The effect of surface heterogeneity between α-quartz and α-cristobalite on adsorption behaviors toward Cu2+ solution

Probing interfacial water structure induced by charge reversal and hydrophobicity of silica surface in the presence of divalent heavy metal ions using sum frequency generation spectroscopy

HYPOTHESIS

Adsorption of divalent heavy metal ions (DHMIs) at the mineral-water interfaces changes interfacial chemical species and charges, interfacial water structure, Stern (SL), and diffuse (DL) layers. These molecular changes can be detected by probing changing orientation and hydrogen-bond network of interfacial water molecules in response to changing local charges and hydrophobicity.

EXPERIMENTS

Sum-frequency generation (SFG) spectroscopy was used to probe changes in vibrational resonances of interfacial OH vs. DHMI concentration and pH. SFG spectra were deconvoluted using the measured surface potential and maximum entropy method in conjunction with the electrical double-layer theory for the SL and DL structures and correlated by hydrophobicity.

FINDINGS

Three surface charge reversals (CRs) were detected at low (CR1), medium (CR2), and high (CR3) pHs. Unlike CR1, SFG signals were minimized at CR2 and CR3 for DHMIs-silica systems highlighting considerable alterations in the structure of interfacial waters due to the inner-sphere sorption of metal hydroxo complexes. SFG results showed "hydrophobic-like" stretching modes at > 3600 cm^-1 for Pb-, Cu-, and Zn-treated silica. However, contact angle measurements revealed the hydrophobization of silica only in the presence of Pb(II), as confirmed by an in-depth SFG analysis of the hydrogen-bond network of the interfacial water molecules in the SL.

Difference in Structure and Electronic Properties of Oxygen Vacancies in α-Quartz and α-Cristobalite Phases of SiO2

α-cristobalite (α-C) is a polymorph of silica, mainly found in space exploration and geochemistry research. Due to similar densities, α-C is often used as a proxy for amorphous SiO2, particularly in computer simulations of SiO2 surfaces and interfaces. However, little is known about the properties of α-C and its basic oxygen defects. Using density functional theory (DFT) simulations we provide a comprehensive report on the properties of perfect structure and oxygen vacancies in α-C. The calculated properties of α-C are compared with those of the better-characterized α-quartz (α-Q). Our results demonstrated that the positively charged O vacancy in α-C is most stable in the dimer configuration, in contrast to α-Q, which favors the puckered configuration. A back-projected configuration was also predicted in both polymorphs. We calculated the optical transition energies and isotropic hyperfine constants for O vacancies in both α-Q and α-C, and compared our findings with the results of previous studies and experiments. This work, thus, offers one of the first in-depth investigations of the properties of oxygen vacancies in α-C.