Environmental Engineering Reference
In-Depth Information
stability, layered structure, high cation exchange capacity (CeC), and so on, have made clays excellent adsorbent materials
toward heavy metal ions (HmI) [13], dyes [14], pesticides [15], and other organic matters [16].
The prominent cations and anions found on the clay surface are Ca
2+
, mg
2+
, H
+
, K
+
, nH
4
+
, na
+
, and sO
4
2−
, Cl
−
, PO
4
3−
, and
nO
3
−
. These ions can be exchanged with other ions relatively easily without affecting the clay mineral structure. The edges and
the faces of clay particles can adsorb anions, cations, and nonionic and polar contaminants from natural water. The contami-
nants accumulate on the clay surface leading to their immobilization through the processes of ion exchange, coordination, or
ion-dipole interactions. sometimes the pollutants can be held through H bonding, van der Waals interactions, or hydrophobic
bonding arising from either strong or weak interactions. The strength of the interactions is determined by various structural and
other features of the clay mineral [13].
There have been various attempts to improve the quality and characteristics of clays by modifying them using different tech-
niques, such as acid activation [17], which can improve their exchangeability, or surface modifications with inorganic materials
such as magnetic nanoparticles [18-20] or organic molecules [21-26]. However, the adsorption selectivity of clay adsorbents
seems to be low due to the ion-exchanging interaction between pristine clays, acid-treated clays, or the magnetic clay nanocom-
posites and HmI. On the contrary, the functional groups introduced via surface modifications with organic molecules can obvi-
ously improve their adsorption selectivity toward HmI [13, 25]; therefore, the surface organo-modification of clay minerals has
become increasingly important for improving the practical applications of clays and clay minerals.
In the present chapter, recent advances in the preparation of the organo-clay nanohybrids via surface modification with small
organic molecules or polymers, as well as their adsorption performance toward the toxic HmI, are reviewed. The surface mod-
ification techniques and the adsorption selectivity of the resultant organo-clay nanohybrid adsorbents are emphasized. And the
practical application prospects of the organo-clay nanohybrid adsorbents are also prospected.
16.2
OrganO-mOdifiCatiOn with small mOleCules
16.2.1
electrostatic interactions or intermolecular forces
due to high CeC and exchangeable cations, ion exchange with organic cations has been used to modify the surface properties
of clay minerals. Organic cation intercalation or modification plays an important role in clay/polymer nanocomposite formation
by providing a hydrophobic environment for the intragallery adsorption of the polymer precursor [27].
16.2.1.1 Quaternary Ammonium Salts or Amines
Wang et al. developed a simple two-step approach to design several
modified clays for selective removal and recovery of heavy metals via ion exchange and hydrophobic anchoring of several
surfactants such as long-chain alkyldiamines (cetylbenzyldimethylammonium, CBdA), long-chain dialkylamines (alkyl-
1,3-diaminopropane, dT), and long-chain carboxylic acids (palmitic acid, PA) onto clay matrices (hectorite (Hect.) or montmo-
rillonite (mmT)) [28]. The adsorption capacities and affinity constants of the organo-modified clays were found to approach
those of the commercial chelating resin (Chelex 100, Bio-Rad). Adsorption had been shown to be mainly through chemical
complexation rather than ion exchange. The maximum adsorption capacity of the modified clay toward Cd(II) ion was found to
be 42 ± 0.8 mg/g clay with affinity constant of 3.0 ± 0.1 mg/l. The adsorption of the metal ions was pH-dependent, so pH can act
as a molecular switch to regenerate the modified clay complex adsorbents.
Bhattacharyya et al. prepared the organo-clay adsorbent for Cd(II) [29] and Cu(II) ions [30] via the immobilization of tetra-
butylammonium bromide (TBA) onto the surfaces of kaolin and mmT. After calcination of the TBA-modified mmT (TBA-
mmT), it also had a higher adsorption capacity than the corresponding clay. The calcined TBA-mmT had only 44, 41, and 42%
of the adsorption capacity of the parent mmT with respect to fe(III), Co(II), and ni(II), indicating an actual decrease in the
number of adsorption sites for taking up metal ions [31].
2-Oxyhydrazino-
N
-(2-methylen-yl-hydroxyphenyl)pyridinium (OmHP) ion was immobilized onto na-mmT clay, and the
modified clay (OmHP-mmT) was used in the removal of Cu(II) [32]. It showed good removal efficiency and selectivity toward
Cu(II) at pH 3.0-8.0 and stirring time 10 min with a removal capacity of 119 meq/100 g. most common ions did not interfere
with the removal process except fe
3+
. And the adsorbed Cu(II) could be quantitatively recovered by 10 ml 1% thiourea in
0.1 mol l
−1
HCl with 100-fold preconcentration factor. The same group had also modified bentonite (BnT) with methylene blue
(mB) [33]. The modified clay (mB-BnT) showed good selectivity toward Hg
2+
with an extraction capacity of 37 mequiv./100 g
in the presence of I
−
, giving rise to a ratio of mB/Hg
2+
/I
−
1:1:3 in the clay phase. And the adsorbed Hg
2+
could be quantitatively
recovered by ammonia buffer (0.05 m nH
4
Cl/nH
4
OH) at pH 8.5.
The unmodified clay minerals show no affinity for chromate due to their negatively charged surfaces. Prakash et al. had
modified the surfaces of the clay mineral such as kaolinite (kaolin), mmT, and pillared mmT with hexadecyltrimethylammo-
nium bromide (HdTmAB) for the removal of the chromate [34]. It was found that mmT could adsorb a quantity of HdTmAB
