PNNL

Abstract

Interfacial reactions drive all elemental cycling on Earth and play a pivotal role in human life such as agriculture, water purification, energy production, environmental contaminant remediation, and nuclear waste repository management. Chemistry at such interfaces has been a focus of research for several decades, with the most recent comprehensive review published in 1999 (Brown et al., Chemical Reviews). Here we critically review research advances on structure, reactivity, and dynamic and coupled processes at oxide- and silicate-water interfaces in the last 20+ years and discuss the future opportunities. First, since the onset of the 21st century, strides towards a detailed understanding of interfaces submerged in aqueous solutions have been enabled by the development techniques with near-atomic measurement resolution utilizing tunable highflux focused ultrafast laser and X-ray sources, as well as nano-fabrication approaches that enable transmission electron microscopy in a liquid cell. This leap into atomic- and nm-scale measurements uncovered scale-dependent chemical phenomena, with reaction thermodynamics, kinetics, and pathways deviating from previous observations made on the larger systems. The second key advance is new experimental evidence for what scientists have hypothesized but could not test previously: namely that interfacial chemical reactions are frequently driven by “anomalies”, or “non-idealities” - such as defects, nanoconfinement, and unanticipated chemical structures. Third, advances in computational chemistry have allowed insight to move beyond simple schematics and gain a molecular model of these complex interfaces. These advances were facilitated by our ability to simulate with greater accuracy systems in concert with their corresponding high-resolution experimental observations. In combination with surface-sensitive measurements, we have gained knowledge of the interfacial structure and dynamics, including the underlying oxide surface and the immediately adjacent water and aqueous ions, which enable us to better define what constitutes the oxide-water interface. This critical review discusses how science is progressing from understanding the ideal oxide- and silicate-water interfaces to more realistic systems, focusing on accomplishments in the last 20 years, and identifying the remaining unknowns, outstanding challenges, and future opportunities for the community to address and/or to seek in the decades to come. We anticipate that the next 20 years will focus on understanding and predicting dynamic transient and reactive structures over greater spatial and temporal ranges as well as systems of greater structural and chemical complexity. Closer collaborations of theoretical and experimental experts across disciplines are critical to achieving this great aspiration.