5:15 PM - 6:45 PM
[MZZ45-P13] Redox reaction of Fe in smectite and its role in soil
Keywords:Smectite, Soil, Fe
The redox reactions of iron between Fe(II) and Fe(III) play an important role as electron acceptors and electron donors in the biogeochemical cycle, and this study focused on the redox reactions of Fe in paddy soils, where the redox state changes significantly during a year. The Fe in paddy soils can be largely divided into Fe (hydro) oxides and structural Fe in phyllosilicates. However, it is very difficult to capture the redox reactions of structural Fe(II)/(III) in phyllosilicates in the complex mixture system of paddy soils, therefore only a single redox state in l phyllosilicates ( e.g., Stucki, 2011; Neumann et al., 2011) or Fe (hydro) oxides in soils have been focused on individually. The aim of this study is to analyze in detail the variation of structural Fe(II)/Fe(III) ratios in phyllosilicates in paddy soils.
We mainly discussed analytical methods for Fe(II)/Fe(III) ratios in phyllosilicates. Linear Combination Fitting (LCF) of X-ray Absorption Fine Structure (XAFS) spectra including X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) was successfully employed for the Fe speciation by assuming 4 end members including (i) ferrihydrite, (ii) Fe(II) in unweathered biotite and (iii) Fe(III) in montmorillonite which represent Fe(II) and Fe(III) in phyllosilicate, respectively, and (iv) Fe2+ adsorbed on strongly acidic cation exchange resin that corresponds to outer-sphere complex of Fe2+ adsorbed on various adsorbents in the soil. The validity of the Fe speciation by XAFS was confirmed by other methods such as Mössbauer spectroscopy and sequential extraction techniques.
Paddy soil was incubated in a simulated paddy field soil system (microcosm), and the changes in the redox state of Fe species (dissolution of Fe(water) oxide and change in the valence of structural Fe in phyllosilicate (phy)) were analyzed in detail using the 4 endmembers determined in Chapter 2. The Fe(II)-phy/total phy increased dramatically during the first 2~3 days of incubation, indicating that most of the Fe(III)-phy with the potential to be reduced (about 15% of total Fe; an amount comparable to ferrihydrite, which is present in about 20% of total Fe) was reduced within 5~6 days. This indicated the importance of Fe(III)-smectite as a reducible Fe species in soils.
As an application of the development of the XAFS method for determining structural Fe(II)/Fe(III) ratios in phyllosilicate, the reactions that reduce structural Fe(III) in phyllosilicates in paddy soils were divided into biotic and abiotic reactions, and each was discussed. For biotic reactions, we focused on Fe reduction and nitrogen fixation by Fe-reducing bacteria in paddy soil reported in a previous study (Masuda et al., 2017, 2021) and investigated the availability of structural Fe(III) in the phyllosilicate in a pure culture system. From pure culture experiments of iron-reducing bacteria, it was confirmed that the iron-reducing bacteria Geomonas terrae utilizes structural Fe(III) in the phyllosilicate (SWy-2) as an electron acceptor and the nitrogen fixation activity was enhanced. In addition, the phyllosilicate adsorbs Fe2+ produced by the reduction of ferrihydrite, suggesting that it may sustain the availability of ferrihydrite by iron-reducing bacteria. For abiotic reactions, electron transfer between structural Fe(III) in phyllosilicates and Fe2+ (Schaefer et al., 2011; Neumann et al., 2013; Latta et al., 2017; Zhang et al., 2019) was analyzed. We obtained evidence using µ-XRF-XANES, normal XANES, and STEM that structural Fe(III) in phyllosilicates exchange electrons with adsorbed Fe2+, which itself is reduced and Fe2+ is oxidized to Fe (hydro) oxides such as ferrihydrite.
From the above, it is concluded that the redox reaction of Fe in the phyllosilicate structure in paddy soils contributes significantly to biological metabolism such as microbial respiration and nitrogen fixation, and to the adsorption of Fe2+ and precipitation of Fe (hydro) oxides such as ferrihydrite.
We mainly discussed analytical methods for Fe(II)/Fe(III) ratios in phyllosilicates. Linear Combination Fitting (LCF) of X-ray Absorption Fine Structure (XAFS) spectra including X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) was successfully employed for the Fe speciation by assuming 4 end members including (i) ferrihydrite, (ii) Fe(II) in unweathered biotite and (iii) Fe(III) in montmorillonite which represent Fe(II) and Fe(III) in phyllosilicate, respectively, and (iv) Fe2+ adsorbed on strongly acidic cation exchange resin that corresponds to outer-sphere complex of Fe2+ adsorbed on various adsorbents in the soil. The validity of the Fe speciation by XAFS was confirmed by other methods such as Mössbauer spectroscopy and sequential extraction techniques.
Paddy soil was incubated in a simulated paddy field soil system (microcosm), and the changes in the redox state of Fe species (dissolution of Fe(water) oxide and change in the valence of structural Fe in phyllosilicate (phy)) were analyzed in detail using the 4 endmembers determined in Chapter 2. The Fe(II)-phy/total phy increased dramatically during the first 2~3 days of incubation, indicating that most of the Fe(III)-phy with the potential to be reduced (about 15% of total Fe; an amount comparable to ferrihydrite, which is present in about 20% of total Fe) was reduced within 5~6 days. This indicated the importance of Fe(III)-smectite as a reducible Fe species in soils.
As an application of the development of the XAFS method for determining structural Fe(II)/Fe(III) ratios in phyllosilicate, the reactions that reduce structural Fe(III) in phyllosilicates in paddy soils were divided into biotic and abiotic reactions, and each was discussed. For biotic reactions, we focused on Fe reduction and nitrogen fixation by Fe-reducing bacteria in paddy soil reported in a previous study (Masuda et al., 2017, 2021) and investigated the availability of structural Fe(III) in the phyllosilicate in a pure culture system. From pure culture experiments of iron-reducing bacteria, it was confirmed that the iron-reducing bacteria Geomonas terrae utilizes structural Fe(III) in the phyllosilicate (SWy-2) as an electron acceptor and the nitrogen fixation activity was enhanced. In addition, the phyllosilicate adsorbs Fe2+ produced by the reduction of ferrihydrite, suggesting that it may sustain the availability of ferrihydrite by iron-reducing bacteria. For abiotic reactions, electron transfer between structural Fe(III) in phyllosilicates and Fe2+ (Schaefer et al., 2011; Neumann et al., 2013; Latta et al., 2017; Zhang et al., 2019) was analyzed. We obtained evidence using µ-XRF-XANES, normal XANES, and STEM that structural Fe(III) in phyllosilicates exchange electrons with adsorbed Fe2+, which itself is reduced and Fe2+ is oxidized to Fe (hydro) oxides such as ferrihydrite.
From the above, it is concluded that the redox reaction of Fe in the phyllosilicate structure in paddy soils contributes significantly to biological metabolism such as microbial respiration and nitrogen fixation, and to the adsorption of Fe2+ and precipitation of Fe (hydro) oxides such as ferrihydrite.