Kinetics and Mechanisms of Nanosilver Oxysulfidation

Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States.
Environmental Science & Technology (Impact Factor: 5.33). 08/2011; 45(17):7345-53. DOI: 10.1021/es201539s
Source: PubMed


Among the many new engineered nanomaterials, nanosilver is one of the highest priority cases for environmental risk assessment. Recent analysis of field samples from water treatment facilities suggests that silver is converted to silver sulfide, whose very low solubility may limit the bioavailability and adverse impact of silver in the environment. The present study demonstrates that silver nanoparticles react with dissolved sulfide species (H(2)S, HS(-)) under relevant but controlled laboratory conditions to produce silver sulfide nanostructures similar to those observed in the field. The reaction is tracked by time-resolved sulfide depletion measurements to yield quantitative reaction rates and stoichiometries. The reaction requires dissolved oxygen, and it is sensitive to pH and natural organic matter. Focused-ion-beam analysis of surface films reveals an irregular coarse-grained sulfide phase that allows deep (>1 μm) conversion of silver surfaces without passivation. At high sulfide concentrations, nanosilver oxysulfidation occurs by a direct particle-fluid reaction. At low sulfide concentration, quantitative kinetic analysis suggests a mechanistic switch to an oxidative dissolution/precipitation mechanism, in which the biologically active Ag(+) ion is generated as an intermediate. The environmental transformation pathways for nanosilver will vary depending on the media-specific competing rates of oxidative dissolution and direct oxysulfidation.

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Available from: Jingyu Liu, Dec 11, 2014
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    • "Overall, the results indicate that nanoAg and Ag þ interact distinctly with microbes in a complex activated sludge environment. Liu et al. (2011) suggested direct sulfidation of nanoAg at high sulfide concentrations without dissolution to Ag þ as intermediate. "
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    ABSTRACT: As nanomaterials in consumer products increasingly enter wastewater treatment plants, there is concern that they may have adverse effects on biological wastewater treatment. Effects of silver (nanoAg), zero-valent iron (NZVI), titanium dioxide (nanoTiO2) and cerium dioxide (nanoCeO2) nanomaterials on nitrification and microbial community structure were examined in duplicate lab-scale nitrifying sequencing batch reactors (SBRs) relative to control SBRs that received no nanomaterials or ionic/bulk analogs. Nitrification function was not measurably inhibited in the SBRs by any of the materials as dosing was initiated at 0.1 mg/L and sequentially increased every 14 days to 1, 10, and 20 mg/L. However, SBRs rapidly lost nitrification function when the Ag(+) experiment was repeated at a continuous high load of 20 mg/L. Shifts in microbial community structure and decreased microbial diversity were associated with both sequential and high loading of nanoAg and Ag(+), with more pronounced effects for Ag(+). Bacteroidetes became more dominant in SBRs dosed with Ag(+), while Proteobacteria became more dominant in SBRs dosed with nanoAg. The two forms of silver also had distinct effects on specific bacterial genera. A decrease in nitrification gene markers (amoA) was observed in SBRs dosed with nanoAg and Ag(+). In contrast, impacts of NZVI, nanoTiO2, nanoCeO2 and their analogs on microbial community structure and nitrification gene markers were limited. TEM-EDS analysis indicated that a large portion of nanoAg remained dispersed in the activated sludge and formed Ag-S complexes, while NZVI, nanoTiO2 and nanoCeO2 were mostly aggregated and chemically unmodified. Overall, this study suggests a high threshold of the four nanomaterials in terms of exerting adverse effects on nitrification function. However, distinct microbial community responses to nanoAg indicate potential long-term effects. Copyright © 2014 Elsevier Ltd. All rights reserved.
    Full-text · Article · Mar 2015 · Water Research
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    • "), the UV–vis absorbance of both cit-Ag NPs and Tween-Ag NPs decreases rapidly following the reaction of Na 2 S, followed by a slower decrease in the UV–vis absorbance over time. It has been suggested that humic acid slightly increases sulfidation rates of Ag NPs (Liu et al., 2011 "
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    ABSTRACT: Silver nanoparticles (Ag NPs) are susceptible to transformations in environmental and biological media such as aggregation, oxidation, dissolution, chlorination, sulfidation, formation/replacement of surface coatings following interaction with natural organic matter (NOM). This paper investigates the impact of surface coating and Suwannee River fulvic acid (SRFA) on the transformations and behavior of Ag NPs (citrate coated and Tween coated; cit-Ag NPs and Tween-Ag NPs, respectively), following reaction with different concentrations of Na2S solution (as a source of sulfide species, H2S and HS-). These transformations and the dominant mechanisms of transformations were investigated using UV-vis and scanning transmission electron microscopy coupled with electron energy loss spectroscopy. Here, we have shown that Ag NP surface coating impacts their dissolution following dilution in ultrahigh purity water, with higher extent of dissolution of Tween-Ag NPs compared with cit-Ag NPs. Tween-Ag NPs are susceptible to dissolution following their sulfidation at low S/Ag molar ratio. Suwannee River fulvic acid (SRFA) slows down the dissolution of Tween-Ag NPs at low sulfide concentrations and reduces the aggregation of cit-Ag NP in the presence of sodium sulfide. Sulfidation appears to occur by direct interaction of sulfide species with Ag NPs rather than by indirect reaction of sulfide with dissolved Ag species subsequent to dissolution. Furthermore, the sulfidation process results in the formation of partially sulfidized Ag NPs containing unreacted (metallic) subgrains at the edge of the NPs for Tween-Ag NPs in the presence of high sulfide concentration (2000 nM Na2S), which occurred to less extent at lower Na2S concentration for Tween-Ag NPs and at all concentrations of Na2S for cit-Ag NPs. Thus, sulfidized Ag NPs may preserve some of the properties of the Ag NPs such as their potential to shed Ag+ ions and their toxic potential of Ag NPs.
    Full-text · Article · Jan 2015 · Science of The Total Environment
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    • "Transformations of the NMs will affect their fate, transport, and toxic properties. For example, many metallic or metal oxide NPs (e.g., Ag, ZnO, CuO) may ultimately become sulfidized in the environment (Liu et al., 2011; Ma et al., 2013). Sulfidation of the metal or metal oxide NPs changes their aggregation state, surface chemistry and charge, as well as their ability to release potentially toxic ions such as Zn 2þ or Ag þ (Levard et al., 2011a), and therefore their persistence and toxicity. "
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    ABSTRACT: Nanomaterials (NMs) will undergo a variety of transformations in the natural environment that can significantly change their transport behavior, ultimate fate, and toxicity. Chemical transformations include oxidation-reduction reactions, ligation, and dissolution (which are often associated with a redox reaction). Physical transformations include aggregation or disaggregation and adsorption of naturally occurring macromolecules. Biological interactions and biouptake of NMs can result in further physicochemical transformations. The current state of knowledge on the effects of these transformations on NM properties and hence their behavior, persistence, and risk in the environment is presented. The implications of these transformations for detection and characterization of NMs in biological and environmental matrices are also discussed. Finally, challenges for risk assessment of NMs due to their environmental transformations are identified.
    Full-text · Article · Dec 2014 · Frontiers of Nanoscience
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