1 Introduction

Pyrogenic carbon is a series of carbonaceous residue derived from incomplete combustion of biomass and fossil fuel, which represents one of the largest organic carbon pools on the earth and plays a critical role in the global carbon cycle and transformation of contaminants (Chen et al. 2022). Pyrogenic carbon can be viewed as a heterogeneous mixture of thermally altered biomacromolecules, the configuration of which is highly dependent on the chemistry of original precursor (Knicker et al. 2008). Nitrogen (N) is an essential element for plant growth and present mainly in the form of peptides in biomass. During biomass burning, a majority of the organic N is rearranged and incorporated into the polyaromatic carbon matrix in the form of heterocyclic aromatic N, i.e., black N (BN) (Knicker 2010). Not only composition and structure but also (photo)electrochemistry of pyrogenic carbon could be modified by the incorporated heteroaromatic N (Liu et al. 2019; Wu et al. 2021). Nevertheless, the heterocyclic-N induced modification to the role of pyrogenic carbon in critical biogeochemical processes is much less explored.

Characterized by a heteroatomic nature and lower graphitic carbon content, BN is to an extent more active in chemical oxidation than black carbon (de la Rosa and Knicker 2011). In a recent study we found that dissolved BN (DBN), the water-soluble fraction of BN (size < 0.45 μm), was more easily photo-transformed than dissolved black carbon (DBC) and the half-life of DBN produced at 500 °C was two times shorter than that of the DBC counterpart, mainly ascribed to the generation of not only reactive oxygen species (ROS) but also reactive nitrogen species (RNS) in DBN under simulated solar irradiation (Lian et al. 2021). However, the intensity of sunlight decreases rapidly with depth of water for many aquatic ecosystems (Zepp and Cline 1977; Häder et al. 2015). The catalytic transformation of microcontaminants by DBN in the dark also needs scrutiny and has more profound implications in the deep water and sediment. Hence, an interesting question confronting us is how and to what extent DBN induces the catalytic degradation of organic contaminants in subsurface waters, where quite a few natural reductants and electron shuttles prevail.

Sulfides such as H2S, HS−, and S2− are mainly derived from the biological sulfate reduction and viewed as one of the most widely distributed nucleophiles and reductants in subsurface waters (Barton et al. 2014; Ding et al. 2022). Pyrogenic carbon has been reported to effectively mediate the transformation of certain sorbed organic pollutants (e.g., nitrobenzene and azo dyes) by sulfides (Weber and Quicker 2018; Ren et al. 2019). The proposed mechanisms responsible for this pyrogenic carbon-mediated transformation mainly include: (i) the oxygen-containing groups (especially quinone moieties) in pyrogenic carbon accelerate the electron transfer from sulfides to target contaminants (Kemper et al. 2008; Yu et al. 2011); (ii) the conductive graphitic regions in pyrogenic carbon promote electron transfer from sulfides to sorbed organic compounds (Ding and Xu 2016; Oh et al. 2005); and (iii) sulfides are oxidized on carbon surfaces to reactive sulfur species that serve as potent nucleophiles promoting the transformation of contaminants (Xu et al. 2013; Ding et al. 2018). Yet, little is known regarding the possible synergies between heterocyclic aromatic N and sorbed sulfides on the transformation of microcontaminants. A recent study found that high pyrolytic temperature resulted in large specific surface area, more pores, and thus high catalytic capacity of N-doped biochar for the dichlorination of carbon tetrachloride in sulfide-containing aqueous solutions (Ding et al. 2022). Relative to N-doped biochar, however, DBN has typical colloidal properties including less porous, high dispersible, and more surface O/N-containing groups (Lian et al. 2021). Meanwhile, the formation and temperature-dependent decomposition of N species endow the graphene sheets in DBN with a defect-rich feature (Yang et al. 2016; Wan et al. 2023), favorable to the generation of delocalized/unpaired electrons and persistent free radicals (PFR) (Wan et al. 2021). The synergistic effect of colloidal properties and heterocyclic N might provide DBN with a distinct reaction pathway in catalyzing dichlorination by sulfides, which has received little attention in previous studies. Given the ubiquitous presence of DBN in dissolved organic matter (DOM) (Jaffe et al. 2012; Maie et al. 2006), unraveling the reactivity of DBN is crucial to expanding our understanding in the transformation and fate of microcontaminants in aquatic ecosystems.

Herein, we extracted DBNs from N-rich pyrogenic carbon prepared at different charring temperatures. Chlorophenols including 2-chlorophenol (2-CP), 3-chlorophenol (3-CP), and 4-chlorophenol (4-CP) were selected as the targeted organic compounds because they are typical microcontaminants and widely detected in the environmental medium (Yang et al. 2021). The objectives of the present work were to (i) explore the differences between DBN and DBC in the catalyzing transformation of chlorophenols by sulfides and (ii) reveal the unique dichlorination mechanisms in the DBN-sulfide catalytic system on the basis of degradation products, electrochemical analysis, and electron paramagnetic resonance (EPR)/free radical quenching results. This study also compared the efficiency between DBC and DBN in the degradation of chlorophenols. Our findings provide mechanistic insights into the crucial role of heteroaromatic N for dissolved pyrogenic carbon in regulating the transformation of organic contaminants in water and soil systems.

2 Materials and methods

2.1 Preparation of BN

BN samples were prepared through slow pyrolysis of N-rich green algae biomass (oven-dried at 60 °C before use) at different charring temperatures (350, 450, 550, 650, or 750 °C) for 3 h in a lab-scale tubular furnace under oxygen-limited conditions (Lian et al. 2021). The obtained solids were ground to pass through a 0.125 mm mesh and labeled as BBN350–BBN750, respectively, where the numbers indicated the temperature. For comparison, BC was produced from wood shavings as a control with the same pyrolytic procedure but only at 450 °C, labelled as BC450. These pyrogenic carbons were exposed to air for 2 months to complete chemisorption of oxygen.

2.2 Extraction of DBN

An aliquot (50 g) of BN was added into 500 mL deionized water and stirred magnetically for 72 h in the dark at 25 °C, followed by 1.0 h sonication at 100 W (Fu et al. 2016). The stirred suspension was passed through a glass fiber membrane (0.7 μm). The retained BN on the membrane was stirred, sonicated, and filtrated three times to ensure that most of the desired fractions could pass through the membrane. The filtrate was collected and freeze-dried to obtain DBN samples. DBN obtained at different temperatures was labeled as DBN350–DBN750, respectively. DBC450 was also extracted with the same procedure. The stock suspensions (100mg L–1) of DBN and DBC450 were prepared in deionized water and stored at 4 °C in the dark before use. The total organic carbon (TOC) of DBN samples was measured by a TOC analyzer (Elementar, Germany), which was in the range of 24.2 to 40.1 mg TOC/L with various charring temperatures.

2.3 Characterization of BN, DBN, and DBC

The bulk elemental composition of BN and DBN was measured with an Elemental Analyzer (Unicube, Elementar, Germany), and their trace elements were determined with an inductively coupled plasma mass spectrometer (ICP-MS) (iCAP-TQ, Thermo-Fisher, USA) after microwave-assisted nitric acid digestion. The surface chemical composition of DBN was determined by X-ray photoelectron spectrometer (XPS) (ESCALAB250Xi, Thermo-Fisher, USA). The N1s spectra were fitted with pyridine-N, pyrrolic-N, and/or pyridone-N, graphitic-N, and oxidized-N peaks, where the positions were around 398.1–398.4 eV, 399.1–400.5 eV, 400.8–401.3 eV, 402.8 eV, respectively (Lian et al. 2016). The functional groups of DBN were characterized by Fourier transform infrared (FTIR) spectrometer (Nexus 870, Nicolet, USA) with 4 cm−1 resolution in the range of 400 and 4000 cm−1. The graphitization tendency was determined by Raman spectrometer (RM 2000, Wotton-under-Edge, UK). Solid-state 13C cross-polarization magic angle spinning nuclear magnetic resonance (CPMAS NMR) spectra were recorded on an Agilent NMR vnmrs600 spectrometer under the following conditions: spinning rate, 10.0 kHz; contact time, 3.5 μs; and pulse delay time, 2.0 s. The hydrodynamic diameter of DBN samples after sonication was measured by a dynamic light scattering analyzer (ZEN3600, Malvren, UK). The electron accepting capacity (EAC) and electron donor capacity (EDC) of DBN and DBC were measured by the mediated electrochemical reduction and mediated electrochemical oxidation methods used in the literature (Klüpfel et al. 2014; Aeschbacher et al. 2010). The electron exchange capacity (EEC) of DBN and DBC was calculated from the sum of EAC and EDC. The specific procedures are provided in the Text S1.

2.4 Degradation experiment of chlorophenols

A certain amount of DBN was well mixed with 19 mL of 20 mM pH 7 phosphate buffer in 20 mL brown glass vials capped with Teflon-lined septa. The vials received a certain volume (< 0.1%) of chlorophenol (purity, 99%) stock solutions in methanol by syringe injection through the vial cap septum. The mixture was magnetically stirred at 30 rpm for 24 h in the dark to achieve an apparent sorption equilibrium. Then, the mixtures were purged with nitrogen gas for 30 min and 1 mL of sodium sulfide nonahydrate (Na2S·9H2O) stock solution (0.1 M) was injected into the vials by syringe through the cap septum to eliminate the headspace. Note that the Na2S·9H2O stock solution was made freshly before use by dissolving Na2S·9H2O in N2-purged phosphate buffer. The final concentrations of DBN, chlorophenol, and sulfides were 100 mg L–1, 5.0 mM, and 50 mM, respectively. The same degradation experiment was also carried out using DBC450 as a mediator for comparison with DBN. The final pH of the solution was 6.9 ± 0.2. During the rotation, triplicates of the vials were sacrificed for analysis at predetermined intervals (0, 24, 48, 72, 96, 120 h). One mL aliquant of the aqueous solution was sampled from the sacrificed vials and immediately mixed with 0.5 mL ethanol to terminate the free radical reactions (if any). Then, the solutions were centrifuged at 4000×g for 30 min using a membrane with a pore size of 3 kDa (Millipore Amicon, Ultra, USA) to separate from solids and used for the determination of aqueous concentrations of chlorophenols and sulfides by high-performance liquid chromatography (HPLC, Agilent 1200, USA) and inductively coupled plasma mass spectrometry (ICP-MS) (iCAP-TQ, Thermo-Fisher, USA), respectively. The concentration of chloride ions (Cl−) produced during the degradation was determined by the mercury thiocyanate method (EPA method 8113) (Lokesh et al. 2020). The detailed analysis methods are presented in the Text S2.

The effect of sulfide concentration on the degradation was also examined, wherein the concentration of sulfides was set from 10 to 100 mM in the reaction systems as mentioned above. All the degradation experiments were performed in triplicate. The main degradation products of chlorophenols were identified by liquid chromatography-mass spectrometer (LC–MS/MS) (Agilent, USA) equipped with an electrospray ionization source. Molecular formulas and structures of degradation products were determined based on accurate m/z analysis using Agilent software. All mathematically possible formulas for each ion were calculated with a mass tolerance of ± 5 ppm. The detailed parameters can be found in the Text S3.

2.5 Sorption experiment of sulfides by DBN

The sorption capacity of DBN for sulfides might greatly affect the degradation of chlorophenols in the DBN-sulfide system. Thus, a single-point sorption experiment was conducted by mixing a certain amount of DBN with 19 mL of phosphate buffer in 20 mL vials as mentioned above. Then, the mixtures were purged with nitrogen gas for 30 min, and 1 mL of Na2S·9H2O stock solution was injected into vials. The final concentrations of DBN and sulfides were 100 mg L–1 and 50 mM, respectively. These vials were stirred in the dark for 48 h to achieve apparent sorption equilibrium and then the mixtures were centrifuged and filtered using the 3 kDa pore size membrane. The concentration of sulfides in the filtered solution was determined by ICP-MS. The vials containing sulfides but without DBN addition were used as control. The mass of sorbed sulfides was calculated by mass balance. All experiments were performed in triplicate. Quality control and quality assurance during the analysis process were conducted using a standard reference material (Shrub Leaves, GBW07602, provided by Nanjing Elida Biotechnology Co., Ltd).

To further explore the reactions between sulfides and DBN, the bound S species on the surfaces of DBN was examined by XPS. Both the sorption experiment and XPS analysis were also carried out using DBC450 as a sorbent for comparison.

2.6 Electron paramagnetic resonance and quenching experiments

To explore whether PFRs were generated from the DBN samples in aqueous suspensions, 0.15 g of the sample was loaded into a quartz tube and then detected using an EPR spectrometer (Bruker A300, Germany) at room temperature. The production of ROS and RNS from DBN suspensions containing various sulfide concentrations (10–100 mM) was probed by EPR coupling with a spin-trapping technique. Specifically, 5,5-dimethyl-1-pyrroline-N-oxide (DMPO,  > 97%) was used to trap ⋅OH and O2⋅−; while 2,2,6,6-tetramethyl-4-piperidone (TEMP) was used to detect the production of 1O2 as spin-trapping agents, respectively. Also, the production of nitrogen oxide radical (NO⋅) was determined using an iron complex of ferrous irons and sodium diethyldithiocarbamate trihydrate (DETC, 99%), (DETC)2–Fe(II), as a chelating agent, and the generated NO–(DETC)2–Fe(II) complex in the reaction of (DETC)2–Fe(II) and NO⋅ was determined by EPR spectrometer (Zhu et al. 2022). More detailed measurements and parameters can be found in the Text S4.

A quenching experiment was carried out to explore the possible effect of various free radicals in the degradation of 2-CP. Isopropanol (IPV), 1,4-benzoquinone, NaN3, and flavanone as quenching agents for ⋅OH, O2⋅−, 1O2, and NO⋅ were added to the reaction systems, respectively. To further explore the forming mechanism of NO⋅, an important intermediate peroxynitrite (ONOO−) was determined by an established fluorescent probe technique with a commonly used fluorescent dye 2,7-dichlorodihydrofluorescein diacetate (DCFH2) as an indicator (300 μm), which could react with ONOO− to produce a highly fluorescent oxidized species (i.e., DCF) (McQuade and Lippard 2010). Briefly, DCFH2 was dissolved in anhydrous dimethyl sulfoxide to prepare a 5 mM of stock solution. Before the observation, the stock solution was diluted by PBS buffer to a working concentration of 1–10 μM and observed under a fluorescence microscope (Eclipse Ni-U, Nikon, Japan), with the excitation/emission wavelength of 504/529 nm.

3 Results and discussion

3.1 Structural properties of BN and DBN derived from N-enriched biomass

The elemental analysis suggests that BN derived from algae biomass contained relatively high content of N, which exhibited a decreasing trend from 12.53% (BN350) to 8.69% (BN750) with increasing charring temperature. The BN samples also contained a range of metal and mineral elements such as Fe, Ca, K, and Na, but their respective contents were remarkably lower (< 2.5%) (Table S1). Comparatively, the N contents of BN are higher than many other N-doped carbon materials prepared by N-doping techniques such as the pyrolysis of crop straw in an ammonia atmosphere (the N content in the obtained samples was in the range of 0.36–7.72%) (Wu et al. 2021) and ammonium nitrate soaking followed by pyrolysis, where the N content in obtained carbons was reported less than 10% (Zhu et al. 2018). Besides green algae in this study, direct pyrolysis of N-rich biomass such as plants and leaf litter can also produce BC with nonnegligible N content (Wagner et al. 2015), indicating that BN is ubiquitous in water/soil environments and the N-containing groups might alter electrochemical characteristics including EDC, EAC, and electron transfer capacity of carbon matrices. The surface N content in DBN extracted from BN ranged from 2.31 to 7.56% based on the XPS results (Table S2). The deconvolution of high-resolution spectra for N1s suggests that the N species in DBN mainly included pyridinic-N (398.25 eV), pyrrolic-N (399.8 eV), graphitic-N (401.1 eV), and oxidized-N (402.8 eV) (Lian et al. 2016). As shown in Figure S1, pyridinic- and pyrrolic-N accounted for over 43 atom% for all the DBN samples except DBN650 (31.79%). Comparatively, DBN750 possessed the highest content of graphitic N (41.68%) due to the increased condensation of carbon matrix; while the samples prepared at moderate temperatures (450–650 °C) exhibited significantly higher yield of oxidized-N, which played a critical role in enhancing the yield of N-containing free radicals (Zhu et al. 2020) (see more discussion below).

FTIR spectra reflect the variation of functionalities in DBN samples, where the N-associated structures are labelled in the Fig. 1a. The peak at 3412 cm−1 was generated by N–H stretching, while the N–H strain appeared at 1631 cm−1. The peak at 1404 cm−1 was denoted as C(sp2)=N stretching, and the peak at 1127 cm−1 can be identified as C(sp2)–N stretching vibration (Bai et al. 2014). The peak intensity of N-containing groups enhanced initially and then decreased when the charring temperature increased from 350 to 750 °C. The solid-state 13C NMR spectra (Fig. 1b) were employed to illustrate the molecular structure of DBN and each peak was relatively quantified to determine the respective content of functional moieties (Table S3). Three evident characteristic peaks of O/N-alkyls (40–90 ppm), aromatic C–C (90–150 ppm), and carboxyl/ester/quinone (165–200 ppm) were observed and their percentages increased with pyrolytic temperature and roughly peaked at 550 °C, consistent with the trend of FTIR data. Additionally, the Raman spectra of DBN (Fig. 1c) showed that all the samples had peaks at 1336–1351 cm−1 and 1565–1595 cm−1, which could be identified as D peak and G peak, respectively (Geng et al. 2011). The relative intensity ratio of D to G bands (ID/IG), a measure of disorder degree, decreased from 1.065 (DBN350) to 0.889 (DBN550), indicating the formation of graphitic domains; however, with further increasing pyrolytic temperature, the ratio increased to 0.984 (DBN750), probably due to enhanced vacancy defects caused by the breakdown of graphitic domains at higher temperature (Wan et al. 2023).

Fig. 1
figure 1

Characterization of DBN samples prepared at different temperatures. (a) FTIR; (b) 13C NMR; and (c) Raman spectra

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Heterocyclic N structures are reported to improve the electron shuttling capacity of carbon matrixes and thus favor the redox-involved environmental processes such as microbial Fe(III) reduction (Wu et al. 2021) and transformation of Cr(VI). The EAC, EDC, and EEC values of DBN samples were in the range of 2.6 to 3.0 mmol e−/g, 1.5 to 1.9 mmol e−/g, and 4.1 to 4.8 mmol e−/g, respectively (Table S4), which roughly decreased with charring temperature. Owing to the higher electronegativity of the doped-N, all the DBNs possessed higher EAC than EDC values. For comparison, the EDC, EAC, and EEC of DBC450 were also determined (Table S4), which exhibited similar EDC values (1.9 mmol e−/g) but significantly (p < 0.05) lower EAC (2.2 mmol e−/g) and EEC (4.1 mmol e−/g) than those of DBN450 (2.8 mmol e−/g for EAC and 4.7 mmol e−/g for EEC). These results suggest that N enrichment enhanced the electron accepting capacity of DBN, which might benefit the acquisition of electrons from sulfides.

3.2 DBN-mediated catalytic dichlorination in sulfide-containing aqueous solutions

The catalytic activities of DBN350–750 and DBC450 were evaluated by monitoring the dichlorination of CPs in the presence of sulfides (Fig. 2). About 39.1% of 2-CP was removed within 120 h by DBN450 in the presence of sulfides and followed by DBN550 (34.7%), DBN650 (21.8%), DBN750 (18.7%), and DBN350 (16.0%). Note that sulfides or DBN450 alone exhibited negligible removal of 2-CP (< 4.2%), indicative of the important synergistic role of DBN and sulfides in the degradation of 2-CP. In contrast, only 12.9% of 2-CP was removed by the combination of DBC450 and sulfides, which further reflected the important catalytic role of heterocyclic N in pyrogenic carbon. However, the degradations of 3-CP and 4-CP were no more than 11.92% and 9.92% in the period of 120 h by all the DBN samples (Fig. 2b and c), much lower those that of 2-CP, indicating the nonnegligible effect of molecular structure of CPs in their degradation (see more discussion below). The degradation kinetics of 2-CP can be well-fitted by the pseudo-first-order model (Table S5) and the highest kinetic constants (Ka) of degradation was 4.1 × 10–3 h−1 (DBN450), which was comparable to that of N-doped graphene (7.61 × 10–3 h−1) and N-doped carbon nanotubes (5.5 × 10–3 h−1) in the chlorine elimination of hexachloroethane in sulfide-containing solutions (Liu et al. 2021), reflecting the remarkable catalytic degradation potential of DBN in the presence of reduced sulfur species (e.g., S2−). It is generally believed that the degradation efficiency of graphitized carbons could be greatly improved by the doped N (especially pyridine-N and pyrrolic-N) due to the facilitated electron transfer (Wu et al. 2021). However, no significantly positive correlation between Ka values and the content of pyridine-N/pyrrolic-N or the total N content in DBN samples was observed (Figures S2–S4). Similarly, the Ka values were also not positively correlated with the EAC or EDC of DBN (Figures S5 and S6). These observations suggest that besides the facilitated electron shuttling by N-doping, other mechanisms might play more important roles in the removal of CPs in the present work.

Fig. 2
figure 2

The degradation kinetics of 5.0 mM chlorophenols (CPs) by different DBN or DBC with 50 mM sulfides for 5 days: (a) 2-CP; (b) 3-CP; (c) 4-CP. Controls also contained 50 mM sulfides in the absence of DBN, or DBN in the absence of sulfides indicating no degradation. Error bars represent the standard deviation of triplicates

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Multiple lines of existing evidence indicate that the sorption of sulfides to conductive carbon surfaces could enhance the degradation of organic contaminants by increasing the electron transfer efficiency (Ding et al. 2018; Xu et al. 2020). Thus, the sorption of DBN samples for sulfides was conducted and the results implied that DBN450 had a significantly (p < 0.05) higher sorption capacity than other samples (Fig. 3a), which was probably due to its higher content of active sites for sulfides. The molecular structure of char materials is highly regulated by carbonization temperature, which generally develops from transition phases to turbostratic ones in the temperature range from 300 to 1000 °C (Keiluweit et al. 2010; Yoo et al. 2018). The medium-temperature chars (e.g., 450–550 °C) possess not only abundant oxyl groups but also newly formed graphitic phases. Noncovalent interactions such as hydrogen bonding and electrostatic interaction may occur between the oxyl groups and negatively charged sulfides at pH 7.0 considering the pKa values of carboxyl and phenolic hydroxyl groups are around 4.3 and 9.8, respectively (Zhao et al. 2015). For graphitic domains, the positively charged C atoms triggered by N-doping may sorb and/or interact with sulfides to form intermediate structures (e.g., C–S–S group), which was identified by K-edge X-ray absorption spectroscopy (Liu et al. 2021). As a result, the medium-temperature DBN exhibited the highest sorption affinity for sulfides. Furthermore, DBN450 possessed the highest colloidal stability in aqueous solutions among the samples with the hydroynamic diameter closely centered at 500 nm during 120 min, evidenced by the aggregation kinetics (Figure S7). The high dispersion capacity endowed the DBN450 more active sites for the binding of sulfides, which was favorable to the sorption of sulfides. More importantly, XPS results suggested that the S2p spectra of DBN450 and DBN650 after sulfide attachment displayed not only a pair of peaks located at 163.6 eV that was probably attributed to S(II) or S0 but also a new pair of peaks at 168.4 eV that was from high valance sulfur such as S(IV)/(VI) (Sun et al. 2022). In contrast, only low valance sulfur peaks at 163.6 eV can be clearly identified in the S2p spectra of DBC550 after sulfide sorption (Figure S8). These variations indicate that the attached sulfides could be oxidized by DBN.

Fig. 3
figure 3

(a) The sorption capacity of DBN for sulfides; (b) correlation between the sulfide sorption and removal efficiency of 2-CP; (c) effect of sulfide concentrations on the degradation of 2-CP, where the concentration of Na2S·9H2O was 50.0 mM and pH was 7.0 ± 0.2

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The removal efficiency of 2-CP was closely related to the sorption of DBN samples for sulfides (R2 = 0.991, p < 0.05) (Fig. 3b), which demonstrated that the binding of sulfides might be one of the most important rate-limiting steps for the decay of CPs by DBN under the reducing conditions. To further explore the contribution of sulfides, the degradation of 2-CP by DBN450 in the presence of various sulfide concentrations (10–100 mM) was compared (Fig. 3c). It is clear that the decay of 2-CP increased initially to reach peak at 50 mM of sulfides and then dropped with higher sulfide concentrations. This indicates that excess sulfides might inhibit the degradation of 2-CP by occupying more active sites for 2-CP and/or consuming oxidizing substances. Besides the binding of sulfides, the sorption affinity of DBN for 2-CP could also influence the removal efficiency (Machado et al. 2020; Channei et al. 2020). Hence, the sorption of 2-CP by DBN samples without the addition of sulfides was conducted and the results showed that the sorption capacity at the end of the experiment was comparable among the DBN samples (Figure S9), which further highlights the critical role of sulfide attachment in facilitating the catalytic degradation of 2-CP.

3.3 Importance of ROS and RNS in the transformation of 2-CP

Besides activating the neighboring C atoms (Kong et al. 2014), the embedded N is favorable to the formation of carbon-centered radicals and their electron donating capacity (Zhu et al. 2020). In the present study, the binding of reduced sulfur species (e.g., S2−) might further increase the electron density of DBN by providing an extra electron donor. Thus, we proposed that the presence of sulfides could contribute to the production of ROS by favoring electron transfer from carbon matrix or PFRs to the chemisorbed oxygen on DBN surfaces. Specifically, the quinone moiety of DBN accepted electrons from sulfides to be reduced to hydroquinone. The formed hydroquinone subsequently donated electrons to oxygen molecules and was converted into quinone (Yang et al. 2017). This reversible redox reactions between quinone and hydroquinone allow electrons to continuously transfer from sulfides to oxygen via DBN (like an electron shuttle), which promotes the production of ROS. The EPR spectra of DBN350-750 indicated that DBN450 possessed the highest EPR signal intensity, followed by DBN350, DBN550, DBN650, and DBN750 (Fig. 4a). The corresponding g-values of the EPR signals were ~ 2.002, which can be designated as carbon-centered radical species (Lachocki et al. 1989). The significantly reduced intensity of EPR signals at higher pyrolytic temperatures was probably due to the elimination of oxygen functionalities and increased graphitization (Zhu et al. 2018), which is consistent with previous studies. A positive correlation between the PFR yield of DBN samples (except DBN350) and the Ka values of 2-CP (R2 = 0.868, Figure S10) is indicative of the great potential of PFRs in promoting the degradation of 2-CP. For DBN350, however, the much lower sorption affinity for sulfides resulted in its decreased catalytic degradation for 2-CP although it had relatively high content of PFR.

Fig. 4
figure 4

(a) EPR spectra of DBN350–DBN750; (b–d) the effect of sulfide concentrations (10, 50, 100 mM of Na2S·9H2O) on the formation of O2⋅−,·OH, 1O2 trapped by DMPO and TEMP in the DBN450/sulfide system; (e) the generation of NO⋅ using (DETC)2–Fe(II) as a spin-trapping agent; (f) the effect of different quenching agents on the degradation of 2-CP by DBN450/sulfide system, where the concentration of quenching agents was 10 mM

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DBN450 was selected as a representative of DBN samples and used for the determination of ROS in the aqueous suspension containing different concentrations of sulfides (Fig. 4b–d). Spin-trapping agents were employed to detect the production of ROS. Three characteristic peaks with intensities of 1:1:1 of TEMP-1O2 adducts, signals of DMPO-O2⋅− adducts with six typical peaks, and typical four-line (1:2:2:1) signals of DMPO-OH adducts were detected, demonstrating the generation of 1O2, O2⋅−, and ⋅OH. Notably, the yields of these ROS were significantly (p < 0.05) higher in the suspensions added with 50 or 100 mmolL–1 Na2S than that with 10 mmol L–1 Na2S, indicating that the production of ROS was closely related to the participation of sulfides. Multiple lines of evidences highlight the pivotal role of ROS in the transformation of organic contaminants both in water/wastewater treatments and exploring their environmental fate (Nidheesh et al. 2021; Fang et al. 2017; Ruan et al. 2019). Hence, we believed that the produced free radicals played a critical role in the dichlorination of CPs in the sulfide-containing aqueous system. To quantitatively determine the contributions of ROS to the decay of CPs, IPV, NaN3, and p-benzoquinone (Wang and Wang 2020) were used as quenching agents for ⋅OH, 1O2, and O2⋅−, respectively, in the representative reaction system of DBN450 and 2-CP (Fig. 4f). The reduction for the degradation of 2-CP followed an order of NaN3 (42.6%), IPV (25%), and p-benzoquinone (13.09%) after the quenching treatment. As a result, it seems that the non-radical mechanism accounted for at least 50% of the 2-CP removal. It is reported that the non-radical mechanism was more prevalent in the degradation of compounds by high-temperature BCs (e.g., 800 °C) due to their higher size of polyaromatic sheets and/or vacancies (Wan et al. 2021; Pignatello et al. 2017). Thus, for the DBN450, we inferred that other mechanism(s) might contribute more to the removal of CPs. Besides ROS, evident isotropic EPR signals of (DETC)2–Fe(II)–NO spin adduct was also identified (Fig. 4e), reflecting the generation of NO⋅, a typical RNS, from DBN450 using (DETC)2–Fe(II) as a spin-trapping agent. It was reported that RNS was effective in degrading various organic contaminants including bisphenol A, ibuprofen, estradiol, and ethinylestradiol (Wu et al. 2019; Wang et al. 2019). To explore the possible contribution of NO⋅ to the decay of 2-CP, the quenching experiment was carried out simultaneously using flavanone as the quenching agent (Wang et al. 2019). We noted that the decrease of 2-CP degradation was up to 72.2% after the quenching of NO⋅, significantly higher than that caused by ROS (Fig. 4f). Consistently, the degradation efficiency was positively related to the content of NO⋅ in DBN samples (Figure S11, R2 = 0.74), implying the dominating contribution of RNS in the decay of 2-CP. Based on the EPR results and ROS/RNS quenching experiments, it was concluded that the contributions to the removal of 2-CP by DBN450 followed an order of RNS > ROS > non-radical mechanism.

An interesting question confronting us is how the RNS was produced in the reactive systems. It is frequently reported that RNS such as NO⋅ and NO2⋅ can be formed through the UV photolysis of nitrate/nitrite or as secondary radicals from the reaction of ⋅OH with nitrite (⋅OH + NO2− → OH− + NO2⋅) (Wu et al. 2019). A recent work developed a photocleaved O2-released nanoplatform to switch ROS into RNS by combining with nitric oxides for repressing hypoxic breast tumor (Luo et al. 2021). We noticed that the DBN samples except DBN750 possessed considerable content of N-oxides as shown by the XPS results (Figure S1), which was likely to react with the generated ROS to form RNS. To justify the hypothesis, the generation of peroxynitrite (ONOO−/ONOOH), an important intermediate to produce multiple RNS (e.g., ONOOH → ⋅OH + NO2⋅) (Bartesaghi and Radi 2018), was monitored in the mixture of DBN450 and sulfides using DCFH2 as a fluorescent probe (Fig. 5). It was shown that extraordinarily stronger green fluorescence signals launched from DBN450 surfaces could be detected than those from the control (without sulfides), which visually evidenced the presence of ONOO−/ONOOH. However, it should be noted that the oxidization of DCFH2 to DCF was not specifically initiated by ONOO−, the conversion of which was also reported using other oxidants such as O2⋅−, CO3⋅−, and NO2⋅ (Wardman 2007). Nevertheless, DCFH2 has very low reactivity with O2⋅− and hydrogen peroxide relative to ONOO− (Wrona et al. 2005). Furthermore, only ROS and RNS as two major oxidants presented in the DBN450/sulfides system, and thus we believed that RNS (i.e., ONOO−) were mainly responsible for the oxidative fluorescence of DCFH2. The overall results indicate that ROS (e.g., ⋅OH, 1O2, and O2⋅−) generated by the electron transfer from DBN to the chemisorbed oxygen on DBN might be the “primary” radicals and RNS (e.g., ONOO−/ONOOH, NO⋅ and/or NO2⋅) resulted from the reactions between ROS and N-oxides in DBN might be the “secondary” radicals, both of which lead to the decay of CPs. By contrast, the non-radical mechanisms (e.g., degradation by reactive sulfur intermediates) only play a minor role in this process. The related mechanisms are illustrated in Fig. 6.

Fig. 5
figure 5

The fluorescence image of DCF in the DBN450/sulfide system (Ex/Em = 504/529 nm). The green fluoresce of DCF was generated by RNS oxidization of DCFH2, where (a) is the bright field, (b) is the dark field, and (c) is the merge channel. The control added with only DBN450 but without sulfides is presented in (d), (e), and (f). (d) bright field; (e) dark field; (f) merge channel

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Fig. 6
figure 6

The contributions of different mechanisms to the decay of chlorophenols. (I) the attached sulfides were oxidized by DBN to form reactive sulfur intermediate structures; (II) the attached sulfides transferred electrons to chemisorbed oxygen via DBN forming ROS (e.g., O2⋅− and ·OH) as the “primary” radicals; and (III) the formed ROS reacted with N-oxides in DBN to generate RNS (e.g., ONOO−/ONOOH, NO⋅) serving as the “secondary” radicals

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3.4 By-product analysis and possible reaction pathway

Based on the analysis of degradation intermediates by LC–MS/MS, the main fragmentations (m/z) and chemical structures are summarized in Figure S12. The proposed degradation pathways of 2-CP by the DBN450/sulfides system were mainly involved substitution, benzene ring cleavage, and hydroxylation reactions. The possible intermediate products were mainly composed of aromatic halides (e.g., 2,4,6-trichlorophenol and chlorohydroquinone), alcohols (phenol, hydroquinone, pyrocatechol, phloroglucinol, crotonyl alcohol, hydrobenzoquinone), and ketones (1,4-benzoquinone). The concentration of Cl− generated in the reaction between DBN450 and 2-CP in the sulfide-containing solutions was monitored during the degradation process. The generation of Cl− was fast at the beginning and more than 66.67% of Cl− was produced in the initial 24 h (Figure S13), which indicated that dichlorination was probably the first step in the degradation of 2-CP and then followed by the cleavage of benzene ring. The removal of TOC was also determined and ~ 22% removal was completed at the end of the experiment. Recalling that the decay of 2-CP by DBN450 was around 40% (Fig. 2), we concluded that the tested CPs were not necessarily decomposed completely and a portion of the intermediate products might retain the benzene ring structure (mainly quinone chemicals) with a nonnegligible toxicity potential (Gu et al. 2022). Thus, the impact of DBN-induced transformations of CPs on their ecotoxicity, i.e., secondary toxicity, also needs investigation in exploring the role of DBN in the biogeochemical processes. The possible reaction pathway for 2-CP degradation is presented in Fig. 7.

Fig. 7
figure 7

Purposed degradation pathway of 2-CP in the DBN450/sulfide system

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It is notable that the degradation efficiency of 2-CP was significantly (p < 0.05) higher than that of 3-CP and 4-CP (Fig. 2). This could probably be explained by the sorption pattern of CPs by DBN samples and reactivity of CPs due to the substituent of Cl. We noticed that the binding affinity of 2-CP to DBN prepared at different temperatures (350–750 °C) was similar (Figure S8), which indicated that surface functionality of DBN might play a more important role than its polyaromatic structure in the sorption. Otherwise, if the π-electron interactions dominated the sorption of CPs (Zhu et al. 2004), the binding affinity would have enhanced with the increasing pyrolytic temperature due to larger polyaromatic structures in the high-temperature DBN. Thus, the hydroxyl group in CP molecular structure might be the preferred site for the binding with DBN and consequently the adjacent C–Cl bonds at ortho positions were more easily to be attacked by ROS and RNS. Additionally, Cl substituent on the aromatic ring could also influence the reactivity of CPs. The electron-donating effect of hydroxyl group is more significant to the Cl at ortho positions than that at meta and para positions and thus the C–Cl bonds at ortho positions could contribute more electrons to ROS/RNS during the reactions.

4 Environmental implications

We are among the first to investigate the catalytic transformation of organic contaminants mediated by DBN in the sulfide-containing aqueous solutions. The present study demonstrates that DBN is a ubiquitous but overlooked mediator in the water and soil environments, widely involved in the abiotic transformation of organic contaminants especially in the presence of reduced sulfur species. Different from previously reported mechanisms, we noticed that in the DBN-sulfide system the sulfides were favorable to the production of ROS on DBN and the generated ROS reacted with N-oxides in DBN to produce RNS, which played a dominating role in the decay of CPs. The generation of RNS is widely observed in the atmosphere, air–water interface, and surface water (Simoes et al. 2021; Rana and Guzman 2022). This work provides new information that RNS might also be formed in deep water and DBN probably possesses higher catalytic activity than we expected through reacting with reduced sulfur species to produce RNS in anaerobic environments, which would have profound implications for many critical biogeochemical processes including the transformations of contaminants and minerals, microbial community dynamics, and the elemental cycling in terrestrial/aquatic systems as well as for scalable applications in the wastewater treatment. Additionally, we also found that the benzene ring structures in the tested chlorophenols could not be completely decomposed in the DBN-sulfide systems in a relatively short time (120 h), and thus the secondary toxicity caused by contaminant transformation in the reaction with DBN also needs more investigations.