Previous   studies   of   membrane   distillation (MD) have shown that superhydrophobic membranes experience  dramatically  less  inorganic  and  particulate  fouling.  However,  little  explanation  for  this improved  performance  has  been  given  in  the  literature.  Furthermore,  studies  comparing  membrane superhydrophobicity  and  biofouling  are  lacking,  though  superhydrophobic  surfaces  are  known  to  be more  vulnerable  to  biofouling  than  other  types. In non-membrane  surfaces,  visible  air  layers  on superhydrophobic  surfaces  have  been  correlated  with
significant  decreases  in  biofouling.  Therefore,  it was  proposed  here  to  use  superhydrophobic  MD  membranes  with  periodic  introduction  of  air  to maintain  an  air  layer  on  the  membrane  surface. Superhydrophobic  membranes  were  created  with initiated chemical vapor deposition (iCVD) of a fluorinated compound, perfluorodecyl acrylate (PFDA). The  substrate  membrane  was  PVDF.    To  test  MD  fouling,  an  MD  membrane  was  placed  on  top  of  a fouling solution, with a heater and stirrer to caus
e evaporation of water through the membrane. Results were analyzed with foulant mass measurements.  Alginate gel fouling was examined, as this compound is  a  common  proxy  for  biological  fouling  in  ocean  w
ater. The  introduction  of  air  layers  was  found  to dramatically  decrease  foulant  adhesion  to  the  membrane,  by  95-97%. Membrane superhydrophobicity made a much smaller impact in reducing fouling. 
Keywords 
 membrane distillation, superhydrophobic surfaces, alginate, air layers, anti-foulin

Chung, Hyung Won

Gonzalez, Jocelyn V.

Lienhard, John H.

Servi, Amelia T

Swaminathan, Jaichander

Van Belleghem, Sarah M.

Warsinger, David Elan Martin

DSpace@MIT

1                                     2015 © American Water Works Association  AWWA ACE15 Conference Proceedings  All Rights Reserved The Combined Effect of Air Layers and Membrane Superhydrophobicity on Biofouling in Membrane Distillation  David E. M. Warsingera, Jocelyn Gonzalez a, Sarah Van Belleghem a, Amelia Servia, Jaichander Swaminathana, Hyung Won Chunga, John H. Lienhard Va* a   Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge MA 02139-4307 USA *    Corresponding Author: lienhard@mit.edu  Previous studies of membrane distillation (MD) have shown that superhydrophobic membranes experience dramatically less inorganic and particulate fouling. However, little explanation for this improved performance has been given in the literature. Furthermore, studies comparing membrane superhydrophobicity and biofouling are lacking, though superhydrophobic surfaces are known to be more vulnerable to biofouling than other types.  In non-membrane surfaces, visible air layers on superhydrophobic surfaces have been correlated with significant decreases in biofouling.  Therefore, it was proposed here to use superhydrophobic MD membranes with periodic introduction of air to maintain an air layer on the membrane surface.  Superhydrophobic membranes were created with initiated chemical vapor deposition (iCVD) of a fluorinated compound, perfluorodecyl acrylate (PFDA).  The substrate membrane was PVDF.  To test MD fouling, an MD membrane was placed on top of a fouling solution, with a heater and stirrer to cause evaporation of water through the membrane. Results were analyzed with foulant mass measurements.  Alginate gel fouling was examined, as this compound is a common proxy for biological fouling in ocean water.  The introduction of air layers was found to dramatically decrease foulant adhesion to the membrane, by 95-97%.  Membrane superhydrophobicity made a much smaller impact in reducing fouling.  Keywords  membrane distillation, superhydrophobic surfaces, alginate, air layers, anti-fouling                                      2015 © American Water Works Association  AWWA ACE15 Conference Proceedings  All Rights Reserved 2Nomenclature  α  foulant sticking efficiency [-] δc  fouling layer average thickness [m]  contact angle [°] ω stirrer rotation rate [rpm] A  experimental pre-exponential factor Am membrane area [cm2] CX salt concentration [g/mL]  ṁp condensate flux [kg/hr] N  number of particles per unit volume [mol/L]  	 induction time [s] T temperature [K] p permeate velocity [m/s]                                          2015 © American Water Works Association  AWWA ACE15 Conference Proceedings  All Rights Reserved 31. Introduction 1.1 Fouling in Membrane Distillation Membrane distillation desalination uses a porous hydrophobic membrane that passes water vapor while rejecting liquid water [1]. Studies have found MD to be more resistant to fouling than other membrane-based desalination processes, such as reverse osmosis (RO) [2, 3]. The mechanism for this resistance, however, is poorly understood [4]. Fouling on the membrane blocks the surface, which reduces permeate flux and therefore hinders MD performance [5]. In past studies, superhydrophobic MD membranes have been shown to be resistant to inorganic scaling [6, 7] and particulate fouling [8]. Past studies on MD have also found that nucleation in the bulk feed fluid contributes significantly to MD fouling [9].   Superhydrophobic surfaces submerged in seawater with air layers have exhibited minimal biofouling compared to other surfaces [10]. In seawater applications, the remains of algae often cause biological fouling. The anionic polysaccharide alginate is a common model for studying algal fouling [11]. Alginate forms a viscous gummy gel on membrane surfaces, significantly impairing mass transport [12, 13]. In the present study, alginate foulants were tested in a beaker-based MD setup with the introduction of air bubbles saturated with water vapor via syringe. This method was applied to either regular hydrophobic or coated superhydrophobic MD membranes. The mass of foulant deposited was measured to determine the effectiveness of air recharging for fouling reduction on hydrophobic and superhydrophobic MD membranes.         1.2 Fouling Deposition Theory  The deposition of particles on a membrane measured as the change in the fouling layer thickness can be modeled as a function of the particles convected towards the surface.  This model has been previously used for RO membranes [14]: =    (1.1)                                      2015 © American Water Works Association  AWWA ACE15 Conference Proceedings  All Rights Reserved 4where t is time elapsed, δc is the fouling layer average thickness, α is the “foulant sticking efficiency,”   is the salt mass concentration, and p is the permeate velocity passing through the membrane. An air layer on the feed side of the membrane can block parts of the membrane surface from foulants and increase effective hydrophobicity, thus decreasing α, the foulant sticking efficiency 1.3 Alginate Fouling Alginate is an anionic polysaccharide, and a major component of the cell walls of algae found in seawater. When exposed to sufficient concentrations of calcium, the calcium replaces the sodium in the alginate. This causes it to form crosslinks, since the +2 charge on calcium ions can form 2 bonds.  The cross-linked long molecule chains turn the solution into a gel [15].   Figure 1. Alginate gel formation in the presence of calcium ions, from [16]                                      2015 © American Water Works Association  AWWA ACE15 Conference Proceedings  All Rights Reserved 5Superhydrophobic surfaces such as those in membrane distillation are known to be more fouling resistant to salts, but more vulnerable to biological fouling [17].  Therefore, improving resistance to biological fouling on these surfaces is a major research goal.  2. Methodology 2.1  Static Membrane Distillation Setup A “static” MD membrane apparatus was used to examine the periodic introduction of air layers to reduce fouling. The membrane divides a hot alginate solution from ambient air.  This feed side is designed to have typical MD conditions, while allowing for quick tests and precise weight measurement.  In a static MD setup as seen in Figure 2, a membrane floats on the surface of foulant-containing water.  The membrane is flush with the edges of the plastic beaker to avoid water getting on top of the membrane and direct evaporation. Insulation minimized heat loss and excess heating of the air. In this study, fouling was measured by weighing the membrane before and after the study.  The entire apparatus was placed within a fume hood to maintain consistent operating conditions (Table 2). The system was monitored with a thermocouple temperature probe and humidity meter.                                       2015 © American Water Works Association  AWWA ACE15 Conference Proceedings  All Rights Reserved 6 Figure 2.  ”Static MD” experimental setup of evaporation through an MD membrane for biofouling    Figure 3. Air was introduced to the system via syringe containing hot saturated air.  For air recharging with a syringe as seen in Figure 3, saturated air at room temperature was injected into the system every 5 minutes.  Each time 5 mL of air was added over ~10 seconds (Table 1), for 4 different trials. The addition of air caused the membrane to lift off the surface.   Table 1. Experimental conditions for air recharging of MD membranes Foulant weight [%wt] Air Injection Frequency [min] Test A Test B Test C Test D 0.04% Alginate  0.029% CaCl2 10 hydrophobic, control hydrophobic, air introduction super-hydrophobic, control super-hydrophobic, air introduction FanMD MembraneFoulant SolutionStirrerHot PlateInsulationSyringe injecting air bubbles                                     2015 © American Water Works Association  AWWA ACE15 Conference Proceedings  All Rights Reserved 7 In seawater desalination including reverse osmosis, alginate gel layers significantly contribute to creating a biofilm on membrane surfaces.  The system was tested with of alginate sufficient to produce a fouling gel layer.  The alginate was procured from Sigma-Aldrich. All cases used the same PVDF membrane: the superhydrophobic trials coated this membrane with PPFDA. Table 2. Experimental Variable Values.   Variables Symbol Values Uncertainty temperature Tf,in 60°C ±2°C humidity RH 31% ±4% condensate flux ṁp 5 LMH ±0.4 LMH stirrer rotation ω 60 rpm ±1 rpm 2.2 Superhydrophobic Membrane Preparation and Testing  The MD membranes used are polyvinylidene fluoride (PVDF) membranes produced by Millipore.  They have a 0.2 μm pore size, and a part number of ISEQ 000 10.  These membranes were made superhydrophobic with a coating by initiated chemical vapor deposition (iCVD) of poly-(1H,1H,2H,2H-perfluorodecyl acrylate) (PPFDA) [18, 19, 20]. The superhydrophobic membranes were prepared in a reactor using a process described previously [21]. For the coating, a PFDA monomer (97% Sigma-Aldrich) and t-butyl peroxide initiator (TBPO) (98% Sigma-Aldrich) were used. The monomer and initiator were added into the chamber at rates of 0.03 and 1.0 cm3/min (sccm), respectively. The PPFDA film was 200 nm thick.   The contact angles were measured with a goniometer (model 590, Ramé-Hart) and images were acquired with the software DropImage.  An automatic dispenser increased or decreased the drop volume to measure advancing and receding contact angles.   Scanning electron microscope (SEM) images with a JEOL 6010a machine verified the conformity of the superhydrophobic coating.                                       2015 © American Water Works Association  AWWA ACE15 Conference Proceedings  All Rights Reserved 83. Results and Discussion 3.1 Fabricated Superhydrophobic Membrane Properties The PPFDA significantly improved the contact angles of the membranes, as seen in Table 3. Notably, receding contact angles increased the most, by 78°. This result aligns with the literature, where receding contact angles are small for mildly superhydrophobic surfaces, due to pinning of water to the surface.  PPFDA provided superior contact angles because it contains more fluorine than PVDF. Additionally, Sidechains also form structures that prevent the fluorine atoms from oriented away from water molecules. The surface roughness overall plays a significant role in increasing surface superhydrophobicity. SEM confirmed that the coating had only a minimal effect on the surface roughness, allowing for an accurate comparison between the membranes.   Table 3. Summary of MD Membrane Contact Angles Used In This Study.  Membrane Static contact angle (°) Superhydrophobic  157 Hydrophobic 125    Figure 4.  Membrane SEM. The PPFDA coating from iCVD has a minimal effect on the membrane surface structure and does not clog pores                                      2015 © American Water Works Association  AWWA ACE15 Conference Proceedings  All Rights Reserved 9 Figure 5.  Photograph of submerged superhydrophobic MD membrane. The membrane appears reflective due to the air layer on the membrane surface.                                        2015 © American Water Works Association  AWWA ACE15 Conference Proceedings  All Rights Reserved 10 3.2 Fouling Results The mass of alginate adhered to the membrane was used to quantify the extent of fouling, as seen in Figure 6.   Figure 6.  Weight of Salt Adhered to MD Membrane, 0.04% Alginate, CaCl2. Alginate fouling was found to have been reduced with the introduction of air on both hydrophobic (“H”) and superhydrophobic (“Super H”) membrane surfaces.  Maintaining air layers dramatically and consistently reduced fouling of alginate on the membrane surface. The gel layers form rapidly when the concentration of calcium is sufficient. Therefore it is not expected that the air affects the formation of gel itself.  Instead, the air acts to significantly reduce the adhesion of the gel to the membrane.   It is worth mentioning that no other fouling occurred here, since the salt added, CaCl2, is extremely soluble.  Notably, biological fouling can reduce membrane hydrophobicity and increase surface energy, 01234567H,noneH,airSuper H,noneSuper H,airAlginate Adhered [mg]                                     2015 © American Water Works Association  AWWA ACE15 Conference Proceedings  All Rights Reserved 11 so avoiding biological scale may also help reduce break through and reduce fouling of salts on the membrane. 4. Conclusion The sustained presence of air layers was proven to be extremely effective in reducing biofouling. The air layers reduced the mass of alginate adhered by 95-97%. The introduction of air layers impairs the adhesion of the alginate gel to the membrane surface, as gel is still present in the bulk solution.  The deposition of the PPFDA coating via iCVD effectively created a very hydrophobic membrane, with a static contact angle of 157°.  The superhydrophobic membrane without air recharging had only slightly less alginate fouling than the uncoated membrane without air recharging. This differs from results for salt precipitation scaling, which have shown superhydrophobic membranes to significantly reduce scaling. Notably, membrane distillation membranes are known to be resistant to inorganic fouling but vulnerable to biofouling, so superhydrophobicity and air layers may be extremely valuable in reducing fouling on MD membranes.     5. ACKNOWLEDGEMENT This work was funded by the Cooperative Agreement between the Masdar Institute of Science and Technology (Masdar University), Abu Dhabi, UAE and the Massachusetts Institute of Technology (MIT), Cambridge, MA, USA, Reference No. 02/MI/MI/CP/11/07633/GEN/G/00. This work made use of the Cornell Center for Materials Research Shared Facilities which are supported through the NSF MRSEC program (DMR-1120296). The authors would like to thank Allen Myerson, You Peng, Emily Tow, McCall Huston, and Grace Connors for their contributions to this work.                                        2015 © American Water Works Association  AWWA ACE15 Conference Proceedings  All Rights Reserved 12 6. References [1] D. Warsinger, J. Swaminathan, L. Maswadeh, and J. H. Lienhard V, “Superhydrophobic condenser surfaces for air gap membrane distillation,” Journal of Membrane Science, vol. 492, pp. 578–587, 2015. [2] D. M. Warsinger, J. Swaminathan, E. Guillen-Burrieza, H. A. Arafat, and J. H. 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Lienhard V, Combining air recharging and membrane superhydrophobicity for fouling prevention in membrane distillation, Journal of Membrane Science, vol. 505, pp. 241–252, 2016  

The Combined Effect of Air Layers and Membrane Superhydrophobicity on Biofouling in Membrane Distillation

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The Combined Effect of Air Layers and Membrane Superhydrophobicity on Biofouling in Membrane Distillation

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