Effects of freshwater Synechococcus sp. cyanobacteria pH buffering on CaCO3 precipitation: Implications for CO2 sequestration

Abstract

In the present study, a mixed-flow steady-state bio-reactor was designed to biomineralize CO2 as a consequence of photosynthesis from active Synechococcus sp. Dissolved CO2, generated by constant air bubbling of inorganic and cyanobacteria stock solutions, was the only source of inorganic carbon. The release of hydroxide ion by cyanobacteria from photosynthesis maintained highly alkaline pH conditions. In the presence of Ca2+ and carbonate species, this led to calcite supersaturation under steady state conditions. Ca2+ remained constant throughout the experiments showing the presence of steady state conditions. Similarly, the Synechococcus sp. biomass concentration remained stable within uncertainty. A gradual pH decrease was observed for the highest Ca2+ condition coinciding with the formation of CaCO3. The high degree of supersaturation, under steady-state conditions, contributed to the stabilization of calcite and maintained a constant driving force for the mineral nucleation and growth. For the highest Ca2+ condition a fast crystal growth rate was consistent with rapid calcite precipitation as suggested further by affinity calculations. Although saturation state based kinetic precipitation models cannot accurately reflect the controls on crystal growth kinetics or reliably predict growth mechanisms, the relatively reaction orders obtained from modeling of calcite precipitation rates as function of decreasing carbonate concentration suggest that the precipitation occurred via surface-controlled rate determining reactions. These high reaction orders support in addition the hypothesis that crystal growth proceeded through complex surface controlled mechanisms. In conclusion, the steady state supersaturated conditions generated by a constant cyanobacteria biomass and metabolic activity strongly suggest that these microorganisms could be used for the development of efficient CO2 sequestration methods in a controlled large-scale environment.