Harnessing High-Valent Metals for Catalytic Oxidation: Next-Gen Strategies in Water Remediation and Circular Chemistry

Abstract

High-valent metal species (iron, manganese, cobalt, copper, and ruthenium) based advanced oxidation processes (AOPs) have emerged as sustainable technologies for water remediation. These processes offer high selectivity, electron transfer efficiency, and compatibility with circular chemistry principles compared to conventional systems. This comprehensive review discusses recent advances in the synthesis, stabilization, and catalytic applications of high-valent metals in aqueous environments. This study highlights their dual functionality, not only as conventional oxidants but also as mechanistic mediators within redox cycles that underpin next-generation AOPs. In this review, the formation mechanisms of these species in various oxidant systems are critically evaluated, highlighting the significance of ligand design, supramolecular confinement, and single-atom engineering in enhancing their stability. The integration of high-valent metal-based AOPs into photocatalysis, sonocatalysis, and electrochemical regeneration is explored through a newly proposed classification framework, highlighting their potential in the development of energy efficient hybrid systems. In addition, this work addresses the critical yet underexplored area of environmental fate, elucidating the post-oxidation transformation pathways of high-valent species, with particular attention to their implications for metal recovery and nutrient valorization. This review highlights the potential of high-valent metal-based AOPs as a promising approach for zero wastewater treatment within circular economies. Future frontiers, including bioinspired catalyst design, machine learning-guided optimization, and closed loop reactor engineering, will bridge the gap between laboratory research and real-world applications.

1. Introduction

Water pollution is currently a serious worldwide concern caused largely by the haphazard release of many contaminants originating from industrial, agricultural, and domestic activities [1]. Industrial wastewater is one of the major contributors because it tends to transport a mix of synthetic chemicals, heavy metals, and organics that cannot be degraded easily [2,3,4,5]. Pharmaceutical compounds are also frequently detected in both surface water and groundwater due to improper disposal, leading to environmental residues that pose serious concerns [6,7]. Moreover, synthetic dyes (textile, leather, printing, carpet, and paper sectors) are also chemically persistent and tend to accumulate in aquatic environments, resulting in several environmental problems and health issues (carcinogenic) [6,8,9,10]. Similarly, agricultural chemicals can contaminate water bodies and negatively affect aquatic life [11]. The combined presence of such diverse contaminants has led to complex pollutant mixtures, and many of these so-called emerging contaminants are not yet addressed and continue to raise significant environmental concerns. This alarming trend has prompted extensive research and policy initiatives aimed at mitigating the risks posed by pollutants [12].

Common treatment methods include aerobic biological treatments, ion exchange, membrane-based separations, adsorption, coagulation, and a range of AOPs such as photocatalysis, sonolysis, ozonation, electrocatalysis, electro-Fenton, and photo-Fenton processes [1,13,14]. Conventional wastewater treatment processes are found to be insufficient for the mineralization of recalcitrant contaminants. For example, biological treatments, such as the activated sludge process, rely on microbial degradation but are ineffective against chemically stable or toxic substances that inhibit microbial activity [15]. Adsorption using activated carbon (AC)/biochar can remove some contaminants, but lacks specificity and requires frequent replacement [16,17]. Chlorination, although effective against pathogens, can produce harmful byproducts [9]. Recent studies on photothermal heating and photocatalysis reveal that integrating these two processes can significantly enhance pollutant degradation efficiency. The synergistic interaction (photothermal heating and photocatalysis) enhances reaction kinetics and solar energy utilization [18]. Green synthesis approaches also offer an ecofriendly route for developing catalysts [19]. These methods face practical challenges, including complex instrumentation, high costs, and significant energy demands, which limit their feasibility.

Among available technologies, AOPs have gained significant attention for their ability to break down persistent organic pollutants. AOPs that generate highly reactive species to drive the oxidative degradation of water pollutants have emerged as the most compelling and sustainable alternative to the conventional wastewater treatment methodologies [20]. Traditional AOPs, including the Fenton process, ozonation, and persulfate-based systems, predominantly rely on non-selective oxidative species, such as hydroxyl and sulfate radicals. Similarly, variety of material g-C₃N₄, TiO₂, CdS, Fe₂O₃, SiO₂, SnO₂, ZnO, ZnS, and BiOI have been utilized in AOPs for effective pollutant removal [6,21,22,23]. Despite the high oxidation potential, they suffer from rapid recombination, scavenging by background anions, and limited electron efficiency [21,22]. These limitations reduce their effectiveness in real world water matrices and increase operational costs, necessitating the development of more selective and efficient oxidation strategies [24].

Recent studies highlight that redox reversibility in metal oxides can boost catalytic performance, and one study combining Ag with Co₃O₄ enhanced both oxygen reduction and oxygen evolution activities compared to pristine material [25]. Similarly, another investigation on CoFe₂O₄ nanomaterials using nano impact electrochemistry allowed the study of oxygen evolution reaction kinetics at the single particle level [26]. Heterocubane complexes [Co³⁺₄O₄] are also gaining attention and emerging as key models designing active material for catalysis [27]. Recently, high-valent metal-oxo species (HMOS, for example, Fe(IV)=O, Fe(V)=O, Mn(V)=O, Co(IV)=O, and Ru(VIII)) have emerged as promising oxidants in next-generation AOPs, owing to their unique reactivity, high selectivity, and resistance to interference [28,29,30,31]. In contrast to products of radical-based reactions, high-valent metals employ defined, non-radical mechanisms, such as oxygen-atom transfer (OAT) and hydride abstraction or electrophilic addition, allowing for the degradation of electron-rich organic functionalities with minimal side products [32,33]. For example, Fe(VI) and Fe(V) species generated in Fe(VI), or Fe(II)-dependent systems, have been shown to oxidize sulfonamides, phenols, and antibiotics like sulfamethoxazole (SMX), notably over a wide pH range [34,35]. Similarly, Co(IV) formed in Co(II)/peroxymonosulfate (PMS) systems is thus described to demonstrate greater oxidation power in comparison to Fe(IV) due to its larger redox potential and stability towards acidic-neutral conditions [36,37,38]. The experiments have been carried out on several catalytic systems in which the high-valent metals have been detected in the Fe(II)/peracetic acid (PAA), Co(II)/PAA, Fe(II)/periodate, and Mn(II)/PAA systems by applying characterization methods like EPR (electron paramagnetic resonance), XAS (extended X-ray absorption spectroscopy), and isotopic labeling (¹⁸O) [32,39,40,41]. Cu(III)-mediated oxidation reactions exceeded the performance of small radical-dominated systems (27% Co₃O₄/PMS and 26% ¹O₂) and showed an unrivaled electron efficiency of high-valent metal pathways and used 77% of the available electrons to mineralize contaminants [28,42,43]. This efficiency can be further boosted in the non-radical systems, where direct electron transfers between pollutants and the catalyst/oxidant occurs, and hence energy is not lost easily [37].

The main challenges are creating high-valent metal species in a way that is controlled, stable, and can be used on a large scale. Limited lifetimes, pH sensitivity, and metal leaching vulnerabilities, especially in homogeneous systems, reduce the practical applicability of such systems in the long term [30]. In an attempt to solve these problems, in recent years, considerable efforts have been devoted to the study of heterogeneous catalysts: metal–organic frameworks (MOFs), transition metal sulfides (TMSs), and single atom catalysts (SACs), which could be more stable, easier to reuse, and accessible to active sites [44,45]. The development of multiple approaches has been used to generate and stabilize the high-valent states in the form of defect engineering, ligand modulation, and electrochemical regeneration, allowing prolonged catalytic cycles to operate [30]. In addition to breaking down pollutants, high-valent metal-based AOPs support the idea of circular chemistry. They help to separate heavy metals from organic complexes, making it possible to both remove contaminants and reuse heavy metals in industrial wastewater management processes [46]. Moreover, the possibility to filter these systems allows transforming waste organics into value products according to the principles of green synthesis and upcycling wastes [32,47]. The amount of literature on high-valent metal-based systems and persulfate indicates widespread research on high-valent metal-based AOPs. Up to the present, there have been few studies that examine the overall study of the high-valent metal applications in the treatment of water and circular chemistry [32].

The current review provides a new insight into the high-valent metal-oxo species and their use as not only strong oxidants but also as dynamic catalysts in closed loop reagent cycles. Unlike the previous reviews that tend to separate the reaction mechanisms and environmental applications as distinct fields, this one fills the gaps by integrating the knowledge of coordination chemistry, catalyst design, and green engineering. The article presents a comprehensive review of the recent advancements in the mechanism of formation, reaction pathways, catalytic, and environmental utilization of high-valent metals in AOPs, with a vision towards scalability, sustainability, and integration into the circular water economy. It also covers the new classification scheme for hybrid AOPs, provides an extensive comparison of metal species redox behavior, and connects how these processes may contribute towards the advancement of circular water treatment strategies. By incorporating emerging approaches such as machine learning, nano engineering, and bioinspired materials, the review points toward promising directions for future research. It aims to serve as a valuable resource for advancing interdisciplinary solutions in water purification and environmental remediation.

2. Advancing High-Valent Metals in AOPs: From Oxidants to Reaction Mediators

Traditionally, reactive oxygen species (ROS) such as hydroxyl (•OH) and sulfate (SO₄•⁻) radicals have been recognized as the major species for the degradation of pollutants in wastewater. Recent findings indicate that the catalytic cycles are guided by the presence of the high-valent metal-oxo species, such as Co(IV), Mn(V), Fe(VI), and Ru(VIII), which increases efficiency and selectivity through the oxidative conversion reactions [28,32,48]. The degradation of pollutants through high-valent transition metal-oxo species is illustrated in Figure 1, showing the activation, transformation, and reduction steps involving Fe(IV), Co(IV), and Cu(III) intermediates. This figure also shows catalyst stabilization and the reuse mechanisms, which promote a circular and sustainable process of water treatment.

Figure 1. High−valent metal-oxo pathways for pollutant degradation and catalyst recovery in sustainable water treatment.

Their interesting features being purely electronic and redox reversible have shaped their current role as key mediators in the AOPs. Fe(VI) is no exception and in that regard will demonstrate itself to be a dual functional reagent in wastewater treatment with respect to coagulant behavior and strong oxidant behavior as its standard reduction potential is in the range of ~0.7–2.2 V, which will be varied with respect to pH [32,49]. Fe(VI) generates reactive intermediates, Fe(V) and Fe(IV), which are spectroscopically confirmed by using the X-ray absorption near edge structure (XANES), as it recovers stepwise [34,40]. These intermediate products are not simply byproducts but are crucial elements in the redox cascade to provide extra oxidative potential and enhance the efficiency of electron use. Here, Co(IV), which generates during the activation of PMS, has greater redox potential than Fe(IV) and offers excellent performance in C–H bond activation and oxygen-atom transfer reactions, as well as oxidation of recalcitrant organic compounds, especially for the activation of dysfunctional C–H bonds that are commonly a bottleneck step in the degradation of known pollutants in the environment [29,42,50].

Recent research suggests that the least frequently reported species, Mn(V) and Ru(VIII), are the most selective and stable compared to the other species in terms of selectivity and stability. The Mn(V)=O species generated in the Mn(II)/PAA systems have been detected using labeling experiments with ¹⁸O. The Mn(V) species are highly electrophilic and react most readily with highly electron-rich contaminants (phenols and sulfonamides) [32,51,52]. Ruthenium, in its highest oxidation state (typically in the form of RuO₄), exhibits strong oxidative power and has been explored for AOPs. Although the standard reduction potential of RuO₄ is typically reported around +0.59 to +1.4 V vs. SHE, depending on the specific redox couple conditions, RuO₄ remains a potent oxidant capable of degrading and mineralizing persistent organic pollutants under mild aqueous conditions [31,48,53]. The strong oxidation characteristics of this metal have rendered it a candidate for electrochemically driven AOPs for the total mineralization of contaminants such as endocrine-disrupting compounds and pharmaceuticals, even at modest potentials. In addition, its high-valent states have been stabilized via ligand engineering and coordination on a site in single-atom catalysts (SACs), where the localized electronic environment changes the distinct character of the metal to prevent over-oxidation or decomposition [54]. Catalytic systems can regenerate high-valent species either through chemical oxidants (iodosylbenzene or peracids) that promote oxygen-atom transfer or via electrochemical oxidation processes that re-oxidize the reduced metal at an electrode surface. For example, even a small amount of Cu(II) can help improve the Fe(III)/PMS system by turning Fe(III) back into Fe(II), which boosts the production of Fe(IV) and keeps the catalytic process going [55]. High-valent metals shift from single-use oxidants to reusable catalysts. They make complex reactions happen more easily and efficiently. This emerging field fits well within two principles of green chemistry: atom economy and energy efficiency. High-valent metals, through their use of oxidants, are more effective than traditional radical AOPs due to slower radical pathways and faster direct electron transfer. For example, it has been shown that Cu(III) processes reach about 77% oxidant consumption, while radical-based AOPs like Fenton operate around 15% effectiveness [43,56]. A comparison of redox potentials, times, mechanisms of action, and competencies of electron transfer of Co(IV), Mn(V), Fe(VI), and Ru(VIII) across diverse oxidant systems (PMS, PAA, periodate) can be found in Table 1.

The recent importance of AOPs research has increasingly moved investigations away from the synthesis and spectroscopic characterization of high-valent species to engineering their catalytic efficiency. This transition of high-valent metals from oxidants to mechanistic mediators offers possibilities for efficient and sustainable wastewater remediation technologies. Not only do these hybrid designs serve to increase catalytic efficiency, but they also serve to increase the selectivity towards the more electron-rich contaminants, reducing undesired peripheral reactions. At the atomic level, nano engineering has provided a new ability to control the environment of high-valent atoms comprising metals. Isolated, well-defined, active sites for maximum atom utilization can be found in the M1–N–C structures (M = Fe, Co, and Mn) of single-atom catalysts (SACs), thus inhibiting dimerization or over-oxidation [57,58]. When defect engineering is applied later, the inclusion of oxygen vacancies, carbon vacancies, or heteroatom doping, also modifies the electronic structure of the metal center and promotes heterolytic O–O bond cleavage across oxidants, such as PMS and PAA, to enable high-valent metal pathways in place of the radical route [59,60].

Table 1. Comparison of important parameters of high-value metal-oxo species in AOPs.

Metal-Oxo Species	Approx. Redox Potential (E0, V vs. NHE)	Primary Formation Pathway	Dominant Reaction Mechanism	Lifetime in Aqueous Solution	Electron Efficiency	Reference
Fe(IV)=O	0.7–1.0 (pH-dependent)	Fe(II)/PMS, Fe(II)/PAA, Fe(VI) reduction	OAT, hydride abstraction	Milliseconds to seconds	Moderate (40–60%)	[41]
Fe(VI)=O	~1.7–2.0	Fe(III)/PAA, Fe(VI) activation	Electrophilic attack, OAT	Sub milliseconds	Low to moderate	[61]
Mn(V)=O	~1.5–1.8	Mn(II)/periodate, Mn(II)/PAA	Selective OAT, electron transfer	Milliseconds	High (>70%)	[62]
Co(IV)=O	~1.8–2.1	Co(II)/PMS, Co(II)/PAA	OAT, C–H bond activation	Seconds (ligand-stabilized)	High (60–75%)	[63,64]
Cu(III)	>2.0	Cu(II)/PMS with activating ligands	Direct electron transfer	Transient (stabilized by ligands)	Very high (~77%)	[65]
Ru(VIII)	~1.3–1.6	Electrochemical oxidation, RuO4−/O3	Selective oxidation, radical generation	Very short	Moderate	[53]

Table 1. Comparison of important parameters of high-value metal-oxo species in AOPs.

Metal-Oxo Species	Approx. Redox Potential (E0, V vs. NHE)	Primary Formation Pathway	Dominant Reaction Mechanism	Lifetime in Aqueous Solution	Electron Efficiency	Reference
Fe(IV)=O	0.7–1.0 (pH-dependent)	Fe(II)/PMS, Fe(II)/PAA, Fe(VI) reduction	OAT, hydride abstraction	Milliseconds to seconds	Moderate (40–60%)	[41]
Fe(VI)=O	~1.7–2.0	Fe(III)/PAA, Fe(VI) activation	Electrophilic attack, OAT	Sub milliseconds	Low to moderate	[61]
Mn(V)=O	~1.5–1.8	Mn(II)/periodate, Mn(II)/PAA	Selective OAT, electron transfer	Milliseconds	High (>70%)	[62]
Co(IV)=O	~1.8–2.1	Co(II)/PMS, Co(II)/PAA	OAT, C–H bond activation	Seconds (ligand-stabilized)	High (60–75%)	[63,64]
Cu(III)	>2.0	Cu(II)/PMS with activating ligands	Direct electron transfer	Transient (stabilized by ligands)	Very high (~77%)	[65]
Ru(VIII)	~1.3–1.6	Electrochemical oxidation, RuO4−/O3	Selective oxidation, radical generation	Very short	Moderate	[53]

3. Emergent Synthesis Pathways for Stable High-Valent Metal Complexes

Metal species of high valency, like Fe(IV), Co(IV), Mn(V), and Ru(VIII), are unstable and short lived species in AOPs, limiting their practical use. The species are thermodynamically potent; their issue is generally restricted, with minute catalytic turnover, little oxidative power because of rapid auto decomposition, over reduction, or hydrolysis in aqueous solvents [28,40]. Many of these problems have been solved using new synthetic chemistry methods. These use special binders that carefully control and stabilize high-valent metal species, even under practical working conditions. Ligand engineering includes the introduction of regular histidine-like N-donor sites [66]. In MOF or carbon supports, it is now replicating these motifs specifically to stabilize the Mn(V)=O and Fe(IV)=O species [67]. As an example, Co–N–C catalysts based on MOFs with a laccase-like active site incorporating pyridinic nitrogen vacancies enable two-electron activation of PMS and high-valent cobalt oxo reaction products formation [68,69].

The recent synthesis of the ligands of polydentate nitrogen and oxygen donors has been mentioned to be a breakthrough, where the high-valent metals are thermodynamically and kinetically stabilized. Tetra amido macrocyclic ligands (TAMLs), pentadentate N–bases ligands (N₄Py and Bn-TPEN), and N–heterocyclic carbene (NHC) ligands have been reported to destabilize the dimers, but stabilize the Fe(IV)=O and Mn(V)=O species by altering the electron density and degradation by a particular solvent [70,71,72,73]. Tetradentate N-donor ligands stabilized Fe(IV)=O complexes, boosting their oxidation potential (relative to complexes without ligands). Additionally, these ligands extend the lifetime of the complexes in aqueous solutions, enabling more effective degradation of pollutants [74]. In a parallel fashion, Co(IV) species formed in Co(II)/PMS systems are stabilized by interaction with nitrogen atoms of pyridine and graphite within the nitrogen-doped carbon matrices, which increases electron mobility and suppresses metal leaching [63]. MOFs, self-assembled cages, and supramolecular structures are also becoming potentially useful materials to trap and stabilize high-valency compounds. The well-defined cavity and tunable environments in these structures exclude bulk free water and other competing nucleophiles from the reactive metal-oxo centers and their accessibility. Various MOF-based materials, including pristine MOFs (ZIF-67, MIL-53(Fe), MIL-88A(Fe)), and MOF-derived structures (FexCo_3−xO₄ nanocages and Co@C nanoboxes), have been used as catalysts in sulfate radical-based AOPs (SR-AOPs) for wastewater treatment. Their porous structures enhance mass transfer and pollutant removal, while spatial confinement helps reduce metal ion leaching compared to traditional catalysts [75]. XAS studies in situ prove the stability of Fe(IV)=O clusters in the MOF matrices under conditions of PAA activation, and therefore show the stabilizing capability of the crystalline framework [32]. Figure 2 compares three types of SACs architectures, ligand-stabilized, cage-confined, and MOF-derived, illustrating their distinct coordination environments and electron transfer pathways for generating high-valent metal species. These structural variations play a very important part in the catalytic activity and stability of the metal centers. As seen in Figure 2, ligand-stabilized metal center manganese (Mn) is coordinated by organic ligands (such as nitrogen N donor groups). These ligands create a stable coordination environment, helping to stabilize the single atom and influence its electronic structure and catalytic behavior. This approach allows precise control over the active sites, enhancing selectivity and efficient redox transformations in water treatment and other catalysis technologies. In Figure 2, the cage-confined, single metal atom is encapsulated within a molecular cage. The single metal atom (Mn) is confined within a molecular cage, which restricts its movement. This prevents aggregation while still allowing reactant molecules to access the active center. Such confinement enhances catalytic stability and catalytic selectivity, improves recyclability, and promotes selective transformations in catalytic pollutant breakdown in the AOPs process and supports resource recovery processes. As shown in Figure 2, in the MOF-derived structure, the single atom (Ru) is derived from the metal–organic framework (MOF), and after pyrolysis the structure maintains a coordination environment provided by the MOF linkers and metal nodes. The MOF-derived structure offers uniform dispersion of single atoms and high stability with high surface area. This MOF-derived structure makes them excellent for sustainable oxidation reduction reactions and in wastewater treatment. In addition, the second avenue to heterogeneous stabilization of the high-valence states is the SACs, which are prepared by a pyrolysis pathway.

Figure 2. A comparative schematic of (a) ligand-stabilized, (b) cage−confined, and (c) MOF−derived SAC architectures.

These materials can embed isolated metal atoms (Fe, Co, and Mn) into nitrogen-doped carbon lattices prepared with MOF-precursory units to afford robust M = N₄ coordination sites that permit persulfate or PAA activation to develop or sustain M(n + 2)=O higher-valence species [76]. The heterolytic O–O bond cleavage of oxidants such as PMS is encouraged by the electron pulling character of the carbon matrix that leads to a preference for two-electron transfer processes that produce higher-valent metal-oxo species rather than radicals [77]. The schematic illustrates (Figure 2) how different stabilization and confinement approaches to ligand coordination, cage encapsulation, and MOF derivation affect the stability, structural design, and catalytic efficiency. Table 2 shows the comparison of the performance of high-valent metal-based AOPs.

Table 2. Comparison of performance of high valent metal based AOPs.

AOP System	Target Pollutant	Degradation Efficiency (%)	Mineralization Efficiency (% TOC Removal)	Dominant ROS	Energy Intensity (kWh/m3)	Key Advantages	Key Limitations	Reference
Fe(VI)/UV	Sulfamethoxazole	98%	65%	Fe(V)/Fe(IV)	0.8	No radical scavenging; dual oxidation–coagulation	High Fe(VI) synthesis cost	[35]
Mn(V)/Periodate	Phenol	95%	70%	Mn(V)=O	0.5	High selectivity; low chloride interference	Requires ligand stabilization (e.g., NTA)	[62]
Co(IV)/PMS	Bisphenol A	99%	78%	Co(IV)=O	1.2	High oxidation potential; fast kinetics	Cobalt leaching at low pH	[78]
Cu(III)/PAA	Carbamazepine	97%	60%	Cu(III)	1.0	High electron efficiency (77%)	Limited oxidant stability	[28]
Fe(IV)/PMS-LDH	Sulfamethoxazole	96%	68%	Fe(IV)=O	0.9	Heterogeneous, reusable, neutral pH operation	Complex synthesis of LDH supports	[79]
Ru(VIII)/O3	Diclofenac	94%	82%	Ru(VIII), •OH	2.1	Complete mineralization	High cost; rare metal	[48]
UV/H2O2 (Radical AOP)	Atrazine	80%	45%	•OH	1.5	Mature technology; scalable	Radical scavenging by NOM; low selectivity	[80]

A fundamental gap in the current literature is the absence of a systematic review linking the structural stability of synthetic high-valent complexes to their performance in advanced oxidation processes (AOPs). Although parameters such as lifetime, turnover frequency (TOF), and coordination number are often reported, they have not been correlated with key AOP performance metrics. These include contaminant degradation rates, oxidant utilization efficiency, and sensitivity to matrix effects. Recent advances in ligand and framework design have contributed to improved stability. However, achieving molecular level control remains a major challenge for scalable and energy efficient water treatment applications. This limitation is largely due to the lack of well-established structure activity relationships (SARs). Although stabilized Fe(IV) in TAML systems demonstrates great reactivity with phenols, the method reported for its synthesis is carried out through multi step organic pathways with low atom economy, which opens up the possibility of concerns over cost and environmental impact [74].

Future work needs to fill the gap between synthetic chemistry and environmental engineering to provide predictive models that deliver ligand denticity/metal spin state/coordination geometry to AOP efficiency. Combining computational screening (DFT predicted redox potentials) with high-throughput experimental validation may both improve the pace at which next-generation stabilizing matrices, including bio inspired porphyrinoids or microbially produced ligands, can be discovered that balance reactivity, durability, and sustainability [32].

4. Synergistic Hybrid AOPs: Integration of High-Valent Metals with Photocatalysis, Sonocatalysis, and Fenton-like Systems

The growing demand for efficient, scalable, and sustainable water remediation technologies has driven the development of hybrid AOPs. In these systems, high-valent metal species are purposefully integrated with complementary energy sources (such as light, ultrasound, or electrochemical potential) to enhance oxidative performance. These synergistic systems overcome these constraints of stand-alone AOPs by promoting high-valent metal-oxo reagent generation, stability, and reactivity (Fe(IV), Mn(V), Cu(III), and Co(IV)) as well as effecting enhanced oxidant utilization and reducing secondary pollution [28,81]. The generation of high-valent metal species has been observed (as shown in Figure 3) in both homogeneous and heterogeneous low-valent metal catalyzed AOPs.

Figure 3. Schematic representation of the formation of high−valent metal species (Fe(IV), Co(IV), Mn(V), and Cu(III)) during the advanced oxidation processes (AOPs) (Reproduced with permission Copyright 2023, Elsevier) [28].

The different transition metals lead to different formation pathways. These pathways can be classified into one-step, as well as two-step, mechanisms (dependent upon different catalytic approaches) [28]. The recent developments have made it important to recategorize the hybrid AOPs in terms of the type of activation mechanism and the characteristics of high-valent metal participation, therefore generating a new categorization system [82].

To better understand different catalytic approaches, three-tier classification framework groups advanced the catalytic systems. (a) Type I (photo enhanced, using light to drive oxidation reactions), (b) Type II (sonocatalytically driven, where ultrasonic waves generate reactive species), and (c) Type III (multi oxidant Fenton-like approach, combining several reactive oxygen species). This framework offers a systematic approach to assess and compare different catalytic strategies for wastewater treatment. In Type I, the energy input generated by the photocatalysis process is used to promote electron transfer from materials like g-C₃N₄, SiO₂, TiO₂, CeO₂, and MoS₂ to the metal centers and enhance the regeneration of low-valent metal states such as Fe(II) and Co(II). As an example, photoexcitation electrons of a GO/g-C₃N4/MoS₂ composite system can be used to reduce Co(III) to Co(II), to activate PMS to produce Co(IV)=O, and improve the degradation of sulfamethoxazole with visible light to a great extent [83,84,85,86]. In this type of heterojunction, the charge carriers are spatially separated which inhibits charge recombination, boosting quantum efficiency and making functioning under solar irradiation possible. System Type II uses ultrasound to create localized hot spots (>5000 K) and micro turbulence in order to accelerate homolytic cleavage of oxidants (such as hydrogen peroxide H₂O₂ and peracetic acid PAA) and increase mass transfer to the catalyst surfaces. Sonolysis has been found to increase the rate of formation of Mn(V)=O species in Mn(II)/PAA systems, largely by the enhancement of the ligand exchange and accommodation of transient intermediates [32]. The degradation of bisphenol A was unusually high (98% in 30 min) in a hybrid sonolysis/Fenton/UV system, as compared to any single technique, because of both concomitant •OH generation and formation of Fe(IV) under ultrasonic cavitation [87]. Type III systems are the most modern category, where multiple chemical oxidants (H₂O₂, PMS, PAA) can be combined with Fenton-like catalysts to discriminate between high-valent metal pathways and radical pathways. As one case in point, the Fe(II)/PAA system produced Fe(IV) through the mode of a nucleophilic attack, whilst the addition of PMS changed the prevailing mode to producing Co(IV) or Fe(IV), based on the catalyst blend used [88]. Recently reported CN supported Fe SACs (Fe/CN) synthesized by a one-step thermal polymerization method for efficient MB treatment as a model pollutant. The experimental and theoretical analyses reveal that densely distributed Fe-N₄ sites drive their unique catalytic behavior. The efficient material (Fe/CN) achieved 95% removal within 25 min, significantly outperforming CN support, Fe₃O₄, and homogeneous Fe²⁺/Fe³⁺ catalysts. The reaction followed first order kinetics, highlighting the superior Fenton-like activity of Fe/CN. Catalytic efficiency was enhanced by increasing the Fe loading up to 1 mmol (Fe single atoms as active sites) but declined at higher loadings (this may be due to clustering). Notably, Fe/CN exhibited very low Fe leaching (0.015 mg/L), well below regulatory limits. This result highlights its high efficiency and environmental safety, making it a promising heterogeneous Fenton-like catalyst. Figure 4a–i presents the catalytic performance and reactive species analysis of the Fe/CN/H₂O₂ system. It also shows MB degradation, following pseudo first order kinetics, with the impact of the scavenger effect on degradation, as well as the system capacity to degrade DPBF. The ESR spectrum confirmed the generation of radicals (¹O₂ and •OH). The experiments with DMSO and phenol highlight their roles in MB degradation. Additionally, the conversion of PMSO to PMSO₂ demonstrates effective reactive oxygen species formation, which further highlighting the importance of the Fenton-like activity of Fe/CN [89].

Figure 4. (a) MB degradation efficiency, (b) pseudo first order kinetic graph, (c) effects of different scavengers, (d) degradation of 1,3−Diphenylisobenzofuran (DPBF), (e) ESR spectra, (f) effect of DMSO on MB degradation, (g) PMSO reduction and PMSO₂ generation, (h) ESR spectra, (i) effect of phenol on MB degradation (Reproduced with permission Copyright 2025, Elsevier) [89].

The evaluation of hybrid AOP systems requires a systematic method to optimize their operational efficiency. Nonetheless, there are problematic aspects of hybrid AOPs that include energy demand and toxic byproducts. Whereas high-valent metal pathways are more efficient as far as electron efficiency goes (77% in Cu(III)-based systems), the total energy consumption in hybrid systems (especially UV or ultrasound-based) may be cost-prohibitive on scale [90]. Also, partial mineralization has the potential to cause the buildup of transformation products whose ecotoxicity is unknown. Partial oxidation may turn out to be more persistent and have more genotoxic intermediates when compared with the parent compound, such as sulfonamide antibiotics [46]. Thus, the development in the future should include life cycle assessment (LCA) and high-resolution mass spectrometry (HRMS) as a live byproduct analytical technique.

A new technology is using artificial intelligence (AI) to optimize the catalyst–oxidant pairings and parameters of operation. Machine learning models trained using descriptors derived by DFT (d-band center, oxidation potential, coordination number) have begun to predict suitable combinations (CoCu₁₀/PMS or FeS₂/PAA) to pollutants with exceptional accuracy, in combination with experimental kinetics [91]. Figure 5 represents the AI-based decision matrix (to analyze and select the most effective treatment configuration) that is used to pick the best hybrid AOP settings, depending on the structure of contaminants, characteristics of water matrices, and energy input limitations. By applying this structured approach, this method allows the development of custom treatment plans as it combines both data-driven and sophisticated oxidation technologies.

Figure 5. Showing an example of an AOP configuration selection decision-making tool selected by an AI-generated contour according to contaminant structure, water matrix, and energy input constraint.

The influencing factor on next-generation hybrid AOPs is high-valent metal catalysis, which integrates with renewable energy and closed loop oxidant regeneration to enable enhanced efficiency and long-term sustainability. Table 3 shows comparisons of energy consumption (kWh/m³), mineralization points (% COD removal), and toxicity decrease (through bioassays) in a sample of hybrid systems. It should be acknowledged that the factors (efficiency, primary energy input, COD/TOC, dominant reactive species, key transformation byproducts, ecotoxicity change) presented originate from studies conducted under varied experimental conditions. Therefore, the comparison aims to illustrate general mechanistic behaviors and catalytic performance rather than provide direct quantitative equivalence.

Table 3. Energy consumption and byproduct profiles of hybrid AOP systems.

Hybrid AOP System	Primary Energy Input	Specific Energy Consumption (kWh/m3)	Mineralization Efficiency (% COD/TOC)	Dominant Reactive Species	Key Transformation Byproducts	Ecotoxicity Change (Post-Treatment)	References
Fe(II)/PAA/UV	UV-C (254 nm)	0.75	68%	Fe(IV)=O, Fe(V)=O	Hydroquinone, benzoquinone (from phenol)	Decreased (Microtox assay)	[92]
Co3O4/PMS/Sonication	Ultrasound (20 kHz)	1.8	75%	Co(IV)=O, SO4•−	Low MW carboxylic acids (oxalic, acetic)	Significantly decreased	[93,94]
Mn(II)/Periodate/O3	Ozone generation	2.1	70%	Mn(V)=O, •OH	Iodate (IO3−), aldehydes	Slight decrease	[29]
Fe(III)/PMS/Cu(II)	Chemical (no external energy)	0.05	55%	Fe (IV)=O, Cu(III)	Chlorinated organics (low yield)	Stable or slightly decreased	[55]
g-C3N4/Co-N-C/PMS/Visible Light	Visible light (λ > 420 nm)	0.4	72%	Co(IV)=O, non-radical electron transfer	Aromatic ring-opening products	Decreased	[95]
Conventional Fenton (Fe2+/H2O2)	Chemical (no external energy)	0.03	40%	•OH	Halogenated DBPs (trihalomethanes)	Increased (in halide-rich water)	[96]
UV/H2O2	UV-C (254 nm)	1.5	45%	•OH	Ketones, aldehydes, carboxylic acids	Variable	[97]

5. Sustainability Perspectives on the Environmental Fate and Transformation of High-Valent Metal Species

Although the chemical and biological reactivity of high-valent metal species in AOPs is increasingly well understood, their post-reaction environmental fate remains significantly underexplored [28,98]. Gaining detailed insight into their transformation pathways, stability, and final speciation following electron transfer is essential for evaluating both their sustainability in catalytic applications and their potential environmental risks [28,29,32]. The rich redox cascades that mediated reducing high-valent metals differ fundamentally, in structure, reactivity, and duration, from the persistent radicals or non-degradable organic pollutants to which they are often compared, and the consequences of these transformation products in the environment are scarcely mentioned in the existing literature [99,100]. This section brings together the discussion on the generation and transformation of byproducts’ compounds during oxidation processes from high-valent metal. It offers a comprehensive perspective on their reactivity, stability, and impact on the environment. Through this approach, this section links earlier system specific findings to a broader application for water quality management and environmentally sustainable remediation strategies.

Once the oxidation products of target contaminants are formed, high-valent metal ions are reduced by one-electron or two-electron transfer pathways in the formation of lower-valent metal ions and hydrolysis products. As an example, Fe(VI) is reduced successively to Fe(V), then Fe(IV), and finally Fe(III), which then can hydrolyze to form Fe(OH)₃ or ferrihydrite in neutral and higher pH environments [51,101]. Such Fe(III) (oxy)hydroxides are not inert waste products and have much coagulation capability and can adsorb or precipitate inorganic anions (phosphate, arsenate) and colloidal organic matter, thus helping clarify the water [97,102]. Such dual capability has nevertheless brought about an appreciable balancing act, as much as there are the advantages of Fe(VI) having combined coagulation and oxidation advantages [103], there are secondary water quality issues to consider, such as the resulting turbidity and increment of iron residuals, and even potentially troublesome byproducts created by the oxidation of the Mn(II), Mn(III), and so on [51]. Correspondingly, Mn(V) species can be produced in Mn(II)/periodate systems or Mn(II)/PAA systems and are quickly reduced to Mn(III) and Mn(II), and the Mn(III) is frequently stabilized by organic ligands, including nitrilotriacetic acid (NTA) or pyrophosphate [62,104]. Whereas the major form, Mn(II), is comparatively non-toxic and naturally commonplace in the aquatic environment, the intermediate, Mn(III), can act as a powerful oxidant or reductant depending on the surroundings, which could lead to redox cycling in other substances, such as Fe and C. Furthermore, there are long-term exposure bioavailability concerns over the mobilization of Mn oxides in sediments that could be a source of neurotoxicity in drinking water supplies and animal feeds if forms of Mn oxides are disproportionately converted to Mn(III) during formation under field conditions [105]. The unanswered, but critical, question is how the reduced metal species could be well recycled or regenerated in functional systems. Recent work indicates that electrochemical/chemical re-oxidation of Fe(III) to Fe(VI) is likely on anodic electrodes (Ni-Sb-SnO₂), which provides a route to catalytic closure [106]. Figure 6 demonstrates the life cycle of Fe(VI), its early electrochemical generation, usage in oxidation process, coagulation, and possible recovery.

Figure 6. Schematic overview of the Fe(VI) redox cycle in AOPs.

This diagram presents a detailed overview of the redox cycling mechanism of high-valent metal (iron species (Fe(VI)) utilized in AOP for the catalysis process. The process begins with the electrochemical formation of strong oxidant Fe(VI) that facilitates the breakdown of organic contaminants and during reactions converts them into intermediate oxidation states (Fe(V), Fe(IV), Fe(II), and Fe(III) hydroxides). This facilitation contributes to contaminant elimination by coagulation and adsorption. The Fe(III) can be regenerated to Fe(VI) by electrochemical or chemical reactions, establishing a closed loop system. This cyclic regeneration pathway enhances catalytic activity, contributes to the overall sustainability, conserves resources, and improves the efficiency of water treatment applications. This kind of process is energy intensive and is still limited to laboratories, and further improvements are required for industrial sectors.

Further complexities in the environmental fate of high-valent metal species arise from their interactions with natural organic matter (NOM) and constituents of real wastewater matrices. NOM acts both as a scavenger and a reductant, facilitating the microbial reduction in species (Fe(VI) and Mn(V)) by serving as an electron donor [32,107]. While such interactions may lower the concentration of highly oxidizing species available for pollutant degradation, they also reflect a form of natural attenuation that limits the persistence and mobility of these species. Radicals (•OH) are non-selective and react readily with halides and natural organic matter (NOM), forming halogenated byproducts, whereas high-valent metal-oxo species show minimal direct reactivity with halides under typical AOP conditions; however, oxidants (H₂O₂ and PMS) can indirectly generate reactive halogen intermediates (HOCl, Br₂•⁻) under acidic or high halide conditions [28,38].

The resistance to durability and the bioaccumulation of residual metal ions, especially Co(II) and Cu(II), should be taken seriously. As a result, future research will need to combine ecotoxicological measurements with material flow analysis to differentiate between catabolically recycled metals and effluent-derived metals [108,109]. Synthesizing the above, the environmental fate of high-valent metal species extends beyond their oxidative activity to include post-reduction speciation, coagulation phenomena, and regeneration dynamics. To enable their sustainable application in water treatment, it is essential to adopt a comprehensive perspective that integrates both reaction chemistry and environmental impact assessment.

6. High-Valent Metal AOPs in the Circular Economy: Toward Zero Wastewater Treatment

Enhancing circular water energy systems with advanced oxidation processing (AOPs) is a paradigm shift in both processes and a product-oriented (scarcity-based) treatment to resource-recovery-based treatment. High-valent metal-mediated AOPs, which are catalyzed by species including Fe(IV), Mn(V), and Co(IV), are well suited to drive this paradigm shift because of their rigor and selectivity, stability in complex matrices, and dual function versatility with respect to oxidation of the contaminant as well as the synthesis of a co-product [32,110,111]. In contrast to typical conventional radical-based pathways, whereby organics are mineralized blindly via the production of CO₂ and H₂O, high-valent metal mechanisms typically occur via controlled, step-wise oxidations in which functional groups are preserved, and hence the possibility to recover valuable intermediates or otherwise recycle waste streams. The formation of recoverable forms of inorganic metals and oxidized organic ligands by complexification and conversion of heavy metal–organic complexes, which are ubiquitous in industrial effluents (tannery effluents, electroplating effluents, and pharmaceutical manufacturing effluents), constitutes one of the primary benefits of high-valent metal AOPs [28,46]. As an example, Cr(III)-organic complexes, comprising more than 60% of chromium in tannery wastewater and highly recalcitrant to conventional treatment, may be broken down effectively through Fe(VI) or Mn(V) mediated oxidation to free Cr(III) that can then be precipitated and reused [32]. Equally, degradation of EDTA- or citrate-complexed Cu(II) removes the organic ligand and produces bioavailable Cu²⁺, which is recoverable by electrochemical deposition or adsorption [46]. This dual approach is in line with green chemistry postulates and principles by transforming hazardous waste into reusable material.

In addition to metal recovery, high-valent metal AOPs may provide a window of opportunity to valorize organic carbon. The specific oxidation of resistant organic pollutants (dyes, phenols, pharmaceuticals) is primarily proceeded by hydroxyl radical (•OH) induced reactions and high-valent metal mediated pathways. Reactive oxygen species attack electron-rich sites on the pollutant molecules, initiating successive steps (hydroxylation, dealkylation, and aromatic ring cleavage). These oxidation routes generate oxygenated compounds (aldehydes, carboxylic acids, and quinones), which hold potential value as valuable chemical feedstocks [6,30,32,38]. These processes simultaneously achieve pollutant degradation and create useful intermediates for sustainable production [112,113,114]. To illustrate this, low molecular weight organic acids, such as oxalic acid and acetic acid, are produced through the partial oxidation of bisphenol. Co(IV)=O can unlock the production of energy using microbial fuel cells [115]. Moreover, the coagulating potential of the Fe(III) hydroxides, produced as the end products of the Fe(VI) reduction reaction, allows the simultaneous removal of nutrients by means of phosphorus precipitation as vivianite or by means of its fixation to iron oxides, which represents a possibility of recovery of nutrients using wastewater [40]. Closed loop catalyst management is required to accomplish actual circularity. Although homogeneous systems are difficult to recover, heterogeneous platforms (including SACs, TMSs, and MOF-derived composites) are both highly reusable and have limited leaching. As an example, hierarchical reactors made of Co₃O₄ remain catalytically active and over 90 percent effective even after five cycles in the PMS activation process, exhibiting almost negligible cobalt leaching (<0.1 mg/L), and thus can be used over a long period of time [95]. SnO₂-Sb based anodes have emerged as promising materials for electrochemical wastewater treatment (due to their high oxidative potential and stability). During electrochemical operation, the incorporated tin (Sb) and antimony (Sb) elements can shift to higher oxidation states (Sn⁴⁺/Sn⁶⁺, Sb⁵⁺/Sb⁶⁺), generating surface-bound metal oxygen species (that serve as potent oxidants) [116,117,118]. These active sites drive the generation of reactive oxygen species (O₃), hydroxyl radicals (•OH), and H₂O₂, collectively (synergistic effects) enhancing the breakdown of organic pollutants. These electrodes offer scalable, sustainable solutions for degrading pollutants in industrial effluents [19,118,119]. Together with renewable energy sources, such as solar or wind-driven electrolyzers, this solution enables energy self-sufficient applications in the field of water treatment with a small carbon footprint [28]. Despite recent advancements, the widespread implementation of high-valent metal-based AOPs remains limited to the pilot scale. This constraint is primarily due to the lack of rigorous pilot-scale validation and comprehensive techno economic assessments. Most existing studies are confined to laboratory settings using model pollutants, with minimal data available on mineralization efficiency, byproduct toxicity, and the overall scalability of these processes [32]. Figure 7 describes a closed loop circular system that combines the high-valent metal AOPs with catalyst regeneration, recycling of the metal, and valorization of the organic compounds.

Figure 7. Closed loop circular system for advanced oxidation and resource recovery.

7. Nano-Engineered, Bioinspired, and Machine Learned High-Valent Systems

The next-generation high-valent metal-based AOPs at the interface of nano engineering, bioinspired design, and artificial intelligence will enter a phase where they can be explored and applied beyond an empirical catalyst design, which has been a major approach towards their development. As the limitations of trial and error approaches continue to be unveiled, scientists will increasingly embrace approaches that are predictive and rational in nature, thus allowing a much greater control over the generation, stability, and reactivity of high-valent redox-active compounds (Fe(IV)). These developments offer not only improved efficiencies for degradation processes, but also tunable selectivity, reduced energy consumption, and increased scalability to real world, strip water treatment.

A systematic revolution from these developments is the application of machine learning (ML) and quantitative structure activity relationship (QSAR) models to predict and optimize the redox behavior of the metal centers to coordinate metal ions’ high valency. While classical approaches, such as density functional theory (DFT), offer valuable mechanistic insights grounded in empirical data, they are often computationally prohibitive for high-throughput screening. In contrast, ML algorithms trained on datasets encompassing metal coordination environments, ligand electronegativity, oxidation states, and reaction kinetics can rapidly predict optimal catalyst oxidant combinations for the selective degradation of target pollutants [60]. Specifically, a DFT-augmented ML model was developed recently to predict the formation energy and oxidation potential of Fe(IV)=O with an overall precision of >90 percent within nitrogen-doped carbon matrices to enable the design of tailored redox window catalysts [60].

Bioinspired mimicry also enlarges the design space by adopting nature to come up with evolutionary solutions to selective oxidation. Multicopper oxidases of manganese-oxidizing bacteria (MnOB) and cytochrome P450 systems use carefully tuned metal chelates to generate and stabilize high-valent intermediates in the very non-oxidative conditions encountered in the ambient environment [120,121]. Figure 8 shows a technology known as radar forecast that displays the maturity of select innovations such as ML guided catalyst design, single-atom systems, and bio hybrid reactors. It is based on already existing publication rates/trends and pilot scale confirmations of efficacy. Figure 8 also illustrates a schematic representation of an AI-driven matrix conceptual framework for a decision based system designed to select the most effective hybrid AOP configurations. The conceptual framework model considers several influencing parameters factors (chemical structure of the target contaminant, characteristics and properties of the water matrix). These variables parameters are assessed through a computational decision mechanism (AI-based decision engine) that interprets the most suitable hybrid AOP configuration. According to specific treatment requirement objectives, the AI-based decision engine system may recommend integrated techniques (O₃ combined with ultraviolet (UV) irradiation, electrochemical oxidation processes, the Fenton process coupled with photocatalysis, and plasma-based AOPs). This systematic intelligent selection approach improves treatment technology, reduces energy requirements, and supports sustainable water purification by adapting AOP designs to real environmental conditions.

Figure 8. Technology radar forecast for emerging AOP innovations (2025–2035).

8. Challenges and Future Prospective

Although high-valent metal-based advanced oxidation processes (AOPs) show great promise for treating contaminated water, several real-world challenges still stand in the way of their practical use. One major issue is that high-valent metal species like Fe(IV), Mn(V), and Co(IV) are often short lived and unstable in water. They tend to break down or lose their effectiveness quickly unless carefully stabilized. While techniques like ligand design, supramolecular confinement, and single-atom catalyst (SAC) engineering have helped extend their stability, applying these methods at scale is still costly and technically complex.

Despite advancements, challenges still exist in scaling more complex engineered systems, and, thus, the selection of high-valent metal SACs for AOPs will likely remain limited. SACs are often made via high-temperature pyrolysis, which often induces agglomeration of the metalloid and non-uniform distributions of sites. Secondly, the long-term stability of bioinspired ligands in oxidative and acidic environments will need to be ascertained.

Another hurdle is the balance between selectivity and versatility. These metal-based systems are highly effective at targeting certain types of pollutants, particularly electron-rich ones, but may struggle to degrade tougher or more water repellent compounds.

In some systems, metal ions, for example, cobalt or copper, can transfer into the water. The presence of these ions can create potential issues for secondary pollution or compliance with environmental regulations. The demand for energy gives a reason for concern. Characteristically, these approaches depend on numerous types of energy input, such as ultrasound, electricity, or UV light. When using an energy input that is not derived from a renewable source, these contributions are not only expensive, but they can also add to our carbon footprint.

Moreover, there are harmful byproducts that can be produced by incomplete pollutant degradation (aldehydes, quinones) that can be harmful to ecological health. Scientifically, the clear research gaps are the absence of clear structure activity relationships that connect the design of a catalyst to its effectiveness, which is based on trial and error and not knowledge-based design. Scientists are regularly left to design better systems.

The practical application of AOPs is often limited due to the recyclability and long-term stability. Multiple cycles can cause structural alterations, loss of active sites, instability, and catalyst fouling in high-valent metal complexes, which can further reduce performance. Designing robust and regenerable catalysts is essential to maintain activity for water treatment processes.

Catalytic oxidation by high-valent metals is a promising and sustainable approach for water remediation and other industrial applications. Future research should aim to focus on the detailed reaction mechanisms of high-valent metal species, enhance catalyst stability, and design effective regeneration methods for the real wastewater environments on the industrial and domestic scale. Integrating these catalytic oxidations into photocatalytic, electrochemical, and synergetic treatment systems can enhance oxidation efficiency and energy utilization in industrial sectors.

Most research studies to date are still accomplished in a laboratory using simple pollutants and a controlled environment. There is a clear gap in assessing the efficiency of these systems at pilot scale in complex real wastewater to know how these systems act in actuality. To overcome these gaps, pilot scale studies and renewable energy will be needed. Until these practical, financial, and environmental questions are addressed, the potential of high-valent metal AOPs remains just out of reach.

Future investigations should focus on developing synergetic (bimetallic and hybrid catalytic) systems that combine high-valent metal reactivity with carbon-based and semiconductor supports. Although laboratory outcomes have been promising, challenges (limited scalability data and a lack of real wastewater testing) continue to hinder practical applications.

Advancing this field directly supports the United Nations (UN) Sustainable Development Goals (Goal 6: Clean Water and Sanitation, Goal 12: Responsible Consumption and Production), supporting a circular and sustainable environmental future. Lastly, lab-level advancements will need to be relevant to the field in future studies by collectively using accelerated aging tests to demonstrate operational conditions with in situ characterizations (operando XAS or EPR), and data-driven modeling techniques such as a life-cycle assessment. Overall, the merging of nano engineering, biology, chemistry, physics, and data-based modeling is evolving the landscape of AOPs of high-valent metals. By using prediction over replication, and innovation over quick mechanisms, we can achieve the elusive answer to sustainable smart systems of water treatments.

9. Conclusions

Metal agents with high oxidation states, such as Fe(IV), Mn(V), Co(IV), and Cu(III), are providing great advances in AOPs that are now being driven away from the radical-based systems of the past, and toward more selective, effective, and safer strategies. These metals will undergo a different oxidative pathway to break down persistent, electron-rich contaminants by transitioning through aggregate transformations, such as oxygen-atom transfer and hydride abstraction, instead of relying on generating unwanted products. New advances in stabilizing these reactive species, with a switch towards the direction of the future in the appearance of distributed reactive sites with ligand design, and the use of single-atom catalysts and other metal–organic frameworks, will allow for unique innovations in treating complicated water contaminants. Recent reviews have highlighted that the integration of high-valent metal species with energy-assisted processes, such as photocatalysis, sonocatalysis, and electrochemical regeneration, not only enhances treatment performance but also supports circular models of water treatment through improved resource recovery and sustainability. These hybrid systems do more than just degrade pollutants; they also enable the recovery of valuable resources, such as nutrients and trace metals, turning waste into something useful. However, several hurdles still need to be addressed. The short lifespan of high-valent species in water, the risk of metal leaching, the energy demands of certain systems, and the lack of clear links between the catalyst structure and performance are ongoing challenges. Moreover, most current research is limited to lab-scale experiments using model pollutants, leaving a gap in our understanding of how these systems perform in real, complex wastewater environments. Looking to the future, promising solutions are emerging. Bioinspired catalysts and microbially derived ligands offer potential for stabilizing high-valent species under mild conditions, while data-driven approaches like ML could speed up the discovery of better catalyst–oxidant combinations. These innovations may help to overcome the current limitations and bring these technologies closer to practical, large-scale use. Ultimately, by treating high-valent metals not just as one-time-use oxidants but as reusable, central players in catalytic cycles, we move closer to achieving sustainable and circular water treatment systems. With the strategic integration of chemical, engineering, and environmental principles, AOPs hold significant potential to address global challenges in water pollution control and resource recovery.