Birnessite Reduction by Iron(II) Organic Complexes.
Jonathan L. Edwards and Christopher J. Matocha. Univ of Kentucky, N-122G Agricultural Science North, Lexington, KY 40546-0091
Reduction and oxidation of iron and manganese may proceed by various pathways in soil environments. Manganese(III,IV) oxide minerals are known to be reduced by dissolved iron(II). However, little is known about interactions of dissolved iron(II)-organic complexes with manganese(III,IV) oxide minerals. The objective of this study was to investigate the reactivity of a dissolved iron(II)-organic complex and uncomplexed (hexaaquo) iron (II) with birnessite, a layer type manganese oxide mineral. Salicylic acid was chosen as the model organic ligand. Birnessite was synthesized (average oxidation state of 3.47) and used in stirred-batch reactions. Experiments were performed at ten degree Celsius, pH 4, one mmol/L dissolved iron(II), and two birnessite concentrations (0.05 and 0.25 g/L). Birnessite reduction by hexaaquo iron(II) was rapid, characterized by approximately 30% loss of iron(II) from solution in one minute. Dissolved manganese release during hexaaquo iron(II) oxidation was non-stoichiometric at both birnessite concentrations, in agreement with previous studies. This may be attributed to secondary precipitation reactions or manganese readsorption to unreacted birnessite. The reduction of birnessite by dissolved iron(II)-salicyclic acid complexes was more rapid, with a 90% loss of iron(II) from solution in one minute. In addition, dissolved iron(II)-salicyclic acid experiments resulted in stoichiometric release of manganese to solution. The presence of salicyclic acid stabilized iron(III) in solution when compared with the hexaaquo iron(II) experiments. Coordination of oxygen atoms in salicyclic acid to iron lowers the reduction potential of the iron(II)-iron(III) couple, making iron(II) a better reductant than hexaaquo iron(II). This explains the greater reactivity of dissolved iron(II)-salicyclic acid complexes. Additional kinetic experiments will be performed to elucidate the mechanism. An important consequence of iron(II)-organic complex oxidation by manganese(III,IV) oxide minerals is that the resulting dissolved Fe(III)-organic complexes could behave as kinetically labile oxidants.