A Novel Equilibrium Passive Sampler for Methylmercury and Other Advances in Monitoring and Activated Carbon Remediation for Mercury and PCBs
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Date
2018-01-01
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Department
Chemical, Biochemical & Environmental Engineering
Program
Engineering, Civil and Environmental
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Distribution Rights granted to UMBC by the author.
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Access limited to the UMBC community. Item may possibly be obtained via Interlibrary Loan thorugh a local library, pending author/copyright holder's permission.
This item is likely protected under Title 17 of the U.S. Copyright Law. Unless on a Creative Commons license, for uses protected by Copyright Law, contact the copyright holder or the author.
Abstract
The ecological and human health risks posed by persistent sediment contaminants like polychlorinated biphenyls (PCBs) and methylmercury (MeHg) are often controlled by their bioavailability at low trophic levels. Benthic and epibenthic organisms can serve as a conduit for the contaminants to enter aquatic food webs, where they are subject to a large degree of biomagnification. As a result, benthic bioavailability is of central importance. This dissertations describes a series of projects linked by the common goal of advancing bioavailability science to improve the quality of contaminated site risk assessment and management. First, cutting-edge environmental engineering technologies were employed to monitor and reduce the bioavailability of PCBs. The principles underlying these technologies were then adapted and applied to the closely related but uniquely challenging problem of mercury, resulting in the development of a novel equilibrium passive sampler for MeHg and critical advances in the use of activated carbon (AC) for in situ mercury remediation. To begin, a multi-year, pilot-scale study was performed to evaluate the persistence and efficacy of several different AC-based sediment amendments in the unique setting of an intertidal, estuarine marsh overrun with Phragmites australis reeds. Black carbon and PCB measurements showed that all three tested amendments remained in place through three years of daily tidal cycling and an unusually disruptive storm event. All three reduced bioaccumulation of PCBs by benthic and epibenthic organisms, with the most effective being a product containing a finer-grained AC. The passive sampling effort for this project employed several different polymer types and performance reference compound (PRC) adjustment methods, enabling direct comparisons that can inform ongoing method development and standardization. The results indicated good agreement among disparate PRC methods, but also identified AC amendment as a potential confounding factor for a newer method based on modeling diffusive transport through sediments. For mercury and MeHg remediation, a project was undertaken to explain the large variability observed in early tests of AC amendment for mercury-impacted sediments. AC-water partitioning coefficients for mercury species complexed with dissolved organic matter (DOM) were measured and found to be far smaller than those previously reported for chloride complexes. Accordingly, the positive effect of AC on inorganic mercury partitioning in microcosms of slurried soil was attenuated by the addition of exogenous DOM. However, no effect on MeHg partitioning was observed. These results highlight the importance of considering site chemistry in remediation design. Lastly, a novel passive sampling device for MeHg in sediment and soil porewaters was developed. This project seeks to address the need for a reliable, equilibrium sampling strategy capable of predicting MeHg bioavailability. Prospective materials were tested under increasingly realistic environmental conditions and several proved capable of concentrating aqueous MeHg by three to four orders of magnitude, permitting good prediction of porewater concentrations in a variety of matrices including salt marsh soils with and without AC amendment. One sampling material, a suspension of AC particles in agarose gels, underwent additional testing that found reasonably rapid, internally diffusive, and reversible MeHg accumulation. Sampler-water partitioning was in the range of 103.0�104.0 L kg-1, comparable to sediment-water partitioning at many sites. In a preliminary bioaccumulation study, sampler uptake was correlated with concentrations in the benthic amphipod L. plumulosus, which were reduced by amendment with AC. The project constitutes a successful proof of concept and provides the basis for further refinement. Future work will seek to optimize the design and preparation of samplers and develop a more detailed understanding of the dynamics of MeHg exchange among samplers and various ligands to facilitate better interpretation of their measurements.