The kinetics of O2(aq) reduction by structural ferrous iron in naturally occurring ferrous silicate minerals

Batch reactor experiments were developed to measure the kinetic reactivity and the reduction capacity of fracture-filling solids collected from the Hard Rock Laboratory (HRL) at Äspö, Sweden. These properties were experimentally studied using wet-chemical methods in order to assess the ability of these fracture fillings to consume O2(aq) and thus provide a redox buffer against oxidising disturbances to a geological repository for spent nuclear fuel. Experimental data was described by a second-order rate law of the formRate=k[O2(aq)][Fe(II)s],where Fe(II)s refers to reactive structural Fe(II) sites hosted at the surface of fracture-filling minerals. Parameter values for the second-order rate constant (k, L mol−1 s−1) were estimated from time series data for [O2(aq)], decrease, and for existing data from previously published studies. The majority of the values of k were found to fall between those for the Fe mono-hydroxo (Fe(OH)+, log k = 1.4) and the Fe di-hydro (Fe(OH)2, log k = 6.9) complexes in solution. An existing free energy relationship for outer-sphere electron transfer kinetics for Fe(II) species oxidation in aqueous solution indicates that the corresponding redox potentials for structural Fe(II) sites are similar to values previously determined from thermodynamic considerations.