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

TLDR

Engineered nanomaterials such as ZnO and CeO₂ nanoparticles are increasingly used, raising concerns about their environmental release, potential accumulation in crop edible tissues, and the lack of studies on the full life cycle of plants grown in NP‑contaminated soil. Soybean seeds were cultivated in organic farm soil amended with 500 mg kg⁻¹ ZnO NPs or 1 000 mg kg⁻¹ CeO₂ NPs, and at harvest synchrotron μ‑XRF and μ‑XANES analyses were performed on plant tissues, including pods, to identify the chemical forms of Ce and Zn. The analyses revealed no ZnO NPs in the tissues but detected O‑bound Zn resembling Zn‑citrate, while Ce remained largely as CeO₂ NPs with a minor fraction of Ce(IV) reduced to Ce(III), marking the first report of these compounds in the edible parts of soybean grown in NP‑amended soil.

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.