Theory – Contact angle
Hydrophilic membranes are known to suffer less from membrane fouling in water treatment in general. It is attributed to the hydration of membrane surface by water molecules, which prevents foulant from direct contact with membrane surface. In fact, hydrophilic membranes have functional groups that provide high polarity to the surface that attract polar molecules such as water.
On the contrary, there is little electrostatic interaction between hydrophobic membranes and water molecules since the weak van der Waals force alone is not enough to hold water molecules on membrane surface. As a result, there is no physical barrier for macromolecules for being adsorbed on membrane surface (Liu, 2011).
Contact angle is the most commonly used parameter to indicate membrane’s hydrophilicity or hydrophobicity, where the higher the contact angle is, the higher the hydrophobicity is. The contact angle must be less than 90o to allow spontaneous water intrusion to the pore without extra pressure as shown in Fig. 1a. (Note: contact angle can be defined as the angle in the opposite side of q depending on literature) In theory, if the contact angle is higher than 90o, extra pressure is required to initiate permeation.
In practical situation, however, actual membrane surface is neither perfectly smooth nor uniform in molecular level. As results, it may take some time to wet all the pores even if the contact angle is less than 90o. At the same time, even if contact angle is higher than 90o, pores will be wet eventually as consequence of 1) defects on membrane surface, 2) water vapour condensation in pore, 3) fouling of pore entrance by macromolecules or inorganic precipitates, etc.
Fig. 1. The effect of equilibrium contact angle, θ, on the pore wetting phenomenon.
Contact angle of virgin membrane is basically decided by the building material of the membrane, but it can widely vary depending on chemical additives, solvents, manufacturing method, process condition, surface roughness, etc. For instance, pure PVDF has a natural contact angle of around 89o at smooth surface as shown in Table 1, but contact angle can be raised to even 148o if membrane is made using alcohol as coagulant via wet immersion method (Kuo, 2008). In addition, measuring contact angle of MF/UF membranes is not easy in some cases, where water drop sips through relatively large pores in short period of time due to the voluntary pore wetting. Surface roughness and the pore size also affect the accuracy of the contact angle measurement. As a result, contact angles found from literature vary quite significantly depending on the experimental condition employed to obtain the values.
Contact angle also changes by oxidative membrane cleanings using NaOCl, H2O2, etc. (Levisky, 2011 ). When PVDF membranes were cleaned with NaOCl, contact angle decreased from 92.2o to 75o-80o depending on the exposure level to chlorine. Contact angle decrease was more significant with PES (polyethersulfone) membrane of which contact angle decreased from 100.6o to 63o-76o.
Table 1. Contact angle of various materials
|Pure Material||Contact angle (approximate)|
|Polyvinyl alcohol (PVA)||60|
Note: There are two definitions of contact angle depending on which side of the angle is measured. If the contact angle in one definition is θ1 and the other is θ2, these two are in the relation of θ1= 180o – θ2.
All the MF/UF membranes used for bulk water treatment in commercial scale have hydrphilic surfaces that allow spontaneous pore wetting. It is not only due to the concerns over membrane fouling, but also due to the impracticality of wetting membranes prior to using them on site.
Liquid entry pressure (LEP), which is the lowest pressure that triggers water passage through hydrophobic membrane, can be calculated using Eq. (1).
LEP : Liquid entry pressure (Pa)
B = capillary constant ( 0-1)
d = defect diameter ( m )
γ = surface tension at the air-liquid interface ( 0.0728 N/m at 20 oC )
θ = liquid-membrane contact angle (radian)
Effect of contact angle on membrane fouling
It has been widely perceived that hydrophilic membranes are less susceptible to initial organic fouling than hydrophobic counterparts in water treatment. It was attributed to the lower hydrophobic-hydrophobic interaction between membrane and naturally occurring organic materials (Kabsch-Korbutowicz, 1999; Ho, 2006; Kang, 2006). When the critical flux of two homemade PVDF membranes with identical pore size (0.1) and porosity (15%) but different contact angles (77o vs 101o) were compared under MBR conditions, the hydrophilic membrane with lower contact angle resulted almost 100% higher critical flux than the hydrophobic one. It was postulated that more macromolecules were adsorbed on and inside of the hydrophobic membrane (van der Marel, 2010). On the contrary, the results presented by other researchers have shown that the surface hydrophilicity alone was not necessarily correlated with membrane fouling propensity (Knoell, 1999;Vrijenhoek, 2001; Hobbs, 2006).
It is very hard to explain how such discrepancy occurred without knowing the exact experimental conditions, but it appears that the contact angle (or hydrophilic) effects are observed mostly in lab scale experiment. In fact, in the experiment mentioned above (van der Marel, 2010), a membrane with high contact angle (101o) that required artificial pore wetting was used as a hydrophobic membrane. However, no known commercial MF/UF membranes used for water treatment have such hydrophobic surface as shown in Table 2. The most hydrophobic commercial membrane has a contact angle of 82o. Meanwhile the contact angles of twenty commonly used commercial RO/NF membranes range 38o-73o (Norberg, 2006). In any case, using hydrophobic membranes with a contact angle larger than 90o is not practical in large scale process since initial pore wetting incurs enormous burden for both capital and operational costs. Therefore, contact angle effect on membrane performance is not clearly visible in practical situation.
In one lab study performed with yeast cells as a foulant, contact angle effect on flux was apparent in the very beginning of the filtration cycle until monolayer of yeast cells was formed (Kang, 2006). In fact, it is easy to imagine the interaction between yeast cells and bare membrane surface is a crucial factor affecting the first mono-layer formation on membrane. However, once the mono-layer is formed, the interaction between free foulants and the mono-layer is no longer affected by the membrane’s hydrophilicity. Therefore, the surface chemistry of the membrane is completely masked by the multi-layers, thus the benefits of hydrophilicity becomes obscure during the long-term filtration (Le-Clech, 2006).
It has been also reported anecdotally that hydrophilic membranes are easier to be cleaned. If cleanings were performed at mild conditions using water jet and base/acid, foulants detachment might be easier from hydrophilic surfaces due to the weaker hydrophobic interaction between the foulants and membrane. This phenomenon might be observed commonly in industrial membrane processes, but there is no enough supporting data as far as MBR in concerned. In fact, membrane cleaning is typically performed under very harsh conditions using strong oxidants such as 1,000 – 5,000 mg/L of NaOCl so that most organic molecules adsorbed on membrane surface are oxidized regardless of their affinity on membrane surface.
Table 2. Contact angle of commercial membranes
|Chlorinated PE (MF)||Kubota||Not Measurable1)|
|Regenerated cellulose (UF)||Microdyne||54.83)|
|Aromatic polyaramide (UF)||Microdyne||66.23)|
|Polyester, PETE (MF)||Osmonics||664)|
|Polycarbonate, PCTE (MF)||Osmonics||664)|
|MCE (MF, GSWP)||Millipore||495)|
© Seong Hoon Yoon