Date of Thesis

Summer 2019

Thesis Type

Masters Thesis (Bucknell Access Only)

Degree Type

Master of Science

Major

Chemistry

First Advisor

Molly McGuire

Second Advisor

Ellen Herman

Keywords

abandoned mine drainage, AMD, colloids, iron oxyhydroxides, natural organic matter, Shamokin Creek Basin

Abstract

Abandoned mine drainage (AMD) is a consequence of coal mining and is the result of the oxidation of sulfide minerals associated with the coal. AMD water is rich in iron oxyhydroxide colloids, which are capable of adsorbing trace metals and contaminants and transporting them in the environment. Previous studies have demonstrated that organic carbon may reverse the overall positive net charge of iron oxyhydroxide colloids, thereby stabilizing them. However, most studies on the relationship between organic carbon and iron-bearing colloids have been in the context of uncontaminated water systems, not AMD. To better understand if similar processes are at play in AMD, two longitudinal studies were performed in the Shamokin Creek watershed within the western middle anthracite coalfield in Pennsylvania. Organic carbon data have been compared in conjunction with dissolved iron, pH, dissolved oxygen, discharge, metal load, and conductivity field measurements. It was found that while the concentration of dissolved free ferrous iron decreases along the longitudinal flow path, the concentration of organic carbon remains relatively consistent, leading to the next question: how much iron is associated with organic carbon and being transported along the longitudinal path? Samples have been collected and preserved for total iron analyses to aid in answering this question. Although the mineralogy of bulk AMD sediment has been previously characterized as a mixture of various iron oxyhydroxides, the mechanism of sediment deposition is unclear as competing homogeneous and heterogeneous processes may be involved. Dissolved ferrous iron in AMD is oxidized upon mixing with surface water and can form suspended colloidal iron oxyhydroxide precipitates that may aggregate and deposit to the streambed. Additionally, heterogeneous nucleation of iron oxyhydroxide precipitation may occur directly on the stream bed. To investigate the role of bed materials on sediment deposition, substrates pre-characterized with two-dimensional Raman mapping were placed in AMD impacted streams and allowed to collect sediment for a period of days. Substrates with varying mineral composition were selected to explore whether common minerals – or specific anomalies on their surfaces – provide nucleation sites for the heterogeneous precipitation of AMD sediment. After collection, the surfaces were re-mapped to assess the influence of common minerals found at AMD sites on the nucleation of iron oxyhydroxides. Similar peaks were observed in the Raman spectra of the deposited AMD sediment on the surface of all substrates at each individual site, indicating no mineral preference among substrates analyzed for heterogeneous precipitation. The same type of AMD sediment, ferrihydrite, was observed at three sites that varied in pH, indicating that the pH in this range may not play a role in influencing the type of AMD iron oxyhydroxide that precipitates. Homogeneous precipitation processes were encouraged by surface roughness and dominated across all the substrates used at each site.

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