Flowing water energizes minerals

A collaborative research team from the Max Planck Institute for Polymer Research (MPIP) in Germany and the University of Namur in Belgium discovered a fundamental, yet unnoticed, phenomenon that motion of water along a mineral surface changes the charge of that surface. The researchers published their finding in Science.

The international research team in Mainz led by Mischa Bonn studied how moving water, like in riverbeds or creeks, affects mineral surfaces and their dissolution. Remarkably, water flow along fluorite and glass surfaces makes these surfaces more positively charged. In the case of fluorite, a 100-fold increase in acid concentration was required to induce similar effects in static water.

Water molecules as reporters

Surfaces of minerals acquire a charge when immersed in water, as part of the minerals can be released from the surface as charged ions. This was known, but that moving water can change the surface charge was entirely unexpected. The research team in Mainz measured the surface charge of immersed minerals using the water molecules directly at the interface as reporters.

Water molecules have a positive and a negative end, and align toward the surface, depending on the surface charge. The interfacial water molecules were interrogated by overlapping two laser pulses of different color at the liquid-mineral interface, whereby a new color can be generated that provides extremely specific information about the interfacial region.

In this manner, both the orientation (pointing up- or downwards) and the number of oriented water molecules can be directly measured, which provides direct access to the surface charge.

The experiments show that flowing a liquid in contact with minerals induces a preferential dissolution of specific mineral constituents. In the case of fluorite, negatively charged ions are preferentially dissolved while positively charged ions remain at the mineral surface. The researchers were even able to show that the sign of the surface charge can be controlled with flow, so that water molecules could be made to reorient, depending on the presence or absence of flow.

The observed phenomenon seems to be rather ubiquitous in geology. Particularly because this phenomenon occurs not only for fluorite, but also for silica surfaces – silicates constitute more than half of the minerals in the earth crust. “These new insights on the fundamentals of mineral dissolution force us to reconsider well-established theories in weathering and environmental sciences to take into account changes in surface charge in addition to well-documented surface erosion.”, explains Mischa Bonn.

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