3.1. NPs characterization
3.1.1. FTIR analysis
FTIR analysis was performed to examine the molecular bonds of the synthesized Fe2O3 and Fe2O3@glutamine NPs (Bahmani et al. 2020; Rahmatmand et al. 2016). Based on Fig. 2, the vibration of Fe-O was revealed between 540-560 cm-1, and the peaks between 3100-3400 cm-1 were related to N-H bonds due to the presence of amino groups in the structure of Fe2O3@glutamine (Branca et al. 2016; Esmaeilpour et al. 2018; Hampton et al. 2010; Patel et al. 2009; Rahmatmand et al. 2016; Singh 2008; Zandahvifard et al. 2021).
3.1.2. TEM and SEM analyses
The size and morphology of NPs were investigated using TEM and SEM analyses, respectively. According to Fig.3, all synthesized and functionalized NPs had a spherical structure. Most Fe2O3 NPs had the average diameters of 5-13 nm. Besides, Fe2O3@glutamine NPs were more significant in diameter in comparison to Fe2O3 NPs with an average diameter of 9-17 nm. The NPs characteristics were shown in Table 2.
Table 2
NPs Characterization.
Name
|
Color
|
Morphology
|
Average particle size, nm
|
Fe2O3
|
Dark brown
|
Spherical
|
5-13
|
Fe2O3@glutamine
|
Dark brown
|
Spherical
|
9-17
|
3.1.3. Stability of NPs
Generally, there is a high tendency for NPs to agglomerate and sediment in the based solvent. According to the literature, ZP analysis is a well-known approach to determine the stability of NPs (Liao et al. 2009; Shadanfar et al. 2021; Tso et al. 2010; Wang et al. 2016). Based on ZP results (Ali et al. 2018), NPs will have high, acceptable, medium, and weak stability if the absolute ZP is more than 60 mV, between 30-60 mV, between 20-30 mV, and between 0-20 mV, respectively. Table 3 presents the ZP of nanofluids in different concentrations. Regarding the pH of NMP solution (5.5) and Table 3, NPs did not have a real effect on the pH of the solution, proving that the ZP was not affected by pH.
Table 3
ZP analysis for Fe2O3 and Fe2O3@glutamine at different concentrations.
Nano fluid
|
pH
|
Absolute ZP after 8 hours (mV)(±2)
|
Status
|
Fe2O3 (0.01 wt.%)
|
5.5
|
40.1
|
Stable
|
Fe2O3 (0.025 wt.%)
|
5.5
|
29
|
Stable
|
Fe2O3 (0.05 wt.%)
|
5.5
|
21.9
|
Stable
|
Fe2O3 (0.075 wt.%)
|
5.6
|
17.6
|
Unstable
|
Fe2O3@glutamine (0.01 wt.%)
|
5.5
|
53
|
Stable
|
Fe2O3@glutamine (0.025 wt.%)
|
5.5
|
44.2
|
Stable
|
Fe2O3@glutamine (0.05 wt.%)
|
5.6
|
35.5
|
Stable
|
Fe2O3@glutamine (0.075 wt.%)
|
5.6
|
23.8
|
Unstable
|
As shown in Table 2, Fe2O3 NPs had hydrophobic characteristics and were stable up to approximately 0.025 wt.%. Beyond this weight percentage of NPs, the ZP was less than the stability of nanofluids (lower than 30 mV). Fe2O3@glutamine NPs were stable as twice as Fe2O3 NPs and could be dispersed in NMP solution to the maximum of 0.05 wt.% without plummeting into the unstable range of zeta potential.
3.2. CO2 absorption
3.2.1. CO2 absorption mechanisms
CO2 absorption can be improved using NPs according to different absorption mechanisms. In one of these mechanisms, the CO2 mass transfer in the base fluid will be facilitated as a result of Brownian motions of NPs, acting as micro-mixers in the solution (Brownian motion theory). The high surface area of NPs is another reason for the CO2 absorption of nano solutions. Further, micro-movements of NPs can cause the CO2 molecules to be taken on the NPs surfaces and transferred to the bulk solution (Shuttle effect theory). According to the last literacy, irregular movements of NPs break large CO2 bubbles into smaller ones, increasing the bubbles’ surface area, and consequently the mass transfer of CO2 (Bubble breaking theory) (Krishnamurthy et al. 2006; Nabipour et al. 2017; Zare et al. 2019). The schematic of the defined mechanisms is shown in Fig. 4.
In this study, it was tried to increase the CO2 absorption capacity of the Fe2O3/NMP nano solution using glutamine amino acid. The absorption capacity of the fundamental solution was improved using NH2 agents as chemical absorbents of CO2. Therefore, the modified Fe2O3 NPs increased the CO2 absorption in both chemical and physical mechanisms.
3.2.2. Fe2O3 nanofluids
Fig.5 illustrates the quantity of CO2 absorption (α) in NMP solution and Fe2O3 nano solution including different weight percentages of NP (0.01, 0.025, 0.05, and 0.075 wt.%) at various initial pressures (20, 30, and 40 bar). Results revealed that pressure had an affirmative effect on the equilibrium absorption of CO2 in such a way that the maximum amount was obtained at 40 bar. For all examined pressures, the optimum weight percentage of Fe2O3 NPs was found to be 0.025 wt.%. Furthermore, the maximum amount of CO2 equilibrium absorption for NMP solution and Fe2O3 nano solution was 1.21 and 1.32 mol.kg-1, respectively. Therefore, Fe2O3 nanofluids increased the CO2 absorption of the base fluid to the maximum of 9.14% at the optimum concentration of NPs concentration. It proves that the NPs using only physical mechanisms like Brownian motion or grazing effect (Rahmatmand et al. 2016) can slightly improve the CO2 absorption, which is not effective in industrial conditions.
Although it was expected to have more CO2 absorption at a higher weight percentage of NPs, there was lower CO2 absorption at NPs concentration more than 0.025 wt.%. This can be credited to the high NPs tendency to agglomerate and sediment at their high weight percentages (Peyravi et al. 2015; Rahmatmand et al. 2016). The instability of Fe2O3 nanofluids was discussed in the previous section (3.1.3).
3.2.3. Fe2O3@glutamine
Fig. 6 shows the equilibrium absorption of CO2 in Fe2O3@glutamine nanosolutions at the pressures of 20, 30, and 40 bar. As can be seen, similar to Fe2O3, the highest CO2 absorption was observed at 40 bar for Fe2O3@glutamine nano solution. Furthermore, the optimum weight percentage of NPs was found to be 0.05 wt.% in which the CO2 absorption was increased to the maximum of 19.41% in comparison to the NMP solution.
3.2.4. Comparison of CO2 absorption capacities for Fe2O3 and Fe2O3@glutamine nano solutions in optimum concentrations
Considering Fig. 7, Fe2O3@glutamine nano solution had more ability in absorbing CO2 than Fe2O3 nano solution which can be attributed to both physical mechanisms (Brownian motion, grazing effect, etc.) and chemical reactions among CO2 molecules and amino groups existing in the glutamine structure. In addition, due to the hydrophilic properties of amino acids (Elhambakhsh et al. 2020a) Fe2O3@glutamine NPs could be employed at higher concentrations (0.05 wt.%) than bare Fe2O3 NPs. Therefore, Fe2O3@glutamine nano solutions have a higher potential to increase CO2 absorption than Fe2O3 nano solutions.
The details of CO2 absorption experiments in each time are mentioned in supplementary information section (Table S1-S9).