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.
    Water Research 03/2015; 68:87–97. DOI:10.1016/j.watres.2014.09.008 · 5.53 Impact Factor
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    • "Manufactured NPs from consumer products will likely enter sewer and wastewater treatment plant (WWTP) systems where NPs are removed from the wastewaters via flocculation and sorption to sludge (Kaegi et al., 2011; Kiser et al., 2009). Detailed investigations reveal that Ag 0 NPs undergo a series of transformations, which involve oxidation (Ag 0 /Ag þ ) followed by sulphidation (Doolette et al., 2013; Kaegi et al., 2011; Liu et al., 2011). This process has been confirmed to occur regardless of the Ag starting material (Doolette et al., 2013; Kim et al., 2010; Lombi et al., 2013). "
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    ABSTRACT: Manufactured nanoparticles (NPs) present in consumer products could enter soils through re-use of biosolids. Among these NPs are those based on silver (Ag), which are found sulphidised (e.g. silver sulphide, Ag2S) in biosolids. Herein, our aim was to examine the release of retained Ag(0) and Ag2S NPs in soils and biosolids as facilitated by environmentally and agriculturally relevant ligands. Under natural soil conditions, exemplified by potassium nitrate and humic acid experiments, release of Ag retained in soil was limited. The highest total Ag release was facilitated by ligands that simulated root exudates (citrate) or fertilisers (thiosulphate). Released Ag was predominantly present in the colloidal phase (>3 kDa-< 0.45 μm); intact NPs only identified in Ag2S-NP extracts. For biosolids containing nanoparticulate-Ag-S, release was also enhanced by thiosulphate, though mostly as colloidal-Ag - not intact NPs. These results suggest that exposure to NPs as a result of its release from soils or biosolids will be low.
    Environmental Pollution 07/2014; 193C:102-110. DOI:10.1016/j.envpol.2014.06.008 · 4.14 Impact Factor
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    • "Instead, the bulk of the spectrum is comprised of near-equal portions of Ag 2 S(s) and Ag-sorbed HA phases. Sulfidation (Kaegi et al. 2011; Kim et al. 2010; Levard et al. 2011; Liu et al. 2011) was previously suggested to be the dominant reaction path for AgNPs in reducing systems. However, our experiment showed the competitive ligand effects (humic acids) on the fate of Ag(I)/AgNPs in soils. "
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    ABSTRACT: Residence time effects on phase transformation of silver nanoparticles (AgNPs) (15-50 nm, with and without polyvinylpyrrolidone (PVP) coating) were investigated in reducing soils using experimental geochemistry and synchrotron-based x-ray techniques. After 30 days of anaerobic incubation, a substantial fraction of PVP-coated AgNPs (15 nm) were transformed into Ag2S and or humic acid (HA) complexed Ag(I), whereas only the HA fraction was dominant in uncoated AgNPs (50 nm). Several investigations recently reported that sulfidation of AgNPs to Ag2S was the predominant mechanism controlling the fate of AgNP in soil-water environments. However, this investigation showed each AgNP underwent particle-specific chemical transformations to different end compounds after 30 days. Considering the small contribution of Ag(I) dissolution from all AgNPs (less than 5 %), we concluded that changes in solid-state chemical speciation of sorbed AgNPs was promoted by particle-specific interactions of NPs in soil chemical constituents, suggesting a critical role of soil absorbents in predicting the fate of AgNPs in terrestrial environments.
    Environmental Science and Pollution Research 03/2014; 21(13). DOI:10.1007/s11356-014-2743-9 · 2.83 Impact Factor
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