# Preparation of fouling resistant and highly perm-selective novel PSf/GO-vanillin nanofiltration membrane for efficient water purification

Sudesh Yadav, Ibrar Ibrar, Akshaya Samal, Ali Altaee, Sébastien Déon, John Zhou, Noreddine Ghaffour

► To cite this version:

Sudesh Yadav, Ibrar Ibrar, Akshaya Samal, Ali Altaee, Sébastien Déon, et al. Preparation of fouling resistant and highly perm-selective novel PSf/GO-vanillin nanofiltration membrane for efficient water purification. Journal of Hazardous Materials, 2022, 421, pp.126744. 10.1016/j.jhazmat.2021.126744. hal-03374742

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Preparation of fouling resistant and highly perm-selective novel PSf/GO-vanillin nanofiltration membrane for efficient water purification.

<table><tr><td>Item Type</td><td>Article</td></tr><tr><td>Authors</td><td>Yadav, Sudesh;Ibrar, Ibrar;Samal, Akshaya K;Altaee, Ali;Déon, Sébastien;Zhou, John;Ghaffour, NorEddine</td></tr><tr><td>Citation</td><td>Yadav, S., Ibrar, I., Samal, A. K., Altaee, A., Déon, S., Zhou, J., & Ghaffour, N. (2022). Preparation of fouling resistant and highly perm-selective novel PSf/GO-vanillin nanofiltration membrane for efficient water purification. Journal of Hazardous Materials, 421, 126744. doi:10.1016/j.jhazmat.2021.126744</td></tr><tr><td>Eprint version</td><td>Post-print</td></tr><tr><td>DOI</td><td>10.1016/j.jhazmat.2021.126744</td></tr><tr><td>Publisher</td><td>Elsevier BV</td></tr><tr><td>Journal</td><td>Journal of hazardous materials</td></tr><tr><td>Rights</td><td>NOTICE: this is the author's version of a work that was accepted for publication in Journal of hazardous materials. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Journal of hazardous materials, [421, , (2021-08-01)] DOI: 10.1016/j.jhazmat.2021.126744. © 2021. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/</td></tr><tr><td>Download date</td><td>2025-07-25 14:09:23</td></tr></table>
1 Preparation of fouling resistant and highly perm-selective novel PSf/GO-vanillin

2 nanofiltration membrane for efficient water purification

3 Sudesh Yadav¹, Ibrar Ibrar¹, Akshaya K. Samal², Ali Altaee¹, #, Sébastien Déon³, John Zhou¹,

4 Noreddine Ghaffour⁴

5 1: Centre for Green Technology, School of Civil and Environmental Engineering, University of

6 Technology Sydney, 15 Broadway, NSW, 2007, Australia

7 2: Centre for Nano and Material Sciences, Jain University, Jain Global Campus, Ramanagara,

8 Bangalore - 562112, India

9 3: Institut UTINAM (UMR CNRS 6213), Université de Bourgogne-Franche-Comté, 16 Route de

10 Gray, 25030 Besançon Cedex, France

11 4: King Abdullah University of Science and Technology (KAUST), Water Desalination and Reuse

12 Center (WDRC), Biological and Environmental Science and Engineering (BESE), 23955-6900

13 Thuwal, Saudi Arabia

14 # Corresponding author email address: ali.altaee@uts.edu.au

15

16 Graphical abstract

17

<figure><img src="image_3.png" /></figure>
19 Abstract

To meet the rising global demand for water, it is necessary to develop membranes capable of efficiently purifying contaminated water sources. Herein, we report a series of novel polysulfone (PSf)/GO-vanillin nanofiltration membranes highly permeable, selective, and fouling resistant. The membranes are composed of two-dimensional (2D) graphite oxide (GO) layers embedded with vanillin as porogen and PSf as the base polymer. There is a growing interest in addressing the synergistic effect of GO and vanillin on improving the permeability and antifouling characteristics of membranes. Various spectroscopic and microscopic techniques were used to perform detailed physicochemical and morphological analyses. The optimized PSf<sub>16</sub>/GO<sub>0.15</sub>-vanillin<sub>0.8</sub> membrane demonstrated 92.5% and 25.4% rejection rate for 2000 ppm magnesium sulphate (MgSO<sub>4</sub>) and sodium chloride (NaCl) solutions respectively. Antifouling results showed over 99% rejection for BSA and 93.57% flux recovery ratio (FRR). Experimental work evaluated the antifouling characteristics of prepared membranes to treat landfill leachate wastewater. The results showed 84-90% rejection for magnesium (Mg<sup>+2</sup>) and calcium (Ca<sup>+2</sup>) with 90.32 FRR. The study experimentally demonstrated that adding GO and vanillin to the polymeric matrix significantly improves fouling resistance and membrane performance. Future research will focus on molecular sieving for industrial separations and other niche applications using mixed matrix membranes.

**Keywords:** PSf/GO-vanillin membrane; mixed matrix membrane; nanofiltration; fouling resistant; and salt rejection

39 1. Introduction

Water scarcity and pollution require advanced water purification technologies to produce potable water from non-common wastewater sources [1, 2]. Hazardous wastewaters, such as industrial and landfill leachate wastewaters, pose a threat to the ecosystem when discharged without proper treatment due to the high concentration of dissolved ions, heavy metals, pesticides and organic matters [3]. Despite the strict environmental regulations, deliberate and accidental waste release are on the rise [3, 4]. Conventional primary and secondary treatments have shown limited efficiency towards emerging pollutants. Tertiary treatments such as adsorption, photocatalysis, and membrane-based processes have been proven successful [5]. While membrane processes have the advantage over traditional treatment
technologies, they currently experience setbacks represented by fouling and membrane
permselectivity [5, 6].

At present, polyamide-based thin-film composite (TFC) membranes occupy the highest portion in reverse osmosis (RO) and nanofiltration (NF) membranes technology for desalination and wastewater treatment [7]. Still, TFC membranes have some shortcomings, including low permeability, hydrophobicity, and fouling tendency [8]. NF membranes effectively remove hazardous heavy metal ions and organic substances from solutions [9, 10], but they are prone to fouling and exhibit low rejection of monovalent salts [11]. Thus, efforts are made in fabricating novel NF membranes to overcome fouling by introducing diverse nano-fillers, such as silver (Ag) [12], titanium oxide (TiO₂) [13], metal-organic frameworks (MOFs) [14], graphene quantum dots [15], molybdenum disulfide (MoS₂) [16], and other organic additives [17]. Although previous research has demonstrated the effectiveness of these materials to tune the free volume characteristics of polymers, boundary defects between polymers and nanomaterials are a challenging problem [18]. Further, many studies have failed in providing information on the interfacial compatibility and gradual detachment of nanomaterials over extended periods [19].

The development of specific molecular separation membranes is gaining considerable attention. As a result, new research studies are focused on the fabrication of membranes with specific molecular weight cut-offs by altering the surface characteristics, pore size distribution, and changing solute diffusion parameters. In general, artificial grafting, surface modification, chemical cross-linking, and in-situ physical blending are suggested for enhancing membrane surface properties such as wettability, membrane fouling, pore diameter, and surface zeta potential [20, 21]. Among the techniques mentioned above, physical blending is the most effective strategy due to its suitability for large-scale production. For instance, Yuan et al. prepared polyaniline (PANI)/polysulfone (PSf) membrane with better water permeability and resistance to fouling [22]. For bovine serum albumin (BSA), humic acid (HA), and sodium alginate (SA), the flux recovery ratio (FRR) was up to 65.3 %, 67.9 %, and 70.1 %, respectively [22]. In another study, PANI-graphene oxide nanofillers were used to prepare polyvinylidene fluoride (PVDF) composite membrane for removing Allura red (AR) and methyl orange (MO) dye from textile effluents [23]. PANI-GO nanofillers reduced the water contact angle (WCA) to 56.11° and increased the pure water flux (PWF) from 112 to 454
80 LMH. With 95 % rejection of MO dye and 98 % of AR dye, the FRR reached 94 % [23]. Recently
81 Shu et al. used micron-sized 2-dimensional MOF (BUT-203) nanosheets to prepare mixed
82 matrix membrane for dye desalination rejection [24]. The findings suggested that chemical
83 cross-linking between polymers and nanofillers improves the membrane performance,
84 whereas dense separation layer and filler-loading limits the membrane performance [23, 24].
85 Pandey et al. developed fouling resistant MXene ($Ti_3C_2T_x$/cellulose acetate (CA) covalently
86 cross-linked membrane, which showed PWF of 256.85 L m⁻² h⁻¹ bar⁻¹ and over 92% rejection
87 for rhodamine B (RhB) dye [25]. The 10% @$Ti_3C_2T_x$/CA membrane demonstrated outstanding
88 antibacterial properties against E. coli and B. subtilis bacteria [25]. They achieved an ideal
89 separation by optimizing the filler-loading and adjusting the membrane pore size. However,
90 current NF membranes have poor antifouling properties and low water flux, which increases
91 energy consumption, making them unsuitable for treating complex wastewaters such as
92 landfill leachate. Thus, it is important to develop novel antifouling and high perm-selective NF
93 membranes for water and wastewater treatment.

94 Graphite oxide (GO) is a promising material widely used in membrane fabrication as a
95 nanofiller to improve water flux and reduce membrane biofouling due to its chlorine
96 tolerance and antimicrobial properties [26]. For instance, Ganesh et al. developed a mixed
97 matrix membrane with GO for salt rejection [27]. At 4 bar pressure, the prepared 2000 ppm
98 GO/PSf membrane showed near 48 LMH water flux and 74 % sodium sulfate ($Na_2SO_4$)
99 rejection [27]. Wu et al. prepared hybrid silica nanoparticles ($SiO_2$)-GO/PSf membranes and
100 achieved 98% rejection for egg albumin [28]. The prepared membranes showed 72% FRR with
101 enhanced antifouling properties [28]. In our previous work, an antifouling PSf/vanillin
102 composite membrane was prepared and achieved 99% rejection for BSA with 88.55% FRR
103 [17]. Vanillin imparts a negative surface charge and hydrophilic properties to the membrane
104 due to high polar surface area (46.53 Å²) and functionalized phenolic ring [17, 29]. The current
105 work investigates the synergistic effect of 2D GO nanolayers and vanillin on membrane
106 permeability and selectivity using PSf as the base polymer. To the best of the authors'
107 knowledge, no study has reported GO-vanillin membrane performance for wastewater
108 treatment. The study presented an experimental work on a novel PSf/GO-vanillin membrane
109 for landfill wastewater treatment. The current study proposed cross-linked GO/vanillin to
110 enhance the wettability and fouling resistance of hydrophobic PSf membranes. The
antifouling and hydrophilicity properties of vanillin and GO will impart special features to the fabricated membrane for landfill wastewater treatment. Antifouling studies were performed using landfill leachate wastewater and BSA (200mg/L) solution. Magnesium sulphate (MgSO₄) and sodium chloride (NaCl) were used as model salts for salt rejection studies.

## 2. Materials

### 2.1. Chemicals

In this study, all chemicals are of analytical grade and have not been further purified. N-Methyl-2-pyrrolidone (NMP), PSf (≈ 35kDa), and BSA (Agarose Electrophoresis: > 98%; Nitrogen: 14.5-16.5% and pH (1% in 0.15 M NaCl): 6.5-7.5) were obtained from Sigma-Aldrich Australia. GO produced by Hummer's method (SKU: GTOP0003; brownish-yellow powder;~99% purity; 0.5-5 µm lateral size; 1-3 nm thickness; and ≤46.0 wt.% oxygen content) was purchased from Advanced Chemicals Supplier (ACS) Material, LLC., CA 91106, USA. Hydrochloric acid (HCl) was purchased from Merck. Sodium hydroxide (NaOH), NaCl, and MgSO₄ used in the rejection test were procured from Chem-supply, Australia. Throughout the experiments, deionized (DI) water (Milli-Q®, Merck) was used in the membrane fabrication process. DI water was used to prepare the standard NaCl, MgSO₄, HCl, NaOH, and BSA aqueous solutions for filtration tests. Biologically treated landfill leachate wastewater was collected from Hurstville Golf Course, NSW, Australia (Table 1).

<table><caption>Table 1: Characteristics of landfill leachate wastewater used for this study.</caption><thead><tr><td>Characteristic</td><td>Value</td></tr></thead><tbody><tr><td>Appearance</td><td>Dark Yellowish-brown colour</td></tr><tr><td>pH<sup>a</sup></td><td>7.5 ± 0.3</td></tr><tr><td>Ammonia, mg/L</td><td>≤ 0.5</td></tr><tr><td>Total suspended solids, mg/L</td><td>27-117</td></tr><tr><td>Total dissolved solids (TDS)<sup>a</sup>, mg/L</td><td>3300</td></tr><tr><td>Total organic carbon (TOC)<sup>b</sup>, mg/L</td><td>120.5 ± 3.6</td></tr><tr><td>Total Iron, mg/L</td><td>3.5-5.2</td></tr><tr><td>Sodium (Na)<sup>c</sup>, mg/L</td><td>142.05</td></tr><tr><td>Magnesium (Mg)<sup>c</sup>, mg/L</td><td>125.3</td></tr><tr><td>Potassium (K)<sup>c</sup>, mg/L</td><td>39.5</td></tr></tbody></table>
a: measured using LAQUA PC210 conductivity metre; b: measured using analytikjena TOC; and c: measured using Agilent Technologies 7900 ICP-MS.

## 2.2. Membrane fabrication

Non-solvent induced phase separation technique was applied for fabricating PSf-GO/vanillin composite membranes, using N-Methyl-2-pyrrolidone (NMP) solvent, PSf base polymer, DI water non-solvent, and vanillin as a pore-forming agent. Initially, a measured amount of GO was exfoliated in NMP using Powersonic, Digital Ultrasonic Bath for an hour. The optimized vanillin concentration, 0.8 g, was dissolved in GO-NMP dispersion for an hour using a magnetic stirrer [17]. Then, PSf was gradually added to the GO-vanillin-NMP solution at 60 °C. To obtain a homogeneous casting solution, the casting solution was stirred for 24 hours, and an ultrasonication bath was used to release the trapped air bubbles. A glass plate was used to case the prepared solution, using a stainless steel casting knife of 200 µm thickness before transferring to a DI water coagulation bath (Figure 1). Finally, the PSf/GO-vanillin composite membranes were transferred to a freshly prepared coagulation bath. The liquid-liquid demixing for PSf-GO/vanillin membranes is shown in Figure 2. The membranes left there for 24 hours to ensure that the NMP solvent was completely removed. The chemical composition of obtained membranes is tabulated in Table 2. The membrane fabrication process was performed under ambient conditions of ~52% humidity and temperature at 22 °C.

<figure><img src="image_4.png" /></figure>
**Figure 1:** An illustration of the fabrication process for PSf-GO/vanillin composite membrane via in-situ cross-linking and blending.

**Table 2:** Chemical composition of casting solution used for fabrication of PSf-GO/vanillin composite membranes.

<table><thead><tr><td>Membrane</td><td>PSf (g)</td><td>NMP (mL)</td><td>Vanillin (g)</td><td>Graphite oxide (mg)</td></tr></thead><tbody><tr><td>M<sub>1</sub></td><td>3.2</td><td>16</td><td>0.8</td><td>0</td></tr><tr><td>M<sub>2</sub></td><td>3.2</td><td>16</td><td>0.8</td><td>50</td></tr><tr><td>M<sub>3</sub></td><td>3.2</td><td>16</td><td>0.8</td><td>100</td></tr><tr><td>M<sub>4</sub></td><td>3.2</td><td>16</td><td>0.8</td><td>150</td></tr><tr><td>M<sub>5</sub></td><td>3.2</td><td>16</td><td>0.8</td><td>200</td></tr></tbody></table>

<figure><img src="image_5.png" /></figure>

**Figure 2:** Schematic illustration for the liquid-liquid demixing for PSf-GO/vanillin composite membrane.

## 2.3. Physiochemical characterization

The obtained membranes were characterised by various analytical tools mentioned in **Table**

3. Water uptake (WU) capacity was determined using previously dried membrane strips of 2
160 × 2 cm² surface area. After recording dried membrane weight (W<sub>d</sub>), membrane strips were 161 then immersed 24 hours in DI water at 22 ± 2 °C. After carefully removing excess surface water 162 with absorbent paper, wet membranes were weighed (W<sub>w</sub>). Finally, WU was determined 163 using Equation 1.

$$164 \qquad WU = \frac{(W_w - W_d)}{W_d} \times 100 \qquad (1)$$

165 Ion exchange capacity (IEC) of PSf-GO/vanillin membranes was measured using the back 166 titration method. Previously dried and weighed membrane strips (2 × 2 cm²) were immersed 167 in 0.5 M HCl solution for 24 hours at 22 ± 2 °C. Then, the membrane strips were removed from 168 the HCl solution, and the residual HCl solution was titrated with 0.05 M NaOH solution. Finally, 169 IEC was measured using Equation 2, where C<sub>1</sub>, C<sub>2</sub>, and V<sub>1</sub>, V<sub>2</sub> are the concentration and volume 170 of HCl and NaOH, respectively.

$$171 \qquad IEC = \frac{C_1 V_1 - C_2 V_2}{W_d} \qquad (2)$$

<table><caption>172 **Table 3:** Various analytical tools used to characterise PSf-GO/vanillin composite membranes.</caption><thead><tr><th>Analytical tool</th><th>Characteristics studied</th></tr></thead><tbody><tr><td>Surface zeta (ζ) potential</td><td>To measure the surface charge of the membrane at interfaces</td></tr><tr><td>Fourier transform infrared (FT-IR)</td><td>For functional group and chemical structure analysis</td></tr><tr><td>Water contact angle (WCA)</td><td>For determining the wettability of the membrane's surface</td></tr><tr><td>Field emission scanning electron microscopy (FESEM)</td><td>To observe the surface morphological structures and cross-sectional area at the nanoscale</td></tr></tbody></table>

173

174 **2.4. Membrane porosity and pore radius**

175 The overall membrane porosity ε, which takes account of the various layers (i.e. skin and support 176 layers), was calculated from the weights of wet and dry membranes using Equation 3 [30]:
$$177 \quad \varepsilon = \frac{\frac{w_w - w_d}{\rho_w}}{\frac{w_w - w_d + w_d}{\rho_w \rho_p}} \quad (3)$$

178 The densities of water and polymer are denoted by $\rho_w$ and $\rho_p$ in (g/cm³), respectively.

179 The overall membrane pore radius ($r_m$) was estimated from pure water flux and porosity values with the Guerout-Elford-Ferry Equation 4 [31]:

$$181 \quad r_m = \sqrt{\frac{(2.9-1.75\varepsilon) 8 \eta L Q}{\varepsilon A \Delta P}} \quad (4)$$

182 The skin layer means pore radius was assessed by fitting salt rejection curves $R_{salt} = f(J_v)$ with a 183 usual transport model based on the coupling between Steric and Donnan exclusion at the 184 pore/solution interfaces and the extended Nernst-Planck equation (Equation 5) for transport 185 description. The various model equations were already presented in the literature [32, 33], and 186 the original methodology to estimate pore size from salt rejection was also detailed in a previous 187 study [17]. Briefly, it consists of adjusting the mean pore radius to fit experimental rejection of 188 $MgSO_4$, knowing the hydraulic permeability of the membrane is estimated from water flux 189 (considering Darcy's law) and membrane charge density from the zeta potential (considering the 190 Gouy-Chapman theory) [34, 35].

2.5. Pure water flux and salt rejection studies

192 Filtration experiments were performed using a Sterlitech Company (USA) HP4750 dead-end 193 filtration cell with a 0.00146 m² membrane surface area (A). The processing volume of the 194 filtration cell was 300 mL. To ensure a constant permeate state at 5 bar operating pressure, 195 prepared membranes were compacted for 60 mins prior to initial readings using DI water feed 196 solution. After compaction, pure water flux ($J_w$) was measured in litres per square meter per 197 hour (L/m2h) for two hours using Equation 5, where the volume of permeate (V) was 198 calculated in litres (L) and permeation time (t) in hours.

$$199 \quad J_w = \frac{V}{A \times t} \quad (5)$$
The salt rejection studies were performed using 2000 ppm MgSO₄ and NaCl solutions as feed for two hours. Every 10 minutes, the permeate rate was calculated using Equation 5. Three trials of each set of experiments were conducted, and the average value is reported. The concentrations of feed (Cf) and permeate (Cp) were measured by a conductivity metre (LAQUA PC210), and the rejection rate was calculated using Equation 6. To minimize concentration polarisation, a magnetic stirrer was used to mix feed solutions during the operation. Cf and Cp for BSA solution and landfill leachate wastewater were measured using UV-Visible spectrophotometer and inductively coupled plasma-mass spectrometry (ICP-MS).

$$ \text{Rejection (\%)} = \left( 1 - \frac{C_p}{C_f} \right) \times 100 \qquad (6) $$

## 2.6. Antifouling studies

The antifouling properties of the PSf-GO/vanillin composite membranes were investigated using above mentioned filtration cell. Antifouling studies were conducted using 200 mg/L of BSA feed solution and landfill leachate wastewater as feed solutions. All experiments were performed at a constant 5 bar pressure with 14.6 cm² active surface area. After measuring the water flux (J₁) of DI water for two hours, the BSA solution was the feed solution to measure the water flux (Jₚ) in the dead-end experiment. Then, the membranes were washed with DI water, and water flux decline (J₂) was measured using a DI water feed solution. Similar experiments were performed for landfill leachate wastewater. Reversible (Rr), irreversible (Rir), total fouling (Rt), and FRR were determined using Equations 7 to 10.

$$ R_r (\%) = \left( \frac{J_2 - J_p}{J_1} \right) \times 100 \qquad (7) $$

$$ R_{ir} (\%) = \left( \frac{J_1 - J_2}{J_1} \right) \times 100 \qquad (8) $$

$$ R_t (\%) = \left( 1 - \frac{J_p}{J_1} \right) \times 100 \qquad (9) $$

$$ FRR (\%) = \left( \frac{J_2}{J_1} \right) \times 100 \qquad (10) $$

# 3. Results

## 3.1. Functional group analysis
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FT-IR analysis was performed to study the functional groups of the prepared membranes. The spectrum shows the percentage of infrared radiation that passes through the sample versus a wavelength function related to covalent bonding. **Figure 3a** shows the FT-IR spectra for the pristine PSf, PSf-vanillin, and PSf/Go-vanillin membranes from 1000cm⁻¹ to 4000cm⁻¹. FT-IR spectra of PSf-vanillin and PSf/Go-vanillin membranes confirmed the existence of polar functional groups on the surface. In the FT-IR, each stretching and bending vibration occurs with a characteristic frequency. The stretching vibration of the hydroxyl (-OH) group for vanillin and GO, which is stronger for PSf/Go-vanillin membranes than PSf-vanillin membranes, is represented by the broad absorption and high-frequency area from 3024cm⁻¹ to 3695cm⁻¹ [17, 36]. This peak is noticeably absent in PSf membranes, as the PSf polymer lacks any carboxylic acid or hydroxyl groups [37]. Additionally, the increase in intensity from 3024cm⁻¹ to 3695cm⁻¹ indicates a strong intermolecular hydrogen bond between GO and vanillin. PSf-vanillin and PSf/GO-vanillin membranes showed a low transmittance C-H stretching peak at 2968cm⁻¹. Pristine GO has a peak around 1615cm⁻¹ that corresponds to the bending vibration of -OH groups, but the peak shifts to 1643cm⁻¹ with GO-vanillin [38]. The change in peak position could be due to intermolecular hydrogen bonding between vanillin and GO, as shown in **Figure 3b-d**. **Figure 3b-d** shows the intermolecular hydrogen bonding epoxy, carboxyl, and hydroxyl groups of GO and vanillin. The peaks at 1242cm⁻¹ and 1643cm⁻¹ represent C-O-C stretching and C=C skeletal vibration of the GO, respectively [39]. Compared to pristine PSf membranes, the change in frequency of peaks for PSf-vanillin and PSf/Go-vanillin membranes could be due to mixing and redistribution of energy states, yielding new energy levels because of intermolecular hydrogen bonding between PSf, vanillin, and GO, as shown in **Figure 3c**. All membranes showed peaks for base polymer i.e. PSf at 1103cm⁻¹ (S=O stretching), 1149cm⁻¹ (O-S-O symmetric stretching), 1489cm⁻¹, and 1581cm⁻¹ for aromatic ring stretching [4, 40, 41]. The noise from 1500cm⁻¹ to 1700cm⁻¹ might be because of strong π-π bond interactions from the aromatic rings of vanillin, GO, and PSf [4, 19].
<figure><img src="image_6.png" /><figcaption>**Figure 3:** a) FT-IR spectra for PSf, PSf-vanillin, and PSf/GO-vanillin membranes, b-d) intermolecular hydrogen bonding between GO and vanillin molecules, e) functional groups and chemical structure of vanillin and f) intermolecular hydrogen bonding between PSf, GO, and vanillin molecules.</figcaption></figure>

## 3.2. Physiochemical characterization

It is worth noting that the membrane's IEC and WU are intricately related, and thus a systematic study of this relationship is necessary. The term "IEC" refers to the total polar functional groups on the membrane surface, and it measures their ionic conductivity [42]. **Figure 4a** shows the IEC and WU for PSf/GO-vanillin membranes. A larger IEC would increase
the membrane permeability and selectivity [43]. As expected, the incorporation of high polar surface area vanillin (46.53 Å²) and hydrophilic GO increases the WU and IEC capacity of the prepared membranes [29]. With an increase in GO concentration, WU and IEC increases, and the M₄ membrane showed the highest WU and IEC of 125.4% and 3.23 mmol/g, respectively. When an excess GO was added to the casting solution, a decrease in WU and IEC was noticed for the M₅ membrane, probably, due to the agglomeration of GO flakes in the M₅ membrane, which prevents the polar functional groups from being accessible. The increase in WU and IEC for PSf/GO-vanillin membranes is justified as following:

a) The addition of polar functional groups (carboxyl (-COOH), hydroxyl (-OH), aldehyde (-CHO), and epoxy (-O-)) gives an additional negative charge to the membranes.

b) GO, and vanillin have the ability to bound water (H₂O) molecules via intermolecular hydrogen bonding.

c) Vanillin imparts porosity to the membranes, which enhances the water absorption on the membrane surface.

Indeed, an increase in IEC leads to an increase in WU, leading to a higher permeation rate. The increase in permeation rate is due to the increase in the diffusion rate of protons and hydroxide ions via the Grotthuss mechanism [44]. When polar functional groups exist on the membrane surface, protons are transferred by hydrogen bonding from one water (H₂O) molecule to the next [45]. Similarly, the high mobility of hydroxyl ions (OH⁻) in H₂O is due to the rapid transfer of H⁺ from H₂O to OH⁻ along with a series of intramolecular hydrogen-bonded H₂O molecules [4, 45, 46].

**Figure 4b** shows the WCA measurements for PSf/GO-vanillin membranes; an upsurge in wetting tendency decreases the membrane WCA and surface tension. The WCA provides important information about solid-liquid integration, membrane surface roughness, and hydrophilicity. The hydrophilicity of the membranes increases with increasing the GO concentration. The M₁ membrane exhibited the highest WCA of 61.4°, with adding more GO to the casting solution, WCA decreases to 56.03°, 55.63°, 53.23° and 50.31° for M₂, M₃, M₄, and M₅, respectively. It is worth noting that GO significantly reduces the WCA, as the pristine PSf membrane exhibits a WCA of over 70° [17, 47]. Additionally, a lower WCA results in higher surface energy and interfacial tension, creating a strong, attractive force that pulls the liquid molecules down. As a result, low WCA results in a high permeation rate.
Electrostatic interactions between charged ions in the feed solution and the membrane's skin layer are critical for understanding the rejection mechanism [48]. Figure 4c shows the surface zeta potential measurements for PSf/GO-vanillin membranes. As a result of carboxylic (-COOH), hydroxyl (-OH), and sulfonyl (-O=S=O-) groups from PSf, GO, and vanillin molecules, the fabricated membranes had a negative zeta potential [17]. The surface zeta potential increased with the increasing GO concentration from -20.52 mV to -25.07 mV for M₁ and M₄ membranes. However, due to the agglomeration of GO flakes, a slight decrease in surface zeta potential was observed for the M₅ membrane (-24.12 mV). This increase in negative zeta potential promotes salt rejection and inhibits foulant adsorption on the membrane surface [48, 49].

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /><figcaption>**Figure 4:** a) WU (water uptake) and IEC (ion exchange capacity) b) WCA (water contact angle), c) surface zeta potential measurements for PSf/GO-vanillin membranes, and d) schematic illustration for the membrane fabrication process.</figcaption></figure>

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### 3.3. Porosity and mean pore radius

The membrane thickness $L$, the overall porosity $\varepsilon$, the overall membrane pore radius $r_m$ and the mean pore radius of the skin layer $r_p$ obtained for the various membranes are summarized in Table 4.

<table><caption>**Table 4:** Structural properties for PSf/GO-vanillin composite membranes.</caption><thead><tr><th>Membrane</th><th>L (μm)</th><th>ε</th><th>r<sub>m</sub> (nm)</th><th>r<sub>p</sub> (nm)</th></tr></thead><tbody><tr><td>M<sub>1</sub></td><td>97</td><td>0.54</td><td>7.05</td><td>0.55</td></tr><tr><td>M<sub>2</sub></td><td>108</td><td>0.56</td><td>7.71</td><td>0.54</td></tr><tr><td>M<sub>3</sub></td><td>113</td><td>0.59</td><td>7.9</td><td>0.54</td></tr><tr><td>M<sub>4</sub></td><td>121</td><td>0.61</td><td>8.36</td><td>0.53</td></tr><tr><td>M<sub>5</sub></td><td>101</td><td>0.56</td><td>10.85</td><td>0.84</td></tr></tbody></table>

As shown in **Table 4**, the thickness and overall porosity of the prepared membranes increases as the GO content increases ($M_4 > M_3 > M_2 > M_1$), except for the highest GO content ($M_5$) for which values are lower. This seems to show that it exists a limit GO amount above which both thickness and porosity collapse.

The mean pore radius of membrane calculated by the Guerout-Elford-Ferry equation from overall porosities shows the same trend, i.e. radius increases with GO content. However, it should be emphasized that the mean pore radius of $M_5$ is markedly higher than that of $M_1$ to $M_4$ membranes. The limit value observed with thickness and porosity does not have a noticeable impact on the overall mean pore radius.

The skin layer properties were specifically studied from mean pore radii assessed by fitting $MgSO_4$ rejection values. The obtained values are typical of NF membranes. It can be concluded that the amount of GO does not have a considerable effect on the pore size of the skin layer (0.54 ± 0.01 nm) up to 150 mg. Indeed, the $M_5$ membrane exhibits a strongly higher pore radius,
327 which means that the limit value of GO amount observable on overall membrane properties
328 (mainly support layers) also impacts the skin layer. This strong increases in skin layer pore size
329 is confirmed by the strongly higher permeability obtained with this membrane in the section
330 devoted to performances (Section 3.5).

331 **3.4. Morphological analysis**

332 **Figure 5** shows the membrane morphology (top surface (a-e), a bottom surface (f-j), and
333 cross-section (k-o) for PSf/GO-vanillin membranes. The influence of GO and vanillin on
334 membrane morphology was investigated. Prepared membranes are integrally skinned
335 asymmetric with the dense top layer and microporous sublayer. The top-surface morphology
336 reveals that wrinkles are forming as GO loading increases. The wrinkled structure could be
337 caused by water molecules accumulating between the PSf polymeric chains and GO layers
338 (Figure 5b-e) [50]. These water molecules gradually drain and form wrinkles during the
339 membrane formation process. Instantaneous liquid-liquid demixing was observed for the M₁
340 membrane, whereas a slight delay in liquid-liquid demixing was observed after adding GO to
341 the casting solution (Figure 2). As a result, the M₁ membrane has finger-like cross-sectional
342 channels (Figure 5k), whereas M₂–M₅ membranes have a sponge-like structure, as evident by
343 FESEM analysis (Figure 5l-0). Additionally, the sponge-like structure could be a result of cross-
344 linking between GO and vanillin molecules. The solubility of vanillin in the solvent and non-
345 solvent used for membrane fabrication results in a porous network on the bottom surface of
346 the prepared membranes (Figure 5f-j).
<figure><img src="image_10.png" /><figcaption>**Figure 5:** Field emission scanning electron microscope (FESEM) images for PSf/GO-vanillin composite membranes.</figcaption></figure>

### 3.5. Performance studies

#### 3.5.1. Pure water flux
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The pure water flux (PWF) of PSf-vanillin and PSf/GO-vanillin composite membranes is shown in **Figure 6a**. The addition of GO to the PSf-vanillin casting solution increases the membrane's hydrophilicity, resulting in increased water molecule sorption on the membrane surface. The WU and WCA studies showed a similar pattern. The addition of GO improved the porous network and provided polar functional groups to the membrane surface, increasing PWF. A PWF was also observed as the top layer thickness decreased from $M_1$ to $M_5$ membranes. The PWF increased up to ≈ 91 LMH for the $M_5$ membrane. The PSf-vanillin ($M_1$) membrane had the lowest PWF, ranging from 39-35 LMH. The slight decrease in PWF across all membranes during operation could be due to membrane pore compression/collapse, resulting in reduced water passage. Furthermore, the PWF could decrease due to the gradual loss of free volume.

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### 3.5.2. Salt rejection studies

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The 2000 ppm NaCl and $MgSO_4$ aqueous solutions were used as feed solutions for salt rejection studies and passed perpendicularly through the membrane surface at constant 5 bar pressure. **Figures 6b and 6c** show water flux of PSf-vanillin and PSf/GO-vanillin membranes using NaCl and $MgSO_4$. Experimental results showed that permeate flux increased 2.5 fold from ≈ 35 to ≈ 88 LMH for the $M_1$ and $M_5$ membrane, respectively. The decline in permeate flux during operation could be ascribed to surface/pore adsorption due to electrostatic interaction between membrane and foulants. Also, permeate flux may be reduced due to salt molecule accumulation on the membrane surface, increasing resistance. However, continuous stirring was used to minimize concentration polarization effects throughout the experiments. Additionally, increasing the water flux stimulates salt molecule convection to the membrane, resulting in a decline in salt rejection of up to 61.04 ± 2.98 for $MgSO_4$ and 10.14 ± 2.97 for NaCl for the $M_5$ membrane. The $M_4$ membrane showed the highest rejection of 92.51 ± 2.73 and 25.43 ± 3.12 for $MgSO_4$ and NaCl, respectively. Compared to the $M_1$ (PSf-vanillin) membrane, $M_3$ and $M_4$ (PSf/GO-vanillin) membranes showed a 4% higher rejection for divalent ions, which could be due to the higher negative surface charge of $M_3$ and $M_4$ membranes due to the presence of GO moieties (**Figure 6d**). In contrast, the commercial UA 60 membrane showed the rejection rate of 77.13 ± 3.12 and 12.02 ± 2.94 for $MgSO_4$ and NaCl, respectively [17]. All membranes showed better rejection for divalent ions compared to the monovalent ions. The rejection results can be justified as follows:
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a) The PSf/GO-vanillin membrane charge and pore radius are vital in rejecting MgSO₄ and NaCl solutions. Thus, the rejection mechanism in PSf/GO-vanillin membranes is governed by both sieving and Donnan effects.

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b) The sulphate (SO<sup>-2</sup>) and chloride (Cl<sup>-</sup>) ions are better repelled by the negatively charged PSf/GO-vanillin membranes, and thus the bulk of these anions remains on the feed side.

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c) As the SO<sup>-2</sup> anion has a larger ionic radius (0.242 nm) than the Cl<sup>-</sup> anion (0.181), the prepared membranes showed better MgSO₄ rejection.

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<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

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**Figure 6:** a) Pure water flux, b) and c) permeate flux of 2000 ppm sodium chloride (NaCl) and magnesium sulphate (MgSO₄) solutions respectively, and d) rejection rate of NaCl and MgSO₄ with PSf/GO-vanillin composite membranes.

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### 3.6. Antifouling studies

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Membrane fouling by organic, inorganic and inert matters causes a sharp decline in the permeate flux. Technically, fouling is caused by physical and chemical interactions between the foulants and the membrane surface. As most foulants are non-polar or hydrophobic, preparing hydrophilic membranes is the most promising way to avoid fouling. Both vanillin
and GO are known for their hydrophilicity, anti-biofouling ability, and antibacterial properties [17, 51]. Antifouling properties of PSf/GO-vanillin membranes were evaluated using BSA and landfill leachate wastewater feed solutions. **Figure 7a** and **7d** show the permeation flux for BSA and landfill leachate wastewater using PSf/GO-vanillin membranes. As expected, the permeation rate decreased for all membranes when BSA and landfill leachate wastewater is the feed solutions. Compared to the PSf-vanillin $M_1$ membrane, the water flux of $M_2$ to $M_5$ PSf/GO-vanillin membranes had better antifouling properties, manifested by higher water flux recovery (**Figure 7**). For the BSA solution, the water flux decreased from 39.84 LMH to 23.87 LMH for $M_1$, 44.09 LMH to 31.76 LMH for $M_2$, 47.67 LMH, and 33.65 for $M_3$, 51.45 LMH for $M_4$ and 91.64 LMH to 75.43 LMH for the $M_5$ membrane. When landfill leachate wastewater was the feed solution, water flux dropped to 21.58, 29.85, 31.99, 34.56, and 72 LMH for $M_1$-$M_5$ membranes, respectively. In both cases, the water flux decreased due to foulant deposition on the membrane surface. Initially, foulants are deposited via convective deposition; later, foulants are chemically attached to the foulant layer [17]. BSA molecules form intermolecular disulphide linkages with the foulant layer [52]. Overall, the water flux decline was higher in the experiments with landfill feed solution. The higher water flux decline in landfill feed solution could be attributed to the interaction between organic and inorganic matters that exacerbated the fouling mechanism. $M_1$ membrane showed the highest $R_t$ (39.2% and 50 %) for BSA and landfill leachate wastewater, respectively (**Figure 7b and e**). In the $M_5$ membrane experiments using BSA and landfill leachate wastewater, the $R_t$ decrease was 14.9% and 19.98%, while the FRR was 93.57% and 90.32% (**Figure 7c and f**), respectively. Obviously, casting solution contains GO enhanced the antifouling properties of the fabricated membranes, and a higher water flux recovery was achieved for $M_2$-$M_5$ membranes. The $M_1$-$M_4$ membranes had a rejection rate of over 99% for BSA; however, the $M_5$ membrane had a lower rejection rate due to the porous network structure, as shown in **Figure 5j**. When landfill leachate wastewater was the feed solution, the rejection rate for divalent ions ($Mg^{+2}$ and $Ca^{+2}$) was higher than monovalent ions ($Na^+$ and $K^+$) (**Table 5**). Except for the $M_5$ membrane, all other prepared membranes showed 84 to 90% rejection rate of $Ca^{+2}$ and $Mg^{+2}$ ions and 20 to 27% rejection rate of $Na^+$ and $K^+$ anions. A similar trend was observed for the salt rejection studies (**Section 3.5.2**). The PSf<sub>16</sub>/GO<sub>0.15</sub>-vanillin<sub>0.8</sub> membrane showed better antifouling and salt rejection performance than the commercial UA60 nanofiltration membrane. The antifouling properties of prepared membranes are justified as following:
431

a) The addition of GO increases the hydrophilicity of the membranes, resulting in a reduction in the interaction between fouling matters and the membrane surface, thus reducing $R_t$, $R_r$, and $R_{ir}$.

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b) As $R_t$, $R_r$, and $R_{ir}$ are inversely proportional to water flux recovery. The prepared membranes demonstrated an increased FRR with increasing GO concentration.

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c) These findings suggest that the negatively charged PSf/GO-vanillin membranes inhibited the aggregation of negatively charged BSA and fouling matters on the surface. Consequently, fouling matters could be easily removed by DI water washing.

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Despite the higher permeation flux and antifouling properties of the $M_5$ membrane, the $M_4$ membrane demonstrated excellent rejection rates and water flux. $M_4$ membrane, therefore, is the best performance membrane when used for landfill leachate wastewater treatment compared to other membranes. For feed solution containing mainly divalent ions, the $M_4$ membrane could be an alternative to the $M_1$ membrane due to the superior water flux and antifouling property. In other words, the $M_4$ membrane could have potential applications for industrial wastewater treatment to remove heavy metals. These results indicated that PSf/GO-vanillin membranes had improved antifouling properties compared to previously reported PSf-vanillin membranes [17]. The use of vanillin results in an eco-friendly and cost-effective additive for the membrane fabrication process. Asymmetric membranes using GO and vanillin provide an easy, cost-effective and versatile approach for preparing highly permeable and fouling resistant membranes.

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<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /></figure>

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<caption>**Figure 7:** Flux measurements (a, d), fouling measurements (b, e) and flux recovery ratio (c, f) for BSA and landfill leachate wastewater, respectively, with PSf/GO-vanillin composite membranes.</caption>
<table><caption>**Table 5:** Concentration and rejection rate of different ions in landfill leachate wastewater with PSf/GO-vanillin membranes.</caption><thead><tr><th>Ion</th><th>Initial concentration (mg/L)</th><th>Rejection rate for M<sub>1</sub> (%)</th><th>Rejection rate for M<sub>2</sub> (%)</th><th>Rejection rate for M<sub>3</sub> (%)</th><th>Rejection rate for M<sub>4</sub> (%)</th><th>Rejection rate for M<sub>5</sub> (%)</th></tr></thead><tbody><tr><td>Sodium (Na<sup>+</sup>)</td><td>142.05 ± 5</td><td>21.12</td><td>22.42</td><td>23.56</td><td>23.52</td><td>8.2</td></tr><tr><td>Magnesium (Mg<sup>+2</sup>)</td><td>125.3 ± 5</td><td>84.87</td><td>88.21</td><td>89.75</td><td>90.12</td><td>60</td></tr><tr><td>Potassium (K<sup>+</sup>)</td><td>39.5 ± 5</td><td>23.4</td><td>25.45</td><td>26.95</td><td>27.02</td><td>10.2</td></tr><tr><td>Calcium (Ca<sup>+2</sup>)</td><td>65.4 ± 5</td><td>85.24</td><td>88.85</td><td>89.25</td><td>90.2</td><td>62.45</td></tr><tr><td>Total organic carbon (TOC)</td><td>120.5 ± 3.6</td><td>85.3</td><td>88.8</td><td>90.2</td><td>92.7</td><td>68.7</td></tr></tbody></table>

## 4. Conclusions

A fouling resistant and highly perm-selective PSf/GO-vanillin nanofiltration membrane was fabricated for wastewater purification and recovery. The 2D GO layers and vanillin help to retain the structural integrity of the PSf based mixed matrix membranes. The present study concludes with the following key points:

a) The addition of 2D GO layers improves the membranes wetting, rejection, and antifouling properties.

b) Combining GO and vanillin enhances the negative surface zeta potential for the prepared membranes, and as a result, a higher rejection rate was observed for divalent ions.

c) The PSf/vanillin ($M_1$) membrane had a PWF of 39 LMH, but adding GO to the casting solution increased the PWF of the $M_5$ membrane to 91 LMH.

d) The optimized PSf<sub>16</sub>/GO<sub>0.15</sub>-vanillin<sub>0.8</sub> membrane showed 92.5% rejection for MgSO<sub>4</sub> solution, nearly 29% higher than the commercial UA 60 membrane, which rejects up to 77.13%.

e) The prepared membrane demonstrated a significantly higher FRR of 93.57% and 90.32% for BSA and landfill leachate wastewater, respectively.
<figure><img src="image_21.png" /></figure>

f) The PSf/GO-vanillin membranes are potential candidates for robust and energy-efficient water treatment because of their high flux and fouling resistance.

<figure><img src="image_22.png" /></figure>

<figure><img src="image_23.png" /></figure>

<figure><img src="image_24.png" /></figure>

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1

Preparation of fouling resistant and highly perm-selective novel PSf/GO-vanillin

2

nanofiltration membrane for efficient water purification

3

Sudesh Yadav¹, Ibrar Ibrar¹, Akshaya K. Samal², Ali Altaee¹, #, Sébastien Déon³, John Zhou¹,

4

Noreddine Ghaffour⁴

5

1: Centre for Green Technology, School of Civil and Environmental Engineering, University of

6

Technology Sydney, 15 Broadway, NSW, 2007, Australia

7

2: Centre for Nano and Material Sciences, Jain University, Jain Global Campus, Ramanagara,

8

Bangalore - 562112, India

9

3: Institut UTINAM (UMR CNRS 6213), Université de Bourgogne-Franche-Comté, 16 Route de

10

Gray, 25030 Besançon Cedex, France

11

4: King Abdullah University of Science and Technology (KAUST), Water Desalination and Reuse

12

Center (WDRC), Biological and Environmental Science and Engineering (BESE), 23955-6900

13

Thuwal, Saudi Arabia

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15

\# Corresponding author email address: ali.altaee@uts.edu.au

16

17

Graphical abstract

18

<figure><img src="image_69.png" /></figure>
20 Abstract

To meet the rising global demand for water, it is necessary to develop membranes capable of efficiently purifying contaminated water sources. Herein, we report a series of novel polysulfone (PSf)/GO-vanillin nanofiltration membranes highly permeable, selective, and fouling resistant. The membranes are composed of two-dimensional (2D) graphite oxide (GO) layers embedded with vanillin as porogen and PSf as the base polymer. There is a growing interest in addressing the synergistic effect of GO and vanillin on improving the permeability and antifouling characteristics of membranes. Various spectroscopic and microscopic techniques were used to perform detailed physicochemical and morphological analyses. The optimized PSf~16~/GO~0.15~-vanillin~0.8~ membrane demonstrated 92.5% and 25.4% rejection rate for 2000 ppm magnesium sulphate (MgSO~4~) and sodium chloride (NaCl) solutions respectively. Antifouling results showed over 99% rejection for BSA and 93.57% flux recovery ratio (FRR). Experimental work evaluated the antifouling characteristics of prepared membranes to treat landfill leachate wastewater. The results showed 84-90% rejection for magnesium (Mg~+2~) and calcium (Ca~+2~) with 90.32 FRR. The study experimentally demonstrated that adding GO and vanillin to the polymeric matrix significantly improves fouling resistance and membrane performance. Future research will focus on molecular sieving for industrial separations and other niche applications using mixed matrix membranes.

**Keywords:** PSf/GO-vanillin membrane; mixed matrix membrane; nanofiltration; fouling resistant; and salt rejection

40 1. Introduction

Water scarcity and pollution require advanced water purification technologies to produce potable water from non-common wastewater sources [1, 2]. Hazardous wastewaters, such as industrial and landfill leachate wastewaters, pose a threat to the ecosystem when discharged without proper treatment due to the high concentration of dissolved ions, heavy metals, pesticides and organic matters [3]. Despite the strict environmental regulations, deliberate and accidental waste release are on the rise [3, 4]. Conventional primary and secondary treatments have shown limited efficiency towards emerging pollutants. Tertiary treatments such as adsorption, photocatalysis, and membrane-based processes have been proven successful [5]. While membrane processes have the advantage over traditional treatment
technologies, they currently experience setbacks represented by fouling and membrane
permselectivity [5, 6].

At present, polyamide-based thin-film composite (TFC) membranes occupy the highest portion in reverse osmosis (RO) and nanofiltration (NF) membranes technology for desalination and wastewater treatment [7]. Still, TFC membranes have some shortcomings, including low permeability, hydrophobicity, and fouling tendency [8]. NF membranes effectively remove hazardous heavy metal ions and organic substances from solutions [9, 10], but they are prone to fouling and exhibit low rejection of monovalent salts [11]. Thus, efforts are made in fabricating novel NF membranes to overcome fouling by introducing diverse nano-fillers, such as silver (Ag) [12], titanium oxide (TiO₂) [13], metal-organic frameworks (MOFs) [14], graphene quantum dots [15], molybdenum disulfide (MoS₂) [16], and other organic additives [17]. Although previous research has demonstrated the effectiveness of these materials to tune the free volume characteristics of polymers, boundary defects between polymers and nanomaterials are a challenging problem [18]. Further, many studies have failed in providing information on the interfacial compatibility and gradual detachment of nanomaterials over extended periods [19].

The development of specific molecular separation membranes is gaining considerable attention. As a result, new research studies are focused on the fabrication of membranes with specific molecular weight cut-offs by altering the surface characteristics, pore size distribution, and changing solute diffusion parameters. In general, artificial grafting, surface modification, chemical cross-linking, and in-situ physical blending are suggested for enhancing membrane surface properties such as wettability, membrane fouling, pore diameter, and surface zeta potential [20, 21]. Among the techniques mentioned above, physical blending is the most effective strategy due to its suitability for large-scale production. For instance, Yuan et al. prepared polyaniline (PANI)/polysulfone (PSf) membrane with better water permeability and resistance to fouling [22]. For bovine serum albumin (BSA), humic acid (HA), and sodium alginate (SA), the flux recovery ratio (FRR) was up to 65.3 %, 67.9 %, and 70.1 %, respectively [22]. In another study, PANI-graphene oxide nanofillers were used to prepare polyvinylidene fluoride (PVDF) composite membrane for removing Allura red (AR) and methyl orange (MO) dye from textile effluents [23]. PANI-GO nanofillers reduced the water contact angle (WCA) to 56.11° and increased the pure water flux (PWF) from 112 to 454
LMH. With 95 % rejection of MO dye and 98 % of AR dye, the FRR reached 94 % [23]. Recently Shu et al. used micron-sized 2-dimensional MOF (BUT-203) nanosheets to prepare mixed matrix membrane for dye desalination rejection [24]. The findings suggested that chemical cross-linking between polymers and nanofillers improves the membrane performance, whereas dense separation layer and filler-loading limits the membrane performance [23, 24]. Pandey et al. developed fouling resistant MXene ($Ti_3C_2T_x$/cellulose acetate (CA) covalently cross-linked membrane, which showed PWF of 256.85 L m⁻² h⁻¹ bar⁻¹ and over 92% rejection for rhodamine B (RhB) dye [25]. The 10% $@Ti_3C_2T_x$/CA membrane demonstrated outstanding antibacterial properties against E. coli and B. subtilis bacteria [25]. They achieved an ideal separation by optimizing the filler-loading and adjusting the membrane pore size. However, current NF membranes have poor antifouling properties and low water flux, which increases energy consumption, making them unsuitable for treating complex wastewaters such as landfill leachate. Thus, it is important to develop novel antifouling and high perm-selective NF membranes for water and wastewater treatment.

Graphite oxide (GO) is a promising material widely used in membrane fabrication as a nanofiller to improve water flux and reduce membrane biofouling due to its chlorine tolerance and antimicrobial properties [26]. For instance, Ganesh et al. developed a mixed matrix membrane with GO for salt rejection [27]. At 4 bar pressure, the prepared 2000 ppm GO/PSf membrane showed near 48 LMH water flux and 74 % sodium sulfate ($Na_2SO_4$) rejection [27]. Wu et al. prepared hybrid silica nanoparticles ($SiO_2$)-GO/PSf membranes and achieved 98% rejection for egg albumin [28]. The prepared membranes showed 72% FRR with enhanced antifouling properties [28]. In our previous work, an antifouling PSf/vanillin composite membrane was prepared and achieved 99% rejection for BSA with 88.55% FRR [17]. Vanillin imparts a negative surface charge and hydrophilic properties to the membrane due to high polar surface area (46.53 Å²) and functionalized phenolic ring [17, 29]. The current work investigates the synergistic effect of 2D GO nanolayers and vanillin on membrane permeability and selectivity using PSf as the base polymer. To the best of the authors' knowledge, no study has reported GO-vanillin membrane performance for wastewater treatment. The study presented an experimental work on a novel PSf/GO-vanillin membrane for landfill wastewater treatment. The current study proposed cross-linked GO/vanillin to enhance the wettability and fouling resistance of hydrophobic PSf membranes. The
antifouling and hydrophilicity properties of vanillin and GO will impart special features to the fabricated membrane for landfill wastewater treatment. Antifouling studies were performed using landfill leachate wastewater and BSA (200mg/L) solution. Magnesium sulphate (MgSO₄) and sodium chloride (NaCl) were used as model salts for salt rejection studies.

## 2. Materials

### 2.1. Chemicals

In this study, all chemicals are of analytical grade and have not been further purified. N-Methyl-2-pyrrolidone (NMP), PSf (≈ 35kDa), and BSA (Agarose Electrophoresis: > 98%; Nitrogen: 14.5-16.5% and pH (1% in 0.15 M NaCl): 6.5-7.5) were obtained from Sigma-Aldrich Australia. GO produced by Hummer's method (SKU: GTOP0003; brownish-yellow powder;~99% purity; 0.5-5 µm lateral size; 1-3 nm thickness; and ≤46.0 wt.% oxygen content) was purchased from Advanced Chemicals Supplier (ACS) Material, LLC., CA 91106, USA. Hydrochloric acid (HCl) was purchased from Merck. Sodium hydroxide (NaOH), NaCl, and MgSO₄ used in the rejection test were procured from Chem-supply, Australia. Throughout the experiments, deionized (DI) water (Milli-Q®, Merck) was used in the membrane fabrication process. DI water was used to prepare the standard NaCl, MgSO₄, HCl, NaOH, and BSA aqueous solutions for filtration tests. Biologically treated landfill leachate wastewater was collected from Hurstville Golf Course, NSW, Australia (Table 1).

<table><caption>**Table 1:** Characteristics of landfill leachate wastewater used for this study.</caption><thead><tr><td>Characteristic</td><td>Value</td></tr></thead><tbody><tr><td>Appearance</td><td>Dark Yellowish-brown colour</td></tr><tr><td>pH<sup>a</sup></td><td>7.5 ± 0.3</td></tr><tr><td>Ammonia, mg/L</td><td>≤ 0.5</td></tr><tr><td>Total suspended solids, mg/L</td><td>27-117</td></tr><tr><td>Total dissolved solids (TDS)<sup>a</sup>, mg/L</td><td>3300</td></tr><tr><td>Total organic carbon (TOC)<sup>b</sup>, mg/L</td><td>120.5 ± 3.6</td></tr><tr><td>Total Iron, mg/L</td><td>3.5-5.2</td></tr><tr><td>Sodium (Na)<sup>c</sup>, mg/L</td><td>142.05</td></tr><tr><td>Magnesium (Mg)<sup>c</sup>, mg/L</td><td>125.3</td></tr><tr><td>Potassium (K)<sup>c</sup>, mg/L</td><td>39.5</td></tr></tbody></table>
a: measured using LAQUA PC210 conductivity metre; b: measured using analytikjena TOC; and c: measured using Agilent Technologies 7900 ICP-MS.

## 2.2. Membrane fabrication

Non-solvent induced phase separation technique was applied for fabricating PSf-GO/vanillin composite membranes, using N-Methyl-2-pyrrolidone (NMP) solvent, PSf base polymer, DI water non-solvent, and vanillin as a pore-forming agent. Initially, a measured amount of GO was exfoliated in NMP using Powersonic, Digital Ultrasonic Bath for an hour. The optimized vanillin concentration, 0.8 g, was dissolved in GO-NMP dispersion for an hour using a magnetic stirrer [17]. Then, PSf was gradually added to the GO-vanillin-NMP solution at 60 °C. To obtain a homogeneous casting solution, the casting solution was stirred for 24 hours, and an ultrasonication bath was used to release the trapped air bubbles. A glass plate was used to case the prepared solution, using a stainless steel casting knife of 200 µm thickness before transferring to a DI water coagulation bath (Figure 1). Finally, the PSf/GO-vanillin composite membranes were transferred to a freshly prepared coagulation bath. **The liquid-liquid demixing for PSf-GO/vanillin membranes is shown in Figure 2.** The membranes left there for 24 hours to ensure that the NMP solvent was completely removed. The chemical composition of obtained membranes is tabulated in **Table 2**. The membrane fabrication process was performed under ambient conditions of ~52% humidity and temperature at 22 °C.

<figure><img src="image_70.png" /></figure>
**Figure 1:** An illustration of the fabrication process for PSf-GO/vanillin composite membrane via in-situ cross-linking and blending.

**Table 2:** Chemical composition of casting solution used for fabrication of PSf-GO/vanillin composite membranes.

<table><thead><tr><th>Membrane</th><th>PSf (g)</th><th>NMP (mL)</th><th>Vanillin (g)</th><th>Graphite oxide (mg)</th></tr></thead><tbody><tr><td>M<sub>1</sub></td><td>3.2</td><td>16</td><td>0.8</td><td>0</td></tr><tr><td>M<sub>2</sub></td><td>3.2</td><td>16</td><td>0.8</td><td>50</td></tr><tr><td>M<sub>3</sub></td><td>3.2</td><td>16</td><td>0.8</td><td>100</td></tr><tr><td>M<sub>4</sub></td><td>3.2</td><td>16</td><td>0.8</td><td>150</td></tr><tr><td>M<sub>5</sub></td><td>3.2</td><td>16</td><td>0.8</td><td>200</td></tr></tbody></table>

<figure><img src="image_71.png" /></figure>

**Figure 2:** Schematic illustration for the liquid-liquid demixing for PSf-GO/vanillin composite membrane.

## 2.3. Physiochemical characterization

The obtained membranes were characterised by various analytical tools mentioned in **Table 3**. Water uptake (WU) capacity was determined using previously dried membrane strips of 2
161 × 2 cm² surface area. After recording dried membrane weight (W<sub>d</sub>), membrane strips were 162 then immersed 24 hours in DI water at 22 ± 2 °C. After carefully removing excess surface water 163 with absorbent paper, wet membranes were weighed (W<sub>w</sub>). Finally, WU was determined 164 using Equation 1.

$$165 \qquad WU = \frac{(W_w - W_d)}{W_d} \times 100 \qquad (1)$$

166 Ion exchange capacity (IEC) of PSf-GO/vanillin membranes was measured using the back 167 titration method. Previously dried and weighed membrane strips (2 × 2 cm²) were immersed 168 in 0.5 M HCl solution for 24 hours at 22 ± 2 °C. Then, the membrane strips were removed from 169 the HCl solution, and the residual HCl solution was titrated with 0.05 M NaOH solution. Finally, 170 IEC was measured using Equation 2, where C₁, C₂, and V₁, V₂ are the concentration and volume 171 of HCl and NaOH, respectively.

$$172 \qquad IEC = \frac{C_1 V_1 - C_2 V_2}{W_d} \qquad (2)$$

<table><caption>173 **Table 3:** Various analytical tools used to characterise PSf-GO/vanillin composite membranes.</caption><thead><tr><td>Analytical tool</td><td>Characteristics studied</td></tr></thead><tbody><tr><td>Surface zeta (ζ) potential</td><td>To measure the surface charge of the membrane at interfaces</td></tr><tr><td>Fourier transform infrared (FT-IR)</td><td>For functional group and chemical structure analysis</td></tr><tr><td>Water contact angle (WCA)</td><td>For determining the wettability of the membrane's surface</td></tr><tr><td>Field emission scanning electron microscopy (FESEM)</td><td>To observe the surface morphological structures and cross-sectional area at the nanoscale</td></tr></tbody></table>

174

175 **2.4. Membrane porosity and pore radius**

176 The overall membrane porosity ε, which takes account of the various layers (i.e. skin and support 177 layers), was calculated from the weights of wet and dry membranes using Equation 3 [30]:
$$178 \qquad \epsilon = \frac{\frac{w_w - w_d}{\rho_w}}{\frac{w_w - w_d + w_d}{\rho_w \rho_p}} \qquad (3)$$

179 The densities of water and polymer are denoted by $\rho_w$ and $\rho_p$ in (g/cm³), respectively.

180 The overall membrane pore radius ($r_m$) was estimated from pure water flux and porosity values with the Guerout-Elford-Ferry Equation 4 [31]:

$$182 \qquad r_m = \sqrt{\frac{(2.9-1.75\epsilon) 8 \eta L Q}{\epsilon A \Delta P}} \qquad (4)$$

183 The skin layer means pore radius was assessed by fitting salt rejection curves $R_{salt} = f(J_v)$ with a 184 usual transport model based on the coupling between Steric and Donnan exclusion at the 185 pore/solution interfaces and the extended Nernst-Planck equation (Equation 5) for transport 186 description. The various model equations were already presented in the literature [32, 33], and 187 the original methodology to estimate pore size from salt rejection was also detailed in a previous 188 study [17]. Briefly, it consists of adjusting the mean pore radius to fit experimental rejection of 189 $MgSO_4$, knowing the hydraulic permeability of the membrane is estimated from water flux 190 (considering Darcy's law) and membrane charge density from the zeta potential (considering the 191 Gouy-Chapman theory) [34, 35].

192 **2.5. Pure water flux and salt rejection studies**

193 Filtration experiments were performed using a Sterlitech Company (USA) HP4750 dead-end 194 filtration cell with a 0.00146 m² membrane surface area (A). The processing volume of the 195 filtration cell was 300 mL. To ensure a constant permeate state at 5 bar operating pressure, 196 prepared membranes were compacted for 60 mins prior to initial readings using DI water feed 197 solution. After compaction, pure water flux ($J_w$) was measured in litres per square meter per 198 hour (L/m2h) for two hours using Equation 5, where the volume of permeate (V) was 199 calculated in litres (L) and permeation time (t) in hours.

$$200 \qquad J_w = \frac{V}{A \times t} \qquad (5)$$
The salt rejection studies were performed using 2000 ppm MgSO₄ and NaCl solutions as feed for two hours. Every 10 minutes, the permeate rate was calculated using Equation 5. Three trials of each set of experiments were conducted, and the average value is reported. The concentrations of feed (Cf) and permeate (Cp) were measured by a conductivity metre (LAQUA PC210), and the rejection rate was calculated using Equation 6. To minimize concentration polarisation, a magnetic stirrer was used to mix feed solutions during the operation. Cf and Cp for BSA solution and landfill leachate wastewater were measured using UV-Visible spectrophotometer and inductively coupled plasma-mass spectrometry (ICP-MS).

$$209 \quad Rejection (\%) = \left(1 - \frac{C_p}{C_f}\right) \times 100 \qquad (6)$$

## 2.6. Antifouling studies

The antifouling properties of the PSf-GO/vanillin composite membranes were investigated using above mentioned filtration cell. Antifouling studies were conducted using 200 mg/L of BSA feed solution and landfill leachate wastewater as feed solutions. All experiments were performed at a constant 5 bar pressure with 14.6 cm² active surface area. After measuring the water flux (J₁) of DI water for two hours, the BSA solution was the feed solution to measure the water flux (Jₚ) in the dead-end experiment. Then, the membranes were washed with DI water, and water flux decline (J₂) was measured using a DI water feed solution. Similar experiments were performed for landfill leachate wastewater. Reversible (Rr), irreversible (Rir), total fouling (Rt), and FRR were determined using Equations 7 to 10.

$$220 \quad R_r (\%) = \left( \frac{J_2 - J_p}{J_1} \right) \times 100 \qquad (7)$$

$$221 \quad R_{ir} (\%) = \left( \frac{J_1 - J_2}{J_1} \right) \times 100 \qquad (8)$$

$$222 \quad R_t (\%) = \left(1 - \frac{J_p}{J_1}\right) \times 100 \qquad (9)$$

$$223 \quad FRR (\%) = \left(\frac{J_2}{J_1}\right) \times 100 \qquad (10)$$

# 3. Results

## 3.1. Functional group analysis
FT-IR analysis was performed to study the functional groups of the prepared membranes. The spectrum shows the percentage of infrared radiation that passes through the sample versus a wavelength function related to covalent bonding. **Figure 3a** shows the FT-IR spectra for the pristine PSf, PSf-vanillin, and PSf/Go-vanillin membranes from 1000cm⁻¹ to 4000cm⁻¹. FT-IR spectra of PSf-vanillin and PSf/Go-vanillin membranes confirmed the existence of polar functional groups on the surface. In the FT-IR, each stretching and bending vibration occurs with a characteristic frequency. The stretching vibration of the hydroxyl (-OH) group for vanillin and GO, which is stronger for PSf/Go-vanillin membranes than PSf-vanillin membranes, is represented by the broad absorption and high-frequency area from 3024cm⁻¹ to 3695cm⁻¹ [17, 36]. This peak is noticeably absent in PSf membranes, as the PSf polymer lacks any carboxylic acid or hydroxyl groups [37]. Additionally, the increase in intensity from 3024cm⁻¹ to 3695cm⁻¹ indicates a strong intermolecular hydrogen bond between GO and vanillin. PSf-vanillin and PSf/GO-vanillin membranes showed a low transmittance C-H stretching peak at 2968cm⁻¹. Pristine GO has a peak around 1615cm⁻¹ that corresponds to the bending vibration of -OH groups, but the peak shifts to 1643cm⁻¹ with GO-vanillin [38]. The change in peak position could be due to intermolecular hydrogen bonding between vanillin and GO, as shown in **Figure 3b-d**. **Figure 3b-d** shows the intermolecular hydrogen bonding epoxy, carboxyl, and hydroxyl groups of GO and vanillin. The peaks at 1242cm⁻¹ and 1643cm⁻¹ represent C-O-C stretching and C=C skeletal vibration of the GO, respectively [39]. Compared to pristine PSf membranes, the change in frequency of peaks for PSf-vanillin and PSf/Go-vanillin membranes could be due to mixing and redistribution of energy states, yielding new energy levels because of intermolecular hydrogen bonding between PSf, vanillin, and GO, as shown in **Figure 3c**. All membranes showed peaks for base polymer i.e. PSf at 1103cm⁻¹ (S=O stretching), 1149cm⁻¹ (O-S-O symmetric stretching), 1489cm⁻¹, and 1581cm⁻¹ for aromatic ring stretching [4, 40, 41]. The noise from 1500cm⁻¹ to 1700cm⁻¹ might be because of strong π-π bond interactions from the aromatic rings of vanillin, GO, and PSf [4, 19].
<figure><img src="image_72.png" /><figcaption>**Figure 3:** a) FT-IR spectra for PSf, PSf-vanillin, and PSf/GO-vanillin membranes, b-d) intermolecular hydrogen bonding between GO and vanillin molecules, e) **functional groups and chemical structure of vanillin** and f) intermolecular hydrogen bonding between PSf, GO, and vanillin molecules.</figcaption></figure>

## 3.2. Physiochemical characterization

It is worth noting that the membrane's IEC and WU are intricately related, and thus a systematic study of this relationship is necessary. The term "IEC" refers to the total polar functional groups on the membrane surface, and it measures their ionic conductivity [42]. **Figure 4a** shows the IEC and WU for PSf/GO-vanillin membranes. A larger IEC would increase
263

the membrane permeability and selectivity [43]. As expected, the incorporation of high polar surface area vanillin (46.53 Å²) and hydrophilic GO increases the WU and IEC capacity of the prepared membranes [29]. With an increase in GO concentration, WU and IEC increases, and the M₄ membrane showed the highest WU and IEC of 125.4% and 3.23 mmol/g, respectively.

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When an excess GO was added to the casting solution, a decrease in WU and IEC was noticed for the M₅ membrane, probably, due to the agglomeration of GO flakes in the M₅ membrane, which prevents the polar functional groups from being accessible. The increase in WU and IEC for PSf/GO-vanillin membranes is justified as following:

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a) The addition of polar functional groups (carboxyl (-COOH), hydroxyl (-OH), aldehyde (-CHO), and epoxy (-O-)) gives an additional negative charge to the membranes.

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b) GO, and vanillin have the ability to bound water (H₂O) molecules via intermolecular hydrogen bonding.

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c) Vanillin imparts porosity to the membranes, which enhances the water absorption on the membrane surface.

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Indeed, an increase in IEC leads to an increase in WU, leading to a higher permeation rate. The increase in permeation rate is due to the increase in the diffusion rate of protons and hydroxide ions via the Grotthuss mechanism [44]. When polar functional groups exist on the membrane surface, protons are transferred by hydrogen bonding from one water (H₂O) molecule to the next [45]. Similarly, the high mobility of hydroxyl ions (OH⁻) in H₂O is due to the rapid transfer of H⁺ from H₂O to OH⁻ along with a series of intramolecular hydrogen-bonded H₂O molecules [4, 45, 46].

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**Figure 4b** shows the WCA measurements for PSf/GO-vanillin membranes; an upsurge in wetting tendency decreases the membrane WCA and surface tension. The WCA provides important information about solid-liquid integration, membrane surface roughness, and hydrophilicity. The hydrophilicity of the membranes increases with increasing the GO concentration. The M₁ membrane exhibited the highest WCA of 61.4°, with adding more GO to the casting solution, WCA decreases to 56.03°, 55.63°, 53.23° and 50.31° for M₂, M₃, M₄, and M₅, respectively. It is worth noting that GO significantly reduces the WCA, as the pristine PSf membrane exhibits a WCA of over 70° [17, 47]. Additionally, a lower WCA results in higher surface energy and interfacial tension, creating a strong, attractive force that pulls the liquid molecules down. As a result, low WCA results in a high permeation rate.

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Electrostatic interactions between charged ions in the feed solution and the membrane's skin layer are critical for understanding the rejection mechanism [48]. Figure 4c shows the surface zeta potential measurements for PSf/GO-vanillin membranes. As a result of carboxylic (-COOH), hydroxyl (-OH), and sulfonyl (-O=S=O-) groups from PSf, GO, and vanillin molecules, the fabricated membranes had a negative zeta potential [17]. The surface zeta potential increased with the increasing GO concentration from -20.52 mV to -25.07 mV for M₁ and M₄ membranes. However, due to the agglomeration of GO flakes, a slight decrease in surface zeta potential was observed for the M₅ membrane (-24.12 mV). This increase in negative zeta potential promotes salt rejection and inhibits foulant adsorption on the membrane surface [48, 49].

<figure><img src="image_73.png" /></figure>

<figure><img src="image_74.png" /></figure>

<figure><img src="image_75.png" /><figcaption>**Figure 4:** a) WU (water uptake) and IEC (ion exchange capacity) b) WCA (water contact angle), c) surface zeta potential measurements for PSf/GO-vanillin membranes, and d) schematic illustration for the membrane fabrication process.</figcaption></figure>

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308
### 3.3. Porosity and mean pore radius

The membrane thickness $L$, the overall porosity $\varepsilon$, the overall membrane pore radius $r_m$ and the mean pore radius of the skin layer $r_p$ obtained for the various membranes are summarized in Table 4.

<table><caption>**Table 4:** Structural properties for PSf/GO-vanillin composite membranes.</caption><thead><tr><th>Membrane</th><th>L (μm)</th><th>ε</th><th>r<sub>m</sub> (nm)</th><th>r<sub>p</sub> (nm)</th></tr></thead><tbody><tr><td>M<sub>1</sub></td><td>97</td><td>0.54</td><td>7.05</td><td>0.55</td></tr><tr><td>M<sub>2</sub></td><td>108</td><td>0.56</td><td>7.71</td><td>0.54</td></tr><tr><td>M<sub>3</sub></td><td>113</td><td>0.59</td><td>7.9</td><td>0.54</td></tr><tr><td>M<sub>4</sub></td><td>121</td><td>0.61</td><td>8.36</td><td>0.53</td></tr><tr><td>M<sub>5</sub></td><td>101</td><td>0.56</td><td>10.85</td><td>0.84</td></tr></tbody></table>

As shown in **Table 4**, the thickness and overall porosity of the prepared membranes increases as the GO content increases ($M_4 > M_3 > M_2 > M_1$), except for the highest GO content ($M_5$) for which values are lower. This seems to show that it exists a limit GO amount above which both thickness and porosity collapse.

The mean pore radius of membrane calculated by the Guerout-Elford-Ferry equation from overall porosities shows the same trend, i.e. radius increases with GO content. However, it should be emphasized that the mean pore radius of $M_5$ is markedly higher than that of $M_1$ to $M_4$ membranes. The limit value observed with thickness and porosity does not have a noticeable impact on the overall mean pore radius.

The skin layer properties were specifically studied from mean pore radii assessed by fitting $MgSO_4$ rejection values. The obtained values are typical of NF membranes. It can be concluded that the amount of GO does not have a considerable effect on the pore size of the skin layer (0.54 ± 0.01 nm) up to 150 mg. Indeed, the $M_5$ membrane exhibits a strongly higher pore radius,
328 which means that the limit value of GO amount observable on overall membrane properties
329 (mainly support layers) also impacts the skin layer. This strong increases in skin layer pore size
330 is confirmed by the strongly higher permeability obtained with this membrane in the section
331 devoted to performances (Section 3.5).

332 **3.4. Morphological analysis**

333 **Figure 5** shows the membrane morphology (top surface (a-e), a bottom surface (f-j), and
334 cross-section (k-o) for PSf/GO-vanillin membranes. The influence of GO and vanillin on
335 membrane morphology was investigated. Prepared membranes are integrally skinned
336 asymmetric with the dense top layer and microporous sublayer. The top-surface morphology
337 reveals that wrinkles are forming as GO loading increases. The wrinkled structure could be
338 caused by water molecules accumulating between the PSf polymeric chains and GO layers
339 (Figure 5b-e) [50]. These water molecules gradually drain and form wrinkles during the
340 membrane formation process. Instantaneous liquid-liquid demixing was observed for the M₁
341 membrane, whereas a slight delay in liquid-liquid demixing was observed after adding GO to
342 the casting solution (Figure 2). As a result, the M₁ membrane has finger-like cross-sectional
343 channels (Figure 5k), whereas M₂–M₅ membranes have a sponge-like structure, as evident by
344 FESEM analysis (Figure 5l-0). Additionally, the sponge-like structure could be a result of cross-
345 linking between GO and vanillin molecules. The solubility of vanillin in the solvent and non-
346 solvent used for membrane fabrication results in a porous network on the bottom surface of
347 the prepared membranes (Figure 5f-j).
<figure><img src="image_76.png" /><figcaption>**Figure 5:** Field emission scanning electron microscope (FESEM) images for PSf/GO-vanillin composite membranes.</figcaption></figure>

### 3.5. Performance studies

#### 3.5.1. Pure water flux
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The pure water flux (PWF) of PSf-vanillin and PSf/GO-vanillin composite membranes is shown in **Figure 6a**. The addition of GO to the PSf-vanillin casting solution increases the membrane's hydrophilicity, resulting in increased water molecule sorption on the membrane surface. The WU and WCA studies showed a similar pattern. The addition of GO improved the porous network and provided polar functional groups to the membrane surface, increasing PWF. A PWF was also observed as the top layer thickness decreased from $M_1$ to $M_5$ membranes. The PWF increased up to ≈ 91 LMH for the $M_5$ membrane. The PSf-vanillin ($M_1$) membrane had the lowest PWF, ranging from 39-35 LMH. The slight decrease in PWF across all membranes during operation could be due to membrane pore compression/collapse, resulting in reduced water passage. Furthermore, the PWF could decrease due to the gradual loss of free volume.

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### 3.5.2. Salt rejection studies

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The 2000 ppm NaCl and $MgSO_4$ aqueous solutions were used as feed solutions for salt rejection studies and passed perpendicularly through the membrane surface at constant 5 bar pressure. **Figures 6b and 6c** show water flux of PSf-vanillin and PSf/GO-vanillin membranes using NaCl and $MgSO_4$. Experimental results showed that permeate flux increased 2.5 fold from ≈ 35 to ≈ 88 LMH for the $M_1$ and $M_5$ membrane, respectively. The decline in permeate flux during operation could be ascribed to surface/pore adsorption due to electrostatic interaction between membrane and foulants. Also, permeate flux may be reduced due to salt molecule accumulation on the membrane surface, increasing resistance. However, continuous stirring was used to minimize concentration polarization effects throughout the experiments. Additionally, increasing the water flux stimulates salt molecule convection to the membrane, resulting in a decline in salt rejection of up to 61.04 ± 2.98 for $MgSO_4$ and 10.14 ± 2.97 for NaCl for the $M_5$ membrane. The $M_4$ membrane showed the highest rejection of 92.51 ± 2.73 and 25.43 ± 3.12 for $MgSO_4$ and NaCl, respectively. Compared to the $M_1$ (PSf-vanillin) membrane, $M_3$ and $M_4$ (PSf/GO-vanillin) membranes showed a 4% higher rejection for divalent ions, which could be due to the higher negative surface charge of $M_3$ and $M_4$ membranes due to the presence of GO moieties (**Figure 6d**). In contrast, the commercial UA 60 membrane showed the rejection rate of 77.13 ± 3.12 and 12.02 ± 2.94 for $MgSO_4$ and NaCl, respectively [17]. All membranes showed better rejection for divalent ions compared to the monovalent ions. The rejection results can be justified as follows:
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a) The PSf/GO-vanillin membrane charge and pore radius are vital in rejecting MgSO₄ and NaCl solutions. Thus, the rejection mechanism in PSf/GO-vanillin membranes is governed by both sieving and Donnan effects.

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b) The sulphate (SO<sup>-2</sup>) and chloride (Cl<sup>-</sup>) ions are better repelled by the negatively charged PSf/GO-vanillin membranes, and thus the bulk of these anions remains on the feed side.

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c) As the SO<sup>-2</sup> anion has a larger ionic radius (0.242 nm) than the Cl<sup>-</sup> anion (0.181), the prepared membranes showed better MgSO₄ rejection.

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<figure><img src="image_77.png" /></figure>

<figure><img src="image_78.png" /></figure>

<figure><img src="image_79.png" /></figure>

<figure><img src="image_80.png" /></figure>

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<caption>**Figure 6:** a) Pure water flux, b) and c) permeate flux of 2000 ppm sodium chloride (NaCl) and magnesium sulphate (MgSO₄) solutions respectively, and d) rejection rate of NaCl and MgSO₄ with PSf/GO-vanillin composite membranes.</caption>

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### 3.6. Antifouling studies

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Membrane fouling by organic, inorganic and inert matters causes a sharp decline in the permeate flux. Technically, fouling is caused by physical and chemical interactions between the foulants and the membrane surface. As most foulants are non-polar or hydrophobic, preparing hydrophilic membranes is the most promising way to avoid fouling. Both vanillin
and GO are known for their hydrophilicity, anti-biofouling ability, and antibacterial properties [17, 51]. Antifouling properties of PSf/GO-vanillin membranes were evaluated using BSA and landfill leachate wastewater feed solutions. **Figure 7a** and **7d** show the permeation flux for BSA and landfill leachate wastewater using PSf/GO-vanillin membranes. As expected, the permeation rate decreased for all membranes when BSA and landfill leachate wastewater is the feed solutions. Compared to the PSf-vanillin $M_1$ membrane, the water flux of $M_2$ to $M_5$ PSf/GO-vanillin membranes had better antifouling properties, manifested by higher water flux recovery (**Figure 7**). For the BSA solution, the water flux decreased from 39.84 LMH to 23.87 LMH for $M_1$, 44.09 LMH to 31.76 LMH for $M_2$, 47.67 LMH, and 33.65 for $M_3$, 51.45 LMH for $M_4$ and 91.64 LMH to 75.43 LMH for the $M_5$ membrane. When landfill leachate wastewater was the feed solution, water flux dropped to 21.58, 29.85, 31.99, 34.56, and 72 LMH for $M_1$-$M_5$ membranes, respectively. In both cases, the water flux decreased due to foulant deposition on the membrane surface. Initially, foulants are deposited via convective deposition; later, foulants are chemically attached to the foulant layer [17]. BSA molecules form intermolecular disulphide linkages with the foulant layer [52]. Overall, the water flux decline was higher in the experiments with landfill feed solution. The higher water flux decline in landfill feed solution could be attributed to the interaction between organic and inorganic matters that exacerbated the fouling mechanism. $M_1$ membrane showed the highest $R_t$ (39.2% and 50 %) for BSA and landfill leachate wastewater, respectively (**Figure 7b and e**). In the $M_5$ membrane experiments using BSA and landfill leachate wastewater, the $R_t$ decrease was 14.9% and 19.98%, while the FRR was 93.57% and 90.32% (**Figure 7c and f**), respectively. Obviously, casting solution contains GO enhanced the antifouling properties of the fabricated membranes, and a higher water flux recovery was achieved for $M_2$-$M_5$ membranes. The $M_1$-$M_4$ membranes had a rejection rate of over 99% for BSA; however, the $M_5$ membrane had a lower rejection rate due to the porous network structure, as shown in **Figure 5j**. When landfill leachate wastewater was the feed solution, the rejection rate for divalent ions ($Mg^{+2}$ and $Ca^{+2}$) was higher than monovalent ions ($Na^+$ and $K^+$) (**Table 5**). Except for the $M_5$ membrane, all other prepared membranes showed 84 to 90% rejection rate of $Ca^{+2}$ and $Mg^{+2}$ ions and 20 to 27% rejection rate of $Na^+$ and $K^+$ anions. A similar trend was observed for the salt rejection studies (**Section 3.5.2**). The PSf<sub>16</sub>/GO<sub>0.15</sub>-vanillin<sub>0.8</sub> membrane showed better antifouling and salt rejection performance than the commercial UA60 nanofiltration membrane. The antifouling properties of prepared membranes are justified as following:
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a) The addition of GO increases the hydrophilicity of the membranes, resulting in a reduction in the interaction between fouling matters and the membrane surface, thus reducing $R_t$, $R_r$, and $R_{ir}$.

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b) As $R_t$, $R_r$, and $R_{ir}$ are inversely proportional to water flux recovery. The prepared membranes demonstrated an increased FRR with increasing GO concentration.

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c) These findings suggest that the negatively charged PSf/GO-vanillin membranes inhibited the aggregation of negatively charged BSA and fouling matters on the surface. Consequently, fouling matters could be easily removed by DI water washing.

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Despite the higher permeation flux and antifouling properties of the $M_5$ membrane, the $M_4$ membrane demonstrated excellent rejection rates and water flux. $M_4$ membrane, therefore, is the best performance membrane when used for landfill leachate wastewater treatment compared to other membranes. For feed solution containing mainly divalent ions, the $M_4$ membrane could be an alternative to the $M_1$ membrane due to the superior water flux and antifouling property. In other words, the $M_4$ membrane could have potential applications for industrial wastewater treatment to remove heavy metals. These results indicated that PSf/GO-vanillin membranes had improved antifouling properties compared to previously reported PSf-vanillin membranes [17]. The use of vanillin results in an eco-friendly and cost-effective additive for the membrane fabrication process. Asymmetric membranes using GO and vanillin provide an easy, cost-effective and versatile approach for preparing highly permeable and fouling resistant membranes.

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<figure><img src="image_81.png" /></figure>

<figure><img src="image_82.png" /></figure>

<figure><img src="image_83.png" /></figure>

<figure><img src="image_84.png" /></figure>

<figure><img src="image_85.png" /></figure>

<figure><img src="image_86.png" /><figcaption>**Figure 7:** Flux measurements (a, d), fouling measurements (b, e) and flux recovery ratio (c, f) for BSA and landfill leachate wastewater, respectively, with PSf/GO-vanillin composite membranes.</figcaption></figure>

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<table><caption>**Table 5:** Concentration and rejection rate of different ions in landfill leachate wastewater with PSf/GO-vanillin membranes.</caption><thead><tr><th>Ion</th><th>Initial<br>concentration<br>(mg/L)</th><th>Rejection<br>rate for<br>M<sub>1</sub> (%)</th><th>Rejection<br>rate for<br>M<sub>2</sub> (%)</th><th>Rejection<br>rate for<br>M<sub>3</sub> (%)</th><th>Rejection<br>rate for<br>M<sub>4</sub> (%)</th><th>Rejection<br>rate for<br>M<sub>5</sub> (%)</th></tr></thead><tbody><tr><td>Sodium (Na<sup>+</sup>)</td><td>142.05 ± 5</td><td>21.12</td><td>22.42</td><td>23.56</td><td>23.52</td><td>8.2</td></tr><tr><td>Magnesium<br>(Mg<sup>+2</sup>)</td><td>125.3 ± 5</td><td>84.87</td><td>88.21</td><td>89.75</td><td>90.12</td><td>60</td></tr><tr><td>Potassium<br>(K<sup>+</sup>)</td><td>39.5 ± 5</td><td>23.4</td><td>25.45</td><td>26.95</td><td>27.02</td><td>10.2</td></tr><tr><td>Calcium<br>(Ca<sup>+2</sup>)</td><td>65.4 ± 5</td><td>85.24</td><td>88.85</td><td>89.25</td><td>90.2</td><td>62.45</td></tr><tr><td>Total organic<br>carbon (TOC)</td><td>120.5 ± 3.6</td><td>85.3</td><td>88.8</td><td>90.2</td><td>92.7</td><td>68.7</td></tr></tbody></table>

## 4. Conclusions

A fouling resistant and highly perm-selective PSf/GO-vanillin nanofiltration membrane was fabricated for wastewater purification and recovery. The 2D GO layers and vanillin help to retain the structural integrity of the PSf based mixed matrix membranes. The present study concludes with the following key points:

a) The addition of 2D GO layers improves the membranes wetting, rejection, and antifouling properties.

b) Combining GO and vanillin enhances the negative surface zeta potential for the prepared membranes, and as a result, a higher rejection rate was observed for divalent ions.

c) The PSf/vanillin ($M_1$) membrane had a PWF of 39 LMH, but adding GO to the casting solution increased the PWF of the $M_5$ membrane to 91 LMH.

d) The optimized PSf<sub>16</sub>/GO<sub>0.15</sub>-vanillin<sub>0.8</sub> membrane showed 92.5% rejection for MgSO<sub>4</sub> solution, nearly 29% higher than the commercial UA 60 membrane, which rejects up to 77.13%.

e) The prepared membrane demonstrated a significantly higher FRR of 93.57% and 90.32% for BSA and landfill leachate wastewater, respectively.
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f) The PSf/GO-vanillin membranes are potential candidates for robust and energy-efficient water treatment because of their high flux and fouling resistance.

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**Abstract**

To meet the rising global demand for water, it is necessary to develop membranes capable of efficiently purifying contaminated water sources. Herein, we report a series of novel polysulfone (PSf)/GO-vanillin nanofiltration membranes highly permeable, selective, and fouling resistant. The membranes are composed of two-dimensional (2D) graphite oxide (GO) layers embedded with vanillin as porogen and PSf as the base polymer. There is a growing interest in addressing the synergistic effect of GO and vanillin on improving the permeability and antifouling characteristics of membranes. Various spectroscopic and microscopic techniques were used to perform detailed physicochemical and morphological analyses. The optimized PSf<sub>16</sub>/GO<sub>0.15</sub>-vanillin<sub>0.8</sub> membrane demonstrated 92.5% and 25.4% rejection rate for 2000 ppm magnesium sulphate (MgSO<sub>4</sub>) and sodium chloride (NaCl) solutions respectively. Antifouling results showed over 99% rejection for BSA and 93.57% flux recovery ratio (FRR). Experimental work evaluated the antifouling characteristics of prepared membranes to treat landfill leachate wastewater. The results showed 84-90% rejection for magnesium (Mg<sup>+2</sup>) and calcium (Ca<sup>+2</sup>) with 90.32 FRR. The study experimentally demonstrated that adding GO and vanillin to the polymeric matrix significantly improves fouling resistance and membrane performance. Future research will focus on molecular sieving for industrial separations and other niche applications using mixed matrix membranes.

**Keywords:** PSf/GO-vanillin membrane; mixed matrix membrane; nanofiltration; fouling resistant; and salt rejection
**Sudesh Yadav:** Conceptualization, Methodology, Data curation, Visualization, Investigation

Writing- Original draft preparation, Ibrar Ibrar, Akshaya K. Samal, Sébastien Déon, John

Zhou, Noreddine Ghaffour: Formal analysis, Visualization, Investigation, Ali Altaee:

Supervision, Project administration, Resources, Funding acquisition, Validation, Writing-Reviewing and Editing
**Declaration of interests**

[x] The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

[ ] The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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