Effects of γ radiation on microstructure of montmorillonite

Abstract

Bentonite will be exposed to γ radiation during geological disposal of high-level radioactive waste. Montmorillonite is the main mineral component of bentonite, and the study on how γ radiation affects the microstructure can help us comprehend how bentonite works and how it evolves. Montmorillonite extracted from GaoMiaoZi (GMZ) bentonite was irradiated by a ⁶⁰Co source (average energy is 1.25 MeV) with a dose rate of 2.88 kGy·h⁻¹ and absorbed dose of 1.0, 2.0, and 3.0 MGy. The samples were then tested by X-ray diffraction, synchronous thermal analysis, Mössbauer spectroscopy, infrared spectroscopy, Raman spectroscopy, and a Four-bar mass spectrometer. The results showed that: γ radiation reduced the d₀₀₁-value, and the average grain size decreased from 5.6 to 4.5 nm. The partial Si-O bonds of the tetrahedral structure, Al-O bonds of the octahedral structure, and hydroxyl (OH) groups in the montmorillonite structure were destroyed, and the mass loss of the hydroxyl (OH) groups decreased from 2.64 to 2.30%. The unit cell charge of montmorillonite rose as a result of H radicals produced by the radiolysis of water entering the crystal structure and converting Fe³⁺ to Fe²⁺. The content of Fe³⁺ decreased from 91 to 83%, while the content of Fe²⁺ increased from 9 to 17%. γ radiation damaged the partial microstructure of montmorillonite and induced the changes in the valence state of structural ion.