Iron speciation in hydrothermal solutions - an experimental and theoretical study

Abstract

The speciation of iron(III) in hydrothermal solution was studied experimentally and theoretically using density-functional-theory calculations. UV-Vis spectrophotometric measurements were made on iron(III) solutions as a function of pH and chlorine concentrations at 25-200°C together with solubility and potentiometric measurements at 25°C. For the UV-Vis measurements, the strong ligand-to-metal charge transitions at wavelengths below 400 nm were used to obtain number of absorbing species, molar absorptivities (ε), and equilibrium formation constants using principle component analysis of the spectra. For the solubility and potentiometric measurements, the measured iron concentrations and pH values were fitted to a given speciation model including hydrolysis and chloro complexation. Density-functional-theory calculations of the ground state geometries were further conducted to constrain the iron(III) speciation. In acid chloride free solutions Fe3+, occupying octahedral coordination, hydrolysis to form FeOH2+, Fe(OH)2 +, Fe(OH)3(aq) and Fe(OH)4 - with increasing pH. With increasing chloride concentration and temperature and acid pH values, iron(III) chloride complexes become increasingly important forming FeCl2+, FeCl2 +, FeCl3(aq) and FeCl4 - with increasing chlorine concentration. The progressive addition of Cl- to the Fe3+ ion lead to complex geometry changes from octahedral to tetrahedral coordination of the FeCl4 - ion. The equilibrium formation constants derived here together with literature values were subsequently used to demonstrate the aqueous speciation of iron in hydrothermal solutions as a function of temperature, fluid composition (pH and Cl concentration) and redox state.