Biological oxidation of sulfur compounds in artificially degraded wood

Marine and lacustrine archaeological waterlogged wood encounters serious problems after excavation due to the accumulation of sulfur and iron compounds during burial. Exposure of these compounds to oxygen results in precipitation of salts and acidification, which can lead to serious structural damage, and ultimately the loss of important cultural heritage. In this study, we evaluated the capacity of the bacterium Thiobacillus denitrificans to transform sulfur compounds commonly found in waterlogged wooden objects, to more readily extractable compounds thereby eliminating the threat of degradation. Oak samples, impregnated with a solution containing iron(II) and sulfides, were used to assess the efficiency of the bacterial treatment. The model wood samples were characterized before and after treatment using different techniques such as ESEM-EDS, micro-Raman spectroscopy, XRD and Sy-XRF mapping. Before treatment, mackinawite (FeS) and mineral sulfur (alpha-S-8) were detected in the impregnated wood. After treatment with T. denitrificans, even though some mineral sulfur remained in the samples, the predominant phase corresponded to oxidized sulfur. This demonstrates that T. denitrificans was able to use the reduced sulfur compounds present in the wood samples as an energy source, thereby producing more soluble oxidized sulfur compounds. In addition, non-invasive techniques such as Fourier transform infrared (FTIR) spectroscopy, were carried out to assess the consequences of the biological treatment on the wood structure. No negative effect on the wood was detected after the treatment in comparison with the reference impregnated wood. This study demonstrates the feasibility of a biotechnological procedure for the preventive extraction of sulfur species from archaeological waterlogged wood.