| The objective of this project was to define the mechanisms, equilibria, and extent of sorption of aqueous uranium onto hydroxyapatite (Ca5(PO 4)3(OH)) for ranges of pH, ionic strength, dissolved uranium concentration, atmospheric carbon dioxide concentration, dissolved bicarbonate concentrations, and solid:liquid ratio/solid surface area. This work was motivated by the need to immobilize uranium in water and soil to reduce its mobility in water and soil to prevent contamination of water supplies and biological systems. Applying hydroxyapatite to in-situ treatment of uranium-bearing ground water and water filtration could be an effective, low cost technology. Hydroxyapatite quickly, effectively, and reversibly sorbed uranium at a high capacity by inner-sphere complexation over a wide range of conditions.; The results indicate that at aqueous uranium concentrations below 10--20 mug/L: (1) equilibrium sorption of uranium to hydroxyapatite occurs in hours, regardless of pH; (2) in ambient and CO2-free atmospheres, over 98% of initial uranium is sorbed to hydroxyapatite, (3) in waters in equilibrium with higher air CO2 concentrations, sorption removed over 97% of aqueous uranium, except above pH 9, where aqueous uranium concentrations were reduced by less than 40%, and (4) at near-neutral pH, bicarbonate alkalinities in excess of 300 mg/L slightly retarded sorption of uranium to hydroxyapatite, relative to lower alkalinities.; Uranium sorption and precipitation are reversible. The reversibility of these reactions requires that in situ treatment be carefully monitored to avoid breakthrough and desorption of uranium into ground water. This reversibility implies that sorption and precipitation of dissolved uranium to hydroxyapatite are equilibrium processes.; There is a small but measurable effect of ionic strength on steady-state dissolved uranium concentrations in the presence of hydroxyapatite.; This research verified that one m2 of hydroxyapatite can sorb over 7.53 x 10-6 moles or 1.8 mg of uranium in agreement with calculations based on phosphate and calcium oxide site densities on the unit cell. This work is significant because small masses of hydroxyapatite can sorb appreciable masses of uranium quickly over a wide range of chemistries. Preliminary work with ground water containing 260 mug/L of uranium and cow bone char indicates that its sorptive capacity is appreciably less than pure hydroxyapatite. (Abstract shortened by UMI.)... |
