Pore-Matching Adsorption of Fullerenols by Cuttlebone Material with Suitable Pore Size

Abstract

Fullerenols (PHFs) are emerging pollutants with health risks, yet conventional adsorbents suffer from small pore sizes, limited effective pore quantities, and low unit area adsorption capacities. This study used cuttlebone (CB), a fishery waste, to prepare carbonized CB (CCB), acid-modified CB (HCB), and a combined carbonized and acidified CB (HCCB) as adsorbents for PHF. These modified adsorbent materials showed increases in effective pores (50–200 nm) for PHF adsorption: 33% (CCB), 23% (HCB), and 41% (HCCB). The initial adsorption rate and adsorption capacity of CCB for PHF increased with higher pyrolysis temperatures due to reduced electrostatic repulsion, increased π–π interactions, and the transformation of aragonite into calcite of CB calcium carbonate skeleton. Acid modification enhances pore filling and thus capacity but slows kinetics via longer intraparticle diffusion; both effects intensify with higher acid concentration and longer treatment. The unit area adsorption capacities of CCB, HCB, and HCCB for PHF were 3.11, 1.18, and 2.11 mg/m², much higher than traditional materials. This research effectively removes PHF and upgrades marine waste into a high-value adsorbent, practicing “pollution control through waste utilization.” The results have both pollution control and circular economy value, expanding sustainable and low-cost material paths, promoting the integration of multiple disciplines such as environmental engineering, materials science, and marine science, and providing new ideas for the treatment of nanomaterial pollutants.

Keywords

cuttlebone effective pores fullerenol unit area adsorption capacities

Introduction

As an important member of the carbon nanomaterial family, fullerenes and their derivatives have demonstrated unique interdisciplinary value in fields such as environmental remediation (e.g., catalytic degradation of pollutants), biomedicine (e.g., design of drug carriers), and functional materials (e.g., flexible electronic devices) (Mousavi et al., 2017; Zhang et al., 2019). At present, with the expansion of production and application of fullerene-based carbon nanomaterials, they inevitably enter the environment. Although pristine fullerenes are highly hydrophobic (Mchedlov-Petrossyan et al., 2023), their hydroxylated derivatives—fullerenol (PHF)—exhibit high aqueous solubility (Liu et al., 2018), thereby increasing their mobility in soils and in surface and groundwaters. By sorbing or complexing environmental pollutants (e.g., polycyclic aromatic hydrocarbons and metal ions) and interacting with exposed organisms (e.g., zebrafish and wheat), PHF can act as carrier phases, potentially promoting cotransport and altering the bioavailability of associated contaminants (Gai et al., 2011; Wang et al., 2016; Yuan and Peng, 2017; Shi et al., 2020). Numerous studies have shown that PHF in the environment exerts toxic effects on cells, microorganisms, aquatic organisms, and terrestrial animals (Henry et al., 2007; Charykov et al., 2016; Nakamura et al., 2018; Mchedlov-Petrossyan et al., 2023). The toxic mechanism of PHF mainly involved inducing oxidative stress and mitochondrial dysfunction, and it even has genotoxicity for some cells (such as lymphocytes and somatic cells) (Dhawan et al., 2006; Trpkovic et al., 2012; Yang et al., 2016). PHF can also enter the human body directly through drug ingestion, skin contact, and other pathways, posing a threat to human health (Bolshakova et al., 2019; Chen et al., 2025). Long-term exposure to PHF may lead to liver and kidney damage as well as neurological disorders, making it a new type of environmental health risk factor. Therefore, it is urgent to study efficient removal methods of PHF.

Unlike the handling of other pollutants, PHFs are cytotoxic and antioxidant, so they are not suitable for biodegradation and chemical oxidation (Bolshakova et al., 2019; Mchedlov-Petrossyan et al., 2023; Chen et al., 2025). Consequently, many researchers try to use adsorption methods to remove PHF. Chen et al. used montmorillonite (Mt) to adsorb PHF (initial concentration of 50–400 mg/L), achieving an adsorption capacity of 16 mg/g after 24 h of saturation (Chen et al., 2016). Zhu et al. utilized activated carbon (AC) as an adsorbent to remove PHF from water, with the specific surface area (SSA) of 1300 m²/g, resulting in an adsorption capacity of 28 mg/g. However, when considering the SSA, the unit adsorption capacity of AC for PHF was only 0.02 mg/m² (Zhu et al., 2018). The PHF tends to self-assemble into nanoscale aggregates (∼20–100 nm) in environmental media from single molecules (∼1.4 nm), significantly weakening the effectiveness of traditional adsorbents based on micropores (Georgieva et al., 2013; Zhu et al., 2018). In existing studies, both Mt and AC can achieve PHF adsorption, but the adsorption capacity and unit area efficiency are limited, and the micropores are difficult to “accommodate” the size of the nanoscale aggregates, which suggests that we need to develop new adsorbents with pore diameters that “match” the size of PHF.

We have noticed marine waste—cuttlebone (CB). The CB is a naturally occurring porous mineral material, mainly composed of aragonite-type calcium carbonate, forming a tough layered framework with characteristic micrometer-scale large pore structures (Xu et al., 2021; Yang et al., 2020). The CB can be used as a natural adsorbent for organic pollutants in the environment (Khazri et al., 2016; Malakootian and Shiri, 2021). Compared with the nanoscale pore size of other traditional adsorbents, the pore sizes of CB are larger, falling within the micron scale (Darwish et al., 2020). Consequently, its SSA is relatively small, leading to suboptimal adsorption effects for certain pollutants, but this feature may be advantageous for the adsorption of the gathered PHF. Therefore, unlike traditional adsorbents targeting molecular-level pollutants, this study will attempt a “size-matching” strategy, leveraging the unique micrometer-scale pore structure of marine waste CB to capture PHF aggregates (20–100 nm). Moreover, our research group’s recent studies have shown that due to its carbon membrane, large pores, and fluid channels, CB exhibited excellent adsorption capacity for DNA macromolecules, being 3–10 times that of traditional adsorbents (Shi et al., 2024). This lays the foundation for the effective adsorption of aggregated PHF by using CB in this study.

Furthermore, the pore structure and surface chemical properties of the original CB are fixed, resulting in a limited number of effective adsorption sites, which makes it difficult to meet actual application needs. Therefore, it is necessary to modify the CB according to specific application requirements, adjusting its pore structure and surface chemical properties to enhance its selective adsorption of target pollutant molecules. While CB modification remains an underexplored research area, established methodologies for functionalizing conventional porous adsorbents offer valuable insights for developing analogous strategies. The CB consists of a calcium carbonate core encapsulated by an organic film, which undergoes carbonization during pyrolysis. Notably, calcium carbonate maintains structural stability below 800°C, as demonstrated by Sarin et al. (2011), with the organic film transforming into carbonaceous material under these conditions. For the inorganic skeleton of CB, the pore structure of CB is enhanced by acid treatment, and hydrochloric acid does not react with organic film (Jung et al., 2018).

In this study, CB was used as raw material to prepare modified CB by pyrolysis and acidification, aiming to enhance the adsorption performance for PHF. The effects of various modification methods and conditions on the physicochemical properties of CB and its adsorption behavior toward PHF were investigated. By analyzing the adsorption characteristics of PHF on the modified CB, the adsorption mechanism was explored. This study is significant for the efficient removal of PHF; at the same time, the marine waste was transformed into high-value-added environmental functional materials, realizing the green circular concept of “using waste to treat pollution.” It has opened up a new path for the development of new adsorption materials that are sustainable, low-cost, and environment-friendly and has dual value in promoting the innovation of environmental pollution control technologies and the development of circular economy. It also promoted the interdisciplinary integration of various fields, such as materials science, environmental engineering, and marine biology, providing innovative ideas for addressing the treatment of new types of nanomaterial pollutants.

Materials and Methods

Materials

The CB was obtained from Bozhou Cuttlefish Bone Biotechnology Co., Ltd. (Bozhou, China), while PHF was purchased from Suzhou Hengqiu Graphene Technology Co., Ltd. (Suzhou, China) Hydrochloric acid (HCl, analytically pure) was procured from Tianjin Damao Chemical Reagent Factory (Tianjin, China). All other reagents used in the experiment were obtained from Aladdin Industrial Co. Ltd. (Shanghai, China). The solutions used in the experiment were prepared using Milli-Q ultrapure water. The PHF solution was freshly prepared and used immediately. It should be stored in the dark for no more than 24 h.

Preparation of CB adsorption materials

Raw material pretreatment: The CB raw materials are repeatedly washed with deionized water until the washing solution becomes clear to remove surface impurities and soluble salts. Then, they are placed in a forced-air drying oven and dried at 105°C until a constant weight is achieved (typically ≥12 h). The dried CB samples are cut into cylindrical pieces with a height of ∼0.5 cm using a stainless steel punch with an inner diameter of 0.5 cm and are set aside for future use.

Preparation of carbonized CB (CCB): Place the pretreated CB cylinder in the constant temperature zone of the tube furnace. Under the protection of high-purity nitrogen gas (purity ≥99.999%), raise the temperature at a rate of 10°C/min to the target temperatures (400°C, 500°C, 600°C) and hold the temperature for 2 h. After the thermal decomposition is completed, the samples are naturally cooled to room temperature under the condition of continuous nitrogen gas supply. The obtained samples are labeled as CCB400, CCB500, and CCB600.

Preparation of acid-modified CB (HCB): The CB samples were immersed in vials containing HCl solutions with concentrations of 0.1, 0.001, and 0.00001 mol/L at a solid-to-liquid (CB:HCl) ratio of 1:100 (g/g). After shaking for 15 min at a speed of 150 r/min, they were labeled as HCB-1, HCB-3, and HCB-5, respectively. As in the above method, CB was placed in a 0.1 mol/L HCl solution and shaken for 7, 30, and 60 min, which were recorded as HCB-7, HCB-30, and HCB-60, respectively. In addition, unless otherwise specified, the HCB mentioned in the article was HCB-30.

Preparation of acidification combined with pyrolytically modified CB (HCCB): The CCB600 was added into vials containing 0.1, 0.001, and 0.00001 mol/L HCl solution at a solid-to-liquid (CB:HCl) ratio of 1:100 (g/g) and then was shaken for 15 min (150 r/min). They were recorded as HCCB600-1, HCCB600-3, and HCCB600-5, respectively. The CCB600 was placed in 0.1 mol/L HCl solution and shaken for 7, 30, and 60 min, which were recorded as HCCB600-7, HCCB600-30, and HCCB600-60, respectively. Furthermore, CCB400 and CCB500 were added to 0.1 mol/L HCl solution and shaken for 30 min, which were recorded as HCCB400 and HCCB500, respectively. In addition, HCCB400, HCCB500, and HCCB600-30 are collectively referred to as HCCB.

Characterization and analysis

The surface morphology and microstructure of the CB adsorption materials were characterized by scanning electron microscope (SEM, Quanta FEG 250, Thermo Fisher Scientific, USA) using secondary electron imaging at 10 kV, a working distance of 5.4–5.7 mm, and a magnification of ×10,000. The elemental compositions were determined by an element analyzer (MicroCube, Elementar, Germany), while SSA and pore diameter distribution were assessed through specific surface area (BET) and barret-joyner-halenda (BJH) methods, respectively (Autosorb-1C, Quantachrome, USA). The surface potential of the CB adsorption materials was detected using the Zeta potential analyzer (ZetaPlus, Brookhaven, USA), and the measurement was repeated 10 times. After removing the extreme values, the average value was calculated. Fourier transform infrared spectroscopy (FTIR, 640-IR, Variance, USA) was used to characterize the surface functional groups of CB, CCB, and HCCB, and the potassium bromide pressing method was adopted. The surface elements of the CB adsorption materials were determined using the multifunctional scanning imaging X-ray photoelectron spectroscopy (XPS, PHI5000 Versaprobe-II, Japan ULVAC-PHI), three parallel samples were set for each sample, and the average value was calculated. The X-ray diffraction (XRD) phase analysis of the main components of hematite and thermally modified hematite was conducted using the TD5000 X-ray diffractometer, within the 2θ angle range of 5–60°, with a step size of 0.020° and a point measurement time of 1 s.

Adsorption experiments

The adsorption kinetics experiments were carried out in 40 mL brown bottles. The 25 mg CB adsorption material was weighed into each bottle, and 25 mg/L PHF solution was added. The bottles were shaken at room temperature (298 K). Samples were taken at 1, 2, 3, 5, 7, 9, 12, 24, 36, 48, 60, 72, 84, and 96 h. During sampling, centrifuge the bottle for 5 min at 3000 r/min. Take an appropriate amount of the supernatant and pass it through a 0.45 μm water-based microporous filter membrane. Use a UV–visible spectrophotometer (UV-2600, Shimadzu, Japan) to determine its concentration at the maximum absorption wavelength of PHF (272 nm).

The adsorption isotherm experiments of PHF on CB adsorption material were carried out at temperatures of 278, 298, and 313 K. The 25 mg sample of CB adsorption material was weighed into a 40 mL brown bottle, and PHF solutions with initial concentration ranging from 5 to 70 mg/L were added. Place the reaction bottle in a constant temperature shaking incubator set to the desired temperature, and shake at 150 rpm until adsorption equilibrium is reached. After reaching adsorption equilibrium, process the supernatant according to the centrifugation and filtration steps mentioned above and determine the PHF concentration using a UV spectrophotometer at 272 nm.

Data analysis and processing methods

The pseudo-first-order and pseudo-second-order kinetic models were used to fit the adsorption kinetic data. The Langmuir and Freundlich models were used to fit the adsorption isotherm data of PHF on CB adsorption materials. The thermodynamic parameters were calculated using the adsorption isotherm data. The specific processing methods are described in Text S1. All ﬁgures, standard deviation, kinetics model ﬁtting procedures, coeffcient of determination (R²), and other statistics were performed using OriginPro 2021 software. The standard errors of the ﬁtting coeffcients were with a 95% conﬁdence interval. Error bars represent standard errors (n = 3).

Results and Discussion

Characteristics of CB adsorption materials

To investigate the particle size and aggregation characteristics of PHF, atomic force microscopy was performed at different concentrations of PHF (Fig. 1). As the concentration increases, PHF would aggregate to form large particles with sizes ranging from ∼28 to 95 nm, significantly larger than other organic pollutants. Studies have shown that an adsorbent with pore size 1.7–3 times larger than the pollutant size results in better adsorption (Tang et al., 2018). Thus, in Supplementary Fig. S1 and Supplementary Table S1, the pore distribution of different CB adsorption materials was analyzed. The pores between ∼50 and 200 nm in the CB adsorption materials are more effective for PHF adsorption. This result clearly defined the material design concept of “using the aggregation scale of pollutants in the aqueous phase as the basis for pore size matching,” which is one of the important innovations of this study. From a broader perspective of application, the design strategy centered on “aggregation scale—pore size matching” was not only applicable to pollutants like PHF that tend to form nano-clusters but could also be extended to microplastics and hydrophobic polycyclic aromatic hydrocarbons, etc., which were “large in volume and prone to aggregation” targets. With increasing pyrolysis temperature, the effective pores of CCB increased, the SSA decreased, and the average pore size and pore volume increased. This result was attributed to the transformation of the CB crystal form from unstable aragonite to stable calcite after pyrolysis modification, as confirmed by XRD analysis (Supplementary Fig. S2). The mineral phase in CB and CCB400 samples was aragonite, while calcite was the only mineral phase in CCB500 and CCB600 samples. At higher pyrolysis temperatures, graphite lamellar condensed, leading to the formation of a microcrystalline structure and larger pores (Tan et al., 2021). Compared with HCCB, the effective pore of HCB increased more significantly after acid modification. With increasing acid concentration and treatment duration, the effective pores and pore volume of HCB increased significantly. This indicated that the tandem modification route of “pyrolysis-induced phase transition and aromatization, acid leaching selective pore opening” could be used to synergistically construct a macroporous network that was compatible with PHF aggregates. This path had replicability and resource potential: for the high-value utilization of calcium-rich biomass or CaCO₃ by-products (such as shells), it could be transformed into water treatment materials with hierarchical pore structures and active sites and extended to various scenarios for the treatment of “large-sized/easy-to-aggregate” organic pollutants.

FIG. 1.

AFM images and height images of different concentrations (A) 5 mg/L; (B) 20 mg/L; (C) 50 mg/L; (D) 70 mg/L, of PHF. AFM, atomic force microscopy; PHF, fullerenols.

To study the surface morphology characteristics of CB prepared by different modification methods, the CB adsorption materials were analyzed by SEM (Fig. 2). With increasing pyrolysis temperature, the organic film coating CCB underwent carbonization and fissuring, driving pore development and progressively exposing the underlying calcium carbonate framework. This evolution was corroborated by XPS surface elemental analysis (Supplementary Table S2). After acid modification, the surface structure of HCB and HCCB became more fragmented and even collapsed. The HCB surface exhibited a dense pore structure with small size and deep pores in the calcium carbonate skeleton. In contrast, HCCB surface formed a relatively larger and collapsed pore structure.

FIG. 2.

SEM image of CB adsorption materials. CB, cuttlebone.

The FTIR analysis revealed that the infrared spectra of CB exhibited no significant alterations before and after modification, with the characteristic functional groups remaining unchanged (Supplementary Fig. S3A and B). However, upon adsorption of PHF by the CB adsorption materials, the intensity of carbonate ion absorption bands at 2524, 1800, 1424, and 1472 cm⁻¹ was notably diminished (Supplementary Fig. S3C and D). This observation indicated a quantitative reduction in carbonate ion concentration following PHF adsorption. This result might be due to the hydroxyl oxygen atom on the PHF surface coordinating with the calcium atom on the calcite surface of CB adsorption material, replacing the oxygen vacancy of carbonate ion (Olsen et al., 2019). In addition, it was found that the C = C peak of the aromatic ring at 1620 cm⁻¹ weakened, indicating that π–π conjugation might occur with the heterocyclic ring of PHF during the adsorption of PHF (Yu et al., 2021).

Adsorption kinetics of PHF on CB adsorption materials

The adsorption kinetics of PHF on CB adsorption materials was shown in Figure 3. The adsorption process was divided into two stages, including the prefast adsorption stage (Stage I: 0–12 h) and postslow adsorption and equilibrium stage (Stage II: 12–96 h). In Stage I, a concentration gradient was established between the high-concentration PHF solution and the low-concentration PHF on CB adsorption materials, which facilitated the diffusion of PHF molecules toward the surface and pores of the CB material, thereby significantly increasing PHF adsorption capacity and accelerating the adsorption rate. In Stage II, PHF adsorption on CCB and HCCB materials reached equilibrium after 60 and 72 h, respectively. In addition, under the same adsorption time, compared with CB, both pyrolytic and acid modification could significantly enhance the adsorption of PHF. In comparison to CCB, both HCB and HCCB exhibited higher adsorption capacities for PHF, mainly because the effective pores and pore volume of HCB and HCCB increased after acid modification, providing more adsorption sites. Moreover, the adsorption capacity of CCB and HCCB increased with the rise of pyrolysis temperature, as higher pyrolysis temperature led to a higher carbonization degree of organic films with more aromatic structures, allowing CCB and HCCB to adsorb more PHF by the π–π interaction.

FIG. 3.

Kinetics of PHF adsorption on CB adsorption materials.

To more accurately and intuitively compare the adsorption rate of PHF on CB adsorption materials, pseudo-first-order and pseudo-second-order kinetic models were used to fit the adsorption kinetics data separately. The fitting results were shown in Supplementary Table S3. The results indicated that the R² values for the pseudo-second-order kinetic model all exceeded 0.97, indicating a superior fit. Therefore, the adsorption of PHF onto CB adsorption materials was characterized by a chemical adsorption process on heterogeneous surfaces (Wang et al., 2024). Compared with pyrolytic modification, acid-modified materials exhibited lower adsorption rates for PHF. Combined with the BET results and SEM images of CB adsorption materials, it could be observed that the surface structures of CB and CCB after acid modification were more fragmented, with more effective pores and larger pore volume. Therefore, the filling of effective pores by PHF required a longer time, ultimately slowing down the overall adsorption rate.

Adsorption isotherm analysis of CB adsorption materials for PHF

To study the adsorption characteristics of PHF on CB adsorption materials, the adsorption isotherm was measured and fitted. As can be seen from Figure 4A, compared with CB and CCB, HCB and HCCB had a higher adsorption capacity for PHF. CB was composed of calcium carbonate and some organic matter (Checa et al., 2015). When CB materials were treated with hydrochloric acid, the following reaction occurred:

{CaCO}_{3} + 2HCl = {CaCl}_{2} + H_{2} O + {CO}_{2} ↑

FIG. 4.

(A) Adsorption isotherm of PHF on CB adsorption materials; (B) the adsorption isotherm fitting curve of PHF on pyrolytically modified materials; (C) the adsorption isotherm fitting curve of PHF on acid-modified materials.

Under the action of hydrochloric acid, the surfaces of CB materials were corroded, forming pores on the surface of the calcium carbonate skeleton, increasing the porosity. Hydrochloric acid also entered the existing pores, corroding the inner walls of the pores and expanding the pores (Yang et al., 2020). Moreover, the generation of CO₂ gas could open some previously blocked pores, also increasing the porosity. The synergy of “pore formation–pore expansion–pore channel unclogging” enabled the acid-modified CB to have a larger pore volume, a higher proportion of effective pores, and a higher SSA (Supplementary Table S1), thereby enhancing the accessibility sites and mass transfer efficiency of PHF and explaining the higher adsorption capacity of HCB and HCCB. This mechanism has universal reference significance for the resource-based acid modification of biomass or by-products containing CaCO₃ (such as shells, coral sands).

Langmuir and Freundlich models were used to fit the adsorption isotherms of PHF on CB adsorption materials (Fig. 4B and C); the fitting parameters could be seen in Supplementary Table S4. Both the Freundlich and Langmuir models exhibited good fitting performance for the adsorption isotherms of PHF by CB and CCB (R² > 0.97). The Langmuir model fitting results showed that the maximum theoretical adsorption capacities of CB, CCB400, CCB500, and CCB600 were 14.17, 20.05, 22.22, and 22.29 mg/g, respectively. In the Freundlich model, with the increase of pyrolysis temperature, the heterogeneity factor (n) decreased from 0.50 to 0.44, indicating enhanced heterogeneity (Zhu et al., 2018; Tang et al., 2020). The adsorption affinity coefficient (K_F) increased from 1.78 to 2.99, indicating enhanced adsorption affinity (Zhang et al., 2018; Raif et al., 2025). As the pyrolysis temperature increased, the degree of carbonization was enhanced, resulting in a more heterogeneous distribution of adsorption sites and a more developed pore structure. For HCB and HCCB, only the Freundlich model provided a good fit to the adsorption isotherm data, indicating that the adsorption of PHF onto these materials was a nonlinear process involving pore filling (Ahn et al., 2005; Laskar and Hashisho, 2020).

Effect of acidification time and concentration on adsorption of PHF by HCB

To study the effect of acid treatment time and acid concentration on the adsorption capacity of acid-modified CB, the adsorption isotherms of PHF on HCB were determined when the acid treatment time was 7, 30, and 60 min and the acid concentration was pH = 1, pH = 3, and pH = 5 (Supplementary Fig. S4). The adsorption capacity of PHF onto HCB increased with increasing acid concentration and prolonged acid treatment time. Based on the Langmuir model fitting results, the maximum theoretical adsorption capacity increased from 14.17 mg/g for CB to 15.21 mg/g for HCB-1 and 18.93 mg/g for HCB-60. This enhancement was ascribed to the formation of more efficient pores and larger pore volumes at higher acid concentrations and longer treatment durations, as supported by data in Supplementary Table S1. Figure 5 provided a 3D response surface analysis to further evaluate the effects of acidification time, acidification concentration, initial PHF concentration, and their interactions on PHF adsorption by HCB. Generally, small or no interaction between two factors indicates a flat response surface. On the contrary, obvious interaction will lead to a curved response surface (Song et al., 2020). In Figure 5A, the interaction between initial PHF concentration and HCB acidification concentration was significant (p < 0.05). When the initial PHF concentration was fixed, the adsorption capacity of HCB significantly increased with the increase of acidification concentration in the range of 0.00001–0.078 mol/L and then decreased. Figure 5B and C reflected the response surfaces of HCB acidification time versus initial PHF concentration and HCB acidification concentration, respectively. However, the software analysis found that the interaction between two factors was not significant (p > 0.05). According to the ridge maximum analysis of the response surface, the optimal adsorption capacity of HCB for PHF occurs when the initial PHF concentration is 70 mg/L, the acidification time is 45 min, and the acidification concentration is 0.078 mol/L. This finding provides the optimal parameters for future material preparation. A quantitative correlation between “process parameters—pore structure—adsorption performance” was established, and the optimal process window was determined. The nonmonotonic effect of acid concentration (excessive acid etching leading to pore wall collapse or site loss) was revealed, providing an operable parameter range for scalable production.

FIG. 5.

Response surface plots for the effect of (A) acidification concentration and initial PHF concentration; (B) acidification time and acidification concentration; and (C) initial PHF concentration and acidification time.

Thermodynamic analysis of PHF adsorption on CB adsorption materials

As shown in Figure 6, with the increase of ambient temperature (278, 298, and 313 K), the Gibbs free energy (ΔG) of the adsorption reaction on CB adsorption materials gradually decreased, indicating that the adsorption on CB material was more likely to occur at a higher ambient temperature (Shuai et al., 2022).

FIG. 6.

At different ambient temperatures, the ΔG of PHF on adsorption materials (A) CB and CCB600; (B) HCB and HCCB600, varies with Qe.

With the increase of PHF concentration, the negative ΔG became a positive ΔG, indicating that the adsorption reaction changed from spontaneous to nonspontaneous (Fan et al., 2016). In addition, compared with CB and CCB600, HCB and HCCB600 had a lower ΔG value, indicating that the acid treatment facilitated the adsorption process of PHF and made the adsorption thermodynamics more favorable. To better explain the adsorption law, the enthalpy change (ΔH) and entropy change (ΔS) are calculated. In Supplementary Fig. S5A, ΔH values of CB adsorption materials were positive, indicating that the adsorption processes of PHF on CB adsorption materials were endothermic, which could well explain the increased adsorption of PHF with increasing temperatures. Compared with CB and CCB600, the variation amplitudes of ΔH for HCB and HCCB600 were smaller as the adsorption capacities increased. This result indicated that achieving a higher adsorption capacity for PHF required less heat as the driving force for HCB and HCCB600. In Supplementary Fig. S5B, the ΔS values of all reactions were positive, indicating that the overall chaos of the reaction system increased, which is more conducive to the adsorption (Mao et al., 2021; Shuai et al., 2022). From an application perspective, the HCB/HCCB system achieving higher capacity at a lower enthalpy cost could reduce the energy consumption burden of temperature regulation, providing significant energy efficiency advantages; this principle also held relevance for other “large-sized/aggregate-prone” organic pollutants.

Adsorption mechanisms of PHF on CB adsorption materials

The surface properties and basic components of CB adsorption materials determined their adsorption mechanism. As shown in Supplementary Fig. S6, all the materials were negatively charged on their surfaces, while PHF molecules could be deprotonated by surface hydroxyl groups to form PHF anions (Liu et al., 2018). Therefore, in the adsorption system of this study, electrostatic repulsion was a key factor affecting the adsorption of PHF on CB adsorption materials. The negative charge density significantly decreased with the increased pyrolysis temperature. This result could well explain the higher adsorption of PHF on CCB prepared at the high pyrolysis temperatures.

For acid-modified CB adsorption materials, with the increase of acid concentration and treatment time, the effective pores and pore volumes of both HCB and HCCB increased, greatly enhancing their pore-filling effect of PHF. In addition, there were 24 hydroxyl groups on the PHF surface, which could coordinate with the calcium atoms or oxygen atoms on the surface of CB adsorption materials that were not fully coordinated (Olsen et al., 2019). The hydrogen bond was also important for interactions between the –NH or –COOH groups on the surface of acid-modified materials and the hydroxyl groups of PHF molecule (Chen et al., 2016). Therefore, the adsorption mechanism of the acid-modified CB adsorption material was characterized by the synergy of “porous filling + site coordination + hydrogen bonding.”

For pyrolysis-modified CB adsorption materials, after pyrolysis treatment, the organic films on the surface were carbonized. With the increasing pyrolysis temperature, the stretching vibration peaks of aromatic C=C and C–C bonds were enhanced (Supplementary Fig. S3), and the unsaturation degree of the carbon structure and the number of π electrons on the material surface also increased (Ling et al., 2018). PHF had an unsaturated carbon structure similar to fullerenes, containing π electrons (Ahn et al., 2005), so the pyrolysis-modified CB adsorption materials could adsorb PHF through π–π interactions, and the adsorption affinity increased with the increasing pyrolysis temperature (Yu et al., 2021). In addition, CB adsorption materials transformed from aragonite-type calcium carbonate to calcite after pyrolysis (Florek et al., 2009). Compared with aragonite, the most common {10.4} faces in calcite crystals were neutral and had less electrostatic repulsion with the PHF (Ataman et al., 2016). Studies had shown that PHF could directly interact with the {01.8} faces of calcite, thereby embedding PHF inside (Calvaresi et al., 2011). It could be seen from the XRD pattern (Supplementary Fig. S2) that {10.4} and {01.8} crystal faces of calcite appeared on the CB adsorption materials after high-temperature pyrolysis, and the diffraction peaks of two faces were relatively strong, which was also a major factor for the adsorption of PHF by CCB.

Therefore, in the context of “overall electrostatic repulsion,” a “multichannel synergy” mechanism pathway was proposed and verified, namely, through the crystal phase transformation (calcite→aragonite) to reduce repulsion, thermal decomposition aromatization to enhance π–π interaction, acid leaching to open pores and promote pore filling, and supplemented by Ca/O site coordination and hydrogen bond interaction, all of which jointly achieve efficient adsorption of anion-type and easily aggregating PHF. This mechanism integrates mineral sites, carbon domain π electrons, and hierarchical pore structure, providing a structural–functional design principle for aggregated-state pollutants.

Comparison of unit area adsorption capacity of PHF between CB adsorption materials in this study and traditional material

The saturation adsorption capacity of PHF on CB adsorption materials was standardized by SSA. The PHF adsorption capacities per unit area of CB adsorption materials were 1.15 mg/m² (CB), 1.70 mg/m² (CCB400), 2.75 mg/m² (CCB500), and 3.11 mg/m² (CCB600) and 1.18 mg/m² (HCB), 1.56 mg/m² (HCCB400), 2.11 mg/m² (HCCB500), and 2.10 mg/m² (HCCB600). Both pyrolysis modification and acid modification increased the adsorption capacity per unit area. In addition, for both CCB and HCCB, PHF adsorption capacities per unit area increased with the increase of pyrolysis temperature, because the higher pyrolysis temperature induced the more effective pores and more aromatic structures on the surface (Supplementary Fig. S1 and Supplementary Table S1). However, it is worth noting that although the effective pores of HCCB were more than that of CCB, the PHF adsorption capacity per unit area of HCCB was lower than that of CCB. This phenomenon was probably resulted from the destruction of organic carbon film by acid modification (Cera et al., 2024), which inhibited the π–π interaction between the surface organic carbon film and PHF.

All the CB adsorption materials in this study showed a higher unit area adsorption capacity for PHF than traditional materials (Supplementary Fig. S7).

Currently known PHF adsorbents include AC, calcined layered double hydroxides (LDH), ferrihydrite (Fh), Mt, and polyethyleneimine-modified montmorillonite (PEI-Mt). Zhu et al. (2018) added 0.02 g of AC (with an SSA of 1300 m²/g) to a 20 mL PHF solution (initial concentration of 5–100 mg/L) for adsorption removal of PHF, achieving a saturated adsorption capacity of 28 mg/g and a unit area adsorption capacity of 0.02 mg/m². They also used LDH (with an SSA of 56 m²/g) for PHF adsorption, with a unit area adsorption capacity of 0.84 mg/m². Liu et al. (2018) used Fh (with an SSA of 309 m²/g) for PHF adsorption, obtaining a unit area adsorption capacity of only 0.22 mg/m² despite the strong ligand exchange interaction between PHF and Fh. Chen et al. (2016) added 0.1 g of Ca-Mt to a 20 mL PHF solution (initial concentration of 50–400 mg/L) and reached saturation after 24 h. The maximum unit area adsorption capacity was 0.24 mg/m². To further enhance adsorption performance, they used Mt modified with 10% PEI (PEI-Mt). The SSA of cationic organic-modified Mt could reach ∼300 m²/g (Zhu et al., 2016). The unit area adsorption capacity of PEI-Mt could be raised to 0.71 mg/m². In the aforementioned studies, the PHF adsorbents had large SSAs, but the obtained unit area adsorption capacities were small, indicating that during the adsorption process, most of the pores were not effectively utilized, and the pore accessibility was poor (Yu et al., 2021). This study focused on the special structure and aggregation properties of PHF. Using large-pore CB, and through the combined regulation of pore structure and surface chemistry through thermal decomposition and acidification, a maximum adsorption capacity of 3.11 mg/m² per unit area was achieved, which was significantly higher than that of previously reported materials, demonstrating the comprehensive advantages in pore size matching, site coordination, and energy efficiency utilization.

Conclusion

This study used CB as raw material and prepared three CB-derived materials (CCB, HCB, and HCCB) for adsorption of PHF through pyrolysis and acidification. The initial adsorption rate, adsorption capacity, and adsorption affinity of CCB for PHF increased with the increase of pyrolysis temperature due to the reduced electrostatic repulsion, the increased π–π interaction, and the transformation of aragonite into calcite of CB calcium carbonate skeleton. After acid modification, HCB and HCCB had an enhanced pore-filling effect for the higher adsorption capacity and the lower adsorption rate of PHF, which were further strengthened with the increase of acid concentration and acid treatment time. Among these CB adsorption materials studied, the one with the best performance could achieve 3.11 mg/m² for PHF, which is much higher than that of literatures reported. This study provided environment-friendly and high-performance materials for the adsorption and removal of PHF.

Authors’ Contributions

S.S.: Conceptualization, formal analysis, investigation, and writing—original draft. X.L.: Investigation, supervision, and writing—review and editing. H.Y.: Experimental operation, investigation, and formal analysis. H.Z.: Investigation, formal analysis, data curation, methodology, and visualization. L.S.: Data curation, resources, supervision, and review and editing. D.Z.: Conceptualization, resources, supervision, review and editing, and funding acquisition.

Data Availability

The datasets used or analyzed during the current study are available from the corresponding author upon reasonable request.

Footnotes

Author Disclosure Statement

The authors have no competing interests to declare that are relevant to the content of this article.

Funding Information

We would like to thank the funding of the Taishan Scholar Foundation of Shandong Province (tsqn202306276), National Natural Science Foundation of China (42167030), Natural Science Foundation of Shandong Province, China (ZR2024QD197, 2025TSGCCZZB0897), and Linyi University High-level Talents (Doctor) Research Foundation (Z6122037, Z6124036).

Supplemental Material

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