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

In this study, the structures and acidity constants (p Ka'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 Ka 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.