Environmental Pollution 148 (2007) 529e538www.elsevier.com/locate/envpol

Effect of condensed organic matter on solvent extraction andaqueous leaching of polycyclic aromatic hydrocarbons

in soils and sediments

Yong Ran a,*, Ke Sun a, Xiaoxuan Ma a, Guohui Wang b,Peter Grathwohl b, Eddy Y. Zeng a

a State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences,Gunagzhou 510640, P.R. China

b Center of Applied Geosciences, University of Tubingen, Tubingen, Germany

Received 26 June 2006; accepted 26 November 2006

Kerogen carbon and aged organic matter is important for the extraction and distributionof native PAHs in the soils and sediments.

Abstract

The contents of nonhydrolyzable organic matter (NHC) and black carbon (BC) were measured in soils and sediments from the Pearl RiverDelta, South China. Polycyclic aromatic hydrocarbons (PAHs) were extracted respectively by Soxhlet and an accelerated solvent extractiondevice (ASE) using different solvents. In addition, sequential aqueous leaching at different temperatures was carried out. The PAH contentextracted with the sequential three solvent ASE is two times higher than that using the Soxhlet extraction method. The relationship of thePAH content with the NHC content is very significant. The PAH concentrations measured at various temperature steps fit well to the Van’tHoff equation and the enthalpy was estimated. The investigation indicates that condensed organic matter such as kerogen carbon, aged organicmatter, and BC is relevant for the extraction and distribution of native PAHs in the investigated field soils and sediments.� 2006 Published by Elsevier Ltd.

Keywords: Condensed organic matter; Polycyclic aromatic hydrocarbons; Accelerated solvent extraction; Aqueous leaching; Soils; Sediments

1. Introduction

Sorption and desorption of organic contaminants playa major role on their distribution, transport, bioavailability,degradation, etc. in the environment. Therefore, interactionmechanisms of such compounds with soil and sedimentorganic matter (SOM) have been one of the environmentalresearch focuses. As SOM in soils and sediments differs inpolarity, elemental composition, aromaticity, condensation,and degree of diagenetic evolution from a loose polymer to

* Corresponding author. Tel.: þ86 20 8529 0263; fax: þ86 20 8529 0706.

E-mail address: yran@gig.ac.cn (Y. Ran).

0269-7491/$ - see front matter � 2006 Published by Elsevier Ltd.

doi:10.1016/j.envpol.2006.11.028

condensed structures, it is not unexpected that heterogeneityof SOM dominates different sorptive properties for HOCs(hydrophobic organic compounds) (Grathwohl, 1990; Brus-seau and Rao, 1989; Kleineidam et al., 1999; McGinleyet al., 1993; Pignatello and Xing, 1996; Karapanagioti et al.,2000; Accardi-Dey and Gschwend, 2002; Allen-King et al.,2002; Ran et al., 2002; Cornelissen et al., 2005). Structurallyand/or chemically different SOM constituents in soils and sed-iments interact differently with HOCs in terms of bindingenergies and rates of associated sorption and desorption.SOM has been characterized as comprising dual domains orcomponents that exhibit distinctly different sorption reactiv-ities (McGinley et al., 1993; Young and Weber, 1995; Pigna-tello and Xing, 1996; Huang and Weber, 1997; Xing and

530 Y. Ran et al. / Environmental Pollution 148 (2007) 529e538

Pignatello, 1997). Extensive and nonlinear sorption wasobserved for HOCs in several inert, condensed, and aromaticmaterials that are physically and/or chemically distinguishableand identifiable, such as various types of kerogen, coals andblack carbon (Grathwohl, 1990; Kleineidam et al., 1999;McGinley et al., 1993; Huang and Weber, 1997; Ghoshet al., 2000, 2003; Karapanagioti et al., 2000; Allen-Kinget al., 2002; Jonker and Koelmans, 2002; Ran et al., 2002,2003, 2004; Yang et al., 2004; Cornelissen et al., 2005). Iden-tification and quantification of these types of condensed SOMassociated with soils and sediments is key to interpretation ofdramatically different observations on HOCs sorption.

Various physical and chemical methods have been used toidentify black carbon (BC) and kerogen carbon (KC). Exten-sive evaluations of methods for BC quantification in soilsand sediments are available (Gustafsson et al., 1997; Middel-burg et al., 1999; Gelinas et al., 2001a; Schmidt et al., 2001;Song et al., 2002; Chun et al., 2004; Nguyen et al., 2004).However, the investigation on the quantification of KC in soilsand sediments is quite limited, which is mainly restricted toorganic-petrographic analyses, chemical treatment, and NMRspectroscopy (Kleineidam et al., 1999; Karapanagioti et al.,2000; Gelinas et al., 2001a; Ran et al., 2002, 2003; Songet al., 2002). KC is organic carbon disseminated in sedimentsnot soluble in acid, base and organic solvents (note that coalparticles in sediments now qualify as kerogen). In this paper,nonhydrolyzable residue remained after the treatment withtrifluoroacetic acid (TFA) and HCl is considered to approxi-mate, but not equal, the molecularly uncharacterized organiccompound (OC) that constitutes a significant component ofresistant organic matter preserved in sediments (Hedges et al.,2000; Komada et al., 2005). So, in our fractionation scheme,nonhydrolyzable organic matter (NHC) deducted from BC isoperationally similar to kerogen.

Kerogen is most abundant organic matter in the Earth’ssedimentary rock formations (Tissot and Welte, 1984).Although most kerogen is deeply buried in reservoirs (Tissotand Welte, 1984), it can still be present in significantamounts in soils and sediments. For example, Ran et al.(2003) found that Borden aquifer SOM (OC content0.02%) contains kerogen carbon. Song et al. (2002) reported24e48% kerogen in one soil and three sediments. Unfortu-nately, the KC quantification method may suffer from opera-tional shortcomings, such as fine SOM particle loss andincomplete amorphous organic carbon (AOC) removal(Song et al., 2002; Ran et al., 2003). Recently, new separa-tion and characterization methods have been developed todifferentiate the fractions of SOM. Soils and sedimentshave been treated with TFA and HCl to remove easily hydro-lyzable organic matter (Gelinas et al., 2001a), which includesyoung organic matter (hydrolyzable sugars and amino acids,etc.) (Komada et al., 2005) and is operationally defined asAOC in this paper. The 13C NMR spectrum of the TFA-treated SOM was similar to that of kerogen, and of fastrelaxation fraction of SOM distinguished by proton spinrelaxation editing techniques (Gelinas et al., 2001b). Hence,this kind of separation method and technique may be

applicable to distinguishing the contribution of differentSOM fractions to distribution and fate of HOCs in soilsand sediments.

This study investigates the effect of condensed SOM (i.e.NHC) such as kerogen and black carbon in soils and recentsediments on aqueous leaching and solvent extraction of poly-cyclic aromatic hydrocarbons (PAHs). We hypothesize thatbesides black carbon, nonhydrolyzable organic carbon suchas kerogen and aged organic matters in soils and sedimentsoriginating from alluvial parent materials is also importantfor the distribution and fate of PAHs. We compare the sequen-tial or single solvent extraction of PAHs by accelerated solventextraction device (ASE) using different solvents with that ofSoxhlet. The aqueous leaching of PAHs at different tempera-tures (25 �C to 150 �C, high pressure using ASE) was alsoinvestigated and compared with the solid-liquid distributioncoefficients of PAHs, where low temperatures and low concen-trations of PAHs (at ng/L level) were present.

2. Materials and methods

2.1. Sample collection and measurement of OC fraction

Two moderately contaminated surface (0e20 cm) soils (HP04 and HP05)

near the urban Guangzhou area and five surface sediments (0e20 cm) (C01eC05) from the Pearl River estuary were collected (for geographic location see

Fig. 1). The soil and sediment samples were freeze-dried, pulverized, and

passed through a 0.25-mm sieve. Nonhydrolyzable organic carbon and black

carbon contents were measured according to references (Gelinas et al.,

2001a) and are listed in Table 1. Briefly, the samples were treated sequentially

with hydrochloric (HCl) and hydrofluoric (HF) acid, trifluoroacetic acid (TFA)

at various concentrations and temperature, and the decarbonated samples were

also heated at 375 �C for 24 h under air. These treatments removed carbonates

and silicate minerals, easily hydrolyzable organic matter, or kerogen and hu-

min, respectively. The original and treated samples were then analyzed for

C, H, N, and O using an Elementar Vario ELIII or a Heraeus CHN-O-RAPID

elemental analyzer.

2.2. ASE extraction with solvents and water

The ASE (Dionex 300) (Idstein, Germany) is an automated system for

extracting organic compounds from a variety of solid samples. The ASE accel-

erates the traditional extraction process by using solvents at elevated

temperatures under a pressure of 100 bar. Fifteen grams of soil or sediment

samples were mixed with equal weight of quartz sand to achieve an improved

permeability. The quartz sand was pre-cleaned by ultrasonic extraction with

methanol three times. The mixture was packed into the extraction cell with

glass-fiber filters (1 mm pore diameter) at each end in order to prevent fine par-

ticles eluting into the collection bottles. The successive ASE extractions (ASE

Sum) employed acetone at 100 �C followed by two times toluene at 150 �C.

Additionally, routine ASE extraction (ASE STD) using 1:1 acetone/hexane

at 120 �C two times was performed for comparison. In both extraction

methods, the time for each extraction was 5 min as usually employed during

the ASE extraction. After extraction the extracts was flushed into a collection

bottle and the packed cell was rinsed again with 60% volume of the same

solvent and flushed into the same collection bottle, and finally purged with

gaseous nitrogen for 1 min before the next step. Aqueous leaching was per-

formed sequentially with Millipore water at different temperatures (ranging

from 27 �C to 100 �C or from 27 �C to 150 �C). The equilibrium static leach-

ing time for each temperature step was 99 min. In preliminary experiments it

was shown that equilibrium times of 30 or 99 min do not lead to different

concentrations. After leaching at each temperature step, 1 min purging with

nitrogen was carried out before the next step.

531Y. Ran et al. / Environmental Pollution 148 (2007) 529e538

Fig. 1. Sampling locations for the two soils (HP04, HP05) and five sediments in the Pearl River Delta region (C01eC05), China.

2.3. PAH analysis

A mixture of deuterated PAH surrogate standards (naphthalene-d8, ace-

naphthene-d10, phenanthrene-d10, chrysene-d12, and perylene-d12) was added

to each 15-g subsample and the spiked sample was Soxhlet-extracted for

48 h with redistilled dichloromethane (DCM). The extract was concentrated,

solvent-exchanged to hexane, and purified using a 1:2 aluminum/silica glass

column. Procedures for extraction, separation, and analysis of PAHs are

detailed elsewhere (Mai et al., 2003; Chen et al., 2005). Recoveries (avera-

ge � standard deviation; n ¼ 14) of the surrogate standards were

36.5 � 11.5 for naphthalene-d8, 68.6 � 11.1 for acenaphthene-d10, 101 � 8.5

for phenanthrene-d10, 91.8 � 9.2 for chrysene-d12, and 70.6 � 9.5% for pery-

lene-d12. Concentrations of PAHs were not surrogate recovery corrected.

For ASE extraction, the organic solvent extracts from the preceding section

were spiked with known quantities of the above five deuterated PAHs as inter-

nal standards, concentrated and solvent-exchanged to hexane, and purified

using the same procedure described above. For aqueous ASE extraction, the

aqueous extracts were also spiked with known quantities of the deuterated

Table 1

Percentages of NHC, BC, and AOC accounting for the total organic carbon

contents (OC) in the soil and sediment samples

Samples OC % NHC BC AOCa

C01 1.02 � 0.079b 35.4 15.3 64.6

C02 0.336 � 0.031 31.4 7.16 68.6

C03 0.618 � 0.020 34.3 17.3 65.7

C04 0.680 � 0.036 34.9 4.64 65.1

C05 0.536 � 0.027 25.6 4.73 74.4

HP04 1.77 � 0.140 73.8 12.5 26.2

HP05 3.35 � 0.177 64.8 11.7 35.2

a Equal to 100 � NHC.b Represents averages and standard deviations of the original organic

carbons.

PAH compounds as internal standards, extracted with cyclohexane, and

concentrated to 0.1 ml.

PAHs were quantified in the selective ion monitoring mode using a Hew-

lettePackard 6890 GC-5973 MS equipped with a 30-m DB-5MS capillary col-

umn. The column temperature was programmed from 65 �C (held for 4 min),

ramped to 270 �C at 10 �C/min (held for 10 min), and further increased to

310 �C at 10 �C/min and held for 6.5 min. The carrier gas was helium at

a constant flow rate of 1.0 ml/min. Sample of 1 ml was injected with a Hew-

lettePackard 7683 autosampler. The standard solution containing 16 PAH

components (naphthalene, acenaphthylene, acenaphthene, fluorene, phenan-

threne, anthracene, fluoranthene, pyrene, benzo[a]anthracene, chrysene,

benzo[b,k]fluoranthene, benzo[a]pyrene, indenzo[1,2,3-cd]pyrene, dibenzo[a,h]

anthracene, and benzo[g,h,i]perylene) and the five deuterated PAH surrogate

standards were quantified using the internal calibration method.

2.4. Data analysis

Distribution coefficients Kd (ml/g) (Kd ¼ CS/CW) were calculated from

contaminant concentration in the solid CS (ng/g) and the aqueous concentra-

tion CW (ng/ml) obtained in the leaching experiments with ASE at different

temperatures. With increasing temperature, the sorption equilibrium shifts

toward the aqueous phase and thus the Kd value decreases. The effect of tem-

perature on Kd can be described with the Van’t Hoff equation:

DH� ¼ �R d lnð1=KdÞ=dð1=TÞ ð1Þ

where DH�

is the desorption enthalpy (kJ/mol), R the universal gas constant

(kJ/mol. k), Kd the sorption distribution coefficient (ml/g), and T the temper-

ature (K). Since CS is almost constant during the sequential aqueous leaching

at different temperatures, a plot ln CW versus (1/RT ) can be used to calculate

the enthalpy DH�

from the slope of the linear regression.

Although the sorption isotherms of PAH congeners are nonlinear if a wide

range of aqueous concentration is considered, they approximate to be linear at

narrow range of aqueous concentrations. We define that the hydrolysable

organic carbon as AOC and nonhydrolyzable organic carbon as condensed

532 Y. Ran et al. / Environmental Pollution 148 (2007) 529e538

organic carbon. In order to quantify the contribution from AOC and NHC to

sorption capacity, the following two OC phase equation was used to estimate

their respective roles:

CS ¼ fAOCKAOCCW þ fNHCKNHCCW ð2Þ

fAOC and fNHC are AOC and NHC contents in the samples, and KAOC and KNHC

are the AOC and NHC normalized solid-liquid distribution coefficients. The

CS (mg/kg) was determined by the solvent ASE extraction (sum) and the CW

(mg/l) was from the leaching solution at 25 �C, and KOC ¼ Kd/fOC ¼ CS/Cw/

fOC. KAOC was estimated from the reported equation (log KOC ¼0.99KOW � 0.34) by Karickhoff (1981).

3. Results and discussion

3.1. Organic carbon fractions

NHC accounts for 25.6e35.4% and 64.8e73.8% of thetotal organic carbon in the estuary sediments and in the urbansoils, respectively (Table 1). The NHC percentages are close tovalues of the acid (6 M HCl) insoluble carbon (43e69%) re-ported for other coastal and marine sediments (Wang et al.,1998; Hwang et al., 2005; Komada et al., 2005). They furtherindicated that the acid insoluble carbon contained considerableancient carbons using carbon isotope techniques. These an-cient organic matters include bitumen or kerogens found insedimentary rocks that have been thermally matured, or theentrainment of terrigeneous organic matter pre-aged as a resultof long residence times within the drainage basin. The terrige-neous organic matter also contains chemically recognizablebiochemicals, such as for example lignin, albeit in a more ex-tensively degraded and altered state (Goni et al., 2005). How-ever, quantitatively discriminating between the incorporationof fossile OC and the entrainment of terrigeneous organic mat-ter from old soils or paleosols is difficult (Goni et al., 2005).

The detailed NHC characterization will be provided ina forthcoming paper. It is noted that the 13C NMR spectrumof the isolated NHC only has aliphatic and aromatic carbonpeaks with neglecting O-containing function groups. This isvery similar to that of the same fraction in the sedimentsreported by Gelinas et al. (2001b). The NHC fraction was re-ported to be rich in polymethylene carbon, and similar inabundance and chemical composition to nonprotein alkylcarbon and slow relaxation fraction of SOM distinguishedby proton spin relaxation editing techniques in the OC-richocean sediment samples (Gelinas et al., 2001b). This nonhy-drolyzable, chemically unique, and spatially separate poly-methylene component is also compositionally similar to cellwall-derived algaenans, the nonhydrolyzable algal sapropel,and kerogen from type I oil shales. The nonprotein alkylcarbon consisted of 20.4e35.8% of the OC in cases whereOC was higher than 1% in the several ocean sediments (Gel-inas et al., 2001b). The soil samples show much higher NHCcontent presumably because the samples are from the vicinityof a coal-powered electricity plant and the atmospheric coalparticles deposition can be expected. Schmidt et al. (1996)reported major influence of lignite coal emissions from a bri-quette factory on the soil organic matter. They found that thecontent of organic carbon originating from brown coal

contamination in bulk soil and size fractions contributed to51e97% of OC using 14C dating. Song et al. (2002) reportedthat the kerogen and BC fractions amounted to 57.8e80.6%of the OC contents in one soil and three sediments. However,as these fractions have obvious oxygen-containing peaks inthe 13C NMR spectra analysis (Song et al., 2002), KC andBC are possibly overestimated.

The soot/black carbons in the soils and sediments rangefrom 4.6% to 17.3% of OC (Table 1). The estimated BC con-tents are comparable to the reported values in the literature.For instance, charcoal was reported to account for 5e45%of OC for soils (Schmidt et al., 2001; Song et al., 2002),and BC contents are within ranges of 3e13% of OC in the sed-iments of New England harbors (Gustafsson and Gschwend,1998), and 15e30% of OC in marine sediments (Middelburget al., 1999).

Based on the difference between the OC and NHC contents,the amorphous, expanded organic carbon (AOC) contents canbe estimated (Table 1). The calculation shows that the AOCcontents in the sediments (64.6e74.4%) are higher than thosein the soils (26.2e35.2%). The above observations and discus-sion indicate the importance of the condensed organic carbonssuch as kerogen, black carbon, aged terrigeneous organicmatter in the NHC fractions from the investigated soils andsediments.

3.2. Total PAH content and itsrelationship with OC fractions

The PAHs extracted by the successive solvent extraction(ASE Sum) ranges from 342 ng/g to 1310 ng/g in the soils,and from 100 ng/g to 359 ng/g in the sediments (Table 2).As the distance from the Pearl River estuary increases fromC01 to C05 sediment sample, PAH contents generally de-crease, reflecting the influence of atmospheric depositionand river transport on the distribution of PAHs. IndividualPAHs content varies considerably within the samples. Thecontents of three-ring acenaphthylene, acenaphthene and fluo-rene are very low or not detectable in the samples. Five PAHs(phenanthrene, fluranthene, pyrene, chrysene and benzo[b]-fluranthene) dominate the overall PAHs in the investigatedsamples. The fluoranthene, chrysene and benzo[b,j,k]fluoran-thene dominated-PAH distribution is characteristic of indus-trial and traffic-contaminated soils and sediments (Chenet al., 2005).

Another feature is that ASE Sum extracted more PAHcongeners than did the mixed solvent ASE (ASE STD) orthe Soxhlet extraction. In fact, the second and third extrac-tion steps in the ASE Sum (toluene 1 and 2) still extractedconsiderable PAH amounts compared to the first acetone ex-traction step (Table 2, Fig. 2a). This suggests that a portionof PAHs adsorbed in the NHC and black carbons is hard tobe extracted by the mixed 1:1 acetone/hexane solvent orSoxhlet extraction. The total PAHs determined by theASE Sum method are about 2.11 times the amount in theSoxhlet-extracted samples, where the PAHs extracted byASE with acetone, toluene 1, and toluene 2 respectively

533Y. Ran et al. / Environmental Pollution 148 (2007) 529e538

Table 2

PAH congeners and total PAHs (mg/kg) in the soils and sediments extracted in different solvents by accelerated solvent extraction and by Soxhlet extraction

Sample/solvent Nap Ace Phe Ant Fth Py BaA Chr B(b,k)F BaP Indeno DahA BghiP PAHs

HP04 toluene2 20.4 0.08 12.4 1.05 5.06 2.66 2.07 5.57 9.44 1.09 2.97 0.65 2.22 65.7

Toluene 1 66.9 0.42 44.4 2.91 33.8 17.7 10.0 25.2 32.4 7.78 9.65 2.28 9.49 263

Acetone 79.7 2.17 54.0 3.43 63.4 47.2 21.2 37.2 60.0 21.2 19.7 4.56 22.1 436

ASE sum 167 2.66 111 7.39 102 67.5 33.3 68.0 102 30.0 32.3 7.48 33.8 764

ASE STD 96.4 1.95 66.9 4.28 71.2 50.8 27.0 38.8 81.7 34.3 43.7 9.85 30.1 557

Soxhlet 21.7 1.83 59.5 4.21 56.2 44.9 23.8 68.0 59.9 24.1 15.6 11.0 18.7 410

HP05 toluene2 19.1 0.16 15.9 2.05 12.7 6.01 3.57 9.21 3.33 2.07 2.76 0.53 2.21 79.6

Toluene 1 48.8 0.53 38.5 2.79 35.7 18.4 10.1 22.4 31 7.78 12.2 2.55 9.84 241

Acetone 74.5 4.96 91.9 9.53 166 125 64.1 88.2 164 62.9 68.7 14.4 54.7 989

ASE sum 142 5.66 146 14.4 215 149 77.9 120 199 72.8 83.6 17.5 66.7 1310

ASE STD 82.1 4.01 92.2 7.62 145 103 58.7 82.7 176 66.3 86.9 19.5 54.6 978

Soxhlet 24.0 2.74 71.6 6.36 101 77.8 39.2 70.9 83.7 31.5 26.6 20.0 36.6 592

C01 toluene2 7.41 0.27 7.49 0.65 2.89 2.85 0.91 2.25 2.88 0.81 0.88 0.21 0.96 30.5

Toluene 1 15.1 0.77 22.1 2.77 14.0 13.6 4.79 8.02 11.5 3.74 3.36 0.83 4.42 105

Acetone 25.5 2.13 49.3 4.55 29.5 30.6 9.14 15.3 24.9 8.70 9.77 2.15 11.8 223

ASE sum 48.0 3.17 78.9 7.97 46.3 47.0 14.8 25.6 39.3 13.3 14.0 3.19 17.2 359

ASE STD 31.9 2.34 59.4 5.67 36.1 36.0 10.7 16.9 33.4 12.2 15.3 3.49 13.2 277

Soxhlet 13.8 n.d. 17.9 1.38 13.8 8.26 2.75 8.26 9.64 2.75 2.75 n.d. 2.75 84.0

C02 toluene2 4.07 0.14 2.71 0.16 0.74 0.59 0.17 0.37 0.57 0.12 0.28 0.04 0.22 10.2

Toluene 1 4.59 0.20 4.83 0.65 4.35 2.82 1.26 1.73 3.25 0.83 1.42 0.68 1.25 27.9

Acetone 12.6 0.87 13.1 1.25 8.74 6.37 1.99 3.40 6.44 1.70 2.34 0.43 3.03 62.3

ASE sum 21.3 1.21 20.6 2.05 13.8 9.78 3.42 5.50 10.3 2.66 4.04 1.16 4.50 100

ASE STD 13.8 0.55 14.7 1.20 11.4 8.49 2.69 4.01 9.25 2.77 4.42 0.69 3.52 77.5

Soxhlet 7.38 n.d. 16.2 1.30 8.04 6.68 1.30 5.37 7.12 1.30 1.92 n.d. 2.60 59.2

C03 toluene2 5.60 0.12 2.41 0.36 2.38 1.16 0.50 0.91 1.98 0.37 0.66 0.08 0.49 17.0

Toluene 1 5.18 0.13 5.94 1.18 9.57 4.56 2.26 3.13 6.83 1.51 2.90 0.37 2.19 45.8

Acetone 13.5 0.75 13.8 1.45 15.6 7.98 3.10 5.18 12.2 2.80 4.84 0.72 5.37 87.3

ASE sum 24.3 1.01 22.1 2.99 27.6 13.7 5.86 9.22 21.0 4.68 8.40 1.17 8.06 150

Soxhlet 11.6 n.d. 19.0 1.36 17.0 8.85 3.41 6.63 14.3 4.07 4.07 n.d. 4.76 95.0

C04 toluene2 4.90 0.12 2.19 0.33 2.50 1.17 0.54 0.95 1.96 0.38 0.69 0.09 0.47 16.3

Toluene 1 4.58 0.17 5.67 1.08 9.30 4.11 2.01 2.88 6.68 1.44 2.78 0.38 2.22 43.3

Acetone 10.7 0.59 12.7 1.40 17.8 8.48 3.19 5.56 12.9 2.55 4.06 0.69 4.78 85.4

ASE sum 20.2 0.88 20.5 2.81 29.7 13.8 5.73 9.38 21.5 4.37 7.53 1.15 7.46 145

ASE STD 12.0 0.76 19.6 2.04 24.8 17.1 4.73 6.72 17.6 4.32 8.52 1.23 5.96 125

Soxhlet 12.2 n.d. 18.7 1.35 15.0 8.36 4.06 6.36 14.9 6.77 5.42 n.d. 6.77 99.9

C05 toluene2 4.02 0.05 1.54 0.19 1.09 0.52 0.21 0.42 0.95 0.19 0.50 0.06 0.34 10.1

Toluene 1 4.10 0.13 4.73 0.79 7.14 3.24 1.60 2.31 5.05 1.03 1.87 0.26 1.46 33.7

Acetone 11.6 0.55 12.8 1.11 13.5 6.61 2.31 4.29 9.91 1.95 2.85 0.74 3.80 72.0

ASE sum 19.7 0.73 19.0 2.10 21.8 10.4 4.11 7.03 15.9 3.16 5.22 1.06 5.60 116

ASE STD 14.7 0.38 14.9 1.40 15.9 7.51 2.90 4.90 13.1 2.89 6.36 0.93 4.50 90.3

Soxhlet 7.87 n.d. 12.5 0.65 10.5 5.91 1.32 5.91 11.2 1.32 2.62 n.d. 3.29 63.0

n.d., not determined.

account for about 1.54, 0.44, and 0.13 of the total 2.11 fac-tor. The routine ASE (ASE STD)-extracted PAHs with 1:1hexane/acetone is about 1.53 times the Soxhlet-extractedPAHs, similar to the acetone-extracted fraction in the ASESum method. The above result demonstrates that ASE isa very effective, quick extraction method for contaminatedmaterials with higher contents of condensed SOM whenthe operating variables such as sequential solvents and tem-perature are optimized. This result is also consistent withother observations that the ASE method is more effectivethan the Soxhlet extraction method (Bandh et al., 2000;Hubert et al., 2000).

Some studies have shown that the environmental distribu-tion of PAHs correlates better with BC contents than withOC levels in lake sediment cores (Gustafsson et al., 1997;Buckley et al., 2004). The same was found for PCDD/Fs inmarine suspended particulate matter and surface sediments(Persson et al., 2002). In addition, the particle fraction contain-ing the coal and BC, although a minor fraction on a massbasis, was observed to contain the majority of sorbed organiccompounds (Rockne et al., 2002; Ghosh et al., 2000; 2003;Hong et al., 2003). In this study assuming that PAHs sorptionon a sediment or soil is dominated by NHC, a positive corre-lation between the spatial distributions of NHC and PAH

534 Y. Ran et al. / Environmental Pollution 148 (2007) 529e538

0

400

800

1200

1600

2000

520 650

Toluene2Toluene1AcetonASE SumASE STD

y = 6.78 + 0.13x R= 0.972

y = 20.1 + 0.441x R= 0.924

y = -28.7 + 1.54x R= 0.959

y = -1.82 + 2.11x R= 0.978

y = 17.3 + 1.53x R= 0.975

PA

Hs n

g/g

Soxhlet PAHs ng/g

(a)

0

400

800

1200

1600

2000

SoxhletToluene2Toluene1AcetonASE SumASE STD

y = -108 + 563x R= 0.815

y = -8.62 + 75.6x R= 0.819

y = -18.1 + 231x R= 0.701

y = -286 + 1.03e+03x R= 0.932

y = -313 + 1.34e+03x R= 0.9

y = -208 + 978x R= 0.905

PA

Hs n

g/g

AOC %

(d)

0

400

800

1200

1600

2000

0 0.24 0.48 0.72 0.96 1.20 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

SoxhletToluene2Toluene1AcetonASE SumASE STD

y = 0.616 + 1.46e+03x R= 0.93

y = 5.4 + 200x R= 0.954

y = 17.6+ 665x R= 0.886

y = -53.4 + 2.43e+03x R= 0.966

y = -30.5 + 3.3e+03x R= 0.974

y = 12.3+ 2.39e+03x R= 0.989

PA

Hs n

g/g

BC %

(c)

0

400

800

1200

1600

2000

0 130 260 390 0 0.5 1 1.5 2 2.5

SoxhletToluene2Toluene1AcetonASE SumASE STD

y = 28.6 + 269x R= 0.992

y = 10.3 + 35.1x R= 0.97

y = 33.6 + 117x R= 0.906

y = 5.27 + 429x R= 0.987

y = 49.2 + 582x R= 0.995

y = 48.9 + 426x R= 0.994

PA

Hs n

g/g

NHC %

(b)

Fig. 2. Relationships among the PAHs contents extracted by the different solvent ASE method and by the Soxhlet method (a), NHC (b), BC (c), and AOC (d) in the

soils and sediments.

concentrations should be expected for PAHs that have thesame source as the NHC (e.g., pyrogenic and petrogenicPAHs). Likewise, AOC-dominated sorption should be accom-panied by positive correlations between AOC and PAHs. Sta-tistical analysis exhibits that the PAH contents extracted byASE and Soxhlet in this study are very significantly correlatedwith the NHC contents with correlation coefficients rangingfrom 0.906 to 0.995 (Fig. 2b). However, the correlationbetween PAHs and BC or AOC shows much larger scatter,and is poorer than the correlation between PAHs and NHC(Fig. 2c,d). As NHC is an important composition of the totalorganic carbons (26e74%), the above result indicates thatbesides the black carbon, NHC is also very important for theconcentration of PAHs in soils and sediments.

3.3. Aqueous ASE extraction, enthalpy,and solidewater distribution of PAHs

The PAH concentrations in the aqueous leachates increasewith the temperatures. For example, the aqueous

concentrations (Cw) of the PAHs without Nap increase from0.07 to 1.05 ng/ml when the extraction temperature increasesfrom 25 to 100 �C for the HP04 soil (Fig. 3).

The increasing PAH concentrations with increasing temper-atures can be described by the Van’t Hoff equation (Fig. 3),where the desorption enthalpy DH can be determined fromthe slope of the regression line in a semi-log plot (Table 3).For the soils and sediments investigated in this study,enthalpies (DH ) between �27.0 kJ/mol and �64.4 kJ/molwere observed. The average enthalpy values for PAHs (with-out Nap), Phe, Ant, Fth, and Py are �40.8, �36.8, �46.8,�53.4, and �52.6 kJ/mol in the six investigated samples.With increasing molecular weight and hydrophobicity, the en-thalpy of PAH congeners appears higher. Our observation is con-sistent with other investigations. Wang (2006) reported �23 to�34 kJ/mol for Phe on two spiked soil samples. Kleineidamet al. (2004) recently presented DH-values in the rangefrom �23 to �38 kJ/mol for desorption of phenanthrenefrom aquifer materials which were artificially contaminatedand aged for up to 1010 days before desorption. In addition,

535Y. Ran et al. / Environmental Pollution 148 (2007) 529e538

0.001

0.01

0.1

1

10 y = 2.38e+03 * e^(-27.4x) R= 0.986

y = 206 * e^(-23.1x) R= 0.973

y = 2.28e+03 * e^(-39.8x) R= 0.963

y = 9.31e+04 * e^(-44.2x) R= 0.993

y = 2.02e+05 * e^(-48.5x) R= 0.989

(e)

C03

0.001

0.01

0.1

1

10

0.3 0.35 0.4

PAHsPheAntFthPy

PAHsPheAntFthPy

PAHsPheAntFthPy

PAHsPheAntFthPy

Cw

n

g/m

l

C02

(d)

0.001

0.01

0.1

1

10

0.32 0.33 0.34 0.35 0.36 0.37 0.38 0.39

y = 3.29e+05 * e^(-42.2x) R= 0.986

y = 5.9e+03 * e^(-33x) R= 0.988

y = 996 * e^(-35.7x) R= 0.995

y = 1.06e+05 * e^(-45.1x) R= 0.989

y = 4.4e+05 * e^(-49.4x) R= 0.996

C01

(C)

0.001

0.01

0.1

1

10

0.3 0.32 0.34 0.36 0.38 0.4 0.42

PAHsPheAntFthPy

y = 5.76e+06 * e^(-48.4x) R= 0.98

y = 1.41e+06 * e^(-47.6x) R= 0.991

y = 1.18e+06 * e^(-53.1x) R= 0.961

y = 4.19e+07 * e^(-60.6x) R= 0.949

y = 2.75e+07 * e^(-60.2x) R= 0.954

y = 2.57e+04 * e^(-34.6x) R= 0.984

y = 5.34e+03 * e^(-32.9x) R= 0.982

y = 718 * e^(-35x) R= 0.929

y = 8.12e+05 * e^(-51.2x) R= 0.996

y = 1.33e+06 * e^(-53.7x) R= 0.998

Cw

n

g/m

l

1/(RT) [kJ mol-1

]

HP05

(b)

0.01

0.1

1

10

0.32 0.34 0.36 0.38 0.4 0.42

PAHsPheFthPy

y = 1.05e+04 * e^(-29.2x) R= 0.976

y = 3.52e+03 * e^(-28.8x) R= 0.978

y = 5.21e+04 * e^(-39.4x) R= 0.968

y = 6.8e+03 * e^(-34.3x) R= 0.977

1/(RT) [kJ mol-1

]

1/(RT) [kJ mol-1

]1/(RT) [kJ mol-1

]

1/(RT) [kJ mol-1

]1/(RT) [kJ mol-1

]

(a)

HP04

0.01

0.1

1

10

0.28 0.3 0.340.32 0.36 0.38 0.40.28 0.3 0.340.32 0.36 0.38 0.4 0.42 0.44

y = 3.3e+04 * e^(-35.1x) R= 0.985

y = 7.69e+03 * e^(-34.6x) R= 0.978

y = 1.48e+05 * e^(-52.4x) R= 0.989

y = 3.83e+07 * e^(-62.7x) R= 0.998

y = 1.57e+06 * e^(-54.4x) R= 0.993

Cw

n

g/m

l

Cw

n

g/m

lC

w n

g/m

lC

w n

g/m

l

C04

(f)

Fig. 3. Plots of the Van’t Hoff equation (2) for the 2 soils and 4 sediments.

536 Y. Ran et al. / Environmental Pollution 148 (2007) 529e538

Table 3

Enthalpy of PAHs in soils and sediments

Sample PAHs Phe Ant Fth Py

HP04 �35.1 � 3.23 �37.2 � 3.00 n.d. �56.8 � 7.07 �45.4 � 4.14

HP05 �44.1 � 5.5 �43.3 � 3.23 �48.8 � 4.46 �39.2 � 5.41 �45.0 � 6.99

C01 �44.9 � 5.34 �36.3 � 3.15 �35.2 � 2.72 �54.7 � 4.59 �56.5 � 2.49

C02 �44.7 � 4.05 �36.0 � 3.90 �52.9 � 9.94 �59.4 � 1.97 �59.5 � 1.99

C03 �30.8 � 3.62 �27.0 � 4.13 �46.3 � 8.36 �45.8 � 4.00 �46.4 � 5.41

C04 �45.2 � 4.15 �40.7 � 4.56 �50.7 � 5.06 �64.4 � 2.79 �62.5 � 4.44

Average �40.8 � 6.24 �36.8 � 5.56 �46.8 � 6.91 �53.4 � 9.27 �52.6 � 7.86

n.d., not determined.

Henzler et al. (2002) studied desorption of PAHs from demo-lition waste and sediments and derived DH-values in therange of �39 to �78.3 kJ/mol for PAHs (Phe, and Fth).

The log KOC and log KNHC values of Nap, Ace, Phe, Ant,Fth, and Py are very well correlated with log KOW in the sam-ples at 25 �C (Fig. 4). The regression lines are close to eachother and the difference among them is only up to 0.3 logunit for the investigated six samples. The KNHCelog KOW

regression line is about 0.25 log unit higher than thelog KOC-log KOW correlation line. The KNHC and log KOC

values of the six PAH congeners are respectively 1.2e1.6log unit and 1.0e1.3 log unit higher than that reported by Kar-ickhoff (1981). Moreover, as the average aqueous concentra-tions (ng/ml) of Nap, Ace, Phe, Ant, Fth, and Py for two orthree duplicate measurements at 25 �C were 0.20e0.47,0.005e0.009, 0.026e0.044, 0.003e0.005, 0.004e0.012,

y = 1.79+ 0.77x R= 0.986

y = 1.24+ 0.886x R= 0.974

y = 1.93+ 0.733x R= 0.993

y = -0.349 + 0.99x R= 1

(a)

y = 1.98 + 0.697x R= 0.979

y = 0.761 + 0.981x R= 0.984

y = 0.972 + 0.918x R= 0.983

y = -0.349 + 0.99x R= 1

3.2

4

4.8

5.6

6.4

lo

gK

OC

(b)

C02C03C04Karickhoff

C02C03C04Karickhoff

y = 2.27 + 0.722x R= 0.978

y = 1.38 + 0.937x R= 0.988

y = 1.48 + 0.901x R= 0.979

y = -0.349 + 0.99x R= 1

3.2

4

4.8

5.6

6.4

lo

gK

NH

C

3.2

4

4.8

5.6

6.4

lo

gK

OC

3.2

4

4.8

5.6

6.4

lo

gK

NH

C

logKOW

(d)

3 3.5 4 4.5 5 5.5logK

OW

3 3.5 4 4.5 5 5.5

logKOW

3 3.5 4 4.5 5 5.5logK

OW

3 3.5 4 4.5 5 5.5

HP04HP05C01Karickhoff

HP04HP05C01Karickhoff

y = 1.78+ 0.793x R= 0.97

y = 1.53+ 0.86x R= 0.99

y = 2.28+ 0.714x R= 0.995

y = -0.349 + 0.99x R= 1

(c)

Fig. 4. The KOC and KNHC values of Nap, Ace, Phe, Ant, Fth, and Py in the two soils and four sediments and its correlation with KOW. The KNHC values are the

NHC-normalized solidewater distribution coefficients corrected for the AOC contribution based on equation (2).

537Y. Ran et al. / Environmental Pollution 148 (2007) 529e538

respectively, having a very narrow range in the six samples,the adsorption mechanism of HOCs plays a dominating roleat these very low aqueous concentrations as prior investiga-tions demonstrated (Xia and Ball, 1999; Ran et al., 2004).Xia and Ball (1999) reported that adsorption is dominatingwhen the organic solute concentration is lower than 10% ofits solubility. Based on equation (2), the contribution of parti-tioning from the AOC fraction to the total sorption is lowerthan 10%. Therefore, the higher KNHC or KOC values are obvi-ously associated with the condensed organic carbons such asNHC.

It appears that sorption to NHC can explain a 1e1.6 ordersof magnitude increase in KOC, and that this increase is depen-dent on the total solute concentration. At very low concentra-tions in the field samples, the elevation in KOC is morepronounced due to the nonlinear sorption phenomenon. InNHC-rich soils and sediments, however, the increase in KOC

may be strong.

4. Conclusion

Our investigation demonstrates that NHC is an importantcomposition of the total organic carbons and is obviouslyhigher than BC. The PAH content extracted with the sequen-tial three solvent ASE is two times that using the Soxhlet ex-traction method, suggesting that a portion of PAHs adsorbed inthe NHC and black carbons is hard to be extracted by themixed 1:1 acetone/hexane solvent or Soxhlet extraction. Therelationship of the PAH contents with the NHC contents is sig-nificant, and is better than that with the BC contents or theamorphous organic carbon (AOC) contents, suggesting thatcondensed organic carbon such as kerogen and aged organicmatter, besides the black carbon, is also very important tothe distribution of PAHs in the field soils and sediments.The PAH concentrations measured at various temperaturesteps fit well to the Van’t Hoff equation. The average enthalpyvalues for PAHs, Phe, Ant, Fth, and Py are �40.8, �36.8,�46.8, �53.4, and �52.6 kJ/mol in the investigated samples.With increasing molecular weight and hydrophobicity, the en-thalpy of PAH congeners appears higher. The NHC and OCnormalized solid-solution distribution coefficients (log KNHC

and log KOC) of the six PAH congeners are respectively1.2e1.6 log unit and 1.0e1.3 log unit higher than the previ-ously reported KOC values. The higher KNHC or KOC valuesthan predicted are obviously associated with NHC, indicatingthe condensed SOM-dominated adsorption in the investigatedsoils and sediments.

Acknowledgments

The authors would like to thank Tongshou Xiang and HuizhiZhang in the Guangzhou Institute of Geochemistry, and RenateRiehle, and Renate Seelig in the hydrogeochemistry laboratory,Tubingen University for their technical assistance. This studywas funded by the National Natural Science Foundation ofChina (NNSFC) (40572174), NNSFC-the German ResearchFoundation (DFG) Collaborative Research Project, the

Excellent Youth Fund of the Guangzhou Institute of Geochem-istry, Chinese Academy of Sciences.

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