Thermal decomposition of volcanic glass (rhyolite): Kinetic deconvolution of dehydration and dehydroxylation process

Thermal decomposition of hydrous volcanic glass occurs through the release of different water species under overlapping processes over wide temperature range. Its investigation is of practical interest since it constitutes integral processing part towards its valorization as source for the production of high-quality porous material for various applications. The study presents investigation of thermal decomposition of hydrous rhyolite through the non-isothermal solid-state kinetics approach. Rhyolite decomposition occurs through three partially overlapping processes, where loosely held and chemically bound water, as well as hydroxyl release at different temperature regions and through different mechanisms. The separation of overlapped thermal curves was done through peak deconvolution method using Frazier-Suzuki equation. Subsequently, the isoconversional (model-free) Friedman, generalized master-plots and Kissinger methods were applied for the determination of apparent activation energy (Ea), reaction model (f(a)) and pre-exponential factor (A) for each individual reaction step considered. Using the kinetic triplet values of each process, the kinetic rate equations were combined allowing precise simulation of the dehydration and dihydroxylation processes. A comparison of model results with thermogravimetric (TG) data, as well as data from the literature, showed the satisfactory accuracy of the model in the simulation of the process and the successful prediction of each water type fraction, during the process evolution. Spectroscopy techniques in UV―VIS and NIR (near infra-red) spectral ranges were applied to raw rhyolite and sample with different water content allowed calculation of color coordinates and its correlation with dehydration and dehydroxylation degrees, and also identification of water species.