Abstract:
Soluble Fe(II) is an important reductant in anoxic surficial fluids, due to fast redox kinetics of the Fe(II)/Fe(III) couple. However, its availability is limited in neutral and alkaline pH range by the solubility of FeS(s), Fe3(PO4)2, FeCO3(s) and other Fe(II) rich minerals. The adsorption of Fe(II) on a variety of mineral phases has been studied. It is shown that, provided enough surface area is available, adsorption is completed before the onset of precipitation, leading to Fe(II) surface species which are able to reduce compounds present in solution in a very efficient way. The abiotic reduction of a variety of N rich compounds (nitrites and nitrobenzenes) by sorbed Fe(II) has been reported in the literature. The observed initial rate of such reduction reactions is shown to be proportional to the ≡FeIIIOFeIIOH° species concentration, in the same manner that the homogeneous oxygenation rate of Fe(II) is proportional to Fe(OH)2°(aq) concentration. The electron transfer in these reactions, appears to occur dominantly via an outer sphere mechanism. In contrast, the abiotic reduction of inorganics, such as U(VI) and Tc(VII), by sorbed Fe(II) involves inner sphere electron transfer mechanism. In the case of uranium reduction, three kinetic steps can be distinguished: the adsorption of the uranyl ion (formation of an inner sphere surface complex), followed by two reductive steps which lead to the formation of a UO2/Fe(OH)3 mixed solid phase. These surface-catalysed reduction reactions may have led to the formation of uranium mineral ores and to the removal of uranium from reducing surface waters.
