Synthesis and Characterization of Cellulose Acetate Membranes with Self-Indicating Properties by Changing the Membrane Surface Color for Separation of Gd(III)

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Article Synthesis and Characterization of Cellulose Acetate Membranes with Self-Indicating Properties by Changing the Membrane Surface Color for Separation of Gd(III)

Oana Steluta Serbanescu 1, Andreea Madalina Pandele 1,2, Florin Miculescu 3 and Stefan Ioan Voicu 1,2,*

1 Department of Analytical Chemistry and Environmental Engineering, Faculty of Applied Chemistry and Materials Science, Gheorghe Polizu 1-7, 011061 Bucharest, Romania; [email protected] (O.S.S.); [email protected] (A.M.P.) 2 Advanced Polymers Materials Group, Gheorghe Polizu 1-7, 011061 Bucharest, Romania 3 Faculty of Materials Science, University Politehnica of Bucharest, Splaiul Independentei 313, 060042 Bucharest, Romania; [email protected] * Correspondence: [email protected] or [email protected]; Tel.: +40-721165757

Received: 13 April 2020; Accepted: 8 May 2020; Published: 10 May 2020

Abstract: This study presents a new, revolutionary, and easy method for evaluating the separation process through a membrane that is based on changing the color of the membrane surface during the separation process. For this purpose, a cellulose acetate membrane surface was modiﬁed in several steps: initially with amino propyl triethoxysilane, followed by glutaraldehyde reaction and calmagite immobilization. Calmagite was chosen for its dual role as a molecule that will complex and retain Gd(III) and also as an indicator for Gd(III). At the contact with the membrane surface, calmagite will actively complex and retain Gd(III), and it will change the color of the membrane surface during the complexation process, showing that the separation occurred. The synthesized materials were characterized by Fourier transform infrared spectroscopy (FT-IR), thermal analysis (TGA-DTA), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy, demonstrating the synthesis of membrane material with self-indicating properties. In addition, in the separation of the Gd(III) process, in which a solution of gadolinium nitrate was used as a source and as a moderator in nuclear reactors, the membrane changed its color from blue to pink. The membrane performances were tested by Induced Coupled Plasma–Mass Spectrometry (ICP-MS) analyses showing a separation process eﬃciency of 86% relative to the initial feed solution.

Keywords: cellulose acetate; surface modiﬁcation; Gd(III) retention; self-indicating properties

1. Introduction Gadolinium-based complexes and salts are used for two main applications: as contrast agent components for clinical and research magnetic resonance imaging (MRI) examinations [1,2] and poison for nuclear reactors in order to control the nuclear reaction [3]. The problem of Gd(III) toxicity is given by the fact that traces of this element or other complexes based on Gd(III) can remain in the brain, causing toxic eﬀects [4,5]. Besides the fact that it is a highly toxic element, one of the biggest advantages of using this element in its currently known application is given by the practical situation that it cannot be present in environment, because contrast MRI agents are subject to very strict regulations, and also the water that is used as the nuclear reactors’ moderator never goes into the environment due to the same strict regulations. From these reasons, the removal of Gd(III) from water is limited

Coatings 2020, 10, 468; doi:10.3390/coatings10050468 www.mdpi.com/journal/coatings Coatings 2020, 10, 468 2 of 15 to the single practical application of regulating the concentration of Gd(III) from the water used to moderate the nuclear reactions. For this application, one single solution is applied: ion exchange resins. Most used resins are based on polystyrene [6], with performances varying from 150 to 178 mg Gd(III)/g of resin. Higher yields for retention can be obtained using mixed bed (MB) resin column consisting of Strong Acid Cation (SAC) resin and Strong Base Anion (SBA) resin [7], achieving a retention of 250 mg Gd(III)/g of resin. New systems based on functionalized mesoporous silica [8,9] or composites with magnetic nanoparticles [10] have been tested, but ion exchange remains the technical solution with the highest retention. Until now, even if we are considering a separation process, no membrane materials have been reported for the removal of Gd(III). We will propose here a revolutionary membrane material that not only has the ability to separate Gd(III) from water used to moderate the nuclear reactions, but also with the remarkable property of changing its color, visually indicating the separation of Gd(III). Membrane materials are unique among all the materials known and used at this time because, besides their structural and functional characteristics and properties, they possess a unique property: selectivity [11]. A membrane is a functional material that acts essentially as a barrier to a particular species of ions, molecules, and particles, but it is permeable to the rest of the species in a multicomponent system [12]. The development of membrane separation processes was based on both technical factors such as separation selectivity, technology reliability, energy efficiency, and productivity, as well as economic ones such as the cost of plant, equipment [13–15], operating and maintenance [16], and the also price of the raw materials [17]. Membrane and membrane processes constitute an interdisciplinary field whose explosive evolution has been determined by the unlimited involvement of materials science, mathematics, physics, physical chemistry, biology, and engineering in every stage from theory to industrial process. A proof of this is the multiple applications in various fields such as biomedical sciences: hemodialysis [18–20], protein separation [21,22], osteointegration [23–55], fuel cells [26–28], environmental decontamination [29]. After performing a membrane separation process, a very important step is the evaluation of the separation efficiency. For this, different methods of analysis are applied to the solution with separated species, such as UV-Vis spectroscopy [30], atomic abortions [31], and Induced Coupled Plasma–Mass Spectrometry ICP-MS [32]. The present study was conducted for Gd(III) separation from aqueous solutions. Gadolinium nitrate is one of the most used moderators for nuclear reactions which, in the recent year, successfully replaced boron for this type of application [7,33–35]. The basic idea of the current research was to have an easy method to remove gadolinium from aqueous solutions and also the possibility that the membrane would indicate the gadolinium separation without adjacent analysis. Since the solutions used in nuclear power are very pure solutions, the possibility of having other interferences to be separated in the system has been eliminated from the start [36]. The excess of gadolinium must be separated in order to control the nuclear reaction at desired parameters. The underlying principle of this research was to find a substance that simultaneously exhibits two roles: separating the gadolinium from aqueous solutions and to indicate that gadolinium was separated by changing a membrane property that could be slightly observed, changing the color of the membrane surface. For this purpose, an indicator has been searched that fulfills both conditions and can be immobilized on the membrane surface. Calmagite is an indicator for metal cations with the molecular formula presented in Figure1. The operating principle of this indicator is based on the complexation reaction of metal cations generated by the two hydroxyl groups and the two nitrogen atoms [37]. The novelty degree of this research is given by the fact that without additional instrumental analysis methods, the efficiency of separation can be observed visibly by the operator through the change of color of the membrane surface. Coatings 2020, 10, 468 3 of 15 Coatings 2020, 10, x FOR PEER REVIEW 3 of 15

FigureFigure 1.1. MolecularMolecular formulaformula ofof calmagite.calmagite. 2. Materials and Methods 2. Materials and Methods In order to functionalize the membrane surface, we have used our previous experience in the In order to functionalize the membrane surface, we have used our previous experience in the modification of the cellulose acetate membranes surface, which implies the following steps: the modification of the cellulose acetate membranes surface, which implies the following steps: the hydrolysis of acetyl groups to increase the number of hydroxyl groups at the membrane surface, hydrolysis of acetyl groups to increase the number of hydroxyl groups at the membrane surface, aminopropyl triethoxy silane (APTES) immobilization, and glutaraldehyde (GA) linkages to the aminopropyl triethoxy silane (APTES) immobilization, and glutaraldehyde (GA) linkages to the amino groups of APTES, followed by immobilization of calmagite. Calmagite immobilization was amino groups of APTES, followed by immobilization of calmagite. Calmagite immobilization was performed at the –OH group, similar to the resveratrol immobilization previously reported [38]. performed at the –OH group, similar to the resveratrol immobilization previously reported [38]. The The immobilization method was originally developed for the immobilization of large molecules such as immobilization method was originally developed for the immobilization of large molecules such as proteins to improve the response of osteoblasts in the bone regeneration process when metal implants proteins to improve the response of osteoblasts in the bone regeneration process when metal implants are mounted (the membrane is at the interface between the metallic implant and the bone) [39]. are mounted (the membrane is at the interface between the metallic implant and the bone) [39]. In this study, for the modification of the membrane’s surface, asymmetric commercial cellulose In this study, for the modification of the membrane’s surface, asymmetric commercial cellulose acetate (CA) membranes with 0.45 µm porosity on the active surface were used, which were purchased acetate (CA) membranes with 0.45 μm porosity on the active surface were used, which were from Lab Box. For the modification of the CA surface with APTES (Sigma Aldrich, St. Louis, MO, purchased from Lab Box. For the modification of the CA surface with APTES (Sigma Aldrich, St. USA, analytic reagent) and GA (Merck, Kenilworth, NJ, USA, analytic reagent), our protocol follows Louis, MO, USA, analytic reagent) and GA (Merck, Kenilworth, NJ, USA, analytic reagent), our the steps previously reported in the literature for sericin and resveratrol immobilization [40,41]. protocol follows the steps previously reported in the literature for sericin and resveratrol For the partial hydrolysis of the membranes, the membranes were treated with a sodium hydroxide immobilization [40,41]. For the partial hydrolysis of the membranes, the membranes were treated solution, the APTES immobilization reaction being carried out in a weak basic catalysis using a 0.1 N with a sodium hydroxide solution, the APTES immobilization reaction being carried out in a weak sodium hydroxide solution. After the reaction, the membranes were washed with deionized water for basic catalysis using a 0.1 N sodium hydroxide solution. After the reaction, the membranes were removing unreacted APTES. For the immobilization of calmagite (Sigma Aldrich, analytical purity), washed with deionized water for removing unreacted APTES. For the immobilization of calmagite glutaraldehyde was used as a linker molecule between APTES and calmagite. After the reaction has (Sigma Aldrich, analytical purity), glutaraldehyde was used as a linker molecule between APTES and been completed, the membranes are washed and stored in cold, ultra-pure water to avoid the formation calmagite. After the reaction has been completed, the membranes are washed and stored in cold, and proliferation of microorganisms on the surface. ultra-pure water to avoid the formation and proliferation of microorganisms on the surface. The synthesized membranes were analyzed by infrared spectroscopy with Fourier transform The synthesized membranes were analyzed by infrared spectroscopy with Fourier transform FT-IR using a Bruker Vertex 70 (Bruker, Billerica, MA, USA) equipment with a diamond ATR device in FT-IR using a Bruker Vertex1 70 (Bruker, Billerica, MA, USA) equipment with a diamond ATR device the range of 600–4000 cm− . The spectra were recorded as an average of 32 successive measurements, in the range of 600–4000 cm−1. The spectra were recorded as an average of 32 successive measurements, eliminating bands of noise, atmospheric CO2, and atmospheric water vapor. Raman spectra were eliminating bands of noise, atmospheric CO2, and atmospheric water vapor. Raman spectra were registered on a DXR Raman Microscope from Thermo Scientific (Thermo Fischer, Waltham, MA, USA) registered on a DXR Raman Microscope from Thermo Scientific (Thermo Fischer, Waltham, MA, USA) using a 633 nm laser line and a number of 10 scans. The laser beam was focused with a 10X objective using a 633 nm laser line and a number of 10 scans. The laser beam was focused with a 10X objective of the Raman microscope. of the Raman microscope. Thermal analysis (TGA) curves were registered on Q500 TA Instruments equipment (TA Instruments, Thermal analysis (TGA) curves were registered on Q500 TA Instruments equipment (TA New Castle, DE, USA), using nitrogen atmosphere from room temperature to 700 ◦C and a heating rate Instruments, New Castle, DE, USA), using nitrogen atmosphere from room temperature to 700 °C of 10 ◦C/min. and a heating rate of 10 °C/min. The surface structures for CA and modified CA were studied by X-ray photoelectron spectroscopy The surface structures for CA and modified CA were studied by X-ray photoelectron (XPS) using a K-Alpha instrument from Thermo Scientific (Thermo Fischer, Waltham, MA, USA), with a spectroscopy (XPS) using a K-Alpha instrument from Thermo Scientific9 (Thermo Fischer, Waltham, monochromated Al Kα source (1486.6 eV), at a bass pressure of 2 10− mbar. The absolute calibration MA, USA), with a monochromated Al Kα source (1486.6 eV), at× a bass pressure of 2 × 10−9 mbar. The of the binding energy scale was performed using Au 4f7/2 (reference binding energy 83.96 eV), Ag 3d5/2 absolute calibration of the binding energy scale was performed using Au 4f7/2 (reference binding (reference binding energy 368.21 eV), Cu 2p3/2 (reference binding energy 932.62 eV). The pass energy energy 83,96 eV), Ag 3d5/2 (reference binding energy 368,21 eV), Cu 2p3/2 (reference binding energy for the survey spectra was 200 eV [41–43]. The fastest scan rate for survey spectra was 1 eV, and the 932,62 eV). The pass energy for the survey spectra was 200 eV [41–43]. The fastest scan rate for survey lowest for high-resolution spectra was 0.1 eV. spectra was 1 eV, and the lowest for high-resolution spectra was 0.1 eV.

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The morphological characterization of the pure CA and functionalized CA membranes was made using a Philips XL 30 ESEM TMP scanning electron microscopy (SEM) (FEI/Philips, Hillsboro, OR, USA). The flows were measured on a Sartorius module, using 47 mm filtering membranes and 250 mL of distilled water. The water flux (Wf ) expressed in L m 2 h 1 was calculated by the following formula (Equation (1)): · − · − V W f = (1) A t · where Wf is the water flux, V is the volume (L) of the feed solution, A is the area (m2) of the membrane, and t is the time (h). Gadolinium retention was studied using a gadolinium nitrate feed solution in ultrapure water (1 g/L), the analysis being performed on the filtered solutions using the ICP-MS technique, using a Bruker Instrument (Bruker, Billerica, MA, USA). Retention was calculated using the following formula:

C R = 100 F 100 (2) − CP ∗ where R is the Gd(III) retention (%), and C and C are the concentrations (g L 1) of Gd(III) in the feed F P · − and the permeate, respectively.

3. Results SEM microscopy (Figure2) revealed some important aspects regarding the morphological changes of synthesized membranes surface. Unlike cellulose acetate standard membrane, the APTES functionalized membrane presents a low surface porosity due to the cross-linking effect induced by covalent interaction between APTES and free hydroxyl groups at the membrane surface. In addition, in the case of an APTES functionalized membrane, on the porous surface, a series of agglomerations are observed due to the APTES polymerization in the presence of traces of water in the alcohol during the functionalization reaction. As a result of the calmagite immobilization, the surface fiber structure increases greatly, which is most probably due to the rejection of calmagite molecules (a large number of uninvolved electrons in nitrogen atoms and oxygen atoms from sulfonic acid groups). After gadolinium filtering, the surface morphology changes in the fibers approaching direction. This is due to the complexation reaction that takes place between the non-participating electrons from the nitrogen atoms and the hydroxyl groups in the adjacent molecules, which also results in a cross-linking and tightening of the surface. Moreover, on the porous surface, we can observe the presence of large crystals of gadolinium, which probably formed around growth sites that were born in the initial calmagite complexation. As can be seen from Figure2, the optical aspect of the synthesized membranes revealed that the basic principle from which the idea of this research started works. In the membrane functionalization phases of hydrolysis—the immobilization of APTES and glutaraldehyde—the surface of the membrane remains white. After calmagite immobilization, the surface of the membrane can be seen in a dark blue color. After the filtration of gadolinium nitrate, a radical change in color from dark blue to pink can be seen, which is visible to the naked eye; besides the separating properties, the membrane also exhibit self-indicating properties. Coatings 2020, 10, 468 5 of 15 Coatings 2020, 10, x FOR PEER REVIEW 5 of 15

Figure 2.2.Scanning Scanning Electron Electron Microscopy Microscopy of synthesized of synt membraneshesized (activemembranes surface and(active porous surface surface) and and opticalporous aspect surface) with evolutionand optical of color aspect changing with on evol theution surface of from color dark changing blue for functionalized on the surface membrane from withdark calmagiteblue for functionalized (CA/APTES/CAL) membrane before and with after calmagite filtration of (CA/APTES/CAL) Gd(III) to pink (CA /beforeAPTES /andCAL after/FIL) wherefiltration FIL—filtration. of Gd(III) APTES: to Aminopropylpink (CA/APTES/CAL/FIL) triethoxy silane, CA: where Cellulose FIL—filtration. acetate. APTES: Aminopropyl triethoxy silane, CA: Cellulose acetate. FT-IR spectra (Figure3) exhibit major changes, proving the functionalization of the used membranes.FT-IR spectra Thus, after(Figure the 3) partial exhibit hydrolysis major change of celluloses, proving acetate the membranes, functionalization in order of to the increase used themembranes. number of Thus, hydroxyl after the groups partial at thehydrolysis surface of cellulose the membrane, acetate the membranes, band specific in order for C to= Oincrease decreases the innumber intensity of hydroxyl because thegroups number at the of surface acetyl groups of the membrane, have been reducedthe band during specific the for hydrolysis C=O decreases process. in Inintensity addition, because functionalization the number withof acetyl APTES groups causes have a movement been reduced of all during of the bandsthe hydrolysis due to the process. presence In ofaddition, Si atoms. functionalization The immobilization with APTES of calmagite causes causeda move anment intensity of all of modification the bands due of to all the bands presence (due toof functionalSi atoms. The groups immobilization changes in of relationcalmagite to caused the total an intensity membrane modification surface: theof all presence bands (due of –Nto functional=N– and –SO H), as well as their slight displacement (for the same reasons). The 1750 cm 1 band, assigned to groups3 changes in relation to the total membrane surface: the presence of –N=N– and− –SO3H), as well as Ctheir=O groups,slight displacement has the highest (for intensity the same in reasons). cellulose The acetate, 1750 whichcm−1 band, is highly assigned diminished to C=O aftergroups, hydrolysis has the withhighest sodium intensity hydroxide. in cellulose The bandacetate, is still which present is highly in the diminished case of other after membranes, hydrolysis which with indicates sodium thathydroxide. the hydrolysis The band of the is membranestill present is notin the complete, case of leaving other fewermembranes, C=O groups which behind. indicates In addition,that the 1 anotherhydrolysis decrease of the inmembrane intensity is can not be complete, observed leav foring the fewer 1250 cm C=O− bandgroups assigned behind. to In the addition, etheric another groups C–O.decrease This in is intensity also explained can be observed by the partial for the hydrolysis 1250 cm−1 of band acetyl assigned groups to in the cellulose etheric acetate,groups C–O. but it This can alsois also be explained due to the by hydrolysis the partial of hydr the polymerolysis of acetyl chains, groups as indicated in cellulos by thee acetate, thermal but analysis. it can also However, be due theto the formation hydrolysis of of new the ether polymer connections chains, as also indicated occurs by by the immobilizing thermal analysis. APTES However, on the surfacethe formation of the membrane.of new ether The connections decreased also intensity occurs of by the immobili band canzing be APTES explained on bythe breaking surface of the the multiple membrane. bindings The ratherdecreased than intensity forming newof the bindings, band can as well be asexplained by steric by hindrance breaking generated the multiple by APTES’s bindings high rather molecular than 1 volume.forming Othernew bindings, changes as occur well in as the by 3200–3600 steric hindrance cm− region generated that areby APTES’s specific for high hydrogen molecular bonding volume. in Other changes occur in the 3200–3600 cm−1 region that are specific for hydrogen bonding in water

Coatings 2020, 10, 468 6 of 15 Coatings 2020, 10, x FOR PEER REVIEW 6 of 15 Coatings 2020, 10, x FOR PEER REVIEW 6 of 15 and hydroxyl groups. In the case of cellulose acetate after treatment with sodium hydroxide (to waterand hydroxyl and hydroxyl groups. groups. In the In case the caseof cellulose of cellulose acetate acetate after after treatment treatment with with sodium sodium hydroxide hydroxide (to increase the number of –OH groups), band intensity increases due to the increase in hydroxyl groups (toincrease increase the the number number of of –OH –OH groups), groups), band band intensity intensity increases increases due due to to the the increase increase in in hydroxyl hydroxyl groups groups number. Intensity is also very high for the CA/APTES/GA membrane because of the cross-linking number.number. IntensityIntensity isis alsoalso veryvery highhigh forfor thethe CACA//APTES/GAAPTES/GA membranemembrane because of the cross-linking effect on the membrane and high water retention [44–47]. eﬀeffectect on on the the membrane membrane and and high high water water retention retention [44[44–47].–47].

FigureFigureFigure 3. 3. 3. FourierFourier Fourier transform transform infraredinfrared spectroscopyspectroscopy spectroscopy (FT-IR) (FT-IR) spectra spectra ofof of synthesizedsynthesized synthesized CA, CA, CA/APTES, CACA/APTES,/APTES, CA/APTES/GA, and CA/APTES/CAL membranes with details for the range 600–20001 cm−1−1 (left) CACA/APTES/GA,/APTES/GA, and and CA CA/APTES/CAL/APTES/CAL membranes membranes with with details details for for the the range range 600–2000 600–2000 cm− cm(left (left)) and 1 −−11 500–4000andand 500–4000 500–4000 cm− ( cmright cm ). (right).(right). GA: Glutaraldehyde. GA:GA: Glutaraldehyde.Glutaraldehyde.

CalmagiteCalmagiteCalmagite immobilization immobilizationimmobilization onon thethethe functionalizedfunctionalizedfunctionalized membrane surface surface can can be be observed observedobserved in inin the thethe IR IRIR spectrumspectrumspectrum ofof of thethe the CACA/APTES/GA/CAL CA/APTES/GA/CAL/APTES/GA/CAL membrane membranemembrane where,where, besidesbesides thethethe cellulosecellulose acetateacetate corresponding corresponding −11 bands,bands,bands, oneone one can cancan notice noticenotice the thethe presencepresencepresence ofofof two twotwo additional additional bandsbands atatat 157915791579 andandand 152015201520 cmcmcm−−1 fromfromfrom the thethe CAL CALCAL structure.structure.structure. TheseThese These peakspeaks peaks correspond correspondcorrespond to toto the thethe stretching stretchingstretching vibrationvibration of ofof the thethe C C=OC=O=O andand NHNH22 amide amideamide bonds bondsbonds and andand areareare consistentconsistent consistent withwith with thethe the valuesvalues values reported reportedreported in inin the the literature literature [[48].48]. TheseTheseThese resultsresults results werewere were further furtherfurther supported supportedsupported byby theirtheir respectiverespective RamanRamanRaman spectraspectra (Figure(Figure 44). 4).). Several Several new new peakspeakspeaks attributedattributed attributed to toto the thethe presencepresencepresence ofofof newnewnew functionalfunctional groupsgroups ononon thethethe pristinepristine membrane membrane were werewere clearly clearlyclearly visible.visible.visible. TheThe The intensitiesintensities intensities of ofof the thethe pristine pristinepristine membrane membranemembrane peaks peaks were were higher higher after afterafter functionalization, functionalization,functionalization, resulting resulting resulting in aninin an enhanced an enhanced enhanced surface surface surface showed showed showed by byresonanceby resonanceresonance scattering scattering [40 ].[40].

FigureFigure 4. 4.Raman Raman spectra spectra of synthesized of synthesized CA, CA CA,/APTES, CA/A CAPTES,/APTES CA/APTES/GA,/GA, and CA/APTES and /CA/APTES/CALCAL membranes. Figure 4. Raman spectra of synthesized CA, CA/APTES, CA/APTES/GA, and CA/APTES/CAL membranes. membranes.

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More proof that the functionalization process takes place is shown in the XPS survey curves depicted in Figure5 5.. TheThe XPSXPS analysisanalysis waswas performedperformed inin orderorder toto observeobserve thethe modificationmodification ofof thethe chemical composition of of CA CA surface surface after after the the functi functionalizationonalization process. process. The The XPS XPS survey survey spectrum spectrum of ofCA CA exhibits exhibits two two peaks peaks assigned assigned to C to 1s C and 1s andO 1s O located 1s located at 284.72 at 284.72 eV and eV 531.34 and 531.34 eV, respectively. eV, respectively. The Thefunctionalization functionalization of the of CA the CAmembrane’s membrane’s surface surface leads leads to some to some changes changes both both in the in the atomic atomic percent percent of ofC 1s C 1sand and O O1s 1sand and also also to tothe the appearance appearance of of some some new new peaks peaks in in the the XPS XPS survey survey spectra. spectra. For For example, example, in the XPSXPS surveysurvey spectraspectra ofof CACA/APTES,/APTES, the the percent percent of of C C 1s 1s increases increases from from 45.16% 45.16% to to 64.96% 64.96% and and the the O 1sO 1s percent percent decreases decreases from from 44.8% 44.8% to to 20.49%. 20.49%. In In addition, addition, beside beside the the CA CA peak,peak, aa newnew peakpeak locatedlocated at 102.12 eV assigned to to Si Si 2p 2p (8.64%) (8.64%) could could be be observed observed and and this this indicates indicates that that the the linking linking of ofAPTES APTES to tothe the membrane membrane surface. surface. The The further further functionalization functionalization step step involves involves the the reaction reaction with with GA, which exhibits the following atomic percent: 62.69% 62.69% for for C C 1s, 28.64% for O 1s, 5.34% for N 1s, and 3.7% Si 2p. The immobilization of calmagite is demonstrated by an increase of O 1s percent to 25.04% and a decrease of C 1s atomic percent to 63.27%, and this is due to the orientation orientation of the calmagite calmagite molecules at the surface, which can be recorded by XPS equipm equipment.ent. In addition, the N 1s 1s atomic atomic percent percent decrease to 3.24% is due to the the shield shield of of the the steric steric planarit planarityy induced induced by the the polarity polarity of of the the aromatic aromatic naphthalene naphthalene rings. The XPS results prove that the modificationmodification process waswas achieved.achieved. In order to to prove prove the the functionalization functionalization of of CA, CA, the the C C 1s 1s (Figure (Figure 6),6), O O 1s 1s (Figure (Figure 7),7), N N 1s, 1s, and and Si Si2p 2p(Figure (Figure 8) 8high-resolution) high-resolution spectra spectra were were recorded recorded fo forr the the samples. samples. According According to to Figure 66,, afterafter the the APTES functionalization, a peak at 281 eV appears,appears, which isis attributedattributed toto C-Si.C-Si. In addition, in the CACA/APTES/APTES sample, the peak at 288 eV is attributedattributed to a C–N bond. The intensity of the O–C–O peak (at a bindingbinding energy of 289 eV) is increasedincreased in the case of CACA/APTES/APTES sample due to the increase of these bonds after the immobilization of APTES. The next step of of functionalization functionalization (reaction (reaction with with GA) GA) was demonstrated by thethe presencepresence ofof aa newnew peakpeak atat 288288 eV,eV, whichwhich isis assignedassigned toto CC=N=N[ [49–51].49–51]. In addition, O 1s spectrum (Figure 77)) cancan bebe deconvoluteddeconvoluted toto threethree peakspeaks centeredcentered atat 530,530, 531,531, and 532 assigned to OO=C,=C, O–C, O–C, and and O–H O–H for for the the CA CA sample, sample, respectively. respectively. In In the the case case of of CA/APTES, CA/APTES, the appearance of an O–Si specificspecific peak at 529 eVeV suggestssuggests the immobilizationimmobilization at the surface of the membrane of APTES. After the immobilizationimmobilization of calmagite, OO=S=S at at 534 534 eV, eV, this bond being present only in the structure of the used indicatorindicator [[49–51].49–51].

Figure 5. X-rayX-ray photoelectron photoelectron spectroscopy spectroscopy (XPS) survey survey analysis analysis of of CA, CA, CA/APTES, CA/APTES, CA/APTES/GA, CA/APTES/GA, and CACA/APTES/CAL/APTES/CAL membranesmembranes withwith markedmarked interestinterest peakspeaks forfor CC 1s,1s, OO 1s,1s, SiSi 2p,2p, andand NN 1s.1s.

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FigureFigure 6. 6.The The ﬁttedfitted CC 1s spectra for synthesized synthesized samples. samples.

FigureFigure 7. 7.The The ﬁttedfitted OO 1s spectra for synthesized synthesized samples. samples.

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InIn Figure Figure8 ,8, deconvoluted deconvoluted spectra spectra for for N N 1s 1s show show thethe presencepresence ofof N–CN–C bondsbonds inin CACA/APTES,/APTES, CACA/APTES/GA,/APTES/GA, respectively respectively CACA/APTES/GA/CAL,/APTES/GA/CAL, atat 399399 eVeV andand NN=C=C bonds in in CA/APTES/GA CA/APTES/GA and and CACA/APTES/GA/CAL/APTES/GA/CAL samples, samples, at 402at 402 eV [eV52 ].[52]. Related Rela toted deconvoluted to deconvoluted spectra spectra for Si 2p,for weSi 2p, can we observe can Si–Oobserve (at 103 Si–O eV) (at and 103 Si–C eV) bondsand Si–C (at 102bonds eV) (at from 102 APTES eV) from used APTES as linkers used andas linkers present and in CApresent/APTES, in CACA/APTES,/APTES/GA, CA/APTES/GA, and CA/APTES and/GA CA/APTES/GA/CAL/CAL samples [53]. samples [53].

FigureFigure 8. 8.The The ﬁttedfitted NN 1s1s andand SiSi 2p2p spectraspectra for synthesized samples.

ForFor industrial industrial applications, applications, thethe thermalthermal stability of the material is is also also an an important important parameter. parameter. ConsideringConsidering this, this, the the thermal thermal stabilitystability andand thethe degradationdegradation profile profile of the pure CA CA and and modified modified CA CA membranesmembranes were were assessed assessed by by thermal thermal analysisanalysis (Figure(Figure9 9).). TheThe purepure CACA membranesmembranes showshow the typical decompositiondecomposition profile profile with with two two degradation degradation stepssteps attributedattributed to the evaporation of the the solvent solvent and and to to thethe degradation degradation of of the the CA CA polymer polymer chain chain [ [30].30]. InIn thethe case of the modified modified CA CA membranes, membranes, the the thermal thermal degradationdegradation profiles profiles illustrateillustrate aa similarsimilar trendtrend asas forfor thethe purepure CACA membrane, which means means that that the the synthesizedsynthesized materials materials are are thermally thermally stable. stable. FigureFigure9 9 show show anan increaseincrease ofof thethe residualresidual massmass in the case of the CA/APTES membrane, which could be attributed to the SiO2 moieties from APTES structure

Coatings 2020, 10, 468 10 of 15 Coatings 2020, 10, x FOR PEER REVIEW 10 of 15 ofand the might CA/APTES prove that membrane, the immobilization which could of be the attributed silane-coupling to the SiO agent2 moieties on the from CA surface APTES was structure achieved and might[54]. prove that the immobilization of the silane-coupling agent on the CA surface was achieved [54]. TheThe lowestlowest residualresidual mass mass (2%) (2%) was was registered registered in in the the case case of of the the CA CA/APTES/GA/CAL/APTES/GA/CAL and and is dueis due to theto the thermal thermal degradation degradation of theof the organic organic moistures moisture attacheds attached on theon CAthe CA membrane’s membrane’s surface. surface. There There are manyare many studies studies that demonstratethat demonstrate that thethat degradation the degradation temperature temperature depends depends on the functionalon the functional groups graftedgroups ongrafted the cellulose on the cellulose acetate surfaceacetate [surface55]. [55].

FigureFigure 9.9. TGA ( A)–)– Differential Diﬀerential Thermal Analysis DTA ( B) spectra of of CA, CA, CA/A CA/APTES,PTES, CA/APTES/GA, CA/APTES/GA, andand CACA/APTES/CAL/APTES/CAL membranes.membranes.

The temperature at which the mass loss is 5% (Td ) was recorded in order to show the thermal The temperature at which the mass loss is 5% (Td5%5%) was recorded in order to show the thermal stabilitystability of of the the as-synthesized as-synthesized membrane. membrane. The dataThe reporteddata reported in the Tablein the1 indicate Table 1 a indicate significant a increasesignificant of theincrease thermal of stabilitythe thermal of the stability modified of CA the membrane modified after CA eachmembrane modification after step.each Themodification enhancement step. of The the thermalenhancement stability of of the the CAthermal/APTES stability membrane of the is assigned CA/APTES to the membrane chemical interaction is assigned between to the the hydroxylchemical groupsinteraction from between CA and the APTES. hydroxyl Moreover, groups this from increase CA an ofd theAPTES. thermal Moreover, stability this could increase be a consequence of the thermal of thestability slight could cross-linking be a consequence effect induced of afterthe immobilizationslight cross-linking of APTES effect onto induced the CA after surface immobilization supported also of byAPTES SEM imagesonto the [56 CA]. In surface the case supported of CA/APTES also/ GA,by SEM the enhancementimages [56]. ofIn thethe thermal case of stabilityCA/APTES/GA, corresponds the toenhancement the strong interlinks of the obtainedthermal instability the CA /APTEScorresponds/GA membrane, to the strong as has beeninterlinks noticed obtained in our previous in the studiesCA/APTES/GA [41]. The membrane, highest thermal as has stability been noticed was registered in our inprevious the case studies of the membrane[41]. The highest with calmagite thermal immobilizedstability was on registered the CA surface. in the case This of could the membrane be mainly assigned with calmagite to the highly immobilized thermal on stable the naphthaleneCA surface. ringsThis could and strong be mainly interlinks assigned built onto the cellulosehighly thermal acetate stable surface. naphthalene Similar behavior rings and was strong also observed interlinks by Abdel-Nabybuilt on the etcellulose al. when acetate they succeededsurface. Similar in chemically behavior modifying was also cellulose observed acetate by Abdel-Naby with diallylamine et al. when [57]. Thethey Derivative succeeded Thermogravimetry in chemically modifying DTG results cellulose show noacetate significant with changes.diallylamine [57]. The Derivative Thermogravimetry DTG results show no significant changes. Table 1. The wt (%), Td5% and the DTG values for the CA membrane before and after the modiﬁcation process. Three diﬀerent samples were analyzed for calculating the standard deviation. Table 1. The wt (%), Td5% and the DTG values for the CA membrane before and after the modification

process. Three differentSample samples Name were analyzed wt for (%) calculating Td5% (the◦C) standard DTG deviation. (◦C) CA 89 1 204 3 433 1 Sample Name wt ±(%) Td5%± (°C) DTG± (°C) CA/APTES 86 1 281 3 431 1 CA 89 ±± 1 204± ± 3 433± ± 1 CA/APTES/GA 88 1 352 3 430 1 ±± ±± ± ± CA/CA/APTESAPTES/GA/CAL 9886 11 372 281 3 3 433 431 1 1 CA/APTES/GA 88 ±± 1 352± ± 3 430± ± 1 CA/APTES/GA/CAL 98 ± 1 372 ± 3 433 ± 1 The thermal behavior of the membrane is not radically modified, indicating the stability of the synthesized during the transformation process. The small changes can be explained by The thermal behavior of the membrane is not radically modified, indicating the stability of the multiple chemical modifications of the synthesized membranes, which have an effect not only on the synthesized during the transformation process. The small changes can be explained by multiple immobilization of calmagite, but also on the partial hydrolysis of the polymer that constitutes the chemical modifications of the synthesized membranes, which have an effect not only on the material. The highest thermal stability in the case of CA/APTES/GA/CAL is attributed to the presence immobilization of calmagite, but also on the partial hydrolysis of the polymer that constitutes the of covalent bonds in the entire structure of the material. material. The highest thermal stability in the case of CA/APTES/GA/CAL is attributed to the presence Flows through synthesized membranes (Figure 10) were also studied, and membrane retention of covalent bonds in the entire structure of the material. capacity was evaluated by five successive passes of 500 mL of 1 g/L aqueous solution of gadolinium Flows through synthesized membranes (Figure 10) were also studied, and membrane retention capacity was evaluated by five successive passes of 500 mL of 1 g/L aqueous solution of gadolinium

Coatings 2020, 10, 468 11 of 15 Coatings 2020, 10, x FOR PEER REVIEW 11 of 15

Coatings 2020, 10, x FOR PEER REVIEW 2 11 of 15 nitrate.nitrate. InIn thethe case case of of neat neat cellulose cellulose acetate acetate membrane, membrane, initial initial water water flow flow was was about about 160 L 160/m h,L/m but2h, after but 2 functionalizationnitrate.after functionalization In the case with of neat calmagite, with cellulose calmagite, the flowacetate the increased flowmembrane, increased to approximately initial to approximately water 600 flow L/ mwas h.600 about This L/m can 1602h. be ThisL/m explained 2canh, but be byafterexplained the functionalization hydrophilicity by the hydrophilicity of with calmagite calmagite, of molecule, calmagite the flow which molecule, increased also induces whichto approximately a also higher induces hydrophilic 600 a higherL/m character2h. Thishydrophilic can to thebe membraneexplainedcharacter toby material. the the membrane hydrophilicity After the material. first of recirculationcalmagite After the molecule,firs cycle,t recirculation the which decrease also cycle, of induces flow the candecrease a behigher explained of hydrophilicflow bycan the be hydrodynamiccharacterexplained to by the the stabilization membrane hydrodynamic ofmaterial. polymer stabilization After layers the of inside firspolymert recirculation the membrane layers inside cycle, structure. the the membrane decrease structure.of flow can be explainedTotalTotal by retentionretention the hydrodynamic (Figure(Figure 1111),), stabilization assessedassessed byby of ICP-MS,ICP-MS, polymer waswas layers 86%86% inside forfor thethe the functionalizedfunctionalizedmembrane structure. membrane, comparedcomparedTotal toretentionto 64%64% for for the(Figure the cellulose cellulose 11), assessed acetate acetate standard standardby ICP-MS, membrane. membrane. was 86% This This for di ffthedifferenceerence functionalized can can be explainedbe explained membrane, by theby complexationcomparedthe complexation to 64% reaction forreaction the of calmagite cellulose of calmagite indicatoracetate indicator standard immobilized immobilized membrane. at the surface Thisat the difference ofsurface the membrane. of can the be membrane. explained In addition, byIn consequentlytheaddition, complexation consequently with reaction higher with values of higher calmagite of retention,values indicator of theretention, changeimmobilized the in surfacechange at the colorin surface occurs, colorof visuallythe occurs, membrane. indicating visually In theaddition,indicating retention consequently the of retention Gd(III). withFurther of Gd(III).higher studies valuesFurther related of studretention, toies this related new the change generation to this in new surface of membranegeneration color occurs, materialsof membrane visually will envisageindicatingmaterials and will the validate envisageretention calibration and of validateGd(III). methods Furthercalibration for stud visually methodsies related indicating for visuallyto this the concentration newindicating generation the of concentration retainedof membrane cation of dependingmaterialsretained cation will on envisage the depending color and of the on validate surface.the color calibration of the surface. methods for visually indicating the concentration of retained cation depending on the color of the surface.

Figure 10. Water flows through CA and CA/APTES/GA/CAL membranes. Five different Figure 10. Water ﬂows through CA and CA/APTES/GA/CAL membranes. Five diﬀerent measurements measurements for calculating standard error. forFigure calculating 10. Water standard flows error. through CA and CA/APTES/GA/CAL membranes. Five different measurements for calculating standard error.

Figure 11. Gd(III) retention (%) related to feed solution through CA and CA/APTES/GA/CAL Figuremembranes. 11. Gd(III) Five different retention measuremen (%) relatedts forto calculatingfeed solution standard through error. CA and CA/APTES/GA/CAL Figure 11. Gd(III) retention (%) related to feed solution through CA and CA/APTES/GA/CAL membranes. Five different measurements for calculating standard error. 4. Conclusionsmembranes. Five diﬀerent measurements for calculating standard error. 4. Conclusions A new, revolutionary, and easy method for evaluating the separation process through a membraneA new,new, was revolutionary, revolutionary, presented, and based and easy oneasy method the methodfact for that evaluating foreach evaluating separated the separation chemicalthe separation process compound throughprocess will a membranethroughreact with a wasmembranean immobilized presented, was based presented, indicator on the at based fact the that surface on each the offact separated the that membrane, ea chemicalch separated which compound chemical will change will compound react the with surface anwill immobilized color react of with the indicatoranmembrane. immobilized at Calmagite the indicator surface was of at chosen thethe membrane,surface for its of dual the which membrane,role as will a molecule change which the thatwill surface willchange complex color the surface ofand the retain color membrane. Gd(III) of the membrane.and also as anCalmagite indicator was for chosenGd(III). for At itsthe dual contact role with as a moleculethe membrane that will surface, complex calmagite and retain will actively Gd(III) andcomplex also asand an indicatorretain Gd(III), for Gd(III). and Atit willthe contactchange with the thecolor membrane of the membrane surface, calmagite surface will during actively the complex and retain Gd(III), and it will change the color of the membrane surface during the

Coatings 2020, 10, 468 12 of 15

Calmagite was chosen for its dual role as a molecule that will complex and retain Gd(III) and also as an indicator for Gd(III). At the contact with the membrane surface, calmagite will actively complex and retain Gd(III), and it will change the color of the membrane surface during the complexation process, showing that the separation occurred. This was achieved by the functionalization of a cellulose acetate polymeric membrane in three steps, with amino propyl triethoxysilane, glutaraldehid, e and calmagite. The synthesized membranes showed good thermal stability (due to the multiple functionalization steps (which induces a cross-linking eﬀect to the entire material). In addition, synthesized membranes were characterized by FT-IR, Raman, and XPS in order to prove the functionalization reactions. Moreover, in the separation of the Gd(III) process, having gadolinium nitrate as a source that was used as a moderator in nuclear reactors, the membrane changed its color from blue to pink, and ICP-MS analyses showed a separation process eﬃciency of 86% relative to the initial feed solution after 5 cycles of recirculation.

Author Contributions: Conceptualisation, S.I.V. and Y.Y.; methodology, O.S.S., A.M.P., F.M., and S.I.V.; validation, A.M.P. and F.M.; formal analysis, A.M.P. and F.M.; investigation, A.M.P. and F.M.; resources, S.I.V.; data curation, S.I.V. and A.M.P.; writing—original draft preparation, O.S.S. and S.I.V.; project administration, S.I.V.; funding acquisition, S.I.V. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by UEFISCDI, grant No. PN-III-P1-1.1-TE-2016-0542. Acknowledgments: The authors gratefully acknowledge the financial support through project TE 20/2018, PN-III-P1-1.1-TE-2016-0542, a new generation of membrane systems with visual control of the separation process efficiency based on the modification of membrane color surface, financed by UEFISCDI Romania. Conflicts of Interest: The authors declare no conflict of interest.

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