Effect of cyanobacteria Synechococcus PCC 7942 on carbonation kinetics of olivine at 20 °C I.A. Bundeleva a,⇑,1 , B. Ménez a , T. Augé b , F. Bodénan b , N. Recham c , F. Guyot a,d a Institut de Physique du Globe de Paris – Sorbonne Paris Cité, Université Paris Diderot, CNRS UMR 7154, 1 rue Jussieu, 75238 Paris, France b Bureau de Recherches Géologiques et Minières (BRGM), 3 avenue Claude Guillemin, 45060 Orléans, France c LRCS, Université de Picardie Jules Verne, Université de Picardie Jules Verne, CNRS UMR 7314, 33 Rue Saint Leu, 80039 Amiens, France d IMPMC, 4 place Jussieu, 75005 Paris, France article info Article history: Available online 14 February 2014 Keywords: Olivine Carbonation Cyanobacteria Silicate dissolution CO 2 mineral storage Industrial slag abstract By accelerating the naturally-occurring carbonation of magnesian silicates, it would be possible to sequester some of the anthropogenic excess of CO 2 in more geologically-stable solid magnesium carbon- ates. 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 Ò 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 micros- copy along with Raman spectroscopy. The results show that, at ambient temperature, cyanobacteria Synechococcus can accelerate silicate dissolution (i.e. Mg 2+ release) and then magnesium carbonate nucle- ation and precipitation by adsorption on the produced exopolymeric substances and local pH increase during photosynthesis, respectively. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction The carbonation reaction consists in dissolution of silicate min- eral (e.g. wollastonite – CaSiO 3 , olivine as forsterite – Mg 2 SiO 4 ), fol- lowed by carbonate precipitation, fixing CO 2 into a stable solid phase (e.g. calcite – CaCO 3 , magnesite – MgCO 3 ). In accordance, by enhancing the naturally-occurring carbonation of magnesium silicate minerals, of large occurrence on Earth, it would be thus possible to sequester important amount of anthropogenic carbon dioxide into geologically-stable magnesite. In addition to large out- crops of mafic and ultramafic rocks that can be found onshore in ophiolitic massifs, or offshore in the oceanic crust, Mg(Ca)-rich alkaline industrial residues (slag) represent a possible feedstock for mineral CO 2 sequestration. These materials are cheap, available near large point sources of CO 2 , and tend to react relatively rapidly with CO 2 due to their chemical instability (Huijgen et al., 2005). If magnesite forms directly from magnesian silicates at elevated temperature (e.g. Huijgen et al., 2005; Zevenhoven et al. 2008; Muriithi et al., 2013), at lower temperatures compatible with microbial activity (i.e. below 100 °C), the initial precipitate is rather a hydrated intermediate phase. Depending on its (OH)/(H 2 O) ratio, it can be either hydromagnesite (Mg 5 (CO 3 ) 4 (OH) 2 4H 2 O), nesqueh- onite (Mg(HCO 3 )(OH)2(H 2 O)) (Shirokova et al., 2011; Mavromatis et al. 2012), or any other hydrated Mg-carbonates, depending on the conditions of reaction (e.g. Hanchen et al., 2008). These inter- mediate minerals would then transform into stable magnesite through diagenetic processes. The dissolution of silicates and their replacement by carbonates are both strongly pH-dependent processes. Silicates are increas- ingly soluble with decreasing pH whereas carbonate precipitation occurs only in alkaline medium in the presence of bicarbonate/car- bonate ions ðHCO  3 =CO 2 3 Þ. Two carbonation pathways were exper- imentally investigated up to now: (i) a direct one in single reactor where silicates can dissolve by reaction with carbonic acid and magnesium carbonates precipitate from solution, typically at 185 °C and 150 bar, (e.g. Gerdemann et al., 2007); (ii) an indirect one involving two separate steps where silicates are first dissolved by a strong acid and then the solution is basified and mixed with CO 2 (e.g. Munz et al., 2009). In all cases, the dissolution rate of http://dx.doi.org/10.1016/j.mineng.2014.01.019 0892-6875/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +33 (0)3 80 39 63 69. E-mail address: irina.bundeleva@u-bourgogne.fr (I.A. Bundeleva). 1 Present address: Université de Bourgogne, UFR Sciences de la Vie, de la Terre et de l’Environnement (SVTE), CNRS UMR 6282, 6, bd Gabriel, 21000 Dijon, France. Minerals Engineering 59 (2014) 2–11 Contents lists available at ScienceDirect Minerals Engineering journal homepage: www.elsevier.com/locate/mineng