Indications for pedogenic formation of perylene in a terrestrial soil profile: Depth distribution and first results from stable carbon isotope ratios

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Applied Geochemistry 22 (2007) 2652–2663

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AppliedGeochemistry

Indications for pedogenic formation of perylene in aterrestrial soil profile: Depth distribution and first results

from stable carbon isotope ratios

Tilman Gocht a,*, Johannes A.C. Barth a, Michaela Epp a, Maik Jochmann a,b,Michaela Blessing a, Torsten C. Schmidt b, Peter Grathwohl a

a University of Tuebingen, Center for Applied Geosciences, Sigwarstrasse 10, 72076 Tuebingen, Germanyb University of Duisburg-Essen, Chair of Instrumental Analysis, Lotharstrasse 1, 47048 Duisburg, Germany

Available online 21 June 2007

Abstract

Concentrations and isotope compositions of polycyclic aromatic hydrocarbons (PAHs) were determined in natural soilsof Southern Germany. In selected profiles perylene concentrations increased with soil depth when compared to the otherPAH compounds present. However, its low solubility made vertical transport by seepage water unlikely. Therefore twomechanisms are discussed that could have caused the unusual distribution of perylene in these soils:

(a) Atmospheric deposition of combustion-derived (i.e. pyrogenic) perylene in the top-soil and

(b) in situ generation in the sub-soil of these specific terrestrial environments. This could have been caused by microbialactivities or other catalytic processes yet unknown.

In order to distinguish between pyrogenic and natural
generation compound-specific 13C/12C ratios (d13C) were com-pared between perylene and other PAHs in samples from the top-soil and sub-soil. Despite successful clean-up of theextracts, low perylene concentrations and peak overlaps with benzo(e)pyrene and benzo(a)pyrene prevented determinationof a unique d13C value for perylene in the upper horizon. However, the d13C value of perylene in the sub-soil was 5.7 per-mille more negative than other equal-mass PAHs (with m/z of 252) in the top-soil, which rather supports in situ generationof perylene in the sub-soil.� 2007 Elsevier Ltd. All rights reserved.
1. Introduction

Polycyclic aromatic hydrocarbons (PAHs) aretoxic and partly carcinogenic pollutants that areintroduced into the environment mainly from the

0883-2927/$ - see front matter � 2007 Elsevier Ltd. All rights reserveddoi:10.1016/j.apgeochem.2007.06.004

* Corresponding author.E-mail address: [email protected] (T. Gocht).

combustion of fossil fuels. Today, most PAHs arereleased from anthropogenic sources (power sta-tions, traffic, household heating) rather than naturalones (volcanic activity or natural fires; Li et al., 2001).Due to their physico-chemical properties, PAHs tendto accumulate in soils and sediments (Jones et al.,1989; Palm et al., 2004) and are often ubiquitouslydistributed even in remote areas (Brun et al., 2004).

.

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T. Gocht et al. / Applied Geochemistry 22 (2007) 2652–2663 2653

Apart from the various pyrogenic sources, thereare strong indications of natural low temperatureprocesses that produce perylene, a PAH that con-sists of five rings. For instance, perylene is presentin deeper parts of lake and marine sediments, wherecombustion-derived PAHs are almost absent(Aizenshtat, 1973; Wakeham et al., 1980; Sillimanet al., 1998, 2001; Arzayus et al., 2001; Gochtet al., 2001; Lima et al., 2003; Chen et al., 2006).Apart from sedimentary environments, unusual per-ylene distributions have also been observed in ter-mite nests in tropical top-soils (Wilcke et al., 2002).

Recent evaluations of pollutants in the environ-ment have increasingly applied isotopes as a toolfor identification and differentiation of sources.For instance, stable carbon isotope ratios(13C/12C) either act as a natural label or can helpto explain and to quantify turnover mechanismswhen shifts in the ratio occur. This has become par-ticularly useful in compound specific isotope tech-niques that are able to determine the isotope ratioof each separated peak. Today, compound specificisotope ratios are applied for the assessment of deg-radation as well as different sources of organic pol-lutants (Schmidt et al., 2004; Elsner et al., 2005).

Here the authors present the first PAH data thatstrongly indicate natural perylene formation in sub-soils of terrestrial environments. To the authors’knowledge such findings have not been previouslyreported. The main objective was to rule out thepossibility of vertical transport of perylene with

Fig. 1. Study area (Seebach catchmen

seepage water based on sorption isotherms. Thisleads to plausible indications of true in situ genera-tion in the sub-soil based on compound specific iso-tope results. This technique follows the successfulwork of Wilcke et al. (2002), who differentiated bio-genic perylene in tropical termite nests from pyro-lytic perylene in temperate soil.

The selected study area (Fig. 1) is ideal for thispurpose because of its exposure to higher altitudesof the Black Forest. Increased atmospheric deposi-tion and soil concentrations of PAHs have been pre-viously recorded at this location (Gocht et al.,2007a,b). This offers a suitable contrast to naturallyproduced perylene of deeper layers. This study ofin situ production of perylene is very valuable as itdemonstrates that the terrestrial cycling of anthro-pogenic pollutants is more complex than previouslythought.

2. Materials and methods

2.1. Study area

The study area is located in a small catchment ofthe Northern Black Forest in Germany. Old, pre-dominantly spruce forests, aged up to 200 years,cover more than 90% of this area. Geologically,rocks of the ‘‘Bunter Sandstone Formation’’ charac-terize the region. On top of this bedrock, most of thehills are covered by periglacial debris. Soils were

t) in the Northern Black Forest.

2654 T. Gocht et al. / Applied Geochemistry 22 (2007) 2652–2663

almost exclusively developed during and after thelast ice age 10,000 to 15,000 years ago. Based onland-use analyses, active local anthropogenicPAH-sources (household heating, traffic) are verylimited. Only one residential settlement is situatedin the area and traffic on the forest tracks isrestricted to forest officials. Thus, the main inputof PAHs is attributed to atmospheric deposition,probably due to emissions from the highly industri-alized Upper Rhine Valley to the West of thecatchment.

2.2. Sampling and extractions

Deposition rates of perylene and other PAHswere determined using a passive sampler. Thisdevice was extensively tested in the field and vali-dated for PAHs (German Industrial Standard,DIN 19739-2, 2003). It consists of a borosilicateglass funnel that attaches to an adsorption cartridgepacked with Amberlite� IRA-743. More detailsabout the sampler are described by Gocht et al.(2007a). In the field, the sampling systems werehoused in an aluminum box. The cartridges wereable to trap wet and dry deposition by adsorptionand filtration of particles. After sampling periodsof about 60 days, the cartridges were replaced bynew ones and the funnels were rinsed with 200 mLacetone to remove residual amounts of PAHs fromthe glass surfaces. This sampling scheme was main-tained over a period of 2 years. During each sam-pling campaign, extra cartridges with adsorbermaterial were carried along as blanks and treatedin the same manner as sample cartridges in orderto quantify potential background contaminationfrom transport and handling. After transfer to thelaboratory, the cartridges were sequentially solventextracted in four steps (50 mL for each) with thesame acetone that was used in the field for rinsingthe funnels. Five deuterated PAHs were added tothe extracts (naphthalene-d8, acenaphthene-d10,phenanthrene-d10, chrysene-d12 and perylene-d12)as surrogate standards. For the removal of co-extracted water residues, 10 mL cyclohexane andabout 2 L Millipore-water were added in order totransfer the PAHs into the cyclohexane phase byliquid–liquid extraction. After 48 h, the cyclohexanewas first separated, then reduced to about 1 mLunder a gentle stream of N2 and finally further puri-fied using column chromatography with coupledpolar (Al2O3, 5% deactivated) and non-polar(SiO2, 5% deactivated) adsorbents. The mobile

phases for the latter step were mixtures of n-hexaneand dichloromethane.

Soil samples were taken from typical soil profilesin the study area (Podzols and Planosols). The maincharacteristic of Podzols is marked by downwardstransport of humic substances as well as dissolvedFe and Mn. This is attributed to highly acidic con-ditions. On the other hand, Planosols form pondwater above a layer of restricted permeability withhydromorphic properties.

For sampling, pits were dug and soils were sam-pled from the different horizons using stainless steelshovels and spoons. The soil samples were air-driedat room temperature, passed through a 2-mm stain-less steel sieve and then homogenized. All furtheranalyses were carried out on samples of <2 mm size.Data on soil properties such as pH-values andorganic carbon content are reported by Gochtet al. (2007b).

PAHs were sequentially extracted with an accel-erated solvent extractor (ASE 300, Dionex), twicewith acetone at 100 �C and then with toluene at150 �C. Surrogate standards were added and thevolume of the extracts was reduced to about 1 mLusing a rotary evaporator. Specific clean-up forthe removal of co-eluting non-target compoundswas carried out using Al2O3 (5% deactivated) andSiO2 (5% deactivated) as described above. Blanksconsisting of pre-extracted quartz sand were treatedin the same manner as the soil samples to accountfor possible background contamination during theextraction and subsequent analyses.

The quantification of PAHs was carried out usingan HP 5890 gas chromatograph (carrier gas: He;column: DB-5 Zebron phenomenex, 30 m, 0.25 mmI.D.) that was equipped with an HP 7673 autosamplerand an HP 5972 mass spectrometer. The followingPAHs were measured: naphthalene, acenaphthylene,acenaphthene, fluorene, phenanthrene, anthra-cene, fluoranthene, pyrene, benz(a)anthracene,chrysene, benzo(b+k)fluoranthene, benzo(a)pyrene,benzo(e)pyrene, perylene, indeno(1,2,3-cd)pyrene,dibenz(a,h)anthracene, benzo(ghi)perylene.

2.3. Sorption isotherms

In order to assess leaching from the top-soil as apossible source for PAHs in the sub-soil, equilib-rium sorption batch experiments were conductedwith phenanthrene as a probe compound. Phenan-threne was selected as a proxy compound forperylene for three reasons. First, it has a much


higher solubility than perylene (phenanthrene:1.27 mg L�1, perylene: 0.0004 mg L�1; Verschueren,1996) and if leaching tests with this compoundremain negative, it can be concluded that perylenewill not be detectable in leachate water either. Sec-ond, phenanthrene is a well-established probe com-pound in equilibrium sorption studies. Its aqueoussolubility allows measurements that cover severalorders of magnitude in concentration at reasonablesolid–water ratios. Third, due to their physico-chemical properties, compounds such as perylenehave a very high affinity for adsorption on solid sur-faces such as glass walls and caps of the batch reac-tors. Hence, mass balances of such compoundsoften show substantial losses from the system,which makes the interpretation of these data diffi-cult. Moreover, many studies on sorption of hydro-phobic organic compounds have includedphenanthrene in their compound list, thus enablingcomparison of the results with the literature (e.g.,Karapanagioti et al., 2000; Kleineidam et al.,1999, 2002, 2004). In contrast, no data on sorptionof perylene based on batch studies are available inthe scientific literature.

For leaching from the top-soil, the horizon withthe highest storage capacity for PAHs was selected.This corresponded to the humic layer at the top ofthe profile (i.e. the Oa horizon, according to theFAO soil classification; FAO, 1998). For fast equil-ibration between aqueous and solid phase concen-trations, the sample was pulverized for 30–40 minin a zirconium oxide planet ball mill (Laborette,Fritsch). After this treatment, the grain size of at

Fig. 2. Depth distribution of perylene and the sum of all other PAHs inForest.

least 80% of the pulverized material was <0.063mm. Concentrated stock solutions of phenanthrenein methanol were prepared and spiked to water thatwas de-ionized, degassed and filter-sterilized. Aque-ous concentrations of phenanthrene ranged from 20to 1000 lg L�1 and methanol concentrations in theaqueous solutions were always kept lower than0.5%, a level at which methanol has no measurableco-solvent effect (Kleineidam et al., 1999). All batchexperiments were conducted in triplicate in 10 to100 mL crimp-top glass reaction vials sealed withPTFE-lined butyl rubber septa (Alltech). The vialscontained 0.002 to 0.07 g of the pulverized samplesand were filled with the spiked aqueous solutionsleaving a headspace of less than 10% of their totalvolumes. The samples were constantly moved on ahorizontal shaking table for seven days at 20 �C inthe dark. Subsequently, samples were centrifugedfor 20 min at 1500 rpm and an aliquot of 0.5 to6 mL of the supernatant water was withdrawn, syr-inge-filtered with Al oxide units (Anodisc), andextracted with cyclohexane. Extracts were analyzedusing an HPLC-system (Waters) with a fluorescencedetector. Blank reference vials without solids wereprepared and treated identically.

2.4. Compound specific stable isotope ratios

Carbon stable isotope ratios of PAHs were deter-mined in samples of the Planosol that showedincreasing concentrations of perylene with depth(Fig. 2). The humic Oa-horizon with the highestPAH-concentrations and the sub-soil with the

terrestrial soil profiles of the Seebach catchment, Northern Black


highest perylene concentrations was chosen. Intotal, 300 g of the humic horizon and 100 g of thesub-soil sample were extracted in two cycles withcyclohexane at 100 �C using the ASE Dionex 300method. Again, clean-up was conducted usingAl2O3 (5% deactivated) and SiO2 (5% deactivated)as described above. However, based on these treat-ments alone a good baseline separation of PAHpeaks could not be achieved. In order to fulfill thisprerequisite for compound specific isotope measure-ments, the extracts were further purified by meansof column chromatography with 100% activatedSiO2 (heated at 250 �C over night). For extractsfrom the humic layer, glass columns with a diameterof 1 cm were packed with the activated SiO2 to aheight of 17 cm and washed with 40 mL of n-hex-ane. Then the extracts were applied to the columnand the alkanes were eluted with 40 mL of n-hexane.This eluate was not considered for further analysis.The remaining PAHs were then eluted with 100 mLof a 9:1 mixture of n-hexane and dichloromethane.This extract was reduced to about 1 mL using arotary evaporator. The extracts of the sub-soil con-tained only small amounts of non-target com-pounds and were treated differently in order tosave solvents and adsorbents. Instead of large glasscolumns, 230 mm Pasteur pipettes were filled withthe activated SiO2 (4 cm). After washing with4 mL of n-hexane, the extracts were applied to thecolumn, the alkanes were removed with 4 mL of n-hexane and the PAHs (in this horizon almost exclu-sively perylene) were eluted with 10 mL of the 9:1mixture of n-hexane and dichloromethane. Thisfraction was again reduced to about 1 mL under agentle stream of N2.

Analyses of the compound specific 13C/12C ratioswere carried out on a gas chromatograph isotoperatio mass spectrometer (GC-IRMS) equipped witha large-volume injection device. A trace GC(Thermo Finnigan, Milan, Italy) was coupled toan isotope ratio mass spectrometer (DeltaPLUS XP,Thermo Finnigan MAT, Bremen, Germany) via acombustion interface (GC Combustion III, ThermoFinnigan MAT, Bremen, Germany) that had a tem-perature of 940 �C. The GC was equipped with aprogrammable temperature vaporization (PTV)injector (Optic 3, ATAS GL International B.V.,Veldhoven, The Netherlands). For analytical sepa-ration a DB5-MS capillary column (30 m, 0.25i.d., 0.25 lm film thickness; J&W Scientific, US)was used. Helium was used as carrier gas with aconstant flow of 2 mL min�1. For 1-lL splitless

injections, a liner with 1 mm I.D. (ATAS GL Inter-national B.V. Veldhoven, The Netherlands) wasused. The injector temperature was held at 300 �Cand splitless time and transfer flow were 60 s and3 mL min�1. The temperature program used wasisothermal for 4 min at 60 �C and then ramped with10 �C min�1 to 310 �C, where it was held 7 min. TheGC was equipped with a CombiPAL Autosampler(Chromtech, Idstein, Germany), allowing liquidinjections up to 250 lL. Large-volume injections(LVI) were performed with a packed 8270 liner(ATAS GL International B.V. Veldhoven, TheNetherlands), which was developed for EPAmethod 8270. The samples were injected with a flowrate of 25 lL s�1 into the liner bed. The initial injec-tor temperature was 60 �C and after injection thesolvent was removed from the sorption materialby a helium sweep flow of 1 mL min�1 throughthe open split. By this measure, the solvent levelwas reduced down to 10% of the initial solvent con-tent. For thermal desorption, the injector washeated in splitless mode to 300 �C with a ramp of15 �C min�1. The transfer time was 60 s with atransfer flow rate of 3 mL min�1. After transfer,the split unit was opened and possible residues wereremoved with a split flow of 25 mL min�1. The pre-cision of triplicate compound specific isotope mea-surements was better than ± 0.5 permille.

2.5. Quality control and quality assurance

PAHs were identified by retention times match-ing those of individual standards running underidentical instrumental conditions. Only peaks witha signal-to-noise ratio above 3 were considered forquantification. Detection limits of single PAHs werein the range of 1–5 pg lL�1, and the compound-spe-cific coefficient of variation (as a measure of analyt-ical precision) was within 2%, based on 3 injectionsof the standard solution. All reported data wereblank corrected, based on the field controls (atmo-spheric deposition) and the laboratory blanks (soilsamples). A second deuterated PAH standard(fluoranthene-d10) was added as internal standardto each extract prior to the GC-MS measurementsand recoveries of the surrogate standards werecalculated. They ranged from 35 to 66% for theatmospheric deposition and 72–106% for the soilsamples. Relatively low recoveries of atmosphericdeposition samples are mainly related to incompletetransfer of the pollutants from the acetone–water mixture to the cyclohexane phase during


liquid–liquid extraction. This could be improved byrepeated liquid–liquid extractions. Nevertheless,with the surrogate standards distributing in thesame way between the different phases as the ana-lytes, this did not affect the final result.

In order to assess the heterogeneity of PAH con-centrations in the soils, homogenized samples of onesoil profile were subdivided into 16 portions foreach horizon. Three randomly selected sub-portionswere extracted and analyzed in parallel and the coef-ficient of variation was calculated for each PAH. Ingeneral, it was less than 20%, thus indicating fairlygood homogenization of the samples.

The various purification steps by column chro-matography described above were investigated forfractionation effects of the stable isotope composi-tion of perylene. Nevertheless, triplicate analysesof standard solutions containing perylene beforeand after their application to the columns revealedno significant differences in the d13C-values and var-ied less than 0.5 permille.

3. Results and discussion

3.1. Depth profiles of perylene

The results of all 16 EPA-PAHs in the soils arereported elsewhere (Gocht et al., 2007b). As shownin Fig. 2, the PAHs accumulated particularly in thehumic layers at the top of the soil profiles, whereasthe lowest concentrations occurred in the sub-soil.A sharp decrease in the concentration profileoccurred at the boundary between the humic layersand the mineral soil. Similar depth distributions ofthe EPA-PAHs have also been obtained in otherrural soils of Southern Germany (Gocht et al.,2007b), which is in agreement with previously pub-lished PAH concentrations in forest soils of South-ern Germany (Krauss et al., 2000).

In the Podzol, the depth distribution of perylenefollowed this general trend and even dropped belowthe detection limit in the sub-soil. Similar depth pro-files for perylene have also been found in other ruralsoils (Gocht, 2005). However, in the case of the Dys-tric Planosol, a very different depth distribution ofperylene was observed. Here the concentrations ofthe other PAHs decreased sharply at the boundarybetween humic layers and mineral horizons. How-ever, at about 1 m below the surface only peryleneincreased and even exceeded concentrations in thetop humic layers (Fig. 2). It remains open, why suchunique perylene depth profiles can only be found in

the Dystric Planosol. Presumably, this type of soiloffers material and conditions that enable significantin situ production of perylene. Unless the precursormaterials and mechanisms of this process are fullyknown, it is difficult to find the precise reason forwhy one soil favors perylene production over theother. Since in situ formation of perylene has veryoften been described in sub-water environmentsunder anoxic conditions, it is assumed that specificredox conditions due to ponded seepage water area prerequisite for perylene formation in terrestrialsoils. However, at the moment this is merely a sug-gestion that needs to be investigated in futureresearch.

According to literature data, individual perylenepeaks occur in deeper parts of profiles in sedimentbodies of marine and lake environments (Aizensh-tat, 1973; Hites et al., 1980; Tan and Heit, 1981;Wakeham et al., 1980; Silliman et al., 1998, 2001;Arzayus et al., 2001; Lima et al., 2003; Chenet al., 2006). Similar trends have been reported forfloodplain sediments with increasing contents ofperylene at depths where other parental PAHs wereabsent (Guggenberger et al., 1996; Gocht et al.,2001). Most of these studies suggest in situ forma-tion of perylene under anaerobic conditions fromso far unknown organic precursors (Aizenshtat,1973; Wakeham et al., 1980; Silliman et al., 1998).However, in sedimentary environments, the forma-tion of perylene could also be located somewhereelse in the watershed and the compound could havebeen subsequently incorporated into the sedimentbodies due to synsedimentary deposition. In terres-trial soil profiles, elevated perylene concentrationshave also been found in top-soils of tropical rainforests and linked to relatively high concentrationsin termite nests (Wilcke et al., 2002; Krauss et al.,2005). So far, such unusual perylene concentrationsin terrestrial soils of temperate regions have notbeen reported. In particular, such high peryleneconcentrations in rural sub-soils have not beendescribed elsewhere.

In order to assess specific inputs of perylene tothe Dystric Planosol, the distribution patterns (fin-gerprints) of PAHs were evaluated for the soil hori-zons of the top-soils and compared with those of theatmospheric deposition that was recorded continu-ously for 2 years at a monitoring station near thesoil profile (c.f. Section 2.2). These were comparedto the sub-soil samples with the unexpected highperylene concentrations (Fig. 3). The PAH distribu-tion patterns were determined by calculating the

Fig. 3. PAH distribution pattern (low volatile compounds) ofatmospheric deposition (n = 9), humic layers and top-soil (n = 3),and sub-soil samples (n = 2) of the Dystric Planosol, NorthernBlack Forest. The error bars indicate 1 standard deviation fromthe arithmetic means. The data on the atmospheric depositionexpress the percentage of all PAHs displayed on the x-axis. Theseare mean percentages (not deposition rates) from nine samplingevents over the period of 2 years. In the soils, these percentageswere calculated for the two humic layers and the top-soil as wellas for the two sub-soil horizons that contain increasing amountsof perylene of one soil profile (the Dystric Planosol shown inFig. 2).


percentages of the individual PAHs in relation tothe sum of low-volatile PAHs for each samplingperiod for atmospheric deposition and for each soilhorizon. This made measurements in the differentcompartments (i.e. soils and atmosphere) compara-ble. The expression in percent was also useful fordata sets with large quantitative variabilities thatwere due to decreasing concentrations within onesoil profile. Note that only the low-volatile PAHswith similar physico-chemical properties to perylenewere considered in these calculations. This furtherenhanced comparison of results from the differentcompartments. Fig. 3 shows that the compositionaldistribution of the samples at the top of the soil pro-file nicely matches with those of atmospheric depo-sition. Furthermore, perylene is obviously a lessimportant component in atmospheric depositionand in the top-soil samples. In contrast, it occursas the only compound in the sub-soils. Hence, the

accumulation of PAHs in the top-soils must bestrongly related to atmospheric deposition. On theother hand, the origin of elevated perylene concen-trations in the sub-soil remains unresolved. In thefollowing section, translocation due to leachingfrom the top-soil as a possible source of perylenein the sub-soil is discussed.

3.2. Calculation of leaching time

Fluxes (F) of pollutants through the unsaturatedzone can be calculated using:

F ¼ CwQ ð1Þwhere Cw denotes the aqueous concentration of thetarget compound in seepage water (mg L�1) and Q

the groundwater recharge (mm a�1). Water budgetmeasurements in the catchment from 1985 to 1992yield a mean annual groundwater recharge of about730 mm based on hydrograph separation at thecatchment outlet (Hinderer et al., 1998). Since Cw

was not directly measured, it was estimated basedon the equilibrium sorption experiments by takinginto account the actual concentration of perylenein the compartment with the highest PAH storagecapacity, i.e. the humic layer. If leaching of perylenefrom the top-soil was responsible for the unexpectedhigh concentrations in the sub-soil, the compoundmust have passed this horizon. Since sorption ofhydrophobic organic compounds such as peryleneis strongly related to organic carbon (Kleineidamet al., 1999; Allen-King et al., 2002), this compart-ment provides the highest ‘‘resistance’’ againsttransport of perylene with seepage water and there-fore should control the flux of perylene in the soilprofile.

The equilibrium sorptive uptake of phenanthrenehas been frequently used as a proxy compound forPAHs in previous sorption studies (e.g. Kleineidamet al., 1999, 2002, 2004; Rugner et al., 1999; Karapa-nagioti et al., 2000; Liang et al., 2006). In thesestudies, the data were best fitted using the Freund-

lich-approach that considers a non-linear shape ofthe sorption isotherm:

Cs ¼ KFrC1=nw ð2Þ

where Cs denotes the equilibrium concentration of acompound in the solid phase (mg kg�1), KFr the Fre-

undlich sorption coefficient (L kg�1), Cw the equilib-rium aqueous concentration of the compound(mg L�1), and 1/n the Freundlich exponent (–). KFr

and 1/n are provided by a regression analysis of

Table 1Calculation of the required time to transfer perylene from the topof the Dystric Planosol to the sub-soil assuming equilibriumtransport

Compartment Parameter Source

Flux

Humic layer Cs (lg kg�1) 19.5 This studySeepage water Cw (mg L�1) 3.93 E-08 Calculated

(Eq. (2))Groundwater

rechargeQ (mm yr�1) 730 Hinderer et al.,

1998Flux F (mg m�2 yr�1) 3.0 E-05 Calculated

(Eq. (1))

Storage

Sub-soil Cs (lg kg�1) 57.1 This studySub-soil q (kg m�3) 1.470 Gocht, 2005Sub-soil d (m) 0.1 MeasuredSub-soil S (mg m�2) 8.4 Calculated

(Eq. (3))

Time

Soil profile t (a) �300,000 Calculated(Eq. (4))


the Freundlich isotherm (data not shown). Com-pared with recently published data on sorption ofhydrophobic organic compounds, these valuesmatch well with those reported for sorption to‘‘young’’ (i.e. thermally unaltered) organic mattersuch as peat (Kleineidam et al., 2002). Insertingthe actual solid phase concentration of perylene intoEq. (2), the equilibrium aqueous concentration wascalculated and the respective annual fluxes weredetermined using Eq. (1) (Table 1).

In order to compare the actual perylene concen-trations in the sub-soil with the fluxes, the storagesS (mg m�2) of perylene were calculated using thefollowing equation:

S ¼ Csqd ð3Þwhere q denotes the dry bulk density of the sub-soil(kg m�3) and d the thickness of the horizon (m). Fi-nally, the required leaching time t (a) can be calcu-lated, relating the actual storage to the flux:

t ¼ SF �1 ð4ÞBased on this approach, the required leaching timeto transfer perylene with seepage water from thetop-soil to the sub-soil amounted to about 300, 000years (Table 1). Since the actual perylene concentra-tion in the humic layer is below the lowest concentra-tions used in the batch experiments, this should betaken as a rough estimate to assess leaching as asource for perylene.

Leaching of perylene can therefore be ruled outbecause the calculated time for this process exceedsthe age of formation of the present soils. Note thatmost of the soils in the study area were developed inthe debris that formed during the last ice age 10,000to 15,000 years ago. Leaching would also requirethat atmospherically deposited perylene is preferen-tially transported or protected against degradationcompared to the other PAHs present. Since neitherof these propositions is reasonable, other sourcesmust be taken into account and in the following sec-tion in situ-formation of perylene will be discussed.This is supported by new compound specific mea-surements of stable carbon isotope ratios.

3.3. Compound specific stable carbon isotope ratios

Two different sources are assumed to be respon-sible for the distribution of perylene in the DystricPlanosol:

(a) Atmospheric deposition of pyrogenic peryleneat the top, and

(b) in situ-formation of perylene from biogenicprecursors in the sub-soil.

The assessment of these different sources wasaddressed by compound specific stable carbon iso-tope ratio measurements (d13C).

In particular, the extracts of the humic horizoncontain large quantities of co-extracted non-targetsubstances, which were removed using columnchromatography techniques described above. As aconsequence, baseline separation could be achievedfor both the extracts of the sub-soil and the humichorizon (Fig. 4). However, since perylene concentra-tions are quite low compared to other low-volatilePAHs in the humic layer (Fig. 3), it was not possibleto separate the perylene peak from the precedingbenzo(a+e)pyrene peak in the GC-IRMS measure-ments (Fig. 5). At low injection volumes, the massof perylene on column was too low to obtain aquantifiable signal on the IRMS, but with increas-ing injection volumes the peaks became broaderand moved closer together resulting in overlaps.This made it difficult to derive reliable compoundspecific isotope ratios. Note that only isotope ratiosof unique and identifiable peaks were considered,which provided less numerous but reliable analyticalresults. In contrast, a distinct perylene peak wasobtained for the sub-soil sample (Fig. 4). Thed13C-values are also summarized in Table 2. In the

Fig. 4. Chromatograms of the GC-IRMS measurements from the humic layer (upper left (a) using injection volume of 1 lL and upperright (b) using 25 lL) and the sub-soil (lower image (c), injection volume 125 lL).

Fig. 5. Literature data of d13C-values of perylene in different environments (bars) compared to sub-soil perylene of this study (cross) (seeabove-mentioned references for further information ).

Table 2Compound specific d13C-values (&) of selected low-volatilePAHs

Compartment B(b+k)fa B(e+a)pb Perc

Standard – – �24.0Humic layer �23.8 �23.9 (�23.4)d

Sub-soil – – �29.5

The standard deviation of all isotope measurements was alwaysbetter than ± 0.5&.

a Benzo(b+k)fluoranthene.b Benzo(e+a)pyrene.c Perylene.d Derived from a peak close to the detection limit (1 lL injec-

tion, Fig. 4.)


humic horizon, the d13C-values of the selected low-volatile PAHs were in the same range, and a morenegative d13C-value was found for perylene in thesub-soil. Unfortunately, due to the weak signal ofperylene in the humic horizon, different sourcesfor perylene cannot be conclusively determined inthe different soil horizons based on the GC-IRMSmeasurements alone. Nevertheless, it is remarkablethat the isotope signal of perylene in the sub-soilwas up to 5.7 permille more negative than thebenzo(b+k)fluoranthene and benzo(e+a)pyrene inthe top humic layer (Table 2). Since direct compar-ison to perylene was difficult, this is no direct proof,


but the noticeably different signal strongly indicatesa different source compared to the atmosphericdeposition. If so, either the precursor material ofthe in situ-produced perylene was isotopicallydepleted compared to top-soil material or the syn-thesis caused an isotope shift towards more negativevalues.

The d13C-values of perylene are in line with liter-ature data (Fig. 5). Silliman et al. (2000) reportedconsistently lower d13C-values of perylene in sub-surface marine sediments when compared to thoseof total organic carbon. They concluded that peryl-ene was probably a product of in situ formation.However, these values are still in the range of pyro-genic perylene reported by other authors (Fig. 5).Wilcke et al. (2002) determined significantly morenegative d13C-values of perylene in tropical termitenests than in urban top-soils that were contami-nated with pyrogenic PAHs. In their study, d13C-values of pyrogenic benzo(b+k)fluoranthenes ofthe same urban top-soils ranged between �25.4and �24.4 permille, while those of pyrogenic ben-zo(a+e)pyrenes were around �25.3 permille. Thesevalues are also close to those obtained in the humichorizons of this study (Table 2). Slightly more neg-ative d13C-values were determined for perylene inthe urban top-soil samples (Fig. 5, Wilcke et al.,2002). Similarly, in surface sediments of a lake withdiffuse input of predominantly combustion-derivedPAHs, d13C-values of perylene were found to be 1to 2 permille more negative than benzo(a+e)pyrenesand benzo(k+f)fluoranthenes (Smirnov et al., 1998).In this study, d13C-values of perylene rangedbetween �26.2 and �30.0 permille (Fig. 5). Sincethese values of pyrogenic perylene are more negativethan those reported for the sub-surface marine sed-iments (as a product of in situ formation), it is notpossible to distinguish the different sources of peryl-ene in different study areas based on compound spe-cific isotope ratios alone. Consequently, one has tobe careful in concluding in situ formation of peryl-ene in the terrestrial sub-soil based on the fewd13C-values presented here.

4. Conclusions

For the first time, indications for in situ-forma-tion of perylene were found in a terrestrial sub-soilprofile, an environment in which synsedimentarydeposition of perylene can be ruled out. Leachingof the compound with seepage water is also unlikelywhen considering the length of the calculated leach-

ing times. Hence, in situ formation of perylene fromterrestrial precursor materials with significantlyhigher solubilities than perylene is the most likelyscenario.

No threshold can currently be derived withrespect to d13C measurements in order to distinguishbetween combustion-derived and naturally pro-duced perylene. Silliman et al. (2000) attributedthe surprisingly negative d13C-values to naturalperylene generation. Based on the high carbon iso-tope variability of 4 permille within one sedimentcore, they concluded that in situ formation of peryl-ene must be the product of various terrigenous andmarine organic materials. The present results alsoindicate natural perylene formation in terrestrialsub-soils, which is related to precursors yetunknown.

The low organic carbon content of about 0.3%(Gocht, 2005) is very promising for the further char-acterization of the organic matter. This is combinedwith the expectation that the precursors must bepresent at the same location. In contrast to sedimen-tary environments, no transport from the surround-ing watershed needs to be taken into account in thisscenario. Thus, further investigations of the DystricPlanosol should aim at the mechanistic understand-ing of the processes that control the in situ-forma-tion of perylene. Such work holds promise toreveal new aspects in the cycling of PAHs. Further-more, the authors plan to extend the investigation ofthe isotope carbon composition of perylene to themeasurement of radiocarbon in order to furtherassess the different sources of top-soil and sub-soilperylene.

Acknowledgements

This work was supported by grants fromBWPLUS (Lebensgrundlage Umwelt und ihreSicherung, Contract Number BWR22006), state ofBaden-Wuerttemberg, Germany, the EuropeanIntegrated Project Aqua Terra (Contract Number505428GOCE), and the Ministry of Science, Re-search, and the Arts of Baden-Wurttemberg (Con-tract Number AZ 33-7533. 18-15-02/87).

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Documents

Indications for pedogenic formation of perylene in a terrestrial soil profile: Depth distribution and first results from stable carbon isotope ratios