Effects of bound phosphate on the bioreduction of lepidocrocite (γ-FeOOH) and maghemite (γ-Fe₂O₃) and formation of secondary minerals

Natural Fe^III oxides typically contain a range of trace elements including P. Although solution phase and adsorbed P (as phosphate) have been shown to impact the bioreduction of Fe^III oxides and the formation of "biogenic" secondary minerals, little is known about the potential effects of occluded/incorporated phosphate. We have examined the bioreduction of Fe^III oxides (lepidocrocite (γ-FeOOH) and maghemite (γ-Fe₂O₃)) containing 0-3 mass% P as "bound" (a term we use to include both adsorbed and occluded/incorporated) phosphate. Kinetic dissolution studies showed congruent release of Fe and P, suggesting that the phosphate in these materials was incorporated within the particles; however, 53% or 86% of the total phosphate associated with the lepidocrocites containing 0.7 or 3 mass% P, respectively, was extracted with 0.1 M NaOH and can be considered to be adsorbed, both to exterior surfaces and within micropores. In the absence of phosphate, lepidocrocite was rapidly reduced to magnetite by Shewanella putrefaciens CN32, and over time the magnetite was partially transformed to ferrous hydroxy carbonate (FHC). The presence of 0.2-0.7 mass% P significantly inhibited the initial reduction of lepidocrocite but ultimately resulted in greater Fe ^II production and the formation of carbonate green rust. The bioreduction of maghemite with and without bound phosphate resulted in solid-state conversion to magnetite, with subsequent formation of FHC. We also examined the potential redox cycling of green rust under alternating Fe ^III-reducing and oxic conditions. Oxidation of biogenic green rust by O₂ resulted in conversion to ferric green rust, which was readily reduced back to green rust by S. putrefaciens CN32. These results indicate the potential for cycling of green rust between reduced and oxidized forms under redox dynamics similar to those encountered in environments that alternate between iron-reducing and oxic conditions, and they are consistent with the identification of green rust in soils/sediments with seasonal redox cycling.