Fuel 79 (2000) 57–67
www.elsevier.com/locate/fuel
Trace elements in coal derived liquids: analysis by ICP-MS and
Mössbauer spectroscopy
R. Richaud a, H. Lachas a, M.-J. Lazaro a, L.J. Clarke b, K.E. Jarvis b, A.A. Herod a,*, T.C. Gibb c,
R. Kandiyoti a
b
a
Department of Chemical Engineering and Chemical Technology, Imperial College (University of London), London SW7 2BY, UK
NERC ICP-MS Facility, TH Huxley School, Imperial College, University of London, Silwood Park, Buckhurst Road, Ascot, Berks, SL5 7TE, UK
c
School of Chemistry, University of Leeds, Leeds LS2 9JT, UK
Accepted 15 June 1999
Abstract
Concentrations of trace elements in coal derived liquids have been investigated by inductively coupled plasma-mass spectrometry (ICPMS) and by Mössbauer spectroscopy. Liquefaction extracts prepared from the Argonne Premium Coals and a coal tar pitch have been
examined. Microwave digestion in concentrated nitric acid has been shown as a suitable method for determining trace element concentrations
in coal derived liquids by ICP-MS—for sample sizes as small as 3–20 mg. High concentrations of Fe were found for all extract samples
(,265–1474 ppm). Ti, Cr, Mn, Co, Ga, Sb, Cs and Ba were measurable. Concentration distributions of trace elements found in the extracts
bore little relation to the corresponding distributions in the original coals. The proportions of individual trace elements present in the original
coals and found in the extracts, varied widely. Mössbauer spectroscopy of the extracts indicated that the high Fe-concentrations corresponded
to the presence of organometallic-Fe compounds—and not to pyritic iron. There is evidence suggesting the presence of material derived from
iron-storage proteins such as ferritin, but final proof is lacking. Our data suggest that other metallic ions detected in these coal derived liquids
may be present in association with the organic material. Concentrations of paramagnetic metal species were found to be of the same order of
magnitude as ESR spin-densities already found in coal liquids. Both types of paramagnetic species are suspected of causing loss of signal in
solid state 13C NMR. q 1999 Elsevier Science Ltd. All rights reserved.
Keywords: Argonne coals; Liquefaction; Pitch; Trace elements; Mössbauer spectra
1. Introduction
Trace elements are known as constituents of coal and
cause environmental and corrosional problems in combustion and gasification systems [1,2]. Some of these trace
elements are thought to be associated with the organic
matter [1]. Despite the extraction of Fe and Ga porphyrins
from coals [3–7], trace metals are less well known as constituents of organic liquids derived from coal. Recently,
extracts of Point of Ayr coal have been shown to contain
trace elements thought to be in organic association with
carbonaceous material—rather than as suspended ash particles [8,9]. Changes were observed in trace element levels as
a function of digestion pressure; non-catalytic hydrocracking of the extracts did not remove the trace elements from
the coal solution.
* Corresponding author. Tel.: 1 44-207-589-5111; fax: 1 44-207-5945604.
E-mail address: [email protected] (A.A. Herod)
On the contrary, occurrences of organometallic
compounds, such as porphyrins, in sediments [10–12],
petroleum crude and petroleum products have been studied
in some detail [13–19]. Geo-porphyrin fractions have been
prepared by a variety of methods to provide a concentrated
sample for analysis by tandem mass spectrometry [11],
plasma desorption mass spectrometry [13], electrospray
mass spectrometry [12], heated-probe mass spectrometry
[14] and—in at least one case—high resolution MS [18].
Atomic emission spectrometry (ICP-AES) has been used to
observe organometallics in heavy petroleum residue fractions separated by size exclusion chromatography (SEC)
[15]; hydrotreating was observed to reduce the size and
concentration of vanadium compounds in coal extracts but
did not remove them from the products. Vanadyl porphyrins
in asphalts and kerogens have been concentrated by SEC for
analysis by ESR [16]. GC with AES detection [19,17] has
been applied to metalloporphyrins in crude oils; the results
suggest that levels of metal concentrations detected by
preparative SEC correspond to only a small fraction of all
0016-2361/00/$ - see front matter q 1999 Elsevier Science Ltd. All rights reserved.
PII: S0016-236 1(99)00129-5
58
R. Richaud et al. / Fuel 79 (2000) 57–67
metalloporphyrins present in the crudes. Similarly the roles
of metals in biological systems are well known [20] and
research continues into enzymes and their metal systems
(e.g. cf. [21–23]).
In our recent work, Mössbauer spectra of a Point of Ayr
coal extract showed the presence of Fe complexes with high
and low spin—which were distinct from pyritic iron identified in the spectrum of the original coal [24,25]. When the
extract was fractionated by planar chromatography [24], the
Mössbauer spectra of the fractions gave spectra which were
similar to those of the whole extract; thus the Fe environments in larger and smaller molecular mass fractions of the
extract [24–29] were found to be similar—but the structures could not be identified as necessarily consisting of
porphyrins.
In the present study, the presence of trace elements in coal
derived liquids has been investigated by ICP-MS (inductively coupled plasma-mass spectrometry) and by Mössbauer spectroscopy. The samples examined include
liquefaction extracts prepared using the rank ordered set
of coals from the Argonne PCSP [30] and a coal tar pitch.
The latter sample, used as the standard coal derived liquid at
the Imperial College laboratory, has previously been extensively examined by mass spectrometry and size exclusion
chromatography [26,28,29]. Its trace element content has
been investigated using acid digestion with microwave heating followed by ICP-MS [31]. The organometallic iron
content of the pitch was not examined by Mössbauer spectroscopy because the iron concentration was too low to
generate adequate signal.
2. Experimental
2.1. Samples
The Argonne coal samples [30,32], ranging from a lignite
to a semi-anthracite have been extracted with tetralin in a
‘flowing solvent liquefaction reactor’ at 4508C, as described
elsewhere [33–35]. Excess solvent tetralin and solvent
derived products were removed from the reaction mixture
by rotary evaporation followed by precipitating the coalderived product as pentane insoluble material. Most tetralin-derived products are known to have remained in the
pentane soluble fraction [33]. In every case, the pentane
insoluble material was a viscous liquid and depending on
the sample, corresponded to between 40 and 90% of the dry,
ash free coal [34]. The coal tar pitch sample is the (.4508C
boiling) distillation residue of tar produced during the high
temperature coking of coal. Results of these analyses have
been compared with values for a commercially available
standard heavy fuel oil with certified trace element concentrations (SRM 1634c; Promochem UK); however, certified
values were available only for a subset of the elements (V,
Co, Ni, As, Se, Ba) determined in this study. Certified reference materials consisting of oil or coal-liquid samples with a
wider range of certified trace element values do not appear
to be commercially available.
2.2. Sample digestion
Due to the small amounts of sample prepared by the
liquefaction procedure [34,35], extract sample sizes
between 3 and 20 mg have been used for the trace element
analyses. These amounts are considerably smaller than
sample sizes normally associated with this type of analysis
(e.g. ,500 mg); the development and verification of procedures used for processing these small sample quantities of
coal derived liquids have been described in detail elsewhere
[36,37]. Briefly, an MLS-1200 Milestone Microwave Oven
was used for the digestions. 1 ml of Aristar grade 70% v/v
nitric acid was added to the sample held in a 7 ml perfluoroalkane vessel. The vessel was placed inside a Teflon
pressure bomb containing 10 ml of acid to ensure generation
of the correct pressure during the digestion. Deionised,
.18 MV, water was used in these analyses.
In previous work on solids containing silicates (e.g. coal,
ash), microwave digestion in nitric acid was shown to give
incomplete recoveries for some trace elements [36–38].
Unlike HF, nitric acid cannot destroy silicates to release
all the trace elements present, into solution. However, in
the current work, the samples consisted of coal extracts
and a coal tar pitch with very low ash contents, typically
of the order of 0.1%. This mineral matter is thought to
consist mostly of metals in organic association, rather than
of silicates. It is thought that digestion in concentrated nitric
acid should be capable of destroying organic structures—
releasing carbon as CO2 and removing elements in organic
association into acid solution. The method is easier to
handle than ‘wet-ashing’ and other high-temperature digestion methods and has been shown to provide more realistic
values for the more volatile elements, e.g. selenium and
arsenic. Verification of the procedure was made against
the heavy fuel oil with certified values for trace element
concentrations (see above).
2.3. ICP-MS method
A Fisons Instrument PQ II Plus quadrupole mass spectrometer with an ICP ion source was used for the analyses.
Samples were diluted with 2% nitric acid solution before
introduction into the instrument. The method has been
previously described [36–38]. Standard solutions of individual elements were used to calibrate the instrument.
Reagent blanks were used to determine background readings on the day of the analysis and ‘limits of quantitation’
(LOQ) defined as 10 times the standard deviation of the
blank runs were calculated. The dilution ratio for the current
solutions was approximately 5000, although it was possible
to run more concentrated solutions resulting in lower LOQs
and greater sensitivity. The elements quantified by the
method included Al, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Ga,
R. Richaud et al. / Fuel 79 (2000) 57–67
59
absorption obtained was low, even for samples with
.1000 ppm of the iron-bearing compounds. Therefore it
was not possible to obtain useful spectra for samples with
either a low iron content ,500 ppm and/or samples that
were only available in small quantity. Nevertheless, useful
results were obtained for five of the liquefaction extracts
(Blind Canyon, Lewiston–Stockton, Pittsburg, Upper Freeport and Wyodak). The low recoilless fractions at room
temperature are not uncommon for iron-bearing proteins,
and this may be compounded by the resin-like consistency
of the materials. One of the samples (Blind Canyon) was
also cooled to 78 K, but this did not give any significant
increase in the absorption intensity. It is suggested that
there was some incipient magnetic broadening taking
place, which was counteracting the expected increase in
recoilless fraction.
The raw data have been displayed by joining the data
points of the spectra (Fig. 1), as this gives a clearer visualisation of the very weak absorptions. Each spectrum was
also smoothed 20 times using a 25-point quadratic smoothing function [40]. The compound lineshape of the spectrum
has been observed to change comparatively slowly and the
smoothing function was chosen to give a useful reduction in
the signal to noise level, whilst minimising tendencies to
cause distortion (Fig. 2).
3. Results and discussion
3.1. Elemental analyses
Fig. 1. Mössbauer spectra at room temperature for extracts from: (a) Upper
Freeport; (b) Pittsburgh; (c) Blind Canyon; (d) Wyodak–Anderson; (e)
Lewiston–Stockton, pentane insolubles. The individual points have been
joined together to give a clearer visual representation of the data.
Ge, As, Zr, Mo, Sb, Cs, Tl and Pb at levels ranging from less
than 1 to more than 1000 ppm.
2.4. Mössbauer spectroscopy
Mössbauer data were collected in the transmission mode
at room temperature using a conventional apparatus [39]:
the source matrix was 57Co/Rh and isomer shifts were determined relative to the spectrum of metallic iron at room
temperature. The percentage absorption was of the order
0.05%, necessitating long counting times (typically several
weeks with up to 10 8 counts per channel). The baseline was
rigorously corrected for geometric effects using a blank of
similar quality, which also served to confirm that the
observed absorptions were a genuine property of the absorbers and that there was no absorption from trace iron impurities in the counter windows.
Because the samples contain a substantial organic
component with a comparatively low absorption coefficient
for gamma rays, it was possible to prepare uniform absorbers containing the undiluted material. However, the 57Fe
Table 1 presents results from trace element analyses by
ICP-MS of the fuel oil (with certified reference values) and
of the coal tar pitch samples. Duplicate digestion runs
(followed by ICP-MS analysis) have been carried out for
both samples—using small sample sizes (10–20 mg). These
experiments have been carried out to assess the method to
be used in the preparation of the Argonne coal extracts for
analysis.
For some elements the LOQ were larger than concentrations in the digest solutions. LOQ was defined as the
concentration corresponding to 10 times the standard deviation of signal for a blank solution run on the same day as the
unknown solutions. Values below the LOQ but in excess of
the limit of detection, LOD (taken as three times the standard deviation), are by definition of poor accuracy. In these
cases, the measured values can be considered as no more
than indications of concentration levels of the particular
elements in the samples.
In Table 1, values for Co and Ni were found to be in good
agreement with the certified values. The V values from our
work were lower than the certified value by about 30%; if
this difference is confirmed by further work, it would imply
that not all the vanadium can be extracted by the concentrated nitric acid based method. The ‘advised’ (rather than
certified) value for Ba was lower than that from our analysis.
60
R. Richaud et al. / Fuel 79 (2000) 57–67
Fig. 2. The Mössbauer spectra at room temperature shown in Fig. 1
smoothed to reduce random noise for extracts from: (a) Upper Freeport;
(b) Pittsburgh; (c) Blind Canyon; (d) Wyodak; (e) Lewiston–Stockton,
pentane insolubles.
the fuel oil standard, have been assumed to be the best
available estimates for this sample in the absence of a
more suitable standard reference material. Values greater
than the LOQ have been shown with 95% confidence limits,
values less than the LOQ have been shown as either approximate (,) levels or as less than the LOQ (,LOQ value).
Table 2 presents trace element contents of the pitch and of
the pentane insoluble fractions prepared from liquefaction
extracts of the eight Argonne coals, showing concentrations
of a range of trace elements between m=z 7 (Li) and m=z 238
(U). Only one solution was prepared for each sample.
Concentrations which were below LOQ values have been
indicated as ,LOQ values and any other determinations
greater than the LOQ, shown as concentrations ^1 standard
deviation.
This set of analyses have been carried out using sample
sizes ranging from 2.3 to 5.6 mg, to test whether the method
would be applicable to the examination of trace elements in
such samples, e.g. of pyrolysis tars produced in a wire-mesh
pyrolyser, a small fixed-bed or a fluidised bed reactor, where
coal samples of mass between 5 and 100 mg may be examined [41–43]. Of the six elements quantified in the pitch
(Table 2), those for Pb, Ga, As are in good agreement
while Zn appears low with respect to Table 1 compared
with Table 2. Ge and Tl data are not available and other
elements are ,LOQ in the extract. High LOQs for some
elements in Table 2 result from reagent contamination; for
some elements e.g. Al, polyatomic ion interferences (involving carbon–nitrogen ions [31]) are thought likely to have
contributed to the high LOQ (42.4 ppm). An ICP-MS mass
spectrum from the pitch sample is shown in Fig. 3 and
shows the presence of Zn, Ga, Ge, As, Tl and Pb.
3.2. Coal extracts
Certified values for As and Se were below the LOQ: the
values from this work for the isotopes As 75 and Se 82 were
consistent with certified values although much lower than
the LOQ.
Concentrations for six additional trace elements detected
in the fuel oil but not quantified by the suppliers (B, Cr, Mn,
Cu, Zn, and Pb) were found to be greater than or no less than
half the determined values of the LOQ. Of these, values
found to be slightly below the LOQ have been shown as
rounded to the nearest ppm—as an ‘indication’ of the
concentration. However, where the measured values were
found to be much lower than the LOQ but .LOD, the result
has been shown as less than the LOQ (,LOQ value).
Results for two Cu isotopes (Cu 63 and 65 of relative abundances 70 2 30%) have been included to allow evaluation
of interferences; our results for the relatively low Cu
concentrations are internally consistent within 50%. Values
for the non-certified elements Be, Ga, Mo, Cd, Sn and Sb
were much lower than the LOQ. The results of Table 1 cover
a selected range of elements and the range of elemental
concentrations in the pitch sample, run at the same time as
Considering the high concentrations of Fe across the
board in the coal extracts (,265–1474 ppm), it is possible
that these relate to contact with reactor walls during the
liquefaction operation; however, the low Cr, Mo and Ni
contents of the extracts do not permit corrosion to be cited
as the main factor. If that was the case, Fe concentrations
may have been expected to be more similar for the eight
extract samples prepared under identical conditions. As
explained below, Mössbauer spectroscopy of the extracts
indicates the observed high Fe-concentrations to correspond
to the presence of organometallic-Fe compounds—and not
to pyritic iron (see below). However, the the data on its own
does not allow distinguishing between organometallic iron
in the original coal and compounds which may have formed
from pyritic iron during the coal extraction process. In addition to Fe, other elements quantified were Ti, Cr, Mn, Co,
Ga, Sb, Cs and Ba.
Many of the elements listed in Table 2 are below the limit
of quantification. To improve the LOQ in the sample several
options are possible. The first is the digestion of a larger
sample mass although dissolved solids still limit the dilution
R. Richaud et al. / Fuel 79 (2000) 57–67
61
Table 1
Trace element determinations in digests of pitch and standard reference material SRM 1634c (fuel oil) by microwave extraction
Sample
Mass (g)
Volume (ml)
Dilution
Dilution factor
Element a
Be 9
B 11
V 51
Cr 52
Mn 55
Co 59
Ni 60
Cu 63
Cu 65
Zn 66
Ga 71
As 75
Se 77
Se 82
Mo 95
Cd 111
Sn 118
Sb 121
Ba 137
Pb 208
a
1634c
1634c
0.011
25
2
4545
LOQ for 1634c
0.42
12.00
0.35
2.09
0.58
0.42
5.39
1.52
1.27
4.25
0.22
1.46
14.62
14.59
1.27
1.29
1.10
0.42
0.42
0.35
0.0206
25
4
4854
1634c 1st rep.
, 0.5
,5
17.4 ^ 2.0
3.0 ^ 0.6
, 0.3
0.17 ^ 0.04
17.8 ^ 2.9
2.4 ^ 0.3
2.7 ^ 0.4
6.4 ^ 1.4
, 0.3
, 1.5
, 15
, 15
4.7 ^ 0.8
, 1.3
, 1.1
4.0 ^ 4.1
5.7 ^ 0.4
1.1 ^ 0.5
1634c 2nd rep.
, 0.5
, 10
19.4 ^ 0.4
2.8 ^ 0.3
, 0.4
0.14 ^ 0.05
18.9 ^ 0.8
1.1 ^ 0.2
1.6 ^ 0.4
5.2 ^ 1.3
, 0.3
, 1.5
, 15
, 15
, 1.3
, 1.3
, 1.1
, 0.5
4.0 ^ 0.5
1.2 ^ 0.1
1634c Cert.Val
28.2 ^ 0.4
0.15 ^ 0.01
17.5 ^ 0.2
0.14 ^ 0.01
0.102 ^ 0.004
0.102 ^ 0.004
[1.8]
LOQ for Pitch
0.42
12.00
0.35
2.09
0.58
0.42
5.39
1.52
1.27
4.25
0.22
1.46
14.62
14.59
1.27
1.29
1.10
0.42
0.42
0.35
Pitch
Pitch
0.0103
25
2
4854
Pitch 1st rep.
, 0.42
12
, 0.35
3.9 ^ 0.2
2.8 ^ 0.2
, 0.4
,6
8.8 ^ 0.3
8.7 ^ 0.6
172 ^ 11
5.9 ^ 0.3
8.0 ^ 0.1
, 15
, 15
, 1.3
2.5 ^ 0.1
2.4 ^ 0.4
0.8 ^ 0.1
0.5 ^ 0.1
184 ^ 14
0.0102
25
2
4902
Pitch 2nd rep.
, 0.42
, 10
, 0.35
4.5 ^ 0.3
3.8 ^ 0.2
0.46 ^ 0.11
,6
2.1 ^ 0.5
2.0 ^ 0.9
164 ^ 12
6.4 ^ 0.2
8.8 ^ 1.2
, 15
, 15
, 1.3
2.4 ^ 0.4
3.0 ^ 0.2
0.9 ^ 0.3
, 0.42
210 ^ 12
Mass …m=z† of isotope used to estimate elemental concentration.
factor which can be used to avoid causing clogging of the
ICP-MS sampling cone orifice.
Pb could not be quantified in the set of extracts from the
Argonne coals, which cover a fairly wide spectrum of rank
and properties, in each case, the Pb level was less than the
LOQ. The relatively high Pb concentrations encountered in
the pitch sample would appear, therefore, to be related to
equipment corrosion—during the coking or tar condensation/distillation stages. Nevertheless freak high Pb concentrations in the coking coals used at the Avenue Coking Plant
(source of the pitch sample) cannot be entirely ruled out—
although it does not, at present, appear possible to verify
this. A study of a coal tar pitch by ICP-AES following
microwave digestion [44] indicated a Pb concentration of
108 ppm (mg/Kg), which tends to confirm the plant as the
source of lead.
3.3. Mössbauer spectroscopy
Mössbauer spectra were recorded for five samples as
shown in Figs. 1 and 2. From inspection of both diagrams, the raw and smoothed Mössbauer data, respectively,
absorptions may be observed at ca. 20.2 and 2.5 mm s 21.
Although the data have relatively low signal to noise ratios,
the peaks appear to show a clear relationship with the iron
environments observed previously in the Point of Ayr coal
extracts and its fractions [24,25]. All five spectra appear to
be similar. The existence of an absorption at ca. 2.5 mm s 21
is unambiguous evidence of the presence of high-spin Fe 21
species. This absorption is the high velocity line of a quadrupole doublet with an overall splitting of the order of 2.8–
3.0 mm s 21. The higher intensity at lower velocities in the
spectrum indicates the existence of a second species which
by analogy with the Point of Ayr data may be a quadrupole
doublet from a high-spin Fe 31 compound. There are,
however, other possibilities; low-spin Fe 21 and Fe 31
compounds would also absorb in this region. It would be
difficult to make a clear distinction between these three
possibilities even with better quality data when (as in
these cases) the number of actual absorbing compounds
has the potential to be large.
The identification of the iron environments involved is
not easy. It is tempting to believe that the iron-bearing residues relate to an organic iron system, which was present in
large quantities in the original ecosystem. However, there is
always the possibility that some major change has occurred
which can obscure this relationship, or indeed that the
species observed are artefacts with only a tenuous relationship to their origin. It is even possible that a major source of
iron did not survive the transformation into coal in any
recognisable form.
Although iron porphyrins are an obvious choice, the
Mössbauer evidence do not really support this possibility.
From the study of heme proteins [45], it is clear that the
62
Table 2
Trace element determinations in digests of pitch and liquefaction extracts of the Argonne set of coals. Values in mg/kg (ppm)
LOQ in solid
Pitch
Beulah Zap
Wyodak–Anderson
Blind canyon
Illinois
Pittsburgh
Lewiston–Stockton
Upper Freeport
Pocahontas
Li
Be
B
Mg
Al
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
Ga
Ge
As
Se
Rd
Sr
Y
Zr
Nb
Mo
Cd
Sn
Sb
Cs
Ba
W
Tl
Pb
Bi
Th
U
1.20
0.89
70.9
19.0
42.4
2.10
2.33
1.16
3.62
0.77
254
0.43
13.1
24.8
37.5
0.15
2.6
3.89
14.8
0.85
0.80
0.18
0.87
2.73
5.76
2.23
3.20
0.52
0.29
2.07
8.79
0.80
4.21
9.24
1.88
0.05
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
47.9 ^ 1.27
5.86 ^ 0.04
6.32 ^ 0.46
7.60 ^ 0.37
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
6.83 ^ 0.13
191 ^ 0.58
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
11.1 ^ 0.78
0.83 ^ 0.10
593 ^ 27.6
, LOQ
, LOQ
, LOQ
, LOQ
0.16 ^ 0.04
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
2.35 ^ 0.20
, LOQ
14.5 ^ 2.60
4.24 ^ 0.49
875 ^ 238
0.55 ^ 0.11
, LOQ
, LOQ
, LOQ
0.46 ^ 0.11
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
1.29 ^ 0.14
0.41 ^ 0.07
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
2.41 ^ 0.44
, LOQ
13.2 ^ 0.90
4.01 ^ 0.28
1195 ^ 52.7
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
13.0 ^ 0.42
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
5.22 ^ 0.39
, LOQ
7.74 ^ 0.41
, LOQ
314 ^ 20.4
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
16.3 ^ 1.38
, LOQ
13.6 ^ 5.52
3.59 ^ 0.08
1183 ^ 34.3
, LOQ
, LOQ
, LOQ
, LOQ
0.62 ^ 0.05
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
13.2 ^ 0.76
, LOQ
36.4 ^ 0.35
5.56 ^ 0.21
1320 ^ 21.5
2.09 ^ 0.11
, LOQ
, LOQ
, LOQ
0.97 ^ 0.08
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
0.83 ^ 0.02
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
8.44 ^ 7.38
, LOQ
17.5 ^ 2.46
8.74 ^ 0.26
1474 ^ 52.8
, LOQ
, LOQ
, LOQ
, LOQ
0.72 ^ 0.07
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
0.79 ^ 0.08
, LOQ
9.57 ^ 0.48
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
265 ^ 634
, LOQ
, LOQ
, LOQ
, LOQ
0.20 ^ 0.22
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
5.83 ^ 5.20
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
, LOQ
R. Richaud et al. / Fuel 79 (2000) 57–67
Element
R. Richaud et al. / Fuel 79 (2000) 57–67
Fig. 3. Mass spectra for the pitch digest with selected parts of the mass range shown with the scale expanded. At m=z 54–58, the argon polyatomic ion peaks are identified. Zn, Ga, Ge and As are readily identified
between m=z 64 and 75. Thallium and lead are seen at high mass. The characteristic fingerprint for Pb is easily seen.
63
64
R. Richaud et al. / Fuel 79 (2000) 57–67
planar heme unit causes substantial quadrupole splitting
regardless of the other ligands involved. However, in the
case of high-spin Fe 21, this distorted geometry results in a
smaller quadrupole splitting than would be usual in a more
symmetrical environment. Thus in deoxyhemoglobin, the
splitting is only 2.4 mm s 21 at 4.2 K and decreases with
increase in temperature. Similarly the isomer shift of
0.9 mm s 21 will also decrease with increasing temperature
and is in any case low because of the bonding to nitrogen in
the plane.
Similarly the iron–sulphur proteins are unlikely to be the
major component. The reduced rubredoxin features Fe 21 coordinated to four sulphur ligands, but while the quadrupole
splitting is large (3.16 mm s 21 at 77 K), the isomer shift is
very low (0.65 mm s 21 at the same temperature) because of
the covalency in the Fe–S bonds. The active centres in
ferredoxins containing more than one iron atom also seem
to be unable to match the Fe 21 species that is observed in
these extracts.
The soluble protein molecule ferritin is similar in form
and function in species ranging from bacteria to plants,
fungi and mammals. It is able to store excess iron (up to
25% by weight) and release it on demand, there being typically 2–3000 iron atoms per molecule as a hydrous ferric
oxide associated with phosphate, and surrounded by a
protein envelope. Each Fe 31 iron is probably six-co-ordinated to oxygen. Electrons can be carried inside the protein
cavity by Fe 21 ions migrating through channels in the
protein envelope. All or part of the Fe 31 ions can be reduced
by electrochemical methods, and similar reduction can be
achieved by chemical reduction with e.g. S2O422 ions under
anaerobic conditions [46]. The Mössbauer parameters of the
two iron states seem largely unaffected by the degree of
reduction. The Fe 21 ions give a slightly broadened quadrupole at 100 K with an isomer shift of 1 1.27 mm s 21 and a
quadrupole splitting of 2.85 mm s 21. The oxidised form is
typical of high-spin Fe 31 with equivalent values of 0.45 and
0.72 mm s 21, respectively. (Note that the isomer shift will
decrease by some 0.2–0.3 mm s 21 at room temperature
because of the second-order Doppler shift). In the context
of coal extracts, the core of ferritin contains a large amount
of iron, which can be largely reduced under anaerobic
conditions and may be comparatively protected from subsequent change by the surrounding protein envelope. Partial
degradation or change in the polarity of the protein envelope
are unlikely to have any effect on the iron core.
The observed Mössbauer parameters for reduced ferritin
are sufficiently close to those observed in the extracts that
this substance must be considered as a serious contender,
and is to be preferred to porphyrins or iron–sulphur
proteins. It should be noted that the high-spin Fe 21 and
Fe 31 would then be part of the same protein system and
their ratio could vary from sample to sample. Least-squares
curve analysis of such low absorptions is not very rewarding
because the minima are exceptionally shallow and the standard deviations large. However, as an exploratory exercise,
a model was created with splittings fixed at the ferritin
values and an encouraging level of agreement with the
observed absorptions was obtained. The extracts described
earlier [24,25] could be fitted quite well in the same way.
However, it is also possible that there is a mixture of
many species including porphyrins in quantities, which
cannot be detected from present data, and also other major
sources of iron such as photosynthetic organisms; finally,
various enzymes cannot be completely excluded. In room
temperature measurements, species with very low recoilless
fractions might not be detected at all, even though they may
be a large fraction of the total iron.
In conclusion, there is some evidence to support the
presence of a significant quantity of iron-bearing material
derived from the iron-storage proteins such as ferritin, but
final proof is lacking and the possibility remains that quantities of other important material classes were not detected.
3.4. General discussion
Although the sample sizes used in this work are very
small, the work indicates that metallic ions are in association with the organic material extracted by liquefaction of
coals in tetralin or by pyrolysis, in the case of pitch sample.
The variable proportions of these elements in the pitch and
extracts from a rank series of coals indicates that the chemical environments are different in each sample. In addition,
the Mössbauer spectra show that the environment of Fe is
organic rather than the inorganic (e.g. iron pyrites), as
shown previously for Point of Ayr coal liquefaction extracts
[24,25]. Iron porphyrins are known to be present in extracts
from coals, but with few side chains on the porphyrin ring
structure. Both heme- and non-heme-iron proteins are
known in nature with molecular masses ranging from
12,000 up to 110,000 in pea cytochrome c6 [20]; in addition,
protective groups based on alkyl imidazole groups may be
arranged around the porphyrin centre to limit access of
approaching ligands as “picket fence porphyrin complexes”
[20,47]. Size exclusion chromatograms of the coal extracts
(cf. [Ref. 33, Fig. 3]) indicate that the majority of the coal
extract corresponds to material excluded from the column
porosity and is therefore of large molecular size, indicated
to be equivalent in elution behaviour to polystyrenes of
molecular mass greater than about 20,000 u and extending
up to 1.85 million u. The coal tar pitch has been examined
previously by size exclusion chromatography and has been
shown to contain a wide range of molecular sizes as judged
by elution behaviour in comparison with polystyrenes
[26,28,29].
In the coal extracts and pitch, the structures involving iron
or any of the other metals are not known although the
protein ferritin is indicated as a possible structure; however,
metals are known to be present in biological systems and it
is not surprising that some metal functionalities such as
porphyrins or ferritin may have survived through coalification and pyrolysis or extraction. The survival of biological
R. Richaud et al. / Fuel 79 (2000) 57–67
65
Table 3
Trace elements in Argonne coals
Element
Be a
Al
Ti
V
Cr a
Mn a
Fe
Co a
Ni a
Cu
Zn
Ga
Ge
As a
Se a
Rb
Sr
Zr
Cd a
Sb a
Ba
La
Ce
Sm
Yb
Pb a
a
Argonne coals (ppm levels-micrograms per gram)
Beulah–Zap
Wyodak–Anderson
Blind Canyon
Illinois
Pittsburgh
Lewiston–Stockton
Upper-Freeport
Pocahonta
0.18
3600
200
–
2.5
80
6000
0.8
1.3
9
5
–
–
3.3
0.6
4
680
18
0.05
0.15
855
5
11
0.4
0.3
1.5
0.25
4800
440
14
6.2
20
3300
1.6
4.9
12
18
3
–
3.5
2.4
13
273
19
0.09
0.19
320
6
17
1
0.4
3.0
0.13
3300
200
–
4.9
4.0
3200
0.8
3.3
8
6
–
–
1.2
1.3
3
66
18
0.06
0.11
32
6
10
0.5
0.2
1.6
0.76
20 000
920
52
31
77
25 000
4.3
18
12
210
7
9
3.2
4.1
18
36
22
0.61
0.88
110
10
26
1.2
0.5
6.5
0.77
17 000
910
24
14
17
13 000
2.4
8.5
8
17
5
3
4.5
1.7
8
62
20
0.06
0.23
48
7
14
1.1
0.5
2.9
1.9
35 000
2300
–
40
15
4000
8.1
16
30
12
–
–
6.9
6.2
36
73
103
0.08
0.52
170
18
45
3.5
1.6
12
1.5
20 000
1000
41
20
41
20 400
5.0
15
18
29
10
4
11
1.9
20
56
28
0.07
0.53
67
13
29
2.0
0.9
7.4
0.80
8500
820
18
14
16
6000
3.8
6.7
18
10
4
–
6.9
3.0
5
95
20
0.08
0.54
262
6
11
1.2
0.6
2.4
From Ref. [1].
marker chemicals (biomarkers) in coals and their observation in pyrolysis tars is well documented [48]. The presence
of organic iron in large and small molecules of a coal digest
has been demonstrated [24].
Trace element reference values for the untreated Argonne
coals are listed in Table 3 (from [1,32]). Values quoted in
the references have been averaged and rounded [1,32]. The
concentrations of trace elements in the extracts bear little
relation to those of the original coals and do not vary in the
same manner with the sequence of coals. There was no
obvious relationship between concentrations of elements
with coal rank; this is as expected since the set of coals
are geologically distinct and their trace elements would be
(probably) mostly derived from the mineral matter which
would be adventitious in origin within the seam [1].
For Fe, the concentrations in the extracts were between
37% (Blind Canyon) and 1.2% (Illinois) of those of the
original coals and allowing for the incomplete extraction
(40–90% extraction), these values become 34 and 1.1%,
respectively. For many elements, the proportion extracted
was very small, 0.5–1.5% for Ti, 2–12% for Ga and 2 and
11% for Ba. For other elements however, such as Cr, Mn,
Co and Sb, the concentrations in the extracts corresponded
to a large proportion of that present in the coal and in some
cases, was even in excess of the concentration in the coal.
This greater concentration in the extracts may result from
the problems of accurately measuring low concentrations
(0.1–0.9 ppm) of trace elements such as Sb or may be a
result of contamination of the extracts. Clearly, measurements based on more concentrated solutions of trace
elements from the extracts are required to resolve this
problem.
For many elements present in the coals such as Be, Al, V,
Ni, Cu, Ge, As, Se, Rb, Sr, Zr, Cd and Pb, the concentrations
in the coals were low and the concentrations in the extracts
were below the level of detection for the particular element.
Of particular importance is the observation that the liquefaction extracts of the lower rank coals, which may be
expected to contain the most oxygen as carboxylic acid
groups, do not contain the highest concentrations of trace
elements. This implies that carboxylic acid group coordination of the trace elements is not the major form of organometallic grouping in these extracts.
This work has implications for the examination of coal
extracts by NMR methods and by electron spin resonance
spectroscopy since the concentrations of paramagnetic
metal species are shown to be of the order of 10 18 or 10 19
atoms per gram of sample, which is of the same order of
magnitude as the number of unpaired electron spins identified in coals in ESR studies [49–51]; these paramagnetic
species are thought to cause loss of signal in solid state 13C
NMR [52,53].
66
R. Richaud et al. / Fuel 79 (2000) 57–67
4. Conclusions
Concentrations of trace elements in coal liquids derived
from the Argonne PCSP set [30] and a coal tar pitch have
been investigated by inductively coupled plasma-mass spectrometry (ICP-MS) and by Mössbauer spectroscopy.
1. Microwave digestion in concentrated nitric acid was used
for the determination of trace element concentrations in
coal derived liquids by ICP-MS with sample sizes as
small as 3–20 mg.
2. Signal levels were too low for quantification for many of
the elements. High concentrations of Fe were found for
all extract samples (,265–1474 ppm). Other elements
detected at measurable concentration levels were Ti,
Cr, Mn, Co, Ga, Sb, Cs and Ba.
3. Concentrations of trace elements in the extracts bore
little relation to corresponding distributions in the original coals. The extracts from low rank coals contained less
trace elements than those for high rank coals.
4. Mössbauer spectroscopy of the extracts indicated that the
high Fe-concentrations corresponded to organometallicFe compounds such as the iron-storage protein ferritin
and not to pyritic iron. The other metallic ions detected
are probably associated with the organic material.
5. A relatively high Pb content was detected in the coal tar
pitch, but none was detected in the set of extracts from
the Argonne coals (,4.2 ppm).
6. The concentrations of paramagnetic metal species were
of the order of 10 18 or 10 19 atoms per gram of sample,
similar to the number of spins per gram found in coal
derived liquids by ESR.
Acknowledgements
Support for this work by the British Coal Utilisation
Research Association (BCURA) and the UK Department
of Trade and Industry under BCURA Contract No. B44 is
gratefully acknowledged. The authors would like to thank
the European Community for a postdoctoral grant for M.-J.
Lazaro (Marie Curie Research Grant; Non-Nuclear Energy
Programme).
References
[1] Davidson RM, Clarke LB. Trace elements in coal IEAPER/21. IEA
Coal Research, London 1996.
[2] Finkelman RB. Fuel Process Technol 1994;39:21.
[3] Bonnett R, Czechowski F. Nature 1980;283:465.
[4] Bonnett R, Czechowski F, Hughes PS. Chem Geol 1991;91:193.
[5] Bonnett R, Burke PJ, Dewey CR, Fairbrother AE, Major HJ. Fuel
1991;70:1227.
[6] Bonnett R, Czechowski F, Latos-Grazynski L. Energy and Fuels
1990;4:710.
[7] Pretorius W, Foulkes M, Ebdon L, Rowland S. J High Res Chromatogr 1993;16:157.
[8] Cloke M. Fuel 1986;65:417.
[9] Cloke M, Hamilton S, Wright JP. Fuel 1987;66:1685.
[10] Lash TD. Energy and Fuels 1993;7:166.
[11] Laycock JD, Yost RA, Wang L, Quirke JME. Energy and Fuels
1995;9:1079.
[12] Van Berkel GJ, Quinones MA, Quirke JME. Energy and Fuels
1993;7:411.
[13] Wood KV, Bonham CC. Energy and Fuels 1993;7:97.
[14] Ali MF, Perzanowski H, Bukhari A, Al-Haji AA. Energy and Fuels
1993;7:179.
[15] Pearson CD, Green JB. Fuel 1989;68:465.
[16] Premovic PI, Jovanovic LS. Fuel 1997;76:267.
[17] Zeng Y, Uden PC. J High Res Chromatogr 1994;17:223.
[18] Grigsby RD, Green JB. Energy and Fuels 1997;11:602.
[19] Zeng Y, Uden PC. J High Res Chromatogr 1994;17:217.
[20] Kendrick MJ, May MT, Plishka MJ, Robinson KD. Metals in biological systems, Chichester, UK: Ellis Horwood, 1992.
[21] Happe RP, Roseboom W, Pierik AJ, Albracht SPJ, Bagley KA. Nature
1997;385:126.
[22] Kanyo ZF, Scolnick LR, Ash DE, Christianson DW. Nature
1996;383:554.
[23] Volbeda A, Charon M-H, Piras C, Hatchikian EC, Frey M, FontecillaCamps JC. Nature 1995;373:580.
[24] Herod AJ, Gibb TC, Herod AA, Shearman J, Dubau C, Zhang S-F,
Kandiyoti R. J Planar Chromatogr 1996;9:361–367.
[25] Herod AJ, Gibb TC, Herod AA, Zhang SF, Xu B, Kandiyoti R. Fuel
1995;75:437.
[26] Lazaro M-J, Herod AA, Cocksedge MJ, Domin M, Kandiyoti R. Fuel
1997;76:1225.
[27] Herod AA, Lazaro M-J, Rahman M, Domin M, Cocksedge MJ,
Kandiyoti R. Rapid Commun Mass Spectrom 1997;11:1627.
[28] Herod AA, Johnson BR, Bartle KD, Carter DM, Cocksedge MJ,
Domin M, Kandiyoti R. Rapid Commun Mass Spectrom
1995;9:1446.
[29] Domin M, Moreea R, Lazaro M-J, Herod AA, Kandiyoti R. Rapid
Commun Mass Spectrom 1997;11:638.
[30] Vorres KS. Energy and Fuels 1990;4:420.
[31] Jarvis KE, Gray AL, Houk RS. Handbook of inductively coupled
plasma mass spectrometry, London: Blackie, 1992.
[32] Vorres KS. User’s Handbook for the Argonne Premium Coal Sample
Program, ANL/PCSP-93/1 Argonne National Laboratory, Argonne,
Illinois, 1993.
[33] Zhang S-F, Xu B, Herod AA, Kimber GM, Dugwell DR, Kandiyoti R.
Fuel 1996;75:1557–67.
[34] Xu B, Kandiyoti R. Energy and Fuels 1996;10:1115.
[35] Xu B, Dix M, Kandiyoti R. Rev Sci Instrum 1995;66:3966–75.
[36] Richaud R, Lachas H, Healey AE, Haines J, Reed G, Herod AA,
Dugwell DR, Kandiyoti R. In: Ziegler A, van Heek KH, Klein J,
Wanzl W, editors. Proceedings of the 9th ICCS, 7–12 September,
Essen, Germany, 1997. p. 457.
[37] Lachas H, Richaud R, Herod AA, Dugwell DR, Kandiyoti R. In:
Ziegler A, van Heek KH, Klein J, Wanzl W, editors. Proceedings
of the 9th ICCS, 7–12 September, Essen, Germany, 1997. p. 429.
[38] Lachas H, Richaud R, Jarvis KE, Herod AA, Dugwell DR, Kandiyoti
R. The Analyst 1999;124:177.
[39] Herod AJ. PhD Thesis. University of Leeds, 1995.
[40] Sawitzky A, Golay MJE. Anal Chem 1964;36:1627.
[41] Pindoria RV, Lim J-Y, Hawkes JE, Lazaro M-J, Herod AA, Kandiyoti
R. Fuel 1997;76:1013.
[42] Madrali ES, Wu F, Xu B, Herod AA, Kandiyoti R. Fuel 1995;9:269.
[43] Megaritis A, Messenbock RC, Collot A-G, Zhuo Y, Dugwell DR,
Kandiyoti R. Fuel 1998;77:1411.
[44] Thomsen M, Kainrath P. Atomic Spectrosc 1998;19(2):60.
[45] Dickson DPE. Mössbauer spectroscopy applied to inorganic chemistry, 1. New York: Plenum Press, 1984, chap. 12, p. 339–89.
[46] Frankel RB, Papaefthymiou GC, Watt GD. Mössbauer spectroscopy
applied to inorganic chemistry, 2. New York: Plenum Press, 1987,
chap. 6, p. 273–89.
R. Richaud et al. / Fuel 79 (2000) 57–67
[47] Shibue T, Kambe H, Nishide H, Akashi S, Dohmae N, Ohashi Y. Eur
Mass Spectrom 1997;3:291.
[48] Scott AC, Fleet AJ. Coal, coal-bearing strata as oil-prone source
rocks? Geological Society Special Publication No 77. London: The
Geological Society, 1994.
[49] Chen X, McManus H, Kevan L. In: Botto RE, Sanada Y, editors.
Magnetic resonance of carbonaceous solids, Advances in Chemistry
Series, 229. Washington, DC: American Chemical Society, 1993. p.
451, chap. 24.
67
[50] Fowler TG, Kandiyoti R, Bartle KD, Snape CE. Carbon 1989;27:197.
[51] Fowler TG, Bartle KD, Kandiyoti R. Energy and Fuels 1989;3:515.
[52] Wind RA, Maciel GE, Botto RE. In: Botto RE, Sanada Y, editors.
Magnetic resonance of carbonaceous solids, Advances in Chemistry
Series, 229. Washington, DC: American Chemical Society, 1993. p.
451, chap. 1.
[53] Cook RL, Langford CH, Yamdagni R, Preston CM. Anal Chem
1996;68:3979.