metrics associated with the dataset provide further insight into the significance of AOP research. With a total citation count of 39,152 and an average of 81.4 citations per document, the academic influence of publications in this field is noteworthy. However, a troubling trend is observed in the declining average citations over recent years, suggesting potential saturation and emphasizing the need for a renewed focus on producing high-quality, innovative research.

Key themes emerging from this analysis indicate the high oxidative efficiency of AOPs, which enables effective degradation of contaminants that often resist traditional treatment methods. Techniques such as Fenton and photocatalytic processes are particularly prominent, showcasing their importance in the effective removal of persistent pollutants. Furthermore, the study underlines the critical need to address emerging contaminants, including pharmaceuticals and micro-pollutants, through advanced treatment methodologies, which is becoming increasingly essential for ensuring water quality and safety.

The interdisciplinary nature of AOP research is reflected in the distribution of scholarly work across various subject areas, with Environmental Science, Chemistry, and Chemical Engineering contributing significantly to the literature. This collaborative framework is vital for optimizing treatment processes and understanding the complex dynamics of water contaminants. Prominent journals such as “Water Research” and “Chemical Engineering Journal” not only disseminate vital findings but also highlight the importance of rigorous peer review and publication standards that further advance the field.

Despite the promising capabilities of AOPs, the study also identifies several critical challenges that may hinder their broader application. High operational costs, potential formation of hazardous byproducts, and variability in treatment effectiveness due to differing water matrix compositions pose significant barriers to implementation. These challenges necessitate strategic improvements and innovations that focus on reducing costs, enhancing operational efficiency, and ensuring environmental safety.

Looking to the future, the research community must prioritize ongoing collaboration and innovation to maximize the practical application of AOPs in real-world contexts. Addressing the current decline in citation averages underscores the importance of maintaining high research quality and relevant contributions to the field. AOP technologies hold considerable promises for mitigating water pollution, but their full potential can only be realized through a commitment to developing sustainable practices and interdisciplinary partnerships that enhance their effectiveness in addressing the evolving challenges of water treatment and environmental sustainability.