Enhanced H2O2 activation and sulfamethoxazole degradation by Fe-impregnated biochar

Journal Pre-proofs Enhanced H2O2 activation and sulfamethoxazole degradation by Fe-impregnated biochar Xiaoying Zhang, Peizhe Sun, Kajia Wei, Xia Huang, Xiaoyuan Zhang PII: DOI: Reference:

S1385-8947(19)33336-4 https://doi.org/10.1016/j.cej.2019.123921 CEJ 123921

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Chemical Engineering Journal

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23 July 2019 6 December 2019 22 December 2019

Please cite this article as: X. Zhang, P. Sun, K. Wei, X. Huang, X. Zhang, Enhanced H2O2 activation and sulfamethoxazole degradation by Fe-impregnated biochar, Chemical Engineering Journal (2019), doi: https:// doi.org/10.1016/j.cej.2019.123921

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Enhanced H2O2 activation and sulfamethoxazole degradation by Feimpregnated biochar Xiaoying Zhanga,b, Peizhe Sunb,*, Kajia Weia, Xia Huanga, Xiaoyuan Zhanga,*

a

State Key Joint Laboratory of Environment Simulation and Pollution Control, School of

Environment, Tsinghua University, Beijing, 100084, P.R.China b

School of Environmental Science and Engineering, Tianjin University, Tianjin, 300072,

P.R.China

*Corresponding Authors. Email: [email protected] (P Sun); [email protected] (X Zhang).

Abstract: Some uses of biochar/H2O2 in the removal of organic pollutants from sewage have made the advanced oxidation of pharmaceuticals and personal care products by modified biochar possible. However, little is known about the performance of modified biochar and the mechanism of this process. Herein, the activation of H2O2 by Fe-impregnated biochar (FBC, produced from maize straw, wheat straw and peanut shell) was studied, and the removal of sulfamethoxazole (SMX) was specifically investigated. The results show that Fe-impregnated biochar had the ability to activate H2O2 and degrade SMX and its performance was better than that of the original biochar. With the increase of pyrolysis temperature from 350°C to 700°C, Fe-impregnated maize straw (1 g/L) enhanced the decomposition of H2O2 (3 mM) from 66% to 99% within 2 h. The degradation of SMX (10 μM) by FM700/H2O2 within 2 h reached 100%, which was approximately 3 times of that of the original biochar. The mechanism of activationdegradation was elucidated by electron paramagnetic resonance (EPR). The results show that the interaction of H2O2 with C-OH promoted the generation of HO• and that HO• and nonradical sites jointly attacked SMX to degrade it. Impregnation with Fe not only increased the specific surface area of the biochar but also provided more active sites to activate H2O2 and degrade SMX. Key words: Fe-impregnated biochar; sulfamethoxazole; activation-degradation; persistent free radicals; non-free radicals

1. Introduction Biochar is an emerging carbonaceous material produced by pyrolysis of low-cost biomass under anaerobic conditions. Due to its porous structure and large surface area, biochar has many potential environmental applications[1-9]. Previous studies have demonstrated that biochar contains a variety of functional groups, such as aromatic structures, carboxyl groups, and hydroxyl groups, which can activate peroxides and degrade organic pollutants (Supporting Information, SI, Table S1)[10-27]. For instance, 93% of 13.7 μM sulfamethazine was removed from aqueous solution by a biochar/H2O2 system within 2 h[11]; 60% of 10 μM sulfamethazine was removed within 1 h[17]. Therefore, biochar is expected to act as an activator of H2O2 to achieve the cost-effective and efficient removal of pharmaceuticals and personal care products (PPCPs) in wastewater. Retrospective studies have demonstrated that biochar modified by metal can remove contaminants more efficiently than the original biochar. For example, As(V) sorption by FeMn biochar composites was 3.44 g/kg, which was much higher than that of the unmodified biochar (0.2 g/kg)[28]; biochar loaded with Fe3+ had a strong ability to remove Cr(VI) (99.64% within 25 min)[29]. Iron is a non-toxic metal that is abundant in natural minerals[30]. To date, a multitude of Fe-modified biochars have been developed, and their biomass sources include water hyacinth, rice husk[31], orange peel[32], bagasse[33] and plant stems[34]; all of these biochars have shown good adsorption and catalytic performance[35, 36]. However, limited information is available on the process by which Fe-modified biochar activates H2O2 and converts organic pollutants, which is also called an advanced oxidation process. Meanwhile,

most of the previous studies have not strictly distinguished adsorption and degradation during the process. The degradation process is more meaningful for the removal of contaminants because desorption and further treatment are not required. Through accurate quantification of degradation, the efficiency of pollutant removal is reflected more directly. In recent years, the mechanism of pollutant degradation related to biochar/H2O2 has received increasing attention[17, 37-39]. In general, the mechanism involves two steps: activation of H2O2 and degradation of pollutants. There are some persistent free radicals (PFRs) in biochar that can exist in humid air for hours or even days[40] and play an important role in the activation of H2O2 to degrade pollutants. It has been reported that the mechanism of H2O2 activation by biochar can be divided into three categories: single-electron reduction by PFRs[41], activation by transition metals and non-radical reactions[42]. When H2O2 was added to a biochar/H2O2 system, the formation of •OH was detected, while the PFR concentration decreased, which proved that electron transfer from PFRs to H2O2 was the most likely mechanism by which the biochar activated H2O2 to form •OH[13]. In addition, studies on biochar-metal nanoparticles have shown that transition metal ions supported on the surface of biochar can react with H2O2 to produce •OH[43, 44]. In the process of degrading p-nitrophenol by a biochar/H2O2 system, Yang et al. proposed three possible pathways for pollutant degradation: reaction with •OH, reaction with PFRs, and reaction with non-radical sites[15]. The mechanism by which •OH interacts with contaminants has been widely described, whereas the interaction of contaminants with PFRs or non-radical sites still requires further investigation. In this paper, one of the most commonly used sulfonamide antibiotics, sulfamethoxazole

(SMX), was selected as a model pollutant[45-48]. Fe-impregnated biochar was synthesized and used to activate H2O2. The removal and degradation (by total extraction) of SMX were investigated. The possible pathways of catalytic oxidation and dominant species were discussed, and the influencing factors of the reaction system were studied. Overall, this study aimed to provide new ideas for the development of biochar materials and applications in PPCP removal.

2. Materials and Methods 2.1. Materials Sulfamethoxazole, titanium(IV) oxysulfate solution (15%) and chromatography-grade methanol were purchased from Sigma-Aldrich (St. Louis, MO, USA). Hydrogen peroxide (H2O2, 30%) was purchased from Aladdin (Shanghai, China). Hydrochloric acid (HCl) was obtained from Chemical Reagent Fifth Plant (Tianjin, China). Sodium thiosulfate (Na2S2O3) was obtained from Titan Scientific Corporation Ltd (Shanghai, China). Other chemicals were of analytical grade and purchased from Kermel Chemical Reagent Corporation Ltd (Tianjin, China). Ultrapure water (resistivity>18 MΩ∙cm-1) was used in this study. 2.2. Preparation of biochar and Fe-impregnated biochar In this work, three raw materials, maize straw, wheat straw and peanut shell, were pyrolyzed at 350°C and 700°C. These different types of biochar (BC) were labelled as M350, M700, W350, W700, P350 and P700, respectively. To achieve higher degradation rates, Fe was used to modify all types of BC, and the various kinds of Fe-impregnated biochar (FBC) were correspondingly labelled as FM350, FM700, FW350, FW700, FP350 and FP700. Fig. 1 illustrates the process of BC and FBC preparation. More details about the preparation steps can

be found in SI, Text S1. In order to demonstrate the results clearly, abbreviation for different samples can be found in SI, Table S2. 2.3. Activation of H2O2 and degradation of SMX H2O2 activation experiments were performed in 10-mL brown glass vials. 10 mg of biochar was added to 9.94 mL of phosphate buffer (PBS, 5 mM, pH 5) and shaken at 200 rpm for 10 min at 25℃ to hydrate. Subsequently, 0.06 mL H2O2 (the final concentration was 3 mM) was quickly added and shaken at 200 rpm for 2 h at 25℃. Samples were taken periodically and filtered through 0.45 μm glass microfibre filters. All experiments were carried out in duplicate. SMX degradation experiments were performed in 10-mL brown glass vials. First, 10 mg biochar was added to PBS (5 mM, pH 5) and shaken at 200 rpm for 10 min at 25℃ to hydrate. Subsequently, 0.1 mL SMX (the final concentration was 10 μM) and a certain amount of H2O2 were quickly added to initiate the reaction. The mixture was shaken at 200 rpm for 2 h at 25℃. Periodically, 20 μL of 0.1 M Na2S2O3 solution was added to the 1.0 mL suspension to quench the reaction. To quantify the total concentration of SMX in the solid and liquid phases, the quenched suspension was extracted with a mixture of 1.0 mL sodium bicarbonate and methanol (volume ratio=1:9) for 30 min and then filtered through a filter (0.45 μm) for analysis. All experiments were carried out in duplicate. Control experiments without H2O2 or biochar were conducted under the same reaction conditions. 2.4. Characterization and analytical methods The surface morphology characteristics of the BC and FBC samples were analysed by a scanning electron microscope (SEM) (S-4800, Hitachi, Japan) at an acceleration voltage of 5.0

kV. The specific surface area (SBET) of the samples was determined using the BrunauerEmmett-Teller (BET) (Autosorb-iQ2-MP, Quantachrome, USA) nitrogen adsorption technique. The point of zero charge (pHpzc) of the BC and FBC samples was calculated from the zeta potential of the sample suspensions at different pH values, which were measured using a zeta potentiometer (Delsa Nano C, Beckman Coulter, USA). The Fe and Ca contents of the BC and FBC samples were measured by inductively coupled plasma emission spectroscopy (ICP-OES) (PE-Optima 8000, USA). The surface functional groups of the samples were analysed by Fourier transform infrared spectroscopy (FTIR) (IRAffinity-1S, Shimadzu, Japan). The structures of the compounds on the surface of the FBC were analysed by X-ray photoelectron spectroscopy (XPS) (250 XI, Thermo, UK). Electron paramagnetic resonance (EPR) (FA-200, JEOL, Japan) was used to measure PFRs on the surface of the samples and free radicals generated during the experiments, which were combined with 5,5-dimethyl-1-pyrroline-Noxide (DMPO) for detection. The concentrations of SMX were determined by a high-performance liquid chromatography (HPLC) instrument (e2695, Waters, USA) equipped with a Zorbax RX-C18 column (2.1 mm × 150 mm, 5 μm) and a UV detector at 254 nm. The mobile phase consisted of 0.1% phosphoric acid solution and methanol, and gradient elution was used at a flow rate of 1.0 mL/min. The sample injection volume was 10 μL, and the column temperature was maintained at 30°C. The concentrations of H2O2 were determined by colorimetry. Titanyl sulfate was used as a colorimetric reagent and determined by UV-vis spectrophotometer. During measurements, 1

mL 0.5 M sulfuric acid solution and 1 mL sample (water was used instead in the blank experiment) were added to a 10 mL volumetric flask, and then 0.1 mL titanyl sulfate (15 wt%) was added. The mixture was diluted with ultrapure water to a final volume of 10 mL. The absorbance was measured at 410 nm.

3. Results and Discussion 3.1. Activation of H2O2 and degradation of SMX in the FBC/H2O2 system As shown in Fig. 2 (a), with the increase of pyrolysis temperature from 350°C to 700°C, FM, FW and FP (1 g/L) enhanced the decomposition of H2O2 (3 mM) within 2 h from 66%, 56% and 59% to 99%, 100% and 69%, respectively. Similarly, when the pyrolysis temperature of unmodified BC was increased, the decomposition of H2O2 also increased. The results may be related to the larger specific surface area and the greater number of active sites of FBC and BC at 700°C than at 350°C. After modification of M350, W350 and P350, the decomposition of H2O2 was improved from 15%, 1% and 0.4% to 66%, 56% and 59%, respectively. After modification of M700, W700 and P700, the decomposition of H2O2 was increased from 59%, 51% and 30% to 99%, 100% and 69%, respectively. These results demonstrate a significant increase in the ability of modified biochar to decompose H2O2, probably because the larger specific surface area after Fe impregnation provided more active sites. Moreover, iron precipitates can also partially decompose H2O2. FM had the best performance among all the FBC samples in decomposing H2O2, and M had the best performance among all the BC samples. In addition, maize straw is one of the most common agricultural wastes in northern China, which is easy to obtain. It is of great significance

to environmental protection, resource conservation and sustainable development of agricultural economy by preparing biochar to recycle maize straw resources. To further explore the decomposition mechanisms of H2O2 by BC and FBC, the decomposition kinetics of H2O2 by maize straw biochar (i.e., M and FM) was investigated. The results of the kinetic experiment are illustrated in Fig. 2 (b). A total of 99% of 3 mM H2O2 was decomposed within 2 h by 1.0 g/L FM700 at pH 5 and 25°C, 66% was decomposed by FM350, and only 15% was decomposed by M350. Moreover, the decomposition of H2O2 in the biochar-free control group was less than 3%. The experimental results of M700 were similar to those of FM350. Using the data in Fig. 2 (b), the kinetic fit curve of H2O2 decomposition by biochar was drawn. As shown in Fig. S1 (a), the processes of H2O2 decomposition by FM350, FM700, M350 and M700 followed the pseudo-first-order kinetic model (R2＞0.96), and the concentration of H2O2 at different times can be described by the following equation: [H 2 O 2 ]=[H 2 O 2 ]0 exp(-kd t )

(2)

where kd is the pseudo-first-order rate constant, [H2O2]0 is the initial concentration of H2O2, and [H2O2] is the concentration of H2O2 at reaction time t. Therefore, kd can be determined by linear regression of the ln([H2O2]/[H2O2]0)-t diagram. Fig. S1 (a) shows that the pseudo-firstorder rate constant of H2O2 decomposition by FM700 was 0.059 min-1, which was significantly higher than the values of 0.008 min-1 for FM350 and 0.007 min-1 for M700. Therefore, increasing the pyrolysis temperature and loading Fe can promote the decomposition of H2O2 by biochar. As shown in Fig. 3, in the presence of DMPO, FM/H2O2 formed a four-line spectrum with

an intensity ratio of 1:2:2:1, and its hyperfine splitting constant aH = aN = 14.89 G, demonstrating the ability of modified biochar to activate H2O2 and produce •OH. However, minimal •OH was produced in the M/H2O2 system. To investigate the performance of the FBC/H2O2 system for degrading and adsorbing SMX, the concentration of SMX remaining in different systems (solid and liquid phases) and liquid phase after 2 h of reaction was determined. Removal of SMX = Degradation of SMX =

[SMX]0 -[SMX]L [SMX]0

(3)

[SMX]0 -[SMX]S+L [SMX]0

Adsorption of SMX = Removal of SMX  Degradation of SMX 

(4) [SMX]S+L -[SMX]L [SMX]0

(5)

where [SMX]0 is the initial concentration of SMX, [SMX]L is the concentration of SMX remaining in liquid phase and [SMX]S+L is the concentration of SMX remaining in solid phase and liquid phase (by total extraction) at reaction time t. As shown in Fig. 4 (a), the removal of SMX were 100% within 2 h by different kinds of FBC, except for 86% by FP350. After modification, the ability of biochar to remove SMX was significantly improved, about 3 times that of original biochar. Owing to its strong adsorption capacity, biochar always has a good removal effect on many pollutants in waterbodies (SI, Table S1). The results of total removal could not reflect the degradation of pollutants in the system. Therefore, total extraction by the mixture of sodium bicarbonate and methanol was used to determine the remaining SMX in the solid and liquid phases in this study. Compared with adsorption, degradation contributed more to SMX removal in the FBC/H2O2 system. Especially in the FBC700 system, SMX was completely degraded within 2h. The degradation of SMX by FM350, FW350 and FP350 was

63%, 63% and 62%, while the adsorption was only 37%, 37% and 25%, respectively. With the increase of pyrolysis temperature from 350°C to 700°C, FM, FW and FP enhanced the degradation of SMX from 63%, 63% and 62% to 100%. Similar situations were observed in the unmodified biochar experiments, indicating that pyrolysis temperature had an effect on the ability of biochar to activate H2O2 (Fig. 2 (a)) and degrade SMX (Fig. 4 (a)). After modification of M350, the degradation of SMX was improved from 15% to 63%. Similarly, after modification of W350 and P350, the degradation of SMX was improved from 13% and 21% to 63% and 62%, respectively. After modification of M700, W700 and P700, the degradation of SMX was increased from 36%, 28% and 24% all to 100%, respectively. No intermediates were observed in HPLC within 2 h (SI, Fig. S9). According to Fig. 4 (a), six kinds of FBC samples had similar degradation performance, but FM had the best performance in SMX removal (adsorption and degradation). Maize straw is cost-effective and easy to obtain. Using maize straw to prepare biochar can reduce environmental pollution. Moreover, combined with the decomposition of H2O2 in Fig. 2, FM that had the best performance in H2O2 activation were selected for subsequent experiments. The degradation kinetics of SMX within 2 h are illustrated in Fig. 4 (b). SMX (10 μM) was completely degraded within 2 h by FM700/H2O2 at pH 5 and 25°C. 63% of SMX was degraded by FM350/H2O2, and only 36% and 45% was degraded by FM350 and FM700. Moreover, the degradation of SMX by H2O2 in the biochar-free control group was less than 23%. Overall, the FBC/H2O2 system had a good degradation effect on SMX, and its catalytic degradation ability was approximately 2 to 4 times that of the original biochar. High-

temperature-modified biochar could degrade SMX without H2O2, indicating that this kind of biochar already contained a certain amount of active sites that can directly degrade SMX. 3.2. Characterization of FBC In section 3.1, the performance of maize straw biochar was better than that of wheat straw and peanut shell biochar. This section discusses the physical and chemical properties of maize straw biochar. Table 1. Physical and chemical characteristics of biochar Elemental analysis

SBET Sample

pHpzc (m2/g)

M

C (Atomic %)

O (Atomic %)

Fe (g/kg)

Ca (g/kg)

40.02

38.35

0.21

2.63

M350

2.1

1.30

77.39

21.95

9.39

12.3

M700

81.8

2.54

88.02

11.6

12.03

16.1

FM350

11.5

10.53

66.39

22.85

125

3.92

FM700

470.5

8.21

66.54

21.47

198

5.40

42.89

41.92

0.37

2.89

W FW350

18.4

10.69

63.37

28.7

129

3.62

FW700

549.7

6.26

76.51

19.78

170

3.84

32.86

32.56

0.32

1.55

P FP350

15.2

9.29

66.53

24.72

114

1.68

FP700

104.6

10.15

75.58

19.02

204

5.37

Fig. 1 shows photographs of M350, M700, FM350 and FM700. M350 was brown and easy to aggregate and clot. M700 was dark grey, and its adhesion was significantly weaker than that of M350. FM350 and FM700, made by Fe impregnation, were black and exhibited almost no agglomeration. According to the BET results, increases in calcination temperature or Fe impregnation led to a significant increase in the specific surface area of the biochar. The specific surface areas of FM350, FM700, M350 and M700 in Table 1 indicate that some microporous structures appeared in the biochar due to an increase in calcination temperature or Fe impregnation[49]. Since the specific surface area is an important indicator of the adsorption capacity of adsorbents, the large surface area of biochars made through Fe impregnation and at high temperature may provide a large amount of active sites and improve the adsorption and activation capability of biochar. The results of H2O2 activation demonstrate the catalytic capacity of modified biochar was significantly increased than that of original biochar. However, FP700 with the highest Fe content showed poor properties of activating H2O2. Probably because the specific surface area of FP700 was only one fifth of that of FM700 and FW700. In addition to Fe, FP700 contained fewer other active sites resulting in the low decomposition rate of H2O2. The biochar prepared in this study mainly contained two elements—C and O. With increasing temperature, the C content increased, the O content decreased, and thus the O/C molar ratio decreased. Therefore, the molar mass of the samples decreased, and the original metal contents relatively increased at high temperatures in the samples. In Table 1, the Fe and Ca contents in M700 were higher than that in M350, and Ca content in all FBC samples

increased. The FTIR spectra (SI, Fig. S2) demonstrate that there were some oxygen-containing functional groups in the biochar, such as -OH at 3408 cm-1 and aromatic C=O at 1602 cm-1. The peak at 2922 cm-1 was the stretching vibration peak of C-H, and the peak disappeared after Fe impregnation. Moreover, in the fingerprint region of FM350, a weak peak at 679 cm-1 was observed, suggesting that Fe-O was formed between the iron precipitates and BC. The SEM images in Fig. 5 illustrate the structure and morphology of BC and FBC, which have been magnified 10,000 times. Many studies have proven that biochar is a porous material [50, 51]. In this study, the body of the biochar was observed to have an ordered honeycomb structure (SI, Fig. S1) with a pore diameter of 5 to 10 μm. It is apparent that Fe impregnation accounted for the formation of iron precipitates on the surface of the biochar (Fig. 5 (a) and (b)). Interestingly, under high-temperature conditions, the iron precipitates were embedded in the biochar matrix and distributed uniformly, indicating good mechanical bonding between the biochar matrix and iron precipitates at high temperature. EDS point scanning suggested that FBC contained Fe and Ca (SI, Fig. S4). The metal content in a sample can be accurately quantified by ICP-OES. As shown in Table 1, Fe impregnation led to a significant increase in Fe concentration, and the effect of impregnation under high-temperature conditions was better than that under low-temperature conditions. It is probably due to that the gasification is usually proportional to the calcination temperature. The vaporization of raw material in gasification increased under high temperature, and thereby Fe concentration increased on the surface of biochar. The XPS spectra of FM350 and FM700 are shown in Fig. 6, which clearly demonstrate

the changes in the oxygen-containing functional groups of the biochar and the valence of Fe on the surface of the biochar. In Fig. 6 (a) and (b), the photoelectron peak of C 1s is divided into three, and the corresponding binding energies were approximately 284.6, 286.0 and 288.0 eV, representing C-C, C-O and O-C=O, respectively. As the pyrolysis temperature increased, the C-C content increased, whereas the C-O content decreased. No photoelectron peak was detected at 283.2 eV, indicating the absence of iron carbide in the sample[52]; that is, Fe was not directly combined with the biochar. C 1s XPS spectra of FW and FP had similar trends with FM (SI, Fig. S5 (a)-(d)). Fig. 6 (c) shows the photoelectron peak of FM350 Fe 2p. The peaks with binding energies of 711.6 and 724.7 eV were formed from the spin-orbit (j-j) coupling of octahedral trivalent Fe, which proved the existence of Fe2O3. The binding energies of the two satellite peaks were approximately 9 eV higher than the corresponding Fe 2p peaks [53]. The peaks with binding energies of 713.8 and 727.3 eV may represent tetrahedral trivalent Fe. However, these two peaks were not common because they did not have separate satellite peaks[54]. Similar trends were observed for FM700 (Fig. 6 (d)) and other samples (SI, Fig. S5 (e)-(h)). 3.3. Factors affecting the FBC/H2O2 system Fig. 7 (a) reflects the effect of H2O2 concentration changes on the FBC/H2O2 system. After adding H2O2, the degradation ability of the system was significantly improved. With increasing H2O2 concentration, the degradation of SMX first increased and then decreased, reaching a maximum when 3 mM H2O2 was added. The decomposition of H2O2 gradually decreased as the H2O2 concentration increased. The decomposition reached 87%, and the lowest value was

only 62%. Combining the trends of the two curves, it can be inferred that there is an optimum H2O2 concentration in the degradation system, possibly due to the increased scavenging effect at higher H2O2 concentrations. Moreover, it can be seen intuitively that SMX had a high utilization efficiency of H2O2 when initial concentration of H2O2 is 3 mM. Therefore, in order to obtain higher utilization, it is necessary to operate the experiment at suitable initial concentration. To explore the effect of pH on the FBC/H2O2 system, activation-degradation experiments were carried out at pH 3, 5, 7.4 and 9. The results are illustrated in Fig. 7 (b). The degradation of SMX reached a maximum of 63% at pH 5, and 65% of 3 mM H2O2 decomposed. Combined with the pHpzc data for FM350, under the experimental conditions, FM350 was positively charged. We know that the pKa values of SMX are 1.39/5.81[47] . SMX is mainly in neutral form—SMX0—at pH 3 and 5 and is mainly in anionic form—SMX-—at pH 7.4 and 9. Therefore, when the pH was increased from 3 to 9, there was always a strong electrostatic attraction between FM350 and SMX, which shows that the degradation reaction is not a complete adsorption-dependent reaction. Under the same conditions, the trend of H2O2 decomposition was not exactly the same as that of the degradation of SMX. The reason is that H2O2 is weakly acidic. Under alkaline conditions, in addition to decomposing to generate free radicals, H2O2 can easily ionize and produce oxygen, which increases the consumption pathways of H2O2[55]. Furthermore, it can be seen from the figure that after Fe was supported on the surface of the biochar, the pH requirement of the system was no longer as harsh as that of the normal Fenton reaction. 33% of 10 μM SMX was degraded at pH 3 and 25% was

degraded at pH 7.4. A significant drop occurred only at pH 9, demonstrating that the pH of the system can be broadened to the range of 3 to 7.4. In addition, the leaching Fe under different initial pH was measured and the results were also shown in Fig. 7 (b). At pH 3, 5, 7.4 and 9, the leaching Fe was 0.207, 0.0329, 0.288 and 0.230 g/kg, respectively, far less than the leaching amount of iron functionalized biochar prepared by Jiang et al. (30.1, 25.5, 10.9 and 3.60 g/kg at pH 3, 5, 7 and 9, respectively)[56], indicating that our biochar matrix had a good mechanical bond with iron precipitates. The low amount of leaching Fe at pH 5 may be beneficial for SMX degradation, because the loading iron may play a greater role in FBC/H2O2. 3.4. Possible mechanism of H2O2 activation and SMX degradation by FBC EPR measurements proved that the surface of the original BC contained PFRs (Table 2), which is consistent with previous literature reports[13]. According to previous reports, the gfactors for C-centred free radicals such as aromatic radicals are < 2.0030, and O-centred free radicals such as semi-steroidal free radicals have g-factors greater than 2.0040. Materials containing both types of radicals have g-factors in the range of 2.0030 to 2.0040. In this study, the g-factors of six kinds of biochar samples ranged from 2.0029 to 2.0034, and the line width ΔHp-p ranged from 3.24 to 6.52 G, indicating that both C-centred and O-centred PFRs were on the surface of the biochar. With increasing pyrolysis temperature, O-centred PFRs shifted to Ccentred PFRs. This study found that the concentration of PFRs decreased as the pyrolysis temperature increased. When the preparation temperature was lower than 500℃ (mediumtemperature condition), the concentration of PFRs increased as the pyrolysis temperature

increased[13, 57]. But our highest preparation temperature was 700℃ (high-temperature condition), which is quite different from 500℃. The oxygen functional group was eliminated and the carbon condensation/ graphitization (Fig. 6 (a) and (b)) was achieved under 700℃[18], so the biochar contained less PFRs. Therefore, an activation-degradation mechanism different from that in previous studies is proposed in this study. Table 2. g-Factor, line width and relative concentration of PFRs in biochar

a

Sample

g-factor

Line width (G)

Relative concentrationa

M350

2.0034

6.08

340.14

M700

2.0029

3.24

128.67

W350

2.0033

6.11

269.13

W700

2.0030

5.20

41.67

P350

2.0034

6.52

281.92

P700

2.0033

--

127.08

Relative quantity relationship of PFRs concentration in different biochars.

At present, the researchers propose that the mechanisms of H2O2 activation by biochar mainly include activation by PFRs, transition metals and non-free radicals. According to the results of this study, the decomposition ratio of H2O2 by unmodified biochar (Fig. 4) had no direct quantitative relationship with the PFR concentration on the surface of biochar (Table 2), and •OH was hardly produced in the M/H2O2 system (Fig. 3), which indicate that the activation of H2O2 is not achieved by single-electron reduction of PFRs. After Fe impregnation, the Fe

content on the surface of biochar increased, and the decomposition of H2O2 was greatly enhanced. However, the amount of •OH produced was not proportional to the Fe content, which proved that the non-free radicals were the main active species. During the pyrolysis process, Fe may protect oxygen-containing functional groups, such as C-OH, and the O content of FM increased. As shown in Fig. 6, the C-OH content of FM350 was higher than that of FM700, so the ability of FM350 to activate H2O2 and produce •OH was strong. However, the measured concentration of •OH was not high, and there was almost no •OH production in the M350 and M700 systems. We infer that the ability of C-OH to activate H2O2 and produce •OH is limited. In the methylene blue experiment (SI, Fig. S6), the presence or absence of H2O2 had almost no effect on the degradation, which proved that there was negligible O2•- in the H2O2 decomposition products, and the reactions may follow equations (5)-(7)[15, 42]. In addition, H2O2 decomposition was faster than SMX degradation, suggesting that H2O2 had other decomposition pathways in addition to activation and ROS production. It can be inferred that the by-product of H2O2 decomposition is O2 (equation (8))[58]. C-OH+H 2 O 2  CO+OH+H 2 O

(6)

H 2 O 2 +CO   OH  +OH - +C +

(7)

H 2 O 2 +C +  HO 2  +H + +C 

(8)

Fe 3+ +HO 2   Fe 2+ +O 2 +H +

(9)

The degradation mechanisms of pollutants reported in the literature include three types— pollutants are attacked by •OH, PFRs or non-free radicals and then degraded. Among these mechanisms, the reaction between •OH and SMX has been widely recognized. The above

mentioned reports state that C-OH can promote the production of •OH, so the effect of C-OH on SMX is likely to be either direct or indirect. However, the degradation of SMX was not proportional to the concentration of PFRs, C-OH and •OH produced, implying that there may be other non-free radical sites on the surface of biochar that can directly react with SMX. The amount of these sites increases with increasing pyrolysis temperature, probably because the larger specific surface area under high temperature provided more active sites. Fig. 8 depicts a possible mechanism by which FBC activates H2O2 to degrade SMX. H2O2 reacts with C-OH to form HO•, HO2• and alkyl radicals. HO• and other non-free radical sites on the surface of biochar can attack SMX and degrade it into products. The main function of the Fe supported on the surface of biochar is to inhibit the decomposition of C-OH so that the surface of FBC contains more C-OH, thereby producing more HO•. In addition, Fe3+ can react with HO2• to form O2, which accelerates the decomposition of H2O2. Combined with this mechanism, it can be seen that the surface of unmodified biochar has less C-OH and the ability to activate H2O2 is limited, which is not conducive to the removal of SMX. The addition of Fe significantly increased the efficiency of degradation. Future research efforts could focus on increasing the utilization efficiency of H2O2, thereby increasing the amount of HO• produced and degradation rate.

4. Conclusions In conclusion, Fe-impregnated biochar can activate H2O2 and degrade SMX, and its reaction performance is better than that of the original biochar. After modification, the decomposition of H2O2 by maize straw biochar prepared at 700°C increased from 58% to 99%,

and the degradation of SMX increased from 36% to 100%. The reaction of H2O2 with C-OH in biochar promotes the production of HO•, which cooperates with non-radical sites to attack SMX and degrade it. Impregnation with Fe not only increased the specific surface area of the biochar but also provided more active sites to activate H2O2 and degrade SMX. The mechanism of SMX degradation by the FBC/H2O2 system proposed in this study provides a new idea for the further treatment of antibiotics in wastewater.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 51778326 and 51708401), the Major Science and Technology Program for Water Pollution Control and Treatment of China (Grant No. 2017ZX07202003) and Natural Science Foundation of Tianjin (No. 17JCYBJC42300).

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Highlights Fe-impregnated biochars were prepared from maize straw, wheat straw and peanut shell. Degradation (excluding adsorption) of SMX was studied through extraction method. Fe-impregnated biochar had the ability to activate H2O2 and degrade SMX. The activation-degradation mechanism is proposed in the system. C-OH and other non-radical sites play key roles in the system.

Declaration of Interest Statement We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

Fig. 1. Preparation of biochar and photographs of M350, M700, FM350 and FM700.

(a)

(b)

Fig. 2. Decomposition of H2O2 by biochar and Fe-impregnated biochar within 2 h. (a) Decomposition (%) of H2O2; (b) kinetics of H2O2 decomposition by (Fe-impregnated) maize straw biochar. Reaction conditions: initial concentration [BC] = 1 g/L; [H2O2]0 = 3 mM; T = 25℃; pH = 5 (5 mM PBS).

• *

• * •

• * •

* •

Fig. 3. EPR spectra of H2O2 (yellow), FM350/H2O2 (black), FM700/H2O2 (red), M350/H2O2 (blue) and M700/H2O2 (green) in the presence of DMPO at 20 min. *: DMPO-OH; •: DMPO-R. Reaction conditions: initial concentration [BC] = 1 g/L; [H2O2]0 = 3 mM; T = 25℃; pH = 5 (5 mM PBS).

(a)

(b)

Fig. 4. H2O2 Activation for SMX degradation by biochar and Fe-impregnated biochar within 2 h. (a) Removal (%) of SMX; (b) kinetics of SMX degradation by FM350/H2O2, FM700/H2O2, FM350 and FM700. Reaction conditions: initial concentration [BC] = 1 g/L; [SMX]0 = 10 μM; [H2O2]0 = 3 mM; T = 25℃; pH = 5 (5 mM PBS).

Fig. 5. SEM images of biochar. (a) FM350; (b) FM700; (c) M350; (d) M700.

(a)

(b)

(c)

(d)

Fig. 6. C 1s XPS spectra of (a) FM350 and (b) FM700; Fe 2p XPS spectra of (c) FM350 and (d) FM700.

(a)

(b)

Fig. 7. Factors of H2O2 decomposition and SMX degradation in the system of Fe-impregnated biochar/ H2O2. (a) The effect of H2O2 concentration, pH = 5 (5 mM PBS); (b) the effect of pH, [H2O2]0 = 3 mM. Reaction conditions: initial concentration [FM350] = 1 g/L; [SMX]0 = 10 μM; T = 25℃.

Fig. 8. Possible mechanism of FBC activating H2O2 for degradation of SMX.

Supporting Information Enhanced H2O2 activation and sulfamethoxazole degradation by Feimpregnated biochar Xiaoying Zhanga,b, Peizhe Sunb,*, Kajia Weia, Xia Huanga, Xiaoyuan Zhanga,*

a

State Key Joint Laboratory of Environment Simulation and Pollution Control, School of

Environment, Tsinghua University, Beijing, 100084, P.R.China b

School of Environmental Science and Engineering, Tianjin University, Tianjin, 300072,

P.R.China

*Corresponding Authors. Email: [email protected] (P Sun); [email protected] (X Zhang).

14 pages, 1 text, 2 tables and 8 figures.

Text S1. Preparation of Biochar and Fe-impregnated Biochar. Biochar (BC). Maize straw, wheat straw and peanut shell were washed and oven-dried overnight at 80℃, grinded through a 60-mesh sieve, and then transferred to a tube furnace for anaerobic pyrolysis (0.3 L/min N2). The samples were heated to the target temperature at 10 ℃/min and maintained for 2 h, then cooled to room temperature. Fe-impregnated biochar (FBC). Maize straw, wheat straw and peanut shell were first impregnated in FeCl3 solution (Fe: BC = 1:5). After ultrasonication for 30 min, the samples vigorously stirred for 2 h. The mixture was dried at 80℃ for 8 h and then pyrolyzed in a tube furnace. The obtained samples were washed by ultrapure water to remove the impurities and dried at 105℃ for 5 h. All samples were placed in sealed containers and stored in the dark prior to use.

Table S1. Biochar activating peroxide for degradation of organic compounds Concentration Biomass

Pollutant

Oxidant

pH

Biochar

Pollutant

Oxidant

(g/L)

(μM)

(mM)

Removal

Reference

Pine needles, wheat straw and

2-Chlorobiphenyl

H2O2

7.4

1

10.6

10

95-99% (2 h)

[1]

Wheat straw

Sulfamethazine

H2O2

7.4

1

13.7

10

93% (2 h)

[2]

Cotton stalks

Sulfamethoxazole

H2O2

9.0

1

10

1

60% (1 h)

[3]

Persulfate

7.4

1

3.9

8

50% (2 h)

[4]

maize straw

2,4,4’Pine needles Trichlorobiphenyl Pig manure

Tetracycline

H2O2

7.4

0.5

67.5

5

92% (2 h)

[5]

Maize straw

Tetracycline

H2O2

7.4

0.5

67.5

5

71% (2 h)

[5]

Bamboo

Tetracycline

H2O2

7.4

0.5

67.5

5

55% (2 h)

[5]

Maize straw

Sulfamethazine

H2O2

3.0

1

35.9

20

32% (2 h)

[6]

Reed

Orange G

Persulfate

9.5

0.2

111

2

40% (2 h)

[7]

Bisphenol A

Persulfate

7.0

0.3

131

0.4

95% (1 h)

[8]

Pine needles

1,4-Dioxane

Persulfate

6.5

1

20

8

45% (2 h)

[9]

Sawdust of poplar

Clofibric acid

Persulfate

4.0

0.5

100

10

97.8% (1 h)

[10]

Peanut shell

Sulfamethoxazole

Persulfate

7.2

0.1

2

0.5

12% (1 h)

[11]

Wheat straw

Sulfamethoxazole

Persulfate

5.0

0.5

39.5

2

8% (1 h)

[12]

Sludge

Sulfamethoxazole

Persulfate

5.0

2.0

40

1.5

92% (2 h)

[13]

Old corrugated containers

Table S2. Abbreviation for different samples Abbreviation

Sample

Abbreviation

Sample

BC

Biochar

FBC

Fe-impregnated biochar

BC350

Biochar at 350℃

FBC350

Fe-impregnated biochar at 350℃

BC700

biochar at 700℃

FBC700

Fe-impregnated at 700℃

M

Maize straw biochar

FM

Fe-impregnated maize straw biochar

M350

Maize straw biochar at 350℃

FM350

Fe-impregnated maize straw biochar at 350℃

M700

Maize straw biochar at 700℃

FM700

Fe-impregnated maize straw biochar at 700℃

W

Wheat straw biochar

FW

Fe-impregnated wheat straw biochar

W350

Wheat straw biochar at 350℃

FW350

Fe-impregnated wheat straw biochar at 350℃

W700

Wheat straw biochar at 700℃

FW700

Fe-impregnated wheat straw biochar at 700℃

P

Peanut shell biochar

FP

Fe-impregnated peanut shell biochar

P350

Peanut shell biochar at 350℃

FP350

Fe-impregnated peanut shell biochar at 350℃

P700

Peanut shell biochar at 700℃

FP700

Fe-impregnated peanut shell biochar at 700℃

(a)

(b)

Fig. S1. Kinetics fitting curve of (a) H2O2 decomposition and (b) SMX degradation by (Feimpregnated) maize straw biochar. Reaction conditions: initial concentration [BC] = 1 g/L; [SMX]0 = 10 μM; [H2O2]0 = 3 mM; T = 25℃; pH = 5 (5 mM PBS).

Fig. S2. FTIR spectra of selected biochar (FM350, FM700, M350 and M700) used in this study.

Fig. S3. SEM image of FM700.

(a) Element CK OK Ca K Fe K

Wt% 40.39 19.39 1.37 38.85

At% 63.39 22.85 0.64 13.11

Element CK OK Ca K Fe K

Wt% 57.37 26.95 0.21 15.46

At% 70.83 24.98 0.08 4.11

(b)

Fig. S4. EDS spectra of (a) FM350 and (b) FM700.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Fig. S5. C 1s XPS spectra of (a) FW350, (b) FW700, (c) FP350 and (d) FP700; Fe 2p XPS spectra of (e) FW350, (f) FW700, (g) FP350 and (h) FP700.

Fig. S6. Kinetics of methylene blue degradation by M700/H2O2 and W700/H2O2 within 2 h. Reaction conditions: initial concentration [BC] = 1 g/L; [MB]0 = 100 mg/L; [H2O2]0 = 3 mM; T = 25℃; pH = 5 (5 mM PBS).

Fig. S7. Kinetics of SMX removal by Fe-impregnated maize straw biochar. Reaction conditions: initial concentration [BC] = 1 g/L; [SMX]0 = 10 μM; [H2O2]0 = 3 mM; T = 25℃; pH = 5 (5 mM PBS).

(a)

(b)

Fig. S8. Kinetics of H2O2 decomposition by (a) (Fe-impregnated) wheat straw biochar and (b) (Feimpregnated) peanut shell biochar within 2 h. Reaction conditions: initial concentration [BC] = 1 g/L; [H2O2]0 = 3 mM; T = 25℃; pH = 5 (5 mM PBS).

(a)

(b)

(c)

(d)

(e)

Fig. S9. HPLC results at 220 nm at (a) 10 min; (b) 20 min; (c) 40 min; (d) 60 min; (e) 120 min. Reaction conditions: initial concentration [FM350] = 1 g/L; [SMX]0 = 10 μM; [H2O2]0 = 3 mM; T = 25℃; pH = 5 (5 mM PBS).

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