A microstructural study of acid-activated montmorillonite from Choushan, China

Bentonite samples from Choushan treated at various acid concentrations were studied using chemical analysis, XRD, Si-29 and Al-27 MAS NMR to investigate the microstructure of the activated montmorillonites. Two types of structural units were formed during the activation: (1) (SiO)(3)SiOH, in which up to 15% of Si is bound; (2) Q(4)(0Al), a major building unit when the acid concentration is >10%. A significant Al-27 signal at 55.0 ppm was recorded for both untreated montmorillonite and (to a much greater extent) in acid-treated inommorillonites. This was interpreted as arising from four-fold coordinated Al located in the 'bulk' octahedral sheet of montmorillonite. In the course of activation, the removal of one of a pair of octahedral Al ions from montmorillonite removes two hydroxyl groups and leaves the other Al of the pair in four-fold coordination.