Transformations and long-term fate of engineered nanomaterials must be measured in realistic complex natural systems to accurately assess the risks that they may pose. Here, we determine the long-term behavior of poly(vinylpyrrolidone)-coated silver nanoparticles (AgNPs) in freshwater mesocosms simulating an emergent wetland environment. AgNPs were either applied to the water column or to the terrestrial soils. The distribution of silver among water, solids, and biota, and Ag speciation in soils and sediment was determined 18 months after dosing. Most (70 wt %) of the added Ag resided in the soils and sediments, and largely remained in the compartment in which they were dosed. However, some movement between soil and sediment was observed. Movement of AgNPs from terrestrial soils to sediments was more facile than from sediments to soils, suggesting that erosion and runoff is a potential pathway for AgNPs to enter waterways. The AgNPs in terrestrial soils were transformed to Ag 2 S (∼52%), whereas AgNPs in the subaquatic sediment were present as Ag 2 S (55%) and Ag-sulfhydryl compounds (27%). Despite significant sulfidation of the AgNPs, a fraction of the added Ag resided in the terrestrial plant biomass (∼3 wt % for the terrestrially dosed mesocosm), and relatively high body burdens of Ag (0.5−3.3 μg Ag/g wet weight) were found in mosquito fish and chironomids in both mesocosms. Thus, Ag from the NPs remained bioavailable even after partial sulfidation and when water column total Ag concentrations are low (<0.002 mg/L).

Speciation of mercury(II) in the aquatic environment and coordination to natural organic matter (NOM) and sulfides governs the bioavailability and mobility of mercury in water and sediment. While previous studies on aqueous Hg(II) speciation have focused on competitive binding of dissolved species, the purpose of this study was to explore the potential for HgS nanoparticles that coprecipitate with NOM in solution. Dynamic light scattering was used to monitor the size of HgS colloids growing over time. The results indicated that humic substances decreased observed growth rates of particles and stabilized aggregates smaller than 0.2 microm for at least 8 h. Thiol-containing organic acids such as cysteine and thioglycolate also decreased growth of HgS particles. Growth rates were also monitored as a function of monovalent electrolyte concentration, humic type, and humic concentration. HgS particles that formed in the presence of humics and thiolates were able to pass through conventional filters (<0.2 micro/m) and appeared to consist of aggregates of nanocrystals in TEM images. Furthermore, 96% of HgS aggregates were removed from aqueous suspension when exposed to octanol, indicating that the particles could be incorrectly identified as dissolved complexes (e.g., HgS(0)(aq)) in bioavailability models. Hg speciation calculations were conducted to consider lower Hg concentrations observed in sediment porewater. While the calculations depended on Hg binding constants that can vary by orders of magnitude, the results indicated that HgS(s) could be oversaturated in filtered porewater, particularly at low dissolved sulfide levels (micromolar or lower). These insights suggest that nanoparticulate HgS can exist in surface waters and porewater of contaminated sediments as a result of kinetically hindered aggregation/precipitation reactions. Further studies are neededto addressthe importance of nanoscale HgS particles for governing the reactivity and bioavailability of mercury in the environment.

An investigation of the potential environmental and health impacts in the immediate aftermath of one of the largest coal ash spills in U.S. history at the Tennessee Valley Authority (TVA) Kingston coal-burning power plant has revealed three major findings. First, the surface release of coal ash with high levels of toxic elements (As = 75 mg/kg; Hg = 150 μg/kg) and radioactivity (226Ra + 228Ra = 8 pCi/g) to the environment has the potential to generate resuspended ambient fine particles (<10 μm) containing these toxics into the atmosphere that may pose a health risk to local communities. Second, leaching of contaminants from the coal ash caused contamination of surface waters in areas of restricted water exchange, but only trace levels were found in the downstream Emory and Clinch Rivers due to river dilution. Third, the accumulation of Hg- and As-rich coal ash in river sediments has the potential to have an impact on the ecological system in the downstream rivers by fish poisoning and methylmercury formation in anaerobic river sediments.

An 18 month investigation of the environmental impacts of the Tennessee Valley Authority (TVA) coal ash spill in Kingston, Tennessee combined with leaching experiments on the spilled TVA coal ash have revealed that leachable coal ash contaminants (LCACs), particularly arsenic, selenium, boron, strontium, and barium, have different effects on the quality of impacted environments. While LCACs levels in the downstream river water are relatively low and below the EPA drinking water and ecological thresholds, elevated levels were found in surface water with restricted water exchange and in pore water extracted from the river sediments downstream from the spill. The high concentration of arsenic (up to 2000 µg/L) is associated with some degree of anoxic conditions and predominance of the reduced arsenic species (arsenite) in the pore waters. Laboratory leaching simulations show that the pH and ash/water ratio control the LCACs' abundance and geochemical composition of the impacted water. These results have important implications for the prediction of the fate and migration of LCACs in the environment, particularly for the storage of coal combustion residues (CCRs) in holding ponds and landfills, and any potential CCRs effluents leakage into lakes, rivers, and other aquatic systems.

Nanoparticulate metal sulfides such as ZnS can influence the transport and bioavailability of pollutant metals in anaerobic environments. The aim of this work was to investigate how the composition of dissolved natural organic matter (NOM) influences the stability of zinc sulfide nanoparticles as they nucleate and aggregate in water with dissolved NOM. We compared NOM fractions that were isolated from several surface waters and represented a range of characteristics including molecular weight, type of carbon, and ligand density. Dynamic light scattering was employed to monitor the growth and aggregation of Zn-S-NOM nanoparticles in supersaturated solutions containing dissolved aquatic humic substances. The NOM was observed to reduce particle growth rates, depending on solution variables such as type and concentration of NOM, monovalent electrolyte concentration, and pH. The rates of growth increased with increasing ionic strength, indicating that observed growth rates primarily represented aggregation of charged Zn-S-NOM particles. Furthermore, the observed rates decreased with increasing molecular weight and aromatic content of the NOM fractions, while carboxylate and reduced sulfur content had little effect. Differences between NOM were likely due to properties that increased electrosteric hindrances for aggregation. Overall, results of this study suggest that the composition and source of NOM are key factors that contribute to the stabilization and persistence of zinc sulfide nanoparticles in the aquatic environment.

Mercury stable isotope abundances were used to trace transport of Hg-impacted river sediment near a coal ash spill at Harriman, Tennessee, USA. δ(202)Hg values for Kingston coal ash released into the Emory River in 2008 are significantly negative (-1.78 ± 0.35‰), whereas sediments of the Clinch River, into which the Emory River flows, are contaminated by an additional Hg source (potentially from the Y-12 complex near Oak Ridge, Tennessee) with near-zero values (-0.23 ± 0.16‰). Nominally uncontaminated Emory River sediments (12 miles upstream from the Emory-Clinch confluence) have intermediate values (-1.17 ± 0.13‰) and contain lower Hg concentrations. Emory River mile 10 sediments, possibly impacted by an old paper mill has δ(202)Hg values of -0.47 ± 0.04‰. A mixing model, using δ(202)Hg values and Hg concentrations, yielded estimates of the relative contributions of coal ash, Clinch River, and Emory River sediments for a suite of 71 sediment samples taken over a 30 month time period from 13 locations. Emory River samples, with two exceptions, are unaffected by Clinch River sediment, despite occasional upstream flow from the Clinch River. As expected, Clinch River sediment below its confluence with the Emory River are affected by Kingston coal ash; however, the relative contribution of the coal ash varies among sampling sites.

Particle-specific properties, including size and chemical speciation, affect the reactivity of mercury (Hg) in natural systems (e.g., dissolution or methylation). Here, terrestrial, river, and marine sediments were size-fractionated and characterized to correlate particle-specific properties of Hg-bearing solids with their bioavailability potential and measured biomethylation. Marine sediments contained ∼20−50% of the total Hg in the <0.5 μm size fraction, compared to only 0.5 and 3.0% in this size fraction for terrestrial and river sediments, respectively. X-ray absorption spectroscopy (XAS) analysis indicated that metacinnabar (β-HgS) was the main mercury species in a marine sediment, whereas organic Hg-thiol (Hg(SR) 2 ) was the main mercury species in a terrestrial sediment. Single-particle inductively coupled plasma time-of-flight mass spectrometry analysis of the marine sediment suggests that half of the Hg in the <0.5 μm size fraction existed as individual nanoparticles, which were β-HgS based on XAS analyses. Glutathione-extractable mercury was higher for samples containing Hg(SR) 2 species than β-HgS species and correlated well with the amount of Hg biomethylation. This particle-scale understanding of how Hg speciation and particle size affect mercury bioavailability potential helps explain the heterogeneity in Hg methylation in natural sediments.

The Tennessee Valley Authority Kingston coal ash spill in December 2008 deposited approximately 4.1 million m(3) of fly ash and bottom ash into the Emory and Clinch River system (Harriman, Tennessee, U.S.A.). The objective of this study was to investigate the impact of the ash on surface water and sediment quality over an eighteen month period after the spill, with a specific focus on mercury and methylmercury in sediments. Our results indicated that surface water quality was not impaired with respect to total mercury concentrations. However, in the sediments of the Emory River near the coal ash spill, total mercury concentrations were 3- to 4-times greater than sediments several miles upstream of the ash spill. Similarly, methylmercury content in the Emory and Clinch River sediments near the ash spill were slightly elevated (up to a factor of 3) at certain locations compared to upstream sediments. Up to 2% of the total mercury in sediments containing coal ash was present as methylmercury. Mercury isotope composition and sediment geochemical data suggested that elevated methylmercury concentrations occurred in regions where native sediments were mixed with coal ash (e.g., less than 28% as coal ash in the Emory River). This coal ash may have provided substrates (such as sulfate) that stimulated biomethylation of mercury. The production of methylmercury in these areas is a concern because this neurotoxic organomercury compound can be highly bioaccumulative. Future risk assessments of coal ash spills should consider not only the leaching potential of mercury from the wastes but also the potential for methylmercury production in receiving waters.