Browsing by Subject "Selenium"

Coal fired power plants generate approximately 45% of the electricity produced in the United States every year, and each year, over 100 million tons of coal ash are produced as a by-product of electricity generation. Coal ash is a solid waste made up principally of bottom ash, fly ash, and flue gas desulfurization materials. The chemical composition of coal ash varies depending on the feed coal source, combustion parameters, and the presence and type of air pollution control devices that remove contaminants from the flue gas into the solid waste stream. Although a significant portion of coal ash waste is recycled, the majority of coal ash is disposed in landfills and holding ponds. Coal ash impoundments have a long history of environmental degradation, which includes: contaminant leaching into groundwater, the discharge of contaminant-laden effluent into surface waters, and catastrophic impoundment failures and ash spills. Despite these known problems, coal ash is not considered a hazardous waste, and thus is not subject to stringent disposal requirements. The current coal ash management system is based on risk assessments of coal ash that do not include environmental parameters that have a profound impact on coal ash contaminant mobility, particularly for the toxic elements such as mercury, arsenic, and selenium. This dissertation research focused on the biogeochemical transformations of mercury, arsenic, and selenium associated with coal ash materials in an effort to: (1) define the key environmental parameters controlling mercury, arsenic, and selenium fate during disposal and ash spills; and (2) delineate the relationship between coal ash characteristics, environmental parameters, and leaching potential.

The impact of coal ash on mercury transformations in anaerobic systems was assessed using anaerobic sediment-ash microcosms to mimic an ash spill into a benthic aquatic system. Anaerobic sediments are the primary zones for the microbial conversion of inorganic mercury to methyl mercury (MeHg), a process that is mediated by anaerobic bacteria, particularly sulfate reducing bacteria (SRB). MeHg is a potent neurotoxin that biomagnifies up the aquatic food chain, presenting a human health risk-- especially to children and pregnant women. The results of the sediment-ash microcosm experiments indicated negligible net production of MeHg in microcosms with no ash and in microcosms amended with the low-sulfate/low-Hg ash. In contrast, microcosms amended with sulfate and mercury-rich ash showed increases in MeHg concentrations that were two to three times greater than control microcosms without ash. The enhancement MeHg production in the microcosms was likely due to large quantities of leachable sulfate that stimulated the activity of methylating bacteria. Overall, these results highlight the importance of considering both the geochemical conditions of the receiving environment and the chemical composition of the coal ash in assessing the MeHg potential of coal ash.

The hypothesis that sulfate-rich coal ash can change sediment microbial communities, enhancing MeHg production, was tested by analyzing coal ash impacts on the SRB community in the sediment-ash microcosms using Terminal Restriction Fragment Length Polymorphism (T-RFLP), Quantitative Polymerase Chain Reaction (q-PCR), and Reverse Transcription-qPCR (RT-qPCR). Coal ash did not appear to cause significant changes to the structure of the overall bacterial community, though results showed that it may have caused a decrease in the evenness for species distribution for both SRB and the overall microbial community. During the five-day incubation experiment, the coal ash had a temporary significant effect on SRB abundance during the first one to two days of the experiment and a more sustained effect on SRB activity. This stimulation of SRB population growth and activity also corresponded with increasing net MeHg production. Overall, results indicate that coal ash amendments do not cause large shifts in the overall microbial community or the SRB community, but results indicate that there are connections between SRB abundance/activity and MeHg production. More research is needed to determine how coal ash directly impacts Hg methylating microorganisms, which include diverse array of microorganisms outside of SRB.

The effect of aerobic and anaerobic conditions on arsenic and selenium leaching from coal ash in an ash spill scenario was also assessed using sediment-ash microcosms. The fate of arsenic and selenium associated with coal ash is of particular concern due to the leachability of these elements at neutral pH and their tendency to bioaccumulate in aquatic organisms. Both the redox speciation of arsenic and selenium, and the pH of the aquatic system, are known to influence leaching into the environment, yet current environmental risk assessments of coal ash focus on pH alone as the primary driving force for arsenic and selenium leaching from coal ash and do not take into account the effects of anaerobic conditions and microbial activity. In this research, total dissolved concentrations of arsenic and selenium, dissolved speciation of arsenic, and solid phase speciation of selenium were monitored to determine the biogeochemical transformations and leaching of arsenic and selenium under differing redox conditions. The results from the sediment-ash microcosm studies showed that redox potential was the major determinant of arsenic and selenium mobility in the microcosm systems with greater arsenic leaching occurring in anaerobic microcosms and greater selenium leaching occurring in aerobic microcosms. Furthermore, the experiments provided clues to how coal ash influences the geochemistry of the benthic environment and how these influences affect the speciation and longer term solubility of arsenic and selenium.

Finally, experiments were conducted to determine how differing CaO, SO3, and Fe2O3 concentrations in coal ash affect the release of arsenic and selenium from sediment-ash mixtures in a simulated ash spill environment. Aerobic and anaerobic sediment-ash microcosms were constructed to mimic an ash spill into a benthic aquatic system, and a variety of coal ash materials were tested as amendments, including seven fly ashes, one lime-treated fly ash sample, and two FGD samples. Results showed that, in most cases, the sediment in the microcosm buffered the system at neutral, which counteracted leaching impacts of differing CaO and SO3 concentrations in the microcosms. Regardless of ash material, leaching of selenium was greater under aerobic conditions and was correlated with the total selenium content of the microcosm. Maximum leaching of arsenic occurred in anaerobic microcosms for some ash materials and in aerobic microcosms for other materials, suggesting that ash material chemistry played a significant role in controlling arsenic mobility. In both aerobic and anaerobic microcosms, dissolved arsenic concentration was correlated with total arsenic content of the ash material and in anaerobic microcosms, dissolved arsenic concentrations also correlated with the total iron content of the ash material. Overall, the results of these experiments showed that arsenic and selenium release under environmentally relevant conditions cannot be predicted by the CaO and SO3 content of the ash material. Rather, the total arsenic, total selenium content, and total iron content of the ash material are good predictors of the worst case environmental leaching scenario.

These investigations illuminated two major conclusions: (1) microbial activity and differing redox conditions are key in determining the impact of coal ash on the environment and in determining the mobility of coal ash contaminants, and (2) coal ash characteristics, such as sulfate and iron content, can change the redox chemistry and microbial activity of the surrounding environment, further influencing the fate of ash contaminants. This work will be useful in designing a framework that accurately predicts the leaching potential of ash contaminants under environmentally relevant conditions. The results will also be helpful in developing treatment technologies for ash impoundment effluent, guiding decisions on ash pond closure and remediation, and in designing long-term monitoring plans and remediation strategies for ash-impacted sites.

An increasingly large fraction of Earth’s surface has been reshaped and contaminated by humans, leading experts to suggest we’ve entered into a new geologic epoch – the Anthropocene. Nowhere are these changes more obvious than in mining-impacted landscapes. Mining reshapes landscapes, liberates trace elements, and alters the fate, transport, and transformation of trace elements. In this dissertation, I examine the extent to which mining both alters landscape features and mobilizes trace elements, and how these paired changes together determine the bioavailability of toxic trace elements. Specifically, in this dissertation, I focus on the mobilization and transformation of selenium (Se) and mercury (Hg) from mountaintop mining (MTM) in West Virginia; the biogeochemical interactions between Se and Hg in ecosystems and organisms; and the fate of Hg derived from artisanal and small-scale gold mining (ASGM) in Senegal and Peru.

In chapter 2, I examine the fate of Hg and Se from MTM of coal. Coal is naturally enriched in trace elements, including Hg and Se. Alkaline mine drainage from MTM – the dominant form of surface coal mining in Appalachia, USA – releases large quantities of Se into streams draining mined catchments, resulting in elevated bioaccumulation of Se in aquatic and riparian organisms. Yet, the release of Hg into these streams from MTM has not yet been studied. I measured total Hg, methyl Hg (MeHg), and Se in stream water, sediment, biofilm, cranefly larvae, and riparian spiders in alkaline streams (pH range: 6.9-8.4) across a mining gradient (0-98% watershed mined) in central Appalachia. Hg concentrations ranged from below detection limit (BDL)-6.9 ng/L in unfiltered water, BDL-0.05 μg/g in bulk sediment, 0.016-0.098 μg/g in biofilm, 0.038-0.11 μg/g in cranefly larvae, and 0.046-0.25 μg/g in riparian spiders. In contrast to Se, I found that Hg concentrations in all environmental compartments were not related to the proportion of the watershed mined, suggesting that Hg is not being released from, nor bioaccumulating within, MTM-VF watersheds. I also did not find clear evidence for a reduction in Hg methylation or bioaccumulation under elevated Se concentrations: water, sediment, biofilm, and riparian spiders exhibited no relationship between Hg and Se; only cranefly larvae exhibited a negative relationship (p=0.0002, r2=0.42). I suggest that the type of surface mining matrix rock, with resultant alkaline or acid mine drainage, is important for the speciation, mobility, and bioaccumulation of trace elements within watersheds affected by mining activities.

In chapter 3, I examine evidence for an interaction between Hg and Se. Hg is a pervasive environmental pollutant and contaminant of concern for both people and wildlife that has been a focus of environmental remediation efforts for decades. A growing body of literature has motivated calls for revising Hg consumption advisories to co-consider Se levels in seafood and implies that remediating aquatic ecosystems with ecosystem-scale Se additions could be a robust solution to Hg contamination. Provided that elevated Se concentrations are also known toxicological threats to aquatic animals, I performed a literature search to evaluate the strength of evidence supporting three assertions underpinning the ameliorating benefits of Se: (1) dietary Se reduces MeHg toxicity in consumers; (2) environmental Se reduces Hg bioaccumulation and biomagnification in aquatic food webs; and (3) Se inhibits Hg bioavailability to, and/or MeHg production by, microbial communities. Limited or ambiguous support for each criterion indicates that many scientific uncertainties and gaps remain regarding Se mediation of Hg behavior and toxicity in abiotic and biotic compartments. Significantly more information is needed to provide a strong scientific basis for modifying current fish consumption advisories on the basis of Se:Hg ratios or for applying Se amendments to remediate Hg-contaminated ecosystems.

In chapter 4, I examine evidence for Hg and Se interaction at the base of the food web. Hg, a potent neurotoxin, can biomagnify through food webs once converted into MeHg. Some studies have found that Se exposure may reduce MeHg bioaccumulation and toxicity, though this pattern is not universal. Se itself can also be toxic at elevated levels. We experimentally manipulated the relative concentrations of dietary MeHg and Se (as selenomethionine [SeMet]) for an aquatic grazer (the mayfly, Neocloeon triangulifer) and its food source (diatoms). Under low MeHg treatment (0.2 ng/L), diatoms exhibited a quadratic pattern, with decreasing diatom MeHg concentration up to 2.0 g Se/L and increasing MeHg accumulation at higher SeMet concentrations. Under high MeHg treatment (2 ng/L), SeMet concentrations had no effect on diatom MeHg concentrations. Mayfly MeHg concentrations and biomagnification factors (concentration of MeHg in mayflies: concentration of MeHg in diatoms) declined with SeMet addition only in the high MeHg treatment. Mayfly biomagnification factors decreased from 5.3 to 3.3 in the high MeHg treatment, while the biomagnification factor was constant with an average of 4.9 in the low MeHg treatment. The benefit of reduced MeHg biomagnification was offset by non-lethal effects and high mortality associated with ‘protective’ levels of SeMet exposure. Mayfly larvae escape behavior (i.e., startle response) was greatly reduced at early exposure days. Larvae took nearly twice as long for all to metamorphose to adults at high Se concentrations. The minimum number of days to emergence did not differ by SeMet exposure, with an average of 13 days. We measured an LC50SeMet for mayflies of 3.9 μg Se/L, with complete mortality at concentrations ≥6.0 μg Se/L. High reproductive mortality occurred at elevated SeMet exposures, with only 0-18% emergence at ≥4.12 g Se/L. Collectively our results suggest that while there is some evidence that Se can reduce MeHg accumulation at the base of the food web at specific exposure levels of SeMet and MeHg, Se is also toxic to mayflies and could lead to negative effects that extend across ecosystem boundaries.

In chapter 5, I examine the fate of total and MeHg from ASGM in Senegal. The largest source of global Hg anthropogenic inputs to the environment is derived from ASGM activities in developing countries. While our understanding of global Hg emissions from ASGM is growing, there is limited empirical documentation about the levels of total Hg (THg) and MeHg contamination near ASGM sites. I measured THg and MeHg concentrations in soil (n=119) sediment (n=22), and water (n=25) from four ASGM villages and one non-ASGM reference village in Senegal, West Africa with active ASGM. Nearly all samples had THg and MeHg concentrations that exceeded the reference village concentrations and USEPA regulatory standards. The highest median THg concentrations were found in huts where Hg-gold amalgams were burned (7.5 μg/g), while the highest median MeHg concentrations and percent Hg as MeHg were found in river sediments (4.2 ng/g, 0.41%). Median river water concentrations of THg and MeHg were also elevated compared to values at the reference site (22 ng THg/L, 0.037 ng MeHg/L in ASGM sites). This study provides direct evidence that Hg from ASGM is entering both the terrestrial and aquatic ecosystems where it is converted to the neurotoxic and bioavailable form of MeHg in soils, sediment, and water.

In chapter 6, I examine pathways of Hg deposition and storage from ASGM in the Peruvian Amazon. Hg emissions from ASGM now exceed coal combustion as the largest global source of Hg to the atmosphere and are being released into some of the most biodiverse ecosystems on Earth. Hg, following microbial conversion to MeHg, is a potent neurotoxin with deleterious impacts on people and wildlife. However, while we know ASGM is an important source of Hg to the atmosphere, we know very little about the fate of this source of Hg. Here, I examine Hg deposition and storage in the Peruvian Amazon by analyzing THg and MeHg in atmospheric, precipitation, leaf, and soil samples from remote and mining-impacted areas. I found that intact forests in the Peruvian Amazon near ASGM receive extremely high inputs of Hg in throughfall (71 µg m-2 yr-1) and litterfall (66 μg m-2 yr-1) and have accumulated significant quantities of soil Hg (9100 μg Hg m-2 within the top five cm). My findings show for the first time that intact forests near ASGM are intercepting high levels of Hg deposition, and that songbirds inhabiting these forests have elevated levels of mercury. Our findings raise important questions about how mercury pollution may constrain modern and future conservation efforts in these ecosystems.

In chapter 7, I examine the combined effects of landscape change and Hg loading from ASGM in the Peruvian Amazon. ASGM is the largest global source of anthropogenic Hg emissions. However, little is known about how effectively Hg released from ASGM is converted into the bioavailable form of MeHg in ASGM-altered landscapes. Through examination of ASGM-impacted river basins in Peru, I show that lake area in heavily mined watersheds has increased by 670% between 1985 and 2018, and that lakes in this area convert Hg into MeHg at net rates 5-7 times greater than rivers. These results suggest that synergistic increases in lake area and Hg loading associated with ASGM are significantly increasing exposure risk for people and wildlife. Similarly dramatic increases in lake area in other ASGM hotspots suggest that ‘hydroscape’ (hydrological landscape) alteration is an important and previously unrecognized component of Hg risk from ASGM.

In chapter 8, I develop several of the emergent themes that connect the distinct elements of this dissertation research. Here I develop a conceptual framework for merging perspectives from geochemistry, landscape ecology, and toxicology to understand the movement, fate and impact of toxic trace elements in the natural world. In the Anthropocene, we typically study the increasing mobilization of toxic trace elements and the changing land cover of our planet as separate issues. Yet the way we alter our landscapes plays a critical role in the likelihood that any particular place will retain, sequester, and alter the transport and bioavailability of trace elements to people and wildlife. The goal of this chapter is to provide examples that demonstrate that the risk of contaminant exposure is not merely a function of loading, but arises through interactions among loading, landscape capture, and biological transformation, all of which are simultaneously altered by human activities. I posit that successful prevention and mitigation of trace element toxicity requires a merging of these diverse perspectives and traditions.

Selenium (Se) is a micronutrient necessary for the function of a variety of important enzymes; Se also exhibits a narrow range in concentrations between essentiality and toxicity. Oviparous vertebrates such as birds and fish are especially sensitive to Se toxicity, which causes reproductive impairment and defects in embryo development. Selenium occurs naturally in the Earth's crust, but it can be mobilized by a variety of anthropogenic activities, including agricultural practices, coal burning, and mining.

Mountaintop removal/valley fill (MTR/VF) coal mining is a form of surface mining found throughout central Appalachia in the United States that involves blasting off the tops of mountains to access underlying coal seams. Spoil rock from the mountain is placed into adjacent valleys, forming valley fills, which bury stream headwaters and negatively impact surface water quality. This research focused on the biological impacts of Se leached from MTR/VF coal mining operations located around the Mud River, West Virginia.

In order to assess the status of Se in a lotic (flowing) system such as the Mud River, surface water, insects, and fish samples including creek chub (Semotilus atromaculatus) and green sunfish (Lepomis cyanellus) were collected from a mining impacted site as well as from a reference site not impacted by mining. Analysis of samples from the mined site showed increased conductivity and Se in the surface waters compared to the reference site in addition to increased concentrations of Se in insects and fish. Histological analysis of mined site fish gills showed a lack of normal parasites, suggesting parasite populations may be disrupted due to poor water quality. X-ray absorption near edge spectroscopy techniques were used to determine the speciation of Se in insect and creek chub samples. Insects contained approximately 40-50% inorganic Se (selenate and selenite) and 50-60% organic Se (Se-methionine and Se-cystine) while fish tissues contained lower proportions of inorganic Se than insects, instead having higher proportions of organic Se in the forms of methyl-Se-cysteine, Se-cystine, and Se-methionine.

Otoliths, calcified inner ear structures, were also collected from Mud River creek chubs and green sunfish and analyzed for Se content using laser ablation inductively couple mass spectrometry (LA-ICP-MS). Significant differences were found between the two species of fish, based on the concentrations of otolith Se. Green sunfish otoliths from all sites contained background or low concentrations of otolith Se (< 1 µg/g) that were not significantly different between mined and unmined sites. In contrast creek chub otoliths from the historically mined site contained much higher (≥ 5 µg/g, up to approximately 68 µg/g) concentrations of Se than for the same species in the unmined site or for the green sunfish. Otolith Se concentrations were related to muscle Se concentrations for creek chubs (R2 = 0.54, p = 0.0002 for the last 20% of the otolith Se versus muscle Se) while no relationship was observed for green sunfish.

Additional experiments using biofilms grown in the Mud River showed increased Se in mined site biofilms compared to the reference site. When we fed fathead minnows (Pimephales promelas) on these biofilms in the laboratory they accumulated higher concentrations of Se in liver and ovary tissues compared to fathead minnows fed on reference site biofilms. No differences in Se accumulation were found in muscle from either treatment group. Biofilms were also centrifuged and separated into filamentous green algae and the remaining diatom fraction. The majority of Se was found in the diatom fraction with only about 1/3rd of total biofilm Se concentration present in the filamentous green algae fraction

Finally, zebrafish (Danio rerio) embryos were exposed to aqueous Se in the form of selenate, selenite, and L-selenomethionine in an attempt to determine if oxidative stress plays a role in selenium embryo toxicity. Selenate and selenite exposure did not induce embryo deformities (lordosis and craniofacial malformation). L-selenomethionine, however, induced significantly higher deformity rates at 100 µg/L compared to controls. Antioxidant rescue of L-selenomethionime induced deformities was attempted in embryos using N-acetylcysteine (NAC). Pretreatment with NAC significantly reduced deformities in the zebrafish embryos secondarily treated with L-selenomethionine, suggesting that oxidative stress may play a role in Se toxicity. Selenite exposure also induced a 6.6-fold increase in glutathione-S-transferase pi class 2 gene expression, which is involved in xenobiotic transformation. No changes in gene expression were observed for selenate or L-selenomethionine-exposed embryos.

The findings in this dissertation contribute to the understanding of how Se bioaccumulates in a lotic system and is transferred through a simulated foodweb in addition to further exploring oxidative stress as a potential mechanism for Se-induced embryo toxicity. Future studies should continue to pursue the role of oxidative stress and other mechanisms in Se toxicity and the biotransformation of Se in aquatic ecosystems.

Selenium (Se) is an essential trace element and metalloid involved in several key metabolic activities: protection against oxidative damage, regulation of immune and thyroid function, and fertility. Several recent lines of evidence from epidemiology, genetic, and transgenic animal studies suggest that Se may play a protective role in Osteoarthritis (OA). However, the exact protective mechanism of Se is still unclear.

In this study, we hypothesized that Se exerts its chondroprotective benefit via an anti-oxidative and anti-inflammatory effect mediated by specific selenoproteins that neutralize cytokine-induced inflammatory responses in chondrocytes. We established an in vitro system for studying the effect of Se in the chondrosarcoma cell line SW-1353 and in human primary chondrocytes. Selenomethionine (SeMet) induced gene expression and enzyme activity of both antioxidative enzymes glutathione peroxidase (GPX) and thioredoxin reductase (TR) in SW-1353 cells. Our data suggest that Se may be protective against oxidative stress through regulation of the activity of these antioxidative enzymes.

As IL-1β is one of the primary pro-inflammatory cytokines contributing to the progression in OA, we next investigated the effect of Se on the gene expression induced by physiological doses of IL-1β. SeMet inhibited IL-1β induced catabolic gene expression of matrix metalloproteinase 1 (MMP1) and MMP13 as well as total MMP activity in chondrocytes. Similarly, SeMet inhibited chondrocyte gene expression of IL-1β induced nitric oxide synthase (iNOS) and cyclooxygenase (COX2) with corresponding reductions in nitric oxide (NO) and prostaglandin E2 (PGE2) production. In addition, SeMet pretreatment attenuated the IL-1β induced activation of p38 MAPK but not the ERK, JNK or NFkB pathways. Taken together, our results suggest that Se inhibits IL-1β induced expression of inflammatory and catabolic genes, partly through inhibition of IL-1β cell signaling.

Since Se may function through selenoproteins, we evaluated the role of three specific major selenoproteins, GPX1, TR1 and DIO2, in modifying the inflammatory response stimulated by IL-1β in chondrocytes by RNA interference. Based on RNA interference results, DIO2 and TR1 mediated the inhibitory effect of SeMet on IL-1β induced COX2 gene expression, while GPX1 did not show a significant inhibitory effect on Se. Depletion of DIO2 increased the IL-1β induced COX2 gene expression. This suggests that DIO2 may negatively modulate the IL-1β response. Our data also suggest that part of this inhibitory effect of DIO2 could be through regulation of IL-1β gene expression itself. These results highlight a potential new role of DIO2 in modulating the inflammatory response in chondrocytes

In summary, the result of this study suggests that Se may exert its chondroprotective effect through specific selenoproteins which neutralize oxidative stress and modify the inflammatory response in chondrocytes.