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A novel in situ membrane cleaning method using periodic electrolysis
Journal of Membrane Science 471 (2014) 149–154
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
A novel in situ membrane cleaning method using periodic electrolysis Raed Hashaikeh b,n, Boor Singh Lalia b, Victor Kochkodan a, Nidal Hilal a,nn a
Centre for Water Advanced Technologies and Environmental Research (CWATER), College of Engineering, Swansea University, Swansea SA2 8PP, UK Institute Center for Water Advanced Technology and Environmental Research (iWATER), Masdar Institute of Science and Technology, P.O. Box 54224, Abu Dhabi, United Arab Emirates
b
art ic l e i nf o
a b s t r a c t
Article history: Received 15 May 2014 Received in revised form 7 August 2014 Accepted 9 August 2014 Available online 19 August 2014
Membrane fouling is the major problem during the practical application of membrane separation processes in industry and water treatment. Therefore a search for novel efficient methods of membrane cleaning is currently of crucial importance for membrane-based technologies. The paper describes a new method of membrane cleaning, which is based on periodic electrolysis using a novel electrically conductive membrane to remove/prevent membrane fouling. The membrane consists of a thin electrically conductive layer of multi-walled carbon nanotubes (MWCNTs) deposited on the membrane's surface. The deposited MWCNTs allow the membrane to function as a cathode in an electrochemical system that includes the electrically conductive membrane, the salt water as an electrolyte and a stainless steel counter anode. The efficiency of the cleaning procedure in the flux recovery has been proved with typical bio- and inorganic membrane foulants such as CaCO3 and yeast suspensions. The cleaning mechanism during the electrolysis process is explained by the evolution of gases forming micro-bubbles at the membrane surface which remove the foulant material out from the membrane. The proposed method enables in situ membrane self-cleaning, thus providing a non-destructive, continuous and renewable approach for the mitigation of the different types of membrane fouling. & 2014 Elsevier B.V. All rights reserved.
Keywords: Membranes Electrolysis (Bio)fouling Self-cleaning MWCNT Desalination
1. Introduction During the last few decades, pressure-driven membrane processes such as reverse osmosis, nanofiltration, ultrafiltration and microfiltration have been widely used in water treatment, biotechnology, the food industry, medicine and other fields [1]. However, the main problem arising upon the operation of the membrane units is membrane fouling, which seriously hampers the application of membrane technologies [2]. Membrane fouling is an extremely complex phenomenon that has not yet been defined precisely. In general, the term is used to describe the undesirable deposition of retained particles, colloids, macromolecules, salts, etc., at the membrane surface or inside the pores. Depending on the membrane process and chemical/ biological nature of foulants, several types of fouling can occur in membrane systems, e.g. inorganic fouling or scaling, organic fouling colloidal fouling and biofouling [3–6]. Membrane fouling, as well as its prevention, has been a subject of many studies since the early 1960s when industrial membrane n
Corresponding author. Corresponding author. E-mail addresses: [email protected] (R. Hashaikeh), [email protected] (N. Hilal). nn
http://dx.doi.org/10.1016/j.memsci.2014.08.017 0376-7388/& 2014 Elsevier B.V. All rights reserved.
separation processes emerged. Membrane fouling can be partially controlled by the selection of an appropriate membrane, adjustment of the operating conditions in a membrane element, including hydrodynamics and operating pressure, and appropriate pretreatment of the feed solutions [7,8]. Unfortunately, very often these actions are not sufficient to cope with fouling. As fouling progresses, membrane flux declines; higher operating pressures and thus more energy must be expended to achieve the desired throughput. Usually, membrane cleaning is applied to remove the foulants and restore the membrane flux. A number of physical and chemical techniques have been used for membrane cleaning including backwashing, pulsing, forward flushing with air, sonification and chemical cleaning [9–12]. However, these operations fail to recover the membrane flux without deteriorating the membrane material and interrupt the water treatment process [13]. In many cases, the membrane elements must be replaced, which sharply increases the treatment costs [8]. Therefore the development of novel efficient methods of membrane cleaning at lower cost is of crucial importance for the wider use of membrane technologies in industry and water treatment. Electrolytic cleaning is a common method used for metal surface cleaning before galvanizing and electroplating [14]. Electrolytic cleaning is an electrolysis process in which a direct voltage
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R. Hashaikeh et al. / Journal of Membrane Science 471 (2014) 149–154
is applied between an anode and a cathode dipped in an electrolyte that consists of free ions for conducting the electric current [15]. During the electrolysis process gases are produced and bubbled at the electrodes' surfaces. The generation and evolution of gases has been found to be effective in removing the tiniest remaining amounts of dirt from the surface of electrically conductive materials [14–17]. The application of this technique to pressure-driven membrane processes is hindered by the inherent insulating nature of polymeric membranes. Over the years, attempts have been made to use electrically conducting polymers to form membranes for different purposes [18,19]. However, traditional conducting polymers are notoriously difficult to process, and membranes made from these materials suffer from low selectivity, low flux, and often, low conductivity [19,20]. Recently, it has been reported that carbon nanotubes (CNTs) possess good electrical conductivity [21]. Because of their electrical properties CNTs have been proposed as functional electrodes in electrochemical systems such as: oxygen reduction in fuel cells; hydrogen production; photo-assisted water electrolysis etc [21–24]. Additionally it was shown that CNTs deposited on a porous support layer [25] or embedded into polymer membranes are capable of bacterial inactivation and bio-fouling prevention once an electrical bias is applied [26]. In this paper, we have studied the possibility of using electrolytic cleaning for polymer membranes modified with multi wall carbon nanotubes (MWCNTs) to develop a membrane cleaning process, which might be used in situ in membrane modules. The polymer membrane has been coated with a thin electrically conductive MWCNTs layer and acts as an electrode during the periodic electrolysis. The efficiency of membrane cleaning has been studied during membrane filtration of calcium carbonate and yeast suspensions, which are typical membrane foulants for membrane-based desalination processes.
2. Methods 2.1. Materials Multi-walled carbon nanotubes (MWCNTs) (Cheaptubes.com, USA) (outer diameter 13–18 nm, purity 4 99 wt%), microfiltration polyvilidefluoride Millipore membranes (GSWP, 0.22 mm), sodium lauryl sulfate (SDS, surfactant), sodium chloride (NaCl, Aldrich), yeast (Baker's yeast, DCL, France), calcium carbonate (CaCO3, Aldrich) were used as received.
2.2. MWCNTs coating on the membrane Millipore membranes were coated using vacuum filtration technique reported elsewhere [27]. 0.05 wt% MWCNTs powder was dispersed in an aqueous solution of 1 wt% SDS in water using probe sonicator (Hielscher, UP400S, 400 W, 24 kHz) at 50% amplitude and 0.5 cycle for 10 min to obtain a homogeneous ink-like suspension. Then 4 ml of MWCNTs suspension was filtered through the membrane with an active area of 9.34 cm2 using vacuum filtration at 30 kPa pressure. Thereafter the membrane sample was carefully washed three times with Milli-Q water to remove any excessive MWCNTs from the membrane surface. Optical images of MWCNTs coated microfiltration membranes were shown in Fig. 1(a–c). First the membranes were coated with MWCNTs and then silver electrodes were printed on the coated membrane to improve the electric charge distribution of the membranes (Fig. 1c). Thickness of the MWCNTs coating on the surface was measured from the cross-sectional SEM images and found to be 3–4 μm. After drying the MWCNTs coated membrane at room temperature, silver electrodes were coated on the membrane surface (Fig. 1c) to improve the electrical contacts and charge distribution on the surface of membrane. 2.3. Membrane characterization Scanning electron microscopy (FEI Quanta FEG 250) was used to image the surface and cross-section of the membrane in high vacuum. Prior to imaging, samples were first coated with a thin gold layer of 5–8 nm thickness. Optical images of the membrane surface were obtained by the optical microscope (Olympus-DP21). Surface resistivity of the MWNTs coated membrane was measured by using four point probe (LakeShore, USA) according to Van der Pauw method [28]. In this method, four electrodes were pasted on the membrane surface using silver paint dots. The electrodes were marked as 1–4 in clockwise position and current is passed through 1 and 2 electrodes and potential is measured between 3 and 4 electrodes. Four consecutive measurements were done by applying current between 2 and 3, 3 and 4, 4 and 1 and potential was measured between 4 and 1, 1 and 2 and 2 and 3 respectively. 2.4. A membrane cleaning setup A custom made cross flow filtration setup was used in this study. A schematic design of the filtration setup is shown in Fig. 2. MWCNTs coated surface acts as a negative electrode (cathode) in electrochemical system using stainless steel of diameter 15 mm as
Fig. 1. Optical images of (a) pristine Millipore membrane, (b) coated with MWCNTs using vacuum filtration, (c) MWCNTs coated membrane with silver electrodes.
R. Hashaikeh et al. / Journal of Membrane Science 471 (2014) 149–154
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Fig. 2. A schematic design of the (a) filtration cell and (b) filtration setup used for the experiments.
Fig. 3. SEM images of (a) pristine Millipore membrane and (b) MWCNTs coated membrane and (c) I–V curves of MWCNTs coated membrane using four point probe measurement.
a positive electrode (anode). The electrolysis was performed by using Autolab 302N potentiostat/galvanostat at 2 V for 2–3 min. CaCO3 and yeast suspensions were used in filtration experiments as feed streams. The yeast suspension was prepared by immersion of 50 mg yeast in aqueous NaCl solution with concentration of 10 g/L. CaCO3 suspension was prepared by addition of 100 mg CaCO3 in 10 g/L aqueous NaCl solution. NaCl solution was used to assist the electrolysis process during cleaning of the fouled membrane. The suspensions were filtered through the MWCNTs coated membrane at operating pressure of 1 bar. The permeate was collected and weighted with time of filtration using a weighing balance. The suspension was filtered through the membranes during 30, 40 or 60 min filtration intervals and after each filtration interval the electrolysis process was conducted for 3 min followed by another filtration interval.
3. Results and discussion SEM images of the pristine and coated Millipore membrane are shown in Fig. 3a and b. The mean pore size of the membranes was 0.2 mm. Surface conductivity of the MWNTs coated membrane was measured by a four point probe using Van der Pauw method to measure the surface resistance of the conductors. The current vs.
potential curve is presented in Fig. 3c. The conductivity was calculated by the formula σ ¼1/Rs t; where t is the thickness of the MWCNTs layer on the support membrane and Rs is the sheet resistance of the sample. The thickness of the conducting layer was measured from the cross-section SEM image of the coated membrane. The conductivity was found to be 10 S cm 1. This value is low compared to the conductivity of metallic electrodes which is typically 105–106 S cm 1 [29]. The printed silver electrodes improve the conductivity and the distribution of charge on the modified membrane during electrolysis. Calcium carbonate suspension as a typical inorganic foulant was chosen for filtration studies. Fig. 4 shows the performance of MWCNTs coated membrane during filtration of aqueous CaCO3 suspension without the electrolysis cleaning. As is shown in this figure, normalized flux through the membrane decreased from 100% to 23.5 7 3% after a filtration time of 2.5 h (Fig. 4, red curve). Next, after filtration of the suspension for 30 min, using a fresh membrane, the electrolysis cleaning for 3 min was applied to the membrane and the normalized flux increased from 6373% to 797 3%. Due to efficient surface cleaning a sharp increase in normalized flux was observed after 2.5 h filtration of CaCO3 suspension: 62.6 7 3% (with the electrolysis cleaning) compared to 23.5 73% in control experiment (without the electrolysis cleaning) (Fig. 4).
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Fig. 4. Normalized flux versus filtration time during filtration of CaCO3 suspension through MWCNTs coated membrane: without electrolysis, and with 3 min electrolysis after filtration intervals of every 30, 40 and 60 min. Concentration of CaCO3 suspension is 100 mg /L. Concentration of NaCl is 10 g/L. Normalized flux was determined as Js/Jo where Js and Jo, are membrane fluxes during filtration of CaCO3 suspension and deionized water, respectively. (For interpretation of the references to color in this figure , the reader is referred to the web version of this article.)
Fig. 5. Normalized flux versus filtration time during filtration of yeast suspension through MWCNTs coated membrane: without electrolysis, and with 3 min electrolysis after filtration intervals of every 30, 40 and 60 min. Concentration of Yeast is 50 mg /L. Concentration of NaCl is 10 g/L. Normalized flux was determined as Js/Jo where Js and Jo, are membrane fluxes during filtration of yeast suspension and deionized water, respectively. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
To study the effect of duration of filtration cycle on the flux recovery, electrolysis cleaning was also performed after 40 and 60 min filtration intervals. When comparing filtration cycles of 30, 40 and 60 min, it can be seen that the shorter filtration interval has a higher flux recovery after electrolysis cleaning (Fig. 4). Obviously during a short filtration cycle, a looser fouling layer is deposited on the membrane surface and such a layer can more easily be removed from the membrane surface when compared with the denser fouling layers formed at prolonged filtration. For example, after filtration of CaCO3 suspension for a duration of 2 h using 30, 40 and 60 min filtration intervals followed by electrolysis cleaning, the total normalized flux values were 63, 40 and 30%, respectively. As can be seen the flux values after cleaning are much higher when compared with normalized membrane flux (26%) after 2 h filtration without cleaning. The electrolysis cleaning allows restoration of the membrane flux, however, the recovery of the membrane flux does not exceed 80%. This is due to partial blocking of the membrane pores with CaCO3 particles. The self-cleaning efficiency of the MWCNTs coated membranes towards biofouling was evaluated during filtration of yeast suspension. Normalized flux at an operating pressure of 1 bar as a function of filtration time (without electrolysis) is shown in Fig. 5, red curve. It is seen that permeation flux decreased exponentially with time and found to be 157 3% after 2.5 h filtration. An 18% increase in the permeation flux (from 49 to 67%) was observed after the first electrolysis cleaning as seen in Fig. 5, green curve. A subsequent increase in flux of 27, 20 and 15% (7 3%) was observed after 60, 90 and 120 min filtration respectively when 30 min filtration cycle was used (Fig. 5, a green curve). Similar flux changes were observed when the periodic electrolysis was performed after 40 or 60 min filtration intervals (Fig. 5, the violet and blue curves). For example, permeation flux increases of 16, 15, 14 and 13% (73) were observed after 40, 80, 120 and 160 min filtration time respectively when a 40 min filtration cycle was used (Fig. 5, a violet curve). The flux recovery for MWCNTs coated membranes after electrolytic cleaning was more pronounced when CaCO3 suspension was used as a feed stream compared with filtration of yeast suspension. This finding is due to different ways in which inorganic and biological materials accumulate and adhere to the membrane surface. Inorganic foulants (CaCO3) and biofoulants' intial attachment are mainly determined by the foulants' ability to adhere to the membrane surface, influenced by hydrophobic interactions, hydrogen bonding, London-van der Waals attractions,
and electrostatic interactions [30]. However, biological attachment to surfaces is much more complex than that of inorganic colloids, with a greater range of functional groups available at the surface. Yeast can produce extracellular polysaccharide (EPS), which contributes to adhesion and fouling significantly. As has been previously demonstrated, biofoulants typically have greater attachment forces with surfaces, and yeast has been observed to have an increased adhesion strength after accumulation at the surface for more than several minutes [31]. As a result it is harder to clean the membrane fouled with yeast cells due to their strong adhesion to the membrane surface. The cleaning mechanism during the electrolysis process is caused by the formation of micro-bubbles at the surface of MWCNTs coated membranes. During electrolysis of aqueous NaCl solutions hydrogen gas is produced at the cathode (the conductive membrane) and chlorine gas is produced at the anode (a stainless steel electrode, see a design of membrane cleaning set-up) according to the following equations: 2H2O þ2e -2OH þ H2 2Cl -Cl2 þ 2e The produced hydrogen microbubbles push deposited foulants material out from the membrane surface into the feed stream. Optical images of neat MWCNTs coated membranes, fouled membranes after filtration of CaCO3 and yeast suspension, and the membrane samples after electrolysis cleaning are shown in Fig. 6a–e. As seen due to the efficient cleaning, the membrane images after electrolysis are similar to the image of the initial membrane. The current versus time curve during the electrolysis process is presented in Fig. 6f. It was found that the current increases and reaches a plateau with increased electrolysis time. A low current at the beginning of the process is obviously explained by a high coverage of the membrane surface with the foulant layer. The current increase with electrolysis time and reaches a quazi-steady state after 1–2 min due to the membrane surface cleaning.
4. Conclusions A membrane filtration unit was combined with an electrochemical system. In order for the electrochemical system to operate, the membrane surface which was used as a cathode needed to be
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Fig. 6. Optical images of the (a) neat MWCNTs coated membrane, (b) the fouled membrane after filtration of CaCO3 suspension for 30 min, (c) the membrane after electrolysis cleaning, (d) the fouled membrane after filtration of yeast suspension for 30 min, (e) the membrane after electrolysis cleaning and (f) typical current vs. time curve during electrolysis at constant potential 2 V.
conductive and electrochemically active. Electrically conductive MWCNTs coated polymer membranes have been fabricated using a robust vacuum filtration technique. By operating the electrochemical system periodically, a novel process of membrane cleaning based onelectrolysis with the MWCNTs coated polymer membranes has been developed. The efficiency of the cleaning procedure in the membrane flux recovery has been proven with typical bio- and inorganic membrane foulants such as yeast suspensions and CaCO3. The cleaning mechanism during the electrolysis process is explained by the formation of gas micro-bubbles at the membrane surface which remove the deposited foulants from the membrane into the feed stream. The proposed cleaning method could provide a non-destructive, continuous and renewable approach for the mitigation of the different types of membrane fouling directly in situ in membrane elements.
References [1] R.W. Baker, Membrane Technology and Applications, John Wiley & Sons, Ltd, West Sussex, England, 2004. [2] K. Scot, R. Hughes, Industrial Membrane Separation Technology, Blackie Academics & Professionals, London, 1996. [3] M. Al-Ahmad, F.A. Abdul Aleem, A. Mutiri, A. Ubaisy, Biofuoling in RO membrane systems part 1: fundamentals and control, Desalination 132 (2000) 173–179. [4] J.L. Nilsson, Protein fouling of uf membranes: causes and consequences, Journal of Membrane Science 52 (1990) 121–142. [5] M. Goosen, S. Sablani, H. Al-Hinai, S. Al‐Obeidani, R. Al‐Belushi, D. Jackson, Fouling of reverse osmosis and ultrafiltration membranes: a critical review, Separation Science and Technology 39 (2005) 2261–2297. [6] M. Taniguchi, J.E. Kilduff, G. Belfort, Modes of natural organic matter fouling during ultrafiltration, Environmental Science & Technology 37 (2003) 1676–1683. [7] H.S. Vrouwenvelder, J.A.M. van Paassen, H.C. Folmer, J.A.M.H. Hofman, M.M. Nederlof, D. van der Kooij, Biofouling of membranes for drinking water production, Desalination 118 (1998) 157–166. [8] I.C. Escobar, E.M. Hoek, C.J. Gabelich, F.A. DiGiano, Y.A.L. Goullec, P. Berube, K.J. Howe, Committee report:recent advances and research needs in membrane fouling, Journal of American Water Works Assocciation 97 (2005) 79–89.
[9] V. Kochkodan, D.J. Johnson, N. Hilal, Polymeric membranes: Surface modification for minimizing (bio)colloidal fouling, Advances in Colloid and Interface Science 206 (2014) 116–140. [10] N. Hilal, O.O. Ogunbiyi, N.J. Miles, R. Nigmatullin, Methods employed for control of fouling in MF and UF membranes: a comprehensive review, Separation Science and Technology 40 (2005) 1957–2005. [11] A. Ahmad, N. Che Lah, S. Ismail, B. Ooi, Membrane antifouling methods and alternatives: ultrasound approach, Separation & Purification Reviews 41 (2012) 318–346. [12] K. Majamaa, J.E. Johnson, U. Bertheas, Three steps to control biofouling in reverse osmosis systems, Desalination and Water Treatment 42 (2012) 107–116. [13] A. Simon, J.A. McDonald, S.J. Khan, W.E. Price, L.D. Nghiem, Effects of caustic cleaning on pore size of nanofiltration membranes and their rejection of trace organic chemicals, Journal of Membrane Science 447 (2013) 153–162. [14] V.M. Riabkov, V.L. Steblianko, Electrolytic process for cleaning and coating electrically conducting surfaces, in, Google Patents, 1997. [15] G.H. Markx, D.B. Kell, Dielectric spectroscopy as a tool for the measurement of the formation of biofilms and of their removal by electrolytic cleaning pulses and biocides, Biofouling 2 (1990) 211–227. [16] O. Sumita, Electrolytic cell for producing charged anode water suitable for surface cleaning or treatment, and method for producing the same and use of the same, in, Google Patents, 2006. [17] A.I. Bryan, M.R. Cushman, Device for in situ cleaning a fouled sensor membrane of deposits, in, Google Patents, 1992. [18] J. Mansouri, R. Burford, Novel membranes from conducting polymers, Journal of Membrane Science 87 (1994) 23–34. [19] I.-H. Loh, R.A. Moody, J. Huang, Electrically conductive membranes: synthesis and applications, Journal of Membrane Science 50 (1990) 31–49. [20] R.D. McCullough, The chemistry of conducting polythiophenes, Advanced Materials 10 (1998) 93–116. [21] M.F. De Volder, S.H. Tawfick, R.H. Baughman, A.J. Hart, Carbon nanotubes: present and future commercial applications, Science 339 (2013) 535–539. [22] Y. Li, W. Zhou, H. Wang, L. Xie, Y. Liang, F. Wei, J.-C. Idrobo, S.J. Pennycook, H. Dai, An oxygen reduction electrocatalyst based on carbon nanotubegraphene complexes, Nature Nanotechnology 7 (2012) 394–400. [23] D. Vairavapandian, P. Vichchulada, M.D. Lay, Preparation and modification of carbon nanotubes: review of recent advances and applications in catalysis and sensing, Analytica Chimica Acta 626 (2008) 119–129. [24] H. He, A. Chen, H. Lv, H. Dong, M. Chang, C. Li, Fabrication of TiO o sub 42o / sub 4 film with different morphologies on Ni anode and application in photoassisted water electrolysis, Applied Surface Science, 266, 2013126–131. [25] C.D. Vecitis, M.H. Schnoor, M.S. Rahaman, J.D. Schiffman, M. Elimelech, Electrochemical multiwalled carbon nanotube filter for viral and bacterial removal and inactivation, Environmental Science & Technology 45 (2011) 3672–3679.
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[26] C.-F.o. de Lannoy, D. Jassby, K. Gloe, A.D. Gordon, M.R. Wiesner, Aquatic biofouling prevention by electrically charged nanocomposite polymer thin film membranes, Environmental Science & Technology 47 (2013) 2760–2768. [27] A.S. Brady-Estévez, S. Kang, M. Elimelech, A Single-Walled-Carbon-Nanotube, Filter for removal of viral and bacterial pathogens, Small 4 (2008) 481–484. [28] L.J.V.d. Pauw, A method of measuring specific resistivity and Hall effect of discs of arbitrary shape, Philips Research Reports 13 (1958) 1–9.
[29] R. Tucceri, A review about the surface resistance technique in electrochemistry, Surface Science Reports 56 (2004) 85–157. [30] J.H. Masliyah, S. Bhattacharjee, Electrokinetic and Colloid Transport Phenomena, John Wiley & Sons, 2006. [31] W.R. Bowen, N. Hilal, R.W. Lovitt, C.J. Wright, Direct measurement of the force of adhesion of a single biological cell using an atomic force microscope, Colloids and Surfaces A: Physicochemical and Engineering Aspects 136 (1998) 231–234.
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
A novel in situ membrane cleaning method using periodic electrolysis Raed Hashaikeh b,n, Boor Singh Lalia b, Victor Kochkodan a, Nidal Hilal a,nn a
Centre for Water Advanced Technologies and Environmental Research (CWATER), College of Engineering, Swansea University, Swansea SA2 8PP, UK Institute Center for Water Advanced Technology and Environmental Research (iWATER), Masdar Institute of Science and Technology, P.O. Box 54224, Abu Dhabi, United Arab Emirates
b
art ic l e i nf o
a b s t r a c t
Article history: Received 15 May 2014 Received in revised form 7 August 2014 Accepted 9 August 2014 Available online 19 August 2014
Membrane fouling is the major problem during the practical application of membrane separation processes in industry and water treatment. Therefore a search for novel efficient methods of membrane cleaning is currently of crucial importance for membrane-based technologies. The paper describes a new method of membrane cleaning, which is based on periodic electrolysis using a novel electrically conductive membrane to remove/prevent membrane fouling. The membrane consists of a thin electrically conductive layer of multi-walled carbon nanotubes (MWCNTs) deposited on the membrane's surface. The deposited MWCNTs allow the membrane to function as a cathode in an electrochemical system that includes the electrically conductive membrane, the salt water as an electrolyte and a stainless steel counter anode. The efficiency of the cleaning procedure in the flux recovery has been proved with typical bio- and inorganic membrane foulants such as CaCO3 and yeast suspensions. The cleaning mechanism during the electrolysis process is explained by the evolution of gases forming micro-bubbles at the membrane surface which remove the foulant material out from the membrane. The proposed method enables in situ membrane self-cleaning, thus providing a non-destructive, continuous and renewable approach for the mitigation of the different types of membrane fouling. & 2014 Elsevier B.V. All rights reserved.
Keywords: Membranes Electrolysis (Bio)fouling Self-cleaning MWCNT Desalination
1. Introduction During the last few decades, pressure-driven membrane processes such as reverse osmosis, nanofiltration, ultrafiltration and microfiltration have been widely used in water treatment, biotechnology, the food industry, medicine and other fields [1]. However, the main problem arising upon the operation of the membrane units is membrane fouling, which seriously hampers the application of membrane technologies [2]. Membrane fouling is an extremely complex phenomenon that has not yet been defined precisely. In general, the term is used to describe the undesirable deposition of retained particles, colloids, macromolecules, salts, etc., at the membrane surface or inside the pores. Depending on the membrane process and chemical/ biological nature of foulants, several types of fouling can occur in membrane systems, e.g. inorganic fouling or scaling, organic fouling colloidal fouling and biofouling [3–6]. Membrane fouling, as well as its prevention, has been a subject of many studies since the early 1960s when industrial membrane n
Corresponding author. Corresponding author. E-mail addresses: [email protected] (R. Hashaikeh), [email protected] (N. Hilal). nn
http://dx.doi.org/10.1016/j.memsci.2014.08.017 0376-7388/& 2014 Elsevier B.V. All rights reserved.
separation processes emerged. Membrane fouling can be partially controlled by the selection of an appropriate membrane, adjustment of the operating conditions in a membrane element, including hydrodynamics and operating pressure, and appropriate pretreatment of the feed solutions [7,8]. Unfortunately, very often these actions are not sufficient to cope with fouling. As fouling progresses, membrane flux declines; higher operating pressures and thus more energy must be expended to achieve the desired throughput. Usually, membrane cleaning is applied to remove the foulants and restore the membrane flux. A number of physical and chemical techniques have been used for membrane cleaning including backwashing, pulsing, forward flushing with air, sonification and chemical cleaning [9–12]. However, these operations fail to recover the membrane flux without deteriorating the membrane material and interrupt the water treatment process [13]. In many cases, the membrane elements must be replaced, which sharply increases the treatment costs [8]. Therefore the development of novel efficient methods of membrane cleaning at lower cost is of crucial importance for the wider use of membrane technologies in industry and water treatment. Electrolytic cleaning is a common method used for metal surface cleaning before galvanizing and electroplating [14]. Electrolytic cleaning is an electrolysis process in which a direct voltage
150
R. Hashaikeh et al. / Journal of Membrane Science 471 (2014) 149–154
is applied between an anode and a cathode dipped in an electrolyte that consists of free ions for conducting the electric current [15]. During the electrolysis process gases are produced and bubbled at the electrodes' surfaces. The generation and evolution of gases has been found to be effective in removing the tiniest remaining amounts of dirt from the surface of electrically conductive materials [14–17]. The application of this technique to pressure-driven membrane processes is hindered by the inherent insulating nature of polymeric membranes. Over the years, attempts have been made to use electrically conducting polymers to form membranes for different purposes [18,19]. However, traditional conducting polymers are notoriously difficult to process, and membranes made from these materials suffer from low selectivity, low flux, and often, low conductivity [19,20]. Recently, it has been reported that carbon nanotubes (CNTs) possess good electrical conductivity [21]. Because of their electrical properties CNTs have been proposed as functional electrodes in electrochemical systems such as: oxygen reduction in fuel cells; hydrogen production; photo-assisted water electrolysis etc [21–24]. Additionally it was shown that CNTs deposited on a porous support layer [25] or embedded into polymer membranes are capable of bacterial inactivation and bio-fouling prevention once an electrical bias is applied [26]. In this paper, we have studied the possibility of using electrolytic cleaning for polymer membranes modified with multi wall carbon nanotubes (MWCNTs) to develop a membrane cleaning process, which might be used in situ in membrane modules. The polymer membrane has been coated with a thin electrically conductive MWCNTs layer and acts as an electrode during the periodic electrolysis. The efficiency of membrane cleaning has been studied during membrane filtration of calcium carbonate and yeast suspensions, which are typical membrane foulants for membrane-based desalination processes.
2. Methods 2.1. Materials Multi-walled carbon nanotubes (MWCNTs) (Cheaptubes.com, USA) (outer diameter 13–18 nm, purity 4 99 wt%), microfiltration polyvilidefluoride Millipore membranes (GSWP, 0.22 mm), sodium lauryl sulfate (SDS, surfactant), sodium chloride (NaCl, Aldrich), yeast (Baker's yeast, DCL, France), calcium carbonate (CaCO3, Aldrich) were used as received.
2.2. MWCNTs coating on the membrane Millipore membranes were coated using vacuum filtration technique reported elsewhere [27]. 0.05 wt% MWCNTs powder was dispersed in an aqueous solution of 1 wt% SDS in water using probe sonicator (Hielscher, UP400S, 400 W, 24 kHz) at 50% amplitude and 0.5 cycle for 10 min to obtain a homogeneous ink-like suspension. Then 4 ml of MWCNTs suspension was filtered through the membrane with an active area of 9.34 cm2 using vacuum filtration at 30 kPa pressure. Thereafter the membrane sample was carefully washed three times with Milli-Q water to remove any excessive MWCNTs from the membrane surface. Optical images of MWCNTs coated microfiltration membranes were shown in Fig. 1(a–c). First the membranes were coated with MWCNTs and then silver electrodes were printed on the coated membrane to improve the electric charge distribution of the membranes (Fig. 1c). Thickness of the MWCNTs coating on the surface was measured from the cross-sectional SEM images and found to be 3–4 μm. After drying the MWCNTs coated membrane at room temperature, silver electrodes were coated on the membrane surface (Fig. 1c) to improve the electrical contacts and charge distribution on the surface of membrane. 2.3. Membrane characterization Scanning electron microscopy (FEI Quanta FEG 250) was used to image the surface and cross-section of the membrane in high vacuum. Prior to imaging, samples were first coated with a thin gold layer of 5–8 nm thickness. Optical images of the membrane surface were obtained by the optical microscope (Olympus-DP21). Surface resistivity of the MWNTs coated membrane was measured by using four point probe (LakeShore, USA) according to Van der Pauw method [28]. In this method, four electrodes were pasted on the membrane surface using silver paint dots. The electrodes were marked as 1–4 in clockwise position and current is passed through 1 and 2 electrodes and potential is measured between 3 and 4 electrodes. Four consecutive measurements were done by applying current between 2 and 3, 3 and 4, 4 and 1 and potential was measured between 4 and 1, 1 and 2 and 2 and 3 respectively. 2.4. A membrane cleaning setup A custom made cross flow filtration setup was used in this study. A schematic design of the filtration setup is shown in Fig. 2. MWCNTs coated surface acts as a negative electrode (cathode) in electrochemical system using stainless steel of diameter 15 mm as
Fig. 1. Optical images of (a) pristine Millipore membrane, (b) coated with MWCNTs using vacuum filtration, (c) MWCNTs coated membrane with silver electrodes.
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Fig. 2. A schematic design of the (a) filtration cell and (b) filtration setup used for the experiments.
Fig. 3. SEM images of (a) pristine Millipore membrane and (b) MWCNTs coated membrane and (c) I–V curves of MWCNTs coated membrane using four point probe measurement.
a positive electrode (anode). The electrolysis was performed by using Autolab 302N potentiostat/galvanostat at 2 V for 2–3 min. CaCO3 and yeast suspensions were used in filtration experiments as feed streams. The yeast suspension was prepared by immersion of 50 mg yeast in aqueous NaCl solution with concentration of 10 g/L. CaCO3 suspension was prepared by addition of 100 mg CaCO3 in 10 g/L aqueous NaCl solution. NaCl solution was used to assist the electrolysis process during cleaning of the fouled membrane. The suspensions were filtered through the MWCNTs coated membrane at operating pressure of 1 bar. The permeate was collected and weighted with time of filtration using a weighing balance. The suspension was filtered through the membranes during 30, 40 or 60 min filtration intervals and after each filtration interval the electrolysis process was conducted for 3 min followed by another filtration interval.
3. Results and discussion SEM images of the pristine and coated Millipore membrane are shown in Fig. 3a and b. The mean pore size of the membranes was 0.2 mm. Surface conductivity of the MWNTs coated membrane was measured by a four point probe using Van der Pauw method to measure the surface resistance of the conductors. The current vs.
potential curve is presented in Fig. 3c. The conductivity was calculated by the formula σ ¼1/Rs t; where t is the thickness of the MWCNTs layer on the support membrane and Rs is the sheet resistance of the sample. The thickness of the conducting layer was measured from the cross-section SEM image of the coated membrane. The conductivity was found to be 10 S cm 1. This value is low compared to the conductivity of metallic electrodes which is typically 105–106 S cm 1 [29]. The printed silver electrodes improve the conductivity and the distribution of charge on the modified membrane during electrolysis. Calcium carbonate suspension as a typical inorganic foulant was chosen for filtration studies. Fig. 4 shows the performance of MWCNTs coated membrane during filtration of aqueous CaCO3 suspension without the electrolysis cleaning. As is shown in this figure, normalized flux through the membrane decreased from 100% to 23.5 7 3% after a filtration time of 2.5 h (Fig. 4, red curve). Next, after filtration of the suspension for 30 min, using a fresh membrane, the electrolysis cleaning for 3 min was applied to the membrane and the normalized flux increased from 6373% to 797 3%. Due to efficient surface cleaning a sharp increase in normalized flux was observed after 2.5 h filtration of CaCO3 suspension: 62.6 7 3% (with the electrolysis cleaning) compared to 23.5 73% in control experiment (without the electrolysis cleaning) (Fig. 4).
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Fig. 4. Normalized flux versus filtration time during filtration of CaCO3 suspension through MWCNTs coated membrane: without electrolysis, and with 3 min electrolysis after filtration intervals of every 30, 40 and 60 min. Concentration of CaCO3 suspension is 100 mg /L. Concentration of NaCl is 10 g/L. Normalized flux was determined as Js/Jo where Js and Jo, are membrane fluxes during filtration of CaCO3 suspension and deionized water, respectively. (For interpretation of the references to color in this figure , the reader is referred to the web version of this article.)
Fig. 5. Normalized flux versus filtration time during filtration of yeast suspension through MWCNTs coated membrane: without electrolysis, and with 3 min electrolysis after filtration intervals of every 30, 40 and 60 min. Concentration of Yeast is 50 mg /L. Concentration of NaCl is 10 g/L. Normalized flux was determined as Js/Jo where Js and Jo, are membrane fluxes during filtration of yeast suspension and deionized water, respectively. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
To study the effect of duration of filtration cycle on the flux recovery, electrolysis cleaning was also performed after 40 and 60 min filtration intervals. When comparing filtration cycles of 30, 40 and 60 min, it can be seen that the shorter filtration interval has a higher flux recovery after electrolysis cleaning (Fig. 4). Obviously during a short filtration cycle, a looser fouling layer is deposited on the membrane surface and such a layer can more easily be removed from the membrane surface when compared with the denser fouling layers formed at prolonged filtration. For example, after filtration of CaCO3 suspension for a duration of 2 h using 30, 40 and 60 min filtration intervals followed by electrolysis cleaning, the total normalized flux values were 63, 40 and 30%, respectively. As can be seen the flux values after cleaning are much higher when compared with normalized membrane flux (26%) after 2 h filtration without cleaning. The electrolysis cleaning allows restoration of the membrane flux, however, the recovery of the membrane flux does not exceed 80%. This is due to partial blocking of the membrane pores with CaCO3 particles. The self-cleaning efficiency of the MWCNTs coated membranes towards biofouling was evaluated during filtration of yeast suspension. Normalized flux at an operating pressure of 1 bar as a function of filtration time (without electrolysis) is shown in Fig. 5, red curve. It is seen that permeation flux decreased exponentially with time and found to be 157 3% after 2.5 h filtration. An 18% increase in the permeation flux (from 49 to 67%) was observed after the first electrolysis cleaning as seen in Fig. 5, green curve. A subsequent increase in flux of 27, 20 and 15% (7 3%) was observed after 60, 90 and 120 min filtration respectively when 30 min filtration cycle was used (Fig. 5, a green curve). Similar flux changes were observed when the periodic electrolysis was performed after 40 or 60 min filtration intervals (Fig. 5, the violet and blue curves). For example, permeation flux increases of 16, 15, 14 and 13% (73) were observed after 40, 80, 120 and 160 min filtration time respectively when a 40 min filtration cycle was used (Fig. 5, a violet curve). The flux recovery for MWCNTs coated membranes after electrolytic cleaning was more pronounced when CaCO3 suspension was used as a feed stream compared with filtration of yeast suspension. This finding is due to different ways in which inorganic and biological materials accumulate and adhere to the membrane surface. Inorganic foulants (CaCO3) and biofoulants' intial attachment are mainly determined by the foulants' ability to adhere to the membrane surface, influenced by hydrophobic interactions, hydrogen bonding, London-van der Waals attractions,
and electrostatic interactions [30]. However, biological attachment to surfaces is much more complex than that of inorganic colloids, with a greater range of functional groups available at the surface. Yeast can produce extracellular polysaccharide (EPS), which contributes to adhesion and fouling significantly. As has been previously demonstrated, biofoulants typically have greater attachment forces with surfaces, and yeast has been observed to have an increased adhesion strength after accumulation at the surface for more than several minutes [31]. As a result it is harder to clean the membrane fouled with yeast cells due to their strong adhesion to the membrane surface. The cleaning mechanism during the electrolysis process is caused by the formation of micro-bubbles at the surface of MWCNTs coated membranes. During electrolysis of aqueous NaCl solutions hydrogen gas is produced at the cathode (the conductive membrane) and chlorine gas is produced at the anode (a stainless steel electrode, see a design of membrane cleaning set-up) according to the following equations: 2H2O þ2e -2OH þ H2 2Cl -Cl2 þ 2e The produced hydrogen microbubbles push deposited foulants material out from the membrane surface into the feed stream. Optical images of neat MWCNTs coated membranes, fouled membranes after filtration of CaCO3 and yeast suspension, and the membrane samples after electrolysis cleaning are shown in Fig. 6a–e. As seen due to the efficient cleaning, the membrane images after electrolysis are similar to the image of the initial membrane. The current versus time curve during the electrolysis process is presented in Fig. 6f. It was found that the current increases and reaches a plateau with increased electrolysis time. A low current at the beginning of the process is obviously explained by a high coverage of the membrane surface with the foulant layer. The current increase with electrolysis time and reaches a quazi-steady state after 1–2 min due to the membrane surface cleaning.
4. Conclusions A membrane filtration unit was combined with an electrochemical system. In order for the electrochemical system to operate, the membrane surface which was used as a cathode needed to be
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Fig. 6. Optical images of the (a) neat MWCNTs coated membrane, (b) the fouled membrane after filtration of CaCO3 suspension for 30 min, (c) the membrane after electrolysis cleaning, (d) the fouled membrane after filtration of yeast suspension for 30 min, (e) the membrane after electrolysis cleaning and (f) typical current vs. time curve during electrolysis at constant potential 2 V.
conductive and electrochemically active. Electrically conductive MWCNTs coated polymer membranes have been fabricated using a robust vacuum filtration technique. By operating the electrochemical system periodically, a novel process of membrane cleaning based onelectrolysis with the MWCNTs coated polymer membranes has been developed. The efficiency of the cleaning procedure in the membrane flux recovery has been proven with typical bio- and inorganic membrane foulants such as yeast suspensions and CaCO3. The cleaning mechanism during the electrolysis process is explained by the formation of gas micro-bubbles at the membrane surface which remove the deposited foulants from the membrane into the feed stream. The proposed cleaning method could provide a non-destructive, continuous and renewable approach for the mitigation of the different types of membrane fouling directly in situ in membrane elements.
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