Thermodynamics and phase equilibria in the complex acid mine drainage systems

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Here I propose to study thermodynamics and phase equilibria in the complex systems relevant to acid mine drainage, a common and widespread environmental problem. The systems to study and the minerals of interest for this project are K2O-MgO-FeO-ZnO-Fe2O3-Al2O3-SO3-H2O and the voltaite-group of minerals; MgO-Fe2O3-SO3-H2O and the minerals magnesiocopiapite and botryogen; Fe2O3-SO3-H2O and the minerals ferricopiapite, paracoquimbite, lausenite, kornelite, rhomboclase, and the phase (H3O)Fe(SO4)2. The thermodynamic properties of these phases will be determined by calorimetric experiments: enthalpies of formation by acid-solution calorimetry and heat capacities and entropies by relaxation calorimetry. The phase equilibria will be studied by charge experiments, i.e., the chemical and crystallographic investigation of coexisting liquids and solids, respectively. In this work, the equilibrium will be reached from both supersaturation and undersaturation, thus crosschecking the reliability of the solubility data. The acquired data for rhomboclase and the phase (H3O)Fe(SO4)2 will be optimized by mathematical programming, an approach that critically assesses available thermodynamic data and phase equilibria. The results of the calorimetric and charge experiments for the system Fe2O3-SO3-H2O will be critically compared using an extended Pitzer-model for concentrated Fe(III)-SO4 solutions (Tosca et al. 2007). In the system MgO-Fe2O3-SO3-H2O, we will perform forward simulations to explore the paths of evaporating Mg-Fe(III)-SO4 solutions and determine the conditions under which the different minerals form (thermodynamic modeling). For all systems, we will construct activity-activity diagrams and compare the results to the observations in nature.