The Importance of the Structure and Chemistry of Serpentine Minerals for CO2 Sequestration through Mineral Carbonation

Alicja M Lacinska, Michael T Styles

British Geological Survey

Serpentine minerals have long been recognised as a candidate material for CO₂ sequestration through mineral carbonation (CCSM). They are Mg silicates that provide Mg²⁺ cations for the reaction with CO₂ to form Mg carbonates creating a permanent sink for the ‘greenhouse’ gas. The Mg cations are released from the mineral structure by a combination of pre-treatment processes, all of which ultimately lead to accelerated disintegration of the resource material. The degree of disintegration and hence the amount of Mg released puts constraints on the subsequent carbonation and the efficiency of the overall technology. For the CCSM technology to be implemented on an industrial scale, this process step has to be efficient and relatively fast, providing a high % of cations at an acceptable cost. Studies to accelerate the reaction kinetics of Mg extraction include heat treatment [1], reduction of particle size or the use of various organic or inorganic solvents [2-4] and  bacterial  leaching [5].

A variable that inevitably affects the reaction kinetics is the crystal structure of the parent material and although statements about its importance have been made previously [3], to date no systematic study that compares the CCSM-related reactivity of different serpentine polymorphs and polytypes under the same experimental conditions was undertaken. This is essential for the selection of the most suitable feed material for the process.

The serpentine minerals belong to a group of sheet silicates containing one tetrahedral (silica) and one octahedral (brucite) layer. The large number of possible arrangements between the layers in a crystal leads to the significant structural complexity of the mineral group. The three main serpentine species are lizardite with flat layers, antigorite with a modulated layer structure and chrysotile forming cylindrical tubes. In addition, each of the previously mentioned species can be further structurally modified to give rise to mineral polytypes. Our previous research indicates that polymorphic and polytypic complexity of serpentine minerals controls mineral reactivity but the underpinning reaction mechanism is poorly understood. Accordingly, to develop a better understanding of reaction controls, we carried out a set of laboratory experiments, combined with a detailed mineralogical and crystallographic study, of the purest possible serpentine minerals samples we could obtain. Two sets of experiments were performed, one at conditions relevant for acid pre-treatment for ex situ CCSM, and other at conditions analogous to in situ CO₂ injection into serpentinite and ultramafic rocks.

The data from these experiments and mineralogical analyses show that leaching efficiency of Mg²⁺ is structure dependant, not only between mineral species such as antigorite and chrysotile but also between the lizardite polytypes. Also, based on analysis of reaction pathways and dissolution rates of representative rocks at conditions closely approximated to in situ CCSM, recommendations on the best injection targets for the technique can be made.

Collectively, our data indicate that the selection of starting materials for ex situ CCSM or the siting of CO₂ injection boreholes must take careful consideration of the mineral or rock type involved. This can have a large impact on the efficiency of the carbonation reactions and thus the overall effectiveness of CCSM technology.

1.             Maroto-Valer, M.M., et al., Comparison of physical and chemical activation of serpentine for enhanced CO₂ sequestration. Abstracts of Papers of the American Chemical Society, 2004. 227: p. U1095-U1095.

2.             Park, A.H.A. and L.S. Fan, CO₂ mineral sequestration: physically activated dissolution of serpentine and pH swing process. Chemical Engineering Science, 2004. 59(22-23): p. 5241-5247.

3.             Wang, X. and M.M. Maroto-Valer, Dissolution of serpentine using recyclable ammonium salts for CO₂ mineral carbonation. Fuel, 2011. 90(3): p. 1229-1237.

4.             Pundsack, F.L. and N.J. Somerville, Recovery of silica, iron oxide and magnesium carbonate from the treatment of serpentine with ammonium bisulfate. United States Patent Office, 1967.

5.             Power, I.M., G.M. Dipple, and G. Southam, Bioleaching of Ultramafic Tailings by Acidithiobacillus spp. for CO₂ Sequestration. Environmental Science & Technology, 2010. 44(1): p. 456-462.