title: Acidity constants and redox potentials of uranyl ions in hydrothermal solutions.
abstract: We report a first principles molecular dynamics (FPMD) study of the structures, acidity constants (pKa) and redox potentials (E0) of uranyl (UO22+) from ambient conditions to 573 K. It is found that UO22+keeps five coordination up to 573 K whereas UO2+transforms from 5 to 4-coordinate as temperature increases to 573 K. The FPMD-based vertical energy gap method is used to derive pKas and E0s. The method is validated by comparing with available experimental data (for E0under the ambient conditions and for pKas from ambient conditions to 367 K), with an uncertainty of 1-2 pKaunits and 0.2 V for pKaand E0. The encouraging results demonstrate that the method may be used to predict the pH-Eh diagrams of f-block elements under the conditions of hydrothermal solutions. The results show that the acidity constants of uranyl decrease with temperature and are lower than 3.0 when the temperature is above 473 K, indicating that hydrolytic forms are dominant for U(vi) in the near neutral pH range. The reduction potential increases with temperature, indicating that the reduced state is more significant at higher temperatures.
authors.affiliation: State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, P. R. China. xiandongliu@nju.edu.cn and Department of Chemistry, University of Cambridge, Cambridge CB2 1EW, UK.
authors.forename: Xiandong
authors.initials: X
authors.lastname: Liu
authors.affiliation: College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, P. R. China and Department of Chemistry, University of Aberdeen, Aberdeen AB24, 3UE, UK. Jcheng@abdn.ac.uk.
authors.forename: Jun
authors.initials: J
authors.lastname: Cheng
authors.affiliation: State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, P. R. China. xiandongliu@nju.edu.cn.
authors.forename: Mengjia
authors.initials: M
authors.lastname: He
authors.affiliation: State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, P. R. China. xiandongliu@nju.edu.cn.
authors.forename: Xiancai
authors.initials: X
authors.lastname: Lu
authors.affiliation: State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, P. R. China. xiandongliu@nju.edu.cn.
authors.forename: Rucheng
authors.initials: R
authors.lastname: Wang