CA 6 Plain
6.3.4. Ultrafiltration and fouling studies using BSA
6.3.4.1. Reversible and irreversible fouling study
Membrane fouling can be classified in two types; these are as reversible and irreversible fouling. Reversible adsorption and deposition of protein causes reversible fouling. This type of fouling can be removed by simple hydraulic cleaning. But irreversible protein adsorption causes irreversible fouling that can only be eliminated by chemical cleaning or enzymatic degradation [101]. 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 different molecular weight of PEG based AAP.
The time dependent flux of membranes modified by the blending of different AAPs is shown in Figure 6.15. DI water flux was measured from 0 - 60 min, 180 - 240 min, 360 - 420 min and BSA flux was measured from 60 - 180 min and 240 - 360 min. Water permeation results were showing, a slight loss of flux through initial time of water permeation and after that it remains constant for all the membranes, but during BSA permeation, a severe flux loss was seen in initial permeation for all the membranes. It may be seen from Figure 6.15, as time passes, the difference between the pure water flux of plain membrane and modified membranes (blended with lower molecular weight based AAP) increases. For membrane modified with lower molecular weight based AAP, flux becomes higher compared to other membranes. Membrane containing AAP-1 had the highest flux at the end of experiment. It may be because of the fact that AAP-1 contains more number of hydrophilic –OH group. The more hydrophilic the membrane was, the less decrease in the flux. Initial loss of flux during UF of BSA may be because of the deposition or adsorption of BSA molecules on the surface of membranes or inside the pores. So, the fouling resistance membrane should successfully oppose the deposition or adsorption of foulants to their surface or pore as reported in literatures [108, 162].
Figure 6.15: Effect of Different AAPs on time dependent flux;
millipore water: 0-60 min, 180-240 min and 360-420 min; BSA solution: 60-180 min and 240-360 min.
Figure 6.16: Effect of AAPs on fouling parameters.
For investigating the fouling resistant behaviour of the membrane, Fr and Fir values were calculated from Figure 6.15 by using equations 2.17-2.19 and are shown in Figure 6.16. It can be seen in figure 6.16 that by the addition of hydrophilic AAP-1, AAP-2, AAP-3 and AAP-4 in modified membranes, Fir values are reduced significantly as compared to plain membrane and therefore, the Fr values are increased. Furthermore the sequence of flux recovery ratio and fouling values are consistent with hydrophilicity and BSA rejection trend of membranes.
In this case also, the hydrophilic segment of the AAP could form hydration layer on the membrane surface through hydrogen bonding, exhibiting anti fouling property and efficiently prevent adsorption deposition of foulants. Sinha et al. [82] also observed the similar result by the blending of amphiphilic polyurethane macromolecules with PSF membrane. However, it can be remarked that Fr values increased after addition of amphiphilic AAP. The possible reason could be accumulation of more BSA on membrane surface due to comparatively increased BSA rejection. Even though increased Fr value, the value of total fouling showed a reducing trend due to remarkable decrease in Fir value. So, these trends suggest that anti fouling property, especially irreversible fouling of modified PSF membrane was enhanced appreciably via blending of amphiphilic AAPs.
Figure 6.17: Effect of AAP on flux recovery ratio.
Figure 6.18: Effect of different AAPs on BSA flux and rejection.
Figure 6.18 shows the BSA rejection and flux for different membranes. It is already reported in the preceding section that amphiphilic AAP not only enhances the hydrophilicity but also pore forming capacity. So, the flux as well as BSA rejection was higher for modified membranes. In case of different AAPs, AAP-1 has most number of -OH groups; hence membrane m2had highest flux as well as flux recovery ratio. Second time flux recovery ratio (Flux2RR) for plain membrane m1 is lower than Flux1RR, In case of membrane m2, the difference between Flux1RR and Flux2RR it is not much (Figure 6.17). The main reason is, plain membrane had already some amount of deposited or adsorbed BSA on their surface, which again increased after second round of BSA ultra filtration. Flux2RR value came close to Flux1RR value, for the AAP with the decreased molecular weight of PEG. Similarly for different additive, results are in line with hydrophilicity study of membranes.
Summary
AAP-1 (synthesized by lower molecular weight of PEG) provided more hydrophilic membrane than other AAP prepared by higher molecular weight PEG. Hydrophilicity of the modified membranes m2, m3, m4 and m5 were found to be increased by the addition of amphiphilic AAP. Presence of AAP also increases the ion exchange capacity of membranes.
This chapter comprises three segments; first segment is conclusions, which includes the results drawn from several works presented in this thesis. Second segment covers the winding up of all the chapters and presents the final verdict on all the works done in the thesis under the part called as summary. Third and last section discusses the ideas for the future work.