Synthesis and characterization of p-carboxy phenyl amino maleimide-g-cellulose acetate/ZrO₂ nanocomposite membrane for water desalination

antibacterial efficiency, desalination, graft copolymerization, membrane composite, molecular docking

The reaction of p-carboxy phenyl amino maleimide (CHM) with cellulose acetate (CA) led to the formation of a modified cellulose acetate polymer (MCA), which was characterized by UV/Vis, ¹H NMR, and ¹³C NMR. The active sites of the reaction were the –NH group of (CHM) and the OAc of CA. CA was grafted with (CHM) to build branches on its main chains, using benzoyl peroxide as an initiator. The results of ¹H NMR and ¹³C NMR revealed the presence of (CHM) moieties inside the polymeric matrix. The (CA-g-CHM) ZrO₂ was fabricated into a membrane, using a phase inversion technique. The effect of ZrO₂ content on the water flux was discussed. The SEM/EDS was also used to characterize the membrane contents and morphology. The morphology of the membrane showed the grafted parts and the EDS confirmed the presence of nitrogen atoms in the polymeric matrix. The thermogravimetry (TGA) results showed that the membrane exhibited high thermal stability which would adjust the membrane for the desalination process. The desalination test indicated the removal of NaCl salt by the membrane, as shown by the EDS and ¹H NMR spectroscopy results. The membrane exhibited good antibacterial and antifungal properties.

HIGHLIGHTS

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Synthesis of novel cellulose-based polymeric materials.
Fabrication of membrane by a phase inversion technique.
Desalination test.
Water flux determination.
Water contact angle determination.

INTRODUCTION

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Polymers from renewable resources are of great interest, since these materials are available in large quantities and they exhibit numerous advantages (Bledzki & Gassan 1999; Mohanty et al. 2000; Zhang et al. 2005). Cellulose-based materials are widely used in the fabrication of membranes for water treatment and biomedical applications (Ionita et al. 2016; Xu et al. 2016). These are referred to for their good mechanical strength. According to the crystallinity of the polymeric matrix, the technique for the membrane fabrication was chosen. For highly crystalline polymers, Electrospinning and centrifugal spinning strategies have gained considerable attention among all kinds of techniques to produce membranes (Mamidi et al. 2022). Cellulose acetate (CA) is a semi-crystalline polymer, which exhibits high plasticity. Thus, the polymer could be fabricated into membranes by the phase inversion technique. However, the polymer suffers from poor heat stability at high temperatures, which might limit its use in the desalination process. Chemical modifications were always used to improve the polymer's thermal properties (Abdel-Naby & Aboubshait 2013; Abdel-Naby & Al-Ghamdi 2014a, 2014b). CA was functionalized with aminopropyl triethoxysilane, followed by a reaction of combining free amino groups with cyanuric chloride (Pandele et al. 2020), aiming to improve its thermal properties. Graft copolymerization is considered as the most effective technique to improve the properties of the polymeric materials, as it modifies the polymer by constructing new branches, polymeric in nature, onto the main chains. The new branches afford the polymer with additional functional groups, which enable it to perform the desired application (Zhan et al. 2016).

Poly maleimides are known to exhibit high thermal stability (Agarwal et al. 2003), thus the involvement of such thermal stable moieties in the cellulose acetate matrix would improve its thermal properties. CA derivatives could be cellulose mono, di, or tri acetate. It was reported that the ratio of acetyl to hydroxyl groups determines the properties of the polymer (McCray et al. 1991). The degree of acetylation was known to reduce the permeability of the membrane to water and salt (Reid & Breton 1959).

In the present work, the chemical modification of CA by (CHM) would increase the hydrophilicity of the polymer due to the presence of –COOH groups in the constructed branches by graft copolymerization, which would enhance the water flux and salt extraction processes. Moreover, the series of carboxylic groups through the polymeric matrix would suppress the bacteria and fungi growth. Molecular docking is an effective tool in drug design (Shoichet et al. 2002). It explains the types of binding interactions and calculates the binding energies between the studied compounds and the proteins' active site. Therefore, molecular docking studies have been carried out to investigate the antibacterial and antifungal properties of N-(p-carboxy phenylamino) maleimide compound against the gram-positive bacteria Staphylococcus aureus (Guilloteau et al. 2002), the gram-negative bacteria Pseudomonas aeruginosa (Kreusch et al. 2003a), and Candida albicans (Whitlow et al. 1997).

MATERIALS AND METHODS

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Materials

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Cellulose acetate Mw = 10,000, hydrazino benzoic acid, nano zirconium oxide, maleic anhydride, tetrahydrofuran, dimethyl formamide, and NaCl, all chemicals are purchased from Sigma-Aldrich (Burlington, MA, USA).

Instruments

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Nuclear magnetic resonance (NMR)

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NMR spectra were recorded with a Bruker AVANCE III spectrometer operating at 400 MHz using DMSO d₆.

Scanning electron microscope and energy-dispersive spectrometer (SEM/EDS)

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The sample was analyzed using (Tescan Vega 3) scanning electron microscope with a detector of secondary electron (SE). And detector of energy-dispersive spectrometer, with an electron backscatter diffraction (EBSD). The analysis was carried out at a voltage of 15 keV was selected with a working distance of 10 mm between the specimen and the detector without any polishing or use of a conductive coating to determine the structure, observe the morphology, and analyze the chemical composition of the sample.

Thermogravimetric analysis (TGA)

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The samples were analyzed using a simultaneous thermogravimetric and differential thermal analyzer (DTG-60H) that was programmed at a heating rate of 5 °C/min between 25 and 500 °C, under N₂ atmosphere, to determine the weight losses of the samples at high temperatures.

Inductively coupled plasma (ICP)

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Inductively coupled plasma atomic emission spectrometer (model ICPE-9820) was used to determine the concentrations of the aqueous solutions in ppm.

Energy-dispersive X-ray spectroscopy (EDX)

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Energy-dispersive X-ray fluorescence spectrometry (model EDX8000) was used to determine the elemental analysis of the samples.

Procedures

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Synthesis of monomer (CHM)

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The synthesis of CHM occurred in two steps. Firstly, 7 moles of hydrazino benzoic acid (HBA) was added to 7 moles of maleic anhydride, in THF. The maliamic acid derivative (MA) was precipitated in an ice bath. The product was filtered and washed thoroughly with THF, then left to dry. Secondly, in a dry flask, 1.25 g of (MA) was mixed with 0.25 g of fused sodium acetate and 5 mL of acetic anhydride. The dry system is tightly closed and was allowed to react, in a steam bath, for 30 min. The mixture was then precipitated in ice. The product was filtered and washed with water and then dried at room temperature. Recrystallization of the product occurred from the ethanol water mixture (70:30), mp = 293 °C. The new product was confirmed using UV/Vis spectroscopy (Figure 1).

Figure 1

UV/Vis spectrum of CHM (b) as compared to HBA (a) (concentration = 0.08 mg/L).

The blue broad peak (b) confirmed the formation of a new product. The high wavelength value is due to the highly conjugated structure of the produced monomer.

Synthesis of CA/CHM modified polymer

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In a three necked flask, 0.007 moles of CHM solution in THF was added to 4 g of CA solution. The reaction was allowed to react at 60 °C, for 10 h, under a nitrogen atmosphere. The product was precipitated in cold methanol (300 mL). The synthesized modified polymer was washed with hot methanol to dissolve the unreacted monomer, and then left to dry at room temperature.

Graft copolymerization

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In two necked round bottom flasks, 0.025 mol CA was dissolved in THF. A proper concentration of benzoyl peroxide (0.001M, 0.0015M, and 0.002M) was added. The reaction was allowed to react at a suitable temperature (30, 40, and 60 °C) in an ultrasonic bath of 300-Watt power. A specific (CHM) concentration (0.01M, 0.015M, and 0.02M) was added to the reaction medium after 15 min. After a different interval of time (10, 15, and 20 h), the product was precipitated in cold methanol. The product was then washed using the Soxhlet system. The comparison between the UV spectrum of the modified, graft copolymers (CA-g-CHM) and that of the parent CA, showed additional peaks related to monomer moieties (Figure 2). The intensity of the graft copolymer is higher than that of the modified one, this is attributed to the higher percentage of the grafted branches as compared to that of the modified group.

Figure 2

UV/vis spectrum of CA-g-CHM (c), modified CA (b) as compared to CA (a) (concentration = 0.08 mg/L).

Fabrication of CA-g-CHM/ZrO₂ nanocomposite membrane

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The membrane was fabricated by a phase inversion technique (Arthanareeswaran & Thanikaivelan 2010). 13.19 wt.% of the polymeric solution, in DMF, was prepared and cast by an automatic film applicator, with a thickness of casting knife (150 um) and (speed 4 rpm, temperature 25 °C). The phase inversion occurred in a coagulation bath of distilled water by immersing the steel sheet till the membrane was separated. Finally, the membrane was allowed to dry in the air.

For nanocomposite membrane, a definite wt.% of ZrO₂ (nanopowder, <50 nm particles) was added to the polymeric solution, and left in a sonic bath for (15–30 min) till the solution becomes homogeneous, then casting occurred.

Determination of water flux

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The water flux was calculated using the following formula (Khan et al. 2020):

where ΔV (L) is the permeate volume; T (h) is the filtration time interval; and A (m²) is the area of membrane.

Preparation of stock solution of NaCl salt ions

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A stock solution of NaCl salt (2,000 ppm) was prepared in deionized water, for the investigation of the desalination process.

Surface hydrophilicity and contact angle measurement

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The hydrophilicity of the prepared membrane surfaces was measured as a function of the contact angle measurements using Rame Hart Goniometer, France. A distilled water drop (2 μl) was added to the surface of the membrane (3 cm × 2 cm) using a micro syringe The contact angle was measured by adding the drop of water, at five different positions, within 20 s, on the membrane surface (Abdellah Ali et al. 2020).

Fabrication of the membranes

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The membrane was fabricated by a phase inversion technique (Arthanareeswaran & Thanikaivelan 2010). 13.19 wt.% of the polymeric solution, in DMF, was prepared and cast by an automatic film applicator, with a thickness of casting knife (150 um) and (speed 4 rpm, temperature 25 °C). The phase inversion occurred in a coagulation bath of distilled water by immersing the steel sheet till the membrane was separated. Then leave it soaked overnight. Finally, the membrane was allowed to dry in the air.

For nanocomposite membrane, a definite wt.% of ZrO₂ (nanopowder, <50 nm particles) was added to the polymeric solution, left in a sonic bath for (15–30 min) till the solution becomes homogeneous, and then was dispersed into the polymeric solution in DMF. Afterwards, the casting of the membrane occurred as mentioned above.

Mechanical properties

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In order to evaluate the mechanical properties of membranes, a tensile testing machine (DCP-KZ300, Sichuan, China) was employed to test the tensile strength and elongation-at-break of membranes. The speed of the crosshead was 20 mm/min. The dried membranes were snipped into a rectangle shape with a width of 15 mm and a total length of 100 mm. All of the samples of membranes were tested in ambient conditions (Bai et al. 2012).

Desalination process

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The membrane of area (15.9 cm²), was introduced in the vacuum filtration system. 25 mL of NaCl solution was allowed to pass through the system. The pressure of the vacuum was adjusted (2 bar). After the passage of the whole salt solution, the membrane was dried and investigated using SEM/EDS to confirm the salt extraction into the polymeric matrix.

Also, the conductivity of permeate solution was measured to determine salts rejection (Ma et al. 2017), using the following equation:

where Cond._f is the conductivity of the feed solution and Cond._p is the conductivity of the permeate solution.

Reusability of the membrane

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0.01 M of nitric acid (68–70%) was added to leach out the Cu (II) from the polymeric matrix. During the leaching process, the solid/liquid content was kept at 0.2 g in 10 mL with a slight increase in the temperature for 2 h (He et al. 2016). A magnetic stir bar was also used to mix the solution during the regeneration process.

Antibacterial and antifungi tests

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The efficiency of the synthesized membrane against the growth of gram-positive and gram-negative bacteria and fungi growth are investigated using the following procedure:

0.1 g of the membrane was dissolved in 1 mL of DMSO. The microorganism (bacteria or fungi) was allowed to grow in the Mueller Hinton Agar environment with MC Farland standard solution.

The mixture was homogenized by the vortex. Then, a swap of microorganism solution was put on a dish. Afterwards, a disc diffusion took place. Finally, 5 microliters of the tested membrane solution was added. After some interval of time, the results could be observed from the discs.

Molecular docking

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The 3D structures for the receptor proteins have been downloaded from the protein data bank (PDB) database (www.rcsb.org/pdb) (Berman et al. 2000). The protein ID for the gram-positive bacteria S. aureus is (1LQW) (Guilloteau et al. 2002), for the gram-negative bacteria P. aeruginosa is (1N5N) (Kreusch et al. 2003b), and for the C. albicans is (1AI9) (Whitlow et al. 1997). Chem Sketch (ACD Labs, Toronto, ON, Canada, freeware) version 2.5 (ACD/ChemSketch 2021) was used to gain the compound minimized 3D structure. After that, the structure was checked by PyMOL software version 4.2.0 (Schrodinger Inc., New York, NY, USA) (The PyMOL Molecular Graphics System 2010). Autudock Tools (ADT) version 1.5.6 (Scripps Research, San Diego, CA USA) (Morris et al. 2009) was used for preparing the structures of the compound and proteins for the calculations. To run the molecular docking calculations, the Autodock vina server (version 1.1.2) (Trott & Olson 2010) was used. The results of the binding and the interaction types were investigated using Discovery Studio Visualizer software (BIOVIA Discovery Studio Visualizer v16.1.0.15350 2015).

RESULTS AND DISCUSSION

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The product of the reaction of phenylhydrazine with maleic anhydride was known to depend on the electronegativity of the phenyl substituent (Rubinstein et al. 1971; Abdel-Naby & Al-Dossary 2008). The course of the reaction referred to the formation of amino maleimide if the phenyl substituent was an electron withdrawing group. While the electron donating substituent led to either pyridazinone or amino iso maleimide.

The structure of the N-(p-carboxyl phenylamino) maleimide was confirmed using ¹H NMR spectroscopy (Figure 3(a)) and ¹³C NMR spectroscopy (Figure 3(b)). The proton spectrum confirmed the presence of allylic hydrogen at 7.8 ppm, the carboxylic proton at 11.30 ppm and the NH proton at 6.77 ppm, while the peak corresponding to NH₂ proton was absent. Moreover, ¹³C NMR spectrum confirmed the presence of two types of carbonyl peaks, corresponding to the maleimide carbonyl (double intensity, δ = 172.5) and the carboxylic carbonyl (δ = 171). This result coincides with Rubinstein et al. (1971) as R = COOH, which is an electron withdrawing group. Thus, the phenylamino maleimide would be formed.

Figure 3

(a) 1H NMR spectrum of CHM. (b) 13C NMR spectrum of CHM.

(a) ¹H NMR spectrum of CHM. (b) ¹³C NMR spectrum of CHM.

Cellulose acetate may contain acetate groups in C6 or/and C2. The modification of the polymer occurs through the displacement of the acetate group at C6, out of the pyranose ring, or at C2, inside the pyranose ring. This substitution reaction occurs when protons are available in the reaction medium (Abdel-Naby & Al-Ghamdi 2014a).

The protons enhance the liberation of the acetate group out of the polymer, and thus, the acetic acid molecule will be formed (Abdel-Naby & Aboubshait 2013; Abdel-Naby & Al-Ghamdi 2014a).

The monomer exhibits two acidic protons. One is that of the –COOH group and the other is of the –NH group. As the lone pair of electrons of the –NH was withdrawn by resonance into the benzene ring, as a result of the high electronegativity of the carboxylic group at the para position of the ring, thus the –NH proton is considered as the most acidic proton of the compound. Consequently, the reaction of CA occurred with CHM via its –NH proton and the following product was obtained (Figure 4).

Figure 4

Modification of (CA) by (CHM).

Figure 5(a) shows the ¹H NMR spectrum of the cellulose acetate (MCA) modified by CHM. The benzenic protons appeared around 7.3 ppm, while the –NH proton of the monomer disappeared as a result of the modification reaction. Also, the peak at 2.9 ppm corresponds to the proton on the pyranose ring, alpha to group. Moreover, the ethylenic proton at 7.86 ppm remained untouched.

Figure 5

(a) 1H NMR of modified CA by CHM (MCA). (b) 13C NMR of modified CA by CHM (MCA).

(a) ¹H NMR of modified CA by CHM (MCA). (b) ¹³C NMR of modified CA by CHM (MCA).

The ¹³C NMR spectrum of MCA (Figure 5(b), Table 1) confirmed the bond between the monomer (CHM), via its –NH group and C8 of CA. Also, there are two additional peaks in the carbonyl region, corresponding to the monomer moieties, as compared to the CA spectrum (S1).

Table 1

¹³C NMR spectral characteristic data of MCA modified polymer

Structure	Carbon atom	¹³C NMR (δ ppm)
	1	146.5
	2	172.5
	3	113
	4	140
	5	133
	6	142
	7	171
	8	43

Graft of CA with (CHM)

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As the modification reaction did not disturb the ethylenic bond, graft copolymerization was suggested to increase the number of active sites (–COOH) through the polymeric matrix, which would enhance the desalination process as well as antibacterial activity. The building of poly (CHM) branches occurred via the grafting process, using benzoyl peroxide as the initiator. The copolymerization occurred via two steps. First, the modification in the absence of the initiator and then the initiator was introduced to the reaction medium to start the graft process. Figure 6 represents the suggested graft copolymerization mechanism.

Figure 6

Graft copolymerization of CA with (CHM).

The product of the graft copolymer was confirmed by both ¹H NMR and ¹³C NMR spectra (Figure 7(a) and 7(b)). The proton of –COOH group was distinguished at δ = 11.3. Also, the –NH protons of the repeating units of the constructed branches could be observed at δ = 7–8. Moreover, the disappearance of c-c ethylene carbons at 135 ppm and the appearance of a peak of 31.7 ppm, corresponding to the saturated CH₂ group, confirmed the copolymerization reaction (Table 2).

Table 2

¹³C NMR chemical shifts (δ ppm) characteristics of (CA-g-CHM)

Structure	Carbon atom	¹³C NMR (δ ppm)
	1	31.7
	2	172.5
	3	113
	4	140
	5	133
	6	142
	7	171
	8	49
	9	122

Figure 7

(a) 1H NMR of CA-g- CHM. (b) 13C NMR of CA-g- CHM.

(a) ¹H NMR of CA-g- CHM. (b) ¹³C NMR of CA-g- CHM.

To determine the optimum conditions for graft copolymerization, the factors affecting the percentage of graft (%G) were studied.

Parameters affecting the graft copolymerization

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The percentage of graft was calculated according to the following equation:

The following parameters were studied to determine the required % of graft needed for the membrane fabrication, suitable for the desalination process.

Effect of time

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The effect of various intervals of time on the percentage of grafts of (CHM) onto CA main chains is shown in Figure 8(a). The results revealed that the percentage of graft increased gradually with time due to the building up of the monomer branches, up to 15 h, where a steady state started this occurred due to the consumption of the monomer units into the polymeric branches (Abdel-Naby & Al-Dossary 2008).

Figure 8

(a) Effect of time on the %G. (b) Effect of the monomer concentration on the %G.

Effect of temperature

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To investigate the effect of temperature on the percentage of graft, three values for the temperature were chosen (30, 50, and 60 °C) to carry out the graft copolymerization.

The results revealed that the %G increased with the increase in temperature (Table 3). As the boiling point of THF is recorded as 65 °C, we did not increase the temperature higher than 60 °C.

Table 3

Effect of temperature on the %G

Temperature (°C)	%G
25	0
30	3.2
50	8.1
60	12.6

CA = 0.025 mole, BP = 0.002M, CHM = 0.02M, and time = 15 h

Effect of monomer concentration

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The effect of monomer concentration on the % of the graft is shown in Figure 8(b). The result revealed the increase of %G with the monomer concentration until it reached 0.02M. Afterwards, the increase in monomer concentration reduced the %G. This might be attributed to the enhancement of the formation of homopolymers (Figure 8(b); Abdel-Naby & Aboubshait 2013).

Thus, the optimum reaction conditions were chosen as temperature 60 °C, time 15 h, monomer concentration 0.02M, and initiator concentration 0.002M.

Another confirmation of the graft process is the surface morphology, where the graft parts (round circles) are distinguished through the CA polymeric matrix (Figure 9).

Figure 9

SEM images of the surface morphology of CA-g-CHM as compared to that of CA.

Prior to the suggestion of the graft copolymer for the fabrication of membrane and desalination application, its thermal stability should be investigated. This occurred using thermogravimetry (TGA).

Thermal properties of the graft copolymer

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The maleimide moieties are known to exhibit high thermal stability (Abdel-Naby & Al-Dossary 2008). The TGA results revealed that the increase in the percentage of graft led to the improvement of the thermal stability of the polymer, as shown bythe increase in the value of initial decomposition temperature, T_o, temperature at which the polymer starts to lose parts of its polymeric matrix. The T_o value increased with the increase of percentage of graft until it reached 350 °C for 12.6% graft. For a higher percentage of graft, a reduction of the T_o was observed, due to the decrease in the crystallinity percentage (Abdel-Naby & Al-Ghamdi 2020), as the length of the branches increases the amorphous regions of the polymeric matrix. Also, The CA-g-CHM (12.6%) exhibited less weight loss at high temperatures (500 °C). Thus, the percentage of 12.6% was chosen for the fabrication of the membrane (Figure 10).

Figure 10

TGA of various percentage of CA-g-CHM, (c) 12.6%, (b) 18%, as compared to the parent CA (a).

Another confirmation of the suitability of the CA-g-CHM (12.6%) to the adjustment as desalinating membrane is higher mechanical properties as compared to the CA membrane. Table 4 shows that the CA-g-CHM (12.6%) membrane exhibited the highest tensile strength and elongation at the brake as compared to other graft percentages as well as the parent polymer (CA). This could be attributed to the fact that the higher percentage of graft might affect the crystallinity, by disabling some of the interchain hydrogen bonds.

Table 4

Effect of the percentage of graft on the tensile strength and elongation at the break of CA

Mechanical properties	Tensile strength (Mpa)	Elongation at break (%)
CA	6.9	6.4
CA-g-CHM (12.6)	7.2	7
CA-g-CHM (18)	8	7.5

Fabrication of the membrane

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The desalination of saline water requires a special type of membrane, which exhibits some effective functional groups, acting as removal active sites. In addition, the membrane should exhibit good water flux as well as a small water contact angle.

The real challenge of a desalinating membrane is to express its ability to trap the dissolved sodium chloride salts dissolved in water into the polymeric matrix.

The CA-g-CHM contains branches of monomer units, each of which exhibits one carboxylic group. Most of the carboxylic groups could act as active sites for the extraction of sodium cations from the saline aqueous solution, performing ionic chemical bonds (Figure 11). The unreacted carboxylic groups could also contribute two extra roles. First, to perform hydrogen bonding with water molecules, thus could participate in the enhancement of the water flux. Second, to suppress bacterial growth. For these reasons CA-g-CHM (%G = 12.6), exhibiting high thermal stability, was used as membrane base material, using the phase inversion technique (Wang et al. 2005).

Figure 11

Schematic representation of the desalination reaction by CA-g-CHM.

The addition of dispersed nanoparticles, such as TiO₂, SiO₂, ZrO₂, and Al₂O₃, as dispersed particles through the membrane, highly improves the water flux through the whole membrane as well as reduces the water contact angle (Bottino et al. 2002; Chakrabarty et al. 2008; Abedini et al. 2011). The nano-size ZrO₂ was chosen to afford the large size of the molecules with limited weight percentage. The high percentage of nanoparticles always led to particle agglomerations and blockage of the membrane pores.

The nano ZrO₂ particles were chosen as dispersive particles into the graft polymeric materials to enhance the water flux and thus the desalination process (Voicu et al. 2016).

Water flux

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The water flux was calculated using the equation mentioned in the experimental section. The results revealed that the percentage of zirconium ions affected the water flux (Table 5). The water flux (WF) increased with the increase in ZrO₂ concentration from 1 to 2 wt.%. Increasing the ZrO₂ NPs to 3 wt.% reduced the water flux due to the blocking action, which occurs as a result of the aggregation of ZrO₂ NP (Wang et al. 2005; Abdel-Naby & Al-Ghamdi 2020).

Table 5

WF of CA-g-CHM membrane as function of ZrO₂ concentration

ZrO₂ Concentration (wt.%)	Water flux (L m⁻²h⁻¹)
0	314
1	453.8
2	632.3
3	400.8
4	220.6

The SEM of the membrane with ZrO₂ (2 wt.%) confirmed the distribution of ZrO₂ (2 wt.%) through the whole membrane (Figure 12).

Figure 12

SEM image of the distribution of ZrO2 (2wt.%) through the polymeric matrix of CA-g-CHM membrane, which shows no agglomeration.

SEM image of the distribution of ZrO₂ (2wt.%) through the polymeric matrix of CA-g-CHM membrane, which shows no agglomeration.

Moreover, the water contact angle measurement of the pure CA membrane demonstrated a moderate surface wettability (highest contact angle value). The addition of ZrO₂ nanoparticles enhanced the surface hydrophilicity of the membranes (Table 6) as also reported in the open literature (Vetrivel et al. 2021).

Table 6

Value of contact angles of membranes

Membrane	Contact angle of water droplet (±2)
CA (10%)	72.5
Grafted CA (10%)	57.2
Grafted CA (10%) + ZrO₂ (1wt.%)	52.8
Grafted CA (10%) + ZrO₂ (2wt.%)	46.5
Grafted CA (10%) + ZrO₂ (3wt.%)	66.8

Membrane	Contact angle of water droplet (±2)
CA (10%)	72.5
Grafted CA (10%)	57.2
Grafted CA (10%) + ZrO₂ (1wt.%)	52.8
Grafted CA (10%) + ZrO₂ (2wt.%)	46.5
Grafted CA (10%) + ZrO₂ (3wt.%)	66.8

From the above-mentioned data, a synergism exists between the efficiency of the graft monomer units and the nano ZrO₂ (2 wt.%) additives in the improvement of the water hydrophilicity of cellulose acetate membrane.

Desalination

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The extraction of the NaCl solution occurred according to the method mentioned in the experimental section. The trapped NaCl particles inside the membrane were confirmed by ¹H NMR. Figure 13 shows the spectrum of the membrane before and after the desalination process. The peak corresponding to the carboxylic proton of CHM moieties disappeared, as it was substituted by the sodium ion.

Figure 13

1H NMR of the membrane before (a) and after (b) the removal of NaCl salt.

¹H NMR of the membrane before (a) and after (b) the removal of NaCl salt.

The salt removal percentage was calculated from the following formula:

where C₀ and C_f are the initial and final concentrations of the NaCl salt solution, after passing through the CA-g-CHM/ZrO₂ membrane. The results revealed that the membrane exhibited excellent salt removal efficiency as compared to the low efficiency of CA (Table 7).

Table 7

Desalination efficiency of CA-g-CHM/ZrO₂ as compared to CA

Contact time (hour)	The remaining concentration (ppm) of NaCl (⁠ = 2,000 ppm)
Contact time (hour)	CA	Removal %	CA-g-CHM/ZrO₂	Removal %
1	1,960	2	940	52
2	1,890	5.5	800	60
6	1,810	9.5	510	74.5

Contact time (hour)	The remaining concentration (ppm) of NaCl (⁠ = 2,000 ppm)
Contact time (hour)	CA	Removal %	CA-g-CHM/ZrO₂	Removal %
1	1,960	2	940	52
2	1,890	5.5	800	60
6	1,810	9.5	510	74.5

Another confirmation of the desalination of NaCl salt solution is the characterization by EDS (Figure 14), which showed the presence of Na⁺ and Cl⁻. The presence of NaCl salt could be also distinguished from the SEM as shown in Figure 15.

Figure 14

The EDS of the membrane after extraction of Na+1 and Cl− from its salt solution.

The EDS of the membrane after extraction of Na⁺¹ and Cl⁻ from its salt solution.

Figure 15

SEM image of the membrane after extraction of NaCl from its salt solution.

Additionally, the calculation of salt rejection of CA-g-CHM (12.6%)/ZrO₂ (2%) membrane showed that the highest salt rejection, at pressure 2 bar, as compared to the CA-g-CHM (12.6%) in the absence of zirconium oxide nanoparticles and the unmodified CA (Figure 16).

Figure 16

Salt rejection efficiency of various membranes.

Thus, the membrane CA-g-CHM (12.6%)/ZrO₂ (2%) is the most suitable membrane for the desalination process, as shown by the good water flux and the highest salt rejection efficiency.

One of the most important properties of the desalination membrane is its antibacterial and antifungi efficiencies.

The experimental tests for the effect of the graft copolymer against the bacteria and fungi growth. Unlike the CA membrane, which lacks any antibacterial or antifungi activity, the graft copolymer exhibited excellent antibacterial and antifungi efficiencies (Figure 17). Thus, the CHM moieties gave the graft copolymer the ability to suppress the growth of bacteria and fungi. This is attributed to the presence of carboxylic groups in the CHM moieties through the grafted branches onto the main CA chains (Xu et al. 2018).

Figure 17

Antibacterial and antifungi efficiencies of CHM.

Table 8 presents the binding energies for N-(p-carboxy phenylamino) maleimides compound with the receptors. The compound shows very good inhibition potential as an antibacterial and antifungal drug. The binding energies range from −6.5 to −7.3 kcal/mol. For the gram-positive bacteria S. aureus, the binding energy is −6.5 kcal/mol. The results are due to several interactions: conventional hydrogen bonds with GLU155, LEU112, and GLN65. In addition, Pi-Pi T shaped interaction with HIS154, Pi-donor hydrogen bond with GLY110, and several van der Waals interactions with SER113, CYS111, GLY60, GLY110, GLU109, LEU105, TYR147, VAL151, VAL59, SER57, GLY58, and HIS158. For the gram-negative bacteria P. aeruginosa, the docking results show that the binding energy between this receptor and the compound equals −6.8 kcal/mol. Several interactions contribute to this energy. It has been found three conventional hydrogen bonds with CYS131, GLY91, and CYS92, Pi-Pi T shaped interaction with HIS134, Pi-Alkyl interaction with ILE45, and Amide-Pi stacked interaction with GLY91. In addition, several van der Waals interactions with LEU93, GLY44, GLY46, GLU135, VAL130, GLU90, TYR88, GLN89, TYR99, GLY97, PRO96, and VAL95. The compound shows very promising inhibition potential against C. albicans. The binding energy is −7.3 kcal/mol. It has been found five conventional hydrogen bonds with THR58, GLY114, GLU116, ARG56, and ARG79. Moreover, several interactions, such as carbon hydrogen bond with SER78, Pi-Alkyl with LYS57, Pi-cation with LYS57, and Pi-anion with GLU116. In addition, there are van der Waals interactions with ALA115, LEU77, ILE117, and GLY55 (Figure 18).

Table 8

The binding energies (kcal/mol) of N-(p-carboxy phenylamino) maleimides compound with receptor proteins

Receptors	Binding energies (kcal/mol)	Hydrogen bonds
Staphylococcus aureus	−6.5	GLU155, LEU112, GLN65
Pseudomonas aeruginosa	−6.8	CYS131, GLY91, CYS92
Candida albicans	−7.3	THR58, GLY114, GLU116, ARG56, ARG79

Receptors	Binding energies (kcal/mol)	Hydrogen bonds
Staphylococcus aureus	−6.5	GLU155, LEU112, GLN65
Pseudomonas aeruginosa	−6.8	CYS131, GLY91, CYS92
Candida albicans	−7.3	THR58, GLY114, GLU116, ARG56, ARG79

Figure 18

2D structures and interactions of the compound with the active site of proteins.

Reusability of the membrane

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As it is well known that the adsorption or desorption of metal ions from a material is highly sensitive to the pH of the medium, the variation of the recovery percentage with the volume of nitric acid was studied (Figure 19).

Figure 19

The recovery percentage as a function of the variation of the volume of 0.5 mol/L HNO3 (mL).

The recovery percentage as a function of the variation of the volume of 0.5 mol/L HNO₃ (mL).

The quantitative recovery (>95%) was obtained in the entire range of 0.5 mol/L HNO₃. The maximum recovery was obtained for 6 mL of HNO₃ (99%).

After the cleaning process (Figure 20(a)), the membrane could be used up to 10 times without losing efficiency (Figure 20(b)).

Figure 20

(a) Schematic representation of the cleaning process of the membrane. (b) Reusability of the membranes CA-g-CHM in the presence and absence of ZrO2.

Cellulose acetate, cellulose acetate propionate and cellulose acetate butyrate membranes for water desalination applications

(a) Schematic representation of the cleaning process of the membrane. (b) Reusability of the membranes CA-g-CHM in the presence and absence of ZrO₂.

CONCLUSION

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The graft copolymerization of CA with CHM was characterized by SEM, ¹H NMR, ¹³C NMR, and UV/Vis spectroscopy. The factors affecting graft copolymerization were studied to determine the optimum graft conditions as well as the suitable % of graft for the desalination process. CA-g-CHM (%G = 12.6) exhibiting the highest thermal stability as well as the highest mechanical properties, which enable the grafted copolymers to be used as membrane base material, using the phase inversion technique.

The CA-g-CHM contains branches of monomer units, each of which exhibited one carboxylic group. Most of the carboxylic groups acted as active sites for the extraction of sodium cations from the saline aqueous solution, performing ionic chemical bonds. The unreacted carboxylic groups could also contribute to the performance of hydrogen bonding with water molecules, and thus could participate in the enhancement of the water flux. Also, the grafted copolymer exhibited higher antibacterial and antifungi growth. As compared to CA, which is known to lack any antibacterial activity.

The addition of dispersed ZrO₂ nanoparticles (2 wt.%), as dispersed particles through the CA-g-CHM (12.6%) membrane, highly improves the water flux up to 637.3 through the whole membrane as well as reducing the water contact angle to 46.5 as compared to CA (72.5) and CA-g-CHM (57.5). Additionally, the salt rejection of the CA-g-CHM (12.6%)/ZrO₂ (2%) membrane showed the highest salt rejection, at pressure 2 bar, as compared to the CA-g-CHM (12.6%), in the absence of zirconium oxide nanoparticles, and the unmodified CA. Thus, the performed membrane showed excellent desalination efficiency as compared to CA, and can be reused up to 10 times without losing efficiency. Moreover, the membrane exhibited excellent antibacterial and antifungal efficiencies. These results were confirmed by molecular docking calculations.

ACKNOWLEDGEMENTS

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The authors greatly acknowledge the facilities offered by the water treatment unit, at the basic and applied research center, College of Science, Imam Abdulrahman Bin Faisal University, and the institutional fund offered by the Ministry of Learning and Education, KSA. Project No IF 20-2020.

AUTHOR CONTRIBUTIONS

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Conceptualization, A.S.A.-N. and S.A.; Data curation, A.S.A.-N., B.A.A. and Y.A.; Formal analysis, B.A.A., S.A. and Y.A.; Investigation, A.S.A.-N. and B.A.A.; Methodology, A.S.A.-N., B.A.A. and S.A.; Software, Y.A.; Writing – original draft, A.S.A.-N., S.A. and Y.A.; Writing – review & editing, A.S.A.-N.

DATA AVAILABILITY STATEMENT

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

CONFLICT OF INTEREST

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The authors declare there is no conflict.

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