Development of Red Clay Ultrafiltration Membranes for Oil-Water Separation

In this study, a red clay/nano-activated carbon membrane was investigated for the removal of oil from industrial wastewater. The sintering temperature was minimized using CaF2 powder as a binder. The fabricated membrane was characterized by its mechanical properties, average pore size, and hydrophilicity. A contact angle of 67.3° and membrane spore size of 95.46 nm were obtained. The prepared membrane was tested by a cross-flow filtration process using an oil-water emulsion, and showed a promising permeate flux and oil rejection results. During the separation of oil from water, the flux increased from 191.38 to 284.99 L/m2 on increasing the applied pressure from 3 to 6 bar. In addition, high water permeability was obtained for the fabricated membrane at low operating pressure. However, the membrane flux decreased from 490.28 to 367.32 L/m2·h due to oil deposition on the membrane surface; regardless, the maximum oil rejection was 99.96% at an oil concentration of 80 NTU and a pressure of 5 bar. The fabricated membrane was negatively charged, as were the oil droplets, thereby facilitating membrane purification through backwashing. The obtained ceramic membrane functioned well as a hydrophilic membrane and showed potential for use in oil wastewater treatment.


Introduction
The daily generation of approximately 210 million barrels of water contaminated with oil can incur costs of $45 B in water purification [1]. Recently the membrane technology has been applied to separate mixtures of oil and water, showing greater efficacy than traditional technologies [2,3]. It is well-known that membranes synthesized from polymers are unstable compared to ceramic membranes [4]. Ceramic membranes are often used in industrial applications, such as wastewater treatment, despite the high cost compared to those of polymeric membranes [5], owing to their extraordinary chemical, mechanical, and thermal properties [6]. They have the capability of backwashing, high flux, good toughness, resistance to bacterial growth, and thermal stability [4,7]. Materials like alumina, zirconia, titanium oxide and zeolite materials have been used in ceramic membranes that resist high pH and pressure to separate oil-water mixtures [4,8].
Additionally, microfiltration (MF) and ultrafiltration (UF) have been used as pressuredriven membrane methods [3]. UF is recognized as the most efficient in oil-water separation with significant advantages compared to conventional separation techniques: it requires no additional chemicals and its energy consumption is low [3]. High-flux MF membranes have also been used for oil-water separation; however, they carry the risk of oil penetration [3]. NaA zeolite was deposited on an α-Al 2 O 3 MF membrane by Cui et al. [9] and used in the separation of oil from water. The fabricated membrane had a pore diameter of 1.2 µm and showed 99% oil separation at a flow rate of 85 L/m 2 ·h and pressure of 50 kPa. A porous MF aluminum ceramic membrane was applied by Liu et al. for oil-water separation [10]; they reported 99.98% removal of emulsified oil.
A UF membrane was used to separate oil from water in an oilfield [3] with more than 96% oil rejection. Depositing TiO 2 on the surface of a UF ZrO 2 membrane to separate Table 1 shows that the red clay used in fabricating the membrane contains higher amounts of SiO 2 (47.63%), Al 2 O 3 (24.03%), and Fe 2 O 3 (9.57%) with low amounts of K 2 O (2.08%) and Na 2 O (1.29%). The red clay was collected from the Biadh plant in Riyadh, Saudi Arabia. The role of CaF 2 (Sigma Aldrich, St. Louis, MO, USA) is to minimize the sintering temperature of the red clay membrane and improve the mechanical strength. Activated carbon (diameter = 4 mm; purity 98%) was obtained from Zhengzhou Company (Henan, China). Powdered activated carbon (65 µm) was obtained using a planetary ball mill at 300 rpm for 4 h, and was blended with water for 72 h and then treated ultrasonically (50 min, 540 W) using an SFX550 (Sonifier, Suwanee, GA, USA). Then, the collected suspension was centrifuged at 3500 rpm for 15 min to yield nano-activated carbon of size 91.6 nm (SEM, Figure 1). It is worth mentioning that activated carbon was converted to nanoactivated carbon for the sole purpose of a pore-forming material, i.e., to create nanopores in the body of the ceramic membrane. After sintering at temperatures above 500 • C, all of the activated carbon was burned from the ceramic membrane, which resulted in pore formation in the ceramic membrane (TGA, Figure 2).  Table 1 shows that the red clay used in fabricating the membrane contains higher amounts of SiO2 (47.63%), Al2O3 (24.03%), and Fe2O3 (9.57%) with low amounts of K2O (2.08%) and Na2O (1.29%). The red clay was collected from the Biadh plant in Riyadh, Saudi Arabia. The role of CaF2 (Sigma Aldrich , USA) is to minimize the sintering temperature of the red clay membrane and improve the mechanical strength.

Raw Materials
Activated carbon (diameter = 4 mm; purity 98%) was obtained from Zhengzhou Company (Henan, China). Powdered activated carbon (65 μm) was obtained using a planetary ball mill at 300 rpm for 4 h, and was blended with water for 72 h and then treated ultrasonically (50 min, 540 W) using an SFX550 (Sonifier; Mexico). Then, the collected suspension was centrifuged at 3500 rpm for 15 min to yield nano-activated carbon of size 91.6 nm (SEM, Figure 1). It is worth mentioning that activated carbon was converted to nanoactivated carbon for the sole purpose of a pore-forming material, i.e., to create nanopores in the body of the ceramic membrane. After sintering at temperatures above 500 °C, all of the activated carbon was burned from the ceramic membrane, which resulted in pore formation in the ceramic membrane (TGA, Figure 2).

Membrane Fabrication
The fabricated raw material for the membrane comprised of a powder blend of 90 wt.% red clay, 5 wt.% CaF2 as a binder, and 5 wt.% nano-activated carbon powder as a pore former. The red clay was ground at 250 rpm for 3 h using a planetary ball mill to achieve the particle size of 100 μm. A z-mixer was used to combine the dry raw materials for 4 h. To this mixture, 400 mL water was slowly added. Through wet mixing, we obtained a paste with satisfactory plasticity, which was then fed into the extruder (Length=200 cm, Width= 50 cm, Die diameter= where Ci is the raw oil turbidity and Cc is the turbidity after filtration. The membrane permeability (P) was obtained by equation 4: where Kc is the membrane permeability and ΔP is the applied pressure.

Characterizations
The zeta potential curves of the oil emulsion and membrane are presented in Figure  3. The isoelectric point of the oil droplets emulsion appears at pH 1, where the zeta potential was negative. The isoelectric point of the membrane was located at a slightly higher pH of 1.7, where the zeta potential was also negative. Since identical charges are known to experience electrostatic repulsion, it can be predicted that our fabricated membrane would prevent fouling during backwashing because of the electrostatic repulsion between the oil emulsion and the membrane.
Mechanical testing of the ceramic membranes with and without CaF2 was performed using the three-point bending technique with a crosshead speed of 0.5 mm/min. The membrane without CaF2 showed a lower bending strength of 49.53 MPa than the membrane with CaF2 (54.13 MPa); the stress-strain relationship of both the membranes was linear. Further, a contact angle of 67.3° indicates that the membrane is hydrophilic. The membrane pore size distribution (Figure 4) indicated that a UF membrane was created without using a coating layer. The pore size of this membrane ranged from 40 nm to 110 nm with an average pore size of 96 nm, and 88% of the total pores were smaller than 96 nm. In addition, the measured porosity of the membrane was 32.56%. SEM was used to determine the morphology of the fabricated membrane ( Figure 5). It was noted that the absence

Membrane Fabrication
The fabricated raw material for the membrane comprised of a powder blend of 90 wt.% red clay, 5 wt.% CaF 2 as a binder, and 5 wt.% nano-activated carbon powder as a pore former. The red clay was ground at 250 rpm for 3 h using a planetary ball mill to achieve the particle size of 100 µm.
Az-mixer was used to combine the dry raw materials for 4 h. To this mixture, 400 mL water was slowly added. Through wet mixing, we obtained a paste with satisfactory plasticity, which was then fed into the extruder (Length = 200 cm, Width = 50 cm, Die diameter = 20 cm). Two sintering stages were applied: first, sintering was performed from 30 to 500 • C at an average heating rate of 1.5 • C/min to burn the organic material, thereby creating pores in the prepared membrane. Figure 2 presents the TGA data in an air of the nano-activated carbon powder, clarifying the sintering operation. It is clear from Figure 2 that complete burning of the nano-activated carbon powder was obtained at 450 • C. Subsequently, the produced membrane was densified by sintering in a furnace at temperatures from 400 • C to 1000 • C at a rate of 2 • C/min for 4 h. Scanning electron microscopy (SEM) (model NNL-200, Philips, 1-nm resolution) was used for morphological characterization and microstructural analysis of the fabricated sintered membrane. Samples smaller than 10 mm were obtained by cutting the sintered membrane samples by a cutting machine with a diamond cutting disc. The membrane sample was dried for 24 h in a vacuum oven, and then etched by 1% HF + 1% HNO 3 solution for 30 s. Finally, the membrane sample was coated by a thin gold layer using a sputter coater (SPI Inc., Lakewood, WA, USA) to increase the conductivity of the membrane and to obtain a clear image. SEM images of the membrane sample were obtained by scanning it with a focused beam of electrons, which interact with the electrons in the membrane sample, producing various detectable signals containing information about the sample's surface topography.

Apparent Porosity
The apparent porosity of the ceramic structures was determined by the standard test method (ISO EN 993-1) for ceramic structures using the Archimedes buoyancy technique with dry weights, soaked weights, and immersed weights in water. The membrane sample was dried in an oven at 105 • C for 24 h to eliminate the absorbed water. The dried membrane sample was weighed by the balance and the weight is recorded as M d . Then the membrane sample was placed in a water-filled container for 24 h at room temperature. After that, the membrane sample has weighed and the weight recorded as M w . In addition, the membrane sample was weighed inside water and the weight recorded as M a (suspension weight). Finally, the apparent porosity can be calculated from equation 1:

Contact Angle Measurements
The capillary force liquid weight gain, which occurs when wetting the membrane sample, was precisely measured using a K100 force tensiometer (Kruss, Wissenschaftliche Laborgeräte, Borsteler Chaussee 85, Germany) with a particularly high resolution to obtain reliable and accurate contact angle (θ) data of the membrane sample. Before measuring, the membrane sample was dried in a vacuum oven for 24 h at 100 • C. The main procedure to measure the membrane contact angle using Kruss K100 involves sealing off the open ends of the tubular membrane sample with epoxy resins, hanging the sample on the microbalance in the K100 force tensiometer (Kruss, Wissenschaftliche Laborgeräte, Borsteler Chaussee 85, Germany)), and immersing the sample gradually into deionized water. The rate of immersion has to be adjusted to~6 mm/min. Finally, the contact angle will be calculated from the forces acting on the membrane surface.

Mechanical Test
The mechanical properties of the ceramic membrane were determined by the threepoint bending strength test using a Shimadzu-Universal testing machine (AGS-X, Riverwood Drive Columbia, MD 21046, USA) with a capacity of 5 kN, a total grip distance of 690 mm, a crosshead speed of 0.5 mm/min, a potential of 200 V, and power of 60 Hz. The stress-strain relationship of the membrane was found to be linear. Membrane samples with a length of at least 4 cm were obtained by cutting the samples by a cutting machine with an artificial diamond disc, the result is obtained by using the equation where "δ f "is the bending strength "F" is the flexural load in newton and "L", "d 2 " and "d 1 " is the span length, outer diameter, and inner diameter respectively.

X-Ray Diffraction
The X-ray diffraction technique (diffractometer used: model D8AD VANCE, BRUKER, Billerica, MA, USA) was used to identify the crystalline phases of the membrane sample. In this technique, the scattered intensity of an X-ray beam, generated upon hitting the membrane sample, is measured as a function of incident angle. The membrane sample for XRD analysis was dried for 24 h in a vacuum oven at 105 • C to eliminate any moisture present in the material. Then, the membrane sample was powdered and spread on the glass holder with a gap of 0.5 mm. The holder with the sample was then placed in the X-ray chamber and scanned at a constant temperature and a speed of 2 • /min using CuKα radiation, over a diffraction angle (2θ) range from 10 • to 80 • , with a step size of 10 • . The Joint Committee on Powder Diffraction Standards (JCPDS) diffraction file cards (2001) are used as reference for interpretation of the X-ray patterns obtained in the experiment.

Pore size Distribution Measurements
The pore size distribution of the membrane was determined using a constant-pressure fluid-fluid porometer (IFTS advanced fluid-fluid porometer, Institut de la Filtration et des Techniques Séparatives, Rue Marcel Pagnol, Foulayronnes, France).

Oil Emulsion Characterization
The performance of the prepared ceramic membrane was characterized by implementing it in the separation of oil from an oil-water mixture, where ultra-pure paraffin oil was used as a synthetic oil in the absence of a surfactant. The emulsion was vigorously and continuously mixed by using an agitator for 50 min at 1350 rpm and remained stable for several days unaffected by gravitational forces. The initial concentration of the oil emulsion was measured in terms of turbidity at 64 NTU. In addition, an oil-water mixture was obtained from an Aramco oilfield. Furthermore, a Zetasizer was used to obtain the zeta potential curve and to determine the oil droplet charge. To define the efficiency of the prepared membrane in oil-water separation, a turbidity meter was used to determine the oil turbidity. Figure 2 shows the cross-flow filtration set-up, which consists of a feed tank, pump, pressure gauges, agitator, and a tubular-type ceramic membrane. In the flow circuit, feeds with a volumetric flow rate of 55 L/h at 25 • C and a specific pressure of a known and constant composition (water and oil emulsion) was pumped continuously through the cross-flow ultra-filtration membrane (diameter = 0.5 cm, length = 20 cm) at a specified cross flow pressure of 5 bar. In addition, a secondary agitator was used to provide mixing effects and to ensure emulsion stability. A water tank was used to collect the permeated water, and the weight was determined using a balance to calculate the flux rate of the clean water.

Ultrafiltration Testing
The water flux, J (L/m 2 ·h), was determined from Equation (2): where V is the water volume collected through the pores of the membrane, A is the membrane surface area, and t is the time.
The turbidities of the water permeate and water in the feed tank were measured using a turbidimeter; the oil rejection (R%) was obtained from Equation (3): where C i is the raw oil turbidity and C c is the turbidity after filtration. The membrane permeability (P) was obtained by Equation (4): where K c is the membrane permeability and ∆P is the applied pressure.

Characterizations
The zeta potential curves of the oil emulsion and membrane are presented in Figure 3. The isoelectric point of the oil droplets emulsion appears at pH 1, where the zeta potential was negative. The isoelectric point of the membrane was located at a slightly higher pH of 1.7, where the zeta potential was also negative. Since identical charges are known to experience electrostatic repulsion, it can be predicted that our fabricated membrane would prevent fouling during backwashing because of the electrostatic repulsion between the oil emulsion and the membrane.
Mechanical testing of the ceramic membranes with and without CaF 2 was performed using the three-point bending technique with a crosshead speed of 0.5 mm/min. The membrane without CaF 2 showed a lower bending strength of 49.53 MPa than the membrane with CaF 2 (54.13 MPa); the stress-strain relationship of both the membranes was linear. Further, a contact angle of 67.3 • indicates that the membrane is hydrophilic. The membrane pore size distribution (Figure 4) indicated that a UF membrane was created without using a coating layer. The pore size of this membrane ranged from 40 nm to 110 nm with an average pore size of 96 nm, and 88% of the total pores were smaller than 96 nm. In addition, the measured porosity of the membrane was 32.56%. SEM was used to determine the morphology of the fabricated membrane ( Figure 5). It was noted that the absence of cracks in the fabricated membrane was indicative of its high quality and good material properties in agreement with the results of the three-point bending strength test. 85 wt.% at 1000 °C. The holding time of the sintering procedure after treatment at 1000 °C was 2 h. CaF2 as a binder and nucleating agent to minimize the sintering temperature was used. The sintering temperature of the ceramic membranes with and without CaF2 was performed. The membrane with CaF2 showed a lower sintering temperature of 1000 °C than the membrane without CaF2 (1150 °C ). The TGA data for CaF2 (Figure 2) showed that the residual weight was ~94 wt.%. Crystal water decomposition was found to occur at 500-600 °C , and the standard decomposition or recrystallization possible during heat treatment occurs at 600-1100 °C .   performed. The membrane with CaF2 showed a lower sintering temperature of 1000 °C than the membrane without CaF2 (1150 °C ). The TGA data for CaF2 (Figure 2) showed that the residual weight was ~94 wt.%. Crystal water decomposition was found to occur at 500-600 °C , and the standard decomposition or recrystallization possible during heat treatment occurs at 600-1100 °C .      Figure 6 also presents the TGA data for the red clay paste, with a residual weight of 85 wt.% at 1000 • C. The holding time of the sintering procedure after treatment at 1000 • C was 2 h. CaF 2 as a binder and nucleating agent to minimize the sintering temperature was used. The sintering temperature of the ceramic membranes with and without CaF 2 was performed. The membrane with CaF 2 showed a lower sintering temperature of 1000 • C than the membrane without CaF 2 (1150 • C). The TGA data for CaF 2 (Figure 2) showed that the residual weight was~94 wt.%. Crystal water decomposition was found to occur at 500-600 • C, and the standard decomposition or recrystallization possible during heat treatment occurs at 600-1100 • C.
The XRD pattern of the fabricated membrane (Figure 7), which contains red clay and CaF 2 , shows that red clay is a highly illitic kaolinite-type clay. It includes illite, kaolinite, and hematite, which gives it a red color. Additionally, the clay contains some amount of free quartz. CaF 2 could be seen in the XRD.
Sintering at 1000 • C led to the decomposition of kaolinite and illite (clay minerals). Some amount of CaF 2 was also decomposed, while the remaining was observed in the membrane structure by XRD analysis. The calcium released from the CaF 2 decomposition and the aluminum silicate from the clay minerals were reacted to create an anorthite phase. During sintering, the free quartz and hematite remained stable, as evidenced by the XRD pattern of the sintered membrane. Most of the decomposed kaolinite was transformed to mullite. In addition, large amounts of amorphous phase were present in the sintered membrane, which was determined by the broad peak between 2θ = 20 • and 2θ = 40 • .  The XRD pattern of the fabricated membrane (Figure 7), which contains red clay and CaF2, shows that red clay is a highly illitic kaolinite-type clay. It includes illite, kaolinite, and hematite, which gives it a red color. Additionally, the clay contains some amount of free quartz. CaF2 could be seen in the XRD. Sintering at 1000 °C led to the decomposition of kaolinite and illite (clay minerals). Some amount of CaF2 was also decomposed, while the remaining was observed in the membrane structure by XRD analysis. The calcium released from the CaF2 decomposition and the aluminum silicate from the clay minerals were reacted to create an anorthite phase. During sintering, the free quartz and hematite remained stable, as evidenced by the XRD pattern of the sintered membrane. Most of the decomposed kaolinite was transformed to mullite. In addition, large amounts of amorphous phase were present in the sintered membrane, which was determined by the broad peak between 2θ = 20° and 2θ = 40°.

Evaluation with Synthetic Oil Emulsion
The water flux was obtained and the water permeate was collected for 4 h at different operating pressures (3, 4, 5, and 6 bar, corresponding to 300, 400, 500, and 600 kPa) and using ultra-pure paraffin oil as a feed at 64 NTU concentration. The water flux rate was calculated from Equation (1) and the membrane permeability was determined from Equation (3) (Figure 8). As shown in Figure 8, the flux changed from 191.38 to 284.99 L/m 2 ·h on in the applied pressure from 3 to 6 bar. The data in Figure 9 were fitted with Darcy's and the membrane permeabilities were obtained. High water permeability was o for the fabricated membrane at low operating pressures. The obtained results w parable to those from the RO process [24]. Based on Figure 8, the standard deviati is lower than the mean (179.84), indicating that the data is reliable. In additio confidence level is 185.10-174.58, i.e., we are 90% certain that the mean lies betwe and 174.58 with a small margin of error.
The oil emulsion was tested at 5 bar (500 kPa) to measure the flux over tim feed concentration of 64 NTU. Based on the standards of the industry, UF can ducted in the range 4-7 bar (400-700 kPa) [25]. Hence, to determine a perfect w the process was run at 5 bar (500 kPa). The water flux rate was calculated from 1 and the rejection was determined from equation 2 (Figure 9). As shown in Figure 8, the flux changed from 191.38 to 284.99 L/m 2 ·h on increasing the applied pressure from 3 to 6 bar. The data in Figure 9 were fitted with Darcy's law [23] and the membrane permeabilities were obtained. High water permeability was observed for the fabricated membrane at low operating pressures. The obtained results were comparable to those from the RO process [24]. Based on Figure 8, the standard deviation (5.36) is lower than the mean (179.84), indicating that the data is reliable. In addition, a 90% confidence level is 185.10-174.58, i.e., we are 90% certain that the mean lies between 185.10 and 174.58 with a small margin of error. As shown in Figure 8, the flux changed from 191.38 to 284.99 L/m 2 ·h on increa the applied pressure from 3 to 6 bar. The data in Figure 9 were fitted with Darcy's law and the membrane permeabilities were obtained. High water permeability was obser for the fabricated membrane at low operating pressures. The obtained results were c parable to those from the RO process [24]. Based on Figure 8, the standard deviation (5 is lower than the mean (179.84), indicating that the data is reliable. In addition, a confidence level is 185.10-174.58, i.e., we are 90% certain that the mean lies between 18 and 174.58 with a small margin of error.
The oil emulsion was tested at 5 bar (500 kPa) to measure the flux over time usi feed concentration of 64 NTU. Based on the standards of the industry, UF can be ducted in the range 4-7 bar (400-700 kPa) [25]. Hence, to determine a perfect water f the process was run at 5 bar (500 kPa). The water flux rate was calculated from equa 1 and the rejection was determined from equation 2 ( Figure 9).  Figure 9 shows that the water flux rate decreased over 4 h from 490.28 to 36 L/m 2 ·h. This behavior could be due to a considerable quantity of oil deposited on the m brane over the course of testing, resulting in a decreased flux through membrane foul The oil emulsion was tested at 5 bar (500 kPa) to measure the flux over time using a feed concentration of 64 NTU. Based on the standards of the industry, UF can be conducted in the range 4-7 bar (400-700 kPa) [25]. Hence, to determine a perfect water flux, the process was run at 5 bar (500 kPa). The water flux rate was calculated from equation 1 and the rejection was determined from equation 2 (Figure 9). Figure 9 shows that the water flux rate decreased over 4 h from 490.28 to 367.32 L/m 2 ·h. This behavior could be due to a considerable quantity of oil deposited on the membrane over the course of testing, resulting in a decreased flux through membrane fouling. Nandi et al. [26] observed similar results warranting an efficient cleaning procedure to remove foulants from the membrane.
The standard deviation (3.65) is lower than the mean (404. 25), indicating that the data is reliable. In addition, the 90% confidence level is 404.26-404.24, i.e., we are 90% certain that the mean is between 404.26 and 404.24 with a small margin of error.

Evaluation with Aramco Oil-Contaminated Water
Contaminated water obtained from Aramco was tested at 5 bar (500 kPa) to measure the flux over time using a feed concentration of 80 NTU. The water flux rate was calculated from Equation (1) and the rejection was determined from Equation (2) (Figure 10).
Crystals 2021, 11, x FOR PEER REVIEW Nandi et al. [26] observed similar results warranting an efficient cleaning pr remove foulants from the membrane.
The standard deviation (3.65) is lower than the mean (404.25), indicating th is reliable. In addition, the 90% confidence level is 404.26-404.24, i.e., we are 9 that the mean is between 404.26 and 404.24 with a small margin of error.

Evaluation with Aramco Oil-Contaminated Water
Contaminated water obtained from Aramco was tested at 5 bar (500 kPa) the flux over time using a feed concentration of 80 NTU. The water flux rate was from equation 1 and the rejection was determined from equation 2 ( Figure 10) It was observed that the water flux decreased because of oil deposition on brane surface causing membrane fouling, while the oil rejection changed fro 99.96%. The decrease in the water flux was related to oil precipitation on the m Based on Figure 10, the standard deviation (3.64) is lower than the mean (318.1 ing that the data is reliable; the 90% confidence level is 318.19-318.15.

Cleaning Mechanism of a Fouled Membrane and Cyclic Filtering Test
Membrane fouling has detrimental effects on membrane performance. D water separation, fouling occurs due to the interaction between the membra droplets in the wastewater; the cohesion between the foulant and membrane pends on membrane surface properties, such as its zeta potential and hydroph Here, the fouling problem on the membrane was investigated via a cyclic fil performed for 1 h, and the flux rate was determined. Next, backwashing was for 10 min to clean foulants from the membrane by pushing water mixed with membrane. Then, the water flux was recalculated, and a total of seven experime It was observed that the water flux decreased because of oil deposition on the membrane surface causing membrane fouling, while the oil rejection changed from 99.23 to 99.96%. The decrease in the water flux was related to oil precipitation on the membrane. Based on Figure 10, the standard deviation (3.64) is lower than the mean (318. 19) indicating that the data is reliable; the 90% confidence level is 318.19-318.15.

Cleaning Mechanism of a Fouled Membrane and Cyclic Filtering Test
Membrane fouling has detrimental effects on membrane performance. During oilwater separation, fouling occurs due to the interaction between the membrane and oil droplets in the wastewater; the cohesion between the foulant and membrane surface depends on membrane surface properties, such as its zeta potential and hydrophilicity [23]. Here, the fouling problem on the membrane was investigated via a cyclic filtrating test performed for 1 h, and the flux rate was determined. Next, backwashing was performed for 10 min to clean foulants from the membrane by pushing water mixed with air into the membrane. Then, the water flux was recalculated, and a total of seven experimental cycles were performed ( Figure 11). possible to purify and reform the negatively charged membrane during backwa cause the oil droplets are also negatively charged (Figure 3), yielding suitable w with sufficient cycling. The backwashing potential arising from the repulsion be negatively charged oil droplets and the membrane surface renders the develop brane suitable for use in oil-water separation. Figure 11 shows that the standard deviation (6.24) is lower than the mean therefore, the data is reliable. In addition, the 90% confidence level is 296.32-27 Figure 11. The water flux of seven experimental cycles filtration for oil separation by fab membrane.
The effectiveness of the fabricated membrane was similar to those of memb scribed in the literature [9,[27][28][29][30], and some comparisons are presented in Tab evident that the fabricated membrane presented favorable performance with which is locally available, and an inexpensive material. Table 2 also presents data expensive membrane materials, such as NaA zeolite deposited on α-Al2O3. Add our tubular ceramic membrane has a higher water flux (367.32 L/m 2 h) than th PVDF-UF at the same operating pressure (5-6 bar, 309 L/m 2 h).

Membrane Type Solution
Pressure (bar) Pe Flux [27] cellulose microfiltration membranes Synthetic produced water 3 [28] PAN nanofiber membrane Synthetic produced water 0.1 [29] a-Al2O3 ceramic membrane Synthetic produced water 1.37 [30] Magnesium bentonite hollow fiber ceramic membrane Performing the filtration periodically can prevent the tendency of fouling and maintain a nearly constant flux rate value owing to the repeated backwash purification of the membrane. The plot of the water flux rate as a function of the number of cycles ( Figure 11) revealed that the membrane retained a significant negative surface charge, as shown in the zeta potential plot of the fabricated membrane (Figure 3), resulting in an almost constant water flux. This also allowed for the membrane to be efficiently cleaned. It is possible to purify and reform the negatively charged membrane during backwashing because the oil droplets are also negatively charged (Figure 3), yielding suitable water flux with sufficient cycling. The backwashing potential arising from the repulsion between the negatively charged oil droplets and the membrane surface renders the developed membrane suitable for use in oil-water separation. Figure 11 shows that the standard deviation (6.24) is lower than the mean (287.89); therefore, the data is reliable. In addition, the 90% confidence level is 296.32-279.45.
The effectiveness of the fabricated membrane was similar to those of membranes described in the literature [9,[27][28][29][30], and some comparisons are presented in Table 2. It is evident that the fabricated membrane presented favorable performance with red clay, which is locally available, and an inexpensive material. Table 2 also presents data for some expensive membrane materials, such as NaA zeolite deposited on α-Al 2 O 3 . Additionally, our tubular ceramic membrane has a higher water flux (367.32 L/m 2 h) than the tubular PVDF-UF at the same operating pressure (5-6 bar, 309 L/m 2 h). The practical implication of the membrane fabricated in this study is the clean, oil-free water, which can be used in agriculture and cooling systems. The scientific contribution of this study is the use of new materials, such as CaF 2 and nano-activated carbon, to fabricate a ceramic membrane from a low-cost material like red clay. Our working hypotheses of using low-cost materials for ceramic membrane fabrication were confirmed by the obtained results. However, the membrane fabricated in this study is only limited to separating oil and water. To expand its for use in industry, its performance must be investigated regarding different types of industrial pollutants. In addition, irreversible fouling also requires further research.

Conclusions
An efficient new membrane was fabricated from red clay combined with CaF 2 as a binder and nano-activated carbon as a pore former. These materials were used to fabricate a porous membrane by the extrusion technique, and the membrane was applied for the purification of oil-contaminated water. CaF 2 was used to minimize the sintering temperature of the red clay membrane and increase the mechanical strength of the membrane as a nucleation promoter.
The fabricated membrane was tested using both, a synthetic oil-water emulsion and water produced from an oilfield from Aramco. The fabricated membrane had an average pore size of 95.46 nm; thus, it qualified as a UF membrane. It showed a good bending strength of 54.13 MPa and a contact angle of 67.3 • , indicating hydrophilicity. The performance of the fabricated membrane complied with the standards of the national wastewater. The clean water also met the desired standards.
In the separation of oil from water, the flux was increased upon increasing the applied pressure. High water permeability was obtained for the fabricated membrane under low operating pressure, and this result was fitted with Darcy's law. The membrane flux decreased by oil deposition on the membrane surface; regardless, the maximum oil rejection was 99.96% at the oil concentration of 80 NTU and pressure of 5 bar (500 kPa). The prepared membrane showed high efficiency in removing foulants by the backwash technique because of the charge repulsion forces between the oil molecules and the negatively charged membrane. The fabricated membrane showed good potential applicability in oil-water separation treatments.
Funding: This research received no external funding.