Quantification of Reactive Surface Area of Shales from a Hillslope via 19F MAS NMR of a Fluorine-Containing Probe Molecule
EMSL Project ID
39986
Abstract
To understand natural processes such as nutrient and contaminant cycling as well as carbon sequestration, the rates of regolith formation from rocks and subsequent weathering into secondary minerals must be characterized. However, these rates and mechanisms are difficult to quantitatively predict for several reasons including the challenges of defining an appropriate surface area term. Though the Brunauer-Emmett-Teller (BET) gas adsorption isotherm and geometric surface area based on particle size are techniques frequently utilized to quantify total surface area for minerals, neither of these methods accounts for the chemical reactivity of various surface species. The use of a nuclear magnetic resonance (NMR) proxy for the quantification of reactive surface area is proposed, which would enable scientists to better predict weathering rates in the field. First a fluorine-containing chlorosilane is covalently bonded to the reactive surface hydroxyl sites on minerals, which are proposed to interact primarily with other components in the environment. To quantify the number of the probe molecules that have been covalently attached to the reactive hydroxyl sites, 19F magic angle spinning (MAS) NMR is utilized. Quantification is achieved through a comparison of integrated peak intensities of treated samples with a 19F reference having a known concentration of spins and then incorporating the relevant stoichiometry to calculate the number of reactive hydroxyl sites. In this study, field weathered shales are collected from a two-dimensional, planar transect at the Susquehanna/Shale Hills Observatory (SSHO) located in central Pennsylvania. This temperate-climate, forested catchment has regolith forming on top of homogeneous Rose Hill shale. Samples have been collected in 10 cm increments down to the bedrock at the ridge top, middle slope, and valley floor. The aim is to explore the relationship between soil depth and reactive surface area for these minerals utilizing a 900 MHz solid-state NMR equipped for ultrafast (up to 40 kHz) spinning speeds. These fast spinning speeds are necessary for accuratel spectral interpretation as the natural samples contain a significant amount of paramagnetic cations such as Fe3+. The use of the highest magnetic field possible is crucial, as the surface areas of these samples are very small (less than 1 m2/g as measured by BET), resulting in reduced experimental sensitivity that is overcome by moving to higher fields. In addition to quantifying reactive surface area, the relationship between reactive surface area and particle size will be investigated as each sample has been divided into its sand and silt/clay fractions. Finally, the 19F peak from TFS may contain more than one magnetically-inequivalent site, which can be used to interpret the contribution of reactive surface area that resides within mineral pores. All of these results will be useful in identifying a feasible value for reactive surface area to develop a fuller understanding of short-term weathering rates of shales.
Project Details
Project type
Large-Scale EMSL Research
Start Date
2010-10-06
End Date
2013-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
Related Publications
Sanders RL, NM Washton, and KT Mueller. 2012. "Atomic-level studies of the depletion in reactive sites during clay mineral dissolution." Geochimica et Cosmochimica Acta 92:100-116. doi:10.1016/j.gca.2012.05.038