Green MoS₂ nanosheets as a promising material for decontamination of hexavalent chromium, pharmaceuticals, and microbial pathogen disinfection: spectroscopic study

First, a proper amount of K2Cr2O7 is dissolved in deionized water to prepare a 1000 ppm stock Cr⁶⁺ solution. Diluting this stock solution to produce a working solution with a known Cr⁶⁺ concentration. The initial solution pH is adjusted by using 0.1 N HCl and 0.1 N NaOH. A series of experiments are carried out by varying the contact time, the catalyst dose, and solution pH to examine the Cr⁶⁺ adsorption, and reduction performances of the prepared catalyst. All experiments are performed in duplicate and average results are reported. For typical adsorption runs, 0.2 g. L-1 of (2H/1T) MoS2 catalyst dispersion in a Cr⁶⁺ solution with an initial concentration of 20 ppm at pH 3 has been used. The mixture is stirred for 30 min under the dark condition to reach adsorption–desorption equilibrium. During the adsorption process, 3 mL of suspension is taken out by syringe filter (PTFE 0.45 μm) in regular 5-min intervals to obtain the supernatant.

The Cr⁶⁺ concentration is determined by colorimetry at 540 nm using the diphenyl carbazide technique and by using a UV–vis spectrophotometer at 348 nm [33]. The amounts of total chromium are determined by inductively coupled plasma (ICP) [30]. The concentration of C³⁺ is calculated from Eq. (1) (the total chromium subtracting the Cr⁶⁺ concentration):

Adsorption percentage can be calculated as follows:where C_o and C_t are the concentrations before and after different irradiation times.

After the adsorption process, the contact solution is exposed to the solar simulator. During the photoreduction process, 3 mL of suspension is taken out in a regular 30-min interval by syringe filter to obtain the supernatant. The concentration of Cr⁶⁺ in the supernatant is evaluated by using a UV–vis spectrophotometer and by the diphenyl carbazide method. The changes in Cr⁶⁺ concentration are determined by the formula.

Degradation % is calculated as follows:where C_o and C_t are the concentrations before and after different irradiation times, respectively. The isotherm and kinetic parameters are also calculated for photocatalytic reactions.

The point-of-zero charges (pH_ZPC) of the (2H/1 T) MoS₂ nanosheets are measured using the pH drift method [32]. In this method, the pH_ZPC of the (2H/1 T) MoS₂ photocatalyst is determined by adding 25 mL of 0.1 N NaCl in 50-mL conical high-density polystyrene flasks. A range of initial pH (pH_initial) values of the NaCl solutions and 50 mg of (2H/1 T) MoS₂ is adjusted from 2 to 12 by using 0.1 M of HCl and NaOH. The suspensions are shaken in a shaker and allowed to equilibrate for 24 h. The suspensions are then centrifuged at 5000 rpm for 15 min and the final pH (pH_final) values of the supernatant liquid are recorded. The value of pH_ZPC is the point where the curve of (pH_final – pH_initial) versus pH_initial crosses the line equal to zero.

The acute toxicity level and effective concentration (EC₅₀) of the synthesized molybdenum disulfide (MoS₂) nanosheets are examined using a 2% screening test of Microtox Analyzer 500 (Modern Water Inc., USA). Five concentrations of MoS₂ nanosheets (50, 100, 250, 500, and 1000 µg. mL^-1) are toxicity tested. The tube containing 500 µL of diluent is inoculated with 10 µL of the resuspended bioluminescence Vibrio fischeri bacterium and used as a control, while in another test tube, 10 µL of each concentration of MoS₂ nanosheets and 10 µL of Vibrio fischeri are added to a tube containing 500 µL of diluent. The toxicity level and effective concentration (EC₅₀) are determined within 15 min.

In vitro antimicrobial effects for the five MoS₂ nanomaterial concentrations (50, 100, 250, 500, and 1000 µg. mL^-1) are tested. Six pathogenic microorganisms, including Escherichia coli O157:H7 (ATCC 35,150) and Pseudomonas aeruginosa (ATCC 10,145) as Gram-negative bacteria; Listeria monocytogenes (ATCC 25,152), Staphylococcus aureus (ATCC 43,300), and Enterococcus faecalis (ATCC 43,845) as Gram-positive bacteria; and Candida albicans (ATCC 10,231) as a yeast model, are involved in this assay. One hundred microliters of each refreshed pathogen is spread on the surface of Müller-Hinton agar plates (BBL, Germany). Sterile filter paper discs (6 mm in diameter) are saturated with MoS₂ nanosheets, and after that, the discs are placed on the surface of Müller-Hinton agar plates. The plates are incubated for 24 h at 37 °C. The diameters of inhibition zones were are measured in millimeters (mm) using a measuring ruler, whereas 6 mm in diameter means that the tested material has no antimicrobial effects [34, 35].

E. coli O157:H7, Listeria monocytogenes, and Candida albicans are used in this experiment. These preserved pathogens with 10% glycerol in − 20 °C are refreshed in 10-mL tryptic soy broth tubes (Merck, Germany). The inoculated tubes are incubated at 37 °C for 24–48 h. The grown microbial pathogens are centrifuged at 5000 rpm for 20 min and washed three times to remove any nutrients and/or debris. Preparation of the microbial pathogens is carried out 24 h before each experiment to determine the initial pathogenic counts, according to APHA (2017). Appropriate microbial counts ~ 10⁶–10⁷ CFU.ml^-1 from each pathogen are separately inoculated into five tubes, of which each tube contains 10 mL from 50, 100, 250, 500, and 1000 µg. mL^–1 of MoS₂ nanosheet suspension. The tubes are incubated in the lab temperature and under shaking at 150 rpm. The samples are withdrawn after different contact times (0, 15, 30, 45, 60, and 120 min). The pathogenic counts are determined using the pour plate method according to APHA (2017). The colonies are expressed as a colony-forming unit (CFU.ml^–1). The following equation is used to calculate the removal efficiency:where C_i is the initial pathogen counts at 0 min and C_o is the pathogen counts after exposure to MoS₂ nanosheets.

The representative SEM images and EDX data of obtained sample are shown in Fig. S1a and b. It is shown that the sample has nanosheets layered structure (as the arrow shown in Fig. S1a) with typical small holes on the surface which can act as active adsorption sites that are beneficial to catalytic activity. These folded nanostructures can significantly decrease the surface recombination of the photogenerated electrons and holes and improve the efficiency of collection, transfer, and separation of carriers, which is essential for enhancing photocatalytic activity.

Figure S1b shows the EDX spectrum of the sample; only sulfur and molybdenum atoms are detected besides the O₂ which comes from the alcohol used in measurement without any additional peak for impurities. This confirms that nanosheets contain only molybdenum and sulfur elements. Table S1 also demonstrates that the atomic ratio of Mo and S is 58.28% and 28.29%, which confirms that the product is MoS₂.

The morphology and nanostructure of the (1T/2H) MoS2 nanosheets are detected by transmission electron microscope (TEM), high-resolution Transmission Electron Microscope (HRTEM), and selected area electron diffraction (SAED) to obtain more insight into the crystal structure of the as-prepared (1T/2H) MoS2 nanosheets (Fig. S2 (a, b, c, d.)) Figure S2a and b reveal that the synthesized (1 T/2H) MoS₂ has consisted of multilayered layers of nanosheets. From the HTEM image it is found that the MOS2 nanosheets are stacked densely with clear lattice fringes with an interlayer spacing of 0.62 nm which suggesting the high crystallinity of the 2D networks (Fig. S2(c)). The selected area electron diffraction pattern (SAED) is illustrated in Fig. S2d, the visible diffraction rings could be indexed to (100) and (110) planes of the hexagonal [27]. The SAED pattern illustrates the crystalline nature of the 2D MoS₂, with highly diffused bands (Fig. S2d).

To understand the mechanism of chromium adsorption and adsorption properties and interactions, adsorption isotherms of Cr₂O₇²⁻ solutions with different initial concentrations from 10 to 80 ppm at pH 3 are applied (Fig. 4a, b) [49, 50]. Both the Langmuir and Freundlich models are used to further quantify the experimental adsorption data and evaluate the adsorption capacity and the adsorbent affinity for the heavy metal ions [51]. Their linear equations (Eqs. 6, 7) are as follows:where q_max (mg g⁻¹) is the Langmuir maximum adsorption capacity necessary to form a monolayer on the adsorbent surface, c_e (mg L⁻¹) denotes the equilibrium concentration of the solution, K_L (L mg⁻¹) and K_F (mg g⁻¹) represent the Langmuir and Freundlich constants, respectively, and n is an empirical parameter related to the intensity of adsorption. Table S2 shows the values of the linear regression coefficients (R²) as well as the equivalent calculated parameters (q_max, K_L, K_F, and n). The validity of each model is checked using the correlation coefficient. It is worth observing that applying the Langmuir adsorption isotherm model gives a higher correlation coefficient with a maximum adsorption capacity (q_max) of 106.5 mg g⁻¹ as illustrated in Table S2. It can be concluded that the Langmuir model is suitable for explaining the adsorption of chromium (Fig. 4a). As a result, chromium adsorption on the homogenous surface of MoS₂ is regarded as a single layer [52]. Furthermore, the plot of the Freundlich model is illustrated in Fig. 4b. Table S2 shows the adsorption capacity at a unit concentration (K_F) and the adsorption intensity (1/n). The 1/n value of 0.3 indicates that the adsorption process is favorable.

The data is fitted to various kinetic models, including pseudo-first and second-order models (Fig. 4c, d), to explore the kinetics of the adsorption process (Eqs. 8, 9). Such models provide precise information on the adsorption mechanism as well as the types of adsorption processes that are involved, such as diffusion control, chemical reaction, and mass transport processes.where the pseudo-first and pseudo-second-order rate constants are K₁ and K₂, respectively (g mg⁻¹ min⁻¹). The amounts adsorbed at time t and equilibrium (mg g⁻¹) are given by q_t and q_e, respectively.

The pseudo-first-order did not provide an acceptable fit for the experimental data, as seen in Fig. 3c, even though R² is 0.992 but the q_e is 59 mg g⁻¹ for 20 mg L⁻¹ dye, which is significantly lower than the experimental value of q_e (100 mg g⁻¹). As shown in Fig. 4d, the pseudo-second model as well shows a good-order correlation. Furthermore, the obtained q_e values (e.g., 107 mg g⁻¹, Table S2) are significantly close to the experimental value (100 mg. g⁻¹). These results suggest that the adsorption process is highly dependent on the surface-active sites created by MoS₂ nanosheet interfaces. The adsorption process of hexavalent chromium ions on the surface of (2H/1 T) MoS₂ nanosheets is likely to be dominated by chemisorption, suggesting that the overall rate of the nanosheet adsorption process is controlled by intraparticle diffusion and the boundary layer diffusion by [53].

As is well known, the process of reduction depends on several factors such as mass transfer, diffusion rate control, and chemical reactions [50, 54]. To understand the mechanism of chromium reduction, the fitting of experimental data with the models of pseudo-zero order, first-order kinetic model, and pseudo-second-order kinetic is investigated that can be described using the following equations (Eqs. 10, 11, 12), respectively:where C₀ and C_t are the initial metal concentration and concentration at time t respectively; k₀, k₁, and k₂ are the rate constants of the zero-, first-, and second-order kinetics, respectively.

The rate constant and the corresponding correlation coefficient R² are calculated from the graphs of the three models (Fig. 4e, f, g). According to the R² value, it can predict the equation that fits the experimental data. The half-life time (t_1/2) from each equation has been calculated and compared with the experimental one. The values of the parameters and the correlation coefficients obtained from these three kinetic models have been accumulated in Table S2. In conclusion, the pseudo-first-order model is the most suitable for describing the reduction process, as evidenced by its higher correlation coefficient.

In addition, the photoreduction rate constant (k₁) is calculated to be 0.031 and the calculated t_1/2 is 22 min, which is closed to the experimental ones. This confirms that the first-order kinetic model is a model that describes photocatalytic reduction.

Among the various variables that affect adsorption, the doses of MoS₂ nanosheets are a particularly important factor due to their cost estimation per unit of solution price. The percentage of Cr⁶⁺ ion adsorption from an aqueous medium as a function of the (2H/1 T) MoS₂ nanosheet doses of 0.1, 0.2, 0.25, and 0.4 g. L⁻¹ at solution pH value of 3 is shown in Fig. 5a. It is found that increasing the dose amount of MoS₂ catalyst leads to an increase in the adsorption percentage. After 15 min, the adsorption percentage obtained with 0.1 g. L⁻¹ dose is 34%. Although with doses of 0.2, 0.25, or 0.4 g L⁻¹, the adsorption percent reached 100%, the adsorption percentage for a 0.1 g. L⁻¹ dose reaches 100% after 30 min, as shown in Fig. 5a.

The improvement in Cr⁶⁺ adsorption efficiency is thought to be due to the unique structure of (2H/1 T) MoS₂ nanosheets. The defects on the surface of the nanosheet provide penetrable channels for Cr⁶⁺ ion diffusion. In addition, the high surface area of (2H/1 T) MoS₂ nanosheets and the availability of active sites on their surface facilitate the interaction and are beneficial for the transport of Cr⁶⁺ from its bulk solution to the active site on nanosheets [51, 54, 55].

The comparative process for the different (2H/1 T) MoS₂ nanosheet doses used for Cr⁶⁺ photoreduction under a solar simulator is shown in Fig. 5b. It is found that the photoreduction process is enhanced as well as the Cr³⁺ concentration in the solution increases with increasing the catalyst dose from 0.1 to 0.4 g L⁻¹ [55]. In addition, at a constant MoS₂ dose, the Cr³⁺ concentration increases with increasing contact time. It is found that, with 0.25 and 0.4 g. L⁻¹ doses, the Cr³⁺ concentration retains to 20 ppm after 2 h. This proves that all the Cr⁶⁺ is reduced to Cr³⁺.

Solution pH is an important and controllable parameter that can affect the surface charge, the degree of protonation of the adsorbent, and the degree of ionization of the adsorbate [31]. It can play a key role in the adsorption and photocatalytic reduction efficiency of Cr⁶⁺ [55].

The influence of solution pH on the adsorption efficiency is carried out by adjusting the pH of the initial solution in the range of 2.0–7.0, in which the initial Cr⁶⁺ concentration is kept at 20 ppm with 0.2 g. L⁻¹ catalyst dose. The variation of the Cr⁶⁺ adsorption % at different pH levels is shown in Fig. 5c. It can be shown that, as the solution pH increases up to 3, the percentage of Cr⁶⁺ adsorption % increases. Beyond this value, a sharp decrease is observed in the adsorption %. After 15 min under acidic conditions of pH 2 and pH 3, 75% and 100% Cr⁶⁺ are adsorbed on (2H/1 T) MoS₂ nanosheets, respectively. When the pH level is increased to pH 5 and pH 7, the Cr⁶⁺ adsorption % decreased to 64% and 28% respectively.

The reason why MoS₂ nanosheets perform differently in adsorbing Cr⁶⁺ at different pH values in the solution can be explained by considering the pH value of the point-of-zero charges (ZPC) [56]. The pH at the ZPC (pH_ZPC) of the (2H/1 T) MoS₂ nanosheet surface is important as it indicates the acidity/basicity and the net surface charge in the solution.

As shown in Fig. 5d, the pH_ZPC of (2H/1 T) MoS₂ nanosheets is 2.6; this means that its surface is positively charged at a solution pH value below 2.6. As below 2.6, the Cr⁶⁺ in aqueous solution is found to be in the forms of HCrO₄⁻ and Cr₂O₇²⁻ and HCrO₄⁻ is the main species [31]. So, the high adsorption % at low pH is mainly due to the electrostatic interaction between the anionic forms of Cr⁶⁺ and the surface of MoS₂ with positive charges via high protonation.

When the pH is raised to pH 5, the MoS₂ surface is negatively charged but not high enough, and its coulomb repulsive interaction with Cr⁶⁺ is not strong enough; thus, it shows moderate adsorption (65%). With increasing the solution pH to 7, the MoS₂ negatively charged surface increased. Also, Cr⁶⁺ is gradually shifted to the form of CrO₄²⁻ species which has a highly negatively charged surface. Consequently, this strengthened the electrostatic repulsion between (2H/1 T) MoS₂ nanosheets and Cr⁶⁺, making Cr⁶⁺ adsorption more difficult. It can be concluded that HCrO₄⁻ is better adsorbed than CrO₄⁻² due to its lower free adsorption energy [57].

On the other hand, increasing the pH value increases the concentration of OH ions present in the solution. Therefore, there is competition between the Cr⁶⁺ species and OH ions on the surface of the M, which leads to a decrease in the adsorption efficiency.

Figure 5e depicts the concentration variations of different Cr ion species including Cr⁶⁺ and Cr³⁺ in aqueous solutions over contact time at different pH during the reduction of Cr⁶⁺ using (2H/1 T) MoS₂ nanosheets. It is found that reduction in acidic solutions provides higher photoreduction efficiency than in alkaline solutions. The photoreduction efficiency decreased significantly with increasing pH. As shown in Fig. 5e, all Cr⁶⁺ ions are adsorbed on the (2H/1 T) MoS₂ surface at pH 2 and pH 3 so its concentration is zero, even though the Cr³⁺ concentration raised to a plateau due to the continuous reduction of the adsorbed Cr⁶⁺ on the surface to Cr³⁺, which reached 100%. At pH 5, the concentrations of Cr⁶⁺ in the solution decreased slightly, whereas the Cr³⁺ concentration increased initially to reach a plateau. The Cr⁶⁺ reduction efficiency decreased to 35%. There is no photoreduction observed with pH 7 (Fig. 5e).

The increased photoreduction potential at lower pH can be attributed to an increase in the thermodynamic driving force of electrons from CB to Cr⁶⁺ ions [57, 58]. In addition to the previously mentioned 100% adsorption capacity of Cr⁶⁺ on the MoS₂ surface, the reduction begins at the surface of MoS₂. Meanwhile, increasing the solution pH higher than 3, the driving force for the reduction of molecular oxygen is higher than for the Cr⁶⁺, and as a result, O₂ may compete for the photogenerated electrons and decrease the rate of Cr⁶⁺ photoreduction [31, 51, 55].

The mechanism for removing Cr⁶⁺ ions over MoS₂ nanosheets is dependent on the synergistic effect of adsorption-photocatalysis. The photoreduction activity of (2H/1 T) MoS₂ nanosheets is highly dependent on interfacial reactions. For instance, the reduction of Cr⁶⁺ occurs under three stages, adsorption to the surface-active center, photoreduction, and surface desorption, as shown in Fig. 6. Firstly, Cr⁶⁺ should adsorb on the surface of (2H/1 T) MoS₂. Then, the photogenerated electrons move to the MoS₂ surface and react with Cr⁶⁺. Based on the opposite charge properties of Cr⁶⁺ and Cr³⁺, when Cr⁶⁺ is reduced to Cr³⁺, Cr³⁺ is quickly removed or desorbed from the surface [31, 32]. So, it is vital to promote Cr⁶⁺ surface adsorption as well as Cr³⁺ repulsion to maximize the photoreduction performance.

The molecular structure and function groups of (2H/1 T) MoS₂ nanosheets, MoS₂ nanosheets adsorbed Cr⁶⁺, and after photoreduction are identified using FTIR spectroscopy as revealed in Fig. 7a, b, c. From Fig. 7a for the pristine MoS₂ nanosheets, it should be noted that the peaks at 3398 cm⁻¹, 3138 cm⁻¹, and 1631 cm⁻¹ are related to the bending and stretching vibrations of the hydroxyl group of adsorbed water molecules, respectively. The peaks at 1627 cm⁻¹ and 1133 cm⁻¹ are attributed to the stretching vibration of S–O in MoS₂ nanosheets [18]. The stretching vibration sharp peak of S-Mo-S is represented at 1403 cm⁻¹. However, the band of O–H bending can be found at 1052 cm⁻¹. The characteristic peaks for out-plane vibration of S atoms (S–S) appear at ∼944 cm⁻¹. A peak at 905 cm⁻¹ might be related to the asymmetric vibration of the Mo–O group. A peak of Mo-S stretching vibration mode is found at 597 cm⁻¹. The spectrum of chromium adsorbed MoS₂ also signifies that after being loaded with metal ions (Fig. 7b), the functional groups of MoS₂ are slightly affected in their position and intensity. These wavelength shifts illustrated that there is metal binding action taking place at the surface of the adsorbents [58] [59]. The clear shifts appear from 3398 cm⁻¹ and 3138 cm⁻¹ for MoS₂ to 3737 cm⁻¹ and 3254 cm⁻¹ for Cr-loaded MoS₂ respectively and with the appearance of a new peak at 2916 cm⁻¹ which confirmed that the surface -OH group is one of the functional groups responsible for adsorption [60]. The S–O peak of MoS₂ is slightly shifted from 1133 cm⁻¹ and 1627 cm⁻¹ to 1126.22 cm⁻¹ and 1631 cm⁻¹, indicating that the S-OH group is also responsible for adsorption [61]. The peak of Mo–O stretching vibration at 902 cm⁻¹ shifted to 952 cm⁻¹ which confirms that Mo–O is also responsible for adsorption. Figure 7c illustrates the FTIR spectrum after reduction, which confirmed that there is no change in (2H/1 T) MoS₂ structure after reduction with no observed peak for Cr⁶⁺ or Cr³⁺ on the (2H/1 T) MoS₂ surface.

Moreover, the activity of MoS₂ nanosheets is investigated for photodegradation of pharmaceutical pollutants (cefadroxil, cefotaxime, meloxicam, ibuprofen, and ciprofloxacin) from wastewater. Figure 8a illustrates the photodegradation of pharmaceuticals using sunlight-assisted photocatalysis with a MoS₂ dose of 0.2 g. L⁻¹ in a natural pH (i.e., pH = 7). After 60 min, it is found that the obtained degradation rates are 100%, 95%, 90%, 80%, and 42% for cefadroxil, ciprofloxacin, meloxicam, cefotaxime, and ibuprofen, respectively. Moreover, the complete degradation of ciprofloxacin, meloxicam, and cefotaxime is achieved after 60, 90, and 90 min of sunlight irradiation respectively over MoS₂ nanosheets. Meanwhile, ibuprofen is completely removed after 120 min. The higher degradation rate of cefadroxil, meloxicam, ciprofloxacin, and cefotaxime is attributed to the higher polar surface of these molecules in aqueous, where the values of polar size are 13.3, 13.6, 7.45, and 17.35 nm according to drug banks. The great polar surface area has increased the capability of adsorption for these pharmaceuticals over the MoS₂ nanosheets, which leads to enhancing photocatalytic degradation. Also, the higher pKa values for cefadroxil (7.22), ciprofloxacin (6.3), and cefotaxime (9.84) increased adsorption of this pharmaceutical which is inconsistent with the photodegradation rates. Meanwhile, ibuprofen has a lower polar surface area (3.73 nm) and pKa (4.43) value in an aqueous solution which leads to a decrease in the adsorption affinity over MoS₂ nanosheets and results in a decline in the photodegradation rate. The photodegradation of pharmaceuticals is found to be fitted with pseudo-first-order (PFO) as shown in Fig. 8b. Furthermore, the PFO rate constants are calculated for the degradation of pharmaceutical pollutants and are illustrated in Fig. 8c. The PFO rate constants are found to be 0.014, 0.0268, 0.0384, 0.0463, and 0.0994 min⁻¹ for ibuprofen, cefotaxime, meloxicam, ciprofloxacin, and cefadroxil respectively. The PFO rate constant of cefadroxil is 7.1, 3.7, 2.6, and 2.15 times higher than that of ibuprofen, cefotaxime, meloxicam, and ciprofloxacin, respectively. Likewise, the normalized initial rates of degradation per catalyst dose of pharmaceuticals are calculated and the value is 1.4, 2.68, 3.84, 4.63, and 9.94 mg. L⁻¹.min⁻¹.g⁻¹ for ibuprofen, cefotaxime, meloxicam, ciprofloxacin, and cefadroxil, respectively. Conclusively, the hydrothermal MoS₂ nanosheets are a non-selective proficient photocatalyst for the decontamination of pharmaceutical pollutants.