Medium-term in situ experiment by using organic biocides and titanium dioxide for the mitigation of microbial colonization on stone surfaces
Graphical abstract
Introduction
The growing awareness in conserving built heritage had stimulated an increase in the number of researches addressed to issues regarding stone degradation caused by biocolonisation. Microorganisms colonizing material surfaces can be considered as one of the most dangerous degrading agent for stone monuments exposed outdoors (Urzì, 2004, Scheerer et al., 2009, Adhikary et al., 2015, Karacal et al., 2015, Marano et al., 2016) and thus they need not only to be removed, but the recolonization process must be slowed down. The restoration intervention of such degraded surfaces is composed of different steps. The first one involves the removal of pre-existing alterations (i.e. deposits, crusts, biological patinas). After that, most of time, the stone needs to be consolidated. For this purpose suitable inorganic and organic products should be used. Cracks and fissures have to be filled with compatible products that do not alter the stability and the aesthetics of the monuments. The last stage involves the application of biocide products to avoid the re-colonization process (Urzì and De Leo, 2007, Crisci et al., 2010, Pinna et al., 2012, Sterflinger and Piñar, 2013). The use of organic compounds (Crisci et al., 2010, Urzì et al., 2016) would not assure long-term protection due to the quick degradation of such materials. For this reason, several studies have been carried out on the use of inorganic nanomaterials (Bruno et al., 2014, La Russa et al., 2014). Nanoparticles, if properly dispersed in coatings, can offer outstanding properties and performances. In this context, nano-sized TiO2 is one of the most promising metal oxides, as it has a high stability and photo-reactivity, as well as a broad-spectrum activation antibiosis and low cost. It has been successfully tested as biocide against various microorganisms (Maness et al., 1999, Hur and Koh, 2001, Yu et al., 2003, Rincón and Pulgarin, 2004, Nadtochenko et al., 2006, Gupta et al., 2013). This compound represents a novel eco-friendly alternative for disinfection (Stoyanova et al., 2012). Further its importance is due to the fact that it may decrease the bioreceptivity of treated surface because of its self-cleaning ability (Baneriee et al., 2015) and thus in a contaminated environment or in outdoor conditions, the rate of recolonization can be highly reduced by its application.
When TiO2 is exposed to ultraviolet (UV) light (λ < 400 nm), holes (h+) and excited electrons (e-) are generated. The holes are capable of oxidizing water or hydroxide anions into hydroxyl radicals (-OH) (Zhang et al., 2008). The free radicals produced can decompose a wide range of organic compounds. The antimicrobial efficacy is determined by the competition among the recombination of electrons and holes after the excitation, and their interaction with the bacteria.
Titanium dioxide activity can be enhanced by the addition of small amounts of metals in the materials (Zaleska, 2008, Zhang et al., 2008, Fonseca et al., 2010, Hou et al., 2009, Iliev et al., 2006, Li et al., 2007). In particular, the Ag doping of TiO2 can also cause the denaturation of proteins present in bacterial cell walls and slow down bacterial growth (Maness et al., 1999), leading to synergistic effect.
The use of bare or doped TiO2 as biocide for stone protection has been successfully tested in Cultural Heritage restoration (Quagliarini et al., 2012, Quagliarini et al., 2013, La Russa et al., 2012, La Russa et al., 2016, La Russa et al., 2014, Ruffolo et al., 2013, Munafò et al., 2015). However, such studies lack long- or medium-term in situ experimentation, since they deal mainly with laboratory tests aimed to assess the products’ efficacy and suitability for stone protection.
This paper deals with an eight months medium-term in situ experimentation carried out in the archaeological site of “Villa dei Papiri” within the excavation of Ercolano site (Naples, Italy) (Fig. 1).
Coatings based on TiO2 have been applied in situ. Before these treatments, organic conventional biocide has been used, followed by the application of newly formulated products based on pure and doped TiO2.
Biological monitoring was carried out during this period of time, before and after the treatments with biocide, and after the treatments with TiO2. The mitigation of microbial colonization was evaluated through the comparative analysis of results obtained during the three different sampling campaigns.
Section snippets
Case study
Villa dei Papiri is one of the main examples of architecture in ancient Herculanum, Naples -Italy-, destroyed by the volcanic eruption of 79 A.D. The villa was discovered almost by accident in April 1750 during the digging of a well.
The selected area for the treatment was a southeast wall of the Villa dei Papiri covered with plaster as shown in Fig. 1. The presence of a little underground river leads to constant water rising from the ground, causing a dramatic stone degradation, especially in
Results and discussion
The results of monitoring campaigns carried out during the period of eight months were obtained by the combination of a multi-analytical approach as well as with microscopic and cultural analysis. They clearly showed that the microbial community affecting the southeast wall of Villa dei Papiri underwent drastic changes due to the different treatments as well as to the position of samples taken from the wall.
Conclusions
This paper represents the first report on the testing of TiO2 based biocide directly on a case study. The procedure showed in this study demonstrated that the use of TiO2 based nanobiocides can prolong the effect of the organic biocide for the mitigation treatment and therefore can be used on other buildings and archaeological areas. In fact, before any treatments, areas were colonized by a composite microbial community, in which both phototrophs and chemoorganotrophs were shown. The cleaning
Acknowledgments
This research was funded by POR Calabria FESR project ‘‘NANOPROTECH’’ (NANO PROtection Technology for Cultural Heritage) (J24E07000380005). We would like to thank dott.ssa Giusi De Carlo for her dedication working on the microbiological samples and Mrs. Sherron Collins for her careful reading and improvement of the English text.
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Both Authors contributed equally to the experimental work.