Formation of iron(II) secondary minerals after iron(III) bioreduction in humid tropical forest soils.
EMSL Project ID
11294
Abstract
In natural environments, Fe(III) (hydr)oxide reduction is mainly biotic and commonly related to the activity of dissimilatory iron-reducing bacteria (DIRB). These bacteria can reduce Fe(III) oxides either by direct contact between the organisms and the oxide surface or by indirect mechanisms not involving contact. These latter mechanisms can include (i) "electron shuttling" or (ii) soluble Fe(III) complexation with subsequent reduction. We are investigating Fe(III) reduction in humid tropical forest soils as affected by "electron shuttles," using anthraquinone-2,6-disulfonate (AQDS), a model compound which is frequently used to study mechanism (i). Anoxic incubations of soil suspensions amended with AQDS showed enhanced production of both soluble and particulate forms of Fe(II) as compared to non-amended soil suspensions (Figure 1). We also found that only about 10 % of the Fe(II) produced is in soluble form, most of it being particulate instead, evidently due to Fe2+ adsorption and the formation of Fe(II) solid phases. Thermodynamic calculations indicated that precipitation of siderite and vivianite was favored under anoxic conditions in the soil suspensions. However, X-ray diffraction analysis of residual solids after soil incubation was insufficient to detect Fe(II) secondary minerals. The application of Mossbauer spectroscopy is, therefore, necessary as an alternative method of detection.
Soil samples of < 2 mm particle size are used in our experiments. We shall determine the Fe mineralogy of the soil before incubation by analyzing the entire soil sample as well as soil samples after citrate-ascorbate and dithionite-citrate-bicarbonate extractions. These latter two extractions are used to remove poorly crystalline and total reducible Fe(III) (hydr)oxides, respectively. To evaluate the formation of Fe(II) secondary phases, we will analyze soils incubated with 0 and 150 μM AQDS, both non-extracted and 0.5 M HCl-extracted, which is adequate to extract Fe(II) produced in the soil suspensions.
Project Details
Project type
Exploratory Research
Start Date
2004-09-28
End Date
2007-03-22
Status
Closed
Released Data Link
Team
Principal Investigator