Current State of Research on the Three-Dimensional Particle Electrode System for Treating Organic Pollutants from Wastewater Streams: Particle Electrode, Degradation Mechanism, and Synergy Effects

Abstract

As the demand for effective wastewater treatment continues to rise, the application of three-dimensional (3D) electrochemical particle electrodes for the removal of organic compounds from industrial wastewater has emerged as a promising solution. This approach offers significant advantages, including high treatment efficiency, operational flexibility, high current efficiency, low energy consumption, and the ability to degrade non-biodegradable organic pollutants, ultimately mineralizing them. This review provides a comprehensive and systematic exploration of the research and development of particle electrodes for use in 3D electrochemical reactors in wastewater treatment. The pivotal role of particle electrodes in removing organic contaminants from wastewater was highlighted, with most materials used as particle electrodes characterized by a specific surface area and well-defined porous structure, both of which were thoroughly discussed. Through the synergistic mechanism of adsorption, the particle electrode aids in the breakdown of organic contaminants, demonstrating the 3D particle electrode’s effectiveness in facilitating multiple oxidation mechanisms for organic wastewater treatment. Furthermore, this review categorized various particle electrode types used in 3D electrochemical wastewater treatment based on their primary components or carriers and the presence or absence of catalysts. Finally, the current status and prospects for the development and enhancement of 3D electrode particles were presented. This review offers valuable insights into the application of the 3D electrode process for environmental water treatment.

1. Introduction

The global water environment is increasingly threatened by the escalating presence of pollutants [1]. According to the World Health Organization, water pollution has compromised the safety of drinking water, endangering the health and survival of millions of people worldwide [2,3]. Among these pollutants, persistent organic pollutants (POPs) pose a major challenge in water treatment due to their high chemical stability, strong bioaccumulation potential, and resistance to degradation. These pollutants, which include pesticides, antibiotics, pharmaceuticals, and personal care products (PPCPs), are particularly difficult to remove using conventional biological treatments or physical-chemical adsorption methods [3,4]. Consequently, they persist in the environment, often resulting in toxic ecological effects. More concerning is the fact that certain POPs, such as endocrine-disrupting chemicals, can accumulate in the food chain even at trace concentrations, thereby posing significant risks to human health. As a result, the development of efficient, sustainable, and advanced water treatment technologies has become an urgent priority in the field of environmental engineering [5].

Currently, mainstream wastewater treatment technologies are divided into two main categories: physical-chemical methods (such as coagulation, adsorption, and membrane separation) and biological methods [6,7]. However, these technologies encounter significant challenges in practical application. Physical-chemical methods are often hindered by low treatment efficiency, high operational costs, and the potential for secondary pollution (such as difficulties in regenerating adsorbents and membrane fouling) [8,9]. Biological methods, on the other hand, are limited by microorganisms’ inadequate tolerance to toxic pollutants and extended degradation cycles [6]. Particularly for complex and hydrophobic POPs, conventional processes typically achieve removal efficiencies of less than 30%. While advanced oxidation technologies, such as Fenton oxidation and ozone oxidation, can enhance degradation, their reliance on chemical reagents, harsh reaction conditions (such as pH control), and the formation of byproducts restrict their large-scale application [8,10,11,12,13].

Electrochemical technology, with its environmental compatibility, controllability, and the advantage of not requiring chemical reagents, has attracted considerable attention in water treatment over the past two decades. The core mechanism of electrochemical processes involves triggering direct oxidation of pollutants or indirect radical attacks (e.g., ·OH, ClO⁻) at the electrode interface, achieving efficient mineralization of persistent organic compounds. Compared with traditional methods, electrocatalytic systems offer modular design potential and high energy efficiency, making them particularly suitable for decentralized wastewater treatment applications [14,15]. However, traditional two-dimensional (2D) planar electrodes face limitations, including insufficient active sites, low mass transfer efficiency, and uneven current distribution, resulting in treatment inefficiencies and high energy consumption that fail to meet industrial demands [16]. For instance, the space-time yield (STR) of 2D electrode reactors is typically below 0.5 kg COD/(m³·h), and the current efficiency (CE) is often below 40%, severely limiting [16] the economic feasibility of the technology [17].

To overcome the inherent limitations of 2D electrodes, three-dimensional (3D) electrode technology has been developed by incorporating conductive particles or porous media to construct a three-dimensional reaction interface [18,19,20]. This significantly enhances the electrode’s specific surface area and mass transfer kinetics. Since Backhurst et al. [21] introduced the concept of 3D electrodes in 1969, their structural design has evolved from unordered particle beds to ordered, structured electrodes. Theoretical studies have shown that 3D electrode systems, through polarization effects, can create micro-electric fields on the surface of the particles, enabling the packed particles to function as “microelectrodes” in the reaction. This effectively expands the reaction area to the entire electrolytic cell volume. Experimental data confirm that 3D electrode systems increase the degradation efficiency of typical POPs (such as tetracycline and bisphenol A) by 2 to 3 times compared to 2D systems, achieving COD removal rates of over 50% while reducing energy consumption by 30% to 50% [22,23,24]. Moreover, by coupling electro-adsorption with electrocatalytic oxidation processes, 3D electrodes can simultaneously concentrate and degrade pollutants, further enhancing treatment efficiency [18,19,20]. However, the engineering application of 3D electrode technology still faces multiple challenges. The stability and lifespan of particle electrode materials remain inadequate, particularly in complex water chemistries where passivation or corrosion can occur [1]. The fluid dynamics inside the reactor are complex, and the coupling mechanisms between mass transfer and reaction are not yet fully understood. Additionally, uneven current density distribution in large-scale systems leads to local overpotentials and increased side reactions. Recent research has focused on addressing these challenges by modifying electrode materials (such as doping and functionalizing carbon-based composites), optimizing reactor configurations (such as synergistic designs of fluidized and packed beds), and intelligently controlling process parameters (such as pulsed electric fields and flow matching) [1,25,26].

This review systematically evaluated the potential and effectiveness of 3D particle systems in organic wastewater treatment. Over the past decade, significant advancements have been made in particle electrode innovations, with breakthroughs in the preparation of 3D structured electrodes that have optimized electrocatalytic processes (

Table 1. Progress and identification of significant contributions and innovation of 3D particle electrodes.

Particle Electrode	Identification Basis	Name	Electrode	Characteristic	Significant Advantage
Without a particle electrode	Simple electrolysis reaction	Traditional electrochemical	2D	Without the particle electrode, the electrochemical processes include electrodes, and under appropriate voltage conditions.	Lower removal efficiency and high energy consumption
With a particle electrode	Particle electrode polarizable	Unipolar electrodes	3D	The typical outcome of a chemical reaction in the cathode and anode chambers is the reduction of metal in the cathode chamber and the oxidation of organic materials in the anode chamber [27].	One of the most straightforward processes for the production of hydrogen is water electrolysis. The capacity of water electrolysis to generate hydrogen exclusively from renewable energy sources represents a significant advantage. It is essential to reduce the energy consumption, cost, and maintenance requirements of existing electrolytic systems while simultaneously enhancing their efficacy in removing pollutants to promote the adoption of water electrolysis [1]. The rise in hydroxyl radicals was attributable to electrochemical oxidation and electro-Fenton oxidation, which occurred concurrently in the three-dimensional electrochemical oxidation system [28].
		Polar electrodes		Electrostatic induction charges the particle electrode, thereby transforming it into a polar particle with positive and negative poles, respectively. Subsequently, reactions occur at the cathode and anode terminals [27,29].
With a particle electrode	Particle electrode filling state	Fixed bed electrodes		The fixed bed electrodes are one of the most straightforward processes for the production of hydrogen in water electrolysis. The capacity to generate hydrogen exclusively from renewable energy sources represents a significant advantage. It is imperative to reduce the energy consumption, cost, and maintenance of existing electrolytic processes simultaneously. Enhancing their efficacy in removing pollutants to promote the adoption of water electrolysis.
		Mobile bed electrodes		Characterize to assess the usage, expense, and upkeep needs of current electrolytic systems while also evaluating their ability to remove contaminants to encourage the use of water electrolysis.
With a particle electrode	Current and flow direction of fluid in the reactor	Circulation electrode		The flow of particle electrodes within the reactor enhances the efficiency with which the materials are transmitted.
		Flow-through electrode		The current and the fluid within the reactor both flow in the same direction.
With a particle electrode	Reactor shape	Round, ring, and electrodes, etc.		The direction of the reactor fluid flow is perpendicular to the current direction.
	Connection-mode	Single-stage electrode		Due to the ability to adjust cathode positions, circular and annular electrodes are often observed in the same reactor [29].
		Bipolar electrodes		The power source is connected in parallel to the anode and cathode. The two ends of the electrical supply are affixed to the poles.
With a particle electrode	Electromagnetic-type air pump, Peristaltic pump, Aeration micro-pipes…/3D-ER	Monopolar		The primary electrode is separated into an anode and cathode zone. The particles in the anode area extend the main electrode and increase the reaction area by acquiring the same charge distribution as the main electrode [27,29].	These techniques have the potential to reduce side reactions, enhance the reactor’s overall electrolytic efficiency, promote a uniform potential distribution, and increase time-space productivity. Also, results in stratification and the loss of the catalyst coating on the particle electrodes. An uneven distribution of the feed current and potential may facilitate improved oxidation [30].
		Bipolar		An electric field induces electrostatic induction, which results in the charging of the particle electrodes. At both ends of the charged particles, electrochemical oxidation and reduction reactions occur as the particles transform into independent three-dimensional electrodes on the surface and increase the electrochemical charged particle [31] reaction. These occur at the ends of the bipolar electrode even though there is no direct electrical connection between it and an external power supply [32].
	Membrane particle electrode	Monopolar		Monopolar membrane electrochemical anion exchange membrane and the cation exchange membrane both require either poor-performing cathode catalysts or costly catalysts (such as IrO2 at the anode) for acidic cation exchange membrane systems, as well as problems with bicarbonate formation and crossover in anion exchange membrane cells. This is because both membranes electrolyze CO2 and water [33].	The membrane electrochemical l enhances the performance of the (bi)carbonate electrolyzer. Membranes are appropriate for several uses and allow for the regulation of ion fluxes and concentrations in electrochemical cells. Management of the ions considerable obstacles to the durability, selectivity, and energy efficiency of electrolysis, the chemical engineering opportunities [34].
		Bipolar		Ion-conductive polymers, known as bipolar membranes, are composed of two fixedly charged layers that are bonded to one another, often with the inclusion of a catalyst layer between them. The cation-exchange layer is constituted by one ionomer layer with fixed negative charges. The anion exchange layer also exhibits fixed positive charges. In electrochemical systems, bipolar membranes can function in two distinct ways: forward bias and reverse bias [34,35].

). As a core component of 3D electrode technology, particle electrodes enhance electrocatalytic oxidation efficiency and pollutant removal. This study provides an overview of current research on particle electrodes, analyzing their mechanisms and roles in wastewater treatment. Additionally, it explored methods to optimize 3D electrode reactor performance and evaluated their applications in electrochemical and environmental governance. The review also synthesized current developments, assessed limitations, proposed improvements, and anticipates future research directions, offering guidance for the continued advancement of 3D electrode technology.

2. Three-Dimensional Electrochemical Process

2.1. Three-Dimensional Electrodes

2.1.1. 3D Carbon Aerogel

In recent years, three-dimensional carbon aerogels have attracted significant attention due to their excellent performance in electrochemical wastewater treatment. The exceptional properties of carbon aerogels, such as high specific surface area, multi-level pore structures composed of micropores and mesopores, excellent electrical conductivity, and appropriate electrochemical potential windows, make them an ideal choice as electrode materials [36]. For instance, carbon aerogels (Mo/Fe-KFA) synthesized by Wei et al. [37] have been widely used as electrode materials in electrochemical wastewater treatment, demonstrating considerable application potential. Their porous structure provides them with excellent stability and reactivity. To optimize the properties of carbon aerogels, various preparation methods have been proposed (Figure 1), including chemical vapor deposition, carbonization of porous structural materials, chemical crosslinking, and freeze-casting techniques [6,9]. These methodologies empower precise regulation of the pore structure and surface properties of carbon aerogels, consequently augmenting their electrical conductivity, thermal stability, and electrochemical activity. Most carbon aerogels are composed of interconnected nanoparticles that form a three-dimensional network, with the resulting porous structure typically controlled by the morphology of the particles, which govern the formation of macropores and mesopores, while the microporous features are closely related to the internal porosity of the particles [38]. Additionally, the materials combine the chemical inertness of carbon amd the high specific surface area of aerogels and have low thermal conductivity and electrical conductivity, but carbon aerogels have high electrical conductivity since carbon composition is present [39], which contributes to their broad application potential in various fields. In the field of three-dimensional electrode reactors, carbon aerogels have demonstrated considerable promise. The carbon particles used in this study had a specific surface area of approximately 57.65 m²·g⁻¹, further demonstrating the potential of three-dimensional carbon aerogels in pollutant adsorption applications. Furthermore, nitrogen-doped porous carbon (3D-NPC) catalysts have shown excellent catalytic performance in peroxymonosulfate (PMS)-mediated organic dye degradation and disinfection. Previous investigators demonstrated that the 3D-NPC catalyst could achieve up to 97.2% removal efficiency of Rhodamine B within five minutes, and this efficiency remained stable for up to five hours, highlighting its practical application value in wastewater treatment [40]. In conclusion, three-dimensional carbon aerogel materials, with their exceptional structural properties and broad application potential, have become important materials in electrochemical wastewater treatment and other environmental remediation fields.

2.1.2. 3D Activated Carbon-Based Electrodes

Activated carbon (AC) and granular activated carbon (GAC) are widely used as anode materials in three-dimensional electrodes. Both materials have large specific surface areas and nanopore sizes, which enable them to adsorb organic pollutants, though they differ in conductivity and application. AC, due to its excellent adsorption and catalytic properties, is extensively applied in advanced oxidation process (AOP) systems for water treatment [27]. AC not only adsorbs specific chemicals but also generates strong oxidizing species (e.g., •OH), thereby enhancing the removal efficiency of pollutants [41]. GAC, on the other hand, is frequently used for organic pollutant removal in water due to its higher adsorption capacity and lower conductivity. For example, in three-dimensional electrode reactors, GAC particles can form charged micro-particles under an external voltage [42], which aids in the degradation of organic pollutants [43]. Although both materials have strong adsorption capabilities, the regeneration process of GAC is slower, which limits its cyclic use. To improve regeneration efficiency, some researchers have developed pore-free graphite adsorbents with high conductivity to enhance electrode regeneration [44]. Furthermore, the conductivity and mass transfer properties of three-dimensional particulate electrodes, combined with the electro-oxidative and adsorption capabilities of activated carbon, significantly improve pollutant removal efficiency [45]. However, the formation of a thin steady-state boundary layer on the electrode surface restricts mass transfer, thereby affecting the degradation efficiency of pollutants [37,38]. Overall, both GAC and AC materials demonstrate advantages in electrode applications, though there are differences in regeneration efficiency and conductivity.

2.1.3. 3D Carbon Nanotube/Carbon Fiber Electrodes

Carbon nanotubes (CNTs) are unique one-dimensional (1D) structures and a form of carbon allotrope. Since their discovery in 1991, CNTs have attracted widespread interest in the field of nanostructured materials due to their remarkable properties and broad application potential [46]. CNTs exhibit excellent adsorption capacity, high electrical conductivity, porous network structure, mechanical stability, and outstanding chemical stability and electrocatalytic performance, making them widely used in electrochemical processes [1]. Consequently, CNTs have been used in carbon-based electrodes for pollutant removal and chemical degradation applications. In addition, activated carbon fibers (ACFs), a type of fiber-based carbon material, have significantly improved adsorption kinetics, high porosity, strong volumetric efficiency, and large storage capacity. Specifically, the average fiber diameter, typical surface area, and total pore volume of ACF particles are 12 μm, 1088 m²/g, and 0.517 m³/g, respectively, with a micropore volume of 0.400 cm³/g and a micropore surface area of 874 m²/g [1]. This microporous structure greatly benefits the adsorption and desorption of m-cresol.

Indeed, ACF materials have demonstrated superior performance in the degradation and mineralization of organic pollutants compared with GAC particle electrode systems, especially in terms of conversion rates and total organic carbon (TOC) removal [1]. However, ACF may limit electrocatalytic reactions of •OH, making direct oxidation of the ACF surface the primary pathway for organic pollutant degradation. Activation of certain functional groups on the ACF surface has been shown to improve its efficiency in pollutant electro-oxidation [47], with the use of three-dimensional particle electrode systems demonstrating excellent removal performance (see

Table 2. Three-dimensional particle electrode performance evaluation.

(a) Removal Performance of 2D and 3D Particle Electrode
Carriers	Main Electrodes	Reactor Type	Catalysts	Pollutants	Reaction Conditions	Removal Efficiency (%)	References
TMP	A: titanic C: stainless steel	2D 3D	Fe	BPA	I = 300 mA C0 = 10 mg L−1 pH = 9 t = 55 min	>98%	[48]
GAC	A: titanic C: stainless steel	2D 3D		Reactive Black B	U = 10 V C0 = 100 mg L−1 pH = 3 t = 60 min	61.46% 74.77%	[27]
GAC and PCP		2D 3D	Ceramist particle (PCP)	Heavy oil refinery wastewater	J = 30 mA cm−2 pH not adjusted C0 = 2973 mg L−1 T = 60 °C	30.8% 45.6%	[49]
AC	A: Ti/SnO2+Sb2O5 DSA C: stainless steel	2D 3D	Granular carbon aerogels	Reactive brilliant red X-3B	Ed = 20 V pH = 5.1 C0 = 800 mg L−1 Airflow rate = 0.4 L min−1	20% 95%	[50]
Modified kaolin	A: Ti/SnO2+Sb2O5 C: stainless steel mesh	2D 3D	Ti/Co/SnO2Sb2O5	Sodium dodecylbenzene sulfonate	J = 38.1 mA cm−1 C0 = 750 mg L−1 pH = 3 T = 20 °C	56% 86%	[51]
GAC	A: graphite C: stainless steel mesh	2D 3D	Ti/Co/SnO2Sb2O5	Paper mill wastewater	J = 167 mA cm−2 C0 = 1357 mg L−1 pH = 11 T = 20 °C	45% 86%	[51]
(b) Lists the Carriers and Catalysts Employed in Wastewater Treatment 3D Electrode Reactors.
Carriers	Catalysts	Pollutants	Main electrodes	Reaction conditions		Removal efficiency (%)	References
Al2O3	CuFe2O4	p-nitrophenol (PNP)	A: Ti/RuO2 C: stainless steel	J = 24 mA cm−2 C0 = 150 mg L−1 pH = 10 t = 30 min		90.69%	[52]
TMP	Pd	BPA	A: titanic C: stainless steel	I = 300 mA C0 = 10 mg L−1 pH = 9 t = 55 min		>98%	[48]
γ-Al2O3	Bi-Sn-Sb	Tetracycline		i = 0.1 A C0 = 100 mg L−1 pH = 5.9 t = 180 min		86.0%	[53]
granular activated carbon	Ti/PbO2	COD	A: Ti/PbO2 C: stainless steel	i = 7.8 mA t = 30 min		95%	[36]
Manganese Slag	Cu/Fe	Salicylic acid, Rhodamine B	A: titanium mesh C: carbon fiber	U = 10 V CE = 0.05 M pH = 3 C0 = 0.10 M		76.9%	[54]
MWCNTs	Pd	4-Chloropheno	A: Ti C: Ti	J = 4.0 mA cm−2 C0 = 0.2 mM		100%	[55]
Ni foam	Pd-Fe	Dimetridazole	A: Pt sheets C: Pt sheets	J = 31 mA cm−2 C0 = 50 mg L−1 pH = 3 AFR = 1.0 L min−1		96.5%	[56]
Steel Slag	Mn	Rhodamine B		U = 5 V C0 = 5 mg L−1 pH = 6 DPE = 15 g L−1 CE = 0.15 mol L−1		100%	[57]
Ceramic particle	Cu/Zn	Salicylic acid, Rhodamine B	A: lead alloy C: stainless steel	HRT = 150 min U = 15 V pH = 3 C0 = 0.75 g L−1 CB = 30 g L−1		83.45%	[58]
AC	Cu	Nitrate	A: Ru/Ir/Ti C: Cu/Ti	U = 5 V C0 = 50 mg L−1 HRT = 3 h		96.05%	[59]
EO, FT	B-doped Gr	Iopromide	A: B-doped Gr C: N-doped Gr 3D-B-doped	17 mA cm−2 0.01 M Na2HPO4/NaH2PO4 pH = 7.5 HRT = 3.45 min C0 = 2 μM		91.3% 84.0% 99.0% 88.0%	[60]
EO, FT	-	Diatrizoate, Triclosan, Diclofenac	A: Ti/SnO2-Sb/PbO2 C: stainless steel	22 mA cm−2 WF = 3500 L m−2h COD = 230 mg L−1		60%	[61]

b).

2.1.4. 3D Slag-Based Particle Electrodes

Solid industrial waste, including various types of slag, is a byproduct of the distillation and refining of metals and non-metals in metallurgical processing. Slag is a byproduct of the extraction from ores and typically consists of a glassy mixture of silica, metal oxides, and sulfides [62]. Among these, blast furnace (BF) slag does not contain valuable or toxic metals, making it unnecessary to extract or recover metals through hydrometallurgical treatment unless sourced from specific mines like the Tilley Iron Ore. These slags also contain chromium (Cr) and vanadium (V), which are reported to form hot metals from BF after reduction with coke. Conversely, Cr and V can be oxidized in the basic oxygen furnace (BOF), resulting in the enrichment of the BOF slag and the generation of vanadium slag (VS) byproducts [63]. These types of slags are effective in removing organic pollutants, particularly dyes, from water and wastewater [64]. Researchers have evaluated the adsorption of Red 183, Acid Yellow 99, and Acid Red 183 dyes and studied the adsorption performance of blast furnace slag (BFS) and Furnace Bottom Ash (FBA) using batch equilibrium methods, considering factors such as temperature, pH, ionic conductivity, and initial dye concentration. The results showed that BFS performed poorly in removing Acid Yellow 99 and Acid Red 183 from wastewater [64].

Wastewater treatment using industrial slags is crucial in recycling industrial waste and controlling pollution. Steel slag, manganese slag, and other types of slag have been widely used as raw materials in three-dimensional particulate electrode reactors [1]. Colloidal particles can be removed from aqueous solutions through electrocoagulation under the influence of electrical currents, thus disrupting suspended, emulsified, or dissolved pollutants. Iron slag contains a large amount of metal (and metal-like) oxides, which are used in various heterogeneous catalytic processes [28,65]. For example, Ref. [66] demonstrated the effective use of copper slag in catalyzing the oxidation of phenol in leachate by combining H₂O₂, H₂O₂/UV, and H₂O₂/SSL systems, showing that copper slag exhibits strong photocatalytic activity in Fenton’s process. Their studies indicated that copper slag outperforms steel slag in the Fenton process, achieving the highest phenol mineralization rate (87%) after 300 min using the copper slag/H₂O₂/SSL system [66]. Furthermore, Nengzi et al. [67] evaluated the physicochemical properties of steel slag through XRF, SEM, and XRD analysis, finding that steel slag primarily consists of rough aggregates of SiO₂ (quartz crystal phase), explaining the high adsorption capacity and good conductivity of this slag type. On the other hand, studies have shown that the three-dimensional electrolysis method exhibits significantly higher COD degradation efficiencies compared with two-dimensional electrolysis methods [67].

2.1.5. 3D Biochar-Based Particle Electrodes

According to Tomczyk et al. [68], biochar is characterized by a rich composition of trace elements such as iron (Fe), copper (Cu), zinc (Zn), sodium (Na), magnesium (Mg), and calcium (Ca), along with high biodegradability and organic carbon content. These properties confer upon biochar its remarkable efficacy in diverse environmental and energy-related applications, particularly within three-dimensional electrode systems. The distinct morphology and exceptional adsorption capabilities of biochar render it an optimal electrode material. Petala et al. [69] proposed a three-dimensional electrochemical method using biochar particles derived from pyrolyzed wood sawdust at 550–850 °C. This method was successfully applied to remove organic pollutants such as sulfamethoxazole (SMX), bisphenol A (BPA), propylparaben (PP), and piroxicam (PR) from water. The study demonstrated the significant role of biochar in electrochemical reactions, especially in water treatment and pollutant removal. Characterization of the biochar’s properties was conducted using nitrogen adsorption analysis based on the Brunauer–Emmett–Teller (BET) method, X-ray energy spectroscopy (X-ray), and scanning electron microscopy (SEM), revealing an uneven distribution of biochar structure. The X-ray diffraction (XRD) patterns of different biochar (BCS) samples showed three distinct peaks at 2θ = 20.72°, 21.97°, and 23.43°, corresponding to the crystal planes (210), (004), and (014), respectively [69].

The application value of biochar in three-dimensional electrode systems is attributable to three factors: its excellent conductivity, superior adsorption ability, and capacity to support electrochemical reactions. The results of the SEM analysis demonstrated that the biochar (BC) particles exhibited smooth surfaces, while the iron-manganese modified biochar (FMBC) particles were covered with iron-manganese oxides, which enhanced their catalytic performance [24,48]. Biochar has garnered significant attention in the field of wastewater treatment due to its high conductivity and favorable adsorption properties [70,71]. This material has demonstrated its effectiveness in addressing a wide range of wastewater types [49,50]. As a three-dimensional particle electrode material, biochar significantly enhances electron transfer efficiency and pollutant degradation capacity, thereby improving the overall performance of electrochemical reactions [72]. Notably, iron-manganese-modified biochar has shown remarkable effectiveness in degrading naphthalene (C₁₀H₈) in drinking water. Research indicates that Fe-Mn/biochar in an H₂O₂ system exhibited a naphthalene degradation rate 80.2 times higher than traditional biochar and 2.18 times higher than iron-manganese oxides [73]. This result not only validates the central role of biochar in three-dimensional electrode systems but also highlights its promising applications in environmental pollution control.

2.1.6. 3D Graphite Particle Electrodes

Three-dimensional (3D) graphite particle electrodes have significant applications in wastewater treatment and environmental remediation due to their excellent conductivity and tunable structural properties. Recent studies have demonstrated that graphite, when treated appropriately, can regain its structural integrity and exhibit noteworthy efficacy in the removal of deleterious compounds. The subsequent discussion will delve into a more thorough examination of the properties and potential applications of 3D graphite particle electrodes.

In comparison to spent graphite (SG), graphite subjected to treatment at 500 °C demonstrates significant structural recovery with a reduced prevalence of defects [74]. This characteristic is likely attributable to the effect of fluoride derived from binders, which helps prevent the formation of structural defects in graphite. However, as the temperature increases further, the fluoride may decompose, affecting the graphite’s performance [74]. Graphite subjected to treatment at 800 °C exhibits a comparatively substantial specific surface area (250 mAh/g), minimal electrical resistivity, and a well-developed porous structure, rendering it an effective material for the extraction of noxious compounds from waste [1,54]. These properties endow graphite with the capacity to play a substantial role in environmental purification processes. Moreover, Yi et al. [75] proposed a simple method for recycling graphite, which can be widely applied to used lithium-ion batteries (LIBs). This method involves melting the cathode current collector for four hours at 1673 K in a nitrogen-rich environment, followed by the separation of copper and graphite through ultrasonic vibration and screening techniques. The study revealed that the purity of the recovered copper and graphite was over 80% and 77.53%, respectively, with the recovered graphite purity exceeding 99.5%. Figure S1a shows that the graphite particle sizes range from 10 to 50 μm, while copper particle sizes range from 5 to 700 μm, with most particles falling between 220 and 700 μm. Figure S1b demonstrates the differences in the samples (e.g., water sample, recycled graphite) following ultrasonic vibration and screening. Standard graphite exhibits distinct peaks at 300, 600, and 800 G, as shown in Figure S1c–f. Figure S1g–i further indicates that positive and negative sieve sizes show that the recycled graphite powder passes through the sieve, while the zero-size sieve suggests that the powder does not pass through the sieve [75]. Thus, the AC voltage was 117 V, and the initial concentration of phenol was 2000 ppm. The filler layer was composed of glass beads measuring 3 mm and graphite flakes ranging from 0.2 to 0.8 mm. The study reported a phenol removal rate of 97%, which was achieved using this method [6]. Subsequently, Hongtao pioneered the development of a fluidized-bed electrolytic cell, a pioneering innovation in wastewater treatment. This innovative cell utilized graphite particles as electrode materials, marking a significant advancement in the field. The wastewater exhibited a flow rate of 2.8 L/min, a current density of 10.7 A/L, a void ratio of 70% for the graphite particles within the fluidized bed, and an average current efficiency of 85% [6]. Furthermore, the technology based on a TiO₂/graphite (TiO₂/C) cathode was proposed for the degradation of polyvinyl chloride (PVC), a prevalent microplastic in water. Furthermore, an investigation was conducted into the effects of reaction temperature and initial PVC concentration [76]. In optimal conditions, the dechlorination efficiency of PVC reached 75% [77] after potentiostatic electrolysis at −0.7 V vs. Ag/AgCl for 6 h [57,58].

These findings collectively suggest that appropriately treated graphite particles not only possess high purity and favorable structural characteristics but also exhibit excellent electrochemical performance, making them highly promising for applications in wastewater treatment and environmental remediation.

2.1.7. 3D Metal Electrode

Metal materials are the most commonly used in particle electrodes, playing a crucial role in 3D electrochemical wastewater treatment systems. These materials not only possess excellent electrical conductivity but also have catalytic active sites that facilitate the oxidation-reduction reactions of pollutants. Furthermore, metal materials have been demonstrated to facilitate the generation of metal ions, which are critical for the execution of electro-Fenton reactions and analogous processes. The most frequently utilized metal particle electrode materials encompass metal particles, metal oxides, and metal foams [1]. This section further explores the application and performance of metal oxides and metal foams as particle electrodes.

3D Metal Oxide Particle Electrodes

Surface-protonated metal oxides exhibit unique physicochemical properties, which make them highly effective for various applications [59,60]. For instance, oxide-based solid-state superionic conductors (SICs) are characterized by reduced ionic conductivity but improved air and electrochemical stability [32,61]. According to Kim et al. [78], the combination of solid electrolytes with oxide cathode materials and lithium metal anodes exhibits considerable promise, as evidenced by a thorough examination of their chemical, electrochemical, and mechanical characteristics. Solid-state sodium batteries are an exemplary subject for this discussion, as they utilize a solid electrolyte that is considered to be among the most effective. The electrolyte in question is Na-b-Al₂O₃, which possesses high ionic conductivity, with values ranging from 0.2 to 0.4 S cm⁻¹ at 300 °C [43,44]. However, its applicability is limited by its extremely high sintering temperature (1200–1500 °C). Rojo’s group evaluated the development, applications, and challenges associated with Na-b-Al₂O₃ in high-temperature sodium batteries [65]. Additionally, late-transition metal oxides are commonly found in layered hydroxide phases, which provide large surface areas for catalysts. These oxide systems exhibit varying structures, as late metal cations may have redox-active properties. Metal oxides such as TiO₂, ZnO, WO₃, Fe₂O₃, and BiVO₄ have been widely used in 3D particle electrode systems for wastewater treatment [79].

Chen et al. [80] synthesized a novel 3D flower-like Cu(II) oxide hybrid nickel-aluminum layered double hydroxide (CuO@LDH) microsphere via a hydrothermal method. This microsphere, with an increasing number of catalytic active sites and a specific surface area, was applied to the degradation and adsorption of organic dyes. Studies have shown that iron-coated nickel foam (NF-Fe) particle electrodes exhibit significantly enhanced electrocatalytic activity for the decomposition of H₂O₂ into •OH compared with bare nickel foam (NF) particle electrodes [81]. Additionally, NF-Fe particle electrodes demonstrate efficacy across a broad pH range, spanning from acidic to neutral conditions. This capacity to function over a more extensive pH spectrum enhances the electro-Fenton system’s potential for practical applications, as it operates under conditions that are more conducive to its effectiveness. The combination of bare nickel foam and NF-Fe has been demonstrated to enhance wastewater treatment efficiency. Mohammadi et al. [82] found that when the dosages of Ni foam and NF-Fe were 14 g/L and 6 g L⁻¹, respectively, the removal rate was higher than that achieved when only NF-Fe was used as the particle electrode [82]. The development of 3D metal oxide particle electrodes at high sintering temperatures (1200–1500 °C) improves their surface protonation, surface area, electrical conductivity (0.2–0.4 S/cm at 300 °C), electrochemical stability, and water separation performance, facilitating their wide application across multiple environmental fields. On the other hand, 3D metal foams offer significant advantages as electrode substrates and for ion transport during water electrolysis due to their large active surface area. For example, 3D graphite particle electrodes show an excellent typical highest capacity of 250 mAh/g, as well as a rich electron environment and high conductivity [74]. Additionally, a green and sustainable 3D particle electrode system consisting of nickel-iron layered double hydroxide and mixed activated carbon particles (NiFe-LDH/AC) has been successfully applied in wastewater treatment, demonstrating excellent degradation of organic pollutants [83].

3D Metal Foam-Based Particle Electrodes

Three-dimensional (3D) nickel foam, as an electrode material, shows promising prospects due to its large active surface area and high conductivity within a porous 3D network [81]. Yang et al. [84] pioneered the development of hierarchical copper phosphide nanoarrays (Cu3P), which were subsequently identified as highly effective electrocatalysts for the water separation process. This breakthrough was achieved through the implementation of a template-based synthesis method, which utilized 3D nickel foam (NF) as the substrate. Furthermore, Li et al. [18] proposed a three-step method to develop 3D CoNiCu-LDH@CuO heterostructures. This method creates a trimetallic CoNiCu-MOF with standard dispersion on copper foam and converts it into a 3D CoNiCu-LDH with a micro-flower structure via an in situ Cu(OH)₂ nanorod matrix [18]. Therefore, the electrocatalyst benefits from an advantageous substrate, a large active surface area, an electron-rich environment, and the synergistic effect between the active material and substrate. Due to their hierarchical 3D porous network, 3D nickel foam (NF) and copper foam (Cuf) provide a simple and rapid ion transport channel during water electrolysis [85]. These substrates, which possess substantial electro-active surface areas, render them optimal for the loading of catalysts. Consequently, they are frequently utilized in 3D porous foam-type nanostructures to effectively enhance electrocatalytic activity, thereby improving the kinetics of water separation reactions [85]. Moreover, NF particles have demonstrated good performance in generating strong oxidizing agents in a 3D electro-Fenton system (3DE-EF) [1]. Nickel foams (NFs), due to their high homogeneity, high porosity, and lightweight characteristics, have been widely applied in numerous electrochemical applications [86]. These foam particles are also used to construct particle electrodes with active groups on their surface and porous structures, with pore sizes ranging from 2 to 50 nm. These characteristics are beneficial for enhancing the adsorption and removal of organic pollutants [1]. Das et al. [85] proposed a rapid (60 s) synthesis method to improve the conductivity of substrate/electrocatalyst materials, which can be effectively applied in water electrolysis for sustainable fuel production. In this method, copper foam (Cuf) is electrodeposited onto NF to form a foam structure. Das et al. [85] proposed a novel synthesis method that can produce results in as little as 60 s. This method was developed to improve the conductivity of substrates and electrodes. In a five-year study, Yanshi et al. [87] employed a three-dimensional electro-Fenton system with iron foam serving as a particle electrode for the pretreatment of folic acid (FA) wastewater. The COD and TN removal percentages of FA wastewater reached 43.5% and 70.4%, respectively, during 6 h electrolysis [87]. Furthermore, ref. [88] investigated the design of a neutral three-dimensional electro-Fenton system with foam nickel as particle electrodes for the treatment of rhodamine B wastewater. As demonstrated in the study by Wei Liu et al. [88], the 3D-E-Fenton system exhibited a significantly higher rhodamine B removal efficiency (99%) in comparison to the counterpart three-dimensional electrochemical system (33%) and E-Fenton system (19%) at neutral pH within a 30 min time frame.

2.2. Degradation and Synergistic Effect of 3D Particle Electrodes

In water treatment processes, the choice and combination of catalysts have a significant impact on the removal efficiency of pollutants, especially in complex environments. Studies have shown that the combination of biochar, Fe-Mn binary oxides, and Fe-Mn/biochar exhibits excellent performance in removing naphthalene, with the synergistic effect of these components significantly enhancing the degradation efficiency of the pollutant. Specifically, three-dimensional (3D) electrochemical particle electrodes, by integrating the actions of adsorption filtration and electrochemical oxidation, not only reduce energy consumption during tetracycline (TC) degradation but also significantly enhance the degradation efficiency of TC.

This study investigated the effectiveness of biochar, Fe-Mn binary oxides, and Fe-Mn/biochar in the removal of naphthalene, with a particular focus on the response of H₂O₂ degradation behavior to different catalysts. For example, the adsorption equilibrium between naphthalene and the catalysts was achieved within 60 min under dark conditions. The removal efficiencies of naphthalene using biochar, Fe-Mn binary oxides, and Fe-Mn/biochar were 23.4%, 20.1%, and 48.9%, respectively [73]. These results suggest that Fe-Mn binary oxides and biochar may exhibit a synergistic effect, as evidenced by the significantly higher adsorption performance of Fe-Mn/biochar, indicating that the combination of these components enhances naphthalene removal efficiency [73].

Three-dimensional electrochemical particle electrodes combine the effects of adsorption filtration and electrochemical oxidation, which not only reduce the energy consumption associated with TC degradation but also enhance the degradation efficiency of TC. However, the catalytic reaction is influenced by several factors. For example, Equations (1) and (2) describe potential reactions near the anode, while Equations (3)–(7) describe the main reactions occurring near the cathode [89]. Fe₃O₄, a typical Fenton-like catalyst, can generate •OH from H₂O₂ via the electro-Fenton reaction (Equation (8)) [1]. Li et al. [73] reported low naphthalene removal efficiency under visible light irradiation, indicating that H₂O₂ is not an effective degradant. Catalyst aggregation can reduce the availability of reactants at the active sites of the catalyst, negatively impacting the degradation efficiency of Fe-Mn binary oxides.

The catalytic efficiency of Fe-Mn/biochar was significantly higher than that of Fe-Mn binary oxides, primarily due to the inclusion of biochar, which helps reduce material aggregation and enhances the catalytic effect. Moreover, Fe-Mn/biochar exhibited a stronger ability to catalyze the activation of H₂O₂ compared with biochar alone, thus improving naphthalene removal efficiency. This could be attributed to the increased concentration of persistent free radicals (PFRs) and the activation effects of Fe-Mn binary oxides on H₂O₂ (Equations (9) and (10)). As mentioned above, the combination of biochar and Fe-Mn binary oxides can effectively enhance the catalytic activity for naphthalene degradation [73].

Yuan et al. [90] found that the catalytic performance of as-synthesized SFNs significantly decreased after the removal of inorganic fractions from the nanomaterials. However, the catalytic performance improved in the presence of SiO₂ and Al₂O₃. The study also showed a strong correlation between high concentrations of inorganic components (mainly SiO₂ and Al₂O₃) and the catalytic performance of as-synthesized SFN (Figure S2). According to Yuan et al. [90], adding Al₂O₃ to the synthesized catalysts can enhance the redox cycle of Fe³⁺/Fe²⁺, promote the adsorption of sulfonic dyes, and improve H₂O₂ degradation. The naphthalene decomposition rate significantly increased with the addition of H₂O₂ under visible light conditions [73]. Li et al. [73] reported naphthalene degradation efficiencies of 75.8%, 46.9%, 44.9%, and 11.5% for Fe-Mn/biochar, Fe-Mn/binary oxides, biochar, and H₂O₂, respectively, after a reaction time of 148 min.

$${\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}^{+}+\cdot {\mathrm{OH}+\mathrm{e}}^{-}$$

(1)

$${2\mathrm{H}}_2\mathrm{O}\to {\mathrm{O}}_2{ + 4\mathrm{H}}^{+}{ + 4\mathrm{e}}^{-}$$

(2)

$${2\mathrm{H}}^{+}{ + 2\mathrm{e}}^{-}\to {\mathrm{H}}_2$$

(3)

$${2\mathrm{H}}_2{\mathrm{O}+2\mathrm{e}}^{-}\to {\mathrm{H}}_2{ + 2\mathrm{OH}}^{-}$$

(4)

$${\mathrm{O}}_2{ + 2\mathrm{H}}^{+}{ + \mathrm{e}}^{-}\to {\mathrm{H}}_2{\mathrm{O}}_2$$

(5)

$${\mathrm{H}}_2{\mathrm{O}}_2\to 2\cdot \mathrm{OH}$$

(6)

$${\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{H}}^{+}{+\mathrm{HO}}_2\cdot {+ \mathrm{e}}^{-}$$

(7)

$${\mathrm{H}}_2{\mathrm{O}}_2{ + \mathrm{HO}}_2\cdot \to {\mathrm{O}}_2{ + \mathrm{H}}_2 + \cdot \mathrm{OH}$$

(8)

$${2\mathrm{H}}_2{\mathrm{O}}_2{ + \mathrm{Fe}}_3{\mathrm{O}}_4\to \cdot \mathrm{OH} + \cdot {\mathrm{OOH} + \mathrm{H}}_2{\mathrm{O} + \mathrm{Fe}}_3{\mathrm{O}}_4$$

(9)

$${3\mathrm{Fe}}^{2+}{ + \mathrm{H}}_2{\mathrm{O}}_2\to {2\mathrm{Fe}}^{3+} + \cdot {\mathrm{OH} + \mathrm{OH}}^{-}$$

(10)

Tang et al. [91] evaluated the catalytic activity of MoCoB MG wire catalysts by examining their ability to degrade crystal violet (CV), a commonly used triaryl methane, dye in industrial applications. They modified the cobalt-to-copper stoichiometric ratio in CoxCuy-Layered Double Hydroxides (LDHs), which resulted in selectively aggressive catalytic behavior toward various antibiotics [91]. This led to the development of a range of electrocatalysts, including PbO₂, SnO₂, RuO₂, CeO₂, and boron-doped diamond (BDD), as well as intensive research into antibiotic degradation via electrocatalytic (EC) processes [92]. Additionally, by employing t-BaTiO₃ as a piezo-catalyst, it was found that SnO₂-Sb, acting as a dimensionally stable, anode-like catalyst, enhanced the photocatalytic degradation of metronidazole [75,93]. Under both acidic and alkaline conditions (pH 3–10), the CoxCuy-LDHs/PMS system removed 96.2% of lomefloxacin (LOM, 10 mg L⁻¹) within 30 min, demonstrating excellent stability and efficiency over 10 degradation cycles of lomefloxacin. In comparison, the CoxCuy-LDH/PMS system exhibited more aggressive behavior toward sulfamethoxazole (SMX), achieving over 95.2% removal of SMX (10 mg L⁻¹) within 30 min [94]. The primary degradation mechanism for the CoxCuy-LDH/PMS system is illustrated in Figure S2b. According to Mi et al. [95], the aforementioned intermediate products may undergo mineralization into CO₂, H₂O, and inorganic ions.

3. Mechanisms of 3D Electrochemical Processes

3.1. Three-Dimensional Electrodes

In three-dimensional electrochemical treatment systems, the primary mechanisms for the removal of organic compounds include adsorption, degradation, electro-oxidation, electroreduction, and electrocoagulation. Therefore, electrode materials with high potential characteristics can serve as optimal choices for treating wastewater containing organic substances [96].

3.1.1. Oxidation and Characteristics of Three-Dimensional Particle Anode Systems

Anodization is a prevalent electrochemical process that aims to enhance the layer structure on the surface of metals. This process was first applied in 1923 and subsequently in 1927 by Grower in sulfuric acid electrolytes [97]. Electrochemical anodic oxidation has become a promising wastewater treatment technology due to its strong degradation capability and good environmental compatibility [98]. Nevertheless, a critical evaluation of the performance of three-dimensional particle anodes in anodic oxidation processes is crucial. For example, Song et al. [43] examined anode plates under 2 V and 4 V voltage conditions using SEM-EDS, showing the appearance of abundant white substances in the anodic regions under these voltage conditions. Further EDS analysis revealed that these substances were mainly composed of sulfur and metallic components. Notably, the sulfur on the anode surface may originate from the coal-based electrode rather than the adsorption of SCN⁻ ions [43]. Studies have indicated that organic pollutant degradation through anodic oxidation is a practical and environmentally friendly process [61,62]. The catalytic activity, selectivity, and current efficiency of electrochemical oxidation are influenced by the anode material. Research on the removal of organic pollutants using different anode materials (including thin-film oxides such as PbO₂ and SnO₂) has shown that some anode materials may exhibit shorter lifespans or surface contamination, leading to a decline in catalytic activity [99]. For instance, platinum (Pt) is frequently utilized as an electrode material for the degradation of organic pollutants due to its excellent conductivity and chemical stability [100]. Thus, research developments in environmental electrochemistry and their potential to contribute to a cleaner environment are reviewed here for wastewater treatment applications. Most environmental pollutants can be successfully eliminated or converted to non-toxic materials by one or more processes, including electrochemical oxidation, electrochemical reduction, electrocoagulation, and electrocoagulation/flotation, electrodialysis, and electrochemical advanced oxidation processes. Specific examples of applications for pollutant removal and reclamation of wastewater are given for the different processes, along with research needs and improvements for the commercial application of these electrochemical processes.

Electrochemical technologies for wastewater treatment and resource reclamation [101], Feng [102] studied platinum, boron-doped diamond (BDD), and titanium-ruthenium-tin ternary oxides (Ti/RuO₂-SnO₂) are common electrode materials currently used in commercialization [83]. BDD anodes [103] can achieve over 30% organic pollutant removal efficiency within a four-hour reaction time. Moreover, previous studies have shown that under a four-hour reaction time, current density of 30 mA cm⁻², and NaCl concentration of 0.05 mol L⁻¹, BDD achieved a 52% TOC removal efficiency. In comparison, platinum and Ti/RuO₂-SnO₂ anodes had TOC removal efficiencies below 45% under the same conditions [59,79]. These studies suggest that as the reaction current density increases (20, 40, 60 mA cm⁻²), COD gradually decomposes, reaching removal efficiencies of 50%, 60%, and 70%, respectively. The poor COD removal efficiency after 420 min of reaction could be attributed to delayed oxidation reactions due to lower heterogeneity. Related studies have also suggested that BDD anodes lead to the adsorption of OH during water oxidation (Equation (11)) and the formation of chlorine-active substances (Equations (12)–(15)) [61,80].

$$\mathrm{M}+\mathrm{HO}\to {\mathrm{M}\left(\mathrm{OH}\right)+\mathrm{H}}^{+}{+\mathrm{e}}^{-}$$

(11)

$${2\mathrm{Cl}}^{-}\to {\mathrm{Cl}}_2{\left(\mathrm{aq}\right)+2\mathrm{e}}^{-}$$

(12)

$${\mathrm{Cl}}_2{\left(\mathrm{aq}\right)+\mathrm{H}}_2\mathrm{O}\to {\mathrm{HClO}\left(\mathrm{aq}\right)+\mathrm{H}}^{+}{\left(\mathrm{aq}\right)+\mathrm{Cl}}^{-}\left(\mathrm{aq}\right)$$

(13)

$$\mathrm{HClO}\left(\mathrm{aq}\right)\rightleftharpoons {\mathrm{ClO}}^{-}{\left(\mathrm{aq}\right)+\mathrm{H}}^{+}\left(\mathrm{aq}\right)$$

(14)

$${\mathrm{O}}_2{\left(\mathrm{g}\right)+2\mathrm{H}}^{+}{+2\mathrm{e}}^{-}\to {\mathrm{H}}_2{\mathrm{O}}_2$$

(15)

3.1.2. Characteristics and Reduction Process of Three-Dimensional Cathode Systems

Song et al. [104] analyzed coal-based cathode plates using SEM-EDS, and the results showed that under a 2 V voltage condition, cations such as Ca, K, Na, and Mg in the wastewater adsorbed onto the surface of the porous coal-based cathode. In contrast, metal cyanide network anions (such as Cu, Fe, and Zn) did not adsorb onto the cathode surface. Additionally, under a 4 V voltage condition, the cathode surface pores were covered by a dense layer of material. Spectral analysis revealed that the cathode surface contained certain amounts of Cu, Fe, and Zn elements, which could explain the electro-deposition of metal ions on the cathode surface [104]. However, direct reduction reactions and electro-generation of H₂O₂ can also convert organic wastewater at the cathode. Direct reduction reactions are primarily used in halogenation processes, including hydrogenation reactions with Pd, Ni, and Pt catalysts (Equations (16)–(22)) (where M represents the metal region, and (H)adsM represents chemisorbed hydrogen), as well as direct electron transfer reactions on non-precious metal catalysts (Equation (23)) (where RX represents carbon-halogen molecules) [2]. Therefore, the role of cathode materials in the electrochemical generation of H₂O₂ on the cathode is crucial. H₂O₂ can be generated through oxidation-reduction processes under acidic and alkaline conditions [74,76]. Heckert et al. [105] and Mi et al. [95] employed reduced graphene oxide-Ce/WO₃ nanocomposite-modified carbon felts (CF) to degrade ciprofloxacin using the electro-Fenton method, where H₂O₂ was generated in situ on the cathode by reducing O₂ (Equations (24) and (25)) [76,83]. To study the nitrate (NO₃⁻) reduction process in solid polymer electrolyte/Pt electrodes, various cathode materials were investigated. The metallic composition of the cathode significantly influenced the NO₃⁻/NO₂⁻ electrocatalytic reduction reactions in membrane electrode assembly (MEA) reactors, particularly in terms of [86,87] selectivity [101] and reactivity. Tang et al. [106] and Wu et al. [107] employed a cylindrical three-dimensional biofilm electrode reactor (3D-BERs) with a stainless steel mesh cathode to remove NO₃⁻ and sulfate (SO₄²⁻) from wastewater, achieving removal efficiencies of 29.35% and 88.49%, respectively [84,85]. According to the Box–Behnken design, the Direct Blue 80 (DB80) dye removal efficiency and chemical oxygen demand (COD) degradation under the optimal conditions of initial dye concentration of 35 mg/L, current of 0.09 A, and reaction time of 18 min reached 98.97% and 66.66%, respectively [108]. Electro-Fenton systems equipped with graphite felt (GF) cathodes (either modified or unmodified) have been employed for the decolorization and mineralization of AO7 (an azo dye) [109].

$${\mathrm{RX}+\mathrm{H}}^{+}{ + 2\mathrm{e}}^{-}\to {\mathrm{RH}+\mathrm{X}}^{-}$$

(16)

$${2\mathrm{H}}_2{\mathrm{O}+2\mathrm{e}}^{-}+\mathrm{M}\to {2\left(\mathrm{H}\right)\mathrm{adsM}+2\mathrm{OH}}^{-}$$

(17)

$$\mathrm{R}-\mathrm{X}+\mathrm{M}\rightleftharpoons (\mathrm{R}-\mathrm{X})\mathrm{adsM}$$

(18)

$$(\mathrm{R}-\mathrm{X})\mathrm{adsM}+2\left(\mathrm{H}\right)\mathrm{adsM}\to (\mathrm{R}-\mathrm{H})\mathrm{adsM}+\mathrm{HX}$$

(19)

$$(\mathrm{R}-\mathrm{H})\mathrm{adsM}\rightleftharpoons \mathrm{R}-\mathrm{H}+\mathrm{M}$$

(20)

$${\mathrm{NDMA}+\mathrm{e}}^{-}\to {[\mathrm{NDMA}]}^{-}$$

(21)

$${[\mathrm{NDMA}]}^{-}{ + \mathrm{H}}_3{\mathrm{O}}^{+}{ + 2\mathrm{H}}_2\mathrm{O}\to {\mathrm{DMA}+\mathrm{NO}+3\mathrm{H}}_2\mathrm{O}$$

(22)

$${\mathrm{O}}_2{\left(\mathrm{g}\right) + \mathrm{H}}_2{\mathrm{O}+2\mathrm{e}}^{-}\to {\mathrm{HO}}_2^{-}{ + \mathrm{HO}}^{-}$$

(23)

$${\mathrm{Ce}}^{3+}{ + \mathrm{H}}_2{\mathrm{O}}_2{ + \mathrm{H}}^{+}\to {\mathrm{Ce}}^{4+} + \cdot {\mathrm{OH} + \mathrm{H}}_2\mathrm{O}$$

(24)

$${\mathrm{Ce}}^{3+}{ + \mathrm{H}}_2{\mathrm{O}}_2{ + \mathrm{H}}^{+}\to {\mathrm{Ce}}^{4+} + \cdot \mathrm{OOH} + \cdot \mathrm{H}$$

(25)

3.2. Three-Dimensional Mechanism Removal

Three-dimensional (3D) electrode particles have been widely developed and applied in wastewater treatment, demonstrating significant pollutant removal efficiencies. The basic mechanisms and functions of 3D electrochemical reactions are illustrated in Figure 2. Previous studies have employed various methods to investigate the role of particle electrodes, considering factors such as electrode material, electrolytes, and wastewater types [1]. Typically, particle electrodes composed of α-Fe₂O₃/PAC, Zn-Fe-rich granular sludge carbon, and γ-Al₂O₃ have been commonly used in 3D electrochemical systems due to their large surface areas and porous structures. The adsorption mechanisms of these materials are believed to be critical in wastewater treatment using 3D electrochemical particle electrode systems. However, the decomposition of organic pollutants during the treatment process may enhance their adsorption onto the particle electrode surfaces. The electrocatalytic process involves essential phases, including reactant adsorption, electron transfer, and product desorption [110]. Previous studies have demonstrated that AC-based adsorption can enhance the oxidation of ammonium (NH₄⁺) in wastewater when using α-Fe₂O₃/PAC-based 3D particle electrode reactors [111]. Additionally, electrocatalytic oxidation experiments have been conducted to remove Bisphenol A from wastewater using N₂-doped graphene aerogel electrodes, showcasing the enhancement of Bisphenol A decomposition due to the strong adsorption capacity of particle electrodes [3,84]. Furthermore, studies have shown a positive correlation between pollutant adsorption and removal rates in particle electrode areas [1,91]. Under high electrolyte concentrations [112], a significant amount of ions in aqueous solutions are adsorbed onto both the main and particle electrodes [113]. According to Zhang et al. [99], the pollutant removal mechanism in 3D electrochemical processes is influenced by multiple factors, including the type of wastewater pollutants, electrode materials, and particle electrode types. However, the tertiary treatment processes in 3D electrode technology remain inadequately understood. Therefore, it is essential to understand the treatment mechanisms to ensure high treatment efficiency comprehensively. The main factors controlling the 3D electrode treatment process are the anode-cathode configuration, particle electrodes, and wastewater properties. Similar to 2D electrode systems, 3D electrode systems also utilize anodes and cathodes. Particle electrodes (e.g., AC) can generate strong oxidants (e.g., H₂O₂, Cl₂, and HClO) in situ due to the formation of polarized microelectrodes, thereby enhancing the removal of pollutants [14,92].

3.3. Three-Dimensional Electro-Sorption

Electro-sorption refers to the process of removing ionic pollutants from water by adsorbing counterions onto the surfaces of porous electrodes [110]. The electro-sorption process primarily consists of two phases: the electro-sorption phase and the desorption phase [88,114]. In the electro-sorption phase, ions are adsorbed onto oppositely charged electrodes, resulting in a desalinated flow. In the desorption phase, adsorbed ions are released, generating a solution with high ion concentrations [106]. According to Ma et al. [1], electro-sorption is a phenomenon where no dissolved chemical species adhere to an electrode under the influence of an electric field, facilitating the surface binding of the electrode. The electro-sorption process can occur after the polarization of the electrode is induced by an applied voltage [115], which determines the pollutant removal rates in treatment, reactors [116]. Electrodesorption represents an adsorption process where a particle electrode attracts both positive and negative charges under an applied electrical current, increasing the charge distribution of ions [117].

The surface area and complex pore structure of particle electrodes can significantly enhance the mass transfer efficiency of pollutants [118]. The electric field can lead to the cause the buildup of positive and negative charges on the surfaces of particle electrodes, thereby improving the efficiency of the electro-sorption/adsorption process [92,94]. A previous study demonstrated that the saturation adsorption capacity of UO₂²⁺ reached 113.80 mg g⁻¹ at 1.8 V, with a corresponding electro-sorption rate of 0.32 mg g⁻¹ min⁻¹ [31]. Song et al. [104] highlighted the accumulation of metal complexes with SCN⁻, Na⁺, K^+, Ca⁺, and Mg⁺ on the surface area of 3D electrode particles following the removal of cyanide at a voltage of 2 V. Earlier studies have also assessed the removal of organic pollutants from aqueous solutions using a 3D GAC-based electrochemical particle electrode, demonstrating the combined effects of oxidation and electro-sorption/adsorption on organic pollutants removal [97,119]. Another study emphasized the enhanced interaction between electrodes and chloride (Cl⁻) ions, leading to the formation of electric double layers, which subsequently improved the electro-sorption capacity of Cl⁻ on GAC when the anode potential increased from 0.3 V to 1.2 V [53]. It is noteworthy that the electro-sorption capacities for open circuit potential (OCP) and 0.3 V were relatively similar, as the OCP was closer to 0.3 V (0.28 V) [45].

3.4. 3D Electro-Oxidation/Electroreduction

In 3D electrode reaction systems, anodic degradation of pollutants can occur via two pathways: direct oxidation and indirect oxidation (mediated by hydroxyl radicals) [104,120]. To accurately represent the actual oxidation performance of these two pathways, the contribution of adsorption has been excluded in this section [121]. According to Li et al. [2], the octahedral positions of double-layer hydrogens (LDHs) can accommodate Co(II), Co(III), Fe(II), and Fe(III) due to their unique structural characteristics. These Fe and Co species can undergo reversible oxidation and reduction (electro-oxidation (E0) Co(II)/Fe(III) = 1.04 V). The anodic removal and degradation of organic compounds involve two main techniques: direct electrolysis and indirect electrolysis. In direct electrolysis, electrons interact with impurities deposited on the electrode surface, enabling effective pollutant oxidation without the need for additional chemicals [2]. The oxidation and adsorption of organic pollutants can further decompose their molecular chains through direct charge transfer from the electrode to the anode area, thereby decreasing their molecular weight [122]. Persistent organic pollutants are initially adsorbed onto the anode surface before undergoing direct charge transfer and indirect electrochemical oxidation (Figure S3). The degradation of carbon chains further reduces the molecular weight of organic pollutants [123]. Direct degradation of organic pollutants at the anode is characterized by two main features: excellent conductivity and chemical stability. Diffusion plays a crucial role in the direct oxidation of organic pollutants at the anode. Specifically, the adsorption of organic pollutants from electrolyte solutions to the anode surface is the primary factor negatively affecting oxidation rates [122]. According to Yang et al. [124], it is possible to increase the anode potential to a level significantly higher than that of hydroxyl radicals (EOH = 2.7 V vs. NHE), thus enhancing the direct transfer of electrons from pollutants to electrodes.

Secondary indirect oxidation is a technique for degrading pollutants by creating intermediate compounds with strong oxidizing capabilities through electrode reactions. The primary mechanisms of this process include the generation of strong oxidants on the electrode surface or at the solution interface [122]. Therefore, oxidants produced electrochemically at the anode via indirect electrolysis are used to oxidize chemical oxygen demand (COD) in solutions. Active chlorine is the most commonly used and conventionally generated oxidizing species in wastewater treatment (Equations (2) and (3)) [125]. Anions in water, such as SO₄²⁻ and Cl⁻, are used to indirectly oxidize organic pollutants. When these ions come into contact with the electrodes, they produce potent oxidizing agents, such as peroxydisulfate (S₂O₈²⁻) and active chlorine, which decompose organic pollutants. The second process involves the oxidation and removal of pollutants using a reversible cycle of high and low-valence metal ions (e.g., Fe³⁺/Fe²⁺, Ni³⁺/Ni²⁺, and Co³⁺/Co²⁺) [6].

Depending on the type of anode used, electro-oxidation (EO) can occur directly on organic compounds via electron transfer (Equation (12)) or indirectly through mediated/indirect oxidation involving electrogenerated active species at the anode, such as AC-based physical adsorption physiosorbed hydroxyl radicals M(OH) (Equation (3)) or active oxygen-based chemisorption (oxygen in the lattice of a metal oxide anode (M = O)) (Equation (14)) [106,126]. Ibuprofen (IBP) was effectively removed through both direct and indirect oxidation [14]. Radical scavenging experiments were conducted to identify the primary mechanisms behind IBP removal and clarify the functions of 3D particle electrode reactors. Compared with 2D electrochemical particle electrode reactors, IBP was oxidized more often and continuously both directly and indirectly, in 3D electrochemical particle electrode reactors, thereby increasing the overall IBP removal rate (Figure S4c). Additionally, both radical scavengers inhibited the elimination of IBP in the 3D electrochemical particle electrode reactors at the beginning of the treatment process (Figure S4a,b). Ethanol (EtOH) and Allyl•OH reduced the IBP removal efficiency by 85.90% and 32.2%, respectively [14]. According to Equation (26), organic pollutants can be immediately destroyed through oxidation on the charged particle electrode surfaces. Particle electrodes (e.g., AC) can also remove organic pollutants indirectly by catalyzing the H₂O₂-based decomposition process to produce OH (Equations (27)–(29)) [14].

$$\mathrm{Pollutant}\stackrel{\hspace{1em}\mathrm{Particle}\mathrm{electrode}\mathrm{surface}\hspace{1em}}{\to }{\mathrm{e}}^{-}+{\mathrm{H}}_2\mathrm{O}+\mathrm{oxidation}$$

(26)

$${\mathrm{O}}_2{ + \mathrm{H}}^{+}{ + 2\mathrm{e}}^{-}\to {\mathrm{H}}_2{\mathrm{O}}_2$$

(27)

$${\mathrm{H}}_2{\mathrm{O}}_2\stackrel{\mathrm{Particle}\mathrm{electrode}\mathrm{surface}}{\to }\cdot \mathrm{OH}$$

(28)

$$\mathrm{Pollutant} + \cdot \mathrm{OH}\to {\mathrm{H}}_2{\mathrm{O} + \mathrm{CO}}_2$$

(29)

3.5. Removal Mechanisms of 3D Electrochemical Particle Electrode Systems

The removal mechanisms of 3D electrochemical particle electrode systems in wastewater treatment primarily rely on the synergistic actions of various reaction pathways, including electrochemical oxidation, adsorption, and catalytic reactions. By optimizing reaction conditions and selecting appropriate catalysts, these systems can effectively degrade organic pollutants, particularly by generating reactive oxygen species (ROS) to facilitate pollutant removal. This section provides a detailed explanation of these mechanisms and their interactions.

According to Chen et al. [80], the reaction mechanism of surface-active sites reveals the occurrence of the heterogeneous Fenton reaction catalyzed by CuO@LDH-5. Specifically, Cu⁺ and Ni²⁺ act as catalysts, promoting the decomposition of H₂O₂ and the generation of •OH. Subsequently, Cu⁺ and Ni²⁺ participate in a reduction process with H₂O₂, replenishing the necessary components. Notably, the negligible shift in the computed relative content indicates a reciprocal electron transfer between Cu and Ni, enabling the ongoing redox cycling of Ni³⁺/Ni²⁺ [127], and Cu⁺/Cu²⁺ pairs, which ensures the continuous activation of H₂O₂, an important finding. Ultimately, methylene blue (MB) is degraded by reactive oxygen species (ROS) into smaller molecular products on 3D flower-like microstructures [128], as illustrated in Figure 3a. Additionally, the results of quenching experiments, probe experiments, and electron paramagnetic resonance (EPR) analyses demonstrated that hydroxyl radicals (•OH) and superoxide anions (¹O₂) are the primary reactive oxygen species (ROS) involved in the removal of carbamazepine (CBZ) by N-Mn@BC as particle electrodes. The non-radical oxidation pathway was identified as a crucial degradation mechanism, as shown in Figure 3b of the study by Piao et al. [30]. In summary, the generation of hydroxyl radicals is significantly enhanced when electrochemical oxidation, electro-Fenton oxidation, and the 3D electrochemical oxidation system operate concurrently. Figure 3c depicts the potential mechanism of RhB degradation in the 3D-ER system based on the preceding analysis [129].

Di et al. highlighted that this technique notably increases the electrochemical reaction area and mass transfer efficiency, allowing oxygen to diffuse from the air onto the cathode surface and generating a substantial amount of hydrogen peroxide [130]. The system’s oxidative capacity is enhanced due to the electro-Fenton-like reaction between Fe²⁺ and Cu⁺ in the Fe/Cu@AT particle electrode, along with the generation of •OH [131], which accelerates catalytic activity. Simultaneously, Cu⁺ and Fe²⁺ undergo oxidation, resulting in the formation of Cu²⁺ and Fe³⁺. In comparison to the Fe/Cu@AT 3D-ER system, the AT 3D-ER system exhibits reduced catalytic efficiency, as evidenced by the decreased •OH generation. Furthermore, the concentration of RhB at the micro-interface may increase due to adsorption [129]. Hydroxyl radicals continuously attack the RhB molecules adsorbed on the particle electrode, thereby accelerating the oxidative decomposition of RhB near the active sites of the particle electrode [113,132]. Some of the degraded pollutants undergo further oxidation following desorption and reabsorption. The available Fe³⁺ and Cu²⁺ active sites for heterogeneous reactions are sufficient, as evidenced by the simultaneous reduction in Fe³⁺ and Cu²⁺ to Fe²⁺ and Cu⁺ via electron transport on the microelectrode [133], a process verified in wastewater treatment.

In general, the wastewater treatment pathways of 3D electrochemical particle electrode systems include adsorption, indirect oxidation, direct oxidation, electro-oxidation, electrocatalytic degradation, electro-sorption, and electrocoagulation. The selection of particle electrodes depends on the types of pollutants present in the wastewater. These pathways result in excellent pollutant removal performance, making particle electrodes highly effective for wastewater treatment. Indeed, different electrochemical particle electrode technologies can be employed depending on the types of catalyst materials, wastewater, and electrolytes. The electro-sorption pathway in 3D particle electrode systems can effectively separate ionic organic pollutants from wastewater. Subsequently, ions can be regenerated in the desorption phase of organic pollutants and concentrated in the water stream. In addition to the electro-sorption pathway, oxidation can also be used to remove organic pollutants.

The main oxidation reactions in 3D particle electrode systems can be classified into direct and indirect oxidation. Direct oxidation involves the interaction of electrons with adsorbed organic compounds on the anode surface, leading to direct charge transfer from the electrode, which in turn reduces the molecular weight of the compounds and results in their degradation. On the other hand, indirect oxidation degrades organic pollutants by generating strong oxidative effects on the electrode surface. Acidic pH values can further enhance the phenol oxidation performance of 3D electrochemical particle electrode systems [134].

4. Applications of 3D Particle Electrode Systems in Environmental Management

4.1. Dye Wastewater Treatment

The treatment of organic dye wastewater is a key research focus in environmental science. Three-dimensional particle electrode systems significantly enhance catalytic degradation efficiency due to their high surface area and multi-level pore structures. The synthesis of Cu(II) oxide-nickel aluminum layered double hydroxide (CuO@LDH) microspheres with a three-dimensional flower-like structure via hydrothermal synthesis. Experimental results showed that the degradation efficiency of methylene blue (MB) reached nearly 100% within 120 min, with the apparent rate constant (k) being 20–93 times higher than traditional two-dimensional electrode systems, demonstrating that the three-dimensional structure significantly enhanced mass transfer and radical reaction kinetics [128]. Additionally, the preparation of novel kaolin-based particle electrodes for treating methyl orange wastewater was reported. To further assess its catalytic performance, additional dyes (Acid Orange II, Eeriochrome Blue Black R, and Methylene Blue) were examined, and the results indicated that over 97% decoloration was observed for all tested dyes. This finding suggests that FM-kaolin-450 could function as an effective particle electrode for the removal of color from dye wastewater [135]. Also, Zong et al. [42] designed a polyurethane-reduced graphene oxide@MnO₂ (PU@RGO@MnO₂) three-dimensional composite material, which anchored MnO₂ nanoparticles onto the RGO surface through protonation, forming a multi-level adsorption-catalysis interface. The three-dimensional lattice structure maximized the contact area between the adsorbent and MB solution, achieving an adsorption efficiency of 94%. In addition, the development of a three-dimensional electrochemical reactor (3D-ER) based on γ-Fe₂O₃-carbon nanotube (CNT) composite particles for the treatment of rhodamine B (RhB) dye wastewater was reported. The results indicated that the chemical oxygen demand (COD) removal rate reached 99.61% within 7 min [129]. Then, Ren et al. [136] investigated the efficiency of organic matter (RhB) removal in a three-dimensional electrode reactor (3DER) constructed from repurposed industrial solid waste, i.e., Mn-loaded steel slag, as the catalytic particle electrodes (CPEs). The RhB removal rate exceeded 96% after a 20 min reaction. This research provides information on the potential of the 3DER for removing refractory organics from water. Furthermore, in the study by Tao et al. [137], the electrochemical oxidation degradation of Rhodamine B dye on a boron-doped diamond electrode was investigated, examining the input mode of power attenuation. A comparison was made between the two modes of power attenuation and the conventional direct current regarding the removal efficiency, current efficiency, and energy consumption of treating Rhodamine B. The application of current attenuation in step-Mode A1 resulted in the decomposition of 98.58% of Rhodamine B, accompanied by a COD removal efficiency of 91.47%. The power attenuation mode for the electrochemical treatment of Rhodamine B dyes is poised to emerge as a leading energy conservation approach for the effective management of dye wastewater in the future [138]. Continuously, the preparation of packed granular activated carbon (GAC) supported TiO₂-SiO₂ oxide (TiO₂-SiO₂/GAC) in a three-dimensional electrochemical reactor (3DER) for the removal of acid orange 7 (AO7) dyeing wastewater was conducted. The results obtained under optimal conditions revealed that the decolorization rate and COD removal efficiency of AO7 were 83.20% and 48.95%, respectively [139]. Additionally, Hamed et al. [140] examined the optimization of the 3D electro-Fenton process for the removal of acid orange 10 from aqueous solutions, employing response surface methodology. The maximum attainable efficiency of dye removal was determined to be 99.15%. This 3D electro-Fenton process has been demonstrated to be an excellent option for removing dyes from aqueous solutions.

4.2. Pharmaceutical Pollutant Treatment

The presence of environmental residues of antibiotics and the dissemination of antibiotic-resistance genes have emerged as significant global ecological concerns. Three-dimensional electrode systems, by integrating electrochemical oxidation and radical reactions, have been shown to effectively degrade pharmaceutical pollutants. The utilization of Ti-Sn-Sb@γ-Al₂O₃ particle electrodes in the construction of a three-dimensional electrolytic system for the treatment of oxytetracycline (OTC) is a subject of considerable interest. Under optimal conditions, the OTC removal rate reached 92.0%, with a total organic carbon (TOC) removal rate of 41.0% [141]. Additionally, Wang et al. [142] employed MnFe₂O₄-rGO electrocatalytic films to activate peroxymonosulfate (PMS) or the degradation of total organic carbon (TOC). The resultant data indicate an 88.3% OTC degradation rate. Thus, a diatomite board@dopamine-W@WC (DPW) three-dimensional Fenton catalyst, which achieved a degradation rate of 94.98% for norfloxacin (NOR) under near-neutral pH conditions, was developed. The reaction rate constant (0.2087 min⁻¹) was 2.08 times higher than that of traditional MoS₂/Fenton systems [143]. In addition, Yueyue et al. [143] studied the advanced reduction in Fe³⁺ and the facilitation of the application of cocatalysts by preparing 3D diatomite plate@polydopamine@WC (DPW) cocatalysts using a simple two-step impregnation method to construct a DPW/Fenton system [143] for the degradation of norfloxacin (NOR) [144] at near-neutral pH (5.5). The degradation efficiency was found to be 94.98%. Consequently, this study offers a theoretical framework and technical support for the large-scale preparation and convenient application of DPW, as well as the degradation of toxic organic contaminants. The same organic pollutant has been removed using a nano-zero-valent iron/nickel/peroxysulfate (nZVI/Ni/PS) system, achieving 99% degradation of NOR in 30 min. Its high efficiency was attributed to the reductive action of nZVI and the catalytic activation of Ni, which synergistically promoted the decomposition of peroxysulfate (PS) into SO₄•⁻ and •OH [145]. Continuously, the hierarchical porous carbon nanorods doped with a bimetallic Fe/M (M = Co, Ni, Mn) alloy (FeM@C) are synthesized via simple pyrolysis of bimetallic MOFs (FeM-MIL-88B). The FeM@C composite has been demonstrated to exhibit remarkable efficacy in the electrochemical sensing of pharmaceutical products, acetaminophen (AC) and rutin (RT), as evidenced by case studies conducted in this work. The application of this sensor has yielded successful outcomes in the determination of AC and RT in real drug samples, exhibiting a recovery rate of 93.3–101.6%. This study provides a foundation for the development of effective bimetal alloy nanoparticles and their applications in electrochemical sensing [146]. Also, Caiping et al. [147] examined the application of CuO-doped red mud (CuO/URM) as a particle electrode in a three-dimensional electro-Fenton (3D/EF) system for the degradation of ciprofloxacin (CIP). The optimal degradation efficiency of CIP, estimated to be approximately 80.66%, was attained following an 80 min treatment period with a CuO/URM dosage. The findings indicated that the 3D/EF system catalyzed by CuO/URM particle electrodes constitutes a dependable technology for the disposal of CIP wastewater. Subsequently, Shumin Yang and Yan Feng [89] conducted electrocatalysis research on the degradation of tetracycline in a three-dimensional aeration electrocatalysis reactor (3D-AER) with a flotation-tailings particle electrode (FPE). The removal rate of TC reached 71.52% within 30 min. In addition, Xuyang et al. [148] investigated the synthesis of titanium dioxide nanotubes/graphene aerogel (TNGA) as a particle electrode and its application as a three-dimensional (3D) electrode for the treatment of wastewater containing tetracycline hydrochloride (TCH) at low electrolyte concentrations. The high TCH degradation rate of 90.6% was obtained after 3 h. This study demonstrated that TNGA is an excellent electrocatalytic material for use as a particle electrode in antibiotic elimination.

4.3. Wastewater Treatment

The application of three-dimensional electrode technology in the treatment of refractory wastewater has shown significant advantages. Thus, the application of iron-carbon particle dynamic reactor to treat landfill leachate, with results showing that under an electrical current density of 16 mA cm⁻² and a flow rate of 0.75 L h⁻¹, the removal rates of total organic carbon (TOC), color, COD, and ammonia nitrogen (NH₄⁺-N) reached 60.02%, 96.50%, 64.98%, and 99.46%, respectively [149]. Also, Dayang et al. [150] conducted the effectiveness of a 3D electrode dynamic reactor in removing contaminants from landfill leachate was investigated. The study utilized a continuous flow system. The findings indicate that the three-dimensional electrode dynamic reactor demonstrates considerable promise in the reduction of total organic carbon (TOC), refractory trace organic pollutants, ammonia-nitrogen (NH₃-N), and chroma in landfill leachate. The implementation of a three-dimensional electrode dynamic reactor in this study exemplifies its potential applications in the treatment of landfill leachate [29]. In addition, the treatment of organic wastewater by a synergistic electrocatalysis process that utilizes Ti³⁺ self-doped TiO₂ nanotube arrays as both the cathode and anode. The enhanced removal of pollutants by electro-Fenton (EF) + anodic oxidation (AO) is attributable to the coexistence of •OH oxidation and direct oxidation on the surface of Ti³⁺/TNTAs. The COD of secondary effluent of coking wastewater underwent a significant decrease from 159.3 mg/L to 47.0 mg/L as a result of the EF + AO process within 120 min. This study proposes a novel strategy for constructing an energy-efficient synergistic electrocatalysis process for the elimination of organic pollutants from wastewater [98]. Guo et al. [151] prepared Ni-Fe oxide-modified three-dimensional particle electrodes by pyrolysis and constructed an electro-Fenton (3D/EF) system for electronic wastewater treatment. The system enhanced COD removal by 35% compared with traditional EF systems by synergistically catalyzing the decomposition of H₂O₂ with Ni supported on algal biochar (ABC) through anodic dissolution of Fe²⁺. Thus, the application of the Fe-Cu@needle-shaped coke three-dimensional electrode system, which achieved COD, NH₄⁺-N, total nitrogen (TN), and color, the removal rates at 94.2%, 93.8%, 90.3%, and 99%, respectively, when treating landfill leachate (Figure 4a). Raman spectroscopy indicated that the increased graphitization degree of coke enhanced its electronic conductivity [152]. Also, using a three-dimensional bio-electrochemical reactor (3D-BER) achieved a 78.99% nitrate removal rate in low carbon-nitrogen ratio wastewater, with an energy consumption of only 1.20 kWh m⁻³. Electrochemical impedance spectroscopy (EIS) analysis revealed that the acetylene black-activated carbon-cobalt (Co/AC) composite electrode facilitated denitrification through direct reduction by Co⁰ and indirect reduction by H⁺, achieving a TN (Figure 4b) removal rate of up to 95% in the presence of Cl⁻. Meng et al.’s [153] α-Fe₂O₃/powdered activated carbon (PAC) three-dimensional electrode system achieved a 95.30% removal rate of NH₄⁺-N after 20 min of electrolysis at 20 V [154]. Thus, Fang et al. [155] investigated the significance of electrogenerated Cu(III) in the context of electrocatalytic oxidation reactions for the treatment of COD was thoroughly examined. This investigation focused on the three-dimensional electrode composed of Cu-Sb-Sn and granular activated carbon. The study’s findings underscored the pivotal role of electrogenerated Cu(III) in this particular electrode configuration, highlighting its crucial impact on the efficiency of electrocatalytic oxidation reactions. Consequently, Cu0.0₂-Sb-Sn/GAC demonstrated enhanced activity, exhibiting a current efficiency of 41.1%. According to the findings of the study by Thai Anh Nguyen et al. [156], the integration of granular activated carbon (GAC) within a conventional electrochemical oxidation (EO) system has been demonstrated to enhance the removal efficiency of reactive dyes in wastewater. After the treatment, the functional groups and cyclic structures of the reactive dyes were decomposed and converted into simpler structures. Consequently, 3D EO emerges as a pioneering alternative, demonstrating remarkable efficacy in the removal of reactive dyes from the water matrix. In addition, the systemization of the banana peel biochar (BPB) and elucidated its function as a particle electrode for the removal of carcinogenic methyl violet 2B dye (MV 2B) in a state-of-the-art three-dimensional electrochemical system with titanium (Ti) anode and a graphite (GP) cathode (BPB–Ti/GP). Subsequent liquid chromatography-mass spectroscopy analysis revealed that MV 2B underwent degradation through N-demethylation. This distinctive approach serves to reduce agricultural waste and promote the sustainability of wastewater treatment processes, thereby establishing a foundation for environmentally sustainable solutions [157]. In addition, Weida et al. [158] investigated the use of vanadium titanomagnetite (VTM) as a dual-action three-dimensional particle electrode and activator in combination with electrochemical oxidation to activate and degrade coking wastewater. Subsequently, the impact of varying VTM dosage, potassium persulfate (KPS) dosage, current density, and reaction time on COD, UV, and DOC removal of coking wastewater was examined. Irrespective of the initial pH, the current density is 15 MA/cm. The degradation of COD, DOC, and UV 254 was found to be effective, with percentages of 94.15%, 86.10%, and 57.67%, respectively. In summary, the electrochemical oxidation process coupled with VTM as a particle electrode activating KPS technology demonstrates a substantial coupling effect in enhancing the degradation of coking wastewater.

5. Conclusions, Perspectives, and Challenges

The three-dimensional electrochemical reaction (3D-ER), a type of advanced oxidation technology, offers the notable advantage of operating without the need for additional catalytic agents. This method has demonstrated substantial potential in wastewater treatment and presents a promising outlook for future large-scale applications. In this study, we conducted a systematic investigation encompassing the structural design of 3D electrochemical reactors, fabrication techniques for particle electrodes, critical process parameters, and electrode energy consumption. Compared with traditional two-dimensional (2D) and conventional 3D particle electrode systems, the proposed approach is not only more cost-effective but also facilitates the uniform expansion of the electrochemical reaction zone. Accordingly, this work discusses the synthesis strategies, synergistic mechanisms, and practical applications of 3D particle electrode systems for efficient wastewater remediation.

5.1. Enhanced Organic Pollutant Removal

The 3D electrochemical process relies on the integration of particle electrodes, which are pivotal in the removal of organic pollutants from industrial wastewater. Their three-dimensional structure offers a significantly larger reaction interface than conventional 2D systems, thereby enhancing mass transfer, charge transport, and overall degradation efficiency. Various types of particle electrodes have been developed for specific wastewater types, each characterized by high porosity, low energy consumption, and versatile electrochemical polarity.

5.2. Surface Adsorption and Catalytic Degradation Mechanisms

Particle electrodes in 3D systems exhibit large specific surface areas and well-developed porous structures, which facilitate the adsorption of organic contaminants. Numerous studies have confirmed that the initial adsorption of pollutants onto the particle electrode surface significantly accelerates subsequent degradation reactions. Enhanced surface area contributes directly to improved adsorption capacity and increased pollutant removal rates.

5.3. Electro-Sorption and Charge Interaction Mechanisms

The 3D particle electrode system demonstrates superior electro-sorption capabilities, which play a critical role in the capture and breakdown of ionic organic contaminants. In this process, counterions are adsorbed onto the electrode surface, forming electrostatic interactions with oppositely charged species. Under applied electric fields, the particle electrodes transition into active electrodes, thereby facilitating charge redistribution and enhancing the overall ion adsorption dynamics.

5.4. Generation of Reactive Oxidants

The degradation of organic pollutants in 3D-ER systems is largely driven by the in-situ generation of reactive oxygen species (ROS), including hydroxyl radicals (•OH) and hydrogen peroxide (H₂O₂), through anodic and cathodic electrochemical reactions. These strong oxidizing agents effectively oxidize and mineralize complex organic compounds, leading to high degrees of pollutant removal.

5.5. Synergistic Effects in Binary Oxide-Based Electrode Systems

Synergistic enhancement is frequently observed in 3D particle electrode systems composed of binary metal oxides. These materials not only improve the efficiency of electrochemical oxidation and adsorption processes but also contribute to energy savings and greater stability. The collaborative interactions between redox-active sites and the conductive matrix amplify the degradation kinetics and extend the electrode’s operational lifespan.

Despite the considerable advancements in 3D-ER technology and the excellent electrocatalytic performance of catalyst-loaded particle electrodes, several challenges remain. Developing high-performance particle electrodes capable of simultaneously treating both organic and inorganic pollutants is essential. Furthermore, real-world, large-scale applications of 3D particle electrode systems are still limited.

Future research should prioritize integrating 3D particle electrode systems with membrane-driven wastewater treatment technologies to enhance their practical applicability for domestic sewage treatment. This synergistic approach could potentially address key limitations, such as improving pollutant removal efficiency, mitigating membrane fouling through electrochemical processes, and reducing overall energy consumption. Furthermore, the development of novel 3D particle electrode materials, particularly plastic polystyrene-supported metal-based microsphere composites (e.g., incorporating Ag, Cu, or Fe catalysts), holds significant promise. These tailored composites could enable the simultaneous and efficient removal of complex, recalcitrant pollutant mixtures (e.g., pharmaceuticals, endocrine disruptors, and heavy metals) often found in domestic sewage. Exploring their stability, regeneration potential, and catalytic mechanisms under realistic conditions is crucial. Ultimately, advancing both integration strategies and innovative electrode materials is essential for broadening the scope, effectiveness, and economic viability of advanced electrochemical wastewater treatment technologies in municipal settings.