Biotransformation is a critical factor that may modify the toxicity, behavior, and fate of engineered nanoparticles in the environment. CeO2 nanoparticles (NPs) are generally recognized as stable under environmental and biological conditions. The present study aims to investigate the biotransformation of CeO2 NPs in plant systems. Transmission electron microscopy (TEM) images show needlelike clusters on the epidermis and in the intercellular spaces of cucumber roots after a treatment with 2000 mg/L CeO2 NPs for 21 days. By using a soft X-ray scanning transmission microscopy (STXM) technique, the needlelike clusters were verified to be CePO4. Near edge X-ray absorption fine structure (XANES) spectra show that Ce presented in the roots as CeO2 and CePO4 while in the shoots as CeO2 and cerium carboxylates. Simulated studies indicate that reducing substances (e.g., ascorbic acids) played a key role in the transformation process and organic acids (e.g., citric acids) can promote particle dissolution. We speculate that CeO2 NPs were first absorbed on the root surfaces and partially dissolved with the assistance of the organic acids and reducing substances excreted by the roots. The released Ce(III) ions were precipitated on the root surfaces and in intercellular spaces with phosphate, or form complexes with carboxyl compounds during translocation to the shoots. To the best of our knowledge, this is the first report confirming the biotransformation and in-depth exploring the translocation process of CeO2 NPs in plants.

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Clarifying hydrogen evolution and identifying the active hydrogen species are crucial to the understanding of the electrocatalytic hydrodechlorination (EHDC) mechanism. Here, monodisperse palladium nanoparticles (Pd NPs) are used as a model catalyst to demonstrate the potential-dependent evolutions of three hydrogen species, including adsorbed atomic hydrogen (H*), absorbed atomic hydrogen (H*), and molecular hydrogen (H) on Pd NPs, and then their effect on EHDC of 2,4-dichlorophenol (2,4-DCP). Our results show that H*, H*, and H all emerge at -0.65 V (vs Ag/AgCl) and have increased amounts at more negative potentials, except for H* that exhibits a reversed trend with the potential varying from -0.85 to -0.95 V. Overall, the concentrations of these three species evolve in an order of H* < H* < H in the potential range of -0.65 to -0.85 V, H* < H* < H in -0.85 to -1.00 V, and H* < H < H* in -1.00 to -1.10 V. By correlating the evolution of each hydrogen species with 2,4-DCP EHDC kinetics and efficiency, we find that H* is the active species, H* is inert, while H bubbles are detrimental to the EHDC reaction. Accordingly, for an efficient EHDC reaction, a moderate potential is desired to yield sufficient H* and limit H negative effect. Our work presents a systematic investigation on the reaction mechanism of EHDC on Pd catalysts, which should advance the application of EHDC technology in practical environmental remediation.

Ceria nanoparticles (nano-CeO(2)), due to their widespread applications, have attracted a lot of concern about their toxic effects on both human health and the environment. The present work aimed to evaluate the in vivo effects of nano-CeO(2) (8.5 nm) on Caenorhabditis elegans (C. elegans) at environmental relevant concentrations (molar concentrations ranging from 1 nM to 100 nM). The results indicate that nano-CeO(2) could induce ROS accumulation and oxidative damage in C. elegans, and finally lead to a decreased lifespan. The most surprising thing is that the mean lifespan of nematodes was significantly decreased by 12% even at the exposure level of 1 nM (p < 0.01). In vitro tests suggest that the ability of nano-CeO(2) to catalyze ROS generation was involved in the mechanism for its toxicity to C. elegans. To our best knowledge, this is the first case in which nanoparticles exhibit adverse effects on organisms at such low concentrations (1nM-100 nM). So, our findings indicate the importance of nanotoxicological investigations at environmentally relevant concentrations and will attract more attentions on the risks of NPs exposure.