Batch equilibrations were performed to investigate the ability of hydroxyapatite (Ca{sub}5(PO{sub}4){sub}3OH) to chemically immobilize U in two contaminated sediment samples having different organic carbon contents (123 and 49 g kg{sup}-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{sup}-1, respectively. Apatite addition of 50 gkg{sup}-1 reduced the solubility of U to values less than would be expected if autunite (Ca(UO{sub}2){sub}2(PO{sub}4){sub}2·10H{sub}2O) was the controlling solid phase. A comparison of the two sediment types suggests that aqueous phase U may becontrolled 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, includingwater-soluble, exchangeable, and acid-soluble (pH≈2.55) fractions, to the Mn-occluded fraction (pH≈11.26). This suggests that apatite amendment redirects solid-phase speciation with secondary U phosphates being solubilized due to the lower pH of theMn-occluded 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 insecondary Al/Fe phosphate phases.
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