Alteration of fine fraction of bentonite from Kopernica (Slovakia) under acid treatment: A combined XRD, FTIR, MAS NMR and AES study

Acid activation of clay minerals is one of the most effective methods proposed to produce materials with increased acidity and adsorption properties. In this work fine fraction of bentonite from deposit Kopernica (Slovakia) was treated with 6 mol/L HCl. Variety of methods were employed in order to investigate the structural changes occurring as a result of the acid activation. Quantitative XRD analysis and FTIR revealed a dominant presence of montmorillonite, with K-feldspars, quartz, volcanic glass, opal C, ferrihydrite, biotite and kaolinite admixtures in acid untreated sample. ²⁹Si MAS NMR spectra also confirmed accessory siliceous phases. ²⁷Al MAS NMR showed that approximately 10% of Al was bound in the tetrahedral sites of montmorillonite. Gradual decrease of the Al, Fe and Mg content, an upward shift of the complex Si–O stretching band and the appearance of Q⁴(0Al), Q³(1OH) signals in the ²⁹Si MAS NMR spectra of the extensively reacted materials indicated a substantial modification of the montmorillonite layers and formation of an amorphous silica product. The accessory minerals quartz, K-feldspars and kaolinite were dissolved more slowly. The presence of kaolinite and quartz/K-feldspars in solid product was confirmed even after treatment for 36 h. The acid treatment affected also the surface and acidity characteristics of the samples. The total specific surface area (EGME SSA) gradually decreased with decreasing montmorillonite content in the reaction product. In contrast, the changes in the N₂BET SSA showed that reaction with an acid first increased it to a maximum after treatment for 6 h, but more extensive treatments had a negative effect. After pyridine adsorption the IR spectra of the samples treated for 1 up to 6 h showed, in addition to the bands related to the H-bonded and physisorbed pyridine, also the diagnostic band of pyridinium cation near 1540 cm⁻¹ confirming the presence of strong Brønsted acid sites.