Nanofiltration Composite Membranes Based on KIT-6 and Functionalized

Article Nanoﬁltration Composite Membranes Based on KIT-6 and Functionalized KIT-6 Nanoparticles in a Polymeric Matrix with Enhanced Performances

Gabriela Paun 1, Viorica Parvulescu 2, Elena Neagu 1, Camelia Albu 1, Larisa Ionita 1, Monica Elisabeta Maxim 2, Andrei Munteanu 3, Madalina Ciobanu 2 and Gabriel Lucian Radu 1,*

1 National Institute for Research-Development of Biological Sciences, 060031 Bucharest, Romania; [email protected] (G.P.); [email protected] (E.N.); [email protected] (C.A.); [email protected] (L.I.) 2 Ilie Murgulescu Institute of Physical Chemistry of the Romanian Academy, Spl. Independentei 202, 060021 Bucharest, Romania; [email protected] (V.P.); [email protected] (M.E.M.); [email protected] (M.C.) 3 Apel Laser, 25 Vanatorilor Street, 077135 Mogosoaia, Romania; [email protected] * Correspondence: [email protected]; Tel.: +021-220-0900

Abstract: The nanofiltration composite membranes were obtained by incorporation of KIT-6 ordered mesoporous silica, before and after its functionalization with amine groups, into polyphenylene-ether- ether-sulfone (PPEES) matrix. The incorporation of silica nanoparticles into PPEES polymer matrix was evidenced by FTIR and UV–VIS spectroscopy. SEM images of the membranes cross-section and Citation: Paun, G.; Parvulescu, V.; their surface topology, evidenced by AFM, showed a low effect of KIT-6 silica nanoparticles loading Neagu, E.; Albu, C.; Ionita, L.; Maxim, and functionalization. The performances of the obtained membranes were appraised in permeation M.E.; Munteanu, A.; Ciobanu, M.; of Chaenomeles japonica fruit extracts and the selective separation of phenolic acids and flavonoids. Radu, G.L. Nanofiltration Composite Membranes Based on KIT-6 and The obtained results proved that the PPEES with functionalized KIT-6 nanofiltration membrane, we Functionalized KIT-6 Nanoparticles have prepared, is suitable for the polyphenolic compound’s concentration from the natural extracts. in a Polymeric Matrix with Enhanced Performances. Membranes 2021, 11, Keywords: nanofiltration composite membranes; PPEES; KIT-6; KIT-6 functionalized; selective sepa- 300. https://doi.org/10.3390/ ration membranes11050300

Academic Editor: Wolfgang Samhaber 1. Introduction Particular interest has been given in recent years to finding solutions to improve Received: 31 March 2021 the characteristics of membranes, such as a high permeate flux, rejection, and better an- Accepted: 17 April 2021 tifouling capacity. The most polymeric membranes are hydrophobic nature, which causes Published: 21 April 2021 serious fouling problems, leading to in a permanent flux decline, shortened membrane lifetime, and increasing the maintenance costs [1]. Previous studies indicated that the Publisher’s Note: MDPI stays neutral physical–chemical properties of membrane surface, such as hydrophilicity and roughness, with regard to jurisdictional claims in are major factors influencing membrane fouling. It is generally accepted that hydrophilic published maps and institutional affil- membrane corresponds to lower membrane fouling potential than hydrophobic one be- iations. cause many foulants are hydrophobic compounds [2,3]. In addition, the most hydrophobic nanofiltration membrane provided the lowest permeate flux in the nanofiltration extract [4]. In order to reduce these limitations and to obtain high performance polymeric membranes, [5] dispersed inorganic nanoparticles into a polymer matrix, forming so called Copyright: © 2021 by the authors. polymeric nanocomposite membranes [6–9]. The obtained membranes show superior per- Licensee MDPI, Basel, Switzerland. formances, such as mechanical toughness, thermal properties, permeability, and selectivity, This article is an open access article compared to polymeric membranes. All of these depend on the quality of interface between distributed under the terms and nanoparticles and the polymer. Thus, better performing membranes containing smaller size conditions of the Creative Commons inorganic particles (<20 nm) were obtained [10]. Moreover, the functionalization of these Attribution (CC BY) license (https:// nanoparticles increases their dispersion and the hydrophilic characteristics of the mem- creativecommons.org/licenses/by/ 4.0/). brane. As well, the previous studies demonstrated that inorganic nanoparticles such as

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silica (SiO2)[11–14], zeolites [15,16], metal oxide (Al2O3, TiO2, ZnO) [15,17–23], and carbon nanotubes [23,24] could improve the hydrophilicity of the modified polymer hydrophobic membranes. Silica nanoparticles have chemical compatible and the mechanical stability needed for preparation of polymeric membrane, and their structure can be changed by chemical functionalization [25]. Thus, SBA-15 ordered mesoporous silica was used as nanofiller to improve the water permeability of a polymer matrix [26]. A higher compati- bility and adhesion between inorganic nanoparticles and polymer matrix were obtained by functionalization with amine groups of silica surface in the presence of (3-aminopropyl) triethoxysilane (APTES) as coupling agent. Another method to improve hydrophilicity of the polymer membranes is the addition of polyvinyl pyrrolidone (PVP) or polyethylene glycol (PEG) in different concentrations [27,28]. Polyphenylene ether ether sulfone (PPEES) was studied little until 2015 for the membrane synthesis. Our previous studies showed that PPEES can be successfully applied to obtain ultrafiltration membranes for concentration of the herbal extract [29]. PPEES received in the last time more attention due to the specific properties, such as less hydrophobicity, good solubility, good electronic properties, high thermal resistance and a good chemical stability [30,31]. An interesting application was as cross-linkers, to induce the polymer network with interpenetrating structure, and at the same time, as a spreader to homogeneously disperse SiO2 nanoparticles in nanocomposite membranes [32]. The chemical structure of PPEES is similar to the polysulfone’s structure, so that the properties of these two polymers are similar, but PPEES is much cheaper and therefore, it is preferable to replace polysulfone with poly(1,4-phenylene-ether-sulfone-ether). Here, we report on incorporation of KIT-6 ordered mesoporous silica, before and after its functionalization with amine groups, into polyphenylene-ether-ether-sulfone matrix. KIT-6 was selected as nanofiller due to the properties such as high surface area, ordered porous structure, mesopores with narrow pore sizes, high adsorption capacity, and thermal and hydrothermal stability [33,34]. The properties of membranes and the influence of incorporation and composition of KIT-6 and KIT-6-NH2 nanoparticles on permeation and selectivity of phenolic acids and flavonoids of Chaenomeles japonica fruit extracts were studied.

2. Materials and Methods 2.1. Synthesis of KIT-6 and Functionalized KIT-6-NH2 Mesoporous Silica Nanoparticles KIT-6 silica nanoparticles were prepared using Pluronic 123, as surfactant, by dis- persing in water, hydrochloric acid, and butanol [35]. The silica precursor, i.e., tetraethyl orthosilicate (TEOS) was added under stirring, and the resulted gel was transferred into autoclave to treat hydrothermally for 48 h at 80, 100, or 120 ◦C. The obtained solids were ﬁltered, washed, and treated thermally at 100 ◦C, for drying, and at 550 ◦C to remove the surfactant. The functionalization with amine group of the KIT-6 mesoporous silica surface was accomplished by post-grafting method [36,37] with 3-aminopropyltriethoxysilane (APTES) as coupling agent.

2.2. Preparation of Nanoﬁltration Membranes The nanoﬁltration composite membranes were obtained by phase inversion method. Poly(1,4-phenylene ether-ether-sulfone) (PPEES; Sigma-Aldrich Chemical Company, Inc., St. Louis, USA) was dissolved in 1-Methyl-2-pyrrolidone (NMP; Honeywell, SUA). The silica nanoparticles and polyvinyl pyrrolidone K90 were added in the polymeric solution under continuous stirring to form a casting solution. The resulting homogeneous solution was subjected to sonication for 45 min to remove trapped bubbles, and then, it was placed as thin layer with a “doctor blade” roller. The polymer was precipitated in a bath with deionized water. The obtained nanocomposite membranes were kept for 3–4 h in deionized water to afford complete phase separation. The membranes with various composition Membranes 2021, 11, 300 3 of 13

of inorganic nanoparticles were named “Mn” (n = 0, 1–4). The composition of different casting solutions is presented in Table1.

Table 1. Compositions of the casting solution.

Membrane Type Polymer (wt. %) Silica Nanoparticle (wt. %)

PPEES PVP KIT-6 KIT-6-NH2 M0 20 2 0 0 M1 20 2 1 0 M2 20 2 0 1 M3 20 2 2 0 M4 20 2 0 2

2.3. Characterization of the Obtained Materials

The structural and textural properties of KIT-6 and KIT-6-NH2 nanoparticles were evaluated by X-ray diffraction at small angles (Bruker AXS D8 diffractometer, Mannheim, Germany) and by N2 adsorption–desorption (Micromeritics ASAP 2010, Dresden, Ger- many). The morphology and ordered porous structure were characterized by scanning electron microscopy (SEM with EDX, FEI Quanta 3D FEG), and transmission electron microscopy (TEM, Tecnai 10 G2-F30) from FEI Company Europe, Eindhoven, Netherlands). The morphology of the polymeric and nanocomposite membranes cross-section was analyzed by scanning electron microscopy (SEM-Hitachi model SU1510, Schaumburg, IL, USA). The SEM operating conditions were 15 kV, with a nominal probe current reading of 30 nA, and a working distance of 15 mm. The samples were left uncoated (all images were taken in variable pressure mode at 30 Pa to compensate the surface chargeup effect so that coating with a conductive layer was not necessary) and the SEM was operated in SEM high-vacuum mode (chamber pressure 30 Pa), using an atmosphere of dry nitrogen gas. The surface roughness of the prepared membranes was analyzed by atomic force microscopy (AFM CORE Nanosurf, Liestal, Switzerland). The chemistry of membranes surface was studied by FTIR spectroscopy with Bruker TENSOR 27 instrument (Mannheim, Germany). FTIR spectra were recorded between 400 and 4000 cm−1. The UV–VIS diffuse reflectance spectra were recorded on a JASCO V570 spectrophotometer (ABL&E Jasco Romania SRL, Cluj-Napoca, Romania). As a reference, a certified reflectance standard, spectral, was used, and the measurements were carried out in the range of 60–190 nm. The water contact angle measurement using a Drop Shape Analysis System, model DSA 2 Easy Drop instrument (Krüss GmbH, Germany) is an evaluation of the membrane’s hydrophilicity. The contact angle (θ) values of each sample were measured at five diverse positions of one sample.

2.4. Permeability and Selectivity of Membranes Performances of the prepared membranes were assessed by a KMS Laboratory Cell CF- 1 cross-flow lab-scale filtration unit. The obtained results for nanocomposite membranes were compared with that of the unfilled one. The permeability and selectivity were evaluated using Chaenomeles japonica fruits extract. The permeate flux (J) and rejection rate (Rj) was calculated with equations:

V J = L m−2 h−1 , (1) A · t

where V is the permeate volume (L), A is the effective membrane area (m2), and t is the time (h) necessary for the V litters of permeate to be collected. The experiment ended when a volumetric concentration ratio (feed volume/retentate volume) of 2.5 was reached. Samples of feed solution, permeate, and retentate were collected for further analysis. Membranes 2021, 11, 300 4 of 13

The rejection to solute (R, %) was determined through the formula:

Cp R = 1 − × 100 (%), (2) j C f

where cp and cf represents concentration in permeate and feed solution, respectively. The C. japonica extract was achieved by accelerate solvent extraction using a Dionex ASE 350 System. The ASE conditions were set as: solvent—60% ethanol, temperature— 100 ◦C, static time—10 min, and static cycle—3. The analysis of the extract concentration was performed by determination the phenolic acids and ﬂavonoids using spectrophotometric methods and HPLC/MS method. The phenolic content was seated with Folin–Ciocalteu assay [38] and expressed as chlorogenic acid equivalents (CAE) mg/mL). The total ﬂavonoid content was measured using the AlCl3 colorimetric method as described by Lin [39] and was expressed as rutin equivalents (RE) µg/mL. The HPLC-MS polyphenol measurements were performed by HPLC SHIMADZU system, through a C18 Kromasil 3.5, 2.1 mm × 100 mm column, using a validated HPLC-MS method [40].

3. Results and Discussion 3.1. Characterizations of the Inorganic Nanoparticles The ordered mesoporous structure of KIT-6 (obtained by treatment in autoclave at ◦ Membranes 2021, 11, x FOR PEER REVIEW100 C) and KIT-6-NH2 nanoparticles, with Ia3d symmetry, was evidenced by small5 angle of 14 XRD patterns (Figure1a). Although the ordered porous structure was preserved, a small decrease in XRD peaks intensity, pore volume, and pore size can be observed for KIT6-NH2.

(a) (b)

FigureFigure 1. 1. ((aa)) XRD XRD patterns patterns at at small small angle angle and and ( (bb))N N22 adsorptionadsorption desorption desorption of of KIT-6 KIT-6 me mesoporoussoporous nanoparticles nanoparticles before before and and afterafter functionalization. functionalization.

AnotherThe textural property properties of the wereinorganic characterized nanoparticles by N that2 sorption determines experiments. their adhesion As, we with can polymersee that adsorptionmatrix, structure, branch and of isotherms stability of (Figure the obtained1b) shows membrane a sharp inﬂectionis morphology. between Thus, 0.6 morphologyand 0.8 range was of thethe relativemain property pressure selected which isfor typical the obtained for mesoporous silica nano silicaparticles materials at differ- such entas KIT-6hydrothermal mesoporous treatment silica [temperature.33]. It can also Thus pollable, SEM to images observe (Figure the preservation 2) show larger of KIT-6 and moretexture compacted with H1 hysteresisparticles for loop samples after functionalizationobtained by hydrothermal of silica with treatment amine at group. 80 °C. TheFor highersamples temperature show a narrow (120 °C pore), a sizelarger distribution particle size centered distribution at 7.2 andcan be 6.4 observed. nm. Therefore, the optimumThe adsorption temperature branch of of hydrothermal isotherms (Figure treatment1b) shows is 100 a sharp °C, inﬂectioncondition betweenin which 0.6 a sphericaland 0.8 range morphology of the relative of particles pressure, with thus smaller highlighting and more the uniform preservation sizes (Figure of KIT-6 3a) texture was obtainedwith H1 for hysteresis KIT-6 powder. loop. The samples show a narrow pore size distribution centered at 7.2 and 6.4 nm. The best surface area (780 m2/g) was obtained for sample obtained by ◦ hydrothermal treatmenta at 100 C. The decrease in of volume mesoporesb relative to KIT-6

Figure 2. SEM images of KIT-6 mesoporous silica hydrothermal treated at 80 °C (a) and 120 °C (b).

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(a) (b)

Figure 1. (a) XRD patterns at small angle and (b) N2 adsorption desorption of KIT-6 mesoporous nanoparticles before and silica and the decrease in the pore diameter conﬁrm the location of the grafted species after functionalization. inside the mesopores, and not just on the external surface. AnotherAnother propertyproperty ofof thethe inorganicinorganic nanoparticlesnanoparticles thatthat determinesdeterminestheir theiradhesion adhesionwith with polymerpolymer matrix,matrix, structure,structure, andand stabilitystability ofof thethe obtainedobtained membranemembrane isis morphology.morphology. Thus,Thus, morphology was the main property selected for the obtained silica nanoparticles at different morphology was the main property selected for the obtained silica nanoparticles at differ- hydrothermal treatment temperature. Thus, SEM images (Figure2) show larger and more ent hydrothermal treatment temperature. Thus, SEM images (Figure 2) show larger and compacted particles for samples obtained by hydrothermal treatment at 80 ◦C. For higher more compacted particles for samples obtained by hydrothermal treatment at 80 °C. For temperature (120 ◦C), a larger particle size distribution can be observed. Therefore, the higher temperature (120 °C), a larger particle size distribution can be observed. Therefore, optimum temperature of hydrothermal treatment is 100 ◦C, condition in which a spherical the optimum temperature of hydrothermal treatment is 100 °C, condition in which a morphology of particles with smaller and more uniform sizes (Figure3a) was obtained for spherical morphology of particles with smaller and more uniform sizes (Figure 3a) was KIT-6 powder. obtained for KIT-6 powder.

a b

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FigureFigure 2.2.SEM SEM imagesimages ofof KIT-6KIT-6 mesoporous mesoporous silica silica hydrothermal hydrothermal treated treated at at 80 80◦ C(°C a(a)) and and 120 120◦ °CC( b(b).).

a b

FigureFigure 3. SEM 3. SEM images images (a) and(a) and TEM TEM images images (b) ( ofb) the of the selected selected KIT-6 KIT-6 sample. sample.

AA high high percent percent of of smaller smaller spherical spherical particles particles was was obtained obtained at at higher higher temperature. temperature. FigureFigure 33 evidenceevidence the morphologymorphology (Figure(Figure3 a)3a) and and the the ordered ordered porous porous structure structure(Figure (Figure3b) 3b)of of the the selected selected KIT-6 KIT-6 samples samples for for nanocomposite nanocomposite membrane membrane preparation. preparation. After After functional- func- tionalization,ization, the morphology the morphology remains remains unchanged. unchanged. TEM imagesTEM images show theshow channels the channels with uniform with uniformpore size pore typically size typically for this for type this of mesoporoustype of mesoporous silica. silica.

3.2. Characterizations of the Nanofiltration Composite Membranes SEM micrographs of cross-section of the prepared membranes (Figure 4) exhibit an asymmetric structure, with a finger-like morphology. These results for nanofiltration composite membranes evidenced a uniform surface, denser top layer, and porous sub-layer with macropores for permeation. The SEM images showed that the addition of 1–2% of KIT-6 and KIT-6-NH2 nanoparticles in membranes exhibit an asymmetric structure similar to the base PPEES membrane [41]. Adding KIT silica nanoparticles to PPEES polymeric solution led to slight change in the morphology of channels. Decreasing of number and pore sizes can be observed for samples with higher concentration (2%) of silica nanoparticle. The KIT-6 and KIT-6-NH2 nanoparticles are found close by and outside the pores forming a low-level roughness on composite membrane surface.

M0 M1

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a b

Figure 3. SEM images (a) and TEM images (b) of the selected KIT-6 sample.

A high percent of smaller spherical particles was obtained at higher temperature. Membranes 2021, 11, 300 6 of 13 Figure 3 evidence the morphology (Figure 3a) and the ordered porous structure (Figure 3b) of the selected KIT-6 samples for nanocomposite membrane preparation. After functionalization, the morphology remains unchanged. TEM images show the channels with uniform pore size typically for this type of mesoporous silica. 3.2. Characterizations of the Nanofiltration Composite Membranes SEM3.2. micrographs Characterizations of cross-sectionof the Nanofiltration of the Composite prepared Membranes membranes (Figure4) exhibit an asymmetricSEM structure, micrographs with aof finger-likecross-section morphology. of the prepared These membranes results (Figure for nanofiltration 4) exhibit an compositeasymmetric membranes structure, evidenced with a a finger-like uniform surface,morphology. denser These top results layer, for and nanofiltration porous sub-layer com- with macroporesposite membranes for permeation. evidenced Thea uniform SEM imagessurface, showeddenser top that layer, the and addition porous ofsub-layer 1–2% of KIT-6 andwith KIT-6-NH macropores2 nanoparticles for permeation. in membranesThe SEM imag exhibites showed an asymmetric that the addition structure of 1–2% similar of to the baseKIT-6 PPEES and KIT-6-NH membrane2 nanoparticles [41]. Adding in membranes KIT silica exhibi nanoparticlest an asymmetric to PPEES structure polymeric similar solutionto led the to base slight PPEES change membrane in the [41]. morphology Adding KI ofT silica channels. nanoparticles Decreasing to PPEES of number polymeric and solution led to slight change in the morphology of channels. Decreasing of number and pore sizes can be observed for samples with higher concentration (2%) of silica nanoparticle. pore sizes can be observed for samples with higher concentration (2%) of silica nanopar- The KIT-6 and KIT-6-NH nanoparticles are found close by and outside the pores forming ticle. The KIT-6 and2 KIT-6-NH2 nanoparticles are found close by and outside the pores a low-levelforming roughness a low-level on composite roughness on membrane composite surface.membrane surface.

M0 M1

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M2 M3

M4a M4b

Figure 4. SEM cross-sectional imagesFigure for the4. SEM obtained cross-sectional membranes images withfor the different obtained membranes concentration with different of KIT-6 concentration and KIT-6-NH of 2 nanoparticles. KIT-6 and KIT-6-NH2 nanoparticles. The results of the surface topography of the membranes, obtained by AFM, are presented in Figure 5. These images show, in condition of low silica concentration, insignifi- cant variation of roughness and surface porosity compared with PPEES membrane (M0). The decrease in membrane roughness surface was observed for nanofiltration composite membranes with amine-functionalized KIT-6 nanoparticles (M2, M4a and M4b, where M4a is the cross-sectional image and M4b is the membrane surface image). This was result of better interaction between inorganic nanoparticles and PPEES polymeric matrix.

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The results of the surface topography of the membranes, obtained by AFM, are presented in Figure5. These images show, in condition of low silica concentration, insigniﬁcant variation of roughness and surface porosity compared with PPEES membrane (M0). The decrease in membrane roughness surface was observed for nanoﬁltration composite mem-

Membranes 2021, 11, x FOR PEER REVIEWbranes with amine-functionalized KIT-6 nanoparticles (M2, M4a and M4b, where8 of M4a14 is the cross-sectional image and M4b is the membrane surface image). This was result of better interaction between inorganic nanoparticles and PPEES polymeric matrix.

Figure 5. Cont.

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FigureFigure 5. 5. AtomicAtomic force force microscopy microscopy (AFM) (AFM) images images of of the the obtained obtained membranes membranes with with di differentfferent concentrations concentrations of of KIT-6 KIT-6 and and Figure 5. Atomic force microscopy (AFM) images of the obtained membranes with different concentrations of KIT-6 and KIT-6-NH2 nanoparticles. KIT-6-NH2 nanoparticles. KIT-6-NH2 nanoparticles. TheThe effect effect of silicasilica nanoparticlesnanoparticles on on hydrophilicity hydrophilicity of microporousof microporous PPEES PPEES membranes mem- The effect of silica nanoparticles on hydrophilicity of microporous PPEES mem- braneswas evaluated was evaluated by contact by contact angle angle measurement. measurement. Figure Figure6 presents 6 presents the water the water contact contact angle branes was evaluated by contact angle measurement. Figure 6 presents the water contact angleprofiles profiles for the for prepared the prepared membranes. membranes. A low A water low water contact contact angle indicatesangle indicates a better a surfacebetter angle profiles for the prepared membranes. A low water contact angle indicates a better surfacehydrophilicity hydrophilicity and water and wettability. water wettability. The results The revelated results revelated that all nanofiltration that all nanofiltration composite surface hydrophilicity and water wettability. The results revelated that all nanofiltration compositemembranes membranes had a lower had contact a lower angle contact than PPEESangle than membrane, PPEES and membrane, also, the hydrophilicityand also, the composite membranes had a lower contact angle than PPEES membrane, and also, the hydrophilicityof nanofiltration of compositenanofiltration membranes composite based membranes on KIT-6 based and functionalized on KIT-6 and KIT-6functionalized nanoparti- hydrophilicity of nanofiltration composite membranes based on KIT-6 and functionalized KIT-6cles was nanoparticles improved, whichwas improved, suggests enhancedwhich sugg fluxests and enhanced antifouling flux properties and antifouling of membranes. prop- KIT-6 nanoparticles was improved, which suggests enhanced flux and antifouling prop- ertiesFurthermore, of membranes. a decrease Furthermore, in the contact a decrease anglewas in the obtained contact with angle increasing was obtained the concentra- with in- erties of membranes. Furthermore, a decrease in the contact angle was obtained with in- creasingtion of silicathe concentration nanoparticles. of Our silica result nanoparticles. is in agreement Our result with otheris in agreement similar studies, with whichother creasing the concentration of silica nanoparticles. Our result is in agreement with other similarshowed studies, that the which addition showed of SiO that2 nanoparticles the addition in of the SiO polymeric2 nanoparticles matrix tendedin the polymeric to decrease similar studies, which showed that the addition of SiO2 nanoparticles in the polymeric matrixmembrane tended contact to decrease angles membrane [42]. contact angles [42]. matrix tended to decrease membrane contact angles [42].

Figure 6. The contact angle of polyphenylene-ether-ether-sulfone (PPEES) and nanocomposite Figure 6. The contact angle of polyphenylene-ether-ether-sulfone (PPEES) and nanocomposite membranes. membranes.Figure 6. The contact angle of polyphenylene-ether-ether-sulfone (PPEES) and nanocomposite membranes. Dispersion of the silica into PPEES polymeric matrix was evidenced by FTIR spectra Dispersion of the silica into PPEES polymeric matrix was evidenced by FTIR spectra of theDispersion prepared membranesof the silica into (Figure PPEES7). polymeric matrix was evidenced by FTIR spectra of the prepared membranes (Figure 7). of the prepared membranes (Figure 7).

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Figure 7. FTIR spectra of PPEES (M0) and PPEES with silica nanoparticles (M1 and M2). Figure 7. FTIR spectra of PPEES (M0) and PPEES with silica nanoparticles (M1 and M2). Figure 7. FTIR spectra of PPEES (M0) and PPEES with silica nanoparticles (M1 and M2). TheThe FTIRFTIR spectrum spectrum of of PPEES PPEES and and PPEES PPEES with with silica silica nanoparticles nanoparticles membrane membrane displayed dis- −1 followingplayedThe followingFTIR results: spectrum results: (a) peaks of (a) PPEES at peaks 810 and cm at− 810 1PPEESwas cm assigned withwas assignedsilica to Si-O-Sinanoparticles to Si-O-Si vibration vibrationmembrane and 960 and cmdis- 960−1 playedwascm−1 assigned was following assigned to Si-O results: to stretchingSi-O (a) stretching peaks vibration at vibration810 ofcm SiOH−1 wasof groupSiOH assigned group corresponding to corresponding Si-O-Si tovibration KIT-6, to andKIT-6, and KIT-6- 960 and KIT-6-NH−1 2 appears only on the M1 and M2 membranes; (b) 1107–1072−1 cm−1 was attributed cmNHwas2 appears assigned only to on Si-O the stretching M1 and M2 vibration membranes; of SiOH (b) 1107–1072 group corresponding cm was attributed to KIT-6, toand the −1 −1 KIT-6-NHaromaticto the aromatic2 ring appears vibrations; ring only vibrations; on (c) the 1150 M1 cmand(c) 1150M2corresponded membranes; cm corresponded to (b) the 1107–1072 symmetric to the cm symmetric O=S=O−1 was attributed stretching O=S=O toofstretching the sulfone aromatic group;of sulfone ring (d) vibrations; 1230group; cm (d)−1 was(c)1230 1150 attributed cm −cm1 was−1 correspondedattributed to the aromatic to theto ether aromaticthe andsymmetric 1323ether cm andO=S=O−1 was1323 stretchingattributedcm−1 was ofattributed to sulfone the S=O togroup; stretching the S=O (d) stretching1230 in sulfone; cm−1 wasin (e) sulfone; 1412attributed cm (e)−1 1412correspondedto the cm aromatic−1 corresponded to asymmetricether and to asym-1323 C-H cmbendingmetric−1 was C-H attributed deformation bending to deformationthe of methyl S=O stretching group; of methyl (f) in 1525 sulfone; group; cm−1 (e)(f)(N-H 15251412 bending) cm−1 (N-Hcorresponded indicated bending) the to indicated presence asym- −1 −1 metricofthe -NH presence C-H2 groups bending of -NH in aminodeformation2 groups silica; in aminoof (g) methyl 1590 silica; cm group; (g)was 1590 (f) attributed1525 cm cmwas−1 (N-H toattributed the bending) C=C to aromatic the indicated C=C ringaro- −1 −1 thevibrations;matic presence ring (h) vibrations;of the-NH band2 groups (h) at 1670 the in amino cmband correlatedat silica; 1670 (g) cm with1590 correlated C=Ocm−1 was stretching, with attributed C=O which stretching, to mightthe C=C indicate which aro- maticthemight existence ring indicate vibrations; of the PVP existence in (h) the the matrix; ofband PVP andat in 1670 (i)the 3600–3400 matrix; cm−1 correlated and cm (i)–1 corresponded3600–3400 with C=O cm stretching,–1 to corresponded O-H stretching which to mightvibrations.O-H stretchingindicate the vibrations. existence of PVP in the matrix; and (i) 3600–3400 cm–1 corresponded to O-H stretchingTheThe presencepresence vibrations. ofof inorganicinorganic nanoparticles nanoparticles in in PPEES PPEES matrix matrix and and their their molecular molecular interac- inter- tionsactionsThe were werepresence evaluated evaluated of inorganic by by UV–VIS UV–VIS nanoparticles absorption absorption ofin compositeofPPEES composite matrix membranes membranes and their (Figure molecular (Figure8). 8). interactions were evaluated by UV–VIS absorption of composite membranes (Figure 8).

Figure 8. UV–VIS absorption spectrum of PPEES and nanocomposite membranes. Figure 8. UV–VIS absorption spectrum of PPEES and nanocomposite membranes. FigureThe 8. UV–VIS UV–VIS absorption spectra showspectrum that of PPEES PPEES polymerand nanocomposite and polymer membranes. with silica nanoparticles, with different concentrations, exhibited similar absorption bands at 258 and 298 nm, indi- cating that the center of absorption bands is related to intramolecular and intermolecular

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charge-transfer interactions. The intensities of the absorption bands decrease after the introduction of silica nanoparticles, this being a proof of the interaction between silica and polymer. Furthermore, the strongest interaction can be seen in the case of PPEES polymer and functionalized silica, the strength of the interaction increasing with the amount of functionalized silica (2 wt. %). In order to study the efficiency of composite nanofiltration membranes compared to the PPEES membranes, the water flux, permeate flux, and polyphenols and flavonoid rejections were measured (Table2).

Table 2. Permeation performance and rejection for total polyphenols and ﬂavonoids from Chaenomeles japonica fruits extract for the prepared membranes.

Total Membrane Pure Water Flux Extract Flux a Flavonoid’s Polyphenols Type a (Lm−2h−1) (Lm−2h−1) Rejection (%) Rejection (%) M0 62.3 ± 0.4 4.1 ± 0.04 60.8 ± 0.5 31.4 ± 0.09 M1 75.9 ± 0.5 6.3 ± 0.05 64.8 ± 0.4 55.0 ± 0.3 M2 86.1 ± 0.7 9.1 ± 0.09 79.5 ± 0.6 60.4 ± 0.5 M3 76.1 ± 0.6 7.7 ± 0.06 79.9 ± 0.6 61.8 ± 0.4 M4 87.5 ± 0.6 10.8 ± 0.09 80.9 ± 0.7 63.8 ± 0.5 a Obtained through ﬁltration of pure water and extract, respectively, at 25 ± 1 ◦C and 8 bar.

With increasing KIT-6 or KIT-6-NH2 silica nanoparticles content from 1 to 2 wt. % and introducing amino group, the separation efficiency for total polyphenols and flavonoids was enhanced sharply. As is presented in Table2, the PPEES (M0) and composite nanofiltration membranes (M1–M4) present a higher rejection coefficient to total polyphenols compare with flavonoids. The results from Table2 indicate that membranes with KIT-6- NH2 silica nanoparticles showed highest rejection values for polyphenol, with 33% higher than rejection for M0 and also with 103% higher than rejection of flavonoid compounds for M0 membrane. These types of composite nanofiltration membranes retain the analyzed compounds more efficiently due to better interaction, i.e., hydrogen bonding between amine group of silica nanoparticles and hydroxyl group of polyphenol and flavonoid compounds. The incorporation of KIT-6 and KIT-6-NH2 hydrophilic silica nanoparticles in the polymeric matrix improves the hydrophilicity of the prepared composite membranes, and the flux values increased directly with the quantity of silica nanoparticles from the membrane, which is in agreement with previously published results [37]. Figure9 displays the linear variation of pure water permeation fluxes with the trans- membrane pressure, for the all-prepared membranes. The separation performance of the optimal membrane towards different polyphenolic compounds was further studied. The HPLC data for phenolic acids and flavonoids in the ASE extract and retentate using PPEES (M0) and PPEES with silica nanoparticles (M3 and M4) membranes are presented in Table3. The quantification of these compounds is important because the most phenolic acids and flavones have medicinal importance in treating chronic diseases [38–40]. Recent studies proved the efficiency of nanofiltration for concentration of polyphenols from natural extracts [43–45]. Membranes 2021, 11, x FOR PEER REVIEW 12 of 14 Membranes 2021, 11, 300 11 of 13

Figure 9. Water ﬂux versus applied pressure for membrane (M0) and composite membranes Figure 9. Water flux versus applied pressure for membrane (M0) and composite membranes (M1– M4).(M1–M4).

Table 3. HPLC-MSThe separation analysis performance results for ASE of extract the opti andmal retentate membrane fractions. towards different polyphenolic compounds was further studied.Retentate The HPLC dataRetentate for phenolic acidsRetentate and flavo- ASE Extract Compound [M/z]noids- in the ASE extract and retentate using(M0) PPEES (M0) and(M3) PPEES with silica(M4) nanoparticles (M3 and M4) membranesµg/mL are presentedµg/mL in Table 3. Theµg/mL quantification ofµ g/mLthese com- Ellagic acid [301]pounds is important 1.41because the most pheno 1.72lic acids and 1.74flavones have medicinal 2.30 im- Rutin [609] portance in treating chronic1.05 diseases [38–40]. 1.17 Recent studies 1.35 proved the efficiency 1.63 of nan- Quercetin-3-β-D-qlucosideofiltration for concentration of polyphenols from natural extracts [43–45]. 3.26 4.11 4.12 5.31 (isoquercitrin) [463] The results obtained by HPLC confirm the results presented in Table 2 regarding the Epicatechin [289]efficiency and selectivity9.24 in the concentratio 8.61n of phenolic acids 7.85 and flavonoids 9.17 with KIT- Quercetol [301] 0.43 0.42 0.47 0.52 6-NH2 composite membranes. Myricetin [317] 0.68 0.69 0.70 0.77 Chlorogenic acid [353] 46.62 56.68 69.29 70.09 Luteolin [285] Table 3. HPLC-MS analysis0.25 results for ASE extract 0.26 and retentate fractions. 0.26 0.29 4-Hydroxybenzoic acid [137] 18.03 22.11Retentate 24.66Retentate Retentate 30.66 Sinapic acid [223] 1.09ASE Extract 1.06 1.08 1.12 Compound [M/z]- (M0) (M3) (M4) μg/mL μg/mL μg/mL μg/mL The results obtained by HPLC confirm the results presented in Table2 regarding Ellagic acid [301] 1.41 1.72 1.74 2.30 the efficiency and selectivity in the concentration of phenolic acids and flavonoids with Rutin [609] 1.05 1.17 1.35 1.63 KIT-6-NH composite membranes. Quercetin-3-2 β-D-qlucoside 3.26 4.11 4.12 5.31 4. Conclusions(isoquercitrin) [463] FourEpicatechin composite [289] nanofiltration, 9.24 membranes 8.61 were obtained 7.85by incorporation 9.17 of KIT-6 orderedQuercetol mesoporous [301] silica, before0.43 and after its functionalization0.42 0.47 with amine groups, 0.52 into polyphenylene-ether-ether-sulfoneMyricetin [317] 0.68 (PPEES) matrix.0.69 A low effect 0.70 of loading and functional- 0.77 izationChlorogenic of KIT-6 acid silica [353] nanoparticles 46.62 on the cross-section56.68 of membranes 69.29 and their 70.09 surface topologyLuteolin was evidenced. [285] The evaluation0.25 of Chaenomeles0.26 japonica 0.26fruit extracts’ permeation 0.29 and4-Hydroxybenzoic selectivity of phenolic acid acids and flavonoids shows a 33% higher rejection of phenolic 18.03 22.11 24.66 30.66 acids and also[137] 103% higher rejection of flavonoid compounds for composite membrane withSinapic functionalized acid [223] KIT-6 and highest1.09 loading compared1.06 to PPEES 1.08 membrane. 1.12

Author Contributions: 4. Conclusions Conceptualization, G.P. and V.P.; methodology, G.P., V.P. and M.C.; investigation and analysis: E.N. (UV–VIS analysis), C.A. (HPLC), L.I. (SEM), M.E.M. (contact angle), A.M. (AFM);Four writing—original composite nanofiltration, draft preparation, membranes G.P. and V.P.; were writing—review obtained by incorporation and editing, G.P. of and KIT-6 V.P.; orderedsupervision, mesoporous G.L.R. All silica, authors before have read and and after agreed its functionalization to the published version with amine of the manuscript.groups, into polyphenylene-ether-ether-sulfone (PPEES) matrix. A low effect of loading and functionalization of KIT-6 silica nanoparticles on the cross-section of membranes and their surface

Membranes 2021, 11, 300 12 of 13

Funding: This work was supported by the Core-Program, developed with the support of the Romanian Ministry of Research and Innovation, project 25N/19270101/2019. Institutional Review Board Statement: “Not applicable” for studies not involving humans or ani- mals. Informed Consent Statement: “Not applicable” for studies not involving humans. Data Availability Statement: The data presented in this study are openly available at [doi], reference number. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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