Mechanical grinding mediated lanthanum modified silicon carbide based nitride carbon composite material enhances adsorption of dye wastewater

Abstract

In this work, a novel La_x-g-C₃N₄@β-SiC composite adsorbent was synthesized by thermal polycondensation and mechanical methods. The samples were analyzed by characterization methods such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The La₁₂-g-C₃N₄@β-SiC sample presented a more porous layered morphology with a uniform pore structure. In addition, the application of these materials in the adsorption and removal of new organic pollutants was studied. The results show that La₁₂-g-C₃N₄@β-SiC has a good adsorption effect on both alizarin red S (ARS) and acid fuchsin (AF). Within 2.5 minutes, the adsorption efficiencies of ARS and AF were 93.23% and 70.33% respectively, and the corresponding adsorption capacities were 62.17 mg g⁻¹ and 46.9 mg g⁻¹ respectively. The kinetic and thermodynamic analyses of the adsorption process indicated that the adsorption of ARS and AF by La₁₂-g-C₃N₄@β-SiC conformed to the quasi-second-order kinetics and Langmuir adsorption isotherm model, with monolayer chemical adsorption being the main form. In addition, the composite material demonstrated excellent cycling stability and maintained outstanding adsorption performance even after eight repeated uses.

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1. Introduction

At present, with the rapid development of industries including printing, dyeing, and coatings, the discharge volume of organic wastewater from these sectors is also increasing, posing substantial threats to the aquatic ecological environment and human health.^1–3 As typical acidic water pollutants, Alizarin red and acid fuchsin, feature complex compositions, high chromaticity, and adsorptive recalcitrance.^4,5 Traditional remediation techniques encounter substantial challenges in addressing these pollutants. Consequently, there is an urgent demand for the development of cost effective and highly efficient green materials for their treatment and remediation. The primary methods for treating acidic aqueous pollutants include physical methods, chemical methods, and biological methods.^6–8 Among these, physical adsorption methods have attracted extensive attention owing to their operational simplicity, availability of inexpensive raw materials, high processing efficiency, and eco-friendly properties.^9–11

Notably, carbon nitride (g-C₃N₄), a novel inorganic non-metallic polymeric material developed over the past decade, has exhibited remarkable performance in the adsorption treatment of organic pollutants.^12–15 This efficacy can be attributed to its simple preparation process, abundant raw materials, and resistance to both acidic and alkaline conditions.^16,17 The distinctive electronic structure of g-C₃N₄, combined with its outstanding compatibility with other functional materials, contributes to its remarkable adsorption performance for organic pollutants.^18,19 However, the morphology of pure g-C₃N₄ bulk materials, typically prepared through calcination, often exhibits a layered structure, which presents limitations such as a small specific surface area, a sparse pore structure, and a narrow absorption response wavelength.^20–22 These factors substantially limit the adsorption performance of g-C₃N₄. Enhancing material properties through metal doping and modification has validated as an effective strategy. By doping g-C₃N₄ with metals, additional active sites are introduced, and improves electron transfer efficiency, thereby increasing the material's adsorption capacity. Furthermore, silicon carbide is a cost-effective and widely utilized inorganic semiconductor material, distinguished by its outstanding catalytic activity, particularly in applications such as adsorption, photocatalysis, and energy storage.^23,24 Silicon carbide possessed excellent mechanical strength, appropriate pore structure, and abundant surface hydroxyl functional groups, which effectively enhanced the dispersion and structural stability of the active components in the system, and could also generate a synergistic adsorption effect with the active components, thereby improving the overall adsorption performance.^25–27 Furthermore, β-SiC was also an efficient electron carrier and heterojunction material. By constructing a heterojunction interface with g-C₃N₄, not only could the recombination of photogenerated electron–hole pairs in g-C₃N₄ be effectively suppressed, but also the specific surface area and structural stability of the composite material could be significantly improved.²⁸ Due to its stable structure, excellent surface chemical properties, and pore structure, silicon carbide became a key component for achieving efficient and stable adsorption. Research has demonstrated that the catalytic performance of silicon carbide can be markedly improved when it is combined with carbon nitride.^29,30 This improvement is attributed to the complementary band gaps of carbon nitride and silicon carbide, which facilitate the formation of a type II heterojunction through chemical interactions.³¹ This structural configuration facilitates the separation and transfer of photogenerated electron–hole pairs, thereby improving the performance of the material. In 2017, Huang et al. first synthesized g-C₃N₄/SiC material by attaching silicon carbide to the surface of carbon nitride and studied its photocatalytic hydrogen production performance.³² The results indicated that the BET surface area of the g-C₃N₄/SiC material was 15.0 m² g⁻¹, with a hydrogen production rate of 182 µmol g⁻¹ h⁻¹, which is 3.4 times greater than that of pure g-C₃N₄. Subsequently, Gan et al. designed SiC nanoparticles and g-C₃N₄ nanosheets by constructing type II heterostructures and applied them to the adsorption of methyl orange (MO) wastewater.³¹ The findings revealed that the adsorption rate of MO by this material reached 55% within one hour. Additionally, the Z-shaped heterostructure SiC/g-C₃N₄ composite material prepared by Guo et al.³³ exhibited a adsorption rate of 71.1% for tetracycline within three hours under visible light conditions. Similarly, there have been numerous studies on various typical organic pollutants, including methyl orange and ciprofloxacin, in other silicon carbide and nitride carbon composite materials, including Cu₂O-SiC/g-C₃N₄,³⁴ SiC/g-C₃N₄,³⁵ 3D-SiC/C₃N₄,³⁶ Ni/SiC/CNNS,³⁷ SiCf/g-C₃N₄.³³ Nevertheless, this research is typically coupled with photocatalytic processes, which substantially elevate both energy consumption and operational complexity. Additionally, the SiC adopted in many existing studies is generally commercial-grade micro-nano powder, whose crystalline phase is typically consists of a mixture of α-SiC and β-SiC.³⁸ These works may lack design and performance comparison experiments involving pure nanocrystalline SiC and g-C₃N₄ composite materials. Additionally, limitations such as the relatively small specific surface area of the prepared materials impede the improvement of their functional performance.

In addition, the cage-structured interactions of rare earth metals facilitate the enrichment of organic pollutants on the catalyst surface, highlighting their significant potential in the fabrication of composite materials with high adsorption efficiency. Lanthanide metal ions serve as exceptional materials for the adsorption and adsorption of organic pollutants. This performance arises from the ability of lanthanide metal ions, when used as dopants, to reduce the particle size of the host material to a certain extent, thereby enhancing the specific surface area and catalytic activity of the material.³⁹ Yin et al. immobilized La(OH)₃ nanoparticles on C₃N₄, effectively limiting the aggregation of the material and enhancing its phosphate adsorption capacity, which reached a maximum of 148.35 mg g⁻¹.⁴⁰ Prasanna et al. doped erbium metal into graphite carbonitride, significantly improving the adsorption and adsorption of tetracycline (TC).⁴¹ Additionally, Yu et al. synthesized cerium metal-modified g-C₃N₄ nanosheets using a thermal shrinkage method, achieving over 80% adsorption efficiency for various dyes within 2.5 hours.⁴² From the perspective of energy consumption, adsorption generally proceeds under dark reaction conditions, without the requirement for additional energy input. This feature differs significantly from photocatalytic processes, which depend on light energy as a driving force. In the previous research of our team, we have been dedicated to developing a series of highly efficient catalytic and adsorptive materials. We successively prepared protonated g-C₃N₄/β-SiC composite materials.⁴³ This material demonstrated excellent performance in the photocatalytic adsorption of alizarin red wastewater, with an adsorption efficiency as high as 99.9% and good cycling stability. Furthermore, lanthanum-modified mesoporous g-C₃N₄ was synthesized, significantly enhancing its adsorption and adsorption performance for tetracycline and acid fuchsin.⁴⁴ Furthermore, we enhanced the photocatalytic performance of g-C₃N₄/β-SiC through acid protonation treatment.⁴⁵ It was found that protonation treatment not only increased the specific surface area of the material but also significantly reduced the recombination probability of photogenerated electron–hole pairs. Among the numerous methods for preparing and modifying functional nanomaterials, the mechanochemical method has gained significant attention in recent years due to its simplicity of operation, solvent-free nature, and environmental friendliness, especially in the fields of environmental remediation and the synthesis of adsorption materials. For instance, Fathy et al. used a planetary ball mill to conduct mechanical chemical activation on discarded cathode ray tubes, successfully extracting approximately 85% of lead using ethylenediaminetetraacetic acid (EDTA), demonstrating the high efficiency of this method in waste resource utilization.⁴⁶ This method promoted the solid-phase chemical reactions induced by mechanical force by disrupting the molecular structure within the material, thereby increasing the number of active sites and expanding its application scope in the environment.

On the basis of the aforementioned research progress, this work aimed to combine rare earth metal-doped g-C₃N₄ with structurally stable β-SiC supports to construct La_x-g-C₃N₄@β-SiC composite materials, and systematically investigate their adsorption performance and mechanism for dye wastewater such as alizarin red and acid fuchsin. This work was expected to provide new strategies for the development of efficient water treatment materials. Lanthanum-modified carbon nitride supported silicon carbide mesoporous composites (La_x-g-C₃N₄@β-SiC) were synthesized using melamine treated with dilute sulfuric acid, lanthanum nitrate hexahydrate and nanoscale β-SiC as raw materials by combining mechanochemical and thermal polymerization methods. The structural characteristics and pore size distribution of La_x-g-C₃N₄@β-SiC were analyzed using techniques such as SEM, XPS, and BET. Furthermore, the adsorption performance of the material toward alizarin red and acid fuchsin was systematically investigated via screening and optimization. Meanwhile, the adsorption mechanism of the optimized material La₁₂-g-C₃N₄@β-SiC toward organic pollutants was analyzed using adsorption isotherms, kinetics models, and thermodynamic fitting equations.

2. Experimental section

2.1 Synthesis of g-C₃N₄

Slowly dissolve 10 g of melamine in a mixture of 35 mL of ethylene glycol and 100 mL of dilute sulfuric acid (0.1 mol L⁻¹), and stir for 50 minutes to form a homogeneous solution. After 24 hours of sedimentation, the white precipitate was collected by suction filtration and washed 3 to 4 times with ethanol and deionized water. After being dried overnight in an oven at 60 °C and ground into powder, A white powder protonated precursor can be obtained, labeled as sample A. The above steps are repeated multiple times, and A total of 70–80 g of sample A is collected. Weigh 10 g of the precursor and calcine it in an air atmosphere (calcination conditions: increase the temperature from room temperature to 350 °C at a rate of 2 °C min⁻¹ and hold for 1 hour; continue to increase the temperature at a rate of 5 °C min⁻¹ and hold for 550 °C for 3 hours). After cooling to room temperature, the protonated graphite phase carbon nitride material was obtained and labeled as g-C₃N₄.

2.2 Synthesis of g-C₃N₄@β-SiC

Melamine (5 g) treated with dilute sulfuric acid and β-SiC powder (0.2 g) were simultaneously added to the ball mill jar. They were ball-milled at 600 rpm for 30 minutes in a planetary ball mill, and the powder was removed from the ball mill jar. The powder of the above-mentioned mixture was taken out and transferred to an alumina crucible. It was calcined in an air atmosphere (calcination conditions: heated from room temperature to 350 °C at a rate of 2 °C min⁻¹ and held for 1 h; then heated to 550 °C at a rate of 5 °C min⁻¹ and held for 3 h). After cooling to room temperature, the g-C₃N₄@β-SiC sample was obtained.

2.3 Synthesis of La_x-g-C₃N₄@β-SiC

Lanthanum nitrate hexahydrate (La(NO₃)·6H₂O) of different masses and melamine treated with 5 g of dilute sulfuric acid were simultaneously added to the ball mill jar, and β-SiC powder (0.2 g) was added. The dosage of lanthanum nitrate hexahydrate should mainly be 2% to 16% of melamine treated with 5 g dilute sulfuric acid. The powder was ground at 600 rpm for 30 minutes in a planetary ball mill and then removed from the ball mill jar. The powder of the above-mentioned mixture was taken out and transferred to an alumina crucible. It was calcined in an air atmosphere (calcination conditions: heated from room temperature to 350 °C at a rate of 2 °C min⁻¹ and held for 1 hour; then heated to 550 °C at a rate of 5 °C min⁻¹ and held for 3 hours). After cooling to room temperature, the preparation of lanthanum metal-doped protonated carbon nitride @β-SiC material was obtained. The prepared samples were recorded as: La₂-g-C₃N₄@β-SiC; La₄-g-C₃N₄@β-SiC; La₆-g-C₃N₄@β-SiC; La₈-g-C₃N₄@β-SiC; La₁₀-g-C₃N₄@β-SiC; La₁₂-g-C₃N₄@β-SiC; La₁₄-g-C₃N₄@β-SiC; La₁₆-g-C₃N₄@β-SiC.

2.4 Characterization

X-ray diffraction (XRD) patterns were obtained on a Bruker D8 Advance diffractometer (Germany) with a copper target. Fourier Transform Infrared (FT-IR) spectra were recorded in the range of 400–4000 cm⁻¹ on a Thermo Nicolet IS5 spectrometer. The specific surface area and porosity were analyzed by the Brunauer–Emmett–Teller (BET) method using a Micromeritics ASAP 2420 fully automated analyzer. Morphological analysis was conducted via scanning electron microscopy (SEM) on a ZEISS Sigma 360 instrument (Germany) and by transmission electron microscopy (TEM) along with high-resolution TEM (HRTEM) on an FEI Talos F200x microscope (USA). Surface chemical states were examined by X-ray photoelectron spectroscopy (XPS) on a Thermo Fisher Nexsa system. Photocatalytic performance was evaluated at room temperature by monitoring absorbance changes with a TU-1901 double-beam UV-Vis spectrophotometer (Perse, China). Finally, liquid chromatography-mass spectrometry (LC-MS) data in positive ion mode were collected using a Waters G2-S QTOF instrument.

2.5 Adsorption performance evaluation experiments

Taking tetracycline as the adsorption object as an example, the adsorption properties of g-C₃N₄, g-C₃N₄@β-SiC, and La_x-g-C₃N₄@β-SiC samples. During the adsorption performance test, the experimental device used was a photochemical reactor, and the light source switch of the testing equipment was always kept off throughout the process to ensure that the entire adsorption process was carried out in a dark environment. Evenly disperse 40 mg of the sample in a 50 mL, 20 mg L⁻¹ wastewater solution. Dark react and stir for 30 minutes in a photochemical reaction instrument. During the adsorption process, samples were taken every ten minutes. The absorbance was measured using a UV-visible spectrophotometer at a wavelength of 359 nm. Before the test, the samples were first filtered through a microporous membrane (with a specification of 0.22 µm), and then placed in a 1 cm × 1 cm cuvette. After completing one experimental cycle according to the above steps, the adsorbent samples were subjected to centrifugation for recovery. Then, they were washed three times with deionized water and ethanol, and subsequently dried at 60 °C for use in the next cycle.

3. Results and discussion

3.1 Microscopic morphology and internal structure

The synthetic pathway of the La_x-g-C₃N₄@β-SiC adsorbent is shown in Fig. 1. The crystallinity and structural properties of g-C₃N₄, g-C₃N₄@β-SiC, and La_x-g-C₃N₄@β-SiC adsorbents were analyzed by X-ray powder diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR). As shown in Fig. 2a, the samples after introducing β-SiC and the rare earth metal lanthanum retain the intrinsic characteristics of the original g-C₃N₄ adsorbent. 13.1° and 27.3° in the XRD pattern correspond to the (100) and (002) crystal planes of g-C₃N₄ respectively.^45–48 With the addition of β-SiC and the rare earth metal lanthanum, the peak signals of the (100) and (002) crystal planes gradually decrease. The 35.8°, 41.5°, 60.2°, 72.0°, and 75.5° XRD of g-C₃N₄@β-SiC correspond respectively to the (111), (200), (220), (311), and (222) crystal planes in β-SiC.^45,49–51 With the continuous increase of the doping amount of lanthanum element in rare earth metal, the peaks of each crystal plane of g-C₃N₄@β-SiC gradually decrease. The changes in the signal intensity of the above crystal plane peaks confirmed the successful preparation of g-C₃N₄@β-SiC and La_x-g-C₃N₄@β-SiC.

Fig. 1 The schematic diagram of the preparation process of the sorbent samples.

Fig. 2 (a) XRD of g-C₃N₄, g-C₃N₄@β-SiC, and La_x-g-C₃N₄@β-SiC; (b) FT-IR spectra of g-C₃N₄, g-C3N4@β-SiC, and La_x-g-C₃N₄@β-SiC.

To further explore its internal structural characteristics, Fourier Transform infrared spectroscopy (FT-IR) analysis was carried out (Fig. 2b). The FT-IR spectra confirm the characteristic structure of the g-C₃N₄ adsorbent. A broad absorption peak near 3085.2 cm⁻¹ corresponds to the N–H stretching vibration.^45–48 The region between 1227.0 and 1625.8 cm⁻¹ is associated with the stretching vibrations of C–N and C=N bonds within the aromatic ring system.^52,53 Notably, the distinct peak at 804.2 cm⁻¹ is attributed to the vibrational mode of the triazine ring, a defining feature of g-C₃N₄.^45,52 With the increase of doping amounts of SiC and rare earth metal La, the peak intensity gradually decreases. The FT-IR spectrum of the La_x-g-C₃N₄@β-SiC adsorbent exhibits a distinct feature not present in pure g-C₃N₄: a characteristic peak emerges at 2160.9 cm⁻¹ under higher La doping levels. The intensity of this peak increases progressively with greater La content.^44,53 This signal is attributed to the stretching vibration of a C≡N bond, rather than aromatic C–N. This spectral change suggests that La doping partially disrupts the triazine ring structure of g-C₃N₄.

The microstructure and morphology were analyzed through SEM and HRTEM characterization, and the results are shown in Fig. 3a–f. Fig. 3a shows the SEM image of La₁₂-g-C₃N₄@β-SiC, presenting a distinct snowflake-like flocculent packing structure and the appearance of some small pore structures. From Fig. 3b, it can be seen that the morphology has a certain layered stacking structure and a skeleton structure similar to that, showing a more porous layered structure with different pores of different diameters, which may lead to an increased specific surface area of the material. It also indicates that the introduction of lanthanum and β-SiC has played a certain promoting role in the morphology modification of carbon nitride polymer materials. In addition, the EDS spectra and Mapping diagrams show that related elements such as Si, N, C, and La are all present in the samples, further indicating the successful preparation of La₁₂-g-C₃N₄@β-SiC. Furthermore, through the plasma spectrometer (ICP-OES) test, it was found that the content of lanthanum in the La₁₂-g-C₃N₄@β-SiC composite adsorbent was 11.6%, which corresponded to the doping amount of lanthanum in this composite material.

Fig. 3 (a and b) SEM micrographs of La₁₂-g-C₃N₄@β-SiC; (c) the TEM analysis of La₁₂-g-C₃N₄@β-SiC; (d) the HRTEM analysis of La_x-g-C₃N₄@β-SiC; (e and f) EDS elemental mapping demonstrating homogeneous spatial distribution of Si, N, C, and La in La_x-g-C₃N₄@β-SiC.

The BET characterization analysis of g-C₃N₄, g-C₃N₄@β-SiC, and La_x-g-C₃N₄@β-SiC revealed type IV N₂ adsorption–desorption isotherms of these adsorbent materials with H3 hysteresis loops (Fig. 4a). This is the characteristic of stratified porous materials containing mesopores (2–50 nm) and macropores (>50 nm), and the main pore size distribution range (0–25 nm range) confirms the mesoporous materials (Fig. 4b). With the introduction of β-SiC and La elements, the adsorbent shows a trend of either increasing or decreasing in terms of specific surface area. This phenomenon may occur because β-SiC fills some of the pores of g-C₃N₄ or forms a dense interface with g-C₃N₄, reducing the specific surface area. The composite adsorbent with a low doping amount of La element is slightly lower than the original g-C₃N₄, which might be caused by clogged pores. Under a higher doping amount, the specific surface area is significantly higher than that of the original g-C₃N₄, indicating that the content of high La element further increases the porosity and the pore volume further increases, suggesting that the porosity is enhanced under a higher doping amount, providing sufficient space for pollutant adsorption. In addition, the BET test results showed that the specific surface area was positively correlated with the adsorption performance. When the doping amount of lanthanum nitrate hexahydrate increased from 2% to 12%, the corresponding specific surface area rose from 11.48 m² g⁻¹ to 17.92 m² g⁻¹, and the adsorption capacity of tetracycline pollutants significantly increased from 6.55 mg g⁻¹ to 35.05 mg g⁻¹. This indicated that when the doping amount of lanthanum increased, the specific surface area showed a synchronous growth trend, and the increase in specific surface area was a key factor in improving adsorption performance. Meanwhile, a positive correlation between pore volume and adsorption performance was also observed as the doping amount increased from 2% to 12%, the corresponding pore volume increased from 0.09 cm³ g⁻¹ to 0.14 cm³ g⁻¹, indicating that the increase in pore volume was also an important influencing factor for improving adsorption performance. The nitrogen adsorption–desorption and pore size analysis indicated that the loading amount of lanthanum was the key factor regulating the pore structure of g-C₃N₄@β-SiC, providing a theoretical basis for subsequent adsorption applications.

Fig. 4 (a) The nitrogen adsorption–desorption isotherms of g-C₃N₄, g-C₃N₄@β-SiC, and La_x-g-C₃N₄@β-SiC; (b) the aperture distribution diagrams of g-C₃N₄, g-C₃N₄@β-SiC, and La_x-g-C₃N₄@β-SiC.

In order to deeply analyze the chemical composition and electronic valence state of the La₁₂-g-C₃N₄@β-SiC composite adsorbent, the characteristic peaks of the X-ray photoelectron spectra of C, N, O, and La in La_x-g-C₃N₄@β-SiC were analyzed in this work. Full-spectrum analysis indicated (Fig. 5a) that La₁₂-g-C₃N₄@β-SiC was composed of C, N, O, La, and Si elements. As the content of Si element was lower than the detection limit of the test, the distinct peak signal of the fine spectrum of XPS could not be detected. However, the XPS full spectrum and EDS mapping can also confirm that La₁₂-g-C₃N₄@β-SiC contains Si elements. These intrinsic structural features jointly support the application potential of La₁₂-g-C₃N₄@β-SiC as an efficient adsorbent. In the C 1s fine spectrum (Fig. 5b), the signal peak at the binding energy of 284.80 eV corresponds to the C–C bond (sp² or sp³ hybrid carbon), which are respectively the C–Si covalent skeletons of β-SiC (in β-SiC, carbon and silicon are covalently bonded).^43,45 There are a few C–C defects at the edges and the edge areas of g-C₃N₄ (carbon sites that are not fully conjugated at the edges of graphite-like structures). The peak signal at 286.39 eV belongs to the N–(C)₃ bond and is a characteristic peak of g-C₃N₄. g-C₃N₄ takes the tri-s-triazine ring as the basic unit. Each carbon atom coordinates with three nitrogen atoms to form a conjugated skeleton. The existence of this peak proves that the main structure of g-C₃N₄ has not been damaged. The signal peak at 289.26 eV corresponds to the C=N–C bond.⁴⁴ During the preparation or storage of adsorbents, the surface carbon sites react with oxygen in the air to form polar carbonyl groups. These polar sites can adsorb polar pollutants such as ARS (alizarin red S) and AF (acid fuchsin) through hydrogen bonds and dipole interactions, thereby enhancing the adsorption performance. Fig. 5c shows the fine spectrum of N 1s. The signal peak with a binding energy of 399.62 eV belongs to the low-valent nitrogen or the nitrogen interacting with La. Among them, La³⁺ (the main valence state of lanthanum) is a cation and can form coordination interaction (La–N bond) with the nitrogen atom with strong electronegativity in g-C₃N₄, changing the electron cloud density of nitrogen. This electronic interaction will adjust the surface charge distribution of the adsorbent, which is conducive to the adsorption of anionic pollutants (such as ARS and AF dissociating into anions in water). The signal peaks of 408.41 eV and 404.18 eV correspond respectively to the NO group (nitrogen oxides) and the N–H bond (amino group), which help the adsorbent to fully contact the pollutants in the aqueous solution.^43,45 It can enhance the adsorption capacity by reacting with the sulfonic acid groups (–SO₃H) and hydroxyl groups (–OH) of the pollutants through hydrogen bonds. The signal peak at 406.16 eV corresponds to the N–(C)₃ bond and is a characteristic peak of g-C₃N₄, demonstrating that the conjugated carbon nitride skeleton of g-C₃N₄ remains intact after recombination (introducing β-SiC) and doping (introducing La). The O 1s fine spectrum shows three types of oxygen-containing species (Fig. 5d), and the signal peak at 538.16 eV corresponds to adsorbed water or hydroxyl groups (–OH), promoting the mass transfer process in the aqueous phase. The 535.75 eV signal peak corresponds to C=O bonds (carbonyl groups) or N=O bonds (nitro groups).⁴⁴ These polar oxygen-containing groups can enhance the interaction between adsorbents and polar pollutants and are important “active sites” in the adsorption process. The signal peak at 531.83 eV belongs to the Si–O bond.^45,47 The presence of the Si–O bond not only enhances the hydrophilicity of β-SiC but also forms hydrogen bonds with pollutants through Si–O–H (silanol groups), further strengthening the adsorption capacity. The 3d spectrum of La (Fig. 5e) reveals the valence state and interaction of lanthanum, presenting typical spin–orbit splitting characteristics. Multiple sets of characteristic peaks appeared in the spectrum, and their peak positions were consistent with those of the characteristic peaks of La³⁺ (the stable valence state of lanthanum) in the reported literature, proving that La exists in the material in the form of La³⁺. As a cationic site, La can adsorb anionic pollutants such as ARS and AF through electrostatic interaction. It can also interact with g-C₃N₄ and β-SiC through La–N and La–O bonds to regulate the electronic structure and distribution of surface-active sites of the material, thereby indirectly enhancing the adsorption performance.

Fig. 5 The XPS full spectrum (a), C 1s spectrum (b), N 1s spectrum (c), O 1s spectrum (d), and La 3d spectrum (e) of La₁₂-g-C₃N₄@β-SiC adsorbent.

3.2 Investigation of adsorption performance

Taking tetracycline as the adsorption performance test object, the performance of a total of 10 groups of samples, including g-C₃N₄, g-C₃N₄@β-SiC, and La_x-g-C₃N₄@β-SiC, was investigated. As can be seen from Fig. 6a, with the continuous increase of La doping amount, the adsorption performance gradually improves, especially when the doping amounts are 12%, 14% and 16%, the adsorption effect is more obvious. Considering the economy of the material, La₁₂-g-C₃N₄@β-SiC adsorbent with a doping amount of 12% was adopted as the object for the subsequent adsorption performance investigation. Furthermore, the adsorption performance of the adsorbent for different emerging organic pollutants was investigated (Fig. 6b), and the performance of the La₁₂-g-C₃N₄@β-SiC adsorbent was evaluated to determine whether it has wide applicability. Under the same conditions, 40 mg of La₁₂-g-C₃N₄@β-SiC adsorbent was dispersed to remove 50 mL of 20 mg L⁻¹ of different wastewater. The appearance changes before and after were recorded (Fig. 6c) and the adsorption efficiency. As can be seen from the figure, alizarin red S (ARS) has the best adsorption efficiency, followed by acid fuchsin (AF). Alizarin red S solution was mainly used for subsequent condition screening. Subsequently, the adsorption effects of La₁₂-g-C₃N₄@β-SiC adsorbent on alizarin red (ARS) wastewater with different feed amounts of 50 mL and 20 mg L⁻¹ were investigated by dispersing La₁₂-g-C₃N₄@β-SiC adsorbent at feed amounts of 5 mg, 10 mg, 15 mg, 20 mg, 25 mg, 40 mg, and 60 mg (Fig. 6d). The adsorption rates of 20 mg, 40 mg, and 60 mg were all above 95%. To save the amount of adsorbent used, 20 mg was adopted as the subsequent feeding amount for condition screening. Then, the adsorption efficiency of organic pollutants at different reaction times was investigated with sampling time intervals of 30 seconds, 60 seconds, and 90 seconds (Fig. 6e). The results showed that within a 30-second time interval, the adsorption effect did not change significantly, so short-time sampling had little impact on the change in adsorption effect. The adsorption effect under different pH values was further investigated (Fig. 6f), and the optimal pH value was screened out as 4. Subsequently, the pH value was refined to 4.2–4.8 to investigate its adsorption effect (Fig. 6g), in order to determine the optimal reaction condition as the pH value of 4. To investigate the recovery performance of the adsorbent under these optimal conditions (Fig. 6h), three independent replicate experiments were conducted. The results demonstrated that after 8 consecutive cycles, the adsorbent maintained an average adsorption efficiency of 90%, indicating excellent cyclic stability.

Fig. 6 (a) Screen different proportions; (b) screen different pollutants; (c) appearance changes before and after adsorption of different pollutants; (d) screen different feed quantities; (e) screen different sampling times; (f) investigation of adsorption performance at different pH values; (g) detailed investigation of adsorption performance at pH values ranging from 4.2 to 4.8; (h) evaluation of catalyst reusability.

The XRD and SEM analyses after the cycle confirmed that there were no significant changes in the morphology, structure and internal properties of the material. The surface morphology remained consistent with that before cycling, indicating that its structure demonstrated excellent adsorption and cycling stability during the reaction process (Fig. 7a and b). This result not only verifies the rationality of the composite material design, but also provides a key experimental basis for the subsequent construction of an efficient and stable adsorption system. In addition, we compared the catalyst used in this work with the catalysts that have been reported (Table 1). The adsorption rate of La₁₂-g-C₃N₄@β-SiC could reach 93% within 30 minutes when it was used to adsorb ARS. The adsorption capacity was higher than that of other catalysts. Moreover, the adsorption rate remained stable at 90% after 8 consecutive cycles, and the preparation cost of the adsorbent was relatively low.

Fig. 7 The SEM (a) and XRD (b) graphs of La₁₂-g-C₃N₄@β-SiC adsorbent before and after recycling.

Table 1 Comparison chart of pollutants adsorbed by different catalysts

Entry	Catalysts	Pollutant	Stability	Time	Yield
1	(CoCuLDH)	RAN	8 times	60 min	70% (ref. 54)
2	D-SCR-G	MO	4 times	60 min	89.6% (ref. 55)
3	Mn–Ce–Co-Ox/PPS	PCDD/Fs	—	>6 h	78.01% (ref. 56)
4	g-C3N4/5-rGO/SnO2	RhB	—	120 min	83.2% (ref. 57)
5	Zn2Ti3O8/g-C3N4	RhB	—	90 min	89.0% (ref. 57)
6	Bi2MoO6	RhB	—	60 min	80% (ref. 58)
7	Ag-TiO2	TOL	—	100 min	80% (ref. 59)
8	Graphene oxide-TiO2 (GOT)	DDVP	—	80 min	80% (ref. 60)
9	TiO2-TMAOH	MB	6 times	20 min	80% (ref. 61)
This work	La12-g-C3N4@β-SiC	ARS	8 times	30 min	93.23%

---

3.3 Adsorption mechanical fitting and analysis

The kinetics study was conducted by mixing La₁₂-g-C₃N₄@β-SiC (20 mg) with ARS (50 mL, 20 mg L⁻¹) solution and AF (50 mL, 20 mg L⁻¹) solution. The adsorption kinetics was studied by using the quasi-first-order and quasi-second-order models. The linear forms of the two models are respectively expressed as formulas (1) and (2):In the formula, q_e and q_t represent the adsorption capacities of the adsorbent for ARS and AF at equilibrium and at different times, respectively. The rate constants of K₁ (min⁻¹) and K₂ (g (mg min)⁻¹ for quasi-first-order and quasi-second-order models, respectively.

As shown in Fig. 8(a), the adsorption process could be divided into three stages, namely, rapid adsorption (0–2.5 min), slow adsorption (2.5–10 min), and equilibrium (after 10 min). Initially, ARS was rapidly captured due to the abundant active sites on the La₁₂-g-C₃N₄@β-SiC surface. During the slow adsorption stage, occupied sites increased the surface concentration of ARS, reducing the concentration gradient between the adsorbent and the solution, which weakened the mass transfer driving force and decreased the adsorption rate. Equilibrium was essentially reached after 10 minutes. The adsorption kinetics were further analyzed by fitting the data with pseudo-first-order and pseudo-second-order models, as presented in Fig. 8(b), (c), and Table 2. The results indicated that the pseudo-second-order model provided a better fit, with a higher correlation coefficient (R² = 0.9997). This indicated a positive correlation between the adsorption rate and the square of the number of adsorption sites. Combined with the thermodynamic analysis data, it could be inferred that this adsorption process was a single-layer adsorption and was mainly chemical adsorption.^62–64

Fig. 8 (a) Adsorption kinetics curve (b) quasi-first-order kinetics (c) quasi-second-order kinetics model fitting curve of La₁₂-g-C₃N₄@β-SiC adsorbing ARS wastewater.

Table 2 ARS adsorption kinetics parameters

Conc./(mg g^−1)	Pseudo-first-order			Pseudo-second-order
ARS	qe (mg g^−1)	K1 (min ^−1)	R^2	qe (mg g^−1)	K1 (g (mg min)^−1)	R^2
20	1.1259	0.2012	0.5847	96.1538	0.0022	0.9997

---

The adsorption kinetics of AF by La₁₂-g-C₃N₄@β-SiC are shown in Fig. 9(a), with the entire process reaching completion within 7.5 minutes. The adsorption data were fitted using pseudo-first-order and pseudo-second-order kinetic models, as presented in Fig. 9(b) and (c). Combined with the kinetic parameters in Table 3, the pseudo-second-order model demonstrates a better fit, effectively describing the adsorption kinetics of AF by La₁₂-g-C₃N₄@β-SiC, with a high correlation coefficient (R² = 0.9993).

Fig. 9 (a) Adsorption kinetics curve (b) quasi-first-order kinetics (c) quasi-second-order kinetics model fitting curve of La₁₂-g-C₃N₄@β-SiC adsorbing AF wastewater.

Table 3 AF adsorption kinetics parameters

Conc./(mg g^−1)	Pseudo-first-order			Pseudo-second-order
AF	qe (mg g^−1)	K1 (min^−1)	R^2	qe (mg g^−1)	K1 (g (mg min)^−1)	R^2
20	8.4807	0.3058	0.7973	22.6142	0.0380	0.9993

---

3.4 Adsorption thermodynamic fitting and analysis

To evaluate the adsorption capacity for ARS and AF and characterize their adsorption behaviors, the Langmuir and Freundlich isotherm models were employed as standard analytical methods.

The Langmuir isotherm model is expressed as:

In the Langmuir model equations, q_e (mg g⁻¹) denoted the equilibrium adsorption amount of the adsorbate (ARS or AF); q_m (mg g⁻¹) was the theoretical maximum adsorption capacity. C_e (mg g⁻¹) was the equilibrium solute concentration, and K_L (L mg⁻¹) was the Langmuir constant related to binding affinity. Furthermore, the essential feature of the Langmuir isotherm could be expressed by the dimensionless separation factor R_L, which described the type of isotherms,In the formula, C₀ represented the initial concentration of the dye, and K_L (L mg⁻¹) was the Langmuir constant. The value of R_L determined the feasibility of the adsorption process. The R_L value could indicate whether the process is irreversible (R_L = 0), beneficial (0 < R_L < 1), linear (R_L = 1), or unfavorable (R_L > 1).

The Freundlich adsorption model describes adsorption on heterogeneous surfaces, where the adsorbed molecules may interact.

The Freundlich isotherm model is expressed as:

In the formula, K_F(L mg⁻¹) and n are the Freundlich constants related to adsorption capacity and adsorption intensity, respectively.

Fig. 10 shows the fitting curves of La₁₂-g-C₃N₄@β-SiC adsorption in ARS wastewater: they are respectively the thermodynamic adsorption curve, the Langmuir adsorption isotherm and the Freundlich adsorption isotherm model. Under the condition of 25 °C, the initial concentration of ARS was 5 mg L⁻¹ to 35 mg L⁻¹, and the adsorption results of ARS were simulated by two models. The Langmuir equation and the Freundlich equation were tested, and the most suitable isotherm model was found. The results are shown in Fig. 10, and the Langmuir equation has a relatively high degree of fitting. In addition, Table 4 lists the parameters of the two models. It can also be seen from the correlation coefficient (R²) that the Langmuir isotherm has a good fitting effect, indicating that the adsorption process of La₁₂-g-C₃N₄@β-SiC material on ARS is monolayer adsorption.

Fig. 10 The La₁₂-g-C₃N₄@β-SiC fitting curve of ARS (a) thermodynamic adsorption curve (b) Langmuir adsorption isotherm (c) Freundlich adsorption isotherm model.

Table 4 ARS adsorption isotherm parameters

Conc./(mg L^−1)	Langmuir-isotherm				Freundlich-isotherm
ARS	qmax (mg g^−1)	KL (L mg^−1)	RL	R^2	n	KF (Lmin ^−1)	R^2
20	19.0730	2.6533	0.0185	0.9977	17.7715	3.3155	0.5609

---

Fig. 11 showed the fitting curves of La₁₂-g-C₃N₄@β-SiC adsorption in AF wastewater: they were respectively the thermodynamic adsorption curve, the Langmuir adsorption isotherm and the Freundlich adsorption isotherm model. Under the condition of 25 °C, the initial concentration of AF was 5 mg L⁻¹ to 35 mg L⁻¹, and the adsorption results of AF were simulated by two models. The Langmuir and Freundlich models were used for verification, and the results were shown in Fig. 11. The Langmuir equation had a relatively high degree of fitting. Table 5 listed the relevant parameters. It could be seen from the correlation coefficient (R²) that the Langmuir isotherm had a good fit for AF adsorption, indicating that the AF adsorption process of La₁₂-g-C₃N₄@β-SiC material was monolayer adsorption (Table 5).

Fig. 11 The La₁₂-g-C₃N₄@β-SiC fitting curve of AF (a) thermodynamic adsorption curve (b) Langmuir adsorption isotherm (c) Freundlich adsorption isotherm model.

Table 5 ARS adsorption isotherm parameters

Conc./(mg L^−1)	Langmuir-isotherm				Freundlich-isotherm
AF	qmax (mg g^−1)	KL (L mg^−1)	RL	R^2	n	KF (L min^−1)	R^2
20	20.1898	0.3663	0.1201	0.9919	2.2154	1.9495	0.8757

---

3.5 Adsorption pathways and mechanisms

To further understand the migration and transformation process of acid fuchsin dye in aquatic environments, it is crucial to clarify its adsorption pathways. Based on the data from the intermediate compounds detected by liquid chromatography-mass spectrometry (LC-MS), two possible adsorption pathways were proposed, as shown in Fig. 12. AF formed product A1 (m/z = 406) via decolorization. The π–π stacking effect between A1 and the aromatic ring led to decolorization or deamidation, resulting in product A2 (m/z = 381). A2 further reduced substituents on the aromatic ring to form product A3 (m/z = 274). A3 subsequently underwent ring opening to form product A4 (m/z = 267). As adsorption proceeded, the products eventually mineralized into CO₂ and H₂O. AF formed product B1 (m/z = 416) through modification of substituents. B1 continued to undergo charge transfer interaction with the AF molecule, resulting in ionization. This could be demonstrated by the continuous change in mass-to-charge ratio, namely products B2 (m/z = 340) and B3 (m/z = 338), but no covalent bonds were broken. Ultimately, these adsorption products mineralized into CO₂ and H₂O.

Fig. 12 Possible AF adsorption path diagram.

The possible adsorption mechanism of La_x-g-C₃N₄@β-SiC as the adsorbent for adsorbing AF was shown in Fig. 13. According to the characterization results of SEM and the analysis results of BET, there existed a rich and uniform mesoporous pore size structure in La_x-g-C₃N₄@β-SiC. Therefore, the influence of pore filling might be one of the reasons why the La_x-g-C₃N₄@β-SiC adsorption material could effectively adsorb AF. In addition, according to previous research reports, AF molecules could be tightly bound to the metal active sites on the adsorbent through electrostatic attraction.⁵⁸ Therefore, acidic fuchsin molecules might enhance the adsorption effect by forming coordination bonds with the metal La present in La_x-g-C₃N₄@β-SiC. Previous research results had shown that the hydroxyl groups in the intermediate after adsorption by AF molecules could interact with the nitrogen-containing components of the composite catalyst to form hydrogen bonds.⁴⁴ Furthermore, in the presence of aromatic rings on the adsorbent (La_x-g-C₃N₄@β-SiC) and the adsorbed dye (AF), π–π formed interactions between them (Fig. 13).

Fig. 13 Possible mechanism of AF adsorption by the La_x-g-C₃N₄@β-SiC adsorbent.

4. Conclusion

In this work, the La₁₂-g-C₃N₄@β-SiC composite adsorbent was prepared by thermal polymerization and mechanical methods. The characterization results of XRD, SEM, and XPS indicated that this material has a uniform-pore, layered structure. The composite material exhibited rapid and efficient adsorption capabilities for ARS and AF pollutants. Within 2.5 minutes, the removal rate of ARS reached 93.23%, and the removal rate of AF was 70.33%. The kinetic and thermodynamic analyses conformed to the pseudo-second-order model and Langmuir isotherm equation, consistent with the single-layer chemical adsorption mechanism. After 8 consecutive cycles, the average adsorption efficiency remained at 90%, demonstrating good stability and application potential. This research provides valuable insights into the design and application of composite materials for the removal of environmental pollutants.

Author contributions

Fanxia Zhang and Yuan Min conceived and designed the experiments; Fanxia Zhang operated the experiments; Zhiqiang Wu, Zhiliang Jin, and Jia Yang provided all the reagents and analysis tools; Yuan Min and Zhiqiang Wu completed the writing of this paper.

Conflicts of interest

The authors declare that they have no competing interests.

Data availability

The data supporting the findings of this study have been included within the article.

Acknowledgements

This work was financially supported by the Natural Science Foundation of the Ningxia Hui Autonomous Region (2024AAC03314, 2026AAC030892); the Firstclass Discipline Construction (Education Discipline) in Higher Education Institutions of Ningxia (NXYLXK2021B10), the Major Project of “Leading the Charge with Open Competition” for Undergraduate Education Reform at Ningxia Normal University (BKJXGG202503) and College student innovation and entrepreneurship training program project (G202510753016; S202510753012).

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Footnote

† Equal contribution.

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