Nanometer-Scale Chemical Heterogeneities of Black Carbon Materials and
Their Impacts on PCB Sorption Properties: Soft X-ray Spectromicroscopy
Study

Tae Hyun Yoon^1,2^, Karim Benzerara^1,3^, Sungwoo Ahn^4^, Richard G.
Luthy^4^,

Tolek Tyliszczak^5^, and Gordon E. Brown, Jr^1,6\*^

[^1^S]{.smallcaps}urface & 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^]{.smallcaps}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](mailto:taeyoon@hanyang.ac.kr)

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.

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 (K~om~), 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 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 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.

Experimental details

**Sample preparation**: Three different types of carbonaceous material
were examined with soft x-ray 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 coal-based
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 μl
of each suspension was dropped on 100 nm-thick Si~3~N~4~ 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 CO~2~. 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 L~3~-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
(I~0~) 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. However, no significant changes were
observed. AXis2000 software (ver2.1p)^28^ was used to align image stacks
and extract NEXAFS spectra from raw data of image stack or line scans.

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  π\*~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  π\*~C-OH~ transition) and/or ketonic
groups (originated from the carbon 1s  π\*~C=O~ transition), and
\~288.7 eV for carboxylic groups (from the carbon 1s  π\*~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 top-left 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 heterogeneities in anthracite coal. To obtain more detailed
chemical information on carbon-containing 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 
π\*~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 graphite^31^, amorphous carbon^31^, fullerenes (e.g., C~60~,
C~70~ etc.),^32,33^ and multi-walled carbon nanotubes^34^, the most
feasible assignment for the broad peak at 285.9 eV is a distorted or
curved graphene structure having an intermediate hybridization between
sp^3^ and sp^2^ for carbon. According to Comelli et al.^31^, the C
K-edge spectra of graphite (sp^2^ hybridized carbon) and diamond (sp^3^
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
fullerenes^32,33^ display peaks around 286 eV (e.g., 285.8 and 286.3 eV
for C~60~^32^) as well as the first π\* orbital transition at lower
energy (e.g., 284.4 eV for C~60~^33^). These spectral features 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)
observations^36^ 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 L~2,3~ --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](http://encyclopedia.laborlawtalk.com/Water),
[coal-gas](http://encyclopedia.laborlawtalk.com/Coal-gas)**,** and
[coal-tar](http://encyclopedia.laborlawtalk.com/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 S**1(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, the coke particles
show chemical heterogeneities at the submicrometer scale (**Figure
S1(B)** and S**1(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 m^2^/g) due to its highly porous
microstructure.^2,15^ AC is also known to have a very high organic
carbon-normalized partition coefficient (K~oc~) for HOC compared to
other black carbon materials^37^ (e.g., soot carbon and coal) as well as
NOM^38^ (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 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 (SSA~anthracite~ = 4.2m^2^/g, SSA~coke~ = 3.2
m^2^/g, SSA~AC~ = 1032 m^2^/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 (sp^2^ 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
HRTEM study^36^ 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 m^2^/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 L~3~-edge), which shows
significant contrast within the particle clusters. To further assess the
spatial heterogeneity of PCB within this particle cluster, a Cl
L~3~-edge line scans across points "**a**" and "**b**" were performed,
and Cl L~3~-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 L~3~-edge 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 I~o~). 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 L~3~-edge 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 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 L~3~-
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., PCB-166) 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., π−π interactions or multiple H-H bonding
between HOC and BC materials as proposed based on quantum chemical
simulations by Kubicki and Apitz^39^) 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
studies^8-11,15,16,37,38,40^ have shown that aged black 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.

**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.

Supporting Information

![](media/image1.png){width="5.501388888888889in"
height="4.707638888888889in"}

Figure S1. STXM images of Coke particles measured at (A) E = 286.9 eV
(B) E = 285.7 eV (energy corresponds to the 1s-π\* transition of C=C
bond in aromatic group) (C) E = 286.7 eV (energy corresponds to the
1s-π\* 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)

![](media/image2.jpeg){width="2.795275590551181in"
height="3.6318897637795278in"}![](media/image3.pct){width="2.9220472440944882in"
height="4.35748031496063in"}

**Figure S2.** STXM image of PCB-166 spiked activated carbon particle
measured at (A) E = 200.7 eV (above Cl L~3~-edge) and (B) Cl L~3~-edge
NEXAFS spectra collected from different regions of image (A) indicated
by **a** and **b**.

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![](media/image5.jpeg){width="3.2318897637795274in"
height="4.5in"}![](media/image6.pct){width="2.9736220472440946in"
height="4.407086614173228in"}

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)

![](media/image8.png){width="5.501388888888889in"
height="4.709722222222222in"}

Figure 2. STXM image of activated carbon particle measured at (A) E =
286.8 eV (B) E = 285.1 eV (energy corresponds to 1s-π\* transition of
C=C bond in aromatic group) (C) E = 286.7 eV (energy corresponds to
1s-π\* 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. 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)

![](media/image9.pct){width="1.9590551181102362in"
height="3.416535433070866in"}![](media/image10.jpeg){width="2.4688976377952754in"
height="1.4645669291338583in"}![](media/image11.jpeg){width="2.4484251968503936in"
height="1.499212598425197in"}![](media/image12.pct){width="1.9590551181102362in"
height="3.4161417322834646in"}

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 L~3~-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)