Page 389 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN009K-419 July 19, 2001 20:57

324 Membranes, Synthetic, Applications

compounds (e.g., cyclodextrins) with well-defined cavity film. Limited lifetime is perhaps the most important lia-
sizes or those carrying coordinating functional groups. bility against practical application of this technology. To
A special case of facilitated transport involves the use addressthisproblem,experimentalmembranescontaining
of organic-soluble liquid ion exchangers to recover metal high concentrations of complexation sites in a solid poly-
ions from dilute solutions. Often referred to as coupled meric matrix have been developed. Above a certain critical
transport, this process operates by driving the transport of carrier density, the transport of the complexed species can
the metal complex with the flow of a second species (most take place by site-to-site jumps—a “bucket brigade” ef-
often protons in the form of a pH gradient) in the oppo- fect. Because the complexation sites are an integral part
site direction. As depicted in Fig. 37, coupled transport of the polymer, there is little loss of efficiency so long as
can operate by two mechanisms: (1) cotransport, where the host polymer remains stable.
metal-containing anions permeate in the same direction as
the protons, and (2) countertransport, where metal cations G. Industrial Dialysis, Donnan Dialysis,
and protons (or analogously, metal-containing anions and and Electrodialysis
another anion supplied from the stripping solution) per-
1. Industrial Dialysis
meate in opposite directions. In all cases, the pH of the
external solutions is adjusted to provide favorable con- Dialysis operates by the diffusion of selected solutes
ditions for the complexation and decomplexation reac- across a nonporous membrane from high to low concentra-
tions at the solution–membrane interfaces, and to maintain tion. An early industrial application of dialysis was caus-
the pH gradient as driving force. Very clean separations tic soda recovery from rayon manufacturing. It had been
are possible in extracting metal ions from dilute solu- a viable process because inexpensive but alkali-resistant
tions, or in separating two or more metal ions with differ- cellulose membranes were available that were capable of
ent complexation characteristics. Practical applications in removing polymeric impurities from the caustic. Gradu-
the plating and metal-finishing industry, wastewater treat- ally however, dialysis is being replaced by dynamic mem-
ment, and hydrometallurgical extraction of ores have been brane technology for caustic soda recovery because of the
contemplated. Until recently, however, commercialization latter’s much higher productivity.
of this technology seems to be hampered by the fluc- Dialysis continues to meet certain specialized appli-
tuating prices of metals such as chromium and copper, cations, particularly those in biotechnology and the life
or by uncertainties in the commodity value of uranium sciences. Delicate substances can be separated without
(Ho, 2000). damage because dialysis is typically performed under
Most liquid membranes are less stable than their poly- mild conditions: ambient temperature, no appreciable
meric counterparts. Although the thin liquid film in the transmembrane pressure drop, and low-shear flow. While
membrane corresponds to a short diffusion path and hence slow compared with pressure-driven processes, dialy-
a high mass transfer rate, small amounts of the immobi- sis discriminates small molecules from large ones reli-
lized liquid can be displaced under pressure. Also, the ably because the absence of a pressure gradient across
immobilized liquid may slowly dissolve in the external the membrane prevents convective flow through defects
phases, eventually leading to discontinuities in the liquid in the membrane. This advantage is significant for two

FIGURE 37 Mechanisms of carrier-facilitated immobilized liquid membrane extraction, also referred to as coupled
transport. The species, R, refers to the carrier component responsible for complexation.