1 State Key Laboratory of Environmental Chemistry and Ecotoxicology，Research Center for Eco-Environmental Sciences，Chinese Academy of Sciences，Beijing 100085，China
2 College of Geography and Environmental Sciences，Zhejiang Normal University，Jinhua 321004，China
3 Institute of Environment and Health，Hangzhou Institute for Advanced Study，University of the Chinese Academy of Sciences，Hangzhou 310024，China
4 College of Resources and Environment，University of the Chinese Academy of Sciences，Beijing 100049，China
Polyphenolic compounds are a class of naturally occurring bioactive substances widely found in the environment. Their characteristics，such as low toxicity，low cost，and broad availability，make them become to be widely used chelating agents，reducing agents，and capping agents for treating typical pollutants in water. Currently，polyphenols are extensively used in advanced oxidation processes （AOPs） through the coupling of common transition metal ions and peroxides. However，the chemical mechanisms of polyphenolic substances in water pollution remediation still lack systematic conclusions. This study reviews and summarizes the compositions of homogeneous and heterogeneous systems containing polyphenolic compounds，as well as the pro-oxidant，antioxidant，and chelating-reduction effects exhibited by polyphenols within these systems. It explains the main active species generated by polyphenolic substances under different systems from both radical and non-radical perspectives，along with the corresponding mechanisms for the removal of water pollutants. The dual role of polyphenols as natural redox mediators （RMs） in constructing complex catalytic systems is emphasized，and the effects of external energies such as light，heat，electricity，ultrasound，and plasma on the reaction mechanisms and pollutant degradation effectiveness in these systems are described. Finally，the article looks ahead to the future development directions of polyphenolic compounds in the field of water treatment.
Contents
1 Introduction
2 H2O2/PS/PAA activation
2.1 ROS of H2O2/PS/PAA
2.2 Polyphenols/Fe（Cu） ions/peroxide systems
2.3 Chelation and reduction of polyphenol-metal ions
Wuyuxin Zhu, Linjun Qin, Guorui Liu.
The Mechanisms of Homogeneous and Heterogeneous Reactions Involving Polyphenolic Compounds in the Water Treatment Process.
Progress in Chemistry, 2025, 37(4): 479-507 DOI:10.7536/PC240606
Natural polyphenols are a class of bioactive natural substances existing in plants, fruits, vegetables, or grains. More than 8,000 types of natural polyphenolic compounds and their derivatives have been isolated from nature. Chemically, polyphenols are organic compounds in which two (or more) hydroxyl groups directly connect to benzene rings or fused-ring aromatic hydrocarbons. In fact, polyphenolic compounds not only include molecules with polyphenol structures but also those containing a single phenolic ring, such as phenolic acids and phenolic alcohols. Research on polyphenols originated from the discovery, extraction, and application of plant polyphenols. Plant polyphenols, also known as plant tannins, are major and commonly existing secondary metabolites in plants and one of the important sources of polyphenolic compounds. Initially, they were applied in the leather industry. Later, it was found that polyphenols also have extensive application value in fields such as food preservation, drug delivery, material synthesis, and energy development. Polyphenolic compounds can be classified according to their sources, biological functions, and chemical structures. Based on their sources, they can be divided into plant polyphenols, animal polyphenols, fungal polyphenols, etc. Further classification can specify certain categories or even specific substances. For example, plant-derived polyphenols can be further divided into tea polyphenols, licorice polyphenols, grape polyphenols, pomegranate polyphenols, apple polyphenols, etc. The most common classification method is based on differences in benzene ring structures, carbon atom numbers, and group compositions. Accordingly, polyphenolic compounds can be categorized into phenolic acids, stilbenes, flavonoids, polyphenolic amides, lignans, and other polyphenolic compounds (e.g., coumarins, xanthones, tannins, etc.). Among them, flavonoids can be further divided into flavanols, flavanones, flavonols, anthocyanins, flavones, isoflavones, neoflavonoids, and chalcones based on the position and number of substituents. Polyphenolic compounds can be extracted from natural sources such as fruits, vegetables, legumes, cereals, tea, coffee, and plants. A single substance often contains different types of polyphenols, and the content of various polyphenols usually differs significantly.
The classification and basic physicochemical properties of common natural polyphenols in nature have been summarized in Table 1. Table 1 shows that polyphenolic compounds generally exhibit poor volatility in aqueous solutions and tend to dissolve more readily in organic solvents. Compared with other polyphenols, phenolic acids have slightly lower logarithms of octanol-water partition coefficients (Log Kow). Among them, chlorogenic acid, tannic acid (TA), and ellagic acid (EA) show negative Log Kow values, indicating higher water solubility for these three phenolic acids. This is because the unique carboxyl groups in phenolic acids can ionize under both acidic and alkaline conditions. Rutin, a flavonol compound, demonstrates relatively high solubility in water. Despite its complex structure, the O atom in the hydroxyl group on the C3 ring is substituted by glycosylation, providing additional hydrophilic hydroxyl groups. Furthermore, phenolic amines, especially levodopa and dopamine, exhibit strong water solubility. These compounds have low molecular weights, and their active functional groups—phenolic hydroxyl and amine groups—are easily ionized in water. The additional carboxyl group in levodopa not only enhances its water solubility but also leads to deprotonation under acidic conditions. Coumarins such as esculetin and daphnetin both have Log Kow values near zero, indicating similar affinity at the water-organic solvent interface and ionization under alkaline conditions; the slight differences in their pKa values arise from different substitution positions of the phenolic hydroxyl groups. Some derivatives of hydroxybenzoic acid (HBA) and hydroxycinnamic acid (HCA), such as hydroxytyrosol, protocatechualdehyde, and 3,4-dihydroxyacetophenone, display hydrophilicity comparable to that of some simple phenolic acids. 3,4-Dihydroxyphenylglycol has a Log Kow value of -0.53, showing greater tendency to dissolve in the aqueous phase, with considerable solubility in aqueous solution. Flavones, isoflavones, and flavonols are generally weakly acidic, while neoflavonoids, flavanones, flavanols, flavanonols, chalcones, and dihydrochalcones tend to ionize more easily under neutral to slightly alkaline conditions. Anthocyanins are a special class of compounds that lack pKa values due to the presence of oxonium ions. Overall, apart from simple phenolic acids and phenolic amines, as well as certain flavonoid compounds, most polyphenols have pKa values within the neutral to alkaline range.
Advanced oxidation processes (AOPs) rely on the use of strong oxidants. Commonly used strong oxidants include hydrogen peroxide (H2O2)[28], persulfate (PS) (including peroxymonosulfate (PMS) and peroxydisulfate (PDS))[29], peracetic acid (PAA)[30], and HClO[31]. Various strategies have been adopted to activate these strong oxidants, including thermal activation[32], light irradiation (e.g., ultraviolet light[33-34], visible light[35-36], LED[37]), electrochemical methods[38-41], ultrasound[42-44], alkaline conditions[45], ozone[46], mechanical effects[47], electron shuttles[48-51], metal ions[52], carbonaceous materials[53], reductants and chelating agents[54-56], and organic co-catalysts[57]. Among these approaches for activating strong oxidants, transition-metal catalysis is commonly employed in AOPs. Trace amounts of transition metal ions are present in natural water bodies, industrial wastewater, and urban sewage. Water treatment processes based on •OH, SO4•-, RO•, etc., can proceed spontaneously without additional energy input. However, during the reaction, the reduction rate of transition metal ions (Fe(III) → Fe(II), 0.002–0.01 M-1·s-1) is much lower than their consumption rate (Fe(II) → Fe(III), 76±1.9×105 M-1·s-1), which represents the rate-limiting step in the activation process of strong oxidants[58]. To enhance the reduction rate, researchers have introduced reductants (RAs) and other electron donors to promote the reduction of transition metal ions, thereby inducing the formation of reactive species[59]. Besides carbonaceous materials and zero-valent metals, both naturally occurring and synthetic reductants that reduce transition metal ions are also receiving increasing attention. Examples include hydroxylamine (HA)[55,60-61], amino acids[62-63], pyrophosphate[64], oxalic acid[65-66], citric acid[67], oxaloacetic acid[68], syringaldehyde[69], picolinic acid[70], ascorbic acid (AA)[71], natural polyphenols, and other substances[72-73], which have already been applied in water treatment research. Notably, naturally occurring polyphenolic compounds containing catechol or galloyl structures not only exhibit strong antioxidant properties but also demonstrate excellent chelating capabilities similar to those of ethylenediamine-N,N′-disuccinic acid (EDDS), ethylenediaminetetraacetic acid (EDTA), and nitrilotriacetic acid (NTA), yet with significantly lower toxicity compared to these conventional chelating agents[74-75].
At present, the identified characteristics of polyphenolic compounds generally include oxidation polymerization, coordination complexation, adhesion, biocompatibility, free radical scavenging, light absorption, antioxidant activity, pro-oxidant activity, antibacterial properties, anti-inflammatory effects, anticancer properties, antiviral effects, and radiation protection[76-80]. Additionally, identical polyphenols can simultaneously exhibit both antioxidant and pro-oxidant properties, which specifically depend on concentration and free radical sources[81]. The chelating, free radical scavenging, antioxidant, and pro-oxidant functions of polyphenols are all manifested during the removal of typical water pollutants; however, no article has summarized these aspects. A significant amount of research focuses primarily on the use of polyphenols to enhance metal ion solubility via chelation-reduction properties and their reactions with common peroxides (H2O2, PS, PAA) and prevalent metals (Fe, Cu). In contrast, studies regarding novel oxidants, other metals applied in water pollution control, self-initiated promotion or inhibition effects of polyphenols, and the influence of external energy remain relatively scarce. Besides increasing the solubility of metal ions, the chelation-reduction effect of polyphenols can also facilitate the transition of solid metals from heterogeneous to homogeneous phases, a function that is similarly rarely mentioned. Reaction mechanisms highlighted by the pro-oxidant and antioxidant properties of polyphenols vary according to reaction systems and conditions. However, previous studies have not systematically categorized or summarized the catalytic mechanisms displayed by polyphenols within these fields. This paper emphasizes the pro-oxidant role of polyphenolic compounds in the "polyphenol-pollutant-other substance" system, which is expected to deepen understanding of the homogeneous and heterogeneous reaction mechanisms involving polyphenols in water treatment processes.
2 Activation of Hydrogen Peroxide/Persulfate/Peracetic Acid
AOPs are a new type of water treatment technology based on the generation and utilization of strong oxidants or reactive oxygen species, capable of effectively degrading and removing various recalcitrant organic and inorganic pollutants. Currently, commonly used AOPs include ozone-based advanced oxidation processes (Ozone-based AOP), UV-based advanced oxidation processes (UV-based AOP), sulfate radical-based advanced oxidation processes (Sulfate radical-AOP, SR-AOP), electrochemical advanced oxidation processes (Electrochemical AOP, eAOP), catalytic advanced oxidation processes (Catalytic AOP, cAOP), physical advanced oxidation processes (Physical AOP, pAOP), and other advanced oxidation processes[82]. Among them, cAOP includes Fenton, photo-Fenton, and UV/catalyst; pAOP includes electron beam, ultrasound, plasma, and microwave[82]. Fenton/Fenton-like and SR-AOP have been more frequently reported in AOPs where polyphenols are applied to wastewater treatment, while UV-based AOP and eAOP have been slightly studied, and reports on ozone-based AOP and pAOP are rare. In AOPs, the chemical structures and natural sources of common polyphenolic compounds used for the removal of typical water pollutants are shown in Figure 1.
2.1 Induced Production of Reactive Oxygen Species via Hydrogen Peroxide/Persulfate/Peracetic Acid
•OH is the most common ROS in aqueous solutions, capable of non-selectively reacting with compounds[72]. Whether under natural conditions or in artificially constructed reaction systems, the generation of •OH is fundamental and widespread. In aqueous systems, •OH can be produced through several methods: nitrate (NO3-)/UV, nitrite (NO2-)/UV, H2O2/UV, ozone, Fenton-like/Fenton reactions, photo-Fenton and other Fenton reactions (including electro-Fenton systems, high-frequency ultrasound, zerovalent iron (ZVI) technology), vacuum ultraviolet (VUV) irradiation, and heterogeneous photocatalysis[83]. The Fenton or Fenton-like reactions are commonly employed in wastewater treatment to generate •OH. However, these reactions require acidic conditions (pH<5), and the instability of H2O2 at ambient temperatures limits its application. SR-AOP is a novel advanced oxidation process; its basic principle involves activating PMS or PDS to form SO4•-, which has a broader pH adaptation range (2–8), better selectivity, stronger oxidation capacity (2.6–3.1 V vs NHE), and longer lifetime (30–40 μs), thereby replacing •OH for removing persistent pollutants[45,84–85]. In addition to directly generating the aforementioned SO4•-, SR-AOP can also produce •OH via SO4•-[86]. PAA exhibits strong oxidizing, disinfecting, and sterilizing capabilities with fewer byproducts[87]. Due to the lower O–O bond dissociation energy of PAA (159 kJ·mol-1) compared to that of H2O2 (213 kJ·mol-1) and PS (317 kJ·mol-1), PAA can be more easily and efficiently activated using ultraviolet light, ultrasound, metal ions, and heterogeneous catalysts to generate various oxidative radicals such as •OH and RO•[88–90].
2.2 Formation of Polyphenol/Iron or Copper Ion/Hydrogen Peroxide System
In recent years, numerous studies have indicated that these natural antioxidants can exert pro-oxidant effects in the presence of metal ions[91-92]. Transition metal ion-activated peroxide reaction systems involving natural polyphenols are generally categorized into three types: polyphenols/Fe (Cu)/H2O2, polyphenols/Fe (Cu)/PS, and polyphenols/Fe/PAA systems. As the most common peroxide, H2O2 has been studied for removing contaminants such as bisphenol A (BPA), methylene blue (MB), alachlor, and propranolol (Inderol) within the polyphenols/Fe (Cu)/H2O2 system[93-96]. Light radiation enhances the oxidative capacity of Fenton's reagent; utilizing this synergistic effect significantly promotes contaminant removal in the polyphenols/Fe (Cu)/H2O2 system, thereby reducing H2O2 consumption and improving the reduction rate of Fe(III) to some extent. Polyphenols also find applications in eAOPs; as shown in Fig. 2a, within the solar photoelectron-Fenton (SPEF) system, polyphenols act as natural complexing agents and reaction enhancers for the photocatalytic removal of four typical pharmaceuticals and personal care products (PPCPs) from raw municipal wastewater (MWW)[97]. Because CaO2 is more stable under normal temperature conditions and reacts with water after dissolution, transforming into H2O2 while slowly releasing active oxygen, some studies have used CaO2 as a sustained-release oxidizing agent for pollutant removal[98-99]. The addition of CaO2 can accelerate iron ion reduction and O2•- generation, regulating radical types and concentrations[100]. Even if no H2O2 is initially added to the system, its generation during subsequent reactions can occur due to the presence of dissolved oxygen.
Polyphenols also require iron or copper ions as mediators to react with PS. During the activation of PS, a series of reactive radicals are generated, such as SO5•-, S2O8•-, SO4•-, HO2•/O2•-, and •OH. Radical quenching and trapping experiments have confirmed that SO4•- and •OH are the main ROS produced after PS activation and are typically responsible for oxidizing and removing pollutants. The advantages of SO4•-, including its long lifetime, strong oxidation capacity, and wide pH range, allow combinations with Fe ions, MnO2, high-valent metal ions, semiconductor materials, carbonaceous materials, mesoporous materials, EDTA, hydroxylamine, L-cysteine, oxalic acid, ascorbic acid, etc., to accelerate the rate-limiting steps in the PS activation process and enhance the overall oxidation capacity of the system. Certainly, polyphenolic compounds are often added as natural RAs into metal ion/PS systems to dissolve precipitated hydroxides and maintain the metallic ions in their ionic state, ensuring that these metal ions can still return to their reduced forms while interacting with and activating HSO5- or S2O82-, thus maintaining long-term stable operation of the system. External energies such as light irradiation, electrocatalysis, heating, ultrasound, and ball milling are also commonly applied to PS-containing systems, serving as common approaches to reduce the activation energy of PS. In current polyphenol/Fe (Cu)/PS systems, combinations with light irradiation, electrocatalysis, and others have already been reported; Figure 2b shows a schematic diagram of a typical electrocatalytic system involving polyphenols. The introduction of these external energies is crucial, significantly accelerating the redox rates of Fe(III) or Cu(II) and reducing the time required for pollutant removal[101-102]. Additionally, solid catalysts containing metals have also been used in polyphenol-present PS systems instead of transition metal ions, which facilitates catalyst recovery and reuse. A summary of the current research status on polyphenols/Fe(Cu)/H2O2 and polyphenols/Fe(Cu)/PS systems is presented in Table 2 and Table 3, respectively, summarizing Fenton/Fenton-like reactions and persulfate-based reactions involving polyphenols.
There are few studies on the involvement of polyphenols in activating PAA to generate ROS, and according to current research findings, alkoxyl radicals (RO•) such as CH3C(O)O• and CH3C(O)OO• have minimal impact on contaminant removal. RO• radicals are often consumed through mechanisms like SET and HAT [103]. Since the PAA reagent itself contains a certain amount of H2O2, the main reaction still involves the attack of contaminants by •OH radicals generated from H2O2.
In the system where the aforementioned peroxides exist, polyphenols first chelate and reduce Fe(III) or Cu(II), forming Fe(II) or Cu(I), which subsequently undergo redox reactions with H2O2, PMS/PDS, or PAA in the presence of molecules or ions such as H2O, OH-, and O2. Through processes of radical generation, transfer, and quenching, reactive species including •OH, SO4•-, HO2•/O2•-, 1O2, CH3C(O)O•, and CH3C(O)OO• are ultimately produced. Meanwhile, certain products formed after some polyphenols are attacked by radicals can also participate in the chelation and reduction of transition metal ions[104].
2.3 Chelation and Reduction Effects of Polyphenols-Metal Ions
The actual coordination of polyphenol-metal ions is related to the structure of polyphenolic ligands, the type and valence state of central metal ions, the ratio of metal ions to polyphenols, and changes in environmental pH. Taking iron ions as an example, this paper summarizes the chelation and reduction processes of iron ions with common polyphenolic compounds, as shown in Figure 3.
The ligands that effectively chelate metal ions are deprotonated catechol and gallic acid ester groups in polyphenols. Iron ions possess an octahedral geometry, and under ideal conditions, the catechol and gallic acid ester groups coordinate with iron ions in a 3:1 ratio, as shown in Fig. 3a[105]. Stability constants K can be used to describe the binding stability of one or multiple polyphenolic compounds with metal ions in aqueous solutions. Each deprotonated catechol group of a polyphenol ligand binding with a metal ion yields a K value, and the overall stability constant βn=K1×K2×…×Km (m represents the number of protonated catechol groups; n is the valence state of the metal ion, i.e., Mn+). The logarithmic values (Log) are commonly employed for description, such as Log K1,2,…,m and Log β, hence Log βn=Log K1+Log K2+…+Log Km[105]. When a single catechol coordinates with Fe(II) (n=2), Log K1=7.95, which is much lower than that of coordination with Fe(III) (n=3), where Log K1=20.01[106]. For quercetin (QCR) coordinating with Fe(II), Log K1 and Log K2 are 9.44 and 3.86 respectively, thus Log β2=13.3; when coordinating with Fe(III), Log β3=44.2[105,107]. Rutin, a glycoside form of QCR, has a Log β value of 44.1, indicating that the influence of bulky substituents on the overall stability constant can be negligible[107]. These results indicate that both the valence state of metal ions and the number of polyphenol ligands involved in binding affect the β value. This phenomenon arises because deprotonated polyphenolic ligands are hard Lewis bases, while Fe(III) is a hard Lewis acid, resulting in large β values upon their combination; conversely, Fe(II) is a soft Lewis acid, which does not stably bind with hard Lewis bases, leading to smaller β values. Therefore, polyphenolic ligands stabilize Fe(III) more strongly than Fe(II)[108].
The same polyphenol ligand can coordinate with both Fe(III) and Fe(II), forming Fe(III)-monomeric polyphenol complexes and Fe(II)-monomeric polyphenol complexes[109]. Between the same polyphenol ligand and the same metal ion, multiple coordination modes may also occur. For example, when gallic acid (Gallic acid, GA) coordinates with Fe(III), one molecule of GA can coordinate with Fe(III), as well as two molecules of GA can simultaneously coordinate with Fe(III), forming two and four coordination bonds, respectively[102]. Additionally, various coordination modes may exist between the same polyphenol ligand and iron ions of different valence states. For instance, Fe(III) forms three coordination bonds with the vicinal phenolic hydroxyl groups on the D ring of two molecules of epigallocatechin gallate (Epigallocatechin gallate, EGCG); while two Fe(II) ions form four coordination bonds with the vicinal phenolic hydroxyl groups on the B ring of two molecules of EGCG (see Figure 3b)[110]. These diverse coordination modes depend on the protonation and deprotonation levels of the polyphenol ligand, a parameter closely related to environmental pH values, which can cause discrepancies between actual and theoretical coordination numbers. Changes in solution color can provide an initial indication of acidity or alkalinity and the coordination ratio between polyphenol ligands and metal ions[105].
In the polyphenols/Fe ions/peroxide system, catechol and GA first bind with equimolar Fe(III), during which electrons transfer from the phenolic hydroxyl oxygen to Fe(III), forming intermediate semiquinone radicals (SQ•-) and Fe(II). SQ•- can reduce another equivalent of Fe(III), ultimately producing benzoquinone (BQ) and Fe(II)[102]. While reducing Fe(III) to Fe(II), the polyphenol ligands themselves are oxidized to SQ•-[93]. At lower pH values, SQ•- becomes protonated, forming neutral ligands, resulting in higher crystal field stability of Fe(II)-monophenolic complexes compared to Fe(III)-monophenolic complexes. Under conditions of higher pH, especially when pH > 7, Fe(III)-bisphenolic or Fe(III)-trisphenolic complexes tend to form, leading to an increase in total stability constant β, thereby suppressing the reduction of Fe(III)[111]. Polyphenols play a pro-oxidative role in inhibiting the reduction of Fe(III); however, they are detrimental to subsequent reactions involving peroxides. Moreover, overall in the system, polyphenols inhibit the final generation of ROS, thus exhibiting antioxidant effects. GA can also directly reduce two molecules of Fe(III) to produce BQ without forming intermediates, as shown in Figure 3c[110].
For structurally complex polyphenolic compounds, in addition to the aforementioned functional groups, they often contain alcohol hydroxyl groups. Each oxidation step of alcohol hydroxyl groups requires only one molecule of Fe(III), and does not form SQ•-. Moreover, O2 can assist in the oxidation of alcohol hydroxyl groups and SQ•-, and the generated O2•- can subsequently reduce Fe(III), as shown in Figure 3c[93]. O2 can also promote the auto-oxidation of Fe(II)-polyphenol complexes to rapidly generate Fe(III)-polyphenol complexes, and the oxidation rate of gallate salts is faster than that of catecholate salts (Figure 3d)[105]. Various methods of reducing Fe(III) facilitate the production of more Fe(II) to activate peroxides, while also preventing precipitation under near-neutral conditions.
2.4 Non-radical Reaction
In the same reaction system, although similar polyphenol precursors and catalysts are present, the active species generated in response to different pollutants and external reaction conditions may vary. However, the primary active species responsible for pollutant degradation within this system might not be consistent. A typical example is the polyphenols/Fe (Cu)/PS system, where the reactive oxygen species (ROS) generally considered to dominate pollutant degradation are SO4•- and •OH[112-114]. Nevertheless, in recent years researchers have found that certain non-radical reaction pathways—for instance, valence state transitions of transition metal ions[31,115], 1O2[116-118], and oxygen vacancies (OVs)[119-120]—also play significant roles in pollutant removal[121].
Recent studies have shown that Fe(IV), detected through PMSO probe experiments in the Fe(II)/PDS system, is the primary reactive species rather than the commonly assumed SO4•-[122]. Previous research has confirmed that Fe(IV) can oxidize PMSO to PMSO2, whereas PMSO is converted into biphenyl products by SO4•- (reactions (1)-(2))[122]. In the Fe(II)/PDS system, Fe(VI) generated during the activation of PDS by Fe(II) over a wide pH range can undergo side reactions with Fe(II), PDS, H2O2, and H2O, making the entire system more complex[122-123].
in the activation process of PS involving polyphenols, Fe(IV) undergoes a reverse disproportionation reaction with Fe(II) to regenerate Fe(III), which subsequently chelates and reduces with natural polyphenols again, achieving regeneration of Fe(II). As a common antioxidant sharing the same enediol structure with polyphenols, ascorbic acid (AA) can also substitute polyphenols to activate peroxides such as H2O2, PS, and PAA. The generation and transformation pathways of Fe(IV) in both AA/Fe(III)/PS and AA/Fe(III)/PAA systems have been reported, and the reaction mechanism is illustrated in Fig. 2c[144-145]. Fe(IV) participates in reactions as an excellent electron acceptor during the Fe(II)/Fe(III) redox cycle. Additionally, compared to using a single metal, various metal ions or synergistic activation between metal ions and high-valent salt ions demonstrate better efficiency in generating radicals for contaminant removal, as shown in Fig. 2d[141].
A large number of experimental results indicate that, compared to traditional AOPs, the addition of natural polyphenolic compounds expands the pH range for reaction occurrence from exclusively acidic conditions to neutral and alkaline ranges. Relying on the chelation and reduction ability of polyphenols, the solubility of metal ions increases significantly, thereby greatly enhancing the reaction rate of activated oxidants by dissolved metal ions. The oxidized metal ions can rapidly chelate with polyphenols to regain electrons and are subsequently reduced to lower valence states. Meanwhile, quinone substances formed through polyphenol oxidation can act as redox mediators (RMs), also known as electron shuttles (ESs), between dihydroxycyclohexadienyl radicals (•OH adducts of phenols) and metal ions oxidized by SQ•-[146]. Therefore, polyphenols function as strong chelating ligands that accelerate the redox cycling of metal ions throughout the entire reaction system while simultaneously suppressing the generation of water treatment wastes such as iron sludge, thus enabling efficient recycling of metal ions[147]. By adjusting parameters such as dosage and feeding intervals, practical applications of polyphenols in removing extracellular polymeric substances (EPS) from wastewater can be achieved, ultimately realizing the goal of treating waste with waste[148].
At present, the above-mentioned system has been applied in the removal of pollutants such as heavy metal ions, PPCPs, amino acids, endocrine disrupters (EDCs), dyes, pesticides, bacteria, and flame retardants[124,109-110,142-143].
3 High-Price Metals
Polyphenols, in addition to directly acting on pollutants through the formation of SQ•-, can also react with contaminants via their catalytic valence metal. These reactions are typically reflected in non-radical reaction pathways. In recent years, researchers have conducted extensive studies on the removal of water pollutants by polyphenol-catalyzed redox metal ions and have made numerous new discoveries.
3.1 Fe³⁺
In addition to its role in electron transfer during the redox cycle process, Fe(IV) can directly remove pollutants through SET alone. As shown in Figure 4a of Ding et al.[114], Fe(IV) can react with BPA, and this high-valent iron species can also simultaneously consume CFA. The generation of Fe(IV) results from the double-electron transfer through Fe(II)-PS complexes formed in the Fe(II)/PS system[149]. Li et al.[135] and An et al.[138] also found that Fe(IV) plays an important role in removing atrazine (ATZ) and its key transformation intermediates, deethylatrazine and desisopropylatrazine, using p-benzoquinone (BQ) and methyl-p-benzoquinone (MBQ). The reaction mechanism is illustrated in Figure 4b.
3.2 Copper Ion
On the basis of demonstrating the existence of Cu(III) in the Cu(II)/PS system[150-151], Cai et al.[142] also observed traces of Cu(III) in the EGCG/Cu(II)/PMS system and successfully proposed the generation pathway of Cu(III) through combined analytical methods. Wang et al.[143] found that GA accelerated the reduction of Cu(II) to Cu(I) during the Cu(II)/PMS catalytic degradation of the novel flame retardant tetrabromobisphenol A (Tetrabromobisphenol A, TBBPA). As an intermediate, Cu(I) reacts with PMS via a two-electron transfer process to generate the primary oxidizing agent Cu(III), which can further catalyze the production of the secondary oxidizing agent •OH under acidic conditions. Subsequently, these two reactive species directly act on TBBPA, and the entire reaction process is illustrated in Fig. 4c.
3.3 Manganese Ion
Research on the direct removal of pollutants by high-valent metal species (HVMS), represented by high-valent iron ions (i.e., Fe(IV), Fe(V), Fe(VI)) and high-valent manganese ions (i.e., Mn(III), Mn(IV), Mn(V), Mn(VI), Mn(VII)), is limited. Such studies often use quinone substances as precursors for generating HVMS. As shown in Figure 4d, coupling of high-valent metals can effectively enhance the oxidation potential, not only shortening the time required for the generation of metal ions in the non-radical pathway but also significantly increasing the oxidation and removal rate of pollutants. Dong et al. found that under acidic conditions, the presence of BQ may promote the transformation of Mn(VII) into MnO2(sub)>2, with the in-situ generated MnO2(sub)>2 participating in redox reactions. Some Mn(IV) is reduced to Mn(III), subsequently forming Mn(III)-SQ•- which accelerates BPA degradation, as shown in Figure 4e. More importantly, MnO2(sub)>2 can directly degrade BPA without undergoing a series of valence changes of metal ions or chelation with ligands. BQ serves as an important redox mediator in dissolved organic matter (DOM), capable of catalyzing valence changes among active manganese species across a wide pH range through redox cycling, thereby enhancing the oxidation capacity of Mn(VII) and enabling Mn(II, III, IV, VII) to jointly drive pollutant degradation. Additionally, related studies indicate that these high-valent metal ions can directly remove pollutants by initiating hydroxylation, carboxylation, bond cleavage, and polymerization reactions, providing a foundation for future research on mechanisms of pollutant degradation enhanced by combining polyphenols with HVMS.
4 Solid Catalysts
Natural polyphenols are widely present in nature, and polyphenolic substances are also used as reducing agents, stabilizers, and capping agents in the synthesis of heterogeneous materials. Typical solid catalysts such as zero-valent metals, mono-metallic and multi-metallic compounds, metal-organic complexes, and non-metallic materials can overcome issues associated with ionic metals as homogeneous catalysts, such as difficulties in recovery and strong pH dependence. These solid catalysts are more easily separated and reused from wastewater and exhibit enhanced tolerance under alkaline conditions[149].
4.1 Zero-Valent Metal Monomer
Tea polyphenols (TPs), tannic acid (TA), gallic acid (GA), and pyrogallic acid (pyGA) can reduce Fe(III), Cu(II), and Ag(I) to zero-valent metal monomers. This process generally consists of two stages: adsorption and reduction. Taking the synthesis of Cu0 nanoparticles (NPs) by polyphenols as an example, Dinesh et al.[155] utilized Hibiscus rosa-sinensis extract as a raw material and (CH3COO)2Cu·H2O as a precursor to synthesize Cu0 NPs under ultrasound for degrading 5-fluorouracil and lovastatin. Notably, this study elucidated the process by which tellimagrandin (the main component of Hibiscus rosa-sinensis extract) undergoes structural decomposition into GA and glucose, which subsequently reduce copper ions and coordinate with Cu0 NPs, as shown in Fig. 5a. The mechanism of Cu0 NP synthesis involving polyphenols is as follows: hard ligands (–OH groups) present in hydrolyzed phenolic products reduce soft metal ions (Cu(I, II)) to form Cu0 NPs; subsequently, soft ligands (–CO) coordinate with Cu0 NPs and stabilize them through electrostatic interactions[156-157].
Polyphenols exhibit excellent performance in enhancing the durability of solid catalysts, degrading organic pollutants, and removing heavy metals. Taking the reduction of Cr(VI) by pyGA/nZVI as an example, the catechol groups in pyGA reduce Fe(III) to form nZVI, while pyGA itself coats the surface of nZVI through chelation, forming a compact core-shell structure that effectively prevents corrosion and leaching of nZVI, thus prolonging the catalyst's lifespan[158]. nZVI induces various reactions via electron transfer to achieve Cr(VI) reduction: on the bare nZVI surface, nearly 60% of nZVI reduces surface-adsorbed Cr(VI) to Cr(III), which subsequently combines with OH- in wastewater to form precipitated Cr(OH)3, while the remaining 40% of nZVI reacts with water producing Fe(II) and hydrogen gas. Meanwhile, the Fe(III)/Fe(II) redox pair in the system promotes partial reduction and removal of Cr(VI). Additionally, excess Fe(III) and Cr(III) form CrxFe(1-x)(OH)3 and CrxFe(1-x)OOH in the presence of water[158]. In addition to the above reactions, pyGA-coated nZVI can directly reduce Cr(VI), yielding Cr(III) and either pyGA-oxide or pyGA-hydroxide. The aforementioned mechanisms for Cr(VI) removal are illustrated in Fig. 5b[158]. Hou et al.[159] further investigated the effectiveness of different polyphenol-modified nZVI in reducing Cr(VI), finding that TA-nZVI achieved the highest Cr(VI) removal efficiency within 2 minutes and showed low sensitivity to pH variations and common anions/cations. This is because TA on the nZVI surface reduces Fe(III) to Fe(II), lowering the charge-transfer resistance of nZVI, whereas EGCG-nZVI and pyGA-nZVI show much weaker Fe(III) reduction capabilities compared to TA-nZVI. Cheng et al.[160] used epigallocatechin gallate (EGCG), the most hydroxyl-rich compound in green tea, as a raw material, mixing it with various metal salts including titanium, zinc, nickel, copper, and cobalt to prepare zero-valent metal monomers. Among these, EGCG/Ti exhibited the best adsorption capacity. Using X-ray photoelectron spectroscopy (XPS), they evaluated the effects of titanium salt types, synthesis conditions, and environmental factors on Cr(VI) removal. Changes in the Cr(VI) 2p characteristic peaks before and after adsorption indicated that the mechanism of Cr(VI) removal involved ≡Cr(VI) being reduced to ≡Cr(III), as shown in Fig. 5d[161].
Polyphenol-synthesized Ag NPs and Au NPs have been reported to be effective in removing organic dyes such as methylene blue (MB) and 4-nitrophenol, as well as in the recovery of precious metals; zero-valent metal monomers such as Fe0 and Cu0 have also been applied for activating PS and other peroxides. These zero-valent metals activate PS during their dissolution process, and the dissolved metal ions may subsequently homogeneously catalyze PS. The general reaction processes are shown in equations (3) to (7):
4.2 Single Metal Compounds
Iron-containing minerals such as hematite (α-Fe2O3) and goethite (α-FeOOH) are abundant in the earth's crust, inexpensive, and environmentally benign; thus, they are commonly used as heterogeneous catalysts[162]. In Fe@Fe2O3 core-shell nanowires combined with AA, both H2O2 ads and H2O2 free can generate •OH (•OHads and •OHfree), and •OHads can be transformed into •OHfree through diffusion[166]. To intuitively assess the extent of influence of the heterogeneous Fenton reaction, Sun et al.[167] investigated the individual contributions of homogeneous and heterogeneous Fenton reactions to the overall degradation of alachlor in a heterogeneous AA/Fe3O4/H2O2 system, as shown in Figure 5e, revealing that surface Fenton reactions contributed up to 62.6%, which was the result of AA accelerating Fe(III)/Fe(II) on the Fe3O4 surface. Compared with α-Fe2O3 and α-FeOOH, magnetite (Fe3O4) possesses an inverse spinel crystal structure, whose unique electromagnetic properties mainly benefit from electron transfer between iron ions in octahedra[163]. Currently, Fe3O4 magnetic nano-scaled particles (Fe3O4 MNPs) have been introduced into H2O2 or PS systems for removing typical water pollutants[164-165]. The reaction mechanism at the heterogeneous interface of Fe3O4 MNPs is illustrated in Figure 5c[168]. Application of MNPs has significantly improved catalytic removal efficiency and recyclability, while incorporation of polyphenols further enhances the performance of Fe3O4 MNPs. For example, adding EGCG with high reducing capability to the Fe3O4/PMS system promotes the cycling of Fe(III)/Fe(II) on the surface of Fe3O4 MNPs, resulting in significantly better catalytic efficiency compared to using only Fe3O4 MNPs[169-170]. Li et al.[171] modified Fe3O4 MNPs with GA and found that the main species activating PMS were iron atoms on Fe3O4, rather than the trace dissolved iron ions generally believed. GA-Fe3O4 MNPs have also been preliminarily applied as a novel photocatalyst in pharmaceutical wastewater treatment[172].
Green synthesis methods using polyphenols as raw materials or modified by polyphenol doping have also been applied to the synthesis of metal oxides such as cobalt oxides, ZnO, and TiO2 NPs, as well as for the removal of pollutants in wastewater; however, these solid catalysts have not been used to activate PS. Radical scavenging and quenching experiments, EPR detection, electrochemical analysis, and chromatography-mass spectrometry analyses indicate that the separation of h+ and e- within the metal oxides is a prerequisite for the generation of •OH, HO2•/O2•- species in the system. Anoxic experiments show that O2 serves as an important oxygen source for these ROS. Manganese oxides are widely present in soils, natural minerals, and aquatic environments, and are the main components of manganese-containing minerals such as birnessite, feitknechtite, and manganite. Manganese oxides are chemically active, and their ability to interconvert between different oxidation states plays a significant role in biogeochemical transformation processes. As the primary form of manganese oxides, MnO2 has an oxidation-reduction potential (ORP) of 1.23 V, making it a typical highly reactive oxidizing agent; therefore, discussing the interaction between polyphenols and MnO2 holds greater practical significance. Existing studies suggest that MnO2 can directly degrade pollutants, a process generally consistent with reactions involving other metal-based solid catalysts. However, in this case, Mn(IV) on the solid surface of MnO2 is typically reduced to Mn(II)s and Mn(II)aq, rather than Mn(III), although its behavior can also be influenced by Mn(III) accumulation. Investigating the influence of DOM or humic acids (HAs) under natural conditions on pollutant degradation by MnO2 is a popular topic for explaining environmental transformation processes of pollutants. Research shows that the effects of natural substances such as DOM or HAs on pollutant degradation by MnO2 are not unidirectional; they may simultaneously exhibit dual roles—either promoting or inhibiting degradation—which leads to seemingly contradictory results. To clarify the relationship between these effects and the specific components of DOM or HAs, researchers have employed numerous approaches to correlate them. A common practice involves separating DOM into different molecular weight fractions to assess how the type and concentration of DOM with varying molecular weights affect MnO2-mediated pollutant removal. Yang et al. divided DOM into three groups—DOM50, DOM250, and DOM400—by heating at different temperatures, finding that only DOM250 promoted the degradation process; DOM250 also achieved favorable degradation rates and effectiveness under varying pH conditions. Van Krevelen diagrams obtained via Fourier-transform ion-cyclotron resonance mass spectrometry (FT-ICR-MS) indicated that polyphenolic compounds abundant in DOM250 were key constituents driving MnO2's oxidative degradation activity. Zhong et al. investigated the reaction mechanism of sulfamethoxazole (SMX) degradation under the stabilization and catalytic action of MnO2 using catechol as a representative HA model compound, discovering that o-SQ• played a crucial role in SMX removal. As shown in Fig. 5f, catechol chelates and reduces Mn(IV/III) on the MnO2 surface or Mn(II) formed during MnO2 reductive dissolution, forming Mn(III/II)-o-SQ• (majority) and Mn(II)-o-SQ• (minority) complexes. Subsequently, SMX-catechol adducts are formed through radical-radical coupling and electrophilic radical coupling, achieving SMX removal. In addition, studies have shown that humic substances (HSs) can also act as coupling agents influencing pollutant transformation efficiency and chemical structures. Song et al. used syringaldehyde (SA) and acetosyringone (AS) as simple model compounds of HSs to construct a MnO2-mediator system, significantly enhancing the transformation efficiency of MnO2 toward sulfonamide antibiotics within a neutral pH range. This enhancement occurs because SA and AS undergo oxidation after being chemisorbed onto the MnO2 surface, generating oxygen-centered phenoxy radicals. These phenoxy radicals can attack the —NH2 group on the benzene ring of sulfonamide antibiotics.
Mackinawite (FeS) particles are ubiquitous natural iron-containing minerals. FeS, as a catalyst, is used to activate persulfate (PS) and mineralize p-chloroaniline. Fan et al. proposed a heterogeneous activation mechanism of PS by FeS: ≡Fe(II), controlled by surface reactions and diffusion on FeS, activates PS to generate •OHads and SO4•−ads, which degrade p-chloroaniline via diffusion. Unlike iron oxides, S(II) can regenerate Fe(II) on the Fe(III) surface, promoting the Fe(II)/Fe(III) cycle and reducing the additional consumption of PS. A study on the removal of lindane by green tea extract (GTE) compared the effects of different iron salts or iron-containing minerals (FeSO4, Fe2(SO4)3, Fe3O4, Fe2O3, FeOOH, and FeS) on the reductive capacity of GTE. ORP measurements showed that FeS and other iron-containing substances exhibited weaker reductive ability towards GTE compared with FeSO4; thus, the reaction mechanism between FeS and epigallocatechin gallate (EGCG) was not further explored. Sulfur is a typical multivalent element, and its continuously variable valence states facilitate redox cycling of various metal ions. Ali et al. comprehensively summarized the catalytic and co-catalytic roles of metal sulfides (including MoS2, WS2, transition metal-doped MoS2/WS2, iron sulfides, sulfur-modified zero-valent iron, etc.) in accelerating the M(n+1)+/Mn+ catalyst cycle in both homogeneous and heterogeneous systems. However, studies on polyphenol-modified metal sulfides and phosphides are rare at present. Therefore, the reaction mechanisms between common polyphenols such as gallic acid (GA), ellagic acid (EA), caffeic acid (CFA), and pyrogallic acid (pyGA) and metal compounds containing electron-rich atoms like S and P, as well as their mechanisms in peroxide activation, are promising research topics for future investigation.
Colloidal ferric hydroxide (CFH, Fe(OH)3 colloid) not only inhibits the surface complexation of As(III) onto CFH by humic substances (HSs) and low molecular weight carboxylic acids (CAs), but also prevents the oxidation of As(III) under near-neutral conditions via photo-induced ligand-to-metal charge transfer (LMCT). The extent of inhibition of As(III) oxidation by HSs and CAs varies depending on the type of polyphenols present. As shown in Figure 5g, Wu et al.[185] innovatively proposed that the light-shielding effect and competitive complexation effect between HSs or CAs and As(III) are responsible for the hindered oxidation of As(III). They further estimated the impact of these two effects on the photooxidation of As(III) through absorbance measurements and modified Freundlich models to determine As(III) desorption. Huang et al.[186] successfully established a quantitative structure-activity relationship (QSAR) model between the observed pseudo-first-order rate constant (kobs) and parameters including the complexation of Fe(III) with natural organic matter (NOM) (logKFe-NOM), molecular weight of NOM, and total acidity percentage of NOM using multiple linear regression analysis.
4.3 Polynuclear Metal Compounds
Alamier et al.[187] biosynthesized magnetic NiFe2O4 NPs from aqueous extracts of leaves, which exhibited heterogeneous photocatalytic Fenton-like properties in oxalic acid. The reaction mechanism for the degradation of MB by NiFe2O4 NPs under light irradiation is shown in Fig. 6a and Equations (8)-(19). In the Fenton system, oxalic acid bound to ≡Fe(III) forms C2O4•- under hv, which then reacts with O2 to generate various ROS. Makofane et al.[188] synthesized ZnFe2O4 NPs to remove MB and SMX. When AgNO3 scavenger was used, the degradation behavior of pollutants significantly decreased, indicating that e- dominated the photocatalytic degradation of MB. Artificially introduced surface defects made it difficult for photogenerated e- and photogenerated h+ to recombine; the excited e- paired with oxygen atoms adsorbed on ZnFe2O4 NPs to form •OH, leading to MB mineralization. Metal M, as an active site, can oxidize itself through electron transfer to activate PS. The released e- is captured by O-O or H-O bonds in intermediates, followed by bond cleavage generating SO4•-[189]. The catalytic process of MFe2O4 involves two redox pairs: M(III)/M(II) and Fe(III)/Fe(II). Due to differences in redox potentials, in bimetallic or multimetallic catalytic systems (e.g., Co and Fe, Cu and Fe), electrons typically transfer from lower redox potential M1 in reduced state to higher redox potential M2 in oxidized state[149]. BiFeO3 NPs were prepared using extract from Abelmoschus esculentus L. leaves (ALE). A narrow bandgap energy (2.00 eV) enabled nearly 95% MB photodegradation efficiency by BiFeO3 NPs[190].
+
Sapindus mukorossi seeds extract rich in polyphenols was synthesized into CeO2@ZnO nanocomposites with Zn(NO3)2 and Ce(NO3)3 through stirring, settling, filtration, washing, drying, and high-temperature heating processes. As shown in Fig. 6b, the migration behavior of photogenerated carriers in the composite material conforms to a Z-scheme heterojunction[191-192]. Compared with (R-Vo)TiO2@EA (thermal reduction) and TiO2@EA, the synthesized Nd-Fe-(D-Vo)TiO2@EA (dual-metal doping) exhibited faster separation efficiency of photogenerated carriers, better visible light absorption performance, and higher mineralization rate of ethinyl estradiol (EE)[207]. As illustrated in Fig. 6c, based on the calculation results of electronic work functions, for Nd-Fe-(D-Vo)TiO2@EA, both the lowest unoccupied molecular orbital (LUMO) of EA and the conduction band (CB) of Nd-Fe-(D-Vo)TiO2 can generate abundant photogenerated electrons, while the highest occupied molecular orbital (HOMO) of EA and the valence band (VB) of Nd-Fe-(D-Vo)TiO2 also produce more h+, which cannot be achieved by the other two catalysts[193]. Additionally, the green synthesis-derived nickel oxide-modified zinc hexacyanocobaltate framework (NiO@ZnHCC) formed the commonly seen p-n junction (with ZnHCC and NiO being p-type and n-type, respectively), where electrons transfer from NiO's CB to ZnHCC under solar irradiation, accompanied by Cr(VI) reduction[194].
4.4 Metal-Organic Complexes
Metal-organic complexes (MOCs) have attracted widespread attention in the field of photocatalysis due to their atomically dispersed catalytic sites, tunable coordination polymers, structural diversity, simple and controllable synthesis process, and adjustable optical properties. Xue et al.[37] synthesized MOCs using Fe(III), Bi(III), and Ce(III) as metal centers and ethylamine (EA) as the organic ligand. Among these, EA-Fe MOCs exhibited the best performance in photocatalytic sterilization, photocatalytic reduction (e.g., Cr(VI)), and photocatalytic degradation of organic compounds (e.g., Tetracycline (TC)). Moreover, as a photocatalysis-self-Fenton system, EA-Fe MOCs can spontaneously generate H2O2. Xue et al.[199] prepared amine-modified Fe(III)@PCN-222 nanorods via an impregnation method and anchored them onto a polydopamine (PDA)-modified PVDF membrane for oil-water separation, photo-Fenton catalytic degradation of aromatic pollutants, and bacterial inactivation. During the modification of the PVDF membrane, PDA mainly utilizes its mussel-inspired highly adhesive structure to alter the mechanical properties and hydrophilicity of the material, which is a key step in ensuring material stability[200].
4.5 Carbon-Based Materials
In addition to the green preparation of metallic elements, metal compounds, and metal-organic complexes using natural substances containing polyphenols, preliminary progress has also been made in the synthesis and application of catalysts supported by carbon-based materials. The synthesis of humic acid–iron combined biochar (HAs-Fe@BC) addresses the issue of low Fe(III)/Fe(II) cycle rates in SR-AOPs and synergistically degrades the model pollutant rhodamine B (Rhodamine B, RhB) with PDS. Excellent catalytic activity originates from the SO4- species generated by the HAs-Fe@BC-PDS* composite. The addition of HAs increases the number of C—OH and CO groups on the surface of Fe@BC, which facilitates electron transfer, as shown in Figure 6d[195]. Safajou et al.[196] used mint extract, graphene oxide, and Cu2+ ions to construct an rGO/Cu nanocomposite photocatalyst for degrading RhB and MB. The combination of copper-based semiconductors and rGO sheets slows the recombination rate of electrons and holes; redox-active groups (quinones, hydroquinones, and phenolic substances) on the surface of carbon-based materials act as RMs to form a conjugated π-electron system; the formed persistent free radicals (PFRs) influence the types and distribution of ROS in aqueous systems, as well as the transformation and removal of pollutants, especially when interacting with metals, where PFRs bound to the surface of carbon-based materials become more stable[98,197-198].
4.6 Inorganic Salt-Supported Catalysts
Polyphenol modification has also been applied to inorganic salt-based supported metal catalysts, significantly enhancing the catalytic performance. Binaeian et al.[201] modified TiO2 loaded hexagonal mesoporous silicate (HMS) with natural polyphenol oak gall tannin at different concentrations. Compared with the control TiO2 (P-25) and TiO2-HMS, TiO2-OGTx-HMS exhibited superior anionic dye adsorption and photocatalytic degradation performance. Similar to other complex tannins, the basic structure of oak gall tannin is also GA. As shown in Fig. 6e, numerous ortho phenolic hydroxyl groups enhance the adsorption and chelation ability of oak gall tannin, which makes it easier for titanium atoms with high oxygen affinity to capture oxygen, leading to chelation and forming thermally stable five-membered rings[201]. The amine-functionalized HMS and glutaraldehyde undergo addition reactions with the H atom at the ortho position of the GA carboxyl group, without damaging the catechol structure of oak gall tannin.
At present, in homogeneous-heterogeneous systems containing polyphenols, the redox couples Fe(III)/Fe(II) and Cu(II)/Cu(I) have been more extensively studied, while investigations combining polyphenols with other redox couples such as Mn(III)/Mn(II), Co(III)/Co(II), Mo(IV)/Mo(III)/Mo(II), and non-metal-rich electron atoms like S and P for removing typical pollutants are still at an early stage[149]. The interactions and catalytic mechanisms between polyphenols and spinel-structured bimetallic oxides, perovskite-type catalysts, metal organic complexes, biochar, inorganic catalysts represented by rGO and g-C3N4, and supported metal catalyst based on inorganic salts have also been rarely explored. Successful construction of heterostructures relies on comparisons among LUMO, HOMO, VB, and CB levels; introducing polyphenols can alter these parameters, thereby influencing electron transfer direction, recombination rates of photogenerated carriers, and reaction mechanisms involving H2O, O2, and pollutants. When introducing cocatalysts into homogeneous systems to accelerate heterogeneous reactions and enhance the redox cycling rate of M(n+1)+/Mn+, polyphenols could be considered as potential candidates. The electron-rich structure of polyphenols and their ability to form ligands or complexes with transition metal catalysts fulfill the requirements for serving as organic cocatalysts[149,202].
In addition, in heterogeneous catalysts involving polyphenols, it is necessary to consider the interactions or bonding forms of polyphenols on different crystal planes (i.e., hydrogen bonds, π bonds, hydrophobic interactions, metal coordination, covalent bonds, and electrostatic interactions), as shown in Fig. 6f~h[203]. These differences may ultimately alter the coordination patterns (e.g., formation of bidentate or monodentate ligands) or cause variations in the types and quantities of radicals or active species, thereby affecting pollutant removal efficiency and reaction mechanisms[71,204]. Therefore, factors such as the crystal facets of solid catalysts, polyphenol binding sites and binding modes, and environmental pH should be given more attention in future studies.
Natural polyphenols remove pollutants by reducing high-valent metal ions or directly chelating low-valent metal ions in aqueous environments, ultimately forming SQ•- and/or metal ion-SQ• complexes[152]. In polyphenol/peroxide systems containing transition metal ions, the main reaction involves activation of peroxides by transition metal ions, while the direct degradation of pollutants by SQ•- generated from natural polyphenol oxidation is a secondary reaction that has minimal impact on pollutant removal and is thus often overlooked. Furthermore, the progressive oxidation of polyphenols to quinones via electron loss is a reversible process, and the role of quinones gaining electrons and being reduced should also be considered within the entire system. It has been proposed that both phenoxyl radicals and quinones formed from polyphenols act as natural redox mediators (RMs). These natural RMs can enhance the transformation efficiency of various organic pollutants by laccase, permanganate, ferrate, periodate, MnO2, O2, H2O2, and other oxidants[205].
5.1 Periodates and Permanganates
In addition to the previously mentioned H2O2, PS, and PAA, oxidants applied in the removal of water pollutants over the past two decades also include ClO-, SO32-/HSO3-, IO4-, MnO₄-, etc.[206-208]. AOPs based on IO4- systems have rapidly developed: besides generating typical ROS, the IO3• produced during IO4- reactions can also be utilized for pollutant removal and transformed into non-toxic iodate (IO3-)[30,209-210]. Given these advantages, Yang et al.[211] utilized polyphenolic compounds such as Protocatechuic acid (PCA), Caffeic acid (CFA), and Dopamine hydrochloride (DA) to react with NaIO4, selectively removing SMX, Naproxen (NPX), ATZ, and BPA by producing active species including o-BQ, •OH, IO3•, O2•-, and 1O2. Research on the combined removal of organic pollutants using polyphenols and permanganate has also been conducted following discoveries regarding the influence of natural manganese ions on the environmental behavior of water pollutants[212]. Because IO4- and MnO4- both contain I(VII) and Mn(VII) at their highest oxidation states, catechol structures are oxidized into o-BQ, which subsequently forms complexes and transformation products via Michael addition, thereby reducing the toxicity of pollutants in water, as shown in Fig. 7a, b. Dong et al.[152] studied permanganate (Mn(VII)) and found that under acidic conditions, SQ•- generated from BQ transformation couples with hydrogen substitution products of BPA, thus inhibiting BPA self-coupling while promoting ring-opening reactions of BPA. Additionally, studies indicate that Mn(VII) also participates in the redox cycling process of MnO2/Mn(II)[69].
5.2 Peroxide
The mutual transformation among polyphenols, semiquinone radicals, and quinones can also activate peroxides and remove pollutants. This process generates reactive species such as 1O2 and oxygen-centered radicals. Specifically, the generation pathway of 1O2 can be summarized as a novel non-radical oxidation process[213-214]. Zhou et al.[215] used BQ as a model to identify the main reactive species produced by quinones activating PMS that significantly affect SMX degradation. Quenching experiments combined with EPR and HPLC-MS using chemical probes pointed toward 1O2, while simultaneously clarifying its formation mechanism, as shown in Figure 7c: the decomposition of dioxetane (an intermediate) formed between PMS and BQ is key to catalytically producing 1O2. Humic acid substances (HAs) are structurally complex and redox-active organic compounds that constitute important components of NOM, DOM, and HSs. Studies have shown that low molecular weight fractions (LMWF), accounting for only 2% of total organic carbon in HAs, exhibit much higher reductive capacity than the residual fraction. Fluorescence spectroscopy revealed that the high reducing ability of LMWF mainly originates from quinone-like functional groups[216]. The major polar fractions of HAs consist of functional groups including carboxylic acids, phenols, hydroxyls, carbonyls, and quinolines, among which carboxyl and phenolic groups serve as significant sources of charged and radical sites, playing dominant roles in hydrogen bond formation and metal binding[217]. Radical sites on HAs can be categorized into two types: long-lived and short-lived (transient) radicals, with transitions between them attributed to pH changes[218-219]. Small molecular weight and structurally well-defined organic molecules, such as partially protonated and fully deprotonated radicals of PCA, can respectively act as simplified models of long-lived and short-lived radicals for HA studies[220-222]. Therefore, natural polyphenols, as major components of HAs, also participate in peroxide activation and radical generation. Fang et al.[223] utilized HAs to activate PDS, generating SO4•- and •OH for in situ remediation of soil, sediment, and groundwater contaminated by 2,4,4'-trichlorobiphenyl (PCB28). In this process, the electron-loss oxidation from SQ•- to SQ was crucial for initiating subsequent massive radical production from S2O82-.
The H on the phenolic ring of polyphenols can also be substituted by -CH3, -OCH3, -COOH, -CHO, -NH2, and even more complex groups. Both polyphenolic compounds containing aromatic ketone structures and quinones contain carbonyl groups, which possess redox-active functional groups. These aromatic ketone-containing polyphenols and quinones can react with PDS when they are reduced to form organic free radicals; however, this reduction process to generate free radicals typically requires additional consumption of SO4•-. Another way to activate PDS is through direct electron transfer by the polyphenol itself[224]. The type, position, and number of substituents on the phenolic ring of polyphenols, the source of oxygen atoms, and the inherent properties of peroxides all undeniably increase the complexity of polyphenol-activated peroxide reactions. Quantum chemical calculations predicting reaction pathways are a commonly used method for elucidating radical reaction mechanisms. Gu et al.[213] employed para-benzoquinone and tetrachloro-1,4-benzoquinone (TCBQ) as model compounds to explain the activation mechanisms of quinones toward typical peroxides (i.e., H2O2, PMS, PAA, CH3OOH) via quantum chemical calculations, as shown in Figure 7d. In the presence of p-BQ, peroxides tend to undergo nucleophilic addition preferentially with the carbonyl carbon bearing more positive charge, followed by termination reactions involving HO2• and CH3OO•; whereas PMS and PAA continue to react and produce 1O2. Due to Cl's higher electronegativity, the vinylic carbon becomes more positively charged than the carbonyl carbon. Therefore, when TCBQ acts as a reactant, HO2• preferentially adds to the vinylic carbon, thereby inducing •OH generation. The activation effect of TCBQ on PMS is nearly identical to that of p-BQ.
5.3 Oxygen, Water, and Other Substances
The process of stepwise/direct oxidation of polyphenols to quinones can still occur in the absence of transition metal ions, where the oxygen donors become O2, H2O or other coexisting substances[96,134,225]; meanwhile, ions present in the aqueous phase can also affect the reaction mechanism and pollutant degradation within the system.
Polyphenols/SQ•- react with dissolved oxygen in water through SET to generate HO2•/O2•-, and some of the HO2•/O2•- accepts an H+ to form H2O2 and •OH. These reactive species can also be used for the reduction of metal ions, activation of oxidizing agents, and the formation of other ROS[137,226-227]. Severino et al.[228] combined EPR experimental observations with DFT computational results and found that common TP monomeric phenols such as EGCG are prone to oxidation of the B and D rings under alkaline pH conditions, where the B ring is the primary oxidation site during autoxidation and the D ring is the preferred site for O2•- oxidation. Hajji et al.[229] investigated the slow autoxidation reaction of QCR in neutral solution and found that the autoxidation rates followed this order: copper-QCR complex > QCR > iron-QCR complex, which is due to Fe(II) autoxidizing into Fe(III) under neutral conditions, whereas Cu(I) bound to QCR or its oxide did not undergo significant autoxidation. Polyphenolic compounds are also considered natural organic photosensitizers that can generate radicals through photochemical reactions and initiate quenching or polymerization. Yuann et al.[230] found that exposure to blue light irradiation changed the color of a basic aqueous solution of catechin (CAT), and LC-MS and NBT reduction assays indicated that CAT underwent photosensitive oxidation to form procyanidin dimers and generated O2•-. The addition of AA, GA, or AlCl3 could all inhibit procyanidin production but via different mechanisms: AA and GA act as sacrificial agents preventing CAT oxidation and serve as radical scavengers; AlCl3, acting as a catalyst, promotes cleavage of the dimeric bonds and re-generates CAT. These studies indicate that polyphenolic compounds can undergo autoxidation and produce PFRs and ROS, making it possible to apply polyphenols for contaminant removal. Excited triplet states of organic chromophores in HAs can induce the oxidation of various organic compounds, suggesting that organic pollutants in natural water bodies might undergo degradation reactions under natural light conditions[231]. During pharmaceutical wastewater treatment, HAs can not only reduce dissolved oxygen to produce H2O2 but also form 3HAs* upon sunlight irradiation, inducing the generation of 1O2 and •OH by exciting O2 or interacting with surrounding water molecules[232].
Ozone is an effective disinfectant, but its instability and high toxicity make it unsuitable for disinfection of drinking water. Moreover, ozone hardly reacts with saturated organic compounds, electron-deficient aromatic compounds, and certain functional groups, which significantly limits its application scope.[83] Currently, TPs have been used as a green disinfection method capable of continuous disinfection without generating disinfection by-products to assist ozone disinfection technology, and preliminary attempts have also been made in eliminating antibiotic resistance genes (ARGs) and pathogenic bacteria in drinking water.[233-234] Confocal laser scanning microscopy (CLSM) shows that the number of dead cells resulting from the synergistic treatment of ozone and TPs is much higher than that from individual treatments; however, no research findings on the mechanism of combined disinfection using ozone and TPs at the free radical level have been published yet.[235]
Common anions in natural water bodies include Cl-, Br-, NO3-, SO42-, HCO3-, and HPO42-, which can also affect the removal efficiency of pollutants. NO3- and HCO3-/CO32- are typical radical scavengers; besides quenching ROS such as •OH and SO4•-, they can also bind with SQ•-, thereby inhibiting both the generation of SQ•- and its transformation into HQ. Du et al.[236] investigated the effect of interfering ions on the reduction of Cr(VI) by TPs-coated g-nZVI through the addition of sulfate and phosphate. They found that sulfate triggers the release of Fe(II) and TPs from the surface of g-nZVI due to the corrosion of the Fe0 core, and the released free Fe(II) and TPs subsequently reduce Cr(VI) to Cr(OH)3 and Cr2O3[237]. The formation of Cl• from Cl- is relatively easy; however, the amount of Cl• generated does not have a positive correlation with the concentration of Cl- in the system. At low concentrations, Cl- tends to promote the generation of Cl•, facilitating the degradation and removal of pollutants, while at high concentrations, Cl- scavenges Cl• and initiates chain reactions[236]. Cl• may attack the benzene rings of polyphenols, causing chlorinated substitution. These chlorinated polyphenol byproducts could lead to environmental toxicity and ecological risks.
5.4 Redox Mediator
RMs are reversible redox organic molecules that act as electron carriers in redox reactions among various inorganic and organic compounds. Based on their sources, RMs can be classified into natural RMs and synthetic RMs; natural RMs can be obtained from the surrounding environment and living organisms. Common RMs include anthraquinone-2,6-disulfonate (AQDS), anthraquinone-2-sulfonate (AQS), 2,2'-azinobis(3-ethylbenzothiazoline)-6-sulfonate (ABTS), 1-hydroxybenzotriazole (HBT), pyrroloquinoline quinone (PQQ), riboflavin (RF), neutral red (NR), and cobalamine. Many components of HSs and quinone analogs also have the function of transferring electrons, such as SA, AS, juglone, lawsone, and menaquinone (also known as vitamin K3)[238]. RMs play important roles in mediating biological activities, dealing with environmental pollution, and promoting energy production[239-241]. To effectively transfer electrons within the electron transport chain, theoretically, the standard ORP (E0′) of RMs should lie between the redox potentials of the electron donor and acceptor. However, due to factors such as reaction activation energy, RMs whose E0′ is lower than that of the electron donor or higher than that of the electron acceptor may still function within the system[238].
ABTS functions both as an activator and radical mediators (RMs), commonly used for the activation of peroxides. The electron-deficient ABTS•+ can be regenerated into ABTS by reductants such as humic acids (HAs)[48]. Studies have reported the formation of CAT-ABTS•+ covalent adducts and the scavenging of ABTS•+ by these adducts; however, CAT was treated as a contaminant in those studies, with the focus primarily on its oxidative removal process. Therefore, it is worth considering introducing typical organic pollutants into this system as target removal substances, thereby changing the role of polyphenols within the system, transforming their radical-scavenging effect (i.e., antioxidant activity) into a pro-oxidation effect on the entire system. 2,2,6,6-Tetramethylpiperidinyl-1-oxide (TEMPO) is a typical piperidine-based nitrogen oxide radical, which, besides functioning as a free radical scavenger, singlet oxygen (1O2) quencher, and selective oxidation agent, is also considered as a type of RM[242]. This is because TEMPO can form TEMPO+ by losing electrons from the N—O bond, after which TEMPO+ selectively oxidizes targeted substances and subsequently regenerates TEMPO. Under weak alkaline conditions, dopamine (DA) self-polymerizes through biomimetic adhesion to form polydopamine (PDA). The dense presence of functional groups such as catechol, imine, and amine endows PDA with strong chemical adsorption capabilities, enhancing its hydrophilicity, biocompatibility, and electronegativity. Under light exposure, oxygen atoms on the catechol groups within PDA lose electrons to form PDA•+, which being unstable, can then extract electrons from TEMPO, reverting PDA back to its ground state, as shown in Figure 7f[174]. The successful incorporation of PDA and addition of TEMPO constructed a dual catalytic cycling system based on PDA/PDA•+ and TEMPO/TEMPO+.
The formation of PDA arises from the non-covalent self-assembly of subunits into trimers, as well as from the continuous transformation of covalently coupled subunit products[243]. During the polymerization process, external stimuli such as light radiation cause the catechol group of DA to lose one electron, forming a DA-SQ•- structure. The initial formation of PDA and the formation of the PDA film are illustrated in reactions (20)–(21)[244].
some researchers have also proposed incorporating PDA into the category of RMs[245]. When polyphenolic compounds represented by PDA transition from their reduced forms to oxidized forms, they can catalyze the reduction of non-toxic or toxic inorganic and organic substances. For example, metal ions such as As(V), Cr(VI), Fe(III), Mn(IV), and Hg(II) are respectively reduced to As(III), Cr(III), Fe(II), Mn(II), and Hg0; perchlorate and other high-valence salt ions are reduced to chlorate; and organic compounds such as azo compounds and nitroaromatics are reduced to aromatic amines[238]. When polyphenolic compounds return from their oxidized forms back to their reduced forms, both inorganic and organic substances can be oxidized. For instance, ZVI, titanium citrate (III), sulfides, and cysteine undergo oxidation reactions, generating Fe(III), titanium citrate (IV), S0, polysulfides, and cystine, respectively[238]. As shown in Fig. 7e, common metal ions in geological settings such as As(V), Cr(VI), Fe(III), Cu(II), Mn(IV), and Hg(II) are referred to as redox-sensitive elements (RSE), playing an important role in biogeochemical cycles, because they are easily affected by HSs during redox processes[216,246]. Wang et al.[110] studied EGCG, the primary extract from green tea, and unexpectedly found during their investigation of its role in degrading Lindane under alkaline conditions that the initial step of Lindane removal was triggered by electrons provided through Fe(II) reduced by EGCG[247]. The initial degradation reaction of Lindane resembles the reduction of Cr(VI) in the same system. As a typical aquatic pollutant, Cr(VI) can also act as a strong oxidant, simultaneously reducing itself to Cr(III) while oxidizing polyphenols[248]. Okello et al.[249] used QCR and two synthetic derivatives (quercetin penta-phosphate (QPP) and quercetin sulfonic acid (QSA)) to reduce Cr(VI). Compared with QPP and QSA supplemented with Pd NP catalysts, adding only water-soluble QCR could efficiently reduce Cr(VI).
In fact, the direct reduction of pollutants by polyphenols reflects their antioxidant effects; however, such reactions currently constitute a small proportion in the field of water treatment research. The removal of most pollutants is attributed to the electron-withdrawing behavior of oxidized polyphenols during the reduction process, i.e., the oxidative removal of pollutants. Common organic pollutants generally possess electron-rich structures and act as good electron donors, making reactions between these pollutants and quinone substances formed from polyphenols possible. Moreover, considering that polyphenol–polyphenol oxides are typical natural RMs, constructing continuous catalytic systems involving the redox duality of polyphenols with other types of polyphenols or other RMs, as well as building dual or multiple catalytic systems in combination with solid catalysts, transition metal ions, etc., represents a fundamental approach for developing and applying polyphenols in pollutant removal in the future.
6 External Energy
In addition to reactions with metals, strong oxidizing agents, O2, and H2O, polyphenols can undergo bond cleavage and generate small molecular radicals such as •H, •OH, and •CH3 when excited by external energy. Among these radicals, •OH exhibits strong oxidizing properties and can oxidize and remove pollutants, contradicting the commonly recognized antioxidant effects of polyphenols. Tannins (TPs) have been reported as auxiliary disinfectants for UV disinfection and are now widely applied in the elimination of antibiotic-resistant bacteria[250]. Qin et al.[251] unexpectedly observed the generation of •OH in EGCG aqueous solutions and tea extracts under simulated sunlight conditions. This •OH production occurs through two pathways, as shown in Figure 7g. Specifically, pathway B involves homolytic cleavage of the C—O bond in the B ring of EGCG caused by ultraviolet radiation, forming •OH and carbon-centered PFRs; however, no in-depth explanation has yet been proposed for this phenomenon. Through spectral fitting and quantification of •OH production from EGCG, CAT, epicatechin (EC), epicatechin gallate (ECG), and epigallocatechin (EGC), they found that EGCG and EGC exhibited the lowest •OH yields. Subsequent FT-ICR-MS and EPR analyses coupled with metal oxide measurements further indicated that the •OH generation from EGCG and EGC was attributed to conjugation effects within their B rings[251-252]. Lv et al.[253] explored direct catalytic degradation of pollutants using polyphenols by photodegrading chlorothalonil (CTL) with six flavonoids: cyanidin chloride, kaempferol, morin, quercetin (QCR), galangin, and luteolin. The efficiency of CTL photoreductive dechlorination increased by 6.7–18.3 times. EPR radical trapping and product analysis of CTL transformation suggested that the photosensitization effect of flavonoids might be related to their hydrogen-donating capacity[253]. Rosiak et al.[254] established a relationship describing the change in total radical concentration over time following electron beam irradiation. The lifetimes of QCR radicals and rutin radicals generated during irradiation were determined as 1200 ± 900 h and 93 ± 32 h, respectively. High temperature and pressure can also induce homolytic cleavage of covalent bonds in polyphenols. For example, tannic acid (TA)-assisted hydrothermal treatment (HT) is employed for antibiotic removal and sludge dewatering. The TA+HT system produces approximately three times more •OH than HT alone, with the •OH formation originating from dehydrogenation reactions occurring at the O—H bonds of TA monomers under high temperature and pressure, leading to the generation of SQ•-[255].
In addition to the cleavage of polyphenol bonds and the formation of free radicals induced by electrochemistry, light radiation, and thermal activation, strong external energies such as electron beams, ultrasound, plasma, and microwaves can not only supply energy sufficient to break polyphenol bonds but also induce water molecule decomposition, generating reactive species such as •OH, •H, and hydrated electrons (eaq-). These reactive species subsequently initiate the activation of polyphenols[256-258]. The aforementioned pathways typically utilize polyphenols either for pollutant removal or as auxiliary agents participating in polyphenol-catalyzed advanced oxidation processes to reduce the reaction activation energy. Even in systems containing only polyphenols and typical pollutants, the involvement of external energy does not necessarily lead to C—O bond cleavage and subsequent •OH generation. Apart from effectively introducing external energy into the reaction system, the activation energy required for bond cleavage, polyphenol structure, and steric hindrance are also crucial factors influencing polyphenol bond breaking and radical formation.
7 Conclusion and Prospect
This paper summarizes the pro-oxidant effects of polyphenols, which can be specifically divided into five aspects: (1) In the polyphenol/Fe(Cu) ion/peroxide system, polyphenols can induce activation of peroxides such as H2O2, PS, and PAA through chelation-reduction of conventional transition metal ions. The reactive species generated by the peroxides remove pollutants via radical or non-radical pathways; (2) Side reactions occurring in the aforementioned systems, especially in the polyphenol/Fe(Cu)/per sulfate system, may induce the generation of HVMS. These oxidizing high-valent metal ions can directly oxidize pollutants through electron transfer; (3) When using plant extracts containing large amounts of polyphenols as raw materials for green chemical synthesis and studying the direct doping or modification of solid catalysts with polyphenols, polyphenols act as reducing agents, chelating agents, and capping agents, effectively improving catalyst durability. The coexistence of homogeneous and heterogeneous states within these systems makes the solid-liquid interfacial reaction more complex. The introduction of external energies, such as light radiation, causes h+, e-, or other reactive species to become dominant forces for the direct and indirect removal of pollutants; (4) On one hand, polyphenols directly contact pollutants by forming a balance between polyphenol-semiquinone radicals-quinones. On the other hand, they can bypass the activation of peroxides through the metal redox cycle, shortening the activation pathway of H2O2/PS/PAA. Polyphenols, along with NOM, DOM, HSs, and HAs that are rich in polyphenols, can also induce the generation of reactive species under natural conditions with the aid of environmental water, oxygen, and other substances. Utilizing the dual redox roles of polyphenol-polyphenol oxides to construct continuous catalytic systems where polyphenols are connected in tandem with other types of polyphenols or RMs, as well as parallel dual or even multiple catalytic systems combined with solid catalysts and transition metal ions, has also been preliminarily applied for pollutant removal; (5) Under special conditions such as light irradiation, electrocatalysis, or heating systems, a small fraction of C—O bonds on the catechol groups of polyphenols undergo homolysis, forming highly oxidative •OH that directly attacks pollutants, achieving one-step removal of pollutants; most polyphenols generate SQ•- through hydrogen abstraction (H-abstraction, HAT), followed by gradual production of large quantities of •OH via reactions between SQ•- and O2.
The antioxidant effects of polyphenols are mainly reflected in the reduction and removal process of highly toxic heavy metal ions such as Cr(VI) and Hg(II), as well as in the quenching of reactive species by •H or organic radicals generated from polyphenols themselves, competing with pollutants for photons, occupying active sites on solid matrices, and reducing the number of effective metal ions available for activating oxidants in homogeneous systems. These side effects may inhibit toxicity reduction and effective removal of pollutants under certain specific conditions. Meanwhile, the promoting or inhibiting effects of natural complex matrices such as DOM on pollutant removal remain unclear. Current studies are limited to conventional indicators such as molecular weight and C/O ratios, without focusing on individual polyphenols. The neglect of inhibitory effects by single polyphenols and the difficulty in elucidating mechanisms within complex polyphenol systems have become central contradictions in the study of reaction mechanisms in polyphenol-pollutant systems.
Based on the review of existing research findings, we propose the following prospects:
(1) For polyphenol/Fe (Cu) ion/peroxide systems, studies on the reduction of transition metal ions by polyphenols to activate PAA, CaO2, and other peroxides are limited. Moreover, most studies have focused only on common iron and copper ions, while practical water treatment applications involving manganese-based substances remain at an early research stage. More attention should also be given to other transition metal elements with favorable redox properties. Current studies indicate that the introduction of external energy as well as certain salt solutions or ions can further enhance both the rate and extent of contaminant removal. Therefore, combining multiple AOPs approaches for removing typical water pollutants, even trace organic contaminants (TrOCs), could further expand the application scope of polyphenols.
(2) Polyphenol-polyphenol oxidoreductases are typical natural RMs, and constructing continuous catalytic systems through the dual redox roles of polyphenols—linking one polyphenol to another or to other types of RMs in series—and establishing dual or even multiple catalytic systems in parallel with solid catalysts, transition metal ions, etc., also represents a strategy for regulating pollutant removal.
(3) Using polyphenols alone for pollutant removal is currently an emerging research direction, as the reaction often requires external energy to initiate. However, this process avoids pollution that may be caused by the introduction of other substances. If polyphenol-containing wastewater can be utilized to remove other pollutants, it would be an ideal approach for synergistic treatment, and the underlying mechanisms warrant further investigation.
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