Cation Exchange in Smectites as a New Approach to Mineral Carbonation

Mineral carbonation of alkaline mine residues is a carbon dioxide removal (CDR) strategy that can be employed by the mining industry. Here, we describe the mineralogy and reactivity of processed kimberlites and kimberlite ore from Venetia (South Africa) and Gahcho Kué (Canada) diamond mines, which are smectite-rich (2.3–44.1 wt.%). Whereas, serpentines, olivines, hydrotalcites and brucite have been traditionally used for mineral carbonation, little is known about the reactivity of smectites to CO₂. The smectite from both mines is distributed as a fine-matrix and is saponite, $M_{m+}^{x/m}Mg_3(Al_xSi_{4−x})O_{10}(OH)_2·nH_2O$, where the layer charge deficiency is balanced by labile, hydrated interlayer cations $(M^{m+})$. A positive correlation between cation exchange capacity and saponite content indicates that smectite is the most reactive phase within these ultramafic rocks and that it can be used as a source of labile $Mg^{2+}$ and $Ca^{2+}$ for carbonation reactions. Our work shows that smectites provide the fast reactivity of kimberlite to $CO_2$ in the absence of the highly reactive mineral brucite $[Mg(OH)_2]$. It opens up the possibility of using other, previously inaccessible rock types for mineral carbonation including tailings from smectite-rich sediment-hosted metal deposits and oil sands tailings. We present a decision tree for accelerated mineral carbonation at mines based on this revised understanding of mineralogical controls on carbonation potential.