Environmental Engineering Reference
In-Depth Information
[196]. In this system, the addition of gold species caused the light absorption to shift toward
the red visible region, with the band-gap energy declining. Magnetically recyclable Fe@
Pt core-shell nanoparticles have been proposed as electrocatalysts for ammonia borane
oxidation, which is far more active (by up to 354%) than the commercial Pt/C catalysts
[197]. Different from altering band gaps of the photocatalysts, our groups have proposed a
semiconductor-insulator-semiconductor structure of Fe 3 O 4 @TiO 2 @polypyrrole for degra-
dation of Orange II. Under radiation conditions, the photo-induced internal electric ield,
with electrons accumulated in the outer layer of Fe 3 O 4 microspheres and correspondingly
more holes concentrated in the inner layers of the PPy coating, can greatly enhance the
transfer of the photo-induced electrons to the surface of the PPy coating, resulting in the
enhancement of the separation of photo-induced carriers and more •OH radicals pro-
duced through e paths (unpublished data).
Owing to its narrow band gap, Fe 2 O 3 has also been proposed as a sensitizer of photo-
catalysts, including TiO 2 nanoparticles [187,198]. Electrons in the valence bands of TiO 2 can
be driven into Fe 2 O 3 because of the formation of the built-in ield in an Fe 2 O 3 -TiO 2 hetero-
junction. The increase in the electron-hole recombination time can promote photocatalytic
activity of the composition [199,200].
Recently, a novel photo-Fenton-like system with the existence of iron oxides and oxalate
has been proposed for water treatment. For example, the heterogeneous iron oxide-oxalate
system, with iron oxides mainly acting as a photocatalyst and oxalic acid being excited
to generate electron-hole pairs, has been prepared for the degradation of Orange I [201].
In this system, a strong ligand-to-metal charge transformation ability is attributed to the
degradation of Orange I and the photochemical reduction of the Fe(III) complex is coupled
to a Fenton reaction.
14.4 Conclusions
During the past decades, various iron-based MNMs, such as simple crystals, structure-
and chemical-functionalized MNMs, with a wide range of compositions and tunable sizes,
have been fabricated by developing synthesizing approaches including series of chemical,
physical, and biological methods for their promising applications ranging from biotech-
nology to environmental remediation. In water treatment, employing iron-based MNMs
to adsorb heavy metals and organic pollutants is one of the most attractive and success-
ful applications owing to their high selectivity and effectiveness. What is more, MNMs
used as photocatalysts have also presented their superiority even with a source of visible
light. However, their studies/applications are still at a relatively early stage and many tech-
niques are still at an experimental or pilot stage. More work including advancing the com-
bination of the superior adsorption performance and magnetic properties of iron oxide to
meet the actual large-scale operation is still needed. In addition, many potentially seri-
ous issues concerning their environmental fate and potential impacts on human health
are inevitable because of their potential applications in many frontiers of the environ-
ment. Currently, very little information on their possible background concentrations and
physical-chemical forms have been given as a result of limitations in their separation and
analytical methods. Therefore, it is urgent to develop accurate and robust methodologies
for determining their concentrations and forms in the environment, and then to evaluate
their effects on the environment.
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