The pollution problem resulting from advancements in science and technology is increasingly severe, particularly concerning organic pollution. Photocatalytic technology is considered one of the most effective methods for treating organic pollution due to its cost-effectiveness, simplicity of operation, high efficiency, and versatility. In this study, palygorskite was purified and extracted using techniques such as ultrasonication, high-speed stirring, centrifugation, and others. Molybdenum disulfide (MoS2) was synthesized in situ on the palygorskite surface through hydrothermal synthesis, resulting in palygorskite/MoS2 nanocomposites. The structure and apparent morphology of the palygorskite/MoS2 composites were analyzed using characterization methods such as transmission electron microscopy, x-ray diffraction, Fourier transform infrared spectroscopy, and others. MoS2 interacted with the hydroxyl groups on the palygorskite surface through amino groups, leading to the dispersion of MoS2 nanosheets on the palygorskite surface, forming a unique nanoflower structure. To assess the photocatalytic degradation performance of palygorskite/MoS2 composites, Rhodamine B was employed as the target pollutant. Under conditions of a pH of 6, a reaction time of 170 min, and a solution concentration of 1500 mg/l, palygorskite/MoS2 composites achieved a Rhodamine B removal amount of 371.73 mg/g. Notably, these composites facilitated the degradation of Rhodamine B into intermediate chain-broken products. The findings of this study hold significant implications for the advancement of clay mineral-based photocatalytic composites and the effective removal of organic pollutants.

As science and technology continue to advance, the space occupied by human activities continues to expand, resulting in significant damage to the ecological environment, and the issue of environmental pollution remains a continuous concern. In recent years, the problem of organic wastewater caused by industrial development has gained particular prominence. Taking the printing and dyeing industry as an example, ∼10%–20% of dyes are discharged with wastewater.1 This dye wastewater often exhibits high concentrations, complex compositions, large chroma, and strong toxicity,2 which can impact the self-purification function of water bodies, impede the growth of aquatic organisms, plants, and micro-organisms, and pose significant threats to the ecological environment.3 Furthermore, dye wastewater can enter the human body through the biological chain, harming the human immune system and nervous system and even potentially inducing cancer.4 Consequently, the efficient removal of dye wastewater has become a critical issue in the field of environmental pollution prevention and control. However, traditional physical, chemical, and biological methods (including sedimentation, oxidation, ultrafiltration, etc.) often struggle to balance both cost and efficiency and may introduce new pollutants during the treatment process, leading to secondary pollution. Therefore, the exploration of low-cost and high-efficiency pollutant removal technology remains a continuous societal concern. Recent research and reports have identified photocatalytic technology as one of the most effective dye degradation methods due to its low cost, ease of operation, high efficiency, and other characteristics,5,6 with the key lying in the development of excellent photocatalytic materials.

Molybdenum disulfide (MoS2) is a transitional metal sulfide comprising two layers of sulfur (S) and one layer of molybdenum (Mo), forming a distinct “sandwich” structure with van der Waals forces between the layers. It exhibits three primary crystal structures: 1T (octahedral), 2H (triprism), and 3R (triprism), as depicted in Fig. 1. Among these, the 2H structure is the most thermodynamically stable structure at room temperature. Generally, both 1T and 3R structures tend to transform into the more stable 2H structure, and 2H can also transition to 1T through structural and phase changes.7,8 MoS2 acts as an excellent semiconductor material, with Mo and S sites present on both the base and edges. The active sites at the edges exhibit unsaturated coordination, leading to significantly higher reactivity than those on the base.9 Extensive theoretical calculations and experimental studies consistently demonstrate that Mo and S sites exposed at the edges serve as the primary catalytic activity centers,10 facilitating various reactions, such as hydrogen evolution11 and reduction. Furthermore, MoS2 demonstrates high charge transfer efficiency and exceptional light-absorption properties, making it an outstanding photocatalyst. Currently, MoS2 finds widespread applications in catalytic hydrogen evolution12,13 and catalytic degradation processes.14–17 

FIG. 1.

2H, 3R, and 1T structure diagram of MoS2.

FIG. 1.

2H, 3R, and 1T structure diagram of MoS2.

Close modal

Due to its high surface energy and interlaminar forces, the central active site and interlaminar charge transfer efficiency of bulk MoS2 are limited, resulting in bulk MoS2 having only a small indirect bandgap (1.3 eV) and a low photocatalytic efficiency. However, reducing its size to two dimensions improves MoS2’s bandgap (transforming it into a direct bandgap of 1.9 eV).18 Simultaneously, two-dimensional MoS2 nanosheets offer a larger specific surface area and better chemical affinity, while also exposing more active sites by overcoming interlaminar forces. As a result, MoS2 is often fashioned into a two-dimensional nanostructure when it participates in catalytic reactions. Nevertheless, agglomeration of two-dimensional nanosheets often occurs, limiting the activity of catalytic sites. Therefore, dispersants are often added when preparing two-dimensional nanosheets. In environmental remediation, a composite method is typically used to address the agglomeration problem of MoS2 nanosheets. Excellent carrier materials can not only disperse MoS2 but also greatly improve the catalytic activity of MoS2 sites, thereby enhancing the efficiency of pollutant removal.

Palygorskite (Pal) is considered an excellent carrier material.19 It represents a typical 2:1 chain-layered clay mineral, composed of layered silicate units with a band structure. Each band structure is inversely connected to another along a set of Si–O–Si bonds through a Si–O tetrahedron,20 resulting in a unique chain layer structure.21 Pal generally has a grain diameter of 40–50 nm, with the potential to extend its length to hundreds of nanometers or even micrometers. It exhibits well-defined pores (measuring 0.38 × 0.63 nm2), a high specific surface area, and appropriate cation exchange capacity. Moreover, Pal possesses abundant hydroxyl groups, oxidation sites, and cation reduction sites, which significantly contribute to adsorption and ion exchange processes.22,23 Currently, Pal is employed for adsorption and degradation of organic pollutants.24–26 In photocatalysis, Pal-loaded systems with TiO2,27 BiVO4,28 CeO2,29 etc., exhibit high degradation performance.

In this work, we focus on the in situ synthesis of MoS2 nanosheets on the surface of purified and stripped Pal to enhance the dispersion of MoS2 nanosheets. The MoS2 nanosheets, when dispersed on the Pal surface, form a distinctive nanoflower structure. This unique structure exposes a greater number of active sites, promoting the degradation of Rhodamine B. The outcomes of this study bear significant implications for the development of novel clay mineral-based photocatalytic composites and the efficient removal of organic pollutants.

Thiourea [CS(NH2)2] and sodium hexametaphosphate [(NaPO3)6] were obtained from Tianjin Beichen Founder Reagent Factory, while ammonium molybdate tetrahydrate [(NH4)6Mo7O24 · 4H2O] was procured from Tianjin No.4 Chemical Reagent Factory. Sodium hydroxide (NaOH) was sourced through Tianjin Opusheng Chemical Co. Ltd. Rhodamine B (C28H31ClN2O3) was acquired from Tianjin Institute of Reagents. All the aforementioned substances were of analytical purity. Palygorskite (Pal) was sourced from Mingguang City, Anhui Province.

(NaPO3)6 (3%) and Pal (5%) were added to deionized water, and the mixture was stirred for 2 h. Subsequently, the mixture was transferred to an ultrasonic instrument and subjected to ultrasonic treatment for 2.5 h. Following this, the mixture was allowed to settle for 24 h.

The clear liquid from the upper layer was suctioned using a glue-head dropper. The collected liquid was then placed in a centrifuge tube and centrifuged at 8000 rpm for 5 min. This centrifugation process was repeated twice. Ultimately, the lower layer of the centrifuged products was retrieved, transferred to a freezer, and subjected to freeze-drying for 24 h. The freeze-dried products were ground into powder for subsequent use.

1 g of the purified and stripped Pal sample was placed into a freezer and frozen for 4 h. It was subsequently transferred to a beaker, to which 50 ml of deionized water was added. The mixture underwent ultrasonic treatment for 4 h, followed by mechanical stirring at 1500 rpm for 12 h. To the Pal sample, 1.236 g of ammonium molybdate tetrahydrate and 1.199 g of thiourea were added in a 1:2.25 ratio. The resulting solution was further stirred at 1500 rpm for 20 min to achieve complete mixing. The solution was then poured into a stainless steel reaction kettle and allowed to react at 220 °C for 24 h. After natural cooling, it underwent multiple washes with deionized water. Subsequently, it was placed in an electrically heated blast drying oven and dried at 80 °C for 10 h, resulting in the formation of the Pal/MoS2 composite material.

The sample structure was determined by x-ray diffraction (XRD, model: Bruker D8 ADVANCE, Bruker, Germany) with Cu-Kα radiation generated at 30 mA and 40 kV. The surface functional groups of the sample were determined by Fourier transform infrared spectroscopy (FTIR, model: Nicolet iS5, Thermo Fisher Scientific, USA) in the transmission mode. For the microstructure analysis of the sample, transmission electron microscopy (TEM, model: JEOL JEM-2100, Japan Electronics Co. Ltd., Japan) was employed at an accelerated voltage of 200 kV. The qualitative analysis of the degradation products of Rhodamine B was conducted using liquid chromatography-mass spectrometry (LC-MS, model: Waters ZQ2000, Waters Corporation, USA). Furthermore, the absorbance value of the solution was determined utilizing an ultraviolet-visible spectrophotometer (UV754N, Shanghai Aosi Scientific Instrument Co. Ltd., China).

1 g of Rhodamine B was dissolved in 1 l of distilled water to prepare a 1 g/l reserve solution of Rhodamine B. The photocatalyst dosage (composed of Pal/MoS2 composite material, namely, Pal and MoS2) was set at 0.1 g. The volume of the Rhodamine B solution was 100 ml. The two components were mixed and stirred for 20 min at 20 °C in the dark to eliminate the potential impact of material adsorption on the photocatalytic effect. Subsequently, the photocatalytic experiment was conducted by activating the xenon lamp light source (500 W).

Concentrations of Rhodamine B were set at 5, 10, 25, 50, 100, 200, 500, 750, 1000, and 1500 mg/l. The reaction time was established as follows: 20, 50, 80, 140, 170, and 200 min (including 20 min of stirring in the dark). These settings were chosen to explore the influence of solution concentration and reaction time on the degradation efficiency of Rhodamine B. A Rhodamine B solution containing 500 mg/l was selected for further investigation; pH values of 4, 6, 8, 10, and 12 were employed to assess the impact of solution pH on the degradation efficiency of Rhodamine B over a 60-min period.

In the test, the mass concentration of pollutants in the solution was converted using an ultraviolet spectrophotometer based on absorbance values. The amount of pollutants removed by the material can be calculated using the following formula:
qe=(C0Ce)×Vm,
(1)
where C0 represents the initial mass concentration of the solution (mg/l), Ce represents the mass concentration of the pollutant at equilibrium in the solution (mg/l), qe represents the amount of material removed (mg/g), V represents the volume of the solution (l), and m represents the mass of the added material (g).

Pal/MoS2 composites were synthesized via the hydrothermal method (Fig. 2). Due to the natural electronegativity of the Pal surface, MoO42− cannot be directly bound to it, while CS(NH2)2 can form hydrogen bonds with hydroxyl groups on the Pal surface via amino groups under hydrothermal conditions.30 This, in turn, allows for the nucleation and growth of MoS2 nanosheets on the Pal surface.

FIG. 2.

Composite mechanism of Pal and MoS2.

FIG. 2.

Composite mechanism of Pal and MoS2.

Close modal

1. Structural characteristics of Pal/MoS2 composites

Figure 3 presents the XRD spectrum of Pal, MoS2, and Pal/MoS2 composites and their photocatalytic degradation of Rhodamine B. As observed in the figure, the characteristic diffraction peaks of Pal appear at 2θ = 8.4°, 13.8°, 16.5°, 19.9°, 27.8°, and 34.9°, corresponding to its (110), (200), (130), (040), (400), and (102) crystal planes, respectively.31 Similarly, the characteristic diffraction peaks of MoS2 are visible at 2θ = 14.2°, 33.4°, 39.7°, and 58.9°, corresponding to its (002), (100), (103), and (110) crystal planes, respectively, consistent with the standard card (JCPDS No. 37-1492).30 The XRD spectrum of Pal/MoS2 composites exclusively displays the characteristic diffraction peaks of Pal and MoS2, without the presence of any other impurities. The in situ growth of MoS2 on the surface of Pal does not disrupt the crystal structure of the two materials, resulting in Pal/MoS2 composites of high purity and good crystallinity. In addition, after the photocatalytic degradation of Rhodamine B, the characteristic diffraction peaks of Pal/MoS2 composites exhibit no significant changes, indicating the excellent structural stability of the synthesized composites.

FIG. 3.

XRD patterns of Pal, MoS2, and Pal/MoS2 composites and their degradation of Rhodamine B.

FIG. 3.

XRD patterns of Pal, MoS2, and Pal/MoS2 composites and their degradation of Rhodamine B.

Close modal

Figure 4 displays the FTIR spectra of Pal, MoS2, and Pal/MoS2 composites and their photocatalytic degradation of Rhodamine B. The band at 445 cm−1 in the three materials corresponds to the Mo–S stretching vibration of MoS2.31,32 The band at 470 cm−1 is linked to the bending vibration of the Si–O–Si bond,33 while the bands at 985 and 1031 cm−1 are associated with the stretching vibration of the inner Si–O–Si bond.34 In addition, the bands at 3421 and 1650 cm−1 represent the characteristic stretching vibration of O–H in coordination water and the bending vibration of H–O–H in adsorbed water, respectively. The band at 3615 cm−1 results from the stretching vibration of the hydroxyl groups connected to the octahedral coordination and the bonding of different metal cations (Mg, Fe) in the lattice in Pal.33,35 The characteristic peaks of Pal and MoS2 are present in Pal/MoS2 composites, although the peaks of the composites appear weakened compared to Pal. This is likely due to the MoS2 coating on the Pal surface. Importantly, the characteristic peaks of the composites do not exhibit significant changes after the degradation of Rhodamine B. This suggests that the hydrothermal synthesis produces a structurally stable Pal/MoS2 composite photocatalyst, and the MoS2 nanosheets are effectively combined with the Pal surface, consistent with the XRD results presented in Fig. 3.

FIG. 4.

FTIR spectra of Pal, MoS2, and Pal/MoS2 composites and their degradation of Rhodamine B.

FIG. 4.

FTIR spectra of Pal, MoS2, and Pal/MoS2 composites and their degradation of Rhodamine B.

Close modal

2. Microstructure of Pal/MoS2 composites

Figure 5 depicts the TEM photograph of Pal/MoS2 composites. As observed in the figure, Pal has a diameter of about 20 nm and a length of ∼350–500 nm, while MoS2 nanosheets have a diameter ranging from about 150 to 250 nm. A unique nanoflower structure is formed by a small number of MoS2 nanosheets, which uniformly encase the surface of Pal [Figs. 5(a) and 5(b)]. Simultaneously, Pal, serving as a matrix, mitigates the agglomeration of MoS2 to some extent, facilitating the exposure of its active sites and enhancing the overall photocatalytic performance of the composites. A closer examination at higher magnification through TEM reveals that MoS2 nanoflowers are firmly anchored to the Pal surface, with no disruption to the structure of either component. Furthermore, MoS2 nanosheets exhibit improved dispersion, displaying a distinct layered structure [Figs. 5(c) and 5(d)]. Lattice analysis indicates a lattice spacing of 0.62 nm in the layered MoS2 nanosheets [Figs. 5(e) and 5(f)], corresponding to the (002) crystal plane of hexagonal MoS2.30 This finding further validates the assembly of small-sized (∼200 nm in diameter) MoS2 nanosheets on the Pal fiber surface. Altogether, MoS2 forms a unique nanoflower structure when combined with Pal, and Pal provides an excellent structural framework for MoS2 growth.

FIG. 5.

TEM photos of Pal/MoS2 composites: (a) and (b) 200 nm; (c) and (d) 50 nm; (e) and (f) analysis of MoS2 lattice spacing.

FIG. 5.

TEM photos of Pal/MoS2 composites: (a) and (b) 200 nm; (c) and (d) 50 nm; (e) and (f) analysis of MoS2 lattice spacing.

Close modal

The structures of Pal, MoS2, and Pal/MoS2 were further characterized using Raman spectroscopy. The Raman scattering spectrum of MoS2 is primarily concentrated in the range of 200–500 cm−1, corresponding to three vibration modes: E1g (287 cm−1), E2g1 (380 cm−1), and A1g (404 cm−1).36 Here, A1g represents the relative vibration between the Mo atom and S atom in the vertical layer plane direction, while E2g1 corresponds to the vibration of both Mo and S atoms within the layer plane.37 The hydrothermal synthesis of MoS2 exhibits a distinct peak in the range of 200–500 cm−1, as depicted in Fig. 6(a). In contrast, Pal does not exhibit any noticeable characteristic peak at this position, while the composite material demonstrates a characteristic peak similar to that of MoS2 within the same range. Furthermore, no significant characteristic peak is observed beyond 500 cm−1, indicating uniform coating of in situ generated MoS2 nanosheets on the surface of Pal with excellent coating effectiveness. The difference between A1g and E2g1 vibration modes can be adjusted by varying the lamellar thickness of MoS2; greater lamellar thickness results in an increased disparity between their respective peaks.38 Furthermore, a close analysis of the characteristic peaks of the composite material and MoS2 at 320–440 cm−1 [Fig. 6(b)] reveals that the two vibration modes in the composite material have peak positions similar to those of MoS2, with the peak position difference greater than 23 cm−1, indicating that multiple MoS2 nanosheets were in situ generated on the surface of Pal. Simultaneously, there is an offset observed in the peaks of both vibration modes A1g and E2g1, which may be attributed to lamellar lattice defects resulting from hydrothermal synthesis. The introduction of a small number of lattice defects is beneficial for enhancing the catalytic performance of MoS2’s active sites. In summary, the Pal/MoS2 composites were successfully synthesized via the hydrothermal method, resulting in uniform coating of MoS2 nanosheets on the Pal surface with minimal defects. These findings are consistent with the results obtained from XRD, FT-IR, and TEM analyses.

FIG. 6.

(a) Raman spectra of Pal, MoS2, and Pal/MoS2. (b) Comparison of MoS2 and Pal/MoS2 in the range of 320–440 cm−1.

FIG. 6.

(a) Raman spectra of Pal, MoS2, and Pal/MoS2. (b) Comparison of MoS2 and Pal/MoS2 in the range of 320–440 cm−1.

Close modal

1. Photocatalytic degradation performance of Pal, MoS2, and the Pal/MoS2 composite for Rhodamine B removal

The photocatalytic degradation performance of the Pal/MoS2 composite material was compared to that of Pal and MoS2 for Rhodamine B removal [Fig. 7(a)]. As the concentration of the Rhodamine B solution increased, both the Pal/MoS2 composite material and Pal demonstrated an incremental increase in Rhodamine B removal. At a Rhodamine B concentration of 1000 mg/l, the removal amount of the Pal/MoS2 composite material was 328.78 mg/g, and that of Pal was 228.49 mg/g. Analyzing the reaction rate, it was observed that the Pal/MoS2 composite material continued to exhibit a significant increase in removal efficiency at a Rhodamine B concentration of 1000 mg/l whereas Pal’s removal amount gradually reached a balance. Pure MoS2, used for Rhodamine B removal, showed fluctuations at around 87.58 mg/g. This can be attributed to the agglomeration during the hydrothermal synthesis of MoS2 and the limited availability of photocatalytic active sites. Altogether, the photocatalytic removal performance of the Pal/MoS2 composite material significantly surpassed that of Pal and MoS2. This improvement can be attributed to the combination of Pal and MoS2, where Pal, acting as a framework, effectively dispersed MoS2 nanosheets, exposing more active sites and thereby enhancing the material’s photocatalytic efficiency.

FIG. 7.

(a) Photocatalytic degradation properties of Pal/MoS2 composites. (a) Removal properties of MoS2, Pal, and Pal/MoS2 composites for Rhodamine B. (b) Influence of Pal/MoS2 composites on Rhodamine B removal at different solution concentrations. (c) Removal rate of Rhodamine B by Pal/MoS2 composites at different solution concentrations. (d) Influence of Pal/MoS2 composites on Rhodamine B removal at different reaction times. (e) Influence of reaction time on the removal effect of Rhodamine B. (f) The number of photocatalytic cycles of Pal/MoS2.

FIG. 7.

(a) Photocatalytic degradation properties of Pal/MoS2 composites. (a) Removal properties of MoS2, Pal, and Pal/MoS2 composites for Rhodamine B. (b) Influence of Pal/MoS2 composites on Rhodamine B removal at different solution concentrations. (c) Removal rate of Rhodamine B by Pal/MoS2 composites at different solution concentrations. (d) Influence of Pal/MoS2 composites on Rhodamine B removal at different reaction times. (e) Influence of reaction time on the removal effect of Rhodamine B. (f) The number of photocatalytic cycles of Pal/MoS2.

Close modal

2. The effect of solution concentration

In order to further examine the saturation removal capacity of Rhodamine B by Pal/MoS2 composites, we discussed the influence of solution concentration on the removal capacity [depicted in Fig. 7(b)]. As the concentration of Rhodamine B increased, the removal capacity of Rhodamine B by Pal/MoS2 composites gradually increased. Up to a concentration of 500 mg/l, the removal capacity increased rapidly; beyond 500 mg/l, the removal capacity increased slowly, with the final saturation removal capacity reaching 371.73 mg/g, which was observed at ∼1500 mg/l. Although the removal capacity increased with the increase in solution mass concentration, the removal rates varied at each concentration level. Figure 7(c) illustrates the relationship between different solution mass concentrations and removal rates. The results indicate that removal rates exceeded 99% when the mass concentration of Rhodamine B was between 5 and 100 mg/l. At a mass concentration of 200 mg/l, the removal rate remained at 98%, but as the mass concentration continued to rise, the removal rate began to decrease. Specifically, at a mass concentration of 500 mg/l, the removal rate dropped to 61%. Finally, when the mass concentration of Rhodamine B reached 1500 mg/l, the removal rate was only 25%. The analysis demonstrates that 0.1 g of Pal/MoS2 composites could effectively remove Rhodamine B from solutions with concentrations below 200 mg/l. Furthermore, as the mass concentration of Rhodamine B increased, the removal capacity continued to increase, ultimately reaching 371.73 mg/g, albeit with a reduction in the removal rate to 25%.

3. The effect of reaction time

Figure 7(d) illustrates the influence of reaction time on the removal efficiency of Rhodamine B. As the reaction time increases, the removal of Rhodamine B by Pal/MoS2 composites at concentrations of 100, 200, and 500 mg/l gradually increases. Within 50 min, the removal reaches 98.83 mg/g (100 mg/l), 182.99 mg/g (200 mg/l), and 280.51 mg/g (500 mg/l), accounting for 99.2%, 93.2%, and 91.7% of the total removal amount, respectively. The removal performance decreases significantly after 50 min, which may be attributed to the high initial photocatalytic sites and the fast photocatalytic degradation rate. The final removal equilibrium is reached at 170 min, with the maximum removal amount of rhodamine being 99.64 mg/g (100 mg/l), 196.28 mg/g (200 mg/l), and 305.94 mg/g (500 mg/l).

4. The effect of solution pH

The pH value of the solution significantly impacts material adsorption and photocatalytic performance, as depicted in Fig. 7(e). Under acidic conditions (pH < 7), Pal/MoS2 composites demonstrate superior efficiency in removing Rhodamine B. This phenomenon can be attributed to the favorable environment that acidic conditions provide for electron transfer on the composite’s surface. This, in turn, ensures the availability of sufficient charges for engagement in surface and interlayer reactions, thereby promoting photocatalytic degradation.39 Conversely, as the pH increases under alkaline conditions, the efficiency of Rhodamine B removal gradually deteriorates. This decline can be ascribed to two primary factors: first, alkaline conditions impede the migration of photogenerated electrons, and second, the anionic nature of the composites’ surface under alkaline conditions generates repulsive electrostatic forces, affecting adsorption performance.40 Nevertheless, it is important to note that excessively strong acidity does not yield improved removal efficiency. Specifically, at a pH of 4, the removal amount of Rhodamine B decreases to 273.65 mg/g. This reduction may be attributed to the corrosive effects of a highly acidic environment, which disrupt the structure of Pal/MoS2 composites and hinder the degradation of Rhodamine B. In summary, mildly acidic conditions support the photocatalytic degradation of Rhodamine B, while alkaline conditions inhibit the photocatalytic process. The maximum removal capacity, reaching 281.60 mg/g, is observed at a solution pH value of 6.

5. The effect of reusability

The removal stability of Pal/MoS2 composites was investigated by repeated experiments. The pollutant concentration was selected as 200 mg/l, and the removal time was selected as 80 min. After each photocatalytic experiment was completed, the composite material was washed with deionized water several times and then weighed after drying for the experiment. The comparison results of five repeated uses are shown in Fig. 7(f). The first removal experiment using the composite materials had the best performance, with a removal amount of 199.24 mg/g and a removal rate of 99.62%. From the second time onward, the removal efficiency gradually decreased, reaching the lowest removal rate of 28.6% by the fifth time. The first two removal experiments maintained a removal rate of over 74%, while from the third time onward, the removal rate only remained at around 50%. The decrease in the removal rate can be attributed to the consumption of active sites during the photocatalytic process and the damage to the material structure caused by multiple washings. Although the introduction of some lattice defects by hydrothermal synthesis can enhance the catalytic activity of S sites and improve the first photocatalytic removal effect, these defects make the structure more vulnerable to damage during repeated use in experiments.

6. Dynamic simulation

In order to further investigate the photocatalytic ability of Pal/MoS2 composites, the photocatalytic reaction rate constant can be obtained from the L-H kinetic model,41 as shown in the following equation:
ln(C0/C)=kKt=Kt,
(2)
where C0 and C are the concentration of pollutants at the beginning and the end (mg/l) respectively; t is the reaction time (min); k is the reaction rate constant; K is the Langmuir adsorption equilibrium constant, and K′ = kK is called the pseudo-first-order reaction kinetic constant (min−1). The kinetic fitting curves under three different concentrations (100, 200, and 500 mg/l) were obtained from the experimental data of reaction time and the removal amount of Rhodamine B, as shown in Fig. 8. The pseudo-first-order reaction kinetic constants at each concentration are shown in Table I.
FIG. 8.

Fitting curves of photocatalysis kinetics at different concentrations.

FIG. 8.

Fitting curves of photocatalysis kinetics at different concentrations.

Close modal
TABLE I.

Pseudo-first-order reaction kinetic constants at different concentrations.

Solution concentration (mg/l) 100 200 500 
K′ (min−1) 0.028 51 0.015 78 0.003 12 
Solution concentration (mg/l) 100 200 500 
K′ (min−1) 0.028 51 0.015 78 0.003 12 

From the kinetic constants of pseudo-first-order reactions at different concentrations shown in Table I, it can be seen that the pseudo-first-order reaction kinetic constant at a concentration of 100 mg/l is the largest, indicating that the reaction rate is fastest at this concentration. Although the overall removal rate at high concentrations is higher, the corresponding reaction kinetic constant is lower. This means that the time required to reach the ultimate removal equilibrium is longer at higher concentrations. The main reason is that there are more pollutants enriched at a single active site at high concentrations and that the photocatalytic degradation of these pollutants takes longer.

The removal of Rhodamine B by Pal/MoS2 composites is attributed to the synergistic effect of adsorption and photocatalytic degradation. Pal’s possession of excellent adsorption performance is attributed to the pores along the c-axis direction and its ion exchange property. Acting as a loading matrix, it can function as a pollutant adsorbent, and the accumulated pollutants provide favorable conditions for subsequent photocatalytic degradation. The main photocatalytic groups on the surface of Pal are MoS2 nanosheets. The photocatalytic mechanism is as follows: When the light energy absorbed by MoS2 equals or exceeds the bandgap energy (Eg), electrons (e) transition from the valence band (VB) to the conduction band (CB) and leave electron vacancies (h+) in VB, as presented in Eq. (3). When light energy is converted into chemical energy, hydroxyl radicals (·OH) and superoxide radicals (·O2)42 are generated [Eqs. (4)(9)], and these two free radicals play an important role in the entire process of photocatalytic degradation of Rhodamine B,
MoS2+hvMoS2(ecb+hvb+),
(3)
MoS2(hvb+)+H2OMoS2+H++OH,
(4)
MoS2(hvb+)+OHMoS2+OH,
(5)
MoS2(ecb)+O2MoS2+O2,
(6)
O2+H+HO2,
(7)
O2+H+O2+H2O2,
(8)
H2O2+MoS2(ecb)OH+OH+MoS2,
(9)
RhB+OH+O2/HO2Intermediateproducts.
(10)

Among them, ·O2 can react with H+ to produce the peroxyhydroxyl radical [HO2·, as presented in Eq. (7)], which exhibits strong oxidation. ·OH also has strong oxidation capabilities. These free radicals can further react with Rhodamine B to generate a series of intermediate products, including free radicals and free radical cations,43 thereby achieving degradation. The products before and after the degradation of Rhodamine B by the Pal/MoS2 composite material have undergone further analysis using LC-MS (Fig. 9). After the hydrolysis of Rhodamine B, its m/z remains at ∼433, as shown in Fig. 9(a), with a larger peak observed at m/z = 443.47, where Rhodamine B’s molecular weight constitutes more than 85%. After photocatalytic degradation, distinct peaks emerge at 371.60, 193.35, and 112.62, calculated to be intermediate products subsequent to Rhodamine B’s degradation.44,45 Notably, m/z = 371.60 may form following the removal of the ethyl group from Rhodamine B, while the intermediate products at m/z = 193.35 and 112.62 may arise through the connection of carboxyl benzene generated after the macromolecular chain of Rhodamine B breaks from HO2· [Fig. 9(b)]. These findings indicate that after the photocatalytic degradation of Rhodamine B by the Pal/MoS2 composite material, intermediate products are generated through bond cleavage, leading to a transformation from high to low molecular weight, as presented in Eq. (10).

FIG. 9.

LC-MS spectrum before and after degradation of Rhodamine B: (a) before degradation; (b) after degradation.

FIG. 9.

LC-MS spectrum before and after degradation of Rhodamine B: (a) before degradation; (b) after degradation.

Close modal

We prepared high-purity and well-crystallized Pal/MoS2 composites via hydrothermal synthesis. MoS2 and Pal were combined through hydrogen bonding involving amino and hydroxyl groups, resulting in the dispersion of MoS2 nanosheets on the Pal surface, forming a unique nanoflower structure. Pal effectively dispersed MoS2, thereby enhancing the photocatalytic degradation performance of Rhodamine B.

The Pal/MoS2 composite improved the photocatalytic degradation of Rhodamine B. Notably, the reaction time and solution pH significantly influenced the photocatalytic degradation process. Rhodamine B removal by the Pal/MoS2 composite saturated at 170 min, reaching a maximum removal capacity of 371.73 mg/g. Optimal degradation occurred at a pH of 6 under weak acidic conditions. Remarkably, 0.1 g of the Pal/MoS2 composite could almost completely remediate rhodamine B-polluted solutions below 200 mg/l, meeting the requirements for addressing dye pollution under natural conditions.

Rhodamine B degradation by Pal/MoS2 composites primarily involves adsorption and photocatalytic processes. Photocatalytic degradation breaks down Rhodamine B chains, converting it from having a high molecular weight to a low molecular weight and generating intermediate products such as carboxy-benzene.

This research was jointly supported by the National Natural Science Foundation of China (Grant No. 42202042), the Liaoning Province’s “Xingliao Talent Plan Project (Grant No. XLYC2007105),” the Projects of the Educational Department of Liaoning Province (Grant No. LJKZ0594), and the Foundation of Key Laboratory of Clay Mineral Applied Research of Gansu Province (Grant No. CMAR-2022-02).

The authors have no conflicts to disclose.

Weidong Tian: Writing – original draft (equal). Limei Wu: Writing – review & editing (equal). Ritong Huang: Investigation (equal). Aiqin Wang: Funding acquisition (equal). Yushen Lu: Resources (equal). Ning Tang: Project administration (equal). Lili Gao: Investigation (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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