Insights into chemical stability of Mg-silicates and silica in aqueous systems using 25Mg and 29Si solid-state MAS NMR spectroscopy: Applications for CO2 capture and utilisation

Abstract

This work investigates chemical stability of Mg-silicates and silica in aqueous systems in order to gain in-depth understanding of their dissolution and precipitation behaviour under different conditions. The aim was to utilise the knowledge gained to develop an engineered carbon mineralisation technology for an efficient and cost competitive CO2 capture and utilisation. The results, based firmly on 29Si solid-state MAS NMR spectroscopy, and complementary techniques, demonstrate the influence of Si coordination (Qn) on the extent of Mg-silicates dissolution, as the increase in the number of neighbouring Si atoms in the structure of Mg-silicates reduced dissolution of Mg-silicates. While 100 and 80% of Q1(3 Mg) Mg-silicate dissolved in pH = 5 and 6.5, the extent of dissolution for Q2(2 Mg) was lower at values of 90 and 65% under the same conditions, while Q3(1 Mg) did not actually dissolve in solutions with pH = 5 and 6.5. The results of precipitation studies indicated the effect of Mg solubility on the structure and Mg content of the precipitated phases. While Mg-silicates with all three structures of Q1(3 Mg), Q2(2 Mg) and Q3(1 Mg) precipitated in the mildly alkaline environment (pH = 8.5) with the ratios of 20, 40 and 40% respectively, in concentrated acidic solutions of pH = 0, only pure silica (no Mg content) with Q3(1H)/Q4(0H, 0 Mg) having a ratio of 35 and 65% precipitated. The results of direct and indirect carbon mineralisation experiments showed that only 40 and 57 wt% of Mg content of thermally treated Mg-silicate was extracted respectively, consistent with 29Si NMR analyses, indicating that only intermediate Mg-silicate phases I and II with Q1(3 Mg) and Q2(2 Mg) structures were reactive, while other Mg-silicate phases remained inert. Another reason for limited Mg extraction in direct and indirect carbon mineralisation experiments is related to precipitation of a silica-rich phase/s on the surface of the reacting particles leading to passivation, again consistent with 29Si NMR analyses. This confirmed precipitation of Mg-silicate with a Q3(1 Mg) structure as well as hydrated silica Q3(1H) and silica Q4(0H, 0 Mg) in aqueous environments, similar to carbonation processes.