Environmental Engineering Reference
In-Depth Information
6.2.5
Fractionation and Separation Methods
Fractionation is defi ned by IUPAC to be classifi cation according to physical, for
example size or surface area, or chemical properties, for example hydrophobicity
or chemical partitioning. In this section, discussion is limited to physical fraction-
ation methods.
6.2.5.1
Microfi ltration
Filtration methods are the most simple and common fractionation methods. They
are distinguished based on the pore size or molecular weight cut-off (MWCO) of
the membranes being used: for microfi ltration 0.2- 10
m; ultrafi ltration 1 kDa to
100 MDa; and for nanofi ltration or reverse osmosis approximately 100- 1000 Da.
Common piston fi ltration, or frontal fi ltration, using microfi ltration membranes
(0.2- 1
µ
m) is the most simple and common method, but fi ltration artefacts that
often affect the results have been reported (Buffl e et al. , 1992; Morrison and Benoit,
2001 ). In piston fi ltration, particle concentration gradients are formed above the
fi lter and this results in concentration polarization and pore clogging. This can lead
to agglomeration and increased deposition of both oversized and undersized par-
ticles on the fi lter membranes. The presence of charged natural organic matter, for
example humic substances, often decreases these artefacts since they stabilize the
particles and also condition the membrane and provide a charge stabilization effect.
Nanoparticles without signifi cant stabilization, either natural or synthetic, are prone
to much larger fi ltration artefacts. It has been observed that 100 nm CuO NP in
distilled water can be effectively removed by a 1
µ
µ
m fi ltration due to these effects
(Hassellöv, unpublished data).
There are many fi lter membrane types and pore sizes available, for example
sieve - type fi lters with well defi ned pore size but very small porosity and depth fi lters
with less well defi ned pores, which allow a larger amount of sample to be fi ltered
before clogging artefacts start (Figure 6.4). Sieve-type fi lters are more suitable for
analytical fi ltration but care has to be taken to avoid these artefacts (Morrison and
Benoit, 2001 ).
6.2.5.2
Ultrafi ltration
As been mentioned, ultrafi ltration is based on membranes with very small pores.
Ultrafi ltration can also be used in a piston fi ltration mode in pressurised cells
sometimes with stirring to reduce the concentration polarization. However, a more
common method to reduce concentration polarization and colloid cake formation
is to use a cross-fl ow in a tangential direction to the membrane in a recirculating
fashion. The principle is called cross-fl ow fi ltration and can be used also for micro-
fi ltration but is mainly used for ultrafi ltration (CFUF). The sample is typically
pumped in a recirculating fashion repeatedly over the membrane and in each cycle
a certain part of the sample permeates the membrane (Figure 6.5). Ultrafi ltration
membranes are given a nominal molecular weight cut-off (MWCO) based on typi-
cally 90% rejection of standard molecules, for example proteins. The most com-
monly used MWCO is 1 kDa, which is often used to operationally defi ne the border
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