Uranyl Arsenate Complexes in Aqueous Solution: Insights from First-Principles Molecular Dynamics Simulations

Abstract

In this study, the structures and acidity constants (p K_a's) of uranyl arsenate complexes in solutions have been revealed by using the first principle molecular dynamics technique. The results show that uranyl and arsenate form stable complexes with the U/As ratios of 1:1 and 1:2, and the bidentate complexation between U and As is highly favored. Speciation-pH distributions are derived based on free energy and p K_a calculations, which indicate that for the 1:1 species, UO₂(H₂AsO₄)(H₂O)₃⁺ is the major species at pH < 7, while UO₂(HAsO₄)(H₂O)₃⁰ and UO₂(AsO₄)(H₂O)₃^- dominate in acid-to-alkaline and extreme alkaline pH ranges. For the 1:2 species, UO₂(H₂AsO₄)₂(H₂O)⁰ is dominant under acid-to-neutral pH conditions, while UO₂(HAsO₄)(H₂AsO₄)(H₂O)^-, UO₂(HAsO₄)(HAsO₄)(H₂O)^2-, and UO₂(AsO₄)(HAsO₄)(H₂O)^3- become the major forms in the pH range of 7.2-10.7, 10.7-12.1, and >12.1, respectively.