Monitoring the fate of copper nanoparticles in river biofilms using scanning transmission X-ray microscopy (STXM)

Scanning transmission X-ray microscopy (STXM) at the C 1s and Cu 2p edges was used to study the fate of Cu nanoparticles, including their sorption–dissolution, speciation and distribution in a complex natural river biofilm exposed to 1mgL−1 Cu nanoparticles for 5min, 3days or 7days. The original Cu nanoparticles, ~30nm in diameter, were composed of Cu(I) and Cu(II) species, presumably a Cu(I) core surrounded by oxidized Cu(II). When the Cu nanoparticles were initially added to the biofilm, Cu nanoparticles were occasionally found as individual particles but most frequently were observed as aggregates. The aggregates were in association with diatoms and the extracellular polymeric substances (EPS) of the biofilm. In addition, a new Cu(II) species, as evident from a shift in its peak position (−0.4eV) relative to the Cu nanoparticles, was found to be sorbed by the diatoms after 5min and was still detected after 7days. Evidently, the Cu nanoparticles had undergone dissolution, followed by sorption of ionic Cu(II) by the diatoms. The bacteria and cyanobacteria in the biofilm did not apparently sorb detectable amounts of Cu at any time. C 1s measurements indicated that the Cu(II) from solution was sorbed by the lipid-rich extracellular polymeric substances associated with the diatoms. These observations demonstrated that both the Cu nanoparticles and dissolution products remained associated with the river biofilm through interactions with specific biopolymers up to 7days following exposure. Therefore, for some metal nanomaterials, sorption of the solid phase with subsequent dissolution and re-sorption are significant with respect to their chemodynamics and effects on the biofilm, especially when trophic transfer events are considered. Importantly, these interactions provide an entry point for copper into the aquatic food web.