Importance of Unburned Coal Carbon, Black Carbon, and

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Environ. Sci. Technol. 2005, 39, 764-769

Importance of Unburned Coal Carbon, Black Carbon, and Amorphous Organic Carbon to Phenanthrene Sorption in Sediments GERARD CORNELISSEN† AND O ¨ RJAN GUSTAFSSON* Institute for Applied Environmental Research (ITM), Stockholm University, 10691 Stockholm, Sweden

The aim of this paper was to estimate the contribution to total phenanthrene sorption from unburned coal and black carbon (BC; soot and charcoal) in sediment. We determined sorption isotherms for five Argonne Premium Coal standards over a wide concentration interval (0.0110 000 ng/L). The coals showed strong and nonlinear sorption (carbon-normalized KF ) 5.41-5.96; nF ) 0.680.82). Coal sorption appeared to become more nonlinear with increasing coal maturity. The coal’s specific surface area appeared to influence KF. On the basis of the current coal sorption observations combined with earlier petrographic analyses and BC sorption experiments, we calculated for one particular sediment that coal, BC, and “other” OC were all important to PHE sorption in the environmentally relevant nanogram per liter range. This indicates that it is important to consider strong sorption to coal in the risk assessment of coal-impacted geosorbents (e.g., river beds) where coal is mined/shipped and manufactured gas plant sites.

Introduction Sorption of hydrophobic organic chemicals (HOCs) governs the fate, transport, and ecotoxicological risks of soil- and sediment-bound chemicals. Earlier, sorption was normalized to the total organic carbon (TOC) content, with TOC-water distribution ratios (KTOC) describing sorption strength (e.g., refs 1 and 2). Several findings such as nonlinear sorption isotherms (e.g., refs 3-10), multiphasic desorption kinetics (e.g., ref 11), strongly elevated field KTOC values (e.g., refs 12 and 13), large variations in biota to sediment accumulation factors (e.g., refs 14-16), and incomplete bioremediation in the absence of microbial limitations (e.g., refs 14 and 16) led to the suggestion of a dual-mode sorption concept (4, 11, 17). In this concept, the OM is regarded to be composed of two domains, one showing linear absorption and one showing nonlinear adsorption. The absorption domain has been proposed to consist of amorphous OM such as humic/fulvic substances, lipoproteins, and lignin (4), whereas harder, more condensed moieties such as unburned coal and black carbon (BC; residues from incomplete combustion such as soot and charcoal) make up the adsorption domain (3-11, 17-20). Sorption to both unburned coal and to BC has been shown to be strong and nonlinear (3-7, 9, 10, 21-26). * Corresponding author phone: +46-86747317; fax: +46-86747638; e-mail: [email protected]. † Present address: Norwegian Geotechnical Institute, P.O. Box 3930 Ullevaal Stadion, N-0806 Oslo, Norway. 764

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The focus of the present paper is on unburned coal. Most coals are remnants of terrestrial plants. In response to diagenesis and catagenesis, coal matures in the sequence peat, subbituminous brown coal, bituminous (hard) coal, and anthracite (27). During these progressive stages of coalification, the coal carbon content increases and sorption becomes stronger and more nonlinear (3-5). Coal consists of 50-95% carbon (27); as we normalized all our coal sorption data on coal carbon content, we will use the term “coal carbon” (CC) throughout this paper, analogous to OC and BC. For our purposes, we include only unburned coal in our definition of CC; charcoal is a residue of incomplete combustion and is therefore per definition BC. In the literature, there is some semantic confusion about the term “coal”. In strict organic petrographic terms, coal includes organic material in all stages of maturation, whereas in common terminology (and in many papers in environmental sciences) the term coal is somewhat misleadingly reserved for material in more advanced stages of maturation. As this latter, mature, material is probably the material that shows strong and nonlinear sorption, we will here stick to the common “environmental-chemical” terminology and refer to it in terms of coal and CC. Therefore, CC is here referring to lignite, brown coal, and anthracite, but not peat and charcoal. A model including sorption to BC, unburned CC, and “other” OC is (24)

CS ) fOCKOCCW + fBCKF,BCCWn,BC + fCCKF,CCCWn,CC (1) where CS is the sorbate concentration in the sediment (µg/kg dry weight, dw) and fOC, fBC, and fCC are the sediment mass fractions of OC, BC, and CC, respectively. The TOC content fTOC is the sum of fOC, fBC, and fCC. KOC is the OC-water distribution coefficient (L/kg), CW is the aqueous concentration (µg/L), KF,BC is the Freundlich BC-water distribution coefficient [(µg/kgBC)/(µg/L)n,BC], KF,CC is the Freundlich sorption coefficient of CC [(µg/kgCC)/(µg/L)n,CC], and nF,BC and nF,CC are the Freundlich exponents of BC and CC sorption, respectively. A concentration dependence appears in the BC and CC terms because of the nonlinearity of BC and CC sorption. To assess the relative importance of OC, BC, and CC for sorption, one needs to constrain the parameters in eq 1. The parameters of OC and BC sorption have been constrained earlier (e.g., refs 23 and 24); in addition, a method was described to assess the parameters of the CC sorption term (24). The fCC can be estimated from coal petrographic analyses. In the present study, we measured KF,CC and nF,CC for five easily obtainable standard Argonne National Laboratories (ANL) Premium Coal samples. The advantage of using these coals is that many of their characteristics have been determined in the Argonne Premium Coal Sample Program. Although coal sorption has been determined in several previous studies (3-7, 9, 10), we decided to establish a new data set for these five standard coals. The reasons for this were (i) previous sorption isotherms were often measured in a relatively high (microgram to milligram per liter) concentration range; (ii) sorption data were only available for two of the Argonne Premium Coal samples (5), at relatively high concentrations; and (iii) novel solid-phase extraction (SPE) sorption methods that circumvent the particle concentration effect have not yet been applied to coal sorption. Thus, we applied polyoxymethylene-SPE (26) over a large concentration range to the abovementioned five coals in the 10.1021/es049320z CCC: $30.25

 2005 American Chemical Society Published on Web 12/17/2004

TABLE 1. Characteristics of the Five Selected ANL Standard Coals in Order of Increasing Carbon Content and VRa name

coal rank

Wyodak Illinois Pittsburgh Upper Freeport Pocahontas

subbituminous high-vol. bit.h high-vol. bit. medium-vol. bit. low-vol. bit.

peat soft brown coal/lignite bright brown coal/lignite bituminous hard coal anthracite

ANL no. mesh 2 3 4 1 5

20 100 20 20 20

% C 75.0 77.7 83.2 85.5 91.1

H/C (mol/mol)

O/C (mol/mol)

0.86 0.77 0.77 0.66 0.59

0.18 0.13 0.08 0.08 0.02

∼50 ∼1.0 >0.45 ∼60 0.8-1.0 0.25-0.45 ∼75 0.7-0.9 0.10-0.25 80-90 0.5-0.8 0.04-0.10 90-100 0.25-0.6 95% of the total PHE-d10 in the system was in the coal, this influenced the measured KF,CC values by maximally 0.04 log unit. In the calculation of sorption isotherms, the mass balances were corrected for. All KF,CC values were corrected for the “salt effect” caused by the biocide concentration. The Setchenow equation was used: log K ) log Ksalt - KS[salt] (30), where KS is the Setchenow constant for PAHs (0.30) and [salt] is the molar VOL. 39, NO. 3, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. CC-normalized logarithmic Freundlich isotherms for PHE-d10 in the five tested Argonne Premium Coal standards. Plotted is the CC-normalized content in the coals vs the freely dissolved aqueous concentration CW. Lines obtained by linear regression.

TABLE 2. Freundlich Sorption Parameters for the Five Selected ANL Standard Coals name Wyodak Illinois Pittsburgh

2 3 4

Upper Freeport Pocahontas

1 5

average

log KF,CC (µg/kgCC)/(µg/L)n,CC

nF,CC whole isotherm

this study this study this study ref 5 this study this study

5.54 ( 0.05 5.96 ( 0.11 5.41 ( 0.07 6.50 5.70 ( 0.13 5.59 ( 0.10

0.82 ( 0.02 0.79 ( 0.04 0.74 ( 0.03 0.56 0.68 ( 0.05 0.66 ( 0.04

this study

5.6

0.75

ANL no.

r2

nF,CC CS,PHE < CS,nativea

nF,CC CS,PHE > CS,nativeb

0.995 0.985 0.993

0.89 ( 0.04 0.99 ( 0.05 0.81 ( 0.07

0.75 ( 0.04 0.65 ( 0.01 0.67 ( 0.03

0.964 0.983

0.89 ( 0.14 0.66 ( 0.07

0.58 ( 0.03 0.66 ( 0.08

a

nF,CC at added PHE-d10 concentrations below the total native PAH concentration. b nF,CC at added PHE-d10 concentrations above the total native PAH concentration.

salt concentration (0.03 M). In all cases the salt correction was 0.09 log unit.

Results The sorption isotherms for the five coals are presented in Figure 1. The isotherms were fit to the Freundlich equation:

CCC ) KF,CCCWnF,CC

(2)

where CCC (µg/g of CC) is the CC-normalized concentration in the coal. The Freundlich parameters are presented in Table 2. The Freundlich sorption coefficients KF,CC were 0.5-1 log unit above recent KF,TOC values for five sediments measured with the same POM-SPE technique (4.76-5.40). Sorption isotherm nonlinearity was observed (nF,CC ) 0.66-0.82). This implies that sorption to CC was especially strong at low concentrations; for example, at CW ) 1 ng/L, log KCC ) 6.36.8 whereas log KOC,sediment ) 5.2-5.5 (24). In Table 2, we also report one literature set of sorption parameters for Pittsburgh coal (5). Their KF,CC is larger than currently observed, whereas nF,CC is lower. The reason for 766

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this might be that Johnson et al. (5) determined their isotherm in the 0.2-500 µg/L range, whereas we focused on the picogram to microgram per liter range. At higher concentrations, sorption becomes more nonlinear (and nF,CC lower) because of increasing competition for a limited number of sites. Strongly nonlinear sorption of PHE to mature carbonaceous materials (shales, kerogens, coals) and charcoals has been observed before (3-7, 9, 10). In most cases, KF,CC was between 105 and 106 L/kg, while nF,CC was in the order of 0.4-0.8; most of these measurements were carried out in the 100-100 000 ng/L range, which is higher and narrower than the currently employed one of 0.01-10 000 ng/L. Our currently observed values of KF,CC and nF,CC were in the broad range of the literature ones (Table 2). In Table 2, we also present nF,CC for the low-concentration and high-concentration parts of the sorption isotherms, respectively. At low concentrations, where CS of the added PHE-d10 is below the total native PAH contents of 5-12 mg/ kg dry weight (dw; Table 1), the sorption isotherms were more linear than at the high concentrations (added PHE-d10 above native PAH contents). This difference is significant at

FIGURE 2. Apparent relationship between Freundlich nF,CC and percentage carbon in the coal (%C). Line obtained by linear regression. the 95% confidence level for all coals except Pocahontas (t-test). The observed phenomenon can probably be explained on the basis of competitive site sorption. At added PHE-d10 contents below the native PAH contents, the total PAH concentration and thus the level of competition on the coal do not change with increasing PHE-d10 content. Therefore the remaining sorption capacity remains constant over this low-concentration interval. As soon as the added PHEd10 contents exceed the native PAH contents, competition for the remaining sorption sites starts and the sorption isotherm becomes more nonlinear and nF,CC decreases.

Discussion Sorption and Coal Characteristics. In a competitive coal sorption mechanism, sorption is dependent on capacity (number of sites) and affinity (binding strength). In the case of (nanopore) surface sorption, the SSA is probably a determinant of site capacity. Site affinity probably increases with increasing coal maturity; as the %C increases with maturity, relationships between %C and coal sorption parameters may be expected. In Figure 2, the sorption nonlinearity coefficient nF,CC has been plotted as a function of the coal carbon contents. A clear relationship was observed through linear regression (r2 ) 0.94; p ) 0.007). As %C increases with increasing coal maturity, this showed that coal sorption became more nonlinear as coal maturity increases, at least for the currently studied limited data set of five medium-rank coals. The observed relationship between nF,CC and coal maturity was in line with earlier observations (e.g., refs 4-7 and 9). It is noted that the values of nF,CC should not be regarded as absolute ones because the value of nF tends to vary with the interval over which the isotherm was measured; the larger the concentration interval, the more nonlinear the sorption. There appeared to be no relation between KF,CC and any of the parameters that describe coal maturity (all linear regression p values >0.5). This was remarkable because such relationships have been reported before (3-7, 9). The coal SSA appeared to influence KF,CC (r2 ) 0.67; p ) 0.08) and, to a lesser extent, nF,CC (r2 ) 0.37; p ) 0.27). The SSA-based correlations were relatively poor because the SSA values are not evenly distributed, with constant and low values for coals 1, 4, and 5. Thus, on the basis of the current data set, no conclusions on the relation between SSA and nF and/of KF,CC could be drawn. It can only be speculated that the relatively high SSA (23.5 m2/g) of Illinois coal no. 3 was the cause of its high KF,CC despite its relatively low %C of 77.7%. CC and BC Inclusive Sorption Model. We estimated the roles of BC and CC in PHE-d10 sorption (using eq 1) for Ketelmeer (KET) sediment from The Netherlands (fTOC 5.5%). This sediment is quite heavily contaminated as it stems from the first major sedimentation area of the Rhine River. The shores of this river are densely populated and heavily industrialized. In addition, large-scale coal mining activities used to take place along Rhine River. Much of the runoff

from these industrial and mining areas has sedimented in Lake Ketelmeer. Although the KET sample may be considered representative of many heavily contaminated sediments, the forthcoming section only considers one particular contaminated situation, and it does not necessarily extend to other geosorbents. It is only intended to present a general framework for the assessment of contaminated situations where both BC and coal are present, such as sediments close to coal-mining and shipping activities and soils from coalfired manufactured gas plant sites. For other sites the BC, CC, and OC contents will be different, leading to variation in their respective contributions to sorption. The fBC of KET sediment was constrained to be 0.72% by the CTO-375 method (23). In ref 24 we suggested to use coal petrography for the quantification of fCC, analogous to earlier approaches in refs 6, 7, and 9. Coal petrographic maceral analyses give insight in the origin, morphology, and maturity of organic particles. They are performed by visual inspection of the organic fraction of the sediment, using a coal petrographic microscope (32). The most condensed and “coaly” maceral group, the inertinites, probably represents both BC and coal-like moieties (24, 33, 34). Thus, the difference between the total inertinite content and the BC content probably represents mature, condensed but unburned moieties (i.e., what we defined here as coal; 24). The other maceral groups, huminites and liptinites, are humic, more amorphous materials from woody and herbaceous tissues, respectively (34). Therefore, these will probably show linear, non-elevated sorption, such as the “amorphous organic matter” in refs 4, 5, 11, and 31. Thus, huminites and liptinite were included in the (amorphous) OC. The total inertinite content of KET sediment was 2.3% on a total mass basis (24), so we calculated fCC to be 1.6% (2.3% total inertinites minus 0.7% BC). Consequently, fOC ) fTOC fBC - fCC ) 5.5 - 0.7 - 1.6 ) 3.2%. KET sediment consisted of approximately 89% inorganic matter. The environmental BC sorption parameters KF,BCenv and nF,BC for KET sediment have been determined at 104.7 and 0.54, respectively (23). Note that the first value is for environmental BC sorption in the presence of native PAHs and OM that compete for the limited BC site capacity; purified natural BC from the KET sediment sorbed nine times stronger in the absence of PAHs and OM (23). The coal sorption parameters nF,CC and KF,CCenv can be derived as follows. For nF,CC, we took the average nF,CC for the five currently studied coals (0.75; Table 2). The average KF,CC was 105.6 (Table 2). However, we cannot directly use this value for KET sediment because also for coal we might expect competition between PHE-d10 and native PAHs and OM. The descriptor of the degree of competition for the limited number of coal sorption sites is the Freundlich coefficient nF. For BC, the nF was 0.54 (23); for the ANL coals, the average value of nF was 0.75 (Table 2). Thus, the degree of competition in the ANL coals is smaller than in the BC. An nF value of 0.54 [a sorption nonlinearity of (1 - nF) ) 0.46] corresponds to a factor 9 in sorptive attenuation (as for BC). As the attenuation of the sorption coefficient changes logarithmically with the deviation from linearity (1 - nF; transformed eq 2), the nF value of 0.75 (1 - nF ) 0.25) observed for the ANL coals will correspond to an attenuation factor of 10[log(9) × 0.25/0.46] ) 3.3 (eq 2). Thus, on the basis of the nF values of BC and ANL coals, it could be calculated that environmental competition effects will attenuate KF,CC by a factor of about 3.3. The environmental KF,CCenv can therefore be estimated to be 105.6/3.3 ) 105.1. In Figure 3, the relative importance of BC, CC, and “other” OC in KET sediment is shown. At very low concentrations in the picogram to nanogram per liter range, BC was the most important sorbent for PHE-d10, while all three sorbent compartments are important in the nanogram per liter range. VOL. 39, NO. 3, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Literature Cited

FIGURE 3. Estimated importance of BC (0.7% of total mass), CC (1.6% of total mass), and “other” OC (3.2% of total mass) for sorption of PHE(-d10) in KET sediment as a function of freely dissolved aqueous concentration CW. Estimation on the basis of eq 1; the sorption parameters of environmental BC sorption (fBC, KF,BCenv, and nF,BC) and the fraction of unburned coal (fCC) were constrained in earlier papers (23, 24); the coal sorption parameters are from Table 2. Note that this graph addresses one particular contaminated situation. At high concentrations in the microgram per liter range and above, nonlinear OC sorption started to overwhelm BC and CC sorption because of saturation of the limited number of BC and CC sorption sites. The total native PAH concentration in the KET sediment porewater was around 0.1 µg/L (23). Hence, in that particular contaminated environmental situation, CC, OC, and BC each significantly contribute to sorption. In this approach, it was assumed that the sorption to the ANL coals is dominated by the inertinites, although these coals consist of only 8-11% of inertinites (28). We believe this is a reasonable assumption because inert and condensed materials (inertinites) probably sorb so much stronger than the other materials that they dominate total coal sorption. Moreover, a sensitivity analysis showed that the fractions of PHE in OC, BC, or CC do not change much upon a changing fCC. If fCC were, for example, as much as 50% higher than assumed in Figure 3 because vitrinites should be included in fCC (i.e., 2.4% instead of the assumed 1.6%, resulting in an fOC of 3.2-0.8% ) 2.4%), the contribution of CC would increase by only 5-10% over the whole concentration range. The contribution of OC would decrease to a similar extent, whereas the BC contribution would remain almost unchanged (less than 1% change). Therefore we argue that the assumption that the sediment CC content is equal to its inertinite content does not change the interpretation of the trends represented in Figure 3. In the present paper, we have not discussed the role of kerogen in strong and nonlinear sorption. Whereas coal consists of detritus from terrestrial plants, kerogen is mainly formed by diagenetic alteration of marine phytoplankton and bacteria. Strong and nonlinear sorption has also been observed for kerogens (4, 19). We therefore expect that kerogen may play a similar role in sorption to kerogencontaining soils and sediments as coal in our currently considered KET sediment (Figure 3). We stress that the quantification of BC and CC contributions to total sorption (Figure 3) is still tentative. However, the present approach will aid in the comprehensive and explicit inclusion of strong coal and BC sorption in toxicant fate modeling, which will improve our ability to estimate contaminant risks and transport.

Acknowledgments This study was funded by the European Union (Project ABACUS, Contract EVK1-2001-00094). O ¨ .G. acknowledges a Senior Researcher Fellowship from the Swedish Research Council (629-2002-2309). 768

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Received for review May 5, 2004. Revised manuscript received October 28, 2004. Accepted November 1, 2004. ES049320Z

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