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

Abstract

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 + H 2 S ( aq ) = FeS 2 + H 2 ( g ) (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 H bonds as an S 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 S bond, the breaking of H 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 1. FeS ( s ) → FeS ( aq ) ( fast < ca. 100 ° C > slow ) 2. FeS ( aq ) + H 2 S ( aq ) → { FeS → SH 2 } ( fast ) 3. { FeS → SH 2 } → [ FeS 2 ⋅ H 2 ( occluded ) ] ( fast ) 4. [ FeS 2 ⋅ H 2 ( occluded ) ] → FeS 2 ( s ) + H 2 ( g ) ( slow < ca. 100 ° C > fast ) The product pyrite forms as mixtures of individual cubes (up to 800 nm in size) and sub-spherical aggregates up to 1400 nm in diameter (“protoframboids”) on the surface of aggregates of particulate FeS. The lack of crystal growth observed in the pyrite products through 20 days of reaction at 125°C suggests that growth is nutrient limited. Observations show that initially, pyrite nucleates on the FeS but subsequently nucleation occurs on pre-formed pyrite crystals. Nucleation is rapid and kinetically favored over crystal growth leading to no significant increase in crystal size as the reaction progresses. There is some evidence that the crystal growth mechanism is through screw dislocation growth. HS − cannot be an electron acceptor because of the high positive calculated energy for the LUMO orbital (+8.015 eV). It is not possible to write an electronically balanced redox reaction involving HS − with FeS without the presence of an additional electron acceptor. In contrast, it is possible at high pH values that the complex FeSH + could react with HS − to form pyrite: however, the reaction is very slow and has not been observed experimentally. The net effect of these observations in natural systems is that reduced systems may change from oxidized to reduced merely by changing pH, from H 2S-dominant (oxidizing) at pH < 7 to HS − dominant (reducing) at pH > 7.