Reduction of Birnessite as Influenced by Iron(II) and Salicylate

Manganese (III,IV) (hydr) oxide minerals, and especially birnessite, have long been identified as important sorbents and oxidants of metals and metalloids in soil and sediment environments. Although the oxidation of dissolved ferrous iron (Fe(II)) concurrent with hexagonal birnessite reduction has been recognized for quite some time, fundamental kinetic data are lacking. In addition, the role of dissolved organic carbon (DOC) in affecting this process is unknown. In this study, stirred-batch kinetic experiments involving hexagonal birnessite reduction and dissolved Fe(II) removal were performed under anoxic conditions at pH 4 and 10º C in the presence and absence of salicylate as a model ligand for DOC. The reaction with Fe(II)-only shows a first-order dependence on both initial dissolved Fe(II) and effective birnessite surface site concentration, with an overall second-order rate coefficient (k) of 4.54 × 10−3 ± 8.6 × 10−4 Lm−2 min−1. The addition of salicylate to Fe(II) solutions increased the initial rate of dissolved Mn(II) production by 9.5- (p = 0.006) and 7-fold (p < 0.001) when compared with identical Fe(II)-only treatments. This is likely due in part to the effect of salicylate on the redox potential of the Fe(II)-Fe(III) couple. This study provides further insight into the role of birnessite in Fe(II) oxidation.