Bioremediation of Cultural Heritage: Removal of Sulphates, Nitrates and Organic Substances

Abstract

Among the different pathologies of stone materials, the presence of sulphates and nitrates is the most frequent. In addition, the presence of organic matter on artistic stoneworks can be attributed to inadequate past restorations, to the lysis of microbial cells of the primary surface colonisation, and to the presence of hydrocarbons originating from oil combustion. The latter appears to be a serious danger for the preservation of artworks themselves. Until today, chemical and physical techniques have been largely used to remove pollutants and residual substances from works of art by using surfactants and solubilizing agents. We developed a multiple bioremediation system for the biological removal of sulphates, nitrates and organic matter present on artistic stoneworks utilizing microbial cultures carefully selected and grown on a suitable support. The study consisted of: screening of microorganisms in order to select cultures with a high sulphate-reducing, denitrifying and biodegradative ability of organic matter; setting up of simulated laboratory tests with stone samples artificially enriched with nitrates, sulphates and organic matter; testing appropriate inert matrices on which to immobilize the selected bacterial strains; and testing sulphate, nitrate and organic matter removal from artificially enriched stones as well as from naturally degraded artworks. Sepiolite was used to develop bacterial biofilms with a high active biomass per cm³. However, in order to eliminate or reduce interferences and the release of undesirable ions and contaminant trace elements, Hydrobiogel97, a mixture of polyacrylamide at different molecular weights, appears to be a good carrier. As regards nitrate removal, Pseudomonas aeruginosa and P. stutzeri were selected for their high denitrifying activity. Treatment with colonised sepiolite of stone specimens artificially enriched with nitrates and of real stone samples showed that, after 30 h, a very high percentage (90% and 88%, respectively) of nitrate was removed. For sulphate removal, Desulfovibrio vulgaris and D. desulfuricans were selected and tested in liquid cultures, on stone specimens artificially enriched with sulphates, and on real marble samples. The highest removal efficiency (81%) was achieved on real marble samples after 36 h of treatment. Finally, for the removal of organic matter, bioremediation tests on ancient frescoes (XV century) located near Pisa, altered by the presence of undesirable residual collagen, were carried out before intervention with traditional restoration. The treatment with pure cultures of P. stutzeri, at a temperature of 17–22°C, showed the complete removal of collagen after 8 h. The results confirm the potentiality of bioremediation processes as soft innovative technology based on the use of microorganisms and their metabolic activity in the recovery of degraded artworks.