This study has been undertaken to understand the precise nature of association of lead in high

sulphur Assam coal. Sequential extraction of lead with ammonium acetate, HCl and RuCl 3 solutions

havebeen carriedout. RuCl3 hasbeenusedasasuitable reagent fortheextractionoflead.Itisfound

thattreatment ofsamplewithCCl 4increasestheextractionofleadbyRuCl 3solution. Theamountof

leadinthecoalstudiedvariesbetween43.2and50.4ppm.Ithasbeenfoundthatleadispredominantly

occurring in coals in association with organic matter and needs concerted attention to understand its

bio-availability. The chemical principles involved therein are also discussed.

D 2004 Elsevier B.V. All rights reserved

BARUAH , J

BARUAH , M K

BORA , G C

KOTOKY , P

IR@NEIST - North East Institute of Science and Technology (CSIR)

Fuel Processing Technology 86 (2005) 731–734www.elsevier.com/locate/fuprocShort communicationExtent of lead in high sulphur Assam coalsMrinal K. Baruaha, Probhat Kotokyb,*, Jyotish Baruahb,Gobin C. BorabaDepartment of Chemistry, NNS College, Titabar-785 630, Assam, IndiabGeoscience Division, Regional Research Laboratory, Jorhat-785 006, Assam, IndiaReceived 31 March 2002; accepted 31 May 2004AbstractThis study has been undertaken to understand the precise nature of association of lead in highsulphur Assam coal. Sequential extraction of lead with ammonium acetate, HCl and RuCl3 solutionshave been carried out. RuCl3 has been used as a suitable reagent for the extraction of lead. It is foundthat treatment of sample with CCl4 increases the extraction of lead by RuCl3 solution. The amount oflead in the coal studied varies between 43.2 and 50.4ppm. It has been found that lead is predominantlyoccurring in coals in association with organic matter and needs concerted attention to understand itsbio-availability. The chemical principles involved therein are also discussed.D 2004 Elsevier B.V. All rights reserved.Keywords: Lead; Assam coals; Hazardous elementsEnvironmental awareness has focused studies on the sources of hazardous elements innature and their source reduction. Combustion of coals in various industrial processesrelease ash, which contains various metals, especially the toxic metals, and has been agreat concern for a long time. Lead is one of such metals, which causes health hazards to aconsiderable extent. In view of the importance of the metal, some works need to be carriedout on the mineral matter of the coal.The sources of lead have a profound influence on the association of the metal in coal.Seawater contains very low concentration (0.002–0.03 ppb) of lead [1] and, therefore, this0378-3820/$ -doi:10.1016/j.* CorrespE-mail addsee front matter D 2004 Elsevier B.V. All rights reserved.fuproc.2004.05.015onding author. Tel.: +91 376 320353; fax: +91 376 3370011.ress: probhatk@yahoo.com (P. Kotoky).M.K. Baruah et al. / Fuel Processing Technology 86 (2005) 731–734732source cannot supply high amount of lead occurring in coal. The occurrence of 8–20 ppmof lead in peat-forming environments [2] has led us to believe that surface water [3] (and toa certain extent, ground water) is the main source of lead to the early stages of coalformation, i.e., peat-bog. Weathering of rocks, oxidation of lead minerals [1], etc., are theplausible ways by which the metal can be mobilised. Therefore, it is important to knowhow the metal is associated with the coal mass.Lead, being an element belonging to the third transition series of the periodic table, mayact as good electrophile for reactions with numerous nucleophiles and can even formmultinuclear compounds. The organic matters are known to have many unfilled sites andthese can interact with lead at the peat-bog stage, resulting in the formation of ion-exchangeable, non-exchangeable and chelate compounds. In some cases, they mightundergo redox reactions with the organic matter. On the other hand, the common valencestate of lead (divalent lead) is a borderline acid in Pearson’s hard–soft acids and bases, and,therefore, it has a tendency to react with either hard or soft bases. In reducingenvironments, soft bases such as sulphur containing ligands could form covalent bondswith lead forming lead sulphides or the metal could be associated with sulphide minerals.The occurrence of galena (PbS) in most coals [4] and the association of lead in pyrite [5,6]further supports our contention. Moreover, formation of organic bound Pb–S compoundsis also not unlikely. As no work seems to have been carried out on the nature of associationof lead in high sulphur Assam coals, the following series of experiments have beenundertaken.Three high sulphur coal samples obtained from Tirap, Tikak and Baragolai collieries ofthe Makum coal field (27816V–27818VN and 95843V–95855VE), Assam (India) were taken forthe study. The samples were ground to size 72 BS. The carbon and sulphur contents of thesamples are given in Table 1.The method used for the extraction of gold from coal [7] is used here for the leaching oflead. Lead was sequentially extracted from 0.5 g coal by 20 ml 1 M ammonium acetatesolution at 25 8C for 1 h followed by 20 ml 1 M HCl with constant stirring. The residuewas treated with CCl4 to remove minerals by gravity separation. However, the extent ofremoval of mineral matter by such treatment from coal is not precisely known, but thesample obtained after such treatment is considered here as the mineral free sample for ourstudy. The residue was further treated with 20 ml 0.01 M RuCl3 solution at 80 8C for 1 h toextract the metal. The residue after RuCl3 treatment was made to ash by heating at 750 8CTable 1Elemental composition and chemical fractionation of lead in Assam coalsNo. CoalsampleCarbona(wt.%)Sulphura(wt.%)Ammoniumacetate extract(ppm)HClextract(ppm)RuCl3 extract of Ash after RuCl3extract ofMineral freesample (ppm)Mineral freesample (ppm)1 Tirap coal 75.5 7.7 ND ND 50.4 ND2 Tikak coal 72.7 5.2 ND ND 43.4 ND3 Baragolai coal 73.3 6.4 ND ND 43.2 NDND=not detected (b1 Ag/g).a As received basis.M.K. Baruah et al. / Fuel Processing Technology 86 (2005) 731–734 733for 1 h. The ash was dissolved in a mixture containing HNO3, HF and HClO4 taken in aTeflon bomb and heated at 120 8C for 1 h. Finally, the mixture was treated with H3BO3solution. Lead in each of the fractions was determined repetitively by using a Perkin-Elmer(Model: 2380) Atomic Absorption Spectrophotometer with detection limit 1 Ag/g and theaverage values of replicate analysis are used for the present study.The chemical fractionation procedure involves leaching of ion-exchangeable cations by1 M ammonium acetate solution and the same sample on subsequent extraction with 1 MHCl removes coordinated species (non-exchangeable) [8,9] and acid soluble carbonatesand oxides [8]. Table 1 shows the amount of lead extracted by different reagents. Onleaching with ammonium acetate solution, the metal was not detected signifying that leadis not present in all the coals in ion-exchangeable form. This may be ascribed to the factthat very high concentration and also preferential uptake of other metals might haveoccupied the exchangeable positions in the organic matter and as a result lead was unableto form exchangeable species. Moreover, extraction with HCl could not show the presenceof metal (Table 1). This reveals that HCl was unable to leach the metal associated witheither mineral or organic matter.In order to get a precise information on the nature of association of the lead, the mineralfree sample was treated with RuCl3 solution where a higher amount of lead was observedto be leached out in all the coals. It is apparent that the lead is predominantly associatedwith organic matter as non-exchangeable or chelate form. However, at this stage, it is notclear about the extent of extraction of lead by RuCl3 solution from the residue.The RuCl3 treated samples were made to ash; however, presence of lead was notobserved in them. Absence of the lead in ash suggests that a certain amount of metal mighthave volatilised away at the temperature at which ash was made. A question still remainsas to which form of the metal, mineral or organic bound, might have removed duringashing. As the mineral bound lead is converted to oxides during ashing, these cannot bevolatilised away easily. Therefore, occurrence of organic bound lead is most likely in coalsunder study.The formation of polynuclear complex may be significant for the divalent cations of thetransition metals in approximately neutral solution [10]. At pHs well below 7, divalentlead simply exists as Pbaq2+ while in alkaline solution it is precipitated as the sparinglysoluble hydroxide, Pb(OH)2, a number of polynuclear hydroxo lead cations have beenformed [10]. In the change of acidic to neutral pH, Pb is transferred sequentially as:[Pb(OH)2]+, [Pb2(OH)]3+, [Pb3(OH)4]2+, [Pb4(OH)4]4+ and [Pb6(OH)8]4+. In alkalinemedium precipitation of Pb(OH)2 occurs and on further increase of pH, plumbites areformed.As the peat stage has near neutral pH, it seems likely that there will be considerableconcentration of polynuclear hydroxo cations of lead. The cations, being very reactive[11], can immediately react with organic matter resulting in the formation of very stablecomplex compounds. This is due to the fact that the polynuclear hydroxo lead cationsexists as hydroxo bridged species (which occurs as gel) and when these species interactwith the organic mater (occurs as colloid) a complicated structure of non-exchangeablelead should be formed because of colloid–colloid interaction. This complicated structurehad been transformed into still more complex networks during the coalification processcontrolled by temperature and pressure. On treatment with either ammonium acetateM.K. Baruah et al. / Fuel Processing Technology 86 (2005) 731–734734solution or dilute HCl, leaching of lead is not possible and this could be the reason why wedid not get lead in the extracts with these reagents. On the other hand, on treatment withRuCl3 solution, Ru3+ ion, being an oxidant, can oxidise divalent lead to higher oxidationstate and as a result, the structure of the polynuclear hydroxo-organic matter network issusceptible to break due to involvement of electron transfer process. Again, the RuCl3solution is acidic, therefore, leaching of lead by this reagent is quite possible. This couldbe the reason for the extraction of lead by RuCl3 solution.The Clarke value [12] of distribution of elements in the earth crust for lead is 12.5 ppm.Lead level for Tirap coal is four times higher, whereas its level is three times higher inTikak and Baragolai coals than the Clarke value. As the occurrence of galena in Assamcoals has not been known to us, the presence of lead in minerals is very negligible. Wemay conclude that (a) lead is predominantly occurring in high sulphur Assam coals,studied here, in association with organic matter, and (b) RuCl3 is a suitable reagent for theextraction of lead. If the major fraction of lead is associated with organic matter contents asobserved from the present study, the bioavailability of metals in an environment underchanging pH and Eh conditions need concerted attention. However, Fe–Mn oxide andorganic matter fractions sometimes shows scavenging effect and act as sink for retention oftoxic heavy metals in natural environment.AcknowledgementsOne of the authors (MKB) is grateful to the Dept. of Science and Technology, Govt. ofIndia, for offering the SERC Visiting Fellowship. The Director of Regional ResearchLaboratory (CSIR)-Jorhat is greatly appreciated for allowing us to publish the work. Thecoal samples were obtained from the Coal Survey Laboratory (CFRI), Jorhat and isgratefully acknowledged.References[1] K.H. Wedepohl (Ed.), Handbook of Geochemistry, Springer-Verlag, Heidelberg, 1974, pp. 82-C-0.[2] D.J. Casagrande, L.D. Erchull, Geochim. Cosmochim. Acta 40 (1976) 387–393.[3] F.W. Chandler, Atl. Geol. 34 (1998) 39–56.[4] M.Th. Mackowsky, M. Teichmuller, G.H. Taylor, D. Chandra, R. Teichmuller (Eds.), Stach’s Textbook ofCoal Petrology, Gebrder Berntreager, Berlin, 1982, pp. 153–170.[5] R.N. White, J.V. Smith, D.A. Spears, M.L. Rivers, S.R. Sutton, Fuel 68 (1989) 1480.[6] D.A. Spears, M.R.T. Martinez, S. Lee, Fuel 73 (1994) 1051.[7] M.K. Baruah, P. Kotoky, G.C. Borah, Fuel 77 (1998) 1867.[8] S.A. Benson, P.L. Holm, ACS Div. Fuel Chem. Preprints 28 (1983) 234.[9] R.N. Miller, P.H. Given, Geochim. Cosmochim. Acta 50 (1986) 2033–2043.[10] J. Burgess, Ions in Solutions, Ellis Horwood, Sussex, 1988.[11] C.E. Marshell, The Physical Chemistry and Mineralogy of Soils, John Wiley and Sons, NewYork, 1964.[12] S.R. Taylor, Geochim. Cosmochim. Acta 28 (1964) 1273–1285.

EXTENT OF LEAD IN HIGH-SULFUR ASSAM COALS

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EXTENT OF LEAD IN HIGH-SULFUR ASSAM COALS

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