Retention of ions contained in single electrolyte solutions was widely reported. The influence
of ions size and charge was discussed [140], and a qualitative prediction of the retention sequence
of ions with different valence and size was given [141]. It was observed that the retention of
symmetric electrolytes (i.e. NaCl, KCl, and MgSO4) in diluted solution is governed by the
membrane co-ions due to electrostatic repulsion, and that the retention of asymmetric electrolytes
Chapter I
37
models based on irreversible thermodynamics are built and can successfully describe the retention
of single electrolyte solutions [125,132,143].
However, when more complex solutions are concerned, like multi-electrolytes systems, there
are interactions between the different ions. Then, the retention of ions in mixed solutions cannot
be predicted from the ones obtained with single solutions. An attempt to use an irreversible
thermodynamic model to predict the individual ions retentions in a Cd2+/Ni2+ binary solution
showed that the experimental retention of the less retained ion, Cd2+, is 10% lower than that
predicted by the model [144]. With binary systems containing two co-ions and only one counter-
ion, it was reported that when one of the co-ions is totally retained by the membrane, the retention
of the other decreases significantly. It was attributed to the co-ions competition. For instance,
during NF of a mixture containing NaCl and an organic acid sodium salt, with a molecular weight
(MW) of 700 g.mol-1 and retention higher than 99%, negative retention of Cl- was observed [145].
The same results, i.e., negative retention of the less retained ion, were also reported with ions of
smaller molecular weight (lesser than 200 g.mol-1), especially with solutions containing
multivalent and monovalent ions. As an example, negative retention of Cl- was observed in the
presence of SO42- [146], as well as for Na+ in the presence of Ca2+ [147].
Such interactions of charged solutes in mixed solutions not only depend on operating
conditions such as filtration flux but also on the feed composition, i.e., concentration and
proportions of ions. It was reported that for a mixed solution containing NaCl and an organic acid
sodium salt, the retention of Cl- decreases to more negative values when the proportion of organic
acid salt in the feed increases [14]. This was also observed with solutions containing Na2SO4 and
NaNO3, for which negative values are obtained for the retention of NO3- when the concentration
ratio of SO42-/NO3- in the feed increases from 1 to 5. Again, more negative values are observed
for an increasing concentration ratio up to 10 [136]. Negative retentions of acetate were also
reported during NF of a succinate/acetate binary solution for certain feed compositions, i.e., for
a concentration about 0.7M and an acetate/succinate ratio of 6, while both acetate and succinate
retentions are positive for lower concentrations or lower acetate/succinate ratio [128]. More
complex mixtures containing formate, acetate, lactate, and succinate salts were investigated too.
Chapter I
38
in the presence of a more retained solute, succinate, and that this decrease is more important for
increasing proportion of succinate [148]. An overview of the negative retention phenomenon is
provided, the possible mechanisms behind are discussed. The author concluded that current
experimental results are not sufficient to predict the negative retention phenomenon, and further
investigations are required [149].
Besides the ions, co-existing neutral and charged solutes could also influence the performance
of nanofiltration
Many authors reported the decrease of uncharged solutes retentions in the presence of
inorganic salts. For instance, the retention of glucose decreased in the presence of KCl [150] and
NaCl[104]. On the other hand, the retentions of charged solutes also decrease in the presence of
inorganic salts, like retention of sodium lactate in the presence of NaCl and Na2SO4 [139].
The mechanisms of the solutes retentions decrease in the presence of inorganic salts are not
the same for neutral and charged solutes. The decrease of the retention of uncharged solutes in the
presence of inorganic salts can be due to their dehydration, leading to a smaller hydration size
[124,151]. Another possibility could be due to the swelling of the membrane due to the adsorption
of salts [150]. On the other hand, the influence of inorganic salts on the retention of charged solutes
could occur due to the screening effect and co-ions competition as previously mentioned. The
addition of inorganic salts increases the ionic concentration of the solution, then the electrostatic
interaction between membrane and charged solutes is weakened, and lower retention is observed
[128]. In addition, the existence of more retained co-ions (i.e., SO42-), could compete with the less
retained co-ions (i.e., lactic acid), which leads to a decrease in retention of less retained co-ions
[139].
For organic impurities, the influence on the retention of organic acid is also reported. The
influence of uncharged organic solute such as glucose is negligible on the retention of other solutes,
both for the charged solute, i.e., NaCl [152], and uncharged solute, i.e., xylose [153]. The charged
organic solutes act as the inorganic salts, which could influence solutes retentions by both
Chapter I
39
However, most of the results are obtained in synthetic solutions. For more complex solutions,
especially with real fermentation broth, some unexpected results were reported. For instance, it is
reported that the retention of butyric acid in a real digestion liquor maintained at nearly 100%, for
all the conditions (pH and transmembrane pressure) investigated [6]. Meanwhile, the retentions
of other VFAs (acetic and propionic acids) decrease when the solution pH decreases [6]. Another
publication shows that the retention sequence of acetate and butyrate is Ac<Bu in synthetic
solutions by DL membrane, changed to Bu<Ac when filtrate a real fermentation broth with the
same type of membrane.
The aforementioned results indicate that there could be more complicated solutes interactions
in the real fermentation broth than in synthetic solutions.
I.2.4.2 Solution pH
A thin-film composite membrane (TFCM), like NF-45 membrane, is composed of a thin
nanoporous active layer made of polyamide, then supported by a macroporous layer made of
polysulfone [154]. The membrane charge can come from the dissociation of functional groups,
adsorption of ions from solution, and the adsorption of polyelectrolytes, ionic surfactants, and
charged macromolecules [155]. Among those sources, dissociation of functional groups is
commonly considered as the main contribution of the pH-dependent membrane charge [156,157].
For TFCMs, the solutes separation only involves the polyamide active layer, and the functional
groups (carboxylic and amino groups) on the active layer determine the membrane surface charge
[156,158]. It is commonly observed that when the solution pH is higher than the isoelectric point
(IEP) of the membrane, the membrane surface charge increases with pH. This theory can be
proved by the measurement of the zeta potential of the membrane [159,160]. However, the zeta
potential measurement is solute-dependent and cannot provide a quantitative measurement of the
membrane charge. Recently, heavy-ion probes were used as a new method to determine the
membrane charge, and dissociation of carboxylic functional groups on the active layer is referred
Chapter I
40
For the nanofiltration of strong electrolytes, the charge of solutes is nearly unchanged, the
influence of solution pH on solutes retention can be attributed to the dissociation of membrane
functional groups leading to the variation of the membrane surface charge. For small symmetric
salts, i.e., NaCl and KCl, various reports have shown that when the solution pH becomes higher
than the IEP of the membrane, the retentions of salts increase with pH due to the increase of
membrane charge [150,159,161,162]. However, for weak electrolytes such as organic acids, the
retention mechanism is more sophisticated, as both the charge of membrane and solutes vary with
solution pH [142]. Besides, the size of organic acids can be much larger than the size of inorganic
electrolytes. Thus, the molecular weight (MW) of organic acids needs to be first discussed. When
the MW of an organic acid is higher than the molecular weight cut off (MWCO) of the membrane,
regardless of the solute and membrane charge, high retention is observed. Therefore, the retentions
of organic acids that have MW higher than MWCO of the membrane are independent of solution
pH [163].
For the nanofiltration of small monovalent organic acids, i.e., acetic acid, low retention is
expected for its neutral form, since its MW (60 g.mol-1) is much lower than the MWCO of
nanofiltration membranes (200-500 g.mol-1). When the solution pH increases, the proportion of
dissociated acetic acid increases; meanwhile, the membrane surface charge also increases, then
stronger electrostatic interactions between membrane and solutes are expected, and then higher
retention in diluted solution should be observed. It is reported that the retention of acetic acid
increased from nearly 0% at pH 2.9 to more than 90% when the solution pH increased to 9.1 at
acetic acid concentration of 5g/L (about 83 mM) [164]. Similar results are reported by other
authors [165,166]. Those observations suggested that in diluted solutions, the retention of small
monovalent organic acids with MW lower than MWCO of the membrane is mainly fixed by
charge effect.
Retention observed for a weak electrolyte solution can be considered as the combination of
the retentions of undissociated and dissociated form, and the proportion of those two forms are
determined by dissociation constant and solution pH. Then, if the retentions of those two forms
as a function of solution pH are acquired individually, the retention of the solute can be estimated
Chapter I
41
reported that the mass transfer parameter of undissociated lactic acid is independent of solution
pH, while the mass transfer parameter of dissociated lactic acid linearly increases with pH. The
retention of lactic acid at different pH can be simulated from those two parameters [143]. However,
the authors only investigated a short range of pH values (2.88 to 4.93), and the influence of ionic
concentration is not discussed.
Besides the weak electrolyte in the solution, the functional groups at the membrane surface
can be dissociated when pH changes. It is suggested that an effective dissociation constant (Keff)
can be used to describe the dissociation of membrane functional groups [167]. The pKeff values
for various polyamide films are reported as 5 to 9 [167]. Thereafter, the concentration of
membrane functional groups at the membrane surface is reported to be determined by heavy ions
probes, and the dissociation constant of membrane is simulated [156,157].
As previously introduced, the retention of small weak acids in dilute solution depends on the
dissociation of solute as well as the dissociation of membrane. Thus, it is expected to observe an
increase of the retentions of weak acids with solution pH following an S-shaped dissociation curve.
This S-shaped retention curve was observed in the retention of sulfamethoxazole and ibuprofen
by a loose NF membrane [168], as well as nanofiltration of succinic acid [128] and sulfuric acid
[146,169,170].
The retentions of weak electrolytes in diluted solutions are strongly influenced by pH, and the
variation of the retention versus pH could be linked to the dissociation constant of solute as well
as the effective dissociation constant of membrane.