Nonlinear optical studies of gypsum dissolution mechanisms, surfactant adsorption on gypsum surface and analysis of environmentally related ions

Abstract

Southeastern Montana hosts one of the largest open-air coal mine sites in the world. Federal regulations after finalization of the open-air coal mining activities require reclamation, which creates buried spoils that may serve as aquifers at former mining sites. Once hosting an inland sea, the region's saline sedimentary rocks and soils contribute to the groundwater salinity, though at lower levels prior to mining. However, the creation of spoils through mining pulverizes soils, sediment, and rocks formerly overlying coal deposits, mixing them and increasing their surface area. In infiltrating waters from snow melt and heavy rain, minerals dissolve more readily due to this enhanced surface area in spoils, increasing the salinity in groundwater. A primary water quality concern in regional groundwater is high sulfate concentrations. Sulfate in water originates from weathering and dissolution of soluble secondary salts, such as calcium sulfate (dihydrate, hemihydrate and anhydrite), sodium sulfate and magnesium sulfate. Gypsum (CaSO 4 x 2H 2O) is thought to be a primary source of dissolved sulfate in the ground and surface waters in Southeastern Montana and 'gypsum dissolution' is at the focus of this dissertation. Vibrational sum frequency generation spectroscopy was used to understand the molecular level interactions at the gypsum surface upon interaction with bulk water. Additionally, surfactant adsorption on gypsum surface and its impacts of gypsum dissolution were investigated. Besides optical spectroscopy studies, an efficient and quick water analysis technique was adapted to determine the sulfate concentrations in environmental samples. Two different types of orientations of structural water molecules at the gypsum surface were observed. Results also showed that these water molecules are tightly bound to the surface. Surfactant adsorption was found to be only limited to the surfactants with sulfate headgroups and resulted in suppressed gypsum dissolution. A titration technique is found to be effective and accurate for sulfate analysis of environmental water samples.