1 Introduction

The large amounts of organic waste have caused increasing academic, public, and political concerns in recent years (Sakaguchi et al. 2018). Considering the negative environmental effects of traditional composting and landfilling disposal, anaerobic digestion has been used as a highly efficient and low-cost technology for the treatment of organic waste (Li et al. 2022). However, during anaerobic digestion, only 15–40% of organic compounds in organic waste are utilized for biogas production; the remaining liquid and solid digestate is discharged with effluent (Opatokun et al. 2015). The widespread application of anaerobic digestion is associated with increased amounts of biogas residues which require further management, restricting the popularization and application of this technology (Liu et al. 2020a, b). More public attention has been paid to the utilization of redundant undesirable biogas residues (Aleksandra et al. 2017).

The pyrolysis of biogas residue into biochar is one of the most promising and eco-friendly treatments, which is attracting increasing attention (Stefaniuk et al. 2016). The advantage of producing biochar with biogas residue is the larger amount of digestion residues that can be utilized compared with other disposal methods such as composting and agricultural fertilization (Qian et al. 2022). Microbially unavailable organic matter during anaerobic digestion is well decomposed by pyrolysis and has a high potential for biochar production (Wu et al. 2022). The pyrolysis of biogas residue to produce biochar can also recover energy in the form of biofuels and biogas at various pyrolysis stages (Stefaniuk and Oleszczuk 2015). Therefore, using biogas residue to prepare biochar reduces the environmental impact and improves the economic profitability of anaerobic digestion plants. Thus, pyrolysis has gradually become an essential technology for solid waste disposal with anaerobic digestion (Huang et al. 2022).

Similar to other biochars, biogas residue biochar (BRB), a major product in pyrolysis, has a relatively high surface area and abundant oxygen-containing functional groups, such as carboxyl and carbonyl. Therefore, BRB is also an effective supplement to traditional commercial biochar for environmental remediation. Recent studies have demonstrated that BRB can be used to adsorb Cd and Ni ions from aqueous solutions and remove Hg (II) from wastewater (Aleksandra et al. 2017; Qian et al. 2022). In addition to heavy metals, tetracycline is also efficiently adsorbed on BRB (Sheng et al. 2022). BRB integrated with phosphate from its ash is also used to recover nutrients from piggery biogas slurry (Luo et al. 2022). Meanwhile, BRB is an excellent heterogeneous persulfate activator and shows good removal performance towards tetracycline (Cui et al. 2022). As a result, the utilization of BRB has caught more public attention.

Permanganate (KMnO4) is extensively used to remove contaminants owing to its low cost, ease of use, and wide range of reactivity (Laszakovits et al. 2022). Compared with other oxidants such as ozone (O3), ferrate (FeO42−), and persulfate (S2O82−), the oxidation of KMnO4 is relatively slow. In addition, the oxidation rates of KMnO4 are highly variable because it is a selective oxidant and tends to react with organics containing electron-rich moieties (Laszakovits et al. 2022). This property limits the application of KMnO4 in the removal of organic pollutants, and many studies have focused on exploring relevant methods to improve the reactivity of KMnO4. Previous studies have confirmed that biochar can improve the ability of KMnO4 to oxidatively remove organic pollutants. Tian et al. (2019) found that the removal efficiency of sulfamethoxazole (SMX) increased by 90% using KMnO4 in the presence of biochar, which was assisted by the formation of intermediate manganese species and the in-situ activation of biochar. Another study also found that the oxidation of phenolic pollutants was effectively enhanced in the presence of biochar in KMnO4 solution; the surface area and the total pore volume of biochar increased by 18.9% and 63.1%, respectively, after the reaction with KMnO4, and this process was similar to the chemical activation process in the production of activated carbon (Tian et al. 2022). Therefore, the activation of biochar is important for the removal of contaminants in the KMnO4 system, and the combination of BRB with KMnO4 could be a promising approach to removing environmental pollutants.

Although an increase in the specific surface areas and oxygen-content functional groups of biochar increased after the reaction with KMnO4 was found in previous studies (Jin et al. 2023; Wang et al. 2015), however, there are some contradictory publications on the chemical activation of biochar; the impacts of KMnO4 on the surface area properties of biochar are different (Ghorbani et al. 2023). Other studies have found that the reaction with KMnO4 did not significantly change the specific surface areas and average pore diameters of biochar (Zhang et al. 2020). In addition, metal oxides, such as manganese oxide, formed during the reaction between biochar and KMnO4 might reduce the specific surface areas and oxygen-containing functional groups (Ghorbani et al. 2023), which do not contribute greatly to organic removal.

Normally, BRB is characterized by higher aromatization, higher mesoporosity, and more oxygen-containing functional groups (Pan et al. 2020). In addition, BRB has a higher ash content than biomass-derived biochar (Wang et al. 2022; Liu et al. 2020a, b). These characteristics would affect the application of BRB during organic pollutant removal in the KMnO4 system. However, to the best of our knowledge, research on the removal of organic pollutants by KMnO4 in the presence of BRB has not yet been carried out, and the removal rate of organic contaminants and the mechanism are unclear. In this study, the effect of BRB on KMnO4-mediated removal of organic contaminants was investigated, the removal rate of organic pollutants was determined, the dominant species were recognized, and the properties of BRB before and after the reaction were analyzed in the system. Based on these results, the mechanism of oxidation of organic contaminants using BRB with KMnO4 was elucidated, and we hope to develop a new technology for the utilization of biogas residue.

2 Materials and methods

2.1 Chemicals

The chemical list and detailed preparation of the stock solutions are provided in Additional file 1: Text S1. All solutions were prepared with deionized water produced by a purification system (Millipore, Billerica, MA, USA). BRB was prepared from biogas residue collected from Dongcun, Beijing. Briefly, the biogas residue was collected and dried at 60 °C for 24 h. The dried biogas residue was then converted into a powder using a rotary grinding machine. Pyrolysis was performed in a vertical furnace with a cylindrical quartz tube. The samples were heated to 600 °C at a heating rate of 5 °C min−1 in a nitrogen atmosphere (gas flow of 300 mL min−1) and held at the final temperature for 2 h. The obtained biochar was ground to a powder of less than 100 mesh.

2.2 Experimental procedure

A schematic diagram of BRB preparation and phenol removal in the BRB/KMnO4 system is shown in Fig. 1 (Altaf et al. 2021a, b). The experiment was conducted in triplicate in a 500-mL beaker with magnetic stirring (400 r min−1) at 25 ± 1 °C. The reactions were initiated by adding phenol and KMnO4 simultaneously to solutions containing BRB. The solution was sampled and filtered at given time intervals for further testing. The detailed experimental procedures are described in Additional file 1: Text S2.

Fig. 1
figure 1

Schematic diagram of BRB preparation and phenol removal in BRB/KMnO4 system

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2.3 Characterization

Initially, the ultimate analysis of BRB was conducted with an elemental analyzer (Elementar UNICUBE), and proximate analysis was performed according to previous literature (Altaf et al. 2021a, b), as shown in Additional file 1: Table S1. Scanning electron microscopy (SEM) images of BRB were obtained using a SU8020 SEM (HIT ACHI, Japan). A Fourier transform infrared spectrometer (FT-IR) was used to characterize the surface functional groups. X-ray diffraction (XRD) patterns of BRB before and after the reaction were measured using a Smartlab X-ray diffractometer (Rigaku, Japan) with a Cu target in the 2θ range of 10–90° (9 kW). Transmission electron microscopy (TEM) images were captured with a Tecnai F20 TEM (FEI, USA) equipped with a Bruker super-X EDS. The Brunauer−Emmett−Teller method (BET) was used to measure the average surface areas. The chemical state of each element in the samples was analyzed by X-ray photoelectron spectroscopy (XPS). The reactive radicals were detected by electron paramagnetic resonance (EPR, Elexsys E500 system Bruker) using 5,5-dimethyl-1-pyrroline-n-oxide (DMPO) and 2,2,6,6-tetramethylpiperidine (TEMP) as spin-trapping agents. More characterization techniques and detailed information are shown in Additional file 1: Text S3.

2.4 Analytical methods

The concentration of phenol was determined by high-performance liquid chromatography (HPLC) (Agilent 6850, USA) equipped with a UV detector at a wavelength of 270 nm with a flow rate of 1.0 mL min−1. The mobile phase was composed of methanol and pure water (60:40, v/v). Separation was accomplished with an Atlantic C18 column (4.6 nm × 150 nm, 3 μm; Waters). Electrochemical experiments were performed according to previous studies (Liu et al. 2022; Liu et al. 2021a, b; Wu et al. 2017). The detailed experimental procedures are described in Additional file 1: Text S4. The TOC content of the solution samples was determined by TOC-V CHS (Kyoto, Shimadzu, Japan).

3 Results and discussion

3.1 Enhanced removal of phenol in KMnO4/BRB System

As shown in Fig. 2a, phenol was poorly adsorbed on BRB, with only less than 3% being removed. KMnO4 alone oxidized 25.3% of phenol in 60 min. However, when KMnO4 reacted with phenol in the presence of BRB, approximately 73.4% of phenol was removed after 60 min (Fig. 2a). The co-presence of KMnO4 and BRB significantly enhanced phenol removal. This result is in accordance with previous studies, which showed that biochar significantly enhanced the KMnO4 oxidation of sulfamethoxazole and phenolic pollutants (Tian et al. 2019, 2022). Besides, 45.2% of TOC was removed by the KMnO4/BRB system whereas only 2.1% and 11.2% were removed by the BRB adsorption and KMnO4 oxidation systems, respectively (Additional file 1: Fig. S1). According to previous studies, this might be attributed to the following possible reasons: (1) the adsorption capacity of biochar improved during the reaction process, and more phenol was adsorbed by the used biochar (Tian et al. 2019); (2) owing to the presence of biochar, the oxidation capacity of the KMnO4 system was improved, resulting in more phenol oxidation (Tian et al. 2022); (3) reactive oxygen species (ROS) might participate by oxidizing more phenol (Xu et al. 2017). Besides, other biochars derived from straw and sludge were also used to remove phenol with KMnO4, and the removal rates were 63.4% and 27.2%, respectively (Additional file 1: Fig. S2), which were lower than that with BRB. This suggests that the ability to remove organic pollutants was higher in the BRB system than in other biochars in the presence of KMnO4. This might be due to the different reaction mechanisms between biochar and KMnO4, which led to various removal rates. Therefore, it is urgent to investigate why BRB enhanced the KMnO4 oxidation of phenol.

Fig. 2
figure 2

Phenol content in the system with KMnO4, BRB and KMnO4/BRB (a); KMnO4 consumption in various systems (b). Experimental condition: [phenol]0 = 10 μM, [KMnO4]0 = 100 μM, [BRB] = 100 mg L−1

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To identify the adsorption capacity of BRB, phenol adsorbed on reacted BRB was investigated (Additional file 1: Text S2). The results showed that there was hardly any phenol adsorbed on the reacted BRB, suggesting that phenol was eliminated by relying on oxidative degradation. Biochar contained organic functional groups and could interact with KMnO4, which led to KMnO4 depletion and affected the oxidation capacity. The variation in KMnO4 content was investigated in this study (Fig. 2b). Approximately 14% of KMnO4 was depleted in a solution of KMnO4-containing phenol. However, more than 20% of KMnO4 was decreased in KMnO4/BRB system. This indicates that the reaction rate of KMnO4 with BRB was faster than that with phenol. That is, BRB was more susceptible to reaction with KMnO4 when phenol and BRB coexisted. However, when phenol was added to KMnO4/BRB system, approximately 50% of KMnO4 was consumed, and the depletion rate was much faster. This suggests that the oxidation capacity of the KMnO4 system was improved in the presence of BRB. Thus, further work should be conducted to explore the mechanism of enhancing oxidation capacity and the main mechanism for the removal of phenol in the KMnO4/BRB system.

3.2 Reactive oxygen species generation in KMnO4/BRB system

Previous studies have shown that ROS such as superoxide radical (O2•−), hydroxyl radical (•OH), and singlet oxygen (1O2) might be involved in the oxidation process (Zeng et al. 2022; Han et al. 2022; Yao et al. 2022). Tert-butanol (TBA) was found to be the scavenger for •OH and was used to reveal the role of •OH in phenol removal in the KMnO4/BRB system (Luo et al. 2019). As shown in Fig. 3a, the presence of TBA did not inhibit but rather slightly accelerated phenol degradation. These results imply that •OH was not the oxidative species responsible for the accelerated removal of phenol in this system. TBA might have accelerated phenol removal because it could react with KMnO4, generating intermediate Mn species with high oxidation capacity, such as Mn(VI) and Mn(V), which removed phenol (Ghosh et al. 2014; Bhattacharyya et al. 2015). Similarly, p-benzoquinone (BQ) was commonly used as the scavenger for O2•− and showed no apparent inhibition effect on phenol removal (Fig. 3b), which suggests that O2•− did not play a significant role in the removal of phenol in this system (Wang et al. 2021; Ge et al. 2022). In addition, the effect of 1O2 was investigated by adding furfuryl alcohol (FFA) as the quenching reagent (Peng et al. 2021a, b). Only about 30% of phenol was eliminated in 60 min in the presence of FFA (1000 μM) (Fig. 3c). The apparent inhibition of phenol removal by FFA suggests that 1O2 might be generated. Therefore, EPR analysis was used to identify the generation of 1O2 (Ji et al. 2021). EPR results showed that a weak signal was detected in the KMnO4/BRB system (Fig. 3d), which suggested that 1O2 also did not participate in the accelerated removal of phenol. This result is similar to that of a previous study, which found that FFA almost completely inhibited the removal of phenol, but no adduct of 1O2 was detected (Ren et al. 2019). This phenomenon could be explained by the oxidation potentials. The half-wave potentials of FFA were detected between + 0.176 V and + 0.442 V at different concentrations (0–400 mM), which were lower than that of phenol (around + 0.57 V), suggesting that FFA was more vulnerable to oxidation than phenol in the BRB/KMnO4 system. This might explain why FFA profoundly inhibited phenol oxidation without the generation of 1O2.

Fig. 3
figure 3

Effect of TBA (a), BQ (b), FFA (c) on the removal of phenol in BRB/KMnO4 system and EPR spectra of BRB/KMnO4 (d). Experimental condition: [phenol]0 = 10 μM, [KMnO4]0 = 100 μM, [BRB] = 100 mg L−1

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3.3 Intermediate Mn species in KMnO4/BRB system

A previous study confirmed that BRB reacted with KMnO4, generating reactive intermediate Mn species during the process (Jiang et al. 2009). Generally, the intermediate Mn species formed were Mn(III), Mn(V), and Mn(VI) during the reaction of KMnO4, which had strong oxidation capacity for organic contaminants but had low stability (Sun et al. 2016; Tian et al. 2022).

Commonly, Mn(III) is reported to participate in the removal of organic contaminants (Hu et al. 2017). Mn(III) was highly unstable and would quickly disproportionate to Mn(II) and MnO2 (Eq. 1) (Zhong and Zhang 2020). In addition, it is also complex with biochar (Tian et al. 2019). The presence of a complexing ligand, such as pyrophosphate (PP), could stabilize Mn(III) to form Mn(III)-PP, preventing its spontaneous disproportionation and enhancing its stability (Sun et al. 2015). Mn(III)-PP is a potent oxidant and can oxidize phenolic compounds to produce Mn(II) much faster than MnO4− (Gao et al. 2018). To determine whether intermediate Mn(III) was engaged in this study, PP was the ligand used to investigate the role of Mn(III). When PP was added to KMnO4/BRB system, the removal efficiency of phenol was 89% (Fig. 4a), which was a little higher compared with the system without PP (73.4%). To confirm this, more studies were carried out. Previous literature has revealed that Mn(III)-complex would show a characteristic absorbance peak at 258 nm under UV irradiation (Xu et al. 2018). Figure 4b shows that no marked characteristic peak was observed at 258 nm during the phenol removal process in the presence of PP in the KMnO4/BRB system. This can be explained as follows. When PP was added, it complexed with Mn(III), which formed in situ in the KMnO4/BRB system to form Mn(III)-PP (Eq. 2). Mn(III)-PP then contributed to the oxidation of phenol to form Mn(II) (Eq. 3). Therefore, the phenol removal efficiency was promoted in the presence of PP. Because of the consumption of Mn(III)-PP, no marked characteristic peak was observed under UV irradiation.

$${\text{Mn}}({\text{III}}) \to {\text{MnO}}_{2} + {\text{Mn}}({\text{II}})$$
(1)
$${\text{Mn}}({\text{III}}) + {\text{PP}} \to {\text{Mn}}({\text{III}}){\text{ - PP}}$$
(2)
$${\text{Mn}}({\text{III}}){\text{ - PP}} + {\text{Phenol}} \to {\text{Mn}}({\text{II}}) + {\text{Products}}$$
(3)
Fig. 4
figure 4

Phenol content (a) and variation of UV − vis spectra (b) during the reactions in KMnO4/BRB at the presence of PP. Experimental condition: [phenol]0 = 10 μM, [KMnO4]0 = 100 μM, [BRB] = 100 mg L−1, [PP] = 500 μM

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This result suggests that Mn(III) might be generated during phenol removal in the KMnO4/BRB system. Due to its instability, some Mn(III) disproportionated to Mn(II) and MnO2, while others complexed with BRB, which could also be verified by XPS analysis.

To explore the Mn intermediates of Mn(V) and Mn(VI), methyl phenyl sulfoxide (PMSO), which can be oxidized by Mn(VI) and Mn(V) with the formation of corresponding sulfone product (methyl phenyl sulfone; PMSO2), was used as a probe compound (Gao et al. 2019). As shown in Additional file 1: Fig. S3, the removal efficiency of PMSO was almost the same in both the KMnO4 and KMnO4/BRB systems. Meanwhile, the generation efficiency of PMSO2 was approximately 100% in both systems, demonstrating that PMSO was mainly transformed to PMSO2 in these two systems. The results suggested that Mn intermediate species of Mn(VI)/Mn(V) were generated in the KMnO4/BRB system, but they did not account for the accelerated removal efficiency of phenol, as BRB did not contribute to the formation of Mn(VI)/Mn(V).

Previous studies have found that the MnO2 formed in situ could participate in the elimination of organic contaminants (Jiang et al. 2015). Therefore, the generation of MnO2 and its effect on phenol removal were explored. According to the literature, nonspecific absorbance at < 500 nm corresponds to MnO2, whereas KMnO4 shows a characteristic absorbance peak at 525 nm using UV spectroscopy (Jiang et al. 2010). The absorbance of KMnO4 and MnO2 increased with their concentrations (Additional file 1: Fig. S4), whereas the presence of BRB did not affect their absorbance (Additional file 1: Fig. S5). As shown in Fig. 5a, the full scan spectra of the KMnO4/BRB suspension were detected during the phenol elimination process. The absorbance at 300–400 nm increased while the absorbance peak at 525 nm decreased, which suggested that the presence of BRB leads to the consumption of KMnO4 and the formation of MnO2.

Fig. 5
figure 5

Variation of UV − vis spectra during the reactions in KMnO4/BRB/phenol a suspension b filtration. Reaction conditions: [phenol]0 = 10 μM, [KMnO4]0 = 100 μM, [BRB] = 100 mg L−1

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Studies have found that MnO2 can form complexes with biochar and play an important role in contaminant oxidation (Liu et al. 2021a, b). To explore whether BRB and the formed MnO2 were combined during phenol removal in the KMnO4/BRB system, full-scan spectra of the filtration were obtained under UV irradiation. Interestingly, MnO2 decreased in filtration during the reaction (Fig. 5b), which was the opposite of that in the suspension. This phenomenon might be ascribed to the fact that MnO2 formed in the system was filtered or combined with BRB (MnO2@BRB). To confirm this, filtration during phenol removal by the KMnO4 system was detected in the full scan UV spectra (Additional file 1: Fig. S6). It showed that the absorbance at 300–400 nm increased, which confirmed that MnO2 formed in situ could not be filtered. However, when BRB was added to the KMnO4 system alone, the absorbance at 300–400 nm decreased (Fig. S7), which indicates that the BRB could form a complex with the MnO2. From the above results, it was suggested that MnO2 was formed in the KMnO4/BRB system.

Since MnO2@BRB was formed during phenol removal in the KMnO4/BRB system, and previous studies have found that MnO2 could participate in the elimination of contaminants, the role of MnO2@BRB in enhancing the removal of phenol needs to be clarified. The resulting MnO2@BRB and MnO2 were prepared as previously reported (Peng et al. 2021a, b) and were used for further investigation. When the prepared MnO2@BRB was added alone, the phenol removal rate was less than 5%, suggesting that MnO2@BRB alone could not remove phenol (Fig. 6a). And when the prepared MnO2 (100 μM) was added alone, the removal efficiency of phenol reached 50.4% (Fig. 6a), and the results were the same as those of previous studies, i.e., MnO2 could oxidize organics (Jiang et al. 2015). Besides, when BRB was added to the system containing the prepared MnO2, the removal efficiency of phenol was 28.9%, which was lower than that when MnO2 alone was added (Fig. 6a). These results indicate that the presence of BRB reduced the oxidative ability of MnO2. A possible reason might be that when MnO2 complexed with BRB, the adsorption site of MnO2 was occupied, which inhibited the formation of the precursor complex between MnO2 and phenol, as the formation of the precursor was important for the MnO2 oxidation of pollutants (Alan 1987; Zhang and Huang 2005).

Fig. 6
figure 6

Phenol content in the reaction MnO2, MnO2@BRB and MnO2/BRB (a), phenol content in the reaction MnO2@BRB/KMnO4 (b), effects of MnO2@BRB dosage on the depletion of KMnO4 (c), effects of MnO2@BRB stewing time on phenol content (d). Experimental condition: [phenol]0 = 10 μM, [KMnO4]0 = 100 μM, [MnO2@BRB]0 = 100 μM, [BRB] = 100 mg L−1

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However, when KMnO4 was added to the system containing MnO2@BRB (100 μM), the efficiency of phenol elimination reached 86.1%, which was higher than that of KMnO4 and MnO2@BRB oxidation systems alone (Fig. 6b), as well as the sum of the elimination efficiencies when KMnO4 and MnO2@BRB were oxidized alone. In addition, the variation in KMnO4 content under different dosages of MnO2@BRB in the presence of phenol was examined (Fig. 6c). Approximately 12.7% of KMnO4 was depleted within 60 min when the dosage of MnO2@BRB was 25 μM. The depletion increased when the dosage of MnO2@BRB increased, reaching 29.3% when the dosage of MnO2@BRB was 100 μM. The results suggested that MnO2@BRB formed during the reaction in the KMnO4/BRB system could catalyze KMnO4 to remove phenol. This might account for the enhanced removal of phenol in this system. To explore the stability of the formed MnO2@BRB, an experiment was carried out after the prepared MnO2@BRB was stewed for a long time (Fig. 6d). The results showed that phenol elimination efficiency in KMnO4/MnO2@BRB system was not affected by stewing time and phenol elimination efficiency reached 81.7% after 30 days. This suggests that the formed MnO2@BRB was stable.

3.4 Characteristics of BRB

Previous results suggested that BRB was more reactive to KMnO4 when phenol and BRB coexisted, and the MnO2@BRB formed during the reaction in the KMnO4/BRB system accounted for the enhanced removal of phenol. However, the role of BRB and its characteristics remain unclear. Commonly, biochar could act as a reductant that enhances the removal of organics in the KMnO4/biochar system (Tian et al. 2019). To confirm this, BRB was pre-ozonized and added to the system containing KMnO4 and phenol (Additional file 1: Text S2). Approximately 49.9% of phenol was eliminated (Additional file 1: Fig. S8), which was lower than that in the pristine BRB system. These results suggest that BRB might act as a reductant, and the reduction groups on BRB might contribute to the enhanced removal of phenol in the KMnO4/BRB system. Therefore, the surface morphologies and elemental abundances of the BRB samples before and after the reaction were determined.

FTIR spectra were used to examine the functional groups on BRB (Fig. 7a). Both raw and used BRB showed peaks at 3421, 1623, 1418, and 1037 cm−1, which were assigned to the − OH (of H2O), C=O, C–H, and C−O groups, respectively. The intensities of the C−O and C=O bonds in the used BRB were higher than those in the raw BRB, suggesting that the BRB was oxidized during the reaction in the KMnO4/BRB system. In addition, a new peak at approximately 520 cm−1 was observed for the used BRB, which was associated with the formation of the Mn−O bond (Tian et al. 2019). The results were the same as those of the previous analysis, suggesting that the Mn species complexed with BRB after the reaction. The crystal structure of the BRB samples was determined by XRD. As shown in Fig. 7b, the peak at 26.5° corresponding to the (002) crystal plane of graphite was observed in the BRB before and after the reaction, which is consistent with most pyrolytic carbons (Li et al. 2021a, b). In the XRD pattern of BRB after the reaction, the peak at 2θ = 23.8° was identified, corresponding to the (210) plane of the γ-MnO2 crystalline structure (ICSD #150462) (Hill and Verbaere 2004). This result further suggests the generation of MnO2 and the formation of MnO2@BRB.

Fig. 7
figure 7

FT − IR (a) result and XRD patterns (b) of raw BRB and used BRB

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SEM and TEM analyses were performed to characterize the morphology and elemental distribution on the surface of the raw and used BRB. As shown in Fig. 8a, the surface morphology of the raw BRB was comparatively rough and composed of irregular particles. The surface of the used BRB was relatively smooth and composed of regular particles (Fig. 8b). The morphology of BRB after the reaction was further investigated using HRTEM (Additional file 1: Fig. S9). Clear lattice fringes of 0.37 nm were observed, which were attributed to the (210) plane of γ-MnO2 (Fig. 8c). More significantly, the EDS mapping showed that the distribution of Mn and O elements was uniform on the BRB, which confirmed that γ-MnO2 was highly dispersed on the BRB after the reaction (Fig. 8d–f).

Fig. 8
figure 8

SEM images of raw BRB (a) and used BRB (b), HRTEM (c) and HAADF-STEM (d) images of used BRB, and (e) mapping for Mn (e) and O (f) of used BRB

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The relative surface area and pore volume of the pristine and used BRB were determined by BET analysis and are shown in Table 1. Compared with the pristine BRB, BET surface area and Langmuir surface area of the used BRB increased from 13.89 and 27.94 m2 g−1 to 32.26 and 68.85 m2 g−1, which increased by 132.25% and 146.42%, respectively. Besides, the micropore area of used BRB increased from 3.27 to 3.65 m2 g−1, and mesoporous increased from 11.08 to 27.40 m2 g−1, which increased by 11.92% and 147.31%, respectively. These results suggest that the increase in the specific surface area of BRB after the reaction is mainly due to the increase in the mesoporous specific surface area. While the total pore volumes of the pristine and used BRB were 0.043 m3 g−1 and 0.077 m3 g−1, respectively, the mesoporous volume of the used BRB increased from 0.040 to 0.073 m3 g−1. This result implies that BRB was activated with the formation of pores, which might provide conditions for the adhesion of Mn in the reaction of BRB with KMnO4.

Table 1 Surface area and pore volume of the raw BRB and used BRB
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XPS was further used to analyze the raw and used BRB (Altaf et al. 2022). Figure 9 shows the full scan spectra and high-resolution spectra of C1s, O1s, and Mn 2p of BRB. As presented in the XPS spectra (Fig. 9a), C (284.6 eV) and O (531.8 eV) were detected in the pristine and the used BRB. Mn was detected in the used BRB, which further demonstrated that Mn species were complexed with BRB after the reaction. The high-resolution spectra of C1s for the pristine BRB could be deconvoluted into five signals, which were attributed to C graphite (284.4 eV), C−C (284.9 eV), C−O (285.7 eV), C=O (286.8 eV), and π–π* shake-up (289.7 eV) (Fig. 9b). The C1s high-resolution spectra of the used BRB could also be deconvoluted into five signals attributed to C graphite (284.4 eV), C−C (284.9 eV), C−O (285.7 eV), C=O (286.8 eV), and C=OOH (288.5 eV) (Tian et al. 2019). Compared to the pristine BRB, the relative content of oxygen-containing groups (C−O, C=O, and COOH) in used BRB increased from 10.13% to 10.88%, 1.53% to 4.32%, and 0 to 5.72%, respectively. This phenomenon further confirmed that BRB was oxidized during the reaction and was in agreement with the FT−IR analysis results. The high-resolution spectra of O1s are shown in Fig. 9c. The spectra of O1s for the raw BRB could be fitted into 531.1, 531.8 and 532.7 eV which represent the vibration of C=O, C–O, and O2/H2O, respectively (Luo et al. 2022). After the reaction in the system with KMnO4, three new peaks of binding energies 529.4, 530.1, and 530.8 eV were ascribed to the oxygenated groups of Mn–O, Mn–O–H, and Mn–O–Mn (Zhu et al. 2020; Joemer et al. 2020). This suggests that Mn was loaded in the BRB. Besides, obvious peaks at 642 eV for Mn (2p 3/2) and 654 eV for Mn (2p 1/2) were observed in the full scan spectra of BRB after the reaction (Fig. 9d), and the spectra of Mn 2p 3/2 and 2p 1/2 could be fitted with three peaks, corresponding to Mn(II) (641.38 eV, 652.73 eV), Mn(III) (642.4 eV, 653.4 eV), and Mn(IV) (643.49 eV, 654.38 eV), with proportions of 50.47%, 27.29%, and 22.24%, respectively. This result revealed that Mn species were generated and loaded on the surface of the oxidized BRB during the reaction process. Besides, the significant change in the peak around 346 eV was observed before and after the reaction (Fig. 9a). The peak was attributed to Ca 2p (Li et al. 2021a, b), which commonly existed in the ash of biochar. The reaction of biochar with KMnO4 would consume the ash content and lead to the decrease of Ca (Shang et al. 2020), resulting in the significant change in the peak of around 346 eV after reaction.

Fig. 9
figure 9

XPS spectra of raw and used BRB for full scan spectra (a), high resolution spectra of C1s (b) and O 1 s (c); high resolution spectra of Mn2p of used BRB (d)

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Additionally, previous studies reported that the electron transfer ability of biochar might participate in the removal of contaminants (Liu et al. 2020a, b), and the electrochemical property of BRB was conducted by examining the electron-donating capacity (EDC) and electron-accepting capacity (EAC) to investigate the electron transfer role of BRB in this system. As shown in Additional file 1: Fig. S10, the EDC value decreased from 0.25 to 0.13 mmol e·(g BRB)−1, suggesting that the redox reaction between BRB and KMnO4 occurred. And the EAC value increased from 0.11 to 0.16 mmol e·(g BRB)−1 after the reaction, suggesting that the electron-accepting capacity of BRB increased during the reaction, which might play an important role in the electron transfer during phenol removal in the system. The EDC capacity was related to phenolic hydroxyl groups, and the EAC capacity was ascribed to the C=O groups on BRB (Liu et al. 2021a, b; Wu et al. 2017). During the phenol removal process in the BRB/KMnO4 system, the oxidation of BRB by KMnO4 led to an increase in the electron-accepting C = O group, whereas the electron-donating phenolic hydroxyl group decreased. This was confirmed by the FTIR and XPS results.

3.5 Reaction mechanism

Based on the above discussion, the reaction mechanism of enhanced phenol removal by KMnO4 in the presence of BRB is illustrated in Fig. 10. The removal efficiency of phenol by KMnO4 alone was limited. When BRB was added to the KMnO4 system containing phenol, due to the reductive groups on the surface, BRB first reacted with KMnO4 (Eq. 4). During this process, BRB was oxidized and the number of oxygen-containing functional groups (C−O, C=O, and COOH) increased, whereas KMnO4 was reduced, and intermediate Mn species (Mn (III), MnO2) were produced. Mn(III) was unstable and a part of Mn(III) disproportionated to Mn(II) and MnO2 (Eq. 1). MnO2 was stable and complexed with BRB, leading to the formation of MnO2@BRB (Eq. 5). The newly formed MnO2@BRB was stable and could catalyze KMnO4 to oxidize phenol (Eq. 6). In addition, the increased number of oxygen-containing functional groups on BRB led to an increase in the electron transfer capacity, which might play an important role in phenol removal. Thus, the oxidation capacity of KMnO4 was promoted in the presence of BRB. During the process, the increased oxygen-containing groups provided sites to complex with Mn(III) and Mn(II) (Eqs. 7–8).

$${\text{5BRB}} + 2{\text{MnO}}_{4}^{ - } + 2{\text{H}}^{ + } \to {\text{5BRB - O + MnO}}_{{2}} + {\text{Mn}}({\text{III}}) + {\text{H}}_{2} {\text{O}}$$
(4)
$${\text{MnO}}_{{2}} + {\text{BRB - O}} \to {\text{MnO}}_{{2}} {\text{@BRB}}$$
(5)
$${\text{MnO}}_{4}^{ - } + {\text{Phenol}}\mathop{\longrightarrow}\limits^{{{\text{MnO}}_{2} {\text{@BRB}}}} {\text{Mn(II)}} + {\text{products}} + {\text{CO}}_{{2}} + {\text{H}}_{{2}} {\text{O}}$$
(6)
$${\text{Mn(III)}} + {\text{BRB - O}} \to {\text{BRB - O - Mn(III)}}$$
(7)
$${\text{Mn(II)}} + {\text{BRB - O}} \to {\text{BRB - O - Mn(II)}}$$
(8)
Fig. 10
figure 10

Proposed reaction mechanism of phenol removal by KMnO4/BRB

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3.6 Influencing factors in KMnO4/BRB system

To examine the factors contributing to contaminant removal in the KMnO4/BRB system, the effects of the BRB dosage, KMnO4 content, reused BRB, solution source, and metal ions on the removal of phenol were investigated. The removal efficiency of phenol increased from 53.7% to 95.7% when the dosage of BRB varied from 25 to 200 mg L-1 with 100 μM of KMnO4 (Fig. 11a). Similarly, when BRB dosage was fixed at 100 mg L-1, the removal percentage of phenol increased from 52.3% to 97.1% as the dosage of KMnO4 varied from 25 to 200 μM (Fig. 11b). Increasing the dosage of both BRB and KMnO4 facilitated the removal of phenol in the system. When the used BRB was collected and re-added to the system, the removal rate of phenol decreased with an increase in the number of used BRB cycles (Fig. 11c). The removal efficiency was 55.6% after the fourth cycle, which was still higher than that of KMnO4 oxidation alone. In addition, the removal rate of phenol from different solution sources was investigated by conducting experiments in authentic groundwater from a well and surface water from a pond (both collected in ShunYi District). The results showed that the removal efficiencies of phenol were 76.9% and 77.7%, respectively (Fig. 11d), which were greater than those conducted in deionized water (removal efficiency of phenol was 73.4%). Previous studies found that excessive background constituents such as natural organic matter reacting with KMnO4 to produce more intermediate Mn might explain this phenomenon (He et al. 2009). Metal ions in the environment containing biochar might also affect the removal of organic pollutants (Fang et al. 2015). The removal of phenol in the presence of Al, Fe, Mg, and Cu in the KMnO4/BRB system was investigated separately (Fig. 11e). Except for Al, the coexistence of metal ions had no obvious effect on phenol removal. The enhanced removal of phenol in the presence of Al3+ might be because Al3+ increased the adsorption of phenol in the newly formed MnO2 and hydrated aluminum hydroxide derived from the hydrolysis of Al3+(Guan et al. 2006; Yao and Millero 1996). Furthermore, in addition to phenol, the removal efficiency of other organic pollutants such as chlorophenol, sulfamethoxazole (SMX), and carbamazepine (CMZ) was also investigated in the KMnO4/BRB system (Fig. 11f). The results showed that CMZ and SMX were almost completely removed in 30 min and 40 min, respectively. The removal efficiencies of 4-chlorophenol and 2,4-dichlorophenol reached more than 90% within 60 min. The pollutant removal efficiencies were all higher than that of phenol, suggesting that the KMnO4/BRB system could also be used for the removal of other organic pollutants. Although KMnO4 could be used to remove contaminants, it should be noted that KMnO4 is flammable and explosive, and potential risks might exist during transportation and use. Although compared to other chemical oxidants such as ozone, ferrate, and persulfate, KMnO4 was easier and safer to store and deliver, it was suggested to be stored in a cool and ventilated warehouse and kept away from heat and flame when used to remove organic contaminants.

Fig. 11
figure 11

Effect of BRB dosage (a) and KMnO4 concentration (b) on phenol removal; performance of reused BRB on phenol removal (c); removal of phenol in various solution source (d); removal of phenol in the presence of metal ion (e); removal of various contaminants (f) in KMnO4/BRB system. Experimental condition: [KMnO4]0 = 100 μM, [phenol]0 = 10 μM, [contaminants]0 = 10 μM, [BRB]0 = 100 mg L−1

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3.7 Limitation and future research

Although the results suggested that BRB significantly promoted the organic contaminant removal rate in the presence of KMnO4, the BRB used in the present work was prepared under certain feedstocks and pyrolysis temperatures. The physicochemical characteristics of the BRB derived from various pyrolysis parameters and biogas residues, and their effect on organic contaminant removal were still not clear, which was the limitation of the present work. In the future, more studies need to be conducted under various BRBs prepared from different pyrolysis conditions and feedstocks, and the oxygen functional groups and surface morphology of various BRB should be characterized to investigate the mechanism of organic pollutant removal in the KMnO4/BRB system.

4 Conclusion

This study investigated phenol removal by KMnO4 in the presence of BRB, and the reaction mechanism was illustrated. The results showed that 73.4% of phenol was removed by KMnO4/BRB within 60 min. ROS was not involved in the accelerated removal of phenol in this study. Intermediate Mn species of Mn(III) and Mn(VI)/Mn(V) were formed in the system, but they did not account for the enhanced removal of phenol. Stable MnO2@BRB was formed and catalyzed KMnO4 to remove phenol, which played a critical role in the accelerated removal of organic pollutants in the KMnO4/BRB system. During the process, BRB was oxidized and activated in situ with the formation of pores, and oxygen-containing functional groups increased, which provided sites to complex with Mn(III) and Mn(II). The removal efficiency of phenol by the KMnO4/BRB system was less affected by the co-existing metal ions and natural environment, and a significant removal rate was observed for other pollutants like antibiotics and chlorophenols, suggesting that the KMnO4/BRB system could be used for the removal of other organic pollutants. Overall, this work proposed new methods for the resource utilization of biogas residue, and KMnO4/BRB oxidation technology is a potential alternative to reduce water pollution contaminated by phenolic compounds.