Study on efficient oil-containing wastewater purification with super-hydrophilic PVDF coating

The sustainable, low-energy, and cost-effective treatment of oily wastewater has become an important issue for aquatic ecosystems, environmental protection, and economic development. Improper treatment could have a significant impact on aquatic ecosystems, resulting in eutrophication and fish deaths. The method for preparing PVDF-CC by impregnation involves using polyvinylidene fluoride (PVDF), chitosan, and sodium citrate. Moreover, these performance improvements can be achieved at a lower cost compared to using many other chemical agents in membrane processing.


Introduction
Oil-water separation technology based on water purification membranes has received widespread praise due to its efficiency and environmental friendliness.However, the treatment of shipboard oil and wastewater can be costly because of the complexity of the process and the high energy consumption required for treatment.The high costs, energy consumption, and potential for secondary pollution make shipboard oil and wastewater treatment a challenging and complex process.Membrane separation technology can effectively remove oil particles as small as a few microns in size, which is difficult to achieve by using traditional separation methods.The technology is characterized by excellent mechanical stability and durability.Additionally, the filtration membrane offers advantages in terms of environmental protection, simple structure, and high efficiency for recycling and utilization.
Chitosan (CS) has demonstrated its application potential in the partitioning of oil and water resulting from the separation due to its inherent hydrophilic and lipophilic functional groups.Therefore, we use CS to modify PVDF membranes to obtain better oil-water separation membranes [1][2].However, simply modifying chitosan can cause it to flake off because it is a weakly charged polyelectrolyte.This makes it easy to lose its stability under certain conditions.To address this issue, a solution of sodium citrate is used to solidify chitosan through a salting-out process that promotes the formation of crosslinks between the chitosan chains.This cross-linking process enhances the mechanical strength and stability of the chitosan matrix, enabling it to achieve long-lasting and effective oil-water separation.As a result, modifying chitosan to enhance its surface properties is easily achievable, allowing for the modification of the membrane with the property of superhydrophilicity.Using sodium citrate to introduce CS into PVDF membranes can effectively improve the membranes' separation selectivity, self-cleaning properties, and stability.
In the work, a PVDF composite membrane for oil-water separation was prepared by using a more convenient immersion method due to its material properties.During the preparation process, the PVDF membrane is uniformly coated with a superhydrophilic coating, which further improves its properties of superhydrophilicity.PVDF composite membrane exhibited good hydrophilicity that allows it to form a Hydration layer.When droplets fall on the PVDF-CC membrane, a selective phenomenon is exhibited on the surface, where water droplets can penetrate the hydrophilic layer while oil droplets will be retained.This phenomenon illustrates that the PVDF-CC membrane efficiently separates oil and water, thus keeping water in a separate state.This demonstrates that PVDF-CC can serve as an excellent oil-water separation membrane with high contamination resistance [3].In addition, PVDF-CC polymer film has excellent hydrophilicity, self-cleaning properties, and high filtration efficiency in oil-water separation.Therefore, it can be widely applied in various fields such as environmental protection and wastewater treatment.

Preparation of PVDF-CC membrane
To modify the PVDF membrane into PVDF-CC, an immersion method is employed.After placing it into an ethanol solution, the membrane is cleaned for 10 minutes by using an ultrasonic cleaning agent to filter out surface impurities.Afterward, we keep the PVDF membrane at room temperature for storage.The preparation method of PVDF-CC is shown in Figure 1.After pre-treating the PVDF membrane, we soak it in a 1.5% chitosan solution.Then the PVDF-CC membrane was immersed in a 2 M sodium citrate solution for 5 minutes to prepare the coating by salting-out method.Finally, the PVDF-CC membrane is obtained by using the salting-out method [4][5].

Characterization
The contact angle at ambient temperature was measured by using the JY-82 contact angle goniometer.

Manufacturing and separation of oil-in-water emulsion mixture
To prepare a water-in-oil emulsion, first, cetyl trimethyl ammonium bromide of 0.3 g and lubricating oil of 3 g are added to the distilled water of 150 ml.Next, in the manufacturing process, a mixture is formed by combining the distilled water of 150 ml, cetyltrimethylammonium bromide of 0.5 g, and lubricating oil of 3 g, resulting in the formation of an emulsion where water is dispersed in oil.The resulting oil-in-water emulsion is vigorously stirred by using a homogenizer for approximately 30 minutes until a homogeneous mixture is obtained.The resulting emulsion is a water-in-oil emulsion with a concentration of 2, 000 ± 100 mg/L.The emulsion is analyzed by using an infrared microscope to determine the oil content.For the separation experiments on the water emulsion, a conventional glass filtration device with an effective separation radius of 5 mm has been selected.Membrane flux and separation efficiency are indicators used to measure the characteristics of membranes.In this experiment, the following formulas are used to calculate these two indicators for the water-in-oil emulsion [2].
where E is the separation efficiency (%); A represents the active separation surface (m 2 ); J is the actual flux passing through the membrane; J is the previous flow; P stands for pressure (Mpa); V is the permeate volume (L); ∆T is the duration of effective separation (h).

Wetting property of the sample
Figure 2. The water contact angle of PVDF-CC.The JY-82 contact angle goniometer was utilized for gauging and characterizing the surface wettability of PVDF-CC, as illustrated in Figure 2. The membrane exhibits fast permeation of water droplets, indicating the superhydrophilic nature of the PVDF-CC membrane [6].Therefore, this type of membrane holds immense potential in practical applications, such as oil-water separation.The self-cleaning ability of the PVDF-CC membrane is shown in Figure 3. Oil was sprayed onto the membrane surface by using a syringe.The surface was not contaminated by the oil, and the oil droplets quickly slipped off the PVDF-CC membrane, demonstrating its excellent self-cleaning ability.

Oil/water decantation
The capacity for oil-water separation of the PVDF-CC membrane was verified by testing the separation process through a conventional glass filtration container [7], as depicted in Figure 4.As illustrated in Table 1, PVDF membrane faces challenges when attempting to effectively separate oil from oil-water emulsion [8].In contrast, the PVDF-CC membrane demonstrates superior separation efficiency.This advantage can be validated by contrasting the oil-water emulsion pre-and post-separation in Figure 5.To verify the recyclability of the membrane, the filtration experiment was repeated three times, and the flux reached 296 L/ (m -2 h -1 bar -1 ).Summarizing data yields the results presented in Table 2, which demonstrate that the PVDF-CC membrane achieves a separation efficiency of over 99% [9].These findings indicate that the membrane exhibits both high separation efficiency and excellent reusability, thus confirming its practical viability in the realm of separation of oil and water.
Table 2.The results obtained when employing PVDF-CC as a demulsifier for oil-in-water emulsion.

Removal of methylene blue
Since distinguishing between the oil and water phases in the oil-water mixture during the separation process is challenging, a trace amount of methylene blue was introduced into the solution to impart color to the water phase, thereby aiding in the visual evaluation of the experimental outcomes.An actual experimental separation process is shown in Figure 6.

Conclusion
The ultra-hydrophilicity of the membrane has been successfully modified by using the impregnation method.Moreover, the resulting PVDF-CC membrane exhibits good self-cleaning properties, excellent separation efficiency, and a higher membrane flux [10].

Figure 1 .
Figure 1.Flowchart for the preparation of PVDF-CC.

Figure 3 .
Figure 3.The self-cleaning performance of PVDF-CC.The self-cleaning ability of the PVDF-CC membrane is shown in Figure3.Oil was sprayed onto the membrane surface by using a syringe.The surface was not contaminated by the oil, and the oil droplets quickly slipped off the PVDF-CC membrane, demonstrating its excellent self-cleaning ability.

Figure 4 .
Figure 4.The utilization of PVDF-CC for the emulsified oil separation process.

Figure 5 .
Figure 5.Comparison of water-in-oil emulsion filtration before and after separation.

Figure 6 .
Figure 6.Changes in methylene blue before and after separation from the oil-water mixture.

Table 1 .
The separation efficiency and flow rate of PVDF and PVDF-CC.