PNNL

The Earth’s crust is a complex arrangement of different rock types with different mineral components, and we currently have a limited ability to sense their properties. An emerging technique relies on electrical signals sent underground to identify the minerals and structures below. However, doing this effectively requires understanding how different minerals respond to the applied electric fields.

Researchers from Pacific Northwest National Laboratory (PNNL) are exploring these phenomena in work that was first enabled under the project Induced Spectral Interrogation Technology for the Environment (INSITE) and is now being expanded in the Center for Understanding Subsurface Signals and Permeability (CUSSP). A pair of papers recently published in the Journal of Physical Chemistry C explore what happens at mineral surfaces in water at the microscale.

The first paper, led by PNNL computational scientist Pauline Simonnin, examined the interface where ions dissolved in water meet quartz, the second most abundant mineral in Earth’s crust. This interface is of critical importance to understanding how the ions interact with the surface and move. They looked at two different types of quartz surfaces to see if the orientation of the material affects the ion transport properties. They found that the quartz surface structure had a significant effect on the overall properties of the interface.

A major difference between the two surfaces is the amount of hydroxide groups present. These groups affect the overall chemistry of the surface as they interact with the water and ions. “This work was really establishing our baseline knowledge of these interfaces,” said Simonnin. “Now we can delve into how adding an electric field changes their behavior.”

Manipulating ions on surfaces

Other research is directly probing surfaces with an electric field. PNNL chemist Ben Legg led work that used a newly developed atomic force microscopy (AFM) capability to manipulate ions on the surface of the mineral calcite. By applying a voltage to an AFM probe hovering above their sample, they could attract or repel ions at the surface. The team would then monitor how quickly the ions moved under the influence of the voltage and watch how they relaxed back to their initial state once the voltage was turned off.

They tested how humidity affected the movement of ions because the humidity level affects the amount of water adsorbed on the surface. Going from 5 to 95% humidity dramatically increases the mobility of the surface ions, with an over 100,000-fold change. When the atmosphere is more humid, more water molecules come down onto the surface to create a thicker film of water that helps ions move. The complex data analysis required intensive collaboration between computational and experimental work.

The two studies highlight fundamentals behind ion behavior on common mineral surfaces. The structure of interfacial water and how it interacts with the ions has an important role in these systems. The results provide the understanding necessary for accurate models of the subsurface. These studies represent one step along the way.

“This initial work showed us how we can effectively use AFM to understand ion mobility at a very detailed level,” said Legg. “Now that we’ve developed this capability, we can explore a broader range of surfaces, ions, and processes.”

CUSSP, a new multi-institutional Energy Earthshot Research Center funded through the Department of Energy’s Office of Science, will take the AFM work begun by Legg and extend it to more complex systems, toward ultimately being able to sense and understand chemical reactions at mineral/water interfaces important to subsurface geochemistry.

In addition to Simonnin, the quartz work included PNNL researchers Sebastien Kerisit, Elias Nakouzi, Timothy C. Johnson, and Kevin Rosso. The calcite work was performed by a PNNL team that includes Legg, Yue Zhu, Nakouzi, Johnson, and Rosso.