Combined determination analysis of surface properties evolution towards bentonite by pH treatments

Abstract

Bentonite possesses multiple physicochemical properties because of a large number of charges on the surface. Understanding charges-related surface characteristics at various aqueous solution conditions is critical for bentonite application in various fields, including material modification and environmental restoration. In this study, the evolution of surface properties (surface potential, surface electric field strength, specific surface area, and surface charge numbers) for bentonite samples was investigated using simple ion-exchange experiments and extended combined determination method at different pH conditions. Zeta potential, X-ray diffraction (XRD), and attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) techniques were used to examine changes in bentonite structure and surface functional groups. The results demonstrated that surface potential was considerably negative (larger in absolute value) and more sensitive to pH variation than the zeta potential of bentonite under identical conditions. At multiple pH settings, the surface electric field strength and the numbers of bentonite’s surface charges were inversely proportional to hydrogen ion concentrations. Cations covalently bonded to clay’s surface might reduce negative surface charges and diminish surface electric field strength, particularly as ionic strength increased. Although moderate pH variation has no discernible effect on bentonite interlayer structural alterations, moderate clay dissolution and changes in mineralogical properties may increase the specific surface area of bentonite, particularly when the solvent chemical conditions are acidic. This study comprehensively sheds light on the evolution of surface properties of bentonite clay under acidic, neutral and alkaline conditions, which is helpful in understanding and optimizing its surface performance for material and environmental applications.