See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/262069326 Sulfidation of copper oxide nanoparticles and properties of resulting copper sulfide Article in Environmental science. Nano · June 2014 DOI: 10.1039/C4EN00018H CITATIONS READS 16 170 8 authors, including: John Stegemeier Clément Levard Carnegie Mellon University French National Centre for Scientific Research 12 PUBLICATIONS 94 CITATIONS 57 PUBLICATIONS 1,699 CITATIONS SEE PROFILE SEE PROFILE Clinton William Noack Gregory V Lowry Carnegie Mellon University Carnegie Mellon University 12 PUBLICATIONS 42 CITATIONS 202 PUBLICATIONS 10,656 CITATIONS SEE PROFILE SEE PROFILE Available from: Clinton William Noack Retrieved on: 15 November 2016 Environmental Science Nano View Article Online PAPER View Journal Sulfidation of copper oxide nanoparticles and properties of resulting copper sulfide† Cite this: DOI: 10.1039/c4en00018h Rui Ma,abh John Stegemeier,ab Clément Levard,bc James G. Dale,bd Clinton W. Noack,a Tiffany Yang,ab Gordon E. BrownJr.efg and Gregory V. Lowry*ab Many nanoparticles (NPs) are transformed in the environment, and the properties of the transformed materials must be determined to accurately assess their environmental risk. Sulfidation is expected to alter the speciation and properties of CuO NPs significantly. Here, commercially available 40 nm CuO NPs were characterized and sulfidized in water by inorganic sulfide, and the properties of the resulting products were determined. X-ray absorption spectroscopy, X-ray diffraction, and transmission electron microscopy indi- cate that CuO (tenorite) is sulfidized by inorganic sulfide to several copper sulfide (CuxSy) species including crystalline CuS (covellite), and amorphous (CuxSy) species at ambient temperature. Some Cu(II) was reduced to Cu(I) during sulfidation, coupled with sulfide oxidation to sulfate, resulting in the formation of small amounts of several copper sulfate hydroxide species as well. The extent of sulfidation depends on the sul- fide to CuO molar concentration ratio used. At the highest S/Cu molar ratio of 2.16, 100% sulfidation was not reached in 7 days, as evidenced by the persistence of small amounts of CuO in the NPs. Sulfidation increased the fraction of copper passing a 3 kDa MWCO filter representing soluble forms of Cu and any Received 7th February 2014, small CuxSy clusters compared to the pristine CuO NPs at environmentally relevant neutral pH. This high Accepted 26th April 2014 solubility is a result of oxidative dissolution of CuxSy, formation of relatively more soluble copper sulfate hydroxides, and the formation of small CuS nanoclusters that pass the 3 kDa MWCO filter. These findings DOI: 10.1039/c4en00018h suggest that sulfidation of CuO may increase its apparent solubility and resulting bioavailability and rsc.li/es-nano eco-toxicity attributed to toxic Cu2+. Published on 29 April 2014. Downloaded 20/06/2014 18:01:45. Nano impact Metal and metal oxide nanomaterials that find their way into reducing environments such as wastewater treatment plants or subaquatic sediments may potentially become sulfidized. The properties of the sulfidized materials will control their fate and toxicity so those properties must be determined. This work describes the sulfidation of CuO nanoparticles, and determines the properties of the sulfidized copper product under environmental conditionsthat are relevant to predicting fate and toxicity, e.g. solubility. This enhances our understanding of the behavior of these nanomaterials in important environmental compartments, and better enables predictions of their interactions with natural systems. a Department of Civil & Environmental Engineering, Carnegie Mellon University, USA. E-mail: [email protected] b Center for Environmental Implications of Nanotechnology (CEINT), USA Introduction c Aix-Marseille Université, CNRS, IRD, CEREGE UM34, 13545 Aix en Provence, France d Department of Geosciences, Virginia Tech, USA Copper-based nanoparticles (NPs) are being used in products e Department of Geological & Environmental Sciences, Stanford University, USA or technologies like semiconductors, heat transfer fluids, cata- – f Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator lysts, batteries, solar cells and biocides.1 6 Copper based NPs Laboratory, USA such as CuO and elemental Cu(0) were found to be among the g Department of Photon Science, SLAC National Accelerator Laboratory, top five most reported metal NPs in recent (2011) nanotech- Menlo Park, CA, USA h Ecological Environment Institute, Chinese Academy for Environmental Planning, nology patents, indicating the potential for an increasing 7 Beijing, China use of these NPs. Their widespread uses will likely lead to † Electronic supplementary information (ESI) available: Molar ratios of Cu to S subsequent release into the environment, and will raise used in sulfidation, TGA weight loss over temperature of CuO NPs, peak concern about their potential toxicity. Nano Cu(0) was matching of XRD standard peak from mineral database for S/Cu = 0.22 and S/Cu = 2.16 sulfidized particles, linear combination fitting results and XANES reported to have a thin oxide layer when exposed to air, spectra from XAS, additional TEM images of the sulfidized CuO particles. where the Cu(0) is oxidized to form Cu2O and then ulti- 8 See DOI: 10.1039/c4en00018h mately to CuO in an aerobic environment. Hence, CuO This journal is © The Royal Society of Chemistry 2014 Environ. Sci.: Nano View Article Online Paper Environmental Science: Nano NPs are an environmentally relevant form of copper for sulfide species under environmental conditions (i.e. ambient assessing the risks of nano copper products. temperature, neutral pH, and low ionic strength). The toxicity of CuO NPs to a variety of organisms has been In this study we sulfidized commercially available CuO NPs studied extensively. According to a recent critical review, CuO using different S/Cu molar ratios. The pristine CuO NPs and the NPs are toxic to crustaceans, algae, fish and bacteria, but resulting sulfidized CuxSy/CuO NPs were extensively character- typically requires higher doses than Cu2+ ions to achieve the ized. The objectives of this study were to (1) provide mechanistic same effects. Cu2+ was found to be more toxic to all organ- insights about the transformation of CuO in the presence of sul- isms except for yeast and mammalian cells in vitro.9 Although fide in aqueous solution; and (2) determine the properties and Cu(0) NPs can catalyze the formation of reactive oxygen spe- solubility of the resulting sulfidized nanoparticles. The pristine cies (ROS), leading to toxicity.10,11 CuO dissolution to release and sulfidized particles were characterized using transmission Cu2+ accounted for most of the observed toxicity in vitro and electron microscopy (TEM), dynamic light scattering (DLS), in vivo.12 This indicates that the ability of the NPs to release thermo gravimetric analysis (TGA), X-ray diffraction (XRD), and Cu2+ plays a key role in copper NP toxicity. synchrotron based X-ray absorption spectroscopy (XAS). Their The dissolution of CuO NPs resulting in the release of Cu2+ dissolution rate was determined by measuring ion release by ions is pH dependent. The dissolution minimum is expected at inductively coupled plasma mass spectrometry (ICP-MS). near neutral pH (6–8). However, the solubility is significantly higher at lower pH (4–5). Dissolution of CuO NPs is also promoted Materials and methods by strong ligands such as amino acids,11,13 even at neutral pH. CuO NPs This behavior makes CuO NPs one of the more toxic metal oxides NP in cell culture media (containing amino acids).14 The CuO NPs (40 nm) were purchased from US Research Sulfidation is an important transformation of many metal Nanomaterials, Inc. (Houston, TX). The primary particle size is and metal oxide NPs. This is because sulfidation has been ~40 nm and the particles are generally spherical. The N2-BET − demonstrated to affect NP chemical composition,15,16 to reduce specific surface area provided by the manufacturer is 20 m2 g 1. ion release,16,17 and to significantly decrease the toxicity of The particles were used as received. The manufacturer claims Ag NPs to a range of organism types.15,18 Ag,ZnO,andCuO that the CuO NPs contain no organic capping agent. This was NPs all contain class B soft metals19 and are likely to sulfidize confirmed by thermo gravimetric analysis (TGA) as described once released into the environment. Donner et al. showed below. The crystal structure of the initial CuO NPs is described that 74–92% of the Cu is present as Cu(I)andCu(II) sulfide in the results section. (chalcocite and covellite, respectively) in fresh biosolids that had been amended with CuO NPs, but these inorganic Sulfidation of CuO nanoparticles Cu-sulfide species were transformed to Cu–organic sulfur com- Sulfidation of CuO NPs was conducted following similar pro- 20 16 plexes and Cu(II) sulfide in aged biosolids. Dimkpa et al. cedures as previously described for sulfidation of ZnO NPs. showed that exposure of plant roots to CuO NPs resulted in In order to minimize the oxidation of sulfide by dissolved Published on 29 April 2014. Downloaded 20/06/2014 18:01:45. – bioaccumulation of Cu2S, Cu cysteine complexes, and CuO oxygen in a typical experiment, all solutions were prepared 21 NPs in the plant. Based on these previous findings, it is using N2-purgedDIwaterandN2-purged headspace. The CuO important to understand the sulfidation products