Leidenfrost green synthesis method for MoO₃ and WO₃ nanorods preparation: characterization and methylene blue adsorption ability

Abstract

Environmental pollution is a critical issue due to its impact on humans and other organisms. An important demand nowadays is the need for a green method to synthesize nanoparticles to remove pollutants. Therefore, this study focuses for the first time on synthesizing the MoO₃ and WO₃ nanorods using the green and self-assembled Leidenfrost method. The XRD, SEM, BET and FTIR analyses were used to characterize the yield powder. The XRD results emphasize the formation of WO₃ and MoO₃ in nanoscale with crystallite sizes 46.28 and 53.05 nm and surface area 2.67 and 24.72 m² g⁻¹, respectively. A comparative study uses synthetic nanorods as adsorbents to adsorb methylene blue (MB) in aqueous solutions. A batch adsorption experiment was performed to investigate the effects of adsorbent doses, shaking time, solution pH and dye concentration to remove MB dye. The results demonstrate that the optimal removal was achieved at pH 2 and 10 with 99% for WO₃ and MoO₃, respectively. The experimental isothermal data follow Langmuir for both adsorbents with a maximum adsorption capacity of 102.37 and 151.41 mg g⁻¹ for WO₃ and MoO₃.

Introduction

One of the most important global issues is water pollution and the depletion of freshwater resources, which threatens economic development and ecosystem health [1, 2]. Industrial wastewater, particularly dye wastewater, is highly valued because of its high level of pollution and the challenges associated with effective treatment. A minimum of 120,000 tonnes of dyes were released into water bodies, impacting water quality, animals and plants, and the surrounding environment [3]. Methyl blue (MB) is a cationic dye, one of the most common dyes widely used as the coloring agent for cotton, paper and wood, as well as a coater for paper stock [4]. Also used in rubbers, pesticides, as a disinfector in dyestuffs, varnishes and pharmaceuticals [5]. Despite their multiple application, it causes numerous environmental issues. A tiny amount of dye in the water will impact the transparency and amount of oxygen in the water [6]. Furthermore, methylene blue is photoreactive, which means it can give reactive oxygen species when exposed to sunlight [7]. As reactive oxygen species have unpaired electrons, they are highly reactive chemically and have the potential to destroy cells in organisms. Also, because it has benzene rings, it is hard to biodegrade in nature and will accumulate in water [3, 8]. It is a toxic compound that causes many human diseases such as diarrhea, gastritis, dermatology, mutation, permanent eye damage, nausea, vomiting, mental confusion and cancer [9,10,11]. The removal of dye from the environment is a major requirement.

Various methods have been used for dye removal [5,6,7]. However, the adsorption method is still the best due to its extraordinary removal efficiency, adaptability, simplicity, recyclability and low cost [12,13,14]. The adsorption method for achieving high efficiency requires an adsorbent with high adsorption capacity. Many adsorbents have been used, both natural and synthetic. Of these, nanoparticles are good adsorbents due to their large area-to-volume ratio [15]. Hence, the adsorbent must exhibit high removal capabilities, fast uptake routes, and a robust mechanical structure. Metal oxide nanoparticles have extensive applications in removing different dyes because organic dyes can interact with the transition metal of the metal oxide nanoparticle [16]. Among various metal oxide nanoparticle adsorbents, MoO₃ and WO₃ were selected in this study.

MoO₃ is a transition metal oxide with many applications in the field of the environmental due to its unique properties as chromogen, electronics, mechanical and catalytic properties [17, 18]. It is a common choice for water remediation applications due to its high surface area, energy storage, acid resistance, thermal stability, and low cytotoxicity [18,19,20].

WO₃ is characterized by its flexibility and structural stability in a harsh environment [21, 22]. It has many environmental applications for pollutant removal due to its exceptional sensing, adsorption and/or photocatalytic properties, and is environmentally friendly [23,24,25].

A variety of physical and chemical methods have been used to synthesis metal oxide nanoparticles. Yet some of these methods use a lot of energy, are expensive, need expensive equipment, and are not environmentally friendly. The current trend is towards green synthetic technologies, which reduce the number of steps in the process and greatly reduce the use of costly and hazardous chemicals.

For the first time, the MoO₃ and WO₃ nanorods were synthesized using the Leidenfrost method. The Leidenfrost method synthesized nanoparticles through the salt solution and a heater. The technique involves forming solution droplets on a hot plate at a temperature above 200 °C (depending on the type of liquid and surface), known as the Leidenfrost temperature [26,27,28,29]. The metal oxide nanoparticle formation is illustrated as follows: a drop of water is sprinkled on a hot surface at a surface temperature above the boiling point of the water. The water drop passes through three stages as follows (1) the outer coating of the drop is evaporated as a result of touching the hot surface; (2) due to the evaporation of the outer layer of the drop, the remaining parts of the drop levitated above the hot surface which separates with a zone of the vapour; and (3) the expanded of the water drop on the hot surface leading to the fast evaporation of all droplet layers till drying leaving powder salt on the surface [28]. Understanding how metal oxide nanoparticles are formed necessitates understanding how water molecules are converted to H⁺ and OH⁻ ions via two different mechanisms. The water molecules are ionized due to (1) increasing precursor concentration, which aids in distorting water molecules' hydrogen bonds, thereby making it easier to ionize the water molecules [30]; and (2) heating the water to temperatures above its boiling point [31, 32]. In the Leidenfrost droplet, the second step (the levitated droplet), the water molecule ionized where inside the droplet, a negative charge was observed due to the predominant hydroxide ions [28]. However, the vapor has a positive charge outside the droplet due to the formation of hydronium ions. The Leidenfrost droplet is considered a reactor where the metal ion combines with the hydroxide ions forming a metal hydroxide, eventually turning into metal oxide.

This study aims to green synthesize MoO₃ and WO₃ by self-assembly Leidenfrost method and study the synthesis method effect on the features of the prepared nanorods, then explore the capacity of MoO₃ and WO₃ surfaces on the adsorption of methylene blue dye from wastewater in a comparative study.

Experimental

Materials

Ammonium molybdate tetrahydrate 99.5% (Koch-Light Laboratory Ltd, England), tungsten trioxide 99.8% (Alfa Aesar, Germany), methylene blue dye, NaOH (DOP, TORKIYA), NH₄OH 28% (DOP, TORKIYE) and HCl 37% (DOP, TORKIYA) were used. All the chemicals of analytical grades were used with no further purification.

Synthesis of MoO₃ and WO₃ nanorods

The Leidenfrost method was used to synthesis the adsorbent nanorods. 0.2 M molybdenum and tungsten solutions in 50 ml distilled water have been prepared. To dissolve both salts, 50 ml of NH₄OH conc. or 50 ml of NaOH 2 M solutions were added to molybdenum or tungsten solutions. The salt solution was added drop by drop through a burette to a clean, hot beaker until a white powder of MoO₃ and WO₃ appeared. The resulting powder was analyzed for characterization.

Characterization

The synthesized metal oxide nanorods were investigated by BrukerAXSD8 Germany x-ray diffraction (XRD) Radiation of Cu Kα at λ = 0.154 nm. FE-SEM (Field Emission-Scanning Electron Microscopy) was used (FE-SEM, QUANTAFEG250, The Netherlands) at 20 kV. Burnauer-Emmett-Teller (BET) Surface Area analysis was used to determine the surface area of the metal oxide nanorods beyond samples degassing at 77.35 K (Quanta CHROME NOVA 2000 Series, UK). Fourier transform-infrared (FTIR) analysis was done by Agilent Technologies, Cary 630, via a spectral transmittance measurement at room temperature. The spectral measurements were done in the range 400–4000 cm⁻¹ at 2 cm⁻¹ spectral resolution to characterize and investigate the variation in the functional groups of the adsorbent before and after adsorption. All the characterization was done using the powder form of the synthesized substance.

Batch experiment

The batch experiment was done by adding different doses (0.01–0.15 g) of the metal oxide nanorods to 50 ml of various initial concentrations (10–100 ppm) of MB dye. The resultant solutions were Shacked with a speed of 200 rpm (10–60 min). The pH effect has been investigated in the range of 2–10. After equilibrium, the MB dye concentration was measured with a UV–Vis spectrophotometer (UVmini-1240 SHIMADZU) at a maximum wavelength of 660 nm.

The removal percentage and the amount of MB dye adsorbed at equilibrium q_e were calculated using the Eqs. (1) and (2):

$$\% Removal= \frac{Co-Ce}{Co}\times 100$$

(1)

$$qe= \frac{Co-Ce}{m}\times V$$

(2)

where C_o and Ce are the initial and equilibrium concentrations (mg/L), respectively. m (g) adsorbent weight and V solution volume.

Results and discussion

Characterization of nano adsorbents

XRD analysis

A typical XRD analysis was used to characterize and indicate the compound formation of MoO₃ and WO₃ nanorods by the Leidenfrost method. From Fig. 1a, WO₃ nanoparticles were formed with a hexagonal structure in a pure phase at 2 thetas, 13.82, 23.05, 28.02, 36.70, 49.71, 55.54 and 63.35^o with an average crystallite size 46.28 nm. Notably, no diffraction peaks other than hexagonal WO₃ have been observed.

Fig. 1

XRD pattern of (a) WO₃ nanopowder and (b) MoO₃ nanopowder synthesized using the Leidenfrost

For MoO₃ nanorodes, Fig. 1b, orthorhombic MoO₃ nanoparticles were obtained at 2 theta, 12,27, 33, 45, 46, 49, 52, 54, 55, 56, 57, 58, 64, 67 and 68°. The crystallite size was determined to be 53.05 nm. A diffraction peak of the hexagonal molybdenum oxide hydroxide hydrate second phase was observed, which indicates MoO₃ is not in a pure form. Crystallite size was calculated using the Scherer equation (Eq. 3) as follows:

$$D=\frac{0.9\lambda }{\beta cos\theta },$$

(3)

where D, λ, β and ϴ are crystallite size, X-ray source wavelength, the full width at half maximum of the peak and the angle at which diffraction intensity is maximized.

SEM analysis

The surface morphologies of MoO₃ and WO₃ nanopowder were described in Fig 2a, b. For MoO₃, a nanorods and plate-like morphology were obtained with different particle sizes. At the same time, WO₃ exhibit a rods structure.

Fig. 2

SEM morphology of (a) MoO₃ nanopowder and (b) WO₃ nanopowder synthesized using the Leidenfrost method

Batch experimental

Effect of adsorbent dose

For both adsorbents (MoO₃ and WO₃), the effect of the adsorbent dose was investigated in the range of 0.01–0.15 g. Figure 3 shows that the highest elimination percentage (98%) was achieved by 0.03 and 0.1 g for MoO₃ and WO₃, respectively.

Fig. 3

Effect of adsorbents doses for removal of 50 ppm MB using WO₃ and MoO₃

The percent removal of the MB dye using the WO₃ adsorbent increased as the adsorbent dose increased (0.01–0.1 g) (86–98%). This is explained by the fact that WO₃ has a limited surface area, 2.67 m²g⁻¹, according to BET measurements, implying that increasing the WO₃ dose resulted in more active sites on the WO₃ surface. Due to the WO₃ surface saturation, the increase in the WO₃ amount caused the removal percentage to drop to 96%. In contrast to WO₃, MoO₃ surface saturation occurs faster, with maximum removal of 0.03 g and a gradual percentage reduction to a minimum of 0.15 g with 93%. In comparison, the MoO₃ nanopowder had a higher removal % over the dose range investigated as well as a faster surface saturation with a lower dose than the WO₃ nanopowder. This is owing to the high surface area of the MoO_3, which measures 24.72 m²g⁻¹ according to BET measurements.

Effect of shaking time

The effect of contact time for MB dye adsorption on MoO₃ and WO₃ was studied in the range of 10–60 min. From Fig. 4, it is clear that the amount of the dye adsorbed using the two adsorbents is relatively fast. The WO₃ adsorbent gives a removal percentage ranging from 98% for 10, 30, and 50 min to 99% for 40 and 60 min., so 40 min was chosen as the optimum contact time. In the case of MoO₃ adsorbent, as illustrated in Fig. 4, a gradual increase in the contact time led to a gradual increase of the dye removal percentage to be maximum at 50 min with 99.5%.

Fig. 4

Effect of shaking time on adsorption of 50 ppm methylene blue dye using MoO₃ and WO₃ nanoparticles

By comparing both adsorbents, the WO₃ surface response time of the MB dye is faster than the MoO₃ nanorods. MoO₃ nanopowder has a lower efficiency removal percentage along the examined contact time range than WO₃.

Effect of pH

The effect of pH is an essential parameter that needs to be investigated because of its impact on the charge of the adsorbent surface and the mechanism of adsorbent removal. pH was studied between 2 and 10 for both adsorbents in this study. From Fig. 5, for WO₃ adsorbent, pH 2 shows 99.00%, while for MoO₃ adsorbent, pH 10 gives 99.85% removal for the MB. These results show that the two adsorbents work in two different media where WO₃ gives maximum removal at a very acidic medium, whereas MoO₃ yields the highest percentage in a very basic medium.

Fig. 5

Effect of pH on adsorption process of MB using WO₃ and MoO₃ nanoparticles

For WO₃ nanorods, during all studied pH, high efficiency removal of MB (≥ 90%) was observed. A high percentage of elimination was recorded at pH 2 (99%), decreasing progressively with increasing pH. This is explained by the fact that, according to the previous literature, the point of zero charge (PZC) of WO₃ nanorods almost ranged 2.5 [24, 33,34,35]. Below the PZC, the WO₃ surface has a positive charge; above PZC, the surface has a negative charge. On the other hand, the stability of the WO₃ nanorods is affected by the pH of the solution, which dissociates in highly acidic and basic environments by the H⁺ and OH⁻ ions. Thus, in a highly acidic medium, the WO₃ surface protonated to be as follows [33, 36] (Eqs. 4, 5)

$${\text{WO}}_{{{3}\left( {\text{s}} \right){ }}} {\text{ + H}}^{ + } {\text{ = WO}}_{{2}} {\text{OH}}^{ + }_{{\text{(aq)}}} .$$

(4)

$${\text{WO}}_{{2}} {\text{OH}}^{ + }_{{\text{(aq)}}} {\text{ + H}}^{ + } {\text{ = WO}}_{{2}}^{{2 + }} \left( {{\text{aq}}} \right){\text{ + H}}_{{2}} {\text{O}}{.}$$

(5)

Increasing the pH led to reducing the removal percentage to be minimized at pH10 by 90%. WO₂²⁺ formed on the surface of the WO₃ adsorbent (Eq. 5) gives the surface a positive charge and as known from the previous literature, MB dye is classified as a cationic dye that has a positive charge [8], so a repulsion force between the adsorbent surface and adsorbate is established. Above the point of zero charge, the surface is negatively charged, which facilitates the adsorption of the cationic dye through electrostatic attraction. This is attributed to that WO₃ particles are not chemically stable in stronger alkaline solutions as they tend to dissolve due to alkaline corrosion (Eqs. 6, 7) [24].

$${\text{WO}}_{{{3}\left( {\text{s}} \right){ }}} {\text{ + 2NaOH = Na}}_{{2}} {\text{WO}}_{{4}} {\text{ + H}}_{{2}} {\text{O}}_{{\text{(aq)}}} {.}$$

(6)

$${\text{Na}}_{{2}} {\text{WO}}_{{4}} {\text{ + H}}_{{2}} {\text{O = 2 Na}}^{ + } {\text{ + WO}}_{4}^{2 - } .$$

(7)

On the other hand, for MoO₃ adsorbent, the removal efficiency be minimum at pH 2 (58%) which increased gradually to be maximum at pH 10 (99.85%). The maximum removal was observed in very basic environments. In aqueous solutions, the metal oxide surface adsorbs water molecules that separate to OH⁻ forming M-OH. In alkaline media, deprotonation of the hydroxyl groups on adsorbent surfaces, as shown in Eq. (8) occurs due to the amphoteric performance of most heavy metal hydroxides [33]. The adsorption mechanism on the MoO₃ surface is controlled by electrostatic attraction between the negative surface charge and the MB positive charge [37]. As a result, as the pH increases, the uptake of the dye increases to be maximum at pH 10. These results agree with Rakass, et al., which found pH 11 yields 99% removal of MB [38] and (2), et al., Jiang et al. [39] Li et al. [40] found pH 9 is an optimum condition.

$${\text{M}}-{\text{OH=M}} - {\text{O}} - {\text{ + H}}^{ + } {.}$$

(8)

Comparing the two adsorbents’ surfaces, WO₃ surface has high efficiency removal through all studied pH than MoO₃.

Effect of dye concentration

The effect of MB dye concentration on adsorption was illustrated in Fig. 6. It is clear that, or WO₃, as the MB concentration increase, the percent removal increase to be maximum at 50 ppm with 99.4%, which become nearly constant (99%) for the higher concentrations, means that the WO₃ surface has high vacant active sites and the capacity to adsorb to 100 ppm [41].

Fig. 6

Effect of concentration on Adsorption capacity of MB Dye on WO₃ and MoO₃ nanoparticles

In the case of MoO₃ nanopowder, Fig. 6, it was clear that 10 and 30 ppm achieve the same removal percentage (95%), which becomes a maximum at 50 ppm. Beyond 50 ppm, the percent removal decreases to be minimized (79%) at 100 ppm. Comparing MoO₃ and WO₃, the MB uptakes achieve a higher percentage using WO₃ nanopowder than MoO₃.

FTIR

The FTIR spectra of the prepared WO₃ and MoO₃ nanopowder exhibit a typical vibration in the range of 400–4000 cm⁻¹. The WO₃ and WO₃ + MB FTIR are shown in Fig. 7a. Distinct bands at 624, 770 and 829 cm⁻¹ were attributed to the stretching and bending vibrations for O–W–O and W–O–W in WO₃. A vibration bands were obtained at 1639 and 3435 cm⁻¹, related to OH from the H₂O molecule [42]. On the other hand, after MB adsorption, two new peaks were formed at wavenumbers 1404 and 2360 cm⁻¹, corresponding to the aromatic ring structure of MB and the C–O as well as C=O groups, respectively [43]. FTIR of WO₃ shows bands at wavenumbers 668, cm⁻¹ attributed to O–W–O of WO₃ [44]. The vibration bands at 1625, 3434 and 3729 cm⁻¹ are related to the OH group of the water molecule [45].

Fig. 7

FTIR of (a) WO₃ Nanoparticle before and after MB adsorption. b MoO₃ nanoparticle before and after MB adsorption

The FTIR of MoO₃ and MoO₃ + MB were represented in Fig. 7b. Four characterized peaks were observed at 600, 872, 993, and 3446 cm⁻¹ for MoO₃ nanorods. The peaks at 3446 cm⁻¹ seem to arise from the O–H modes of the water of representation. The strong vibration band is observed at 600 cm⁻¹, corresponding to the stretching vibrations of Mo–O–Mo [46]. The strong peaks at 872 cm⁻¹ and 993 cm⁻¹ indicate the stretching vibrations of Mo=O [47]. A small peak formed at 1631 cm⁻¹ is related to the -OH bending vibration band. After dye adsorption, a small shift was observed for all peaks to a higher wavenumber with an enhanced peak intensity. In the case of MoO₃ + MB, the same four basic peaks as in the MoO₃ nanorods were formed as well as two new peaks at 1433 and 2924 cm⁻¹ were observed, which related to the aromatic range in the MB dye and –CH– aromatic stretching vibration band [48].

Isotherm study

Many different models were used for defining equilibrium experiments for the adsorption of various pollutants on solid surfaces. Freundlich and Langmuir's most popular isotherm models were applied in this study. The isotherm models illustrate the relation between the adsorbate and adsorbent and surface homogeneity.

Freundlich and Langmuir mathematical equations are described in Eqs. (9, 10, respectively), where q_e (mg g⁻¹) the amount of adsorbate adsorbed/gram of adsorbent at equilibrium, C_e (mg L⁻¹) the equilibrium concentration in solution, k_F and K_L are Freundlich and Langmuir constants cooperated to adsorption capacity (L/mg). 1/n an empirical value correlated to adsorption intensity. When plotting log q_e against log C_e, the 1\n and log k_f values can be obtained from the slope and straight-line intercept [49]. For this study, the K_f and 1\n were shown in Table 1. Considering the R² values of the two adsorbents from the isotherm models (Fig. 8; Table 1), it was obvious that the two studied adsorbents follow the Langmuir model (R² = 0.99). The isotherm follows Langmuir meaning that the MB dye adsorption occurs in a homogeneous chemosorption monolayer in adsorbents surfaces. The maximum adsorption capacity (q_m) was 102.37 and 151.41 mg g⁻¹ for WO₃ and MoO₃ nanopowder, respectively.

Table 1 Isotherm model parameter of adsorption of MB dye

Fig. 8

Adsorption isotherms of MB dye onto WO₃ and MoO₃ nanoparticles

$$\mathrm{log}{q}_{e}= \mathrm{log}{K}_{f}+ \frac{1}{n}\mathrm{log}{C}_{e}$$

(9)

$$\frac{{C}_{e}}{{Q}_{e}}=\frac{1}{{q}_{m}}{C}_{e}+ \frac{1}{{q}_{m}{K}_{L}}$$

(10)

The fundamental properties of the Langmuir isotherm can be expressed in terms of a dimensionless constant known as the separation factor or equilibrium parameter, which is given by the following Eq. (11) [50].

$${{{R}}}_{{{L}}}=\frac{1}{1+{{{K}}}_{{{L}}}{{{C}}}_{{{o}}}}$$

(11)

where C₀ (mg/L) is the initial MB concentration and K_L (L/mg) is the Langmuir constant. The value of the R_L illustrates the adsorption isotherm shape and favorability of the adsorption process based on the Langmuir isotherm. The nature of the adsorption process according to the R_L value describe as follow: if R_L > 1 undesirable, R_L = 1 linear, R_L = 0 Irreversible and 0 < R_L < 1 desirable. In this study, for the WO₃ and MoO₃ adsorbents the R_L value range in 0 < R_L < 1 as illustrated in Table 1 meaning that the adsorption o MB dye on the two adsorbent surfaces is desirable.

Kinetic study of dye adsorption

The kinetics of MB adsorption using WO₃ and MoO₃ nanorods were studied by pseudo-first-order Eq. (12) [51] and pseudo-second-order models Eq. (13) [52, 53]:

$$\mathrm{log}\left({q}_{e}-{q}_{t}\right)=\mathrm{log}{q}_{e}-\left(\frac{{K}_{1}}{2.303}\right)t$$

(12)

$$\frac{{{t}}}{{{{q}}}_{{{t}}}}= \frac{1}{{{{K}}}_{2}{{{q}}}_{{{e}}}^{2}}+\frac{{{t}}}{{{{q}}}_{{{e}}}}$$

(13)

where q_e (mg/g) and q_t (mg/g) are the capacity of the adsorbed MO on the adsorbent at equilibrium and at time t; k₁ (min⁻¹) and k₂ (g/mg·min) are the pseudo-first-order and pseudo-second-order rate constant, respectively. T (min) is the adsorption time. In comparing the two adsorbents, Fig. 9 and Table 2, it was clear that the two adsorbents achieve the pseudo-second-order model with R² values of 0.99, meaning that chemosorption adsorption occurs.

Fig. 9

Kinetics adsorption study of MB dye onto WO₃ and MoO₃ nanoparticles

Table 2 Adsorption kinetic parameters of MB adsorption

Table 3 summarizes the findings of this study and compares them to other literature. Table 3 shows that MoO₃ nanorods have the highest adsorption maximum capacity (q_m) followed by WO₃ compared to other adsorbents. Also, high adsorption removal (99%) was obtained with the two adsorbents of this study in a short time (40 for WO₃ and 50 min for MoO₃) relative to other studies except for activated carbon (AC) which recorded 5 min shaking time optimum equilibrium condition with 95% removal.

Table 3 Comparison of the adsorption best conditions of this study with other literature

Conclusion

The environmentally benign Leidenfrost process synthesizes WO₃ and MoO₃ nanorods in succession, with crystallite diameters of 46.28 nm and 53.05 nm, respectively. Comparing the two adsorbents shows that both adsorbents have high removal capacity for MB dye removal. The adsorption equilibrium for both adsorbents follows the Langmuir model with a maximum adsorption capacity of 24.34 and 151.41 mg/g for WO₃ and MoO₃, respectively. The results of the kinetic study indicated that both adsorbents undergo the pseudo-second-order pattern. The adsorption equilibrium takes place in 40 and 50 min with a small dose quantity (0.1 and 0.03 g/50 ml) for WO₃ and MoO₃.