Comparative Study on the Catalytic Ozonation of Biotreated Landfill Leachate Using γ-Al₂O₃-Based Catalysts Loaded with Different Metals

Abstract

Global municipal solid waste (~2B tons/year) affects sustainability, as landfill and incineration face persistent leachate contamination, demanding effective management to advance water recycling and circular economies. Accelerated investigation of hybrid biocatalytic ozonation systems is imperative to enhance contaminant removal efficiency for stringent discharge compliance. This study investigates the catalytic ozonation effects of γ-Al₂O₃-based catalysts loaded with different metals (Cu, Mn, Zn, Y, Ce, Fe, Mg) on the biochemical effluent of landfill leachate. The catalysts were synthesized via a mixed method and subsequently characterized using scanning electron microscopy (SEM) and X-ray diffraction (XRD). Pseudo-second-order kinetics revealed active metal loading’s impact on adsorption capacity, with Cu/γ-Al₂O₃ and Mg/γ-Al₂O₃ achieving the highest Q_e (0.85). To elucidate differential degradation performance among the catalysts, the ozone/oxygen gas mixture was introduced at a controlled flow rate. Experimental results demonstrate that the Cu/γ-Al₂O₃ catalyst, exhibiting optimal comprehensive degradation performance, achieved COD and TOC removal efficiencies of 84.5% and 70.9%, respectively. UV–vis absorbance ratios revealed the following catalytic disparities: Mg/γ-Al₂O₃ achieved the highest aromatic compound removal efficiency; Ce/γ-Al₂O₃ excelled in macromolecular organics degradation. EEM-PARAFAC analysis revealed differential fluorophore removal: Cu/γ-Al₂O₃ exhibited broad efficacy across all five components, while Mg/γ-Al₂O₃ demonstrated optimal removal of C2 and C4, but showed limited efficacy toward C5. These findings provide important insights into selecting catalysts in practical engineering applications for landfill leachate treatment. This study aims to elucidate catalyst formulation-dependent degradation disparities, guiding water quality-specific catalyst selection to ultimately enhance catalytic ozonation efficiency.

1. Introduction

Approximately 2 billion tons of municipal solid waste is generated globally each year, posing a significant challenge to sustainable development in terms of effective waste management [1]. Currently, the primary methods for treating municipal solid waste are sanitary landfilling and incineration. However, landfill leachate can continue to seep out for decades after disposal [2]. Even when incineration processes reinject raw leachate into the furnace, issues such as the emission of harmful substances like dioxins persist [3]. Therefore, both sanitary landfilling and incineration face substantial challenges in managing landfill leachate. The proper treatment of landfill leachate plays a pivotal role in advancing sustainable urban development by effectively addressing solid waste management challenges while concurrently facilitating the recycling and utilization of water resources. This dual benefit mechanism not only mitigates environmental contamination risks but also aligns with the circular economy principles advocated in contemporary urban water resource management systems.

Biological treatment, being a low-cost wastewater treatment method, is widely applied to such wastewater management [4]. However, the effluent from biological treatment of landfill leachate often fails to meet increasingly stringent discharge standards [5]. This may be due to the inadequate removal of recalcitrant organic matter present in the leachate by biological processes. For instance, China’s latest “Pollution Control Standard for Domestic Waste Landfills” stipulates a special discharge standard for landfill leachate, requiring the chemical oxygen demand (COD) to be less than 60 mg/L [6]. Although biological treatment processes can achieve approximately 90% COD removal efficiency, they are still insufficient to meet discharge requirements when treating landfill leachate with COD levels reaching thousands or even tens of thousands [7]. Consequently, integrating multiple treatment processes is considered the optimal solution for landfill leachate management [8].

Currently, the most commonly used treatment process for landfill leachate combines biological treatment with membrane filtration. In China, over 80% of landfill leachate treatment plants employ this method [9]. However, membrane modules not only entail high maintenance and replacement costs but also fail to completely degrade pollutants. Consequently, the concentrates produced by membrane separation require further treatment [10]. In contrast, the combined process of biological treatment and catalytic ozonation effectively addresses this issue [11]. Gautam et al. conducted a comprehensive review of advanced oxidation processes (AOPs) for hazardous waste landfill leachate treatment. Despite the emergence of cutting-edge technologies such as electrocoagulation, electro-Fenton, and photo-Fenton, the catalytic ozonation process remains a reliable option for both pretreatment and post-treatment stages of leachate management [12]. Research by Duan et al. demonstrated that a Mn-Ni loaded carbon felt catalyst in an MBR-catalytic ozonation process achieved a COD removal rate of 99.8%, meeting China’s landfill leachate discharge standards within just 13 days in a cyclic reaction system. Similarly, Zhu et al. utilized a MnCeO_x/γ-Al₂O₃ catalyst to enhance the COD removal rate of landfill leachate from 55.8% (post-biological treatment) to 82.4% [13]. Furthermore, Ma et al. employed a cow dung ash catalyst loaded with nano-Fe₃O₄, reducing the COD of landfill leachate from 1050 mg/L (post-MBR treatment) to approximately 60 mg/L [14]. These findings indicate that the combination of biological treatment and catalytic ozonation holds significant promise for landfill leachate treatment.

However, research on catalyst suitability for landfill leachate remains scarce. Previous studies have primarily focused on optimizing the metal loading ratios of catalysts, without thoroughly investigating the differences among various metal components. Addressing this research gap, our study utilized γ-Al₂O₃ as the catalyst support to prepare seven catalysts with different metal components. Using biologically treated landfill leachate as the research subject, we explored the following: (1) the differences in COD and total organic carbon (TOC) removal efficiencies during adsorption and catalytic ozonation processes using different catalysts; (2) the water quality characteristics of the effluent from catalytic ozonation based on ultraviolet–visible (UV–vis) spectroscopy analysis, thereby revealing the specific degradation preferences of different catalysts; (3) the removal characteristics of different catalysts on C1–C5 components by calculating the content of fluorescent components in the effluent from catalytic ozonation using the excitation-emission matrix–parallel factor analysis (EEM-PARAFAC) method.

2. Materials and Methods

2.1. Landfill Leachate Sample

The landfill leachate samples in this study were collected from a landfill site in Jiangsu Province, China, specifically from the effluent of a sequencing batch reactor (SBR) process. The specific parameters are presented in

Table 1. Landfill leachate water quality data.

Parameter	pH	TOC	DOC	COD	NH3-N	UV254	UV365
Range	7.15–7.25	106–110	100–103	300–330	24.5–26.2	1.613–1.615	0.380–0.382

.

2.2. Preparation of Catalysts

The catalysts used in this study were prepared via a mechanical mixing method, involving γ-Al₂O₃ loaded with single metal components: Cu, Mn, Zn, Y, Ce, Fe, and Mg. All seven catalysts share consistent formulation ratios, with the active metal content constituting approximately 6 wt% of the total catalyst mass. The preparation steps were as follows: (1) a specific amount of γ-Al₂O₃ powder was uniformly mixed with the respective metal oxide (CuO, MnO, Zn, Y₂O₃, CeO₂, Fe₂O₃ and MgO) powders and then pelletized to obtain catalyst particles with diameters ranging from 3 to 5 mm; (2) the pelletized catalysts were subjected to steam treatment for 12 h by placing them on a mesh and introducing steam; (3) subsequently, the catalysts were calcined in a muffle furnace at 550 °C for 4 h. After cooling, they were sealed and stored for future use. Some properties of the catalysts are presented in

Table 2. Partial properties of catalysts.

Support	Active Components	Particle Size (mm)	Pore Volume (cm3/g)	Average Pore Diameter (nm)	BET Surface Area (m2/g)
γ-Al2O3	Cu, Mn, Zn, Y, Ce, Fe, Mg	3–5	0.41–0.45	5.88–7.56	151.44–236.73

.

2.3. Ozonation and Catalytic Ozonation Procedure

The experimental setup is illustrated in Figure 1. An ozone generator, manufactured by Tonglin Ozone (Beijing, China), was utilized, and an electronic flow meter from HORIBA (Tokyo, Japan) was employed to monitor gas flow rates. The catalyst dosage ratio in both adsorption experiments and catalytic ozonation experiments with landfill leachate remained consistent, where 600 mL of landfill leachate and 200 g of catalyst were added to a 1 L reactor. The ozone concentration was maintained at 10 ± 0.1 mg/L, with an O₃/O₂ gas mixture introduced into the reactor at a flow rate of 25 mL/min. Each catalytic ozonation experiment lasted for 4 h, with samples collected every 30 min for subsequent analyses, including TOC, COD, UV–vis spectroscopy, and EEM. To eliminate the influence of catalyst adsorption on the catalytic ozonation process, all catalysts were pre-adsorbed in landfill leachate for 24 h prior to the experiments.

2.4. Analytical Method

Prior to measurement, all water samples were filtered through a 0.45 μm membrane to remove particulate matter. To analyze the EEM data of landfill leachate samples, an F-7000 fluorescence spectrophotometer (Hitachi, Tokyo, Japan) was utilized. The instrument scanned excitation wavelengths from 200 to 600 nm and emission wavelengths from 200 to 600 nm, both with 5 nm increments. Total organic carbon (TOC) was quantified using a TOC-L total organic carbon analyzer (Shimadzu, Kyoto, Japan) following the TC-IC method, wherein TOC is determined by subtracting inorganic carbon (IC) from total carbon (TC) in the aqueous samples. COD was measured using the Chinese National Standard Method (HJ/T 399-2007) [15], with blank controls prepared with ultrapure water and titrated to reduce measurement errors, while a predetermined amount of Hg₂SO₄ was added to landfill leachate samples to eliminate chloride interference. UV–vis absorbance values were obtained using an N4 UV–vis spectrophotometer (INESA, Shanghai, China). Blank correction utilizing ultrapure water was implemented to minimize analytical errors, after which the UV–vis absorbance of experimental samples was quantified. For the parallel factor analysis (PARAFAC) modeling of the EEM data, we employed the DOM Fluor toolbox within MATLAB 2021a. This approach allowed for the extraction of fluorescent components of dissolved organic matter (DOM).

3. Result and Discussion

3.1. Characterization of Catalysts

The SEM images, as depicted in Figure 2, reveal that the surface morphology of the γ-Al₂O₃-based catalysts loaded with six different metals—excluding Ce—appears similar. These catalysts exhibit uneven surfaces with irregularly distributed particles and abundant porous structures. In contrast, the Ce/γ-Al₂O₃ catalyst displays a distinct sheet-like surface morphology.

As shown in Figure 3, all peaks in the XRD patterns of Cu, Zn, and γ-Al₂O₃ samples show complete correspondence with standard PDF references: Cu/γ-Al₂O₃ matches CuO (PDF#89-5898), Zn/γ-Al₂O₃ aligns with ZnO (PDF#80-0075), and γ-Al₂O₃ corresponds to Al₂O₃ (PDF#50-0741). For other samples, unmarked weak peaks persist alongside labeled ones, including the following: Mn/γ-Al₂O₃—all peaks except those at 26.3° and 26.4° match MnO₂ (PDF#89-5171); Ce/γ-Al₂O₃—peaks excluding 37.22° and 67.13° correspond to CeO₂ (PDF#34-0394); Y/γ-Al₂O₃—all peaks except 67.42° conform to Y₂O₃ (PDF#83-0927); Fe/γ-Al₂O₃—peaks other than 29.50°, 30.95°, and 67.33° match Fe₂O₃ (PDF#89-0599); Mg/γ-Al₂O₃—all peaks except 45.42° and 66.75° align with MgO (PDF#87-0652). The unmarked weak diffraction peaks, with intensities below the three-strong-peak criterion for phase identification, may originate from raw material impurities or crystalline defects.

3.2. Catalytic Ozonation Efficiency of Different Catalysts

The adsorption performance of a catalyst is a crucial factor in evaluating its catalytic ozonation efficiency. Previous studies have demonstrated that excellent adsorption properties facilitate the rapid degradation and deep mineralization of pollutants [16]. For instance, Xing et al. investigated the catalytic ozonation of oxytetracycline hydrochloride using Mn-Mg/Al₂O₃ catalysts and found that Mg loading enhanced the electrostatic attraction between oxytetracycline molecules and the catalyst surface, thereby improving the catalyst’s adsorption performance [17]. In this study, adsorption experiments were conducted to determine the adsorption kinetics of catalysts loaded with different metals. The adsorption effects on organic pollutants were monitored by measuring TOC values. The results indicated that the adsorption processes of γ-Al₂O₃-based catalysts conformed to a pseudo-second-order kinetic model (Equation (1)). In this study, qₜ represents the TOC adsorption removal capacity (calculated as 1 − C/C₀) at time t, and q_e denotes the total equilibrium TOC adsorption removal capacity. Accordingly, Equation (1) is reformulated as Equation (2). The observed rate constant (K_obs) and equilibrium adsorption capacity (Q_e) for all catalysts were derived from Equation (2) fitting (

Table 3. The adsorption kinetic constant (K_obs) and total adsorption capacity (Q_e) of different catalysts.

Parameter	Cu	Mn	Zn	Y	Ce	Fe	Mg
Kobs	4.54	3.81	5.24	5.00	5.35	3.98	3.08
Qe	0.85	0.82	0.81	0.76	0.74	0.76	0.85

).

$${\displaystyle \frac{t}{q_t}}={\displaystyle \frac{1}{k{q}_e^2}}+{\displaystyle \frac{1}{q_e}}$$

(1)

$${\displaystyle \frac{t}{1-C/C_0}}={\displaystyle \frac{1}{K_{obs}{Q}_e^2}}+{\displaystyle \frac{1}{Q_e}}$$

(2)

Catalysts such as Ce/γ-Al₂O₃, Zn/γ-Al₂O₃, Y/γ-Al₂O₃, and Cu/γ-Al₂O₃ exhibited higher K_obs values (ranging from 4.54 to 5.35), while Fe/γ-Al₂O₃, Mn/γ-Al₂O₃, and Zn/γ-Al₂O₃ showed lower K_obs values (ranging from 3.08 to 3.98). Cu/γ-Al₂O₃ and Mg/γ-Al₂O₃ had the highest Q_e values (Q_e = 0.85), Mn and Zn had slightly lower Q_e values (ranging from 0.81 to 0.82), and Ce/γ-Al₂O₃, Y/γ-Al₂O₃, and Fe/γ-Al₂O₃ had the lowest Q_e values among the seven catalysts tested (ranging from 0.74 to 0.76). These results demonstrate that Ce/γ-Al₂O₃ and Zn/γ-Al₂O₃ exhibit faster pollutant adsorption, while Cu/γ-Al₂O₃ and Mg/γ-Al₂O₃ achieve higher total adsorption capacities. Thus, the active metal component significantly influences the adsorption performance of the catalysts [18].

In the catalytic ozonation experiments, a controlled ozone dosing strategy was implemented to amplify performance variations among catalysts. As illustrated in Figure 4, the metal-loaded catalysts demonstrated substantial enhancement in COD and TOC removal efficiencies during the 4 h degradation process. Statistical analysis revealed distinct COD removal patterns: the Cu/γ-Al₂O₃ catalyst achieved optimal performance with 84.5% COD reduction (265.2 mg/L removal), followed by Fe/γ-Al₂O₃, Y/γ-Al₂O₃, and Mn/γ-Al₂O₃ systems showing 76.8–79.4% removal efficiency (241.2–249.2 mg/L). Comparatively, Mg/γ-Al₂O₃, Ce/γ-Al₂O₃, and Zn/γ-Al₂O₃ exhibited lower efficiencies of 70.4–71.7% (221.2–225.2 mg/L). TOC elimination displayed narrower variation ranges, with Ce/γ-Al₂O₃, Zn/γ-Al₂O₃, Cu/γ-Al₂O₃, and Mg/γ-Al₂O₃ achieving 70.9–72.8% reduction (76.8–78.9 mg/L), while Y/γ-Al₂O₃, Fe/γ-Al₂O₃, and Mn/γ-Al₂O₃ demonstrated 65.5–67.6% removal (71.0–73.4 mg/L). Notably, the Cu/γ-Al₂O₃ system exhibited superior comprehensive performance, reducing COD from 314 mg/L to 48.8 mg/L (0.64 gO₃/g COD utilization efficiency). This meets the updated contamination control standards for municipal solid waste landfills (GB 16889-2024, COD < 60 mg/L) [6]. Comparative analysis with existing studies (

Table 4. Comparison of the different O₃-based AOPs treatment of landfill leachate.

AOP Process	Initial COD Concentration (mg/L)	COD Removal (%)	Ozone Dosing (g O3/g COD)	Reference
Cu/γ-Al2O3 catalytic ozonation	314	84%	0.64	This work
Lava rock-packed bubble column	400–430	46%	0.60	[19]
MnCeOx/γ-Al2O3 catalytic ozonation	1062	82%	0.43	[13]
GAC/O3	1628–1732	56%	1.42	[20]
Co0.25-NC@Al2O3 catalytic ozonation	110	65%	4.36	[21]

) confirmed its exceptional catalytic ozonation efficiency, suggesting potential for sustainable wastewater treatment applications.

Numerous studies have documented the superior catalytic performance of copper-based systems. Yin et al. [22] demonstrated that incorporating Cu into spinel-type CuAl₂O₄ catalysts enhanced ≡Cu²⁺ and ≡Al³⁺ active sites, which was identified as the critical factor for their exceptional activity. Structural analyses reveal that both γ-Al₂O₃ and spinel phases (e.g., MgAl₂O₄) share closely packed cubic configurations [23], providing a favorable framework for metal dispersion. Comparative studies by Kim et al. [24] on γ-Al₂O₃-supported catalysts further confirmed the highest aromatic hydrocarbon degradation efficiency in Cu/γ-Al₂O₃ systems. Surface characterization by Zhao et al. [25] revealed that Cu-loading modified cordierite substrates through alterations in hydroxyl group density and pH_PZC (pH at point of zero charge), with optimal catalytic activity observed when operating near the catalyst’s pH_PZC [26]. These synergistic effects collectively suggest two mechanistic pathways: (1) structural modulation, where Cu incorporation induces unique metal–support interactions that reshape surface electronic properties; (2) electrochemical alignment, where the pH_PZC adjustment through Cu-loading creates favorable electrostatic conditions for pollutant adsorption at typical wastewater pH levels.

The aforementioned studies substantiate that Cu/γ-Al₂O₃ demonstrates superior holistic contaminant removal performance, likely attributable to its unique surface physicochemical properties. However, significance analysis of the comparative results (Figure 4) reveals that COD and TOC removal metrics alone inadequately resolve degradation disparities among these catalysts, necessitating supplementary characterization through advanced analytical indicators.

3.3. Catalytic Ozonation Characteristics of Different Catalysts

3.3.1. Water Quality Characteristics Based on UV–Vis

UV–vis spectroscopy has been extensively employed to monitor the degradation pathways of abiotic humic substances. Compared to conventional bulk parameters (COD, TOC), the Absorbance Ratio Index (ARI) provides enhanced resolution for differentiating catalytic degradation mechanisms, with its temporal evolution explicitly depicted in Figure 5.

α_210/254

The negative correlation between α_210/254 ratios and aromaticity was attributed to the higher UV absorption efficiency of -NH₄⁺ at 210 nm compared to 254 nm, coupled with the greater absorbance ratio of proteins/amino acids to humic substances at 210 nm [27]. Metal-loaded γ-Al₂O₃-based catalysts demonstrated enhanced removal efficiency for aromatic compounds in landfill leachate through catalytic ozonation. After 4 h treatment, comparable aromaticity reductions were observed for Ce/γ-Al₂O₃, Y/γ-Al₂O₃, Fe/γ-Al₂O₃, Mn/γ-Al₂O₃, and Zn/γ-Al₂O₃ systems (α_210/254 range: 10.45–11.97). Notably, Mg/γ-Al₂O₃ achieved optimal performance (α_210/254 = 15.15), followed by Cu/γ-Al₂O₃ (α_210/254 = 13.54), indicating metal-specific catalytic activation patterns.

α_220/254

The α_220/254 ratio exhibited a positive correlation with non-polarity. Korshin et al. have interpreted this ratio as an indicator of polar functional group abundance on aromatic rings, where dissolved organic matter (DOM) with higher ratios demonstrates greater recalcitrance to removal [28]. Wong et al. observed a significant decline in the 220 nm absorption band of dyeing wastewater during treatment processes, potentially associated with condensed aromatic hydrocarbon degradation and conjugated structure reduction [29]. After 4 h catalytic ozonation, Mg/γ-Al₂O₃, Cu/γ-Al₂O₃, and Zn/γ-Al₂O₃ systems achieved the most pronounced enhancement in α_220/254 ratios (values increased from 1.53 to 15.27, 14.07, and 12.69, respectively), suggesting more effective cleavage of condensed aromatic structures compared to other metal-modified catalysts.

α_254/204

A strong positive correlation was observed between the hydrophobic-to-hydrophilic ratio and α_254/204 values (R² = 0.9837), as demonstrated by Al-Juboori et al., with aromatic constituents predominating in the hydrophobic fractions of dissolved organic carbon (DOC), while non-aromatic components dominated hydrophilic fractions [30]. This analytical approach was further validated by Hur et al. [31], who established α_254/204 as a reliable discriminator for DOM compositional variations (R² = 0.968). As illustrated in Figure 5, all metal-loaded Al₂O₃-based catalysts exhibited comparable aromatic compound degradation efficiencies, with minimal performance discrepancies observed among different metal modifications.

α_365/250

The α_365/250 ratio (commonly termed E₃/E₂) has been widely adopted as a spectroscopic indicator of molecular weight distribution in natural organic matter (NOM), where elevated ratios correspond to higher molecular weight fractions due to enhanced long-wavelength absorption by macromolecular constituents [32,33]. Although Ce/γ-Al₂O₃ demonstrated moderate performance in conventional pollutant removal COD and TOC, it exhibited remarkable efficacy in macromolecular substance degradation, as evidenced by the drastic reduction in α_365/250 values from 0.23 to 0.01 during catalytic ozonation.

α_400/300

The α_400/300 ratio has been established as a robust spectroscopic indicator for assessing humification degree in humic acids, characterized by minimal background interference and excellent reproducibility [34]. As delineated in Figure 5, Cu/γ-Al₂O₃ and Y/γ-Al₂O₃ exhibited optimal humification enhancement during the initial 2 h treatment (α_400/300 reduction from 0.22 to 0.05 and 0.06, respectively). However, this promotional effect attenuated during subsequent 2–4 h operation. Following 4 h degradation, Mn/γ-Al₂O₃, Zn/γ-Al₂O₃, and Ce/γ-Al₂O₃ systems achieved comparatively lower residual ratios (0.01 ≤ α_400/300 ≤ 0.02). Notably, Mn/γ-Al₂O₃ demonstrated superior performance in sustained humification promotion, maintaining effective functionality throughout both 2 h and 4 h degradation phases.

Catalytic performance was metal-dependent, with Mg and Cu systems excelling in aromatic/non-polar compound removal, while Ce and Mn favored macromolecular/humic degradation.

3.3.2. Degradation Characteristics of DOM Based on EEM

The EEM-PARAFAC analysis delineated landfill leachate into five fluorescent components, as illustrated in Figure 6. Component C1 displayed spectral signatures consistent with humic-like substances reported in municipal waste matrices [35,36]. C2 exhibited strong alignment with microbially derived humic acids [37,38]. C3’s fluorescence profile resembled photochemical degradation byproducts and terrestrial humic materials [39,40,41]. C4 was characterized as protein-like substances through reference spectral matching [39,42,43]. Notably, C5 represents a UV humic-like component with limited literature documentation [40,44]. This compositional analysis confirms the predominance of humic-like substances in raw leachate samples, aligning with the characteristic profile of stabilized landfill leachates documented in prior studies [45,46,47].

TOC was employed as a complementary parameter to assess mineralization efficiency during catalytic degradation of fluorescent components. Figure 7 reveals distinct oxidation patterns: conventional ozonation achieved persistent fluorescence reduction with minimal TOC variation (<6.5% reduction) over 4 h, demonstrating molecular ozone’s selective chromophore oxidation capacity. Comparatively, metal-loaded catalytic systems exhibited synergistic removal efficiencies, attributable to enhanced radical-mediated oxidation mechanisms. When analyzing the catalytic effects of different catalysts on fluorescent components, TOC is utilized as an auxiliary indicator to assess the degree of mineralization. As depicted in Figure 7, during a 4 h ozonation degradation process, the fluorescence intensity continually decreased, whereas TOC remained nearly constant. This observation suggests that ozone molecules exhibit oxidative effects on chromophoric groups but are ineffective in fully oxidizing organic pollutants, reflecting the selective oxidation characteristic of ozone. In contrast, ozone catalytic oxidation systems loaded with metals demonstrated significant removal efficiencies for both fluorescent substances and TOC, aligning with the perspective that radicals generated by catalysts possess higher oxidation efficiency than ozone molecules. With prolonged degradation time, the five fluorescent components in the ozone catalytic oxidation system were gradually oxidized and decomposed. During the initial 2 h, TOC degradation was more pronounced; however, after 2 h, the TOC degradation rate gradually slowed, while the content of the five fluorescent components continued to decrease slowly. This finding is consistent with the conclusion by Fu et al. that ozonation can effectively oxidize all fluorescent components [48]. The possible reason is that easily degradable substances have been gradually depleted, and ozone and its generated radicals cannot further achieve complete mineralization [49,50]. As shown in Figure 7, regarding the removal efficiency of fluorescent components, Cu/γ-Al₂O₃ and Mg/γ-Al₂O₃ catalysts outperformed catalysts loaded with other metals.

Figure 8 presents the relative residual concentrations of EEM components after 4 h catalytic ozonation, normalized against the γ-Al₂O₃ carrier baseline. Component-specific analysis revealed metal-dependent degradation patterns: Mg/γ-Al₂O₃ achieved optimal removal efficiencies for C1 (0.68-fold of carrier), C2 (0.59-fold), and C4 (0.53-fold). Cu/γ-Al₂O₃ demonstrated superior C3 reduction (0.71-fold), while Zn/γ-Al₂O₃ showed maximal C5 elimination (0.87-fold). Catalytic performance evaluation against the bare carrier indicated the following:

(1)

Cu/γ-Al₂O₃ outperformed γ-Al₂O₃ across all components (C1–C5);

(2)

Mn/γ-Al₂O₃ and Zn/γ-Al₂O₃ exhibited inferior C1/C2 removal but enhanced C3/C4/C5 degradation;

(3)

Y/γ-Al₂O₃, Ce/γ-Al₂O₃, and Fe/γ-Al₂O₃ improved C4 removal and maintained comparable C3 elimination, yet showed reduced efficacy for C1/C2/C5;

(4)

Mg/γ-Al₂O₃ surpassed the carrier in C1–C4 removal but underperformed in C5 elimination.

Figure 8. EEM component contents after 4 h catalytic ozonation.

Figure 8. EEM component contents after 4 h catalytic ozonation.

4. Conclusions

(1)

The γ-Al₂O₃ catalysts loaded with various metals significantly enhance the removal efficiency of organic pollutants in the biochemical effluent of landfill leachate. Among them, the Cu/γ-Al₂O₃ catalyst exhibits the best overall catalytic performance, achieving the highest removal rates of COD and TOC, making it the preferred catalyst for practical applications.

(2)

The removal efficiencies of different organic components in the biochemical effluent of landfill leachate vary among the catalysts loaded with different metals. The Mg/γ-Al₂O₃ catalyst demonstrates the best removal efficiency for aromatic compounds and non-polar substances, followed by the Cu/γ-Al₂O₃ catalyst. In contrast, the Ce/γ-Al₂O₃ catalyst shows outstanding performance in degrading macromolecular substances.

(3)

The γ-Al₂O₃ catalysts loaded with metals exhibit significant catalytic effects on fluorescent substances in landfill leachate. The Cu/γ-Al₂O₃ catalyst achieves better removal efficiencies for components C1 through C5 compared to the γ-Al₂O₃ support alone, while the Mg/γ-Al₂O₃ catalyst shows the best specific removal effect on component C4.

This study substantiates that the strategic selection of heterogeneous ozone catalysts significantly enhances catalytic ozonation efficiency for advanced landfill leachate treatment, while providing critical insights into addressing urban sustainability challenges through integrated solid waste valorization and water resource circularity.

However, this study conducted comparative analyses of degradation performance among single-metal-loaded catalysts using conventional metrics (COD, TOC, UV, EEM), yet lacked in-depth exploration at the molecular level and suffered from limited catalyst diversity. Future studies should expand to investigate catalytic degradation variations across diverse water matrices and multi-metal formulations, supplemented by advanced analytical techniques (e.g., FT-ICR-MS) for molecular-scale mechanistic elucidation. In addition, the primary constraints impeding the practical implementation of advanced catalytic materials in engineering systems are high catalyst procurement costs and insufficient operational stability. Comprehensive investigations are warranted to develop cost-effective catalytic alternatives and optimize stability enhancement strategies, with the dual objectives of extending catalyst longevity and minimizing lifecycle expenditures.