Dissolution kinetics of synthetic and natural meta-autunite minerals, X sub(3-n) super((n)) super(+ ) [(UO sub(2))(PO sub(4))] sub(2) . xH sub(2)O, under acidic conditions

Mass transport within the uranium geochemical cycle is impacted by the availability of phosphorous. In oxidizing environments, in which the uranyl (UO sub(2) super(2+)) ionic species is typically mobile, formation of sparingly soluble uranyl phosphate minerals exerts a strong influence on uranium transport. Autunite group minerals, X sub(3 - n) super((n)) super(+) [(UO sub(2))(PO sub(4))] sub(2) . xH sub(2)O, 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[(UO sub(2)) sub(2)(PO sub(4)) sub(2)] . 3H sub(2)O, and synthetic sodium meta-autunite, Na sub(2)[(UO sub(2)) sub(2 )(PO sub(4)) sub(2)] . 3H sub(2)O, 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 degree to 70 degree 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, (UO sub(2)) sub(3)(PO sub(4 )) sub(2) . x H sub(2)O, may serve as a secondary phase limiting the migration of uranium in the environment.