The occurrence of uranium(U(VI)) with co-existing arsenic(As(V)) contaminations caused by natural processes and mining is a concern to affected communities, particularly to our partner Native American communities. Here we investigated the simultaneous removal of aqueous U and As after reacting with natural limestone and precipitation with hydroxyapatite (HAp, Ca10(PO4)6(OH)2). In the 31-day removal experiment at acidic conditions (initial pH of 3), 1 mM U and As were removed in the presence of 1 g L-1 limestone by potential co-precipitation of Ca, U, and As. However, U and As species did not precipitate at circumneutral conditions. By adding 1 mM phosphate and 3 mM calcium in solutions, 0.05 mM U was removed through rapid calcium-uranyl-phosphate precipitation. In the experiments with 2 mM PO43- and 10 mM Ca2+ at pH ranging from 7 to 11, removal of As was found to be significantly improved (between 62.7% and 98.4%) at pH around 9. Abundant HAp precipitation in solids was confirmed by XRD and SEM/EDX, and electron microprobe analysis on cross-sections of HAp-coated limestone. Uranium immobilization occurred mainly through co-precipitation of U, Ca, and P. In contrast, As immobilization was mainly performed by the adsorption of HAp through active ion exchange. The results from this study indicate that the HAp-limestone system can achieve feasible immobilization of aqueous U and As, and be potentially used as an economically accessible approach for on-site remediation at contaminated locations.

Removal of uranium(VI) and arsenic(V) mixtures from water by reactions with calcium(Ca)2+, phosphate(PO4)3-, and natural limestone

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