# Analysis of Technologies for Carbon Dioxide Capture from the Air
## 2 DAC Technologies
### Additional Technologies for CO\({}_{2}\) Capture from Air  
In this section, other technologies for CO\({}_{2}\) capture from the air are described. These include membrane separation, carbon mineralization, photocatalysis and cryogenic separation. A few studies have been reported about membrane separation for CO\({}_{2}\) capture from the air, although they are promising for CO\({}_{2}\) separation from more concentrated sources [38; 123; 124]. [PERSON] et al. [45] reported that a DAC based on membrane is implausible due to the very low driving force (the driving force for CO\({}_{2}\) permeation from the air is about 40 kPa, three to four orders of magnitude lower than that required for a capture that is economically feasible) and large volume of sweep gas that is required. Another disadvantage is the trade-off between permeability and selectivity, limiting the general utility, especially when high performances are needed. In the work of [PERSON] et al. [123], an amine liquid membrane with several glycol compounds was considered to capture CO\({}_{2}\) from the air, evaluating the effect of membrane durability, amine concentration and the molecular structure of amine compounds on CO\({}_{2}\) permeability. Diffusion phenomena were described in the work of [PERSON] et al. [38]: N\({}_{2}\) gases have a solution-diffusion mechanism, while CO\({}_{2}\) gases have a facilitated transport mechanism. Moreover, considering a membrane with 6% of PEI, the authors found a great CO\({}_{2}\)/N\({}_{2}\) selectivity (about 300). A simulation has been developed in [PERSON] et al. [124]. Three stages of a polymeric membrane were used to concentrate CO\({}_{2}\) from 0.04% to 40%, with a competitive energy consumption (14.2 kWh/kgCO\({}_{2}\)/day). Other studies showed that high-performance materials (not commercialized and inorganic) could achieve a CO\({}_{2}\) concentration of 12% in a single stage and up to 99% with two stages [39]. However, the costs of systems with two stages were estimated to be in the order of 1000-10,000 $/tonCO\({}_{2}\), with an energy consumption of about 10 GJ/tonCO\({}_{2}\). In mineral carbonation or mineralization, metal oxides (mainly CaO and MgO) present in geological formations, such as \"ultramafic rocks\" (mainly olivine, serpentine, brucite and/or wollastonite), react with atmospheric CO\({}_{2}\) to produce thermodynamically stable mineral carbonates, resulting in a permanent form of CO\({}_{2}\) storage [18]. This is an irreversible chemical reaction, ensuring storage of CO\({}_{2}\), as described in the following reactions (see Equations (32)-(36)) [24]:  
\[CaSiO_{3}+CO_{2}\ \leftrightarrow CaCO_{3} \tag{32}\]  
\[Mg_{s}SiO_{4}+2 CO_{2}\ \leftrightarrow 2 MgCO_{3}+SiO_{2} \tag{33}\]  
\[CaMgSi_{2}O_{6}+2 CO_{2}\ \leftrightarrow CaMg(CO_{3})_{2}+2 SiO_{2} \tag{34}\]  
\[Mg_{3}Si_{2}(OH)_{4}+3 CO_{2}\ \leftrightarrow 3 MgCO_{3}+2 SiO_{2}+2H_{2}O \tag{35}\]  
\[Mg(OH)_{2}+CO_{2}\ \leftrightarrow\ MgCO_{3}+H_{2}O \tag{36}\]  
Overall, these are thermodynamically favored and natural spontaneous reactions that occur during weathering, according to which CO\({}_{2}\) in air reacts with Mg\({}^{-}\) and Ca\({}^{-}\) rich rocks to produce solid carbonates. This technology is called weathering and occurs slowly on a geological time scale [125]. The benefits of this capture route were suggested by [PERSON] and [PERSON] [126] and [PERSON] et al. [127] as the following: its being environmentally safe, the exothermic reactions reducing energy consumption, the abundance of feedstock, permanent CO\({}_{2}\) storage, no monitoring requirements, the generation of products that are thermodynamically stable and large potential sequestration capacity (CO\({}_{2}\) uptake values: 0.62 tons of CO\({}_{2}\)/ton of olivine, 0.76 tons of CO\({}_{2}\)/ton of brucite and ~0.4 to 0.5 tons of CO\({}_{2}\)/ton of pyroxene, serpentine and wollastonite [24]). On the other hand, the main disadvantage of this capture route is that the process is very slow, such that many methods have been proposed to accelerate it [18].