Date Published: June 7, 2019
Publisher: Public Library of Science
Author(s): Isabella Römer, Sophie Marie Briffa, Yadira Arroyo Rojas Dasilva, Dimitri Hapiuk, Vanessa Trouillet, Richard E. Palmer, Eugenia Valsami-Jones, Amitava Mukherjee.
The potential hazard posed by nanomaterials can be significantly influenced by transformations which these materials undergo during their lifecycle, from manufacturing through to disposal. The transformations may depend on the nanomaterials’ own physicochemical properties as well as the environment they are exposed to. This study focuses on the mechanisms of transformation of cerium oxide nanoparticles (CeO2 NPs) in laboratory experiments which simulate potential scenarios in which the NPs are exposed to phosphate-bearing media. We have experimented with the transformation of four different kinds of CeO2 NPs, in order to investigate the effects of nanoparticle size, capping agent (three were uncapped and one was PVP capped) and oxidation state (two consisted mostly of Ce4+ and two were a mix of Ce3+/Ce4+). They were exposed to a reaction solution containing KH2PO4, citric acid and ascorbic acid at pH values of 2.3, 5.5 and 12.3, and concentrations of 1mM and 5mM. The transformations were followed by UV-vis, zeta potential and XRD measurements, which were taken after 7 and 21 days, and by transmission electron microscopy after 21 days. X-ray photoelectron spectroscopy was measured at 5mM concentration after 21 days for some samples. Results show that for pH 5 and 5mM phosphate concentration, CePO4 NPs were formed. Nanoparticles that were mostly Ce4+ did not dissolve at 1mM reagent concentration, and did not produce CePO4 NPs. When PVP was present as a capping agent it proved to be an extra reducing agent, and CePO4 was found under all conditions used. This is the first paper where the transformation of CeO2 NPs in the presence of phosphate has been studied for particles with different size, shapes and capping agents, in a range of different conditions and using many different characterisation methods.
Nanoparticles (NPs) can be defined as materials with at least one dimension between 1 and 100 nm, and that possess unique physicochemical properties that differ from the bulk [1–3]. The global market for nanomaterials (NMs) already exceeds 10 million tons with products underpinned by nanotechnology having a global value of €2 trillion . With employment in the NMs sector at about 400,000 in Europe alone, the industry contributes significantly to the economy and its products are improving the quality of human life . Due to the expanding use of NMs in products their discharge to the environment is rapidly increasing and having knowledge of how they behave and change under different conditions is very important [5,6]. Most published work to date has focused on pristine NPs, which can be structurally and chemically distinct from their aged counterparts and may behave differently and have different toxicity .
We have studied the ageing of different CeO2 NPs under different conditions mimicking exposure to environments where phosphate and a range of pH may be occurring. We found that concentration of the reagents, particle size, oxidation state, capping agent, the presence of organic acids and reducing agent, and pH all have an effect on the formation of CePO4 particles. All CeO2 NPs transformed to CePO4 at 5mM reagent concentration and pH 5. At 1mM concentration and pH 5 the reaction was only complete for PROM-Ce and Ce10. It has been observed that for some plant species (e.g., corn and wheat), deficiency of P can increase the potential phytotoxicity of CeO2 NPs and enhance the accumulation of Ce (mainly in the form of Ce3+) in plants [40,62]. The particle’s oxidation state played a key role in the dissolution of the CeO2 NPs and the formation of CePO4, thus Ce NM-211 and Ce NM-212, which contained mainly Ce4+ and were therefore already fully oxidised, did not dissolve at 1mM pH 5, whereas PROM-Ce and Ce10 (which were a mix of Ce3+ and Ce4+) did. In the case of Ce10 we think that the PVP had an additional reducing effect, thus permitting a more efficient formation of CePO4 particles at every condition used. CeO2 NPs in the environment could follow the type of transformations observed in this work, thus it is very important to assess how NMs transform during ecotoxicological assays. Further studies are needed in environmentally relevant conditions, such as natural and waste waters or soils where natural organic matter is present.