Actively Shaken In-Situ Passive Sampler Platform for Methylmercury and Organics

Dr. Upal Ghosh | University of Maryland Baltimore County

ER-2540

Objective

ER-2540_WinterIPR2019

Accurate bioavailability measurements are needed for improved site risk assessments, proper selection of remedy, and post-remediation monitoring. While freely dissolved porewater concentrations of organic contaminants and aqueous concentrations of bioavailable forms of trace metals in sediments have been demonstrated to be the ideal metrics for assessing bioavailability, accurate measurements of the low environmentally relevant concentrations have been a challenge. Passive sampling for sediment porewater has emerged as a very promising approach, but in situ measurements are complicated by slow mass transfer of strongly hydrophobic compounds. For methylmercury (MeHg), development of passive samplers has been hindered by the lack of an approach that adequately predicts porewater concentrations in complex milieu.

This project is being conducted in two phases. Phase I is complete and initial efforts successfully demonstrated the feasibility of using periodic vibration to enhance mass transfer and accuracy of measurement of strongly hydrophobic compounds and also demonstrated the feasibility of an equilibrium passive sampling approach for MeHg. The results of Phase I studies can be found in the Final Report. In Phase II, key objectives are to 1) optimize the vibration frequency through mathematical modeling and laboratory experiments, 2) demonstrate passive sampling for PCBs, DDT, and dioxin/furan contaminated sediments, 3) field-test the vibrating passive sampling platform; 4) complete development of a passive sampling material that reliably predicts pore water MeHg concentrations in complex matrices, 5) develop enriched stable isotope MeHg spikes as performance reference compounds, 6) verify the predictive capability of MeHg samplers in natural sediments. The ultimate goal is to develop an in situ, actively shaken deployment platform that can accommodate multiple types of passive samplers (for organics and MeHg).

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Technical Approach

In Phase II, laboratory studies will be performed to operationalize a platform for the deployment of passive sampling devices. A key innovation in the design will be the incorporation of vibration devices to disrupt the aqueous boundary layer around the passive samplers to enhance mass transfer and provide accurate porewater measurements. The proof-of-concept demonstrated in Phase I will be made ready for field use through optimization of the vibration frequency, increasing the size of the devices to accommodate larger samplers, demonstration with PCB/DDT/dioxin/furan contaminated sediments, and field-testing. It is anticipated that the platform will be able to incorporate passive samplers for both hydrophobic organic contaminants such as dioxins, furans, DDT, and PCBs, as well as new samplers for MeHg.

For MeHg, the goal is the development of an equilibrium passive sampling approach, for which the Phase I study provided proof of concept. Sampler uptake kinetics and equilibrium partitioning for a variety of naturally occurring MeHg compounds will be evaluated, using enriched Hg stable isotope tracers to follow exchange among compounds and samplers. Polymer design will be further optimized to achieve rapid and reversible equilibrium with the aqueous MeHg pool. Critically, the idea that passive sampler measurement of “freely dissolved” MeHg can be used to predict total aqueous MeHg will be experimentally tested.

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Benefits

This research advances the field of bioavailability assessment by developing an improved approach for in situ measurement of bioavailable concentrations of pollutants in sediment porewater. The research addresses key bottlenecks for in situ measurements by enhancing mass transfer and by promoting the development of a passive equilibrium sampler for MeHg. (Anticipated Project Completion - 2021)

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Points of Contact

Principal Investigator

Dr. Upal Ghosh

University of Maryland Baltimore County

Phone: 410-455-8665

Fax: 410-455-6500

Program Manager

Environmental Restoration

SERDP and ESTCP

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