Superhydrophilic Modification of a Polypropylene Depth Filter for Efficient Oil–water Separation

1. Introduction

Oil–water separation is a crucial process in several industrial and environmental applications. The mixing of oil and water in industrial processes can lead to severe issues, such as equipment corrosion, contamination, and reduced efficiency, resulting in increased operational costs and environmental hazards. Furthermore, industrial and domestic wastewater, and marine oil spills can severely pollute the environment and adversely affect human health and ecosystems [1-5]. Therefore, efficient and sustainable oil–water separation technologies must be developed to meet the growing demand for clean water and protect the environment.

In recent years, superhydrophilic and superhydrophobic surfaces have gained significant attention in the field of oil–water separation [6-14]. Superhydrophilic surfaces have a high affinity for water, allowing the water to spread and form a thin film while repelling oil. In contrast, hydrophobic surfaces have a low affinity for water and a high affinity for oil, forming an oil layer on the surface to repel water. Various methods can be used to produce extremely hydrophilic and hydrophobic surfaces, such as changing the surface roughness of a substrate or applying a wettability modification coating.

Depth filters, which are commercially used to remove impurities from wastewater, comprise tens to hundreds of porous layers. Depth filters are more resistant to impurity penetration and show superior filtering performance at higher pressures than single-membrane filters [15]. However, they are usually made of polypropylene or polyethylene, which have low surface energy and reactivity [16]. Consequently, modifying their surface wettability is difficult. Recently, an extremely hydrophilic depth filter with a polyacrylamide (PAM) hydrogel coating was fabricated by physically coating polyimide(PI) on the depth filter and chemically bonding it with 1-methyl-2-pyrrolidone(NMP)/N′ -dicyclohexylcarbodiimide(DCC) and acrylamide(Am) [17]. However, PI and NMP/DCC have harmful effects on the human body and require several hours to fabricate.

This study presents a superhydrophilic filter that can be quickly fabricated without the use of harmful chemicals. Plasma treatment was used to increase the surface energy of the polypropylene-based depth filter. Subsequently, a hydrogel layer was formed through PAM polymerization to produce an extremely hydrophilic depth filter. The fabricated filter had a contact angle of 0^o with water and showed a high repulsive force with oil in water with an underwater oil contact angle of 142.9^o. By applying this filter to a continuous oil–water separation device, an oil–water separation device that can continuously separate water and oil was manufactured; the amount of oil in the water was less than 15 ppm up to a flow rate of 300 mL/min. These results show that the developed filter has a high economic and environmental value and can quickly separate clean water in various industries and environments where oil–water separation is necessary.

2. Experimental Section

3. Results and Discussion

3.1 Fabrication of the Superhydrophilic Depth Filter

Figure 1(a) illustrates the fabrication method of the polypropylene filter with the polyacrylamide-based hydrogel layer. First, the polypropylene depth filter was treated with plasma to increase its wettability for deep coating. Figures 1b and 1c show the SEM images of the filters treated with pristine and O₂ plasma (OP), respectively. The surface of the filter fiber does not significantly change after plasma treatment. Subsequently, the OP-treated filter was dip-coated in a solution containing APS (a radical source) and BIS (a crosslinker). The alkene moieties of BIS are radical initiators, and radical polymerization occurs via APS. During polymerization, hydrophilic polymer groups surrounded and coated the filter fibers, resulting in a well-coated hydrophilic on the filter surface, as shown in Figure 1(d).

Fig. 1

(a) Schematic of the PAM-coated filter fabrication process (b), (c), and (d) SEM images

3.2 Wetting Characteristics of a Superhydrophilic Depth Filter

The surface components of the substrate were analyzed using FTIR before and after coating with PAM. New peaks at 1538 (C = O), 1652 (C = O), and 3296 cm−1 (N-H) were observed after PAM coating, in contrast to the pristine state, indicating the successful coating of hydrophilic groups on the filter surface (Fig. 2(a)).

Fig. 2

(a) FT-IR spectra of the pristine- and PAM-treated surfaces. (b) Photograph of water droplets during the coating process. (c) Changes in the water contact angle of the filter surface during the coating process

Water contact angles were measured on the filter surface at various fabrication stages. The pristine filter showed high hydrophobicity with a contact angle of 133.4±8.0^o owing to the hydrophobic nature of polypropylene and the micro-sized filter fibers. After OP treatment, the contact angle of the filter (OP_0) was measured to be 0^o owing to the formation of temporary OH groups on the filter surface, which can be attributed to the plasma. However, the hydrophilic property imparted to the filter by plasma treatment was unstable, and the contact angle after 24 h of plasma treatment was 116.6±6.3^o, indicating a partial restoration of the hydrophobicity observed before plasma treatment [18,19]. In the case of the filter with the PAM hydrogel layer formed after plasma treatment, superhydrophilicity with a contact angel of 0^o was observed, possibly because of the hydrophilic nature of the microstructured fibers and PAM-modified filter. Furthermore, the PAM-modified filter had an underwater oil contact angle of 142.9±7.0^o and exhibited a high repulsive force with oil in water.

3.3 Importance of Plasma Treatment

To investigate the effect of plasma treatment on hydrogel coating, we compared filters that were deep-coated with and without plasma treatment. Figures 3(a), 3(b) water droplets on the filters without and with plasma treatment, respectively. We compared their wettability by dropping blue dye on the outside and inside of the filter surfaces. Both filters showed a good hydrophilic coating on the outer surface. However, the filter without plasma treatment showed hydrophobicity on the inside because of the inability of the coating solution to penetrate the filter fibers owing to the hydrophobic nature of polypropylene. Moreover, the SEM image of the filter without plasma treatment suggest that the PAM hydrogel material was barely coated (Figure 3(c)). In contrast, the filter with plasma treatment showed an evenly distributed hydrogel coating on the filter fibers, thereby confirming the necessity of plasma treatment for an evenly coated hydrophilic depth filter.

Fig. 3

Photograph of water droplets on inner and outer surfaces of PAM-treated depth filters (a) without plasma treatment and (b) with plasma treatment. SEM images of the inner surface of PAM-treated depth filters (c) without plasma treatment and (d) with plasma treatment

3.4 Oil–water Separation Using a Superhydrophilic Filter

To evaluate the performance of oil-water separation in a continuous oil–water separation system, the system was designed as depicted in Figure 4. The oil-water mixture is pumped into filter housing 1, and purified water is discharged through a superhydrophilic depth filter. The pressure controller maintains pressure on the hydrophilic filter and discharges the accumulated oil to the top. The oil is purified by the superhydrophobic glass fiber (GF) filter in filter housing 2. To fabricate the superhydrophobic GF filter, GF filter was plasma-treated and coated with octadecyltrichlorosilane using a previously reported method [20]. The mixture that is not filtered by the superhydrophilic PAM and GF filters returns to the original oil–water mixture via the return pipe. The fluid undergoes a circulation process in the system, and returns to the original oil–water mixture through the return pipe, thereby continuously separating water and oil.

Fig. 4

Schematic of the oil-water separation device composed of a superhydrophilic and superhydrophobic filter

An actual photograph of the oil–water separation system is presented in Figure 5(a). In this experiment, the oil–water mixture was separated into water and oil at various flow rates, and the purities of water and oil were measured when 1 L of water and 250 mL of oil were produced. The oil–water mixture before separation and the separated water and oil are shown in Figure 5(b). The purified water had a purity of 5.46 ppm at 100 mL/min, 8.27 ppm at 200 mL/min, and 14.02 ppm at 300 mL/min, demonstrating an excellent separation performance of less than 15 ppm. Additionally, the oil was continuously separated with high purity, with 35.60 ppm at 100 mL/min, 50.04 ppm at 200 mL/min, and 69.82 ppm at 300 mL/min. Therefore, the proposed system based on a superhydrophilic depth filter is suitable for oil–water separation in various applications.

Fig. 5

Photographs of (a) the oil-water separation device used in the study and (b) oil and water before and after filtering. (c) oil content of the filtrated water and water content of the filtrated oil

4. Conclusions

In this study, we successfully developed a superhydrophilic depth filter based on PAM hydrogel coating treated with O₂ plasma (OP) and applied it to an oil–water separation system to obtain high-purity water and oil. Plasma treatment and hydrogel coating showed excellent performances in modifying the wettability of the polymer surface; however, the plasma treatment yielded a temporary coating, while hydrogel coating was not adequately deep. However, when plasma treatment and dip coating were simultaneously performed, excellent hydrophilic properties were imparted to both the inside and outside of the hydrophobic depth filter; thus, an oil–water separation system was developed using this filter. Because the mixture before separation in this study had more water than oil, a large amount of water was primarily removed by the superhydrophilic depth filter. However, if the proportion of oil is greater than that of water, the separation rate may decrease or the content of oil in the separated water may increase. Therefore, if a superhydrophobic depth filter is developed and an oil–water separation system employing the superhydrophilic depth filter is formed, a system capable of selectively and quickly separating high-purity water and oil regardless of the ratio of water to oil can be developed. We expect this oil–water separation system based on wettability-modified depth filters to be applied in various industrial and environmental fields, such as wastewater treatment and marine oil spills.

Acknowledgments

This work was financially supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (NRF-2021R1A2C2008600).

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