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공학석사학위논문 투과성능에서지지층의역할을개량적으로분석하기위한 Polyamide- 양극산화알루미늄옥사이드정삼투막의활용 Application of Polyamide-Anodized Aluminum Oxide Membrane for Understanding of Relation between Support Layer and Permeation Performance 2014 년 8 월 서울대학교대학원 화학생물공학부 홍성필
초록 정삼투공정은기존의수처리기술들에비해낮은에너지소모율을보이기때문에차세대수처리기술로주목받고있다. 그러나정삼투공정에사용되는정삼투막의개발은막내부농도분극현상에의해투과수량확보가어려운상황이다. 막내부농도분극현상의원인이되는정삼투막지지층의염확산에대한저항은지지층의구조적특성에영향을받는다. 일반적으로얇은두께, 높은공극률, 그리고기공의모양이직선에가까울수록낮은저항을갖는이상적인지지층이된다고여겨진다. 본연구에서는, 이러한조건에부합하는양극산화알루미늄옥사이드를최초로지지층으로적용하는데성공하였다. 제조방법으로는이미잘형성된 polyamide 선택층을상업용정삼투막으로부터옮겨오는방법 (Thin film transfer process) 을사용하였다. 또한선택층과지지층계면의접착력을향상시키기위해지지층을 polydopamine으로코팅하였다. 그결과, 크게향상된투과수량을보였고이는지지층의저항이낮아졌기때문에막내농도분극현상이완화되었기때문이라볼수있다. 비록소재와방법의실용적가치는제한적이지만, 기존고분자지지층에비해양극산화알루미늄옥사이드의두께나공극률을조절할 I
수있어앞으로, 지지층구조와정삼투막성능간의관계를보다개 량적인측면에서볼수있는새로운길이열릴것으로기대된다. II
목차 초록... I List of Figures... V List of Tables... X 제 1 장서론... 1 1.1. 정삼투공정... 1 1.2. 박막복합막 (Thin Film Composite membrane, TFC membrane)... 4 1.3. 농도분극현상 (Concentration Polarization, CP)... 9 1.4. 염의확산에대한지지층의저항... 13 1.5. 양극산화알루미늄옥사이드 (Anodized Aluminum Oxide, AAO) 17 1.6. 연구목적... 19 제 2 장연구방법... 21 2.1. Thin film transfer process 를이용한 polyamide 박막과양극산화 알루미나필터를이용한 TFC 정삼투막제조... 21 2.2. Polydopamine 을이용한양극산화알루미나필터코팅... 24 2.3. 하이브리드정삼투막평가... 25 III
2.4. 양극산화알루미나필터지지층의저항값분석... 29 제 3 장결과및고찰... 31 3.1. 하이브리드막의특성... 31 3.1.1. 적외선분광광도계를통한 polyamide 선택층조사... 31 3.1.2. 전자주사현미경을이용한하이브리드정삼투막조사... 31 3.2 하이브리드정삼투막평가... 36 3.2.1. Polysulfone 과양극산화알루미나필터지지층의저항비교... 36 3.2.2. Polydopamine 이코팅된지지층을사용한하이브리드막 평가... 42 제 4 장결론... 47 참고문헌... 49 Abstract... 53 IV
List of Figures Figure 1 Schematic of FO desalination process (Qin, Lay, & Kekre, 2012).... 3 Figure 2 The development of thin film composite reverse osmosis membrane (K. P. Lee, Arnot, & Mattia, 2011)... 6 Figure 3 Structural formula showing the chemistry of thin film composite membrane; m-phenylene-diamine (MPD) and trimesoyl chloride (TMC) are monomers of polyamide (PA)... 7 Figure 4 Cross-sectional TEM images of ESPA-1 samples: (a) stained with uranyl nitrate, x60k; (b) stained with sodiumtungstate, x 60K (Freger, 2003).... 8 Figure 5 Schematics of thin film composite membrane with salt concentration profiles. Left scheme illustrated external concentration polarization (ECP) and right one illustrated internal concentration polarization (ICP); C D,b is the bulk concentration of draw solution, C D,I is the concentration at the interface between support layer and draw solution, C I is the concentration at interface between selective layer and support layer, C F,I is the concentration at the interface between feed solution and selective layer, and C F,b is the concentration at bulk of feed solution.... 12 Figure 6 SEM micrographs displaying the cross-section of V
support membranes cast from (A and D) 9, (B and E) 15, and (C and F) 18 wt% polysulfone concentration in 100% 1-methyl-2-Pyrrolidinone (NMP) or dimethyl formamide (DMF) solvent. The micrographs in the left column were cast from 100% NMP, whereas the right column of micrographs depicts membranes cast from 100% DMF (Tiraferri et al., 2011).... 15 Figure 7 Schematic of an ideal FO membrane with a dense active layer (reject pollutants and salt ions) and a scaffold-like nanofiber support layer. The nanofiber support layer is highly porous and has an interconnected pore structure, which provides direct paths for water and salt diffusion (Song et al., 2011).... 16 Figure 8 SEM images of fabricated anodized aluminum oxide (unpublished, 2014).... 20 Figure 9 Scheme illustrating the basic concept of thin film transfer process. Firstly, polyamide-polysulfone thin film composite membrane (PA-PSf TFC membrane) was immersed in NMP solvent (N-Methyl-2-pyrrolidone). PSf was dissolved by NMP but PA selective layer was remained. After washing step, PA selective layer was scooped up by a new support layer from water. PA selective layer floated with non-wrinkled state because of the high surface tension. PA thin film was combined with two different support materials, anodic alumina filter VI
(a) and polysulfone (b) respectively.... 23 Figure 10 Photo images of polyamide-anodic alumina filter hybrid membrane with alumina filter coated with polydopamine (left) and with bare alumina filter (right). 27 Figure 11 Schematic of cross-flow system for forward osmosis process (Yoon, Baek, Yu, & Yoon, 2013)... 28 Figure 12 ATR-IR spectra of a polysulfone support (black) and polyamide-polysulfone membrane which was remathed via thin film transfer method (red). (a) amide C=O (1650 cm -1 ), (b) aromatic ring (1610 cm -1 ), (c) amide C-N (1540 cm -1 ), respectively (Bui et al., 2011).... 33 Figure 13 SEM images of anodic alumina filters. Front and back side of bare alumina filter ((a) and (b)). Front and back side of anodic alumina filter coated with polydopamine ((c) and (d)). There were almost indifference between morphologies of them.... 34 Figure 14 The Cross sectional SEM image of polyamide (PA)-anodic alumina (AAO) filter hybrid membrane.... 35 Figure 15 The comparison of membrane performances. These two membranes consist of same polyamide (PA) selective layer and different support layer. The water flux of PA selective layer on polysulfone (PSf) support, and PA selective layer on bare anodic aluminum oxide (AAO) VII
filter are 2.8 and 56 LMH (L/m 2 h), respectively.... 39 Figure 16 The concentration profile of different support layer (anodic aluminum oxide support: red line, polysuflone support: black line)... 41 Figure 17 Performances of polyamide-anodic alumina filter coated with polydopamine (PA-pAAO) hybrid membrane (a) and polyamide-bare anodic alumina filter (b). The water permeation flux was represented by black rigid line and the average salt backward diffusion was blue dashed line. Note that selective layers of both were synthesized by interfacial polymerization process in the lab.... 44 Figure 18 Schematic illustration of polydopamine coating effect in water permeation process. Anodic alumina support layer coated with polydopamine which is water permeable polymer contributed to the increase of permeable region (blue dashed lines are permeation ways in (a) and (b)). Water could flow along with additional ways (dashed line in (b))... 45 Figure 19 Performance data of hybrid membrane contained of synthesized polyamide thin film and anodized alumina filter coated with polydopamine (PA-pAAO). The selective layer was facing with draw solution. The average water permeation was about 56 LMH after 90 minutes for stabilization (black line) and the average salt backward diffusion was about 0.26 mol/m 2 h (blue dashed VIII
line).... 46 IX
List of Tables Table 1 Summary of Permeation Flux and Support Layer Resistivity 39 X
제 1 장서론 1.1. 정삼투공정 정삼투압공정은기존의수처리기술의에너지효율을획기적으로낮추어줄차세대수처리기술로서주목받고있다. 이공정은막을이용하는공정으로, 막양쪽에서로다른농도의용액을둠으로발생하는삼투압을구동력으로한다. 일반적으로막의한쪽에고농도의유도용액 (draw solution) 을두고, 반대쪽에는상대적으로저농도인유입용액 (feed solution) 이놓이게된다 (Figure 1). 그결과삼투현상에의해저농도의용액에서순수한물이막을통과해고농도의유도용액으로흘러들어가게된다. 이때발생하는삼투압은별도의에너지를필요로하지않고자연적으로발생하는것이기때문에, 정삼투압공정은에너지소모가거의없다는큰장점을갖는다. 하지만일반적으로, 수처리공정으로서의정삼투공정의생산물은막을통과한깨끗한물이다. 따라서유도용액으로흘러들어온생산수를다시유도용액으로부터분리해내기위한분리 / 회수공정이추가로필요하다. 정삼투공정이실용가능한차세대수처리기술로발전하기위해 1
꼭필요한것이바로막연구이다. 현재상용화되어있는막은그성능에서아직부족한것이사실이다. 따라서현재널리활용되고있는수처리기술인역삼투공정을통해그우수성이입증된박막복합막 (thin film composite membrane, TFC membrane) 을정삼투공정에적용하려는움직임들이있다. 하지만 TFC 막을사용할경우막의성능을크게저하시키는막내농도분극현상이야기되어, 이를극복하는것이막개발의필수적인요소이다. 2
Figure 1 Schematic of FO desalination process (Qin et al. 2012). 3
1.2. 박막복합막 (Thin Film Composite membrane, TFC membrane) 정삼투공정에서반투과성막은유도용액쪽으로흐르는물을통과시켜주는반면, 유도용액속의오염물질이나이온의역확산 (salt reverse diffusion or salt back diffusion) 현상을막아주는역할을한다. 결과적으로, 원수에서물이막을통과해흐르는양은막의투과성능을나타내며, 막을통과하는유량대비염의오염물질이나염또는이온의역확산양이막의선택도 (selectivity) 를나타내주는지표가된다. 막의선택능과투과성이정삼투공정의효율을결정하는중요한요소가되며, 유도용액과원수의농도차이역시삼투압의크기를결정하므로수처리속도에큰영향을주는중요한요소이다. 여러가지막의형태중에서도, 성공적으로상업화된 polyamide TFC 막을정삼투공정에도적용하려는움직임이가장활발하다 (Yip et al. 2010, Tiraferri et al. 2011, Han et al. 2012). Figure 2 에서볼수있듯이, polyamide TFC 막의우수한선택도와투과성은역삼투공정에서이미입증되었다 (Kim et al. 2000). 일반적으로 TFC 막은물질의선택적이동이일어나는선택층과선택층이 4
압력에버틸수있도록지지해주는역할을하는지지층으로구성되어있다. 여러가지소재중에서도우수한 1가이온제거능때문에 Figure 3 에묘사된구조를지닌 fully aromatic polyamide 가선택층으로주로사용되며, 지지층으로는비교적다양한소재가사용되지만상업화된기술에서는 polysulfone이주로사용되고있다. TFC 막을제조하기위해서는 polysulfone 지지층위에 fully aromatic polyamide를박막형태로얻어야한다. 이를위해서계면중합을이용해 polysulfone 지지층위에 fully aromatic polyamide 를합성하며, 이렇게합성된 fully aromatic polyamide 선택층은일반적으로약 500 nm 정도의매우얇은두께를갖는박막형태를갖게된다 (Figure 4) (Freger 2004). 일반적으로선택층에서일어나는물질수송의속도가막을통과하는전체속도를결정하는단계로여겨지며, 따라서합성되는 fully aromatic polyamide 선택층의두께가막성능을결정하는중요한요소로작용한다 (Liu et al. 2008, Alsvik et al. 2013). 5
Figure 2 The development of thin film composite reverse osmosis membrane (Lee et al. 2011) 6
Figure 3 Structural formula showing the chemistry of thin film composite membrane; m-phenylene-diamine (MPD) and trimesoyl chloride (TMC) are monomers of polyamide (PA). 7
Figure 4 Cross-sectional TEM images of ESPA-1 samples: (a) stained with uranyl nitrate, x60k; (b) stained with sodiumtungstate, x 60K (Freger 2003). 8
이후에차세대수처리기술로불리는압력지연공정 (pressure retarded osmosis, PRO) 과정삼투공정이주목받게되자, 현재그우수성이입증된 polyamide TFC 막을이공정들에적합한형태로바꾸어적용하려는노력들이있다 (Han et al. 2012, Yip et al. 2011). 하지만아직상용화에성공한정삼투압 TFC 막은없으며 HTI 사의 cellulose acetate계비대칭막만이상용화되어있다. 이는 TFC 막을사용할경우발생되는막내농도분극현상이막의성능을크게저하시키기때문이다. 1.3. 농도분극현상 (Concentration Polarization, CP) Polyamide TFC 막이정삼투막으로적용되는데가장걸림돌이되는것이농도분극 (concentration polarization, CP) 현상이다. 정삼투공정에서농도분극현상이란, 어떠한이유에의해벌크 (bulk) 용액의농도와막표면또는막내부에존재하는용액의농도가달라지는현상을뜻한다. 이러한현상은크게막외부농도분극현상과막내부농도분극현상으로나뉜다. Figure 5 에서볼수있듯이, 막을통과하여나오는처리된물이유도용액과닿아있는 9
지지층표면에서유도용액과섞여유도용액의원래농도보다떨어지게하는결과를야기한다. 반면원수쪽에서는 polyamide 선택층에의해막을통과하지못하는오염물질이나염 / 이온이계속 polyamide 표면에존재하게되어이곳에있는원수의농도가기존의원수농도보다상승하는결과를낳는다. 다시말하면, 막에걸리는전체농도차이가 C D,b - C F,b 에서 C D,I - C F,I 로감소하게되어삼투압의저해를초래한다. 이러한막외부분극현상은정삼투막에유입되는원수와유도용액을막과평행하게흘려주는유속을조정함으로써어느정도약화시킬수있다. 보다중요한것은바로내부농도분극 (internal membrane concentration polarization, ICP) 현상이다. 한연구에의하면 polyamide TFC 막의지지층에의해발생하는이 ICP 현상은이론적으로기대되는막투과성능의 80 % 나감소시키는것으로보고되었다 (Gray et al. 2006). 막내부농도분극현상은지지층으로들어오는고농도유도용액의농도가벌크의농도보다감소하는현상을의미한다. 지지층이유도용액안의염이지지층안으로확산 (diffusion) 될때저항으로작용하여용액이침투되는속도보다느린속도로침투하기때문에일어난다. 결과적으로 polyamide 선택층에걸리는농도차이가크게감소하므로, 유실되는삼투압도크다. 결국보다나은정삼투막의개발을위해서는이현상을억제할수 10
있어야만한다. 현재이현상의원인은, 정삼투막을구성하고있는다공성지지층이염확산에대한저항을지니고있기때문으로밝혀졌으며지지층의구조를통제함으로써이저항을줄일수있다고보고되고있다. 11
Figure 5 Schematics of thin film composite membrane with salt concentration profiles. Left scheme illustrated external concentration polarization (ECP) and right one illustrated internal concentration polarization (ICP); C D,b is the bulk concentration of draw solution, C D,I is the concentration at the interface between support layer and draw solution, C I is the concentration at interface between selective layer and support layer, C F,I is the concentration at the interface between feed solution and selective layer, and C F,b is the concentration at bulk of feed solution. 12
1.4. 염확산에대한지지층의저항 막이갖는염확산에대한저항은막지지층의구조적특성인공극률, 두께, 기공의비틀림도 (tortuosity) 에의해결정된다는관계는 1981년에 Lee 에의해보고되었다 (Lee et al. 1981). 수학적으로유도된이관계에따르면지지층의두께가얇고, 공극률이높으며, 공극의비틀림도가 1에가까울수록이상적인지지층이된다. 이러한지지층의구조와지지층이갖는저항에대한관계가알려진이후에, 이를면밀히검토해본연구가있었다. 이연구에서는막내농도분극현상을최소화시키는최적화된정삼투막을찾기위해지지층을다양한조건으로합성하여다양한구조를지니게하였고, 그위에 polyamide 선택층을계면중합한 polyamide TFC 막을정삼투공정에적용하였다 (Figure 6). 그결과높은공극률을갖는지지층은확실히막내농도분극현상을줄여주지만, 반대로결함없는 polyamide 선택층의형성에방해요소로작용한다는결론을얻었다 (Tiraferri et al. 2011). 나노섬유를이용한지지층을제조하여지지층의저항을줄이기위 한시도도있었다. 전기방사를통해나노섬유를무작위로뽑아내어 판상으로만든경우, 기존의스폰지같은고분자지지층과는다르게 13
매우높은공극률과상대적으로낮은비틀림도를지닌지지층이된 다 (Figure 7). 그결과, 기존의상용막에비해약 6 배향상된투과 성능을보여줬다 (Song et al. 2011). 14
Figure 6 SEM micrographs displaying the cross-section of support membranes cast from (A and D) 9, (B and E) 15, and (C and F) 18 wt% polysulfone concentration in 100% 1- methyl-2-pyrrolidinone (NMP) or dimethyl formamide (DMF) solvent. The micrographs in the left column were cast from 100% NMP, whereas the right column of micrographs depicts membranes cast from 100% DMF (Tiraferri et al. 2011). 15
Figure 7 Schematic of an ideal FO membrane with a dense active layer (reject pollutants and salt ions) and a scaffold-like nanofiber support layer. The nanofiber support layer is highly porous and has an interconnected pore structure, which provides direct paths for water and salt diffusion (Song et al. 2011). 16
1.5. 양극산화알루미늄옥사이드 (Anodized Aluminum Oxide, AAO) 양극산화알루미늄옥사이드는특정한산용액을전해질로하여알루미늄에일정한전압을인가하여얻는다. 이러한조건에서산화되는알루미늄은일정한크기의기공이벌집모양으로잘정렬된구조를갖게된다 (Figure 8). 전압을인가하는시간을조절해두께를비교적자유롭게조절가능하며, 기공의크기는수십에서수백나노미터에이르기까지다양하게조절이가능할뿐만아니라, 기공을넓히는등의추가공정을통해공극률역시조절이가능하다. 양극산화알루미늄옥사이드는규칙적으로잘정렬된구조가갖는이점때문에다양한곳에서적용되고있다. 특히나노패턴을만드는템플릿 (template) 으로널리활용되고있다 (Crouse et al. 2000). 더불어비틀림도가 1인직선형기공을이용해, 나노튜브를성장시킬때고정시켜주는틀로도이용되고있다. 실재로수나노미터의직경을갖는탄소나노튜브 (Carbon nano tube, CNT) 를수십나노미터의직경을갖는크기로제조하고자할때틀로서활용되었다 (Hu et al. 2001). 뿐만아니라, 다른소재들에비해기공의크기가매우좁은영역 17
으로분포하고있다는특징때문에, 물질을분리하는필터로도활용 되고있다. 이미필터로서는널리상업화가되어사용되는중이다. 본연구에서는양극산화알루미늄옥사이드의구조적특징에주목하였다. 이소재는높은공극률과충분히얇은두께를지니도록조절할수있으며, 무엇보다비틀림도가 1이라는이상적인구조적특징을지니고있다. 이는앞에서언급한지지층이염의확산에대한낮은저항을가질수있는조건에매우적합하다. 이렇게적합한조건을갖춘양극산화알루미늄옥사이드소재를그동안적용하지못했던이유중하나는, 그위에선택층의역할을하는 polyamide 박막이결함없이잘형성되지않기때문이다. 한연구에의하면공극률이 6%, 기공크기가 70 nm인 polysulfone 지지층과공극률이 10%, 기공크기가 150 nm인 polysulfone 지지층위에각각 polyamide 선택층을계면중합하여비교하였을경우, 공극률이높고기공의크기가큰지지층위에보다얇은선택층이형성되어결손부위가형성될가능성이크다고보고하였다 (Singh et al. 2006). 18
1.6. 연구목적 본연구에서는보다우수한성능을지닌정삼투막을제조하기위해기존의고분자재료에서벗어나, 지지층으로서적합한구조를지니고있는양극산화알루미늄옥사이드소재를적용해보았다. 제조방법으로는상업용역삼투막인 polyamide TFC 막으로부터 polyamide 선택층을얻어양극산화알루미나필터위로옮기는방법 (thin film transfer process) 을적용해제조하였고, 또한접착력이있는것으로널리알려진 polydopamine으로지지층을코팅하여두층이잘붙어있을수있도록하였다. 제조된하이브리드막을정삼투공정에적용한결과막내부농도분극의영향이크게줄어들었음을확인하였다. 향상된투과수량과지지층의구조를통해정량적으로분석하여이를확인할수있었다. 따라서본연구를통해양극산화알루미나필터를적용한정삼투막을제조함으로새로운하이브리드 TFC 막의가능성을제시하였다. 이와같은새로운정삼투막은소재와방법적측면에서실용적가치는제한적이지만, 지지층의두께와공극률을조정할수있다는장점이있다. 따라서이를통해지지층의구조와정삼투막의성능간의관계를보다개량적인측면에서살펴볼수있는새로운연구적접근방법을제시할수있을것이라기대된다. 19
Figure 8 SEM images of fabricated anodized aluminum oxide (unpublished, 2014). 20
제 2 장연구방법 2.1. Thin film transfer process 를이용한 polyamide 박막과양극산화알루미나필터를이용한 TFC 정삼투막제조 상용화된 polyamide-polysulfone TFC 역삼투막 (Hydranautics a Nitto Denko, oceancide, CA) 으로부터 polyamide 선택층을분리해내기위해 N-Methyl-2-pyrrolidone (NMP) 으로 polysulfone 지지층을녹인다. NMP는불용성인 fully aromatic polyamide 고분자를팽윤 (swelling) 시키는좋은용매로알려져있다 (Aharoni 1992). 그렇기때문에 polysulfone 지지층이빠르게녹아들어가면서발생되는스트레스에박막형태의 polyamide 선택층이찢어지는현상을방지할수있다. 상용역삼투막을 NMP 용매에넣은뒤, polysulfone이완전히녹기를기다리면, 일정시간이지나 polyamide 박막만남아있게된다. 이것을건져내어다시 NMP 용매에담가잔여 polysulfone을씻어준다. 씻겨진 polyamide 박막을떠서증류수위에두면물의큰표면장력에의해 polyamide 박막 21
이물표면위에주름없이펴진상태로떠있게된다 (Figure 9). 이상태의 polyamide 박막을양극산화알루미나필터위에올린상태로떠낸뒤, 완전히말려주면 polyamide 박막과양극산화알루미나필터가결합된상태의정삼투막이완성된다. 제조과정에서 polyamide 선택층의화학적구조가무너지지않았는지확인하기위해 attenuated total reflectance fourier transform infrared (ATR FT-IR) 을통해조사해보았다. 이때, ATR FT-IR의침투거리에비해 polyamide 박막의두께가너무얇아잘관측이되지않으므로, 동일한제조과정을거친뒤얻어진 polyamide 박막을다시 polysulfone위에올린뒤조사하였다. 또한전자주사현미경 (SEM) 으로제조된하이브리드막의상태를살펴보았다. 22
Figure 9 Scheme illustrating the basic concept of thin film transfer process. Firstly, polyamide-polysulfone thin film composite membrane (PA-PSf TFC membrane) was immersed in NMP solvent (N-Methyl-2-pyrrolidone). PSf was dissolved by NMP but PA selective layer was remained. After washing step, PA selective layer was scooped up by a new support layer from water. PA selective layer floated with non-wrinkled state because of the high surface tension. PA thin film was combined with two different support materials, anodic alumina filter (a) and polysulfone (b) respectively. 23
2.2. Polydopamine 을이용한양극산화알루미나필 터코팅 Polyamide 선택층과양극산화알루미나필터지지층사이의접착력을향상시키기위해 polydopamine을지지층에코팅하였다. polydopamine은다양한표면에쉽게코팅된다는점과, 접착력을지닌코팅제로이미널리알려져있는소재이다 (Lee et al. 2007). 코팅방법은다음과같다. 상온에서 dopamine 2 g/l이녹아있는 Phosphate Buffered Saline (PBS) 0.01 M 수용액에양극산화알루미나필터를수직방향으로넣는다. 24 h이지난뒤꺼내면표면이녹색으로변해있는양극산화알루미나필터를얻을수있다 (Figure 10). 이렇게코팅된알루미나필터와직접 polysulfone 지지층에계면중합으로합성한 polyamide 선택층을결합시켜 (PApAAO) 하이브리드막을제조하였다. 비교를위해직접계면중합한 polyamide 선택층과코팅되어있지않은양극산화알루미나필터를지지층으로이루어진하이브리드막 (PA-bAAO) 과비교하였다. 24
2.3. 하이브리드정삼투막평가 Figure 11 은본실험에서사용한정삼투공정의개념도이다. 막을사이에두고양쪽으로각각고농도의유도용액과저농도의원수가평행하게흐른다. 측면흐름으로외압이발생되는것을막기위해측면유속은 4 cm/s (Re=234) 으로저속운전을하였다. 운전은상온에서진행되었다. 유도용액으로는 1 M의염화나트륨용액을, 원수로는증류수를사용하였다. 막의성능평가는시료당투과수량이안정적으로나오기시작한시점으로부터 15분간격으로총 4회측정하여얻은자료의평균값을사용하여, 총 3~4개의시료로실험을진행하였다. 현재상업용정삼투막은 cellulose acetate 계비대칭형막뿐이기때문에, 지지층이바뀐효과를비교하기위해서는막의형태와소재가적합하지않다. 따라서, 상용정삼투막을비교군으로사용하였으며, 동등한비교를위해부직포를제거하여 polyamidepolysulfone만의상태로동일한환경하에서막성능평가를하였다. 또한상업용이아닌직접 polysulfone위에계면중합으로제조한 polyamide 선택층을지니고있는코팅된양극산화알루미나필터 (paao) 하이브리드막역시동일한환경에서평가하였다. 이때 25
막의방향은선택층이저농도의유입용액과닿아있으며, 방향을바 꾸어서도평가하였다. 26
Figure 10 Photo images of polyamide-anodic alumina filter hybrid membrane with alumina filter coated with polydopamine (left) and with bare alumina filter (right). 27
Figure 11 Schematic of cross-flow system for forward osmosis process (Yoon et al. 2013) 28
2.4. 양극산화알루미나필터지지층의저항값분석 막내부농도분극현상은지지층이갖는염확산에대한저항에 의해발생하며, 지지층이갖는저항은그구조적특성에기인한다. K τ ε (1) 여기에서 K는막의저항, S는 structure parameter로각각정의된다. D는확산계수 (diffusion coefficient), t는지지층의두께, τ 는기공의비틀림도, ε는공극률을의미한다. 식 (1) 의관계는 Lee에의해수학적으로유도된것이며 (Lee et al. 1981), 이를통해이상적인정삼투막지지층은두께가얇고, 비틀림도가 1에가까우며, 공극률이높을수록작다는것을알수있다. 일반적으로고분자지지층의경우두께를제외한구조적특성들을정량적으로측정하기어려우므로 K값을구할때, 식 (1) 을사용할수없다. 따라서아래의실험식을주로이용한다 (McCutcheon and Elimelech 2006). ln,, (2) J w 는막을통과하는유속, π, 는벌크유도용액의삼투압을, π, 29
는벌크의원수에해당하는삼투압을의미한다. A 와 B 는막고유 의특성으로각각투과성능과, 제거성능에대한계수로압력을가하 는역삼투공정에서측정하여얻는값이다. 여기서는제조된하이브리드막의지지층이갖고있는저항을위의두식을모두이용하여분석해보았다. 기존의고분자소재로되어있는지지층의경우식 (1) 과같은구조적특징으로부터저항을분석할수없었다. 왜냐하면비틀림도같은구조적변수를정량적으로측정하는것이매우어렵게때문이다. 따라서기존의경우식 (2) 와같은실험식을통해서만저항에접근할수있었다. 실험식만이용할경우, 투과성능이높아진원인은전부낮아진지지층의저항값으로만표현될수밖에없다. 결국실험을통해얻은투과성능이나실험식으로계산된지지층의저항이타당한것인가에대한결과는제시해주지못한다. 그러나양극산화알루미늄필터지지층의경우는식 (1) 을통해서도접근할수있으며, 따라서이두가지모두를활용하여저항을분석할수있었다. 30
제 3 장결과및고찰 3.1. 하이브리드막의특성 3.1.1. 적외선분광광도계를통한 polyamide 선택층조사 Figure 12 은 polyamide가없는깨끗한 polysulfone 지지층과상용역삼투막으로부터분리한 polyamide 선택층을다시 polysulfone 지지층과결합시킨막에서얻은적외선분광스펙트럼들을비교한것이다. 1540 cm -1, 1650 cm -1 에서나타난피크는각각 amide 결합구조에속해있는 C-N 결합과 carbonyl gourp에해당한다 (Yu et al. 2013). 1610 cm-1의피크는방향족고리를의미한다 (Bui et al. 2011). 따라서선택층의 fully aromatic polyamide 구조가여전히존재함을알수있다. 3.1.2. 전자주사현미경을이용한하이브리드정삼투막조사 전자주사현미경을통해제조된하이브리드정삼투막을살펴보 았다. Figure 13 (a) 와 (b) 는코팅을하지않은양극산화알루미나 31
필터지지층의 SEM 사진이다. 반면 Figure (c) 와 (d) 는 polydopamine이코팅된양극산화알루미나필터지지층의 SEM 사진이다. SEM 으로확인한결과코팅된 polydopamine의두께가충분히얇아코팅으로인한기공의크기나공극률등의변화는미미하여무시할만하다. Figure 14 는제조된하이브리드막단면의 SEM 사진으로양극산화알루미나필터위에 polyamide 박막이잘올라가있음을확인할수있었다. 국소적인부분에서지지층과선택층이일부떨어져있는모습을확인하였는데, 이는두층사이의계면에접착력이부족하거나샘플을제조하는과정에서발생한것으로생각된다. 32
Figure 12 ATR-IR spectra of a polysulfone support (black) and polyamide-polysulfone membrane which was re-mathed via thin film transfer method (red). (a) amide C=O (1650 cm -1 ), (b) aromatic ring (1610 cm -1 ), (c) amide C-N (1540 cm -1 ), respectively (Bui et al. 2011). 33
Figure 13 SEM images of anodic alumina filters. Front and back side of bare alumina filter ((a) and (b)). Front and back side of anodic alumina filter coated with polydopamine ((c) and (d)). There were almost indifference between morphologies of them. 34
Figure 14 The Cross sectional SEM image of polyamide (PA)- anodic alumina (AAO) filter hybrid membrane. 35
3.2 하이브리드정삼투막평가 3.2.1. Polysulfone 과양극산화알루미나필터지지층의저항 비교 Figure 15은제조된하이브리드정삼투막의평가결과이다. 상업용역삼투막을비교군으로둔이유는, 지지층이 polysulfone 일때와양극산화알루미나필터일때를비교하기위해서다. 결과적으로제조된하이브리드정삼투막의투과수량이상업용역삼투막에비해월등히높은이유는양극산화알루미나필터지지층의저항이 polysulfone 지지층의저항에비해매우작기때문이라고생각할수있다. 이를확인하기위해정의식 ( 식 (1)) 과실험식 ( 식 (2)) 을이용해두지지층의저항을각각계산해보았다 (Table 1). 실험식을이용해계산한결과양극산화알루미나필터지지층의저항은 6.2 10 s/m, polysulfone 지지층의저항은 480 10 s/m이다. 식 (1) 을이용해계산한양극산화알루미나가갖는구조에서오는저항값은 8.4 10 s/m이다. 구조적인측면에서봐도 polysulfone 지지층이갖는저항에비해훨씬작은저항값을지닌것을알수있었다. 하이브리드막이높은투과성을보이는동시에, 염의역확산도도 36
큰값을갖는이유는명료하다. Figure 16의검은색선은지지층의저항이클때, 빨간색선은지지층저항이보다작을때의농도분포를의미한다. 즉, 지지층의저항이작아지면선택층과지지층사이계면에서염의농도가상승하게되고, 그결과선택층을통과하게되는염의농도는증가하게된다. 다시말하면선택층의제거성능이같다고하더라도지지층의저항이작아지면염이역으로확산되어나오는양은증가하여, 양극산화알루미나를지지층으로한막의염역확산양이높은값을갖게된다. 하지만선택도 ( 물의투과양과염의역확산양의비율 ) 를비교해보면, 상업용역삼투막의경우는 87 L of water/mol of NaCl, 알루미나지지층을갖는하이브리드막의경우는 109 L of water/mol of NaCl로유사한값을갖는것을알수있다. 따라서제조된하이브리드막은적당한선택도를유지하면서정삼투막으로작동하고있다고볼수있다. 양극산화알루미늄옥사이드를소재의지지층적용이가능하다는사실은다음과같은의미를지닌다. 이소재는두께나공극률그리고기공의크기등구조적특징을비교적자유롭게조절할수있다. 또한본연구에서사용한 thin film transfer process를이용해동일한선택층에서로다른구조를지닌지지층을적용할수있다. 결과적으로지지층구조와막의성능간의관계를보 37
다개량적인측면에서, 정확하게살펴보는것이가능해질것이 라고생각된다. 38
80 0.8 J w Water flux, J w (LMH) 60 40 20 J s 0.6 0.4 0.2 Salt back diffusion, J s (mol/m 2 h) 0 PA-AAO PA-PSf 0.0 Figure 15 The comparison of membrane performances. These two membranes consist of same polyamide (PA) selective layer and different support layer. The water flux of PA selective layer on polysulfone (PSf) support, and PA selective layer on bare anodic aluminum oxide (AAO) filter are 2.8 and 56 LMH (L/m 2 h), respectively. 39
Table 1. Summary of Permeation Flux and Support Layer Resistivity Water flux (LMH) Resistivity (10 4 s/m) PA-AAO a) 62 6.2 PA-AAO b) 48 8.4 Commercial RO 2.8 480 a This data was obtained by evaluation from FO process. Water flux and resistivity values were calculated by Equation (1) b This data was estimated by Equation (2). Water flux and resistivity values were expectation ones, not experimental data. 40
water C D, b C I, AAO C p, AAO C I, PSf C F, b C p, PSf salt Figure 16 The concentration profile of different support layer (anodic aluminum oxide support: red line, polysuflone support: black line) 41
3.2.2. Polydopamine 이코팅된지지층을사용한하이브리드 막평가 선택층과지지층사이의접착력을향상시키기위해 polydopamine을양극산화알루미나필터지지층에코팅하였다. 직접계면중합하여얻어진 polyamide 선택층을사용하여제조된 PA-bAAO와 PA-pAAO 하이브리드막을정삼투공정에적용하여평가한결과는 Figure 17에나타나있다. 두가지모두의경우유도용액의염이역으로확산되는양은유사했다. 그러나투과성능에서큰차이가났다. 지지층에 polydopamine이코팅되어있지않는 PA-bAAO 하이브리드막의경우 7.6 LMH의낮은투과량을보이지만, PA-pAAO 하이브리드막의경우는낮은저항을지니고있는지지층의효과가드러난 54 LMH의투과량을보였다. polydopamine 코팅을하지않은경우, 앞의상업용역삼투막으로부터얻은 polyamide 선택층을사용한경우와똑같은지지층이지만선택층의질이다르기때문에투과성능에차이가나는것이라여겨진다. 한편직접계면중합한 polyamide 선택층을사용하지만서로다른지지층으로되어있는 PA-bAAO와 PA-pAAO의투과성능이차이가나는이유는 polydopamine이물에대해어느정도투과성을지니고있기때문이다 (McCloskey et al. 2010). Figure 18에서설명하고있는것처럼, 지지층과선택층사이에존재하는 42
polydopamine에의해지지층과선택층사이에물이통과할수있는공간이형성된다. 따라서기존에는지지층표면의기공이없는부분에막혀사용하지못하는영역들을통해서도물을통과시킬수있게된다. 이와유사한이유로, 지지층의기공크기와공극률이역삼투공정의막내부물질수송현상에미치는영향이설명된바있다 (Pendergast et al. 2010). 막의방향을바꾸어평가할때 PA-bAAO의경우선택층의일부가찢어지는현상이발생하였다. 이는막의방향을바꾸면삼투압에의해투과수가지지층으로부터선택층이뜯어지는방향으로흘러더큰외력이작용하게되기때문이다. 하지만 PA-pAAO의경우찢어지는현상없이막이작동되는것을확인할수있었다. Figure 19은방향을바꿔선택층이고농도의유도용액과닿아있는상태로막을평가한결과이다. 약 90분이지난뒤안정화가되었으며, 약 200여분동안안정적으로작동하는것을볼수있다. 이는 polydopamine이두층사이의접착력을향상시켰기때문이라할수있다. PA-pAAO의실험결과막의방향을바꾸어도비슷한유속을보인다. 이러한결과는, 지지층의염확산에대한저항이감소하여막내부농도분극현상이굉장히개선된것이라보고되었다 (Song et al. 2011). 43
Figure 17 Performances of polyamide-anodic alumina filter coated with polydopamine (PA-pAAO) hybrid membrane (a) and polyamide-bare anodic alumina filter (b). The water permeation flux was represented by black rigid line and the average salt backward diffusion was blue dashed line. Note that selective layers of both were synthesized by interfacial polymerization process in the lab. 44
Figure 18 Schematic illustration of polydopamine coating effect in water permeation process. Blue dashed lines mean water pathways. Some of water pathways are blocked by top surface of support layer when support layer is not coated (a). However, more water pathways placed between selective layer and support layer can be obtained when support layer is coated with polydopamine which is water permeable polymer (b). 45
Figure 19 Performance data of hybrid membrane contained of synthesized polyamide thin film and anodized alumina filter coated with polydopamine (PA-pAAO). The selective layer was facing with draw solution. The average water permeation was about 56 LMH after 90 minutes for stabilization (black line) and the average salt backward diffusion was about 0.26 mol/m 2 h (blue dashed line). 46
제 4 장결론 본연구에서는기존의 polysulfone 고분자소재가가질수없는구조적인장점을지닌양극산화알루미늄옥사이드소재의필터를처음으로지지층에적용하여정삼투막을제조하였다. 양극산화알루미나필터를정삼투막지지층으로적용시키기위해 fully aromatic polyamide 선택층을상업용역삼투막으로부터얻어내어, 양극산화알루미나필터위로옮기는 thin film transfer process를사용하였다. 또한선택층과지지층사이의접착력을향상시키기위해 polydopamine을양극산화알루미나필터지지층에코팅하였다. 주요결과는제조된하이브리드정삼투막은기존의선택도를유지한채매우향상된투과성능을보였다. 양극산화알루미나필터구조가갖는저항을분석함으로써향상된성능은지지층구조가갖는특징에기인한다는것을알수있었다. 또한 polydopamine 코팅을통해막의내구성을향상시킬수있었고, 코팅되지않은경우보다높은투과성능을보였다. 이이유는기존에는지지층표면에가로막혀있는부분으로물이통과하여흐를수있게되었기때문이다. 비록본연구에서사용된양극산화알루미늄라는소재와선택층 47
을외부로부터취하는 transfer process는대면적화에불리하지만, 기존 polysulfone과구별되어양극산화알루미늄옥사이드두께나공극률을조정할수있어지지층구조와정삼투막성능간의관계를보다정량적으로살펴보는길을제공해줄것으로기대된다. 48
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Abstract Sung Pil Hong School of Chemical and Biological Engineering, Collage of Engineering The graduate school Seoul national University The forward osmosis process has been regarded as a next generation technique in water treatment because of their low energy cost property. However, the development of the higher-permeation membrane for this process has been limited by internal concentration polarization (ICP). This ICP phenomenon is caused by the salt diffusion resistivity of the support layer which is the function of support layer structure. Generally, thin, porous, and straight pore channel are considered as ideal structural features of the support layer for low resistivity. In this experiment, we firstly applied anodic aluminum oxide which has proper structural features as a support layer. To fabricate this hybrid 53
membrane, polyamide selective layer in commercial reverse osmosis membrane was transferred to commercial anodic alumina filter (Thin film transfer process). Additionally, the alumina support layer coated with polydopamine for improving adhesion between selective and support layer. The result of evaluation of fabricated hybrid membranes showed enhanced permeation performance with low support layer resistivity in forward osmosis process. Although the material and fabrication process were not favorable for practical use, this hybrid membrane can open a new investigation approach. Anodic aluminum oxide has been known as a good material with adjustable structure properties and selective layers with similar qualities can be offered by transfer process used in here. In this respect, we believe that this success of applying the anodic alumina filter as a new support layer can offer opportunities us a way for studying the relationship between the structure of support layer and the membrane performance. 54