Molecular characteristics of organic matter derived from sulfonated biochar†

Zhengfeng Jiang^abd, Chen He*^b, Fei Gao^a, Quan Shi*^b, Yang Chen^e, Haimeng Yu^b, Zhimao Zhou^c and Ruoxin Wang^a
aPetrochemical Research Institute, PetroChina Company Limited, Beijing 100195, China
^bState Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China. E-mail: hechen@cup.edu.cn; sq@cup.edu.cn
^cCAS Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China
^dNational Elite Institute of Engineering, CNPC, Beijing 100096, China
^eResearch Center for Atmospheric Environment, Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, China

First published on 30th July 2024

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Environmental significance Sulfonated biochar is an excellent functional carbon material with special surface properties that give it a wide range of potential applications in heavy metal and pollutant adsorption. An in-depth understanding of sulfonated biochar-derived dissolved organic matter (SBC-DOM) is essential to predict its potential impacts on the environment. The technology used to produce sulfonated biochar is quite different from that used for pyrochar and hydrochar, resulting in the unique molecular and environmental properties of SBC-DOM. In this paper, the composition of SBC-DOM was investigated for the first time at the molecular level. The results of this study provide a theoretical basis for evaluating the application prospects of sulfonated biochar.

1 Introduction

Biochar is a carbon-rich solid product obtained through the thermochemical conversion of biomass under oxygen-limited conditions. There are currently two main technologies for producing biochar: hydrothermal carbonization and conventional pyrolytic carbonization.¹ Pyrolysis biochar (pyrochar) is obtained through a thermochemical decomposition process of low moisture biomass in the absence of oxygen at elevated temperatures (300–600 °C) with several hours of residence time.² Hydrothermal carbonization involves the utilization of wet feedstock or the treatment of dry feedstock with water under its sub-critical condition (180–250 °C, 2–6 MPa).³ Unlike pyrochar (PYC) and hydrochar (HYC), sulfonated biochar (SBC) is obtained by sulfonating biomass (rice husk) or defective carbon materials.^4,5 SBC has abundant polar functional groups, including –SO₃H, –OH, –COOH, and so on.⁵ Therefore, SBC exhibits high acidity, hydrophilicity, and excellent heavy metal adsorption capabilities, playing an important role in solid acid catalysis, soil amendment, carbon fixation, and remediation of pollutants in water.^4–7 Generally, the yield of SBC prepared by the in situ functionalization approach (80–250 °C) is low.^5,8 Recently, we utilized rice husk (RH) and alkylated waste sulfuric acid (WSAA) in a one-step carbonization-sulfonation reaction, resulting in a SBC yield of more than 80%.⁴ The complete use of WSAA in the preparation of SBC not only solves the problem of environmental pollution of waste sulfuric acid but also recovers sulfur resources.

Although biochar is a refractory carbonaceous material, its application in a variety of environments may result in the release of dissolved substances by water due to aging, irrigation, rainfall, or leaching.⁹ Biochar-derived dissolved organic matter (B-DOM) plays an important role in controlling the environmental remediation potential of biochar.¹⁰ The molecular structure and composition of B-DOM largely determine its impact on the environment¹⁰ and are influenced by the raw material of biochar, manufacturing processes, and extraction methods.¹¹ However, the molecular characteristics of sulfonated biochar-derived dissolved organic matter (SBC-DOM) and its effects on the environment are not known.

Various techniques such as UV-vis spectroscopy, excitation–emission matrix, and Fourier transform infrared spectroscopy have been used to detect the composition and structure of B-DOM.^9,12 Although extensive efforts have been made to elucidate B-DOM, the challenge also exists due to its very complex molecular composition. Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) has enabled detailed assessment of the molecular composition of B-DOM.^9,13,14 Li et al.¹³ utilized FT-ICR MS to analyze the molecular composition of organic matter derived from soybean straw biochar (450 °C) and found that a high percentage of unique molecular formulae could be detected in each fraction extracted with different polarity solvents. Huang et al.^15,16 conducted a study utilizing FT-ICR MS and spectroscopic techniques to investigate the binding behavior of DOM derived from different biochar with lead and cadmium. Wu et al.¹⁷ characterized the structural changes of hydrochar-derived DOM (HYC-DOM) at different pH values using FT-ICR MS. Song et al.⁹ used FT-ICR MS and spectroscopic techniques to elucidate the complexation sequence of copper with B-DOM molecular compounds. While FT-ICR MS had been extensively utilized for molecular characterization of pyrochar-derived DOM (PYC-DOM) and HYC-DOM, SBC-DOM has scarcely been reported.

In this study, rice husk was used as the biomass feedstock and PYC, HYC, and SBC were prepared under low-temperature conditions. The B-DOMs were extracted using pure water from RH, PYC, HYC, and SBC to simulate the release of organic matter in carbon materials. The molecular characteristics of different B-DOMs were analyzed by FT-ICR-MS. The purpose of this study is (1) to explore the molecular composition features of SBC-DOM and (2) to assess the stability and biodegradation potential of SBC-DOM compared to PYC-DOM and HYC-DOM. This research can provide new insights into the environmental effects and fates of sulfonated biochar-derived DOM fractions in nature.

2 Materials and methods

2.1. Samples and chemicals

The raw biomass material used in this study was rice husk, which was collected from a farmland in Jilin Province, China. The alkylated waste sulfuric acid (WSAA) containing 90.06 wt% sulfuric acid and 7.50 wt% acid-soluble hydrocarbon was collected from a Petroleum Chemical Industry Co., Ltd (Henan, China). Ultrapure water (H₂O, HPLC grade) and methanol (CH₃OH, HPLC grade) were purchased from Fisher Scientific.

2.2. Biochar preparation and characterization

In this study, rice husk was chosen as the biomass raw material, which was oven-dried at 80 °C, then ground and passed through a 20-mesh sieve.¹³ Three carbon materials were prepared from rice husk. The pyrochar was prepared at 300 °C, while the hydrochar and sulfonated biochar were prepared at 180 °C, respectively. The detailed preparation procedure of the sulfonated biochar can be found in Text S1 (see the ESI†), as described by Zhou et al.⁴

The C, H, S, and O contents of the raw biomass material and carbon materials were measured by elemental analysis (Vario EL cube elementar, Germany), and the method of measuring ash content followed a previous study.¹⁸ The surface area and pore development were confirmed using an automatic static physisorption analyzer (ASAP 2420,USA). The morphologies of the three carbon materials were investigated using a field emission scanning electron microscope (SEM, GeminiSEM300, Germany). The quantitative determination of surface acidic groups was performed by Boehm titration.¹⁹ The characteristics of the three carbon materials can be found in Tables S1, S2, and Fig. S1.†

2.3. Organic matter extraction from the raw biomass material and carbon materials

B-DOMs were extracted from RH, PYC, HYC, and SBC according to Zhang et al.²⁰ Briefly, the dried 10 g carbon materials were added to 200 mL ultrapure water, and the turbid liquid was swayed in a bath table at 180 rpm at 25 ± 2 °C for 72 h. Then, the aqueous extracts were filtered through a 0.45 μm hydrophilic filter membrane and stored in the dark at 4 °C until further purification. The dissolved organic carbon (DOC) of the aqueous extracts was measured using a Shimadzu TOC-5000 analyzer.

2.4. FT-ICR-MS analysis

The MS analyses of B-DOMs were carried out using negative ion Apollo II electrospray ionization (ESI) coupled with a 9.4 T Apex Ultra FT-ICR mass spectrometer (Bruker, Germany).²¹ The acidified B-DOM aqueous samples (pH = 2) were desalinated and concentrated using solid-phase-extraction cartridges (Agilent, Bond-Elut-PPL).^22,23 The B-DOM samples were adjusted to a final concentration of about 100 mg L⁻¹ with methanol and then injected into an electrospray source at 250 μL h⁻¹. The spray shield voltage, capillary column entrance voltage, and end voltage were 4.0 kV, 4.5 kV, and −320 V, respectively. Ions accumulated in the collision cell for 0.5 s and were transferred into the ICR cell with a 1.3 ms flight time. A total of 128 scans with m/z 200–800 and a 2 M word size were accumulated. The detected mass peaks with S/N ratios greater than 4 were processed. Molecular formula assignments were limited by the following: ¹²C (0–60),¹H (0–120), ¹⁴N (0–3), ¹⁶O (0–30) and ³²S (0–5) atoms.²⁴ The mass measurement accuracy for singly charged molecular ions was better than ±1.0 ppm.

2.5. Data processing

In this study, H/C, O/C, the aromaticity index (AI_mod) and double-bond equivalent (DBE) were calculated as the basis for composition analysis of B-DOM.^13,14 The calculation formulae are shown in Text S2 of the ESI.† The polarity index (N + O)/C was calculated to represent the abundance of polar functional groups in organic matter.¹⁸ The formulae plotted on the van Krevelen diagram were categorized into seven biomolecular compound groups:²⁵ polycyclic aromatics (PCAs, AI_mod ≥ 0.67), highly aromatic compounds (HAs, 0.66 ≥ AI_mod > 0.50), highly unsaturated compounds (HUs, AI_mod ≤ 0.50 and H/C < 1.5), unsaturated aliphatic compounds without N (UAs, 2.0 > H/C ≥ 1.5 and n = 0), unsaturated aliphatic compounds with N (UAs-N, 2.0 > H/C ≥ 1.5 and n > 0), carbohydrates (H/C ≥ 2.0 and O/C ≥ 0.9), and fatty acids and saturated compounds (FAs & Sat., H/C ≥ 2.0 and O/C < 0.9). The molecular lability index (MLB_L%) and carboxyl-rich alicyclic molecules (CRAM) can be easily used to estimate and compare the lability of organic matter.^26,27

3 Results and discussion

3.1. DOC concentrations and pH values of biochar-extractable organic matter

The DOC concentrations in the water-extracted DOM solutions from various matrices are shown in Table 1. The DOC yield of SBC is 3.45 mg C per g BC, which is similar to that of PYC (3.09 mg C per g BC), but significantly lower than that of HYC (13.23 mg C per g BC) and RH (9.53 mg C per g BC). As a higher H/C ratio of biochar indicates a lower degree of carbonization,²⁸ the carbon materials with higher H/C atomic ratios tend to produce larger DOC concentrations (Tables 1 and S1†).²⁹ In the sulfonation process, the RH was first hydrolyzed by sulfuric acid to sugars, and then the sugars were dehydrated, polymerized, and carbonized to prepare charcoal.⁸ The acid-soluble organic matter (ASOM), which does not exist in the ordinary sulfuric acid, in the alkylation waste acid can polymerize organic matter into larger molecules^30,31 and further carbonize into SBC. The ASOM promotes the polymerization of organic matter during the sulfonation process, thereby reducing the amount of water-extractable organic matter in the SBC. The DOC of HYC is higher than that of PYC, which was consistent with the finding of Song et al.¹¹ It is generally believed that hydrothermal carbonization is relatively weak and a large part of DOM is not completely polymerized or mineralized, which leads to higher DOC extracted from HYC.¹¹

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The pH values of these water-extracted B-DOMs are shown in Table 1. The pH of SBC-DOM is the lowest (3.83). This may be due to the large amount of acidic organic matter containing –SO₃H extracted from the SBC, which makes the pH of SBC-DOM much lower than that of other B-DOMs. During the release of acidic SBC into the soil, the metal ions in the soil are more readily dissolved out and thus more readily be absorbed by plants.³² For example, under low pH conditions, B-DOM can alter the structure of Mn oxide minerals, facilitating the release of Mn(II).¹⁷ This has a positive effect on remediation of heavy metal pollution.

3.2. Molecular characteristics of sulfonated biochar-DOM

The negative-ion ESI FT-ICR MS mass spectra of B-DOM from RH, PYC, HYC, and SBC are shown in Fig. 1. The molecular weight distribution (MWD) of the B-DOM from RH, HYC and PYC is 200–800 Da (Fig. 1a–c), while the MWD of SBC-DOM is 200–600 Da (Fig. 1d). For SBC-DOM, the number of formulae assigned at 500–800 Da accounts for 8% of total assigned formulae (Fig. S2†), while they account for 23%, 22%, and 30% of B-DOMs from RH, PYC and, HYC, respectively. Meanwhile, the intensity-weight average m/z of SBC-DOM is lower than that of other B-DOMs (Table 1). This may be because the acid-soluble organic matter in WSAA promoted the polymerization and carbonization of macromolecular organic matter which is not easily extracted by water.³¹ In general, most DOM molecules with low molecular weight are rapidly mineralized and have low biological stability and high conversion potential.³³ At the same time, low molecular weight DOM has a strong complexing ability to metal ions.³³ Fig. 1e–h show the scale-expanded segments of the real mass spectra at m/z 313. With a mass resolving power of ∼310000, almost all the mass peaks were well resolved and assigned to unambiguous molecular compositions with less than ±1.0 ppm mass errors. The formulae identified in SBC-DOM are mainly CHOS-containing compound formulae (containing C, H, O and S atoms), while CHO-containing compound formulae are dominant in B-DOMs of RH, PYC, and HYC.

The numbers of formulae are 3560, 4899, 4913, and 5752 in samples SBC-, PYC-, HYC-, and RH-DOM, respectively (Fig. 2a). A total of 10839 unique formulae are detected across the whole sample set, and only 289 formulae are shared by all samples (Fig. 2b). The molecular composition of SBC-DOM is very different from that of other B-DOMs. The number of unique formulae of SBC-DOM is 2592, which is 72.8% of its total number of formulae (Fig. 2b). In addition, the numbers of CHOS- and CHONS-containing formulae reach 85% for SBC-DOM (Fig. 2c). The main types of compounds of DOM from RH, HYC, and, PYC are CHO- and CHON-containing compound formulae (over 80%, Fig. 2c), and the number of common formulae of them is 2407. The similarity of the molecular formulae between PYC-DOM and HYC-DOM is as high as 57% (Fig. S3†), but the SBC-DOM molecular formula exhibits less than 10% similar to other B-DOMs.

To further discuss the molecular characterization of SBC-DOM, the formulae of each DOM sample were counted according to heteroatom class species (Fig. 3). The heteroatom classes of SBC-DOM are very different from that of RH-, HYC- and, PYC- DOMs. The predominant heteroatom class species of SBC-DOM detected in negative-ion ESI include O₃S₁–O₁₀S₁ and O₃S₂–O₁₀S₂, among which O₇S₁ and O₆S₂ classes have the highest relative abundance. Organic sulfur compounds are present as S–H, C–S, CS, SO, –SO₃H, and –SO₂– groups.³⁴ The relative abundances of these CHOS-containing compounds decrease with the increasing number of S atoms in the molecules (Fig. S4†). The relative abundance of CHON-, CHONS-, and CHO-containing compounds is less than 15%, 8%, and, 2%, respectively (Fig. S4†). Nevertheless, the CHO containing compounds have the highest abundance in RH-, HYC-, and PYC-DOMs (Fig. 3a–c), including O₂–O₁₃ classes. In the sulfonation process, sulfonic acid groups or other components containing the S element were introduced into the carbon skeleton structure through chemical modification or functionalization technology, resulting in most of the compounds in SBC-DOM being S-containing compounds.⁵

From the van Krevelen diagrams, the O/C and H/C ratios of S-containing compounds in SBC-DOM are 0.1–1.3 and 0.4–2.4, while they are 0.1–0.6 and 0.8–2.2 for RH-, PYC-, and HYC-DOMs (Fig. 4a). The O/C_wa and (N + O)/C_wa in SBC-DOM were significantly higher than those of other B-DOMs (Table 2), indicating a higher content of polar functional groups in SBC-DOM. SBC can be oxidized by strong oxidation-sulfonation agents such as concentrated H₂SO₄, fuming oil, and gaseous SO₃ to produce oxygen functional groups.⁵ The organic matter with a high polarity index (N + O)/C can promote the dissolution of pollutants and improve mobility.³⁵ Additionally, DOM with a high oxygen content (O/C ratio ≥0.4) is prone to complexation with heavy metals.³⁶

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Highly unsaturated compounds (51%) and unsaturated aliphatic compounds (35%) are present in high abundance in SBC-DOM (Fig. 4b and Table S3†). Further, the CHOS-containing compounds in SBC-DOM mainly contained unsaturated aliphatic compounds, lipid compounds and highly unsaturated compounds (Fig. 4c), which is consistent with the findings of Tian et al.¹⁴ Thermal instability mineralizes these CHOS-containing organic compounds into inorganic sulfates, which are taken up by the plant root system from the soil and transported to the stems for uptake and utilization via transpiration flow.³² Moreover, the aromaticity values of CHONS-containing compounds are higher than those of CHOS-containing compounds in SBC-DOM (Fig. 4c), which may be related to compounds containing nitrogen atoms with stronger aromaticity.³⁷ Molecules with N and S are more aromatic,¹⁶ and nitrogen-containing functional groups easily bind with Cu in the environment.⁹

Furthermore, HR-, HYC- and PYC-DOMs have the most abundant CHO- and CHON-containing compounds (Fig. 2c), which were more widely distributed in the VK diagram (Fig. 4a). According to the classification of aromatics, HYC-DOM and PYC-DOM mainly contain highly aromatic compounds, highly unsaturated compounds and unsaturated aliphatic compounds (Fig. 4b and Table S3†).

The differences in the compositions of these B-DOMs were investigated with principal component analysis (PCA). The ordination of the first two principal components (PC1 and PC2, explaining 67.4% and 21.5% of the variance, respectively) explained 88.9% of the total variance (Fig. 5). The degrees of similarity and difference among various DOMs were determined by the linear spacing among the points. There is a shorter distance between HYC-DOM and PYC-DOM, while SBC-DOM exhibits a larger distance from other B-DOMs. The PCA results show large differences in composition between SBC-DOM and other B-DOMs, as well as less differences between HYC-DOM and PYC-DOM. Parameters such as CHOS-containing compounds, CHONS-containing compounds, (N + O)/C and O/C are negatively correlated with the PCA1 axis, and have the strongest correlation with SBC-DOM (Fig. 5). These differences could be related to the molecular structure of the B-DOMs. For SBC-DOM, DBE_wa and AI_mod,wa are lower and the H/C_wa ratio is higher than that of other B-DOMs (Table 2), indicating that SBC-DOM has lower unsaturation properties and a less complex aromatic structure. To summarize, employing the constrained FT-ICR MS method has demonstrated considerable diversity in the composition of SBC-DOM in different B-DOMs.

3.3. Characteristics of oxygen- and sulfur-containing functional groups in sulfonated biochar-DOM

Further investigation is conducted to examine the relationship between the number of oxygen atoms and the DBE value in each molecular formula of DOM. The SBC-DOM consisted primarily of compounds containing 2 to 13 oxygen atoms, accompanied by DBE values ranging from 0 to 15 (Fig. S5b†). The number of oxygen atoms and DBE values of SBC-DOM are lower than those in other B-DOMs (Fig. S5†). This may be related to the fact that during the sulfonation process, sulfuric acid first degrades the biomolecules into small molecules, and then carbonates and dehydrates them, resulting in the conversion of many oxygen atoms into H₂O.⁸

The relationship between DBE_wa and the number of oxygen atoms derived from the molecular formulae of B-DOMs is shown in Fig. 6. The scatter diagram of B-DOMs showed a V-pattern distribution. The rear section of the V-pattern was analyzed by linear analysis to obtain the slope K, y-intercept and R² values.²⁰ The change in the slope of the linear equation indicates an increase or decrease in different types of O atoms in chemical molecules.³⁸ Fig. 6 shows that the number of O atoms and DBE_wa have significant correlation (R²>0.95). The slopes between DBE_wa and number of O atoms decrease sequentially from HYC- to SBC- to PYC-DOMs (Fig. 6a). Normally, the epoxy, acyl or carbonyl group containing one O atom can provide one DBE value, but the hydroxyl cannot.³⁹ In addition, two O atoms in the carboxyl group can contribute to one DBE value.³⁸ It can be speculated that the SBC-DOM contained more unsaturated carboxyl groups than PYC-DOM. This may be related to the fact that the heating with H₂SO₄ simultaneously causes oxidation of the biochar surface and creates additional carboxyl sites which significantly increase the density of oxygen functional groups (by almost ∼1.2–1.3-fold).⁵ In addition, acidic groups were detected at a higher density in SBC (Table S1†).

SBC can be defined as a new carbon material composed of polycyclic aromatic carbon and functional groups such as the –SO₃H group.⁵ In order to explore the existing form of sulfur-containing functional groups in SBC-DOM, we analyzed the relationship between the number of O atoms and DBE_wa of S₁O_x, S₂O_x, and S₃O_x compounds (Fig. 6b). The results show that when the number of O atoms is the same, the DBE_wa value decreases with the increase of the number of S atoms. Since the –SO₃H group does not provide a DBE value, this result suggests that there may be multiple –SO₃H groups in CHOS₂- and CHOS₃-containing compounds. In addition, the slopes for S₁O_x, S₂O_x, and S₃O_x are 1.02, 1.01 and 0.94, respectively. The slope approaching 1.0 indicates that the addition of one oxygen atoms resulted in an increase of ∼one DBE_wa. However, when the number of O atoms is the same, the DBE value does not decrease by 3 for each additional S atom, which indicates that in addition to the –SO₃H group, S elements in CHOS₂ and CHOS₃-containing compounds may also appear in the form of other functional groups, such as C–S, CS, C–SH, –COS, SO and so on.^40,41

3.4. Molecular lability of sulfonated biochar-DOM

The unsaturated aliphatic compounds, carbohydrates and fatty acids with a high H/C ratio are usually defined as microbially bioactive components,¹⁴ while the aromatic compounds and highly unsaturated compounds with generally low H/C ratios are defined as stable and refractory compounds.²⁶ The molecular lability of these four B-DOMs is shown in Fig. 7. The MLB_L% was ranked from high to low: SBC- > RH- > PYC- > HYC-DOM, which indicates that SBC-DOM is more labile than HYC-DOM and PYC-DOM. The high molecular lability of SBC-DOM can be explained by the high proportion of unsaturated aliphatic compounds and fatty acids and sulfonic compounds which have high bioavailability.^9,26 In addition, the organic matter with an H/C ratio >1.2 and O/C ratio >0.5 in soil was significantly positively correlated with biodegradability. Compared with the other three DOMs, the H/C and O/C ratios of SBC-DOM were the highest (Table 2), indicating that SBC-DOM has greater potential for biodegradation.

There are several reasons for the higher liability of SBC-DOM than other B-DOMs. Firstly, the preparation process of SBC is fundamentally different from other biochar. At low temperatures, concentrated sulfuric acid first hydrolyzes biomass macromolecules into small molecules and then promotes the dehydration and polymerization of small molecules into carbon.^4,8 Due to the limit of the aromatization degree at low temperatures, carbon appears to be unstable in this process. Furthermore, the proportion of lipids and aliphatic compounds in SBC-DOM fractions extracted under acidic conditions may be relatively high.⁹ Secondly, the large number of oxygen-containing groups (–SO₃H, –OH, –COOH, etc.) on the surface enhances the hydrophilicity of the SBC, which is much higher than that of the traditional PYC and HYC. This means that water molecules can penetrate deeper into the SBC during the extraction process,⁴² resulting in a more complete extraction of B-DOM. In addition, it has been noted that most of the unstable B-DOM is bonded to the biochar backbone through π–π bonding and intermolecular entanglement.^13,14 However, the SBC surface has many polar functional groups which can enhance the interaction between labile compounds and water molecules, thus making them easier to extract.

Sulfur is commonly used as a soil modifier to mobilize other chemical elements, such as heavy metals, or as a fertilizer in agricultural soils.^32,43 Compared with other DOMs, SBC-DOM can provide more S-element content (Table 2 and Fig. 2c). Sulfur-containing organic molecules have high photochemical instability, and some studies have found that CHOS-containing compounds have faster degradation kinetics than CHO-containing compounds.^44,45

The CRAM% is often related to refractory compounds.²⁷ The SBC-DOM is lower than that of RH-DOM (Fig. 7), which may be due to the hydrolyzation of highly unsaturated compounds (i.e. cellulose, hemicellulose and lignin) in the rice husk into unsaturated aliphatic compounds.⁸ HYC-DOM and PYC-DOM contained a large amount of CRAM (69.67% and 66.74%), indicating that it included a large number of refractory substances. Although approximately 1300 more compounds are derived with HYC-DOM and PYC-DOM than with SBC-DOM, these compounds are mainly polycyclic aromatic and highly aromatic compounds which have high chemical recalcitrance.⁴⁶ The main types of compounds in the unique molecules of SBC-DOM were highly unsaturated compounds and unsaturated aliphatic compounds (Fig. S6†). Therefore, from the bioavailability point of view, SBC-DOM is easily degraded by microorganisms and has less adverse impact on the environment than HYC-DOM and PYC-DOM.

4 Conclusions

In this study, we compared the molecular characteristics of B-DOMs from sulfonated biochar, hydrochar and pyrochar. We found that SBC-DOM has a lower soluble organic carbon concentration. The FT-ICR MS results show that the CHOS- and CHONS-containing compound formulae are predominant in SBC-DOM, while B-DOMs from rice husk, pyrochar, and hydrochar are dominated by CHO-containing compound formulae. Compared to the other three types of DOMs, SBC-DOM has lower values of AI_mod,wa, DBE_wa, and m/z_wa, while higher values of O/C_wa and H/C_wa, and higher proportion of unsaturated aliphatic compounds and lipid compounds. Meanwhile, the biodegradability (MLB_L%) of SBC-DOM is significantly higher than that of other types of B-DOMs, while the relative content of CRAM is lower. Compared to other B-DOMs, SBC-DOM has a stronger polarity, and lower the degrees of aromaticity and unsaturation, which makes it more susceptible to microbial degradation. The results of this study will help us to understand the composition of sulfonated biochar-DOM and help to assess the potential of sulfonated biochar for soil remediation and improvement. However, further studies are needed to investigate the dynamic and qualitative changes of sulfonated biochar-DOM in different water and soil systems, as well as its impact on the environment and the fate of organic and inorganic pollutants.

Data availability

The data supporting this article have been included as part of the ESI.†

Author contributions

Zhengfeng Jiang: investigation, writing – original draft, writing – review & editing; Chen He: data processing, visualization, writing – review & editing; Fei Gao: investigation, supervision; Quan Shi: conceptualization, methodology, software, supervision, writing – review & editing; Yang Chen: methodology, data analysis; Haimeng Yu: conceptualization, methodology; Zhimao Zhou: resources, validation; Ruoxin Wang: conceptualization, supervision.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Science Foundation of China University of Petroleum, Beijing (No. 2462023YJRC003).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4em00233d

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