Environmental Engineering Reference
In-Depth Information
14.3.1 Adsorption Technologies
Among water treatment technologies, adsorption is one of the most promising and fre-
quently used techniques owing to its convenient application and superior eficiency. The
features of an ideal adsorbent often embrace stronger afinity to target compounds/ions
and more binding sites on its surface. Therefore, various MNMs have been synthesized
and modiied for water treatment by altering the particle size, shape, and surface chem-
istry, which have been proven eficient in removing a large number of contaminants that
can be categorized into three groups, i.e., heavy metals, organic pollutants, and radioactive
elements.
14.3.1.1 Sorbents for Heavy Metals
Heavy metal contamination has been of great concern because of its adverse effect on
living creatures and its tendency toward bioaccumulation even at low concentrations.
Therefore, removal of heavy metals from natural and industrial water is extremely urgent
and has attracted considerable attention [136-140]. MNMs are one of the promising candi-
dates for bench-scale research and ield applications [141,142]. To date, large numbers of
MNMs with/without modiication or functionalization proposed have been referred to
removal of heavy metals in different forms.
It has been reported that heavy metals in both anion and cation species can be effectively
removed from aqueous solutions by using simple magnetic nanocrystals. For example,
monodispersed Fe
3
O
4
nanocrystals can be directly used to remove both As(III) and As(V)
species from aqueous solutions via the strong and speciic interactions [143,144]. Fe
3
O
4
nanocrystals were also used to remove Pb(II) ions, with a maximum adsorption capacity
of 36.0 mg/g, much higher than that of low-cost adsorbents [145]. γ-Fe
2
O
3
, as the oxidation
counterpart of Fe
3
O
4
, was also proved to be effective in removing As(V) [146] and Cr(VI)
[133,147]. The removal eficiency for those heavy metal species depends strongly on the
size of the magnetic nanocrystals. The smaller the size of the MNMs, the more favorable it
is for metal ions diffusing from solution onto the active sites of the sorbents, which in turn
increase the removal eficiency. In a typical separation process using simple nanocrystals
as sorbents, the speciicity independently refers to the sorption constant
K
; the pH value is
a key parameter on the adsorption capacity because pH controls the surface charge density
of sorbents and the chemical nature of metallic ions.
For these simple nanocrystals, there are several drawbacks that are unfavorable for appli-
cation in water treatment, i.e., aggregation derives from high surface energy of nanoscale
dimension, easily etching nature in acid and alkaline environment, as well as the application
limitation resulting from the single surface group that has poor selectivity prone to competi-
tion by coexisting interfering ions when used in adsorption [148]. Therefore, MNMs were
developed by being modiied with functional groups (Figure 14.8) (e.g., ligands, surfactants,
polymers, silica, and carbons); designing the size and structure; and altering active func-
tional group numbers to improve their stability, capacity, and selectivity [8,25,142,149-158],
in which surface modiication is a versatile technique favorable not only for protecting mag-
netic cores from oxidation to retain high saturation magnetism for rapid isolation but also for
enhancing their water compatibility and the diffusion rate of heavy metals onto the sorbent
surfaces. What is more, it is convenient to graft different functional groups to meet the pur-
pose of improving selectivity and effectiveness in heavy metal removal.
The mechanisms of adsorption of contaminant by those functionalized MNMs mainly
involve surface site binding, electrostatic interaction, and modiied ligands combination
