ORPIMENT SOLUBILITY EQUILIBRIUM AND ARSENIC SPECIATION FOR A HOT SPRING AT YELLOWSTONE NATIONAL PARK USING REVISED THERMODYNAMIC DATA

A persistent small hot spring in Norris Geyser Basin, Yellowstone National Park, Wyoming precipitates a bright orpiment-yellow clay that forms a layer about 2 centimeters thick. The water has been analyzed for all major and most trace elements including redox species of arsenic, iron, and sulfur. On-site measurements include temperature (43-53^oC), pH (3.3-3.4), specific conductance (1300-1700 microsiemen/cm), Eh (0.093-0.170 V), H₂S (3.5-4.25 mg/L), and As (0.28-0.38 mg/L). Water samples were collected through syringes and filtered through 0.2 micrometer pore size filters and preserved on site. The dissolved arsenic is ³90% As(III). The precipitate consists primarily of elemental sulfur and orpiment. The orpiment grains, identified by microprobe analysis, are up to 45 micrometers in size, but mostly are much finer grained. The water is an acid Na-Cl-SO4 type, but it has an anomalously low As/Cl ratio compared with most other hot springs in the Park, indicating removal of arsenic from the thermal water by precipitation. The analyses have been used to compute the speciation and saturation index (SI) for orpiment solubility using the recently revised arsenic thermodynamic data. Speciation calculations indicate that thioarsenites and arsenous acid predominate. Inclusion of thioarsenites reduces substantially (orders of magnitude) the activity of arsenous acid and the orpiment ion-activity product. The SI for orpiment ranges from -0.4 at 43^oC to -1.5 at 53^oC. Increased undersaturation at the higher temperature indicates that reaction enthalpies may be in error because the water composition is remarkably consistent from one sampling time to the next. The enthalpy values most suspect are those for the thioarsenite species, but the reaction enthalpy for the orpiment Ksp also could be in error.