Effects of organic ligands on supercritical CO2-induced phlogopite dissolution and secondary mineral formation

To evaluate the long-term and short-term risks associated with geologic CO2 sequestration (GCS), we need to understand both the reactions at supercritical CO2 (scCO2)–saline water–rock interfaces, and the environmental factors affecting these interactions. This research investigated the effects of four organic ligands—oxalate, malonate, acetate, and propionate—on the dissolution and surface morphological changes of phlogopite [KMg2.87Si3.07Al1.23O10(F,OH)2] under GCS conditions (95 °C and 102 atm). Phlogopite was chosen as a model clay mineral in potential GCS sites. After CO2 injection, the dissolution of CO2 should cause a decrease in saline water pH, increasing phlogopite dissolution. This effect can be lessened by the buffering capacity of organic ligands. However, in this study, the ligands that formed strong complexes with surface metals (i.e., oxalate) caused phlogopite dissolution rates to increase via ligand-promoted dissolution, although the pH increased. The experimentally observed dissolution rates of phlogopite were in the order of: oxalate > malonate > acetate ≈ propionate. In addition, based on results from ion chromatography, oxalate and malonate concentrations were stable in our reaction systems; however, aqueous acetate and propionate concentrations continuously decreased due to solvent extraction of acetic acid and propionic acid by scCO2 at 95 °C and 102 atm. After 159 h, all of the acetate and propionate were removed from aqueous solutions. Although the aqueous species in the bulk solution were not supersaturated with respect to potential secondary mineral phases, interestingly, in the presence of oxalate, nanoscale precipitation of amorphous silica and fibrous illite was observed at the phlogopite surface only 3 h after CO2 injection. At this early reaction time, illite fibers formed a connected, hexagonal framework on phlogopite basal surfaces, but at a later reaction time, these structures detached from the surface and triggered the formation of dissolution channels. In addition, kaolinite, boehmite, diaspore, and gibbsite were identified as secondary mineral phases. These results provide new information towards understanding organic speciesʹ interactions at scCO2–saline water–rock interfaces in deep saline aquifers.