Marine Harguindeguy
email: marine.harguindeguy@umontpellier.fr

Claire Antonelli
email: claire.antonelli@umontpellier.fr

Marie-Pierre Belleville
email: marie-pierre.belleville@umontpellier.fr

José Sanchez-Marcano
email: jose.sanchez-marcano@umontpellier.fr

Céline Pochat-Bohatier

Gelatin supports with immobilized laccase as sustainable biocatalysts for water treatment

Keywords: Gelatin beads, biocatalysts, Enzyme immobilization, Laccase

Millimeter-size beads of gelatin are manufactured by dripping process to give enzyme supports qualified for micropollutants biodegradation in alternative wastewater treatment.

The bead diameter is dependent on the tip diameter, the gelatin solution viscosity and the swelling of polymer chains in the collecting bath. Chemical cross-linking was performed with glutaraldehyde using optimal concentration to give mechanical and thermal properties suitable for application in stirred reactor in aqueous medium. Laccases from Trametes versicolor are grafted on the gelatin beads with glutaraldehyde. 60% of the initial enzymatic activity, evaluated by the oxidation of 2'-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)diammonium salt (ABTS) is maintained after 10 successive cycles of reaction. Thermal stability at 60°C of immobilized biocatalysts is improved when compared to free enzymes (45% vs 10% of relative activity after 6 hours of incubation). The simplicity of the procedure to form gelatin beads and their properties make them promising bio-based and biodegradable support for enzyme immobilization.

INTRODUCTION

The interest of enzymatic reactions in industrial applications has been increased in recent years because the use of biocatalysts results in green processes which can be carried out without organic solvents and at mild conditions. 1,2 Different areas can benefit from enzymatic bioprocesses due to the high specificity and selectivity of enzymes. They include pharmaceutical, 3,4 food, 5,6 cosmetic industries 7 and environmental applications. 8,9 Nevertheless, the main drawback for full development of enzymatic processes concerns the biocatalyst separation and its reusability which affects the operation costs. The immobilization of enzymes onto a support allows the biocatalyst reuse and enhances its stability. 10 Efficient immobilization, avoiding enzyme leaching, is achieved on support displaying numerous functional groups, large surface area, biocompatibility, no toxicity, and good structural and mechanical stability. 11 Different supports have been studied and applied at industrial scale from inorganic supports like silica, 12 inorganic oxides, 13 minerals, 14 to organic materials as biopolymers 15 or synthetic polymers, 16 and even hybrid materials. 17 Biopolymers like alginate and chitosan are frequently used for enzyme immobilization due to their interesting properties which include biocompatibility, high biodegradability, absence of toxicity and relatively low cost. 15,18,19 Gelatin brings all these properties and allows enzyme immobilization by encapsulation or entrapment. [20][21][START_REF] Schrieber | Gelatine Handbook: Theory and Industrial Practice[END_REF] Gelatin microspheres can be prepared by emulsification, [START_REF] Franssen | [END_REF] electrospinning, 24 spray drying, 25 oil-in-water emulsion 26,27 and water-in-water emulsion. 28 From our knowledge, no study has been already reported on the preparation of millimeter-size beads by dripping in a gelation bath for gelatin. However, dripping process is a very simple process to implement; it has been widely used to prepare alginate beads for various applications. [28][29][30] This paper presents the operating conditions to design biocatalysts made by immobilized laccases onto gelatin beads for applications related to wastewater treatment. The formation mechanism of the beads is based on the coacervation of gelatin droplets in polyethylene glycol solution. The protein precipitate in the nonionic polymer because of excluded volume effect on gelatin. 31 The beads are then used as support for the covalent immobilization of a commercial laccase from Trametes versicolor. This enzyme is able to oxidize a broad variety of substrates using only oxygen as co-substrate and is suitable for environmental remediation by degrading micropollutants in water. 32,33 As long term stability in water and mechanical resistance to shear stresses is required, conditions of beads cross linking is studied and additional material characterizations (swelling capacity, thermal property) are carried out to evaluate sustainability of gelatin beads in aqueous solutions. Finally, laccase are grafted and enzymatic activity and stability of biocatalysts obtained are studied using ABTS oxidation as model reaction for micropollutant biodegradation.

EXPERIMENTAL

Beads elaboration

A proper amount of gelatin is added to 30 g of water in order to achieve final concentrations of 15, 18, 20 and 22 wt-%; the suspension is heated and let to dissolve at controlled temperature (60°C) during 2h. PEG solutions of diverse molecular weights (5, 10, 20, 35 KDa) are prepared in distilled water at a concentration of 40% w/v. They are let to mix during the whole night at 50°C. PEG 10 solutions with various concentrations (20%, 30%, 40%, 50% w/v) are formulated using the same procedure. The dripping process is carried out by adding dropwise a gelatin solution into a polyethylene glycol solution at 50°C via a SP200 syringe pump (flow rate: 4 mL min -1 ). The low affinity between gelatin and PEG allows the droplets to keep their bead shape because gelatin does not mix with PEG. Freezing of the droplets is then carried out by immersion in an ice bath at 5°C so that the gelatin beads undergo a fast gelation. Afterward, they are taken away from the solution and washed thoroughly with water to remove residual traces of PEG. Figure 1 sums up the steps of the elaboration of gelatin beads.

Rheological measurements

The viscosity of gelatin and PEG solutions is measured by the means of a MCR 301 rheometer (Anton Paar) using a cone-and-plate geometry (diameter 50 mm, angle 1°). The measurements are carried out at 60°C for gelatin solutions and 50°C for PEG solutions, with a constant shear rate of 500 s -1 .

Theoretical determination of beads diameter

The size of the beads obtained by dripping technique can be predicted by using the work of Kamaruddin et al (2014), based on Tate's law, with Equation 1, where d T is the tip diameter (0.27.10 -3 m), γ the gelatin surface tension (41 mN.m -1 at 60°C), ρ the gelatin solution density

60°C 50°C 5°C a) b) c)
(kg.m -3 ), g the gravitational constant (9.81m.s -2 ), K LF the liquid lost factor, and K SF the shrinkage lost factor. 29,34 𝑑 𝑝 = 𝐾 𝑆𝐹 𝐾 𝐿𝐹 (

0.06𝑑 𝑇 𝛾 𝜌𝑔 ) 1 3 (1)
K LF was demonstrated to be only dependent to the tip size, and not to the liquid properties, and can be determined using Equation 2.

𝐾 𝐿𝐹 = 2.471 -1.926 * 𝑑 𝑇 (2)
K SF is the shrinkage factor and corresponds to the ratio between the diameter of the bead before and after gelation.

Cross-linking and activation of gelatin beads

The beads are immerged in an aqueous glutaraldehyde (GTA) solution under magnetic stirring (250 rpm), during 1 hour at 25°C. They are then filtered and thoroughly washed with citrate phosphate buffer (pH 7; 50 mmol.L -1 ) to remove the unreacted GTA. To study the influence of GTA concentration on the cross linking ratio, several solutions are prepared: 0.025; 0.05; 0.1; 0.3; 0.5; 1; 2; 3; 4 and 5% v/v.

Cross-linked beads are then immerged in a solution of laccase (Trametes Versicolor)

(3 U.mL -1
) prepared in a citric phosphate buffer solution (pH 7; 50 mmol.L -1 ) during 2 hours at 25°C under magnetic stirring (250 rpm). The beads are once again washed thoroughly with buffer solution (3x50 mL) to remove the enzymes that have not been immobilized on the surface.

Activity assay of laccases and kinetics constants determination

The activity of free and immobilized laccase is determined using a colorimetric assay based on the oxidation of ABTS. 900 µL of ABTS (1 mmol.L -1 , pH 4) are mixed with 100 µL of enzymatic solution (for free enzyme) directly inside a spectrophotometric cell and the reaction is monitored with a spectrophotometer Shimadzu UV-2401 PC at 420 nm along with the software UV Probe version 2.21. One unit of enzyme activity is defined as the amount of enzyme that converted 1 μmol per minute of ABTS to its cation radical at pH 4 and 25°C. The optical density (or absorbance) can be linked to the amount of enzyme with the Beer Lambert law :

𝑂𝐷 = 𝜀 * 𝑙 * 𝑐 (3)
with ε the molar extinction coefficient (0.036 L.µmol -1 .cm -1 ), l the optical path length (1 cm).

In the case of immobilized enzymes, 0.7 g of active beads are immerged in 25 mL of ABTS solution (1 mmol.L -1 , pH 4) and the absorbance of the solution is monitored every minute during 15 minutes. The slope of the curve is then used to determine the rate of ABTS conversion and thus is directly related to enzymatic activity (A).

The immobilization yield (η im ) was determined (equations 4 and 5) by taking into account the enzymatic solution activity before (A i ) and after grafting (A f ), as well as the sum of the activity of the washing solutions ∑ A 𝑤 .

A 𝑖𝑚 = A 𝑖 -(A 𝑓 + ∑ A 𝑤 ) (4) η 𝑖𝑚 = A 𝑖𝑚𝑡ℎ𝑒𝑜 A 𝑖 * 100 (5)
A im : Theoretical immobilized enzymes activity

The expressed activity 𝐴 𝑒𝑥𝑝 (%) is defined by the ratio between the experimental immobilized enzymes activity 𝐴 𝑖𝑚 𝑒𝑥𝑝 (determined by ABTS assay) on the initial enzymatic activity (equation 6) :

𝐴 𝑒𝑥𝑝 = 𝐴 𝑖𝑚 𝑒𝑥𝑝 𝐴 𝑖 * 100 (6)
The pH profiles of the free and immobilized enzymes were determined by measuring the enzymatic activity at 25°C, with 1 mmol.L -1 ABTS solutions prepared at pH 2.

Thermal stability and reusability of the supports

Thermal stability of the free and immobilized enzymes is evaluated by incubating the samples at 60°C for 6 hours. The activity is measured with ABTS as substrate (50 mmol.L -1 ;

pH 4) at 25°C at different time intervals. Enzymatic activity of immobilized enzymes and free enzymes is measured before incubation and is considered as reference value of 100%. The experiment is carried out in triplicate and the average value is displayed.

To determine the reusability of the supports, 0.64 g of active beads at an enzymatic equivalent of 60 U.L -1 is let to react with 25 mL of ABTS (1 mmol.L -1 , pH 4, 25°C) during 10 cycles of 15 minutes. Between each cycles, the beads are washed thoroughly with 50 mL of citrate phosphate buffer (pH 7, 50 mmol.L -1 ). The solution of ABTS is replaced by a fresh one at the end of each cycle. The initial activity was considered as reference value of 100%.

Bead characterizations

FT-IR analysis

Infrared spectra are recorded on a Nicolet 710 Fourier-transform infrared (FTIR)

spectrometer, using a Thermo-Electron attenuated total reflectance (ATR) accessory with a diamond crystal. For each spectrum, 128 scans are collected with a resolution of 4 cm -1 .

Crosslinking ratio determination

In order to evaluate the influence of the GTA concentration for a given reaction time on the crosslinking ratio, determination of the number of residual amino groups in gelatin after crosslinking is carried out. 35,36 1 mg of cross-linked gels (with different concentrations of GTA) are solubilized in 1 mL of Borax during 3 hours at 60°C. Then, 1 mL of 1% v/v TNBS solution is added, and the solutions are stirred and heated at 60°C during 2h30. 3 mL of HCl (1 mol.L -1 ) are added to stop the reaction. After 30 minutes, the solutions are diluted ten times and the absorbances are measured with the spectrophotometer Shimadzu UV-2401 PC at 347 nm. The absorbance of two controls is also measured: one sample of uncrosslinked gelatin (Abs G ) prepared in the exact same way as described before, and one sample of TNBS/Borax/HCl (Abs TNBS ). The crosslinking ratio is calculated according to Equation 8.

𝐶𝑟𝑜𝑠𝑠 𝑙𝑖𝑛𝑘𝑖𝑛𝑔 𝑟𝑎𝑡𝑖𝑜 = 100 * [1 - 100 * (𝐴𝑏𝑠-𝐴𝑏𝑠 𝐺 ) 𝐴𝑏𝑠 𝐺 -𝐴𝑏𝑠 𝑇𝑁𝐵𝑆 ] (7)
3 replicates are carried out for each sample and the average value is reported.

Swelling ratio

Gelatin beads (uncross-linked and cross-linked at various concentrations of GTA), are dried in a desiccator under silica gel for 48 hours until they reach a constant weight (w d ). Beads are then immerged in 15 mL of distilled water at 25°C. From this point, the gels are weighed at different times (w t ) with a Precisa XB 320 M precision balance to see the evolution of the swelling ratio (SW (%), Equation 9). Each experiment is carried out by triplicate and the average value is reported.

𝑆𝑊(%) = 𝑤 𝑑 -𝑤 𝑡 𝑤 𝑑 * 100 (8)

Mechanical properties

Gelatin solutions of several concentrations are poured in cylindrical molds. After 4 hours at room temperature, hydrogel cylinders are then sliced into small samples (0.8 cm of diameter, 2 cm of height). A part of the samples are cross-linked with GTA (0.5%, 1%, 2%, 3%, 4%, 5% v/v) during 1 hour at 25°C.

Uniaxial compression tests are carried out with a dynamic mechanical analysis system (Metravib, 50dB), equipped with the software Dynatest 6.93. Both cross-linked and uncrosslinked hydrogels are tested and are compressed at a speed rate of 10 -5 m.s -1 using a 40 N load cell. The young modulus E, the stress at break σ b and the strain at break ε b are determined. Measurements are carried out with 5 replicates and the mean data are given.

To evaluate the beads resistance in water, the gels are dipped in 15 mL water and the same tests are carried out at different times after immersion. The apparatus was turned upside down so that the gels are immersed in water even during the tests (Figure 2).

FIGURE 2: Dynamic mechanical analysis in immersion mode

Thermal analysis

Thermal properties are determined both on pristine and crosslinked gelatin beads. Their degradation temperature and residual humidity are measured by thermogravimetric analysis (TGA) carried out on a TA Instruments TGA G500 under nitrogen flow of 60 mL.min -1 . The samples are heated from 20°C to 1000°C at a heating rate of 10°C.min -1 . The gels are then analyzed with a differential scanning calorimeter DSC 2920 equipped with cooling accessory RCS90 (TA Instruments). The samples (around 3 mg) are sealed in aluminum TA pan. The instrument is calibrated with pure indium and used an empty pan as reference. The gels are heated from -40°C to 200°C at a 20°C.min -1 heating rate. The value given for each sample is an average of 3 replicates.

RESULTS AND DISCUSSION

Beads elaboration

The first experiments are conducted by dripping a gelatin solution of 22 wt-% in a coagulation bath made with PEG 10 solution with a concentration ranging from 20 to 50 wt-% (Table 1). and closer from each other enhancing the exclusion phenomena between the polymer and the protein. 37 Higher PEG 10 concentration favors protein folding to form beads of smaller diameter. 38 Secondly the velocity of gelatin droplets inside the PEG 10 solution (visual observation) decreases with the rising of PEG concentration, limiting the deformation of droplets along the phase separation process. However, for high concentration of PEG (at 50% w/v), the droplet do not penetrate into the solution. Therefore, a concentration of 40% w/v was chosen. These preliminary experiments lead us to take also into consideration the viscosity of the solutions for further assays. Table 3 shows the influence of the gelatin concentration on the beads formation and diameter. The formation of the beads is here achieved with a PEG 10 solution of 40% w/v. The results

show that if the concentration of the gelatin solution is less than 20 wt-%, the droplet spread out on the surface, making it impossible to form a bead. The viscosity of PEG 10 solution is 60.1 mPa.s (Table 2) and the droplets penetration is possible when gelatin solution viscosity is 78.2 and 133 mPa.s, suggesting that it needs to be superior to the collecting bath viscosity to form beads by dripping. Another point is to consider that when droplet does not penetrate inside the collecting bath, it is because it has not enough energy to break through the surface. This can be related to the density of the gelatin solution which increases with the concentration rising and thus makes the droplet weight high enough to perforate the liquid interface. Additionally, it has also been observed a slight but non significant bead diameter reduction with the increase of gelatin concentration.

A concentration of 22 wt-% gelatin was therefore chosen, and all the optimized parameters for the beads elaboration are compiled in Table 4. Assays are first performed by dripping into a PEG bath with a temperature controlled at 5°C, with the aim of a fast gelation of the gelatin solution. However, at this low temperature, the droplets remain close to the surface of the bath, while when the temperature is controlled at 50°C, the beads penetrate deeply in the bath, which ease their formation and improve their sphericity. The bath temperature is therefore fixed at 50°C along the dripping process.

[Gelatin] (wt-%) 22

PEG M w (KDa) 10

[PEG] (%w/v) 40

Temperature of PEG solution (°C) 50

Syringe pump flowrate (mL.min -1 ) 4

Surface tension γ gelatin/air (mN.m -1 )* 40.7

Surface tension γ PEG/air (mN.m -1 )* 81.7 ± 2.06

Experimental bead diameter Φ experimental (mm) 1.9 ± 0.19

Theoretical bead diameter Φ theorical (mm)** 1.3

The determination of the gelatin solution density at 22 wt-% (Table 3) and the surface tension of the gelatin/air interface (Table 4) is achieved to use Equation 2 and predict the bead diameter by using the method described by Kamaruddin et al (2014) (see Part 2.6.). A theoretical diameter of 1.3 mm is found by using a shrinkage factor equal to 1, thus assuming that the gelatin immersion does not induce any shrinkage of the bead. By comparing this value with the experimental one (1.9 ± 0.19 mm), it appears that the gelation does not induce any shrinkage but on the opposite there is a swelling of the polymer matrix which increases the bead size. We can therefore make the assumption that this theoretical diameter corresponds rather to the bead size before immersion in the PEG solution. A simple calculation of the ratio of the bead volumes after and before gelation, (considering that the bead volume is V = .d p 3 /6) shows that the bead volume of gelatin is three times more important after gelation (with d p = 1.9 mmm) in the PEG solution compared to the bead volume after detachment from the tip (with d p = 1.3 mm). The bead volume before gelation is dependent of the gelatin/air surface tension and the tip diameter. The tip diameter was determined to be 0.54 mm, which is already relatively low, regarding the flowing of the relatively viscous gelatin solution. According to the predictive model and our experimental observations, it can be concluded that the production of gelatin beads using our experimental set-up can only lead to beads with diameter close to 2 mm. The swelling of the gelatin spheres associated to the viscosity of the protein solution make impossible the production of beads with smaller diameter. A reduction of a tip diameter would be necessary but implies also the use of higher pressure to pump the gelatin solution.

The surface tensions of PEG/air and gelatin/air interfaces are calculated by using Tate's law (Table 4). It can be noted that the surface tension of PEG/air interface is twice higher than the one of gelatin/air. Gelatin droplets need to break the PEG/air interface to penetrate into the PEG solution. This can be related to the minimal density (1.045 g.mL -1 ) required for the gelatin solution to penetrate the PEG solution (see Table 3).

FTIR analysis

A typical FTIR spectrum of un-crosslinked gelatin hydrogel (22 wt-%) is presented in Figure 3.

It displays the characteristics bands of gelatin: the large band around 3300 cm -1 corresponds to the amide-A peak (N-H stretching band) as well as free water; the amide I (C=O) is detected from the peak around 1630 cm -1 , the amide II (N-H and C-H bands) is represented by the peak around 1550 cm -1 , and the small peaks at around 1240 cm -1 matches the amide III bonds (C-N, N-H). 39,40 No others bands are present in the spectrum, thus it can be concluded that the polyethylene glycol is correctly washed and do not affect the chemical composition of gelatin beads. It confirms that PEG molecules are not inserted in the gelatin beads. Their use is needed to hinder gelatin mixing in the coagulation bath and thus allows maintaining the bead shape of the droplets.

FIGURE 3: FT-IR spectrum of uncross-linked gelatin hydrogel

Influence of the cross linking ratio on the gelatin beads properties

The cross linking ratio of gelatin beads has been studied in order to maximize the number of active sites where the enzymes can be grafted as well as to improve the mechanical properties of the beads. Gelatin is indeed a material which has the ability to absorb a high quantity of water, and this property can become a problem for the use in aqueous media.

Due to the multiplicity of GTA forms in aqueous solution, it is quite difficult to have a simple explanation for the crosslinking mechanism, as several reactions probably happen at the same time. The most commonly accepted hypothesis in the literature is the reaction between aldehyde groups of GTA with the lysine residues of gelatin to form a Schiff base.

More specifically, the amino groups (-NH 2 ) of lysin residues proceed in a nucleophile Moreover, GTA molecules will undergo a polymerization by aldol condensation, and the imine linkages of the Schiff base formed will thus be stabilized by conjugation. 41 It has been currently reported in the literature that the reaction of GTA and proteins can be monitored by a color modification from white to yellow-orange. This change had been explained by the conjugation of the double bonds which generate light absorption. 42 Gelatin cylindrical hydrogels are cross-linked (from 0% GTA to 3% GTA). Figure 5 displays the enhancement of the color intensity of gelatin cylinders together with the GTA concentration. This result indicates the increase of the imine linkages of the Schiff base formed between aldehyde groups of GTA molecules with the lysine residues of gelatin as explained above. Beside these qualitative observations, TNBS assay 34 is used to determined quantitatively the final crosslinking ratio of the gelatin beads in relation with the concentration of GTA used during the crosslinking step (Figure 6). It can be observed that when the GTA concentration increases, the cross linking ratio increases very rapidly up to 100% for a 2 % of GTA concentration, with small deviation around 100%. Then from 3% no deviation around 100% was observed. With the aim to confirm the results obtained with TNBS assay, cross-linked gels have been immersed in water for 35 h and the swelling ratio is determined with time up to 35h (Figure 7). The gelatin beads present a much lower swelling ability once cross linked. After 24 hours of immersion in water, the beads without cross linking have an important swelling ratio (485%), whereas when cross linked with a GTA solution of 3%, the swelling ratio is reduced to 135%. No significant difference on the swelling ratio between GTA concentrations of 3%, 4% or 5% is observed which confirms the results obtained with TNBS assay (Figure 6): from 3% GTA, it can be considered that the cross-linking reaction between GTA and gelatin is completed.

Previous studies confirm the result that cross linking reduces the capability of a gel to absorb water. 44,45 This property can be explained by the reaction of the GTA with the amine functions of gelatin, which prevents them to interact and form hydrophilic bonds with water molecules, and which reduces the chain mobility.

The mechanical properties of the as-prepared gel are then determined, first on the gels themselves then on the gels after 24h immersion in water and using the immersion mode of the instrument. The Young modulus, stress at break, and strain at break of gelatin hydrogels cross linked with different amount of GTA are given in Table 5. 12.0 ± 0.9 11.0 ± 0.9

The data clearly show that initially, the Young modulus of un-crossliked gelatin is relatively low (0.8 MPa). When the cross linking degree is increased, the Young modulus for gels with and without immersion increases until a plateau reached from 3% GTA (Young modulus  2.1 MPa), even if from 5% it seems to decrease but results are not really significant.

Concerning stress and strain at break, a similar phenomenon is observed. It can be explained by the fact that when the cross linking degree increases, the number of covalent bonds in the gelatin structure increases as well, which strengthen the gel. TNBS assay demonstrated that from 3% GTA the cross-linking ratio was maximum (Figure 5). Bigi (2001) and Talebian (2007) have previously shown that Young modulus and stress at break of gelatin increase with the % of GTA and thus the degree of cross-linking. 36,46 Amounts of GTA higher than 3%

do not improve the mechanical properties of the gel.

As the main application for these gelatin beads is to be used as enzyme immobilization supports in permanent contact with water, it is relevant to consider rather the mechanical properties of the gels after 24 hours of immersion in water. However results do not display significant change and here again the value of 3% GTA seems suit for cross-linking. It can also be concluded that the stiffness of the gelatin beads is increased by GTA cross-linking, allowing a sustainable use in aqueous solutions. The thermal stability of the gels was also evaluated because gelatin is well-known to display a gelation temperature close to ambient conditions. Firstly, it has been observed that the beads resist to immersion in water up to 100°C once cross-linked with 3% GTA. Figure 8 represents a typical thermographic analysis (TGA) curve of uncross-linked gelatin gel (22 wt-%). The first part of the curve coincides with the water evaporation, around 100°C, and the second decrease matches with gelatin degradation, starting at around 200°C. According to the TGA results; the temperature range -40°C to 200°C was selected for the DSC experiments, at a rate of 20°C/min.

DSC scans allow the determination of the triple helix denaturation temperature T d of gelatin gels. Figure 9 displays the thermograph of uncross-linked gelatin (curve labelled nr), and of gelatin gel cross-linked with 1% GTA. The endothermic peak around 100°C for the uncross-linked sample and around 150°C for the cross-linked one matches to melting point, that is to say the denaturation of the triple helix into a random configuration, with rupture of hydrogen bond. The thermal transition T g is quite difficult to see here, probably hidden with the melting peak for the black curve, but it can be guessed around 100°C for the crosslinked gel. This experiment has been made for different samples, not displayed in the same figure, but all the results are shown in Table 6. of the proteic nature of both the enzymes and the support. However the immobilization yield (equations 4 and 5) was found to be 31 ± 6% and the expressed activity (equation 6) is 50 ± 5%. The immobilization yield and the expressed activity are thus inferior to 100%, meaning that grafting conditions may be improved to increase these parameters.

As immobilization can modify the pH and temperature profiles of laccase, the effect of pH and temperature on both free and immobilized laccase activities has been studied, within a range of pH from 2.2 to 6 and a range of temperature from 20 to 55°C. ions partitioning due to binding to the supports. 49 At higher pH, the relative activity decreases strongly for both free and immobilized enzymes. At pH 5, for free enzymes, the relative activity has already decreased until 10% ± 3%, and no activity is observed at pH 6.

For immobilized enzymes, 52% ± 4% of relative activity remains at pH 5 and there is still 10% ± 2% relative activity at pH 6. The immobilized laccase thus shows higher resistance to pH changes in the medium, and similar conclusions can be found in literature with various supports. 13,50 The rest of the experiments will be performed at pH 4 as it is the optimized value for immobilized enzymes.

The figure 10.B) shows that the temperature profiles of both free and immobilized laccases follow a similar pattern, as their activity increase from 20 to 35°C, reach an optimum at 35°C

and decrease until 55°C. The activity of free enzymes decreases to 75% ± 9% of relative activity at 45°C and 32% ± 5% relative activity at 55°C. The decrease of immobilized enzymes is less drastic and retains 98% ± 5% activity at 45°C and 79% ± 4% at 55°C. Immobilization seems to prevent enzymes from structural transformation when confronted to higher temperatures, several studies have already observed this result with diverse immobilization supports. [51][52][53] The kinetics parameters of Michaelis-Menten K m and V max can characterize the biocatalyst activity towards a chosen substrate. Here, the affinity of free and immobilized laccase onto gelatin beads towards ABTS are compared.

The results obtained with Lineweaver-Burk equation are represented in Table 7. The affinity of an enzyme towards a given substrate is higher when K m is low whereas the reactivity increases with V max enhancement. Results show that in the case of ABTS, the K m is lower when laccase is immobilized onto gelatin beads when compared with free enzymes, while V max is slightly higher with immobilized enzymes than with free enzymes. It is important to note that the value of V max depends on the enzymes concentration and can only be compared if the amounts of enzymes used are identical for both enzyme forms. In our case the experiments are carried out at the same enzyme "activity" (30 U.L -1 ) for the both forms of laccases; it is therefore not surprising that the V max values are close. It is thus not possible to conclude whether or not immobilization on gelatin beads improves the reactivity of the laccase of Trametes versicolor. Commonly in the literature is reported that immobilization on inorganic or polymer supports causes diffusional limitations and decreases protein flexibility and then gives a more difficult approach of the substrate to active sites of the enzyme. [54][55][56] However, some studies have noticed a decrease of K m when laccase from Trametes

Versicolor is immobilized on gelatin supports. Asgher M. et al (2017) reported a slight decrease of K m in gelatin entrapped laccase compared to free laccase. 57 De Cazes M. et al (2014) reported a value of K m 5 times lower than free laccase when laccase was covalently immobilized on ceramic membranes coated by a glutaraldehyde crosslinked gelatin layer. 58 Thus, the present results confirm that gelatin seems to have a positive effect probably with less diffusional hindrance than traditional inorganic or polymer supports. One of the advantages of enzyme immobilization is that enzymes can be recovered and reused instead of having to be separated at the end of every reaction cycle. To verify this possibility, ten successive reactions have been carried out with the same beads. At the end of each cycle, the reaction medium is poured; beads are rinsed with water and added to a fresh ABTS solution. Figure 11 shows the relative activity of the immobilized enzyme for each cycle. It can be noticed that the activity globally decreases as the number of cycles increases, especially in the two first cycles, in which an abrupt decrease from 100 to 60% was observed. This decrease of activity would be caused by protein leakage from the beads or enzyme conformational changes leading to denaturation. 59,60 Indeed, it can be assumed that a part of enzymes are physically adsorbed onto the gelatin chains by weak interactions and are then released in the medium along the two first cycles, explaining why after the activity is roughly constant between cycle 3 and 10. However, the activity is then maintained at 60%

until the end of the 10 th cycle, which proves the presence of covalent bonds between laccase and gelatin and the ability of gelatin beads to become efficient enzyme supports.

ABTS

To evaluate the ability of immobilization to enhance enzyme stability, free and immobilized laccase are incubated for 6 hours at 60°C and their activity is measured every hour (Figure 12). It is observed that high temperature affects negatively the activity of enzymes. This result is well-known and can be explained because of the conformational change due to temperature increase. 61 Free enzymes activity decreases indeed to 40% after 1h30 of

Materials

  Gelatin porcine skin type A (MW  50-100 KDa, bloom 300) (CAS 9000-70-8), polyethylene glycol (PEG with molecular weight 5, 10, 20, 35 KDa are denoted PEG 5 , PEG 10 , PEG 20 and PEG 35 respectively) (CAS 25322-68-3), glutaraldehyde aqueous solution grade II (25% w t in H 2 O, CAS : 1111-30-8), laccase (from Trametes Versicolor, CAS 80498-15-32), 2'-Azino-bis(3ethylbenzothiazoline-6-sulfonic acid)diammonium salt (CAS 30931-67-0) (ABTS), 2,4,6-Trinitrobenzenesulfonic acid solution (TNBS) (5% w/v in H 2 O, CAS 2508-19-2), hydrochloric acid (37% w t in H 2 O, CAS 7647-01-0), and borax solution (5g/dl in deionized water) are of ACS reagent grade and purchased from Sigma-Aldrich. Buffer solutions are prepared from citric acid (CAS 77-92-9), and di-sodium hydrogen phosphate dihydrate (CAS 10028-24-7).

FIGURE 1 :

 1 FIGURE 1: Steps of bead elaboration: (a) dissolution of the gelatin solution at 60°C during 2h,

  of two phases.38 So PEG 10 is chosen as intermediate M w for the rest of the experiments.

FIGURE 4 :

 4 FIGURE 4: Nucleophile reaction between lysin residue of gelatin and carbonyl groups of glutaraldehyde

FIGURE 5 :FIGURE 6 :FIGURE 7 :

 567 FIGURE 5: Evolution of the color of the gelatin gels with the crosslinking ratio (left to right:

FIGURE 8 :

 8 FIGURE 8: Typical TGA curve of a gelatin sample (22 wt-%)

FIGURE 9 :

 9 FIGURE 9: DSC thermographs of gelatin un-crosslinked (nr) and cross-linked with 1% GTA

FIGURE 10 :

 10 FIGURE 10: A) pH profiles for free (black curve) and immobilized (red curve) enzymes, at 25 °C and B) Temperature profiles for free (black curve) and immobilized (red curve) enzymes, at pH 4. The figure 10.A) represents the pH profiles for free and immobilized laccase towards ABTS

FIGURE 11 :

 11 FIGURE 11: Relative activity of immobilized enzymes after several cycles of reactions with

  than 10% after 6 hours. With the immobilized enzymes, the activity decreases as well but the effect of temperature is less drastic and activity remains at a correct level (45% of relative activity after 6 hours at 60°C). The immobilization has a stabilizing effect on the enzyme, preventing molecular mobility and deformation of the active site. Other studies have shown similar results with different enzymes supports.51,62

FIGURE 12 :

 12 FIGURE 12: Stability of free or immobilized enzyme towards temperature

TABLE 1 :

 1 Influence of PEG 10 concentration on the gelatin beads formation; [gelatin] = 22 wt-

	%.
	Concentration (%w/v)	20	30	40	50
	Observations	Non spherical beads	Non spherical beads	Spherical beads	Droplets blocked at the surface
	characteristic length (mm)	5.0 ± 0.75	2.7 ± 0.11	1.9 ± 0.19	N/A

At relatively low PEG 10 concentrations (20% and 30% w/v), the beads formed are not spherical. Sphericity is improved when the concentration is increased (40% w/v) and the characteristic length of the objects is 1.9 mm. If PEG 10 is too concentrated (50% w /v), the droplets remain attached at the surface. This behavior can be explained by taking into consideration both physical-chemical and kinetics aspects. First, PEG and gelatin do not display affinity because of large excluded-volumes and the absence of acidic sites on PEG to interact with gelatin. When the concentration increases, the PEG chains are more numerous

TABLE 2 :

 2 The impact of the bath formulation is then investigated, by modifying the size of PEG. Four different PEGs showing molecular weight (Mw) of 5, 10, 20 and 35 kDa are tested and the characteristics of the beads obtained are reported in Table2. Influence of the PEG molecular weight (M w ) on the PEG solution viscosity (η) and At highest M w (i.e 20 and 35 KDa), the viscosity of PEG solutions is 220.5 and 649.5 mPa.s respectively, too high compared to gelatin one for droplets penetration into the polymer solution. They remain blocked at the surface of the bath and it is impossible to recover any beads. PEG 5 and PEG 10 are both efficient to form beads but PEG 5 gives bigger bead diameter. Studies on protein exclusion phenomena by PEG proved that when the

	gelatin beads formation; [PEG] = 40% w/v, [gelatin] = 22 wt-%, gelatin viscosity at 60°C=133
	mPa.s.
	PEG M w (KDa)	PEG η (mPa.s)	Gelatin beads formation	Beads diameter (mm)

The viscosity of the gelatin solution at 22 wt-% and 60°C has been measured and is equal to 133 mPa.s. degree of PEG polymerization (i.e M w ) increases, the mixing entropy is reduced, favoring

TABLE 3 :

 3 Influence of the gelatin concentration on the gelatin beads formation; [PEG 10 ] =

	40% w/v.
	[Gelatin] (wt-%)	Gelatin Density ρ (g.mL -1 )	Gelatin viscosity η at 60°C (mPa.s)	Faisability of beads formation	Beads diameter (mm)
	15	1.034	39.4	Droplets spread out on the liquid surface	N/A
	18	1.039	61.8	Droplets spread out on the liquid surface	N/A
	20	1.045	78.2	Droplets break liquid surface and form beads	2.5 ± 0.48
	22	1.053	133	Droplets break liquid surface and form beads	1.9 ± 0.19

TABLE 4 :

 4 Preparation conditions and bead diameter

* determined by the method described by

Worley (1992)

based on Tate's Law. ** calculated with the method described by

Kamaruddin et al (2014)

.

TABLE 5 :

 5 Comparison of gelatin hydrogels mechanical properties before and after 24 hours

		immersion
		Young modulus (MPa)	Stress at break (kPa)	Strain at break (%)
	[GTA]
		Before	After 24h		After 24h	Before	After 24h
	(%)			Before immersion
		immersion	immersion		immersion	immersion	immersion
	0	0.8 ± 0.05	0.7 ± 0.06	36.3 ± 1.25	20.0 ± 1	10.3 ± 0.5	9.0 ± 0.5
	0.5	1.27 ± 0.06	1.09 ± 0.26	55.3 ± 3.8	40 ± 16.3	10.6 ± 0.5	9.0 ± 0.9
	1	1.32 ± 0.11	1.59 ± 0.10	66.7 ± 2.4	73.3 ± 4.7	11.0 ± 0.2 11.00 ± 0.01
	2	1.62 ± 0.13	1.84 ± 0.16	96 ± 12	156.7 ± 4.7 11.7 ± 0.5	12.0 ± 0.5
					133.3 ±
	3	2.30 ± 0.24	2.01 ± 0.01	227.5 ± 6.0		12.0 ± 0.1	10.0 ± 0.9
					57.0
					223.3 ±
	4	2.13 ± 0.08	2.32 ± 0.22	223.3 ± 20		13.7 ± 0.5	12 .0± 1.2
					57.0
	5	1.92 ± 0.30	1.90 ± 0.19	180 ± 43	183 ± 15

TABLE 6 :

 6 Thermal properties of gelatin hydrogelsAll these characterizations provide evidences of gelatin beads cross-linking and allowed us to select a concentration of GTA of 3% v/v for the rest of the experiments. These conditions allow the preparation of gelatin beads displaying enough stability for further use in aqueous solutions.

	Enzymatic activity, stability, and reusability
	Gelatin beads are a support of choice for enzyme immobilization because gelatin displays
	numerous available amino functional groups, which can react with free aldehyde functions
	of glutaraldehyde without supplementary functionalization, and lead to robust covalent
	bonds. Secondly, this biopolymer is relatively cheap and widely produced at industrial scale.
	Moreover, gelatin is a bio-based polymer so biodegradation can be envisioned when
	enzymes become inactive. The immobilization of laccase from Trametes versicolor is carried
	out as explained in the experimental section, on gelatin beads of 2 mm of diameter and
	cross-linked with 3% of GTA solution.
	The enzymatic activity is measured with ABTS. After immobilization, the beads show an
	activity of 0.1 U.g -1 . The enzyme loading amount could not be directly determined because

TABLE 7 :

 7 Kinetics constants of laccase towards ABTS

		V max (µmol.min -1 )	K m (µmol.L -1 )
	Free enzymes	0.029 ± 0.001 55.6 ± 0.73
	Immobilized enzymes	0.033 ± 0.005 38.0 ± 2.6

ACKNOWLEDGMENTS

The author would like to convey thanks to the Ministry for granting the thesis scholarship to pursue this research. This work was supported by the "Institut Européen des Membranes" through the water project (ENZYCLEAN).


Qazi Sohaib

Amir Muhammad

Mohammad Younas

Mashallah Rezakazemi
email: mashalah.rezakazemi@gmail.com

Stéphanie Druon-Bocquet

José Sanchez-Marcano

Rigorous Non-Isothermal Modelling Approach for Mass and Energy Transport during CO 2 Absorption into Aqueous Solution of Amino Acid Ionic Liquids in Hollow Fiber Membrane Contactors

Keywords: Non-isothermal modelling, CO 2 absorption, Amino acid ionic liquids, Hollow fiber membrane contactor, Chemisorption

Rigorous non-isothermal modeling approach for mass and energy transport during CO2 absorption into aqueous solution of amino acid ionic liquids in hollow fiber membrane contactors

Carbon dioxide (CO 2 ) gas is one of the major contributors to global warming and climate change. The fossil fuel-based power sectors are the largest sources of CO 2 emissions [START_REF] Wang | Post-combustion CO2 capture with chemical absorption: A state-of-the-art review[END_REF].

Therefore, it is of great importance to implement CO 2 capture technologies in power sectors [START_REF] Wang | Post-combustion CO2 capture with chemical absorption: A state-of-the-art review[END_REF][START_REF] Yu | Recent advances in CO2 capture and utilization[END_REF]. Post-combustion carbon capture (PCCC) is one of the widely studied technologies for carbon reduction as it holds the greatest advantage of retrofitting to existing units in power plants [START_REF] Zhao | Status and progress of membrane contactors in post-combustion carbon capture: A state-of-the-art review of new developments[END_REF][START_REF] Sohaib | Post-combustion CO2 capture by coupling [emim] cation based ionic liquids with a membrane contactor; Pseudo-steady-state approach[END_REF]. Some of the main challenges of this process include low flue gas pressure (≈1 atm) and low CO 2 concentration (< 16 %) [START_REF] D'alessandro | Carbon dioxide capture: Prospects for new materials[END_REF]. However, the liquid absorbent based PCCC is still recognized as state of the art carbon capture technology as it can provide a high CO 2 absorption efficiency (> 80 %) even at low concentrations of CO 2 [START_REF] Zhao | Status and progress of membrane contactors in post-combustion carbon capture: A state-of-the-art review of new developments[END_REF][START_REF] Figueroa | Advances in CO2 capture technology-The U.S. Department of Energy's Carbon Sequestration Program[END_REF].

Currently, amine-based absorbents represent more than 90 % of the market and are regarded as a leading technology for CO 2 capture [START_REF] Kohl | Gas purification 5th ed[END_REF]. Amines are widely used absorbents for CO 2 capture due to their high reactivity with CO 2 and high absorption capacity [START_REF] Aghaie | A systematic review on CO 2 capture with ionic liquids: Current status and future prospects[END_REF]. However, these absorbents have several drawbacks, including, high corrosion rate, high construction rate, degradation by some gases like SO 2 or NO 2 and high volatility [START_REF] Yu | A review of CO2 capture by absorption and adsorption[END_REF][START_REF] Chen | Chemical solvent in chemical solvent: A class of hybrid materials for effective capture of CO2[END_REF][START_REF] Sohaib | CO2 capture with room temperature ionic liquids; coupled absorption/desorption and single module absorption in membrane contactor[END_REF]. Alternative absorbents have been proposed like ionic liquids (ILs), which can overcome the drawbacks of conventional absorbents [START_REF] Macdowell | An overview of CO2 capture technologies[END_REF][START_REF] Zhang | Carbon capture with ionic liquids: overview and progress[END_REF][START_REF] Liu | Promoted adsorption of CO2 on amine-impregnated adsorbents by functionalized ionic liquids[END_REF]. Low volatility, negligible vapor pressure, thermal stability, high CO 2 solubility, less toxicity, and low corrosion rates are the main features of ILs [START_REF] Ghandi | A Review of Ionic Liquids, Their Limits and Applications[END_REF][START_REF] Yan | Ionic liquids combined with membrane separation processes: A review[END_REF][START_REF] Papatryfon | CO2 capture efficiency, corrosion properties, and ecotoxicity evaluation of amine solutions involving newly synthesized ionic liquids[END_REF][START_REF] Dai | Membrane absorption using ionic liquid for pre-combustion CO2 capture at elevated pressure and temperature[END_REF][START_REF] Zhang | Highly Efficient CO2 Capture by Polyethylenimine Plus 1-Ethyl-3-Methylimidazolium Acetate Mixed Absorbents[END_REF]. These ILs have been divided into two categories, Room temperature ILs (RTILs) and Task-specific ILs (TSILs) or functionalized ILs. RTILs are conventional ILs that provide typical physical solvent behavior for the gas solubility following Henry's Law. Most of the RTILs have very high CO 2 solubility compared to other gases such as N 2 , O 2 , H 2 and CH 4 [START_REF] Lei | Gas solubility in ionic liquids[END_REF][START_REF] Dai | Combination of ionic liquids with membrane technology: A new approach for CO2separation[END_REF]. TSILs provide very high CO 2 solubility which can reach up to three times of the corresponding alkyl chain RTILs. These ILs are able to absorb CO 2 by both chemical and physical sorption, for which the CO 2 loading increases with an increase in pressure. The chemical absorption of TSILs can be represented by chemical reaction, while the physical absorption of TSILs can be represented using Henry's law constant. TSILs can absorb CO 2 at low partial pressures due to the chemical reactions [START_REF] Shiflett | Phase behavior of carbon dioxide in ionic liquids[END_REF]. Some of the TSILs developed by researchers for CO 2 capture include protic ILs [START_REF] Wang | Carbon dioxide capture by superbasederived protic ionic liquids[END_REF], amino acid-based ILs [START_REF] Ohno | Amino acid ionic liquids[END_REF] and aminebased ILs [START_REF] Wang | Highly efficient CO 2 capture by tunable alkanolamine-based ionic liquids with multidentate cation coordination[END_REF][START_REF] Ren | CO 2 capture from flue gas at high temperatures by new ionic liquids with high capacity[END_REF]. Among these ILs, amino acid-based ILs got more attention recently because of their high CO 2 capacity and high solubility in water [START_REF] Wei | Amino acid salts for CO 2 capture at fl ue gas temperatures[END_REF].

Differential gas-liquid contactors (packed columns, bubble columns and spray towers) and stage-wise gas-liquid contactors (plate columns and rotating disc contactors) are currently used absorbers on an industrial scale. Liquid holdup and gas-liquid interfacial area are the important parameters in the design of this equipment [START_REF] Perry | Perry's Chemical Engineering Handbook[END_REF]. The above-mentioned absorbers face some crucial problems including high energy consumption, flooding, foaming, solvent loss due to degradation and entrainments, equipment corrosion and large footprint. Hollow fiber membrane contactor (HFMC) technology is an alternative to overcome these problems [START_REF] Gabelman | Hollow fiber membrane contactors[END_REF][START_REF] Dindore | Hollow fiber membrane contactor as a gas-liquid model contactor[END_REF][START_REF] Qazi | CO 2 capture in a hollow fiber membrane contactor coupled with ionic liquid : Influence of membrane wetting and process parameters[END_REF][START_REF] Qazi | Mathematical Modelling of CO 2 Absorption with Ionic Liquids in a Membrane Contactor , Study of Absorption Kinetics and Influence of Temperature[END_REF][START_REF] Sohaib | Modelling Pre-Combustion CO2 Capture with Tubular Membrane Contactor Using Ionic Liquids at Elevated Temperatures[END_REF]. Membrane contactors are non-dispersive contacting systems in which the membrane does not provide selectivity by itself and rather works as a barrier between gas and liquid and increases the mass transfer area. The advantages of the membrane contactor include nondispersive mass transfer operations, high known and constant interfacial area, operational flexibility and modularity [START_REF] Perry | Perry's Chemical Engineering Handbook[END_REF][START_REF] Qi | Microporous hollow fibers for gas absorption. II. Mass transfer across the membrane[END_REF][START_REF] Demontigny | Comparing the absorption performance of packed columns and membrane contactors[END_REF][START_REF] Falk-Pedersen | CO 2 Capture with Membrane Contactors[END_REF]. Falk-Pedersen et al. [START_REF] Falk-Pedersen | CO 2 Capture with Membrane Contactors[END_REF] used aqueous solution (30 wt.%) of mono-ethanolamine (MEA) in a membrane contactor for the absorption of exhaust gases from a gas turbine on an offshore installation. and different cations, as shown in Figure 1. The main drawback of the amino acid-based ILs is their very high viscosities (> 200 mPa.s) which also reduces the diffusivity of CO 2 and slows down the absorption process [START_REF] Jiang | Tetraalkylammonium amino acids as functionalized ionic liquids of low viscosity[END_REF][START_REF] Feng | Absorption of CO2 in the aqueous solutions of functionalized ionic liquids and MDEA[END_REF]. As these ILs are highly soluble in water, using the aqueous solutions instead of pure ILs can reduce the viscosity to a very low value. The lower viscosities of these aqueous solutions mitigate the flow and pressure drop related problems in the membrane contactors and also enhances CO 2 diffusivitiy [START_REF] Zhao | Status and progress of membrane contactors in post-combustion carbon capture: A state-of-the-art review of new developments[END_REF]. The aqueous solutions of these ILs can provide high surface tension and a very high contact angle with hydrophobic membranes (used in this study), avoiding pore wetting of the membrane. Moreover, these ILs are very highly reactive providing remarkably high solubilities for CO 2 following both chemisorption and physisorption. The above-mentioned properties of these ILs and their aqueous solutions makes them potential candidates for using with hydrophobic membranes.

For this work, the considered ILs are new to the membrane contactor absorption. Up to the authors knowledge, these ILs have never been studied (experimentally as well as modeling)

for membrane contactor based CO 2 absorption operations. Most of the models developed for the CO 2 absorption in membrane contactors are based on isothermal behavior. In fact, the process is non-isothermal as the energy of dissolution reaction is released and accumulated along the membrane contactor length, particularly for very reactive absorbents as considered for the current study. Thus, the non-isothermal model developed here provides more realistic approach towards the process of CO 2 absorption in reactive absorbents. The isothermal approach developed in the other studies underestimates the process due to not considering the actual absorption process which is non-isothermal. The current model considers a new approach for the non-isothermal behavior of the chemisorption of CO 2 in the amino acidbased reactive ILs used as absorbents in the membrane contactor. Water evaporation and condensation have been neglected by maintaining the liquid side pressure and introducing a dry gas mixture at the inlet. The steady-state model was implemented to investigate various operating conditions for dry gas mixture, unloaded absorbents and partial conversion of absorbents. The model was able to investigate the non-isothermal behavior of the absorption process and its influence on the axial profiles of reaction and absorption kinetics. Simulations were performed to study the influence of absorbent concentration and process temperature on the separation performance of ILs, CO 2 boundary flux and reaction kinetics between CO 2 and ILs. Moreover, experimental data of CO 2 absorption in membrane contactor were presented and compared with the current simulations.

Studied experimental setup and modeling system characteristics

A membrane contactor gas absorption setup suitable for post-combustion CO 2 capture was adopted to be modeled here. A typycal HFMC based post-combustion CO 2 capture setup with liquid on lumen side is presented in Figure 2. The setup has been described by many reserchers in their studies [START_REF] Chabanon | Pushing the limits of intensified CO2 post-combustion capture by gas-liquid absorption through a membrane contactor[END_REF][START_REF] Chabanon | Modeling strategies of membrane contactors for post-combustion carbon capture: A critical comparative study[END_REF][START_REF] Chabanon | Membrane contactors for postcombustion carbon dioxide capture: A comparative study of wetting resistance on long time scales[END_REF][START_REF] Albo | Carbon dioxide capture from flue gases using a cross-flow membrane contactor and the ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate[END_REF][START_REF] Gong | Experiments and simulation of CO2 removal by mixed amines in a hollow fiber membrane module[END_REF], and is very suitable especially for industrial applications [START_REF] Zhao | Status and progress of membrane contactors in post-combustion carbon capture: A state-of-the-art review of new developments[END_REF]. Liquid absorbents flows through the tube side of the membrane contactor.

Gas streams of N 2 and CO 2 are measured by gas flowmeter for introducing a known concentration of post-combustion gas mixture and to control gas flowrate. Gas streams are introduced to a buffer tank first, to uniformly mix the gases. The mixed gas stream is then allowed to flow countercurrently through the shell side of the membrane contactor. CO 2 from the gas stream is absorbed in the IL whose concentration is monitored with a CO 2 analyzer on the gas side outlet of the memberane contactor. To prevent penetration of gas bubbles to the liquid side, liquid side pressure is always kept slightly higher than the gas side. The simplified form of the modeling setup is presented in Figure 3. A microporous hydrophobic membrane contactor (poly-propylene fibers) of parallel configuration was selected for the study whose characteristics, process parameters and operating conditions are presented in Table 1. Four amino acid-functionalized ILs were also selected as absorbents.

The The model is based on the non-isothermal absorption process across the membrane contactor.

CO 2 is considered to be the only species that can cross the gas-liquid interface. The following assumptions were made to solve the model. d) The gas mixture is considered to be dry (no water vapors) e) Neglecting the presence of water evaporation and condensation

Model equations

The following continuity equation is considered for the mass balance over the three domains.

𝜕𝐶 𝑖 𝜕𝑡 = -∇. 𝐶 𝑖 𝑈 -∇. 𝑗 𝑖 + ŕ 𝑖 (1)
Equation ( 1) in the steady-state form can be written as:

0 = -∇. 𝐶 𝑖 𝑈 -∇. 𝑗 𝑖 + ŕ 𝑖 (2)
Based on the above-mentioned assumptions, the steady-state continuity equation ( 2

The thermal balances based on the above-mentioned assumptions are presented below.

𝜆 𝑔 [ 𝜕 2 𝑇 𝑔 𝜕𝑟 2 + 1 𝑟 𝜕𝑇 𝑔 𝜕𝑟 + 𝜕 2 𝑇 𝑔 𝜕𝑧 2 ] = 𝑈 𝑧,𝑔 𝐶 𝑝,𝑔 𝜌 𝑔 𝜕𝑇 𝑔 𝜕𝑧 (6)
𝜆 𝑚,𝑒 [ 𝜕 2 𝑇 𝑚 𝜕𝑟 2 + 1 𝑟 𝜕𝑇 𝑚 𝜕𝑟 + 𝜕 2 𝑇 𝑚 𝜕𝑧 2 ] = 0 ; 𝜆 𝑚,𝑒 = 𝜀𝜆 𝑔 + (1 -𝜀)𝜆 𝑚 (7)
Hagen-Poiseuille equation with no-slip conditions was used for the fully developed velocity profile of the fluid on the lumen, while for the shell side Happel's free surface model was used [START_REF] Happel | Viscous flow relative to arrays of cylinders[END_REF].

𝑈 𝑧,𝑔 = 2𝑢 𝑔 [1 -( 𝑟 𝑟 𝑖 ) 2 ] ( 9
)
𝑈 𝑧,𝑙 = 2𝑢 𝑙 [1 -( 𝑟 𝑒 𝑟 𝑠 ) 2 ] (𝑟 𝑟 𝑠 ⁄ ) 2 -(𝑟 𝑒 𝑟 𝑠 ⁄ ) 2 +2ln (𝑟 𝑒 𝑟 ⁄ ) 3+(𝑟 𝑒 𝑟 𝑠 ⁄ ) 4 -4(𝑟 𝑒 𝑟 𝑠 ⁄ ) 2 +4𝑙𝑛 (𝑟 𝑒 𝑟 𝑠 ⁄ ) , 𝑟 𝑠 = 𝑟 𝑒 √ 1 1-∅ , 1 -∅ = 𝑁𝑟 𝑒 2 𝑟 𝑐 2
(10)

Reaction and absorption kinetics

The show similar reactivity for CO 2 as of primary alkanolamines [START_REF] Jing | Characterization and kinetics of carbon dioxide absorption into aqueous tetramethylammonium glycinate solution[END_REF][START_REF] Kumar | Kinetics of the reaction of CO2 with aqueous potassium salt of taurine and glycine[END_REF][START_REF] Wu | Kinetics of CO 2 absorption into aqueous 1ethyl-3-methylimidazolium glycinate solution[END_REF]. The zwitterion mechanism proposed by Caplow [START_REF] Caplow | Kinetics of Carbamate Formation and Breakdown[END_REF] for primary alkanol amines is considered as a reaction mechanism for amino acid-based ILs here. CO 2 reacts with the amino acid-based ILs via the formation of zwitterion due to the deprotonation by a base present in the aqueous solution [START_REF] Jing | Characterization and kinetics of carbon dioxide absorption into aqueous tetramethylammonium glycinate solution[END_REF].

𝐻 2 𝑁 -𝐶𝐻 2 -𝐶𝑂𝑂 -𝑅 + + 𝐶𝑂 2 ⇌ 𝐾 1 𝐾 2 -𝑂𝑂𝐶 + 𝐻 2 𝑁 -𝐶𝐻 2 -𝐶𝑂𝑂 -𝑅 + (11)
̅ 𝑂𝑂𝐶 + 𝐻 2 𝑁 -𝐶𝐻 2 -𝐶𝑂𝑂 -𝑅 + + 𝐵 𝑖 → 𝐾 𝐵 𝑖 -𝑂𝑂𝐶𝐻𝑁 -𝐶𝐻 2 -𝐶𝑂𝑂 -𝑅 + + 𝐵 𝑖 𝐻 + ( 12
)
𝐵 𝑖 is the base present in the solution to deprotonate zwitterions. Liquid phase equilibrium reactions are as under.

̅ 𝑂𝑂𝐶𝐻𝑁 -𝐶𝐻 2 -𝐶𝑂𝑂 -𝑅 + + 𝐻 2 𝑂 ⇌ 𝐻 2 𝑁 -𝐶𝐻 2 -𝐶𝑂𝑂 -𝑅 + + 𝐻𝐶𝑂 3 - (13)
𝐻 3 𝑁 -𝐶𝐻 2 -𝐶𝑂𝑂 -𝑅 + ⇌ 𝐻 2 𝑁 -𝐶𝐻 2 -𝐶𝑂𝑂 -𝑅 + + 𝐻 + (14)
𝐶𝑂 2 + 𝐻 2 𝑂 ⇌ 𝐻 + + 𝐻𝐶𝑂 3 - ( 15
)
𝐻𝐶𝑂 3 -⇌ 𝐻 + + 𝐶𝑂 3 - ( 16
)
𝐻 2 𝑂 ⇌ 𝐻 + + 𝑂𝐻 - (17)
The criterion to determine whether the reaction completely occurs in the bulk or in the liquid film is based on the Hatta number. Hatta number can be defined as under.

𝐻 𝑎 = √𝐷 𝑖 𝐾 𝑜𝑣 𝐾 𝑙 (18)
Where 𝐾 𝑜𝑣 and 𝐾 𝑙 represent the overall reaction rate constant (s -1 ) and liquid film mass transfer coefficient (m s -1 ). To confirm the fast pseudo-first-order regime, the Hatta number must follow the criterion of 2 < 𝐻 𝑎 ≪ 𝐸 ∞ [START_REF] Jing | Characterization and kinetics of carbon dioxide absorption into aqueous tetramethylammonium glycinate solution[END_REF][START_REF] Danckwerts | Gas-Liquid Reactions[END_REF]. 𝐸 ∞ represents infinite enhancement factor.

When this criterion is fulfilled, the reaction of CO 2 and amino acid based ILs can be considered as in the fast pseudo first order regime which means that change in the concentration of the absorbent is negligible and reaction is of the first order for CO 2 [START_REF] Kierzkowska-Pawlak | Determination of kinetics in gas-liquid reaction systems. An overview[END_REF]. ILs 2. The secondorder reaction rate was temperature-dependent and increased with an increase in temperature.

The kinetic data for all four ILs (at 1M Concentration) were calculated from the Arrhenius plot and are presented in Table 2.

Solubility, phase equilibria, and diffusion coefficients

The interfacial phase equilibrium for the absorption species was defined as follows.

𝐶 𝑖,𝑙 = 𝑚 𝐶 𝑖,𝑔

Where 𝑚 is the dimensionless distribution coefficient for the absorption species which can be presented as below.

𝑚 = 𝑅𝑇𝐻 (20)
where H represents the solubility of the absorption species.

Where ℎ 𝑖 and ℎ 𝑔 represent ion and gas specific constants, respectively and 𝐶 𝑖 is ion concentration. The solubility of CO 2 in water was calculated as under [START_REF] Versteeg | Solubility and Diffusivity of Acid Gases (CO2, N2O) in Aqueous Alkanolamine Solutions[END_REF].

ln 𝐻 * = 𝑎 + 𝑏(𝛿 1 -𝛿 2 ) 2 𝑇 ( 23
)
Where 𝛿(𝐽𝑐𝑚 -3 ) 1 2 represents the solubility parameter, while 𝑎 (𝐽𝑐𝑚 -3 ) -1 and 𝑏(𝐽𝑐𝑚 -3 ) -1

are constants depending on the gas [START_REF] Camper | Low pressure hydrocarbon solubility in room temperature ionic liquids containing imidazolium rings interpreted using regular solution theory[END_REF][START_REF] Barton | Handbook of Solubility Parameters and Other Cohesion Parameters[END_REF].

The solubility of CO 2 , 𝐻(𝑚𝑜𝑙 𝑚 -3 𝑃𝑎 -1 ) in aqueous solutions of [C₆mim][Gly],

[C 2 mim][Gly] and [C 4 mim][Gly] was found from the Henry law using the parameter 𝐻 * [START_REF] Sander | Compilation of Henry's law constants (version 4.0) for water as solvent[END_REF].

𝐻 = 10 𝜌 𝑀 𝐻 * (24)
Diffusivity of CO 2 and N 2 in the gas phase can be found using the correlation developed by Fuller et al. [START_REF] Fuller | A new method for prediction of binary gasphase diffusion coefficients[END_REF].

𝐷 𝑖,𝑔 = 0.01013 𝑇 1.75 (

1 𝑀 𝐶𝑂 2 + 1 𝑀 𝑁 2 ) 0.5 𝑃 [(∑ ῡ 𝐶𝑂 2 ) 1 3 +(∑ ῡ 𝑁 2 ) 1 3 ] 2 (25)
Diffusivity if CO 2 in the membrane can be calculated from the following equation. The membrane was considered as gas-filled and no wetting conditions were implemented here.

Where 𝐷 𝐾𝑛 is the Knudsen diffusion coefficient, which can be found as below.

𝐷 𝐾𝑛 = 1 2 𝑑 𝑝 √ 8𝑅𝑇 𝜋𝑀 (27)
where 𝑑 𝑝 is the membrane pore diameter.

Meshing domains, boundary conditions, and numerical resolution

Meshing and proper discretization of the geometry are very important for the finite element analysis. The current absorption process is mostly controlled by the liquid side resistance.

Therefore, a rigorous absorption model with sufficient discretization is needed. Moreover, interfacial chemical reactions were considered in the current absorption process, as it can also be confirmed from the analysis in Section 2.2.3 and the high values of Hatta number presented in Table 2. Refined mapped (rectangular shaped) meshing was implemented across the symmetrical geometry with more refining along with the interface and inside the membrane. The meshed surface along with the labeled boundary conditions (BCs) is presented in Figure 4. BCs for both mass and energy transport equations are presented in Table 3. The set of 2D model equations presented in the preceding sections are boundary value problems of the partial differential equation system. The model was developed and solved using COMSOL Multiphysics® which uses the finite element method. The solved model was post-processed in MATLAB® and Microsoft Excel.

Results and discussion

Comparison between simulations and experimental data

To validate the model comparison between simulations and experimental data was carried out.

Up to the authors knowledge, the ILs used in the current study have never been implemented in the membrane contactor absorption operations. Thus there is no experimental data available in the literature for the ILs considered in this study. Therefore experimental data of the membrane contactor absorption with IL monoethanolamine glycinate ([MEA][Gly]) was used for the comparison with the current simulations. This IL has the same anion, and almost comparable physicochemical properties as of the ILs used in the simulations here. The experimental setup was adopted from the work of Lu et al. [START_REF] Lu | CO2capture using a functional protic ionic liquid by membrane absorption[END_REF] in which IL [MEA][Gly] was used for CO 2 absorption in membrane contactor. Characteristics of the membrane and operating conditions were adopted from Tables 1 and2, respectively, of their studies.

Simulation were performed for the unloaded solvent C i,l-in =0 at temperature T=289 K, gas side pressure P= 1.08 bar, IL concentration C IL = 0.5 mol L -1 and gas flow rate Q g = 800 ml min -1 .

The liquid flowrate was varied from 80 to 160 ml min -1 , and the normalized CO 2 concentration at the gas side outlet was recorded and reported in Figure 5. This section presents the simulations of CO 2 concentration, IL concentration, and variations of temperature and reaction rate constant over the interfacial boundary of the membrane. As mentioned in the preceding sections, the gas/liquid equilibrium over the interfacial boundary is defined by Henry's Law. Hatta numbers presented in Table 2 have confirmed the reaction for absorption species and ILs to occur at the interface mostly. This number can define whether the reaction occurs in the bulk or the liquid film depending upon its value [START_REF] Kierzkowska-Pawlak | Determination of kinetics in gas-liquid reaction systems. An overview[END_REF]. The steady-state gradual evolution of the axial concentration of CO 2 and IL is presented in Figure 6. Unloaded absorbent was introduced to the tube side at z=0 while the gas mixture having 15 % CO 2 and N 2 (rest to balance) was passed counter currently from the shell side of the membrane contactor. The absorbents were considered to be at chemical equilibrium having homogenous concentration at the inlet of the contactor. A gradual decrease in the concentrations of both gas-phase CO 2 and absorbent can be observed toward the respective outlets due to absorption at the interface. At the inlet of the gas side, a faster drop in the concentration of CO 2 can be observed which demonstrates the effect of CO 2 concentration on the absorption process. The gradual decrease in the concentration of absorbent along with the fiber length shows the enrichment of the absorbent with CO

ILs performance

Separation performances of the ILs were evaluated by comparing the radial profiles of the normalized gas-phase CO 2 concentration and separation efficiency at the specified axial length. Separation efficiency was calculated from the following equation.

𝑆𝑒𝑝𝑎𝑟𝑎𝑡𝑖𝑜𝑛 𝑒𝑓𝑓𝑐𝑖𝑐𝑖𝑛𝑒𝑐𝑦 = (1 - 𝐶 𝑖,𝑔 𝐶 𝑖,𝑔-𝑖𝑛 ) × 100 (27)
Simulations of the gas phase CO 2 concentrations were obtained at 298 K inlet temperature for the relative radial length of the shell (r/r s ). The effect of the IL concentration (C IL ) and process temperature on separation performance was evaluated and reported in Figures 7 and8, respectively.

The concentration of IL has a strong influence on the separation performance of the four ILs.

On one hand, an increase in IL concentration decreases both diffusivity and solubility of CO 2 in the aqueous solution of IL, on the other hand, it increases the reaction rate [START_REF] Feng | Absorption of CO2 in the aqueous solutions of functionalized ionic liquids and MDEA[END_REF][START_REF] Zhou | Characterization and absorption of carbon dioxide into aqueous solution of amino acid ionic liquid [N1111][Gly] and 2-amino-2-methyl-1propanol[END_REF]. The decrease in solubility is in accord with the salting out effect [START_REF] Weisenberger | Estimation of Gas Solubilities in Salt Solutions at Temperatures from 273 K to 363 K[END_REF]. Diffusivity and physical solubility are very important parameters which strongly influence the CO 2 separation in membrane contactors. A decrease in both parameters decreases the separation efficiency. The overall effect can be observed in Figure 9, which shows that generally the efficiency is enhanced by an increase in IL concentration. This confirms the stronger influence of reaction rate as an increase in IL concentration decreases diffusivity and solubility of CO respectively. Thus, it can be concluded that the overall separation performance is the combined effect of all three parameters, diffusivity, solubility and reaction rate. An increase in the CO 2 solubilities has been reported with an increase in the alkyl chain of the imidazolium cation [START_REF] Baltus | Low-Pressure Solubility of Carbon Dioxide in Room-Temperature Ionic Liquids Measured with a Quartz Crystal Microbalance[END_REF][START_REF] Chen | Solubility of CO2 in imidazolium-based tetrafluoroborate ionic liquids[END_REF]. However, the effect of the alkyl chain cation was found to be minor compared to anions [START_REF] Supasitmongkol | High CO2 solubility in ionic liquids and a tetraalkylammonium-based poly(ionic liquid)[END_REF]. This effect seems to be validated in regard to the separation efficiencies of imidazolium cation based ILs reported above. decreases with an increase in temperature [START_REF] Lei | Gas solubility in ionic liquids[END_REF][START_REF] Okoturo | Temperature dependence of viscosity for room temperature ionic liquids[END_REF]. Diffusivity of the CO 2 in ILs and gaseous mixture has proportionality with the temperature. An increase in temperature increases the diffusivity of CO 2 due to a decrease in the viscosity of the ILs. Similar results have been reported in other literature [START_REF] Jing | Characterization and kinetics of carbon dioxide absorption into aqueous tetramethylammonium glycinate solution[END_REF][START_REF] Guo | Kinetics of carbon dioxide absorption into aqueous [Hmim][Gly] solution[END_REF]. Feng et al. [START_REF] Feng | Absorption of CO2 in the aqueous solutions of functionalized ionic liquids and MDEA[END_REF] reported that an increase in temperature increases the reaction rate of the glycine based ILs. The overall effect of temperature on the absorption performance can be observed in Figure 10. An increase can be observed in the separation performance with an increase in temperature. However, the increase is less compared to that observed for IL concentration. An increase in temperature from 298 to 323 The enhancement in the separation efficiency with an increase in IL concentration (higher reaction rates) and temperature (higher reaction rates and diffusivities) also shows the dominance of the chemisorption over the physisorption in the current membrane contactor absorption process. It is noteworthy here that the process was steady-state and the IL entering the contactor was always unloaded. Starting near the contactor length of z/Z=0, the reaction rate is a little higher for low IL concentrations than that of high IL concentrations. Moving along the contactor length the reaction rate becomes very high for the high IL concentrations. The low reaction rates for the high ILs concentration near the liquid inlet (z/Z=0) are due to the very low concentration of CO 2 at the gas outlet (z/Z=0). In the case of high IL concentrations, the CO 2 is absorbed very quickly at the gas side inlet (z/Z=1). The maximal values of the reaction rate for all ILs were recorded near the contactor length at z/Z=0.8. The reaction rate is maximum near this point due to the availability of optimal reactants concentration, as can be seen from Figure 6. Also, the reaction rate is affected by rate constant, which increases along contactor length with increase in temperature and reaches a maximum near that point. Hoff et al. [START_REF] Hoff | Modeling and experimental study of carbon dioxide absorption in aqueous alkanolamine solutions using a membrane contactor[END_REF] and Hoff and Svendsen [START_REF] Hoff | Membrane contactors for CO2 absorption -Application, modeling and mass transfer effects[END_REF] reported that the complex formation after the reaction affects the overall absorption process. However, due to unloaded solvent and steady-state conditions, the reaction mostly occurs near the interface and the effect can be neglected on the overall absorption. Moreover, very high liquid flowrates were used in their studies, which create radial concentration gradients.

K

  Sohaib, Amir Muhammad, Mohammad Younas, Mashallah Rezakazemi, Stéphanie Druon-Bocquet, José Sanchez-Marcano To cite this version: Qazi Sohaib, Amir Muhammad, Mohammad Younas, Mashallah Rezakazemi, Stéphanie Druon-Bocquet, et al.. Rigorous non-isothermal modeling approach for mass and energy transport during CO2 absorption into aqueous solution of amino acid ionic liquids in hollow fiber membrane contactors. Separation and Purification Technology, In press, 254, pp.117644. 10.1016/j.seppur.2020.117644. hal-02930334 1 Introduction

Figure 1 .

 1 Figure 1. Cations and anion of the ILs (a) [N 1111 ] + , (b) [C 2 mim] + , (c) [C 4 mim] + , (d) [C 6 mim] + and (e) [Gly] -.

Figure 2 .

 2 Figure 2. Studied experimental setup; black dashed lines represent gas flow, solid blue lines

  ILs selected were tetramethylammonium glycinate [N 1111 ][Gly] as amino acid-based IL and three imidazolium amino acid ILs namely 1-ethyl-3-methylimidazolium glycinate [C 2 mim][Gly], 1-butyl-3-methylimidazolium glycinate [C 4 mim][Gly] and 1-hexyl-3methylimidazolium glycine [C₆mim][Gly]. Dry gas mixture (CO 2 +N 2 ) and absorbents were considered flowing counter currently on the shell side and lumen side of the membrane contactor, respectively. For industrial operations, the reported CO 2 concentration is about 8-15%[START_REF] Jing | Characterization and kinetics of carbon dioxide absorption into aqueous tetramethylammonium glycinate solution[END_REF]. Thus, 15% CO 2 concentration is considered throughout the model. The absorption species (CO 2 ) transfer from the shell side (by convection and diffusion) to the pores of the membrane (only diffusion in the pores) and then absorbs (chemisorption followed by physisorption) in the absorbent on the lumen side of the membrane contactor.

Figure 3 Table 1 A

 31 Figure 3 Schematic representation of the membrane contactor PCCC setup and the section used for model development

  arrangements for gas and IL flow in membrane contactor c) Gas and liquid flow under laminar conditions with fully developed velocity profile with no radial components Thermodynamics: a) Application of Henry's law for gas-liquid equilibrium on the interface b) The membrane works as a non-selective barrier c) Neglected solubility of N 2 d) Ideal gas behavior Kinetics, mass transfer, and heat transfer a) Reversible reaction kinetics b) Application of Fick's diffusion through porous media for membrane mass transfer, neglecting convective contributions c) Contributions of advection and diffusion to mass balance solved by considering local concentrations and contributions of convection, conduction, and heat of reaction to energy balance solved by local temperatures.

  four ILs selected for the current study, [N 1111 ][Gly], [C 2 mim][Gly], [C 4 mim][Gly] and [C₆mim][Gly] have the same glycine ([Gly]) anion and different cations. The mechanism is similar for the above-mentioned amino acid-based ILs. Glycine based aqueous alkaline salts

[

  N 1111 ][Gly] [51], [C 2 mim][Gly] [54], [C 4 mim][Gly] [58] and [C₆mim][Gly] [59] fulfill the criteria of fast pseudo-first-order reaction regime for a range of concentrations. Enhancement factors, Hatta numbers, and second-order rate constants are presented in Table

  The solubility of [C 2 mim][Gly], [C 4 mim][Gly] and [C₆mim][Gly] was calculated according tothe regular solution theory. For ILs the Henry law constant 𝐻 * (𝑎𝑡𝑚) decreases with an increase in solubility. The regular solution theory is presented below.

Figure 4

 4 Figure 4 Meshed domains and boundaries of membrane contactor for mass and energy transport

  The square dots represent the experimental data of the IL [MEA][Gly] while the lines represent simulations of the four IL. The relative standard deviations between experimental data points and simulations are very low and are in the range of 1.8 % -4.1 %. The highest relative standard deviation of 4.1 % was recorded at Q IL = 100 ml min -1 , while lowest of 1.8 % was recorded at Q IL = 80 ml min -1 . The model seems to be able to closely represent the experimental data. The trends between the experimental data and simulations are identical. Experimental data of [MEA][Gly] shows high separation performance than the simulated ILs, except for the IL [C₆mim][Gly] where the difference in the separation performance is very low. The difference in the separation performance is due to the different physicochemical properties and reactivity of the ILs.

Figure 5 4 . 2

 542 Figure 5 Comparison between the simulations (line) and experimental data (square dots); 𝑄 𝑔 = 800 𝑚𝑙 𝑚𝑖𝑛 -1 , 𝐶 𝑖,𝑙-𝑖𝑛 = 0 𝑚𝑜𝑙 𝐿 -1 , 𝐶 𝐼𝐿-𝑖𝑛 = 0.5 𝑚𝑜𝑙 𝐿 -1 , 𝑇 𝑙-𝑖𝑛 , 𝑇 𝑔-𝑖𝑛 = 289 𝐾

  2 . A similar trend of absorption can be seen for all ILs. ILs [N 1111 ][Gly] and [C 4 mim][Gly] have nearly the same concentration profiles. IL [C₆mim][Gly] can be regarded as the most efficient one while [C 2 mim][Gly] as the least efficient one among all ILs.

Figure 6 Figure 7

 67 Figure 6 Normalized axial concentration distribution of absorption species (CO 2 ) and absorbents over mass transfer interface; 𝑄 𝐼𝐿 = 20 𝑚𝑙 𝑚𝑖𝑛 -1 , 𝑄 𝑔 = 60 𝑚𝑙 𝑚𝑖𝑛 -1 , 𝐶 𝑖,𝑙-𝑖𝑛 = 0 𝑚𝑜𝑙 𝐿 -1 , 𝐶 𝐼𝐿-𝑖𝑛 = 1 𝑚𝑜𝑙 𝐿 -1 , 𝑇 𝑙-𝑖𝑛 , 𝑇 𝑔-𝑖𝑛 = 298 𝐾

Figure 7

 7 Figure 7 Temperature profile of the liquid phase over mass transfer interface; 𝑄 𝐼𝐿 = 20 𝑚𝑙 𝑚𝑖𝑛 -1 , 𝑄 𝑔 = 60 𝑚𝑙 𝑚𝑖𝑛 -1 , 𝐶 𝑖,𝑙-𝑖𝑛 = 0 𝑚𝑜𝑙 𝐿 -1 , 𝐶 𝐼𝐿-𝑖𝑛 = 1 𝑚𝑜𝑙 𝐿 -1 , 𝑇 𝑙-𝑖𝑛 , 𝑇 𝑔-𝑖𝑛 = 298 𝐾

Figure 8

 8 Figure 8 Normalized axial reaction rate constant distribution over mass transfer interface; 𝑄 𝐼𝐿 = 20 𝑚𝑙 𝑚𝑖𝑛 -1 , 𝑄 𝑔 = 60 𝑚𝑙 𝑚𝑖𝑛 -1 , 𝐶 𝑖,𝑙-𝑖𝑛 = 0 𝑚𝑜𝑙 𝐿 -1 , 𝐶 𝐼𝐿-𝑖𝑛 = 1 𝑚𝑜𝑙 𝐿 -1 , 𝑇 𝑙-𝑖𝑛 , 𝑇 𝑔-𝑖𝑛 = 298 𝐾

3 Pa - 1 , 2 .

 312 2 and increases the reaction rate. The lower CO 2 concentration (Figure 7) represents higher separation performance. For IL [N 1111 ][Gly] the efficiency increases by 23 % for increasing IL concentration from 0.3 to 2.0 mol L -1 . For [C 2 mim][Gly] an efficiency increase of 18 % was observed for increasing IL concentration 0.5 to 1.5 mol L -1 . Further increase in IL concentration has decreased efficiency. This phenomenon can be linked with the strong dependence of the membrane contactor absorption process on the physicochemical (viscosity, density and diffusion coefficient) properties of the absorbent. Similarly, for [C 4 mim][Gly] an efficiency increase of 18 % was observed for increasing IL concentration from 0.5 to 2.0 mol L -1 while for [C 6 mim][Gly] the increase was 13 % for an increase in IL concentration from 0.5 to 1.2 mol L -1 . A comparative analysis of the performance of four ILs was carried out at IL concentration of 1 mol L -1 . ILs [N 1111 ][Gly], [C 2 mim][Gly], [C 4 mim][Gly] and [C 6 mim][Gly] have diffusivities of 1.06 x 10 -9 m 2 s -1 , 1.66 x 10 -9 m 2 s -1 , 1.55 x 10 -9 m 2 s -1 and 1.14 x 10 -9 m 2 s -1 , respectively. Thus based on the CO 2 diffusivity, these ILs can be ranked as, [C 2 mim][Gly] > [C 4 mim][Gly] > [C 6 mim][Gly] > [N 1111 ][Gly]. ILs [N 1111 ][Gly], [C 2 mim][Gly], [C 4 mim][Gly] and [C 6 mim][Gly] have physical solubilities of 2.63 x 10 -4 mol m -3 Pa -1 , 2.52 x 10 -4 mol m -48 x 10 -4 mol m -3 Pa -1 and 2.7 x 10 -4 mol m -3 Pa -1 , respectively. Thus based on the CO 2 solubility, these ILs can be ranked as, [C 6 mim][Gly] > [N 1111 ][Gly] > [C 2 mim][Gly] > [C 4 mim][Gly]. Based on the K 2 values from Table 2, ILs can be ranked as [C 6 mim][Gly] > [N 1111 ][Gly] > [C 4 mim][Gly] > [C 2 mim][Gly]. Based on the overall performance at IL concentration of 1 mol L -1 , the four ILs can be ranked as [C 6 mim][Gly] > [C 2 mim][Gly] > [C 4 mim][Gly] > [N 1111 ][Gly]. IL [C₆mim][Gly] was found with the highest efficiency of 85 % among all ILs. While comparing the ILs at 2 mol L -1 the efficiencies were 83, 75 and 75 % for [C 4 mim][Gly], [C 2 mim][Gly] and [N 1111 ][Gly],

Figure 9

 9 Figure 9 Normalized radial profiles of gas-phase CO 2 concentrations for different concentration of ionic liquids (a) [N 1111 ][Gly], (b) [C 2 mim][Gly], (c) [C 4 mim][Gly] and (d) [C₆mim][Gly]; 𝑄 𝐼𝐿 = 20 𝑚𝑙 𝑚𝑖𝑛 -1 , 𝑄 𝑔 = 60 𝑚𝑙 𝑚𝑖𝑛 -1 , 𝐶 𝑖,𝑙-𝑖𝑛 = 0 𝑚𝑜𝑙 𝐿 -1 , 𝑇 𝑙-𝑖𝑛 , 𝑇 𝑔-𝑖𝑛 = 303 𝐾

  has increased the efficiency by nearly 1, 3, 11 and 5 % for ILs [N 1111 ][Gly], [C 2 mim][Gly], [C 4 mim][Gly] and [C₆mim][Gly], respectively. Effect of temperature on the separation performance of IL [N 1111 ][Gly] was almost negligible. Strong influence of temperature can be observed for IL [C 4 mim][Gly].

Figure 10 1 4. 4 Figure 10 .

 101410 Figure 10 Normalized radial profiles of gas phase CO 2 concentrations at different process temperatures for ionic liquids (a) [N 1111 ][Gly], (b) [C 2 mim][Gly], (c) [C 4 mim][Gly] and (d) [C₆mim][Gly]; 𝑄 𝐼𝐿 = 20 𝑚𝑙 𝑚𝑖𝑛 -1 , 𝑄 𝑔 = 60 𝑚𝑙 𝑚𝑖𝑛 -1 , 𝐶 𝑖,𝑙-𝑖𝑛 = 0 𝑚𝑜𝑙 𝐿 -1 , 𝐶 𝐼𝐿-𝑖𝑛 = 1 𝑚𝑜𝑙 𝐿 -1

Figure 11 Figure 12 1 4. 5

 111215 Figure 11 Axial profiles of the CO 2 boundary flux across the mass transfer interface for different concentrations of the ionic liquids (a) [N 1111 ][Gly], (b) [C 2 mim][Gly], (c) [C 4 mim][Gly] and (d) [C₆mim][Gly]; 𝑄 𝐼𝐿 = 20 𝑚𝑙 𝑚𝑖𝑛 -1 , 𝑄 𝑔 = 60 𝑚𝑙 𝑚𝑖𝑛 -1 , 𝐶 𝑖,𝑙-𝑖𝑛 = 0 𝑚𝑜𝑙 𝐿 -1 , 𝑇 𝑙-𝑖𝑛 , 𝑇 𝑔-𝑖𝑛 = 303 𝐾

Figure 13

 13 Figure 13 Axial profiles of the reaction rate across the mass transfer interface for different concentrations of the ionic liquids (a) [N 1111 ][Gly], (b) [C 2 mim][Gly], (c) [C 4 mim][Gly] and (d) [C₆mim][Gly]; 𝑄 𝐼𝐿 = 20 𝑚𝑙 𝑚𝑖𝑛 -1 , 𝑄 𝑔 = 60 𝑚𝑙 𝑚𝑖𝑛 -1 , 𝐶 𝑖,𝑙-𝑖𝑛 = 0 𝑚𝑜𝑙 𝐿 -1 , 𝑇 𝑙-𝑖𝑛 , 𝑇 𝑔-𝑖𝑛 = 303 𝐾

Figure 14 5 Conclusion

 145 Figure 14 Axial profiles of the reaction rate across the mass transfer interface at different process temperatures for the ionic liquids (a) [N 1111 ][Gly], (b) [C 2 mim][Gly], (c) [C 4 mim][Gly] and (d) [C₆mim][Gly]; 𝑄 𝐼𝐿 = 20 𝑚𝑙 𝑚𝑖𝑛 -1 , 𝑄 𝑔 = 60 𝑚𝑙 𝑚𝑖𝑛 -1 , 𝐶 𝑖,𝑙-𝑖𝑛 = 0 𝑚𝑜𝑙 𝐿 -1 , 𝐶 𝐼𝐿-𝑖𝑛 = 1 𝑚𝑜𝑙 𝐿 -1

  The setup was investigated for the cyclic and non-cyclic absorption process, static regeneration process and continuous absorption and regeneration process. The blended (AMP and IL) absorbents were able to achieve higher net and circular absorption capacities compared to the pure IL in the coupling process. To overcome the mass transfer limitations of the CO 2 capture process, various technologies were implemented, including supported ILs and encapsulated ILs. The dual functionality (both ILs and support) of the supported ILs enhances the CO 2 capture process[START_REF] Pan | Ionic Liquids for Chemisorption of CO 2[END_REF]. Recently, Sánchez Fuentes et al.

	mixture of 2-amino-2-methyl-1-propanol (AMP) and ILs 1-butyl-3-methyl-imidazolium
	tetrafluoroborate	[bmim][BF4]	and	1-(3-aminopropyl)-3-methyl-imidazolium
	tetrafluoroborate [apmim][BF4]. [40] developed supported ionic liquid membranes (SILMs) based on three TSILs 1-(2-
	aminoethyl)-3-methylimidazolium trifluoromethanesulfonate	([AEMIm]Tf),	1-(2-
	aminoethyl)-3-methylimidazolium	tetrafluoroborate	([AEMIm]BF 4 )	and
				Serval benefits have been reported
	including 38-42 % savings of operating cost, 35-40 % capital cost reduction, 34-40 %
	equipment weight reduction and 40-50% footprint requirement reduction. Chabanon et al.

[START_REF] Chabanon | Pushing the limits of intensified CO2 post-combustion capture by gas-liquid absorption through a membrane contactor[END_REF]

studied the process intensification of membrane contactor setup with concentrated MEA solutions. The maximum intensification factor was obtained with 70 wt.% MEA concentration at 60 o C. Lu et al. [38] developed a membrane contactor CO 2 absorption setup using aqueous trioctylmethylammonium anthranilate ([TOMA]), for CO 2 /N 2 separation. The SILMs showed very high permselectivity of up to 70. In another work, Zhang et al. [41] developed four diamine-monocarboxylate-based protic ILs for CO 2 /N 2 and CO 2 /CH 4 separation. A very high selectivity of 151 was reported for CO 2 /N 2 . Encapsulation of ILs within a porous material increases the effective mass transfer area for CO 2 absorption and reduces the mass transfer resistance[42]. Recently, Santiago et al. [43] encapsulated amino acid based ILs in porous carbon capsules. The author investigated reaction enthalpies and both physical and chemical CO 2 absorption for all ILs. In another study, Romanos et al. [44] developed novel supported IL phase system by encapsulating amine-functionalized ILs within an envelope of silica nanoparticles. The author reported promising CO 2 /N 2 separation performance and very fast absorption kinetics. In the current study four amino-based TSILs tetramethylammonium glycinate [N 1111 ][Gly], 1ethyl-3-methylimidazolium glycinate [C 2 mim][Gly], 1-butyl-3-methylimidazolium glycinate [C 4 mim][Gly] and 1-hexyl-3-methylimidazolium glycinate [C₆mim][Gly] have been investigated for CO 2 capture in a membrane contactor. These ILs have similar anion ([Gly])

  ) for the gas, membrane and liquid side are presented below.

	𝐷 𝑖,𝑔 [	𝜕 2 𝐶 𝑖,𝑔 𝜕𝑟 2 +	1 𝑟	𝜕𝐶 𝑖,𝑔 𝜕𝑟	+	𝜕 2 𝐶 𝑖,𝑔 𝜕𝑧 2 ] = 𝑈 𝑧,𝑔	𝜕𝐶 𝑖,𝑔 𝜕𝑧	(3)
	𝐷 𝑖,𝑚 [	𝜕 2 𝐶 𝑖,𝑚 𝜕𝑟 2 +	1 𝑟	𝜕𝐶 𝑖,𝑚 𝜕𝑟	+	𝜕 2 𝐶 𝑖,𝑚 𝜕𝑧 2 ] = 0	(4)
	𝐷 𝑖,𝑙 [	𝜕 2 𝐶 𝑖,𝑙 𝜕𝑟 2 +	1 𝑟	𝜕𝐶 𝑖,𝑙 𝜕𝑟	+	𝜕 2 𝐶 𝑖,𝑙 𝜕𝑧 2 ] + ŕ 𝑖 = 𝑈 𝑧,𝑙	𝜕𝐶 𝑖,𝑙 𝜕𝑧

Table 2

 2 Enhancement factor (E), Hatta number (H a ) and rate constant (K 2 ) for the ILs

	IL concentration (mole L -1 )	303 K E	303 K H a	303 K K 2 (L mol -1 s -1 )	K 2 (L mol -1 s -1 )	Reference
	[N 1111 ][Gly]
	0.3	14.56 14.56	242.16	-	[51]
	0.7	34.39 34.39	719.34	-	[51]
	1.0	41.33 41.33	736.93	4.6930 × 10 5 exp (-1856/T)	[51]
	2.0	62.68 62.68	1297.13	-	[51]
	[C 2 mim][Gly]
	0.5	23.67 23.67	296.29	-	[54]
	1.0	51.58 51.58	662.95	6.041 × 10 6 exp (-2801/T)	[54]
	1.5	87.57 87.57	1213.52	-	[54]
	2.0	107.1	107.1	1280.78	-	[54]
		5	5
	[C 4 mim][Gly]
	0.5	28.31 28.31	417.52	-	[58]
	1.0	52.65 52.65	675.51	1.3460 × 10 10 exp (-5038/T)	[58]
	1.5	102.2	102.2	1539.38	-	[58]
		7	7
	2.0	139.4	139.4	1955.60	-	[58]
		9	9
	[C₆mim][Gly]
	0.5	36.02 36.02	759.22	-	[59]
	0.8	48.86 48.86	1091.81	-	[59]

  The aqueous solutions of [N 1111 ][Gly], [C 2 mim][Gly], [C 4 mim][Gly] and [C₆mim][Gly] are electrolyte solutions. Equations developed by Barret[START_REF] Barrett | Gas Absorption on a Sieve Plate[END_REF] and Danckwerts[START_REF] Danckwerts | Gas-Liquid Reactions[END_REF] were used to calculate the diffusivities of CO 2

	in these solutions.
	(𝐷 𝑖,𝑙 𝜇 𝑙	0.8 ) 𝑇 = (𝐷 𝑖,𝑤 𝜇 𝑤	0.8 ) 𝑇	(28)
	log 𝐷 𝑖,𝑤 = -8.1764 +	712.5 𝑇	-	2.591 10 5 𝑇 2

Table 3

 3 Boundary conditions for mass and energy transport

	Boundary Condition	Mass transport	Energy
			transport

Reaction rate (mol m -3 s -1 ) δ Thickness of the membrane (m) R Perfect gas constant (J mol -1 K -1 ) λ Thermal conductivity (W m -1 K -1 )