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Article

Degradation of Tetracycline in Water by Fe-Modified Sterculia Foetida Biochar Activated Peroxodisulfate

by
Yuchen Zhang
1,
Xigai Jia
1,
Ziyang Kang
1,
Xiaoxuan Kang
1,
Ming Ge
1,
Dongbin Zhang
2,
Jilun Wei
2,
Chongqing Wang
3,* and
Zhangxing He
1,4,*
1
College of Chemical Engineering, North China University of Science and Technology, Tangshan 063210, China
2
Tangshan Sanyou Group Xingda Chemical Fiber Co., Ltd., Tangshan 063305, China
3
School of Chemical Engineering, Zhengzhou University, Zhengzhou 450001, China
4
Tangshan Sanyou Group Co., Ltd., Tangshan 063305, China
*
Authors to whom correspondence should be addressed.
Sustainability 2022, 14(19), 12097; https://doi.org/10.3390/su141912097
Submission received: 21 August 2022 / Revised: 15 September 2022 / Accepted: 20 September 2022 / Published: 24 September 2022
(This article belongs to the Special Issue The Effect of Recycling on Sustainability of Solid Waste Management)
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Abstract

:
Tetracycline (TC) is a broad-spectrum antibiotic commonly, made use of in aquaculture and animal husbandry. After entering water bodies, it will represent a major threat to human health. In this study, sterculia foetida biochar (SFC) was readied by the combined hydrothermal pyrolysis (co-HTP) method with sterculia foetida as raw materials. Fen-SFC (Fe2-SFC, Fe3-SFC, and Fe4-SFC) was obtained by doping SFH with different concentrations of FeCl3. Finally, activation of peroxodisulfate (PDS) was achieved, using Fe3-SFC to degrade TC. The degradation of TC obeyed pseudo-second-order kinetics, and the constant of the reaction rate was 0.491 L mg−1 min−1. Radical trapping experiments, EPR test and electrochemical tests evidenced that the high catalytic performance of the Fe3-SFC/PDS system was ascribed to free radical pathway (•OH and SO4•−) and non-radical pathway (1O2 and electron transfer), in which the latter plays a dominant role. This research not only demonstrates a new kind of biochar as an effective catalyst for PS activation, but also offers an avenue for the value-added reuse of sterculia foetida.

1. Introduction

Tetracycline (TC) as a broad-spectrum antibiotic has been diffusely made use of in clinical medicine and animal husbandry, and TC is usually discharged into the environment through medical wastewater and domestic wastewater [1,2,3]. Due to the poor biodegradability of TC, it may accumulate in the environment for a long time [4]. Many studies have indicated that after entering the aquatic environment [5], TC can cause the production of antibiotic resistance genes, boost bacterial resistance and cause ecotoxicity [6,7]. This will endanger human health and ecosystem stability [8]. As a result, an effective technique to degradation of TC in the water bodies is required [9,10].
Advanced oxidation processes (AOP) have shown great efficiency and almost non-selectivity in treating pollutants, when compared to traditional approaches [11,12]. A valid method for degrading organic contaminants in wastewater is the persulfate (PS)-based advanced oxidation process (AOP) [13,14]. Peroxymonosulfate (PMS) and peroxodisulfate (PDS) are two PS oxidants [15]. However, PS alone has a low degradation efficiency for antibiotics [16]. Various methods [17], such as photocatalytic [18], heating [19], UV light [20] andtransition metals [19], can generate reactive oxygen species (ROS) by activating PS, but these methods suffer from energy requirements, metal leaching, and other disadvantages. Research shows PS can be activated by carbon-based materials [21] such as carbon nanotubes (CNT) [22], activated carbon (AC) [23], biochar (BC) [24], and reduced graphene oxide (r-GO) [25], etc. Due to the advantages of abundant raw materials, low cost [26], rich mesoporous structure [27], and surface functional groups [28], BC produced from biochar waste has increasingly become a current research hotspot [23,29,30].
Modified BC doped with transition metals and heteroatoms is often utilized to prepare catalysts to improve the catalytic activity of BC [10,31,32]. Given Fe-based catalysts’ high PS activation efficiency, combining biochar with iron-based materials to improve PS activation efficiency has received extensive attention [29]. As an activator, ferric chloride (FeCl3) [33] can enhance the accessible surface area of carbon materials while maintaining good electrical conductivity [34], promoting the adsorption of pollutants and electron transfer. Luo [29] et al. developed iron-rich magnetic BC (KMBC)-activated PS to efficiently degrade metronidazole, with a degradation rate of 98.4% after 120 min, and discovered that active ingredient content in magnetic BC [35], particularly the content of Fe, (II) was closely related to its activation properties [36,37].
BC was made in this work using sterculia foetida as a raw material [38,39]. The sterculia foetida was first subjected to hydrothermal treatment and then calcined to obtain SFC. Using FeCl3 as the precursor, iron-modified BC with different ratios (Fe2-SFC, Fe3-SFC, and Fe4-SFC) were further prepared. The degradation performance of the produced materials and SFC were compared when they were utilized to degrade TC. The catalysts’ characteristics were examined by approaches for characterization [40] such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, BET surface area measurements, and X-ray photoelectron spectroscopy (XPS). The influences of certain factors (catalyst dosage, oxidizer dosage, solution temperatures, solution pH, concentration of initial, coexisting anions, etc.) on the removal of TC in Fe3-SFC/PDS system were probed. Finally, the mechanism of Fe3-SFC-activated PDS for TC degradation was brought forwards [41].
In this study, the Fe element is doped with sterculia foetida biochar to prepare iron-based biochar catalysts for the first time, which keeps carbon materials’ high electrical conductivity and larger accessible surface area, and provides a more novel and efficient method for the value-added utilization of sterculia foetida. At the same time, the above materials are used as catalysts, and peroxydisulfate activation is used to achieve more efficient TC degradation under non-selective conditions and broaden the removal path of water pollutants.

2. Materials and Methods

2.1. Materials

Sterculia foetida was purchased from the supermarket. Absolute ethanol (purity > 99.7%), sodium hydroxide (NaOH, analytical reagent), sulphury acid (H2SO4, analytical reagent), sodium bicarbonate (NaHCO3, analytical reagent), and sodium phosphate (Na3PO4, analytical reagent) were obtained from Tianjin Yong Da Chemical Reagent Co., Ltd. Tetracycline (TC, analytical reagent) was purchased from Aladdin Industrial Corporation. Ethanol (EtOH, chromatographic grade) was acquired from China Shanghai Aladdin Biochemical Technology Co., Ltd. Anhydrous ferric chloride (FeCl3, analytical reagent) and Sodium persulfate (Na2S2O8, analytical reagent) were obtained from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China) Sodium nitrate Tianjin Bai Shi Chemical Industry Co., Ltd. (Tianjin, China) provided the analytical reagents Tertbutyl alcohol (TBA, C4H10O), sodium chloride (NaCl, analytical reagent), and sodium nitrate (NaNO3, analytical reagent). Alfa Esha (China) Chemical Co., Ltd. (Shanghai, China) supplied the furfuryl alcohol (FFA, C5H6O2, purity > 98%).

2.2. Biochar-Based Catalyst Synthesis

Sterculia foetidas was soaked for 2 h in 1000 mL distilled water. The fruit cores were removed and filtered after the sterculia foetidas were soaked to expand, then diverted to five 100 mL autoclaves and heated for 18 h at 180 °C, and then cooled to room temperature. After washing and filtering the product with distilled water and hydrous ethanol, hydrothermal biochar (SFH) was obtained. It was placed inside a tubular reactor and calcined for 2 h at 800 °C in a N2 environment to produce blank calcined biochar (SFC). After mixing and grinding the mass of SFH and FeCl3 in a certain ratio (1:2; 1:3 and 1:4), it was placed in a tubular reactor to calcine the obtained material under the same conditions, and then soaked in 0.1 mM HCl solution for 12 h, according to the ratio of 0.5 g material to 200 mL hydrochloric acid solution. After filtration, the product was centrifuged and washed with distilled water to get Fe-doped FeCl3-modified biochar (Fe2-SFC; Fe3-SFC and Fe4-SFC). The material preparation method is shown in Scheme 1.

2.3. Characterization

Fourier transform infrared spectroscopy was used to identify the Fourier transform infrared (FT-IR, D-76275 Ettlingen Germany) of SFC and Fe3-SFC as KBr (mass ratio of 1:400) pellets, before and after use. The D8-X-ray diffractometer (Bruker Company, Bremen, Germany) was used to find the obtained sample’s XRD diffraction pattern. To examine the catalyst’s morphology, a scanning electron microscope (SEM, JEOL, JSM-6390LV) was employed. The BET surface area, pore size, and pore volume of the catalyst were determined using a specific surface area pore size analyzer (Beijing Beishi Shi De Instrument Technology Co., Ltd., 3H2000PM1 Beijing China). X-ray photoelectron spectroscopy (Thermo KAlpha XPS America) was used to determine elemental valence of the catalyst. An electron spin resonance spectrometer (EPR) was used to detect the free radicals produced in the BC/PDS system.

2.4. Catalytic Degradation Experiment

In a typical method, 10 mg of biochar catalyst and 100 mL of 20 mg/L TC solution were added into a 250 mL beaker, and ultrasonic treatment was carried out for 1 min. It was quickly transferred to a water bath, stirred at 400 r/min, and 100 mg of PDS was added to the beaker solution to start the degradation reaction after 15 s of stirring. A certain amount of the sample solution (3 mL) was removed in a given time (0, 1, 2, 5, 10, 20, 40, 60 min), and filtered with 0.45 μm polyethersulfone membrane. The absorbance of the filtrate was immediately detected by UV-V spectrophotometer at λ = 359.5 nm.
Without special instructions, all experiments were performed under the conditions of temperature 25 °C, pH 6.93, and initial TC concentration of 20 mg/L, with no adjustment required. The effects of different kinds of biochar on TC degradation were studied by using the above method, and the first-order and second-order kinetics analyses were made. Different biochar concentrations (0.05 g/L, 0.10 g/L and 0.15 g/L), different PDS concentrations (0.5 g/L,1.0 g/L and 1.5 g/L), different pH values (3.01, 6.93 and 11.03), different TC concentrations (10 mg/L, 20 mg/L and 30 mg/L), different temperatures (25 °C, 35 °C and 45 °C) and different anions (Cl−, NO3−, HPO42−, HCO3−) of the TC degradation were investigated. The free radical capture experiment and carbon cycle experiment in the process of TC degradation were tested.

2.5. Electrochemical Test

Preparation of the working electrode was as follows: 20 mg biochar was combined with 2 mL ethanol for 3 h of ultrasonic treatment, then dropped on the glassy carbon electrode, dried for 10 min at 60 °C, and then left to stand at 25 °C for 4 h. Platinum wire electrode was a counter-electrode, saturated calomel electrode was the reference electrode, with 50 mM Na2SO4 as the electrolyte.
Linear sweep voltammetry (LSV), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were implemented at the electrochemical workstation (Chi660e, Shanghai CH Instrument Technology Co., Ltd.). The LSV diagram of SFC under different conditions (separate SFC, SFC + PDS and SFC + PDS + TC) was measured at 0.05 V/s sweep speed and −1.0–1.5 V potential. At the same time, the LSV of Fe3-SFC was measured at the same method and experimental conditions. At −1.5~1.5 V potential and 0.005 V/s scanning speed, the cyclic voltammetry (CV) curves of SFC and Fe3-SFC were measured. EIS was measured with an amplitude of 0.005 V and a range of 10−1~105 Hz in frequency.

3. Results and Discussion

3.1. Characterization

The changes in surface functional groups before and after the reaction of SFC and Fe3-SFC are shown in Figure 1a,b using FT-IR analysis. C-H (610 cm−1) [42], C-OH/C-O-C (1100 cm−1) [43], C-N (1380 cm−1) [44] and -OH/-NH2 (1570, 3420 cm−1) [45] are the main tensile vibration functional groups of SFC, and these functional groups also react with Fe3-SFC. The stretching vibration frequency of C-OH/C-O-C in Fe3-SFC is higher than in SFC, whereas the stretching vibration of C-N, and -OH/-NH2 is more noticeable in SFC. The bands in SFC are slightly different from Fe3-SFC in the FT-IR spectra [46], and the functional groups on the surface of the two materials changed before and after the reaction, indicating that these surface functional groups make an important impact in catalytic degradation [47,48]. The functional groups on its surface will react with PDS, and its main reaction equation is [3]:
C-OH + S2O82−→C-O• + SO4•− + HSO4−
C-O + S2O82−→C-O• + 2SO4•−
Figure 1c shows the Raman spectra of SFC and Fe3-SFC. The D and the G bands of the carbon material are represented by the peaks at 1350 and 1590 cm−1 [49], respectively. SFC and Fe3-SFC have ID/IG values of 0.798 and 0.829, respectively, indicating that Fe3-SFC has more defects than SFC, which can supply more defect sites for PDS to activate the catalytic performance of biochar [50,51].
SEM images of SFC and Fe3-SFC are shown in Figure 2a–d. Although both SFC and Fe3-SFC are composed of granular structures, there are significant differences in material morphology between the two catalysts [52]. Fe3-SFC has larger particles and irregular surface roughness due to the incorporation of a small amount of Fe, while SFC exhibits relatively regular small particle shapes with smooth and more regular surfaces. As shown in Figure 2e–h, Fe3-SFC contains C, N, O and Fe elements, the distribution is relatively uniform, and the content of Fe element is relatively low. Figure 2i,g again shows that both SFC and Fe3-SFC contain the elements C, N and O, and that Fe3-SFC also contains the Fe element.
The crystal structure of the obtained carbon catalyst was investigated using XRD measurements. As shown in Figure 3, the diffraction peaks of SFC, Fe3-SFC and Fe3-SFC used at 2θ are at 20.8°, which corresponds to the (002) graphite structural plane [53]. At the same time, no other peaks were added after the modification of Fe, which indicated that most of Fe did not remain on the surface or inside of biochar, and Fe changed the structure of biochar during calcination. The peaks of Fe3-SFC did not change significantly after use, which proves that Fe3-SFC has good stability.
The N2 adsorption and desorption isotherms of SFC and Fe3-SFC are demonstrated in Figure 4a. SFC and Fe3-SFC are shown as typical type II isotherms [54]. Fe3-SFC can offer more active sites for the activation of PDS, since both its BET surface area and pore volume are greater than those of SFC (Table 1) at 959.2110 m2/g and 1.0152 cm3/g, respectively (Table 1) [52,55]. Figure 4b shows the pore size–distribution curves of SFC and Fe3-SFC. Transition pores and micropores are found in both SFC and Fe3-SFC [56].
The chemical composition and morphology of SFC and Fe3-SFC were analyzed using XPS, shown in Figure 5. According to XPS results, both SFC and Fe3-SFC contain C, N, and O elements, and Fe3-SFC also contains Fe (Figure 5a). The atomic content ratios of C, N, O and Fe in SFC and Fe3-SFC are shown in Figure 5b. It shows that the C content of Fe3-SFC after Fe doping is relatively lower than that of SFC. Meanwhile, the content of N and O increases, which provides more functional groups, and the Fe element is also doped in it, but its content is less than 5%. The C atom content of SFC is higher than that of Fe3-SFC, while N and O are lower than Fe3-SFC. C1s can be deconvolved into distinct peaks, as seen in Figure 5c. Three distinctive peaks in SFC and Fe3-SFC are attributed to C-C/C=C, C-H, and C-OH bonds, respectively, at energies of 284.8, 285.5, and 286.4 eV [53]. The N1s spectra of the two materials can be deconvoluted into three peaks (Figure 5d). These characteristic peaks at 401.09, 399.8, and 398.4 eV can be attributed to graphitic N, pyrrolic N, and pyridine N, respectively [57], and the major peaks for both are graphitic N. Figure 5e shows the XPS spectra of O1s for the two materials. The O1s XPS spectrum of SFC is divided into three peaks: C-OH (531.6 eV), O-H (533.7 eV), and C=O (532.7 eV). Fe3-SFC has one more peak than SFC at 530.4 eV, which is Fe-O [58]. In addition, Figure 5f shows the Fe 2p spectrum, in which the two peaks at roughly 713.7 and 725.5 eV, correspond to Fe3+ 2p3/2 and Fe3+ 2p1/2 in Fe3-SFC [55]. The peak at 711.6 eV belongs to the 2p1/2 peak of Fe2+ [59]. The two peaks at 723.2 and 718.5 eV can be ascribed to Fe2+ and Fe 2p3/2, respectively, indicating the presence of Fe0 species on the Fe3-SFC surface [58].

3.2. TC Degradation

The TC adsorption and catalytic properties of SFC and Fen-SFC were investigated. When comparing Figure 6a,b, it can be shown that after adding PDS, both SFC and Fen-SFC have a greater removal effect. In the instance of PDS, the TC degradation was investigated. After 60 min, the degradation effect of SFC on TC was very limited, at only 19.2%. However, in the Fen-SFC/PDS system, except Fe4-SFC, the TC degradation rate of the other modified catalysts could reach more than 75.0% (Figure 6b). The findings demonstrate that adding FeCl3 considerably improved the capacity to remove TC. In a certain range, the removal rate increased as the FeCl3 level increased. When FeCl3:SFC = 3:1, the highest degradation efficiency can reach 91.5%. When FeCl3:SFC = 4:1, the degradation of catalytic performance is even less than that of blank biochar. This indicates that the amount of FeCl3 has reached saturation, and when FeCl3:SFC = 3:1, excessive FeCl3 may take up reaction sites on the biochar surface and reduce the degradation efficiency. It proved that FeCl3:SFC = 3:1 is the best ratio to remove TC.
Characterization of TC degradation in Fe3-SFC/PDS systems using pseudo-first-order and pseudo-second-order kinetics (Equations (1) and (2)) [60,61].
ln(C0/Ct) = k1t
1/Ct − 1/C0 = k2t
TC concentrations at time 0 and t are represented by C0 and Ct. The first- and second-order rate constants are denoted by k1 and k2, respectively. Equations (1) and (2) were used to monitor the TC-degrading kinetic model in the Fe3-SFC/PDS system (Figure 6c,d). The second-order model fitting’s correlation coefficient (R2) is 0.998, greater than the first-order model’s (0.924) (Table 2). This outcome proves that the second-order reaction kinetics are better suited for TC degradation in the Fe3-SFC/PDS system [61].
Figure 7a shows that Fe3-SFC dosage had a favorable effect on TC degradation, as degradation rate of TC also increased from 81.9% to 92.3% as the amount of Fe3-SFC was enhanced from 0.05 g/L to 0.15 g/L. This suggested that high doses of Fe3-SFC could effectively improve the removal rate of TC, as more Fe3-SFC could certify more active sites for activating PDS, thereby increasing the activation efficiency [62]. Figure 7b depicts the result of PDS concentration on degradation of TC. With an increase in PDS concentration, it was seen that the rate of TC degradation decreased, probably because excessive PDS would react with active oxidizing species to form some substances with low oxidation performance. But the amount of PDS had little impact on the catalytic degradation performance of the Fe3-SFC/PDS system [63]. Figure 7c shows that, as the initial concentration of TC increases, its degradation rate decreased, which indicated that active sites on the Fe3-SFC surface are more easily occupied by TC. Therefore, the contact between PDS and Fe3-SFC is reduced [64].
Within the Fe3-SFC/PDS system, the effect of initial pH ranging from 3.01 to 11.03 on TC degradation is shown in Figure 7d. Over a pH range of 3.01 to 11.03, nearly overlapping TC degradation curves were obtained, indicating that Fe3-SFC was catalytically stable over a wide working pH range [65,66]. When temperature increased from 25 °C to 35 °C, TC degradation rate increased slightly, but was almost unchanged (Figure 7e), which showed that higher temperature could enhance the catalytic performance of the Fe3-SFC/PDS system, which was due to the higher temperature. High temperature could activate PDS to produce more active species, but the effect of this change was relatively weak [61]. Figure 7f shows the effects of Cl−, HPO42−, HCO3−, and NO3− on the degradation efficiency of TC. The existence of four anions showed different degrees of inhibition on the degradation of TC. Cl− and NO3− had only a minor inhibitory effect. Cl− can react with SO4•− to form Cl• with relatively weak oxidizing power [67], and NO3− possessed a quenching effect on •OH [61]. Relatively strong inhibition of TC degradation by HPO42− and HCO3−. HCO3− had a powerful removal effect on SO4•− and •OH, thus decreasing TC degradation efficiency [68]. HPO42− in-solution was also the scavenger of •OH and SO4•− [69]. In addition, HPO42− could form complexes with surface metal ions on Fe3-SFC/PDS, and the active sites on the catalyst surface would be covered by complexes [70], thus hindering the TC degradation.
Cl− + SO4•−→Cl• + SO42−
NO3− + •OH→NO3• + OH−
HCO3− + SO4•−→HCO3• + SO42−
HCO3− + •OH→H2O + CO3•−
HPO42− + SO4•−→HPO4•− + SO42−
HPO42− + •OH→H2O + HPO4•−
The reusability of Fe3-SFC was investigated (Figure 8). Three recovery experiments were performed on Fe3-SFC. After each experiment, recovery, washing and drying of the spent N-BC catalyst were performed. It could be seen that degradation of TC in three runs were 90.5%, 83.9% and 51.4% after 60 min of reaction, respectively. With the increase of recycling times, TC degradation gradually decreases, which might be due to reduction of surface functional groups and coverage of active sites by intermediates, e.g., [66], which indicated that Fe3-SFC had a certain lifetime before being consumed in the reaction. However, the degradation rate after three recycling cycles was 51.4%, indicating that Fe3-SFC still had the ability to remove TC from the aquatic environment [44].
Table 3 compares the degradation of pollutants by Fe3-SFC and other biochar catalysts.

3.3. Reaction Mechanism

To better grasp degradation mechanism of TC and discern active substances in the system, quenching experiments were performed on the Fe3-SFC/PDS system. Ethanol (EtOH) had been reported to be a typical •OH and SO4•− trap, and t-butanol (TBA) was supposed to be able to capture •OH [77,78]. Singlet oxygen (1O2) could be largely trapped by furfuryl alcohol (FFA) [79]. Figure 9a shows that TC degradation efficiency was significantly reduced in the presence of FFA, whereas the degradation efficiency of TC decreased only slightly in the presence of TBA or EtOH. This result demonstrated that the Fe3-SFC/PDS system produced a large amount of 1O2 and a bit of •OH and SO4•−, indicating that main active species for degrading TC was 1O2. In order to further confirm existence of 1O2 in this system, it was measured using EPR technique. A strong signal of TEMP 1O2 was heeded in the Fe3-SFC/PDS-TEMP system, as shown in Figure 9b, indicating the presence of 1O2 in the Fe3-SFC/PDS-TEMP system [61].
Figure 10 shows the LSV curves, CV curves, and EIS analysis of SFC and Fe3-SFC under different conditions. The conduction of biochar could be clarified by electrochemical measurement [80]. Through the LSV diagrams of the two materials (Figure 10a,b), it could be found that the current increases significantly [81] with the increase of PDS solution. This indicates the interaction and electron rearrangement between PDS and biochar-based catalysts [82]. It is worth noting that injecting TC afterward caused another current increase, demonstrating that electron transfer [83] was faster in the PDS/SFC or Fe3-SFC/TC ternary system. The birth material carbon was thought to act as a bridge for the formed current, promoting electron transfer from the TC molecule to the PDS [84]. When the currents of the two systems were compared, it could be said that the current of Fe3-SFC was significantly higher than that of SFC, indicating that Fe3-SFC had a better electron transfer ability than SFC [85].
From Figure 10c, it is worth noting that closed area of Fe3-SFC in the current potential curve is much larger than that of SFC [86], indicating that Fe3-SFC has a large charge storage capacity to accept electrons [87]. The results of LSV and CV further confirmed that catalytic performance of Fe3-SFC for PDS activation was higher than SFC.
EIS tests were performed to further investigate electron transfer ability of two carbon-based catalysts (Figure 10d). The electron transfer ability of biochar-based catalysts is described according to the diameter of the semicircle [88]. The semicircle diameter of Fe3-SFC is significantly smaller than that of SFC. The impedance order of biochar is Fe3-SFC < SFC. This is very consistent with their catalytic performance results. It can be concluded that the incorporation of Fe may improve the electron density and promote the electron flow. In the original structure, doping Fe atoms will help with electron triggering and transfer [89].
In order to facilitate electron transfer between the catalyst and the PDS molecule and facilitate the degradation of organic pollutants, catalytic activation aims to weaken and break down the superoxide O-O link in PDS [61]. XPS spectra of Fe3-SFC before and after the reaction reveal the activation mechanism of PDS (Figure 11). Figure 11a shows that the content of C-C/C=C functional groups in Fe3-SFC decreased from 65.3% to 50.4% after the reaction, which indicated that C-C/C=C was the main reactive functional group to activate PDS [74]. The content of pyridinic N decreased by 18.75% after the reaction, as shown in Figure 11b, indicating that pyridinic N was involved in the activation of PDS [58]. Although graphite N did not change much, previous studies had proved that graphite N could accelerate the transfer of electrons between adjacent ones and destroy the inertia of networks of C, thereby increasing the positive charge of C [61], which was beneficial to pass. PDS was activated by a positively charged nucleophilic reaction that produces 1O2, implying that graphite N was involved in the process. After the reaction, the O-Fe in Fe3-SFC vanished (Figure 11c), which indicated that O-Fe was the main reactive site when Fe3-SFC catalyzed PDS degradation of TC [74]. After the reaction, the Fe2+ bond in the XPS of Fe3-SFC (Figure 11d) disappeared and the Fe2+ 2p1/2 bond area decreased, demonstrating that Fe2+ served as a PDS-activating functional group and was involved in the degradation of TC, which further confirmed that Fe3-SFC was involved in the reaction of ferrous iron [76]. The reason why Fe2+ 2p1/2 still exists at this time is that high-energy Fe2+ is more likely to react [90].
Based on the above analysis, the radical (SO4•− and •OH) and non-radical (1O2 and electron transfer) pathways were used to degrade TC in the Fe3-SFC/PDS system, with the latter playing a dominant role (Figure 9a). O-Fe in Fe3-SFC disappeared after the reaction, according to the O1s-XPS spectra of Fe3-SFC before and after the reaction, showing that O-Fe was the primary reaction site [91]. Relevant studies had shown that the content of active ingredients in magnetic BC, especially Fe (II), was closely related to its activation properties. The Fe2+ bond in the XPS of Fe3-SFC vanished after the reaction, demonstrating that Fe2+ may have acted as a PDS-activating functional group to encourage 1O2 production and was implicated in the degradation of TC. Furthermore, 1O2 was produced by the nucleophilic reaction between the three positively charged C ions surrounding graphite N and PDS. In addition to active species, TC degradation may result from the electron transfer (Figure 10b). Rapid TC degradation might be caused by 1O2 and electron transfer in the Fe3-SFC/PDS oxidation system, whereas •OH and SO4•− played an adjuvant function in TC degradation.
The reversible transformation of Fe2+ and Fe3+ is shown in Figure 12, and the main reactions involved are [92]:
Fe3+ + S2O82−→Fe2+ + S2O8−•
Fe3+ + S2O82−→SO4−• + Fe2+ + SO42−
In the experiments of Xie et al. [93], the Fe concentration in wastewater discharge after Fe-doped catalyst was degraded by TC was lower than the standard limits from environmental requirements for China’s surface water (GB 3838-2002). In this paper, Fe-modified catalysts containing less Fe are used, and the Fe content of the degraded water is low. This will not pollute the environment in a secondary way.

4. Conclusions

In this research, using ferric chloride and sterculia foetida as raw materials, a novel magnetic biochar (Fe3-SFC) was successfully synthesized as a persulfate activator to efficiently degrade TC. The characterization results showed that the Fe3-SFC surface was significantly coated with iron corrosion products but less content, and the surface’s low-valent Fe (II) on the surface participated in the activation of PDS and was converted into high-valent Fe (III). When the dose of Fe3-SFC is increased, the efficiency of TC’s degradation increases, and high TC concentrations will have low degradation efficiencies. PDS concentration (0.5 to 1.5 g/L), solution pH (3.01 to 11.03) and reaction temperature (25 to 45 °C) have little influence on TC degradation. In the presence of HPO42− and HCO3−, the TC degradation in the Fe3-SFC/PDS system was restrained. The degradation mechanism of TC in the Fe3-SFC/PDS system revealed that non-radical pathways involving 1O2, electron transfer, and Fe (II) played a significant role. At the same time, there are some phenomena such as low preparation yield, poor magnetic properties and low recovery rate of Fe3-SFC observed in this experiment. Therefore, the Fe3-SFC synthesized in this study has important practical significance in PS-activated degradation of TC.

Author Contributions

Conceptualization, Y.Z., X.J., Z.K. and Z.H.; writing—original draft preparation, Y.Z.; writing—review and editing, Y.Z., X.J., Z.K. and X.K.; visualization, X.K., M.G. and Z.H.; supervision, C.W., J.W. and Z.H.; project administration, C.W., D.Z. and Z.H.; funding acquisition, Z.H. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Hebei National Science Fund for Distinguished Young Scholars (No. E2019209433), Youth Talent Program of Department of Education of Hebei Province (No. BJ2018020) and Hebei Province High-level Talents Funded Project (No. B2020003030).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

No data reported.

Conflicts of Interest

The authors declare no conflict of interest.

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Scheme 1. Schematic representation of the preparation of SFC and Fen-SFC.
Scheme 1. Schematic representation of the preparation of SFC and Fen-SFC.
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Figure 1. FT-IR spectra of the two materials before and after TC adsorption (a,b); Raman spectra of SFC and Fe3−SFC (c).
Figure 1. FT-IR spectra of the two materials before and after TC adsorption (a,b); Raman spectra of SFC and Fe3−SFC (c).
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Figure 2. SEM images of SFC (a,b) and Fe3-SFC (c,d); EDS mapping of Fe3-SFC (e–h); EDS spectra of SFC (i); EDS spectra of Fe3-SFC (j).
Figure 2. SEM images of SFC (a,b) and Fe3-SFC (c,d); EDS mapping of Fe3-SFC (e–h); EDS spectra of SFC (i); EDS spectra of Fe3-SFC (j).
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Figure 3. XRD patterns of SFC; Fe3-SFC and Fe3-SFC after use.
Figure 3. XRD patterns of SFC; Fe3-SFC and Fe3-SFC after use.
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Figure 4. N2 adsorption–desorption isotherms (a); and pore–size distribution curve (b).
Figure 4. N2 adsorption–desorption isotherms (a); and pore–size distribution curve (b).
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Figure 5. XPS survey scan of SFC and Fe3-SFC (a); proportion of C, N, O, and Fe content (b); C1s peaks of SFC and Fe3-SFC (c); N1s peaks of SFC and Fe3-SFC (d); O1s peaks of SFC and Fe3-SFC (e); Fe2p peaks of SFC and Fe3-SFC (f).
Figure 5. XPS survey scan of SFC and Fe3-SFC (a); proportion of C, N, O, and Fe content (b); C1s peaks of SFC and Fe3-SFC (c); N1s peaks of SFC and Fe3-SFC (d); O1s peaks of SFC and Fe3-SFC (e); Fe2p peaks of SFC and Fe3-SFC (f).
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Figure 6. TC adsorption by the different biochar (a); TC degradation by the different biochar (b) the pseudo-first-order kinetic fitting curve (c); and the pseudo-second-order kinetics fitting curve (d).
Figure 6. TC adsorption by the different biochar (a); TC degradation by the different biochar (b) the pseudo-first-order kinetic fitting curve (c); and the pseudo-second-order kinetics fitting curve (d).
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Figure 7. The influence of catalyst dosage (a); PDS dosage (b); TC concentrations (c); initial pH (d); temperature of reaction (e) and anions (f) on the degradation of TC in the Fe3−SFC/PDS system.
Figure 7. The influence of catalyst dosage (a); PDS dosage (b); TC concentrations (c); initial pH (d); temperature of reaction (e) and anions (f) on the degradation of TC in the Fe3−SFC/PDS system.
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Figure 8. Reusability experiment of Fe3-SFC.
Figure 8. Reusability experiment of Fe3-SFC.
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Figure 9. Effects of different quenchers on TC degradation in Fe3−SFC/PDS system (a) and the EPR spectrum in Fe3−SFC/PDS system with TEMP (b).
Figure 9. Effects of different quenchers on TC degradation in Fe3−SFC/PDS system (a) and the EPR spectrum in Fe3−SFC/PDS system with TEMP (b).
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Figure 10. LSV curve of SFC (a) and Fe3−SFC (b) in different situations; CV curves of SFC and Fe3−SFC (c); EIS analysis of SFC and Fe3−SFC (d).
Figure 10. LSV curve of SFC (a) and Fe3−SFC (b) in different situations; CV curves of SFC and Fe3−SFC (c); EIS analysis of SFC and Fe3−SFC (d).
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Figure 11. XPS spectra of C1s (a); O1s (b); N1s (c) and Fe2p (d) of Fe3-SFC before and after reaction.
Figure 11. XPS spectra of C1s (a); O1s (b); N1s (c) and Fe2p (d) of Fe3-SFC before and after reaction.
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Figure 12. Mechanism of the degradation of TC in Fe3−SFC/PDS system.
Figure 12. Mechanism of the degradation of TC in Fe3−SFC/PDS system.
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Table
Table 1. Specific surface area and pore volume of SFC and Fe3-SFC.
Table 1. Specific surface area and pore volume of SFC and Fe3-SFC.
SampleSurface Area (m2/g)Pore Volume (cm3/g)
SFC558.50500.5649
Fe3-SFC959.21101.0152
Table
Table 2. Fitting results regarding the degradation of TC in Fe3-SFC/PDS system.
Table 2. Fitting results regarding the degradation of TC in Fe3-SFC/PDS system.
Fitting FormulaDynamical EquationkR2
Pseudo-first-order kinetic−ln(C0/Ct) = 0.085t + 0.5490.085 min−10.924
Pseudo-second-order kinetic1/Ct − 1/C0 = 0.491t + 0.7560.491·L mg−1·min−10.998
Table
Table 3. Comparison of Fe3-SFC and other materials in terms of pollutant degradation.
Table 3. Comparison of Fe3-SFC and other materials in terms of pollutant degradation.
CatalystsTarget
Pollutants		Catalyst Dosage
(G/L)		Pollutant
Concentration
(Mg/L)		Persulfatedosage (G/L)Reaction
Time		Degradation
Conditions		D.E. (%)Ref.
Fe3-SFCTetracycline0.1100.41 hpH: 6.93
T: 25 °C		91.50This
work
KOH activation biocharTetracycline0.25023 hpH: 7
T: 25 °C		82.36[66]
Goethite/
biochar		Tetracycline0.5200.11 hpH: 7
T: 25 °C		66.71[64]
N-doped Enteromorpha prolifera-derived magnetic biocharTetracycline0.2500.45 hpH: 4
T: 25 °C		87.2[71]
Passion fruit shell-derived biocharTetracycline0.4200.12 hpH: 7
T: 25 °C		90.91[61]
Black fungus-derived N-doped
biochar		Tetracycline0.3200.41 hpH: 6.9
T: 25 °C		89.8[53]
Fe(II)-rich potassium-doped magnetic biocharMetronidazole0.52010 mM120 minpH: 6.5
T: 25 °C		98.4[72]
Fe, N co-doped biochar (Fe-N-BC)Acid orange0.2201 mM40 minpH: 3
T: 25 °C		100[57]
Composite of iron sulfide and biochar (FeS@BC)Tetracycline0.320010 mM30 minpH: 3.6
T: 25 °C		87.4[73]
Magnetic rape straw biochar (MRSB)Tetracycline hydrochloride1208 mM120 minpH: 5.68
T: 25 °C		98.02[74]
Magnetic biochar was prepared from dewatered piggery sludgeTetracycline0.756.720 mg/L120 minpH: 7
T: 25 °C		66.87[75]
Biochar supported nanosized iron (nFe(0)/BC)Tetracycline0.41001 mM240 minpH: 5
T: 25 °C		97.68[76]
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Zhang, Y.; Jia, X.; Kang, Z.; Kang, X.; Ge, M.; Zhang, D.; Wei, J.; Wang, C.; He, Z. Degradation of Tetracycline in Water by Fe-Modified Sterculia Foetida Biochar Activated Peroxodisulfate. Sustainability 2022, 14, 12097. https://doi.org/10.3390/su141912097

AMA Style

Zhang Y, Jia X, Kang Z, Kang X, Ge M, Zhang D, Wei J, Wang C, He Z. Degradation of Tetracycline in Water by Fe-Modified Sterculia Foetida Biochar Activated Peroxodisulfate. Sustainability. 2022; 14(19):12097. https://doi.org/10.3390/su141912097

Chicago/Turabian Style

Zhang, Yuchen, Xigai Jia, Ziyang Kang, Xiaoxuan Kang, Ming Ge, Dongbin Zhang, Jilun Wei, Chongqing Wang, and Zhangxing He. 2022. "Degradation of Tetracycline in Water by Fe-Modified Sterculia Foetida Biochar Activated Peroxodisulfate" Sustainability 14, no. 19: 12097. https://doi.org/10.3390/su141912097

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