PSU-g-SBMA hollow ﬁ ber membrane for treatment of oily wastewater

Ultra ﬁ ltration membranes can intercept oil particles smaller than 10 μ m, but the membranes are easily contaminated by oil due to their hydrophobicity. To treat various oily wastewaters, we prepared a hydrophilic hollow ﬁ ber membrane (HFM) with anti-fouling property by grafting sulfobetaine methacrylate (SBMA) onto polysulfone (PSU). For six simulated wastewaters containing emulsi ﬁ ed oil at 1,000 mg/L, the PSU-g-SBMA HFM was able to remove 98.5 – 99.7% of oil, higher than that of PSU HFM at 91.1 – 98.9%. The oil concentration in ﬁ ltrate was less than 15 mg/L, which could meet the discharge standard of wastewater. The water ﬂ ux of PSU-g-SBMA HFM can be completely recovered after being washed by rhamnolipid and alkali solution, while the same cleaning process could not recover the PSU HFM. As found, the contact angles of oil droplets on the PSU-g-SBMA membrane were larger than those on PSU membrane, which indicated the improved hydrophilicity by PSU-g-SBMA. For 48 h of ﬁ ltration to soybean and diesel oil/water emulsion, the effect of PSU-g-SBMA HFM was stable and the ﬂ ux could be completely recovered by cleaning. Therefore, we provided a new method for oily wastewater treatment, which can ef ﬁ ciently and energy-saving remove various oil substances in wastewater. Oil contamination on membranes partly by the contact angles of


INTRODUCTION
Oily wastewater comes from a wide range of sources, such as petroleum exploitation, the petrochemical industry, marine transportation, wool and leather manufacturing, dairy processing, pharmaceutical production, and kitchen wastewater. The existing forms of oil in wastewater are of three categories (Tanudjaja et al. 2019): floating oil with particle size .150 μm, disperse oil at 20-150 μm, and emulsified oil at ,20 μm. Floating and dispersed oil can be removed by air flotation and flocculation, but emulsified oil cannot be removed (Tanudjaja et al. 2019). Membrane filtration can remove particles smaller than 10 μm, so membrane technology has been widely used to treat emulsified oily wastewater in recent years (Pulido 2016;Tanudjaja et al. 2019;Bolto et al. 2020).
Ultrafiltration membranes can be used for oil-water emulsion separation, such as polysulfone (PSU), polyvinylidene fluoride (PVDF), cellulose acetate, ceramics, or these after hydrophilic modification (Pulido 2016;Tanudjaja et al. 2019;Bolto et al. 2020). A typical ultrafiltration membrane is for treatment of water containing 10-10,000 mg/L of oil, and is able to remove more than 90% of oil (Tanudjaja et al. 2019;Bolto et al. 2020). The oil concentration in the filtrate is usually less than 30 mg/L when the emulsified oil before filtration is less than 1,000 mg/L (Bolto et al. 2020), which can meet the discharge standard of wastewater (Tanudjaja et al. 2019). However, oil contaminated the surface and pores of the membrane in treatment, so the flux of the membrane decreased sharply to less than 50 LHM even if the membrane has been hydrophilically modified (Bolto et al. 2020). For example, the working flux of a hydrophilic PAN-g-PEO blend membrane is only 23% of the initial flux after 24 hours of treatment by refinery wastewater, while that of an unmodified PAN membrane is only 3% of the initial flux (Asatekin & Mayes 2009). Moreover, the flux of the contaminated membrane could not be completely recovered by cleaning with surfactants and alkali solution, because the oil was adsorbed on both the surface and pores (Chen et al. 2009a(Chen et al. , 2009bZhao et al. 2012;Wang et al. 2014).
In this paper, the blend modified ultrafiltration hollow fiber membrane (HFM) was prepared to improve the hydrophilicity in the membrane pores, so as to resist oil fouling more effectively than the surface modification. Moreover, we also used an efficient and environmentally-friendly biosurfactant rhamnolipid to clean the oil contaminated HFM in collaboration with alkali, since rhamnolipid, a product from microbial fermentation, shows better decontamination effect than conventional surfactants (Long et al. 2014). In addition, the membrane fouling caused by oils from plants, animals and minerals were systematically compared and the possible mechanisms were explored to find the best procedure for treatment of oily wastewater from different sources.

Preparation of PSU-g-SBMA hollow fiber membrane
As shown in Figure 1, CMPSU prepared in the laboratory was dissolved in 50 ml DMSO to a 36 g/L solution, and SBMA was further dissolved in DMSO at the ratio of 4:1 (mol:mol). After nitrogen treatment for 2 h, CuCl and bpy were added at the mass ratio of SBMA: CuCl: bpy ¼ 100:1:2 for 4 h of reaction at 65°C. The reaction was terminated to obtain a DMSO solution containing PSU-g-SBMA copolymer. The grafting rate of SBMA was 75.8% by 1 H NMR. Water Science & Technology Vol 84 No 12,3577 PSU was dissolved into NMP with a concentration of 360 g/L. The solution of PSU-g-SBMA and PSU were then blended in equal volume (v/v ¼ 1:1), then PEG-1,000 at 36 g/L was added. After stirring for 24 hours, PSU-g-SBMA HFM was casted by wet spinning at 25°C and humidity of 50-60% with internal and external water bath according to the previously reported method (Shen et al. 2010). The inner and outer diameters of HFM were set to be 1.2 and 1.4 mm.

Characterizations of HFM
Scanning electron microscopy (SEM, Hitachi, TM-1000) was used to observe the surface and cross-section of HFM. The cross-section of PSU-g-SBMA HFM was obtained by drying the membrane at room temperature and then brittle fracture in liquid nitrogen. The surface and cross-section of the HFM were fixed on the sample table with conductive double-sided adhesive, respectively. After vacuum gold plating, the surface and cross-section of the HFM were observed by SEM.
XPS analysis was conducted on PHI5OOOCESCA system (PHI, USA). The PSU and PSU-g-SBMA HFM was placed in the XPS analysis room to pump at high vacuum. AI is used as the anode target material (1,486.6 ev) with high voltage of 14.0 kv, cathode current of 17.9 mA, dwell time of 100 ms, and energy of 93.9 ev.

PSU-g-SBMA HFM for treatment of oily wastewater
Soybean oil, olive oil, lard oil, gasoline, diesel oil and crude oil were added into pure water at a concentration of 1,000 mg/L and prepared by ultrasonic treatment for 5 h. The oil emulsion could be stable for more than 14 days at room temperature. The HFM modules for ultrafiltration experiments were made of glass tubes (inner diameter ¼ 8 mm, length ¼ 18 cm) with fixed HFMs inside. The device for filtration is shown in Figure 2.
The separation and washing process of oil-water emulsion is 30 min of pretreatment by water at 0.1 MPa before 60 min of ultrafiltration with oil/water emulsion with crossflow rate of 60 L/h. Then, the HFM was subsequently washed by water, 0.01 M NaOH solution (pH ¼ 12), 300 ppm RHA þ 0.01 M NaOH solution at atmospheric pressure. The water flux after each washing was determined.
Soybean oil and diesel oil were selected for long-term filtration by PSU-g-SBMA HFM. When the working flux was reduced to 1/2 of the initial flux, a solution with 300 ppm RHA þ 0.01 M NaOH was used to clean the HFM, and then the feed liquid was replaced by oil/water emulsion to continue filtration for 48 h. The oil contents were detected by an ultraviolet spectrophotometer (Molecular Device M2).

Detection of contact angle for different oils
The membrane was fixed with double-sided adhesive in the glass dish, then deionized water was injected into the dish. 1 μL of oil stained with Oil Red was injected on the membrane surfaces, and the images were adapted by a camera (digital camera 600D, Canon company of Japan; Macro lens EF 100 mm f/2.8, Canon company, Japan). The contact angle is calculated by software (contact angle measuring instrument c2000c1, Shanghai Zhongchen digital technology equipment Co., Ltd).

RESULTS AND DISCUSSION
3.1. Characterizations of PSU-g-SBMA HFM The prepared PSU-g-SBMA HFM is shown in Figure 3(a). The surface and cross-section morphology was observed by SEM. As shown in Figure 3(b), the outer surface of the membrane was flat without pores. The cross-section of the membrane was typically a finger-like structure, indicating the feature of ultrafiltration membrane (Shen et al. 2010;Burts et al. 2020;Plisko et al. 2020). In addition, the inner and outer diameters of the HFM were respectively about 1.2 mm and 1.4 mm, which was consistent with the designed diameters.
For further characterization, the surface of PSU and PSU-g-SBMA HFM was detected by XPS ( Figure 4). As found, the N1s peak of 398.60 eV appeared and the S2p peak at 167.40 eV was more obvious in PSU-g-SBMA HFM, which came from the amino and sulfo groups in the grafted SBMA, respectively. Figure 3(b) and 3(c) showed the C1s peak of PSU and PSU-g-SBMA HFM, respectively. In Figure 4(b), the C-H/C-C peak and C-O/C-S peak were at 284.0 and 285.5 eV, respectively. The new peak at 287.1 eV in Figure 3(c) was the COO peak of grafted SBMA. The results of XPS indicated the existence of SBMA on the surface of PSU-g-SBMA HFM.
In order to investigate the filtration property of the HFMs, the protein adsorption, water flux and BSA ultrafiltration were further detected. As shown in Table 1, the water flux of PSU and PSU-g-SBMA HFM were similar (about 250 LMH). However, the PSU-g-SBMA HFM displayed much lower static adsorption of BSA and fibrin than PSU HFM and other PSU membranes (Shen et al. 2014), indicating its anti-fouling property. The flux recovery rate (FRR) and BSA rejection rate of the two HFM are also listed in Table 1. PSU-g-SBMA HFM had better performance in separation efficiency and anti-fouling effect, which was consistent with the previous reports that SBMA modified ultrafiltration membrane improved the anti-fouling to proteins (Chiao et al. 2020;Shahkaramipour et al. 2020).

Comparison of PSU-g-SBMA and PSU HFMs for treatment of six oily wastewater
To investigate the separating effects of the two HFMs for different oil emulsions, soybean oil/olive oil and lard oil were selected as vegetable oil and animal oil, while gasoline, diesel and crude oil were representative of mineral oil. The anti-fouling and separation performance of the HFMs were investigated by ultrafiltration of oil-water emulsion with oil concentration at 1,000 mg/L. The HFM cleaning was first using deionized water, then using NaOH solution, and finally RHA þ NaOH solution, since RHA has shown the excellent performance on membrane cleaning according to our previous study (Long et al. 2014). The washing process is not shown in Figure 5.
As shown in Figure 5(a) and 5(b), PSU-g-SBMA HFM had higher flux for oil/water emulsion ultrafiltration of two vegetable oils (130-220 LMH) than PSU HFM, and the decrease of flux within 1 h did not exceed 20%. By contrast, the flux of PSU HFM decreased by 72.5% and 33.3%, indicating that it had been seriously contaminated. The water flux of PSU-g-SBMA HFM could not be completely recovered by cleaning with water or NaOH solution, but could be completely recovered by RHA þ NaOH solution. For the PSU HFM, all the washing solutions could not restore more than half of the water flux, suggesting that the contamination on PSU HFM was more serious and difficult to clean. Compared with vegetable oil, lard oil emulsion did not irreversibly foul the two HFMs. Washing with water could completely recover the flux (Figure 5(c)), indicating the fact that the membrane fouling caused by lard oil was easy to remove. This might be due to the high melting point of lard oil, so that the solid particles in the emulsion at room temperature did not easily enter the membrane pores.
Moreover, the fouling of mineral oils (gasoline, diesel oil and crude oil) to PSU-g-SBMA HFM was less than that to PSU HFM ( Figure 5(d)-5(f)). Among them, the fouling caused by gasoline and diesel oil on PSU-g-SBMA HFM could be cleaned by RHA þ NaOH solution (Figure 5(d) and 5(e)), but the fouling by crude oil was only recovered by half after cleaning. Hence, the fouling caused by crude oil was the most serious and difficult to clean. Figure 6 showed the oily wastewater before and after treatment by PSU-g-SBMA HFM. The oil-water emulsion was cloudy before ultrafiltration, indicating the emulsion state of oil. After filtration, the filtrate was completely clear and transparent, suggesting the completely removed oil. In order to determine the removal ratio of oil, we determined the oil content in the filtrate. As shown in Table 2, PSU-g-SBMA HFM removed more than 98.5% of the oil. And the oil in the filtrate was less than 15 mg/L, which had met the EPA's requirement that the maximum oil content of wastewater discharged within 24 hours should not exceed 72 mg/L (Tanudjaja et al. 2019). In contrast, there was 89 mg/L of crude oil in the filtrate by PSU HFM, which could not meet the discharge standard of oily wastewater (Tanudjaja et al. 2019). Comparing different sources of oil, the removal ratio of vegetable and animal oil was higher than that of mineral oil. Vegetable and animal oils are mainly composed of fatty acid triglycerides, while mineral oils are mainly composed of aliphatic and/or aromatic hydrocarbons. Hence, for the higher molecular weight and hydrophilicity of vegetable and animal oils than mineral oils, they were easier to be intercepted by ultrafiltration membrane (Tanudjaja et al. 2019;Bolto et al. 2020).

Optimization of cleaning conditions to remove the crude oil contamination
Because of the severe fouling caused by crude oil, the washing temperature was increased to enhance the washing effect, while the commonly used detergent of sodium dodecyl sulfonate (SDS) was selected to compare the washing effect of RHA. Table 3 showed that the FFR of PSU-g-SBMA HFM was only 55% after washing with NaOH þ RHA and NaOH þ SDS at room temperature, but increased to 100% at 55°C. Thus, high temperature was helpful for cleaning of crude oil, which might relate to the improved fluidity of crude oil and increase on cleaning ability of surfactant at high temperature. Moreover, although SDS and RHA showed similar effects on cleaning, the concentration of SDS (5,000 mg/L) was much higher than that of RHA (300 mg/ L). This was because the critical micelle concentration (CMC) of SDS was very high (up to 5,000 mg/L) (Mitsionis & Vaimakis 2012), while that of RHA was only 5-200 mg/L (Lang & Wullbrandt 1999). Since SDS was difficult to degrade in the environment, the high concentration of SDS would elicit toxicity to environmental organisms (Aguilar-Alberola & Mesquita-Joanes 2012; Cao et al. 2020). By contrast, RHA was not only used in small amount, but also was able to be degraded naturally without environmental pollution (Thakur et al. 2021;Varjani et al. 2021). Therefore, RHA þ NaOH solution has certain advantages on removing the oil contamination on membranes.

Contact angle of different oils on HFMs
To further investigate the anti-fouling mechanism of PSU-g-SBMA HFM to different oils, the contact angle between oil and membrane was detected in aqueous phase. Because the surface of the HFM was not easy to observe, the corresponding flat membranes were used for the detection.
The oil contact angle of the PSU-g-SBMA membrane was larger than that of the PSU membrane (Figure 7(a) and 7(b)), which might explain the better anti-fouling of the PSU-g-SBMA membrane. The oil droplets were easy to peel off from the surface of the PSU-g-SBMA membrane for the large contact angle, but well adhered to the surface of the PSU membrane. The contact angle of the six oils on the PSU-g-SBMA membrane was in rank of crude oil , diesel oil , soybean oil , gasoline ≈ olive oil ≈ lard oil, close to the order of oil contamination (crude oil . diesel oil . gasoline . soybean oil . olive oil . lard oil). This result indicated that the hydrophobicity of oil was an important factor affecting its fouling. However, it was not the only factor to determine the fouling, since the size, molecular weight and melting point of oil droplets affected the contamination as well (Tanudjaja et al. 2019).

PSU-g-SBMA HFM for long-term treatment of oily wastewater
To evaluate the long-term treatment effect of PSU-g-SBMA HFM on oily wastewater, an ultrafiltration experiment of soybean oil and diesel oil was carried out during a treatment of 48 h. When the flux decreased to 50% of the initial flux, the membrane was washed with RHA (300 ppm) þ NaOH (0.01 M). For soybean oil emulsion, the membrane flux decreased to 50% of the initial flux at 6.5-8.5 h. After repeated use of 6 times, it still maintained 94% of the original flux (Figure 8(a)), showing good anti-fouling property. For the diesel oil emulsion, the flux decreased to 50% of initial flux at 5-6 h (Figure 8(b)), indicating that the diesel oil fouled the PSU-g-SBMA HFM more seriously. However, after washing with RHA (300 ppm) þ NaOH (0.01 M) solution, the flux could also recover to more than 95% of the original flux. Hence, the PSU-g-SBMA HFM could recover to the flux through simple cleaning even during long-term ultrafiltration.

CONCLUSION
For six simulated wastewaters containing emulsified oil at 1,000 mg/L, the PSU-g-SBMA HFM removed 98.5-99.7% of oil, higher than that of PSU HFM at 91.1-98.9%. The oil concentration in filtrate was less than 15 mg/L, which could meet the discharge standard of wastewater. The six oils showed the contamination to HFM with the rank of crude oil . diesel oil . gasoline . soybean oil . olive oil . lard oil. The water flux of PSU-g-SBMA HFM could be completely recovered after being washed by rhamnolipid and alkali solution, while the cleaning process could not recover the PSU HFM. As found, the contact angles of oil droplets on the PSU-g-SBMA membrane were larger than those on the PSU membrane, which indicated the improved hydrophilicity by PSU-g-SBMA. The order of contact angle of six oils on the PSU-g-SBMA membrane was crude oil , diesel oil , soybean oil , gasoline ≈ olive oil ≈ lard, close to the rank of oil contamination. Finally, the PSU-g-SBMA HFM was applied for 48 h of filtration to simulated wastewater containing soybean and diesel oil. The results showed that the treatment was stable, and the flux could be completely recovered by cleaning. Therefore, this paper provides a new method for oily wastewater treatment, which can enable efficient and energy-saving removal of various oil substances.

ACKNOWLEDGEMENT
We gratefully acknowledge the financial support of this study by NSFC (National Natural Science Foundation of China, No 22078287).

DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information. Water Science & Technology Vol 84 No 12,3584