Chemistry Reference
In-Depth Information
involve filtration, flocculation, activated charcoal, and ion exchange resins, which
are expensive and can result in toxic exposures of the workers involved. Recently,
due to its chelating capacity for cations, low price, rich resources, and environmen-
tal kindness, Ch has been developed as one of the most popular adsorbents for the
removal of metal ions from aqueous solution and is widely used in water treatment
applications [
126
,
127
]. The capacity of Ch to complex metallic ions is one of its
most important potentialities. However, the adsorption ability of Ch has not been
realized to a satisfying level. Many chemical modifications of Ch have been made
to improve the cation adsorption capacities. For example, carboxymethylation was
prepared and regarded as a simple and effective process to facilitate the adsorption
ability of Ch with heavy metals [
128
,
129
]. Farag and Kareem [
130
] reported that
the adsorption capacities of Pb
2+
ions increased with increasing the degree of
substitution of CMCh at the range of 0.8-1.5 mmol g
1
sample and that the
formation of cross-linking between the amino group and aldehyde group improved
the adsorption capacity up to maximum before a decrease. In addition, Sousa et al.
[
131
] chemically modified Ch with ethylene sulfide under solvent-free conditions to
give the higher capacity for cations removal from aqueous solution. Emara et al.
[
132
] synthesized two modified Ch polymers by reaction of Ch with cynnamoyl
chloride (Ch-Cin) and cinnamoyl isothiocyanate (Ch-ThioCin). The metal uptake
capacity of the two polymers was measured at different pH values. The results
indicated that the order of the overall rate constant
K
a
for Ch-Cin was Fe(III)
>
Cr
(III)
Cr(III).
Jiang and his group [
133
] have prepared poly(sodium 4-styrenesulfonate) (PSS)-
graftedCh by nitroxide-mediated polymerization (NMP) of sodium4-styrenesulfonate
in the presence of Ch-4-hydroxy-2,2,6,6-tetramethyl piperidine-1-oxyl (4-hydroxy-
TEMPO) macro initiator. The results indicated that the Ch-g-PSS could be used as
ion exchanger for the environment protection and that the ion exchanging property of
Ch-g-PSS could be controlled by adjusting the PSS graft content.
The results obtained by Sabaa and his group [
109
] showed that the amount of
metal ions uptake (Ni
2+
and Co
2+
) by Ch is highly increased by grafting with
polyacrylonitrile (PAN) and that this increase is highly pronounced when the nitrile
groups in the grafted samples were converted into the more polar amidoxime
groups. The results also indicated that CMCh-g-PAN, on the other hand, adsorb
less metal ions (Ni
2+
,Co
2+
,Cd
2+
, and Cu
2+
) from aqueous solutions than the parent
CMCh, while their amidoxime derivatives adsorb more metal ions than both the
graft copolymers and CMCh [
110
].
The influence of pH on zinc ion binding ability of both Ch and Ch-grafted
PMMA (Ch-g-PMMA) was investigated by Muzzarelli [
134
] and was found to be
higher at alkaline pH (pH 8 for Ch and pH 10 for Ch-g-PMMA). In addition, the
results obtained by Singh and his group [
135
] have proved that the zinc ions binding
capacity of Ch is markedly increased by the presence of PMMA grafts, which
provide additional metal binding sites. Since hydroxyl groups of Ch are highly
hydrated and are known not to be adsorption sites, the amino groups of Ch and the
ester groups at grafted sites are the sites responsible for metal ion adsorption. Since
at the acidic pH the amino groups remain protonated and are not free for binding,
>
Cu(II), while for Ch-ThioCin, it was Fe(III)
>
Cu(II)
>
