Some common factors that lead to stone decay include air pollution (such as sulfur dioxide and acid rain) that dissolves stone, salts growing within pores of a stone that stress the stone’s tensile strength to turn it into powders, and bio-deterioration. In order to consolidate the stone, various materials and conservation methods have been applied. Most have limitations and disadvantages. Disadvantages include incompatibility with the substrate, alteration of the water transport properties (polymeric consolidant), mechanical bonding (alkoxysilanes to calcium carbonate substrates), and toxicity (Ba(OH)2).
A recent publication by Fawda Jroundiet al. presents a novel and environmentally-friendly approach for the conservation of calcium carbonate (CaCO3) stone. The procedure is based on the isolation of an indigenous community of carbonate-forming (aka carbonatogenic) bacteria, which form calcium carbonate from salt damaged stone, and then those bacteria are cultured and re-applied on to the same stone.
To be specific, the bacteria community was collected from decayed porous bioclastic limestone and identified using molecular methods. It was then inoculated with M-3P medium, a patented nutritive solution, to activate the carbonatogenic bacteria from among the community of bacteria present in a stone. Then all the bacteria were re-applied in-situ on the original same limestone. Meanwhile, two other commonly applied bacteria carbonatogenesis treatments using M.xanthus (a nutritive solution along with stone-isolated single bacteria of proven carbonatogenic ability, so here providing external carbonatogenic bacteria) and M-3P (to activate the carbonatogenic bacteria from amongst the microbial community of the stone, so here providing external nutrition) were tested and compared.
Those two treatments (M.xanthus and M-3P) showed limited consolidation, and the consolidation effect was almost lost in a short time (12 months after treatment). While for self-inoculation bio-treatment, a peeling test shows superior surface consolidation, and the effect was sustained over the duration of the study (24 months). Also, a drilling resistance test after 24 months showed a significant increase in stone cohesion, while no scaling or surface material was observed compared to untreated stone. Importantly, the color change was satisfactory with limited change.
Figure 1. Analysis of in situ bio-treated calcarenite stone. SEM images of untreated calcarenite (a) and calcarenite treated with the bacterial community (b–e). (a) Micrometer-sized calcite (Cc) crystals in the control stone show dissolution pits and are covered by salt (e.g., NaCl) crystals; (b) bacterial calcite (BCc) cement shows nanogranular structure surrounded by EPS; (c) detail of the nanogranular structure of calcite biocement interspersed with EPS; (d) bacterial calcite and abundant EPS covering the calcarenite substrate; (e) detail of mineralized bacterial cells (arrows); (f) XRD patterns of calcarenite before (blue) and after (red) bacterial bio-treatment. Cc, calcite; Gy, gypsum; Qtz, quartz
The consolidation of the substrate was probably due to the formation of aggregates of nano-sized CaCO3 embedded in exopolymeric substances (EPS) without blocking the pores (Figure 1(b-e)). Also after monitoring bacteria population for 24 months, the team determined the culturable bacteria capable of producing acid decreased by 75%, and neither nitrifying or sulfur-oxidizer were detected after the treatment.
The dissolution of calcite crystals after the self-inoculation bio-treatment was evaluated using in-situ AFM analysis to study the effect of salt weathering on the stone. The dissolution rate of CaCO3 was strongly reduced after the treatment, probably due to the formation of a passivating bacterial EPS coating on the substrate.
This article showed that this new self-inoculation bio-treatment is highly effective at consolidating decayed calcareous stones without side effects, and the novel conservation methodology may have broad applications in other stone works. If you have an interest in the details in Fawda Jroundi et al.’s work, please check out the article linked above.