Galip Yiyen, CREWS graduate student with the Powder River Basin team, will defend his Ph.D. in Chemistry beginning with a seminar titled "Nonlinear Optical Studies of Gypsum Dissolution Mechanisms, Surfactant Adsorption on Gypsum Surface and Analysis of Environmentally Related Ions."
Gypsum - CaSO42H2O - is an earth abundant mineral and one of the primary sources of dissolved sulfate in environmental ground and surface waters. Sulfate concentration in groundwater near open-air coal mines can reach to 5000 mg/L, even from retired mines that have passed through the full reclamation process. These concentrations significantly exceed levels considered safe for human (250 mg/L) and livestock (3000 mg/L) consumption. In this research, surfactant adsorption and its effects on gypsum dissolution were examined, and field-applicable sulfate analysis techniques have been developed. Vibrational sum frequency generation (VSFG) spectroscopy experiments show that the anionic surfactant SDS (sodium dodecyl sulfate) adsorbs to gypsum surfaces and forms a highly ordered, anisotropic film; the cationic surfactant DTAC (dodecyl trimethylammonium chloride), in contrast, shows no net polar ordering implying that these surfactants form disordered aggregates on the gypsum surface. Conductometric titration results support these findings and show that SDS solutions suppress gypsum dissolution by ~40% relative to gypsum dissolution in DTAC-containing and pure water solutions. Additional evidence of differences between SDS and DTAC adsorption comes from EDX images. These findings imply that the adsorbed SDS film disrupts water structure at the interface and is responsible for inhibiting gypsum dissolution. Additionally, water structure at gypsum/air and gypsum/water interfaces were investigated with VSFG. At the gypsum/air interface results showed two separate water populations engaged into different intermolecular interactions. These populations were assigned to structural water molecules are tightly bound to gypsum surface. Finally, this research motivated the development of a quick and cost-effective water analysis technique based on conductometric titrations to measure sulfate concentrations in environmental water samples. Results were benchmarked against results from the same samples using widely accepted and federally approved water analysis measurement protocols. Results reported by these different techniques agreed to within 10%. This work contributed to the understanding of adsorption to mineral surfaces, which underpins the froth flotation and mineral recovery.
Research Advisor: Dr. Rob Walker