Page 40 - Membranes for Industrial Wastewater Recovery and Re-Use

20 Membranes for lndustrial Wastewater Recovery and Re-use

Table 2.3 Commercially available phase inversion polymeric membrane materials
Polymera Advantages Disadvantages Processb
CA Chlorine resistant Susceptible to alkaline hydrolysis RO, NF. UF
Inexpensive atpH>6
More fouling resistant than PA Susceptible to biodegradation
Limited thermal and chemical
stability
Limited permselectivity (-9 5%
rejection)
Slightly lower permeability
PA More all-round stability than CA Very limited chlorine tolerance RO, NF
More permselectivity than CA (<0.1 mg1-I)
PAN High resistance to hydrolysis Hydrophobic UF, RO substr.
High resistance to oxidation Requires copolymers to make
less brittle
PSU, PES Very good all-round stability Hydrophobic UF. RO substr.
Mechanically strong
PVDF, Extremely high chemical Highly hydrophobic UF, MF
PTFE stability Limited mechanical stability
High thermal stability Limited intrinsic permeability
Expensive
PEI High chemical stability Hydrophobic UF, RO
Very high thermal stability Less solvent resistant than PVDF substr.
Mechanically strong Poorer alkaline stability than
PSU or PAN
PP Inexpensive Hydrophobic UF, MF

a CA, cellulose acetate (predominantly di- or tri-acetate); PA, polyamide (aromatic): PAN,
polyacrylonitrile: PSU, polysulphone: PES, poly(ether sulphone): PVDF, poly(viny1idene fluoride); PTFE,
polytetrafluoroethane: PEI, polyetherimide; PP, polypropylene.
Most usual application in bold type.

oxidative attack and hydrolytic degradation promoted by extremes of pH, is their
hydrophobicity. This makes them very susceptible to non-specific adsorption
of hydrophobic contaminants in the water, which reduce the permeability of the
membrane. This phenomenon, termed fouling, imposes a very significant
constraint on the operation of all membrane processes. Much developmental
work has been conducted in producing chemically and mechanically robust
membranes with hydrophilic surfaces that are therefore less susceptible to
fouling by common natural hydrophobic materials such as proteins.
Surface modification can take place by four recognised routes:

0 chemical oxidation,
0 organic chemical reaction,
0 plasma treatment, and
0 grafting.
Chemical oxidation, using standard oxidative chemicals such as chromic or
nitric acid, converts surface groups such as alkenes and esters to hydroxyl and