Effectiveness of microwave assisted acid treatment on dioctahedral and trioctahedral smectites. The influence of octahedral composition

The effect of the microwave assisted acid treatment (MAT) on the structure and texture of dioctahedral (montmorillonite and nontronite) and trioctahedral (saponite and stevensite) smectites were studied by SEM, nitrogen adsorption, XRF, XRD and FTIR. The effectiveness of this treatment is notably influenced by the chemical composition of the octahedral sheet. Thus, trioctahedral smectites seem to be very reactive to the microwave assisted acid treatment. In saponite and stevensite this treatment caused in the first 20 min the progressive Mg2 + depletion of the octahedral sheet, the gradual destruction of the smectite structure and the formation of an amorphous silica phase which contributes to a noticeable increase of the BET specific surface area. After 20 min of microwave assisted acid treatment the increase in SSA is of 315 m2/g in saponite and 244 m2/g in stevensite. The assistance of microwaves during the acid treatment allows us to get, in 20 min, specific surface areas equivalent to those obtained after 48 h with conventional heating methods. On the contrary, dioctahedral smectites seems to be less reactive to the microwave assisted acid treatment.