Increase in hydrophilicity of polysulfone membrane using polyethylene glycol methyl ether
4.3. Results and discussion
4.3.4. Ultrafiltration of BSA
Apart from transmembrane pressure, the rejection and flux characteristics of the membranes strongly depend on the structure of the membrane as well as the properties of the feed solution. So, the prepared UF membranes were also characterized by estimating rejection and flux during permeation experiment using BSA solution. To investigate the fouling behaviour, membranes were cleaned after BSA solution ultrafiltration, and the PWF of the cleaned membranes was measured. Figure 4.9 shows the time-dependent flux of membrane during ultrafiltration. It may be seen that the PWF (J ) before UF of the BSA solution changes
marginally, but it decreased drastically at the initial operation of BSA solution ultrafiltration.
It happened due to the deposition or adsorption of protein molecules on membrane surface at the initial BSA ultrafiltration operation [118, 120, 121]. After some time of operation it reaches equilibrium, so that a relatively steady flux (Jp) was obtained in the final operation of BSA ultrafiltration.
Figure 4.9: Effect of molecular weight of PEGME on time dependent flux of membrane during ultrafiltration.
4.3.4.1. Reversible and irreversible fouling
There are two types of membrane fouling, reversible and irreversible. Reversible deposition and adsorption of protein causes reversible fouling, which can be removed by hydraulic cleaning. Whereas irreversible protein adsorption causes irreversible fouling that can only be eliminated by chemical cleaning or enzymatic degradation [116]. To find out these fouling
values, pH of 7 was used during experiments. The summarizing of total fouling (Ft), reversible fouling (Fr) and irreversible fouling (Fir) as a function of molecular weight of PEGME is shown in Figure 4.10. It can be seen that Ft decreases from 0.9 to 0.77 with increase in molecular weight of PEGME from 550 to 5000. The lower value of Ft indicates lower total flux loss, resulting in less protein adsorption or deposition on the membrane surface [116, 120]. From figure it may also be seen that membrane containing higher molecular weight PEGME has lower value of Fir. This is due to the fact that the membrane containing higher molecular weight of PEGME has lower value of persistent protein adsorption on the membrane and initial water flux could be recovered by a simple cleaning methods using deionized water.
Figure 4.10: Effect of molecular weight of PEGME on different fouling parameters.
4.3.4.2. Effect of molecular weight of PEGME
Figure 4.11 shows the effect of molecular weight of PEGME in membrane casting solution on the average flux using BSA solution during 2 h ultrafiltration. The pH of the BSA solution was maintained at 4.8, 7 and 10. It is observed from the figure that with increase in molecular weight of PEGME from 550 to 5000, the flux gradually increases from 0.9 to 9.2 L/m2h, 4.7 to 61.1 L/m2h and 1.5 to 30.5 L/m2h when pH of BSA solution is 4.8, 7 and 10, respectively.
This increasing trend of flux, irrespective of the pH of BSA, may be attributing to the formation of membranes with higher pore density, which is seen in LLDP experiment. It is also clear from FESEM image (Figure 4.1) that more porous membranes are formed as the molecular weight of PEGME increases.
Figure 4.11: Effect of molecular weight of PEGME on BSA flux at different pH.
Figure 4.12 shows the effect of molecular weight of PEGME in membrane casting solution on the rejection of BSA at solution pH of 4.8, 7 and 10. It is observed from the figure that with the increase of molecular weight of PEGME from 550 to 5000, the rejection gradually increases from 29 to 85%, 5.5 to 51 % and 13 to 60 % when pH of BSA solution is 4.8, 7 and 10, respectively. The rejection of protein by ultrafiltration membranes can be explained under the concept of protein adsorption and consequent pore narrowing, as a result of both hydrophobic and electrostatic interactions between the membrane surface and the protein molecules [118, 121, 125]. The morphological structure of the membrane (which includes both top layer and sublayer) in protein transmission or rejection also play an important role. The increasing trend in percentage rejection with increase in molecular weight of PEGME may be because of the resistance offered by the dense and sponge like structure of the membranes (Figure 4.1) with higher molecular weight of PEGME.
4.3.4.3. Effect of pH of BSA
The BSA rejection is very much pH dependent. Rejection is highest and flux is least at isoelectric point (IEP) of BSA solution [121, 125, 130]. BSA molecules have no charge at IEP (i.e. at pH 4.8). The BSA molecules remained in most compact size when get deposited on the membrane surface form least permeable layer [121, 125]. This compact layer is responsible for highest BSA rejection and least flux. At pH 7, BSA molecule bears net negative charge and enlarges due to electrostatic repulsion. These effects would give a more permeable deposited layer and should give a higher flux and lower rejection [121, 125].
Higher rejection and lower flux at pH 10 compare to pH 7 can be explained by Coulomb’s law [118]. As BSA molecules have more negative charged at pH 10 compare to pH 7 and due to the –CH3 group of retained PEGME in membrane, there is a positive inductive effect (+I effect), which causes a positive charge on –CH group. So, positive charge on membrane
surface and negative charge on BSA, leads to more protein adsorption on membrane surface, which cause lower flux and higher rejection compare to pH 7.
Figure 4.12: Effect of molecular weight of PEGME on BSA rejection through prepared membranes at different pH.