A Contaminant Trap as a Tool for Isolating and Measuring the Desorption Resistant Fraction of Soil Pollutants

Published: February 28, 2011

r 2011 American Chemical Society 2932 dx.doi.org/10.1021/es1033124 | Environ. Sci. Technol. 2011, 45, 2932–2937

ARTICLE

pubs.acs.org/est

A Contaminant Trap as a Tool for Isolating and Measuring theDesorption Resistant Fraction of Soil PollutantsPhilipp Mayer,*,† Jannik L. Olsen,†,‡ Varvara Gouliarmou,† Marion Hasinger,§ Romana Kendler,§ andAndreas P. Loibner§

†National Environmental Research Institute, Aarhus University, PO Box 358, 4000 Roskilde, Denmark‡Department of Life Sciences and Chemistry, Roskilde University, PO Box 260, 4000 Roskilde, Denmark§IFA-Tulln, Department for Agrobiotechnology BOKU, University of Natural Resources and Life Sciences, Vienna 3430 Tulln, Austria

bS Supporting Information

ABSTRACT: Bioremediation of contaminated soils often leaves a desorption-resistant pollutant fraction behind in the soil, whichin the present study was isolated with a combination of diffusive carrier and infinite diffusive sink. Such a diffusive sink was made bycasting a composite of silicone and activated carbon into the bottom of a large glass. Field-contaminated soil samples were thensuspended in a cyclodextrin solution and incubated in such glasses for the continuous trapping of PAH molecules during theirrelease from the soil matrix. The PAH concentrations remaining in the soil were determined by exhaustive extraction and comparedwith a biodegradation experiment. The concentration decline in the first soil was faster in the contaminant trap than in thebiodegradation experiment, but the halting of the biodegradation process before reaching the legal threshold level was well indicatedby the contaminant trap. The PAH concentrations in the second soil hardly decreased in the traps at all, in good agreement with thebiodegradation experiment. The PAHs in this soil appeared to be “stuck” by strong sorption. The contaminant trap proved to be apractical approach to the isolation and quantification of the desorption-resistant PAH fraction.

’ INTRODUCTION

Polycyclic aromatic hydrocarbons (PAHs) are priority pollu-tants that are present at measurable concentration in virtually allsoils. High PAH levels often originate from historic soil con-tamination, for instance at manufactured gas plant sites, but alsothe continuous deposition of airborne pyrogenic particles canlead to significant PAH concentrations in urban soils. Biodegra-dation of PAHs has often been observed in soil, and bioremedia-tion has been applied as a cleanup technology. However, evenafter years of soil remediation, for instance by composting, somefraction of the original PAH contamination persists, usually as aresult of strong sorption and slow desorption.1-4 Such adesorption-resistant or recalcitrant fraction is very difficult toremove even by intensive biological and chemical methods,5-7

and if the residual concentrations are still above the regulatorysoil quality criteria, the treated soil remains hazardous waste. Theincomplete bioremediation then causes both technical andeconomic problems.

The first aim of the present studywas to develop and characterizea “contaminant trap” as a simple tool for the isolation and quan-tification of the desorption-resistant PAH fraction, since much ofthis fraction is likely to remain in the soil even after intensivebioremediation. From the perspective of the degrader organism,this fraction can be characterized as “nonaccessible”,8,9 whilefrom the perspective of the soil matrix, it can be characterized as“desorption resistant” 10 or “very slow desorbing”.11 Even thoughthis categorization is useful, it ignores important aspects of thedesorption kinetics from the matrix and the uptake kinetics intotarget organisms.3,12-14 The second aim of the study was theproof of principle testing of these contaminant traps with regard

to bioremediation predictions. Provided that physicochemicalconditions such as pH, temperature, salinity, and electron accep-tor and nutrient supply support the growth of pollutant-degrad-ing microorganisms, the major obstacle to the biodegradation ofPAHs in soil is a lack of their bioaccessibilty.2,6,7,15 As thisdeficiency may result in a decrease of bioremediation perfor-mance or a total lack of PAH removal, it is crucial to have toolsavailable for determining this desorption-resistant fraction ornonaccessible pool in advance.

Two PAH-contaminated industrial soils were incubated in thecontaminant traps to determine the desorption-resistant PAHfractions, which then were compared with the PAH fractionsremaining in the soils after a biodegradation experiment.

’WORKING PRINCIPLE

A soil suspension is incubated in a contaminant trap glass forthe continuous trapping of contaminants during their desorptionfrom the soil matrix. Cyclodextrin serves here as a diffusivecarrier, and a composite of activated carbon and silicone as adiffusive sink. The remaining contaminants in the soil can then bemeasured by conventional extraction and instrumental analysis.The aim of the method is thus to measure the “desorptionresistant” contaminant concentration, and it thus complementsestablished mild extraction and solid-phase extraction methodsthat are directed at the bioaccessible concentrations. 11,16-18

Received: September 30, 2010Accepted: February 4, 2011Revised: January 19, 2011

2933 dx.doi.org/10.1021/es1033124 |Environ. Sci. Technol. 2011, 45, 2932–2937

Environmental Science & Technology ARTICLE

The contaminant trap glass was made by casting a compositeof poly(dimethylsiloxane) (PDMS) and activated carbon (AC)into the bottom of a large glass jar. This composite was expectedto be an effective diffusion sink for PAHs and for many otherhydrophobic organic chemicals. The PDMS provides highpermeability for such chemicals and maintains its physicalintegrity and performance even when brought into contact withvery complex matrixes.19-21 The activated carbon provides veryhigh sorption affinity and capacity, particularly for PAHs.22-24

The trapping of PAHs by this composite has conceptual simila-rities to trapping by Tenax or XAD-4 particles that are mixed intosoil and sediment suspensions.11,25 However, the composite isnot mixed into the soil but remains a separate phase with asmooth surface, so that a soil sample can be easily harvested afterincubation without any phase separation steps. The contaminanttrap also has similarities to passive dosing vials, where PDMSwithout activated carbon is cast into the bottom of a vial.26,27 Themain difference here is that passive dosing applies PDMS as apartitioning source for establishing and maintaining constantfreely dissolved concentrations and chemical activities, whereasthe contaminant trap applies an AC-PDMS composite as apartitioning sink for keeping freely dissolved concentrationsand chemical activities close to zero.

Such traps can be operated and applied inmany different ways.In the present study, we aimed for a nonagitated system in orderto allow for the incubation of many samples and if needed also forextended periods of time (e.g., years). Hydroxypropyl-β-cyclo-dextrin was applied as a diffusive carrier from the soil and to theAC-PDMS composite. In a previous study, the cyclodextrinwas shown to increase the diffusive mass transfer of PAHs by afactor ranging from 3.9 for naphthalene to 1134 for dibenzo-[ah]anthracene.28,29 Additionally, cyclodextrin has also beenshown to enhance the capacity of the solution to receivehydrophobic organic chemicals, a property that has been utilizedin several cyclodextrin extraction methods.17,30

’MATERIALS AND METHODS

The following PAHs were used as analytical standards in thewater/cyclodextrin solution; naphthalene (NAPH, 99%, Fluka),acenaphthene (ACNAP, 99%, EGA), fluorene (FLOUE, 98%,Aldrich), phenanthrene (PHE, 98%, Aldrich), anthracene (ANT,98%, Sigma), fluoranthene (FLUO, 99%, Aldrich), pyrene (PY,99%, EGA), benzo[a]anthracene (B[a]A, 98%, Aldrich), chry-sene (CHRY, 95%, Aldrich), benzo[k]fluoranthene (B[k]F,98%, Aldrich), benzo[a]pyrene (B[a]P, 98%, Aldrich), anddibenzo[ah]anthracene (diB[a,h]A, 97%, Aldrich). Activatedcarbon, ‘activated charcoal, untreated powder 100-400 mesh’from Sigma-Aldrich (Vallensbæk Strand, Denmark), and silicone(Dow Corning SILASTIC 9161 RTV Silicone rubber) with acuring agent (Dow Corning SILASTIC 9162 RTV catalystcontaining ethyl silicate) was used for the AC/PDMS composite.The water/cyclodextrin solution was prepared by adding 140 g ofcyclodextrin (HPCD, hydroxypropyl-β-cyclodextrin >97% pur-ity, Wacker-Chemie, Burghausen, Germany) to a 1 L volumetricflask and filling with demineralized water.Manufacturing of Contaminant Trap Glass. Activated car-

bon was added to the PDMS prepolymer in the ratio 1:10 (mass:mass) and then stirred until it reached a homogeneous gray color.The curing agent was added using an increased dose of 16%(mass/mass) compared to the manufacturer’s instructions(10%) in order to compensate for curing agent binding to the

activated carbon. This curing dose was determined during severalmethod development tests. This mixture was again thoroughlystirred until it reached a homogeneous consistency, which wasfound to be crucial for uniform and predictable polymerization.Large contaminant traps were then made by adding 50 g of thismixture to 500 mL brown pill glasses, and small traps were madeby adding 10 g of this mixture to 120 mL brown pill glasses (bothsupplied by Apodan Nordic A/S, Copenhagen, Denmark). Thecomposite cured at approximately 21 �C, which typically re-quired 1-2 weeks. Twenty five grams of the polymer compositewas cured, and its density was calculated on the basis of its finalvolume andmass to be 1.48( 0.02 g/mL. The surface area (largetrap) in contact with the sample suspension was 82 cm2, and theheight of the composite ranged from 2 to 6 mm due to thecurvature of the glass bottom.Elimination Kinetics from the Solution. An experiment was

carried out with the 500 mL trap glasses to assess the eliminationkinetics of the PAHs from the cyclodextrin solution. A spikesolution was prepared by dissolving 12 PAHs at each 40 mg/L inmethanol, which was then spiked to the cyclodextrin solution inorder to reach initial concentrations of 1000 μg/L for each PAH.Fifty milliliters of this solution was incubated in the traps forthree days (no agitation), and samples of the solution were takenat predetermined time points and measured by HPLC.Capacity of the Trap. Naphthalene was chosen for this

experiment, since it had the lowest elimination in the previousexperiment (worst case) and since even high quantities can bebrought into solutions rather easily (low melting point, highaqueous solubility). Fifty milliliters of cyclodextrin solutions withvery high naphthalene concentrations (20-6000 mg/L) wereincubated in the traps and the concentrations in the solutionsmeasured after 9 and 30 days. The trapping efficiency was testedin this manner at very high naphthalene loadings of the AC/PDMS composite (20-6000 mg/kg composite), which corre-sponds to a loading of the 5 g of activated carbon at approxi-mately 200-60 000 mg/kg AC. Since the maximum soil mass tobe incubated in the traps was equal to the mass of activatedcarbon in the traps (5 g dry weight in the large traps), theseloadings can be compared 1:1 to the desorbable PAHs (in mg/kgsoil dry weight) to be trapped by the composite.Application to Industrial Soils. Once the elimination dy-

namics and the sorptive capacity of the traps were confirmed,contaminant trap experiments were conducted in order to isolateand determine the desorption-resistant PAHs in soil samplesfrom industrial sites. The results were compared to a biodegrada-tion experiment with the same soils in order to investigatewhether the desorption-resistant fraction could act as a predictorfor the lowest achievable PAH concentration by bioremediationmeans (i.e., nonaccessible fraction).Soils. Soils were collected from two historically PAH-con-

taminated sites in Austria: a former storage site for PAH-contaminated materials (soil HOF) and a former manufacturedgas plant (soil WGS). Before use, the soils were sieved to <2 mmand stored in the dark at 4 �C. Detailed soil characteristicsincluding contaminant levels are given in Table S1 (SupportingInformation).Contaminant Trap Experiments. Soil (5 g dry matter) was

transferred into large contaminant trap glasses. Cyclodextrinsolution was added to facilitate mass transfer, and the solutionwas amended with sodium azide (10 g/L), >99% purity, Merck(Darmstadt, Germany) in order to suppress microbial activity.31

The volume of the solution was 20 mL, chosen to match the



volume of nutrient solution in the degradation experiment. Theglasses were shaken gently for a short period to achieve ahomogeneous layer of the slurry and then incubated withoutagitation. The desorption time started when the solution wasapplied to glasses holding the soil samples (T = 0). All con-taminant trap incubations were carried out in triplicate and sincesampling and extraction was destructive, this meant that threeglasses were used for each time point.Samples were taken on days 1, 7, 31, and 92. Slurries were

passed through a filter (5951/2 folded paper filter, diameter110mm; Schleicher& Schuell, Dassel, Germany). The contaminanttrap glass was rinsed several times using demineralized water whichwas passed through the same filter. Filtrates were optically clear, andPAH analysis of filtrates indicated no loss of PAHs.Biodegradation Experiment. The rate and extent of biode-

gradation were investigated using bioslurry treatments which hadbeen published earlier.32 The degradation tests comprise thefollowing in brief: sieved soil (<2mm) was transferred to 100mLflasks and amended with nutrient solution and PAH-degradingbacteria. Slurries were shaken on a rotary shaker to providesufficient oxygen. Sampling took place on days 4, 7, 14, 28, 56, 77,and 92. All degradation experiments were kept at room tempera-ture and carried out in triplicate, meaning that three biodegrada-tion vessels were used for each time point. This was laborious butensured that biodegradation profiles were based on independentmeasurements. Poisoned control vessels were included andindicated less than 13% of abiotic PAH removal (average) fromHOF soil and no removal from WGS soil.Analytical Procedures. Samples of cyclodextrin solutions

from the initial experiments were diluted 1:1 with methanoland then analyzed byHPLCwith fluorescence detection (Agilent1100 system with G1321A FLD; Ex. 260 nm; Em. 350, 420, 440,and 500 nm). The “CP-Ecospher 4 PAH” separation column wasobtained from Varian Inc. (Palo Alto, CA) and was operated at0.5 mL/min (28 �C, 30 μL injection). Methanol (HPLC gradefrom Merck Darmstadt, Germany) and water (SUPER-Q trea-ted, Millipore, Billerica, MA) was used as the mobile phase: 50%methanol at t = 0-2 min, linear gradient from 50 -75%methanol at t = 2-7 min, linear gradient from 75 to 100% att = 7-35 min, and 100% methanol at t = 35-48 min. The PAHconcentrations in the extracts were quantified by a five-pointexternal standard curve. The analysis was generally carried outwithin two weeks post sampling. Signal integration was per-formed with HP Chemstation software (A.06.03, Agilent Tech-nologies, Palo Alto, CA) and manually corrected as necessary.Soil samples from the contaminant traps and the biodegrada-

tion experiment were analyzed as described previously.32,33 Inshort, filters containing soil from the contaminant trap glasseswere air-dried (4 h), transferred to a cellulose extraction thimble(40 � 85 mm; Schleicher & Schuell, Dassel, Germany) andextracted with ethyl acetate using an automated Soxhlet instru-ment (Gerhardt Soxtherm extractor model 2000 automatic,Bonn, Germany). Clean-up of extracts was not required, butthe samples were diluted with acetonitrile prior to HPLCanalysis.33 Individual PAHs were quantified by HPLC reverse-phase analysis using a three-dimensional fluorescence detector.Details are given elsewhere.32

’RESULTS AND DISCUSSION

Elimination Kinetics from the Solution.Within the first dayof incubation, the major fraction of all PAHs was effectively

eliminated from the cyclodextrin solution as shown in Figure 1.For clarity, only seven PAHs are shown in the figure, but alltwelve PAHs that were spiked to the solution yielded similarresults. After 23 h, 97.7% (NAPH) to 98.7% (PYR, FLUO), andafter 32 h, 98.7% (NAPH) to 99.6% (equal for six PAHs), hadbeen removed from the solution. The contaminant trap thusremoved 98-99% of the PAHs from the spiked solution withinjust one day of incubation. Such kinetics are more than sufficientwithin the context of the present study and can be furtherenhanced by agitation such as orbital shaking.34

This experiment was conducted with cyclodextrin concentra-tions of 140 g/L that are normally used for cyclodextrin extrac-tions aiming to determine bacterial (bio)accessibility.17,35 TheAC/PDMS trapping phase was able to remove PAHs from suchsolutions very efficiently (Figure 1). The integration of such aninfinite diffusion sink into a cyclodextrin extraction method willthus maintain a full gradient between the contaminated soil andthe cyclodextrin solution. This maximizes desorption and en-sures that no resorption to the soil occurs, which otherwise canlead to an underestimation of the bioaccessible fraction.36

Capacity of the Trap. Naphthalene was selected for thisexperiment, since the lowest elimination efficiency was obtainedfor this compound in the previous experiment. Trap efficienciesdecreased with increasing naphthalene loading (Figure S1,Supporting Information), but remained above 95% for loadingsof up to 20 g NAPH/kg activated carbon, which is equivalent toan initial naphthalene concentration in the cyclodextrin solutionof 2 g/L. Since the maximum soil mass to be incubated in thetraps was equal to the mass of activated carbon (5 g dry weight inthe large traps), these data suggest that accessibilities of up to 20 gNAPH/kg soil dryweight can be efficiently trapped by the composite.This experiment was conducted with only one PAH, whereas

contaminated soils will contain many PAHs as well as other soilpollutants. On the basis of the results obtained, we expect a largesurplus of trapping capacity as long as the accessibility of the sumof PAHs (e.g., a sum of 7 or 16) remains below 1 g/kg dry weight.Samples with higher accessibility can still be accommodated insuch glasses when reducing the sample mass. Alternatively, asufficient capacity for a given sample can be analytically con-firmed by parallel incubations of traps with 50 and 100 g ofcomposite without changing the surface area for mass transfer. Asufficient trapping capacity for the complete depletion of readily

Figure 1. Elimination kinetics of PAHs from a spiked cyclodextrinsolution. Within the first day of incubation, 98-99% of the PAHs wereremoved from the solution.



desorbable PAHs would then be indicated by similar results inboth traps.37

Application to Industrial Soils. Soil samples from twoindustrial sites were incubated in the traps for the continuousremoval of the desorbed PAHs. The PAHs remaining in the soilwere then measured and compared with those remaining in thesoil during a biodegradation experiment.Individual PAH concentrations before treatment and after the

contaminant trap and biodegradation treatments are shown inFigure 2a and 2b and the sum of PAHs remaining in the soil as afunction of incubation time is shown in Figure 3.In the soil from a former storage site for PAH-contaminated

materials (HOF-soil), the concentrations remaining at the end ofthe incubations were for all measured PAHs lower in the con-taminant trap than in the biodegradation experiment (Figure 2).This was in good agreement with the faster concentration declinein the traps than in the biodegradation experiment (Figure 3),which was most pronounced at the beginning of the experiment(lag phase). Nevertheless, the fact that the biodegradationprocess stopped before reaching the threshold level (e.g., 40 mg/kg dw in Denmark) was well indicated by the contaminant trap.The PAH concentrations in the soil from a former gas-

manufacturing plant (mgp soil, WGS) hardly decreased at allin the contaminant traps, in good agreement with the biodegra-dation experiment. The PAHs in this mgp soil appeared to be“stuck” by strong sorption in accordance with a recent study onanother mgp soil sample.6 For such “stuck soils”, the contaminanttrap is a very simple and effective tool to explain and predict theabsence of biodegradation.

The two tested soils were different in terms of origin, soilcharacteristics, and pollution history. Soil HOF showed betterdegradation and lower residual concentrations in the trapexperiment than WGS soil, even though HOF soil had a highersoil organic carbon (TOC: 46 g/kg dry weight) than WGS soil(TOC: 15 g/kg dry weight). This confirms that origin, type, andcomposition rather than total amount of soil organic carbondetermine the sorption of hydrophobic organic contaminants.38

Taking into account the history of both soils, WGS coming froma former gas-manufacturing site is more likely to contain highlysorptive carbonaceous particles.39 This assumption is also sup-ported by the fact that there is no statistical difference betweendegradation results and sampler performance (tested with two-way Anova and a Bonferoni post test).General Features and Future Developments.Themeasure-

ment end point of the present study is the “desorption-resistantconcentration”. During the practical work with contaminatedsoils, this measurement end point was found to be very robustwith regard to contaminant losses during sample handling,sieving, drying, and storage. Such losses must normally beavoided when measuring the total concentration and even morewhen measuring bioavailability parameters such as freely dis-solved concentration, chemical activity, and accessibility. In fact,such losses will hardly affect the desorption-resistantconcentration.The soil samples leaving the contaminant trap can be extracted

and analyzed with the analytical methods of choice. Internationalstandards and guidelines should be followed when the goal is topredict the remaining PAH concentrations that eventually will beevaluated with such standard methods. Otherwise the extractionmethods should be selected and optimized for complete extractionperformance, particularly for soil samples with a high soot content,which can be difficult to extract even by Soxhlet extraction.40

Is the desorption-resistant fraction also largely nonaccessibleto human and ecological receptors other than microorganisms?This is a very important question that needs to be addressedbefore bioavailability parameters are generally integrated intolegislation. A positive answer to this question would support aparadigm shift for PAH-contaminated soils from “remediation

Figure 2. PAH concentrations after 92 days of incubation in contami-nant traps and in biodegradation experiments for HOF soil from a PAH-contaminated waste disposal site (top) and soil WGS from a gas-manufacturing site (bottom).

Figure 3. PAH concentrations remaining in the soils during incubationsin contaminant traps and in biodegradation experiments for HOF andWGS soils. The Danish guideline value of 40 mg/L was included as anexample of a benchmark that needs to be reached by commercialremediation companies.



for the removal of contaminants” toward “remediation forexposure and risk reduction”.6 In other words, it might besufficient in some cases to let microorganisms remove the readilydesorbing fraction if this efficiently reduces the exposure and riskoriginating from the polluted soil. In this context, the contami-nant trap can be applied to generate soil samples that aredominated by desorption-resistant contaminants. Such materialwould then be ideal for the thorough testing of desorption-resistant contaminants with regard to their (absence of) chemicalactivity, toxicity, digestive uptake, and plant uptake. The humanbioaccessibility could for instance be assessed using theSimulator of the Human Intestinal Microbial Ecosystem(SHIME).41

The contaminant trap might also be useful for testing reme-diation strategies that involve amendments with activated carbonand biochar, which are added to soils and sediments in order tominimize their contaminant bioavailability and mobility.42-44

The amended soils and sediments can then be incubated in thecontaminant traps in order to test and hopefully document thatthe contaminants are converted into a desorption-resistant form.The desorption kinetics and dynamics of the PAHs were

somewhat ignored in the present study. For kinetic studies, thecontaminant trap can be used to maintain a full gradient betweensoil and solution in order to determine well-defined desorptionkinetics in analogy to the Tenax method of Cornelissen.11 Forshort-term kinetics, agitation of the traps would be recom-mended, as this would substantially improve the temporalresolution of the method.34 Alternatively, the composite can justbe brought in contact with intact soil in order to determine thediffusive flux into an infinite sink in analogy to the diffusivegradient in the thin film (DGT) concept that has been developedfor cationic metals.45 However, such applications require furtherdevelopment of the method and particularly an adjustment of thetrapping material. The challenge is to select a sorbent thatprovides sufficient sorption capacity to act as a diffusive sinkwhile at the same time allowing for quantitative back-extractionsof the target analytes.In summary, the contaminant trap is a practical and simple

approach for the continuous stripping of desorbable PAHs,which can be utilized for the isolation and quantification of thedesorption-resistant PAH fraction. This desorption-resistantPAH fraction was in good agreement with the PAH concentra-tions remaining after a biodegradation experiment, and the trapsare thus expected to be useful for the assessment and manage-ment of PAH-contaminated soil. Additional research is needed toinvestigate the exposure and risks originating from the desorp-tion-resistant contaminant fraction.

’ASSOCIATED CONTENT

bS Supporting Information. (1) The composition andcharacteristics of the two industrial soils, (2) the trappingefficiency of the contaminant traps at high naphthalene loadings,and (3) PAH concentrations remaining in soils during theexperiments including the abiotic controls. This material isavailable free of charge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*Tel: þ45 46 30 18 81; fax: þ45 46 30 11 14; e-mail: [email protected].

’ACKNOWLEDGMENT

We thank Margit Fernqvist for her technical assistance. Thisresearch project was financially supported by the Danish Councilfor Strategic Research (REMTEC), the European Commission(MODELPROBE, no. 213161), the Austrian EnvironmentalMinistry/KPC (ISPAK), and the Austrian FFG (SOILDRIFT).

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