Sol-Gel-Derived Hydroxyapatite and its Application to Sorption of Heavy Metals
Hydroxyapatite (HA) microspheres of diameter <70 μm have been synthesized by solgel processing. The starting sols were prepared by ultrasonic mixing of concentrated solutions of calcium acetate (1.7M) with 85% H3PO4, followed by emulsification in dehydrated 2-ethyl-1- hexanol. Drops of emulsion were solidified by extraction of water with this solvent. The final thermal treatment was a 2 h soaking in air at 900°C. Properties such as hydraulic resistance and sedimentation rate, which are important for application in ion-exchangers, were superior for our prepared microspheres in comparison with irregularly shaped commercial HA. Adsorption of the following metals was investigated: U, Zn, Fe, Cu, Ni, Co, Cd, Pb, Mn, Al, Cr, As, Sb, Bi and Mo. Retention was ≈100% for of all the metals studied when pH > 3–4. On average, ≈1/20 moles of metal reacted with 1 mole of HA. Adsorbed metals could be desorbed with efficiencies of 60–90%. In addition, nuclear-waste-saturated beds of HA could be transformed to insoluble ceramics by thermal treatment. The cost of producing HA microspheres was estimated to be comparable to current prices of irregularly shaped commercial hydroxyapatite powders. For radioactive contaminants such as U, for which resorption need not be considered, porous monoliths were produced by use of industrial reagents. The retention capacity was determined to be 30 mg of U per 1 mg of monolith.
A. Deptuła et al., "Sol-Gel-Derived Hydroxyapatite and its Application to Sorption of Heavy Metals", Advances in Science and Technology, Vol. 45, pp. 2198-2203, 2006