Immobilization of uranium in contaminated sediments by hydroxyapatite addition

Batch equilibrations were performed to investigate the ability of hydroxyap atite (Ca-5(PO4)(3)OH) to chemically immobilize U in two contaminated sedim ent samples having different organic carbon contents (123 and 49 g kg(-1), respectively). Apatite additions lowered aqueous U to near proposed drinkin g water standards in batch equilibrations of two distinct sediment strata h aving total U concentrations of 1703 and 2100 mg kg(-1), respectively. Apat ite addition of 50 g kg(-1) reduced the solubility of U to values less than would be expected if autunite (Ca(UO2)(2)(PO4)(2). 10H(2)O) was the contro lling solid phase. A comparison of the two sediment types suggests that aqu eous phase U may be controlled by both the DOC content through complexation and the equilibrium pH for a given apatite application rate. Sequential ch emical extractions demonstrated that apatite amendment transfers U from mor e chemically labile fractions, including water-soluble, exchangeable, and a cid-soluble (pH approximate to 2.55) fractions, to the Mn-occluded fraction (pH approximate to 1.26). This suggests that apatite amendment redirects s olid-phase speciation with secondary U phosphates being solubilized due to the lower pH of the Mn-occluded extractant, despite the lack of significant quantities of Mn oxides within these sediments. Energy dispersive X-ray (E DX) analysis conducted in a transmission electron microscope (TEM) confirme d that apatite amendment sequesters some U in secondary Al/Fe phosphate pha ses.