Environmental Engineering Reference
In-Depth Information
and the subsequent passage of gas bubbles through the solutions. Due to the
surface active nature of the surfactant, the ion-surfactant pairs are concen-
trated at the air/water interface of bubbles which float to the surface of the
solution where they are removed as foams. In general, an ionic surfactant
(known as collector in mineral processing) of opposite charge to the surface-
inactive contaminants is used to induce an electrostatic force between them,
thus forming ion-collector pairs. However, it is possible to use a non-ionic
surfactant capable of forming coordination bonds with contaminants as a
collector [13]. Ion flotation has been widely applied in base metal recovery,
wastewater treatment, removal of radioactive elements from water, and the
recovery of precious metals [14, 15]. The major advantage of ion flotation over
activated carbon adsorption is that air bubbles are relatively inexpensive to
produce and no desorption step is required. However, a stoichiometric ratio of
surfactant molecules to ions to be removed is needed in ion flotation. Therefore,
the process can be quite expensive and may only be used to float ions in
solutions of low concentration.
In summary, each technique reviewed has its own limitations in industrial
applications although they have been practiced to a varying degree. Low
selectivity, complex to operate, high capital and energy costs, and slow separa-
tion kinetics are the commonly inherent shortcomings. In addition, it is also
inefficient in treating waste streams that contain low concentrations of con-
taminants and may fail when handling wastes of complex chemistry. Because
the active materials are difficult to regenerate, these processes generate signifi-
cant amounts of secondary waste.
6.3 Magnetic Composite Sorbents (MCS)
MCS refers to the tailoring of physical, chemical, and surface properties
of magnetic composites to enable selective or non-selective attachment to
the composites of ions, molecules, macromolecules, cells, colloidal particles,
or liquid phases from complex fluid systems [16]. In essence MCS is an inter-
disciplinary subject since it requires an integrated approach involving the
manufacture and surface hybridization of appropriate composites with careful
attention to the constraints imposed by end users. The properties of the com-
posite particles are of critical importance to the successful application of the
technology. The composite particles must fulfill a number of criteria relating to
their shape, size, porosity, mechanical strength, chemical inertness, density,
magnetic properties, wettability, surface charge, surface concentration of reac-
tive groups, cost, ease of manufacture, sterilizability, aggregation properties,
and regeneration [16].
Some application examples of this technology are the use of magnetite
particles to accelerate the coagulation of sewages [17], removal of radio
nuclides from milk by functionalized polymers such as resin with embedded
