© The Author(s) 2024. Published by Science Press and EDP Sciences.

INTRODUCTION

Nuclear energy stands out as an indispensable green energy source due to its reliability and consistency in mitigating a range of environmental issues stemming from substantial greenhouse gas emissions [1,2]. As energy shortages intensify and nuclear energy consumption rises, uranium as a basic raw material for the nuclear industry has garnered considerable attention [3–5]. However, uranium mining, nuclear industry processes, and even nuclear accidents generate substantial volumes of radioactive wastewater containing uranium [6,7]. U(VI) predominantly exists in the form of soluble UO₂²⁺ with significant mobility in aquatic environments. The high mobility of UO₂²⁺ allows it to spread rapidly, exerting a profound impact on the ecological environment [7,8]. The severe chemical toxicity and radiological hazard of uranium can cause damage on various human organs upon contact or exposure, posing a significant threat to human health when it enters the body through the mouth, nose, or skin [7,8]. Consequently, the efficient removal and capture of uranium from complex radioactive wastewater are of vital importance not only to address environmental pollution but also to ensure the safety and sustainable development of nuclear energy.

Various treatment methods of wastewater containing uranium have been developed such as solvent extraction [9], chemical precipitation [9], membrane filtration [10], chemical reduction, and adsorption [6,11–13]. Among them, adsorption is considered as a reliable technique due to the simple operation, low cost, and high efficiency. Many uranium adsorption materials have been reported, including minerals [14,15], metal oxides [16,17], metal chalcogenides [7,18–20], resin [21,22], metal organic frameworks (MOFs) [21–24], porous organic polymers (POPs) [25,26], covalent organic frameworks (COFs) [27–29], and so on. Transition metal dichalcogenides stand out due to their sulfur or selenium-rich surfaces and their highly tunable properties. These materials can effectively track and remove metal ions through the interaction between sulfur or selenium and metal ions [11,18,30]. Molybdenum disulfide (MoS₂), a prominent example of transition metal dichalcogenides, has garnered significant interest due to its remarkable attributes such as a narrow bandgap energy, non-toxicity, biocompatibility, low cost, and easy synthesis [31,32]. More importantly, it is a promising material for removing uranium by the interaction of Lewis soft base S²⁻ with the relatively soft UO₂²⁺ [11,18,30]. However, the previous study indicated that the raw MoS₂ has a low affinity for UO₂²⁺ [18]. To address this limitation, researchers have prepared various MoS₂ composite materials to enhance the U(VI) adsorption performance of MoS₂ by either increasing the number of uranium adsorption sites or improving the material’s hydrophilicity [33–35], such as layered double hydroxide modified MoS₂ [33], phosphonate-functionalized MoS₂ [34], and sodium dodecyl sulfate modified MoS₂ [35]. It is the key to select the appropriate carrier to prepare high-performance composite materials based on molybdenum sulfide.

Collagen fibers (CF), a structural protein known for its mechanical strength and hydrophilicity, have found extensive applications in separation processes due to their fibrous structure, which promotes efficient mass transfer [36,37]. CF is endowed with numerous active functional groups, such as –OH, –CONH₂, –COOH, and –NH₂, that can interact with metal ions, making them an excellent candidate for removing pollutants [7,36]. Consequently, CF emerges as a promising adsorbent for the removal of U(VI) from aqueous solutions. Currently, a variety of CF-based adsorption materials have been developed for uranium capture [7,38–40]. The primary strategies involve: (i) activating and modifying the CF structure to increase the active sites of materials for effective U(VI) adsorption, such as alkaline treatment or grafting organic functional molecules onto CF [39,40]; (ii) loading metal ions or nanoparticles. For example, CF loaded with Ti⁴⁺ [38] or nanoparticles [41], have shown the ability of effective uranium removal from wastewater.

Our strategy is to composite CF with metal chalcogenides. Metal chalcogenides contribute specific adsorption sites for U(VI), while CF enriches the composite with additional functional groups, such as amino and carboxyl groups, which are beneficial to U(VI) capture. Moreover, the hydrophilic nature and fibrous structure of CF are advantageous for promoting mass transfer and thus facilitating U(VI) adsorption. However, only one case of nano-ZnS/alkali-activated CF composite has been reported by us [7]. It indicates that CF serves as an excellent carrier for metal sulfides, enabling the in-situ growth of zinc sulfide on CF to prepare composite material. Additionally, the carboxyl and amino groups of CF can assist in binding uranium, further enhancing the adsorption capacity of the composite material. Compared with bulk zinc sulfide, this composite material exhibits higher uranium removal performance, which proves the correctness of our compositing strategy. Therefore, combining metal chalcogenides with CF can give composite materials the advantages of both metal chalcogenides and CF. However, CF with the characteristic of lower thermal stability is not suitable for in situ growth of other metal chalcogenides such as molybdenum sulfide and molybdenum selenide, as their synthesis temperature is higher. Therefore, there is a pressing need to develop a green, simple, and universally applicable compositing strategy or method to prepare composite materials with transition metal dichalcogenides and CF for higher uranium adsorption performance.

In this work, we proposed a universal preparation strategy for preparing MQ₂-collagen fiber (M = Mo, W; Q = S, Se) composite materials, that is, green, simple and efficient co-ball milling of transition metal dichalcogenides with CF. MQ₂ as an adsorbent is uniformly distributed on the carrier CF after compositing, exposing more active sites. Meanwhile, CF can also provide additional functional groups such as amino carboxyl groups as adsorption sites. The presence of CF makes the material more hydrophilic, and its fibrous structure can promote mass transfer and adsorption of U(VI). Therefore, compared with bulk MQ₂, composite materials exhibit better U(VI) adsorption performance. We take the MoS₂-CF as the representative to study the removal performance of U(VI) in detail. The results show that MoS₂-CF has the saturated U(VI) adsorption capacity of 301 mg g⁻¹, and can selectively remove uranium in the presence of a large amount of alkali and alkali earth metal ions and even in actual water samples. At the same time, the material also shows recycle ability for U(VI) adsorption. The mechanism of uranium adsorption is revealed through various characterizations including Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectra (XPS), and elemental distribution mappings. The efficient uranium adsorption originates from the interaction of amino and carboxyl groups on CF and sulfur on the surface of MoS₂ for U(VI). Overall, our compositing strategy is universal for low-cost and eco-friendly composite materials combining transition metal dichalcogenide with collagen fiber by co-ball milling. The prepared MQ₂-CF composite materials have excellent application potential for highly efficient capture of U(VI) from wastewater.

RESULTS AND DISCUSSION

Characterizations of MoS₂-CF

The composite material (MoS₂-CF) of MoS₂ and CF is constructed using green methods, in which water is the processing solvent, natural polymers (collagen fibers) are as carriers/adsorption enhancers and MoS₂ acts as the adsorption unit. In brief, CF, MoS₂, and solvent water are placed in a ball milling tank for ball milling, and then the ball milling solution is freeze-dried to obtain composite materials (Figure 1). The final adsorbent material is fibrous with an average diameter at the nanoscale. The morphology of MoS₂ that synthesized based on literature can be seen from SEM images as nanospheres formed by stacking nanosheets (Figure 2A). From SEM, it can be seen that CF presents a smooth surface (Figure 2B), while elemental distribution mapping and EDS show that it contains C, H, and N (Figure S1). After compositing, uniform molybdenum sulfide nanoparticles appear on the surface of CF (Figure 2C). The corresponding EDS and elemental distribution mapping images of MoS₂-CF display the presence of Mo and S elements originating from MoS₂, confirming the successful loading of MoS₂ on CF (Figures 2D and S1).

Figure 1 (A) Illustration of self-assembly preparation of MoS2-CF. (B) Photos of MoS2, CF, and MoS2-CF; the enlarged image is the SEM of MoS2-CF.

Figure 2 Characterization of MoS2-CF. SEM of MoS2 (A), CF (B) and MoS2-CF (C). (D) Elemental distribution mapping of MoS2-CF. (E) XPS of CF, MoS2, and MoS2-CF. (F) Water contact angle of CF, MoS2, and MoS2-CF. (G) Zeta potential of CF, MoS2, and MoS2-CF after ball milling.

To gain a deeper understanding of the composite, the adsorbent is further characterized. SEM, elemental distribution mapping, and EDS confirmed the successful synthesis of MoS₂ (Figure S2). The PXRD diffraction peaks and Raman spectroscopic analysis of obtained MoS₂ are consistent with 1T phase of MoS₂ reported in the literature [42,43] (Figure S2). At the same time, after calcination, it can transform into 2H phase MoS₂ (Figure S2d). The PXRD pattern of MoS₂-CF confirms MoS₂ successfully loaded on CF (Figure S3a). Importantly, the characteristic peak position of MoS₂ remains unchanged after compositing, indicating that the structure of MoS₂ is not affected throughout the entire preparation process [44]. FTIR of various materials can provide a deeper understanding of the surface chemical groups present on the materials (Figure S3b). The characteristic peaks of CF are as follows: a broad absorption peak can be seen at 3100–3650 cm⁻¹, which corresponds to the vibrational absorption of O–H and N–H. Vibration absorption of C–H in CF can be observed at 2950 cm⁻¹ and 2880 cm⁻¹. The absorption peaks at 1650 cm⁻¹ and 1546 cm⁻¹ correspond to the C=O vibrational absorption from amides and carboxyl groups, respectively [45,46]. Meanwhile, these corresponding peaks are also observed in the composite material without significant changes. Additionally, XPS of the composite material shows the peaks of S 2p (161.5 eV), Mo 3d_3/2 (230.6 eV), and Mo 3d_5/2 (226.9 eV) (Figure 2E), further confirming the successful loading of MoS₂ onto CF. Compared with the pristine MoS₂, we observed the significant enhancement in the hydrophilicity of the composite material, as evidenced by water contact angle measurements, which is beneficial to the adsorption of U(VI) by MoS₂-CF [33] (Figure 2F). Moreover, FT-IR and PXRD of MoS₂-CF after soaking solutions with different pH values (pH 1–12) do not show significant changes compared to the untreated material, indicating that MoS₂-CF exhibits excellent acid-base stability [7] (Figure S4).

Zeta potential measurements shed light on the compositing mechanism. After ball milling, the single CF exhibits positive Zeta potential (1.0 mV), while MoS₂ shows negative Zeta potential (−30.6 mV) (Figure 2G). The electrostatic interaction between them facilitates the self-assembly of MoS₂ and CF into composite material. The negative Zeta potential of MoS₂-CF (−28.7 mV) suggests that it can enhance the adsorption of U (VI) through Coulomb interaction [12].

Adsorption of uranium

Effect of composite ratio

In the prepared transition metal dichalcogenides and CF composite materials, we chose MoS₂-CF as the representative to conduct a detailed study on the U adsorption performance. Considering the influence of ball milling experimental conditions on the uranium adsorption performance of composite materials, we investigated the effects of zirconium bead diameter, mass, and ball milling time on the uranium adsorption performance of composite materials. It was found that, the composite material prepared with 60 mg of 2 mm diameter zirconium beads had the highest uranium adsorption performance at a ball milling time of 16 hours, reaching 85.3 mg g⁻¹ (Figure S5). Therefore, such ball milling conditions were chosen for the preparation of composite materials. The ratio of components in a composite material significantly influences its adsorption capabilities, prompting us to investigate how the ratio of MoS₂ and CF affects U(VI) adsorption (E). MoS₂ and CF exhibit relatively low adsorption capacities for U(VI), at 12.6 mg g⁻¹ and 34.8 mg g⁻¹, respectively. However, the U(VI) adsorption capacity of MoS₂-CF with the different ratio of MoS₂ and CF (ranging from 4:1 to 1:8) surpasses that of the pristine MoS₂ and CF, which may be derived from the improved hydrophilicity of MoS₂-CF and the accelerated mass transfer process of the fibrous sample of composite. These findings validate the effectiveness of our composite strategy in improving the adsorption capabilities of transition metal chalcogenides. As the proportion of MoS₂ within the composite material increases, so does the number of available adsorption sites provided by MoS₂ for U(VI). This increase in available sites is instrumental in enhancing the adsorption capacity of the composite material for U(VI), resulting in a gradual rise in U(VI) removal capacity. Notably, when the ratio of MoS₂ to CF is 1:5, the adsorption capacity reaches its maximum at 85.3 mg g⁻¹, which is 6.7 times greater than pristine MoS₂. Then, an increase in MoS₂ proportion results in a decrease in U(VI) adsorption capacity of the MoS₂-CF composite. This decline may stem from an excess of MoS₂, which obscures adsorption sites, thereby reducing the overall adsorption capacity. Taking the material cost and adsorption capacity into account, we ultimately selected a composite material with a ratio of 1:5 between MoS₂ and CF for subsequent experiments.

Effect of pH value

As is well known, pH value can significantly impact the speciation of uranium, which in turn affects its adsorption by various adsorbents [47,48]. Consequently, it is crucial to examining the adsorption behavior of U(VI) under different pH conditions. At low pH (pH < 5.0), UO₂²⁺ ions occupy the dominant position. However, at higher pH values, uranium is less likely to remain as uranyl cations and may instead form hydrolytic precipitates [25,39]. With this in mind, we focused our investigation on the adsorption of UO₂²⁺ at pH values less than 5. At a pH of 1, the adsorption capacity of MoS₂-CF is only 3.33 mg g⁻¹, which may be due to a large amount of H⁺ competing with U(VI) for adsorption sites in the solution. Subsequently, as the pH increases, the number of hydrogen ions competing with U(VI) for adsorption sites in the solution decreases, leading to the removal capacity of MoS₂-CF for U(VI) gradually increases, reaching 83.5 mg g⁻¹ at pH of 5 (Figure 3B). Therefore, pH of 5 was selected for subsequent experiments.

Figure 3 Adsorption of MoS2-CF for U(VI). (A) The effect of composite ratio of MoS2 and CF on U(VI) adsorption (C0U = 150 ppm, pH = 5, t = 12 h, m/V = 1 g L−1). (B) The pH effect on adsorption of MoS2-CF (C0U = 200 ppm, t = 12 h, m/V = 1 g L−1). (C) Adsorption kinetics simulation (C0U = 150 ppm, pH = 5, m/V = 1 g L−1). (D) Simulation of adsorption isotherms (C0U = 25–550 ppm, pH = 5, t= 12 h, m/V = 1 g L−1). (E) The effect of different molar ratios coexisting Na+, K+, Mg2+, and Ca2+ on U(VI) adsorption of MoS2-CF (C0U = 10 ppm, pH = 5, t = 12 h, m/V = 1 g L−1). (F) U(VI) adsorption of MoS2-CF in actual water samples (Taken from Fuzhou, Fujian) (C0U = 150 ppm, pH = 5, t = 12 h, m/V = 1 g L−1).

Effect of time

We explored the effect of adsorption time on the capture of U(VI). U(VI) in the solution was rapidly adsorbed by MoS₂-CF within the first 2 hours. As the adsorption process continues, available sites on MoS₂-CF are gradually occupied, resulting in a decelerating adsorption rate and ultimately reaching adsorption equilibrium at 12 hours with the adsorption capacity of MoS₂-CF for U(VI) being 95.7 mg g⁻¹ (Figure 3C) [48]. Therefore, we chose 12 hours as the subsequent adsorption time. Fitting the adsorption kinetics curve using different models (Table S1), we found that the R² of pseudo-second-order kinetic model (R² = 0.985) was higher than pseudo-first-order kinetic model (R² = 0.945) and intra-particle diffusion model (R² = 0.576) (Table S1), which indicating the adsorption process of MoS₂-CF is more in line with pseudo-second-order kinetic model. This result indicates that the adsorption process is chemical adsorption.

Effect of U concentration

In order to obtain the saturated adsorption capacity of MoS₂-CF, we systematically measured the adsorption isotherms of adsorbents through sequencing batch experiments (Figure 3D). The fitting of the adsorption isotherm curve revealed that it aligns more closely with the Langmuir adsorption isotherm model (R² = 0.997) compared to Freundlich adsorption isotherm model (R² = 0.972) (Table S2), indicating the adsorption process is a single-layer adsorption. The saturated adsorption capacity of MoS₂-CF for U(VI) was calculated to be 301.1 mg g⁻¹ by Langmuir adsorption isotherm model (Figure 3D), which is much higher than the reported 2H-MoS₂ (45.7 mg g⁻¹) and sodium dodecyl sulfate modified molybdenum disulfide composites (98.4 mg g⁻¹)[18,35]. This indicated that MoS₂-CF possesses superior U(VI) adsorption performance, which validates the effectiveness of our composite strategy in enhancing the adsorption capacity for U(VI).

Effect of competitive ions and adsorption in actual water samples

In actual water environments, the presence of competing ions can interfere with the adsorption performance of materials. Therefore, it is crucial to explore the adsorption selectivity in the presence of other coexisting ions [7]. Our studies reveal that MoS₂-CF maintains adsorption selectivity even under with high concentrations of monovalent ions including Na⁺ and K⁺. Even in the presence of individual Na⁺ ions at a 1000-fold molar excess to U, the distribution ratio (K_d^U) of MoS₂-CF can remain at 2.8 × 10³ mg L⁻¹ (Figure S6). Similarly, MoS₂-CF maintains high adsorption performance in the presence of high concentrations of individual K⁺ ions with the K_d^U of 2.3 × 10³ mg L⁻¹ with K⁺ ions at the 1000-fold molar excess to U (Figure S7). Under the coexisting divalent Mg²⁺ and Ca²⁺ ions, MoS₂-CF also maintains high adsorption performance. Under 1000-fold molar excess of Mg²⁺ ions, the K_d^U of the MoS₂-CF can still be as high as 3.5 × 10³ mg L⁻¹ (Figure S8), while the K_d^U of MoS₂-CF can remain at 2.3 × 10³ mg L⁻¹ in the condition of Ca/U molar ratio more than 1000 times (Figure S9). These results indicate that MoS₂-CF has excellent resistance to interference from Na⁺, K⁺, Mg²⁺ and Ca²⁺. Furthermore, when 10-fold excess of Na⁺, K⁺, Mg²⁺, and Ca²⁺ are present simultaneously, the K_d^U of MoS₂-CF can still maintain 3.7 × 10³ mg L⁻¹. This demonstrates that the composite material possesses excellent selectivity [49] (Figure 3E). Adsorption experiments on labeled U(VI) in tap water, lake water, and seawater confirm that MoS₂-CF can effectively capture U(VI) in actual water environment (Figure 3F). The adsorption performance of MoS₂-CF in actual water samples is sea water > lake water > tap water. Notably, even in seawater, which contains complex competitive ions, the adsorption capacity of MoS₂-CF for U(VI) still reaches 112.2 mg g⁻¹, likely due to the highest pH of seawater.

Recycle performance of MoS₂-CF

The recycling performance of the material is also crucial. After MoS₂-CF has completed U(VI) adsorption (MoS₂-CF-U), the adsorbed U(VI) can be removed by soaking the MoS₂-CF-U in 0.1 M sodium carbonate solution, and then the desorbed composite material can be reused again for U(VI) adsorption. In the first cycle, the uranium adsorption capacity for U(VI)of MoS₂-CF is 81.2 mg g⁻¹. After the five cycles, there was no significant change in the adsorption performance for U(VI) with the adsorption capacity being 79.4 mg g⁻¹, indicating MoS₂-CF has good cycling performance (Figure S10a). Moreover, no significant changes in the absorption peak were found in the FT-IR after cycling, indicating that the composite material has good recycling stability (Figure S10b).

Adsorption mechanism

Studying the adsorption mechanism of MoS₂-CF for U(VI) is crucial for understanding the adsorption behavior of U(VI) and providing guidance for the preparation of more efficient adsorption materials. From the SEM images of MoS₂-CF-U, it is found that there is no significant change in the surface morphology before and after U(VI) adsorption, suggesting the good stability of MoS₂-CF (Figure 4A). The PXRD of MoS₂-CF and MoS₂-CF-U are consistent (Figure S11a). The presence of U is confirmed by elemental mapping and EDS analysis, indicating successfully U(VI) adsorption by MoS₂-CF (Figure S11b) [50]. XPS analysis provides further evidence for U(VI) adsorption. The peak at 381.8 eV in XPS of MoS₂-CF-U is attributed to U 4f_7/2, confirming the presence of U (Figure 4B and 4C). High-resolution XPS spectra of N 1s, O 1s and S 2p measured for MoS₂-CF-U are presented in Figure 4. The N 1s peak for MoS₂-CF observed at 399.33 eV shifts to 399.57 eV for MoS₂-CF-U, indicating that N coordinates with U (Figure 4E) [7,13]. Similarly, after completing the adsorption of U(VI), the O 1s peaks shifts from 530.75 eV (C–O) and 530.99 eV (C=O) to 531.20 eV and 531.43 eV, respectively, suggesting coordination between carboxyl groups and U (Figure 4F) [46,51]. In addition, we found that the S 2p peaks of MoS₂-CF observed at 160.66 eV, 161.84 eV, 162.11 eV, and 163.26 eV belonging to S–H 2 p_1/2 and S–H 2 p_3/2 and Mo–S 2 p_1/2 and Mo–S 2 p_3/2, respectively. These peaks of MoS₂-CF-U shifted to 160.81 eV, 161.96 eV, 162.30 eV, and 163.45 eV, respectively (Figure 4D). This indicates that the abundant S atoms from the surface of MoS₂ also participate in the interaction for uranium [7]. FT-IR further supports successful uranium adsorption (Figure 4G), with a characteristic UO₂²⁺ absorption peak appearing at 912 cm⁻¹ [13,51]. Additionally, the carboxyl group’s vibration absorption peak shifts from 1546 cm⁻¹ to 1535 cm⁻¹ after UO₂²⁺ absorption, consistent with literature reports of such shifts upon coordination with uranium [13,51]. Based on these findings, we think that the abundant sulfur sites on the surface of MoS₂ and the amino and carboxyl groups from CF work synergistically to coordination with uranium, jointly completing the capture of uranium from aqueous solution.

Figure 4 Characterization of MoS2-CF after U(VI) adsorption. (A) SEM and elemental distribution mappings of MoS2-CF-U. (B) XPS of MoS2-CF and MoS2-CF-U. (C) High-resolution XPS spectra of U 4f of MoS2-CF-U. (D) High-resolution XPS spectra of S 2p of MoS2-CF and MoS2-CF-U. (E) High-resolution XPS spectra of N 1s of MoS2-CF and MoS2-CF-U. (F) High-resolution XPS spectra of O 1s of MoS2-CF and MoS2-CF-U. (G) FT-IR of MoS2-CF and MoS2-CF-U.

Verification of strategy universality

To demonstrate the versatility of our compositing strategy, we selected commercially available transition metal dichalcogenides (MoSe₂, WS₂, and WSe₂) as validation materials for the synthesis of other composite materials and applied them to uranium adsorption. Given MoS₂-CF’s optimal performance at a 1:5 ratio, we applied the same ratio between transition metal dichalcogenides and CF for the synthesis of MoSe₂-CF, WS₂-CF, and WSe₂-CF. The photo of the MoSe₂-CF, WS₂-CF, and WSe₂-CF shows that their color closely resembles their pristine dichalcogenide counterparts (Figure S12). Comprehensive characterizations (including SEM, Mapping, EDS, PXRD, FT-IR, and XPS) of MoSe₂, WS₂, WSe₂, MoSe₂-CF, WS₂-CF, and WSe₂-CF (Figures S13–19) confirm the successful fabrication of the composites. Zeta-potential of MoSe₂, WS₂, WSe₂ MoSe₂-CF, WS₂-CF, and WSe₂-CF were −11.9, −34.0, −24.8, −18.6, −21.6, and −34.4 eV, respectively, suggesting that the assembly mechanism of MoSe₂-CF, WS₂-CF, and WSe₂-CF are electrostatic self-assembly, and their surface negative potential is advantageous for U(VI) adsorption (Figure 5A). Meanwhile, we anticipate that the hydrophilic properties and fibrous structure of the composite materials will enhance their U(VI) adsorption capabilities compared to the pristine dichalcogenides (Figure 5B). Adsorption experiments on these composite materials confirmed that their U(VI) removal capacity exceeds that of the unmodified dichalcogenides, highlighting the improved adsorption performance after compositing (Figure 5C). After the U(VI) adsorption, SEM image of the MoSe₂-CF, WS₂-CF, and WSe₂-CF showed no significant change in the morphology (Figure 5D, 5E, and 5F), but also the appearance of uranium elements in EDS and elemental mapping indicated the successful adsorption of uranium (Figure S20). XPS analysis of the MoSe₂-CF-U (380.9, and 391.8 eV), WS₂-CF-U (381.2, and 392.0 eV), and WSe₂-CF-U (380.7, and 391.5 eV) also showed U 4f absorption peaks, further confirming U(VI) capture (Figure 5G). Their FT-IR also display uranyl absorption peaks around 916 cm⁻¹ (Figure 5H), providing additional evidence of successful U(VI) adsorption [7,13,51]. Therefore, our experiments demonstrate that the compositing strategy is universal and can be effectively applied to prepare a series of MQ₂-CF (M = Mo, W; Q = S, Se) composite materials with enhanced uranium adsorption performance.

Figure 5 Characterization of U(VI) adsorption by MoSe2-CF, WS2-CF, and WSe2-CF. (A) Zeta potential of MoSe2, WS2, WSe2, MoSe2-CF, WS2-CF, and WSe2-CF after ball milling. (B) Water contact angle of MoSe2-CF, WS2-CF, and WSe2-CF. (C) Adsorption of uranium by MoSe2, WS2, WSe2, MoSe2-CF, WS2-CF (C0U = 150 ppm, pH = 5, t = 12 h, m/V = 1 g L−1). SEM and elemental distribution mappings of MoSe2-CF-U (D), WS2-CF-U (E) and WSe2-CF-U (F). (G) XPS of MoSe2-CF-U, WS2-CF-U, and WSe2-CF-U. (H) FT-IR of MoSe2-CF-U, WS2-CF-U, and WSe2-CF-U.

CONCLUSIONS

In this study, we propose a universal compositing strategy for the preparation of transition metal dichalcogenides composites with CF, resulting in a series of MQ₂-collagen fiber (M = Mo, W; Q = S, Se) composite materials designed for efficient uranium adsorption. The composite materials exhibit enhanced hydrophilicity and leverage the fibrous structure of CF to improve uranium adsorption, outperforming the pristine transition metal dichalcogenides. Zeta potential analysis illuminated the self-assembly mechanism between transition metal dichalcogenides and CF, which occurs through electrostatic interactions. Characterization techniques including SEM, EDS, elemental mapping, FT-IR, and XPS revealed the U(VI) adsorption mechanism of the composite materials. The findings suggest that sulfur sites on the surface of MoS₂, along with amino and carboxyl groups from CF, work in synergy to coordinate and capture uranium from aqueous solutions. Our study proposes a straightforward method for fabricating transition metal dichalcogenides composite materials, offering a significant boost to U(VI) adsorption performance, and paves the way for the reuse of leather industry waste as functional materials.

MATERIALS AND METHODS

Materials

All chemicals are commercially available products with analytical grade and used without purification. All solutions were prepared using Milli-Q water (18.2 MΩ cm). MoS₂ was synthesized based on literature. CF are the rough product from the leather factory (Shenzhen Yongshengye Textile Products Trade Co., Ltd.). (UO₂)(NO₃)₂⋅6H₂O (99%, Hubei Jusheng Technology Co., Ltd.), NaCl (AR, Sinopharm Chemical Reagent Co., Ltd), MgCl₂ (99%, Adamas Reagent Co., Ltd), KCl (AR, General-Reagent), CaCl₂⋅6H₂O (98%, Adamas Reagent Co., Ltd), Na₂CO₃ (Shanghai Hongguang Industry Co., Ltd), 1-butyl-3-methylimidazolium thiocyanate ([BMIM]SCN) (98%, Adamas Reagent Co., Ltd), (NH₄)₆Mo₇O₂₄⋅4H₂O (98%, Adamas Reagent Co., Ltd), MoSe₂ (99.99%, Adamas Reagent Co., Ltd), WS₂ (99.9%, Adamas Reagent Co., Ltd), and WSe₂ (99.999%, Adamas Reagent Co., Ltd). MoSe₂, WS₂, and WSe₂ are 2-H phase as verified by powder X-ray diffraction (PXRD) (Figure S27).

Preparation of MoS₂ and MQ₂-CF composite materials (M = Mo, W; Q = S, Se)

MoS₂ was synthesized based on literature [42]. First, 3.50 mmol [BMIM]SCN, 0.05 mmol (NH₄)₆Mo₇O₂₄·4H₂O, and 100 μL HCl were dissolved in 10 mL deionized water under vigorous stirring for 30 min. The obtained homogeneous solution was transferred to a 25 mL Teflon-lined autoclave. Then, the autoclave was sealed and heated at 200 °C for 24 h. The obtained black powder sample was washed with deionized water and ethanol for several times, and then dried in a vacuum at 65 °C for 6 h. The black powder sample is 1T phase of MoS₂ as proved through scanning electron microscopy (SEM), elemental distribution mappings, energy dispersive spectroscopy (EDS), Raman spectroscopy, and PXRD (Figure S2). The synthesized 1T phase of MoS₂ after calcination at 400 °C last for 4 h at N₂ atmosphere can transform into 2H phase of MoS₂ (Figure S2d).

All MQ₂-CF composite materials (M = Mo, W; Q = S, Se) are prepared according to the following steps. In a typical synthesis, 1T phase of MoS₂ and CF (a total of 0.2 g) were placed into PTFE ball mill, then added with 2 mL of water and 60 g of zirconium beads with a diameter of 2 millimeters. The ball milling program was 450 r/min and lasted for 16 hours. Then the products were freeze-dried for 24 hours to obtain the MoS₂-CF. MoSe₂-CF, WS₂-CF, and WSe₂-CF were prepared through the same method. To demonstrate the assembly mechanism of composite materials, 0.2 g MQ₂ (M = Mo, W; Q = S, Se), CF and the mixture of MQ₂ and CF were ball milled separately with 2 mL water. The Zeta potential of these samples after ball milling was measured.

Characterization

The surface morphology, EDS, and elemental distribution mappings were characterized by scanning electron microscopy (JEOL JSM-6700F). Fourier transform infrared spectroscopy (FTIR) was conducted in the 400–4000 cm⁻¹ range by using ATR on a Bruker VERTEX 70 FTIR spectrophotometer. Powder X-ray diffraction (PXRD) patterns were recorded by Rigaku Miniflex-II diffractometer at room temperature. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Fisher ESCALAB 250Xi XPS Spectrometer system. Inductively coupled plasma-mass spectrometry (ICP-MS) was performed on an XSeries II and inductively coupled plasma-optical emission spectrometry (ICP-OES) was studied with a Thermo 7400. The pH values of all solutions were tested by Shanghai Leich E-201F. Contact angle images are recorded on a Zhongchen JC2000DS contact angle meter equipped with a CCD camera. Zeta potential was measured by Zetasizer Nano ZS90 DLS spectrometer (Malvern Inc., UK).

Batch adsorption experiments

In a typical adsorption experiment, composite material was transferred to a 20 mL glass bottle containing aqueous solution of UO₂(NO₃)₂⋅6H₂O (m/V = 1 g L⁻¹). The adsorption efficiency (E), adsorption capacity (q_e), and distribution ratio (K_d) can be calculated by equations (1), (2), and (3) in supporting information, respectively. The pseudo-first-order kinetic model, pseudo-second-order kinetic model, and intra-particle diffusion model can be calculated by equations (4), (5), and (6) in supporting information, respectively. The Langmuir and Freundlich isotherm adsorption models can be calculated by equations (7) and (8) in supporting information, respectively. The mixture was shaken in a shaker for 12 h at 25 °C and then centrifuged for separation. The composite material after U(VI) adsorption (MoS₂-CF-U) was obtained. The supernatant was filtered and diluted for ICP-MS or ICP-OES testing and the solid was washed several times with deionized water drying. The effect of composite ratio, pH, time, U(VI) concentration, coexisting ions, and actual water sample on adsorption were investigated in the study. Moreover, the reuse experiment of MoS₂-CF was also researched. The detailed adsorption experiments were shown in SI. At the same time, two parallel experiments were carried out in each experiment to ensure the repeatability of the experiments.

Data availability

The original data are available from corresponding authors upon reasonable request.

Acknowledgments

We are grateful for the technical support from the Comprehensive Training Platform of the Specialized Laboratory (College of Chemistry, Sichuan University) and Analytical & Testing Center of Sichuan University.

Funding

This work was supported by the National Natural Science Foundation of China (U21A20296, 22376149, 22325605, 22176136, U1867205 and 22076185), and the Key Research and Development Program of Sichuan Province, China (2020YFS0070 and 2021YFH0170).

Author contributions

C.L.: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing - Original draft, Visualization. L.P.L.: Formal analysis, Methodology, Writing - Review & Editing. J.H.H.: Methodology, Investigation. L.Y.: Methodology. J.T.L.: Methodology. C.Q.X.: Supervision, Project administration, Funding acquisition, Investigation. X.Y.H.: Conceptualization, Supervision, Writing - Review & Editing, Project administration, Funding acquisition. M.L.F.: Conceptualization, Supervision, Writing - Review & Editing, Project administration, Funding acquisition, Resources. All authors have given approval to the final version of the manuscript.

Conflict of interest

The authors declare no conflict of interest.

Supplementary information

Supplementary files provided by the authors. Access here

The supporting information is available online at https://doi.org/10.1360/nso/20240029. The supporting materials are published as submitted, without typesetting or editing. The responsibility for scientific accuracy and content remains entirely with the authors.

References

All Figures

	Figure 1 (A) Illustration of self-assembly preparation of MoS2-CF. (B) Photos of MoS2, CF, and MoS2-CF; the enlarged image is the SEM of MoS2-CF.
In the text

	Figure 2 Characterization of MoS2-CF. SEM of MoS2 (A), CF (B) and MoS2-CF (C). (D) Elemental distribution mapping of MoS2-CF. (E) XPS of CF, MoS2, and MoS2-CF. (F) Water contact angle of CF, MoS2, and MoS2-CF. (G) Zeta potential of CF, MoS2, and MoS2-CF after ball milling.
In the text

Figure 3 Adsorption of MoS2-CF for U(VI). (A) The effect of composite ratio of MoS2 and CF on U(VI) adsorption (C0U = 150 ppm, pH = 5, t = 12 h, m/V = 1 g L−1). (B) The pH effect on adsorption of MoS2-CF (C0U = 200 ppm, t = 12 h, m/V = 1 g L−1). (C) Adsorption kinetics simulation (C0U = 150 ppm, pH = 5, m/V = 1 g L−1). (D) Simulation of adsorption isotherms (C0U = 25–550 ppm, pH = 5, t= 12 h, m/V = 1 g L−1). (E) The effect of different molar ratios coexisting Na+, K+, Mg2+, and Ca2+ on U(VI) adsorption of MoS2-CF (C0U = 10 ppm, pH = 5, t = 12 h, m/V = 1 g L−1). (F) U(VI) adsorption of MoS2-CF in actual water samples (Taken from Fuzhou, Fujian) (C0U = 150 ppm, pH = 5, t = 12 h, m/V = 1 g L−1).

In the text

Figure 4 Characterization of MoS2-CF after U(VI) adsorption. (A) SEM and elemental distribution mappings of MoS2-CF-U. (B) XPS of MoS2-CF and MoS2-CF-U. (C) High-resolution XPS spectra of U 4f of MoS2-CF-U. (D) High-resolution XPS spectra of S 2p of MoS2-CF and MoS2-CF-U. (E) High-resolution XPS spectra of N 1s of MoS2-CF and MoS2-CF-U. (F) High-resolution XPS spectra of O 1s of MoS2-CF and MoS2-CF-U. (G) FT-IR of MoS2-CF and MoS2-CF-U.

In the text

Figure 5 Characterization of U(VI) adsorption by MoSe2-CF, WS2-CF, and WSe2-CF. (A) Zeta potential of MoSe2, WS2, WSe2, MoSe2-CF, WS2-CF, and WSe2-CF after ball milling. (B) Water contact angle of MoSe2-CF, WS2-CF, and WSe2-CF. (C) Adsorption of uranium by MoSe2, WS2, WSe2, MoSe2-CF, WS2-CF (C0U = 150 ppm, pH = 5, t = 12 h, m/V = 1 g L−1). SEM and elemental distribution mappings of MoSe2-CF-U (D), WS2-CF-U (E) and WSe2-CF-U (F). (G) XPS of MoSe2-CF-U, WS2-CF-U, and WSe2-CF-U. (H) FT-IR of MoSe2-CF-U, WS2-CF-U, and WSe2-CF-U.

In the text