Vibrational sum frequency generation (VSFG) spectroscopy, conductometric titration measurements, and EDX elemental mapping were used to examine surfactant adsorption to the gypsum (010) surface and assess the effects of surfactant adsorption on gypsum solubility in aqueous solutions. Sodium dodecyl sulfate (SDS) and dodecyltrimethyl­ammonium chloride (DTAC) were used as anionic and cationic surfactants, respectively. Gypsum/SDS interactions result in an ordered precipitate layer on the gypsum surface after water evaporation; gypsum/DTAC interaction did not show a similar effect, despite exposure of gypsum to equivalent amounts of surfactant. VSFG spectra showed that SDS molecules adsorb with their chains parallel to the gypsum surface; spectra from gypsum surfaces treated with DTAC, however, showed no measurable response, implying that these surfactants form disorganized aggregates with no polar ordering. Vibrational data were supported by independent EDX measurements that show a uniform distribution of SDS across the gypsum surface. In contrast, element-specific EDX images showed that DTAC clustered in tightly localized patches that left most of the gypsum surface exposed. The uniform adsorption of SDS on the gypsum surface suppresses long-term dissolution up to 40% when compared to samples exposed to DTAC. Gypsum samples in DTAC-containing solutions lose approximately the same amount of material to dissolution as samples immersed in pure water.

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Conductometric titrations were used to measure sulfate concentrations in ground and surface water samples taken from land reclaimed after open-air coal mining. Sulfate concentrations ranged from 460 mg/L in surface water upstream of the former coal mine’s location to almost 3500 mg/L in groundwater sampled at the spoil site. Data from the titration measurements were benchmarked against EPA-approved ion chromatography (IC) measurements and results agreed to within ±3.6% (averaged over 36 samples) with a range of +10.4 and −11.3%. To test the generality of conductometric titration as a method for measuring dissolved constituents in environmental aquatic systems, additional measurements testing for chloride were performed with surface water samples collected from four different sites in south central and southwest Montana. Chloride concentrations ranged from 2.2 to 12 ppm. Based on measurements with control samples prepared in the laboratory, the environmental sample measurements are believed to be accurate to within ±6.4%. These conductometric titration studies highlight the technique’s simplicity, accuracy, cost effectiveness, and potential to produce rapid results. Additional analyses suggest that even simpler, non-species-specific conductivity data can provide an on-site, rapid assessment of sulfate levels in ground and surface water when historical speciation data are available.