Effect of cyanobacteria Synechococcus PCC 7942 on carbonation kinetics of olivine at 20 °C

By accelerating the naturally-occurring carbonation of magnesian silicates, it would be possible to sequester some of the anthropogenic excess of CO2 in more geologically-stable solid magnesium carbonates. Reaction rates can be accelerated by decreasing the particle size, raising the reaction temperature, increasing the pressure, using a catalyst, and hypothetically, by bacterial addition. We aimed here at assessing quantitatively the added value of photosynthetic microbial activity on the efficiency of Mg-silicates carbonation processes. Synechococcus PCC 7942 (freshwater cyanobacteria) was selected for this study. Two magnesian silicate minerals (substrates) were chosen: a synthetic forsterite with nanometer-sized grains and an industrial ultramafic slag (scoria). All tests were performed at 20 ± 1 °C in closed and sterile 1L Schott® glass bottle reactors. With the aim to elucidate the interaction between mineral phases and bacteria, we used pH and concentration measurements, scanning and transmission electron microscopy along with Raman spectroscopy. The results show that, at ambient temperature, cyanobacteria Synechococcus can accelerate silicate dissolution (i.e. Mg2+ release) and then magnesium carbonate nucleation and precipitation by adsorption on the produced exopolymeric substances and local pH increase during photosynthesis, respectively.