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Article

Synergistic Photoelectrocatalytic Degradation of Tetracycline Using Phosphate-Grafted Mo:BiVO4 Photoanode Coupled with Pd/CMK-3 Cathode for Dual-Functional Activation of Water and Molecular Oxygen

by
Minglei Yang
†,
Zhenhong Xu
†,
Chongjun Tang
,
Shuaijie Wang
,
Zhourong Xiao
and
Fei Ye
*
Hebei Key Laboratory of Heavy Metal Deep-Remediation in Water and Resource Reuse, School of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Coatings 2025, 15(9), 1027; https://doi.org/10.3390/coatings15091027
Submission received: 26 July 2025 / Revised: 24 August 2025 / Accepted: 31 August 2025 / Published: 2 September 2025
(This article belongs to the Special Issue Recent Advances in Surface and Interface Engineering for Environmental Pollutant Remediation and Resource Recovery)
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Abstract

This research introduces a synergistic photoelectrocatalytic (PEC) system designed for the effective degradation of tetracycline (TC), integrating a PO43−-grafted Mo-doped BiVO4 (PO43−-Mo:BiVO4) photoanode with a Pd-loaded ordered mesoporous carbon (Pd/CMK-3) cathode. The incorporation of Mo doping and PO43− modification significantly improved the photoanode’s charge separation efficiency, achieving a photocurrent density of 2.9 mA cm−2, and fine-tuned its band structure to enhance hydroxyl radical (·OH) generation. Meanwhile, the Pd/CMK-3 cathode promoted a two-electron oxygen reduction reaction pathway, producing hydrogen peroxide (H2O2) and facilitating molecular oxygen activation via atomic hydrogen (H*) intermediates. Under optimized conditions—1.0 V vs. Ag/AgCl of anodic potential, pH 6.58, and oxygen saturation—the combined system accomplished 80% TC degradation within 60 min, markedly surpassing the performance of the photoanode (72%) or cathode (71%) alone. Notably, this synergistic approach also reduced energy consumption to 0.0065 kWh m−3, outperforming individual components. Radical quenching experiments and liquid chromatography–mass spectrometry (LC-MS) analysis revealed that the photogenerated holes (h+) and ·OH were the key reactive species responsible for TC mineralization. The system demonstrated remarkable stability, with only a 2.96% decline in activity, and effectively degraded other contaminants, such as phenol, 4-chlorophenol, and ciprofloxacin. This study highlights an energy-efficient PEC strategy that harnesses the combined strengths of anodic oxidation and cathodic molecular oxygen activation to significantly enhance the removal of organic pollutants.

Graphical Abstract

1. Introduction

Tetracycline (TC) is a widely used antibacterial agent in livestock farming. Due to its low biodegradability, high persistence, and tendency for bioaccumulation, conventional municipal wastewater treatment systems struggle to completely remove it [1,2,3,4]. Studies show that partially degraded TC accumulates in animal tissues, posing carcinogenic, teratogenic, and mutagenic risks to humans through the food chain. Moreover, some degradation byproducts exhibit higher toxicity than TC itself [5,6,7]. Thus, developing advanced removal technologies is critical.
Among advanced treatment methods [8,9,10], photoelectrocatalytic (PEC) systems, which integrate photoanodic oxidation with electro-cathodic reduction, have gained significant prominence [11,12,13,14]. These systems utilize solar energy to generate electron-hole pairs, and the application of an external electric field improves charge separation, thereby promoting both cathodic reduction and anodic oxidation processes [15,16]. Importantly, PEC methods provide advantages such as preventing secondary pollution and eliminating the need for chemical oxidants, contributing to their growing popularity and extensive use [17,18]. The success of PEC degradation largely depends on the development of high-performance electrodes: photoanodes must exhibit broad spectral absorption, appropriate band structures, and efficient charge separation [19,20], whereas electro-cathodes require excellent conductivity, strong oxygen reduction activity, and high selectivity [21].
Mo:BiVO4, where Mo is doped into the V site of BiVO4 to boost its bulk conductivity, demonstrates outstanding PEC performance. This is evidenced by its increased photocurrent density and reduced charge recombination rate compared to pristine BiVO4, making it a representative photoanode material [22,23,24,25]. However, its degradation efficiency is constrained by the competing processes of O2 evolution and ·OH radical formation. Zhao et al. demonstrated that attaching phosphate ions as Brønsted acid/base sites onto the surface of photocatalysts enhances the production of ·OH radicals. Similarly, Choi et al. reported that treating Mo:BiVO4 with surface phosphate effectively inhibits the oxidation of H2O2 by reducing its absorption. The incorporation of Brønsted acid/base sites, along with the weakened adsorption of H2O2 molecules, undoubtedly promotes the selective oxidation of water to generate ·OH radicals rather than O2 at the photoanode.
Zhao et al. demonstrated that attaching phosphate ions as Brønsted acid/base sites onto the surface of photocatalysts enhances the production of ·OH radicals. Similarly, Choi et al. reported that treating Mo:BiVO4 with surface phosphate effectively inhibits the oxidation of H2O2 by reducing its absorption. The incorporation of surface phosphate ions as Brønsted acid/base sites, along with the weakened adsorption of H2O2 molecules, undoubtedly promotes the selective oxidation of water to generate ·OH radicals rather than O2 at the photoanode [23,24]. Additionally, in conventional PEC systems, the cathode undergoes H2 evolution (Figure 1a), which contributes negligibly to pollutant degradation [26,27]. Emerging studies reveal that atomic hydrogen (H*)—a key intermediate in H2 evolution—can activate molecular oxygen, thereby enhancing the breakdown of contaminants [28,29,30,31,32]. Consequently, integrating photoanodic oxidation with cathode-generated H* holds promise for boosting overall system efficiency, although such integrated designs have been scarcely explored to date.
To overcome these limitations, we developed a novel PO43−-grafted Mo:BiVO4 photoanode paired with a Pd/CMK-3 cathode (Figure 1b). The choice of the Pd/CMK-3 composite as the cathode material is driven by CMK-3’s stable and outstanding selectivity for the 2-electron oxygen reduction reaction (ORR) producing H2O2, combined with Pd’s excellent ability to generate H* species. This synergy enables the direct use of green and recyclable reductant (H*) to convert the H2O2 formed from O2 reduction into reactive species, thereby enhancing the degradation of TC. The PO43−-grafted Mo:BiVO4 photoanode harnesses photogenerated holes to drive direct or indirect oxidation processes, while the cathode accepts photogenerated electrons and utilizes Pd sites to generate H* [33,34,35]. This H* activates O2, together with in situ-produced H2O2 on the CMK-3 support, synergistically enhancing pollutant degradation. Using TC as the target pollutant, we systematically optimized parameters, including applied anodic bias voltage, pH, and dissolved oxygen (DO) levels. The underlying degradation pathways were elucidated via radical quenching, liquid chromatography–mass spectrometry (LC-MS), and comparative analyses. This research offers a viable and efficient strategy for the treatment of organic pollutants in wastewater.

2. Experimental Section

2.1. Chemical Reagents

Sodium sulfate (Na2SO4), potassium iodide (KI), H2O2 (30 wt.%), and phenol were purchased from Tianjin Damao Chemical Reagent Co (Tianjin, China). Bismuth nitrate pentahydrate (Bi(NO3)3·5H2O), vanadyl acetylacetonate ((C5H7O2)2OV), isopropyl alcohol ((CH3)2CHOH), potassium hexachloropalladate (K2PdCl6), ciprofloxacin (CIP), 4-Chlorophenol (4-CP), and catalase were sourced from Aladdin Reagent Co (Shanghai, China). Isopropanol, EDTA-2Na, and chloroform (CHCl3) were obtained from Kermel Reagent Co (Tianjin, China). p-Benzoquinone (p-BQ) and tetracycline hydrochloride (TC) were sourced from Meryer Chemical Reagent Co., Ltd. (Shanghai, China), while anhydrous ethanol (C2H5OH) and nitric acid (HNO3) were supplied by Tianjin Kaitong Chemical Reagent Co., Ltd. (Tianjin, China). and Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), respectively. Hydrophobic carbon paper and ordered mesoporous carbon CMK-3 were supplied by Shanghai Hesen Electric Co. (Shanghai, China). and Nanjing Jicang Nano Technology Co. (Nanjing, China), respectively. All reagents were analytical grade unless otherwise specified.

2.2. Synthesis of the Surface Phosphate-Treated Mo:BiVO4 Photoanode (PO43−-Mo:BiVO4)

BiVO4 was synthesized via a previously reported electrodeposition method [22,23,24]. In detail, we should prepare a 50 mL of 0.4 mol·L−1 KI solution, with pH adjusted to 1.7 using concentrated HNO3. Then 0.97 g of Bi(NO3)3·5H2O (0.04 mol·L−1) is added and dissolved uniformly via sonication. By further mixing this solution with 20 mL of anhydrous ethanol containing 0.23 mol·L−1 of p-BQ we can obtain the precursor solution. Finally, using fluorine-doped tin oxide (FTO) as the working electrode, the Ag/AgCl electrode as the reference electrode, and a graphite sheet as the counter electrode, perform electrodeposition at −0.1 V for 180 s to obtain a BiOI electrode loaded on the FTO substrate. In the next stage, we further prepare a 0.2 M VO(acac)2/DMSO solution, then pipette 0.15 mL of this solution and drop it on the precursor-obtained BiOI electrode surface. Transfer to a muffle furnace, set the heating rate to 2 °C·min−1, and calcine at 450 °C for 2 h. After cooling to room temperature, immerse the electrode in 1 mol·L−1 NaOH solution for 20 min, rinse with deionized water to remove excess black V2O5, and air-dry naturally to obtain the BiVO4 photoanode.
The method for the preparation of Mo-doped BiVO4 photoanode was the same as that for BiVO4 photoanode synthesis, except that 0.016 g Mo(acac)2 was added into the 0.2 mol·L−1 of VO(acac)2/DMSO solution to form a Mo-containing VO(acac)2 solution [25].
PO43− was also grafted onto the Mo:BiVO4 surface via a simple electrodeposition method [36,37]. The specific procedure is as follows: Using an aqueous solution of 31.25 mmol·L−1 NaH2PO2·H2O and 6.25 mmol·L−1 CH3COONa as the electrodeposition solution, Mo:BiVO4 as the working electrode, the Ag/AgCl electrode as the reference electrode, and a graphite sheet as the counter electrode, perform cyclic voltammetry (CV) scanning in the range of −0.4~0.3 V (vs. RHE) at a scan rate of 50 mV·s−1 for 5 cycles to obtain PO43−-Mo:BiVO4.

2.3. Synthesis of Pd/CMK-3 Cathode

The Pd/CMK-3 cathode catalyst was synthesized via chemical reduction deposition of Pd on the surface of commercial CMK-3 [38]. Specifically, a solution containing 0.0115 g of K2PdCl6 was prepared. This solution was then blended with 20 mL of ethanol and 0.1 g of CMK-3 in a 50 mL beaker, followed by ultrasonic dispersion for 20 min to ensure homogeneity. The mixture was subsequently stirred at 80 °C to evaporate the solvent. The resulting solid was then introduced into 20 mL of a 0.1 mol·L-1 NaBH4 ethanol solution at ambient temperature. After vigorous stirring for 30 min, the product was filtered and thoroughly rinsed with ultrapure water, yielding 3 wt.% Pd/CMK-3. The obtained solid was further dried at 80 °C for 2 h prior to use.
To immobilize Pd/CMK-3 onto he cathode surface, hydrophobic carbon paper served as the catalyst support, which was sectioned into 2 × 3 cm2 rectangular substrates. A homogeneous catalyst ink was formulated by dispersing the catalyst in a mixture of 25 μL Nafion membrane solution, 0.5 mL isopropanol, and 1.2 mL ultrapure water via sonication. This ink was uniformly coated onto the carbon paper at a loading density of 0.5 mg·cm−2, ensuring identical working areas to the anode catalyst. Accelerated drying was achieved under an infrared lamp, resulting in the final electrocatalytic cathode.

2.4. Catalyst Characterization

Microstructural and electrochemical characterization was performed using the following instrumentation: scanning electron microscopy (SEM, SUPRA55 SAPPHIRE, Carl Zeiss AG, Jena, Germany) was employed to observe catalyst morphology and microstructure. Crystalline phase analysis was conducted via X-ray diffraction (XRD, Rigaku SmartLab9, Rigaku Corp., Tokyo, Japan) with Cu Kα radiation (λ = 1.54178 Å). X-ray photoelectron spectroscopy (XPS, ESCALAB250Xi, Thermo Fisher Scientific, Waltham, MA, USA) utilizing Al Kα excitation (1486.6 eV) was used to determine elemental composition, valence states, and valence band binding energies, with all spectra calibrated against the C 1s peak at 284.6 eV. UV–vis diffuse reflectance spectroscopy (DRS, UV-2450, Shimadzu, Kyoto, Japan) was employed to evaluate optical response properties and absorption edges across 250–800 nm. Photoelectrochemical measurements employed a CHI660E workstation (Chenhua Instruments, Shanghai, China) configured with a three-electrode system: an Ag/AgCl reference electrode, a catalyst-loaded FTO/hydrophobic carbon paper working electrode, and a graphite plate counter electrode. Under 100 mW·cm−2 illumination, photoanodes underwent linear sweep voltammetry (LSV) and Mott–Schottky testing. For the CMK-3-based cathodes, cyclic voltammetry (CV) and rotating disk electrode (RDE) measurements were performed in 0.1 M Na2SO4 electrolyte.

2.5. PEC Degradation Experimental Setup

PEC degradation experiments were conducted in a custom single-cell quartz reactor. Prior to initiating the reaction, 29 mL of 0.1 mol·L−1 Na2SO4 and 1 mL of 600 mg·L−1 TC (or other pollutants such as phenol) were added to the reactor, formulating an electrolyte solution containing 20 mg·L−1 TC. The reactor was then positioned parallel to a 300 W Xenon lamp at a distance of 20 cm, with a 420 nm cutoff filter placed between the reactor and the light source.
The PEC degradation employed a three-electrode configuration: an Ag/AgCl electrode as the reference electrode, FTO coated with catalyst powder as the working electrode, and Pd/CMK-3-carbon paper as the counter electrode. Both cathode and photoanode exhibited an electrolyte contact area of 4 cm2, with an inter-electrode distance of 3 cm. Reactions proceeded under illumination for 120 min, with approximately 3 mL samples extracted every 20 min. Samples were filtered through 0.22 μm nylon membranes, and TC concentration was determined by measuring absorbance at λ = 357 nm using a UV-Vis spectrophotometer [39]. The intermediates formed during the degradation of TC were analyzed using a liquid chromatography-mass spectrometry system (HPLC-MS, Agilent 6460 Triple Quadruple MS, Agilient, Santa Clara, CA, USA). Table S1 details the chromatographic conditions and acquisition parameters of the HPLC-MS system for reference. When investigating cathode performance, a separate three-electrode system was configured: an Ag/AgCl reference electrode, a Pd/CMK-3–carbon paper working electrode, and a graphite plate counter electrode, without illumination.

3. Results and Discussion

3.1. Characterization of PO43−-Mo:BiVO4 Photoanode and Pd/CMK-3 Cathode

To confirm the successful preparation of the anode material, X-ray diffraction (XRD) patterns of the PO43−-Mo:BiVO4 photoanode were first tested to analyze its crystal structure. As shown in Figure 2a, characteristic diffraction peaks at 18.7°, 19.0°, 29.0°, and 30.6° were detected, corresponding to the (110), (011), (121), and (040) crystal planes of monoclinic scheelite BiVO4 (PDF#14-0688), indicating that PO43−-Mo:BiVO4 is constructed based on monoclinic BiVO4. No additional diffraction peaks were observed, suggesting that the crystal structure of BiVO4 remained unchanged after Mo doping or PO43− grafting [40]. To further demonstrate the successful incorporation of Mo and PO43− into BiVO4, X-ray photoelectron spectroscopy (XPS) was employed to analyze its chemical composition. The XPS survey spectrum (Figure 2b) confirmed that the PO43−-Mo:BiVO4 photoanode consists of Bi, V, and O elements. Moreover, In the high-resolution Mo 3d XPS spectra, as shown in Figure 2c, a subtle peak near 232 eV—corresponding well to the binding energies of Mo 3d—can be observed and fitted in the Mo 3d spectrum of Mo:BiVO4, while this peak is barely noticeable in the Mo 3d spectrum of PO43−-Mo:BiVO4. The faint signal in Mo:BiVO4 arises from the low Mo doping level (theoretical amount of 1%), while the absence of a detectable peak in PO43−-Mo:BiVO4 results from both the low Mo content and the surface coverage of PO43−. Nonetheless, the successful incorporation of Mo into BiVO4 is confirmed by the elemental analysis data (Table S2): the atomic percentage of Mo in Mo:BiVO4 is measured at 1.15%, closely matching the theoretical value, whereas in PO43−-Mo:BiVO4, it is slightly lower at 0.92%. In contrast, clear characteristic peaks for P 2p were detected (Figure 2d), proving the successful modification of PO43− onto the Mo:BiVO4 surface. However, magnified XPS analysis revealed that compared to pure BiVO4, the binding energies of Bi 4f and V 2p in both Mo:BiVO4 and PO43−-Mo:BiVO4 shifted toward higher energies (Figure S1). This shift occurs because Mo6+ substitutes V5+ sites, reducing electron density in modified BiVO4 and resulting in increased binding energies for Bi 4f and V 2p [25]. These results collectively verify the successful preparation of PO43−-Mo:BiVO4.
To confirm the successful preparation of Pd/CMK-3, SEM characterization was employed to observe the microstructure of the as-prepared sample, revealing distinct metallic particles distributed on a porous honeycomb-structured surface (Figure 2e). XRD analysis offers additional confirmation of the successful synthesis of the Pd/CMK-3 composite. As illustrated in Figure 2f, the XRD spectrum of CMK-3 exhibits a broad peak ranging from 15° to 40°, characteristic of the (002) plane typically observed in carbon materials. In comparison, the Pd/CMK-3 pattern retains this broad peak but also reveals three sharp, well-defined peaks at 40.1°, 46.7°, and 68.1°. These peaks correspond to the (111), (200), and (220) crystal planes of metallic palladium (PDF#46-1043), clearly indicating the effective deposition of Pd species onto the CMK-3 framework.

3.2. Photoelectric Properties of PO43−-Mo:BiVO4 Photoanode and Pd/CMK-3 Cathode

For photoanode materials, three pivotal factors—light absorption capacity, band structure, and charge separation capability—collectively dictate their catalytic efficiency [41]. The light absorption capacity governs photon capture efficiency, the band structure determines redox potential, while charge separation regulates carrier migration from bulk to surface for redox reactions.
To probe the influence of Mo doping and PO43− modification on BiVO4’s light absorption, Diffuse reflectance spectroscopy (DRS) analyses showed that the absorption edges of pristine BiVO4 experienced a minor shift from 512 nm to 502 nm following modification with both Mo doping and PO43− treatment (Mo:BiVO4 and PO43−-Mo:BiVO4). However, a noticeable decrease in light absorption intensity was observed (Figure 3a). This suggests that the slight Mo doping (1%) or PO43− modification does not substantially change the light absorption range but may reduce photon utilization efficiency during the subsequent degradation processes of pristine BiVO4, which could be detrimental. Despite this, further characterization of the band positions confirms that Mo doping and PO43− grafting positively influence the oxidation capability of pristine BiVO4. This conclusion is supported by the energy band alignment diagram presented in Figure 3b. Specifically, based on the band gap values (Eg) derived from Tauc plots (Figure S2a) and the flat-band potentials (Fermi levels) obtained from Mott–Schottky measurements (Figure S2b–d), the Eg of BiVO4 increases from 2.42 eV to 2.47 eV upon both Mo doping and PO43− modification. Concurrently, the Fermi level shifts positively from 0.61 eV in pristine BiVO4 to 0.84 eV in Mo:BiVO4 and further to 0.92 eV in PO43−-Mo:BiVO4. Integrating these values with valence band binding energies measured by XPS (1.39, 1.47, and 1.53 eV as shown in Figure S3), the conduction band minimum (CB) and valence band maximum (VB) of PO43−-Mo:BiVO4 are calculated to be −0.01 V and 2.45 V, respectively [42]. Using the same approach for BiVO4 and Mo:BiVO4, comparative analysis reveals that Mo doping and PO43− modification effectively tune the band structure, thereby enhancing hydroxyl radical (·OH) generation via water oxidation at 2.38 V and hydrogen peroxide (H2O2) production through oxygen reduction at 0.68 V [43] (Figure 3b). Linear sweep voltammetry (LSV) under a 1.0 V vs. Ag/AgCl bias quantified charge separation capabilities: pristine BiVO4 exhibited 1.0 mA·cm−2 photocurrent density, which peaked at 2.6 mA·cm−2 for Mo:BiVO4 before declining at higher doping levels. Further PO43− modification elevated the photocurrent to 2.9 mA·cm−2—a 190% enhancement versus unmodified BiVO4 (Figure 3c)—conclusively demonstrating that both Mo doping and PO43− loading augment light harvesting and charge transfer efficiencies.
For cathodic materials, conductivity, oxygen reduction (ORR) activity, and selectivity constitute critical determinants of pollutant degradation efficiency. Electrochemical impedance spectroscopy (EIS) analyses revealed that Pd/CMK-3 exhibited a shallower low-frequency slope and a marginally reduced high-frequency arc radius compared to pristine CMK-3, demonstrating superior charge transfer kinetics [44] (Figure 3d).
To assess Pd-induced enhancement in ORR activity, cyclic voltammetry (CV) was conducted under N2 and O2 atmospheres. Pd/CMK-3 generated significantly higher current densities than CMK-3 across all potentials. Moreover, the oxygen reduction peak was more pronounced than that of CMK-3, confirming Pd’s catalytic role in accelerating oxygen reduction (Figure 3e). Notably, two oxidation peaks emerged at −0.8~−0.6 V and 0.1~0.3 V (vs. Ag/AgCl) for Pd/CMK-3 under both atmospheres, attributable to H2 oxidation and H* formation [40,45]. Concurrently, a distinct reduction peak at −0.5~−0.4 V (vs. Ag/AgCl) in O2-saturated electrolyte suggested probable ORR activity. Rotating disk electrode (RDE) validation resolved the ORR mechanism: fitted curves yielded electron transfer numbers of 2.85, 2.64, 2.57, and 2.59 at −0.50 V, −0.45 V, −0.40 V, and −0.35 V, respectively, unequivocally indicating a two-electron-dominated pathway (Figure 3f). This selectivity toward H2O2 synthesis facilitates H*-mediated catalytic processes that are critical for pollutant degradation.

3.3. Degradation of TC by PO43−Mo:BiVO4 Photoanode & Pd/CMK-3 Cathode Synergistic System

In the PEC synergistic system, we first investigated the effects of external bias, solution pH, and dissolved oxygen (DO) on TC degradation. As depicted in Figure 4a, increasing the applied anodic bias from 0.5 V to 1.0 V and subsequently to 1.5 V (vs. Ag/AgCl) significantly elevated TC degradation rates from 41% to 75% and further to 80%, which can be ascribed to the promotion of photogenerated charges separation efficiency. However, despite the degradation rate rising from 75% to 80% when bias increased from 1.0 V to 1.5 V, energy consumption analysis revealed a critical trade-off: the system consumed 0.0129 kWh·m−3 at 1.5 V, which is twice the 0.0065 kWh·m−3 consumed at 1.0 V (Table S3). Consequently, 1.0 V was selected as the optimal bias for subsequent experiments, balancing degradation efficiency and energy economics. pH dependency studies demonstrated progressive degradation decline from 84% at pH 2 to 72% at pH 8 (Figure 4b). This attenuation correlates with reduced H+ concentration diminishing H* generation at the cathode, thereby impairing TC degradation efficiency. The critical role of oxygen reduction was further elucidated through DO modulation. To create oxygen-free and oxygen-rich environments, ultrapure gases (99.99%) of N2 and O2 were bubbled at a flow rate of 100 sccm for 15 min before performing the PEC degradation experiments. In contrast, the air condition was established simply by exposing the reactor to ambient air without any gas bubbling. Consequently, under N2 purging, TC degradation plummeted to 53%, markedly lower than rates under air (75%) or O2-saturated (80%) conditions (Figure 4c). This confirms that cathodic oxygen reduction reactions are indispensable for efficient pollutant mineralization.
To investigate the advantages of the synergistic system, TC degradation efficiencies of the synergistic system were compared with those of individual PO43−-Mo:BiVO4 photoanode and Pd/CMK-3 cathode systems under optimized conditions (1.0 V anodic bias, pH = 6.58, O2 saturation). As shown in Figure 5a, TC degradation rates reached 72% for the individual photoanode, 71% for the individual cathode, and 80% for the synergistic system. While the absolute improvement in degradation ratio appears modest, a closer examination of the kinetic rate constants reveals significant advantages. As illustrated in Figure S4, the rate constant derived from first-order kinetics for the synergistic system is 1.32 ± 0.04 min−1, which is 1.3 times and 1.2 times higher than those of the individual cathode (1.03 ± 0.05 min−1) and photoanode (1.08 ± 0.06 min−1) systems, respectively. Additionally, the energy consumption values were measured as 0.0074 kWh·m−3 for the individual photoanode system, 0.0097 kWh·m−3 for the individual cathode system, and 0.0065 kWh·m−3 for the synergistic system (Figure 5b). Notably, the synergistic system achieved energy savings of 12.2% and 33% compared to the individual photoanode and cathode systems, respectively. Therefore, the synergistic system achieves superior TC degradation performance with lower energy consumption.
To evaluate electrode stability, cycling tests were conducted for TC degradation using the PEC synergistic system. As demonstrated in Figure 5c, the TC removal rate decreased by only 2.96% after four consecutive cycles, confirming exceptional operational stability of the synergistic system. Moreover, to verify universality, the system’s efficacy was assessed for degrading organic pollutants beyond TC, including phenol, 4-CP, and CIP. Degradation efficiencies reached 66% (phenol), 67% (4-CP), and 58% (CIP), as quantified in Figure 5d. Although slightly inferior to TC degradation (80%), these results demonstrate the system’s applicability for diverse contaminant remediation.

3.4. TC Degradation Mechanism

To investigate the active species and intermediate evolution during TC degradation in the PEC synergistic system, radical quenching experiments were first performed. As illustrated in Figure 6a, the addition of EDTA-2Na (h+ scavenger) and IPA (·OH scavenger) significantly reduced the TC removal rates from 73% to 15.5% and 56.6%, respectively. In contrast, the introduction of CHCl3 (·O2− scavenger) resulted in minimal changes, indicating that h+ and ·OH are the main reactive species responsible for TC degradation, while ·O2- plays negligible role. Notably, when catalase was added as a H2O2 scavenger, a slight decline in TC degradation was observed during the initial 40 min, mirroring the effect seen with IPA’s suppression of ·OH. However, this inhibitory effect gradually diminished between 40 and 120 min. This behavior can be attributed to the limited direct oxidative capacity of H2O2 on TC; instead, during the first 40 min, H2O2 primarily facilitates ·OH generation through cathodic H* activation [42]. Beyond this period, ·OH production is increasingly supported by anodic water oxidation within the photoelectrochemical (PEC) system, thereby reducing the impact of H2O2 removal. Further quantitative analysis of active species concentrations reinforced these findings: the measured levels were 6.32 μmol·L−1 for ·O2−, 18.81 μmol·L−1 for H2O2, and 21.06 μmol·L−1 for ·OH (Figure 6b), confirming ·OH as the predominant radical, in agreement with the quenching experiments.
Subsequently, to clarify the TC degradation pathway, the intermediate products formed during the process were analyzed via LC-MS (Figure S5). Based on these results, nine intermediates (P1–P9) were identified. Accordingly, two primary pathways for the PEC degradation of TC are proposed, as illustrated in Figure 7. In pathway I, TC undergoes successive hydroxylation by the radical ·OH, transforming into P1 (m/z = 461) and P2 (m/z = 477), which then convert into P3 (m/z = 433) and P4 (m/z = 350) through demethylation, deamidation, and ring-opening reactions. In pathway II, demethylation of the dimethylamino group produces P5 (m/z = 417), which subsequently undergoes ring-opening with loss of –OH to form P6 (m/z = 399) and P7 (m/z = 375). Finally, P4 and P7 are further oxidized by h+, ·O2−, and ·OH to yield P8 (m/z = 202) and P9 (m/z = 274), which are ultimately mineralized into CO2 and H2O.

4. Conclusions

This study addresses the critical challenges of BiVO4—specifically low photogenerated electron-hole pair separation efficiency, poor photocatalytic performance, and inadequate selectivity for ·OH production during water oxidation—by implementing Mo doping and surface PO43− grafting. Simultaneously, the introduction of H*-mediated molecular oxygen activation reactions at the cathode enabled the construction of a synergistic PEC system to collaboratively enhance degradation. This system achieved 80% tetracycline (TC) degradation efficiency under 1.0 VAg/AgCl of anodic bias. Radical quenching experiments and intermediate product analysis confirmed that h+ and ·OH serve as the primary active species in the synergistic PEC system, driving TC fragmentation into low-molecular-weight intermediates (including species with m/z = 202 and 274). This novel PEC architecture provides an effective new strategy for mitigating environmental pollution through efficient contaminant degradation.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/coatings15091027/s1. Figure S1: The high-resolution XPS spectra of (b) Bi 4f, (c) V 2p, and (d) O1s.; Figure S2: UV-Vis absorption spectra (Ahυ)1/2 and hυ relationship diagram of BiVO4, Mo:BiVO4, and PO43−Mo:BiVO4; Figure S3: The price band edge binding energy curve of BiVO4, Mo:BiVO4, and PO43−Mo:BiVO4; Figure S4: Kinetic models of TC degradation in different systems; Figure S5: LC-MS results of PEC degradation of TC solution over synergistic PEC system at 60 min; Table S1: The energy consumption of different voltage drops in the collaborative system; Table S2: The structural information of TC and its degradation intermediates; Table S3: The energy consumption of different voltage drops in the collaborative system.

Author Contributions

Conceptualization, M.Y. and F.Y.; methodology, M.Y. and Z.X. (Zhenhong Xu); software, C.T.; validation, M.Y. and Z.X. (Zhenhong Xu); formal analysis, Z.X. (Zhourong Xiao) and S.W.; investigation, Z.X. (Zhenhong Xu); resources, F.Y.; data curation, M.Y. and Z.X. (Zhenhong Xu); writing—original draft preparation, M.Y.; writing—review and editing, F.Y.; visualization, C.T.; supervision, F.Y.; project administration, F.Y.; funding acquisition, F.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Yanzhao Golden Platform Talent Project (Education Platform) of Hebei Province (No. HJYB202517), the Natural Science Foundation of Hebei Province (No. B2024203026), and the Open Foundation of Key Laboratory of Functional Textile Material and Product, Ministry of Education, Xi’an Polytechnic University, Xi’an, P.R. China 710048 (No. 2024FTMP013).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data is contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic comparison of the PEC system for pollutant degradation. (a) Conventional systems featuring non-selective photoanodic oxidation paired with an ineffective cathodic hydrogen evolution process. (b) The proposed system demonstrating selective photoanodic oxidation synergistically combined with a cathodic H*-mediated molecular oxygen activation process.
Figure 1. Schematic comparison of the PEC system for pollutant degradation. (a) Conventional systems featuring non-selective photoanodic oxidation paired with an ineffective cathodic hydrogen evolution process. (b) The proposed system demonstrating selective photoanodic oxidation synergistically combined with a cathodic H*-mediated molecular oxygen activation process.
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Figure 2. (a) XRD patterns, (b) the survey XPS spectrum and the high-resolution spectra of (c) Mo 3d, (d) P 2p of PO43−-Mo:BiVO4. (e) The SEM images of Pd/CMK-3, (f) XRD patterns of CMK-3 and Pd/CMK-3.
Figure 2. (a) XRD patterns, (b) the survey XPS spectrum and the high-resolution spectra of (c) Mo 3d, (d) P 2p of PO43−-Mo:BiVO4. (e) The SEM images of Pd/CMK-3, (f) XRD patterns of CMK-3 and Pd/CMK-3.
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Figure 3. (a) UV–vis absorption spectra, (b) band alignment diagrams, and (c) linear sweep voltammetry (LSV) of BiVO4, Mo:BiVO4, and PO43−Mo:BiVO4. (d) Electrochemical impedance spectroscopy (EIS) and (e) cyclic voltammetry (CV) under N2/O2 atmospheres of CMK-3 and Pd/CMK-3. (f) Koutecky–levich plots for Pd/CMK-3.
Figure 3. (a) UV–vis absorption spectra, (b) band alignment diagrams, and (c) linear sweep voltammetry (LSV) of BiVO4, Mo:BiVO4, and PO43−Mo:BiVO4. (d) Electrochemical impedance spectroscopy (EIS) and (e) cyclic voltammetry (CV) under N2/O2 atmospheres of CMK-3 and Pd/CMK-3. (f) Koutecky–levich plots for Pd/CMK-3.
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Figure 4. Effect of (a) anode potential (0, 0.5, 1 and 1.5 V vs. Ag/AgCl); (b) solution pH levels adjusted to 2, 4, 6.58, and 8 using 0.1 mol·L−1 H2SO4 or 0.2 mol·L−1 NaOH; (c) dissolved oxygen (DO) of the solution on TC degradation.
Figure 4. Effect of (a) anode potential (0, 0.5, 1 and 1.5 V vs. Ag/AgCl); (b) solution pH levels adjusted to 2, 4, 6.58, and 8 using 0.1 mol·L−1 H2SO4 or 0.2 mol·L−1 NaOH; (c) dissolved oxygen (DO) of the solution on TC degradation.
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Figure 5. Performance of the synergistic PEC system. (a) Comparative degradation efficiency, (b) kinetic constants and energy consumption, (c) TC degradation on synergistic system during four times of cycle test, and (d) degradation of other pollutants using the synergistic system.
Figure 5. Performance of the synergistic PEC system. (a) Comparative degradation efficiency, (b) kinetic constants and energy consumption, (c) TC degradation on synergistic system during four times of cycle test, and (d) degradation of other pollutants using the synergistic system.
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Figure 6. (a) PEC degradation of TC in the presence of different active specie scavengers (EDTA-2Na, IPA, CHCl3 and catalase). (b) Reactive species concentrations (·OH, H2O2 and ·O2−) were quantified as follows: the generated ·OH was quantitatively determined by a probe reaction of salicylic acid (SA) and ·OH [46]; the H2O2 concentration was measured using the I3− method [47]; the concentration of ·O2− was detected using nitroblue tetrazolium, NBT [48].
Figure 6. (a) PEC degradation of TC in the presence of different active specie scavengers (EDTA-2Na, IPA, CHCl3 and catalase). (b) Reactive species concentrations (·OH, H2O2 and ·O2−) were quantified as follows: the generated ·OH was quantitatively determined by a probe reaction of salicylic acid (SA) and ·OH [46]; the H2O2 concentration was measured using the I3− method [47]; the concentration of ·O2− was detected using nitroblue tetrazolium, NBT [48].
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Figure 7. Proposed TC degradation pathway in the synergistic PEC system.
Figure 7. Proposed TC degradation pathway in the synergistic PEC system.
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Yang, M.; Xu, Z.; Tang, C.; Wang, S.; Xiao, Z.; Ye, F. Synergistic Photoelectrocatalytic Degradation of Tetracycline Using Phosphate-Grafted Mo:BiVO4 Photoanode Coupled with Pd/CMK-3 Cathode for Dual-Functional Activation of Water and Molecular Oxygen. Coatings 2025, 15, 1027. https://doi.org/10.3390/coatings15091027

AMA Style

Yang M, Xu Z, Tang C, Wang S, Xiao Z, Ye F. Synergistic Photoelectrocatalytic Degradation of Tetracycline Using Phosphate-Grafted Mo:BiVO4 Photoanode Coupled with Pd/CMK-3 Cathode for Dual-Functional Activation of Water and Molecular Oxygen. Coatings. 2025; 15(9):1027. https://doi.org/10.3390/coatings15091027

Chicago/Turabian Style

Yang, Minglei, Zhenhong Xu, Chongjun Tang, Shuaijie Wang, Zhourong Xiao, and Fei Ye. 2025. "Synergistic Photoelectrocatalytic Degradation of Tetracycline Using Phosphate-Grafted Mo:BiVO4 Photoanode Coupled with Pd/CMK-3 Cathode for Dual-Functional Activation of Water and Molecular Oxygen" Coatings 15, no. 9: 1027. https://doi.org/10.3390/coatings15091027

APA Style

Yang, M., Xu, Z., Tang, C., Wang, S., Xiao, Z., & Ye, F. (2025). Synergistic Photoelectrocatalytic Degradation of Tetracycline Using Phosphate-Grafted Mo:BiVO4 Photoanode Coupled with Pd/CMK-3 Cathode for Dual-Functional Activation of Water and Molecular Oxygen. Coatings, 15(9), 1027. https://doi.org/10.3390/coatings15091027

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