Raman spectroscopy of the system iron(III)–sulfuric acid–water: An approach to Tinto River's (Spain) hydrogeochemistry

Acid mine drainage is formed when pyrite (FeS2) is exposed and reacts with air and water to form sulfuric acid and dissolved iron. Tinto River (Huelva, Spain) is an example of this phenomenon. In this study, Raman spectroscopy has been used to investigate the speciation of the system iron(III)–sulfuric acid–water as an approach to Tinto River's aqueous solutions. The molalities of sulfuric acid (0.09 mol/kg) and iron(III) (0.01–1.5 mol/kg) were chosen to mimic the concentration of the species in Tinto River waters. Raman spectra of the solutions reveal a strong iron(III)–sulfate inner-sphere interaction through the ν1 sulfate band at 981 cm−1 and its shoulder at 1005 cm−1. Iron(III)–sulfate interaction may also be facilitated by hydrogen bonds and monitored in the Raman spectra through the symmetric stretching band of bisulfate at 1052 cm−1 and a shoulder at 1040 cm−1. Other bands in the low-frequency region of the Raman spectra are attributed to the hydrogen-bonded complexes formation as well.