Prototype of a periodically vibrated device used to deploy passive samplers in sediments.

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 chemicals of concern 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 was conducted in two phases. Phase I 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. 

During Phase II, this research focused on advancing passive sampling on two major fronts: 1) use of mathematical modeling and device development to enable short-term passive sampling for hydrophobic pollutants, and 2) development of an equilibrium passive sampler for methylmercury in water and sediment porewater. The proof-of-concept developments in the Phase I project were successfully advanced with further testing and field deployments, paving the way for the extension of passive sampling to these heretofore difficult to measure analytes/conditions.

High resolution depth profile in sediments.

Equilibrium passive sampler for methylmercury​.

Field deployment of vibrating passive sampler.

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 were evaluated, using enriched Hg stable isotope tracers to follow exchange among compounds and samplers. Polymer design was 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 was experimentally tested.

In Phase II, laboratory studies were performed to operationalize a platform for the deployment of passive sampling devices. A key innovation in the design was 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 was 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-impacted sediments, and field-testing.

An equilibrium passive sampler for MeHg was developed that can be used in water or sediment. The goal was to develop a sampler using the equilibrium model concept developed for hydrophobic organic chemicals, while building on the knowledge gained from kinetic passive sampler development for MeHg to date. First, the project team identified sampler materials with the potential to mimic MeHg partitioning into sediments using a range of polymers embedded with suitable sorbents for MeHg. The most promising were activated carbon (AC) embedded in agarose, thiol-SAMMS embedded in agarose, and cysteine-functionalized polyethylene terephthalate. Investigation of the MeHg accumulation mechanism by AC embedded in agarose suggested sampling was kinetically influenced by MeHg interactions with AC particles and not limited by diffusion through the gel for this material. Reversible equilibrium exchange of environmentally relevant MeHg species was demonstrated through a series of dual isotope labeled exchange experiments confirming the feasibility of using isotope labeled MeHg as performance reference compounds. Isotopically labeled MeHgDOM species approached equilibrium in the samplers over 14 d while mass balance was maintained, providing strong evidence that the ag+AC polymer material is capable of equilibrium measurements of environmentally relevant MeHg species within a reasonable deployment time frame. Samplers deployed across the sediment-water interface of sediment microcosms and in the field estimated both overlying water and porewater MeHg concentrations within a factor of two to four of grab sample measurements, based on the average measured sampler-water partitioning coefficients (K_ps) for species of MeHg bound to natural organic matter in adsorption isotherm experiments. 

This project demonstrated through numerical modeling and laboratory/field experiments that short periodic shaking of a passive sampler deployed in static sediment (by incorporating low-cost motors in the sampler platform) greatly enhances the rate of mass transfer and brings the sampler closer to equilibrium. The improvement over static sediment deployment is especially relevant for the high molecular weight compounds such as benzo(a)pyrene, high molecular weight polychlorinated biphenyls, and dioxins. Through careful modeling of mass transfer in a passive sampler, this research showed that the time integration period in a passive sampler is different for different compounds and needs to be considered when interpreting results of long-term deployments. The research also demonstrates the feasibility of short-term water column measurements using passive samplers to capture episodic events like stormflows.

Samplers provided good estimates for MeHg across a range of sediment types. A passive sampler sheet deployed in the sediment could be sectioned into 0.5 cm slices to obtain a high-resolution depth profile of porewater MeHg in sediments that can be valuable for the assessment of transport and remediation effectiveness. Taken together, the results indicate that ag+AC polymers, used as equilibrium samplers, can provide accurate MeHg estimations across many site chemistries, with a simple back calculation based on a standardized K_ps. (Project Completion - 2024)

Jalalizadeh, M. and U. Ghosh. 2017. Analysis of Measurement Errors in Passive Sampling of Porewater PCB Concentrations under Static and Periodically Vibrated Conditions. Environmental Science and Technology, 51(12):7018-7027. doi.org/10.1021/acs.est.7b01020. 

Jalalizadeh, M., U. Ghosh. 2016. In Situ Passive Sampling of Sediment Porewater Enhanced by Periodic Vibration. Environmental Science and Technology, 50(16):8741–8749. doi.org/10.1021/acs.est.6b00531.

Sanders, J.P., A. McBurney, C.C. Gilmour, G. Schwartz, S. Washburn, S.B. Kane Driscoll, S.S. Brown, and U. Ghosh. 2020. Development of a Novel Equilibrium Passive Sampling Device for Methylmercury in Sediment and Soil Porewaters. Environmental Toxicology and Chemistry, 39(2):323–334. doi.org/10.1002/etc.4631.

Washburn, S.J., J. Damond, J.P. Sanders, C.C. Gilmour, and U. Ghosh. 2022. Uptake Mechanisms of a Novel, Activated Carbon-Based Equilibrium Passive Sampler for Estimating Porewater Methylmercury. Environmental Toxicology and Chemistry, 41(9):2052-2064. doi.org/10.1002/etc.5406.

Patents

Ghosh, U. and M. Jalalizadeh. 2020. Actively Shaken In-Situ Passive Sampling Device. US Patent. Pub. No. US 2018/0088008A1.

Student Awards

Damond, J. 2020. Second Place, Best Student Platform Presentation, SETAC Chesapeake Potomac Regional Chapter Annual Virtual Meeting.

Damond, J. 2022. Third Place, SETAC Chesapeake-Potomac and Hudson-Delaware Regional Chapter (CPRC-HDC) Joint Spring Meeting.

Ghosh, O. 2020. Third Place, Best Student Platform Presentation, SETAC Chesapeake Potomac Regional Chapter Annual Virtual Meeting.

Ghosh, O. 2023. Best Student Paper Award at the Battelle International Sediment Conference.

Jalalizadeh, M. 2016. First Place, Student Poster Award, SETAC Hudson Delaware Chapter Meeting.

Jalalizadeh, M. 2017. Third Place, 8th Annual Geosyntec Student Paper Competition.

Sanders, J. 2016. Most Popular Poster, Society of Environmental Toxicology and Chemistry, Chesapeake Potomac Regional Chapter, Virtual Poster Contest.

Sanders, J. 2017. Best Student Poster Award, Society of Environmental Toxicology and Chemistry, Hudson Delaware Chapter meeting.

Sanders, J. 2017. Second Place, 8th Annual Geosyntec Student Paper Competition.

Sanders, J. 2020. Best Student Paper Award from Society of Environmental Toxicology and Chemistry.

Sanders, J. Top 10 Exceptional Papers of 2020 by the Journal of Environmental Toxicology and Chemistry.