Microbial and abiotic approaches to carbonating stevensite: what works and what doesn’t

Although Mg-rich clays are common in sediments, rocks and mineral wastes produced by mining, little is known about how they might contribute to carbon dioxide (CO 2 ) sequestration. The purpose of this study is to use previously tested mineral carbonation techniques at ambient temperature and pressure on the low-temperature Mg-rich clay mineral, stevensite [(Na 2 y · n H2O)(Mg 3– y X y )Si 4 O 10 (OH) 2 ]. This study tests the hypothesis that authigenic 2:1 Mg-clays, while less reactive to CO 2 than other Mg-rich minerals ( e.g., brucite and serpentines), can transform into carbonates under high alkalinity conditions (100–1187 mEq/L, pH = 9–11). Abiotic experiments using either CO 2(g) bubbling or addition of HCl into alkaline lake water and deionized water followed by addition of NaOH to induce a pH swing did not result in carbonation of synthetic stevensite. However, the abiotic experiments gave important insights into the role of silicate dissolution for alkalinity generation and carbon sequestration. Incubation of a natural biofilm with stevensite in highly alkaline lake water resulted in precipitation of the hydrated magnesium carbonate mineral, dypingite [Mg 5 (CO 3 ) 4 (OH) 2 ·∼5H2O], on the surface of synthetic stevensite as observed with X-ray diffraction and scanning electron microscopy. A Mg-rich clay phase was also precipitated in an incubation experiment containing biofilm but without the addition of synthetic stevensite. These results provide important insights and observations that can contribute towards geochemical models of environments where we observe carbonation of clay minerals, such as alkaline lakes, which can serve as natural analogues for industrial carbon sequestration projects.