KINETICS OF PYRITE FORMATION BY THE H2S OXIDATION OF IRON(II) MONOSULFIDE IN AQUEOUS-SOLUTIONS BETWEEN 25 AND 125-DEGREES-C - THE MECHANISM

H2S acts as an oxidizing agent in natural systems and compares with mo lecular oxygen as an electron acceptor. The experimentally determined lowest unoccupied molecular orbital (LUMO) for H2S is -1.1 eV, which m eans that H2S can be an excellent electron acceptor. In the oxidation of Fe(II) monosulfide by H2S in aqueous solutions between 25 and 125 d egrees C, FeS + H2S(aq) = FeS2 + H-2(g) (where FeS is any Fe(II) monos ulfide, H2S(aq) is aqueous H2S, FeS2 is pyrite and H-2(g) is hydrogen gas), FeS is the electron donor (reductant) and aqueous H2S is the ele ctron acceptor (oxidant) and the product of the oxidation is H-2 gas. Because of the relative destabilization of H2S caused by the presence of an antibonding LUMO orbital in a significantly bent molecule, elect rons 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 H2S. The reaction is transport-controlled. The mean A rrhenius energy for the reaction is 33.7 kJ mol(-1). The Arrhenius ene rgy is temperature dependent, which is consistent with electroactive, colloidal FeS being the FeS reactant. MO calculations suggest that the reaction proceeds through a FeS --> SH2 intermediate. The intermediat e allows for the formation of an S-S bond, the breaking of H-S bonds w ith the formation of H-2 and the conversion of Fe(II) from high to low spin. The H-2 and FeS2 formed interact with adsorption of H-2 onto th e FeS2 surface. The reaction mechanism can be summarised 1. FeS(s) --> FeS(aq) (fast < ca. 100 degrees C > slow) 2. FeS(aq) + H2S(aq) --> (F eS --> SH2] (fast) 3. {FeS --> SH2} --> [FeS2 . H-2(occluded)] (fast) 4. [FeS2 . H((2(occluded)) --> FeS2(s) (slow < ca.100 degrees 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 day s of reaction at 125 degrees C suggests that growth is nutrient limite d. Observations show that initially, pyrite nucleates on the FeS but s ubsequently nucleation occurs on pre-formed pyrite crystals. Nucleatio n is rapid and kinetically favored over crystal growth leading to no s ignificant increase in crystal size as the reaction progresses. There is some evidence that the crystal growth mechanism is through screw di slocation growth. HS- cannot be an electron acceptor because of the hi gh positive calculated energy for the LUMO orbital (+8.015 eV). It is not possible to write an electronically balanced redox reaction involv ing HS- with FeS without the presence of an additional electron accept or. In contrast, it is possible at high pH values that the complex FeS H+ 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 fr om oxidized to reduced merely by changing pH, from H2S-dominant (oxidi zing) at pH < 7 to HS- dominant (reducing) at pH > 7. Copyright (C) 19 97 Elsevier Science