Enhancing Aqueous Carbonation of Calcium Silicate through Acid and Base Pretreatments with Implications for Efficient Carbon Mineralization

Abstract

Carbon dioxide (CO 2 ) mineralization based on aqueous carbonation of alkaline earth silicate minerals is a promising route toward large-scale carbon removal. Traditional aqueous carbonation methods largely adopt acidification-based approaches, e.g., using concentrated/pressurized CO 2 or acidic media, to accelerate mineral dissolution and carbonation. In this study, we designed and tested three distinctive routes to evaluate the effect of pretreatments under different pH conditions on aqueous carbonation, using amorphous calcium silicate (CS) as an example system. Pretreating CS with high concentrations (100 mM) of HCl (Route I) or NaOH (Route II and III) enhanced their carbonation degrees. However, NaOH pretreatment overall yielded higher carbonation degrees than the HCl pretreatment, with the highest carbonation degree achieved through Route III, where an extra step is taken after the NaOH pretreatment to remove the solution containing dissolved silica prior to carbonation. The HCl and NaOH pretreatments formed different intermediate silica products on the CS surface. Silica precipitated from the HCl pretreatment had a minimal effect on the carbonation degree. The high Ca/Si ratio intermediate phases formed from the NaOH, on the other hand, can be readily carbonated. In contrast to commonly utilized acidification-based approaches, basification offers a more promising route to accelerate aqueous carbonation as it can mitigate the need for costly pH swing and high-concentration/pressurized CO 2 . The key to aqueous carbonation under basic conditions, as suggested by this study, is the control of aqueous silica species that have a suppressing effect on carbonation. Overall, this study highlights the critical needs for investigations of aqueous mineral carbonation in a broader pH region.

Silica Polymerization Driving Opposite Effects of pH on Aqueous Carbonation Using Crystalline and Amorphous Calcium Silicates

“…Recently, we have reported the possibility to regenerate the hydroxide solution through mineral carbonation under basic conditions. , This indirect carbonation process involves performing the following reaction at high pH (using calcium silicate as an example mineral): , CaSiO 3 + CO 3 2 − + H 2 O → CaCO 3 + 2 OH − + SiO 2 In contrast to the direct carbonation approach, where the mineral directly reacts with gaseous CO 2 in one step, the carbonate ions here are provided as a soluble carbonate salt such as Na 2 CO 3 . Different from the commonly used acidification approach where cations are leached out of the mineral, the acceleration in mineral carbonation is hypothesized to be that basic solutions convert mineral surfaces into Ca-rich CSH, which is then easily carbonated. , The carbonating reaction results in the formation of OH – (eq ) which can be further used to capture CO 2 from the atmosphere (eq ). Based on these reactions, CO 2 capture and mineralization can be achieved all under basic conditions, without large pH swings or energy-intensive sorbent regeneration.…”

“…This suggests that a low pH level could lead to a higher carbonation degree, as the solubility of alkali metal silicate (e.g., diopside and forsterite) is lower at higher pH levels . However, in the case of both amorphous CS and coal fly ash (containing a significant content of amorphous aluminosilicates) samples, an opposite pH dependence in the basic region was observed, with high carbonation efficiency achieved at high pH levels. , We speculated that these different pH dependences of CS carbonation were attributed to the mineral crystallinity. The crystallinity plays a significant role in mineral chemical and physical characteristics. , Well-crystallized CS (CCS) minerals, such as wollastonite and pseudowollastonite, exhibit a highly organized atomic structure.…”

“…12−14 Recently, we have reported the possibility to regenerate the hydroxide solution through mineral carbonation under basic conditions. 15,16 This indirect carbonation process involves performing the following reaction at high pH (using calcium silicate as an example mineral): 17,18…”

“…Different from the commonly used acidification approach where cations are leached out of the mineral, the acceleration in mineral carbonation is hypothesized to be that basic solutions convert mineral surfaces into Ca-rich CSH, which is then easily carbonated. 15,16 The carbonating reaction results in the formation of OH − (eq 2) which can be further used to capture CO 2 from the atmosphere (eq 1). Based on these reactions, CO 2 capture and mineralization can be achieved all under basic conditions, without large pH swings or energy-intensive sorbent regeneration.…”

“…20 However, in the case of both amorphous CS and coal fly ash (containing a significant content of amorphous aluminosilicates) samples, an opposite pH dependence in the basic region was observed, with high carbonation efficiency achieved at high pH levels. 15,16 We speculated that these different pH dependences of CS carbonation were attributed to the mineral crystallinity. The crystallinity plays a significant role in mineral chemical and physical characteristics. )…”