Medium-term in situ experiment by using organic biocides and titanium dioxide for the mitigation of microbial colonization on stone surfaces

Organic biocides are commonly used to reduce the biocolonization on stone surfaces. However, it should be possible to prolong a new re-colonization by the use of a combined application of organic and inorganic active compounds. TiO2, thanks to its high chemical stability, non-toxicity, high photo-reactivity and low cost, make it a potential effective molecule for long-term biocide activity against several biofoulers. In this research, a multi analytical approach, including microscopy, cultural and molecular analyses, has been applied to monitoring the treated surfaces of the southeast wall of Villa dei Papiri in Ercolano. In this study case, organic conventional biocide has been used, followed by the application of newly formulated products based on pure and doped titanium dioxide nanoparticles. As control, one part of the wall was treated only with organic biocides before applying only the binder. Biological sampling was carried out during eight months, before and after the treatments with biocide, and after the treatments with undoped and doped TiO2. The comparative analysis of results showed that microorganisms were drastically reduced after the biocide treatments, while the treatment with bare and doped TiO2 reduced a potential new recolonization. However, the effectiveness of the treatments was dependent on distance from the ground since high humidity reduced the efficiency of the treatments.