The kinetics of O-2(aq) reduction by structural ferrous iron in naturally occurring ferrous silicate minerals

摘要

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 Aspo, Sweden. These properties were experimentally studied using wet-chemical methods in order to assess the ability of these fracture fillings to consume 02(a,) 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 form Rate = k[O-2(aq)][Fe(II)(s)], where Fe(II), 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 [O-2(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.