Environmental Engineering Reference
In-Depth Information
adsorb Ag
+
, Hg
2+
, and Pb
2+
from aqueous solutions [159]. Thiol-functionalized magnetic
mesoporous silica was further developed to remove Pb
2+
and Hg
2+
from aqueous solutions,
with an adsorption capacity of as high as 260 and 91.5 mg/g
of sorbent, respectively [160].
It was found that the removal eficiency of Hg(II) using mercapto-functionalized nano-
magnetic Fe
3
O
4
polymers was highly pH dependent and related to the content of the Fe
3
O
4
core [161]. An amino group was also grafted onto MNPs and proposed for removing Cu(II)
and Cr(VI) from water, with an adsorption capacity of 12.43 and 11.24 mg/g, respectively
[162]. Magnetic chitosan nanocomposites have been synthesized on the basis of amine-
functionalized magnetite nanoparticles and applied for the removal of heavy metals [163].
Generally speaking, all those prepared nanosorbents have presented good reusability and
stability in water treatment, suggesting their potential for practical applications.
However, most of the technologies on iron-based MNMs for removing heavy metals
are still at a relatively early stage. More work is needed to advance knowledge of these
sorbents, including the factors of practical environmental conditions and the supply of the
strong magnetic ield for their transfer from laboratory- to ield-scale application.
14.3.1.2 Sorbents for Organic Contaminants
In the past decades, large numbers of MNMs based on magnetite (Fe
3
O
4
) or maghemite
(γ-Fe
2
O
3
) have been proposed for removing organic compounds from aqueous solutions
[5,142,164]. It has been reported that some MNMs with unique structures exhibit satisfy-
ing performance in removing some organic compounds existing in ionic forms. Azo dye
contaminants, a kind of widespread and severe pollutant in many places worldwide, are
typical target organic compounds for magnetic sorbents. In these cases, it is electrostatic
attraction between the iron oxide surface and the compound species in solution that is
responsible for adsorption [165]. For example, it was reported that Fe
3
O
4
hollow nano-
spheres present an effective removal for red dye, with the maximum adsorption capacity
reaching 90 mg/g [142]. 3-D lower-like iron oxide nanostructures have been proven an
outstanding sorbent for removal of Orange II, with the removal capacity of 43.5 mg/g [63].
Exchange reactions are another mechanism that accounts for the adsorption of some
organic contaminants. Such adsorption processes can often be divided into two steps: sur-
face exchange reactions irst take place until the surface functional sites are fully occupied,
and then during the second step, the contaminants diffuse into the sorbents for further
inner surface interactions [15,166,167]. Sorbents in processes are usually not single mag-
netic nanocrystals, but the composite magnetic sorbents are either modiied with various
functional groups to improve afinity toward contaminants or are constructed with unique
structures to prevent aggregation and supply particular transfer paths. Usually, magnetic
seeding is only used to realize the magnetic susceptibility and the concomitant comfort-
able separation from aqueous solutions, while functional groups play the role of select-
ing and adsorbing organic contaminants. To date, series of organic compounds such as
dyes, pesticides, phenols, and some emerging organic pollutants, including antibiotics and
endocrine-disrupting chemicals, have been proven to fall into such separation processes.
Activated carbon, one of the attractive and inexpensive materials for removing con-
taminants owing to its high surface area, porous structure, and easy functionalization,
is one of the most frequently funtionalized media for preparing multicomponent MNMs
[168,169]. Those magnetic functionalized carbon MNMs have been proven to present good
performance in the removal of organic compounds such as chloroform, phenol, chloroben-
zene, pesticides, and dyes [170]. For example, Fe
3
O
4
@C MNMs prepared using powder-
activated carbon and Fe
3
O
4
nanoparticles can be used to effectively remove amoxicillin
