SREL Reprint #2334




Immobilization of Uranium in Contaminated Sediments by Hydroxyapatite Addition


Advanced Analytical Center for Environmental Sciences, Savannah River Ecology Laboratory, The University of Georgia, P.O. Drawer E, Aiken, South Carolina 29802


Batch equilibrations were performed to investigate the ability of hydroxyapatite (Ca5(PO4)3OH) to chemically immobilize U in two contaminated sediment samples having different organic carbon contents (123 and 49 g kg-1, respectively). Apatite additions lowered aqueous U to near proposed drinking water standards in batch equilibrations of two distinct sediment strata having total U concentrations of 1703 and 2100 mg kg-1, respectively. Apatite addition of 50 g kg-1 reduced the solubility of U to values less than would be expected if autunite (Ca(UO2)2(PO4)2Ěl0H20) was the controlling solid phase. A comparison of the two sediment types suggests that aqueous phase U may be controlled by both the DOC content through complexation and the equilibrium pH for a given apatite application rate. Sequential chemical extractions demonstrated that apatite amendment transfers U from more chemically labile fractions, including water-soluble, exchangeable, and acid soluble (pH ~ 2.55) fractions, to the Mn-occluded fraction (pH ~ 1.26). This suggests that apatite amendment redirects solid-phase speciation with secondary U phosphates being solubilized due to the lower pH of the Mnoccluded extractant, despite the lack of significant quantities of Mn oxides within these sediments. Energy dispersive X-ray (EDX) analysis conducted in a transmission electron microscope (TEM) confirmed that apatite amendment sequesters some U in secondary Al/Fe phosphate phases.

SREL Reprint #2334

Arey, J.S., J.C. Seaman, and P.M. Bertsch. 1999. Immobilization of uranium in contaminated sediments by hydroxyapatite addition. Environmental Science & Technology 33:337-342.

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