Introduction

Recently, there is a worldwide gap between the available clean fresh-waters and the population’s daily water demands (Elkady et al. 2016). Accordingly, municipal water with an acceptable quality has now become a rare commodity. Therefore, the current water stress situation has lightened the role of desalination processes in providing an alternative pure water source to meet the required global water needs. Thus, desalination has been widely expanding and spreading in the last decades to globally cover water needs (Angelakis et al. 2021; Shatat and Riffat 2014; Pistocchi et al. 2020; Elkady and Hassan 2021). Essentially, desalination is the process by which clean water is extracted from brackish or seawater (Huyen et al. 2021). The desalination process could be accomplished either by thermally or membrane driven techniques. Thermal desalination comprises multi-stage flash distillation (MSF), thermal vapor compression (TVC) and multi-effect distillation (MED). On the other side, membrane desalination includes membrane distillation (MD), pervaporation (PV), electrodialysis (ED), forward osmosis (FO) and reverse osmosis (RO) (Ba et al. 2019; Lotfy et al. 2022; Nthunya et al. 2022). For many years, RO has been considered as the prevalent desalination technique. It has been widely spread with around 69% of the total installed desalting plants because of its low energy requirement, lower footprints and relatively low cost compared to other desalination processes (Ritt et al. 2022; Feria-Díaz et al. 2021; Lim et al. 2021).

Accordingly, many researchers were devoted to investigating the enhancement of RO membranes using various materials and techniques. Biomimetic as well as silver nanoparticles (NPs) were impregnated into polyamide (PA) matrix to improve its performance (Huo et al. 2023). Additionally, covalent organic frameworks (COFs) were used to help in fabricating a thin film nanocomposite (TFN) with excellent properties (Qi et al. 2023). TiO2-NPs and polyethylene glycol were added to enhance polyimide based thin film composite (TFC) membrane (Hosseini et al. 2020). Multi-layer graphene oxide was used to modify conventional PA membrane permeability (Su et al. 2023). Another concept focused on studying the effect of solvent mixture and curing temperature on the membrane efficiency (Mokarinezhad et al. 2023). Amongst the most studied polymeric materials, CA is one of the most investigated and applied polymers for membrane fabrication. CA-based membranes are famous for their hydrophilic nature, biocompatibility, good chemical properties, chlorine tolerance, availability, ease of fabrication and low cost (Vatanpour et al. 2022; Lakra et al. 2021; Li et al. 2022a). Nonetheless, it still has some drawbacks that need to be improved to compete other RO membranes, for instance, mechanical stability, salt rejection and pure water permeability (Ghaseminezhad et al. 2019).

In accordance, many membrane enhancement approaches were deliberated to overcome CA membrane defects for different applications (Yang et al. 2019; De Guzman et al. 2021). Those approaches including but not limited to polymeric blending (Jamshaid et al. 2020; El-Gendi et al. 2021; Koriem et al. 2022b), surface grafting (Gebru and Das 2018; Xiang et al. 2019) and nanoparticles impregnation into polymeric materials forming mixed matrix membranes (MMMs) (Wei et al. 2020). Various investigations were considered with impregnating nano-sized materials within CA membranes such as candle soot (Abdelhamid and Khalil 2019), titanium and aluminum oxides (Baniasadi et al. 2021a; Shafiq et al. 2018), iron oxide (Evangeline et al. 2019), graphene oxide (GO) (Xu and Na 2020), silver (Xu et al. 2016), activated carbon (Koriem et al. 2022a) and Metal organic frameworks (MOFs) (Diab et al. 2021). MOFs are a class of hybrid organic-inorganic nano scaled materials with an extreme large surface area. Additionally, as a result to the presence of organic ligands, they have a much better affinity towards polymeric matrices comparing to the inorganic fillers (Zirehpour et al. 2016). Among the known MOFs, a zirconium-based MOF (UiO-66) is prominent for its water affinity, extraordinary chemical and thermal stability. Furthermore, its aperture size was estimated to be 6 Angstrom, making it a competitive candidate for sieving mono and divalent ions from salty waters (Gu et al. 2020; Kadhom et al. 2017; Ma et al. 2017).

The main purpose of the current study is to provide an efficient CA-RO membrane with an enhanced flux using a simple method. The promising results of our previous work (Koriem et al. 2022b) motivated us to further study the effect of MOF incorporation directly into CA membrane. Therefore, zirconium-based nanoparticles were impregnated at different concentrations into CA matrix to study their effect on improving the membrane water flux. Moreover, the performance of the optimum CAU-X membrane was investigated at various operating parameters. To the best of our knowledge, there is no other research concerned with impregnating UiO-66 nanoparticles (UiO-66-NPs) into CA membrane to be used for RO desalination. Consequently, in the current study, the hydrophilic porous zirconium-based UiO-66-NPs were selected to be synthesized and embedded into the CA polymeric cast membrane. The obtained NPs were characterized by various techniques to identify its functional and morphological structure, which in return confirms the successful preparation of the targeted MOF. The fabricated NPs were then incorporated into CA matrices at different concentrations and cast at certain conditions. The effect of UiO-66-NPs impregnation on the characteristics of the neat CA membrane was investigated via several characterization methods. After that, the desalination performance of all the fabricated CA-based membranes was tested with the aid of a lab-scale RO system. Lastly, the effect of operation conditions on the optimum membrane performance was further investigated by changing the initial feed water concentration, operating pressure, and temperature.

Experimental work

Materials

All the purchased chemicals, except of the commercial grade sodium chloride (NaCl, 99%) which was used for the preparation of saline feedwater solutions, were of a high analytical grade quality and used as they were purchased with no other modification or purification. For the synthesis of CA membranes, CA powder with 100,000 g/mol average molecular weight was obtained from Acros Organics. Acetone (\(\ge\)99%) and 1,4-dioxane (\(\ge\)99%) were supplied by Fisher, methanol (\(\ge\)99%) was provided by Sigma Aldrich and glacial acetic acid was purchased from Merck. To synthesize UiO-66-NPs, zirconium (IV) chloride (ZrCl4) (\(\ge\) 99.5%) and dimethylformamide (DMF) (\(\ge\)99%) were delivered by Merck and benzene-1,4-dicarboxylic acid (BDC) (>98%) was supplied by Acros Organics.

Synthesis of UiO-66 nanoparticles

UiO-66-NPs were synthetized via solvothermal method according to the schematic diagram illustrated in Fig. 1. According to literature, firstly, 0.15 g of ZrCl4 was stirred at room temperature and dissolved in 45 ml of DMF. Then, equimolar of BDC (0.11 g) was dissolved at 45 ml of DMF. After that, the two solutions were mixed, and 0.13 ml of distilled water was added as a modulator (Trinh et al. 2017; Zamidi Ahmad et al. 2018). The final solution was transferred to a Teflon cup, where it was well sealed in a stainless-steel autoclave jar. The jar was finally moved to a preheated furnace of 120 °C and left for 24 h (Valenzano et al. 2011). The resultant NPs were left to cool at room temperature, then they were separated using centrifugation at 6000 rpm for 60 min. Finally, the obtained NPs were washed three times using DMF, followed by another 3 times using methanol. The washed UiO-66-NPs were dried in a vacuum oven over night.

Fig. 1
figure 1

Schematic diagram of UiO-66-NPs synthesis via solvothermal method

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Fabrication of neat CA and CA/UiO-66 hybrid membranes

Flat sheet CA-based membranes either the blank or MMMs were fabricated via phase inversion technique and the detailed composition of each membrane is clearly illustrated in Table 1. As previously reported in literature, a dope solution of 14 wt% CA was prepared by dissolving certain amount of CA polymeric powder in a quaternary solvent mixture composed of (dioxane, acetone, acetic acid and methanol) (Morsy et al. 2016; Elkony et al. 2020a; Mohammed Ali et al. 2020). Each component of the mixture plays a vital role in the membrane synthesizing process. Dioxane and acetone were selected for their high CA dissolving ability (Aburideh et al. 2021), while acetic acid and methanol were added as softener and non-solvent (Duarte et al. 2006), respectively. For the synthesis of MMMs, various concentrations of 0.02, 0.05 and 0.1 wt% from UiO-66-NPs, relative to CA weight, were firstly well dispersed in the same quaternary solvent mixture. Then, same mass of CA powder was added to the UiO-66/solvent mixture. The neat CA and hybrid CA/UiO-66 dope solutions were continuously stirred at 20 °C for 12 h, then they were left for specific time in a cold temperature to discard defective micro air bubbles. Each dope solution was then cast at room temperature on a previously cleaned and dried glass plate with the help of an automatic film applicator, which was adjusted at 250 μm thickness and 50 mm/sec speed. Thenceforth, the cast film was left for 30 s to partially evaporate at room temperature before being immersed into a (0–4 °C) distilled water (DW) coagulation bath. The obtained flat sheet membranes were then moved and further washed in another cold DW bath to remove any residual solvents. As a final step, the fabricated membranes were thermally annealed for 10 min at 80 °C and the annealed membranes were preserved under DW for 24 h before being further characterized and tested in RO lab scale unit. The synthesized membranes were named (CAU-X), where X refers to UiO-66-NPs concentration.

Table 1 The composition and phase inversion conditions of neat CA and CA/UiO-66 hybrid membranes
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Characterization techniques of UiO-66-NPs, neat CA membrane and CA/UiO-66 hybrid membranes

All the prepared samples were carefully dried in a drying oven prior to being characterized. The particle size of the obtained UiO-66-NPs was inspected by transmission electron microscope (TEM, JEOL JEM-2100 F). The Brunauer Emmet-Teller (BET) analyzer that is based on nitrogen adsorption/desorption was used to determine the fabricated nanoparticles surface area and pore volume. Samples were degassed under vacuum prior to characterization at 150 °C to remove any residuals or impurities. The functional groups of both MOF nanoparticles as well as the fabricated membranes were determined by Fourier transform infrared (FTIR, Bruker Vertex 60). The selected wave number was in the range of (400–4000 cm−1). Moreover, the crystalline nature of all the samples was investigated with the help of x-ray diffraction (XRD, Shimadzu XRD-6100) with a copper target (λ = 1.54 °A) which operating at 40 kV and 30 mA. A system consists of 1° diverging slit and 0.3 mm receiving slit was used. The diffraction peaks were acquired in a continuous scanning mode with a scanning range varying between 5° and 80° and a scanning speed of 12°/min. Furthermore, the thermal stability of the neat CA and MMMs was studied by thermal gravimetric analysis (TGA Q50). The morphology of NPs and membranes top, bottom and cross-section were investigated by scanning electron microscope (SEM, JEOL JSM-6010 LV). Membrane samples were cut under liquid nitrogen to obtain an obvious structure. All samples were coated with platinum prior to SEM characterization and the accelerating voltage used was 10 kV and 15 kV for UiO-66-NPs and membranes, respectively. To investigate the hydrophilic nature of pristine CA and CA/UiO-66 hybrid membranes, the contact angle of the synthesized membranes was measured with the help of (DSA 100, KRÜSS). For each membrane, various random locations were selected for the contact angle measuring to minimize any possible error. The mechanical strength of the membranes was evaluated by the tensile strength (4468, Instron). The membranes were tested under a 3 mm/min elongation rate.

RO performance evaluation of the neat CA and hybrid CA/UiO-66 membranes

The permeate water flux (PWF) as well as salt rejection (SR) ability were investigated in a bench scale RO crossflow setup (CF042, Sterlitech, USA). Each tested membrane had a rectangular surface area of 42 cm2, which is similar to the active area of the testing cell. A saline solution of 5000 ppm NaCl was fed at room temperature to the cell by a high-pressure pump (HPP) (Hydra-cell) and the pressure was gradually increased until certain pressure was obtained. Later, the effect of initial feed water concentration and the applied pressure on the optimum membrane performance was observed. The solute concentration was detected in the permeate water by a TDS measuring device (Adwa, AD32).

The membrane performance was evaluated based on the rejected salts (R%) and the flux of the produced water (J) as illustrated in Eqs. (1 & 2) (Al-Hobaib et al. 2015; Anjum et al. 2020):

$$R\left(\%\right)=1-\frac{{C}_{p}}{{C}_{i}}$$
(1)

Where, R (%) is the salt rejection of each membrane, Cp and Ci are the concentrations (ppm) of permeated and feed waters, respectively.

$$J= \frac{Q}{A*t}$$
(2)

Where, J (L/m2 h) is the permeated flux, Q (L) is the permeated water volume, A (m2) is the membrane active area, and t (h) is the time.

Results and discussion

Characterization of the synthesized UiO-66 NPs

The characteristics of the fabricated UiO-66 NPs were indicated via various methods. FTIR was used to confirm the successful preparation of UiO-66-NPs by determining the functional groups of the prepared NPs. As shown in Fig. 2a, the peak appeared at 1655 cm−1 is assigned to C = O of terephthalic acid. Moreover, the band observed at 1578 cm−1 give an indication to the carboxylate group of the organic ligand (Zhang et al. 2019; Elhussein et al. 2020). The weak peak at 1502 cm−1 refers to the presence of C = C bond of the benzene ring in BDC (Nasrabadi et al. 2019). In addition, the peaks detected at 743 and 659 cm−1 are attributed to O-H and C-H bonds of the organic BDC ligand. Whereas the shown band at 548 cm−1 is assigned to the (Zr-OC) asymmetric stretch (Koriem et al. 2021).

XRD was investigated in order to understand the crystalline nature of the fabricated NPs and the main diffraction peaks of UiO-66-NPs are illustrated in Fig. 2b. As it can be seen, at 2\({\uptheta }\) the appearance of peaks at 7.24°, 8.42° and 25.62° gives an indication to the successful fabrication of UiO-66. The results also indicated that the nanoparticles of UiO-66 are highly crystalline in nature. The studied crystallographic structure of synthetized UiO-66 was in accordance with the previously mentioned in literature (Molavi et al. 2018; Chen et al. 2018).

SEM images were observed to understand the morphology and size of the prepared UiO-66-NPs. As it can be seen in Fig. 2c, UiO-66 has a very small particle size of around 50 nm. However, large particles are presented, and this might be due to agglomerations caused by the small size.

Thus, TEM images were studied to determine the average particle size of the synthesized UiO-66-NPs. As it is illustrated in Fig. 2d, the obtained particles can be considered homogenous in size and shape. Additionally, the measured average size of UiO-66-NPs based on the TEM images was found to be in the range of 35 nm. Clearly, the prepared particles are nano-sized materials, which is advantageous for many applications (Li and Bie 2017).

BET results showed that the fabricated UiO-66-NPs has a high value of surface area of 1013.4 m2/g and a pore volume of 0.584 cm3/g. Thus, this porous structure is expected to provide water paths for a better permeability. The obtained results agree to previously published results (Su et al. 2017; Mesgarian et al. 2020).

Fig. 2
figure 2

Characterization of the synthesized UiO-66-NPs a FTIR b XRD c SEM and d TEM

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Characterization of the fabricated neat CA and hybrid CA/UiO-66 membranes

FTIR Analysis

The functional groups of the fabricated blank and hybrid membranes were examined by FTIR. As it can be noticed in Fig. 3, for the neat CAU-0 membrane, the wide band between 3670 and 3160 cm−1 represents the cellulosic characteristic OH peak, whilst the recorded value of 2970 cm−1 could be attributed to C-H stretching vibration, while the peak at 1724 cm−1 refers to the C = O stretching vibration.

Fig. 3
figure 3

FTIR spectra of the neat CAU-0 and hybrid CAU-0.02, CAU-0.05 and CAU-0.1membranes

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Additionally, the value at 1332 cm−1 gives an indication to the presence of C–H bending and the bands at 1037 and 1128 cm−1 represent respectively the C–O symmetric and asymmetric vibration (Kumar et al. 2019; Namjoufar et al. 2021; Li et al. 2022b). On the other hand, the addition of UiO-66 NPs did not have a noticeable change on CA functional groups. However, the intensity of OH peak was decreased after UiO-66 impregnation, this might refer that the added NPs have formed a hydrogen bond with the CA polymer (Tanvidkar et al. 2022).

XRD Analysis

XRD patterns of the pristine CAU-0 and mixed CAU-0.02, CAU-0.05 and CAU-0.1 membranes were investigated to verify the impregnation of UiO-66-NPs into CA matrix. Figure 4 illustrates the amorphous nature of the blank CAU-0 membrane. However, with the incorporation of UiO-66 NPs, the intensity of characteristic diffraction peaks of Zr-MOF starts to slightly increase within the composite membranes at around 2θ = 7.2° and 8.4°. Additionally, with the escalating concentration of MOF, the top of the characteristic CA wide peak between 15° and 30° slightly shifted towards the distinctive peak of UiO-66 at 25.8° (Al-Shaeli et al. 2021; Liang et al. 2021). The previous findings prove the successful impregnation of UiO-66 NPs with CA polymeric matrix.

Fig. 4
figure 4

XRD patterns of the neat CAU-0 and hybrid CAU-0.02, CAU-0.05 and CAU-0.1 membranes

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SEM analysis

To inspect the influence of Zr-based MOF on the pristine CA membrane morphological structure, SEM images of each membrane surface, bottom and cross section were examined. As clearly demonstrated in Fig. 5a the surface of the neat CAU-0 membrane consists of a smooth non-porous structure (Peixoto et al. 2020). Nonetheless, this surface started to corrugate to some extent with the addition of UiO-66-NPs. Moreover, small white particles became visible on the surface of CA/UiO-66 hybrid membranes and this might be a confirmation of the presence of UiO-66-NPs in the structure of the cast CAU-X membranes. However, with increasing the concentration of NPs some large agglomeration was found on the surface and this might have a negative effect on the membrane performance.

Fig. 5
figure 5

SEM images of a top b bottom and c cross-section of the neat CAU-0 and hybrid CAU-0.02, CAU-0.05 and CAU-0.1 membranes

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The bottom surface of the neat CAU-0 and hybrid CA/UiO-66 membranes is shown in Fig. 5b. As displayed, no pores were present on the bottom surface of the neat CAU-0 membrane. Yet, some small holes were detected after the impregnation of UiO-66-NPs as shown in CAU-0.05 bottom surface. Same observation for another filler was mentioned elsewhere (Ali et al. 2021b).

The membranes cross-sectional morphologies were clearly identified in Fig. 5c. As it can be seen, the blank CAU-0 membrane had nearly a free macro-void morphology, which might be due to the low exchange rate of solvent and non-solvent (El-Ghaffar et al. 2020). On the other side, an obvious asymmetric structure can be seen in the hybrid CA/UiO-66 membranes. This asymmetric morphology is characterized by the presence of a thin top active layer on a porous-thick layer and the pores of this supporting layer could have a tear or finger like shape (Ali et al. 2021b;Elkony et al. 2020b;Ebrahim et al. 2016). It is clearly illustrated that the addition of UiO-66-NPs influenced the morphology of blank CAU-0 membrane and thus it is expected to provide an alternative water way and in return this will affect the membrane permeability. In addition, from cross-section SEM images, the presence of UiO-66 NPs has developed the shape of the existed finger-like pores. Compared to CAU-0 membrane the impregnation of UiO-66-NPs have enhanced the rate solvent/non-solvent exchange causing the formation of a porous structure (Norahim et al. 2019; Baniasadi et al. 2021b).

Contact angle and membrane porosity

Table 2 demonstrates the effect of UiO-66-NPs impregnation on the hydrophilic nature and porosity of the neat CAU-0 membrane. As demonstrated, the obtained contact angle of the pristine CAU-0 membrane was found to be 58.1°, which is in accordance with the recorded value in literature (Lee 2020; Ali et al. 2021a; Qi et al. 2022). This value has slightly declined for the fabricated hybrid CA/UiO-66 membranes. This could be attributed to the hydrophilic nature of UiO-66 NPs that facilitate the passage of water molecules through the membrane. The measured contact angle of CAU-0.02 and CAU-0.05 was found to be 54.6° and 53.2°, respectively. However, the value was found to be 56.3° for CAU-0.1 membrane. The slight increase in contact angle of the later membrane could be due to the nanoparticles agglomeration, which in return has slightly increased the roughness of the membrane surface (El-Ghaffar et al. 2020;Ali et al. 2021a, 2021b). It could be expected that with enhancing the hydrophilic nature of the CA/UiO-66 MMMs the permeate water flux would also enhance eventually. In addition, the membrane porosity increased gradually from 71.4% for the neat CA membrane to 79.3% for CAU-0.1 hybrid membrane. The increased porosity can be interpreted as the presence of a super hydrophilic nanofiller in the dope polymeric solution increases the exchange rate between solvent and non-solvent phases in the coagulation bath. This rapid exchange could influence the formation of a longer finger like pores (Ma et al. 2020; Emadzadeh et al. 2014).

Table 2 The measured values of contact angle and porosity of CA and CA/UiO-66 hybrid membranes
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Mechanical strength

In order to study the effect of UiO-66 on the mechanical stability of the blank CAU-0 membrane, the tensile strength properties were investigated in Fig. 6. As it can be seen, the addition of UiO-66-NPs has positive impact onto the mechanical strength of CAU-0 membrane. The tensile strength was elevated from 6.05 MPa for CAU-0 up to 10.7 for CAU-0.05 membrane, then it decreased with further UiO-66-NPs addition to be 6.8 MPa for CAU-0.1 membrane. The filler/polymer interaction could be the reason of the increase in the mechanical property of the membrane. However, if the filler concentration has further increased this might cause aggregation of NPs, which in return forms stress points in the membrane structure (Asiri et al. 2022). In addition, as previously illustrated by SEM images, the presence of UiO-66-NPs has affected the porous structure of the hybrid membranes and larger pores were formed with higher MOF-NPs concentration. This also might be the reason behind the lower membrane mechanical stability of CAU-0.1. Same trend was previously reported (Gzara et al. 2016; Zahid et al. 2021).

Fig. 6
figure 6

Tensile strength of the neat CAU-0 and hybrid CAU-0.02, CAU-0.05 and CAU-0.1 membranes

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RO membrane performance

In the experimental work, the permeate water started to flow at 10 bars for the pristine membrane. However, the permeate water started to flow at lower pressure of 8 bars after the impregnation of UiO-66-NPs, same observation was previously reported for other filler (Ali et al. 2021b). The experiments were conducted at 10 bars for all membranes to provide a fair comparison at the same conditions. Figure 7 exhibits a comparison of the performance of blank CA and hybrid CA/UiO-66 membranes at the same initial concentration, operating temperature and operating pressure. As clearly illustrated, the blank CAU-0 membrane has the highest salt rejection and the lowest water flux of 99.8% and 1.14 L/m2h,respectively. These results are in consistent with the previously explained dense structure in SEM images of CAU-0 membrane.

Compared to the blank CAU-0 membrane, the hybrid CAU-X membranes were found to have lower salt rejection and better permeability. This might be caused by the presence of nonselective voids formed between UiO-66-NPs and CA chains (Ali et al. 2021a). As it can be seen, CAU-0.02 membrane’s salt rejection has slightly diminished to 97.6%, while the PWF has almost doubled to be 2.8 L/m2h. Additionally, with further increase in UiO-66 concentration in the membrane matrix, the salt rejection has further decreased by 5.7%, while the PWF has increased by almost 20% for the CAU-0.05 membrane in comparison to CAU-0.02 membrane. Generally, the impregnation of the hydrophilic UiO-66-NPs has enhanced the hydrophilic nature of the membranes, as previously explained by contact angle, and in return the membrane PWF due to the interaction between water molecules and the surface of membrane. Furthermore, the addition of the hydrophilic nanofiller has improved the membrane porosity and pores distribution, as aforementioned in Fig. 5, and consequently the hybrid membrane permeability compared to the neat CAU-0 membrane. Additionally, UiO-66-NPs have increased the membrane pores to some extent causing some salts to escape as well as water molecules to pass. Same observations were previously reported with other nanofillers (Ali et al. 2021a, 2021b;Nyamiati et al. 2021;Shi et al. 2017). Surprisingly, with further addition of UiO-66 the salt rejection was raised from 92% for CAU-0.05 to 93% for CAU-0.1 membrane, while the PWF was slightly reduced from 3.4 L/m2h for CAU-0.05 to 2.8 L/m2h for CAU-0.1 membrane. This might be a result to UiO-66-NPs agglomeration and blocking some membrane pores (Ghaseminezhad et al. 2019). Accordingly, the fabricated CAU-0.05 composite membrane was considered as an optimum membrane for RO operation.

Fig. 7
figure 7

RO performance of the neat CAU-0 and hybrid CAU-0.02, CAU-0.05 and CAU-0.1 membranes operating at feedwater solution of 5000 ppm NaCl, temperature of 25 °C and operating pressure of 10 bar

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The results of this study were compared to previously published research articles that focused on the enhancement of CA-based RO membranes with numerous nanoparticles. As illustrated in Table 3, the blending of UiO-66-NPs with the pristine membrane had a satisfying performance either in salt rejection or permeability compared to some nanoparticles.

Table 3 A comparison of CA-based RO hybrid membranes performance that was previously reported in literature
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Influence of RO operation parameters onto the performance of CAU-0.05 composite membrane

In order to assess the membrane performance under different RO conditions, salt rejection and PWF of CAU-0.05 membrane were measured at diverse feedwater concentrations, pressure and temperatures. As it can be seen in Fig. 8, the membrane salt rejection was affected by the increased feedwater concentration, where the rejection decreased from 96.74 to 72.3% with the increase of feedwater TDS from 1000 to 13,000 ppm. This is due to the increased concentration of feedwater caused an increased osmotic pressure at the surface of the membrane. In this case, a layer of concentrated rejected ions is built at the membrane surface, which impeded the transport of lower molecular solutes. Additionally, the permeate water flux decreased from 3.4 L/m2h to 2 L/m2h with the increase of feedwater salt concentration from 1000 to 13,000 ppm, respectively. This might be explained as an effect for the concentration polarization phenomenon, which means the presence of greater concentration of the rejected ions at the membrane surface than that of the bulk solution. These results agree with previous studies (Alsalhy et al. 2013; Shigidi et al. 2022).

Fig. 8
figure 8

The effect of feedwater concentration on CAU-0.05 RO membrane performance at constant temperature of 25 °C and operating pressure of 10 bar

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For the same feedwater salinity, the effect of the applied pressure on CAU-0.05 membrane performance was illustrated in Fig. 9. It was found that, the PWF increased from 3.4 L/m2h to 10.47 L/m2 h with increasing the pressure from 10 to 30 bars. This increase was expected as the elevated applied pressure forces more water molecules to pass through the semi-permeable membrane. However, with increasing the pressure the membrane salt rejection ability decreased from 92 to 70.5% when the operating pressure raised from 10 to 30 bars. This decline might refer to the concentration polarization and the formation of NaCl salt layer near to the membrane surface, which caused more salts to escape through the membrane. These results are in agreement with literature (Elkony et al. 2020a).

Fig. 9
figure 9

The effect of operating pressure on CAU-0.05 RO membrane performance at constant operating temperature of 25 °C and initial feed concentration of 5000 ppm NaCl

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The temperature of the feedwater is considered one of the most important parameters that affects the RO membrane performance (Gedam 2012). Figure 10 shows the effect of various feedwater temperatures on the performance of CAU-0.05 membrane. As it was demonstrated, the membrane salt rejection declined from 92 to 85.6% as the feedwater temperature increased from 25°C to 40 °C, respectively. On the other hand, the PWF increased as the feedwater temperature rose. This could be explained that as temperature increase the viscosity decrease and this facilitate the water permeation rate through the membrane. Additionally, the salt solubility increases too and thus a higher diffusion rate could take place through the membrane. This trend was previously reported in literature (Gedam 2012; Abdulmuttaleb et al. 2014).

Fig. 10
figure 10

The effect of feedwater temperature on CAU-0.05 RO membrane performance at constant operating pressure of 10 bar and initial feed concentration of 5000 ppm NaCl

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Conclusions

The present work investigated the effect of UiO-66-NPs impregnation on the characteristics and performance of CA blank membrane for brackish water desalination. UiO-66 MOF was fabricated via solvothermal method, while neat CA and hybrid CA/UiO-66 membranes were synthesized by NIPS technique. The data of the characterized samples proved the successful preparation of UiO-66-NPs as well as the successful incorporation of UiO-66 nano-MOF into CA membrane matrix. The RO membrane performance was conducted, and the results revealed that the PWF was enhanced by the addition of UiO-66-NPs. It was found that CAU-0 membrane has a 99.8% and 1.14 LMH salt rejection and permeability, respectively. However, with a small reduction of salt rejection to be almost 97.6%, the permeability incremented to become 2.8 LMH for CAU-0.02 membrane. The same trend continued for CAU-0.05 membrane with a salt rejection of 93% and PWF of 3.4 LMH. With further addition of UiO-66-NPs, salt rejection slightly increased by only 1%, while membrane permeability decreased by around 17% for CAU-0.1 membrane. The optimum membrane was selected to be further tested under various feedwater concentrations, operating temperatures, and pressures. After compromising between the membrane PWF and MSR, the optimum membrane was found to be CAU-0.05 with PWF of 3.4 L/m2h, which is almost three times higher than that of CAU-0 blank membrane. Furthermore, the CAU-0.05 membrane mechanical stability was enhanced compared with the neat CA membrane, which indicates a better pressure stability. The performance results showed that CAU-0.05 membrane can reject salts from brackish water up to 5000 ppm NaCl by 92%. Additionally, the same membrane can reject salts with the same efficiency and even better permeability of 5 LMH when working under pressure up to 15 bar. The optimum operating pressure to maintain the membrane efficiency was found to be 25 °C.