The development of low-cost methods for wastewater treatment and the separation of oil-in-water emulsions is of considerable significance. Recently, natural material-based, inexpensive membranes have become a hot area of research. In this work, natural olive seeds were used to develop a novel ceramic membrane support. With the oil filtration process in place, the choice was reached to utilize the olive kernels’ beneficial qualities best. The process involved blending plastic paste with water and organic ingredients, followed by extruding the resulting paste into a porous tubular. After firing at 200 °C/2 h, the membrane's water permeability and porosity were 1,852 L/h m2 bar and 45%, respectively, and its average pore width varied from 2 to 15 μm. The efficiency of the microfiltration membrane in separating oil-in-water emulsions was assessed using two test solutions containing oil concentrations of 500 and 1,000 mg/L. Under a transmembrane pressure of 1 bar, the membrane exhibited exceptional permeate flux exceeding 200 L/m2 h, along with a high oil rejection rate of over 96% across all feed concentrations.

  • A new ceramic microfiltration membrane based on olive seeds was prepared by extruding and sintering.

  • The membrane's permeability to distilled water is up to 1,852 L/h m2 bar.

  • Oil rejection rates of up to 97.6% were achieved for the 1000 mg/L sample.

  • Valorization of Olive Grains: Transforming this agricultural by-product into valuable resources, such as ceramic membranes.

Membrane technology is gaining a lot of interest due to its possible uses in various production processes (Fatni et al. 2021; Addich et al. 2022a). It offers several advantages, such as a changeable microstructure, pore size distribution, low environmental pollution, and reduced energy consumption at gentler conditions (Agarwalla & Mohanty 2022; Gu et al. 2022). Consequently, numerous researchers are currently working toward the development of novel membrane types and application procedures (Lagdali et al. 2023). Microfiltration is a widely adopted technique for the separation of particles, microorganisms, and colloidal species from suspensions (Najid et al. 2022). It involves the use of a selectively permeable membrane that selectively retains the target species while allowing the desired filtrate to pass through. The process is suitable for numerous uses across multiple sectors (Teow et al. 2022), including biotechnology, pharmaceuticals, food, and beverage (Poli et al. 2022), and wastewater treatment (Safaee et al. 2022), among others (Addich et al. 2022a, 2022b). The effectiveness of the microfiltration process is primarily determined by the size and composition of the microorganisms and particles to be removed, the membrane's pore size, and operating conditions such as pressure, flow rate, and temperature. Overall, microfiltration is a reliable and efficient solution for the separation and purification of suspensions, and its versatility and flexibility make it a popular choice in many applications (Omar et al. 2024).

Ceramic membranes have significant industrial potential. As such, a lot of effort has gone into developing nature-based ceramic membranes using clay materials like sand (Aloulou et al. 2017; Addich et al. 2022a, 2022b), clay (Ouaddari et al. 2019), and phosphate (Mouiya et al. 2018). Membrane processes used for separating oil-in-water emulsions often face the issue of membrane fouling caused by the accumulation of oil phase near the membrane surface (Alftessi et al. 2021; Naseer et al. 2024). This poses a major challenge to increase the efficiency of membrane permeate flux and cleaning procedures during operation. During oil-in-water emulsion filtration, convective permeation flow transports oil droplets near the membrane surface, which can then accumulate on it. The burgeoning expansion of various industries such as oil and gas, petrochemicals, food processing, pharmaceuticals, and metallurgical sectors results in the generation of substantial volumes of oily wastewater effluents. The standard concentration of oil and grease in the produced water from oil fields typically falls within the range of 100–1,000 mg/L or may even exceed these levels, contingent upon the characteristics of the crude oil (Chakrabarty et al. 2008). Substituting conventional polymeric membranes with ceramic microfiltration membranes proves highly efficient owing to their hydrophilic properties and precise, narrow pore size distribution.

The objective of this project is to utilize the extrusion technique for the production and characterization of a ceramic tubular membrane using olive seeds as a raw material. The selection of this particular raw material is justified by the requirement for the development of hybrid ceramic membranes possessing exceptional surface properties and demonstrating effective and sustainable anti-fouling characteristics during cleaning and filtration cycles.

Powder preparation

In this study, we utilized olive seeds obtained from Taroudant, Morocco, to manufacture tubular microfiltration membranes. We ground 50 g of the powder using a mortar crusher (Retsch, France) for 20 min. After drying the powder, we sifted it through a 125 μm sieve to obtain the desired structure for the membrane.

Elaboration of the ceramic membrane

The production of tube-shaped supports involves the use of 125 μm particle size powder and organic additives, including Amijel derived from starch, starch, and methocel. After several tests, the ideal paste composition comprised 82% olive seed powder, 10% Amidon as a porosity agent, 4% plasticizer (methocel), and 4% binder (Amijel). Homogenization of the blend was performed using an electric mixer at 250 (tr/min) for 20 min. Distilled water (28.4% by weight) was progressively added to the solid mixture to achieve the desired plastic paste. The mixture was sealed and aged for 24 h to guarantee full dispersion of organic ingredients and water.

A tubular membrane with a diameter of 6 mm, a thickness of 2 mm, and a length of 15 cm was fabricated through the extrusion process, as illustrated in Figure 1. After drying at room temperature for 24 h, the membrane was sintered in a programmable furnace at temperatures of 200 and 250 °C for 2 h each. These temperatures were determined based on the TGA/DTA findings of the powder.
Figure 1

Extrusion method.

Figure 1

Extrusion method.

Close modal

Characterization techniques

The developed membrane was subjected to a comprehensive characterization process involving several analytical techniques, including X-ray diffraction (XRD), differential thermal analysis (DTA), thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, and scanning electron microscopy (SEM). The XRD analysis provided insights into the crystalline structure of the olive seed powder, while FTIR spectroscopy revealed the functional groups. TGA/DTA analyses provide information on the thermal stability and decomposition behavior of the membrane. Finally, SEM analysis was used to evaluate the morphology and the membrane surface. The combination of these techniques provides a comprehensive understanding of the physicochemical properties of the developed membrane.

To evaluate the chemical resistance of the membrane, for 2 days under standard conditions, it was submerged in concentrated acid (pH = 4) and basic (pH = 9) solutions. After drying the membrane, the net reduction in mass was calculated. The porosity of the membrane was also determined during this process to assess the structural properties required for specific applications. The measurement of this parameter is based on the following equation:
formula
(1)
where mw is the dry weight, mf is the buoyant weight, and ms is the weight when completely saturated with water.

In addition, the three-point bending method of the Shimadzu EZ-LX was used to measure resistance to mechanical force.

Permeability test

A laboratory pilot made of stainless steel, as shown in Figure 2, was utilized to conduct the filtration test. The pilot has a 5-liter feed tank, two manometers, a membrane model, and a circulation pump. The pressure used during the test varied from 0.1 to 0.5 bar. For 30 min of filtration, a constant transmembrane pressure of 0.2 bar was used. A surface area of 28 cm2 was used for the filtration experiments. The membrane support was submerged in clean water for a full day before the filtering test, and distilled water was used to determine the water's permeability.
Figure 2

Schema of the laboratory pilot.

Figure 2

Schema of the laboratory pilot.

Close modal

Characterization of the powder

XRD analysis

The chemical composition of olive seed powder is expressed in weight percentage. Specifically, the powder is composed of 34.25% cellulose, 12.40% hemicellulose, 23.36% lignin, 4.33% fat content, and 3.01% ashes. It is worth noting that lignocellulosic materials can possess crystalline and amorphous forms of cellulose, while the hemicellulose and lignin components have an amorphous structure (Ahmed et al. 2022; Essekri et al. 2023). Furthermore, in Figure 3, the XRD pattern of untreated biomass exhibits two wide peaks about 22.5 and 25.5 at 2θ values, denoting the 101 and 002 diffraction plane in cellulose (Kalyani et al. 2017; Essekri et al. 2023).
Figure 3

X-ray diffractogram of olive seed powder.

Figure 3

X-ray diffractogram of olive seed powder.

Close modal
Three images were found in the TGA/DTA analysis of the powder (Figure 4). The first image was taken between 30 and 140 °C, and the evaporation of water generated a 14.8% weight reduction. During this stage, an endothermic stage was noted at about 60 °C. The second image displayed two exothermic episodes and a weight loss of 79.1% between 220 and 495 °C. The thermal breakdown of carbohydrates (cellulose and lignocellulose) produces the peak at 300 °C. However, the overlapping thermal breakdown of lignocellulosic components (cellulose, hemicellulose, and lignin) is responsible for the peak at 395 °C in the temperature range of 260–495 °C (Hadoun et al. 2013).
Figure 4

Thermal analysis curves (TGA/DTA) of the olive seed powder.

Figure 4

Thermal analysis curves (TGA/DTA) of the olive seed powder.

Close modal

FTIR spectroscopy

The infrared spectrum of the raw material is shown in Figure 5. The observed band at 3,320 cm−1 corresponds to the stretching vibrations of hydroxyl groups, as previously noted in studies (Kumar et al. 2019; Ahmed et al. 2022). The band at 2,915 cm−1 is attributed to the asymmetric stretch of aliphatic C–H (Singh & Rattan 2011; Essekri et al. 2023). According to El-Naggar & Rabei (2020), the carbonyl (C = O) stretching vibration is represented by the absorption band at 1,646 cm−1. The band seen at 1,380 cm−1 represents the C–H deformation mode. Essekri et al. (2023) have indicated that the band at 993 cm−1 is linked to the bending of the aromatic C–H bonds in cellulose.
Figure 5

FTIR spectra olive seed powder.

Figure 5

FTIR spectra olive seed powder.

Close modal

Microfiltration membrane

Characterization

The morphology of the elaborated membrane, which underwent sintering at 200 and 250 °C, was examined using SEM images. Figure 6 presents SEM images of the membrane surface. The results revealed that macro-defects were absent in both cases. However, a porous morphology was detected for the membrane sintered at 200 °C (Figure 6(a)). At 250 °C (Figure 6(b)), the membrane's surface exhibited a homogeneous porous structure.
Figure 6

SEM images of the membrane sintered at (a) 200 °C and (b) 250 °C, and (c) porosity and flexural strength and (d) pore size distribution of the membrane sintered at 200 °C.

Figure 6

SEM images of the membrane sintered at (a) 200 °C and (b) 250 °C, and (c) porosity and flexural strength and (d) pore size distribution of the membrane sintered at 200 °C.

Close modal

The process of sintering temperature determines the porosity of a ceramic membrane. During this process, organic additives are combusted, which creates pores in the membrane. The microstructure of the membrane is considerably affected by the sintering temperature. Sintering begins at 200 °C, causing small fragments to agglomerate. At 250 °C, the duration of the piece is predominately portrayed in Figure 6(b). Analysis of the membrane revealed that 200 °C is the ideal temperature for sintering to create a ceramic membrane from olive seeds. Figure 6(c) illustrates the advancement of porosity and flexural strength of the membrane prepared at both sintering temperatures. The mechanical resistance increases from 0.5 to 0.7 MPa. However, these values are inadequate when compared to the literature. In addition, fritting over 350 °C is not recommended according to TGA/DTA analysis; nonetheless, oils can be filtered by gravitation without applying high pressure. Furthermore, the next study project will focus on incorporating a particular amount of clay to improve the mechanical strength of this membrane.

The researchers utilized ImageJ software to assess the features of the pores in the membranes based on the SEM images acquired (Achiou et al. 2018; Addich et al. 2022a, 2022b). The software was able to measure the diameters of almost 100 pores, and a careful sampling process was necessary to determine the pore size distribution. These values represent the actual porous shape of the support material. From Figure 6(d), it is evident that the ceramic membrane contains pores with diameters ranging from 2 to 15 μm in 75–89% of the support pores.

The results presented in Figure 7 show the percentage of the lost weight of the microfiltration membrane during testing. The membrane experiences minimal weight loss during testing, consistently remaining below 4% for all sintering temperatures. The membrane also displays corrosion resistance.
Figure 7

Chemical resistance of the membrane.

Figure 7

Chemical resistance of the membrane.

Close modal

Permeability

To ensure the optimal performance of the developed ceramic membrane, the membrane's permeability to distilled water must be evaluated beforehand. This is achieved by measuring the volume of liquid permeated over time at varying transmembrane pressure drop values, ranging from 0.1 to 0.2 bar. To ensure the reliability of the results, permeation experiments are conducted to ascertain that the liquid flux remains consistent over time. The permeate flux (F), measured in L/h m2, is related to the hydraulic permeability (Lp) through the following equation:
formula
where ΔP (bar) represents the transmembrane pressure. Figure 8 illustrates the variation of the membrane flux as a function of pressure. The curve of the membrane is linear, indicating a permeability of 1,852 L/h m2 bar. This demonstrates the membrane's ability to efficiently treat large amounts of liquid.
Figure 8

Water permeability.

Figure 8

Water permeability.

Close modal

Oil-in-water emulsion test solutions were prepared using virgin-grade olive oil in deionized (DI) water with 0.01 wt% Tween 80 as the surfactant. Two different oil concentrations, 500 and 1,000 mg/L, were utilized. The oil, surfactant, and DI water were combined in Pyrex glass bottles and subjected to mechanical shearing using a homogenizer (Ultra turax) at 14,000 rpm for 30 min. Subsequently, the obtained emulsion was left to stabilize for 24 h.

The light microscopy images of the prepared oil-in-water emulsion test solutions and their corresponding particle size distribution are shown in Figure 9(a). All the emulsions showed a broad size distribution of oil droplets, with most of the size ranging from 2 to 16 μm.
Figure 9

(a) SEM of the prepared oil-in-water emulsion test solutions, (b) variation of permeate flux, (c) feed and the filtrate water samples, and (d) SEM image of the filtered membrane.

Figure 9

(a) SEM of the prepared oil-in-water emulsion test solutions, (b) variation of permeate flux, (c) feed and the filtrate water samples, and (d) SEM image of the filtered membrane.

Close modal

Figure 9(b) illustrates the variation of permeate flux over time during the filtration process of both emulsions under 1 bar pressure. It is observed that the flux experiences a significant initial decrease, eventually stabilizing at nearly constant values. Initially, water flows easily through the membrane pores, resulting in high flux values. However, as filtration progresses, oil droplets obstruct the pores, leading to a reduction in flux values until reaching a steady state. This rapid decline in flux during the early stages of filtration is analyzed by examining the flux using various membrane fouling models.

The photographs of the feed and the filtrate water samples are shown in Figure 9(c). An SEM image of the filtered membrane sample surface is shown in Figure 9(d). The turbidity values of the water sample before and after the filtration experiments are given in Table 1.

Table 1

Characteristics of the membrane during the filtration of oil-in-water emulsion solutions

Sample (mg/L)Turbidity (NTU)
BeforeAfterFinal oil concentrationPercentage oil rejection (%)
500 385 ± 5.19 6.83 ± 0.49 18 96.4 
1,000 880 ± 4 8.11 ± 0.35 24 97.6 
Sample (mg/L)Turbidity (NTU)
BeforeAfterFinal oil concentrationPercentage oil rejection (%)
500 385 ± 5.19 6.83 ± 0.49 18 96.4 
1,000 880 ± 4 8.11 ± 0.35 24 97.6 

The concentration of oil in the permeate water was determined using UV–visible spectroscopy, and the results are presented in Table 1. It was observed that an increase in the oil concentration in the feed resulted in enhanced oil rejection, consistent with findings reported by Ebrahimi et al. (2010). Oil rejection rates of up to 97.6% were achieved for the 1,000 mg/L sample. When the oil content in the feed is high, the accumulation of oily particles gradually blocks the larger pore channels through which tiny oily particles could penetrate, thereby affecting the rejection performance over time.

This study describes the elaboration of an innovative microfiltration membrane made from olive seeds. The membrane's support was evaluated with various characterization techniques, including SEM. The study findings indicate that the membrane holds significant potential for use in various applications where microfiltration is required. The cost-effectiveness of the membrane production process and its efficient separation capabilities make it an attractive alternative to existing technologies.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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