International Journal of Coal Geology 94 (2012) 214–224

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International Journal of Coal Geology

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Modes of occurrence of trace and minor elements in some Australian coals

K.W. Riley a,⁎, D.H. French a, O.P. Farrell a, R.A. Wood a, F.E. Huggins b

a CSIRO Energy Technology, PO Box 52, North Ryde, NSW, 1670, Australiab CME/CFFS, University of Kentucky, 105 Whalen Building, 533 S. Limestone Street, Lexington, KY 40506–0043, USA

⁎ Corresponding author. Tel.: +61 2 9490 5311; fax:E-mail address: ken.riley@csiro.au (K.W. Riley).

0166-5162/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.coal.2011.06.011

a b s t r a c t

a r t i c l e i n f o

Article history:Received 17 January 2011Received in revised form 14 June 2011Accepted 19 June 2011Available online 3 July 2011

Keywords:Australian coalsElement speciationTrace elements

The modes of occurrence of the trace elements in six Australian coals are reported, together with the nature andpercentages of theminerals present. The trace elements studiedwereAs, B, Be, Bi, Cd, Co, Cr, Cu,Hg,Mn,Mo,Ni, Pb,Sb, Th, Tl, U and Zn, aswell as theminor elements S and Fe. Themodes of occurrencewere determined chemicallyby sequential extraction. For comparison, X-ray absorptionfine structure (XAFS) andnear edge structure (XANES)spectroscopieswereused todetermine themodesof occurrence of As, Pb, Ni, S andZn in four of these six coals and57FeMössbauer spectroscopywas used to estimate the Fe-species (or forms) occurring in the same four coals. Theresults obtained were compared with those published on coals generally in the literature. The integrated resultsprovide the most extensive set of information published to-date on the modes of occurrence of trace elements inAustralian coals.

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© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Although there are numerous papers on the occurrence of traceelements in coal (see for example, Swaine, 1990, 1995), there is still someuncertainty about the assignment of some trace elements. Finkelman(1994, 1995) reports studies into the modes of occurrence of traceelements in coals and ranks the confidence (from one to a maximum often) in the assignment of the likely mode. Not surprisingly, thedetermination of the occurrence of the trace elements present at lowconcentrations in coal is difficult; it should be noted that the traceelements in the low-pyrite coals of Australia are frequently at very lowconcentrations. As well, different researchers use different techniques(Huggins, 2002). In a report on the results of an international inter-laboratory study, Davidson (2000) comments on the often pooragreement between techniques suchas gravity separation and sequentialleaching and the need for confirmation from techniques such as X-rayabsorption near edge structure (XANES) spectrometry.

This paper follows on from an earlier paper on the speciation of Se inAustralian coal samples (Riley et al., 2007). The earlier paper containedinformation on the modes of occurrence of the environmentallyimportant trace element, selenium, whereas this paper containsinformation on the occurrence of a wide ranging number of elementsin the same samples. This should not only provide information that willprove useful to geologists studying the formation and occurrence ofelements in Australian coals, but also provide data that are of use tothose studying the fate of these elements in the utilisation of the coalssampled.

2. Experimental

2.1. Description of the coal samples

In total, six Australian “run of mine” coal samples from Permian andTriassic deposits were analysed (Table 1). Four of these samples werefrom the collection of the Cooperative Research Centre for Coal inSustainable Development (CCSD). This Centre's archivedwebsite can beaccessed at http://pandora.nla.gov.au/index.html (accessedMay, 2011).The two other samples (Cal and Tar; Table 1) were feed coals frompower stations in Queensland (Narukawa et al., 2003). The locations ofthe samples are identified in Fig. 1.

The Warkworth (War) and Great Greta (GG) samples are from theHunter Coalfield and the Newcastle Coalfield, respectively, in thenorthern part of the Permo-Triassic Sydney Basin and the adjacentCranky Corner Basin. The Great Greta sample is from the Early to Mid-PermianGreta CoalMeasures and theWarkworth sample is from the LatePermian Wittingham Coal Measures. Both are bituminous coals, theWarkworth coalhavingavitrinite reflectanceof0.73%and theGreatGretacoal 0.52%. The Greta Coal Measures are a terrestrial coal bearing unitformed during a marine regression (Agnew et al., 1995). The high pyritelevels are indicative ofmarine influence and organic sulphur contents arealso high, further supporting a marine influence. The vitrinite reflectanceis anomalously low in relation to other rank indicators (Wardet al., 2007),againdue tomarine influence on the coal deposit. TheWarkworthmine isa multi-seam operation within the Jerrys Plains Subgroup, a sequence ofterrestrial coals. This sequence formed in an environment of progradingdelta sequences interrupted by marine incursions (Sniffin and Beckett,1995). The coal seams were deposited on the delta plain.

The Curragh (Cur) and Blair Athol (BA) samples are from the Permo-Triassic Bowen Basin in central Queensland. For much of its existence,

Table 1Description of coal samples.

Ident. Coal (location) Description

BA Blair Athol Bituminous; blend of Nos 1, 3 and 4 seams from thePermian Blair Athol Basin, Queensland.

War Warkworth Bituminous; blend of six seams (Vaux, Mt. Arthur,Piercefield, Glen Munro, Woodlands Hill andBlakefield) from the Permian Wittingham CoalMeasures of the Sydney Basin, Hunter Valley, NSW.

GG Great Greta Bituminous; from the Great Greta Colliery; earlyPermian Tangorin seam (marine influenced), CrankyCorner Basin, Hunter Valley, NSW

Cur Curragh Bituminous; from the Cancer, Aries, Castor, Pollux andPisces seams within the late Permian Rangal CoalMeasures of the Bowen Basin, Queensland.

Cal Feed to CallidePower Station

Sub-bituminous; blend of several seams from the LateTriassic Callide Basin, Queensland

Tar Feed to TarongPower Station

Bituminous; blend of three seams from the TriassicTarong Basin, Meandu, Queensland.

215K.W. Riley et al. / International Journal of Coal Geology 94 (2012) 214–224

the Bowen Basin was a region of shallow-water or terrestrialsedimentation. The rank of the coals in the basin is related to Triassicthrust faulting and Cretaceous intrusions. The Blair Athol sample is fromthe Blair Athol CoalMeasures, which are considered to have formed as araised bog intermittently cut by fluvial flood plain deposits (Mallettet al., 1995). The rank is bituminouswith a vitrinite reflectance of 0.60%.The Curragh sample is from the late Permian Rangal Coal Measures,which occur in the northern and central parts of the Bowen Basin. The

Callide

Tarong

Brisbane

Blair Athol Curragh

SydneyGreat Greta

Warkworth

Melbourne

Export Mining Area

Domestic Mining Area

Proven Economic Area

Potential Economic Area

Fig. 1. Location of samples in relation to coalfield areas of eastern Australia.

coal is bituminous, with vitrinite reflectance values varying from 1.22 to1.36%. The coalmeasureswere deposited as freshwater paludal peats ona broad shelf (Mallett et al., 1995).

The Callide Basin occurs in central Queensland and is a Mesozoicintermontane basin of which the Triassic Callide Coal Measures are asignificant stratigraphic unit (Biggs et al., 1995). The rank is sub-bituminous, with vitrinite reflectance varying from 0.46% to 0.53%.The sample used in this study (Cal) is from run of mine production forthe nearby Callide power station.

The Tarong (Tar) sample was taken from the Meandu mine in theTriassic fault-bounded Tarong Basin in southeast Queensland, andrepresents the coal supplied to the nearby Tarong Power Station. Thecoal was deposited in a complex of alluvial fans, fluvial systems andalluvial plains with lacustrine and swamp environments (Pegrem,1995). The coal is of bituminous rank, with a vitrinite reflectance ofapproximately 0.68%.

2.2. Analysis of the coal samples

The moisture contents in the coals were determined by drying at110 °C in a nitrogen-purged oven (Standards Australia, 2000). Theminerals present in the coals were determined by X-ray diffraction(XRD), after ashing in an oxygen plasma instrument at approximately100 °C−150 °C; quantification of the diffractograms was made usingthe Rietveld-based SIROQUANT software package (Taylor, 1991;Wardet al., 2001).

The total concentrations of most of the trace elements weredetermined using the following procedures. After ashing of the coalsat 450 °C, subsamples of the ash were either fused with lithiummetaborate and the fused material dissolved with nitric acid or weredissolvedwith amixture of hydrochloric, nitric andhydrofluoric acids inclosed vessels (prior to measurement of the analyte concentrations, theexcess fluoride was complexed with boric acid). The resulting solutionswere analysed using inductively coupled plasma atomic emissionspectrometry (ICP-AES) and inductively coupled plasma mass spec-trometry (ICP-MS). The exceptions to this approach were Hg, total Sand B. Mercury was determined by cold vapour/atomic fluorescencespectrometry (AFS) after extraction of the coal with aqua regia (ASTM,2006). Both total S and B were determined by ICP-AES following ashingin the presence of Eschka's mixture and extraction with hydrochloricacid (StandardsAustralia, 1997, 1998). The sulphate S and pyritic Sweredetermined using a sequential acid leach procedure with hydrochloricand nitric acids (Standards Australia, 2002) and subsequent measure-ment of soluble S and Fe using ICP-AES.

The modes of occurrence of the trace elements and Fe in the coalswere determined by sequential extraction (and subsequent measure-ment using standards prepared in matrices matched to the extrac-tants). The sequential extraction procedures and operational fractionsused in the study are described below:

a) Soluble: 10 g coal was extracted with 200 mL MilliQ water in aclosed polypropylene bottle; this was placed in an ultrasonic bathfor 10 min to mix the contents; the bottle was then rolled for 18 hat ambient temperature (23°–25 °C). The extract was recovered byfiltration; this was acidified prior to the determinations of thesoluble trace elements.

b) Exchangeable species: thewashed residuewasextracted(by rolling)with 200 mL of 1 M ammonium acetate for 18 h at ambienttemperature (23°–25 °C) to release “exchangeable” trace elements.

c) Carbonate, oxide and monosulfide associated: the washed residuewas extracted (by rolling) with 150 mL of 6 M HCl for 18 h atambient temperature (23°–25 °C) to dissolve the trace elementsassociatedwith the carbonates, oxides andmonosulfides. Thiswouldalso dissolve any Fe3+ species including those thatmay have formedfrom Fe2+ in the previous two steps— this would include any FeSO4.

Table 3Coal analyses — proximate, ultimate and vitrinite reflectance.

Coal BA War GG Cal Cur Tar

Proximate analysis (ad)#Air-dried moisture 7.9 2.8 1.4 8.0 1.3 1.8Ash 8.1 11.9 14.1 21.4 19.9 nd*Volatile Matter 27.0 31.4 43.2 24.5 19.1 nd*Fixed Carbon 57.0 53.9 41.3 46.1 59.7 nd*

Ultimate analysis (daf)##Carbon 83.5 83.7 81.6 78.1 88.1 79.0Hydrogen 4.84 5.45 6.09 4.30 4.57 5.5Nitrogen 1.84 1.81 1.25 1.12 1.70 1.4Sulphur 0.35 0.47 6.29 0.30 0.90 0.7Oxygen 9.50 8.60 4.80 16.20 4.70 13.4

Mean maximum vitrinite reflectance% 0.60 0.73 0.52 0.46 1.31 0.64

nd*: not determined.ad#: air dried basis.daf##: dry ash free basis.

216 K.W. Riley et al. / International Journal of Coal Geology 94 (2012) 214–224

d) Pyritic: thewashed residuewas extracted (by rolling)with 150 mLof 2 M HNO3 to dissolve pyrite; again this was done for 18 h atambient temperature (23°–25 °C).

e) Silica bound: the silicate minerals in the washed residue wereextracted with 25 mL conc, HF and 2.5 mL conc. HCl at 50 °C in aheated ultrasonic bath for 2 h; the extract was diluted to 150 mLwith MilliQ water.

f) Residual: determined by difference (total — sum of extractablephases) indicative of an organic association, shielded components,or occurrence in a resistate mineral phase.

The extraction procedure used is similar to that used by Dale et al.(1999) and also Spears et al. (1998), but it should be noted that theselatter researchers destroyed the organic matter with nitric acid prior todetermining the silicate bound trace elements. Dai et al. (2004) usedboth physical and chemical separation to determine the modes ofoccurrence of an extensive range of elements in a Chinese coal field. Inanother study, Dai et al. (2008) used a scanning electron microscopeequipped with an energy-dispersive X-ray spectrometer (SEM–EDX) tostudy the characteristics of the minerals and to determine thedistribution of some elements in the coal. See also comments byDavidson (2000) and the Concluding Comments of this paper.

In this study, themodesof occurrenceofAs, Pb,Ni, andZn in fourof thecoals (GG, Cur, Cal and Tar) were determined by X-ray absorption finestructure (XAFS) spectroscopy (Huggins and Huffman, 1996), althoughtypically only the X-ray absorption near-edge structure (XANES) portionof the spectrum was utilised for the investigation. This work wascompleted on beam-lines 11–2 of the Stanford Synchrotron ResearchLaboratory (SSRL) at Stanford University, California and X-18B of theNational Synchrotron Light Source (NSLS) at Brookhaven NationalLaboratory, NY. The modes of occurrence of S were also obtained byXAFS spectroscopy at beam-line X-19A at NSLS. The speciation of iron inthe same coals was estimated using 57Fe Mössbauer spectroscopy. Boththe Mössbauer and XAFS techniques were done directly on the as-received coals and not on any extract residues or ash.

3. Results and discussion

Themineral phase percentages were calculated to a coal basis fromthe XRD results obtained on the low temperature ashes and arereported in Table 2. Also reported in Table 2 are the low temperatureash yields; these provide an approximation of the mineral mattercontent of the coals.

Table 2Calculated percentages of mineral phases in coals (from XRD analyses of lowtemperature ashes).

Coal: BA War GG Cur Cal Tar

%Quartz 2.58 4.67 1.00 5.93 4.22 17.90Kaolinite 5.62 5.27 6.05 4.77 19.27 18.30Illite 0.15 2.96 1.37Smectite 0.05 0.17 0.19 0.13Chamosite 4.08Anatase 0.23 0.37Brookite 0.28Boehmite 0.14Hematite 0.17 0.04 0.85Calcite 3.50 2.27Ankerite 0.59Siderite 0.11 0.79 2.72Bassanite 1.92 1.79 0.91Gypsum 2.18Anhydrite 2.61JarositePyrite 0.22 1.05 0.57Total ash yield(low temperature)

8.9 13.9 18.5 21.8 28.3 36.6

The properties (proximate and ultimate analyses and meanvitrinite reflectance) of the coals are listed in Table 3. Note thatthere are differences in the ash yields reported in Tables 2 and 3 (i.e.LTA and 815 °C ash); these are indicative in many instances of waterof hydration, structural hydroxyl groups and sulphur as sulphatebeing retained in the LTA. The concentrations of total, pyritic andorganic sulphur by difference (air dried basis) are listed in Table 4.

The analyses of the extractant solutions (from which the modes ofthe occurrence of the trace elements associated with each phase in thecoals, as definedby the extractingmedium, can be estimated) are given inTables 5–10. The differences between the sum of the extracted traceelements and the total concentration, i.e. the residual concentrations, areoften takenas the concentrationsof organically associated trace elements.It should be noted that, in a few cases, the residual concentrations areexpressed as negative numbers. In these few instances, the initial totalconcentration is less than the sum of the concentrations determinedsequentially. Either there is an error in the determination of the totalconcentration or the sum of the errors associated with the sequentialmeasurements is significant relative to the total concentration of specifictrace elements in the coal. Note also that the minor and trace elementsseen in the residual matter may be present as compounds or in mineralmatter shielded by the residual organic matter.

The results from interpreting the sulphur XANES spectra aresummarised in Table 11, and the results of the analyses by Mössbauerspectrometry together with the% pyritic sulphur (Huffman andHuggins, 1978) are summarised in Table 12.

Thewt.%pyritic sulphur in the coalwasestimated fromtheMössbauerdata for pyrite using the method of Huffman and Huggins (1978). Itshouldbenoted that this estimate is generally ingoodagreementwith thevalues obtained by the chemical extraction procedure based on themethods of StandardsAustralia (Table4).However, this is notparticularlyso for the Cal coal sample, and thus the species identified as “pyrite?” inthis spectrum is in all probability not pyrite but anon-magnetic ironoxideor oxyhydroxide phase. For the GG coal sample, the difference in the

Table 4Concentrations of the sulphur species in the coals.

Coal Sulphur species (% air dried)

Total Pyritic Sulphate Organic

BA 0.29 0.12 0.02 0.15War 0.40 0.07 b0.01 0.33GG 5.32 0.53 0.12 4.67Cur 0.71 0.36 0.08 0.27Cal 0.19 0.02 0.02 0.15Tar 0.23 0.06 0.02 0.15

Table 6Distribution of trace elements (mg/kg, on an air-dried basis) in leachates and residue(by difference) of War.

Total asanalysed

MilliQ 1Macetate

6M HCl 2MHNO3

HF/HCl Residual

As 0.43 b0.01 b0.01 0.17 0.04 0.30 −0.08B 15 0.6 b0.1 2.5 4.0 b0.1 7.9Be 1.8 b0.01 b0.01 0.26 0.10 1.12 0.3Bi 0.19 b0.01 b0.01 0.13 b0.01 b0.01 0.06Cd 0.09 b0.01 b0.01 0.03 b0.01 0.09 −0.03Co 6.4 b0.01 0.06 0.72 0.07 2.3 3.3Cr 6.9 b0.01 b0.01 0.87 0.10 4.2 1.7Cu 7.60 b0.01 0.10 1.7 1.4 1.7 2.7Fe 3140 b0.2 b0.2 2294 283 572 −9Hg 0.015 b0.005 b0.005 b0.005 0.013 b0.005 0.002Mn 25 0.31 1.27 20.8 0.82 3.4 −2Mo 0.64 0.02 b0.01 0.04 0.09 0.03 0.46Ni 7.1 0.02 0.09 0.55 0.32 1.80 4.3Pb 6.8 b0.01 0.39 4.86 0.48 0.87 0.2Sb 0.59 b0.01 b0.01 0.02 0.02 0.24 0.31Th 3.0 b0.01 b0.01 1.57 0.031 0.65 0.8Tl 0.11 b0.01 0.01 b0.01 0.02 0.04 0.04U 1.02 b0.01 0.01 0.25 0.03 0.18 0.54V 29 b0.01 b0.01 2.4 2.2 19.2 5Zn 15 b0.01 2.0 13.1 2.3 3.5 −6

217K.W. Riley et al. / International Journal of Coal Geology 94 (2012) 214–224

pyritic sulphur determinations is possibly due to the different extents ofoxidation of the pyrite to produce sulphate. The pyrite in GG may havebecomemore oxidised at the time theMössbauer spectrumwas obtainedcompared to when the analyses for forms of sulphur were conducted.

4. Discussion

The occurrences of the minor and trace elements are discussedbelow:

SulphurPyrite is common but occurs at low concentrations in the Gondwanacoals of present-day Australia (Taylor et al., 1998). Pyrite has beenidentified in three of the coal samples by XRD (see Tables 2 and 3).However, the results of chemical analysis indicate that pyrite ispresent at low levels in all of the coals. Organic sulphur is high in theGG coal but present at low levels in all of the other coals. XAFS data onthe speciation of the sulphur in four of the coals, GG, Cur, Cal and Tar,were reported and discussed previously (Riley et al., 2007). Alsodiscussed in the earlier paperwere the apparent discrepancies in eachcoal's mineralogy obtained by XRD (Table 2) and the speciation ofsulphur results (Table 4). This is particularly so for the GG coal. It wassuggested that the sulphur-rich minerals, anhydrite, bassanite andgypsum identified as being present in this coal were predominantlyartefacts of the low temperature ashing process (Riley et al., 2007).In agreement with the comments above is the relatively low % S assulphate in the GG coal (Table 11). The XRD analysis of the lowtemperature ash (Table 2) indicates that this coal containssignificant amounts of bassanite and gypsum; although the % S(expressed in absolute terms in Table 4) as sulphate for this coal isthe highest of the six coals, it is suggested that abundant calciumsulphates are formed during the LTA procedure. Electron micro-probe studies by Ward et al. (2007) have indicated significantproportions of Ca (up to 1%) in the macerals of coals from thissuccession; these are also consistent with such a process forbassanite and gypsum formation.It should benoted also that XAFS results are expressed as a percentageof the non-pyritic sulphur in each of the four coal samples (Table 11).The sulphur concentrations in coals from the Great Greta sample arehigh and are predominantly organic; thus the low relative percentage

Table 5Distribution of trace elements (mg/kg, on an air-dried basis) in leachates and residue(by difference) of BA.

Total asanalysed

MilliQ 1Macetate

6M HCl 2MHNO3

HF/HCl Residual

As 2.3 0.01 b0.01 1.22 0.51 b0.01 0.6B 20 2.3 0. 6 3.5 3.0 b0.1 10.6Be 0.73 0.02 b0.01 0.33 0.085 0.23 0.07Bi 0.15 b0.01 b0.01 0.14 0.002 b0.01 0.01Cd 0.07 0.02 0.01 0.02 0.01 0.02 −0.01Co 1.9 0.10 0.02 0.70 0.05 0.56 0.5Cr 5.0 0.01 b0.2 0.84 0.150 3.8 0.2Cu 6.2 0.16 b0.04 0.76 0.5 1.20 3.6Fe 2392 253 0.17 1345 620 124 50Hg 0.058 b0.005 b0.005 b0.005 0.024 b0.005 0.034Mn 9.3 2.06 0.90 6.91 0.03 0.4 −0.7Mo 0.87 b0.01 0.009 0.02 0.18 0.05 0.61Ni 4.30 0.12 b0.01 0.49 0.59 1.60 1.50Pb 5.0 0.01 0.05 4.56 0.23 0.69 −0.5Sb 0.14 b0.01 b0.01 b0.01 0.02 0.07 0.05Th 3.5 b0.01 b0.01 0.81 0.051 1.80 0.8Tl 0.07 0.01 0.01 b0.01 0.01 0.01 0.03U 0.71 b0.01 0.003 0.15 0.05 0.27 0.23V 6.7 b0.01 b0.01 2.48 1.20 2.81 0.2Zn 20 2.65 0.16 14.73 2.46 1.15 −1

TabDist(by

ABBBCCCCFeHMMNPSbThTlUVZ

of sulphate is a consequence of this high organic sulphur concentra-tion. The large quantity of sulphate noted for the Cur coal is consistentwith the significant amounts of iron sulphate identified in theMössbauer spectrum of the same coal, likely as a result of pyriteoxidation. The high relative percentages of elemental sulphur in thecoals are intriguing. These occurrences are not readily explainedunless the elemental sulphur has formed as an oxidation product (seeDavidson, 1993).

IronThe modes of occurrence of iron are of interest as iron-bearingminerals are likely hosts formany of the trace elements. Hydrochloricacid soluble iron is likely to include iron carbonate (e.g. siderite) andironoxides, e.g. ironhydroxyoxidesorhaematite. Theamountof nitricacid extractable iron provides a measure of the pyrite content.As can be seen from the data in Tables 5–10, the majority of theiron in most samples is soluble in HCl. The most likely forms of

le 7ribution of trace elements (mg/kg, on an air-dried basis) in leachates and residuedifference) of GG.

Total asanalysed

MilliQ 1Macetate

6M HCl 2MHNO3

HF/HCl Residual

s 1.7 b0.01 b0.01 1.42 0.22 0.30 −0.286 5.3 1.8 25 17 1.4 35

e 0.95 b0.01 b0.01 0.10 0.04 0.20 0.62i 0.22 b0.01 b0.01 0.19 0.01 b0.01 0.02d 0.10 b0.01 0.025 0.05 0.01 0.03 −0.02o 1.9 0.057 0.070 0.50 0.08 0.59 0.6r 9.4 b0.01 b0.01 2.09 0.16 5.1 2.1u 13 0.06 0.71 5.6 4.0 1.8 1

6613 b0.2 b0.2 5119 1750 112 −368g 0.16 b0.005 b0.005 0.02 0.059 b0.005 0.08n 160 29.9 32 82.66 6.2 16.2 −7o 0.73 0.014 0.009 0.10 0.12 0.09 0.4i 4.0 0.54 0.66 1.42 0.45 0.90 0b 5.0 b0.01 0.019 4.86 0.70 0.33 −0.9

0.25 b0.01 b0.01 0.08 0.02 0.06 0.091.2 b0.01 b0.01 0.40 0.04 0.35 0.410.15 b0.01 0.03 0.05 0.03 0.04 b0.010.57 b0.01 b0.01 0.05 0.03 0.13 0.35

20 0.01 0.02 2.3 2.3 11 5n 7.7 b0.01 b0.01 8.6 1.1 0.68 −2.3

Table 8Distribution of trace elements (mg/kg, on an air-dried basis) in leachates and residue(by difference) of Cur.

Total asanalysed

MilliQ 1Macetate

6M HCl 2MHNO3

HF/HCl Residual

As 0.93 b0.01 b0.01 0.58 0.10 b0.01 0.25B 5.3 0.2 b0.1 b 0.1 b 0.1 b0.1 5.1Be 0.70 b0.01 b0.01 0.16 b0.01 0.21 0.33Bi 0.13 b0.01 b0.01 0.09 b0.01 b0.01 0.04Cd 0.05 b0.01 0.021 0.02 b0.01 b0.01 0.01Co 5.7 0.01 0.07 2.3 0.15 0.68 2.5Cr 8.3 b0.01 b0.01 3.0 0.06 4.9 0.4Cu 13 0.01 0.68 1.0 4.1 3.8 3.4Fe 18473 b0.2 b0.2 15192 1940 485 856Hg 0.022 b0.005 b0.005 b0.005 0.009 b0.005 0.013Mn 150 0.6 24 131 2.4 1.1 −9Mo 0.25 0.01 b0.01 0.10 0.05 b0.01 0.09Ni 10 0.11 0.56 2.6 0.45 1.7 4.6Pb 3.9 b0.01 b0.01 2.7 0.47 0.92 −0.2Sb 0.19 b0.01 b0.01 0.03 0.01 0.06 0.09Th 1.4 b0.01 b0.01 0.71 0.01 0.77 −0.1Tl 0.04 b0.01 b0.01 b0.01 b0.01 b0.01 0.04U 0.47 b0.01 0.02 0.16 b0.01 0.12 0.17V 32 b0.01 0.05 11 0.30 16 5Zn 8.7 b0.01 0.18 11 1.5 0.85 −5.3

Table 10Distribution of trace elements (mg/kg, on an air-dried basis) in leachates and residue(by difference) of Tar.

Total asanalysed

MilliQ 1Macetate

6M HCl 2MHNO3

HF/HCl Residual

As 1.7 0.11 b0.01 0.62 0.14 1.3 −0.5B 12 1.0 b0.1 2.6 3.0 b0.1 5.4Be 1.5 b0.01 b0.01 0.27 0.19 0.78 0.3Bi 0.32 b0.01 b0.01 0.16 b0.01 b0.01 0.16Cd 0.16 b0.01 0.03 0.09 0.01 0.12 −0.1Co 7.2 0.14 0.56 2.7 0.50 1.3 2.0Cr 7.5 b0.01 b0.01 0.55 0.14 4.6 2.2Cu 21 0.04 b0.04 11 2.8 4.1 3.0Fe 646 b0.2 b0.2 214 57 234 141Hg 0.022 b0.005 b0.005 b0.005 0.014 b0.005 0.008Mn 5.5 0.25 1.5 2.7 0.02 0.9 0.2Mo 1.7 0.10 0.33 0.10 0.08 0.12 0.97Ni 4.4 0.09 0.32 3.2 1.2 2.1 −2.5Pb 10 b0.01 0.25 6.9 0.75 1.3 0.8Sb 0.31 b0.01 b0.01 0.02 0.01 0.08 0.2Th 4.6 b0.01 b0.01 1.9 0.04 0.77 1.9Tl 0.28 b0.01 0.04 b0.01 0.05 0.02 0.17U 1.4 b0.01 b0.01 0.20 0.03 0.29 0.88V 65 0.34 0.30 8.9 11 31 14Zn 62 0.78 5.1 51 2.4 1.6 1

218 K.W. Riley et al. / International Journal of Coal Geology 94 (2012) 214–224

HCl-soluble iron are carbonates or oxides, some of whichmay haveformed as a consequence of oxidation of iron sulphates duringextraction (see below, the results of Mössbauer spectroscopy andTable 12). All of the coals contain some pyritic iron (HNO3 soluble),significant proportions of which (N20% of the total iron) arepresent in the BA and GG coals. The BA coal also contains water-soluble iron; this is presumably as Fe2+ and may be indicative ofsome partial oxidation of the coal.

The data from theMössbauer spectroscopy (Table 12) indicate that:

a) The GG coal contains pyrite, szomolnokite (ferrous sulphate),jarosite (potasssium ferric hydroxy sulphate) and possiblymelanterite (ferrous sulphate). The sulphates of iron are likelyproducts of pyrite oxidation. Significant fractions of the iron inthis coal are soluble inHCl and inHNO3. This is consistentwiththe Mössbauer data.

b) The Cur coal contains iron bound to clays, siderite, pyriteand jarosite. The large proportion of iron that is HCl-soluble

Table 9Distribution of trace elements (mg/kg, on an air-dried basis) in leachates and residue(by difference) of Cal.

Total asanalysed

MilliQ 1 Macetate

6 M HCl 2 MHNO3

HF/HCl Residual

As 1.7 b0.01 b0.01 0.19 0.11 0.60 0.8B 18 2.8 0.6 3.5 3 b1 8.1Be 0.82 b0.01 b0.01 0.37 0.13 0.28 0.05Bi 0.28 b0.01 b0.01 0.17 b0.01 b0.01 0.11Cd 0.15 b0.01 b0.01 0.06 0.02 0.11 −0.04Co 6.6 b0.01 0.09 1.8 0.38 1.10 3.2Cr 12 b0.01 b0.2 0.76 0.45 6.4 4.4Cu 19 b0.01 b0.04 10 1.6 3.1 4Fe 22839 b0.2 b0.2 19492 630 765 1952Hg 0.022 b0.005 b0.005 0.011 0.020 0.007 −0.016Mn 439 0.89 29 371 0.26 3.5 34Mo 0.81 b0.01 b0.01 0.02 0.05 0.14 0.6Ni 13 b0.01 0.09 1.7 1.9 3.8 5.5Pb 8.8 b0.01 0.03 6.2 0.61 1.4 0.6Sb 0.21 b0.01 b0.01 0.01 0.02 0.08 0.1Th 3.9 b0.01 b0.01 2.2 0.11 0.66 0.9Tl 0.15 b0.01 0.02 0.01 0.08 0.01 0.03U 1.06 b0.01 b0.01 0.33 0.06 0.22 0.45V 33 b0.01 0.02 13 4.6 13 3Zn 33 0.03 0.22 24 2.0 2.4 5

and the small proportion that is HNO3-soluble are consis-tent with the Mössbauer data.

c) The Cal coal contains haematite, siderite and possible pyrite;the acid extraction results are in agreement with theMössbauer data in that the major portion of the iron inHCl-soluble.

d) The Tar coal contains iron oxyhydroxides and ferrous ironbound in clays. This is consistent with the results of thechemical fractionation in that the iron oxyhydroxideswould be soluble in HCl and the clay bound iron (iron insilicate structures) would be soluble in HF/HCl. Howeverthe results of chemical fractionation indicate that somepyritic ironmay be present. There is also some residual iron;it is unlikely that this is organically associated. There issome uncertainty in the interpretation of the data.

Based on the results reported in Tables 5–10, the followingcomments can be made on the occurrence of each of the traceelements in these Australian coals:

ArsenicAccording to the reports in Wedepohl (1969), As “can probably”substitute for Si, Al, Fe and Ti in the rock forming minerals; it maybe present in high concentrations in magnetite and ilmenite. Themonosulfide, arsenopyrite (FeAsS), is the most abundant oremineral. Finkelman (1994) reports that As is most likely present inthe pyrite in coal, with a possibly minor amount organically bound(confidence level in the assignments of As is 8 out of 10). Swaine(1990) suggests that some As may also be present as arsenate ionsin clays or phosphate minerals. Dale et al. (1999) report that As is

Table 11Distribution of sulphur among the major sulphur forms in four coals (excluding pyriteand other metal sulphides) as determined by XANES spectrometry.

Coal Relative % of the S forms present (other than as sulphides)

Elemental. S Organic sulphide Thiophenic Sulfone Sulphate

GG b3 21 73 b2 6Cur 10 b5 49 3 38Cal 29 b5 48 2 21Tar 5 7 74 b2 14

Table 1257Fe Mössbauer results including % Pyritic S on four Australian coals (note that % Fe isrelative to total Fe and pyritic sulphur is on an air-dried basis).

Coal Mössbauercomponent

IS QS H0 Width % Fe Mineral Pyritic S

Cal 1Q 1.21 1.80 0.26 27 Siderite2Q 0.32 0.63 0.60 16 Pyrite? 0.411M 0.40 −0.07 489 0.44 17 Hematite2M 0.40 −0.07 459 1.20 39 Hematite

Cur 1Q 0.31 0.59 0.28 13 Pyrite 0.302Q 1.23 1.78 0.40 27 Siderite3Q 0.29 1.19 0.29 10 Jarosite4Q 1.15 2.67 0.30 50 Clay/Fe2+

GG 1Q 0.32 0.59 0.31 40 Pyrite 0.282Q 1.27 2.72 0.26 34 Szomolnokite3Q 0.35 1.06 0.34 24 Jarosite4Q 1.27 3.52 0.31 2 Melanterite?

Tar 1Q 0.37 0.78 0.73 74 FeOOH? b0.042Q 1.13 2.64 0.54 26 Clay/Fe2+

Note that “Pyrite?”, “Melanterite?” and “FeOOH?” are probable species in the relevant coalsamples. Note also the components: Q — Quadrupole doublet; M — Magnetic sextet.Mössbauer parameters: IS — isomer shift, relative to metallic iron; QS — quadrupolesplitting; H0 — Magnetic hyperfine splitting in kGauss. Width — full peak width at halfmaximum height.% Fe is the percentage of the total iron in the coal present in the iron-bearingmineral; it is derived from the relative areasunder theMossbauer peaks attributedto each mineral.

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associated with pyrite. Kolker et al. (2000) report that As in UScoals is principally associated with pyrite, as do Ward et al. (1999)for other Australian coal samples. In a related paper, Huggins et al.(2002) used a combination of sequential leaching and XAFS toanalyse a bituminous coal from Ohio. The researchers report thatarsenate is the species leached by HCl and As in the pyrite isleached by HNO3 and that the arsenate in this coal is likely aproduct of pyrite oxidation (Kolker and Huggins, 2007).

Yudovich and Ketris (2005a), in an extensive review of As incoal, suggest that there are three dominant forms, i.e. pyritic,organic, and arsenate.

The results of the sequential leaching (Tables 5–10) do notindicate that the As in these six Australian coals is principallyassociated with pyrite. This may be a consequence of the lowpyrite content of the coals or indicative of oxidation of some of thecoals. The data from sequential leaching and XAFS spectroscopyindicate that:

a) Arsenic in the BA sample is associated with HCl-solubleminerals (probably ankerite and jarosite), with a lesserproportion present in sulphide form (note that pyrite wasnot identified by XRD as being present but the mineral maybe present at low concentrations).

b) Arsenic in the War coal is associated with the silicates(kaolinite, illite, smectite), with a lesser proportion associ-ated with HCl-soluble minerals (probably ankerite) andeven less in sulphide form (again, pyrite was not identifiedby XRD).

c) Arsenic in the GG coal is associatedwith HCl-solubleminerals(calcite, bassanite and gypsum), with a lesser proportionassociatedwith the clays/silicates (kaolinite) and less presentin the sulphide. The data from XAFS measurements indicatethatmost of theAs is present as oxidised forms (i.e. arsenite orarsenate),with less present in thepyrite. Both techniques giveresults that are in agreement.

d) Arsenic in the Cur coal is primarily associated withcarbonate or monosulfide minerals. The results from theXAFS spectroscopy indicate that the As is present primarilyas arsenate, with possibly some arsenite and lesser amountsof As in sulphides. Again the results are in agreement. The

XAFS data indicate that the As that is soluble in HCl is morelikely to be present as an oxidised species rather thanassociated with or as a monosulfide.

e) Arsenic in the Cal feed coal is associated with silicates and isalso present in the residual (organic) matter. The results fromthe XAFS technique again indicate thatmuch of the As is in anoxidised form, and this is in agreement with the evidence forsilicate bound (or associated) As. The signal/noise ratio for theCal coal is of poor quality. Thus it is reasonable to conclude thatAs may be in other forms, i.e. reduced species in sulphides ororganically bound, as indicated by the chemical fractionation.

f) Arsenic in the Tar sample is present in the silicates and in thecarbonates, with minor amounts present in the sulphides(pyrite). The XAFS data support this conclusion.

Note that the negative values for As in three of the coals indicatethat the sum of As found in the six extracts is greater than the total asanalysed. This is probably a consequence of the difficulty of accuratelymeasuring the low concentrations present in the extracts. The sum ofthese amounts also includes the sum of all the errors associated withthe measurements. There is good agreement between the resultsfrom the chemical leaching approach and the results from the XAFSspectroscopy for the four coals studied by both techniques. The XAFSdata enable one to generalise that any HCl soluble As in these coals islikely associatedwith carbonates or oxides rather thanmonosulfides,or may be present as arsenate as a result of pyrite oxidation.

BoronAs mentioned in the earlier paper (Riley et al., 2007), the Bconcentrations in five of the six coals are in the range of 6–20 mg/kg(Tables 5–10) and are indicative of freshwater influences. The GreatGreta coal, GG, contains B at a concentration of 86 mg/kg, indicative ofsome marine influence (Goodarzi and Swaine, 1994).Wedepohl (1969) states that with few exceptions “boron occurs inchemical combination with oxygen” i.e. as borates; the exceptionsare the minerals, ferrucite, NaBF4 and avogadrite, (K, Cs)BF4. Swaine(1990) indicates thatmost of the B is organically associated in coal. Itshould be noted that B also occurs in tourmaline, a highly refractorysilicate mineral with a B concentration of up to approximately 3%.Thus the “organic association” may be indicative of minor amountsof acid-resistant tourmaline present in the residual organic matterfollowing acid extraction. Boyd (2002) studied the volatility of B inone Australian coal and noted that B if present in tourmaline is notvolatile during ashing. The researcher used this fact and leachingbehaviour to identify different modes of occurrence of B in this oneparticular coal. Boyd (2002) also provides a comprehensive reviewof the literature on B in coal.It can be seen from Tables 5–10, that in all the coals, the proportionof B present in the residual “organic” matter is 40% or greater. Intwo of the coals, B in the residual material is approximately 100%of that present. Boron is not present in the HF-soluble silicates inany of the coals.

The results of the sequential leaching indicate that:

a) Boron in the BA coal is in a water soluble form, as well as inthe HCl and HNO3 leachates. The major proportion is foundin the residual matter (organic, shielded, or in a resistatemineral such as tourmaline).

b) Boron in the War coal is distributed in a similar manner tothat in the BA coal. There is less water-soluble B and more B

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occurring in the HNO3 leachate (note that pyrite was notidentified by XRD of the LTA). Themajor proportion is foundin the residual matter (organic, shielded, or in resistateminerals).

c) Boron in the GG coal is associated with HCl-soluble minerals(e.g. calcite, siderite, bassanite and gypsum, ormonosulfides),and also in the HNO3 leachate. The major proportion (approx40%) is found in the residual matter (organic, shielded orresistateminerals). A veryminor proportion is water-soluble;this may be a consequence of the oxidation of pyrite.

d) Boron in the Cur coal is almost exclusively found in theresidual matter (organic, shielded or resistate minerals).There is a minor proportion that is water-soluble.

e) Boron in the Cal coal is almost exclusively found in theresidual matter (organic, shielded or resistate minerals).There is a minor proportion that is water-soluble.

f) Boron in the Tar coal is in a water soluble form, as well as inthe HCl leachate (carbonates or monosulfides) and in theHNO3 leachate (pyrite). The major proportion is found in theresidual matter (organic, shielded or resistate minerals).

BerylliumAccording to Wedepohl (1969), Be is “widely distributed in lowconcentration in rock-forming minerals in which it replaces Si. Inminerals the coordination number is always four”. Both Swaine(1990) and Finkelman (1994) suggest (level of confidence 4) thatBe is associatedwith the organicmatterwith some present in clays.According to Finkelman (1994), there is “abundant evidence in theliterature” that “indicates an organic affinity for Be. Few otherelements are so consistently concentrated in the float fraction inlaboratoryfloat-sink experiments. Moreover, the Be content of coalvaries inversely with ash yield”. Dale et al. (1999) report that Be ispresent in the silicates in the Australian coals studied by that group.

It can be seen from the data in Tables 5–10 that:

a) Beryllium in the BA coal is associated primarily with thesilicates (quartz and clays) and the carbonates/monosulfides(probably siderite). There does appear to be some Beassociated with the pyrite and the hematite.

b) Beryllium in the War coal is primarily associated with thesilicates.

c) Beryllium in the GG coal is present in the residual organicmatter or in a shielded or resistate mineral (possibly beryl).

d) Beryllium in the Cur coal is probably associated with calcite,the silicates and either the residual organic matter, or ashielded or resistate mineral (possibly beryl).

e) Beryllium in the Cal coal is present in phases soluble in HCl(possibly bassanite and gypsum) and also HF (i.e. silicates).There appears to be some Be associated with sulphides.

f) Beryllium in the Tar coal is primarily in the silicates, withsmaller proportions associatedwith carbonates/monosulfides,sulphides, and in the residual material (organic or shielded orresistate mineral).

The association of Be with organic matter is not generally seen inthe results for these coals. Nor is there any relationship between theash yield and the concentrations of Be (see Table 2). Of course, thissurvey is limited. If Bewas associatedwith the organicmatter, then aninverse relationship between ash yield and Be content is possible. Thepresence of the acid resistant mineral beryl, Al2Be3Si6O18, may partlyexplain the “organic” association reported by some researchers. Thismineral would only have to be present at very low levels.

BismuthThere does not appear to be any work published on the directspeciation of Bi in coal. Spears and Tewalt (2009) used indirectmeasurements (i.e. regression analysis) to suggest that Bi is associatedwith the clays in a Late Carboniferous coal from the UK. Bismuthoccurs at very lowconcentrations in theAustralian coals of thepresentstudy, i.e. 0.13–0.32 mg/kg, and this in itself can lead to analyticalerrors. However, it can be seen from the data in Tables 5–10 that:

a) Bismuth in all the coals is present in the HCl extracts; this isindicative of Bi being present as oxides or carbonates, orperhaps present associated with monosulfides.

b) Bismuth is also present in the residual material in all of thecoals. Significant amounts relative to those present in theHCl extract are present in theWar, Cur, Cal and Tar samples.In fact, the Tar sample contains equal amounts of Bi in bothforms (Table 10).

CadmiumFinkelman (1994) reports (level of confidence 8) that Cd ispredominantly associated with sphalerite, ZnS, although it may befound in other sulphides. This is in agreement with the summationof Swaine (1990). Goodarzi (2002) reports that Cd is associatedwith sphalerite in Canadian feed coals. Dale et al. (1999) report Cdas being present in the monosulfides, pyrite and also in thesilicates. Although Finkelman (1994) rates the level of confidenceat 8 for the assertion that Cd is predominantly associated with ZnS,there appears to be some uncertainty about the modes ofoccurrence of Cd in coal. The data reported here (Tables 5–10)indicate that Cd is present in many modes. However, Cd is presentat extremely low concentrations, and there is the possibility oferror in the measurement itself or from contamination. In fact, allthe coal samples in the present study contain Cd at less than0.2 mg/kg and the residual concentration is negative (i.e. total isless than the sum of the concentrations in all the extracts).Although the data set is limited, there is no obvious relationshipbetween the Cd and Zn concentrations in the coal samples, apartfrom the fact that the two coals with the highest concentrations ofCd also have the highest concentrations of Zn. This does notexclude the possibility of Cd being present in sphalerite, as there isno reason to expect that the sphalerite in coals from differentlocations would contain similar concentrations of Cd. Obviously,for there to be a relationship between Cd and sphalerite, there isalso a supposition that Zn is predominantly present as sphalerite.The XAFS data for Zn obtained on the GG, Cur, Cal and Tar coalsindicate that sphalerite is the major form of Zn in only the lattertwo samples, and that Zn may be present as other species (seebelow).

CobaltCobalt is most likely associated with sulphide minerals, but also inclays and in the organic matter (level of confidence 4, Finkelman,1994). Dale et al. (1999) found that Co was associated with thesilicates in the Australian coals studied by that group.Cobalt in the present series of samples (Tables 5–10) is distributedacross all the modes of occurrence as defined by the analyticalscheme. In all coals, Co is associated with silicates and with theresidual matter (organic, shielded or resistate minerals). It is alsopresent in the oxide/carbonate/monosulfide group, and also to alesser extent in pyrite.

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ChromiumFinkelman (1994) states that there “are insufficient data to specifythe modes of occurrence of chromium in coal”. Some organicassociation is suspected (see also Swaine, 1990). Dale et al. (1999)report that Cr is present in the oxide/carbonate/monosulfidegroup, but that it may be present in the silicates and associatedwith the organic matter. Huggins et al. (1999) used XAFSspectroscopy to determine the valency of the Cr present in bothcoal and coal ash (see also Huffman et al., 1994). Those researchersconcluded that Cr was present as Cr3+ in the coal. In a more recentpaper, Huggins and Huffman (2004), state that “chromiumappears to occur in most bituminous coals in only two majorforms: as Cr3+ in organic association and as Cr3+ in illite (althoughthese conclusions were based principally on the findings of studieson coals from the USA).

The data from the sequential leaching (Tables 5–10) indicate that:

a) Chromium in the BA coal is principally associated withthe silicates. Minor proportions are found in the carbonate/monosulfide group and in the residual matter (organicshielded, or a resistate mineral).

b) Chromium in theWar coal is distributed in a similar mannerto that in the BA coal. The major proportion is found in thesilicates and a significant proportion in the residual matter(organic, shielded or resistate minerals). A minor propor-tion is found in the oxide/carbonate/monosulfide group.

c) The distribution of Cr in the GG coal is similar to that in the BAcoal and very similar to the War sample, i.e. the majorproportion is found in the silicates and significant proportionsin the residualmatter (organic, shielded or resistateminerals)and the oxide/carbonate/monosulfide group.

d) Chromium in the Cur coal is found principally associatedwith the silicates. A significant proportion is found in theoxide/carbonate/monosulfide group.

e) The distribution of Cr in the Cal sample is similar to that in theWar coal, i.e. themajor proportion is found in the silicates anda significant proportion in the residual matter (organic,shielded or resistate minerals). A very minor proportion isfound in the oxide/carbonate/monosulfide group.

f) The distribution of Cr in the Tar coal is again similar to that inthe War coal (and thus to that in the Cal).

Although, the data set is limited, the evidence is that Cr isgenerally present in the silicates and in the residual organic matter.Thesefindings are in agreementwith the observationofHuggins andHuffman (2004) and also the comments on anorganic association bySwaine (1990). However, it is reasonable to consider that Cr may bepresent in resistate minerals. It is well known that Cr3+ can replacecations (isomorphous replacement) inhighly resistateminerals suchberyl, corundum, rutile, spinel and tourmaline (Wedepohl, 1969).

CopperCopper is likely to be present in coal as chalcopyrite or othersulphides, and possibly as organically bound species (Swaine, 1990).Dale et al. (1999) report that Cu is present in the oxide/carbonate/monosulfide group, in the pyrite, and in the silicates.The data from the sequential leaching (Tables 5–10) indicate thatCu occurs in many modes in the coals of the present study:

a) Copper in the BA coal is principally found in the residualmatter (organic, shielded or resistate minerals). There issome associated with the silicates and also minor pro-

portions in the carbonate/monosulfide group and in thesulphide (pyrite) fraction. There is a very small portion thatis water-soluble.

b) Similarly, Cu in the War coal is in the residual matter(organic, shielded or resistate minerals). Copper is alsofound in the silicates, the sulphide and the carbonate/monosulfide group.

c) Copper in the GG coal is distributed “evenly” between thesulphide and the carbonates. Significant proportions are alsofound in the silicates, and the carbonate/monosulfide group.A lesser fraction is found in the residual matter (organic,shielded or resistate minerals), and as an ion-exchangeableform (possibly bound to clays).

d) Copper in the Cur coal is evenly distributed in the silicates,the sulphide and the residual matter (organic, shielded orresistate minerals).

e) Copper in the Cal coal is principally in the carbonates, theresidual matter (organic, shielded or resistate minerals), thesilicates and also in the oxide/carbonate/monosulphidegroup and the sulphide fraction.

f) The distribution of Cu in the Tar sample is similar to that in theCal coal, i.e. principally in the oxide/carbonate/monosulfidegroup, the silicates and in the residual matter (organicallybound, shielded or in resistate minerals).

MercuryIn coal, it seems that much of the Hg is associated with pyrite(Finkelman, 1994, level of confidence 6). Swaine (1990) states thatHg is “probably associated with pyrite and sometimes sphalerite,with organically bound Hg still an uncertainty”. Dale et al. (1999)report that Hg is associated with the pyrite and that there is residualHg present after acid extractions; this could be organically bound, ormore likely Hg associated with finely dispersed pyrite, which isprotected by coaly matter during acid leaching. The occurrence ofHg in US coals has been studied by the US Geological Survey (Tewaltet al., 2001), and the point is made that, because of the element'slow concentration, “it is particularly difficult to determine themodesof mercury occurrence in coal”. It is further stated that researcherssuggest much of Hg in coal is associated with pyrite, although otherforms in coal have been reported including organically bound,elemental, and in sulphide and selenide minerals (see Tewalt et al.,2001).Hower andRobertson (2003) report thepresenceofHg in leadselenide in coal. Yudovich and Ketris (2005b) review the geochem-istry of Hg in coal worldwide and suggest that Hg may be associatedwith the clays (silicates), organic matter and sulphides.The data in Tables 5–10 indicate that, in most of the coals of thepresent study, theHg ispredominantly associatedwithpyrite, but thatother forms, including organically bound (or associated or shielded)and elemental Hg, aswell as sulphides and selenides, are also possible.

ManganeseSwaine (1990) states that Mn in coal is associated with carbonateminerals and clays. Finkelman (1994; confidence level 8) indicatesthat most of the Mn is in carbonates, especially siderite and ankerite.Consistent with this, Dale et al. (1999) found Mn present in theoxide/carbonate/monosulfide group. The data in Tables 5–10 indicatethat, in most of the coals for the present study, the Mn ispredominantly in an HCl-soluble form (oxide/carbonate/monosulfidegroup). However some of the Mn is present in association with otherphases. There appears to be some associated with the silicates andsulphides. Some water-soluble Mn is also present in the BA and GGcoals.

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MolybdenumSwaine (1990) states that “the mode of occurrence of molybdenumin coals ranges from mostly inorganic to mostly organic”. Dale et al.(1999) report that Mo is present in the monosulfides, pyrite andpossibly associated with organic matter. The data for the presentstudy (Tables 5–10) indicate that inmost of the coals theMo is foundin the residual material (organically associated, shielded or presentin resistate minerals).

NickelAccording to Finkelman (1994), there is a lack of any direct evidencefor themodes of occurrence of Ni in coal; itmay be either organicallybound or associated with sulphides (level of confidence 2). Theresults of Dale et al. (1999) indicate that Ni is present in both themonosulfides and the organic matter.The results of chemical fractionation in the present study indicatethat:

a) Nickel in the BA coal is associated with the silicates and alsothe residual matter (organic, shielded or in resistateminerals), with a lesser proportion present in the sulphide(pyrite was not identified by XRD but the mineral may bepresent at low concentrations).

b) Nickel in the War coal is associated with the residual matter(organic, shielded or in resistate minerals), with a lesserproportion associated with the silicates (kaolinite, illite,smectite).

c) Nickel in the GG coal is distributed over a number ofmodes, i.e.water soluble, extractable in the oxide/carbonate/monosulfidegroup, pyrite and the silicates. It is not obvious why the Ni is sowidely distributed. It may be a result of some oxidation.

d) Nickel in the Cur coal is primary associatedwith the residualmatter (organic, shielded or in resistate minerals), withlesser proportions in the oxide/carbonate/monosulfidegroup and the silicates.

e) Nickel in theCal sample is distributed over a number ofmodesi.e. in the residual matter (organic, shielded or in resistateminerals), the silicates, pyrite and also in the oxide/carbonate/monosulfide group.

f) Nickel in the Tar coal is present in the oxide/carbonate/mo-nosulfide group, the silicates and pyrite. Some contaminationhas apparently occurredwith this sample during extraction, asthe sum of the Ni in these modes is greater than the analyticalresult for the “total” Ni concentration.

LeadLead in coal occurs as sulphides (galena) or associatedwith sulphideminerals (Finkelman, 1994; level of confidence 8). The presence oflead selenide has been reported in coals (Hower and Robertson,2003), and Dale et al. (1999) report that some of the Pb is present inthe silicates.As can be seen in Tables 5–10, Pb is predominantly in an HCl-soluble form in all the coals in the present study. This is consistentwith it being present as a monosulfide (e.g. galena). There alsoappears to be some (minor) Pb in the silicates and in the sulphides(e.g. pyrite) in all of the coals. The XAFS spectroscopic data indicatethat Pb is present primarily as lead sulphide in the Cur and Tarcoals, and this is consistent with the Pb being soluble in HCl.

AntimonyAccording to Wedepohl (1969), Sb can probably substitute for Fe inmany minerals. It is possibly found in ilmenite; there are also

numbers of Sb-bearing sulphide minerals e.g. stibnite Sb2S3. It ispossible for Sb to occur as a substitute for iron in sulphides such aspyrite. Swaine (1990) states that “It is not clear how Sb occurs incoals, but it is likely that an organic association prevails in manycoals, together with a sulphide association.” According to Finkelman(1994), Sb may be present in pyrite and as accessory sulphides (e.g.stibnite) dispersed through the organic matter. Finkelman (1994)also states that some Sb “may be organically” bound; the level ofconfidence, however, is low (4 out of a possible 10).It is apparent (Tables 5–10) that Sb is primarily in two forms withinthe coals of the present study, a species soluble in HF (i.e. in thesilicates) and an insoluble form (e.g. organically bound, shielded orin acid resistate minerals such as ilmenite).

ThoriumThe scarcity of Th, its ability to substitute for other elements incrystal lattices, and the absence of a geochemical method ofconcentration of the element all combine to render Th a highlydispersed material (Wedepohl, 1969). It is adsorbed into clays andretained in “heavy resistate” minerals. According to Swaine(1990), Th is unlikely to be organically bound. It is also found inminerals such as monazite and zircon. Finkelman (1995) agreesthat Th is likely to be “associated with monazite with minoramounts in xenotime, zircon, and perhaps some clays”.The presence of Th in “resistate” minerals such as monazite andzircon is a likely explanation for its occurrence in the residual organicmatter of the coals studied (Tables 5–10). However, Th is alsopresent in an HCl-soluble phase (oxide/carbonate/monosulfide),and also appears to be associated with the silicates (e.g. clays).

ThalliumFinkelman (1995) suggests that Tl in coal is most likely associatedwith pyrite (although the level of confidence is low, 4 out of 10).Thallium is distributed across most phases in all the coals coveredby the present study (Tables 5–10). Relatively high residual Tl isfound in the Cur and Tar coals. There is no indication from theresults that Tl is predominantly in the HNO3 extract (associatedwith pyrite) in any of the Australian coals studied.

UraniumAccording to Wedepohl (1969), oxidation of U minerals results inthe formation of carbonates, phosphates, vanadates, silicates andsulphates of U. Swaine (1990, pp 170–171) states that in coal “Umay be in the mineral matter but also organically bound to thecoal. In the latter form, it would presumably be volatile”.In the present series of coals (Tables 5–10), U is present at tracelevels, generally in or associated with oxide/carbonate/monosul-fides and in the silicates (quartz and feldspars); it is also apparentthat a high percentage of the U is present in the residual matter ofall the coals after extraction. It may be present in association withorganic matter as suggested by Swaine (1990). Finkelman (1995)states that “Much of the U in coal appears to be organically bound.However, a substantial proportion of the U in high-rank coals maybe associated with accessory minerals such as zircon and rare-earth phosphates”. It is thus also conceivable that U is present inthe resistate mineral, monazite, in the coals of the present study.It is also apparent from examination of the data in Tables 5–10,that both U and Th often have a similar distribution, i.e. in the HCl-soluble and HF-soluble fractions and also in the residual matter.

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The one exception to this is the distributions in the Cur sample, inwhich there is a significant amount of U in the residual matter butlittle or no Th in the same fraction. It is a matter of conjecture, butthis could indicate that the U is associated with the organic phasein this coal rather than being present in a resistate Th/U mineralphase.If U but not Th was organically bound then it is to be expected thatthe ratio of Th to U would vary significantly in those coals where Uwas “organically bound”.

ZincAlthough there are significant errors in the estimation of residual Zn(often the value is negative, and it is well established in analyticalchemistry that Znmaybe anotorious contaminant), it is obvious thatthe predominant occurrence of Zn in all of the coals is in the HCl-soluble phase. Zinc is most likely to occur as sphalerite, ZnS in coal(Swaine, 1990). The results of Dale et al. (1999) are consistent withthis observation. Finkelman (1994 and 1995) gives a confidencelevel of 8 (out of 10) to the probability of Zn occurring as sphalerite.The chemical fractionation data (Tables 5–10) indicate that Zn inthe coals studied is generally present in a phase soluble in HCl. Thisis consistent with it being present as sphalerite. It is also possiblethat the HCl-soluble Zn is associated with carbonates or oxides oradsorbed on to clays (such as illite). Unfortunately, there isevidence that contamination has occurred in some samples duringextraction; the sums of the amounts extracted are significantlygreater than the totals as analysed directly on the coals (e.g. War,GG and Cur).

The data from XANES spectroscopy indicate that:

a) The spectrum from the Zn in the GG coal is quite unlike thatfrom any coal examined by the authors previously. The Zn inthe GG coal is predominantly present as ZnO, with a lesserfraction in the clays (illite) and possibly as other forms(unidentified). Any Zn as the oxide is soluble in HCl and thechemical fractionation results indicate that the bulk of theZn is HCl-soluble. Zinc adsorbed on to illite would also beextracted with HCl.

b) The Zn in the Cur coal is predominantly present as Zn boundto illite, with possibly some Zn in the silicates or otherspecies. Again, the chemical fractionation indicates thatmost of the Zn is extractable with HCl (as is possible if theZn is in chlorite which is present in this coal).

c) The Zn in the Cal coal is predominantly present as ZnS(sphalerite), with a lesser proportion of Zn bound to illite. It ispossible that Zn as sulphate or silicate may be present. Theseresults are consistent with the Zn being extractable with HCl.

d) The Zn in theTar coal is alsopredominantly ZnS,withpossiblysome bound to illite and present as other unidentified forms.

In the case of Zn, the data from XANES enable identification of Znspecies that are soluble in HCl. It is apparent that it is simplistic toassume that all theZnsoluble inHCl ispredominantlyZnS(sphalerite).

5. Concluding remarks

It is possible to criticise most (if not all) results obtained fromstudies of speciation or modes of occurrence of trace elements in coalbased on selective extraction techniques. It may be simply that someof the extraction or separation techniques used do not obtain a “pure”extract of the species of interest. Some of the physical separationtechniques may depend on particle size, i.e. some species may be

shielded by organic matter; this also may be a factor during chemicalseparation. As well, species may change during chemical extraction(e.g. precipitate or oxidise).

In this study, the chemical extraction techniques were used to obtaindata on speciation/occurrence of trace elements (often at very lowconcentrations) in Australian coal samples. However, such data, ifgenerated with care and interpreted with the same care, do provide avery good indication of the speciation/occurrence. There is obviously onesignificant limitation in the technique used, and that is the residualconcentration of a trace element being used to indicate that the traceelement is organically associated. Such an occurrence could equally beinterpreted as being indicative of an association with residual mineralmatter (either shielded by organicmatter or in an acid resistantmineral)or simply an analytical error. The certainty of anydesignation is obviouslyincreased by the use of sophisticated techniques such as XANES/XAFS,although even these techniques may lack the required sensitivity(detection limits) or may not be applicable to some trace elements.

Nevertheless, this compilation of speciation data is the mostcomprehensive ever reported on a suite of Australian coals. These dataprovide information on themodes of occurrence of a range of elements.Suchdataprovide informationon the likely residences of these elementswithin the coals and, apart from being of interest to geologists who arestudying the occurrence of these elements in coals, the data may be ofuse in providing information of the behaviour and impact of theseelements when the sampled coals are utilised.

Acknowledgements

The authors wish to acknowledge the comprehensive workcompleted by the two anonymous reviewers and to Colin Ward andShifeng Dai, and also the financial support provided by the CooperativeResearchCentre for Coal in SustainableDevelopment,whichwas fundedin part by the Cooperative Research Centres Program of the Common-wealth Government of Australia. The XAFS/XANES investigations wereperformed at the National Synchrotron Light Source, BrookhavenNational Laboratory, NY, which is supported by the U.S. Department ofEnergy, Office of Science, Office of Basic Energy Sciences, under ContractNo. DE-AC02-98CH10886.

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