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Article

Carbon Capture Efficiency of Mechanically Activated Australian Halloysite-Rich Kaolin with Varying Iron Impurities and Its Potential Reuse for Removing Dyes from Water

by
Siavash Davoodi
1,
Bhabananda Biswas
1,2,* and
Ravi Naidu
1,2,*
1
Global Centre for Environmental Remediation (GCER), School of Environmental and Life Sciences, University of Newcastle, Callaghan, NSW 2308, Australia
2
crc for Contamination Assessment and Remediation of the Environment, Callaghan, NSW 2308, Australia
*
Authors to whom correspondence should be addressed.
Minerals 2025, 15(4), 399; https://doi.org/10.3390/min15040399
Submission received: 13 March 2025 / Revised: 3 April 2025 / Accepted: 8 April 2025 / Published: 10 April 2025
(This article belongs to the Special Issue Adsorption Properties and Environmental Applications of Clay Minerals)
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Abstract

Sustainable strategies are required to mitigate elevated atmospheric CO2 levels. Achieving that by adsorption, especially by using clay-based adsorbents, drew attention. These are even more promising when these adsorbents are obtained by low-cost modifications. This study evaluates the effect of ball milling on the carbon capture performance of Australian halloysite nanotube (HNT)-rich kaolin samples: one without iron impurities (Hal) and the other with iron impurities (HalFe). The iron was mainly nested within illite/mica minerals in HalFe. Samples were ball-milled for 30 and 60 min, and their CO2 sorption was assessed at various pressures and temperatures. Crystallography, electronic microscopy, and surface area and charge characterization revealed reduced length and increased width of tubular structure following ball milling, leading to higher specific surface area without compromising crystallinity. CO2 sorption of Hal increased 14% at 20 bar and 15 °C after 60 min milling, with a ~300% rise at near-atmospheric pressures. Conversely, milling negatively affected CO2 sorption of HalFe, likely due to iron/illite-mica-related damage during milling. Crystallography, infrared, and thermographic analyses revealed physisorption as the primary sorption mechanism. Since direct disposal of CO2-laden materials is against sustainability principles, these materials were tested for methylene blue removal from aqueous solutions, achieving ~83% (Hal) and ~91% (HalFe) removal efficiencies. This highlights HNTs-rich kaolin clays’ valorization potential for carbon capture and utilization (CCU).

Graphical Abstract

1. Introduction

Human activities, particularly the extensive use of fossil fuels in industrial processes, transportation, and energy production, have significantly increased the concentration of greenhouse gases (GHGs) in the atmosphere. This rise in GHGs, particularly carbon dioxide (CO2), is a primary driver of global warming and climate change, posing serious threats to environmental stability and human health [1,2,3]. The 2023 National Oceanic and Atmospheric Administration’s (NOAA) Global Monitoring Lab annual report indicates that the average global CO2 level has reached an unprecedented high of 419.3 mg/L, representing an increase of 50% compared to pre-industrial revolution [4]. This reality necessitates urgent action to curb emissions and mitigate climate impacts.
Various technologies for CO2 capture have been developed, including absorption, membrane separation, cryogenic separation, and adsorption, each with distinct advantages and limitations. Absorption using chemical solvents, such as amines, is widely used but requires high energy for solvent regeneration and can face issues with solvent degradation. Overcoming these obstacles, the adsorption process using porous adsorbents is particularly advantageous due to its compatibility with existing power plant configurations, minimal modification needs, and lower operational costs compared to absorption methods. Common adsorbents in this process include activated carbon, zeolites, metal-organic frameworks (MOFs), carbon nanotubes (CNTs), and porous carbons [5,6,7,8,9,10]. While these adsorbents exhibit high carbon capture capacities, mainly due to their exceptionally large surface areas, they may often involve complex and costly synthesis processes, leading to efficiency concerns, especially in diffuse polluted areas, such as livestock sheds and organic waste processing facilities. Additionally, they may exhibit poor chemical and thermal stability under harsh operating conditions and may not meet biocompatibility requirements when interfacing with biological systems [2,11,12].
Clays present desirable alternative adsorbents for carbon capture due to their natural abundance, low cost, and favorable textural properties. These materials often possess relatively high surface areas and porosity, potentially enhancing their ability to adsorb gases [13,14]. However, their adsorption capacities may vary considerably based on specific structural and chemical characteristics and can be influenced by various physicochemical activation methods [1,15]. It should be noted that while clays exhibit much less specific surface area and porosity compared to the mentioned synthetic adsorbents, leading to lower CO2 capture efficiencies, they can be as much as 100 times cheaper [11]. Among clay minerals, halloysite nanotubes (HNTs) have emerged as a material of interest due to their tunable surface chemistry, surface area, porosity, excellent mechanical properties, and biocompatibility. The alumina octahedron inside the HNT lumen is responsible for forming an internal surface with Al-OH groups, while the external surface is composed of a tetrahedral silica sheet with Si-O-Si groups, creating opposing charges on the inner and outer surfaces [14,16,17,18].
The existing literature on harnessing these HNT properties primarily focuses on the application of pure HNTs (e.g., HNTs from vendor such as Merck Australia). However, HNTs are naturally found in many deposits as a constituent of kaolin clay among other mineral phases, including kaolinite and impurities such as iron, and collectively they form a mineral assemblage [17,19]. In many cases, HNTs are separated from the kaolin matrix through energy-intensive processes, which are not cost-effective for various applications. Retaining kaolinite and iron impurities might be beneficial for certain applications; for instance, iron may potentially enhance the CO2 sorption performance [20,21,22,23].
Numerous modification techniques have been studied to enhance the CO2 sorption performance of HNTs by altering surface area, pore structure, surface functional groups, and loading additional chemicals and materials inside the halloysite tubes [1,24,25,26,27,28]. However, these modifications may not be cost-effective (e.g., modification following HNTs separation) or environmentally friendly (e.g., use of toxic surfactants to make HNT composite).
Looking at more realistic and feasible approaches, ball milling may be a potential treatment method that can enhance the desirable properties of raw materials to be used in CO2 sorption applications [29,30,31]. This process is environmentally friendly, cost-effective, and often readily available in the clay deposit beneficiation process. The mechanical stress induced during grinding by ball milling can increase the surface area of clay samples by dislodging the particles and reducing particle size. Additionally, it can alter the structural characteristics of materials and activate them through the heat and pressure generated during milling [32,33,34,35]. The outcomes may vary depending on the material feedstock; for example, Biswas et al. [17] reported an extraordinary lumen size of the HNTs collected from Western Australian Noombenberry region, and the grinding outcomes of these materials might be different than that for commercially available Sigma-Aldrich HNTs. While ball milling seems promising grinding techniques for enhancing materials properties, there is currently no research on its application within kaolin matrices where high-lumen HNTs are predominant mineral species. Moreover, these deposits commonly include varieties of kaolin that are either iron-poor or iron-rich; the elevated amount of iron was linked to mica/illite mineral species in the kaolin clay [17]. This was a relatively new deposit, and prior knowledge on the simple activation of these assemblages via ball milling for CO2 sorption is lacking.
To address these research gaps, the present study aims to (i) evaluate the CO2 sorption potential of newly discovered Australian halloysite-rich kaolin with varying iron impurities under different conditions, (ii) investigate the effect of low-energy ball milling on their properties and evaluate their CO2 sorption performance, (iii) explore the mechanisms involved in the sorption process, and (iv) subsequently utilize the CO2-sorbed materials for the removal of a model dye (e.g., methylene blue) from aqueous environments. Our hypothesis posits that simple low-energy ball milling can potentially enhance the CO2 sorption performance of the adsorbent, making it useful for areas with diffused emissions, with the prospect of further utilizing the laden materials in other remediation applications, thus promoting a circular economy.

2. Materials and Methods

2.1. Minerals and Their Geological Sources

Two natural kaolin clay samples, provided by ESG Minerals from the Cloud Nine halloysite-kaolin deposit situated ~350 km to the east of Perth and to the southeast of Merredin in Western Australia, were used in this study. Figure S1a shows the exact location of the Cloud Nine deposit [36], and Figure S1b illustrates the physical appearance of the collected raw clay samples without any processing and purification. Halloysite was the dominant mineral species for both samples, with one of them without iron impurity and the other with iron (Fe) impurity. The iron can be in its oxide form. However, for ease of reference in this article, the samples were named Hal and HalFe after their main mineral species and impurities. These samples were passed through a 150-micron test sieve; the passed portions were collected and denoted as Hal150 and HalFe150, respectively. The percentage of halloysite, kaolinite, and iron oxide content of raw samples is listed in Table 1. A complete and thorough characterization of these samples was previously studied by the authors and published elsewhere [17].

2.2. Mechanical Activation by Ball Milling

A total of 20 g of Hal150 and HalFe150 were treated using a planetary ball milling machine (PQ-N2, Across International, Bayswater, Australia) with zirconia beads at 400 rpm for 30 and 60 min with a sample-to-bead weight ratio of 1:1, switching between forward and reverse mode every 15 min. The resulting materials were denoted Hal30M, HalFe30M, Hal60M, and HalFe60M, respectively, and stored in a vacuum desiccator for further analysis.

2.3. Material Characterization

The morphological study of the materials before and after ball milling was performed using scanning electron microscopy (SEM) (Sigma VP FESEM, Zeiss, Oberkochen, Germany) and transmission electron microscopy (TEM) (JEM-2100 Plus 200kV LaB6 TEM, JEOL, Peabody, MA, USA). For SEM, powdered samples were poured over carbon tape and coated with a thin layer of platinum (4 nm). For TEM, the ethanol-dispersed samples were placed onto lacey carbon film and dried prior to the imaging. To analyze the potential changes in halloysite nanotube dimensions, ImageJ software (version 1.54F) was employed to count 10–20 random particles in each sample. To determine the zeta potential value as a proxy of surface charges, samples were dispersed in Milli-Q water (0.02 wt.%) and measured at natural pH using a zeta potential/nanoparticle analyzer (NanoPlus-HD, Particulate Systems, Norcross, GA, USA). X-ray diffraction (XRD) analysis was conducted using an Empyrean diffractometer (PANalytical, Almelo, The Netherlands) with Cu Kα radiation (λ = 1.54060 Å). The measurements were performed in a 2θ range from 5° to 70° with a step size of 0.039° and a current of 40 mA at 45 kv to study the potential changes in crystallinity of the samples due to ball milling. The X’pert HighScore software (version 4.7), coupled with the PDF-4/Minerals 2022, ICDD database, was utilized for mineral phase identification of the materials [37]. The N2 adsorption-desorption isotherms of the samples at 77.4 K were obtained (Tristar 3020, Micromeritics, Norcross, GA, USA). The Brunauer–Emmett–Teller (BET) model was used to determine the specific surface area and pore size/volume [38], and the Barrett–Joyner–Halenda (BJH) model was used to determine the pore size distribution of the samples [39]. The BET specific surface area was calculated using adsorption data within the relative pressure (P/P0) range of approximately 0.01–0.29. Samples were degassed at 105 °C for 12 h prior to analysis.

2.4. CO2 Sorption Experiments

A high-pressure volumetric gas sorption analyzer (iSorb HP2, Anton Paar, Ashland, VA, USA) equipped with furnace and circulator water bath systems for temperature control was used to perform gas sorption experiments at pressures ranging from 0.1 to 20 bar and temperature points of 15, 25, 35, and 45 °C. The samples (~0.7 g) were degassed at 105 °C for 12 h prior to CO2 sorption experiments. Gas sorption data were interpreted using the Helmholtz equation embedded in the instrument software interface, specifically utilizing the Schmidt–Wenzel equation. This equation is defined in terms of Helmholtz residual free energy and is an improvement of the Benedict–Webb–Rubin equation for CO2. Provided by Span and Wagner, this equation accurately describes the fluid region for CO2 from the triple-point temperature to 1100 K, and pressures up to 800 MPa [40].

2.5. Post-Sorption Analysis

To better understand the CO2 sorption nature of both halloysite samples, whether physisorption or chemisorption plays the major role, selected CO2-laden samples were further characterized. Fourier transform infrared spectroscopy (FTIR) (Agilent-660/620), X-ray fluorescence spectroscopy (XRF) (Epsilon 1, PANalytical, Almelo, The Netherlands), X-ray diffraction (XRD), and thermogravimetric analysis (TGA) (Mettler-Toledo GmbH, Greifensee, Switzerland) were used for this purpose.

2.6. Utilization of Laden Materials

CO2-laden materials with the highest CO2 sorption capacity from each Hal and HalFe groups were selected and further evaluated for dye removal from aqueous environments, and their performance was compared to their non-CO2-laden counterparts to investigate the impact of sorbed CO2 on dye removal efficiency. The experiments were conducted using methylene blue (MB) as the adsorbate and ideal experimental conditions as reported elsewhere (pH 8, initial MB concentration of 10 ppm, and adsorbent dosage of 1 g L−1) [41]. In this process, 30 mg of selected materials were mixed with 30 mL of 10 ppm MB solution on a shaker at 150 strokes/min and 25 °C for 24 h. The mixture was centrifuged at 4500 rpm (RCF = 4415 g) for 15 min, and the concentration of residual MB in the supernatant was measured using a UV–Vis spectrophotometer (UV-3600 plus, Shimadzu, Kyoto, Japan) at λmax = 661 nm, and the dye-laden adsorbents were collected for further FTIR analysis.

3. Results and Discussion

3.1. Material Characterization

3.1.1. Morphological Analysis

The Hal150 and HalFe150 morphologies are characterized primarily by tubular forms of halloysite structures accompanied by platy-shaped kaolinite (Figure 1). The HNTs had open-ended cylindrical shapes with hollow lumens in the center along the tubes’ length (Figure 2). The average lengths of halloysite nanotubes in the Hal150 and HalFe150 samples were ~2.06 and 2.05 µm, respectively, with average widths of 157.24 nm and 126.54 nm.
Both Hal150 and HalFe150 samples show a reduction in length after 30 min of ball milling, measuring about ~1.62 µm and ~0.94 µm, respectively, and an increase in the width of halloysite nanotubes, suggesting their fragmentation into smaller tubes and gradual flattening. Kaolinite sheets were also broken down into smaller pieces during ball milling. This trend continued after further milling until reaching 60 min, resulting in even shorter and wider tubes (HNT length/width ~1.21 µm/245.8 nm, and ~0.85 µm/189.71 nm, respectively). This alteration in morphology may be attributed to two phenomena: (1) the unfolding of tubes, which leads to an increase in width, and (2) flattening of tubes due to the mechanical force of ball milling, contributing to their wider appearance in two-dimensional images.
The observed reduction in tube length and increase in tube width due to ball milling is consistent with the existing literature [42].
In this case, HalFe seems to be more severely impacted by ball milling than Hal, with a significantly large proportion of nanotubes being crushed and flattened, compromising their hollow tubular structure. It can be argued that the iron crystals present in the HalFe structure, which exhibits higher hardness, may serve as an additional milling medium, thereby enhancing milling efficiency by facilitating greater energy transfer from the balls to the material being ground [34].

3.1.2. Surface Area and Pore Volume

According to the International Union of Pure and Applied Chemistry (IUPAC), the N2 adsorption/desorption isotherm curves for the Hal and HalFe samples, both before and after ball milling treatment (Figure 3a,b), fall into the type II physisorption isotherm classification, exhibiting associated H3 hysteresis loops [43]. BET model indicates the specific surface area (SSA) of Hal samples increased by 15% and 26% after ball milling for 30 min and 60 min, respectively. HalFe samples also showed increases in SSA of 6.5% and 12% in the same milling durations (Figure 3e). These results suggest that ball milling is effective in enhancing the physical properties of these materials as adsorbents, which is consistent with the existing literature [30,31]. The reason why HalFe samples exhibited lower enhancement in terms of SSA compared to Hal under identical milling conditions can be attributed to the more substantial impact of ball milling on HalFe samples. As evidenced in SEM and TEM images, the hollow tubular structure of nanotubes in the HalFe samples was severely damaged, resulting in a reduced increase of SSA for HalFe samples, less than half that of Hal sample.
Figure 3c,d present the pore size distribution for Hal and HalFe samples based on the BJH model, revealing that both materials predominantly consist of mesopores (pore diameter > 2 nm), with the highest peak intensities observed at ~2.8 nm [43]. The pore size ranges of ~2–3 nm and ~10–20 nm correspond to the surface pores and lumens of the nanotubes, respectively [42]. Notable upward shifts in the entire distribution curve for Hal samples following ball milling suggest the formation of new surface pores, unclogging of previously closed pores, and possible fragmentation and the unfolding of nanotubes, leading to an increase in pore volume and specific surface area and enhanced mesoporosity [44]. In contrast, for HalFe samples, the upward shifts primarily occurred in the ~2–3 nm region (surface pores), accompanied by a slight downward shift in the ~9–18 nm range (lumen). This indicates damage to the hollow tubular structure of halloysite during ball milling by partially collapsing and flattening of tubes, meaning larger lumen pores were compromised while smaller surface pores became relatively more accessible. This observation aligns with the increasing trend of SSA from ball milling and the fact that the SSA increase for Hal samples was nearly double that of HalFe samples.

3.1.3. Zeta Potential for Surface Charge Values

As shown in Figure 3f, a downward trend in zeta potential was observed for Hal samples subjected to ball milling. Low-energy ball milling typically increases the surface area of the particles by breaking them down into smaller sizes, as confirmed by SEM and BET analyses. This process exposes more surface functional groups, leading to an increase in the negative surface charge [35].
However, the zeta potential for HalFe becomes less negative after 30 min of milling and shows minimal change thereafter. The HalFe sample contains both Fe2+ and Fe3+ [17]. It can be argued that the mechanical force and pressure exerted during ball milling may have induced an oxidation process in the iron content of HalFe, resulting in an increased Fe3+/Fe2+ ratio. A similar phenomenon has also been reported for montmorillonite [45].

3.1.4. XRD Analysis

The patterns for both Hal150 and HalFe150 exhibit sharp, well-defined peaks, indicating a highly crystalline structure, as shown in Figure 4a,b. The sharp peaks at 2θ = 12.3° (7.1 Å) correspond to the (001) reflection plane, confirming the predominance of the kaolin group. While quartz and feldspar are present as impurities in both samples, mica/illite minerals (2θ = 8.8°) are found exclusively in HalFe150, which appears to be associated with the presence of iron oxides within the mineral assemblage [17]. Overall, the sharp and high-intensity peaks of the treated materials indicate that the crystalline structure of both Hal and HalFe samples remains intact and has not deteriorated during the short-term ball milling.

3.1.5. Overall Changes in Milled Samples and Implications

Overall, milling energy resulted in varied damages to the morphology, particularly HNT tubes, while maintaining the crystallinity of both HNT and kaolinite in the samples. The significant changes that occurred in the physicochemical properties of the materials due to ball milling are summarized in Table 2.

3.2. CO2 Sorption Experiments

As shown in Figure 5a,b, the maximum CO2 sorption capacities of both Hal150 and HalFe150 were 8.70 mg/g and 12.37 mg/g, respectively, achieved at 15 °C under a pressure of 20 bar. The CO2 sorption performance for Hal150 was significantly influenced by temperature, with a marked decrease observed as the temperature rose. This trend resulted in a reduction of more than half at 45 °C compared to 15 °C. This relationship between temperature and adsorption rate can be attributed to the dominance of physisorption and the exothermic nature of CO2 sorption [46]. A similar downward trend was also observed for HalFe150, except for the CO2 sorption amount at 35 °C, which was higher than that at 25 °C but decreased at 45 °C. This behavior, which is contradictory to the general exothermic nature of CO2 physisorption, may be attributed to the presence of iron impurities in HalFe150, potentially creating additional active sites that exhibit temperature-dependent interaction with CO2 [22,47]. This means while physisorption remains dominant, additional interactions involving iron sites may become slightly more favorable at 35 °C, leading to a higher CO2 sorption capacity compared to 25 °C. Overall, HalFe150 demonstrated superior CO2 sorption capacity and operational temperature conditions compared to Hal150, which is consistent with the former having a higher specific surface area and better pore characteristics than the latter.
Subsequent evaluation of the treated materials under the optimal temperature of 15 °C revealed their CO2 sorption performance. As shown in Figure 5c,d, Hal150 and Hal30M demonstrated fairly similar performance in identical experimental conditions. However, Hal60M showed a notable enhancement in CO2 sorption performance, with a value of almost 14% higher than Hal150. More importantly, the CO2 sorption performance of Hal60M in the atmospheric pressure range (inset, Figure 5c) showed a drastic 3-fold increase compared to Hal150. This finding is of particular significance, showing the enhanced potential of Hal60M for applications in diffuse polluted areas such as livestock sheds.
Consistent with previous observations, milling for 30 min did not affect the CO2 sorption performance of HalFe. Nevertheless, further milling of HalFe for 60 min caused a considerable drop of ~14% in sorption capacity compared to the untreated HalFe150. This reduction in performance for treated HalFe can be attributed to severe damage to the halloysite hollow tubular structure due to prolonged milling in contrast to treated Hal, as evidenced by SEM and TEM images. The reduction in adsorption capacity due to prolonged ball milling has also been reported for non-kaolin clays, such as montmorillonite [48,49]. In our case, these iron impurities were inherent properties of the clay material, and altered functional groups associated with an increased Fe3⁺/Fe2⁺ ratio of milled HalFe might have contributed to the reduced CO2 sorption performance. However, the precise role of different states of iron in the clay matrix on CO2 sorption warrants further study.

3.3. Mechanism of CO2 Sorption on Raw and Milled Materials

To further investigate the nature of CO2 sorption, Hal60M and HalFe150, which exhibited the highest sorption performance within their respective groups, were selected for post-sorption analysis.
Thermogravimetric analysis (TGA) of Hal60M and HalFe150, both before and after CO2 sorption, is presented in Figure 6, along with their corresponding DTG curves. The CO2-sorbed samples labelled Hal60M-AS and HalFe150-AS demonstrated greater weight loss during complete combustion, accounting for 0.6% and 1.6%, respectively, compared to their feedstock counterpart. This increase in weight loss confirms the presence of CO2 in the materials. Notably, HalFe150-AS exhibited approximately three times the weight loss of Hal60M-AS, consistent with the results of sorption experiments that indicated HalFe150 enhanced CO2 sorption efficiency (see Figure 5).
Hal60M and Hal60M-AS showed a single thermal event around 510 °C, attributed to the dehydroxylation of aluminol groups [50,51,52,53]. HalFe150 and HalFe150-AS displayed an additional thermal event near 275 °C, which can be associated with the oxidation and decomposition of the iron content [54,55]. In contrast to HalFe150, HalFe150-AS showed roughly 0.5% weight loss at the 200–370 °C range, which can be attributed to the desorption of CO2. The temperature at which CO2 desorption occurs can vary depending on the specific adsorbent materials used and their properties and is typically in the range of 50–150 °C [56,57,58]. In our case, the weight loss happened in an elevated temperature range, which may be attributed to the presence of stronger interactions between CO2 and the iron-containing surface sites in HalFe150. However, the absence of carbonate formation can suggest the iron has only facilitated the surface adsorption of CO2 onto HalFe150 [47]. Detailed analyses of the thermal events for both materials are provided in Figures S2 and S3.
In the case of CO2 sorption inside the interlayer space of materials, the (001) diffraction plane d-spacing is expected to increase, which would cause a shift in the respective peak [59]. Additionally, the formation of new peaks may suggest the occurrence of chemisorption, for example, through the formation of carbonates or other chemically bound species. However, as shown in Figure 7, no new peaks or shifts in existing peaks were detected after sorption for both materials, which suggests the interlayer spacing was not engaged during CO2 sorption experiments, and that the physisorption played the major part in the CO2 sorption.
Further evidence to support the dominance of physisorption comes from the XRF profiles of selected samples before and after CO2 sorption (Table S1), showing for both Hal60M and HalFe150 no carbonates were detected after CO2 sorption, defying the chemisorption pathway through the formation of carbonates. This result indicates that physisorption was the primary mechanism of CO2 sorption in these materials.
The FTIR spectra of the selected materials (Figure S4) remained unchanged after CO2 sorption, with no new peaks observed in the carbonate-related bands at 1460, 880, and 712 cm−1 [60]. This finding confirms XRF results, indicating that carbonates were not formed during CO2 sorption experiments and highlighting the predominance of physisorption.

3.4. Utilization of Milled and CO2-Laden Materials

Both Hal60M and HalFe150 demonstrated excellent MB removal performance, achieving removal efficiencies of approximately 89% and 93%, respectively (Figure 8a).
The appearance of two new peaks at 1390 cm−1 and 1340 cm−1 in the FTIR spectra of the materials after contact with MB, which correspond to C-H and C-N, respectively, can be attributed to MB sorption (Figure 8b) [61,62]. Surprisingly, their CO2-laden counterparts also exhibited strong MB removal performance, with only a slight reduction of 2%–5%. This decrease can be attributed to the occupation of some adsorption sites by CO2 [63]. Islam et al. [64] reported that a synthesized zeolite, after CO2 capture, showed better MB sorption performance compared to its non-CO2 sorbed counterpart. They suggested that because the mechanism of CO2 sorption involved chemisorption through the formation of carbonates, this enhanced MB sorption was due to the strong complexation of the formed carbonates with MB. In contrast, the dominant mechanism of CO2 sorption in the present study was physisorption, which resulted in a slight decrease in MB removal efficiency following CO2 sorption. Nevertheless, the materials still demonstrated significant potential for efficient MB removal, indicating that rather than disposal of the CO2-sorbed (spent) adsorbent materials, they can be further utilized effectively in remediation applications, thus promoting a circular economy. The changes in appearance of adsorbent materials and MB solution before and after MB removal are shown in Figure 8c.
It is worth noting that other experimental and environmental parameters, such as temperature, pH, and adsorbent dosage, may influence the MB sorption performance of these materials. This suggests the need for further in-depth research to optimize the MB removal process using CO2-laden materials, with the goal of achieving even better performance.

4. Conclusions

The effectiveness of ball milling as a sustainable and cost-efficient approach for enhancing the carbon capture performance of halloysite-rich kaolin clay was studied. In this case, grinding by ball milling altered the structural and physicochemical properties of both iron-poor (Hal) and iron-rich (HalFe) halloysite samples. It notably enhanced the surface area and mesoporosity of Hal sample, especially after 60 min of milling, leading to a significant 3-fold improvement in CO2 sorption capacity in near-atmospheric conditions. In contrast, the CO2 sorption performance of HalFe decreased after milling due to crystal damage of the HNT tubular morphology. In either case, physisorption predominates for CO2 onto the studied halloysite samples.
One would argue that studied halloysite-rich kaolin clay samples, without chemical activation, showed limited CO2 sorption capacity compared to many chemically synthesized clay-based composites and commercial adsorbents such as activated carbon and zeolites. However, good adsorption at near atmospheric pressure and the effective utilization of CO2-laden materials for dye removal from aqueous solutions highlighted the dual functionality of these potentially sustainable and cost-efficient materials. Despite that, we identified a few limitations of this study, including the role of moisture content in materials for CO2 sorption and reusability of CO2-sorbed dye-laden material. Studying the long-term stability of the captured CO2 within the materials and life cycle assessment of the used materials could have also been useful for assessing the effective sequestration, potential utilization, and the overall cost-benefit aspects of this approach. Future research should focus on these aspects to obtain a broader understanding of the studied material in the carbon capture and utilization area.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min15040399/s1, Figure S1: Cloud Nine Halloysite-Kaolin location and physical appearance of raw clay samples as received without any processing; Figure S2: Detailed thermal analysis of Hal60M and Hal60M-AS; Figure S3: Detailed thermal analysis of HalFe150 and HalFe150-AS; Figure S4: FTIR spectra of Hal60M and HalFe150 before and after CO2 sorption; Table S1: XRF analysis of Hal60M and HalFe150 before and after CO2 sorption experiments.

Author Contributions

Conceptualization, S.D. and B.B.; Methodology, S.D. and B.B.; Formal Analysis, S.D.; Investigation, S.D.; Resources, B.B. and R.N.; Writing—Original Draft Preparation, S.D.; Writing—Review and Editing, B.B. and R.N.; Visualization, S.D.; Supervision, B.B. and R.N.; Project Administration, B.B. and R.N.; Funding Acquisition, B.B. and R.N. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding authors.

Acknowledgments

First author acknowledges ESG Minerals Australia for supplying raw clay matrix from Western Australian deposit, and also thanks crcCARE for industry liaison and logistic support. All authors acknowledge Balu Thomboare and Laurence N Warr, Institute of Geography and Geology, University of Greifswald, for performing TEM analysis. TEM facility was funded by the Deutsche Forschungsgemeinschaft (DFG project # 42802702; https://gepris.dfg.de/gepris/projekt/428027021 accessed on 15 October 2024), and SEM and XRD facilities supported by crcCARE (https://crccare.com/), operated by Saianand Gopalan.

Conflicts of Interest

The authors declare no conflict of interest.

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Minerals 15 00399 g001
Figure 1. SEM images of Hal and HalFe samples prior to and after ball milling for 30 and 60 min. symbol “+” indicates increase, and “−” is the decrease of the value.
Figure 1. SEM images of Hal and HalFe samples prior to and after ball milling for 30 and 60 min. symbol “+” indicates increase, and “−” is the decrease of the value.
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Figure 2. TEM micrographs of Hal and HalFe samples prior to and after ball milling for 60 min.
Figure 2. TEM micrographs of Hal and HalFe samples prior to and after ball milling for 60 min.
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Figure 3. N2 adsorption/desorption isotherms (a,b), BJH pore size distribution (c,d), BET specific surface area (e), and zeta potential (f) of pristine and treated Hal and HalFe samples.
Figure 3. N2 adsorption/desorption isotherms (a,b), BJH pore size distribution (c,d), BET specific surface area (e), and zeta potential (f) of pristine and treated Hal and HalFe samples.
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Figure 4. XRD patterns of (a) Hal and (b) HalFe samples prior to and after ball milling. * = kaolin; Φ = K-feldspar; θ = quartz; Δ = mica/illite. Two inset figures are magnified d001 regions of mica/illite d001 (8.7–9° 2θ) and halloysite-kaolinite (12–13° 2θ), respectively.
Figure 4. XRD patterns of (a) Hal and (b) HalFe samples prior to and after ball milling. * = kaolin; Φ = K-feldspar; θ = quartz; Δ = mica/illite. Two inset figures are magnified d001 regions of mica/illite d001 (8.7–9° 2θ) and halloysite-kaolinite (12–13° 2θ), respectively.
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Figure 5. CO2 sorption isotherm data of (a) pristine Hal and (b) HalFe at various pressure and temperature points, and ball milling treated (c) Hal and (d) HalFe samples at 15 °C and 0.1–20 bar.
Figure 5. CO2 sorption isotherm data of (a) pristine Hal and (b) HalFe at various pressure and temperature points, and ball milling treated (c) Hal and (d) HalFe samples at 15 °C and 0.1–20 bar.
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Figure 6. Thermogravimetric analysis of (a) Hal60M and (b) HalFe150 before and after CO2 sorption (AS: after sorption).
Figure 6. Thermogravimetric analysis of (a) Hal60M and (b) HalFe150 before and after CO2 sorption (AS: after sorption).
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Figure 7. XRD patterns of Hal60M and HalFe150 before and after CO2 sorption (AS: after sorption).
Figure 7. XRD patterns of Hal60M and HalFe150 before and after CO2 sorption (AS: after sorption).
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Figure 8. (a) MB removal efficiency of selected materials (AS: After CO2 sorption), (b) FTIR spectra of CO2-laden materials and their non-CO2 sorbed counterparts before and after MB sorption, and (c) changes in appearance of selected materials and MB solution after MB removal.
Figure 8. (a) MB removal efficiency of selected materials (AS: After CO2 sorption), (b) FTIR spectra of CO2-laden materials and their non-CO2 sorbed counterparts before and after MB sorption, and (c) changes in appearance of selected materials and MB solution after MB removal.
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Table 1. Supplier-provided mineral composition of the kaolin samples [17].
Table 1. Supplier-provided mineral composition of the kaolin samples [17].
Halloysite (%)Kaolinite (%)Iron Oxide (%)Major Kaolin Clay Species
Hal58200.32Halloysite
HalFe42315.4Halloysite
Table 2. Summary of the changes to materials’ properties.
Table 2. Summary of the changes to materials’ properties.
HalHalFe
30M60M30M60M
MorphologyTube length~21% ↓~41% ↓~54% ↓~58% ↓
Tube width-~56% ↑-~50% ↑
Tubular structureMostly intact, with some tube breakage and flatteningMore damaged, with flattened and compromised tubular structures
Zeta potential↓↓↑↑
Surface area15% ↑26% ↑6.5% ↑12% ↑
Pore characteristicsImproved mesoporositySlightly improved mesoporosity
CrystallinityRetained √Retained √
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Davoodi, S.; Biswas, B.; Naidu, R. Carbon Capture Efficiency of Mechanically Activated Australian Halloysite-Rich Kaolin with Varying Iron Impurities and Its Potential Reuse for Removing Dyes from Water. Minerals 2025, 15, 399. https://doi.org/10.3390/min15040399

AMA Style

Davoodi S, Biswas B, Naidu R. Carbon Capture Efficiency of Mechanically Activated Australian Halloysite-Rich Kaolin with Varying Iron Impurities and Its Potential Reuse for Removing Dyes from Water. Minerals. 2025; 15(4):399. https://doi.org/10.3390/min15040399

Chicago/Turabian Style

Davoodi, Siavash, Bhabananda Biswas, and Ravi Naidu. 2025. "Carbon Capture Efficiency of Mechanically Activated Australian Halloysite-Rich Kaolin with Varying Iron Impurities and Its Potential Reuse for Removing Dyes from Water" Minerals 15, no. 4: 399. https://doi.org/10.3390/min15040399

APA Style

Davoodi, S., Biswas, B., & Naidu, R. (2025). Carbon Capture Efficiency of Mechanically Activated Australian Halloysite-Rich Kaolin with Varying Iron Impurities and Its Potential Reuse for Removing Dyes from Water. Minerals, 15(4), 399. https://doi.org/10.3390/min15040399

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