Net-0 experimental procedure to obtain Ca‑carbonate bio-cement via microbially induced precipitation in a fluvial natural lab

Despite laboratory reproduced microbially induced Ca‑carbonate precipitation is currently used for various sustainable bio-engineering applications, an inevitable environmental impact because of the waste materials and chemicals involved in lab activities still occurs. Here is presented a new and alternative net-0 experimental procedure to obtain a Ca‑carbonate bio-cement, developed in a natural fluvial laboratory (Parmenta stream) where microbial-mediated calcite precipitates, forming typical tufa deposits. The experiment consisted in the treating of a common multimineral sand (0,355–1 mm in grain size) with the stream flowing water for 4 months, associated with a parallel control monitoring of the precipitation process. During this time interval, thanks to the development and mediation of the microbial biofilm community, calcite precipitated with a variable daily rate from 0,49 to 2,94 μm/day, showing typical hierarchical nano- to micro-crystalline morphological features. This process brought to the gradual formation of a calcite bio-cement around the sand particles, starting from scattered tens of μm-size crystal aggregates to continuous crusts up to ca. 200 μm thick. The internal friction angle of the sand showed a total increase of ca. 20 % (i.e. 5,9° from 28,6° to 34,5°), with the major growth of ca. 16 % (i.e. 4,8° from 28,6° to 33,4°) after just 1 month; this indicates that just few amounts of new precipitates are enough to obtain the 80 % of such increase, most probably due to the roughness rise of the grains. Whereas, the cohesion showed a constant reduction trough time up to a total of ca. -64 % (from 0,17 Kg/cm2 to 0,06 Kg/cm2); this probably due to a reduction of the grains' specific surface on which the electrostatic attractions act.