Agriculture Reference
In-Depth Information
the NF membrane has five characteristics: separation range between RO and UF,
aperture of
1 nm (in general, 1-2 nm), suitable for separating 200-1,000 mole-
cules with relative molecular weight, membrane surface with negative charge, and
monovalent ion removal rate of
>
<
90 % and divalent and polyvalent ion removal
>
rate of
90 %.
The sieving effect and charge repulsion are the primary separation mechanism of
the NF membrane, and the membrane retains organic pollutants and multivalent
salts in water. The important mechanical principle underlying separation using the
NF membrane is the electrical effect—an electrostatic effect emerges between the
negatively charged groups in the NF membrane and electrolyte ions with different
charge strengths, resulting in varied ion retention rates. In diverse systems
containing different valence ions, the NF membrane selectivity for different ions
is not the same due to the Donnan effect, and the proportion of different ions
passing through the NF membrane is not similar; multivalent ions can be prevented
from passing through the NF membrane, which can maintain high desalting per-
formance under low pressure. Typically, the NF membrane is negatively charged
under neutral and alkaline conditions and positively charged under acidic condi-
tions. The charge on the surface of the NF membrane has a significant role in the
separation of ions. In general, the negatively charged NF membrane is composed of
polymer-containing sulfonic acid group (-SO
3
H) or carboxyl group (-COOH) or a
polymer film into which the negatively charged group has been introduced. On the
other hand, the positively charged NF membrane is composed of materials mainly
containing amino group (-NH
2
), and colloidal particles and bacterial toxins in
water get adsorbed onto the surface of the membrane. Moreover, the positively
charged NF membrane can be used to separate positively charged amino acids,
proteins, and other substances and can be employed for clean production during
cathodic electrophoretic painting process because the positively charged NF mem-
brane exerts repulsive force on the particles with similar electric property. The
interception of neutral molecules (such as glucose and other organic molecules) by
positively or negatively charged NF membrane is caused by the molecular sieve
effect of nanoscale micropores, and the interception rate is associated with the
molecular particle size, operating pressure, concentration, and molecular charac-
teristics. On the other hand, the separation of inorganic ions and charged organic
substances by the NF membrane is affected not only by the particle size but also by
the chemical potential and potential gradient. The mass transfer process is signifi-
cantly affected by the surface charge and the Donnan effect among the charged
molecules (ions), and if the charge on the surface of the ions and NF membrane is
higher, then the removal rate of the contaminants will also be higher. Therefore,
different charged NF membranes exhibit varied interception capacities for different
valence ions. Furthermore, the ion selectivity of negatively charged NF membrane
increases with the increasing pH,
indicating that pH affects the removal of
pollutants.
NF membranes are widely used, particularly in the field of water treatment,
because of its stability, nontoxicity, and energy-efficient nature. In recent years,
basic research and practical engineering studies on NF technology have achieved
