Dissolution and carbonation of a serpentinite: Inferences from acid attack and high P–T experiments performed in aqueous solutions at variable salinity

Abstract

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 solution 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 solutions 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 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: log rate = - 0.45 ( ± 0.09 ) × pH - 10.01 ( ± 0.30 ) . The initial dissolution rate (for 25 °C) by acid attack obtained in this work is consistent with this relationship. 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, suggesting 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.