Peggy A. O’Day

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.

Immobilization of Hg(II) by Coprecipitation in Sulfate-Cement Systems

Uptake and molecular speciation of dissolved Hg during formation of Al- or Fe-ettringite-type and high-pH phases were investigated in coprecipitation and sorption experiments of sulfate-cement treatments used for soil and sediment remediation. Ettringite and minor gypsum were identified by XRD as primary phases in Al systems, whereas gypsum and ferrihydrite were the main products in Hg–Fe precipitates. Characterization of Hg–Al solids by bulk Hg EXAFS, electron microprobe, and microfocused-XRF mapping indicated coordination of Hg by Cl ligands, multiple Hg and Cl backscattering atoms, and concentration of Hg as small particles. Thermodynamic predictions agreed with experimental observations for bulk phases, but Hg speciation indicated lack of equilibration with the final solution. Results suggest physical encapsulation of Hg as a polynuclear chloromercury(II) salt in ettringite as the primary immobilization mechanism. In Hg–Fe solids, structural characterization indicated Hg coordination by O atoms only and Fe backscattering atoms that is consistent with inner-sphere complexation of Hg(OH)20 coprecipitated with ferrihydrite. Precipitation of ferrihydrite removed Hg from solution, but the resulting solid was sufficiently hydrated to allow equilibration of sorbed Hg species with the aqueous solution. Electron microprobe XRF characterization of sorption samples with low Hg concentration reacted with cement and FeSO4 amendment indicated correlation of Hg and Fe, supporting the interpretation of Hg removal by precipitation of an Fe(III) oxide phase.

Cesium and strontium incorporation into zeolite-type phases during homogeneous nucleation from caustic solutions

Abstract Formation of faujasite- and sodalite/cancrinite-type phases associated with caustic waste reactions in the environment may structurally incorporate contaminant species such as radioactive Sr2+ and Cs+, and thus provide a mechanism of attenuation. To investigate mineral evolution and structural incorporation of cations in simplified experiments, aluminosilicate solids were precipitated homogeneously at room temperature from batch solutions containing a 1:1 molal ratio of Si to Al and 10-3 molal Sr and/or Cs, and aged for 30 or 548 days. Syntheses were done with solutions in equilibrium with atmospheric CO2 and with gas-purged solutions. Experimental products were characterized by bulk chemical analyses, chemical extractions, XRD, SEM/TEM, TGA, solid-state 27Al NMR, and Sr EXAFS. Chemical analysis showed that solids had a 1:1 Al:Si molar ratio, and that Sr was sequestered at higher amounts than Cs. After 30 days of aging in purged solutions, XRD showed that zeolite X (faujasite-type) was the only crystalline product. After aging 30 and 548 days in solutions equilibrated with atmospheric CO2, a mixture of sodalite, cancrinite, and minor zeolite X were produced. Surface areas of solids at 30 days were much lower than published values for zeolite phases synthesized at high temperature, although particle aging produced more crystalline and less aggregated phases with higher bulk surface areas. Characterization of products by 27Al NMR indicated only tetrahedrally coordinated Al. Measured isotropic shifts of primary resonances did not change substantially with precipitate aging although the primary mineral phase changed from zeolite X to sodalite/cancrinite, indicating local ordering of Al-Si tetrahedra. Analysis of reaction products by Sr EXAFS suggested Sr bonding in hexagonal prisms and six-membered rings of the supercages of zeolite X that may be more site specific than those of monovalent cations. For samples aged for 548 days, interatomic distances from Sr-EXAFS are consistent with partial Sr dehydration and bonding to framework oxygen atoms in sodalite cages or in large channels in cancrinite. Incorporation of Sr into both faujasite and sodalite/cancrinite phases is favored over Cs during room-temperature synthesis, possibly because of increased cation site competition between Cs+ and Na+. Results of this study help to constrain cation incorporation into sodalite/cancrinite mineral assemblages that form at caustic waste-impacted field sites and may aid in the predictive modeling of contaminant release.

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

Abstract 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 6 h, their expression was unchanged except for GCLC, which was reduced compared with controls. At 18 h, 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 3 h. The nuclear translocation of Nrf2 occurred later than that for NF‐&kgr;B p65 protein and the induction of proinflammatory cytokines (TNF&agr; and IL‐1&bgr;). NF‐&kgr;B inhibitor SN50 prevented the reduction of GCLC at 6 h and abolished the induction of antioxidant genes at 18 h by SiO2‐Fe, but did not affect the basal and sulforaphane‐induced expression of antioxidant genes, suggesting that NF‐&kgr;B 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‐&kgr;B‐Nrf2 interactions. Graphical abstract Figure. No Caption available. HighlightsSilica‐iron nanoparticles induce pro‐inflammatory cytokines in human macrophages.Nrf2–regulated antioxidant genes show delayed induction by silica‐iron nPM.NF‐&kgr;B inhibition blocked induction of cytokines by silica‐iron nPM.NF‐&kgr;B inhibition blocked Nrf2–regulated antioxidant gene induction.