Fe(II)-induced transformation of iron minerals in soil ferromanganese nodules

Abstract

Recrystallization of iron (hydr)oxides by ferrous iron [Fe(II)] is an important part of the biogeochemical cycling of iron in earth surface environments. Ferromanganese nodules are complex assemblages of natural Fe and Mn minerals, which are widely found in soils and marine environments, and critically impact the geochemical behavior of trace metals and nutrients. However, little is known regarding the Fe(II)-induced recrystallization of ferromanganese nodules, in comparison to pure iron (hydr)oxides. Accordingly, the objective of this study was to elucidate the reaction processes of aqueous Fe(II) with ferromanganese nodules, and to use complementary spectroscopic techniques to characterize the products. Goethite in the nodule that underwent phase transformation to magnetite or Fe(II) oxidation to form magnetite has not been reported in previous studies with respect to pure goethite. 57Fe-enriched isotope tracer experiments confirmed Fe atom exchange between aqueous Fe(II) and structural Fe(III) of the nodule and was enhanced at higher pH conditions. The coexistence of Mn oxides and other minerals possibly make the goethite in the nodule more reactive for phase transformation to magnetite than pure synthetic goethite. In addition, Fe(III) precipitates were formed due to Fe(II) oxidation by Mn(III) of the nodule, and magnetite was likely consequently produced through further Fe(II)-induced reaction. Considering the enrichment of trace metals and nutrients in ferromanganese nodules, the observed Fe(II)-induced recrystallization of nodules in the study is expected to exert important effects on the geochemical behavior of the elements, especially in nodule-enriched soils.