# Hydrophobic cordierite-based capillary membranes applied to desalination

Sharath Jayakumar, Philippe Trens, Mihai Barboiu, Sophie Annick Cerneaux

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Sharath Jayakumar, Philippe Trens, Mihai Barboiu, Sophie Annick Cerneaux. Hydrophobic cordierite-based capillary membranes applied to desalination. Separation and Purification Technology, 2025, 359, pp.130679. 10.1016/j.seppur.2024.130679. hal-04846799

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# Hydrophobic cordierite-based capillary membranes applied to desalination

Sharath Jayakumar<sup>a</sup>, Philippe Trens<sup>b</sup>, Mihail Barboiu<sup>a</sup>, Sophie Cerneaux<sup>a,*</sup>

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

## ARTICLE INFO

Editor: B. Van der Bruggen

**Keywords:**

Capillary ceramic membranes

Chemical grafting

Hydrophobicity

Air gap membrane distillation

## ABSTRACT

Hydrophobic ceramic membranes are widely used in water treatment, increasingly in desalination using membrane distillation process. Indeed, due to their ability to perform in harsh pH, temperature, and pressure conditions, they may replace hydrophobic polymer membranes. Many examples exist in the literature using porous ceramic membranes in a tubular configuration; however, a few of them reports on capillary porous membranes that may be further applied to desalination by membrane distillation. The development of membrane supports based on low cost powders instead of more expensive alumina, titania or other oxides will help decrease the production cost of porous capillary membranes. Novel cordierite capillary membrane supports were fabricated by extrusion using various cost efficient and bio-sourced pore-forming agents such as corn starch, maize flour, and rice husk. The capillary supports were further coated with one to three filtration layers, namely zirconia (8 m²/g), zirconia (43 m²/g), and γ-alumina to yield membranes with an active layer pore size of 9 nm. The different membranes were finally chemically modified using three alkoxysilanes, namely 1H,1H,2H,2H-perfluorooctyltriethoxysilane, 1H,1H,2H,2H-perfluorodecyltriethoxysilane and dodecyltriethoxysilane to bring hydrophobicity to the membranes to be applied to air gap membrane distillation (AGMD). The chemical modification of the mineral capillary membranes was evidenced by infrared spectroscopy, water contact angle measurement, and N₂ and water vapors sorption experiments. Thermogravimetric and gas sorption analyses allowed us to approximate the number of grafted molecules and their temperature resistance. The pure water permeability of the hydrophilic non-modified corn starch-based membrane was 597 L/(m²·h.bar) that was higher than the membranes prepared with maize flour (143 L/(m²·h.bar)) and rice husk (302 L/(m²·h.bar)). AGMD of a NaCl solution (35 g/L) yielded a permeate flux between 3–8 L/(m²·h). The membranes grafted with 1H,1H,2H,2H-perfluorodecyltriethoxysilane gave a higher rejection of 95 % due to higher contact angle and Liquid Entry Pressure of water (LEP<sub>w</sub>).

## 1. Introduction

Over 71 % of the Earth's surface is covered by water, among which 97 % represent saline water from the oceans. Freshwater resources account for only 3 % of the total water present on the Earth's surface, out of which 30 % is only available for drinking as groundwater, and the remaining 70 % are trapped in ice caps and glaciers [1]. There is an increasing demand for freshwater due to the overconsumption of water for domestic purposes, the increase in the world population, and climate changes, which are related to the accelerated growth rate and has further complicated the water stress problem [2]. In order to find solutions to the water shortage, membrane-based separation processes were developed and appeared to be worldwide technologies as they are

cost-effective, simple to operate, and energy-efficient in most cases [3]. There are several membrane-based desalination technologies being implemented such as reverse osmosis [4], nanofiltration [5], and more recently membrane distillation (MD) [6]. Among them, membrane distillation happens to possess several attractive features, such as low operating temperature and no transmembrane pressure. These features make MD advantageous compared to other techniques due to its low operating cost and possibly lower energy consumption due to the use of alternative energy sources like solar energy [7,8]. The main drawbacks of the MD process that may be encountered are however a low permeate flux, membrane pore wetting, and membrane fouling [9].

Membrane distillation using ceramic membranes is an emerging membrane-based desalination technology for purification of saline

<footnote><sup>a</sup> Institut Européen des Membranes, UMR 5635, Univ Montpellier, ENSCM, CNRS, Place Eugene Bataillon, Adaptive Supramolecular Nanosystems Group, Place Eugene Bataillon, 34095 Montpellier, France</footnote>

<footnote><sup>b</sup> Institut Charles Gerhardt, UMR 5253, CNRS, ENSCM, Univ Montpellier, Pôle Chimie Balard Recherche, 1919 route de Mende, 34293 Montpellier, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: sophie.cerneaux@umontpellier.fr (S. Cerneaux).</footnote>
water. It is a thermally driven separation process based on the trans-membrane vapor pressure difference brought about by the temperature difference occurring between both sides of the membrane. Therefore, only vapor molecules are transported across the membrane that needs to be hydrophobic, porous and should have a low thermal conductivity to prevent heat loss. The feed solution is heated continuously in order to obtain a high distillate flux [10]. On comparing with the conventional thermal distillation process, MD can be performed at relatively lower temperature between 50 °C and 90 °C without heating the feed solution to its boiling point. Hence it leads to the use of energy efficient thermal processes like the use of solar energy, geothermal energy, heat energy from the industrial waste streams and so on [11–13]. Only volatile vapors are expected to pass through the hydrophobic membrane pores due to the water vapor high surface tension [14]. The main factors that govern the performance of the membranes in MD are the hydrophobicity, the membrane pore size, the membrane wall thickness and the porosity that invariably affect the water flux and the salt rejection [15]. There are several modes in which membrane distillation can be performed, such as direct contact membrane distillation (DCMD), air gap membrane distillation (AGMD), sweep gas membrane distillation (SGMD), and vacuum membrane distillation (VMD) [16–18]. The differences between these systems rely on the way of recovering the permeate, using a cooling system, a sweeping gas or vacuum, respectively.

Membranes can be prepared using different kinds of materials, like oxide materials, which consist of alumina [19], titania [20], zirconia [21], and polymeric materials that are mainly cellulose acetate [22], polytetrafluoroethylene (PTFE) [23], polyvinylidene fluoride (PVDF) [24], and a combination of both ceramic and polymeric materials [25]. Ceramic membranes possess several advantages over polymer membranes because of their high thermal stability, better mechanical strength, chemical resistance, and anti-fouling properties, particularly when working in harsh conditions not bearable by polymer membranes [26].

Ceramic membranes are usually prepared in an asymmetric configuration with multilayer coatings over a ceramic support in order to have high permeability and selectivity with an optimum pore size distribution [18]. Hence, the basis for preparing an asymmetric ceramic membrane will be the ceramic support that should possess excellent mechanical strength properties with a porosity sufficient to yield a high permeability. Any cracks or deformation in the membrane support will ultimately lead to defects in the intermediate layers, thus affecting the overall membrane performances [27]. The mechanical strength and the membrane performance of the ceramic membrane are influenced by several factors like the pore size distribution, porosity, wall thickness of the membrane, sintering conditions, powder particle size, etc. [28]. Since ceramic membranes are generally hydrophilic in nature, they need to be hydrophobic to process desalination by MD. Among the several ways used to achieve a hydrophobic surface, the most commonly used techniques are the chemical grafting of molecules containing long hydrophobic functional groups and the creation of rough surfaces by utilizing techniques such as plasma etching in the presence of PTFE [29], nano texturing [30], and anodic oxidation of aluminum [31].

Membrane modification by chemical grafting consists of functionalizing the surface with low surface free energy agents such as alkoxysilanes since they are able to react with free hydroxyl groups of the hydrophilic membrane to create resistant chemical bonds with the oxide surface. Hence, the hydrophilic membrane becomes hydrophobic at the end of the functionalization process [32]. The grafting will also have an impact on the properties of the membrane thereby affecting its performance [33]. The researchers used commercial tubular alumina 100 nm and titania 75 nm membranes to perform their study with 10/5 mm of outer/inner diameters (O.D./I.D.). A better AGMD efficiency ($\Delta T = 85^{\circ}C$) was demonstrated for the C8-modified alumina membranes yielding a LEP<sub>w</sub> of 3 bar and a pure water flux of 4.8 L/(m<sup>2</sup>.h). J. Schnittger et al. recently studied the influence of the material selection

and layers characteristics of hydrophobic ceramic membranes (grafted with C6F) when used in MD (in direct contact and vacuum modes) [34]. They compared the efficiency of single channel tubular membranes (10/7 mm O.D./I.D.) made from Al<sub>2</sub>O<sub>3</sub>, TiO<sub>2</sub>, cordierite and mixed oxides with wall thicknesses of 1.5 and 1.0 mm for alumina and titania, respectively. When tested in direct contact MD, the TiO<sub>2</sub> membrane exhibited the highest permeate flux of 4.2 L/(m<sup>2</sup>.h) followed by the mixed oxide support coated with the TiO<sub>2</sub> and ZrO<sub>2</sub> layers (~3 L/(m<sup>2</sup>.h)). For the Al<sub>2</sub>O<sub>3</sub> membranes, the cordierite and the mixed oxide supports coated with alumina membrane layers, significantly lower permeate fluxes between 0.8 and 1.5 L/(m<sup>2</sup>.h) were achieved.

In this work, for the first time several capillary ceramic supports were fabricated from cordierite by extrusion and coated with various filtration layers. Several membranes were prepared by coating the cordierite capillary supports with membrane filtration layers of zirconia (8 m<sup>2</sup>/g), which is the microfiltration (MF) layer followed by an ultrafiltration (UF) layer of zirconia (43 m<sup>2</sup>/g) and finally with a third layer of gamma alumina (γ-Al<sub>2</sub>O<sub>3</sub>), aimed to be a nanofiltration (NF) layer. The capillary membrane supports were prepared using different pore forming agents such as corn starch, maize flour and rice husk to define the best conditions for the filtration layers depositions. The membranes morphology, mechanical resistance and pure water permeability were determined after coating the different filtration layers. In a second step, the membranes were rendered hydrophobic by grafting alkoxysilane groups of different chain length and nature. Finally, the membranes performance was tested in air gap membrane distillation (AGMD) and both the water flux and the NaCl rejection were calculated.

## 2. Experimental section

### 2.1. Materials

Cordierite clay that is composed of 2Al<sub>2</sub>O<sub>3</sub>.5SiO<sub>2</sub>.2MgO with a particle size of 30 µm was purchased from IMERYS France. Amijel (CPlus 12,076 Cerestar, France) and methocel (The Dow Chemical Company) were cellulosic derivatives used as plasticizers and binders in the paste formulation. Corn starch (RG 03,408 Cerestar, France), maize flour (Maizena), and risk husk (Cameroon) were used as bio-sourced pore forming agents. Zirconia powders presenting a specific surface area (SSA) of 8 m<sup>2</sup>/g and 43 m<sup>2</sup>/g were used to prepare the micro and ultrafiltration layers, respectively. These powders purchased from CEZUS were characterized by an average grain diameter of 0.65 µm and 0.22 µm, respectively. Gamma alumina (γ-Al<sub>2</sub>O<sub>3</sub>) powder was synthesized by sol gel in distilled water (18 MΩ cm<sup>-1</sup>) using boehmite (PLURAL SB, Condea Chemie GmbH, Germany) as starting material to prepare the nanofiltration filtration layer. The nitric acid, poly(vinyl alcohol) (PVA), poly(ethylene glycol) with an average M<sub>n</sub> of 300 g/mol (PEG300) were obtained from Sigma Aldrich, France. The hydroxy ethyl cellulose (HEC) was purchased from Fluka, France and the Dolapix CE64 from Zschimmer & Schwarz, Germany. The chemical grafting of both powders and membranes was done using the organosilanes reagents 1H,1H,2H,2H-perfluorooctyltriethoxysilane (C6F); 1H,1H,2H,2H-perfluorodecyltriethoxysilane (C8F) from Sigma Aldrich, France and dodecyltriethoxysilane (C12) from Fisher Scientific, France. Absolute ethanol (Carlo Erba Reagents SAS, France) was used as received.

### 2.2. Preparation of the capillary supports

The ceramic pastes were formulated using cordierite, a pore forming agent being either corn starch, maize flour or rice husk, and amijel and methocel used as plasticizers and binders, respectively. All these raw materials were mixed thoroughly in a mixer and the required amount of distilled water was added to blend the 100 g of powders in order to obtain processable ceramic pastes. The weight fraction of the different components was varied to shed in light their influence on the support characteristics, keeping the load of amijel and methocel constant at 4 g
each as mentioned in Table 1.

The capillary cordierite supports M1, M2, M3, M4 and M7 were prepared by a conventional extrusion process using a home-made apparatus that produces hollow supports of 3.5/2.0 mm outer/inner dimensions (O.D/I.D). The extruded capillary supports were dried at room temperature with the help of rotating rollers for 24 h and then dried in an oven at 50 °C for 24 h before heat treatment. The sintering step is important in this process where the tubular ceramic supports were subjected to high temperature by a gradual increase to bring the mechanical strength to the support, to prevent deformation in shape and formation of cracks but also to liberate the porosity. The applied temperature schedule consisted first in an increase from room temperature to 300 °C at a rate of 90 °C per hour. After one hour of dwelling, the temperature was raised to 1275 °C at a rate of 300 °C per hour. Finally, after a dwelling time of one hour at 1275 °C, the muffle furnace was cooled down to room temperature to fulfill the sintering process. The same overall process was applied to prepare the flat ceramic supports M5, M6, M8 and M9, calendered at a thickness of 3 mm. It should be noted here that hollow tubular capillary supports were impossible to obtain in these conditions since they were not straight or twisted.

## 2.3. Coating of the membrane filtration layers over the capillary ceramic supports

The capillary cordierite supports were coated with two and three different oxides layers to decrease the pore size of the final active layer by slip casting of slurries containing the raw materials such as zirconia (8 m²/g), zirconia (43 m²/g) and γ-alumina (200 m²/g), respectively. Each filtration layer slurry was prepared separately. For the preparation of the MF slurry, 5 g of zirconia (8 m²/g) was suspended in 15 mL of PVA (12 wt% in water) and 30 mL of an aqueous solution at 0.2 wt% of Dolapix and stirred at 400 rpm for 4 h at 35 °C. For the UF slurry, 0.75 g of zirconia (43 m²/g) was mixed with 31.25 mL of HEC (2 wt% in water), 2 mL of PEG300 and 16 mL of Dolapix (0.1 wt% in water) and stirred at 250 rpm for 3 h at 30 °C. For the NF slurry, 2 g of boehmite was added to 30 mL of distilled water and stirred vigorously for 2 h. Then 2.4 mL of 2M nitric acid was added and stirred gently for 5 min before the addition of 12.9 mL of PVA (12 wt% in water) and stirred for another 1 h.

The tubular capillary support was fitted with a flexible tube at one end and fixed to a vertical stand. The slurry was injected through the other end of the tube with the help of a syringe to form the coating inside the capillary with an induction time of 5 min. The excess slurry was removed from the support and the coated capillaries were dried in an oven at 50 °C overnight prior to being subjected to a sintering process to a maximum temperature of 1150 °C, 750 °C and 540 °C for the MF, UF, and NF layers coated membranes, respectively. The same slurries were deposited by tape casting over the flat cordierite supports and subjected to the same heat treatment.

For characterization purposes, capillary membranes were prepared by coating one zirconia MF layer alone, with 2 layers as MF and UF zirconia layers and finally with the MF and UF zirconia layers followed

<table><caption>**Table 1**

Composition of the raw ceramic pastes yielding the cordierite capillary supports M1 to M9.</caption><thead><tr><th>Membrane</th><th>Cordierite (g)</th><th>Corn starch (g)</th><th>Maize flour (g)</th><th>Rice husk (g)</th></tr></thead><tbody><tr><td>M1</td><td>87</td><td>5</td><td>—</td><td>—</td></tr><tr><td>M2</td><td>82</td><td>10</td><td>—</td><td>—</td></tr><tr><td>M3</td><td>77</td><td>15</td><td>—</td><td>—</td></tr><tr><td>M4</td><td>87</td><td>—</td><td>5</td><td>—</td></tr><tr><td>M5*</td><td>82</td><td>—</td><td>10</td><td>—</td></tr><tr><td>M6*</td><td>77</td><td>—</td><td>15</td><td>—</td></tr><tr><td>M7</td><td>87</td><td>—</td><td>—</td><td>5</td></tr><tr><td>M8*</td><td>82</td><td>—</td><td>—</td><td>10</td></tr><tr><td>M9*</td><td>77</td><td>—</td><td>—</td><td>15</td></tr></tbody></table>

by the NF alumina layer (3 layers). In addition, the different slurries were dried and subjected to the same heat treatment than the corresponding tubular membranes for gas sorption analyses before and after grafting.

## 2.4. Chemical modification of the different membranes

Prior to grafting, the tubular and flat ceramic membranes were cleaned by ultrasonication for 10 min in ethanol and dried in an oven at 50 °C for 12 h. The grafting solution at a concentration of 0.1M was prepared by mixing the grafting compounds 1H,1H,2H,2H-perfluorocetyltriethoxysilane (C6F), 1H,1H,2H,2H-perfluorodecyltriethoxysilane (C8F) and dodecyltriethoxysilane (C12) with the required amount of ethanol. The experiment was carried out under an inert atmosphere to avoid contamination of the solution and prevent early hydrolysis and polycondensation reactions of the triethoxysilane groups. For better grafting efficiency, the reaction was performed under reflux at 70 °C for 5 h. Finally, the modified membranes were dried for 12 h in an oven at 80 °C. Fig. 1 illustrates the process of chemical grafting of the hydrophobic alkoxysilane groups on the membranes. The water droplets are shown on the outside; however, the coating is also hydrophobic on the inside of the capillary since the contact angle was measured on the corresponding flat membranes.

Modified powders were also prepared for characterization purposes. After grafting, they were recovered by filtration and washed with ethanol prior being dried at 50 °C for 12h. For 15 cm-long tubular cordierite supports with both ZrO₂ or Al₂O₃ as active layers, 150 mL of the grafting solution was required to maintain the same molecular ratio between the organic and the ceramic parts.

The hydrophobic feature of the modified ceramic membranes was thus evidenced by the measurement of the water contact angle on the corresponding flat membranes and by the determination of the Liquid Entry Pressure with water (LEP<sub>w</sub>) using a tangential filtration pilot and grafted capillary membranes. The modified zirconia and alumina powders were characterized by thermogravimetric analyses (TGA), infrared spectroscopy (ATR-IR) and N₂ and water vapor sorption analyses before and after grafting.

## 2.5. Characterization techniques

### 2.5.1. Determination of the membrane supports pore size

Mercury porosimetry (Micromeritics, AutoPore IV 9500) was used to determine the pore size of the capillary membrane supports by applying a high pressure on the sample immersed in mercury. Since the pressure applied to the mercury to intrude the pores of the membrane is inversely proportional to the pore size of the membrane support, larger the pores, smaller will be the applied pressure [35,36]. The pore size distribution was calculated using the AutoPore IV 9500 software.

#### 2.5.1.1. Mechanical resistance of the membrane supports.
The mechanical strength of the flat membrane supports with different compositions was determined by the three-point bending method (Universal LLOYD instruments, LRX apparatus) using a load of 50N, a span of 30 mm, and a cross-head speed of 10 mm min⁻¹. The membranes were cut at the dimensions of 40 mm length, 10 mm width and 3 mm thickness and placed over the span. The flexural strength can be theoretically calculated using equation (1);

$$ \sigma_f = \frac{3FL}{2bh^2} \qquad (1) $$

where σ<sub>f</sub> is the flexural strength (MPa), F is the force acting on the support (N), L is the span length (mm), b is the width of the test beam (mm), and h is the thickness of the test beam (mm) [37].

#### 2.5.1.2. Morphological and structural analyses of the membranes.
The top

<footnote>* Only flat sheet supports were prepared for mechanical strength testing.</footnote>
$$R = -(CF_2)_5CF_3 \text{ noted C6F; } -(CF_2)_7CF_3 \text{ noted as C8F; } -(CH_2)_9CH_3 \text{ noted C12} \\ R' = -CH_2CH_3$$

<figure><img src="image_4.png" /><figcaption>Fig. 1. Schematic representation of the chemical grafting of alkoxysilane groups on the membrane and picture of water droplets over a modified capillary.</figcaption></figure>

surface and the cross-section of the membranes were analyzed using a Scanning Electron Microscopy (SEM) (Hitachi S4800, Hitachi High-Tech Corporation, Japan) operating at an acceleration voltage of 2 kV. The samples were mounted on a carbon tape fixed on a metallic support and subjected to sputtering before the SEM analysis. The chemical composition of the ceramic membranes was analyzed using energy-dispersive X-ray spectroscopy (Detector: Oxford Instruments X-Max AZTEC, UK). The X-ray diffraction (XRD) patterns of the raw mineral powders were obtained using a X'pert-Pro Pan Analytical Diffractometer in a Bragg-Brentano configuration using the Cu Kα anticathode (40 kV, 20 mA, λ = 1.54 Å).

### 2.5.1.3. Pure water permeability of the hydrophilic capillary membranes.
The pure water permeability tests were performed on the tubular ceramic membranes before grafting them with hydrophobic functional groups. The main factors influencing the permeability of the membrane were the type of membrane material used, the pore size, the porosity and the thickness of the active layer [38]. The 15 cm-long capillary membranes were placed in a home-made module of a tangential filtration pilot [39] (Figure S 2) and tested with pure water (18 MΩ cm⁻¹) thermostated at 20°C at varying nitrogen pressure. The permeability coefficient (Lp) of the membranes was obtained by plotting a linear curve of the pure water flux (Jv) as a function of the increasing transmembrane pressure (Δp) (Equation (2)) ranging from 1 to 5 bar after a stationary state was achieved at each pressure [40].

$$J_v = L_p \Delta p \qquad (2)$$

### 2.5.1.4. Characterization of the chemical modification of the membranes.
The different membrane filtration powders obtained from the prepared slurries along with the hydrophobic corresponding powders were characterized by Thermogravimetric Analysis (TGA-SDT Q600 TA Instruments, USA) under a nitrogen flow from ambient temperature to 1000 °C at a rate of 10 °C/min. The objectives were to confirm the presence of the hydrophobic moieties and to define the temperature of decomposition of the organic parts. The weight loss (wt.%) was found to vary depending on the nature and chain length of the alkoxysilane

groups used for the grafting [41]. The presence of the functional groups or the chemical structure of the grafted alkoxysilane groups over the membrane filtration layer were also analyzed by Attenuated Total Reflectance-Fourier Transform Infrared Spectrometry (ATR-IR) (ThermoFisher Scientific, USA) in the range of 500–4000 cm⁻¹.

The N₂ adsorption/desorption analyses (ASAP 2020) using the BET (Branauer–Emmett–Teller) and BJH (Barrett–Joyner–Halenda) models were performed to determine the specific surface area, the pore size and distribution of the oxide powders before and after grafting [42]. The main motive behind using this characterization technique was to explain how the hydrophobization process will affect the membrane morphology [43]. Before beginning the measurements, sample degassing was performed at 90 °C in order to remove any adsorbed molecules.

In addition, some Dynamic Vapor Sorption (DVS) of water (Surface Measurements Systems, UK) were performed to determine water vapor sorption isotherms at different relative pressures. Indeed, the aim was here to try to correlate the sorption parameters with the hydrophobicity of the materials and therefore with their MD performances. A steady flow of dry nitrogen gas, at a rate of 200 mL/min, blended with a water-containing nitrogen stream, maintained a consistent partial pressure of water vapor. Experimental runs commenced from 0 % partial pressure, gradually ascending in 5 % increments before reversing back to 0 %. This systematic approach, executed within a nitrogen stream, delineated the complete sorption isotherm. Each cycle spanned approximately 3 days, with equilibrium defined by the slope of the relative mass variation versus time curve. This equilibrium criterion was taken as 0.004 %/min over a 10 min interval. All experiments were conducted at a constant temperature of 25 °C.

Another way of characterizing the hydrophobicity is to measure the angle made by 5 µL water droplets over flat modified ceramic membranes, called the contact angle (θ). The measurements were taken at different places on the membrane surface to report an average value.

Finally, the efficiency of grafting hydrophobic alkoxysilane groups on the tubular membranes was determined by measuring the Liquid Entry Pressure of water (LEPw) using the same laboratory setup and module (Figure S 2) as for the water permeability measurement of the hydrophilic membranes [39]. Indeed, this value corresponds to the
nitrogen pressure that needs to be applied to the feed solution to observe the first drop of water permeating through the hydrophobic membrane on the downstream. The pure water thermostated at 20°C was circulated in the pilot loop using a circulation pump. The Laplace-Young equation is often used to define the LEPw relating the surface tension (γL), the contact angle (θ) and the pore radius (rmax) of the membrane. An additional pore geometry coefficient can be taken into account to better characterize the relationship between the pores and the LEPw value. The LEPw is directly proportional to the contact angle and inversely proportional to the pore radius. The LEPw increases with the increase of the contact angle, which is due to a high hydrophobicity, and decreases when the pore size of the membrane is larger due to more pore wetting [44]. This implies that a membrane with a high LEPw will have a higher hydrophobicity and less pore wetting probably due to a lower pore size, which may thus result in a lower water flux during membrane distillation [39].

## 2.6. Application to desalination by air gap membrane distillation (AGMD)

The efficiency in the separation process of the prepared tubular grafted ceramic membranes can be evaluated by performing Air Gap Membrane Distillation (AGMD). The experiment was carried out on a laboratory-scale membrane distillation pilot. The 15 cm-long capillary membranes were fitted in a home-made module and connected to the AGMD system described in [33]. The feed solution used for the experiment was a sodium chloride solution (35 g/L). The temperature of the feed solution was maintained at 85 °C and the cooling water was circulated outside the module at a temperature of 5 °C to maintain a temperature difference across the membranes of 80 °C. The collected permeate was weighed on a balance to measure the permeate flux and the permeate salt concentration was checked by measuring its conductivity using a VWR Phenomenal MU 6100H conductometer. The permeate flux was calculated using the equation (3),

$$J_w = \frac{\Delta m}{A.t} \qquad (3)$$

where Jw represents the water permeate flux (kg/(m² h)), Δm is the permeate weight (kg), A is the effective membrane surface area (m²) and t is the experiment time duration (h) [45].

The salt rejection R (%) was calculated using the equation (4),

$$R = \left( 1 - \frac{C_p}{C_f} \right) \times 100 \qquad (4)$$

where Cp and Cf are the NaCl concentrations in the distilled permeate and the feed, respectively [46].

# 3. Results and discussion

## 3.1. Characterization of the ceramic membranes

Cordierite capillary supports were prepared using three different pore forming agents namely corn starch, maize flour, and rice husk to evidence their influence on the pores size and distribution of the corresponding supports. The pore size distribution obtained from mercury porosimetry analyses was found to be narrow for membrane supports prepared using corn starch and maize flour, centered at 5.0 ± 1.4 µm and 4.8 ± 1.4 µm, respectively (Figure S 1). For the rice husk-based membranes, the peak appears broader at 3.5 ± 1.8 µm that may be due to a more heterogeneous distribution of rice husk in the membrane. Taking into account the error range for all the measurements, it seems that the pore size distribution of the different capillary supports is not drastically affected by the pore forming agent nature. In addition, the overall porosity of the supports is almost similar for the corn starch and the maize flour with values of 29 and 28 %, respectively and slightly

lower for the rice husk of 17 %, even though an error of 5% has to be taken into account for the porosity values. However, one need to keep in mind that the presence of large voids in the supports will not be detected using this technique. The γ-alumina membranes deposited on the cordierite capillaries prepared with the corn starch as pore forming agent shows a higher porosity of 47 % compared to that prepared using maize flour and rice husk of 36 % and 22 %, respectively.

By changing the amount of the pore forming agents during the support preparation from 5 wt% to 10 wt% and 15 wt%, there is not only a change in the pore size of the sintered capillary supports but also in their mechanical resistance (Fig. 2). Indeed, for the corn starch-based supports, a decrease of the mechanical resistance was observed as the flexural strength decreases from 14.1 ± 1.5 MPa for 5 wt% (M1) to 10.5 ± 0.9 MPa at 10 wt% (M2) and 8.2 ± 2.2 MPa for 15 wt% (M3). The same behavior is noticed for the supports prepared with the maize flour with a flexural strength of 13.5 ± 2.5 MPa for 5 wt% (M4) going down to 5.2 ± 0.6 MPa and 4.1 ± 0.4 MPa for 10 wt% (M5) and 15 wt% (M6), respectively. This decrease is probably due to the fact that when the amount of pore forming agent is increased, the amount of cordierite used is thus reduced that has for impact to improve the mechanical resistance. Regarding the rice husk-based membrane supports, a different behavior is observed. Indeed, the flexural strength is increasing simultaneously with the pore forming agent content, starting at 12.3 ± 2.0 MPa for M7 and going up to 19.3 ± 0.9 MPa for M9 prepared with 15 wt% of risk husk. It may be inferred that the presence of silica and alumina in the raw rice husk induces the formation of a mullite phase upon the heat treatment that has for direct impact to provide additional strength to the membrane support but also to drastically decrease both the pore size of the support and the overall porosity. Unfortunately, 10 wt% and 15 wt% of rice husk in the membrane support formulation were unsuitable for the extrusion of the capillary ceramic membrane, hence 5 wt% of rice husk was used in the paste formulation (M7). The minimum flexural strength required to yield strong enough supports being at least 10 MPa, the membrane supports M3, M5 and M6 were not considered for the membranes performance testing. Therefore, only the membranes prepared with 5 wt% of pore forming agents will be used in the filtration experiments, namely membranes M1, M4 and M7.

The SEM micrographs allowed us to measure the wall thickness of the sintered capillary support M1 (Figs. 3-D1) and gave us information on the morphology of the ceramic supports prepared with different pore forming agents. From picture A3 in Fig. 3, it is obvious that the capillary supports prepared with risk husk as pore forming agent present large macrovoids compared to the supports prepared using corn starch (Figs. 3-A1) or maize flour (Figs. 3-A2). Additional images B1 to B3 and C1 to C3 gave information on the presence and the average thickness (4 µm) of

<figure><img src="image_5.png" /><figcaption>Fig. 2. Flexural strength of the capillary membrane supports prepared with different fractions of pore forming agents as mentioned in Table 1.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** SEM micrographs of capillary M1 prepared with corn starch (A1), with the MF and UF zirconia layers (B1) and the three filtration layers (C1); capillary M4 with maize flour (A2), with the MF and UF zirconia layers (B2) and the three filtration layers (C2); and capillary M7 with rice husk (A3), with the MF and UF zirconia layers (B3) and the three filtration layers (C3). D1 represents the cross-section of the capillary support M1 and pictures D2 and D3 illustrate the EDX elemental mapping of the capillary membrane M4 with the zirconia layers and the capillary membrane M1 with the zirconia and alumina layers, respectively.</figcaption></figure>

the two zirconia layers and the two zirconia plus the gamma alumina layers, respectively. For further evidence of the presence of those layers, the EDX mapping was performed to better illustrate the elemental composition and repartition of the elements in the membranes M4 and M1 as depicted in Figs. 3-D2 and D3, respectively.

The XRD pattern (Fig. 4A) of the raw cordierite powder sintered at 1275 °C had diffraction peaks at 10.3°, 18.2°, 21.6°, 26.4°, 28.3°, 29.5°, 33.8°, which correspond to the Miller planes of crystalline cordierite at (100), (110), (102), (112), (202), (211) and (212), respectively [47]. Zirconia powder (8 m²/g) (Fig. 4B) that was used to coat the MF layer was sintered at 1150 °C and shows diffraction peaks at 23.9°, 28.1°,

30.1°, 31.2°, 33.9°, 50.0°, 60.0° with Miller planes at (110), (−111), (011), (111), (002), (112) and (121), respectively. The Miller planes (110), (−111), (111), and (002) were in the monoclinic phase and the (011), (112) and (121) ones were in the tetragonal phase. The presence of a tetragonal phase in the compound can cause an increase in the strength and toughness of the final material [48]; For the 43 m²/g zirconia powder sintered at 750 °C, the diffraction peaks found at 30.0°, 35.2°, 50.0°, and 60.0° correspond to the (111), (200), (220), and (311) planes that all belong to the tetragonal phase [49]. For the gamma alumina (650 °C) coated as the NF layer, the diffraction peaks at 36.8°, 46.0°, and 66.1° correspond to the (311), (400), and (440) Miller planes
<figure><img src="image_7.png" /><figcaption>Fig. 4. XRD patterns of cordierite (A), zirconia 8 m²/g (B), zirconia 43 m²/g (C), and gamma alumina (D).</figcaption></figure>

[50].

Before performing the chemical grafting of hydrophobic triethox-
yalkylsilanes, the pure water permeability Lp of the different mem-
branes prepared with corn starch (M1), maize flour (M4), and rice husk
(M7) as pore forming agents and coated with the two MF and UF zirconia
layers or with the zirconia and alumina layers (three layers) was
measured. From Fig. 5, the two zirconia layers deposited on the rice
husk-based cordierite capillary supports M7 (left) show a higher Lp of
1636 L/(m²·h.bar) compared to the other capillary supports M1 (962 L/
(m²·h.bar)) and M4 (577 L/(m²·h.bar)). The permeability measurements
were repeated after coating the third alumina layer that was expected to
decrease the porosity and pore size of the final membrane. Indeed, it was
found that the pure water permeability of all the membranes decreased
due to the decrease in the membrane pore size. However, for the
membranes having three filtration layers (Fig. 5-right), a slightly
different trend is now observed. The water permeability of 597 L/(m²·h·

bar) for M1 prepared with corn starch was found to be higher compared
to membranes M4 (143 L/(m²·h.bar)) and M7 (302 L/(m²·h.bar)). As
seen on the SEM images of M7, the large macrovoids that were present
may have been partially blocked by the last alumina layer, diminishing
the average pore size and thus the permeability. In any case, it appears
from these experiments that the lowest permeability results were ob-
tained for the capillaries prepared with the maize flour even though the
pore size and overall porosity were satisfying.

After chemical modification with perfluoroalkyltriethoxysilanes (C6F, C8F) and dodecyltriethoxysilane (C12), we tried to evidence the influence of the chain length and nature of the organic parts on the performances of the membranes. First, the presence of the grafted alkoxysilane groups on the last filtration layer made of gamma alumina powder can be confirmed by performing ATR-IR analyses. In addition, these analyses were repeated after a thermal treatment up to 1000 °C, performed during the thermogravimetric analyses on the same samples

<figure><img src="image_8.png" /><figcaption>Fig. 5. Evolution of the water flux as a function of the applied entry pressure for the capillary membranes prepared with corn starch (M1), maize flour (M4) and rice husk (M7) as pore forming agents and coated with the zirconia MF and UF layers (left), and the MF, UF, and alumina layers (right).</figcaption></figure>
to evaluate the impact of the temperature on the chemical bonds resistance as shown in Fig. 6. It appears that the temperature applied to the samples during the TGA measurements (1000 °C) had for impact to degrade the C12 chain, since the disappearance of the vibration band of the C–H groups at 2920 cm⁻¹ is observed. The same observation may be done for the C6F and C8F groups that were decomposed upon heat treatment since the C-F vibration bonds at around 1200 cm⁻¹ were absent on the right part of Fig. 6. In addition, the vibration band centered at 900 cm⁻¹ is enlarged from left to right that may be attributed to the degradation of the Al-O-Si bonds resulting in the formation of a silica layer in addition to the alumina layer.

To go further in the characterization of the grafting over the different powders of zirconia and alumina, nitrogen and water vapor sorption analyses were performed. The nitrogen sorption isotherms are presented in Fig. 7. In the case of zirconia-based materials, the sorption isotherms belong to the type II, typical for non-porous or microporous materials, whereas for alumina-based materials, type IV sorption isotherms have been obtained, typical for mesoporous materials. From the BET transform of the sorption isotherms, the C<sub>BET</sub> parameter can be extracted. This parameter C depends on the pore diameter, surface porosity, and adsorbate/adsorbent affinity. A low value of C<sub>BET</sub> is thus obtained in large pores materials, with low surface interaction. The value of C<sub>BET</sub> decreased from unmodified membranes to modified membranes with C6F, C8F and C12 as observed in Table 2. It can be concluded that the surface grafting disables the surface active sites such as Zr-OH or Al-OH groups.

In addition, water vapor sorption measurements (Fig. 8) were performed to anticipate what may occur during the AGMD experiments. Indeed, water vapor has a strong affinity for the zirconia and alumina hydrophilic layers as evidenced by the high Henry constant K<sub>H</sub> as shown in Table 2. Upon grafting, the values drastically decrease, which sheds light on a weaker affinity for the water vapor. The K<sub>H</sub> value decreases with an increase in the length of the hydrophobic chain from C6F to C8F. However, the alumina layer grafted with C12 seems to have a higher K<sub>H</sub> value despite the long hydrophobic chain because the C12 chain contains only –CH<sub>2</sub> group compared to –CF<sub>2</sub> groups of C6F and C8F.

To correlate the results from these sorption analyses in terms of number of molecules present on the modified surfaces, further TGA experiments were performed. As seen in Fig. 9, the first zone from room temperature to 200 °C is characteristic of the solvent or water evaporation with a slight weight loss of 3.0 % for all the modified powders except for the bare alumina that should encounter hydroxyl degradation as well. The second area ranging from 200 °C to 1000 °C should be representative of the weight loss encountered upon the C6F, C8F and C12 degradation. Hence, it can be assumed that these organic parts were stable up to 200 °C and that the molecules will be unable to be released in the water during the AGMD experiments. The highest weight loss of 38.4 % was observed for C8F that is higher than C12 despite C12 having a longer chain length. This may be attributed to the presence of the –CF<sub>2</sub>

groups compared to C12 containing only –CH₂ groups. When looking at the C6F-modified powders, a weight loss of 26 % was obtained, which was consistent with the size of the fluorinated chain compared to C8F. From Table 3, it was clear that long C12 chains cover more surface than the fluorinated chains of C6F and C8F. It might be because the fluorinated chains are more hindered and thus they are less present on the surface of gamma alumina.

The efficiency of the hydrophobic grafting over the capillary membranes can also be determined by measuring the Liquid Entry Pressure of water (LEP<sub>w</sub>). Generally, ceramic membranes are hydrophilic in nature and can easily allow water to penetrate through the pores of the membrane. Hence, their LEP<sub>w</sub> was 0 bar because there was no pressure difference during water transport in the system. The obtained LEP<sub>w</sub> values of the hydrophobized capillary membranes were found to change with the variation in the hydrophobic chain length and with the type of pore forming agent used during the preparation of the membrane. As shown in Fig. 10, the LEP<sub>w</sub> values at 20 °C of the C8F-grafted membranes was found to be higher whatever the pore forming agent used to prepare the capillary membranes. When looking at the membranes modified with C6F, the same trend is observed than for the C8F molecules with the corn starch-based membranes having a higher LEP<sub>w</sub> than the maize flour-based and rice husk-based membranes. In addition, the LEP<sub>w</sub> was found to decrease to 2 bar and less for the C12-grafted membranes despite the long –CH₂ chains. Membranes prepared with corn starch M1 and maize flour M4 as pore forming agents, grafted with the C8F molecules were found to be more hydrophobic than the rice husk-based membranes M7 with LEP<sub>w</sub> of 5.5 bar and 4 bar, respectively. Finally, these results are in total accordance with the contact angles measured over the flat modified supports.

The contact angle of the C8F-grafted membranes M1 of 145.5° was found to be higher due to the long –CF₂ chains and higher molecular weight of the hydrophobic part. The contact angles of C6F (125.5°) and C12 (124.5°) grafted membranes M4 were almost similar due to the presence of –CF₂ groups in C6F and long chain –CH₂ groups in C12 and reached 137° for C8F grafting. The rice husk-based membranes M7 had comparatively lesser contact angle values of 112–114° for the C6F and C12-modified membranes despite smaller pore size that may be due to the presence of macrovoids in the membrane and went up to 130.5° for the C8F grafting.

## 3.2. Hydrophobic capillary membranes performance in AGMD

One of the main features of the membranes being applied to membrane distillation experiments is the non-pore wettability of the pore walls by the feed solution. In addition, the LEP<sub>w</sub> of the membranes should be considerably high to ensure sufficient hydrophobicity and repelling interaction with the fluid. Finally, the membranes should have good mechanical resistance in order to withstand high temperature and pressure, and a porosity of minimum 25 % to ensure better water flux

<figure><img src="image_9.png" /><figcaption>Fig. 6. ATR-IR spectra of bare and grafted alumina powders (left) before TGA and (right) after TGA experiments.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Fig. 7. Nitrogen adsorption/desorption isotherm of (A) bare zirconia powder, (B) zirconia grafted with C6F, (C) zirconia grafted with C8F, (D) zirconia grafted with C12, (E) bare alumina layer, (F) alumina grafted with C6F, (G) alumina grafted with C8F, and (H) alumina grafted with C12.</figcaption></figure>

upon cooling.

The temperature difference (ΔT) was maintained at 80 °C and each experiment was carried out for a period of 6 h. In all the experiments performed in AGMD, an increase of the permeate flux is observed at the first stage of the experiment to stabilize after around 2 h. With the time, the temperature at the inside interface of the capillary within the

module is slightly lower due to heat exchange with the cold part of the outer side and a slight heating is thus noted on the permeate side of the capillary. The slight heat exchange at the membrane walls has faintly affects the temperature gradient inducing the stabilization of the permeate flux. A similar behavior was noticed for polymer membranes in AGMD of highly saline solutions [51]. It was found that membranes
<caption>**Table 2**</caption>

Evaluation of the BET specific surface area (SSA), the C<sub>BET</sub> parameter, the pore size obtained from N<sub>2</sub> adsorption isotherms, and the Henry's constant K<sub>H</sub> from the water vapor adsorption isotherms for both the zirconia and alumina layers.

<table><thead><tr><th rowspan="2"></th><th colspan="4">Zirconia UF layer</th><th colspan="4">Gamma alumina layer</th></tr><tr><th>BET SSA (m<sup>2</sup>/g)</th><th>C<sub>BET</sub></th><th>Average pore size (nm)</th><th>K<sub>H</sub>/H<sub>2</sub>O (mg/g)</th><th>BET SSA (m<sup>2</sup>/g)</th><th>C<sub>BET</sub></th><th>Average pore size (nm)</th><th>K<sub>H</sub>/H<sub>2</sub>O (mg/g)</th></tr></thead><tbody><tr><td>bare</td><td>40.7</td><td>103</td><td>16</td><td>110</td><td>187.7</td><td>102</td><td>9.1</td><td>481</td></tr><tr><td>C6F</td><td>25.4</td><td>23</td><td>28</td><td>46</td><td>169.1</td><td>28</td><td>5.7</td><td>247</td></tr><tr><td>C8F</td><td>13.1</td><td>18</td><td>41</td><td>33</td><td>48.5</td><td>30</td><td>6.7</td><td>109</td></tr><tr><td>C12</td><td>23.4</td><td>19</td><td>50</td><td>24</td><td>121.7</td><td>26</td><td>7.4</td><td>184</td></tr></tbody></table>

<figure><img src="image_11.png" /><figcaption>**Fig. 8.** Water vapor adsorption/desorption isotherms for zirconia powders (left) and alumina powders (right) with and without grafting.</figcaption></figure>

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

<figure><img src="image_13.png" /><figcaption>**Fig. 9.** Weight loss of unmodified and modified alumina membranes using TGA.</figcaption></figure>

<caption>**Table 3**</caption>

Calculation of the number of molecules per nm² grafted onto the oxide surface.

<table><thead><tr><th>Grafted molecules</th><th>M<sub>w</sub> of hydrophobic part (g/mol)</th><th>Number of molecules per nm<sup>2</sup></th></tr></thead><tbody><tr><td>C6F</td><td>347</td><td>2.39</td></tr><tr><td>C8F</td><td>447</td><td>2.75</td></tr><tr><td>C12</td><td>169</td><td>2.89</td></tr></tbody></table>

modified by C8F had better rejection compared to C6F and C12-modified membranes as shown in Fig. 11. These results were in accordance with the LEP<sub>w</sub> values and the contact angles obtained during the membranes characterization. Similarly, the maize flour and rice husk-based membranes yielded higher flux compared to the corn starch-based membranes because the latter had higher LEP<sub>w</sub> values. The

NaCl rejection of rice husk based membranes was lower because of a lower hydrophobicity as obtained from the LEP<sub>w</sub> and due to the presence of macrovoids in the membrane. Among the membranes selected for AGMD, membranes with corn starch M1 as pore forming agent and modified with C8F showed the best performance with a rejection rate over 95 % and a permeate flux of 3 L/(m²·h·bar). Since the wall thickness of the capillary is low compared to ceramic membranes reported in the literature, it can be inferred that a larger heat loss by conduction occurs during the MD experiments. To overcome this, we would prepare other capillaries with increased wall thickness using the same formulations as described in this work, expecting that better results will be obtained.

In order to compare the results with the change in temperature difference (ΔT), AGMD was performed for gamma alumina M1 membranes with corn starch as pore forming agent grafted with C8F with a ΔT of 70 °C, 60 °C, 55 °C, and 50 °C. It was found that the water flux of the membranes decreased from 2.8 L/(m²·h) for 70 °C to 2.0 L/(m²·h) for 60 °C as shown in Fig. 12. By lowering the temperature gradient, it is observed that even lower water flux are obtained down to 1.0 L/(m²·h) for 50 °C. Similarly, the NaCl rejection decreased from 89 % to 66 % with the lowering of the driving force of the system. This shows that there is a strong influence of the temperature difference applied on the transport properties of the membranes. In addition, it remains possible to increase the temperature difference to 90°C to obtained better performances in AGMD.

In the literature, are mainly found electrospun and commercially available membranes that are further chemically modified and they are usually characterized by 10/7 mm O.D./I.D dimensions. Voigt et al. [52] prepared capillary membranes in alpha alumina, coated with additional filtration layers of TiO<sub>2</sub>, with dimensions of 2.9/1.9 mm O.D./I.D. by extrusion and sintered at 1700 °C. The UF layer thickness was 7 µm with 5 nm pore size and the pure water permeability of their membrane was 160 L/(m²·h·bar). These results are comparable with the prepared capillary of 3.5/2 mm O.D./I.D. dimensions that is characterized by a pore size of 9 nm and a water permeability of 143 L/(m²·h·bar). The improvement of using hydrophobic capillary membranes in AGMD
<figure><img src="image_14.png" /><figcaption>Fig. 10. Contact angle (left) and LEP~w~ (right) of the alumina membranes prepared with different pore forming agents and grafted with C6F, C8F and C12.</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>Fig. 11. Water flux and NaCl rejection of gamma alumina-coated membranes prepared with (A) corn starch, (B) maize flour and (C) rice husk as pore forming agents obtained in AGMD at a $\Delta T$ of 80 °C.</figcaption></figure>

could yield to even better performances since the packing density inside the filtration module could enable a flux enhancement and probably a better rejection of salt. Indeed, instead of one 10/7 mm O.D./I.D

membrane, it was possible to place three ceramic capillaries inside the AGMD module. These experiments are under progress; however, engineering issues are being faced with the fixation of the three capillaries
<figure><img src="image_16.png" /><figcaption>**Fig. 12.** Performances in AGMD for gamma alumina membranes prepared with corn starch as pore forming agent and grafted with C8F at a ΔT of 70 °C, 60 °C, 55 °C, 50 °C.</figcaption></figure>

inside the module and have observed leakage of the feed at the module entry. The design of a new and more adapted module is expected soon.

## 4. Conclusion

With the objective to increase the volume to ratio packing density in the module of MD, capillary ceramic membranes of 3.5/2.0 mm outer/inner dimensions were designed by extrusion of pastes. To decrease the production cost of the capillary ceramic membranes, cordierite clay was used to prepare the supports and three different pore forming agents such as corn starch, maize flour and rice husk that are economically attractive and bio-sourced. To decrease the pore size of the capillary supports to avoid pore wetting, two and three filtration layers made of zirconia (8 m²/g), zirconia (43 m²/g) and gamma-alumina (γ-Al₂O₃) were coated to yield an active filtration layer with 9 nm pore size, in the range of low ultrafiltration characterized by a Henry's constant of 481 mg/g. The capillary membranes are characterized by a mechanical strength of 14 MPa and a pure water permeability of 597 L/(m².h.bar) before grafting. The different capillary membranes were made hydrophobic by chemical grafting of three alkoxysilanes, namely 1H,1H,2H,2H-perfluorooctyltriethoxysilane (C6F), 1H,1H,2H,2H-perfluorodecyltriethoxysilane (C8F) and dodecyltriethoxysilane (C12).

The length of the hydrophobic chain had an impact on the LEP~w~ and contact angles of the modified capillary membranes. Membranes modified with the C8F molecules had a higher LEP~w~ value of 5.5 bar compared to the other membranes. The specific surface area of the modified membranes decreased upon grafting showing lower interaction of the nitrogen and water vapors with the zirconia and alumina oxides layers. The presence of the grafted hydrophobic alkoxysilane groups was also confirmed by ATR-IR spectroscopy and the grafted chemicals were shown to be stable in the temperature range of the membrane distillation experiments and up to 200 °C.

The performances of all the modified hydrophobic capillary membranes were evaluated in air gap membrane distillation of a saline solution at 35g/L. The impact of the grafted membranes formulation and characteristics on the separation process was demonstrated. We found that capillary membranes M1 prepared with corn starch as pore forming agent and modified with C8F had a high salt rejection over 95 % at a ΔT of 80 °C that could be improved by increasing the driving force of the system. Indeed, a higher ΔT will be applied and capillaries with thicker wall thicknesses will be prepared. We will also play on the grafting solution concentration to improve further the hydrophobic barrier in order to reduce the heat loss through conduction. The permeate flux of 3 L/m²·h can also be improved by using more than one capillary membrane in the module, that is under progress. In addition, trying to avoid perfluorinated molecules, other attempts are to be done to find other ways

to improve the contact angle of the capillary (145.5°) that will thus help to improve the MD performances. Finally, these capillary membranes were applied to oil/water separation to enlarge their potential application on water treatment that will be the subject on another publication.

## CRediT authorship contribution statement

**Sharath Jayakumar:** Methodology, Formal analysis, Investigation, Data curation, Writing – original draft. **Philippe Trens:** Writing – review & editing. **Mihail Barboiu:** Conceptualization, Resources, Funding acquisition, Supervision, Validation, Writing – review & editing. **Sophie Cerneaux:** Conceptualization, Methodology, Supervision, Validation, Project administration, Writing – review & editing.

## Declaration of competing interest

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.

## Acknowledgement

The authors would like to acknowledge the IEM workshop for their involvement in the multi-capillary module conception and the French government for financial support.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.seppur.2024.130679.

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