W. Richard Bowen and Nidal Hilal 4
W. Richard Bowen and Nidal Hilal 4
W. Richard Bowen and Nidal Hilal 4
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166 5. AFM AND DEvELOPMENT OF (BIO)FOULINg-REsIsTANT MEMBRANEs<br />
mean observed adhesion forces <strong>and</strong> the number of snap-in events measured<br />
for each membrane are also tabulated in Table 5.6, based on the measurements<br />
taken from nine different points on each membrane surface.<br />
All measurements for P-P membrane contained snap-in events, but this<br />
decreased as the SPEEK content of the membrane increased. For the S5-20<br />
membrane, containing 5% SPEEK, no snap-in event was measured at all.<br />
As the SPEEK content of the membranes is increased, a substantive <strong>and</strong><br />
systematic decrease in mean observed adhesion forces is seen, being a factor<br />
of almost 40 between P-P <strong>and</strong> S5-20 membranes. These trends together<br />
show the profound influence on the surface properties of the membranes<br />
of the incorporation of small amounts of SPEEK. It is also notable that the<br />
presence of snap-in events in some cases in the S-series membranes, except<br />
for S5-20, <strong>and</strong> the large st<strong>and</strong>ard deviation values show that some variability<br />
in the surface properties of the membranes exist.<br />
AFM has also been used to help in the assessment of fouling by<br />
humic acid (HA) of SPEEK-modified polysulphone membranes [30].<br />
HAs are heterogeneous materials, containing three main types of functional<br />
groups: carboxylic acids, phenolic acids <strong>and</strong> methoxycarbonyls.<br />
They are mostly negatively charged for pH values above pH 2.8 [31].<br />
Measurements were carried out with the S5-20 SPEEK membrane, <strong>and</strong><br />
an aromatic PES membrane (ES404), which were chosen as a comparable<br />
commercially available membrane [30].<br />
The effects of a deposited HA layer on membrane filtration will<br />
depend to some extent on its physical state. Images of membranes<br />
ES404 <strong>and</strong> S5-20 prior to use in filtering HA from water are shown in<br />
Figure 5.23(a). Before use, the S5-20 membrane has a rougher surface,<br />
visible in the images, <strong>and</strong> also indicated by a greater rms roughness.<br />
Figure 5.23(b) shows images of the two same membranes after 2 h filtering<br />
HA from model water. For the ES404 membrane the deposit is compact<br />
with occasional larger spheroidal aggregates. The rms surface roughness<br />
was greater by a factor of approximately 1.9 compared to before filtration.<br />
For the S5-20 SPEEK membrane, the deposit is much greater due to<br />
a higher flux [30]. The deposit is also less compact <strong>and</strong> more irregular than<br />
that seen for the ES404 membrane, <strong>and</strong> the rms surface roughness has<br />
increased by a factor of approximately 5.6 compared to before filtration.<br />
Both membranes were rinsed subsequent to filtration. Recovered membranes<br />
are shown in Figure 5.23(c). For the ES404 membrane, the surface<br />
roughness is similar to the fouled membrane, suggesting that rinsing has<br />
had little effect on removing the fouling aggregates. For the S5-20 membrane,<br />
however, the roughness value is greatly reduced from the fouled<br />
membrane, but has not quite returned to the value seen for the unused<br />
membrane. This suggests that most, but not all, of the fouling material<br />
has been removed by simple rinsing.