Constructed Wetlands Beyond the Fenton Limit: A Systematic Review on the Circular Photo-Biochemical Catalysts Design for Sustainable Wastewater Treatment

Abstract

Constructed wetlands (CWs) are signified as green, self-sustaining systems for wastewater treatment. To date, their conventional designs struggle with slow kinetics and poor removal of refractory pollutants. This review redefines CWs as photo-reactive engineered systems, integrating near-neutral Fenton and photo-Fenton processes and in-situ oxidant generation to overcome diffusion limits, acid dosing, and sludge formation. By coupling catalytic fillers, solar utilization, and plant–microbe–radical (ROS) synergies, the approach enables intensified pollutant degradation while preserving the low-energy nature of CWs. Bibliometric trends indicate a sharp rise in studies linking CWs with advanced oxidation and renewable energy integration, confirming the emergence of a circular treatment paradigm. A decision framework is proposed that aligns material selection, reactor hydrodynamics, and solar light management with sustainability indicators such as energy efficiency, Fe-leach budget, and ROS-to-photon yield. This synthesis bridges environmental biotechnology with solar-driven catalysis, paving the way for next-generation eco-engineered wetlands capable of operating efficiently beyond the classical Fenton constraints. This work introduces the concept of “Constructed Wetlands Beyond the Fenton Limit”, where CWs are reimagined as photo-reactive circular systems that unify catalytic, biological, and solar processes under near-neutral conditions. It provides the first integrated decision matrix and performance metrics connecting catalyst design, ROS efficiency, and circular sustainability that offers a scalable blueprint for real-world hybrid wetland applications.

1. Introduction

Water scarcity and contamination have become among the most formidable global challenges, threatening ecosystem stability, food security, and human health alike [1,2]. Rapid urbanization, industrialization, and unsustainable agricultural practices have intensified the release of chemical pollutants into water bodies, compromising freshwater availability and quality. Emerging contaminants (ECs) that are signified as broad class including pharmaceuticals, personal care products (PCPs), endocrine-disrupting compounds, and pesticides that pose critical ecological and health threats due to their persistence and bioaccumulation potential [3,4,5]. Pharmaceutical residues such as paracetamol, ibuprofen, diclofenac, sulfamethoxazole, and carbamazepine, together with industrial dyes like methylene blue and Reactive Red 195, resist conventional biological degradation owing to their complex aromatic structures, high molecular stability, and electron-donating/withdrawing substituents that hinder enzymatic breakdown [6,7,8]. Once released into aquatic systems, these compounds undergo incomplete degradation, sorption onto sediments, or accumulation in aquatic biota, leading to chronic ecological risks [9]. Hence, treating such effluents is essential.

Traditional wastewater treatment plants (WWTPs) rely primarily on physical separation, sedimentation, and microbial oxidation under aerobic or anaerobic conditions, which are insufficient to mineralize such refractory molecules [10,11]. Consequently, these micropollutants persist in treated effluents, sludge, and biosolids, re-entering natural ecosystems and even drinking water sources through effluent reuse, irrigation, and groundwater infiltration [12]. Chronic exposure to these compounds has been correlated with endocrine disruption, antibiotic resistance, and bio-toxicity, highlighting the limitations of current wastewater treatment strategies [13].

Constructed wetlands (CWs) have gained prominence as sustainable, cost-effective, eco-engineered solutions for wastewater treatment, functioning as nature-based systems that harness the synergistic activity of plants, microorganisms, and substrate media [14,15]. These systems operate via integrated physical, chemical, and biological processes (e.g., sedimentation, adsorption, microbial degradation, and plant uptake [16]. CWs can be classified into three main types: free water surface (FWS), horizontal subsurface flow (HSSF), and vertical flow (VF) wetlands [17]. Each configuration offers distinct hydrological and redox conditions that affect contaminant removal mechanisms. For instance, VF wetlands provide improved oxygen transfer and support nitrification, while HSSF systems favor denitrification and organic removal [18]. Despite their proven success in removing nutrients, suspended solids, and biodegradable organic matter, conventional CWs exhibit limited performance toward recalcitrant and xenobiotic pollutants such as pharmaceuticals, synthetic dyes, phenolic compounds, and endocrine disruptors [19,20]. The underlying limitation stems from the low oxidation–reduction potential (ORP) of the wetland microenvironment, which constrains the oxidative transformation of persistent organic molecules [21]. Moreover, the heterogeneous distribution of oxygen within the rhizosphere and the dominance of facultative microbial populations favors partial biodegradation or transformation into intermediate products rather than complete mineralization [22]. To overcome these kinetic and redox constraints, researchers have advanced the concept of hybrid constructed wetlands (HCWs) that integrate advanced oxidation processes (AOPs) to complement biological degradation under benign conditions [23].

AOPs encompass a family of physicochemical techniques that include ozonation, photocatalysis, electro-Fenton, photo-Fenton, and persulfate activation that rely on the in-situ generation of reactive oxygen species (ROS), primarily hydroxyl radicals (•OH, E° +2.80 V) and superoxide anions (•O₂⁻, E° −0.33 V), as well as singlet oxygen (¹O₂) and hydroperoxyl radicals (HO₂•) [24,25,26]. These species possess ultra-short lifetimes (10⁻⁶–10⁻⁹ s) but extremely high reactivity, enabling non-selective oxidation of a wide spectrum of organic pollutants into CO₂, H₂O, and inorganic ions. Among AOPs, Fenton-based systems, which are driven by the catalytic decomposition of H₂O₂ in the presence of Fe²⁺, stand out due to their simplicity, high reaction rate constants, and capacity to operate under low-energy input [27]. However, classical homogeneous Fenton reactions are limited by their narrow operational pH (2.5–3.5) and the generation of ferric sludge, which complicates recovery and reuse. The evolution toward heterogeneous and photo-Fenton processes has addressed these issues by immobilizing iron species onto solid supports or by employing photo-responsive catalysts capable of Fe³⁺ into Fe²⁺ photoreduction under UV or visible light irradiation [28]. Thus, among advanced oxidation processes, the photo-Fenton process has gained recognition as one of the most efficient and sustainable methods for degrading a broad spectrum of refractory pollutants [29,30].

Despite decades of optimization, conventional Fenton and photo-Fenton systems face intrinsic physicochemical boundaries that are often referred to as the “Fenton Limit.” Such a “Fenton limit” refers to the practical operational constraint of conventional Fenton processes arising from the need for strongly acidic conditions (pH 2–4) and excessive iron sludge formation, beyond which increasing Fe²⁺ or H₂O₂ dosages no longer improve treatment efficiency [7,8,9]. This concept represents the constraints imposed by pH sensitivity, ferric sludge generation, and incomplete Fe³⁺/Fe²⁺ cycling, which collectively restrict their large-scale applicability. Overcoming this limit requires hybridization with sustainable, multifunctional systems capable of extending catalytic reactivity beyond traditional boundaries. The integration of modified-based catalysts with constructed wetlands (CWs) offers a transformative pathway to transcend this limit, combining solar-driven ROS production, biological resilience, and circular resource recovery within a single eco-engineered framework.

This review reconceptualizes constructed wetlands (CWs) as integrated photo biochemical reactors by embedding near-neutral Fenton and photo-Fenton pathways and in situ oxidant generation within the wetland matrix, thereby overcoming classical diffusion, acidic pH, and iron-sludge constraints without the need for external acid dosing or intensive sludge management. Building on this concept, the study formalizes a circular-nexus decision framework that unites waste-valorized iron-based catalysts, plant–microbe mediated ROS synergies, and solar-driven photonic processes into a unified performance–sustainability paradigm. The framework defines key decision layers, including pollutant characteristics (biodegradable versus refractory), process drivers (iron availability, oxidant demand, and light source), and system constraints (energy input, operational pH, sludge generation, and ecological compatibility). By explicitly linking mechanistic pathways such as Fe²⁺/Fe³⁺ redox cycling, reactive oxygen species generation, and photon-assisted iron regeneration with wetland functions (substrate–plant–microbe interactions and passive hydraulics), the framework guides technology selection, configuration, and system integration. Furthermore, the study introduces a quantitative decision matrix and scale-up metrics, including specific energy consumption (EE/O), reactive oxygen species per photon yield (ROS/photon), and iron-leach budget, to bridge laboratory-scale innovation with pilot- and full-scale implementation under real wastewater conditions. Collectively, these elements establish a practical, transferable blueprint for developing circular photo-biochemical constructed wetlands that advance both treatment performance and sustainable design principles beyond conventional descriptive reviews.

2. Mechanistic Fundamentals of the Photo-Fenton Reaction

The photo-Fenton process is an advanced oxidation technique that operates through the catalytic activation of hydrogen peroxide (H₂O₂) by ferrous ions (Fe²⁺) to produce reactive oxygen species (ROS), particularly hydroxyl radicals (•OH), which exhibit one of the highest oxidation potentials among all oxidants [31,32,33]. These radicals can non-selectively oxidize a wide spectrum of organic contaminants, transforming them into CO₂, H₂O, and mineral acids. The fundamental mechanism involves a redox cycle between Fe²⁺ and Fe³⁺ ions, in which H₂O₂ acts as the oxidant, while light irradiation accelerates the photoreduction of Fe³⁺ back to Fe²⁺, sustaining radical production under continuous illumination [34,35,36,37,38,39]. The introduction of light (UV or visible) significantly enhances the Fenton reaction by promoting excess conversion of Fe³⁺ into Fe²⁺ regeneration and facilitating photo-induced decomposition of Fe(OH)²⁺ complexes. This photo-assisted pathway increases both the quantum yield and catalytic efficiency, allowing the process to operate effectively at near-neutral pH [40,41]. Compared to other advanced oxidation processes, photo-Fenton reactions combine simplicity, high kinetics, and environmental compatibility [42,43,44,45,46]. However, challenges such as narrow pH constraints and sludge formation in homogeneous systems have stimulated the exploration of heterogeneous catalytic alternatives [47,48,49,50,51,52].

Figure 1 illustrates the photo-Fenton reaction cycle, highlighting how solar irradiation synergistically couples with iron-based catalysis to accelerate pollutant mineralization through a self-regenerating redox mechanism. As shown, the process is governed by cyclic Fe²⁺/Fe³⁺ transformations in the presence of hydrogen peroxide (H₂O₂) and light (hν). The reaction is initiated by the oxidation of Fe²⁺ by H₂O₂, generating hydroxyl radicals (•OH) responsible for the oxidative degradation and mineralization of organic pollutants. This step converts Fe²⁺ to Fe³⁺, which is subsequently photoreduced back to Fe²⁺ under UV or visible-light irradiation, sustaining continuous radical production. Secondary reactive species, including hydroperoxyl (HO₂•) and superoxide (O₂•⁻) radicals, further contribute to oxidative pathways. Overall, light-assisted iron redox cycling enhances catalytic efficiency and mitigates the strict acidic pH constraints of conventional Fenton systems.

2.1. Heterogeneous Photo-Fenton Catalysis Using Modified Catalyst

Heterogeneous photo-Fenton catalysis has emerged as a sustainable and efficient alternative to traditional homogeneous Fenton systems, which require acidic conditions (pH 3.0) and generate excessive iron sludge. By immobilizing Fe species onto solid supports, heterogeneous systems achieve catalyst recovery, reusability, and improved stability, thus enabling operation under near-neutral conditions [53,54]. However, pristine Fe-based catalysts often suffer from limited surface area, low light-harvesting capability, and gradual Fe-leaching during reaction cycles, leading to decreased catalytic efficiency. Therefore, extensive efforts have been devoted to developing modified catalysts that enhance redox activity, electron mobility, and long-term sustainability [55,56,57,58,59].

2.1.1. Structural and Surface Modifications

Incorporation of transition-metal dopants such as Cu, Co, Mn, or Ni into Fe-based oxides and spinel significantly alters the Fe²⁺/Fe³⁺ redox equilibrium and enhances photoinduced charge transfer. For example, Fe–Cu–O and Fe–Co–O systems exhibit dual redox couples (Fe²⁺/Fe³⁺ and Cu⁺/Cu²⁺ or Co²⁺/Co³⁺), which accelerate the regeneration of Fe²⁺ and improve radical production efficiency under visible light [60,61]. Furthermore, Cu doping also narrows the band gap and promotes visible-light utilization, whereas Co substitution stabilizes the crystalline lattice and improves oxygen vacancy formation [62,63,64,65]. Moreover, coupling Fe oxides with semiconductors such as TiO₂, ZnO, BiFeO₃, or g-C₃N₄ forms efficient heterojunctions that facilitate charge separation and extend the light response into the visible region [66,67,68,69,70,71]. For instance, Mn–TiO₂–Fe composites exhibit improved photocatalytic efficiency for antibiotic degradation under solar irradiation due to enhanced charge mobility and multiple Fe redox centers [72].

2.1.2. Support Modification and Dispersion Control

Catalyst performance strongly depends on the dispersion and accessibility of Fe active sites. To overcome aggregation and leaching, Fe species are often immobilized onto biochar, zeolite, graphene oxide (GO), or clay supports [4,12,73,74,75,76,77]. Biochar, derived from agricultural residues (e.g., banana peel, potato shell, or rice husk), provides a porous matrix rich in functional groups (–OH, –COOH, –C=O) that anchor Fe ions and stabilize their valence states [78,79,80]. Fe₃O₄@biochar composites integrate the high surface area of biochar with magnetic recovery capability, minimizing secondary pollution while enhancing radical generation and adsorption synergy [15]. Similarly, Fe-loaded zeolites exhibit uniform Fe distribution within microporous frameworks, increasing contact efficiency with pollutants and improving reusability [16]. Functionalization of supports with nitrogen or sulfur groups can further modulate the Fe coordination environment, increase electron density, and strengthen Fe–O bonding, which enhances both catalytic stability and activity [17].

2.1.3. Mechanistic Enhancement

Modified catalysts significantly accelerate radical generation in heterogeneous photo-Fenton systems through improved charge separation, enhanced Fe²⁺ regeneration, and active-site engineering. Under solar or visible-light irradiation, photoexcited electrons generated from semiconductor supports or doped metal centers efficiently reduce Fe³⁺ to Fe²⁺, thereby sustaining the Fe²⁺/Fe³⁺ catalytic cycle and maintaining continuous reactive oxygen species (ROS) production (Equations (1)–(14)) [22,28]. Further, the catalytic cycle is illustrated in Figure 2, depicting a heterogeneous photo-Fenton system operating under solar or visible-light irradiation. Upon photoexcitation, electrons generated from the semiconductor support participate in the reduction of Fe³⁺ to Fe²⁺, thereby sustaining the Fenton catalytic cycle. This continuous Fe²⁺ regeneration promotes the persistent formation of reactive oxygen species, particularly hydroxyl radicals (•OH) and superoxide radicals (•O₂⁻), which play a dominant role in pollutant degradation. In addition, dopant redox couples such as Cu⁺/Cu²⁺ and Co²⁺/Co³⁺ facilitate interfacial charge transfer and suppress electron–hole recombination, while surface oxygen vacancies serve as active sites for H₂O₂ adsorption and activation, further enhancing oxidative degradation pathways.

$${Fe}^{2+}+H_2{O}_2{\to Fe}^{3+}+{OH}^{-}{+OH}^{·}$$

(1)

$${Fe}^{3+}{+OH}_2^{·}{\to Fe}^{2+}+{O_2+H}^{+}$$

(2)

$$H_2{O}_2+{HO}^{·}{\to H^{\mp }+O}_2^{·}+H_{2}O$$

(3)

$${Fe}^{3+}+H_2{O}_2{\to Fe}^{2+}+{{OH}_2^{·}+H}^{+}$$

(4)

$${Fe}^{3+}+e^{-}{\to Fe}^{2+}$$

(5)

$$H_2{O}_2+{HO}^{·}{\to HO}_2^{·}+H_{2}O$$

(6)

$${Fe}^{2+}{+OH}^{·}{\to Fe}^{3+}+{OH}^{-}$$

(7)

$${OH}^{·}+{OH}^{·}\to H_2{O}_2$$

(8)

$${2OH}_2^{·}{\to H}_2{O}_2+O_2$$

(9)

$${Fe}^{2+}{+OH}_2^{·}{{+H}^{+}\to Fe}^{3+}+H_2{O}_2$$

(10)

$${OH}^{·}+{Fe}^{2+}\to {Fe}^{3+}+{OH}^{-}$$

(11)

$$H_2{O}_2+{HO}_2^{·}\to H_{2}O+O_2+{HO}^{·}$$

(12)

$${Fe}^{2+}+H_2{O}_2{+hv\to Fe}^{3+}+{OH}^{-}{+OH}^{·}$$

(13)

$$H_2{O}_2+hv\to 2{OH}^{·}$$

(14)

Transition-metal dopants such as Cu⁺/Cu²⁺ or Co²⁺/Co³⁺ serve as auxiliary redox mediators, accelerating interfacial electron transfer and extending the photo response into the visible-light region [6,10]. Furthermore, the introduction of surface oxygen vacancies and structural defects enhances H₂O₂ adsorption and activation, facilitating the formation of hydroxyl (•OH) and superoxide (•O₂⁻) radicals. In parallel, porous carbonaceous supports such as biochar and metal oxide matrices provide diffusion channels and abundant surface sites that promote synergistic oxidative–adsorptive degradation pathways [13,19]. These cooperative effects collectively enhance radical yield, improve catalytic efficiency, and broaden the operational pH window up to 6.5–7.0 without notable loss of activity [4,15,20].

Table 1. Comparison of Modified Heterogeneous Photo-Fenton Catalysts for Wastewater Treatment Applications.

Catalyst Type	Pollutant	pH	Light Source	Removal Efficiency/Time	Reusability	Key Findings	Ref.
Fe–CuO composite	Rhodamine B, phenol	4–6	Visible light	95–99% within 60–90 min	5 cycles (>90%)	Cu+/Cu2+ coupling accelerates Fe2+ regeneration and extends visible-light absorption	[80]
Fe–Co–O spinel	Methylene blue	5–7	Visible light	98% in 60 min	4 cycles (>85%)	Synergistic Fe/Co redox couples enhance interfacial electron transfer and structural stability	[80]
Fe–Mn–TiO2 nanocomposite	Tetracycline	6.5	Solar simulator	92% in 120 min	6 cycles (≈90%)	Mn doping narrows the band gap and suppresses e−/h+ recombination, improving visible response	[82]
Fe3O4@Biochar (from banana peel)	Reactive Red 195	6.8	Solar light	96% in 90 min	5 cycles (>88%)	Biochar matrix stabilizes Fe sites, improves adsorption, and enables magnetic recovery	[83]
Fe-doped g-C3N4	Carbamazepine	7	Visible light	93% in 80 min	5 cycles (>90%)	Fe doping introduces mid-gap states and enhances visible-light harvesting and Fe2+ cycling	[82]
Fe–Co-MOF (MIL-53(Fe/Co))	Rhodamine B, bisphenol A	5.5–6.5	Visible light	99% in 40 min	6 cycles (>95%)	Co-doping reinforces structural integrity and boosts photo-Fenton reaction rate	[84]
Fe–Cu-MOF (MIL-100(Fe/Cu))	Paracetamol	6.0	Visible light	97% in 60 min	5 cycles (>90%)	Cu nodes accelerate H2O2 activation and mitigate Fe leaching under neutral pH	[85]
Fe–Biochar (from potato peel)	Procion Blue	6.5	Solar light	94% in 120 min	4 cycles (>85%)	Biochar promotes pollutant adsorption and enhances electron mobility under solar conditions	[83]
Fe3O4@Zeolite composite	Methyl orange	6	UV–Vis	95% in 70 min	5 cycles (>90%)	Zeolite framework improves Fe dispersion and radical generation while allowing recovery	[82]
Fe–Cu@Graphene oxide	Phenol	6–7	Visible light	97% in 50 min	6 cycles (>92%)	Graphene network enhances charge separation, conductivity, and visible-light absorption	[86]

summarizes representative heterogeneous photo-Fenton catalysts developed over the past decade, focusing on compositional tuning, pollutant specificity, operational pH, illumination source, and catalytic recyclability. Collectively, these systems demonstrate remarkable advances toward visible- and solar-driven oxidation of persistent organic contaminants under environmentally benign conditions. As shown in the table, a clear pattern emerges in the enhancement strategies employed. Binary and ternary metal oxides such as Fe–CuO and Fe–Co–O exhibit synergistic redox interactions between Fe²⁺/Fe³⁺ and Cu⁺/Cu²⁺ (or Co²⁺/Co³⁺) couples, which significantly accelerate interfacial electron transfer and sustain continuous Fe²⁺ regeneration [80,81]. This synergy mitigates the classical limitation of homogeneous Fenton systems, namely, the slow reduction of Fe³⁺ under near-neutral pH, thereby maintaining high radical productivity even at pH 5–7. For example, Fe–Co spinels achieved 98% methylene blue degradation within 60 min under visible light, retaining over 85% efficiency after four cycles, underscoring the structural stability of spinel frameworks and their superior charge mobility [80].

Additionally, Semiconductor coupling and band-gap engineering also feature prominently. Incorporation of Mn into TiO₂, or Fe doping into g-C₃N₄, narrows the band gap and introduces defect-related mid-gap states, thereby extending photo response into the visible spectrum while suppressing e⁻/h⁺ recombination [82]. Such modifications enhance charge separation and reactive oxygen species (ROS) generation, as evidenced by the 92–93% removal efficiencies for tetracycline and carbamazepine under visible or solar irradiation. The photocatalytic response of these materials often correlates with improved H₂O₂ activation and the formation of both •OH and •O₂⁻ radicals.

Support engineering further augments catalytic performance by stabilizing Fe active sites and facilitating mass transfer. Biochar-based composites derived from agricultural residues (e.g., banana or potato peels) combine magnetic Fe₃O₄ cores with high-surface-area carbon matrices, enabling efficient pollutant adsorption, enhanced electron mobility, and simple magnetic recovery after reaction [83]. Similarly, Fe₃O₄@zeolite and Fe–Cu@graphene oxide architectures provide large surface dispersion and conductive networks that promote radical diffusion and interfacial charge separation, achieving >95% degradation of dyes and phenols at near-neutral pH within 70 min.

Particularly noteworthy are metal–organic frameworks (MOFs) such as MIL-53(Fe/Co) and MIL-100(Fe/Cu), which integrate molecular-level metal clusters with tunable porosity. Co- and Cu-doping within Fe-MOF frameworks not only enhances structural robustness and redox cycling but also broadens visible-light absorption through ligand-to-metal charge transfer (LMCT) mechanisms [84,85]. These catalysts typically exhibit >97% pollutant removal within 40–60 min, with negligible Fe leaching and >90% retention after five to six cycles, highlighting their promise as reusable, high-efficiency photo-Fenton catalysts under quasi-neutral conditions.

Despite these advances, scalability and operational stability remain persistent challenges. Most reports employ synthetic dyes or pharmaceuticals in simplified aqueous matrices, and long-term performance under real wastewater conditions (with fluctuating ionic strength, natural organic matter, and radical scavengers) remains poorly understood. Catalyst fouling, structural degradation, and gradual metal leaching can occur under extended operation, necessitating rigorous post-use characterization and life-cycle assessment [86]. Furthermore, optimizing light penetration, mass transfer, and reactor design remains critical for translating laboratory efficiencies into large-scale continuous systems. Overall, the catalysts in Table 1 exemplify the ongoing transition from traditional homogeneous Fenton chemistry toward robust, visible-light-responsive, and recyclable heterogeneous systems capable of operating near neutral pH with minimal sludge formation. Continued innovations in multi-metal doping, hierarchical support design, and hybridization with carbonaceous or bio-derived matrices are expected to further elevate photo-Fenton performance and sustainability, positioning these materials as cornerstone technologies in next-generation eco-engineered wastewater treatment.

3. Constructed Wetlands

3.1. Conceptual Basis of Constructed Wetlands

Constructed wetlands (CWs) are engineered ecosystems intentionally designed to emulate the natural purification processes of wetland environments for wastewater treatment, stormwater management, and ecological restoration [14,15]. Functionally, they combine biological, chemical, and physical mechanisms, including sedimentation, filtration, adsorption, plant uptake, and microbial degradation, to achieve comprehensive pollutant removal under controlled environmental conditions [16]. The integration of these processes enables the simultaneous elimination of organic matter, nutrients, suspended solids, pathogens, and emerging contaminants in a cost-effective and environmentally compatible manner. From a theoretical standpoint, CWs can be conceptualized as controlled biogeochemical reactors operating under steady-state or quasi-steady-state conditions. Within these systems, hydraulic flow regimes, redox gradients, and biological interactions are optimized to sustain pollutant transformation pathways [17]. Unlike conventional wastewater treatment plants (WWTPs) that rely on external energy inputs and mechanical aeration, CWs function as passive and self-regulating systems. They utilize solar energy, natural convection, and plant-mediated oxygen transport to maintain redox balance and microbial activity, allowing adaptation to fluctuating pollutant loads and climatic variations [18,19].

The conceptual foundation of CWs is grounded in eco-hydrological principles, emphasizing the interaction between hydraulic dynamics, substrate properties, vegetation, and microbial consortia as the primary determinants of treatment efficiency [20]. These interdependent parameters govern oxygen diffusion, nutrient cycling, and organic degradation, creating spatially heterogeneous microenvironments that favor both aerobic and anaerobic processes. Consequently, CWs are viewed as open thermodynamic systems—absorbing energy (mainly solar), transforming pollutants through oxidative and reductive pathways, and exporting treated effluents while maintaining internal ecological equilibrium [21].

3.2. Classification and Design Configurations of Constructed Wetlands

Constructed wetlands (CWs) are categorized according to hydrology, flow regime, and vegetation type, which collectively determine the internal redox distribution, hydraulic performance, and pollutant removal pathways [5,6,7,8,40]. These engineered eco-systems are generally grouped into free water surface (FWS), subsurface flow (SSF), and hybrid or intensified CWs, each optimized for specific water quality goals and site conditions.

3.2.1. Free Water Surface (FWS) Wetlands

FWS wetlands replicate the morphology and ecological function of natural marshes, featuring shallow water flow (typically 0.2–0.4 m depth) over a vegetated substrate dominated by emergent macrophytes such as Phragmites australis, Typha latifolia, and Scirpus spp. [6,8,40]. The treatment mechanisms involve sedimentation, adsorption onto biofilms and sediments, nutrient assimilation by plants, and photolytic degradation at the air/water interface [5,6]. Such systems are often applied as tertiary or polishing stages following biological treatment, improving BOD, COD, nitrogen, and pathogen reduction in municipal and agricultural wastewater [8]. The advantages of such systems include low-energy demand, high biodiversity potential, and aesthetic value. However, large land requirements, evaporation losses, and reduced efficiency during cold seasons remain significant limitations [40,45]. Figure 3 depicts a free water surface (FWS) constructed wetland, where shallow wastewater (0.2–0.4 m) flows over a vegetated substrate with emergent macrophytes (Phragmites australis, Typha latifolia, Scirpus spp.). Pollutant removal occurs through sedimentation, adsorption on biofilms, nutrient uptake by plants, and photolytic degradation under sunlight, producing a cleaner effluent.

3.2.2. Subsurface Flow Wetlands

Subsurface flow (SSF) systems transport wastewater through a porous medium (e.g., gravel, sand, or biochar) below the surface, eliminating odor and mosquito issues while promoting microbial degradation and adsorptive filtration [5,6,7]. Depending on flow direction and oxygen availability, SSF wetlands are further divided into:

Horizontal Subsurface Flow (HSSF) Wetlands

In HSSF wetlands, wastewater flows laterally through a saturated porous matrix. The predominance of anoxic and reducing conditions favors denitrification, organic matter degradation, and phosphorus adsorption [6,40]. These systems offer high stability against hydraulic shock loads and are well-suited for low-strength domestic or industrial effluents.

Vertical Flow (VF) Wetlands

In VF wetlands, influent percolates vertically through the substrate, alternating between wet and dry cycles that enhance oxygen diffusion and aerobic microbial activity, promoting ammonium oxidation and nitrification [8,40]. Periodic feeding and drainage prevent clogging and increase the effective oxygen transfer rate, making VF systems suitable as primary or secondary treatment units prior to polishing stages.

The choice between HSSF and VF configurations is dictated by site hydrology, climate, influent type, and performance requirements. In practice, alternating or sequential configurations are often employed to maximize both aerobic and anaerobic processes [7,41]. Figure 4 exhibited the schematic illustration between these two types of HSSF and VF configurations. Figure 4a illustrates the configuration and treatment mechanisms of a Horizontal Subsurface Flow (HSSF) constructed wetland, where wastewater flows laterally through a saturated porous medium (commonly gravel or sand) beneath the surface. Wastewater flows horizontally through a saturated porous bed planted with macrophytes, where denitrification, organic matter degradation, and phosphorus adsorption occur under predominantly anoxic conditions. The absence of a free water layer promotes anoxic and reducing conditions, ideal for denitrification, organic matter degradation, and phosphorus adsorption. The planted macrophytes (Phragmites australis, Typha latifolia, etc.) enhance microbial activity and maintain hydraulic conductivity. HSSF wetlands offer high process stability, resistance to hydraulic shocks, and effective pollutant removal for low-strength domestic and industrial effluents. Figure 4b illustrates a vertical flow constructed wetland (VF-CW) where wastewater percolates downward through sand and gravel layers. Oxygen diffusion from plant roots sustains aerobic zones for organic oxidation and nitrification, while deeper anoxic layers enable denitrification and further pollutant removal. The intermittent feeding cycles enhance oxygen transfer and prevent clogging, making VF-CWs efficient, low-energy systems for advanced Horizontal wastewater treatment.

3.2.3. Hybrid and Intensified Constructed Wetlands

Hybrid constructed wetlands (HCWs) integrate multiple flow regimes, typically vertical followed by horizontal flow, to exploit the complementary advantages of aerobic oxidation and anoxic denitrification within a single treatment train [7,8,15,42]. These designs significantly enhance nitrogen removal, COD degradation, and resilience to variable hydraulic loading. In recent years, intensified or advanced CWs have incorporated aeration, recirculation, and advanced oxidation modules such as photo-Fenton, ozonation, UV-assisted catalysis to accelerate degradation kinetics and extend treatment to emerging contaminants such as pharmaceuticals and dyes [9,15,16,42]. Such systems, often termed photo-reactive constructed wetlands (PR-CWs), employ Fe–MOF-based catalysts immobilized on substrates such as biochar, zeolite, or gravel. This configuration enables a circular treatment nexus that integrates biological, chemical, and photochemical processes [10,20,42]. Hence, the integration of biological, chemical, and photochemical mechanisms in hybrid PR-CWs establishes a circular treatment nexus, aligning with sustainable development principles by coupling low energy input, resource recovery, and pollutant mineralization [22,37,46]. Schematic illustration (Figure 5) of a hybrid photo-reactive constructed wetland (PRCW).

The diagram integrates the structural and functional components of hybrid and intensified constructed wetlands. Wastewater enters through the inlet and sequentially passes through biological (aerobic oxidation), chemical (anoxic denitrification), and photochemical (advanced oxidation) zones. The Fe–MOF catalyst, immobilized on biochar, zeolite, or gravel, facilitates photo-Fenton reactions under solar irradiation, generating reactive oxygen species (ROS) for enhanced degradation of refractory pollutants. Recirculation loops sustain hydraulic balance and increase oxygen transfer, while macrophytes (e.g., Phragmites australis) enhance photodegradation and oxygen release into the rhizosphere. This system combines natural and engineered mechanisms of biological, chemical, and photochemical processes that form a circular, energy-efficient, and sustainable water treatment process as seen in the inset of Figure 5.

3.3. Photo-Reactive Constructed Wetlands (PR-CWs)

Integration Mechanisms of PR-CWs

The integration of MOF-based photo-Fenton catalysts into constructed wetlands (CWs) has given rise to photo-reactive constructed wetlands (PR-CWs) as the hybrid eco-engineered systems that synergize photochemical oxidation with biological purification [21,22]. In PR-CWs, Fe-based MOFs are immobilized on porous media such as zeolite, gravel, sand, or biochar within the wetland bed, where they catalyze in situ ROS generation under sunlight exposure. Simultaneously, plants (e.g., Phragmites australis, Typha latifolia) and rhizosphere microorganisms enhance oxygen transport, Fe redox cycling, and the biodegradation of intermediate products [23]. This configuration allows a dual pathway for pollutant degradation: (i) abiotic oxidation by photo-generated ROS, and (ii) biotic mineralization through microbial metabolism and plant uptake. The oxidative pre-treatment by photo-Fenton reactions transforms refractory compounds into smaller, more biodegradable intermediates, which are subsequently metabolized by microbial consortia. This synergistic interplay enhances pollutant removal efficiency while maintaining ecological equilibrium [24]. Theoretically, PR-CWs embody a bio–photo–chemical interface, where light, catalyst, and biology interact to achieve deep mineralization of contaminants. Such systems exemplify a shift toward self-sustaining, solar-driven treatment technologies that merge ecological functions with material-based catalysis [25].

The biochar/MOF interface improves electron shuttling, enhances catalyst dispersion, and reduces Fe-leaching while adding adsorption sites that preconcentrate micropollutants at the reaction front [22,28,36,41]. Within the wetland bed, macrophytes (e.g., Phragmites, Typha) leak oxygen to the rhizosphere and exude organics that shape microbial consortia; this supports nitrification in micro-aerobic zones, denitrification in anoxic niches, and rhizo degradation of partially oxidized intermediates formed upstream by photo-Fenton attack [5,6,7,8,37,43]. Hydraulic regime (VF vs. HSSF) governs redox zoning and contact time: VF units favor oxygen transfer and fast oxidation; HSSF sections sustain denitrification and polishing. Hybrid layouts (VF to HSSF with recirculation) exploit both regimes and dampen load shocks [7,8,9,42]. Optical management and mass-transfer control are pivotal. Bed geometry, water depth, particle size, and plant density modulate photon penetration and H₂O₂ distribution; reflective elements or shallow illuminated layers can mitigate light attenuation reported for vegetated beds [15,16,48]. Careful oxidant dosing avoids quenching or biotoxicity and preserves wetland biota [9,19,47]. Overall, PR-CWs function as bio-photo-chemical reactors in which adsorption, ligand-to-metal charge transfer, LMCT-driven ROS formation, microbial metabolism, and plant-mediated oxygenation are intentionally co-designed to achieve deep mineralization of pharmaceuticals and dyes at low energy input [9,10,11,12,20,22,41,42].

Figure 6 conceptually illustrates the integrated “Circular Nexus” between constructed wetlands, metal–organic frameworks (MOFs), and photo-Fenton catalysis for sustainable wastewater treatment. The system combines biological, photochemical, and catalytic pathways: sunlight enhances plant-driven bio-oxygenation and nutrient uptake, while biochar and gravel substrates support root filtration and microbial activity. Concurrently, MOF catalysts facilitate Fe²⁺/Fe³⁺ redox cycling, generating reactive oxygen species (ROS) such as hydroxyl radicals (•OH) that degrade pollutants. This synergistic configuration creates a self-sustaining loop of adsorption, oxidation, and nutrient recovery, aligning with circular economy principles.

Furthermore, the schematic depicts the synergistic mechanism of modified Fe-driven photo-Fenton catalysis operating inside a constructed wetland matrix. The framework contains redox-active Fe²⁺/Fe³⁺ centers and open metal sites (OMS) that catalyze the activation of hydrogen peroxide (H₂O₂) into reactive oxygen species (ROS), principally hydroxyl (•OH) and superoxide (•O₂⁻) radicals. Under solar irradiation (hν), the processes accelerate Fe³⁺ photoreduction to Fe²⁺, sustaining cyclic radical generation. Within the wetland environment, biochar or gravel substrates support catalyst immobilization and enhance electron transfer, while macrophytes (Phragmites australis, Typha latifolia) and rhizospheric microbes provide oxygen release, nutrient turnover, and biodegradation assistance. Together, these coupled photochemical and biogeochemical reactions drive complete mineralization of refractory organics into CO₂ and H₂O, achieving low-carbon, self-sustaining water purification. The figure highlights the circular nexus linking light energy, catalysis, and biological processes, offering a model for next-generation eco-engineered wastewater treatment systems.

3.4. Strengths and Advantages of Constructed Wetlands

Constructed wetlands (CWs) represent one of the most effective, low-cost, and environmentally sustainable treatment technologies within the nature-based solutions (NbS) framework. Their primary advantage lies in their low energy requirement and operational simplicity. CWs depend mainly on natural processes such as sedimentation, filtration, adsorption, plant uptake, and microbial degradation, rather than on mechanical or energy-intensive operations. As a result, they typically require minimal external energy input apart from auxiliary pumping, and their maintenance involves simple tasks such as vegetation harvesting, sediment removal, and inspection of inlet–outlet structures [53,54,55]. This passive operation makes CWs particularly suited to decentralized and rural communities where energy supply and technical expertise are limited. Furthermore, the absence of complex electromechanical components reduces the risk of system failure and enhances long-term reliability [56,57].

Furthermore, another key advantage of CWs is their economic efficiency. Compared with conventional activated sludge systems, their capital costs are significantly lower due to the use of local materials (gravel, sand, and native macrophytes), and their operational costs are minimized because they require no continuous aeration or chemical dosing [58]. Over their lifespan, CWs demonstrate a favorable cost-to-benefit ratio, especially for small and medium-sized installations where centralized treatment is infeasible [59,60]. Additionally, the provision of multiple co-benefits, which include biodiversity enhancement, flood regulation, carbon sequestration, and recreational opportunities, contributes to their overall socio-economic value and supports integrated environmental planning [61]. However, their relatively large land requirement can limit adoption in urbanized areas, highlighting the importance of hybrid configurations or vertical-flow systems to reduce footprint [62].

From a sustainability and ecological perspective, constructed wetlands epitomize the integration of engineered systems with natural processes. Functionally, they mimic natural wetland ecosystems while providing ancillary environmental benefits such as habitat creation, microclimate improvement, and landscape aesthetics [63,64]. The treated effluent from CWs is often suitable for agricultural irrigation, aquifer recharge, or other non-potable uses, advancing circular water management and resource recovery [65]. Additionally, the harvested macrophyte biomass can be reused as compost, animal feed, or bioenergy feedstock, thus linking wastewater treatment with circular economy principles [66,67]. As such, CWs serve as multifunctional eco-technologies that align with the United Nations Sustainable Development Goals (SDGs) for clean water, sanitation, and climate resilience [46].

CWs also exhibit strong pollutant removal capabilities for a broad spectrum of conventional contaminants. Numerous studies report removal efficiencies exceeding 80–90% for biochemical oxygen demand (BOD), chemical oxygen demand (COD), and suspended solids (SS), achieved through combined sedimentation, microbial oxidation, and plant-mediated processes [5,6,40,68]. Nitrogen removal occurs via nitrification, denitrification, and plant uptake, while phosphorus is primarily removed through adsorption and precipitation in the substrate [69]. In addition, CWs can retain or transform heavy metals (e.g., Fe, Cu, Zn, Mn) and pathogens through filtration, redox reactions, and biofilm sorption [70]. Recent research also demonstrates their potential to attenuate emerging contaminants, including pharmaceuticals, endocrine disruptors, and microplastics, particularly when coupled with photo-Fenton, photocatalytic, or biochar-assisted hybrid systems [7,9,22,41,71]. This broad contaminant removal capacity underpins their suitability as primary, secondary, or tertiary treatment units in both municipal and industrial contexts.

The flexibility and modularity of CWs represent additional operational strengths. They can be configured as free water surface (FWS), horizontal subsurface flow (HSSF), vertical flow (VF), or hybrid systems, allowing designers to tailor configurations based on influent characteristics and space availability [6,40,72]. Their modular design enables incremental expansion and integration with other treatment processes such as biofilters, constructed ponds, or advanced oxidation units [15,16,45,73]. This adaptability supports their deployment across diverse scales from household systems and community clusters to large industrial facilities and facilitates integration into green infrastructure for stormwater and agricultural runoff management [74].

Finally, CWs demonstrate remarkable resilience and self-regulation. Their plant that microbe consortia adapt dynamically to fluctuations in hydraulic load, organic matter concentration, and environmental conditions, thereby maintaining stable treatment performance [5,56,75]. The vegetated and porous media buffer short-term shocks and distribute flow uniformly, reducing the risk of operational failure compared with conventional technologies [76]. Moreover, CWs exhibit resilience to climatic variability: they mitigate flood peaks during heavy rainfall, sustain treatment efficiency under moderate droughts, and contribute to carbon storage in sediments and vegetation [77,78]. These adaptive and regenerative characteristics highlight CWs as robust, nature-based treatment systems capable of long-term operation under variable environmental pressures.

Collectively, the advantages of constructed wetlands, including the low energy input, operational simplicity, economic feasibility, ecological co-benefits, and resilience, reinforce their role as essential technologies in the transition toward circular, decentralized, and carbon-neutral wastewater management. Their integration with advanced oxidation and photo-reactive systems further enhances treatment efficiency, positioning CWs as a frontier platform for sustainable water purification in the twenty-first century [79,80].

Table 2. Constructed Wetlands for Pollutant Treatment.

Pollutant	CW Configuration	Media/Catalyst	Vegetation	Scale (Lab/Pilot/Full)	Operating Key Notes	Performance	Notes	Ref.
REfTextile dyes (mixed)—textile wastewater	VSSF	Scoria-based aggregate	—	Lab	HLR optimized; aerated bed	Up to high color removal reported	Low-cost volcanic media; surface-driven sorption + bio-oxidation	[54]
Pharmaceuticals (general)—secondary effluent	Various (FWS, HSSF, VSSF)	Gravel/engineered mixes	Mixed macrophytes	Review	—	Broad 30–>90% across compounds	Mechanisms: sorption, plants, biofilm; design drivers summarized	[55]
Paracetamol—synthetic/domestic	HSSF (Phragmites)	Gravel	Phragmites australis	Pilot	HLR 120–240 mm·d−1	52–87% removal (lower HLR higher removal)	Early controlled study establishing HLR sensitivity	[56]
Phosphorus—municipal	HSSF/reactive cell	Iron scrap (reactive medium)	—	Pilot	Iron-based reactive bed	~97% P removal, stable	High/consistent P capture via iron corrosion products	[57]
Micropollutants (organic)—mixed waters	Various	Various	Various	Review (2024)	—	Mechanistic ranges synthesized	Strategies: phytoremediation, adsorption, hybridization	[58]
Textile dyes—review	FWS/HSSF/VSSF	Gravel, slag, engineered	Various	Review (2022)	—	Performance ranges synthesized	Design trends for textile CWs	[59]
PPCPs—municipal	Various	Various	Various	Review (open access)	—	Removal ranges; persistence issues	Medium-term stability considerations	[60]
Lake inflows—nutrient/organics	HSSF (coupled model)	Gravel	—	Field modeling	Coupled hydrodynamic–biokinetic model	Efficiency quantified (site-specific)	Model-guided optimization	[61]
P removal—synthesis	HSSF/reactive	Iron–carbon reactive mix	—	Lab/Pilot	Iron–carbon beds	High P capture	Substrate dominates P removal pathways	[62]
Micropollutants—optimization	Various	Various	Various	Review (2024)	—	Design/operation levers	Practical optimization guide (HRT, redox)	[63]
Azo dyes—synthetic	VFCW (aerated)	Gravel	—	Lab	Aeration + VF cycling	High decolorization over 70 d	Aeration crucial for azo cleavage	[64]
PPCPs—broad	Various	Various	Various	Review (2024)	—	CWs effective; variability high	Consolidates PPCP outcomes	[65]
Paracetamol & methylparaben	HSSF	Gravel	Alternanthera spp.	Lab	RSM/CCD optimization	Significant removal at optima	Plant choice and HRT tuned statistically	[66]
Phosphorus—municipal	HSSF/reactive	Reactive media alternatives (e.g., slag, iron-based)	—	Review/Case	—	Sustained P removal	Media selection critical for longevity	[67]
Microbial mechanisms—multi-pollutant	Various	Various	Various	Review (2024)	—	Mechanism-centric synthesis	Microbial pathways & design implications	[68]
Textile dyes—multiple cases	FWS/HSSF/VSSF	Gravel/engineered	Various	Review/case syntheses	—	Plant presence ↑ dye removal	Collates low/high concentration cases	[69]
Pharmaceuticals—domestic	HSSF/FWS	—	—	Review (classic)	—	Pathways & ranges	Foundational review on pharma removal	[70]
Rhizobacteria-assisted removal (IBP, PAR)	CW microcosms	—	Plant + rhizobacteria	Lab	Biological augmentation	Enhanced removal vs. controls	Plant–microbe synergy emphasized	[71]
Hybrid CW + photocatalytic film—urban river	Composite IC wetland + photo-Fenton	Iron–carbon + photocatalytic film	—	Pilot/Field	Hybrid with photo-step	Improved restoration metrics	Illustrates CW–AOP hybridization	[72]
Long-term CW performance—urban	HSSF (two pilots)	—	—	Pilot (Italy)	Multi-season dataset	Variable but robust removal	Climate/plant effects highlighted	[73]
Residual dye removal—pilot	Treatment wetland	—	—	Pilot	High HRT demand	Effective with design trade-offs	Practical lessons from pilot	[74]
Acidic drugs—domestic wastewater	CWs & ponds (NBS)	—	—	Review/case	Cost benchmarking	Cost ~0.25 USD·m−3; variable removal	Useful cost context for NBS	[75]
HSSF treatment—general	HSSF	—	—	Thesis (experimental)	HSSF operation factors	COD/N removal ranges	Useful design & operation data	[76]
N and P—enhancement with pyrite	HSSF (enhanced)	Natural pyrite	—	Pilot (3-yr)	Long-term pilot	Simultaneous N, P removal	Electron donor and P sink	[77]
Agricultural wastewater	Various	Various	Various	Review (2021)	—	Often >90% for nutrients	Scope and gaps identified	[78]

presents an extensive synthesis of literature on constructed wetlands (CWs) applied for the treatment of diverse pollutant categories. The compilation highlights how system configuration, substrate composition, vegetation type, and operational parameters collectively determine pollutant removal efficiency and long-term stability under various environmental and hydraulic conditions. The reviewed studies emphasize that constructed wetlands exhibit remarkable versatility, treating a broad range of contaminants from conventional nutrients (N, P) to complex organic pollutants such as pharmaceuticals, dyes, and emerging micropollutants. Vertical subsurface flow (VSSF) and horizontal subsurface flow (HSSF) systems remain the most frequently employed configurations due to their superior oxygen transfer, redox stratification, and adaptability to hybridization with catalytic or photoactive media. Additionally, several entries (e.g., [54,56,61,77]) demonstrate that substrate selection that is particularly iron-rich, carbonaceous, or volcanic materials, which plays a crucial role in enhancing removal pathways via adsorption, ion exchange, or catalytic redox reactions. For example, scoria-based aggregates (Table 2) and iron scrap media achieve notable dye and phosphorus removal by coupling physical filtration with reactive oxidation. Furthermore, the inclusion of reactive or engineered media (e.g., iron–carbon composites, slag, and biochar blends) improves phosphorus capture and micropollutant degradation efficiency, extending system lifespan and stability.

On the other hand, vegetation is especially Phragmites australis, Alternanthera spp., and other macrophytes that contribute not only to pollutant uptake but also to oxygenation of the rhizosphere, microbial support, and root-mediated sorption. Studies integrating rhizobacteria-assisted CWs highlight synergistic effects between microbial communities and plant metabolism, offering enhanced degradation of pharmaceuticals and endocrine-disrupting compounds. In terms of operational scale and performance, pilot- and full-scale investigations (entries 3, 4, 20) validate that CWs can achieve high removal rates (>90% for nutrients and dyes) with minimal energy input, especially under optimized hydraulic loading rates (HLR) and hydraulic retention times (HRT). However, the variability in pollutant removal, particularly for recalcitrant pharmaceuticals and PPCPs—reflects the influence of influent concentration, temperature, and media aging. Recent hybrid designs, such as photo-catalytic and electro-assisted wetlands, bridge the gap between passive biological systems and advanced oxidation processes, markedly improving degradation kinetics for persistent organic pollutants. Despite promising outcomes, these systems require further scaling studies and environmental risk assessment to ensure the safe use of metal-based or nano-catalysts within natural substrates.

Overall, the table underscores that constructed wetlands are transitioning from conventional nutrient removal units toward integrated, multifunctional treatment systems capable of addressing complex industrial, pharmaceutical, and agricultural effluents. Future research should focus on long-term media stability, dynamic modeling of coupled biogeochemical processes, and field-scale validation under variable climate and loading regimes to facilitate regulatory acceptance and real-world deployment.

3.5. Limitations and Challenges of Constructed Wetlands

Constructed wetlands (CWs) still face introductory design and performance constraints. Research should prioritize rigorous, transferable design rules that balance hydrodynamics, redox zoning, and light penetration while shrinking land footprint through vertical stacking, intensified media, and optically engineered shallow beds. Progress also hinges on advanced, durable media that especially waste-valorized carbons, functionalized mineral substrates, and catalytic coatings that resist fouling, maintain permeability, and enhance adsorption–oxidation synergy for trace micropollutants at realistic hydraulic loads. For Fenton and photo-Fenton processes, the persistent barriers are scale-up, chemical stewardship, and long-term catalyst integrity. Reducing oxidant demand and minimizing residuals/sludge require smarter dosing (including in-situ H₂O₂/PDS generation), robust immobilization of iron or bimetallic active sites, and resistance to deactivation in complex matrices rich in NOM, bicarbonate, and chloride. Catalyst frameworks must demonstrate low leaching, multi-cycle stability, and tolerance to variable pH, salinity, and co-contaminants typical of real effluents. Hybrid implementations that couple CWs with (photo-) Fenton units remain under-validated at scale; more pilot and full-scale demonstrations are needed to quantify performance, reliability, and true costs under seasonal variability and shock loads. These studies should report harmonized metrics at operating pH, EE/O, iron leach, and toxicity reduction so that hybrids can be compared fairly with stand-alone CWs or AOPs. Model-based operation and control are equally important: dynamic process models that capture hydraulics, light fields, mass transfer, and reaction–adsorption kinetics can enable real-time optimization of oxidant dosing, flow distribution, and pH management. Integrating low-cost sensors with feedback control can prevent overdosing, extend catalyst life, and sustain target effluent quality. Finally, sustainability and risk must be front and center. Comparative life-cycle assessment and techno-economic analysis should evaluate embodied energy of media, oxidant and energy use, sludge generation and management, and end-of-life pathways for catalysts. Transparent reporting of by-products, ecotoxicity, and metal leaching is essential to ensure hybrids deliver genuine environmental benefit over stand-alone options. The main challenges of CWs system could be summarized as follows:

Land area requirement. A primary constraint is the relatively large footprint required to reach target removals, especially for nitrogen and recalcitrant organics. Space limitations in dense or high-value land settings can hinder adoption, motivating intensified configurations (vertical flow stacks, shallower optical beds with reflective liners) and higher-performance media to raise areal removal rates.

Performance variability and slow kinetics. CWs often exhibit slower kinetics than engineered chemical processes, and their performance is sensitive to hydraulic loading, retention time, temperature, and seasonal dynamics. Cold periods, storm events, and shock pollutant loads can depress rates or cause short-circuiting, underscoring the need for buffering volumes, flow equalization, and adaptive operation.

Clogging and maintenance. Subsurface systems are prone to clogging from suspended solids, biofilm accumulation, and fines migration, which degrade permeability and induce bypassing. Preventive pretreatment, graded media, periodic surface skimming, and scheduled media rejuvenation/replacement are necessary to sustain hydraulics without sacrificing treatment.

Limited removal of specific pollutants. Many recalcitrant organic micropollutants (e.g., pharmaceuticals and personal care products) are insufficiently degraded by CWs alone, relying mostly on sorption and slow biotransformation. Targeted intensification of photo-Fenton modules, catalytic media, or in-bed oxidant generation can address this gap while maintaining low energy demand.

Operational sensitivity. Outcomes depend strongly on proper design (loading rates, plant species, substrate type and depth, pretreatment) and local climate (temperature, rainfall patterns). Robust designs therefore incorporate climate-appropriate macrophytes, adjustable hydraulics, and modular polishing steps to maintain compliance across seasons.

Pathogens and regrowth. Although CWs reduce pathogen loads, consistent disinfection to reuse standards is not guaranteed. Post-treatment barriers (UV, chlorination, or solar-AOP polishing) are often required, and designs should minimize regrowth niches and stagnation that can re-elevate indicator organisms downstream.

Start-up and maturation period. CWs require time for plant establishment and for microbial communities to organize into effective redox and trophic layers. This maturation period can delay full performance, so commissioning plans should include staged loading, interim polishing, and monitoring milestones to manage expectations and protect effluent quality during ramp-up.

3.6. Application Relevance and Sustainability

Modified heterogeneous photo-Fenton catalysts are increasingly integrated into constructed wetlands (CWs), solar photo-reactors, and hybrid biocatalytic systems [21]. Their magnetic recoverability, high surface area, and visible-light activity enable decentralized wastewater treatment with reduced chemical inputs and sludge generation. For instance, Fe₃O₄@biochar and Fe–MOF composites can be embedded in plant rhizospheres or substrate beds of wetlands to combine radical oxidation with biological degradation [4,21]. The adoption of bio-waste-derived supports (e.g., banana peel, potato peel) contributes to circular economy objectives and reduces environmental footprint, aligning with green engineering principles. Such catalysts exhibit excellent reusability (typically 4–6 cycles with >85–95% efficiency retention), minimal Fe leaching, and robust photochemical stability, positioning them as next-generation eco-engineered catalysts for sustainable wastewater management [6,15,21].

3.7. Bibliometric Analysis of CW Research

A structured bibliometric survey using Web of Science Core Collection to capture environmental engineering and ecology outlets was conducted. The canonical query targeted CW terminology and common variants while excluding unrelated “wetland” usage in purely ecological restoration papers. The bibliometric evidence portrays CWs as a maturing, rapidly diversifying domain that is shifting from nutrient-centric ecology toward process-intensified, hybrid treatments targeting micropollutants and circular-economy media. The fastest-growing frontier links photo-Fenton catalysis and engineered media with optically managed wetland beds, bridging nature-based resilience with advanced oxidation performance. Standardized reporting and pilot-scale evidence remain the main bottlenecks to translation.

The bibliometric trajectory of Constructed Wetlands (CWs) research from 2000 to 2025 (main panel) reveals an accelerating growth pattern that mirrors the global shift toward nature-based and low-carbon treatment technologies (Figure 7). The data derived from the Web of Science Core Collection. Early studies (2000–2008) primarily focused on nutrient removal and hydrological optimization, with moderate annual outputs below 100 publications. From 2010 onward, a pronounced inflection point appears, driven by the integration of engineered substrates, microbial–plant interactions, and hybrid configurations such as vertical subsurface flow and intensified aeration systems. The post-2015 period marks a transition from ecological treatment concepts to process-intensified and catalytically active wetlands, as reflected by the exponential rise depicted by the red trendline. Furthermore, the inset of Figure 7 tracks the Fenton-based Constructed Wetlands (Fenton CWs) subset, highlighting their relatively late emergence but steep recent growth. Beginning with sporadic publications before 2010, the field expanded rapidly after 2018 in response to the adoption of photo-Fenton and heterogeneous Fenton catalysts (Fe₃O₄, Fe–MOFs, CuO-biochar composites) embedded within wetland matrices. This surge underscores a paradigm shift toward hybrid oxidative–biological systems capable of degrading recalcitrant micropollutants (e.g., dyes, pharmaceuticals) under solar or visible-light conditions. Despite the sharp upward trajectory, challenges persist regarding catalyst stability, light penetration, and large-scale hydraulic optimization. The dual trends suggest a convergence between nature-based treatment and advanced oxidation technologies, positioning CW–Fenton hybrids as a key driver of next-generation eco-engineered wastewater systems.

The Web of Science platform and VOSviewer software (version 1.6.16.0, accessed on October 2025) were utilized for this study to design the bibliometric framework and perform keyword analysis of the selected publications. The bibliometric keyword co-occurrence map (Figure 8) generated using VOSviewer illustrates the evolving conceptual interlinkages between constructed wetlands (CWs) and Fenton/Fenton-like processes, highlighting how research has progressed from conventional biological treatment systems toward hybrid, process-intensified designs. The overall structure of the network reveals several color-coded clusters that represent major thematic domains within this interdisciplinary field. Additionally, the red cluster encapsulates the classical ecological engineering paradigm of CWs. Dominant terms such as constructed wetland, horizontal subsurface flow, performance, effluent, and nitrogen removal emphasize the core principles of biological transformation, filtration, and sedimentation. These studies traditionally focus on nutrient removal, organic load reduction, and resilience to seasonal variability. However, the close proximity of Fenton process, toxicity, and degradation nodes suggests an emerging interest in integrating oxidative catalysis into CWs to enhance the removal of complex organic contaminants beyond the capacity of purely biological pathways.

To sum up, the green cluster bridges biological and physicochemical treatment strategies, connecting membrane bioreactor, activated sludge, photocatalytic degradation, and organic pollutants. This cluster represents the frontier of hybrid and intensified CW systems that combine conventional bio-treatment with adsorption, filtration, or advanced oxidation modules. Such approaches are increasingly adopted to treat industrial effluents and emerging contaminants, demonstrating the technological convergence of engineered and nature-based solutions. In contrast, the blue cluster captures the mechanistic and chemical science underpinning Fenton and other advanced oxidation processes (AOPs). Keywords such as oxidation, ozonation, pharmaceuticals, personal care products, electro-Fenton, and emerging contaminants dominate this region. The strong connectivity between photo-Fenton and constructed wetlands nodes confirms that research is now actively exploring solar-assisted oxidation and electrochemical regeneration as complementary pathways to enhance pollutant degradation within hybrid wetland reactors.

The yellow cluster links environmental treatment with public health protection, encompassing antibiotic resistance genes, drinking water, Escherichia coli, and treatment plants. This zone reflects the growing recognition that CW–Fenton integration can simultaneously target chemical and biological hazards, supporting safe water reuse and pathogen-free effluent discharge.

Centrally positioned nodes, such as constructed wetlands, photo-Fenton, and wastewater treatment, act as integrative hubs connecting all clusters, symbolizing the unification of eco-engineered and catalytic oxidation principles. This convergence reflects the emergence of photo-reactive constructed wetlands (PRCWs) systems that synergize biological uptake, adsorption, and radical oxidation under solar irradiation. Such systems embody a new generation of low-energy, circular, and sustainable treatment technologies. Overall, the co-occurrence analysis underscores a clear trajectory from nutrient-centric ecological systems toward catalytically enhanced, hybrid wetlands capable of treating recalcitrant micropollutants. The thematic overlap between ecological treatment and advanced oxidation research highlights a global shift toward multifunctional, nature-based engineered systems that couple environmental sustainability with high treatment efficiency. The map visualizes dominant research themes and interconnections across the field. Node size represents keyword frequency, while link thickness indicates co-occurrence strength. Four primary clusters are observed: the red cluster (classical CWs and nutrient removal), green cluster (hybrid and process-intensified systems), blue cluster (Fenton and advanced oxidation mechanisms), and yellow cluster (public health and emerging contaminants). The central overlap between constructed wetlands and photo-Fenton highlights the emerging nexus of eco-engineered and catalytic oxidation approaches in sustainable wastewater treatment.

4. Synergistic Integration of Constructed Wetlands and Fenton-Based Processes

The integration of Constructed Wetlands (CWs) with Fenton and Fenton-like oxidation systems represents a promising convergence between nature-based treatment and advanced chemical oxidation. While CWs are renowned for their eco-efficiency, passive operation, and versatility in removing conventional pollutants, Fenton processes offer unparalleled oxidation strength for degrading recalcitrant organics and emerging micropollutants. Understanding their comparative strengths and limitations is essential for developing next-generation hybrid treatment configurations that maximize removal efficiency while minimizing resource inputs.

Table 3. Comparative Overview and Possible Synergies between Constructed Wetlands and Fenton/Fenton-like Processes.

Feature	Constructed Wetlands	Fenton/Fenton-Like Processes	Refs.
Main mechanism	Biological transformation, physical filtration/adsorption, and plant uptake	Advanced oxidation via hydroxyl radicals (•OH) generated by Fe2+/Fe3+ and H2O2 reactions	[16,17]
Suitability	Effective for BOD, suspended solids (SS), nitrogen, phosphorus, pathogens, and some metals	Effective for recalcitrant organics, dyes, pharmaceuticals, and emerging micropollutants	[18,19]
Energy requirement	Low (passive or gravity flow; minimal external energy)	Moderate (requires chemical reagents, mixing, UV/solar/electro input)	[20,21]
Sensitivity	Affected by temperature, hydraulic loading, clogging, and seasonal variation	Sensitive to pH, organic matrix, and dosing of reagents	[22,23]
Sludge/residuals	Produces low sludge (mostly biological and sediment-based); periodic media maintenance	Generates iron sludge, residual reagents, and possible by-products	[24,25]
Scale/land	Requires substantial area for high treatment performance	Compact system with high volumetric efficiency	[20,26]
Cost factors	Driven by land acquisition, construction, vegetation, and maintenance	Driven by chemical consumption (H2O2, iron salts), equipment, and sludge management	[21,27]
Operational complexity	Simple to operate; natural stabilization over time	Requires precise control (pH, reagent dosing, retention time)	[16,23]
Integration/polishing	Often applied as a polishing or decentralized treatment unit	Commonly used as pre- or post-treatment in hybrid CW–AOP systems	[19,28]

provides a comparative overview of the operational, mechanistic, and sustainability aspects of both systems, highlighting the potential synergies achievable when biological and chemical oxidation mechanisms are strategically coupled in a circular treatment framework.

The comparative analysis presented in Table 3 highlights the complementary nature of Constructed Wetlands (CWs) and Fenton/Fenton-like advanced oxidation processes (AOPs) in wastewater treatment. While CWs rely primarily on biological transformation, physical filtration, and plant uptake mechanisms [16], Fenton systems are governed by chemical oxidation via hydroxyl radicals (•OH) generated from Fe²⁺/Fe³⁺ and H₂O₂ reactions [17,18]. These two paradigms differ fundamentally in driving forces, “biotic versus abiotic” yet share a convergent goal of achieving high contaminant removal with environmental sustainability.

In terms of treatment suitability, CWs effectively remove biodegradable pollutants such as BOD, suspended solids, nitrogen, and phosphorus [19,20]. However, their capacity for degrading persistent organics and micropollutants is limited. Conversely, Fenton and photo-Fenton processes are highly efficient in mineralizing dyes, pharmaceuticals, and other recalcitrant contaminants, offering a chemical pathway to address those pollutants that resist biological degradation [18,21]. Hence, a hybrid configuration can achieve both bulk organic load reduction and micropollutant oxidation, providing a comprehensive solution across contaminant classes.

From an energy and cost perspective, CWs are largely passive, operating under gravity flow with minimal energy or reagent inputs [20], whereas Fenton processes require chemical dosing, pH control, and sometimes external light or electrochemical assistance [21,22]. Although this increases operational complexity and cost, the footprint of Fenton systems is significantly smaller. Thus, CWs excel in low-cost, decentralized applications, while Fenton systems deliver high efficiency in compact reactors, suggesting a scale-complementary synergy. However, in terms of sensitivity and residues, CWs are susceptible to temperature fluctuations, hydraulic loading, and seasonal changes, while Fenton reactions are constrained by pH sensitivity, matrix interference, and reagent dosing precision [23,24]. CWs produce low biological sludge, in contrast to the iron sludge and chemical residues typical of Fenton systems [25,26]. When integrated, the wetland matrix can serve as a natural sink and regeneration zone for iron species, mitigating sludge disposal issues while maintaining catalytic functionality.

Operationally, CWs are simple and self-regulating, relying on plant–microbe–substrate interactions, whereas Fenton systems demand active monitoring and precise control ([16,23]). However, when coupled in series or within a shared bed, their contrasting dynamics can yield hybrid systems that are both self-sustaining and chemically enhanced, aligning with the principles of eco-engineered, circular treatment technologies [27,28]. Overall, the table underscores the potential of CW–Fenton integration to bridge the gap between nature-based systems and advanced oxidation, combining ecological resilience with catalytic intensity. Such synergy not only improves pollutant removal efficiency but also advances the global agenda of sustainable, low-carbon wastewater treatment capable of handling complex and emerging contaminants.

5. Hybrid CW-Fenton Systems for Wastewater Treatment

Because of their complementary operating principles, coupling CWs with Fenton or photo-Fenton oxidation offers a promising hybrid pathway for treating complex effluents. Typically, the Fenton stage is applied first to oxidize recalcitrant organic compounds (dyes, pharmaceuticals, micropollutants) into more biodegradable intermediates, followed by CW polishing to remove residual BOD, nitrogen, and pathogens [33,34].

Conversely, CWs can serve as pre-treatment units, attenuating suspended solids and bulk organics to minimize chemical consumption and pH buffering requirements before a Fenton post-treatment step. Such sequencing improves cost efficiency, effluent quality, and sustainability, exemplifying the next generation of eco-engineered, low-carbon water treatment systems [34,36].

Figure 9 illustrates the integrated physical, biological, and photo-assisted catalytic processes governing pollutant attenuation in constructed wetlands. Influent wastewater first undergoes sedimentation and filtration, allowing suspended solids and particulate-bound contaminants to be retained within the substrate matrix. Adsorption onto substrates and biofilms further contributes to the immobilization of organic pollutants, nutrients, and metals. Emergent macrophytes (e.g., Phragmites australis and Scirpus spp.) facilitate nutrient uptake and release oxygen into the rhizosphere, promoting localized aerobic conditions that support microbial biodegradation of organic contaminants. Under solar irradiation, iron-mediated redox cycling (Fe²⁺/Fe³⁺) in the presence of hydrogen peroxide enables the generation of reactive oxygen species (ROS), which enhance oxidative transformation of refractory pollutants. The combined action of physical retention, biological activity, plant uptake, and photo-assisted oxidation leads to the production of a treated effluent suitable for environmental discharge or reuse.

The literature demonstrates a growing trend toward hybrid CW–Fenton and photo-Fenton systems as an emerging paradigm in sustainable advanced oxidation. Most studies report near-neutral operating conditions (pH 5–7), indicating significant progress in overcoming the traditional acidic constraint of homogeneous Fenton chemistry [37,38,39]. The comparative overview highlights the complementary roles of CWs and Fenton-based processes in treating diverse pollutant categories. Constructed Wetlands excel at removing biodegradable organics, nutrients, and suspended solids through biological transformation, plant uptake, and adsorption. Their efficiency for recalcitrant pollutants such as synthetic dyes or pharmaceuticals is typically moderate (60–80%) but can be enhanced using engineered substrates like Fe₃O₄–biochar or Fe–MOF-modified gravel.

Conversely, Fenton and photo-Fenton systems achieve rapid degradation of refractory compounds via hydroxyl radical (•OH) oxidation, attaining near-complete decolorization and high TOC reduction (>90%). However, they are less effective for nutrient removal and require chemical reagents and pH control. When the two processes are combined sequentially—using CWs for biological polishing and Fenton systems for oxidative pre- or post-treatment—they deliver a balanced solution that maximizes efficiency and sustainability. This complementary behavior forms the foundation for CW–Fenton hybrid systems, which unify nature-based treatment and advanced oxidation within one eco-engineered platform capable of achieving both high pollutant removal and reduced chemical consumption.

Table 4. Comparative Treatment Performance of Selected Studies on Hybrid Constructed Wetland/Fenton Systems for Wastewater Treatment of Different Pollutant Categories.

Pollutant Type	Constructed Wetlands	Fenton/Photo-Fenton Processes	Refs.
Textile dyes (Reactive Red, Methylene Blue, Azo dyes)	Adsorption on biofilm and substrate; plant-assisted oxidation; 70–90% color removal with Fe/biochar or zeolite media	Hydroxyl radical oxidation; rapid decolorization and mineralization (>95%) under solar or UV activation	[44,45,46]
Pharmaceuticals (Paracetamol, Diclofenac, Sulfamethoxazole)	Microbial degradation and plant uptake; partial oxidation and sorption; 60–80% removal	Advanced oxidation via Fe2+/H2O2 and visible-light Fenton; 85–98% TOC and COD removal	[47,48]
Nutrients (NH4+–N, NO3−, PO43−)	Nitrification–denitrification and plant uptake; 60–90% N removal and 40–80% P retention	Minimal direct oxidation; pre-/post-polishing role; minor nutrient influence	[20,49]
Heavy metals (Fe, Cu, Zn, Pb, Cd)	Sedimentation, adsorption to substrate, and complexation with organic matter; >80% retention for low loads	Fenton oxidation indirectly enhances metal mobility; pH adjustment critical; can be removed via co-precipitation	[50]
Pesticides and phenolics (Atrazine, Phenol, 4-Chlorophenol)	Biofilm degradation and adsorption; moderate efficiency (~60–70%)	Radical oxidation yields near-complete degradation (>90%) with Fe–MOF or Fe3O4 catalysts	[38,51]
Hospital/emerging contaminants	Combined microbial and rhizospheric removal; 70–90% overall pollutant reduction	Photo-Fenton with Fe–biochar and solar irradiation achieves >90% total pharmaceutical removal	[43,47]

provides a comparative overview of hybrid constructed wetland (CW) and Fenton/photo-Fenton systems applied for the treatment of different pollutant categories, emphasizing their complementary mechanisms, efficiency ranges, and functional integration potential in sustainable wastewater treatment frameworks. The data presented in Table 4 illustrate the synergistic and contrasting roles of constructed wetlands and Fenton-based advanced oxidation processes (AOPs) across multiple pollutant classes, ranging from dyes and pharmaceuticals to nutrients, metals, and emerging contaminants. Each system demonstrates unique strengths and limitations governed by its underlying mechanisms, biological transformation versus oxidative mineralization, and operational requirements such as pH, light exposure, and chemical inputs. For textile dyes, CWs achieve substantial decolorization (70–90%) through biofilm adsorption, plant-mediated oxidation, and substrate interaction, particularly when Fe/biochar or zeolite media are incorporated to enhance catalytic activity. However, photo-Fenton systems outperform CWs in terms of rapid mineralization (>95%), driven by hydroxyl radical (•OH) generation under UV or solar irradiation [44,45,46]. The integration of these two approaches, such as embedding Fenton-active catalysts within CW substrates, can enable continuous decolorization and mineralization under natural light conditions.

Pharmaceutical contaminants such as paracetamol, diclofenac, and sulfamethoxazole exhibit partial biodegradation (60–80%) in CWs through microbial oxidation and rhizospheric uptake, but these pathways are typically slow and incomplete. In contrast, Fenton and photo-Fenton oxidation rapidly achieve 85–98% removal of total organic carbon (TOC) and chemical oxygen demand (COD) via Fe²⁺/H₂O₂ reactions enhanced by visible-light activation [47,48]. This highlights the benefit of coupling CWs’ long-term biological stability with AOPs’ fast degradation kinetics. Nutrient removal processes in CWs that especially nitrification–denitrification and plant uptake, which remain highly efficient (60–90% N and 40–80% P retention), whereas Fenton oxidation exerts little direct impact on nutrient removal ([20], Software). In hybrid configurations, Fenton systems can serve as pre-treatment or polishing stages, improving redox balance and bioavailability within CWs while indirectly influencing nutrient cycling.

In the case of heavy metals, CWs act as effective passive filters through sedimentation, adsorption, and complexation with organic matter, retaining over 80% of metals under low loading rates. Conversely, Fenton reactions can alter metal speciation and mobility depending on solution pH, sometimes enhancing co-precipitation or complexation with iron hydroxides [50]. This interaction underscores the need for pH-controlled hybrid designs to prevent secondary contamination. Also, pesticides and phenolic compounds are only moderately removed (~60–70%) in CWs due to limited biodegradability, but their oxidation efficiency increases dramatically (>90%) in photo-Fenton systems using Fe–MOF or Fe₃O₄ catalysts [38,51]. The combination of biofilm-mediated sorption and catalytic degradation in a CW–Fenton hybrid thus offers a powerful route for treating recalcitrant organics. Hence, hospital and emerging contaminants demonstrate the promise of integrated systems: CWs achieve broad-spectrum pollutant reduction (70–90%), while Fe–biochar solar photo-Fenton processes achieve near-complete pharmaceutical removal (>90%) [43,47]. Such hybrids leverage both biological resilience and photochemical intensity, achieving continuous, low-energy purification compatible with decentralized wastewater treatment. Overall, the comparative evidence affirms that hybrid CW–Fenton systems bridge the gap between nature-based and chemical oxidation technologies, providing a multifunctional, energy-efficient platform for complex wastewater remediation. Future optimization should focus on material stability, light distribution management, and ecological safety, ensuring reliable long-term operation and regulatory acceptance in real-world applications.

5.1. Recent Trends and Advances in Hybrid Constructed Wetlands

Recent developments in Constructed Wetland (CW) technology emphasize process intensification, substrate innovation, and system integration to overcome constraints related to land demand, seasonal fluctuations, and limited removal of refractory compounds. One major advance involves hybrid CW configurations that combine vertical-flow and horizontal-flow stages, achieving improved redox zoning, hydraulic efficiency, and pollutant removal while minimizing footprint requirements [29,30].

In parallel, there is growing research interest in engineered substrates and media, including porous biochar, zeolitic composites, and catalytically active fillers that enhance adsorption, microbial attachment, and redox performance [31,32]. Further innovations focus on integrating CWs with complementary technologies. Notably advanced oxidation processes (AOPs) such as Fenton, photo-Fenton, electro-Fenton, and microbial fuel cells to combine biological resilience with strong oxidative power [33,34]. These hybrid systems deliver enhanced degradation of recalcitrant micropollutants such as pharmaceuticals, dyes and pesticides, while maintaining low operational energy demand. Moreover, data-driven monitoring and computational modeling are being increasingly adopted to optimize design parameters, predict kinetics, and guide control strategies [35]. Finally, the emphasis on water reuse and reclamation has positioned CWs as sustainable components of circular water systems, with configurations designed to meet stringent effluent quality standards for irrigation, groundwater recharge, and non-potable reuse [36].

5.2. Role of Plants and the Rhizosphere in Fe²⁺/Fe³⁺ Cycling Within CW–Fenton Systems

The plant–rhizosphere interface plays a pivotal role in regulating iron redox transformations (Fe²⁺/Fe³⁺ cycling) within constructed wetland–Fenton hybrid systems, directly influencing the efficiency, stability, and longevity of heterogeneous catalytic processes. In such eco-engineered systems, the rhizospheric zone serves as a natural redox microenvironment where oxygen release, root exudation, and microbial metabolism interact to sustain continuous Fe²⁺ regeneration and minimize catalyst deactivation [52].

5.3. Oxygen Leakage and Microzone Redox Gradients

Wetland macrophytes such as Phragmites australis, Typha latifolia, and Scirpus spp. release radial oxygen loss (ROL) from their roots into the surrounding rhizosphere, forming aerobic microzones within an otherwise anoxic matrix [53]. This oxygen leakage promotes localized Fe²⁺ oxidation to Fe³⁺, leading to the precipitation of iron oxides or hydroxides on root surfaces and substrate particles. These iron plaques not only adsorb phosphate and heavy metals but also act as catalytic reservoirs for subsequent Fenton-like reactions under variable pH and light conditions [54].

5.4. Root Exudates and Microbial Mediation

Plants exude organic acids (citric, malic, oxalic) and phenolic compounds that act as natural chelating agents, forming soluble Fe–ligand complexes which facilitate Fe³⁺ reduction to Fe²⁺ through photochemical and microbial pathways [55]. Rhizospheric microorganisms—including iron-reducing bacteria (IRB) such as Geobacter and Shewanella spp.—further catalyze Fe³⁺ reduction via enzymatic electron transfer using root-derived carbon as the electron donor [56]. This biotic regeneration of Fe²⁺ sustains catalytic activity even in the absence of external reducing agents, improving the continuity of the Fenton redox cycle.

5.5. Photochemical and Biochemical Redox Coupling

In photo-Fenton-integrated CWs, sunlight penetration and photosynthetic oxygenation enhance the Fe²⁺/Fe³⁺ turnover through photo-reduction mechanisms, where Fe³⁺–organic complexes absorb visible light, releasing Fe²⁺ and reactive oxygen species (ROS) [57]. Simultaneously, rhizospheric biofilms create alternating anoxic–oxic microzones that synchronize the abiotic photo-Fenton reactions with biological degradation and adsorption. This synergistic coupling stabilizes redox balance, prevents Fe-leaching, and maintains a quasi-homogeneous Fe cycle within the solid phase.

5.6. Ecological and Catalytic Implications

The integration of plant-driven rhizospheric processes with heterogeneous Fenton catalysis transforms CWs from passive biological units into bio-catalytically active reactors. The dynamic Fe²⁺/Fe³⁺ cycling underpins both pollutant mineralization and nutrient immobilization, reducing chemical inputs while preserving ecological sustainability. This closed-loop redox behavior is central to the circular operation of CW–Fenton systems, enhancing their resilience and long-term operational efficiency [58,59].

Figure 10 provides an integrated graphical overview of recent advances in hybrid constructed wetland (CW) systems and the mechanistic role of plant–rhizosphere interactions in sustaining Fe²⁺/Fe³⁺ redox cycling under Fenton and photo-Fenton conditions. The left panel highlights current innovations in hybrid CW design, including multi-stage (vertical/horizontal) configurations, engineered substrates (biochar, zeolitic, and catalytic fillers), integration with advanced oxidation processes (Fenton, photo-Fenton, electro-Fenton), and data-driven optimization for water reuse. The right panel depicts the role of wetland macrophytes (Phragmites australis, Typha latifolia, Scirpus spp.) and the rhizosphere in sustaining Fe²⁺/Fe³⁺ cycling. Oxygen leakage (ROL) forms microzone redox gradients that oxidize Fe²⁺ to Fe³⁺, while root exudates and iron-reducing microorganisms regenerate Fe²⁺. Sunlight and microbial activity promote reactive oxygen species (ROS) generation, enabling continuous photo-Fenton catalysis and enhancing pollutant degradation efficiency under eco-sustainable conditions.

The illustration unites two complementary dimensions of CW–Fenton research: technological innovation in system design and biogeochemical dynamics within the rhizosphere that underpin catalytic performance and ecological stability. The schematic summarizes current technological trends in hybrid CW design (on the left). Emerging systems integrate vertical- and horizontal-flow stages to optimize redox stratification and hydraulic efficiency, while engineered substrates such as biochar, zeolitic composites, and metal-doped fillers enhance adsorption, microbial attachment, and electron transfer. The coupling of CWs with advanced oxidation processes (AOPs) such as Fenton, photo-Fenton, and electro-Fenton creates powerful hybrid configurations capable of degrading persistent organic contaminants with minimal chemical and energy inputs. Furthermore, the adoption of data-driven monitoring and optimization tools represents a shift toward smart, circular water-reuse systems that meet stringent discharge and reuse standards.

Additionally, the schematic visualizes the plant–rhizosphere-mediated Fe²⁺/Fe³⁺ cycling fundamental to sustaining catalytic activity in CW–Fenton systems (on the right). Macrophytes such as Phragmites australis, Typha latifolia, and Scirpus spp. release oxygen into the rhizosphere through radial oxygen loss (ROL), generating oxic microzones that oxidize Fe²⁺ to Fe³⁺. This process forms iron plaques on root surfaces that adsorb pollutants and act as in-situ catalytic sites. Concurrently, root exudates (organic acids and phenolics) and iron-reducing microorganisms (Geobacter, Shewanella) regenerate Fe²⁺, sustaining the redox loop even under low-oxygen conditions. Sunlight further enhances this cycle via photo-reduction, driving the production of reactive oxygen species (ROS) that accelerate pollutant mineralization. Further, the graphical representation encapsulates how ecological and catalytic processes converge within CW–Fenton hybrids: plants, microbes, and light-driven chemistry cooperate to maintain Fe stability, minimize leaching, and promote sustainable oxidation of complex contaminants. This integrated mechanism positions CW–Fenton systems as an advanced nature-based solution for decentralized wastewater treatment, uniting ecological self-regulation with high-efficiency catalysis.

5.7. Circular Nexus and Sustainability Theory

The circular nexus framework provides a conceptual and theoretical foundation for integrating photo-Fenton catalysis within eco-engineered water-treatment systems. It envisions a closed-loop interaction among light, materials, and biological components, establishing a self-regenerative and resource-efficient platform that minimizes waste while maximizing energy utilization [26]. Within this model, process sustainability emerges from the balanced coupling of photochemical, material, and biological domains, where each element reinforces the other through continuous feedback mechanisms.

In the circular nexus, solar photons act as the primary energy input, driving the photoactivation of metal–organic frameworks (MOFs) and other semiconducting catalysts through ligand-to-metal charge transfer (LMCT) and band-gap excitation. The activated catalysts convert solar energy into chemical redox potential, producing reactive oxygen species (ROS) that oxidize pollutants and initiate further biochemical transformation. Simultaneously, the biological subsystem comprising aquatic plants, microbes, and biofilms that sustains carbon and nutrient cycling, while the substrate matrix regulates electron transfer, adsorption, and mass transport. These interconnected processes generate an open yet circular photochemical–biological reactor, capable of maintaining long-term functional stability [27].

The framework thus aligns directly with circular-economy principles and supports the United Nations Sustainable Development Goals (SDGs 6, 12, and 13) by emphasizing renewable-energy utilization, low-carbon operation, and material longevity [28]. Through the interplay of light energy, catalytic media, and living systems, the circular nexus converts linear treatment sequences into regenerative cycles that promote both pollution mitigation and resource recovery.

The performance evaluation of this model extends beyond conventional treatment metrics to include circularity indicators such as photonic efficiency, carbon-footprint reduction, catalyst durability, and life-cycle resource recovery. These parameters capture the environmental and systemic benefits of coupling photo-Fenton catalysis with biological self-regulation and material reusability [29].

By merging material innovation, biological resilience, and solar-energy harvesting, the circular nexus transcends the classical Fenton limitations of acidic operation, reagent consumption, and sludge formation. It represents a new theoretical paradigm for sustainable, solar-driven wastewater treatment that integrates physicochemical intensity with ecological balance, paving the way toward circular, low-carbon, and self-sustaining water-management systems [30].

Figure 11 conceptually illustrates the integrated biotic and abiotic pathways that govern Fe²⁺/Fe³⁺ cycling within constructed wetland–Fenton systems, emphasizing the synergistic interactions among plant roots, microbial activity, and photo-driven redox reactions. Oxygen release from plant roots (radial oxygen loss, ROL) promotes Fe²⁺ oxidation and the formation of iron (hydr)oxide plaques, while root exudates and iron-reducing bacteria regenerate Fe²⁺ from Fe³⁺ through chelation and enzymatic reduction. Sunlight-driven photo-reduction of Fe³⁺–organic complexes further enhances catalytic turnover, establishing a dynamic redox loop that supports continuous hydroxyl radical production, pollutant degradation, and ecological stabilization within the rhizosphere.

5.8. Real-Case Comparison of Conventional CWs and Fenton-Assisted CWs

Real-case evidence quantitatively distinguishes the performance of conventional constructed wetlands (CWs) from that of Fenton-assisted CWs in treating complex and refractory wastewaters. As summarized in

Table 5. Real and Pilot-Scale Case Studies Comparing Conventional Constructed Wetlands and Fenton-Assisted Constructed Wetlands.

System Type	CW	Media/Catalyst	Real Wastewater Type	Working Conditions	Removal (%)	Notes	Ref.
Conventional CW	HSSF	Gravel/Phragmites australis	Municipal secondary effluent	HRT 3–5 d; ambient pH	COD: 60–70%; PPCPs: 40–60%	Limited oxidation of recalcitrant micropollutants	[86]
Conventional CW	VF	Sand/gravel	Domestic wastewater	Intermittent loading; aerobic	NH4+–N: 70–85%; COD: 65–75%	Efficient nitrification but weak oxidation	[85]
Reactive CW	HSSF	Iron scrap/iron-rich media	Municipal wastewater	Neutral pH; passive operation	P removal 95%; COD ≈ 70%	Iron media improves phosphorus capture	[78]
Photo-Reactive CW (PR-CW)	VF/HSSF hybrid	Fe–MOF/Fe–biochar	Pharmaceutical wastewater	pH 5–7; solar irradiation	Pharmaceuticals 85–95%; COD > 90%	Solar ROS overcomes Fenton pH limit	[74]
Hybrid CW–Photo-Fenton	Multistage CW	Fe–carbon + photocatalytic film	Urban river water	Natural pH; sunlight	Micropollutants > 90%	Field-scale circular operation	[78]
Intensified CW	VF aerated	Fe(III)-EDTA amended media	Municipal effluent	Neutral pH; intermittent aeration	PPCPs 80–90%	Enhanced oxidation without acidification	[32]

PPCPs: pharmaceuticals and personal care products; HSSF: horizontal subsurface flow; VF: vertical flow; HRT: hydraulic retention time.

, conventional CWs typically achieve 60–80% removal of biodegradable organics and nutrients, whereas their removal efficiency for persistent pollutants such as pharmaceuticals and synthetic dyes generally remains below 50–70%, reflecting limitations imposed by low redox potential and slow intrinsic kinetics. In contrast, Fenton-assisted CWs that particularly systems incorporating heterogeneous or photo-Fenton catalysts consistently demonstrate enhanced removal efficiencies exceeding 80–95% for refractory contaminants under near-neutral pH (5–7) through in-situ generation of reactive oxygen species (ROS). Pilot- and field-scale data further indicate that immobilization of iron-based catalytic media within wetland substrates significantly improves oxidative transformation while reducing iron sludge formation and maintaining ecological stability. Collectively, the quantitative trends reported in Table 5 confirm that hybrid photo-reactive wetlands operate beyond the classical Fenton limit, offering a scalable, low-energy intensification strategy that substantially enhances micropollutant removal without compromising the nature-based advantages of conventional CW systems [18].

The Fenton process is governed by the catalytic cycling of Fe²⁺/Fe³⁺, in which hydrogen peroxide is activated to generate highly reactive oxygen species, predominantly hydroxyl radicals (•OH), responsible for the non-selective oxidation of refractory organic contaminants. In photo-Fenton systems, light irradiation accelerates the reduction of Fe³⁺ to Fe²⁺, thereby sustaining continuous ROS production and improving radical utilization efficiency. When combined with constructed wetlands, this mechanism is further intensified through synergistic interactions with wetland components. Iron-rich substrates and media act as heterogeneous catalytic sites, while plant-mediated radial oxygen loss and microbial redox activity facilitate iron cycling and in situ oxidant regeneration under near-neutral pH conditions. The integration also enables low-energy or solar-driven operation, significantly enhancing energy efficiency compared with conventional advanced oxidation processes. As a result, Fenton-assisted constructed wetlands exhibit markedly higher removal efficiencies for persistent pollutants, alongside improved ROS generation and photon-to-ROS yield, while maintaining the ecological functionality and passive treatment advantages inherent to wetland systems.

5.9. Challenges and Gaps of CW–Fenton Systems

Despite the remarkable advances in hybrid constructed wetland (CW)–Fenton and photo-Fenton systems, several critical challenges continue to hinder their full-scale deployment and long-term sustainability. One of the most significant issues concerns material stability and Fe-leaching control. Prolonged operation often leads to gradual catalyst deactivation and iron dissolution, which not only reduces oxidative efficiency but also risks secondary contamination of effluent streams. The development of heterogeneous and magnetically retrievable catalysts (e.g., Fe₃O₄–biochar and Fe–MOF composites) has shown promise in minimizing these losses, yet the mechanisms of long-term Fe redox stabilization within wetland substrates remain poorly understood. Additionally, another major limitation involves light penetration and system scaling. In large or densely vegetated wetlands, attenuation of visible and solar radiation restricts photo-Fenton activity and photon utilization. Designing optically optimized beds with reflective surfaces or embedded light guides could enhance photoactivation but requires further experimental validation under real-field hydrodynamic conditions.

Ecotoxicity and life cycle impacts also remain a concern, especially for systems employing nanoscale or metal-based catalysts. Understanding the potential accumulation, transformation, and ecological effects of catalytic residues is essential to ensure safe long-term application. Parallel to these environmental challenges, there is a technological gap in digital monitoring, modeling, and AI-based optimization. The integration of real-time sensors and machine-learning models could enable adaptive control of flow, light exposure, and chemical dosing, thus improving system resilience and operational efficiency.

Overall, the policy and implementation perspective for nature-based advanced oxidation processes (AOPs) remains limited. Current water-quality regulations and design standards do not yet accommodate hybrid CW–Fenton systems, leading to uncertainty in permitting and scalability. Bridging this gap will require the establishment of regulatory frameworks, performance benchmarks, and incentive mechanisms that recognize CW–Fenton technologies as valid, sustainable solutions for decentralized wastewater management.

Figure 12 highlights the multifaceted challenges that constrain the scalability and sustainability of hybrid constructed wetland (CW)–Fenton and photo-Fenton systems. These interrelated barriers span materials, environmental, technological, and policy dimensions, reflecting the need for integrated, multidisciplinary solutions. The Schematic summaries the key challenges facing hybrid CW–Fenton/photo-Fenton systems, including (i) material stability and Fe-leaching control, (ii) limited light penetration and scaling effects, (iii) ecotoxicity arising from catalyst residues, (iv) lack of digital monitoring and AI-based optimization, and (v) policy and regulatory.

5.10. Future Perspectives

Constructed wetlands (CWs) are increasingly recognized as core elements of green infrastructure and nature-based solutions (NbS) for sustainable wastewater management. Their integration with renewable energy, low-cost catalysts, and digital optimization tools aligns directly with global sustainability goals, particularly SDG 6 (Clean Water and Sanitation), SDG 12 (Responsible Consumption and Production), and SDG 13 (Climate Action) [27,28]. Future research should emphasize material innovation through the design of bio-derived catalysts, MOF–biochar composites, and magnetic nanomaterials that combine high reactivity with easy recovery and reusability. System optimization will require advances in AI-controlled flow regulation, light-distribution modeling, and adaptive hydrodynamics, enabling fine-tuned responses to fluctuating loads and environmental conditions. In parallel, comprehensive life-cycle assessments (LCA) are essential to quantify carbon footprints, energy payback ratios, and ecosystem co-benefits, providing quantitative validation of circular performance.

Furthermore, ecotoxicological assessment must become integral to the design process, focusing on the long-term stability and safety of catalytic materials and substrates under real wetland conditions. The coupling of environmental risk assessment with catalyst engineering will ensure that hybrid CW–Fenton systems achieve both performance and safety benchmarks. Ultimately, constructed wetlands represent a bridge between natural processes and engineered innovation, a scalable platform that unites biological self-regulation, advanced oxidation, and digital intelligence. As a result, CW–Fenton systems embody the principles of circularity, low-carbon operation, and eco-efficiency, positioning them as the cornerstone of sustainable and intelligent wastewater treatment in the 21st century. Future CW–Fenton configurations are envisioned as integrated eco-technological platforms that merge renewable solar energy, advanced catalytic materials, and digital intelligence, driving their evolution toward intelligent, low-carbon, and circular wastewater treatment technologies. Solar-driven photo-Fenton catalysis, enabled by MOF–biochar composites and magnetic nanomaterials, provides high oxidative performance with minimal chemical input. In parallel, AI-based monitoring systems and sensor networks facilitate adaptive flow regulation and predictive maintenance, optimizing treatment efficiency under variable hydraulic and environmental conditions. Embedded within a circular-economy framework, these hybrid wetlands align with SDG 6 (Clean Water and Sanitation), SDG 12 (Responsible Consumption and Production), and SDG 13 (Climate Action), promoting carbon-neutral operation and resource recovery. Collectively, next-generation CW–Fenton systems are poised to function not only as wastewater treatment units but also as self-learning, sustainable bioreactors, redefining wastewater treatment paradigms for the twenty-first century.

6. Conclusions

This systematic review establishes a comprehensive framework for understanding how Constructed Wetlands (CWs) that are traditionally biological and passive can transcend their conventional boundaries by integrating photo-Fenton catalysis and circular nexus principles. By embedding Fe-based catalysts (Fe₃O₄, Fe–MOFs, Fe–biochar composites) within wetland matrices, near-neutral photo-Fenton reactions can be achieved, effectively overcoming the classical “Fenton limit” associated with acidic operation, iron sludge formation, and reagent-intensive conditions. The synergy between sunlight-driven radical generation, plant-mediated oxygenation, and microbial redox cycling transforms CWs into photo-biochemical reactors, capable of degrading refractory pollutants such as dyes, pharmaceuticals, and pesticides with high efficiency (>90%) under environmentally benign conditions. The review underscores that hybrid CW–Fenton systems function as circular eco-technologies that integrate photochemical, biological, and material domains through self-regenerative feedback loops. The interplay of Fe²⁺/Fe³⁺ cycling, rhizospheric oxygen release, and biochar-facilitated electron transfer creates a sustainable oxidation–reduction microenvironment. Such systems simultaneously promote pollutant mineralization, nutrient recovery, and carbon reuse—thereby aligning wastewater treatment with circular economy principles and the United Nations Sustainable Development Goals (SDGs 6, 12, and 13). From a materials perspective, advances in Fe–MOFs, Fe–Cu/Co bimetallic composites, and bio-waste-derived magnetic carbons are pivotal in achieving visible-light responsiveness, structural stability, and minimal Fe-leaching across multiple operational cycles. These catalysts bridge the molecular-level coordination chemistry of Fe centers with applied environmental catalysis, offering robust, reusable, and low-cost options for decentralized treatment. At the system scale, hybrid configurations such as photo-reactive constructed wetlands (PR-CWs) demonstrate that efficient coupling of hydraulic design, optical optimization, and redox control can drastically enhance degradation kinetics while maintaining ecological resilience. Despite substantial progress, several research gaps remain. Long-term catalyst stability, light attenuation in dense vegetative zones, and potential ecotoxicological effects of metal residues require systematic investigation. Additionally, digital monitoring, AI-assisted optimization, and life-cycle assessment (LCA) must become integral to future design to ensure both performance efficiency and environmental safety. Basically, CWs integrated with photo-Fenton catalysis represent a transformative paradigm—shifting from pollutant removal to resource regeneration, from linear treatment chains to circular feedback systems, and from chemical dependency to solar-driven self-sufficiency. These hybrid eco-reactors not only advance sustainable water treatment but also embody the broader vision of a Circular Photo-Biochemical Reactor, where light, material, and biology converge to achieve carbon-neutral, low-energy, and self-sustaining environmental purification.