Dissolution and carbonation of a serpentinite: Inferences from acid attack and high P-T experiments performed in aqueous solutions at variable salinity |
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Authors: | Andrea Orlando Daniele BorriniLuigi Marini |
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Institution: | 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 |
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Abstract: | Dissolution experiments on a serpentinite were performed at 70 °C, 0.1 MPa, in H2SO4 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 solution and supercritical CO2 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 solutions by increasing their pH failed since Mg- and NH4- 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 CO2 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 H2SO4 solution.Through a thorough review of previous experimental investigations on the dissolution kinetics of serpentine minerals the authors propose adopting: (i) the log rate mole m−2 s−1] value of −12.08 ± 0.16 (1σ), as representative of the neutral dissolution mechanism at 25 °C and (ii) the following relationship for the acidic dissolution mechanism at 25 °C: |
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