Thermodynamics and phase equilibria in the complex acid mine drainage systems
Final Report Abstract
In this project, we have derived internally consistent thermodynamic data for several key sulfates that control solubility and mobility of elements in the acid mine drainage systems. These data can be incorporated into numerical models that simulate formation of acid mine drainage. The focus of the work was the calorimetric determination of enthalpies of formation, low-temperature heat capacities and entropies that could be combined into Gibbs free energies of formation and solubility products. A data set for the acidic sulfates rhomboclase [(H5O2)Fe(SO4)2·2H2O] and the phase (H3O)Fe(SO4)2 was derived and the results tested against simple and complex (Pitzer) activity-molality models. The Pitzer model, even though in its extended form, does not perform satisfactorily and should not be used for modeling. Further NMR experiments and ab initio calculations elucidated the details of the crystal structures of these two phases. Thermodynamic sets were derived also for minerals from the copiapite- [(Mg,Fe2+,Fe3+2/3,Al2/3)Fe4(SO4)6(OH)2·20H2O] and voltaite [K2Fe5 2+Fe3 3+Al(SO4)12·18H2O] group and for botryogen MgFe(SO4)2(OH)·7H2O. Laboratory experiments defined the compositions of the aqueous solutions co-existing with these minerals and help to support the thermodynamic data, if appropriate activity-molality models for such solutions exist. Furthermore, we addressed the kinetics of conversion of the X-ray amorphous ferrihydrite into crystalline iron oxides. Ferrihydrite with different loads of arsenate and phosphate shows significant deceleration of the transformation rate. Such rates should be taken into considerations when the binding ability of acid mine drainage in time is being predicted.
Publications
- Crystal chemistry, Mössbauer spectroscopy, and thermodynamic properties of botryogen. Neues Jahrbuch für Mineralogie, Abhandlungen 2016, 193/2, 147-159
Juraj Majzlan, Jakub Plášil, Edgar Dachs, Artur Benisek, Christian Bender Koch
(See online at https://doi.org/10.1127/njma/2016/0299) - Thermodynamics and crystal chemistry of rhomboclase, (H5O2)Fe(SO4)2·2H2O, and the phase (H3O)Fe(SO4)2 and implications for acid mine drainage. American Mineralogist 2017, 102, 643-654
Juraj Majzlan, Klaus-Dieter Grevel, Boris Kiefer, Ulla Gro Nielsen, Elisabeth Grube, Edgar Dachs, Artur Benisek, Mary Anne White, Michel B Johnson
(See online at https://doi.org/10.2138/am-2017-5909)
