Remediation of Waterbodies: Status and Challenges in Photocatalytic Nitrate Reduction to N₂—Implications for Recirculating Aquaculture Systems and Nitrogen Sensing

Abstract

Nitrate pollution in freshwater has become an increasing concern for both environmental sustainability and human health, especially in water reuse systems and intensive aquaculture. Photocatalytic reduction in nitrate to nitrogen gas (N₂) represents a promising low-chemical treatment strategy that can operate under sunlight or LED irradiation, and in general, enable nitrate removal without generating concentrated waste streams. Over the past decade, the development of advanced photocatalytic materials, including heterojunction semiconductors, plasmonic catalysts, and single-atom co-catalysts, has significantly enhanced visible-light absorption and overall photocatalytic performance. Despite these advances in photocatalyst design and synthesis, several critical challenges still limit the large-scale implementation of photocatalytic nitrate reduction to N₂. First, selectivity toward N₂ remains limited, as competing reaction pathways often lead to the formation of undesirable byproducts, such as nitrite (NO₂⁻), ammonium (NH₄⁺), and nitrous oxide (N₂O). Second, nitrogen reaction pathways are often uncertain, because many studies lack isotopic labeling or nitrogen mass balances, making it difficult to verify that the detected N₂ originates from nitrate reduction. Third, practical implementation is restricted by several technical challenges, including catalyst fouling or leaching, limitations in reactor design, excessive addition of hole scavengers, and the relatively high energy demand associated with indoor LED-driven systems. This review critically surveys advances from 2015 to 2025 in photocatalytic materials and reaction mechanisms for nitrate conversion to N₂. It highlights best practices for reliable product quantification and reaction pathway validation, and evaluates the feasibility of integrating these systems into recirculating aquaculture systems (RAS), where effective nitrate management is essential. In addition, the potential role of modern inline nitrate sensors (optical and electrochemical) and automated process control is discussed, outlining pathways toward hybrid photocatalytic–biological nitrate removal systems for sustainable aquaculture applications.

1. Introduction

Nitrate (NO₃⁻) pollution in surface water and groundwater is a persistent global issue, primarily driven by agricultural runoff, wastewater discharge, and concentrated effluents from intensive livestock and aquaculture operations (Figure 1a,b). Elevated nitrate levels contribute to eutrophication and harmful algal blooms, which cascade, leading to hypoxia and biodiversity loss in aquatic ecosystems [1,2]. Such an event caused severe ecological damage in the multifunctional hydro-accumulation lakes Stara Moravica and Svetićevo in Serbia, the reservoirs that are primarily used for irrigation and recreational fishing, where rapidly advancing eutrophication during October and November 2023 resulted in extensive fish mortality (Figure 1c,d) [3]. Although designated for protection under national and international laws, lakes Palić and Ludaš of Serbia also remain in a persistently critical ecological state due to eutrophication [4,5,6,7,8]. Based on measurements of total nitrogen (TN), total phosphorus (TP), chlorophyll-a (Chl-a), Secchi depth, and other water-quality parameters between 2011 and 2021, both of these lakes have been classified as “extremely eutrophic” [9]. This eutrophication poses a substantial threat to the ecological integrity of these water bodies and may, under continued nutrient loading, lead to further degradation of ecosystem function and loss of biodiversity. Furthermore, hydrobiological and ichthyofaunal monitoring conducted in 2018 and 2019 by Miljanović and co-authors, as part of comprehensive scientific investigations supporting the preparation of the Annual Management Programs for the fisheries areas Palić (within the Nature Park “Palić”) and “Ludaš” (within the Special Nature Reserve “Ludaško jezero”) for the 2018–2027 period, commissioned by the protected-area manager, the Public Enterprise “Palić–Ludaš” (Kanjiški put 17a, Palić, Serbia), in accordance with national legal requirements, also revealed a pronounced restructuring of the fish community, characterized by strong dominance of non-native taxa. This structural simplification of the ichthyofauna is consistent with, and further supports, the advanced eutrophication reported for these ecosystems in Refs. [8,9,10]. The Annual Management Programs document that the invasive Prussian carp (Carassius gibelio; locally referred to as “babuška” or silver crucian carp) has undergone a significant increase under the prevailing eutrophic conditions and represented the vast majority of total fish abundance. In Lake Palić, C. gibelio accounted for 79.75% of all recorded individuals, accompanied by Pseudorasbora parva (amur chebachok) at 18.35%, Lepomis gibbosus (pumpkinseed sunfish) at 1.27%, and Sander lucioperca (pike-perch) at 0.63% [11]. A similar pattern was observed in Lake Ludaš, where C. gibelio constituted 82.43% of individuals, with additional contributions from P. parva (14.29%), common carp (Cyprinus carpio, 1.35%), rudd (Scardinius erythrophthalmus, 0.97%), bleak (Alburnus alburnus, 0.58%), and pike (Esox lucius, 0.39%) [12]. This imbalance reduces trophic complexity, undermines native biodiversity, and accelerates water-quality deterioration through intensified nutrient recycling and sediment resuspension. Excess nitrogen loading, together with elevated phosphorus concentrations, is a central driver of this degradation, promoting persistent eutrophication, harmful algal growth, and conditions favorable to invasive, hypoxia-tolerant fish species. The primary sources of these nutrient pressures in lakes Palić and Ludaš include prolonged inputs of insufficiently treated municipal wastewater from the nearby city of Subotica, Serbia, direct discharge of untreated sewage from nearby settlements, and diffuse nutrient pollution, particularly nitrate- and ammonium-rich runoff, from agricultural land in the surrounding catchment [10]. Soil and water analyses within the lakes’ buffer zones further revealed elevated concentrations of phosphorus, reactive nitrogen species, and even heavy metals, underscoring that existing protection measures have been insufficient to prevent ongoing contamination [10]. Given the status of Lake Palić as a Nature Park, and Lake Ludaš as a Wetland of International Importance (Ramsar site), the observed eutrophication and biotic imbalance reflect systemic failure in nutrient-load control and land-use management, rather than natural or temporary fluctuations. Restoration of ecological integrity will therefore require not only general improvements in wastewater management but targeted reduction of nitrogen inputs, including advanced nitrogen-removal processes in municipal wastewater treatment, strict control of nitrate- and ammonium-rich agricultural runoff, and the expansion of buffer systems (e.g., riparian strips, constructed wetlands) designed specifically to intercept reactive nitrogen. Complementary in-lake ecological interventions, such as biomanipulation and habitat restoration, will be necessary to re-establish balanced aquatic communities once external nitrogen loading has been substantially confined. It is also well documented that chronic exposure to nitrate-contaminated drinking water affects human health, causing methemoglobinemia in infants and the potential formation of carcinogenic N-nitroso compounds, which have been linked to certain cancers [13,14]. Owing to these risks, international regulatory agencies such as the World Health Organization (WHO), the European Union (EU), and the U.S. Environmental Protection Agency (EPA) have established strict limits for nitrate concentrations in drinking water: 50 mg/L as NO₃⁻ (WHO/EU) and 10 mg/L as nitrogen (U.S. EPA) [15].

Figure 1. (a) Schematic representation of major nitrate (NO₃⁻) pollution sources, including agriculture, wastewater, and livestock, and their environmental impacts, such as algal blooms, oxygen depletion, and ecological damage in freshwater systems. (b) Photograph of the polluted Vidrenjak River flowing through the town of Tutin, Serbia. (c) Geographic coordinates and map location of Stara Moravica and Svetićevo Lakes in the Vojvodina region of Serbia. (d) Photograph illustrating advanced eutrophication observed in October–November 2023 at these hydro-accumulation lakes, which led to a large-scale fish kill.

Traditional nitrate removal methods, including biological denitrification, ion exchange, and reverse osmosis (Figure 2), have been well reviewed in the literature [16,17,18,19,20]. Although effective, these methods are often complex to operate, require a lot of energy or chemicals, and can produce additional waste, such as brine (liquid waste streams rich in nitrate and salts from reverse osmosis or ion-exchange regeneration) or used resins (spent ion-exchange materials requiring disposal) [20]. Photocatalytic nitrate reduction has emerged as a compelling alternative or final refining step for nitrate remediation because, in theory, it only harnesses light energy to produce strong reductants at semiconductor photocatalyst surfaces and thereby converts nitrate directly into molecular N₂, which minimizes the reagent inputs and concentrated waste production [21,22,23,24,25] (Figure 2). Although the photocatalytic nitrate reduction to N₂ is recognized as a promising method for removing nitrate from water, several technical challenges limit its real-world use [26]. Catalysts can lose activity over time because reaction intermediates like nitrite or ammonium, as well as other substances in water, block active sites on the catalyst surface [26]. Many photocatalysts rely on ultraviolet light and suffer from fast recombination of electrons and holes, which significantly reduces energy efficiency. This limitation often necessitates the use of organic hole scavengers (sacrificial reductants) such as formic acid, sodium oxalate, citric acid, methanol, ethanol, sulfite, and, in some cases, in situ-generated electrons under bias or with co-catalysts. These species effectively capture photogenerated holes, suppress recombination, and generate reactive intermediate radicals (e.g., carbon dioxide radical anions, CO₂^•−), which act as strong reducing agents [21]. In addition, reactions can produce unwanted byproducts such as nitrite and ammonium instead of harmless nitrogen gas, and fouling from organic matter or suspended solids further reduces light penetration and catalytic performance [26]. To overcome these issues, researchers are developing more stable catalysts that work under visible light, suppress electron–hole recombination, resist fouling, and can be regenerated [26]. Reactor designs that improve light distribution and mass transfer, as well as hybrid systems combining photocatalysis with biological or electrochemical processes, can help maintain efficiency, control product formation, and enable practical application in real water treatment systems [26]. Early photocatalysis research, which was mainly focused on removal of organic pollutants by oxidative processes, used UV-active TiO₂ (Figure 2), but in the last decade, this focus has shifted to development of visible-light active materials, such as heterojunctions [27,28,29,30,31], doped oxides, plasmonic metal–semiconductor hybrids, and single-atom catalysts, which absorb sunlight better and improve charge separation and surface reactions [32,33,34]. Green synthetic routes for oxide nanomaterials that are also used as photocatalysts, together with scalable immobilization strategies, are actively being developed to lower environmental impact and enable pilot-scale photocatalytic applications [35,36,37].

Figure 2. Illustrative comparison of conventional nitrate removal and photocatalytic nitrate reduction. Ion exchange and reverse osmosis remove NO₃⁻ but generate secondary wastes (brine and spent resins). Photocatalytic reduction uses light-activated semiconductors (e.g., TiO₂) to generate e⁻/h⁺ pairs that convert nitrate directly to N₂ and H₂O without producing concentrated waste streams.

Although oxidation and reduction proceed concurrently in semiconductor photocatalysis upon light excitation, which generates electron–hole pairs [38] (Figure 2), photocatalytic materials optimized for oxidative pollutant degradation, such as those commonly used for the photocatalytic removal of organic contaminants [27,28,29,30,31,32,33,34], cannot be assumed to facilitate selective reduction pathways, such as nitrate-to-N₂ conversion.

In general, the feasibility of nitrate reduction to N₂ is governed by three principal factors: (i) the position of the photocatalyst conduction band relative to the redox potentials of NO₃⁻ reduction to N₂ [39] (Table 1); (ii) efficient photogenerated charge separation; and (iii) favorable surface reaction pathways that suppress competing reactions [40].

Table 1. Theoretical reduction potentials (E⁰) of nitrate at pH 7 (25 °C) [41,42].

Half-Reaction	Electrons (e−)	Protons (H+)	E0 at pH 7 (V vs. SHE)
NO3− + 4H+ + 3e− → NO (nitric oxide) + 2H2O	3	4	+0.35 V
NO3− + 10H+ + 8e− → NH4+ (ammonium) + 3H2O	8	10	+0.36 V
NO3− + 2H+ + 2e− → NO2− (nitrite) + H2O	2	2	+0.43 V
2NO3− + 12H+ + 10e− → N2 (nitrogen) + 6H2O	10	12	+0.75
2NO3− + 12H+ + 10e− → N2O (nitrous oxide) + 6H2O	10	12	+0.76

For nitrate-to-N₂ conversion at neutral pH~7, the photocatalysts has to have conduction band (CB) potential that is sufficiently negative compared to reduction potential of nitrate to N₂ (+0.75 V vs. SHE at pH 7 and 25 °C, Table 1) to thermodynamically drive nitrate reduction, while the valence band must remain positive enough to sustain oxidation of the corresponding electron donor [43,44]. Therefore, a photocatalyst previously employed for oxidative applications may be adapted for selective nitrate reduction, provided that its band energetics are verified, competing pathways (e.g., hydrogen evolution or partial reduction to NH₄⁺) are controlled, and appropriate surface or co-catalyst modifications are implemented [45,46]. In principle, any semiconductor fulfilling the above-mentioned energetic and kinetic criteria can drive nitrate reduction [43,44,45,46,47,48]. An overview of commonly used metal-oxide photocatalysts in oxidative photocatalysis, but with the ability for nitrate reduction under near-neutral conditions, including conduction band potentials, suitability for N₂, NO₂⁻, or NH₄⁺ formation, important properties, and references used, is summarized in Table 2.

Table 2. Metal-oxide photocatalysts suitable for nitrate reduction at near-neutral pH (~7): conduction band potentials, target nitrogenous products, and key design considerations with references used.

Catalyst	CB vs. SHE (pH 7)	Suitable for N2, NO2−, NH4+?	Comments/References
TiO2 (anatase)	−0.3 to −0.5 V	yes	Widely used, stable, co-catalysts improve selectivity [38,40]
WO3	+0.1 V	partially	Needs heterojunctions for full N2 reduction [49,50]
ZnO	−0.4 to −0.5 V	yes	Good electron transfer; less stable in acidic conditions [39]
BiVO4	~0 V	partially	Visible light active; needs co-catalyst/heterojunction [51]
Doped/heterojunction oxides	varies	yes	Extended absorption, better charge separation, tunable selectivity [38]

The strategies to achieve efficient nitrate-to-N₂ reduction are largely analogous to those used in efficient photocatalytic degradation of organic contaminants by oxidative processes: (1) construction of Z-scheme or heterojunction systems to enhance charge separation; (2) co-catalyst engineering to reduce over-potentials and improve selectivity; (3) integration into photo-electrochemical configurations where electrons are supplemented by an external bias; and ultimately (4) the development of systems that eliminate the need for sacrificial additives (e.g., formic acid, sodium oxalate, citric acid, methanol, ethanol, and sulfite) [43,44,45,46,47,48]. The last-mentioned dependence on hole scavengers highlights a fundamental challenge: achieving efficient, selective nitrate-to-N₂ reduction using water as the sole electron donor remains difficult due to sluggish oxygen evolution kinetics and charge recombination [26]. Frequently employed formic acid is effective in suppressing electron–hole recombination and promoting selective nitrate reduction to N₂, but its limitations remain [43]: (1) It requires continuous addition in stoichiometric or excess amounts; (2) large-scale implementation would increase operational costs; (3) it introduces additional organic carbon into the system, potentially leading to secondary treatment requirements; and (4) its use shifts the process away from being a truly sustainable or “green” standalone nitrate remediation strategy.

In general, in photocatalytic nitrate reduction, nitrate (NO₃⁻) can be transformed into different nitrogen-containing products depending on which reaction pathways dominate on the catalyst surface. The principal products are nitrogen gas (N₂), the desired end product; ammonium (NH₄⁺), an undesired reduced form that remains a pollutant; and nitrous oxide (N₂O), a greenhouse gas formed during incomplete reduction. Product selectivity is determined by competing surface reactions driven by photogenerated electrons (e⁻) from the photocatalyst [52,53].

Depending on reaction conditions, such as pH, light intensity, catalyst composition, surface structure, and the presence of co-existing species, electrons may follow distinct pathways. In the ideal pathway, controlled multi-electron transfer promotes N–N bond formation on the catalyst surface, yielding molecular nitrogen according to [43]:

2NO₃⁻ + 12H⁺ + 10e⁻→ N₂ + 6H₂O

(1)

This pathway is considered optimal because N₂ is chemically inert, readily removed from water, and does not generate secondary pollutants.

Alternatively, under conditions of excessive or poorly controlled electron transfer, nitrate may undergo over-reduction to the −3 oxidation state of nitrogen, forming ammonium:

NO₃⁻ + 10H⁺ + 8e⁻→ NH₄⁺ + 3H₂O

(2)

This pathway is undesirable because NH₄⁺ remains in the aqueous phase, contributes to eutrophication, and can cause secondary water-quality problems.

A third pathway involves partial reduction, in which the reaction terminates prematurely, leading to the formation of nitrous oxide:

2NO₃⁻ + 10H⁺ + 8e⁻→ N₂O + 5H₂O

(3)

The formation of N₂O is indicative of poor selectivity and insufficient control over the reaction pathway, and is particularly problematic due to its strong greenhouse gas effect.

Consequently, the final products depend on how photogenerated electrons are shared among these competing pathways. Even small changes in conditions of pH, light, catalyst type, surface defects, co-catalysts, or ions greatly affect product selectivity [52] (Figure 3).

Figure 3. Schematic representation of photocatalytic nitrate (NO₃⁻) reduction pathways on a catalyst surface. Photogenerated electrons (e⁻) drive competing reactions leading to nitrogen gas (N₂, ideal outcome), nitrous oxide (N₂O, partial reduction), or ammonium (NH₄⁺, over-reduction). The final product distribution depends on catalyst type, reaction conditions (e.g., pH), and electron flow.

Because N₂ is naturally abundant and reaction intermediates are short-lived, proving that N₂ actually comes from nitrate usually requires ¹⁵N isotopic labeling, full nitrogen mass balances, and gas monitoring for N₂O and NO. Many studies miss one or more of these steps, making results hard to compare [54,55,56].

Several practical engineering challenges are still unresolved: catalyst geometry and placement, spatial light distribution, hydrodynamics, fouling in chemically and biologically complex waters, previously mentioned reliance on sacrificial agents, and the substantial energy demand associated with indoor LED illumination all influence whether a laboratory-scale photocatalyst can be translated into reliable performance at larger scales.

Therefore, many reported photocatalytic nitrate reduction to N₂ systems remain proof-of-concept studies (Table 3).

Table 3. Summary of primary technical challenges in photocatalytic nitrate reduction to N₂, along with corresponding photocatalyst materials or approaches, their mechanisms for addressing each challenge, explicit identification of the reducing/electron donor agents employed, and supporting literature references.

Challenge	Photocatalysts/Approach [Reference]	Mechanism/Benefit	Reducing Agent/Electron Donor
Low N2 selectivity and high sacrificial-agent demand	SrFexTi1−xO3/TiO2 heterojunction (Fe-doped perovskite with TiO2) [43]	Lewis-acid sites enhance nitrate adsorption; improved charge separation promotes selective NO3−→N2 reduction	Formic acid (HCOOH)
Low N2 selectivity, catalyst instability (Cu2O reduction), high bias in electrocatalysis	Cu2O nanorod arrays on Cu foam; photoelectrocatalytic system [44]	Light-generated electrons assist nitrate reduction and lower electrical bias; improved stability and N2 selectivity	Photogenerated electrons supplied through external electrochemical bias)
Low N2 selectivity	ZnCr2S4 spinel with metal co-catalysts (Pt, Pd) [45]	Visible-light activity; improved electron transfer promotes NO3−→N2	Sodium oxalate (hole scavenger)
Rapid electron–hole recombination	Heterojunction catalysts (TiO2/WO3, TiO2/graphene), noble metal co-catalysts (Ag, Pt), Ag@SrTiO3-TiO2 [46,47]	Promote charge separation, increase electron lifetime	Hydrated electrons (eaq¯) generated in situ under irradiation
Low product selectivity in complex matrices	Tailored morphology & co-catalysts [46]	Optimize intermediate adsorption and reaction pathway control	Formic acid, citric acid alcohols
Low solar efficiency (UV-only catalysts)	Doped TiO2 (N, C, or S doping), MoS2, g-C3N4 [45,48]	Extend light absorption into the visible spectrum	Photogenerated electrons in MoS2 conduction band (no external sacrificial donor; solar-driven)
Performance variability & scaling	Standardized testing & in situ characterization [53]	Understand mechanisms, optimize conditions for real water systems	Photogenerated electrons

Photocatalytic nitrate removal could benefit further by integrating the photocatalytic reactor with sensors and automated control systems (Figure 4). Sensors continuously monitor key water-quality parameters, including nitrate concentration, pH, dissolved oxygen, and temperature, providing real-time process data [57,58,59]. This data is analyzed using control algorithms and predictive models to assess reaction progress and identify optimal operating conditions. Automated actuators then adjust variables such as light intensity, hydraulic flow rates, and catalyst exposure to promote selective reduction in nitrate to nitrogen gas (N₂) while minimizing undesired byproducts. A central control platform, or digital twin, supervises the system, simulates operating scenarios, and enables predictive maintenance, ensuring stable and efficient performance under variable water quality. Overall, the integration of sensing, modeling, and automation enables a smart, adaptive system that improves nitrate removal, reduces energy consumption, and prolongs catalyst lifetime.

Figure 4. Schematic of a smart photocatalytic nitrate removal system. Sensors continuously measure water quality (nitrate, pH, oxygen, and temperature), and models use this data to predict and optimize reactor conditions. Automated controls adjust light, flow, and other parameters, while a central system monitors performance and signals maintenance, enabling real-time, efficient, and selective nitrate removal.

Therefore, the application of semiconductor photocatalysis as a low-cost and environmentally sustainable water treatment technology in recirculating aquaculture systems (RAS) represents a logical and controllable intermediate step toward future deployment of photocatalytic nitrate reduction to N₂ in more complex and dynamic open-water environments, such as lakes and rivers, where benthic invertebrate communities and aquatic biodiversity, well-established indicators of ecosystem health, are particularly sensitive to water-quality perturbations [60,61]. Within RAS, operational conditions can be precisely regulated, enabling systematic evaluation of catalyst performance, stability, and treatment efficacy before translation to large-scale ecological systems. RAS constitute a particularly relevant test bed because they require stringent nitrate management and operate under controlled, yet realistic, conditions that bridge laboratory studies and natural environments. In RAS, ammonia excreted by fish is converted to nitrate through nitrification. While nitrate is less toxic than ammonia or nitrite, its long-term accumulation can reduce fish growth, impair immune function, affect reproduction, and limit opportunities for water reuse [62]. Traditional nitrate control in RAS relies on water exchange, heterotrophic or autotrophic denitrification reactors (which need added carbon or special conditions), or membrane treatments, each having advantages and disadvantages in terms of cost, space requirements, or operational complexity [63]. An integrated treatment framework can be envisioned in which a final photocatalytic reactor is implemented as a polishing step after solids removal and biofiltration, with the potential to simultaneously reduce both nitrate and residual nitrite to N₂ gas. Although laboratory-scale studies frequently rely on organic reductants like formic acid to achieve high nitrate removal efficiencies [21,26], sensor-controlled operation may enable the use of water-splitting-assisted reduction pathways to reach safe nitrate levels without the addition of external organic carbon [3]. Such an approach could reduce water exchange requirements and support higher levels of water reuse. However, for practical implementation, catalyst–reactor systems would need to be engineered with consideration for fouling resistance, operational stability, ease of maintenance, and low energy demand. These aspects remain closely linked to current scalability challenges in photocatalytic nitrate-to-N₂ reduction, including competing side reactions, incomplete conversion in complex water matrices, and challenges in maintaining high selectivity toward N₂ [26,52,64]. Sensor technology is another key enabler. Modern UV/LED optical nitrate probes, compact electrochemical sensors, and portable analyzers now provide near-real-time monitoring with high enough sensitivity to support adaptive, alternating photocatalytic treatment, reducing energy use and catalyst contamination [58,59]. Standardization and guidelines for in situ nutrient sensors help integrate sensor data into farm management and research, improving reproducibility and decision-making [57].

This review summarizes research from 2015 to 2025 on photocatalytic nitrate reduction to N₂, with a focus on reaction mechanisms, appropriate methods for evaluating performance, and the current readiness of the technology for real-world applications. We further examine the implications of these findings for RAS and discuss how sensor-controlled, hybrid treatment systems could facilitate the transition of photocatalysis from laboratory studies to full-scale aquaculture operations. Key research gaps are identified, particularly the need to verify product selectivity in real water matrices and to conduct pilot-scale demonstrations. Finally, we propose a research roadmap emphasizing standardized testing protocols, pilot studies under realistic RAS conditions, and integration with automated, sensor-based process control.

2. Literature Search Strategy

A comprehensive literature survey was conducted using scientific papers, book chapters, technical reports, and reference monographs published in English, as well as relevant regional sources. Indexed and non-indexed journals were screened through major online databases, including Google Scholar, PubMed, Scopus, Web of Science, Semantic Scholar, ResearchGate, and ScienceDirect. Additional information was retrieved from open-access platforms and institutional repositories with a focus on photocatalytic nitrate reduction to N₂, catalyst development, mechanistic studies, and applications in RAS. This review summarizes research published between 2015 and 2025, during which 127 publications were identified as directly relevant to the scope of the topic, covering catalyst materials, reaction pathways, engineering aspects, and sensing technologies. The reference lists of all selected papers were further examined to ensure completeness and to obtain additional data necessary for a more accurate and comprehensive analysis of current advancements and research gaps.

3. Photocatalytic Nitrate Reduction Mechanisms: Current Status andPerspectives

3.1. Reaction Pathways

In short, the photocatalytic reduction in nitrate (NO₃⁻) is an intrinsically complex, multi-electron and multi-proton process [26,65], in which the desired outcome is the conversion of nitrate into dinitrogen (N₂). Two main reaction pathways are generally observed: the denitrification sequence (NO₃⁻ → NO₂⁻ → NO → N₂O → N₂) and the competing ammonification route leading to NH₄⁺ [65,66,67]. The choice of pathway, and hence the product distribution, depends strongly on catalyst surface chemistry, proton/electron flux, aqueous matrix conditions (pH, ionic strength, and sacrificial agents), and light irradiation characteristics [21,65,66]. The key conditions favoring N₂ formation are outlined below.

First, selective photocatalytic reduction in nitrate to N₂ requires carefully controlled reaction conditions that favor stepwise denitrification while suppressing highly reducing, proton-rich environments that promote NH₄⁺ formation [64,68,69,70]. Mildly acidic to near-neutral pH (≈5.5–7) is optimal, as it provides sufficient protons for multi-electron transfer without inducing over-reduction. At lower pH (<4–5), NH₄⁺ formation predominates, whereas at higher pH (>7.5–8), weaker nitrate adsorption and competition from OH⁻ lead to increased accumulation of NO₂⁻ and N₂O. Buffered systems that stabilize NO₃⁻ and NO₂⁻ while moderating proton availability are therefore most favorable for N–N coupling [71].

Second, electron delivery must also be matched to the kinetics of N–N bond formation. Insufficient electron flux results in accumulation of intermediates such as NO₂⁻ or N₂O, while excessive flux, caused by high light intensity or strong reductants, drives over-reduction to NH₄⁺. Optimal performance is achieved under moderate photon flux, controlled sacrificial donor concentrations, and with catalysts capable of transient electron storage or buffering [71].

Third, catalyst surface properties play a critical role in stabilizing N–N intermediates. Efficient co-adsorption of species such as NO and N₂O is observed on mixed metal oxides (e.g., Fe–Ti, Cu–Ti, and Ag–Ti), materials with redox-active centers (Fe²⁺/Fe³⁺, Cu⁺/Cu²⁺), and catalysts with moderate oxygen-vacancy densities [64,66,68,69,70]. In contrast, highly reducing single-metal sites or surfaces with strong hydrogen adsorption tend to favor NH₄⁺ formation. Because N₂ generation proceeds via a bimolecular surface reaction, it is enhanced by adjacent active sites, heterojunctions, stepped facets, or metal–oxide interfaces. An optimal defect density is essential: insufficient defects limit nitrate activation, whereas excessive defects promote uncontrolled electron transfer.

Fourth, co-catalysts should primarily enhance charge separation and stabilize reaction intermediates rather than promote hydrogen evolution. Low loadings of noble metals (e.g., Ag, Pd) improve electron transport and reduce charge recombination through Schottky junction formation, even at minimal metal content [68]. Transition metals (e.g., Cu, Fe) further contribute by stabilizing nitrogen-containing intermediates [72]. In contrast, high loadings of strong hydrogen-evolution catalysts (e.g., Pt) divert electrons toward H₂ formation and proton-coupled hydrogenation pathways, thereby reducing selectivity toward N₂ and promoting ammonification [73].

Finally, reactor design must ensure sufficient residence time for complete multi-electron conversion, favoring immobilized catalysts and low hydraulic loading. Minimizing competitive adsorption by species such as OH⁻, HCO₃⁻, or SO₄²⁻ further enhances N₂ selectivity, which explains the superior performance typically observed in pure or weakly mineralized water matrices [71].

Table 4 summarizes representative studies that achieve the selective nitrate reduction to N₂, highlighting the roles of pH, catalyst composition, light intensity, and reactor design.

Table 4. Examples of photocatalytic nitrate reduction with high N₂ selectivity (Pathway 1), including catalyst systems, reaction conditions, and reported selectivity outcomes.

Catalyst/Reaction Conditions	N2 Selectivity/Performance	Key Notes/Mechanistic Factors [References]
Ilmenite (FeTiO3)/UV light, oxalic acid as hole scavenger, N2 atmosphere, C2O42−/NO3− molar ratio ≥ stoichiometric	>93% N2 selectivity, up to >97% with excess oxalate; negligible NH4+ formed	Controlled hole scavenger and stoichiometric reducing agent promote multi-electron transfer to N2 via NO2− intermediate; avoids NH4+ pathway [64]
ZnCr2S4 with Pd&Pt co-catalysts/ZnCr2S4 loaded with 0.5 wt% Pd and 0.5 wt% Pt; Hg lamp UV illumination; 100 ppm NO3− aqueous	~65% differential N2 selectivity in early stages; possible approach to higher N2 yield over time	High electron density facilitates deep multi-electron reduction; selectivity increases with irradiation and catalyst design [45]
Ag/SiO2@crystalline TiO2 core–shell/5 wt% Ag/SiO2@TiO2, light scattering & SPR, nitrate~2000 mg/L; UV/visible	~93.6% N2 selectivity with ~95.8% nitrate removal in ~4 h	Ag acts as electron sink/promotes charge separation; surface plasmon effects improve electron availability for N–N coupling [68]
Cu0/Cu2O–PANI–CNT composite/Cu0/Cu2O-PANI-CN, neutral pH, no external hole scavenger	100% N2 selectivity with ~100% NO3− removal	Dual active components: Cu phase reduces NO3−→NO2−, PANI facilitates further reduction to N2; neutral pH helps avoid over-hydrogenation [69]
g-C3N4/TiO2&ZnO composites/nanocomposite surfaces under UV/visible, pH not strongly acidic	N2 selectivity rising to ~90–95% during reaction	Composite design enhances charge separation and overlapping absorption, promoting stepwise reduction over side reactions [70]
Nd&N co-doped TiO2 with hole scavenger (HCOOH), visible illumination	~85% N2 selectivity, high NO3− conversion	Dopant and hole scavenger combination improves adsorption and suppresses undesired pathways [71]

These analytic best practices form a cornerstone of transitioning from lab demonstration to practical application.

3.2. Catalyst Classes

Over the last decade, catalyst development for nitrate to N₂ photocatalytic reduction has progressed rapidly, with three major classes emerging: (i) doped/modified metal-oxide semiconductors (e.g., TiO₂, CuFe₂O₄, ZnO, and BiVO₄) [21,23,74,75,76]; (ii) heterojunction and Z-scheme systems [47,76]; and (iii) nanomaterials and plasmonic/metal-supported and single-atom co-catalysts [68,70,77]. Furthermore, mechanistic tools such as operando spectroscopy, DFT modeling, and isotopic tracing are increasingly used to link catalyst properties to reaction performance [78].

(i) Doped/Modified Metal Oxides

Traditional photocatalysts such as TiO₂ have been modified via doping (Zn, Fe, N, etc.) to increase visible-light absorption, improve charge separation, and tailor adsorption properties [79,80]. Studies on doped TiO₂ have reported partial nitrate conversion under UVC or visible-light irradiation in the presence of organic hole scavengers, highlighting how sacrificial agents interact with dopant-induced electronic states to influence reduction pathways [79]. Collectively, these findings indicate that while doping can enhance activity, it is rarely sufficient on its own; matrix effects, catalyst fouling, and photon and hydraulic limitations remain key barriers to scalable implementation.

Likewise, recent advances in semiconductor-based photocatalysts continue to emphasize ZnO nanostructures due to their strong redox potential, high charge-carrier mobility, and compatibility with composite architectures, which make them attractive for aqueous nitrate removal and reduction [74,81]. In this context, a biogenic ZnO nanophotocatalyst (NPc), synthesized via a green precipitation route using an aqueous extract of the edible mushroom Cyclocybe aegerita (V. Brig.) Vizzini 2014 and zinc acetate as precursors has shown promising optoelectronic properties [75]. The use of mushroom-derived extracts is particularly advantageous because basidiomycetes are rich in phenolic compounds, polysaccharides, proteins, and other redox-active metabolites capable of acting simultaneously as reducing, stabilizing, and capping agents during nanoparticle formation [82,83,84]. Moreover, the biochemical complexity of fungal extracts can modulate nucleation kinetics and particle morphology, yielding nanostructures with enhanced surface reactivity and improved charge-transfer characteristics. Edible and medicinal mushrooms have increasingly been recognized as efficient biofactories for metal and metal-oxide nanoparticle synthesis, combining sustainability, low toxicity, and intrinsic biofunctionalization potential [82,83,84]. The material was prepared using mild, green-chemistry conditions (80 °C, 2 h, and pH 12 adjusted with 2 M NaOH) [85], and exhibits a band alignment thermodynamically favorable for both photocatalytic denitrification and nitrification. Specifically, conduction-band electrons can reduce nitrate and nitrite intermediates toward N₂, whereas valence-band holes, and the associated ^•OH radicals, are capable of oxidizing ammonia, enabling complete nitrogen transformation under appropriate conditions [75]. To translate these properties into practical denitrification performance, immobilization of ZnO on functional supports is increasingly pursued to improve dispersion, suppress particle agglomeration, and enable catalyst recovery after use [86]. Incorporation of ZnO into polymeric or biopolymer matrices, such as alginate, can preserve photocatalytic activity while improving material stability and reusability. Immobilized ZnO systems have repeatedly shown higher nitrate removal efficiencies and greater suitability for continuous-flow reactors than suspended ZnO powders [81,86,87]. In parallel, ZnO-containing composite photocatalysts (e.g., g-C₃N₄/TiO₂/ZnO) have demonstrated high nitrate conversion and improved selectivity toward N₂, confirming that ZnO can effectively participate in the sequential reduction pathway from nitrate via nitrite to N₂, under photocatalytic conditions [70].

(ii) Heterojunction and Z-Scheme Systems

To further improve charge separation and direct electron transfer, heterojunction photocatalysts (such as BiVO₄/NH₂-MIL-101(Fe) Z-scheme) have been applied to nitrate reduction. In one study, the NH₂-MIL-101(Fe)/BiVO₄ heterojunction achieved ~94.8% NO₃⁻ removal and ~93.4% N₂ selectivity under UV irradiation (100 mg N/L initial, 50 min) [88,89]. The internal electric field of the Z-scheme promoted efficient charge separation and migration of photogenerated electrons toward nitrate reduction sites. While these Z-scheme architectures deliver high selectivity under controlled laboratory conditions, their effectiveness in realistic, chemically complex water matrices remains limited.

(iii) Plasmonic/Metal-Supported and Single-Atom Co-Catalysts

Moving toward atomic-scale catalyst engineering, plasmonic metals (Ag, Au) loaded on semiconductors, i.e., plasmonic photocatalysts and single-atom co-catalysts, are increasingly exploited due to their ability to tune surface adsorption, enhance localized electromagnetic fields, and improve interfacial electron-transfer kinetics [34]. Plasmonic catalysts contain metals capable of supporting collective oscillations of conduction-band electrons (plasmons) when excited by electromagnetic radiation, a phenomenon known as localized surface plasmon resonance (LSPR). The decay of these plasmons generates energetic charge carriers (hot electrons and hot holes) as well as strong localized electromagnetic fields, which can significantly accelerate surface chemical reactions.

For instance, as mentioned earlier, the Ag–TiO₂/formic acid system [21] achieved approximately 82% N₂ selectivity under visible-light irradiation (50 mg-N/L NO₃⁻, 120 min). However, mechanistic analysis highlighted that CO₂^•− radicals generated from HCOOH played a dominant role in nitrate reduction, rather than the direct participation of photogenerated electrons.

Single-atom metal catalysts, widely studied in electrocatalysis, are now also being explored in photocatalytic systems [34]. Theoretical studies suggest that key descriptors, such as the binding strength of intermediates and their surface coverage, strongly influence reaction pathways and N₂ selectivity [80].

3.3. Mechanistic Diagnostics

Recent reviews highlight three core elements of mechanistic diagnostics: (1) reliable quantification of N₂ using ¹⁵N labeling or mass spectrometry; (2) detection of key intermediates (NO₂⁻, NO, and N₂O) in both aqueous and gas phases; and (3) standardized reporting of photon-flux and catalyst loading. Advances in mechanistic understanding show that reaction selectivity is governed by the interplay of reactive species (^•e⁻, ^•H, and CO₂^•−), intermediate lifetimes, and surface adsorption/desorption kinetics. For example, an ilmenite-based photocatalytic study showed that bicarbonate and Ca²⁺ significantly reduce performance by interfering with NO₃⁻ adsorption and with the hole scavenger oxalate, demonstrating the critical importance of water matrix effects [66]. Moreover, the presence of matrix ions was shown to modify the pH and displace nitrate from surface adsorption sites, ultimately lowering N₂ selectivity [80]. Furthermore, surface fouling, catalyst deactivation, and back-oxidation of reduced intermediates (e.g., NO₂⁻ → NO₃⁻) remain under-reported but critical challenges. Several studies recommend periodic catalyst regeneration and cleaning to enable operation under realistic water reuse conditions [26,65]. Developing reactors that allow direct sampling of gaseous products (N₂, N₂O) is also essential for verifying nitrogen selectivity. Increasingly, the reporting of apparent quantum yields (AQY) and photon flux (in µmol photons s⁻¹) is expected to validate photocatalytic performance claims [45]. Nitrate reduction to N₂ is governed by the availability of photogenerated electrons, interfacial charge-transfer processes, and suppression of competing oxidative reactions [52]. For example, the performance of immobilized ZnO/activated carbon alginate nanocomposite catalysts is controlled by interfacial charge-transfer efficiency, the availability of photogenerated redox equivalents, and the suppression of competing oxidative pathways [86]. Confinement of ZnO nanoparticles within hydrogel networks modifies local mass transfer conditions and can shift the balance between electron-driven nitrate reduction and hole-mediated oxidation of sacrificial species or water [86]. Similarly, ZnO-based composites supported on zeolites or biopolymers have also been shown to promote selective nitrate conversion by stabilizing reactive intermediates and improving electron accessibility [90]. Overall, photocatalytic nitrate-to-N₂ research has progressed substantially over the past decade, achieving selectivity under controlled laboratory conditions and improving mechanistic understanding of catalyst surface structure, light absorption, and reaction environment control product distribution. However, translation to real-water systems and continuous operation remains a gap. Specifically, achieving >90% selectivity to N₂ under high nitrate loading, in complex ionic matrices, over extended cycles with stable catalyst performance, remains rare. While new diagnostic practices (isotopic labeling, full mass balances) are emerging, inconsistency in reporting remains. For practical applications, including nitrate removal in RAS, effective integration of catalyst design, reactor engineering, fouling control, and sensor-based process monitoring will be essential.

4. Integration in Recirculating Aquaculture Systems (RAS)

In RAS, the nitrogen cycle begins with fish metabolic excretion (primarily as ammonium, NH₄⁺), which is oxidized in biofilters through nitrification (NH₄⁺ → NO₂⁻ → NO₃⁻). Because nitrate is relatively stable, it tends to accumulate and is typically removed by water exchange, which conflicts with the closed-loop, minimal-discharge goals of modern RAS. While nitrate is less acutely toxic than ammonia or nitrite, prolonged exposure to elevated concentrations can negatively affect growth, immunity, osmoregulation, and reproduction of cultured species. Consequently, effective nitrate control has become a key requirement for sustainable RAS operation [91,92,93,94].

4.1. Nitrogen Cycle in RAS

In a typical RAS, feed-derived nitrogen is converted by nitrifiers into nitrate. Without active removal, nitrate builds up, requiring either water replacement or dedicated removal units. Traditional nitrate removal approaches include denitrification reactors using heterotrophic bacteria (requiring organic carbon) or autotrophic denitrifiers (e.g., hydrogenotrophic systems). For instance, a hydrogenotrophic denitrification system applied in RAS removed nitrate at rates up to 504 g N/m³× d (504 g of nitrogen, as nitrate, per cubic meter of reactor volume per day) at 1 h retention time (each “packet” of water stays in the reactor for only 1 h before exiting), demonstrating a highly efficient treatment process [95]. Solid-phase denitrification reactors that use biodegradable carbon have also been implemented in RAS and have shown effective nitrate removal and associated shifts in microbial community structure [96]. These biological methods are proven but have limitations: carbon dosing increases operational cost and risk of heterotrophic overgrowth, while autotrophic reactors often require a specialized supply of H₂ or thiosulfate and careful control [97].

4.2. Photocatalysis in RAS

Photocatalytic nitrate reduction offers a compelling alternative or complement to biological removal in RAS [98]. The photocatalysis that converts NO₃⁻ directly to N₂ without added organics aligns with the “zero discharge” and low-organics goals of intensive aquaculture. However, successful integration into RAS requires addressing several practical constraints: complex water matrices as RAS waters contain dissolved organic carbon (DOC), suspended solids, salts (e.g., chloride, sulfate), and biofilms that can foul catalyst surfaces, block light penetration, or scavenge reactive radicals. These matrix effects degrade photocatalyst performance relative to pure laboratory water [29,46].

Since RAS typically operate with high water turnover and short hydraulic retention time, photocatalytic reactors must be designed for continuous flow. Photocatalytic reactors should therefore maximize photon efficiency, minimize dead zones, and be compatible with recirculating water. Immobilized catalysts (thin films, coated packing) combined with modular LED illumination are promising strategies to achieve effective nitrate and nitrite removal under these conditions [99]. Integrating a photocatalytic reactor downstream of the nitrification stage offers a means to simultaneously reduce both nitrate and residual nitrite to N₂ gas, addressing end-of-pipe nitrogen removal that biological processes alone cannot achieve in RAS [21,45,64,68] (Figure 5).

Figure 5. Schematic of the integrated photocatalytic reactor for simultaneous reduction of nitrate (NO₃⁻) and nitrite (NO₂⁻) to N₂. UV/visible light generates electrons and holes on the catalyst surface, driving stepwise reduction through intermediates (NO₂⁻, NO, and N₂O) to N₂. Hole scavengers suppress competing oxidation reactions, while minor byproducts such as N₂O and water are also formed.

The photocatalytic reduction of nitrate (NO₃⁻) to N₂ proceeds stepwise via nitrite (NO₂⁻) as direct 5-electron reduction in NO₃⁻ to N₂ is thermodynamically unfavorable and has not been experimentally observed. Initial 2-electron transfer reduces surface-bound NO₃⁻ to NO₂⁻, which either desorbs or is further reduced at adjacent sites. Transient NO₂⁻ is consistently detected even when N₂ is the final product, making simultaneous removal of nitrate and nitrite critical in systems such as RAS to maintain high N₂ selectivity and avoid byproducts like NH₄⁺ or N₂O [64]. Therefore, the general multi-electron pathway can be described: (1) adsorption of NO₃⁻ on the catalyst surface; (2) reduction to NO₂⁻ (NO₃⁻ + 2e⁻ + 2H⁺ → NO₂⁻ + H₂O); (3) stepwise reduction through intermediates such as NO, N₂O, and surface-bound nitrogen radicals (N^•); and (4) recombination of N^• to form N₂ (Figure 6). This mechanism has been confirmed on TiO₂, ZnO, BiVO₄, and other doped semiconductors, often using ¹⁵N isotopic labeling to track intermediate formation [45].

Figure 6. Schematic of the thermodynamically favorable photocatalytic reduction of nitrate (NO₃⁻) to N₂. Nitrate absorbs on the immobilized photocatalyst surface, where UV/visible light generates electrons and holes that drive its stepwise reduction to NO₂⁻ and further subsequent intermediates. Surface-bound nitrogen radicals (N^•) then recombine to form N₂.

Furthermore, catalyst stability under RAS operational conditions requires materials that resist leaching, photo-corrosion, and fouling, while maintaining activity across variable pH, temperature, and ionic strength. In addition, regular cleaning/regeneration must be feasible without system disruption [100]. In this context, ZnO-based composites, particularly those immobilized in biopolymer matrices such as alginate or chitin, have demonstrated promising performance in aquaculture-like wastewater streams [90]. For example, chitin/ZnO composites were used to treat aquaculture wastewater under UV illumination, achieving substantial removal of nitrogenous contaminants under controlled conditions [90]. More recently, ZnO/activated carbon alginate beads maintained catalytic performance in fixed-bed flow reactors, highlighting their relevance for integration into RAS side-stream treatment where physical containment and hydraulic stability are essential [87]. The demonstrated feasibility of immobilized ZnO within polymeric scaffolds, such as the porous matrices, suggests that structured zinc oxide–sodium alginate (ZnO/SA) composites could be integrated into existing RAS filtration modules (e.g., denitrification units, polishing reactors, or degassing chambers). Their combined photocatalysis and antimicrobial action offer a hybrid strategy for nitrate reduction, organic matter removal, and microbial control without additional chemical inputs. Such integration aligns with key RAS design requirements, including minimal nanoparticle release, ease of replacement, and compatibility with continuous operation.

Next to mention is the energy and nutrient footprint that must meet all requirements since indoor RAS typically lack direct sunlight and rely on LED or lamp sources, so the energy cost of photocatalysis must be justified by operational gains (e.g., reduced water exchange, improved fish performance). For that, a techno-economic analysis is needed.

As noted above, the most practical implementation is likely a hybrid treatment concept that integrates photocatalysis with established biological treatments. In this configuration, a photocatalytic unit partially reduces nitrate to moderate levels, while a downstream biological denitrifier finishes the removal. This hybrid approach combines the strengths of both technologies, reduces the need for external organic carbon, and enables smaller reactor volumes [101].

In practice, a feasible RAS configuration could use a dedicated polishing loop: after solids removal and nitrification, water is passed through a photocatalyst unit whenever sensors detect nitrate above a set threshold, and then returned to the fish tank. Continuous sensor monitoring and minimal water exchange maintain nitrate within safe levels, reducing system footprint and improving overall sustainability [102].

5. Nitrogen-Sensing Technologies for RAS

Advanced sensing is essential for efficient control of nitrate removal in RAS, particularly when photocatalytic polishing is applied. Continuous or high-frequency monitoring of nitrogen species enables precise reactor operation, prevents over-treatment, and ensures the health and welfare of the cultured fish.

5.1. Ion-Selective Electrodes (ISEs)

ISEs for nitrate (NO₃⁻) and ammonium (NH₄⁺) are widely used because of their simplicity and low cost, enabling continuous ion monitoring through selective membranes. Nevertheless, in RAS, they face practical challenges, including membrane fouling by organics and biofilms, drift over time, calibration requirements, and interference from high ionic strength or turbidity [103]. While valuable for routine monitoring, ISEs alone may not satisfy the accuracy, long-term stability, and maintenance requirements of a sensor-driven photocatalytic control system.

5.2. UV/LED Spectroscopic Sensors

Spectroscopic sensors measure nitrite by its characteristic UV absorbance (~200–220 nm) or by using LEDs tuned for nitrite/nitrate detection. They enable non-consumptive, inline monitoring and are well-suited for RAS operation. For example, compact UV/LED analyzers designed for RAS applications (Aquamonitrix^®, Carlow, Ireland) provide real-time nitrate and nitrite measurements with minimal maintenance and reagent use [59]. These sensors, when coupled with wireless data transmission, enable high-frequency monitoring and provide data for automated processcontrol.

5.3. Electrochemical and Optical Biosensors

Electrochemical sensors (e.g., copper electrodes, polyaniline coatings) and optical biosensors have been developed for multi-parameter monitoring in aquatic systems, including nitrate, ammonium, pH, and temperature [104,105]. One integrated platform achieved detection limits of 0.84 mg/L for nitrate and 0.02 mg/L for ammonium, and its deployment in a RAS demonstrated practical feasibility [57]. These sensors offer high sensitivity and multi-parameter capabilities, but challenges remain with fouling, long-term stability, and cost in commercial RAS applications.

5.4. Gaseous N₂O/NO Monitoring

Incomplete nitrate reduction, whether biological or photocatalytic, can produce and release nitrous oxide (N₂O) or nitric oxide (NO), both of which are environmentally and regulatory relevant [97,106]. Therefore, inline or periodic gas-phase monitoring of N₂O using methods such as chemiluminescence, non-dispersive infrared spectroscopy (NDIR), or gas chromatography (GC) is increasingly recommended, especially in systems employing photocatalytic nitrate removal, to ensure a net reduction in greenhouse-gas emissions [107].

5.5. Integrated Sensor Networks and Control

The most effective approach uses an integrated sensor network, including nitrate, nitrite, ammonium, pH, oxidation–reduction potential (ORP), linked to a digital management system that activates treatment units (photocatalyst, biological reactor, and water exchange) when concentrations exceed set thresholds. Such a sensor–actuator framework enables adaptive control, reduces energy/reactant consumption, prevents over-treatment, and maintains optimal water quality for fish [108]. Real-time monitoring also supports data logging and analytics to support continuous improvement of RAS operations (Figure 7).

Figure 7. Schematic illustration of proposed integrated nitrogen-sensing and removal in a recirculating aquaculture system (RAS).

6. Challenges and Future Directions

6.1. Selectivity and Product Control

Achieving high selectivity toward N₂ (>90%) in realistic water matrices remains a major challenge. Competition pathways that yield ammonium (NH₄⁺) or nitrous oxide (N₂O) continue to limit the effectiveness of photocatalytic nitrate reduction [26]. The presence of matrix ions and organic matter often reduces performance, highlighting the need for future catalyst design that resists fouling, promotes N–N coupling, and operates reliably in turbid, saline, or biofouling-prone waters. Long-term demonstrations of stable >90% N₂ selectivity under real-world conditions are still limited.

6.2. Standardization of Metrics and Reporting

A major barrier to progress is the inconsistent reporting of catalyst performance: many studies omit critical parameters such as photon flux, spectral distribution, catalyst loading, reactor geometry, or nitrogen mass balance [26]. Without standardized protocols, including ¹⁵N tracing and gas-phase N₂/N₂O measurement, meaningful comparison and scale-up remain difficult [109,110]. Future research should adopt common benchmarking frameworks, incorporating apparent quantum efficiency (AQE), full nitrogen budgets, and testing in realistic water matrices.

6.3. Engineering and System Integration

Transitioning from batch laboratory tests to continuous, flow-through RAS-compatible modules is challenging. Key engineering challenges include ensuring uniform irradiation in immobilized systems, minimizing hydraulic dead zones, managing catalyst cleaning and regeneration, balancing energy input from LEDs or sunlight, and fitting treatment modules within RAS layout constraints [111]. Techno-economic analyses comparing photocatalytic treatment with water exchange, biological denitrification, and membrane technologies are needed to define viable applications such as zero-discharge RAS, high-value species production, or farms with limited space [112]. A pilot demonstration of a combined photocatalytic–ozonation system in a small RAS aquarium [113] illustrates a proof-of-concept for integrating advanced oxidation processes into recirculating systems.

6.4. Hybrid Systems and Adaptive Management

Instead of a “one-size-fits-all” photocatalyst approach, current evidence favors hybrid treatment trains: a photocatalytic polishing unit upstream of a biological denitrifier or membrane system, activated only when nitrate exceeds a set threshold and controlled by inline sensors [114]. This design reduces carbon dosing, water exchange, and energy consumption while leveraging established microbial processes. Sensor-driven control loops that operate only when needed optimize efficiency and maintenance requirements [115]. Designing materials that support both microbial and photocatalytic activity representsa promising direction for future development.

6.5. Sensor and Automation Integration

The full potential of photocatalytic nitrate removal can be realized only when the technology is integrated with advanced sensing and automation. Real-time monitoring of key water-quality parameters enables adaptive control of photocatalytic units, including on-demand activation, early detection of catalyst fouling or sensor drift, and continuous optimization of energy use and supplementary reagents. Recent advances in electrochemical and optical sensors for nitrate monitoring [116], Raman-based rapid nitrate detection [117], and integrated multi-analyte platforms capable of simultaneously measuring nitrate, ammonium, pH, and temperature [57] demonstrate the feasibility of continuous, high-resolution monitoring in aquaculture systems. Furthermore, Internet of Things (IoT)- enabled sensor networks for long-term nitrate/nitrite tracking in wastewater [118] highlight the potential for scalable deployment in RAS. Expanding monitoring to include gas-phase N₂O and integrating these data with machine-learning predictive models enables proactive water-quality management, enhancing system stability and reducing contamination risks. TiO₂-based photocatalytic systems particularly underscore the benefits of coupling treatment processes with real-time sensing for optimal performance [119].

6.6. Environmental and Regulatory Considerations

As photocatalytic systems progress from laboratory studies to practical deployment, it is essential to evaluate potential unintended environmental impacts. Key issues include possible N₂O emissions during nitrogen conversion, catalyst leaching or nanoparticle release, and related ecotoxicity risks [120,121], which are the overall energy and material demands identified through life-cycle assessment [122,123]. Consequently, field trials must incorporate a comprehensive accounting of greenhouse gases and catalyst stability verification, as well as evaluation of regulatory compliance under existing aquaculture and wastewater treatment regulations. Effective scale-up will also require structured engagement with aquaculture regulators, industry partners, and standards organizations to establish acceptable performance metrics, maintenance and monitoring requirements, and guidelines for safe, long-term operation of photocatalytic treatment systems.

6.7. Pilot Testing and Real-Water Validation

A major gap in photocatalysis research is the lack of long-term pilot studies performed under realistic RAS operating conditions. Most studies do not account for practical factors such asorganic loading, biofilm development, hydraulic variability, and seasonal temperature changes, all of which strongly affect treatment performance and system stability [124]. Future research should therefore prioritize demonstration in operating RAS facilities, with continuous monitoring of nitrate-removal kinetics, catalyst durability and leaching, biofouling and maintenance requirements, fish-health, and techno-economic feasibility [125,126]. Furthermore, testingphotocatalytic reactors using real, naturally variable influent waters, rather than idealized laboratory matrices, is essential for quantifying true performance, revealingfailure modes, and establishing operational limits for scale-up [126,127].

7. Conclusions

Photocatalysis has emerged as an efficient and environmentally friendly approach for the remediation of natural waters, particularly for nitrate removal. Recent advances in catalyst engineering, including metal-doped semiconductors, carbon-based composites, and heterojunction systems, have significantly improved light utilization, charge separation, and nitrate reduction efficiency. These developments, together with progress in mechanistic understanding and sensing technologies, highlight the growing potential of photocatalytic processes for practical water treatment applications.

However, several limitations continue to hinder large-scale implementation. Catalyst deactivation, low selectivity toward desirable nitrogen products, and reduced activity in complex water matrices remain major challenges. Furthermore, scaling photocatalytic systems requires improved reactor designs, optimized light distribution, and better energy efficiency. Coupling photocatalysis with complementary technologies such as biological treatments, adsorption, or membranes may offer synergistic benefits.

Overall, photocatalysis represents a promising and rapidly evolving strategy for sustainable water remediation. Continued interdisciplinary research focused on catalyst stability, process integration, and real-water performance will be essential to enable the transition of photocatalytic nitrate reduction from laboratory studies to reliable, field-scale applications.