1. Introduction
Discharges of organic wastewater containing ammonia (NH
3) and dissolved organic matter (DOC) can cause the deterioration of water bodies [[
1], [
2], [
3]], which results in water eutrophication and the death of aquatic organisms [
4,
5]. Traditional water-treatment methods for denitrification and decarbonization are commonly used in municipal wastewater treatment systems, however, they have limitations in carbon source supplementation and nitrogen removal efficiency [[
6], [
7], [
8]]. Consequently, there is a compelling requirement to develop highly efficient and easy-to-operate methods to achieve simultaneous denitrification and decarbonization of wastewater.
Electrochemical technologies are environmentally friendly and low-pollution water treatment methods [
9,
10]. The electrochemical oxidation of ammonia can be divided into direct and indirect processes [
11]. During direct oxidation, ammonia adsorbed on the anode surface is converted to nitrogen (N
2) through electron transfer [
12]. During indirect oxidation, reactive oxygen species (ROS) are produced on the electrode surface and the absorbed ammonia is oxidized to nitrogen [
13,
14]. Moreover, chloride ions in the effluent can be oxidized to form reactive chlorine species (RCS), which enhance ammonia oxidation and accelerate nitrogen removal [
15,
16]. During the RCS synthesis process, various chlorine radicals and reactive species such as hypochlorous acid (HOCl), chlorine free radicals (·Cl), and chlorine oxygen free radicals (·ClO) are generated near the anode [
17,
18]. Because of the high redox potential of ·ClO, it reacts rapidly with electron-rich compounds [
19,
20], and its reaction rate with NH
3 is two to three orders of magnitude higher than that of hydroxyl radical (·OH) and ·Cl, respectively [
21]. In the region near the anode, ·ClO is usually produced during the scavenging of ·OH by HClO or free chlorine (ClO
−) [
18,
22]. Therefore, controlling the conversion of ·OH and ·Cl to ·ClO by regulating the active sites of the catalyst is crucial for achieving efficient denitrification.
Furthermore, although extensive research has been performed on the application of electrochemical ammonia oxidation (EAOR) technology for ammonia-nitrogen removal in water treatment, challenges remain in extending its use to the entire wastewater treatment industry [
23]. The lifespan of the free radicals formed on the electrode surface is short, and is limited to a thin layer less than 1 μm from the anode surface [
24]. Therefore, it is essential to design advanced low-pollution surfaces to ensure high reaction kinetics, particularly for the specific
in situ generation of the target radical ·ClO [
25]. Augmentation of electrocatalytic potency necessitates chemical reagent circumvention, which concomitantly truncates hydraulic retention duration, mitigates intra-reactor transport constraints, and elevates Faradaic yield coefficients within the electron-transfer regime [
26]. Incorporating catalysts into highly electro-reactive membranes (EMs) used in flow-through operations could potentially address the challenges related to limited reaction rates and N
2 selectivity when treating NH
3-laden wastewater [
27]. Porous membranes have proven to be ideal electrode substrates for enhancing the electrocatalytic performance [
28,
29]. Continuous flow through EMs with high electrode surface areas can enhance mass transport and maximize the catalyst utilization efficiency [
30]. These porous substrates spatially confine the reactants and electrify the active sites at the micrometer and nanometer scales. The efficiency of the spatial generation of ·ClO can also be improved owing to the spatial confinement of the membrane pores and channels, which facilitates faster rates of
in situ radical generation within the membrane pores [
31]. Considering their dual selectivity and solute separation functionality, EMs can be easily integrated into existing systems, such as electrochemical oxidation systems and capacitive deionization (CDI), or used as on-the-spot devices for water treatment [
15,
32]. However, few examples of EMs for simultaneous denitrification and decarbonization of wastewater have been successfully introduced.
In this study, an electrocatalytic membrane for ultraefficient ·ClO production is proposed. The electro-reactive membrane (RuO2@PbO2-M) features electrochemically reactive surfaces formed by reverse electrodepositing β-lead dioxide (β-PbO2) and coating the ruthenium dioxide (RuO2) supports. The efficacy of in situ generated ·ClO is evaluated for simultaneous denitrification and decarbonization during elector-filtration using acetaminophen (APAP) as a simulated contaminant. The rapid conversion mechanism of EAOR is further investigated. The co-existence of ·ClO and its precursors was confirmed, and their spatiotemporal distribution was investigated throughout the RuO2@PbO2-M during the electro-filtration. The treatment mechanism of RuO2@PbO2-M was investigated for the efficient purification of organic wastewater containing NH3 using an electro-filtration system. This study provides a new research approach for the catalytic oxidation of an electrode to treat organic ammonia wastewater.
2. Material and methods
2.1. Materials
Analytical-grade sodium sulfate (Na2SO4), ammonium sulfate ((NH4)2SO4), sodium chloride (NaCl), sodium fluoride (NaF), lead nitrate (Pb(NO3)2), nitric acid (HNO3), and ruthenium(III) chloride (RuCl3) were purchased from Sinopharm Chemical Reagent Co., Ltd., China. Nitrobenzene (NB), tert-butanol (TBA), and sodium bicarbonate (NaHCO3) were purchased from Aladdin Industrial Corporation (China). All the solutions were prepared using deionized water.
2.2. Preparation of the RuO2@PbO2-M
Commercial 300 mesh titanium powder (purity > 99.5%) was evenly distributed in a crucible and placed in a vacuum drying oven at 60 oC for 2 h. After drying, the titanium (Ti) powder was weighed and spread in a mold with a diameter of 47 nm, at a pressing pressure of 35 bar (1 bar = 105 Pa) for a pressing time of 15 min. It was then calcined at high temperature (550 oC) for 2 h under an inert argon atmosphere to obtain Ti-membrane (M). The Ti-M was placed in an acidic solution (0.1 mol·L−1 HNO3, 0.5 mol·L−1 Pb(NO3)2, and 0.04 mol·L−1 NaF). A layer of β-PbO2 was electro-deposited on the surface of Ti-M by applying a current density of 60 mA·cm−2 for 20 min, followed by 30 mA·cm−2 for 40 min, the product was named lead dioxide membrane (PbO2-M). Span 80 and Tween 80 were mixed in a mass ratio of 8:2, and organic supports with a carbon framework were prepared through pyrolysis under nitrogen protection. The RuCl3 gel was mixed with the organic supports and coated onto PbO2-M using a brushing method, and the carbon framework was removed by high-temperature (muffle furnace calcination, hold 1 h) calcination to produce the final membrane, RuO2@PbO2-M.
2.3. Analytical methods
The phase and crystallinity were measured using X-ray diffraction (XRD; D/max 200 PC, Rigaku, Japan). The morphologies of the prepared electrodes were characterized by scanning electron microscopy (SEM; Hitachi SU8010, Hitachi, Japan) and energy-dispersive X-ray spectroscopy (EDS; Hitachi SU8010). Valence band characteristics were interrogated via high-resolution monochromatic X-ray photoelectron spectrometry (Kratos AXIS UltraDLD instrumentation, Japan) for quantitative compositional profiling at near-surface domains.
Free chlorine concentrations in the system were determined using the
N,
N-diethyl-
p-phenylenediamine (DPD) method [
33]. Nitrate (NO
3−) and nitrite (NO
2−) ions were analyzed using ion chromatography (Dionex, USA), and NH
3 was detected with Nessler reagent via a UV-visible spectrophotometer set to 420 nm [
34]. The electrochemical behavior of RuO
2@PbO
2-M was tesetted using linear sweep voltammetry (LSV). Total nitrogen (TN) removal levels were measured with a multi N/C 3100 TOC/TN analyzer (Analytik Jena, Germany). NB, TBA, and NaHCO
3 were measured using high-performance liquid chromatography (HPLC; LC-20AT, Shimadzu, Japan) wih a C18 column. Detection wavelengths were set at 266 nm for NB, 227 nm for TBA, and 230 nm for NaHCO
3 [
35].
2.4. Evaluation of denitrification and decarbonization performance of RuO2@PbO2-M
The denitrification and decarbonization performance of RuO2@PbO2-M was evaluated using (NH4)2SO4 and APAP as model pollutants in a laboratory-scale dead-end filtration setup. A round membrane coupon with an effective area of 17.34 cm2 was prepared. Next, a terminal filtration tank was used with deionized water passing through for 30 min at a residence time of 1.12 min to achieve a stable permeate flux.
2.5. Determination of the concentration of radical species
NB, TBA, and NaHCO3 were used as the probes to quantify the steady-state concentrations of ·OH, ·Cl, and ·ClO in the RuO2@PbO2-M elector-filtration system.
3. Results and Discussion
3.1. Synthesis and characterization of RuO2@PbO2-M nanostructures on Ti-M
Nanostructured PbO
2 and RuO
2 supports was synthesized on Ti-M to enhance the electrocatalytic activity and electrode stability of EM, PbO
2-M as illustrated in
Fig. 1(a). Next, the RuO
2 supports were seeded onto the as-built PbO
2-M scaffold by templating to produce RuO
2@PbO
2-M (
Fig. 1(a)).
The surface morphology and structure of the electrodes were characterized using scanning electron microscopy. The result showed that the surface of the RuO
2@PbO
2-M gel coating was uniform and dense without discernible boundaries between the PbO
2 grains. EDS mapping was used to evaluate the elemental distribution patterns of the EMs. As illustrated in
Fig. 1(b), the functionalization of PbO
2 and RuO
2 resulted in a uniform distribution of Ti (blue) along with Pb (green) and Ru (purple), thereby confirming the effective dispersion of RuO
2 and PbO
2 on the RuO
2@PbO
2-M composite. The XRD peaks of Ti-M were ascribed only to titanium anatase (JCPDS NO. 73-0851).
Fig. 1(c) displays the XRD patterns of RuO
2@PbO
2-M, with diffraction peaks at 25.4°, 36.2°, and 49.0° corresponding to the (1 1 0), (1 0 1), and (2 1 1) planes of the β-PbO
2 phase (JCPDS NO. 42-1121), respectively. It has been reported that β-PbO
2 is primarily electrodeposited in acidic electrolytes containing fluorides [
36]. Furthermore, the presence of a RuO
2 diffraction peak at 53.3° for RuO
2@PbO
2-M was confirmed by comparison with a reference standard (JCPDS NO. 04-1209).
This series of EMs designs provides a unique research object for studying a relatively constant surface-to-volume ratio (SVR) while significantly altering the cross-sectional dimensions of the EMs flow channels. The preparation of the PbO
2 and RuO
2 functional layers caused a reduction in the pore size of the EMs, which consequently resulted in a decrease in the porosity (
Fig. 1(d)). Contrariwise, RuO
2@PbO
2-M nanocomposites featuring reduced mesoporous dimensions demand substantially augmented hydraulic potential gradients—quantified via the viscous flux dependence principle (Hagen-Poiseuille formalism)—when benchmarked against macroporous Ti-M substrates in isoperistaltic filtration paradigms (
Fig. 1(e)).
Fig. 1(f) shows the chemical composition of the RuO
2@PbO
2-M determined by XPS. Analysis was performed to assess the surface chemical composition and valence states of the RuO
2@PbO
2-M electrode. The O 1s spectrum exhibited an asymmetric peak, deconvoluted into two components at binding energies of 531.0 and 529.2 eV, corresponding to O-Pb and O-Ru interactions, respectively [
37,
38]. Characteristic peaks at 137.6 and 142.5 eV were identified for the Pb 4f
7/2 and Pb 4f
5/2 orbitals, which indicates the presence of Pb
2+. Additionally, Ru 3d exhibited peaks at 280.6 and 284.8 eV, corresponding to 3d
5/2 and 3d
3/2 of Ru
4+. The presence of O-Ru bonds suggests surface oxidation [
39], which explains the absence of Ru peaks in the XRD patterns of Ti-M. The synthesis and characterization of RuO
2@PbO
2-M demonstrated its improved electrocatalytic activity and stability.
3.2. Enhanced electrochemical performance of the RuO2@PbO2-M electrodes
In this study, the electrochemical behavior of the Ti-M, PbO
2-M, and RuO
2@PbO
2-M electrodes was investigated using a standard three-electrode electrochemical cell, as shown in
Fig. 2(a). The measured oxygen evolution potentials (OEP) of Ti-M, PbO
2-M, and RuO
2@PbO
2-M were 1.64, 1.65, and 1.76 eV, respectively. Notably, the incorporation of Ru significantly enhanced the OEP of the electrodes [
40]. Elevated OEPs are beneficial because they inhibit undesired oxygen evolution reactions, facilitate radical formation, and improve electrocatalytic oxidation efficiency, thereby contributing to reduced energy consumption [
41].
Fig. 2(b) shows the Nyquist plots for the different electrodes, with detailed simulation results provided in the Table S1 in Appendix A. The inset shows the equivalent-circuit model, Within the constructed equivalent circuit topology,
Rs explicitly quantifies bulk electrolytic ohmic dissipation,
Qdl and
Rct jointly epitomize Helmholtz-plane stored charge density and activation-controlled charge traversal barrier, respectively, while the
Qads and
Rads coupling models sorptive phase accumulation dynamics and metastable intermediate species transport impedance at electrochemical interphases, respectively. The analysis reveals that the first high-frequency semicircle is consistently smaller than the second low-frequency semicircle for Ti-M, PbO
2-M, and RuO
2@PbO
2-M, which indicates that the kinetics of these electrodes are primarily governed by the slower adsorption and desorption processes of intermediates (that is,
Rct << Rads). Furthermore, PbO
2-M exhibited a smaller high-frequency semicircle than Ti-M, suggesting that RuO
2@PbO
2-M exhibited an accelerated charge-transfer rate [
42]. A corresponding trend was observed for
Rads, whereas
Qads exhibited an inverse relationship. A lower
Rads and higher
Qads typically signify the generation of more radicals at the electrode surface, which is advantageous for the electrocatalytic oxidation of pollutants [
43]. Furthermore, the pronounced variation in
Rads relative to
Rct highlights the critical role of RuO
2 modified PbO
2-M in regulating the surface structural characteristics.
Figs. 2(c) and
(d) present the Tafel slopes and electrochemically active surface area (ECSA) curves, respectively, demonstrating that RuO
2@PbO
2-M possesses a greater number of active sites and enhanced electrocatalytic activity [
44].
3.3. Enhanced ammonia oxidation performance of electro-filtration systems using RuO2@PbO2-M anodes
In this study, the ammonia oxidation performances of electro-filtration systems was investigated using various electrode materials: Ti-M, PbO2-M, and RuO2@PbO2-M. A concentration of 100 mg·L−1 (NH4)2SO4 was selected as the simulated pollutant for performance evaluation. Notably, a decrease in the flow rate increased the time spent within the pores, analogous to the extension of the reaction time in a batch reactor.
As illustrated in
Fig. 3(a), significant decomposition of NH
3-N occurred within a residence time (
tR) of 1 min. Control experiments indicated that Ti-M initially removed only a small portion of NH
3-N, likely owing to physical adsorption during the electro-filtration process.
Fig. 3(b) shows that the NH
3-N degradation kinetics induced by 0.25RuO
2@PbO
2-M were significantly faster than those induced by Ti-M and PbO
2-M. The rate constant for the probe reaction in the EMs with the largest pore size, Ti-M, was measured at 0.25 s
−1 (
Fig. 3(b)). In contrast, when the RuO
2 concentration was 0.25 wt% (that is, 0.25RuO
2@PbO
2-M), the rate constant significantly increased to 2.7.0 s
−1, manifesting an undecimal throughput augmentation factor relative to batch processing kinetics. The half-life (
τH) of the reaction of the electro-filtration system is inversely proportional to
Kobs, and decreased from approximately 120 s for Ti-M to 36 s for PbO
2-M, and down to approximately 14 s for 0.25RuO
2@PbO
2-M. Notably, even when these rate constants were normalized using the SVR, the kinetic enhancement remained substantial.
The Sankey diagram in
Fig. 3(c) illustrates the mass flow of ammonia oxidation through different EMs, and clearly depicts the conversion pathways of NH
3-N. This is consistent with the results shown in
Fig. 3(a), which indicate that 0.25RuO
2@PbO
2-M exhibited both high NH
3-N removal efficiency and effective N
2 selectivity, while producing a lower yield of NO
3−-N. Additionally, statistical calculations of energy consumption (EnC) and average current efficiency (ACE) was performed for ammonia removal using composite membranes I-V, including Ti-M, PbO
2-M, 0.2RuO
2@PbO
2-M, 0.25RuO
2@PbO
2-M, and 0.3RuO
2@PbO
2-M. These findings suggest that 0.25RuO
2@PbO
2-M offers significant advantages as an electro-filtration anode, characterized by a high current efficiency and low energy consumption (
Fig. 3(d)).
3.4. Experiment on factors influencing optimization under electro-filtration conditions
This study investigated the impacts of various operational parameters on the nitrogen removal performance of a catalyst 0.25RuO2@PbO2-M, utilizing a controlled variable approach. Key parameters examined include current density, pH, chloride ion (Cl−) concentration, and initial ammonia concentration. Single electro-filtration mode was employed with 0.25RuO2@PbO2-M to maintain a membrane pore retention time of 1.12 min. Continuous electro-filtration was performed for 1 h to assess the efficiency of ammonia removal under the specified conditions.
As shown in
Fig. 4(a), an increase in the current density enhanced the removal of NH
3-N. At higher current densities, more active chlorine, ·Cl, and ·OH were generated, which significantly boosted the removal rate and denitrification efficiency of NH
3-N. Conversely, lower current densities resulted in reduced generation of ·OH and HClO at the electrode surface. However, excessively high current densities can cause intensified side reactions, such as oxygen evolution, which oxidizes ·OH to H
2O and decrease the concentration of ·ClO radicals in the solution. This reduction hampers denitrification [
45,
46]. Additionally, electro-filtration degradation experiments were performed with initial NH
3-N concentrations ranging from 0 to 300 mg·L
-1 (
Fig. 4(b)). It was determined that the NH
3-N initial concentration is positively correlated with the degradation rate.
When the chloride ion concentration was 0 mmol·L
-1, the solution lacked chloride ions, resulting in a NH
3-N removal rate of only 12.6% within 60 min (
Fig. 4(c)). It is believed that ·OH is the primary active species generated by the electrode, which catalytically oxidizes NH
3-N in the solution to NO
3-. However, as the chloride ion concentration increased from 50 to 300 mmol·L
−1, the removal rate of NH
3-N dramatically increased from 12.6% to 99.7%. This indicated that the presence of chloride ions significantly enhanced the removal of NH
3-N in the RuO
2@PbO
2-M system. Chloride ions quickly ionize near the anode, producing active chlorine, which facilitates the degradation of NH
3-N because of the rapid increase in the active species. As the amount of active chlorine in the solution increased, the concentrations of OCl
- and ·Cl also increased. However, when the concentration reached a certain value, the ·Cl concentration gradually decreased. As ·OH reacts with ·Cl, the concentration of ·Cl decreased while that of ·ClO increased. This is because, in the presence of ·OH, the synergistic oxidation of active oxygen species groups generates ·ClO at a much faster rate than the reaction of ·Cl with active chlorine to form ·ClO. Therefore, the RuO
2@PbO
2-M electrode system exhibited enhanced oxidative activity.
From the changing pattern of the data in
Fig. 4(d), it was observed that under acidic conditions, the increased concentration of H
+ in the solution promoted the reduction of H
+ to generate hydrogen gas (H
2), thereby accelerating the rates of electron and mass transfer [
47]. In contrast, under strongly alkaline conditions, the denitrification efficiency decreased from 96.3% at pH 3 to 3.9% at pH 9. This decline may be attributed to corrosion of the 0.25RuO
2@PbO
2-M electrode surface by the highly alkaline solution, which diminishes the oxidative capability of the electrode. In summary, the optimal operating conditions were a current density of 20 mA·cm
-2, chloride ion concentration of 100 mg·L
-1, acidic pH, and retention time of 1.12 min, which resulted in an NH
3-N removal efficiency of approximately 99.7%.
3.5. Role of the reactive radicals in NH3-N removal during electro-filtration
To elucidate the roles of ·OH, ·Cl, and ·ClO in the effective removal of NH
3-N during the electro-filtration process utilizing RuO
2@PbO
2-M, quenching experiments were performed with specific scavengers, as shown in
Fig. 5(a). NB, TBA, and NaHCO
3 served as specific scavengers of ·OH, ·Cl, and ·ClO, respectively [
48,
49]. Notably, the efficient quenching observed with NaHCO
3 underscores the predominant role of ·ClO in NH
3-N removal. In contrast, NB only inhibited NH
3-N conversion by 23%, indicating that ·OH is not the principal radical facilitating the direct transformation of NH
3 to N
2.
Furthermore, electron paramagnetic resonance (EPR) spectroscopy was employed using 5,5-dimethyl-1-pyrroline
N-oxide (DMPO) as a spin-trap reagent to identify the oxidative radicals involved (
Fig. 5(b)). Signals corresponding to DMPO-·OH confirmed the generation of ·OH, while DMPO-·ClO signal and additional low-intensity peaks suggested the presence of ·Cl [
50]. The intensities of ·OH and ·ClO signals in 0.25 RuO
2@PbO
2-M system were significantly higher than those in the PbO
2-M system, which indicates the crucial role of ·ClO radical-driven oxidation in RuO
2@PbO
2-M. To further investigate the involvement of ·OH in ·ClO generation, the degradation of NH
3-N by HClO in the presence and absence of the PbO
2-M photoanode was compared. The results demonstrated a significant increase in degradation rates when PbO
2-M was employed, suggesting that HClO is converted to the more reactive ·ClO.
As illustrated in
Fig. 5(c), the oxidation degradation of NH
3-N at the RuO
2@PbO
2-M electrode inhibited chlorite oxidation, thereby reducing the active chlorine consumption. Consequently, the concentration of active chlorine rises over time, which enhances ·OCl generation. The spatial distribution of ·OH indicates that elevated ·OH concentrations are advantageous for pollutant degradation. Under indirect oxidation conditions with added chloride ions, RuO
2@PbO
2-M electrodes facilitate the rapid reaction between ClO
- with ·OH and ·Cl, which promotes the formation ·ClO radicals. However, as ·ClO generation increases, a portion is subsequently consumed.
3.6. Simultaneous denitrification and decarbonization of amino-containing pollutants in an electrified flow-through membrane system
APAP was selected as the model for assessing the simultaneous denitrification and decarbonization capabilities of RuO
2@PbO
2-M through
in situ ·ClO generation within an electrified flow-through membrane filtration system, as illustrated in
Fig. 6(a). Manifests 1.12-minute void-phase confinement duration per unidirectionality cycle, volumetrically mapped through orthogonal permeation vector partitioning in membranous tortuosity domains. The interwoven architecture of RuO
2@PbO
2-M, synthesized using a templating method, effectively immobilized the Ru and Pb catalysts within the Ti-M framework. This design promoted the simultaneous exposure of active sites while maintaining the structural integrity and conductivity (
Fig. 6(b), left). This unique configuration enhances the oxidative activity towards NH
3-N and chemical oxygen demand (COD) during the electro-filtration process, leveraging convection-enhanced mass transfer. The convective movement of water through the EMs reduces the diffusion boundary layer to a scale comparable to the pore radius, which significantly improves both the mass transport rates and the Faradaic efficiency of electrocatalytic oxidation.
Fig. 6(b), right illustrates two key steps in simultaneous denitrification and decarbonization, highlighting that ·ClO formation serves as the rate-determining step (orange box). The proposed pathway for concurrent nitrogen and carbon removal is depicted in the gray box in
Fig. 6(b). Ruthenium is widely recognized for its catalytic efficacy in the electrochemical reduction of NH
3 to N
2 because of its high activity in the initial charge-transfer step of the rate-determining reaction; however, it does not mineralize organic pollutants. This study demonstrates the structural advantages and feasibility of achieving efficient selective oxidation along with simultaneous nitrogen and carbon removal by circulating electro-filtration under chlorination conditions.
The efficacy of RuO
2@PbO
2-M for the removal of four prevalent amine contaminants, lysine (LYS), anamorelin (ANI), APAP, and tetracycline (TC), from industrial and pharmaceutical wastewater was investigated (
Fig. 6(c)). APAP, a widely used antipyretic and analgesic, is an emerging phenolic pollutant in aquatic environments that poses significant risks to liver, metabolic, and neurological health upon human exposure [
51]. Organic pollutants containing amino groups with different carbon-to-nitrogen (C/N) ratios was selected. Notably, the ammonia-nitrogen removal rate decreased as the C/N ratio increased, which highlights the complex interactions in the treatment process. ·OH exhibited a preferential reactivity towards organic carbon over denitrification, with C-C single bonds degrading more readily than benzene rings, thereby rendering polycyclic organic pollutants more susceptible to degradation. Specifically, the removal rate of NH
3-N from total carbon was 20%, whereas the COD removal efficiency reached 80%. This indicates that ·ClO primarily targets carbon during simultaneous denitrification and decarbonization, with reactive species subsequently engaging NH
3-N, contingent on sufficient ·ClO concentrations.
Five comparative experiments were performed to evaluate the NH
3-N oxidation and APAP removal efficiencies of different EMs under controlled conditions. As shown in
Fig. 6(d), a clear relationship exists between the EMs and the residual NH
3-N concentrations in the permeate after 1 h. Notably, at the RuO
2 doping ratio of 0.25 wt%, the selectivity for N
2 increased (
Fig. 6(d), right). The removal efficiencies for APAP was ranked as follows: 0.25RuO
2@PbO
2-M > 0.3RuO
2@PbO
2-M > 0.2RuO
2@PbO
2-M > PbO
2-M > Ti-M. These variations reflect the distinct reactive species produced by each EMs, with the synergistic effects of oxidizing agents, such as ·OH and ·Cl, playing a crucial role in NH
3-N removal. In electro-filtration systems dominated by ·OH, the concurrent presence of ammonia nitrogen and organic compounds favors the oxidation of the latter, inhibiting NH
3-N removal. Conversely, in the presence of chloride ions, ·Cl effectively targets the nitrogen atoms of NH
3-N, enhancing its removal through chlorine-mediated reactions [
51].
Liquid chromatography-mass spectrometry (LC-MS; Waters e2695, Walters, USA) was employed to identify the intermediates formed during APAP degradation and elucidate the potential degradation pathways. NaCl served as the electrolyte, and ·OH were identified as the primary active species. Theoretical calculations indicated that the carbon sites on the aromatic ring, particularly C6 and C9, as well as the N4 position, exhibit significant susceptibility to attack by reactive species (Fig. S1 in Appendix A). APAP mineralization initiates via radical adduction trajectories targeting the biphenylic linkage (C5-C10) or heterocyclic N4 nucleus, precipitating three divergent pathways: peri-carbon oxygenation at electrophic loci (C5/C10), azotic center nucleophilic displacement, and acetyl fission as corroborated computationally (Fig. S1). These initial reactions yield various intermediates, including p-aminophenol and hydroquinone, as well as their hydroxylated derivatives. This degradation profile aligns with the findings of other advanced oxidation processes (AOPs). The intermediates identified by LC-MS analysis (M1-M11) are listed in Table S2 in Appendix A. The proposed degradation pathways are shown in
Fig. 6(e).
An intermediate with a mass-to-charge ratio (
m/
z) of 300 was also identified, which was hypothesized to be an APAP dimer (M2) [
52]. This dimer may undergo transformations such as deacetylation, hydroxylation, and hydroxyl substitution, resulting in products M4 through M5. Alternatively, degradation may occur through ring-opening reactions, yielding simpler intermediates characterized by a single aromatic structure (e.g., M6). Ultimately, these intermediates may be further oxidized into low-molecular-weight organic compounds (e.g., M9) and completely mineralized CO
2, H
2O, and inorganic nitrogenous ions (NH
3 and NO
3-). Alternatively, M6 can yield M7 and M10 through chlorination and oxidation reactions, during which ammonia is oxidized to N
2 [
53]. AOPs and EAOR are recognized as effective technologies for converting bio-refractory organic compounds, such as APAP, into benign inorganic end products.
3.7. Advanced electro-filtration system for high-concentration ammonia nitrogen wastewater treatment.
Fig. 7 illustrates the treatment process for high-concentration NH
3-N wastewater. In this system, industrial raw water is introduced into an electro-filtration unit through mixing and primary settling tanks, utilizing pumps for efficient transfer. The tertiary electro-filtration system employs a coupling of point addition and filtration to ensure that the treated effluent meets established quality standards prior to discharge. The effluent was categorized into three stages: 1st, 2nd, and 3rd. After 36 h of continuous operation, RuO
2@PbO
2-M demonstrated low energy consumption and high current density (
Fig. 7(e), with an average EnC value of approximately 0.06 kWh·m
−3, corresponding to an operational cost of approximately 0.00435 CNY·t
−1), which results in significantly reduced operational costs compared to the current benchmarks, both domestically and internationally. Metal-ion leaching experiments performed at various filtration stages (
Fig. 7(c)) indicated that increased impact forces enhanced water flow across the membrane, which caused elevated Pb
2+ leaching from the electrode material. However, the system stability over time correlated with a reduction in metal leaching. After 84 h of operation, the COD of the effluent decreased from 1570 to 225 mg·L
-1, while the ammonia nitrogen concentrations reduced from 503.50 to 4.84 mg·L
-1 through secondary electro-filtration, both achieving composite discharge standards. In summary, the system effectively reduced NH
3-N and COD, ensuring efficient compliance with the discharge standards.
4. Conclusions
This study presents the synthesis of RuO2@PbO2-M, a novel electrocatalyst that facilitates the ultra-efficient production of ·ClO, thereby significantly enhancing Faradic efficiency and electro-catalytic performance compared to traditional materials. The simultaneous processes of denitrification and decarbonization was investigated using a single-pass electro-filtration system to elucidate both the feasibility and underlying mechanisms of these processes. The experimental results demonstrated that with a retention time of merely 1.2 min, the removal rates for NH3-N reached 100%. Furthermore, treating actual high-ammonia wastewater yielded a COD removal rate of 68% and a TN removal rate of 99.6% over a continuous 70 h operational period. The role of ·ClO in ammonia removal was also investigated, which provided critical insights that could guide future research on electrocatalytic oxidation methodologies. The implications of this research are significant because it proposes a holistic approach to wastewater management that effectively targets ammonia and organic carbon pollutants. This study advances the field of sustainable water treatment technologies and highlights the potential of our electro-filtration system to contribute significantly to environmental engineering practices.
CRediT authorship contribution statement
Bin Zhao: Writing - original draft, Investigation, Formal analysis, Data curation. Jialin Yang: Writing - original draft, Formal analysis, Data curation. Ruiping Liu: Validation, Supervision, Funding acquisition. Jiuhui Qu: Validation, Supervision. Meng Sun: Validation, Supervision, Investigation, Funding acquisition, Conceptualization, Writing - review & editing, Writing - original draft.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (52270043), the National Key Research and Development Program of China (2023YFE0113800 and 2024YFC3715000), and the Natural Science Foundation of Beijing Municipality (8242030).
Appendix A. Supplementary material
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.eng.2025.07.016.
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