Photocatalytic Performance of Ag₃PO_4/BiVO₄ P-N Type Heterojunction for Treatment of Landfill Leachate Tailwater

Abstract

A novel Ag₃PO₄/BiVO₄ heterojunction was synthesized via a combined hydrothermal–in situ precipitation method. With an optimal Bi:Ag molar ratio of 1:2 and after calcination at 200 °C for 22 h, 0.9 g of this composite reduced the chemical oxygen demand (COD) of landfill leachate tailwater from 232 mg·L⁻¹ to 142 mg·L⁻¹ and its UV₂₅₄ absorbance from 0.22 to 0.156 under visible light irradiation within 140 min. The material exhibited a bandgap of 2.56 eV, along with enhanced visible-light absorption and improved charge-carrier separation efficiency. In the Ag₃PO₄/BiVO₄/peroxymonosulfate (PMS)/visible light system, using 0.5 g of catalyst and 2.0 g·L⁻¹ of PMS at pH 11 reduced the COD from 242 mg·L⁻¹ to 138 mg·L⁻¹. A subsequent two-stage treatment process, integrating the Ag₃PO₄/BiVO₄/PMS/vis and P25/UV process, achieved a final tailwater COD of 90 mg·L⁻¹—meeting standard discharge limits—and a 69.5% removal of humic-like substances. The heterojunction catalyst retained its activity over four consecutive cycles. Radical quenching experiments and electron paramagnetic resonance (EPR) spectroscopy identified photogenerated holes (h⁺), hydroxyl radicals(·OH), and sulfate radicals (SO₄⁻·) as the primary reactive species. Gas chromatography–mass spectrometry (GC–MS) analysis identified intermediate organic compounds and proposed plausible degradation pathways. These results support a reaction mechanism in which h⁺ oxidizes H₂O to generate ·OH, while PMS accepts electrons to produce SO₄⁻· and further ·OH radicals, leading to effective pollutant mineralization. Collectively, this solar-driven, sulfate radical-based advanced oxidation process offers an energy-efficient strategy with reduced chemical consumption for the sustainable treatment of refractory wastewater.

1. Introduction

Landfill leachate is a highly concentrated, complex organic wastewater generated from the decomposition of solid waste [1,2]. Typically dark in color, it is produced through microbial activity and the percolation of water through landfill sites [3]. Its treatment represents a significant challenge in environmental management [4]. Leachate characteristics evolve with landfill age, typically categorized into three phases: young (<5 years), intermediate (5–10 years), and mature (>10 years), with key parameters summarized in Table 1. Treatment is complicated by the presence of refractory organic compounds, such as humic and fulvic acids, an imbalanced nutrient profile, and elevated heavy metal concentrations. The analytical methods and corresponding standards for water quality detection are listed in Table 2.

Conventional treatment often combines biological and physicochemical methods [5]. While biological processes effectively remove biodegradable matter at relatively low cost, the resulting tailwater retains recalcitrant organics. Techniques like adsorption [6] and membrane filtration [7] can remove these under mild conditions but often transfer pollutants rather than degrade them. Methods that degrade recalcitrant organics, such as catalytic wet oxidation [8] and supercritical oxidation [9], require extreme temperature and pressure. Others, like electrochemical and ultrasonic treatments, are energy-intensive. Consequently, research focuses on integrated catalytic systems for multi-pollutant removal, exemplified by a Ce-Fe₂O₃/Al₂O₃-based electro-Fenton process that concurrently degrades organics and reduces Cr(VI) [10]. Recent studies further highlight the effectiveness of advanced catalytic materials and heterojunction designs in addressing complex environmental challenges. For instance, innovations in CO₂ photoreduction via S-scheme heterojunctions [11] and enhanced ion transport in energy storage materials [12] underscore the critical role of material engineering in improving charge dynamics and catalytic efficiency [13].

To address this, photocatalysis offers a solar-driven solution. Enhancing charge separation is key, often achieved by coupling semiconductors to form heterojunctions [14]. Among visible-light photocatalysts, metal vanadates (MVO₄) are promising due to their visible-light response, narrow bandgap, and stability [15]. Bismuth vanadate (BiVO₄) is especially attractive owing to its availability, non-toxicity, and visible-light absorption [16]. However, BiVO₄ suffers from rapid charge recombination and limited surface area. Strategies to mitigate these issues include morphology control, doping, and constructing heterojunctions with other semiconductors [17], which is particularly effective for enhancing charge separation and activity [18].

Silver phosphate (Ag₃PO₄) also exhibits strong visible-light activity but is hampered by photo-corrosion and solubility [19,20]. Coupling Ag₃PO₄ with BiVO₄ [21], to form a heterostructure, is therefore a promising strategy to synergize their advantages, enhancing both charge separation and photocatalytic stability [22].

Advanced oxidation processes (AOPs) based on reactive radicals like hydroxyl (•OH) and sulfate (SO₄⁻·) are effective for degrading refractory organics into harmless end products like CO₂ and H₂O, achieving mineralization rather than phase transfer [23]. Peroxymonosulfate (PMS) is a stable, non-toxic oxidant. To generate radicals like SO₄⁻·, its O–O bond must be activated. Photoactivation using sunlight is particularly attractive due to its low energy cost and sustainability. While UV light activates PMS efficiently, developing visible-light-driven systems is essential for maximizing solar energy use. Furthermore, for practical application, catalyst stability and reusability are as critical as initial activity for reducing long-term operational costs and material footprint [24].

Based on the complementary properties of Ag₃PO₄ and BiVO₄, we propose the following hypothesis: the Ag₃PO₄/BiVO₄ heterojunction will effectively activate peroxymonosulfate (PMS) under visible light, generating reactive radicals that synergistically degrade refractory organics in mature landfill leachate. This system is hypothesized to achieve superior removal of both bulk organic content (COD) and aromatic/humic-like substances (UV₂₅₄). The enhanced performance is expected to result from improved charge separation at the heterojunction interface, which promotes the generation of key radical species. This hypothesis is tested by optimizing photocatalytic performance and elucidating the reaction mechanism via radical identification, photoelectrochemical analysis, and catalyst stability assessment.

This work demonstrates a more energy-efficient and sustainable advanced oxidation process for the treatment of complex landfill leachate.

Table 1. Water quality characteristics of leachate at different landfill times [3].

Leachate Type	Young	Intermediate	Mature
Landfill time (year)	<5	5–10	>10
pH	6.6–7.5	7.0–8.0	>7.5
COD (mg·L−1)	10,000–30,000	3000–10,000	<3000
BOD5/COD	0.4–0.6	0.2–0.4	<0.2
C/N	5–10	3–4	<3
Organic Components	80%VFA	5–30%VFA, Humic, fulvic acids	Humic, fulvic acids
Biodegradability	easy	middle	low

Table 1. Water quality characteristics of leachate at different landfill times [3].

Leachate Type	Young	Intermediate	Mature
Landfill time (year)	<5	5–10	>10
pH	6.6–7.5	7.0–8.0	>7.5
COD (mg·L−1)	10,000–30,000	3000–10,000	<3000
BOD5/COD	0.4–0.6	0.2–0.4	<0.2
C/N	5–10	3–4	<3
Organic Components	80%VFA	5–30%VFA, Humic, fulvic acids	Humic, fulvic acids
Biodegradability	easy	middle	low

Table 2. Index and method of water quality detection.

Parameter	Analytical Method	Standard Method
pH	Glass electrode method	CJ/T 428-2013 [25]
COD	Fast digestion-spectrophotometric method	HJ 924-2017 [26]
TN	Alkaline potassium persulfate digestion UV spectrophotometric method	HJ 636-2012 [27]
NH4+-N	Nessler’s reagent spectrophotometric method	HJ 535-2009 [28]
TP	Ammonium molybdate spectrophotometric method	GB 11893-89 [29]

Table 2. Index and method of water quality detection.

Parameter	Analytical Method	Standard Method
pH	Glass electrode method	CJ/T 428-2013 [25]
COD	Fast digestion-spectrophotometric method	HJ 924-2017 [26]
TN	Alkaline potassium persulfate digestion UV spectrophotometric method	HJ 636-2012 [27]
NH4+-N	Nessler’s reagent spectrophotometric method	HJ 535-2009 [28]
TP	Ammonium molybdate spectrophotometric method	GB 11893-89 [29]

2. Experimental Section

2.1. Photocatalytic Reactor Setup

The photocatalytic experiments were conducted using a setup comprising a 300 W xenon lamp (CEL-HXF300, Beijing Zhongjiao Jinyuan Technology Co., Ltd., Beijing, China) as the visible light source, equipped with a 420 nm cutoff filter. The system also included a power supply, a lifting platform, and a thermostatic water bath with magnetic stirring to maintain constant temperature and mixing. The crystal composition of all the photocatalyst samples was recorded using an X-ray diffractometer (XRD) (X’Pert3 Powder, PANalytical B.V., Almelo, The Netherlands). The morphology of the Ag₃PO₄, BiVO₄, and Ag₃PO₄/BiVO₄ was observed on scanning electron microscopy (SEM) (JSM-7900F plus JEOL Ltd., Tokyo, Japan). The elemental composition and chemical state of Ag₃PO₄, BiVO₄, and Ag₃PO₄/BiVO₄ were determined with X-ray photoelectron spectroscopy system (XPS) (Thermo Fisher Scientific, Loughborough, UK). The functional group of Ag₃PO₄, BiVO₄ and Ag₃PO₄/BiVO₄ was studied by Fourier-transform infrared (FT-IR) spectroscopy (PerkinElmer, Waltham, MA, USA). Using a Brunauer–Emmett–Teller (BET) analyzer (Micromeritics ASAP 2460, Micromeritics Instrument Corp., Norcross, GA, USA), the specific surface area was ascertained. Electrochemical impedance spectroscopy (EIS) (CHI660E, Shanghai Chenhua Instrument Co., Ltd., Shanghai, China), and ultraviolet–visible diffuse reflectance spectra (UV–Vis DRS) (Lambda750, PerkinElmer, Waltham, MA, USA) were used to examine the optical and photoelectrical capabilities. An F98 fluorescence spectrophotometer was used to measure the EEM fluorescence spectrum (Linguang, Shanghai, China). Total organic carbon (TOC) was tested using a total organic carbon analyzer (TOC-L, Shimadzu Corporation, Kyoto, Japan).

2.2. Synthesis of BiVO₄ and Ag₃PO₄/BiVO₄

BiVO₄ was synthesized via a hydrothermal method. Briefly, 10 mmol of Bi(NO₃)₃·5H₂O and 10 mmol of NH₄VO₃ were separately dissolved in 40 mL and 30 mL of deionized water under ultrasonication for 30 min, followed by 30 min of stirring. The NH₄VO₃ solution was slowly added to the Bi(NO₃)₃·5H₂O solution with continuous magnetic stirring for 30 min. The resulting mixture was transferred to a 100 mL Teflon-lined autoclave and heated at 180 °C for 24 h. The product was cooled, washed repeatedly with deionized water and ethanol, dried at 60 °C for 12 h, and ground into a fine powder.

The Ag₃PO₄/BiVO₄ composite was prepared via in situ precipitation. First, 3 mmol of AgNO₃ was dissolved in 75 mL of ultrapure water. Next, 1.944 g of the as-prepared BiVO₄ powder was dispersed in this solution under ultrasonication for 10 min and stirring for 20 min. Separately, 1 mmol of Na₃PO₄·12H₂O was dissolved in 25 mL of ultrapure water. This phosphate solution was then added dropwise to the AgNO₃/BiVO₄ suspension under continuous stirring, leading to the in situ formation of Ag₃PO₄ on the BiVO₄ surface. The mixture was stirred in the dark for 5 h. The resulting solid was collected by filtration, washed thoroughly with ultrapure water and ethanol, dried at 60 °C for 12 h, and ground, yielding the composite with a nominal Bi/Ag molar ratio of 2:1. Composites with other Bi/Ag ratios (4:1, 1:1, 1:2) were synthesized analogously by adjusting the mass of BiVO₄ added.

2.3. Standard Photocatalytic Procedure for the Ag₃PO₄/BiVO₄/PMS/Vis System

A volume of 200 mL of landfill leachate tailwater was placed in a 500 mL beaker. Specific amounts of the Ag₃PO₄/BiVO₄ catalyst and peroxymonosulfate (PMS) were added (see specific experimental sections for details). The suspension was magnetically stirred in the dark for 30 min to establish adsorption–desorption equilibrium. The photocatalytic reaction was then initiated by turning on the 300W xenon lamp (λ ≥ 420 nm). The light source was positioned at a distance of 15 cm from the solution. The reaction proceeded at room temperature with continuous stirring. At predetermined time intervals, aliquots were withdrawn, immediately filtered through a 0.45 μm membrane to remove catalyst particles, and analyzed for chemical oxygen demand (COD) and UV₂₅₄ absorbance. Unless otherwise stated, all quantitative experiments were performed in triplicate, with data presented as mean ± standard deviation (SD).

2.4. Two-Stage Photocatalytic Treatment

2.4.1. First-Stage Photocatalytic Conditions

For the first stage, 200 mL of raw leachate (pH adjusted to 11) was treated with 0.5 g of Ag₃PO₄/BiVO₄ and 2.0 g·L⁻¹ of PMS. Following a 30 min dark adsorption period, the mixture was irradiated with the xenon lamp for 60 min. The catalyst was then removed by filtration (0.45 μm). The filtrate was analyzed for COD and UV₂₅₄. and used as the influent for the second stage.

2.4.2. Second-Stage Photocatalytic Conditions

The tailwater from the first stage was subjected to two different secondary treatments: (i) The Ag₃PO₄/BiVO₄/PMS/vis system: 200 mL of the first-stage tailwater was treated with 0.5 g of fresh and 2.0 g·L⁻¹ PMS under visible light (pH unadjusted). (ii) The P25/UV system: 200 mL of the same tailwater was treated with 0.3 g of commercial TiO₂ (P25, Evonik Industries AG, Essen, Germany) under UV light irradiation (pH unadjusted).

2.5. Analytical Methods and Activity Assessment

Landfill leachate tailwater contains high concentrations of recalcitrant organic pollutants and is characterized by elevated chemical oxygen demand (COD) and slow intrinsic biodegradability. Consequently, COD serves as a primary indicator for evaluating the quality of leachate tailwater [30]. As reported in the literature, dissolved organic matter (DOM) constitutes a major fraction of landfill leachate tailwater and can be classified into humic substances, fulvic acids, and hydrophilic organic compounds. Among these, humic substances represent more than 70% of the DOM in treated leachate tailwater. With the maturation of leachate, the proportion of humic-like substances in DOM generally increases [31,32]. Therefore, UV₂₅₄ was selected as a secondary representative indicator to assess the degradation efficiency of organic matter in landfill leachate tailwater.

2.5.1. Photocatalytic Activity Assessment

The photocatalytic activity was assessed by monitoring the removal of COD and UV₂₅₄ from the leachate. Experiments generally followed the procedure outlined in Section 2.3. For optimization experiments (e.g., catalyst dosage, PMS concentration, pH), a single-factor approach was used, varying one parameter while holding others constant. Degradation efficiency was calculated based on the measured COD and UV₂₅₄ values relative to the initial concentrations. The conventional water quality characteristics of landfill leachate tailwater are shown in

Table 3. Conventional indicators of landfill leachate tailwater.

Parameter	pH	COD (mg·L−1)	TN (mg·L−1)	TP (mg·L−1)	NH4+-N (mg·L−1)
Value	7.3–8.0	200–270	1467–1563	21–27	302–349

.

2.5.2. Radical Scavenging Experiment

Because chemical oxidant demand (COD) measurements can be interfered with by radical scavengers, and because UV₂₅₄ is specific to humic substances, a model pollutant was used to isolate the radical effects. Rhodamine B (RhB) was therefore selected as the probe compound [33]. For these experiments, a 100 mL RhB solution (20 mg·L⁻¹) was used with a specified catalyst mass in a 250 mL beaker. The procedure followed that in Section 2.3: a 30 min dark adsorption period followed by visible light irradiation with continuous stirring. Samples were taken at intervals, filtered (0.45 μm), and the RhB concentration was determined via its absorbance at 554 nm. The degradation efficiency (η) at time t was calculated using Equation (1), where absorbance values were used directly proportional to concentration:

$${\mathsf{\eta }=(\mathit{C}}_{\mathit{O}}-{\mathit{C}}_{\mathit{t}}{)/\mathit{C}}_0$$

(1)

$\mathsf{\eta }$ is the degradation rate of RhB;

${\mathit{C}}_{\mathit{O}}$ is the initial absorbance of RhB;

${\mathit{C}}_{\mathit{t}}$ is the absorbance after RhB degradation at different time points.

In the radical trapping experiments, ethylenediaminetetraacetic acid disodium salt (EDTA-Na₂) was employed as the scavenger for h⁺, p-benzoquinone (BQ) for ·O₂⁻, isopropanol (IPA) for ·OH, and methanol (MeOH) for both SO₄⁻· and ·OH, to quench the corresponding reactive species. The dosages of each scavenger are listed in

Table 4. Amounts of trapping agent added in the free radical scavenging experiment.

RhB	Catalyst Dosage	Scavenger	Target Species	Scavenger Dosage
100 mL	0.025 g B1A2 + 0.02 gPMS	EDTA-Na2	h+	0.01 g
		BQ	·O2−	0.05 g
		IPA	·OH	0.5 mL
		MeOH	SO4−·	0.5 mL

.

2.5.3. Photocatalyst Stability and Recycling Tests

Catalyst stability was assessed through consecutive recycling tests. After each photocatalytic run, the used catalyst was recovered by filtration, washed thoroughly with ultrapure water and ethanol, dried, and ground before reuse. Performance was monitored by analyzing COD and UV₂₅₄ removal in each cycle.

The specific recycling conditions were: 200 mL of landfill leachate, 0.5 g of Ag₃PO₄/BiVO₄ composite, 2.0 g·L⁻¹ PMS, pH 11, with a 30 min dark period followed by 60 min of visible light irradiation. Post-reaction, the catalyst was separated, washed with water and ethanol, dried at 120 °C for 8 h, ground, and reused.

2.6. Investigation of Degradation Mechanism

2.6.1. Electron Paramagnetic Resonance (EPR)

EPR spectroscopy (Bruker EMXplus-6/1, Billerica, MA, USA) was used to directly detect radical species generated during photocatalysis. Spin-trapping experiments were performed using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) to identify hydroxyl radicals (·OH) and sulfate radicals (SO₄⁻·).

2.6.2. Gas Chromatography-Mass Spectrometry (GC-MS)

Gas chromatography–mass spectrometry (GC-MS) was employed to analyze variations in the composition and content of organic compounds in the leachate tailwater before and after photocatalytic treatment. The analysis was performed using an Elite-5MS quartz capillary column (30 m × 0.25 mm, 0.25 μm film thickness) with helium as the carrier gas at a constant flow rate of 1 mL/min. The injector temperature was set to 280 °C in splitless mode with an injection volume of 1 μL. The column temperature program was as follows: initial temperature of 40 °C held for 5 min, increased to 200 °C at 8 °C/min and held for 10 min, then raised to 260 °C at 8 °C/min and held for 7 min. The mass spectrometer operated in electron impact (EI) mode with an ion source temperature of 250 °C and electron energy of 70 eV. The solvent delay was 5 min, and the mass scan range was 33–550 m/z [34].

Qualitative and Quantitative Methods:

Qualitative identification of detected compounds was based on matching mass spectra against the NIST11 database and comparing retention times, while excluding column bleed peaks. Quantitative analysis was performed using the area normalization method, where the relative content of each identified compound was calculated as the percentage of its peak area relative to the total area of all identified peaks. The formula used is as follows:

$$\mathit{Ci}={\displaystyle \frac{\mathit{Ai}}{\mathit{A}1+\mathit{A}2+\dots +\mathit{Ai}+\dots +\mathit{An}}}\text{×}100\%$$

(2)

$\mathit{Ci}$ represents the content of an individual identified compound (expressed in %);

$\mathit{Ai}$ denotes the chromatographic peak area corresponding to the identified compound;

n is the total number of identified compounds.

3. Results and Discussion

3.1. Characterization

SEM images (Figure 1a–d) reveal that pristine BiVO₄ consists of regular, faceted blocks, while Ag₃PO₄ forms uniformly dispersed granules [35]. In the Ag₃PO₄/BiVO₄ composite (Figure 1c,d), Ag₃PO₄ particles are deposited on the surface of BiVO₄, confirming the successful formation of the composite [36]. Figure 1e presents the XRD patterns. The diffraction peaks of the synthesized BiVO₄ are consistent with the monoclinic phase (JCPDS-14-0688) [37], with high intensity, indicating good crystallinity and phase purity. Characteristic peaks at 2θ = 18.95°, 28.80°, 34.47°, 35.20°, 39.75°, 42.45°, 47.26°, and 53.97° correspond to the (110), (−121), (121), (200), (002), (−112), (051), (042), and (222) planes of BiVO₄, respectively [38]. The composite patterns clearly show the coexistence of Ag₃PO₄ (JCPDS 01-084-0192) and BiVO₄ phases without any impurity peaks [39]. As the Ag₃PO₄ content increases, the intensity of its (210) peak at 2θ = 33.39° grows, confirming successful composite formation [40].

FTIR spectra (Figure 1f) show a broad band around 3430 cm⁻¹ in BiVO₄, attributed to O–H stretching from adsorbed water [41]. In the composite, bands at ~1650 cm⁻¹ and between 730–850 cm⁻¹ are assigned to O–H bending and V–O stretching vibrations of V₃(VO₄) tetrahedra, respectively [42]. Critically, new peaks appear at ~560, 1100, and 1373 cm⁻¹, corresponding to O=P–O, P–O–P, and P=O vibrations, confirming the successful incorporation of Ag₃PO₄.

3.2. Textural Properties (BET Analysis)

Nitrogen adsorption–desorption analysis (Figure 2) shows that both BiVO₄ and the Ag₃PO₄/BiVO₄ composite exhibit Type IV isotherms with an H3 hysteresis loop, characteristic of mesoporous materials [43]. The composite’s specific surface area (0.975 m²/g) is 1.54 times greater than that of pristine BiVO₄. The average pore diameter also increased from 12.7 nm to 17.2 nm. This increased surface area and expanded pore structure provide more active sites and facilitate the transport of reactants and charge carriers, which is beneficial for photocatalytic performance [44].

3.3. X-Ray Photoelectron Spectroscopy (XPS) Analysis

XPS was used to analyze surface composition and chemical states. The survey scan (Figure 3a) confirms the presence of Bi, V, and O in BiVO₄, with Ag and P appearing in the composite. In the Bi 4f region (Figure 3b), peaks at 163.9 eV (4f_5/2) and 158.6 eV (4f_7/2) [45]. The V 2p spectrum (Figure 3c) shows the expected doublet for V5⁺ [46]. The Ag 3d spectrum (Figure 3d) displays peaks at 373.3 eV and 367.3 eV, confirming Ag⁺ [47]. The P 2p peak at 132.1 eV (Figure 3e) is characteristic of PO₄³⁻. The O 1s spectra (Figure 3f) can be deconvoluted into contributions from lattice oxygen (Bi–O/V–O) and surface hydroxyl groups (O–H); the composite shows an additional component from P–O bonds [48]. A minor C 1s peak is attributed to adventitious carbon [49].

3.4. Optical and Photoelectrochemical Properties

The optical properties were studied using UV-Vis diffuse reflectance spectroscopy (DRS). As shown in Figure 4a, all samples exhibit strong visible-light absorption with edges at ~547 nm (BiVO₄), ~620 nm (Ag₃PO₄), and ~580 nm (Ag₃PO₄/BiVO₄) [15]. The composite demonstrates enhanced absorption across the visible range [50,51].

Tauc plots (Figure 4b–d) give direct band gaps of 2.14 eV (BiVO₄), 2.2 eV (Ag₃PO₄), and 2.56 eV for the composite. The increased band gap of the composite may influence charge carrier dynamics [52].

Photoelectrochemical measurements corroborate these findings. The Ag₃PO₄/BiVO₄ composite exhibits a stronger and more stable photocurrent (Figure 4f) and a smaller semicircle in electrochemical impedance spectra (EIS, Figure 4g), indicating lower charge-transfer resistance and more efficient charge separation [53,54]. Furthermore, its photoluminescence (PL) intensity is significantly quenched (Figure 4h), confirming suppressed electron–hole recombination due to effective interfacial charge transfer within the heterojunction [55].

3.5. Optimization of Synthesis Conditions for Ag₃PO₄/BiVO₄

During photocatalytic treatment, all systems employing BiVO₄ and Ag₃PO₄/BiVO₄ showed a distinct reversal in UV₂₅₄ removal upon irradiation. A measurable reduction occurred during the initial dark adsorption phase, indicating adsorption of humic substances onto the catalyst [56]. However, UV₂₅₄ values consistently increased after light exposure, sometimes exceeding the initial leachate absorbance. This phenomenon is attributed to two interrelated processes. First, irradiation can partially degrade adsorbed humics, generating intermediates like aromatic compounds or quinones that may absorb more strongly at 254 nm [57]. Second, photocorrosion of BiVO₄ may dissolve surface species, releasing ions (e.g., Bi³⁺, VO₄³⁻) or soluble Bi-organic complexes that absorb UV light. For Ag₃PO₄/BiVO₄, light-induced reduction of Ag₃PO₄ and release of phosphate or Ag species could amplify this effect [57,58].

As shown in Figure 5a,b, the Ag₃PO₄/BiVO₄ composite exhibited optimal performance at a Bi/Ag molar ratio of 1:2 [59]. Figure 5c,d shows that heating durations of 20 h and 22 h yielded superior COD removal, with the 22 h sample achieving the highest UV₂₅₄ removal. Synthesis time influences Ag₃PO₄ loading; an optimal balance between active sites and aggregation is crucial for charge transfer. Therefore, 22 h was selected as the optimal duration. As shown in Figure 5e, COD removal increased with calcination temperature up to 200 °C. A further increase to 220 °C decreased performance, as moderate temperature promotes Ag₃PO₄ deposition, while excessive heat may cause decomposition. Similarly, Figure 5f shows optimal UV₂₅₄ removal at 200 °C, with absorbance declining from 0.395 to 0.295.

The photocatalytic reaction time significantly influences degradation efficiency. As shown in Figure 5g, COD removal increased within the first 120 min of irradiation, reaching a maximum reduction from 256 mg·L⁻¹ to 184 mg·L⁻¹. Beyond this point, removal plateaued and showed a slight rebound. This may be due to the depletion of reactive species, re-adsorption or re-polymerization of intermediates, and evaporation effects [60,61]. Similarly, UV₂₅₄ removal (Figure 5h) peaked at 80 min before gradually increasing. Consequently, 120 min was selected as the optimal reaction time.

Catalyst Dosage and Reaction Temperature: As shown in Figure 5i,j, a catalyst dosage of 0.9 g provided the best balance between adsorption and photocatalytic activity; higher dosages led to light shielding and aggregation [62]. Regarding temperature, the system performed optimally at 33 °C (Figure 5k,l), achieving the highest COD reduction (from 265 mg·L⁻¹ to 181 mg·L⁻¹). This temperature balances enhanced radical generation against factors like decreased dissolved oxygen [63].

Effect of pH: As shown in Figure 5m,n, pH significantly influenced performance. The highest adsorption and photocatalytic COD removal occurred at pH 10. UV₂₅₄ removal improved with pH, peaking at pH 12. This difference arises because UV₂₅₄ primarily tracks humic substances, while COD measures a broader organic range. Although high pH favors humic degradation, excessively alkaline conditions (e.g., pH 12) can decompose Ag₃PO₄/BiVO₄ and destabilize the heterojunction [64]. Therefore, pH 10 was selected as the optimal balance between activity and catalyst stability, achieving a reduction from 232 mg·L⁻¹ to 142 mg·L⁻¹ in COD and from 0.22 to 0.156 in UV₂₅₄ removal.

In summary, the optimal synthesis conditions were a Bi:Ag molar ratio of 1:2, calcined at 200 °C for 22 h. The optimal treatment parameters were 0.9 g catalyst per 200 mL leachate, pH 10, and 33 °C. Under these conditions, after 140 min of irradiation, COD decreased from 232 mg·L⁻¹ to 142 mg·L⁻¹ and UV₂₅₄ from 0.22 to 0.156.

3.6. Optimization of the Ag₃PO₄/BiVO₄/PMS/Vis

As shown in Figure 6a,b, COD and UV₂₅₄ removal improved with increasing PMS concentration up to 2.0 g·L⁻¹ but declined at 3.0 g·L⁻¹. Moderate PMS enhances radical generation, while excess PMS scavenges radicals via sacrificial reactions, reducing efficiency [65] (Equation (3)).

$${{\mathrm{SO}}_4}^{-}·+{{\mathrm{HSO}}_5}^{-} \to {{\mathrm{SO}}_5}^{-}·+{{\mathrm{SO}}_4}^{2-}·+ {\mathrm{H}}^{+}/{\mathrm{OH}}^{-}$$

(3)

As shown in Figure 6c,d, increasing the catalyst dosage from 0.3 g to 0.5 g enhanced removal due to increased radical generation. A further increase to 0.6 g reduced efficiency due to light shielding and agglomeration [63]; thus, 0.5 g was optimal. Regarding pH (Figure 6e,f), COD removal improved from pH 3 to 9 but decreased above pH 11. While high pH promotes organic flocculation [66], excessive alkalinity (>9.4) converts SO₄⁻· to less reactive species (e.g., SO₅²⁻ and ·OH). The optimum pH was 11, achieving a reduction from 242 mg·L⁻¹ to 138 mg·L⁻¹ in COD and from 0.454 to 0.187 in UV₂₅₄ (58.8% removal).

3.7. Two-Stage Photocatalytic Treatment

The performance of a dual-stage Ag₃PO₄/BiVO₄/PMS/vis system is shown in Figure 7a,b. In the first stage, COD decreased from 194 mg·L⁻¹ to 115 mg·L⁻¹ after dark adsorption and to 113 mg·L⁻¹ after photocatalysis. In the second stage, COD dropped to 97 mg·L⁻¹ after dark adsorption but rebounded to 112 mg·L⁻¹ after illumination, indicating negligible net removal. This rebound is attributed to analytical interference: residual PMS can be oxidized by the dichromate used in COD measurement, leading to overestimation [33,67]. Control experiments in ultrapure water (0.5 g catalyst, 2.0 g·L⁻¹ PMS) confirmed this positive bias, showing a significant artificial COD increase (Figure 7c,d). This interference makes the system unsuitable for a second treatment stage where pollutant concentrations are low.

In contrast, a hybrid Ag₃PO₄/BiVO₄/PMS/vis + P25/UV system was effective (Figure 7e,f). The final COD reached 90 mg·L⁻¹, meeting the Chinese discharge limit (GB 16889-2008). Fluctuations during treatment reflect the complex leachate matrix [68].

UV₂₅₄ showed a continuous decline from 0.38 to 0.116 (69.5% removal), with the P25/UV stage being particularly effective for humic substances.

In summary, the sequential system effectively removed COD and UV₂₅₄, meeting discharge standards while mitigating catalyst deactivation via stage-specific treatment.

Catalyst Stability: As shown in Figure 8a, the catalyst maintained good activity over four cycles, with COD removal decreasing from 28% to 24%. The gradual decline is attributed to the coverage of active sites by small, recalcitrant organic molecules (e.g., siloxanes), which hinder charge carrier adsorption.

XRD analysis (Figure 8b) showed no significant change in crystallinity after recycling, indicating structural stability. Minor peak shifts likely result from adsorbed organic matter.

SEM images (Figure 8c) reveal partial photocorrosion of Ag₃PO₄ and coverage by organic residues (e.g., cyclic siloxanes). This surface fouling, combined with minor structural alteration, contributes to the observed activity loss.

3.8. Kinetic Analysis

The degradation kinetics of CODcr and UV₂₅₄-absorbing organics in landfill leachate were fitted by the pseudo-first-order model: ln(C₀/Ct) = kobs, where C₀ and Ct are initial and instantaneous CODcr/UV₂₅₄ values, t is reaction time, and k_oβs (min⁻¹) is the apparent rate constant.

Kinetic parameters are listed in

Table 5. Pseudo-first-order kinetic parameters for CODcr and UV₂₅₄ removal.

Catalyst System	CODcrkobs min−1	R2	UV254kobs min−1	R2
Two-stage catalytic system	0.00314	1.000	0.00855	0.623
Ag3PO4/BiVO4	0.00152	0.567	0.00477	0.695
Ag3PO4	0.00120	0.446	0.00438	0.847
BiVO4	0.000657	0.868	0.000512	0.595

. The corresponding pseudo-first-order kinetic fitting curves are illustrated in Figure 9. The two-stage system exhibited the highest kobs for both CODcr (0.00314 min⁻¹, R² = 1.000) and UV₂₅₄ (0.00855 min⁻¹, R² = 0.623). All catalysts followed the same reactivity order: Two-stage system > Ag₃PO₄/BiVO₄ > Ag₃PO₄ > BiVO₄ [69,70].

Ag₃PO₄/BiVO₄ showed higher kobs than single catalysts, indicating effective charge separation. Relatively low R² values were attributed to a complex leachate matrix and intermediate accumulation. Consistent kinetic trends between CODcr and UV₂₅₄ confirmed the heterojunction’s enhanced activity and the two-stage system’s synergistic effect.

3.9. Mechanism Analysis of Ag₃PO₄/BiVO₄/PMS/Vis Photocatalysis

Radical quenching experiments were conducted using EDTA-2Na as a scavenger for holes (h⁺), BQ for superoxide radicals (·O₂⁻), IPA for hydroxyl radicals (·OH), and MeOH for both ·OH and sulfate radicals (SO₄⁻·). As shown in Figure 10, the degradation efficiency of RhB reached 100% within 60 min in the absence of any scavenger (control group). The addition of EDTA-2Na reduced the degradation efficiency to 62.6%, indicating a 37.4% decline attributed to the quenching of h⁺, which participates not only in direct oxidation but also in the generation of ·O₂⁻, ·OH, and singlet oxygen (¹O₂) through reactions with dissolved oxygen, water, or PMS. This confirms the presence of h⁺ in the Ag₃PO₄/BiVO₄/PMS/vis system.

The BQ quenching group showed a decrease in RhB degradation to 84.7%, corresponding to a 15.3% reduction, suggesting that ·O₂⁻ is not a dominant reactive species. This is consistent with the conduction band (CB) potential of Ag₃PO₄/BiVO₄, which is insufficiently negative to reduce O₂ to ·O₂⁻ (E(O₂/·O₂⁻) = −0.33 eV) [71].

In the presence of IPA, the degradation efficiency dropped to 57.7%, indicating that ·OH contributes significantly to RhB degradation (42.3% reduction). The formation of ·OH is favored due to the sufficiently positive valence band (VB) potential of Ag₃PO₄/BiVO₄ relative to the H₂O/·OH redox couple (2.27 eV) [72].

The most pronounced inhibition was observed with MeOH, which quenches both ·OH and SO₄⁻·, confirming their crucial roles in the degradation process. SO₄⁻· is generated via the cleavage of the O–O bond in PMS by photogenerated electrons.

These results demonstrate that h⁺ ·OH, and SO₄⁻· are the primary reactive species in the Ag₃PO₄/BiVO₄/PMS/vis system, whereas ·O₂⁻ plays a minor role.

To further verify the generation of ·OH and SO₄⁻·, EPR spectroscopy was performed using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin-trapping agent. Samples were prepared by dispersing 1 mg/mL of catalyst in deionized water (for ·OH) or methanol (for SO₄⁻·), followed by the addition of 200 mM DMPO.

As shown in Figure 10a, no EPR signal was detected in the dark, indicating no ·OH formation under dark conditions. Characteristic DMPO ·OH adduct signals appeared after 5 min and intensified after 10 min of irradiation, confirming light-induced ·OH generation with increasing concentration over time.

Similarly, Figure 10b shows no SO₄⁻· signal in the dark, whereas distinct DMPO-SO₄⁻· signals were observed after 5 min and further enhanced after 10 min of irradiation, demonstrating the light-dependent generation and accumulation of SO₄⁻· in the system.

The 3D-EEM spectra of the landfill leachate tailwater were divided into five characteristic regions [73] (Figure 11a,b): Region I (tyrosine-like, Ex/Em = 230–250/230–330), Region II (tryptophan-like, Ex/Em = 230–250/330–380), Region III (fulvic-like, Ex/Em = 230–250/380–545), Region IV (microbial by-products, Ex/Em = 250–380/250–380), and Region V (humic-like, Ex/Em = 250–520/380–545). Higher contour intensity corresponds to higher concentrations of the corresponding organic fraction.

As shown in Figure 11a, the fluorescence contour of the raw leachate (Ex/Em = 250–400/300–500) primarily overlapped with Regions IV and V, with the peak located in Region V, indicating that humic-like substances constituted the dominant organic fraction. These substances, characterized by aromatic and conjugated double-bond structures [74], are typically resistant to biodegradation. After photocatalytic treatment using the Ag₃PO₄/BiVO₄/PMS/vis system, the 3D-EEM spectrum (Figure 11b) retained similar contour regions (IV and V), but the fluorescence intensity significantly decreased, with the peak position remaining unchanged. This suggests effective degradation of both microbial by-products and humic-like substances into smaller molecular intermediates, short-chain alkanes, CO₂, and H₂O by photogenerated reactive species (h⁺ ·OH, and SO₄⁻·).

The UV–Vis absorption spectrum (Figure 11c) further supported these findings. The raw leachate exhibited strong absorption in the 250–420 nm region, characteristic of aromatic and conjugated systems with high molecular complexity [75]. After photocatalytic treatment, the absorbance in this region markedly decreased, indicating the breakdown of conjugated double bonds, carboxyl groups, and polycyclic aromatic structures into simpler, non-conjugated molecules. The reduction in organic content was attributed to oxidative reactions with reactive oxygen species (¹O₂ SO₄⁻·, and ·OH) generated during photocatalysis [72].

In summary, the combined 3D-EEM and UV–Vis analyses confirm that the Ag₃PO₄/BiVO₄/PMS/vis system effectively degrades recalcitrant humic-like and microbial-derived organics in landfill leachate tailwater via photocatalytic oxidation.

GC-MS analysis was performed to investigate changes in organic composition before and after photocatalytic treatment of landfill leachate tailwater. The total ion chromatograms (TICs) are shown in Figure 12. Compound identification was conducted using the NIST database, with column bleed peaks such as polydimethylsiloxane excluded. Quantification was based on the area normalization method. The main organic constituents detected before and after treatment are summarized in Table 4 and Table 5, respectively.

As shown in Figure 12a, the raw leachate tailwater contained 301 detectable peaks, corresponding to a wide range of organic compounds, including alkanes (C1-C28), esters, biphenyls, phenols, oxalates, ketones, ethers, aldehydes, and alcohols. Alkanes constituted the dominant group. The 16 most abundant compounds identified in the raw tailwater (

Table 6. Main organic compounds before photocatalytic treatment of leachate tailwater.

Serial Number	Retention Time (min)	Organic Compound	Match Factor	Relative Content (%)
1	12.78	3,3-Dimethylhexane	78	2.81
2	13.73	3,6-Dimethyldecane	72	1.8
3	15.66	Dodecane	95	1.64
4	17.13	2,6,11-Trimethyldodecane	81	3.32
5	17.38	4-tert-Butylphenol	96	1.93
6	17.48	Tridecane	96	1.86
7	17.93	2,6,11-Trimethyldodecane	90	2.18
8	18.94	Biphenyl	95	12.77
9	20.7	2,-Methyl-6-propyldodecane	78	3.22
10	20.96	2,4-Diethylphenol	96	2.03
11	21.38	Hexadecane	90	2.44
12	23.83	2-Bromododecane	86	2.84
13	24.41	Heneicosane	91	2.08
14	26.93	2-Bromododecane	91	1.78
15	27.7	Dibutyl phthalate	68	2.12
16	40.23	2,2-Diethylphenol	96	0.87

) comprised 12 alkanes, 3 phenolic compounds, and 1 ester. Biphenyl was the most abundant component (12.77%). The prevalence of alkanes and aromatic hydrocarbons is consistent with previous studies on leachate composition [55], and their recalcitrant nature correlates with the low biodegradability of landfill tailwater.

After photocatalytic treatment (Figure 12b), 299 peaks remained, with alkanes still predominant. The 16 major compounds in the treated tailwater (

Table 7. Main organic compounds after photocatalytic treatment of leachate tailwater.

Serial Number	Retention Time (min)	Organic Compound	Match Factor	Relative Content (%)
1	12.78	4,7-dimethylundecane	83	3.32
2	13.73	3,7-dimethyldecane	64	2.10
3	13.86	1H-Benzotriazole	47	1.86
4	15.65	Dodecane	95	1.49
5	17.13	2,6,11-trimethylhexadecane	81	3.88
6	17.38	4-tert-Butylphenol	96	1.95
7	17.47	Tridecane	96	1.66
8	17.93	2,6,11-trimethylhexadecane	91	2.51
9	20.7	1-Bromotetradecane	78	3.71
10	20.96	2,4-Dimethylphenol	96	2.16
11	21.38	Heneicosane	90	2.85
12	23.83	Hexadecane	91	3.09
13	24.41	Heneicosane	90	2.44
14	26.93	Heneicosane	86	2.06
15	27.7	Heptadecane	89	2.37
16	40.23	2,2′-Methylenebis(phenol)	89	1.18

) included 12 alkanes, 4 phenolic compounds, and benzotriazole. The most abundant compound was 2,6,11-trimethylhexadecane (3.88%).

Figure 12c showed the TOC removal efficiency, which was used to evaluate the mineralization behavior of the synthesized photocatalyst. TOC decreased from 155.0 mg·L⁻¹ to 76.5 mg·L⁻¹, and the mineralization rate reaches 50.64%, indicating that Hybrid Ag₃PO₄/BiVO₄/PMS/vis + P25/UV has a certain mineralization.

By comparing the TICs (Figure 12a,b) and compositional data (Table 6 and Table 7), the following transformation pathways during photocatalysis were inferred:

1. 3,3-Dimethylhexane was degraded into smaller molecules, while benzotriazole was formed, indicating both C-C bond cleavage and cyclization.

2. 2,6,11-Trimethylhexadecane underwent C-C bond scission, yielding smaller molecules, H₂O, CO₂, and 2,6,11-trimethyldodecane.

3. 2-Bromotetradecane was degraded to 2-bromododecane, CO₂, and H₂O.

4. Heneicosane, a long-chain alkane, was cleaved into hexadecane and other low-molecular-weight compounds.

5. Heptadecane was partially cleaved, while tetrabutyl titanate was newly formed.

6. 2,2′-Methylenebis(phenol) underwent aromatic ring opening to form 2,2-diethylphenol, CO_2, and H₂O.

These results demonstrate that the Ag₃PO₄/BiVO₄/PMS/vis system effectively decomposes complex and recalcitrant organic compounds in landfill leachate tailwater through C-C bond cleavage, ring-opening, and oxidative mineralization.

Based on the results, a mechanism for PMS activation by Ag₃PO₄/BiVO₄ to degrade leachate tailwater is proposed (Figure 13). The photocatalytic activity depends on bandgap energy and alignment. The determined Eg values are 2.14 eV (BiVO₄), 2.2 eV (Ag₃PO₄), and 2.56 eV (Ag₃PO₄/BiVO₄). The valence (EVB) and conduction (ECB) band potentials can be estimated using the following empirical formulas:

E_VB = X − E^e + 0.5Eg

(4)

E_CB= E_VB − Eg

(5)

X represents the absolute electronegativity of the semiconductor.

E^e denotes the energy of free electrons on the hydrogen scale, approximately 4.5 eV [76],

Eg is the optical bandgap,

E_vB and E_CB are the valence band and conduction band potentials, respectively.

Based on the calculated band structures, the valence band (EVB) and conduction band (ECB) potentials of BiVO₄ are determined to be 2.61 eV and 0.45 eV, while those of Ag₃PO₄ are 2.559 eV and 0.359 eV, respectively. The matched band alignment enables the formation of an efficient Ag₃PO₄/BiVO₄ p–n heterojunction. Under visible-light irradiation, photogenerated electrons in the CB of Ag₃PO₄ migrate to the CB of BiVO₄, while holes in the VB of BiVO₄ transfer to the VB of Ag₃PO₄, driven by the built-in electric field at the interface. This charge separation effectively suppresses electron–hole recombination and enhances the utilization of photogenerated carriers [77].

Although the CB potentials of both semiconductors are insufficient to reduce O₂ to ·O₂⁻ [78], their sufficiently positive VB potentials allow the oxidation of H₂O to ·OH [72]. Furthermore, the accumulated holes (h⁺) in the VB of Ag₃PO₄ directly participate in the degradation of organic pollutants in the leachate, thereby improving the overall photocatalytic performance.

$${\mathrm{Ag}}_3{\mathrm{PO}}_4+\mathrm{h}\mathsf{\nu }\to {\mathrm{e}}^{-}\left({\mathrm{Ag}}_3{\mathrm{PO}}_4\right)+{\mathrm{h}}^{+}\left({\mathrm{Ag}}_3{\mathrm{PO}}_4\right)$$

(6)

$${\mathrm{BiVO}}_4+\mathrm{h}\mathsf{\nu }\to {\mathrm{e}}^{-}\left({\mathrm{BiVO}}_4\right)+{\mathrm{h}}^{+}\left({\mathrm{BiVO}}_4\right)$$

(7)

$${\mathrm{e}}^{-}\left({\mathrm{Ag}}_3{\mathrm{PO}}_4\right)\to {\mathrm{e}}^{-}\left({\mathrm{BiVO}}_4\right)$$

(8)

$${\mathrm{h}}^{+}\left({\mathrm{BiVO}}_4\right)\to {\mathrm{h}}^{+}\left({\mathrm{Ag}}_3{\mathrm{PO}}_4\right)$$

(9)

$${\mathrm{h}}^{+}\left({\mathrm{Ag}}_3{\mathrm{PO}}_4\right)+{\mathrm{H}}_2\mathrm{O}\to · \mathrm{OH}+{\mathrm{H}}^{+}$$

(10)

The introduction of PMS into the Ag₃PO₄/BiVO₄ system enables multiple roles: PMS acts as an electron acceptor, capturing photogenerated electrons and cleaving the O-O bond to produce SO₄⁻· and ·OH (Equations (11)–(14)) [72]. Simultaneously, PMS can also serve as an electron donor, reacting with photogenerated holes to generate ¹O₂ (Equations (15) and (16)) [79]. Additionally, a small amount of ¹O₂ is formed through the self-decomposition of PMS (Equation (17)) [74].

$${{\mathrm{HSO}}_5}^{-}+{\mathrm{e}}^{-}\to {{\mathrm{SO}}_4}^{-}·+{\mathrm{OH}}^{-}$$

(11)

$${{\mathrm{HSO}}_5}^{-}+{\mathrm{e}}^{-}\to {{\mathrm{SO}}_4}^{2-}+·\mathrm{OH}$$

(12)

$${{\mathrm{SO}}_4}^{-}·+{\mathrm{H}}_2\mathrm{O}\to {{\mathrm{SO}}_4}^{2-}+·\mathrm{OH}+{\mathrm{H}}^{+}$$

(13)

$${{\mathrm{SO}}_4}^{-}·+{\mathrm{OH}}^{-}\to {{\mathrm{SO}}_4}^{2-}+·\mathrm{OH}$$

(14)

$${{\mathrm{HSO}}_5}^{-}+{\mathrm{h}}^{+}\to {{\mathrm{SO}}_5}^{-}·+{\mathrm{H}}^{+}$$

(15)

$${{\mathrm{SO}}_5}^{-}·+{\mathrm{H}}_2\mathrm{O} \to {{\mathrm{HSO}}_4}^{-}+{}^1\mathrm{O}_2$$

(16)

$${{\mathrm{HSO}}_5}^{-}+{{\mathrm{SO}}_5}^{2-}\to {{\mathrm{HSO}}_4}^{-}+{{\mathrm{SO}}_4}^{2-}+{}^1\mathrm{O}_2$$

(17)

In the Ag₃PO₄/BiVO₄/PMS/vis system, reactive species including h⁺,·OH, SO₄⁻·, and ¹O₂ were identified. These species collectively degrade organic compounds in landfill leachate tailwater, converting large molecules into smaller intermediates and ultimately achieving mineralization (Equation (18)). Based on the above analysis, a schematic mechanism illustrating the generation of reactive species in the Ag₃PO₄/BiVO₄/PMS/vis system is proposed.

$${\mathrm{h}}^{+}/·\mathrm{OH}/{{\mathrm{SO}}_4}^{-}·/{}^1\mathrm{O}_2+\mathrm{Organic} \mathrm{pollutants}\to \mathrm{Small} \mathrm{molecular} \mathrm{intermediates}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{CO}}_2$$

(18)

This mechanism underscores that the system’s efficacy stems from the intrinsic heterojunction design combined with PMS activation, avoiding the need for external physical fields or atomic-scale engineering. Compared to advanced strategies like piezoelectric-enhanced [80] or single-atom catalysis [81], which offer high efficiency in model systems, our approach provides a robust, potentially more scalable pathway for treating complex real wastewater, prioritizing broad-spectrum oxidation and operational simplicity.

To further highlight the innovation and advantages of this study,

Table 8. Comparison of This Study with Reported Ag₃PO₄/BiVO₄-Based Photocatalytic Systems.

Comparison Items	Reported Ag3PO4/BiVO4-Based Photocatalytic Systems	This Study
Target Pollutants	Simulated dyes (e.g., methylene blue MB, reactive blue KN-R), simple phenols (4-chlorophenol, 4-nitrophenol), single antibiotics (levofloxacin) and other model organic pollutants [42,49,82]	Mature actual landfill leachate tailwater (complex components: humic substances, refractory organic matter, ammonia nitrogen, salts, microbial metabolites, etc.), not a simulated pollutant system
Reaction System	Mainly simple photocatalytic systems; a few are coupled with H2O2, enzyme catalysis or photoelectrocatalysis, no relevant reports on coupling with peroxymonosulfate (PMS) activation [82]; mainly rely on active species such as ·OH, h+, and O2− to degrade pollutants [34,42,49,82]	Ag3PO4/BiVO4 heterojunction photocatalysis + peroxymonosulfate (PMS) activation synergistic system, which combines the heterojunction with PMS activation for the first time to synergistically generate strong oxidizing species and improve degradation efficiency
Application Scenarios	Laboratory simulated wastewater and simple water distribution systems with few environmental interference factors; no application cases of complex actual wastewater treatment [34,42,49]	Real, high chroma, high organic matter, high salinity landfill leachate advanced treatment scenarios with complex working conditions, which are consistent with actual engineering application needs
Performance Evaluation Indicators	Mainly decolorization rate, degradation rate of single model pollutants, first-order kinetic constants, and catalyst cycle stability; no key compliance indicators for actual wastewater are involved [34,42,49]	COD removal rate, stable effluent COD compliance value (≤90 mg·L−1), mineralization degree, process stability, and improvement of biodegradability of actual tailwater, focusing on the compliance requirements of actual wastewater advanced treatment
Treatment Process	Single-stage batch photocatalytic process, with the core goal of “improving the degradation efficiency of model pollutants” and no clear process design optimization [34,42,49]	Two-stage synergistic catalytic process, with the engineering goal of “up-to-standard discharge of actual tailwater”, and the process concept is reported for the first time in similar studies
Core Objectives	Verify the synthesis feasibility, photocatalytic activity, carrier separation efficiency and catalytic mechanism of Ag3PO4/BiVO4 heterojunction [34,42,49]	Achieve stable compliance of effluent COD ≤ 90 mg·L−1 for actual landfill leachate tailwater, solve the problem of advanced treatment of complex actual wastewater, and promote the engineering application of the heterojunction system
Main Findings	The construction of Ag3PO4/BiVO4 heterojunction can effectively promote the separation of photogenerated carriers, improve the visible light absorption efficiency and photocatalytic degradation activity; some composite systems can improve the cycle stability of catalysts [34,42,49,83]	Ag3PO4/BiVO4 heterojunction can efficiently activate PMS to generate strong oxidizing species; the two-stage process can significantly improve the pollutant mineralization efficiency and system stability under complex matrix, and achieve strict compliance discharge
Research Limitations	Only applicable to simulated wastewater, difficult to directly use for complex actual wastewater treatment; no engineering compliance process design; some systems have problems such as catalyst activity attenuation and difficult recovery [34,42,49]	Scale-up tests and long-term operation verification are still needed; the free radical competition mechanism in complex matrix needs further optimization; the large-scale preparation and recovery process of catalysts needs subsequent improvement

compares the proposed Ag₃PO₄/BiVO₄-PMS synergistic system with the reported Ag₃PO₄/BiVO₄-based photocatalytic systems in terms of target pollutants, reaction systems, performance indicators, main findings, and limitations.

It can be clearly seen from Table 8 that this study is not a simple repetition of the synthesis and performance testing of known catalysts but expands the material system to a more challenging actual wastewater treatment scenario and proposes an innovative process scheme around strict effluent standards.

4. Conclusions

This study confirms the proposed hypothesis that a visible-light-driven Ag₃PO₄/BiVO₄/PMS system can achieve synergistic and enhanced degradation of refractory organics in landfill leachate, attributable to the heterojunction-enhanced charge separation and radical generation.

In this study, an Ag₃PO₄/BiVO₄ heterojunction was synthesized via a combined hydrothermal and in situ precipitation method. The optimal synthesis conditions were a Bi:Ag molar ratio of 1:2 and calcination at 200 °C for 22 h. Under visible light, 0.9 g of the optimized composite reduced the COD of landfill leachate from 232 mg·L⁻¹ to 142 mg·L⁻¹ and its UV₂₅₄ absorbance from 0.22 to 0.156 within 140 min (pH 10, 33 °C). Material characterization showed that incorporating Ag₃PO₄ increased the specific surface area, enhanced visible-light absorption, and suppressed charge carrier recombination. The composite exhibited a bandgap of 2.56 eV and demonstrated superior charge separation efficiency and lower charge-transfer resistance compared to its individual components.

An Ag₃PO₄/BiVO₄/PMS/visible light system was subsequently optimized, achieving a COD reduction from 242 mg·L⁻¹ to 138 mg·L⁻¹ and a UV₂₅₄ reduction from 0.454 to 0.187 using 0.5 g catalyst, 2.0 g·L⁻¹ PMS, and pH 11. To meet stringent discharge standards, a two-stage process integrating this system with a P25 TiO₂/UV treatment was designed. This strategy mitigated catalyst deactivation from adsorption saturation, yielding a final tailwater COD of 90 mg·L⁻¹ and a 69.5% removal of humic-like substances. The composite catalyst showed good stability over multiple cycles, with a minor activity loss due to surface fouling by small organic molecules.

Mechanistic studies using radical quenching and EPR spectroscopy identified photogenerated holes (h⁺), hydroxyl radicals (·OH), and sulfate radicals (SO₄⁻·) as the primary reactive species. GC–MS analysis revealed hundreds of organic compounds, primarily alkanes, esters, and phenols, and allowed the inference of key degradation pathways. The proposed mechanism involves light-induced charge separation, where h⁺ oxidizes H₂O to ·OH, and PMS acts as an electron acceptor to generate SO₄⁻· and further ·OH, leading to the effective degradation and mineralization of refractory pollutants.

In conclusion, an effective Ag₃PO₄/BiVO₄/PMS/visible light system was developed for landfill leachate treatment. This work contributes to sustainable wastewater treatment by leveraging visible light and promoting pollutant mineralization. While advanced strategies employ hybrid physical fields [80,84] or atomic-scale engineering [81] to maximize performance, this study presents a complementary approach emphasizing chemical synergy, catalyst stability, and process integration under ambient conditions. This paradigm offers a practical and scalable pathway for the remediation of complex wastewater, aligning with key sustainability objectives for reducing the environmental impact of water treatment technologies.