**About the Editors**

### **Dionysios (Dion) Demetriou Dionysiou**

Dr. Dionysios (Dion) D. Dionysiou is currently a Herman Schneider Professor of Environmental Engineering and Distinguished Research Professor (STEMM) at the University of Cincinnati. He has served as a UNESCO co-Chair Professor on "Water Access and Sustainability". He teaches courses and performs research in the areas of drinking water quality and treatment, advanced unit operations for water treatment, advanced oxidation technologies and nanotechnologies, and physical-chemical processes for water quality control. He has received funding from NSF, US EPA, NASA, NOAA/CICEET, USGS, USDA, Ohio Sea Grant, USAID, and DuPont. He is currently one of the Executive Editors of Chemical Engineering Journal, Editor-in-Chief of the Journal of Environmental Engineering (ASCE), co-Editor of Current Opinion in Chemical Engineering, and member of the Editorial Boards of several other journals. Dr. Dionysiou is the author or co-author of over 630 refereed journal publications, over 87 conference proceedings, 39 book chapter publications, 38 editorials, and more than 650 presentations. He has edited/co-edited seven books on water quality, water reuse, ferrates, photocatalysis and treatment of harmful algal blooms and cyanotoxins. Dr. Dionysiou's work received over 71,000 citations with an H factor of 137 (Google Scholar). He is a Highly Cited Researcher (in Engineering, Environment/Ecology, and Chemistry based on Clarivate Analytics, Web of Science, 2018, 2019, 2020, 2021 and 2022, and in Environmental Science and Engineering and Chemical Engineering based of Shanghai Ranking's Global Ranking of Academic Subjects by Elsevier, 2016).

### **Yujue Wang**

Dr. Yujue Wang is currently a full professor at the School of Environment, Tsinghua University. He has served as a Vice Director of Beijing Key Laboratory for Emerging Organic Contaminants Control since 2017, and is an associate editor of the journal *Water* and editorial board of *Journal of Porous Materials*. Dr. Wang's research mainly focuses on advanced oxidation processes and physicochemical processes for water and wastewater treatment. He has authored or co-authored more than 150 referred journal papers, which have been cited more than 9400 times with an H factor of 60 (Google Scholar). In addition, Dr. Wang is the author or co-author of three book chapters and more than 50 presentations. Dr. Wang is ranked as the world's top 2% scientist by Stanford University in 2019, 2020, and 2021. His research has been granted several distinguished awards, including the First Prize of Natural Science of Ministry of Education of China, the Gold Medal of International Exhibition of Inventions of Geneva, and Cutting-edge Academic Achievements of Beijing.

### **Huijiao Wang**

Dr. Huijiao Wang is currently an associate professor at School of Chemical and Environmental Engineering, China University of Mining and Technology (Beijing). Her main research area is advanced oxidation technologies and kinetic modelling for water and wastewater treatment. Dr. Wang is the author or co-author of more than 30 referred journal papers, and her work has received more than 1400 citations with an H factor of 20 (Google Scholar). Additionally, she is the editorial board member of the journal *Water*, and has received the funding of China National Postdoctoral Program for Innovative Talents.

## *Editorial* **Advanced Oxidation Processes for Removal of Emerging Contaminants in Water**

**Huijiao Wang 1,\*, Yujue Wang 2,\* and Dionysios D. Dionysiou 3,\***


**Abstract:** This Special Issue includes manuscripts on mechanistic understanding, development, and implementation of advanced oxidation processes (AOPs) for the removal of contaminants of emerging concern in water and wastewater treatment. The main goal was successfully achieved under the joint effort of authors, anonymous reviewers, and editorial managers. Totally, one review and 15 research papers are included in the Special Issue. These are mainly focused on catalyst synthesis, reactor design, treatment performance, kinetic modeling, reaction mechanisms, and by-product formation during electrochemical, photocatalytic, plasma, persulfate, chlorine, ozone-based, and Fenton-related AOPs at different scales. This Special Issue received attention from researchers from different parts of the world such as Argentina, Brazil, Canada, China, Germany, India, Mexico, and the USA. The guest editors are happy to see that all papers presented are innovative and meaningful, and hope that this Special Issue can promote mechanistic understanding and engineering applications of AOPs for the removal of contaminants of emerging concern in water.

**Keywords:** advanced oxidation; concern; contaminant; electrochemical; emerging; hydroxyl radical; oxidation; ozonation; persulfate; photocatalysis; radical; UV; chlorine; UV/chlorine; Fenton; plasma; water

### **1. Introduction**

Efficient and cost-effective removal of various contaminants in water matrices is a major challenge in water and wastewater treatment [1,2]. In this regard, advanced oxidation processes (AOPs) have been considered a promising option because highly reactive radicals such as hydroxyl and sulfate radicals generated in AOPs can effectively oxidize a broad range of emerging contaminants; other radicals such as reactive chlorine and nitrogen radicals can also play significant roles [1,3–5]. Nevertheless, the practical application of AOPs is challenged by the high energy demand, formation of harmful oxidation byproducts, difficulty in scaling up, etc. [3,6,7]. Therefore, both novel mechanistic understanding and improved engineering designs are needed to overcome these challenges and thus bridge academic research with practical applications.

In this Special Issue, we attempted to focus on the mechanistic understanding, development, and implementation of AOPs for the removal of emerging contaminants in water and wastewater treatment. Ozone-, UV-, H2O2-, chlorine-, persulfate-based AOPs, electricity-driven AOPs, and photocatalytic AOPs were among the technologies of interest in this Special Issue. Relevant topics of interest included reaction kinetics, catalyst fabrication, model simulation, theoretical calculations, by-product formation, and degradation mechanisms. Topics on reactor design, economic evaluation, and experiments at different scales (lab- and pilot-scale) were also of interest.

**Citation:** Wang, H.; Wang, Y.; Dionysiou, D.D. Advanced Oxidation Processes for Removal of Emerging Contaminants in Water. *Water* **2023**, *15*, 398. https://doi.org/10.3390/ w15030398

Received: 3 January 2023 Revised: 6 January 2023 Accepted: 9 January 2023 Published: 18 January 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

### **2. Summary of This Special Issue**

Two papers were published on the topic of persulfate technology. In Liu et al. [8], the authors investigated the degradation of acyclovir and atenolol by the UV/peroxydisulfate (UV/PDS) process from the perspective of degradation kinetics, model simulations, and reaction pathway. Results show that the UV/PDS process could effectively generate sulfate radicals (SO<sup>4</sup> •−) and hydroxyl radicals (•OH) to remove the two micropollutants, with SO<sup>4</sup> •− playing a more significant role in the process. In the other study, Mo et al. [9] focused on the synthesis of a copper-magnesium oxide/carbon nitride composite (CM/g-C3N4) to activate peroxymonosulfate (PMS). The CM/g-C3N<sup>4</sup> presented superior catalytic performance and reusability for PMS activation and Rhodamine B (RhB) degradation, and SO<sup>4</sup> •− and singlet oxygen (1O2) were found to be important for RhB removal. Therefore, different oxidant species can be generated by various activation methods during the persulfate oxidation process, which is beneficial for the degradation of micropollutants with varying reactivities.

The electrochemical oxidation process is another effective technology for micropollutant removal. For instance, Yanagida et al. [10] used the boron-doped diamond (BDD) electrode to electrochemically oxidize PFAS in contaminated water and then scaled up the technology for the treatment of 189 L of PFOA and PFOS-contaminated water. LC/MS/MS analysis results show that micrograms per liter (ppb) PFAS could be easily degraded by BDD electrochemical oxidation. Considering the great importance of electrode material, da Silva et al. [11] evaluated the performance of three anodes (Ni/BDD, Ti/Pt, Ti/RuO2) to treat groundwater contaminated by petroleum-derived fuel, with the Ti/RuO<sup>2</sup> anode achieving the highest chemical oxygen demand (COD) degradation efficiency and lowest energy consumption. Besides, a pilot-flow plant was established to further verify the viability of electrochemical treatment at a larger scale.

In addition to electrochemical oxidation, a heterogeneous electro-Fenton (HEF) process using MnFe2O4-GO catalyst was employed to remove Rhodamine B from aqueous solution [12]. This study focused on the efficiency of electrodes and catalyst, as well as their application in real textile wastewater treatment. Significant color reduction and obvious biodegradability enhancement were observed after treatment. Two other Fenton-related studies were also reported in this Special Issue. In the study of Olea-Mejia et al. [13], a Cu2O/Al2O<sup>3</sup> catalyst was synthesized to improve Bisphenol A (BPA) oxidation and mineralization during the photo-Fenton process driven by UV radiation and visible light. Besides, Lin et al. [14] applied the Fenton process to treat acrylic manufacturing wastewater. The results show that total organic carbon and nitrogen can be effectively removed to meet related discharge standards, providing a successful example of industrial wastewater treatment by the Fenton technology.

Three papers on the photocatalytic process were included in this Special Issue. In Juárez-Cortazar et al. [15], TiO<sup>2</sup> was doped with metal waste (door key) to improve its photocatalytic efficiency, and a synergistic effect of the dopants and TiO<sup>2</sup> was achieved for diclofenac mineralization. Meanwhile, Mehling et al. [16] investigated the energetic efficiency of TiO<sup>2</sup> photocatalysis from a different perspective, i.e., reactor design. Three reactor systems were evaluated, with catalyst arrangement and irradiation power identified as the major influencing parameters on energy consumption performance. Other than TiO<sup>2</sup> doping and reactor design, Manassero et al. [17] focused more on radiation modeling and kinetics in different photocatalytic reactors. In their study, a strategy was proposed to obtain intrinsic kinetic parameters independent of reactor geometry, reactor size, and irradiation conditions. The results indicate that the radiation model can be employed for photocatalytic reactor design, optimization, and scaling-up, thus bridging the gap between laboratory experiments and real applications.

Ozone-based AOP, as a promising research and development option, was investigated from two different perspectives in this Special Issue. In the study of Luo et al. [18], ozonation was combined with electro-coagulation (i.e., the electro-hybrid ozonation-coagulation process) to remove surfactant and microplastics from laundry wastewater. In addition, Zhang et al. [19] prepared a mesoporous CeO<sup>2</sup> by the nano-casting method and applied the catalyst for the catalytic ozonation of atrazine. The well-ordered mesoporous structure, high surface area, and redox Ce3+/Ce4+ cycling contributed to the superior activity of the synthetical CeO2. Both studies present the effectiveness and important role of ozone-based AOPs in the removal of emerging contaminants in water.

Plasma technology was also reviewed and studied in this Special Issue. In the research paper of Liu et al. [20], a novel reactor was designed for simulated dye wastewater treatment by plasma in the presence of various catalysts, and the results show that the plasma/PS/Fe2+ system achieved the best synergy and highest removal rate. In the review paper of He et al. [21], they summarized recent research progress on non-thermal plasma technique for remediation of water and soil contaminated by emerging organic pollutants in terms of pollutant degradation mechanism, the synergy of non-thermal plasma with other techniques, bottlenecks, and suggestions to promote plasma technology toward practical applications.

Besides, one paper investigated the removal of emerging contaminants by novel material adsorption [22]. Specifically, a carbon material was derived from the nitrogen-rich bio-based metal-organic framework (MOF) and was evaluated as an absorbent for pharmaceutical elimination from the water environment. The high surface area and abundant mesoporous structure of the obtained MOF contributed greatly to hydrophobic pharmaceutical removal.

In addition to contaminant removal, the study of Li et al. [23] paid attention to disinfection by-product (DBP) formation during medium-pressure UV/chlorine AOP. Results show that DBP formation is highly dependent on the precursor activity, solution pH, and the presence of Br−. The authors suggest that the UV/chlorine-induced change in total chlorine demand might be taken as an indicator to predict the change in DBP formation potential.

**Author Contributions:** Writing—original draft preparation, H.W.; writing—review and editing, D.D.D., Y.W. and H.W.; supervision, D.D.D. and Y.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** Thanks to all who have contributed to the Special Issue, the authors, anonymous reviewers, as well as the editorial managers. All the guest editors are very happy with the review process and management of the Special Issue and offer their special thanks.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**


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