Dissolution and carbonation of a serpentinite: Inferences from acid attack and high P–T experiments performed in aqueous solutions at variable salinity Andrea Orlando a,⇑ , Daniele Borrini b , Luigi Marini c a C.N.R., Istituto di Geoscienze e Georisorse, U.O.S. di Firenze, Via G. La Pira, 4, I-50121 Firenze, Italy b Dipartimento di Scienze della Terra, Università degli Studi di Firenze, Via G. La Pira, 4, I-50121 Firenze, Italy c Consultant in Applied Geochemistry, Via A. Fratti 253, I-55049 Viareggio (LU), Italy article info Article history: Received 2 February 2011 Accepted 18 June 2011 Available online 24 June 2011 Editorial handling by R. Fuge abstract Dissolution experiments on a serpentinite were performed at 70 °C, 0.1 MPa, in H 2 SO 4 solution, in open and closed systems, in order to evaluate the overall dissolution rate of mineral components over different times (4, 9 and 24 h). In addition, the serpentinite powder was reacted with a NaCl-bearing aqueous solu- tion and supercritical CO 2 for 24 h at higher pressures (9–30 MPa) and temperatures (250–300 °C) either in a stirred reactor or in an externally-heated pressure vessel to assess both the dissolution rate of serpentinite minerals and the progress of the carbonation reaction. Results show that, at 0.1 MPa, MgO extraction from serpentinite ranges from 82% to 98% and dissolution rate varies from 8.5 10 10 mole m 2 s 1 to 4.2 10 9 mole m 2 s 1 . Attempts to obtain carbonates from the Mg-rich solu- tions by increasing their pH failed since Mg- and NH 4 - bearing sulfates promptly precipitated. On the other hand, at higher pressures, significant crystallization (5.0–10.4 wt%) of Ca- and Fe-bearing magnesite was accomplished at 30 MPa and 300 °C using 100 g L 1 NaCl aqueous solutions. The corresponding amount of CO 2 sequestered by crystallization of carbonates is 9.4–15.9 mole%. Dissolution rate (from 6.3 10 11 mole m 2 s 1 to 1.3 10 10 mole m 2 s 1 ) is lower than that obtained at 0.1 MPa and 70 °C but it is related to pH values much higher (3.3–4.4) than that (0.65) calculated for the H 2 SO 4 solution. Through a thorough review of previous experimental investigations on the dissolution kinetics of ser- pentine minerals the authors propose adopting: (i) the log rate [mole m 2 s 1 ] value of 12.08 ± 0.16 (1r), as representative of the neutral dissolution mechanism at 25 °C and (ii) the following relationship for the acidic dissolution mechanism at 25 °C: log rate ¼0:45ð0:09Þ pH 10:01ð0:30Þ: The initial dissolution rate (for 25 °C) by acid attack obtained in this work is consistent with this relation- ship. In contrast, the average dissolution rate (for 25 °C) determined in this study through the pressure- vessel experiments is 4.5 orders of magnitude lower than that computed through this equation, suggest- ing that silica armoring of serpentinite grains played a significant role in these experiments. Overall, the obtained data may improve both the planning of ex-situ mineral carbonation using the CO 2 separated from biogas and the modeling of in-situ mineral carbonation. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction In order to mitigate the rise of atmospheric CO 2 in the near fu- ture, one of the most efficient solutions consists of mineral trap- ping, in which stable carbonates are produced through the reaction between CO 2 and silicate minerals (e.g. Seifritz, 1990; Lackner et al., 1995; Oelkers et al., 2008). Generally, this approach concerns Mg 2+ -, Ca 2+ -, and Fe 2+ -bearing silicates such as olivine, serpentine, and pyroxenes, which are mainly found in igneous ultramafic rocks, variably affected by alteration processes. The pro- cess can be considered as the result of two consecutive steps: (1) destabilization or dissolution of primary silicate minerals with consequent release of metal cations and (2) reaction between me- tal cations and CO 2 to form carbonates and by-products. This can be represented through the following overall reaction, where M indicates divalent cations: M x Si y O ðxþ2yzÞ OH 2z þ xCO 2 ¡ xMCO 3 þ ySiO 2 þ zH 2 O: ð1Þ Many parameters, such as T, P, pH, pCO 2 and salinity control the kinetics of these steps, which is usually very slow, at least when mineral trapping takes place underground, where CO 2 interacts with geological formations (in situ mineral carbonation). In this case, mineral trapping is usually considered to occur over long times, in the order of >10 1 –10 4 a (e.g., Bachu, 2008; Gaus, 2010), as also suggested by geochemical modeling, in spite of the large 0883-2927/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2011.06.023 ⇑ Corresponding author. Tel.: +39 055 2757510; fax: +39 055 290312. E-mail address: orlando@igg.cnr.it (A. Orlando). Applied Geochemistry 26 (2011) 1569–1583 Contents lists available at ScienceDirect Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem