Unveiling the interaction of uranyl and arsenate: Insights into the formation mechanisms of uranyl arsenate minerals

Abstract

Uranyl arsenate minerals, which exhibit low solubility, serve as major sinks for U and As, playing a crucial role in controlling the mobility of U and As in the environment. However, the specific mechanisms underlying the formation of uranyl arsenate minerals have remained largely elusive. Herein, the formation pathway of the non-charged UO₂(H₂AsO₄)₂·nH₂O⁰ complex was investigated to elucidate the early formation of the UO₂(H₂AsO₄)₂·nH₂O mineral (where n represents the stoichiometric number of H₂O), a representative uranyl arsenate mineral. Based on the combination experiments of U(VI) and As(V), our findings underscore the significant dependence of UO₂(H₂AsO₄)₂·nH₂O⁰ formation on solution pH (4.0–10.0). Density functional theory (DFT) calculations reveal a two-step reaction involving two distinct pathways (Pathway 1 and Pathway 2) for the formation of UO₂(H₂AsO₄)₂·nH₂O, and the intermediate was confirmed by in situ Raman and fluorescence spectroscopy. Specifically, the hydroxyl‑connected uranyl (UO₂OH⁺) reacts with the protonated arsenate (H₂AsO₄^-) species to form the intermediate UO₂HAsO₄·H₂O (Pathway 1) or UO₂OHH₂AsO₄ (Pathway 2) with a U/As ratio of 1:1. Meanwhile, all the transition states also were obtained and the energy barrier suggested that the UO₂(H₂AsO₄)₂·2H₂O⁰ formed by Pathway 1 is thermodynamically favored over Pathway 2, and may serve as the primary fundamental structural unit or precursor for the early formation of the UO₂(H₂AsO₄)₂·nH₂O mineral phase. Altogether, this study contributes to advancing the understanding of the formation of uranyl arsenate minerals at the molecular scale and provides a theoretical basis for predicting and regulating uranium and arsenic mobilization in their coexisting environment.