Hybrid membranes made from industrial waste for the separation of oil emulsions^#

Abstract

The development of sustainable membranes from polymer waste is a promising strategy for wastewater treatment. This study investigated the effect of zinc oxide (ZnO) incorporation on recycled polyamide 66 (PA66) membranes prepared by the phase inversion method for oily emulsion separation. Membranes were characterized by atomic force microscopy, porosity, water absorption, pore size, contact angle, and permeation tests. ZnO promoted structural modifications, increasing roughness (140.87 nm), water absorption (76.6%), porosity (60.5%), and mean pore radius (1.85 µm) compared with pure PA66. The membrane containing 3 wt.% ZnO exhibited the highest stabilized water permeate flux (159.82 L.m^-2.h^-1). All membranes achieved color and turbidity removals of up to 99.88% and oil rejection above 95%, producing permeates that complied with current discharge regulations. These results demonstrate that recycled PA66/ZnO hybrid membranes are promising, sustainable candidates for efficient oily wastewater treatment.

Keywords:
hybrid membranes; phase inversion; polyamide 66 waste; treatment of oily emulsions

1. Introduction

Water and wastewater treatment relies on a combination of conventional unit operations, including screening, sedimentation, coagulation, flocculation, filtration, ion exchange, adsorption, biological treatment, disinfection, oxidation, chemical precipitation, and membrane-based separations. Among these technologies, membrane processes have attracted increasing attention because they offer high separation efficiency, compact system design, and the ability to remove a wide range of contaminants[¹]. However, effluent treatment relying solely on conventional processes is often associated with high operational costs and significant energy consumption. Consequently, there is a growing need for research focused on the development of advanced water purification technologies that reduce energy demand while minimizing environmental impacts. In this context, membrane separation processes represent a promising alternative, offering high efficiency, compact system design, and the potential to replace or complement conventional treatment steps in a more sustainable manner.

In this scenario, membrane separation processes (MSPs) stand out due to their energy efficiency, environmentally friendly nature, operational simplicity, and ability to partially or fully replace conventional treatment processes. In addition, membrane technologies enable the recovery of high-value products and offer significant flexibility in system design and in the development of hybrid treatment processes[²]. A membrane is defined as a selective barrier that separates two phases and restricts, either totally or partially, the transport of one or more chemical species present in these phases[³].

The application of MSPs depends on their specific field of use, including chemical and pharmaceutical industries, water and wastewater treatment, medical applications, among others. Membrane-based processes encompass a wide range of separation techniques, such as microfiltration, ultrafiltration, reverse osmosis, dialysis, electrodialysis, gas permeation, and pervaporation[⁴,⁵]. Among these, microfiltration membranes have proven to be an effective solution for the treatment of industrial effluents and wastewater, primarily due to their pore sizes, which typically range from 0.1 to 10 µm[⁶].

Commercial membranes are generally classified into two distinct categories based on their constituent materials: organic polymeric membranes and inorganic membranes, which are typically based on metals, glass, or ceramic materials. In practice, organic membranes are more widely used due to their lower production costs compared to inorganic membranes, thereby offering greater potential for market expansion and industrial application[⁷,⁸].

Phase inversion is one of the most widely employed and versatile techniques for the fabrication of asymmetric polymeric membranes, including polyamides[⁹,¹⁰]. In phase inversion processes, the polymer undergoes a transformation from a liquid solution to a solid state. This method allows extensive control over membrane morphology, which is strongly influenced by the choice of solvent and nonsolvent systems employed during membrane preparation.

The increasing generation of oily effluents from industrial activities has intensified the search for efficient and sustainable technologies for oil-water separation. Among the various techniques available, membrane separation processes stand out for their high efficiency, operational simplicity, and low energy consumption. In this context, the development of polymeric membranes obtained by phase inversion has received considerable attention due to the versatility of the technique in producing porous structures with controlled properties.

Recent studies have highlighted the advances in membrane technologies for wastewater treatment, including the removal of heavy metals, the separation of oil-water emulsions, and fouling control through the incorporation of nanomaterials. Membranes modified with nanocomposites, such as MoS₂/g-C₃N₄ and Ag-CuO, have demonstrated significant improvements in filtration performance and separation efficiency. Furthermore, adsorption processes have been used as complementary technologies to increase contaminant removal. These advances reinforce the importance of developing new materials for environmental applications, highlighting the relevance of the present study, which uses PA66 waste and ZnO nanoparticles to obtain hybrid membranes for the treatment of oil-in-water emulsions[¹¹-¹⁴].

Polymers such as poly(lactic acid) (PLA), poly(butylene adipate-co-terephthalate) (PBAT), and polyethersulfone (PES) show great potential for the manufacture of these membranes, due to their biodegradability, mechanical strength, chemical stability, and processability. Furthermore, the use of polymer blends and nanocomposites has proven to be an effective strategy for optimizing properties such as porosity, hydrophilicity, permeability, mechanical strength, and fouling resistance. Thus, the combination of different polymer matrices and nanoparticles through the phase inversion technique allows for the production of membranes with superior performance in oil-water separation, contributing to wastewater treatment and the preservation of water resources[¹⁵-¹⁸].

The most widely used membranes are produced from synthetic polymers such as polyamides, polyethersulfone, polyacrylonitrile, polysulfones, among others[¹⁹]. Polyamides are particularly favored due to their excellent separation performance, exhibiting high water flux combined with effective salt rejection, enhanced stability over a wide pH and temperature range, fouling resistance, antibacterial effects, and a favorable balance between water permeability and solute selectivity[²⁰,²¹].

Hybrid membranes are capable of combining inorganic and organic components during membrane formation or polymerization, offering significant advantages for water and wastewater treatment applications[²²]. The incorporation of inorganic nanoparticles, such as clay minerals[²³], zinc oxide (ZnO), among others, into polymer solutions to fabricate hybrid membranes represents an effective strategy to enhance physicochemical properties. In addition, such hybrid systems have demonstrated improved performance across various separation processes, including the desalination of brackish and saline waters[²⁴,²⁵].

The development of membranes from industrial polymeric waste brings significant advances to the state of the art in the area of membrane separation that are related to reducing industrial polymeric waste, promoting the reuse of materials that would otherwise be discarded; having a lower dependence on virgin polymers, reducing costs and environmental impacts and adopting greener processes[²⁶,²⁷]. In addition, the preparation of hybrids membranes allows, in many cases, to find a relationship between low cost, due to the use of filler, and high level of performance, which can result in synergy between the properties of the individual components. This fact demonstrates the importance of studying the insertion of polymer composite membranes to improve properties and consequently obtain greater efficiency in membrane separation processes.

Waste generated from the spinning of synthetic polyamide fibers, when used in textile manufacturing, is often reused within the production process; however, when improperly disposed of in the environment, such materials exhibit a degradation time of approximately 30 years[²⁸-³⁰]. Thus, the development of hybrid microfiltration membranes combining polyamide waste and inorganic additives represents an innovative approach aligned with the principles of sustainability, circular economy, and environmental efficiency. Therefore, the present study aims to utilize industrial waste derived from synthetic polyamide fibers for the fabrication of organic-inorganic composite microfiltration membranes through the phase inversion technique, with properties tailored for the treatment of oily emulsions.

2. Materials and Methods

2.1 Materials

The polymeric matrix was obtained from synthetic Nylon® fibers (polyamide 66, PA66) derived from industrial waste generated during the manufacture of nylon yarns for automotive tire cord fabrics. The material was supplied by an industrial facility located in Camaçari, Bahia, Brazil. Zinc oxide (ZnO, 98% purity, Vetec Laboratory Products Ltd.) was used as the inorganic filler for the preparation of the hybrid membranes. Formic acid (CH₂O₂, 85%, Vetec Laboratory Products Ltd.) was employed as the solvent, while potassium chloride (KCl, analytical grade) was used as a pore-forming agent during membrane fabrication. Unless otherwise specified, all other chemicals used in this study were of analytical grade. The n-hexane (C₆H₁₄, 95%, Exodus Scientific Laboratory Products). The oily emulsions were prepared using crude oil obtained from the Guamaré oil field, located in the onshore portion of the Potiguar Basin, in the state of Rio Grande do Norte, Brazil, between the municipalities of Macau and Guamaré, approximately 150 km northwest of Natal, in the Northeast region of the country.

2.2 Membrane preparation

The solutions were prepared containing 20% by mass of total solids (PA66 and ZnO) and 80% by mass of CH₂O₂, as shown in Table 1. Additionally, KCl was incorporated at a proportion of 10% by mass relative to the total amount of solids, acting as a porogenic agent during the phase inversion process. After membrane formation, the KCl was eliminated during the coagulation and washing steps, and is not considered a component of the final membrane composition. Therefore, the final composition of the membranes was considered only in terms of the PA66 polymer matrix and the incorporated ZnO nanoparticles, while KCl acted additive.

Table 1
Composition of polyamide membranes and their hybrids.

Membranes	PA66 (g)	ZnO (g)	CH2O2 (g)
Pure PA66	20.0	0.0	80.0
PA66/1% ZnO	19.8	0.2	80.0
PA66/3% ZnO	19.4	0.6	80.0
PA66/5% ZnO	19.0	1.0	80.0

The solutions were prepared at room temperature (25 °C) under an average relative humidity of approximately 65%. The prepared solutions were molded onto glass plates using glass rods with a gap of approximately 0.3 mm, value refers to the nominal thickness of the deposited liquid film and the final membrane thickness is illustrated in Table 2. Immediately after molding, the films were immersed in a non-solvent coagulation bath (water) at room temperature, monitored by a thermometer, ensuring complete submersion of the plates. The membranes remained in the non-solvent bath until phase separation and precipitation were complete, taking approximately 3 min. Then, the membranes were detached from the glass plates and carefully washed with distilled water for 60 min.

Table 2
Thicknesses wet and dry of membranes of polyamide and their hybrids.

Membranes	Wet averages (µm)	Standard deviation	Dry averages (µm)	Standard deviation
Pure PA66	246.67	1.15 x 10-3	193.33	2.08 x 10-3
PA66/1% ZnO	263.33	6.24 x 10-3	201.35	8.62 x 10-3
PA66/3% ZnO	152.54	1.73 x 10-3	105.82	1.02 x 10-3
PA66/5% ZnO	193.33	1.78 x 10-3	182.04	1.25 x 10-3

Membranes intended for permeate flux measurements were stored by immersion in a mixture containing 10% n-hexane and 90% water until testing. In contrast, membranes used for other characterizations were dried at room temperature under the ambient relative humidity conditions present during drying. The storage of membranes prior to flux measurements aimed to prevent pore collapse caused by capillary forces during drying, as water exhibits relatively high surface tension.

2.3 Preparation of oily emulsions

To prepare the oily emulsions, 50, 100, and 200 mg of oil (crude petroleum) were accurately weighed using an analytical balance and individually transferred to 1 liters (L) beakers, followed by the addition of 1 L of water to each beaker. Each mixture was then separately subjected to high-shear mixing at a rotational speed of 22,000 revolutions per minute (rpm) for 15 min. Immediately after homogenization, the emulsions were used to feed the flow system. Table 3 illustrates the intrinsic physicochemical characteristics of the crude oil at a temperature of 25 °C.

Table 3
Intrinsic physicochemical characteristics of the crude oil.

Physicochemical characteristics	Density (g.cm-3)	API degree (°API)	Sulfur content (%)	Viscosity (mPa.s)
Crude oil	0.89	27.49	0.29	177.53 ± 0.46

2.4 Characterization of membranes and oily emulsions

2.4.1 Atomic force microscopy

Atomic force microscopy (AFM) was performed using an SPM 9700 instrument (Shimadzu, Kyoto, Japan) to evaluate the membrane morphology. Surface topography and relative roughness of the membranes were examined in dynamic mode at a scanning rate of 1 Hz. The membranes were mounted on a sample holder and analyzed over a scanning area of 10×10 µm² using the SPM Manager software. The obtained topographic images were used to calculate the average surface roughness of the selected membrane areas. The topographic images obtained by AFM provide essential information on the surface morphology of the membranes, as the depressions observed in the images indicate the degree of membrane porosity, which directly influences filtration performance, in addition to providing data on surface roughness. The roughness parameters evaluated include Ra (nm), Rz (nm), and Rzjis (nm), with Ra being the primary parameter. Ra corresponds to the average roughness and is calculated according to Equation 1[³¹]:

$Ra=\frac{1}{L_x{L}_y}\underset{0}{\overset{L_y}{{\displaystyle \int }}}\underset{0}{\overset{L_x}{{\displaystyle \int }}}\left|f\left(x,y\right)\right|dxdy$ (1)

Where Lx and Ly represent the dimensions of the analyzed surface area, and f(x, y) corresponds to the surface profile relative to the mean plane. The parameter Rz defines the maximum height, where Z is the difference between the highest peak and the lowest valley within the analyzed area. Rzjis represents the ten-point mean roughness, which corresponds to the average height difference between the five highest peaks and the five deepest valleys on the surface.

2.4.2 Water absorption

The water absorption capacity was measured by placing membranes with different masses in contact with water. The system was maintained at a temperature of 24 °C. The membranes were weighed before and after 24 h of water absorption. The density of the membranes was determined following the procedures established by the ASTM D792-08 standard[³²], also with triple replication to ensure consistency in the results.The percentage of water content was measured by the difference in weight between the dry and wet membranes, expressed through Equation 2[³³].

$Waterabsorption\left(\%\right)=\frac{W_w-W_d}{W_d}X100\%$ (2)

Where W_w (g) and W_d (g) are the wet and dry membrane masses, respectively. Measurements were made and average values ​​will be reported to minimize errors.

2.4.3 Porosity

Porosity was determined by immersing the membranes in water for 24 h. The initial mass of the membrane, the absorbed mass of water, the relative volume of the membrane and the density of the penetrating liquid, which is water, were taken into account. The experiment was performed in triplicate, expressed by Equation 3[³⁴].

$$\epsilon =\left[\frac{\left(W_w-W_d\right)}{V_m\cdot {\rho }_a}\right]$$ (3)

Where W_w (g) and W_d (g) correspond to the wet and dry membrane masses, respectively; V_m (cm³) represents the membrane volume; and ρ_a (g.cm^-3) is the density of water at 25 °C.

2.4.4 Average pore radius

The average pore radius rm was determined using the Guerout-Elford-Ferry equation, as per Equation 4[³⁵], based on porosity and pure water permeation rate data. The experiment was performed in triplicate.

$r_m=\sqrt{\frac{\left(\mathrm{2,9}-\mathrm{1,75}\epsilon \right).8.l.\eta .Q}{\epsilon .A.\Delta P}}$ (4)

Where η is the viscosity of water (8.9 × 10^-4 Pa.s), l is the membrane thickness (m), Q is the water permeation rate (m³.s^-1), A is the effective membrane area (m²), and ΔP is the transmembrane pressure (Pa).

2.4.5 Optical Microscopy (OM)

Optical microscopy analysis was performed using a LEICA M750 instrument with a built-in CCD camera and Leica lenses. This was used to obtain real images with polarized light of the oil emulsions at concentrations of 50, 100, and 200 mg.L^-1. After the analyses, ten droplet measurements were taken at each concentration to evaluate the average size of the oil droplets present in the emulsions[³⁶].

2.4.6 Flow measurements

For the water and effluent flow measurement tests, a perpendicular filtration cell coupled to a filtration system was used to measure the permeate. The membranes were subjected to permeability tests at a pressure of 1.0 bar. The effluents were prepared at a concentration of 50, 100 e 200 mg.L^-1. Membrane performance can be evaluated through the permeate flux and of a given solute present in the feed solution. The volumetric flow (J) for the hybrid membranes with their respective variations was determined through Equation 4[³⁷]:

$J=\frac{PermeateVolume\left(L\right)}{Time\left(h\right).MembraneArea\left(m^2\right)}$ (4)

2.4.7 Physicochemical parameters of the oily effluents

The physicochemical parameters of the oily effluents analyzed were turbidity and apparent color. Turbidity measurements were performed using an AP200 WT turbidimeter, and the apparent color was obtained using the AquaColor colorimeter from Policontrol. The methodology employed to determine the physicochemical parameters of the water before and after the membrane separation process followed the procedures recommended in the Standard Methods for the Examination of Water and Wastewater[³⁸].

2.4.8 Oil and Grease Content (TOG)

The concentration of oil present in the aqueous phase was determined by measuring absorbance using a GTA-96 UV-visible spectrophotometer, in a range of 190-1000 nm. Initially, an absorbance-concentration calibration curve was obtained using known concentration oil standards in n-hexane PA. The samples read in this equipment were previously extracted with n-hexane[³⁹,⁴⁰]. Considering the instrument used for data collection, uncertainty analysis was performed for the spectrophotometer components in the following areas: wavelength range (190 - 1000 nm), transmittance range (0 - 200%T), absorbance range (0.3 - 3.0 Å), shown in Table 4. In the experiments, uncertainties of type B are assumed, since they are associated with systematic errors and can be estimated using other sources, such as calibration certificates and manufacturer specifications.

Table 4
Range, precision and uncertainty of each component of the measuring instrument.

Instrument	Range	Precision	Uncertainty
Wavelength range (nm)	190 - 1000	± 1.0	0.06
Transmittance range (% T)	0 - 200	± 0.5	0.29
Absorbance range (A)	0.3 - 3.0	± 0.01	0.006

Thus, the uncertainty of each device was determined according to Equation 5. It should also be noted that all devices were calibrated according to the manufacturer's recommendations[⁴¹]. The specifications of the instrument used in this study are listed in Table 2, including range, accuracy, and uncertainty.

$u=\frac{a}{\surd 3}$ (5)

Where, $u$ corresponds to the uncertainty of the instrument and is the standard precision of the instrument.

The reliability of the results was evaluated through uncertainty analysis applied to the experimental data, since this procedure allows quantifying the errors associated with the measurements. This analysis considers factors such as instrumental precision, the variability of the experimental data, and possible calibration errors that may affect the accuracy of the results. Table 4 presents the individual uncertainties of the components, highlighting the need to also consider the uncertainties associated with derived and combined parameters, such as the concentration of oil present in the oily emulsion, as they are directly influenced by the instrumental measurements[⁴²].

2.4.9 Membrane Selectivity

The selectivity of the membranes was estimated by the rejection coefficient R (%) or yield, calculated based on the quotient of the oil concentrations in the permeate C_p (mg.L^-1) and in the feed C_o (mg.L^-1), expressed by Equation 5[⁴³]:

$R\left(\%\right)=\left[\frac{\left(C_0-C_P\right)}{C_0}\right]x100$ (5)

3. Results and Discussions

3.1 Atomic force microscopy

AFM was used to investigate variations in surface morphology under these conditions. One of the main parameters closely related to the antifouling capacity of membranes is roughness. Figure 1 illustrates the surface and phase images of the 3D AFM at 10x10 µm size of pure PA66 membranes and hybrid membranes with 1, 3, and 5% ZnO with KCl.

Figure 1
Surface images obtained by AFM of pure PA66 membranes and hybrid membranes with 1, 3, and 5% ZnO, analyzing: (a) height and (b) roughness.

The presence of different percentages of ZnO in the membrane can also alter the surface morphology. The membrane surfaces (Figure 1) are presented by two distinct regions: light and dark regions. The light regions correspond to the higher areas related to heights/valleys/pores, and the dark regions to areas of low topographic intensity. The greater number of upper and lower topographic areas can result in greater roughness of the membrane surface, which can be considered a sign of increased pore diameter on the top surface based on the membrane topography[⁴⁴]. According to Figure 1, it was found that ZnO tended to project onto the surface of the PA66 membrane during coagulation. It also caused the polymer to solidify near the surface, so that large local peaks were formed. It is important to note that upon reaching the membrane surface, depending on the thermodynamic condition during phase inversion, the amount of roughness tends to increase. Furthermore, disruption of the PA66 chain homogeneity on the surface may occur, which could be another reason for the high roughness value in hybrid membranes, resulting in increased permeability and attributing this relationship to the increase in the effective area of ​​the membranes in the case of higher roughness[⁴⁵].

The surface roughness parameters of the membranes are presented in Table 5, where Ra is the arithmetic mean roughness, Rz is the maximum height, and Rzjis is the 10-point average roughness. The results in Table 3 indicated that all roughness parameters of the membranes with added ZnO were superior to those of the pure membrane. Therefore, it was found that by adding inorganic particles to the solution, the membranes exhibited greater roughness compared to the membrane without the additive[⁴⁶-⁴⁸].

Table 5
Roughness parameters (Ra, Rz and Rzjis) of the surface of pure PA66 membranes and hybrid membranes with 1, 3 and 5% ZnO with the addition of KCl salt.

Membranes	Ra (nm)	Rz (nm)	Rzjis (nm)
Pure PA66	76.97	792.21	391.54
PA66/1% ZnO	80.77	928.50	462.19
PA66/3% ZnO	101.44	1048.00	508.28
PA66/5% ZnO	140.87	1566.00	780.97

AFM results demonstrate that the incorporation of ZnO promoted a progressive increase in the surface roughness of PA66 membranes. The average roughness (Ra) values ​​increased from 76.97 nm for the pure membrane to 80.77, 101.44, and 140.87 nm in the membranes containing 1%, 3%, and 5% ZnO, respectively. A similar trend was observed for the Rz and Rzjis parameters, which showed significant increases with increasing ZnO concentration. The Rz parameter increased from 792.21 nm to 1566 nm, while Rzjis went from 391.54 nm to 780.97 nm for the pure membrane and the membrane containing 5% ZnO, respectively. The increase in surface roughness can be attributed to the incorporation of ZnO into the polymer matrix, promoting modifications in the surface topography and increasing the structural heterogeneity of the membranes. This behavior suggests that the introduction of ZnO influenced the phase inversion process, favoring the formation of regions with greater surface irregularity. Furthermore, the presence of ZnO may have contributed to the formation of protrusions and depressions on the membrane surface, resulting in the higher roughness values ​​observed for formulations with higher zinc oxide content.

From a performance perspective, the increase in roughness can be considered beneficial, since it is frequently associated with an increase in available surface area and greater interaction between the membrane and the permeant fluid. This behavior may contribute to the higher fluxes observed in the hybrid membranes. The results therefore indicate that the incorporation of ZnO significantly modified the surface morphology of the membranes, contributing to the improvement of their functional properties and reinforcing the potential of these membranes for applications in separation processes.

It is important to emphasize that potassium chloride (KCl) was added to all solutions for the preparation of pure and hybrid PA66 membranes, with the intention of KCl acting as a porogenic agent during the preparation of the membranes by phase inversion, favoring the formation of a more porous structure. Its presence influences the exchange kinetics between solvent and non-solvent during coagulation, promoting faster unmixing and increasing the void fraction in the polymer matrix. After its removal during the washing steps, the salt leaves spaces that contribute to increased porosity and permeability of the membrane. In the case of PA66/ZnO membranes, the increase in porosity can be attributed to the combined effect of KCl, acting as a porogenic agent, and ZnO, which, due to its hydrophilic character, can accelerate phase inversion and favor the formation of a more open structure.

3.2 Water absorption

The Figure 2 illustrates the results obtained for the water absorption of pure PA66 membranes and hybrid membranes with 1, 3, and 5% ZnO with the addition of the salt. This water absorption parameter was studied to confirm whether the incorporation of ZnO influenced the hydrophilic properties of the PA66 membrane.

Figure 2
Water absorption of pure PA66 membranes and hybrid membranes with 1, 3, and 5% ZnO.

From Figure 2, the absorption of 66.9±0.78% was observed for the pure PA66 membrane, and for the membranes with 1, 3, and 5% ZnO, the values ​​were 67.6±1.79%; 76.6±2.32% and 72.1±1.91%, respectively. Higher water absorption was obtained when a higher percentage of ZnO was introduced, probably due to the hydrophilic nature of this inorganic compound. Furthermore, the greatest increase in water absorption was 76.6% for the membrane with 3% by weight of ZnO, which may also be related to a more uniform distribution of ZnO particles throughout the surface and transverse structure of this membrane[⁴⁹,⁵⁰]. Regarding the PA66/5%ZnO membrane, a decrease in water absorption was observed when compared to the PA66/3%ZnO membrane, due to the formation of agglomerates, indicating that optimal absorption conditions were achieved in the composition with 3% ZnO.

The incorporation of ZnO nanoparticles influences the phase inversion process due to their hydrophilic character and their interaction with the polymer/solvent/non-solvent system. During coagulation, the particles favor the diffusion of water into the polymer solution and accelerate the exit of the solvent, promoting a higher unmixing rate. Furthermore, the presence of ZnO can reduce the thermodynamic stability of the system, favoring a faster and more intense phase separation. As a consequence, a more porous structure with greater pore interconnectivity is formed, contributing to increased membrane permeability. However, at higher concentrations, the tendency for particle agglomeration and the increase in the viscosity of solution can hinder solvent-non-solvent exchange and compromise the uniformity of the formed structure, resulting in reduced water absorption and hydraulic performance.

3.3 Porosity

Figure 3 illustrates the porosities of pure PA66 membranes and hybrids with 1, 3 and 5% ZnO with added salt.

Figure 3
Porosity of pure PA66 membranes and hybrid membranes with 1, 3, and 5% ZnO.

From Figure 3, it can be observed that the porosity of the pure PA66 membrane was 46.5±0.71%, gradually increasing to 49.0±0.62%, 55.6±1.04%, and 60.5±1.80% for the membranes containing 1, 3, and 5% ZnO by mass, respectively. Considering the magnitude of these differences and the limited overlap between the intervals associated with the standard deviations, the results indicate a consistent trend of increased porosity with the incorporation of ZnO. This behavior suggests that the incorporation of ZnO influenced the phase inversion process, favoring a faster separation between solvent and non-solvent and resulting in the formation of more porous structures. The hydrophilic nature of the ZnO nanoparticles may have accelerated the diffusion of water into the polymer solution during coagulation, promoting changes in membrane morphology and increasing the volume fraction of voids[⁵¹].

It is also observed that the membrane containing 5% ZnO showed higher porosity but lower water absorption compared to the other hybrid membranes. This behavior may be associated with the agglomeration of nanoparticles at higher concentrations, affecting the uniformity of the porous structure formed. Furthermore, the presence of macrovoids or finger-like structures may contribute to increased overall porosity without necessarily resulting in greater water retention, since absorption also depends on pore connectivity and the interaction between water and the membrane surface. Similar results were reported by[⁵²], who observed the formation of macrovoids in polyamide 6 membranes obtained by phase inversion.

3.4 Average pore radius

The average pore radius of pure PA66 membranes and hybrid membranes with 1, 3, and 5% ZnO can be observed in Figure 4.

Figure 4
Average pore radius of pure PA66 membranes and hybrid membranes with 1, 3, and 5% ZnO.

As shown in Figure 4, an average pore radius of 1.24±0.06 µm was observed for the pure PA66 membrane, and for the membranes with 1, 3 and 5% ZnO, the values ​​were 1.40±0.05 µm, 1.85±0.09 µm, e 1.66±0.05 µm, respectively. The average pore radius obtained resulted in an average pore diameter of 2.48±0.06 µm for the pure PA66 membrane, and for the membranes with 1, 3, and 5% ZnO, the values ​​were 2.80±0.05 µm, 3.70±0.09 µm, and 3.32±0.05 µm, respectively. It can be observed from Figure 4 that the PA66/3% ZnO and PA66/5% ZnO membranes showed a decrease in the average pore radius size when compared to the pure PA66 membrane, probably due to the addition of ZnO causing a change in the pore diameter of the PA66 membranes[⁵³].

It is important to emphasize that in hybrid membranes, the reduction from large pores to smaller pores occurs due to the interactions between PA66 and ZnO. In general, when a higher percentage of ZnO was added to the PA66/5%ZnO membrane, agglomerations and partial blockage of the pores by the particles may have occurred, decreasing the average pore size. This behavior of decreasing average pore radius with increasing percentage of inorganic nanoparticles was also identified by[⁵⁴] when they prepared nanocomposite membranes with a polyethersulfone matrix with hydrated ferric oxide and halloysite nanotubes.

3.5 Optical microscopy

The images in Figure 5 obtained by optical microscopy refer to oil droplets on the scale of 20 μm, inherent to oily emulsions at concentrations of 50, 100 and 200 mg.L^-1.

Figure 5
Optical microscopy with the distribution of diameters of oily emulsions at concentrations of: (a) 50 mg.L^-1, (b) 100 mg.L^-1 and (c) 200 mg.L^-1.

Figure 5 and Table 6 shows the measurements of oil droplet diameters on the outermost surfaces of the oily emulsions, obtaining average droplet diameters of 6.94 µm, 6.86 µm, and 6.87 µm for concentrations of 50, 100, and 200 mg.L^-1, respectively. In the innermost region, it was possible to visualize very small and distorted droplets that cannot be clearly seen due to light reflection on the outer surface of the larger oil droplets present in the emulsions. Furthermore, it was observed that the average pore diameter obtained from their average radii was 2.48 µm, µm, 2.8 µm, 3.7 µm, and 2.48 µm for the pure PA66 membranes and the hybrid membranes with 1%, 3%, and 5% ZnO, respectively. These values ​​are smaller than the size of the oil droplets, proving that the prepared membranes have adequate pore sizes to separate the oil present, regardless of the concentration of these emulsions[⁵⁵,⁵⁶].

Table 6
Diameter of oil droplets at concentrations of 50, 100, and 200 mg.L^-1, prepared with a stirring speed of 15,000 rpm and a stirring time of 30 min.

Oily emulsions	Average (µm)	Standard deviation	Minimum (µm)	Q1 (µm)	Median (µm)	Q3 (µm)	Maximum (µm)
50 mg.L-1	6.94	1.24	5.14	5.61	7.17	7.35	9.41
100 mg.L-1	6.86	0.73	6.01	6.20	7.71	7.64	8.00
200 mg.L-1	6.87	0.79	5.61	6.42	6.74	7.46	8.02

3.6 Flow measurements

Figure 6 shows the water flux measurements for pure PA66 membranes and hybrid membranes with 1, 3, and 5% ZnO with the addition of KCl salt, at a pressure of 1.0 bar.

Figure 6
Water permeate flux for pure PA66 membranes and their hybrids with 1, 3 and 5% ZnO at a pressure of 1.0 bar.

Figure 6 shows the results of permeate water flux in the membranes, observing a gradual reduction in flux during the first 20 min of the test, followed by its stabilization. This behavior is characteristic of polymeric membranes and can be attributed mainly to the structural compaction promoted by the pressure applied during filtration, as well as to the accommodation of the porous structure when subjected to continuous water flow. Additionally, the hydrophilic nature of PA66 and the presence of ZnO favor the interaction between water and the polymeric matrix, as evidenced by the water absorption results. This interaction can promote a slight swelling of the amorphous regions of the polymer, increasing the resistance to water transport through the membrane during the initial moments of filtration. However, permeate flux is not governed exclusively by water absorption, but mainly by structural characteristics of the membrane, such as porosity, pore size and connectivity, and tortuosity of transport channels. After the initial period of compaction and structural accommodation, the system reaches an equilibrium condition, resulting in the stabilization of permeate flux. This behavior demonstrates that water absorption is related to the membrane's affinity for the permeant, while permeation performance is predominantly controlled by the porous structure developed during the phase inversion process[⁵⁷].

Table 7 shows the initial, stabilized, and final water permeate fluxes of pure PA66 membranes and their hybrids as a function of a pressure of 1.0 bar. Therefore, in general, an increase in the initial, stabilized, and final flux values ​​of the membranes with 1, 3, and 5% ZnO was observed compared to the pure PA66 membrane, due to structural modifications in the membrane, increasing its hydrophilicity and favoring the formation of a more porous structure. Furthermore, the presence of ZnO contributed to reducing resistance to water transport, showing potential to minimize fouling due to its intrinsic hydrophilic and antimicrobial properties[⁵⁸]. In processes that use the pressure gradient as a driving force, the permeate flux obtained is directly proportional to the pressure gradient itself, which is normally observed in microfiltration processes for lower pressures up to 2 bar.

Table 7
Values ​​of initial, stabilized, and final fluxes of pure PA66 membranes and hybrid membranes with 1, 3, and 5% ZnO at a pressure of 1.0 bar.

Membranes	Initial (L.m-2.h-1)	Stabilized (L.m-2.h-1)	Final (L.m-2.h-1)
Pure PA66	156.31	94.81	89.16
PA66/1% ZnO	263.21	155.21	146.02
PA66/3% ZnO	336.37	159.82	148.60
PA66/5% ZnO	180.06	117.63	112.59

The comparison between the initial and final fluxes of the membranes highlights the effect of ZnO incorporation on the hydraulic performance of the system. The pristine PA66 membrane exhibited an initial flux of 156.31 L.m^-2.h^-1 and a final flux of 89.16 L.m^-2.h^-1, corresponding to a reduction of approximately 42.9% throughout the filtration test. In contrast, the ZnO-modified membranes showed higher flux values during the entire filtration process. The membrane containing 1 wt.% ZnO presented an initial flux of 263.21 L.m^-2.h^-1 and a final flux of 146.02 L.m^-2.h^-1, whereas the membrane containing 3 wt.% ZnO achieved the highest initial flux (336.37 L.m^-2.h^-1) and a final flux of 148.60 L.m^-2.h^-1.

Although a decrease in flux was observed over time for all membranes, the ZnO-containing membranes maintained significantly higher final fluxes than the pristine PA66 membrane. This behavior suggests that the incorporation of ZnO promoted favorable changes in the membrane structure and surface properties, resulting in enhanced water permeability. However, for the membrane containing 5 wt.% ZnO, a reduction in performance was observed, with an initial flux of 180.06 L.m^-2.h^-1 and a final flux of 112.59 L.m^-2.h^-1. This result indicates that excessive nanoparticle loading may lead to particle agglomeration and partial pore blockage, negatively affecting hydraulic performance. Overall, the results demonstrate that ZnO incorporation at concentrations up to 3 wt.% was effective in improving permeate flux, with the membrane containing 3 wt.% ZnO exhibiting the best permeability performance.

In general, the water flux for the PA66/ZnO membranes was greater than the flux obtained for the pure polyamide 66 membrane, confirming that the increase in the size of the surface pores seen in the porosity facilitated water absorption. Similar results were obtained by[⁵⁸], as they verified a significant increase in the permeate flux of water in nanocomposite membranes containing inorganic fillers compared to membranes without additives[⁵⁹]. Based on the values ​​obtained in Figure 4, it was found that the PA66/5% ZnO membrane showed lower permeate flux values ​​when compared to the other hybrid membranes, probably due to the formation of agglomerates along its microstructure, resulting in a decrease in water flux[⁵⁹]. These agglomerates are confirmed and justified in the AFM analysis.

The addition of ZnO promoted the formation of a rougher, more irregular, and heterogeneous surface. This behavior is directly related to the porosity results, which increased from 46.5% to 60.5% with increasing ZnO concentration. During the phase inversion process, the presence of ZnO particles can accelerate solvent/non-solvent exchange, favoring the formation of a more porous structure and a rougher surface topography. The simultaneous increase in roughness and porosity contributed to the increased water flux of the hybrid membranes compared to the pure PA66 membrane, since a more porous structure offers less hydraulic resistance to water transport.

This effect was particularly evident for the membranes containing 1% and 3% ZnO, which showed the highest stabilized flux values. However, although the PA66/5% ZnO membrane showed the highest roughness and porosity values, its flux was lower than that observed for the PA66/3% ZnO membrane. This result suggests that, at higher concentrations, ZnO agglomerates may form, increasing the tortuosity of transport channels and reducing the effective connectivity of pores. Therefore, the results indicate that permeation does not depend exclusively on porosity or surface roughness, but also on the distribution, interconnectivity, and morphology of pores formed during phase inversion.

The volumetric flow measurements of the oil-in-water emulsion separation (J) of the membranes were plotted in reference to the water volumetric flow (J₀), i.e., J/J₀. The influence of the oil concentration on the flux factor (J/J₀) of the membranes can be observed in Figure 7.

Figure 7
Flow factor (J/J₀) of pure PA66 membrane and hybrid membranes with 1, 3, and 5% ZnO, at concentrations of 50, 100, and 200 mg.L^-1 and at a pressure of 1.0 bar.

From Figure 7, it was possible to observe that there was a decrease in the J/J₀ ratio, which is related to the increase in the concentration of oil contained in the effluent, causing greater clogging of the pores, with a probable concentration of polarization near the surface of the membranes that is established rapidly and may cause a sharp initial drop in permeate flux, thus leading to fouling, hindering the passage of oil at higher concentrations and even obstructing the pores present[⁶⁰].

In general, fouling is the set of phenomena capable of causing a drop in permeate flux when working with a solution or suspension. The extent of the phenomenon depends on the nature of the solution, but also, and more markedly, on the operating conditions of the system. In particular, in microfiltration processes, the decline in flux is very large, and can reach up to 5% of the pure water flux value. The PA66/3% ZnO membrane showed the best J/J₀ flux factor ratio, regardless of the oil emulsion concentration. This may indicate that the optimal conditions for ZnO addition were achieved, since membrane permeability is also associated with water absorption; in this case, it presented the highest water absorption value of 76.6±2.32%, and also showed a high porosity of 55.6±1.04%, justifying its high performance compared to the other compositions[⁶¹,⁶²].

The observed flux drop during filtration is mainly associated with fouling phenomena, caused by the deposition of oil droplets and other contaminants on the surface and inside the pores of the membrane. This accumulation increases resistance to water transport, gradually reducing the permeate flux. Membranes containing ZnO showed less flux reduction compared to pure PA66, indicating greater resistance to fouling. This behavior can be attributed to the increased hydrophilicity, porosity, and permeability promoted by the incorporation of ZnO, which favors water transport and minimizes oil deposition. Thus, the higher stabilized flux values ​​observed for the hybrid membranes demonstrate the positive influence of ZnO on filtration performance and mitigation of fouling effects.

3.7 Physicochemical parameters of oily effluents

Table 8 shows the turbidity, both of the effluent using pure and hybrid PA66 membranes at a pressure of 1.0 bar and concentrations of 50, 100, and 200 mg.L^-1.

Table 8
Values ​​of turbidity of pure PA66 membrane and hybrid membranes with 1, 3, and 5% ZnO in concentrations of 50, 100, and 200 mg.L^-1 of oily emulsions.

Membranes	Concentration (mg.L-1)	Initial turbidity (NTU)	Final turbidity (NTU)	Removal (%)
Pure PA66	50	63.7	0.34 ± 0.05	99.46
Pure PA66	100	101.3	0.94 ± 0.09	99.07
Pure PA66	200	154.2	0.37 ± 0.02	99.76
PA66/1% ZnO	50	63.7	0.51 ± 0.09	99.20
PA66/1% ZnO	100	101.3	0.37 ± 0.09	99.63
PA66/1% ZnO	200	154.2	0.35 ± 0.04	99.77
PA66/3% ZnO	50	63.7	0.31 ± 0.07	99.51
PA66/3% ZnO	100	101.3	0.35 ± 0.17	99.65
PA66/3% ZnO	200	154.2	0.18 ± 0.15	99.88
PA66/5% ZnO	50	63.7	0.32 ± 0.10	99.49
PA66/5% ZnO	100	101.3	0.18 ± 0.04	99.82
PA66/5% ZnO	200	154.2	0.31 ± 0.18	99.80

Turbidity measures suspended solids that affect the transparency of a solution and the efficiency of water purification. Turbidity analysis will assess the concentration of suspended particles from oil that was not retained by the microfiltration process[⁶³,⁶⁴]. Table 8 illustrates the turbidity values ​​of the oily emulsions present in the permeate after flow tests with the effluents, thus obtaining values ​​lower than 0.94 NTU, which are below the maximum permitted value of 5.0 NTU, and in accordance with Annex 11 of Ordinance GM/MS No. 888 of the Ministry of Health[⁶⁵].

Table 9 presents the color removal of effluents from pure and hybrid PA66 membranes at the respective concentrations (50, 100 and 200 mg.L^-1) and pressure of 1.0 bar.

Table 9
Values ​​of color of pure PA66 membrane and hybrid membranes with 1, 3, and 5% ZnO in concentrations of 50, 100, and 200 mg.L^-1 of oily emulsions.

Membranes	Concentration (mg.L-1)	Initial color (Pt-Co)	Final color (Pt-Co)	Removal (%)
Pure PA66	50	142.3	1.80 ± 0.20	98.74
Pure PA66	100	163.0	4.70 ± 1.10	97.08
Pure PA66	200	203.0	0.60 ± 0.20	99.70
PA66/1% ZnO	50	142.3	1.60 ± 0.10	98.88
PA66/1% ZnO	100	163.0	2.00 ± 0.47	98.77
PA66/1% ZnO	200	203.0	0.86 ± 0.47	99.57
PA66/3% ZnO	50	142.3	0.60 ± 0.10	99.58
PA66/3% ZnO	100	163.0	1.00 ± 0.35	99.39
PA66/3% ZnO	200	203.0	0.70 ± 0.35	99.66
PA66/5% ZnO	50	142.3	1.90 ± 0.17	98.67
PA66/5% ZnO	100	163.0	1.43 ± 0.15	99.12
PA66/5% ZnO	200	203.0	1.43 ± 0.15	99.29

In general, we can observe in Table 9 that, regardless of the oil concentration present in the oily emulsion, color removal was above 97%, with the pure PA66 membrane showing the lowest removal value (97.08%) at a concentration of 100 mg.L^-1, and the highest removal was for the PA66/1% ZnO membrane at a concentration of 50 mg.L^-1, achieving a color removal of 99.88% in treated water. Therefore, values ​​lower than 4.7 Pt-Co were obtained, which are below the maximum permitted value of 15 Pt-Co and are in accordance with Annex 11 of Ordinance GM/MS No. 888 of the Ministry of Health[⁶⁵].

3.8 Membrane selectivity

Table 10 shows the oil concentration in the permeate (C_P), in the feed suspension (C₀) and the rejection coefficient, using oily emulsion, for the pure PA6 membranes and their hybrids 1, 3, 5% w/w of ZnO.

Table 10
Oil concentration in the feed suspension (C₀), in the permeate (C_P) and the rejection coefficient for all membranes at a pressure of 1.0 bar using oily emulsion.

Membranes	C0 (mg.L-1)	CP (mg.L-1)	Rejection (%)
Pure PA66	50	1.88	96.24
	100	0.00	100.00
	200	0.00	100.00
PA66/1% ZnO	50	2.40	95.20
	100	0.00	100.00
	200	1.70	99.14
PA66/3% ZnO	50	0.00	100.00
	100	0.00	100.00
	200	0.00	100.00
PA66/5% ZnO	50	0.00	100.00
	100	0.00	100.00
	200	0.00	100.00

The results presented indicate high contaminant rejection efficiency for all membranes evaluated, regardless of the initial solution concentration. However, it is observed that the incorporation of ZnO promoted improvements in separation performance, mainly for lower contaminant concentrations. The pure PA66 membrane showed a yield of 96.24% for the initial concentration of 50 mg.L^-1, reaching 100% removal for concentrations of 100 and 200 mg.L^-1. The membrane containing 1% ZnO exhibited similar behavior, with yields of 95.20% for mg.L^-1 and 99.14% for 200 mg.L^-1. On the other hand, the membranes containing 3% and 5% ZnO achieved complete removal for all concentrations evaluated, resulting in zero permeate concentrations[⁶⁶].

The results demonstrated that all membranes exhibited high separation efficiency, with yields exceeding 95%, highlighting the ability of the PA66 matrix to promote contaminant retention. It is noteworthy that the incorporation of ZnO did not compromise removal efficiency, even while promoting significant changes in the structural properties and hydraulic performance of the membranes. The results demonstrated that all membranes exhibited high separation efficiency, with yields exceeding 95%, highlighting the ability of the PA66 matrix to promote contaminant retention. It is noteworthy that the incorporation of ZnO did not compromise removal efficiency, even while promoting significant changes in the structural properties and hydraulic performance of the membranes.

Furthermore, the hybrid membranes showed higher permeate fluxes than the pure PA66 membrane, indicating that the incorporation of ZnO enabled increased permeability without compromising selectivity[⁶⁷]. This result is particularly relevant for applications in membrane separation processes, since the simultaneous increase in flux and removal efficiency represents one of the main technological challenges in the field. Thus, the results obtained demonstrate the potential of PA66/ZnO membranes for water treatment, combining high productivity and separation performance.

For all membranes the rejection coefficient was higher for the feed oil concentrations of 100 and 200 mg.L^-1. Also due to the formation of oil polarization concentration close to the membranes surface, leading to clogging and obstructing thus the passage of oil, mainly at the highest concentration. The polarization concentration is inherent in any selective transport process and contributes to the formation of a layer of polarization which provides additional resistance to mass transfer of solvent through the membrane, causing a drop in the flux permeated[⁶⁸,⁶⁹].

In general, all the studied membranes in the separation process of oil/water emulsions at the concentrations (50, 100 and 200 mg.L^-1) achieved a significant reduction of oil in the permeate, meeting the requirements set by the resolution 430 of the National Environment Council environment, in Brazil, which states that the content of oils and greases in water must have a maximum concentration of 20 mg.L^-1 to be discarded in the environment[⁷⁰,⁷¹]. It is important to highlight that excellent results were obtained in separating the oil contained in the effluents using an industrial waste product of polyamide 66, directing this waste towards the production of membranes, demonstrating that it is a viable, low-cost and sustainable technology when compared to other conventional effluent treatment processes.

4. Conclusions

The results obtained demonstrated the viability of producing hybrid membranes from polyamide 66 synthetic fiber waste for application in the separation of oil-water emulsions, contributing to the development of sustainable alternatives for the treatment of oily wastewater. The incorporation of ZnO promoted significant changes in the morphological properties of the membranes, resulting in increased porosity, hydrophilicity, and permeate flux, without compromising separation efficiency. Among the formulations evaluated, the membrane containing 3% ZnO showed the best overall performance, combining high flux and permeability values ​​with excellent capacity for removing oil droplets present in the emulsions studied. Thus, the results highlight the potential of using polyamide 66 waste as a raw material for the production of high-performance microfiltration membranes, adding value to an industrial waste that would otherwise be discarded and contributing to the principles of the circular economy. Furthermore, the study demonstrates that the controlled incorporation of ZnO constitutes an effective strategy to improve the structural and functional properties of membranes, opening perspectives for future investigations aimed at optimizing these membranes and their application in industrial-scale oily wastewater treatment systems.

6. Acknowledgements

The authors acknowledge the Brazilian National Council for Scientific and Technological Development (CNPq), the Coordination for the Improvement of Higher Education Personnel (CAPES), the Bahia State Research Support Foundation (FAPESB) and he grant #2393/2023-d, Paraiba State Research Support Foundation (FAPESQ) for their financial support.

• #
  This paper has been partially presented at the 18th Brazilian Polymer Congress, held in Campos do Jordão, SP, 19 - 23/Oct/2025
• Data Availability:
  All data supporting the findings of this study are included in this article.
• How to cite:
  Souza, J. E. S., Ferreira, D. O. J., Lima, C. A. P., Medeiros, V. N., Araújo, E. M., & Medeiros, K. M. (2026). Hybrid membranes made from industrial waste for the separation of oil emulsions. Polímeros: Ciência e Tecnologia, 36(4), e20260042. https://doi.org/10.1590/0104-1428.20260008

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Edited by

• Editor-in-Chief:
  Sebastião V. Canevarolo

Data availability

All data supporting the findings of this study are included in this article.

Publication Dates

• Publication in this collection
  31 Aug 2026
• Date of issue
  2026

History

• Received
  21 Feb 2026
• Reviewed
  27 July 2026
• Accepted
  28 July 2026