29Si solid state MAS NMR study on leaching behaviors and chemical stability of different Mg-silicate structures for CO2 sequestration

Silicon is one of the most earth abundant elements, and thus, the fate and reactivity of silicate materials are often important for various energy and environmental technologies including carbon sequestration, where CO2 is captured and stored as a thermodynamically stable solid carbonate phase. Thus, understanding the structures and chemistries of different silicate phases has become an important research aim. In this study, the changes in the silicate structures (Q(0)-Q(4)) of heat-treated Mg-bearing mineral (serpentine) exposed to a CO2-water system (carbonic acid) was investigated using Si-29 MAS NMR, XRPD and ICP-OES and the identified structures were employed to explain complex leaching behaviors of silicate materials. The Si-29 MAS NMR and XRPD analysis indicated that the heat-treated serpentine is a mixture of amorphous (Q(1): dehydroxylate I, Q(2): enstatite, Q(4): silica) and crystalline (Q(0): forsterite, Q(3): dehydroxylate II and serpentine) phase, while natural serpentine mineral has single crystalline Q(3) silicate structure. The leaching experiments showed that both Mg and Si in the amorphous silicate structures (Q(1): dehydroxylate I, Q(2): enstatite) are more soluble than those in crystalline phase (Q(0): forsterite, Q(3): dehydroxylate II and serpentine). Therefore, tuning the silicate structure towards Q(1) and Q(2) would significantly improve carbon sequestration potential of silicate minerals, whereas silicate materials with Q(3) structure would provide great chemical stabilities in acidic conditions. The solubilities of silicate structures were in the order of Q(1) (dehydroxylate I) > Q(2) (enstatite) >> Q(0) (forsterite) > Q(3) (dehydroxylate II) > Q(3) (serpentine) and this finding can be used to better design a wide range of energy and environmental materials and reaction systems.