Re-evaluation of stability and toxicity of silver sulfide nanoparticle in environmental water: Oxidative dissolution by manganese oxide☆
Graphical abstract
Introduction
Silver sulfide nanoparticle (Ag2S-NP) is used in optical and electronic devices due to its special physicochemical properties (Zhu and Xu, 2011; Jia et al., 2014), resulting in an inevitable release of Ag2S-NP during manufacturing, use and disposal of the devices. Besides the direct discharge, a buildup of Ag2S-NP in the natural environment is associated with silver nanoparticle (Ag-NP), one of the most promising engineered nanomaterials currently (Hassan et al., 2018; Tosco and Sethi, 2018); Ag-NP can be converted almost entirely into Ag2S-NP in wastewater treatment systems within short time (Yin et al., 2017; Zou et al., 2017). Consequently, Ag2S-NP is released into the aquatic environment through effluent discharge (Kim et al., 2010; Kaegi et al., 2011; Kent et al., 2014). Ag2S-NP was reported to be physicochemically stable and biologically inert, resulting in an emerging consensus about high stability of Ag2S-NP in the natural environment (Levard et al., 2012; Reinsch et al., 2012; Levard et al., 2013; Khaksar et al., 2015; Yin et al., 2017). However, our research group recently documented that the stability of Ag2S-NP in environmental water might be overestimated: measurable dissolution of Ag2S-NP and occurrence of Ag-NP were observed in an aqueous solution of Ag2S-NP containing Fe3+ at environmentally relevant concentrations under visible light irradiation conditions (Li et al., 2016a; Li et al., 2016b). In addition, Thalmann et al. (2015) reported that Ag2S-NP could be oxidized and further transformed to silver chloride particles in the ozonation-treated wastewater treatment plant effluent. Although the overestimated stability of Ag2S-NP in environmental water is recognized, studies on potential transformations are still very few and in particular the stability of Ag2S-NP in environmental processes remain unexplored.
Manganese is an important element involved in many redox reactions in naturally environmental processes. Among manganese species, Mn(IV) and Mn(II) are the most prevalent forms in the environment, and especially Mn(IV) is the thermodynamically favored oxidation state in environmental waters (Webb et al., 2005; Hocking et al., 2011; Huynh et al., 2015; Marafatto et al., 2015). Previous work has documented that manganese(IV) oxide, one of the strongest naturally occurring oxidants in the environment, was capable of oxidizing several classes of inorganic and organic contaminants (Lin et al., 2009; Ying et al., 2011; Landrot et al., 2012). Recently, Johnson et al. (2016) reported that dissolved sulfide (i.e., Na2S) as an electron donor could induce colloidal manganese(IV) oxide reduction under an anaerobic conditions. Given precautionary principles, some questions rose: can silver be remobilized from Ag2S-NP by manganese oxide in environmental water? If so, what is the effect of environmental factors? How can Ag2S-NP undergo dissolution by manganese oxide? And what is the impact of Ag2S-NP dissolution by manganese oxide on aquatic organisms? These concerns are important for researchers to systematically understand potential transformations and risks of Ag2S-NP in the aquatic environment. However, little is known about these concerns up to now.
In this context, the objective of this work was to investigate the potential transformation of Ag2S-NP in aqueous solution containing manganese oxide at environmentally relevant concentrations. The effects of environmental factors like natural organic matter (NOM), organic acids and inorganic salts on the Ag2S-NP dissolution by manganese oxide were systematically examined. Specifically, we further proposed the pathway of Ag2S-NP dissolution by manganese oxide on the basis of experimental data. Finally, the potential toxicity of Ag2S-NP dissolution by manganese oxide was evaluated through zebra fish (Danio rerio) embryo development tests. Thus, the results of this study can improve our understanding of Ag2S-NP stability in the natural environment.
Section snippets
Materials
In the present study, ultrapure water (UPW) from a Direct-Q-system (Millipore, Billerica, USA) with a resistivity of 18.2 MΩ/cm was used for preparation of all solutions. The reagents except for the NOM were all purchased from Sigma-Aldrich (St. Luis, USA). NOM (2R101N) from the Suwannee River was purchased from the International Humic Substance Society (Denver, USA). Here a NOM stock solution with the determined content of 100 mg/L dissolved organic carbon (DOC) (TOC-L total organic carbon
Characterization of the prepared manganese oxide
The UV–vis spectrum of the prepared manganese oxide showed a broad absorption band ranging between 250 and 600 nm with a peak position of ∼411 nm (Fig. 1a), which is similar with a characteristic of MnO2 synthesized in a recent work (Li et al., 2015). Additionally, Fig. 1b shows the XRD pattern of the prepared manganese oxide. Although a broad diffraction peak located at around 22.7° was observed, which might be attributed to a glass substrate used due to the low amounts of sample,
Conclusions
Here we found that Ag2S-NP could undergo dissolution through reactions with MnO2 in aqueous solution, being contradictory with the previous consensus about the high stability of Ag2S-NP, which would further shed light on the stability, fate and transformation of Ag2S-NP in the natural environment. Considering the fact that manganese oxide has been recently applied to degrade various organic pollutants in water (Lin et al., 2014; Dang et al., 2016), the dissolution of Ag2S-NP by MnO2 would be
Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgements
We thank the Zhejiang Province of Natural Science Foundation (LY18B070011, LQ16B070003), the National Natural Science Foundation of China (21806141, 21806143) and Science Foundation of State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences (KF2017-04) for financial support. The authors thank the anonymous reviewers for their valuable comments and suggestions on this work.
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