Amine-Functionalized and KOH-Activated Metakaolin for CO₂ Capture: Surface Engineering, Kinetics, and Regeneration

396 Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 396-418 p ISSN: 2635-3342; e ISSN: 2635-3350 Original Research Article Amine-Functionalized and KOH-Activated Metakaolin for CO₂ Capture: Surface Engineering, Kinetics, and Regeneration Udochukwu, E.C. Federal University Otuoke, PMB 126, Yenagoa, Bayelsa State, Nigeria. [email protected] http://doi.org/10.5281/zenodo.18061379 ARTICLE INFORMATION ABSTRACT Article history: Received 08 Sep. 2025 Revised 11 Oct. 2025 Accepted 20 Oct. 2025 Available online 30 Dec. 2025 This work presents a systematic evaluation of metakaolin-based sorbents modified via KOH activation and amine functionalization for low-concentration CO₂ capture. The materials were engineered to enhance surface area, pore architecture, and functional group density, enabling effective adsorption under realistic post-combustion and ambient conditions. Adsorption kinetics showed that KOH-activated metakaolin followed pseudo-first-order behaviour, indicating physisorption, whereas amine-functionalized samples aligned with pseudo-second-order kinetics, reflective of chemisorptive interactions. Under 1000 ppm CO₂, the KOH-activated variant exhibited a maximum uptake of 1.42 mmol·g⁻¹, outperforming raw and amine-grafted counterparts. In humid environments of 65 % RH, the amine-functionalized sample improved uptake by 22 %, reaching 1.06 mmol·g⁻¹, demonstrating strong affinity in moisture-rich conditions. Both sorbents displayed over 88% capacity retention after five regeneration cycles at 100 °C, confirming low-energy desorption and structural resilience. These findings position metakaolin as a scalable, cost-effective, and dual-mode sorbent platform for carbon capture applications, particularly in decentralized or humid operational settings. © 2025 RJEES. All rights reserved. Keywords: Metakaolin CO₂ capture KOH activation Amine functionalization Dual adsorption mechanism Direct air capture (DAC) 1. INTRODUCTION The continuous rise in atmospheric carbon dioxide (CO₂) levels, now exceeding 420 ppm, has heightened the global urgency to develop effective and scalable mitigation technologies (Nunes, 2023). Among various approaches, post-combustion carbon capture (PCCC) is considered the most deployable, given its compatibility with existing fossil-fuel-based infrastructure (Hanson, Nwakile and Hammed, 2025). Currently, chemical absorption using monoethanolamine (MEA) dominates commercial applications, with notable examples like the Boundary Dam in Canada and Petra Nova in the United States achieving capture efficiencies above 90% (North and Styring, 2019; Elsayed, 2024; Gibbins, Samson and Lucquiaud, 2024). However, widespread adoption of MEA-based systems is 397 E.C. Udochukwu / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 396-418 hindered by significant drawbacks, including high regeneration energy requirements of about 3.5 GJ/ton CO₂, solvent degradation, and corrosive byproducts, which is most cases, raise long-term operational and economic concerns (Kennedy, 2020; Buvik et al., 2021; Awadh, 2023). In response, research has pivoted towards next-generation solid sorbents such as metal-organic frameworks (MOFs), zeolites, covalent organic frameworks (COFs), and ionic liquids. While these materials offer high surface areas and tunable functionalities, their practical use is often limited by hydrothermal instability, complex synthesis routes, and elevated production costs (Chen et al., 2024; Amjad et al., 2025; Wang et al., 2025). This highlights the ongoing need for thermally stable, costefficient, and environmentally benign sorbents that can operate under real-world flue gas conditions, particularly those involving humidity, temperature variation, and trace contaminants (Ali et al., 2023; Gendron and Zakharova, 2024). Mineral-based sorbents, especially those derived from natural clays and industrial residues, have emerged as promising candidates due to their abundance, low toxicity, and robust thermal properties. One such material, metakaolin, a calcined form of kaolinite (Al₂Si₂O₅(OH)₄), has shown promise due to its high surface reactivity, tunable porosity, and Lewis acidic/basic functional sites created upon thermal dehydroxylation at 600–800 °C (Abdalqadir et al., 2024; Gendron and Zakharova, 2024). Although traditionally applied in cementitious systems and geopolymer matrices, its physicochemical structure makes it a viable candidate for CO₂ capture via both physisorption and chemisorption (Zewdie, Bizualem and Nurie, 2024; Ghaemi, Khoshraftar and Pouresmaeil, 2025). Recent studies have begun to explore metakaolin as a standalone CO₂ adsorbent under post-combustion conditions. For example, Pouresmaeil, Ghaemi, & Norouzbeigi, (2025) demonstrated that aminefunctionalization of metakaolinite led to a 41.9% increase in CO₂ uptake, emphasizing the potential for enhanced chemisorption performance. In another study, Liu et al., (2021) reported that geopolymer templates based on metakaolin, when modified with amines, exhibited significantly improved capture capacities and cycling stability. Paşabeyoğlu, (2025) also confirmed the value of thermally calcined metakaolin as a sustainable and tunable CO₂ sorbent platform. Additionally Ghaemi et al., (2025) and Hashmi et al., (2025) showed that mesoporous structuring and ion exchange modifications substantially reduce diffusion limitations, improving CO₂ accessibility and capture kinetics. Acid etching, which enhances surface hydroxylation and introduces microstructural defects favourable for chemisorption, was also shown to contribute to increased CO₂ affinity and durability over multiple adsorption–desorption cycles Zhou et al., (2021). Furthermore, metakaolin’s thermal regeneration profile is significantly less energy-intensive than MEA-based systems, with desorption occurring below 150 °C and minimal structural degradation over repeated cycles (Ding et al., 2023; Wang et al., 2023). This suggests strong potential for integration into modular and decentralized CCS units, particularly in developing economies with kaolinite-rich reserves. The modest energy requirement for kaolin calcination further supports its application in offgrid or small-scale emissions sources, such as brick kilns, flaring sites, and cement plants, which are often excluded from large-scale CCS networks (Cao et al., 2021). Despite these advantages, the adsorptive behaviour of metakaolin remains under-characterized compared to commercial solid sorbents. There is a clear research gap concerning the influence of calcination temperature, pore network architecture, surface hydroxylation, and chemical functionalization on CO₂ uptake and cyclic stability. In particular, the mechanistic understanding of CO₂/surface interactions and moisture co-adsorption effects remains nascent. This study aims to systematically investigate the adsorption performance of synthesized metakaolin under laboratory-simulated flue gas conditions. Emphasis is placed on correlating calcination parameters, textural properties, and surface chemistry with CO₂ uptake behaviour. The goal is to provide foundational data that inform the design, scalability, and deployment of metakaolin-based sorbents as viable low-cost alternatives for post-combustion carbon capture in diverse industrial contexts. 398 E.C. Udochukwu / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 396-418 2. MATERIALS AND METHODS 2.1. Synthesis of Metakaolin The raw kaolin was sourced from Isiala Ngwa in Abia State, Nigeria (Udochukwu, 2025). Prior to calcination, the kaolin was oven-dried at 105 °C for 24 hours to remove residual moisture. Calcination was performed in a muffle furnace under atmospheric conditions. A 100 g portion of dried kaolin was placed in an alumina crucible and heated to a target temperature of 750 °C. The sample was held at this temperature for 3 hours with a ramp rate of 10 °C/min. This temperature range was selected based on literature indicating optimal dehydroxylation of kaolinite to form amorphous metakaolin occurs between 650–800 °C (Khaled et al., 2023; Zunino and Scrivener, 2024). The calcined product was allowed to cool naturally to room temperature within the furnace. The resulting metakaolin was gently ground using a mortar and pestle and passed through a 125 µm sieve to ensure uniform particle size. 2.2. Chemical Activation of Metakaolin To enhance CO₂ affinity, the metakaolin was modified via surface functionalization and pore structure tuning. Selected samples were treated with amine-containing silanes (APTES) through a wet impregnation method, followed by drying at 80 °C for 6 hours. Other samples were thermochemically activated with KOH at a 1:2 weight ratio (precursor: KOH), followed by heating at 600 °C under nitrogen for 2 hours, then washed with dilute HCl and deionized water until neutral pH. 2.3. Characterization Techniques The surface area and pore structure of the three metakaolin samples (MK-Raw, MK-KOH and MK-NH2) were analysed using N₂ adsorption/desorption isotherms at 77 K (BET and BJH methods). Total pore volume, micropore volume, and average pore diameter were determined to correlate with CO₂ adsorption performance. Fourier-transform infrared spectroscopy (FTIR) was used to characterize surface functional groups and elemental composition. Thermal stability was assessed using thermogravimetric analysis (TGA) from room temperature to 800 °C under nitrogen. 2.4. CO₂ Generation Setup To simulate realistic low-emission environments and assess the CO₂ adsorption performance of tailored metakaolin, a controlled gas generation setup was employed. The system was designed to deliver a consistent and adjustable flow of CO₂ at concentrations similar to dilute emission sources, such as indoor air or ambient industrial exhaust. A certified gas cylinder containing a known CO₂/N₂ mixture was used as the base gas supply. To allow adjustable CO₂ levels, a mass flow controller (MFC) was added, enabling controlled mixing of pure CO₂ and nitrogen to produce concentrations between 400 ppm and 2%. The gas flow was kept steady at 100 mL·min⁻¹, and all tests were carried out at 25 °C under atmospheric pressure. When needed, the gas stream was passed through a humidifier to simulate real-world moisture conditions. Temperature and humidity were continuously tracked using calibrated sensors to ensure consistent test conditions. Each metakaolin sample was packed into a fixed-bed column, and the gas passed through a pre-conditioning unit to stabilize flow and temperature before reaching the sample. This setup ensured that adsorption data were reliable and not affected by fluctuations. Overall, the system allowed precise control of gas composition, flow, and humidity, making it suitable for testing CO₂ capture under realistic, low-emission conditions. 2.5. Adsorption Experiments The experiments were conducted in a lab-scale fixed-bed column system. The system included mass flow controllers for precise control of gas composition, a humidifier, and a stainless-steel adsorption column (inner diameter: 10 mm, length: 100 mm). A schematic diagram of the setup is provided in Figure 1. Each adsorbent sample of 1 g was thermally pre-activated at 120 °C under dry nitrogen flow of 100 mL·min⁻¹ for 3 hours. The sample was then packed into the column with quartz wool at both ends to ensure uniform gas distribution. The inlet gas mixture was composed of CO₂ of 1000 ppm balanced with nitrogen. In humidified runs, the gas stream was passed through a temperature-controlled bubbler to achieve 70% relative humidity. The gas flow rate was fixed at 100 mL·min⁻¹, and experiments were conducted at room temperature (25 °C) and atmospheric pressure. The CO₂ concentration at the outlet was continuously monitored using a nondispersive infrared (NDIR) analyser (LI-COR LI-820). Breakthrough curves were recorded until the outlet 399 E.C. Udochukwu / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 396-418 CO₂ concentration reached 95% of the inlet value. From the area above the breakthrough curve, the dynamic adsorption capacity (mmol·g⁻¹) was calculated using Equation 1. 𝑞 = 𝐹 𝑚∫(𝐶𝑖𝑛 − 𝐶𝑜𝑢𝑡)𝑑𝑡 𝑡𝑏 0 (1) where q is the adsorption capacity, F is the flow rate, m is the mass of the adsorbent, Cₐ values are inlet/outlet CO₂ concentrations, and tb is the breakthrough time. After each adsorption run, the column was purged with pure nitrogen at 100 °C for 1 hour to desorb CO₂. This adsorption/desorption cycle was repeated five times for each sample. The capacity retention across cycles was used to assess the stability and reusability of the adsorbents. Figure 1: Experimental set-up 3. RESULTS AND DISCUSSION 3.1. MK-KOH Characterization Analysis Before and After Carbon Capture 3.1.1. BET surface area analysis for MK-KOH The BET surface area analysis of metakaolin before and after carbon capture shows a significant change in the textural properties of the material. Before carbon capture (Figure 2a), the BET plot indicates a surface area of 21.57 m²/g, derived from a slope of 157.65 and an intercept of 3.77. The high correlation coefficient (r = 0.99988) confirms excellent linearity, suggesting that the adsorption data fit well within the BET model range (typically 0.05 < P/P₀ < 0.30). The calculated C constant of 42.8 reflects moderate adsorbate– adsorbent interactions, consistent with typical metakaolin surfaces dominated by amorphous aluminosilicate phases (Zhang et al., 2020). After carbon capture (Figure 2b), the BET surface area increased substantially to 57.32 m²/g, nearly a 2.7fold rise. The slope decreased to 60.52, while the intercept dropped sharply to 0.249, indicating a notable shift in adsorption behaviour. The C constant rose to 252.78, suggesting much stronger adsorbate surface interactions, possibly due to the formation of new active sites or enhanced surface polarity following CO₂ chemisorption or carbonation reactions (Liang et al., 2021). Again, the linear fit remains excellent (r = 0.99949), confirming that the BET model remains valid for both samples. This marked increase in surface area after CO₂ capture implies significant structural or morphological modification of the metakaolin. One plausible explanation is that the carbonation process introduced micro and mesoporous features, possibly through partial dissolution–reprecipitation reactions forming carbonate or bicarbonate species on the surface (Liang et al., 2021). These changes likely improved the material’s textural openness and exposed additional adsorption sites. Similar increases in BET surface area have been 400 E.C. Udochukwu / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 396-418 reported for aluminosilicate-based sorbents undergoing CO₂ activation or thermal treatment (Zhan et al., 2019). Overall, the BET data indicate that carbon capture not only altered the surface chemistry of metakaolin but also enhanced its physical adsorption capacity. The increase in surface area and the higher C constant both point toward improved surface heterogeneity and stronger gas–solid interactions, which could enhance subsequent CO₂ adsorption–desorption cycling performance. Figure 2a: BET surface area analysis for MK-KOH Figure 2b: BET analysis for MK-KOH after carbon capture The Dubinin–Astakhov (DA) pore size distributions of metakaolin before and after carbon capture reveal notable changes in the microporous characteristics of the material. For the original metakaolin (Figure 3a), the DA plot shows a micropore volume of 0.015 cm³/g with a modal pore diameter of 2.09 nm, placing it at the boundary between microand mesoporous regions. The characteristic adsorption energy (E) is relatively low at 1.97 kJ/mol, indicating relatively weak adsorbate surface interactions, which aligns with the modest BET surface area (21.57 m²/g) previously discussed. This pore distribution suggests a structure composed primarily of small, loosely connected micropores typical of thermally activated clays, where dehydroxylation of kaolinite produces a disordered, yet moderately porous, aluminosilicate matrix (Granados-Reyes, Salagre and Cesteros, 2014; Zhang et al., 2020). After carbon capture, significant textural evolution is observed (Figure 3b). The micropore volume increases more than twofold to 0.037 cm³/g, while the modal pore diameter slightly decreases to 1.83 nm. This shift toward smaller pores, coupled with the increase in adsorption energy to 2.92 kJ/mol, suggests a densification of the pore network and the creation of more energetically favourable adsorption sites. Such structural refinement is consistent with CO₂ chemisorption or carbonation processes, which can lead to surface reconstruction, formation of carbonate phases, and the generation of new microporous domains (Felekoğlu 401 E.C. Udochukwu / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 396-418 et al., 2006). The increase in both micropore volume and adsorption energy indicates enhanced accessibility and stronger affinity for adsorbates changes that likely contribute to the higher BET surface area (57.32 m²/g) observed after carbon capture. The narrowing of the pore size distribution and the shift toward smaller pores also point to the development of more uniform micropores, which can improve CO₂ adsorption capacity and stability during cycling (Zhan et al., 2019). Overall, the DA results confirm that the carbon capture process not only alters the chemical nature of metakaolin surfaces but also induces a textural transformation, increasing the microporosity and strengthening adsorption potential. These enhancement of both its chemical and physical structure makes it a suitable adsorbent for capturing carbon dioxide, especially in post-combustion capture scenarios. Figure 3a: BET surface area analysis (DA plot) for MK-KOH Figure 3b: BET analysis (DA plot) for MK-KOH after carbon capture 3.1.2. FTIR analysis of MK-KOH before and after CO₂ capture The FTIR spectra of metakaolin before and after carbon capture (Figures 4a and 4b) show similar overall profiles, which is expected since both samples originate from the same aluminosilicate framework. However, some subtle but important differences can be observed that help explain the structural and chemical transformations occurring during CO₂ interaction. Both spectra exhibit characteristic metakaolin absorption bands, including broad OH stretching vibrations around 3697–3388 cm⁻¹, which are attributed to surface hydroxyl groups and residual structural water (Krukowski et al., 2015). These bands are typical of dehydroxylated kaolinite phases that retain limited hydrogen bonding after calcination (Gonçalves and Pereira, 2013). The band near 1638 cm⁻¹ corresponds to H–O–H bending vibrations of adsorbed water molecules, indicating that even after activation, the surface retains some moisture affinity (Liang et al., 2021). The strong band centred near 1000 cm⁻¹ (with sub-bands at 980 and 909 cm⁻¹) represents the asymmetric stretching vibrations of Si–O–Si and Si–O–Al bonds, confirming the formation of the 402 E.C. Udochukwu / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 396-418 amorphous aluminosilicate framework that defines metakaolin’s reactive nature (Palomo et al., 1999; Zhang et al., 2020). This could arise from CO₂ adsorption forming bicarbonate or carbonate complexes on aluminarich sites (Zhan et al., 2019). Similarly, minor changes in the 1000–900 cm⁻¹ region could indicate Si–O–C or Al–O–C bond formation, implying that CO₂ interacted chemically rather than just physisorbing onto the surface (Frías, Villar and Savastano, 2011). These spectral observations align well with the BET and DA analyses, which showed increases in surface area and micropore volume after carbon capture. The creation of new reactive sites and surface species likely enhanced porosity and adsorption potential. The FTIR data, therefore, supports the conclusion that CO₂ capture by metakaolin involves both physical adsorption in newly formed micropores and chemical surface modification through carbonate or bicarbonate species. The FTIR comparison confirms that while the overall aluminosilicate framework of metakaolin remains stable, carbon capture introduces subtle chemical changes on the surface, enhancing its textural and adsorption properties. This combination of structural preservation and surface reactivity makes metakaolin a promising material for cyclic CO₂ adsorption or catalytic applications. Figure 4a: FTIR analysis for MK-KOH Figure 4b: FTIR analysis for MK-KOH after carbon capture 3.1.3. TGA analysis of MK-KOH before CO₂ capture The thermogravimetric analysis (TGA) of metakaolin before and after carbon capture provides important insights into the material’s thermal stability and compositional changes following CO₂ interaction. Both 403 E.C. Udochukwu / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 396-418 curves display a similar overall weight-loss pattern, but distinct differences in the magnitude and nature of decomposition steps indicate that carbon capture alters the thermal behaviour of metakaolin (Zhang et al., 2021). For the original metakaolin sample, the TGA curve (Figure 5a) shows three main weight-loss regions. The first minor weight loss below 150 °C corresponds to the removal of physically adsorbed moisture and surface-bound water (Alarcon-Ruiz et al., 2005). A more noticeable mass decrease between 200–400 °C is likely associated with dehydroxylation of residual kaolinite-like phases, where structural hydroxyl groups are released as water vapour. The sharp weight loss from 400–550 °C represents the most significant decomposition stage, typically linked to the release of chemically bound hydroxyls and possible breakdown of intermediate aluminosilicate phases formed during kaolinite dehydroxylation (Adeniyi et al., 2020). After carbon capture (Figure 5b), the overall thermal profile remains similar but shows slightly greater total weight loss and a more pronounced drop in the 400–550 °C region. This suggests that the post-capture metakaolin contains additional thermally labile species, likely related to surface carbonates or bicarbonates formed during CO₂ adsorption. These species typically decompose in this temperature range, releasing CO₂ gas, which aligns with the observed increase in weight loss (Ueno et al., 2018). The first region (below 150 °C) also shows a small rise in mass loss, indicating enhanced surface hydration due to the formation of new hydroxyl or carbonate groups, consistent with FTIR findings that showed stronger OH stretching bands. The material’s final residue remains around 18%, confirming that the fundamental aluminosilicate structure is maintained despite surface modifications. Overall, the TGA comparison reveals that carbon capture introduces chemically bound CO₂-derived species, which slightly reduce the thermal stability but enhance surface reactivity. These changes align with the BET and DA results, which showed increased surface area and microporosity after capture, factors that promote CO₂ uptake but can also make the structure more reactive under heat (Zhang et al., 2020). In summary, the TGA data support the conclusion that CO₂ capture by metakaolin involves surface carbonation and hydration processes rather than major structural breakdown. The post-capture material maintains good thermal stability, making it potentially suitable for reusable sorbent applications or catalytic processes that require moderate thermal regeneration (Wang et al., 2025). Figure 5a: TGA for MK-KOH 404 E.C. Udochukwu / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 396-418 Figure 5b: TGA for MK-KOH after carbon capture The thermal decomposition behaviour of metakaolin, before and after carbon capture, was investigated using thermogravimetric analysis (TGA) and differential thermal analysis (DTA), as shown in Figures 6a and b, respectively. Both samples exhibit a characteristic single-stage mass loss occurring between approximately 300 °C and 600 °C; however, the extent, onset temperature, and derivative mass loss differ significantly between the untreated and carbon-captured samples, confirming the influence of CO₂ uptake on the material's thermal profile. In the raw metakaolin sample (Figure 6a), the TGA curve (red, dashed) shows a gradual weight loss beginning near 300 °C, with a total mass reduction of approximately 80% by 900 °C. The major decomposition event is centred near 400 °C, consistent with the dehydroxylation of structural – OH groups from the aluminosilicate framework. The derivative weight curve (blue) shows a broad, symmetrical peak at this temperature, with a maximum weight loss rate of approximately 3.5 %/min. This thermal behaviour is typical of metakaolin and reflects the removal of bound water and minor decomposition of residual organic matter or volatile impurities (Liu et al., 2020). In contrast, the carbon-captured metakaolin (Figure 6b) displays a more abrupt and higher magnitude weight loss, with the TGA curve indicating a reduction of nearly 85% over the same temperature range. Notably, the onset of major weight loss occurs at a lower temperature (280–300 °C), suggesting reduced thermal stability due to the incorporation of additional volatile species (e.g. bicarbonates, surface carbonates). The derivative curve remains centered around 400 °C, but exhibits a sharper and narrower peak, indicating a more intense and focused decomposition event. The greater weight loss and sharper DTA signal in the carbon captured sample can be attributed to the presence of CO₂ containing species, such as physically adsorbed CO₂, bicarbonates, or surface carbonates formed during the carbon capture process. These phases decompose more readily upon heating, releasing gaseous CO₂ and contributing to the observed thermal behaviour (Baudin et al., 2023; Igami et al., 2025). The shift to an earlier onset temperature and steeper derivative weight loss further supports the presence of thermally labile carbonate phases, which are either absent or less abundant in the untreated metakaolin. Additionally, the residual mass at 900 °C is slightly lower for the carbon-captured sample (15 %) compared to the untreated sample (20 %), reinforcing the conclusion that more volatile material is released during heating. This trend aligns with previous studies on CO₂ loaded aluminosilicates, where carbonation increases overall mass loss in TGA and modifies the thermal stability window of the material (Kemp, Lewis and Rushton, 2022; Baudin et al., 2023). The thermal behaviour of metakaolin is significantly altered following carbon capture. The increased total weight loss, earlier decomposition onset, and sharper derivative peak indicate the successful incorporation of CO₂ species, most likely as bicarbonates or surface carbonates. These species decompose readily during heating, thereby lowering thermal stability and increasing the rate of mass loss. These results not only confirm the CO₂ uptake capacity of metakaolin but also provide valuable insight into the thermochemical changes induced by the carbon capture process, insights that are critical for evaluating the suitability of metakaolin-based materials in carbon capture, utilization, and storage (CCUS) systems. 411 E.C. Udochukwu / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 396-418 3.4. Effect of Humidity on Adsorption Behaviour Humidity played a key role in modulating CO₂ uptake, especially in functionalized samples. Under 65% RH, amine-functionalized metakaolin exhibited a 22% increase in CO₂ capacity compared to dry conditions, likely due to the formation of carbamates and bicarbonates facilitated by water. In contrast, unmodified samples showed a decline in capacity, reflecting the competitive adsorption of water. The KOH-activated sample displayed moderate sensitivity to moisture, with a 15% decrease in capacity, indicating a degree of hydrothermal stability. These findings suggest that appropriate surface modification strategies can enhance performance in humid environments typical of real-world capture conditions. 3.5. Regeneration and Reusability Thermal desorption under mild conditions (100 °C, N₂ flow) was sufficient to restore adsorption capacity over multiple cycles as shown in Figure 12. The KOH-activated and amine-functionalized samples retained 90% of their initial capacity after five adsorption/desorption cycles. No significant textural degradation was observed post-cycling, as confirmed by BET and FTIR analysis. This stability under low-temperature regeneration is advantageous for reducing energy input in DAC systems and supports the potential of tailored metakaolin as a practical adsorbent for continuous operation. Figure 12: Adsorption capacity vs cycle number curve 3.6. Adsorption Mechanism Considerations The performance trends point to a dual adsorption mechanism. Physical adsorption dominated in KOHactivated samples, supported by high surface area and mesopore volume. In contrast, chemisorption was more prominent in amine-functionalized variants, especially under humid conditions where water facilitates the formation of reactive species. The coexistence of both mechanisms offers a tunable platform for optimizing adsorbents across a range of environmental conditions. 3.7. Advanced Material Chemistry and Kinetic Perspectives The structural and chemical configuration of metakaolin plays a critical role in its CO₂ adsorption performance. Upon calcination of kaolinite at 650 - 800 °C, dehydroxylation transforms it into a disordered, amorphous aluminosilicate with high reactivity. This transformation enhances surface hydroxyl group availability and porosity, both essential for adsorption applications. Recent investigations by Geng, Guo, Zhang, Cheng, & Di, (2022) into sol-gel-derived CaO adsorbents modified with metakaolin reported a significant stabilization of the reactive phase. Incorporating 10 wt% metakaolin reduced sintering, preserved nanocrystalline CaO (27 nm), and significantly improved surface area of 18.64 m²/g. These features led to enhanced CO₂ capacities, sustaining 0.51–0.53 g CO₂/g after 30 carbonation/regeneration cycles. Metakaolin’s influence as a sintering barrier and reactive matrix support thus provides thermal resilience and structural continuity under cycling. Parallel developments in functionalized metakaolin geopolymers (Mirković et al., 2023) demonstrate the synergistic role of polyethylenimine (PEI) grafting. Here, chemisorption via amine groups facilitates CO₂ 412 E.C. Udochukwu / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 396-418 uptake (up to 0.69 mmol/g), particularly under humid conditions, reflecting enhanced affinity through carbamate and bicarbonate formation. The intrinsic reactivity and amine-functional compatibility of metakaolin positions it as a robust platform for hybrid sorbents. From a structural perspective, metakaolin’s porous matrix contains a blend of micropores and mesopores, as confirmed in this study (BET: 21.6–57.3 m²/g, pore diameter 2–5 nm). The interplay of pore confinement, reactive aluminosilicate sites, and accessible hydroxyl groups enables both physical adsorption and chemical binding, with implications for kinetic performance. Kinetic analyses of similar clay-based and amine-functionalized systems typically align with pseudo-first-order or pseudo-second-order models, depending on dominant mechanisms. In physisorption-dominant systems (e.g., KOH-activated metakaolin), fast initial uptake reflects abundant surface area, but performance plateaus due to pore saturation. In contrast, PEI-functionalized samples exhibit slower but sustained uptake driven by CO₂ binding to amine groups, consistent with chemisorptive behaviour and higher energy interaction profiles. Moreover, studies on CaO systems suggest reaction-limiting regimes vary by temperature. At 400 °C, partial carbonation proceeds through pore filling, while at 600 °C, external CaCO₃ layers impede further adsorption. In metakaolin-modified CaO, finer dispersions mitigate this limitation, sustaining deeper carbonation layers and enhancing utilization. The practical implication of these insights is the potential to engineer metakaolin-based adsorbents with tailored kinetics. For instance, combining PEI-functionalization (chemically selective binding) with a porous CaO-matrix supported on metakaolin may offer dual-mode sorption, optimal under variable CO₂ concentrations and humidity levels. 3.8. Comparative Benchmarking with Commercial Sorbents To contextualize the performance of metakaolin-based sorbents, a comparison with leading commercial materials is essential as shown in Table 2. Common industrial sorbents include activated carbons, zeolites, amine-based solvents, and metal-organic frameworks (MOFs). Each of these exhibits distinct advantages and trade-offs in terms of CO₂ capacity, thermal stability, moisture sensitivity, and regeneration energy. Table 2: Comparative benchmarking of this study with commercial sorbents Sorbent type Typical CO₂ capacity (mmol/g) Optimal conditions Regeneration temp (°C) Humidity tolerance Cost & scalability Reference Aminebased solvents 3.0–7.0 40–60 °C, dry/humid 100–150 High Moderate, corrosive Zhou et al. (2021) Zeolites 2.0–4.0 50 °C, dry 150–200 Low Low-cost, moisture sensitive Amjad et al. (2025) Activated Carbon 0.5–2.0 25–100 °C, dry 150–200 Moderate Cheap, readily available (Jedli et al. 2024) MOFs (MgMOF-74) 4.0–9.0 25–60 °C, dry/humid 80–200 Variable High cost, low stability Liu et al. (2021) MK-KOH 1.2–1.5 25 °C, dry to 65% RH 100 Moderate Low-cost, stable This study MK-NH₂ 0.8–1.1 25 °C, humid favourable 100 High Functionalizati on cost This study 3.9. Comparative Kinetics of Metakaolin and Commercial Sorbents The rate and mechanism of CO₂ uptake are crucial for evaluating adsorbent efficacy. Table 3 summarizes the dominant kinetic models, time to equilibrium, and mechanistic insights for each sorbent. Compared to commercial adsorbents, metakaolin variants offer competitive kinetics, especially in low-energy DAC scenarios. While activated carbon is faster, it lacks long-term moisture resistance. Amine-functionalized metakaolin (MK-NH₂) shows chemisorption-like behavior, improving under humid conditions. The pseudosecond-order fit for NH₂-functionalized materials confirms chemical interaction as the rate-limiting step. 413 E.C. Udochukwu / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 396-418 These insights validate metakaolin’s use in hybrid systems where balance between speed and selectivity is essential. Table 3: Comparative kinetics of metakaolin and commercial sorbents Sorbent type Dominant kinetic model Time to equilibrium Mechanism Reference Amine solvents Pseudo-2nd order 10–15 min Chemisorption (slow) Zhou et al., (2021) Zeolites Pseudo-1st + intraparticle diff 5–10 min Physisorption (fast) Zentou et al., (2025) Activated Carbon Pseudo-1st order 3–8 min Surface-limited physisorption Jedli et al., (2024) MOFs Dual-model 5–20 min Site-specific + diffusion Wang et al., (2023) MK-KOH Pseudo-1st order 6–9 min Surface-limited physisorption This study MK-NH₂ Pseudo-2nd order 10–13 min Chemisorption (with H₂O aid) This study 3.10. Reaction Kinetics and Model Application in this Study To characterize the adsorption behaviour of metakaolin-based adsorbents, kinetic models were applied to experimental CO₂ breakthrough data. The uptake kinetics were evaluated using pseudo-first-order and pseudo-second-order models as shown in Table 4. MK-KOH and MK-Raw show excellent fit to pseudofirst-order kinetics, indicating surface-limited physisorption. MK-NH₂ exhibits better fit with the pseudosecond-order model, suggesting chemisorption behaviour due to interaction with amine groups. All R² values exceed 0.98, confirming strong statistical validity of the fits. This detailed kinetic modelling provides deeper insight into the mechanisms of CO₂ adsorption in metakaolin-based sorbents, allowing for predictive design and operational tuning for real-world carbon capture applications. Table 4: Reaction kinetics and model application in this study Sample qe₁ (1st) k₁ (1/min) R²₁ qe₂ (2nd) k₂ (g/mmol·min) R²₂ MK-Raw 0.261 0.321 0.9951 0.257 0.915 0.9902 MK-KOH 0.715 0.273 0.9962 0.701 0.732 0.9878 MK-NH₂ 0.662 0.189 0.9827 0.648 0.951 0.9946 3.11. Comparison of Recent Metakaolin-Based Sorbents for CO₂ Capture with this Study Table 5 offers a comparative assessment of contemporary metakaolin-derived sorbents applied in CO₂ capture, situating the present research alongside key developments in the field. Prior investigations have largely explored geopolymer–zeolite systems (Papa et al., 2023), zeolitic frameworks (Oliveira & Cecilia, 2023), and polymer-enhanced hybrids (Mirković et al., 2023), achieving CO₂ uptake levels typically between 1.5 and 3.0 mmol·g⁻¹ in CO₂-enriched conditions and showing excellent regeneration (>90%). More advanced structures such as CaO–metakaolin composites (Wu et al., 2025) report higher uptake values (above 10 mmol·g⁻¹) but are constrained by high-temperature cycling, which limits feasibility for direct air capture (DAC). Alternative synthesis techniques, including microwave-assisted crystallization (Oliveira & Cecilia, 2023) and solar-driven calcination (Pasabeyoğlu, 2025), have improved sustainability profiles but remain largely assessed under non-dilute CO₂ conditions. The present work distinguishes itself by focusing explicitly on low-concentration CO₂ environments (400– 1000 ppm) relevant to DAC, an area that remains underexplored. Although unmodified metakaolin exhibited limited adsorption (0.23 mmol·g⁻¹), straightforward modifications such as KOH activation and amine grafting enhanced performance to 1.42 mmol·g⁻¹ and 0.87 mmol·g⁻¹, respectively. All tested sorbents demonstrated stable regeneration (>90%) under mild desorption conditions (100 °C, N₂ atmosphere), suggesting suitability for energy-efficient scalable DAC operations. Notably, under humid conditions (65% RH), amine-functionalized metakaolin not only retained but improved its uptake, underscoring its robustness in real-world ambient scenarios. In summary, while previous efforts have primarily addressed CO₂ capture 414 E.C. Udochukwu / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 396-418 under industrial or high-concentration settings, this study contributes to filling a critical gap by proposing a viable and adaptable metakaolin platform tailored for ambient, decentralized CO₂ capture applications. Table 5: Comparison of recent metakaolin-based sorbents for CO₂ capture with this study Study Sorbent type Synthesis / modification CO₂ uptake (mmol·g⁻¹) Conditions Regeneration (%) Key insight This Study (2025) Raw, KOHactivated, Aminefunctionalized Metakaolin Calcination (750 °C), KOH impregnation, APTES amine functionalization 0.23 (Raw), 1.42 (KOH), 0.87 (Amine) @1000 ppm 400–1000 ppm, 25 °C, dry & humid (65% RH) 95.1 (Raw), 91.3 (KOH), 92.4 (Amine) First systematic DAC-focused evaluation; mild regeneration viable Papa et al. (2023) Geopolymer/Zeol ite NaA composites Geopolymerization + zeolite crystallization 2.5–3.0 Lowpressure CO₂, 25–50 °C 90% over cycles Hybrid zeolite– geopolymer enhances microporosity (Oliveira et al. (2023) Zeolite A from metakaolinite Microwaveassisted hydrothermal synthesis 1.5–2.2 Ambient T adsorption 90% retention Microwave lowers energy footprint Mirko vić et al. (2023) Metakaolin– Brushite– Polymer hybrids (PEI/amine) Polymer embedding + amine functionalization 2.0–2.3 CO₂/N₂ streams, 25 °C 90% under humid cycles Amine functionalization boosts uptake in moisture Chen et al. (2022) Porous geopolymer solid amine sorbent Low-energy synthesis + amine impregnation 2.0–2.5 Flue-gas simulated CO₂, ambient T 90% with low energy input Pore control enhances chemisorption Geng et al. (2024) CaO–Metakaolin composites CaO sorbent with 10 wt% metakaolin 10–12 (CaO enhanced) High-T CO₂ looping 95% Metakaolin improves antisintering in CaO sorbents Paşabeyoğlu, (2025) Solar-calcined metakaolin/halloy site zeolites Solar calcination + hydrothermal crystallization 1.5–2.5 Lab CO₂ adsorption 90% Eco-friendly calcination pathway Vegere et al. (2019) Alkali-activated metakaolin Alkaline activation 1.0–1.5 CO₂ adsorption, biogas upgrading 90% Zeolite-like binders act as CO₂ adsorbents 4. CONCLUSION This study offers a comprehensive evaluation of metakaolin modified through KOH activation and amine functionalization, highlighting its potential as a dual-mode CO₂ adsorbent. Key findings indicate that surface engineering significantly enhances CO₂ uptake, moisture tolerance, and regeneration stability. KOHactivated metakaolin achieved a maximum adsorption capacity of 1.42 mmol·g⁻¹ at 1000 ppm, while aminefunctionalized samples reached 1.06 mmol·g⁻¹ under humid conditions, showing enhanced chemisorption behaviour. Kinetic modelling confirmed distinct uptake mechanisms in the form of pseudo-first-order behaviour for KOH-activated samples, indicative of physisorption, and pseudo-second-order kinetics for amine-functionalized materials, confirming chemisorptive interactions. From a performance standpoint, metakaolin-based sorbents demonstrated competitive or superior behaviour compared to commercial alternatives such as activated carbon, zeolites, and certain MOFs, especially under low-concentration and moisture-rich conditions. Their stable capacity over five adsorption–desorption cycles, with over 88% retention, reflects strong reusability and thermal integrity under mild regeneration temperatures of 100 °C. 415 E.C. Udochukwu / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 396-418 In terms of cost and scalability, metakaolin offers a major advantage. Derived from abundant, locally available kaolin clay, it requires relatively low processing energy (calcination at 750 °C) and uses inexpensive activating agents like KOH or APTES. Compared to high-cost MOFs and solvent-based systems, the simplified synthesis route and raw material availability position metakaolin as a low-cost and economically viable option for large-scale or decentralized carbon capture systems, including applications in developing regions with kaolinite-rich deposits. These results underscore the feasibility of using metakaolin in both post-combustion and direct air capture (DAC) technologies, particularly where cost, moisture tolerance, and operational simplicity are critical. Its dual adsorption capability, balancing physisorption and chemisorption offers flexibility across varied flue gas conditions. Future research should explore dual-functionalization approaches that combine alkali activation with metal oxide doping (e.g., MgO, CaO) to further enhance CO₂ affinity. Additionally, tailoring the pore structure through templating methods may improve gas diffusion and surface accessibility. 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