Environmental Engineering Reference
In-Depth Information
In order to increase the adsorbed amounts of Zn
2+
and Cu
2+
, mmT was previously intercalated with ethylenediamine (edA-mmT)
[48]. edA caused an increase in the adsorbed copper ions of about 28% due to the formation of [Cu(edA)
2
]
2+
in the interlayer.
However, the adsorbed amounts of the Zn
2+
ions on the edA-mmT was three- to fourfold higher than CeC, but no complex
with edA was observed. It was resulted from the expansion of a space between the mmT layers, which facilitates the interca-
lation of the zinc ions and nitrate compensating the excessive positive charge.
Amphoteric surfactants-modified clay exhibited the potential for simultaneously adsorbing heavy metals and organic
pollutants. Boyd et al. used the carboxylic group-bearing carboxydecyltriethylammonium [(HOOC)C
10
H
20
n(C
2
H
5
)
3
]
+
(CdTeA) cations to substitute exchangeable inorganic cations of the mmT [49]. The dual sorptive properties of the
CdTeA-mmT toward Pb
2+
and chlorobenzene were compared with those of its nonfunctionalized analogue, decyltri-
methylammonium (dTmA)-mmT, and with na-mmT. The results indicated that CdTeA-mmT possessed the dual
sorptive properties for both heavy metals and organic contaminants with the noncompetitive nature of the sorption
processes.
An amphoteric biologically based ligand, cysteine (Cys), was also used for the modification of the BnT (n-Ben). The mod-
ified sorbent (Cys-Ben) demonstrated affinity for soft and moderately soft HmI, such as Cd(II) and Pb(II), probably as a result
of the soft basic character of the thiol ligand side chains [50]. Cys-Ben was found to be effective for metal binding with capac-
ities of 0.503 and 0.525 mmol/g for Pb(II) and Cd(II), respectively, much higher than that of n-Ben.
2-(3-(2-aminoethylthio)propylthio)ethanamine (AePe) was used as the ligand for Hect. and mmT to increase the che-
lating affinity toward Hg(II) ions, due to the presence of both n and s donor atoms in the molecule and the ease of syn-
thesis [51]. The x-ray diffraction (xRd) patterns indicated that AePe was mainly grafted on the external surface of
mmT, while AePe was grafted on both the external and interlayer surfaces of Hect. The AePe-modified clay minerals
were good chelating materials for Hg(II) ions, compared to the unmodified clay minerals. The adsorption capacity of
AePe-mmT and AePe-Hect. for Hg(II) was 46.1 and 54.7 mg/g, respectively, for solution containing 140 mg/l Hg(II)
ions (pH 4).
16.2.1.2 Other N-Containing Molecules
The organic chelating agents for HmI have also been immobilized onto the clay
minerals as adsorbents. de Leon et al. revealed the potential of pillared clay as an adsorbent for the removal of metal ions from
effluents [52]. They pillared a Brazilian BnT with 1.10-phenanthroline via the chemical adsorption process for the removal of
Cu
2+
ions. The pillared clay showed a maximum uptake of Cu
2+
ions at approximately 110 mg/g of adsorbent, which represented
more than 10 times the uptake capacity of the untreated BnT. This Cu
2+
ion adsorption was independent of medium pH, and it
was very high when compared with other known adsorbents. However, the adsorbed Cu
2+
ions could not be desorbed from the
pillared clay in either acidic or basic solutions.
2,2′-dipyridyl was used as a complexation agent to be immobilized onto BnT via ion exchange as an adsorbent in the
removal of copper(II) ions [53]. The results indicated that the maximum adsorption capacity was 54.07 mg/g from the Langmuir
isotherm model at 50°C. The thermodynamic parameters indicated that the adsorption process is spontaneous, endothermic,
and chemical in nature.
8-Hydroxy quinolin was modified onto BnT by intercalation of 8-hydroxy quinolinium ion for the removal of Pb(II) [54],
Cd(II) [55], and Cu(II) [56] ions. The maximum adsorption capacity was 142.94, 61.35, and 56.55 mg/g, respectively, for the
three HmI from the Langmuir isotherm model.
In order to enhance the adsorption selectivity toward Hg(II) ions, some sulfur-containing molecules such as 2-mercaptoben-
zothiazole [57] and 2-mercaptobenzimidazole (mBI) [58] were impregnated onto the clay surface for the removal of some HmI
from water samples. The adsorption of Hg(II) increased with increasing pH and reached a plateau in the pH range 4.0-8.0. The
removal of Hg(II) was found to be >99% at an initial concentration of 50 mg/l. The adsorbents could also be used to remove
Cd(II) and Pb(II) from wastewaters.
16.2.1.3 Anionic Surfactants
Ion exchange with cationic surfactants has been widely used to alter the surface properties of
the clay minerals in order to improve sorption ability. Relatively few studies have been made for sorption of heavy metal cations
on clays modified with anionic surfactants such as sodium dodecyl sulfate (sds). The sorption of metal ions by naturally avail-
able clays did not receive much attention, perhaps due to the weak binding strength between them. since BnT clays have a
tendency to swell, large anionic species such as sds can easily enter and become fixed strongly in the interlayer region of, for
example, mmT [59]. In order to adsorb metal cations and form complexes, the clay surface must possess negatively charged
sites or there should be a replacement of weakly held counterions in the solution. sorption of metal cations on anionic surfac-
tant-modified clays is due to the formation of a surface cation complex [60]. Lin and Juang modified the naturally available
mmT by the anionic surfactant sds, which penetrated into the interlamellar region of the clay by the expansion of clay sheets
in the c-axis, for the removal of Cu
2+
and Zn
2+
ions from aqueous solutions [61].
