Environmental Engineering Reference
In-Depth Information
advantages [6-8]. During a standard magnetic separation process, these MNMs are used
for ixing speciic metal species/molecules or for degrading toxic pollutants; a magnetic
ield is utilized as the main force for easily isolating these MNMs and the tagged contami-
nants from aqueous solutions [9-14]. The puriication process does not generate secondary
waste, and these MNMs can often be recycled.
The magnetic component of most of the MNMs used for treating wastewater is magnetite
Fe
3
O
4
(or its oxidation counterpart γ-Fe
2
O
3
). In recent years, the preparation and utilization of
these iron-based MNMs with novel properties and functions have been widely studied owing
to the easy synthesis, coating or modiication, and the ability to control on an atomic scale
[15,16]. Since the practical applications depend on the properties of these MNMs involving
chemical stability, water compatibility, and the afinity to the target compounds/ions, they are
often modiied/functionalized with a series of media with suitable functional groups, such as
silicon, phosphoric acids, carboxylic acid, and amine [15-19]. In addition, it has been proven
that the size distribution, morphology, magnetic properties, and surface chemistry of MNMs
depend on the preparation methods and surface coating media [20]. Therefore, a variety of
synthesis approaches have been developed for preparing MNMs of high quality [16,21-24].
The synthesis approaches can be divided into three classes: (i) physical methods involving
gas-phase deposition, electron beam lithography, pulsed laser ablation, laser-induced pyrol-
ysis, powder ball billing, and aerosol, etc.; (ii) chemical methods involving coprecipitation,
microemulsion, hydrothermal and electrochemical deposition, sonochemical and thermal
decomposition, etc.; and (iii) biological methods mediated by fungi, bacteria, and protein, etc.
Many of the prepared iron-based MNMs have presented promising potential for industrial-
scale wastewater treatment at both laboratory- and ield-scale tests [25,26].
In this chapter, a brief introduction on the preparation of iron-based MNMs and their
applications as nanosorbents and photocatalysts for water treatment have been given. In
addition, the likely fates of MNMs discharged into the environment are also discussed.
14.2 Synthesis of Iron-Based MNMs
Given their unique characteristics and great potential for water treatment on an industrial
scale, many efforts have been made for synthesizing iron-based MNMs of high quality and
of particular usage by developing synthesis approaches involving chemical, physical, and
biological methods. Figure 14.1 presents the three most important fabrication approaches
published for synthesizing superparamagnetic iron oxide nanoparticles (SPIONs), sum-
marized by Mahmoudi et al. [27]. Consider that the applications of iron-based MNMs in
water treatment greatly depend on the particle size, shape, and surface chemistry [20], as
well as the degree of the structural defects or impurities present in the NMs [28]. Various
preparation methods have been exploited to meet the application demand. Hydrolysis of
ferrous salts in alkaline solution is the most simple and fundamental method for prepar-
ing magnetic Fe
3
O
4
and γ-Fe
2
O
3
, the products of which are usually dificult to control in
size and structure. Therefore, hydrothermal and solvothermal methods and many other
chemically and physically based methods were developed for easily controlling the size
distribution and tailoring the structure to extend the application spectrum of MNMs.
Generally, iron-based MNMs can be divided into three categories, including simple nano-
crystals, structurally functionalized MNMs, and chemically functionalized MNMs—the
preparation approaches of these MNMs are introduced in detail in the following section.
