The interfacial processes controlling carbon dioxide mineralisation in magnesium and calcium silicates

Abstract

• Mechanical passivation is not caused solely by silica formation. • Bicarbonate and sodium ions buffer pH and mitigate passivating silica formation. • Confined water films facilitate coupled dissolution-re-precipitation in wet scCO 2 . • Reaction in wet scCO 2 generates carbonates at lower temperature conditions. • Non-uniform petrophysical response is exhibited in mineralised whole rocks. Nanometre to micrometre scale interfacial processes control CO 2 mineralisation in silicate rocks targeted for carbon sequestration. Understanding the chemical mechanisms prompted by the addition of CO 2 into aqueous-rock systems is necessary to design and manage industrial scale mineralisation operations. This work presents a synthesis of the past two decades of research on chemical processes taking place at the solid mineral interface, and how they drive or inhibit ex-situ or in-situ mineralisation. Studies cited in this review focus on samples representative of mafic or ultramafic rocks, their constituent mineral phases, and the calcium silicate wollastonite. Key findings include 1) Mechanical passivation is not caused solely by silica formation, a variety of chemical species can inhibit reactions by forming pervasive layers, including the target carbonate phase. 2) The functionality of engineered carrier solutions primarily derives from sodium bicarbonate which provide a pH buffering effect, excess CO 3 2– ions, and limit mass transport resistance from Si-rich passivating compounds. 3) Non-uniform mineralisation is exhibited in whole rocks and can be attributed to the inherent pore characteristics and heterogeneous distribution of grain sizes that produce micro-environments of preferential carbonation in natural mafic rock samples. 4) Thin water films facilitate a coupled dissolution-re-precipitation mechanism in water-bearing supercritical (CO 2 -rich) environments. 5) Mineralisation in both aqueous and supercritical CO 2 systems generates product layers of comparable morphology, consisting of interstratified carbonate-silica growths. Yet, experiments involving a wet supercritical CO 2 phase tend to generate carbonates at lower temperature conditions indicating enhanced reactivity or a more porous passivating layer.

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