Surprisingly highly reactive silica that dissolves rapidly in dilute alkali (NaOH) solution even at ambient temperatures (25 °C)

Abstract

In this study, the silica with special structure that rapidly dissolves in dilute alkali solutions even at room temperature (25 °C), which challenges the traditional concept of the high stability of silica properties, is reported. After treatment in 2 mol/L NaOH lixivium at 25 °C for 2 h, the active silica is completely dissolved, which has not been observed previously for other silica types (except for silica gel). Such silica can be obtained in large quantities from the acid leaching residue of kaolinite whose stable silica species are converted into a highly reactive phase through the double activation of both the breakage of the Si–O–Al bond and the subsequent spontaneous polycondensation of the Q3 silicon oxide into the Q4 silica structure. Simultaneous condensation/polymerization reactions occur either between the two adjacent silica tetrahedra in the same silica layer or between two tetrahedra from different layers, forming two types of highly active silica with 29Si nuclear magnetic resonance chemical shifts at −115.87 ppm and −106.96 ppm. The Si–O bond strength of the active silica is significantly lower than those of raw kaolinite and other silica species. Research and development of this active silica resource is beneficial to improving the energy-intensive status of the silica industry from the source.