title: Effect of Reaction Pathway on the Extent and Mechanism of Uranium(VI) Immobilization with Calcium and Phosphate.
abstract: 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.
authors.affiliation: Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis , St. Louis, Missouri 63130, United States.
authors.forename: Vrajesh S
authors.initials: VS
authors.lastname: Mehta
authors.affiliation: Department of Earth and Planetary Sciences, Washington University in St. Louis , St. Louis, Missouri 63130, United States.
authors.forename: Fabien
authors.initials: F
authors.lastname: Maillot
authors.affiliation: Department of Fundamental and Computational Sciences Directorate, Pacific Northwest National Laboratory , Richland, Washington 99352, United States.
authors.forename: Zheming
authors.initials: Z
authors.lastname: Wang
authors.affiliation: Department of Earth and Planetary Sciences, Washington University in St. Louis , St. Louis, Missouri 63130, United States.
authors.forename: Jeffrey G
authors.initials: JG
authors.lastname: Catalano
authors.affiliation: Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis , St. Louis, Missouri 63130, United States.
authors.forename: Daniel E
authors.initials: DE
authors.lastname: Giammar
mesh_terms: D000327:Adsorption; D002118:Calcium / Q000737:chemistry*; D002130:Calcium Phosphates / Q000737:chemistry; D052918:Environmental Restoration and Remediation; D060587:Groundwater / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D010710:Phosphates / Q000737:chemistry*; D013050:Spectrometry, Fluorescence; D014501:Uranium / Q000737:chemistry* / Q000302:isolation & purification; D014874:Water Pollutants, Chemical / Q000737:chemistry / Q000302:isolation & purification; D056928:X-Ray Absorption Spectroscopy
chemical_list: D002130:Calcium Phosphates; D010710:Phosphates; D014874:Water Pollutants, Chemical; C519480:amorphous calcium phosphate; D014501:Uranium; C020243:calcium phosphate; D002118:Calcium