Introduction

The world's continuously expanding population has significantly increased demand for pure drinking water in recent decades. However, as urbanization and industrialization continue, the availability of traditional sources of drinking water reduced (Li et al. 2018). Pressure-driven membranes are the most common, safe, and advanced technique for water treatment due to the limitation of phase change during separation, simple operation, low energy requirement, higher efficiency, and high filtration rate (Wang et al. 2019; Jamil et al. 2018). Thus desalination has therefore been significantly growing and spreading in recent decades to meet the world's water demands (Wu and Field 2019). The most common membrane filtering technologies used for water softening and desalination are reverse osmosis and nanofiltration. Nevertheless, these processes had some limitations as high tendency of surface clogging, high applied pressure, less permeability.

Cellulose acetate (CA) or polysulfone covered with a thick layer of polyamide thin film are the two most often utilized polymeric materials in the production of RO membranes. Surface functionalization improves the permeability, stability, and water quality of RO membranes, which lowers energy consumption and operating costs. The primary factors influencing water affinity and the mechanism of interactions with salts are the surface characteristics of RO membranes, such as hydrophilicity, water permeability, salt rejection, water flow, and surface roughness (Chen et al. 2018; Choi et al. 2016; Ismail et al. 2017).

In order to construct materials hierarchically from the bottom up and significantly improve performance as well as add new functionality to existing products, nanotechnology, which literally refers to the manipulation of materials and processes engineered to the molecular scale of 1–100 nm, is an emerging field. The changes and developments investigated in this area have pushed seawater desalination forward as a more practical and sustainable choice (Chung and Hu 1997; Liu et al. 2016).

Using nanoparticles in membrane preparation leads to high-performance membranes in terms of high productivity and high rejection which is considered the next generation of low-pressure membranes with low energy consumption (Fausey et al. 2019).

Using nanoparticles such as chitosan, clay-like materials, alumina, zeolite, and manganese-based sorbents enhances the membrane surface and hydrophilic properties.

Zirconium oxide (ZrO2) has nontoxicity, hydration, high porosity, and affordability, so it garnered greater attention as an additive for membrane fabrication. ZrO2 is considered more stable than metal oxides (Yu et al. 2019) and metal–organic frameworks (Mansor et al. 2021). He et al. (He et al. 2016) fabricated hollow fiber blend membranes using polysulfone (PSf) mixed with ZrO2 nanoparticles by phase inversion technique. Where the membranes exhibited hydrophilic nature, porosity, and rejection percentage for arsenic in the feed from 51.65 to 92.65 ppb.

Zheng et al. (Zheng et al. 2011) Zirconia and polyvinylidene fluoride (PVDF) were used to create mixed matrix membranes that had increased hydrophilicity, porosity, pure water permeability, and arsenic removal capabilities (Maximous et al. 2010). Blend polyethersulfone membrane with ZrO2-modified enhanced porosity, hydrophilicity, pure water permeability, and antifouling properties.

Incorporation of zirconium oxide nanoparticles with cellulose acetate provides membranes with good performance and antifouling properties. Accordingly, the addition of an adsorptive zirconium oxide nanoparticle to blend the membrane of polyvinyl alcohol/cellulose acetate can enhance the membrane performance in terms of water desalting.

In this work, RO membranes were prepared using the ZrO2 along with PVA/CA by phase inversion technique. The blending ratio of polymers was investigated. The membranes were characterized by FTIR analysis, porosity, contact angle measurement, pure water permeability, and antifouling studies.

Materials and methods

Materials and chemicals

Cellulose acetate (Mwt = 100,000 gmol−1) and polyvinyl alcohol (Mwt = 90,000 gmol−1) were obtained from Sigma Aldrich. Ethanol and N-methylpyridine were purchased from Acros Organics (USA). 1,2,3,4 butane tetracarboxylic acid (BTCA) was gotten from Merck Germany (purity > 99%). ZrOCl2.8H2O, KOH, sodium hydroxide, and bovine serum albumin were purchased from Sigma Aldrich.

Synthesis of ZrO2 nanoparticles

0.1 M of ZrOCl2.8H2O was dissolved in 100 ml of distilled water under stirring. Then, 0.2 M of KOH was added to the prepared solution. After that the solution was put in a stainless steel Teflon lined sterilized capacity of 100 ml and heated in the oven at 180 °C for 16 h. The resulting precipitates were washed with distilled water and ethanol. The final product was dried for 3 h under a vacuum at 80 °C.

Polymeric membranes fabrication

The membranes were fabricated by immersion phase inversion method. ZrO2 NPs were first dispersed in NMP as the main solvent and stirred for 1 h at 50 kHz. Cellulose acetate was added in a percentage of 24 wt% with 0.4 wt% of PVA on the NPs solution as shown in Table 1. The mixture was stirred for 12 h. The solutions were cast on nonwoven polyester fabric support with a thickness of 200 μm then instantly dipped into a distilled water coagulating bath to form a membrane.

Table 1 Polymeric composition of the prepared membranes
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Characterization techniques for ZrO2NPs

Philips powder diffractometer, X-ray diffraction (XRD) was used to characterize NPs. TESCAN VEGA3 Czech Republic was used for scanning electron microscopy (SEM) analysis with broker EDS.

The nanostructures and surface charge of ZrO2 were determined by high-resolution transmission electron microscopy (HRTEM; JOEL) and INTENSITY-Weighted GAUSSIAN DISTRIBUTION Analysis (Solid Particle) using Particle Sizing Systems, Inc. Santa Barbara, Calif., USA.

Characterization methods for the prepared membranes

Mechanical Testing (Stress at break (σ), and elongation at break (ε)) was tested using an H5KS universal tensile testing machine, Scanning electron microscope (QUANTA FEG250) was used to determine the membrane morphology.

The membrane porosity was determined using the mass loss of the wet membrane after drying. The wet membrane sample weight was cut in a species area of 1cmx 1 cm, then soaked in water for 24 h. After that, the sample weight was measured and then dried in the oven for 24 h at 80 °C. The porosity is calculated according to Eq. 1, where the porosity was ε:

$$\upvarepsilon =\frac{{M}_{w- {M}_{d}}}{ALP}$$
(1)

where Mw (g) is the wet membrane weight, Md (g) is the dry membrane weight, and A, L, ρ are the wet membrane effective area (cm2). The experiments were repeated for five times to minimize experimental error.

Surface wettability assessment

Contact angle measurements were used to ascertain the samples' wettability. Data Physics Instrument OCA 15E sessile drop technique was used to determine the Contact angle. For all trials, the volume of the droplets was adjusted to 10 µL, and a photo was taken right away after each drop was deposited. The calculated contact angle was the mean of 5 measurements.

Evaluation of membrane performance

The RO performance of the manufactured membranes was assessed in a cross-flow filtration system with an effective filtration area of 192 cm2. A 2000 ppm NaCl aqueous solution was used for all RO penetration tests, which were carried out at 15 bar.

RO membranes need to be pre-pressurized for two hours to obtain a stable condition before testing.

The water flux (Jw) is calculated by Eq. (2)(Wu and Field 2019):

$$\text{Flux}=\frac{Q}{AT}$$
(2)

Q, A, and are permeate volume (L), filtration membrane area (m2), and permeation time (h), respectively.

The salt rejection is calculated by Eq. (3)

$$\text{Removl} \left(\%\right)=\left(1-\left(\frac{{C}_{P}}{{C}_{F}}\right)\right)100$$
(3)

where CF and CP are the feed and permeate concentrations which were measured using a conductivity meter (4510 conductivity meter -JEN WAY).

The fouling and durability properties of the prepared membranes were studied using 1000 mg L−1 of BSA under 15 bars for 3 cycles. The process was carried out for 6 h and its permeation flux was calculated (Jp, L/m2h) (Karkooti et al. 2018). Then, the used membrane was washed and the second water flux (Jw2) was measured. The flux recovery ratio (FRR) and fouling resistance such as total fouling ratio (Rt), irreversible fouling ratio (Rir), and reversible fouling ratio (Rr) were calculated from the following equations (Roy Choudhury et al. 2018; Mansor et al. 2020):

$${R}_{r}\%=\left(\frac{{J}_{wc- {J}_{p}}}{{J}_{wI}}\right)100$$
(4)
$${R}_{irr}\%=\left(\frac{{J}_{wI- {J}_{Wc}}}{{J}_{wI}}\right)100$$
(5)
$$\text{FRR}=\left(\frac{{J}_{wc}}{{J}_{wI}}\right)100$$
(6)

Agilent Cary 100 UV/Vis spectrophotometer was used to measure the HA concentration. The long-term experiment was carried out using the optimum RO membrane using 2000 mg L−1BSA aqueous solution as the feed solution.

Results and discussion

Characterization of the prepared nano-ZrO2

Figure 1 illustrates the X-ray diffraction (XRD) of the synthesized ZrO2 nanoparticles. It was observed that the diffraction peaks pattern indicates a good nature for crystals with tetragonal structure (Sagadevan et al. 2016). The diffraction peaks are in good agreement with those of the standard data (JCPDS 37–1484). The strongest diffraction peak at around 28.59, 31.8, 50 and 59.7°2Th corresponds to the (101), (002), (112) and (211) planes. Almost all the diffraction peaks are matched JCPDS card.

Fig. 1
figure 1

X-ray diffraction of zirconium oxide nanoparticles

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Figure 2 shows the morphologies of prepared ZrO2 nanoparticles by SEM. According to the nature of the particles, there were aggregating or overlapping of the particles. The SEM images indicate a homogeneous spherical shape with a small size and random distribution (Sagadevan et al. 2016). Figure 2b indicates the Energy Dispersive X-ray spectroscopy (EDAX) for ZrO2 which exhibits signals of Zr, O and C which was an indication of the successful preparation of ZrO2.

Fig. 2
figure 2

a. SEM image of nanocrystalline ZrO2 and b EDAx of the prepared ZrO2

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High-Resolution Transmission Electron Microscope (HR-TEM) was used to explain textural and nanostructure properties. The prepared ZrO2 appears as nanospheres that has size less than 20 nm in regular distribution way as shown in Fig. 3a, b. Also the size distribution was determined and based on this classification, the pore diameter of the ZrO2 can be classified as mesopores according to the IUPAC where the class distribution for mesopores (2 < d < 50 nm) as presented in Fig. 3c.

Fig. 3
figure 3

a, b TEM images of Spheres ZrO2 c particle size distribution of ZrO2

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Characterization of the prepared RO membranes

Morphology of the RO membranes

Figure 4 shows the membranes cross sections morphologies. As it can be seen, the blank CA/PVA membrane had clear macrovoid morphology, which might be due to the low exchange rate of NMP and water. On the other hand, an obvious asymmetric structure can be seen in the prepared membranes with different ZrO2 nanoparticles concentrations (0.7 wt% and 0.5 wt%). This asymmetric morphology is characterized by the presence of a dense top active layer and sublayer containing a compacted structure over the nonwoven polyester fabric support for an embedding ratio of 0.5 wt%. ZrO2.That means using 0.5 wt%. ZrO2 in casting solutions form a dense top layer for the prepared membrane. However increasing the percentage of ZrO2 to.

Fig. 4
figure 4

a SEM images for the prepared membrane without ZrO2 addition b SEM images for the prepared RO membrane with 0.5 wt% of ZrO2 and c SEM images for the prepared RO membrane with 0.7 wt% of ZrO2

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0.7 wt % exhibits the formation of a big finger-like macro void structure compared with the smallest holes El-Manharawy and Hafez, 2011 with CA/PVA membrane loaded with 0.5 wt%. ZrO2. Due to interface instabilities that cause finger-like voids to emerge, viscous fingering in the presence of crosslinkers appears. The features of the membrane, such as water flux, are significantly influenced by the relative thickness of finger-like and sponge-like regions. (Elkony et al. 2020). The presence of more macro-voids and finger-like structures supports the idea that ZrO2 may function as a pore former.

After switching the NMP and water inside the membrane and solidifying the polymer with the rich phase, which produced finger-like voids, the polymer's lean phase can be viewed as the seed of aggregation and developed around the NPs, forming macro finger-like pores (Liu et al. 2016).

By adding a high ratio of NPs to the casting solution, the liquid–liquid phase separation occurs more quickly, the NPs aggregate, and the casting solution's viscosity rises, producing membranes with a more porous structure. (Arthanareeswaran et al. 2008). Weak sites in sub-layers experience greater stress relaxation than weak areas in skin layers, resulting in finger-like structures that are visibly visible at 0.7% weight concentration of NPs (Reuvers and Smolders 1987). The phase inversion process ideally produces pores with extended finger shapes, high surface energy, and solvent evaporation (Pusch 1977; Zhang et al. 2012).

Hydrophilicity of fabricated membranes

The water contact angle of the blank PVA/CA and ZrO2 nanocomposite membranes are shown in Fig. 5. The water contact angle values were 53, 59,62,69and 52° for the blank PVA/CA, and modified membranes with ZrO2 0.1 wt%, 0.3 wt%, 0.5 wt%, 0.7 wt%, respectively. The nanocomposite membranes of ZrO2 0.5 wt% exhibits increasing in the contact angle compared to the blank PVA/CA but without reaching the hydrophobic range.

Fig. 5
figure 5

Impact of ZrO2 weight on the water contact angle for the neat membrane and the modified

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It seems that the presence of ZrO2 in the polymeric matrix leads to a reduction in interface energy due to the migration of NPs toward the top membrane surface during membrane formation. So the surface hydrophilicity and water contact angle of the prepared membranes were changed according to NPs percentage (Kumar et al. 2013). Similar observations were also reported for the TiO2 nanoparticles in the PES mix matrix membrane (Ghaemi et al. 2018; Safarpour et al. 2015).

Mechanical properties

Figure 6 indicates the mechanical properties of the prepared membranes which have been enhanced due to the addition of ZrO2 to the membrane making it more crystalline, and the crystalline membrane is stronger than the amorphous ones (Isawi et al. 2016). Crystalline membrane has regular, distinct shape and repeating arrangement in contrast with random structure in amorphous one that missing uniformity.

Fig. 6
figure 6

Impact of ZrO2 weight on the Stress and extension of the PVA/CA membranes

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The increase in mechanical properties for the manufactured membranes by increasing the ratio of ZrO2 NPs from 0% to 0.7% wt was validated by the values of the stress at maximum load (N/m2). This could be attributed to good interaction between CA chains and ZrO2 NPs via electrostatic and hydrogen bonding (Martinez et al. 2013; Huang et al. 2015).

The fabricated Membrane with the embedding ratio of 0.5 wt% has the highest stress strength of 9.2 N/m2. The results indicate that significant increase in the tensile strength of ZrO2-PVA/CA in comparison with blank PVA/CA membrane due to the homogenous dispersion of nano-ZrO2 clusters and the interfacial interaction of zirconium with hydroxyl groups of the PVA.

Also, elongation was improved from 10.5 mm to 19 mm for the blank PVA/CA and 0.5 ZrO2 -PVA/CA membranes, respectively. However, there was a reduction in elongation at increasing ZrO2 to 0.7 wt% due to the stiffness of nano-ZrO2 (Singh et al. 2013).

However, the mechanical properties are still high with 0.7%wt. but the optimum embedding ratio from the other study was 0.5% wt. This resulted from the practical number of NPs filling and reacting with PES (Razmjou et al. 2011; characterization of PVA et al. 2012).

Impact of NPs loading amount on separation performance

The laboratory reverse osmosis system was used to test the performance of the modified PVA/CA membranes that had been produced. Through the evaluation of both salt rejection and water flow for a feed solution of 2,000 mg/L NaCl at the applied pressure of 15 bars for 5 h, the impact of embedding ZrO2 on the membrane performance was investigated. The effect of ZrO2 wt% on the salt rejection of the membranes is shown in Fig. 7. A With raising the concentration of ZrO2, salt rejection (%) and water flux increased at the lowest NPs loading amount (0. 1 wt%). This improvement in salt rejection was brought about by the deposition of ZrO2 nanoparticles on the CA/PVA membrane's surface, which may have plug pinholes and other defects, and improved salt rejection. This conclusion was consistent with previous research (Turgman-Cohen et al. 2013). The result demonstrates that 0.5% ZrO2 was the appropriate concentration, providing more salt rejection (97%) than the un-modified CA/PVA membrane owing to the presence of the dense layer that act as a selective layer for solutes from desalted water. Then, at the greatest loading of 0.7 wt%, the salt separation exhibits a drop that reaches 86%. These alterations that result are most likely attributable to changes in polymeric structure and potential voids between NPs and polymeric solution. Additionally, the variation in NaCl permeation and rejection performance is caused by changes in the NPs' degree of aggregation that causing the formation of a porous structure as provide with SEM images, the aggregation of ZrO2 NPs has affected negatively on the membrane selectivity.

Fig. 7
figure 7

Impact of ZrO2 NPs loading amount on separation performance

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With 0.1 wt% ZrO2 NPs, the permeate flux of the un-modified membrane increased from 20 to 27 LMH. The permeate flux steadily reduced to 19 LMH after increasing the loading amount to 0.3 wt%, and it reached 12.5 LMH after raising the loading amount to 0.5 wt% of NPs. The molecular sieve effects and the advantageous nano-gaps could be replaced by the interstitial on selected defects created at the interfaces. (Rahimpour et al. 2011). The inorganic/organic hybrid nature of the material was found to have good compatibility with polymers, according to the results (Batool et al. 2021).

Desalination performance

Figure 8 illustrates the water permeability and salt rejection of the fabricated membranes, which were tested under an applied pressure of ~ 15 bars using a feed mixture of 2000 mg l−1 NaCl and 1000 mg l−1 BSA. The results indicate that using 0.5 wt% of ZrO2 nano-sphere in the matrix of the polymeric membrane provides the highest membrane performance, where the separation percentage reaches to 99%. However, the membrane characterization indicates a thicker top layer that can exhibit more selectivity as a result, higher salt separation. Increasing ZrO2 percentage up to 0.7 wt% exhibits a reduction on membrane separation percentage. The high connections between metal oxide atoms, increasing percentage of them lead to high agglomeration and bad nanoparticles distributions, which leading to membrane defects that provides decline separation percentage and increasing in water permeability.

Fig. 8
figure 8

Performance of RO membrane in the presence of 1000 mg L−1 BSA as the feed solution

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Fouling behavior for the prepared membranes

Figure 9 illustrates fouling properties of the membrane, where 1000 mg L−1 of BSA solution was used as a foulant feeding for 72 h under 15 bar. The results indicate that the reversible and irreversible fouling ratio (Rr) for the membrane of 0.5% ZrO2 was lower than un-modified PVA/CA. The flux recovery ratio reach to 94.7% for 0.5% ZrO2, Rir ratio was 5.3% and Rr ratio was 16.5%. The enhancement of the antifouling properties of membrane was related to easily washing of the membrane surface and capable of mitigation of the deposition of BSA on the surface of the membrane.

Fig. 9
figure 9

a Fouling resistance for the RO membrane in terms of Rir/RT and Rr/RT b: Flux recovery ratio, Reversible and irreversible fouling for the prepared RO membranes

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Further, the antifouling behavior of ZrO2-PVA/CA has been depended on zeta potential value. The synthesized ZrO2 has a negative charge − 19.23 mV (Ordóñez et al. 2020).As well, The BSA molecules have negatively charged (− 30 mV) (nanoparticles and effect of surfactant on dispersion and stability 2020). Both BSA molecules and membrane surface loaded with ZrO2 were negatively charged, and thereby the rejected protein molecules was very easy to wash so the membrane surface has highly antifouling properties.

Long-time performance and durability properties

Long term experiment was carried out on the optimum membrane as shown in Fig. 10 for 72 h at 15 bar using 1000 mg L−1 HA as feed solution. The permeate flux was fixed during the filtration time and with a slight increase in separation percentage during process time. The recorded flux remained closely constant during time (flux 12.5 LMH), then slightly reduced due to cake formation over the membrane surface. The results indicate that the prepared membrane exhibits stable performance during long-term operation. Durability and reusability of this membrane was studied for 3cycles as illustrated in Fig. 10b. In the first cycle, the water flux of the membrane was 16 LMH and after using HA solution the flux reached to 12.5 LMH. After cleaning by DI-water solution for 30 min, the permeate flux of the membrane were, respectively, recovered to 15.8 LMH indicating that the membrane exhibits higher flux recovery. In the second cycle and third cycle after operation process and cleaning, the membrane still provides slightly reduction of water flux with average flux of 12.5 LMH. So, the modified membrane with 0.5% ZrO2 provides high flux recovery leading to good durability property and good reusability for the membrane.

Fig. 10
figure 10

a Long-time performance of RO membrane (0.5 wt% ZrO2) using 1000 mg L − 1 HA as the feed solution and b Performance of RO membrane (0.5 wt% ZrO2) in terms of flux before washing and after using 1000 mg L − 1 HA as the feed solution

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Chlorine resistance of the RO membrane

The chlorine resistance test was carried out on the ZrO2 CA/PVA at three pH conditions (4.0, 7.0 and 10.0). The results indicate that there were no any observations for degradation of the ZrO2 CA/ PVA membrane at pH 4.0, 7.0 and 10. However, the membrane was stable after NaOCl solution exposure of 25–100 ppm for 20 h in comparison with other literatures, which indicated failure of membrane due to degradation within a short time leading to rapidly reduction in salt rejection, increasing water flux and altered surface morphology. After testing the effect of chlorine on membrane performance, the effect of salt separation is tested as shown in Fig. 11. The results indicate that the salt separation percentage was stable even the chlorine concentration increase, and at different pH. So, the ZrO2 nanoparticles on the membrane surface offers chlorine resistant and chemically stable membrane property, which protects the membrane from chlorine and fouling attack. The effect of different pH on ZrO2 degradation, where (Fig. 11b) demonstrates that ZrO2 modified membrane provides low degradation rate during process time, which means incorporation of ZrO2 on membrane matrix provides positive effect on the stability and degradation behavior. The compatibility between CA and ZrO2 nanoparticles was excellent due to increase in the rigidity of polymer chains between CA and ZrO2 nanoparticles (Wang et al. 2022).

Fig. 11
figure 11

a Performances of Modified RO membranes after chlorine exposure in terms of flux and salt rejection. b The stability studies modified RO membranes in terms of weight loss

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Removal of hardness

These membranes' separating capabilities have been utilized to treat hard water and create soft water, which is used in many industrial applications (Yu et al. 2016). Alkalinity dominates low-chloride water types, but sulfate takes up an excessive amount of space in high-chloride types. When the salinity of the water is higher, the sodium ion and chloride are largely proportionate, but when the salinity is lower, it appears that calcium and magnesium can be tolerated more readily than sodium and chloride (Nanda et al. 2008).

The efficiency of the modified membranes was determined by using real brackish water. The physico-chemical characteristics of feed real water and permeate purified water was determined as illustrated in Table 2. The level of the overall cations responsible on the water salinity and hardness was very high in the effluent. However the level of these cations in permeate after the filtration process using the modified membrane were very low. The permeate water was very soft less than 0.3 mgl−1. Also, Iron and Manganese ions were completely removed. The level of sulfate and bicarbonate of real water in permeate after membrane filtration was very low. The tested modified membrane completely removed hardness and lowered the concentration of and sulfate and conductivity. This is owed to the fact that this RO membrane is capable of removing both monovalent and multivalent ions from brackish water and this is the main feature of RO membrane.

Table 2 Composition of brackish water
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The results of this work were compared to previously published research articles that deal with the enhancement of RO membranes based on cellulose acetate with different nanoparticles. As showed in Table 3, the loading of ZrO2 NPs with the neat membrane had a satisfying performance either in salt rejection or permeability compared to the used zeolite, silica, graphitic carbon nitride, titanium oxide and zirconium-based MOF nanoparticles.

Table 3 Performance of some RO membranes modified with different nanoparticles
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Conclusion

In this work, mixed matrix flat sheet membranes was prepared using cellulose acetate and poly vinyl alcohol (PVA) with different additive percentage (0.1, 0.3, 0.5 and 0.7 wt%) of zirconium oxide (ZrO2) nanoparticle. The synthesized ZrO2 nanoparticle was characterized by, SEM, TEM and XRD. The fabricated CA/PVA/ ZrO2 membranes were characterized by SEM and mechanical properties. The results indicate that the prepared membranes by 0.5 wt% of ZrO2 in PVA/CA provide the best membrane performance. While there was a reduction in NaCl removal at using the excessive dosages of ZrO2 to 0.7 wt%. The antifouling behavior was tested on the prepared membranes using bovine serum albumin (BSA), which exhibits good antifouling properties, where The flux recovery ratio reach to 94.7% for 0.5% ZrO2. Long term experiment was carried out on prepared membranes by 0.5 wt% of ZrO2 in PVA/CA matrix membrane and it provides high flux recovery leading to good durability property and good reusability for the membrane. The chlorine resistance test was carried out on the optimum membrane at different pH and the results indicate that it offers chlorine resistant and chemically stable property, which protects the membrane from chlorine and fouling attack. The membranes were tested in hardness removal and exhibit good performance.