Modeling description and spectroscopic evidence of surface acid–base properties of natural illites

Abstract

The acid–base properties of natural illites from different areas were studied by potentiometric titrations. The acidimetric supernatant was regarded as the system blank to calculate the surface site concentration due to consideration of substrate dissolution during the prolonged acidic titration. The following surface complexation model could give a good interpretation of the surface acid–base reactions of the aqueous illites: >SOH↔> SO − +H + , p K a int =3.73∼ 4.06, Al 3+ +H 4 SiO 4 +2H 2 O↔ Al(OH) 2 (OSi(OH) 3 )+3H + , p K f2 =9.85∼ 10.71, >SOH+Al 3+ +H 4 SiO 4 ↔ >SOAl(OSi(OH) 3 ) + +2H + ,pK SC =3.31∼ 3.40. Both experimental and optimized data revealed some similarities in the acid–base behaviors of the different illites. Based on matching results with the standard XRD patterns, the changes in intensities of the diagnostic 0 0 l diffraction lines indicated that the formation of surface Al–Si complexes or precipitates appeared preferable to the bulk deposition of Al(OH) 3 solid. Similar features were also reflected in the microscopic Raman spectra, where the vibrational peaks of Si–O and Al–O bonds diminished following acid attack, then rose again after hydroxide back titration. The varied ratio of signal intensity between IVAl and VIAl species present in 27Al MAS NMR spectra, together with the nearly stable BET surface areas after acidic titration, showed that the basal planes in the layer structure of natural illite contributed to the dissociation of structural components as well as the edge faces. The combined spectroscopic evidence plus the acid–base titration results suggested that the interactions of illite surface sites with silicic acid and aluminum species, released from illite substrate, should be considered in the modeling description.