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Attapulgite as an eco-friendly adsorbent in the treatment of real radioactive wastewater


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Operators cannot ignore the radiation hazards arising from nuclear weapons. In this study, batch adsorption experiments were investigated to remove the radioactive isotope Cs-137 from the real radioactive wastewater. The attapulgite natural clay mineral was characterized and adopted as an adsorbent in a batch adsorption system. Equilibrium was reached after 2 h with a Cs-137 removal efficiency of 97% for attapulgite. The kinetics of Cs-137 adsorption on the attapulgite clay surface were evaluated. The pseudo-second-order kinetic model produced an excellent fit with the experimental kinetic data for attapulgite, indicating that attapulgite was the best adsorption medium.
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Attapulgite as an eco-friendly adsorbent in the treatment of real radioactive
Wasan Muslima, Salam Al-Nasrib, Talib M. Albayati c,*and Issam Salihd
Iraqi Geological Survey/Ministry of Industry and Minerals
Iraqi Atomic Energy Commission (IAEC)/Radiation and Nuclear Safety Directorate, Baghdad, Iraq
Department of Chemical Engineering, University of Technology Iraq, 52 Alsinaa St., P.O. Box 35010, Baghdad, Iraq
Department of Chemical Engineering and Petroleum Industries, Al-Mustaqbal University, Babylon 51001, Iraq
*Corresponding author. E-mail:
TMA, 0000-0001-5619-7760
Operators cannot ignore the radiation hazards arising from nuclear weapons. In this study, batch adsorption experiments were
investigated to remove the radioactive isotope Cs-137 from the real radioactive wastewater. The attapulgite natural clay mineral
was characterized and adopted as an adsorbent in a batch adsorption system. Equilibrium was reached after 2 h with a Cs-137
removal efciency of 97% for attapulgite. The kinetics of Cs-137 adsorption on the attapulgite clay surface were evaluated. The
pseudo-second-order kinetic model produced an excellent t with the experimental kinetic data for attapulgite, indicating that
attapulgite was the best adsorption medium.
Key words: attapulgite, cesium adsorption, clay minerals, Cs-137, wastewater remediation
Very cheap attapulgite clay was used in a batch adsorption system.
Iraqi attapulgite natural clay proved as an efcient adsorbent for the removal of Cs-137.
Natural clay was modied and manufactured from a locally available material.
The real samples of radioactive wastewater containing 137Cs have been treated.
This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying,
adaptation and redistribution, provided the original work is properly cited (
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Radioactive wastewater is one of the riskiest pollutants generated by energy stations, medical programs, and
diverse extractive industries worldwide (Paranhos Gazineu et al. 2005). Effective treatments need to be cost-effec-
tive and safely reduce the volume of aqueous waste (International Atomic Energy Agency 2002;Cherif et al.
2017) containing long-lived beta/gamma activity stored in large tanks under nuclear sites. Cs-137 has a radio-
active half-life of about 30 years and very high solubility in liquid systems, and incorporates into both the soil
environment and aquatic organisms (Al-Alawy & Mzher 2019;Ahmed 2022). Liquids contaminated with Cs-
137 are a potential environmental problem. High radioactivity aqueous wastes with long-lived radionuclides
may be treated using different treatment technologies, including ion exchange/sorption, chemical precipitation,
and/or evaporation, reverse osmosis, ltration, and solvent extraction (IAEA 1999). Many studies have found that
adsorption is a good technique for removing radioactive materials from wastewater, with high activity and low
operating cost (Alardhi et al. 2020;Kadhum et al. 2021;Ali et al. 2022a). The best media in the treatment of
industrial wastewater were clay minerals whose features make them optimal adsorbents due to their low pro-
duction cost, ready availability, non-toxic nature, high specic surfaces, excellent adsorption properties, and
great potential for ion exchange (Al-Ani & Sarapää; 2008). Clay minerals adsorb cesium to balance the negative
charge on the aluminosilicate structure caused by the counter-ions (e.g., Na, Ca, Mg, or K) as adsorption sites on
the clay sheet surface, interlayers between sheets, and broken bonds at the edges of clay crystals (Wilson 2007;
Yuan et al. 2013). Attapulgite is the rock name of palygorskite, a hydrated MgAl silicate material that has a 2:1
inverted structure, i.e., the apices of the silica tetrahedrons are regularly inverted along the a-axis. This results in
parallel channels throughout the particles, which give these minerals a high internal specic surface containing
exchangeable cations and water (Stewart & Mollins 1996). Large cation exchange capacities (CECs) and high
total uptake of cesium occur when the interlayer sites are available for adsorption, as has been recorded in
the cases of montmorillonite and palygorskite (Adebowale et al. 2006;Ohnuki & Kozai 2013;Okumura et al.
2013;Ali et al. 2022b). Several studies have inspected the mechanism of cesium adsorption by ion exchange
with different potential sites on mineral surfaces and studied the effect of the structural characteristics of these
clay minerals (Cornell 1993;Park et al. 2019;Zabulonov et al. 2021). Other research examined the parameters
that inuence adsorption, adsorption isotherms, thermodynamics, and kinetics for many clay minerals (Sheha &
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Metwally 2007;Hadadi et al. 2009;Akalin et al. 2018;Semenkova et al. 2018; Al-Alawy et al. 2020; Muslim et al.
In this work, natural attapulgite clay minerals from the Western Desert of Iraq were selected as potential low-
cost, readily available, environmentally friendly adsorbents adopted for use in a batch adsorption system. Attapul-
gite was implemented to treat real radioactive wastewater containing Cs-137 that has been accumulated since
1991 underneath the Al-Tuwaitha Nuclear Research Center near Baghdad, Iraq. The inuence of various vari-
ables on the adsorption process was investigated along with its isotherms and kinetics.
2.1. Clay mineral preparation and characterization
Clays have characteristics that depend on their geological formation and mining location. Deposits of attapulgite
occur in Wadi Bashira in Iraqs Western Desert. The representative sample was crushed in a jaw crusher (Retsch
BB 1, Germany) and then milled in a rotating cylinder ball mill to pass a 75-μm sieve opening. Wet chemical analysis
to identify the attapulgites chemical composition was done in the Central Laboratories Department, Iraqi Geologi-
cal Survey. X-ray diffraction (XRD) mineralogical analyses were performed using the Ital structure model MPD
3000 (Spain, Al Razi Metallurgical Center, Tehran, Iran). Scanning electron microscopy (SEM) and energy-disper-
sive X-ray spectroscopy (EDX) were employed to investigate claysmorphologies with the MIRA3 TESCAN
instrument (Australia, Al Razi Metallurgical Center, Tehran, Iran). Particle size distribution analysis was done
using a Brookhaven Instruments (USA) 90Plus particle size analyzer (Nanotechnology Center, UOT, Iraq). The
mineralsspecic surfaces (SSA) and CECs were obtained from technical reports of the Iraqi Geological Survey
(Baghdad, Iraq). Fourier-transform infrared (FT-IR) spectroscopy analyses were run with a Bomem MB-Series
FT-IR Spectrometer (France) and operated according to ASTM E 1252-98(21) to specify the functional groups.
2.2. Radioactive wastewater sample preparation
The radioactive wastewater samples were taken from a reservoir underneath the destroyed Radiochemical Lab-
oratories (RCL) at the Al-Tuwaitha site (Iraq). The gamma spectroscopy analysis was conducted using a closed-
end, coaxial, p-type model (GEM65P4-95/ORTEC (USA, Al-Tuwaitha site, Iraq) high purity germanium detector
(HPGe), yielding high-level waste (HLW) containing radioactive cesium (Cs-137) with a specic activity of
4.5 GBq/L (Ibrahim et al. 2018). As per the appropriate safety procedure, the sample was diluted with distilled
water to a safe limit to be handled within the laboratory. The activity was reduced to about 6,372 Bq/L, which
is considered the initial activity concentration.
2.3. Batch adsorption experiments
Batch mode experiments were carried out to evaluate the use of the clay to adsorb Cs-137 from the radioactive
wastewater. In glass containers, 0.1 g of clay was added to 30 ml radioactive wastewater samples, with Cs-137
activity concentration of 6,372 Bq/L and pH 6. The sample containers were shaken at 200 rpm at room tempera-
ture (25 °C) for different mixing times (i.e., 0.5, 1, 1.5, 2, and 3 h). Solid particles were separated from the solution
by centrifugation rather than ltration, using lter paper, to avoid the adsorption of contaminants onto the lter
paper. Filtrate samples (20 ml) were put into a Marinelli beaker to measure the cesium radioactivity concen-
tration after treatment using gamma spectroscopy (HPGe detector). The Cs-137 (μg/L) concentrations in the
ltrates were estimated using Equations (1)(4) (Knoll & Wegst 1980).
Specific Activity(SA) ¼
Aav w
w¼SA m
Aav (2)
where Aav is Avogadros number (6.02 10
nuclei/mol), λis the radioisotope decay constant (s
), mis the
atomic weight (g/mol), and wis the weight (g).
half life ¼0:693
Cs isotope concentration in filtrate (C)¼w
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The clay was investigated by studying the removal efciency (R%), adsorption capacity, q
(mg/g), and adsorp-
tion coefcient, K
(L/g), respectively, of the Cs-137 isotope at equilibrium, using Equations (5)(7) (Abbood et al.
R%¼concentration of adsorbed cesium
initial concentration of cesium ¼C0Ce
100 (5)
qe¼amount of cesium adsorbed
amount of adsorbent ¼(C0Ce)V
where C
and Care the initial and equilibrium concentrations of radioactive cesium (mg/L), Vis the solution
volume (L), and Mis the weight of the clay mineral (g).
2.4. Adsorption kinetics
The Cs-137 adsorption mechanism on the clay surfaces was investigated using the contact time data. Three lin-
earized adsorption kinetics models were used to evaluate the experimental results pseudo-rst-order (Lagergren
model), pseudo-second-order (Ho model), and intraparticle diffusion (WeberMorris model) which are rep-
resented by Equations (8)(10), respectively (Al-Jaaf et al. 2022;Jabbar et al. 2022).
where qand q
are the adsorption capacity (mg/g) at equilibrium and time t(min), respectively; K
and K
adsorption rate constants of the pseudo-rst-order (min
) and pseudo-second-order (g/mg·min), respectively; K
is the intraparticle diffusion rate (mg/g·min
) constant, and Cis the diffusion intraparticle constant (Khadim
et al. 2022).
3.1. Clay mineral characterization
The results of the chemical and mineralogical analyses of the attapulgite are shown in Table 1 and Figure 1.
The attapulgite sample contains predominantly montmorillonite (smectite) associated with palygorskite as the
main minerals, in addition to impurities like silica, calcite, and gypsum, as shown in Table 1 (Al-Ajeel et al. 2008).
The montmorillonite is considered a Ca-montmorillonite on the basis of the ratio of (Na
O) to (CaO þ
MgO), which is approaching 0.136 (Abdou et al. 2013).
In Figure 1, the XRD analyses illuminate a convergence for attapulgite, in which the major peaks are palygors-
kite at 20° and diffraction angles (2θ), and the minor mineral is montmorillonite with 2θat 20.5° and 7°. In fact,
the sample is a montmorillonite-rich, palygorskite clay. The results show clearly that the clays have not been sub-
ject to any purication or modication processes (Al-Alawy et al. 2020).
Figure 2 displays the SEM images for the attapulgite clay. As can be seen, the original attapulgite structure con-
sisted of blocks, channels, and ribbon-likesheets, while after adsorption, the framework collapsed with the
disorder in the layered structure, and the particles were almost at (Muslim et al. 2022).
Table 1 |Chemical analyses of the clays
Chemical composition SiO
b (%) Al
(%) Fe
(%) CaO (%) MgO (%) SO
(%) LOI (%) Na
O (%) K
O (%) Cl (%)
Attapulgite 40.1 9.6 3.38 19.64 4.36 0.32 20.5 0.8 0.29
LOI, loss on ignition.
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EDX qualitative elemental composition analysis, by identifying the materials crystal structure, was achieved at
ain Figure 2, after adsorption on the attapulgite clay surface. The EDX results are shown in Figure 3.
The EDX spectrum detects the attapulgite clay surfaces after adsorption and the presence of Cs-137 of the
radioactive wastewater, as shown in Figure 3. The particle size test was based on dynamic light scattering
(DLS), and the clays mean particle size is presented in Figure 4.
According to the particle size analysis, attapulgite exhibited small particle size. The surface areas were
measured using the BET method. Attapulgites specic surface and cation exchange capacity are given in Table 2.
The infrared spectra for attapulgite are shown in Figure 5. The spectrum after adsorption (Ab) shows the
stretching vibration of AlOHAl, MgOH, and/or H
O at 3,427 cm
and the stretching vibration of SiOat
1,104 cm
, while the SiOH stretching vibration appeared at 1,029 cm
. Moreover, the spectrum showed
the stretching vibration bands of SiOSi and SiOAl at 767 and 778 cm
. The change in the spectra appeared
clearly when compared to the reading before the reaction, as shown in the spectrum before adsorption (Aa), with
a solid shift in the vibration of SiO, SiOH, and SiOSi, which appeared at 1,104, 1,026, and 870/772 cm
(Muslim et al. 2022).
Figure 2 |SEM images for attapulgite. arepresents a known point on the clay surface analyzed after adsorption.
Figure 1 |XRD patterns of attapulgite.
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3.2. Batch adsorption results
3.2.1. Activity concentration and removal (%)
The results of the batch adsorption experiments are shown in Figure 6(a). The maximum reduction in Cs-137
activity was achieved at 177 Bq/L during the 2 h required for cesium uptake to reach equilibrium for attapulgite.
The proportional removal (%) was determined using Equation (5) and is presented in Figure 6(b), which shows
that equilibrium was reached quickly, achieving 80% adsorption after 1 h. However, at an equilibrium time of 2 h,
attapulgite removal efciency reached 97%. The montmorillonite (smectite) content in attapulgite, as shown in
Figure 1, leads, rstly, to the reduction in attapulgite particle size. Also, attapulgite has high CEC, which rep-
resents the existence of active adsorption sites (Table 2). Secondly, montmorillonites existence in
attapulgite causes the absorption of a signicant amount of water (expandable clays), making the process
a combination of absorption and adsorption (called sorption), which boosted cesium uptake from the waste-
water (Park et al. 2019).
Figure 3 |EDX analysis of the attapulgite clay after adsorption.
Figure 4 |Particle size analysis for attapulgite.
Table 2 |Mean particle size and specic surface of attapulgite
Sample Mean particle diameter (nm) St. Deviation Density (g/cm
) CEC (meq/100 g) SSA (m
A41.2 1.42 2.4 14.08 60.7
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3.2.2. Mechanisms of Cs sorption
The extent of Cs adsorption on montmorillonite and attapulgite depends on the mineralsion exchange site types,
which is characterized by the function and availability of the interlayer site type (III) that offers high CECs and
cesium uptake. In attapulgite, the active sites are only in the planar (basal) surface (type I) and the edges of the
interlayers (type II), which both show low CECs compared to type III. In kaolinite, the ion exchange capability is
due to broken bonds at the edges of the clay plates and hydroxyl groups on the basal lamellar. The results indicate
Figure 5 |FT-IR spectra of attapulgite (Ab) before adsorption and (Aa) after adsorption.
Figure 6 |Effect of contact time on (a) activity concentration and (b) removal (%) of Cs-137 from radioactive wastewater for
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that Cs is adsorbed not only at the frayed edgesites but also at other sites where the adsorption is reversible, as
reported by Erten et al. (1988),Comans et al. (1991),Comans & Hockley (1992), and Shahwan et al. (1999). One
is instantaneous and reversible on a timescale of a few days or less. The other is irreversible, occurs at longer
times, and is caused by Cs migration into the interlayers. Slow Cs migration into interlayers was also proposed
by Evans et al. (1983). These were in accord with the extent of cesium adsorption (desorption) by attapulgite
after 2 h in the results as claried in Figure 6(b), because some of the cesium sites are reversible on attapulgites
basal planes. The results of this study are agreement with the mechanism of adsorption (Ali et al. 2023;Khader
et al. 2023).
3.2.3. Adsorption capacity and distribution coefcient
The adsorption capacity (q
) and distribution coefcient (K
) for the attapulgite at different contact times were
calculated from Equations (6) and (7), respectively, and are shown in Figure 7(a) and 7(b), respectively. Clearly,
the prime K
and q
for attapulgite after 2 h of contact time were caused by the very low initial Cs-137 concen-
tration, because sorption increased sharply then (Missana et al. 2014;Baborová et al. 2018).
3.3. Adsorption kinetics
The results from the kinetic models for attapulgite pseudo-rst-order (Equation (8)), pseudo-second-order
(Equation (9)), and intraparticle diffusion (Equation (10)) are displayed in Figure 8(a)8(c), respectively. The
Cs-137 adsorption mechanisms for attapulgite better t the pseudo-second-order kinetic model with a high
regression coefcient of 0.9971, which is higher than the pseudo-rst-order model value of 0.9919.
The q
predicted for attapulgite by the pseudo-rst-order model approached the experimental q
, as shown
in Table 3. The intraparticle diffusion adsorption kinetic model is based on the assumption that the rate-control-
ling step may involve valence forces through ion exchange, substitution, or complexation (Wei et al. 2019;
Al-Rahmani et al. 2020).
Since the plot of q
versus t
in the intraparticle model, as shown in Figure 8(c), did not pass the origin, intra-
particle diffusion did not wholly affect the adsorption process. Also, the diffusion models correlation coefcient
) for attapulgite was lower than that of the pseudo-second-order (0.867), as shown in Table 3. The suitability of
the pseudo-second-order model with the experimental result means that adsorption is controlled by ion exchange,
in which electrostatic interactions play a signicant part ( Jiaojiao et al. 2009;Xiang et al. 2014).
3.4. Comparative study
The research focus is the exploration of novel and effective adsorbents for Cs removal. The widespread explora-
tion of advanced functional materials for nuclide pollution control is driven by increasingly severe environmental
problems. Researchers have extensively explored and designed adsorbents for Cs removal. A comparison
between the results of this study and previous studies is illustrated in Table 4 and suggests that attapulgite is a
Figure 7 |(a) Adsorption capacity (q
) and (b) adsorption distribution coefcient (K
) for attapulgite.
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strong, stable, and efcient sorbent for Cs-137 removal. Moreover, attapulgite is easily applied as an adsorbent
with a natural, low-cost, eco-friendly, and simple batch sorption process compared with synthesized adsorbents,
such as zeolites, composites, and bio-sorbents for Cs-137 radioactive decontamination. Excellent Cs-137
Figure 8 |Adsorption kinetic models of attapulgite: (a) pseudo-rst-order adsorption kinetic model, (b) pseudo-second-order
adsorption kinetic model, and (c) intraparticle diffusion model.
Table 3 |Kinetic model adsorption parameters for attapulgite
Experimental Pseudo-rst-order model
Intraparticle diffusion
(mg/g) q
(mg/g) R
(mg/g) R
C(mg/g) R
Attapulgite 0.58E-6 0.55E-6 0.991 0.45E-6 0.9971 0.3 0.867
Table 4 |Adsorption capacities of Cs-137 by various adsorbents
Adsorption capacity Q
Removal efciency
Equilibrium time
(h) References
Nanoclusters Microparticles 45.87 99.7 6 Yang et al. (2016)
Nanocomposites with graphene
55.56 90 12 Yang et al. (2014)
Nanoparticles 96.00 NA 24 Thammawong et al.
Nanocomposites 280.82 NA 24 Jang & Lee (2016)
Nanoparticles with PEG 274.70 64.8 1 Qian et al. (2017)
Microparticles 16.30 97.0 10 min Wang et al. (2020)
Attapulgite NA 97 2 This study
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adsorption efciencies were achieved by attapulgite (97%) for a 2-h equilibrium time without functionalization or
treatment of its surface.
Attapulgite had a small particle size, a high specic surface, better cation exchangeability, and an effective func-
tional site. Excellent adsorption efciencies of Cs-137 were achieved by attapulgite (97%) for a 2-h equilibrium
time. The high adsorption efciencies achieved in this study resulted from using low Cs-137 radioactivity concen-
trations (6.372 KBq/L). The kinetics of Cs-137 adsorption on attapulgite were evaluated. The pseudo-second-
order kinetic model produces a good t with the experimental data. According to the results, the local raw atta-
pulgite was suitable clay and should be selected to manage the removal of the Cs-137 from wastewater. The
attapulgite adsorbents proved to be promising materials for removing Cs-137 because they are inexpensive, avail-
able, and effective.
We gratefully acknowledge the scientic support of the Department of Chemical Engineering, University of Tech-
nology-Iraq; Iraqi Atomic Energy Commission (IAEC)/Radiation and Nuclear Safety Directorate, Baghdad, Iraq,
and the Iraqi Geological Survey/Ministry of Industry and Minerals, and the Department of Chemical and Pet-
roleum Industries Engineering at Al-Mustaqbal University College in Babylon, Iraq.
All relevant data are included in the paper or its Supplementary Information.
The authors declare there is no conict.
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