Retraction Retraction: 3D printed cellulose-based high-efficiency oil-water separation mesh ( IOP Conf. Ser.: Earth Environ. Sci. 514 022088)

. In this study, a high-performance oil-water separation mesh based on cellulose was fabricated by Direct Ink Writing (DIW) 3D printing technology. The 3D printing process can be undergo in a natural environment by using a cellulose acetate/ethyl acetate solution as a DIW printing ink. This oil-water separation mesh prepared has various advantages, such as uniform size, controllable aperture, simple and easy printing process, and rapid prototyping. The super high separation efficiency of 96 % and the high flux of 400 000 L·m -2 ·h -1 can be realized when the average pore size of the 3D printed super-hydrophilic cellulose mesh is 280 μm or less. Moreover, oil-water separation mesh prepared based on highly hydrophilic cellulose are chemically resistant to extreme acidity and alkalinity. It is also capable of separating oils of various viscosities. In addition, the cellulose mesh can cope with the mechanical force caused by high-frequency vibration, and maintain excellent oil-water separation efficiency in the external force field. Therefore, this highly efficient oil-water separation mesh can be used in oil-water separation processes in a variety of applications.


Introduction
Industry plays an important role in country's economic development. However, such industry activities will generate a large amount of oily wastewater 1 , which has become an extremely common pollutant in the world, and also causes serious environmental problems worldwide. A variety of oil-water separation methods and new materials have been invented so far to solve these problems. For example, using porous materials, such as sponge 2-3 , foam 4 and textile products [5][6] to prevent emergencies such as oil leakage and oil leakage. However, this porous material can only absorb a small amount of oil, and it has a very low oil-water separation efficiency along with the water absorption process. Furthermore, the reprocessing and reuse of the waste oil absorbed by porous materials is an extremely complicated process that requires a lot of time and money, so in most cases it is incinerated or buried for disposal, thereby generating some toxic gases and causing the consumption of land resources 7 .
Mesh filtration method has attracted widespread attention in both academia and industry due to its intuitive oil-water separation mechanism and high oil-water separation efficiency 7 . The large pore size of the mesh and the gravity of the liquid make it very easy to pass through the mesh. Therefore, the mesh filtration has a large flux, which effectively shortens the oil-water separation time and can significantly reduce the energy consumption of oil-water separation 8 . An important consideration in mesh filter is the wettability of the liquid, that is, the ability of the liquid to spread on a solid surface. At present, by R e t r a c t e d changing the nanostructure or microstructure of the solid surface, the super-wetting properties of many materials can be constructed. However, the super-wetness constructed by the micro-structure depends to a large extent on the micro-structure of the solid surface, if the micro-structure is destroyed, the superwetability of the solid surface disappears [9][10] . Moreover, this complex solid surface requires complicated preparation processes, which increases the cost of oil-water separation. Therefore, it is an effective idea to find a material that has super-wetability but independent on the complex micro-topological structure of the solid surface. As one of the environmentally friendly materials, cellulose has a wide range of sources and exists in all plant bodies. Due to cellulose contains a large number of hydroxyl groups, it has good hydrophilicity and excellent oleophobicity in the aqueous phase. This makes it independent on the complex surface micro-or nano-topology. 11 In this study, an oil-water separation mesh based on cellulose with higher efficiency was fabricated by using Direct Ink Writing (DIW) 3D printing technology. Because cellulose structure is rich in a large number of hydroxyl groups, it is difficult to dissolve in water or other organic solvents. Therefore, cellulose acetate was used as raw material and ethyl acetate was used as a solvent to prepare cellulose acetate/ethyl acetate 3D printing "ink". After the mesh is formed, it is immersed in a sodium hydroxide solution at pH 13 to hydrolysis. The final ultra-hydrophilic cellulose mesh filter membrane has excellent properties of high throughput and high separation efficiency, and it still maintains stability in a strong acid and alkyl environment.

Materials
Cellulose acetate (CA, number average molecular weight: 50 kDa, purchased from Sigma-Aldrich); trimethyl-terminated polydimethylsiloxane (PDMS, number average molecular mass: 5-7 kDa, purchased from Gelest); sodium hydroxide, hydrochloric acid, ethyl acetate, methanol, n-hexadecimal reagents such as alkane, cyclohexane and xylene were purchased from Sigma-Aldrich. All the above materials and reagents were used without further purification.
2.1.1. D printing. Cellulose acetate was first dissolved in ethyl acetate solvent at 80 °C with the concentration of 5 wt%, 10 wt%, 15 wt%, and 20 wt% respectively. After it is completely dissolved, cooling down to room temperature and stirring until use. Fill the prepared "ink" into a syringe with a Luer-lock conical tip. The capacity of the syringe is 10 mL. A three-axis positioning single-extrusion DIW printer (Allevi 2 Bioprinter, Allevi) is used for printing. The pattern is controlled by CAD software (Auto CAD, AutoCAD). An air compression device is used to control the extrusion pressure of the syringe and the "ink" flow rate. The print rate is controlled at 40 mm/s, and the print substrate is glass. After printing is completed, dry under natural conditions for 24 hours to allow the ethyl acetate to completely evaporate. Then, the printed mesh filter membrane is immersed in a sodium hydroxide solution of pH 13 for 6 hours to perform a hydrolysis reaction, thereby achieving the conversion of cellulose acetate to cellulose. After the hydrolysis, the mesh filter membrane was washed three times with distilled water and soaked in deionized water for 24 hours to further remove the sodium hydroxide solution.

Oil-water separation
The vertical device is selected for oil-water separation test, as shown in Figure 3. The area where the liquid is in contact with the mesh filter membrane is 4.9 cm 2 , and the flux (Flux) is defined as the volume (liter, L) of water passing through a unit area (square meter, m 2 ) per unit time (hour, h). The volume ratio of oil to water is 1: 1 and the total volume is 10 mL. After the oil-water separating, collect all the remaining oil and measure its volume with a cylinder. Then the oil-water separation efficiency can be calculated by the following formula (Equation 1): R e t r a c t e d Where Vc is the volume of the collected oil, and Vi is the volume of the original oil.

Characterization
The rheological properties of the "ink" are measured by a rheometer (Discovery Hybrid Rheometer) at 25 °C. The frequency of the oscillating mode is 6.2832 rad/s. The contact angle (CA) was measured by a light and shadow camera from VCA Optima. The water used was deionized water, and the test conditions were room temperature and humidity of about 50 %. The Nicolet 6700 spectrometer infrared spectrometer (FTIR) was used to characterize the chemical structure of cellulose acetate before and after hydrolysis, with a resolution of 4 cm -1 and a total of 32 measurements. Figure 1 shows the modulus-stress relationship of cellulose acetate/ethyl acetate solutions with different concentrations. The storage modulus is greater than the loss modulus in all solutions, indicating the solution has more obvious elastic behavior. With shear stress increasing, the storage modulus and loss modulus increase slightly. After the shear stress increases to a certain threshold, the storage modulus together with loss modulus dropping largely, suggesting the solution no longer maintains the elastic behavior. Then loss modulus is slightly higher than the storage modulus, indicating the solution begins to flow, it exhibits more obvious viscous behavior. For the solution with concentration of 20 wt%, both the storage modulus and loss modulus decrease rapidly, means the solution is very sensitive to stress. Therefore, in this study, a 20 wt% solution was selected as the "ink" for DIW printing.  R e t r a c t e d In this study, a two-layered cross-printing mode was used to prepare mesh filter. The printed mesh filter is shown in Figure 2a-b. The optical microscope shows uniform pore size distribution, suggesting the advantages of high precision and rapid prototyping of 3D printing. In this study, mesh filters with pore sizes of 200 μm, 280 μm, 360 μm, 440 μm, and 600 μm were printed respectively. Due to low hydroxyl group content in the cellulose acetate molecules, the cellulose acetate mesh filter needs to be subjected to a hydrolysis reaction to enhance its hydrophilicity. The reaction process was schematic illustrated in Figure 2c, and the FTIR spectrum was shown in Figure 2d. The hydroxyl content in cellulose acetate is very small (ν(OH) peak intensity is very weak). However, the hydroxyl content significantly increases after hydrolysis, and the spectral absorption peaks related to acetate are all weakened, such as the stretching vibration peak of the carbonyl group (ν (C=O)), the deformation vibration peaks of the C-O group and the C-H group in the acetate group (δ(C-O) and δ(C-H)). It demonstrates the cellulose acetate has undergone a conversion to cellulose. In addition, the C-O-C group of the bridged structure in the cellulose structure did not change at all, indicating that the main chain structure of the cellulose was not destroyed during the hydrolysis process, and the excellent mechanical properties were still maintained.

Results and discussion
The vertical oil-water separation device is shown in Figure 3a. Before testing, the mesh filter was immersed in water for 3 minutes so that water were adsorbed on the cellulose surface to form a hydration layer, thereby preventing oil substances from passing through the mesh. [12][13][14] The oily substance selected here is n-hexadecane. First, add a certain amount of n-hexadecane to the vertical device with the pressure smaller than the extrusion pressure. After adding water to n-hexadecane, the water phase with higher density will sink to the bottom and pass through the mesh filter, finally be collected in a beaker. Maintain a certain level of liquid and record the flux. At this time, the n-hexadecane is still above the mesh filter membrane. After the water has passed through the mesh filter membrane, the n-hexadecane can be collected to calculate the oil-water efficiency (Equation 1). Figure 3. (a) The scene of the oil-water separation in a vertical device, the n-hexadecane and water are dyed red and blue with dyes respectively. The pore size of the mesh filter membrane used in this device is 280 μm. (i) Only n-hexadecane is filled in the device; (ii) Water is poured into this vertical device; (iii) Water can be collected in a beaker through a mesh filter membrane. (b) The relationship of flux, oil-water separation efficiency with the pore diameter of mesh filter, the pressure above the mesh was kept at 300 Pa during the measurement.
The water flux and oil-water separation efficiency are shown in Figure 3b. The flux increases together with pore size, for example, water flux can reach to 365 000 L·m -2 ·h -1 and further increase to 400 000 L·m -2 ·h -1 with the pore size increase from 200 μm to 280 μm. The flux of 400 000 L·m -2 ·h -1 is 10 000 times the water flux of industrial ultra-thin filter membranes 15 , and still maintains more than 90% oil-water separation efficiency, so our cellulose oil-water separation mesh filter prepared by 3D printing technique has many advantages such as high throughput, high separation efficiency, simple preparation and easy operation, and no pollution to the environment. When the pore size is further increased, although the water flux can be further improved, the oil-water separation efficiency is reduced.  The pore size is 280 μm.
In order to confirm the environmental stability of this cellulose mesh filter, the ability to resist mechanical forces, chemical stability against acid, alkyl, and different oils were measured. The oil-water separation efficiency of the cellulose mesh filter membrane before and after the ultrasonic vibration treatment was shown in Figure 4a. After severing mechanical shock, the cellulose mesh filter membrane still has an oil-water separation efficiency of up to 95%. Generally, the oil-water separation membrane prepared through the coating is difficult to withstand high-frequency oscillations due to high-frequency oscillations can cause the hydrophilic or hydrophobic coatings to fall off, resulting in a significant decrease in the oil-water separation efficiency.
Moreover, the complicated chemical environment will also have a great impact on the separation efficiency of the oil-water separation membrane. Figure 4b shows the separation efficiency variations when the cellulose mesh were immersed into deionized water, hydrochloric acid solution (pH~1), sodium hydroxide solution (pH ~ 13), and neutral water (pH~7). After one day of medium immersion, separation efficiency did not change significantly, especially in acidic conditions, it could still be maintained above 95%. However, in alkaline conditions, the oil-water separation efficiency slightly decreases. Here, it is believed that the alkaline solution has a better interaction with the oil substance, and can cause the oil substance to generate suspended particles 16 with a smaller diameter in the water phase. Moreover, it has been reported that there is a good interaction between hydrophobic hydrocarbons and hydroxide ions, so that there is a negative charge [17][18] at the interface of water and hydrophobic hydrocarbons, and this negative charge will be further reduced. The size of the small oil droplets makes it easy to pass through the grid, leading to a reduction in the efficiency of oil-water separation.
In addition, the separation efficiency in different oils was studied. The oils here used include linear aliphatic n-hexadecane, high molecular weight polydimethylsiloxane (PDMS), cyclic organic liquid cyclohexane and aromatic xylene. Among them, PDMS has the highest viscosity (97 cP), and cyclohexane has the lowest viscosity (1 cP). However, it exhibit high separation efficiency in both PDMS and cyclohexane. Suggesting the viscosity of oil substances is not the most important factor for oil-water separation efficiency. In xylene, a relatively low separation efficiency was obtained, which is mainly due to the formation of π…H-O interactions between aromatic compounds and water molecules. This interaction increases the fiber affinity between the mesh filter and the aromatic compound, results in a small decrease in the oil-water separation efficiency.

Conclusion
In this study, a cellulose mesh filter with high throughput and high oil-water separation efficiency was fabricated by 3D printing technology. The higher flux of 400 000 L·m -2 ·h -1 is 10 000 times the water flux of the industrial ultra-thin filter. Compared with the traditional oil-water separation membrane that requires surface treatment, the 3D printed cellulose mesh filter no longer needs to consider the problem of coating uniformity and the interaction between the coating and the substrate, and has good environmental stability. Involving the ability to resist mechanical shock, chemical stability against acid and alkyl, and the ability to effectively separate a variety of oils. The 3D printed cellulose mesh filter is expected to realize large-scale production and application in the industry in the future.