REMOVAL OF CONTAMINANTS FROM GROUNDWATER THROUGH ION EXCHANGE OR SURFACE ADSORPTION USING HYDROXYAPATITE

High surface area (>200 m²/g) crystals composed of nanosize (30-50nm) hydroxyapatite (HA; Ca₅(PO₄)₃OH) are being investigated as an alternative in cleaning up contaminated groundwater and soil by removing toxic wastes and radionuclides such as U, Pu, Pb and ⁹⁰Sr. Static batch tests were carried out at 2, 4, 8, 24 and 120 hour time periods by reacting HA with individual solutions of Sr, Pb and Ce to evaluate the ability of HA in removing these potential contaminants. Cerium is being used as a surrogate for Pu due to their similar electron shell structures. Chemical analyses on leachate solutions and post-reaction digested solids were conducted by Inductively Coupled Plasma-Mass Spectroscopy to determine the extent of ion removal. Removal of these ions is expected to occur via ionic substitution or surface adsorption onto the HA solids. Adsorption results by HA appear to be effective based on the tests conducted. The concentration of Sr²⁺ was reduced from 1.0x10^-2 to 7.57x10^-6 molar within 24 hours. The Pb²⁺ tests showed ~99.99% ion removal, from 1.9x10^-5 to 1.2x10^-9 molar in eight hours and achieved its maximum removal potential within 24 hours. The Ce⁴⁺ concentration dropped from 1.0x10^-2 to 2.88x10^-6 molar after reacting with HA for two hours. Ion removal selectivity is based upon the ionic radii and valence charge. The Sr²⁺ and Ca²⁺ in the HA structure both have a similar valence charge which leads to preferential substitution of Ca²⁺ by Sr²⁺. Even though Ce⁴⁺ and Ca²⁺ have different valence charge, a small difference in ionic radii between both ions may allow Ca²⁺ (0.99 Å) to be readily substituted for Ce⁴⁺ (1.01 Å) in HA crystal structure. Substitution in HA crystals could also occur through charge coupled substitution. Charge differences between Ca²⁺ and the ion that tries to occupy Ca²⁺ position in the crystal structure will require charge compensation to maintain charge balance (e.g., Ce³⁺ + Si⁴⁺ Û Ca²⁺ + P⁵⁺). The potential for this mechanism was tested by mixing various ratios of Ce³⁺, Ce⁴⁺, Cs⁺ , Na and Si in solution to provide charge and size compensating ions. Results indicate improved retention of both Ce³⁺ and Ce⁴⁺. The adsorption potential of HA displayed promising results and should be considered as a viable option in cleaning contaminants from groundwater.