Browsing by Subject "Mercury sulfide"

Mercury (Hg) is a toxic pollutant whose speciation will highly impact its fate and transformation in the environment. One understudied form is that of liquid elemental mercury (Hg0L), a dense non-aqueous phase liquid which, upon release to the environment, can persist in soil for decades. Over time, this residual Hg0L is gradually released to groundwater and eventually surface water, serving as a long-term source of contamination. Hg that reaches natural waters can eventually be transformed into methylmercury (MeHg), a powerful neurotoxin that can biomagnify in aquatic food chains and endanger human health. The presence of Hg0L in the environment brings up a number of interrelated topics relevant in Hg science, namely mobilization, reactivity, and exposure. Hg0L contamination exists worldwide due to the historic and current use of Hg0L in various industrial and mining practices. Many former Hg processing sites, such as chlor alkali plants, contain Hg-contaminated soils. Today, however, artisanal and small-scale gold mining (ASGM) is the primary utilizer of Hg0L. This largely unregulated, rudimentary mining process uses large quantities of Hg0L to separate and amalgamate gold from excavated soils. This gold-Hg amalgam is later heated to evaporate the Hg0L, resulting in large losses of Hg to the atmosphere. As a result of the ongoing and past use of Hg0L, threats of Hg exposure due to contaminated soil environments will remain prevalent. This dissertation addresses three questions in order to better understand Hg0L in the environment and reliable ways to assess human exposure to the metal: (1) How does Hg0L interact with sulfide, and what impact do these interactions have on Hg0L dissolution and mobility? (2) What is the Hg speciation in streambank soils downstream of a site with historical Hg0L soil contamination, and what is the potential for future mobility and transformation of Hg in these soils? (3) How does exposure to Hg0L impact the efficacy of hair total Hg (THg) as a proxy for MeHg exposure in ASGM communities? These questions are explored through the lens of two different sites: Madre de Dios, Peru, a region contaminated from rapidly growing ASGM activity, and East Fork Poplar Creek (EFPC), a superfund site with historical Hg0L contamination in Oak Ridge, TN. The spillage of Hg0L, with a density that is 13 times the density of liquid water, results in deep penetration of the metal into soils and entrapment of Hg0L droplets in soil pore spaces. Over time, this residual Hg0L can undergo various chemical and biological transformations that will ultimately impact its fate in the subsurface. Of particular interest is the corrosion of Hg0L via reaction with reduced inorganic sulfur species to form mercury sulfide (HgS), a process that enables long term sequestration of mercury in soils and generally reduces its mobility and bioavailability. The second chapter of this dissertation examines the natural corrosion of Hg0L in the presence of sulfide by quantifying rates of Hg release under different sulfide doses and aging environments. For droplets aged in ambient air, no differences in Hg release were observed among all sulfide doses. However, for droplets aged in the presence of a strong oxidant (H2O2) and reacted under low sulfide doses, we observed an increase in Hg release relative to droplets aged in air. However, the release of Hg from droplets aged in H2O2 was suppressed upon addition of adequate sulfide. These results suggest two critical factors dictate the corrosion of Hg0L in the presence of sulfide: surface oxidation of the droplet and sufficient sulfide dose. Surface oxidation controls the release of Hg into solution; without adequate oxidation, no Hg will be available to be sequestered through precipitation of HgS. However, in the case of an oxidized Hg0L surface, sufficient sulfide is needed to prevent large amounts of Hg from being released into solution. These results suggest that the mobilization of Hg0L will largely depend on aging conditions in the subsurface. Hg mobilized from the dissolution of Hg0L will likely be transported to other environmental compartments downstream. For sites located near alluvial systems, bank soils are one likely receptor of this mobilized Hg. The solid-phase speciation of Hg in these receiving streambanks are critical because erosion and flooding events may mobilize Hg into the adjacent waterbody and eventually be methylated. Therefore in Chapter 3 of this dissertation, we characterized mercury speciation in Hg-enriched soils (100-1100 mg/kg Hg) collected from the incised bank of the East Fork Poplar Creek (EFPC) in Oak Ridge, TN (USA). A combination of characterization techniques were used including scanning electron microscopy, X-ray absorption fine structure spectroscopy, and transmission electron spectroscopy. Altogether the data demonstrated the predominance of nanoparticulate HgS with crystal lattice defects in the streambank soils of this industrially impacted stream. The results of this work support further investigation of the impact of these nanocrystalline lattice defects on particle surface reactivity, including Hg dissolution rates and bioavailability at key microbial interfaces. Increased availability of Hg to methylators may give rise to hotspots of MeHg bioaccumulation. In order to identify and monitor human populations with elevated MeHg exposure, total mercury content (THg) in hair is used as a proxy for chronic MeHg exposure. However, while this biomarker may be useful for populations who are primarily exposed to Hg as dietary MeHg, recent studies have indicated that for those in mixed exposure communities such as ASGM areas, THg in hair may comprise a substantial proportion of inorganic mercury (iHg). With the enactment of the Minamata Convention on Mercury prompting renewed efforts for biomonitoring of vulnerable populations, the efficacy of hair THg as a proxy for MeHg exposures to populations needs to be evaluated for ASGM settings. Chapter four investigates the efficacy of hair THg as an indicator of MeHg exposure by quantifying both THg and MeHg contents in hair from a representative subset of participants in a large, population-level mercury exposure assessment near ASGM areas in Madre de Dios (MDD), Peru. We observed that 100% of hair samples contained MeHg, and 86% had levels reflective of dietary exposure, or non-occupational exposure. For a small subset of individuals living in mining towns (N = 15; ~20% of within mining sample), we observed the opposite result: hair THg mostly comprised of inorganic mercury. However, this subset did not influence population level trends; hair MeHg-THg correlations were high (r >0.7) for all communities, regardless of location or nativity. Our results support the use of hair THg for monitoring of MeHg exposure of populations in ASGM settings in addition to urine, the current recommended biomarker for ASGM communities. Altogether, this research seeks to improve our risk characterization of environments and populations impacted by soil Hg0L contamination by addressing gaps in our knowledge on Hg0L mobilization, downstream reactivity, and exposure evaluation. This work will contribute to our understanding of the formation and identification of biological Hg hotspots resulting from Hg0L contaminated sites. Additionally, our enhanced understanding of hair biomarkers will help countries with active ASGM sites develop monitoring programs to identify vulnerable populations and assess the effectiveness of programs designed to decrease environmental Hg release.

Mercury contamination in aquatic ecosystems is a concern as anaerobic aquatic sediments are the primary regions of methylmercury production in freshwater and coastal regions. Methlymercury is a bioaccumulative neurotoxin, and human exposure to methylmercury can result in impaired functioning of the central nervous system and developmental disabilities in children. To minimize the risk of human exposure to methylmercury, it is important to be knowledgeable of the various sources which can supply mercury to aquatic ecosystems as well as have a complete understanding of the biogeochemical processes which are involved in methylmercury production in aquatic systems. In this dissertation work, both mercury biogeochemical speciation in anaerobic aquatic sediments and sources of mercury to aquatic systems were addressed.

The biogeochemical speciation of mercury is a critical factor which influences the fate and transformation of mercury in aquatic environments. In anaerobic sediments, mercury chemical speciation is controlled by reduced sulfur groups, such as inorganic sulfide and reduced sulfur moieties in dissolved organic matter (DOM). The formation of mercury sulfide nanoparticles through stabilization by dissolved organic matter (DOM) was investigated in precipitation studies using dynamic light scattering. Mercury sulfide nanoparticles (particle diameter < 100 nm) were stabilized through precipitation reactions that were kinetically hindered by DOM. To further investigate the interaction between DOM and metal sulfides, similar precipitation studies were performed using zinc sulfide and a number of DOM isolates (humic and fulvic acids) representing a range of DOM properties. The results of these experiments suggest that the mechanism of metal sulfide particle stabilization may be electrostatic or electrosteric, depending on the nature of the DOM molecule.

The mercury that is methylated in aquatic systems enters these environments via a number of sources, including atmospheric deposition, landscape runoff and other industrial and municipal activities. In two separate field studies, two potential sources of mercury to aquatic systems were investigated: landscape runoff and coal combustion products. The mercury loading to aquatic environments from these sources and their potential for transformation to methylmercury were investigated.

Landscape runoff from a Duke University campus catchment (Durham, NC) was identified as a source of mercury to a stream-wetland. The source of mercury to the runoff was likely from a `legacy' source of mercury; the historic application of mercury fungicide compounds to turf grass during the 20th century. Downstream of the point where the runoff was discharged to the stream-wetland, methylmercury concentrations were detected in stream sediments (up to 11% of total mercury), suggesting that this legacy mercury could be transformed to methylmercury.

The environmental impact of coal combustion products (CCPs) with respect to mercury and methylmercury was also investigated in a river system (Roane County, TN) that was inundated with fly ash and bottom ash from the Tennessee Valley Authority Kingston coal ash spill in 2008. Elevated total mercury and methylmercury sediment concentrations (relative to upstream sediments) were detected in regions impacted by the ash spill, and our biogeochemical data suggested that the ash may have stimulated methylmercury production in river sediments.

The results of this dissertation work address the formation of mercury sulfide (along with zinc sulfide) nanoparticles in anaerobic aquatic sediments. In the current mercury methylation paradigm, dissolved mercury species such as Hg(SH)02(aq) and HgS0(aq) are assumed to be the only mercury species that are available for methylation. The results of this dissertation work suggests that in previous studies, HgS0(aq) may have been mistaken as mercury sulfide nanoparticles which may be formed in under supersaturated conditions (with respect to HgS(s)) where DOM is present. Mercury sulfide nanoparticles are a mercury biogeochemical species that has been largely ignored in the research literature and whose role in the mercury biogeochemical cycle and in mercury methylation remains to be investigated.

This dissertation work also identifies potential sources of mercury to aquatic systems, namely, landscape runoff and CCPs. Atmospheric deposition is currently considered to be the major source of mercury to inland aquatic water bodies compared to sources such as landscape runoff and CCPs. However, in the watershed studied in this dissertation, landscape runoff was identified as a larger source of mercury than atmospheric deposition, suggesting that these so-called `minor' sources may actually be major sources of mercury to watersheds depending on land usage, and should be considered in watershed models. Furthermore, the environmental hazards of mercury-associated with CCPs has typically been determined through leaching experiments, such as the Toxicity Characteristic Leaching Procedure (TCLP), which are not representative of environmental conditions and do not predict that CCPs may influence mercury methylation in aquatic sediments. Thus, in this dissertation work, we suggest that leaching protocols such as the TCLP should be re-evaluated.

Overall, this dissertation work will be useful in future studies examining mercury speciation and bioavailability to methylating bacteria in aquatic sediments, and the formation of metal sulfide nanoparticles in aquatic systems. Additionally, data on sources of mercury will be useful in developing policies for the regulation of these sources and in assessing the risk to human health from mercury methylation.