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10.3390/cleantechnol8010023
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Article

Evaluating Halloysite-Rich Kaolin/Biopolymer Composites for Enhanced Carbon Capture—A Study of Isotherms and Mechanisms

by
Siavash Davoodi
1,*,
Bhabananda Biswas
1,2,*,
Laurence N. Warr
3,
Balu R. Thombare
3 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
crcCARE Pty Ltd., Callaghan, NSW 2308, Australia
3
Institute of Geography and Geology, University of Greifswald, 17489 Greifswald, Germany
*
Authors to whom correspondence should be addressed.
Clean Technol. 2026, 8(1), 23; https://doi.org/10.3390/cleantechnol8010023
Submission received: 1 December 2025 / Revised: 24 January 2026 / Accepted: 28 January 2026 / Published: 3 February 2026
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Abstract

Anthropogenic CO2 emissions have accelerated climate change, prompting the need for effective capture technologies. Adsorption using clay-based sorbents offers an eco-friendly alternative, although performance often requires enhancement. This study explored mechanochemical modification of two halloysite-rich kaolin clay samples—iron-poor (Hal) and iron-rich (HalFe)—using locust bean gum and quillaja saponin and compared their CO2 uptake with the calcined counterparts (CHal, CHalFe). All samples were characterized using standard techniques, and their CO2 uptake was measured volumetrically across 0.1–20 bar and 15–35 °C. Modified sorbents showed enhanced mesoporosity and binding sites, increasing CO2 uptake by up to 26% at 20 bar (11.85 mg/g) and 125% at 1 bar (2.25 mg/g). Calcination, however, reduced surface area and sorption capacity. Isosteric heat values remained within the physisorption range, as supported by FTIR, XRF, and XPS, which showed no bulk carbonate formation. These sorbents show lower CO2 uptakes than conventional ones. Yet their low costs, abundance, biocompatibility, and solvent-free synthesis indicate strong potential for large-scale applications, especially for low-pressure implementations such as landfills. Further detailed studies on kinetics, thermodynamics, and sorbent regeneration are needed. Spent sorbents can potentially be repurposed for subsequent use in other applications, e.g., water treatment, construction materials, thereby minimizing waste production and supporting circular economy principles.

Graphical Abstract

1. Introduction

Global warming is driven by rising greenhouse gas (GHG) levels in the atmosphere, particularly CO2 from burning fossil fuels and industrial activities, and is considered a major environmental challenge [1,2]. Effective CO2 mitigation strategies are essential, with capture technologies such as absorption [3], adsorption [4], membrane separation [5], and cryogenic distillation [6] being explored. Among these, adsorption stands out for its operational simplicity, high efficiency, lower energy demand, durability under harsh conditions, and cost-effectiveness compared to absorption [7].
Currently, physical adsorption using porous materials such as activated carbons [8], carbon nanotubes (CNTs) [9], zeolites [10], and metal–organic frameworks (MOFs) [11] is widely applied due to their high surface areas and adsorption capacities. However, their large-scale use is limited by high costs, complex synthesis, moisture sensitivity, and regeneration challenges [12,13]. Moreover, in low but steady GHG emission settings (e.g., livestock or waste facilities), these materials may be impractical due to cost and containment issues. In contrast, clay minerals offer a promising low-cost alternative owing to their natural abundance, environmental friendliness, biocompatibility, and ease of modification [14]. Unlike synthesized porous adsorbents, where the main application is cyclic adsorption–regeneration, clays offer the potential to reuse spent clay adsorbents together with the captured CO2 in other applications. These include construction materials, for example as supplementary cementitious materials [15], where the release of pre-sorbed CO2 during cement hydration can promote carbonate formation, enabling permanent CO2 storage with enhancing mortar strength [16].
Among clay minerals, the nanotubular polymorph of halloysite (Hly), a dioctahedral 1:1 kaolinite-group clay mineral, popularly known as halloysite nanotubes (HNTs) has attracted attention for CO2 capture because of their unique tubular aluminosilicate structure with a hollow lumen and oppositely charged surfaces. The alumina-rich inner surface is positively charged, while the silica-rich outer surface is negatively charged across a wide pH range, enabling selective and targeted surface modification. As a result, HNTs also provide biocompatibility, tunable surface chemistry, and excellent mechanical properties [17,18,19]. Large deposits of halloysite nanotubes have been found worldwide in countries, such as Australia, New Zealand, USA, Brazil, and Mexico, with an annual global supply of tens of thousands of tons and a low price of ~$4/kg, contrasting with synthetic nanotubes such as CNTs at much higher prices of nearly $500/kg [20].
HNTs-related studies mainly utilize pure HNTs from commercial sources. However, in natural deposits, these minerals typically occur in kaolin lithologies alongside minerals like kaolinite (Kln) and impurities such as iron oxides, hydroxides, and sulfides, which form complex assemblages [21]. Separating HNTs from kaolin deposits often requires energy-intensive, costly processes, while retaining Kln and iron may be advantageous; for example, iron minerals such as hematite (Fe2O3), magnetite (Fe3O4), and wustite (FeO) can potentially enhance CO2 sorption performance through formation of siderite (FeCO3) [22,23,24].
Nonetheless, clays exhibit inherent limitations, including natural impurities and lower surface area and porosity compared to synthetic and commercial adsorbents, which can adversely impact their CO2 adsorption capacity [25]. However, the cost of clays is significantly lower than that of synthetic adsorbents such as MOFs and zeolites, even as low as 1% of the cost, when factoring in both the synthesis and application, especially at large scale [12].
To overcome the intrinsic limitations of clays, various modification techniques have been explored, including calcination where the material undergoes phase transition, acid or alkali etching, and surface functionalization using organic and inorganic compounds [21,26]. For example, Chen and Lu [27] showed the CO2 sorption capacity of kaolinite increased to 3.4 mg/g after it was treated with 3M H2SO4 for 10 h, compared to ~0 mg/g for pristine kaolinite. Pajdak, et al. [12], studied the effect of calcination at 600 °C on the CO2 sorption capacity of halloysite, showing an increase to 0.63 mmol/g from 0.54 mmol/g. For carbon capture applications, surface functionalization, typically using amines, has been widely studied to enhance the CO2 sorption performance of HNTs [14]. For instance, according to Wang, et al. [14], amino-bifunctionalized acid thermally activated halloysite nanotubes exhibited much higher CO2 sorption performance compared to the non-functionalized sample (1.03 mmol/g vs. 0.076 mmol/g). However, the large amounts of solvents and toxic reagents required in these methods are costly and also pose environmental safety risks once the end products are retired or disposed [28,29]. To tackle these obstacles, it is important to explore alternative modification approaches where solvent consumption is minimal to none, and the modifying agents are ideally biocompatible.
Ball milling is a sustainable, solvent-free mechanochemical technique that facilitates chemical transformations via mechanical energy (compression, shear, friction) often with simplicity and low energy consumption [29,30,31]. It is especially beneficial for synthesizing pharmaceutical materials where solvent avoidance is critical [28,31]. This method offers precise control over particle size and morphology, enhances surface area and reactivity, and is effective for producing nanomaterials and improving properties of carbon-based materials such as biochars. Its scalability and the uniform product distribution highlight its economic viability. Ball milling also aids in forming polymer–clay nanocomposites by resolving filler–matrix compatibility issues [29,30,31,32,33].
Biopolymers are gaining attention for CO2 capture due to their renewable, sustainable nature. They form biocomposites with nanofillers like graphene, CNTs, MOFs, and zeolites, enhancing mechanical strength, surface area, and porosity for improved CO2 adsorption [34,35]. Functional groups (e.g., amino, hydroxyl) grafted onto adsorbents further facilitate CO2 capture via weak interactions [36,37].
Locust bean gum (LBG), a non-ionic polysaccharide from Ceratonia siliqua seeds [38,39,40], offers high hydrophilicity, biocompatibility, salt and pH resistance, and mechanical strength, making it suitable for composite synthesis [41]. On the other hand, saponins (SPN), which are non-ionic biosurfactants from over 500 plant species, have amphiphilic structures that also provide strong surface activity. Beyond food and industrial uses, LBG and SPN are applied in nanoparticle synthesis, biopharmaceuticals, edible films, cosmetics, energy sectors, and soil stabilization, underscoring its versatility and extensive applicability [38,39,42]. Their biodegradability makes them ideal for eco-friendly uses like soil remediation, heavy metal removal, and hydrophobic pollutant recycling [43,44]. From a sustainability perspective, the fact that both LBG and SPN are bio-based and sourced from renewable agricultural value chains also supports the valorization of agricultural by-products and reduces reliance on non-renewable fossil-derived polymers.
Despite the promising properties of LBG and SPN, their application in environmental remediation, particularly as adsorbent surface modifiers and in biocomposite synthesis, remains underexplored. They were mostly used in aqueous medium, such as LBG/iron oxide nanocomposites for removing methylene blue and methyl violet [45], graphene oxide/LBG aerogels for Rhodamine-B and Indigo Carmine [41], chitosan/saponin–bentonite composite for methyl orange and Cr (VI) [46], organo–bentonite with saponin for methylene blue [47], and saponin-modified diatomite for azithromycin [44]. Recent studies have demonstrated the efficacy of biopolymer–clay composites for CO2 capture using chitosan and other polysaccharides, typically relying on amine-rich chemistries or wet-processing routes [34,36]. However, the application of LBG and SPN, which are non-ionic biopolymers unlike cationic chitosan, via a solvent-free mechanochemical route for CO2 capture remains unexplored.
Addressing the identified research gaps, this study aims to (i) evaluate the use of LBG and saponin for modifying HNTs-rich kaolin for CO2 sorption via ball milling, (ii) characterize the modified materials and assess their CO2 sorption performance, and (iii) investigate the underlying CO2 sorption mechanisms. Additionally, the effect of calcination, which is a conventional clay treatment method, on the CO2 sorption performance of halloysite-rich kaolin was evaluated and compared. We hypothesize that ball milling-assisted modification with these biopolymers can enhance CO2 uptake without additional chemicals or solvents, enabling potential utilization of CO2-laden materials in other applications.

2. Materials and Methods

2.1. Materials

Two variations of kaolin clays originating from the Cloud Nine deposit were supplied by ESG Minerals (Perth, Australia) and used in this study. These two variants mainly differed by the level of iron impurity present within their matrices [21,48]. For clarity, the variant with negligible iron impurities is denoted Hal, while the iron-bearing variant is denoted HalFe throughout this article. The supplier-reported proportions of halloysite, kaolinite, and iron oxide are summarized in Table 1, with halloysite being the dominant phase in both materials. A complete characterization of these raw clays has been published previously by the authors [21,48]. Prior to processing, both clays were dry-sieved through a 150 µm test sieve, and the <150 µm fraction was used throughout this study.
Locust bean gum (LBG) and Quillaja saponin (SPN) with 25-30% sapogenin content were purchased from Sigma-Aldrich (St. Louis, USA) and used as received. The chemical structures of LBG and SPN are depicted in Figure S1 [49,50].

2.2. Modification Procedure

A 10 g portion of each Hal and HalFe sample was mixed with 0.5 g of LBG or SPN and processed in a planetary ball mill (Across International PQ-N2, Bayswater, Australia) at 400 rpm for 30 min. This procedure used 1 L grade 304 stainless steel grinding jars, 8 mm zirconia beads, and a sample-to-bead weight ratio of 1:5. Milling alternated between forward and reverse rotation every 15 min, and a 30 min cooling-down pause between cycles. The entire milling operation was performed under normal atmosphere (air), without any pre-drying or pre-mixing. Final products were labeled as Hal/LBG, HalFe/LBG, Hal/SPN, and HalFe/SPN based on combining clay and the modifying agents. The biopolymer dosage (~5%) was chosen to allow potential use of spent adsorbents as supplementary cementitious materials, such that partial cement replacement at 10 wt.% [15] results in a final biopolymer content of ~0.5% in mortar, reported as the optimal concentration for concrete [51]. It should be noted that the ~5% biopolymer dosage was selected in the present study as a representative case study rather than an optimized loading, and that the observed trends may be dosage-dependent.
Additionally, Hal and HalFe samples were thermally treated (calcined) in a muffle furnace at 800 °C for 3 h, following Haw, et al. [15], and the resulting materials were denoted as CHal and CHalFe, respectively.

2.3. Material Characterization

Morphological and microstructural analysis of materials was conducted using scanning electron microscopy (SEM) (Zeiss Sigma VP FESEM, Oberkochen, Germany) and transmission electron microscopy (TEM) (JEM-2100 Plus 200kV LaB6 TEM, JEOL, Peabody, USA). Powdered samples were mounted on carbon tape and coated with a thin platinum layer (~4 nm) prior to SEM imaging. For TEM sample preparation, dispersed particles were deposited on a lacey carbon support and dried before imaging. The potential changes in halloysite nanotube dimensions of each sample were measured using ImageJ (v1.54F) from TEM micrographs by analyzing 10–20 particles from multiple distinct, non-overlapping fields of view to ensure representativeness. To avoid selection bias, all clearly distinguishable nanotubes within a given field of view were measured, and the mean value reported. Surface charge was assessed via zeta potential measurements, with samples dispersed in deionized water (0.02 wt.%) using a NanoPlus-HD analyzer (Particulate Systems, Norcross, USA).
Crystallographic changes were evaluated by X-ray diffraction (XRD) using an Empyrean diffractometer (PANalytical, Almelo, The Netherlands) with Cu Kα radiation (λ = 1.54060 Å). Data were acquired over 2θ = 5–70° with a step size of 0.039°, operating at 45 kV and 40 mA. X’Pert HighScore (v4.7) with the PDF-4/Minerals 2022 (ICDD) database was used for phase identification and FWHM measurement using Pseudo Voight fitting function across the whole 2θ range, with weighted profile residuals (Rwp) reported as an indication of fit quality [52].
The N2 adsorption−desorption isotherms were obtained at 77.4 K using a surface area analyzer (Micromeritics Tristar 3020, Norcross, USA). The specific surface area and pore size/volume were determined using the Brunauer–Emmett–Teller (BET) model [53], and the pore size distribution was assessed with the Barrett–Joyner–Halenda (BJH) model. Although BJH model has limitations, particularly in the microporous region (<2 nm), it is applicable here as the dominant porosity of the materials lies in the mesoporous range [54]. BET surface areas were computed from adsorption data in the relative pressure range P/P0 ≈ 0.01–0.29. Prior to analysis, raw materials were degassed at 105 °C for 12 h, whereas modified samples were degassed at 60 °C for 24 h. Fourier transform infrared spectroscopy (FTIR) (Agilent Cary 660/620, Santa Clara, USA) was employed to evaluate potential changes in surface functional groups of materials after modification, and thermogravimetric analysis (TGA) (Mettler-Toledo GmbH, Greifensee, Switzerland) to confirm polymer loading and assess the composites thermal stability.
X-ray photoelectron spectroscopy (XPS) (Kratos AXIS Ultra DLD, Manchester, UK) with monochromatic Al Kα at 1486.6 eV was used for survey and high-resolution scans of surface O, C, Si, Al, and Fe in raw, calcined, and LBG/SPN modified samples before and after CO2 adsorption. CasaXPS software (version 2.3.26PR1.0) was used for XPS data processing, with C1s = 284.8 eV calibration for adventitious carbon [55].

2.4. CO2 Sorption Experiments

CO2 sorption measurements were conducted using a volumetric high-pressure gas sorption analyzer (Anton Paar iSorb HP2, Ashland, USA), equipped with a furnace and circulator water bath for temperature control. Isotherms were measured over 0.1–20 bar at 15, 25, and 35 °C to investigate the CO2 capture performance of materials at both atmospheric and high pressures. For each pressure point within the target pressure range, 10 equilibrium points with an interval of 60 s were collected before reporting the corresponding CO2 uptake value. Approximately 0.7 g of sample used per run was degassed at 105 °C for 12 h for unmodified materials and at 60 °C for 24 h for modified materials prior to CO2 sorption experiments. The instrument’s software (iSorbHPwin v3.1.14) interpreted the gas sorption data using the integrated Helmholtz equation of state, specifically the Schmidt–Wenzel formulation and the Span–Wagner description for CO2. This description covers temperatures from the triple point up to high-temperature regimes (1100 K) and pressures extending to 800 MPa [56]. CO2-sorbed materials were then further analyzed via X-ray fluorescence (XRF) (Epsilon 1, PANalytical, Almelo, The Netherlands), FTIR, and XPS to better understand the mechanism of CO2 sorption and to determine whether physisorption or chemisorption is dominant.

2.5. Isosteric Heat of Adsorption

Isosteric heat of adsorption (Qst) is a key thermodynamic parameter that quantifies the heat released or absorbed during the uptake and is defined at a given loading of the sorbate. Its importance in the design and optimization of gas sorption systems arises from the fact that the adsorbent can heat up due to the energy released during uptake, potentially affecting the overall performance [57]. Qst (kJ/mol) was determined using the Clausius–Clapeyron equation embedded in the instrument’s software interface [58] based on pore volume values obtained from BET and absolute sorption isotherm data. This method utilizes the experimental isotherms (via spline interpolation) to generate isosteres at constant surface coverages, allowing for the precise calculation of adsorption enthalpy as a function of loading without assuming a specific isotherm model (e.g., Langmuir or Freundlich), and thereby avoiding potential errors associated with model fitting.

3. Results and Discussion

3.1. Material Characterization

3.1.1. Morphological Analysis

The pristine Hal and HalFe samples exhibited the expected tubular halloysite morphology, accompanied by stacked plate-like structures attributed to kaolinite (Figure 1) [21,48].
Ball milling-assisted modification using SPN and LBG induced morphological and microstructural changes in both Hal and HalFe samples. Both LBG/SPN modified samples exhibited noticeable fragmentation of HNTs and kaolinite sheets, rough surfaces, and irregular aggregation and agglomeration of particles. This can be attributed to the mechanical force of ball milling breaking down the particles [48], combined with the biopolymers covering particles surfaces and their acting as glue to hold particles together, thereby creating aggregates [44].
TEM images support the SEM findings, confirming the breakage and fragmentation of tubes and aggregation of particles after LBG/SPN modification (Figure 2). The mean tube lengths of the modified samples were 1.16 µm (Hal/LBG), 1.19 µm (Hal/SPN), 0.96 µm (HalFe/LBG), and 0.98 µm (HalFe/SPN), compared with ~2.0 µm for the pristine materials [48]. Corresponding mean widths were 158, 155, 126, and 125 nm, respectively, significantly smaller than those of the purely ball-milled samples reported previously (Hal60M ≈ 246 nm; HalFe60M ≈ 190 nm) [48]. While biopolymer-assisted milling shortened the nanotubes by roughly 40–50%, it limited lateral deformation and prevented the extensive flattening seen in unmodified milled samples. This effect may be due to the presence of LBG and SPN creating a viscous, cushioning micro-environment between particles during milling, thereby reducing interparticle friction and the amount of intracrystalline strain. Such polymer-assisted milling has been reported to limit filler breakage and preserve morphology in polymer–clay and polymer–cellulose nanocomposites [31,32,59,60]. Moreover, TEM images show a low contrast matrix material that appears to be covering the modified samples’ particles and forming agglomerates, which suggests the successful formation of composites [61].
In the case of calcination at 800 °C, CHal and CHalFe mostly preserved their tubular morphology, though with apparent denser packing, and without visible signs of fragmentation (See microscopy images in Figure S2). This is consistent with other studies, confirming that the tubular morphology of HNTs remains unaffected at temperatures below 900 °C [15], whereas metakaolinite particles undergo agglomeration during calcination [62].

3.1.2. Pore Characteristics and Surface Area

The adsorption data show that all tested materials have well-developed and open mesoporous networks (2–50 nm pores). This is confirmed by the characteristic Type IV adsorption curves and by the hysteresis loops caused by gas condensation and evaporation inside those pores (Figure 3a,b) [63].
Following ball mill-assisted modification, Hal/LBG showed a significant increase of nearly 60% and 56% in SSA and cumulative pore volume, respectively, compared to raw Hal (Figure 3c,d). This increase can be attributed to the fragmentation of particles during milling, which increased the external surface area, while preserving the hollow tubular structure of nanotubes and the lumen, as observed earlier in TEM images. The corresponding particle size distribution (PSD) curves also displayed significantly higher mesopore volumes, especially in the 4–6 nm range, indicating newly accessible mesoporous structures formed or stabilized by LBG (Figure 3e,f). In contrast, Hal/SPN exhibited severe pore blockage, drastically reducing SSA (67%), pore volume (80%), and mesopore accessibility, as evidenced by a significantly suppressed PSD curve [64]. On the other hand, both LBG/SPN modified HalFe samples showed a minor increase in SSA (~15%). Interestingly, in terms of enhancing SSA and cumulative pore volume, the LBG paired best with Hal (Hal/LBG), while SPN performed best with HalFe (HalFe/SPN).
Calcination slightly decreased SSA and cumulative pore volume, likely due to partial particle sintering or distortion [65]. The corresponding PSD curves showed minor decreases in mesopore volumes but retained similar pore size distribution, with peaks around 4–6 nm.

3.1.3. XRD Analysis

Raw Hal and HalFe samples exhibited sharp and well-defined reflections (Figure 4), indicating a highly crystalline structure. The diffraction peaks at 7.1 Å correspond to the (001) plane, confirming that kaolin minerals are the dominant phases. It should be noted that this peak represents the combined contribution of halloysite/kaolinite, and that the individual contributions of each component in the random powder orientation could not be effectively separated [66] Although both samples contain quartz and feldspar as impurities, mica/illite minerals (10 Å) were detected only in HalFe and are associated with the presence of iron oxides in its assemblage [21].
Biopolymer modifications via ball milling introduced minor changes. LBG/SPN modified samples exhibited reduced intensities and broadening in ∼7.1 Å halloysite/kaolinite reflections, as evidenced by an increase in the corresponding full width at half maximum (FWHM) values (Table 2), without creating new peaks, indicating reduced crystallinity due to mechanical milling and/or surface coverage by LBG and SPN. However, quartz peaks remained distinct and were unaffected.
Calcination at 800 °C led to significant structural changes, notably the disappearance of 7.1 Å halloysite/kaolinite reflections, suggesting the conversion of kaolinite into amorphous metakaolin. Thermal treat did not, however, affect the quartz reflections, indicative of the stability of this mineral at elevated temperatures.

3.1.4. Zeta Potential Analysis

According to zeta potential measurements (Figure 5), raw Hal and HalFe exhibited relatively stable negative surface charges. Both Hal/LBG and HalFe/LBG displayed a substantial decrease in surface charge negativity, suggesting successful coverage and shielding of the charged clay mineral surfaces by LBG, consistent with the findings of Mikofsky, et al. [67]. HalFe/LBG exhibited even lower negativity, suggesting enhanced LBG coverage of HalFe particles during milling. This may have been achieved due to presence of iron oxide crystals in HalFe matrix serving as an additional milling medium, enhancing surface coating efficiency [68].
Similarly, Hal/SPN showed a reduction in negative charge after modification, which is consistent with the literature [64]. However, the zeta potential for HalFe/SPN remained nearly identical to the raw HalFe sample (~21 mV), possibly suggesting adsorption without complete charge shielding [67].
Calcination had minor impacts on zeta potential, slightly increasing negativity for CHal while reducing it for CHalFe. This contrasting behavior can be explained by the increased proportion of positive surface sites as a result of increased Fe3+/Fe2+ ratio in CHalFe (0.55) compared to HalFe (0.46), which may have been caused by an induced oxidation in iron content during calcination (Table S1) [48].

3.1.5. Fourier Transform Infrared Spectroscopy

The FTIR spectra of raw, LBG/SPN modified, and calcined Hal and HalFe samples are presented in Figure 6. Raw Hal and HalFe samples displayed characteristic bands of halloysite and kaolinite, similar to those reported in the existing literature [21,60,69]. The bands at approximately 3695 and 3622 cm−1 are ascribed to the O–H stretching vibrations of surface and inner hydroxyl groups, and the band at 910 cm−1 refers to O–H deformation of hydroxyl groups. Additionally, the bands at around 3444 cm−1 and 1638 cm−1 are associated with O-H stretching and deformation of water, both absorbed and crystallization water. The bands at around 1030 and 754 cm−1 are associated with the vertical and in-plane stretching of Si–O groups, and the band at 1115 cm−1 is related to the apical Si–O groups. Moreover, the bands at 540 and 470 cm−1 are attributed to Si–O–Al and Si–O–Si [27,69].
Both SPN and LBG modifications introduced clear organic signatures, notably C–H stretching bands in the 2900–2850 cm−1 region. Hal/SPN and HalFe/SPN displayed an additional peak at 1731 cm−1, corresponding to stretching C=O [70,71,72]. Mineral-specific bands remained largely intact, confirming the modifications occurred through surface adsorption without structural alteration of clay minerals. Overall, FTIR results support the successful integration of biopolymer coatings onto the surfaces of Hal and HalFe samples, influencing surface chemistry without disrupting the inherent clay structure.
After calcination of Hal and HalFe at 800 °C, the sharp O–H stretching bands at 3695 and 3622 cm−1 disappeared, indicating loss of structural hydroxyls. In the fingerprint region, the Al–OH bending at 910 cm−1 was suppressed, indicating disruption of the octahedral sheet, while the Si–O stretching bands at 1115 and 1030 cm−1 collapsed into one weaker, broadened band. Low-wavenumber bands at 754, 690, 540, and 470 cm−1 were also attenuated or disappeared.

3.1.6. Thermogravimetric Analysis

Figure 7 shows the thermogravimetric profiles of raw and LBG/SPN modified Hal and HalFe. All samples followed the typical three-step pattern seen for aluminosilicate clays with organic additives: (i) low-temperature water loss, (ii) mid-temperature organic decomposition, and (iii) high-temperature dehydroxylation of the clay mineral’s structure. Other studies of organo/biopolymer clays report the same sequence: dehydration up to ~200 °C, organic losses in the ~200–400/450 °C range, and structural losses at ~430–650 °C depending on the clay mineral type [73,74,75].
The first mass-loss step (<~150–200 °C) is attributed to the evaporation of physically adsorbed water [76]. It is relatively small for the raw clays and varies after modification, and its magnitude often depends on surface chemistry/hydrophilicity after modification, as the organic molecules can change how much moisture the surface holds [73].
The second region (~200–400 °C) corresponds to the breakdown of the biopolymers [77]. Relative to raw Hal and HalFe, their LBG/SPN composites show a clear extra mass loss in this window, with DTG peaks at ~276–278 °C, confirming the presence of organics. Similar DTG peaks are reported for surfactant-modified montmorillonite, kaolinite, and halloysite across ~230–420 °C, with the number of peaks and their positions reflecting whether organics are on external surfaces or interlayers and how strongly they are bound [74]. LBG composites generally show a slightly larger weight loss than SPN composites, suggesting either higher LBG uptake or slightly lower thermal stability of LBG in these composites.
The third and main thermal event (~450–650 °C) is the structural dehydroxylation of the aluminosilicate lattice, with DTG peaks observed at approximately 500–510 °C [78]. It should be noted that in LBG/SPN composites, part of the organic decomposition tail can overlap with the early portion of the dehydroxylation stage.
At the end of the analysis, the raw Hal and HalFe leave higher residues than the composites. The lower residues of the composites directly indicate the combustion of the added organics, which is consistent with the extra mass losses in the second stage. LBG contributes slightly more organic matter than SPN, while Hal tends to retain a little more than HalFe, suggesting that Fe impurities in HalFe may alter the degradation pathway.

3.2. CO2 Sorption Experiments

As shown in Figure 8, across all tested materials, CO2 uptake peaked at 15 °C and decreased with increasing temperature to 25 °C and 35 °C, with a steep initial rise and near-linear growth at higher pressures, suggesting exothermic physisorption to be the dominant sorption mechanism [79].
LBG/SPN modification increased the CO2 uptake capacity of both Hal and HalFe, with steeper initial rises at low pressure and higher uptakes at 20 bar (Figure 8a–d). This increase can be attributed to (i) the addition of LBG/SPN functional groups (e.g., hydroxyl groups) providing additional weak interaction sites for CO2 without causing strong chemisorption and (ii) biopolymer coating stabilizing particle dispersion, which can help preserve external/lumen accessibility. In terms of CO2 sorption enhancement, Hal/LBG showed the highest increase (+26%) reaching 11.85 mg/g CO2 uptake, followed by Hal/SPN with 10.47 mg/g (+17%), HalFe/SPN with 14.79 mg/g (+16%), and HalFe/LBG with 13.85 mg/g (+9%) compared to their pristine counterparts at 20 bar. However, the absolute CO2 uptake amounts for LBG/SPN modified HalFe samples are notably higher than those of modified Hal, which can be attributed to the presence of iron oxide impurities in HalFe-related materials, creating additional active sites that interact with CO2 [80]. Interestingly, modified samples exhibited far more significant CO2 sorption enhancements at atmospheric pressure (1 bar) compared to raw samples, with Hal/LBG showing a 125% increase reaching 2.25 mg/g, followed by Hal/SPN with 1.96 mg/g (+97%), HalFe/SPN with 3.32 mg/g (+48%), and HalFe/LBG with 3.07 mg/g (+37%). This feature highlights the potential application of modified Hal and HalFe in low concentration but persistent emission sources, such as landfills and farmhouses.
Compared to our previous work on the sole effect of ball milling on CO2 capture performance of Hal and HalFe samples [48], ball mill-assisted LBG/SPN modification resulted in much higher CO2 uptakes than pure ball milling. Under similar milling times (30 min) and sorption temperatures (15 °C), Hal/LBG showed a 41% enhancement in CO2 uptake, followed by 25% for HalFe/SPN, 24% for Hal/SPN, and 15% for HalFe/LBG. Interestingly, the CO2 uptake enhancement of LBG/SPN-modified HalFe contrasts with our earlier finding that ball milling alone reduced CO2 adsorption of HalFe by damaging the hollow tubular structure of HNTs. In the present study, however, the hollow tubular structure of HNTs in LBG/SPN-modified HalFe samples remained largely intact (as evidenced by TEM), contributing to their enhanced CO2 sorption performance.
After calcination, the CO2 sorption performance of CHal and CHalFe at 15 °C displayed drops of 17% and 41%, respectively (Figure 8e,f), compared to their raw counterparts, which is consistent with their reduced mesoporosity as evidenced in BET results. It can also be attributed to the reported thermal effects in kaolin-group minerals at the calcination conditions used in this study. Heating at 800 °C removes adsorbed and structural water and dehydroxylates the layers, forming metakaolin/metahalloysite [81]. This reduces the density of surface –OH groups that weakly bond with CO2 and promotes aggregation, which is consistent with the observations made in FTIR and BET analyses. Contrastingly, in another study by Pajdak, et al. [12], calcination of halloysite (from halloysite open pit mine in Dunino, Poland) at 600 °C led to increased SSA and CO2 sorption capacity. However, aside from using a different calcination temperature, their halloysite feed material was also varied in nature from the ones used in the present study, as it was mainly consisted of nanotubes and nanoplates with an SSA of 62.7 m2/g—more than 6 times higher than ours—and Fe2O3 content of ~18%. These differences may have been responsible for the contrasting results.

3.3. Isosteric Heat of Adsorption

The isosteric heat of CO2 adsorption (Qst) decreased with loading across all samples (Figure 9), which is consistent with heterogeneous site-energy distributions in porous adsorbents where high-energy sites fill first and progressively weaker sites are engaged at higher coverages [82,83].
LBG/SPN modified Hal start from moderately high Qst and maintain larger heats across the working region, which can be ascribed to the polymers’ dense hydroxyl functionalities and the CO2 quadrupole, supporting dipole–quadrupole and H-bonding interactions and also creating mild interfacial confinement at coated external/interparticle regions that stabilizes the adsorbate [84]. Modified HalFe samples, especially HalFe/SPN, showed the most persistent Qst. This behavior, which is in agreement with the reported CO2 sorption isotherm data, can be attributed to the presence of additional active sites that interact with CO2 in iron-rich materials [48].
CHal showed the highest low-coverage Qst, which can be attributed to dehydroxylation of halloysite/kaolin layers and formation of defects, exposing under-coordinated surface atoms and increasing basicity [85]. However, the rapid decline of Qst indicates these strong sites are quite scarce. CHalFe started lower at ~23.1 kJ/mol at 0.56 mg/g and declined to ~14.3 kJ/mol at 4.5 mg/g, suggesting a reduction in either the number or the strength of the high-energy sites after calcination that may be due to the presence of iron impurities.

3.4. CO2 Sorption Mechanism

To better understand the nature of CO2 sorption by modified and calcined Hal and HalFe samples, CO2-loaded adsorbents were further examined after CO2 sorption experiments using XRF, FTIR, and XPS analyses, and the results were compared to their pre-sorption counterparts.
XRF profiles collected from materials after CO2 sorption experiments (Table S2) revealed that no carbonates were detected, suggesting physisorption is the dominant mechanism involved in CO2 sorption. However, XRF analysis is bulk-sensitive in nature and the lack of carbonate detection in analysis results does not defy the formation of trace surface carbonate species [86]. Similarly, comparing FTIR spectra of calcined and modified materials before and after CO2 sorption (Figure S3) showed no new peaks in carbonate-related region at 1460, 880, and 712 cm−1 [87], further emphasizing the dominance of physisorption through weak van der Waals attraction [88], and dipole–quadrupole and H-bond interactions with hydroxyl functional groups without formation of new chemical species [89,90].
From high-resolution XPS scans, C 1s O–C=O component at ~289 eV, O 1s carbonate-related component at ~531.5 eV, and O/Si ratios were compared between modified and calcined samples before (Figures S4–S6, Table S1) and after CO2 sorption (Figure S7, Table S3) to further evaluate potential formation of surface carbonate. Observing an increase in either or both the components, preferably together with a slight rise in O/Si, can be indicative of carbonate formation. From comparative results in Table 3, HalFe/SPN and HalFe/LBG exhibit substantial O 1s carbonate growth (+13.1 and +9.2, respectively), suggesting formation of carbonate even with C 1s O–C=O remaining unchanged. Hal/SPN also shows a positive O 1s carbonate shift (+8.2). In contrast, Hal/LBG and CHal showed no signs of carbonate formation.
Interestingly, LBG/SPN modified and calcined HalFe samples showed the clearest signs of carbonate formation, especially in the O 1s region, with a positive Δ in the O 1s carbonate-related component (+5.6 to +13.1). This suggests that the Fe content provides active sites that stabilize carbonate upon being exposed to CO2 [24]. HalFe/SPN showed the highest O 1s response, suggesting that the SPN-modified local environment may further stabilize surface carbonate, which is consistent with our findings in Section 3.3. However, FTIR and XRF could not show any indication of formation of carbonate or other new chemical species. This suggests that the scale of formed carbonate was quite insignificant and well below the detection limit of these instruments, ruling out chemisorption as an influential sorption mechanism. This is also consistent with our previous work, where XRD, XRF, and FTIR analyses did not detect carbonate formation on purely ball milled Hal and HalFe samples after CO2 sorption [48], and Chen and Lu′s [27] study on CO2 adsorption of kaolinite, where XRD and FTIR analyses did not detect formation of carbonate.

4. Conclusions

Using simple, low-cost and industry-friendly approaches, this study evaluated the potential for CO2 capture enhancement in two naturally occurring halloysite-rich kaolin clays: one iron-poor (Hal), and the other iron-rich (HalFe). Ball mill-assisted modification of these minerals with LBG or SPN notably improved the CO2 uptakes. For instance, Hal/LBG exhibited a 60% increase in specific surface area (reaching 15.52 m2/g) and a 56% rise in cumulative pore volume compared to raw Hal, as confirmed by N2 adsorption-desorption isotherms. This structural enhancement translated to a +26% increase in CO2 uptake compared to its raw counterpart (Hal) (reaching 11.85 mg/g at 20 bar), which is also 41% higher than purely ball-milled raw Hal in similar milling and sorption conditions. More interestingly, it led to a 125% increase in CO2 uptake at atmospheric pressure (1 bar), reaching 2.25 mg/g. These results highlight their potential application in diffused emission sources, such as farmhouses and landfills. Mechanistically, milling with biopolymers helped open or preserve mesopores and prevented excessive particle aggregation, while introducing additional –OH groups that enhance CO2 uptake via weak van der Waals attractions, dipole–quadrupole attractions, and H-bonding with CO2 molecules. The lack of formation of carbonates or other new chemical species after CO2 sorption, as evidenced by FTIR, XRF, and XPS analyses, suggests the dominance of physisorption.
Many carbon capture methods are effective but expensive or complex. Our approach leverages abundant natural clays, natural renewable biopolymers, and solvent-free processing delivering improved CO2 sorption performance across both high- and low-pressure regimes, which is potentially useful for diffuse low-concentration sources. Moreover, the biocompatibility of the raw materials and modifying agents supports the potential repurposing of spent adsorbents along with the captured CO2 in other applications such as developing construction materials orsoil amendments. This approach can reduce waste production and associated costs while promoting circular economy.
However, this study has certain limitations, including the absence of data on biopolymer dosage optimization, adsorption kinetics, stability of the biopolymer coating, cyclic regenerability of adsorbents, the impact of moisture and mixed gas environments on CO2 capture performance, and in-depth quantitative/correlational analysis to explain the contrasting behavior of Hal/SPN. Future work should address these gaps through targeted experiments under realistic conditions to further validate and optimize the approach.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/cleantechnol8010023/s1, Figure S1: Chemical structures of (a) locust bean gum (LBG), and (b) quillaja saponin (SPN); Figure S2: SEM (a) and TEM (b) micrographs of CHal and CHalFe; Figure S3: FTIR spectra of calcined and modified materials before and after CO2 sorption; Figure S4: High-resolution XPS spectra of raw, calcined, and modified Hal (O1s, C1s, and Al 2p); Figure S5; High-resolution XPS spectra of raw, calcined, and modified HalFe (O1s, C1s, and Al 2p); Figure S6: High-resolution XPS spectra of raw, calcined, and modified HalFe (Fe 2p); Table S1: XPS high-resolution components’ fitting data of raw, calcined, and modified materials; Table S2: XRF profiles of calcined and modified samples after CO2 sorption; Figure S7: High-resolution XPS spectra of calcined and modified Hal and HalFe after CO2 sorption; Table S3: XPS high-resolution components’ fitting data of calcined and modified materials after CO2 sorption.

Author Contributions

Conceptualization, S.D. and B.B.; Methodology, S.D. and B.B.; Formal Analysis, S.D.; Investigation, S.D.; Resources, B.B., R.N., L.N.W. and B.R.T.; Writing—Original Draft Preparation, S.D.; Writing—Review and Editing, S.D., B.B., R.N., L.N.W. and B.R.T.; 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 and Supplementary Material. Further inquiries can be directed to the corresponding authors.

Acknowledgments

The authors acknowledge ESG Minerals Australia for supplying raw clay samples from Cloud Nine deposit and thank crcCARE (https://crccare.com/ accessed on 17 November 2025) for industry liaison, logistic support, PhD fellowship, and providing SEM and XRD facilities, operated by Saianand Gopalan. The authors also acknowledge Future Industries Institute, University of South Australia for providing XPS facilities and thank The Deutsche Forschungsgemeinschaft (DFG), Germany for financing the JEOL 2100 Plus TEM microscope used in this study (DFG project number 428027021).

Conflicts of Interest

Ravi Naidu is employed by the University of Newcastle, Australia, and is seconded to crcCARE Pty Ltd. Bhabananda Biswas was solely employed by the University of Newcastle, Australia, at the time this research was conducted. The other authors declare that the research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. SEM images of raw and LBG/SPN modified Hal and HalFe samples.
Figure 1. SEM images of raw and LBG/SPN modified Hal and HalFe samples.
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Figure 2. TEM images of raw and LBG/SPN modified Hal and HalFe samples.
Figure 2. TEM images of raw and LBG/SPN modified Hal and HalFe samples.
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Figure 3. N2 adsorption-desorption isotherms (a,b), BET specific surface area (c), cumulative pore volume (d), and BJH pore size distribution curves (e,f).
Figure 3. N2 adsorption-desorption isotherms (a,b), BET specific surface area (c), cumulative pore volume (d), and BJH pore size distribution curves (e,f).
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Figure 4. XRD diffraction patterns of pristine, modified, and calcined (a) Hal, and (b) HalFe samples. (major minerals phases: * = halloysite/kaolinite; Φ = K-feldspar; θ = quartz; ∆ = mica/illite).
Figure 4. XRD diffraction patterns of pristine, modified, and calcined (a) Hal, and (b) HalFe samples. (major minerals phases: * = halloysite/kaolinite; Φ = K-feldspar; θ = quartz; ∆ = mica/illite).
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Figure 5. Zeta potential of raw, modified, and calcined materials.
Figure 5. Zeta potential of raw, modified, and calcined materials.
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Figure 6. Comparative FTIR spectra of raw, modified, and calcined samples, as shown in (a–f) (cut and magnified sections correspond to 2800–3000 cm−1 regions).
Figure 6. Comparative FTIR spectra of raw, modified, and calcined samples, as shown in (a–f) (cut and magnified sections correspond to 2800–3000 cm−1 regions).
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Figure 7. Comparative TGA analysis of raw and modified (a) Hal and (b) HalFe.
Figure 7. Comparative TGA analysis of raw and modified (a) Hal and (b) HalFe.
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Figure 8. CO2 sorption isotherm data of LBG/SPN modified Hal (a,b) and HalFe (c,d), and calcined Hal (e) and HalFe (f), at various pressures and temperature points. Corresponding data for pristine Hal and HalFe at 15 °C are shown as dashed lines for reference. The values stated at the end of each curve represent the corresponding CO2 uptake capacities at 20 bar.
Figure 8. CO2 sorption isotherm data of LBG/SPN modified Hal (a,b) and HalFe (c,d), and calcined Hal (e) and HalFe (f), at various pressures and temperature points. Corresponding data for pristine Hal and HalFe at 15 °C are shown as dashed lines for reference. The values stated at the end of each curve represent the corresponding CO2 uptake capacities at 20 bar.
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Figure 9. Isosteric heat of CO2 adsorption for calcined and LBG/SPN modified (a) Hal and (b) HalFe. Data corresponding to raw Hal and HalFe are also included for reference.
Figure 9. Isosteric heat of CO2 adsorption for calcined and LBG/SPN modified (a) Hal and (b) HalFe. Data corresponding to raw Hal and HalFe are also included for reference.
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Table 1. Mineral composition of the kaolin samples (provided by the supplier) [21].
Table 1. Mineral composition of the kaolin samples (provided by the supplier) [21].
SamplesHly (%)Kln (%)Iron Oxide (%)Major Kaolin Species
Hal58200.32HNTs
HalFe42315.4HNTs
Table 2. FWHM values for 7.1 Å halloysite/kaolinite reflections.
Table 2. FWHM values for 7.1 Å halloysite/kaolinite reflections.
SamplesHalHalFeHal/LBGHal/SPNHalFe/LBGHalFe/SPN
FWHM (°2θ)0.19480.23330.24530.24200.26030.2606
Rwp7.365.796.537.106.014.80
Table 3. Changes in carbonate-related components from high-resolution scans and elemental O/Si ratios from survey scans.
Table 3. Changes in carbonate-related components from high-resolution scans and elemental O/Si ratios from survey scans.
PairΔ C 1s O–C=OΔ O 1s CarbonateΔ O/Si
Hal/LBG, Hal/LBG-CO2−10.3−3.6−0.07
Hal/SPN, Hal/SPN-CO20.0+8.2+0.10
HalFe/LBG, HalFe/LBG-CO20.0+9.2+0.06
HalFe/SPN, HalFe/SPN-CO20.0+13.1−0.01
CHal, CHal-CO20.0+1.2+0.02
CHalFe, CHalFe-CO2+7.4+5.6+0.05
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Davoodi, S.; Biswas, B.; Warr, L.N.; Thombare, B.R.; Naidu, R. Evaluating Halloysite-Rich Kaolin/Biopolymer Composites for Enhanced Carbon Capture—A Study of Isotherms and Mechanisms. Clean Technol. 2026, 8, 23. https://doi.org/10.3390/cleantechnol8010023

AMA Style

Davoodi S, Biswas B, Warr LN, Thombare BR, Naidu R. Evaluating Halloysite-Rich Kaolin/Biopolymer Composites for Enhanced Carbon Capture—A Study of Isotherms and Mechanisms. Clean Technologies. 2026; 8(1):23. https://doi.org/10.3390/cleantechnol8010023

Chicago/Turabian Style

Davoodi, Siavash, Bhabananda Biswas, Laurence N. Warr, Balu R. Thombare, and Ravi Naidu. 2026. "Evaluating Halloysite-Rich Kaolin/Biopolymer Composites for Enhanced Carbon Capture—A Study of Isotherms and Mechanisms" Clean Technologies 8, no. 1: 23. https://doi.org/10.3390/cleantechnol8010023

APA Style

Davoodi, S., Biswas, B., Warr, L. N., Thombare, B. R., & Naidu, R. (2026). Evaluating Halloysite-Rich Kaolin/Biopolymer Composites for Enhanced Carbon Capture—A Study of Isotherms and Mechanisms. Clean Technologies, 8(1), 23. https://doi.org/10.3390/cleantechnol8010023

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