Sulfidation Mechanism for Zinc Oxide Nanoparticles and the Effect of Sulfidation on Their Solubility

Abstract

Environmental transformations of nanoparticles (NPs) affect their properties and toxicity potential. Sulfidation is an important transformation process affecting the fate of NPs containing metal cations with an affinity for sulfide. Here, the extent and mechanism of sulfidation of ZnO NPs were investigated, and the properties of resulting products were carefully characterized. Synchrotron X-ray absorption spectroscopy and X-ray diffraction analysis reveal that transformation of ZnO to ZnS occurs readily at ambient temperature in the presence of inorganic sulfide. The extent of sulfidation depends on sulfide concentration, and close to 100% conversion can be obtained in 5 days given sufficient addition of sulfide. X-ray diffraction and transmission electron microscopy showed formation of primarily ZnS NPs smaller than 5 nm, indicating that sulfidation of ZnO NPs occurs by a dissolution and reprecipitation mechanism. The solubility of partially sulfidized ZnO NPs is controlled by the remaining ZnO core and not quenched by a ZnS shell formed as was observed for partially sulfidized Ag NPs. Sulfidation also led to NP aggregation and a decrease of surface charge. These changes suggest that sulfidation of ZnO NPs alters the behavior, fate, and toxicity of ZnO NPs in the environment. The reactivity and fate of the resulting <5 nm ZnS particles remains to be determined.

Emerging patterns for engineered nanomaterials in the environment: a review of fate and toxicity studies

“…For ZnO, close to 100 % was converted to ZnS within 5 days in the presence of sufficient sulfide concentrations (Ma et al 2013). Similarly, for Ag, in systems with a ratio of S/Ag as low as 0.2, more than 15 % of the Ag is found as Ag 2 S within 24 h (Levard et al 2011).…”

“…Similarly, for Ag, in systems with a ratio of S/Ag as low as 0.2, more than 15 % of the Ag is found as Ag 2 S within 24 h (Levard et al 2011). The presence of sulfides may also cause rapid aggregation and sedimentation (Ma et al 2013). This concurs with the fact that the point-of-zero charge increases with increasing sulfur for Ag, though this will not necessarily lead to an increase in aggregation (Levard et al 2011).…”

“…The asterisks indicate if most studies used to estimate the rate of dissolution of a specific ENM and water type used a coated ENM anaerobic conditions, but if oxygen is completely absent, the results of Liu et al (2011) indicate that sulfidation will actually be suppressed. In anaerobic waters, ZnO NPs can be transformed to ZnS through the dissolution and reprecipitation of zinc ions (Ma et al 2013), Cd NPs can be transformed to CdS (Cabot et al 2008), and PVP-coated Ag NPs may be transformed to silver sulfide (Ag 2 S) sometimes via oxysulfidation (Levard et al 2011;Liu et al 2011;Reinsch et al 2012). Near complete conversion of Ag to Ag 2 S was found in wastewater treatment plant effluent, indicating that Ag 2 S will be formed in sulfur rich, reducing environments (Kim et al 2010).…”

Sulfidation as a Natural Antidote to Metallic Nanoparticles Is Overestimated: CuO Sulfidation Yields CuS Nanoparticles with Increased Toxicity in Medaka (Oryzias latipes) Embryos

“…This is an indirect indication that dissolution characteristics play an important role in the increased toxicity of CuS-NPs relative to CuO-NPs in medaka embryos, which contrasts with previous studies on that sulfidation reduced dissolution of metallic NPs due to the lower solubility of metal sulfides. [19][20][21][22]42 Also, it contradicts the solubility products (K sp ) of CuS (8 × 10 −37 ) and CuO (1 × 10 −7 ) in solutions. As shown in Figure 4, CuS-NPs show higher extent of dissolution relative to CuO-NPs could be attributed to not only the oxidation dissolution of the formed CuS-NPs, but also the presence of CuS clusters.…”

Chemical transformation of zinc oxide nanoparticles as a result of interaction with hydroxyapatite

“…Recently, much attention has been paid to chemical transformations of ENPs in the environment, which will result in largely unknown end products and affect the fate and potential toxicity of ENPs in the environment [4]. For example, studies have demonstrated the transformation of ZnO NPs to ZnS in the presence of sodium sulfide (Na 2 S) or hydrogen sulfide (H 2 S) [12,13]. Our previous research [14] has confirmed that phosphate ions can induce the transformation of ZnO NPs to tetrahydrate Zn 3 (PO 4 ) 2 in aqueous solutions at room temperature and under neutral pH conditions.…”