Gypsum - CaSO<sub>4</sub>2H<sub>2</sub>O - 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, significantly exceeding 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. Vibrational sum frequency generation (VSFG) spectroscopy experiments show that the anionic surfactant SDS (sodium dodecyl sulfate) adsorbs to form 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. Conductometric titration supports these findings and show that SDS solutions suppress gypsum dissolution by ~40% relative to DTAC and pure water solutions. Additional evidence of differences between SDS and DTAC adsorption comes from EDX images. EDX mapping shows that SDS was uniformly able to cover the surface, but DTAC adsorption only seems to happen at rough areas on gypsum surface forming patches and leaving large expanses of the gypsum exposed to solution. These findings imply that the adsorbed SDS film disrupts water structure at the interface and impedes loss to dissolution.

Surfactant adsorption to gypsum surfaces and the effects on solubility in aqueous solutions

ACS Spring 2022

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