Antimicrobial chitosan/TPP-based coatings for the prevention of biodeterioration of outdoor stone sculptures

Outdoor stone sculptures are prone to accelerated deterioration caused by the proliferation of microorganisms in the stone. The physical, chemical and mechanical action of microorganisms on stone heritage causes aesthetic and structural changes that devalue the artworks over time. Developing more sustainable and ecological alternatives for their preventive conservation is necessary to reduce the negative environmental and human health impacts of currently used toxic biocides. Chitosan-based coatings cross-linked with citric acid and sodium tripolyphosphate (TPP) were developed for application to stone sculptures, as antimicrobial protection to inhibit the growth of degrading microorganisms. After polymerisation of the formulations, the resulting films were only partially soluble while remaining permeable to water vapour. These characteristics offer the possibility to reverse the conservation treatments, or to re-treat in regular periods, as part of programmed conservation strategies. Bacteria and fungi commonly found in stone microbiomes that contribute to biodeterioration processes were inhibited, particularly the pigment-producing yeast Rhodotorula spp., which causes discolouration of stone surfaces. The most interesting coating was successfully tested on granite, limestone and marble samples, and its presence on the stones' surfaces was confirmed by FTIR and SEM analyses. The chitosan-based coating caused no visible colour changes to the stones and reduced the wettability of granite and limestone, thus representing a potential antimicrobial protective layer for stone cultural heritage.