Removal of Attached Zinc Oxide and Titanium Dioxide Nanoparticles from Spinach Leaves by Rinsing in the Absence and Presence of Preexisting Surface Extracellular Polymeric Substances (EPS)

Abstract

Interest in the use of nanoparticles in agriculture has grown in recent years due to their potential abilities across a range of applications that could increase agricultural production, improve the efficiency of nutrient delivery, or improve pest management. However, as with any application of nanomaterials, concern exists about potential risks to human health. Because many applications might result in the attachment of nanoparticles to produce surfaces, it is important to understand the conditions under which rinsing is likely to remove nanoparticles from surfaces and the degree to which they can be removed. This work explored the rinsing removal of two types of nanoparticles, titanium dioxide (TiO2) and zinc oxide (ZnO), from spinach leaf surfaces in the absence and presence of biofilms based on extracellular polymeric substances (EPS). A hypothesis driving the work was that the presence of biofilms might enhance the retention of nanoparticles. The work combined experiments to determine surface energy parameters for fresh and rotten spinach, for use in extended DLVO (xDLVO) calculations, as well as direct rinsing experiments to explore nanoparticle removal from spinach surfaces. Nanoparticles were quantified using backscattered scanning electron microscopy using techniques developed for the work. Results of xDLVO calculations suggest that the presence of biofilms may actually be likely to reduce the retention of nanoparticles by produce surfaces, although this effect was not apparent in rinsing experiments, which exhibited similar removal of high-concentration TiO2 from spinach leaves. Overall, nanoparticles deposited from high-concentration suspensions were found to be removed to a greater degree by rinsing, while those deposited from low-concentration suspensions exhibited no apparent release, even under conditions where release might be favored.

Keywords

• titanium dioxide
• zinc oxide
• agriculture
• spinach
• biofilms