Introduction
Olivine is one of the prime candidate materials for CO2 conversion and capture because it is very abundant in the earths crust and can be converted into environmentally benign and thermodynamically stable MgCO3 [1]. However under natural weathering conditions (exposure to atmosphere or seawater) the conversion of olivine rock is very slow (hundreds of years). This process can be accelerated by using reactors at elevated temperature and pressure and/or by increasing the reactive surface area via grinding into small olivine particles [2].
Experiments with cm-scale Olivine crystals in our lab confirmed the strong effect of pH and temperature on dissolution rate [3], but also some unresolved mysteries were revealed. Evidence was found for the formation of a (passivating) Surface Alteration Layer (SAL) and also strong increases in surface rough-ness were observed. The question is, to what extent these phenomena play a role if instead of cm-scale crystals, 10-100 μm-scale grains are used. Crystal planes will be less prominent, and dissolution-induced erosion could make the grains smoother instead of rougher. At this smaller scale also the diffusion of released ions will be faster, with possible consequences for the formation of a SAL.
LEFT: SEM image of small Olivine grains obtained after milling [1]. Right: Confo-cal Raman microscope, here used for examining a large olivine pebble.
Grinding the latter into 10-100 μm-scale grains should strongly enhance the dis-solution rate while still allowing optical microscopy.
Research Objectives
The main question to be addressed is: How do olivine micro-grains dissolve in acid? In particular: How fast do they dissolve? How does the shrink-rate depend on grain size? What happens to the shape? These questions will be approached via optical microscopy in a micro-container. High magnification microscopy allows studying the size and shape of the grain while it is dissolving. The small dimensions of the surrounding liquid (e.g. a microfluidic container) will ensure rapid homogenization via diffusion. Mg2+ (and Fe3+) and H+ concentrations will evolve as the dissolution proceeds, and can be detected in certain ranges with fluorescence- or Raman-based microscopies. Ex-situ characterization of pristine or partially dissolved grains with e.g. SEM, AFM, profilometry can be provided as service measurements.
Learning Objectives
In addition to the standard learning objectives for a Bachelor’s project (research planning, academic writing, data presenting, etc.), you will learn how to:
· explore some unchartered terrain in the context of in situ dissolution of micro grains
· perform mineral dissolution and optical microscopy experiments in the lab
· interpret the measured morphologies, ion concentrations in the context of dissolution and diffusion
Contact Information
· Supervision: Prof. Dr. Frieder Mugele (f.mugele@utwente.nl)
References
1. Rigopoulos, I, et al. Carbon dioxide storage in olivine basalts: effect of ball milling process. Powder technology 273 (2015): 220-229.
2. White, Arthur F., and Susan L. Brantley, eds. Chemical weathering rates of silicate minerals. Vol. 31. Walter de Gruyter GmbH & Co KG, 2018.
3. Oelkers, Eric H., et al. Olivine dissolution rates: A critical review. Chem Geology 500 (2018): 1-19.