The surface structure of a-uranophane and its interaction with Eu(III) – An integrated computational and fluorescence spectroscopy study Jadwiga Kuta a , Zheming Wang b , Katy Wisuri a , Matthew C.F. Wander a , Nathalie A. Wall a , Aurora E. Clark a,⇑ a Department of Chemistry, Washington State University, Pullman, WA 99164, United States b Fundamental and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, WA 99352, United States Received 9 June 2012; accepted in revised form 30 October 2012; Available online 15 November 2012 Abstract Uranophane is a rare U(VI) secondary silicate mineral that is relevant to the long-term performance of high level nuclear waste repositories. It can be formed under oxidizing conditions, potentially acting as an additional barrier to radionuclide migration through the accessible environment via mineral sorption reactions. To help understand the mechanisms involved in such sorption, a combination of theoretical calculations (classical molecular dynamics and ab initio density functional the- ory), and experimental work (sorption and laser induced fluorescence spectroscopy studies), have been employed to investi- gate the uranophane|water interface as well as the interfacial reactivity of the U(VI) silicate toward acidic conditions and radionuclide ion sorption. The combination of theoretical and experimental sorption studies help identify the molecular struc- ture of the surface-sorbed species. Interfacial water is found to orient primarily with the hydrogen-atoms directed towards the negatively charged surface, with sorption sites belonging to three different groups: (1) those involving uranyl oxygen, (2) involving uranyl and silica hydroxyl oxygen atoms, and (3) involving hydroxyl hydrogen. Under basic conditions, deproto- nation of the Si–OH groups is predicted to be responsible for uranophane dissolution, while protonation of bridging oxygens is likely responsible for acidic dissolution. Stable inner-sphere sorbed Eu(III) species are observed both experimentally and computationally. Moreover, Eu(III) is found to react with both protonated and deprotonated surface sites, indicating that radionuclides may sorb to the surface of uranophane under a broad range of pH conditions that alter the relative concentra- tions of protonated and deprotonated surface sites. This is in stark contrast to prior observations regarding other silicate min- erals such as quartz, where sorption under basic conditions is enhanced. Ó 2012 Elsevier Ltd. All rights reserved. 1. INTRODUCTION Uranium silicates may play a major role in the long-term dosage of nuclear waste repositories and the continued study of the behavior of these minerals is warranted. The understanding of these mineral phases is crucial for an accurate quantification of a nuclear waste repository source term – defined as the radioactivity released to the accessible environment, because they will impact the mobilities of uranium, neptunium, and other radionuclides present at the site. a-Uranophane, Ca(UO 2 ) 2 (SiO 3 OH) 2 5H 2 O, also known as uranotile or hydrated calcium uranyl (VI) silicate, is a rare secondary mineral formed in nature by the oxida- tion of uranium-bearing primary minerals, such as urani- nite (Wronkiewicz et al., 1992, 1996). Also, uranophane and other uranyl silicates may form on a geologic timescale in the close vicinity of high level waste repositories, as dissolved uranium species in spent nuclear fuel interact with environmentally ubiquitous silicate mineral phases 0016-7037/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.gca.2012.10.056 ⇑ Corresponding author. E-mail address: auclark@wsu.edu (A.E. Clark). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 103 (2013) 184–196