Nanomembranes - Nanotechnologies for Water Environment Applications - page 374

solutes was applicable for ions, but not vice versa. Rejection of organics may be
determined by size and charge as well as the same parameters that govern ion rejection,
including the membrane “pore” size, charge, pH, ion charge and size, flux and pressure,
concentration, solute-solute interactions, composition of mixtures, and speciation. In
addition, factors such as molecular conformation and structure may play a role.
Agui et al. (1992) found that pH and ionic strength showed important effects on
humic substances (HSs) removal using a RO membrane. Rejection was higher at neutral
pH (90%), rather than acidic pH (60 - 75%), and the rejection was concentration
dependent. The reasons for this solvent dependent behavior were attributed as adsorption,
hydrogen-bonding, or electrostatic attraction. However, the reported rejections are
relatively low for RO. Braghetta (1995) reported a reduction in rejection of DOC at low
pH and high ionic strength for a sulfonated polysulphone hollow fiber NF membrane
(MWCO = 1 kDa). Sorption on the hydrophobic domain (membrane polymer-organic
solute interaction) is regarded as an important phenomenon which may influence the
rejection behavior. Hydrophobicity of organic compounds was further found to be an
important factor affecting adsorption of organics onto the membrane surface (Kiso et al.,
2001). Adsorption inside the pores or at the membrane surface narrows the pores. When
the molecules have a similar size as the pores, permeation can lead to pore blocking, a
phenomenon that can be enhanced or caused by adsorption. For the filtration of non-
macromolecular components with NF, this phenomenon has not yet been described (Van
der Bruggen and Vandecasteele, 2001). Interactions in multi-component solution may
enhance rejection by membrane due to the effect of coupling of different components
known as frictional coupling. Studies have shown that systems of electrolytes-
nonelectrolytes or electric solutions containing weak acids exhibits strong interaction
effects (Scltanieh and Sahebdelfar, 2001). Ohlenbusch et al. (2000) showed that the
interaction between phenolic compounds and natural organic matters was dependent on
the characteristics of both the contaminant and the NOM.
A complete study on organics and salt rejection by NF in synthetic surface water
solutions conducted by Schäfer (2001) further confirmed the importance of the effects of
the aforementioned factors on organics rejection by NF. Fulvic acid (FA) was used for
rejection experiments, as FA is smaller than HA. Therefore, the rejection due to the size
of this compound should be lowest. Preliminary experiments were carried out with TFC-
SR membrane to test the impact of background salt composition on flux and rejection.
The solution contains 5 mg/L DOC FA, under pH 8, with the presence of NaHCO 3 ,
CaCl 2 , NaCl or their mixtures. The results showed that FA rejection is not influenced
significantly by the salts environment. This indicated that size exclusion is the dominant
mechanism and that the structure of the molecule (curled at a high salt concentration or
linear at a low salt concentration) is not of importance. Effects of the organic type,
concentration and pH were studied by introducing FA, HA and NOM at concentration
levels of 2.5-7.5 mg/L DOC to the solution under the pH of 4.5, 8 and 10 (Figure 12.4).