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Cross validation of two partitioning-based sampling approaches in mesocosms containing PCB contaminated field sediment, biota, and activated carbon amendment Stine Nørgaard Schmidt, Alice P. Wang, Philip T. Gidley, Allyson H. Wooley, Guilherme R. Lotufo, Robert M. Burgess, Upal Ghosh, Loretta A. Fernandez, and Philipp Mayer Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b01909 • Publication Date (Web): 02 Aug 2017 Downloaded from http://pubs.acs.org on August 3, 2017

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Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Environmental Science & Technology

In situ sampling

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Ex situ equilibrium sampling

ACS Paragon Plus Environment


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TITLE:

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Cross validation of two partitioning-based sampling approaches in mesocosms containing

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PCB contaminated field sediment, biota, and activated carbon amendment

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AUTHORS:

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Stine N. Schmidt,*1 Alice P. Wang,2 Philip T. Gidley,3 Allyson H. Wooley,3 Guilherme R.

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Lotufo,3 Robert M. Burgess,4 Upal Ghosh,5 Loretta A. Fernandez,*2 and Philipp Mayer1

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AFFILIATIONS:

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1

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Lyngby, Denmark; 2Northeastern University, Department of Civil and Environmental

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Engineering, Boston, MA, USA; 3US Army Corps of Engineers, Engineer Research and

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Development Center, Vicksburg, MS, USA; 4US Environmental Protection Agency,

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NHEERL/Atlantic Ecology Division, Narragansett, RI, USA; 5University of Maryland

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Baltimore County, Department of Chemical, Biochemical, and Environmental Engineering,

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Baltimore, MD, USA

Technical University of Denmark, Department of Environmental Engineering, Kgs.

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CORRESPONDING AUTHORS (*):

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Stine N. Schmidt, Technical University of Denmark, Department of Environmental

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Engineering, Kgs. Lyngby, Denmark. Phone: (+45) 45251425. E-mail: [email protected]

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and Loretta A. Fernandez, Northeastern University, Department of Civil and Environmental

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Engineering, Boston, MA, USA. Phone: (+1) 617 373 5461. E-mail: [email protected]

1 ACS Paragon Plus Environment

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ABSTRACT The Gold Standard for determining freely dissolved concentrations (Cfree) of

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hydrophobic organic compounds in sediment interstitial water would be in situ deployment

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combined with equilibrium sampling, which is generally difficult to achieve. In the present

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study, ex situ equilibrium sampling with multiple thicknesses of silicone and in situ pre-

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equilibrium sampling with low density polyethylene (LDPE) loaded with performance

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reference compounds were applied independently to measure polychlorinated biphenyls

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(PCBs) in mesocosms with (1) New Bedford Harbor sediment (MA, USA), (2) sediment

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and biota, and (3) activated carbon amended sediment and biota. The aim was to cross

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validate the two different sampling approaches. Around 100 PCB congeners were

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quantified in the two sampling polymers, and the results confirmed the good precision of

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both methods and were in overall good agreement with recently published silicone to

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LDPE partition ratios. Further, the methods yielded Cfree in good agreement for all three

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experiments. The average ratio between Cfree determined by the two methods was factor

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1.4±0.3 (range: 0.6-2.0), and the results thus cross-validated the two sampling

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approaches. For future investigations, specific aims and requirements in terms of

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application, data treatment, and data quality requirements should dictate the selection of

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the most appropriate partitioning-based sampling approach.



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INTRODUCTION Worldwide, enormous amounts of sediment contaminated with hydrophobic organic

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compounds (HOCs) require assessment and remediation.1,2 The first step is often to

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measure the extent of the contamination in terms of concentration and spatial distribution.

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Historically, exhaustive extraction methods have been applied to determine total

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concentrations (Ctotal). However, Ctotal is poorly related to the actual exposure and thereby

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the potential risk for bioaccumulation and adverse effects caused by sediment associated

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contaminants.3,4 Therefore, partitioning-based sampling methods are increasingly used for

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determining freely dissolved concentrations (Cfree),5,6 which quantify effective

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concentrations for diffusive mass transfer and partitioning7 and thus provide a basis for

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quantitative thermodynamic exposure assessments.

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Within the last two decades, several partitioning-based sampling methods have been

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developed, tested, and applied for determining the Cfree of HOCs in sediment interstitial

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water.6 They fall into two categories: (1) The equilibrium sampling approach is to

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equilibrate a polymer with the sediment, which generally requires ex situ incubation of very

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thin polymers under agitated conditions in the laboratory.8,9,10 The advantage of this

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approach is that equilibrium partitioning is a simple and well-defined regime, whereas its

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limitation is that it does not necessarily provide in situ levels of Cfree.7 (2) The in situ pre-

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equilibrium sampling approach is to place a polymer within the sediment on site and then

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infer equilibrium concentrations through the use of performance reference compounds

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(PRCs) or time series measurements.11,12,13 While equilibration is still achievable, it

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typically requires sampling times of months, and for larger HOCs even years, which is

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often impractical and can lead to considerable delays in project conduct and decision

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making. The advantage of this approach is its potential for capturing in situ levels of Cfree.


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However, this is a more complicated approach as it requires additional steps such as mass

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transfer modeling and initial estimates of contaminant levels and partitioning behavior in

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sediment to determine appropriate sampler sizes and quantities of PRCs.14 The use of

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PRCs and mass transfer models can also introduce error and uncertainty.7

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The Gold Standard for determining Cfree of HOCs in sediment interstitial water would

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be in situ deployment combined with equilibrium sampling (i.e., “in situ equilibrium

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sampling”), which is generally difficult to achieve. However, two promising but still

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developing approaches may make in situ equilibrium sampling feasible and practical: in

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situ vibration of the sampler15 and in situ sampling with multiple thicknesses of silicone.16,17

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During the interim, the strategy of the present study was to compare ex situ equilibrium

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sampling with in situ pre-equilibrium sampling in mesocosm experiments with sediment

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contaminated with polychlorinated biphenyls (PCBs). The aims were (1) to quantify the

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magnitude, precision, and relationship of equilibrium concentrations in two polymers that

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were deployed in two different ways and (2) then to compare Cfree determined by the two

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methods. Ex situ equilibrium sampling with multiple thicknesses of silicone and in situ

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sampling with low density polyethylene (LDPE) loaded with PRCs were applied. Despite

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differences in their working principles, the null hypothesis was that the two sampling

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approaches would yield (1) equilibrium concentrations in silicone and LDPE in good

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agreement with recently reported silicone to LDPE partition ratios and (2) similar Cfree

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values. This cross validation on two levels would make the two approaches truly

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compatible, complementary, and well aligned.

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The present study is part of a project that combines quantitative thermodynamic

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exposure assessments and bioaccumulation studies to assess sediment restoration

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techniques based on amendment with activated carbon (AC). A range of mesocosm



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experiments were prepared with PCB contaminated sediment from New Bedford Harbor

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(NBH, MA, USA), which contains total PCB concentrations in the hundreds of mg kg-1 (dw)

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primarily from contamination by Aroclors 1242 and 1254.18 The mesocosms were used for

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a systematic comparison of the two sampling approaches under controlled conditions to

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avoid boundary effects and lateral transport commonly noted in field plot studies.19 The

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two approaches were applied in experimental setups with biota, AC amendment, and

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ongoing PCB contamination of the sediment. The masses of PCBs accumulated in both

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sampling polymers were measured with the same precise analytical method, providing the

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analysis of up to 130 congeners. In this way, the basis for comparing the two sampling

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approaches was free of biases caused by differing analytical laboratories.

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EXPERIMENTAL SECTION This paper includes results from three mesocosm experiments with sediment

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(Experiment 1), sediment and biota (Experiment 2), and AC-amended sediment and biota

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(Experiment 3). Sediment was collected from NBH on two occasions and stored in steel

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drums at 2.8-4.0°C for about a year. Sediment from three drums was combined,

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homogenized, and stored for days to months before use. Prior to starting Experiment 3,

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the experimental sediment was amended with 4.3% AC (dw) during careful mixing for a

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month (see page S2-3, SI). Experimental mesocosms were prepared in 52-L glass aquaria

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measuring 51×25×41 cm (length×width×height, Glasscages.com, USA), and 13-14 kg of

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wet sediment was added to each of six aquaria for each of the three experiments.

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Reconstituted seawater (30‰) was prepared by dissolving Instant Ocean® in water

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treated by reverse osmosis filtration and then slowly added to fill the aquaria. The

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mesocosms were kept at 20°C using a water bath, covered by acrylic lids, and left to


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consolidate for 8-14 d before the experiments started. After consolidation, the sediment

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layer was approximately 6 cm thick. In Experiments 2 and 3, five worms (Nereis virens,

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~3.5 g ww each), 15 clams (Mercenaria mercenaria, ~10.6 g ww each), and five fish

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(Cyprinodon variegatus, ~1.4 g ww each) were added to each mesocosm, with the fish

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being held in mesh, stainless steel cages to prevent them from directly contacting and

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bioturbating the sediment. During the 90-d experiments, diffused air was continuously

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supplied (via an aquarium air stone) to aerate the surface water. Gravel bed treatment

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systems were operated to maintain low ammonia levels, and the surface water was re-

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circulated with an overall flow rate of 17.3±9.5 L h-1 (range: 7.9-46.1 L h-1, n=53) in the 18

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mesocosms. Temperature, pH, dissolved oxygen, and salinity were monitored daily, and

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ammonia levels were monitored at least once a week. The mesocosm experiments were

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conducted with a photoperiod of 16 h light and 8 h dark.

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In each experiment, three replicate mesocosms received PCB-spiked sediment input

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three times a week for the full period of 90 d to simulate ongoing PCB contamination of the

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sediment. Correspondingly, three replicate mesocosms received un-spiked sediment. The

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sediment used for inputs was collected from a relatively clean area (Bayou Lafourche near

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Leeville, LA, USA, see page S3). Part of the sediment was spiked with the “input” PCB

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congeners 13, 54, and 173 (Ultra Scientific, USA). The spiking was performed following

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the shell coating method described by Northcott and Jones (2000) and Lotufo et al

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(2001).20,21 For each of the three congeners, an acetone solution was prepared and

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pipetted into the same 2-L glass jar. The jar was rolled horizontally (5.5 rpm) without lid

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until complete evaporation of the acetone, leaving the compounds coated on the inner jar

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surface. Then, 700 g of wet sediment was added, the jar sealed and left to roll horizontally

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at 5.5 rpm for two weeks before use as PCB-spiked input sediment. Four batches of PCB-



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spiked sediment were prepared and used in Experiments 1-3. The weighted average

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concentrations of PCBs 13, 54, and 173 were 9.3, 12.5, and 2.8 mg kg-1 (dw), respectively

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(see page S4). In the same way, part of the sediment was rolled for two weeks in the

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absence of added PCBs before use as un-spiked input sediment (the three congeners

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were not detected in this material). The sediment inputs were introduced to the 18

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mesocosms manually by pre-mixing 5 mL sediment (containing approximately 1.35 g dry

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sediment particles) with water from the respective mesocosm to form a uniform and

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dispersible plume in the mesocosm surface water. Water circulation through the treatment

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systems was stopped just before adding the input material and switched off for 7 h to allow

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the introduced sediment to settle.

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In situ sampling with LDPE. Strips were prepared from a LDPE sheet with a

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thickness of 25.4 µm (ACE Hardware Corp., USA) and measured 2.5×15.2 cm. First, the

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strips were pre-cleaned by two overnight methylene chloride rinses, followed by two

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methanol rinses, and two additional rinses with laboratory water treated by reverse

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osmosis filtration, ion exchange, and AC filtration (EMD Millipore, Germany). Then, the

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strips were loaded with four PRCs (rare PCB congeners 14, 35, 73, and 122, Ultra

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Scientific, USA). PCB standard solutions of 100 mg L-1 in hexane (Ultra Scientific, USA)

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were combined, and the majority of hexane evaporated under nitrogen. The PRCs were

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then re-dissolved in methanol (J.T. Baker, USA) to reach a final concentration of 5 mg L-1

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for each of the compounds in the mixture. Each strip was loaded separately by adding it to

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a 500-mL amber jar filled with laboratory water spiked with 200 µL of the PRC mixture. The

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jars were sealed with aluminum foil lined caps and placed on an orbital shaker for six

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weeks (80 rpm) in accordance with a necessary loading time, as calculated from the

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MATLAB mass transfer model.22 After loading, the nominal mass of each PRC was 1000


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ng per strip and the precise pre-exposure concentrations (t0) were measured at the time of

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sampler analysis, as described below.

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At day 0, one loaded strip was inserted into the sediment bed in each of the 18

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mesocosms (supported by a stainless steel frame). The strips were deployed in the

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sediment for the full period of 90 d and then recovered for analysis. Each strip was rinsed

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with deionized water to remove adhering sediment particles, dried with lint free tissue, and

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cut into five approximately equally-sized horizontal segments. One segment, the part

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sampling the interstitial water in the top 2-3 cm of sediment, was analyzed per mesocosm.

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The segments weighed 18.6±2.4 mg (n=18) and were extracted individually in 5 mL

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hexane (95%, Fisher Scientific, USA) overnight before chemical analysis, as described

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below. T0 samples (PRC-loaded, unexposed samplers, n=4) were rinsed and extracted,

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correspondingly.

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Cfree (ng L-1) were calculated from the PCB concentrations in LDPE when

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extrapolated to equilibrium (CLDPE∞, ng kg-1) and compound specific LDPE to water

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partition ratios (KLDPE:water, L kg-1):

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C

177

(1)

= :

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CLDPE∞ were determined from the concentrations measured in LDPE after deployment

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(CLDPE(t), ng kg-1) and an adjustment for the fractional equilibration of the target PCBs (feq,

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unitless):

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C

=

()

(2)

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feq values for the target PCBs were calculated from the fractional equilibration of the PRCs,

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feq PRC (unitless):

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f

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=

! ( " )#

!(

)

! ( " )

(3)

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where CLDPE PRC (t0) is the initial PRC concentration in the sampler (ng kg-1) and

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CLDPE PRC(t) is the PRC concentration in the sampler after deployment (ng kg-1). PRC

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calculation software is available through the Environmental Strategic Technology

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Certification Program.23 This MATLAB graphical user interface runs a Laplace-space

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mathematical model of Fickian diffusion through polymeric and porous media. After

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entering feq PRC values for a given LDPE strip segment, compound specific KLDPE:water

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values, deployment time (90 d), the default sediment porosity (0.7), and thickness of the

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LDPE (25.4 µm), the feq for each target PCB was calculated.23 One PRC, PCB 14, was not

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used in the calculation of feq as it was completely depleted from the samplers. KLDPE:water (L

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kg-1) values were calculated for each PCB congener from its octanol to water partition ratio

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(Kow)24 using a previously published linear free energy relationship based on compiled data

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for 22 polycyclic aromatic hydrocarbons (PAHs) and 110 PCBs (r2=0.89, RSME=0.29,

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Table S1):22

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log K

:()

= 0.97 × log K /( − 0.07

(4)

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Ex situ equilibrium sampling with silicone. Non-depletive equilibrium sampling

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was conducted in silicone coated jars with multiple coating thicknesses.9 These equilibrium

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sampling devices (ESDs) consisted of 120-mL, clear glass jars coated with µm-thin layers


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of silicone (Dow Corning® 1-2577 conformal coating, Diatom A/S, Denmark) on their inner

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vertical surface. The ESDs were coated and pre-cleaned in the following manner: Silicone

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stock solution, as received from the supplier, was weighed into a disposable bottle and

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diluted in pentane. The bottle was sealed and the silicone solution homogenized at 500

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rpm on an orbital shaker for at least 1 h. To produce different coating thicknesses, the

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concentration of silicone was adjusted in the pentane solution, taking into consideration

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the percentage of silicone in the silicone stock solution, the density of the silicone, and the

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area of the inner vertical surface of the glass jars. Then, the glass jars (without lids) were

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placed horizontally on a roller mixer, and a 2-mL aliquot of the well-mixed silicone solution

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was gently added to each jar while rolling (60 rpm). The silicone solution quickly dispersed

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evenly to cover the full inner height of each jar (i.e., from the bottom to the rim). After

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application, the jars were left to roll for 30 min until all organic solvent had evaporated,

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leaving the silicone coated on the inner vertical surface. The jars were left at room

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temperature overnight for the silicone to cure, before final curing at 100°C for 2 h. The jars

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were then pre-cleaned in three steps with 5-mL aliquots of ethyl acetate, acetone, and

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ethanol during horizontal rolling of the sealed ESDs for at least 30 min (60 rpm). After

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removal of the ethanol and evaporation of residual solvent, the mass of silicone in each jar

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was determined gravimetrically by weighing before and after coating and pre-cleaning. For

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this study, ESDs were prepared with three coating thicknesses: (1) 0.76±0.04 µm (6.5±0.3

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mg silicone, n=19), (2) 1.62±0.05 µm (13.7±0.4 mg silicone, n=19), and (3) 3.13±0.13 µm

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(26.6±1.1 mg silicone, n=18). Just before use, the silicone was pre-cleaned with hexane.

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At termination of the mesocosm experiments (day 90), most of the surface water was

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siphoned off and sediment was collected for equilibrium sampling. Three sediment cores

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(with a diameter of 7.2 cm) were taken from each of the 18 mesocosms, and the top 2-3



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cm of sediment was transferred to ESDs coated with 0.76, 1.62, and 3.13 µm silicone,

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respectively. Approximately 80 g wet sediment was added to each jar. Small amounts of

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surface water from the given mesocosm was added to the nearly filled ESDs to obtain

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viscous sediment slurries, which allowed good contact between the sediment and silicone

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during sampling, while at the same time, keeping the water content at a minimum to

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ensure fast PCB mass transfer from sediment to silicone. The ESDs were sealed with

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PTFE-lined plastic lids and rolled horizontally at 19 rpm for 14 d (at room temperature).

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After equilibration, the coated jars were emptied and rinsed with several portions of Milli-Q

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water to remove any sediment particles adhering to the silicone, and the silicone was

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gently dried using lint free tissue. The silicone was then extracted with 2 mL hexane (95%,

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Fisher Scientific, USA) during horizontal rolling (12-14 rpm) of the ESD for at least 30 min.

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Again, the ESDs were sealed with clean PTFE-lined lids. The hexane was collected, and a

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fresh portion of 2 mL hexane added to each jar. The extraction was repeated, the two

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extracts combined, and the volume adjusted to exactly 4 mL before chemical analysis, as

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described below.

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Cfree (ng L-1) were determined via concentrations in silicone at equilibrium with the

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sediment (Csilicone⇌sediment, ng kg-1) and experimentally derived silicone to water partition

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ratios (Ksilicone:water, L kg-1):25

251 252

C

=

2343567 ⇌2 839 7

(5)

2343567 :

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The Csilicone⇌sediment was determined in the following manner: The mass of PCB (ng) was

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plotted against silicone mass (g) in GraphPad Prism 5.0 software (GraphPad Software,

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Inc., USA). Best estimate of the PCB concentration in silicone was determined via linear


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regression through the origin (i.e., the slope, ng g-1) and supplied with a r2 value and

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standard error of the mean (SEM, ng g-1) by the software. For each concentration, the

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relative standard error (RSE, %) was calculated as SEM over Csilicone⇌sediment × 100. For

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PCBs with 8-10 chlorine atoms (i.e., PCBs 194-209), data was excluded for jars with the

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thickest silicone coating due to disequilibrium. Concentrations were deemed valid when

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the regression described more than 60% of the variation in the data set (r2>0.60) and the

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RSE on the concentration was less than 20%. Concentrations were categorized as highly

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precise when r2>0.70 and RSELOD was included in the data treatments.

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Before the experiments started, NBH sediment was screened in order to select

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appropriate internal standards and PCB congeners to be used as PRCs and ongoing input

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congeners. These compounds were selected to avoid interference with native PCBs in the

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sediment in terms of presence and analytical separation.

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RESULTS AND DISCUSSION Results from the two sampling approaches were used to quantify the magnitude,

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precision, and relationship of equilibrium concentrations in the two polymers and to

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compare Cfree determined by the two methods.

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Equilibrium polymer concentrations. A total of 111, 110, and 76 congeners were

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quantified in silicone for the experiments with sediment only, sediment and biota (+biota),

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and AC amended sediment and biota (+AC), respectively. Of these, 97% (sediment), 93%

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(+biota), and 83% (+AC) were deemed valid with respectively 94%, 92%, and 90% of the

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valid concentrations fulfilling the extended validity criteria (r2>0.70 and RSE

Cross Validation of Two Partitioning-Based ... - ACS Publications

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