PhD Defence: Bo C. Christiansen

Cand. scient. Bo C. Christiansen will give a public lecture entitled: "Green rust: What is it? And what does it mean to us?" as part of the defence of his thesis: "Structure and reactions of GRNa, SO4 and the occurrence of GR in nature" followed by an oral defence. The defence is open to the public. The thesis was submitted in August 2008.

Evaluation Committee. 
• Professor Rod Ewing, Department of Geological Sciences, The University of Michigan, Michigan, USA.
• Professor Liane Benning, Earth and Biosphere Institute, School of Earth and Environment, University of Leeds, Leeds, United Kingdom. 
• Professor Klaus Bechgaard (Chairman), Department of Chemistry an Nano-Science Center , University of Copenhagen, Copenhagen, Denmark. 
 
Supervisors:
Professor Susan Stipp, Geochemistry, Research Leader, NanoGeoScience, Department of Chemistry and Nano-Science Center, University of Copenhagen, Copenhagen, Denmark.
Associate Professor Tonci Balic-Zunic, Department of Geography and Geology, University of Copenhagen, Copenhagen, Denmark.

Abstract

Pollution of our ground waters is an increasing threat to society. An ability to reduce and immobilise toxic elements by simple and cheap measures is thus of great interest. The ferrous-iron containing compound, green rust, is a good candidate for water treatment. The reactive compounds are easy to prepare and the end products, for example ochre, pose no threat to the environment. In this thesis, I report on the identification of green rust in nature, on its structure and on its reaction with redox-sensitive toxic compounds. It is essential to know the structure and composition of these compounds and to understand their formation and transformation processes, so model predictions about groundwater quality can be made reliably. In these studies, I showed that the sulphate containing green rust also contains a monovalent cation. This opens up for the possibility of exchange reactions of not only anions but also cations, as observed in normal clay minerals. The Fe(II) in the structure makes green rust very reactive toward redox-sensitive elements. My studies of the interaction of green rust with the contaminants neptunium (Np5+) and selenium (Se4+) showed that green rust can reduce both elements and immobilise them either by sorption or by reduction to a discrete phase. Green rust is difficult to observe in nature because it is only stable where O2 levels are low and when extracted without the greatest care it reacts faster than it can be analysed. I collected groundwater samples from Bornholm, Denmark and Äspö, Sweden and with special preparation, I could demonstrate that green rust was there. This proves the occurrence of green rust in groundwater systems and because of its reactivity, it is now clear how important it is to include it in groundwater modelling of contaminant mobilisation.