Reduction of Cr(VI) by Crop-Residue-Derived Black Carbon

Oct 23, 2009 - Burning crop residues is a common postharvest practice on farmland, leading to the accumulation of black carbon (BC) in the soil. To un...
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Environ. Sci. Technol. 2009 43, 8801–8806

Reduction of Cr(VI) by Crop-Residue-Derived Black Carbon N A I - H U A H S U , † S H A N - L I W A N G , * ,† YU-CHI LIN,† G. DANIEL SHENG,‡ AND JYH-FU LEE§ Department of Soil and Environmental Sciences, National Chung Hsing University, Taichung 40227, Taiwan, College of Biological and Environmental Engineering, Zhejiang University of Technology, Hangzhou 310032, China, and National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan

Received June 26, 2009. Revised manuscript received September 30, 2009. Accepted October 12, 2009.

Burning crop residues is a common postharvest practice on farmland, leading to the accumulation of black carbon (BC) in the soil. To understand the potential role of BC in immobilizing toxic Cr(VI) in soil, this study evaluated the Cr(VI) sorption kinetics at pH levels ranging from 3 to 7 and examined the reaction mechanism of Cr(VI) with BC derived from burning rice straw. The BC samples, after reacting with Cr(VI), were analyzed using Cr K-edge X-ray absorption spectroscopy. The results showed that Cr(VI) was sorbed and subsequently reduced to Cr(III), which was bound to the BC surface through surface complexation and precipitation. As indicated by the diffuse reflection infrared Fourier transform spectra, the phenolic groups of BC are the dominant drivers of Cr(VI) reduction, giving rise to carbonyl/carboxyl groups on the BC surface. The reaction rate of Cr(VI) with BC increased from 10-3.62 to 10-1.65 h-1 as pH was decreased from 7 to 3 because Cr(VI) sorption and reduction both occur faster at low pH. These results suggest that BC derived from burning crop residue is an effective reductant for Cr(VI) and may play an important role in determining the fate of Cr(VI) in BC-rich farmland soils.

Introduction Cr(VI) poses a serious threat to the ecosystem and to public health because of its high toxicity and mobility. Cr(VI) is commonly present as relatively soluble oxyanions (i.e., CrO42-, HCrO4-, and Cr2O72-) (1). These anions have a high mobility in the soil profile due to the fact that soil particles are predominately negatively charged and the electrostatic repulsion excludes these anions from adsorption (2). One of the major mechanisms of immobilizing Cr(VI) in soil is the reduction of Cr(VI) to Cr(III) (2, 3). Unlike Cr(VI), Cr(III) is strongly retained on soil particles and has a low solubility in the common pH range of soil (2, 3). Because of the low toxicity and mobility of Cr(III), the reductive transformation of Cr(VI) to Cr(III) is an important natural attenuation process for Cr(VI) released into the environment (4). The major reductants include Fe(II), S(II), and organic matter (2, 5, 6), and their reactions with Cr(VI) have been the subject of intensive * Corresponding author e-mail: [email protected]; phone: +886 4 22840373, ext. 3406; fax: +886 4 22862043. † National Chung Hsing University. ‡ Zhejiang University of Technology. § National Synchrotron Radiation Research Center. 10.1021/es901872x CCC: $40.75

Published on Web 10/23/2009

 2009 American Chemical Society

research. In particular, the reduction rates of Cr(VI) have been reported for Fe(II) (7-9) and S(II) (10, 11) and organic matter (12-15) in aqueous media, and aqueous Fe(II) and S(II) species are more reactive than organic matter in terms of Cr(VI) reduction rate. However, Fe(II) and S(II) are generally more appreciable under reducing conditions. In surface soil under oxic conditions, Fe(II) and S(II) are not present in significant concentrations and may not contribute to the Cr(VI) reduction to any observable extent. Soil organic matter (SOM) is therefore considered to be quantitatively the predominant reductant for Cr(VI) in surface soil (2, 6). SOM is made up of the decomposition residues of onceliving organisms and has a diverse and heterogeneous structure and a nonstoichiometric composition. A substantial portion of SOM is in the form of humic substance (HS), which can be further operationally separated into three fractions, namely fulvic acid (FA), humic acid (HA), and humin, on the basis of their solubility in acid and base. In areas where plant residue is burned or field fire occurs, a portion of SOM may consist of black carbon (BC) resulting from incomplete burning of biomass (16-18). Combustion of fossil fuel can also contribute to the BC content in the environment (18, 19) but this form of BC is not considered here. BC is a ubiquitous form of carbon comprising a range of materials from polyaromatic compounds to graphitic carbon (18-20) and may play an important role in the sorption and retention of pollutants (21, 22). For example, recent studies have proved that BC arising from burning crop residue is an effective sorbent for organic contaminants, such as chlorophenol (23), diuron (24), and MCPA (25) and for inorganic contaminants, such as Pb (26). Our preliminary studies showed that BC is also effective in reducing Cr(VI) in soil, which, to the best of our knowledge, has not been fully reported. This is mainly because traditional measurements of SOM do not distinguish the contribution of BC to the Cr(VI) reduction of SOM from that of the materials resulting from biodegradation of organic matter (20, 27). Cr(VI) reduction has been investigated for the subcomponents of SOM such as FA (12) and HA (13-15). Considering the fact that BC composes of 1-80% of SOM (18-20) (in particular, of the aromatic fraction of SOM (28-30) that is responsible for Cr(VI) reduction (15, 31, 32)), Cr(VI) reduction by BC merits scientific attention. Understanding the interactions between Cr(VI) and BC will allow scientists to predict the fate of toxic Cr(VI) in soils where BC is present. Thus, in this work, we used BC derived from rice straw as a model BC to aim at determining the sorption kinetics and mechanism of Cr(VI) reduction by BC. Rice straw is a byproduct of rice production that plays a prominent role in the world food supply, and represents a common lignocellulose-based crop residue. Burning rice straw leads to carbonization of ligocellulosic material and partial oxidation of resultant BC, giving rise to oxygencontaining surface functional groups attached to peripheral aromatic units in BC (18, 19). This work will clarify the specific chemical interactions between Cr(VI) and the functional groups on the adsorbing surface in hope that the information obtained will further our understanding of the role of BC derived from crop residues in determining the fate and mobility of Cr(VI) in contaminated farmland soils.

Materials and Methods Preparation of Black Carbon. Rice straw was collected from the Agricultural Research Institute in Taichung County, Taiwan. To simulate field burning, rice straw was dried at 40 °C for 1 week and then burned on a steel plate in open air. The temperature measured during the burning reached as VOL. 43, NO. 23, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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high as 300 °C, due to the combustion and decomposition of cellulose, hemicellulose, and lignin in rice straw (33). The resulting material was washed 4 times with a 1 M HCl solution and then 4 times with a 1:1 mixture of 1 M HCl and 1 M HF. This procedure has been proven effective in removing silica and inorganic salts without causing any discernible change in the surface property of BC (34). Afterward, the BC solid was equilibrated with a 1 M KCl solution at pH 7 for 24 h and then placed in a cellulose-ester dialysis tube to equilibrate with an excessive volume of deionized water outside the tube. The deionized water was replaced with fresh deionized water daily until the electrical conductance of the water after equilibration reached 30 µS cm-1 or lower. The suspension in the dialysis tube was centrifuged and the collected BC solid was then freeze-dried and stored in a plastic bottle prior to use. The elemental composition, point of zero charge (PZC), surface area, porosity, and surface functionality of the resultant BC are provided in the Supporting Information (SI). Cr(VI) Sorption of BC. A 1-g sample of BC was added into 500 mL of 1 mM KCrO4 solution in 0.01 M KCl in a waterjacked reaction vessel. During the reaction, the vessel was covered with aluminum foil and connected to a water bath so the reaction proceeded in the dark and at 25 °C. The pH of the suspension was maintained constant at 3, 4, 5, 6, or 7 by adding 0.01 M KOH or HCl solution. At given time intervals, a 10-mL aliquot was withdrawn from the vessel with a syringe and passed through a 0.45-µm (pore size) membrane filter. The Cr(VI) concentrations in the filtrate were determined using the s-diphenylcarbazide method (35). At the end of the experiment, the suspension was filtered and the residual solids were washed 3 times with 200 mL of deionized water. A portion of each of the collected residual solids was treated with 1 M H3PO4 (solids to solution ratio ) 1:10) to extract any sorbed Cr(VI) on BC. The rest of the collected solids was freeze-dried and analyzed by Cr K-edge X-ray absorption spectroscopy (XAS) and diffuse reflection infrared Fourier transform (DRIFT) spectroscopy. Cr K-Edge XAS. Cr K-edge XAS measurements were conducted at the Beamline 17C1 of the National Synchrotron Radiation Research Center in Hsinchu, Taiwan. The Cr K-edge XAS spectra of the samples were obtained on fluorescent mode using a Lytle detector, and at least two scans were obtained for each sample. Spectral analysis was accomplished using Athena (36) following the procedure suggested by Kelly et al. (37). All spectra were calibrated to the edge of metallic Cr at 5989 eV. The scans for each sample were averaged, followed by background removal and normalization. The spectra of pure Cr2O3 and K2CrO4 chemicals were also obtained to serve as the reference standards for the Cr(III) and Cr(VI) oxidation states. For selected samples, the extended X-ray absorption fine structure (EXAFS) data were extracted from the normalized XAS spectra and converted to their χ(k) functions. The k3-weighted χ(k) function was then Fourier transformed with the Hanning window (dk ) 1) and the resulting Fourier transform (FT) magnitude was fitted using Arthemis (36) and the theoretical parameters from Feff 8.2 (38). To generate the theoretical parameters of the Cr-O and Cr-Cr shells, the structural parameters of R-Cr2O3 were obtained from the inorganic crystal structure database and entered into Feff 8.20. Fitted parameters, including the coordination number (CN), interatomic distance (R), Fermi shift (E0) and Debye-Waller factor (σ2), were initially set with reasonable guesses. For the amplitude reduction function (S02), a value of 0.73, obtained by fitting the experimental spectrum of R-Cr2O3, was used as the starting value and was allowed to vary only at the final fitting step. The E0 value was allowed to float by no more than (5 eV. The fitting process was iterated until the calculated value of the error parameter 8802

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FIGURE 1. Cr(VI) sorption kinetics of BC at pH 3-7.

FIGURE 2. Cr K-edge XANES for K2CrO4, Cr2O3, and Cr sorbed on BC at pH 3-7. ∑(χexp - χfit)2/∑(χexp)2 < 0.01, where χexp and χfit are the experimental and calculated χ(k) functions, respectively. DRIFT Spectroscopy. DRIFT spectra of BCs after reacting with 0, 0.5, 1, and 5 mM Cr(VI) were acquired on a ThermoNicolet Nexus FTIR spectrometer equipped with a liquid nitrogen cooled MCT detector. Each sample of 0.01 g was randomly distributed in 0.20 g of KBr powder. DRIFT spectra were obtained by a coaddition of 512 individual scans with a resolution of 4 cm-1 and then converted into Kubelka-Munk units.

Results and Discussion Cr(VI) Sorption on BC. Figure 1 shows the Cr(VI) sorption kinetics of BC at 25 °C in the common pH range of the soil (pH 3-7). The rate of Cr(VI) sorption decreases with increasing solution pH. To clarify the mechanisms behind Cr(VI) sorption on BC, the oxidation state of Cr sorbed on BC was determined using Cr K-edge XANES (Figure 2). The XANES of K2CrO4 and Cr2O3 used as the standards of Cr(VI) and Cr(III), respectively, are also shown for comparison. The XANES spectrum of K2CrO4 exhibited a strong pre-edge feature at ∆E ) 3.0 eV. This pre-edge feature is a distinct character of Cr(VI) in a tetrahedral coordination; it can be used to identify Cr(VI) in a sample because, comparatively,

FIGURE 3. (a) Fourier transform magnitude of the K-edge EXAFS for Cr sorbed on BC at pH 3 (without phase shift correction) and (b) the corresponding k3-weighted spectrum. Open circles and the solid line represent the experimental and fitted data, respectively. (c) Interatomic distances for Cr(III) sorbed on BC derived by EXAFS analysis.

TABLE 1. Fit Parameters of EXAFS for Cr Sorbed on BC at pH 3 Based on the Feff Phase and Amplitude Functions

c

atomic pairs

CNa

R (Å)b

σ2 (Å2)c

Cr(III)-O Cr(III)-Cr(III)

6.0 ( 0.2 0.5 ( 0.1

1.99 ( 0.02 2.97 ( 0.02

0.0030 0.0022

a CN: coordination number. b R: interatomic distance. σ2: Debye-Waller factor (disorder parameter).

the pre-edge region of Cr(III) in Cr2O3 exhibits two weak peaks at ∆E ) 1.5 and 4.2 eV (Figure 2). Because we saw no apparent presence of the pre-edge feature of Cr(VI) in the XANES spectra for Cr sorbed on BC at pH 3-7 and saw the weak pre-edge features of Cr(III), we conclude that Cr(III) was the predominant oxidation state of Cr sorbed on BC. This was consistent with the result of phosphate extraction, which showed that no detectable Cr(VI) could be extracted from the BC samples collected after Cr(VI) sorption (data not shown). Because Cr(III) was not originally present in the system, the predominant occurrence of Cr(III) on the BC surface indicated that Cr(VI) was sorbed and subsequently reduced to Cr(III) on BC. The local structure of Cr sorbed on BC was further determined using EXAFS. Figure 3a shows the experimental and fitted Fourier Transform (FT) magnitudes derived from the EXAFS spectrum of Cr sorbed on BC at pH 3, where the maximal Cr sorption of BC occurred (Figure 1). The spectrum was fitted over k ) 3.7-13.6 Å-1 (Figure 3b), using the theoretical parameters of the Cr(III)-O and Cr(III)-Cr(III) shells within R ) 1-3 Å. A reasonable fit to the experimental data was obtained with the value of ∑(χexp - χfit)2/∑(χexp)2 ) 0.009 and the fitted structural parameters are listed in Table 1. The strongest peak in the FT magnitude was attributed to the Cr(III)-O shell (Figure 3a) and the corresponding CN and R were 6.0 ((0.2) and 1.99 ((0.02) Å, respectively (Table 1 and Figure 3c). In general, Cr(III) is octahedrally coordinated with a Cr-O distance = 2 Å, while Cr(VI) is tetrahedrally coordinated with a Cr-O distance = 1.6 Å (39). Thus, the CN and R values of the Cr-O shell were consistent with the coordination environment of Cr(III). For the Cr(III)-Cr(III) shell, the CN and R were determined to be 0.5 ((0.1) and 2.97 ((0.02) Å (Table 1). Both Cr-O and Cr-Cr distances agreed well with those reported for Cr(III) sorbed on silica and iron oxides and Cr hydroxide precipitates (40, 41). If Cr(III) forms surface complexes with the surface functional groups of BC but does not form surface precipitates on BC, the Cr(III)-Cr(III) shell cannot be detected. Thus, the

FIGURE 4. DRIFT spectra of BC after reacting with (a) 0, (b) 0.5, (c) 1, and (d) 5 mM Cr(VI) solutions at pH 3. existence of the Cr(III)-Cr(III) shell revealed the occurrence of Cr(III) precipitation on the surface of BC. Nonetheless, the corresponding coordination number of 0.5 for the Cr(III)-Cr(III) shell was much lower than those in the edgesharing geometry in Cr oxides and hydroxides (e.g., 4 in R-Cr2O3 (42) and 6 in R-CrOOH (43)). Because XAS probes the average state of an element in a sample, the reasonable explanation for the low CN of the Cr(III)-Cr(III) shell is that a portion of Cr(III) on the BC surface formed a precipitate while the rest bound to the BC surface through surface complexation. The formation of Cr(III) surface precipitate is expected to be enhanced with increasing pH due to the decrease in the solubility of Cr(III) at higher pH (44). To identify the surface functional groups of BC responsible for Cr(VI) reduction, DRIFT spectra in the fingerprint region were obtained for BC after reacting with 0, 0.5, 1, and 5 mM Cr(VI) solutions at pH 3 (Figure 4). The DRIFT spectrum of the unreacted BC showed bands at 1708, 1595, 1434, 1212, 1103, 902, and 802 cm-1. The band at 1708 cm-1 was assigned to the free carboxyl groups (45-47). The intense 1595 cm-1 band is a superposition of the bands corresponding to the stretching vibrations of conjugated CdC bonds and carbonyl groups (45-48). The band at 1434 cm-1 may be attributed to the stretching of the ionic or chelated carboxylic groups, and the 1212 cm-1 band may be attributed to the overlapping of C-O stretching and O-H bending of the phenolic groups. The C-O stretching of esters is seen at 1103 cm-1 (45-47). The conjugated CdC vibrations contribute to the bands at 902 and 802 cm-1; the 802 cm-1 band also has contributions from the aromatic C-H in-plane vibrations (49). The DRIFT VOL. 43, NO. 23, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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spectra of BC showed that the oxygen-containing functional groups were prevalent in the structure of BC, which was consistent with the result of the Boehm titration (Table S1). After BC was reacted with Cr(VI) of various concentrations, the intensity of the band at 1212 cm-1 decreased, while those at 1595 and 1434 cm-1 increased (Figure 4). The decrease in the intensity of the band at 1212 cm-1 may be attributed to the oxidation of the phenolic groups on the BC surface. As suggested by Elovitz and Fish (32), the oxidation of phenols leads to the formation of quinones containing carbonyl groups. Previous studies also showed that partial oxidation of carbons gives rise to the formation of carbonyl/carboxyl groups on the carbon surfaces (47, 50, 51). Thus, the increasing intensities at 1595 and 1434 cm-1 after the reaction of BC with Cr(VI) may be attributed to the consequent formations of carbonyl and carboxyl groups, respectively, on the BC surface. The formation of ionic or chelated carboxyl groups at 1434 cm-1 also indicates the binding of Cr(III) ion to the groups. Thus, the carboxyl groups provide binding sites for Cr(III) ions resulting from Cr(VI) reduction and therefore facilitate Cr(III) sorption on the BC surface. Examination of Figure 1 showed that the reduction of Cr(VI) by BC preferentially happens at lower pH. There are two explanations for the increase in Cr(VI) reduction with decreasing pH: (1) Decreasing pH results in an increase in the redox potential of the Cr(VI)/Cr(III) couple. Thus, the reduction of Cr(VI) is accelerated at lower pH. (2) Formation of an inner- or outer-sphere complex between the electron donor and acceptor in a redox reaction is the prerequisite for subsequent electron transfer between the redox pair (52). For example, Elovitz and Fish (32) suggested that the reduction of Cr(VI) by phenol proceeds through the formation of a chromate-phenol ester intermediate and the subsequent electron transfer from phenol to Cr(VI) results in quinone and Cr(III). Thus, Cr(VI) sorption is the determinant step for the occurrence of the subsequent reduction of Cr(VI) by the BC surface. In other words, Cr(VI) is bound to the BC surface (predominately, the phenolic groups on BC) and then reduced to Cr(III). Thus, the dependence of the reaction rate on pH also results from the effects of pH on the Cr(VI) sorption of BC due to the charge properties of both sorbate and sorbent. The pKa1 and pKa2 of H2CrO4 are 0.86 and 6.51, respectively (6). Thus, HCrO4- is the predominant Cr(VI) species at 0.86 < pH < 6.51, and CrO42- is predominant at pH > 6.51. Meanwhile, the oxygen-containing functional groups present on the BC surface are also subject to protonation/deprotonation depending on pH. At pH values lower than PZC (i.e., 3.7, Table S1), the surface of BC has a net positive charge; the electrostatic attraction between anionic Cr(VI) and cationic BC surface may enhance the sorption of Cr(VI) on BC and thus the subsequent reduction of Cr(VI) by BC. As the pH was increased, the buildup of negative charge on the BC surface resulted in a prevailing electrostatic repulsion between Cr(VI) and BC, which consequently reduced the Cr(VI) sorption of BC and slowed the subsequent electron transfer from BC to Cr(VI). Cr(VI) Reduction Rate Constant. The reduction of Cr(VI) by BC can be described by pseudo-first-order kinetics with respect to the Cr(VI) concentration, as indicated by the linear relationship (r2 > 0.92) between the logarithmic concentration of Cr(VI) and reaction time (Figure 5 and Table 2). Although this approach was empirical in considering the surface heterogeneity of BC, it allowed us to obtain a concentrationindependent parameter, the reaction half-life, that is advantageous in comparing the Cr reduction rate of BC with that of SOM reported in the literature (5, 12-15). The pseudofirst-order rate constant of Cr(VI) reduction by BC lies in the range of 10-1.56 to 10-3.62 h-1 at pH 3-7. The rate constant at pH 3 for BC is about an order of magnitude higher than the corresponding values reported for SOM, including humic 8804

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FIGURE 5. Logarithmic plot of Cr(VI) sorption kinetics of BC at pH 3-7.

TABLE 2. Pseudo-First-Order Rate Constants and Corresponding Half-Lives of the Cr(VI) Reduction by BC at pH 3-7 Compared with Those by Soil Organic Matter (SOM) Reported in the Literature sorbent

pH log k (h-1)

r2

half life (h)

0.992 0.978 0.979 0.922 0.911

25 116 381 803 2886 256

3 4 5 6 7 humic acid 3.2

-1.56 -2.22 -2.74 -3.06 -3.62 -2.57

4 5.1 humic acid 3

-2.67 -2.70 -2.82

325 348 462

NOM NOM

-2.10 -2.36

88 158

BC

3 3

ref this work

Fukushima et al. (1997) Gu and Chen (2003)

acid (Table 2). The half-life of Cr(VI) reduction by BC was determined to be 25 h, which is much shorter than those for SOM in a range from 88 to 462 h (Table 2). At pH 5, the rate constant of 10-2.74 h-1 for BC is on the same order of magnitude as the value of 10-2.70 h-1 reported for humic acid by Fukushima et al. (14). Thus, the rate constant and half-life reported here indicate that BC is an effective reductant for Cr(VI) and can reduce Cr(VI) more readily than SOM at low pH. SOM has been considered the predominant reductant for Cr(VI) in surface soil but the Cr(VI) reduction of SOM only accelerates at low pH or by heating. As suggested by Gu and Chen (15), conformational change of SOM may result in less reactive sites available for Cr(VI) reduction because the structure of SOM is flexible and can be coiled and aggregated in response to the change of solution condition, such as pH. Comparatively, the structure of BC is considered rigid; the surface functional groups attached to the peripheral aromatic units are always exposed to the aqueous surroundings and are therefore available for Cr(VI) reduction. This may explain the higher effectiveness of BC in Cr(VI) reduction compared with SOM. In addition, since BC may make up the aromatic portion of SOM to a large extent (28-30), these results imply that BC may dominate the other components of SOM with respect to the extent and rate of Cr(VI) reduction in soil. Environmental Significance. BC is an effective reductant for Cr(VI) due to its reactive surface functional groups and large surface area. The mechanism of Cr(VI) interaction with BC has two parts: the sorption of Cr(VI) and the subsequent reduction of sorbed Cr(VI) to Cr(III). The resultant Cr(III) is

bound to the BC surface through surface complexation and precipitation. Considering the long-term and continuous buildup of BC in soil where field fire or crop-residue burning frequently occurs, BC may become an important driver for Cr(VI) reduction in soil and therefore plays a significant role in reducing the toxicity and mobility of Cr(VI) in BC-rich soils. In other words, in the presence of BC, Cr(VI) poses a reduced environmental risk. However, the contribution of BC to the Cr(VI) reduction of SOM has not been discriminated from those of the other SOM components, such as humic substances. Thus, the extent and rate of Cr(VI) reduction reported for SOM and its subcomponents in previous studies may potentially include the contribution of BC. Without considering the contribution of BC to the total Cr(VI) reduction of SOM, the contribution of non-BC fraction of SOM can be significantly overestimated. Thus, the Cr(VI) reduction of SOM reported in the literature may need to be re-examined to differentiate the Cr(VI) reduction capability of the non-BC elements in SOM from that of BC in order to better understand the influence of each individual component of the soil on the fate of Cr(VI) in the environment.

Acknowledgments This work was financially supported by the National Science Council of Taiwan under Project NSC97-2313-B-005-024MY3 and by the Ministry of Education of Taiwan under the ATU plan. This research was carried out (in part) at the National Synchrotron Radiation Research Center at Hsinchu, Taiwan.

Supporting Information Available Measurement and description of the elemental composition, point of zero charge (PZC), surface area, porosity, and surface functionality for the BC. This information is available free of charge via the Internet at http://pubs.acs.org.

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