Alzheimer's disease ferroptotic associations with oxidative damage and neuronal loss.

The role of reactive iron in Alzheimer's Disease (AD) remains unresolved. Little is known of how AD may alter iron transport, glutathione-mediated oxidative repair, and their associations with ApoE alleles. Postmortem brain intravascular blood was minimized by washing minced brain (n=24/group). HNE from iron-associated lipid peroxidation increased in AD prefrontal cortex by 50% for whole tissue and in subcellular lipid rafts, where Aß-peptides are produced. HNE correlated with iron storage ferritin light chain (FTL; r=0.35); both were higher in ApoE4. Iron chelation by DFO in EFAD mice decreased HNE consistent with ferroptosis. Neuronal and synaptic loss in AD was inversely correlated to FTL (r=-0.55). AD decreased levels of ferroptosis suppressor protein 1, glutamate cysteine ligase modulator subunit (GCLM), and lipid raft glutathione peroxidase 4 (GPx4), mitigators of ferroptosis. These findings provide a mechanistic framework for iron-associated neurodegeneration during AD by impaired lipid peroxidation repair mechanisms involving glutathione.

Multimodal X-ray nano-spectromicroscopy analysis of chemically heterogeneous systems.

Understanding the nanoscale chemical speciation of heterogeneous systems in their native environment is critical for several disciplines such as life and environmental sciences, biogeochemistry, and materials science. Synchrotron-based X-ray spectromicroscopy tools are widely used to understand the chemistry and morphology of complex material systems owing to their high penetration depth and sensitivity. The multidimensional (4D+) structure of spectromicroscopy data poses visualization and data-reduction challenges. This paper reports the strategies for the visualization and analysis of spectromicroscopy data. We created a new graphical user interface and data analysis platform named XMIDAS (X-ray multimodal image data analysis software) to visualize spectromicroscopy data from both image and spectrum representations. The interactive data analysis toolkit combined conventional analysis methods with well-established machine learning classification algorithms (e.g. nonnegative matrix factorization) for data reduction. The data visualization and analysis methodologies were then defined and optimized using a model particle aggregate with known chemical composition. Nanoprobe-based X-ray fluorescence (nano-XRF) and X-ray absorption near edge structure (nano-XANES) spectromicroscopy techniques were used to probe elemental and chemical state information of the aggregate sample. We illustrated the complete chemical speciation methodology of the model particle by using XMIDAS. Next, we demonstrated the application of this approach in detecting and characterizing nanoparticles associated with alveolar macrophages. Our multimodal approach combining nano-XRF, nano-XANES, and differential phase-contrast imaging efficiently visualizes the chemistry of localized nanostructure with the morphology. We believe that the optimized data-reduction strategies and tool development will facilitate the analysis of complex biological and environmental samples using X-ray spectromicroscopy techniques.

Effects of flow on uranium speciation in soils impacted by acidic waste fluids.

Radioactive acidic liquid waste is a common byproduct of uranium (U) and plutonium (Pu) enrichment and recycling processes whose accidental and planned release has led to a significant input of U into soils and sediments across the world, including at the U.S. DOE's Hanford site (WA, USA). Because of the particularly hazardous nature of U, it is important to predict its speciation when introduced into soils and sediments by acidic waste fluids. Of fundamental importance are the coupled effects of acid-driven mineral transformation and reactive transport on U speciation. To evaluate the effect of waste-fluid residence time and co-associated dissolved phosphate concentrations on U speciation in impacted soils and sediments, uncontaminated surface materials (from the Hanford Site) were reacted with U-containing synthetic acidic waste fluids (pH 2) amended with dissolved phosphate concentrations in both batch (no flow) and flow-through column systems for 7-365 days. By comparing dissolved U behavior and solid phase speciation as a function of flow regimen, we found that the availability of proton-promoted dissolution products (such as Si) to sequester U into uranyl silicates was dependent on waste fluid-sediment contact time as uranyl silicates were not detected in short contact time flow-through systems but were detected in no-flow, long contact time, reactors. Moreover, the dominance of uranyl phosphate as neoprecipitate U scavenger (principally in the form of meta-ankoleite) in phosphate amended systems confirmed the importance of phosphate amendments for an efficient sequestration of U in the soils and sediments. Overall, our experiments suggest that the formation of uranyl silicates in soils impacted by acidic waste fluids is likely to be limited unless reaction products are allowed to accumulate in soil pores, highlighting the importance of investigating soil U speciation in flow-through, transport-driven systems as opposed to no-flow, batch systems. This study provides insights into uranium speciation and its potential changes under acidic conditions for better prediction of risks and subsequent development of efficient remediation strategies.

Iron Speciation in Respirable Particulate Matter and Implications for Human Health.

Particulate matter (PM) air pollution poses a major global health risk, but the role of iron (Fe) is not clearly defined because chemistry at the particle-cell interface is often not considered. Detailed spectromicroscopy characterizations of PM2.5 samples from the San Joaquin Valley, CA identified major Fe-bearing components and estimated their relative proportions. Iron in ambient PM2.5 was present in spatially and temporally variable mixtures, mostly as Fe(III) oxides and phyllosilicates, but with significant fractions of metallic iron (Fe(0)), Fe(II,III) oxide, and Fe(III) bonded to organic carbon. Fe(0) was present as aggregated, nm-sized particles that comprised up to ∼30% of the Fe spectral fraction. Mixtures reflect anthropogenic and geogenic particles subjected to environmental weathering, but reduced Fe in PM originates from anthropogenic sources, likely as abrasion products. Possible mechanistic pathways involving Fe(0) particles and mixtures of Fe(II) and Fe(III) surface species may generate hydrogen peroxide and oxygen-centered radical species (hydroxyl, hydroperoxyl, or superoxide) in Fenton-type reactions. From a health perspective, PM mixtures with reduced and oxidized Fe will have a disproportionate effect in cellular response after inhalation because of their tendency to shuttle electrons and produce oxidants and electrophiles that induce inflammation and oxidative stress.

Phosphate controls uranium release from acidic waste-weathered Hanford sediments.

Mineral dissolution and secondary phase precipitation may control the fate of inorganic contaminants introduced to soils and sediments during liquid waste discharges. When the solutions are aggressive enough to induce transformation of native minerals, incorporated contaminants may be released during dissolution due to percolation of meteoric waters. This study evaluated the release of uranium (U) from Hanford sediments that had been previously reacted for 180 or 365 days with liquid waste solutions containing U with and without 3 mM dissolved phosphate at pH 2 and 3. Flow-through column experiments were conducted under continuous saturated flow with a simulated background porewater (BPW; pH ~7) for 22 d. Up to 5% of the total U was released from the sediments reacted under PO4-free conditions, attributable to the dissolution of becquerelite and boltwoodite formed during weathering. Contrastingly, negligible U was released from PO4-reacted sediments, where meta-ankoleite was identified as the main U-mineral phase. Linear combination fits of U LIII-edge EXAFS spectra of sediments before and after BPW leaching and thermodynamic calculations suggest that the formed becquerelite and meta-ankoleite transformed into schoepite and a phosphuranylite-type phase, respectively. These results demonstrate the stabilization of U as recalcitrant uranyl minerals formed in sediments and highlight the key role of PO4 in U release at contaminated sites.

Iron Speciation in Particulate Matter (PM2.5) from Urban Los Angeles Using Spectro-microscopy Methods.

The speciation, oxidation states, and relative abundance of iron (Fe) phases in PM2.5 samples from two locations in urban Los Angeles were investigated using a combination of bulk and spatially resolved, element-specific spectroscopy and microscopy methods. Synchrotron X-ray absorption spectroscopy (XAS) of bulk samples in situ (i.e., without extraction or digestion) was used to quantify the relative fractions of major Fe phases, which were corroborated by spatially resolved spectro-microscopy measurements. Ferrihydrite (amorphous Fe(III)-hydroxide) comprised the largest Fe fraction (34-52%), with hematite (α-Fe2O3; 13-23%) and magnetite (Fe3O4; 10-24%) identified as major crystalline oxide components. An Fe-bearing phyllosilicate fraction (16-23%) was fit best with a reference spectrum of a natural illite/smectite mineral, and metallic Fe(0) was a relatively small (2-6%) but easily identified component. Sizes, morphologies, oxidation state, and trace element compositions of Fe-bearing PM from electron microscopy, electron energy loss spectroscopy (EELS), and scanning transmission X-ray microscopy (STXM) revealed variable and heterogeneous mixtures of Fe species and phases, often associated with carbonaceous material with evidence of surface oxidation. Ferrihydrite (or related Fe(III) hydroxide phases) was ubiquitous in PM samples. It forms as an oxidation or surface alteration product of crystalline Fe phases, and also occurs as coatings or nanoparticles dispersed with other phases as a result of environmental dissolution and re-precipitation reactions. The prevalence of ferrihydrite (and adsorbed Fe(III) has likely been underestimated in studies of ambient PM because it is non-crystalline, non-magnetic, more soluble than crystalline phases, and found in complex mixtures. Review of potential sources of different particle types suggests that the majority of Fe-bearing PM from these urban sites originates from anthropogenic activities, primarily abrasion products from vehicle braking systems and engine emissions from combustion and/or wear. These variable mixtures have a high probability for electron transfer reactions between Fe, redox-active metals such as copper, and reactive carbon species such as quinones. Our findings suggest the need to assess biological responses of specific Fe-bearing phases both individually and in combination to unravel mechanisms of adverse health effects of particulate Fe.

Effects of mercury, organic carbon, and microbial inhibition on methylmercury cycling at the profundal sediment-water interface of a sulfate-rich hypereutrophic reservoir.

Methylmercury (MeHg) produced by anaerobic bacteria in lakes and reservoirs, poses a threat to ecosystem and human health due to its ability to bioaccumulate in aquatic food webs. This study used 48-hr microcosm incubations of profundal sediment and bottom water from a sulfate-rich, hypereutrophic reservoir to assess seasonal patterns of MeHg cycling under various treatments. Treatments included addition of air, Hg(II), organic carbon, and microbial inhibitors. Both aeration and sodium molybdate, a sulfate-reducing bacteria (SRB) inhibitor, generally decreased MeHg concentration in microcosm water, likely by inhibiting SRB activity. The methanogenic inhibitor bromoethanesulfonate increased MeHg concentration 2- to 4- fold, suggesting that methanogens were potent demethylators. Pyruvate increased MeHg concentration under moderately reduced conditions, likely by stimulating SRB, but decreased it under highly reduced conditions, likely by stimulating methanogens. Acetate increased MeHg concentration, likely due to the stimulation of acetotrophic SRB. Results suggest that iron-reducing bacteria (IRB) were not especially prominent methylators and MeHg production at the sediment-water interface is elevated under moderately reduced conditions corresponding with SRB activity. In contrast, it is suppressed under oxic conditions due to low SRB activity, and under highly reduced conditions (<-100 mV) due to enhanced demethylation by methanogens.

Anaerobic Dissolution Rates of U(IV)-Oxide by Abiotic and Nitrate-Dependent Bacterial Pathways.

The long-term stability of U(IV) solid phases in anaerobic aquifers depends upon their reactivity in the presence of oxidizing chemical species and microbial catalysts. We performed flow-through column experiments under anaerobic conditions to investigate the mechanisms and dissolution rates of biogenic, noncrystalline UO2(s) by chemical oxidants (nitrate and/or nitrite) or by Thiobacillus denitrificans, a widespread, denitrifying, chemolithoautotrophic model bacterium. Dissolution rates of UO2(s) with dissolved nitrite were approximately 5 to 10 times greater than with nitrate alone. In the presence of wild-type T. denitrificans and nitrate, UO2(s) dissolution rates were similar to those of abiotic experiments with nitrite (from 1.15 × 10-14 to 4.94 × 10-13 mol m-2 s-1). Experiments with a T. dentrificans mutant strain defective in U(IV) oxidation supported microbially mediated U(IV) oxidation. X-ray absorption spectroscopy (XAS) analysis of post-reaction solids showed the presence of mononuclear U(VI) species rather than a solid U(VI) phase. At steady-state U release, kinetic and spectroscopic results suggest detachment of oxidized U(VI) from the UO2(s) surface as the rate-determining step rather than electron transfer or ion diffusion. Under anaerobic conditions, production of nitrite by nitrate-reducing microorganisms and enzymatically catalyzed, nitrate-dependent U(IV) oxidation are likely dual processes by which reduced U solids may be oxidized and mobilized in the aqueous phase.

Dissolved Carbonate and pH Control the Dissolution of Uranyl Phosphate Minerals in Flow-Through Porous Media.

Uranyl phosphate minerals represent an important secondary source of uranium release at contaminated sites. In flow-through column experiments with background porewater (BPW) of typical freshwater aquifer composition (pH 7.0, ∼0.2 mM total carbonate (TC)), dissolution of K-ankoleite (KUO2PO4·3H2O), Na-autunite (NaUO2PO4·3H2O), and Ca-autunite (Ca(UO2)2(PO4)2·6H2O) was controlled by mineral solubility at steady-state U release. Effluent concentrations indicated exchange with BPW cations, and postreaction characterization showed alteration of the initial mineral composition, changes in structure (decreased crystallinity, increased disorder, and distortion of U-P mineral sheets) and possible neoformation of phases of similar structure. Increasing the BPW pH and TC to 8.1-8.2 and 2.2-3.7 mM, respectively, resulted in mineral undersaturation and produced ca. 2 orders-of-magnitude higher U and P release without reaching steady state. Minerals incorporated less BPW cations into their structures compared to low carbonate BPW experiments but showed structural disorder and distortion. Faster dissolution rates were attributed to the formation of binary and ternary uranyl carbonate complexes that accelerate the rate-determining step of uranyl detachment from the uranyl-phosphate layered structure. Calculated dissolution rates (log Rs between -8.95 and -10.32 mol m-2 s-1), accounting for reaction and transport in porous media, were similar to dissolution rates of other classes of uranyl minerals. In undersaturated solutions, dissolution rates for uranyl phosphate, oxyhydroxide, and silicate minerals can be predicted within 1-2 orders-of-magnitude from pH ∼5-10 on the basis of pH/carbonate concentration.

Phosphorus Speciation in Atmospherically Deposited Particulate Matter and Implications for Terrestrial Ecosystem Productivity.

Chemical forms of phosphorus (P) in airborne particulate matter (PM) are poorly known and do not correlate with solubility or extraction measurements commonly used to infer speciation. We used P X-ray absorption near-edge structure (XANES) and 31P nuclear magnetic resonance (NMR) spectroscopies to determine P species in PM collected at four mountain sites (Colorado and California). Organic P species dominated samples from high elevations, with organic P estimated at 65-100% of total P in bulk samples by XANES and 79-88% in extracted fractions (62-84% of total P) by NMR regardless of particle size (≥10 or 1-10 µm). Phosphorus monoester and diester organic species were dominant and present in about equal proportions, with low fractions of inorganic P species. By comparison, PM from low elevation contained mixtures of organic and inorganic P, with organic P estimated at 30-60% of total P. Intercontinental PM transport determined from radiogenic lead (Pb) isotopes varied from 0 to 59% (mean 37%) Asian-sourced Pb at high elevation, whereas stronger regional PM inputs were found at low elevation. Airborne flux of bioavailable P to high-elevation ecosystems may be twice as high as estimated by global models, which will disproportionately affect net primary productivity.

Gibbsite (100) and Kaolinite (100) Sorption of Cadmium(II): A Density Functional Theory and XANES Study of Structures and Energies.

Due to the potential toxicity of cadmium (Cd2+) and its presence in various waste products found in the environment, it is necessary to develop methods to attenuate and remediate Cd2+ waste. Sorption of Cd2+ to mineral surfaces is a potential route to accomplish this goal. This work focused on improving our molecular-scale understanding of the chemistry of Cd2+ interactions with gibbsite and kaolinite mineral surfaces. Plane-wave density functional theory (DFT) energy minimization calculations and molecular dynamics simulations were used to study the adsorption energies and the nature of the bonds between Cd2+ and the mineral surfaces for possible inner- and outer-sphere surface complexes. Models resulting from the DFT calculations were used to calculate theoretical XANES spectra that were compared with experimental Cd LIII XANES of aqueous Cd2+ as a proxy for outer-sphere Cd2+ hydrated complexes associated with the mineral surfaces. These studies suggest that Cd2+ would favorably bond to the (100) surfaces of both kaolinite and gibbsite through a bidentate mononuclear interaction. However, the results indicate that mixtures of surface complexes on these minerals are likely.

Surface characterization and chemical speciation of adsorbed iron(iii) on oxidized carbon nanoparticles.

Carbonaceous nanomaterials represent a significant portion of ultra-fine airborne particulate matter, and iron is the most abundant transition metal in air particles. Owing to their high surface area and atmospheric oxidation, carbon nanoparticles (CNP) are enriched with surface carbonyl functional groups and act as a host for metals and small molecules. Using a synthetic model, concentration-dependent changes in the chemical speciation of iron adsorbed on oxidized carbon surfaces were investigated by a combination of X-ray and electron microscopic and spectroscopic methods. Carbon K-edge absorption spectra demonstrated that the CNP surface was enriched with carboxylic acid groups after chemical oxidation but that microporosity was unchanged. Oxidized CNP showed a high affinity for sorption of Fe(iii) from solution (75-95% uptake) and spectroscopic measurements confirmed a 3+ oxidation state of Fe on CNP irrespective of surface loading. The bonding of adsorbed Fe(iii) at variable loadings was determined by iron K-edge X-ray absorption spectroscopy. At low loadings (3 and 10 µmol Fe m-2 CNP), mononuclear Fe was octahedrally coordinated to oxygen atoms of carboxylate groups. As Fe surface coverage increased (21 and 31 µmol Fe m-2 CNP), Fe-Fe backscatters were observed at interatomic distances indicating iron (oxy)hydroxide particle formation on CNP. Electron-donating surface carboxylate groups on CNP coordinated and stabilized mononuclear Fe(iii). Saturation of high-affinity sites may have promoted hydroxide particle nucleation at higher loading, demonstrating that the chemical form of reactive metal ions may change with surface concentration and degree of CNP surface oxidation. Model systems such as those discussed here, with controlled surface properties and known chemical speciation of adsorbed metals, are needed to establish structure-activity models for toxicity assessments of environmentally relevant nanoparticles.

Manganese(iv) oxide amendments reduce methylmercury concentrations in sediment porewater.

Manganese(iv) oxide (pyrolusite, birnessite) mineral amendments can reduce dissolved MeHg concentrations in sediment theoretically by inhibiting microbial sulfate reduction, which is a major methylation pathway in sediments. Anaerobic sediment slurry microcosms in which Hg methylation was stimulated by addition of labile organic carbon (acetate) and HgCl2 showed that manganese(iv) oxide reduced the percent MeHg in slurry porewater (filtered), by 1-2 orders of magnitude relative to controls. Sediment-water mesocosms with pyrolusite or birnessite either directly mixed into the top 5 cm or applied in a thin (5 cm) sand layer over sediment showed reductions in percent MeHg in porewater of 66-69% for pyrolusite and 81-89% for birnessite amendment. A thin sand layer alone resulted in 65% reduction. CO2 respirometry experiments showed that the amendments stimulated microbial activity. Microbial community census by PCR and DNA sequencing indicated that the addition of Mn(iv) oxides did not significantly alter the indigenous sediment microbial community structure, although a small increase in abundance of iron and manganese reducers was observed after a 2 week incubation period. The mechanism of decreasing MeHg relative to Hg concentrations in porewater likely involved an increase in the importance of Mn(iv) reduction (relative to sulfate reduction) in heterotrophic microbial metabolism in the sediments amended with Mn(iv) oxides. Manganese reduction was confirmed as the predominant biogeochemical redox process by microelectrode voltammetry profiling of the sediment microcosms, although adsorption to Mn oxide surfaces, enhanced MeHg demethylation, and abiotic reduction of Mn(iv) also may have been involved in reducing percent MeHg and suppressing net MeHg production. These results represent a novel approach for mitigating MeHg impacts from sediments with potential applicability to a range of aquatic settings including intertidal zones, tidal marshes, seasonal wetlands, reservoirs, and lakes.

Characterization of manganese oxide amendments for in situ remediation of mercury-contaminated sediments.

Addition of Mn(iv)-oxide phases pyrolusite or birnessite was investigated as a remedial amendment for Hg-contaminated sediments. Because inorganic Hg methylation is a byproduct of bacterial sulfate reduction, reaction of Mn(iv) oxide with pore water should poise sediment oxidation potential at a level higher than favorable for Hg methylation. Changes in Mn(iv)-oxide mineralogy and oxidation state over time were investigated in sediment tank mesocosm experiments in which Mn(iv)-oxide amendment was either mixed into Hg-contaminated sediment or applied as a thin-layer sand cap on top of sediment. Mesocosms were sampled between 4 and 15 months of operation and solid phases were characterized by X-ray absorption spectroscopy (XAS). For pyrolusite-amended sediments, Mn(iv) oxide was altered to a mixture of Mn(iii)-oxyhydroxide and Mn, Fe(iii, ii)-oxide phases, with a progressive increase in the Mn(ii)-carbonate fraction over time as mesocosm sediments became more reduced. For birnessite-amended sediments, both Mn(iii) oxyhydroxide and Mn(ii) carbonate were identified at 4 months, indicating a faster rate of Mn reduction compared to pyrolusite. After 15 months of reaction, birnessite was converted completely to Mn(ii) carbonate, whereas residual Mn, Fe(iii, ii)-oxide phases were still present in addition to Mn(ii) carbonate in the pyrolusite mesocosm. In the thin-layer sand cap mesocosms, no changes in either pyrolusite or birnessite XAS spectra were observed after 10 months of reaction. Equilibrium phase relationships support the interpretation of mineral redox buffering by mixed-valent (Mn, Fe)(iii, ii)-oxide phases. Results suggest that amendment longevity for redox buffering can be controlled by adjusting the mass and type of Mn(iv) oxide applied, mineral crystallinity, surface area, and particle size. For a given site, amendment capping versus mixing with sediment should be evaluated to determine the optimum treatment approach, which may vary depending on application constraints, rate of Mn(iv) oxide transformation, and frequency of reapplication to maintain desired oxidation state and pH.

Uranium Release from Acidic Weathered Hanford Sediments: Single-Pass Flow-Through and Column Experiments.

The reaction of acidic radioactive waste with sediments can induce mineral transformation reactions that, in turn, control contaminant fate. Here, sediment weathering by synthetic uranium-containing acid solutions was investigated using bench-scale experiments to simulate waste disposal conditions at Hanford's cribs (Hanford, WA). During acid weathering, the presence of phosphate exerted a strong influence over uranium mineralogy and a rapidly precipitated, crystalline uranium phosphate phase (meta-ankoleite [K(UO2)(PO4)·3H2O]) was identified using spectroscopic and diffraction-based techniques. In phosphate-free system, uranium oxyhydroxide minerals such as K-compreignacite [K2(UO2)6O4(OH)6·7H2O] were formed. Single-pass flow-through (SPFT) and column leaching experiments using synthetic Hanford pore water showed that uranium precipitated as meta-ankoleite during acid weathering was strongly retained in the sediments, with an average release rate of 2.67 × 10-12 mol g-1 s-1. In the absence of phosphate, uranium release was controlled by dissolution of uranium oxyhydroxide (compreignacite-type) mineral with a release rate of 1.05-2.42 × 10-10 mol g-1 s-1. The uranium mineralogy and release rates determined for both systems in this study support the development of accurate U-release models for the prediction of contaminant transport. These results suggest that phosphate minerals may be a good candidate for uranium remediation approaches at contaminated sites.

Delayed Nrf2-regulated antioxidant gene induction in response to silica nanoparticles.

Silica nanoparticles with iron on their surface cause the production of oxidants and stimulate an inflammatory response in macrophages. Nuclear factor erythroid-derived 2 - like factor 2 (Nrf2) signaling and its regulated antioxidant genes play critical roles in maintaining redox homeostasis. In this study we investigated the regulation of four representative Nrf2-regulated antioxidant genes; i.e., glutamate cysteine ligase (GCL) catalytic subunit (GCLC), GCL modifier subunit (GCLM), heme oxygenase 1 (HO-1), and NAD(P)H:quinone oxidoreductase-1 (NQO-1), by iron-coated silica nanoparticles (SiO2-Fe) in human THP-1 macrophages. We found that the expression of these four antioxidant genes was modified by SiO2-Fe in a time-dependent manner. At 6h, their expression was unchanged except for GCLC, which was reduced compared with controls. At 18h, the expression of these antioxidant genes was significantly increased compared with controls. In contrast, the Nrf2 activator sulforaphane induced all antioxidant genes at as early as 3h. The nuclear translocation of Nrf2 occurred later than that for NF-κB p65 protein and the induction of proinflammatory cytokines (TNFα and IL-1ß). NF-κB inhibitor SN50 prevented the reduction of GCLC at 6h and abolished the induction of antioxidant genes at 18h by SiO2-Fe, but did not affect the basal and sulforaphane-induced expression of antioxidant genes, suggesting that NF-κB signaling plays a key role in the induction of Nrf2-mediated genes in response to SiO2-Fe. Consistently, SN50 inhibited the nuclear translocation of Nrf2 caused by SiO2-Fe. In addition, Nrf2 silencing decreased the basal and SiO2-induced expression of the four reprehensive antioxidant genes. Taken together, these data indicated that SiO2-Fe induced a delayed response of Nrf2-regulated antioxidant genes, likely through NF-κB-Nrf2 interactions.

Mechanism of Hg(II) Immobilization in Sediments by Sulfate-Cement Amendment.

Reactive amendments such as Portland and super-sulfate cements offer a promising technology for immobilizing metalloid contaminants such as mercury (Hg) in soils and sediments through sequestration in less bioavailable solid forms. Tidal marsh sediments were reacted with dissolved Hg(II) in synthetic seawater and fresh water solutions, treated with Portland cement and FeSO4 amendment, and aged for up to 90 days. Reacted solids were analyzed with bulk sequential extraction methods and characterized by powder X-ray diffraction (XRD), electron microscopy, and synchrotron X-ray absorption spectroscopy at the Hg LIII- and S K-edge. In amended sediments, XRD, SEM and sulfur K-edge XANES indicated formation of gypsum in seawater experiments or ettringite-type (Ca6Al2(SO4)3(OH)12.26H2O) phases in fresh water experiments, depending on the final solution pH (seawater ∼8.5; freshwater ∼10.5). Analysis of Hg EXAFS spectra showed Cl and Hg ligands in the first- and second-coordination shells at distances characteristic of a polynuclear chloromercury(II) salt, perhaps as a nanoparticulate phase, in both seawater and fresh water experiments. In addition to the chloromercury species, a smaller fraction (∼20-25%) of Hg was bonded to O atoms in fresh water sample spectra, suggesting the presence of a minor sorbed Hg fraction. In the absence of amendment treatment, Hg sorption and resistance to extraction can be accounted for by relatively strong binding by reduced S species present in the marsh sediment detected by S XANES. Thermodynamic calculations predict stable aqueous Hg-Cl species at seawater final pH, but higher final pH in fresh water favors aqueous Hg-hydroxide species. The difference in Hg coordination between aqueous and solid phases suggests that the initial Hg-Cl coordination was stabilized in the cement hydration products and did not re-equilibrate with the bulk solution with aging. Collectively, results suggest physical encapsulation of Hg as a polynuclear chloromercury(II) salt as the primary immobilization mechanism.

Combining single-particle inductively coupled plasma mass spectrometry and X-ray absorption spectroscopy to evaluate the release of colloidal arsenic from environmental samples.

Detection and sizing of natural colloids involved in the release and transport of toxic metals and metalloids is essential to understand and model their environmental effects. Single-particle inductively coupled plasma mass spectrometry (SP-ICP-MS) was applied for the detection of arsenic-bearing particles released from mine wastes. Arsenic-bearing particles were detected in leachates from mine wastes, with a mass-per-particle detection limit of 0.64 ng of arsenic. Conversion of the mass-per-particle information provided by SP-ICP-MS into size information requires knowledge of the nature of the particles; therefore, synchrotron-based X-ray absorption spectroscopy (XAS) was used to identify scorodite (FeAsO4·2H2O) as the main species in the colloidal particles isolated by ultrafiltration. The size of the scorodite particles detected in the leachates was below 300-350 nm, in good agreement with the values obtained by TEM. The size of the particles detected by SP-ICP-MS was determined as the average edge of scorodite crystals, which show a rhombic dipyramidal form, achieving a size detection limit of 117 nm. The combined use of SP-ICP-MS and XAS allowed detection, identification, and size determination of scorodite particles released from mine wastes, suggesting their potential to transport arsenic. Graphical abstract Analytical approach for the detection and size characterization of As-bearing particles by SP-ICP-MS and XAS in environmental samples.

Arsenic speciation in the dispersible colloidal fraction of soils from a mine-impacted creek.

Arsenic and iron speciation in the dispersible colloid fraction (DCF; 10-1000 nm) from an As-rich mine waste pile, sediments of a streambed that collects runoff from waste pile, the streambed subsoil, and the sediments of a downstream pond were investigated by combining asymmetrical-flow field-flow fractionation (AsFlFFF)/inductively-coupled plasma-mass spectrometry (ICP-MS), transmission electron microscopy (TEM) and X-ray absorption (XAS) spectroscopy. Calcium, Fe and As (Fe/As molar ratio ∼ 1) were the main components of the DCF from waste pile. TEM/EDS and As and Fe XAS analysis revealed the presence of nanoparticle scorodite in this same DCF, as well as Fe nanoparticles in all samples downstream of the waste pile. Arsenic and Fe XAS showed As(V) adsorbed onto nanoparticulate ferrihydrite in the DCF of downstream samples. Micro-X-ray fluorescence indicated a strong correlation between Fe and As in phyllosilicate/Fe(3+) (oxi) hydroxide aggregates from the sediment pond. Fractionation analysis showed the mean particle size of the DCF from the streambed sample to be smaller than that of the streambed subsoil and sediment ponds samples. These results show that an important and variable fraction of As may be bound to dispersible colloids that can be released from contaminated soils and transported downstream in natural systems.

Influence of phosphate and silica on U(VI) precipitation from acidic and neutralized wastewaters.

Uranium speciation and physical-chemical characteristics were studied in solids precipitated from synthetic acidic to circumneutral wastewaters in the presence and absence of dissolved silica and phosphate to examine thermodynamic and kinetic controls on phase formation. Composition of synthetic wastewater was based on disposal sites 216-U-8 and 216-U-12 Cribs at the Hanford site (WA, USA). In the absence of dissolved silica or phosphate, crystalline or amorphous uranyl oxide hydrates, either compreignacite or meta-schoepite, precipitated at pH 5 or 7 after 30 d of reaction, in agreement with thermodynamic calculations. In the presence of 1 mM dissolved silica representative of groundwater concentrations, amorphous phases dominated by compreignacite precipitated rapidly at pH 5 or 7 as a metastable phase and formation of poorly crystalline boltwoodite, the thermodynamically stable uranyl silicate phase, was slow. In the presence of phosphate (3 mM), meta-ankoleite initially precipitated as the primary phase at pH 3, 5, or 7 regardless of the presence of 1 mM dissolved silica. Analysis of precipitates by U LIII-edge extended X-ray absorption fine structure (EXAFS) indicated that "autunite-type" sheets of meta-ankoleite transformed to "phosphuranylite-type" sheets after 30 d of reaction, probably due to Ca substitution in the structure. Low solubility of uranyl phosphate phases limits dissolved U(VI) concentrations but differences in particle size, crystallinity, and precipitate composition vary with pH and base cation concentration, which will influence the thermodynamic and kinetic stability of these phases.