The effect of ?-irradiation on the structure and subsequent thermal decomposition of brucite

Abstract

The effect of γ-irradiation on the structure, phase composition and kinetics of isothermal decomposition of natural textural brucite Mg(OH)2 has been investigated by Mn2+ electron paramagnetic resonance (EPR), proton magnetic resonance (PMR), X-ray diffraction (XRD) and weight loss methods. Starting from a 106-Gy dose, γ-irradiation (60Co, 13.8 Gys−1) is found to stimulate the formation of a new phase in the brucite structure, namely basic magnesium carbonate. The carbonate phase is assumed to form in brucite under γ-irradiation accordingly to the scheme \(\) (in the brucite structure). There is also a possibility that γ-irradiation forms particles with high reaction ability, CO−2 radicals and/or CO molecules, which can react with the brucite structure. Preliminary γ-irradiation (9.75 × 107 Gy) slows down the subsequent isothermal dehydroxylation of natural brucite, which can be explained by the formation of the new carbonate phase in the Mg(OH)2 structure. Dehydroxylation kinetics of both original and irradiated samples are interpreted by a two-stage nucleation model at 623, 648, 673, 698 and 723 K. The reaction rate is limited by the first nucleation stage rate (proton transition from an OH group near the reaction interface on a freed vacant orbital of an oxygen ion of the OH group in the nearest elementary cell, i.e., formation of a structured water molecule). The second-stage rate (water molecule removal from the structure and proton migration from the residual hydroxyl inside the structure) is about 1 order of magnitude higher. The activation energy of the limiting stage is 194 and 163 kJ mol−1 for the original and irradiated samples, respectively. Non-linear Arrhenius dependencies for the first-stage rate constants are related to the potential barrier reduction due to thermal fluctuations of large structural zones (with radii of about 20 and 81 A in original and irradiated samples, respectively), whose ions form this barrier.