Application of Electrochemical Oxidation for Urea Removal: A Review

Abstract

The consistent quality control of ultrapure water (UPW) in semiconductor manufacturing depends on removing trace organonitrogen compounds such as urea. Due to its high solubility, chemical stability, and neutral polarity, urea is inadequately removed by conventional processes. Even at low concentrations, it elevates total organic carbon (TOC) and reduces electrical resistivity. The use of reclaimed water as a sustainable feed stream amplifies this challenge because its nitrogen content is variable and persistent. Conventional methods such as reverse osmosis, ultraviolet oxidation, and ion exchange remain limited in treating urea due to its uncharged, low-molecular-weight nature. This review examines the performance and limitations of these processes and explores electrochemical oxidation (EO) as an alternative. Advances in EO are analyzed with attention to degradation pathways, electrode design, reaction selectivity, and operational parameters. Integrated systems combining EO with membrane filtration, adsorption, or chemical oxidation are also reviewed. Although EO shows promise for selectively degrading urea, its application in UPW production is still in its early stages. Challenges such as low conductivity, byproduct formation, and energy efficiency must be addressed. The paper first discusses urea in reclaimed water and associated removal challenges, then examines both conventional and emerging treatment technologies. Subsequent sections delve into the mechanisms and optimization of EO, including electrode materials and operational parameters. The review concludes with a summary of main findings and a discussion of future research directions, aiming to provide a comprehensive foundation for validating EO as a viable technology for producing UPW from reclaimed water.

1. Introduction

1.1. Global Demand for Water Reuse and Ultra-Pure Water

Ultra-pure water (UPW) is an extremely high-purity form of water that contains almost no electrolytes other than hydrogen and hydroxide ions. It is primarily utilized in industries such as semiconductor manufacturing, photovoltaics, pharmaceuticals, and biotechnology. Theoretically, the ideal specific resistivity of UPW at 25 °C is estimated to be approximately 18.3 MΩ·cm, a value derived based on ionic mobility [1]. According to this criterion, conventional distilled water, reverse osmosis permeates, and high-purity water with specific resistivity less than 10 MΩ·cm are not classified as UPW [1]. The objective of UPW production is to eliminate all forms of contaminants, including pathogenic microorganisms, suspended solids, radioactive substances, and organic and inorganic pollutants [2,3]. Although the ultimate goal is to completely remove all substances except for water molecules, this level of purity is practically unattainable. Therefore, acceptable purity levels vary depending on the industry, and the semiconductor sector imposes the most stringent quality requirements for UPW [3,4]. As semiconductor technologies continue to advance, the requirements for UPW quality have become increasingly rigorous. This trend underscores the critical importance of precisely controlling the quality of effluent throughout the entire UPW production process. The semiconductor industry involves the precise integration of numerous microscopic transistors onto wafers, and UPW is required in large volumes at every stage of production. For instance, manufacturing a 30 cm wafer requires approximately 8300 L of water, and a single semiconductor fabrication facility may consume between 30 and 60 million liters of UPW per day [3]. Accordingly, the semiconductor industry is widely regarded as a water-intensive sector, and the increased demand for UPW driven by technological advancements inevitably results in a surge in raw water consumption. Securing sufficient volumes of raw water thus becomes a critical challenge that directly affects the sustainability of UPW-based industrial operations.

Meanwhile, the intensifying global water scarcity has drawn attention to the potential use of alternative water sources [5]. Various strategies, such as water reuse, rainwater, and seawater desalination, have been actively explored. Among these, reuse water is recognized as having high potential in terms of supply stability and treatment capacity [6,7]. In particular, utilizing advanced-treated municipal wastewater in UPW production processes has emerged as a practical and sustainable solution. This approach offers both environmental and economic benefits by reducing dependence on potable water and improving water reuse efficiency [8,9,10]. Recently, attempts have been made to enhance the removal efficiency of residual urea in UPW production through pH-swing-based chlorination processes. In that study, monochlorourea was initially formed under acidic conditions, followed by a transition to neutral and alkaline pH to induce sequential conversion to polychlorinated urea species and subsequent hydrolysis, thereby accelerating urea oxidation [11]. Sim [12] introduced a demonstration case established by Taiwan Semiconductor Manufacturing Company (TSMC) at the Southern Taiwan Science Park (STSP) (Figure 1). This facility has a capacity of 20,000 cubic meters per day and supplies treated water for semiconductor operations through a treatment train consisting of biological treatment, ultrafiltration (UF), and reverse osmosis (RO). The internal recycling rate of the system reportedly exceeds 85% [12]. This large-scale commercial implementation demonstrates the potential of reuse water to meet the demand for UPW and is recognized as a leading example of water circulation strategy in the semiconductor industry. However, among the components of the treated effluent, urea and other nitrogenous organic contaminants are known to affect critical water quality parameters such as electrical resistivity and total organic carbon (TOC) content. These constituents are still considered key targets for improvement in removal performance.

Previous studies have demonstrated that residual TOC levels in UPW systems are closely related to the presence of urea in the feedwater, identifying urea as one of the primary contributors to TOC [2]. Choi and Chung [2] conducted a year-long monitoring study at a full-scale UPW production facility and reported that urea concentrations in the raw water varied seasonally between 5 and 50 μg/L. This variability had a direct impact on the TOC concentration in the final effluent. Notably, a positive correlation of approximately 0.1 mg/L TOC per μg/L of urea was observed between the urea concentration in the feedwater and the TOC level at the point-of-use (POU), indicating that urea is not effectively removed throughout the UPW treatment process. A regression analysis indicated that TOC levels increased by approximately 0.1 mg/L for every 1 μg/L increase in urea concentration, with a high correlation (Pearson r = 0.92). According to the study, conventional unit operations, including two-stage RO, were only capable of removing approximately 50% of the influent urea. Furthermore, ultraviolet (UV) oxidation processes designed for TOC reduction achieved less than 10% removal of urea. These limitations are considered a major factor preventing the final effluent from meeting UPW TOC quality standards. In addition, reuse water contains a complex mixture of contaminants beyond urea, including ammonia, phosphate, microorganisms, and suspended solids. The quality characteristics of reuse water vary by region, and these variations are known to influence the overall removal efficiency of urea and other organonitrogen compounds in UPW production. Therefore, in industries that require ultra-high-water purity, such as semiconductor manufacturing [4], conventional physicochemical processes alone are insufficient to achieve effective TOC control and maintain high-purity water quality in UPW production systems [13]. An integrated approach incorporating advanced oxidation processes and supplementary electrochemical water treatment (EWT) technologies is necessary to efficiently remove urea and other nitrogenous organic contaminants [14,15,16].

1.2. Managing Urea in Water Reuse for UPW Applications

Urea is a nitrogenous organic compound that enters wastewater through various pathways, primarily from domestic, agricultural, and industrial activities [14]. Human urine is one of the major sources of urea, with adults typically excreting an average of 1.5 L of urine per day containing urea concentrations of approximately 22 to 23 g/L [2,17]. Due to these characteristics, domestic wastewater inherently contains high levels of urea. In rural areas, urea applied as fertilizer can infiltrate surface and groundwater and ultimately reach river systems [13]. In addition, industrial facilities that use urea as a raw material, such as pure urea manufacturing plants and urea formaldehyde resin production facilities, discharge wastewater with high urea concentrations. Without proper treatment, such effluents may directly impact the environment [18]. When urea enters aquatic environments from these diverse sources, it increases nitrogen concentrations in the water, leading to eutrophication. This stimulates algal blooms, and the toxic compounds produced by cyanobacteria, known as cyanotoxins, can pose serious threats to both human health and aquatic ecosystems [19,20]. Moreover, urea can react with chlorine during water treatment processes to form harmful DBPs such as trichloramine (NCl₃), raising safety concerns in drinking water production [21,22]. In UPW production systems that use reuse water as a feed source, even trace amounts of urea can result in TOC concentrations that exceed quality standards. Therefore, the need for effective urea removal is further emphasized [2].

According to a recent 20-year publication trend analysis using the ScienceDirect API (Figure 2), the number of studies retrieved with the keyword combination “urea, wastewater, and electrochemical oxidation” has increased sharply since 2018, reaching its peak in 2024. This trend reflects growing academic interest in the subject. Meanwhile, although the semiconductor industry is experiencing a surge in demand for UPW and a growing need for advanced water treatment technologies through treated water reuse, studies that directly link urea and UPW remain exceptionally rare. A keyword search on ScienceDirect using both “urea” and “UPW” revealed that, over the past decade, there were only a handful of publications on this topic. To examine the relationship between UPW production and the semiconductor industry, this study conducted a literature survey using the keyword combination “semiconductor and ultrapure water.” The search returned 289 articles and only 35 review papers, suggesting that while a considerable body of work exists in the form of industrial reports and application-focused journals, academically structured reviews or meta-analyses remain scarce. This highlights a significant disconnect between industrial needs and academic response. In particular, documented applications of electrochemical urea removal technologies in advanced water treatment systems for the semiconductor sector are rare, which underscores the novelty and contribution of the present review.

Table 1. Overview of Recent Studies on Urea Removal Technologies (2016–2024).

Study Topic and Application Area	Year	Description of Research Focus	Urea Removal Technologies Reviewed	Ref.
Urea decomposition using electrochemical approaches	2016	Analysis of oxidation mechanisms for urea degradation	Adsorption, thermal hydrolysis, enzymatic hydrolysis, biological treatment, and electrochemical oxidation	[21]
Uremic toxin removal in artificial kidney systems	2017	Medical application—urea removal from patients’ blood	Adsorption	[23]
Uremic toxin removal in artificial kidney systems	2020	Integration of enzymatic and electrochemical treatments	Adsorption, enzymatic hydrolysis, and electrochemical oxidation	[24]
Urea removal from municipal and reclaimed wastewater	2021	Environmental engineering focus on municipal reuse systems	Adsorption, thermal hydrolysis, biological treatment, and electrochemical oxidation	[25]
Removal of low-molecular-weight organics in UPW systems	2021	Industrial UPW purification and enhancement strategies	Adsorption, reverse osmosis, photocatalysis, photolysis, and electrochemical oxidation	[10]
Sequential electrocoagulation and chemical coagulation	2023	Enhanced urea removal from synthetic and domestic wastewater	Electrocoagulation (EC), chemical coagulation (CC), combined process (CC–EC)	[26]
Electrooxidation of urea for hydrogen and N2 recovery	2024	High N2 selectivity via asymmetric Ni–O–Ti active sites	Electrochemical oxidation	[21]
PAC-assisted SR-AOP for urea elimination in reclaimed water	2024	PMS activation using powdered activated carbon	Sulfate radical-based advanced oxidation process (SR-AOP)	[27]

summarizes representative studies on urea removal technologies published between 2016 and 2024, highlighting their application areas, target removal methods, and key technical features.

This review aims to provide a comprehensive overview of recent advancements and the practical applicability of electrochemical oxidation technologies for the effective removal of urea in water reuse systems. In advanced water treatment systems that produce UPW from raw water containing high concentrations of nitrogenous and organic compounds, electrochemical processes have attracted considerable attention due to their environmental compatibility and potential for resource recovery. In ultra-precision industries such as semiconductor manufacturing, the removal of trace-level contaminants is critical, and urea removal is regarded as a core technology to meet stringent water quality standards. However, practical applications of electrochemical water treatment technologies specifically designed for urea removal in UPW production using reuse water as a feed source remain highly limited. This review seeks to address this research gap by comparing the urea removal performance of various water treatment technologies and establishing the foundation for applying electrochemical oxidation, which has recently emerged as a promising approach, to UPW production systems (Figure 3). To this end, the review focuses on analyzing the reaction mechanisms, electrode materials, operational parameters, and both laboratory and field-scale applications of electrochemical technologies for urea treatment. It also proposes technical alternatives to overcome the limitations of conventional treatment processes. Furthermore, by discussing future directions in urea removal and the potential for integration into hybrid treatment systems, this work aims to contribute to the development of sustainable water resource management and environmentally friendly purification technologies.

2. Urea in Reuse Water and Its Removal Challenges

Urea is a non-toxic and relatively weak base, though it exhibits stronger basicity than most other amide compounds. It is a stable organic compound that is highly soluble in water and undergoes hydrolysis under acidic or basic conditions or in the presence of urease, showing reactivity under various environmental conditions [2,21]. Urea exists in different structural forms in gaseous and solid states. In the gaseous phase, it adopts a non-planar geometry due to pyramidalization at the nitrogen atoms. In contrast, the solid-state structure is characterized by a planar arrangement stabilized through eight hydrogen bonds formed among six neighboring molecules. In aqueous solution, urea exhibits structural flexibility, allowing rapid interconversion between these configurations [28]. These molecular characteristics influence the behavior of urea during water treatment processes and are important considerations when selecting appropriate removal strategies. A summary of urea’s physicochemical properties is presented in

Table 2. Categorized Summary of Physicochemical Properties of Urea (CH₄N₂O).

Category	Property	Value and Description
Basic Molecular Info.	Chemical formula	CH4N2O (Carbamide)
	Molecular weight	60.06 g/mol
	Density	1.32 g/cm3 (measured at 20 °C)
	Molecular dimensions	Approximately 0.56 × 0.63 × 0.30 nm (x, y, z)
Aqueous Behavior	pH of 10% aqueous solution	Around 7.2 (near-neutral)
	Acid dissociation constant (pK ₐ at 298 K)	0.1 (indicates weak acidity)
	Water–octanol partition coefficient	−2.11 (reflecting strong hydrophilicity)
Thermodynamic Properties	Standard enthalpy of combustion $(∆H_{℃.solid})$	631.4 kJ/mol
	Gibbs free energy of formation $(∆G_{℉})$	−38.5 kJ/mol
	Total energy (approximate)	−645.0 kJ/mol
Electronic Properties	Ionization energy (IE)	Ranges from 9.70 to 10.33 eV

[13,29,30].

Although urea removal technologies have been continuously developed based on various process principles and physicochemical properties, many of them remain at a pre-commercial stage. Some approaches have demonstrated significant potential under specific conditions, particularly in terms of contaminant removal or resource recovery (e.g., nitrogen and water). However, challenges such as high operational costs, system complexity, and scalability limitations have constrained their broader application. Urea can be removed directly through adsorption or membrane-based processes, or it can be transformed into harmless gases such as CO₂, H₂O, and N₂ via electrochemical oxidation or biological conversion. Alternatively, it may be converted into ionic species such as NH₄⁺ or nitrate (NO₃⁻), which can then be removed in subsequent treatment stages [14,31]. However, under practical reaction conditions, urea nitrogen may not be fully converted to N₂, but instead partially oxidized into intermediate species such as NO₃⁻, NO₂⁻, NH₃, and CN⁻. Despite this, quantitative assessments of the distribution and behavior of these byproducts remain limited in the current literature. Nevertheless, many of these processes are energy-intensive or rely on biological treatment processes that require continuous maintenance, resulting in technical and economic limitations during full-scale operation [13]. To address these challenges, recent efforts have focused on hybrid systems that combine electrochemical oxidation with membrane processes, UV-based advanced oxidation with biological treatments, or forward osmosis with membrane distillation (FO–MD). Accordingly, this section reviews the working principles and representative studies of conventional urea removal technologies and provides a comprehensive evaluation of emerging hybrid processes, including recent trends and the limitations associated with each approach.

2.1. Membrane-Based Separation

RO-based ultrapure water production processes are often combined with ion exchange (IX) or granular activated carbon (GAC) to achieve high-purity industrial water [1,32,33].

In current water treatment systems for UPW production, hybrid processes typically combine RO with IX or GAC treatment. Specifically, RO–IX and RO–GAC configurations have been widely applied in nuclear power plants for the reliable production of high-purity industrial water and have demonstrated both field applicability and operational stability in practice [34,35,36]. However, these physicochemical combinations still exhibit limited effectiveness in removing certain trace contaminants such as urea, which is characterized by its small molecular size and nonionic properties.

Accordingly, recent approaches have proposed the immobilization of urease enzymes onto conventional RO membranes. Choi [33] investigated an enzyme-assisted strategy to remove urea, which is typically difficult to eliminate in UPW systems. In their study, urease was immobilized onto the polyamide layer of a commercial RO membrane (BW30) via EDC/NHS chemical crosslinking, enabling the biological degradation of urea. As a result, the urease-coated membrane exhibited approximately 27.9 percent higher urea removal efficiency compared to the unmodified membrane, and efficiency increased up to 65.9 percent under a two-stage RO process. While these results demonstrate notable performance improvements, enzyme-assisted RO membranes involve additional operational complexity, potential enzyme replacement costs, and slightly higher energy demand due to the optimal pH and temperature requirements for enzyme activity. Therefore, large-scale UPW applications must carefully consider these cost–energy trade-offs alongside removal efficiency gains [33]. Urease, a biological catalyst that hydrolyzes urea into ammonia and carbon dioxide, improves removal efficiency through two mechanisms when attached to the membrane surface. First, it serves as a physical barrier that restricts urea permeation across the membrane. Second, it facilitates direct enzymatic decomposition of urea. In a related development, Abusultan [37] proposed a hybrid process that integrates ion exchange resins with bipolar membrane electrodialysis (BMED) for the effective removal and recovery of residual ionic species (e.g., calcium, magnesium, ammonium) present in RO permeate [37]. This configuration allows for the selective separation of specific ions using ion exchange membranes, while simultaneously enabling the generation of acid and base streams through the BMED system (Figure 4).

Aung [38] proposed an integrated process combining electrodialysis (ED) with an ammonia-selective recovery system (EAS) for the selective extraction of ammonia from anaerobic digestate. This hybrid system demonstrated effective enrichment and recovery of ammonium (NH₄⁺), achieving an experimental recovery rate exceeding 99% [38]. Wang [36] developed a pilot-scale system for UPW production tailored for semiconductor manufacturing and quantitatively analyzed the removal characteristics of low-molecular-weight organic compounds (LMWOCs) (Figure 5). The full treatment system consisted of ten sequential stages, achieving a 99.4% reduction in the initial dissolved organic carbon (DOC) concentration from 1.42 mg/L. Of the total removal, 85.3% was attributed to the pretreatment stages and 13.7% to the make-up stages. However, the polishing stage contributed only 0.4% to overall DOC removal, and recalcitrant compounds such as trichloromethane (TCM), dibromochloromethane (DBCM), and urea persisted even after RO and UV treatment, accounting for approximately 56.5% of the final DOC [36]. This study highlighted the limitations of current advanced treatment systems by quantitatively identifying the residual refractory organics, thereby underscoring the need for enhancement in UPW production processes. In particular, the results demonstrated that conventional RO and UV-based processes are insufficient for removing electrically neutral and polar compounds such as urea and certain trihalomethanes (THMs). Accordingly, future research should focus on optimizing treatment processes for enhanced selectivity, either through the integrated application of advanced oxidation processes (AOP), such as electrochemical oxidation (EO), or through the use of selective adsorbents.

2.2. Adsorption-Based Techniques

Adsorption is one of the most extensively studied water treatment technologies for urea removal, primarily due to its simple operational principles, low energy consumption, and regenerability [21]. The adsorption mechanism of urea is described as a multistep pathway involving film diffusion, intraparticle diffusion, and surface binding, with film diffusion typically being the rate-limiting step in most systems [39]. In general, adsorption is categorized into physical adsorption, which is based on weak, nonspecific van der Waals forces, and chemical adsorption, which involves the formation of strong covalent or coordination bonds with specific functional groups or metal ions [40]. Physical adsorption is characterized by low activation energy (<20 kJ/mol) and is reversible, whereas chemical adsorption requires significantly higher activation energy (40–400 kJ/mol), is irreversible, and highly specific [40]. The adsorption of urea is an exothermic process with a negative enthalpy change, thus being more favorable at lower temperatures [41]. In addition, when the surface of the adsorbent carries a positive charge under conditions below its point of zero charge (pH < pH_zpc), adsorption performance can be enhanced through electrostatic interactions with the positively charged amino groups of urea molecules present in alkaline environments [42]. The performance of adsorption-based urea removal technologies is largely governed by the surface structure of the material, the characteristics of functional groups, and operational parameters such as pH and temperature. For future application in the UPW industry, continued in-depth research is required to enhance regenerability and selectivity, optimize treatment processes, and ensure scalability.

2.2.1. Carbon-Based Adsorbents

Carbon-based adsorbents have been extensively studied as effective materials for urea removal in water treatment due to their high specific surface area, porous structures, and diverse surface functional groups. These materials are considered highly promising for application in precision water treatment systems such as UPW production, owing to their simple operation, low energy consumption, and regenerability (

Table 3. Urea adsorption capacities of different carbonaceous adsorbents [44,45,46,47].

Adsorbent Type	BET Surface Area (m2/g)	qmax (mg/g)	Adsorption Mechanism
Commercial GAC	950	47.2	Physisorption
Activated Carbon Fiber	1300	63.5	Mixed (physisorption + chemisorption)
HNO3-treated AC	1100	58.4	Chemisorption (surface –OH/–COOH enriched)
Coconut shell-derived AC	920	42.1	Physisorption
Spherical AC (SAC)	1050	49.8	Mixed; reusable over 5 cycles
Biochar (Bamboo)	450	35.7	Physisorption

). Recent studies have actively compared and analyzed the urea adsorption characteristics and mechanisms of various materials, including commercial GAC, activated carbon fibers (ACF), biochar, and spherical activated carbon (SAC) [43]. However, further investigation is required to fully validate the efficiency of adsorption-based processes.

GAC possesses a relatively broad pore structure and a specific surface area ranging from 500 to 1500 m²/g [47], exhibiting a moderate adsorption capacity with a reported qₘₐₓ of approximately 47.2 mg/g [46]. It has been reported to maintain relatively stable adsorption performance under neutral to alkaline conditions. However, since most of its micropores are smaller than the molecular size of urea, access to the internal pores may be limited, thereby reducing the effective utilization of its high surface area. The presence of oxygen-containing surface functional groups (e.g., –OH, –COOH) promotes hydrogen bonding and electrostatic interactions with urea, contributing to enhanced adsorption performance [13]. In comparison, ACF exhibits a significantly higher BET surface area exceeding 1300 m²/g and possesses a uniform microporous structure, resulting in superior adsorption capacity and faster kinetics. ACF operates via a dual adsorption mechanism involving both physisorption and chemisorption, and the presence or absence of surface functional groups modulates its selectivity toward urea. Notably, acid treatment with agents such as HNO₃ can increase surface acidity and functional group density, thereby enhancing chemisorption and improving overall adsorption efficiency [47]. Such modified activated carbons allow for precise control over not only physical pore structures but also surface charge and functional properties, enabling tailored application in advanced water treatment processes.

From the perspectives of sustainability and cost-effectiveness, biomass-derived biochar has received considerable attention [13]. Biochar produced from various precursors such as bamboo shoots, coconut shells, and palm shells exhibits diverse surface properties and pore structures depending on production conditions, including pyrolysis temperature and activation methods. Although it generally shows lower adsorption capacity than GAC (qₘₐₓ < 40 mg/g) [46], it offers advantages in terms of low production cost and reusability. However, some types of biochar suffer from low surface area and nonspecific interactions, which limit their selectivity for urea removal. In addition, the dominant adsorption mechanisms remain unclear, indicating that further studies are required before commercial application is feasible [48]. Meanwhile, spherical activated carbon (SAC) has drawn attention due to its high mechanical strength, abrasion resistance, and minimal performance loss during repeated use. In experimental studies, SAC maintained over 90% urea removal efficiency even after more than five adsorption–desorption cycles, demonstrating its suitability for long-term operation in high-durability precision water treatment systems [45].

2.2.2. Inorganic Adsorbents

In the category of inorganic adsorbents, mesoporous silica materials such as SBA-15 and MCM-41, as well as zeolites including 3A, 5A, and 13X, have been applied for urea removal. SBA-15, characterized by large pore diameters (2–50 nm) and tunable pore structures, facilitates the incorporation of functional groups. Silica surfaces functionalized with amine groups exhibit strong adsorption capacities through hydrogen bonding or dipole interactions with urea molecules [23,48,49,50]. Zeolites contain exchangeable metal cations (e.g., Na⁺, Ca²⁺) that balance the negative charge of the framework, and the type of cation influences the affinity toward urea [51,52,53]. Notably, zeolite 3A demonstrated a high removal efficiency of 67.8%, which is attributed to its small pore size, indicating the importance of molecular sieving effects in the adsorption of urea [54].

Recently, high-efficiency urea removal using precision porous materials, such as inorganic carbon-based adsorbents and metal–organic frameworks (MOFs), has attracted increasing attention. Kim [34] applied a peroxymonosulfate (PMS)-based sulfate radical advanced oxidation process (SR-AOP) for treating reuse water intended for UPW production. Compared to the low urea removal efficiencies (10–50%) observed in conventional RO and activated carbon (AC) systems, the combination of powdered activated carbon (PAC) and PMS achieved near-complete removal [34]. This enhanced performance was attributed to the oxygenated functional groups and defect structures on the PAC surface, which facilitated the generation of reactive radicals (SO₄•⁻, •OH, O₂•⁻), with the system operating stably over a wide pH range and without metal leaching. Similarly, Zhang [46] reported urea removal efficiencies of 100% and 90% using PMS and peroxydisulfate (PDS) systems catalyzed by inorganic GAC and biochar, respectively. Notably, the GAC–PMS system achieved complete removal even at a minimum dose of 0.2 g/L, demonstrating its applicability for UPW pretreatment [46]. For biochar, thermal treatment temperature had a significant impact on adsorption performance, with 900 °C-treated samples achieving approximately 70% removal. In another study, Esfahani [42] investigated urea adsorption using ultrasonically post-treated MIL-100(Fe) MOF and reported a maximum adsorption capacity of 3321 mg/g, with over 85% removal within 2 min and retention of over 90% efficiency after four regeneration cycles [42]. The MOF exhibited rapid and effective urea adsorption through physical interactions such as dipole–dipole forces, π–NH bonding, and pore filling, and its applicability in real water matrices was also evaluated. In addition, a variety of inorganic nanomaterials such as alumina, boron nitride (BN) nanotubes, metal-functionalized silicon nanowires, and MXene-based titanium carbides have been investigated as potential adsorbents for urea removal. Nevertheless, the majority of these studies have been limited to molecular simulations, and further experimental research is necessary to verify their practical applicability [55,56,57]. While MOF and PMS-based AOP systems show promising results, but since most studies have been conducted in synthetic water, their performance under actual UPW feedwater conditions has not yet been fully verified and requires further investigation.

2.2.3. Polymeric Adsorbents

Biopolymeric materials derived from biomass-based polysaccharides have attracted attention as environmentally friendly water treatment media due to their excellent biodegradability, biocompatibility, low cost, and renewability. Among them, chitosan functions as a hydrophilic polymer in aqueous solution and provides potential for interaction with urea owing to its molecular structure enriched with amino and hydroxyl groups [58]. However, urea adsorption onto chitosan alone relies primarily on hydrogen bonding, resulting in limited binding strength. Therefore, strategies to enhance coordination through the incorporation of transition metal ions such as Cu²⁺, Ni²⁺, and Zn²⁺ have been proposed [59]. These metal ions form coordination bonds with the oxygen atoms of urea that can donate electron pairs, exhibiting stronger interactions than hydrogen bonding [60]. Nevertheless, in aqueous environments, the amino groups of chitosan can become hydrated or stabilized by internal hydrogen bonds, thereby reducing the efficiency of metal coordination. To address this, crosslinking reactions using glutaraldehyde have been employed. The imine groups (–C=N–) formed during this process not only serve as coordination sites for metal ions but also provide structural stability [61]. In particular, glutaraldehyde-crosslinked chitosan complexed with Cu²⁺ has demonstrated a chemical adsorption energy of approximately 8 kJ/mol and urea removal capacities exceeding 200 mg/g [62]. However, chitosan-based adsorbents are reported to face technical limitations for large-scale applications due to their water solubility and the cost associated with the crosslinking process.

Meanwhile, polysaccharide polymers such as cellulose and starch can be chemically oxidized to introduce aldehyde (–CHO) and carboxyl (–COOH) groups, which react with the amino groups (–NH₂) of urea to form imine bonds [13]. Although the steric hindrance created after bonding may limit the formation of additional hydrogen bonds, auxiliary hydrogen bonding from oxygen atoms in the polymer backbone can still contribute to the overall adsorption strength [23]. In fact, oxidized cellulose and starch derivatives have shown urea adsorption capacities in the range of 60 to 100 mg/g, which exceed those of commercial activated carbon in several studies [63]. Accordingly, chitosan and oxidized polysaccharide-based polymeric adsorbents exhibit diverse urea removal performances and mechanisms depending on their functionalization strategies. These materials show significant potential for application in filtration systems for UPW pretreatment or in polishing stages for low-concentration reuse water. The functionalization methods, binding mechanisms, and adsorption capacity ranges for each type of polymer are summarized in

Table 4. Polymeric adsorbents and functionalization strategies for urea removal.

Polymeric Adsorbent	Water Matrix/Condition	Urea Removal Performance	Key Features and Significance	Ref.
Chitosan/Cu(II) Affinity Membrane	Simulated reclaimed water, 25 °C, pH 7	110 mg/g (30 min)	Selective urea binding via Cu2+ coordination; applicable as a pre-filtration membrane for UPW	[58]
Glutaraldehyde-crosslinked Chitosan (Cu2+ complex)	Model reclaimed water, 298 K	205 mg/g (kobs ↑ with crosslinking)	Cu2+ chelation and crosslinking improve both adsorption kinetics and capacity	[59]
Chitosan-coated Dialdehyde Cellulose (CDAC)	Reuse water + Urease-assisted	65 mg/g (20 min)	Imine bond formation via –CHO groups; hybrid enzymatic–adsorptive urea removal system	[64]
Oxidized Starch Nanoparticles (oxy-SNPs)	200 mg/L urea in reclaimed water, 25 °C	95% removal (4 h) qmax = 185 mg/g	Optimized –CHO and –COOH content; follows Langmuir isotherm model	[63]
Dialdehyde Cellulose–Chitosan Hybrid/Acrylic Acid Graft	Simulated semiconductor reuse water	150 mg/g (60 min)	Schiff base interaction + –COOH/–NH2 binding; >92% capacity retention after 5 reuse cycles	[65]

.

2.3. Advanced Oxidation Processes

UV-based AOPs remove organic contaminants either through direct photolysis or via the generation of reactive radicals such as hydroxyl radicals (•OH), chlorine radicals (Cl•), and sulfate radicals (SO₄•⁻), which are produced by the photodecomposition of oxidants including hydrogen peroxide (H₂O₂), chlorine (Cl₂), and persulfate (S₂O₈²⁻) under UV irradiation [66,67]. However, urea exhibits extremely low absorbance at 254 nm (λₘₐₓ = 245 nm, ε = 0.0172/M·cm), rendering direct UV photolysis ineffective for breaking its chemical bonds [22]. Indeed, UV-C treatment using medium- and low-pressure mercury lamps has been reported to achieve urea removal efficiencies of less than 10 percent, highlighting the limitations of UV-only processes [10,68]. Consequently, the addition of oxidants is deemed essential for effective treatment. Although the oxidation pathway of urea has not been fully elucidated, it is hypothesized that hydroxyl radical-mediated oxidation initiates with hydrogen abstraction from the amino group of urea, ultimately leading to its degradation into carbon dioxide (Equation (1)) [69].

Representative Oxidation Mechanism of Urea:

CO(NH₂)₂ + 16•OH → HCO₂⁻ + 2NO₃⁻ + H₂O + 2H⁺

(1)

UV/Cl₂ and UV/H₂O₂ processes generate Cl• and •OH radicals, respectively. In the UV/Cl₂ system, urea is initially converted into N-chlorourea, which is subsequently degraded [22]. However, this process may lead to the substantial formation of trichloramine (NCl₃) under low pH conditions, raising concerns regarding radical scavenging and secondary pollution [70].

H₂NCO(NH₂) + Cl₂ → NHClCO(NH₂) + HCl

(2)

NHClCO(NH₂) + hν → NH₄⁺ + NO₃⁻ + CO₂

(3)

In the case of the UV/H₂O₂ process, excessive H₂O₂ can scavenge •OH radicals, thereby reducing oxidation efficiency. Moreover, the molar absorption coefficient of H₂O₂ (ε ≈ 18.6/M·cm, 254 nm) is relatively low, which limits its photolysis efficiency [71]. In contrast, the UV/S₂O₈²⁻ process, which utilizes persulfate as an oxidant, has demonstrated urea removal efficiencies exceeding 97%. In contrast, the UV/S₂O₈²⁻ process, which utilizes persulfate as an oxidant, has demonstrated urea removal efficiencies exceeding 97%. This is attributed to the higher reactivity and selectivity of SO₄•⁻ toward compounds containing amide groups (–CONH₂), compared to •OH [15]. In particular, vacuum ultraviolet (VUV) at 185 nm can induce photo-dissociation of water molecules, generating substantial amounts of •OH and SO₄•⁻ radicals, thereby achieving higher treatment efficiencies than UV-C at 254 nm (Equations (4) and (5)) [2,72].

$${\mathrm{S}}_2{\mathrm{O}}_8^{2-}+hv \to 2\mathrm{S}{\mathrm{O}}_4^{•-}$$

(4)

$$\mathrm{S}{\mathrm{O}}_4^{-}+H_{2}O\to H_4^{+}\mathrm{S}{\mathrm{O}}_4^{2-}+•OH$$

(5)

Choi and Chung’s research team at the Samsung Engineering R&D Center in South Korea evaluated the applicability of AOP to overcome the limitations in removing refractory organic nitrogen compounds such as urea in UPW production systems [2]. Although existing UPW systems include various treatment steps such as RO, it has been reported that seasonal increases in urea concentration in the feed water lead to elevated TOC levels in the final product water, thereby compromising water quality stability. The study experimentally demonstrated that the UV/persulfate process, which combines 185 nm UV irradiation with sodium persulfate (Na₂S₂O₈), effectively decomposes urea. While UV irradiation alone achieved less than 10% removal, co-injection of 20 mmol/L persulfate increased the removal efficiency to 90%. This was attributed to the oxidation of the urea amino group (–NH₂) to NO₃⁻ by sulfate radicals (SO₄•⁻) generated from the UV-activated persulfate (Figure 6). Long [73] compared various UV-based urea oxidation processes and reported the following order of removal efficiency: VUV/K₂S₂O₈ > UV/K₂S₂O₈ > VUV > VUV/H₂O₂ > UV/H₂O₂ > UV ≈ UV/Na₂SO₃. These findings indicate that combining VUV (185 nm) with persulfate (S₂O₈²⁻) yields the most effective photooxidation performance for urea. This is attributed to the higher selectivity and oxidation potential of SO₄•⁻ toward amide-functional compounds (–CONH₂) compared to •OH. UV-based AOPs for urea removal have recently advanced rapidly, and

Table 5. Application and demonstration of AOP for urea removal in reuse and wastewater for ultra-pure water production.

Applied AOP and Key Conditions	Water Source	Urea Removal Performance	Remarks	Ref.
UV/Persulfate (PS), continuous column (25 °C, 254 nm)	Semiconductor UPW system, 2nd RO feed	>95% (20 µg/L, 30 min)	Reaction rate k ≈ 0.15/min; minimal sulfate/peroxide side reactions link	[46]
Persulfate + UV (UV/S2O82−), continuous column, >20 μmol/L persulfate, 254 nm	UPW production process, low TOC UV unit feed water	Urea removal efficiency increased from 9% to 90% with >20 μmol/L persulfate, 30 min	UV dose is a critical factor	[2]
VUV/UV/Cl2 (185+254 nm, acidic pH 5)	RO permeates (TOC < 50 µg/L)	100% (10 µg/L, 10 min)	Urea-N → N2 84%, NO3− < 1 µg/L, no BrO3− link	[74]
PAC-assisted PMS (SR-AOP, 0.2 g/L PAC, 2.0 g/L PMS)	Reclaimed water (2nd MF effluent)	100% (30 µg/L, 5 min)	Stable at pH 5–9, CO32−/PO43− suppressed, Cl− promoted link	[27]
O3/NaBr rapid bromination AOP	US Intel UPW system	<1 µg/L (initial 20 µg/L)	First industrial-scale application, TOC spike suppression link	[75]

summarizes the recent research results.

In addition, photocatalyst-based AOPs offer a reaction environment where simultaneous oxidation and reduction can occur, as photoexcitation of the catalyst generates electron–hole (e⁻/h⁺) pairs [76]. The physicochemical properties of the photocatalyst and the wavelength (energy) of the light source play a crucial role in determining the efficiency of the reaction. Figure 7 schematically illustrates the photocatalytic oxidation pathway of urea, in which the compound is converted into inorganic nitrogen species. The photocatalytic degradation of urea is primarily driven by •OH generated via the holes (h⁺), and this process can be effectively enhanced through surface modification of TiO₂. Metal doping (e.g., Pt, Pd, Au) and anionic complexation (e.g., F⁻) facilitate hole transfer and increase •OH production, thereby improving urea removal efficiency. In particular, the co-doping of Pd and F⁻ has been reported to maximize hole transfer via a synergistic effect [77,78]. Kim [27] proposed a UV-photocatalytic process using a TiO₂–ZnO nanocomposite for urea removal in UPW production. In their study, the TiO₂–ZnO photocatalyst was activated under 365 nm UV-A irradiation, leading to the oxidative decomposition of urea, with ammonium (NH₄⁺) and NO₃⁻ detected as the primary nitrogen-containing products. In particular, ZnO enhanced electron transfer characteristics, effectively suppressing the electron–hole recombination of the photocatalyst and maintaining high catalytic activity through synergistic interaction with TiO₂. This process achieved a maximum urea removal efficiency of 92.4% and demonstrated applicability in fixed-bed reactors, indicating its strong potential for industrial-scale implementation [27]. These studies collectively highlight the inherent limitations of UV-based urea removal due to its low light absorption efficiency. Therefore, further development of high-performance systems capable of complete mineralization of urea into N₂ and CO₂ is required. Such systems should integrate various energy sources, surface modification techniques, and optimized oxidant combinations to enhance treatment efficiency [13].

3. Electrochemical Urea Oxidation

Recently, the advancement of EWT technologies has emerged as a promising alternative for urea removal in UPW production and reuse water treatment [21,79,80,81]. According to existing literature, various electrochemical and chemical treatment methods have been actively developed for urea removal; however, their direct application to UPW manufacturing remains extremely limited to date. Urea is a representative organic nitrogen compound that is difficult to remove in conventional water treatment processes due to its high solubility and chemical stability. Recently, the advancement of EWT technologies has emerged as a promising alternative for urea removal in UPW production and reuse water treatment [21,79,80,81].

According to existing literature, various electrochemical and chemical treatment methods have been actively developed for urea removal; however, their direct application to UPW manufacturing remains extremely limited to date. Therefore, this section highlights electrochemical oxidation (EO) as the core technological focus of this review, evaluating its mechanistic pathways, electrode configurations, and performance constraints, particularly under the low-conductivity and high-purity requirements of UPW production systems. EO is recognized as a promising treatment technology capable of achieving both high energy efficiency and environmental sustainability. Accordingly, further in-depth research is required to explore the potential applicability of this approach to UPW systems.

3.1. Electrochemical Oxidation for Urea Decomposition

EO is a water treatment process in which pollutants are degraded either through direct electron transfer at the surface of the anode or through reactive oxidative species generated within the system. The overall efficiency of EO is strongly governed by several operational factors, including current density, electrode composition, and the mass transfer rate of the target contaminant [82]. For urea, EO involves two predominant degradation pathways: (1) direct electron transfer (DET) to the anode surface, and (2) indirect oxidation facilitated by reactive chlorine species (RCS), such as Cl₂, hypochlorous acid (HOCl), and hypochlorite (ClO⁻), which are generated through the anodic oxidation of chloride ions (Cl⁻) [31]. In most real-world water treatment systems, the indirect oxidation mechanism via RCS tends to be the dominant route. Chlorine produced at the anode is subsequently transformed in the aqueous phase into HOCl and ClO⁻, both of which serve as active oxidants for urea. In this process, urea is predominantly mineralized into environmentally harmless gases such as N₂ and CO₂ [21,83] (Figure 8).

The direct oxidation pathway can proceed via •OH radicals generated on high OER electrodes such as BDD and PbO₂, or through surface redox reactions occurring on metal oxide electrodes such as Ni(OH)₂/NiOOH (

Table 6. Direct vs. indirect anodic oxidation.

Pathway	Governing Step	Representative Andes	Key Reaction(s)	Typical By-Products	Ref.
Indirect (RCS-mediated)	Cl− → Cl2 → HOCl/ClO−	RuO2/Ti DSA, MMO, graphite	NH2CONH2 + 3 HOCl → N2 + CO2 + 3 Cl− + H2O	ClO3−, ClO4−, chloramine	[21]
Direct electron transfer	Direct electron exchange between pollutant and electrode surface	Active anode	H2NCONH2 + H2O → N2 + 3H2 + CO2 + 6e−	Minimal by-products (matrix dependent)	[21,83]
Direct physisorbed OH−	Water discharge on high-OER anode	BDD, PbO2, SnO2–Sb	OH- + NH2CONH2 → N2 + CO2 + H2O	NO3−, NO2−	[86]
Direct chemisorbed Mox + 1	Surface redox cycle (Ni(OH)2/NiOOH)	Ni–Pt foam, Co-doped steel	NiOOH + NH2CONH2 → Ni(OH)2 + N2 + CO2	NO3−, NH4+ (minor)	[16]

) [14,84,85]. This EO-based urea removal process offers advantages, including selective generation of N₂/CO₂ and minimal secondary pollution. However, under high Cl⁻ concentrations, there is a potential risk of forming harmful byproducts such as ClO₃⁻ and ClO₄⁻, which necessitates careful optimization of electrode materials and operational parameters. It has also been observed that nitrogen transformation pathways differ significantly with pH and applied potential, impacting both removal efficiency and nitrogen speciation [80].

3.2. Optimizing Electrodes and Processes for Urea Decomposition

Electrode materials and cell configuration are critical factors that directly influence the electrochemical oxidation pathway of urea, energy efficiency, and the formation of byproducts [13]. The dominant oxidation mechanism at the electrode surface varies depending on the electrode material, which in turn affects both the removal efficiency and reaction selectivity. For instance, graphite electrodes primarily promote indirect oxidation via RCS, such as HOCl and ClO⁻, which are formed through the oxidation of Cl⁻. The energy efficiency and scalability of the process are highly dependent on the cell structure and electrode arrangement. El Gheriany [14] proposed a cell design incorporating a horizontal dual-anode configuration with integrated mixing and cooling functions to improve current distribution and mixing efficiency while reducing energy consumption [14] (Figure 9).

RuO₂/Ti-based dimensionally stable anodes (DSAs) also generate RCS in the presence of Cl⁻ with high stability, and their oxidation performance improves with increasing Cl⁻ and urea concentrations and elevated temperatures [84]. Recent pilot studies have reported that modifying DSAs with antimony-doped tin oxide (Sb–SnO₂) layers can expand the anodic potential window and increase •OH generation, thereby reducing dependence on RCS and minimizing chlorate formation [10]. DSAs are regarded as practical electrodes due to their high durability and excellent stability in chlorine evolution. Mixed metal oxide (MMO) electrodes are capable of facilitating both indirect oxidation and direct oxidation via •OH radicals. A synergistic effect has been observed when MMO electrodes are combined with photoelectrocatalytic systems [80]. BDD electrodes, which predominantly operate through the direct oxidation pathway, generate •OH radicals at high oxidation potentials. The tendency for byproduct formation, such as NO₃⁻ and ClO₃⁻, differs depending on whether the system is configured as a divided or undivided cell [21,80,85,87]. Suzuki [86] reported a hybrid system combining BDD electrodes with mesoporous TiO₂ photocatalysts, which achieved nearly 99% removal efficiency of urea in synthetic urine containing approximately 20,000 mg/L of urea. The system employed Kr-Br excimer ultraviolet irradiation at a wavelength of 207 nm to initiate the reaction, and NH₄⁺ was identified as the main nitrogenous product, indicating that partial mineralization rather than complete photooxidation of urea was dominant [86]. Jermakka [88] developed an EO treatment system aimed at selectively oxidizing organic matter in human urine while preserving total ammonia nitrogen (TAN). To achieve this, they employed a dual-chamber electrochemical reactor based on BDD electrodes along with a reagent-free pH control system to treat both synthetic and real human urine under various pH conditions. The results showed that at pH ≤ 3, more than 90% of the organic matter was removed while 79% of TAN was preserved, whereas at pH 8.4, TAN was almost completely oxidized. Furthermore, they confirmed that when the Cl⁻ to TAN ratio exceeded 0.2, rapid oxidation dominated, whereas below this threshold, slow direct oxidation was the prevailing pathway [88]. In addition, DFT modeling of BDD surfaces under varying electrolyte compositions has revealed that lattice termination (hydrogen- vs. oxygen-terminated) can alter adsorption energies of urea and intermediates, thereby shifting the preferred oxidation route [80].

While BDD electrodes offer advantages in terms of corrosion resistance and longevity, their high manufacturing cost remains a notable limitation. In nickel-based electrodes (e.g., Ni, Ni–Pt), redox transitions between Ni(OH)₂ and NiOOH occur under alkaline conditions, with NiOOH serving as the primary active species responsible for urea oxidation [14,87]. Ni-based electrodes demonstrate high oxidation efficiency even in the absence of Cl⁻. In particular, Urbańczyk [21] reported that Ni–Pt sintered electrodes exhibited significantly superior performance compared to Ti/Pt or pure Ni electrodes. Recently, Zhan [16] demonstrated, through density functional theory (DFT) calculations and isotope-labeling experiments, that asymmetric Ni–O–Ti active sites could enhance N₂ selectivity up to 99% compared to conventional Ni–O–Ni structures. This improvement was attributed to the suppression of C–N bond cleavage and the dominance of intramolecular N–N coupling pathways.

Akkari [83] proposed a wastewater treatment approach using Ni-based catalysts (NiO/NiOOH) for EO of urea, where urea was converted into N₂ and carbonate (CO₃²⁻), while simultaneously producing hydrogen (H₂). The EO reaction predominantly occurs on NiOOH; however, the accumulation of byproducts such as cyanate (OCN⁻), nitrite (NO₂⁻), and ammonia (NH₃) can lead to catalyst deactivation. Importantly, this deactivation is primarily attributed to the slow regeneration rate of NiOOH, which limits the sustained availability of catalytically active Ni³⁺ species. Such regeneration delays, combined with the surface fouling caused by adsorbed intermediates, progressively reduce the number of active sites and hinder long-term operation. This study employed operando electrochemical impedance spectroscopy (EIS) and ultraviolet–visible Spectroelectrochemistry (UV–Vis SEC) to analyze the deactivation mechanisms, identifying the slow regeneration rate of NiOOH as the primary cause [83]. Yang and colleagues at Zhengzhou University conducted a comprehensive review of the urea oxidation reaction (UOR) using Ni-based catalysts. Their study compared various Ni oxides and hydroxides, metal- and nonmetal-doped catalysts, and different catalyst support materials. They also elucidated the mechanisms of both direct and indirect oxidation pathways through theoretical and experimental approaches. In particular, they confirmed that highly active Ni³⁺ ions in the NiOOH/Ni(OH)₂ system act as the central catalytic sites in the indirect oxidation pathway. These Ni-based catalysts possess dual functionality by enabling the electrochemical oxidation of urea for simultaneous hydrogen generation and urea-containing wastewater treatment. Accordingly, they are considered promising alternatives to noble-metal catalysts for practical applications [89].

From a kinetic perspective, most electrode systems follow pseudo-first-order kinetics. Several studies have demonstrated that multivariable optimization using response surface methodology (RSM) enables the design of optimal trade-offs between urea removal efficiency and energy consumption by adjusting parameters such as current density, pH, and Cl⁻ concentration [80,84]. Moreover, coupling EO with intermittent polarity reversal has been proposed as a strategy to mitigate electrode fouling and extend operational lifespan in continuous-flow applications [79]. Therefore, the selection of electrode materials and the design of cell architecture should be regarded as critical factors that influence not only the reaction efficiency but also byproduct minimization, system scalability, and the feasibility of integrating into reuse-based UPW production systems. A comparative summary of the oxidation mechanisms, reaction trends, major byproducts, and energy efficiency associated with major electrodes used for the electrochemical oxidation of urea is provided in

Table 7. Summary of urea removal characteristics by electrochemical oxidation using various anode materials.

Material	Characteristic	Reaction Mechanism	Optimal Conditions	Product	Refs.
Nickel/Nickel Oxide	High activity in alkaline media; robust; widely studied	Direct oxidation on anode via Ni(OH)2/NiOOH redox; catalyzes urea oxidation	Alkaline solution (e.g., 1 M KOH), 0.4–0.65 V vs. Hg/HgO	N2, CO2	[90,91,92,93]
Dimensional Stable Anode	Chemically stable, high Cl2 evolution efficiency	Indirect oxidation via active chlorine (Cl2, HOCl, ClO−) generated at anode	Dilute chloride (100–400 ppm), 0.6–2.5 A, 16–34.5 °C	N2, CO2, minor NO3−	[84]
Boron-Doped Diamond	Chemically inert, high O2 overpotential, generates hydroxyl radicals	Direct oxidation via •OH; also mediates RCS if Cl− present	Acidic–neutral pH, 3–4 V vs. SCE, divided/undivided cell	N2 (with Cl−), NO3−	[86,93,94]
Ti/Pt, Ti/Pt–Ir, IrO2, RuO2	Noble metal/oxide coatings, good for RCS formation	Indirect oxidation via Cl2/HOCl/ClO−; also some direct oxidation	Neutral–alkaline, moderate current densities	N2, CO2	[95,96]
Graphite	Traditional, inexpensive, high O2 overpotential	Indirect oxidation via Cl2/HOCl/ClO− (chlorine-mediated)	0.5–3% NaCl, pH 5–7.6, 1.77–5.31 mA/cm2	N2, CO2	[97,98,99]
Doped Mixed Metal Oxide	High durability, low O2 evolution, can be photo-assisted	Direct and indirect oxidation (•OH, RCS); enhanced by photoelectrocatalysis	7.46 mA/cm2, 1.11 g/L NaCl, pH 3.25, ~43 min	9–12 V, 3–4.5 cm electrode gap, NaCl/CaCl2	[80]
Aluminum, Titanium	Used in electrocoagulation, cost-effective	Removal via metal hydroxide floc formation, limited direct oxidation	9–12 V, 3–4.5 cm electrode gap, NaCl/CaCl2	Flocs, minor N2, CO2	[100]

.

3.3. Limitations of EO Application and Future Research Direction

To date, EO technologies have primarily demonstrated the effectiveness of urea removal under laboratory conditions using synthetic solutions or high-conductivity reclaimed water. However, research targeting EO application for the production of UPW remains extremely limited. In practice, several technical barriers must be overcome to implement EO in actual UPW systems. First, under the low-conductivity conditions characteristic of UPW (<1 µS/cm), decreased electrical conductivity leads to increased cell voltage and reduced energy efficiency, posing significant technical limitations [94]. This phenomenon intensifies the risk of forming oxidative byproducts (e.g., ClO₃⁻, ClO₄⁻) during EO reactions, which can negatively impact system stability and long-term sustainability [87]. Moreover, EO processes may generate additional oxidation byproducts such as cyanate, nitrite, and nitrate, and even trace concentrations of these species can adversely affect downstream polishing systems if the high purity standards required in UPW systems are not met. Therefore, understanding the long-term behavior, formation mechanisms, and residual levels of EO-derived byproducts under real-world conductivity environments and developing effective mitigation strategies remain critical research needs for applying EO in UPW production.

To date, most EO-related studies have been conducted under synthetic laboratory conditions or with reclaimed water of relatively high conductivity, and pilot-scale investigations under UPW-relevant conditions are virtually nonexistent. In real industrial settings, the presence of diverse organic and inorganic interfering substances can alter EO reaction efficiency and byproduct formation pathways. Furthermore, EO system performance is highly dependent on electrode configuration, material, current density, and electrolyte composition, making it difficult to assess commercial feasibility based solely on conceptual or lab-scale results. Accordingly, long-term, field-based pilot-scale studies are essential for evaluating the industrial applicability of EO processes. These studies should comprehensively assess treatment efficiency, reaction characteristics, and byproduct formation and control under a range of realistic operating conditions. Recently, however, Liu [101] conducted a pilot-scale study applying a BDD (boron-doped diamond) anode-based EO process to treat landfill leachate (LL) and nanofiltration concentrate leachate (NCLL). This study comprehensively evaluated the technical and economic feasibility of BDD-EO technology for treating complex wastewater containing high concentrations of refractory organic matter (DOM), ammonium nitrogen (NH₄⁺-N), and heavy metals. The authors reported removal efficiencies exceeding 70–90% for COD, TOC, and NH₄⁺-N, while the concentration of disinfection byproducts (DBPs) generated during treatment remained below 1 mg/g TOC. Additionally, a 30-day continuous operation trial demonstrated that the BDD-EO process maintained stable performance across various influent conditions [101]. These findings suggest that the BDD-EO process, owing to its high efficiency, low sludge production, and minimal chemical requirements, holds promise as an environmentally friendly advanced treatment technology. Moreover, its potential extension to the removal of trace-level refractory contaminants such as urea indicates a high level of applicability to semiconductor process water, reclaimed water, and high-precision wastewater treatment, thereby contributing to enhanced water reuse and recovery in industrial settings.

Another major limitation of the EO process is its high energy consumption. Previous studies have reported energy demands in the range of 0.4–1.2 kWh/g urea for EO-based urea removal [102], based primarily on high-concentration urea conditions. Under the low-concentration conditions required for UPW production, oxidation efficiency may be lower and energy demand relatively higher, thereby limiting the economic feasibility of the technology. Thus, evaluating the practical commercial viability of EO-based urea removal requires an energy efficiency analysis based on actual UPW operating conditions, as well as a comprehensive techno-economic (LCC) assessment that includes operating cost estimation, maintenance requirements, and electrode replacement cycles.

4. Conclusions and Future Perspectives

4.1. Future Work

Despite the promising role of EO in urea removal, several critical limitations remain. These include (1) the high •OH demand for complete degradation, (2) the need for advanced catalyst and electrode material development, (3) high energy consumption associated with EO, and (4) the necessity of additional steps to remove supporting electrolytes in electrocatalytic systems.

Notably, in electrocatalytic oxidation using active anodes, •OH typically reacts first with the electrode surface rather than directly with urea, requiring highly reactive anodes that still may not achieve complete urea mineralization. Furthermore, when reuse water is considered as a feedstock for UPW production, additional complexity arises. Although reuse water is economically favorable compared to municipal tap water, it requires advanced treatment barriers to meet UPW standards. Therefore, practical implementation should focus on overcoming these limitations.

4.1.1. Optimization Under Low-Conductivity Conditions Is Critical

UPW typically exhibits conductivity below 1 μS/cm; substantial electrolyte resistance leads to high cell voltage and low energy efficiency. To address this, strategies such as narrow interelectrode spacing (<1 mm), high-surface-area porous electrodes, and pulsed or bipolar power supply modes should be explored. These approaches can enhance mass transfer and reduce energy demand, thereby improving EO applicability in energy-sensitive industries like semiconductor manufacturing.

4.1.2. Hybrid Integration Strategies

EO can be combined with complementary processes such as biological treatment, membrane filtration, adsorption, and photocatalysis. In addition to standalone EO optimization, future research should explore its integration with other AOPs, particularly photoelectrocatalysis (PEC). PEC enhances EO performance by generating more reactive radicals (e.g., •OH, O₂•⁻) while reducing energy consumption through light-assisted activation. Its compatibility with low-conductivity water and potential use of solar energy make PEC a promising candidate for UPW applications. Such integrated systems have the potential to simultaneously reduce energy consumption, enhance selectivity, and improve overall process stability.

4.1.3. Pilot-Scale Validation

Pilot-scale validation under real reuse water conditions remains insufficient. Most studies rely on synthetic matrices and short-term tests, offering limited insights into long-term stability, electrode durability, and system maintenance. Field-scale demonstrations are critical to assess the feasibility of EO systems in continuous UPW production.

4.1.4. Techno-Economic and Environmental Assessment

Future developments should include life-cycle assessments and cost–benefit analyses to quantitatively compare EO with other technologies. These evaluations will clarify EO’s competitiveness and scalability for industrial applications such as semiconductor and display manufacturing, where reliability and cost-efficiency are essential.

4.2. Conclusions

This review systematically examines recent research trends and application potentials of EO as an advanced treatment technology for urea removal in reuse water systems. With the growing demand for technologies that can selectively remove trace contaminants such as urea and organic nitrogen compounds, which are difficult to eliminate using conventional processes, EO has gained increasing attention as a promising solution. In response to this technological need, this review provides a comprehensive analysis of EO-based urea removal strategies, focusing on their applicability and current progress in the context of reuse water treatment aimed at achieving UPW quality standards.

Conventional AOPs such as ozonation, UV/Fenton, and photocatalysis have shown high efficiency in the removal of general organic compounds. However, many studies have reported limited removal performance for organic nitrogen compounds such as urea. Although adsorption processes offer advantages including operational simplicity and low cost, they exhibit low affinity toward hydrophilic, nonpolar, and uncharged molecules like urea, resulting in limited adsorption capacity. It has also been reported that membrane-based processes are ineffective for urea removal, as urea, due to its low molecular weight and low polarity, readily permeates conventional microfiltration and reverse osmosis membranes. In contrast, EO involves both direct oxidation and indirect oxidation via RCS, and demonstrates relatively high selectivity and rapid reactivity for urea removal through electrochemical condition control. EO also offers practical advantages, as it allows for modular system design and maintains stable treatment performance under varying water quality conditions through precise electrochemical regulation. These characteristics suggest that EO can function effectively as a polishing step or as a specialized contaminant removal module in hybrid system configurations. However, most studies to date have been limited to laboratory-scale experiments using synthetic water, and long-term performance validation and system optimization under actual reuse water conditions remain at an early stage. Further research should address technical challenges related to the scale-up of EO systems, the refinement of process design, the development of strategies to minimize reaction byproducts, and the enhancement of energy efficiency, all of which are essential from a practical implementation perspective. In addition, pilot-scale validation, long-term performance evaluation under diverse water quality conditions, and assessments of integration with other advanced treatment processes will be necessary. If these efforts are carried out in parallel, EO is expected to play an increasingly important role as an advanced treatment technology for producing UPW from reuse water.