Behavior and fate of industrial zinc oxide nanoparticles in a carbonate-rich river water.

The present study precisely describes the solubility patterns of commercial uncoated and organic coated ZnO NPs (nc-NPs and c-NPs, respectively) in a natural carbonate-rich water and the physicochemical processes involved. NPs transformation rates were determined with the Donnan Membrane approach (DMT, to obtain Zn(2+) concentration) and ultrafiltration (i.e. Zn(2+) and Zn bound to small organic ligands) and modeled with VMinteQ. XPS measurements evidenced the presence on native nc-NPs of a Zn(OH)2 layer which accounts for almost 22% of total Zn. This Zn(OH)2 phase is more soluble than ZnO, and could control the early dissolution steps of the nc-NPs in our system. Indeed, nc-NPs display a fast (<1 h) dissolution step reaching 19 μM Zn in solution (<1% of the total initial zinc concentration). Comparatively, c-NPs progressively release zinc during the first 48 h, to finally reach a maximum of 197 μM (10% of total Zn), which is 10 times the maximum value measured for nc-NPs. Over the long term, dissolved Zn concentrations decrease in both systems, corresponding to the neoformation of carbonate phases observed by TEM imaging. The kinetic modeling allows highlighting two different ranges of time, corresponding to (i) first 10h with a fast precipitation (kp(')=-182.10(-4)) related to a highly oversaturated solution with respect to carbonate zinc mineral and (ii) a second slower precipitation step (kp(')=-8.10(-4)), related to the embedding of NPs in the precipitated carbonate matrix. The steady state is reached after 3 months of interaction.