Iron is a very common element in nature that typically form solid Fe-oxides as rock weather and Fe(II) oxidises to Fe(III). These Fe-oxides are typically nanoparticulate and therefore have a high surface area, where reactions occur between the minerals and compounds in solution.
We are interested in the behaviour of the Fe-oxides in the presence of Fe(II), both in the form of dissolved Fe(II) and as mixed-valent Fe-oxides such as green rust. In such systems, a number of of interesting redox reaction occur, including electron transfer from surface-bound Fe(II) to the Fe-oxides and reduction of a range of contaminants by electrons transferred from the Fe-oxide itself or from surface bound Fe(II).
Green rust is a reactive corrosion product. It is a layered double hydroxide, composed of Fe(II)/Fe(III) hydroxide sheets, that are separated by interlayers with water, anions, and, somtimes, cations. We have studied the structure, stability and reactivity for sulphate bearing green rust, which could occur during corrosion in the suphate rich ground waters at some of the potential sites for a high level Swedish radioactive waste repository.
Our laboratory studies show that 1) green rust sulphate contains interlayer cations, such as Na, similar to other layered double hydroxide with 11 Å basal plane distance. This impacts the thermodynamic stability of the compound, and will most likely affect the exchange kinetics of anions and confer cation exchange capacity to the green rust. 2) Na- and sulphate containing green rust reacts readily with Np and Se, which has a radioactive isotope that could be released should the repository fail. In addition, green rust undergoes topotactic transformation to the Fe(III)-oxyhydroxide goethite upon its oxidation by the contaminant chromate. In the process, chromate becomes reduced to Cr(III) and incorporated in the formed goethite. Studies on green rust conducted in the field identified carbonate bearing green rust in deep situated groundwater at the Äspö Hard Rock Laboratory.
In Sweden, the radioactive waste is to be stored at about 500 m depth, where conditions are anaerobic. The spent fuel rods will be placed in thick copper canisters, which corrode very slowly in the absence of oxygen. However, during the 100.000 year life time of the repository, glaciations may occur, which could drastically alter the flow conditions and introduce oxidising water in the deeper subsurface. To probe if prior glaciations have resulted in oxidation of the subsurface, a paleo-redox indicator was developed based on Fe-oxide mineralogy and their Fe isotope composition. In the first 100 m closest to the surface, Fe-oxides mineralogy and Fe isotope composition varied, indicating low-temperature genesis. In contrast, deeper situated Fe-oxides existed as hematite and showed little variation in Fe isotope composition. The results indicate that oxidising waters have penetrated to about 100 meters at the repository model site, the Äspö Hard Rock Laboratory.
– Dideriksen K., Christiansen B., Baker J., Frandsen C., Balic-Zunic T., Tullborg E., Morup S., and Stipp S.
Fe-oxide fracture fillings as a palaeo-redox indicator: Structure, crystal form and Fe isotope composition. Chemical Geology, 2007, 244, 330-343.
– Dideriksen K., Christiansen B., Frandsen C., Balic-Zunic T., Morup S., and Stipp S.
Paleo-redox boundaries in fractured granite.
Geochimica et Cosmochimica Acta, 2010, 74, 2866-2880.
Green Rust Structure:
– Christiansen B., Balic-Zunic T., Dideriksen K., and Stipp S.
Identification of Green Rust in Groundwater. Environmental Science & Technology, 2009, 43, 3436-3441; http://dx.doi.org/10.1021/es8011047
– Christiansen B., Balic-Zunic T., Petit P., Frandsen C., Morup S., Geckeis H., Katerinopoulou A., and Stipp S. Composition and structure of an iron-bearing, layered double hydroxide (LDH) – Green rust sodium sulphate, Geochimica et Cosmochimica Acta, 2009, 73, 3579-3592; http://dx.doi.org/10.1016/j.gca.2009.03.032
– Christiansen B., Dideriksen K., Skovbjerg L., Nedel S., and Stipp S.
On Fougerite, Clays and Clay Minerals, 2011, 59, 3-9; http://dx.doi.org/10.1346/CCMN.2011.0590102
– Davesne E., Dideriksen K., Christiansen B., Sonne M., Ayala-Luis K., Koch C., Hansen H., and Stipp S. Free energy of formation for green rust sodium sulphate ((NaFe6Fe3III)-Fe-II(OH)(18)(SO4)(2(s))), Geochimica et Cosmochimica Acta, 2010, 74, 6451-6467.
Green rust interaction with trace elements, Cr, Se, Ce, Np:
– Skovbjerg L., Stipp S., Utsunomiya S., and Ewing R. The mechanisms of reduction of hexavalent chromium by green rust sodium sulphate: Formation of Cr-goethite, Geochimica et Cosmochimica Acta, 2006, 70, 3582-3592; http://dx.doi.org/10.1016/j.gca.2006.02.017
– Skovbjerg L., Christiansen B., Nedel S., Dideriksen K., and Stipp S.
The role of green rust in the migration of radionuclides: An overview of processes that can control mobility of radioactive elements in the environment using as examples Np, Se and Cr, Radiochimica Acta, 2010, 98, 607-612; http://dx.doi.org/10.1524/ract.2010.1760
– Nedel S., Dideriksen K., Christiansen B., Bovet N., and Stipp S.
Uptake and Release of Cerium During Fe-Oxide Formation and Transformation in Fe(II) Solutions, Environmental Science & Technology, 2010, 44, 4493-4498; http://dx.doi.org/10.1021/es9031503
Dideriksen K., Christiansen B., Baker J., Frandsen C., Balic-Zunic T., Tullborg E., Morup S., and Stipp S. Fe-oxide fracture fillings as a palaeo-redox indicator: Structure, crystal form and Fe isotope composition.
Christiansen B., Balic-Zunic T., Petit P., Frandsen C., Morup S., Geckeis H., Katerinopoulou A., and Stipp S. Composition and structure of an iron-bearing, layered double hydroxide (LDH) – Green rust sodium sulphate.
Skovbjerg L., Christiansen B., Nedel S., Dideriksen K., and Stipp S. The role of green rust in the migration of radionuclides: An overview of processes that can control mobility of radioactive elements in the environment using as examples Np, Se and Cr.