The kinetics of radical formation on mechanically activated kaolinite surfaces

Abstract

Kaolinite is a versatile material with many applications in various fields such as in cosmetics or wastewater treatment. Here, we report the first evaluation of mechanoradicals formation on activated kaolinite powders, and its kinetics, via wet-grinding in ethanol using a free-radical scavenger (2,2-diphenyl-1-picrylhydrazyl; DPPH). Increasing the duration of ball milling of two reference kaolinite samples resulted in increased DPPH consumption due to an increase in the surface area of the processed powders. The correlation between the increasing rates of the surface areas and DPPH consumption yielded information on the density of radical species that may react with ethanol to form H radicals on the newly created surfaces of the kaolinite powders. The radical density was much lower than that reported previously for quartz, possibly due to the anisotropic nature of the kaolinite crystal structure. The actual surfaces that take part in the radical formation reaction were mainly newly created edge surfaces rather than basal surfaces. The kaolinite sample with fewer defects was more resistant to delamination, and thus its edge-to-basal surface-area ratio was also higher, which may have resulted in higher reactivity during mechanical processing. This knowledge is key for industrial applications of processed kaolinite powders.