Page 41 - Membranes for Industrial Wastewater Recovery and Re-Use

Membrane technology 2 1

carboxylate groups, respectively, which are more hydrophilic and normally
negatively charged. Oxidative chemicals can also be used to promote grafting of
hydrophilic monomers onto the membrane surface. Plasma treatment involves
the generation of a plasma by continuous electrical discharge in a gas containing
oxygen or water (to produce oxygen-containing functional groups), or nitrogen,
ammonia or amines (to produce nitrogen-containing functional groups). Plasma
treatment offers the advantages of being fairly controllable, since the reaction
conditions can be adjusted to produce the required degree of reaction, and
demands no hazardous liquids.
Chemical reaction and grafting add an organic layer to the surface of
the polymer, rather than functionalising it in the way that the above
oxidative reactions do. The surface can also be modified by specific organic
chemical reactions which rely on the chemical reactivity of the polymer.
Polysulphone, for example, will react with chloroether to produce the ortho-
chloromethylated polymer (Guiver et al., 1993). Polymer grafting can be
promoted chemically or thermally but more usually involves irradiation in the
presence of a suitable initiator and, invariably, vinyl or acrylic monomers.
Ion exchange membranes, whose most important property is permeability to
counter-ions and impermeability to co-ions, can be produced either by
functionalisation of a membrane, as mentioned above, or by immobilising
powdered ion exchange resins in an inert resin matrix. Functionalisation
produces a homogenous material that, as with many membranes, requires fixing
on a suitable material to give it mechanical strength. The powder immobilisation
method is relatively inexpensive but the materials produced are heterogeneous,
less selective for counter-ions and have inferior mass transport properties than
the equivalent more expensive homogeneous materials.

Recent developments. membrane materials
The search for new membrane materials with superior properties with respect to
permeability and resistance to chemical, thermal and biological attack is
ongoing. Of greatest interest is fouling resistance, and this demands the
development of membranes with a low affinity to pre-identified foulants in
the feedstock. The ease with which this can be achieved is to a large extent
dependent on the heterogeneity of the feedwater matrix, since the membrane
surface cannot be modified so as to be able to repel contaminants of different
charges and hydrophobicities. In general natural foulants tend to carry a
negative charge, which then demands that the membrane also carries a negative
charge to repel the foulant. It should be pointed out, however, that some filters
are surface modified to carry a positive charge (e.g. the Pall “Posidyne” nylon-
based filter) to attract negatively charged suspended materials, but these filters
are actually depth filters designed to retain particles rather than reject them.
One application that has received a great deal of interest is the removal of
natural organic matter (NOM) from upland surface waters for potable water
production using nanofiltration membranes. NOM contains a number of
different fractions of which humic acids are one. Humic acids, which contain
phenolic and carboxylate functional groups, tend to be negatively charged in