In Situ Quantification of Surface pH of Dry Soils and Minerals: An Innovative Approach Combining Indicators and Diffuse Reflectance Spectroscopy

Most natural soils and minerals are typically dry or semidry, and their surface characteristics and interfacial interactions are distinct from those of slurry systems. The surface acidity of dry soils and minerals cannot be measured by using traditional methods such as potentiometric and temperature-programmed desorption methods. In this article, we describe the development of an analytical method based on a combination of acid-base indicators and diffuse reflectance spectroscopy (DRS). The surface pH (pH_surf) values of dry clay minerals and metal oxides were obtained, which were 1.13-3.14 units lower than that of slurry systems. This result was confirmed by the pollutant degradation activity on the mineral surface with different water contents. The potentiometric titration and surface complexation models (SCM) were used to reveal the acidity characteristics of different minerals. The pH_surf values of soils from seven provinces in China were determined using three methods: (1) real soil samples with different colors were diluted in BaSO₄, and pH_surf values were determined using DRS; (2) the pH_surf values of soil samples were also calculated using the pH_surf vs slurry pH curves, which were constructed using the pH_surf of six metal oxides and clay minerals obtained using DRS and their slurry pH values; (3) the pH_surf values of soil samples were determined using the pH values through extension from the slurry pH vs soil-water ratio plot. The pH_surf values calculated using the former two DRS methods were consistent and provided a better measure of the surface acidity of dry soil than slurry pH and its extension method. This review presents an in situ method for the quantitative surface-acidity measurement of dry soils and minerals and provides insights into acidity and interface research in other low-moisture systems.