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Article

Study on Photocatalytic Peroxone Process for Treating Organic Pollutants in Leachate Based on Modified Carbon Quantum Dots

by
Shuo Wu
,
Nuo Meng
,
Lin Ma
,
Xiguo Zhang
,
Shihu Ding
and
Wei Wang
*
MOE Key Laboratory of Pollution Processes and Environmental Criteria, Tianjin Key Laboratory of Environmental Technology for Complex Trans-Media Pollution, Tianjin Key Laboratory of Environmental Remediation and Pollution Control, College of Environmental Science and Engineering, Nankai University, Tianjin 300350, China
*
Author to whom correspondence should be addressed.
Catalysts 2025, 15(9), 903; https://doi.org/10.3390/catal15090903
Submission received: 14 August 2025 / Revised: 12 September 2025 / Accepted: 16 September 2025 / Published: 18 September 2025
(This article belongs to the Special Issue Environmental Catalysis and Nanomaterials for Water Pollution Control)
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Abstract

This study couples a carbon quantum dot photocatalyst with a proton relay installed (EDTA-CQDs) for efficient hydrogen peroxide (H2O2) production with an ozone (O3) system. In situ activation of O3 is achieved by the photogenerated H2O2, which integrates the photocatalytic hydrogen peroxide production (PHP) and advanced oxidation processes (AOPs) to form a new photocatalytic peroxone (H2O2/O3) system, achieving highly efficient solar-driven degradation of recalcitrant organic pollutants in landfill leachate without the addition of external H2O2. The composite system exhibits efficient degradation ability for various typical pollutants in landfill leachate, among which the degradation percentage of 100 mg L−1 hydroquinone (HQ) reaches 97% within 30 min. This is due to the synergistic effects of O3 oxidation, photoactivation of O3, activation of O3 by EDTA-CQDs, and activation of O3 by in situ-generated H2O2. In the EDTA-CQD-based H2O2/O3 system, free radicals can be dynamically regenerated after the addition of pollutants, achieving sustained and efficient degradation. Therefore, in the treatment of actual leachate, the removal percentages of COD, TOC, and UV254 are nearly 90%, 70%, and 55%, respectively, demonstrating the significant advantage of this system in treating high-concentration recalcitrant organic pollutants in wastewater of complex quality.

Graphical Abstract

1. Introduction

Landfilling stands as one of the most widely adopted methods for managing urban solid waste. In landfill treatment, solid waste is decomposed by natural bacterial communities through a series of physical, chemical, and biological processes, resulting in highly contaminated wastewater termed “leachate” that poses severe environmental risks and significant threats to surface and groundwater systems. Therefore, it is crucial to treat such wastewater before its discharge [1,2,3,4,5].
Due to poor biodegradability, conventional treatment methods often have low removal efficiency and high operating costs for pollutants in landfill leachate [6,7,8]. In recent years, advanced oxidation technologies (AOPs) based on the highly oxidative hydroxyl radical (·OH) have attracted widespread attention for the treatment of recalcitrant wastewater. Among them, the Fenton process, UV/TiO2, ozone oxidation, and electrochemical oxidation have been used for the treatment of leachate [1,9,10,11]. The ozone oxidation process has achieved especially good results [10,12,13,14]. However, in these studies, ·OH is mostly generated through the catalytic oxidation of ozone to decompose organic compounds. Metal catalysts are expensive and prone to deactivation, and high salt solutions can cause metal corrosion and agglomeration. These issues to some extent limit the large-scale application of catalytic ozonation technology.
At present, the peroxone process (H2O2/ O3) is the most direct and convenient method to improve the ozone oxidation process [15,16,17]. H2O2 can induce rapid decomposition of O3 and promote the generation of ·OH, which is an economical and feasible way to transform simple ozone oxidation into advanced oxidation processes [18,19]. This process utilizes the synergistic action of H2O2 and O3 to efficiently generate reactive radicals under neutral to alkaline conditions, breaking through the strong acid dependence of the Fenton system and significantly expanding its application scope. The peroxone process has excellent mineralization ability for refractory organics such as humic acid and antibiotics, and can effectively decolorize, which is highly consistent with the water quality characteristics of leachate [20,21,22,23,24]. However, the traditional peroxone process relies on the exogenous addition of H2O2, while direct addition of H2O2 is costly and poses transportation safety hazards. Adding too much H2O2 at once can compete with organic components and cause quenching of ·OH, resulting in waste of active ingredients. In addition, the low solubility of O3 will limit the utilization rate of oxidants [25,26,27].
To address these technological bottlenecks, this study proposes a synergistic process combining photocatalytic H2O2 production with in situ O3 activation. Photocatalytic H2O2 production (PHP), as a green and effective method, is considered a promising alternative to chemical H2O2 production. Under light radiation, the excited photogenerated electrons of a catalyst can reduce oxygen through the proton-coupled electron transfer (PCET) step to generate H2O2. The continuously produced H2O2 can activate O3 in situ to generate free radicals, ensuring the steady dissolution and efficient utilization of O3. Meanwhile, the ultraviolet light in the light source of PHP can catalyze ozone oxidation. In addition, the O2 generated by O3 decomposition can serve as an oxygen source for PHP reaction, further promoting the generation of H2O2. Only O2 and H2O are generated after the oxidation reaction, which does not cause pollution to the environment. Therefore, from both the perspective of oxidation efficiency and environmental safety, it is an ideal choice for the treatment of leachate wastewater.
A large amount of existing research has provided multiple approaches (such as defect modification, element doping, morphology control, etc.) [28,29,30] to enhance the separation and transport of photoinduced electron, but there is limited research on enhancing the photocatalytic performance of oxygen reduction through promoting proton extraction and efficient transfer of photocatalysts. In our previous research [31], a strategy was developed to promote H2O2 photosynthesis by enhancing the proton supply. The carboxyl groups (-COOH) of chelating agent ethylenediaminetetraacetic acid (EDTA) are linked to the amino groups (-NH2) on the surface of carbon quantum dots (CQDs). The amide bonds (-CO-NH-) formed act as a proton relay to activate the proton-supplying groups on the surface of the catalyst and improve the proton utilization in solution, promoting the two-electron reduction of oxygen through the PCET reaction, with a H2O2 selectivity of up to 94%, while avoiding the use of organic sacrificial agents.
CQD, as a novel carbon-based photocatalyst, can effectively avoid the problem of metal ion leaching during the catalytic process. Due to the presence of graphene sp2 hybrid carbon networks, CQDs may not only serve as a photocatalyst but also simultaneously initiate a carbon-catalyzed ozonation process [32,33,34,35,36]. The conjugated π system within the graphite carbon framework can accelerate the electron transfer rate, thereby enhancing ozone activation and redox reactions with organic compounds. This rapid charge transfer also produces a synergistic effect with other active sites, such as defects, oxygen-containing functional groups, and doped heteroatoms, triggering free-radical/non-free-radical pathways and promoting redox reaction activity [37]. When the PHP system based on modified CQDs is combined with ozone, it is still unknown whether the photocatalytic and catalytic ozonation effects of CQDs, as well as the photocatalytic ozonation effect, can cooperate synergistically, and in what way the pollutants will be oxidized and degraded in this composite system.
There is currently no report in the existing publications on the construction of an in situ peroxone system using carbon-based materials for photocatalytic H2O2 production. The simultaneous introduction of self-produced H2O2, non-metallic catalysis, and photoactivation may create a more efficient advanced oxidation process, solve key problems such as weak ozone oxidation ability, low production of ·OH, and narrow applicable pH range, and fill the research gap of CQD photocatalyst-enhanced ozonation technology for treating high-salinity and high-COD landfill leachate. Therefore, a study was conducted on the in situ photocatalysis-enhanced O3 treatment of organic pollutants in leachate based on the modified CQDs here. Using EDTA-modified carbon quantum dots (EDTA-CQDs) developed as photocatalysts, an integrated “photocatalytic H2O2/O3” system was constructed by incorporating in situ-generated H2O2 instead of external chemical reagents with O3 to enhance the free radical chain reactions under simulated solar radiation. Targeting common phenolic pollutants (hydroquinone, HQ), novel recalcitrant organic antibiotics (sulfamethoxazole, SMX; tetracycline, TC), and antiepileptic drugs (carbamazepine, CBZ) in landfill leachate, this study investigated the performance of catalytic degradation and effects of process parameters, revealed the free radical generation patterns, multiple reaction mechanisms, and pollutant degradation pathways, and further verified the treatment efficiency of the composite system for actual landfill leachate. Among these pollutants, HQ (a monocyclic phenol) represents small-molecule organics, while SMX (with a sulfonamide group), TC (a polycyclic tetracycline), and CBZ (a tricyclic heterocycle) embody recalcitrant structures, including heterocyclic rings, amide bonds, and sulfonic acid groups, which are prevalent in landfill leachate (e.g., refractory pharmaceuticals and industrial intermediates). The selection of reference substances covers diverse organic pollutant structures with different degradability, which can verify the catalytic universality of EDTA-CQDs. The research aims to overcome the limitations of traditional ozonation oxidation processes by developing a low-cost, efficient, and environmentally friendly advanced treatment technology for leachate, providing theoretical foundations and technical support for sustainable landfill leachate management.

2. Results and Discussion

2.1. Degradation of Hydroquinone in Different Systems

Our previous research confirmed the successful introduction of proton relay on the surface of CQDs through the glycol solvothermal method and subsequent amidation reaction. The obtained EDTA-modified CQDs can increase the photogenerated H2O2 yield of CQDs from 512 to 2200 μ mol g−1 h−1 without the need for aeration and sacrificial agents, which is much higher than other reported carbon-based photocatalysts [31]. EDTA-CQDs have an appropriate band structure (valence band: 1.67eV; conduction band: −1.04eV) and therefore can be excited by light to generate electrons and holes. The photogenerated electrons transfer to the surface of the material and can reduce O2 to generate H2O2 through two-step single-electron reactions with the proton provided by the hydroxyl groups of CQD itself and the amide bonds formed on the CQD surface. In addition, the amide bonds can obtain protons from the solution and promote intramolecular proton transfer, acting as proton relays. Adequate proton supply ensures effective electron transfer and O2 utilization, promoting the production of H2O2 through the PCET reaction. The efficient degradation performance of the EDTA-CQD-based H2O2/O3 system was observed by simultaneously adding O3 and the target pollutant hydroquinone to the photocatalytic system, demonstrating the existence of synergistic effects.
To investigate the contributions and interactions of various components in the EDTA-CQD-based H2O2/O3 system, degradation experiments were conducted on light irradiation, O3 oxidation, photoactivated O3 oxidation, EDTA-CQD adsorption, EDTA-CQD-activated O3 oxidation, EDTA-CQD-based H2O2 oxidation, and the EDTA-CQD-based H2O2/O3 system (Figure 1). In the single degradation system, the removal efficiency of HQ by light irradiation, EDTA-CQD adsorption, and EDTA-CQD-generated H2O2 is very low (<10%) within 30 min. The stability of H2O2 is high, and it is difficult to decompose spontaneously to produce highly active free radicals. The structurally stable HQ required strong oxidizing species (e.g., ·OH) to attack its benzene rings or hydroxyl groups for effective degradation. Light irradiation and unactivated H2O2 cannot provide a sufficient driving force. The photochemical activity of EDTA-CQDs depends on photogenerated carriers, and active species cannot be produced without the participation of light. Additionally, their surface adsorption is limited. The direct O3 oxidation achieved a 60% degradation percentage for HQ.
Light activation primarily accelerates O3 decomposition to generate ·OH and other free radicals, enhancing the indirect oxidation pathway. As an energy source, light triggers the following free radical chain reactions:
O3 + hν → O2 + O(3P)
O3 + hν → O2 + O(1D)
O(3P)/O(1D) + H2O → 2·OH
In the photoreactive O3 system, light radiation generates free radicals through a series of chain reactions. Firstly, O3 absorbs light energy and decomposes into ground-state oxygen atoms (O(3P)) or excited-state oxygen atoms (O(1D)). These oxygen atoms then react with water to form ·OH. Subsequently, ·OH can react with O3 to generate peroxy radicals (HO2·), which in turn react with O3 to regenerate ·OH, forming a continuous free radical chain. These chain reactions enable sustained production and accumulation of ·OH, improving the pollutant degradation efficiency through indirect oxidation pathways. In this study, the degradation percentage of HQ only increased to 66% after irradiation, which may be due to the use of a low-power simulated solar light source in the experiment, with insufficient ultraviolet radiation.
This study revealed that EDTA-CQDs in the absence of light irradiation can also activate O3, achieving a 76% degradation percentage of HQ in this system. The surface of EDTA-CQDs contains functional groups such as hydroxyl (-OH), carboxyl (-COOH), and amino (-NH2) groups, which can adsorb O3 molecules through hydrogen bonding or electrostatic interactions. Through density functional theory (DFT) calculations, Li et al. [38] found that the -OH sites on the surface of CQDs have a binding energy of −0.65 eV for O3, significantly lower than the decomposition barrier (approximately 1.0 eV) of O3, thus enabling O3 decomposition. The adsorbed O3 may react with the graphene sp2 hybrid carbon networks and undergo heterolysis or homolysis on the surface of EDTA-CQDs, generating ·OH and ·O2− [39]. The generated ·O2− can further react with O3 to regenerate ·OH, significantly increasing the concentration of free radicals and improving the oxidation efficiency.
Studies also demonstrate that -COOH-modified carbon quantum dots (COOH-CQDs) exhibit higher catalytic activity than –OH-modified carbon quantum dots (OH-CQDs). Zhang et al. [39] reported that, under pH 7 conditions, the decomposition rate constant of O3 by COOH-CQDs reached 0.052 min−1, which is 2.3 times higher than that of OH-CQDs. This enhancement stems from the acidic environment generated by -COOH groups, which facilitates the protonation of O3 (O3 + H+ → HO3+), thereby reducing the energy barrier of decomposition. The initial pH value in this experiment is 6, and HQ can be degraded to produce acidic intermediates. Therefore, the degradation percentage of the EDTA-CQDs/O3 system increased by 16% compared to the O3 system.
At an EDTA-CQDs dosage of 0.2 g/L, the yield of H2O2 under radiation for 30 min was 20 mg/L, and no ·OH was detected, indicating that H2O2 did not decompose only in the presence of radiation. This also explains the extremely low HQ removal in this system. When O3 and HQ were added to the above system, H2O2 was almost completely consumed at 30 min of reaction, verifying its role in activating O3. The yield of ·OH was approximately 27 μM at this time. It can be seen that, in the EDTA-CQD-photogenerated H2O2 /O3 system, the efficient degradation (95%) of HQ is due to the coupling of multiple mechanisms, including O3 oxidation, O3 activation by light, O3 activation by EDTA-CQDs, and O3 activation by photogenerated H2O2.
When using the same amount of H2O2 (20 mg/L) and O3 to form a peroxone system, it was found that, even under light irradiation, the removal of HQ was only 80%. This demonstrates that the O2 generated by the decomposition of O3 in the EDTA-CQD-based H2O2/O3 system serves as an oxygen source for the EDTA-CQD-mediated O2 reduction reaction to promote the in situ production of H2O2. Meanwhile, the rapid consumption of H2O2 and production of ·OH also prove that the generated H2O2 can be fully utilized in situ to activate O3. These avoid various drawbacks of adding exogenous H2O2 in traditional peroxone systems.
The reusability investigation of EDTA-CQDs found that the catalyst maintained high HQ removal efficiency (95%, 88%, and 86%) in three cycles of reaction. This is consistent with the stable H2O2 production performance (100%, 92%, and 95%) of EDTA-CQDs. As demonstrated by the characterizations in our previous article [31], the structural composition and surface functional group did not undergo significant changes after multiple uses, indicating that the EDTA-CQDs remained stable during the photocatalytic process.

2.2. Degradation of Hydroquinone Under Different Reaction Conditions

Subsequently, the reaction conditions were adjusted by using 3 mol/L NaOH/HCl to alter the initial pH of the reaction system. As shown in Figure 2, in the EDTA-CQD photogenerated H2O2/ O3 system, the initial pH of the reaction affected the degradation rate, but, in the pH range of 1–12, the degradation percentage of each system exceeded 90% after 60 min of reaction. The acidic environment can provide more protons and promote the formation of H2O2, accelerating the degradation process in the initial stage of the reaction. H2O2 rapidly generated and decomposed to produce free radicals, activating O3 for degradation. However, an excessively low pH environment (pH 1) inhibited O3 decomposition, resulting in a lower reaction rate. In a strongly alkaline condition (pH 12), O3 self-decomposition dominated initially, while ·OH radicals were quenched under alkaline conditions, hindering oxidation reactions. However, the intermediate products of HQ degradation are acidic, so, as the reaction progressed, the system pH gradually decreased, and a certain degradation efficiency was maintained. Within the pH range of 5–9, H2O2 and O3 exhibited efficient synergistic interactions, with dual reaction mechanisms of direct attack by O3 and oxidation by free radicals derived from O3 decomposition. It can be seen that the composite system exhibits high degradation ability in a wide pH range (1–12) by coordinating H2O2 production and O3 decomposition. It not only utilizes the advantages of an acidic environment for H2O2 generation but also has a strengthening effect on the two degradation mechanisms of O3 (direct oxidation by O3 and indirect oxidation by ·OH), exhibiting strong adaptability to complex pH conditions in water treatment. This approach provides a solution for handling pollutants with varying pH levels in practical applications.
Comparing the effects of different HQ concentrations and EDTA-CQD catalyst dosage on degradation rates (Figure 3), it was found that a 50-fold increase in HQ concentration resulted in only a 30% decrease in degradation efficiency. The degradation efficiency of HQ at 10 mg/L approached 100% within 10 min, while 500 mg/L HQ also achieved a degradation percentage of 70% within 30 min. This indicates that the EDTA-CQD-based H2O2/O3 system has strong oxidation capacity, and it can provide sufficient oxidants for high concentrations of HQ. EDTA-CQDs serve as both photocatalytic material and O3 activation material. Increasing their dosage can provide more active sites, promoting H2O2 generation and O3 decomposition to boost oxidation capacity. When the dosage of EDTA-CQDs increased from 0.05 g/L to 0.2 g/L, the degradation efficiency increased by about 25%, but the further increase in dosage had little effect on improving the degradation performance, which may be due to the insufficient light absorption caused by the excessive dosage. Therefore, the reaction conditions for the EDTA-CQD-based H2O2/O3 system in the subsequent experiments were set at T = 25 °C, Xe = 26.54 mW cm−2, O3 = 0.1 L h−1, and EDTA-CQDs = 0.2 g L−1. Under these conditions, the degradation percentage of 100 mg/L HQ reached nearly 97% after 30 min of reaction.

2.3. Degradation Effect of EDTA-CQD-Based H2O2/O3 System on Emerging Pollutants

To verify the universality of the EDTA-CQD-based H2O2/O3 system for the treatment of various new organic pollutants, three environmentally persistent contaminants with significant structural differences were tested (Figure 4). Among them, CBZ contains nitrogen heterocycles and conjugated structures, TC has polycyclic frameworks and polar functional groups, and SMX contains sulfonamide groups and heterocycles. They were used to comprehensively investigate the degradation ability of the EDTA-CQD-based H2O2/O3 system on nitrogen-containing heterocycles, polycyclic structures, and polar functional groups. The experiments verified the effectiveness of the composite system in cleaving various chemical bonds, including C-N bonds, C-O bonds, and π bonds in the benzene ring, thereby confirming its broad applicability in treating complex wastewater.
The experimental results indicated that the photodegradation percentage of CBZ by photoactivated O3 within 30 min was only 43%, significantly lower than HQ (66%), while the composite system achieved a degradation percentage of 89%. This is due to the strong chemical stability of N-heterocycles (such as diazepines) of CBZ. Compared with HQ, CBZ lacks an obvious high electron cloud density region, making it difficult to be degraded by O3 through direct electrophilic attack. It needs to rely on the indirect oxidation path of · OH to attack the conjugated double bond or side chain first, and then it gradually degrades the heterocycle, resulting in a longer reaction path and time.
The polycyclic amide structure of TC and the sulfonamide heterocycles of SMX exhibit low sensitivity to direct electrophilic attack by O3, and also rely on indirect oxidation by ·OH radicals. In photoactivated O3 systems, UV-driven O3 decomposition better accommodates the stepwise oxidation requirements of TC/SMX. The negative charges on the surface of EDTA-CQDs may generate electrostatic repulsion with the polar groups of TC/SMX (e.g., carboxyl groups of TC and sulfonamide groups of SMX), weakening the adsorption of pollutants by the catalyst and reducing the contact efficiency between the free radicals and contaminants in the EDTA-CQDs/O3 system. Therefore, unlike HQ degradation, the photoactivated O3 degradation of TC/SMX outperformed EDTA-CQD-activated O3. These three pollutants showed high degradation efficiency in the EDTA-CQDs/O3/light composite system, demonstrating the high treatment efficiency of the ternary system for organic pollutants with different types, structures, and environmental hazards, and its potential to be applied in actual wastewater.
Additionally, this study examined the degradation performance of different concentrations of CBZ, TC, and SMX (in the range of 10–500 mg/L). In practical landfill leachate, the content of these pollutants ranges from hundreds to thousands of mg/L, and they are often diluted before AOP treatment. The concentration gradient selected in this study (10–500 mg/L) falls within the common diluted range. Within the concentration range under investigation, these pollutants exhibit good degradation performance. For example, the degradation efficiency values of CBZ and TC are both above 90% at 100 mg/L, and about 70% degradation can still be achieved even at 500 mg/L. This confirms that the EDTA-CQD-based H2O2/O3 system has strong adaptability to fluctuations in pollutant concentration, which is critical for dealing with the changes in the composition of actual landfill leachate.

2.4. Mechanism Analysis of HQ Degradation by EDTA-CQD-Based H2O2/O3 System

An analysis of reactive oxygen species (ROS) in different reaction systems using EPR spin trapping revealed that the system without O3 (Figure 5a,b) showed no ·OH signals, indicating that the H2O2 generated by EDTA-CQD photocatalysis did not decompose into highly reactive ·OH radicals. After adding pollutants, there was no significant change in the signal of 1O2, suggesting that 1O2 is not the primary species responsible for HQ degradation. In the photo-EDTA-CQDs/O3 system, the observed decrease in 1O2 signals after the addition of pollutant may result from the quenching effects caused by the massive generation of ·OH radicals.
·OH + 1O2 → H2O + O2
In the photoactivated O3 system, the production of ·OH is relatively low and completely consumed after HQ addition. The ·OH signals in the EDTA-CQD-based H2O2/O3 composite system are stronger initially and rapidly decrease upon HQ addition, then increase and recover within 5 min. This may be due to the rapid reaction between the phenolic hydroxyl groups of HQ and ·OH after the addition of pollutant, which accelerates the activation of O3 by EDTA-CQDs and H2O2, and sustains ·OH generation. The high ·OH yield in the composite system ensures the continuous oxidation of HQ, enabling direct O3 attack combined with free radical oxidation. Simultaneously, the sustained free radical production results in significantly higher degradation efficiency compared to the photoactivated O3 system.
In the photo-O3 and photo-EDTA-CQDs/O3 systems, HO2· and O2−exhibited opposite trends of change (Figure 5c,d). In the photo-O3 system, HO2· and O2− originate from the photolysis of O3 (Equation (5)). The direct electrophilic addition of HQ to O3 consumes O3 and inhibits its decomposition, resulting in a significant reduction in HO2· and O2- signal values after adding HQ.
O3 + OH− →HO2·+ ·O2−
In the EDTA-CQD-based H2O2/O3 composite system, the signals of HO2· and ·O2− are enhanced after the addition of HQ. This may be because HO2· and ·O2− are also intermediate products of EDTA-CQD photocatalytic production of H2O2. After the addition of HQ, H2O2 consumption leads to the continuous generation of photogenerated electrons on EDTA-CQDs, which continues to reduce O2 to generate HO2· and ·O2−.
According to the above results, the catalytic degradation mechanisms of EDTA-CQD-based H2O2/O3 composite system are as follows:
O3 + hν → ·OH + O2
EDTA-CQDs + O3 → EDTA-CQDs + ·OH + ·O2−
H2O2 → HO2· + H+
O3 + ·OH → HO2 · + O2
O3 + HO2· → ·HO + ·O2− + O2
O3 + ·O2− → ·O3− + O2
·O3− + H2O → ·HO + HO− + O2
·OH + R-H→R· + H2O
R· + O3 → ROO· → CO2 + H2O + low molecular weight product

2.5. Analysis of the Degradation Path of HQ in the EDTA-CQD-Based H2O2/O3 System

The degradation products of HQ were analyzed using liquid chromatography–mass spectrometry. Under the action of EDTA-CQD-based H2O2/O3, HQ primarily generated low-polarity compounds such as p-benzoquinone and o-benzoquinone within 5 min, among which the more unstable o-benzoquinone disappeared after 20 min of reaction. Starting from 10 min, malic acid, fumaric acid, and oxalic acid appeared, indicating that the benzoquinone substances underwent ring-opening reactions to form unsaturated carboxylic acids followed by further oxidation. At 20 min, formic acid and acetic acid were detected, which are presumed to be degradation products of oxalic acid.
Therefore, the degradation path of HQ in the EDTA-CQD-based H2O2/O3 system is speculated as follows (Figure 6): (i) O3 electrophilic attacks the adjacent or para carbon atoms of the phenolic hydroxyl group on hydroquinone (the electron cloud density of the adjacent and para positions is higher), forming covalent bonds and rapidly rebounding to form p-benzoquinone. The ·OH radical is added to the adjacent carbon atom of the phenolic hydroxyl group through an addition reaction, and the intermediate formed by the addition reaction undergoes further oxidation while triggering electron rearrangement within the molecule. During the electronic rearrangement process, the chemical bonds on the benzene ring break and recombine, gradually transforming the original hydroquinone structure into the ortho-benzoquinone structure. Phenolic hydroxyl is oxidized to carbonyl (C=O), and the position of the double bond on the benzene ring also changes accordingly, generating o-benzoquinone. (ii) The p-benzoquinone is added by ·OH radicals to form a dihydroxy intermediate, which then undergoes intramolecular electron rearrangement to form fumaric acid (trans-butenedioic acid). The o-benzoquinone is attacked by · OH at the double bond position (alpha position) to form an epoxidation intermediate, which is then hydrolyzed to form maleic acid (cis-butenedioic acid). (iii) Maleic acid (cis) is more easily hydrolyzed and decarboxylated to produce acetic acid, or oxidized and cleaved to produce oxalic acid and formic acid. Fumaric acid (trans) tends to undergo oxidative decarboxylation to directly generate acetic acid, or to generate oxalic acid and formic acid under strong hydrolysis conditions. The final products are all low-molecular-weight carboxylic acids, which can be further mineralized into CO2 and H2O.

2.6. Treatment Effect of EDTA-CQD-Based H2O2/O3 System on Landfill Leachate

When treating landfill leachate wastewater (no reference substances were artificially added) under light irradiation, the EDTA-CQD-based H2O2/O3 system consistently outperformed the O3 system in COD removal efficiency (Figure 7). After 250 min of reaction, the O3/EDTA-CQD system achieved nearly 90% COD removal, compared to only 50% for the O3 system. This demonstrates that the synergistic effect of photogenerated H2O2 catalyzed by EDTA-CQDs and O3 significantly enhanced free radical production and accelerated the oxidation of organic pollutants. The rapid increase in COD removal rate in the initial stage (within 60 min) was attributed to the photocatalytic H2O2 generation, which promoted O3 decomposition and generated abundant free radicals to rapidly attack organic compounds. In contrast, the O3 only system exhibited slower reaction rates due to its limited direct oxidizing capacity, resulting in lower degradation efficiency for complex wastewater components.
TOC reflects the total organic carbon content. The O3/EDTA-CQD system achieved a 70% removal percentage of TOC after 60 min, while the O3 system alone only reached 35%. This demonstrates that the photocatalytic system based on EDTA-CQDs not only degrades organic matter by activating O3 but also promotes deep mineralization (conversion to CO2 and H2O). In contrast, the photoactivation ability of O3 is limited, and standalone O3 struggles to disrupt the stable structure of organic compounds and has insufficient mineralization capacity.
UV254 can be used to characterize the organic compounds containing conjugated double bonds (e.g., aromatic compounds and humic substances) in landfill leachate. The O3/EDTA-CQD system achieved a UV254 removal percentage exceeding 50% at 60 min, compared to only 30% in the standalone O3 system. This demonstrates that the composite system is more efficient in treating refractory aromatic organic matter. The ·OH radicals generated by the activation of O3 in the EDTA-CQD photocatalytic system can specifically destroy the conjugated structures.
The COD of the landfill leachate used in this study reaches 6800 mg/L. Even after dilution 10 times, it remains classified as high-concentration organic wastewater. The O3/EDTA-CQD system demonstrates superior interference resistance in treating complex-water-quality wastewater compared to the single-O3 system, showing better performance in handling high-concentration and refractory organic compounds. This breakthrough provides material and technical support for practical engineering applications.
The EDTA-CQD-mediated in situ H2O2/O3 system proposed in this study exhibits some significant advantages over the latest AOPs. Compared with metal-catalyzed homogeneous/heterogeneous Fenton-like systems [40,41], and electro-Fenton process [42], it reduces metal consumption and dissolution, operates over a wider pH range, eliminates frequent pH adjustments, and reduces secondary pollution. Unlike AOPs that rely on high-energy ultraviolet radiation (such as UV/H2O2 and UV/O3) [43], this system only uses low-power simulated solar light, which is more cost-effective and sustainable. Compared with other carbon-based photocatalytic materials [44], the modified CQDs used in this study integrate efficient H2O2 production and O3 activation capabilities, thus achieving high-performance degradation of multi-refractory organic pollutants.
In conventional methods for treating landfill leachate, biological treatment (e.g., activated sludge process) is sensitive to toxic refractory organics, resulting in low treatment efficiency and unstable system operation [45]. Physicochemical treatments (e.g., coagulation–sedimentation) can only remove less than 50% of COD (focusing on suspended solids and macromolecular organics) and are ineffective for dissolved refractory organics [46,47]. In contrast, this system directly decomposes these organic pollutants via the strong oxidizing capacity of ·OH, thus retaining high degradation and mineralization ability for high-salt and high-COD landfill leachate. In terms of cost, biological treatment requires aeration, nutrient addition, and sludge disposal, and physicochemical treatment demands large amounts of coagulants, resulting in high energy and chemical costs. Although the EDTA-CQD-based H2O2/O3 system has initial investments (O3 generation equipment and catalyst synthesis), the extremely low catalyst dosage, recycling of oxygen, and solar-driven in situ H2O2 production ensure its long-term cost-effectiveness due to high efficiency and low consumption. The comparison of this study and other technologies is shown in Table 1.

3. Experiment

3.1. Reagents and Materials

Citric acid, potassium titanium oxalate, and deuterium oxide were purchased from Sanjiang Sairuida Technology Co., Ltd. (Tianjin, China). Urea, ethylene glycol, 5,5-dimethyl-1-pyrrolidinone N-oxide, hydroquinone, sulfamethoxazole, and carbamazepine were procured from Titan Technology Co., Ltd. (Shanghai, China). Hydrochloric acid, sodium hydroxide, tetracycline hydrochloride, ethylenediaminetetraacetic acid, hypoxynyl triacetic acid, ethylenediamine-N,N’-dipersulfonate, N-hydroxyasparamide (NHS), 1,3-dimethylaminopropyl-3-ethylcarbodiimide hydrochloride (EDC), and 2-(N-morpholinyl)ethanesulfonic acid (MES) were obtained from Solomon Biotechnology Co., Ltd. (Tianjin, China). All reagents are of analytical grade and require no further purification. Oxygen (99.99%) and high-purity nitrogen (99.99%) were supplied by Air Liquide Tianjin (Tianjin, China).

3.2. Preparation of EDTA-CQDs

Firstly, CQDs with rich amino groups on the surface were synthesized via solvothermal method. Under continuous magnetic stirring, 0.6 g citric acid and 0.4 g urea were completely dissolved in 30 mL ethylene glycol for 2 h. The resulting solution was transferred to a 50 mL stainless steel autoclave lined with PTFE and heated at 180 °C for 7 h. The black solid product was filtered using a 0.22 μm membrane to remove larger particles, then dialyzed in deionized water for 24 h (MWCO 1000) to eliminate precursor compounds. After dialysis, the material underwent freeze-drying for approximately 30 h to obtain powdered CQDs.
Subsequently, 25 mg CQD powder was added to 25 mL MES buffer solution (pH = 6). After ultrasonication for 20 min, the solution was purged with nitrogen for 30 min to remove dissolved oxygen. The mixture was placed in an ice bath and sequentially added 100 mg EDC and 50 mg NHS. Then, 25 mg EDTA was added, along with a small amount of NaOH solution (l mol/L) to adjust pH to 7.4. Under nitrogen protection, continuous stirring was performed for 24 h to ensure complete reaction between EDTA and CQDs. Following the reaction, the mixture was dialyzed with deionized water for 48 h to remove unreacted substances and buffer, then freeze-dried to obtain EDTA-CQD powder.

3.3. Evaluation of Degradation Effect

To evaluate the degradation efficiency of organic pollutants in landfill leachate by photogenerated H2O2/O3 system based on EDTA-CQD catalyst, hydroquinone (HQ), a common phenolic pollutant in landfill leachate, was selected as the primary target. Degradation experiments were conducted under xenon lamp-simulated irradiation. In each experiment, 5 mg of EDTA-CQDs was dispersed into 50 mL of HQ solution (100 mg/L). At the same time, the non-illumination system, ozone oxidation system, and catalytic ozonation system were used as controls. Samples were collected at designated intervals, diluted tenfold after passing through 0.22 μm filter membranes, and analyzed by high-performance liquid chromatography (Ultimate-3000, Thermo Fisher, Waltham, MA, USA) to measure the pollutant concentration. Additionally, several emerging pollutants, tetracycline (TC), carbamazepine (CBZ), and sulfamethoxazole (SMX), were selected to validate the selectivity of the composite system for organic contaminants.

3.4. Determination of Reactive Oxygen Species (ROS)

In order to identify the reactive oxygen species (ROS) that play a role in the degradation of HQ in the EDTA-CQD-based H2O2/O3 system, electron paramagnetic resonance (EPR) spectrometer (EMX EPR, Bruker, Rheinstetten, Germany) was used to detect the experimental groups under different reaction conditions before and after the addition of pollutant, and the reaction mechanism was analyzed according to the variation in reaction peak intensity [55,56]. Dimethyl sulfoxide (DMSO) was used as the probe to trap hydroxyl radical, and then the generated formaldehyde reacted with 2,4-dinitrophenylhydrazine (DNPH) to form hydrazine (HCHO-DNPH), which was analyzed by HPLC (Ultimate-3000, Thermo Fisher, Waltham, MA, USA) to calculate the concentration of hydroxyl radical.

3.5. Degradation Pathway Analysis

To investigate the degradation pathways and intermediate products of organic pollutants, liquid chromatography–mass spectrometry (Orbitrap Fusion, Thermo Fisher, Waltham, MA, USA) was employed to detect the solution during the reaction process. The analysis conditions are as follows: the mass spectrometer is equipped with an electrospray ionization source (ESI), and the positive and negative ion modes are collected simultaneously; the scanning range is set at m/z 50–1500 with a resolution of 70,000 (m/z < 200) and a mass accuracy ≤ 5 ppm. The data acquisition is performed using data-dependent scanning (DDA) mode, and Xcalibur software (Data acquisition and processing software for mass spectrometry, v4.2, Thermo Fisher, Waltham, MA, USA, 2018) is used for data processing, matching retention times and precise molecular weights (error ≤ 10 ppm) [1,57,58].

3.6. Construction of Actual Wastewater Treatment System for Landfill lLeachate

The experimental setup for treating landfill leachate using the EDTA-CQD-based H2O2/O3 system is illustrated in Figure 8. The reaction system consists of an ozone generator (producing O3 gas), an ozone monitor (for real-time concentration tracking), an aeration diffuser (delivering ozone bubbles uniformly into the reactor), an exhaust port (for gas discharge and composition monitoring), an injection port (adding EDTA-CQD catalyst), a sampling port (collecting post-reaction samples for analysis), and a 300W xenon lamp (simulating sunlight). The reaction device measures 25 cm in height with a 6 cm inner diameter, designed to process 700 mL of water per cycle, theoretically. The operation method is as follows: the ozone gas generated by the generator forms bubbles through the diffuser, which are evenly distributed in the leachate; meanwhile, EDTA-CQDs are injected via the injection port to generate H2O2 and catalyze the activation of O3 under light irradiation, effectively degrading pollutants in the leachate. COD, TOC, and UV254 parameters are periodically sampled through the sampling port for analysis to evaluate the treatment efficiency.
A DR3900 spectrophotometer (Hach, Loveland, CO, USA) was used to measure the absorbance at 620 nm and calculate the COD of sample (mg/L). The TOC value was obtained through the TOC analyzer (TOC-L-CPN, Kyoto, Japan Shimadzu). The absorbance at 254 nm was recorded by scanning the sample on a Lambda 35 spectrophotometer (PerkinElmer, Waltham, MA, USA) and converted to the concentration of aromatic organic compounds per unit volume.
The landfill leachate was collected from the Taihuan Renewable Resources Utilization Co., Ltd. (Tianjin, China). After sampling, centrifugation (3000 rpm, 15 min) was performed to remove suspended solids, preventing equipment clogging and microbial interference. Excess heavy metals and high-concentration ammonia nitrogen were removed using the Na2S precipitation method and blow-off method, respectively. Subsequently, 0.22 μm filter membranes were employed for microbial inactivation filtration. Following pretreatment, baseline parameters, including COD, TOC, and ammonia nitrogen, were measured, and then the samples were stored at 4 °C for refrigeration [58,59,60,61,62,63,64]. The water quality parameters are as follows (Table 2):
Considering the influence of deep color of water samples on photocatalytic reaction, the raw water samples were diluted 10 times before degradation.

4. Conclusions

This study focuses on the limitations of the current ozone oxidation treatment of wastewater and the in situ application of photogenerated H2O2 in ozone oxidation. Based on a modified CQD (EDTA-CQD), a photocatalytic H2O2/O3 composite system was designed for the effective treatment of multi-component refractory wastewater from landfill leachate. The EDTA-CQD-based H2O2/O3 system exhibited exceptional degradation efficiency for various typical contaminants in landfill leachate, including HQ, CBZ, TC, and SMX. The composite system achieved a degradation percentage of 97% for high-concentration HQ within 30 min through multiple synergistic mechanisms, including O3 oxidation, photoactivation of O3, activation of O3 by EDTA-CQDs, and activation of O3 by in situ-produced of H2O2. By analyzing the chain reaction of O3 activated by light, a catalyst, and H2O2 in the EDTA-CQD-based H2O2/O3 system, the pathway and mechanism of O3 direct oxidation and free radical indirect oxidation degradation of pollutants have been demonstrated. Using this system to treat actual leachate, key indicators such as COD, TOC, and UV254 were significantly reduced, verifying that the coupling of in situ photogenerated H2O2 by EDTA-CQDs and O3 can improve activation performance and free radical yield, achieving efficient treatment of multi-component refractory wastewater.

Author Contributions

Conceptualization, S.W. and N.M.; methodology, S.W., L.M. and N.M.; writing—original draft preparation, S.W.; data curation, X.Z. and S.D.; writing—review and editing, supervision, and funding acquisition, W.W. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by National Natural Science Foundation of China (No. 22376107 and U23B20165), Natural Science Foundation of Tianjin, China (No. 24JCYBJC01640), Tianjin Key Research and Development Plan of China (22YFYSHZ003000), and the NCC Fund (NCC2022-PY-01).

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. HQ degradation efficiency in different systems. Reaction conditions: initial Ph = 6, T = 25 °C, HQ = 100 mg L−1, EDTA-CQDs = 0.2 g/L, Xe = 26.54 mW cm−2, O3 = 0.1 L/h (80%).
Figure 1. HQ degradation efficiency in different systems. Reaction conditions: initial Ph = 6, T = 25 °C, HQ = 100 mg L−1, EDTA-CQDs = 0.2 g/L, Xe = 26.54 mW cm−2, O3 = 0.1 L/h (80%).
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Catalysts 15 00903 g002
Figure 2. HQ degradation efficiency of the EDTA-CQD-based H2O2/O3 system under different pH. Reaction conditions: T = 25 °C, HQ = 100 mg L−1, EDTA-CQDs = 0.2 g/L, Xe = 26.54 mW cm−2, O3 = 0.1 L/h (80%).
Figure 2. HQ degradation efficiency of the EDTA-CQD-based H2O2/O3 system under different pH. Reaction conditions: T = 25 °C, HQ = 100 mg L−1, EDTA-CQDs = 0.2 g/L, Xe = 26.54 mW cm−2, O3 = 0.1 L/h (80%).
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Catalysts 15 00903 g003
Figure 3. HQ degradation efficiency of EDTA-CQD-based H2O2/O3 system under different HQ concentrations (a) and EDTA-CQDs dosages (b). Reaction conditions: T = 25 °C, Xe = 26.54 mW cm−2, O3 = 0.1 L/h (80%).
Figure 3. HQ degradation efficiency of EDTA-CQD-based H2O2/O3 system under different HQ concentrations (a) and EDTA-CQDs dosages (b). Reaction conditions: T = 25 °C, Xe = 26.54 mW cm−2, O3 = 0.1 L/h (80%).
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Catalysts 15 00903 g004
Figure 4. Degradation efficiency of carbamazepine CBZ (a), tetracycline TC (b), and sulfamethoxazole SMX (c) by the EDTA-CQD-based H2O2/O3 system. Reaction conditions: initial pH = 6, T = 25 °C, pollutant =100 mg L−1, EDTA-CQDs = 0.2 g L−1, Xe = 26.54 mW cm−2, O3 = 0.1 L h−1 (80%).
Figure 4. Degradation efficiency of carbamazepine CBZ (a), tetracycline TC (b), and sulfamethoxazole SMX (c) by the EDTA-CQD-based H2O2/O3 system. Reaction conditions: initial pH = 6, T = 25 °C, pollutant =100 mg L−1, EDTA-CQDs = 0.2 g L−1, Xe = 26.54 mW cm−2, O3 = 0.1 L h−1 (80%).
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Catalysts 15 00903 g005
Figure 5. EPR spectra of 1O2 (a), ·OH (b), HO2· (c), and ·O2− (d) in the EDTA-CQDs, O3, and EDTA-CQDs/O3 systems before and after the addition of pollutants under irradiation. Reaction conditions: initial pH = 6, T = 25 °C, pollutant =100 mg L−1, EDTA-CQDs = 0.2 g L−1, Xe = 26.54 mW cm−2, O3 = 0.1 L h−1 (80%).
Figure 5. EPR spectra of 1O2 (a), ·OH (b), HO2· (c), and ·O2− (d) in the EDTA-CQDs, O3, and EDTA-CQDs/O3 systems before and after the addition of pollutants under irradiation. Reaction conditions: initial pH = 6, T = 25 °C, pollutant =100 mg L−1, EDTA-CQDs = 0.2 g L−1, Xe = 26.54 mW cm−2, O3 = 0.1 L h−1 (80%).
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Figure 6. Degradation pathway of HQ in the EDTA-CQD-based H2O2/O3 system.
Figure 6. Degradation pathway of HQ in the EDTA-CQD-based H2O2/O3 system.
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Catalysts 15 00903 g007
Figure 7. Removal percentages of COD (a), TOC (b), and UV254 (c) in landfill leachate treated with O3 and EDTA-CQDs/O3 under illumination conditions. Raw water parameters: COD = 6800 mg/L, TOC = 1772.6 mg/L, pH = 6.54 (diluted 10 times); reaction conditions: initial pH = 7.54, T=25 °C, EDTA-CQDs = 0.2 g/L, Xe = 26.54 mW·cm−2, O3 = 0.1 L/h (80%).
Figure 7. Removal percentages of COD (a), TOC (b), and UV254 (c) in landfill leachate treated with O3 and EDTA-CQDs/O3 under illumination conditions. Raw water parameters: COD = 6800 mg/L, TOC = 1772.6 mg/L, pH = 6.54 (diluted 10 times); reaction conditions: initial pH = 7.54, T=25 °C, EDTA-CQDs = 0.2 g/L, Xe = 26.54 mW·cm−2, O3 = 0.1 L/h (80%).
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Figure 8. Schematic diagram of the reaction device of EDTA-CQD-based H2O2/O3 system.
Figure 8. Schematic diagram of the reaction device of EDTA-CQD-based H2O2/O3 system.
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Table 1. Comparison of various technologies for landfill leachate treatment.
Table 1. Comparison of various technologies for landfill leachate treatment.
Landfill LeachateProcessReaction
Conditions		Degradation EfficiencyReferences
COD: 680 mg L−1
TOC: 177 mg L−1		EDTA-CQD-based H2O2/O3 systemEDTA-CQDs: 0.2 g/L
Xe: 27 mW cm−2
O3: 0.1 L/h
pH = 7.54 		90%
70% 		This study
COD: 255 mg L−1UV-TiO2 photocatalysis combination with aged waste reactorsTiO2: 4 mg L−1
pH = 8.88		32.5% [48]
COD: 11378 mg L−1Integration of electrocoagulation and ozonation process Current: 100 mA
O3: 400 mg /h		80% [49]
COD: 600 mg L−1P-type TiO2 nanoparticle photocatalysisFluorescent lamp: 36 W
Si-doped TiO2: 3.5 mg/mL
pH = 6 		85% [50]
COD: 24,000 g O2 dm−3
TOC: 21 g L−1		Combination TiO2/Ag nanocomposite photocatalysis and biological treatment using Candida tropicalis strainVisible light: 33W cm−2
TiO2/Ag: 0.937 g dm−3
pH = 4.31		90%
85% 		[51]
COD: 7920 g O2 L−3
Coupling of photocatalysis and biological aerobic processUV light: 20–60 kJ/L
TiO2: 100–600 mg L−1
H2O2: 300 mg L−1		68%[52]
COD: 1950 mg L−1
TOC: 590 mg L−1		Photocatalytic degradation by Ag-TiO2 nanoparticlesUVC lamp: 25 W
Aeration: 6 L/min
pH = 4		62%
55%		[53]
COD: 2029 ppmPhotocatalytic degradation by Boron-doped TiO2TiO2: 2.5 g L−1
UV lamp: 33 W
pH = 4.4		59% [54]
Table 2. Water quality parameters of landfill leachate.
Table 2. Water quality parameters of landfill leachate.
ProjectUnitValue
pH/7.54
ConductivitymS cm−136.9
TDSmg L−11690
SSmg L−18
NH4+-Nmg L−167.92
TNmg L−1258.39
CODmg L−16800
TOCmg L−11772.6
Cl−mg L−12722.16
SO42−mg L−122,271.68
Ca2+g L−128.483
Mg2+g L−162.986
Total ironmg L−124.98
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Wu, S.; Meng, N.; Ma, L.; Zhang, X.; Ding, S.; Wang, W. Study on Photocatalytic Peroxone Process for Treating Organic Pollutants in Leachate Based on Modified Carbon Quantum Dots. Catalysts 2025, 15, 903. https://doi.org/10.3390/catal15090903

AMA Style

Wu S, Meng N, Ma L, Zhang X, Ding S, Wang W. Study on Photocatalytic Peroxone Process for Treating Organic Pollutants in Leachate Based on Modified Carbon Quantum Dots. Catalysts. 2025; 15(9):903. https://doi.org/10.3390/catal15090903

Chicago/Turabian Style

Wu, Shuo, Nuo Meng, Lin Ma, Xiguo Zhang, Shihu Ding, and Wei Wang. 2025. "Study on Photocatalytic Peroxone Process for Treating Organic Pollutants in Leachate Based on Modified Carbon Quantum Dots" Catalysts 15, no. 9: 903. https://doi.org/10.3390/catal15090903

APA Style

Wu, S., Meng, N., Ma, L., Zhang, X., Ding, S., & Wang, W. (2025). Study on Photocatalytic Peroxone Process for Treating Organic Pollutants in Leachate Based on Modified Carbon Quantum Dots. Catalysts, 15(9), 903. https://doi.org/10.3390/catal15090903

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