Environmental Engineering Reference
In-Depth Information
accumulation, or adsorption of foulants (colloidal particles, organic matter, biological sub-
stances, inorganic salts) onto membrane surfaces and/or within the membrane, deteriorat-
ing the performance of the membranes over time. Consequently, to improve the overall
performance of membranes, new components, usually nanomaterials, are added to the
bulk of the membrane material by various physical or chemical methods. This usually
modiies the morphology and structure of the membranes. Therefore, to gain complete
understanding of the modiied membranes, speciic methods are used and have been cat-
egorized as follows
45
:
• Characterization of composition: Fourier transform infrared spectroscopy, nuclear
magnetic resonance, x-ray photoelectron spectroscopy, differential scanning calo-
rimetry, transmission electron microscopy (TEM).
• Characterization of morphology and structure: scanning electron microscopy,
atomic force microscopy, microtomy followed by TEM.
• Characterization of performance: surface hydrophilicity/hydrophobicity by con-
tact angle measurements, permeability and selectivity, antifouling properties.
20.3.3 Removal of Heavy Metals, Organometallics, and
Metalloids Using Nanomembranes
Pollution of water by heavy metals such as mercury (Hg), cadmium (Cd), arsenic (As),
chromium (Cr), thallium (Tl), lead (Pb), selenium (Se), and zinc (Zn) is a serious problem
for many countries, including South Africa. The term “heavy metal” refers to any metal-
lic chemical element that has a relatively high density and is toxic or poisonous at low
concentrations. These metals are dangerous because they tend to bioaccumulate. Thus,
no matter how low in concentration, they still pose serious health problems. The applica-
tion of nanotechnology-enabled membranes for the removal of these metals is undergoing
immense research.
The various materials and methods used to fabricate the membranes and their eficien-
cies can be observed in a number of studies.
48-63
A number of important points can be
drawn from these studies:
• Membranes can effectively be used for the removal of heavy metals from various
water sources including wastewater. Depending on the materials used, eficien-
cies of up to 100% can be removed.
50
• There is an increase in research toward the incorporation of other nanomaterials/
nanoparticles (e.g., TiO
2
, SiO
2
, carbon nanotubes, cyclodextrin) into membranes.
Addition of the nanomaterials improves the antifouling properties of the mem-
branes and sometimes the mechanical strength.
49, 51- 65
• Many of the new membranes have been tested in a laboratory scale. A few studies
have shown that the membranes are effective at a large scale.
50,59
• Electrospinning and phase inversion methods are the most widely used technolo-
gies for the preparation of nanostructured membranes.
• Most of the studies have shown that the pH of the solution treated is crucial
because it affects the adsorption capacity of membranes.
