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Analysis of measurement errors in passive sampling of porewater PCB concentrations under static and periodically vibrated conditions Mehregan Jalalizadeh, and Upal Ghosh Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 24 May 2017 Downloaded from http://pubs.acs.org on May 27, 2017
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Environmental Science & Technology
Analysis of measurement errors in passive sampling of porewater PCB concentrations under static and periodically vibrated conditions
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Mehregan Jalalizadeh and Upal Ghosh*
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Department of Chemical, Biochemical, and Environmental Engineering
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University of Maryland Baltimore County
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Baltimore, MD 21250
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*
Corresponding author:
[email protected]; 410-455-8665; Fax: 410-455-6500
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ABSTRACT
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Although the field of passive sampling to measure freely dissolved concentrations in sediment
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porewater has been sufficiently advanced for organic compounds in the low- to mid-range of
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hydrophobicity, in situ passive sampling of strongly hydrophobic polychlorinated biphenyls
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(PCBs) is still challenged by slow approach to equilibrium. Periodic vibration of polyethylene
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(PE) passive samplers during exposure has been previously shown to enhance the mass transfer
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of polycyclic aromatic hydrocarbons (PAHs) from sediment into PE. Herein, we used a new
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vibrating platform, developed based on our earlier platform design, to demonstrate the
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effectiveness of periodic vibration for large molecular weight PCBs such as hexa-, hepta-, and
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octachloro-PCBs. Uptake of PCBs in PE after 7, 14, 28, and 56 days under different vibration
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modes was compared to that under static and mixed laboratory deployments. All PCBs reached
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within 95 – 100% of equilibrium after 56 days of deployment in the system vibrated briefly
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every 2 min, while none of the congeners achieved more than 50% of equilibrium in static
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deployment for the same period. Periodic vibration also increased the dissipation rate of four
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performance reference compounds (PRCs) from passive samplers. Higher fractional loss of
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PRCs and closer approach to equilibrium in the vibrated deployment resulted in estimation of
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corrected porewater concentrations that were statistically indistinguishable from the true
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equilibrium values even after a short 7-day deployment. Porewater concentrations of the strongly
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hydrophobic PCB congeners were overestimated by up to an order of magnitude in the static
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passive sampler after the same deployment time.
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Environmental Science & Technology
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INTRODUCTION
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Reliable bioavailability measurements of hydrophobic organic compounds (HOCs) such as
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polychlorinated biphenyls (PCBs), and polychorinated dibenzo-p-dioxins/dibenzofurans
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(PCDD/Fs) are needed for improved sediment risk assessments, proper selection of remedy, and
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post-remediation monitoring. Several studies have shown that freely dissolved concentrations
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(Cfree) of organics in sediment porewater can be related to toxicity and bioaccumulation.1,2
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Passive sampling for the measurement of Cfree of organic pollutants in sediment porewater has
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emerged as a very promising approach, but in situ measurements are challenged by slow mass
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transfer of strongly hydrophobic compounds. The slow mass transfer results in under-
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equilibrated passive sampler measurements that need to be corrected for determining Cfree. For a
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passive sampler deployed in stagnant sediment, mass transfer of the contaminants is controlled
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by transport through the polymer and/or sediment phase, depending on the sediment sorption
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characteristics and diffusivity of the pollutants in sediment porewater and polymer.3 For HOCs,
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the overall kinetics is often controlled by transport through the sediment phase.3-5Under this
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condition, correction for non-equilibrium requires estimation of site specific sorption
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characteristics (e.g., retarded diffusion). The previously developed methods determined site-
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specific sorption characteristics from the loss kinetics of selected performance reference
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compounds (PRCs) and calculated corrected Cfree from non-equilibrium polymer
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concentrations.3,6 However, even with these methods, estimating Cfree of strongly hydrophobic
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compounds is prone to errors. Extrapolations based on the loss kinetics of strongly hydrophobic
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PRCs is challenging, since long exposure times are required to measure PRC levels that are with
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statistical significance different from the initially spiked PRC levels.6 For example, Choi et al.4
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observed only 20% reduction in concentration of PCB 192 from 17 µm PE after 265 days of
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deployment in sediment. Also, high imprecisions involved in the measurement of fractional loss
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of PCB 192, caused a wide confidence interval for the estimated sediment sorption
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characteristics for this congener. In addition to the mentioned challenges with using the PRC-
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correction methods, it is not clear yet whether the retardation factor for HOCs that diffuse from
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sediment into the passive sampler is identical to the retardation factor for the PRCs that diffuse
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out of the passive sampler and into the sediment. Apell and Gschwend5 argued that PCBs exhibit
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the same retardation factor through the sediment when diffusing into and out of PE passive
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samplers. However, anisotropic exchange kinetics of PCBs and DDTs has been observed for PE 3
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and PDMS passive samplers in more recent studies.4,7 The mechanisms involved in anisotropic
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exchange kinetics are not well understood, but possibly depend on the sediment properties.4
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Enhancing the overall kinetics by reducing the mass transfer resistance in sediment overcomes
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the mentioned shortcomings of PRC-correction methods. This assumption is tested further in the
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present study.
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In our previous work,8 we addressed the challenge of mass transfer limitation by using a
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vibrating platform for the deployment of passive sampling devices in sediments. We
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demonstrated through laboratory measurements and numerical modeling that the platform can
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greatly enhance the rate of mass transfer of 16 PAHs (3.4
