Environmental Engineering Reference
In-Depth Information
solvation in the polymer layer may decrease separation eficiency. The results did not
agree with simple models of solvent lux such as the Hagen-Poiseuille equation. Overall,
the data indicate the importance of pilot-scale measurements to provide effective evalua-
tion of technology in a real-world scale application.
Molecular-scale polymer growth and nanoscale self-assembly for structure formation
show great promise in the future. Polymers are inherently “soft,” meaning they can be
relatively homogeneous in composition while tolerating relatively high degrees of imper-
fection. The soft nature also implies that the structural arrangement can be altered by
changes in solutes, electrical potentials, and mechanical forces creating an opportunity for
changes in functionality and self-cleaning. Additional functionality can be built by adding
nanoparticles to the membrane material to either degrade contaminants or inactive bacte-
ria. Polymer-nanocomposite membranes can be developed by incorporating either mag-
nesium oxide (MgO)
33
or silver nanoparticles
34
as antibacterial agents, and zero-valent iron
for the removal of halogenated hydrocarbons, radionuclides, and organic compounds.
35
Nanocomposite materials can also be used to reduce fouling by increasing the hydrophi-
licity of the membrane. Polyethersulfone UF membranes dip-coated with titania nanopar-
ticles were created by Luo et al., which demonstrated a signiicant drop and contact angle
and wetting performance.
36
These studies demonstrate the creative potential available in
polymer-nanocomposite materials for membrane-based separations.
7.3.2 Hydrophobic Mesh Coatings
High lux and resistance to fouling are key to achieving systems that can be suficiently
robust for ship bilge water treatment. A strategy toward this end is to use large pore open-
ings and tailor the surface chemistry and/or nanostructure to control hydrophobicity.
Development of artiicial superhydrophobic surfaces
37
based on “rough” nanostructured
surfaces has been accomplished using a variety of approaches, including template synthe-
sis,
38
phase separation,
39
and electrodeposition.
40
These types of approaches can be used
to modify nonwoven materials for rapid oil-water separation.
41
A notable example is from
the Wang group, which created smart textiles and polyurethane sponges that are able
to switch between superoleophilic and superoleophobic by grafting pH-responsive block
copolymers to these materials.
42
Poly(2-vinylpyridine) in conjunction with polydimethyl-
siloxane were used to create surfaces that have switchable oil wettability due to the pro-
tonation state of the pyridine group (Figure 7.3). At neutral pH, the pyridine groups are
deprotonated and the surface is oleophilic, whereas at pH <2 the pyridines are protonated
and the surface becomes hydrophilic. The resulting nonwoven material can be used for
remarkably fast and switchable oil-water separations, which was demonstrated by the
separation of gasoline-water mixtures (Figure 7.4). One of the most attractive features of
this technology is the use of inexpensive cellulose and polypropylene textiles that show
good promise for large-scale production and application to bilge water treatment and
other oily-water separations.
Functionalization of Nomax
®
alumina fabric with hydrophilic cysteic acid surface-
stabilized alumina nanoparticles were reported from the Barron group with the similar goal
of creating high-lux materials for water while preventing the lux of oil through the fabric
mesh.
43
Cysteic acid (Figure 7.5) functionalizes the alumina surface via the carboxylate
group, and at neutral pH the molecule exists in a zwitterion form that is hydrophilic. The
resulting membrane systems were capable of screening phase-separated hydrocarbons
from water with high tribological endurance and stability across a pH range from 2 to 12.
A similar and promising approach modifying polyester membranes and stainless-steel
