Student Theses

Date of Award

Fall 9-1-2021

Document Type

Thesis

Degree Name

Master of Arts (MA)

Department

Earth Science

Language

English

First Advisor

Jacquelyn Bracco

Abstract

Abstract

Ion desorption processes control the retention of ions, such as Pb2+, at barite surfaces, whereas ion adsorption processes are a likely initial step in the incorporation of divalent cations, such as Sr2+ and Pb2+, into barite. Based on previous studies on samples dominated by flat terraces with relatively few defects, Sr2+ and Pb2+ adsorb and incorporate at the barite (001)-water interface (Bracco et al., 2019; Bracco et al., 2020). Here, I measured how susceptible lead (Pb) is to desorption and the role surface variability plays in its sorption behavior. First, I measured desorption of Pb at the barite (001) - water interface using in situ X-ray reflectivity (XR) to understand the effects of Pb on the barite (001) surface structure and resonant anomalous X-ray reflectivity (RAXR) to resolve the amount of sorbed and desorbed Pb. My results show that ~50% of the sorbed lead (Pb) species desorb after reacting the barite sample with Pb free solution (BSS) for 0.5 hr. Three additional measurements made in BSS indicate that amount of desorbed Pb increases over time Pb-free barite saturated solution (BSS). However, even after reacting for 12 hours, 31% of the sorbed Pb still remains. The desorption experiments suggest that at [Pb] ≥ 225µM incorporated species are resistant to desorption. While incorporation of Pb is a relatively fast process, the reverse process (exchange of Ba for incorporated Pb) is slow. The incorporation of Pb within the barite surface leads to stabilization of Pb, which is partially irreversible, further making barite an ideal host mineral for Pb sequestration.

To study how surface variability affects sorption, I then used in situ x-ray reflectivity to measure sorption of Sr2+ and Pb2+ as a function of two parameters correlated with the number of steps on a mineral surface: surface domain size and the Robinson roughness parameter (β). A variety of samples were measured in situ, including cleaved samples with varying degrees of surface roughness and samples grown in solutions supersaturated with respect to barite to create steps on the surface. Pb2+ sorption was similar for all samples with β < 0.1. Similar analyses were made in the presence of Sr2+, but reacting the samples with solutions that increased the roughness decreased the sorption as compared with freshly cleaved samples. For both ions, there was little difference in the average position of the sorbed ions and the ratio of incorporated to adsorbed species for all samples. Together, these results emphasize parameters such as surface roughness and changing chemical conditions play an important role in controlling the extent of ion adsorption at mineral surfaces, which can affect differences in sorption measured on minerals in laboratory versus field settings.

Available for download on Sunday, August 28, 2022

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