Browsing by Author "Hsu-Kim, Heileen"
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Item Open Access A Python-Based Program for Estimating Biological Surface Acidity by Using a Non-Electrostatic Adsorption Model(2023) Li, HaotianThe objective of this research is to develop an open-source Python-based surface complexation modeling program to estimate the acidity of biological surface. Several computer software already exists with such function installed, such as ProtoFit and FITEQL. However, these programs lack capabilities in constraining fitting parameters, resulting in model fits that are not necessarily justified by the input data. Here, a new Python-based model algorithm was developed to estimate surface acidity and protonation constants for biological surfaces. The program was developed based on the algorithm of ProtoFit, and improved to allow for user-defined boundary conditions for model fitting parameters. This model was tested on potentiometric pH titration data for suspensions of Pseudomonas fluorescens and Bacillus subtilis bacterial cells and suspensions of colloidal particles (e.g., extracellular vesicles) that were isolated from cell cultures. Model testing was also performed for titration data collected for aqueous buffer solutions with known chemical species and concentration. The estimated surface acidities from Python script and ProtoFit are compared, and error analysis was conducted. Error analysis showed that the Python script modeled the titration data with lower curve-fitting error than models by ProtoFit, which suggests a better optimization performance in Python script. However, the model comparisons for the aqueous buffer titrations (for which acidity constants were known) did not showing such trend. We believe that is because experimental error is much larger than model error in our setups. Therefore, variance-based sensitivity analysis was further conducted on the Python script, and the result shows that the titrant concentration (N_tit) and adsorbent mass (M_ads) were two variables that contributed the most variance in our model output.
Item Open Access Antagonistic Growth Effects of Mercury and Selenium in Caenorhabditis elegans Are Chemical-Species-Dependent and Do Not Depend on Internal Hg/Se Ratios.(Environ Sci Technol, 2016-03-15) Wyatt, Lauren H; Diringer, Sarah E; Rogers, Laura A; Hsu-Kim, Heileen; Pan, William K; Meyer, Joel NThe relationship between mercury (Hg) and selenium (Se) toxicity is complex, with coexposure reported to reduce, increase, and have no effect on toxicity. Different interactions may be related to chemical compound, but this has not been systematically examined. Our goal was to assess the interactive effects between the two elements on growth in the nematode Caenorhabditis elegans, focusing on inorganic and organic Hg (HgCl2 and MeHgCl) and Se (selenomethionine, sodium selenite, and sodium selenate) compounds. We utilized aqueous Hg/Se dosing molar ratios that were either above, below, or equal to 1 and measured the internal nematode total Hg and Se concentrations for the highest concentrations of each Se compound. Observed interactions were complicated, differed between Se and Hg compounds, and included greater-than-additive, additive, and less-than-additive growth impacts. Biologically significant interactions were only observed when the dosing Se solution concentration was 100-25,000 times greater than the dosing Hg concentration. Mitigation of growth impacts was not predictable on the basis of internal Hg/Se molar ratio; improved growth was observed at some internal Hg/Se molar ratios both above and below 1. These findings suggest that future assessments of the Hg and Se relationship should incorporate chemical compound into the evaluation.Item Open Access Boron and strontium isotopic characterization of coal combustion residuals: validation of new environmental tracers.(Environ Sci Technol, 2014-12-16) Ruhl, Laura S; Dwyer, Gary S; Hsu-Kim, Heileen; Hower, James C; Vengosh, AvnerIn the U.S., coal fired power plants produce over 136 million tons of coal combustion residuals (CCRs) annually. CCRs are enriched in toxic elements, and their leachates can have significant impacts on water quality. Here we report the boron and strontium isotopic ratios of leaching experiments on CCRs from a variety of coal sources (Appalachian, Illinois, and Powder River Basins). CCR leachates had a mostly negative δ(11)B, ranging from -17.6 to +6.3‰, and (87)Sr/(86)Sr ranging from 0.70975 to 0.71251. Additionally, we utilized these isotopic ratios for tracing CCR contaminants in different environments: (1) the 2008 Tennessee Valley Authority (TVA) coal ash spill affected waters; (2) CCR effluents from power plants in Tennessee and North Carolina; (3) lakes and rivers affected by CCR effluents in North Carolina; and (4) porewater extracted from sediments in lakes affected by CCRs. The boron isotopes measured in these environments had a distinctive negative δ(11)B signature relative to background waters. In contrast (87)Sr/(86)Sr ratios in CCRs were not always exclusively different from background, limiting their use as a CCR tracer. This investigation demonstrates the validity of the combined geochemical and isotopic approach as a unique and practical identification method for delineating and evaluating the environmental impact of CCRs.Item Open Access Comparing Metal Collection between Unmodified and Resin-impregnated Polyurethane Foam(2023-04-25) Hao, ZhihengIndoor dust could contain numerous contaminants including metals from various origins. Polyurethane foam (PUF) has been used to measure trace metals in the ambient environment but has not been tested as a personal sampler. Moreover, metal-chelating materials such as Chelex-100 resin have a greater affinity to metals over other materials. Thus, this study aims at comparing the performance of metal collection by PUF and PUF modified by Chelex-100 resin and metal-neutral resin (XAD-4) by conducting one experiment at a fixed site and two experiments involving PUF inserted into wristbands. Of 9 measured metals, results show that unamended PUF samplers performs better in general, whereas PUF coated with Chelex-100 resin has greater abilities only for specific metals such as Cu, Mn, and Ni. In addition, PUF infused with XAD-4 resin does not show improvement in metal collection.Item Unknown Deposition of silver nanoparticles in geochemically heterogeneous porous media: predicting affinity from surface composition analysis.(2011) Lin, ShihongThe transport of uncoated silver nanoparticles (AgNPs) in a porous medium composed of silica glass beads modified with a partial coverage of iron oxide (hematite) was studied and compared to that in a porous medium composed of unmodified glass beads (GB). At a pH lower than the point of zero charge (PZC) of hematite, the affinity of AgNPs for a hematite-coated glass bead (FeO-GB) surface was significantly higher than that for an uncoated surface. There was a linear correlation between the average nanoparticle affinity for media composed of mixtures of FeO-GB and GB collectors and the relative composition of those media as quantified by the attachment efficiency over a range of mixing mass ratios of the two types of collectors, so that the average AgNPs affinity for these media is readily predicted from the mass (or surface) weighted average of affinities for each of the surface types. X-ray photoelectron spectroscopy (XPS) was used to quantify the composition of the collector surface as a basis for predicting the affinity between the nanoparticles for a heterogeneous collector surface. A correlation was also observed between the local abundances of AgNPs and FeO on the collector surface.Item Unknown From Source to Surveillance: An Assessment of Liquid Elemental Mercury Mobilization in Soil, Downstream Reactivity, and Biomarkers of Exposure(2021) Koenigsmark, FayeMercury (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.
Item Unknown Legacy source of mercury in an urban stream-wetland ecosystem in central North Carolina, USA.(Chemosphere, 2015-11) Deonarine, Amrika; Hsu-Kim, Heileen; Zhang, Tong; Cai, Yong; Richardson, Curtis JIn the United States, aquatic mercury contamination originates from point and non-point sources to watersheds. Here, we studied the contribution of mercury in urban runoff derived from historically contaminated soils and the subsequent production of methylmercury in a stream-wetland complex (Durham, North Carolina), the receiving water of this runoff. Our results demonstrated that the mercury originated from the leachate of grass-covered athletic fields. A fraction of mercury in this soil existed as phenylmercury, suggesting that mercurial anti-fungal compounds were historically applied to this soil. Further downstream in the anaerobic sediments of the stream-wetland complex, a fraction (up to 9%) of mercury was converted to methylmercury, the bioaccumulative form of the metal. Importantly, the concentrations of total mercury and methylmercury were reduced to background levels within the stream-wetland complex. Overall, this work provides an example of a legacy source of mercury that should be considered in urban watershed models and watershed management.Item Unknown Microbe-Mineral Interactions: Mercury Homogenization, Bacterial Colloid Surface Acidity, and Protocol Transfeminism.(2023) Wadle, Austin JacobMicroorganisms, humans, and minerals are in constant interaction. Whether within human biology or in the extra-human extracellular environment. Environmental engineers are interested historically in minerals that have particular consequence for environmental health. This work seeks to explore an interstitialality between environmental engineering and our subject matter as well as the people who perform that work. This interstitialality is represented in a variety of ways in this research and will largely take place in three case studies that are defined by the follow questions: (1) What are the relative rates of homogenization of environmentally relevant mercury (Hg) in a freshwater wetland? (2) What do bacterial nanoparticles contribute to the pH buffering of the extracellular environment as compared to their parent cells? (3) Can we incorporate lessons from queer and feminist science studies into environmental engineering and what does a transfeminist protocol look like in environmental engineering? The first being uh an application of a range of Hg species into wetland simulated mesocosms freshwater, here Hg is transformed and homogenized in the interstitial spaces between cells, the mineral portions of soil, the water column and finally other forms of biota. Much of these transformations are mediated bacterially, specifically the transformation of inorganic species Hg into monomethylmercury (MeHg). These wetland mesocosms were dosed with isotopically labelled Hg, here forward referred to as endmembers. We performed experiments in simulated freshwater wetland mesocosms that were dosed with four isotopically labeled mercury forms: two dissolved (Hg2+ and Hg-humic acid) and two particulate (nano-HgS and Hg adsorbed to FeS). Over the course of one year, we monitored the four Hg isotope endmembers for their relative distribution between surface water, sediment, and fish in the wetland mesocosms, partitioning between soluble and particulate forms, reactivity by sequential extractions, and uptake flux in diffusive gradient in thin-film (DGT) passive samplers. We observed that the four isotope spikes were relatively similar in concentration (ca. 3000 ng/L) immediately after spike addition. At 1 to 3 months after dosing, Hg concentrations were 1 to 50 ng/L and were greater for the initially dissolved isotope endmembers than the initially particulate endmembers. In contrast, the Hg isotope endmembers in surface sediments were similar in concentrations for all time periods after spike addition. However, the uptake fluxes of the Hg in DGT samplers, which is a measure of the reactive fraction of souble Hg, were generally greater for initially dissolved Hg endmembers and lower for the initially particulate endmembers. At one year post-dosing, the DGT-uptake fluxes were converging toward similar values between the Hg isotope endmembers. However, the relative distribution of isotope endmembers were still significantly different in both the water column and sediment (p<0.01 according to one-way ANOVA analysis). For MeHg concentrations in surface sediment and fish, the relative contributions by each endmember were significantly different at all sampling time points, including the final one year time point. Altogether, these results provide insights to the timescales for distribution for different Hg species that enter a wetland ecosystem. While these inputs attain homogeneity in concentration in the primary ecosystem storage compartment (i.e., sediments) within weeks after addition, these input pools remain differentiated for more than one year in terms of reactivity for passive samplers, methylmercury concentration and bioaccumulation. The second case study examines another interstitial space focused on the cell surface and extracellular environment with respect to proton buffering. The acid-base characteristics of bacterial surfaces are relevant for key processes such as cation exchange and pH buffering at the cell envelope. Microorganisms also produce nanoparticulate soft material such as extracellular vesicles (EVs) and strands of flagellin. Given their high surface area to volume ratio, these biogenic particles may contribute additional surfaces for proton buffering and cation exchange for the external cellular environment. In this work, biocolloid suspensions were isolated from two ubiquitous environmental bacteria, the gram-negative Pseudomonas fluorescens and the gram-positive Bacillus subtilis. These resulting colloidal suspensions were evaluated via alkalimetric titrations and surface proton modeling of the data to derive surface acidity constants and site concentrations. Biocolloids isolated from P. fluorescens cultures contributed 2.18 (+/- 0.65) µmole/g cell surface acidity. This proton exchange density corresponded to 1 surface acid site for every 113 surface sites at the cell envelope of the whole cell. In contrast, biocolloids isolated from B. subtilis contributed surface acidity of 3.89 (+/- 0.82) µmole/g cell, which corresponded to a ratio of 1 site to every 64 cell surface sites. While lower than two orders of magnitude in total surface acidity, we note that these particles have measurable contributions to the extracellular environmental proton buffering and cation exchange. Additionally, even between these two species, surface active biocolloid production can vary by a factor of 2. Surface acidity and cation exchange have profound impacts on bacterial and human health, sequestering toxic metals as one example. Given these results, further study into the general and more specific surface functionality is needed to constrain the possible functions these nanoparticles could perform in pristine and engineered environments. The final exploration in this chapter lends itself to a more theoretical lens, while still residing in a highly interstitial space. The technical study of extracellular vesicles from bacteria prompted further questions about how scholars relate to their organisms. This prompted a discursive intervention in environmental engineering and science studies. I propose that EVs can be a productive object for theoretical exploration using frameworks critical to feminist science studies and queer of color critique. I sought to disrupt masculinist interactions with extracellular vesicles in biotechnological context and to prompt the science studies community to explore the theory making capacity of microbial (largely bacterial and fungal) extracellular vesicles. Feminist and anticolonial studies have shown that protocol can be a productive starting point for materially enacting theoretical goals around ethics. I propose a transfeminist protocol for the isolation of biocolloids from Pseudomonas fluorescens and Bacillus subtilis. Reflecting my own lived reality while culturing these bacteria, this protocol assisted me in knowing my organisms and their capacity for surface acidity as well as knowing myself and the role of an environmental engineer. I hope for this work to inspire deeper interactions and questioning. These three case studies show how microbe-mineral-human interactions profoundly shape material and theoretical conditions. In tandem, the research here shows several paths for these interactions. One shows how different species of Hg can be introduced into freshwater wetland environments through anthropogenic activities. Those environments then themselves react and transform that Hg and resulting partitioning and fate of that Hg has long lasting impacts. The second shows the contribution of biogenic soft nanoparticles like EVs and strands of flagellin to the extracellular pH buffering capacity. This capacity is influenced by the surfaces of the bacteria that produce them, as well as mineral surfaces and dissolved molecules. These materials have capacity for buffering protons and sorbing metals. Finally, I offer a perspective to pull environmental engineers closer to the organisms that we study. The materiality of the organisms that I studied show me that we are already very intimately connected. If we acknowledge this connection, point to it, and take agency while allowing our organisms to express their own—we have much to gain.
Item Open Access Recovery of Rare Earth Elements from Coal Combustion Ash: Survey, Extraction, and Speciation(2018) Taggart, RossThis research explores the beneficial reuse of coal combustion fly ash as a source of rare earth elements (REE). We characterized fly ashes of varied geological origin, tested several extraction methods and parameters, and investigated REE location and speciation in fly ash. Total REE content in a broad sample of U.S. fly ashes were quantified using HF/HNO3 digestion, Na2O2 sintering, and HNO3 digestion. If was found that Appalachian Basin coal ashes had significantly higher total REE content than Illinois Basin or Powder River Basin ashes. However, Powder River Basin ashes had higher HNO3-extractable REE content. Sinter-based extraction methods were tested for REE recovery from fly ash. Optimal sintering conditions were found to be a 1:1 NaOH-ash ratio and 1-2 M HNO3 leaching solution. Bulk and microscale Y speciation in fly ash were compared using sequential selective extractions and x-ray absorption spectroscopy. Bulk speciation suggested Y entrained in the aluminosilicate glass phase while microscale speciation resembled trace yttrium minerals.
Item Open Access Recovery of Rare Earth Elements From Waste Materials: Leaching and Separation Using Supported Liquid Membranes(2022) Middleton, AndrewRare earth elements (REEs; defined as the stable lanthanides, yttrium, and scandium) are ubiquitous in a wide range of modern technologies; however, their global market supply is dominated by a single source and has been subject to major disruptions in recent years. This has led to an interest in finding alternative, non-traditional sources of REEs. Potential non-traditional sources include secondary waste materials such as coal combustion residual (CCR), acid mine drainage (AMD), and electronic wastes (e-waste), among others.While methods for processing and purification of REEs from traditional ores are reasonably well established, there are not well-established processing and purification methods for low-grade, non-traditional sources. This is in part because non-traditional sources have a complex and diverse chemical composition and relatively low REE concentrations. This work focused on understanding the factors controlling REE recovery in two steps of the recovery process: 1) acid leaching of REEs from solid wastes and 2) separation using a supported liquid membrane (SLM) process. The waste materials used in this research were CCR, coal processing refuse, and AMD. The overall aim of this work was to understand how the diverse chemistry of waste materials affects REE recovery during leaching and purification. Specifically, this research addressed the following aims: 1. Identify the factors that control REE solubility (e.g., pH and major element concentration) in acid leachates of coal fly ashes and coal residue.
2. Quantify the effects of various leachate characteristics such as major metal concentrations and feed pH on REE mass transfer in SLMs using synthetic feeds that mimic real feedstocks from low-grade waste materials.3. Evaluate the effectiveness of SLMs as a method for selective separation of REEs from real AMD feeds from sites distinct water chemistries and identify feed characteristics that are predictive of SLM performance. The aim of Chapter 2 of this research was to understand the connection between REE solubility, pH, and major elemental components of leachates for coal by-products. One of the early steps in recovering REEs from coal by-products is often acid leaching, which can result in low pH leachates with complex aqueous chemistry. To accomplish the aim of this chapter, we investigated the effects of solids concentration (i.e., pulp density) and pH adjustment on REE solubility in acid leachates of coal fly ashes from the Powder River Basin (PRB) and Appalachian Basin in the United States, and a coal processing refuse from the Southwestern U.S. For PRB ashes, the concentrations of soluble REEs generally increased with increasing pulp density; however, at pulp density values above 80-100 g/L, the soluble REE concentrations in the leachates were markedly lower. Similarly, the soluble concentrations of other major solutes (Fe, Al, Si) that leached from PRB fly ashes were also non-linear with pulp density. These major elements tended to reach maximum concentration values at 60-70 g/L pulp density. In contrast, for the Appalachian fly ashes and the coal by-product, soluble concentrations of REE and major elements in leachates increased linearly with pulp density. Chemical equilibria calculations of mineral saturation indices indicated that trends in soluble REE concentrations could be explained by saturation conditions for Fe and Al-(hydr)oxides and possibility sulfate minerals, but not lanthanide hydroxides. Furthermore, pH adjustment of the acid leachates showed that REEs and many major solutes were removed from solution at pH values above 4.5, also consistent with Fe- and Al-(hydr)oxide precipitation. These results highlight the importance of understanding the chemical composition of leachates when designing REE recovery processes for low-grade geologic feedstocks and that precipitation of hydr(oxide) or sulfate minerals of major elements rather than discreet formation of REE mineral phases could be used for process optimization. The aim of Chapter 3 of this work was to quantify the effects of three different competitive metals (Fe(III), Fe(II), and Al) and feed pH on the mass transfer and recovery of two rare earth elements (REE) (Nd and Er) using SLMs. SLMs are a promising alternative to solvent exchange processes that combine two different unit operations (extraction and stripping) into one. To our knowledge, there are no studies that have systematically assessed the effects of individual metals on REE mass transfer using SLMs with feeds that have metals concentrations relevant to low-grade waste materials. Previous work has suggested that competition by non-REE metals for metal binding sites at the membrane interface can decrease REE recovery when using CCR leachates. To accomplish the aim of this chapter, we used simulated feeds that are representative of low-grade feedstocks of REEs, such as coal fly ash and acid mine drainage (AMD). The simulated feeds consisted of either Nd or Er as the REE of interest and either Fe(III), Fe(II), or Al as the competitive metal. The feeds had relatively low REE concentrations (0.01-100 µmol/L) compared to the competitive metals, Fe and Al (0.017-35 mmol/L), and had pH values from 1 to 3.5. The results showed that at sufficiently high concentrations of competitive metals REE mass transfer could be impeded. Further, the concentration at which REE mass transfer was inhibited was lowest for Fe(III) compared to Fe(II) and Al. The results also showed that feed pH is a major driver of REE transfer across the membrane, with feeds at higher pH having higher mass transfer. Additionally, we were able to show that the observed decreases in REE mass transfer were not due to unwanted loss mechanisms in the bulk feed (e.g., coprecipitation with mineral phases formed over the lifetime of the reactor) or due to mineral formation on the membrane surface blocking sorption sites at the feed-membrane interface. The results of this study will help to provide a framework for predicting how REE mass transfer will be affected when using real feeds with complex aqueous chemistries and multiple competitive metals. In the fourth chapter of this research, we evaluated the recovery of REEs from a real potential feedstock, AMD, using SLMs. The major aims for this study were to: 1) assess the effectiveness of SLM-based REE separation from AMD samples representing a spectrum of aqueous compositions; 2) determine the effects of AMD storage and holding time on separation performance; and 3) assess the impact of AMD pre-treatment (e.g., filtration and pH adjustment) on REE recovery. The results showed that relative separation fluxes of REE by SLM correlated with AMD characteristics such as pH and major ions such as Fe and Ca. The purity of acid strippant product, expressed as REE dry weigh content, depended on the initial REE concentrations in the AMD. Additionally, Fe(II) oxidation during the aging of AMD samples significantly decreased REE mass transfer by SLM separation. However, filtration of freshly collected AMD limited Fe(II) oxidation, enabling flexibility in feed stock storage time for separation of AMD. Pre-treatment of AMD samples by pH adjustment did not substantially improve separation performance. Overall, this study provides a framework for applying SLMs for REE recovery from AMD sources by establishing primary water quality parameters that influence separation flux and product purity. Such insights are needed to support a mechanistic understanding of critical metals separation by SLM for complex and nontraditional feedstocks such as AMD wastes. In total, this research provides insight on key feed characteristics that control recovery of REEs from low-grade waste materials during two stages of the recovery process: acid leaching and separation using SLMs. The results from this research can be used to help engineer more efficient recovery processes that could help low-grade waste materials become economically viable sources of REEs.
Item Open Access Residential metal contamination and potential health risks of exposure in adobe brick houses in Potosí, Bolivia.(The Science of the total environment, 2016-08) McEwen, Abigail R; Hsu-Kim, Heileen; Robins, Nicholas A; Hagan, Nicole A; Halabi, Susan; Barras, Olivo; Richter, Daniel deB; Vandenberg, John JPotosí, Bolivia, is the site of centuries of historic and present-day mining of the Cerro Rico, a mountain known for its rich polymetallic deposits, and was the site of large-scale Colonial era silver refining operations. In this study, the concentrations of several metal and metalloid elements were quantified in adobe brick, dirt floor, and surface dust samples from 49 houses in Potosí. Median concentrations of total mercury (Hg), lead (Pb), and arsenic (As) were significantly greater than concentrations measured in Sucre, Bolivia, a non-mining town, and exceeded US-based soil screening levels. Adobe brick samples were further analyzed for bioaccessible concentrations of trace elements using a simulated gastric fluid (GF) extraction. Median GF extractable concentrations of Hg, As, and Pb were 0.085, 13.9, and 32.2% of the total element concentration, respectively. Total and GF extractable concentrations of Hg, As, and Pb were used to estimate exposure and potential health risks to children following incidental ingestion of adobe brick particles. Risks were assessed using a range of potential ingestion rates (50-1000mg/day). Overall, the results of the risk assessment show that the majority of households sampled contained concentrations of bioaccessible Pb and As, but not Hg, that represent a potential health risk. Even at the lowest ingestion rate considered, the majority of households exceeded the risk threshold for Pb, indicating that the concentrations of this metal are of particular concern. To our knowledge, this is the first study to quantify key trace elements in building materials in adobe brick houses and the results indicate that these houses are a potential source of exposure to metals and metalloids in South American mining communities. Additional studies are needed to fully characterize personal exposure and to understand potential adverse health outcomes within the community.Item Open Access Silver toxicity across salinity gradients: the role of dissolved silver chloride species (AgCl x ) in Atlantic killifish (Fundulus heteroclitus) and medaka (Oryzias latipes) early life-stage toxicity.(Ecotoxicology, 2016-08) Matson, Cole W; Bone, Audrey J; Auffan, Mélanie; Lindberg, T Ty; Arnold, Mariah C; Hsu-Kim, Heileen; Wiesner, Mark R; Di Giulio, Richard TThe influence of salinity on Ag toxicity was investigated in Atlantic killifish (Fundulus heteroclitus) early life-stages. Embryo mortality was significantly reduced as salinity increased and Ag(+) was converted to AgCl(solid). However, as salinity continued to rise (>5 ‰), toxicity increased to a level at least as high as observed for Ag(+) in deionized water. Rather than correlating with Ag(+), Fundulus embryo toxicity was better explained (R(2) = 0.96) by total dissolved Ag (Ag(+), AgCl2 (-), AgCl3 (2-), AgCl4 (3-)). Complementary experiments were conducted with medaka (Oryzias latipes) embryos to determine if this pattern was consistent among evolutionarily divergent euryhaline species. Contrary to Fundulus data, medaka toxicity data were best explained by Ag(+) concentrations (R(2) = 0.94), suggesting that differing ionoregulatory physiology may drive observed differences. Fundulus larvae were also tested, and toxicity did increase at higher salinities, but did not track predicted silver speciation. Alternatively, toxicity began to increase only at salinities above the isosmotic point, suggesting that shifts in osmoregulatory strategy at higher salinities might be an important factor. Na(+) dysregulation was confirmed as the mechanism of toxicity in Ag-exposed Fundulus larvae at both low and high salinities. While Ag uptake was highest at low salinities for both Fundulus embryos and larvae, uptake was not predictive of toxicity.Item Open Access Size-dependent Fate of Nanoceria in Large Scale Simulated Wetlands(2017) Cooper, Jane L.Nanoceria, or cerium (IV) oxide, is used widely in industry for its catalytic and physical properties, thus its release into the environment is eminent. The environmental risk of nanoceria is still unclear, but understanding its environmental fate could help to inform future studies. To understand its fate, large scale simulated wetlands were constructed and dosed weekly. Nanoceria of primary particle sizes ~4nm (sm-CeO2) and ~140nm (lg-CeO2) were dosed into these mesocosms, 750 mg total over nine months. Single particle ICP-MS (spICP-MS) was employed with microsecond dwell times (0.1ms) to understand particulate cerium in surface water. spICP-MS proved ineffective as a comparison tool between treatments, since sm-CeO2 was below instrument detection, and further data processing could not amend the issue. However, comparisons between nanoceria stocks and mesocosm samples could be conducted. Mesocosm water dosed with lg-CeO2 exhibited some aggregation of its smaller fraction.
At the end of 9-months, elemental analyses showed that nanoceria size did not affect the mass of cerium in surface water, as mesocosms treated with lg-CeO2 contained 4.0 ± 1.4 mg of Ce and sm-CeO2 treated waters contained 5.6± 6.7mg Ce. Greater biological uptake did occur when treated with smaller particles, as shown in elevated root concentration (sm-CeO2: 67 ± 14 ng g-1 ; lg-CeO2: 21 ± 10 ng g-1) and total Ce in Egeria densa biomass (sm-CeO2: 22 ± 1.4 ; lg-CeO2: 2.9 ± 0.5 mg). The environmental compartments in this study accounted for a small fraction of dosed nanoceria (< 5%), so assumed nanoceria fate is the sediment.
Item Open Access Sources and Biogeochemical Transformation of Mercury in Aquatic Ecosystems(2011) Deonarine, AmrikaMercury 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.