Quaternized modified waste wood-derived porous carbon electrode for highly selective capacitive deionization denitrification

For tackling the critical challenge of poor selectivity in nitrate pollution control within industrialized recirculating aquaculture systems, this study develops a cetyltrimethylammonium bromide-modified self-supporting wood-derived porous carbon composite (CTA-3DPC) as a highly selective capacitive deionization electrode material. The CTA-3DPC features a vertically aligned hierarchical porous structure, which facilitates solution mass transfer during the electrosorption process. When employed as the anode in electrosorption, CTA-3DPC demonstrates strong selectivity for NO 3 − over competing anions such as Cl −, SO 4 2‒, and H 2 PO 4 −. The adsorption capacity of CTA-3DPC for NO 3 − reaches 40.67 mg g -1, significantly surpassing those for Cl − (8.56 mg g -1), H 2 PO 4 − (18.1 mg g -1), and SO 4 2‒ (23.07 mg g -1). Notably, CTA-3DPC achieves a NO 3 − /Cl − selectivity coefficient of 13.23, substantially exceeding previously reported values. This high selectivity is attributed to the electrostatic loading of CTAB onto the negatively charged wood-derived activated carbon surface, where the hydrophilic quaternary ammonium cations orient outward, forming a dense and stable positively charged layer at the carbon/electrolyte interface. This layer reduces the potential of zero charge on the material surface and favors the selective adsorption of NO 3 − due to its planar structure and weak solvation effects. This work provides a novel strategy for fabricating biomass-based electrodes with high nitrate selectivity.

The global expansion of intensive recirculating aquaculture systems (RAS) represents a critical strategy for meeting the rising demand for aquatic protein while mitigating the environmental footprint of traditional open-water farming. By filtering and reusing water, RAS significantly reduces effluent discharge and land use (Xiao et al., 2019). However, this closed-loop configuration intensifies the accumulation of metabolic waste products, particularly nitrogenous compounds like ammonia and nitrite, which are converted to less toxic nitrate (NO 3 −) through biological nitrification. While NO 3 − is less acutely toxic, its persistent accumulation in RAS water poses a chronic threat to aquatic animal health, leading to reduced growth rates, increased susceptibility to disease, and elevated mortality. Consequently, maintaining NO 3 − concentrations below safe thresholds (typically <50 mg L -1 as NO 3 -N) is paramount for sustainable, high-density aquaculture (Kang et al., 2023; Schram et al., 2014). This necessity drives the demand for efficient, energy-conscious, and scalable nitrate removal technologies within RAS water treatment loops.

Currently, the primary methods for NO 3 − removal include biological, chemical, and physical approaches (Zirrahi et al., 2024; Arumugham et al., 2024; Wu et al., 2021). While these methods have shown success in specific application scenarios, they exhibit significant limitations in RAS. High salinity (especially in marine aquaculture), low carbon-to-nitrogen ratios, and saturated dissolved oxygen levels severely inhibit biological denitrification. Chemical reduction requires active metals or H 2 as reducing agents, which may lead to secondary pollution. Physical treatment methods such as ion exchange, reverse osmosis, and electrodialysis lack selectivity for target ions, often resulting in the simultaneous removal of all charged ions, which leads to high energy consumption and resource wastage. Therefore, developing a technology capable of selectively extracting nitrate from complex saline matrices is a pivotal challenge for advancing RAS environmental and economic sustainability.

Capacitive deionization (CDI) has emerged as a promising, energy-efficient alternative for brackish water desalination and targeted ion removal (Sun et al., 2023; Chen et al., 2024). Operating on the principle of electrosorption, CDI employs a low electrical potential (typically 0.8–1.8 V) to drive ions from the feedwater into the electrical double layers (EDLs) formed at the surfaces of porous carbon electrodes. During regeneration, ions are released back into a concentrate stream by short-circuiting or reversing the voltage, enabling electrode reuse without chemical additives (Vafakhah et al., 2020). The carbon footprint of CDI is intrinsically linked to the salt adsorption capacity (SAC) and charge efficiency of its electrode materials (Liang et al., 2022; Ratajczak et al., 2019). While traditional activated carbons offer high surface area and good conductivity, they are inherently non-selective, adsorbing ions primarily based on their hydrated radius and charge in the EDL, which is unfavorable for targeting nitrate in the presence of abundant Cl − (Lu et al., 2025; Zheng et al., 2023a; Wang et al., 2024). To impart selectivity, researchers have explored surface functionalization of carbon electrodes. Introducing positive charges or specific binding sites can preferentially attract target anions (Cen et al., 2021; Chen et al., 2022). For instance, coating carbon with cationic polymers (e.g., polyethyleneimine) or grafting quaternary ammonium groups has shown promise in enhancing nitrate affinity (Oyarzun et al., 2018; Gan et al., 2019). However, these modifications often involve complex synthesis or can degrade conductivity. Furthermore, these materials frequently face issues with high cost, scalability, dissolution, or slow kinetics. Thus, there remains a compelling need to develop low-cost, robust, and highly selec This study not only presents tive electrode materials that combine the advantages of carbon electrode (stability, conductivity) with precise ionic selectivity.

Biomass-derived activated carbons present an attractive, sustainable platform for CDI electrodes. Waste wood, an abundant and renewable resource, can be converted into highly porous carbons through thermochemical activation (Zheng L. et al., 2025; Al-Labadi et al., 2025). The resulting three-dimensional, hierarchically porous structure-integrating macro-, meso-, and micropores, is ideal for CDI. Macropores serve as ion-buffering reservoirs, mesopores facilitate rapid ion transport, and micropores provide immense surface area for EDL formation (He et al., 2022; Zheng et al., 2023b). However, the natural surface chemistry of wood-derived carbon is often negatively charged due to oxygen-containing groups (carboxyl, hydroxyl), which is not inherently selective for anions. Surface engineering is therefore essential to transform this versatile carbon scaffold into a nitrate-selective substrate.

Cationic surfactants, such as cetyltrimethylammonium bromide (CTAB), offer a straightforward and effective route for such engineering (Tsai et al., 2025). CTAB possesses a hydrophilic quaternary ammonium head (positively charged) and a long hydrophobic alkyl tail. It can be electrostatically anchored onto negatively charged carbon surfaces. Upon modification, the cationic head groups orient outward, creating a dense, stable layer of positive charge at the carbon/electrolyte interface. This modified interface can electrostatically favor the adsorption of anions. Crucially, the affinity for different anions can be modulated by the specific properties of this interfacial layer and the inherent characteristics of the anions (e.g., charge density, hydration shell) (Gamaethiralalage et al., 2021). This provides a mechanism to achieve selectivity, particularly for nitrate, which may exhibit favorable interactions with the tailored cationic interface compared to other anions like chloride.

This work addresses the critical selectivity challenge in RAS nitrate management through the development of a novel, high-performance CDI electrode material: a CTAB-modified, self-supporting wood-derived activated carbon monolith with a three-dimensional hierarchical porous structure (CTA-3DPC). Selective nitrate removal from multi-ion solutions simulating aquaculture wastewater was achieved through the unique combination of two key features: (i) a 3D hierarchical porous architecture, which provides high capacity and fast kinetics, and (ii) a surface-engineered, positively charged interface, which imparts anion selectivity. We hypothesized that this synergistic design would yield an electrode with unparalleled performance for nitrate capture. The electrosorption performance is rigorously evaluated in both single- and mixed-anion solutions to assess its capacity, kinetics, and selectivity. Furthermore, the enhanced hydrophilicity post-modification improves electrode wettability and ion accessibility. This study presents a novel electrode material for addressing the critical challenge of nitrate selectivity in aquaculture wastewater treatment. The work establishes a general and sustainable strategy for the design of selective ion-capture electrodes, with a focus on understanding the fundamental mechanisms governing nitrate selectivity in capacitive deionization systems.

Materials

Waste basswood blocks (3.0 cm × 1.0 cm × 0.1 cm) were collected from a forest farm located in the Changbai Mountains, Jilin Province, China. Sodium nitrate (NaNO 3, 99.5%), sodium chloride (NaCl, 99%), sodium dihydrogen phosphate (NaH 2 PO 4, 99%), and sodium sulfate (Na 2 SO 4, 99%) were obtained from Sinopharm Chemical Reagent Co., Ltd. Cetyltrimethylammonium bromide (CTAB) was purchased from Tianjin Kermel Chemical Reagent Development Center. Zinc chloride (ZnCl 2, 98%) and hydrochloric acid (HCl, 37%) were supplied by Shanghai Lingfeng Chemical Reagent Co., Ltd. All chemicals were used as received without further purification unless otherwise stated. All solutions prepared during the experiment were made with deionized water (resistivity of approximately 18.2 MΩ cm).

Fabrication of CTA-3DPC

The preparation process of CTA-3DPC is illustrated in Figure 1. First, waste basswood blocks were placed in a tube furnace under nitrogen protection (with a constant N 2 flow rate of 100 mL min -1) and carbonized at 1,000 °C for 6 h to obtain charcoal blocks. Subsequently, the obtained charcoal pieces were immersed in a 6 M ZnCl 2 solution (mass ratio of charcoal to ZnCl 2 solution = 1:2) overnight, then taken out and dried in air at 100 °C for at least 12 h. Afterwards, the resulting product was activated in a tube furnace at 850 °C for 2 h under a nitrogen atmosphere to yield black activated carbon (AC) pieces. The obtained AC pieces were immersed in a 0.5 M HCl solution for cleaning, followed by repeated rinsing with boiling water until the rinsing solution became neutral, thus obtaining three-dimensional porous activated carbon (3DPC) samples. Next, 0.009 mol of CTAB was dissolved in 20 mL of absolute ethanol to form a clear and transparent solution, and the 3DPC samples were immersed in the aforementioned solution for 12 h. After being taken out, the samples were washed several times with distilled water and absolute ethanol in sequence, and finally dried at 120 °C for 12 h to obtain the final product, CTA-3DPC.

Materials characterization

The morphology of the samples was examined by scanning electron microscopy (FESEM, S-4800, Hitachi, Japan). X-ray diffraction (XRD, D/Max-2500) was used to characterize the crystal structure of the materials. The degree of graphitization was determined using a Raman spectrometer (LabRAM Aramis, Horiba Jobin Yvon S.A.S., France). Fourier transform infrared spectroscopy was performed on a Thermo Nicolet IS5 FT-IR spectrometer. The pore structure of the materials, including specific surface area and pore size distribution, was analyzed under liquid nitrogen conditions using a Brunauer–Emmett–Teller (BET) analyzer (Microtrac BEL Corp, Japan). Contact angle measurements were carried out using a contact angle goniometer (DSA30, KRUSS, Germany).

Electrochemical characterization

The electrochemical performance of the prepared electrode materials was characterized using an electrochemical workstation (CHI 660E, CH Instrument, Inc., Shanghai, China) in a 1.0 mol L -1 NaNO 3 solution. A three-electrode system was employed, consisting of an Ag/AgCl reference electrode, a Pt counter electrode, and the prepared material (3DPC or CTA-3DPC; dimensions: 1.0 cm × 1.0 cm × 0.1 cm, loading: 30.0 mg cm -2) as the working electrode. The tests conducted included cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). CV measurements were performed within a potential range of −0.1 V–0.9 V at various scan rates, while EIS was carried out over a frequency range from 100 kHz to 0.01 Hz with an amplitude perturbation of 5.0 mV.

The specific capacitance (C sp, F g ‒1) was derived from the CV data using Equation 1 (Wang et al., 2021): where IdV represents the integrated area of the CV curve; ΔV (V) and v (V s ‒1) denote the potential scanning window and scan rate, respectively; and m (g) corresponds to the mass of the active material in the working electrode.

The NO 3 − ion diffusion coefficient (D NO3‒, cm ‒2 s ‒1) can be determined based on EIS results using Equations 2,3, (Wang et al., 2021): where R and T are the ideal gas constant (8.314 J K −1 mol ‒1) and the absolute temperature (298 K), respectively; F is the Faraday constant (96,485 C mol -1); c is the mass density of F − within the electrode (mol cm ‒3); A denotes the surface area of the working electrode (cm 2); σ represents the Warburg factor (Ω s ‒0.5); Z re is the real part impedance (Ω); and ω is the frequency (s ‒1).

The potential of zero charge (E pzc) was determined via a differential capacitance method. The measurement was conducted in 5 mM NaNO 3, NaCl, Na 2 SO 4, and NaH 2 PO 4 solution at a frequency of 10 mHz, using a potential sweep from 40 to 240 mV with an AC amplitude of 10 mV.

Denitrification experiments

The denitrification performance of the prepared electrodes was evaluated using a CDI setup. The system was configured with 3DPC as the cathode (active material loading: 30.0 mg cm -2) and CTA-3DPC as the anode (active material loading: 30.0 mg cm -2). Denitrification tests were conducted in a batch-mode asymmetric CDI configuration: a 50 mL NaNO 3 solution was used to simulate aquaculture wastewater, with a controlled flow rate of 20 mL min -1 and a constant operating voltage of 1.2 V. The NO 3 − concentration in the simulated aquaculture water was set to 100 mg L -1. Effluent water samples were taken every 2 min and the NO 3 − concentration was measured using ion chromatography (IC, ICS-5000, DIONEX, United States). The electrosorption experiments for Cl −, SO 4 2‒, and H 2 PO 4 − solutions were conducted using the same method as for NO 3 −.

The performance indicators of the electrode during the tests were recorded and NO 3 − adsorption capacity (NAC, mg g ‒1) were subsequently calculated using Equation 4 (Wang et al., 2021): where C 0 (mg L ‒1) and C t (mg L ‒1) are the concentrations of NO 3 − at the operation time of 0 and t min, respectively; V (L) represents the total volume of NaNO 3 solution; m (g) is the mass of the active material in the working electrode.

Selective denitrification tests: 50 mL of binary mixed solutions of NO 3 − /Cl −, NO 3 − /SO 4 2‒, and NO 3 − /H 2 PO 4 − were prepared separately (with Cl −, SO 4 2‒, and H 2 PO 4 − concentrations all at 100 mg L -1, and NO 3 − concentration at 5 mg L -1). Denitrification experiments were conducted according to the CDI denitrification testing method. The concentrations of various ions in the solution before and after denitrification were measured using ion chromatograph, and the ion selectivity coefficient (Selectivity, S) was calculated using Equation 5.

Where C i0 (mg L ‒1) and C it (mg L ‒1) represent the concentrations of the target ion at the initial time (0 min) and at time (t min), respectively, while C j0 (mg L ‒1) and C jt (mg L ‒1) denote the concentrations of the competing ion at the initial time (0 min) and at time (t min), respectively.

Physicochemical characterizations of CTA-3DPC

Morphology and structure

The surface morphology of the prepared CTA-3DPC sample is shown in Figure 2. After activation with ZnCl 2, the CTA-3DPC surface retains a porous structure composed mainly of two types of macropores at the micrometer scale, with pore sizes ranging from 100 to 200 μm for larger pores and approximately 10 μm for smaller pores. This porous architecture facilitates solution mass transfer during the electrosorption process (Figure 2a). The cross-sectional morphology of CTA-3DPC further reveals the presence of vertically aligned and interconnected channels within the material. The pore walls are uniformly decorated with numerous pores across multiple scales. The presence of micropores and mesopores (collectively referred to as nanopores) is further confirmed by the N 2 adsorption-desorption isotherm analysis and NLDFT pore size distribution results, which reveal abundant pores in the 1–50 nm range. These nanopores connect the vertical channels and further enhance the mass transfer efficiency of the solution (Figures 2b,c). Elemental mapping via EDS demonstrates that nitrogen is homogeneously distributed over the 3DPC substrate (composed of carbon and oxygen elements), confirming the successful loading of CTAB onto the 3DPC surface (Figure 2d).

Furthermore, the chemical composition of the synthesized materials was analyzed using XRD and Raman spectroscopy. As shown in Supplementary Figure S1, the XRD patterns of 3DPC and CTA-3DPC exhibit only two broad diffraction peaks at 24.6° and 44.2°, corresponding to the (002) and (101) crystal planes of hexagonal graphite (JCPDS card no. 41–1,487) (Zhang et al., 2022), indicating that both 3DPC and CTA-3DPC possess an amorphous structure. As shown in Figure 3a, the Raman spectra of all samples display a D band at 1,346 cm ‒1 (associated with sp 2 -hybridized carbon atoms in amorphous carbon, representing disordered structural features) and a G band at 1,600 cm ‒1 (attributed to sp 3 -hybridized carbon atoms in graphene-like carbon, reflecting crystalline structural features) (Nie et al., 2021). A higher I D /I G ratio indicates greater surface defects and structural disorder in the material. The I D /I G ratio of CTA-3DPC is 1.097, which is higher than that of 3DPC (1.022), suggesting that CTA-3DPC possesses more surface defects and a more complex surface structure (Kim et al., 2017). The increase in surface defects of CTA-3DPC is beneficial for the formation of adsorption sites, thereby enhancing ion transport efficiency during the adsorption process.

Figure 3b shows the comparison of FT-IR spectra for the three samples. Compared to the unmodified 3DPC, CTA-3DPC exhibits significantly enhanced absorption peaks at 2,850 cm ‒1 and 2,922 cm ‒1, corresponding to the asymmetric and symmetric stretching vibrations of the -CH 2 - groups in the CTAB alkyl chain, respectively (Lou et al., 2024). The absorption peak at 1,470 cm ‒1 is attributed to the scissoring vibration of methylene groups. Additionally, two weak absorption peaks at 910 cm ‒1 and 960 cm ‒1 correspond to the stretching vibrations of C-N bonds, further confirming the presence of quaternary ammonium cations (Lou et al., 2024). At the same time, the broad O-H peak observed in 3DPC at around 1,103 cm ‒1 shows weakening and shifting, indicating electrostatic interactions and hydrogen bonding between the quaternary ammonium cations of CTAB and the oxygen-containing functional groups on the activated carbon surface (Deng et al., 2024; Miao et al., 2024). This spectral change is consistent with electrostatic interactions between the negatively charged oxygen groups on the carbon surface and the positively charged quaternary ammonium heads of CTAB. Such interfacial complexation perturbs the local vibrational environment of these functional groups, leading to peak broadening and intensity reduction. These results confirm that CTAB has been successfully anchored onto the wood-derived activated carbon surface, forming a composite structure with hydrophobic alkyl chains and a positively charged interface.

The pore structures of 3DPC and CTA-3DPC were characterized by nitrogen adsorption-desorption isotherm measurements. Both 3DPC and CTA-3DPC exhibit Type IV isotherms with distinct hysteresis loops, indicating the presence of abundant cage-like pore structures in both materials (Figure 3c) (Wu et al., 2020). Furthermore, micropores, mesopores, and macropores coexist in 3DPC and CTA-3DPC. Specifically, in the low relative pressure region (P/P 0 < 0.1), a sharp increase in nitrogen uptake is observed for both materials, confirming the existence of numerous micropores. In the intermediate relative pressure region (0.2 < P/P 0 < 0.9), the trend of the isotherm further supports the presence of mesopores. In the high relative pressure region (0.9 < P/P 0 < 1.0), a slight rise in the isotherm suggests the presence of a small amount of macropores (Kim et al., 2020). Thus, 3DPC and CTA-3DPC both possess hierarchical pore structures comprising micropores, mesopores, and macropores. The results obtained from the NLDFT analysis further confirm the coexistence of micropores, mesopores, and macropores in both materials. Among these, micropores and mesopores constitute the dominant pore framework, which provides high specific surface area and abundant adsorption sites, thereby enhancing the electric double-layer capacitance. At the same time, mesopores help reduce ion transport resistance and mitigate electric double-layer stacking. Macropores act as buffer zones for the solution, facilitating ion migration to the inner surface of the electrode material and improving ion transport efficiency (Xiao et al., 2025). According to the BET results, the specific surface area of CTA-3DPC (922.2 m 2 g -1) is slightly lower than that of 3DPC (1,233.5 m 2 g -1), which can be attributed to partial pore blockage caused by CTAB loading; however, the overall impact is limited. As shown in Supplementary Figure S2, the water contact angle of the CTA-3DPC is 92.13°, which is lower than that of the 3DPC (105.06°). This demonstrates that after CTAB loading, the hydrophilicity of the material is significantly improved, thereby accelerating ion diffusion rates.

Electrochemical characterization

Electrochemical characterization serves as an essential tool for evaluating the capacitive performance of the prepared materials and analyzing the ion transport process at the solution/electrode interface. As observed from the cyclic voltammetry (CV) curves (Figure 4a), both 3DPC and CTA-3DPC exhibit nearly rectangular shapes without distinct redox peaks, indicating typical electric double-layer capacitive behavior. The CV curve of CTA-3DPC encloses a significantly larger area compared to that of 3DPC, with a specific capacitance of 167.2 F g -1, substantially higher than that of 3DPC (69.46 F g -1). To further investigate the capacitive behavior, CV tests were conducted at various scan rates (2–100 mV s -1), allowing calculation of the specific capacitance at each scan rate (Figures 4b,c). The specific capacitance of both materials decreases with increasing scan rate, yet CTA-3DPC consistently shows higher specific capacitance across the entire scan rate range compared to 3DPC, a characteristic advantageous for CDI applications (Figure 4d).

Furthermore, electrochemical impedance spectroscopy was employed to investigate the resistance characteristics and charge transfer behavior of the tested samples (Figure 4e). From the EIS results, the bulk resistance (R s) can be derived from the high-frequency intercept of the curve with the real axis, and the charge transfer resistance (R ct) can be estimated from the diameter of the semicircle in the high-frequency region (Zhang S. et al., 2025). The calculated R s (3.12 Ω) and R ct (3.47 Ω) values for CTA-3DPC are higher than those for 3DPC (R s = 3.01Ω, R ct = 1.61Ω), indicating that the introduction of CTAB increases both the bulk resistance and the charge transfer resistance of the electrode material due to the insulating nature of CTAB. Using the low-frequency region of the Nyquist plots, the diffusion coefficient of NO 3 − ions (D NO3‒) for the electrodes was calculated. Despite its lower conductivity, CTA-3DPC exhibits a relatively high D NO3‒ value (2.32 × 10 –12 cm 2 s -1), suggesting that the reduced conductivity does not significantly hinder the diffusion process of NO 3 − ions (Wang et al., 2021). Although CTAB loading decreases the conductivity of CTA-3DPC, the abundant NO 3 − -specific active sites introduced on its surface enhance its adsorption capacity.

Denitrification performance and mechanism

Denitrification performance

To evaluate the CDI denitrification performance of the CTA-3DPC electrode, tests were conducted in solutions containing NO 3 −, Cl −, SO 4 2‒, and H 2 PO 4 − anions, each at a concentration of 100 mg L -1. As shown in Figure 5a, the NO 3 − adsorption capacity rapidly reached 31.75 mg g -1 within the first 50 min, and further increased to 40.67 mg g -1 after 70 min, achieving a high adsorption capacity. In contrast, the CTA-3DPC electrode exhibited limited adsorption performance toward the other anions, with the adsorption process showing a gradual increase and final capacities of 8.55 mg g -1 for Cl −, 18.1 mg g -1 for H 2 PO 4 −, and 23.07 mg g -1 for SO 4 2‒, all significantly lower than that for NO 3 −. Subsequently, the influence of applied voltage on the CDI desalination performance was systematically investigated. Figure 5b presents the CDI performance under different applied voltages, with the initial concentration maintained at 100 mg L -1. Increasing the applied voltage thickens the electrical double layer (EDL) and enhances the Coulombic interaction between salt ions and the electrode, thereby improving the electrosorption capacity (Xiao et al., 2025). Specifically, the NO 3 − adsorption capacities of the CTA-3DPC electrode at 1.0, 1.2, 1.4, 1.6, and 1.8 V were 36.51, 40.67, 48.4, 55.41, and 59.6 mg g -1, respectively. These values were substantially higher than those obtained for solutions containing other anions under the same applied voltages, reaffirming the superior performance of the CTA-3DPC electrode in CDI-based nitrate removal.

In practical aquaculture water, competing anions such as Cl −, SO 4 2‒, and H 2 PO 4 − will compete with NO 3 − for adsorption sites on the electrode. To further investigate the selectivity of 3DPC and CTA-3DPC for NO 3 − in the presence of competing ions, both electrode materials were applied for selective nitrate removal via CDI in binary mixed solutions of NO 3 − /SO 4 2‒, NO 3 − /H 2 PO 4 −, and NO 3 − /Cl − (with Cl −, SO 4 2‒, and H 2 PO 4 − concentrations fixed at 100 mg L -1, and NO 3 − concentration at 5 mg L -1). The removal performance of the different materials for NO 3 − was evaluated, as shown in Figure 5c. The results indicate that the 3DPC electrode exhibited selectivity coefficients for NO 3 − of 3 ± 0.13, 5 ± 0.15, and 4.2 ± 0.18 in the presence of Cl −, SO 4 2‒, and H 2 PO 4 −, respectively, demonstrating only marginal selectivity for NO 3 − under competitive conditions. In contrast, the selectivity coefficients of CTA-3DPC for NO 3 − were significantly higher, reaching 10 ± 0.15, 11 ± 0.31, and 13 ± 0.23, representing a 2–3 fold enhancement compared to 3DPC. This confirms the superior selectivity of CTA-3DPC for NO 3 − across different competing anions. The improved selectivity is attributed to the introduction of quaternary ammonium cations from the hydrophilic head of CTAB, which forms a dense and stable positively charged layer on the wood-derived activated carbon surface, thereby favoring the selective adsorption of NO 3 −.

Overall, the competitive order of anions in inhibiting NO 3 − removal was SO 4 2‒ > Cl − > H 2 PO 4 −. These observations can be explained by factors such as ionic valence, hydrated ionic radius, hydration energy, electronegativity, and the pore structure of the electrode. Compared to monovalent anions, divalent SO 4 2‒ experiences a stronger electric field force and exhibits greater electrophoretic mobility, leading to stronger competitiveness-consistent with previous reports of higher removal rates for divalent anions relative to monovalent ones. For ions with the same valence, a smaller hydrated ionic radius generally corresponds to lower electromigration resistance and easier adsorption onto the electrode. The hydrated ionic radii of Cl −, SO 4 2‒, and H 2 PO 4 − are Cl − (0.332 nm) < NO 3 − (0.335 nm) < H 2 PO 4 − (0.392 nm) (Gamaethiralalage et al., 2021). Given its smallest hydrated radius, Cl − experiences the lowest electromigration resistance, facilitating its adsorption onto the electrode and thus exerting stronger competitive adsorption against NO 3 −.

Figure 5d and Table 1 summarizes the reported selectivity coefficients of various electrode materials for NO 3 − in NO 3 − /Cl − binary solutions at a working voltage of 1.2 V. The CTA-3DPC electrode demonstrated the highest selectivity coefficient among all compared materials, highlighting its excellent NO 3 − selectivity in the presence of competing Cl − and verifying the practical feasibility of CTA-3DPC-based CDI for selective nitrate removal (Chen et al., 2022; Kim et al., 2019; Sun et al., 2018; Tsai et al., 2021; Wang et al., 2020; Hawks et al., 2019).

Denitrification mechanism

To gain a deeper understanding of the high-performance denitrification capability of CTA-3DPC, the kinetic behavior of NO 3 − intercalation and its capacitive contribution were analyzed through CV tests at various scan rates (Bi et al., 2025; Ardizzone et al., 1990). Consequently, we separately calculated the contributions from “non-diffusion-controlled (capacitive)” and “diffusion-controlled” processes to quantify the electrode’s response mechanisms. The classic Trasatti method was employed to quantitatively distinguish between diffusion-controlled and non-diffusion-controlled contributions (Fleischmann et al., 2020). Surface capacitance was obtained by extrapolating CV curves at high scan rates, while total capacitance was derived similarly at low scan rates (Figures 6a,b). The non-diffusion-controlled contribution ratios for CTA-3DPC and 3DPC were 89% and 68%, respectively (Figure 6c). This clearly demonstrates that the synergistic effect of quaternary ammonium cations and the three-dimensional porous structure in CTA-3DPC enhances the pseudocapacitive (non-diffusion-limited) contribution, facilitating rapid and reversible NO 3 − ion reaction kinetics, thereby ensuring its high-performance denitrification capability.

CDI is an interfacial process where the E pzc plays a crucial role, significantly affecting the desalination performance and cycling stability of the material. Noticeable ion adsorption on the electrode surface occurs only when the applied potential deviates from the E pzc. Specifically, in the absence of an applied potential, the intrinsic surface potential of the electrode spontaneously adsorbs ions from the solution. When the applied potential equals the E pzc, the counter-charge on the electrode surface drives ions to desorb, resulting in a desorption effect (Duan et al., 2022). The E pzc values of CTA-3DPC in the presence of Cl −, SO 4 2‒, H 2 PO 4 −, and NO 3 − are 0.13 V, 0.27 V, −0.1 V, and −0.15 V, respectively, all lower than those of 3DPC. This is attributed to the introduction of CTAB, which incorporates positively charged functional groups onto the material surface, thereby lowering the potential of zero charge (Figure 6d). NO 3 − ion, due to its low hydration energy and anisotropic solvation effects, can enter the pores at a relatively low potential (−0.15 V), resulting in the lowest E pzc. Cl − ion, with its high hydration energy and weaker isotropic solvation, requires stronger electrode–ion interactions, leading to a higher E pzc of 0.13 V SO 4 2‒ ion, with the highest hydration energy and larger hydrated diameter, requires substantial dehydration to access the pore structure, corresponding to the highest surface charge density and thus the highest E pzc (0.27 V) (Wu et al., 2016; Wang et al., 2023). This feature favors the selective adsorption of NO 3 − in the presence of competing ions.

To address the critical challenge of poor selectivity in nitrate removal from industrial recirculating aquaculture wastewater, this study successfully developed a CTA-3DPC composites as a high-performance capacitive deionization electrode. The material possesses a vertically aligned hierarchical porous structure, significantly enhancing ion mass transfer during the electrosorption process. When used as the anode, CTA-3DPC exhibits exceptional adsorption capacity for NO 3 −, with a capacity of 40.67 mg g -1, substantially higher than those for Cl − (8.56 mg g -1), H 2 PO 4 − (18.1 mg g -1), and SO 4 2‒ (23.07 mg g -1). Notably, it achieves a high NO 3 − /Cl − selectivity coefficient of 13.23 in binary systems, ranking among the highest values reported to date. Mechanistic studies reveal that CTAB forms a dense positively charged layer on the carbon surface via electrostatic interactions, lowering the potential of zero charge. The synergy between this modified interface and the planar structure as well as weak solvation characteristics of NO 3 − is key to the high selectivity. This work provides a novel strategy for designing cost-effective and efficient biomass-based nitrate-selective electrodes, demonstrating promising potential for precise denitrification in aquaculture wastewater treatment.