A thermal and mass spectrometric study of synthetic johannite [Cu(UO 2) 2(SO 4) 2(OH) 2] · 8H 2O

Abstract

The course of the thermal decomposition of synthetic johannite, [Cu(UO 2) 2(SO 4) 2-(OH) 2· 8H 2O, was investigated by means of simultaneous TG-DTA-MS (thermogravimetry-differential thermal analysis-mass spectrometry). The dehydration and dehydroxylation processes (60–450°C) fall into four stages, the temperatures of which are influenced by the transport phenomena. Firstly, the four water molecules not coordinated by Cu 2+ cations are released, followed by release of the water molecules (in two steps) that are coordinated by Cu 2+ cations. Then, dehydroxylation takes place which may partly overlap with the last stage of dehydration. Further heating of the fully anhydrous compound results in a release of small amounts of oxygen at approx. 490 and 520°C, accompanied by corresponding exotherms on the DTA curve. This suggests the formation of new crystalline phases such as α-UO 2SO 4, β-UO 2SO 4 and UO 3− x . Decomposition of the anhydrous compound (CuO·2UO 3·2SO 3 or CuO+2UO 2SO 4) is characterized by the evolution of SO 3 (in two steps) and oxygen. Small exotherms at 655 and 688°C in argon and air, respectively, mark the onset of SO 3 evolution and can be assigned to the formation of crystalline CuU 3O 10. The presence of oxygen, formed both in the course of the decomposition reaction and as a result of SO 3 dissociation, causes the inhibitive influence of the oxygen content in the atmosphere over the sample on the course of decomposition.