Super Hydrophilic Nanoporous AC/WS2 QDs/SSM for Highly Efficient Oil-Water Mixture Separation

INTRODUCTION
Effluents containing oil-water mixture are produced in many industries such as metalworking, oil refining, petrochemical industries, food industries, leather industries and metal polishing. The sources of these effluents include cooling fluids in metalworking processes, namely metal rolling, metal cutting and wire making, washing baths for cleaning metal parts and car wash effluents [1-3]. A filter surface that has two properties, i.e. hydrophilic and super lipophilic, is suitable for separating water and oil. Such a filter can be produced by the accumulation of activated carbon on metal mesh [4,5]. Separation methods of oil-water mixture include adsorption and Nano filter methods that both use porous substrate, but due to low efficiency, low adsorption rate and secondary pollution, nanomaterials can be used to improve their performance. In meantime, carbon and activated carbon (AC) filters are a good option. AC is a unique material due to its high surface area, high porosity, high adsorption capacity, surface reactivation capability, and low cost. Its important applications are in purification, recycling and separation [6,7]. Carbon materials also have the property of being friendly; therefore, AC has a high lipophilic property owing to its capillary properties. Research has also shown that decreasing surface roughness prevents water leakage. Ergo, the manufacture of metal mesh in which AC is used, can be used to separate oil from water. Metal mesh has high mechanical strength, mass production and high filtration flux [8,9]. Nanomaterials has attracted the attention of many researchers due to their unique physical and chemical properties [10]. Wang et al., prepared flower-like CdS with SDBS surfactant that make the CdS crystals assemble together and also studied its optical properties [11]. Wang et al., fabricated nanostructured CuO films with indium tin oxide - coated glass substrates by electrodeposition method and studied its optical properties [12]. 
Wang et al., synthesized flower-like ZnO nanostructures assembled by nanosheets and discussion its structural and optical characterization [13]. Yang et al., prepared CdS/TiO2 nanotube and discussed its photocatalytic property [14]. Yang et al., synthesized a series of Zn-doped dendritic-like CdS structures by a simple hydrothermal method that enhanced photocatalytic activity [15]. Wang et al., fabricated diverse CuO nanostructures by a hydrothermal method and investigated their photocatalytic properties [16]. Among these, the QDs are colloidal semiconductor nanocrystals that have high quantum efficiency, wide absorption spectrum, narrow emission with adjustable capability and color stability. The energy gap of these materials depends on several factors, including the type of bond, the bond strength, and the particle size. The phenomenon of quantum constraints causes a wide range of features in these nanoparticles [17]. QDs have different absorption and diffusion properties depending on their size. Among these nanomaterials are families of chemical compounds that are known as dichalcogens intermediate metals and having shown interesting properties [18-19]. Super hydrophilic materials have also significantly contributed to progress in surface science. The recent extensive research of super hydrophilic coating has been inspired from lotus leaf wetting, known as the “lotus effect” [20-22]. Among these, Nano porous AC/WS2 is one of the super-hydrophilic materials with the property of separating oil from water. WS2 is a semiconductor with indirect band gap. This compound has a hexagonal structure and a spatial group of P63/mmc. WS2 has a layer structure in which each layer of tungsten atoms is covalently bonded to S atoms. S-W-S joints are weaker adjacent layers and are made of van der Waals force. The stacked layers in this structure are such that each layer is located along the axis of accumulation of W atoms with a period of rotation after the S atoms, which causes the layer to deviate completely from the flat state [23-26].  Krasian et al., studied a hybrid of 2D materials (MoS2 and WS2) as an effective performance enhancer for poly (lactic acid) fibrous mats in oil adsorption and oil/water separation. According to the results, effective separation of surfactant-stabilized oil/water emulsion was obtained with ~70% flux recovery over separating cycles [27]. Zhou et al., delved into electroplating of non-fluorinated superhydrophobic Ni/WC/WS2 composite coatings with high abrasive resistance. Hardened by WC and lubricated by WS2 inclusions, the superhydrophobic coatings showed remarkable abrasive resistance with a bearing capacity ≥10,000 mm abrasion length [28]. Cheng et al., investigated hybrid 2D WS2/GO nanofiltration membranes for finely molecular sieving. The hydrated hybrid WS2/GO membrane with 15 wt% GO (GO15WS2) exhibited high rejection (>90%) for dyes and ions with a hydrated radius above 4.9 Å [29]. In this study, i.e. nanoporous AC/WS2 QDs superhydrophilic composite, reproducibility and fast time were prepared and coated on the mesh for easier collection with one-step hydrothermal method, then They were studied for separation of oil-water mixture. The effect of pH and solvent of different water and solvent ratios were investigated in separation. It should be noted that the stabilization of AC/WS2 QDs nanoparticles on metal mesh and its application in separating oil-water mixture could be a research innovation.
Nevertheless, there are still many limitations in the synthesis of such filters which have applied uses on manufacturing scale for either industrial or very small particles, small enough to separate single step water-oil stratified. Moreover, they have a cost-effective, good collection and easy construction method for this purpose.
In this study, nanoporous AC/WS2 QDs superhydrophilic composite, reproducibility, high selectivity, good flux and fast time were prepared and coated on the mesh for more resistance and easier collection with low cost and one-step hydrothermal method for investigating separation of oil-water mixture. It should be noted that the stabilization of AC/WS2 QDs nanoparticles on metal mesh and its application in separating oil-water mixture is a research innovation. 

MATERIALS AND METHODS 
The raw walnut shells were collected from Tuyserkan, Hamedan province, Iran and East Azerbaijan mines, Iran respectively.  Sodium tungstate 99.99% (CAS Number: 13472-45-2), thiourea 99.99% (CAS Number: 62-56-6), Hydrochloric acid 37% (CAS Number: 7647-01-0), potassium hydroxide 99.99% (CAS Number: 1310-58-3), ethanol 96% (CAS Number:  64-17-5), n-Hexane 99% 99% (CAS Number: 110-54-3), toluene, anhydrous, 99.9% (CAS Number: 108-88-3), ethyl acetate anhydrous 99.8% (CAS Number: 141-78-6), Dimethyl ether ≥99.9% (CAS Number: 115-10-6), Dimethyl carbonate anhydrous, ≥99% (CAS Number: 616-38-6) and 1,2-Dichloroethane anhydrous, 99.8%; CAS Number: 107-06-2,  diesel and gasoline were prepared, 0.4 micron stainless steel mesh filter was provided by Iran Steel Company.

Nano porous AC synthesis method
Walnut shell was washed, dried and ground. Then, KOH was added following the ratio of 1 to 3. It was placed in a nitrogen atmosphere at 800 oC for 1 h. The resulting powder was then washed with 1 mM hydrochloric acid and dried at 120 oC in an oven (Fig. 1).

AC/WS2 QDs/SSM synthesis method
0.88 g of Na2WO4, 1.14 g of thiourea and 0.1 g of Nano porous AC with 30 mL of distilled water and 100 mL of 97% ethanol and metal mesh with a diameter of 5 cm were placed in the autoclave. It was heated for 12 hours at 200 oC in the oven (Fig. 2).

Oil-water mixture separation
First, we put two flange-shaped glass cylinders inside the Teflon and placed the silicone gasket on the bottom cylinder. Then, the mesh, pre-soaked with water, was placed on the washer and the second washer was placed on the mesh; the top cylinder and Teflon were easily wrapped in the lower set to completely seal the two cylinders mesh and washers. Various samples of pure water, petrol, gasoline, hexane, toluene, ethyl acetate, dibutyl ether, dichloroethane, and dimethyl carbonate with volumetric ratios of 20/20, 40/20 20/40 are mixed in pH 5, 7 9 were prepared and the mesh filter was rejected (Fig. 3).
The water contents before and after Nano filter were measured using the Karl Fischer moisture titration method.
The oil absorption capacity was measured by immersing, the Nano porous CA/WS2 QDs/SSM in different oils until saturation. 
The kinetic of oil sorption was performed by recording oil sorption at a certain time interval (2 s) by placing AC/WS2 QDs/SSM on the surface of the oil. The time span of the experiment was 2 s to 14 S.
The kinetic of sorption by AC/WS2 QDs/SSM is presented in Table 2. The sorption of all oils increases with sorption time until saturation. The kinetic of oil sorption can be described by pseudo-first, second order and Intra particle diffusion kinetic model [30]. The equations and values of rate constants are listed in Table 2.

RESULTS AND DISCUSSION
X-ray diffraction analysis
X-ray diffraction (XRD) analysis was used to identify the structure of the material. Fig. 4 shows the scattering pattern of XRD Nano porous AC, WS2 and Nano porous AC/WS2 QDs. In Nano porous AC, the broad (002) diffraction peak (2θ = 15-30º) can be attributed to the amorphous carbon structures. The weak and broad (101) diffraction peak (2θ = 40-50º) is due to the axis of the graphite structure.
In Nano porous AC/WS2 QDs, the peaks at angles 24o and 43o are related to AC and WS2. The diffraction peaks at 2θ value of 15.45o, 29.75o, 30.61o and 51.16o were observed, ascribed to the (002), (004), (100) and (105), the results showed that indexed to the hexagonal WS2 phase with cell constants of a = 3.154 Å and c = 12.362 Å, which is consistent with the standard JCPDS card no. 08-0237. 
The crystalline size of the prepared Nano porous AC, WS2 and Nano porous AC/WS2 QDs are calculated using Scherrer’s formula:
D signifies crystallite size, λ denotes the cathode wavelength (nm), K is constant equal to 0.89, β FWHM (rad) and θ angle are corresponding to the desired peak.
Table 1 shows the calculated crystalline and lattice parameters of the prepared Nano porous AC, WS2 and Nano porous AC/WS2 QDs. 

Proposed Mechanism
Functional groups in the molecular level recover the wettability of the surface. Generally, the hydrophilicity is affected by decreasing the carbon length and functional groups shape [31]. Therefore, Ac is a substrate for WS2 QDs, according to the FTIR spectrum advent peak at 700 cm-1 and 1100 corresponding to the S-C (Fig. 5). The main pattern is played by WS2 in this matrix. The interaction between the AC/ WS2 QDs surface and hydrocarbon molecules (oil) decreases due to molecular defect of S atoms at the substrate surface (Fig. 13), hence the access of the S atomics to hydrocarbon (oil) molecular decreases and cannot form a bond between sulfur–carbon all of the substrate. This could result in enhancement the interfacial tension, causing the surface to become super hydrophilic.
Fourier-transform infrared spectroscopy 
Fig. 5 shows the FTIR spectrum of the prepared Nano porous AC peak at 1095.38 cm -1 assigned to C-O-C, at 1628.13 cm-1 attributed to C=O and in the peak range of 3442.50 cm-1 corresponding to the tensile vibration of OH.
The FT-IR spectra of Nano porous AC/WS2 QDs in the range of 442.53 cm-1 for W-S tensile vibration, in the range of 623.79 cm-1 for C-S, in the range 10.968 and 1405.29 cm-1, corresponds to C-O-C, in the range 1623.02 cm-1 corresponds to C= O, and in the peak range 3427.17 cm-1 corresponds to the tensile vibration of OH (Fig. 5).

Photoluminescence analysis
During photoluminescence, the solution of WS2 QDs was obtained by excitation at wavelengths of 300 to 460 nm and in the range of 10 nm. This result is consistent with the spectrum obtained in Fig. 3. The response peaks of the photovoltaic spectrum show a red shift as the wavelength of the action increases. According to this shift, the responses move to higher wavelengths (360 to 420 nm). This indicates a non-uniform distribution of particle size and the effects of quantum constraints. As the stimulus wavelength increases, the intensity of the response increases, reaching its maximum at 340 nm, and then decreases. Therefore, the maximum response intensity in ultraviolet excitation is 340 nm, which is 412 nm blue. The built-in sample has a high intensity. After a few months, the emission spectrum was measured again and the same results were obtained. Therefore, the built-in quantum dots are stable and do not lose their photovoltaic properties (Fig. 6) [32,33].
QDs are nanoparticles with unique light properties that are capable of emitting light of different colors, and the color of the light depends on the size of their crystal. In this study, the light emission color of WS2 QDs is blue, indicating that the majority of our QDs sizes are within 2-5 nm range; large blue-shift is expected, as reported in literature (Fig. 7).

SEM analysis 
SEM analysis was used to evaluate the morphology and particle size. In the Fig. 8, Nano porous AC, Nano porous AC/WS2 QDs and Nano porous AC/WS2 QDs SSM are respectively shown. The results of WS2 morphology are rod, and Nano porous AC/WS2 particles are well established on the mesh (Fig. 8). Based on the results, the diameter of AC nanoparticles is about 19 nm and the diameter of the rod like WS2 is about 8 nm.

X-ray energy spectroscopy
X-ray energy spectroscopy is an analytical method used to structurally analyze or chemically characterize a sample. In this study, EDS/Map analysis was used to evaluate the weight percentage of AC/WS2 elements. According to the results, the weight percentages of C, S and W were 46.14, 20.89 and 32.43, respectively (Fig. 9).
The Nano porous AC and Nano porous AC/WS2 QDs TEM images are shown in Fig. 10. Nano porous AC TEM image are shown in dimensions of 50 nm. According to the results, the particles are completely uniform and nanoscale. Based on Fig. 10 (b), it can be observed that these Nano Rod are assembled, which are about 100 nm in length and 10 nm in Diameter, respectively. According to the result of TEM, the particle size and morphology of Nano porous AC and Nano porous AC/WS2 QDs are in agreement with the result of XRD and SEM.
 
BET-BJH Analysis
The measurement of specific surface area, diameter, volume and size distribution of material cavities have very important applications. These methods include absorption-based, diffraction-based, and imaging-based methods. Among these methods, the BET method was introduced in 1938, based on absorption, which is much simpler, more accurate and more reliable than other methods. The basis of this method is based on measuring the amount of nitrogen gas absorbed and absorbed by the surface of the material under consideration and at a liquid nitrogen temperature of 77 k. In this way, the cell containing the sample to be analyzed is placed in a liquid nitrogen tank, then with a gradual increase in nitrogen gas pressure with a high purity percentage during different stages, the amount of gas volume absorption by the material is calculated. The results showed that Nano porous AC and Nano porous AC/WS2 QDs have surface area of 2736.36 and ​​301.76 (m2/g), the total volume of cavities are 1.43 and 0.182 cm3/g , and the average cavities are 20.64 A0 and 2.41 nm (Fig. 11).

Contact angle analysis
Using the contact angle measurement system, the amount of hydrophilicity and hydrophobicity, can be investigated.
To examine the hydrophilicity property of Nano porous AC/WS2 QDs, contact angle analysis was performed. As seen in Fig. 10, the contact angle of the Nano porous AC/WS2 QDs is under 90 °; therefore, the AC/WS2 QDs is hydrophil. The sample AC/WS2 QDs/SSM with distinguished wettability towards water and oil could be reckoned as a good separation membrane for oil-water mixture separation.

Number of Atomic Vacancy
The cyclic voltametric (CV) diagram (Fig. 13) shows that flow of the curve of AC/MoS2 QDs is very low by 0.8 µA, indicating that there is no structural defect in the molybdenum-sulfide composition due to the approximate molar ratio of 1:2 molybdenum to sulfide. Molybdenum atoms are all covalently bonded to sulfide atoms, and all sulfur atoms interact with hydrocarbons, blocking the passage of water and making the substrate super hydrophilic super oleophobic.

Thickness of layers
AFM analysis was carried out to deals with the effect of Nano porous AC/WS2 QDs and AC/WS2 QDs/SSM on the surface roughness. Fig. 14 indicates the AFM micrographs and the surface topography of the AC/WS2 QDs coated-mesh. AFM results reveal that the Root Mean Square (RMS) roughness (Rq) are 6 µm and 7 µm for AC/WS2 QDs and AC/WS2 QDs/SSM, respectively. Therefore, the surface roughness of AC/WS2 QDs/SSM is higher than of AC/WS2 QDs. Decreasing the surface roughness has led to the conversion of the surface from hydrophilic AC/WS2 to superhydrophilic AC/WS2 QDs/SSM. Furthermore, the micrographs confirm the micro/nanoscale roughness that is the consequence of packing uniform particles of the coating together tightly.  
As the AFM analysis shows, the thickness of the tungsten composite layer is about 6 nm, and according to the Young- Dupre equation [34], which is used to calculate the water contact angle, following as the layer thickness decreases, the contact angle decreases likewise and the surface becomes super hydrophilic.
The contact angle of water is calculated according to the Young-Dupre equation:

                                          
                                                                                             
Where wθ denotes the contact angle of water, and wγ is the surface tension of water and W (h) is the solid-liquid interfacial energy obtained from the following relation:

C is the repulsion between liquid and solid, h is the separation fraction, A is the Hammaker constant, and d is the thickness of the AC/WS2 QDs. As the thickness of the WS2 layer d decreases, the energy between the solid-liquid surface W (h) increases; as a result, the water contact angle decreases and the surface becomes hydrophilic.

Effect of solvent type, water and solvent ratio on separation efficiency
The amount of water in the oil-water mixture is one of the most effective parameters for the emulsification and stability of the emulsion. As the amount of water increases, so does the amount of unstable coarse emulsion, and as a result of separation, there is an increase. Also, the level of bipolar attraction is highly dependent on the size of the droplets and the distance between the droplets; therefore, in an emulsion that is the same size as the droplets, the distance between the proportional droplets and the inverse of the volume fraction is dispersed. Observing the results obtained from the experiments of the first stage, we conclude that the solvent type is also effective in the amount of its separation from water, because if we compare the results of the separation in pH = 7 (according to the solvent type) to the one used, it can be seen that the heavier the molecular weight of the solvent, the lower the separation value.  The effect of petrol, gasoline, hexane, toluene, ethyl acetate, dibutyl ether, dichloroethane (dichloromethane), and dimethyl carbonate was investigated. Based on the results of petrol with a water-to-solvent ratio of 20/20 at pH = 7 at a time of 14 seconds, 99.99% was separated by AC/WS2 mesh. Also, in order to investigate the effect of solvent of different water and solvent ratios, volumetric ratios of 20/20, 40/20 and 20/40 were examined as a mixture at pH = 7. The results showed that by increasing the solvent value, the separation efficiency decreased, while by increasing the amount of water, no effect was observed on the separation efficiency (Fig. 15 a) petrol, b) gasoline, c) hexane, d) toluene, e) ethyl acetate, f) dibutyl ether, g) dichloroethane, and h) dimethyl carbonate).

pH effect
One of the effective parameters in the stability and instability of oil-water mixture is pH. At very high and low pH values, emulsions tend to be more stable, while moderate pH values cause emulsion instability. The optimal pH value in the purification of oil-water mixture in gasoline is about 7. In fact, mixtures are more stable at a very high or low pH values (Fig. 16).

Recycling ability
Various samples of pure water, petrol, gasoline and toluene with volumetric ratios of 40/20 were prepared by mixing the pH 7 at 25 oC and the mesh filter was rejected. The results showed that the repeatability cycle for AC / WS2 porous QDs / SSM with 99.99% efficiency and over 6000 L m−2 h−1 flux is cycle 25 and its duration is up to 30 seconds.
The experimental data were fitted using kinetic equations [35]. Logergren has defined a pseudo first-order rate equation in the following form: 

                                                                                                                         
Where qe and qt (mol.g-1) are the amount of adsorbed material at equilibrium and time t, respectively, and k1 (min-1) is the rate constant of pseudo first-order adsorption. 
The Elovich equation is another rate equation based on adsorbent capacity. In this equation, by using the boundary conditions qt = 0 at t = 0 and qt= qt at t = t, we can obtain the following equation:                                                                                                            

                                
                                                                                    
Where α denotes the initial adsorption rate constant (mol.g-1 min-1) and  signifies β desorption rate constant (g. mol-1) during the experiment.  The Elovich equation is commonly used to determine the kinetics of liquid adsorption on heterogenous solids.
Blanchard (1984) , for the first time reported a pseudo second-order rate equation for ion exchange reactions. McKay and Ho defined the linear form of the pseudo second-order rate equation as follows: 

Where k2 (g.moL-1.min-1) is the rate constant of pseudo second-order adsorption.
 Based on results, Blanchard kinetic model has the highest agreement with experimental data (Table 2). 
The experimental data were fitted using kinetic equations. Based on results, Blanchard kinetic model has the highest agreement with experimental data (Table 2) and the results of this research are shown with the research of others in Table 3.

CONCLUSION
In this study, by Nano filter and the use of a mesh as a hydrophilic with contact angle below 90° and petroleum-based substrate, the oil mixture was separated from the water. Observing the results obtained from the experiments of the first stage, we concluded that the with different type of solvents in the different amount of its for separation from water. Based on the results of petrol with a water-to-solvent ratio of 40/20 at pH = 7 and at a time of 14 seconds, 99.99% was separated by Nano porous AC/WS2 QDs/SSM and over 6000 flux L m−2 h−1.The results showed that with the Nano porous AC/WS2 QDs/SSM, the repeatability cycle is 25 times and its duration to 30 seconds and Blanchard kinetic model was in good agreement with the experimental data. As shown in this study, the stabilization of AC/WS2 QDs nanoparticles on metal mesh and its application in the separation of oil from water is an efficient and inexpensive materials with high flux, high repeatability and simple method of research innovation which can be used in future studies even on industrial scale.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.