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Silica minerals are a kind of important minerals and widespread on the earth’s surface. They play an irreplaceable role in the whole geochemistry and environment processes. The diversity in the crystal structure of SiO2 polymorphs might lead to the heterogeneity in their surface microstructures and properties. As two common SiO2 polymorph minerals in soil and sediments, α-quartz and α-cristobalite have been studied for the effects of their surface heterogeneity on adsorption behaviors toward crystal violet (CV) by batch adsorption experiments in different specific surface areas (SSAs) and at different pH values and temperatures, as well as by X-ray photoelectron spectroscopy (XPS) investigation. Owing to the larger surface site density, the saturated adsorption amount of α-quartz was larger than that of α-cristobalite. It was also indicated by the larger slope of adsorption lines as a function of SSA. The adsorption capacity of both increased with increasing pH and temperature. In the thermodynamic study, the negative ΔG indicated that the adsorption of CV on the surface was spontaneous and the positive ΔH suggested that the reaction was endothermic. The well-fitted Langmuir adsorption isotherms suggested that the CV adsorption was monolayer adsorption. The adsorption interaction force was mainly involved in electrostatic attraction force between the negatively charged surface reactive sites and positively charged N atoms in the dimethylamino groups of CV. The XPS spectra of N 1s showed that the stoichiometric ratio of Nlow/Nhigh changed from lower than 2:1 to about 2:1 as the adsorption changed from the unsaturated to saturated state. The change reflected that the spatial arrangement of adsorbed CV monomer on the mineral surface could be readjusted by lifting the average tilt angle between the average plane of the CV monomer and the sample surface during the adsorption process. Surface heterogeneity of α-quartz and α-cristobalite controlled the different distributions and postures of adsorbed CV monomers on the surface. The CV monomers adsorbed on α-quartz presented a larger average tilt angle because of its larger surface reactive site density, while α-cristobalite did conversely.

The dissolution and precipitation of silica minerals in rocks, soils, and sediments are essential processes of material transformations near the Earth’s surface. In this study, the dissolution of α-quartz and α-cristobalite are investigated at 25 °C in HNO3, NaOH, KCl, and MgCl2 solutions. The amounts of silicon release from α-quartz in HNO3 and electrolyte solutions are larger than those from α-cristobalite, and the circumstance is in consistence with the density of surface silanols. In NaOH solutions, the amounts of silicon release increase significantly and the maximum amount is about 30 times higher than that in acid and electrolyte solutions. Moreover, the maximum silicon release is inversely proportional to the density of surface silanols, resulting in a lower silicon release from α-quartz in comparison to that from α-cristobalite. It could be deduced that there is a possible correlation with structural defects, for instance, oxygen vacancies, demonstrated by the electron paramagnetic resonance (EPR) spectrum. By analyzing the surface composition of samples after dissolution, the result of X-ray photoelectron spectroscopy (XPS) confirms that no stable amorphous silica layer form on the surface of α-quartz and α-cristobalite under the current experimental conditions, along with the verification of high-resolution transmission electron microscope (TEM). Furthermore, the surface species of α-quartz and α-cristobalite after reacting with different solutions are also qualitatively determined by XPS.

Mineral surface microstructure and properties not only exert an important influence on the surface dissolution and growth of minerals, but also control the reaction rate and model between mineral and heavy metal ions in environment. The surface difference of silica polymorphs had been verified in some previous studies. To further study the effect of surface heterogeneity between α-quartz and α-cristobalite on adsorption behaviors toward heavy metal ions, the adsorption characteristics of Cu2+ ions were investigated by adsorption experiments. Cu2+ ions were predominantly adsorbed on α-quartz because of its larger surface site density. The reaction between α-quartz/α-cristobalite and Cu2+ ions was an endothermic process. The increase of pH could prompt the formation of Cu precipitation on the surface, corresponding to the declined intensity of EPR signal of g=2.08/2.05 at a higher pH. Cu2+ ions were bonded with surface oxygen atoms to form an inner-sphere structure at lower pH and initial concentration solutions. The intensity loss of g=2.08 in α-quartz in a higher initial concentration demonstrated the formation of Cu(II) dimeric/polynuclear species, resulting from the larger surface site density. While, from the increased EPR signal of g=2.08, mononuclear adsorption was still the predominant form on α-cristobalite under the same reaction conditions.