Methods for determination of coal carbon in reclaimed minesoils: A review

Author's personal copy

Review

Methods for determination of coal carbon in reclaimed minesoils: A review

David A.N. Ussiri a,⁎, Pierre-Andre Jacinthe b, Rattan Lal a

a Carbon Management and Sequestration Center, School of Environment and Natural Resources, The Ohio State University, 2021 Coffey Rd, Columbus, OH 43210, USAb Department of Earth Sciences, Indiana University Purdue University, 723 West Michigan Street, Indianapolis, IN 46202, USA

a b s t r a c ta r t i c l e i n f o

Article history:Received 28 January 2013Received in revised form 3 September 2013Accepted 16 September 2013Available online 11 October 2013

Keywords:Reclaimed minesoilsGeogenic organic carbonCarbon sequestrationRecent organic carbonRadiocarbon isotopesCoal carbon contamination

Organic carbon (OC) of the minesoils reclaimed from coal mining often contains carbon (C) associated with coalparticles frommining and the reclamation activities. This C, collectively referred to as geogenic OC (formed as aproduct of geological processes),must be quantified in order to accurately determine the pools and sequestrationpotential of OC that originates from recent vegetation input. Reclaimed mined lands can provide significant sinkfor atmospheric carbon dioxide (CO2) through C assimilation in vegetation biomass, formation and accumulationof SOM. However, the validity of the reported C sequestration potentials inminesoils reclaimed from coalminingactivities is questionable due to inability to quantitatively determine the different C sources that may be presentin these soils. Due to its high C content, coal particles present in these soils may lead to overestimation of poolsand sequestration rates, and can also represent a large C background against which small changes in recent OCmust be measured. This is a methodological challenge which must be overcome in reclaimed minesoils (RMS).Standard procedures for quantifying soil organic C (SOC) cannot distinguish geogenic C and recent OC fromplant biomass. Appropriate soil C analysis in RMSmust differentiate between inorganic C (carbonates), geogenicOC and recent OC from decomposition of plant biomass. Therefore, the purpose of this review is to collate andsynthesize the available information on the existing techniques for separating geogenic and recent OC, and quan-tifying these OC fractions in RMS. Methods for quantifying geogenic OC in RMS have been grouped into micro-scopic, thermal, chemical, spectroscopic, isotopic, and combination of some of these methods. The majorlimitation of thermal and chemical methods is the overlap in sensitivity between some types of coal and recentOM fractions. Most of the spectroscopic techniques are semi-quantitative, and generally yield less accurate esti-mates of geogenic OC. Radiocarbon analysis is one of the most reliable methods for quantifying geogenic OC inRMS. However, the need for specialized instrumentation, advanced computational skills, and high analyticalcosts precludes its adoption for routine soil analysis. Additional research is needed to further evaluate the existingtechniques, develop some reliable and cost-effective methods, and ultimately propose standard geogenic OCquantification methods that can be widely adopted.

© 2013 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1562. Nature and properties of coal and its effects in organic carbon quantification in reclaimed mined soils . . . . . . . . . . . . . . . . . . . . . 157

2.1. Nature and properties of coal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1572.1.1. Coal formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1572.1.2. Chemical and physical properties of coal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157

2.2. Nature and properties of reclaimed minesoils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1572.2.1. Coal mining techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1572.2.2. Mined land reclamation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1572.2.3. Properties of minesoils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158

Geoderma 214–215 (2014) 155–167

Abbreviations: SOC, soil organic carbon; OC, organic carbon; RMS, reclaimed minesoils; OM, organic matter; SOM, soil organic matter; TOC, total organic carbon; TC, total carbon; IC,inorganic carbon; BC, black carbon; EC, elemental carbon; TG, thermogravimetric; DTG, derivative thermogravimetric; CPMAS NMR, cross polarizationmagic angle spin nuclear magneticresonance.⁎ Corresponding author at: Illinois State Geological Survey, Prairie Research Institute, University of Illinois at Urbana-Champaign, 615 E Peabody Drive, Champaign, IL 61820,

USA. Tel.: +1 217 265 6425.E-mail address: [email protected] (D.A.N. Ussiri).

0016-7061/$ – see front matter © 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.geoderma.2013.09.015

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3. Analytical methods for quantifying carbon of different sources in reclaimed mined lands . . . . . . . . . . . . . . . . . . . . . . . . . . . 1583.1. Inorganic carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1583.2. Geogenic and recent soil organic carbon separation and quantification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158

3.2.1. Optical and microscopic methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1593.2.2. Thermal oxidation methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1603.2.3. Chemical methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1603.2.4. Spectroscopic characterization methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1623.2.5. Isotopic methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162

4. Geogenic organic carbon distribution in reclaimed minesoils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1645. Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1656. Research needs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165

1. Introduction

Minesoil are soils formed on landscapes altered by surface miningactivities. Sometimes referred to as spoils or anthropogenic soils(Sencindiver and Ammons, 2000), these drastically disturbed soilsexhibit profile characteristics, physical, chemical, and biological con-ditions that reflect anthropogenic perturbations rather than naturalsoil forming processes (McSweetney and Jansen, 1984). Minesoilsare often characterized by heterogeneous mixtures of rock frag-ments and sediment materials. Compared to native soils, the volumeof rock fragments in minesoils can be as high as 67% of the total soilvolume (Ashby et al., 1984; Ciolkosz et al., 1985; Thurman andSencindiver, 1986).

In addition to recent organic carbon (OC) from humification of plantresidues, minesoils formed from reclamation of surface mining of coaloften contain varying quantities of coal particles distributed throughoutthe soil profile depending on mining and reclamation techniques(Insam and Domsch, 1988; Roberts et al., 1988; Rumpel et al.,1998a,b; Schafer et al., 1980; Schmidt et al., 1996; Stroo and Jencks,1982; Ussiri and Lal, 2008a). The organic carbon (OC) of coal parti-cles is termed geogenic OC (i.e., OC which has been subjected to geo-logical processes). Microbial decomposition of recent soil organicmatter (SOM) from plant litter and other pedogenic processes leadto mixing of recently formed SOM with coal particles. For minesoilsdeveloped on calcareous parent materials (siltstone, sandstone, andlimestone), inorganic C may also occur in significant quantities.Therefore, total carbon (TC) in reclaimed minesoils (RMS) can be amixture of: (1) inorganic carbon (IC) originating from parent mate-rial, (2) geogenic OC from coal particles incorporated during miningand reclamation operations, and (3) plant-derived recent soil organiccarbon (recent OC). Because of the dark color of humus and its intimatemixing with fine coal particles, these two sources of carbonaceous ma-terials cannot be distinguished by visual means.

RMS can be a significant sink for atmospheric carbon dioxide (CO2)through CO2 assimilation into aboveground biomass and accumulationof recent SOM. Soil organic carbon (SOC) sequestration rates rangingbetween 0.45 and 3.0 Mg C ha−1 yr−1 during the first two decadesfollowing reclamation have been reported (Akala and Lal, 2001;Shukla et al., 2004; also see reviews by Shrestha and Lal, 2006;Shrestha et al., 2009; Sperow, 2006; Ussiri and Lal, 2005). Other re-ports have shown that SOC storage in RMS sometimes exceeds thatof the adjacent undisturbed soils (Fettweis et al., 2005; Shukla et al.,2005; Ussiri et al., 2006a). However, the accuracy of SOC pools and se-questration rates which have been reported is questionable due to fail-ure, inmany studies, to account for the contribution of coal C to the totalSOC pools in RMS. Standard SOC determination methods such as drycombustion do not distinguish between recent and geogenic OC, andmay lead to overestimation of SOC pools and sequestration rates inRMS. Quantifying recent SOC accrued from biochemical processing ofplant materials (litter, dead roots, and root exudates) is crucial to un-derstanding SOC dynamics, and accurate determination of SOC pools

and sequestration rates in RMS. Therefore, development of reliable an-alytical techniques capable of overcoming these challenges is urgentlyneeded.

In this review, geogenic OC collectively represents lithifiedplant debris at different degrees of coalification formed by biologi-cal and geological processes over millions of years. It differs from“elemental C” (also called black carbon (BC) in atmospheric stud-ies, Andreae and Gelencser, 2006) in that BC results from incom-plete combustion (i.e., pyrogenic processes) of biomass and fossilfuel (char) and its condensates (soot) (Elmquist et al., 2006). Al-though geogenic OC and BC have some chemical and structuralsimilarities (Currie et al., 2002; Gustafsson et al., 2001) and alsocan co-occur in soil and sediment samples (Goldberg, 1985), it isimportant to stress their differences based on their origin. A compre-hensive review on the geochemistry of BC and measuring techniqueswas published (Masiello, 2004), and an inter-laboratory comparisonof methods using reference materials has been conducted recently(Hammes et al., 2007). Therefore, methods exclusive to BC quantifica-tion will not be covered in the present review.

Geogenic OC occurs in soils at specific sites associated with coalmining and utilization activities (Rumpel et al., 1998b; Ussiri andLal, 2008a), including deposition of airborne coal particles near coalprocessing and coal fired power plants (Schmidt et al., 2000), andsoils ameliorated with coal combustion ash (Schmidt et al., 1999).Coal is formed from peat by diagenetic processes in geologic timescale. It is a continuum representing different chemical structuresand reactivity ranging from lignite to anthracite and graphite. Simi-lar to pyrogenic BC, there are no generally accepted and standardizedmethods for quantifying geogenic OC in the environment.

Research effort to quantify the contribution of geogenic C to theSOC in RMS remains limited (Jacinthe et al., 2009; Maharaj et al.,2007a; Rumpel et al., 1998a,b; Ussiri and Lal, 2008a). During thelast few decades, several methods have been proposed for separatingand quantifying geogenic OC content in soils and sediments. Theseinclude methods based on microscopic, spectroscopic, and isotopicanalyses of samples, and methodologies that rely on the resistancebetween coal and recent OC to thermal and chemical oxidation todistinguish geogenic from recent OC and quantify geogenic OC(Jacinthe et al., 2009; Maharaj et al., 2007a; Rumpel et al., 1998b,2001; Ussiri and Lal, 2008a). The objectives of this review are to col-late, summarize and synthesize available information on methodsfor quantifying coal C, its distribution in RMS, and identify researchpriorities for developing and standardizing coal C quantificationtechniques in RMS. First, we will briefly describe general propertiesof coal and its geology to distinguish it from pyrogenic BC and recentOC that evolved from decomposition of recent vegetation biomass.We will also briefly describe mining and land reclamation tech-niques in order to provide the proper context to understand the for-mation and unique properties of RMS, including contamination bycoal particles. The bulk of the review will focus on available tech-niques for geogenic C quantification. The last section will outline

156 D.A.N. Ussiri et al. / Geoderma 214–215 (2014) 155–167


research needed to develop standardized techniques for geogenic Cdetermination in RMS. Building fromour initial evaluation ofmethodol-ogies for determination of geogenic C in RMS of the Eastern UnitedStates (Jacinthe et al., 2009), the present review ismore comprehensive,both in terms of its scope and content, and covers awider range of tech-niques applicable to coal of all ranks.

2. Nature and properties of coal and its effects in organic carbonquantification in reclaimed mined soils

2.1. Nature and properties of coal

2.1.1. Coal formationCoal is a combustible sedimentary rock formed by lithification of

plant debris, and is composed primarily of C and variable quantitiesof other elements. It is a complex mixture of plant remains that accu-mulated in water-logged environments and transformed bymicrobi-al and other diagenetic processes over geological time scale (Wardand Suárez-Ruiz, 2008). Increased heat and pressure at greaterdepths of burial, tectonic action, and prolonged time of burial causedphysical and chemical changes known as coalification and result inthe formation of peat, and then coals of variable ranks (Hatcher andClifford, 1997;Ward, 2003;Ward and Suárez-Ruiz, 2008). During anox-ic decomposition processes, less complex molecules, such as proteinsand cellulose are preferentially degraded while lignin and other aro-matic moieties are preserved (Schmidt et al., 2000; Stout et al., 1988).Many plant biopolymers survive coalification relatively unaltered, andothers are only partially altered. The impact of heat and pressure alsovaries, yielding smaller molecules in some cases and more complexmacromolecules in other instances (Hatcher and Clifford, 1997). Al-though peat and coal can be viewed as macromolecular structures(Hatcher and Clifford, 1997; Mukhopadhyay and Hatcher, 1993),their chemical composition is widely variable. This is a reflection ofthe large diversity of vegetation (both vascular and nonvascular)contributing plant remains to coal-formation, the variable degree ofdegradation, and different physical and chemical processes influencingcoalification in various depositional environments.

2.1.2. Chemical and physical properties of coalVariations in geological settings, type of peat-forming plants,

paleo-climate and paleo-hydrological regime are major external fac-tors influencing coal formation (Suggate and Dickinson, 2004). In ad-dition, pressure and temperature play a significant role in modifyingthe depositional environment and the chemical structure of depositedbiopolymers, leading to formation of coals of various ranks (Bates andHatcher, 1989; Crowell, 2002; Hatcher and Clifford, 1997; Wilsonet al., 1987). Pressure affects compactness, porosity and moisture con-tent of the coal (van Krevelen, 1993). The length of time in formation,commonly referred to as ‘organic maturity’ is the additional factor de-termining coal quality.

Coal properties can be expressed using three independent pa-rameters namely rank, type, and grade. Rank reflects the degree ofchange undergone by coal as it matures from peat to anthracite. Itdetermines the physical and chemical properties of coal. Coals oflower ranks are strongly hydrophilic and have high intrinsic moisturecontent. Type refers to the nature of plant debris fromwhich the originalpeat was derived. Grade is related to the extent to which the accumula-tion of plant debris has remained free of contamination by inorganicminerals during the coalification process (Ward, 2003). Coalification ismarked by progressive decrease in moisture and volatile functionalgroups, with consequent increase in C content (Ward and Suárez-Ruiz,2008). As the rank increases, coal becomes more hydrophobic probablydue to gradual elimination of polar groups (i.e., OH− and COOH−) duringcoalification (van Krevelen, 1993). However, there exists a signifi-cant variation in properties among coals of the same rank, broughtabout by differences in the assemblage of plant constituents that

formed the original peat-forming vegetation and other factors suchas geological settings, age and coalification history (Given, 1984)noted above. The aromaticity of coal increases with increase inrank (Smith et al., 1993).

Themain chemical elements in coal are C, hydrogen (H), oxygen (O),nitrogen (N), sulfur (S), and various trace elements. Depending on itsorigin and rank, C concentration can be more than 50% by weight andover 70% by volume (Crowell, 2002). Both N and S are generally minorcomponents, usually occurring in concentrations b2% by weight (Glickand Davis, 1991). Coal also contains a wide range of other inorganicmatter and elements, including metals in trace amounts. Many ofthese were probably inherited from the original plant tissues, fixedby chemical processes (carboxylation, metalation, and chelation) ortrapped during peat accumulation (Filby and van Berkel, 1987;Given and Miller, 1987; Miller and Given, 1987).

2.2. Nature and properties of reclaimed minesoils

2.2.1. Coal mining techniquesCoal is mined by either surface or undergroundmethods depending

on the geology of the coal deposit. In addition to coal seam location ingeologic formation, mining method is also dictated by other factors,including uniformity and pitch of the seam, nature, and thicknessof the overburden above the coal seam, as well as geomorphologyof land surface. Surface mining, also known as opencast mining, isthe method of choice when (a) the coal seam is relatively shallow(40 to 50 m), (b) the overburden consists of unconsolidated material,or (c) the geomorphology allows easy removal of the earth and rockstrata (overburden) (Hansen, 1990).

The first step in surface mining involves the removal of vegetation,scraping, and stockpiling of topsoil for post-mining land reclamation.Then, using explosives, the overlying overburden of subsoil and rock isbroken up into a heterogeneousmass of freshly blasted rocks and un-consolidated materials known as “mine spoils”. The spoil is removedin order to expose the coal seamwhich is then drilled into, fractured,and systematically mined in strips. Surface coal mining, therefore,causes drastic land disturbance and degradation, and generateslarge volume of mine spoils. For most sites, complete restorationcannot be achieved through natural recolonization and secondarysuccession, and therefore, necessitates managerial reclamation. Inrecognition of that necessity, the surface mining control and recla-mation act of 1977 (SMCRA) – which mandates the restoration ofstrip mined areas to conditions similar to pre-mining state – wasadopted in the USA. Therefore, after coal removal, the mining pit isbackfilled with spoil, the land surface is graded, and the stockpiledtopsoil is spread on the surface prior to establishing a vegetationcover. Application of lime and chemical fertilizers is sometimes madeto correct soil acidity and address nutrient deficiencies, respectively.

2.2.2. Mined land reclamationReclamation of mined soils is the process of returning the land to

a useful state. It involves restoration of land topography to its origi-nal contour by grading the overburden materials, followed by recon-struction of suitable rooting media for vegetation establishment(Barnhisel and Hower, 1997). Reclaimed and/or re-vegetated minespoils are herein referred to as reclaimed minesoils (RMS) whetheror not topsoil application was made during reclamation phase. RMSare manmade or anthropic soils (Lal et al., 2004), and are often lessweathered than pre-mined soils. Depending on mineral composi-tion, the weathering of mine spoils can be rapid, and lead to the de-velopment of soils with properties that differ greatly from theoriginal undisturbed soil (Sencindiver and Ammons, 2000). Surfacemining causes drastic perturbations of the original soil profile, andthe level of disturbance generally exceeds the natural resilience ofsoils, leading to severe landscape degradation. In addition, severe lossof SOM leads to decline in soil quality and functions (Lal, 1997). The

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SOM losses can be attributed to lack of inputs from plant litter, mechan-ical mixing of A, B, and C horizons during removal and handling of over-burden material, soil erosion, leaching and accelerated decompositionof SOM in stockpiled topsoil. Themajor purpose of the reclamation pro-cess is to establish a stable landscape that is less prone to erosion andcould support an adequate vegetation cover.

2.2.3. Properties of minesoilsRMS are often characterized by low levels of key nutrients, unfavor-

able pH, poor soil structure, high bulk density (1.55–1.86 Mg m−3),high rock fragments (33–45%), lowwater holding capacity, and lowbio-mass productivity (Haering et al., 2000; Indorante and Jansen, 1981;Shrestha and Lal, 2006). However, available data suggest that fully re-stored minesoils have high potential for C sequestration compared toother terrestrial soils, especially during the first two decades followingrestoration (Akala and Lal, 2001; Sperow, 2006; Ussiri and Lal, 2005;Ussiri et al., 2006b). Carbon sequestration in minesoils refers to a grad-ual accretion of SOC pools resulting from decomposition of vegetationbiomass. Proper reclamation and post-reclamation management mayfurther enhance C sequestration and increase the economic value ofRMS (Shrestha et al., 2009). However, there is a great uncertainty re-garding pools and rates of C sequestration reported from RMS. This un-certainty stems largely from the presence of coal particles in RMS andfailure in some studies to correct for coal C contribution to SOC inRMS. Because of its high content of C and light weight, coal particleseven in small amount can introduce large errors in measurement of re-cent SOC pools in RMS. Carbon-rich coal particles can form a large OCbackground, against which small increments of recent OC must bemeasured. It is difficult to detect small increments in recent OC accu-mulation and assess accurate C sequestration rates against a largebackground of OC introduced by coal particles. Analytical techniquesare therefore, needed to address this challenge.

3. Analytical methods for quantifying carbon of different sources inreclaimed mined lands

Forms of the total C pool in RMS include: (a) inorganic C (IC) orcarbonates present as a part of mineralogical composition of soilsand overburden, (b) recent SOC resulting from decomposition ofplant materials into humic substances, and (c) geogenic OC (or coalC) which is of geological origin introduced during coal mining andreclamation activities.

3.1. Inorganic carbon

The inorganic C (IC) pool in soils includes predominantly carbonateminerals — calcite (CaCO3) and dolomite (CaMg(CO3)2). Calcite is usu-ally the dominant form in active pedogenic environments (Doner andLynn, 1977). These minerals usually control the activities of Ca2+ andMg2+ in soil solution, and occur in neutral to alkaline soils. However,solid phase carbonates in the form of nodules are also known to existin some acid soil environments (Loeppert and Suarez, 1996).

The IC concentration is usually quantified by acid dissolution (Eqs.(1), (2)):

CaCO3 þ 2Hþ→Ca

2þ þ CO2 þ H2O ð1Þ

CaMgðCO3Þ2 þ 4Hþ→Ca

2þ þMg2þ þ 2CO2 þ 2H2O: ð2Þ

The CO2 produced can be quantified by gravimetric (Allison andMoodie, 1965), titrimetric (Bundy, 1972), manometric (Presley, 1975)or gas chromatographic method (Ussiri et al., 2006b). The presence ofMnO2 in soils can interfere with the acid dissolution procedure due toits influence on SOM oxidation (Allison and Moodie, 1965), therebyoverestimating the IC concentration. Addition of FeCl2 or FeSO4 to the

acid is recommended to minimize SOM decomposition during theacid treatment (Loeppert and Suarez, 1996).

An alternative IC quantification method involves determination oftotal C of soil samples by dry combustion using elemental CN analyzerbefore and after acid dissolution of carbonates (Horvath et al., 2005;Midwood and Boutton, 1998). Concentration of IC is then determinedby difference.

3.2. Geogenic and recent soil organic carbon separation and quantification

The SOC content occurring in coal mining landscapes is a mixtureof geogenic OC and recent OC. Although some overlaps exist in thechemistry of recent SOM and coal, there are dissimilarities that canbe exploited to distinguish them and quantify the contribution ofeach source to total OC pool of RMS. These differences include chem-ical structure, functional group composition, chemical reactivity, andheat resistance (Rumpel et al., 2001). For example, recent OM ismainly composed of reactive and oxidizable compounds such as carbo-hydrates, protein, lignin and lipids (Rumpel et al., 2000). In contrast,coal exhibits highly aromatic and condensed structure as a result ofdecomposition and polymerization during the coalification process.Changes in structure during coalification involve a loss of proteinsand carbohydrate which are easily destroyed (Wilson et al., 1987).Although microbial decomposition of geogenic C in minesoils hasbeen reported (Cohen and Gabriele, 1982; Rumpel and Kogel-Knabner,2004; Waschkies and Huttl, 1999), coal is considered highly resistant tochemical and biological degradation (Rumpel et al., 2000; Schmidt et al.,2000).Microbial degradation of coal is also limited due to the inaccessibil-ity to microorganisms of the small pores in the coal matrix (Cohen andGabriele, 1982). Willmann and Fakoussa (1997) noted that microbialdegradation of brown coal in soils depends on the presence of an eas-ily available source of OC. Based on the functional group compositionand reactivity of coal, several methods originally developed for quan-tifying BC in soils and sediments have been adopted for the determina-tion of geogenic OC in RMS. A summary of analytical methods forgeogenic OC quantification are presented in Table 1.

Geogenic OC quantification techniques in soil can be grouped into 6general categories, including: (i) optical and microscopic, (ii) thermal,(iii) chemical, (iv) spectroscopic, (v) molecular markers, and (vi) isoto-pic. Optical and microscopic methods rely on coal optical propertiessuch as blackness (Ghosh et al., 2000; Karapanagioti et al., 2001;Wik and Renberg, 1996). Thermal oxidation methods are based onthe ability to oxidize recent OC under specific heating conditions(temperature, oxygen), and then measure the remaining C after theheat treatment (Gelinas et al., 2001; Schmidt et al., 2001). Chemicaloxidation involves treatment of the samples with chemical reagentsto remove the recent OC fractions and measuring the remaining C afterthe chemical treatment (Schmidt et al., 2001; Wu et al., 1999). Spec-troscopic methods are based on spectroscopic analysis of samples toidentify infrared bands or nuclear magnetic resonance (NMR) regionscharacteristic to coal, generally after oxidative removal of non-coalorganics by a chemical pre-treatment (Rumpel et al., 2001; Schmidtet al., 1996). In the molecular approach, functional groups and/or clas-ses of organic compounds associated with coal are first identified and,based on the concentration of these chemical markers the coal contentof the environment samples is determined (Brodowski et al., 2005b;Glaser et al., 1998). The effectiveness of different methods in relationto coal rank, reactivity and/or structure is presented in Fig. 1. It is impor-tant to note that several of these methods rely on the assumption thatgeogenic and recent OC can be distinguished on the basis of their opti-cal, chemical and/or spectroscopic properties. Some techniques mea-sure different regions within the chemical reactivity continuum ofcoal. In practice, violation of these assumptions occurs to a varying de-gree depending on sample composition and method. Therefore, a com-bination of methods can be used to increase the confidence andaccuracy of the results. Some of these techniques are labor-intensive



and may require a level of instrumentation not readily available inmany typical soil laboratories.

3.2.1. Optical and microscopic methodsOptical and microscopic methods are used because optical proper-

ties are diagnostic of blackmaterials (Jones et al., 1992), and use less so-phisticated equipment and expertise. A microscope is a simple, directobservational tool which allows the user to see the features of interestand draw the appropriate inferences. The morphology (particle size,shape) surface characteristics and distribution of particles can be deter-mined using microscopy (Griffin and Goldberg, 1981), and throughcomparison of this information with standard reference materials,the identification and quantification of coal particles can be made(Fernandes et al., 2003). Coal particles can be quantified in situ onpetrographic thin sheets or polished sections. Prior to microscopicanalysis, coal can be concentrated by sieving or chemical treatments(Table 1). Sieving involves sequential washing through a series of

sieves of different mesh sizes and subsequent separation for identifi-cation and counting of particles. For a better discrimination of coalparticles during microscopic analysis, chemical pre-treatments of RMSsamples are sometimes carried out. These chemical treatments includedeflocculation of soil aggregates (generally with KOH) and removal ofcarbonates and silicates (with HCl and HF, respectively). Particles(recalcitrant OM and coal) surviving these pretreatments arethen suspended in oil and mounted on glass slides for microscopicobservation. Microscopic examination can be performed using eitherbrightfield light microscope, scanning electron microscope (SEM)(Rumpel et al., 1998b) or transmission electron microscope (TEM) de-pending on the particle size. Coal particles can be distinguished frommineral particles based on shape and color, and estimated distributionbased on an area (Rumpel et al., 1998b).

Scanning electron microscopy has been successfully used foridentifying BC in environmental samples, including coal and the fos-sil fuel combustion residues (Griffin and Goldberg, 1981; Jones and

Table 1Summarized methods for coal carbon determination.

Method Material analyzed Pretreatment Oxidation/treatment Geogenic C determination References

Optical/microscopy Sediment Sieving, deflocculating, removalof carbonates by HCl

None Light microscope, electronmicroscope (SEM or TEM)

Griffin and Goldberg (1981),Fernandes et al. (2003),Brodowski et al. (2005a)

Thermal oxidation Soil Sieving (b2 mm), removal ofcarbonates with 1 M HCltreatment

Thermal, heated in air at 375 °Cfor 24 h

C and N elemental analysesof the residual or massbalance

Gustafsson et al. (1997),Schmidt et al. (2001)

Thermal oxidation Soil Sieving (b2 mm), removal ofcarbonates and silicates with1 M HCl, and 10% HF, respec-tively

Thermal, heated at 375 °C for24 h

C and N elemental analysesof the residual or massbalance

Schmidt et al. (2001)

Thermal oxidation andacid hydrolysis

Soils As above 6 M TFA at 100 °C for 18 h, 6 MHCl at 110 °C for 24 h, thermaloxidation at 375 °C for 24 h

C and N elemental analysesof the residual

Schmidt et al. (2001)

Chemical oxidation Sediments HCl, HF Hydrogen peroxide Counting Kershaw (1986)Chemical oxidation Sediments HCl, HF Cr2O7

2−, H2SO4, heated at 50–60 °C for 60 h

Counting, mass loss Odegaard (1993), Wolbachand Anders (1989), Lim andCashier (1996)

Chemical oxidation Coal HCl, HF H2O2, KOH Infra-red spectroscopy Smith et al. (1993)Chemical oxidation Sediments HCl, HF Cr2O7

2−, H2SO4, heated to 60 °C,72 h + KOH, H2O2, heated to60 °C at 8 h

Elemental C, N and Hanalyzer

Bird and Grocke (1997)

Chemical oxidation Sediments HCl, HF Hot nitric acid (HNO3) GC, combustion - loss onignition, mass spectrometer(MS)

Verado (1997), Winkler(1985)

Chemical oxidation Soils None Sodium peroxodisulfate(Na2S2O8) with NaHCO3 buffer

Elemental C, N analyses ofthe residual

Mieier and Menegatti (1997),Menegatti et al. (1999)

Chemical oxidation Soils None Sodium chlorite + aceticacid + DI water

Elemental C, N analysis ofthe residual, 13C NMR spec-troscopy

Simpson and Hatcher (2004)

Chemical extraction–thermal oxidation

Soils HCl, HF NaOH extraction, 65% HNO3, 1%HCl, thermal treatment, 340 °Cin oxygen or air

Elemental C, N analysis ofthe residual, mass balance

Kuhlbusch (1995), Schmidtet al. (2001), Jacinthe et al.(2009)

Chemical extraction,chemical oxidation,thermal oxidation

Soils Sieve (b2 mm), removal ofcarbonates

0.5 M NaOH extraction, 60%HNO3 oxidation, hydrolysis, 10%HF, thermal oxidation at 340 °Cfor 3 h

Elemental C and N analysesof the residual,mass balance

Ussiri and Lal (2008a)

Specific molecularbiomarker

Soils Sieve (b2 mm), removal ofcarbonates and silicate with HCland HF treatment, respectively

65% HNO3 170 °C for 8 h,pressure filtered, washed withDI water, freeze dried

Benzene–carboxylic acid asmolecular biomarkersanalyzed by GC with FIDdetector

Glaser et al. (1998)

Ultraviolet photo-oxidation

Soils Physical fractionation Ultraviolet photo-oxidation 13C CPMAS NMR spectros-copy

Skjemstad et al. (2002)

Specific molecularmarker

Soils Digestion with 4 Mtrifluoroacetic acid (TFA) at104 °C

Digestion with 65% HNO3 at170 °C for 8 h. BC wasconverted to benzene poly-carboxylic acid (BPCA)

BPCA quantified with gaschromatograph (flameionization detector)

Glaser et al. (1998),Brodowski et al. (2005b)

13C NMR spectroscopy Soils Grinding, 10% HF treatment toremove paramagnetic materials

None Relative abundance offunctional groups associatedwith coal by NMR spectros-copy

Skjemstad et al. (2002),Simpson and Hatcher (2004),Rumpel et al. (2000)

Radiocarbon activitymeasurement

Soils Removal of carbonates,grinding and sieving through250 μm

None Accelerator massspectroscopy (AMS),14C activity

Rumpel et al. (2003), Fettweiset al. (2005), Ussiri and Lal(2008a)



Chaloner, 1991). In complex matrices such as RMS, which includeseveral forms of OC and various minerals however, coupling SEMwith energy dispersive X-ray spectroscopy enables detailed analysisof morphology and surface characteristics (Brodowski et al., 2005a).If the SEM is coupled with an elemental analyzer, coal particles can beidentified and the amount present can be quantified based on composi-tion (C, H and N) and element ratios (Fernandes et al., 2003). Thisprocedure is time-consuming and its accuracy relies on correct dif-ferentiation between coal and other dark-colored (opaque) debrisin the soils. Further, optical methods provide particle size and othermorphological details that allow inference to be made regardingcoal type. Despite their potential, however, optical methods are gen-erally qualitative, and not well suited for quantitative assessment ofgeogenic OC and C sequestration potential in RMS. In addition, mi-croscopic technique can only detect relatively larger particles andcannot detect coal degradation products.

3.2.2. Thermal oxidation methodsThermal oxidation methods rely on the difference in heat resistance

between recently-humified SOM and coal particles. Soil samples areheated in a stream of air or O2 at a predetermined temperature toconvert all combustible OC to CO2 (Schmidt et al., 2001) which canthen be measured. Recent SOM includes relatively easily oxidizableC fractions such as carbohydrates, proteins, and lipids. It is generallyassumed that these OC fractions will be oxidized and volatilized,whereas high rank coal such as bituminous and anthracite will resistthe heat treatment. Thus, by adjusting the combustion temperatureand duration, recent OC can be oxidized and the remaining geogenicOC can subsequently be quantified using elemental C analysis or Cmass balance (Table 1). The efficiency of the thermal oxidationmethod varies with coal rank. If the RMS contain low-rank browncoal particles, the thermal oxidation method is not suitable, becausesome of the coal particles will be oxidized along with recent OM frac-tions, leading to underestimation of geogenic C concentration.

An example of thermal analysis technique which allows for de-termination of physical properties of OM being oxidized (mass,temperature and enthalpy) as the sample is subjected to a con-trolled temperature program was developed by Maharaj et al.(2007a,b). In this approach, the sample is heated incrementally ina temperature controlled thermogravimetric (TG) analyzer, andchanges in mass, temperature and enthalpy of the sample arerecorded during the thermal oxidation process (Wendlandt, 1986).The time derivative of weight change in relation to temperature

provides information on the stability and composition of the samplebeing investigated (Tan et al., 1986). Specific thermal events as indicat-ed by inflections and peaks on the TG curve (thermal profile) are as-sociated with different types of materials in the sample beinganalyzed (Maharaj et al., 2007b). Weight loss at temperaturesN200 °C is generally attributed to loss of various forms of moisture.Loss of weight between 250 °C and 395 °C is attributed to SOM oxi-dation, 400 °C to 520 °C to coal oxidation, and above 600 °C to de-composition of carbonate minerals (Maharaj et al., 2007a). Othervariations of thermal oxidation approach are summarized in Table 1.

As stated above, thermal oxidation methods are based on the as-sumption that temperature conditions can be selected to thermallyoxidize recent OC fractions while geogenic OC remains un-oxidized(Schmidt and Noack, 2000). However, due to close associations andincorporation of coal particles within the macromolecular structureof SOM, selective oxidation of recent SOM remains a methodologicalchallenge. In addition, both recent SOM and coal particles comprise acontinuum of OM compounds that overlap in their thermal resis-tance and chemistry (Rumpel and Kogel-Knabner, 2002). Less refrac-tory C compounds associated with coal particles can be oxidizedalong with recent OC while highly refractory recent OC remainsunoxidized at the selected temperature. This overlap may be lessproblematic if high rank coal particles (bituminous and anthracite)are the sources of coal particles present in RMS. Therefore, there ex-ists no single, universally-accepted temperature limit for separatingOM from these sources. As a result, operationally defined tempera-ture cutoffs are set empirically, and probably vary with the rank ofcoal particles present in RMS (Fig. 1).

3.2.3. Chemical methodsChemical pretreatment methods have been successfully used to

remove SOM in order to expose mineral phase prior to analysis(Mikutta et al., 2005). Such analyses include particle size distribution(Gee and Bauder, 1986), composition, specific surface area (Kahleet al., 2003), and characterization of organically bound metals. Re-moval of SOM is also required in isolating BC in soils and sedimentsfor further assessment and quantification (Kuhlbusch, 1995; Schmidtand Noack, 2000; Schmidt et al., 2001; Simpson and Hatcher, 2004).These are the premises on which chemical methods for quantifyinggeogenic C have been developed.

Geogenic OC and pyrogenic BC in soils is generally considered resis-tant to chemical oxidation and microbial mineralization due to theirpredominantly aromatic structure (Schmidt and Noack, 2000). Chemi-cal methods for quantifying geogenic OC in soils and sediments involveoxidation/digestion of recentOC, followed by the assessment of residualC. Soil or sediment samples must be deflocculated to expose occludedOM, and pretreated to remove carbonates and silicates. The presenceof carbonates and silicates interfereswith the chemical oxidants and re-duces the efficiency of SOM removal (Anderson, 1963). In calcareoussoils for example, carbonate coatings favor occlusion and thus physicalprotection of OM against chemical reaction (Mikutta et al., 2005).Carbonates are removed by dilute acid reaction (see above), whilehydrofluoric acid (HF; usually ≤10% by volume) is commonly usedfor demineralization prior to chemical treatment (Table 1). The re-cent OC, comprising humified SOM, is removed by chemical oxidantssuch as sodium hypochlorite (NaOCl), hydrogen peroxide (H2O2),potassium dichromate (K2Cr2O4), sulfuric acid (H2SO4), nitric acid(HNO3), disodium peroxodisulfate (Na2S2O8), permanganate (KMnO4),or ultraviolet light (Schmidt et al., 2001). Mild alkaline solutions(i.e., b0.5 MNaOH) have also been used to extract recalcitrant recentOC (Jacinthe et al., 2009; Kuhlbusch, 1995; Ussiri and Lal, 2008a).Some of these reagents are often used in combination and at differ-ent temperatures, to increase the efficiency of recent OM removal.The remaining resistant OC fraction can be characterized by differentmethods, including elemental, microscopic, and spectroscopicmethods.Spectroscopic methods have the potential of distinguishing geogenic

Peat

LigniteSub-

bituminous

Brown coal

Coa

l/or

gani

cm

atte

r ty

peC

oal

rank Low High

Rea

ctiv

ity Highlyreactive inert

CPMAS 13C NMR

Isotopic methods

Molecular markers

Chemical methods

Thermal methods

Geo

geni

c or

gani

c ca

rbon

quan

tific

atio

n m

etho

ds

Semi-anthracite

AnthraciteBituminous

Fig. 1. Coal reactivity suitable methods of analysis continuum in the reclaimed minesoils.



OC and unoxidized recalcitrant OC based on differences in functionalgroups. Accurate quantification of geogenic OC concentration in soils re-quires complete oxidation of recent SOM without removing geogenicOC. Reaction with coal can cause underestimation, and incompleteremoval of SOM could lead to overestimation of geogenic OC concen-tration in RMS. Summary of chemical methods applicable forgeogenic OC quantification in soils is presented in Table 1.

The efficiency of recent SOM removal from soils by chemical re-agents is determined by reaction conditions (temperature, time ofcontact, addition of catalysts) and RMS properties (soil pH, mineral-ogy, SOC concentration and quality) (Mikutta et al., 2005). Variousoxidation protocols have been reported (Table 1), and much uncer-tainty remains regarding the effects of oxidative reagents on both re-cent and geogenic OC fractions. Geogenic OC is a continuum oforganic material ranging in chemical oxidation resistance from lessresistant brown coal and lignite to highly resistant bituminous coal,anthracite, and graphite. This is also true for recent SOM pool. Resis-tance of coal to chemical treatment is rank-dependent. Coals oflower rank (e.g., brown coal) are less resistant to chemical oxidationand may be oxidized along with recent OM, and therefore, in RMScontaminated with low-rank coal particles, chemical methods couldunderestimate geogenic OC. In contrast, recalcitrant but recent OMmay remain after the chemical treatment and that could lead to theoverestimation of geogenic OC. Selected chemical methods arediscussed below with an emphasis on reaction conditions and effi-ciency of recent OC removal. Results for several samples analyzedusing some chemical methods by our lab (Carbon Management andSequestration Center (C-MASC)) at The Ohio State University (OSU)are presented in Table 2.

3.2.3.1. Hydrogen peroxide. Most sample pretreatment protocols rec-ommend using 10 to 50% (wt/wt) H2O2 (Table 1). Increasing the con-centration of H2O2 has little effect on the removal efficiency of recentOC, probably due to decomposition of H2O2, and presence of chemi-cally stable andmineral protected organic compounds. Further, H2O2

is thermodynamically unstable, and decomposes into O2 and H2O(Padrieck et al., 1992). The decomposition rate increases with in-crease in pH (Xiang and Lee, 2000). In addition, increase in temperatureaccelerates the decomposition of H2O2, but shortens the reaction timenecessary to oxidize SOM, whereas, at lower temperatures the contact

time needs to be extended (Schultz et al., 1999). Use of H2O2 with ace-tate buffer (pH 5) prevents the acidic conditions resulting from forma-tion of acid oxidation products which reduces oxidation effectiveness ofH2O2 (Pennell et al., 1995). Chemical degradation of humic substancesby H2O2 yields water soluble low-molecular-weight compounds e.g.formic acid, acetic acid, oxalic acid, malonic acid, phenols, and benzenecarboxylic acids (Goldstone et al., 2002; Xiang and Lee, 2000), whichcan be removed by rinsing with deionized water (DI). The extent ofC removal by H2O2 (20 to 94%) varies with soil type and particlesize (Kahle et al., 2003; Schultz et al., 1999). In addition to high soilpH, the presence of manganese oxides, carbonates, and other protec-tive mineral phases also reduces the SOM removal efficiency. TheSOM removal efficiency can be improved by using Na2P2O7 as a dis-persing agent (Simon et al., 1992). Pyrophosphate releases occludedSOM, and displaces SOM adsorbed onto mineral surfaces (Celi et al.,2000; Varadachari et al., 1995). Tests with Ohio bituminous coalsamples at C-MASC laboratory showed that bituminous coal is resis-tant to the H2O2 treatment, but that up to 98% of the recent OC poolcan be oxidized (Table 2). These results suggest a greater potentialfor H2O2 oxidation technique, especially if geogenic OC determina-tion can be combined with spectroscopic techniques.

3.2.3.2. Sodium hypochlorite (NaOCl). The protocols for NaOCl oxidationof OM involve the treatment of soil samples with 2.5 to 6% NaOCl(Mikutta et al., 2005; Simpson and Hatcher, 2004). Simpson andHatcher (2004) used 1 g of soil, 5 g of sodium chlorite, 200 ml of DIwater and 5 ml of acetic acid, and reaction time of 2 h at room tem-perature, while Mikutta et al. (2005) recommended oxidation atpH 9.5 with boiling for 15 min. In both cases, oxidation was repeatedthree consecutive times. The short reaction time is used in order tominimize NaOCl decomposition (Mikutta et al., 2005). Heat-inducedchanges to OM structures can be minimized if the NaOCl treatment isapplied at room temperature, but extended contact time would beneeded for higher efficiency (Kaizer et al., 2002). The NaOCl treatmentdoes not require additional dispersing reagent because of the dispersionprovided by NaOCl (Mikutta et al., 2005). The NaOCl reaction with OCproduces aliphatic carboxylic acids, which can be removed by DIwater rinsing. The efficiency of recent OC removal with NaOClranges from 77 to 95% of the initial recent OC content (Kaiser andGuggenberger, 2003). The NaOC oxidation method was tested

Table 2Comparative results of the selected chemical methods tested for coal C quantification at Carbon Management and Sequestration Center (C-MASC).

Samplea Sample properties Residual carbon

Depth N C C/N ratio Chemi-thermal methodb Hydrogen peroxide (H2O2, 30%) Sodium hypochloride (NaOCl) Disodium persulfate (Na2S2O8)

g kg−1 g kg−1-

Coal and soil artificial mixturesCoal – 11.7 594 50.7 590 (99.3)c 576 (97.0) 589.1 (99.2) 600.9 (101.2)M0 – 2.4 25.7 9.6 1.3 (5.1) 2.6 (10.1) 2.1 (8.2) 0.8 (3.1)M1 – 2.6 33.2 12.5 8.9 (26.8) 11.2 (33.7) 6.6 (19.9) 9.4 (28.3)M2 – 2.6 39.1 14.9 16.5 (42.2) 18 (46.0) 14.9 (38.1) 17.5 (44.8)M3 – 2.9 57.4 18.5 33.9 (59.1) 36.4 (63.4) 31.8 (55.4) 34.9 (60.8)

MinesoilsRMS1 0–5 8.3 80.3 9.7 0.4 (0.5) 4.0 (5.0) 0.9 (1.1) 3.0 (3.7)RMS2 5–10 2.6 22.4 8.6 0.4 (1.8) 1.1 (4.9) 1.1 (4.9) 1.4 (6.3)RMS3 10–20 1.6 13.1 8 0.3 (2.3) 1.1 (8.4) 0.7 (5.3) 1.3 (9.9)RMS41 20–30 1.1 11.4 10.8 0.8 (7.0) 1.2 (10.5) 1.2 (10.5) 1.7 (14.9)RMS42 20–30 0.8 23.2 28.8 16.5 (71.1) 18.1 (78.0) 17.2 (74.1) 18.1 (78.0)RMS5 30–40 1 6.9 7.2 3.2 (46.4) 3.6 (52.2) 2.7 (39.1) 2.8 (40.6)RMS6 40–50 0.7 6.9 10.4 6.2 (89.9) 3.2 (46.4) 5.3 (76.8) 3.2 (46.4)RMS71 30–40 1.1 27.9 26.6 25.4 (91.0) 26.9 (96.4) 24.8 (88.9) 26.2 (93.9)RMS72 30–40 0.6 15.7 24.7 11.1 (70.7) 14.3 (91.1) 15 (95.5) 15.1 (96.2)RMS9 40–50 0.8 18.9 23 16.9 (89.4) 18.1 (95.8) 17.4 (92.1) 18.1 (95.8)

a M0 = coal-free soil (S0), M1 = S0 + 1.3% coal, M2 = S0 + 2.25% coal, M3 = S0 + 5.25% coal; RMS = reclaimed minesoils.b Chemi-thermal method as described in Ussiri and Lal (2008a).c Numbers in bracket are percentage of soil organic carbon remaining after treatment.



with bituminous coal in our laboratory, and results demonstrated thepotential of themethod in RMS containing high rank coal particles if re-action conditions are optimized (Table 2).

3.2.3.3. Disodium persulfate. This procedure was originally developedas a single-step method for the removal of SOM from soils and sedi-ment prior to claymineralogical studies (Meier andMenegatti, 1997;Menegatti et al., 1999). Thermal decomposition of Na2S2O8 does notdecrease C removal efficiency at temperatures N80 °C. The NaHCO3

buffer can be used to maintain pH of the reaction environment be-tween 7 and 8 (Kiem and Kogel-Knabner, 2002; Menegatti et al.,1999) and to prevent acid mediated mineral dissolution. Hot washwith formic acid has been employed to remove salts (Mieier andMenegatti, 1997). Meier and Menegatti (1997) argued that the effi-ciency of Na2S2O8 to remove SOM is superior to that of H2O2 andNaOCl. An evaluation conducted in our laboratory supports thatstatement, and demonstrated the resistance of coal to Na2S2O8 treat-ment (Table 2). The Na2S2O8 treatment is one of the promisingchemical methods for recent OC removal in RMS. Further researchis needed to optimize reaction conditions for RMS samples, however.

3.2.3.4. Ultraviolet oxidation. High-energy ultraviolet (UV) photo-oxidation in the presence of O2 removes the labile fraction of SOMfrom soils (Skjemstad et al., 1993, 1996). Skjemstad et al. (1996) ob-served that pre-treatment of silt and clay fraction with 2% HF prior toUV photo-oxidation significantly increased the SOM removal efficiencyof the UV treatment. Rumpel et al. (2000) showed that lignite-derived Cis more resistant to high-energy UV photo-oxidation treatment than re-cent OC. Therefore, high-energy UV photo-oxidation can be used to ox-idize recent OC and enable assessment of geogenic OC by spectroscopicmethods (Rumpel et al., 2000). As was found with other oxidation pro-cedures, some recent SOM fractions may survive the UV treatment. Theamount of recent SOM resistant to UV photo-oxidation depends on theassociation of recent SOM with soil minerals. Also, low rank geogenicOC such as lignite can be partly oxidized by UV, which may underesti-mate geogenic OC content (Rumpel et al., 2000). Sample pretreatmentsteps such as sample grinding and demineralization with HF willincrease the efficiency of UV photo-oxidation. Application of 13CCPMAS NMR spectroscopy after UV oxidation enables quantificationof geogenic OC in the presence of residual recalcitrant OC (Rumpelet al., 2000).

3.2.4. Spectroscopic characterization methodsSpectroscopic characterization methods relate geogenic OC to

relative abundance of specific functional groups in OM as revealedby spectral analysis. Two methods commonly applicable to geogenicOC determination are infrared (IR) and NMR spectroscopy. The IRspectroscopy is based on the specific response of organic groups toinfrared light in different regions of the IR spectrum. NMR spectros-copy identifies regions characteristic of coal structures and estimatescoal concentration in the RMS samples based on the relative strengthof the identified bands.

3.2.4.1. Infrared spectroscopy. Fourier transformed infrared (FTIR)spectroscopy and diffuse reflectance Fourier transform (DRIFT) IRspectroscopy analyses are commonly used to examine complex or-ganic substances in the solid state. Interpretation and quantificationof SOM spectra information can be achieved through multivariatedata analysis (Luinge et al., 1993) and predictivemodeling using par-tial least squares (PLS). Rumpel et al. (2001) used DRIFT IR to esti-mate the contribution of lignite C in RMS of Lusatia district. Thespectra showed that lignite C and recent OC occur in intimate mix-tures in these soils with extreme overlap of the IR peaks, makingthe assignment of signal to lignite difficult. A PLS prediction modelwas used to quantify lignite C based on sample chemistry and struc-tural differences between recent SOM, lignite and RMS samples as

revealed by DRIFT IR spectroscopy. Estimate of lignite C in RMS de-termined by IR spectroscopy was highly correlated with that obtainedusing 14C activity measurements (Rumpel et al., 2001).

3.2.4.2. Nuclear magnetic resonance (NMR). Solid state 13C CPMAS NMRtechnique is commonly employed to obtain structural information ofSOM and infer the relative contribution of different C sources in thesample based on functional group distribution (Conte et al., 1997;Wilson, 1987). It is a non-destructive technique for characterizingchemical structure of organic materials. The peak intensity ofNMR spectroscopy is approximately proportional to the fraction of Crepresented by the structural fingerprint assigned to the abscissa ofthe spectrum (chemical shift). Therefore, the 13C NMRmethod can pro-vide quantitative information about the structure. However, for macro-molecules or complex mixtures of macromolecules such RMS, the NMRspectra becomes a complex assemblage of overlapping peaks whereonly the average structural entities can be discerned (Wilson, 1987).

While recent SOM mainly includes carbohydrates, protein, lignin,and lipids; coal predominantly contains aliphatic and aromatic struc-tures (Hatcher and Clifford, 1997; Rumpel et al., 1998b). Therefore,the composition of geogenic OM in RMS as determined by its chemicalstructure differs considerably from that of recent SOM. 13C CPMASNMRspectroscopy was used to investigate structural transformations of OMoccurring during coalification (Bates and Hatcher, 1989; Wilson et al.,1987). It has also been successfully applied for the quantification of lig-nite C contribution to soils contaminated by lignite mining operations(Kogel-Knabner, 1997; Rumpel et al., 1998a,b, 1999; Schmidt et al.,1996). Schmidt et al. (1996) suggested that, due to the distinct domi-nance of aliphatic (alkyl C) and aromatic C in brown coal, NMR spectralinformation associated with these functional groups can be used as afingerprint for brown coal presence in RMS. These authors proposedthe use of the signal intensity ratio (A) as an indicator of the presenceof lignite in soils, where A is computed as:

A ¼ alkyl Cþ aromatic Cð ÞO� alkyl Cþ carboxylic CÞ : ð3Þ

The value of A is typically between 0.8 and 1.2 for non-contaminatedsoils and between 1.6 and 2.2 for soils contaminated with brown coal(Schmidt et al., 1996). Thus, the value of A increases as lignite contri-bution increases. Rumpel et al. (1998b) studied the composition ofOM in lignite-rich RMS sites under oak (Querns spp) and Scots pine(Pinus spp) using 13C and 15N CPMAS NMR spectroscopy and obtainedevidence of four types of C in these soils: (1) lignite inherent to parentmaterial resulting from lignite mining, (2) OM derived from decompo-sition of plant residues, (3) carbonaceous particles from air-bornelignite particles emanating from coal-fired plants, and (4) lignitefrom land amelioration with lignite ash. It is important to note, how-ever, that the 13C CPMAS NMR analysis lacks sensitivity in RMS con-taining low amounts of coal particles, requiring long measurementtimes in order to acquire quantifiable spectrum. Overall, NMR spec-troscopy should be considered a semi-quantitative method.

3.2.5. Isotopic methods

3.2.5.1. Radiocarbon approach. Radiocarbon or radioactive C (14C) is un-stable isotope of C present in environment in low quantities (10−9%).The radiocarbonmethod is based on the rate of decay of this radioactiveisotope of C. The half-life of 14C is 5730 years. As a result, coalwhichwasformedmillions of years ago does not have 14C activity (“dead carbon”)(Rumpel et al., 2000). In contrast, recent OM contains 14C activity (“ac-tive carbon”) due to plant uptake of 14CO2 from the atmosphere. Aftercorrection for isotopic fractionation, plant biomass has the same 14C ac-tivity as the atmospheric CO2, assimilated and incorporated into theirtissue during photosynthesis. Therefore, 14C concentration in living



plant tissue is in equilibriumwith 14CO2 in the atmosphere. Once the tis-sue dies, and metabolic function ceases, there is no replenishment of14C, only radioactive decay occurs. By measuring the 14C concentrationof the organic material, a number of decay events per gram of C canbe calculated. Comparison of measured 14C activity with that of modernsample and application of half-life of 14C radioisotope allows death dateand therefore, age of OM to be calculated.

The SOC in RMS consists of two pools: one with variable 14C activity(recent OC) and one without 14C activity (coal), Fig. 2. The radiocarbonapproach for determination of geogenic OC in RMS is based on differ-ence in 14C activity between these C sources. The relative proportionof geogenic C can be obtained by mass balance using the measured14C activity in RMS sample:

F ¼ 1− ASOC

Arecent

� �� 100 ð4Þ

where, F is the fraction of geogenic OC (%), ASOC is the measured 14Cconcentration of RMS, and Arecent OC is the 14C concentration of recentOC derived from plant material. However, we do not know the rela-tive 14C concentration of recent SOM (i.e., Arecent OC), because the rel-ative 14C activity in recent OM fraction is not well constrained due tovariation in atmospheric 14CO2 concentration during the past 50 to60 years. The 14C concentration of the atmosphere increased around1955 due to early nuclear weapon testing, and then increased again in1962/1963 (Manning et al., 1990; Rafter and Stout, 1970). Since 1963,14C concentration in the atmospheric has progressively decreased. Asa result, the 14C concentration in plant tissues has also systematicallychanged. The 14C concentration of recent SOM synthesized from the at-mospheric CO2 and added to soil OM changed systematically from yearto year because of the instability of 14CO2 concentration stated above.These variations in atmospheric 14C concentration must be consideredin these computations (Eq. (4)). 14Cmeasurement of soils yields the ap-parent mean residence time (MRT) of SOM, which is defined as 1/k, k isthe first-order decay constant for OM decomposition (Paul and Clark,1996). Exact numerical age of SOM is difficult to establish, since SOMconsists of continuum of SOC at different ages, stages of decomposition,and humification ranging from months to millennia. Rumpel et al.(2003) developed amodeling approach for estimating 14C activity of re-cent OM in RMS by using time series approach. This approach usedRothamsted model of SOC to calculate the age distribution of soil OCand the existing records of atmospheric 14CO2.

The 14C activity of the RMS sample is determined by acceleratormass spectroscopy (AMS) after removing carbonates by HCl treatment(Morgenroth et al., 2004). In radiocarbon dating, year 1950 is year

0 before present (BP) by convention (Taylor, 1987). The proportionof geogenic C and recent OC can be calculated using Eqs. (5) and (6):

Recent OC %ð Þ ¼ ASOC

ArecentOC� SOC ð5Þ

Geogenic C %ð Þ ¼ ASOC

ArecentOC� SOC ð6Þ

where, SOC is the organic C concentration of RMS.Analysis of lignite containing soils in Germany showed very old 14C

ages, which was generated by lignite with no 14C activity (Rumpelet al., 2003). Radiocarbon method showed that lignite C accounted forup to 83% in the Oh and Ai horizons and 80–93% of total OC in the top0–5 cm depth. In contrast, the C in the subsoil was exclusively geogenicOC in these soils (Rumpel et al., 2003). Using radiocarbon analysis at dif-ferent sites, Chabbi et al. (2006) observed lignite C contribution rangingfrom 20 to 80% of the total organic C. Similarly, in Ohio, USA, radio-carbon analysis revealed that the top 10 cm layer of the sitesmined for bituminous coal and reclaimed with topsoil applicationwith grass planting 28 years agowas free from geogenic C due to storedtopsoil application (Ussiri and Lal, 2008a). Contribution of coal C in-creasedwith increase in depth from0 at the top 10 cm(i.e., applied top-soil) to 92% for 30 to 40 cm layer dominated by mine spoils (Ussiri andLal, 2008a,b).

RMS data from Germany and USA revealed that in general, geogenicOC distribution is highly variable both spatially and by depth. Therefore,for accurate quantification of geogenic OC in RMS large number ofsamples must be analyzed. However, radiocarbon method uses a so-phisticated instrumentation not readily available in typical soil andenvironmental laboratory. Therefore, samples must be submitted toa specialized analytical laboratory, and analytical cost limits thenumber of samples which can be analyzed.

3.2.5.2. 13C isotope ratio method. The 13C isotopes represent approxi-mately 1.11% of Earth's C pool (Craig, 1957). Biological materials dis-criminate against the heavier isotope (13C) during biological reactionprocesses (Blair et al., 1985; Galimov, 1985). Therefore, natural iso-topic variation in plants is the result of C isotope fractionation duringphotosynthesis (Farquhar et al., 1989), largely due to biochemicalproperties of the primary CO2-fixing enzymes and limitation to13CO2 diffusion through leaf stomata (O'Leary, 1989). Terrestrialplants are divided into three major photosynthesis types, namely:(1) C3 plants in which the carboxylating enzyme discriminatesagainst 13CO2 during photosynthesis, resulting in low δ13C composi-tion (−35 to −22‰; mean −27‰), (2) C4 plants that possessphosphoenol pyruvate carboxylase enzyme system that is less dis-criminatory against 13CO2, and consequently have a relatively higherδ13C signature (−18 to −8‰; mean -12‰, Fig. 3), and (3) plantswith Crassulacean acid metabolism (CAM) which are able to switchbetween C3 and C4 photosynthetic pathways depending on environmen-tal conditions, and have δ13C ranging from −28 to −10‰ (Griffiths,1992). The 13C natural abundance of SOM reflects that of the vegetationfromwhich it originates, and this has been used to determine the contri-bution of vegetation communities to SOC pools, especially where C3 veg-etation has been replaced by C4 vegetation and vice versa (Balesdentet al., 1993; Bernoux et al., 1998; Desjardins et al., 1994).

The 13C isotope technique is also a potential tool for estimating coal Cand recent OC contribution in RMS. Lignin and lipids are generallydepleted in 13C, whereas cellulose and proteins are enriched in 13C(Preston et al., 2006). These variations can be used in certain casesas indicators for selective preservation of specific classes of organiccompounds during OC decomposition. Microbial respiration also dis-criminates against 13C, leading to relative enrichment of 13C in plantmaterials resistant to microbial mineralization (Hobbie et al., 1999).The δ13C value of the atmosphere has varied over the geological time(Friedli et al., 1984), and decreased over the past 150 years as a result of

Total organic Carbon in Reclaimed Minesoils(Geogenic C + Recent C)

Coal in “spoil” Coal dustCoal in

ameliorationash

OM in appliedorganic materials

(eg mulch,compost)

Plant litter,faunal detritus

14C activity = 0 pMC 14C activity 115 - 116 pMC

Total organic carbonin Minesoils

HumificationTransformationProtection

Mixing

Fig. 2. Soil organic carbon sources and radiocarbon activity in reclaimed minesoils.



increased fossil fuel burning (Friedli et al., 1986). Coal C is normallyenriched in 13C compared to plant material with C3 photosynthesispathway (Fig. 3) probably due to microbial discrimination against theheavier isotope. Plant remains underwent extensive transformationand decomposition prior to geological coalification process (Stachet al., 1982; Whiticar, 1996). The variation of C isotope in coal is lim-ited to a range of about 5‰ between δ13C−22 and−27‰ (mean =24.1‰, Fig. 3) (Compston, 1960, Holmes et al., 1981; Redding et al.,1980) and does not change significantly with rank or maturity ofcoal (Whiticar, 1996).

A simple two-compartment model can be used to estimate theproportions of coal C and recent OC in RMS. The differences in 13Csignature between coal C and recent vegetation have been used to es-timate geogenic OC in RMS (Chabbi et al., 2006). Using 13C isotopes, it

was possible to get an indication of the contribution of lignite C alongtransect of the rehabilitated minesoils ranging from forest to sub-merged lake sediment at Lusatia mining district in Germany (Chabbiet al., 2006). In these soils, the 13C isotope ratio was strongly relatedtoOMcomposition, and lignin content and composition. In themixturescontaining minesoils and sediments, the δ13C composition was suffi-ciently different to allow the discrimination of geogenic and recent OCsources (Chabbi et al., 2007, 2008). Similar results have also been re-ported for minesoils contaminated with bituminous coal C in theEastern USA (Ussiri and Lal, 2008a,b).

4. Geogenic organic carbon distribution in reclaimed minesoils

Depending on mining and reclamation techniques, the amount ofgeogenic OC can be substantial (Table 3). In addition, coal can makeup a large portion of SOM in RMS, soils adjacent to coal processing in-dustries, and in lands ameliorated with coal combustion byproductssuch as coal ash (Schmidt et al., 1999; Zikeli et al., 2005). Coal con-tamination contributed to the irregular depth distribution of SOC inRMS (Thurman and Sencindiver, 1986). In Lusatia mining district(Germany), lignite-C accounted for 25 to 99% of the total SOC in rehabil-itated and reforested RMS (Rumpel et al., 1998b, 2001). Geogenic OCaccounted for up to 80% of the OC in the A horizon of cultivatedMollisol contaminated by brown coal emissions from a briquette fac-tory (Schmidt et al., 1996). Chabbi et al. (2006) reported a lignitecontribution of 20–80% of total SOC open-cast mined soils and sedi-ment of Lusatia (Germany). Generally, the concentration of geogenicC increases with increase in soil depth, and nearly 100% of OC in the Chorizon (mine spoil) can be of geogenic origin (Table 3).

Age since reclamation had little effect on the relative abundanceof geogenic OC in RMS probably due to its biochemical recalcitranceof geogenic OC (Robertson and Morgan, 1995). For example, Rumpelet al. (2003) showed that lignite accounted for 80 to 93% of the totalC in the top 0–5 cm mineral horizon of the age-chronosequence se-ries of minesoils ranging from 11 to 32 years old at Lusatian miningdistrict in Germany. Rumpel et al. (1998b) observed that after36 years of restoration, more than half of the OC in 0–5 cm soillayer under red oak (Querus spp) was derived from decomposingplant material. In areas heavily contaminated by fly-ash, soot and lig-nite dust, lignite-derived material accounted for up to 40% of OC inthe forest floor (Rumpel et al., 1998a) and up to 80% of the soil C ofan agricultural soil (Schmidt et al., 1996). Ussiri and Lal (2008a) de-termined the geogenic OC content of RMS restored with topsoil

0-10 10-20-30-40-50

Dia

gene

tic /

Fos

sil C

Ter

rest

rial C

Atm

osph

eric

C

PD

B s

tand

ard

AtmosphericCO2

C4 Plants

C3 Plants

Coal

Petroleum/Crude oil

Naturalgas

-27‰

-12‰

-24.1‰

δ13C (‰)

-8‰

( -35 to -22‰)

(-18 to -8‰)

( -27 to -22‰)

-30 to -20‰

-16 to -10‰

Photosynthesis

Diagenetic transformations ingeological timescale

Fig. 3. Carbon isotope ratios of fossil C and major terrestrial plant groups.Modified from Whiticar (1996).

Table 3Distribution of geogenic organic carbon in reclaimed minesoils.

Age since reclamation Horizon/soil depth Total OC (g/kg) Geogenic C (g/kg) Geogenic C (% of TOC) Reference

36 year-old red oak Oh (forest floor) 224 56 25 Rumpel et al. (1998b),Rumpel et al. (2000)Ai (0–5 cm) 110 54 49

Cv (1 m) 37 36.9 9620 year-old Scots pine Ai1 (0–1 cm) 78 75.4 69

Ai2 (1–3 cm) 45 37 8322 year-old pine F 350–483 10–28 3–6 Fettweis et al. (2005)

H 116–346 6–49 5.2–140–4 15–72 10–66 67–924–10 22–83 20–78 91–94

15–36-year old pine Forest floor 89.7–298 16.5–49 14.3–45.4 Rumpel et al. (2003)Mineral soil 20–91 8–71 79.1–91.3

11-year old pine Of (1) 234 96 41 Rumpel et al. (1999)Ai 0–2 102 86 84Cv 100 36 36 100

17-year old pine Of (2 cm) 320 96 30Ai1 (0–1) 80 54 68Ai2 (1–3) 46 37.7 82Cv (3–100) 18 Nd

32-year old pine Ai (0–5) 180 115 64Cv (5–100) 36 35.6 99

28-year old grassland 0–50 cm 3.6–155.8 0–24.4 0–92 Ussiri and Lal (2008a)



application using radiocarbon analysis and chemi-thermal methodand observed that geogenic OC accounted for 2 to 40% of the totalOC in the top 30 cm soil layer, while in the subsoil (below 30 cmdepth) geogenic C accounted for up to 92% of the total organic C.The geogenic C concentration increased with increase in soildepth for ripped RMS reclaimed by topsoil application. In contrast,in overburden restored without topsoil application, the concentra-tion of geogenic OC was found to decrease with increased soil depth(Jacinthe and Lal, 2007). The pool of geogenic C was twice as muchin plowed than in no-till (27.4 vs 12.5% of TOC) reclaimed croplandssuggesting remobilization of overburden materials during tillageoperations (Jacinthe and Lal, 2009). Overall, the reviewed data sug-gests that the distribution of geogenic OC in coal-contaminatedRMS can be highly variable; therefore, intensive sampling is neces-sary in order to obtain adequate measure of SOC in these restoredecosystems.

5. Summary and conclusions

Minesoils contain geogenic OC associated with coal particles and re-cent OC derived fromplant debris. In order to obtain accurate estimationsof SOC pool and sequestration rates in RMS,methods are needed to quan-tify the relative contribution of these sources to total SOC in RMS. Coal iscomposed of a continuumof geologically altered organic compounds lith-ified over millions of years ago and preserved in depositional basins.Compared to recent OM, coal is highly aromatic and hydrophobic in na-ture. Because of these characteristics, coal-derived C is relatively inert tomicrobial degradation and highly resistant to thermal and chemical oxi-dation. However, the degree of resistance depends on coal rank. Thesedifferences in properties form the basis for the various techniques for es-timating geogenic OC and recent SOC concentration in RMS. Thermal ox-idation and chemical reactivity techniques involve the removal of recentOC by heat and various chemical treatments and/or extraction agents,followed by assessment of refractory OC, mainly geogenic OC. Themajor limitation of these methods is their inability to remove highly re-calcitrant recent OC. The 13C NMR spectroscopy method generally differ-entiates between recent SOC and geogenic OC based on relativeabundance of specific functional groups characteristic of coal or recentOC. While structural and compositional differences between geogenicand recent SOC are fairly well understood, overlaps exist in the spectralsignature of theseC sources, and thatmakes it a challenge to quantitative-ly estimate geogenic OC by spectroscopic methods. Radiocarbon mea-surements are based on the assumption that coal is composed of C thatwas deposited several millions of years ago and, therefore, without 14Cactivity (i.e., “dead carbon”). Radiocarbon analysis is one of themost reli-able methods for quantitative measurement of geogenic OC in RMS.However, high analytical costs and requirement of specialized analyticalinstrument as well as specialized expertise preclude adoption of thismethod for routine soil analytical laboratory.

Although various methods have been employed in quantifyinggeogenic OC in RMS, there are still no standardmethods for that purpose.The radiocarbon (14C) method is the closet to be a referencemethod, buthigh analytical costs preclude its wide adoption. While the thermal andchemical methods can be used for routine soil analysis, they have severalshortcomings including incomplete oxidation of recent SOC and variableefficiency of the method depending on reaction conditions and rank ofthe coal particles in RMS. Nonetheless, with further research, they offera great potential for methods that could be widely adopted for geogenicC determination in RMS. Systematic inter-laboratory comparison of anal-ysis of well-defined standardmaterials could be crucial step towards thatgoal.

6. Research needs

An accurate quantification of newly sequestered SOC in restoredlands disturbed by coal mining has been a challenge due to the lack of

proven and standardized techniques for determination of geogenic Ccontent (in order to correct for coal contamination). Althoughsome progress has been made recently on developing techniquesfor quantifying geogenic C in RMS, the applicability of these methodsto coals of different ranks and soils of different mineralogy is not wellestablished. Currently, radiocarbon analysis technique is the onlyunambiguous and reliable technique of differentiating geogenic Cfrom recent OC from RMS. Additional research is needed, specificallywith regard to the following:

• Refine, calibrate, and standardize the promising methodologies forgeogenic C quantification,

• Establish reference samples and conduct inter-laboratory compari-sons as a way to validate and standardize the techniques,

• Develop sampling protocols, which can enable credible assessment ofgeogenic C in RMS, and

• Evaluate the effects of soil physical and mineralogical properties onthe selectivity and efficiency of C removal by chemical and thermaltreatments.

Acknowledgments

This work was jointly supported by funds from the United StatesDepartment of Energy (DoE) award no: DE-FC26-04NT42208 andthe Ohio Coal Development Office (OCDO) project D-07-03. The au-thors thank two anonymous reviewers for their valuable commentsand suggestions.

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Methods for determination of coal carbon in reclaimed minesoils: A review