Research Analytical Characterization of Contaminated Soils from Former Manufactured Gas Plants F R A N K H A E S E L E R , †,‡ D E N I S B L A N C H E T , † VINCENT DRUELLE,§ P E T E R W E R N E R , ‡,| A N D J E A N - P A U L V A N D E C A S T E E L E * ,† Institut Franc¸ ais du Pe´trole, Division Chimie et Physicochimie Applique´es, 92852 Rueil Malmaison, Cedex, France, DVGW-Technologiezentrum Wasser, Karlsruherstrasse 84, 76139 Karlsruhe, Germany, Gaz De France, Direction de la Recherche, CERSTA, 361 Avenue du Pre´sident Wilson, 93211 La Plaine-Saint-Denis, Cedex, France, and Institut fu ¨r Abfallwirtschaft und Altlasten, Technische Universita¨t Dresden, 01062 Dresden, Germany
Detailed analytical characterization of the organic matter (OM) of aged polluted soils from five former manufactured gas plants (MGP) and of two coal tars was completed. It was aimed at obtaining information relevant to the physicochemical state of the polycyclic aromatic hydrocarbon (PAH) pollutants and to their in-situ evolution in time. Overall characterization of total OM (essentially polluting OM) was carried out directly on soil samples with or without prior extraction with solvent. It involved a technique of pyrolysis/oxidation coupled to flame ionization/thermal conductivity detection (Rock Eval). Extracts in solvent were fractionated by liquid chromatography into saturated hydrocarbons, PAH, and resins, the first two fractions being further characterized by gas chromatography and mass spectrometry. The compositions of OM of soils were found to be very similar. A total of 28% of organic carbon, including all PAH, was extractable by solvent. The compositions of coal tars were qualitatively similar to those of OM of MGP soils but with a higher proportion (48%) of total extractable OM and of PAH, in particular lower PAH. Contamination of MGP soils appeared essentially as coal tar having undergone natural attenuation. The constant association of PAH with heavy OM in MGP soils is important with respect to the mobility and bioaccessibility of these pollutants.
Introduction Polycyclic aromatic hydrocarbons (PAH) are compounds that originate from many pyrolysis processes and from petroleum. They are widespread environmental pollutants that cause concern because of their genotoxic properties (1). The environmental importance of these hydrocarbons is highlighted by the presence of 16 unsubstituted PAH in the priority * Corresponding author e-mail:
[email protected]; telephone: 33 1 47 52 64 85; fax: 33 1 47 52 70 01. † Institut Franc ¸ ais du Pe´trole. ‡ DVGW-Technologiezentrum Wasser. § Gaz De France, Direction de la Recherche. | Institut fu ¨ r Abfallwirtschaft und Altlasten. 10.1021/es9805829 CCC: $18.00 Published on Web 01/29/1999
1999 American Chemical Society
list defined by the U.S. Environmental Protection Agency (EPA). In coal pyrolysis sites such as the former manufactured gas plants (MGP) or coke production plants, coal tar is an acknowledged major source of PAH (2). The presence of coal tar particles however is not easily observable in all PAHcontaminated soil samples, and some authors have suggested that a portion of contaminating PAH could be adsorbed on soil (3). Furthermore, published studies related to the remediation of such soils focused on the PAH-containing fraction that could be extracted by solvents (2-5). In the present study, we first aimed at an overall characterization of the organic matter (OM) of aged polluted soils of former MGP, involving both the fraction extractable by solvents and the nonextractable fraction. Characterization of the nonsolvent-extractable OM involved the use of a pyrolysis/oxidation technique (Rock Eval) performed directly on soil samples. We then determined with the same techniques the composition of coal tar for comparison with the composition of OM present in contaminated soils. The final goal was to obtain information relevant to the physicochemical state of the pollutants (association of PAH with coal tar), to the diversity of their composition among sites, and to their in-situ evolution in time (natural attenuation).
Materials and Methods Composition and Extraction of Soils and Coal Tars. The five soil samples used originated from five different former MGP sites and presented various PAH concentration levels. The soil composition (gravel, sand, and silt-clay) were respectively (in percent) for soil SA, 22/66/12; for soil VC, 18/60/22; for soil LH, 28/56/16; for soil LC, 32/41/27; and for soil G, 35/40/25. Prior to analysis, each soil sample was very thoroughly homogenized by mixing before and after sieving through a 2-mm sieve. The water content was determined on portions of homogenized samples after 24 h drying at 105 °C. The extraction of OM was carried out on 10-50 g of homogenized soil (depending on pollutant concentration) with 100 mL of a cyclohexane-acetone (85/15, v/v) mixture. Prior to extraction, the samples were dried with Na2SO4. Extraction was carried out successively three times on a rotary shaker at 60 rpm during 2 h. After each extraction, the soil was allowed to settle for 10 h before pipetting most of the supernatant solvent. The extracts were stored at 4 °C in amber glass flasks closed with Teflon screw caps until analysis. Analyses were performed on extracts of each step. The extraction yield of each step (typically 95%, 5%, 0%) was calculated taking into account the remaining solvent in the extracted soil (determined gravimetrically). The coal tars were sampled in tanks found on former MGP sites. Elemental composition was performed according to ASTM method D5291 for carbon, hydrogen, and nitrogen; ASTM method D5622 for oxygen; and ASTM method D1552 for sulfur. For extraction, 2 or 3 g of coal tar was accurately weighed and homogenized by successive additions of small volumes (0.2 mL) of cyclohexane-acetone and dichloromethane. The fraction of coal tar that was insoluble in solvent was removed by filtration on a glass fiber filter (GFF, Whatman, Maidstone, U.K.). To quantify the extracted OM, portions of soil and of coal tar extracts were weighed after gentle solvent evaporation. For the samples containing volatile compounds (essentially coal tars), the retained mass was when two successive weighings (12 h intervals) presented a loss lower than 0.3 mg. VOL. 33, NO. 6, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Fractionation of Extracts. For fractionation into saturated hydrocarbons, aromatics, and resins (SAR), the cyclohexaneacetone extract was washed three times with distilled water to eliminate acetone. Some OM that was not soluble in cyclohexane precipitated during this operation. The precipitate was removed, dried, and gravimetrically quantified and is referred to as “other extractable OM”. The different hydrocarbon families in the cyclohexane extract were then fractionated by liquid chromatography on a silica gel minicolumn using a technique adapted from ASTM method D2007 and based on step increases of eluent polarity. The chromatographic system was made up of a glass Pasteur pipet containing 1 g of activated (4 h at 400 °C) silica gel Si 60, 70-230 mesh (Merck, Darmstadt, Germany) topped with a solvent glass reservoir. The gel was conditioned with dichloromethane and cyclohexane and loaded with the extract. The saturated hydrocarbons were eluted first with cyclohexane and then with cyclohexane-dichloromethane (4/1, v/v). The elution solvent was evaporated, and the fraction was dissolved in dichloromethane. The aromatic hydrocarbons were eluted by cyclohexane-dichloromethane (3/2, v/v). The saturated and aromatic fractions were analyzed by gas chromatography (GC). The polar compounds, thereafter called resins, were eluted with dichloromethanemethanol (1/1, v/v), then with methanol, and finally with ethyl acetate. This fraction was gravimetrically determined after complete solvent evaporation. Gas Chromatography. GC analyses of the saturated and aromatic fractions were carried out with a 3400 chromatograph (from Varian Instruments, San Fernando, CA) equipped with an autosampler (Varian 8200), an on-column injector, and a flame ionization detector (FID) also from Varian. The hydrocarbons were separated on a 60-m glass capillary column DB 5 (J&W, Folsom, CA) with a 0.25 mm internal diameter and coated with a 0.1 µm thick methyl-phenylsilicone film. Operating conditions were as follows: injection volume, 1 µL; detector temperature, 300 °C; injector temperature program, 30 °C hold for 0.5 min, 30-300 °C rise at 150 °C/min, 300 °C hold for 100 min; column temperature program, 50 °C hold for 5 min, 50-300 °C rise at 2 °C/min, 300 °C hold for 20 min. Helium was used as a carrier gas at a flow rate of 0.9 mL/min. For each PAH, the detection limit was 0.5 ng/µL solvent. The data were acquired on an informatic system (HP 1000, Hewlett-Packard) and integrated by the software LAS (Hewlett-Packard, Sunnyvale, CA). The internal standard (undecane) was used only for checking the quality of injection. The chromatographic signal was integrated after subtraction of the baseline obtained by periodical injections of solvent. The calibration was done for each EPA PAH by external calibration, using a certified mixture at concentrations between 1 and 100 ng/µL. All PAH had nearly the same response factor, and their average value was used for quantifying the unknown peaks. The quality of the whole chromatographic operation was checked periodically by injecting known standards and solvent. The sum of all the peaks was used in order to express the concentrations of total saturated and total aromatic hydrocarbons. Gas Chromatography-Mass Spectrometry (GC-MS). A high-resolution double-focusing mass spectrometer was used (Autospec, Micromass, Manchester, U.K.). Conditions were as follows: 70 eV electron ionization, resolution of 1000, and scan speed of 0.5 s/decade. The source temperature was 270 °C, and the mass range covered was 26-650 amu. The identification of compounds was performed according to spectrum libraries (NIST and Wiley). Identification criteria were retention time, reference compounds, and satisfactory spectrum match when compared to library spectra. Rock Eval Analyses. A Rock Eval III apparatus (Vinci Technologies, Rueil Malmaison, France) was used to characterize total soil and coal tar OM. This technique was initially 826
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developed to provide geochemical information on the nature and state of maturity of the OM content (kerogen) of rocks in relation to their potential for petroleum formation (6) and has been recently used for the characterization of soils contaminated with petroleum products (7). It involves two successive analysis cycles: pyrolysis and oxidation performed on the same sample. The pyrolysis cycle coupled to FID was performed in a temperature-programmed oven (200 °C hold for 5 min; 200-600 °C rise at 10 °C/min) that was flushed with nitrogen. During the temperature rise, different characteristic peaks were observed: S1 corresponding to volatile distillable hydrocarbons (T e 200 °C), S2a corresponding to other distillable hydrocarbons (200 °C e T e 350 °C), and S2b corresponding to nondistillable hydrocarbons subjected to thermal cracking (350 °C e T e 600 °C). The oxidation cycle (7 min at 600 °C in an oven flushed with oxygen) coupled to thermal conductivity detection yielded another peak called S4. During this step, the residual nondistillable organic carbon (coked carbon) was oxidized to CO2. The total organic carbon (TOC) was defined as the sum of peaks S1, S2, and S4. The calibration for FID response (pyrolysis step) was performed with petroleum hydrocarbons containing 83% carbon, and the response factor used was corrected in order to take into account the carbon content of PAH (95%). The CO2 response given by the thermal conductivity detector (oxidation step) was calibrated by injecting known amounts of CO2. For the five soil samples, Rock Eval analyses were performed using 130-200 mg of air-dried soil on nonextracted portions and on portions extracted three times with solvent. Analyses were also performed on coal tars, humic substances, and vegetal charcoal. Chemicals. The calibration standard containing the 16 EPA PAH for GC-FID calibration was the TCL Polynuclear Aromatic Hydrocarbons Mix (ref 4-8905) from Supelco (Bellefonte, PA). The humic substances (IHSS Soil Humic Acid Reference 1R102H, IHSS Peat Humic Acid Reference 1R103H, IHSS Leonardite Humic Acid Reference 1R104H and IHSS Summit Hill Humic Acid Reference 1R106H) were provided by the International Humic Substances Society (St. Paul, MN).
Results and Discussion Five soils of different former MGP were characterized. The studies concerned the total OM of solvent extracted and nonextracted soils as well as soil extracts. The composition of two coal tars was studied using the same techniques. Characterization of Total Organic Matter of Soils. Total OM (essentially polluting OM) of soils was studied by Rock Eval analysis. As shown in Table 1, very different TOC values (ranging from 18.2 to 101.4 g/kg dry soil) were observed in the samples studied. The TOC value given by the Rock Eval technique represented the whole organic carbon content of the analyzed sample. The OM of five samples of different MGP sites was characterized by the presence of a distillable fraction (S1 and S2a peaks) associated to a nondistillable one (S2b and S4 peaks). In the same conditions, the humic substances and vegetal charcoal samples tested were not detected in pyrolysis fractions. The Rock Eval data for solvent extracted soils showed that most of the distillable fraction was removed. The nondistillable fractions were the main constituents of the nonextractable OM, although they were removed in part by extraction. In all samples, a residual S1 peak was observed after extraction. We assumed that this peak corresponded to solvent trapped in extracted soil. Chromatographic Characterization of Soil Extracts. Fractionation of the cyclohexane-acetone extracts was performed by liquid chromatography. The results presented in Table 2 show a similar distribution (expressed in percent of extracted OM) of the different fractions for all soil
TABLE 1. Distribution of Organic Matter in Extracted and Nonextracted Contaminated Soils from Former MGP As Measured by Rock Eval Analysisa organic matter content (g of C/kg dry soil) in S1
S2a
S2b
S4
TOCb
14.2 (1.4) 1.4 (0.4) 3.4 (0.9) 0.3 (0.3) 0.7 (0.3) 0.5 (0.1) 0.2 (0.2) 0.3 (0.2) 0.3 (0.2) 0.1 (0.0)
23.1 (1.7) 7.2 (1.8) 10.2 (2.8) 2.5 (0.5) 2.2 (0.6) 0.4 (0.0) 0.9 (0.8) 0.7 (0.1) 0.9 (0.3) 0.3 (0.1)
8.1 (1.8) 5.5 (1.3) 8.3 (1.9) 3.3 (0.4) 2.2 (0.6) 1.1 (0.2) 2.8 (0.8) 1.5 (0.4) 1.5 (1.0) 0.6 (0.2)
56.0 (6.7) 47.7 (3.5) 44.4 (4.4) 38.0 (3.7) 25.1 (6.2) 21.3 (0.6) 24.2 (2.5) 20.1 (3.1) 15.5 (2.9) 8.7 (1.2)
101.4 61.9 66.3 44.1 30.1 23.2 28.2 22.6 18.2 9.8
soil SA VC LH LC G a
nonextracted solvent extracted nonextracted solvent extracted nonextracted solvent extracted nonextracted solvent extracted nonextracted solvent extracted
Standard deviation of four analyses in parentheses. bTotal organic carbon ) S1 + S2a + S2b + S4.
TABLE 2. Content of Saturated Hydrocarbons, Aromatic Hydrocarbons, and Resins in Soil Extracts from Former MGP concentrationa (g/kg dry soil)
extracted OM
soil
saturatedb
aromaticsb
resinsc
sum SARd
in solventc
Rock Eval datae
SA VC LH LC G
0.2 (0.0) 0.6 (0.0) 0.0 (0.0) 0.0 (0.1) 0.3 (0.0)
14.4 (0.3) 5.7 (0.4) 2.9 (0.4) 1.9 (0.2) 2.7 (0.1)
13.7 (0.8) 11.8 (2.0) 5.5 (0.9) 2.9 (0.7) 2.8 (0.2)
28.6 18.5 8.8 5.2 5.9
32.6 20.0 7.6 6.5 5.7
39.5 22.1 6.9 5.6 8.4
a Mean of three independent analyses. Standard deviation of three analyses in parentheses. b Determination by GC-FID. c Gravimetric determination. d Sum SAR ) saturated hydrocarbons + aromatics + resins. e Rock Eval data: TOCnonsolvent extracted soil - TOCsolvent extracted soil.
samples: there always were traces of saturated hydrocarbons while the aromatic fraction represented 35-50%. The resins represented about 50-65% of the total. Similar results have been reported in the literature (4, 8). The extracted OM determined in solvent was in good accordance with the extracted OM evaluated by the Rock Eval technique (difference in TOC values of solvent extracted and nonextracted soils). Thus, the different analytical determinations used in this study for the characterization of OM of polluted soils from former MGP sites were in good concordance. This allows to conclude that peaks S1 and S2a, which are the Rock Eval fractions mainly extracted by solvent (Table 1), are mostly constituted of aromatic hydrocarbons and resins. The saturated and aromatic fractions of soil extracts separated by liquid chromatography were studied by GCFID analysis. GC chromatograms of the aromatic fractions showed about 300 different peaks. A GC-MS analysis led to the identification of 84 compounds. The identified compounds of the aromatic fraction were essentially unsubstituted PAH, methyl-, dimethyl-, ethyl-, and phenyl-substituted PAH, and also several S- and O-containing heterocyclic PAH. All these identified compounds corresponded to structures previously described by authors working on the identification of PAH in solvent extracts of coal tars and of polluted soil (8-11). GC-MS analysis of the saturated fraction was also performed. The chromatograms presented many unresolved peaks, centered around C20, that were typically isoalkanes. They also showed four (C14, C16, C17, and C18) main isoalkanes. The C17 and the C18 compounds were respectively identified as pristane and phytane. Significant concentrations of linear alkanes ranging from C15 to C27 were also observed. These major compounds could not originate from pyrolysis and consequently must have been present in coal before gas manufacturing. Novotny et al. (12) also reported the presence of saturated hydrocarbons in a fractionated coal tar extract. The concentrations of PAH in the aromatic fraction of soil extracts are presented in Table 3. The analyzed samples
showed a similar PAH distribution centered on the 4-, 5-, and 6-cycle PAH. Only soil SA did present a higher concentration in 2- and 3-cycle PAH. An interesting point is the proportion of the 16 EPA PAH with respect to other polyaromatics. Clearly, EPA PAH were individually the most abundant compounds, representing from 22 to 43% of the total aromatic hydrocarbons and justifying their position as target compounds to be analyzed in coal tar pollutions (8, 10). However, it appears advisable to also take into account the other PAH that constitute the major part of total PAH. The results of the different characterization techniques used in this study (Rock Eval and SAR fractionation) are synthesized in Figure 1 for the five soils studied. The relative contributions of EPA PAH, other PAH, resins, and other extractable OM to TOC are presented. It appears that the studied soils showed a high degree of similarity in the overall distribution of the polluting OM regardless of their different origins and concentration levels. The whole results also showed that solvent extraction was quite efficient (strong reduction of S1 and S2a peaks after solvent extraction). Furthermore, we observed no consistent differences between various extraction methods, namely, cold extraction, Soxhlet, microwave, and extraction under pressure (140 bar) at 120 °C (data not shown). These observations are in accordance with reports of other authors (13-16). Thus solvent extraction was not hindered by the presence of heavy OM. It appears that PAH extraction from MGP soils has to be considered mainly as a partitioning between a solvent and a dense nonaqueous phase liquid, which is very favorable to the solvent. In these conditions, the reproducibility of analyses depended essentially on the degree of homogenization of the soil sample. PAH pollution in MGP sites thus appears very different from that of ubiquitous PAH released in the atmosphere by various pyrolysis sources (17) and which lead after deposition to PAH sorbed or trapped on humic substances by still poorly understood mechanisms. Solvent extraction of the latter PAH was reported to be difficult (18-20), in particular in older samples. VOL. 33, NO. 6, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 3. PAH Distribution in Contaminated Soils from Former MGP concentrationa (mg/kg dry soil) SA naphthalene sum of 2 cycles
477 (7) 477 (7)
acenaphthylene acenaphthene fluorene phenanthrene anthracene sum of 3 cycles
172 (4) 35 (6) 167 (11) 680 (6) 216 (5) 1,270 (20)
fluoranthene pyrene benz[a]anthracene chrysene sum of 4 cycles
1,036 (4) 826 (5) 432 (16) 421 (19) 2,715 (36)
benzo[b]fluoranthene Belnzo[k]fluoranthene benzo[a]pyrene indeno[c,d]pyrene dibenz[a,h]anthracene benzo[g,h,i]perylene sum of 5 and 6 cycles
511 (62) 256 (62) 426 (20) 277 (27) 91 (18) 254 (24) 1 815 (45)
sum of 16 EPA PAH total aromatic hydrocarbons
6 277 (90) 14 650 (261)
VC 8b
3 (0) 3 (0)
20b
7 (0) 2 (1) 6 (2) 66 (3) 30 (1) 111 (5)
43b
210 (12) 167 (12) 152 (27) 132 (15) 661 (57)
29b
164 (44) 112 (19) 122 (7) 82 (8) 49 (12) 73 (5) 602 (49)
43c
1 377 (106) 5 706 (340)
LH 0b
10 (0) 10 (0)
8b
8 (0) 3 (0) 16 (0) 89 (1) 53 (0) 169 (1)
48b
201 (7) 147 (6) 86 (6) 86 (7) 520 (25)
44b
86 (6) 81 (5) 97 (8) 66 (10) 13 (3) 55 (7) 398 (39)
24c
1 097 (66) 2 630 (257)
LC 1b
8 (1) 8 (1)
15b
1 (0) 6 (1) 9 (1) 42 (4) 47 (1) 105 (7)
47b
72 (6) 57 (5) 47 (4) 55 (4) 231 (18)
36b
48 (1) 47 (4) 51 (1) 40 (1) 16 (1) 35 (1) 237 (9)
42c
a Mean of three independent analyses; standard deviation of three analyses in parentheses. the total aromatic hydrocarbons.
b
G 1b
7 (2) 7 (2)
1b
18b
6 (3) 6 (4) 10 (3) 24 (3) 76 (7) 122 (18)
24b
40b
46 (2) 46 (3) 32 (3) 41 (2) 165 (6)
33b
41b
51 (12) 40 (17) 42 (7) 29 (1) 17 (7) 34 (2) 213 (40)
42b
28c
581 (35) 2 061 (154)
507 (35) 2 335 (571)
22c
In percent of the 16 EPA PAHs. c In percent of
TABLE 4. Distribution of Organic Matter in Coal Tars As Measured by Rock Eval Analysis concentrationa (g of C/kg of coal tar) Rock Eval fractions
coal tar G
coal tar N
S1 S2a S2b S4 TOC
162.2 (10)b 122.0 (15)b 0.0 (0)b 345.4 (25)b 629.7 (46)b
373.7 (1)b 158.4 (4)b 1.8 (1)b 344.0 (23)b 877.9 (26)b
a Mean of four independent analyses. analyses.
FIGURE 1. Relative contribution of OM constituents of MGP soils. Expressed in percent of TOC. Characterization of Coal Tars. The same analytical approach used for the characterization of polluted MGP soils was also applied to two coal tars found in closed tanks of disused MGP sites. As shown in Table 4, the two coal tars presented a large distillable fraction, characterized by the presence of S1 and S2a peaks. Furthermore, both coal tars had an important (34%) nondistillable fraction consisting only of peak S4 (absence of S2b). The TOC values given by the Rock Eval technique and expressed in mass percentages were respectively 62.9 and 87.9% for coal tars G and N. These results were in acceptable accordance with the elemental composition of the coal tars (for coal tar G: C ) 58.2%, H ) 7.1%, O ) 28.9%, N ) 0.8%, and S ) 0.4%; for coal tar N: C ) 78.6%, H ) 5.3%, O ) 12.8%, N ) 1.2%, and S ) 0.5%). As shown by the elemental composition, the coal tars were not only composed of hydrocarbons but contained significant and variable amounts of other elements (30% oxygen in coal 828
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b
Standard deviation of four
tar G and 13% in coal tar N). The presence of various proportions of oxygen in coal tars has been already reported (21, 22). An important part (about 50%) of both coal tars was insoluble in the solvents used (cyclohexane-acetone and dichloromethane). The insoluble fraction was mainly composed of nondistillable compounds as shown by Rock Eval analysis (peak S4 constituted more than 90% of this fraction). A comparison between Rock Eval determination (sum of S1 and S2a) and gravimetric data of solvent extracts for coal tar G showed that the dichloromethane and the cyclohexaneacetone extracts were equivalent as this tar is well extracted by both solvents (301 and 322 g/kg of coal tar, respectively). Furthermore, these values were in good accordance with the Rock Eval data (296 g/kg of coal tar). The same comparison for coal tar N showed that cyclohexane-acetone could only solubilize 388 g/kg while dichloromethane solubilized 547 g/kg, a value in much better accordance with the Rock Eval data of 554 g/kg. Variable solubilities of coal tar in different solvents are reported in the literature (10, 21), this property being used for coal tar characterization. Our data showed that for coal tar N the distillable fraction (as determined by Rock Eval analyses) corresponded to the dichloromethane extractable fraction. In any case, for each coal tar, the PAH concentrations (including the non-EPA PAH) analyzed by
TABLE 5. SAR Fractionation and PAH Distribution in Coal Tars in Cyclohexane-Acetone (85/15, v/v) Extracts of Coal Tars concentrationa (g/kg of coal tar) coal tar G
c
coal tar N
total saturated HC total aromatic HC resins
1.2 226.0 33.2
0.0 270.0 71.7
naphthalene sum of 3 cycles sum of 4 cycles sum of 5 and 6 cycles sum of 16 EPA PAH
37.1 (35)b 33.7 (31)b 24.0 (22)b 12.8 (12)b 107.5 (48)c
56.2 (34)b 49.2 (30)b 39.6 (24)b 19.1 (12)b 164.1 (61)c
a Mean of two independent analyses. b In percent of the 16 EPA PAHs. In percent of the total aromatic hydrocarbons.
identification of 140 compounds. In addition to the compounds identified in soil extracts, we observed other lower hydrocarbons (mono- and diaromatics) and N-containing heterocyclic PAH. Comparison of the Composition of Coal Tars and Contaminated MGP Soils. A synthesis of the results of the different characterization techniques used in this study (Rock Eval, SAR fractionation, and GC-FID) is presented in Figure 2 for coal tars and for former MGP soils. It appears that the distribution of OM was similar for coal tars and polluted soils from former MGP. Nonextractable OM was present in both cases although in higher amounts in MGP soils. Nevertheless, coal tars presented lower proportions of nonextractable OM and higher proportions of hydrocarbons. The nature of the PAH was similar, but coal tars contained higher proportions of hydrocarbons with 1-, 2-, and 3-cycles. This shows a clear relationship between coal tars and soil pollutants from former MGP. The characterization of total OM thus shows that the pollution of former MGP originated from coal tars. The PAH were systematically associated to a heavy nonextractable polluting OM fraction resistant to solvent extraction, and we found no evidence of PAH presence in another form. The coal tar pollution of a MGP site appeared to have undergone an evolution in time. The differences in composition observed are in line with natural attenuation of polluting OM resulting in particular from evaporation, leaching (23), and biodegradation (24) of the constituents. This process of natural attenuation, which may differ according to site conditions, should be considered in risk assessment of MGP sites. Adsorption of PAH in soil particles has been suggested to be the cause of the limited biodegradability of these pollutants in MGP soils (5, 25). Because of its obvious implications on the mobility and biodegradability of PAH, the systematic association of PAH with a large and fairly constant proportion of heavy nonextractable OM, demonstrated here, appears as one of the most important features of MGP soils. The analytical tools used in this study allow a more complete characterization of polluting OM than previously published methods and can be fruitfully applied to other types of pollutions such as coking plants or sites polluted by creosote or diesel oil.
Acknowledgments We thank IFP colleagues Ve´ronique Bardin for participation to PAH analyses, Jean Ducreux for advice on SAR fractionation, Anne Fafet for GC-MS identification, and Eric Lafargue and Jean Espitalie´ for useful discussions on Rock Eval characterization.
Literature Cited FIGURE 2. Compared distribution of OM in coal tars (4a) and polluted MGP soils (4b). Expressed in percent of TOC. GC-FID in both extraction solvents dichloromethane and cyclohexane-acetone were very similar (data not shown). These results indicate that the fraction resistant to cyclohexane-acetone solubilization was not the hydrocarbon fraction but other compounds, probably polar compounds, which were more soluble in dichloromethane. In fact, as shown in Table 5, both coal tars had a very similar PAH distribution when considering the relative concentrations of the 16 EPA PAH or of the total aromatic hydrocarbons. The nature of the pollutants was the same in coal tars and in polluted soils, but their distribution was quite different with coal tars being richer in 2- and 3-cycle PAH. GC-MS analysis of the aromatic fraction of coal tar G led to the
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Received for review April 23, 1998. Revised manuscript received November 30, 1998. Accepted December 14, 1998. ES9805829
