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Workreport 2018-30

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Name:

Microbial Release of Iron from Olkiluoto Rock Minerals

Writer:

Johansson, L., Stahlén, J., Taborowski, T., Pedersen, K., Lukkari, S., Marmo, J.

Language:

English

Page count:

110

Summary:

The ferrous iron (Fe2+) and dissolved sulfide contents in Olkiluoto groundwater samples show an inverse relationship, which supports the significance of iron sulfide precipitation in limiting the concurrent dissolved occurrence of these species. The objective of this study was to investigate if iron silicate minerals that include ferric iron (Fe3+), and are found in Olkiluoto, can serve as electron acceptors for iron reducing bacteria (IRB). The project was planned and executed in four steps over a three year period from 2015 to 2017. The first step comprised laboratory bottle experiments with different minerals, electron donors and groundwater samples from several drillholes in the underground rock characterisation facility ONKALO in Olkiluoto. The second and third steps included pressure cell experiments mimicking in situ conditions where minerals were exposed to groundwater bacteria for attachment and growth. The final step investigated the effect of sulfide on iron silicate minerals in the laboratory.

Biotite, garnet, and chlorite concentrates, used in this study, have been prepared from migmatised gneiss and chlorite schist. These minerals are the major iron carriers in the rock. The samples were provided by Posiva and derived from ONKALO, Olkiluoto. For secondary crushing, high voltage pulsing was applied using the Swiss SELFRAG technology. It recovered fairly coarse and clean, monomineralic particles for further laboratory work. Recovery of the final mineral concentrates was based on sieving, heavy liquid separation and various magnets. Mineral and chemical compositions, elements distribution and mineral liberation of the concentrates were determined. Also Fe(II)/Fe(III) ratios of the concentrates were analysed. The quality of the concentrates was feasible from the bacterial leaching point of view due to the fact that biotite, garnet, and chlorite show no significant mixing with each other between mineral specific concentrates. The content and liberation percentage for each mineral concentrates are as follows: biotite 82/50, garnet 96/87 and chlorite 49/28. Mössbauer measurements gave a result of 85.5 % ferrous component for both biotite and chlorite and a full 100% for garnet. However, some Fe oxide pigment and/or inclusion in garnet grains are possible.

In the laboratory bottle experiments, bottles were loaded with the minerals garnet or biotite, a cultivation medium was added with various electron donors and then they were inoculated with groundwater collected from 5 drillholes in ONKALO, representing different groundwater chemistries. Several bottles were also prepared with ferrihydrite (FeOOH) which has been used for cultivation of IRB from Olkiluoto samples. The bottles were incubated at 18 °C, and after 28 and 56 days the amounts of Fe2+ and sulfide were measured.

To investigate if bacteria in ONK-PVA6 or ONK-KR15 groundwater can release Fe(III) from iron silicate minerals separated from Olkiluoto rocks and reduce it to Fe2+, two pressure cell experiments were performed. In the first experiment, pressure cells were loaded with garnet, biotite, or chlorite, and connected in series with sulfate-rich ONK-PVA6 groundwater circulating from a packed-off section to allow attachment and microcolony formation by bacteria on the minerals before electron donors were added in the laboratory. The pressure cells were incubated at 20 °C and sampled after 8−9 weeks. The second pressure cell experiment followed the layout of the first experiment, but instead of using ONK-PVA6 groundwater, the pressure cells were connected in series with sulfate-poor ONK-KR15 groundwater. The pressure cell experiments indicated that sulfide per se can influence the release of Fe2+ from Fe(III) in garnet, biotite, and chlorite. These minerals were exposed to sulfide under bacteria-free laboratory conditions and the effect on release of Fe2+, the loss of free sulfide, and the formation of elemental sulfur was investigated.

There were differences in the amount of released Fe2+ as a function of offered carbon and energy sources. Short organic acids and H2 resulted in the highest amounts of Fe2+, of which some was precipitated as FeS. There were also differences in the amount of released Fe2+ in laboratory bottles as a function of inoculation groundwater. All three tested minerals extracted from Olkiluoto rock supported microbial iron reduction, in which cultures with garnets released larger amounts of dissolved Fe2+ than biotite and chlorite, while biotite had higher amounts of acid-extractable Fe2+. Sulfate reduction occurred in the sample series that were inoculated with groundwater known to contain sulfate reducing bacteria, if sulfate was available. The results show two possible mitigation processes of sulfide in groundwater: 1) iron reducing bacteria use, for example, acetate and/or H2 to reduce Fe(III) in iron silicate minerals to Fe2+ that precipitate with sulfide; 2) sulfide reacts with Fe(III) in minerals and form sulfur and Fe2+ which precipitates with excess sulfide.

Keywords:

Ferrous iron, sulfide, biotite, garnet, chlorite, iron reduction, bacteria, groundwater

File(s):

WR 2018-30_web (pdf) (23.8 MB)


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