Fig. 14 indicates that the changing tendency of the total membrane
resistance with time through PVDF and PVDF–PVP membranes.
As seen in Fig. 14, plots of the total membrane resistance
with varying time for the PVDF and PVDF–PVP membranes all
yielded concave-down curves, suggesting that the external fouling
was the main mechanism for fouling which was also demonstrated
by the results of the antifouling experiments. Thus the permeation
flux might be recovered to the original value before ultrafiltration
without any loss after mild cleaning. On the other hand, the total
membrane resistance of the modified PVDF membranes was less
than that of the original PVDF membranes. For example, the membrane
resistance of the PVDF70K–PVP membrane only amounted
to 1/3 that of PVDF70K membrane, and the membrane resistance
of the PVDF100K–PVP membrane decreased 10% that of PVDF100K membrane. This was mainly due to the grafted PVP and
hydroxyl groups onto the modified membrane surface which
increased the membrane penetration quality and consequently
decreased the resistance of membranes.
Table 4 indicates the flux recovery of fouled membranes after
membrane cleaning by pure water and 3 wt.% NaOH aqueous solution.
As shown in Table 4, the flux recovery of PVDF–PVP membranes
after cleaned by pure water and 3 wt.% NaOH aqueous
solution were both obviously higher than that of original PVDF
membranes. Viewed from the contact angle, it was attributed that
the grafted PVP layer accelerated the water permeation flux and
repelled the oil at the same time. Accordingly, the hydrophilic performance
of the modified PVDF membranes improved and the
oleophilic property decreased reciprocally. Thus the antifouling
capability of PVDF–PVP membranes was improved greatly.
Besides, the membrane cleaning effects using 3 wt.% NaOH aqueous
solution were much superior to those using pure water.
Fig. 14 indicates that the changing tendency of the total membraneresistance with time through PVDF and PVDF–PVP membranes.As seen in Fig. 14, plots of the total membrane resistancewith varying time for the PVDF and PVDF–PVP membranes allyielded concave-down curves, suggesting that the external foulingwas the main mechanism for fouling which was also demonstratedby the results of the antifouling experiments. Thus the permeationflux might be recovered to the original value before ultrafiltrationwithout any loss after mild cleaning. On the other hand, the totalmembrane resistance of the modified PVDF membranes was lessthan that of the original PVDF membranes. For example, the membraneresistance of the PVDF70K–PVP membrane only amountedto 1/3 that of PVDF70K membrane, and the membrane resistanceof the PVDF100K–PVP membrane decreased 10% that of PVDF100K membrane. This was mainly due to the grafted PVP andhydroxyl groups onto the modified membrane surface whichincreased the membrane penetration quality and consequentlydecreased the resistance of membranes.Table 4 indicates the flux recovery of fouled membranes aftermembrane cleaning by pure water and 3 wt.% NaOH aqueous solution.As shown in Table 4, the flux recovery of PVDF–PVP membranesafter cleaned by pure water and 3 wt.% NaOH aqueoussolution were both obviously higher than that of original PVDFmembranes. Viewed from the contact angle, it was attributed thatthe grafted PVP layer accelerated the water permeation flux andrepelled the oil at the same time. Accordingly, the hydrophilic performanceof the modified PVDF membranes improved and theoleophilic property decreased reciprocally. Thus the antifoulingcapability of PVDF–PVP membranes was improved greatly.Besides, the membrane cleaning effects using 3 wt.% NaOH aqueoussolution were much superior to those using pure water.
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