Enhanced Contaminant Desorption Induced by Phosphate Mineral Additions to Sediment

Apatite, Ca10(PO4)6(OH,F)2, has been successfully used as a soil amendment at numerous sites to immobilize metals and radionuclides. Such sites commonly contain multiple contaminants; the impact of apatite on these contaminants is expected to vary greatly. The objective of this study was to determine the influence of apatite on nontargeted sediment contaminants. Laboratory batch experiments were conducted under oxidized (several weekly wet/dry cycles) and reduced (water-saturated) conditions with a sediment collected from a wetland contaminated with numerous metals and radionuclides. Apatite additions resulted in the significant (p ≤ 0.05) reduction of porewater Cd, Co, Hg, Pb, and U concentrations. However, apatite additions also resulted in the enhanced desorption of As, Se, and Th. Increases in porewater As and Se concentrations were the result of phosphate competitive exchange and not to the 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 C concentrations under oxidized (E h = 497 mV) but not reduced (E h = 65 mV) conditions. In the oxidized system, the leachate from the apatite treatment had a brown coloration and contained 226 mg L-1 organic C, as compared to 141 mg L-1 in the unamended samples. The desorbed organic C likely contained significant quantities of Th. This conclusion was supported by (i) the observation that porewater Th partitioned to hydrophobic resins, (ii) thermodynamic calculations which predicted that essentially all 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 Th concentrations. Sediment additions of zero-valent iron particles along with the apatite eliminated the enhanced desorption of As, Se, and Th observed when only apatite was added. This study underscores the importance of monitoring the influence of sediment amendments on nontarget contaminants and provides examples of how the sediment additions of apatite can effectively immobilize some contaminants while enhancing the mobility of others.