| Abstract
| - Apatite, Ca10(PO4)6(OH,F)2, has been successfully used asa soil amendment at numerous sites to immobilize metals andradionuclides. Such sites commonly contain multiplecontaminants; the impact of apatite on these contaminantsis expected to vary greatly. The objective of this studywas to determine the influence of apatite on nontargetedsediment contaminants. Laboratory batch experimentswere conducted under oxidized (several weekly wet/drycycles) and reduced (water-saturated) conditions with asediment collected from a wetland contaminated withnumerous metals and radionuclides. Apatite additions resultedin the significant (p ≤ 0.05) reduction of porewater Cd,Co, Hg, Pb, and U concentrations. However, apatite additionsalso resulted in the enhanced desorption of As, Se, andTh. Increases in porewater As and Se concentrations werethe result of phosphate competitive exchange and not tothe release of these contaminants directly from the apatite,which contained 29 mg kg-1 As and 0.2 mg kg-1 Se.Apatite additions increased porewater Th and organic Cconcentrations under oxidized (Eh = 497 mV) but not reduced(Eh = 65 mV) conditions. In the oxidized system, theleachate from the apatite treatment had a brown colorationand contained 226 mg L-1 organic C, as compared to141 mg L-1 in the unamended samples. The desorbed organicC likely contained significant quantities of Th. Thisconclusion was supported by (i) the observation thatporewater Th partitioned to hydrophobic resins, (ii)thermodynamic calculations which predicted that essentiallyall porewater Th existed as organic matter complexes,and (iii) there were significant correlations (r = 0.91, n =8, p ≤ 0.01) between porewater organic C and Thconcentrations. Sediment additions of zero-valent ironparticles along with the apatite eliminated the enhanceddesorption of As, Se, and Th observed when only apatite wasadded. This study underscores the importance ofmonitoring the influence of sediment amendments onnontarget contaminants and provides examples of howthe sediment additions of apatite can effectively immobilizesome contaminants while enhancing the mobility ofothers.
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