Bio-deposition of a calcium carbonate layer on degraded limestone by Bacillus species

To obtain a restoring and protective calcite layer on degraded limestone, five different strains of the Bacillus sphaericus group and one strain of Bacillus lentus were tested for their ureolytic driven calcium carbonate precipitation. Although all the Bacillus strains were capable of depositing calcium carbonate, differences occurred in the amount of precipitated calcium carbonate on agar plate colonies. Seven parameters involved in the process were examined: calcite deposition on limestone cubes, pH increase, urea degrading capacity, extracellular polymeric substances (EPS)-production, biofilm formation, ζ-potential and deposition of dense crystal layers. The strain selection for optimal deposition of a dense CaCO₃ layer on limestone, was based on decrease in water absorption rate by treated limestone. Not all of the bacterial strains were effective in the restoration of deteriorated Euville limestone. The best calcite precipitating strains were characterised by high ureolytic efficiency, homogeneous calcite deposition on limestone cubes and a very negative ζ-potential.