W. Richard Bowen and Nidal Hilal 4

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