An Electron Microscope Study of Biomineralisation for Geotechnical Engineering Purposes

Abstract

Directed biomineralisation, or using microorganisms to cause the formation of minerals, has been proposed as an effective method for permeability reduction and ground improvement. Where the precipitated mineral is a carbonate, heavy metal carbonates (e.g. otavite, malachite/azurite, cerussite, smithsonite, clearcreekite) can form, locking in heavy metal contamination. Where calcium carbonate forms there is an additional benefit of a high pH which, due to the buffering effect, can greatly reduce the mobility of heavy metal ions. Seven bacteria obtained from the soil & landfill leachate environments in Suzhou China, were induced to precipitate calcium carbonate under laboratory conditions within a medium consisting of a calcium source, urea and nutrient broth in a conical flask. Trials within clean sand columns resulted in a permeability which was 1/5 of that of a non-microbial column in addition to relative increases in strength of X3-5. On this basis, it is suggested that some geotechnical works using biomineralisation may be achieved without requiring external sources bacteria. This may be achieved either by isolating and growing the bacteria for application in the ground, or where growth can be achieved, by stimulating the bacteria in situ. An electron microscope assessment of the mineral structures formed by the bacteria indicates that a variety of different crystal forms are generated by the biomineralisation process. Some crystal structures, especially the open crystal structures, are of less use for engineering purposes. This indicates that not all bacteria that can precipitate carbonates would be of use for achieving geotechnical aims.