Effect of Reaction Pathway on the Extent and Mechanism of Uranium(VI) Immobilization with Calcium and Phosphate

Phosphate addition to subsurface environments contaminated with uranium can be used as an in situ remediation approach. Batch experiments were conducted to evaluate the dependence of the extent and mechanism of uranium uptake on the pathway for reaction with calcium phosphates. At pH 4.0 and 6.0 uranium uptake from solution occurred via autunite (Ca(UO₂)₂(PO₄)₂) precipitation irrespective of the starting forms of calcium and phosphate. At pH 7.5, a condition at which calcium phosphate solids could form, the uptake mechanism depended on the nature of the calcium and phosphate as determined by X-ray absorption spectroscopy and laser-induced fluorescence spectroscopy. When dissolved uranium, calcium, and phosphate were added simultaneously, uranium was structurally incorporated into a newly formed amorphous calcium phosphate solid. Adsorption was the dominant removal mechanism for uranium contacted with preformed amorphous calcium phosphate solids. When U(VI) was added to a suspension containing amorphous calcium phosphate solids as well as dissolved calcium and phosphate, then removal occurred through precipitation (57 ± 4%) of autunite and adsorption (43 ± 4%) onto calcium phosphate. Dissolved uranium, calcium, and phosphate concentrations with saturation index calculations helped identify removal mechanisms and determine thermodynamically favorable solid phases.