Environmental Engineering Reference
In-Depth Information
Tetrahedral
Octahedral
O
Tetrahedral
O
1.81
Sr
2+
O
O
O
O
O
O
O
O
Sr
2+
DCH18C
Na
+
Na
+
1.42 nm
-Na
+
O
O
Ben-Crown
Bentonite
Ben-Crown
figure 16.1
Intercalation of crown ether in bentonite (BnT), leading to propping of the layers and the ion exchange behavior of
Ben-Crown.
Humic acid-modified Ca-mmT was prepared to adsorb copper (II), cadmium (II), and chromium (III) ions from aqueous
solutions [62]. Batch experiments were carried out to investigate the possible adsorption mechanisms of the metal ions on
humic acid-modified Ca-mmT. The results showed that the adsorption capacities of the modified clay for the metal ions were
improved significantly as compared to that of the raw clay. The maximum adsorption capacities followed the order of
Cr(III) > Cu(II) > Cd(II) for both materials.
16.2.1.4 Neutral Molecules
Crown ether is known to complex with numerous metals and is widely used for the adsorption
of HmI. They can also be intercalated between the layers of 2:1 BnT saturated with alkaline or alkaline earth cations.
dicyclohexano-18-crown-6 was modified to BnT for the adsorption of cesium and strontium (fig. 16.1) [63]. The extraction
data were fitted by the Langmuir adsorption model, and the apparent experimental exchange capacity obtained by linear regres-
sion analysis was in good agreement with the amount of crown ether (0.22 mmol/g) intercalated in BnT.
Liu et al. modified BnT with 4′-methylbenzo-15-crown-5 for the adsorption of HmI (Cu
2+
and Pb
2+
) [64]. for BnT modi-
fied with mB15C5 (mB15C5-BnT) and natural BnT (n-BnT), the equilibrium data closely fitted the Langmuir model and
showed the affinity order Pb
2+
> Cu
2+
, and the adsorption capacity of mB15C5-BnT was higher than that of n-BnT for Pb
2+
and Cu
2+
. The adsorption of Cu
2+
and Pb
2+
increases with increasing pH, and the adsorption of Cu
2+
and Pb
2+
reached a
maximum at pH 3.5-6.
16.2.2
Covalent modifications
The most widely used surface covalent organo-modification method is to modify the clay with functional silanes. Celis et al.
functionalized sepiolite by covalently grafting 3-mercaptopropyltrimethoxysilane (mPs) to the surface silanol groups of the
clay, whereas mmT was functionalized by replacement of the interlayer inorganic cation (na
+
) by 2-mercaptoethylammonium
(meAm) cations [65]. The functionalized clays adsorbed most of the Hg(II) ions present in solution up to saturation and were
also good adsorbents of Pb(II) at low metal ion concentrations (i.e., <0.02 mm). They were, however, less effective toward
Pb(II) and Zn(II) at high metal ion concentrations. The presence of nanO
3
or Ca(nO
3
)
2
as background electrolytes at concen-
trations ranging from 0.001 to 0.1 m did not alter the great adsorption capacity of functionalized sepiolite for Hg(II).
The BnT from Campina grande (Paraíba, PB), Brazil, was modified by acid treatment followed by immobilization of ligands
containing thiol (-sH) groups by covalent grafting with surface and interlayer silanol groups with mPs under anhydrous con-
ditions (fig. 16.2) [66]. The unmodified BnT adsorbed silver ions in negligible amounts (0.06-0.08 mmol/g of freeze-dried
BnT) due to the cation exchange mechanism. The functionalized samples revealed a high affinity toward Ag
+
(0.85-1.03 mmol/g
of clay), about 10 times higher than the unmodified one. The results suggested that the mechanism of adsorption involves pri-
marily silver ion complexation by the thiol groups instead of cation exchange.
natural and acid-activated sepiolite samples were functionalized with mPs, and the functionalization of sepiolites by grafting
silane reagents occurs mainly on the surface, whereby their crystalline structure remains unchanged [67]. The efficiency of the
adsorbents in Cr(VI) removal from aqueous solutions follows the order: functionalized acid-activated sepiolite > functionalized
natural sepiolite > acid-activated sepiolite > natural sepiolite. The adsorption capacity was strongly dependent on the pH of the
solution from which the adsorption occurred. maximum Cr(VI) removal was ca. 8.0 mg Cr(VI) per gram of functionalized
