Controllable synthesis and adsorption mechanism of flower-like MoS2/g-C3N4 nanocomposites for the removal of methylene blue in water

The treatment of dye-bearing wastes by adsorption using low-cost synthetic adsorbents is urgently needed and challenging because it offers the dual benefit of water treatment and waste management. Herein, the flower-like MoS2/g-C3N4 nanocomposites with a size range of 200–400 nm and a specific surface area of 70.656 m2/g were synthesized as new methylene blue (MB) adsorbent materials. Their adsorption mechanisms and influences of pH, adsorbent dosage, concentration, and temperature on the adsorption properties were investigated by batch sorption experiments. Adsorption isotherm data were fitted with the Langmuir model, and the adsorption kinetic characteristics conform to the quasi second-order kinetic equation. According to thermodynamic data, methylene blue (MB) adsorption was endothermic and spontaneous, exhibiting a maximum adsorption capacity of 278.4 mg/g at 45 °C. Additionally, the impact of MB effluent on the germination of chickpea and wheat was studied contrastively with treatment of the tested materials. The results indicated that adequate adsorption of flower-like MoS2/g-C3N4 nanocomposite could potentially treat wastewater from the textile industry to be used for irrigation and solve the problem of textile effluent disposal.


Introduction
Global population growth and industrial development have caused an increasing demand for textiles and clothing, which has resulted in the continual discharge of harmful pollutants into waterways, posing severe environmental problems (Pan et al. 2016). According to statistics, about 80% of the wastewater generated by textile, leather, and paper industry every year is discharged into the environment without any treatment, which has caused serious environmental damage and has become a serious hidden danger to the safety of water supply around the world. Especially, that has become a major problem that threatens the quality of drinking water supplies across the globe (Carolin et al. 2017;Katheresan et al. 2018;Oladipo and Gazi 2014).
Organic dye is a major source of wastewater from the dye and dye intermediate production industry, which is composed of mother liquor of various products and intermediate crystallization, materials lost in the production process, and sewage washed out into the ground. Therefore, dyes, especially organic dyes, are often present in high concentrations in these wastewater streams. Biological toxicity is a common characteristic of most organic dyes, which may harm or even kill aquatic organisms. Presently, printing and dyeing wastewater can be treated using photocatalytic degradation (Paul et al. 2019a), adsorption (Fronczak 2020), biological treatment , filtration (Zanacic et al. 2016), and membrane technology (Farhat et al. 2016). Adsorption, with its lowcost, high efficiency, simplicity, and lack of secondary pollution, has gradually become an indispensable method for treating the wastewater in printing and dyeing industries (Gao et al. 2016;Wang et al. 2019). Treatment of dye wastewater by adsorption is greatly influenced by the structure and surface morphology of the adsorbent, so the selection of adsorbents with exceptional adsorption performance is imperative to achieving efficiency in dye wastewater treatment.
The preparation and modification of adsorption materials, especially carbon materials, have received a great deal of attention. It is the most widely used adsorbent due to its large surface area and good adsorption performance (Chen et al. 2017a;Manilo et al. 2016;Murray and Ormeci 2018). Throughout the years, increasing consideration has been given to cheap and readily available materials with a large specific surface area, unique structural properties, and exceptional chemical stability. Graphite phase carbon nitride is a new type of non-metallic graphite semiconductor material, which has the advantages of good stability, metal-free "earth-abundant nature," and many adsorption sites (Wu et al. 2018;Basharnavaz et al. 2020). It showed excellent application prospects in the field of adsorption (Hu et al. 2015;Zhang et al. 2016). However, the bulk g-C 3 N 4 synthesized directly from organic precursors often has a smaller specific surface area (less than 20 m 2 · g −1 ) and meager mass diffusion transfer rate. From a practical perspective, particularly in adsorption, establishing of controlled porosity and morphology at the nanoscale in bulk g-C 3 N 4 is essential to maximize its efficiency (Iqbal et al. 2018). In 2013, Galen D. Stucky and co-workers synthesized carbon nitride mesoporous hollow spheres by changing the precursor materials, which greatly improved the specific surface area and adsorption sites (Jun et al. 2013). Transition metal sulfides like MoS 2 form two-dimensional layered structures with each layer of molybdenum atoms sandwiched between two layers of sulfur atoms. MoS 2 possesses high thermochemical stability; a large specific surface area with excessive sulfides (S 2-) can adsorb various heavy metals and organic pollutants through electrostatic, hydrophobic, or chemical complex interactions Liu et al. 2019). For instance, Zhang et al. successfully embedded MoS 2 into the crosslinked chitosan to recover Au(III) through electrostatic attraction and coordination (Zhao et al. 2020a). Ibrahim M. Alarifi and colleagues synthesized and used MoS 2 as an adsorbent to adsorb Congo red to achieve the maximum adsorption capacity of 80.64 mg/g at 50 °C (Alarifi et al. 2021). Nevertheless, it is difficult to use pure MoS 2 as an adsorbent because it is easily agglomerated, limiting its active sites, thereby decreasing its performance. Designing and constructing special architectures are an important strategy for exposing more surface active sites for adsorption. It is well established that two-dimensional (2D) materials can be constructed by exfoliation, chemical vapor deposition, and synthetic wet chemistry methods (Tan et al. 2017). There is, however, a big challenge involved in the large-scale preparation of nanosheets, since nanosheets restack and reunite to reduce the active surface sites (Mi et al. 2015). As a result of the above-mentioned problem, a 3D flower-like heterostructure construction composed of 1 3 Vol.: (0123456789) 2D nanosheets is a particularly important for adsorption applications since it not only enhances structural stability but also maximizes the exposure of surface active sites. Herein, based on the structural similarity, we assembled flower-like g-C 3 N 4 as a supporting matrix material for MoS 2 to prepare MoS 2 /g-C 3 N 4 nanocomposites to combine their excellent properties for the adsorption of methylene blue effluents.
Methylene blue (MB) has been widely used for decades as a common organic dye and is extensively used in industrial production, biological, and scientific research. Studying the adsorption of adsorbent on MB waste liquid can provide an economical and effective method for water pollution treatment. As a means of achieving a novel adsorbent with good adsorption performance on methylene blue, we synthesized flower-like MoS 2 /g-C 3 N 4 nanocomposites via hydrothermal reaction. According to the experimental results, the flower-like MoS 2 /g-C 3 N 4 nanocomposites can adsorb MB strongly, especially at 45 °C, where a maximum adsorption capacity of 278.4 mg/g was observed. This adsorption capacity is better than many adsorbents reported previously. In addition, the adsorption of MB on the nanocomposites has good recyclability as well as good stability after five cycles. We studied the factors affecting MB adsorption (pH, adsorbent dose, and original MB concentration), the adsorption isotherm model, and adsorption kinetics and thermodynamics. To demonstrate the applicability of the nanocomposites in wastewater treatment, MB-containing water treated with the nanocomposites is used to water the seeds of wheat and chickpea plants to examine their effects on plant growth.

Materials
Melamine, cyanuric acid, and sodium molybdate dihydrate were purchased from Aladdin Chemical Reagent Co., Ltd (Shanghai, China). Dimethyl sulfoxide and thiourea were procured from Guangdong Guanghua Sci-Tech Co., Ltd (Guangdong, China). While polyvinyl pyrrolidone was bought from Sigma-Aldrich (The USA). All chemicals used here were of analytical grade and without further purification. Deionized (DI) water was used in all the experiments.
Preparation of g-C 3 N 4 hollow sphere The nanocomposite was synthesized based on Refs Jun et al. (2013); Zhao et al. (2020b). Firstly, 1 g of melamine was added to 50 ml dimethyl sulfoxide (DMSO) and 1 g of cyanuric acid to 25 ml DMSO, stirred for 30 min to form a solution, and labeled it as solution A and B. Then, we added solution B into solution A dropwise and continued stirring for 30 min to form a white precipitate. After the reaction, the supernatant was discarded, and the precipitate washed, centrifuged, and dried at 50 °C for 6 h. Finally, the resultant solid grind in agate and mortar was labeled as MCA and calcined at 550 °C for 3 h in muffle furnace to obtain spherical g-C 3 N 4 .
Preparation of MoS 2 /g-C 3 N 4 nanocomposites First, 0.29 g of sodium molybdate dihydrate (Na 2 MoO 4 · 2H 2 O) was added to 40 mL water and formed a solution. Subsequently, 0.005 g of polyvinyl pyrrolidone (PVP) was added to the solution and magnetically stirred for 10 min. After that, hydrochloric acid was added to adjust the solution pH to 1. Then, 1.44 g of thiourea was added with the help of ultrasonic dispersion to form a solution. In a typical synthesis procedure of MoS 2 /g-C 3 N 4 nanocomposite, 0.192 g of the as-prepared g-C 3 N 4 sample was dispersed into the above solution and sonicated for another 2 h in a probe sonicator. Finally, the reaction mixture was transferred to a 100 mL hydrothermal Teflon-lined autoclave reactor (Scheme 1), and the reaction was conducted for 18 h in an oven at 220 °C. After the reaction, the product was thoroughly washed with water and centrifuged and dried at 50 °C for 6 h to obtain MoS 2 /g-C 3 N 4 . Pure MoS 2 can be obtained without adding g-C 3 N 4 .
The study of plant growth First, 10 seeds of wheat and chickpea of the same shape and size were selected and sprouted in wet cotton over 24 h. The seeds were then planted in the test tube with the same amount of MB solution (40 mg/L) and the treated solution (MB solution adsorbed by MoS 2 /g-C 3 N 4 nanocomposites). A 15-day follow-up study was conducted to determine these plants' root and bud development (Gunture et al. 2020a(Gunture et al. , b, 2019.

Characterization of materials
The crystal phase of as-prepared adsorbents was characterized by XRD (Bruker D2 PHASER X) using Cu kα as the irradiation source (λ = 1.5418 Å). The chemical composition and electronic state of the adsorbents were studied by XPS (Kratos Axis Ultra DLD). The morphologies and nanostructures of the as-prepared adsorbents were analyzed by HRTEM (FEI Talos F200X TEM) and FESEM (FEI Verios G4). The Brunauer-Emmett-Teller (BET) specific surface areas of the as-synthesized nanocomposite adsorbents were calculated by N 2 adsorption-desorption isotherms analyzed by Beishide Instrument Technology (3H-2000PS2). The absorbance of the solution was measured using a UV-Vis spectrophotometer made by Shanghai Jinghua Instrument.
Adsorption procedure Different masses of MoS 2 /g-C 3 N 4 nanocomposites were added into a 50 mL MB solution. The original concentration C 0 of the MB solutions was varied between 20 and 100 mg/L. After that, the solution in the round bottom flask was heated at different temperatures (15 °C, 25 °C, 35 °C, and 45 °C) with stirring at 300 rpm for about 60 min (until adsorption equilibrium was reached). The pH of the solution was adjusted to between 1 and 11 by adding 0.1 mol/L HCl or 0.1 mol/L NaOH. We took the supernatant from the adsorption phase after centrifugation. Then, the absorbance of MB at the maximum absorption wavelength (664 nm) was measured by UV spectrophotometer. After the adsorption equilibrium was achieved, the equilibrium concentrations of Ce (mg/L), the removal efficiency (η%), and equilibrium adsorption capacity (q e ) of MB solution were counted by these formulae: Scheme 1 Schematic diagram for the flower-like MoS 2 /g-C 3 N 4 preparation 1 3 Vol.: (0123456789) where V (L) was the volume of MB solution, m (g) was the weight of MoS 2 /g-C 3 N 4 nanocomposites, and q e (mg/g) was the adsorption capacity at equilibrium.

Structure characterization
The powder XRD patterns of the g-C 3 N 4 , MoS 2 , and MoS 2 /g-C 3 N 4 nanocomposites are shown in Fig. 1. In the typical XRD pattern, all diffraction peaks of g-C 3 N 4 correspond to the standard card (JCPDS 87-1526). The peak value at 13.2° corresponds to the (100) crystal planes, caused by the periodic arrangement of stacking units between layers of g-C 3 N 4 (Paul and Nehra 2021). The peak value at 27.5° corresponds to the (002) crystal planes, which is the characteristic peak of in-plane structural repeating unit of graphite-like layered structure of g-C 3 N 4 (Paul et al. 2019b  (1) respectively (Ali et al. 2019). As can be seen from the typical XRD pattern of a MoS 2 /g-C 3 N 4 nanocomposites, the peak value of MoS 2 is clearly visible, but that of g-C 3 N 4 is very small, which has been attributed to the low content of g-C 3 N 4 in the nanocomposites. Another possible reason for the weakness of the (002) peak of g-C 3 N 4 is that it may coincide or interfere with the (100) peaks of MoS 2 . Nevertheless, the diffraction peak intensity of MoS 2 decreases with the addition of g-C 3 N 4 , especially at the 15.1° peak. This indicates that the addition of g-C 3 N 4 further limits the aggregation of the molybdenum disulfide layer (Chen et al. 2017b). XPS analysis further determined the chemical state and elemental composition of the MoS 2 /g-C 3 N 4 nanocomposites. Figure 2a illustrated the XPS survey spectrum which confirmed the existence of C, N, Mo, and S elements in MoS 2 /g-C 3 N 4 nanocomposites. High-resolution spectra were examined on the C1s, N1s, Mo3d, and S2p regions. Figure 2b represents the C1s spectrum, where peak at 284.1 eV corresponds to the C-C pure bonding in aromatic rings, while the peaks positioned at 285.8 eV and 288.0 eV correspond to N-C = N and C-NH 2 bonding, respectively (Yi et al. 2017). In the N1s spectrum of Fig. 2c, three peaks positioned at 398.6 eV, 399.1 eV, and 403.8 eV belong to C = N-C, N-(C) 3 , and N-H structures, respectively (Paul et al. 2020). In addition, the two peaks in the Mo 3d spectrum (Fig. 2d) at around 228.95 and 232.13 eV correspond to Mo 3d 5/2 and Mo 3d 3/2 , respectively, which belong to the Mo 4+ species. There was another peak at 226.01 eV, which corresponds to the existence of S 2-. A pair of peaks is observed at 161.87 and 163.20 eV in the S 2p spectrum (Fig. 2e), which correspond to S 2p 3/2 and S 2p 1/2 , respectively, indicating the existence of S 2- . These studies confirmed the successful preparation of MoS 2 /g-C 3 N 4 nanocomposites.
The morphology and nanostructures of precursor MCA, pure g-C 3 N 4 , pure MoS 2 , and MoS 2 /g-C 3 N 4 nanocomposites were directly analyzed by SEM and TEM (Fig. 3). For precursor MCA (Fig. 3a), FESEM analysis showed the spherical shape morphology, while pure g-C 3 N 4 (Fig. 3b) possessed the hollow sphere shaped, indicating that the morphology of the supramolecular precursor (MCA) is different from that g-C 3 N 4 . Figure 3c shows pure MoS 2 and displays 3D flowerlike morphologies in  Figure 3d indicates that the morphology of the MoS 2 /g-C 3 N 4 nanocomposites basically keeps the appearance of MoS 2 , which size is about 200-400 nm. As for the TEM images of MoS 2 /g-C 3 N 4 nanocomposite in Fig. 3e, MoS 2 was loaded on the g-C 3 N 4 , which reveal nanocomposites formation between MoS 2 and g-C 3 N 4 . Figure 3f shows the presence of well-defined lattice fringes of 0.62 nm; this can be attributed to the (002) crystal plane of MoS 2 (Monga et al. 2020). Figure S1 showed the N 2 adsorption desorption isotherm of the as-prepared products. Since there was almost no adsorption limitation at the high-pressure stage, the adsorption isotherm of MoS 2 /g-C 3 N 4 nanocomposites was determined to be type IV isotherm with H3 type hysteresis loops, closely related to the capillary condensation phenomenon of slit pores generated by the stacking of lamellar particles Zhu et al. 2020). The pore size of MoS 2 /g-C 3 N 4 nanocomposites was mainly between 2 and 17 nm, indicating the nanocomposite was mesoporous. It can be seen in Table S1, the BET surface area, average pore size, and pore volume of MoS 2 /g-C 3 N 4 nanocomposites were 70.656 m 2 /g, 15.4494 nm, and 0.2729 cm 3 /g, respectively. Using these parameters, it is anticipated that the mesoporous MoS 2 /g-C 3 N 4 nanocomposites could have excellent adsorption characteristics for MB.

Effect of pH value
The pH value greatly influences on the ionization degree of MB molecules and the surface charge of adsorbent in the solution (Mantasha et al. 2020). In order to study the effect of pH value on MB adsorption, we set the pH value from 1 to 11. In Fig. S2, the MB adsorption efficiency of MoS 2 /g-C 3 N 4 nanocomposites increased with the pH value in the range of 1 to 11. The surface charge of MoS 2 /g-C 3 N 4 nanocomposites is shown in Fig. S3. At relatively low pH, the proton ions in the solution may compete with the cationic MB for active sites on the MoS 2 /g-C 3 N 4 nanocomposites, which inhibits the adsorption of MB on the synthetic adsorbent. MoS 2 /g-C 3 N 4 nanocomposites had lower surface charges with an increase in pH, leading to a stronger electrostatic attraction between MB and adsorbents. The results showed that a higher pH value led to greater adsorption efficiency. Based on the results in Fig. S4, the MoS 2 /g-C 3 N 4 nanocomposites exhibited higher adsorption efficiency at pH = 7, compared to both g-C 3 N 4 and MoS 2 .

Effect of adsorbent dosage
In order to test the effect of the dose of MoS 2 /g-C 3 N 4 nanocomposites on adsorption efficiency and capacity, different amounts of nanocomposites were added to MB solution. Figure S5 showed that as the adsorbent dosage is increased, the adsorption efficiency of MB increases at first but then mostly remains unchanged, resulting in a decrease in the equilibrium adsorption capacity of MB from 251.2 to 99.7 mg/g. A critical point in the adsorption process is the equilibrium state, where the adsorbent can no longer adsorb MB. It is evident from the above that a high MB adsorption rate can be achieved by using a small quantity of adsorbent. Considering the removal efficiency and practicability, we selected 10 mg adsorbent for the following experiments.

Effect of MB concentration
A range of concentrations of MB solution was used to test the as-synthesized nanocomposite adsorption capacity and efficiency. According to Fig. S6, when MB concentration increased from 20 to 100 mg/L, the equilibrium adsorption capacity jumped from 98.8 to 227.4 mg/g, consistent with le Chatelier's principle. There is a possibility that as MB concentration increases, the adsorbate in solution has a stronger driving force to overcome the mass transfer resistance between solution and adsorbent (Zhao et al. 2020c). Eventually, as the MB concentration increased, its adsorption rate dropped from 98.80 to 45.48%, which was due to the saturation of the active center in the adsorbent at a certain concentration.

Effect of adsorption temperature
In this work, the temperatures were set at 15 °C, 25 °C, 35 °C, and 45 °C, respectively. According to Fig. S7, temperature appeared to influence MB adsorption. As the temperature increased from 15 to 45 °C, the adsorption efficiency increased significantly from 86.84 to 98.17%. In order to calculate the depth of diffusion in the nanocomposites, the temperature must be increased as the Brownian motion will accelerate when the temperature increases. However, lowering the temperature will decrease the diffusion in the nanocomposites (Fu et al. 2017). Considering the adsorption efficiency and practicability, the following experiments were carried out at 25 °C.

Recyclability of MoS 2 /g-C 3 N 4 for the MB adsorption
Recycling and stability of adsorbents can greatly improve efficiency and reduce its cost significantly. In this study, MB was desorbed with 0.1 mol/L NaOH. Figure S8 showed the MB adsorption efficiency of MoS 2 /g-C 3 N 4 nanocomposites for five consecutive cycles. According to the results, the adsorption efficiency of MB decreased gradually as the adsorbate could not be completely desorbed and failed to occupy some active adsorption sites. A maximum of 73.58% of the MB was adsorbed on MoS 2 /g-C 3 N 4 nanocomposite despite the five cycles of adsorption, indicating the excellent reusability of the adsorbent. Figure 4a showed the effect of contact time on adsorption capacity. Increasing the contact time led to an increase in adsorption capacity that increased rapidly at the first then gradually slowed down until adsorption equilibrium was achieved after 60 min. In this study, pseudo-first-order, pseudo-second-order kinetics, and intra-particle diffusion models were used to analyze the adsorption process (Tehrani and Zare-Dorabei 2016). The calculation formula of the pseudo-first-order kinetics model was as follows:

Adsorption kinetics
(3) ln q e − q t = lnq e − tk 1 1 3 Vol.: (0123456789) The calculation formula of the pseudo-secondorder model was as follows: The calculation formula of the intra-particle diffusion model was as follows: where q e (mg/g) is described above, q t (mg/g) represents the adsorption capacity at any contact time, and t (min) is the adsorption time. k 1 , k 2 , and k 3 are the (4) (5) q t = k 3 t 0.5 + C pseudo-first-order, pseudo-second-order, and intraparticle diffusion model rate constants, respectively. C (mg/g) is the intercept obtained by fitting the intraparticle diffusion model.
The linear fitting results of the two kinetic models are shown in Fig. 4. It illustrated that the pseudo-second-order model (Fig. 4c) is better than the pseudo-first-order model (Fig. 4b) in describing the process of adsorption kinetics, as demonstrated by the higher R 2 (R 2 = 0.9999) value. In addition, the calculated results of the two kinetic models are shown in Table S2. The theoretical equilibrium adsorption capacity q e (cal) of the pseudo-second-order model is more consistent with the experimental data q e (exp), which indicates that the adsorption of MB on MoS 2 /g-C 3 N 4 nanocomposites conforms to the pseudo-secondorder model and the adsorption process is mainly controlled by chemical adsorption (Fang et al. 2018;Guan et al. 2017).
The model of intra-particle diffusion after fitting is shown in Fig. 4d. In this case, the plot was nonlinear, indicating that intra-particle diffusion was not the only factor limiting the particle diffusion. Adsorption of MB can be accomplished in three ways. The first stage was from the solution to the surface of MoS 2 /g-C 3 N 4 nanocomposites. The second stage was the diffusion of MB inside MoS 2 /g-C 3 N 4 nanocomposites. The third stage was the final balance stage. Based on the above analysis, it is clear that intra-particle diffusion and surface adsorption occur simultaneously, which has a significant impact on the adsorption of MB.
Adsorption isotherm Figure S9 showed the MB adsorption isotherms of MoS 2 /g-C 3 N 4 nanocomposite at different temperatures. The experimental data were fitted into the Langmuir model for monolayer adsorption (Eq. (6)) and Friedrich model of multilayer adsorption (Eq. (7)) (Gunture et al. 2020b). They can be expressed as where Ce (mg/L) was the equilibrium concentration of MB, q m (mg/g) was the theoretical maximum adsorption capacity, and K L (L/mg) and K F were Langmuir constant and Friedrich constant, respectively. n was the constant related to the surface inhomogeneity of adsorbent (Zhao et al. 2020c).
(6) C e q e = C e q m + 1 K L q m (7) log q e = log K F + 1 n log C e  The fitting results of the two isotherm models are shown in Fig. 5. The data in Table 1 showed that R 2 values in the Langmuir model (Fig. 5a) were bigger than those in the Friedrich model (Fig. 5b). Therefore, the adsorption of MB on MoS 2 /g-C 3 N 4 nanocomposites followed the Langmuir isotherm model. At 45 °C, q max is 278.4 mg/g, which indicates that MoS 2 /g-C 3 N 4 nanocomposites have a higher MB adsorption effect. The comparison of q max of different adsorbents is given in Table S3. Therefore, the adsorption capacity of MoS 2 /g-C 3 N 4 nanocomposites for MB was similar or higher than that of other commonly used adsorbents.
We used the separation coefficient R L to evaluate the feasibility of adsorption by the following equation: where C 0 (mg/L) and K L (L/mg) are as defined above. As shown in Table S4, 0 < R L < 1, indicating that the MB adsorption process was favorable (Mohammadnejad et al. 2018).

Adsorption thermodynamics
At different temperatures, we used various thermodynamic parameters to understand the adsorption process. The thermodynamic parameters can be obtained by the following expression: where K L is the equilibrium distribution coefficient, R is the molar gas constant (8.314 J/mol·K), T (K) is the temperature, ΔG θ is the Gibbs-free energy change, ΔH θ is the enthalpy change, and ΔS θ is entropy change. Figure 6 showed the plot of ln K L against 1/T. From Table 2, we can see that the value of ΔG θ was negative, indicating that the adsorption process was a spontaneous process at different temperatures, and the increase in temperature was conducive to the adsorption of MB. The value of ΔH θ was positive, indicating that the adsorption was an endothermic process. In addition, the value of ΔS θ was positive, indicating that the disorder of the solid-liquid interface increases during the adsorption process.
Adsorption mechanism Figure S10 showed the possible adsorption mechanism of MoS 2 /g-C 3 N 4 . From the above results, we can conclude that the adsorption of MB by MoS 2 /g-C 3 N 4 belongs to the Langmuir monolayer adsorption model. Adsorption rate decreased as adsorption capacity increased. In fact, based on the molecular structure, MB is a cationic dye, while MoS 2 /g-C 3 N 4 is negatively charged in the solution, so there is an electrostatic interaction between MB and MoS 2 /g-C 3 N 4 nanocomposite (Fig. S3). As shown in Fig. S10, MB and MoS 2 /g-C 3 N 4 both contain aromatic rings and possible stronger π-π interaction. Furthermore, both MoS 2 /g-C 3 N 4 and MB contain nitrogen atoms,  allowing hydrogen bonds to form between them. The above analysis indicated that the adsorption of MB was mainly driven by electrostatic interactions, together with π-π interaction and hydrogen bond.

Ecological assessment of treated wastewater
In order to evaluate the ecological characteristics of the treated MB solution, untreated MB solution and the treated solution were used to culture the seeds of wheat and chickpea, respectively (Gunture et al. 2020b). The growth of germinated wheat and chickpea seeds was analyzed in the next 15 days. As shown in Fig. 7a, the average length of wheat seedlings grown in untreated MB solution is 14.7 cm, 17.2 cm lower than the average in treated solutions. In case of chickpea (Fig. 7b), the average length grown in untreated MB and treated MB solution is 21.4 cm and 43.6 cm, respectively. These results showed that untreated MB solution had strong inhibition on root and bud germination of both plants. When compared with untreated MB solution, the treated one showed a great improvement in growth of root and bud parts of wheat and chickpea.

Conclusions
In this study, we designed and synthesized MoS 2 /g-C 3 N 4 nanocomposites for MB adsorption.
The experimental results indicated that removing MB from water by adsorption is feasible. The factors affecting the adsorption performance of MB (pH of solution, adsorption dose, MB concentration, and adsorption temperature) were studied in detail. The adsorption process followed the Langmuir monolayer adsorption isotherm and pseudo-second-order kinetic models. Thermodynamic data showed that adsorption is a spontaneous process, and the increase of temperature is conducive to the adsorption of MB. MB can reach the adsorption equilibrium within 60 min, and the maximum adsorption capacity of MB was 278.4 mg/g at 45 °C. In addition, when the treated wastewater was used to water wheat and chickpea seeds, we found that the treated wastewater allowed these crops to grow better than the untreated MB solution. The present study indicated that the synthesized MoS 2 /g-C 3 N 4 nanocomposites have good application prospects for wastewater treatment and environmental remediation.

Conflict of interest
The authors declare no competing interests.