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

•Cyanobacteria Synechococcus accelerates olivine dissolution at 20 °C.•Carbonation occurs in 3 stages (i.e. silicate dissolution; nucleation on exopolymeric substances; magnesian carbonate growth).•The carbonation reaction is strongly controlled by surface reactions and local saturation states.

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.

Graphical abstractProposed mechanism for low-temperature silicate carbonation enhanced by cyanobacterial photosynthetic activity: (A) EPS formation and accumulation during bacterial activity; (B) silicate dissolution likely enhanced by EPS. Magnesium and silicon are adsorbed on the EPS due to charge affinities (Dupraz et al., 2009); (C) Supersaturations with respect to Mg-carbonates are locally reached thanks to the photosynthetic activity of the cyanobacteria that increases pH and alkalinity, thus inducing carbonate precipitation close to the bacterial cells in the EPS.Figure optionsDownload full-size imageDownload as PowerPoint slide