Novel Hydroxyapatite Beads for the Adsorption of Radionuclides from Decommissioned Nuclear Power Plant Sites  
[PERSON]  
1 Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Diehak-ro, Yuseong-gu, Daejeon 34141, Korea; [EMAIL_ADDRESS] (T.N.T. ); [EMAIL_ADDRESS] (J.-K.); [EMAIL_ADDRESS] (J.-K.); [EMAIL_ADDRESS] (J.-K.); [EMAIL_ADDRESS] (J.-K.); [EMAIL_ADDRESS] (J.-K.); [EMAIL_ADDRESS] (J.-K.); [EMAIL_ADDRESS] (S.O.) [PERSON]  
1 Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Diehak-ro, Yuseong-gu, Daejeon 34141, Korea; [EMAIL_ADDRESS] (T.N.T. ); [EMAIL_ADDRESS] (J.-K.); [EMAIL_ADDRESS] (J.-K.); [EMAIL_ADDRESS] (J.-K.); [EMAIL_ADDRESS] (J.-K.); [EMAIL_ADDRESS] (J.-  
of such radionuclides, various methods have been proposed and developed to effectively control radionuclide ions in contaminated water, including reverse osmosis, membrane separation, adsorption, biological treatment, and electrodialysis [3, 5, 7, 8]. Among them, adsorption has been widely investigated as the most cost-effective, environmentally friendly method and the easiest technique for industrial application [9, 10]. In this regard, various types of adsorbents, including biomass, clays, activated carbon, hydroxyapatite, metal particulates, and metal oxides functionalized with bio-based substances have been used to separate contaminants during water treatment [1, 5, 11]. Of them, hydroxyapatite (Ca\({}_{10}\)(PO\({}_{4}\))\({}_{6}\)(OH)\({}_{2}\)), a calcium phosphate, has emerged as one of the promising environmental remediation materials with low cost, nontoxicity, biocompatibility, and biodegradability properties [12]. The popularity lies in its particular structure and unique properties, such as its ion exchange capability, adsorptive ability, acid-base adjustability, and thermal stability [12, 13, 14, 15, 16]. The feasibility of hydroxyapatite (HdA) nanoparticles as effective adsorbents is well documented for the adsorption of various multi-valent metal ions, such as lead, cobalt, nickel, copper, zinc, cadmium, strontium, and uranium [1, 17, 18, 19]. Indeed, the HdA has a versatile structure, which allows foreign ions to substitute Ca-ions on the framework without any distortion [20]. Nevertheless, the use of HdA nanoparticles in the industrial field is limited due to their difficulties in separating them from effluent and the possibility of causing secondary pollution [12]. The auxiliary separation steps, such as ultrafiltration or the sedimentation process, requires a significant amount of time, additional investment, and operation costs [21]. In addition, the application of powdered hydroxyapatite (p-HdA) for the continuous-flow column is impossible due to the significant head loss that is induced by the small size of HdA powders. Taking into account these issues, the fabrication of macro-sized HdA granules will enable easier separation in suspended applications, as well as lower head-loss during continuous-flow column applications. For biomedical applications, several methods have been employed to fabricate a macro-structured HdA, such as granulating via centrifugation and vibration, sintering, casting in plaster models, and spraying [22, 23, 24]. Regarding environmental applications, only a few studies have been developed for water treatment so far. [PERSON] et al. [25, 26] fabricated porous HdA ceramic beads (4.0-5.0 mm) by mixing the p-HdA and solute starch and sintering at 1200 \({}^{\circ}\)C for 2 h to adsorb the fluoride. [PERSON] et al. [27] mixed the p-HdA and polyvinyl alcohol (PVA) and calinated at 600 \({}^{\circ}\)C for 4 h to obtain HdA pellets (2.0 \(\times\) 10.0 mm) with a maximum lead adsorption capacity of 7.99 mg/g.