Surface Selective Studies of Olivine Dissolution by 1H-29Si and 25Mg MAS NMR
EMSL Project ID
21594
Abstract
Mineral trapping is a proposed mechanism for sequestering atmospheric carbon dioxide, allowing permanent confinement of this greenhouse gas. However, the sequestration process requires readily accessible cations that can form stable carbonate phases. Magnesium silicate minerals are proposed as one source for these cations, with dissolved magnesium reacting with carbonate to form magnesite (MgCO3). The simplest magnesium silicate mineral is olivine and its dissolution has been the subject of many previous studies due to its fast reaction kinetics and simple structure, hence making olivine ideal for laboratory dissolution studies. However, while there is a healthy literature precedent, the mechanism of olivine dissolution is still debated and under study in our Center for Environmental Kinetics Analysis. To better elucidate the changing chemical environment on the surface of forsterite (the magnesium end member of olivine) we have chosen solid-state nuclear magnetic resonance (NMR) as our primary analytical technique based on its molecular level information content. Since the chemistry of dissolution is confined to the solid/liquid interface, surface selective analytical techniques are required. Cross polarization NMR techniques will provide direct information on the dissolution mechanism, as protons are confined to the forsterite surface or near-surface layers. In previous (moderate field) studies of acid weathered forsterite, 29Si MAS NMR has confirmed the Q0 speciation of the bulk while 1H-29Si CP MAS NMR at 9.4 T has been unsuccessful in providing an observable signal after accumulation of a significant number of transients. This has been attributed to the low surface area of these materials, so that the low overall abundance of protons provides only a limited (small) CPMAS signal. Therefore, accumulating signal at the highest magnetic field possible will increase the sensitivity of the NMR experiments to these surface environments, and we propose to carry out these experiments at PNNL. In concurrent studies, 25Mg MAS experiments at 11.7 T have also been unsuccessful in providing a sufficient signal to noise ratio. However, data from previous studies conducted at PNNL at 21.1 T have been quite successful, indicating the presence of a second magnesium phase forming on the olivine surface (detected with one-pulse NMR experiments rather than CPMAS NMR). Utilizing the higher fields available at the PNNL we will be able to proceed in our advanced understanding of this second magnesium environment and follow the changing forsterite surface during the dissolution process.
Project Details
Project type
Capability Research
Start Date
2006-11-15
End Date
2007-07-18
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members