Skip to main content
NCBI home page
ACS Omega logoLink to ACS Omega
. 2026 Apr 14;11(16):24298–24308. doi: 10.1021/acsomega.5c13314

Dicyandiamide-Modified Ultrahighly Cross-Linked Polystyrene Resin for Selective CL-20 Adsorption from Explosive Waste Acid

Na Lin a,b,c, Jiaqi Lu a,b,c, Liying Wang a,b,c, Zhifei Hao a,b,c, Qihang Zhao a,b,c, Bing Bai a,b,c, Zhengfang Ye d,*, Yongfeng Zhang a,b,c,*
PMCID: PMC13129839  PMID: 42077908

Abstract

A major environmental concern associated with the industrial production of hexanitrohexaazaisowurtzitane (CL-20), a potent high-explosive compound, is the generation of large volumes of strongly acidic waste. During the process, a considerable portion of CL-20 enters the waste stream, thereby complicating its recovery and causing significant resource loss. A dicyandiamide-modified highly cross-linked resin is presented for the preferential adsorption of CL-20 from real acidic waste liquids, enabling resource recovery. Strong interactions are formed between the amino groups and the nitro group (NO2) and nitrogen atoms in CL-20 through hydrogen bonding, electrostatic attraction, and nitrogen–nitrogen interactions, which enhances the adsorption capacity. The adsorption process was well-described by pseudo-second-order kinetics and the Langmuir adsorption isotherm, as determined through kinetic and thermodynamic analyses. Under a fixed strongly acidic condition (nitric–sulfuric mixed acid system, pH = 0.17), a maximum adsorption capacity of 30.68 mg·g–1 was achieved. Evaluation of the thermodynamic parameters revealed that the adsorption was spontaneous and endothermic. Meanwhile, ethyl acetate enabled highly efficient desorption, with efficiency reaching up to 100%. X-ray photoelectron spectroscopy (XPS) and molecular dynamics simulations demonstrated that the adsorption is primarily driven by hydrogen bonding and polar interactions. Based on these findings, the modified resin is posited as a promising material for the effective adsorption and recovery of CL-20 from waste acid.

graphic file with name ao5c13314_0010.jpg

graphic file with name ao5c13314_0008.jpg

1. Introduction

The processes of nitrification, crystallization, and reprocessing in the pyrotechnic explosives industry generate large volumes of highly acidic waste streams, primarily mother liquors and wash waters, which contain high-energy nitrogen-containing compounds. Beyond posing a dual threat of ecotoxicity and potential safety hazards, − the presence of these compounds in waste streams constitutes a loss of valuable energetic materials. Due to its high density and large heat of explosion, the caged polynitroamine CL-20 has garnered considerable attention among these compounds. , However, recovering CL-20 from real production wastewater is particularly challenging due to the strongly acidic conditions, high salinity, and coexistence of competing components like nitrate, sulfate, other nitro compounds, and organic byproducts, all of which severely hinder its capture and enrichment. Conventional approaches including solvent extraction, activated carbon adsorption, and general-purpose ion exchange typically exhibit limitations such as insufficient selectivity, suppressed capacity under acidic conditions, or poor regenerability. Consequently, there is an urgent need for adsorbents that combine acid stability, high surface area, and targeted molecular interaction sites.

Macroporous resins are widely used for extracting metals, aromatic compounds, and other substances from industrial wastewater. For instance, Wang et al. functionalized an ultrahigh cross-linked resin using modifiers such as pyrrolidone and 2-imidazolone to effectively remove aniline and its derivatives from wastewater. In a study on tungsten smelting wastewater, Zeng et al. examined the adsorption behavior of low-concentration WO3 from acidic solution using a D318-type macroporous resin. Furthermore, Zhao et al. designed and synthesized a functionalized poly­(tertiary amine) macroporous resin (TA12-MR), which demonstrated high efficiency in the recovery of tungsten and molybdenum from secondary resources, showing promise as a reusability.

Hyper-cross-linked resins (HCRs) provide a promising platform for the removal of trace organic pollutants from complex matrices, owing to their rigid three-dimensional porous networks, abundant micro- and mesopores, and excellent chemical stability. − As microporous polymers, ultrahigh cross-linked resins can be synthesized without relying on monomers with unique functional groups. Instead, they are typically prepared by interlinking organic structures using external cross-linking agents. Large-scale production of these resins from inexpensive organic monomers is achieved through simple Friedel–Crafts alkylation reactions, offering a cost-effective approach. Nonetheless, unmodified HCRs rely mainly on hydrophobic interactions and weak nonspecific intermolecular forces, and therefore exhibit limited affinity and molecular recognition toward polar and structurally rigid polynitroamines such as CL-20. Moreover, systematic studies that correlate surface functionalization, adsorption behavior, and regeneration performance under acidic conditions are still scarce.

Dicyandiamide (DCD) has a high nitrogen content and during pyrolysis, nitrogen functional groups can be significantly doped into the carbon skeleton, while Dicyandiamide decomposes readily at lower temperatures allowing gradual release of nitrogen and better control of the development of the porous structure. Furthermore, DCD contains guanidine/imine-type nitrogen functionalities that can be introduced as high-density basic nitrogen sites through postmodificatio. − These sites, capable of forming multipoint hydrogen bonds, undergo partial protonation under acidic conditions. This process enhances both hydrogen-bonding/dipole–dipole interactions and “hard acid–hard base” (HSAB) type interactions with the strongly electron-accepting polynitro groups of CL-20. When combined with the hydrophobic microporous environment of HCR, such multifunctionalization is expected to promote the preferential enrichment of CL-20.

Building on the rationale that protonated sites enhance interactions with CL-20s polynitro groups through hydrogen bonding and HSAB principles, we designed and prepared a dicyandiamide-modified ultrahighly cross-linked resin (DCD-HCR). This resin is specifically tailored for efficient and regenerable adsorption and recovery of CL-20 from strongly acidic, high-salt mother liquors. Using a scalable HCR matrix as the backbone, DCD-related nitrogenous functional groups were introduced via a postmodification strategy to produce adsorbents with three target features: (i) high specific surface area with accessible pore channels, (ii) chemical stability in strong acid, and (iii) dense, protonatable multipoint interaction sites capable of complementing CL-20s electronic and structural characteristics.

Focusing on structure–property–mechanism relationships, the modified materials’ surface chemistry and porosity were characterized by FT-IR, XPS, and N2 adsorption–desorption analyses. Adsorption kinetics and thermodynamics were used to elucidate rate-controlling steps and the governing kinetic model. Mild and effective desorption systems were screened, followed by the execution of multiple adsorption–desorption cycles to evaluate regenerability, with postcycle characterization employed to verify structural stability. Subsequently, the material’s prospects for engineering application in the treatment of highly acidic waste streams and the recovery of target products were thoroughly discussed. The objective of this work is to demonstrate that Dicyandiamide (DCD) functionalization endows ultrahighly cross-linked resins (HCRs) with a multipoint interaction microenvironment tailored for CL-20. This design complements the electron-deficient nature of CL-20s polynitro groups while preserving the favorable pore architecture and acid stability inherent to HCRs. Consequently, the DCD-HCR enables high-capacity and recyclable recovery of CL-20 from strongly acidic and complex matrices. The findings provide a sustainable materials-chemistry pathway for the deep treatment of acidic waste liquors and resource recovery in the pyrotechnic explosives industry, and they offer design principles for functionalized porous adsorbents applicable to other complex and extreme matrices.

This work addresses the longstanding challenge of selectively recovering CL-20 from strongly acidic and complex waste streams by rationally integrating material design, surface functionalization, and adsorption–regeneration evaluation. A Dicyandiamide-modified ultrahighly cross-linked resin (DCD-HCR) was constructed to introduce dense, protonatable nitrogen functionalities into a robust HCR framework, thereby creating a multipoint interaction microenvironment tailored for the highly polar and structurally rigid CL-20 molecule. In this study, the adsorption performance of the aminated resin was systematically investigated under strongly acidic conditions, and the highest adsorption capacity was observed at pH = 0.17, corresponding to a nitric–sulfuric mixed acid system. Through comprehensive structural characterization, adsorption kinetics and thermodynamics analyses, and regeneration studies under acidic conditions, this study elucidates the structure–property–mechanism relationships governing CL-20 adsorption. The results demonstrate that DCD functionalization effectively overcomes the limitations of unmodified HCRs and provides a viable strategy for the efficient and recyclable recovery of energetic compounds from extreme acidic matrices.

2. Experimental Section

2.1. Experimental Materials

Chloromethyl polystyrene resin (CMPS), hereafter referred to as chloroballs, was purchased from Zhengzhou Aino Chemical Technology Co. 1,2-Dichloroethane, dicyandiamine, anhydrous aluminum chloride, anhydrous ethanol, and acetonitrile were obtained from Macklin Biochemicals Ltd., Shanghai, China. Deionized water was prepared in-house in the laboratory. Acetonitrile used was of chromatographic grade, and all other organic reagents were of analytical grade.

The industrial waste acid and CL-20 used in this study were developed by Beijing University of Technology, Beijing, China. The purity of CL-20 was ≥ 99%. The primary component of the waste acid is a nitro-sulfur mixed acid, as detailed in Table S1, which exhibits extreme acidity and considerable recovery potential.

2.2. Preparation Methods

The chloromethyl polystyrene resin (chloroballs) was added to 1,2-dichloroethane and subjected to mechanical stirring at 150 rpm overnight. The mixture was then heated to 343 K (70 °C) by gradually adding a Lewis acid catalyst. The reaction was maintained under reflux at this temperature to carry out the Friedel–Crafts reaction. Upon completion of the reaction, the product was washed and dried multiple times using a 1:1 mixture (v/v) of deionized water and ethanol, until a constant weight was achieved. The ultrahigh cross-linking resin (HCR) was obtained. Typically, the Friedel–Crafts cross-linking was carried out using AlCl3 as the catalyst (AlCl3/monomer mass ratio = 1g:5g), with a solid-to-liquid ratio of 0.1 g mL–1.

For the preparation of the adsorbent, Dicyandiamide-modified ultrahigh cross-linked resin (DCD-HCR), the ultrahigh cross-linked resin was allowed to swell under mechanical stirring at 150 rpm overnigh Dicyandiamide, in varying molar amounts, was then added, and the reaction was carried out at 313 K (40 °C) for 6 h. After the reaction, the resin was washed with ethanol and deionized water, and then dried to yield the Dicyandiamide-modified ultrahigh cross-linking resin. The preparation equation is shown in Scheme . Operating conditions are shown in Table S2.

1. Preparation Process of DCD-HCR.

1

Open in a new tab

2.3. Analytical Methods

A Specific Surface Area and Pore Size Analyzer (model 3H-2000PS2; Beijing Best Instrument Technology Co., Ltd.) was used to determine specific surface area, pore volume, and pore size. Prior to testing, polymers were degassed under vacuum at 100 °C for 3 h. Specific surface area was measured by the BET method, and the overall pore characteristics were analyzed by the BJH method. The elemental composition of the adsorbent was characterized by XPS using a Thermo Scientific alpha energy spectrometer. Surface morphology was observed by field-emission scanning electron microscopy (FE-SEM; SU8020, Hitachi, Japan). FT-IR spectra were recorded on a Nicolet Nexus 670 using KBr pellets; the scan range was 4000–400 cm–1 with a resolution of 0.09 cm–1.

2.4. HPLC Detection Method

The concentration of CL-20 in solution was quantified by dual ternary liquid chromatography (U3000UPLC, Dionex, USA). The mobile phase was methanol/water (50/50, v/v) at a flow rate of 1 mL min–1. Separation was performed on a Prontosil C18 Ace-EPS column (4.6 × 250 mm, 5 μm). The detection wavelength was 230 nm, and the elution (retention) time was 7.503 min. Actual samples were diluted with acetonitrile to within the calibration range, filtered through a 0.45 μm organic membrane, and analyzed for CL-20.

2.5. Adsorption Measurement Method

Spent acid (100 mL) was added to a 250 mL conical flask, followed by 0.1, 0.3, and 1 g of resin (separately). Flasks were shaken in a thermostatic shaker at 25 °C and 200 rpm for 12 h. After adsorption, the supernatant was filtered through a 0.45 μm organic membrane, diluted with acetonitrile to an appropriate concentration, and analyzed by liquid chromatography. The adsorption capacity was calculated as

qt=V(C0−Ct)M 1

where C 0 and C t are the initial concentration and the concentration at time t (mg L–1), V is the solution volume (L), and M is the resin mass (g).

For the adsorption isotherm, 0.5 g of resin was added to flasks containing waste acid at different concentrations (10-fold, 7-fold, 5-fold, 2-fold, and 1-fold dilutions). Each flask contained 100 mL of waste acid and was shaken for 12 h in a thermostatic oscillator at 298.15, 308.15, or 318.15 K and 200 rpm, after which concentrations were determined.

For adsorption kinetics, 0.6 g of resin was added to 200 mL of CL-20 waste acid solution and shaken at 25 °C and 200 rpm. Intermittent sampling (2 mL) was conducted at 0, 2, 5, 10, 15, 20, 30, 40, 60, 120, 240, 360, 480, 600, and 720 min. Each sample was immediately filtered (0.45 μm), diluted as needed, and analyzed by liquid chromatography.

2.6. Resin Desorption and Repeatability Experiments

Desorption Experiments

Methanol, ethanol, acetone, and ethyl acetate were used as desorption solvents. Industrial waste acid (200 mL) and 0.5 g of resin were placed in a 250 mL conical flask and shaken at 25 °C and 200 rpm for 8 h. The remaining CL-20 concentration in the spent acid was measured. The spent acid was then filtered off, 10 mL of desorption solvent was added to the resin, and desorption proceeded under the same conditions for 12 h. The mixture was filtered, and CL-20 in the desorbed solution was quantified.

Repeatability Experiments

A mixture of 200 mL spent acid and 1 g resin was adsorbed to equilibrium. After removing the spent acid, 10 mL of desorbent was added for desorption. The resin and desorbent were retained for subsequent cycles, with fresh spent acid added each time. After each cycle, the resin was rinsed with deionized water. Four cycles were performed, and the CL-20 concentrations in the spent acid and desorbent were measured after each cycle.

3. Results and Discussion

3.1. Material Characterization

SEM images of the pristine CMPS and modified resins are shown in Figure . The pristine CMPS exhibits a relatively smooth and dense surface, with no discernible open pore structure at the microscale, indicating a compact polymer morphology. After post-cross-linking and subsequent amination, the resin surface becomes noticeably rougher, accompanied by the appearance of surface irregularities and cracks. These morphological changes reflect increased structural heterogeneity and exposure of internal porous domains rather than the direct formation of new pores.

1.

1

Open in a new tab

SEM images of CMPS (a, d), ultrahigh cross-linked resin from Friedel–Crafts alkylation (b, e), and DCD-HCR from amination (c, f).

Importantly, cross-sectional SEM images further reveal that the interior of the post-cross-linked and DCD-modified resins consists of a well-developed and interconnected porous framework extending throughout the resin matrix (Figure S1). This observation confirms that post-cross-linking induces a rigid three-dimensional porous skeleton, while subsequent Dicyandiamide modification improves pore accessibility without disrupting the internal structure. The combined surface and cross-sectional SEM results provide direct morphological evidence for the formation of an interconnected porous network, consistent with the porosity evolution observed in nitrogen adsorption measurements (Figure S2).

Quantitative pore structure parameters derived from nitrogen adsorption–desorption measurements are summarized in Table S3. The pristine CMPS exhibits a very low BET surface area of 20.47 m2 g–1, a pore volume of 0.180 cm3 g–1, and a large average pore diameter of 35.20 nm, indicating a loosely packed structure dominated by macropores and large mesopores. After post-cross-linking, the BET surface area increases dramatically to 623.09 m2 g–1, accompanied by an increase in pore volume to 0.449 cm3 g–1 and a sharp decrease in average pore size to 2.88 nm. This pronounced change reflects the formation of a rigid and highly cross-linked polymer framework through the introduction of methylene bridges, giving rise to an intrinsic micro/mesoporous network and substantially enhancing the internal surface area.

Following Dicyandiamide modification, the BET surface area further increases to 850.29 m2 g–1, while the pore volume rises to 0.638 cm3 g–1 and the average pore diameter remains nearly unchanged at approximately 3.00 nm. Notably, this additional increase in surface area and pore volume does not indicate the generation of new pore structures. Instead, it is mainly attributed to improved accessibility and more effective utilization of the intrinsic micro- and small mesopores formed during the post-cross-linking process. The incorporation of Dicyandiamide functional groups likely facilitates the opening of partially blocked pore entrances and alters the surface chemistry, thereby increasing the fraction of accessible pores.

Nitrogen adsorption–desorption isotherms (Figure S2) further corroborate the pore structure evolution. The pristine CMPS shows minimal nitrogen uptake over the entire relative pressure range, with only a slight increase and an inconspicuous hysteresis loop at P/P0 > 0.8, indicating limited porosity dominated by large mesopores and macropores. In contrast, the post-cross-linked resin exhibits a significant uptake at low relative pressures (P/P0 < 0.1), characteristic of abundant micropores, along with a distinct hysteresis loop in the intermediate pressure region, reflecting the presence of mesoporous domains. After Dicyandiamide modification, DCD-HCR displays a more pronounced, nearly rectangular H1-type hysteresis loop in the P/P0 range of 0.4–0.9, suggesting more accessible and uniform mesoporous channels. The pore-size distribution of DCD-HCR is mainly concentrated in the range of 2–10 nm, indicating an enhanced contribution from small mesopores.

Overall, these results demonstrate that the intrinsic micro/mesoporous framework of the resin is primarily constructed during the post-cross-linking step, while subsequent Dicyandiamide modification further improves pore accessibility and effective surface utilization without generating new pore structures. This hierarchical porous architecture provides a structural basis for the enhanced adsorption performance of DCD-HCR toward CL-20.

FT-IR spectra (Figure a) of CMPS, the ultrahighly cross-linked resin and the Dicyandiamide-modified resin are largely similar, indicating no obvious change in the polymer backbone. Relative to HCR resin, the Dicyandiamide-modified sample shows clear N–H stretching at ∼ 3440 cm–1 and N–H bending at ∼ 1630 cm–118,. − The C–Cl band at 1263, 674 cm–1 in CMPS is markedly reduced after modification. ,,

2.

2

Open in a new tab

(a) FT-IR spectrum comparison of resin modification. (b) High-resolution XPS N 1s spectra of the resin before and after modification.

From the high-resolution N 1s XPS spectra presented in Figure b, the unmodified CMPS resin exhibits a negligible nitrogen signal, confirming the absence of nitrogen-containing functional groups in its polymeric framework. In contrast, the DCD-HCR resin displays two well-resolved peaks at binding energies of 399.86 and 402.04 eV, which are assigned to N–H and CN/CN species, respectively. XPS survey spectra reveal that the pristine CMPS and HCR mainly consist of C and O, while no nitrogen signal is detected. After Dicyandiamide modification, a clear N signal appears with an atomic percentage of 3.39 at. %, accompanied by an increase in oxygen content. Detailed data are provided in Table S4. The emergence of these characteristic nitrogen signals in DCD-HCR confirms the successful incorporation of nitrogen-containing functional groups into the resin matrix via Dicyandiamide modification. As shown in Figure S3, slight shifts in the C 1s spectra are observed, indicating changes in the local electronic environment of carbon atoms due to the introduction of nitrogen species. These results demonstrate that the amination reaction effectively introduced nitrogen-containing functionalities into the polymer architecture. ,

The surface charge properties of DCD-HCR were investigated by zeta potential measurements as a function of solution pH, and the results are shown in Figure S4. As the pH increased from 2 to 10, the zeta potential of DCD-HCR gradually decreased from positive to negative values, indicating a progressive transition of the surface charge. Under strongly acidic conditions (pH = 2), DCD-HCR exhibited a high positive zeta potential, which can be attributed to the extensive protonation of nitrogen-containing functional groups introduced by Dicyandiamide modification. With increasing pH, gradual deprotonation occurred, leading to a continuous decrease in surface positive charge and a reversal to negative values at higher pH. The point of zero charge (pHpzc) can be estimated to lie between pH 5 and 6.

This pH-dependent surface charge behavior provides direct evidence that DCD-HCR possesses a positively charged surface under strong acidic environments, which is favorable for enhancing electrostatic interactions with highly polar CL-20 molecules. Consequently, the zeta potential results strongly support the adsorption behavior observed under acidic conditions and corroborate the proposed adsorption mechanism.

3.2. Adsorption Kinetics

The adsorption capacities of the three resins are shown in Figure a. The adsorption capacity of the Dicyandiamide-modified resin is three times that of the original CMPS, confirming the effectiveness of the modification. The adsorption process of DCD-HCR will be further analyzed through kinetic fitting.

3.

3

Open in a new tab

(a) Comparison of the adsorption behavior of three types of resins; (b) adsorption kinetic curves of DCD-HCR; (c) the process diagram of the membrane diffusion simulation of DCD-HCR.

The experimental data were fitted to the pseudo-first-order, pseudo-second-order, and intraparticle diffusion (IPD) kinetic models, whose mathematical expressions are provided in Computational Methods. − To evaluate the adsorption performance of DCD-HCR resin, adsorption kinetic experiments were conducted. Adsorption kinetics describes the quantitative relationship between contact time and adsorption at constant temperature and concentration. As shown in Figure b, the adsorption capacity of DCD-HCR for CL-20 increased rapidly during the initial 60 min and then gradually approached equilibrium. This rapid uptake at the early stage is attributed to the high availability of active adsorption sites and the elevated concentration of CL-20 in the solution. As time progressed, the adsorption rate decreased, and equilibrium was achieved at approximately 240 min.

The fitted parameters for the kinetic models are listed in Table , and the IPD parameters are summarized in Table .

1. Parameter Fitting of DCD-HCR Adsorption Kinetics Model.

Adsorbents DCD-HCR
pseudo-first-order model K1 0.492 ± 0.0397
qe 19.592 ± 0.256
R2 0.894
pseudo-second-order mode K2 0.043 ± 0.0017
qe 20.313 ± 0.086
R2 0.991
Open in a new tab

2. Parameter Fits for the Intraparticle Diffusion Model.

Adsorbents DCD-HCR
rapid diffusion k 1 3.081 ± 0.311
C1 8.754 ± 0.702
R2 0.980
slower diffusion k 2 0.193 ± 0.024
C2 18.613 ± 0.122
R2 0.956
equilibrium stage k 3 0.020 ± 0.002
C3 19.726 ± 0.031
R2 0.958
Open in a new tab

For DCD-HCR, the correlation coefficients (R2) of the pseudo-first-order and pseudo-second-order models were 0.868 and 0.985, respectively. The pseudo-second-order model therefore provided a better fit, indicating that it more accurately represents the adsorption process of CL-20 on DCD-HCR. The theoretical equilibrium adsorption capacity (20.334 mg g–1) agreed well with the experimental value (20.252 mg g–1). It should be noted that the pseudo-second-order model is an empirical kinetic model. The good agreement with this model mainly reflects the dependence of adsorption rate on the availability of surface adsorption sites, rather than providing direct evidence for a specific adsorption mechanism.

As shown in Figure S5, the IPD fitting curve consists of three distinct linear segments, corresponding to different diffusion stages, where k 1 > k 2 > k 3, The first stage (0–20 min) represents rapid diffusion, followed by a slower diffusion phase (20–240 min), and finally an equilibrium stage (after 240 min). The IPD plot does not pass through the origin, indicating that intraparticle diffusion is not the sole rate-controlling step. The initial stage is dominated by surface adsorption and film diffusion, followed by a slower intraparticle diffusion phase and eventual equilibrium. The specific diffusion process is simulated as shown in Figure c.

3.3. Adsorption Isotherms

To further analyze the adsorption mechanism, the equilibrium data were fitted using the Langmuir, Freundlich, and Temkin isotherm models, as described in the Computational Methods. The adsorption isotherm behavior of CL-20 on DCD-HCR was evaluated based on these models. −

The Langmuir model provides the best overall fit across the investigated temperature range, with correlation coefficients (R2) exceeding 0.95 at all three temperatures (Figure a; Table S5). This result indicates that the adsorption of CL-20 on DCD-HCR is predominantly governed by monolayer adsorption on a finite number of effective adsorption sites within the investigated concentration range. Based on the Langmuir model, the maximum adsorption capacity (q max) at 25 °C was determined to be 30.68 mg g–1 and is taken as the representative maximum adsorption capacity of the resin.

4.

4

Open in a new tab

(a) Langmuir isotherm and Freundlich adsorption isotherm curves of DCD-HCR. (b) Temkin adsorption isotherm curves of DCD-HCR. (c) Thermodynamic parameters fitting the linear relationship between ln K and 1/T. (d) Desorption of ethyl acetate, acetone, methanol, and ethanol. (e) Cyclic removal rate of ethyl acetate, acetone, methanol, and ethanol. (f) Cyclic adsorption capacity of ethyl acetate, acetone, methanol, and ethanol.

The strong correlation with the Langmuir model reflects progressive saturation of effective adsorption sites during adsorption. Although the hypercross-linked polymeric resin possesses intrinsic surface heterogeneity, the Freundlich model exhibits comparatively poorer fitting performance, indicating that multilayer adsorption or strong heterogeneity effects are not dominant under the present conditions. ,

To assess the practical performance of DCD-HCR, its CL-20 adsorption capacity was compared with those of previously reported adsorbents (Table S6). DCD-HCR shows a competitive maximum adsorption capacity (qm = 30.68 mg g–1) under strongly acidic conditions, comparable to or exceeding that of conventional macroporous resins reported for CL-20 removal from waste acid. Despite some materials exhibiting higher adsorption capacities, the stable performance of DCD-HCR in a strongly acidic mixed-acid system highlights its robustness and practical potential for CL-20 recovery from industrial waste acid streams.

To further elucidate the energetic evolution during adsorption, the Temkin model was applied as a complementary analysis. The Temkin equilibrium constant (KT) increases with temperature, indicating enhanced adsorption affinity at elevated temperatures, consistent with the endothermic nature of the adsorption process (ΔH > 0). In contrast, the Temkin constant bT decreases with increasing temperature, suggesting that energetic differences among adsorption sites become less significant as thermal energy increases.

Accordingly, the fitting quality of the Temkin model gradually decreases with increasing temperature, indicating that coverage-dependent energetic effects are more pronounced at lower temperatures, whereas adsorption at elevated temperatures is increasingly dominated by site saturation. This trend is consistent with thermodynamic results showing increasingly negative Gibbs free energy changes with rising temperature, confirming the spontaneous and endothermic nature of CL-20 adsorption on DCD-HCR.

Overall, these results demonstrate that CL-20 adsorption on DCD-HCR is dominated by monolayer adsorption on effective sites, with coverage-dependent energetic effects at lower temperatures and site saturation effects prevailing at elevated temperatures.

3.4. Adsorption Thermodynamics

The thermodynamic behavior of CL-20 adsorption on DCD-HCR resin was analyzed by evaluating the enthalpy change (ΔH), entropy change (ΔS), and Gibbs free energy change (ΔG). Its fundamental equation is as shown in Computational Methods. , The calculated values are summarized in Table , The thermodynamic parameters fitting the linear relationship between ln K and 1/T are shown in Figure c. At 298.15, 303.15, and 308.15 K, ΔG < 0 was calculated in the actual adsorption process, and the reaction is spontaneous, indicating that the adsorption process was spontaneous under the studied temperature conditions

3. Thermodynamic Parameters of CL-20 Adsorption on DCD-HCR.

Adsorbent ΔG (kJ/mol) ΔH (kJ/mol) ΔS (kJ/mol)
DCD-HCR 298 K –22.86 58.55 0.27
303 K –24.31
308 K –25.59
Open in a new tab

The positive ΔH value confirms that the adsorption is endothermic, requiring heat input to proceed. This suggests that the interaction between CL-20 and the ultrahighly cross-linked resin (HCR) primarily involves weak forces such as van der Waals interactions and hydrogen bonding. Consequently, the process depends on external heat absorption to enhance molecular motion and facilitate adsorption. Additionally, the positive ΔS value indicates that the adsorption process increases system disorder. This rise in entropy may result from greater molecular freedom of the adsorbed CL-20 molecules or from the rearrangement of surrounding water and solute molecules, leading to higher randomness in the system.

Amine modification of the ultrahighly cross-linked resin enhances surface polarity and hydrophilicity by introducing amino functional groups. This structural modification aligns with the adsorption isotherm results, where both the equilibrium concentration and adsorption capacity increased with temperature.

3.5. Desorption and Reproducibility Experiments

During the adsorption of CL-20 by DCD-HCR, desorption plays a crucial role in determining the regeneration efficiency and recyclability of the adsorbent. To evaluate the effect of different solvents on desorption performance, four commonly used solvents (ethyl acetate, acetone, ethanol, and methanol) were tested. The desorption efficiency and capacity retention were calculated using the following equations:

D(%)=CdVd(c0−ce)V×100% 2

where C d is the concentration of CL-20 in the desorbent (mg L–1), V d is the desorbent volume (L), C 0 and C e are the initial and equilibrium concentrations of CL-20 in the waste acid (mg L–1), and V is the volume of waste acid (L).

As shown in Figure d, among the tested solvents, ethyl acetate exhibited the highest desorption efficiency, achieving 100%. This superior performance may be attributed to its strong solvating ability, which effectively disrupts the physical adsorption forces between CL-20 and the resin. Acetone also showed high desorption efficiency (up to 85%), while methanol and ethanol achieved lower efficiencies of 75% and 50%, respectively, offering a trade-off between cost, environmental impact, and desorption performance.

Four adsorption–desorption cycles were conducted under identical conditions to evaluate the regeneration stability of the resin. As shown in Figure e,f, the adsorption capacity of the resin gradually decreased with increasing cycle number. The initial adsorption capacity (q1) ranged from 29.6 to 30.07 mg g–1, while by the fourth cycle it decreased to 5.47–19.7 mg g–1.

The decrease in adsorption capacity can be mainly attributed to incomplete desorption caused by strong interactions between the adsorbate and the resin, as well as partial pore blockage due to the accumulation of residual adsorbates on the resin surface. Considering that the regeneration was performed using organic solvents at 25 °C for 12 h, which are mild conditions, structural degradation of the resin is unlikely to be the dominant reason for the observed capacity decay.

Resins regenerated with ethyl acetate maintained the highest performance, effectively adsorbing and desorbing CL-20 across multiple cycles. Minimal changes in adsorption capacity were observed after two cycles, indicating that two adsorption–desorption cycles represent the optimal regeneration frequency. Even after three cycles, the adsorption capacity of the resin was 24.72 mg g–1, and the capacity retention rate of the resin still remained above 80%, confirming its excellent stability and reusability. Based on these findings, ethyl acetate can be regarded as the optimal desorbing agent.

4. Adsorption Mechanism

Figure a shows the XPS survey spectra of DCD-HCR before and after CL-20 adsorption. A pronounced increase in the N signal after adsorption confirms the successful uptake of nitrogen-containing nitro compounds. To further elucidate the adsorption mechanism at the molecular level, high-resolution XPS (HR-XPS) spectra of the C 1s and N 1s regions were analyzed, and the corresponding fitting parameters are summarized in Table S7.

5.

5

Open in a new tab

(a) XPS full spectrum of the resin before and after adsorption; (b) comparison of N 1s high-resolution spectra of DCD-HCR before and after adsorption.

As shown in Figure b, the N 1s spectrum of DCD-HCR before adsorption exhibits two main peaks at 399.86 eV (N–H) and 402.04 eV (CN/C–N+), indicating a relatively uniform nitrogen chemical environment on the resin surface. After adsorption, two additional peaks appear at 402.94 and 406.13 eV, which can be assigned to −NO or charge-transfer-related species and −NO2 groups, respectively. Meanwhile, the primary CN peak shifts slightly to lower binding energy (401.92 eV). The emergence of high-binding-energy nitro-related peaks and the binding-energy shift of nitrogen species indicate electron density redistribution, suggesting charge transfer and hydrogen-bond interactions between CL-20 molecules and nitrogen-containing functional groups on the resin surface.

The C 1s spectra (Figure S6) further support these interactions. Prior to adsorption, peaks at 284.8 eV (C–C/CC), 284.7 eV (C–N), and 286.56 eV (CN) confirm a carbon framework enriched with nitrogen-containing functionalities, while the π→π* satellite peak at 291.18 eV indicates a conjugated structure within the resin matrix. After adsorption, all C 1s peaks exhibit slight shifts toward higher binding energies, reflecting changes in the local electronic environment of carbon atoms due to interactions with CL-20 molecules.

Consistent with the XPS results, FT-IR spectra (Figure S7) show the appearance of characteristic nitro stretching vibrations at 1292 and 1560 cm–1 after adsorption, confirming the presence of CL-20 on the resin surface. In addition, the broadening of amine-related bands at 1630 and 3440 cm–1 suggests the formation of hydrogen bonding between amino groups of DCD-HCR and nitro groups of CL-20.

Based on these surface characterization results, CN/C–N and amino (N–H) groups are identified as the primary active sites for CL-20 adsorption. Hydrogen bonding, together with dipole–dipole and van der Waals interactions associated with the highly polar nitro groups of CL-20, plays a major role in stabilizing the adsorbed molecules on the resin surface.

Combining the kinetic, isotherm, thermodynamic, and surface characterization results, a schematic adsorption mechanism is proposed and illustrated in Figure . CL-20 molecules first migrate from the bulk solution to the external surface of DCD-HCR through film diffusion, followed by intraparticle diffusion within the interconnected porous structure. The pseudo-second-order kinetic behavior indicates that surface interactions significantly influence the overall adsorption rate.

6.

6

Open in a new tab

Schematic illustration of the adsorption mechanism of CL-20 on DCD-HCR under strongly acidic conditions.

Under strongly acidic conditions, nitrogen-containing functional groups on DCD-HCR are extensively protonated, leading to a positively charged surface. This protonation facilitates electrostatic interactions with the highly polar nitro groups of CL-20, while hydrogen bonding and other polar intermolecular interactions cooperatively stabilize CL-20 molecules within the resin pores. The predominance of Langmuir-type adsorption behavior suggests monolayer adsorption on effective sites, whereas the Temkin analysis reflects coverage-dependent energetic changes during adsorption. Thermodynamic results further confirm that the adsorption process is spontaneous and endothermic.

5. Conclusions

An ultrahighly cross-linked resin (DCD-HCR) was successfully synthesized via Friedel–Crafts alkylation followed by Dicyandiamide amination. The resin exhibits a high specific surface area, a well-developed porous structure, and abundant nitrogen-containing functional groups, providing favorable adsorption sites for CL-20.

Adsorption experiments demonstrate that the adsorption behavior of CL-20 on DCD-HCR is dependent on initial concentration, contact time, and temperature, with particularly strong adsorption affinity under strongly acidic conditions. Kinetic analysis shows that the adsorption process follows a pseudo-second-order model, indicating the significant involvement of surface interactions. Adsorption isotherm analysis reveals that the adsorption behavior is predominantly described by the Langmuir model, suggesting monolayer adsorption on a finite number of effective sites, while the Temkin model reflects coverage-dependent energetic variations during adsorption.

Thermodynamic analysis confirms that the adsorption of CL-20 on DCD-HCR is a spontaneous and endothermic process, with enhanced favorability at elevated temperatures. XPS and FT-IR analyses indicate that nitrogen-containing functional groups, especially CN/C–N and amino (N–H) moieties, act as the primary active sites, with hydrogen bonding, electrostatic interactions, and other polar intermolecular forces jointly contributing to the adsorption mechanism.

Regeneration studies demonstrate that ethyl acetate is an effective desorption solvent, enabling stable adsorption performance over multiple adsorption–desorption cycles under mild conditions. Overall, owing to its simple synthesis, robust adsorption performance in strongly acidic media, and good regenerability, DCD-HCR shows promising potential for the adsorption and recovery of energetic compounds such as CL-20 from acidic waste streams. Further studies on long-term stability and process optimization are required to support practical applications.

Supplementary Material

ao5c13314_si_001.pdf (1.2MB, pdf)

Acknowledgments

The authors would like to express their sincere gratitude to Prof. Yongfeng Zhang, Prof. Zhengfang Ye, and Prof. Liying Wang for their insightful advice, constructive comments, and careful guidance throughout the preparation of this manuscript.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c13314.

  • Computational methods for adsorption kinetics, adsorption isotherms, and thermodynamic parameters; composition of waste acid (Table S1); detailed parameters for the preparation and modification of adsorbents (Table S2); physical properties of the pore structure including surface area and pore volume (Table S3); elemental composition of CMPS, HCR, and DCD-HCR determined by XPS in atomic percentage (Table S4); parameter fitting results of isothermal adsorption models for CL-20 on DCD-HCR (Table S5); comparison of CL-20 adsorption performance between DCD-HCR and previously reported adsorbents (Table S6); fitting parameters of C 1s and N 1s XPS peaks before and after adsorption (Table S7); SEM images at various magnifications revealing the internal porous cross-section of the resin (Figure S1); nitrogen adsorption–desorption isotherms and pore size distribution curves (Figure S2); high-resolution XPS C 1s spectra of the resin before and after modification (Figure S3); zeta potential of DCD-HCR as a function of solution pH (Figure S4); fitting results of the membrane diffusion model for DCD-HCR (Figure S5); comparison of high-resolution C 1s spectra before and after adsorption (Figure S6); Fourier Transform Infrared (FT-IR) spectra comparison of DCD-HCR before and after adsorption (Figure S7) (PDF)

N.L.: Investigation, Writing – original draft, Formal analysis. J.L.: Investigation. L.W.: Review. Z.H.: Investigation. Q.Z.: Investigation. B.B.: Investigation. Z.Y.: Investigation. Y.Z.: Investigation, Writing – review and editing, Formal analysis.

The authors declare no competing financial interest.

References

  1. Balakrishnan V. K., Monteil-Rivera F., Gautier M. A., Hawari J.. Sorption and Stability of the Polycyclic Nitramine Explosive CL-20 in Soil. J. Environ. Qual. 2004;33(4):1362–1368. doi: 10.2134/jeq2004.1362. [DOI] [PubMed] [Google Scholar]
  2. Hawari J., Deschamps S., Beaulieu C., Paquet L., Halasz A.. Photodegradation of CL-20: insights into the mechanisms of initial reactions and environmental fate. Water Res. 2004;38(19):4055–4064. doi: 10.1016/j.watres.2004.06.032. [DOI] [PubMed] [Google Scholar]
  3. Szecsody J. E., Girvin D. C., Devary B. J., Campbell J. A.. Sorption and oxic degradation of the explosive CL-20 during transport in subsurface sediments. Chemosphere. 2004;56(6):593–610. doi: 10.1016/j.chemosphere.2004.04.028. [DOI] [PubMed] [Google Scholar]
  4. Karakaya P., Christodoulatos C., Koutsospyros A., Balas W., Nicolich S., Sidhoum M.. Biodegradation of the High Explosive Hexanitrohexaazaisowurtzitane (CL-20) Int. J. Env. Res. Public Health. 2009;6(4):1371–1392. doi: 10.3390/ijerph6041371. [DOI] [PMC free article] [PubMed] [Google Scholar]
  5. Larson S. L., Felt D. R., Davis J. L., Escalon L.. Analysis of CL-20 in Environmental Matrices: Water and Soil. J. Chromatogr. Sci. 2002;40(4):201–207. doi: 10.1093/chromsci/40.4.201. [DOI] [PubMed] [Google Scholar]
  6. Ma S., Tan Y., Liu N., Li C., Wang X., Ma J., Zhao Q., Liu J., Song J., Qin J., Ye Z.. Removal of CL-20 from waste acid using macroporous resin: Stabilization of waste acid and recovery of CL-20. J. Hazard. Mater. 2025;495:138995. doi: 10.1016/j.jhazmat.2025.138995. [DOI] [PubMed] [Google Scholar]
  7. Tan Y., Ma S., Liu N., Wang X., Li C., Ma J., Wu K., Zhang M., Liu J., Song J., Qin J., Zhao Q., Ye Z.. Study on the adsorption mechanism of CL-20 in waste acid using silicon-based materials. Environ. Res. 2025;277:121595. doi: 10.1016/j.envres.2025.121595. [DOI] [PubMed] [Google Scholar]
  8. Wang T., Shen C., Wang N., Dai J., Liu Z., Fei Z J C.. Physicochemical, S A; Aspects, E, Adsorption of 3-Aminoacetanilide from aqueous solution by chemically modified hyper-crosslinked resins: Adsorption equilibrium, thermodynamics and selectivity. Colloids Surf. Physicochem. Eng. Aspects. 2019;575:346–351. doi: 10.1016/j.colsurfa.2019.05.029. [DOI] [Google Scholar]
  9. Zeng X., Zeng B., Liang B., Zhang K., Huang L., Liu X., Huang W.. Recovery of Low-Concentration Tungsten from Acidic Solution Using D318 Macroporous Resin. Molecules. 2024;29(20):4946. doi: 10.3390/molecules29204946. [DOI] [PMC free article] [PubMed] [Google Scholar]
  10. Zhao M., Cui C.. High adsorption of fulvic acid by amino modified styrene-type macroporous resin and evaluation of its mechanism. Water Sci. Technol. 2020;81(5):1000–1010. doi: 10.2166/wst.2020.187. [DOI] [PubMed] [Google Scholar]
  11. Castaldo R., Gentile G., Avella M., Carfagna C., Ambrogi V.. Microporous Hyper-Crosslinked Polystyrenes and Nanocomposites with High Adsorption Properties: A Review. Polymers. 2017;9(12):651. doi: 10.3390/polym9120651. [DOI] [PMC free article] [PubMed] [Google Scholar]
  12. Nandamol P. S., Porel M.. Guanidinium-based ionic porous organic polymer as a propitious material for inordinate uptake of permanganate ions from water. Polym. Chem. 2025;16(6):687–695. doi: 10.1039/D4PY01329H. [DOI] [Google Scholar]
  13. Chu S., Feng X., Liu C., Wu H., Liu X.. Advances in Chelating Resins for Adsorption of Heavy Metal Ions. Ind. Eng. Chem. Res. 2022;61(31):11309–11328. doi: 10.1021/acs.iecr.2c01353. [DOI] [Google Scholar]
  14. Tan L., Tan B.. Hypercrosslinked porous polymer materials: design, synthesis, and applications. Chem. Soc. Rev. 2017;46:3322–3356. doi: 10.1039/C6CS00851H. [DOI] [PubMed] [Google Scholar]
  15. Ma X., Li Y., Ye Z., Yang L., Zhou L., Wang L.. Novel chelating resin with cyanoguanidine group: Useful recyclable materials for Hg­(II) removal in aqueous environment. J. Hazard. Mater. 2011;185(2–3):1348–1354. doi: 10.1016/j.jhazmat.2010.10.054. [DOI] [PubMed] [Google Scholar]
  16. Owsik I. A., Kolarz B. N., Jermakowicz-Bartkowiak D., Jezierska J.. Synthesis and characterization of resins with ligands derived from Dicyandiamide. Polymer. 2003;44(19):5547–5526. doi: 10.1016/S0032-3861(03)00582-2. [DOI] [Google Scholar]
  17. Chang X., Sun X., Yang Z., Mi H., Guo F., Su Z., Zheng W., Qiu J.. Dicyandiamide assisted highly interconnected hierarchical pore structure in coal tar pitch-derived carbon towards high-performance Zn-ion hybrid capacitor. Carbon. 2025;235:120019. doi: 10.1016/j.carbon.2025.120019. [DOI] [Google Scholar]
  18. Karyab H., Hamidi Z., Ghotbinia F., Khani Z. M., Nazeri N.. Application of the central composite design and response surface methodology for optimization of reactive color removal from aqueous solutions using Dicyandiamide-formaldehyde resin modified by ammonium chloride. Desalin. Water Treat. 2023;308:217–228. doi: 10.5004/dwt.2023.29829. [DOI] [Google Scholar]
  19. Abdolhossein Rejali N., Dinari M.. Highly efficient adsorption of direct Scarlet dye using guanidinium-based covalent organic polymer. Sci. Rep. 2025;15(1):13768. doi: 10.1038/s41598-025-98812-4. [DOI] [PMC free article] [PubMed] [Google Scholar]
  20. Lai W., Zhang K., Shao P., Yang L., Ding L., Pavlostathis S. G., Shi H., Zou L., Liang D., Luo X.. Optimization of adsorption configuration by DFT calculation for design of adsorbent: A case study of palladium ion-imprinted polymers. J. Hazard. Mater. 2019;379:120791. doi: 10.1016/j.jhazmat.2019.120791. [DOI] [PubMed] [Google Scholar]
  21. Huang J., Wang X., Patil P. D., Tang J., Chen L., Liu Y.-N.. Synthesis, characterization and adsorption properties of an amide-modified hyper-cross-linked resin. RSC Adv. 2014;4(77):41172–41178. doi: 10.1039/C4RA07322C. [DOI] [Google Scholar]
  22. Leofanti G., Padovan M., Tozzola G., Venturelli B.. Surface area and pore texture of catalysts. Catal. Today. 1998;41(1):207–219. doi: 10.1016/S0920-5861(98)00050-9. [DOI] [Google Scholar]
  23. Li Y., Sun N., Li L., Zhao N., Xiao F., Wei W., Sun Y., Huang W.. Grafting of Amines on Ethanol-Extracted SBA-15 for CO2 Adsorption. Materials. 2013;6:981–999. doi: 10.3390/ma6030981. [DOI] [PMC free article] [PubMed] [Google Scholar]
  24. Moradi M. R., Torkashvand A., Ramezanipour Penchah H., Ghaemi A.. Amine functionalized benzene based hypercrosslinked polymer as an adsorbent for CO(2)/N(2) adsorption. Sci. Rep. 2023;13(1):9214. doi: 10.1038/s41598-023-36434-4. [DOI] [PMC free article] [PubMed] [Google Scholar]
  25. Castaldo R., Avolio R., Cocca M., Errico M. E., Avella M., Gentile G.. Amino-functionalized hyper-crosslinked resins for enhanced adsorption of carbon dioxide and polar dyes. Chem. Eng. J. 2021;418:129463. doi: 10.1016/j.cej.2021.129463. [DOI] [Google Scholar]
  26. Lv H., Peng L., Li W., Zhao L., Zhang M.. Efficient and selective capture of Au­(III) from PCBs by pentaethylenehexamine-modified chloromethylated polystyrene beads. Environ. Sci. Pollut Res. Int. 2023;30(53):113494–113503. doi: 10.1007/s11356-023-30446-3. [DOI] [PubMed] [Google Scholar]
  27. Wang H., Wang Y., Sun X., Hu H., Peng Q.. Two functional post-cross-linked polystyrene resins: Effect of structure on the enhanced removal of benzene sulfonic acid. Colloids Surf. Physicochem. Eng. Aspects. 2020;588:124398. doi: 10.1016/j.colsurfa.2019.124398. [DOI] [Google Scholar]
  28. Su P., Chen S., Yang Z., Zhong N., Jiang C., Li W.. Vapor-phase postsynthetic amination of hypercrosslinked polymers for efficient iodine capture. Chin. Chem. Lett. 2024;35(9):109357. doi: 10.1016/j.cclet.2023.109357. [DOI] [Google Scholar]
  29. Azizian S.. Kinetic models of sorption: a theoretical analysis. J. Colloid Interface Sci. 2004;276(1):47–52. doi: 10.1016/j.jcis.2004.03.048. [DOI] [PubMed] [Google Scholar]
  30. Ho Y. S., McKay G.. Pseudo-second order model for sorption processes. Proc. Biochem. 1999;34(5):451–465. doi: 10.1016/S0032-9592(98)00112-5. [DOI] [Google Scholar]
  31. Adams I., F.W A., Hassan L. G., S.A M., Itodo H.. Intraparticle Diffusion and Intraparticulate Diffusivities of Herbicide on Derived Activated Carbon. Researcher. 2010;2:74. [Google Scholar]
  32. Lagergren S.. About the Theory of So-Called Adsorption of Soluble Substances. K. Sven. Vetenskapsakad. Handl. 1898;24:1–39. [Google Scholar]
  33. Ho Y. S., Mckay G.. Pseudo-second order model for sorption processes. Process Biochem. 1999;34(5):451–465. doi: 10.1016/S0032-9592(98)00112-5. [DOI] [Google Scholar]
  34. Weber W. J. Jr., Morris J. C.. Kinetics of adsorption on carbon from solution. Journal of the Sanitary Engineering Division, ASCE. 1963;89:31–59. doi: 10.1061/JSEDAI.0000430. [DOI] [Google Scholar]
  35. Bi M., Wei X., Lou Q., Di D., Pei D., Liu B.. Preparation of p-phenylenediamine modified polymer brush grafted PS-DVB materials for highly selective adsorption of flavonoids. Colloids Surf. Physicochem. Eng. Aspects. 2023;679:132525. doi: 10.1016/j.colsurfa.2023.132525. [DOI] [Google Scholar]
  36. Simonin J.-P.. Intraparticle diffusion-adsorption model to describe liquid–solid adsorption kinetics. Rev. Mex. Ing. Chim. 2016;15:161–173. [Google Scholar]
  37. Langmuir I.. The adsorption of gases on plane surfaces of glass, mica, and platinum. J. Am. Chem. Soc. 1916;38:2221–2295. doi: 10.1021/ja02268a002. [DOI] [Google Scholar]
  38. Murphy O. P., Vashishtha M., Palanisamy P., Kumar K. V.. A Review on the Adsorption Isotherms and Design Calculations for the Optimization of Adsorbent Mass and Contact Time. ACS Omega. 2023;8(20):17407–17430. doi: 10.1021/acsomega.2c08155. [DOI] [PMC free article] [PubMed] [Google Scholar]
  39. Freundlich H.. Über die Adsorption in Lösungen. Z. Phys. Chem. 1907;57:385–470. doi: 10.1515/zpch-1907-5723. [DOI] [Google Scholar]
  40. Temkin M. J., Pyzhev V.. Recent modifications of Langmuir isotherms. Acta Physiochim. URSS. 1940;12:217. [Google Scholar]
  41. Erdem Yayayürük A., Çankaya N., Yayayürük O.. Advancing eco-friendly adsorption: a sustainable approach for rapid and efficient methylene blue removal. Polym. Bull. 2025;82(8):3207–3223. doi: 10.1007/s00289-025-05662-1. [DOI] [Google Scholar]
  42. Patiha, Heraldy E., Hidayat Y., Firdaus M.. The langmuir isotherm adsorption equation: The monolayer approach. IOP Conf. Ser.: Mater. Sci. Eng. 2016;107:012067. doi: 10.1088/1757-899X/107/1/012067. [DOI] [Google Scholar]
  43. Ghosal P. S., Gupta A. K.. Determination of thermodynamic parameters from Langmuir isotherm constant-revisited. J. Mol. Liq. 2017;225:137–146. doi: 10.1016/j.molliq.2016.11.058. [DOI] [Google Scholar]
  44. Chen J., Ren J., Ye C., Li L., Yang C., Qiu T.. Highly selective removal of 2,4-dinitrotoluene for industrial wastewater treatment through hyper-cross-linked resins. J. Cleaner Prod. 2021;288:125128. doi: 10.1016/j.jclepro.2020.125128. [DOI] [Google Scholar]
  45. Waheed A., Kazi I. W., Manzar M. S., Ahmad T., Mansha M., Ullah N., Ahmed Blaisi N. I.. Ultrahigh and efficient removal of Methyl orange, Eriochrom Black T and acid Blue 92 by triazine based cross-linked polyamine resin: Synthesis, isotherm and kinetic studies. Colloids Surf. Physicochem. Eng. Aspects. 2020;607:125472. doi: 10.1016/j.colsurfa.2020.125472. [DOI] [Google Scholar]
  46. Shi T., Ma L., Xi X., Nie Z.. Preparation of functional polytertiary amine macroporous resin and its adsorption and separation properties for tungsten and molybdenum. Sep. Purif. Technol. 2024;332:125759. doi: 10.1016/j.seppur.2023.125759. [DOI] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

ao5c13314_si_001.pdf (1.2MB, pdf)

Articles from ACS Omega are provided here courtesy of American Chemical Society

RESOURCES