KINETICS OF PYRITE FORMATION BY THE H2S OXIDATION OF IRON (II) MONOSULFIDE IN AQUEOUS SOLUTIONS BETWEEN 25 AND 125OC: THE RATE EQUATION

Abstract

A kinetic study of the reactionFeS(s) + H2S(aq) = FeS2 + H2(g) where FeS is precipitated FeS, H2S(aq) is aqueous H2S, FeS2 is pyrite, and H2(g) is hydrogen gas, shows that the rate between 25 and 125°C can be described by the equationdFeS2/dt = k(FeS) (cH2S(aq)) where k the second order rate constant varies between 1.03 x 10-4 L mol-1 s-1 at 25°C and 3.20 x 10-3 L mol-1 s-1 at 125°C. The rate constant shows a sigmoidal temperature dependence with an average Arrhenius activation energy of 35 kJ mol-1.The reaction is surprisingly fast at ambient temperatures with up to 50% reaction being completed within one day. The direct dependence of the rate on cH2S(aq) means that the rate is pH dependent for any fixed total sulfide concentration. In typical sulfidic aquatic systems and sediments 9 x 10-13 to 9 x 10-8 mol FeS2 per L sediment will be formed each day by this process. This is equivalent to approximately 3 x 10-10 to 3 x 10-5 mol FeS2 per g sediment per year. At pH = 7, for the same total sulfide and FeS constraints, the rate of pyrite formation 1.5 x 10-9 to 1.5 x 10-4 mol FeS2 per g sediment per year. In hydrothermal systems, such as deep ocean vents, the rate of pyrite formation by oxidation of FeS by H2S at 125°C assuming a typical H2S concentration of 1 mM is 3.2 x 10-6 mol L-1 s-1 per mol FeS. A 1 million tonne pyrite deposit could form from a solution containing 1 mmol FeS and 1 mmol H2S by this process in 1000 years at a flow rate of 0.3 Ls-1. The fluid would have a H2 concentration of 3 x 10-9 M.The process is by far the most rapid of the pyrite-forming reactions hitherto identified. Alternative pyrite-forming processes involving HS-, rather than H2S, as the reaction requires an additional oxidising agent to maintain electron balance. These pathways may involve reactants such as polysulfides or intermediaries such as greigite, Fe3S4. In natural systems, therefore, the H2S process will tend to be favored in strictly anoxic environments. In transitional environments, with limited molecular oxygen contents, pyrite formation through the polysulfide pathway or a solid state process may become more important. However, the generally low concentrations of polysulfides and greigite observed in natural pyrite-forming environments might suggest that these processes are always subordinate. The fast nature of the H2S oxidation process suggest that it makes a significant contribution to the global sulfur cycle.The process provides a straightforward explanation of a number of observations regarding pyrite formation in natural systems, including pyrite formation in strictly anoxic environments and rapid pyrite formation. Hydrogen generation through this process in aquatic and sedimentary systems provides an important alternative metabolite for microorganisms.