V Sarapulova

E Nevakshenova

X Nebavskaya

A Kozmai

D Aleshkina

G Pourcelly

V Nikonenko

N Pismenskaya

Characterization of bulk and surface properties of anion-exchange membranes in initial stages

Keywords: Anion exchange membrane, fouling, wine, voltammetry, contact angle, surface charge, surface morphology, electrical resistance, water splitting, electroconvection

Characterization of bulk and surface properties of anion-exchange membranes in initial stages of fouling by red wine

V. Sarapulova, E. Nevakshenova, X. Nebavskaya, A. Kozmai, D. Aleshkina, G.

charge are measured. In addition, the AMX-Sb membrane is characterized by voltammetry and pH-metry. It is found that polyphenols act un important role in membrane fouling. Initially, it is relatively small and mobile anthocyanins, which penetrate inside the membrane; then they are followed by larger and slower tannins and/or anthocyanin-tannin complexes. Polyphenols together with polysaccharides and other wine constituents form colloidal aggregates, which fill the membrane pores and are deposited by islets on the surface as a foulant layer. The appearance of this layer increases hydrophilicity of the surface while reducing its charge. The membrane conductivity decreases with increasing the duration of membrane contact with wine. However, the effect of this contact on the limiting current density, ilim, overlimiting transfer and water splitting is unexpected. In early stages of fouling, ilim of the AMX-Sb membrane increases and water splitting is found suppressed, electroconvection is essentially enhanced. The latter should be due to the isle-type structure of the foulant layer: surface electrical heterogeneity promotes electroconvection. However, the contact of the membrane with wine for several tens of hours results in formation of all-over foulant layer stimulating water splitting and reducing electroconvection.

Introduction

Winemaking is multi-tonnage production, whose efficiency is rapidly progressing due to introduction of membrane technologies [START_REF] Rayess | Membrane technologies in wine industry: an overview[END_REF][START_REF] Tamime | Membrane processing: dairy and beverage applications[END_REF]. Membrane processes play an important role in improvement of wine quality: pH correction, dealcoholization, clarification and stabilization of wine. They gradually replace the traditional methods in the wine industry [START_REF] Rayess | Membrane technologies in wine industry: an overview[END_REF][START_REF] Corti | Tartaric stabilization of wines: comparison between electrodialysis and contact cold treatment [Estabilización tartárica en vinos: Comparación entre electrodiálisis y tratamiento de frío por contacto[END_REF][START_REF] De Pinho | Membrane processes in must and wine industries[END_REF]. First, the use of membrane processes allows decreasing the production costs by transition to continuous, controlled and automated techniques, as well as by the reduction of energy consumption and wine losses. Second, the membrane technologies are more environmentally friendly, since they do not require the addition of chemicals to wine; they also reduce the problem of utilization of wastes [START_REF] Daufin | Recent and emerging applications of membrane processes in the food and dairy industry[END_REF].

The greatest progress is achieved in the field of clarification and stabilization of wine by micro-and nanofiltration (MF and NF) [START_REF] Gautier | Une nouvelle approche révolutionnaire et performante de la filtration tangentielle des vins[END_REF][START_REF] Vernhet | Innovations dans la microfiltration des vins: membranes et procédés[END_REF]. In this field, "stability" denotes the lack of any adverse physical, chemical or organoleptic changes during some determined period of wine storage [START_REF] Thoukis | Chemistry of wine stabilization: a review[END_REF]. Such adverse changes include [START_REF] Thoukis | Chemistry of wine stabilization: a review[END_REF][START_REF] Ribéreau -Gayon | Handbook of enology: the chemistry of wine, Stabilization and Treatments[END_REF] the darkening or other changes in color, appearance of undesirable odours and tastes, turbidity and precipitation. The main components of such sediments are crystals of hydrotartrates of potassium and calcium.

Tartrate stability of wines can be successfully achieved via electrodialysis (ED) [1,10].

The advantage of this method is decrease of losses of wine and valuable components (catechins, anthocyanins and leucoanthocyanins) compared with cold stabilization and ultrafiltration [START_REF] Riponi | Essais de stabilisation tartrique des vins au moyen de l'électrodialyse[END_REF][START_REF] Low | Economic evaluation of alternative technologies for tartrate stabilisation of wines[END_REF]. Adaptation of ED to stabilization of wine and grape juice is being developed since 1970s [START_REF] Audinos | Application of electrodialysis to the elimination of certain grape juice and wine components[END_REF][START_REF] Paronetto | Some tests on tartrate stabilization of musts and wines by electrodialysis[END_REF], and the industrial applications of this method have appeared in 1990s [START_REF] Lasanta | Tartrate stabilization of wines[END_REF][START_REF] Moutounet | les acquisitions r´ecentes dans les traitements physiques du vin[END_REF].

Wine circulates through the desalination compartments of an electrodialyzer while pure water, which intakes the ions to be removed, circulates through the concentration compartments. The desalination degree, which is controlled by wine conductance, is usually 15 -20% for young, 20 -30% for dessert and 5 -15% for mature wines [START_REF] Gonçalves | Wine tartaric stabilization by electrodialysis and its assessment by the saturation temperature[END_REF][START_REF] Benítez | Comparison of electrodialysis and cold treatment on an industrial scale for tartrate stabilization of sherry wines[END_REF]. Use of electrodialysis with bipolar membranes allows conducting the tartrate stabilization with simultaneous pH correction of wines and fruit juices [START_REF] Rayess | Membrane technologies in wine industry: an overview[END_REF][START_REF] Serre | Deacidification of cranberry juice by electrodialysis: impact of membrane types and configurations on acid migration and juice physicochemical characteristics[END_REF].

Another rapidly developing fields for ED application in food and beverages industry are treatment of wastewaters, creation of water recycle systems [START_REF] Cassano | Membrane technologies for water treatment and reuse in the food and beverage industries[END_REF][START_REF] Arvanitoyannis | Wine waste treatment methodology[END_REF], as well as extraction of antioxidants (such as anthocyanins, proanthocyanidins and ellagitannins) from the winery wastes or increase of their concentration in fruit juices [START_REF] Bazinet | Antioxidants, mechanisms, and recovery by membrane processes[END_REF][START_REF] Giacobbo | Sequential pressure-driven membrane operations to recover and fractionate polyphenols and polysaccharides from second racking wine lees[END_REF]. The problem of membrane fouling is crucial in these applications. According to Mikhaylin and Bazinet [24], for the membrane processes in food industry, the costs of membrane regeneration and replacement amount to 20 -30% (pressure driven processes) to 40 -50% (electrically driven processes) of the total costs. Hence, the control of fouling and the development of methods for its prevention play a very important role in successful exploitation of membrane modules in food and beverage industry.

To solve this problem, a good understanding of mechanisms of this phenomenon is required.

Wine is a very complicated medium, which contains more than 600 components [START_REF] Jackson | Wine science: principles and applications[END_REF],

among them there are polyphenols (anthocyanins, tannins, etc.), polysaccharides including pectins, amino acids and proteins. Interactions between wine components are very complicated.

However, colloidal interactions involving polyphenols are highlighted in literature as playing a crucial role in wine stability, clarification process and taste [START_REF] Poncet-Legrand | Flavan-3-ol Aggregation in Model Ethanolic Solutions: Incidence of Polyphenol Structure, Concentration, Ethanol Content, and Ionic Strength[END_REF], as well as in membrane fouling [START_REF] Ulbricht | Fouling in microfiltration of wine: The influence of the membrane polymer on adsorption of polyphenols and polysaccharides[END_REF][START_REF] Madaeni | Chemical cleaning of reverse osmosis membranes[END_REF][START_REF] Cifuentes-Araya | How pulse modes affect proton-barriers and anion-exchange membrane mineral fouling during consecutive electrodialysis treatments[END_REF][START_REF] Ghalloussi | Ageing of ion-exchange membranes in electrodialysis: A structural and physicochemical investigation[END_REF].

Interactions between wine components result in formation of different types of species, which can be classified as follows: solute molecules as ions, organic acids and sugars (<1 nm), molecules with colloidal behavior as polyphenols, polysaccharides and proteins (between 1 nm and 1 μm) and particles as microorganisms, tartaric crystals and organic precipitates (>1 μm) Audinos [START_REF] Audinos | Fouling of ion-selective membranes during electrodialysis of grape must[END_REF] proposed the hypothesis that the most probable cause for the loss of performance of ED apparatuses and the growth in energy consumption in the tartrate stabilization of wine is the fouling of membrane stack with polyphenols. However, the details of this process, the type of interactions and the kinetics of fouling were not considered. The aging of IEMs during their long-term use in ED of food solutions is studied in Refs. [START_REF] Ghalloussi | Ageing of ion-exchange membranes in electrodialysis: A structural and physicochemical investigation[END_REF][START_REF] Garcia-Vasquez | Evolution of anion-exchange membrane properties in a full scale electrodialysis stack[END_REF][START_REF] Ghalloussi | Ageing of ion-exchange membranes used in an electrodialysis for food industry: SEM, EDX and limiting current investigations[END_REF][START_REF] Garcia-Vasquez | Effects of acidbase cleaning procedure on structure and properties of anion-exchange membranes used in electrodialysis[END_REF]. It is found that in the case of cation-exchange membrane (CEMs), the use of membranes causes a decrease of their ion-exchange capacity, water uptake, electrical conductivity and thickness, while the mechanical strength changes insignificantly. In the case of AEM (which possesses the same aromatic matrix), the membrane thickness and water uptake increase, while the ionexchange capacity and electrical conductivity change slightly and the mechanical strength decreases dramatically. The hydrophilicity of CEMs decreases while the hydrophilicity of AEMs increases. It is suggested [START_REF] Ghalloussi | Ageing of ion-exchange membranes in electrodialysis: A structural and physicochemical investigation[END_REF][START_REF] Garcia-Vasquez | Evolution of anion-exchange membrane properties in a full scale electrodialysis stack[END_REF] that one of the reasons for the difference in behavior of CEMs and AEMs is the formation of organic colloidal particles inside the membrane pores; these particles interact differently with the fixed groups and matrix of CEMs and AEMs. As far as we know, the effect of fouling on the electrochemical behavior of the membranes, which contacted with wine, was not studied yet.

In this paper, a comprehensive study of the kinetics of anion exchange membranes fouling in initial stages caused by their contact with red wine is presented for the first time.

Different methods of surface visualization (AFM and optical microscopy), analysis of the chemical structure (ATR FTIR spectroscopy) as well as membrane physico-chemical (ionexchange capacity, water content, thickness) and electrochemical (voltammetry, electrokinetic measurements, pH-metry) characterization are applied. Initial stages of fouling are investigated and a possible overall scenario of this process is suggested. Besides of the description of membrane performance degradation, we show that in early stages of fouling, an improvement of certain membrane properties occurs: water splitting is partially suppressed and electroconvection is increased.

Experimental

Membranes and solutions

A homogeneous AMX-Sb anion-exchange membrane (Astom, Japan) is selected for this study, as it is frequently used in ED treatment of liquids in food industry (processing of whey, wine, juices, etc.). This membrane is manufactured by the paste method [START_REF]Astom corporation ion exchange membranes[END_REF]. The ion-exchange matrix of AMX-Sb consists of a copolymer of styrene and divinylbenzene. The fixed groups are mainly quaternary ammonium bases [START_REF]Astom corporation ion exchange membranes[END_REF]. The membrane contains also an inert filler: the granules of polyvinyl chloride, whose diameter is about to 60 nm. Along with the AMX-Sb membrane, some experiments are made with a heterogeneous anion-exchange MA-41P membrane (Schekinoazot, Russia). MA-41P is a heterogeneous macroporous membrane produced by hot pressing of powdered AV-17-2P anion exchange macroporous resin and low pressure polyethylene acting as an inert binder. The capron net serves as a reinforcing cloth. The AV-17-2P resin is a regularly crosslinked copolymer of divynilbenzene and styrene with fixed quaternary ammonium bases as functional groups [59]. About 80% of the MA-41P membrane surface is covered with polyethylene. The resin particles, which protrude through the polyethylene, provide ion conduction of the membrane. The main characteristics of this membrane are presented in Table 1 Distilled water (the electrical conductivity was 0.5 μS cm -1 at 25°C, pH = 5.5), solid NaCl (analytical grade, OJSC Vekton) and red dry Mourvèdre/Syrah/Grenache wine (pH = 3.5) were used in the experiments.

Before the study, AMX-Sb and MA-41P membranes underwent standard salt pretreatment [62] and then were equilibrated with a 0.02 M NaCl solution. Then each membrane sheet was divided into several samples. One sample was left pristine for the comparison, while the others were fouled by red wine in a two-chamber dialysis flow cell. The red wine circulated through one of the chambers for a certain time, and a 0.02 M NaCl solution (pH 6.0 ± 0.5), through the other; no current was applied. Thus, the fouling of the membrane occurred only from one of its sides. The symbol related to the samples fouled with wine is indicated with subscript "w", the time of contact in hours is marked with a number in the subscript. Thus, "AMX-Sbw10" denotes the sample, which was in contact with wine during 10 hours; "AMX-Sbw72" relates to the sample, which was in contact with wine during 72 hours.

Methods

The water contact angle of surface of a swollen membrane sample was determined as a function of time of its contact with wine. The sessile drop technique according to the procedure described in [START_REF] Belashova | Overlimiting mass transfer through cation-exchange membranes modified by Nafion film and carbon nanotubes[END_REF] was applied. The test liquid was distilled water. The contact angles reported in this paper were registered 20 seconds after the application of a test drop.

The charge density of the surface of a swollen AEM was calculated from zeta potential, which in turn was found from tangential streaming potential measurements. We employed a laboratory-made gap cell similar to that applied in Anton Paar SurPASS 3. Note that the latter is used, in particular, for the measurements of this kind in the case of ion-exchange membranes [START_REF] Sedkaoui | A new lateral method for characterizing the electrical conductivity of ion-exchange membranes[END_REF]. The construction and distinguishing features of our set-up are described in detail by Sabbatovskii et al. [65]; the cell was equipped with two Ag/AgCl electrodes and connected to a GW Instek multimeter. The measurements of tangential streaming potential and its use for calculation of the space charge density are presented in our earlier publication [START_REF] Nebavskaya | Impact of ion exchange membrane surface charge and hydrophobicity on electroconvection at underlimiting and overlimiting currents[END_REF]. Two identical rectangular AEM samples formed a slit channel with 25 mm × 2.5 mm × 70 µm dimensions (length×width×height). Prior to streaming potential measurements, the fouled membranes were stored for 24 hours in a 0.02 M NaCl solution (pH=6.7) or in a 0.02 M NaCl solution, pH of which was lowered to 3.5 ± 0.1 by addition of HCl. Soaking in a 0.02 M NaCl solution was aimed at two goals: transition of the membrane to the Cl -form and stabilization of its properties. Auxiliary measurements have shown that the electrical conductivity of a fouled in wine membrane rapidly changes after the first hour of soaking. Then the rate of conductivity variation slows down and becomes quite low after 24 h. of soaking. However, the conductivity does not return to its value of the pristine membrane. Hence, we concluded that 24 hours are sufficient for relative stabilization of membrane properties, but are not enough for restoration of membrane properties. The use of solutions with different pH was made in order to study the response of the surface charge on the pH of the bathing solution.

The measurements of streaming potential were carried out at 20 °C using a 0.02 M NaCl (pH 6.7± 0.1) solution pumped with a linear flow rate of 40 -70 cm s -1 , when the pressure drop between the channel inlet and outlet was in the range from 0.125 bar to 0.625 bar.

The visualization of surface and cross-sections of swollen AMX-Sb membranes was carried out using a SOPTOP CX40M optical microscope (China) with a set of 5x, 10x, 20x, 50x and 10x objectives and a digital eyepiece camera and another optical microscope Altami BIO-2 (Russia) with a set of 4x, 10x, 40x and 100x objectives and a digital eyepiece camera. One side of the membrane under study contacted with an anthocyanin solution (10 mg L -1 , pH 3.5 ± 0.1) for 10 hours or with wine (pH 3.5 ± 0.1) for 3, 10 or 72 hours in a dialysis cell, as described in Section 2.1. Each membrane was cut into four pieces before visualization. The first piece was investigated immediately while the others were soaked for an hour in a buffer solution at pH 1.7 ± 0.1 (the second piece), a buffer solution at pH 6.7 ± 0.1 (third piece) or a buffer solution at pH 9.2 ± 0.1 (the fourth piece) and then photographed.

The morphology and surface relief of the air-dry AMX-Sbw samples were studied by the AFM method in a contactless mode (JEOL 5400 microscope, Japan, JEOL 5400 software).

Different parts of the sample surface were scanned each at least 10 time. The obtained data were processed using statistical analysis.

The chemical composition of the air-dried AEM surface and air-dried wine (both at 45 °C) were studied by ATR-FTIR spectroscopy using the disturbed total internal reflection method with a Vertex-70 spectrometer (Bruker Optics, Germany) in the range 4500 -450 cm -1 . The IR spectra were processed using OPUS ™ software.

The current-voltage characteristics, the counterion transport numbers in the membrane samples, the difference in pH between the desalination channel outlet and inlet were measured using an ED flow cell with an Autolab PGSTAT100N potentiostat/galvanostat.

The flow-through four-compartment ED cell as well as overall setup and the procedure of data registration and processing are described in detail in [START_REF] Belova | The effect of anion-exchange membrane surface properties on mechanisms of overlimiting mass transfer[END_REF][START_REF] Belashova | Overlimiting mass transfer through cation-exchange membranes modified by Nafion film and carbon nanotubes[END_REF]. The central desalination compartment of the cell was formed by an AEM under investigation and an auxiliary MK-40 cation-exchange membrane, which separated the central compartment from the (platinum) cathode compartment. Another auxiliary AEM membrane (an AMX-Sb one) was placed between the AEM under study and the platinum anode. The parameters of the desalination compartment were as follows: the intermembrane distance, h, was 6.5 mm; the linear flow rate of the 0.02 M NaCl solution, V, was 0.4 cm s -1 ; the polarized area of the membrane was 2 × 2 cm 2 ; pH of the solution was equal to 6.0 ±0.2. The investigations were carried out at 20 ± 1 °C.

The theoretical value of the limiting current density was calculated using the Lévêque equation obtained in the framework of the convection-diffusion model [68]:

        -         - = 2 . 0 47 . 1 ) ( 3 / 1 2 lim LD V h t T h FDC i Cl Сl theor (1)
where L is the length of the desalination channel (2 cm); C is the molar NaCl concentration at the channel entrance; Cl T and Cl t are the transport numbers of the Cl -ion in the membrane and the solution, respectively; D is the diffusion coefficient of NaCl in solution at infinite dilution; F is the Faraday constant.

The application of Eq. ( 1) for the membrane system under study gives lim theor i =2.9 mA cm -2 .

When comparing different membrane systems, instead of the total potential drop, Δφ, the reduced potential drop, Δφ' , is used [69]:

ef iR ϕ ϕ ′ ∆ = ∆ - (2)
where the effective resistance of the membrane system at current density i → 0,

0 / ef i R i ϕ → = ∂∆ ∂ (Ohm cm 2 ),

Results and discussions

Characteristics of membrane surface and volume

Morphology and surface relief of air-dry samples

Fig. 1 shows the AFM images of air-dry samples of the pristine AMX-Sb and the AMX-Sbw72 sample, which was in contact with wine by one of its sides for 72 hours. The results of statistical processing of 10 topographic images obtained for each of the samples are given in Table 2. Sratio is the roughness factor, defined as the ratio of the real surface area to the geometric area, the latter is the projection of the real surface on a plane parallel to the macroscopic visible phase boundary considered as ideally flat [71], Rq is the root mean square deviation of the assessed profile (ISO 18115-2:2013),

In the case of AMX-Sb (Fig. 1a), sharp peaks and valleys are randomly distributed over the area of the sample. An average width of the local protrusions is about 60 nm, which corresponds to the average linear size of the PVC granules involved in the membrane structure formation.

The local protrusions on the AMX-Sbw72 surface (Fig. 1b) have a more rounded shape.

The values of all roughness parameters increase in comparison to those for the pristine membrane (Table 2). For example, the value of Rz, which is the average value of the absolute values of the heights of the five highest peaks and the depths of the five deepest valleys [72],

increases by a factor of 1.5. This change in the surface relief is similar to that observed in other studies [START_REF] Suwal | Presence of free amino acids in protein hydrolysate during electroseparation of peptides: impact on system efficiency and membrane physicochemical properties[END_REF][START_REF] Krisilova | Estimation of effect of amino acid's sorption on surface state of ion-exchange membranes using atomic-force microscopy data[END_REF] for the membranes fouled with proteins and aromatic amino acids. Some parts of the AMX-Sbw72 surface contain the subjects, which are similar to rod-shaped bacteria (Fig. 1c).

The surface relief in the immediate vicinity of these subjects appears similar to that of the pristine membrane. Some bulk properties of AMX-Sb and MA-41P membranes after their contact with wine are shown in Table 3. As for the membrane thickness, this parameter increases by 1.5 % and 5.9 % (in comparison with the pristine membranes, Table 1) in the cases of AMX-Sb and MA-41P membranes, respectively, when the time of soaking is 10 h. This increase in thickness is 3.0% (AMX-Sbw72) and 7.2% (MA-41Pw72), when the time of soaking is 72 h (Figs. 2b,d,f). The optic images of pristine and fouled membranes are given in Figs. 2, 3 and4. The images of AMX-Sbw (Fig. 2) are taken immediately after disassembling the dialysis cell (where the membrane was fouled) and rinsing the membrane with distilled water. Hence, it is presumed that the observed coloration corresponds to the pH of wine, which was 3.5. The images of fouled MA-41P membrane (Figs. 3,4) are obtained after soaking the previously fouled samples in three buffer solutions with different pH values (1.68, 6.68 or 9.18) for 1 h, as described in Section 2.2.

Optical microscopy of swollen samples

The surface of fouled AMX-Sbw membrane is covered with spots, which are pale ruby after the contact with wine (pH = 3.5, see Fig. 2c) for 10 h, and become red-brown after 72 h of the contact (Fig. 2e). The color of the bulk of AMX-Sbw membrane undergoes similar changes:

it is pale ruby after 10 h of contact with wine (Fig. 2d), and dense deep purple after 72 h of contact (Fig. 2f). Following the hypothesis presented above, the first color (Fig. 2d) can be attributed to the anthocyanins, and the second one (Fig. 2f), to the complexes of anthocyanins with tannins and polysaccharides [START_REF] Simoes Costa | Astringency quantification in wine: comparison of the electronictongue and FT-MIR spectroscopy[END_REF]. Special side illumination of the cross-section (Fig. 2f) reveals a tawny colored layer, which has thickness of several micrometers and which fairly evenly covers the surface of the AMX-Sbw72 membrane. The way the color of this layer changes depending on the direction of illumination suggests the openwork structure of this layer and its high water content. After 10 h (Figs. 2с and2d) of contact with wine, this foulant layer is seen also, however, it is not continuous, but rather island-like.

Non-fouled membrane

Fouled membranes

Color of anthocyanin solution depending on pH Optical images of samples obtained at different pH values (Figs. 3b,c,d,4) allow tracing the kinetics of fouling and get an idea of the composition of the foulants within the membranes and on their surface. It is important to note that in the first few hours of contact of the heterogeneous membrane with wine, the foulants are localized only on the ion-exchange particles and are nearly absent on the surface occupied by polyethylene (Figs. 3b,c,4 c).

Increasing the time of contact with wine leads to the expansion of the spots of the foulant around the ion-exchange particles (Figs. 3d,4d). The formation of such an island-like structure is apparently facilitated by electrostatic interactions of the wine components with the fixed groups of ion-exchange material, as well as by π-π interactions of the aromatic fragments of membrane matrix with polyphenols. Most likely, similar processes (i.e. preferential adsorption of wine components on the ion exchange material) also occur in the case of the homogeneous AMX-Sb membrane, which is a composite of an aromatic ion-exchange polymer and an aliphatic binder (polyvinyl chloride). Then this explains the island-like formation of foulant aggregates, which is observed in the first few hours of contact with wine on the surface of both homogeneous (Fig. 2c), and heterogeneous (Figs. 3b,c,4c) membranes. Anthocyanins are rather small molecules (with a molecular mass close to 200 g mole -1 ), hence, they can rapidly penetrate into an ion-exchange membrane. However, it can be seen that after 10 h of the contact with wine, the coloration of MА-41P surface shifts so that more brownish shades appear (Fig. 4c,d) in comparison with those in the case of pure anthocyanins (Fig. 4b).

This change in color may reflect the appearance of tannins and anthocyanin-tannin adducts (such as catechins), which are yellow and brown, respectively [START_REF] Simoes Costa | Astringency quantification in wine: comparison of the electronictongue and FT-MIR spectroscopy[END_REF]. The color of MА-41P bulk (Fig. 3) corresponds to a color related to the anthocyanin solution with a pH shifted to the alkaline region in comparison with the solution, in which the fouled membrane was stored before taking the photos. This can be explained by the Donnan exclusion of H + ions as co-ions from an anionexchange membrane [79]: the pH of the AEM inner solution is always more alkaline for 2 -3 pH unities in comparison with the external solution [START_REF] Langevin | Ion-exchange membrane fouling by peptides: A phenomenon governed by electrostatic interactions[END_REF]. Indeed, the greenish-yellow color of the bulk of MА-41Pw10 sample stored in the solution with 6.68 (Fig. 3b) relates rather well to the color of the anthocyanin solution with pH=11.0 (Fig. 3e).

It is seen that after a short contact with wine, there is a near-surface layer in the MА-41P membrane, whose color is more intensive than the bulk; with increasing time of contact the thickness of this layer increases. After 3 h of the contact, the color of this layer is not uniform in the MА-41P. One could say, that there are regions where the depth of anthocyanin penetration into the membrane is greater and the regions with higher resistance to the anthocyanin diffusion.

This difference in the depth of anthocyanin penetration can be explained by the fact that the diffusion is possible through the ion-exchange particles and the interstices between these particles and polyethylene inclusions, while polyethylene is not permeable. With increasing time of the contact, the color of the membrane surface as well as the bulk becomes more intensive, testifying higher concentration of the pigment.

Results of ATR-FTIR spectroscopy

FTIR spectroscopy gives additional information about the chemical nature of foulants and their interactions with an anion-exchange membrane. A number of peaks are recorded in the IR spectra of the AMX-Sbw72 surface in the range from 1768 cm -1 to 770 cm -1 , which are absent in case of the pristine membrane, but are typical for wine (Fig. 5). In publications [78, 80] on the identification of wine composition, this entire range is listed as fingerprint area. The exact identification of all the wine components in the membrane is quite complicated. Therefore, we will pay attention only to some of them, which could contribute to a better understanding of fouling. In the range 1768 -1478 cm -1 , there are absorption bands of amides, as well as the peaks characteristic for carbonyl groups of esters. The accumulation of organic acids inside the membrane leads to the appearance of C=O bands at about 1715 cm -1 , (Fig. 5, insertion a) and a peak at around 1167 cm -1 , which corresponds to -COOH groups (Fig.

5, insertion c).

The 1617 cm -1 absorption band, which is identified in the FTIR spectrum of the pristine AMX-Sb, is typical for stretching vibrations of the C = C bond of benzene rings of divinylbenzene in the polymer matrix [START_REF] Ghalloussi | Ageing of ion-exchange membranes in electrodialysis: A structural and physicochemical investigation[END_REF]. In the case of AMX-Sbw72, this peak shifts to a value of 1604 cm -1 . Such absorption band is identified in wine spectra and is typical for stretching vibrations of the C = C bonds of benzene rings of polyphenols (anthocyanins, tannins, etc.) (Fig. 5a). Besides, in the C=C spectral region (1650 -1450 cm -1 ), the peaks at 1450 cm -1 , 1475 cm -1 , 1480 cm -1 and 1510 cm -1 are recorded for the pristine as well for the fouled membranes. They all correspond to C=C stretching bands in aromatic rings in the polymer matrix.

The peaks in the range of 1478 -1185 cm -1 indicate the presence of amides, carboxyl proteins, nucleotides (DNA and RNA) and phospholipids (Fig. 5, insertion b). The peak of 1115 cm -1 is close to that found in wine where it is attributed to polysaccharides (glucans and mannans) [START_REF] Simoes Costa | Astringency quantification in wine: comparison of the electronictongue and FT-MIR spectroscopy[END_REF][START_REF] Silva | Application of FTIR-ATR to Moscatel dessert wines for prediction of total phenolic and flavonoid contents and antioxidant capacity[END_REF] (Fig. 5,insertion c). However, in the case of wine (Fig. 5), this peak is recorded at 1105 cm -1 . Evidently, the shift of this peak indicates the interaction of polysaccharides with the membrane matrix.

Both for wine and for the membranes under study (both dried at 45 °C), we observe a peak in the 3300-3400 cm -1 range, which corresponds to the absorption band of the covalent -OH bond (Fig. 5). The intensity of this peak is minimal for the initial membrane, in which trace amounts of water are apparently present, and maximal for the sample of dried wine. The latter contains not only trace amounts of water, but also a significant amount of -OH groups in polysaccharides, anthocyanins and tannins. In the case of AMX-Sbw72, the intensity of this peak decreases in comparison with the wine, but increases in comparison with the initial membrane, which indicates the presence of polysaccharides, anthocyanins and tannins in the fouled membrane. The shift of the peak recorded for the AMX-Sbw72 sample to the region of higher wavenumber (50 cm -1 ) compared to the wine suggests that in the membrane, the -OH groups of the foulants form hydrogen bonds with each other and with the fixed groups of ion-exchange material.

Contact angles and surface charge of swollen AEMs

The decrease in contact angle of the AMX-Sb and MA-41P membranes caused by their contact with wine (Fig. 6) evidences the adsorption of hydrated components of wine. The main changes of the contact angle in the case of AMX-Sb occur within the first 30 hours; more time is needed to stabilize the contact angle for the MA-41P membrane. The stabilization of the contact angle should mean that the formation of a relatively dense near-surface foulant layer is over, and the surface properties no longer change. As Fig. 2c shows, short contact with wine leads to formation of an island-like foulant layer. A longer contact causes an increase in the surface fraction occupied by the foulant layer, hence an increase in hydrophilicity of the overall membrane surface. Since only a small part (about 20%) of the heterogeneous MA-41P membrane surface contains ion-exchange material, which adsorbs the fouling wine constituents and thus serves as initiator of the fouling process, it takes more time to form a continuous foulant layer in the case of MA-41P.

Since only a small part (about 20%) of the heterogeneous MA-41P membrane surface contains ion-exchange material, which adsorbs the fouling wine constituents and thus serves as initiator of the fouling process, it takes more time to form a continuous foulant layer in the case of MA-41P. The molecules contained in wine and forming the foulant layer (polyphenols, tannins, polysaccharides, proteins and other) are large. They include hydrophobic and hydrophilic parts. It seems that the hydrophobic parts can easier penetrate into the ion-exchange material, while the hydrophilic parts remain on the surface. Thus, the membrane surface becomes more hydrophilic. Apparently, the AV-17-2P anion-exchange macroporous resin contained in the MA-41P membrane allows greater sorption of the hydrophobic parts of the foulant molecules than the AMX-Sb membrane, which explains higher hydrophilization of the MA-41P surface during its contact with wine. The surface charge of the sample, which contacted with wine for 72 hours, is essentially smaller in absolute value than that of the pristine membrane (Fig. 7). The sign of charge of the AMX-Sbw72 membrane surface depends on the pH of the solution, where the membrane was stored in after its contact with wine: if the fouled membrane was kept in acidic media (pH = 3.5)

prior to electrokinetic measurements, then the sign of its charge remains the same as it was for the pristine membrane (positive); and when the membrane was kept in neutral solution (pH = 6.7), its surface charge changes to negative.

The surface charge of the fouled membrane is apparently determined by the foulant layer Analysis of the data presented above allowed us to suggest the following hypothesis about the sequence of events occurring when an anion-exchange membrane is fouled with wine.

During the first hours of contact, strongly hydrated and relatively small organic molecules penetrate into the membrane pores, causing their stretching, an increase in membrane thickness and a sharp decrease of contact angle. These molecules may include anions of organic acids.

Judging by the observed changes in coloration and their dependence on pH, anthocyanins (which are rather mobile with their molecular mass close to 200 g mole -1 ) might be a significant part of these molecules. When the pores are expanded, tannins, proteins and other high molecular substances with low diffusivity can enter the membrane, causing a further increase in its thickness. The presence of chromene cycles, which are characteristic of anthocyanins and condensed tannins (catechins), at membrane / solution interface, may cause the observed changes in surface charge. In acidic medium, the chromene cycles exist in form of positively charged flavylium cations [START_REF] Ribéreau-Gayon | Phenolic Compounds[END_REF], which could contribute to the positive charge of the foulant layer. In neutral medium, these cycles transform into chalcone or quinonoid pseudo-bases, which do not possess electric charge [START_REF] Ribéreau-Gayon | Phenolic Compounds[END_REF]. A small negative charge of AMX-Sbw72 surface is presumed to be caused by partially dissociated hydroxyl and carboxyl groups, which are present in the formed surficial colloidal aggregates. The subsequent formation of branched strongly hydrated colloidal structures in large pores and on the AEM surface is apparently similar to those observed in the case of micro-and ultrafiltration membranes fouled with wine

Current-voltage curves and pH changes

CVCs of studied samples as well as differences in pH between the desalination channel Table 4. Characteristics of I-V curves, diffusion layer thickness and water splitting rate. The latter is expressed by the OH -ion transport number in the AEM. The effective diffusion layer thickness, δΝ, found using Eq. ( 3), is given for different values of the reduced potential drop, Δφ', (Eq. ( 2), to compare with δLev=239 µm, calculated using Eq. ( 1). As it can be seen from Fig. 8a and Table 4, a short contact (10 h) of the AMX-Sb membrane with wine leads to an increase in the values of ilim exp . As well, the plateau length decreases and the plateau slope increases, which signifies that the overlimiting conductance increases. At the same time, the pH of the outlet solution is higher than that in the case of the non-fouled AMX-Sb membrane. The variation of pH between the inlet and outlet solutions is caused by the difference in the rate of water splitting at the AEM and CEM. Water splitting at the depleted interface of the AEM generates H + and OH -ions; the OH -ions pass through the AEM into the concentrate compartment, while the H + ions go towards the bulk of desalination channel. The depleted interface of the CEM generates the H + ions, which pass through the CEM into the concentrate compartment, and OH -ions, which go towards the bulk of desalination channel. Fig. 8c shows that at low current densities, the outlet solution pH is higher than the inlet solution pH. This signifies that water splitting rate at the CEM is higher than that at the AEM.

The cause is that the limiting current density at the CEM in NaCl solutions is about 1.5 times lower than that at the AEM, which is due to the lower diffusivity of Na + compared to Cl -. Hence, the limiting current density at the CEM is reached, when i / ilim theor for the AEM is about 0.6. As a heterogeneous MK-40 membrane is used as the CEM in our ED cell, the local limiting current density at the conductive surface regions is attained even at lower values of i / ilim theor (funnel effect [START_REF] Rubinstein | Ion-exchange funneling in thin-film coating modification of heterogeneous electrodialysis membranes[END_REF]). However, when the value of i / ilim theor is sufficiently high, the desalted solution is acidified (Fig. 8c), which is due to higher water splitting at the AEM. The latter is explained by the fact that at a relatively high concentration polarization the water splitting rate is governed by Thus, examining Fig. 8c allows one to conclude that the water splitting rate at the AMX-Sbw10 is lower than that at the AMX-Sb membrane, since the CEM membrane forming the desalination channel together with the AEM under study is the same (MK-40) in both cases. This conclusion is confirmed by the measurement of the effective transport numbers of OH -ions, TOH, in the AEMs. (The method of TOH determination is described in Section 2.2.) The value of TOH, found at i / ilim theor = 1.5 (∆ϕ'=0.5 V) is equal to 0.16 ± 0.05 for the AMX-Sb and 0.09 ± 0.05 for the AMX-Sbw10 (Table 4).

The observed changes in the CVC, in the absence of intensive water splitting, may occur only due to enhanced mass transfer caused by current induced convection. As the electrolyte concentration in the feed solution is low (0.02 M NaCl), the expected contribution of gravitational convection is low [START_REF] Rubinstein | Electric field effects in and around ion-exchange membranes[END_REF][START_REF] Zabolotsky | On the role of gravitational convection in the transfer enhancement of salt ions in the course of dilute solution electrodialysis[END_REF]. However, electroconvection in similar conditions could enhance mass transfer significantly [START_REF] Korzhova | Effect of surface hydrophobization on chronopotentiometric behavior of an AMX anion-exchange membrane at overlimiting currents[END_REF][START_REF] Nebavskaya | Impact of ion exchange membrane surface charge and hydrophobicity on electroconvection at underlimiting and overlimiting currents[END_REF]. The effect of electroconvection on the mass transfer rate is often evaluated by the value of effective thickness of diffusion boundary layer, δN, entering the following equation [START_REF] Urtenov | Decoupling of the Nernst-Planck and Poisson equations. Application to a membrane system at overlimiting currents[END_REF]:

H 2 Cl Cl Cl Cl N H FD C D i i D δ = + (3)
where Cl i and H i are the partial current densities of Cl -and H + in the depleted diffusion layer, respectively, Cl D -= 2.04 10 -5 cm 2 s -1 and H D = 9.34 10 -5 cm 2 s -1 are the diffusion coefficients of Cl -and H + in solution, respectively. The absolute values are used in Eq. ( 3) in order to avoid the confusion with the sign of Cl i and H i . Eq. ( 3) is deduced with taking into account the exaltation current due to the flux of H + ions generated at the AEM depleted surface, which "exalts" the Cl -transport from the feed solution bulk. The increase ("exaltation") in the Cl - current density is caused by the electrostatic forces of attraction between the H + ions produced in water splitting and located near the depleted surface and the Cl -ions in the bulk feed solution becomes steeper and oscillations of potential drop appear (Fig. 7a). (Note that in the experiment, the current density is swept with a constant rate of 35.7 mA h -1 ). One can see that the beginning of the steeper slope occurs at a higher current density in the case of AMX-Sbw10, hence, the current (and mass transfer) increase owing to electroosmosis of the first kind is essentially higher in the case of AMX-Sbw10 when compared to the AMX-Sb. Apparently it is due to the surface heterogeneity induced by the deposition of wine components' aggregates. Perhaps, an increase in surface waviness discussed in Section 3.1.1 ( A longer contact of the AMX-Sb membrane with wine causes a significant decrease in the ilim exp value (the characteristics of AMX-Sbw72 in Fig. 8a and Table 4). It is accompanied by an increase in the plateau length and a decrease in its slope on the CVC as well as by a decrease in the overlimiting current density compared to those obtained for the AMX-Sb and AMX-Sbw10 membranes (Fig. 8a, Table 4). The reason for the decrease in ilim exp is most likely the reduction of electroconvection. This reduction may be caused by a significant decrease in the surface charge (Fig. 7), which leads to a decrease in electroosmosis of the first kind [95,97,102]

important at i≤ilim exp [91,66Ошибка! Закладка не определена.]. Another cause may be due to transformation of the surface structure from an island-like one (after 10 h of fouling) to a homogeneous one: nearly all the surface of the AMX-Sbw72 membrane is covered with the deposit of wine components' aggregates (Fig. 2e). In addition, intensive water splitting at the AMX-Sbw72 surface can also reduce electroconvection: the appearance of H + ions in the depleted solution near the interface decreases the space charge density (negative in the case of AEM)

[103].

A strong acidification of the desalination channel outlet solution in the case of AMX-Sbw72 membrane evidences that an intensive water splitting at the AMX-Sbw72 surface starts immediately after the limiting current density is reached (Fig. 8b). The cause for increasing water splitting rate may be formation of a bipolar membrane structure due to the foulant layer.

Fig. 7 shows that in the case where the AMX-Sbw72 membrane is bathed in a 0.02 M NaCl solution (which is used in voltammetric measurements), the charge of its surface is negative, while it is positive for the pristine membrane. Note that the membrane bulk remains pristine after 72 h of the contact with wine. The negative surface charge may be due to native structures of microorganisms found on the surface (Fig. 1с), which contain negatively charged carboxylic and phosphoric acid groups in nucleotides (DNA and RNA) and phospholipids. Carboxyl groups are contained also in foulant constituents such as organic acids and proteins. The FTIR analysis (section 3.1.3.) shows that carboxylic and phosphoric groups are absent in the pristine membrane, while they appear in the near-surface layer of AMX-Sbw72 membrane. In addition to their negative charge, the carboxylic and phosphoric groups are known as effective catalyzers of Thus, voltammetry and pH measurements show intensification of electroconvection and weakening of water splitting in the case of a short-term contact of the AMX-Sb membrane with wine. However, a long-term contact of this membrane with wine leads to suppression of electroconvection and an increase in water splitting.

Conclusion

The contact of AEMs with wine results in deterioration of membrane properties. A longterm contact leads to a decrease in ion-exchange capacity and conductivity, while the water content and membrane thickness increase [START_REF] Ghalloussi | Ageing of ion-exchange membranes in electrodialysis: A structural and physicochemical investigation[END_REF][START_REF] Garcia-Vasquez | Evolution of anion-exchange membrane properties in a full scale electrodialysis stack[END_REF]. However, a short-term contact has an unexpected effect. While a 10-hour contact with wine causes a two-fold increase in the AMX-Sb membrane ohmic resistance, some electrochemical properties of the fouled membrane are improved: water splitting at the membrane surface is partially suppressed, electroconvection is enhanced, as well as the mass transfer rate in overlimiting current regime. This improvement is governed by changes of the surface properties and is apparently due to the foulant openwork layer formed by aggregates of wine constituents on the membrane surface. The aggregates are highly hydrated colloidal structures, which involve, according to the FTIR analysis, anthocyanins, tannins, amino acids, polysaccharides and proteins. Their surface contains the groups, which are negatively charged at a neutral pH, while can be protonated and become positively charged at a low pH.

The island-like structure of the foulant layer leads to the appearance of tangential electric force producing electroosmotic lateral flow due to appearance of electric heterogeneity.

Increasing surface roughness/waviness can also contribute to enhancing electroosmosis. The wine is a very complicated system containing a large number of components.

However, the components, which mainly affect the behavior of ion-exchange membranes, can be now specified; they are polyphenols and polysaccharides as suggested in literature [START_REF] Poncet-Legrand | Flavan-3-ol Aggregation in Model Ethanolic Solutions: Incidence of Polyphenol Structure, Concentration, Ethanol Content, and Ionic Strength[END_REF][START_REF] Ulbricht | Fouling in microfiltration of wine: The influence of the membrane polymer on adsorption of polyphenols and polysaccharides[END_REF][START_REF] Madaeni | Chemical cleaning of reverse osmosis membranes[END_REF][START_REF] Cifuentes-Araya | How pulse modes affect proton-barriers and anion-exchange membrane mineral fouling during consecutive electrodialysis treatments[END_REF][START_REF] Audinos | Fouling of ion-selective membranes during electrodialysis of grape must[END_REF] and confirmed in our study. Nevertheless, many important details of the impact of different components remain unclear. It seems that the best strategy, as it was suggested by an anonymous Reviewer of our paper, is to study the membrane behavior separately using each of the main

[ 31 ]

 31 . A number of studies were carried out in order to identify the species responsible for membrane fouling and the mechanism of fouling.Membrane fouling in pressure driven processes is studied[START_REF] Ulbricht | Fouling in microfiltration of wine: The influence of the membrane polymer on adsorption of polyphenols and polysaccharides[END_REF][START_REF] Aimar | Slow colloidal aggregation and membrane fouling[END_REF][START_REF] Vernhet | Relative impact of major wine polysaccharides on the performances of an organic microfiltration membrane[END_REF][START_REF] Blanpain | Investigation of fouling mechanisms governing permeate flux in the crossflow microfiltration of beer[END_REF][START_REF] Susanto | Fouling behavior of aqueous solutions of polyphenolic compounds during ultrafiltration[END_REF][START_REF] Lappin-Scott | Bacterial biofilms and surface fouling[END_REF][START_REF] Zularisam | Role of natural organic matter (NOM), colloidal particles, and solution chemistry on ultrafiltration performance[END_REF] in more details than that occurring in electrodialysis. It is shown that the protein molecules participate in intermolecular interactions with each other and in hydrophobic interactions with the membrane material[START_REF] Koehler | Intermolecular forces between a protein and a hydrophilic modified polysulfone film with relevance to filtration[END_REF]. At pH values between 3 and 3.8, typical for wine, hydroxyl functional groups of polysaccharides act as Lewis and Brønsted-Lowry bases. They serve as donors of electron pairs and acceptors of protons [33], and this particular property mainly determines the interactions between the polysaccharides and membranes. Polyphenols are amphiphilic molecules with hydrophobic aromatic rings and hydrophilic phenolic hydroxyl groups. These substances play crucial role in membrane fouling not only in winemaking but also in production of other drinks (juices and beers) [34,39]. The aggregates of polyphenols and polysaccharides present in red wine have a strong contribution to adsorptive fouling, the interaction between polyphenols and the membrane surface is the main factor governing the fouling [27]. An important role is due to the π-π (stacking) interactions [27] between phenol rings of polyphenols and aromatic groups (e.g. styrene) of membrane matrix. Electrostatic [27,35,40] intermolecular and foulant-membrane material interactions are also possible. Colloidal state of organic molecules formed due to the interactions between polyphenols, polysaccharides, amino acids and proteins is supported by weak van der Waals attraction forces and electrostatic repulsion forces [35,41]. The fouling leads to a significant increase in hydraulic resistance and to a decrease in productive capacity of UF and MF modules [27]. The reduction of membrane permeability caused by fouling can occur as blockage of UF and MF membrane pores by colloidal particles [42,43]. The majority of organic substances mentioned above are nutrient for microbes, hence the organic fouling is frequently accompanied by biofouling [24,36,44]. It should be noted that some microorganisms can be adsorbed at membrane surface as early as during the first hours of operation of membrane modules [45]. The source of microbial contamination can be piping, storage tanks, pretreatment systems [46], where microorganisms such as those responsible for brewing [47] found their habitat. As for the fouling of ion-exchange membranes, the majority of studies focuses on the interactions of ion-exchange membranes and proteins or amino acids [24,48,49,50]. It is found that the change in pH of treated solutions enhances fouling [49,51], when the amino acids and peptides gain the charge opposite to that of the fixed groups of membranes. If the foulants are aromatic, then an AEM, which possesses an aliphatic matrix, is fouled weaker than a membrane with an aromatic matrix [52,53].

  includes the ohmic resistance of the space (membrane + solution) between the measuring electrodes and the diffusion resistance of the enriched and depleted diffusion boundary layers (DBLs). The value of Ref is found by an extrapolation using the experimental current-voltage curves (CVC). The use of Δφ' allows eliminating errors associated with possible changes in the distance between the surface of the membrane and the tips of Luggin capillaries during assembly and disassembly of membrane stack. As well, this excludes the effect of changes in the membrane ohmic resistance caused by its fouling on the shape of the CVC.The method of measurement of the ion transport numbers in IEM in the case of competition between the salt counterion and the H + (or OH -) ions generated in water splitting reaction at the membrane/depleted solution interface is described in Ref.[START_REF] Pismenskaya | Enhancing ion transfer in overlimiting electrodialysis of dilute solutions by modifying the surface of heterogeneous ion-exchange membranes[END_REF]. The method is applied in relatively dilute solutions where the transport of salt co-ion through the membrane can be neglected. The idea is in the measurement of the mass-transfer rate through the membrane under study in batch mode where an intermediate tank is used in the desalting stream. When the desalted solution is acidified with time, a NaOH solution is added into the tank in order to maintain pH=7 there. An acid is added, if the solution in the tank becomes alkaline. The mass transfer rate is found by the rate of the salt concentration decrease in the tank taking into account that the salt concentration in the tank varies due to ion transfer through IEMs in the ED cell and due to addition of alkaline (or acid). The transport number of counterion is determined from the ratio of its flux density through the membrane and the current density measured with an amperemeter.

Fig. 1 .

 1 Fig. 1. AFM images of air-dried samples of AMX-Sb (а) and AMX-Sbw72 (b, c); the image shown in (c) reveals the presence of subjects similar to bacteria.

Fig. 2 Fig. 2 .

 22 Fig.2show optical micrographs of the swollen pristine and fouled AMX-Sb membranes.The pristine AMX-Sb membrane is found reddish-orange in the pH range from 1 to 10 (Figs.2a and 2b), the threads of reinforcing cloth may be seen. The size of "mesh" formed by the reinforcing cloth is (380 × 480) ± 10 μm. The valleys on the surface are observed in the areas where the threads are close to the membrane surface (Fig.3b), and the protrusions are observed where the threads are distanced from the surface. It is explained by the fact that the threads obstruct swelling of the membrane material [75]. Thus, the surface of the swollen AMX-Sb membrane and its fouled samples is wavy; the difference between the highest and lowest points of the surface, b, is about 20 ± 2 μm. The parameters above characterizing the waviness of the AMX-Sb surface are close to those found in other publications [76,66].

Fig. 3 .

 3 Fig. 3. Optical images of the surface and cross-section (a, b, c, d) of the swollen MA-41P membrane; (e) shows the color of an anthocyanin water solution as a function of its pH. The first line (b) shows the colors of the membrane after contact with wine for 3 h and then stored in buffer solutions with pH=1.68 or 6.68 or 9.18 for 1 h. The second (c) and third (d) lines present similar images in the cases where the membrane was contact with wine for 10 h and 72 h, respectively. The distance between the tips of the arrow in (a) corresponds to the membrane thickness.

Fig. 4 .

 4 Fig. 4. Optical images of the surface of the swollen MA-41P membrane: pristine membrane in

Fig. 5 .

 5 Fig. 5. FTIR spectra of the pristine membrane surface, the surface of AMX-Sbw72 and the wine.

Fig. 6 .

 6 Fig. 6. Dependence of the contact angle of AMX-Sb (1) and MA-41P (2) membranes upon the duration of its soaking in wine.

7 Fig. 7 .

 77 Fig. 7. Surface charge of the pristine AMX-Sb membrane equilibrated with a 0.02 M NaCl solution (pH=6.7) and the sample, which contacted with wine for 72 hours and then was stored for 24 hours in NaCl solutions with pH=6.7 or pH=3.5

  [27,33,34,35,36,38,39,40,45]. As it was shown by Bazinet et al. [49,50], the main interactions governing the fouling of anion-exchange membrane by organic acids presented in wine and juices, are of the electrostatic nature. Some of the hydroxyl groups of tannins and the carboxyl groups of organic acids in wine are deprotonated due to elevated pH of the internal solution, which causes a negative charge of these acids [82]. Additionally, there are chemical interactions, which have an essential impact on the AEM behavior in wine and juices. The aromatic matrix of studied membranes affects the introduction of polyphenols into the AEM volume as well as their adsorption on the AEM surface due to π-π (stacking) interactions between the benzene rings (present in membrane matrix) and phenol rings (components of anthocyanins and tannins) [27]. Besides, the hydrogen bonds can be formed between tertiary or secondary amino groups of an AEM [76] and hydroxyl (carboxyl) groups of anthocyanins, tannins and other components of wine [83].

inlet and outlet are shown in Fig. 8 .Fig. 8 .

 88 Fig. 8. I-V curves for a pristine AMX-Sb membrane and the samples contacted with wine for 10 h (AMX-Sbw10) and 72 h (AMX-Sbw72) (a), as well as the pH difference between the desalination compartment outlet and inlet vs i / ilim theor (c); (b) defines the characteristic values of the I-V curve. A 0.02 M NaCl was used as the feed solution.

  the catalytic participation of membrane functional groups, which accelerate protonationdeprotonation reactions of water [86;87]. The MK-40 membrane contains sulfo-groups characterized by a very low catalytic activity towards water splitting reaction [86,87]. The AMX-Sb membrane contains mainly quaternary ammonium, but a certain amount of tertiary and secondary amino groups [88], the latter showing a strong catalytic activity towards water splitting [86,87].

Fig. 8a shows

  that this protrusion disappears when hydrophilic wine components' aggregates form on the AMX-Sb surface. However, the deposition of hydrophobic spots on the AMX surface results in an increase of this protrusion. It can be so important that a current density equal to 1.2 ilim theor passes at ∆ϕ' ≈20 mV, and a current density equal to 1.4 ilim theor passes at ∆ϕ' ≈20 mV[START_REF] Korzhova | Effect of surface hydrophobization on chronopotentiometric behavior of an AMX anion-exchange membrane at overlimiting currents[END_REF]. A local maximum on the chronopotentiogram of such a modified membrane arises at ∆ϕ' ≈20 -30 mV followed by a local minimum, which can drop to ∆ϕ' ≈ 1 mV[START_REF] Korzhova | Effect of surface hydrophobization on chronopotentiometric behavior of an AMX anion-exchange membrane at overlimiting currents[END_REF], making one to assume that these early oscillations are due to equilibrium electroconvective instability recently predicted theoretically byRubinstein and Zaltzman [96].

water splitting reaction [ 86 , 104 ].

 86104 Similar effects of increasing water splitting caused by the contact of IEMs with protein-containing solution are described in literature by Park et al. [105], who studied the impact of BSA, and by Slouka et al. [106], who shown that DNA molecules on the surface of an AEM contributed to increasing water splitting and electroconvective vortex suppression. The formation of a bipolar membrane structure is also possible under electric current due to precipitation of hydroxides of multivalent cations on the depleted membrane surface [107].

  Electroconvection develops despite the noticeable hydrophilization of the AMX-Sb surface by the highly hydrated aggregates of wine constituents. The intensity of electroconvection at relatively high voltage is comparable with the case where the low-conducting hydrophobic spots are introduced on the AMX-Sb membrane surface[START_REF] Korzhova | Effect of surface hydrophobization on chronopotentiometric behavior of an AMX anion-exchange membrane at overlimiting currents[END_REF]. However, at low voltage (∆ϕ'<40 mV), electroconvection in the case of hydrophobic spots is essentially more intensive than electroconvection developed due to the hydrophilic aggregates of the foulant. Hence, our results confirm the results of other authors [108,109] that the membrane surface heterogeneity, within certain limits, together with surface waviness [66,97], enhances electroconvection. An important new finding consists in the fact that the degree of hydrophobicity affects electroconvection mainly at low voltages. With increasing voltage, electroconvection tends to be unstable, then the degree of hydrophobicity acts a secondary role.The color of membrane bulk in initial stages of membrane fouling by wine is determined by the anthocyanin species, which have relatively low molecular mass. At later stages, larger molecules of tannins, proteins and other wine constituents penetrate into the membrane leading to essential loss of conductivity. The Donnan exclusion of the H + ions as co-ions from the membrane shifts pH of the internal solution to alkaline region, which causes deprotonation of anthocyanins and tannins and their transformation into negatively charged forms. These changes in the anthocyanin and tannin form can be observed due to changes in the membrane bulk color.The shift in the charge of sorbed species leads to a partial loss of membrane exchange capacity caused by the electrostatic interactions of negatively charged groups of sorbed species and positively charged fixed groups of the AEM. The subsequent contact of the membrane with wine leads to increasing and thickening of the foulant layer. It becomes rather uniformly distributed over the surface and 2 -3 µm thick after 72 hours of contact of the AMX-Sb with wine. The presence of this layer, as well as biofouling of the AMX-Sbw72 simple lead to an increase in water splitting and a reduction of electroconvection.

  components. With that, it should be taken into account that due to different chemical affinity between different components and the membrane matrix, the membrane behavior in a mixture of wine components could have some features in comparison to the cases where only individual components are present. This affinity may be due to π-π (stacking) interactions [27] between phenol rings of polyphenols and aromatic groups of membrane matrix, van der Waals attraction forces [35, 41] and other kinds of interactions. This way could lead to clearer answers about the mechanisms of membrane fouling in wine and other complicated organic products, kinetics of species transport and other questions.

Table 1 .

 1 . Main properties of AMX-Sb and MA-41P membranes

	Property

b Our measurements. c [

60]. d [61]

Table 2 .

 2 Surface roughness parameters of the pristine and fouled membranes

	Parameter	Membrane
		AMX-Sb	AMX-Sbw72
	Rz, nm	89 ± 20	133 ± 22
	Sratio	1.03 ± 0.01	1.06 ± 0.01
	Rq, nm	19 ± 5	27 ± 4
	Rz is the ten point height of irregularities (ISO 25178-2-2014),

Table 3 .

 3 Variation of bulk properties of AMX-Sb and MA-41P membranes after their

	contact with wine
	Membrane	Thickness,	Exchange capacity	Water content,	Conductivity
		µm	(swollen membrane),	g H2O (g dry membrane) -1	in 0.02 M
			meq g -1		NaCl solution,
					S m -1
	MA-41P	527	1.24	0.52	0.41
	MA-41Pw10	558	-	-	0.13
	MA-41Pw72	565	1.18	0.53	0.06
	AMX-Sb	136	1.30	0.20	0.28
	AMX-Sbw10	138	-	-	0.06
	AMX-Sbw72	140	0.77	0.27	0.03

[92,93].

  The diffusion layer of effective thickness δN, acts a similar role as the Nernst diffusion layer, which controls the mass transfer rate at underlimiting current density[START_REF] Nikonenko | Competition between diffusion and electroconvection at an ion-selective surface in intensive current regimes[END_REF].

	The value of δN may be found from Eq. (3), when the partial current densities, Cl i and
	H i , are known from the experiment. Cl i is expressed as	over Cl i T	lim	, H i as	over OH i T	lim	, where	over i	lim
	is a given overlimiting current density, while Cl T and OH T are the effective transport number of
	Cl -and OH -ions in the membrane found for this current density;	Cl T	+ OH T	=	1	(

Table 4 )

 4 . It is taken into account that the partial current density of H + ions in the depleted diffusion layer is equal to the partial current density of OH -ions in the AEM. The estimation of δN shows that its value at low concentration polarization (CP) (∆ϕ'=0.1 V) is close to that found using the Lévêque equation[START_REF] Rayess | Membrane technologies in wine industry: an overview[END_REF]. However, δN for AMX-Sbw10 is lower than that for AMX-Sb. At higher CP (i=1.5 ilim theor ) the difference between these values increases. It can be suggested basing on the results of Refs. [66,91] that at low CP electroconvection occurs as electroosmosis of the first kind [95,96], while at higher CP, it is nonequilibrium unstable electroconvection of Rubinstein-Zaltzman mode [97] with a great contribution of electroosmosis of the second kind [98,99]. The onset of electroconvection instability occurs at ∆ϕ' close to 0.4 V, where the slope of I-V curve

Table 2 )

 2 may also contribute to in enhancement of electroconvection. This electric and geometric heterogeneity leads to the appearance of

tangential electric force enhancing electroosmosis [100,

[START_REF] Mishchuk | Electro-osmosis of the second kind near the heterogeneous ion-exchange membrane[END_REF][START_REF] Rubinstein | Equilibrium electroconvective instability[END_REF]

. It is interesting that the water contact angle decreases with increasing degree of surface fouling; that signifies that the low conducting spots of wine components' aggregates are of hydrophilic nature. When comparing with the case where the low conducting spots on the AMX membrane surface are hydrophobic [66,91], one can state that there is no essential difference in the behavior of the modified membranes at ∆ϕ' > 0.4 V, where electroosmosis of the first kind is well developed and passes to electroosmosis of the second kind. However, in the initial stages of electroconvection when electroosmosis of the first kind only becomes noticeable (∆ϕ' ≈ 20 -50 mV), the difference is essential. One can see that on the I-V curve of the AMX-Sb membrane in the above mentioned interval of ∆ϕ' there is a small protrusion directed to the vertical axis and characterized by the fact that with increasing current density, the value of ∆ϕ' decreases instead of normally expected increase. We link this decrease in ∆ϕ' with onset of electroconvection bringing fresh solution from the bulk to the depleted interface and evacuating from this interface depleted solution [66Ошибка! Закладка не определена.,91].

Corresponding author: v_nikonenko@mail.ru, nikon@chem.kubsu.ru (V.V. Nikonenko).

Acknowledgements

The research was supported by Russian Science Foundation (project № 17-19-10305). The authors thank also the Center for Collective Use of the Kuban State University "Diagnostics of the structure and properties of nanomaterials" and "Eco-analytical Center of Kuban State University" for the scientific equipment provided.


Imane Argui

Maxime Gueriau

Samia Ainouz

Towards a Mixed-Reality framework for autonomous driving*

Testing autonomous driving algorithms on mobile systems in simulation is an essential step to validate the models and train the system for a large set of (possibly unpredictable and critical) situations. Yet, the transfer of the model from simulation to reality is challenging due to the reality gap (i.e., discrepancies between reality and simulation models). Mixed-reality environments enable testing models on real vehicles without taking financial and safety risks. Additionally, it can reduce the development costs of the system by providing faster testing and debugging for mobile robots. This paper proposes a preliminary work towards a mixed-reality framework for autonomous navigation based on RGB-D cameras. The aim is to represent the objects in two environments within a single display using an augmentation strategy. We tested a first prototype by introducing a differential robot able to navigate in its environment, visualize augmented objects and detect them correctly using a pre-trained model based on Faster R-CNN.

I. INTRODUCTION

Autonomous driving (AD) is expected to contribute to the emergence of safe, cost-effective and efficient alternatives to existing transportation systems. AD-enabled vehicles have been used in a wide range of applications: urban driving (cars, taxis, trucks), healthcare industry (ambulances), farming (tractors, irrigators, buggies), industries (forklifts, car crash testing vehicles). However, deploying a fully Autonomous Vehicle (AV) remains challenging as it requires prior training in various and repeated testing conditions. Deep learning algorithms have been used in order to adapt to different situations (including unpredictable or risky scenarios). Such approaches allow to train intelligent vehicles to autonomous navigation, obstacle avoidance, mapping, object detection, etc. Ideally, a fully autonomous vehicle should be able to adapt its actions continuously in response to the changes occurring in its environment. The process of learning and training for autonomous vehicles can be summarized in three steps:

1) Pre-training of the learning model in simulation;

2) Implementation of the pre-trained model on real-life robots/cars; 3) Fine-tuning of model parameters on real vehicles. Running tests in simulation is an essential step for model verification and validation. During this stage, the vehicle also needs to be trained to face critical, possibly dangerous and/or expensive situations. Even with the latest developments on simulation tools proposing high graphics and numerous options to reproduce real-world conditions, one *This work was supported by the ANR RAIMo under grant reference ANR-20-CHIA-0021 1 The authors are with Normandie Univ, INSA Rouen, UNIROUEN, LITIS, 76801 Saint Etienne du Rouvray, France firstname.lastname@insa-rouen.fr classical problem persists: the reality gap during the transfer of models from simulation to reality. This is generally caused by discrepancies between conditions in simulation and reality (vehicle dynamics, sensor data, road and weather conditions). On the other hand, experiments on real vehicles and environments are necessary to evaluate and validate the model and obtain realistic results. Nonetheless, real world experiments can be very cost-consuming as they may require expensive materials or special facilities.

A promising alternative for this problem is to combine the advantages of simulation with real-world conditions. This technique is referred to in literature as Mixed-Reality (MR) testing. It was first introduced in [START_REF] Milgram | A taxonomy of mixed reality visual displays[END_REF], where the authors defined the mixed-reality environment as an environment where real world and virtual world objects belong together to a single environment. Figure 1, known as Milgram's Continuum, illustrates a taxonomy for the change of real/virtual environments to yield a new world where real and virtual elements can co-exist. MR can be defined as the space between a purely physical world and purely virtual world. Fig. 1: Virtuality continuum [START_REF] Milgram | A taxonomy of mixed reality visual displays[END_REF] Training vehicles to autonomous navigation in a mixedreality environment allows to run experiments repeatedly to train an agent to take the convenient action for a particular situation. Moreover, it can contribute to reducing most of the safety risks by replacing actual obstacles with virtual ones. It also simplifies debugging by testing directly on a real vehicle. Virtual information such as maps and sensor data can be mixed with information collected from the physical environment and visualized in a unique world. In this work, we focus on vision-based autonomous systems as they rely on the detection and recognition of complex roadside information (traffic lights, traffic signs, etc.) and moving users (cars, pedestrians, etc.). The idea is to enable these systems to be trained to face risky situations in a safer environment where virtual elements can be added to their perception. This paper describes a research effort towards the development of a MR framework based on vision. We introduce a strategy to augment a vehicle agent's perception by adding objects modeled in a second (virtual) environment. The potential of the proposed framework as well as the results of the developed augmentation strategy are demonstrated in simulation.

II. RELATED WORK

Recently, Mixed-Reality simulations have been explored to tackle research problems in different domains. The ambition behind this interest is to leverage existing visual and spatial skills to expand user interaction possibilities. In one of the first applications of MR [START_REF] Billinghurst | The magicbook: a transitional ar interface[END_REF], the authors proposed a MR interface for a user-experience based on a real book. Using an augmented-reality display, the book is augmented with virtual elements, allowing the users to view a shared Mixed Reality object. Later on, MR has been applied for different purposes. Several studies review the use of MR: its applications and current challenges [START_REF] Rokhsaritalemi | A review on mixed reality: Current trends, challenges and prospects[END_REF], the applications of MR in healthcare higher education [START_REF] Stretton | Exploring mobile mixed reality in healthcare higher education: A systematic review[END_REF], and the opportunities of using MR simulations in teaching and learning [START_REF] Murphy | Learning with avatars: Exploring mixed reality simulations for next-generation teaching and learning[END_REF]. In robotics, MR was used first to facilitate prototyping for virtual humanoids amongst real obstacles [START_REF] Stilman | Augmented reality for robot development and experimentation[END_REF]. MR was applied for mobile robotics initially in Chen et al. [START_REF] -H. Chen | Mixed reality simulation for mobile robots[END_REF]. The aim of this research was to integrate virtual obstacles in the real world in order to provide a safe simulation environment. Using LiDAR, the authors mixed the data received from the virtual and the real world. More recent works focused on introducing dynamic obstacles for self-driving vehicles: (i) in [START_REF] Zofka | The sleepwalker framework: Verification and validation of autonomous vehicles by mixed reality LiDAR stimulation[END_REF], the authors used LiDAR to detect a virtual pedestrian. The experiments were performed in the real environment augmented by virtual elements. (ii) In [START_REF] Mitchell | Multi-vehicle mixed-reality reinforcement learning for autonomous multi-lane driving[END_REF], the mobile robot is trained in a multi-vehicle multi-lane environment. The real robot was learning autonomous driving with 16 virtual agents simulating a background traffic. (iii) Another application of introducing virtual elements was presented in [START_REF] Baruffa | Mixed reality autonomous vehicle simulation: Implementation of a hardware-in-theloop architecture at a miniature scale[END_REF], where the agent followed a simulated bus using ArUco markers. The real robot performed the same actions as its virtual twin.

To the best of our knowledge, MR environments developed and proposed for self-driving vehicles rely on augmenting data received from LiDAR. Although LiDAR has been considered lately as an essential part of self-driving vehicles proving its accuracy and efficiency, it remains unable to interpret roadway information like landmarks and drivable paths, to face difficulties in bad weather conditions, and has expensive initial and maintenance costs. On the other hand, the depth cameras have gained more importance lately. They are less expensive, and create high-definition mapping data by identifying target object shape, appearance, and texture. They not only capture objects but also the landmarks, drivable paths among other data making it a reliable sensor. Moreover, they can gain a high-resolution image of distant objects, thus making them able to see objects that can't be perceived by low-resolution LiDAR. Therefore, the main focus of this work is to develop a mixed-reality framework based on RGB-D cameras where an agent navigates autonomously in an environment and perceives objects present in both virtual and real worlds.

III. MIXED-REALITY FRAMEWORK

This paper presents a preliminary work that aims at demonstrating the potential of the framework. The architecture of the framework is illustrated in Figure 2. The main idea is to control a mobile robot in environment A (real world), and enable it to perceive the fusion of two environments (real and virtual). In this paper, we present the first steps of testing on simulation by having two virtual worlds. The tests were established on simulation. To obtain two distinct environments in ROS, we created namespace groups to run in parallel two simulations in the environments A and B. The robot in the environment A is controlled by the differential driving plugin in Gazebo, and its digital twin in the environment B reproduced the same behavior in this environment (section III-B). The augmentation strategy described in section III-C is executed on the images received from camera A and camera B before applying the object detection algorithm to the output result.

A. Simulation tools

The simulation environment used for the framework is the Open-Source robot simulation Gazebo [START_REF] Koenig | Design and use paradigms for gazebo, an open-source multi-robot simulator[END_REF], known for its high flexibility and its seamless integration with the middleware ROS [START_REF] Quigley | Ros: an open-source robot operating system[END_REF]. It also already supports a large selection of mobile robots and manipulators, as well as their sensors and control algorithms. Two environments were created, where the robot and its twin can navigate. The communication between different nodes is set up by ROS.

B. Control and digital twining

In order to control the robot, we used existing Gazebo plugins. They give a greater functionality to the robot model and can link to ROS messages and service calls for sensor output and motor input. The plugin used for driving provides a basic controller for 2-wheeled robot in Gazebo. In order to mirror the robot's behavior in the second environment, we used a simplified digital twining method by sending the linear and angular velocity of robot A to robot B.

C. Augmentation strategy

The characteristics of the depth camera are described in URDF files (Unified Robotic Description Format) that can be interpreted in Gazebo. The images received from the camera were converted to OpenCv format using the CvBridge library. To enable the robot to perceive the fusion of two environments, it is necessary to compare the two images, calculate the difference between them, and add this difference to the initial image the robot will perceive. To achieve this augmentation, we designed an algorithm that enables us to combine the output of the camera A (the initial image where we will add the difference I A ) and the output of the camera B (I B ). union between the two images is applied pixelwise. Since I B is the output of the camera in the virtual world, we are only interested in the obstacles and objects, the background is irrelevent, therefore, it's equal 0. I r is the result the robot is going to perceive, as computed in Equation 1. Fig. 2: Overview of the proposed framework enabling the physical coupling of two virtual robots and the fusion of their perception using a vision-based strategy.

I r = I A ∪ (I B > 0) * I B ( 1
)
This strategy allows the robot to see, in the same image, objects appearing in the two environments. The pixelar position is assigned to the center of the objects to determine their location in the scene. At this stage of the work, occlusions and light conditions are still not considered.

IV. EXPERIMENTATIONS

The MR framework developed in this paper is based on a differential wheeled robot integrating a depth camera. The tools and elements used as well as the results of the tests are described in this section.

A. Experimental setup

The framework presented in this paper is based on a differential wheeled robot illustrated in Figure 3. Its movement is based on two separately driven wheels placed on either sides of the robot. The direction can be controlled by varying the rate of rotation of the wheels. The kinematic model for the robot can be expressed in Equation 2. Fig. 3: Kinematic model of a differential drive mobile robot P = (x r , y r , θ r ) is the pose of the robot, v r is the linear velocity and ω r is the angular velocity of the robot. As mentioned in section II, this work is mainly based on depth cameras, enabling the agent to get an additional understanding of a scene that doesn't require a human intervention. This is especially important for tasks like autonomous navigation and obstacle avoidance. The camera used during this work is the Intel Realsense D435. It's an active stereo depth camera that can get up to 848x480 at up to 90 FPS [START_REF] Ahn | Analysis and noise modeling of the intel realsense d435 for mobile robots[END_REF]. It provides distance measurement along with RGB data. This sensor has beneficial characteristics in terms of resolution, frames per second, form factor, weight and price range.

In order to verify and complete the framework, we decided to test object detection algorithms on the results of the fusion. The algorithm used for this task is the FASTER R-CNN [START_REF] Ren | Faster r-cnn: Towards realtime object detection with region proposal networks[END_REF]. This network is an extension of FAST R-CNN, and as its name suggests, it is faster due to the region proposal network (RPN). The architecture of this network is a combination of two modules: RPN is in charge of generating region proposals and FAST R-CNN ensures the multiple object detection in the proposed regions. Since the goal of this implementation was to test the augmentation strategy proposed, we used the pre-trained network on MS-COCO 2017 [START_REF] Lin | Microsoft coco: Common objects in context[END_REF], a largescale object detection, segmentation, and captioning dataset. It contains 328.000 images of everyday objects and humans.

B. Tests of the augmentation strategy

In order to verify the augmentation strategy presented in section III-C, we carried out a first test in an offline setup (i.e., using various pre-recorded images). Figure 4 presents the fusion of two images acquired from simulation (image 1left, and image 2 -center) and the result of the fusionright. In the initial images, we place different elements (pedestrian, car, stop sign, etc.) in different positions (each row in Figure 4 represents a different setup). After the application of the algorithm, we can observe that the objects present in the initial images (1 & 2) are placed in their exact position within a single image (result of the fusion). Since these images have the same background, and are both virtual, we decided to go a step further, and test the fusion of one real image and one virtual image. The real images represent a traffic road. Results of this second test are presented in Figure 5. The most-right images (resulting from the fusion) seem to accurately combine the original images (1 & 2). The virtual elements were added in the real images in the same shape, size and position. The results of the offline testing

C. Application of the detection algorithm

Once the environment was set-up and coupled to our RGB-D augmentation strategy, the next step was to complete the framework by implementing an object detection algorithm and run additional on-line tests to check the validity of the fusion. The robot will navigate in the environment A, perceive the fusion of the two environments and start detecting the objects surrounding it (e.g., stop sign, buses, stop light, etc.).

We started testing the algorithm online using Gazebo and ROS. We created a ROS node that subscribes to the data received from camera A and camera B, applies the fusion strategy, and sends the output to the robot. The object detection algorithm was tested on the resulting images after the fusion. The robot navigates in the environment A, perceives the fusion of the two environments (A and B), and detects the objects in this image as shown in the figure 6 where it recognizes the bus and the stop sign. To test the robustness of the fusion and the detection algorithm, we decided to overlap two elements. The results are represented in figure 7 In this paper, we proposed a preliminary work towards a MR framework for autonomous navigation based on RGB-D cameras. The main motivation behind this research is to fill the gap between tests in simulation and real-world, to limit safety risks and to be able to run an experiment several times in order for the agent to learn and train on a given task. A fusion algorithm has been proposed to mix two virtual environments so that the robot can perceive one single environment with different obstacles. After numerous tests in offline and online settings, an object detection model has been introduced to complete the framework. The particularity of this framework is that it relies on depth-cameras, enabling the robot to detect objects, but also landmarks and drivable paths. We aim in the future to introduce a real robot able to navigate in a real world and interact with physical and virtual obstacles. Additional improvements will be considered for future work: the improvement of digital twining by sending velocity and position to avoid drifts, the introduction of dynamic obstacles in the virtual environment could help to evaluate the agent's behavior and train it to avoid critical situations like unexpected pedestrian crossing or accidents. Learning-based approaches require a substantial amount of data for learning and training the agent to autonomous navigation and obstacle avoidance, the training of these models in the MR framework will lead to a faster learning by generating a diverse amount of data. Finally, the introduction of a virtual learning agent would enable us to investigate the communication between the agents and the interaction in a multi-agent MR environment.

Fig. 4 :Fig. 5 :Fig. 6 :

 456 Fig. 4: Results of the fusion of Image 1 (virtual) and Image 2 (virtual)

  . Image (a) and image (b) are the initial images. Image (c) is the result of the fusion of the images. The pedestrian and the truck are overlapped. The image (d) is the result of the object detection algorithm applied. It detected the pedestrian and the truck.

Fig. 7 :

 7 Fig. 7: Results of the fusion and object detection algorithm