Environmental Engineering Reference
In-Depth Information
To further improve compatibility, Huiqing Wu et al. introduced reactive amine groups
onto mesoporous silica nanoparticles to allow covalent incorporation into a piperazine
TMC-based separating layer [35]. These TFN membranes gave higher initial and sustained
lux rates, as well as improved fouling stability compared with TFC controls.
In an alternate line of research, Huiqing Wu et al. form TFN membranes with the poly-
amide polymer phase being replaced with a polyester by the use of triethanolamine as
aqueous reactant and multiwalled carbon nanotubes as the nanoparticle [36]. Further
reinements in a follow-up article describe the determination of preferred surfactants to
help disperse the nanotubes [37]. The resulting membranes had good lux and sulfate
selectivity, which they speculate may be of interest in the chloralkali industry.
As new improvements and application areas of TFN membranes are identiied and
developed, the pace of research in this area is expected to rapidly increase.
16.3 Industrial TFN Development
Permeability and fouling resistance are primary drivers of cost in membrane-based water
treatment systems. These performance metrics directly inluence the energy intensity
and capital expenditures of an RO plant, and therefore, the economics of desalination. At
70%-80% of the total expense of RO-desalinated water, energy consumption and capital
expenditures are the primary reason why desalination remains expensive compared with
many other freshwater sources. As a result, the commercial value of TFN technology was
readily apparent and licensed shortly after discovery by Hoek et al. to NanoH2O Inc.
16.3.1 Nanocomposite Membrane Development
Commercial development of TFN technology for SWRO involved the identiication of sev-
eral technical failure modes speciic to TFNs, and then developing methods to detect and
prevent the onset of those failure modes. The importance of defect reduction can quickly
be realized by recognizing the 1000-fold difference in permeability between the support-
ing polysulfone and the thin ilm layer. In an industry where 99.7% rejection of dissolved
ions is expected, a parts-per-million areal defect frequency in the thin ilm will render the
membrane unsuitable for use as the low through defect is ampliied by the locally higher
defect low rate.
Figure 16.3 illustrates one such failure mode. Scanning electron microscopy (SEM) imag-
ing of a membrane (“B” in Table 16.1) surface reveals that nanoparticles have aggregated
in the organic solution and deposited on the surface of the membrane. Identiication of the
deposited nanoparticles was conirmed with energy dispersive x-ray element mapping. In
contrast to the nanoparticles shown in Figure 16.2, this aggregate is not contained within
the barrier layer but instead sits on the surface. Although its appearance is supericially
similar to that of a fouling layer, its high porosity leads to minimal resistance to low.
However, the decreased concentration of nanoparticle incorporated within the ilm mini-
mizes any permeability increase as shown in Table 16.1.
A second such failure mode illustrates a secondary aggregation-based failure mode, in
this case smaller aggregates that end up incorporated within the ilm. In Figure 16.4, a
TEM image of membrane C shows smaller aggregates contained within a TFN membrane.
