Environmental Science Nano PAPER Cite this: DOI: 10.1039/d4en00014e Received 5th January 2024, Accepted 21st June 2024 DOI: 10.1039/d4en00014e rsc.li/es-nano Overlooked impact of surface hydroxylation on the solubility of less-soluble compounds: a case study of CeO 2 † Tatiana V. Plakhova, a Anna Yu. Romanchuk, a Anastasia D. Konyukhova, a Irina F. Seregina, a Alexander E. Baranchikov, b Roman D. Svetogorov, ac Maxwell W. Terban, d Vladimir K. Ivanov be and Stepan N. Kalmykov * a Unexpectedly, the solubility of CeO 2 nanoparticles (NPs) at 25 °C does not depend on particle size, but is significantly affected by the sample's thermal pre-treatment. The classical interpretation of NPs' solubility proposed by the Gibbs–Thompson or Kelvin equations fails to describe the experimental data on CeO 2 solubility obtained in this study. Thermal treatment did not change the samples' morphological characteristics, while slightly affecting NP hydroxylation and local crystallinity. The differences in the solubility of dried and non-treated CeO 2 particles were most noticeable at pH < 4, and dissolved cerium concentration was much lower in the case of the dried sample. After prolonged storage (up to 4.5 years) of CeO 2 NPs in aqueous media, the solubility of dried samples gradually increased, while for non-treated samples it remained unchanged. Based on the example of CeO 2 , the dissolution laws of other less soluble nanomaterials should be reconsidered. Introduction Rapid advances in nanoscience and nanotechnology, in the 21st century, have created a growing demand for nanomaterials possessing unique properties for various fields of application, from catalysts and accumulators to pharmaceuticals. A decrease in particle size to a few nanometres leads to a significant increase in surface-to- volume ratio and the emergence of unusual effects responsible for size-specific properties, e.g., optical, magnetic and catalytic. 1–4 Mass production of nanomaterials inevitably results in their release into the environment. The impact of engineered nanoparticles (NPs) on biosystems remains debatable and is probably strongly related to their physicochemical properties. 5–7 Living organisms can easily absorb NPs through ingestion, respiration, or both, increasing NPs' exposure to the ecosystem. 8 In addition, various analytical techniques have revealed that a very large number of NPs could exist in the environment as a result of natural NP production. 9 NPs can be dissolved in biological media, leading to the release of toxic metal ions. 10 Thus, the dissolution of natural and engineered NPs can have significant consequences for ecotoxicity. Environ. Sci.: Nano This journal is © The Royal Society of Chemistry 2024 a Lomonosov Moscow State University, Department of Chemistry, Leninskie Gory 1/ 3, 119991 Moscow, Russia. E-mail: stepan@radio.chem.msu.ru b Kurnakov Institute of General and Inorganic Chemistry of the Russian Academy of Sciences, Leninskii prosp. 31, 117901 Moscow, Russia c National Research Centre “Kurchatov Institute”, Akademika Kurchatova pl. 1, 123182 Moscow, Russia d Max Planck Institute for Solid State Research, Heisenbergstraße 1, 70569 Stuttgart, Germany e National Research University Higher School of Economics, Myasnitskaya st. 20, 101000 Moscow, Russia † Electronic supplementary information (ESI) available: HRTEM images and ED data; XRD data; Ce L 3 edge HERFD-XAS spectra; PDF G(r) experimental data and fit results; refined parameter values; dissolution curves; dissolution rate constants; comparison of different centrifugation. See DOI: https://doi.org/ 10.1039/d4en00014e Environmental significance In view of the high demand for CeO 2 -based materials, understanding their dissolution behaviour is imperative. Ceria nanoparticles, whether freshly made and fully hydroxylated or dried, may exhibit distinct environmental behaviours, as surface hydroxylation is crucial in determining their catalytic and biological properties. Unexpectedly, hydroxylated CeO 2 nanoparticles exhibit greater long-term stability than dried ones which significantly advances our understanding of nanoscale materials' dynamics in environmental science. Analogies between cerium dioxide and actinide dioxides are vital for environmental considerations, emphasising the necessity of comprehending the influence of water on the long-term behaviour of these materials and the potential spread of radionuclides through groundwater systems. Published on 05 July 2024. Downloaded by KIT Library on 7/11/2024 8:52:29 AM. View Article Online View Journal