Recently published in the peer reviewed journal Environmental Science: Nano, a new NanoFASE paper by researchers Alexander Gogos and Ralf Kägi, Eawag, Switzerland, and colleagues, looks at how fast copper oxide nanoparticles become sulfidized in wastewater treatment.

"This paper features the first results from our investigations (in liaison with NanoFASE scientists led by Frank Kammer and Antonia Praetorius at UNIVIE) on the transformation reactions that take place in wastewater treatment (i.e. the sulfidation of copper oxide nanoparticles)" explains the lead author. "We used controlled batch experiments, to provide a better understanding of the kinetics of this reaction as well as underlying reaction mechanisms. Overall we could show that the reaction is very fast, with half-life times in the order of minutes; so we can expect that copper oxide nanoparticles, which are released to the wastewater stream, will likely be discharged as copper sulfides to surface waters."

The original publication is available at: http://doi.org/10.1039/C7EN00309A

Abstract:

Sulfidation of copper oxide nanoparticles (CuO NPs) in urban wastewater systems is expected to influence their impact on the environment. However, the kinetics of this reaction has not been studied to date and the reaction mechanism remains largely unexplored. We therefore investigated the sulfidation kinetics of CuO NPs reacted with bisulfide (HS−) at concentrations relevant to wastewater systems. Pristine CuO NPs (50 nm, 7.7 μM) were reacted with HS− (26.4–105.6 μM) in oxic solutions buffered to pH 8.0. The reaction progress was monitored using silver nitrate to quench the reaction and selectively dissolve the copper sulfides (CuxS) and zincon to spectrophotometrically quantify the released Cu2+. In addition, the reaction products were characterized at selected time points using analytical electron microscopy and X-ray absorption spectroscopy (XAS). The sulfidation rate of the CuO NPs was best described by a pseudo first order rate law and the corresponding half-life times ranged between 1 and 6 minutes. XAS results showed that crystalline CuO NPs rapidly transformed into amorphous CuxS and gradually into crystalline CuS (covellite). The comparable size of pristine and transformed primary particles, the similar morphology of their aggregates, and the initial formation of CuO–CuxS core–shell structures revealed by analytical electron microscopy suggest that the initial sulfidation occurred via a direct conversion reaction mechanism. Our findings suggest that CuO NPs released from various sources into wastewater will rapidly transform into amorphous CuxS and eventually recrystallize into covellite.