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Enhanced accessibility of PAHs and heterocyclic PAHs in industrially contaminated soil after passive dosing of a competitive sorbate Stefan Humel, Stine Nørgaard Schmidt, Marion SumetzbergerHasinger, Philipp Mayer, and Andreas Paul Loibner Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b01198 • Publication Date (Web): 05 Jun 2017 Downloaded from http://pubs.acs.org on June 8, 2017
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Environmental Science & Technology
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Enhanced accessibility of PAHs and heterocyclic
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PAHs in industrially contaminated soil after passive
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dosing of a competitive sorbate
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Stefan Humel,† Stine N. Schmidt,‡ Marion Sumetzberger-Hasinger,† Philipp Mayer,‡ and
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Andreas P. Loibner*†
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†
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Austria
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‡
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Lyngby, Denmark
IFA-Tulln, BOKU - University of Natural Resources and Life Sciences, Vienna, 3430 Tulln,
Department of Environmental Engineering, Technical University of Denmark, 2800 Kgs.
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* corresponding author, BOKU - University of Natural Resources and Life Sciences, Vienna,
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IFA-Tulln, Konrad-Lorenz-Straße 20, 3430 Tulln, Austria, phone: +43 1 47654 97470, fax: +43
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1 47654-97409, email:
[email protected] 13 14
KEYWORDS: soil, polycyclic aromatic hydrocarbons, bioavailability, passive dosing,
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competitive sorption
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Abstract graphic:
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ABSTRACT
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To assess the exposure to polycyclic aromatic hydrocarbons (PAHs), it is important to
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understand the binding mechanisms between specific soil constituents and the organic pollutant.
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In this study, sorptive bioaccessibility extraction (SBE) was applied to quantify the accessible
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PAH fraction in industrially contaminated soil with and without passive dosing of a competitive
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sorbate. SBE experiments revealed an accessible PAH fraction of 41±1% (∑16 US EPA PAHs +
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5 further PAHs). The passive dosing of toluene below its saturation level revealed competitive
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binding and resulted in an average increase of the accessible fraction to 49±2%, whereby
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primarily the accessibility of higher molecular weight PAHs (log Kow >6) was affected.
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Competitive binding was verified using the same soil with only desorption resistant PAHs
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present. In this experiment, passive dosing of toluene resulted in desorption of 13±0.4% PAH.
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We explain increased PAH desorption after addition of toluene by competitive adsorption to high
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affinity sorption sites while acknowledging that toluene could additionally have increased PAH
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mobility within the soil matrix. Findings suggest that the presence of co-pollutants at
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contaminated sites deserves specific considerations as these may increase accessibility and
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thereby exposure and mobility of PAHs.
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INTRODUCTION
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Polycyclic aromatic hydrocarbons (PAHs) are hydrophobic pollutants that are found
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ubiquitously in the environment. They can be classified based on their origin as (i) petrogenic
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PAHs derived from liquid organic fuels before combustion, (ii) pyrogenic PAHs are derived
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from incomplete combustion processes either on particles such as soot or in non-aqueous phase
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liquids (NAPL) as formed during coal gasification, and (iii) biogenic PAHs produced by certain
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aquatic and terrestrial microorganisms.1,2 Since the beginning of industrialization, industrial
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production and economic development have been based on processing and combustion of fossil
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fuels. Consequently, PAHs and other petrogenic and pyrogenic products (e.g. mono-aromatics)
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are frequently found in the environment.3–5 PAHs, as prioritised by the US EPA,6 may be found
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in environmental samples together with contaminants such as heterocyclic PAHs containing
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nitrogen, sulphur or oxygen atoms and alkylated PAHs.7–9 Some of these compounds are potent
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toxins and therefore face specific considerations.10
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When hydrophobic contaminants like PAHs are introduced into the terrestrial environment,
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they tend to sorb to hydrophobic regions in soil organic matter (SOM).11–13 SOM comprises
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diverse amorphous organic carbon phases that contain more or less converted residues of
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biomass (plants, animals and microorganisms). It may also contain a fraction of combustion-
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derived matter known as black carbon.14–16 Whereas absorption of PAHs is suggested to be
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dominant for the amorphous organic carbon phase, adsorption to surfaces with high affinity for
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PAHs is considered to occur for carbonaceous materials like black carbon, especially at
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environmentally relevant low contaminant concentrations.14,17–19 Such carbonaceous materials
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are frequently present in industrially contaminated soils from combustion related manufacturing
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sites.20–22 Whereas absorption by organic carbon involves a partitioning process into the organic
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matter that is described by linear models, presuming infinite sorption sites,23,24 adsorption to
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black carbon is described by nonlinear models and characterized by a limited number of sorption
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sites.14,25–27 Strong sorption of PAHs in soil and other processes, such as physical encapsulation
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or tar oil alterations,12,22,28–30 result in a reduction in the accessible fraction of the contaminants
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and thereby decreasing exposure.31 PAHs that are resistant to desorption are expected to be
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adsorbed to high affinity sites, trapped in soil pores or partially occluded in soot or weathered tar
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matrices.14,20,32 High affinity adsorption sites would be subject to competition in the presence of
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co-sorbates such as mono-aromatic substances,33 a group of contaminants that is often found
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together with PAHs at industrial sites.3–5 Competitive sorption is considered a process in which
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two sorbates aim for the same sorption site. In contrast to extraction, the competing sorbates are
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dissolved or matrix-associated and no free phase of a sorbate is present. Competition for
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adsorption sites has already been investigated with substance classes such as PAHs,27,34,35 mono-
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aromatic hydrocarbons,36,37 chlorinated hydrocarbons and aromatic pharmaceuticals.38–40
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Competition was also observed between chlorinated benzene and different natural acids,41
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between different hydrophobic organic compounds including PAHs,42 between toluene and
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naphthalene as well as between PAHs and surfactants,33,43,44 indicating that also different
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substance classes may compete for the same sorption sites. However, all of these studies were
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performed with polluted river sediments or artificially spiked soils (or carbon nanotubes and
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activated carbon), which may result in a competitive behavior different from field-contaminated
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soils.45,46
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Competitive sorption may change the accessibility of PAHs in soil. To quantify the accessible
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fraction of PAHs, several methods have been developed using selected solvents to establish mild
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extraction conditions.47–50 More recent methods integrate a silicone based sorptive sink for
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capturing pollutant molecules that have desorbed from the soil.51–55 Such sorptive
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bioaccessibility extraction (SBE) methods provide high extraction capacity and resemble
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environmental conditions very well as they are based on aqueous desorption processes and do
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not modify the soil matrix.53 SBE directly measures the accessible fraction of PAHs as it
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quantifies the amount that is desorbing from soil.
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In this study, competition for high affinity adsorption sites between toluene and PAHs was
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investigated in industrially contaminated soil. The aim was to identify the retention mechanism
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for the desorption resistant PAHs, with the working hypothesis that the addition of toluene as
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competitive sorbate can enhance PAH release and accessibility, which in turn would confirm
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PAH adsorption to high affinity sites. Additionally, the study aimed to quantify the expected
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enhancement of PAH bioaccessibility by competitive sorbates as quantitative data would impact
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the assessment and management of environmental risk. The technical working hypothesis was
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that a silicone rod simultaneously can serve as analytical sorptive sink for the PAHs and as
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passive dosing phase for the competitive sorbate (e.g. toluene).
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MATERIAL AND METHODS
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Soil. The industrially contaminated soil was sampled at a landfill in Lower Austria and
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originated from an industrial site in Vienna, Austria. The soil as received contained residues of
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tar paper, wood, bricks and concrete. The soil was sieved 6, it accelerated the
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desorption of all PAHs. Differences in the desorption rate were observed for PAHs with a log
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Kow
