Iron-based nanomaterials have high technological impacts on various pro-environmental applications, including wastewater treatment using the co-precipitation method. The purpose of this research was to identify the changes of iron nanomaterial's structure caused by the presence of selenium, a typical water contaminant, which might affect the removal when the iron co-precipitation method is used. Therefore, we have investigated the maturation of co-precipitated nanosized ferric oxyhydroxides under alkaline conditions and their thermal transformation into hematite in the presence of selenite and selenate with high concentrations. Since the association of selenium with precipitates surfaces has been proven to be weak, the mineralogy of the system was affected insignificantly, and the goethite was identified as an only ferric phase in all treatments. However, the morphology and the crystallinity of ferric oxyhydroxides was slightly altered. Selenium affected the structural order of precipitates, especially at the initial phase of co-precipitation. Still, the crystal integrity and homogeneity increased with time almost constantly, regardless of the treatment. The thermal transformation into well crystalized hematite was more pronounced in the presence of selenite, while selenate-treated and selenium-free samples indicated the presence of highly disordered fraction. This highlights that the aftermath of selenium release does not result in destabilization of ferric phases; however, since weak interactions of selenium are dominant at alkaline conditions with goethite's surfaces, it still poses a high risk for the environment. The findings of this study should be applicable in waters affected by mining and metallurgical operations.

Department of Environmental Engineering Faculty of Mining and Geology VŠB Technical University of Ostrava 17 Listopadu 15 2172 70800 Ostrava Poruba Czech Republic

Department of Nuclear Reactors Faculty of Nuclear Sciences and Physical Engineering Czech Technical University Prague 5 Holešovičkách 2 18000 Prague Czech Republic

Faculty of Mathematics and Physics Charles University 5 Holešovičkách 2 18000 Prague Czech Republic

Institute of Laboratory Research on Geomaterials Faculty of Natural Sciences Comenius University in Bratislava Mlynská Dolina Ilkovičova 6 84215 Bratislava Slovakia

Institute of Nuclear and Physical Engineering Slovak Technical University in Bratislava Ilkovičova 3 81219 Bratislava Slovakia

Polymer Institute Slovak Academy of Sciences Dúbravská Cesta 9 84541 Bratislava Slovakia

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