Protection of metal artifacts with the formation of metal–oxalates complexes by Beauveria bassiana
, Joseph, Edith, Cario, Sylvie, Simon, Anaële, Wörle, Marie, Mazzeo, Rocco, Junier, Pilar, Job, Daniel
Several fungi present high tolerance to toxic metals and some are able to transform metals into metal-oxalate complexes. In this study, the ability of Beauveria bassiana to produce copper oxalates was evaluated. Growth performance was tested on various copper-containing media. B. bassiana proved highly resistant to copper, tolerating concentrations of up to 20 g L-1, and precipitating copper oxalates on all media tested. Chromatographic analyses showed that this species produced oxalic acid as sole metal chelator. The production of metal-oxalates can be used in the restoration and conservation of archeological and modern metal artifacts. The production of copper oxalates was confirmed directly using metallic pieces (both archeological and modern). The conversion of corrosion products into copper oxalates was demonstrated as well. In order to assess whether the capability of B. bassiana to produce metal-oxalates could be applied to other metals, iron and silver were tested as well. Iron appears to be directly sequestered in the wall of the fungal hyphae forming oxalates. However, the formation of a homogeneous layer on the object is not yet optimal. On silver, a co-precipitation of copper and silver oxalates occurred. As this greenish patina would not be acceptable on silver objects, silver reduction was explored as a tarnishing remediation. First experiments showed the transformation of silver nitrate into nanoparticles of elemental silver by an unknown extracellular mechanism. The production of copper oxalates is immediately applicable for the conservation of copper-based artifacts. For iron and silver this is not yet the case. However, the vast ability of B. bassiana to transform toxic metals using different immobilization mechanisms seems to offer considerable possibilities for industrial applications, such as the bioremediation of contaminated soils or the green synthesis of chemicals.
Assessment of a biological approach for the protection of copper alloys artefacts
2013, Joseph, Edith, Letardi, Paola, Junier, Pilar, Simon, Anaele, Comensoli, Lucrezia, Job, Daniel, Wörle, Marie
We propose an innovative treatment by which the protection of copper-based artefacts can be provided by naturally occurring microorganisms. The properties of some fungi were exploited for the transformation of existing corrosion patinas into copper oxalates. The latter are known to be insoluble and chemically stable. Within the earlier EU-ARTECH and BAHAMAS projects, very promising results were obtained with an almost 100% conversion from copper hydroxysulfates and hydroxychlorides into copper oxalates. A fungal strain was used which had been isolated from vineyard soil heavily contaminated with copper. Further scientific investigations were carried out to determine the parameters of the process and allowing the formation of a reproducible and homogeneous patina of copper oxalates, called biopatina to highlight its biological origin. Particular attention was paid to the efficacy, durability and impact on colour of the newly developed treatment. Different copper and bronze coupons with either urban or marine patinas were prepared. Several analytical techniques were used for the characterisation of the patinas: Fourier Transform InfraRed microspectroscopy (µFTIR), colourimetry and Electrochemical Impedance Spectroscopy (EIS). The coupons were treated with either the biological treatment or reference materials (wax: Cosmoloid H80; silane: Dynasylan® F8263) and exposed to atmospheric corrosion (ISMAR-SMS Genoa Harbour, corrosivity class 5) in December 2011. The long-term behaviour and performance of the treatments under study was monitored and compared over a one year period using the same complement of analytical techniques used for the characterisation of the original patinas. These first measurements suggested a different weathering behaviour of the biopatina. In fact, in comparison to the silane and wax treatments the biopatina showed a lower colour variation, and a corrosion stabilisation process seemed to be in progress. A deeper analysis of colour and corrosion rate variation from different application methods was also achieved. The complete assessment of the different treatments will be finished this year.