Degassing N₂ from the Direct Oxidation of Total Ammonia in Mariculture Using a Three-Dimensional Electrode System

Abstract

Elevated levels of total ammonia nitrogen (TAN) are recognized as a primary contributor to acute toxicity in aquatic organisms across freshwater aquaculture and mariculture environments. Existing technologies for TAN removal from wastewater are constrained by complex processes, high energy consumption, and an inability to meet discharge standards in a single step. Conventional electrochemical routes often over-oxidize TAN to nitrate, which undermines the goal of achieving truly harmless wastewater. Herein, we use a three-dimensional (3D) electrochemical system packed with particulate electrodes to realize the “TAN to N₂” in one step. The design exploits a synergistic mechanism in which anodic ·OH and HClO cooperatively oxidize TAN while cathodic sites concurrently reduce nitrate nitrogen, turning NH₄⁺ directly to N₂ without nitrate accumulation. The 3D electrochemical system is particularly suitable for marine aquaculture wastewater, especially when addressing the low TAN concentration characteristic. Results show that the 3D system increased N₂ selectivity from 67.90% to 92.06% while stabilizing wastewater pH within a mildly alkaline window. The system operates in situ, enabling direct recycle of culture water and offering a new technological paradigm for harmless, on-site treatment and resource recovery from mariculture wastewater.

1. Introduction

Total ammonia nitrogen (TAN) has been widely recognized as the primary pollutant in wastewater [1,2,3,4,5]. Particularly in aquaculture systems, even trace amounts of TAN can induce neurological symptoms for the aquatic organism [6,7]. Persistently elevated TAN concentrations trigger excessive phytoplankton growth, frequently leading to harmful algal blooms and rapid water quality deterioration [8]. Furthermore, prolonged exposure to high concentrations of nitrate and nitrite can exert toxic effects on aquatic organisms, such as causing organ damage in fish and inducing systemic pathologies [9,10].

The strategies for removing TAN from aquaculture wastewater include physical methods, biological methods, breakpoint chlorination, and electrochemical oxidation. Physicochemical methods encompass adsorption, filtration, ion exchange, and other techniques. Cheng et al. [11] achieved a maximum TAN adsorption of 1.909 mg·g⁻¹ using pyrolytic biochar from Eupatorium adenophorum (300 °C) in a coexisting ammonia–phosphorus system. They rely on functional materials such as adsorbents and membrane components, whose performance is highly sensitive to salinity, organic matter, temperature, and pH [11,12]. Biological methods typically convert TAN into nitrate through microbial nitrification. Terada et al. [13] employed a Membrane Aerated Biofilm Reactor (MABR) system, utilizing ammonia-oxidizing bacteria (AOB) within the biofilm to treat wastewater with a TAN concentration of 100 mg·L⁻¹. After continuous operation for 70 days, the TAN was ultimately converted into nitrate. Nevertheless, the oxidation efficiency of biological methods is constrained by microbial activity, resulting in extended treatment cycles [13]. Breakpoint chlorination employs sodium hypochlorite to remove TAN. Zhang et al. [14] employed a combined Ultraviolet (UV)/HClO process to achieve 50% TAN removal at a Cl₂/N molar ratio of 0.8 and TAN concentration of 1.00 mg·L⁻¹. The products included NO₃⁻ and trace amounts of NO₂⁻, with minimal disinfection byproducts generated. Breakpoint chlorination consumes large amounts of reagents, operates with low efficiency, and requires extended treatment cycles. When operating, pH may change due to the generation of H⁺ [15], which affects oxidation removal effectiveness, as it varies with pH [16]. None of the above methods truly achieves pollutant removal; they merely convert TAN into adsorbed forms or oxidize it into nitrate or nitrite, which also pose toxicity to aquatic environments and threaten the survival of aquatic organisms. Moreover, achieving direct liquid-phase oxidation of TAN to N₂ without causing secondary pollution has significant practical value.

Electrochemical oxidation (EO) technology offers simple operation, high pollutant removal efficiency, and adaptability to complex water conditions such as low temperatures and high salinity. It can indirectly oxidize TAN by generating HClO, theoretically converting TAN into N₂. However, in actual wastewater treatment (including seawater), TAN is often over-oxidized to nitrite and nitrate, leading to the accumulation of toxic byproducts [17]. Díaz et al. [18] employed boron-doped diamond (BDD) electrodes to oxidize Cl⁻ into HClO for treating TAN in aquaculture wastewater. Conducted at current densities of 10–50 A·m⁻², the study demonstrated poor TAN removal efficiency and generated significant byproducts, resulting in nitrate accumulation and toxicity. Therefore, improvements to existing methods are needed to further enhance N₂ selectivity and reduce the formation of byproducts such as nitrate.

Seawater contains abundant Cl⁻, which can be oxidized to generate HClO for TAN oxidation. Electrochemical oxidation of TAN using HClO offers advantages of “low cost, high efficiency, and strong adaptability” [19,20]. Commercial Ti/RuO₂–IrO₂ anodes exhibit low initial chlorine evolution overpotentials, effectively suppressing oxygen evolution reaction (OER) to promote chloride oxidation and enhance chlorine evolution selectivity, which is an excellent chlorine-evolving electrode [21]. Furthermore, during electrochemical TAN oxidation, ·OH radicals can directly participate in the oxidation reaction, thereby reducing the formation of harmful intermediate products [22]. SnO₂ exhibits high conductivity and OER catalytic activity, generating highly reactive species like ·OH to accelerate pollutant oxidation and degradation [23]. Incorporating Sb into SnO₂ alters its structure, enhances conductivity, and provides active sites that promote electrode adsorption of target pollutants [24].

Therefore, we employed a commercial Ti/RuO₂–IrO₂ anode and a Ti mesh cathode as the base electrochemical cell to construct a SnO₂-Sb₂O₃@GAC (granular activated carbon) 3D electrode system for the efficient oxidative removal of TAN. During the treatment of mariculture wastewater, no additional reagents are required; the electrode system operates continuously and stably using only the inherent substances in seawater (NaCl, H₂O). The synergistic action of in situ-generated HClO and hydroxyl radicals rapidly and efficiently oxidizes TAN into N₂, directly converting pollutants into volatile, non-polluting gases and preventing secondary water contamination. This research provides theoretical and technological support for high-efficiency recycling of mariculture wastewater.

2. Materials and Methods

2.1. Methods for Measuring Related Substances and Sources of Chemical Reagents

TAN was quantified spectrophotometrically with Nessler’s reagent following the Chinese national standard HJ 535-2009 [25]. Nitrate nitrogen was determined by Ultraviolet (UV) spectrophotometry according to HJ/T 346-2007 [26], whereas nitrite nitrogen was assayed by the N-(1-naphthyl) ethylenediamine photometric method specified in GB 7493-1987 [27]. Total and free residual chlorine were measured by the N,N-diethyl-1,4-phenylenediamine (DPD) spectrophotometric procedure described in HJ 586-2010 [28]. Combined chlorine was calculated as the difference between total and free chlorine concentrations. The relevant instruments used in our experiment are listed in Table S1. The sources and purity of the relevant chemical reagents are shown in Table S2. Commercial Ti/RuO₂–IrO₂ electrodes were purchased from Schulte Industrial Technology (Suzhou, China).

2.2. Synthesis of Granular Electrodes

SnO₂-Sb₂O₃@GAC granules were prepared by an impregnation–calcination route. Briefly, GAC was pretreated by rinsing five times with deionized water and dried at 105 °C for 4 h. A precursor solution containing SnCl₄·5H₂O (0.5 mol·L⁻¹) and SbCl₃ (0.05 mol·L⁻¹) was prepared in a mixed solvent of n-butanol (490 mL) and concentrated HCl (10 mL). The dried GAC was immersed in the precursor solution and magnetically stirred for 4 h to ensure uniform coating. After impregnation, the slurry was filtered, and the solids were dried at 105 °C for 4 h, then calcined in a muffle furnace at 400 °C for 4 h under static air. The above sequence was repeated three times to yield the final SnO₂-Sb₂O₃@GAC granular electrodes. Air treatment at 400 °C causes some degree of pore structure shrinkage or blockage, but does not completely destroy its porosity, retaining a high specific surface area [23,24,29,30,31,32,33,34].

2.3. Electrochemical Testing

All experiments were carried out in a rectangular polymethyl methacrylate (PMMA) electrolytic cell with an effective volume of 500 mL. Grooves (2 mm apart) machined on the inner side walls accommodate the electrodes, allowing a maximum inter-electrode gap of 4 cm.

For two-dimensional (2D) electrode tests, a dimensionally stable Ti/RuO₂–IrO₂ anode (100 × 100 × 1 mm, geometric area 100 cm²) was employed, with a Ti mesh cathode of identical dimensions. The pair was connected to a Direct Current (DC) regulated power supply. A 3D electrode system was constructed by packing the prepared particles between the anode and cathode (Figure 1).

Synthetic freshwater aquaculture wastewater was prepared by dissolving (NH₄)₂SO₄, NaCl, and Na₂SO₄ in deionized water. The Na₂SO₄ dosage was adjusted to maintain a constant conductivity. Based on the TAN levels reported for intensive fish-pond wastewater [35,36,37,38], initial TAN concentrations of 10 mg·L⁻¹ and 50 mg·L⁻¹ were selected. Chloride was introduced as NaCl at gradient concentrations of 500, 1250, and 1875 mg·L⁻¹. An inter-electrode distance of 4 cm was adopted, and electrolysis was conducted at a constant current density of 5 mA·cm⁻².

Artificial seawater was prepared by dissolving commercial marine salt in deionized water to a salinity of 33‰. The detailed ionic composition is given in Table S3. Ammonia was supplied as (NH₄)₂SO₄, and all other experimental operations and parameters were identical to those in the freshwater system. To simulate real marine-culture wastewater, a synthetic solution was formulated according to the pollutant concentrations documented in the literature by adding NaNO₃, NaNO₂, and (NH₄)₂SO₄ to the artificial seawater, yielding initial concentrations of 20 mg·L⁻¹ NH₄⁺-N, 10 mg·L⁻¹ NO₃⁻-N, and 2 mg·L⁻¹ NO₂⁻-N. The initial pH was 8.44, and electrolysis was performed at a constant current density of 5 mA·cm⁻².

2.4. Formulas for Relevant Parameters

The calculation method for pollutant removal efficiency $\eta (\%)$ is as follows:

$$\eta ={\displaystyle \frac{100\times \left(c_0-c_t\right)}{c_0}}$$

(1)

where $c_0$ (mg·L⁻¹) is the initial concentration, and $c_t$ (mg·L⁻¹) is the concentration after electrolysis time t (min).

Energy consumption (kWh·m⁻³) is calculated using the following formula, with units representing the electrical energy consumed per unit volume of TAN removed from aquaculture wastewater:

$$W={\displaystyle \frac{U\times I\times t}{m\times 60}}\times \left(c_0-c_t\right)$$

(2)

where $U$ (V) is the voltage between electrodes, $I$ (A) is the current intensity through the electrode plates, $t$ (min) is the electrolysis time, $m$ (mg) is the amount of TAN removed during electrolysis time $t$, and $c_0$ (g·m⁻³) is the initial concentration, and $c_t$ (g·m⁻³) is the concentration after electrolysis time $t$.

The current density (mA·cm⁻²) is calculated using the following equation, where the unit represents the proportion of electricity consumption used for TAN removal relative to total electricity consumption:

$$CE={\displaystyle \frac{100\times m\times 3\times F}{14\times I\times t\times 60\times 1000}}$$

(3)

where $m$ (mg) is the amount of TAN removed during electrolysis time $t$ (min), $F$ (C·mol⁻¹) is the faraday constant, whose value is 96,485, and $I$ (A) is the current intensity through the electrode plates.

The method of calculating the N₂ Selection Efficiency $\mathrm{ɑ}(\%)$ is given by the following equation:

$$\mathrm{ɑ}=1-{\displaystyle \frac{c_1+c_2+c_3}{c_0}}\times 100\%$$

(4)

where $c_0$ (mg·L⁻¹) represents the initial NH₄⁺-N concentration, $c_1$ (mg·L⁻¹) denotes the NO₃⁻-N concentration in the wastewater, $c_2$ (mg·L⁻¹) indicates the NO₂⁻-N concentration in the wastewater, and $c_3$ (mg·L⁻¹) signifies the NH₂Cl-N concentration in the wastewater (expressed as NH₂Cl).

As demonstrated by some related studies [39,40,41], introducing HClO oxidizes TAN under various conditions (high salinity, acidity, presence of recalcitrant organic matter). Quantitative detection of NOx (including NO and N₂O) revealed concentrations below the detection limit, rendering them negligible. Therefore, in this work, it can be concluded that other nitrogen-containing gaseous byproducts were negligible compared with N₂.

3. Results and Discussion

3.1. Characterization of Granular Electrode Materials

The microstructure of pristine GAC, the fresh electrode, was examined by scanning electron microscopy (SEM). Low- and high-magnification micrographs of the pristine GAC (Figure 2a) reveal a relatively smooth surface perforated by abundant macropores. After deposition, the SnO₂-Sb₂O₃@GAC surface is uniformly decorated with densely packed bipyramidal crystallites with a size of ~100 nm (Figure 2b,c), yielding a markedly rougher morphology than the bare carbon.

The transmission electron microscopy (TEM) image in Figure 2d resolves the microstructure of the particulate electrode. The activated-carbon matrix appears as a featureless, lattice-fringe-free matrix, consistent with amorphous carbon. Discrete crystallites exhibiting well-defined lattice fringes are anchored on this support. Inter-planar spacings of 0.26 nm and 0.33 nm measured directly from the high-resolution micrograph correspond to the (101) and (110) planes of the SnO₂-Sb₂O₃. Energy-dispersive X-ray elemental maps (Figure 2e) further reveal a homogeneous distribution of Sn and Sb across the carbon surface. Confirming the successful and uniform decoration of the activated carbon with SnO₂-Sb₂O₃.

Figure 2f presents the X-ray diffraction (XRD) patterns of GAC and the as-prepared SnO₂-Sb₂O₃@GAC particle electrode. The diffractogram of SnO₂-Sb₂O₃@GAC exhibits characteristic reflections of a tin–antimony oxide phase (PDF#88-2348), confirming the successful formation of Sb₂O₃.

N₂ adsorption–desorption isotherm of SnO₂-Sb₂O₃@GAC was conducted to evaluate the surface area and the pore structure of the as-prepared materials in Figure 2g, showing the isotherm of type IV with an H4 hysteresis loop in the mid-to-high relative-pressure region, indicating the coexistence of micro- and meso-pores. The specific surface area is calculated to be 1.42 × 10³ m²·g⁻¹, and the total pore volume and mean pore width are 1.15 cm³·g⁻¹ and 1.62 nm, respectively, placing the material at the micropore–mesopore boundary. This hierarchical porosity endows the particle electrode with both high surface area and ample void volume, facilitating rapid molecular diffusion and efficient guest accommodation.

3.2. Mechanism and Effect of TAN Oxidation in Electrode Systems

Electrochemical oxidation of TAN proceeds by two pathways: direct oxidation and indirect oxidation. Direct oxidation relies on the direct electron transfer from ammonia species at the anode surface [42]. Theoretically, NH₄⁺-N can be converted to N₂ by losing three electrons at the anode. However, the weak adsorption of NH₄⁺ on most electrode surfaces makes direct oxidation kinetically unfavorable [43,44]. To verify the contribution of the direct pathway, a synthetic wastewater containing 70 mg·L⁻¹ NH₄⁺-N (prepared with (NH₄)₂SO₄ and Na₂SO₄) was electrolyzed for 120 min at a constant current using a Ti/RuO₂–IrO₂ anode and a Ti-mesh cathode. The concentration of NH₄⁺-N decreased only from 71.31 to 67.71 mg·L⁻¹, corresponding to a removal efficiency of 5.04% (Figure S1). This marginal reduction indicates that direct oxidation makes a negligible contribution to ammonia removal. Consequently, indirect oxidation was adopted as the principal strategy for subsequent TAN treatment.

Indirect TAN oxidation is achieved by introducing an electroactive species that is converted on the anode into a potent oxidant, which then diffuses into the bulk solution and reacts homogeneously with NH₄⁺-N. Cl⁻ was added to the system to ensure that it could be electro-transformed into Cl₂ on the Ti/RuO₂–IrO₂ anode by the chlorine-evolution reaction (CER), and the dissolved Cl₂ rapidly hydrolyses to produce ClO⁻/HClO, which subsequently oxidizes ammonia. CV curves of the Ti/RuO₂–IrO₂ electrode in 0.5 M NaCl and in artificial seawater (Figure 3a,b) show a marked anodic current increase at ~1.2 V (vs. RHE), corresponding to CER, followed by a cathodic peak at the same potential, according to the Pourbaix diagram of CER and OER in 0.5 M NaCl, this phenomenon can be assigned to HClO reduction. Upon the addition of 0.25 M (NH₄)₂SO₄ to both solutions, the reduction peak vanished, indicating that TAN was rapidly consumed by hypochlorous acid. These results confirm that HClO generated from Cl⁻ oxidation is the key mediator of TAN oxidation. The reactions occurring at the anode, cathode, and in the bulk are as follows:

Anodic reaction:

2Cl⁻ → Cl₂↑ + 2e⁻

(5)

Cathodic reaction:

2H₂O + 2e⁻ → H₂↑ + 2OH⁻

(6)

Physical phase reaction:

Cl₂ + H₂O → HClO + H⁺ + Cl⁻

(7)

$$\mathrm{HClO}+{\displaystyle \frac{2}{3}}\mathrm{NH}_4{}^{+}\to {\displaystyle \frac{1}{3}}{\mathrm{N}}_2\uparrow +{\mathrm{H}}_2\mathrm{O}+{\displaystyle \frac{5}{3}{\mathrm{H}}^{+}+{\mathrm{Cl}}^{-}}$$

(8)

4HClO + NH₄⁺ → NO₃⁻ + H₂O + 6H⁺ + 4Cl⁻

(9)

HClO + NH₄⁺ → NH₂Cl + H₂O + H⁺

(10)

In a 2D electrolytic system, the Ti/RuO₂–IrO₂ anode exhibits high activity toward both chlorine evolution and ammonia oxidation. However, this is accompanied by the formation of bound chlorine (chloramines) and NO₃⁻-N as undesired byproducts. During the oxidation of TAN by the strongly oxidizing HClO, the concentration of NO₂⁻-N remains below the detection limit and is therefore regarded as negligible [45]. Consistently, the wastewater nitrite concentration was found to be extremely low throughout the experiments. Under conditions of fixed anode surface area, the rate of HClO generation depends on the bulk chloride concentration. We conducted three sets of experiments with Cl⁻ concentrations of 500, 1250, and 1875 mg·L⁻¹ according to the relevant multiples of 2.5 and 1.5. When the concentration of Cl⁻ is 500 mg·L⁻¹, the low rate of HClO generation leads to extended electrolysis time, reduced current efficiency, and increased energy consumption. Raising the concentration of Cl⁻ to 1875 mg·L⁻¹ accelerates HClO production and achieves rapid ammonia removal. Yet a substantial fraction of the electrical charge is diverted to chlorine generation. Despite improvements in electrical efficiency, energy consumption remains high. The concentration of NH₂Cl, a byproduct in the wastewater, reached as high as 7.74 mg·L⁻¹, which is significantly higher compared to other experiments. Therefore, the concentration of Cl⁻ was not further increased in the freshwater system. Concurrently, partial over-oxidation of TAN to nitrate nitrogen occurs, diminishing the selectivity toward N₂ (Figure 4). Reducing the initial TAN concentration from 50 to 10 mg·L⁻¹ does not alter the observed removal trends (Figure S3).

According to Reactions (7–10), the oxidation of NH₄⁺ by HClO releases stoichiometric amounts of H⁺. Although cathodic reduction simultaneously generates OH⁻, the net proton balance remains positive, stabilizing the bulk pH in a strongly acidic window of 3.00–4.00. In freshwater systems, the 2D electrode system achieves selective oxidation of TAN to N₂ with minimal NaCl addition. The strongly acidic wastewater fails to meet water quality standards for recirculating aquaculture systems and poses a threat to fish survival. Next, we conducted TAN removal tests in simulated mariculture wastewater. When initial TAN concentrations were 50 and 10 mg·L⁻¹ (Figure 5), the concentration of nitrate nitrogen in wastewater reached 10.12 mg·L⁻¹ and 2.82 mg·L⁻¹, respectively. The former exceeded the wastewater standard limit, resulting in a final N₂ selection rate of only 67.90% and 76.16% (Figure 5a). The underlying reason is that in seawater systems, the 2D electrode system generates a large amount of HClO instantaneously during TAN treatment. This excessively oxidizes TAN into nitrate, leading to nitrate accumulation and reduced N₂ selectivity. Excessive nitrate accumulation can also poison fish, and the resulting acidification of the water is ultimately unsuitable for fish survival. Therefore, improvements to the 2D electrode system are necessary: While HClO exhibits excellent TAN oxidation activity, excessively high concentrations readily drive over-oxidation of TAN, diminishing the efficiency of directed N₂ conversion. Building upon the original chlorine oxidation approach, we introduced novel reactive species to partially replace chlorine’s oxidative function.

To elucidate the oxidation mechanism of TAN in the 3D granular-electrode system, a 50 mg·L⁻¹ NH₄⁺-N solution (prepared from (NH₄)₂SO₄) containing 0.1 M Na₂SO₄ as supporting electrolyte was electrolyzed at 5 mA·cm⁻², while the temporal decay of NH₄⁺-N was monitored. An identical solution was then spiked with 30 mM tert-butanol (TBA) to scavenge ·OH radicals, and the electrolysis was repeated under the same conditions. As shown in Figure S2, the removal efficiency of NH₄⁺-N was 14.7% in the presence of TBA, significantly lower than the 82.3% achieved in its absence. This pronounced inhibition indicates that TBA suppresses ·OH generation, thereby hindering TAN oxidation, confirming the pivotal role of ·OH radicals in the catalytic removal of NH₄⁺-N in the 3D electrode system.

In addition, to verify the relative oxidation contribution rate of ·OH compared to HClO, we conducted quenching experiments under different salinities (Figure 3). The seawater used in the experiments was configured with a salinity of 33‰, corresponding to a NaCl concentration of approximately 16,000 mg·L⁻¹. On this basis, two NaCl concentration gradients (8000 mg·L⁻¹ and 2000 mg·L⁻¹) were established. Na₂SO₄ was added to maintain consistent electrical conductivity across all groups, followed by electrolytic treatment at a current density of 5 mA·cm⁻². In each group of experiments, ·OH was quenched with TBA to retain only the oxidative effect of HClO on TAN; meanwhile, an experimental group with the combined oxidation of TAN by ·OH and HClO was established. The difference between the two groups represents the mass of TAN oxidized by ·OH. Data at the 10-min point were analyzed for all three groups, and the results indicated that the higher the NaCl concentration, the greater the oxidation contribution ratio of HClO rose. This is attributed to the fact that an increase in Cl⁻ concentration promotes the generation of HClO, while the generation rate of ·OH shows no significant change. However, when the NaCl concentration reaches a certain level, the maximum concentration of HClO that the system can produce per unit time reaches a limit. At this point, the oxidation contribution ratio of HClO to ·OH tends to stabilize, with HClO consistently occupying a dominant position.

The reaction area and active sites of the 2D electrode system are limited to two electrode plates. After filling with granular electrodes, these particles can form many microelectrolytic cells under the influence of an electric field, significantly increasing the reaction area, shortening the mass transfer distance between pollutants and the electrode, and enhancing the reaction kinetics. This approach significantly accelerates the electrochemical oxidation of target pollutants on the particle electrode surfaces while substantially enhancing mass transfer efficiency, thereby addressing limitations such as restricted mass transfer capacity in a 2D electrode system [29,33]. Moreover, the SnO₂-Sb₂O₃ metal oxides loaded onto the particle electrode surface exhibit high catalytic activity, promoting the generation of highly oxidative ·OH radicals. These ·OH radicals synergistically interact with HClO produced by the Ti/RuO₂–IrO₂ anode, achieving deep removal of TAN. Under applied electric fields, each particle polarizes into a bipolar microelectrode: The cathode-facing side of the particle acts as an anode driving water oxidation (2H₂O → H₂O₂/·OH + 2H⁺ + 2e⁻), while the anode-facing side functions as a cathode promoting nitrate reduction (2NO₃⁻ + 12H⁺ + 10e⁻ → N₂ + 6H₂O). The overall reaction schematic is shown in Figure 1.

Firstly, we tested the TAN treatment performance of the 3D electrode system in freshwater systems. In freshwater, the 3D system utilizes the synergistic action of the strong oxidizing species ·OH and HClO to degrade TAN. After ·OH partially replaced HClO, the H⁺ accumulation rate decreased, maintaining the wastewater pH within a neutral to weakly alkaline range (the endpoint pH of the 2D system was significantly lower than the initial value, while the 3D system remained near neutral).

Compared with a 2D electrode system, a 3D electrode system filled with granular electrodes exhibits increased particle-to-particle contact, which induces short-circuit currents. These short-circuit currents reduce the system’s effective reaction current. At low ion concentrations, the efficiency of active substance generation decreases while the occurrence of other side reactions increases [46]. At lower Cl⁻ concentrations (500 mg·L⁻¹, 1250 mg·L⁻¹), the presence of short-circuit currents reduces system current efficiency and increases energy consumption, further diminishing active substance generation efficiency and TAN oxidation effectiveness. Higher concentrations of byproducts like chloramines lead to reduced N₂ selectivity. As Cl⁻ concentration further increases, the 3D electrode system remains affected by short-circuit current, resulting in reduced effective reaction current. However, at this concentration, more HClO is generated, which is sufficient to effectively oxidize TAN. During this process, ·OH partially replaces HClO in oxidizing TAN, reducing the formation of chloramine byproducts and thereby improving N₂ selectivity.

When the initial TAN concentration decreased to 10 mg·L⁻¹, the Cl⁻/NH₄⁺ molar ratio correspondingly increased, and the TAN removal times for both systems showed no difference (Figure S2). The corresponding patterns for the 3D electrode system were identical to those observed at a Cl⁻ concentration of 1875 mg·L⁻¹. At this TAN concentration, the 3D electrode system demonstrated superior TAN removal performance compared to the 2D electrode system. The wastewater pH remained neutral to weakly alkaline, resulting in higher wastewater quality. Although the 3D electrode system exhibits lower current efficiency and higher energy consumption, it resolves wastewater acidification issues, making the discharge water meet relevant standards.

Secondly, we tested its TAN treatment performance in a seawater system. At an initial TAN concentration of 50 mg·L⁻¹, nitrate nitrogen concentration first peaked, then continuously declined during the reaction. This turning point precisely coincided with the moment TAN was nearly fully oxidized (Figure 5a). This phenomenon stems from the continuous reduction of nitrate (NO₃⁻ → N₂) within the cathodic polarization zone on the granular electrode surface. Since the oxidation rate of TAN exceeds the reduction rate of nitrate, NH₄⁺ conversion occurs preferentially. Once NH₄⁺ is depleted, nitrate reduction becomes the dominant pathway, resulting in the characteristic “rise–fall” curve pattern. Although the short-circuit current of the system reduced current efficiency and increased energy consumption per unit (Figure 5b), both residual nitrate and HClO concentrations in the final wastewater decreased, with N₂ selectivity improving from 67.90% to 92.06%. Consequently, the 3D structure demonstrated more pronounced performance advantages in seawater, exhibiting outstanding TAN removal efficiency. However, in freshwater environments, corresponding nitrate reduction is negligible. This is due to the influence of Cl⁻ and nitrate concentrations. Lan et al. [47] systematically investigated the effect of Cl⁻ concentration on nitrate reduction in simulated wastewater by gradually increasing the NaCl concentration from 0.001 mol·L⁻¹ to 0.1 mol·L⁻¹. Results showed that as Cl⁻ concentration increased, nitrate removal efficiency within a fixed period of time exhibited an upward trend, indicating that Cl⁻ promotes nitrate reduction. Furthermore, increasing nitrate concentration enhances its contact with active sites on granular electrodes, thereby promoting reduction. This is evidenced by the linear increase in nitrate removal efficiency when the initial nitrate concentration rose from 14.8 mg·L⁻¹ to 39.8 mg·L⁻¹, indicating that higher nitrate concentrations are more conducive to the reduction reaction.

Therefore, it can be indicated that in freshwater systems, the low concentration of Cl⁻ results in a weaker promotion of nitrate reduction. Simultaneously, the nitrate concentration generated in freshwater systems remains consistently low, preventing it from contacting active sites on granular electrodes. These dual factors lead to negligible nitrate reduction. In contrast, seawater systems feature high Cl⁻ concentrations and generate significantly higher nitrate concentrations. This environment further promotes nitrate reduction at the cathode region of granular electrodes, resulting in a more pronounced reduction phenomenon in seawater.

Subsequently, when the initial TAN concentration was reduced to 10 mg·L⁻¹, the characteristic pattern of nitrate nitrogen first increasing and then decreasing was still observed in seawater, with rapid oxidation of TAN similarly achieved. This result aligns with findings under higher TAN (50 mg·L⁻¹) conditions, demonstrating that the N₂ selectivity of the 3D electrode system significantly outperforms that of 2D systems (Figure 5c,d). The particulate electrodes simultaneously achieve efficient NH₄⁺ oxidation and directed NO₃⁻-N reduction, making the 3D electrode system more advantageous for treating mariculture wastewater with low TAN concentrations.

Finally, we investigated and compared the advantages of the 3D electrode system over other methods in treating TAN (

Table 1. Comparison of Different Methods.

Methods	Craftsmanship	Initial TAN Concentration	TAN Removal Efficiency	Final Product
Physical Method [11]	EBC Adsorption	50.0 mg·L−1	1.909 mg·g−1	within EBC
Biological Method [13]	MABR (nitrification)	100.0 mg·L−1	Nearly 100%	NO3−
Breakpoint Chlorination [14]	254 nm UV with HClO	1.0 mg·L−1	50%	NO3−, NO2−
Electrochemical Oxidation [48]	Ti/PbO2 Electrodes	60.0 mg·L−1	Nearly 100%	N2 (87.00%), NO3−, NH2Cl
This work	SnO2-Sb2O3@GAC granular electrode	50.0 mg·L−1	Nearly 100%	N2 (92.60%), NO3− (1.96%), NH2Cl (5.44%)

). The electrochemical method achieves faster oxidation rates and higher removal efficiency for TAN, typically achieving near-complete removal. Unlike biological or physical methods, it is unaffected by toxic substances such as antibiotics and phenols present in water bodies, making it adaptable to diverse and complex aquatic environments. However, while traditional electrochemical methods also primarily produce N₂ as the main product, they generate significant byproducts such as nitrates. The 3D electrode system incorporates granular electrodes. These particles form numerous microelectrolytic cells under the influence of an electric field, significantly increasing the electrode reaction area. Compared to the 2D electrode system, this enhances mass transfer efficiency while simultaneously achieving efficient oxidation of NH₄⁺ and reduction of NO₃⁻-N, further improving nitrogen N₂ selectivity.

3.3. Three-Dimensional Electrode System Treats Simulated Real Marine Aquaculture TAN Wastewater

As demonstrated in the preceding section, a 3D electrode system in seawater can simultaneously achieve high N₂ selectivity and selective NO₃⁻-N reduction, while maintaining a weakly alkaline pH. Previous batch experiments have afforded nearly complete removal of NH₄⁺-N and NO₃⁻-N without detectable accumulation of NO₂⁻-N. Nevertheless, in real marine-culture wastewater, nitrite toxicity to fish and invertebrates remains a critical concern. To verify the practical applicability of the 3D electrode system, we therefore challenged it with a synthetic seawater matrix that closely mimics actual aquaculture wastewater. Simulated wastewater was prepared by dissolving NaNO₃, NaNO₂, and (NH₄)₂SO₄ in artificial seawater solution, with initial concentrations of 20 mg·L⁻¹ NH₄⁺-N, 10 mg·L⁻¹ NO₃⁻-N, and 2 mg·L⁻¹ NO₂⁻-N (pH 8.44). Electrolysis was performed at a constant current density of 5 mA·cm⁻². As shown in Figure 6a, NO₃⁻-N was rapidly reduced on the granular surfaces, while NO₂⁻-N declined sharply within the first 10 min. When NH₄⁺ oxidation approached completion, the NO₃⁻-N profile exhibited a clear inflection and continued to decrease thereafter. concomitantly, NH₄⁺ and NO₂⁻-N concentrations rose slightly, indicating ongoing NO₃⁻ → NO₂⁻ → N₂ cascade reduction in the cathodic micro-domains of the granules. After 45 min of treatment, removal efficiencies reached 98.14% for NH₄⁺-N, 65.94% for NO₃⁻-N, and 81.12% for NO₂⁻-N (Figure 6b), with all final concentrations below the respective emission limits. The wastewater pH stabilized at 7.50, satisfying water-reuse criteria for marine aquaculture. These results demonstrate that the 3D granular-electrode system can integrate high-efficiency NH₄⁺ oxidation with deep NO₃⁻ reduction, directing both species selectively toward N₂ and enabling single-step treatment and recycling of real seawater culture wastewater with robust nitrogen removal and water-quality security.

However, real seawater is not a simple electrolyte solution but a highly complex mixture: besides cations and anions such as Na⁺, Mg²⁺, Ca²⁺, Cl⁻, and SO₄²⁻, it is also rich in dissolved organic matter (DOM), active microorganisms, colloidal particles, and trace elements like bromine and iodine. Among these, chloramines readily react with certain organic compounds to form nitrogenous disinfection byproducts (N-DBPs) such as haloacetic acids (HAAs), halo-N-methylamines (HNMs), and nitrosamines (NAs). Sun et al. [49] investigated the relationship between chloramine concentration and N-DBP levels. When chloramine concentrations ranged from 2.57 to 25.73 mg·L⁻¹, the formation of N-DBPs from antibiotic precursors such as levofloxacin (LEV) and sulfamethoxazole (SMX) showed a significant and sustained increase, the formation of N-DBPs from antibiotic precursors like levofloxacin LEV and SMX significantly and continuously increased. This concentration range represents the critical dose range where chloramines promote DBP formation. In contrast, the amine concentrations generated by the 3D electrode system in seawater ranged from 0.96 to 2.72 mg·L⁻¹, falling out of the lower limit of the range reported in the literature. This suggests that the amine concentrations produced by this system are insufficient to drive substantial formation of disinfection byproducts, thereby minimizing toxic effects.

Furthermore, although long-term stability testing of the granular electrodes in simulated wastewater demonstrated sustained high-efficiency TAN removal performance across multiple experimental cycles, the composition of real seawater is highly complex. Validation of the long-term stability of granular electrodes under field conditions.

4. Conclusions

This study focuses on the efficient conversion of TAN into N₂ using a 3D electrode system, achieving a one-step transformation of pollutants from the liquid phase to the gas phase. It simultaneously suppresses the formation of toxic byproducts such as nitrate and nitrite, eliminates secondary pollution, and effectively stabilizes the pH of the wastewater. In marine aquaculture environments, this process enables efficient, low-consumption, and stable recycling of mariculture wastewater without the need for additional chemical additives, which is highly promising for the future development of a new paradigm for pollutant control in marine aquaculture.