Advanced Oxidation Process: Applications and Prospects

The generation of waste has increased significantly over the last 50 years. This has affected the quality of the air, water, and soil, leading to climate change, the effects of which are increasingly being felt by the world’s population. Faced with this scenario, society is being forced to look for solutions to maintain the health of the planet and of humans. In this sense, advanced oxidation technologies (AOTs) present solutions that are difficult to achieve through conventional technologies, given the complexity of the composition of the waste generated by industries that increasingly pollute. The scientific community is working to develop AOTs from emerging to well-established technologies, as they are a greener option and fit well into the processes that lead to a circular economy. This Topic is dedicated to understanding the current situation concerning AOTs, and what these technologies can offer us in the search for effective environmental solutions to reduce or eliminate waste generated by different industrial sectors. The papers that make up the Topic and correspond to different technologies involving advanced oxidation processes (AOPs) are as follows.

Lu et al. [1] studied the degradation effect of heat/peroxymonosulfate (PMS) on atrazine (ATZ). The results show that the heat/PMS degradation for ATZ is 96.28% when the phosphate buffer (PB) pH, temperature, PMS dosage, ATZ concentration, and reaction time are 7, 50 °C, 400 μmol/L, 2.5 μmol/L, and 60 min. Kim et al. [2] aimed to enhance the catalytic activity of a coal-based powdered activated carbon (PAC) via thermal treatment. They suggest that a simple thermal treatment can significantly change the characteristics of a PAC, to improve the removal of organic micropollutants. These changes in properties, and how they affect performance, can provide important information regarding the improvement of carbonaceous catalysts. Govindan et al. [3] examined the relationship between the intrinsic structure of a carbocatalyst and the catalytic activity of peroxomonosulfate (PMS) activation in acetaminophen degradation.

Wang et al. [4] demonstrated the potential utility of ferrate(VI)-based advanced oxidation processes for the degradation of the UV filter benzophenone-4 (BP-4). Zhu et al. [5] investigated the degradation of cetirizine (CTZ), a representative antihistamine, under UV/chlorine treatment. Liu and Sun [6] developed an efficient process for treating refractory 2,4-dichlorophenoxyacetic acid with radio frequencies.

Akter et al. [7] investigated the consequences of ozone dosage rates on the qualitative changes in organic compounds and nitrogen in anaerobic digestion effluent during the ozone process. Rafiee et al. [8] worked to reduce the chemical oxygen demand (COD) of an aeration effluent with an initial COD of 13,004 mg/L. In this study, an optimization process was conducted in order to find the quantities of H₂O₂, O₃, and UV irradiance required to reduce the COD of the effluent to the lowest possible.

Zhang et al. [9] studied doxycycline (DOX), a typical antibiotic, and its removal using potassium ferrate (Fe(VI)) and montmorillonite, and investigated the effect of the Fe(VI) dosage, reaction time, initial pH value, montmorillonite dosage, adsorption pH, time, and temperature on DOX removal.

Kastanek et al. [10] show the indispensability of the Fenton reaction in relation to environmental issues, as it represents the basis of all advanced oxidation processes around the idea of oxidative hydroxide radicals. The study aims to not only summarize the current knowledge of the Fenton process and identify its advantages, but also address the problems that remain to be solved. Zhang et al. [11] indicate that Cu and Co have shown a superior catalytic performance to those of other transition elements, and that layered double hydroxides (LDHs) have presented advantages over other heterogeneous Fenton catalysts. Manduca Artiles et al. [12] report the degradation of diazepam (DZP) in aqueous media via gamma radiation, high-frequency ultrasound, and UV radiation (artificial–solar), as well as the results when each process is intensified using oxidizing agents (H₂O₂ and Fenton reagent). Zhang et al. [13] fabricated three different types of manganese dioxide (MnO₂), with rod-like, needle-like, and mixed morphologies, via a hydrothermal method, changing the preparation conditions and adding metal ions, which were utilized as an activator of persulfate (PS) to remove aqueous dyes. In their review, do Nascimento et al. [14] comment on the importance of choosing the best synthesis method and experimental conditions to modify the structural, morphological, and electronic characteristics of semiconductors and, more specifically, tin oxide (SnO₂), as these parameters may be a determinant of a better performance in various applications, including photocatalysis. Suchanek et al. [15] have developed a new methodology for a broader assessment of the photocatalytic removal of NOx species (NO₂, HONO, and NO) from the air. The study provides important suggestions concerning the suitability of NO and NO₂ as test molecules, with NO being more suitable. Silerio-Vázquez et al. [16] analyzed a solar heterogeneous photocatalytic (HP) process for arsenite (AsIII) oxidation and coliform disinfection from a real groundwater matrix employing two reactors—a flat-plate reactor (FPR) and a compound parabolic collector (CPC)—with and without added hydrogen peroxide (H₂O₂). Arouca et al. [17] evaluated the effectiveness of white-light photolysis (WLP) via an advanced oxidation process (AOP) for removing polycyclic aromatic hydrocarbons (PAHs) from proximity firefighting protective clothing (PFPC), while maintaining the integrity of the fabric fibers. Experiments were carried out, with variations in the reaction time and the concentration of H₂O₂. With WLP (without H₂O₂), it was possible to remove more than 73% of the PAHs tested from the outer layer of PFPC in 3 days. Díez et al. [18] carried out the synthesis and characterization of novel graphene oxide coupled to TiO₂ (GO-TiO₂) in order to better understand the performance of this photocatalyst compared to the well-known TiO₂ (P25) from Degussa. Checa et al. [19] synthesized and characterized a magnetic graphene oxide titania (FeGOTi) catalyst for application in primidone removal from water, and checked for any possible enhancements. This compound was chosen because its presence is expected in urban wastewater, and its direct reaction with ozone is relatively low. Jorge et al. [20] investigated the degradation of a model agro-industrial wastewater phenolic compound (caffeic acid, CA) via a UV-A-Fenton system. According to the results, the UV-A-Fenton process at pH 3.0 achieved the highest CA degradation rate. Navidpour et al. [21] present a review of the photocatalytic processes and the mechanisms, reaction kinetics, and optical and electrical properties of semiconductors, and the unique characteristics of titanium as the most widely used photocatalyst; and compare the photocatalytic activity between different titania phases (anatase, rutile, and brookite) and between colorful and white TiO₂ nanoparticles. Haketa et al. [22] designed novel Au nano-particle catalysts immobilized on both titanium(IV)- and alkylthiol-functionalized SBA-15 type mesoporous silicates. The combination of Au nano-particle catalysts and other species that activate H₂O₂ served as an aerobic oxidation catalyst applicable to various substrates, including alkenes and alkanes.

Issa et al. [23] used the boron-doped diamond (BDD) anode, combined with a gas diffusion electrode (GDE) as a cathode, to constitute an attractive type of electrolysis system for the treatment of wastewater to remove organic pollutants. With this approach, they studied the treatment of synthetic wastewater, simulating the vacuum toilet sewage on trains via a BDD-GDE reactor, where the kinetics were presented as the abatement of the chemical oxygen demand (COD) over time.

Přibilová et al. [24] trialed the possibility of using persulfate to lower the amount of emerging contaminants released into the environment. The main disadvantage of sulfur-based AOPs is the need for activation. The authors investigated an economically and environmentally friendly solution based on hydrodynamic cavitation, which does not require heating or the additional activation of chemical substances.

Jorge et al. [25] proposed a new approach to winery wastewater treatment: (1) the application of the coagulation–flocculation–decantation (CFD) process with an organic coagulant based on almond skin extract (ASE), (2) the treatment of the organic recalcitrant matter through sulfate radical advanced oxidation processes (SR-AOPs), and (3) the evaluation of the efficiency of the combined CFD with UV-A, UV-C, and ultrasound (US) reactors.

Liu et al. [26] studied the migration/change characteristics of the centerline of the channel of the Yangtze River in Zhenjiang–Yangzhou; these characteristics are crucial for a comprehensive understanding of the river. In this study, a detailed calculation method is proposed to extract the channel centerline in Zhenjiang–Yangzhou, using old maps and remote sensing satellite maps, and to dissect it into seven parts.

Tincu et al. [27] studied mercury (Hg) exposure through a fish-based diet to evaluate the correlations between Hg blood concentrations and specific biomarkers for oxidative stress that can lead to neurological and cardiovascular diseases through the exacerbation of oxidative stress.

The scientific contributions to this Topic have shed light on the state of the art of emerging advanced oxidation technologies, which show promise for being incorporated into the industrial sector in the not-too-distant future.

We would like to express our gratitude to MDPI Editorial, through the journals Catalysts, Processes, Sci, International Journal of Environmental Research and Public Health, and Water, for offering us the opportunity to serve as Guest Editors and contribute to this detailed exploration of advanced oxidation technologies. In addition, we would like to thank all the authors who shared their research and the referees for their invaluable contributions.