Capture of carbon dioxide by biologically induced precipitationof calcium carbonates by anoxygenic phototrophic sulfur bacteria

Tesfa1,2*, R. Todor1, M. Carrier1, S. Dubos1, M. Peyre-Lavigne1, L. Shirokova2, M. Sperandio1, L. Menjot, A. Karen O.S. Pokrovsky2, C. Dumas11 TBI, Université de Toulouse, CNRS, INRAE, INSA Toulouse, France.2 Géosciences Environnement Toulouse (GET) – Research Institute for Development [IRD]: Toulouse University, CNRS, Toulouse, France*Corresponding author: marawittesfa.research@yahoo.com To reduce atmospheric carbon (CO2) level, the presented study aims to understand the biological processes that entrap CO2 by calcium carbonate precipitation (CaCO3). Two biological mechanisms precipitating carbonates occur naturally [1]: (i) an active mechanism, where bacteria precipitate carbonates thanks to their metabolism and (ii) a passive mechanism, where microorganisms change the chemical environment by increasing the pH and/or producing exopolymers, sequentially inducing precipitation. In the presented study, the precipitation induced by anoxygenic phototrophic sulfur bacteria (APSB) was studied, pure cultures of Allochromatium Vinosum as a model microorganism.To understand this biologically induced precipitation, we attempted to reproduce the chemical environment in a lab-controlled reactor which allowed to characterize the nature of precipitated minerals, quantify their yield and rates of formation to deduce their carbon capturing capacities. These experiments were conducted in small batch and semi-continuous bioreactors, containing A. Vinosum with its inorganic growth media (vitamins, trace elements, inorganic energy source, sodium carbonates and chloride calcium). The growth media was a strictly inorganic substrate to prevent heterotrophy. To optimize carbonate precipitation and pinpoint its driving variables, some parameters such as the concentration of bacteria, elements from the growth media (Sulfate, phosphate, Magnesium) and the incubation time were modified. The chemical environment was then monitored (pH, COD, inorganic carbon, ions…) and precipitates were collected subsequently to filtration, weighted and analyzed (XRD, SEM).The incubation variation time displayed two different precipitation phases: rapid, reaching chemical equilibria within one hour, and slow, reaching equilibria within 15 days.We hypothesize the rapid kinetics was chemically driven and the slow kinetics depended on A. Vinosum growth cycle. The presence of phosphate was also shown to induce calcium phosphate precipitation as apatite, competing with CaCO3 precipitation. Previous studies showed that CaCO3 precipitation occurs when bacteria have an organic energy source [2]. Because the aim here is to reduce CO2, by working in an inorganic growth media to precipitate carbonates with solely inorganic carbon sources, CaCO3 precipitation was challenging and the yields of carbonate precipitation were lower than in traditional experiments with organic-rich media. Improving the seal on the air tight bioreactors resulted in a better CaCO3 precipitation yield. The work in progress aim to optimize the precipitation and consequently CO2 capture by decoupling bacterial growth phase from the mineral precipitation phase by working with separate reactors.[1] Dupraz, C., Visscher, P.T., 2005. Trends Microbiol. 13, 429–438.[2] Bundeleva, I.A., Shirokova, L.S., Bénézeth, P., Pokrovsky, O.S., Kompantseva, E.I., Balor, S., 2012. Chem. Geol. 291, 116–131.