In Situ Synchrotron X-ray Fluorescence Mapping and Speciation of CeO2and ZnO Nanoparticles in Soil Cultivated Soybean (Glycine max)

Abstract

With the increased use of engineered nanomaterials such as ZnO and CeO₂ nanoparticles (NPs), these materials will inevitably be released into the environment, with unknown consequences. In addition, the potential storage of these NPs or their biotransformed products in edible/reproductive organs of crop plants can cause them to enter into the food chain and the next plant generation. Few reports thus far have addressed the entire life cycle of plants grown in NP-contaminated soil. Soybean ( Glycine max ) seeds were germinated and grown to full maturity in organic farm soil amended with either ZnO NPs at 500 mg/kg or CeO₂ NPs at 1000 mg/kg. At harvest, synchrotron μ-XRF and μ-XANES analyses were performed on soybean tissues, including pods, to determine the forms of Ce and Zn in NP-treated plants. The X-ray absorption spectroscopy studies showed no presence of ZnO NPs within tissues. However, μ-XANES data showed O-bound Zn, in a form resembling Zn-citrate, which could be an important Zn complex in the soybean grains. On the other hand, the synchrotron μ-XANES results showed that Ce remained mostly as CeO₂ NPs within the plant. The data also showed that a small percentage of Ce(IV), the oxidation state of Ce in CeO₂ NPs, was biotransformed to Ce(III). To our knowledge, this is the first report on the presence of CeO₂ and Zn compounds in the reproductive/edible portion of the soybean plant grown in farm soil with CeO₂ and ZnO NPs.