Investigation of tetracycline degradation by activating persulfate with lotus leaf biochar: source of active substance and toxicity evaluation

With the continuous acceleration of the social industrialization process, the production and use of antibiotics have been increasing year by year, leading to increasingly serious pollution of the ecological environment, which has become a key issue of global concern (Huang et al. 2023). Tetracycline (TC) is the most widely used antibiotic, and it is extensively used in livestock farming, aquaculture, and human healthcare (Hang et al. 2024). In China, the use of TC in animal husbandry, aquaculture, and human healthcare accounts for about 50, 10, and 12%, respectively (Zhang et al. 2015; Shao et al. 2021b; Cherian et al. 2023). This results in the detection of TC in groundwater, natural water bodies, and municipal sewage, among other water sources. This could lead to the creation of highly resistant super bacteria, spreading widely in the ecological environment, and posing a threat to the safety of human and other organisms' lives (Li et al. 2024). For example, Anthony et al. (2018) described that the release of antibiotics into the environment at sub-lethal concentrations leads to the emergence of antibiotic resistance and the selection of antibiotic resistance genes. Rayan (2023) explained that bacteria use quorum sensing to form biofilms, where antibiotic resistance genes are transferred from antibiotic-resistant bacteria to susceptible strains, allowing them to exchange resistance genes and develop resistance mechanisms. So, developing an efficient and environmentally friendly way to treat antibiotics in water environments is of great significance.

At present, the methods for dealing with antibiotic pollution include adsorption method (Zhang et al. 2021), membrane filtration (Zhang et al. 2024b), photocatalytic method (Zheng et al. 2024), electrocatalytic method (Xie et al. 2024), persulfate method (Song et al. 2023), biofilm method (Zhang et al. 2024a), etc. Among numerous water treatment processes, the advanced oxidation process (AOP) based on persulfate (PDS) stands out. Due to the various activation methods available for PDS in PDS-AOPs, whether they are physical activation methods (thermal decomposition, ultrasonic waves, ultraviolet light, etc.) or chemical activation methods (transition metals, alkaline conditions, carbon catalysis, etc.) (Liu et al. 2021b; Yin et al. 2021; Li et al. 2022), they can effectively activate PDS and generate active species (ROS) with strong oxidation capabilities, namely ·OH, ⁠, and ⁠. They can all efficiently degrade organic compounds in the environment. However, the essence of physical activation is to transfer external energy to PDS, causing the breakage of the O–O bond and the generation of highly oxidative ROS. The multi-level transformation of this energy not only incurs high costs but also results in significant energy loss (Yin et al. 2021). In chemical activation, the issue of ion leaching after transition metal activation is particularly severe, with high toxicity and difficult application. Therefore, non-metals (carbon-based materials) show unique advantages in activating PDS due to their extremely environmentally friendly and controllable electron structure distribution (Peng et al. 2021). For example, Li et al. (2020) used carbon nanotubes for PDS activation to achieve efficient oxidation of benzyl alcohols. Indrawirawan et al. (2015) utilized ordered mesoporous carbon as a metal-free catalyst for PDS to facilitate the oxidative degradation of phenol. Hirani et al. (2023) used graphene materials to activate PDS to accomplish efficient removal of antibiotics from aqueous environments. Gao et al. (2022) used three different carbon materials to activate PDS to investigate the mechanism of efficient degradation of antibiotics by activating PDS with different carbon materials. This is due to the fact that carbon-based materials have been widely used by researchers for PDS-AOPs based on their advantages of low cost, non-toxicity, acid and alkali resistance, large specific surface area, scalable surface charge, and selective control of pollutants in specific waters (Xiao et al. 2020; Hao et al. 2023).

Currently, carbon-based materials that activate PDS include nanodiamonds, reduced graphene oxide, carbon nanotubes, and biochar (BC), and their excellent activation effects on PDS have been widely studied (Yun et al. 2018; Ren et al. 2020). However, some carbon-based materials are very expensive and cannot be widely used on a large scale, while BC materials have gained favor among many researchers. BC is the pyrolysis product of agricultural, forestry, and livestock waste, with low cost, wide availability, and strong application prospects (El-Bestawy et al. 2023; Seo et al. 2023; Singh & Verma 2024; Yan et al. 2024). Most researchers modify BC during its production to enhance its catalytic effect on PDS, neglecting the original catalytic performance of BC materials. They introduce metallic elements (Fe, Cu, Co, Ni, Mn, Ce, etc.) or non-metallic elements (N, S, P, B, etc.) into biochar to form modified biochar with the introduced elements as the active site, so as to achieve efficient activation of PDS. For example, the B-modified sludge biochar prepared by Kang et al. (2024), the Co-doped water hyacinth biochar developed by Yi et al. (2024), and the Co/N modified biochar obtained by Zhu et al. (2022) had all achieved the purpose of efficient activation of PDS oxidative degradation of pollutants. However, some researchers have also prepared original BC catalysts with excellent catalytic performance. For example, fish scale N, P co-doped BC prepared by He et al. (He et al. 2023), traditional Chinese medicine residue BC prepared by Jia et al. (2023), and N-doped digestate BC prepared by Liu et al. (2021a). The catalytic performance of BC prepared varies with different BC precursors. Some original BC possess unique elements (B, N, S, and P) that give them excellent catalytic properties, while others exhibit outstanding catalytic performance due to their large specific surface area and unique graphitic or defective structures. Therefore, the selection and preparation of biochar precursors are also crucial factors in activating PDS for degrading pollutants (Song et al. 2022). Lotus leaf resources are extremely abundant in China, with a cultivation area of 9–11 million mu annually. Every year, the catering industry uses about 7% of the lotus leaves, while the pharmaceutical industry uses about 5%. The remaining over 80% of lotus leaves are not properly disposed of and end up rotting in the lotus ponds. In addition, lotus leaves contain nitrogen elements inside, and their surfaces have ordered three-dimensional micro-nano structures, which have a strong binding capacity to effectively bind pollutants. This makes them an ideal material for preparing biochar (Song et al. 2023).

In this study, lotus leaf biochar (LLBC) with different pyrolysis temperatures (500–700 °C) was prepared using withered lotus leaves as a biomass source. The catalyst was used to activate PDS to degrade TC, and the mechanism of TC degradation in the LLBC/PDS system was deeply analyzed. The active sites of LLBC activating PDS and the sources of TC active substances degraded by the LLBC/PDS system were explored in detail through experiments and related characterization. The possible degradation pathways of TC were speculated and the toxicity of degradation products was predicted. In addition, this paper promotes a new understanding of the oxidation process of the biochar/peroxydisulfate system in practical water treatment by testing the feasibility of the LLBC/PDS system for practical applications using different water quality bases and continuous flow column experiments.

Preparation of LLBC: The lotus leaf samples were picked from the lotus pond in Wujin District, Changzhou City, Jiangsu Province. First, the lotus leaf samples were cleaned with ultrasonic pool for 1 h, then washed with ultrapure water for three times, and the lotus leaves were cut into small pieces with scissors and put into a tinfoil tray, and then dried at 105 °C in a blast drying oven (24 h). Next, the dried lotus leaves were put into a crusher for crushing and passed through a 100-mesh screen. Then, the prepared lotus leaf biomass was placed in a quartz boat before being put into a tube furnace for pyrolysis. The tube furnace was ramped up to 500, 600, and 700 °C at a ramping rate of 5 °C/min, and held at the three temperatures for 2 h. The obtained LLBC were labeled as 500, 600, and 700 °C LLBC, respectively. Finally, the resulting LLBC was ground and washed three times each with 1% dilute hydrochloric acid, ultrapure water, ethanol, and ultrapure water. The washed LLBC was dried in a blast drying oven at 105 °C for 12 h.

In this study, the experiments were carried out by adding PDS for oxidative degradation after the adsorption equilibrium of TC by LLBC. All experiments were performed in a constant temperature shaking chamber at 25 °C, away from light.

In the pre-adsorption experiment, the volume of the contaminant (TC = 10 mg/L) reaction solution was 50 mL, and 20 mg of 500, 600, and 700 °C LLBC were added to the reaction solution for 3 h adsorption equilibrium experiment test (Supplementary Figure S1(a)). Every 30 min, 3 mL of the supernatant was removed from the reaction device, and the supernatant was filtered through a 0.22 μm membrane to detect the concentration of TC using high-performance liquid chromatography.

In the experiment of catalytic degradation of TC in the LLBC/PDS system, first, 20 mg of LLBC was added to 50 mL of pollutant reaction solution (TC = 10 mg/L). Wait for 30 min to make the adsorption state of LLBC on TC in the thermostatic shaking chamber was allowed to reach saturation equilibrium (Supplementary Figure S1(a)). Then, 5 mL of PDS (40 mM) solution was added to the adsorption equilibrium reaction solution, and the experiment of LLBC/PDS system degradation of TC was started. Every 30 min, 3 mL of the supernatant was removed from the reaction device, passed through a 0.22 μm filter, the filtrate was placed into a 10 mL centrifuge tube, and 3 mL of methanol solution was added to the centrifuge tube for ROS quenching to prevent further oxidative degradation of TC by PDS. After each experiment, the catalyst was recovered by a centrifuge method, and the recovered catalyst was washed with ethanol and ultrapure water for three times, and then put into a blast drying oven to dry at 105 °C for 12 h. After that, the catalyst was used for the cyclic experiments.

This is shown in Supplementary Figure S2(c), weighing 100 mg of LLBC filled into the pre-prepared column for the continuous flow column experiment. To prevent LLBC from leaking during the experiment, the ends of the columns were therefore sealed using absorbent sponges. In the continuous flow catalytic degradation experiments, to ensure a mixed flow rate of 0.5 mL/min of TC and PDS in the flow column, we injected TC and PDS into larger flasks separately and controlled the effluent at the same flow rate (0.25 mL/min), and finally mixed the effluent together at a flow rate of 0.5 mL/min through the column filled with LLBC. To ensure that the TC and PDS in the continuous flow column were consistent with the experimental conditions in the beaker (TC = 10 mg/L, PDS = 4 mM), the concentrations of TC and PDS in the large flask were adjusted to TC = 20 mg/L and PDS = 8 mM, respectively. In continuous flow adsorption experiments, ultrapure water was used to replace the PDS solution and passed through the column containing LLBC in the same manner as described above. Finally, the concentration of TC in the effluent was measured at a fixed time interval.

The materials used in this study, analytical grade reagents, testing methods, characterization analyses, and characterization operations, and more details can be found in Supplementary Text S1–S5.

Raman spectroscopy is used to detect the structural defects and graphitization degree (sp²C) of catalysts. Typically, the D band (1,350 cm⁻¹) reflects the degree of structural defects in the catalyst, while the G band (1,570 cm⁻¹) reflects the level of graphitization in the catalyst (Miao et al. 2022). The intensity ratio of the D band to the G band (I_D/I_G) is an important parameter that reflects the degree of atomic structural defects in the catalyst. As shown in Figure 2(b), the ratio of I_D/I_G (500 °C LLBC = 0.65, 600 °C LLBC = 0.74, and 700 °C LLBC = 0.93) gradually increases with the temperature rise. This indicates that the increase in pyrolysis temperature promotes the formation of structural defects in LLBC. A higher degree of defects typically favors the activation of PDS. Therefore, the defect structure may be one of the sources of ROS in the LLBC/PDS system (He et al. 2023).

X-ray diffraction (XRD) is a powerful technique to characterize the chemical composition, crystal structure, and grain size of catalysts in a fast, accurate, and efficient way. In this study, XRD was used to detect the chemical composition and possible crystal structure of LLBC. The results indicate that all LLBC exhibit strong crystal vibrations, which are in complete agreement with the vibration trajectory of KCl (PDF-#04-0587) (Figure 2(c)). This may be due to the potassium salt inherent in the lotus leaf itself. In LLBC, the diffraction peak at 29.39° is formed due to the recrystallization of the waxy layer of the lotus leaf during pyrolysis. The main elements of the wax layer of the lotus leaf are C, O, and H elements. Therefore, after pyrolysis and recrystallization, it may form amorphous carbon corresponding to the (200) plane with a peak intensity of 4,000 a.u. (Ensikat et al. 2006; Nishimura et al. 2016; Liu et al. 2020). Amorphous carbon belongs to a hybrid approximation of sp³/sp² hybridization, which is a quasi-amorphous form and can be viewed as graphite-like carbon. Moreover, with the increase in temperature, the content of amorphous carbon also increases. This result is similar to the results shown by Raman, indicating the presence of graphite-like carbon in LLBC. In addition, by observing the XRD spectra of the LLBC/PDS system before and after the reaction, a significant decrease in the content of graphite-like carbon can be observed (Figure 2(d)). Therefore, graphite-like carbon may also be one of the active sites for activating PDS.

Fourier transform infrared spectroscopy (FT-IR) is suitable for detecting the type of functional groups contained in a material and determining the possible chemical structure to evaluate the properties of the material. In this study, FT-IR was used to analyze the characteristic absorbance of LLBC to determine the functional groups that may exist on its surface. As shown in Figure 2(e), the strong peak vibration at 3,423 cm⁻¹ is caused by the stretching of O–H bonds. The fluctuation at 1,577 cm⁻¹ is due to the C = C stretching of the aromatic components or the C = O stretching of the conjugate ketones (C = C/C = O), where the peak fluctuations become weaker with increasing temperature. The strong peak oscillations at 1,437 and 1,028 cm⁻¹ are caused by the C = C stretching of aliphatic groups and the symmetrical stretching of C–C/C–O. At 873 cm⁻¹, it is caused by the strong peak vibration of the C = O bond (Janu et al. 2021; Amirchand et al. 2023; Seo et al. 2023). Among them, a significant decrease in the intensity of the peak vibration can be observed after the reaction of the C = C and C = O bonds of 1,437 and 873 m⁻¹ (Figure 2(f)). This clearly suggests that the C = C and C = O bonds contribute to the TC oxidative degradation. C = C is also known as sp²C. It has been reported that sp²C can mediate the electron transfer of contaminants to PDS by forming charge-transfer complexes (Shao et al. 2021a). That is, the electrons of TC are transferred to the surface of PDS through C = C to produce sulfate radicals (⁠⁠) and hydroxyl radicals (·OH). and ·OH generate singlet oxygen (¹O₂) through the contact between the electrons transported by C = C and the superoxide free radical (⁠⁠) in the reaction solution to complete the oxidative degradation of TC. Since C = O is an electron-rich group, PDS can attract electrons on the surface of carbon-based materials to form and ·OH, and react with in aqueous solution to produce ¹O₂ (Li et al. 2020). That is, PDS directly reacts in aqueous solution by seizing electrons of C = O to produce ⁠, ·OH and ¹O₂ thus complete the oxidative degradation of TC. Therefore, the C = C and C = O bonds may be among of the active sites for catalytic degradation of TC in the LLBC/PDS system, while the changes in the C = C bond reaction in FT-IR previous to and following the response show consistent results with XRD.

Different dosages of LLBC were used to activate PDS to explore the optimal usage amount of LLBC. As demonstrated in Figure 3(b), the eliminate efficiency of TC was significantly enhanced when the LLBC dose was raised from 0 to 1 g/L, and the elimination efficiency of TC reached a maximum of 88.22% when LLBC = 1 g/L. However, when the amount of LLBC was increased from 0 to 0.4 g/L, the pseudo-first-order kinetic K of TC degradation in the LLBC/PDS system was continuously enhanced, from K = 0.00104 min⁻¹ to K = 0.00868 min⁻¹ (Supplementary Figure S1(c)). When the amount of LLBC increased from 0.4 to 1 g/L, the pseudo-first-order kinetic constant K decreased from K = 0.00868 min⁻¹ to K = 0.00665 min⁻¹. This may be due to the increased dosage of LLBC, which leads to an increase in ROS within the LLBC/PDS system. The phenomenon of mutual quenching between ROS may ultimately result in limited enhancement of TC removal efficiency. Considering the economic benefits, subsequent exploratory experiments will continue to use an LLBC dosage of 0.4 g/L, as when LLBC is 0.4 g/L, the eliminate efficiency of TC is 86.58%, and even at LLBC = 1 g/L, the eliminate efficiency of TC is only 88.22%.

The dosage of oxidants is also among the factors considered. The TC degradation curves corresponding to different concentrations of PDS and the degradation kinetics constants K are shown in Figure 3(c) and Supplementary Figure S1(d). When the level of oxidant PDS increased from 0 to 4 mmol/L, the eliminate efficiency of TC increased from 30.50 to 86.58%, and the pseudo-first-order kinetic constant K increased from K = 0.00025 min⁻¹ to K = 0.00868 min⁻¹. This is due to the fact that more PDS is activated by LLBC to produce more ROS. However, when the PDS dosage was increased to 10 mM, the removal rate of TC decreased to 85.55%, and the pseudo-first-order kinetic constant K decreased to K = 0.00749 min⁻¹. This phenomenon is due to the limited catalytic sites on LLBC or the excessive generation of ROS by excess PDS, leading to their mutual quenching. Therefore, in subsequent exploratory experiments, the amount of PDS used is 0.4 g/L.

pH is a key factor affecting the catalytic performance of materials, so the effect of pH on the catalyst was studied by using TC solutions with different initial pH values. In the LLBC/PDS system at different pH values, when the reaction solution is neutral, the maximum removal rate reaches 86.57%, with a pseudo-first-order rate constant of K = 0.00868 min⁻¹. However, whether under alkaline or acidic conditions, the removal efficiency of TC shows a decreasing trend. Specifically, at pH = 3, the removal rate of TC is 74.44% (K = 0.00680 min⁻¹), at pH = 5, the removal rate is 75.35% (K = 0.00697 min⁻¹), at pH = 9, the removal rate is 76.05% (K = 0.00320 min⁻¹), and at pH = 11, the removal rate is 77.35% (K = 0.00331 min⁻¹) (Figure 3(d) and Supplementary Figure S1(e)). The phenomenon may be caused by the following reasons:

(Ⅰ) The Zeta potential of the LLBC/PDS system cannot effectively accelerate the charge-transfer process under both acidic and alkaline conditions, thus failing to promote the effective binding and activation with PDS (Wang et al. 2023). (Ⅱ) The pH_pzc of LLBC is 5.30 (Supplementary Figure S2(a)), which allows us to conclude that the LLBC surface has a significant bit of negative energy at pH > 5.30 and a significant bit of positive energy at pH < 5.30. When the pH of the TC is pH < 3.3, TC is stored in the solution in the form of TC⁺, and there is also a large amount of positive charge on the surface of LLBC at this time. This leads to the presence of mutual repulsive electrostatic forces between LLBC and TC, resulting in a decrease in the removal efficiency of TC. Similarly, when the acidity and alkalinity of the TC reaction solution is pH > 7.7, the existence of TC is mostly in the shape of TC⁻ and TC²⁻, and there are also a substantial amount of negative ions on the surface of LLBC at this time. This results in the electrostatic repulsion between LLBC and PDS, TC, which hinders efficient catalysis and reduces the removal efficiency of TC (Duan et al. 2022). (Ⅲ) LLBC has the best activation effect at pH = 7, which may be owing to LLBC's positive energy surface, TC⁰, the main form of TC, at this time, the mutual attraction between LLBC, PDS, and TC is completely dependent on van der Waals force and hydrogen bonding force (Guo et al. 2024). PDS exists in anionic form both under alkaline and acidic conditions. Therefore, when pH < 5.30, the attraction between PDS and LLBC may rely on van der Waals forces and intermolecular forces, etc. When pH > 5.30, the charge on the surface of LLBC is in the form of a positive charge, and the attraction between PDS and LLBC may be due to electrostatic forces of mutual attraction in addition to van der Waals forces and intermolecular forces. Although different pH levels can affect the elimination efficiency of TC, the removal rate of TC still exceeds 75% under different acidic and alkaline conditions. Therefore, it can be inferred that the LLBC/PDS system has a wide pH tolerance.

This work studied the effects of various inorganic anions on the elimination of TC. Inorganic anions are typically used as free radical scavengers, interfering with the catalytic degradation performance during the oxidation process of free radicals. As shown in Figure 3(e), had the greatest impact on the LLBC/PDS system, but the TC removal rate was still 81.65%. The reason for the decrease in the TC elimination effect might be a result of the reality that breaks the acid-base balance of the LLBC/PDS, which makes the pH of the LLBC/PDS system low, which undoubtedly inhibits the TC removal effect of the LLBC/PDS system (Zhuo et al. 2023). This is consistent with the results of the exploration experiments with different initial pH values mentioned above. The impact of the other anions on the entire system is minimal, or even negligible. The aforementioned findings demonstrate how well the LLBC/PDS can fend against the effects of outside anions.

The cyclic experiments investigated the reusability of the LLBC/PDS for degrading TC. After five cycles of recycling, the elimination rate of TC by LLBC showed a significant downward trend (Figure 3(f)). The removal rate of TC was 63.33% after five cycles of LLBC. The decrease in catalytic efficiency may be due to the large number of TC intermediates occupying the surface pore size of LLBC, thereby reducing the electron transfer efficiency and the consumption of active sites. In addition, key active sites (e.g., defects, C = C, etc.) may undergo irreversible changes after the reaction. Overall, LLBC exhibits excellent stability and reusability.

In order to determine the actual utility of LLBC in the actual entity, tap water, natural water, and secondary sedimentation tank effluent were used as TC solvents to configure contaminants with a concentration that matched the laboratory (TC = 10 mg/L in all water quality bases; TP_{(Tap water)} = 0.046 mg/L, TP_{(Natural water bodies)} = 0.081 mg/L, TP_{(The water of the second sedimentation tank)} = 0.382 mg/L). The results showed that the three water-based solvents had an impact on the catalytic elimination efficiency of the LLBC/PDS (Figure 3(g)), and the degradation efficiency was 79.03, 70.54, and 68.45%. Compared with the experimental results of ultrapure water (86.59%), the actual water contains different concentrations of anions, cations, microorganisms, and other organic substances, which will affect the LLBC/PDS/TC. The oxidative elimination of TC by the LLBC/PDS system in tap water showed a difference of 7.56% in removal efficiency compared with that of ultrapure water. This may be due to the presence of dissolved metal ions (Fe, Mn) and other inorganic substances in tap water that also react with the oxidant and consume the oxidant (Marotta et al. 2012). Tap water also contains a large number of anions, which will preferentially react quickly with the free radicals generated, thereby inhibiting their further oxidation of TC, etc., resulting in the reduction of the oxidative elimination effect of TC. The oxidative elimination of TC by the LLBC/PDS system in natural water bodies showed a difference in removal efficiency of 16.05% compared with that of ultrapure water. This is not only due to the natural water body containing a large number of inorganic ions and dissolved minerals interference, but also may be due to the natural water body contains a large number of organic matter (humic acid, fulvic acid, etc.), as well as the water body contains particles, suspended solids will affect the LLBC and other reaction media, change the effective contact surface of the reactants, and then affect the oxidative degradation efficiency of the LLBC/PDS system to TC (Liu et al. 2022). The oxidative elimination of TC by the LLBC/PDS system in the effluent of the secondary sedimentation tank showed a difference in removal efficiency of 18.14% compared with that of ultrapure water. In addition to the residual organic matter, dissolved inorganics, and suspended solids particles in the water body, there are also residual microorganisms and biological metabolites that can affect the LLBC/PDS system. The composition of the secondary sedimentation tank effluent is also relatively complex and may contain pollutants from different sources, which can lead to competitive reactions in the oxidation process of the LLBC/PDS system, resulting in a reduction in the efficiency of TC oxidative degradation (Niu et al. 2023). By measuring the COD concentration before and after the reaction, it was found that the COD₀ before the reaction was 36.17 mg/L, and the COD was 13.67 mg/L after the reaction (Supplementary Figure S2(d)). This also confirms that in addition to oxidizing and degrading TC in the water, the LLBC/PDS system also oxidizes other substances in the effluent of the secondary sedimentation tank. In order to improve the effect of LLBC in actual water treatment, the pore size of LLBC can be adjusted by the template method to form an exclusion effect, so that LLBC selects target pollutants with small size to adsorb to the surface of the catalyst, and organic matter and other interference items with large size are blocked outside the pore size (Hu et al. 2023a). Alternatively, by using molten salt, while changing the pore size of the LLBC, the N element inside the LLBC is induced to become a new active site for catalyzing PDS, which strengthens its ability to remove pollutants (Xie et al. 2020).

In this study, the continuous flow column experiment was used to simulate and test whether the LLBC/PDS/TC system can be used with the real continuous operation project. The results of the column experiment (Figure 3(h)) show that the effluent concentration of TC increases dramatically at 180 min in the adsorption experiment of the column without PDS. In the catalytic system containing LLBC and PDS, the effluent concentration of TC reached a stable level at 540 min, the LLBC-activated PDS failed at this time, and ROS could not be produced to degrade TC, and the TC removal rate was 24.90%. At 270 min, the TC effluent concentration rose sharply, with a removal rate of 61.56%. Compared with other biochar/persulfate systems (Supplementary Table S2), the LLBC/PDS system did not show significant differences in the catalytic degradation of TC in ultrapure water. However, it was prioritized over other catalytic systems in different water quality bases and continuous flow column experiments to evaluate its potential for practical engineering applications. The aforementioned findings show that the LLBC/PDS system not only has the ability to efficiently treat organic pollutants in actual water bodies but also has the potential to be applied in continuous flow engineering.

As shown in Figure 4(a), the sp²C, sp³C, C–O, C = O, and π–π* satellite peaks were matched to 284.40, 284.89, 285.90, 287.92, and 293.06 eV in the high-resolution spectra of C1s prior to the reaction. Interestingly, the π–π* satellite peak in C1s (Figure 4(b)) disappeared after the reaction. The formation of the π–π* satellite peak has been reported to be due to the presence of a large number of π electrons around sp²C (C = C) and C = O inside biochar (Oyekunle et al. 2021). The disappearance of the π–π* satellite peak after the reaction indicated that sp²C (C = C) and C = O were involved in the process of TC degradation in the LLBC/PDS. This also confirms the most direct evidence of electron transfer occurring in the eliminate process of TC in the LLBC/PDS (Kang et al. 2023). Following the response, the binding energies of sp²C, sp³C, C–O, and C = O in the catalyst did not change significantly, but the peak areas of these bonds showed some changes. The area occupied by sp²C decreased from 25.87 to 21.90%, the area occupied by sp³C decreased from 39.46 to 37.79%, and the region captured by C = O decreased from 9.66 to 9.57%. As shown in Figure 4(c), in the high-resolution spectrum of O1s before the reaction, C = O and C–O were assigned to 531.54 and 533.15 eV, respectively. Following the response, the binding energy of C = O (Figure 4(d)) increased to 531.74 eV, and its area percentage decreased from 59.38 to 37.30%. The binding energy of C–O decreased to 533.11 eV, and its area percentage increased from 40.62 to 62.70%. This indicates that electron enrichment may occur at the C = O site, and C = O is consumed during the electron transfer process. The XPS results of C1s and O1s mentioned above are consistent with the changes in C = C and C = O shown in the FT-IR previous to and following the response (Figure 2(b)). This undoubtedly confirms the involvement of sp²C (C = C) and C = O in the activation of PDS, making them the main active sites.

To verify the accuracy of the above quenching experiments, this study further used ESR to confirm the generated ROS. From Figure 5(b)–5(d), it can be seen that when only PDS is present in the catalytic system and capture agents such as DMPO and TEMP are added to the system, there are no characteristic signals of any ROS. However, after adding DMPO and TEMP to the LLBC/PDS system and then performing ROS signal detection, significant fluctuations in ROS signals can be clearly observed. Among them, caught ⁠, OH, and signals exhibit classical peak vibrations, and the peak vibrations increase gradually with time (Figure 5(c) and 5(d); Hu et al. 2023b). This shows that ⁠, ·OH, and do exist in the LLBC/PDS system. TEMP was employed to detect the presence of ¹O₂ in the LLBC/PDS. A strong signal of the typical triple peak of TEMP-¹O₂ was found, and the triple peak signal became stronger and stronger with the increase of time (Figure 5(b)). The above ESR detection was in line with the capture experiment's findings, which undoubtedly indicated that TC in LLBC/PDS was degraded by the joint action of multiple ROS.

Since the quenching experiment results show that electrochemical testing was performed to confirm further the feasibility of a process for transferring electrons in the LLBC/PDS system. When LLBC is used as a bare electrode as a working electrode, the electrochemical impedance (EIS) value is 40 Ω in a solution with sodium sulfate as the electrolyte, and the EIS value is reduced to 30 Ω when PDS was incorporated into the electrolyte in drops, which makes electron transfer possible in the electronic LLBC/PDS system (Figure 5(e); Kang et al. 2023). The decrease in the value of EIS means that the ability to transport electrons in the LLBC/PDS system is stronger, which is more conducive to the electron transfer between LLBC, PDS, and TC, so as to achieve the purpose of LLBC to efficiently activate PDS and degrade TC (Zhou et al. 2024). Through the timing current I ∼ T response, it is found that when PDS and TC are added to the system at 100 and 200 s, respectively, very sharp current fluctuations are generated. This is due to the formation of an LLBC-PDS* metastable complex on the electrode surface at the moment of PDS addition, and the TC provides electrons for LLBC-PDS* (Figure 5(f)). This is the most direct evidence of electron transfer between the LLBC surface, PDS, and TC (Miao et al. 2022). In addition, LLBC was tested by cyclic voltammetry (CV, Figure 5(g)) and linear scanning voltammetry (LSV, Figure 5(h)). The results show that compared with the electrolyte without PDS, the electrolyte with PDS increases the CV current density, but the enhancement effect is not significant. However, in the LSV test, it was found that the electrolyte's current density in a solution that contains PDS was effectively enhanced as contrasted with the electrolyte lacking PDS, and the current density was further enhanced after the addition of TC. The above results prove that the electron transfer mechanism between LLBC, PDS, and TC is real.

The results of quenching experiment, ESR test, and electrochemical test showed that there was a variety of ROS in the LLBC/PDS system. Among them, the non-free radical ¹O₂ plays a major role in the degradation of TC, the free radical plays a secondary role, and the free radical ⁠, ·OH, and electron transfer mechanisms are also present in LLBC/PDS systems and also play a role in the oxidative elimination of TC.

Based on the above analysis and discussion of XPS and FT-IR, as well as the verification of quenching experiments, ESR, and electrochemical characterization, the elimination of TC was completed by the LLBC/PDS system under the combined action of free radicals and non-free radicals. Among them, the ROS in the free radical pathway is ⁠, ·OH, and ⁠, and the non-free radical pathways are ¹O₂ and electron transfer mechanisms, respectively.

In this study, LLBC was prepared and applied to AOPs of activated PDS for oxidative degradation of TC. The results indicate that the increase in the pyrolysis temperature of LLBC leads to a transition in the dominant carbon configuration from sp³C hybridization to sp²C hybridization in LLBC. With the increase in pyrolysis temperature, the defect structure, sp²C, and functional group structure within LLBC are all enhanced. Therefore, the oxidative elimination efficiency of TC in the LLBC/PDS system is also improved. Within the LLBC/PDS/TC, the ROS detected by capture the experiment, ESR technology, and electrochemical test included ⁠, ·OH, ⁠, ¹O₂, and electron transfer mechanisms, among which ¹O₂ is dominant in the elimination of TC. Furthermore, this work identified 29 degradation companion products of TC and proposed five potential routes of deterioration based on this. Simultaneously, using the T.E.S.T. toxicity prediction software, the toxicity of all intermediates was predicted, confirming that the toxicity of TC can be successfully decreased using the LLBC/PDS system. In conclusion, this study offers fresh perspectives on the cooperative oxidation of organic pollutants by reactive oxygen species and surface complexes, and the lotus leaf resources have also been rationally utilized.

This work was supported by the National Natural Science Foundation of China [No. 21477050]. Besides, the author would like to thank the Analysis and Testing Center of Changzhou University (atc.cczu.edu.cn/main.htm) for their help in FT-IR, XRD, ESR, and BET testing and Shiyanjia Lab (www.shiyanjia.com) for related testing works on XPS and electrochemical workstation. Thanks to eceshi (www.eceshi.com) for its help in the SEM test. Chemical computing and software services are from the High-Performance Computing Cluster System of Changzhou University (HPCCS-CCZU).

J.X.: Conceptualization, Data curation, Writing – original draft. J.S.: Conceptualization, Writing – review and editing. H.G.: Data curation, Investigation. L.W.: Conceptualization, Funding acquisition, Writing – review and editing.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.