Organic radicals for selective micropollutant removal in complex water

INTRODUCTION

Advanced oxidation processes have the capability to generate abundant inorganic radicals with high one-electron reduction potentials, such as hydroxyl radical (HO^•, 1.9-2.7 V) and sulfate radical (SO₄^•-, 2.5-3.1 V), which are extensively employed for the degradation of micropollutants in water, including endocrine disrupting chemicals, antibiotics, and pesticides^[1-3]. However, in real water bodies, complex matrices containing anions and dissolved organic matter (DOM) can interfere with the reactions of inorganic radicals. DOM is a heterogeneous mixture of organic compounds derived from humic and fulvic substances as well as anthropogenic activities, and it is rich in oxygen-containing functional groups such as phenolic hydroxyls and carboxyls. Both DOM and anions are typically present at mg·L^-1 levels, which can decrease the degradation efficiency of micropollutants present at ng·L^-1-μg·L^-1 levels^[4-6]. For example, the reaction rate of DOM with HO^• is ~10⁸ M_C^-1·s^-1, while the reaction rates of anions such as Cl^- and CO₃^2- with HO^• are ~10⁹ M^-1·s^-1 under neutral conditions^[6,7]. Notably, all kinetic parameters in this perspective are obtained at room temperature. To enhance the degradation efficiency of micropollutants, substantial amounts of oxidants and significant energy inputs are needed, thereby leading to elevated operational costs and carbon footprints^[8]. These limitations highlight an urgent need to develop oxidation strategies that are more resistant to matrix interference while maintaining high selectivity and efficiency for micropollutant degradation.

Organic radicals have recently emerged as promising alternatives or complements to conventional inorganic radicals in water treatment, primarily comprising O-centered, N-centered, C-centered, and S-centered radicals^[4,9,10]. Compared with inorganic radicals, organic radicals can be generated under milder oxidative conditions, and many exhibit high selectivity toward electron-rich micropollutants with improved tolerance to matrix effects, thereby offering intrinsic anti-interference advantages^[11,12]. For example, peracetic acid activation can be activated under ultraviolet (UV) irradiation or metal catalysis to generate acetyloxyl radical [CH₃C(O)O^•] and peroxyacetyl radical [CH₃C(O)OO^•], which are capable of selectively degrading micropollutants in water with reduced susceptibility to inorganic anions^[13]. In addition, various natural micromolecular redox mediators (a class of organic radical precursors), such as syringaldehyde, acetosyringone, violuric acid, 4-hydroxybenzoic acid, and catechol, can be transformed into organic radicals under redox conditions, including UV irradiation, enzyme-catalyzed oxidation, and metal redox processes. These transformations generate radicals such as phenoxy radicals (PhO^•) or semiquinone radicals (SQ^•-), which enable micropollutant degradation through mediated electron-transfer or radical-addition pathways^{[11,12,14-16]}. Notably, although macromolecular DOM has traditionally been regarded as a radical scavenger, it can also serve as a precursor of organic radicals (e.g., PhO^• and ROO^•) under photochemical or oxidative conditions, generating oxidative organic radicals that further degrade micropollutants^[4,17]. These findings open new possibilities for harnessing naturally abundant components in water matrices for micropollutant degradation.

Despite growing interest, the reactions between organic radicals and micropollutants remain insufficiently understood, with limited knowledge of their reaction selectivity and the potential application in water treatment. A deeper and systematic investigation is therefore required to fully elucidate their advantages and to enable their rational application in water treatment. This perspective highlights the unique advantages and potential applications of organic radicals in treating micropollutants in water, with a particular focus on their reaction with micropollutants and their compatibility with green and low-carbon treatment strategies.

SELECTIVE REACTIONS OF ORGANIC RADICALS AND MICROPOLLUTANTS

Compared with inorganic radicals such as HO^• and SO₄^•-, organic radicals generally exhibit moderate redox potentials (E⁰, Table 1). With the exception of certain alkoxy radicals, the E⁰ of most organic radicals is lower than the upper potential limit of oxide anions (approximately 1.6 V), thereby precluding the oxidation of other anions in aqueous systems by organic radicals [Figure 1]^[30]. In addition, our previous study shows that in systems containing SQ^•-, anions such as Cl^-, CO₃^2-, and Br^- do not affect its reaction with anilines, further confirming the resistance of organic radicals to anion quenching^[11]. Inorganic cations such as Na⁺, K⁺, Ca²⁺ and Mg²⁺ present in effluent at mg·L^-1 levels show negligible reactivity toward radicals because they neither readily gain nor donate electrons^[31]. Although SQ^•- can react with transition metal cations such as Fe²⁺ and Cu²⁺ at rates of 10⁵-10⁸ M^-1·s^-1, and ROO^• reacts with Fe²⁺ at ~10⁶ M^-1·s^-1 under acidic conditions, the relatively low concentrations of transition metal cations in effluent (approximately μg·L^-1) limit their quenching effect on organic radicals^[32-34]. For DOM, the second-order rate constants of it with ROO^• are around 10⁵-10⁶ M_C^-1·s^-1, which are significantly lower than those of HO^• with DOM (~10⁸ M_C^-1·s^-1) under neutral conditions^[7,35]. These properties reduce quenching by background constituents and contribute to the relatively long lifetimes and higher stability of organic radicals in water (μs-s, much longer than lifetimes of inorganic radicals, ns-μs)^[1,36,37]. Notably, the E⁰ of organic radicals is tunable and can be strongly modulated by molecular structure, substituents, and the surrounding chemical environment (e.g., pH and temperature)^[38,39]. These properties regulate organic radicals to preferentially react with electron-rich moieties commonly present in micropollutants, including aromatic rings, heterocycles, and electron-donating substituents^[32]. Thus, organic radicals provide intrinsic advantages for the selective removal of micropollutants in complex water matrices.

Figure 1. E⁰ of representative organic radicals.

The reaction kinetics between organic radicals and micropollutants differ markedly from those involving inorganic radicals. While HO^• typically reacts with various micropollutants (e.g., phenols and alcohols) at near diffusion-controlled rates (10⁸-10¹⁰ M^-1·s^-1, pH 1-9), organic radicals tend to exhibit compound-specific and structure-dependent rate constants^[40]. For example, for certain micropollutants containing electron-donating structures such as phenols, amines, and thiols, the second-order reaction rate constants of PhO^• with them can reach up to 10⁷-10⁹ M^-1·s^-1 under neutral and alkaline conditions, whereas reactions with alcohols, olefins, and fatty acid esters are slower, with rate constants of less than 10⁴ M^-1·s^-1[4,32]. These kinetics are closely related to the electronic properties of target micropollutants, including electron density distribution, ionization state, and substituent effects, which are influenced by adjacent electronegative atoms or the resonance stabilization of PhO^•[11]. Importantly, although the intrinsic reaction rates of organic radicals may be lower than those of inorganic radicals, their effective reactivity in real water systems can be comparable or even superior due to reduced competition from inorganic ions and DOM. This feature highlights the importance of evaluating organic radical-driven kinetics under environmentally relevant conditions.

Organic radicals can react with micropollutants through multiple pathways, including electron transfer, hydrogen atom abstraction, radical addition, and coupling or polymerization reactions [Figure 2]^[11,32,34]. In many cases, electron transfer dominates the initial interaction with electron-rich micropollutants, generating radical cation intermediates that subsequently undergo structural rearrangement or bond cleavage. In contrast, addition and polymerization reactions often proceed with high selectivity (10⁷-10⁹ M^-1·s^-1, pH 1–14) and improved controllability^{[11,32,34,41,42]}. These characteristics not only facilitate targeted micropollutant transformation but also suggest the possibility of converting micropollutants into high-value products during wastewater treatment. For instance, the rapid polymerization of PhO^• can be leveraged to convert phenolic compounds in water into recoverable polymers, providing a promising strategy for transforming pollutants into value-added resources^[41,42].

Figure 2. Differences between homogeneous and heterogeneous organic radical water treatment systems. DOM: Dissolved organic matter; UV: ultraviolet.

APPLICATIONS OF ORGANIC RADICALS IN WATER TREATMENT

While organic radicals exhibit selective degradation of micropollutants in real water, their inherent solubility in water presents potential toxicity risks, limiting their direct application for micropollutant degradation. For example, peracetic acid can trigger the formation of disinfection by-products in water containing halide ions^[43]. Similarly, the addition of redox mediators not only poses the risk of transformation into more toxic by-products, but some of these compounds can also generate colored transformation products, thereby adversely affecting water colorimetry and turbidity^[44].

A feasible strategy to circumvent these limitations is the immobilization of organic radical precursors, such as 2,2’-azino-bis (3-ethylbenzothiazoline-6-sulfonate), (2,2,6,6-tetramethylpiperidin-1-yl) oxyl and 9-azabicyclo [3,3,1] nonan-3-one-9-oxyl (ABNO), onto engineered nanosystems such as chitosan, cellulose, silica, and ferrite nanoparticles^[45,46]. Immobilization can reduce direct exposure of soluble mediators to the aquatic environment, mitigate leakage-related toxicity, and enable more controllable organic radical generation at solid-water interfaces [Figure 2]. For example, Zhang et al. successfully covalently immobilized ABNO onto the surface of silica. Upon the addition of permanganate, the immobilized ABNO is rapidly activated to generate oxygen-centered ABNO radicals, capable of efficiently degrading micropollutants^[45]. Nevertheless, the deployment of immobilized redox mediators faces challenges related to recyclability. Repeated activation cycles can weaken the active sites of the mediators through chemical transformation or surface passivation, resulting in decreased re-generability of redox mediators and gradual material deactivation.

Constructing regenerable organic radical precursors offers an alternative strategy to overcome these limitations. Under solar irradiation, quinone compounds can be efficiently converted into hydroquinone and SQ^•-[47]. Hydroquinone derivatives exhibit strong reducing ability and can be readily oxidized by molecular oxygen, generating SQ^•- and quinone species^[47]. Ye et al. synthesized donor-acceptor ligand structures containing anthraquinone derivatives^[48,49]. Specifically, upon light irradiation, the material undergoes charge-carrier separation, generating long-lived oxygen-centered organic radicals (SQ-like radical). These organic radicals subsequently react with micropollutants, converting into anthrahydroquinone-type species. The anthrahydroquinone moieties are then oxidized by molecular oxygen, regenerating the original structure. This closed photoredox cycle demonstrates high stability and reusability (≥ 50 cycles) under light irradiation while effectively suppressing secondary by-product accumulation, thus enhancing the environmental compatibility and operational safety of organic radical-based water treatment technologies.

SUMMARY AND PERSPECTIVE

In summary, organic radicals hold significant potential for degrading micropollutants in complex water bodies. However, several aspects still require further investigation.

The reaction kinetics of organic radicals with micropollutants remain poorly understood, and the transient spectrum is currently the primary experimental technique used to quantify these reactions^[11]. However, due to the substantial structural diversity of organic radicals, even subtle variations in functional groups can lead to significant changes in reactivity. Consequently, experimentally determining reaction kinetics is a complex, labor-intensive, and potentially unending process. This can be achieved by constructing comprehensive and standardized kinetic databases, systematically integrating molecular descriptors derived from quantum chemical calculations with structural features, and developing graph neural network models coupled with physics-informed or hybrid modeling frameworks to capture both molecular structure and underlying reaction mechanisms^[50]. In addition, rigorous cross-validation and independent experimental validation are necessary to ensure model reliability and generalizability. Such efforts would facilitate the rational design of more selective and efficient organic radical-based processes for water treatment applications.

In addition, DOM is an important source of organic radicals in real water matrices, and the targeted utilization of DOM-derived organic radicals holds great promise for water treatment applications. Previous studies have shown that SQ^•- in DOM can complex with MnO₂ or Zn²⁺, enhancing the stability and persistence of SQ^•-, which in turn improves the degradation of sulfonamide micropollutants^[51,52]. Future research should prioritize optimizing targeted strategies to enhance the generation of DOM-derived organic radicals while suppressing their self-decay. Specifically, controlled photoactivation approaches and catalytic mineral interfaces can be engineered to promote the selective formation of organic radicals^[17,51]. Surface complexation strategies may be employed to stabilize organic radicals and prolong their lifetimes^[51]. In addition, systematic optimization of solution chemistry parameters, such as pH and temperature, can further regulate radical concentrations.

Finally, although immobilized organic radical precursors or radical-generating materials show clear potential for application in packed-bed reactors and catalytic membrane systems, their practical performance is governed by a critical trade-off between radical reactivity and material stability^[45]. In immobilized organic radical systems, reactions mainly occur at the solid-liquid interface, requiring micropollutants to diffuse to the material surface before degradation. Under high-flow or high-throughput conditions, mass transfer limitations can markedly reduce treatment efficiency, particularly in packed-bed or fixed-bed reactors. Addressing this issue requires rational material and reactor design to enhance mass transport while maintaining radical reactivity and structural stability. Effective strategies include constructing hierarchical micro-meso-macroporous architectures to facilitate micropollutant diffusion, increasing specific surface area without compromising mechanical strength, and optimizing the spatial distribution of active sites to minimize radical self-quenching^[53,54]. In addition, flow-through catalytic membranes can reduce boundary layer resistance, and fluidized-bed or structured monolith reactors can improve hydrodynamic performance^[55].