Towards a Mechanistic Understanding of Autunite Dissolution

Uranyl-phosphate phases are secondary U minerals formed during the oxic weathering of primary UO₂ deposits. The most common naturally-occurring uranyl-phosphates, the autunite group minerals (X_{3 - n} ⁽ⁿ⁾⁺ [(UO₂)(PO₄)]₂ · xH₂O), have been identified as the long-term U-controlling phases in many systems of environmental interest. The autunite-type sheet contains uranyl square bipyramids linked to phosphate tetrahedra, in which each equatorial vertex of the uranyl bipyramid is shared with a different phosphate tetrahedron, and each tetrahedron is in turn linked to four distinct uranyl polyhedra.  Because of the crystallographic distribution of periodic bond chains in the autunite structure, we propose that the dissolution rate of autunite minerals measured from the sheet edge will be significantly greater than that measured on the basal autunite face (001). Previous results have also shown that pH-sensitive differences in interlayer cation release rate (e.g., Na⁺ or Ca²⁺) may derive from different reaction mechanisms (ion exchange versus matrix dissolution). In order to test this working hypothesis, we have conducted dissolution experiments using single crystals of natural autunite mounted in buffered flow-through reactors over a range of circumneutral to alkaline pH conditions and variable temperatures ≤ 70°C. Using techniques previously employed for silicate and carbonate minerals, dissolution rates were measured using high resolution vertical scanning interferometry by maintaining pristine reference surfaces under the conditions of interest. This approach has been used to quantify the difference in autunite mineral dissolution rates as measured from the edge versus the face of the mineral. These results were then compared with results from standard experiments using powder materials.