Chemical Engineering Journal 162 (2010) 97–105
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Chemical Engineering Journal
journal homepage: www.elsevier.com/locate/cej
Thallium adsorption onto polyacryamide–aluminosilicate composites: A Tl
isotope tracer study
Zeynep Mine S ¸ enol, Ulvi Ulusoy
∗
Cumhuriyet University, Department of Chemistry, Sivas 58140, Turkey
article info
Article history:
Received 11 February 2010
Received in revised form 4 May 2010
Accepted 5 May 2010
Keywords:
Thallium
Adsorption
Aluminosilicate
Composite
Isotope tracer
abstract
Adsorptive features of the composites of polyacrylamide (PAAm) and bentonite (B), and zeolite (Z) were
investigated for Tl
+
and Tl
3+
.
Langmuir monolayer adsorption capacities for the corresponding ions were 1.85 and 0.97 mol kg
-1
for PAAm–Z and 0.36 and 0.16 mol kg
-1
for PAAm–B. The values of enthalpy and entropy changes were
positive for both composites and ions. The compatibility of the second order adsorption kinetics implied
that the rate-controlling step was concentration dependent; the sorption process was ion exchange.
High adsorption rate was found for both composites; the time required for adsorption of half of Tl
+
concentrations was 7 min.
In the presence of both ions, PAAm–B was more selective for Tl
3+
than PAAm–Z. The reusability tests
for Tl
+
for five uses proved that the composites were reusable after complete recovery of the loaded ion.
The values of Tl
+
adsorption onto PAAm–Z from solutions containing Fe
3+
, Pb
2+
, Zn
2+
confirmed that the
effect of the presence of these ions on Tl
+
extraction was not significant. Although Tl
+
sorption decreased
with increasing ionic strength (CaCl
2
) of the medium, the results of adsorption from sea water containing
5 × 10
-6
mol L
-1
(1 mg L
-1
) of Tl
+
ascertained that the composites still adsorbed about 10% of Tl.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Thallium (Tl) classified in the group III A of the periodic chart
has two comparatively stable state ions, monovalent (Tl
+
; thal-
lous) and trivalent (Tl
3+
; thallic). Tl
+
resembles to group I cations
(Pb
2+
, Hg
2
2+
and Ag
+
) and also to K
+
, Rb
+
and Cs
+
of the alkali-metal
ions [1]. Monovalent Tl is more stable than trivalent Tl analogues
in aqua solution at neutral pH. The dissolved Tl in sea water
(10–100 pmol kg
-1
) contains only about 1–5% of Tl in trivalent
state. In rocks, it is concentrated in plagioclase or in K
+
-minerals
such as K-feldspars and biotite. Thallium is observed to be a part of
sulfide melts and it is enriched in some sulfide minerals [2].
Thallium in both states is toxic and acts as a cumulative poi-
son. High Tl
+
concentration in the blood agglutinates and lyses
erythrocytes. Tl
3+
has a strong capacity to complex with citrates,
glutamates and albumin. Lethal dose (LD
100
) of Tl
+
by subcutaneous
or intravenous injection is 12–15 mg kg
-1
in dogs, 20–25 mg kg
-1
in rats and 8–10 mg kg
-1
in humans [3,4]. Tl is introduced into the
environment mainly as waste from the production of Zn, Cd and
Pb and by combustion of coal [5]. The U.S. Environmental Protec-
Abbreviations: B, bentonite; Z, zeolite; PAAm, polyacrylamide; PAAm–B/Z, the
composites of polyacrylamide and bentonite/zeolite; DR, Dubinin–Radushkevich.
∗
Corresponding author. Tel.: +90 346 2191010x1623; fax: +90 346 2191186.
E-mail address: ulusoy@cumhuriyet.edu.tr (U. Ulusoy).
tion Agency (EPA) standards for Tl is 2 gL
-1
in drinking water,
4 gL
-1
in sea water, 140 gL
-1
in wastewaters and 0.1 mg m
-3
in
stack gas.
Industrial uses of Tl and its compounds are in the manufac-
ture of imitation jewelry, low-temperature thermometers, ceramic
semiconductor materials, scintillation counters for radioactivity
measurements and optical lenses. Tl compounds are used in IR
spectrometers, optical systems, semiconductor and laser industry,
scintillographic imaging, super conductivity, coloring glass and as a
molecular probe to emulate the biological function of alkali-metal
ions [6].
Because of the toxicity and the industrial importance,
removal/recovery of Tl in waste and contaminated water has
been among major concerns of process industries [7]. The EPA
indicated that only two industrial removal technologies exist for
recovering thallium from process solutions: oxidative precipita-
tion of thallichydroxide and reductive cementation of thallium
using elemental zinc as the precipitant the best one of which is
the chemical oxidation of thallium followed by chemical pre-
cipitation with hydroxide compounds, settling, and filtration
(http://www.epa.gov/minewastetechnology/annual/annual2004/
mwtp2004annualrpt.pdf, last visited in 29.04.2010). Due to the
strong tendency of Tl
3+
to form a complex, the interest of inves-
tigations is generally given to its extraction by complex forming
reagents as reported by Nascimento and Schwedt [8], Gidwani et al.
[9], Chung et al. [7], Zhang et al. [10], and Rajesh and Subramanian
1385-8947/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cej.2010.05.005