Inner sphere Fe-OSO₃ coordination, iron(III) and sulfate hydration in aqueous solutions observed by polarized Raman...

The oxidative degradation of pyrite and other sulfide minerals in abandoned coal mines produces acid mine drainage, leading to contamination of natural water with high concentrations of iron(III) and sulfate. In aqueous environments, Fe(III) readily forms hydrated complexes and undergoes hydrolysis, resulting in complex speciation that complicates the chemistry of acidic sulfate waters. Thermodynamic speciation modeling reveals substantial formation of mixed ligand iron-sulfate and iron-bisulfate outer-sphere complexation and suggests only minor inner-sphere Fe-OSO₃ interactions. Yet, some studies suggest significant inner-sphere coordination. In this work we address this knowledge gap. Here we investigate ferric sulfate solutions (0.01-1.00 mol kg^-1) up to the solubility limit using polarized Raman spectroscopy. We investigate solid state and solution phase iron(III) sulfate to evaluate sulfate and hydration vibrational signatures using sodium sulfate and magnesium sulfate as reference salts. By analyzing sulfate stretching, sulfate bending, low frequency Fe-O modes, and the O-H stretching region, we identify clear spectral indicators of Fe-O-S coordination. The spectra reveal evidence of inner-sphere interactions between Fe³⁺ and SO₄^2-, observed as perturbations to the ν₁ symmetric stretching mode of sulfate. Specifically, the emergence of a concentration dependent peak near ∼1010 cm^-1, along with symmetry lowering features in isotropic and anisotropic spectra, corresponds to a blue-shifted component of the sulfate symmetric stretch, consistent with Fe-O-S coordination. This feature was distinguished from protonation of sulfate to form bisulfate through analysis of the ∼1040 cm^-1 band assigned to the symmetric S-O stretching vibration of HSO₄^-. Additional perturbations to the hydrogen-bonding network were observed in both the low frequency region, associated with metal-oxygen vibrations, and the high frequency region corresponding to O-H stretching modes of water. From this research, the identification of stable Fe(III)-sulfate complexation in aqueous solutions may provide important insight toward development of new methodologies for chemical degradation within acid mine drainage environments.