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Environ Sci Pollut Res (2014) 21:6162–6169 DOI 10.1007/s11356-014-2533-4

RESEARCH ARTICLE

Reduction of nitrobenzene with sulfides catalyzed by the black carbons from crop-residue ashes Wenwen Gong & Xinhui Liu & Li Tao & Wei Xue & Wenjun Fu & Dengmiao Cheng

Received: 10 October 2013 / Accepted: 7 January 2014 / Published online: 29 January 2014 # Springer-Verlag Berlin Heidelberg 2014

Abstract In this paper, three types of black carbons (BCs) named R-BC, W-BC, and C-BC were derived from rice straw ashes, wheat straw ashes, and corn straw ashes, respectively. Under room temperature and in an anaerobic aqueous solution, these three types of BCs could catalyze the reduction of nitrobenzene (NB) by sulfides rather than only act as the superabsorbent. The catalytic activities of BCs derived from different crop-residue ashes were very different and in the order of R-BC > W-BC > C-BC, since the reaction rate constants (kobs) of NB with the BCs in the presence of 3 mM sulfides were 0.0186, 0.0063, and 0.0051 h−1, respectively. The key catalytic active sites for NB reduction were evaluated, with four types of modified BCs and two types of tailored graphite as the model catalysts. The results indicated that BCs probably had two types of active sites for NB reduction, the microscopic graphene moieties and the surface oxygen functional groups. Since the sulfides and BCs often coexist in the environment, this BC-catalyzed reduction technology of NACs may be applied as an in situ remediation technique without the need for reagent addition. Keywords Black carbons . Crop-residue ashes . Catalytic reduction . Active sites . Nitrobenzene . Sulfides

Introduction The black carbon (BCs) is widespread in the environment, accounting for 2–18 % of total organic carbon (TOC) in the Responsible editor: Bingcai Pan W. Gong : X. Liu (*) : L. Tao : W. Xue : W. Fu : D. Cheng State Key Laboratory of Water Environment Simulation, School of Environment, Beijing Normal University, Beijing 100875, People’s Republic of China e-mail: [email protected]

soils and sediments, and even 30–45 % of TOC in fireimpacted soils (Cornelissen et al. 2005). The ashes from field burning of agricultural crop residues is one of the primary sources of BCs in many farming communities since this practice is convenient and cost-efficient for immediate land clearing and land-use change (Cui et al. 2009). Particulate matters (ashes) resulting from such burning may contain a lot of BCs due to the combustive carbonization in the burning process (Yang and Sheng 2003; Sheng et al. 2005; Cui et al. 2009). It has been reported that approximately 730 Tg of biomass are burned in Asia every year, 34 % of which was due to the burning of agricultural crop residues in the field (Streets et al. 2003). As a large agricultural country, China is rich in the crop straw, the total output of which reached 622 Tg in 2002, with an annual increasing rate of 1.4 % (Zeng et al. 2007; Cui et al. 2009). Half of these crop residues were burned in the field, which led to the generation and accumulation of BCs derived from the crop residue ashes into the soils and sediments (Masiello 1998; Zhong et al. 2003; Forbes et al. 2006). A number of studies have demonstrated that BCs from crop-residue ashes have a very high affinity and capacity for adsorbing organic contaminants, including the nitro aromatic compounds (NACs), polycyclic aromatic hydrocarbons, and pesticides, and consequently have great impacts on the transportation, transformation, and bioavailability of organic contaminants in the aquatic environment (Yang and Sheng 2003; Brändli et al. 2008; Min et al. 2008; Chen et al. 2009; Sun et al. 2012). Besides, some recent studies have suggested that some kinds of BCs could act as effective redox mediators to catalyze the reductive transition process of some redoxsensitive compounds (Kemper et al. 2008; Xu et al. 2010). For example, it has been reported that NACs could be rapidly destroyed in the presence of a reductant and BCs, such as activated carbon, graphite, soot, and charcoals (Oh and Chiu 2009; Yu et al. 2011, 2012; Amezquita-Garcia et al. 2013). Since there are widespread environmental pollutants that have

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aromatic nitro groups, and the sulfides and BCs often coexist in the environment, understanding the reduction kinetics and mechanisms is of great significance to the in situ removals of these toxic chemicals. BCs derived from different starting materials under different conditions may possess different chemical compositions and surface properties and differ greatly in the catalytic activities or catalytic mechanisms. For example, Kemper et al. (2008) reported that 4.5 μM research department explosive (RDX) was nearly completely removed by 3 mM sulfide within 3 h, 16 h, and 17 h in the presence of 2 g/L activated carbon, 2 g/L pine char, and 4 g/L graphite, respectively. Besides, 2 g/L of the uncharred woods did not significantly adsorb RDX or mediate RDX removal over 16 h. In addition, Oh and Chiu (2009) demonstrated the obvious differences in nitrocompound destruction rates by sulfides in the presence of different BCs, including graphite, activated carbon, and diesel soot. Therefore, it is essential to extensively characterize each single source of BCs in order to address the catalytic activities of BCs. However, there is clearly a lack of information on the catalytic effects of the BCs derived from crop-residue ashes which are widespread in the environment, and the key active sites of catalytic reduction have not been determined unequivocally due to the inherent heterogeneous and inscrutable characteristics of these carbon materials. One objective of this study was to investigate the catalytic potential of the BCs from crop-residue ashes in an anaerobic aqueous solution. Nitrobenzene (NB) was chosen as the representative target compound, and three types of BCs derived from crop-residue ashes of three different starting materials were selected to investigate their catalytic activities on the reduction of NB in anaerobic sulfides aqueous solution at the room temperature. The other objective was to further analyze the catalytic mechanism of BC-mediated reduction reaction. In this study, four types of modified BCs and two types of tailored graphite samples were derived from different treatments. Changes in their surface properties were measured and compared with the variation in catalytic activities during NB reduction.

Materials and methods Materials NB (99 %) and aniline (AN, 99.8 %) were reagent grade and were purchased from J&K chemical. Sodium hydrosulfide was obtained from Alfa Aesar (Ward Hill, MA). Sulfides stock solutions, which included H2S, S2−, and HS−, were freshly prepared by dissolving a certain amount of sodium hydrosulfide in deoxygenated deionized water. Chemicals used in the derivatization reactions, including the trifluoroacetic anhydride (TFAA), trifluoroethyl hydrazine (TFH), di-tertbutylcarbodiimide (DTBC), and trifluoroethanol

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(TFE), were purchased from Aldrich. All other chemicals were of analytical grade or higher purity and were used without further purification. Preparation of BCs from crop-residue ashes Three types of crop residue ash-derived BCs (R-BC, W-BC, and C-BC) were prepared from the rice (Oryza sativa L.) straw, wheat (Triticum aestivum L.) straw, and corn (Zea mays L.) straw collected from Hebei Province in China, respectively. The three types of straw account for approximately 76.1 % of the total crop straw resources in China (Zeng et al. 2007). To simulate the field burning process, the straws were burned on a steel plate in the open filed under uncontrolled conditions in a windless afternoon (Yang and Sheng 2003). The ashes was collected, grounded, and passed through a 20-mesh sieve, respectively. The initial W-BC was subjected to four modifications with the use of different chemical agents to change the surface characters. To remove the soluble salts in the ashes, sample of W-BC was washed several times with deionized water (Huang et al. 2011) and named W-BC1. To remove mineral materials (primarily silicate), the water-washed BC (W-BC1) was treated with a 1 M:1 M HCl/HF solution for several times (Chen and Huang 2011), and the demineralized sample was named W-BC2. The third type of BC (W-BC3) was obtained by treating W-BC2 with oxidative acid solution (H2SO4 + HNO3) (Teng et al. 2001). Finally, the fourth type of BC (W-BC4) was obtained by treating W-BC2 with methanol and chloroform (1:1, v/v) for 24 h to remove the extractable native organic matter (EOM) (Chen and Huang 2011). All the treated BC samples were separated from water by vacuum filtration, and then washed with distilled water for six to eight times until the filtrate became neutral. All residues were ovendried at 80 °C for 1–2 days and were mechanically ground with a mortar and pestle to less than 165 μm before used. Graphite oxide (GO) was synthetized from high-purity graphite powder through liquid oxidation based on the Hummers method (Hummers and Offeman 1958) as follows: 0.5 g of graphite was added into the mixture of 3 g of potassium permanganate, 0.5 g of sodium nitrate, and 25 mL of sulfuric acid at room temperature, followed by heating to 35 °C with a water bath and then stirring for 15 min. Deionized water (40 mL) was added into the vessel with stirring, followed by heating to 90 °C for over 30 min, and finally 3 mL H2O2 (30 %) was gradually added. The obtained sample was filleted and the residues were washed with deionized water for several times, and then the solid samples were separated from water by the centrifugal treatment and dried in a vacuum furnace at 50 °C. Derivatization of GO was carried out by Langley and Fairbrother (2007) and the derivative sample was named DGO. The oxygen functional groups of GO were derivatized using derivatizing agents as follows. The hydroxyl groups were derivatized using TFAA, the carbonyl groups were

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derivatized using TFH, and the derivatization of carboxylic acid groups was accomplished using TFE, pyridine, and DTBC in a volume ratio of 9:4:8, respectively. Characterization of samples The percentages (weight %) of carbon, hydrogen, and nitrogen were measured using an EA 112 CHN elemental analyzer (Thermo Finnigan). The surface areas of all the samples were measured based on N2 adsorption at 77 K by the Brunauer– Emmett–Teller (BET) method using an Autosorb-1-MP surface area analyzer of American Quantachrome Company. The functional groups of the BCs were qualitatively determined using a Fourier transform infrared (FTIR) spectrometer (NEXUS 870, Thermo Nicolet, USA), recording the spectrum scope from 4,000 to 400 cm−1 with a resolution of 4 cm−1. The surface acidity and basicity was determined using the Boehm’s titration method (Boehm 1996). A JSM-6390 scanning electron microscope (SEM) was used to determine the structure and the morphology of the samples. Experimental procedures All experiments were controlled using a 20.0 mM phosphate buffer solution and performed in triplicate. The solutions were purged with nitrogen gas (99.9 %) for 2 h before adding the sulfides and organic compounds to remove oxygen from the solutions. The solutions were transferred to an anaerobic glove box (Bel-Art, Pequannock, NJ) under N2 and decanted into 20.0-mL vials containing 0.01 g pre-weighed BCs. Then, NB (5.5 μmol) was spiked into each vial, yielding an aqueous concentration of 0.275 mM. Sulfide stock solution (0.06 mmol) was added to each bottle as the reductant, and the sulfides concentration of reaction solution was usually 3 mM. Finally, all vials were placed onto a rotating bed at 180 rpm and 25 °C. The preliminary sorption experiment indicated that the aqueous concentration of NB rapidly decreased from 0.275 to 0.184 mM within 12 h due to its adsorption onto BCs (data not shown). In order to overcome the impact of adsorption and ensure a high recovery of NB and the product AN, the solutions that contained BCs were separated by centrifugation. The BCs were sequentially extracted three times with 5 mL of methanol, and then the extracted solutions were mixed with the supernatant solution for analyses. Therefore, the results are presented as the total mass of NB or AN retrieved from both phases. Using these processing methods, the recovery tests of NB and AN indicated that the recoveries of NB and AN were 85─92 and 90─95 %, respectively. Analysis Periodically, samples were sacrificed for analysis. All samples were analyzed using an Agilent 1100GC HPLC equipped with

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a Diomand C-18 column (200 × 4.6 mm, 5 μm). A methanolwater mixture (65/35, v/v) was used as the mobile phase at a flow rate of 1.0 mL/min. The wavelength of UV detector was set at 260 and 233 nm for NB and AN, respectively. The injection volume of all the samples was 20.0 μL. The retention times for NB and AN were 5.60 and 2.85 min, respectively.

Results and discussion Characterization of BCs from crop-residue ashes The selected physicochemical properties of the three original BCs from crop-residue ashes (R-BC, W-BC, and C-BC) are given in Table 1. There was little difference in the contents of carbon element of the three original BCs, which were low (48.5–58.1 %). Nevertheless, the values of BET surface areas varied widely. The sample of R-BC had the largest BET surface area, reaching 217.8 m2/g, which was approximately 6.4 and 2.5 times of that of C-BC (34.2 m2/g) and W-BC (88.2 m2/g), respectively. SEM observations were made in order to get visualized information on the morphologies of the three original BCs from crop-residue ashes (Fig. 1). Distinct morphologies among different BCs produced from different starting materials were observed. The samples of R-BC and W-BC were structurally porous, while the surface of C-BC was relatively smoother. The initial sample of W-BC was subjected to four modifications with the use of different chemical agents, and the changes of the elementary composition and surface chemistry of the original and treated samples are shown in Table 1. Firstly, there was no obvious change in the physicochemical characteristics of W-BC1 after water washing. Secondly, there was noticeable increase in the carbon contents, surface areas, and the total concentration of acid groups of W-BC2 prepared by HCl/HF treatment in comparison to the original BCs, and this was mainly attributed to the removal of mineral materials (primarily silica) in the BCs (Chen and Huang 2011). Thirdly, there was no significant difference in the physicochemical characteristics between the W-BC2 and the W-BC4, which was obtained by treating W-BC2 with the organic solvent, indicating that there was little EOM in the wheat straw ashderived BC, and the treatment with the organic solvent had little influence on the structural modification. Lastly, the total concentration of oxygen functional groups increased from 0.97 to 3.01 mmol/g when W-BC2 was oxidized with H2SO4 and HNO3, mainly owing to the formation of carboxylic and lactones groups. The concentration of carbonyl groups (two carbonyl groups = quinone groups) of W-BC3 was approximately 2.7 and 1.9 times higher than the values of the original sample W-BC and the acid treating sample WBC2, respectively.

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Table 1 Selected physicochemical properties of the carbon materials Samples

SBET a (m2/g)

Composition (%)

C

H

N

R-BC C-BC W-BC W-BC1 W-BC2 W-BC3

58.1 52.2 48.5 51.1 74.2 74.1

2.1 4.5 1.4 1.3 2.1 2.7

0.3 0.6 0.2 0.2 1.1 1.7

W-BC4 Graphite GO D-GO

76.6 98.5 64.2 78.1

2.4 0.4 3.8 5.6

1.2 0.0 0.0 2.3

a

Surface oxygen functional groups (mmol/g) Carboxylic group

Lactone group

Phenol group

Carbonyl group

Total

217.8 34.2 88.2 89.4 104.6 176.4

0.08 0.30 0.01 0.02 0.10 0.90

0.04 0.10 0.06 0.04 0.05 0.63

0.18 0.10 0.25 0.28 0.45 0.78

0.40 0.46 0.26 0.28 0.37 0.70

0.70 0.96 0.58 0.62 0.97 3.01

102.4 9.08 71.0 77.4

0.15 0.00 0.60 0.07

0.22 0.00 0.34 0.32

0.47 0.00 1.12 0.15

0.31 0.00 0.30 0.03

1.15 0.00 3.67 0.57

BET surface area

Figure 2 shows the FTIR spectra of the original sample (WBC) and the four types of treated samples (W-BC1–W-BC4), which could be used to prove the presence of oxygenated surface functional groups (e.g., quinone groups) on the surface of BCs. The FTIR spectra showed there was significant change in the oxygenated surface functional groups of acidoxidized W-BC3. The increase in the intensity of bands ranging from 1,740 to 1,600 cm−1 (C=O stretching vibrations in quinones, ketones, aldehydes, lactones, anhydrides, and carboxylic acids) can be attributed to the formation of carboxylic, lactonic, and quinones groups (Amezquita-Garcia et al. 2013). This is consistent with the results obtained with Boehm titrations method (see Table 1). BC catalytic reduction of NB The BC catalytic reduction of NB by sulfides is shown in Fig. 3. The results of the blank control group, which included neither sulfides nor BCs, showed that there was no discernible NB decay within the experimental period. The HPLC results of the blank control group and the reductant-free control group only containing NB and BCs showed that there was no reduction product during the experimental period (data not shown). In the BC-free control group containing only NB and 3 mM sulfides, the concentration of NB decreased slightly by approximately 12 % over 72 h at pH 7.0, which indicated that the aqueous sulfides had limited reactivity for the reductive decay of NB. However, the results of the group with both sulfides and R-BC showed that NB reduction was greatly enhanced in the presence of both 0.5 g/L R-BC and 3 mM sulfides (decreased by approximately 75 % over 72 h).

The pathway of NB reduction (Eq. 1) involves three successive two-electron steps and protonation steps (Larsen et al. 2000; Schwarzenbach et al. 2003). 2e

2e

2e

2H

2H

2H

ArNO2 →þ ArNO →þ ArNHOH →þ ArNH2

ð1Þ

It has been reported that BCs (such as graphite) have the ability to transfer electron and atomic hydrogen (Oh et al. 2002, 2004). Therefore, BCs probably promote the reduction process of NB by accelerating the transfer of electron and atomic hydrogen from the reductant to NB. With the decrease of NB, the concentration of the final product AN increased, reaching approximately 72 % of the initial NB mass after 72 h (Fig. 3). When 0.275 mM NB was mixed with 0.5 g/L BCs derived from crop-residue ashes and 3 mM sulfides at pH 7.0, 1 mol of AN formed per mole of NB destructed, indicating that the product and reactant remained mass balance. Figure 4 illustrates the variation of NB conversion with time for reactions catalyzed by three different BCs derived from crop-residue ashes (R-BC, W-BC, and C-BC) at pH 7.0 and 25 °C. The plots of ln(NB/ NB0) versus time were linear (R2 ranging from 0.883 to 0.953), indicating that all of the BCs prepared from the three starting materials had the ability to mediate the reduction of NB in the presence of a reductant, and the reaction was pseudo-first order in the presence of 0.5 g/L BCs from crop-residue ashes and 3 mM sulfides at pH 7.0 and 25 °C. The observed pseudo-first-order reaction rate constant (kobs) under the given conditions was calculated using the equation ln(NB/NB0) = −kobs t. Based on this equation, the largest kobs value of 0.0186 h−1 (in the presence of 0.5 g/L R-BC and 3 mM sulfides) was approximately 11 times greater than the kobs value (0.0016 h−1) obtained in the presence of only 3 mM sulfides. Undoubtedly, the BCs produced from different original materials

C-O

W-BC W-BC1

%Tansmitance

C-H

C=O C=C

O-H

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W-BC2

W-BC3 W-BC4

4000 3500

2000

1500

1000

500

-1

Wavenumber (cm )

Fig. 2 FTIR spectra of the original sample (W-BC) and four types of treated samples (W-BC1 ~ W-BC4)

great variations in the catalytic activities of different type of BCs from different sources (Kemper et al. 2008; Oh and Chiu 2009; Xu et al. 2010; Yu et al. 2011, 2012; Amezquita-Garcia et al. 2013). In this experiment, the kobs values of NB catalytic reduction with three crop residue ash-derived BCs in the presence of 3 mM sulfides were 0.0186, 0.0063, and 0.0051 h−1, respectively. The data indicated that their catalytic activities were in the order of R-BC > W-BC > C-BC. As is known, the catalytic reaction over carbon materials is a surface reaction, and therefore the highest catalytic activity of R-BC is most likely due to its largest surface areas (Table 1). Catalytic active sites of crop residue ash-derived BCs It is difficult to determine the dominant structure controlling BCmediate reduction of NB using these BCs since multiple structural characteristics may change simultaneously. Fortunately, chemical treatments provide a way to modify given structural characteristics of BCs and allow of further insight into the BC catalytic mechanisms. The original sample W-BC and four types of modified BCs (W-BC1–W-BC4) were selected as catalysts, 0.30

Fig. 1 Scanning electron micrographs of three original crop residue ashderived BCs with a magnification of ×5,000: a R-BC; b W-BC; c C-BC

had different chemical compositions and surface properties, such as elemental compositions, surface areas, surface functional groups, and surface structures. Previous studies assessed different types of BCs, such as graphite, activated carbon, diesel soot, and wood char, in terms of their ability to mediate the reductive transformation of NACs, and the results indicated that there are

Concentration (mM)

0.25 0.20 0.15 0.10 0.05

NB, blank control NB, reductant-free control (R-BC only) NB, BC-free control (sulfides only) NB, with both R-BC and sulfides AN

0.00 0

10

20

30

40

50

60

70

80

Time (h)

Fig. 3 Reduction of NB by sulfides (3 mM) in the presence of R-BC (0.5 g/L) at pH 7.0 and 25 °C

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0.00

ln (NB/NB0)

-0.30 -0.60 R-BC W-BC C-BC

-0.90 -1.20 -1.50 -1.80 0

10

20

30

40

50

60

70

Time (h)

Fig. 4 Plot of ln([NB]/[NB]0) versus time for the reduction of NB by sulfides (3.0 mM) in the presence of three crop residue ash-derived BCs (R-BC, W-BC, C-BC) at pH 7.0 and 25 °C

and the changes in their surface properties were compared with the variation in catalytic activities during NB reduction in order to gain insight into the catalytic active sites. The destruction of NB by sulfides over carbon catalysts was a surface reaction. Consequently, the surface areas played a dominant role in mediating this reaction. For surface reactions, the rate constant should be surface area-based rather than mass-based, and therefore the surface area normalized rate constant (kSA) of NB reduction was acquired using the equation kSA = kobs / (surface area of carbon catalyst). The kSA values of NB reduction by 3 mM sulfides over different carbon catalysts are shown in Fig. 5. It is of interest to notice that the BCs derived from the same initial material and treated using different chemical methods, with the exception of W-BC3, had roughly equal catalytic activities compared with the initial BCs (kSA = 1.12 × 10−4 g/(m2 h)). This means the soluble salts (removed by water 2.00

washing) and EOM (removed by organic solvent extracting) had no obvious effects on the reductive reaction of NB. The slight increase of kSA of NB decay over W-BC2 (1.30 × 10−4 g/(m2 h)) may be because the progress of HCl/HF treatment removed the mineral materials on the surface of BCs and then exposed more active sites for this reaction. On the contrary, W-BC3 (obtained via oxidation by H2SO4 and HNO3) exhibited much less catalytic effects (kSA = 0.42 × 10−4 g/(m2 h)) on NB decay than the original BCs and that obtained using other treatments. As is known, the drastic oxidation process could alter the structural properties of carbon materials, such as the surface area (see Table 1), pore volume, mean pore size, and chemical bond (Teng et al. 2001; Valdes et al. 2002; Laura and Langley 2007). These changes of the structural properties might have great impacts on the catalytic reduction which is a surface reaction. In addition, some researchers considered that oxidation by nitric acid removed, the inner amorphous carbon of BCs (Kamegawa et al. 1998; Chen and Huang 2011), and therefore the possible involvement of amorphous carbon cannot be ruled out. However, H2SO4 and HNO3 are strong oxidizing reagents which can result in the formation of oxygen functional groups on carbon surface during the treatment of carbon with these acids (Teng et al. 2001; Zhou et al. 2012), and this can be supported by the data in Table 1 and Fig. 2. The total surface oxygen functional groups of W-BC increased from 0.58 to 3.01 mmol/g after drastic oxidation by H2SO4 and HNO3. Therefore, it is difficult to determine the changes of which carbon properties contributed to the decrease of kSA of NB decay over W-BC3. In order to exclude the effects of the multiple and inscrutable structural characteristics of BCs, sheet graphite was chosen as the model catalyst, and two types of tailored samples (GO and D-GO) were prepared. First, their elemental composition, BETN2 surface area, and content of surface oxygen functional groups were characterized (Table 1). Then the catalytic effects of the three model catalysts on the decay of NB were evaluated

2

1.50 1.25

-4

kSA(10 g/m *h)

1.75

1.00 0.75 0.50 0.25

-G O D

G O

G ra ph ite

W -B C W -B C 1 W -B C 2 W -B C 3 W -B C 4

0.00

Fig. 5 The surface area normalized reduction rate constant (kSA) of NB decay by 3 mM sulfides in 20 mM phosphate buffer at pH 7.0 and 25 °C over 0.5 g/L different carbon catalysts

Fig. 6 The conceptual schematic diagram for BC-mediated reduction of NB by sulfides

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in the presence of sulfides at pH 7.0 and 25 °C (Fig. 5). Obviously, the kSA of graphite (1.76 × 10−4 g/(m2 h)) was far greater than that of GO (0.34 × 10−4 L/(m2 h)) as well as D-GO (0.22 × 10−4 L/(m2 h)), which indicated that changes of the physicochemical properties of graphite during the wet oxidation progress and/or derivatization process weakened the catalytic activity of graphite. Sorption studies have shown that NACs were adsorbed primarily to the electron-rich edge sites of graphene sheets in BCs through the π-π electron donor-acceptor (EDA) interactions (Zhu and Pignatello 2005). The high catalytic activity of graphite which contained little oxygen indicated that the microscopic graphene moieties played an important role in NB decay and were the major active sites for the reduction of NB. Besides, it is considered that the oxidation process could break the π-π conjugation in the original graphite structure and result in a decrease of the electrical conductivity and an amorphous structure with short-range atomic order (Chen and McCreery 1996; Lee and Seo 2011). The decrease of catalytic effects of GO compared to the original graphite (approximately 81 % decrease) was likely due to the deceleration of delocalized π-electrons in the graphene regions. However, there are rich oxygen functional groups (such as hydroxyl, epoxide, carboxyl, and carbonyl) that are randomly distributed in the graphene structure of GO (Stankovich et al. 2006). Previous researches have demonstrated that the redoxmediating compounds, such as quinones, could act as electron carriers in the abiotic reduction of nitro aromatic compounds and azo dyes in the presence of sulfide (Dunnivant et al. 1992; Van der Zee et al. 2003). In this experiment, to further investigate the involvement of oxygenated functional groups, the carbonyl, carboxylic acid, and hydroxyl groups on GO were deactivated with fluorinated derivatization agents. The derivative sample D-GO showed slightly less catalytic activity than GO (Fig. 5), indicating that the oxygen functional groups also contributed to the catalytic capability for NB decay, although the contribution was relatively small. The experimental results indicated that BCs probably had two types of active sites for NB reduction, the microscopic graphene moieties were the major active sites, and the surface oxygen functional groups were the minor active sites (Fig. 6).

Conclusions The ashes from field burning of crop residues is one of the primary sources for BCs in the environment and has great influence on the environmental behaviors of the organic compounds. This study demonstrated that the three types of BCs derived from crop-residue ashes were able to mediate the reductive transformation of NB by sulfides in anaerobic conditions from an environmental point of view. Besides, two types of active sites, graphene moieties and surface oxygen

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functional groups, contributed to the catalytic activities of BCs for NB reduction. One of the advantages of this catalytic reduction of NB is that the reaction is performed in mild conditions, room temperature and atmospheric pressure. Although we provided preliminary evidence that NACs reduction by sulfides could be accelerated by the BCs derived from crop-residue ashes in a given system, further work would be required to investigate the other catalytic reduction processes (such as the catalytic reductive dechlorination of chlorinated organic pollutants) by various natural reductants in the presence of native BCs. Acknowledgments The research was financially supported by the National Science Foundation for Innovative Research Group (51121003), Major State Basic Research Development Program (2013CB430405), National Natural Science Foundation of China (20977009), and the Fundamental Research Funds for the Central Universities.

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