| Since the cessation of open-pit mining activities from the Berkeley Pit located in Butte, MT in 1982, the pit had gradually filled with metal-laden acidic mine water. The purpose of this investigation was to estimate and apply thermodynamic theory to a group of iron-sulfate solids that might equilibrate with the Berkeley Pit water. Thermodynamic data was incomplete for acid mine affected water, and the effects of Cu cementation on the formation of iron oxide and sulfate solids had not been analyzed. Ferric iron compounds such as Ferrihydrite, Goethite, Schwertmannite, Potassium Jarosite and Potassium Hydronium Jarosite were modelled in Stabcal using variables such as temperature, Eh from field records or from a redox couple, pH and chemical composition. Water quality records were analyzed from field data collected by the MBMG since 1987. Computation of saturation indices using log Q data showed that the Berkeley pit water was under-saturated with Ferrihydrite and over-saturated with Goethite and K-Jarosite. In addition, commonly expressed variables for thermodynamic data, such as dG25C, S25C, Cp using the Maier Kelley equation, were estimated by using a weighted multiple regression model for Schwertmannite, KH-Jarosite, and K-Jarosite.;Schwertmannite had a dG25C = -978 kcal, S25C = -6049 cal, H25C = -2964 kcal and Cp values of [a = 161273488, b = -378690560, c = -43316576]. Schwertmannite also had a dGrex at 25°C= -95.794 kcal. KH-Jarosite had a dG25C =-786 kcal, S25C = 5118 cal, H25C = 586 kcal and Cp values of [a = -145717088, b = 342660160, c = 39022976]. K-Jarosite had a dG 25C = -765 kcal/mol, S25C of -1139 cal, H25C = -1249 kcal and Cp values of [a = -46482960, b = 107543872, c = 112836016]. Thermodynamic quantities such as dGformation of the species, dGrex and log K from reactions involving Fe3+and Fe2+ ions were also tabulated. Based on the tabulated data, Eh-pH diagrams were constructed and a titration simulation was performed to determine the acidities of selected samples. An analysis of the effect of copper cementation on water chemistry over the last decade was also conducted. |
