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By
Scott A. Stout, Stephen Emsbo-Mattingly, Allen D. Uhler and
Kevin McCarthy
Introduction
Coal and its by-products have had a tremendous influence in
the industrial history of the United States and many other
countries. From its utility in the production of coke in the
smelting of iron since the mid-1800's, to its use in producing
steam for transportation and machinery in the late 1800's to
early 1900's, to its modern use in the production of
electricity, this one substance exceeds or rivals petroleum in
the influenced it has had the industrial history of the U.S.
Environmental forensic investigations commonly encounter the
hydrocarbons derived from coal-derived liquids, such as coal
tar, creosote, or naphthalene oil (e.g., Emsbo-Mattingly et
al., 2000). However, the operational history of many
industrial sites also included the storage of 'lump' coal as a
feedstock in the production of, for example, coke,
manufactured gas, or steam. As such, some of these properties,
as well as surrounding properties and waterways contain solid
coal particles that were spilled, blown, washed or otherwise
distributed within area's soils and sediments. In some areas,
naturally occurring coal particles eroded from sedimentary
rock outcroppings containing coal seams could occur in nearby
sediments. While these solid coal particles, some as small as
a few microns, are not contaminants per se under CERCLA, their
presence in soils and sediments can increase the
concentrations of extractable total hydrocarbons (TPH) or
polycyclic aromatic hydrocarbons (PAH) contained in those
soils or sediments (see below), and thereby, affect forensic
interpretations and property management decisions. Therefore,
it is important to recognize and consider the influence of
particulate coal in forensic and other environmental site
investigations.
In this paper, we discuss some microscopic (petrographic) and
chemical
techniques that might permit the recognition of particulate
coal's influence
on soil and sediment characterization, provide some relevant
petrographic
and chemical data surrounding the character of different coal
ranks.
What is Coal?
Coal is a sedimentary rock predominantly comprised of the
lithified
remains of plants that accumulated in ancient mires, marshes,
and swamps. The characteristics of any given coal will depend
upon a combination of the original biological input materials
(coal type) and the degree to which the coal-bearing strata
have been altered by the affects of pressure and temperature
associated with burial over geologic time (coal rank; Table
1).
On a molecular level, coal is comprised of two phases; (1) a
non-crystalline, macromolecular, three-dimensional,
cross-linked (largely
aromatic) 'network phase' and (2) a multitude of relatively
small
molecules within the network, i.e., the 'mobile phase' (e.g.,
Given, 1987; Haenel, 1992). The latter is of particular
interest in environmental studies
since some fraction of a coal's mobile phase includes
hydrocarbons that can be solvent-extracted using conventional
extraction methods (e.g., EPA Method 3550). A coal's type and
rank will influence the concentration (and composition) of the
solvent-extractable, mobile phase compounds (see below).
Of critical importance to environmental investigations is the
fact that
these mobile phase hydrocarbon extracted from particulate coal
will;
(1) increase the TPH and PAH concentrations of soils or
sediments, and
(2) mix with any petroleum or other forms of hydrocarbon
contamination in the soils or sediments.
Either of these influences can affect the result of an
environmental
assessment by increasing the concentration of regulatory
metrics (TPH or PAH) or by confounding chemical fingerprinting
of 'true' contaminants that may be present. In either case, it
is important for the forensic
investigator to be able to recognize the presence of
particulate coal in
soils and sediments and accommodate for its influence in their
interpretations.
Recognizing Coal Particles in Soils and Sediments
Organic Petrography
A simple but often overlooked method by which particulate coal
in soils
and sediments can be recognized is to look at the samples
during their
collection and prior to extraction/laboratory analysis.
Sometimes opaque
black particles are readily visible indicating that
particulate coal (or
other form of solid organic particles, e.g., soot or coke) may
be present.
More sophisticated visual inspection of the samples can be
performed using organic petrology, i.e., the microscopic study
of organic matter (Taylor et al., 1998). Soils and sediments
can be visually examined using a variety of microscopic
methods, but reflected light microscopy is most common. Soils
and sediments are dried and embedded in epoxy pellets and
polished in order to provide a smooth reflective surface of
the mineral grains and any coal particles that may be present.
These polished pellets are examined under reflected white (unpolarized
and polarized) or uv light to reveal characteristic
microscopic features of coal (Figure 1). A trained eye can
readily identify coal particles among the mineral grains.
Additional quantitative analysis including point counting of
maceral composition (ASTM D 2799) or vitrinite reflectance
measurements ASTM D 2798) can provide valuable information as
to the coal type and rank, respectively. (The latter is based
upon the increase in vitrinite reflectance that occurs with
increases in coal rank). Organic petrographic analysis allows
the organic petrographer to (1) definitively recognize the
presence of particulate coal in soils or sediments and (2) in
some instances, determine the presence of different coal types
and/or ranks, which in some investigations could have forensic
implications (e.g., Hower et al., 2000).
Chemical Fingerprinting
Conventional chemical fingerprinting can also aid in the
recognition of
particulate coal in soil and sediments provided the right type
of
'fingerprinting' data are available. This includes
high-resolution gas
chromatograms and quantitative PAH data. Figure 2 shows
examples of each of these for two coals of different rank that
were serially extracted using dichloromethane in the same
manner that soils and sediments would be extracted. The
hydrocarbons liberated during this procedure represent the
extractable 'mobile phase' described above.
The lignite A (low rank) coal exhibits an unusual
'fingerprint' that is
dominated by peaks within the 'diesel range' (Figure 2). Mass
spectral
analysis of these peaks reveals them to be predominantly
comprised of
various hydrocarbons (diterpanes) derived from labdanoid-type
of plant
resins, common in Rocky Mountain and other coal basins (e.g.,
Anderson and Crelling, 1994). The PAH extracted from this
lignite included a full range of compounds dominated by
perylene (per) and various alkylated phenanthrenes (P1-P4) and
fluoranthenes/pyrenes (FP1-FP3; Figure 2). The high volatile
bituminous (higher rank) coal exhibits a very different
chromatographic fingerprint that is dominated by peaks
attributable to various alkylated naphthalenes and n-alkanes,
the latter exhibiting a characteristic odd-even predominance
within the C25-C31 range. An unresolved complex mixture (UCM
'hump') is evident in the chromatogram, but confined to the
C25-C35 range (Figure 2). The PAH extracted from the high
volatile bituminous coal exhibit a predominance of alkylated
naphthalenes (as had been evident in the gas chromatogram)
that exhibit a 'bell-shaped' pattern often attributed to
petroleum contamination. However, clearly these PAH (78,300 ug/kg)
are
unassociated with petroleum contamination and, if left
unrecognized, could be easily misinterpreted as a petroleum
contaminant (e.g., diesel fuel). Thus, gas chromatograms of
the extractable (total) hydrocarbons and PAH from soils and
sediments suspected of containing coal particles must be
carefully examined.
TPH and PAH Yields from Coals of Different Rank
The different concentrations of extractable TPH and TPAH in
these two
coals (Fig. 2) exemplify a phenomenon long known to the coal
chemistry, namely, the concentration of extractable
hydrocarbons changes with coal rank (e.g., Radke et al.,
1980). Lower rank and higher rank coals generally yield lower
concentrations of extractable hydrocarbons due to the amount
of 'mobile phase' present (versus macromolecular network).
This phenomenon in demonstrated in Figure 3 that shows the
extractable TPH and PAH obtained from a series of coals of
different rank. In this graph rank is expressed by the
vitrinite reflectance of the coals (see above). With
increasing rank the coals show an increase in the extractable
TPH and TPAH, which reaches a maximum around a vitrinite
reflectance of ~0.6. This generally corresponds to a coal rank
of high volatile bituminous coal, where a maximum proportion
of 'mobile phase' exists. At higher ranks the yields of
extractable TPH and TPAH decrease due to the increasing
polymerization of the coal's 'network phase', which
incorporates or physically traps the extractable hydrocarbons.
Thus, the impact that particulate coal can have on the
concentration of TPH and TPAH in soils containing coal will,
to a degree, depend on the rank of the coal(s) that is
present.
Conclusion
Particulate coal can occur in soils and sediments at or near
many historic and active industrial sites that used (use) coal
as a feedstock to the production of coke, manufactured gas, or
steam (for locomotion, machinery, or electricity). The
presence of coal in soils and sediments can increase the
concentrations of TPH and PAH extracted from these matrices.
If unrecognized, these coal-derived hydrocarbons could
confound interpretations surrounding the source(s) of the
hydrocarbons (e.g., confused with petroleum) and regulatory
decisions made based upon apparent concentrations. The latter
is particularly important since particulate coal is not a
CERCLA contaminant. Environmental forensic investigations at
sites where coal was historically used may need to employ
organic petrographic and advanced chemical fingerprinting
methods in order to defensibly recognize the presence of
particulate coal, and accommodate for its impact on the site.
Images
Figure 1
click to
enlarge
Figure 2
click to enlarge
Figure 3
click to enlarge
Table 1
click to enlarge
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