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Chen J.,China University of Geosciences | Chen J.,Rice University | Xiu Z.,Rice University | Lowry G.V.,Carnegie Mellon University | And 2 more authors.
Water Research | Year: 2011

Nano-scale zero-valent iron (NZVI) particles are increasingly used to remediate aquifers contaminated with hazardous oxidized pollutants such as trichloroethylene (TCE). However, the high reduction potential of NZVI can result in toxicity to indigenous bacteria and hinder their participation in the cleanup process. Here, we report on the mitigation of the bactericidal activity of NZVI towards gram-negative Escherichia coli and gram-positive Bacillus subtilis in the presence of Suwannee River humic acids (SRHA), which were used as a model for natural organic matter (NOM). B. subtilis was more tolerant to NZVI (1 g/L) than E. coli in aerobic bicarbonate-buffered medium. SRHA (10 mg/L) significantly mitigated toxicity, and survival rates after 4 h exposure increased to similar levels observed for controls not exposed to NZVI. TEM images showed that the surface of NZVI and E. coli was surrounded by a visible floccus. This decreased the zeta potential of NZVI from -30 to -45 mV and apparently exerted electrosteric hindrance to minimize direct contact with bacteria, which mitigated toxicity. H2 production during anaerobic NZVI corrosion was not significantly hindered by SRHA (p > 0.05), However, NZVI reactivity towards TCE (20 mg/L), assessed by the first-order dechlorination rate coefficient, decreased by 23%. Overall, these results suggest that the presence of NOM offers a tradeoff for NZVI-based remediation, with higher potential for concurrent or sequential bioremediation at the expense of partially inhibited abiotic reactivity with the target contaminant (TCE). © 2010 Elsevier Ltd.

Reinsch B.C.,Carnegie Mellon University | Reinsch B.C.,CEINT Inc | Forsberg B.,University of Minnesota | Penn R.L.,University of Minnesota | And 3 more authors.
Environmental Science and Technology | Year: 2010

Nanoscale zerovalent iron (NZVI) that was aged in simulated groundwater was evaluated for alterations in composition and speciation over 6 months to understand the possible transformations NZVI could undergo in natural waters. NZVI was exposed to 10 mN of various common groundwater anions (Cl-, NO3-, SO42-, HPO4 2-, and HCO3-) or to dissolved oxygen (saturated, ∼9 mg/L). Fresh and exposed NZVI samples, along with Fe-oxide model compounds, were then analyzed using synchrotron radiation X-ray absorption spectroscopy (XAS) to yield both relative oxidation state, using the X-ray absorption near edge structure (XANES), and quantitative speciation information regarding the types and proportions of mineral species present, from analysis of the extended X-ray absorption fine structure (EXAFS). Over 1 month of aging the dissolved anions inhibited the oxidation of the NZVI to varying degrees. Aging for 6 months, however, resulted in average oxidation states that were similar to each other regardless of the anion used, except for nitrate. Nitrate passivated the NZVI surface such that even after 6 months of aging the particles retained nearly the same mineral and Fe0 content as fresh NZVI. Linear least-squares combination fitting (LCF) of the EXAFS spectra for 1 month-aged samples indicated that the oxidized particles remain predominantly a binary phase system containing Fe0 and Fe3O4, while the 6 month aged samples contained additional mineral phases such as vivianite (Fe3(PO4)28H2O) and iron sulfate species, possibly schwertmannite (Fe3+16O 16(OH,SO4)12-1310-12H2O). The presence of these additional mineral species was confirmed using synchrotron-based X-ray diffraction (XRD). NZVI exposed to water saturated with dissolved oxygen showed a rapid (<24 h) loss of Fe0 and evolved both magnetite and maghemite (γ-Fe2O3) within the oxide layer. These findings have implications toward the eventual fate, transport, and toxicity of NZVI used for groundwater remediation. © 2010 American Chemical Society.

Xiao Y.,Duke University | Xiao Y.,CEINT Inc | Wiesner M.R.,Duke University | Wiesner M.R.,CEINT Inc
Journal of Hazardous Materials | Year: 2012

The surface chemistry of nanoparticles, including their hydrophobicity, is a key determinant of their fate, transport and toxicity. Engineered NPs often have surface coatings that control the surface chemistry of NPs and may dominate the effects of the nanoparticle core. Suitable characterization methods for surface hydrophobicity at the nano-scale are needed. Three types of methods, surface adsorption, affinity coefficient and contact angle, were investigated in this study with seven carbon and metal based NPs with and without coatings. The adsorption of hydrophobic molecules, Rose Bengal dye and naphthalene, on NPs was used as one measure of hydrophobicity and was compared with the relative affinity of NPs for octanol or water phases, analogous to the determination of octanol-water partition coefficients for organic molecules. The sessile drop method was adapted for measuring contact angle of a thin film of NPs. Results for these three methods were qualitatively in agreement. Aqueous-nC 60 and tetrahydrofuran-nC 60 were observed to be more hydrophobic than nano-Ag coated with polyvinylpyrrolidone or gum arabic, followed by nano-Ag or nano-Au with citrate-functionalized surfaces. Fullerol was shown to be the least hydrophobic of seven NPs tested. The advantages and limitations of each method were also discussed. © 2012 Elsevier B.V.

Lin S.,Duke University | Lin S.,CEINT Inc | Wiesner M.R.,Duke University | Wiesner M.R.,CEINT Inc
Environmental Science and Technology | Year: 2012

Theoretical and experimental approaches were employed to study the effect of aggregation on the affinity between nanoparticles (NPs) and a flat surface that is quantified by the attachment efficiency. Computer simulations were used to generate virtual aggregates formed via either diffusion limited cluster aggregation or reaction limited cluster aggregation. The colloidal interactions between the simulated aggregates and a flat surface were evaluated based on the surface element integration approach. It was found that the strength of colloidal interaction for the aggregated NPs was on the same order of magnitude as those for the primary particles and was significantly weaker than that for an equivalent sphere defined by the gyration radius of the aggregate. The results from the deposition experiments using quartz crystal microbalance suggested that the attachment efficiencies (unfavorable deposition) for aggregated NPs were higher at the initial stage but later became similar to that of the primary NPs when equilibrium deposition was reached. The high initial affinity was postulated to be attributable to secondary minimum deposition. These results suggest that it is the size of the primary particles, not that of the aggregates, that determines the strength of the colloidal interaction between the aggregate and an environmental surface. © 2012 American Chemical Society.

de Lannoy C.-F.,Duke University | de Lannoy C.-F.,CEINT Inc | Soyer E.,Marmara University | Wiesner M.R.,Duke University | Wiesner M.R.,CEINT Inc
Journal of Membrane Science | Year: 2013

Multiwalled carbon nanotubes (CNTs) carboxylated to varying weight percentages were added to polysulfone membranes. The degree to which the CNTs were retained within the membrane during membrane production, operation and cleaning was examined as a function of the extent of CNT carboxylation. The effects of CNTs on polymeric membranes - increases in tensile strength, changes in surface hydrophilicity, and changes in membrane permeability - were evaluated as a function of CNT carboxylation, which was found to be coupled to CNT retention within the membrane. It was found that CNTs functionalized to a higher degree form more homogeneous polymer solutions, which lead to greater improvements in the aforementioned membrane characteristics. However, CNTs functionalized to a higher degree were also found to more-readily leave the membrane during immersion precipitation and membrane cleaning. Therefore, a balance was discovered between the benefits associated with increased dispersibility and hydrophilicity, and the disadvantages associated with decreased retention, increased leaching, and decreased strength of CNTs with greater carboxylation.© 2013.

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