Mechanochemical synthesis of autunites – a new route to remediation of uranium Chenhao Wu University of Edinburgh, UK Uranium (U), both radioactive and chemotoxic, is vital for use in nuclear power plants worldwide. However, activities associated with the mining and processing of uranium ores are responsible for widespread uranium contamination 1 . Autunite family are secondary uranium minerals with general formula M z+ [(UO 2 )(XO 4 )] z ·nH 2 O (M = Na + , K + , Cu 2+ , Ca 2+ and Mg 2+ etc.; X = P/As). These minerals have been recognised as important phases for controlling uranium mobility in the environment due to their extremely low solubility under circumneutral pH conditions 2 . Here we present a new mechanochemical synthesis of autunite family phases by simply hand grinding the reagents with mortar and pestle. Previous studies of uranium remediation focused on precipitating uranium from contaminated water to form autunite phases, however, this method provides a treatment of the uranium contamination in solid form, especially in mine tailings. In this experiment, stochiometric uranyl salts [UO 2 (NO 3 ) 2 ·6H 2 O, UO 2 SO 4 ·3.5H 2 O and UO 2 (CH 3 CO 2 ) 2 ·2H 2 O], hydrogen phosphate salts [NaH 2 PO 4 , K 2 HPO 4 and (NH 4 ) 2 HPO 4 ] and nitrate salts [Cu(NO 3 ) 2 ·3H 2 O, NaNO 3 , KNO 3 , Ca(NO 3 ) 2 ·4H 2 O and Mg(NO 3 ) 2 ] are mixed and hand-ground to obtain the autunite phases. The resulting samples are characterised by X-ray powder diffraction (XRPD), Fourier-transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The XRPD and SEM results showed that Cu-autunite, Cu[(UO 2 )(PO 4 )] 2 ·8H 2 O, has the highest crystallinity compared with other autunite analogues (sodium, potassium, calcium and magnesium) synthesised in the experiment. The result is consistent with the previous studies which report that the Cu-autunite phase is more stable due to the unique Jahn-Teller distortion of Cu 3 . The result also showed that the products synthesised using UO 2 (NO 3 ) 2 ·6H 2 O contain the least reagents residue compared with those using UO 2 SO 4 ·3.5H 2 O and UO 2 (CH 3 CO 2 ) 2 ·2H 2 O, which indicates that the type of uranium salt affects the efficiency of the reaction. The results of the experiments with different phosphate sources show that the Cu-autunite phase is more favourable to form even when other competing cations (Na + , K + , NH 4 + ) exist. The successful synthesis of autunite phases shows the possibility of applying the mechanochemical method in uranium remediation. With further study, this mechanochemical treatment of uranium waste could provide an efficient, low-cost, and solvent-free strategy with global application. References 1. V. Balaram, A. Rani and D. P. S. Rathore, Geosystems and Geoenvironment , 2022, 1 , 100043. 2. C. L. Corkhill, D. E. Crean, D. J. Bailey, C. Makepeace, M. C. Stennett, R. Tappero, D. Grolimund and N. C. Hyatt, npj Materials Degradation , 2017, 1 , 19. 3. E. A. Dzik, H. L. Lobeck, L. Zhang and P. C. Burns, The Journal of Chemical Thermodynamics , 2017, 114 , 165-171.
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