Removal of uranium from groundwater by bone powder

Permeable Reactive Barriers (PRB) is a promising technology for groundwater decontamination. U-contaminated sites by activities related to nuclear fuel cycle are a problem of increasing regulatory concern. Much effort has been devoted to the development of cost-effective solutions for site clean-up. The use of phosphate-based materials, such as hydroxyapatite (HAP), have proved to be an effective alternative for the remediation of uranium-contaminated groundwater. The use of phosphates is particularly advantageous due to their environmental compatibility, cost-effectiveness, and long-term stability under subsurface conditions. The application of phosphate-based technologies has been explored under different environmental conditions and at different scales, ranging from laboratory-scale experiments to field trials.

The goal of this study was to determine sorption characteristics of animal bone for uranium removal. For that purpose, isotherm batch tests were conducted for 7 days under room conditions, using adsorbent dosages of 1, 2 and 3 g/l. Water with an average uranium concentration of 5 ± 1 ppm was used for the tests. This value tries to reflect the U values measured in mining areas, which show a great variability related to the evolution of each natural system. Factors affecting the adsorption, such as adsorbent dosage or reaction time, were evaluated. Prior to testing, the animal bone was pretreated to remove organic matter. After cleaning with a hydrogen peroxide solution, the bone was crushed and sieved to a particle size of less than 50 µm to improve its sorption capacity. Uranium concentration in solution was measured by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). According to the results obtained, U-removal efficiencies calculated for 1, 2 and 3 g_sorbent/l were 98.4%, 99.5% and 99.8%, respectively.

Uranium sorption on bone powder seems to reach equilibrium within the first 60 minutes of reaction, pointing to a fast sorption kinetics.  Langmuir and Freundlich equations were used to describe the adsorption mechanism. The maximum adsorption capacity (Q_max) according to Langmuir model was found to be 8.39 mg/g. Kinetic data of bone powder are in good agreement with a pseudo-second-order kinetic model, showing high performance of uranium removal within 5 min.

In order to elucidate the U-retention mechanisms, a complete physicochemical and microstructural characterization of the bone powder before and after reaction was performed. Animal bone powder seems to be a crystalline mesoporous solid with a mean pore size of 14 nm. According to FTIR analysis, characteristic absorption bands corresponding to carbonate and phosphate functional groups were centred at 1418 and 1455 cm^-1, in the case of carbonate, and 470, 562-605, 961 and 1031 cm⁻¹ for phosphate. These results confirm the presence of phosphate groups (e.g. from hydroxyapatite) and some carbonate. U (VI) is easily removed from aqueous solution due to its high affinity for phosphates.

Based on the results obtained, bone powder exhibited advantageous features towards uranium adsorption, since its high phosphate content facilities U removal, which is fast and greater that 99% according to the tests performed.