MAIA - Microbes for Archaeological Iron Artworks

Titre du projet

Description

Archaeological iron artefacts encounter serious post-excavation problems when contaminated with salts. In fact, once excavated, the exposure to a higher oxygen concentration and lower relative humidity renders the corrosion crust formed during burial not longer stable. In particular, the process is induced by chloride ions, present as an acidic ferric chloride solution, and leads to the formation of iron oxyhydroxides FeO(OH), specially akaganéite. Results of this ongoing corrosion can then been observed as flakes, cracks and finally loose of shape on the object. So far, the treatment adopted for the stabilization of archaeological iron artefacts is the use of alkaline immersion baths. This method is based on the slow diffusion of the chloride ions from the objects to the alkaline solution. The solution has to be changed regularly when the chloride ions concentration stops increasing. The treatment is then stopped after about 3 months when the chloride ions concentration is lower than 20 ppm in the solution. This approach is extremely labor- and time-consuming and large quantities of solution need to be neutralized and processed afterwards. Moreover, there is no evidence that chloride ions are not remaining in the object, as only the chloride ions in solution are measured. Therefore, the aim of this project is to develop and evaluate novel conservation methods, which could be more effectives than the desalination treatments currently used. In order to allow a better extraction of the chloride ions, we should consider two aspects during the treatment: to stop the corrosion, for example by removing oxygen or using alkaline pH, and to increase the porosity of the corrosion crust with the formation of low molar volume compounds. We propose here to exploit the unique properties of some microorganisms for the stabilization of archeological iron. To this purpose, three different strategies will be adopted either leading to the formation of stable compounds of low molar volume or using chloride-translocation properties. First, we will test some species of fungi that have been reported for their ability to transform metal compounds into metal oxalates, known to be chemically stable compounds of low molar volume. We already obtained promising results with the precipitation of iron oxalates by Beauveria bassiana. We observed also that this strain is able to grow in alkaline conditions that permit the passivation of the iron surface during treatment. However, many aspects of the possible application still need to be carefully studied. Therefore, we will carry on our investigation in order to better understand the mechanisms involved into the precipitation of iron oxalates and also optimize the application procedure on corroded iron standards. The same approach will be exploited to precipitate magnetite, another very stable compound of low molar volume. We propose here to evaluate B. bassiana and other microorganisms that have been reported in the literature for the biosynthesis of magnetite nanoparticles. In particular, we would like to explore the possibilities offered by the fungal strains Fusarium oxysporum or Verticillium sp. but also extend the study to iron-reducing and magnetotactic bacteria, which also form magnetite under anaerobic and neutral conditions. Instead of use pH > 9 to stop corrosion, these bacteria will allow us to work at neutral pH without corroding the object as oxygen will no longer be present. Finally, in order to enhance the removal of chloride ions outer the iron object, we propose to test the possible translocation of the chloride ions by fungi. In particularly chlorine-rich environments, some species of fungi develop strategies for detoxify their surrounding environment from the toxic chlorine. These halophiles can be isolated and cultured for example from natural places such as salt lakes. In addition, white-rot fungi are also studied in bioremediation for allowing chlorine migration. Based on the results achieved, we could contribute to the development of a synergetic microbial consortium specially designed for the removal of chloride ions and the simultaneously formation of stable iron compounds. Particular attention will be devoted to the efficiency and impact on metallographic structure of the proposed treatment to overcome the problems associated with the treatments in use nowadays. Real samples will be also included in the studies in order to validate the new methodology. This research issue presents innovative aspects in biogeochemistry of micro-organisms and conservation science. A key point for its successful achievement is the creation of an interdisciplinary research partnership, which brings together experts from the fields of chemistry, microbiology and metal conservation.

Chercheur principal

Joseph, Edith

Comensoli, Lucrezia

Kooli, Wafa

Statut

Completed

Date de début

1 Janvier 2013

Date de fin

31 Décembre 2015

Organisations

Institut de biologie