Dissolution kinetics of synthetic and natural meta‐autunite minerals, X3−n(n)+ [(UO2)(PO4)]2 · xH2O, under acidic conditions

Abstract

Mass transport within the uranium geochemical cycle is impacted by the availability of phosphorous. In oxidizing environments, in which the uranyl (UO22+) ionic species is typically mobile, formation of sparingly soluble uranyl phosphate minerals exerts a strong influence on uranium transport. Autunite group minerals, X3 − n(n)+ [(UO2)(PO4)]2 · xH2O, have been identified as the long‐term uranium‐controlling phases in many systems of geochemical interest. Anthropogenic operations related to uranium mining operations have created acidic environments exposing uranyl phosphate minerals to low‐pH groundwaters. Investigations regarding the dissolution behavior of autunite group minerals under acidic conditions have not been reported; consequently, knowledge of the longevity of uranium‐controlling solids is incomplete. The purpose of this investigation was threefold: (1) to quantify the dissolution kinetics of natural calcium meta‐autunite, Ca[(UO2)2(PO4)2] · 3H2O, and synthetic sodium meta‐autunite, Na2[(UO2)2(PO4)2] · 3H2O, under acidic conditions; (2) to measure the effect of temperature and pH on meta‐autunite mineral dissolution; and (3) to investigate the formation of secondary uranyl phosphate phases as long‐term controls on uranium migration. Single‐pass flow‐through (SPFT) dissolution tests were conducted over the pH range of 2 to 5 and from 5° to 70°C. Results presented here illustrate meta‐autunite dissolution kinetics are strongly dependent on pH but are relatively insensitive to temperature variations. In addition, the formation of secondary uranyl‐phosphate phases such as uranyl phosphate, (UO2)3(PO4)2 · x H2O, may serve as a secondary phase limiting the migration of uranium in the environment.