## 1 Introduction  
The use of nanoparticles has been increasing over the last few years. Nanoscale material application includes a variety of different areas (e.g., electronic, biomedical, pharmaceutical, cosmetic, energy, environmental, etc.). Its increasing use is posing a threat to the environment and human beings that can be exposed by inhalation, absorption through the dermal system and ingestion through the gastrointestinal system [1]. It has been proved that their presence in the ecosystem may have negative effects on organisms' reproduction, mobility and survival [2], especially in mammals. Additionally, toxicological effects might be caused by their biomimetics properties and high ability of distribution and bioaccumulation in soil, water and foods [3]. Cerium is the rare-earth element (REE) considered by the authors of this article, belongs to the group of and is included in the wider group of REEs [4]. The toxicity of the REE, which are not essential to plants, is still under investigation. However, commercial REE fertilizers are used for their proven positive effect on the yield of several crop species [5]. Nonetheless, it is noteworthy that annual long-term applications of insecticides and herbicides and the excessive fertilization were identified as a significant source of potentially toxic trace elements such as heavy metals and REEs that can modify the natural geochemistry of agricultural soils [6]. Severalauthors have proven that REEs can be absorbed and accumulated by plants, and some have linked such a phenomenon with the similarity of the atomic radii that they share with Ca [7]. Some authors [8] have reported the biochemical and physiological aspects of REEs in plants in a review that also described the history of the application of REEs in agriculture that started in China in the year 1972 after decades of research. In water, when such elements are not in their ionic form but occur as nanoparticles, the scenario changes radically, and it gets more complicated to understand and predict the interactions and fate of the elements in the environment. In experiments using CeO\({}_{2}\) nanoparticles (CeO\({}_{2}\)NPs) [9], it was noticed that 250 mg L\({}^{-1}\) was sufficient to show toxicity in beans (_Phaseolus vulgaris_ L. var. khoman, pinto beans). The authors noticed that the treatment with CeO\({}_{2}\)NPs induced a reduction in Mo accumulation and a parallel overaccumulation of K, regardless of the tissue considered, as well as a modification of the phytohormones levels. Negative results were also observed by some authors [10], who noticed that the growth of shoots and chlorophyll synthesis were inhibited by CeO\({}_{2}\)NPs in glycine max, and by others [11], who observed chromosomal aberrations in _Allium sativum_ L. root cells as a consequence of high Ce concentration exposure that could be associated with the inhibition of root growth. Additionally, it has been reported that CeO\({}_{2}\)NPs were taken up without any modification into the plant tissues of soybean seedlings and, also, in roots of corn seedlings [12]. Other authors observed such a phenomenon in plants of _Coriandrum sativum_ L. [12], _Cucumis sativus_ L. [13] and _Solanum lycopersicon_ L. [14]. Going up the food chain, animals have been proven to undergo accumulation. Investigating the effects of the combined exposure to diesel exhaust particles (DEPs) and CeO\({}_{2}\) on the pulmonary system in a rat model (specific pathogen-free male Sprague-Dawley rats exposed via a single intratracheal instillation), it was observed that CeO\({}_{2}\) induces a sustained inflammatory response [15]. CeO\({}_{2}\) activated alveolar macrophage and lymphocyte secretion of the proinflammatory cytokine IL-12. Other studies showed effects on the nematode _Caenorhabditis elegans_[16], the bacteria _Escherichia coli_[17] and several others. The fate of nanoparticles in natural soil is not yet entirely clear. It is certain that many factors influence their fate and behavior.