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Article

Activation of Peracetic Acid by Waste Tea Residue-Derived Biochar for Bisphenol A Elimination: Synergetic Adsorption and Non-Radical Oxidation

by
Shijun Zhu
1,2,
Xinchen Zhang
1,
Shangming Shen
1,
Yang Wang
1,
Yongshu Hu
1,
Hao Yang
1,
Wenbin Liu
3,
Xiaoyan Ma
1,2 and
Jing Deng
1,2,*
1
Zhejiang Key Laboratory of Green Construction and Intelligent Operation & Maintenance for Coastal Infrastructure, College of Civil Engineering, Zhejiang University of Technology, Hangzhou 310023, China
2
Taizhou Research Institute of Intelligent Construction on Coastal Soft Soil, Zhejiang University of Technology, Taizhou 318000, China
3
Department of Environment, Yangtze Delta Region Institute of Tsinghua University, Jiaxing 314006, China
*
Author to whom correspondence should be addressed.
Catalysts 2026, 16(5), 420; https://doi.org/10.3390/catal16050420
Submission received: 26 March 2026 / Revised: 10 April 2026 / Accepted: 14 April 2026 / Published: 3 May 2026
(This article belongs to the Special Issue Novel Advanced Oxidation Processes for Catalytic Degradation of Emerging Contaminants)
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Abstract

Biochar (BC)-activated peracetic acid (PAA)-based advanced oxidation processes (AOPs) were increasingly considered as cost-efficient and eco-friendly water treatment technologies for the removal of organic pollutants. However, the specific role of intrinsic carbon, nitrogen species and structure properties played in activation mechanism is still vague. In this study, the waste tea residues-based biochar (WTBC) was prepared by thermal carbonization and applied to activate PAA for the degradation of bisphenol A (BPA). The product carbonized at 800 °C (WTBC800) possessed larger specific surface area (342.57 m2/g), more abundant porous structure and massive defects state (ID/IG = 3.53), and exhibited a superior activation performance with 83.7% BPA removal within 120 min. Adsorption and non-radical oxidation pathways [e.g., the mediated electron transfer process (ETP) and singlet oxygen (1O2) generation] were evidenced to play the dominant roles in the BPA degradation through the formation of metastable complex WTBC-PAA*. The graphitic carbon, functional nitrogen species, defects structure and persistent free radicals (PFRs) in WTBC were proposed to contribute to the activation of PAA. Overall, relatively higher dosages of WTBC (0–0.5 g/L) and PAA (0–1.5 mM) facilitated the BPA degradation. The solution pH and water matrix (e.g., Cl−, NO3−, HCO3− and SO42−) presented a negligible effect on the BPA degradation in WTBC/PAA system. This study not only proposes a sustainable approach for organic pollutants removal in wastewater, but also promotes the resource re-utilization of agricultural waste.

Graphical Abstract

1. Introduction

Advanced oxidation processes (AOPs) have emerged as promising technologies for the removal of recalcitrant organic pollutants from water. AOPs generate highly reactive species, such as hydroxyl radicals (•OH), sulfate radicals (SO4•−), and singlet oxygen (1O2), which can non-selectively oxidize a wide range of organic contaminants. Compared to conventional biological or physicochemical methods, AOPs offer the advantage of complete mineralization of persistent organic pollutants without phase transfer [1].
Peracetic acid (PAA, CH3C(O)OOH) has been widely utilized in water treatment processes as an emerging oxidant due to its high redox potential (Eh0 = 1.06–1.96 V), good efficiency for decontamination and minimal generation of toxic byproducts [1,2]. However, the oxidation efficiency of sole PAA in removing organic pollutants from water is still limited and requires activation strategies. Studies have reported that PAA possesses an asymmetrical structure with lower O–O bond energy (159 kJ/mol) compared to PMS (317 kJ/mol) and H2O2 (213 kJ/mol), which renders PAA more vulnerable to hemolytic cleavage during the activation process to generate reactive species (e.g., •OH, •CH3, and 1O2) [3]. Various activation strategies such as thermal energy [4], light irradiation [5], ultrasound [6], transition metal ions [7] and metal oxides [8] have been utilized to activate PAA for the environmental remediation. However, the energy-intensive operation mode and inevitable metal leaching problems limit their application in practical water treatment [9].
Bisphenol A (BPA) is a typical endocrine-disrupting chemical (EDC) widely used in the production of polycarbonate plastics and epoxy resins. Due to its widespread application, BPA has been frequently detected in various environmental matrices, including surface water, groundwater, and wastewater, with concentrations ranging from ng/L to μg/L [10]. BPA exhibits reproductive and developmental toxicity, and its endocrine-disrupting effects (e.g., estrogenic activity) have raised significant concerns for both aquatic organisms and human health. Moreover, BPA is relatively persistent in aquatic environments and is not easily mineralized by conventional biological treatment processes, making it an ideal model contaminant for advanced oxidation studies.
Metal-free carbonaceous materials presented unique potential owing to the nature of environmentally friendly, cost-effective and easily available. Biochar, generated from pyrolysis and modification of biomass, has been reported as a porous carbon-rich material for the activation of various peroxides [11]. Waste tea residue (WTR) is a typical agricultural biomass and more than 5 million tons of WTR are produced each year across the world [12]. Considering its abundant lignin and total cellulose content, the waste tea residues-based biochar (WTBC) would have a multi-porous reticular structure and contain appreciable aromatic functional groups via a facile carbonthermal reduction strategy [13]. Therefore, WTBC inherently function as highly effective porous bio-adsorbents, demonstrating multi-tiered physical adsorption capabilities [14,15]. Meanwhile, the heteroatom-doped porous WTBC rich in some elements (e.g., N, O) can be obtained without additional doping and immobilization, which would also have a great potential in accelerating the peroxides activation [16]. The porosity structure of WTBC with high specific surface area could improve the adsorption of PAA and target pollutants and also increase collision frequency between PAA molecules and catalytic sites, thus promoting the diffusion of reaction intermediates and improving the overall oxidation efficiencies [17]. As a result, the adsorption and catalytic oxidation will work synergistically and promote each other to achieve better removal of micropollutants in PAA activation system. Consequently, WTR can be efficiently recycled and converted into biochar. Utilizing it as a highly promising and sustainable adsorption-enhanced PAA activator owns great potential to broaden the application in wastewater treatment.
Recently, biochar-based materials have been widely investigated as catalysts for the activation of various oxidants, including persulfate (PS), peracetic acid (PAA), for the degradation of organic pollutants. For example, several studies have reported that biochar can effectively activate persulfate to generate reactive species for BPA removal via non-radical pathways such as singlet oxygen (1O2) and electron transfer [4,8]. Regarding PAA activation, biochar has also been shown to effectively activate PAA for emerging contaminant removal, with surface defects and persistent free radicals (PFRs) serving as key active sites [9]. However, the application of biochar-derived from waste tea residues for PAA activation in BPA removal has rarely been reported.
As diverse biomass source and inherent properties can significantly affect the physical characteristics (i.e., structure, function) and chemical compositions (i.e., ash content, fixed carbon, volatile matter) of biochar, different biochar-based materials display varying catalytic capability and the generation of reactive species [18]. Whether the biochar derived from WTR exhibits higher porous structure or more abundant defects structure during the pyrolysis process with different temperature is rarely studied still now. The persistent free radicals (PFRs) might present unpaired electron domains and exhibit stronger electron transfer capacity, which could promote the catalytic performance of carbonaceous materials [19]. In addition, the oxygen-containing functional groups (e.g., C=O, -OH) and surface defects structure may also take place during the carbonization process [20]. However, whether these active sites in surface of biochar could contribute to the facilitation of electron transfer process (ETP) or generation of reactive oxygen species (ROS) in PAA activation process still remains ambiguous. Therefore, the activation mechanisms particularly such as the non-radical process and the synergistic effect of adsorption/catalysis in the WTBC/PAA system needs to be further explored.
In this study, WTR were adopted as raw materials to prepare biochar for the activation of PAA. BPA, a typical micropollutant in natural aquatic environment, was selected as the target organic contaminant. The physicochemical properties of WTBC obtained at different carbonization temperatures were systematically analyzed to correlate the relationship between the structural features and catalytic properties. The adsorption efficiency and catalytic performance of BPA in this system under various conditions including WTBC dosages, PAA concentrations and initial solution pH values were studied. Then, the effects of various water matrices including HCO3−, Cl−, NO3−, and SO42− on BPA removal were evaluated. Furthermore, the key active sites (e.g., PFRs, functional groups and detects structure) for PAA activation were revealed, and a novel catalytic mechanism especially for the ETPs was unraveled in the WTBC/PAA system according to the specific detection and analysis. In addition, the possible removal pathways of BPA in WTBC/PAA system were proposed and the toxicity of intermediates was evaluated. This study would provide a potential strategy for wastewater remediation by PAA activation, and meanwhile achieve the waste reclamation through reutilization of WTR.

2. Results and Discussion

2.1. Characterization of WTBC

The morphologies and distribution of major elements of WTBC was observed by SEM and TEM-EDS. Figure 1b–e depict the surface structure and morphology of WTR, WTBC300, WTBC500 and WTBC800, respectively. The surface of WTR was coarse and irregular without significant porous structure. Nevertheless, with the pyrolysis temperature increases, the obvious wrinkled structure and grooves on the surface of WTBC were observed. The macropores tunnel with neatly arranged honeycomb-like property on the inner-surface was observed for WTBC800, suggesting that the decomposition of organic substances and reassembling the solid matrix occurred under carbonization modification. The TEM image (Figure 1f) further shows that WTBC800 features larger channels that are interspersed with numerous micropores, suggesting an irregular pore distribution and a highly porous surface structure. The elemental composition of the material surface was analyzed by EDS (Figure 1g), indicating that carbon (C) and oxygen (O) are the primary elements, with a small amount of nitrogen (N) dispersed across the surface.
The microporous structure was further confirmed by N2 sorption analysis. Figure 2a and Table S2 show that adsorption and desorption curves of WTBC800 exhibit hysteresis loops, which belongs to the IV adsorption isotherm. This phenomenon indicates the presence of hierarchical pores including micropores (<2 nm) and mesopores (2–50 nm) in WTBC800, which is consistent with the pore size distribution determined using the Barrett–Joyner–Halenda (BJH) equation. However, no clear hysteresis loop was observed in the WTR and the other WTBC samples. As the pyrolysis temperature increased, the specific surface area and pore volume of the biochar gradually increased from 0.190 m2/g and 0.016 m3/g to 342.57 m2/g and 0.153 cm3/g, respectively. The average pore size of commercial WTBC800 is 3.85 nm. The result of the BET experiment demonstrates that higher pyrolysis temperatures promote the development of a high specific surface area and a well-developed pore structure possibly due to the release of volatile gases (i.e., NH3, water vapor) from the decomposition of organic matter in the waste tea residue. While, the pores with various sizes were created and the carbon material was remained. Notably, the specific surface area of WTBC exhibited a slight increase at pyrolysis temperatures of 300 °C and 500 °C, but rose sharply from 3.45 m2/g to 342.58 m2/g when the temperature reached 800 °C. This substantial enhancement is likely due to the formation or decomposition of calcium salts. Additionally, as the pyrolysis temperature increased, the average pore size of WTBC gradually decreased, suggesting that higher temperatures favor the formation of micropores. The combination of an optimal pore structure and a high specific surface area enables the creation of numerous active sites, while also improving mass transport and charge transfer kinetics, significantly boosting catalytic performance [21].
The XRD patterns of different WTBC samples are displayed in Figure 2b. A broad peak at around 2θ values of 22° to 26° (002) appeared in WTR and all WTBC samples, implying the irregularity and disordered carbon layer and parallel stacking of sheet-like graphene of WTBC [22]. Notably, another broad peak, detected at 2θ ≈ 43° assigned to (100) plane of the graphite structure [23], was identified for WTBC800, indicating that more graphite layers formation were developed as carbonization temperature raised. A sharp diffraction peak at around 2θ = 29.5° corresponding to CaCO3 appears when the pyrolysis temperature increased from 300 °C to 500 °C. Nevertheless, the WTBC800 sample only displayed a weak diffraction pattern, indicating that the chemical bonds in calcium salts were subsequently broke down during the carbonization process. The formation of its high specific surface area and extensive pore network could be largely attributed to this pyrolysis modification. As shown in Figure 2c, various functional groups were generated on the surface of WTBC after pyrolysis process. The FTIR spectra of these samples present the peaks of –OH stretching (3432 cm−1), C–H vibrations in carbohydrates (2923 cm−1 and 2852 cm−1), aromatic C=O/C=C stretching of carboxylic acids (1617 cm−1), C–O (1437 cm−1), and O–C–O vibrations (1059 cm−1). The intensity of these functional groups exhibits a decline trend with increasing pyrolysis temperature. This results can be attributed to the following factors: (1) the thermal decomposition of organic components (e.g., cellulose and lignin) in tea waste, leading to the gradual breakdown of oxygen-containing functional groups (e.g., O–H, C=O, and C–O), aliphatic functional groups (e.g., C–H), as well as the reduction in C=C functional groups; and (2) the release of volatile organic compounds (e.g., H2O, CO2, CO, and CH4) during pyrolysis process. The reduction in oxygen-containing functional groups (e.g., O–H and C=O) decreases the surface polarity of biochar but enhances its hydrophobicity, which may influence its interactions with polar molecules such as PAA and BPA. Although decomposition of oxygen functional groups may diminish the generation of free radicals (e.g., ·OH and RO·), defects states in the carbon framework would be generated, thus providing coordinatively unsaturated carbon sites with partially localized electrons to mediate non-radical pathways (e.g., ETPs and 1O2 generation) [2]. In addition, the formation of porous structures could facilitate the adsorption ability of WTBC.
The Raman spectra of WTBC are presented in Figure 2d. The D peaks (1357–1370 cm−1) and G peaks (1585–1591 cm−1) represent the disordered carbon (sp3) and graphitic carbon (sp2), respectively [24]. The intensity ratio of these peaks (ID/IG) indicate the degree of defect and graphitization of WTBC samples [25]. Compared to WTR (ID/IG = 1.7), WTBC samples displayed the higher ID/IG ratio of 3.16, 3.41 and 3.53 with the pyrolysis temperature increasing from 300 °C, 500 °C to 800 °C, suggesting that the presence of more defects in WTBC800 was enhanced likely due to atomic rearrangements during the carbonization process. These defects, such as carbon defects, nitrogen vacancies and edge defects, are widely recognized as catalytic sites for the PAA activation. The higher defect density suggests the presence of abundant active sites on WTBC800, which may contribute to its catalytic performance in PAA activation. In addition, the degree of graphitization and defects states are also important factor for ETPs between the donor and the electron acceptor [2].
Carbon defects induced the unstable electronic state, namely delocalized and unpaired electrons could transfer to PAA to accomplish activation. The unpaired electron configuration, denoted as PFRs in biochar materials, was commonly identified as the dominant active site in peroxide activation processes. As depicted in Figure 2e, the formation of PFRs in WTR and WTBCs were determined by EPR analysis. No signal intensity was detected for WTR and WTBC300. Notably, the intensities of PFRs increased remarkably as carbonization temperature raised from 500 °C to 800 °C. This phenomenon indicates that the PFRs generation was promoted with increase in the carbonization temperature owing to the improved thermal chemical bonds breakages of organic matters in WTR. The intensities of PFRs followed the order of WTBC800 ≫ WTBC500 > WTBC300 ≈ WTR, which presented a positive correlation between BPA removal and PFRs intensity. These results definitely confirmed that PFRs served as the dominant active sites in WTBC for mediating the PAA activation during the BPA removal process. In addition, EPR signals (g factor = 2.001–2.005 shown in Figure S1) can also be attributed to the unpaired electrons trapped by surface defects and vacancies [26]. WTBC800 exhibits significantly stronger intensity relative spin concentration compared to other WTBC samples, suggesting more plenty of vacancies were developed in graphitic carbon framework. This conclusion is highly consistent with the Raman results. Notably, the EPR spectrum of WTBC800 not only exhibited significantly higher intensity than that of WTBC500, but also displayed a distinct spectral shape. A new species with a very narrow linewidth was observed in WTBC800, which was absent in WTBC500. This narrow linewidth suggests the formation of more localized and stable carbon-centered persistent free radicals (PFRs) at higher pyrolysis temperature (800 °C), whereas the broader signals observed in WTBC500 may be attributed to oxygen-associated radicals (e.g., semiquinone-type radicals) that are more susceptible to environmental perturbations.
XPS analysis was conducted to investigate the surface chemical composition of WTR, fresh WTBC800 and used WTBC800. As describe in Figure 2f, the tea residue-derived char is primarily composed of C and O, and N1s was observed in fresh WTBC800. Further, hyperfine XPS spectra of WTR were illustrated in Figure S2. The C1s spectrum of the WTR could be deconvoluted into four distinct peaks at 284.7, 285.8, 286.7 and 288.3 eV, corresponding to C–C/C=C, C–OH/C–O–C, C=O, and COOH functional groups, respectively [9,27]. In N1s spectra, the WTR exhibited a distinct peak at 400.5 eV corresponding to pyrrolic N. The hyperfine XPS spectra of WTBC before and after reaction were discussed in Section 3.3.

2.2. PAA Activation Performance and Catalytic Oxidation

The removal efficiency of BPA in different treatment systems is displayed in Figure 3a. The removal of BPA was negligible by WTR alone, PAA alone and WTR/PAA system, indicating that neither WTR nor PAA alone could achieve the BPA removal, and WTR could not effectively activate PAA to degrade BPA. To evaluate the effect of carbonization temperature on the property of as-prepared WTBC products, the adsorption and activation capability for the BPA removal were compared (Figure S3). For WTBC300 and WTBC500, no obvious adsorption or activation capacity was observed for the BPA removal. In contrast, in the WTBC800 alone system, the adsorption capacity for BPA is apparent with approximately 54.2% of removal efficiency within 120 min. Previous studies have reported that the π–π electron acceptor structure, graphitized structures and hydrogen bonding between BPA (-OH) and oxygen-containing functional groups (-COOH) on the surface of carbon materials [28] are also possible adsorption mechanisms for BPA by WTBC800. In addition, the combination of WTBC800 and PAA could further improve the removal efficiency of BPA with approximately 83.7%, which indicated that the better BPA removal in WTBC800/PAA system might be ascribed to the adsorption capacity and catalytic oxidation. Therefore, both pre-adsorption and PAA activation by WTBC800 contributed to the BPA removal in this study.
Additionally, kinetic analysis of BPA degradation in WTBC/PAA process was performed using a pseudo-first-order model. As shown in Figure 3b, the WTBC800/PAA system exhibited remarkable catalytic activity with an apparent rate constant (kobs) of 0.037 min−1, outperforming other WTBC samples (with kobs range at 0.0002–0.009 min−1) by one to two orders of magnitude. This substantial enhancement in catalytic efficiency can be attributed to the thermally induced structural modifications through 800 °C pyrolysis process, which facilitated the generation of highly active sites in the carbon matrix.
Since the PAA solution also contains H2O2 and CH3COOH, the catalytic performance of H2O2 by WTBC800 was investigated to rule out the effect of co-existed H2O2. As shown in Figure 3c, only 50.6% of BPA was removed in the WTBC800/H2O2 system, which was even lower than that by the WTBC800 alone. This result suggested that the presence of H2O2 would desorb BPA from WTBC800 due to the relatively weak Van der Waals strength and the occupation of active sites in WTBC by H2O2 quenching effect of organic radicals [7,29], leading to the lessened adsorption and activation of PAA. To further verify the activation of PAA by WTBC800, the decomposition of PAA overreaction time was also detected in two processes. As depicted in Figure 3d, in the absence of WTBC800, the PAA concentration remained essentially unchanged in the PAA/BPA system. However, in the presence of WTBC800, the PAA concentration decreased significantly within 10 min, suggesting that WTBC800 could accelerate the decomposition of PAA and might produce reactive species through PAA activation process.
The effect of WTBC dosages from 0.1 to 0.5 g/L on the BPA removal with an initial PAA concentration of 1.5 mM was investigated. As shown in Figure 3e, a significant enhancement of the BPA removal from 29.6% to 83.7% was observed when WTBC dosage increasing from 0.1 to 0.4 g/L, and the BPA removal efficiency kept nearly stable (≈83%) with further elevating WTBC dose from 0.4 to 0.5 g/L. The relevant kobs augmented from 0.003 to 0.037 min−1 and exhibited a significant positive correlation with WTBC dosage (Figure S4). This dose-dependent behavior could be explained that the proportional increase in catalyst dose could provide more active sites and facilitate both the activation of PAA and adsorption of organic contaminants, thus improving the overall treatment performance.
As depicted in Figure 3f, the BPA removal efficiency rose from 54% to 83.7% when the initial PAA concentration was increased from 0 to 1.5 mM. This enhancement can be attributed to the increased availability of reactive oxidants generated from higher PAA concentrations, which promoted the degradation of BPA in the WTBC800/PAA system. However, a further increase in PAA concentration to 2.0 mM resulted in a diminished BPA removal efficiency. This phenomenon may be explained by the complete occupation of active sites on the WTBC surface at the higher PAA concentration, potentially limiting the overall oxidative capacity in system.
The influence of pH on the BPA removal in the WTBC800/PAA system was investigated (Figure S5a). The degradation efficiency of BPA exhibited minimal dependence on initial pH, with the degradation efficiency after 120 min following the order of pH 3 (72.6%) < pH 9 (75.9%) < pH 5 (77.3%) < pH 7 (77.9%) < pH 11 (79.4%). The results demonstrate that the WTBC/PAA system achieves better BPA removal under strongly alkaline conditions (pH = 11), while exhibiting the least effectiveness under strongly acidic conditions (pH = 3). This phenomenon may be attributed to the higher activation energy required for PAA activation, which hinders its reactivity under acidic condition. As reported, pH changes altered the stability of PAA. PAA remains stable under acid-base conditions, whereas it undergoes self-decomposition when the solution pH exceeds its dissociation constant (pKa = 8.2) of PAA [20]. When pH raised to 9.0, the PAA with deprotonated species (PAA−) were the predominant species. In addition, the O–O bond length in CH3C(O)OO− is 1.3922 Å, whereas it is 1.5318 Å in CH3C(O)OOH [28]. Consequently, the O-O bond energy in CH3C(O)OOH is higher than that in CH3C(O)OO−, indicating that the anionic form of PAA is more readily activated. Therefore, increasing pH could facilitate PAA activation and enhance the BPA removal.
The co-existed anion anions and dissolved organic matter (DOM) as important factors in aquatic environment probably exert a different impact on catalytic oxidation performance. Therefore, the bicarbonate (HCO3−), chloride (Cl−), nitrate (NO3−), sulfate (SO42−) and humic acid (HA, as a representative of DOM) were selected to evaluate the effect on the BPA removal in WTBC/PAA system. As shown in Figure S5b, the presence of these anions only slightly inhibited the removal of BPA in the WTBC800/PAA system, which could be attributed to two primary factors. (1) The inorganic ions preferentially adsorbs onto the catalyst surface due to the small ionic radius and strong polarizability, and then occupies the positively charged surfaces and more active sites of WTBC, thus inhibiting the PAA activation and suppressing the generation of reactive species [9]; (2) low concentrations of the above anion as a nucleophile may attack the peroxy bond (–O–O–) of PAA and promote its non-selective decomposition, leading to the formation of less reactive products (e.g., HCO4−, ClO−) [30]. Similarly, the presence of HA (10 mg/L) only slightly inhibited BPA removal, with the degradation efficiency slightly decreasing from 83.7% to 78.8% at 120 min, indicating that the inhibitory effect of DOM is limited. In fact, both inorganic ions and HA have a negligible influence on BPA degradation in WTBC/PAA process, demonstrating that the WTBC/PAA system has good tolerance to coexisting substances and holds great potential for practical wastewater treatment applications.
The reusability of WTBC800 was evaluated by performing three consecutive degradation cycles. As shown in Figure S6, the BPA removal efficiency decreased slightly from 82.7% in the first cycle to 79.8% in the second cycle and 77.8% in the third cycle. This gradual decrease may be attributed to the occupation of active sites by adsorbed intermediates or minor catalyst loss during recovery. Nevertheless, WTBC800 maintained ~78% BPA removal after three cycles, demonstrating good reusability and stability for potential practical applications.

2.3. Contribution of Reactive Species to BPA Removal

According to prior research, PAA-based AOPs primarily eliminate pollutants via radical and non-radical pathways. The radical pathways including hydroxyl radicals (HO·) and organic radicals (e.g., CH3C(O)OO·, CH3C(O)O·, CH3·, and CH3OO·) always play an important role in the degradation of organic pollutants in aquatic solution [31,32]. Whereas the non-radical pathway mainly depends on electron transfer processes (ETP) [33] and 1O2 [4]. To elucidate the reaction mechanisms of the WTBC/PAA system, this study systematically investigated the radical pathway.
Typically, TBA is identified as a specific quencher for HO·, with a reaction rate constant of (3.8–7.6) × 108 M−1s−1 [34]. However, TBA exhibits low reactivity toward other radicals in the PAA-based AOPs system, such as CH3C(O)O· and CH3C(O)OO·. MeOH can be used to quench both RO· and HO·. As shown in Figure 4a, quenching experiments with excess TBA (100 mM) and MeOH (100 mM) illustrated that both scavengers posed weak inhibitory effects on the BPA removal (MeOH slightly weaker than TBA), the relevant kobs augmented from 0.0371 to 0.024 and 0.025 min−1 (Figure 4b). Considering that TBA and MeOH could hardly react with PAA and exerted little impact on the reaction between WTBC and PAA, this result indicated that radicals such as HO· and RO· are not the primary reactive species contributing to the BPA removal in the WTBC/PAA system. However, the inhibitory effect of MeOH was slightly lower than that of TBA, which might be attributed to the higher hydrophilicity of MeOH (polarity index of MeOH: 5.1 > TBA: 4.2) [35].
In the peroxide (i.e., persulfate and H2O2) activation process by carbon-based catalyst, 1O2 has been shown to play a key role in degrading organic pollutants through non-radical mechanisms [36]. In addition, a previous study has established the generation of 1O2 by PAA activation [37]. To further clarify the reaction mechanisms of the WTBC800/PAA system, L-histidine (L-his, k = 3 × 107 M−1s−1) [38] was selected as a specific quencher for 1O2 to assess its role in BPA degradation and explore the non-radical pathway. As shown in Figure 4a, adding excess L-his (10 mM) markedly suppressed the BPA degradation with removal efficiency reducing from 82.1% to 21.3%, and the relevant kobs dropped from 0.037 to 0.003 min−1 (Figure 4b). It was concluded that 1O2 are the main reactive species contributing to the BPA removal in the WTBC800/PAA system.
To further validate the potential generation of free radicals in the WTBC/PAA system, EPR tests were conducted using DMPO as a spin-trapping agent. It is noteworthy that DMPO serves not only as a trap for HO· and ·O2− but is also susceptible to oxidation by RO·, yielding a stable nitroxide adduct and DMPOX, which exhibits a diagnostic EPR signature [9]. The EPR spectra obtained in this study (Figure 4c), however, show no detectable signal characteristic of DMPOX, effectively ruling out the presence of RO· in appreciable concentrations. Additionally, the characteristic quartet signal associated with DMPO-HO· adducts was only faintly discernible. Hence, these EPR observations provide direct evidence that neither HO· nor RO· plays a substantial role in the oxidation process. Conversely, when TEMP as a selective probe for 1O2 was introduced (Figure 4c), a well-defined triplet signal with the expected 1:1:1 intensity pattern emerged in the EPR spectrum, confirming the formation of 1O2 under the experimental conditions [28]. The congruence between the quenching experiments and EPR data leads to the compelling inference that BPA degradation in the WTBC800/PAA system proceeds mainly via non-radical pathways, with 1O2 being the predominant oxidizing agent. The marginal inhibition noted in scavenger experiments likely stems from incidental quenching of 1O2 or other non-target interactions at the elevated scavenger concentrations used, rather than from suppression of a dominant free-radical mechanism [39]. Similar non-radical mechanisms, particularly the dominant role of singlet oxygen (1O2), have also been reported in biochar-mediated persulfate activation systems for the degradation of persistent organic pollutants such as lindane and β-endosulfan [40,41]. These studies provide additional evidence that non-radical pathways can be mediated by biochar carbon matrices, which is in line with our observations in the WTBC/PAA system. Therefore, it is reasonable to propose that the non-radical process rather than the free radical oxidation would contribute to BPA removal in the WTBC/PAA system. The generation of 1O2 in this system may proceed via surface-mediated reactions, including electron transfer from adsorbed PAA to dissolved O2 or self-decomposition of PAA on the carbon surface.

2.4. Electron Transfer Process

Surface complex-mediated ETP is another non-radical pathway in carbon-based advanced oxidation processes [42]. Generally, the sp2 carbons with conjugated π structure can promote adsorption of organic contaminants via π–π interactions and also facilitate free-flowing electrons migrating from the adsorbed pollutants to the surface-activated PAA [2]. Therefore, a series of electrochemical analysis methods were performed to examine the potential contribution of ETP for the BPA degradation in the WTBC/PAA system.
As depicted in Figure 4d, the in situ open-circuit potential (OCP) curves of the WTBC-coated electrode increased immediately after the addition of PAA and gradually achieved a plateau, indicating the formation of the WTBC-PAA complex (WTBC-PAA*) [43]. Following the stabilization of the OCP, the subsequent addition of BPA led to a further increase in OCP, which manifests the occurrence of ETP between the WTBC-PAA* and BPA.
The electrical conductivity of WTBC obtained at different carbonization temperatures was compared by electrochemical impedance spectroscopy (EIS) [44]. Figure 4e illustrates the impedance characteristics of electrodes with varying BC loadings. The semicircular region observed in the high-frequency range corresponds to the charge transfer resistance at the electrode–electrolyte interface [23]. The WTBC800 presented a smaller semicircular arc in Nyquist plots than WTBC500 and WTBC300 counterparts, revealing the superior electron transfer capability. This notable decrease in electrochemical impedance directly correlates with the enhanced charge transfer ability, which significantly promote the activation process of PAA. This scenario implied that WTBC800 has potential as an electron transfer mediator in this system. Elevated pyrolysis temperatures can further increase the graphitization level and reduce functional groups in WTBC800, which has potential as an electron transfer mediator to improve ETP from organic pollutants to WTBC-PAA*.
As shown in Figure 4f, the addition of PAA during linear-sweep voltammetry (LSV) analysis led to a current increase, further suggesting the formation of a metastable reactive complex [45]. However, a notable current rise occurred when both BPA and PAA were added together, indicating current flow on the WTBC surface in the presence of both BPA and PAA. Consequently, it is also anticipated that WTBC can act as an electron transfer mediator between BPA and PAA. The WTBC with a large number of surface defects possesses a unique interfacial property to boost electrons accumulation in carbonaceous matrix. These active sites in WTBC could enhance ETP through the non-radical process.

2.5. Removal Mechanism of BPA by WTBC/PAA System

XPS analysis was used for observing the components and functional group changes in fresh and used WTBC. As shown in Figure 5a, a prominent π–π* peak emerged at 290.5 eV in WTBC800 during the carbonization process, indicating the formation of aromatic ring structures. Meanwhile, the peak intensities of these groups (C–O, C=O, O–C=O) gradually diminished, suggesting that the oxygen-containing functional groups were decomposed during the pyrolysis. The stability of peak at 248.76 eV after the reaction confirms the structural integrity of the carbon matrix throughout the catalytic process. From Figure 5b, the N1s spectra of WTBC800 could resolve into three peaks. New emerging peaks at 398.5 eV and 401.2 eV are attributed to functional nitrogen species including pyridinic N and graphitic N, respectively [46]. The relative content of nitrogen-containing functional groups showed insignificant change before and after the reaction. XPS analysis of the O1s spectrum revealed three characteristic peaks at approximately 531.4, 532.4, and 533.6 eV, corresponding to C=O, C–O, and C–OH groups, respectively (Figure 5c). An oxygen content of 18.45% was quantified, confirming the abundance of oxygen-containing functional groups in WTBC. Previous studies have identified that C–OH serving as electron-donating groups could facilitate the adsorption and electron shuttle over the carbon surface through increasing the electron density of the sp2-C structure [47]. Comparative analysis of the O1s spectra between fresh and used WTBC800 revealed a reduction in the relative percentages of C–OH groups from 44.1% to 19.7%. This result implies that surface oxidation reactions possibly occurred on WTBC800 during BPA degradation, likely involving the production of ROS and process of electron transfer. These findings further underscore the crucial role of graphic carbon, functional nitrogen species and oxygen-containing functional groups in the adsorption performance and activity of WTBC for the BPA removal.
Based on the aforementioned discussion, the possible reaction and BPA degradation mechanisms in WTBC/PAA system are elucidated in Figure 6. Adsorption and non-radical oxidation pathways were evidenced to play the dominant roles in the BPA degradation through the formation of metastable complex WTBC-PAA*. Moreover, the functional N, defects state and surface PFRs on WTBC could promote the mediated ETPs.

2.6. Degradation Pathways of BPA in WTBC/PAA System

HPLC-MS/MS analysis was employed to detect possible intermediate generated during the degradation of BPA. Based on the 13 identified byproducts and previous studies (Figure S6), a schematic of degradation pathways for BPA in the WTBC800/PAA system is proposed including two main pathways: coupling reactions (Pathway I) and β-scission (Pathway II) (Figure 7a) [48,49]. In Pathway I, BPA and its degradation intermediates undergo coupling reactions to form polycyclic compounds (P1–P2). In Pathway II, BPA is transformed into intermediates P3, P4, and P8 through β-scission mechanism. Then, P5, P6, P7 and P9 were generated by substitution, hydroxylation, and addition reactions. Subsequently, these intermediates are further degraded into small molecular compounds, including P10 (m/z = 100), P11 (m/z = 101), P12 (m/z = 117) and P13 (m/z = 113) through ring-opening reactions. Ultimately, the intermediates are mineralized into CO2 and H2O by reactive oxygen species (ROS) produced in the WTBC800/PAA system.
The ecological toxicity parameters of BPA and its intermediates products were evaluated using the Toxicity Estimation Software Tool Version 5.1 (T.E.S.T.) based on quantitative structure-activity relationship (QSAR) modeling [50,51]. The assessment focused on four key endpoints: bioconcentration factor, Oral rat LD50, developmental toxicity and mutagenicity. Comparative analysis revealed distinct trends in toxicity reduction during BPA degradation (Figure 7b). Most intermediates except for P1 and P8 exhibited significantly lower bioconcentration factor values than parent BPA. Acute toxicity assessment (Figure 7c) demonstrated that all intermediates except P3 and P12 possessed higher LC50 values than BPA (1.74 mol/kg), indicating that acute toxicity of the most intermediates reduced after degradation process. Similarly, the majority of transformation products showed decreased developmental toxicity (Figure 7d). Mutagenicity screening classified all intermediates as negative except for P9 and P10 (Figure 7e). Based on the above results, WTBC800/PAA-mediated degradation generally reduced the overall toxicity of BPA, though certain intermediates retain measurable toxicological potential.

3. Materials and Methods

3.1. Chemicals and Regents

PAA was purchased from Sinopharm Chemical Reagent Co., Ltd., Hangzhou, China. BPA was obtained from Aladdin, Shanghai, China. Other detailed information of the chemicals involved was listed in Table S1.

3.2. Biochar Preparation and Characterization

The WTBC was synthesized from the discarded tea residues using a straightforward pyrolysis process (Figure 1a). Initially, the waste tea residues (WTR) were thoroughly washed with deionized water and subsequently dried in an oven at 60 °C. The dried raw samples were then pulverized and sieved through a 100-mesh screen. The resulting tea residue particles (5 g) were placed in a ceramic boat and subjected to pyrolysis in a tube furnace under a nitrogen atmosphere (100 mL/min). The temperature was increased at a rate of 5 °C/min, and the samples were carbonized at series of temperatures (300, 500, and 800 °C) for 120 min. Then, these samples were designated as WTBC300, WTBC500, and WTBC800, respectively. The characterization methods for the as-prepared WTBC samples are detailed in Text S2.

3.3. Experimental Procedure

The experimental procedures were conducted by using a standardized setup consisting of 100 mL glass beakers maintained at 25 °C with continuous magnetic agitation at 500 rpm. For each trial, PAA was added to 100 mL of BPA solution (5 mg/L), followed immediately by the addition of a predetermined quantity of WTBC to commence the reaction process. Samples were collected at regular intervals with a 1 mL syringe, subjected to microfiltration using 0.22 μm pore-size filters, and immediately deactivated with sodium thiosulfate solution (0.1 M concentration) to terminate the chemical process. Quantitative analysis of BPA remaining in the treated samples was then performed to determine the removal efficiency.

3.4. Characterization

The WTBC was characterized with scanning electron microscopy (SEM), High-Resolution Transmission Electron Microscope (HR-TEM), Brunuer–Emmett–Teller (BET) analyzer, X-ray diffractometer (XRD), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS). The detailed analysis methods were provided in Text S1.

3.5. Determination for BPA and Its Degradation Intermediates

BPA and its degradation intermediates were determined by high-performance liquid chromatography (HPLC) and chromatography-mass spectrometry (HPLC-MS/MS). The specific operations were provided in the Supplementary Materials Text S2.

4. Conclusions

This study utilized waste tea residue as a raw material to prepare biochar through pyrolysis, which was subsequently employed to activate PAA for the degradation of BPA in aqueous solutions. Through systematic experiments and characterization analyses, the high efficiency and underlying mechanisms of the WTBC/PAA system in BPA removal were clarified. WTBC, characterized by its high specific surface area, well-developed porous structure, and abundant surface functional groups, effectively activated PAA to generate 1O2. The low impedance of WTBC facilitated its role as an efficient electron transfer medium in the system. Non-radical pathways (1O2, ETPs) played a dominant role in BPA degradation. The defects structure and PFRs act as important active sites in WTBC/PAA* complex for the BPA degradation. Additionally, the hierarchical porous structure and carbon defects of WTBC not only enhanced its adsorption capacity for BPA, but also promoted the catalytic activation of PAA. This synergistic effect of adsorption and catalysis significantly improved the overall BPA removal efficiency. The WTBC/PAA oxidation system demonstrates remarkable operational stability across a wide pH range and maintains excellent performance in the presence of various aqueous constituents. This innovative approach, which transforms WTR into functional biochar materials, represents a sustainable and economically viable strategy for environmental remediation while simultaneously establishing a novel pathway for the valorization of agro-industrial byproducts. The WTBC/PAA system avoids issues associated with traditional advanced oxidation technologies, such as high energy consumption, secondary pollution, and metal ion leaching, showcasing broad application prospects. This study demonstrates the high efficiency and practicality of WTBC in PAA activation for the BPA degradation, offering new insights for the development of non-radical advanced oxidation technologies.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal16050420/s1, Table S1: Chemicals and reagents; Table S2: Surface porosity of and ID/IG of raw tea residue, WTBC300, WTBC500 and WTBC800; Table S3: Details of the eluents and detection wavelengths of HPLC; Table S4: Chromatographic conditions for gradient elution of BPA transformation products; Figure S1: The g factors for WTR, WTBC300, WTBC500 and WTBC800; Figure S2: The high-resolution XPS C1s (a), N1s (b) and O1s (c) spectra of WTR; Figure S3: Removal of BPA by different processes. (Conditions: [BPA] = 5mg/L, [WTBC] = 0.4 g/L, [PAA] = 1.5 mM, temperature = 25 °C, pH = 7.2 ± 0.2); Figure S4: Fitting of the BPA removal kinetics under different WTBC dosages (Conditions: [BPA] = 5mg/L, [PAA] = 1.5 mM, temperature = 25 °C, pH = 7.2 ± 0.2); Figure S5: The effects of (a) initial pH and (d) influence of coexisting anions on the BPA degradation in WTBC/PAA system (Conditions: [BPA] = 5 mg/L, [WTBC] = 0.4 g/L, [PAA] = 1.5 mM, [Cl−] = [NO3−] = [HCO3−] = [SO42−] = 10 mM, temperature = 25 °C, pH = 7.2 ± 0.2); Figure S6: The EPR spectra of WTBCs samples; Figure S7: Reusability tests on WTBC/PAA for BPA removal; Figure S8: LC-MS profiles of the intermediates during BPA degradation in the BC800/PAA system; Text S1: Characterizations; Text S2: The specific operation methods for BPA and its degradation intermediates.

Author Contributions

Conceptualization, X.M.; Methodology, S.S.; Software, Y.H.; Formal analysis, Y.W. and H.Y.; Investigation, X.Z.; Resources, W.L.; Writing—original draft, X.Z.; Writing—review and editing, S.Z.; Supervision, S.Z. and J.D.; Project administration, J.D.; Funding acquisition, S.Z. All authors have read and agreed to the published version of the manuscript.

Funding

National Natural Science Foundation of China (No. 42307065); Natural Science Foundation of Zhejiang Province (No. LQ23E080020); Zhejiang Provincial “Jianbing Lingyan+X” Science and Technology Program (No. 2025C02097).

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

This research was also supported by Zhejiang Key Laboratory of Green Construction and Intelligent Operation & Maintenance for Coastal Infrastructure (Zhejiang University of Technology); Engineering Research Center of Ministry of Education for Renewable Energy Infrastructure Construction Technology.

Conflicts of Interest

The authors declare no conflicts of interest.

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Catalysts 16 00420 g001
Figure 1. (a) The schematic illustration of WTBC preparation; SEM image of (b) WTR, (c) WTBC300, (d) WTBC500 and (e) WTBC800; (f) TEM and (g) EDS elemental mapping of WTBC800.
Figure 1. (a) The schematic illustration of WTBC preparation; SEM image of (b) WTR, (c) WTBC300, (d) WTBC500 and (e) WTBC800; (f) TEM and (g) EDS elemental mapping of WTBC800.
Catalysts 16 00420 g001
Catalysts 16 00420 g002
Figure 2. (a) The N2 adsorption-desorption isotherms by BET method; (b) the XRD spectra, (c) FTIR spectra, (d) Raman spectra and (e) EPR spectra of WTBCs samples; (f) XPS surveys of the WTR and WTBC800 before and after reaction.
Figure 2. (a) The N2 adsorption-desorption isotherms by BET method; (b) the XRD spectra, (c) FTIR spectra, (d) Raman spectra and (e) EPR spectra of WTBCs samples; (f) XPS surveys of the WTR and WTBC800 before and after reaction.
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Catalysts 16 00420 g003
Figure 3. (a,c) The removal of BPA in different systems; (b) fitting of the BPA removal kinetics; (d) PAA decomposition by different processes; (e) WTBC dosages evaluation; (f) PAA dosage determination in WTBC/PAA system. Conditions: [BPA] = 5 mg/L, [WTBC] = 0.4 g/L, [PAA] = 1.5 mM, [H2O2] = 1.5 mM, temperature = 25 °C, initial pH = 7.2 ± 0.2.
Figure 3. (a,c) The removal of BPA in different systems; (b) fitting of the BPA removal kinetics; (d) PAA decomposition by different processes; (e) WTBC dosages evaluation; (f) PAA dosage determination in WTBC/PAA system. Conditions: [BPA] = 5 mg/L, [WTBC] = 0.4 g/L, [PAA] = 1.5 mM, [H2O2] = 1.5 mM, temperature = 25 °C, initial pH = 7.2 ± 0.2.
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Catalysts 16 00420 g004
Figure 4. (a) Quenching effects of TBA, MeOH, and L-histidine scavengers on the BPA degradation; (b) fitting of BPA removal kinetics under different quenching conditions; (c) EPR spectra using DMPO and TEMPO as a spin-trapping agent in the WTBC/PAA system. (d) open-circuit potential (OCP) curves for the WTBC; (e) electrochemical impedance spectroscopic (EIS) analysis of WTBC loaded electrode; (f) linear-sweep voltammograms (LSV) under different conditions for BPA removal in WTBC/PAA system.
Figure 4. (a) Quenching effects of TBA, MeOH, and L-histidine scavengers on the BPA degradation; (b) fitting of BPA removal kinetics under different quenching conditions; (c) EPR spectra using DMPO and TEMPO as a spin-trapping agent in the WTBC/PAA system. (d) open-circuit potential (OCP) curves for the WTBC; (e) electrochemical impedance spectroscopic (EIS) analysis of WTBC loaded electrode; (f) linear-sweep voltammograms (LSV) under different conditions for BPA removal in WTBC/PAA system.
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Catalysts 16 00420 g005
Figure 5. High-resolution XPS (a) C1s, (b) N1s and (c) O1s spectra of fresh and used WTBC800.
Figure 5. High-resolution XPS (a) C1s, (b) N1s and (c) O1s spectra of fresh and used WTBC800.
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Catalysts 16 00420 g006
Figure 6. Schematic diagram of possible reaction mechanisms in WTBC/PAA system.
Figure 6. Schematic diagram of possible reaction mechanisms in WTBC/PAA system.
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Figure 7. (a) Pathways for the BPA degradation by WTBC800/PAA system; (b) bioconcentration factor, (c) oral rat LD50 (d) developmental toxicity and (e) mutagenicity of BPA and degradation intermediates in the WTBC800/PAA system.
Figure 7. (a) Pathways for the BPA degradation by WTBC800/PAA system; (b) bioconcentration factor, (c) oral rat LD50 (d) developmental toxicity and (e) mutagenicity of BPA and degradation intermediates in the WTBC800/PAA system.
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MDPI and ACS Style

Zhu, S.; Zhang, X.; Shen, S.; Wang, Y.; Hu, Y.; Yang, H.; Liu, W.; Ma, X.; Deng, J. Activation of Peracetic Acid by Waste Tea Residue-Derived Biochar for Bisphenol A Elimination: Synergetic Adsorption and Non-Radical Oxidation. Catalysts 2026, 16, 420. https://doi.org/10.3390/catal16050420

AMA Style

Zhu S, Zhang X, Shen S, Wang Y, Hu Y, Yang H, Liu W, Ma X, Deng J. Activation of Peracetic Acid by Waste Tea Residue-Derived Biochar for Bisphenol A Elimination: Synergetic Adsorption and Non-Radical Oxidation. Catalysts. 2026; 16(5):420. https://doi.org/10.3390/catal16050420

Chicago/Turabian Style

Zhu, Shijun, Xinchen Zhang, Shangming Shen, Yang Wang, Yongshu Hu, Hao Yang, Wenbin Liu, Xiaoyan Ma, and Jing Deng. 2026. "Activation of Peracetic Acid by Waste Tea Residue-Derived Biochar for Bisphenol A Elimination: Synergetic Adsorption and Non-Radical Oxidation" Catalysts 16, no. 5: 420. https://doi.org/10.3390/catal16050420

APA Style

Zhu, S., Zhang, X., Shen, S., Wang, Y., Hu, Y., Yang, H., Liu, W., Ma, X., & Deng, J. (2026). Activation of Peracetic Acid by Waste Tea Residue-Derived Biochar for Bisphenol A Elimination: Synergetic Adsorption and Non-Radical Oxidation. Catalysts, 16(5), 420. https://doi.org/10.3390/catal16050420

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