Flume Experiment: Effect Of Motion on pH and Weathering


This experiment was carried out using a large recirculating flume filled with coarse sand made up approximately ~30% of olivine with all fine-grained sediments removed prior to the start.

The water was moved at a speed to model basic ocean currents of 40-60 cm/sec. When the water flows, the olivine is transported and tumbled as can be observed in the accompanying animated image.

Video courtesy of Prof. dr. Poppe de Boer (Personal Communication)

As can be seen in the graph below, when the water is refreshed the pH is measured at 8-8.05. As the olivine weathering occurs, the alkalinity of the reaction causes the water’s pH to rise to ~8.3 pH. When the current is stopped, a silica coating rapidly forms on the olivine, which greatly slows down the reaction. Without the olivine reacting and with CO2 continuing to enter through the open-air by diffusion, the pH drops down to ~8.1 pH.

The non-connected points in the above graph are when the current was stopped. During the first break of 2 days, as grain collisions stop, the pH of the water falls. When the water is refreshed and motion resumes, the pH is seen to rapidly rise as the olivine tumbles and breaks down.

This mechanical activation is crucial for the acceleration of the weathering rate. This is why Project Vesta plans to use high-energy, tropical beaches to weather olivine and also why any discussion of the olivine weathering rate that doesn’t include motion does not apply to our project.

Last updated on May 18, 2020
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