Uranyl phosphate (MUO2PO4, M = Na+, K+, NH4+) precipitation for uranium sequestering: formation and physicochemical characterisation

Uranyl phosphate synthesis (MUO 2 PO 4 , M = Na + , K + , NH 4 + ) by diffusion method, direct and pH-controlled precipitation has been performed and compared. Chemical and thermogravimetric analysis revealed compound stoichiometry to be in good agreement with ideal mineral phases (sodium uranyl phosphate (NaUO 2 PO 4 ⋅xH 2 O), meta-ankoleite (KUO 2 PO 4 ⋅3H 2 O) and uramphite ((NH 4 )UO 2 PO 4 ⋅3H 2 O)) except for sodium and ammonium cations. Layered platelet-type crystal structures associated with uranyl phosphates were confirmed. Some long-range crystal structure was observed under direct precipitation conditions. No visible evidence of long-range crystal structures for samples formed during pH-controlled precipitation was found due to dilute formation conditions and rapid nucleation kinetics.