Soft X-ray Spectromicroscopy Study of

Nanometer-Scale Chemical Heterogeneities of Black Carbon Materials and Their Impacts on PCB Sorption Properties: Soft X-ray Spectromicroscopy Study Tae Hyun Yoon1,2, Karim Benzerara1,3, Sungwoo Ahn4, Richard G. Luthy4, Tolek Tyliszczak5, and Gordon E. Brown, Jr1,6* 1 Surface & Aqueous Geochemistry Group, Department of Geological & Environmental Sciences, Stanford University, Stanford, CA 94305-2115, USA 2 Department of Chemistry, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul, 133-791, Korea 3 Laboratoire de Minéralogie-Cristallographie, UMR 7590 CNRS and Institut de Physique du Globe de Paris, 4 place Jussieu, 75252 Paris Cedex, France 4 Department of Civil & Environmental Engineering, Stanford University, Stanford, CA 94305-4020, USA 5 Chemical Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA 6 Stanford Synchrotron Radiation Laboratory, SLAC, MS 69, 2575 Sand Hill Road, Menlo Park, CA 94025, USA Manuscript # ES06 0173+ Revised Version Resubmitted to Environmental Science & Technology April 28, 2006 * Corresponding author: E-mail: gordon@pangea.stanford.edu -1- Abstract Synchrotron-based soft x-ray spectromicroscopy was used to probe nanometer-scale chemical heterogeneities of black carbon (BC) materials, including anthracite coal, coke, and activated carbon (AC), and to study their impact on the partitioning of one type of polychlorinated biphenyls (PCB-166 - 2,3,4,4',5,6 hexachloro biphenyl) onto AC particles. Various carbon species (e.g., aromatic, ketonic/phenolic, and carboxylic functional groups) were found in all of the BC materials examined, and impurities (e.g., carbonate and potassium ions in anthracite coal) were identified in nanometer-scale regions of these samples. We show that these chemical heterogeneities in AC particles influence their sorption of hydrophobic organic compounds (HOCs). PCB-166 was found to accumulate preferentially on AC particles with the highest content of aromatic functionalities. These new findings from x-ray spectromicroscopy have the following implications for the role of BC materials in the environment: (1) the functional groups of BC materials vary on a 25-nanometer scale, and so does the abundance of the HOCs; (2) molecular-level characterization of HOC sorption preferences on AC will lead to an improved understanding of AC sorption properties for the remediation of HOCs in soils and sediments. -2- Introduction Carbonaceous materials (CM), including all non-carbonate carbon-containing matter (e.g., natural organic matter (NOM) and black carbon (BC)),1 are ubiquitous in the environment (e.g., humic substances in soils and aquatic environments, air-borne particulate matter from diesel fuel combustion and biomass burning, and various types of coals as energy sources) and have a broad range of industrial applications (e.g., activated carbon as adsorbents for water and gas purification processes and carbon fiber as a reinforcing component in composite materials).2 One of the important environmental issues related to carbonaceous materials is their ability to sorb hydrophobic organic compounds (HOCs), such as PCBs (polychlorinated biphenyls) and PAHs (polycyclic aromatic hydrocarbons) in soils and sediments.3 Partitioning of these toxic pollutants between CM and aqueous solutions is one of the most critical factors determining their bioavailability and mobility. A better understanding of such partitioning is required in order to design remediation strategies for HOC’s in both groundwater and surface water resources.1,3 Until recently, this partitioning behavior was often predicted by simple linear sorption isotherm models based on an equilibrium partitioning approach using organic matter partitioning parameters (Kom), which assume CM to be chemically homogeneous. This simple and widely used modeling paradigm generally works well at high concentrations of non-polar organic compounds in “normal” soil and sediment organic matter. However, this approach often overpredicts the concentrations of PAHs and PCBs in the aqueous phase, and deviation from a linear sorption isotherm is most significant at low total HOC concentrations in soils and sediments containing thermally altered carbonaceous materials (e.g., soots, coals, and kerosene).4-6 Recently, a new modeling paradigm has been suggested that considers CM to be a heterogeneous material composed of black carbon and amorphous organic carbon.1,3,7-14 This -3- new paradigm describes the sorption of HOCs onto CM as a combination of (1) surface adsorption to black carbon materials (non-linear term) and (2) phase partitioning onto amorphous organic carbon (linear term), and it provides better predictive capability.1,3, 7-14 As a result of this improved modeling approach, a more quantitative assessment of the chemical heterogeneities in CM is needed. The sorption of HOCs on BC components has been compared with the sorption of HOCs on other components of sediments to better understand their sorption preferences. These studies show that HOCs are preferentially sorbed onto BC relative to other organic or mineral components.4-6 In addition, the presence of BC was found to make HOCs less bioavailable in soils and sediment.15-17 However, because BC comes from different sources and has experienced a range of diagenesis and weathering conditions, 2,18 BC found in soils and sediments probably has significant chemical heterogeneities. Such heterogeneities may cause significant changes in the sorption mechanism and strength of binding of HOCs in soils and sediments containing BC. Current understanding of the nature of BC components, their distribution within BC particles, and their relationship to the sorption properties of pollutants, especially HOCs, is limited because only a few molecular-level studies combining both spectroscopy and microscopy have been performed, mainly due to the lack of appropriate tools to probe chemical heterogeneities at the nanometer scale under in situ conditions. Synchrotron-based soft x-ray spectromicroscopy (i.e., scanning transmission x-ray microscopy (STXM) and near edge x-ray absorption fine structure (NEXAFS) spectroscopy) are well suited to address questions about chemical heterogeneities of BC at the nanometer spatial scale and the impact of such heterogeneities on HOC sorption properties. STXM is one of the few methods capable of characterizing particles ~ 30 nm in diameter in the presence of water -4- and/or under atmospheric conditions. NEXAFS spectroscopy combined with STXM imaging on environmental and biological samples can provide chemical information at this spatial scale with very high spectral resolution (< 0.1 eV at C K-edge).19,20 In addition, the ability to tune the x-ray energy from a synchrotron light source enables element-specific mapping of many environmentally relevant species.19,20 The potential of soft x-ray spectromicroscopy for studying BC was first recognized by Cody et al.21,22, who demonstrated the presence of multiple components within various coal materials at the submicrometer scale, although with limited spectral resolution (~ 0.3 eV). Recent soft x-ray spectroscopy studies on BC have been performed using STXM instruments with much improved spectral resolution (0.05 ~ 0.1 eV), confirming the existence of various components in BC as well as characterizing them spectroscopically.23-26 For example, Lehmann et al.25 recently studied the chemical heterogeneities of BC particles collected from a Brazilian soil using synchrotron-based soft x-ray spectromicroscopy. They found chemical heterogeneities within single BC particles, which had highly aromatic cores and strongly oxidized surface regions containing a high content of carboxyl and carbonyl functional groups. The highly aromatic core region corresponds to the original BC. However, it is not clear whether the strongly oxidized surface regions resulted from oxidation of the original BC particles by microbial or abiotic processes or from the sorption of NOM layers on the BC particles. The purpose of the present study is to investigate chemical heterogeneities of BC components in carbonaceous materials using three representative samples (i.e., anthracite coal, coke, and activated carbon) which have only trace amounts of volatile organic matter and have not been exposed to soil or sediment environments. We will show that chemical heterogeneities affect the distribution of HOCs (i.e., PCB-166) in these materials. -5- Experimental details Sample preparation: Three different types of carbonaceous material were examined with soft xray spectromicroscopy in this study. Coke was obtained from Ispat Inland Inc. (East Chicago, IL); anthracite coal was purchased from George L. Throop Company (Pasadena, CA); and coalbased activated carbon (TOG 50x200) was purchased from Calgon Carbon Corporation (Pittsburgh, PA). These raw materials were finely ground and prepared for STXM experiments as follows. Powdered carbonaceous materials were suspended in DI water (~0.5 g/L), and approximately 1 l of each suspension was dropped on 100 nm-thick Si3N4 membranes (Silson Ltd.), which were then air-dried. A PCB-spiked activated carbon sample was prepared in the following way. PCB-166 (2,3,4,4',5,6 hexachloro biphenyl) was obtained from Ultra Scientific (North Kingstown, RI). 25 mg of fine activated carbon powder was placed in a 14-ml vial and immersed in approximately 8 ml of hexane. 500 ml of PCB-166, dissolved in hexane at 100 mg/ml concentration, was spiked and the sample was equilibrated overnight with occasional shaking. Hexane was completely evaporated under a gentle stream of nitrogen gas after the overnight equilibration, and the vial containing the dried PCB-spiked activated carbon was rolled on a horizontal roller for an additional 48 to 72 hours for further equilibration until the sample was prepared for STXM analysis. STXM experiments: STXM studies were performed at the Advanced Light Source (ALS) on branch line 11.0.2.2 with the synchrotron storage ring operating at 1.9 GeV and 200-400 mA stored current. More details about this branch line can be found in Bluhm et al..27 Energy calibrations were made using the well-resolved C 3p Rydberg peak at 294.96 eV of gaseous CO2. -6- The STXM sample chamber was filled with He to minimize attenuation of the soft x-rays. The detector used in all measurements was a Si photodiode or a PMT with a phosphor scintillator. Image stacks or line/point scans of carbonaceous materials were performed to collect C K-edge or Cl L3-edge NEXAFS spectra. Stacks of images or lines were taken by scanning in the x-y direction (image stack) or x direction (line scan) of selected sample areas at each energy increment over the energy range of interest; here, x refers to the horizontal direction, y to the vertical direction, and the x-y plane to the plane perpendicular to the x-ray beam. Normalization and background correction of the NEXAFS spectra were performed by dividing each spectrum by a second spectrum (I0) taken at a location on the sample at which the element of interest was absent. Possible sample changes caused by x-ray radiation were also monitored by looking for changes in the carbon K-edge spectra over the course of each experiment. significant changes were observed. However, no AXis2000 software (ver2.1p)28 was used to align image stacks and extract NEXAFS spectra from raw data of image stack or line scans. -7- Results and Discussion Chemical Heterogeneities of Black Carbon at the Nanometer Scale: Anthracite (coal): Anthracite is the highest rank of coal. It is hard and brittle with black lustrous features, and contains a high percentage of fixed carbon and a low percentage of volatile matter. Due to its composition, we believe that anthracite coal may represents BC particles with minimum contamination by NOM inclusions. Carbon K-edge NEXAFS spectral features of various carbon materials have been previously studied.23-26,29,30 According to this earlier work, aromatic groups typically show a characteristic peak at 284.9 to 285.5 eV (from the carbon 1s  *C=C electronic transition of the aromatic C=C double bonds), while more oxidized carbon functional moieties have characteristic electronic transitions at higher energies: ~286.7 eV for phenolic (from the carbon 1s  *C-OH transition) and/or ketonic groups (originated from the carbon 1s  *C=O transition), and ~288.7 eV for carboxylic groups (from the carbon 1s  *C=O transition). To probe the spatial distribution of these reduced and oxidized carbon functional groups, we collected STXM images at 285.1 eV (Figure 1(A), corresponding to aromatic groups) and 286.7 eV (Figure 1(B), corresponding to ketonic or phenolic groups) on the same region of anthracite coal particles. These two STXM images collected show dramatic differences in contrast. In the image taken at 285.1 eV, a large, a dark cluster (with cracks developed between particles) is visible on the topleft side of Figure 1(A), indicating a relatively high concentration of aromatic groups in this region. However, in the image taken at 286.7 eV, significant x-ray absorption caused by other particles is observed in most of Figure 1(B), indicating the presence of more oxidized forms of carbon in this region. These differences in x-ray contrast indicate the presence of chemical -8- heterogeneities in anthracite coal. To obtain more detailed chemical information on carboncontaining functional groups present in the area depicted in Figure 1, we also collected a stack of STXM images over the C K-edge energy range. Spectra from different areas in the anthracite coal particles of Figure 1 were extracted and are presented in Figure 1(C). These C K-edge NEXAFS spectra show various chemical species in this region, as revealed by absorption peaks at 284.7, 285.9, 286.7, 288.7, 290.0, 297.4 and 299.9 eV. As discussed earlier, the peaks at 286.7 and 288.7eV correspond to ketonic (or phenolic) and carboxylic functional groups, respectively. These spectral features due to oxidized carbon were dominantly observed in the C K-edge NEXAFS spectrum from region “c”, suggesting that these smaller particles probably have more hydrophilic character. In contrast, the C K-edge NEXAFS spectrum from region “a” contains additional peaks at 284.7, 285.9 and 290.0 eV. Although slightly lower in energy than expected for aromatic carbon, the peak at 284.7 eV can be assigned to the carbon 1s  *C=C electronic transition of the aromatic group, while the peak at 290.0 eV can be assigned to carbonate impurities included in this hard coal. The origin of the peak at 285.9 eV, however, is not as clear. Based on previous C K-edge NEXAFS studies on various carbon materials such as graphite31, amorphous carbon31, fullerenes (e.g., C60, C70 etc.),32,33 and multi-walled carbon nanotubes34, the most feasible assignment for the broad peak at 285.9 eV is a distorted or curved graphene structure having an intermediate hybridization between sp3 and sp2 for carbon. According to Comelli et al.31, the C K-edge spectra of graphite (sp2 hybridized carbon) and diamond (sp3 hybridized carbon) show only a single intense electronic transition in the 280 to 290eV energy range (285 eV for graphite and 290 eV for diamond), while those of distorted graphene structures such as fullerenes32,33 display peaks around 286 eV (e.g., 285.8 and 286.3 eV for C6032) as well as the first * orbital transition at lower energy (e.g., 284.4 eV for C6033). These spectral features -9- originate from the mixed nature of carbon hybridization of these materials, resulting from the distortion of the planar graphene layer.35 Recent high resolution transmission electron microscopy (HRTEM) observations36 also support our suggestion of highly distorted local structures in anthracite coal. From these peak assignments, we propose that region “a” (Fig. 1(B)) represents an anthracite coal component having a slightly oxidized, highly aromatic nature, resulting in hydrophobic characteristics. NEXAFS spectra collected for region “b” (within the crack developed into the anthracite coal particle cluster) show a doublet located at 297.4 and 299.9 eV in addition to the other spectral features observed in regions “a” and “c”. This doublet feature can be assigned to potassium L2,3 –edge absorption, which is consistent with the inclusion of inorganic impurities associated with the highly oxidized components of anthracite coal or carbonate impurities within the crack of the particle cluster. Coke: We also performed similar measurements on the coke particles. Coke is a solid carbonaceous residue derived from low-ash, low-sulfur bituminous coal from which the volatile constituents (including water, coal-gas, and coal-tar) are driven off by baking in an airless oven at temperatures as high as 1,000°C; therefore, these coke particles are almost free of volatiles (<0.4 % for this coke sample as measured by ASTM D3172 method). A STXM image collected at 286.9 eV on a finely ground coke powder is shown in Figure S1(A) (see supporting information), and two additional STXM images obtained from the same sample region at two different energies are also presented Figure S1(B) and S1(C). Carbon K-edge NEXAFS spectra extracted from different areas of the image stack (see Figure S1(A); areas indicated by red lines and labeled a, b, c, d, e, f, and g) are presented in Figure S1(D) to show chemical heterogeneities of these coke particles. Similar to the anthracite coal particles, - 10 - the coke particles show chemical heterogeneities at the submicrometer scale (Figure S1(B) and S1(C)); however, impurities (e.g., carbonate and potassium found in anthracite coal) were rarely observed, and distorted forms of carbon with mixed hybridization structures were less distinct in the coke particles. The NEXAFS spectrum from region a (Figure S1(A)) is dominated by spectral features representative of aromatic moieties, whereas the spectral features of regions b, c, and d indicate mixtures of aromatic, ketonic/phenolic, and carboxylic functional groups. In contrast, regions e, f, and g, which are not apparent in Figure S1(B) but are visible in Figure S1(C), show spectral features dominated by ketonic/phenolic and carboxylic functional groups with only a minor contribution from aromatic functional groups. Because there is only a small amount of volatile organic carbon in this coke material (<0.4 %), this highly oxidized region is most likely due to highly oxidized surface regions of coke particles, rather than to volatile NOM inclusions as previously reported by Lehmann et al.25 for a black carbon particle from a Brazilian soil. Activated Carbon (AC): The Calgon AC used in this experiment is a BC made from coal that was pyrolized and oxidized under controlled conditions. This activated carbon material is known to have an extremely small amount of extractable organic compounds and an exceptionally high surface area (1,032 m2/g) due to its highly porous microstructure.2,15 AC is also known to have a very high organic carbon-normalized partition coefficient (Koc) for HOC compared to other black carbon materials37 (e.g., soot carbon and coal) as well as NOM38 (e.g., humic acid). STXM images and NEXAFS spectra collected on a finely ground activated carbon powder are shown in Figure 2. Experimental methods similar to those used for the anthracite coal and coke samples were used to collect STXM images and carbon K-edge NEXAFS spectra - 11 - on the AC sample. STXM images collected at 285.1 eV, which corresponds to the x-ray absorption resonance of aromatic groups, show only few small dark spots, indicating that regions dominated by aromatic groups represent only a small portion of the mapped area (Figure 2(B)). However, at 286.7 eV, which corresponds to the absorption energy of ketonic C=O or phenolic Ar-OH bonds (see Figure 2(C)), the other AC particles display high contrast. Carbon K-edge NEXAFS spectra (Figure 2(D)) clearly show ketonic and/or phenolic groups (peak at 286.7eV) and carboxylic groups (peak at 288.7eV) in these AC particles, whereas only a small portion of the mapped area (regions a, b, and c) shows highly aromatic spectral features. Compared to the micrometer sized aromatic-rich domains widely distributed in the anthracite coal (Figure 1) and coke (Figure S1) samples, aromatic-rich regions in the AC powder were found only in a few locations with diameters less than a few hundred nanometers. This difference is probably due to the highly porous and highly oxidized nature of the AC samples compared to the anthracite coal and coke samples (SSAanthracite = 4.2m2/g, SSAcoke = 3.2 m2/g, SSAAC = 1032 m2/g)15. The FWHM of the C K-edge peak corresponding to the aromatic component of the AC sample is narrower than that of the coke particle (Figure S1). Moreover, the spectral features associated with the distorted graphene layers observed in anthracite coal (Figure 1) were rarely found in the C K-edge NEXAFS of this AC, indicating that aromatic groups in the AC sample are more homogeneously distributed than those in the anthracite coal and coke examined. The NEXAFS spectral features in the reduced carbon regions of activated carbon suggest that these particles contain relatively well-ordered stacks of graphene layers (sp2 hybridization) rather than highly distorted and curved structures containing mixed hybridization states (e.g., fullerenes, carbon nanotubes). This difference is probably due to the high temperature pyrolysis and activation procedure used to prepare the highly nanoporous structures of activated carbon. A recent - 12 - HRTEM study36 also reported similar observations on the changes of local structures in BC materials; highly disordered local structures were observed in anthracite raw materials and more ordered local structures were found in heat-treated (750° and 1000°C) and moderately activated (SSA ~ 1000 m2/g) anthracite materials. We have shown that all three types of BC material examined in this study have chemical heterogeneities at the nanometer scale. Some areas have highly aromatic character with only minor oxidation, while other areas contain highly oxidized functional groups (e.g., carboxylic, ketonic, and phenolic groups), which can be ascribed to the various acidic and basic properties of these carbonaceous materials. In addition, depending on the conditions they have experienced, BC materials display various levels of chemical heterogeneities including different types of reduced carbon (e.g., highly distorted graphene layers and well-ordered graphite layer) and impurities (e.g., carbonate and inorganic salts). Because the chemical heterogeneities of BC materials may result in different regions having different binding affinities for HOCs, probably caused by different adsorption mechanisms, we also studied the impact of these chemical heterogeneities on the partitioning of HOC (i.e., PCB-166) in activated carbon, using a simplified model laboratory system. Heterogeneous Partitioning of PCB on Activated Carbon: Additional experiments were conducted to assess the impact of chemical heterogeneities on the distribution of PCB in BC material. We used PCB-166-spiked AC powders and soft x-ray spectromicroscopy measurements at the carbon K-edge and the chlorine L-edge to probe the spatial heterogeneities of different carbon functional groups and PCB-166, respectively. Figure S2(A) (see supporting information) displays a STXM image taken at 200.7 eV (above Cl L3edge), which shows significant contrast within the particle clusters. To further assess the spatial - 13 - heterogeneity of PCB within this particle cluster, a Cl L3-edge line scans across points “a” and “b” were performed, and Cl L3-edge NEXAFS spectra for each region were collected. Since there was no detectable Cl signature in the unspiked AC powders, the Cl signatures in the Cl L3edge NEXAFS spectra (Figure S2(B)) provide evidence for significant enrichment of Cl (i.e., PCB-166) in region “a” compared to adjacent region “b” (the background absorption, i.e., optical densities, caused by black carbon materials is similar - 1.05 for region “a”, 0.91 for region “b”). However, for the region shown in Figure S2(A), C K-edge spectra with reasonable spectral quality could not be obtained due to saturation of the C absorption signal caused by particles that were too thick (high quality NEXAFS spectra can only be collected for a sample with a thickness resulting in 30 to 80 % transmission of Io). Other regions with smaller AC particle sizes and similar PCB enrichments were searched for and were found within the activated carbon particle clusters in several regions of our sample. Figures 3(A) and 3(B) show two of these areas where most of the AC particles have diameters less than 200 nm, and corresponding optical densities at the C K-edge are within an appropriate range for C K-edge NEXAFS spectroscopy. However, due to the smaller thicknesses of these AC particles, much less Cl absorption was detected even for the PCB-enriched particles, and point scans with much longer acquisition times (500 ms/pt) were needed to obtain reasonable S/N ratios in the Cl L3edge spectra. Several regions of the AC sample spiked with PCB-166 showed correlations between the local chemical properties of AC particles and their PCB sorption properties. For example, as can be seen in Figure 3(C), the C K-edge NEXAFS spectra of regions “a” and “b” show quite different chemical characteristics. For region “a” we observed intense peaks from aromatic groups (285.1eV) and ketonic/phenolic groups (286.7 eV) as well as a very small contribution - 14 - from carboxylic groups, whereas the C K-edge spectrum of region “b” (see Figure 3(C)) is dominated by ketonic/phenolic (286.7 eV) and carboxylic (288.7 eV) groups, with only a minor contribution from aromatic groups. Cl L3- edge NEXAFS spectra from the same regions (Figure 3(D)) show that there are positive correlations between local chemical properties of AC particles and their PCB sorption properties. As shown in Figure 3(D), significantly more Cl (i.e., PCB166) was detected in the AC particles with more aromatic carbon moieties, whereas a negligible amount of PCB sorption was observed for the AC particles with a high content of carboxylic functional groups. Because of the small number of AC particles examined in this study on which PCB-166 was sorbed and because we used only one type of PCB, we are hesitant to generalize the correlation found between aromatic-rich regions on BC materials and PCB concentrations in these same regions, which suggests preferential sorption of PCB on regions of BC rich in aromatic functional groups. Nonetheless, these are the first direct observations of PCB partitioning onto a BC material at the 25-100 nanometer scale, and they show that PCB-166 is preferentially accumulated in regions with high proportions of aromatic carbon moieties on the AC particles examined. However, determination of detailed HOC sorption mechanisms (e.g.,  interactions or multiple H-H bonding between HOC and BC materials as proposed based on quantum chemical simulations by Kubicki and Apitz39) is not possible from our spectra. Further spectroscopic studies combined with theoretical modeling of HOC sorption mechanisms are needed to address this issue. Implications of Heterogeneous Partitioning of PCB in Black Carbon Materials: The dominant roles played by black carbon materials in the sequestration of PAHs and PCBs in sediments are well known. For example, recent studies8-11,15,16,37,38,40 have shown that aged black - 15 - carbon particles are mainly responsible for the accumulation of PAHs and PCBs in sediments, and this association makes HOCs less available to the biosphere. In this study, we show that black carbon materials have chemical heterogeneities at the 25-100 nanometer scale, which can vary in extent and types of chemical moieties depending on the type of black carbon materials. More importantly, the spatial distribution of local chemical heterogeneities also determines spatial chemical affinities of BC for HOCs and affects their partitioning between the chemically heterogeneous components of BC. These new findings have several implications for future studies of black carbon materials. (1) Clearly, specific surface area is a very important factor, but the sequestration of HOCs by black carbon materials in sediments can vary depending on the extent of chemical heterogeneities and the ratios of oxidized vs. reduced carbon between components of black carbon materials, which also influence the sorption properties of HOCs onto black carbon materials. Thus, in addition to specific surface area and pore size distribution, the ratios of BC components as well as the total BC content of soils and sediments need to be characterized to understand HOC sorption behavior in these geomedia. (2) More systematic molecular-level studies of the relationship between PCB sorption properties and the surface chemistry of BC components will enable us to understand the types and mechanisms of PCB sorption on BC materials. This molecular-level understanding will also aid in the development of engineered BC material for remediation purposes (e.g., functionalized activated carbon). (3) Soft x-ray spectromicroscopy holds significant promise for characterization of nanometer-sized black carbon particles in atmospheric and aqueous environments (e.g., soot particles from diesel exhaust), which may play important roles in the transport of HOC pollutants as well as their bioavailability in nature. - 16 - Acknowledgments. We gratefully acknowledge the support of NSF Grant CHE-0431425 (GEB and THY) (Stanford Environmental Molecular Science Institute) as well as support from the Woods Institute for the Environment at Stanford University (GEB and KB), the Ford Fund (SA), and the DoD’s Strategic Environmental Research and Development Program (RGL). We also thank Dr. D. Smithenry and Y. M. Cho of Stanford University for helping with the PCB-spiked sample preparation. The work at the ALS and ALS BL 11.0.2 was supported in part by the Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences, and Division of Chemical Sciences, Geosciences, and Biosciences of the U.S. Department of Energy at Lawrence Berkeley National Laboratory under contract No. DE-AC03-76SF00098. - 17 - Supporting Information (A) (D) (B) (C) Figure S1. STXM images of Coke particles measured at (A) E = 286.9 eV (B) E = 285.7 eV (energy corresponds to the 1s-* transition of C=C bond in aromatic group) (C) E = 286.7 eV (energy corresponds to the 1s-* transition of ketone C=O or phenolic Ar-OH bond), and (D) C K-edge NEXAFS spectra collected from different areas of image (A) indicated by a, b, c, d, e, f, and g. (Energy positions for vertical dashed lines are 285.1, 286.7, 288.7 and 292.5 eV) - 18 - 1 .4 (A) 1 .3 (B) 1 .2 O p tic a l D e n s ty a 1 .1 a b 1 .0 b 0 .9 0 .8 196 198 200 202 204 206 208 210 E n e rg y (e V ) Figure S2. STXM image of PCB-166 spiked activated carbon particle measured at (A) E = 200.7 eV (above Cl L3-edge) and (B) Cl L3-edge NEXAFS spectra collected from different regions of image (A) indicated by a and b. - 19 - References 1. Allen-King, R. M.; Grathwohl, P.; Ball, W.P. New modeling paradigms for the sorption of hydrophobic organic chemicals to heterogeneous carbonaceous matter in soils, sediments, and rocks. Adv. Water Resources 2002, 25, 985. 2. Goldberg, E. D. Black Carbon in the Environment: Properties and Distribution. Wiley: Chichester, UK, 1985. 3. Luthy, R. G.; Aiken, G. R.; Brusseau, M. L.; Cunnimgham, S. D.; Gschwend, P. M.; Pignatello, J. J.; Reinhard, M.; Traina, S. J.; Weber, W. J. Jr.; Westall, J.C. Sequestration of hydrophobic organic contaminants by geosorbents. Environ. Sci. Technol. 1997, 31, 3341. 4. McGroddy, S. E.; Farrington, J. W. Sediment porewater partitioning of polycyclic aromatic hydrocarbons in three cores from Boston Harbor, Massachusetts. Environ. Sci. 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Chem. 2005, 24, 2185 - 23 - (A) 1.6 1.4 c 1.2 1.0 b 0.8 (B) b 0.6 a 0.4 a c 0.2 0.0 (C) 280 285 290 295 300 305 -0.2 E n e rg y (e V ) Figure 1. STXM image of anthracite coal particles measured at (A) E = 285.1 eV and (B) E = 286.7 eV. (C) C K-edge NEXAFS spectra collected from different regions of image (B) indicated by a, b, and c. (Energy positions for the vertical dashed lines are 284.7, 285.9, 286.7, 288.7, 290.0, 297.4, and 299.9 eV) - 24 - (A) (D) (B) (C) Figure 2. STXM image of activated carbon particle measured at (A) E = 286.8 eV (B) E = 285.1 eV (energy corresponds to 1s-* transition of C=C bond in aromatic group) (C) E = 286.7 eV (energy corresponds to 1s-* transition of ketone C=O or phenolic Ar-OH bond), and (D) C Kedge NEXAFS spectra collected from different areas of image (A) indicated by a, b, c, d, e, f, and g. Big dark spot located at the bottom left region of STXM images are due to uncrushed big particle. In this region, most of the x-ray was absorbed and NEXAFS analysis of this region is impossible. (Energy positions for the vertical dashed lines are 285.0, 286.7, 288.7, and 292.5 eV) - 25 - 2 .0 0 .3 (A) a 1 .5 (C) 0 .2 (D) a 1 .0 0 .1 a (B) 0 .5 b 0 .0 b b 0 .0 280 285 290 295 300 305 195 200 205 210 E n e rg y (e V ) E n e rg y (e V ) Figure 3. STXM image of PCB-166 spiked activated carbon particle measured at (A) E = 285.2 eV and (B) E=288.7 eV. (C) Carbon K-edge and (D) Chlorine L3-edge NEXAFS spectra collected from different regions of image (A) and (B) indicated by a and b. (Energy positions for the vertical dashed lines are 285.1, 286.7, and 288.7 eV) - 26 -