Peroxymonosulfate Activation by Rice-Husk-Derived Biochar (RBC) for the Degradation of Sulfamethoxazole: The Key Role of Hydroxyl Groups

Abstract

In this work, rice-husk-derived biochar (RBC) was synthesized by using simple one-step pyrolysis strategies and served as catalysts to activate peroxymonosulfate (PMS) for degrading sulfamethoxazole (SMX). When the annealing temperature (T) = 800 °C, RBC₈₀₀ exhibits the typical hardwood structure with several micropores and mesoporous. Furthermore, RBC₈₀₀ obtains more defect sites than RBC₆₀₀, RBC₇₀₀, and RBC₉₀₀. In the RBC₈₀₀/PMS system, the removal rate of the SMX reached 92.0% under optimal conditions. The kinetic reaction rate constant (k_obs) of the RBC₈₀₀/PMS system was 0.009 min⁻¹, which was about 1.50, 1.28, and 4.50 times that of the RBC₆₀₀/PMS (k_obs = 0.006 min⁻¹), RBC₇₀₀/PMS (k_obs = 0.007 min⁻¹), and RBC₉₀₀/PMS (k_obs = 0.002 min⁻¹) systems, respectively. In the RBC₈₀₀/PMS system, sulfate radical (SO₄^•−) is the main active species. Compared with other active sites, the hydroxyl group (C-OH) on the surface of RBC₈₀₀ interacts more strongly with PMS, which is more likely to promote the stretching of the O-O bond of the PMS, thus breaking into the activated state and significantly reducing the activation energy required for reaction. The degradation intermediates of SMX were speculated, and the toxicity analysis was conducted. Generally, this work reveals in depth the interaction between reactive sites of biochar-based catalysts and PMS at the molecular level.

1. Introduction

Recently, pharmaceuticals and personal care products (PPCPs) have been widely adopted to treat a variety of human and animal diseases [1]. Among them, sulfamethoxazole (SMX) as a typical PPCPs can effectively prevent coccidiosis, diarrhea, gastroenteritis, and other bacterial diseases [2]. SMX is first consumed by humans or animals and enters the wastewater treatment system with excreta. However, most conventional treatment processes cannot degrade SMX effectively, so residual SMX enters the natural water environment with treated municipal wastewater [3]. Residual SMX in aquatic environments may increase the resistance of pathogenic microorganisms, causing negative effects on aquatic life [4]. Therefore, it is important to develop an effective method for eliminating residual SMX in the aquatic environment.

The hydroxyl radical-based advanced oxidation processes (HR-AOPs) are the effective method for degrading emerging contaminants, and their mechanisms are largely dependent on the hydroxyl radical (^•OH) [5]. However, HR-AOPs also have many shortcomings, such as the difficulty of transporting and storing hydrogen peroxide (H₂O₂), a narrow pH application range, and higher energy consumption [6]. Recently, persulfate-based advanced oxidation processes (PS-AOPs) have been favored by researchers due to higher oxidizability and wider applicability under the environments (pH = 2.0–8.0) [5]. There are lots of methods for peroxydisulfate (PDS) and peroxymonosulfate (PMS) activation to generate reactive oxygen species (ROSs), such as ultraviolet irradiation, heat, and transition metals [7]. However, these activation methods often have problems such as extensive energy consumption, high cost, metal leaching, etc., which limits their application prospects in actual wastewater [8].

Biochar (BC) has become a research hotspot recently [9]. Biomass feedstock, such as sludge, corncob, wood chips and shrimp shell, is rich and cheap [10]. Besides, BC has abundant functional groups as well as well-developed porous structure, which obtains significant advantages for the synthesis of carbonaceous catalysts [11]. Qi et al. reported that the Enteromorpha based graphene-like biochar (EGB) was synthesized as a PDS activator for the SMX removal [12]. Liu et al. reported that boron-doped graphitic porous biochar (B-KBC) was synthesized and used as catalysts to activate PDS for removing SMX [13]. Besides, Zhao et al. systematically reported the research progress of the PMS activation by biochar-based catalysts [11]. Although some studies have been carried out on the degradation of micropollutants via activating PS by pristine BC, the interaction mechanism between surface active sites of BC and PS is still unclear. In addition, the pyrolysis temperature (T) largely determines the structure and physicochemical properties of catalysts, while the specific surface area (SSAs), micropore structure and graphitization degree of BC will affect the removal rate of organic micropollutants in PS-AOPs [11]. Herein, the structure-activity relationship between the physicochemical properties of catalysts and the catalytic activity needs to be further revealed.

Herein, raw biochar (RBC_T) derived from rice husk was prepared as a PMS activator for the elimination of SMX. The difference of physicochemical properties of RBC_T at different pyrolysis temperatures was investigated, and the structure-activity relationship between physicochemical properties and catalytic activity was established. Then, the ROSs in the RBC₈₀₀/PMS system were identified. The degradation pathway of SMX was predicted, and the interaction mechanism between active sites and PMS was revealed by density functional theory (DFT) calculation. The study proposes a deep understanding of PMS activation mechanism and demonstrates great potential of the biochar-based catalysts toward sewage treatment.

2. Results and Discussion

2.1. Characterization of RBC₈₀₀

The morphology of RBC_T were observed by scanning electron microscopy (SEM). As displayed in Figure 1a–d, RBC₆₀₀ shows the clumpy structure and incomplete porous structure. When the pyrolysis temperature is 700 °C, RBC₇₀₀ shows a plate structure, and the pore structure has initially developed. RBC₈₀₀ derived from rice husks exhibits a typical hardwood structure, with lots of micropores and mesoporous. When the pyrolysis temperature is 900 °C, the carbon network collapses. It can be seen that the porous structure of RBC is relatively developed when the pyrolysis temperature is 800 °C, which is conducive to the catalytic oxidation process. However, excessive pyrolysis temperature (T ≥ 900 °C) may destroy the well-developed porous structure.

As depicted in Figure 1e,f, energy-dispersive X-ray spectroscopy (EDS) analyzed the elemental content of RBC₈₀₀, in which C content accounted for 65.3%, O content accounted for 24.0%, Si content accounted for 5.5%, N content accounted for 0.2% and Fe content accounted for 5.0%. Besides, Fe element is uniformly distributed on RBC₈₀₀ surface. These results indicate that rice husk-derived RBC₈₀₀ naturally contains a small amount of Fe element without additional iron source.

Furthermore, the ultrastructure of crystal lattice of RBC₈₀₀ was observed in the high-resolution transmission electron microscope (HRTEM) and selected area electron diffraction (SAED) pattern, as depicted in Figure 2a,b. RBC₈₀₀ is composed of graphitized (sp²C) and disordered carbon (sp³C) structures, and the degree of graphitization and disordered of the catalyst need to be further determined by Raman spectroscopy. The Miller indices of (111) and (220) is consistent with the X-ray diffraction (XRD) results.

XRD patterns of RBC_T are depicted in Figure 3a. Obviously, the pronounced diffraction peak at 21.60° was attributed to the (111) plane of SiO₂ [14]. Two other weak peaks at 35.6° and 56.2°, corresponding to the (220) and (331) planes, agree well with the crystalline phase of calcite (JCPDS 27-0605).

FT-IR spectra show the functional groups of RBC_T (Figure 3b). The peaks at 1250 cm⁻¹, 1622 cm⁻¹, and 3419 cm⁻¹ are respectively attributed to the stretching vibration of C-O, C=O and -OH groups [15]. Xin et al. reported that the content of functional groups can be qualitatively judged by the absorption peak intensity [6]. These results show that the annealing temperature has a crucial effect on the formation of oxygen-containing functional groups. The content of C=O and -OH groups on RBC₈₀₀ surface is significantly higher than that on RBC₆₀₀, RBC₇₀₀ and RBC₉₀₀, while the C=O and -OH groups on the surface of catalyst can promote electron transfer processes and activate PMS to generate ROSs [16].

N₂ adsorption-desorption isotherms (Figure 3c) indicate the specific surface areas (SSAs) of RBC₈₀₀ (194.86 m²⋅g⁻¹) are larger than that of RBC₆₀₀ (65.00 m²⋅g⁻¹), RBC₇₀₀ (124.11 m²⋅g⁻¹) and RBC₉₀₀ (102.71 m²⋅g⁻¹) (Table S1). The appearance of both D-band and G-band of RBC_T reveals the co-presence of disordered and crystalline graphite structures [17] (Figure 3d). Furthermore, the ratio of I_D/I_G is the key parameter to indicate the defective degree of RBC_T [18,19]. The I_D/I_G value was obtained by calculating the intensity of ration of D peak to G peak [13]. The I_D/I_G is 0.67, 0.98, 1.07, and 0.88 for RBC₆₀₀, RBC₇₀₀, RBC₈₀₀, and RBC₉₀₀, respectively. The result demonstrates that RBC₈₀₀ has obtained abundant defects (vacancy and edge defects) during pyrolysis, which is conducive to catalytic oxidation [20].

2.2. Catalytic Oxidation of SMX

The adsorption process accords with Langmuir model, suggesting the monolayer adsorption of SMX on RBC₈₀₀ surface (Figure S1). In the pure RBC_T system, 23.0%, 27.7%, 32.0%, and 29.0% of SMX could be adsorbed onto RBC₆₀₀, RBC₇₀₀, RBC₈₀₀, and RBC₉₀₀ within 200 min, respectively (Figure 4a). RBC₈₀₀ has higher SMX adsorption capacities, due to its larger SSAs and developed porous structure [21].

In the RBC800/PMS system, the degradation efficiency of SMX was 67.3% within 40 min, indicating that BC can activate PMS to a certain extent [22]. In the presence of PMS, the removal rate of RBC₆₀₀, RBC₇₀₀, RBC₈₀₀, and RBC₉₀₀ within 200 min was 77.0%, 80.0%, 92.0%, and 79.0%, respectively (Figure 4b). The removal process of SMX followed a first-order kinetics behavior, and k_obs is the reaction rate constant. The k_obs of RBC₈₀₀/PMS system was 0.009 min⁻¹, being about 1.50, 1.28, and 4.50 times that of RBC₆₀₀/PMS, RBC₇₀₀/PMS, and RBC₉₀₀/PMS systems, respectively (Table S2).

It indicated that the annealing temperature (T) can affect the catalytic performances of catalysts. Within a certain range (T < 900 °C), the increase of pyrolysis temperature is conducive to the development of micropores and promotes the conversion of the organic phase with poor crystallinity into graphitic carbon structure, thus enhancing the catalytic performance of catalysts [23]. However, when the pyrolysis temperature is too high (T = 900 °C), the collapse of the carbon skeleton causes a partial loss of the defects, leading to a decrease in the catalytic activity of RBC_T [21]. The PPCPs degradation rates in different systems were studied in Table S3. Besides, the effect of various factors on SMX degra-dation were investigated (Figure S2). The point of zero charge (pH_pzc) of RBC₈₀₀ was displayed in Figure S3. To explore the catalytic performance of RBC₈₀₀ in depth, the residual PMS concentration was detected by the ABTs colorimetric method. The decomposition rate of PMS in the RBC₈₀₀/PMS system was 74.9% within 200 min (Figure S4). It was suggested that the favorable degradation rate of SMX in the RBC₈₀₀/PMS system could be due to the rapid decomposition of PMS.

Co-existing ions can affect the SMX degradation by interacting with ROSs [24]. As shown in Figure S5, typical anions (HCO₃⁻, Cl⁻, H₂PO₄⁻) and humic acid (HA) may restrain the SMX elimination. When 5.0 mM and 10.0 mM Cl⁻ were present in the RBC₈₀₀/PMS system, SMX elimination rate decreased from 92.0% to 73.0% and 58.0%, respectively, within 200 min. The k_obs decreased from 0.009 min⁻¹ to 0.003 min⁻¹ and 0.002 min⁻¹, which was due to the generation of Cl^•, Cl₂, and HOCl (Equations (1)–(5)) [25]. When increasing the H₂PO₄⁻ concentration to 5.0 mM and 10.0 mM, the SMX removal rate decreased to 70.0% and 57.7%, respectively. Similarly, when HCO₃⁻ concentration was increased from 0 to 5.0 mM and 10.0 mM, the SMX elimination rate decreased from 92.0% to 66.3% and 60.0%, respectively. These results reveal that H₂PO₄⁻ and HCO₃⁻ possess a quenching effect on reactive radicals (Equations (6)–(9)) [26]. When 5.0 mg⋅L⁻¹ and 10.0 mg⋅L⁻¹ of HA was added into the RBC₈₀₀/PMS system, k_obs decreased from 0.009 min⁻¹ to 0.003 min⁻¹ and 0.002 min⁻¹, respectively. The π-π stacking effect of HA can lead to competing adsorption between HA and PMS, thus hinder the production of active species [27].

$$\mathrm{C}{\mathrm{l}}^{-}+\mathrm{S}{\mathrm{O}}_4^{·-}\to \mathrm{S}{\mathrm{O}}_4^{2-}+{\mathrm{C}\mathrm{l}}^{\cdot }$$

(1)

$$\mathrm{H}\mathrm{S}{\mathrm{O}}_5^{-}+\mathrm{C}{\mathrm{l}}^{-}\to \mathrm{S}{\mathrm{O}}_4^{2-}+\mathrm{H}\mathrm{O}\mathrm{C}\mathrm{l}$$

(2)

$$\mathrm{H}\mathrm{S}{\mathrm{O}}_5^{-}+2\mathrm{C}{\mathrm{l}}^{-}+{\mathrm{H}}^{+}\to \mathrm{S}{\mathrm{O}}_4^{2-}+{\mathrm{C}\mathrm{l}}_2+{\mathrm{H}}_2\mathrm{O}$$

(3)

$$\mathrm{C}{\mathrm{l}}^{-}+·\mathrm{O}\mathrm{H}\to \mathrm{H}\mathrm{O}\mathrm{C}{\mathrm{l}}^{·-}$$

(4)

$$\mathrm{H}\mathrm{O}\mathrm{C}{\mathrm{l}}^{·-}+{\mathrm{H}}^{+}\to {\mathrm{C}\mathrm{l}}^{\cdot }+{\mathrm{H}}_2\mathrm{O}$$

(5)

$$\mathrm{O}\mathrm{H}+{\mathrm{H}}_2\mathrm{P}{\mathrm{O}}_4^{-}\to {\mathrm{H}}_2\mathrm{P}{\mathrm{O}}_4^{·}+\mathrm{O}{\mathrm{H}}^{-}$$

(6)

$$\mathrm{S}{\mathrm{O}}_4^{·-}+{\mathrm{H}}_2\mathrm{P}{\mathrm{O}}_4^{-}\to {\mathrm{H}}_2\mathrm{P}{\mathrm{O}}_4^{·}+\mathrm{S}{\mathrm{O}}_4^{2-}$$

(7)

$$\mathrm{H}\mathrm{C}{\mathrm{O}}_3^{-}{+}^{·}\mathrm{O}\mathrm{H}\to {\mathrm{H}}_2\mathrm{O}+\mathrm{C}{\mathrm{O}}_3^{2-}$$

(8)

$$\mathrm{H}\mathrm{C}{\mathrm{O}}_3^{-}+\mathrm{S}{\mathrm{O}}_4^{·-}\to \mathrm{S}{\mathrm{O}}_4^{2-}+\mathrm{H}\mathrm{C}{\mathrm{O}}_3^{·}$$

(9)

To study the universal applicability of RBC₈₀₀ in real waterbody, more tests were conducted in various water matrices. The characteristics of different water matrices were listed in Table S4. The removal rate of SMX in tap water (79.0%) was lower than that obtained for deionized water (92.0%) (Figure S6a). Besides, the elimination rate decreased in river water, which might be due to the presence of certain level of ions in the river water [28].

To explore the reusability of RBC₈₀₀, five consecutive degradation tests were performed (Figure S6b). After four cycles, the elimination rate of SMX within 200 min decreased from 92.0% to 66.3%. This could be due to the sedimentation of intermediates on the surface of RBC₈₀₀, which occupied the defective sites for activating PMS [15]. To recover the porous structure of passivated RBC₈₀₀, a thermal treatment (annealing at 450 °C under N₂ flow for 2 h) was applied. The result showed that the catalytic activity was partially recovered with 74.0% within 200 min in the RBC₈₀₀/PMS system.

To further explore the wide applicability of RBC₈₀₀, the degradation experiments of different pollutants were carried out (Figure S6c). In the RBC₈₀₀/PMS system, the degradation rates of CEX, CIP, CMP and TCS within 200 min were 87.3%, 91.1%, 90.2% and 89.6%, respectively, and the mineralization rates were 67.4%, 72.3%, 73.7% and 76.1%, respectively. These results showed that the admirable universality of RBC₈₀₀ as an efficient PMS activator to remove a broad array of typical PPCPs. As displayed in Figure S6d, the RBC₈₀₀/PMS system had a satisfactory mineralization performance on SMX with a TOC elimination rate of 79.0% within 200 min.

2.3. Mechanism Discussion

2.3.1. Identification of ROSs

Quenching tests were conducted to first to determine the dominant ROSs responsible for SMX degradation process [13,29]. Methanol (MeOH) was regarded as a scavenger of ^•OH ((1.6–7.7) × 10⁷ M⁻¹s⁻¹) and SO₄^•− ((1.2–2.8) × 10⁷ M⁻¹s⁻¹) [30]. In contrast, Tert-butanol (TBA) was used as a scavenger to quench ^•OH (6.0 × 10⁸ M⁻¹s⁻¹) [31]. p-benzoquinone (p-BQ) was used to inhibit O₂^•− (9.6 × 10⁸ M⁻¹s⁻¹) [24]. The SMX elimination rates decreased from 92.0% to 37.0%, 70.0%, 84.0%, and 52.0% within 200 min after adding MeOH (0.5 M), TBA (0.5 M), p-BQ (20.0 mM), and FFA (0.5 M), respectively (Figure 5). The results showed that MeOH had an obvious inhibitory effect on SMX degradation in the RBC₈₀₀/PMS system, while TBA had a mild inhibitory effect on SMX degradation. Therefore, SO₄^•− may be the main ROSs, leading the degradation process of SMX. In addition, O₂^•− had little effect on the degradation of SMX, while ^•OH and ¹O₂ were also involved in the degradation of SMX.

ESR was used to monitor the presence of main active species in the RBC₈₀₀/PMS system using 2,2,6,6-tetramethyl-4-piperidinol (TEMP) and 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as spin-trapping agents [32]. When RBC₈₀₀, PMS and TEMP were added to the system, a typical triplet signal with the intensity ratio of 1:1:1 verified the existence of ¹O₂. The self-decomposition of PMS could produce a small amount of ¹O₂ [33] (Figure 6a). After adding SMX to the RBC₈₀₀/PMS system, the peak intensity of TEMP-¹O₂ was significantly weakened, which indicated that ¹O₂ played a role in the degradation process of SMX. A spectrum with seven main peaks belonging to DMPO-X was observed in the RBC₈₀₀/PMS system, suggesting that RBC₈₀₀ activated PMS to produce SO₄^•− and ^•OH (Figure 6b). After adding SMX to the RBC₈₀₀/PMS system, DMPO-^•OH and DMPO-SO₄^•− signals were significantly weakened, which indicated that ^•OH and SO₄^•− played a vital role in the degradation process of SMX. According to the quenching experiment and ESR test results, ¹O₂, SO₄^•− and ^•OH were produced in the RBC₈₀₀/PMS system, and SO₄^•− played a dominant role in the removal of SMX.

2.3.2. Reaction Mechanism

PMS activation mechanism was further investigated in this work. First, ^•OH and SO₄^•− generated from the destruction of the O-O bond of the PMS by free-flowing π electrons on the sp² hybrid carbon of the RBC₈₀₀ (Equations (10) and (11)). Secondly, ¹O₂ could be produced by the self-decomposition of PMS (Equations (12) and (13)) [33]. According to FT-IR and XPS results, the surface of RBC₈₀₀ contained C-OH, COOH, C=O and other oxygen-containing functional groups, which could be used as the active center of RBC₈₀₀. Two characteristic peaks attributed to C 1s and O 1s were observed on the full XPS spectra with binding energies at 285.1 eV and 531.1 eV, respectively (Figure 7a). The O1s XPS spectra can be decomposed into three components (Figure 7b). The peak at 531.8 eV and 533.3 eV could be assigned to C=O and COOH, respectively. A peak centered at 534.1 eV corresponds to C-OH. After the reaction, the content of COOH decreased from 43.7% to 34.2%, suggesting that COOH, as the active site in the catalytic reaction, activated PMS to produce SO₄^•−. In addition, C-OH on the surface of RBC₈₀₀ can also activate PMS to produce SO₄^•−, and the conversion between SO₄^•− and ^•OH can be flexible (Equations (14)–(17)) [16]. Due to electrostatic attraction and the interaction between electron donors and acceptors, the adsorption of RBC₈₀₀ facilitated the uniform distribution of SMX molecules on the surface of the carbon matrix, promoting contact with ROSs [34].

$$\mathsf{\pi }–\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{n}\mathrm{s}+\mathrm{H}\mathrm{S}{\mathrm{O}}_5^{-}\to \mathrm{S}{\mathrm{O}}_4^{·-}+\mathrm{O}{\mathrm{H}}^{-}$$

(10)

$$\mathsf{\pi }–\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{n}\mathrm{s}+\mathrm{H}\mathrm{S}{\mathrm{O}}_5^{-}{\to }^{·}\mathrm{O}\mathrm{H}+\mathrm{S}{\mathrm{O}}_4^{2-}$$

(11)

$$\mathrm{H}\mathrm{S}{\mathrm{O}}_5^{-}\to \mathrm{S}{\mathrm{O}}_5^{2-}+{\mathrm{H}}^{+}$$

(12)

$$\mathrm{H}\mathrm{S}{\mathrm{O}}_5^{-}+\mathrm{S}{\mathrm{O}}_5^{2-}\to \mathrm{S}{\mathrm{O}}_4^{2-}+\mathrm{H}\mathrm{S}{\mathrm{O}}_4^{-}+1{\mathrm{O}}_2$$

(13)

$$\mathrm{B}\mathrm{C}–\mathrm{O}\mathrm{O}\mathrm{H}+\mathrm{H}\mathrm{S}{\mathrm{O}}_5^{-}\to \mathrm{S}{\mathrm{O}}_4^{·-}+\mathrm{B}\mathrm{C}–\mathrm{O}{\mathrm{O}}^{\cdot }+{\mathrm{H}}_2\mathrm{O}$$

(14)

$$\mathrm{B}\mathrm{C}–\mathrm{O}\mathrm{H}+\mathrm{H}\mathrm{S}{\mathrm{O}}_5^{-}\to \mathrm{S}{\mathrm{O}}_4^{·-}+\mathrm{B}\mathrm{C}–{\mathrm{O}}^{\cdot }+{\mathrm{H}}_2\mathrm{O}$$

(15)

$$\mathrm{S}{\mathrm{O}}_4^{·-}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}^{+}+\mathrm{S}{\mathrm{O}}_4^{2-}{+}^{·}\mathrm{O}\mathrm{H}$$

(16)

$${}^{·}\mathrm{O}\mathrm{H}/\mathrm{S}{\mathrm{O}}_4^{·-}/1{\mathrm{O}}_2+\mathrm{S}\mathrm{M}\mathrm{X}\to \mathrm{i}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{m}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{s}\to \mathrm{d}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{d}\mathrm{e}\mathrm{d} \mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{s}+\mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}$$

(17)

Based on the above analysis results, sp² hybrid carbon on the RBC₈₀₀ surface and oxygen-containing functional groups such as COOH, C-OH and C=O could be used as the active sites, but the interaction mechanism between PMS and these active sites has not been deeply revealed. Therefore, density functional theory (DFT) calculations were adopted to further explore the activation mechanism of PMS on the surface of RBC₈₀₀.

The adsorption processes of PMS on sp² hybrid carbon network (C/PMS), carbon network edge containing COOH functional group (COOH/PMS), carbon network edge containing C-OH functional group (C-OH/PMS) and carbon network edge containing C=O functional group (C=O/PMS) were studied. The top and side views of all the optimized configurations are shown in Figure 8. The shortest distance (D) between PMS and sp² hybrid carbon network, COOH, C-OH and C=O were 2.18 Å, 1.46 Å, 1.43 Å and 2.01 Å, respectively (Table S5). Compared with other configurations, the C-OH/PMS configuration has the smallest D value, suggesting that there may be a strong interaction between C-OH and PMS.

The essence of PMS activation is the breaking of O-O bond. The O-O bond length (l_o-o) of PMS at sp² hybrid carbon network, COOH, C-OH and C=O is 1.454 Å, 1.464 Å, 1.468 Å and 1.458 Å, respectively. These results indicate that oxygen-containing functional groups can act as reactive sites and promote the stretching of O-O bond of PMS. The l_o-o of PMS is the longest at C-OH, suggesting that the C-OH functional group could largely promote the stretching of the O-O bond of PMS, thus breaking into the activated state.

The energy barrier required for the reaction is calculated by optimizing the reactants and transition states. The energy barriers corresponding to the C/PMS, COOH/PMS, C-OH/PMS and C=O/PMS configurations are 35.16 kcal/mol, 31.93 kcal/mol, 28.12 kcal/mol and 32.28 kcal/mol, respectively. Obviously, the transition state energy barrier corresponding to the C-OH/PMS configuration is low, indicating that the C-OH functional group can significantly reduce the activation energy required for the reaction, and the catalytic activity of this active site is high. Based on the above discussion, Figure 9 displays the degradation mechanism of SMX in the RBC₈₀₀/PMS system.

2.4. Degradation Pathways of SMX

The intermediates of SMX degradation were investigated by UPLC-TOF/MS (Figures S7 and S8 and Table S6). In pathway I, under the attack of ^•OH, electrophilic addition reaction occurred on the C atom of the isoxazole ring of SMX to produce product I (m/z = 288) [2]. In addition, SO₄^•− may also attack the olefin double bond on the isoxazole ring of the SMX molecule and form product I [35]. Subsequently, ^•OH further attacks the S-N bond, transforming product I into product II (m/z = 133) and product III (m/z = 190). Surface charge distribution of SMX manifests that negatively charge regions are concentrated around the O and N atoms (Figure S9). Via the attack of ROSs, the N-O bond on the isoxazole ring breaks to produce product IV (m/z = 117), which is eventually mineralized into CO₂ and H₂O. In pathway II, under the attack of ^•OH, SMX molecules first undergo nitration reaction to produce product VI (m/z = 284), then break S-C bond on SMX molecules and undergo hydroxylation reaction to produce product VII (m/z = 143), and then undergo benzene ring opening reaction and transform into small molecular organic matter, and eventually mineralized into CO₂ and H₂O.

2.5. Toxicity Assessment

The bioaccumulation factor and developmental toxicity of SMX as well as its intermediate products were estimated by Toxicity Estimation Software Tool (T.E.S.T) (5.1.1.0). T.E.S.T has a simple interface and easy operation, and the software is equipped with a guide, which is easy to use. Users can draw and load the structure of chemical substances by CAS number, SMILES code, substance name, InChi code, DTXSID or manually, then select the prediction end point and method, and change the output path of the result. The software automatically generates a result report after the prediction. The bioaccumulation factor of SMX was 17.11 mg/L. The bioaccumulation factor of P(I), P(II), P(III), P(IV), P(V), P(VI), P(VII) and P(VIII) were 0.67, 0.59, 1.89, 0.44, 0.49, 17.06, 7.04 and 1.34 mg/L, respectively (Figure 10a). The developmental toxicity of SMX was 0.85 mg/L. The developmental toxicity of P(I), P(II), P(III), P(IV), P(V), P(VI), P(VII) and P(VIII) were 0.79, 0.54, 0.60, 0.72, 0.66, 0.78, 0.57 and 0.50 mg/L, respectively (Figure 10b). The bioaccumulation factor and developmental toxicity of all intermediates was lower than that of SMX. According to the results of toxicity analysis, the comprehensive environmental risk of SMX was decreased in the RBC₈₀₀/PMS system.

3. Materials and Methods

3.1. Preparation of Catalysts

Firstly, Rice husk was washed, oven-dried at 100 °C for 12 h, ground, and then passed through a 100-mesh sieve to acquire thin powders for further use. BC was prepared by pyrolysis at a constant calcination temperature under N₂ atmosphere (5 °C/min of heating rate) for 2 h. The resulting powers are further ground, sieved and then stored in a ziplock bag. To further remove ash, soluble salts and other impurities from biochar, and increase the content of acidic functional groups on the BC surface, 100.0 g of the collected sample was added to 300.0 mL of 0.1 M hydrochloric acid (HCl, 36.0–38.0%) solution. The mixture was stirred for 24 h, then filtered, and the biochar is washed with ultra-pure water for several times to stabilize its pH to 6.0. The biochar was dried at 100–105 °C for 12 h after cleaning, and the resulting composities were denoted as RBC_T (T = 600, 700, 800, 900 °C).

3.2. Reaction Procedures

Degradation tests were conducted in 50 mL vials to investigate the catalytic activity of RBC_T. The range of catalyst dosing was 0.2–1.0 g/L, the pH was 3.0–11.0, the PMS concentration was 0.2–1.0 mM, and the reaction temperature was set to 25 °C. The batch experiments were carried out at 150 rpm, and 1.0 mL reaction solution was taken periodically and filtered through a 0.22 μm filter. The redundant reaction was restrained by adding 1 mL Na₂S₂O₃ (0.2 M). All tests were conducted in triplicate, and the final values were averaged.

4. Conclusions

Raw biochar (RBC_T) was prepared by single one-step pyrolysis strategies using green and cheap rice husk biomass as precursor. The structure-activity relationship between physicochemical properties of RBC_T and catalytic activity is discussed in detail. The pyrolysis temperature (T) has a crucial influence on the morphology and structural characteristics of RBC_T, which determines the catalytic performance of RBC_T. When T < 800 °C, the porous structure of RBC_T is not fully developed, and when T > 800 °C, the carbon skeleton of RBC_T collapses. When T = 800 °C, RBC₈₀₀ exhibits a typical hardwood structure with micropores and mesoporous. In addition, RBC₈₀₀ obtains more defect sites than RBC₆₀₀, RBC₇₀₀, and RBC₉₀₀. Therefore, under optimal conditions, the elimination rate of SMX in the RBC₈₀₀/PMS system within 200 min was 92.0%, k_obs is 0.009 min⁻¹, which is 1.8, 1.6 and 1.5 times that of the RBC₆₀₀/PMS, RBC₇₀₀/PMS and RBC₉₀₀/PMS systems. Compared with similar reports, rice husk-derived biochar has obtained higher degradation performance [36]. Radical quenching, ESR and XPS analysis manifested that the main ROSs in the RBC₈₀₀/PMS system were measured to be SO₄^•−. According to the DFT calculation results, compared with other active sites, the C-OH functional group on the surface of RBC₈₀₀ interacts more strongly with PMS, which is more likely to promote the stretching of the O-O bond of PMS, thus breaking into the activated state and significantly reducing the activation energy required for reaction. This work reveals in depth the interaction mechanism between active sites of biochar-based catalysts and PMS at the molecular level. Besides, the structure-activity relationship between the physicochemical properties of catalysts and the catalytic activity were revealed.