Environmental Engineering Reference
In-Depth Information
some of the atoms or irons in the material have a net magnetic moment due to unpaired
electrons in partially filled orbitals. However, the individual magnetic moments do not
interact magnetically, and the magnetization is zero when the magnetic field is removed.
In the presence of a field, there is now a partial alignment of the atomic magnetic
moments in the direction of the field, resulting in a net positive magnetization and
susceptibility. The atomic moments in ferromagnetic materials exhibit very strong
interactions. These interactions are produced by electronic exchange forces and result in
a parallel or antiparallel alignment of atomic moments. For ionic compounds, such as
oxides, more complex forms of magnetic ordering can occur as a result of the crystal
structure.
Another type of magnetic ordering is called ferrimagnetism. The magnetic
structure is composed of two magnetic sublattices (called A and B) separated by
oxygens. The exchange interactions are mediated by the oxygen anions. When this
happens, the interactions are called indirect or superexchange interactions. The strongest
superexchange interactions result in an antiparallel alignment of spins between A and B
sublattices. In ferrimagnets, the magnetic moments of sublattices are not equal and result
in a net magnetic moment. It exhibits spontaneous magnetization and weaker magnetic
property than ferromagnetism. If the A and B sublattice moments are exactly equal but
opposite, the net moment is zero. This type of magnetic ordering is called
antiferromagnetism with no collective magnetic interactions and out of magnetic order.
Ferromagnetic and ferrimagnetic materials are usually considered as being
magnetic, and the remaining three are so weak in magnetism that they are usually
thought of as nonmagnetic (McCurrie, 1994). Thus, in order to use the magnetic
property to separate the particles from water, the material with stronger magnetic
property should be chosen as adsorbent. Nevertheless, if the magnetic property is too
strong, the nanoparticles will agglomerate by their magnetic attraction in the solution
which leads to part but not all of the surface area being exposed, thereby lowering the
adsorption capacity. Thus, ferrimagnetic nanoparticles probably are the best choice for
the adsorption of heavy metals.
With the latest developments in nanotechnology, various types of magnetic
nanoparticles have been successfully synthesized and have received considerable
attention to solve environmental problems. Wang and Zhang (1997) conducted batch
studies on the dechlorination of trichloroethylene (TCE) and polychlorinated biphenyls
(PCBs) using nanoscale iron particles and the results showed that these nanoscale
particles could quickly and completely dechlorinate several chlorinated aliphatic
compounds and a mixture of PCBs at a relatively low iron nanoparticles to solution ratio
(2-5 g/100 mL). Watson and Cressey (2000) observed that the biologically produced
iron sulfide nanoparticles had surface area of 400-500 m 2 /g and the adsorption capacity
for Cu and Cd was up to 200 mg metal /g adsorbent. Watson and coworkers (2001)
 
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