Chemical Engineering Journal 171 (2011) 1276–1286
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Chemical Engineering Journal
jo u r n al hom epage: www.elsevier.com/locate/cej
Adsorption of heavy metals in acid to alkaline environments by montmorillonite
and Ca-montmorillonite
Liberto de Pablo
a,∗
, M. Lourdes Chávez
b
, Mohamed Abatal
a
a
Instituto de Geología, Universidad Nacional Autónoma de México, 04510 México, DF, Mexico
b
Facultad de Química, Universidad Nacional Autónoma de México, 04510 México, DF, Mexico
a r t i c l e i n f o
Article history:
Received 10 March 2011
Received in revised form 11 May 2011
Accepted 12 May 2011
Keywords:
Heavy metals adsorption
Montmorillonite adsorption
Ca-montmorillonite adsorption
a b s t r a c t
Adsorption of Hg
2+
, Cr
3+
, Pb
2+
, Cu
2+
, Zn
2+
, Ba
2+
, Ni
2+
, Mn
2+
, Cd
2+
, Ba
2+
and Ag
+
by Ca-montmorillonite
from monocationic solutions followed second-order kinetics, attaining removals q
e
of Pb
2+
6.187 × 10
2
,
Hg
2+
4.369 × 10
2
, Cu
2+
2.045 × 10
2
, Zn
2+
1.576 × 10
2
, Ag
+
1.375 × 10
2
, Cr
3+
7.917 × 10
1
, Ni
2+
7.300 × 10
1
,
Ba
2+
6.815 × 10
1
, Cd
2+
2.003 × 10
1
and Mn
2+
1.632 × 10
1
mg g
-1
, greater than those by montmorillonite.
Equilibrium adsorption by montmorillonite from bimodal metal + Ca
2+
solutions followed the Langmuir
model with adsorptions of Zn
2+
3.477 × 10
1
, Ba
2+
6.984, Hg
2+
3.681 × 10
1
, Ni
2+
4.849, Mn
2+
3.820 and Cd
2+
6.978 mg g
-1
, and by Ca-montmorillonite of Zn
2+
3.523 × 10
1
, Ba
2+
1.073 × 10
1
and Ni
2+
5.969 mg g
-1
.
Within the pH range established by acid cationic solutions and near neutral montmorillonite and Ca-
montmorillonite, at pHs below 5 precipitate oxides and hydroxides of Cr
3+
, Pb
2+
and Cu
2+
while at higher
pHs Hg
2
2+
, Zn
2+
, Ba
2+
, Cd
2+
, Ni
2+
, Mn
2+
and Ag
+
remain in solution and are adsorbed as simple and hydroxyl
cations by the clay, keeping the selectivity sequence Hg
2
2+
> Zn
2+
> Ba
2+
> Cd
2+
> Ni
2+
> Mn
2+
.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Clay minerals have well-recognized adsorption capabilities
that have extensive industrial, technological, agricultural, and
environmental applications. The smectite minerals, namely mont-
morillonite, have been broadly investigated as adsorbents of heavy
metals from acidic solutions. The adsorption of these metals from
weak acid to alkaline calcic solutions is not as well documented.
Therefore, the present study examines adsorption by montmoril-
lonite and Ca-montmorillonite of Pb
2+
, Cu
2+
, Cr
3+
, Zn
2+
, Ba
2+
, Hg
2+
,
Ni
2+
, Mn
2+
, Cd
2+
and Ag
+
from weak acid to alkaline solutions.
Heavy metals, whether they are simple ions normal in acidic
environments or hydroxyl ions common in weakly acidic to alkaline
media, are a serious environmental problem because of their toxi-
city and abundance. Excessive accumulation of metal ions in living
organisms, including the human body, and ecological systems can
cause decreased stabilization and vegetation of soils, health disor-
ders, and physical, chemical, and biological damage. Heavy metals
are non-biodegradable, making them accumulate to toxic levels in
∗
Corresponding author at: Instituto de Geología, Universidad Nacional Autónoma
de México, Ciudad Universitaria, México, 04510 DF, Mexico. Tel.: +52 555 6224284;
fax: +52 555 6224317.
E-mail address: liberto@servidor.unam.mx (L. de Pablo).
soil available for plant and animal uptake when they are released
into water and soil fluids. This process allows metals to enter the
food chain, which ends with human consumption [1–3].
The adsorption of metals from aqueous acidic solutions
by clay minerals has been extensively investigated [4–14].
Generally, clay minerals adsorb metals from aqueous solu-
tions through cation replacement or by forming surface func-
tional groups that normally observe the selectivity rule of
Ca > Hg > Pb > Cu > Mg > Zn > Co > Ni > Cd. Unlike with acidic solu-
tions, the adsorption of heavy metals from alkaline solutions has
not been investigated intensely. Studies have shown that addi-
tion of Ca
2+
promotes alkalinity and retention of Cu
2+
and Cd
2+
[15]. The technique also improves adsorption by calcareous soil
of Ni
2+
, Co
2+
and Cu
2+
from metal-Ca binary solutions, maintain-
ing the affinity series Pb > Cu > Ni > Cd > Zn [16]. Bhattacharyya and
Gupta [17] described the removal and immobilization of Cd
2+
, Pb
2+
and Cu
2+
by montmorillonite in acid to slightly alkaline media and
that of Cd
2+
, Co
2+
, Cu
2+
, Pb
2+
and Ni
2+
by acid-activated montmo-
rillonite. Anomalous adsorption of metal, which occurs in excess
of the cation exchange capacity (CEC) of clay, has been investi-
gated for some metals, e.g., Cu
2+
and Zn
2+
by H-montmorillonite
[18] and Cu
2+
by Na- and Ca-montmorillonite [19] and by Cheto
clay [20]. Adsorption by montmorillonite was associated with
swelling [2,21,22]. Adsorption under acidic conditions with tran-
sition from mono- to di-hydrated stages [23] has been shown to
1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.cej.2011.05.055