Mos2-Cysteine Nanofiltration Membrane for Lead Removal

chemengineering

Article MoS2-Cysteine Nanoﬁltration Membrane for Lead Removal

Jaewon Jang 1,* , Sang-Soo Chee 2,* , Yesol Kang 1 and Suhun Kim 1

1 School of Earth Sciences and Environmental Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju 61005, Korea; [email protected] (Y.K.); [email protected] (S.K.) 2 Nanomaterials and Nanotechnology Center, Korea Institute of Ceramic Engineering and Technology (KICET), Jinju-si 52851, Korea * Correspondence: [email protected] (J.J.); [email protected] (S.-S.C.); Tel.: +82-62-715-2477 (J.J.); +82-55-792-2684 (S.-S.C.)

Abstract: To overcome the limitations of polymers, such as the trade-off relationship between water permeance and solute rejection, as well as the difﬁculty of functionalization, research on nanomaterials is being actively conducted. One of the representative nanomaterials is graphene, which has a two-dimensional shape and chemical tunability. Graphene is usually used in the form of graphene oxide in the water treatment ﬁeld because it has advantages such as high water permeance and functionality on its surface. However, there is a problem in that it lacks physical stability under

water-contacted conditions due to the high hydrophilicity. To overcome this problem, MoS2, which has a similar shape to graphene and hydrophobicity, can be a new option. In this study, bulk MoS2 was dispersed in a mixed solvent of acetone/isopropyl alcohol, and MoS2 nanosheet was obtained by applying sonic energy to exfoliate. In addition, Cysteine was functionalized in MoS2 with a mild reaction. When the nanoﬁltration (NF) performance of the membrane was compared under various

conditions, the composite membrane incorporated by Cysteine 10 wt % (vs. MoS2) showed the best NF performances.

Citation: Jang, J.; Chee, S.-S.; Kang, Y.; Keywords: molybdenum disulﬁde; cysteine; lead; heavy metal; wastewater; nanoﬁltration

Kim, S. MoS2-Cysteine Nanoﬁltration Membrane for Lead Removal. ChemEngineering 2021, 5, 41. https://doi.org/10.3390/ 1. Introduction chemengineering5030041 The importance of technology to purify polluted water is growing because many water intake sources for obtaining clean water are being polluted due to the continuous popu- Academic Editor: Alírio E. Rodrigues lation growth and industrial development in various countries [1,2]. Among pollutants, heavy metals such as lead, cadmium, mercury, and chromium can accumulate in the human Received: 29 June 2021 body and cause serious problems such as Minamata disease, Itai-Itai disease, and cancer [3]. Accepted: 27 July 2021 Published: 1 August 2021 In addition, many industrial wastewaters containing heavy metals must be discharged into nature after removing those heavy metals. Most heavy metals harm the human body

Publisher’s Note: MDPI stays neutral or organic life, but lead is especially damaging. Lead is emitted from various industries with regard to jurisdictional claims in and causes a problem even if a small amount, as low as 0.01–0.5 ppm, is accumulated in published maps and institutional affil- the body [4,5]. According to US EPA (the United States Environmental Protection Agency) iations. standards, the dissolved lead concentration in wastewater must not exceed 5.0 ppm [6]. To remove heavy metal ions contained in water, there are various methods, such as forward osmosis (FO), reverse osmosis (RO), nanofiltration (NF), chemical precipitation, ion exchange, and adsorption [7–9]. Among the various methods, precipitation, agglomeration, ion exchange, and adsorption have been widely used to remove ions [10–14]. However, Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. the process that requires a chemical reaction has the disadvantages of having high costs This article is an open access article and a long process time. Moreover, it needs the post-treatment process. Therefore, there distributed under the terms and is a need to more actively utilize inexpensive, efficient, and contaminant-free processes conditions of the Creative Commons such as RO, NF, UF, MF, etc. Among them, the NF process can effectively reject inorganic Attribution (CC BY) license (https:// ions by the size exclusion principle and the electrostatic interaction mechanism (Donnan creativecommons.org/licenses/by/ exclusion) between the membrane and ions, and can improve the water flux by controlling 4.0/). the process conditions so that it consumes less energy than the RO process [15].

ChemEngineering 2021, 5, 41. https://doi.org/10.3390/chemengineering5030041 https://www.mdpi.com/journal/chemengineering ChemEngineering 2021, 5, 41 2 of 9

There are many studies that removed lead ions or other heavy metal ions that have been conducted using NF membranes. When a solution containing Pb2+ and Co2+ ions was passed through a commercial NF membrane, AFC30, the rejections were 84% and 64%, respectively, at pH 5 [16], and the rejections of AFC40 against Co(NO3)2 and Pb(NO3)2 were 76% and 60% at pH 6, respectively [17,18]. Another commercial membrane, NF 270, showed a rejection of 60–70% for Pb2+ and Co2+ at pH 5 [19]. On the other hand, NF90, an aromatic polyamide Thin-film composite membrane produced by Dow-FilmTec, showed a high rejection of 91–94% for Pb2+ at pH 5–6 because of the size exclusion effect which resulted from the small pore size [20]. The NF90 membrane is a tight NF membrane with a molecular weight cut-off (MWCO) of about 90 Da, and has a maximum operating pressure, flow rate, and temperature of 4137 kPa, 3.6 m3 h−1, and 45 ◦C, respectively. It is also very robust with a usable pH range of 1 to 12 [20]. In addition, there are various tailor- made membrane research cases. T.S. Chung et al. reported the hollow fiber membrane composed of polybenzimidazole/polyethersulfone dual-layer for the rejection of Pb2+ (93%, at pH 2.2) and Cd2+ (95%, pH 5) [21]. Moreover, they reported the NF membrane with a pentablock copolymer selective layer. It exhibited excellent rejection efficiency (>98%) for Pb2+ and Cd2+ at a pH range of 5.3–5.8 [22]. Moreover, PEI-based membranes showed better rejection for heavy metals than PIP-based membranes, but the PEI-based membranes had a lower permeate of 1 LMH bar−1 than the PIP-based membranes (6–10 LMH bar−1). The above cases have their own merits, but it is difficult to change membrane properties by engineering polymers without applying hetero material. To overcome the trade-off relationship between water permeance and solute rejection, therefore, the focus of this study is the fabrication of 2D nanomaterial-based membrane using MoS2 to maintain a certain level of water permeance while gaining a high level of lead ion rejection. The exfoliated MoS2 nanosheet has several advantages when used in water treatment membranes: (1) MoS2 nanosheets are not detached easily when applied to the water treatment process because it does not have a hydrophilic functional group, (2) free from the phenomenon of a decrease in permeance due to the attraction between hydrophilic functional groups and water molecules, (3) it receives less resistance from the movement of water molecules because the surface of the MoS2 nanosheet is relatively smooth compared to graphene oxide (GO), and (4) the degree of deformation or compression under pressure is less than GO because the out-of-plane of the MoS2 nanosheet is rigid [23–27]. Therefore, if these features are well utilized, NF membranes with high solute rejection with good water permeance can be manufactured. This performance is meaningful because it cannot be realized with conventional commercial membranes. This study focused on improving the lead ion rejection of the membrane to obtain lead- free water after a single NF process. Instead of controlling the pore size of the membrane, a method of improving the rejection performance for lead ions via electrostatic interaction by controlling the surface characteristics is adopted. Considering the trade-off relationship between water permeance and solute rejection, the appropriate membrane thickness and operating pressure were selected. In addition, the composition of the membrane showing the optimal permeance and rejection values was studied.

2. Materials and Methods 2.1. Materials

Bulk MoS2 powder (99%, metal basis), starting material, was purchased from Alfa Aesar (Ward Hill, MA, USA). Acetone (99.5%) and isopropyl alcohol (IPA) (99.5%) were obtained from DUKSAN (Danwon-gu, Ansan, Republic of Korea). L-Cysteine (97%) and N,N-Dimethylformamide (DMF, ACS reagent, ≥99.8%) were acquired from Sigma-Aldrich. Polytetraﬂuoroethylene (PTFE, 0.1 microns, 47 mm, STERLITECH, Kent, WA, USA) was used as the ﬁltering membrane for rinsing MoS2 nanosheets. In the NF test, lead (II) nitrate (ACS reagent, ≥99.0%, Sigma-Aldrich, Saint Louis, MO, USA) and polyethersulfone (PES, 1000 Da, 47 mm, NP010, Microdyn Nadir, Wiesbaden, Germany) were used as a feed solute and supporting membrane, respectively. ChemEngineering 2021, 5, x FOR PEER REVIEW 3 of 10

was used as the filtering membrane for rinsing MoS2 nanosheets. In the NF test, lead (II) nitrate (ACS reagent, ≥99.0%, Sigma-Aldrich, Saint Louis, MO, USA) and polyethersulfone (PES, 1000 Da, 47 mm, NP010, Microdyn Nadir, Wiesbaden, Germany) were used as ChemEngineering 2021, 5, 41 3 of 9 a feed solute and supporting membrane, respectively.

2.2. Synthesis of Cysteine-Functionalized MoS2 Nanosheets 2.2. Synthesis of Cysteine-Functionalized MoS Nanosheets Bulk MoS2 is dispersed in a mixed 2solvent to exfoliate. Bulk MoS2 powder (200 mg) wasBulk added MoS to2 theis dispersed prepared in acetone-IPA a mixed solvent (8:2) to mi exfoliate.xture solvent Bulk MoS and2 sonicatedpowder (200 in a mg) bath for was10 h. added After to precipitating the prepared acetone-IPAthe powders (8:2) that mixture have not solvent been and exfoliated sonicated for in 1 a bathday, forthe 10 superna- h. Aftertant precipitatingis collected and the powderscentrifuged that (3000 have notRPM, been 30 exfoliated min). After for 1collecting day, thesupernatant the supernatant, iscarry collected out vacuum and centrifuged filtration (3000 through RPM, a 30PTFE min). membrane After collecting filter and the supernatant,then wash with carry ethanol out vacuum ﬁltration through a PTFE membrane ﬁlter and then wash with ethanol and and DI water. MoS2 nanosheets were vacuum dried at 50 °C for 1 day and then collected. DI water. MoS nanosheets were vacuum dried at 50 ◦C for 1 day and then collected. 2 −1 The MoS2 nanosheet was redispersed in DMF to have a concentration of 100 ug −mL1 (50 The MoS2 nanosheet was redispersed in DMF to have a concentration of 100 ug mL mL), and Cysteine was added to become 1, 2, 5, 10, 20, 40 wt % based on the MoS2 mass. (50 mL), and Cysteine was added to become 1, 2, 5, 10, 20, 40 wt % based on the MoS2 This MoS2-Cysteine mixture was reacted by oil-bath at 70 °C◦ for 30 min to obtain Cys- mass. This MoS2-Cysteine mixture was reacted by oil-bath at 70 C for 30 min to obtain 2 2 Cys-functionalizedfunctionalized MoS MoS (MoS2 (MoS-Cys)2-Cys) nanosheets nanosheets (Scheme (Scheme 1).1). The The synthesized synthesized compositescomposites were werenamed named MC0, MC0, MC1, MC1, MC2, MC2, MC5, MC5, MC10, MC10, MC20, MC20, and and MC40 MC40 according according to tothe the Cysteine Cysteine content content0–40 wt 0–40 %. wt %.

SchemeScheme 1. 1.A A procedure procedure of of the the MoS MoS2 exfoliation2 exfoliation and and Cys-functionalized Cys-functionalized MoS2 MoSsynthesis.2 synthesis. 2.3. Fabrication of the NF Membrane 2.3. Fabrication of the NF Membrane NF membranes for the test were fabricated by the facile vacuum filtration process NF membranes for the test were fabricated by the facile vacuum filtration process (Figure S1). Pristine MoS2 or MoS2-Cys dispersions were filtered on the PES flat sheet membrane(Figure S1). (47 Pristine mm in diameter),MoS2 or MoS and2-Cys the membranes dispersions were were dried filter ated 50 on◦C the in a PES vacuum flat sheet state.membrane The volume (47 mm of in dispersions diameter), was and determined the memb accordingranes were to dried the target at 50 thickness °C in a vacuum of −1 thestate. final The product. volume Based of dispersions on the pristine was determin MoS2 dispersion,ed according if 1 mLto the of thetarget 100 thicknessµg mL of the solutionfinal product. is filtered, Based an activeon the layer pristine with MoS a thickness2 dispersion, of about if 1 100 mL nm of wasthe 100 obtained. μg mL If−1 the solution volumeis filtered, of the an solution active layer is increased with a bythickness 1 mL, the of finalabout thickness 100 nm increaseswas obtained. by about If the 100 volume nm. of Thicknessthe solution control is increased was possible by 1 undermL, the the final same thickness conditions increases with pristine by about MoS 1002 nanosheet nm. Thickness even when Cysteine is functionalized, and the concentration of Cysteine has a little effect control was possible under the same conditions with pristine MoS2 nanosheet even when onCysteine the thickness is functionalized, change. and the concentration of Cysteine has a little effect on the thick- 2.4.ness Characterizations change.

The MoS2 nanosheet was analyzed by X-ray diffraction (XRD, D/Max–2500, Rigaku, Akishima-shi, Tokyo, Japan) and Raman spectroscopy (LabRAM HR, Horiba-Jobin-Yvon Co., Bensheim, Germany) with a LASER of 514 nm using the dried membrane form, and UV-Vis spectrometer (S-3100, SCINCO Co., Ltd., Gangnam-gu, Seoul, Korea) was carried −1 out using the MoS2 dispersion in acetone-IPA mixture solvent (>100 µg mL ) to conﬁrm the exfoliation state. Characterizations of the MoS2-Cys nanosheet were carried out by ChemEngineering 2021, 5, 41 4 of 9

Fourier transform infrared spectroscopy (FTIR, Nicolet iS10 FTIR Spectrometer, Thermo Fisher Scientific, Waltham, MA, USA) with preparing pellet using MoS2-Cys and KBr at a 1:100 wt % ratio, and X-ray photoelectron spectroscopy (XPS, Multilab 2000, Thermo Fisher Scientific, Waltham, MA, USA) was carried out using the dried membrane form. In addition, analysis of the membranes was performed by a Zeta potential instrument (ELSZ-2, Otsuka Electronics Co., Ltd., Hirakata-shi, Osaka, Japan), field-emission scanning electron microscopy (FESEM, JSM-7500F, JEOL, Akishima, Tokyo, Japan), and atomic force microscopy (AFM, XE-100, Park Systems, Gwanggyo-ro, Suwon, Korea) using the membrane that MoS2-based material deposited onto the PES support.

2.5. NF Performances An NF test was conducted using a lab-scale dead-end cell (HP4750, high pressure stirred cell, STERLITECH, Kent, WA, USA) under the pressure of 1–9 bar. The membranes were used after cutting to a circular shape to ﬁt into the dead-end cell kit holder. The effective membrane area was 14.6 cm2. Before all tests, pre-compaction was performed with DI water at 5 bar for 1 h. In the performance test, a Pb(NO3)2 aqueous solution of 1000 ppm was used as a feed solution after adjusting pH until 6 by adding 0.1 M NaOH aqueous solution. Water permeance (WP) and the rejection of Pb2+ ions were calculated using Equations (1) and (2) below, respectively:

Q WP = (1) A∆P

where Q is the ﬂow rate (L h−1) of permeate, A is the effective surface area (m2), ∆p is the transmembrane pressure (bar). ! Cp R = 1 − × 100% (2) Cf

where Cp and Cf are the concentrations of lead in permeate and feed, respectively.

3. Results 3.1. Synthesis of MoS2 Nanosheet

A physical property analysis was performed to check whether MoS2 was well exfoliated or not. In the XRD pattern, it can be seen that the bulk MoS2 pattern is in good agreement with the reference (JCPDS card 87-2416). After MoS2 exfoliation, only the peak related to the (002) plane remained, and other peaks disappeared so that it was conﬁrmed that the MoS2 nanosheets were obtained (Figure1a) and agree with the literatures [ 28,29]. Moreover, the same type of UV-Vis spectrum and absorption peaks as those previously reported in MoS2 nanosheet-related literature were observed after exfoliation, while no absorption peak was observed in the spectrum of bulk MoS2 (Figure1b). Peaks 1 and 2 refer to the electron transition between the higher density of state regions, and Peaks 3 and 4 refer to direct transitions of electrons from the valence band to the conduction band [30,31]. In addi- 1 −1 tion, in the Raman analysis results, E2g and A1g peaks were blue-shifted (∆p = 26.69 cm ) −1 and red-shifted (∆p = 24.90 cm ), respectively (Figure S2), after MoS2 exfoliation. As the 1 in-plane vibration between Mo and S atoms is strengthened, E2g is blue-shifted, and van der Waals interaction in the out-of-plane direction of S atoms is reduced, resulting in the redshift of A1g [32]. When these results were considered together, it was conﬁrmed that MoS2 was well exfoliated in the form of nanosheets. ChemEngineering 2021, 5, x FOR PEER REVIEW 5 of 10 ChemEngineering 2021, 5, x FOR PEER REVIEW 5 of 10

blue-shifted, and van der Waals interaction in the out-of-plane direction of S atoms is re- blue-shifted, and van der Waals interaction in the out-of-plane direction of S atoms is re- ChemEngineering 2021, 5, 41 duced, resulting in the redshift of 𝐴 [32]. When these results were considered together,5 of 9 duced, resulting in the redshift of 𝐴 [32]. When these results were considered together, it was confirmed that MoS2 was well exfoliated in the form of nanosheets. it was confirmed that MoS2 was well exfoliated in the form of nanosheets. (a) (b) (a)Bulk MoS2 (b) 1 Bulk MoS2 BulkExfoliated MoS2 MoS2 1 2 Bulk MoS2 Exfoliated MoS2 ExfoliatedMoS2 JCPDS MoS 87-24162 2 Exfoliated MoS2 MoS2 JCPDS 87-2416

3 3 4 4 Intensity (a. u.) (a. Intensity Absorption (a. u.) Intensity (a. u.) (a. Intensity Absorption (a. u.)

20 40 60 80 400 500 600 700 800 20 40 60 80 400 500 600 700 800 2 Theta (degree) Wavelength (nm) 2 Theta (degree) Wavelength (nm) Figure 1. (a) XRD patterns and (b) UV-Vis spectra of the bulk MoS2 and exfoliated MoS2 nanosheet. Figure 1. ((aa)) XRD XRD patterns patterns and and (b) UV-Vis UV-Vis spectraspectra ofof thethe bulkbulk MoSMoS22 and exfoliated MoS22 nanosheet. 3.2. Physical/Chemical Analysis of Cysteine-Functionalized MoS2 3.2. Physical/Chemical Analysis of Cysteine-Functionalized MoS2 All the the analysis analysis results results for for the the MoS MoS2-Cys-Cys membrane membrane provided provided those those for MC10 for MC10 as rep- as All the analysis results for the MoS2-Cys2 membrane provided those for MC10 as representative. By checking the functional group of materials through ATR, the existence or resentative.representative. By Bychecking checking the the functional functional grou groupp of ofmaterials materials through through ATR, ATR, the the existence existence or bonding state was analyzed (Figure2 2).). MoSMoS2 nanosheetnanosheet showed the general spectral shape 2 bonding state was analyzed (Figure 2). MoS nanosheet showed−−11 the general−−1 spectral shape of chemically exfoliatedexfoliated MoSMoS22 withwith bandsbands of of 3500–3200 3500–3200 cm cm andand 16131613 cmcm resultingresulting from of chemically exfoliated MoS2 with bands of 3500–3200 cm−1 and 1613 cm−1 resulting from the –OH group, and thethe bandband ofof 1200–9001200–900 cmcm–1–1 attributed to overlapping the stretching the –OH group, and the band of 1200–900 cm–1 attributed to overlapping the stretching vibrations ofof Mo=OMo = andO and S=O S [33 =, 34O]. [33,34]. Through Through this result, this the result, presence the ofpresence MoS2 nanosheets of MoS2 vibrations of Mo = O and S = O [33,34]. Through this result, the presence of MoS2 nanosheetscan be conﬁrmed, can be andconfirmed, it can be and seen it thatcan mildbe seen oxidation that mild occurred oxidation in the occurred process in of the exfoliat- pro- nanosheets can be confirmed, and it can be seen that mild oxidation occurred in the pro- cessing MoSof exfoliating2 through MoS a solution2 through process. a solution In the process. case of Cysteine, In the case major of Cysteine, bands were major observed bands cess of exfoliating MoS2 through a solution process. In the case of Cysteine, major bands wereat 3436, observed 2950, 2515,at 3436, 2020, 2950, and 2515, 1525–1315 2020, and cm 1525–1315−1, and these cm−1 corresponded, and these corresponded to –OH, –NH to –, were observed at 3436, 2950, 2515, 2020, and 1525–1315 cm−1, and these corresponded to 2– OH,and S–H–NH stretching2, and S–H vibrations; stretching N–H vibrations; stretching N–H vibration; stretching and vibration; the –COOH and group the –COOH [35–37]. OH, –NH2, and S–H stretching vibrations; N–H stretching vibration; and the –COOH groupIn addition, [35–37]. in In the addition, case of MoSin the2-Cys, case of it wasMoS2 conﬁrmed-Cys, it was that confirmed the Mo=O that and the S=OMo = stretch O and group [35–37]. In addition, in the–1 case of MoS2-Cys, it was confirmed that the Mo = O and Svibration = O stretch bands vibration at 1200–900 bands at cm 1200–900derived cm–1 from derived MoS from2 were MoS strengthened2 were strengthened compared com- to S = O stretch vibration bands at 1200–900 cm–1 derived from MoS2 were strengthened com- paredthat of to the that Cysteine. of the Cysteine. Considering Considering that additional that additional oxidation oxidation cannot occurcannot in occur the process in the pared to that of the Cysteine. Considering that additional oxidation cannot occur in the processof functionalizing of functionaliz Cysteineing Cysteine on MoS 2on, it MoS can2 be, it analyzed can be analyzed that bonds that between bonds between the oxygen the process of functionalizing Cysteine on MoS2, it can be analyzed that bonds between the oxygenfunctional functional groups ofgroups Cysteine of Cysteine and the and components the components of MoS2 ofare MoS formed.2 are formed. oxygen functional groups of Cysteine and the components of MoS2 are formed.

MoS2 nanosheet MoS nanosheet Cysteine2 Cysteine Cysteine‒MoS2 Cysteine‒MoS2 Tranmittance (a. u.)

Tranmittance (a. u.) 3436 3436 2020 2950 25152020 15251315 13151200‒900 2950 2515 1525 1200‒900 4000 3500 3000 2500 2000 1500 1000 500 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm-1) -1 Wavenumber (cm ) Figure 2. FTIR spectra of the MoS 2 nanosheet,nanosheet, Cysteine, Cysteine, and and Cysteine-functionalized Cysteine-functionalized MoS 22.. Figure 2. FTIR spectra of the MoS2 nanosheet, Cysteine, and Cysteine-functionalized MoS2.

The status of MoS2-Cys-Cys was double-checked through XPS analysis. In In Figure Figure 33a,b,a,b, The status of MoS2-Cys was double-checked through XPS analysis. In Figure 3a,b, general peaks peaks of of the the chemically chemically exfoliated exfoliated MoS MoS2 were2 were observed observed from from the Mo the 3d Mo core-level 3d core- general peaks of the chemically exfoliated MoS2 were observed from the Mo 3d core-level spectralevel spectra of MoS of2 MoSand MoS2 and2-Cys MoS [38,39].2-Cys [ 38Note,39]. that, Note even that, af eventer the after functionalization the functionalization of Cys- spectra of MoS2 and MoS2-Cys [38,39]. Note that, even after the functionalization of Cys- teineof Cysteine on MoS on2, it MoS can2 be, it seen can bethat seen additional that additional oxidation oxidation did not occur did not because occur the because intensity the teine on MoS2, it can be seen that additional oxidation did not occur because the intensity ofintensity the peak of resulting the peak resultingfrom MoO from3 did MoO not 3increasedid not at increase all. On atthe all. other On thehand, other the hand, intensity the ofintensity the peak of theresulting S 2s spectrum from MoO next3 did to the not Mo increase 3d spectrum at all. On was the enhanced other hand, after Cysteinethe intensity was combined. In addition, in the S 2p spectra of MoS2 and MoS2-Cys (Figure3c,d), it can be seen that there was no additional oxidation during the functionalization process because the S 2p1/2 and S 2p3/2 peaks do not change even after combining Cysteine to MoS2. Cysteine-related peaks appeared at 164.5 eV and 163.6 eV (Figure3d and Figure S3) [38,40]. ChemEngineering 2021, 5, x FOR PEER REVIEW 6 of 10

of the S 2s spectrum next to the Mo 3d spectrum was enhanced after Cysteine was combined. In addition, in the S 2p spectra of MoS2 and MoS2-Cys (Figure 3c,d), it can be seen ChemEngineering 2021, 5, 41 that there was no additional oxidation during the functionalization process because6 of the 9 S 2p1/2 and S 2p3/2 peaks do not change even after combining Cysteine to MoS2. Cysteine- related peaks appeared at 164.5 eV and 163.6 eV (Figures 3d and S3) [38,40]. Combining these results with the above FTIR results, it can be said that an interaction between MoS2 Combining these results with the above FTIR results, it can be said that an interaction and Cysteine has occurred. between MoS2 and Cysteine has occurred.

(a) (b) Mo 3d Mo 3d

2H MoS2 2H MoS2 Intensity (a. u.) Intensity (a. u.) (a. Intensity S 2s Cys S 2s MoO3 MoO3

237 234 231 228 225 237 234 231 228 225 Binding energy (eV) Binding energy (eV) (c) (d) S 2p S 2p S 2p 3/2 S 2p 3/2

S 2p1/2

S 2p1/2 Intensity (a. u.) Intensity(a. u.) Cys

165 164 163 162 161 160 166 165 164 163 162 161 160 Binding energy (eV) Binding energy (eV)

FigureFigure 3. Mo 3d XPSXPS core-levelcore-level spectra spectra of of the the (a ()a MoS) MoS2 nanosheet2 nanosheet and and (b) ( MoSb) MoS2-Cys2-Cys nanosheet. nanosheet. S 2p S 2p XPS core-level spectra of the (c) MoS2 nanosheet and (d) MoS2-Cys nanosheet. XPS core-level spectra of the (c) MoS2 nanosheet and (d) MoS2-Cys nanosheet.

In addition, thethe morphologymorphology and and nanosheet nanosheet size size were were checked checked out out (Figure (Figure S4). S4). In theIn the SEMSEM images, itit cancan bebe seenseen that that the the pristine pristine MoS MoS2 membrane2 membrane has has amorphology a morphology in thein the form form ofof stacked MoSMoS22 nanosheets,nanosheets,and and the the membrane membrane composed composed of of MoS MoS2-Cys2-Cys also also had had almost almost thethe same morphologymorphology (Figure(Figures S4a,b). S4a,b). Moreover, Moreover, when when individual individual MoS MoS2 and2 and MoS MoS2-Cys2-Cys nanosheetsnanosheets werewere measured measured by by AFM, AFM, the the lateral latera sizesl sizes of nanosheets of nanosheets were 0.957were and0.957 0.937 and um, 0.937 respectively (Figure S4c,d). The thickness of MoS nanosheet was about 1.4 nm, and about um, respectively (Figure S4c,d). The thickness 2of MoS2 nanosheet was about 1.4 nm, and 0.1 nm thicker for MoS2-Cys than MoS2, although there were slight variations. This slight about 0.1 nm thicker for MoS2-Cys than MoS2, although there were slight variations. This difference may be due to the presence of Cys molecules. slight difference may be due to the presence of Cys molecules. 3.3. Nanofiltration Performances 3.3. Nanofiltration Performances The NF performance of the MoS2-Cys membrane for the lead dissolved water was tested.The First, NF weperformance searched forof the optimalMoS2-Cys active membrane layer thickness for the andlead operating dissolved pressure water was tested.using the First, pristine we searched MoS2 nanosheets. for the optimal When the active thickness layer ofthickness the MoS 2andnanosheet operating layer pressure was usingcontrolled the pristine in the range MoS of2 nanosheets. 100–500 nm underWhen constant the thickness pressure of (5the bar), MoS the2 nanosheet water permeance layer was controlleddecreases as in the the thickness range of increases, 100–500 nm and under when theconstant thickness pressure increases (5 bar), from the 100 water to 200 perme- nm, ancethe Pb decreases2+ rejection as the was thickness greatly improved. increases, However,and when thethe Pbthickness2+ rejection increases was no from longer 100 to 200significantly nm, the Pb improved2+ rejection even was though greatly the improved. thickness wasHowever, increased the toPb 300–5002+ rejection nm was so that no longer the significantlyoptimal thickness improved was determined even though to the be 200thickness nm (Figure was 4increaseda). In addition, to 300–500 when nm observing so that the optimalthe NF performancesthickness was at determined various operating to be 200 pressures nm (Figure from 4a). 1 to In 9 addition, bar with thewhen thickness observing thefixed NF at performances 200 nm, the water at various permeance operating increased pressures and the from rejection 1 to decreased 9 bar with as increasingthe thickness fixedoperating at 200 pressure nm, the (Figurewater permeance4b). However, increased since itand was the confirmed rejection thatdecreased the decrease as increasing in rejection becomes magnified when the pressure is higher than 5 bar, the operating pressure operating pressure (Figure 4b). However, since it was confirmed that the decrease in re- was set to 5 bar. jection becomes magnified when the pressure is higher than 5 bar, the operating pressure was set to 5 bar. ChemEngineering 2021, 5, x FOR PEER REVIEW 7 of 10 ChemEngineeringChemEngineering 20212021,, 55,, 41x FOR PEER REVIEW 7 7of of 10 9

(a) (b) (a) (b)40 100 40 5 bar 100 40 T = 200 nm 100 T = 200 nm 40 5 bar 100 90 90 30 90 30 90 30 30 80 80 20 80 20 80 20 70 20 70 Rejection (%) Rejection (%) Rejection 70 70 10 Rejection (%) 10 (%) Rejection 60 60 10 10 60 60 Water permeance (LMH/bar) Water permeance (LMH/bar) permeance Water Water permeance (LMH/bar) Water permeance (LMH/bar) permeance Water 0 50 0 50 0 100 200 300 400 500 50 0024681050 100 200 300 400 500 0246810 Thickness (nm) Applied pressure (bar) Thickness (nm) Applied pressure (bar) Figure 4. Water permeance and Pb2+ ion rejection of the MoS2 nanosheet membrane according to (a) 2+ 2+ 2 MoSFigure2 layer 4. Water Waterthickness permeance permeance and (b) andapplied and Pb Pb pressureionion rejection rejection (the oferror the of bars theMoS MoS we nanosheetre2 calculatednanosheet membrane based membrane on according the accordingrepeated to (a ) experiments).toMoS (a)2 layer MoS2 thicknesslayer thickness and (b) and applied (b) applied pressure pressure (the error (the bars error were bars calculated were calculated based on based the repeated on the repeatedexperiments). experiments). Based on the previous test results, the thickness of the active layer was 200 nm and Based on the previous test results, the thicknessthickness of the activeactive layerlayer waswas 200200 nmnm andand the operating pressure was fixed at 5 bar, and the NF performances of the MoS2-based the operating pressure was fixedfixed atat 55 bar,bar, andand thethe NFNF performancesperformances ofof thethe MoSMoS2-based-based membranes were evaluated according to the Cysteine content using the Pb(NO3)2 aqueous2 membranes werewere evaluatedevaluated accordingaccording toto thethe CysteineCysteine contentcontent usingusing thethe Pb(NOPb(NO3))2 aqueous solution-adjusted pH 6 (Figure 5). The water permeance of the MoS2 nanosheet 3membrane2 wassolution-adjusted 22.3 LMH bar −pH pH1, and 6 6 (Figure (Figureas the 5). 5Cysteine). The The water water content permeance permeance increased of the of from the MoS MoS1 2wt nanosheet %2 nanosheet to 10 membranewt %, mem- the −1 −1 waterbranewas 22.3 permeance was LMH 22.3 LMHbar also, barand gradually as, and the asCysteineimproved, the Cysteine content obtaining content increased the increased maximum from from1 wt permeance 1% wt to% 10to wt10 of %, wt56.7 the % , thewater water permeance−1 permeance also alsogradually gradually improved, improved, obtaining obtaining the maximum the maximum permeance permeance of 56.7 of LMH bar in MC10.− This phenomenon resulted from increasing the content of hydrophilic 56.7LMH LMH bar−1 barin MC10.1 in MC10. This phenomenon This phenomenon resulted resulted from increasing from increasing the content the of content hydrophilic of hy- Cysteine functional groups on the MoS2 (MoS2-Cys itself was still hydrophobic) (Figure drophilic Cysteine functional groups on the MoS (MoS -Cys itself was still hydrophobic) S5a).Cysteine When functional the concentration groups on of theCysteine MoS2 (MoSwas in2-Cyscreased2 itself2 to was20 and still 40 hydrophobic) wt %, it seems (Figure that (Figure S5a). When the concentration of Cysteine was increased to 20 and 40 wt %, it theS5a). excessive When the Cysteine concentration present ofin Cysteinethe membra wasne in hinderscreased theto 20mobility and 40 of wt water %, it moleculesseems that seemsthe excessive that the Cysteine excessive present Cysteine in the present membra in thene membranehinders the hinders mobility the of mobility water2+ molecules of water regardless of whether it is hydrophilic or not. In addition, the rejection of Pb ions con-2+ molecules regardless of whether it is hydrophilic or not. In addition, the rejection2+ of Pb tinuedregardless to increase of whether as the it Cysteine is hydrophilic content or increased, not. In addition, but MC20 the and rejection MC40 of showed Pb ions a very con- ions continued to increase as the Cysteine content increased, but MC20 and MC40 showed smalltinued increase to increase in rejection as the Cysteine so that the content optimal increased, condition but can MC20 be determined and MC40 showedas MC10 a with very a very small increase in rejection so that the optimal condition can be determined as MC10 small increase in rejection so that the optimal condition2+ can be determined as MC10 with considering water permeance. The rejection of Pb ions2+ was improved after combining withconsidering considering water water permeance. permeance. The rejection The rejection of Pb2+ of ions Pb wasions improved was improved after combining after com- the Cysteine to MoS2 via the Donnan exclusion effect because there is no reason for the biningthe Cysteine the Cysteine to MoS to2 via MoS the2 viaDonnan the Donnan exclusion exclusion effect because effect because there isthere no reason is no reasonfor the dramatic change in the overall pore size or structure of the MoS2 membrane by the intro- fordramatic the dramatic change changein the overall in the pore overall size pore or structure size or structure of the MoS of2 themembrane MoS2 membrane by the intro- by ductionthe introduction of Cysteine. of Cysteine. In fact, when In fact, checking when checking the Zeta the potential Zeta potential of MoS of2 MoSnanosheetnanosheet and duction of Cysteine. In fact, when checking the Zeta potential of MoS2 nanosheet2 and MC10and MC10 membranes membranes under under various various pH pHconditions, conditions, MC10 MC10 showed showed positive positive Zeta Zeta at at pH pH 6 6 whereMC10 themembranes NF test was under performed various (FigurepH conditions, S5b). Therefore, MC10 showed it was possiblepositive toZeta effectively at pH 6 where thethe NFNF testtest waswas performedperformed (Figure(Figure S5b).S5b). Therefore, it waswas possiblepossible toto effectivelyeffectively improve the rejection of Pb2+2+ ions by controlling the electrochemical state of the material improve the rejection of Pb 2+ ions by controlling the electrochemical state of the material surfaceimprove without the rejection changing of Pbthe physicalions by controllingstructure of the the electrochemical membrane or the state state of ofthe the material water surface without changing the physical structurestructure ofof thethe membranemembrane oror thethe statestate ofof thethe water channel. In addition, the residual Pb metal was confirmed from the MoS2-Cys membrane channel. In addition, the residual Pb metal was confirmed from the MoS2-Cys membrane channel. In addition, the residual Pb metal was confirmed from the MoS2-Cys membrane surfacesurface after after nanofiltration nanofiltration (Figure (Figure S6). S6). surface after nanofiltration (Figure S6). 80 80

) 100 -1

) 100 -1 60 90 60 90 80 40 80 40 70 70 (%) Rejection 20 (%) Rejection 20 60

Waterpermeance (LMHbar 60 Waterpermeance (LMHbar 0 50 0 MC0 MC1 MC2 MC5 MC10 MC20 MC40 50 MC0 MC1 MC2 MC5 MC10 MC20 MC40 Membrane Membrane FigureFigure 5. 5. NFNF performance performance MoS MoS2-based2-based membrane membrane according according to to Cysteine Cysteine concentrations. concentrations. Figure 5. NF performance MoS2-based membrane according to Cysteine concentrations. ChemEngineering 2021, 5, 41 8 of 9

4. Conclusions

Bulk MoS2 was exfoliated in a nanosheet state based on a simple and industrially applicable solution process, and Cysteine was incorporated therein. When an appropriate 2+ amount of Cysteine was applied to MoS2, both the water permeance and Pb rejection were improved, overcoming the trade-off relationship between solvent permeance and solute rejection in the NF system. In particular, since the NF performance was improved by controlling the electrochemical properties of the surface without changing the physical structure or state of the membrane, it is quite valuable in that it can enhance the efﬁciency of the fabrication process. However, in order to improve the completeness of this research in the future, it is necessary to study how the NF performance changes under various pH conditions or how good the long-term operation stability is. In addition, it would be worthy of checking the rejection performance of not only the Pb2+ ions but also the other heavy metal ions that are frequently released from the industry.

Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/chemengineering5030041/s1, Figure S1: A fabrication process of MoS2-Cys NF membranes, Figure S2: Raman spectra of the bulk MoS2 and exfoliated MoS2 nanosheet, Figure S3: S 2p XPS core-level spectra of the Cysteine, Figure S4: SEM images of (a) pristine MoS2 and (b) MoS2-Cys membranes. AFM results of (c) pristine MoS2 and (d) MoS2-Cys membranes. Figure S5: (a) Water contact angle according to Cysteine concentration and (b) Zeta potential of the MoS2 nanosheet and MC10 in various pH. Author Contributions: Conceptualization, J.J.; methodology, J.J. and S.-S.C.; validation, J.J. and S.-S.C.; formal analysis, J.J., Y.K. and S.K.; investigation, J.J., Y.K. and S.K.; resources, J.J. and S.-S.C.; data curation, J.J. and S.K.; writing—original draft preparation, J.J.; writing—review and editing, J.J. and S.-S.C.; visualization, Y.K. and S.K.; supervision, J.J.; project administration, J.J. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the GIST Research Project and the GIST Research Institute(GRI) grant funded by the GIST in 2021. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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