Kinetics of pyrite formation by the H 2S oxidation of iron (II) monosulfide in aqueous solutions between 25 and 125°C: The mechanism

H ₂S acts as an oxidizing agent in natural systems and compares with molecular oxygen as an electron acceptor. The experimentally determined lowest unoccupied molecular orbital (LUMO) for H ₂S is -1.1 eV, which means that H ₂S can be an excellent electron acceptor. In the oxidation of Fe(II) monosulfide by H ₂S in aqueous solutions between 25 and 125°C, FeS+ S=FeS2₊ (where FeS is any Fe(II) monosulfide, H ₂S _(aq) is aqueous H ₂S, FeS ₂ is pyrite and H _2(g) is hydrogen gas), FeS is the electron donor (reductant) and aqueous H ₂S is the electron acceptor (oxidant) and the product of the oxidation is H ₂ gas. Because of the relative destabilization of H ₂S caused by the presence of an antibonding LUMO orbital in a significantly bent molecule, electrons added to this LUMO orbital cause a weakening of both S sbnd H bonds as an S sbnd S bond forms. This allows the hydrogen atoms to combine to form H ₂ because of their proximity and favorable interaction based on the original LUMO of H ₂S. The reaction is transport-controlled. The mean Arrhenius energy for the reaction is 33.7 kJ mol ^-1. The Arrhenius energy is temperature dependent, which is consistent with electroactive, colloidal FeS being the FeS reactant. MO calculations suggest that the reaction proceeds through a FeS → SH ₂ intermediate. The intermediate allows for the formation of an S sbnd S bond, the breaking of H sbnd S bonds with the formation of H ₂ and the conversion of Fe(II) from high to low spin. The H ₂ and FeS ₂ formed interact with adsorption of H ₂ onto the FeS ₂ surface. The reaction mechanism can be summarised