MICROBIALLY MEDIATED Ca-CARBONATE PRECIPITATION IN TUFA-FORMING FLUVIAL SYSTEMS: A NATURAL LAB TO DEVELOP SUSTAINABLE BIO-INSPIRED MATERIALS

For many decades fossil and recent microbial mediated carbonates were the focus of numerous studies. Recently these bio-sedimentary systems inspired an innovative approach to produce sustainable biomaterials, to reduce the harmful impact on the natural environment due to industrial activities, such as microbially induced Ca-carbonate precipitate (MICP). MICP has been studied for many applications including environmental remediation, production of construction material, cations removal in wastewater, and carbon sequestration. However, even if MICP could represents a good alternative to the utilization of natural resources and to reduce CO2 and NO2 emission of industrial activities, the process produces an inevitable environmental impact as the waste and chemicals involved in lab activities. With the aim to develop a bio-cement for increasing the geotechnical properties of a common multimineral sand, a more sustainable process of MICP was tested introducing the sand in the water flow of a tufa-forming river with active microbial Ca-carbonate deposition. The tufa deposits are covered by a lithifying biofilm composed of a microbial community including autotrophic and heterotrophic bacteria, algae, viruses, and extracellular polymeric substances. Biominerals forming the tufa deposit replace the organic substrates starting with an amorphous phase rich in Ca, Si, and other cations, followed by massive precipitation of fibrous to polyhedral Ca-carbonate crystals and subordinately lamellar/fibrous Mg-clay crystals. During the 16 weeks of the experiment, the biofilm colonized rapidly all the surface of the sand grains, which were gradually encrusted by neoformed Ca-carbonate biominerals showing the same crystal structures and composition of those forming the tufa deposit. Moreover, the sand showed a progressive increase of the internal friction angle from 28,6° to 35,4°, with a trend very similar to MICP lab experiments that used a single bacterial species. This suggests that is possible to use natural Ca-carbonate biominerals, induced by natural microbial communities, for engineering applications with very sustainable procedures.