Engineering a Positively Charged Surface on g‑C₃N₄ via CTAB Modification for Ultrahigh Capacity and pH-Robust Adsorption of Congo Red

Abstract

Congo red (CR) poses significant environmental and health risks, making its effective removal a critical objective in wastewater treatment. To address this challenge, this research modified bulk g-C₃N₄ (BCN) with the surfactant cetyltrimethylammonium bromide (CTAB) via a hydrothermal method. The resulting adsorbent, 0.2CTAB/CN, demonstrated an optimal performance in CR removal. 0.2CTAB/CN exhibited a zeta potential of +25.1 mV, in contrast to −17.7 mV for BCN. This positive surface charge promotes effective adsorption of the anionic dye CR, achieving a removal efficiency exceeding 98% over a broad pH range (pH of 1–12). The maximum adsorption capacity of 0.2CTAB/CN for CR, as fitted by the Langmuir model (R ² = 0.981), reached 1945 mg/g at 313 K, surpassing most reported CN-based adsorbents. Importantly, the modified adsorbent maintained excellent CR removal performance not only in ultrapure water (98.6%) but also exhibited comparable efficiency in complex water matrices, including tap water (98.5%) and natural river water (98.8%), demonstrating its potential for practical wastewater treatment applications. This research provides valuable insights into the design of high-performance adsorbents and offers fundamental support for the advanced treatment of wastewater containing anionic dyes.

1. Introduction

Congo red (CR), a typical toxic anionic azo dye, is widely used in printing, paper, and textiles. , Its molecular structure consists of six benzene rings, contributing to high stability and resistance to biodegradation. CR is a recognized carcinogen that can have serious, even fatal, effects on a number of organs, including the eyes, skin, respiratory system, and reproductive system, despite its high water solubility. , Hence, efficiently eliminating CR from aqueous environments is of critical importance.

Adsorption has emerged as one of the most promising techniques for treating dye-contaminated wastewater due to its simplicity, cost-effectiveness, and high removal efficiency. − Although numerous adsorbents (e.g., MOF compounds, inorganic–organic hybrid materials) have been developed for CR removal, many possess superior properties, though some still face limitations such as moderate adsorption capacity, poor performance in complex water matrices, or impractical synthesis procedures. − Graphitic carbon nitride (g-C₃N₄, CN), a metal-free polymeric semiconductor, exhibits great potential in adsorption applications owing to its abundant surface functional groups, remarkable chemical stability, nontoxicity, and a distinctive layered structure. − The adsorption performance of CN stems from multiple synergistic mechanisms: electrostatic attraction through protonatable amino groups, hydrogen bonding with polar dye molecules, and π–π stacking between its electron-rich tri-s-triazine units and aromatic pollutants. − These combined interactions enable highly efficient contaminant removal. Recent studies have employed various modification strategies such as acid treatment, thermal oxidation exfoliation, and surface functionalization to improve the adsorption capacity of CN. − Among these approaches, surfactant-assisted modulation of CN’s morphology and surface chemistry has emerged as a promising modification strategy. ,

The cationic surfactant cetyltrimethylammonium bromide (CTAB) is composed of a hydrophilic quaternary ammonium group and a hydrophobic hexadecyl chain. In nanomaterial synthesis, CTAB serves as a multifunctional template: (i) its amphiphilic nature reduces surface energy to inhibit particle aggregation, (ii) the long alkyl chain induces steric hindrance to control morphology, and (iii) it guides the creation of mesopores, increasing specific surface area (SSA). ,, Previous studies have confirmed that CTAB-modified nanomaterials (e.g., silica, graphene oxide) exhibit significantly improved adsorption performance for pollutants. − This surface engineering strategy is particularly relevant for CN, whose native negatively charged surface limits its affinity for anionic pollutants like CR. The introduction of CTAB is therefore designed to functionalize the CN surface with cationic quaternary ammonium groups, converting electrostatic repulsion into attraction and creating high-energy adsorption sites specifically tailored for anionic dye capture.

In this paper, CTAB was introduced during the hydrothermal modification of bulk CN (BCN) to modulate its interlayer spacing, SSA, and surface charge. To comprehensively understand the adsorption behavior, detailed investigations, including adsorption kinetics, isotherms, and thermodynamics, were conducted to elucidate the interaction mechanisms between the modified CN and CR. Furthermore, to evaluate the practical applicability of the adsorbent, its CR removal performance was systematically assessed across various aqueous matrices.

2. Experimental Section

2.1. Reagents

All chemical reagents, including urea, anhydrous ethanol, CR, rhodamine B (RhB), methyl orange (MO), methylene blue (MB), HNO₃, and NaOH, were supplied from Sinopharm Chemical Reagent (Shanghai, China). Crystal violet (CV) was purchased from Yonghua Chemical Co., Ltd. (Suzhou, China). CTAB was acquired from Macklin Biochemical (Shanghai, China). All chemicals are analytically pure.

2.2. Adsorbent Synthesis

2.2.1. Synthesis of Bulk g-C₃N₄

BCN was prepared according to a previously reported procedure. Twenty g of urea was calcined in a crucible using a muffle furnace at 550 °C for 2 h, with a heating rate of 10 °C/min. The product was cooled to room temperature and then alternately cleaned with ethanol and deionized water. The obtained material was then dried in an oven and ground into a fine powder, yielding BCN as a yellow powder with a yield of approximately 4%.

2.2.2. Synthesis of CTAB-Modified g-C₃N₄

CTAB-modified CN was synthesized via a hydrothermal process (Scheme ). After dispersing 0.2 g of the as-synthesized BCN in 60 mL of deionized water, the mixture was agitated for 5 min at 300 rpm. Subsequently, different amounts of CTAB were added (mass ratios of CTAB/BCN = 0, 0.1, 0.2, 0.3, and 0.5), followed by continuous stirring for 30 min. The mixture was transferred into a Teflon-lined stainless steel autoclave and reacted at 170 °C for 8 h. The resulting product was cooled to room temperature, washed alternately with deionized water and ethanol three times, and then dried in an oven. Finally, the solid was ground into a fine powder. The samples after the hydrothermal process with different CTAB loadings were labeled as xCTAB/CN (x = 0, 0.1, 0.2, 0.3, and 0.5).

1. Schematic Illustration of the Synthesis Process of BCN and xCTAB/CN (Prepared with a CTAB/BCN Mass Ratio of x, x = 0, 0.1, 0.2, 0.3, and 0.5).

2.3. Characterization

X-ray diffraction (XRD) analysis was conducted on a Shimadzu XRD-6100 diffractometer (Kyoto, Japan) with Cu Kα radiation (λ = 1.5406 Å), scanning a 2θ range from 10° to 80° with a step size of 0.02°. Fourier-transform infrared (FTIR) spectroscopy was conducted on a Shimadzu IRTracer-100 spectrometer, employing the KBr pellet method, over the wavenumber range of 4000 to 400 cm^–1. Morphological and elemental distribution analyses were carried out using a GeminiSEM 360 field-emission scanning electron microscope (SEM) (Carl Zeiss, Germany) equipped with an energy-dispersive X-ray (EDX) detector, operated at an accelerating voltage of 15 kV. For SEM observation, the powder sample was directly mounted on conductive carbon tape and sputter-coated with gold to enhance conductivity. Transmission electron microscopy (TEM) observations were performed on a JEM-F200 instrument (JEOL Ltd., Japan) operated at 200 kV. The sample was prepared by dispersing the powder in ethanol via ultrasonication for 5 min, after which a drop of the suspension was placed onto a copper grid and dried at room temperature prior to analysis. Atomic force microscopy (AFM) imaging was conducted using a Dimension Icon atomic force microscope (Bruker, Germany) in tapping mode. The sample was prepared by dispersing the powder in ethanol via ultrasonication for 5 min, followed by dropping the suspension onto a freshly cleaved mica substrate and drying at room temperature before measurement. Surface elemental composition was examined by X-ray photoelectron spectroscopy (XPS) using a Thermo Scientific K-Alpha spectrometer (USA) with monochromatic Al Kα radiation. SSA was determined from nitrogen adsorption–desorption isotherms recorded at 77 K with an ASAP 2460 analyzer (Micromeritics, USA) based on the Brunauer–Emmett–Teller (BET) method. Zeta potential and particle size were assessed with a Zetasizer Nano ZS (Malvern Panalytical, UK).

2.4. Congo Red Adsorption Assay

The adsorption assays were conducted by adding 5–50 mg of the synthesized adsorbent to 50 mL of CR aqueous solutions (50–500 mg/L). Unless otherwise noted, the mixtures were agitated in the dark at 293 K for 180 min. The solution pH was adjusted from 1 to 12 using HNO₃ or NaOH, starting from an initial pH of approximately 6.3. At designated time points, 1 mL samples were collected and centrifuged at 10,000g for 5 min. The supernatant was analyzed for CR concentration using a UV-2600 spectrophotometer (Shimadzu) at 498 nm. , The cyclic experiment involves desorbing the adsorbent after the adsorption test with 15% diluted ammonia–water. The collected powder samples are rinsed alternately with water and ethanol before conducting further adsorption tests.

3. Results and Discussion

3.1. Structure and Composition Characterization of Absorbents

XRD and FTIR analyses were used to describe the physical structural information on the CTAB-modified CN. Two typical peaks are shown in the XRD patterns of every sample (Figure a). The peak at 13.3° is attributed to the (100) plane of CN, corresponding to in-plane structural packing of melon units, whereas the reflection at 27.7° corresponds to the (002) plane, indicative of interplanar stacking of aromatic systems. ,, After hydrothermal treatment, the intensity of the (002) peak increases significantly, suggesting an improvement in crystallinity or enhanced hydrogen bonding interactions. , When CTAB is introduced during hydrothermal treatment, the (002) peak shifts by approximately 0.2° toward lower angles and exhibits decreased intensity relative to the sample without CTAB (0CTAB/CN), whose (002) peak position remains unchanged from that of BCN (Figure b). This suggests that CTAB expands the interlayer spacing, leading to partial delamination of the layered structure. Notably, two new peaks emerge at 10.9° and 23.0°, which are consistent with melem-related structures, as widely reported. With increasing CTAB concentration, the peak at 23.0° gradually intensifies. The observed structural transformations can be attributed to hydrothermal-induced hydrolysis of BCN, which disrupts the in-plane long-range ordering. The addition of CTAB further intensifies this process through its alkaline catalytic effect, leading to cleavage of in-plane melon chains and subsequent generation of increased melem units.

1.

(a) XRD patterns and (b) enlarged view of (002) peak of BCN and xCTAB/CN (prepared with a CTAB/BCN mass ratio of x, x = 0, 0.1, 0.2, 0.3, and 0.5). (c) FTIR spectra of BCN and xCTAB/CN (x = 0, 0.1, 0.2, 0.3, and 0.5). (d) XPS spectra of C 1s peaks, (e) N 1s peaks, and (f) Br 3d peaks of BCN and xCTAB/CN (x = 0, 0.1, 0.2, 0.3, and 0.5).

Figure c illustrates that the FTIR spectra of both bulk and hydrothermally treated CN samples display distinct vibrational features. The broad absorption band between 3000 and 3600 cm^–1 corresponds to N–H/O–H stretching vibrations from structural NH/–NH₂ groups and adsorbed water molecules. , Multiple intense peaks in the 1200–1700 cm^–1 region arise from skeletal vibrations of C–N and CN heterocycles. , The characteristic peak at 809 cm^–1, attributed to tri-s-triazine ring vibrations, weakens after hydrothermal treatment, suggesting a partial breakdown of the tri-s-triazine units (Figure S1). , Importantly, the overall spectral profiles remain remarkably similar, confirming that the hydrothermal treatment preserves the fundamental molecular structure of CN while inducing subtle changes in its structural arrangement.

The surface composition and valence states of BCN and xCTAB/CN (x = 0, 0.1, 0.2, 0.3, and 0.5) were investigated by XPS analysis. The survey spectra of all samples exhibit characteristic peaks of C, N, and O elements (Figure S2), where the minor O 1s peak originates from surface-adsorbed H₂O or O₂. Notably, while the Br signal was not prominent in the survey spectra due to its low concentration, targeted high-resolution scans were performed to verify the incorporation of CTAB. The high-resolution C 1s spectra (Figure d) were fitted with five components centered at binding energies of 284.9, 286.9, 288.2, 288.4, and 293.5 eV. The prominent peak observed at 284.9 eV is associated with graphitic carbon (C–C), while the peak at 286.9 eV is linked to C–N bonds. The signal at 288.2 eV arises from sp²-hybridized carbon (N–CN) in the triazine rings, whereas the peak at 288.4 eV indicates the presence of C–O bonds due to surface oxidation. − Additionally, the π–π* transition satellite peak appears at 293.7 eV. , For the N 1s spectra (Figure e), four distinct peaks were identified at 398.6, 399.3, 401.2, and 404.2 eV, assigned to sp²-hybridized nitrogen in triazine rings (CN–C), tertiary nitrogen (N–(C)₃), free amino groups (C–N–H), and π–π* excitation, respectively. ,, Crucially, high-resolution Br 3d spectra were acquired to probe the incorporation of CTAB (Figure f). Distinct Br 3d peaks can be observed in the 0.2CTAB/CN, 0.3CTAB/CN, and 0.5CTAB/CN samples. Quantitative XPS analysis reveals that the concentrations of Br in the 0.1CTAB/CN, 0.2CTAB/CN, 0.3CTAB/CN, and 0.5CTAB/CN samples are 0.16 at. %, 0.21 at. %, 0.29 at. %, and 0.36 at. %, respectively. The intensity of these peaks exhibits a positive correlation with the CTAB content, providing direct spectroscopic evidence for the successful formation of a CTAB/CN composite. Notably, these residual Br levels are substantially lower than the stoichiometric equivalent of the incorporated CTA⁺ cations, indicating that the majority of Br^– counterions were effectively removed during the washing process. The trace amounts of remaining bromine are likely attributable to ions physically entrapped within the bulk structure or strongly adsorbed species that were inaccessible to the washing solvent.

The morphology of the materials was examined by SEM and TEM. As shown in Figure a–c, BCN, 0CTAB/CN, and 0.2CTAB/CN all exhibit a sheet-like structure. Notably, the sheets of 0CTAB/CN and especially 0.2CTAB/CN are significantly smaller than those of BCN, which is consistent with TEM observations (Figure d–f). SEM images of 0.3CTAB/CN and 0.5CTAB/CN (Figure S3) reveal morphologies similar to that of 0.2CTAB/CN, all displaying short sheet-like structures. However, with increasing CTAB content, these sheets tend to form larger aggregates. This reduction in lateral size aligns with the XRD results, indicating that hydrothermal treatment disrupts the long-range in-plane order of CN.

2.

SEM images of (a) BCN, (b) 0CTAB/CN (prepared with a CTAB/BCN mass ratio of 0), and (c) 0.2CTAB/CN (prepared with a CTAB/BCN mass ratio of 0.2). TEM images of (d) BCN, (e) 0CTAB/CN, and (f) 0.2CTAB/CN. AFM 2D surface images of (g) BCN, (h) 0CTAB/CN, and (i) 0.2CTAB/CN. AFM height images of (j) BCN, (k) 0CTAB/CN, and (l) 0.2CTAB/CN.

To further elucidate the elemental distribution in the modified samples, EDX analysis was performed on the 0.2CTAB/CN sample. The corresponding elemental mapping (Figure S4) clearly reveals the homogeneous distribution of C, N, and trace amounts of Br across the sheet-like structure. However, consistent with the quantitative XPS results, the signal for Br in the mapping is extremely faint (Figure S4e), and the quantified atomic concentration obtained from EDX is approximately 0 at. %. This corroborates the XPS findings that the vast majority of Br^– counterions were effectively removed during the washing process.

AFM 2D and 3D images provide additional confirmation of the layered structure (Figures g–i and S5), with measured thicknesses of 47, 25, and 3 nm for BCN, 0CTAB/CN, and 0.2CTAB/CN, respectively (Figure j–l). The decrease in thickness can be attributed to the hydrothermal process, which induces tighter stacking of CN layers, reducing the interlayer spacing in 0CTAB/CN. The addition of CTAB promotes exfoliation, leading to a more pronounced decrease in thickness for 0.2CTAB/CN.

The following were done to find the SSA of BCN, 0CTAB/CN, and 0.2CTAB/CN using BET analysis. All three materials exhibited mesoporous characteristics with type IV isotherms and hysteresis loops (Figure S6). The SSA of 0CTAB/CN (69 m²/g) was found to be lower than that of BCN (83 m²/g) (Table S1), which could be due to the tighter interlayer stacking observed in XRD and AFM analyses, leading to decreased interlayer spacing. In contrast, 0.2CTAB/CN showed an increased SSA of 88 m²/g (Table S1), likely due to the addition of CTAB promoting interlayer exfoliation. A larger SSA can offer greater accessibility to active sites for adsorption and surface reactions.

3.2. Surface Charge and Particle Size Analysis

The surface charge plays a crucial role in determining the electrostatic adsorption properties of the materials. As illustrated in Figure a, the zeta potential measurements reveal that BCN has a negative surface charge of −17.7 mV. After hydrothermal treatment in the absence of CTAB, the resulting material (0CTAB/CN) exhibits a zeta potential of −1.13 mV, which may result from the effect of amino group protonation. − Notably, the CTAB-modified samples (0.1–0.5CTAB/CN) demonstrate positive zeta potentials of 20.4, 25.1, 25.7, and 27.4 mV, respectively. This marked reversal in surface charge can be attributed to the collective effect of the CTAB-induced exposure of additional amino groups and the intrinsic positively charged quaternary ammonium groups from CTAB itself.

3.

(a) Zeta potential and (b) particle size of BCN and xCTAB/CN (prepared with a CTAB/BCN mass ratio of x, x = 0, 0.1, 0.2, 0.3, and 0.5). (c) Removal efficiency of CR by BCN and xCTAB/CN (x = 0, 0.1, 0.2, 0.3, and 0.5). (d) Removal efficiency of various dyes by 0.2CTAB/CN. (e) Effect of pH on CR adsorption and the zeta potential of 0.2CTAB/CN. (f) Effect of 0.2CTAB/CN dosage on CR adsorption capacity and removal efficiency. In panel b, *indicates that p-value is below 0.05. Error bars represent the standard deviation of triplicate experiments.

Particle size analysis revealed that BCN exhibited an average hydrodynamic diameter of 264 ± 45 nm (Figures b and S7). Following hydrothermal treatment, a progressive decrease in hydrodynamic diameter was observed with increasing CTAB content. The minimum average hydrodynamic diameter of 120 ± 14 nm was achieved at a CTAB/BCN mass ratio of 0.2. This significant size reduction can be attributed to the CTAB-induced disruption of in-plane long-range ordering and enhanced interlayer exfoliation of CN, which was directly visualized through XRD, SEM, and AFM characterizations. However, an increase in CTAB concentration beyond the ideal ratio led to hydrodynamic diameter enlargement. The increase in hydrodynamic diameter at higher CTAB concentrations maybe attributed to bridge-flocculation, where excess CTAB or micelles link multiple nanosheets together via hydrophobic interactions, leading to aggregation.

3.3. Adsorption Experiments

The as-prepared CN samples (10 mg) were used for the adsorption of CR (Figure c). The BCN exhibited a removal efficiency of only 35.7% for 100 mg/L CR. After hydrothermal treatment, all five modified samples showed improved CR removal efficiency. This enhancement can be attributed to the increased surface positive charge of the hydrothermally treated CN, which facilitates the adsorption of the anionic dye CR. Furthermore, when compared to 0CTAB/CN, the addition of CTAB in hydrothermal treatment increased the CR removal efficiency even more. 0.2CTAB/CN exhibited the highest efficiency, reaching 98.6%, which is approximately 2.8 times that of BCN. Since the difference in CR removal efficiency between 0.2CTAB/CN and 0.3CTAB/CN for 100 mg/L CR was negligible, their adsorption performance for 200 mg/L CR was further investigated (Figure S8). The data clearly showed that 0.2CTAB/CN had better adsorption ability than 0.3CTAB/CN. The optimal CR adsorption performance of 0.2CTAB/CN can be attributed to the synergistic effect of its high surface positive zeta potential and reduced average hydrodynamic diameter. Therefore, 0.2CTAB/CN was selected as the adsorbent for subsequent studies.

The adsorption performance of 0.2CTAB/CN toward different dyes (CR, MO, RhB, MB, CV) was systematically evaluated (Figures d and S9). The results revealed that 0.2CTAB/CN exhibited negligible adsorption for cationic dyes (RhB, MB, and CV), which can be attributed to its inherently positive zeta potential. In contrast, 0.2CTAB/CN demonstrated a higher adsorption capacity for the anionic dye CR compared to MO. This difference arises from several factors: first, CR (696.68 g/mol) has a higher molecular weight than MO (327.33 g/mol), contributing to stronger interactions. Additionally, CR contains more sulfonic acid groups than MO, which not only enhances electrostatic attraction with 0.2CTAB/CN but also facilitates hydrogen bonding. Furthermore, CR contains more aromatic moieties than MO, which can engage in enhanced π–π stacking with the electron-rich CN framework. These combined effects lead to the superior adsorption of CR over MO by 0.2CTAB/CN.

Since aqueous pH changes may have a significant effect on adsorption processes, the effect of solution pH (range from 1 to 12) on the adsorption performance of 0.2CTAB/CN toward CR was methodically examined (Figure e). Remarkably, the 0.2CTAB/CN nanosheets maintained exceptional removal efficiency (>98%) for CR across the entire pH spectrum (pH 1–12). This outstanding pH-independent adsorption behavior can be attributed to the consistently positive zeta potential of 0.2CTAB/CN across the entire tested pH range (Figure e). These findings demonstrate the robust adsorption capability of 0.2CTAB/CN and its potential for practical applications where pH variations may occur.

The adsorption performance of 0.2CTAB/CN with different dosages (5 mg, 10 mg, 20 mg, 30 mg, and 50 mg) toward CR was investigated (Figure f). The findings showed that as the dosage of the adsorbent increased, the CR removal efficiency steadily enhanced, whereas the adsorption capacity (q _e) diminished. The trends of removal efficiency and adsorption capacity intersected near 10 mg, suggesting that this dosage may represent an optimal balance between adsorption efficiency and capacity under the given experimental conditions. Thus, 10 mg was used as the best adsorbent dosage for additional research.

3.4. Kinetic, Isotherm, and Thermodynamic Modeling

The adsorption kinetics indicated that the adsorption capacity of 0.2CTAB/CN for CR rose sharply within the initial 20 min, then slowed progressively, attaining equilibrium within 40 min (Figure a).

4.

Adsorption kinetics and isotherm modeling of CR on 0.2CTAB/CN (prepared with a CTAB/BCN mass ratio of 0.2). Kinetics study: (a) effect of contact time on adsorption capacity and removal efficiency; linear fitting curves for the (b) pseudo-first-order, (c) pseudo-second-order, and (d) Weber–Morris intraparticle diffusion models. Isotherm study: (e) Freundlich and (f) Langmuir model fitting at different temperatures. Error bars represent the standard deviation of triplicate experiments.

To further investigate the kinetics, the experimental data were modeled with both pseudo-first-order (eq ), pseudo-second-order (eq ), and two-compartmental (eq ) kinetic equations. ,

$$\ln (q_{\mathrm{e}}-q_t)=\ln q_{\mathrm{e}}-k_{1}t$$ 1

$$t/q_t=1/({\mathrm{k}}_2{q}_{\mathrm{e}}^2)+t/q_{\mathrm{e}}$$ 2

$$q_t=q_{\mathrm{f}}(1-{\mathrm{e}}^{-k_{\mathrm{f}}t})+q_{\mathrm{s}}(1-{\mathrm{e}}^{-k_{\mathrm{s}}t})$$ 3

where k ₁ (min^–1) and k ₂ (g/mg/min) are the rate constants for the pseudo-first-order and pseudo-second-order models, respectively. t (min) is the adsorption time. q _t and q _e are the adsorption capacity at time t and at equilibrium, respectively. In eq , q _f and q _s represent the contributions from surface adsorption and bulk absorption, respectively. k _f and k _s are the corresponding rate constants. The total equilibrium capacity is given by q _e = q _f+ q _s.

Linear regression analysis indicated that the pseudo-second-order model provided a better fit to the data, with a correlation coefficient (R ²) of 0.999 (Table ), substantially exceeding that of the pseudo-first-order model (R ² = 0.996) (Figure b,c). Furthermore, the theoretical equilibrium adsorption capacity calculated by the pseudo-second-order model showed better agreement with the experimental values compared to the pseudo-first-order model. These results clearly indicate that the pseudo-second-order model provides a more appropriate description of the adsorption process. To gain deeper insight, the two-compartment (double-exponential) kinetic model was further applied. This model provided a significantly better fit (R ² = 0.997) than the pseudo-first-order model, revealing a combined mechanism involving fast surface adsorption followed by slow bulk absorption. The fitted parameters gave q _f = 55 mg/g (11.1%) and q _s = 441 mg/g (88.9%), indicating that bulk absorption is the dominant mechanism.

1. Pseudo-First- and Pseudo-Second-Order Kinetic Parameters for the Adsorption of CR by 0.2CTAB/CN (Prepared with a CTAB/BCN Mass Ratio of 0.2).

model	parameter
pseudo-first-order	k 1 (min–1)	0.121
	q e (mg/g)	496
	R 2	0.996
pseudo-second-order	k 2 (g/mg/min)	0.00223
	q e (mg/g)	500
	R 2	0.999
two-compartmental	q f (mg/g)	55
	k f (min–1)	83.817
	q s (mg/g)	441
	k s (min–1)	0.115
	q e (mg/g)	496
	R 2	0.997

To explain the adsorption mechanism, the diffusion process of CR was analyzed using the Weber–Morris intraparticle diffusion model (eq )

$$q_t=k_{\mathrm{w}}{t}^{1/2}+C$$ 4

where k _w (mg/ (g·min^1/2)) represents the intraparticle diffusion rate constant and C (mg/g) denotes the boundary layer thickness constant.

As illustrated in Figure c, the relationship between q _t and t _1/2 exhibited multilinear characteristics, indicating the involvement of multiple processes in the adsorption mechanism. The slope k _w reflects the rate of each adsorption step, with the decreasing rates (k _w1 > k _w2 > k _w3) characterizing distinct adsorption stages (Table ). The adsorption process of CR onto 0.2CTAB/CN proceeds through three distinct stages. In the initial stage, CR molecules rapidly diffuse through the boundary layer from the aqueous solution to the 0.2CTAB/CN’s external surface, exhibiting high mass transfer rates. During the intermediate stage, the molecules penetrate into the internal pores of 0.2CTAB/CN and bind with active sites, while the increasing boundary layer thickness constant (C) indicates growing intraparticle diffusion resistance. , In the final stage, the declining concentration of residual dye molecules results in reduced mass transfer rates until equilibrium is achieved, where adsorption and desorption processes occur simultaneously on both the surface and internal pores of the adsorbent.

2. Intra-particle Diffusion Modeling Parameters for CR Adsorption by 0.2CTAB/CN (Prepared with a CTAB/BCN Mass Ratio of 0.2).

model	parameter
Weber and Morris	k w1 (mg/(g·min1/2))	103.61
	k w2 (mg/(g·min1/2))	47.83
	k w3 (mg/(g·min1/2))	0.33
	C 1 (mg/g)	1.19
	C 2 (mg/g)	222.99
	C 3 (mg/g)	491.74

To assess the impact of temperature and adsorbate concentration on the performance of 0.2CTAB/CN, isotherm analysis was conducted (Figure e,f). As the initial CR concentration increased from 50 to 600 mg/L, the q _e significantly increased. This enhancement can be attributed to the accelerated diffusion of CR molecules within the mesoporous structure of 0.2CTAB/CN, driven by a stronger concentration gradient force. Regarding temperature effects, the adsorption efficiency followed the order: 313 K > 303 K > 293 K, indicating that the adsorption process is endothermic and favors higher temperatures.

To further investigate the adsorption mechanism and determine the theoretical maximum adsorption capacity, the isotherm data were fitted using the Freundlich and Langmuir models (eqs and , respectively). ,

$$q_{\mathrm{e}}=K_{\mathrm{F}}{C}_e^{1/n}$$ 5

where K _F (L/mg) and n are empirical Freundlich constants. C _e is the equilibrium CR concentration (mg/L).

$$q_{\mathrm{e}}=\frac{q_{\mathrm{m}}{K}_{\mathrm{L}}{C}_{\mathrm{e}}}{1+K_{\mathrm{L}}{C}_{\mathrm{e}}}$$ 6

where q _m (mg/g) is the theoretical maximum adsorption capacity. K _L (L/mg) is the Langmuir equilibrium constant.

In addition, the dimensionless separation factor (R _L) was calculated using eq to assess adsorption favorability

$$R_{\mathrm{L}}=\frac{1}{1+K_{\mathrm{L}}{C}_0}$$ 7

where C ₀ is the initial CR concentration (mg/L).

In all tested temperatures, the Langmuir model exhibited higher linear regression coefficients (R ² = 0.973–0.981) than the Freundlich model (R ² = 0.896–0.921) (Figure e,f and Table ). The Langmuir model suggests monolayer adsorption on a homogeneous surface, while the Freundlich model implies multilayer adsorption on a heterogeneous surface. , The fact that both models describe the experimental data reasonably well highlights the complexity of CR adsorption onto 0.2CTAB/CN. Specifically, the adsorption process involves monolayer coverage at specific binding sites, accompanied by additional multilayer adsorption on irregular surface domains. The R _L values consistently ranged between 0 and 1, confirming favorable adsorption under the experimental conditions. The q _m derived from the Langmuir model at 293 K was 1689 mg/g. The adsorption process being endothermic, both theoretical and experimental q _m values rose with increasing temperature, reaching 1704 mg/g at 303 K and 1945 mg/g at 313 K. These results surpass most reported CN-based adsorbents (Table S2) and commercial activated carbon. The adsorption isotherm of MO on 0.2CTAB/CN at 293 K was also investigated for comparison (Figure S10). The process also better followed the Langmuir model (R ² = 0.913), with a q _m of 141 mg/g, which is significantly lower than that for CR (Table S3). This further confirms the superior affinity and adsorption performance of 0.2CTAB/CN toward CR.

3. Isotherm Modeling Parameters for CR Adsorption by 0.2CTAB/CN (Prepared with a CTAB/BCN Mass Ratio of 0.2).

		temperature (K)
isotherm model	parameter	293	303	313
Freundlich	K F (mg/g)/(mg/L) n	57.955	61.277	52.292
	1/n	0.486	0.480	0.517
	R 2	0.907	0.896	0.921
Langmuir	K L (L/mg)	0.00455	0.00470	0.00393
	q m (mg/g)	1689	1704	1945
	R L	0.268–0.815	0.262–0.810	0.298–0.836
	R 2	0.973	0.970	0.981

The thermodynamic parameters, namely Gibbs free energy (ΔG ⁰), standard enthalpy (ΔH ⁰), and standard entropy (ΔS ⁰), were determined according to the following expressions

$$\mathrm{\Delta }{G}^0=-RT\ln b$$ 8

$$\mathrm{\Delta }{H}^0=-R{T}_2{T}_1/(T_2-T_1)\ln (b_2/b_1)$$ 9

$$\mathrm{\Delta }{S}^0=(\mathrm{\Delta }{H}^0-\mathrm{\Delta }{G}^0)/T$$ 10

here, R denotes the universal gas constant (8.314 J/mol/K), T represents the absolute temperature (K), and b was calculated from the Langmuir constant K _L after unit conversion. K _L (L/mg) was converted to a molar-basis constant (L/mol). Because the standard concentration is 1 mol/L, b becomes dimensionless and is used directly in eq .

The ΔG ^0 values ranged from −19.6 to −20.6 kJ/mol, demonstrating that the adsorption of CR onto 0.2CTAB/CN was thermodynamically spontaneous in the temperature range of 293–313 K (Table ). The positive ΔS ⁰ values (84.3 to 86.8 J/mol/K) indicated an increase in the randomness during adsorption. The weakly positive ΔH ⁰ value (5.8 kJ/mol) further confirmed the endothermic nature of the process, suggesting that energy input was required for CR desolvation and surface reorganization. The small magnitude of ΔH ⁰, combined with the observed thermodynamic parameters, points to a physical adsorption mechanism dominating the process.

4. Thermodynamic Parameters for CR Adsorption by 0.2CTAB/CN (Prepared with a CTAB/BCN Mass Ratio of 0.2).

temperature (K)	ΔG 0 (kJ/mol)	ΔS 0 (J/mol/K)	ΔH 0 (kJ/mol)
293	–19.6	86.8	5.8
303	–20.4	86.4
313	–20.6	84.3

3.5. Evaluation in Different Water Matrices and Coexisting Substance Effects

Dye-containing wastewater typically contains various anions, cations, and organic compounds that may significantly influence dye adsorption. To assess the practical applicability of 0.2CTAB/CN, its CR adsorption performance was evaluated in different water matrices, including deionized water, tap water, and Yangtze River water (Figure a) (30°36′27″ north latitude and 114°18′33″ east longitude). Remarkably, the removal efficiencies of CR remained consistently high across all tested water samples (98.6%, 98.5%, and 98.8%, respectively), demonstrating the material’s robustness under environmentally relevant conditions.

5.

(a) Removal efficiency of CR by 0.2CTAB/CN (prepared with a CTAB/BCN mass ratio of 0.2) in different water matrices and the effects of (b) cations, (c) anions, and (d) organic compounds on the adsorption of CR. Error bars represent the standard deviation of triplicate experiments.

The adsorption behavior of CR was further systematically investigated in the presence of coexisting substances. As depicted in Figure b, the CR removal efficiency gradually increased with increasing concentrations (0.01–0.1 mol/L) of K⁺, Mg²⁺, and Ca²⁺, reaching higher values compared to the control without cation addition at 0.10 mol/L. This improvement can be explained by three synergistic mechanisms: (1) salting-out effects that decreased CR solubility and promoted precipitation; (2) cation-induced molecular aggregation of CR forming colloidal particles; and (3) reduction of electrostatic repulsion between CR molecules. In contrast, Na⁺ showed negligible effects on CR adsorption under identical conditions. As shown in Figure c, the coexisting anions demonstrated concentration-dependent inhibitory effects in the following order: Cl^– < NO₃ ^– < SO₄ ^2– ≤ CO₃ ^2– < PO₄ ^3–. This trend primarily resulted from competitive adsorption at active sites, with CO₃ ^2– and PO₄ ^3– additionally interfering with the protonation of NH₂ groups on 0.2CTAB/CN nanosheets. The evaluation of organic compounds (Figure d) revealed differential impacts, where glucose and sodium citrate caused marginal reductions in adsorption capacity, while ascorbic acid and humic acid demonstrated more pronounced inhibitory effects. This phenomenon occurred because these compounds introduced additional anions into the system that competed with CR molecules for adsorption sites, thereby lowering the overall removal efficiency. ,

3.6. Reusability and Adsorption Mechanism

The cyclic stability of an adsorbent is a crucial performance parameter for its practical application. Figure a illustrates the removal efficiency of CR by 0.2CTAB/CN across successive cycles. The CR removal rate dropped from the initial 98.5% to 46.7% after the third cycle. The decline in performance is linked to the inadequate desorption of CR molecules during the regeneration process. To further investigate the structural changes, FTIR analysis was conducted on both pristine CR and 0.2CTAB/CN before and after the adsorption–desorption cycles (Figure b). The FTIR spectrum of CR exhibited characteristic bands corresponding to its functional groups: a prominent peak at 3470 cm^–1 assigned to –NH₂ stretching vibrations, a peak at 1585 cm^–1 associated with the NN bond, along with bands at 1172 cm^–1 and 1061 cm^–1 attributed to the asymmetric stretching and symmetric vibrations of –SO₃ ^– in aromatic sulfonates, respectively. , A small peak observed at 833 cm^–1 was related to vibrational modes of p-disubstituted benzene rings. After three cycles, the FTIR spectrum of 0.2CTAB/CN shows several new features in the 1000–1200 cm^–1 region that coincide with the characteristic –SO₃ ^– bands of CR, suggesting the presence of residual dye molecules on the adsorbent. Notably, the band originally located at 1061 cm^–1 shifted to a lower wavenumber of 1036 cm^–1. This shift confirms the formation of a chemical bond between CR and 0.2CTAB/CN, further explaining the incomplete desorption and the observed decline in regeneration efficiency. To improve reusability in future work, magnetic components could be incorporated into the composite to facilitate rapid separation and minimize adsorbent loss during regeneration. The cyclic stability could also be enhanced by optimizing the desorption process, for instance by employing a stronger eluent to disrupt the chemical bonding between the dye and adsorbent.

6.

(a) Adsorption capacity over three consecutive cycles. (b) FTIR spectra of original 0.2CTAB/CN (prepared with a CTAB/BCN mass ratio of 0.2), 0.2CTAB/CN after three cycles, and pure CR. (c) Proposed mechanism for CR adsorption on 0.2CTAB/CN.

The adsorption mechanism of CR onto 0.2CTAB/CN is proposed and illustrated in Figure c. Following a hydrothermal treatment in an aqueous CTAB solution, the layered structure of BCN was effectively exfoliated into nanosheets, accompanied by a disruption of the in-plane long-range order (Figures and ). The resultant 0.2CTAB/CN nanosheets possess a strong positive surface charge (Figure a), which originates from both the protonated amino groups on the CN and the positively charged quaternary ammonium groups of CTAB, thereby enabling highly efficient electrostatic attraction toward the anionic dye CR. Furthermore, π–π stacking form between the electron-rich s-triazine or heptazine units of 0.2CTAB/CN and the aromatic rings of the CR molecule. Additionally, hydrogen bonding is established between the amino groups on 0.2CTAB/CN and the sulfonate groups on CR. Beyond these primary interactions, a significant adsorption contribution arises from the hydrophobic long-chain alkyl tails of CTAB. Specifically, the hexadecyl chain of CTAB forms hydrophobic domains on the surface of the CN nanosheets. The large hydrophobic aromatic rings of the CR molecule can strongly associate with these alkyl chains via hydrophobic interactions, guided by the “like dissolves like” principle. Concurrently, substantial van der Waals forces between the alkyl chains and the aromatic backbones of CR further reinforce the adsorption. The synergistic combination of electrostatic attraction, π–π stacking, hydrogen bonding, hydrophobic interactions, and van der Waals forces collectively contributes to the substantial and robust adsorption of CR onto the 0.2CTAB/CN nanosheets. The synergy of these multiple interactions, although advantageous for achieving high adsorption efficiency, inevitably makes the desorption process difficult, thereby accounting for the poor recyclability observed in Figure a.

4. Conclusions

This study demonstrates that CTAB-modified CN serves as a highly efficient and practical adsorbent for CR removal. The optimal performance was achieved at a mass ratio of CTAB to BCN is 0.2 (0.2CTAB/CN), which exhibited a maximum adsorption capacity according to the Langmuir isotherm model for CR of 1945 mg/g at 313 K, higher than most reported CN-based adsorbents. The positive surface charge and abundant active sites of 0.2CTAB/CN ensure efficient CR removal over a broad pH range (1–12). The CR adsorption by 0.2CTAB/CN was found to follow pseudo-second-order kinetics and spontaneous processes. More importantly, while many adsorbents perform well only under idealized laboratory conditions, 0.2CTAB/CN maintained high removal efficiency not only in ultrapure water but also in complex real water matrices such as tap water and river water. This exceptional performance underscores the material’s significant potential for immediate, single-use treatment applications targeting anionic dyes in industrial wastewater.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c02590.

• FTIR spectra in the region of the tri-s-triazine ring vibrations for BCN and xCTAB/CN (prepared with a CTAB/BCN mass ratio of x, x = 0, 0.1, 0.2, 0.3, and 0.5); survey XPS spectra of BCN and xCTAB/CN; SEM images of 0.3CTAB/CN and 0.5CTAB/CN; SEM image and the corresponding EDX results of 0.2CTAB/CN; AFM 3D surface images and BET results of BCN, 0CTAB/CN, and 0.2CTAB/CN; particle size distribution of BCN and xCTAB/CN; removal efficiency of CR by 0.2CTAB/CN; photographs of dye solutions before and after 3 h adsorption by 0.2CTAB/CN; adsorption isotherms of MO on 0.2CTAB/CN; adsorption capacities of different g-C₃N₄-based adsorbents for CR (PDF)

Junhao Fang: Investigation. Fuwenjie Tang: Investigation. Ziao Zhang: Investigation. Feng Gao: Investigation. Yun Mao: Investigation, Funding acquisition. Yangyue Pan: Investigation. Xingxing Li: Resources. Pei Liu: Resources. Mengying Xu: Writing-review and editing, Writing-original draft, Supervision, Resources, Project administration, Methodology, Funding acquisition, Conceptualization.

The authors declare no competing financial interest.