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Sorption of cesium and strontium ions by natural zeolite and management of produced secondary waste
Journal Pre-proof Sorption of cesium and strontium ions by natural zeolite and management of produced secondary waste Tayebe Abdollahi, Jafar Towfighi, Hadi Rezaei-Vahidian
PII: DOI: Reference:
S2352-1864(19)30346-3 https://doi.org/10.1016/j.eti.2019.100592 ETI 100592
To appear in:
Environmental Technology & Innovation
Received date : 15 July 2019 Revised date : 17 December 2019 Accepted date : 20 December 2019 Please cite this article as: T. Abdollahi, J. Towfighi and H. Rezaei-Vahidian, Sorption of cesium and strontium ions by natural zeolite and management of produced secondary waste. Environmental Technology & Innovation (2019), doi: https://doi.org/10.1016/j.eti.2019.100592. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Elsevier Ltd. All rights reserved.
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Sorption of cesium and strontium ions by natural zeolite and management of produced
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secondary waste
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Tayebe Abdollahi1*, Jafar Towfighi1, Hadi Rezaei-Vahidian2
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4 1. Department of Chemical Engineering, Tarbiat Modares University, P.O. Box 14115-116, Tehran, Iran.
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2. Space transportation research institute, Iranian space research center, Tehran, Iran.
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E-mail: [email protected] (Tayebe Abdollahi), [email protected] (Jafar Towfighi) and [email protected] (Hadi Rezaei-Vahidian).
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*Corresponding authors: Tel/Fax.: +9821 82883311, E-mail addresses: [email protected]
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Abstract
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The aim of this work is sorptive removal of cesium and strontium ions from aqueous solution
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using natural zeolite and management of produced secondary waste. Characterization of the
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natural zeolite was performed by X-ray diffraction (XRD), X-ray fluorescence (XRF), Brunauer–
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Emmett–Teller (BET), and field emission scanning electron microscopy (FE-SEM). The
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effective parameters on the sorption process such as initial solution pH, adsorbent dosage, and
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concentration of cesium and strontium ions were optimized by using experimental design
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method. Based on the results, removal efficiency for cesium ion was about 67.8% at the optimum
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condition of adsorbent dosage of 0.40 g, pH = 7.23 and Cs+ concentration of 10 mg L-1, and for
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the strontium ion was about 93.5% at the optimum condition of adsorbent dosage of 0.3 g, pH =
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7.9 and Sr2+ concentration of 10 mg L-1. Kinetic studies showed that the removal process follow
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pseudo-second-order kinetic model for both ions. The sorption results showed that the data for
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cesium and strontium ions were fitted by Freundlich isotherm better than Langmuir isotherm.
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Finally, to manage secondary waste produced from the sorption process, the cesium and
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strontium ions stabilized in zeolite structure and related tests such as compressive strength and
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leaching were evaluated. The results of waste management section showed that the stabilization
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process was performed as well by heat preparation process.
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Keywords: Adsorption; Waste management; Response Surface Methodology; Stabilization;
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Thermal treatment.
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1. Introduction Zeolites are naturally hydrated aluminosilicate minerals with selective unique ion-exchange
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and a known resistance to radiation. The structures of zeolites consist of three-dimensional
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frameworks of SiO4 and AlO4 tetrahedral. The aluminium ion is small enough to fill the position
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in the center of the tetrahedron of four oxygen atoms and the isomorphous replacement of Si4+ by
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Al3+ produces a negative charge in the lattice that the net negative charge is balanced by the
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exchangeable cation. These cations are exchangeable with certain cations in solutions such as
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lead, cadmium, zinc, and manganese (Barrer, 1978). These properties have encouraged extensive
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studies into their use in the treatment of radioactive waste. This was originally recognized by
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Ames in the course of his pioneering research on the environmental application of clinoptilolite
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(Ames Jr, 1960) and has since then been confirmed by other investigators with other zeolite
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minerals, e.g., mordenite (Munthali et al., 2015), phillipsite (Cappelletti et al., 2011), and
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chabazite (Nakai et al., 2013). The radionuclides of cesium and strontium are the most abundant
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in the suite of nuclear fission products that are usually released into wastewater (Shenber and
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Johanson, 1992).
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Sorption processes as a physical water treatment methods transfer aqueous pollutant to
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surface of adsorbent and produce secondary waste. The produced solid secondary waste must be
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managed. The adsorbed pollutant can be desorbed and be released into the environment. One of
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the effective methods to prevent this problem is fixing of the pollutant in a solid media to reduce
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its diffusion. Many materials such as zeolites and phosphate-induced products, and by-products
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such as quarry waste, and sewage sludge were used in the metal immobilization (Cao et al.,
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2003; Ciccu et al., 2003; Guo et al., 2006). Immobilization of Cs-containing zeolitic tuffs in
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cement matrices has been the subject of several investigations, which pointed out the safety of
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the procedure (Bosch et al., 2004). Optimization of a process variables is accomplished to attain the maximum process
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efficiency. In conventional methods, one variable is used at a time to examine the influence of
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operational parameters. In this method a high number of experiments should be performed and
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the interactive effects between variables cannot examine (Bezerra et al., 2008). Recently,
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researchers use the multivariate statistical techniques to optimize the effective parameters by a
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minimum number of experiments. Response surface methodology (RSM) is an effective
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experimental design method for modelling and analysis of processes (Ghafari et al., 2014).
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The present work deals with a series of experiments to assess the utility of natural zeolite for
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the removal of Cs+ and Sr2+ ions from aqueous solution. The natural zeolite obtained from Semnan
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province desert in Iran. To optimize the process central composite design (CCD) has been used.
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The effect of operational parameters of adsorbent dosage, pH of the solution, and initial
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concentration of the ions on the sorption process have been optimized. Also kinetic and isotherm
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investigations have been assessed for both ions. Finally, the secondary produced solid waste has
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been managed so that the Cs+ and Sr2+ ions were fixed in zeolite structure and related tests such
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as compressive strength and leaching were evaluated.
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2. Material and methods
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2.1. Zeolite adsorbent
The natural adsorbent, examined in our work, originated from the Arshe mine of Semnan
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province in Iran. Before use, the zeolite samples were passed through a 0.07 mm sieve (200
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mesh). Characterizations of the adsorbent were investigated using X-ray fluorescence (XRF),
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Brunauer–Emmett–Teller (BET), and field emission scanning electron microscopy (FE-SEM).
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The phases of the natural adsorbent were determined by XRD analyses with an automated
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PHILIPS-binary diffractometer using Cu Kα radiation at 35 kV and 30 mA over the range (2θ) of
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4–55. 2.2. Chemicals and Instruments
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All chemicals were obtained from Merck Ltd. and used without further purification.
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Strontium and cesium ion were supplied as strontium and cesium chloride, from Merck
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Company. Distilled water was utilized to prepare solutions. The measurement of pH was
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performed using a pH meter CG841 model (Schott Ltd. Co., Germany). FE-SEM images were
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acquired using an electron microscopy VEGA model (TESCAN Ltd. Co., Czech Republic). The
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XRD and XRF analysis were carried out using an Explorer Model (GNR Ltd. Co., USA) and
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ARL PERFOMIR' x model (Thermo electron Ltd. Co., USA), respectively. Brunauer–Emmett–
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Teller (BET) analysis was carried out using a Model Nova 2200 e (Quantachrome Ltd. Co.,
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USA). Sr2+ ion concentration was determined using Inductively Coupled Plasma-Atomic
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Emission Spectrometry Liberty 150 AX Turbo (Varian Ltd. Co., USA) and cesium ion
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concentration was determined using Atomic Absorption Spectroscopy model (Spectra AA220
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Varian Ltd. Co., USA).
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2.3. Sorption experiments
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The sorption of Cs+ and Sr2+ ions on natural zeolite was carried out using batch techniques at
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room temperature. To this aim, a known amount of zeolite samples was mixed with 40 ml of
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solution having 10 mg L-1 Cs+ or Sr2+after adjusting solution pH. The suspensions were stirred at
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170 rpm for 10 min and then centrifuged at 3000 rpm to separate adsorbent.
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2.4. Waste management procedure
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In order to investigate stabilization of cesium and strontium ions in the utilized adsorbent
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structure, 150 g of adsorbent with 70 ml solution containing 10 mg L-1 of the cesium or strontium
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ions were thoroughly mixed in the optimum condition obtained from sorption process such that a
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uniform mortar was achieved. The obtained mortar was poured into a cubic container with
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dimensions of 5 × 5 × 5 cm. Then, the samples were dried and the stabilization tests were
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performed.
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2.5. Design of experiment
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RSM as a statistical technique is useful to optimize effective factors which creates a
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mathematical model to describe the process (Boyacı, 2005). In this technique selection of the
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effective factors and their levels is the main step of the process. In this study the effect of dosage
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of zeolite, initial pH, and concentration of Cs+ or Sr2+ ions on removal efficiency was studied
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using central composite design (CCD) as one of the tools of RSM. Using CCD, for three factors,
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20 experiments were designed and performed. Design of experiments was done by Design
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Expert version 10 software and the model statistical analysis was performed by analysis of
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variance (ANOVA).
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3. Result and discussion
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3.1. Adsorbent Characterization
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Chemical analysis of the natural adsorbent by XRF has been presented in Table 1 of
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supporting information. As can be seen the natural adsorbent contain a complement of
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exchangeable sodium, potassium, magnesium and calcium ions. The BET analysis is commonly
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used for determining surface areas of adsorbent. The BET surface area of the adsorbent was
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measured about 21.3912 m2/g. FE-SEM image (a) and the energy dispersive X-ray spectroscopy
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(EDXS) (b) of the natural adsorbent were demonstrated in Fig. 1 of supporting information. The
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elemental compositions of O, Al, Si, Mg, Na, Cl, Ca and Fe in the natural adsorbent were 47.30,
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7.13, 37.10, 1.72, 2.33, 2.01, 0.20 and 2.21 wt%, respectively. The crystalline phase of the
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natural adsorbent was determined using an X-ray diffraction pattern (Fig. 2 of supporting
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information). The apparent peaks at the 2θ of 7.6865º, 9.6453º, 16.7689º, 19.6489º, 21.7916º,
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26.4497º, 28.1002º, 29.8023º, 31.7978º, 34.8631º, and 49.9380º indicate the presence of
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montmorillonite (JCPDS-00-011-0303), heulandite (JCPDS-00-014-0248), and muscovite
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(JCPDS-00-046-1311) in the natural adsorbent. In the pHzpc the surface charge of the adsorbent
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is zero. The surface of the adsorbent has a positive charge in the pH value below pHzpc and has a
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negative charge in the pH above pHzpc (Mohan et al., 2011). To determine pHzpc, 40 ml sodium
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chloride solutions (0.01 M) were prepared and their pH were set in the range of 2.5-12.5 using
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0.1 mol L−1 HCl and/or 0.1 mol L−1 NaOH. Then 0.4 g of the natural zeolite was added into the
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solution and stirred for 48 hours and then pH of the solutions was measured. The results of pHzpc
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analysis show that the surface charge of the natural zeolite is zero at pH 7.5.
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3.2. Experimental Design
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Definition of effective parameters with appropriate range is one of the main step in the
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experimental design. To this aim some relative reports was evaluated and some preliminary tests
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were performed. The experiments were done at room temperature (25 °C). The effect of mesh
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size of the adsorbent was evaluated and the results were presented in Fig. 3 of supporting
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information. As can be seen from the figure the process efficiency increased with increasing of
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mesh size for both ions. So the mesh number of 200 was selected as a suitable size of the
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adsorbent particles. In the following, in order to find suitable time of the process some
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experiments were performed at medium condition of the parameters using mesh number of 200
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for adsorbent that the results were demonstrated in Fig. 4 of supporting information. As can be
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seen, the efficiency of the process for both cesium and strontium has been slightly increased
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after 10 min of the process. Considering that the experiments of this section were done at
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medium condition of the parameters, so the 10 min was selected as a suitable time for the
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sorption process.
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To design the experiments, the main factors such as initial pH of the solution, adsorbent
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dosage, and Cs+ or Sr2+ ion concentration were selected as operational parameters and efficiency
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of the process was elected as response. Based on related literatures and preliminary performed
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tests the range of the variables and their levels were determined that are presented in Table 2 and
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3 of supporting information for cesium and strontium, respectively. Twenty tests, designed by
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CCD methodology were presented in Table 4 and 5 of supporting information for cesium and
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strontium ions, respectively.
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The designed experiments were done and regression analysis for the process indicated that
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the data can be modeled by a second order polynomial equation as:
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𝑅𝐸
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0.030
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𝑅𝐸
17.369
145.038
𝐴𝑑𝑠.
196.48
11.53
𝐴𝑑𝑠.
2.116
𝐶𝑠
7.927
𝑝𝐻
112.89
𝑝𝐻
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0.548
68.02
𝑆𝑟
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𝐶𝑠
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𝐴𝑑𝑠. 4.06
0.06
𝐴𝑑𝑠. (1)
𝑆𝑟
𝑆𝑟
4.489
𝑝𝐻
3.40
𝐴𝑑𝑠. (2)
The ANOVA for the models was explored so that, values of “Prob>F” (p-value) less than
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0.05 denote the model terms are significant and the values larger than 0.10 specify they are not
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significant. In the present work, for cesium ion removal, the initial pH of the solution, dosage of
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zeolite, and Cs+ ion concentration were the significant model terms which were held. The
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analysis of variance for the reduced quadratic model for removal of Cs+ ion has been presented
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in table 6 of supporting information. For this model, F-value and p-value were 356.87 and less
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than 0.0001, respectively. As a result, this model was significant. Also, the Pred R-Squared of
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0.97 is in reasonable agreement with the “Adj R-Squared” of 0.96. “Adeq Precision” measures
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the signal to noise ratio and a ratio greater than 4 is desirable. For the reduced model, this ratio is
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31.92 which indicate an adequate signal.
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In the case of Sr2+ ion, pH, [Sr2+], and Adsorbent dosage were also the significant model
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terms. The analysis of variance for the reduced quadratic model for sorption of Sr2+ ion has been
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presented in table 7 of supporting information. The Model F-value of 47.85 and the p-value less
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than 0.0001 implies the model is significant. In this work, the Pred R-Squared of 0.9566 is in
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reasonable agreement with the “Adj R-Squared” of 0.9366. Also, for the reduced model, the
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Adeq precision was 23.79 which indicate an adequate signal.
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To evaluate the adequacy of the model graphically, diagnostic plots could be utilized. Most
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of the plots display residuals, which show the difference between experimental and predicted
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responses. In the DOE software, normal probability, residuals versus predicted, residuals versus
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run, and actual versus predicted are widely used to evaluate the adequacy of the model
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(Montgomery, 1997; Trinh and Kang, 2011). The diagnostic plots for cesium and strontium ions
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sorption model were demonstrated in Fig. 5 and 6 of supporting information.
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The 3D response surface curves were utilized to illuminate the interaction of the variables. In
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these figures the effect of two variables were assessed, while the other parameter was held at
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zero level (Gao et al., 2009; Su et al., 2010). The 3D plots of variables for the sorption of Cs+
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and Sr2+ ions have been shown in Fig. 1. As can be seen from the figure, the removal efficiency
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is increased with increasing of adsorbent dosage for both ions. When the amount of adsorbent is
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risen, the number of active sorption sites on the sorbents is increased and considering that the
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initial concentration of the ions is constant, the removal efficiency will be increased. Also, increasing of pollutant concentration with a fixed value of adsorbent, decreases
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removal efficiency and the removal efficiency is increased with increasing initial pH, that is due
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to the increasing of negative charge on the surface of the adsorbent, based on the results of the
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pHzpc analysis.
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Fig 1. Response surface graph of the variation of the sorption efficiency; (A) Sr2+, (B) [Cs+].
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Optimization study of the experimental results was performed by the software. In this study,
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all parameters were set in range and the goal of response was set at maximize. Under the setting,
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for Cs+ ion, the software predicted approximately 67.8% sorption efficiency at the optimum
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values of adsorbent dosage of 0.40 g, pH = 7.23 and Cs+ concentration of 10 mg L-1. Also for
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Sr2+ ion, the software predicted approximately 93.5% sorption efficiency at the optimum values
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of adsorbent dosage of 0.3 g, pH = 7.9 and Sr2+ concentration of 10 mg L-1. In order to evaluate
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accuracy of the model prediction, some experiments were performed at the optimum conditions
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and 71.8% and 92.6% sorption efficiency were practically obtained for Cs+ and Sr2+ ions,
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respectively.
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3.3. Isotherm studies
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Temperature has a prominent effect on the sorption capacity of adsorbents. Sorption
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equilibrium is generally defined by an isotherm equation whose parameters state the surface
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properties and affinity of the sorbent, at a fixed temperature and pH. An sorption isotherm
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illustrates the relationship between the amount of adsorbate on the adsorbent (Perić et al., 2004).
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In order to isotherm studies, 40 mL of the cesium or strontium ions solution with different
11
concentrations (5–650 mg/L) were agitated with optimum amount of adsorbent and optimum pH
12
for each ion at 25 °C. After 10 min contact time, the solution was filtered and the concentration
13
of Cs+ and Sr2+ ions were determined. The regular sorption isotherm models of Langmuir and
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Freundlich were selected to fit the obtained isotherm data.
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The Langmuir sorption isotherm model supposes that the adsorbent surface is homogeneous
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and mono-layer sorption occurs on the surface of adsorbent. The linearized form of the Langmuir
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equation can be expressed as:
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(3)
where qe is the amount of cesium or strontium ions adsorbed per unit weight of adsorbent (mg/g),
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Ce the equilibrium concentration of the cesium or strontium ions in the equilibrium solution
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(mg/L), b and K are the Langmuir constants that relate to energy of sorption. The numerical
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value of constants b and K is obtained by the slope and intercept of the plot of (1/qe) versus Ce.
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The Freundlich sorption isotherm, mostly fits the experimental data over a wide range of
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concentrations. In this model, it is supposed that the multilayer sorption occurs on the adsorbent
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with heterogeneous surface. The Freundlich equation may be written as: 𝑙𝑜𝑔 𝐶
(4)
where Kf is constant indicative of the relative sorption capacity of zeolite (mg/g) and 1/n is
5
the constant indicative of the intensity of the sorption process.
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𝑙𝑜𝑔 𝐾
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𝑙𝑜𝑔 𝑞
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Fitting of the sorption data for cesium and strontium ions were assessed by Langmuir and
8
Freundlich models. The results showed that the data were fitted by Freundlich better that
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Langmuir for both ions. The results of Langmuir fitting were presented in the related section and
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Fig. 7 of supporting information. Fitting of the data by Freundlich isotherm model was
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demonstrated in Fig. 2 for both ions. As can be seen cesium and strontium sorption data were
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fitted by Freundlich isotherm model with a regression coefficient of 0.98 and 0.99, respectively.
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The numerical values of the constants 1/n and Kf are computed from the slope and the intercepts,
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such that for cesium ion n and Kf were obtained 1.29 and 0.256, respectively, and for strontium
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ion n and Kf were obtained 1.29 and 0.165, respectively.
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2.0
2.0
Sr2
1.0 0.5 0.0 ‐0.5 ‐1.0
0.5
1.0
1.5
2.0
2.5
1.0 0.5 0.0 0.0
3.0
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y = 0.7758x ‐ 0.7817 R² = 0.9902
1.5
y = 0.7722x ‐ 0.5913 R² = 0.9811
log qe (mg/g)
log qe (mg/g)
1.5
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Cs+
0.5
1.0
1.5
2.0
2.5
‐0.5 ‐1.0
log Ce (mg/l)
log Ce (mg/l)
Fig. 2. Freundlich linear sorption isotherm plots of cesium and strontium ions on the
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adsorbent.
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3.4. Kinetic studies Kinetic studies were performed at obtained optimum condition and room temperature (25
2 3
°C). The amount of ion sorbed at a time t, qt (mg/g), was calculated as: 𝑞
(5)
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where C0 and Ct are the initial and equilibrium concentrations (mg/L) of cesium or strontium
6
ions, V is the volume (L), and m is the weight (g) of the adsorbent. Fitting of the data were
7
assessed by linear pseudo-first-order and pseudo-second-order kinetic models. The pseudo-first-
8
order kinetic model fitting was demonstrated in Fig. 8 of supporting information. Plotting of t/qe
9
versus time express pseudo-second-order kinetic models (Eq. 6) that the results show that the
10
data were well fitted by pseudo-second-order kinetic model (Fig. 3) with regression coefficient
11
of 0.999 and 0.999 for cesium and strontium ions, respectively.
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𝑡
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200
250
50 0 0
50
100
140 120 100 80 60 40 20 0
150
0
time (min)
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y = 1.54656x + 1.20680 R² = 0.99999
160 t/qt (min g/mg)
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t/qt (min g/mg)
150
180
Ce+
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y = 1.62154x + 1.33838 R² = 0.99997
200
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(6)
50 time (min)
100
150
Fig 3. Pseudo-second-order kinetic model plot for the sorption of cesium and strontium ions
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on the adsorbent.
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3.5. Secondary waste management Production of secondary solid waste is the main challenge in the sorption process. Because
3
in the sorption method the pollutant is transferred from aqueous media on the surface of
4
adsorbent and after releasing the secondary waste in the environment, desorption will be
5
occurred. So the produced secondary waste must be managed. To this aim, stabilization of the
6
cesium and strontium ions in the adsorbent structure was studied. The morphology and
7
composition of the adsorbent after doing the sorption process were evaluated by FESEM-EDXS
8
analysis. Fig. 4 and 5 show FE-SEM image (a) and EDXS (b) of the adsorbent for cesium and
9
strontium, respectively. The elemental compositions of O, Al, Si, K, Ca, and Cs+ were 58.68,
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5.28, 30.56, 3.06, 2.02 and 0.40 wt%, respectively, for cesium sorption and the elemental
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compositions of O, Al, Si, K, Ca, and Sr2+ were 35.47, 7.48, 52.54, 2.14, 2.01 and 0.36 wt%,
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respectively, for strontium adsorption.
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Fig. 4. (a) FE-SEM image of the adsorbent after exposures to cesium and (b) its EDXS spectrum (vacuum, 10-5 Pa; lens current, 1.8 A; voltage, 25 kV).
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(b) (b)
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Fig 5. (a) FE-SEM image of the adsorbent after exposures to strontium and (b) its EDXS spectrum (vacuum, 10-5 Pa; lens current, 1.8 A; voltage, 25 kV)
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3.5.1. Stabilization in room temperature
Stabilization of cesium and strontium ions in the adsorbent structure was performed at room
5
temperature and the tests of compressive strength and leaching were evaluated. To perform
6
compressive strength test, some blocs were prepared and dried at room temperature such that
7
after 24 h the bloc samples were bring out from the mold and dried. Then the compressive
8
strength test was performed and the results show that after 3 days it reached to 0.993 and 0.568
9
MPa for dried adsorbent containing cesium and strontium ions, respectively.
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The possibility of preventing pollutant diffusion in to the environment was assessed by
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leaching tests of back-exchange and availability test of the samples. In order to perform back-
12
exchange test the samples were placed in sodium chloride solution with concentration of 10 mg
13
L-1 and stirred for 24 h with 170 rpm. Availability test of the samples were performed by placing
14
of the samples in deionized water (solid-to-liquid ratio by weight equal to 1/50) and the test was
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performed in two stages. At first, the solution was stirred for 3 h with 170 rpm and the pH of the
16
solution was kept constant at 7.0. The process was then renewed after separation of solid from
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solution, in the same conditions, except the pH was kept constant at 4.0 (Bosch et al., 2004).
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After doing the experiments the solution was filtrated and released cesium and strontium ions
3
concentration was determined. The amount of released cesium ion of the samples for back-
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exchange and availability tests were 9.09 and 9.91 mg L-1, respectively. Also the amount of
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released strontium ion for back-exchange and availability tests were 9.13 and 9.94 mg L-1,
6
respectively. The leaching results show that fixing of cesium and strontium ions is not performed
7
as well. Given that the results of compressive strength and stabilization were not desirable; heat
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preparation of the samples was performed to stabilize strontium ion in the zeolite structure.
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3.5.2. Stabilization with heat preparation
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To perform heat preparation of the samples, at first, some blocs were prepared and dried at
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room temperature for 24 h and heat prepared at temperatures of 700, 900, and 1100 °C for 2, 6,
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and 12 h. Then the samples were cooled to room temperature and compressive strength tests
13
were evaluated that the results have been demonstrated in table 1. The results show that
14
compressive strength of the samples which heat prepared at 700 and 900 °C is mildly increased
15
with passing of heat preparation time from 2 to 12 h. The trend of compressive strength variation
16
with heat preparation at 1100 °C is different from 700 and 900 °C. As can be seen the
17
compressive strength of the samples containing cesium and strontium ions at 1100 °C is 15.04
18
and 15.87 MPa after 2 h and it decreased to 5.65 and 5.15 MPa after 12 h heat preparation time.
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It can be duo to the restructuring of the samples in high temperature with longtime of heat
20
preparation. In order to investigate restructuring of the samples, XRD analysis was performed for
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the samples with 2 h heat prepared at 1100 °C containing cesium and strontium ions.
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The XRD pattern in comparison with the standard XRD patterns of matched compound for
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sample containing cesium ion was demonstrated in Fig. 6. Presence of iron oxide, sodalite, 16
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pollucite and cesium aluminum silicate is confirmed by XRD analysis. Also formation of the
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cesium aluminum silicate which is confirmed via good accordance of the XRD pattern of the
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sample (Reference code: 00-041-0569), imply that the cesium ion placed in molecular structure. Also the XRD pattern of the sample containing strontium ion in comparison with the
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standard XRD patterns of matched compound was demonstrated in Fig. 7. This XRD analysis
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confirms presence of iron oxide, strontium aluminum oxide, strontium aluminum silicate, and
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sodalite. Presence of strontium aluminum oxide and strontium aluminum silicate imply that the
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strontium ion placed in molecular structure.
Given that the cesium and strontium ions placed in a molecular structure, its leaching should
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be decreased. In the following leaching experiments for heat prepared samples were evaluated.
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Table 1. Compressive strength of the heat prepared samples (Cross section of blocs =
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0.002025 m2).
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Compressive strength of the samples containing Cs+ (MPa) 200 °C 700 °C 1100 °C 2.62 5.31 15.04 2.75 6.2 7.38 2.86 6.35 5.65
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Compressive strength of the samples containing Sr2+ (MPa) 200 °C 700 °C 1100 °C 2.5 6.57 15.87 2.91 6.79 5.56 2.92 7.43 5.15
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Fig. 6. The XRD patterns of the sample containing cesium ion heat prepared at1100 °C for 2 h with related standards patterns.
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Fig 7. The XRD patterns of the sample containing strontium heat prepared at1100 °C for 2 h with related standards patterns.
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To evaluate leaching test, the samples with higher compressive strength were selected such
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that the sample with 12 h heat preparation time for 700 and 900 °C and 2 h for 1100 °C were
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evaluated by back-exchange and availability tests same as previous section. The results of leaching tests of back-exchange and availability test were presented in table 2.
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As can be seen from the table, leaching results show that stabilization of cesium and strontium
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ions in all temperatures has been well performed. The results show that releasing cesium and
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strontium ions has not been observed in the sample heat prepared at 1100 °C. considering the
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obtained results, it can be concluded that heat preparation in 1100 °C for 2 h well stabilize
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cesium and strontium ions in zeolite structure.
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Table 2. Leaching of cesium and strontium ions for heat prepared samples. Total leached cesium ion (mg L-1) Total leached strontium ion (mg L-1) T °C Back-exchange Availability tests Back-exchange Availability tests 3.27 0.81 1.69 1.2 700
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900
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1100
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0.08
1.28
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4. Conclusions
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The present work showed that Iranian natural zeolite is an efficient reactive matter to adsorb
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cesium and strontium ions from aqueous media. Based on the results, the following major
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conclusions can be drawn:
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A second order reduced polynomial model, established by CCD method, is able to describe the sorption of cesium and strontium ions by Iranian natural zeolite.
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The natural adsorbent can adsorb cesium and strontium ions about 67.8 and 93.5%,
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respectively.
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The sorption data for cesium and strontium ions follow Freundlich isotherm model.
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The sorption data were well fitted with pseudo-second order kinetic model for both ions.
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Secondary waste management studies showed that heat preparation in 1100 °C for 2 h well stabilize cesium and strontium ions in the natural zeolite structure.
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The cesium and strontium ions can be well stabilized in the natural zeolite structure with well compressive strength without any leaching.
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References
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Ames Jr, L., 1960. The cation sieve properties of clinoptilolite. Am. Mineralogist 45. Barrer, R.M., 1978. Zeolites and clay minerals as sorbents and molecular sieves. Academic press. Bezerra, M.A., Santelli, R.E., Oliveira, E.P., Villar, L.S., Escaleira, L.A., 2008. Response surface methodology (RSM) as a tool for optimization in analytical chemistry. Talanta 76(5), 965-977. Bosch, P., Caputo, D., Liguori, B., Colella, C., 2004. Safe trapping of Cs in heat-treated zeolite matrices. J. Nucl. Mater. 324(2), 183-188. Boyacı, I.H., 2005. A new approach for determination of enzyme kinetic constants using response surface methodology. Biochem. Eng. J. 25(1), 55-62. Cao, R.X., Ma, L.Q., Chen, M., Singh, S.P., Harris, W.G., 2003. Phosphate-induced metal immobilization in a contaminated site. Environ. Pollut. 122(1), 19-28. Cappelletti, P., Rapisardo, G., De Gennaro, B., Colella, A., Langella, A., Graziano, S.F., Bish, D.L., De Gennaro, M., 2011. Immobilization of Cs and Sr in aluminosilicate matrices derived from natural zeolites. J. Nucl. Mater. 414(3), 451-457. Ciccu, R., Ghiani, M., Serci, A., Fadda, S., Peretti, R., Zucca, A., 2003. Heavy metal immobilization in the mining-contaminated soils using various industrial wastes. Miner. Eng. 16(3), 187-192. Gao, H., Liu, M., Liu, J., Dai, H., Zhou, X., Liu, X., Zhuo, Y., Zhang, W., Zhang, L., 2009. Medium optimization for the production of avermectin B1a by Streptomyces avermitilis 14-12A using response surface methodology. Biores. Technol. 100(17), 4012-4016. Ghafari, E., Costa, H., Júlio, E., 2014. RSM-based model to predict the performance of selfcompacting UHPC reinforced with hybrid steel micro-fibers. Cons. Build. Mater. 66, 375-383. Guo, G., Zhou, Q., Ma, L.Q., 2006. Availability and assessment of fixing additives for the in situ remediation of heavy metal contaminated soils: a review. Environ. Monit. Assess. 116(1), 513-528. Mohan, D., Sarswat, A., Singh, V.K., Alexandre-Franco, M., Pittman, C.U., 2011. Development of magnetic activated carbon from almond shells for trinitrophenol removal from water. Chem. Eng. J. 172(2), 1111-1125. Montgomery, D.C., 1997. Design and analysis of experiments, 5 ed. Wiley New York. Munthali, M., Johan, E., Aono, H., Matsue, N., 2015. Cs+ and Sr 2+ adsorption selectivity of zeolites in relation to radioactive decontamination. J. Asian Ceram. Societies 3(3), 245-250. Nakai, T., Wakabayashi, S., Mimura, H., Niibori, Y., Kurosaki, F., Matsukura, M., Tanigawa, H., Ishizaki, E., 2013. Evaluation of adsorption properties for Cs and Sr selective adsorbents-13171. WM Symposia, 1628 E. Southern Avenue, Suite 9-332, Tempe, AZ 85282 (United States). Perić, J., Trgo, M., Medvidović, N.V., 2004. Removal of zinc, copper and lead by natural zeolite—a comparison of adsorption isotherms. Water res. 38(7), 1893-1899.
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Shenber, M., Johanson, K., 1992. Influence of zeolite on the availability of radiocaesium in soil to plants. Sci. total environ. 113(3), 287-295. Su, J.-J., Zhou, Q., Zhang, H.-Y., Li, Y.-Q., Huang, X.-Q., Xu, Y.-Q., 2010. Medium optimization for phenazine-1-carboxylic acid production by a gacA qscR double mutant of Pseudomonas sp. M18 using response surface methodology. Biores. tech. 101(11), 4089-4095. Trinh, T.K., Kang, L.S., 2011. Response surface methodological approach to optimize the coagulation–flocculation process in drinking water treatment. Chem. Eng. Res. Des. 89(7), 1126-1135.
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Tayebe Abdollahi December 17, 2019.
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Jafar Towfighi December 17, 2019.
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Hadi Rezaei-Vahidian December 17, 2019.
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Declaration of interests
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☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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PII: DOI: Reference:
S2352-1864(19)30346-3 https://doi.org/10.1016/j.eti.2019.100592 ETI 100592
To appear in:
Environmental Technology & Innovation
Received date : 15 July 2019 Revised date : 17 December 2019 Accepted date : 20 December 2019 Please cite this article as: T. Abdollahi, J. Towfighi and H. Rezaei-Vahidian, Sorption of cesium and strontium ions by natural zeolite and management of produced secondary waste. Environmental Technology & Innovation (2019), doi: https://doi.org/10.1016/j.eti.2019.100592. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Sorption of cesium and strontium ions by natural zeolite and management of produced
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secondary waste
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Tayebe Abdollahi1*, Jafar Towfighi1, Hadi Rezaei-Vahidian2
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4 1. Department of Chemical Engineering, Tarbiat Modares University, P.O. Box 14115-116, Tehran, Iran.
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2. Space transportation research institute, Iranian space research center, Tehran, Iran.
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E-mail: [email protected] (Tayebe Abdollahi), [email protected] (Jafar Towfighi) and [email protected] (Hadi Rezaei-Vahidian).
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*Corresponding authors: Tel/Fax.: +9821 82883311, E-mail addresses: [email protected]
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Abstract
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The aim of this work is sorptive removal of cesium and strontium ions from aqueous solution
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using natural zeolite and management of produced secondary waste. Characterization of the
4
natural zeolite was performed by X-ray diffraction (XRD), X-ray fluorescence (XRF), Brunauer–
5
Emmett–Teller (BET), and field emission scanning electron microscopy (FE-SEM). The
6
effective parameters on the sorption process such as initial solution pH, adsorbent dosage, and
7
concentration of cesium and strontium ions were optimized by using experimental design
8
method. Based on the results, removal efficiency for cesium ion was about 67.8% at the optimum
9
condition of adsorbent dosage of 0.40 g, pH = 7.23 and Cs+ concentration of 10 mg L-1, and for
10
the strontium ion was about 93.5% at the optimum condition of adsorbent dosage of 0.3 g, pH =
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7.9 and Sr2+ concentration of 10 mg L-1. Kinetic studies showed that the removal process follow
12
pseudo-second-order kinetic model for both ions. The sorption results showed that the data for
13
cesium and strontium ions were fitted by Freundlich isotherm better than Langmuir isotherm.
14
Finally, to manage secondary waste produced from the sorption process, the cesium and
15
strontium ions stabilized in zeolite structure and related tests such as compressive strength and
16
leaching were evaluated. The results of waste management section showed that the stabilization
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process was performed as well by heat preparation process.
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Keywords: Adsorption; Waste management; Response Surface Methodology; Stabilization;
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Thermal treatment.
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1. Introduction Zeolites are naturally hydrated aluminosilicate minerals with selective unique ion-exchange
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and a known resistance to radiation. The structures of zeolites consist of three-dimensional
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frameworks of SiO4 and AlO4 tetrahedral. The aluminium ion is small enough to fill the position
5
in the center of the tetrahedron of four oxygen atoms and the isomorphous replacement of Si4+ by
6
Al3+ produces a negative charge in the lattice that the net negative charge is balanced by the
7
exchangeable cation. These cations are exchangeable with certain cations in solutions such as
8
lead, cadmium, zinc, and manganese (Barrer, 1978). These properties have encouraged extensive
9
studies into their use in the treatment of radioactive waste. This was originally recognized by
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Ames in the course of his pioneering research on the environmental application of clinoptilolite
11
(Ames Jr, 1960) and has since then been confirmed by other investigators with other zeolite
12
minerals, e.g., mordenite (Munthali et al., 2015), phillipsite (Cappelletti et al., 2011), and
13
chabazite (Nakai et al., 2013). The radionuclides of cesium and strontium are the most abundant
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in the suite of nuclear fission products that are usually released into wastewater (Shenber and
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Johanson, 1992).
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Sorption processes as a physical water treatment methods transfer aqueous pollutant to
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surface of adsorbent and produce secondary waste. The produced solid secondary waste must be
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managed. The adsorbed pollutant can be desorbed and be released into the environment. One of
19
the effective methods to prevent this problem is fixing of the pollutant in a solid media to reduce
20
its diffusion. Many materials such as zeolites and phosphate-induced products, and by-products
21
such as quarry waste, and sewage sludge were used in the metal immobilization (Cao et al.,
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2003; Ciccu et al., 2003; Guo et al., 2006). Immobilization of Cs-containing zeolitic tuffs in
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cement matrices has been the subject of several investigations, which pointed out the safety of
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the procedure (Bosch et al., 2004). Optimization of a process variables is accomplished to attain the maximum process
4
efficiency. In conventional methods, one variable is used at a time to examine the influence of
5
operational parameters. In this method a high number of experiments should be performed and
6
the interactive effects between variables cannot examine (Bezerra et al., 2008). Recently,
7
researchers use the multivariate statistical techniques to optimize the effective parameters by a
8
minimum number of experiments. Response surface methodology (RSM) is an effective
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experimental design method for modelling and analysis of processes (Ghafari et al., 2014).
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The present work deals with a series of experiments to assess the utility of natural zeolite for
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the removal of Cs+ and Sr2+ ions from aqueous solution. The natural zeolite obtained from Semnan
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province desert in Iran. To optimize the process central composite design (CCD) has been used.
13
The effect of operational parameters of adsorbent dosage, pH of the solution, and initial
14
concentration of the ions on the sorption process have been optimized. Also kinetic and isotherm
15
investigations have been assessed for both ions. Finally, the secondary produced solid waste has
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been managed so that the Cs+ and Sr2+ ions were fixed in zeolite structure and related tests such
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as compressive strength and leaching were evaluated.
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2. Material and methods
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2.1. Zeolite adsorbent
The natural adsorbent, examined in our work, originated from the Arshe mine of Semnan
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province in Iran. Before use, the zeolite samples were passed through a 0.07 mm sieve (200
22
mesh). Characterizations of the adsorbent were investigated using X-ray fluorescence (XRF),
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Brunauer–Emmett–Teller (BET), and field emission scanning electron microscopy (FE-SEM).
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The phases of the natural adsorbent were determined by XRD analyses with an automated
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PHILIPS-binary diffractometer using Cu Kα radiation at 35 kV and 30 mA over the range (2θ) of
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4–55. 2.2. Chemicals and Instruments
5
All chemicals were obtained from Merck Ltd. and used without further purification.
6
Strontium and cesium ion were supplied as strontium and cesium chloride, from Merck
7
Company. Distilled water was utilized to prepare solutions. The measurement of pH was
8
performed using a pH meter CG841 model (Schott Ltd. Co., Germany). FE-SEM images were
9
acquired using an electron microscopy VEGA model (TESCAN Ltd. Co., Czech Republic). The
10
XRD and XRF analysis were carried out using an Explorer Model (GNR Ltd. Co., USA) and
11
ARL PERFOMIR' x model (Thermo electron Ltd. Co., USA), respectively. Brunauer–Emmett–
12
Teller (BET) analysis was carried out using a Model Nova 2200 e (Quantachrome Ltd. Co.,
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USA). Sr2+ ion concentration was determined using Inductively Coupled Plasma-Atomic
14
Emission Spectrometry Liberty 150 AX Turbo (Varian Ltd. Co., USA) and cesium ion
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concentration was determined using Atomic Absorption Spectroscopy model (Spectra AA220
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Varian Ltd. Co., USA).
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2.3. Sorption experiments
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The sorption of Cs+ and Sr2+ ions on natural zeolite was carried out using batch techniques at
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room temperature. To this aim, a known amount of zeolite samples was mixed with 40 ml of
20
solution having 10 mg L-1 Cs+ or Sr2+after adjusting solution pH. The suspensions were stirred at
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170 rpm for 10 min and then centrifuged at 3000 rpm to separate adsorbent.
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2.4. Waste management procedure
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In order to investigate stabilization of cesium and strontium ions in the utilized adsorbent
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structure, 150 g of adsorbent with 70 ml solution containing 10 mg L-1 of the cesium or strontium
3
ions were thoroughly mixed in the optimum condition obtained from sorption process such that a
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uniform mortar was achieved. The obtained mortar was poured into a cubic container with
5
dimensions of 5 × 5 × 5 cm. Then, the samples were dried and the stabilization tests were
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performed.
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2.5. Design of experiment
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RSM as a statistical technique is useful to optimize effective factors which creates a
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mathematical model to describe the process (Boyacı, 2005). In this technique selection of the
10
effective factors and their levels is the main step of the process. In this study the effect of dosage
11
of zeolite, initial pH, and concentration of Cs+ or Sr2+ ions on removal efficiency was studied
12
using central composite design (CCD) as one of the tools of RSM. Using CCD, for three factors,
13
20 experiments were designed and performed. Design of experiments was done by Design
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Expert version 10 software and the model statistical analysis was performed by analysis of
15
variance (ANOVA).
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3. Result and discussion
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3.1. Adsorbent Characterization
18
Chemical analysis of the natural adsorbent by XRF has been presented in Table 1 of
19
supporting information. As can be seen the natural adsorbent contain a complement of
20
exchangeable sodium, potassium, magnesium and calcium ions. The BET analysis is commonly
21
used for determining surface areas of adsorbent. The BET surface area of the adsorbent was
22
measured about 21.3912 m2/g. FE-SEM image (a) and the energy dispersive X-ray spectroscopy
23
(EDXS) (b) of the natural adsorbent were demonstrated in Fig. 1 of supporting information. The
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elemental compositions of O, Al, Si, Mg, Na, Cl, Ca and Fe in the natural adsorbent were 47.30,
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7.13, 37.10, 1.72, 2.33, 2.01, 0.20 and 2.21 wt%, respectively. The crystalline phase of the
3
natural adsorbent was determined using an X-ray diffraction pattern (Fig. 2 of supporting
4
information). The apparent peaks at the 2θ of 7.6865º, 9.6453º, 16.7689º, 19.6489º, 21.7916º,
5
26.4497º, 28.1002º, 29.8023º, 31.7978º, 34.8631º, and 49.9380º indicate the presence of
6
montmorillonite (JCPDS-00-011-0303), heulandite (JCPDS-00-014-0248), and muscovite
7
(JCPDS-00-046-1311) in the natural adsorbent. In the pHzpc the surface charge of the adsorbent
8
is zero. The surface of the adsorbent has a positive charge in the pH value below pHzpc and has a
9
negative charge in the pH above pHzpc (Mohan et al., 2011). To determine pHzpc, 40 ml sodium
10
chloride solutions (0.01 M) were prepared and their pH were set in the range of 2.5-12.5 using
11
0.1 mol L−1 HCl and/or 0.1 mol L−1 NaOH. Then 0.4 g of the natural zeolite was added into the
12
solution and stirred for 48 hours and then pH of the solutions was measured. The results of pHzpc
13
analysis show that the surface charge of the natural zeolite is zero at pH 7.5.
14
3.2. Experimental Design
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Definition of effective parameters with appropriate range is one of the main step in the
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experimental design. To this aim some relative reports was evaluated and some preliminary tests
17
were performed. The experiments were done at room temperature (25 °C). The effect of mesh
18
size of the adsorbent was evaluated and the results were presented in Fig. 3 of supporting
19
information. As can be seen from the figure the process efficiency increased with increasing of
20
mesh size for both ions. So the mesh number of 200 was selected as a suitable size of the
21
adsorbent particles. In the following, in order to find suitable time of the process some
22
experiments were performed at medium condition of the parameters using mesh number of 200
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for adsorbent that the results were demonstrated in Fig. 4 of supporting information. As can be
2
seen, the efficiency of the process for both cesium and strontium has been slightly increased
3
after 10 min of the process. Considering that the experiments of this section were done at
4
medium condition of the parameters, so the 10 min was selected as a suitable time for the
5
sorption process.
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To design the experiments, the main factors such as initial pH of the solution, adsorbent
7
dosage, and Cs+ or Sr2+ ion concentration were selected as operational parameters and efficiency
8
of the process was elected as response. Based on related literatures and preliminary performed
9
tests the range of the variables and their levels were determined that are presented in Table 2 and
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3 of supporting information for cesium and strontium, respectively. Twenty tests, designed by
11
CCD methodology were presented in Table 4 and 5 of supporting information for cesium and
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strontium ions, respectively.
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The designed experiments were done and regression analysis for the process indicated that
13
the data can be modeled by a second order polynomial equation as:
15
𝑅𝐸
16
0.030
17
𝑅𝐸
17.369
145.038
𝐴𝑑𝑠.
196.48
11.53
𝐴𝑑𝑠.
2.116
𝐶𝑠
7.927
𝑝𝐻
112.89
𝑝𝐻
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0.548
68.02
𝑆𝑟
18
𝐶𝑠
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14
𝐴𝑑𝑠. 4.06
0.06
𝐴𝑑𝑠. (1)
𝑆𝑟
𝑆𝑟
4.489
𝑝𝐻
3.40
𝐴𝑑𝑠. (2)
The ANOVA for the models was explored so that, values of “Prob>F” (p-value) less than
20
0.05 denote the model terms are significant and the values larger than 0.10 specify they are not
21
significant. In the present work, for cesium ion removal, the initial pH of the solution, dosage of
22
zeolite, and Cs+ ion concentration were the significant model terms which were held. The
23
analysis of variance for the reduced quadratic model for removal of Cs+ ion has been presented
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in table 6 of supporting information. For this model, F-value and p-value were 356.87 and less
2
than 0.0001, respectively. As a result, this model was significant. Also, the Pred R-Squared of
3
0.97 is in reasonable agreement with the “Adj R-Squared” of 0.96. “Adeq Precision” measures
4
the signal to noise ratio and a ratio greater than 4 is desirable. For the reduced model, this ratio is
5
31.92 which indicate an adequate signal.
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In the case of Sr2+ ion, pH, [Sr2+], and Adsorbent dosage were also the significant model
7
terms. The analysis of variance for the reduced quadratic model for sorption of Sr2+ ion has been
8
presented in table 7 of supporting information. The Model F-value of 47.85 and the p-value less
9
than 0.0001 implies the model is significant. In this work, the Pred R-Squared of 0.9566 is in
10
reasonable agreement with the “Adj R-Squared” of 0.9366. Also, for the reduced model, the
11
Adeq precision was 23.79 which indicate an adequate signal.
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To evaluate the adequacy of the model graphically, diagnostic plots could be utilized. Most
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of the plots display residuals, which show the difference between experimental and predicted
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responses. In the DOE software, normal probability, residuals versus predicted, residuals versus
15
run, and actual versus predicted are widely used to evaluate the adequacy of the model
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(Montgomery, 1997; Trinh and Kang, 2011). The diagnostic plots for cesium and strontium ions
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sorption model were demonstrated in Fig. 5 and 6 of supporting information.
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The 3D response surface curves were utilized to illuminate the interaction of the variables. In
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these figures the effect of two variables were assessed, while the other parameter was held at
20
zero level (Gao et al., 2009; Su et al., 2010). The 3D plots of variables for the sorption of Cs+
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and Sr2+ ions have been shown in Fig. 1. As can be seen from the figure, the removal efficiency
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is increased with increasing of adsorbent dosage for both ions. When the amount of adsorbent is
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risen, the number of active sorption sites on the sorbents is increased and considering that the
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initial concentration of the ions is constant, the removal efficiency will be increased. Also, increasing of pollutant concentration with a fixed value of adsorbent, decreases
4
removal efficiency and the removal efficiency is increased with increasing initial pH, that is due
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to the increasing of negative charge on the surface of the adsorbent, based on the results of the
6
pHzpc analysis.
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Fig 1. Response surface graph of the variation of the sorption efficiency; (A) Sr2+, (B) [Cs+].
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Optimization study of the experimental results was performed by the software. In this study,
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all parameters were set in range and the goal of response was set at maximize. Under the setting,
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for Cs+ ion, the software predicted approximately 67.8% sorption efficiency at the optimum
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values of adsorbent dosage of 0.40 g, pH = 7.23 and Cs+ concentration of 10 mg L-1. Also for
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Sr2+ ion, the software predicted approximately 93.5% sorption efficiency at the optimum values
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of adsorbent dosage of 0.3 g, pH = 7.9 and Sr2+ concentration of 10 mg L-1. In order to evaluate
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accuracy of the model prediction, some experiments were performed at the optimum conditions
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and 71.8% and 92.6% sorption efficiency were practically obtained for Cs+ and Sr2+ ions,
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respectively.
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3.3. Isotherm studies
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Temperature has a prominent effect on the sorption capacity of adsorbents. Sorption
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equilibrium is generally defined by an isotherm equation whose parameters state the surface
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properties and affinity of the sorbent, at a fixed temperature and pH. An sorption isotherm
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illustrates the relationship between the amount of adsorbate on the adsorbent (Perić et al., 2004).
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In order to isotherm studies, 40 mL of the cesium or strontium ions solution with different
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concentrations (5–650 mg/L) were agitated with optimum amount of adsorbent and optimum pH
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for each ion at 25 °C. After 10 min contact time, the solution was filtered and the concentration
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of Cs+ and Sr2+ ions were determined. The regular sorption isotherm models of Langmuir and
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Freundlich were selected to fit the obtained isotherm data.
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The Langmuir sorption isotherm model supposes that the adsorbent surface is homogeneous
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and mono-layer sorption occurs on the surface of adsorbent. The linearized form of the Langmuir
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equation can be expressed as:
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(3)
where qe is the amount of cesium or strontium ions adsorbed per unit weight of adsorbent (mg/g),
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Ce the equilibrium concentration of the cesium or strontium ions in the equilibrium solution
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(mg/L), b and K are the Langmuir constants that relate to energy of sorption. The numerical
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value of constants b and K is obtained by the slope and intercept of the plot of (1/qe) versus Ce.
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The Freundlich sorption isotherm, mostly fits the experimental data over a wide range of
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concentrations. In this model, it is supposed that the multilayer sorption occurs on the adsorbent
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with heterogeneous surface. The Freundlich equation may be written as: 𝑙𝑜𝑔 𝐶
(4)
where Kf is constant indicative of the relative sorption capacity of zeolite (mg/g) and 1/n is
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the constant indicative of the intensity of the sorption process.
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𝑙𝑜𝑔 𝐾
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𝑙𝑜𝑔 𝑞
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Fitting of the sorption data for cesium and strontium ions were assessed by Langmuir and
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Freundlich models. The results showed that the data were fitted by Freundlich better that
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Langmuir for both ions. The results of Langmuir fitting were presented in the related section and
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Fig. 7 of supporting information. Fitting of the data by Freundlich isotherm model was
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demonstrated in Fig. 2 for both ions. As can be seen cesium and strontium sorption data were
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fitted by Freundlich isotherm model with a regression coefficient of 0.98 and 0.99, respectively.
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The numerical values of the constants 1/n and Kf are computed from the slope and the intercepts,
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such that for cesium ion n and Kf were obtained 1.29 and 0.256, respectively, and for strontium
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ion n and Kf were obtained 1.29 and 0.165, respectively.
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2.0
2.0
Sr2
1.0 0.5 0.0 ‐0.5 ‐1.0
0.5
1.0
1.5
2.0
2.5
1.0 0.5 0.0 0.0
3.0
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y = 0.7758x ‐ 0.7817 R² = 0.9902
1.5
y = 0.7722x ‐ 0.5913 R² = 0.9811
log qe (mg/g)
log qe (mg/g)
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Cs+
0.5
1.0
1.5
2.0
2.5
‐0.5 ‐1.0
log Ce (mg/l)
log Ce (mg/l)
Fig. 2. Freundlich linear sorption isotherm plots of cesium and strontium ions on the
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adsorbent.
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3.4. Kinetic studies Kinetic studies were performed at obtained optimum condition and room temperature (25
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°C). The amount of ion sorbed at a time t, qt (mg/g), was calculated as: 𝑞
(5)
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where C0 and Ct are the initial and equilibrium concentrations (mg/L) of cesium or strontium
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ions, V is the volume (L), and m is the weight (g) of the adsorbent. Fitting of the data were
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assessed by linear pseudo-first-order and pseudo-second-order kinetic models. The pseudo-first-
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order kinetic model fitting was demonstrated in Fig. 8 of supporting information. Plotting of t/qe
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versus time express pseudo-second-order kinetic models (Eq. 6) that the results show that the
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data were well fitted by pseudo-second-order kinetic model (Fig. 3) with regression coefficient
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of 0.999 and 0.999 for cesium and strontium ions, respectively.
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𝑡
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200
250
50 0 0
50
100
140 120 100 80 60 40 20 0
150
0
time (min)
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y = 1.54656x + 1.20680 R² = 0.99999
160 t/qt (min g/mg)
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t/qt (min g/mg)
150
180
Ce+
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y = 1.62154x + 1.33838 R² = 0.99997
200
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(6)
50 time (min)
100
150
Fig 3. Pseudo-second-order kinetic model plot for the sorption of cesium and strontium ions
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on the adsorbent.
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3.5. Secondary waste management Production of secondary solid waste is the main challenge in the sorption process. Because
3
in the sorption method the pollutant is transferred from aqueous media on the surface of
4
adsorbent and after releasing the secondary waste in the environment, desorption will be
5
occurred. So the produced secondary waste must be managed. To this aim, stabilization of the
6
cesium and strontium ions in the adsorbent structure was studied. The morphology and
7
composition of the adsorbent after doing the sorption process were evaluated by FESEM-EDXS
8
analysis. Fig. 4 and 5 show FE-SEM image (a) and EDXS (b) of the adsorbent for cesium and
9
strontium, respectively. The elemental compositions of O, Al, Si, K, Ca, and Cs+ were 58.68,
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5.28, 30.56, 3.06, 2.02 and 0.40 wt%, respectively, for cesium sorption and the elemental
11
compositions of O, Al, Si, K, Ca, and Sr2+ were 35.47, 7.48, 52.54, 2.14, 2.01 and 0.36 wt%,
12
respectively, for strontium adsorption.
(b)
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Fig. 4. (a) FE-SEM image of the adsorbent after exposures to cesium and (b) its EDXS spectrum (vacuum, 10-5 Pa; lens current, 1.8 A; voltage, 25 kV).
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(b) (b)
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Fig 5. (a) FE-SEM image of the adsorbent after exposures to strontium and (b) its EDXS spectrum (vacuum, 10-5 Pa; lens current, 1.8 A; voltage, 25 kV)
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3.5.1. Stabilization in room temperature
Stabilization of cesium and strontium ions in the adsorbent structure was performed at room
5
temperature and the tests of compressive strength and leaching were evaluated. To perform
6
compressive strength test, some blocs were prepared and dried at room temperature such that
7
after 24 h the bloc samples were bring out from the mold and dried. Then the compressive
8
strength test was performed and the results show that after 3 days it reached to 0.993 and 0.568
9
MPa for dried adsorbent containing cesium and strontium ions, respectively.
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The possibility of preventing pollutant diffusion in to the environment was assessed by
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leaching tests of back-exchange and availability test of the samples. In order to perform back-
12
exchange test the samples were placed in sodium chloride solution with concentration of 10 mg
13
L-1 and stirred for 24 h with 170 rpm. Availability test of the samples were performed by placing
14
of the samples in deionized water (solid-to-liquid ratio by weight equal to 1/50) and the test was
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performed in two stages. At first, the solution was stirred for 3 h with 170 rpm and the pH of the
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solution was kept constant at 7.0. The process was then renewed after separation of solid from
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solution, in the same conditions, except the pH was kept constant at 4.0 (Bosch et al., 2004).
2
After doing the experiments the solution was filtrated and released cesium and strontium ions
3
concentration was determined. The amount of released cesium ion of the samples for back-
4
exchange and availability tests were 9.09 and 9.91 mg L-1, respectively. Also the amount of
5
released strontium ion for back-exchange and availability tests were 9.13 and 9.94 mg L-1,
6
respectively. The leaching results show that fixing of cesium and strontium ions is not performed
7
as well. Given that the results of compressive strength and stabilization were not desirable; heat
8
preparation of the samples was performed to stabilize strontium ion in the zeolite structure.
9
3.5.2. Stabilization with heat preparation
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To perform heat preparation of the samples, at first, some blocs were prepared and dried at
11
room temperature for 24 h and heat prepared at temperatures of 700, 900, and 1100 °C for 2, 6,
12
and 12 h. Then the samples were cooled to room temperature and compressive strength tests
13
were evaluated that the results have been demonstrated in table 1. The results show that
14
compressive strength of the samples which heat prepared at 700 and 900 °C is mildly increased
15
with passing of heat preparation time from 2 to 12 h. The trend of compressive strength variation
16
with heat preparation at 1100 °C is different from 700 and 900 °C. As can be seen the
17
compressive strength of the samples containing cesium and strontium ions at 1100 °C is 15.04
18
and 15.87 MPa after 2 h and it decreased to 5.65 and 5.15 MPa after 12 h heat preparation time.
19
It can be duo to the restructuring of the samples in high temperature with longtime of heat
20
preparation. In order to investigate restructuring of the samples, XRD analysis was performed for
21
the samples with 2 h heat prepared at 1100 °C containing cesium and strontium ions.
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The XRD pattern in comparison with the standard XRD patterns of matched compound for
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sample containing cesium ion was demonstrated in Fig. 6. Presence of iron oxide, sodalite, 16
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pollucite and cesium aluminum silicate is confirmed by XRD analysis. Also formation of the
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cesium aluminum silicate which is confirmed via good accordance of the XRD pattern of the
3
sample (Reference code: 00-041-0569), imply that the cesium ion placed in molecular structure. Also the XRD pattern of the sample containing strontium ion in comparison with the
5
standard XRD patterns of matched compound was demonstrated in Fig. 7. This XRD analysis
6
confirms presence of iron oxide, strontium aluminum oxide, strontium aluminum silicate, and
7
sodalite. Presence of strontium aluminum oxide and strontium aluminum silicate imply that the
8
strontium ion placed in molecular structure.
Given that the cesium and strontium ions placed in a molecular structure, its leaching should
9
be decreased. In the following leaching experiments for heat prepared samples were evaluated.
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Table 1. Compressive strength of the heat prepared samples (Cross section of blocs =
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0.002025 m2).
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Compressive strength of the samples containing Cs+ (MPa) 200 °C 700 °C 1100 °C 2.62 5.31 15.04 2.75 6.2 7.38 2.86 6.35 5.65
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Time (day)
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Compressive strength of the samples containing Sr2+ (MPa) 200 °C 700 °C 1100 °C 2.5 6.57 15.87 2.91 6.79 5.56 2.92 7.43 5.15
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Fig. 6. The XRD patterns of the sample containing cesium ion heat prepared at1100 °C for 2 h with related standards patterns.
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Fig 7. The XRD patterns of the sample containing strontium heat prepared at1100 °C for 2 h with related standards patterns.
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To evaluate leaching test, the samples with higher compressive strength were selected such
2
that the sample with 12 h heat preparation time for 700 and 900 °C and 2 h for 1100 °C were
3
evaluated by back-exchange and availability tests same as previous section. The results of leaching tests of back-exchange and availability test were presented in table 2.
5
As can be seen from the table, leaching results show that stabilization of cesium and strontium
6
ions in all temperatures has been well performed. The results show that releasing cesium and
7
strontium ions has not been observed in the sample heat prepared at 1100 °C. considering the
8
obtained results, it can be concluded that heat preparation in 1100 °C for 2 h well stabilize
9
cesium and strontium ions in zeolite structure.
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Table 2. Leaching of cesium and strontium ions for heat prepared samples. Total leached cesium ion (mg L-1) Total leached strontium ion (mg L-1) T °C Back-exchange Availability tests Back-exchange Availability tests 3.27 0.81 1.69 1.2 700
10
900
0.08
1100
0.0
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1.1
0.08
1.28
0.0
4. Conclusions
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The present work showed that Iranian natural zeolite is an efficient reactive matter to adsorb
14
cesium and strontium ions from aqueous media. Based on the results, the following major
15
conclusions can be drawn:
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A second order reduced polynomial model, established by CCD method, is able to describe the sorption of cesium and strontium ions by Iranian natural zeolite.
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The natural adsorbent can adsorb cesium and strontium ions about 67.8 and 93.5%,
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respectively.
19 20
The sorption data for cesium and strontium ions follow Freundlich isotherm model.
21
The sorption data were well fitted with pseudo-second order kinetic model for both ions.
22
Secondary waste management studies showed that heat preparation in 1100 °C for 2 h well stabilize cesium and strontium ions in the natural zeolite structure.
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The cesium and strontium ions can be well stabilized in the natural zeolite structure with well compressive strength without any leaching.
2 3
References
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Ames Jr, L., 1960. The cation sieve properties of clinoptilolite. Am. Mineralogist 45. Barrer, R.M., 1978. Zeolites and clay minerals as sorbents and molecular sieves. Academic press. Bezerra, M.A., Santelli, R.E., Oliveira, E.P., Villar, L.S., Escaleira, L.A., 2008. Response surface methodology (RSM) as a tool for optimization in analytical chemistry. Talanta 76(5), 965-977. Bosch, P., Caputo, D., Liguori, B., Colella, C., 2004. Safe trapping of Cs in heat-treated zeolite matrices. J. Nucl. Mater. 324(2), 183-188. Boyacı, I.H., 2005. A new approach for determination of enzyme kinetic constants using response surface methodology. Biochem. Eng. J. 25(1), 55-62. Cao, R.X., Ma, L.Q., Chen, M., Singh, S.P., Harris, W.G., 2003. Phosphate-induced metal immobilization in a contaminated site. Environ. Pollut. 122(1), 19-28. Cappelletti, P., Rapisardo, G., De Gennaro, B., Colella, A., Langella, A., Graziano, S.F., Bish, D.L., De Gennaro, M., 2011. Immobilization of Cs and Sr in aluminosilicate matrices derived from natural zeolites. J. Nucl. Mater. 414(3), 451-457. Ciccu, R., Ghiani, M., Serci, A., Fadda, S., Peretti, R., Zucca, A., 2003. Heavy metal immobilization in the mining-contaminated soils using various industrial wastes. Miner. Eng. 16(3), 187-192. Gao, H., Liu, M., Liu, J., Dai, H., Zhou, X., Liu, X., Zhuo, Y., Zhang, W., Zhang, L., 2009. Medium optimization for the production of avermectin B1a by Streptomyces avermitilis 14-12A using response surface methodology. Biores. Technol. 100(17), 4012-4016. Ghafari, E., Costa, H., Júlio, E., 2014. RSM-based model to predict the performance of selfcompacting UHPC reinforced with hybrid steel micro-fibers. Cons. Build. Mater. 66, 375-383. Guo, G., Zhou, Q., Ma, L.Q., 2006. Availability and assessment of fixing additives for the in situ remediation of heavy metal contaminated soils: a review. Environ. Monit. Assess. 116(1), 513-528. Mohan, D., Sarswat, A., Singh, V.K., Alexandre-Franco, M., Pittman, C.U., 2011. Development of magnetic activated carbon from almond shells for trinitrophenol removal from water. Chem. Eng. J. 172(2), 1111-1125. Montgomery, D.C., 1997. Design and analysis of experiments, 5 ed. Wiley New York. Munthali, M., Johan, E., Aono, H., Matsue, N., 2015. Cs+ and Sr 2+ adsorption selectivity of zeolites in relation to radioactive decontamination. J. Asian Ceram. Societies 3(3), 245-250. Nakai, T., Wakabayashi, S., Mimura, H., Niibori, Y., Kurosaki, F., Matsukura, M., Tanigawa, H., Ishizaki, E., 2013. Evaluation of adsorption properties for Cs and Sr selective adsorbents-13171. WM Symposia, 1628 E. Southern Avenue, Suite 9-332, Tempe, AZ 85282 (United States). Perić, J., Trgo, M., Medvidović, N.V., 2004. Removal of zinc, copper and lead by natural zeolite—a comparison of adsorption isotherms. Water res. 38(7), 1893-1899.
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Shenber, M., Johanson, K., 1992. Influence of zeolite on the availability of radiocaesium in soil to plants. Sci. total environ. 113(3), 287-295. Su, J.-J., Zhou, Q., Zhang, H.-Y., Li, Y.-Q., Huang, X.-Q., Xu, Y.-Q., 2010. Medium optimization for phenazine-1-carboxylic acid production by a gacA qscR double mutant of Pseudomonas sp. M18 using response surface methodology. Biores. tech. 101(11), 4089-4095. Trinh, T.K., Kang, L.S., 2011. Response surface methodological approach to optimize the coagulation–flocculation process in drinking water treatment. Chem. Eng. Res. Des. 89(7), 1126-1135.
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1 We confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us.
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We confirm that we have given due consideration to the protection of intellectual property associated with this work and that there are no impediments to publication, including the timing of publication, with respect to intellectual property. In so doing we confirm that we have followed the regulations of our institutions concerning intellectual property.
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We understand that the Corresponding Author is the sole contact for the Editorial process (including Editorial Manager and direct communications with the office). He is responsible for communicating with the other authors about progress, submissions of revisions and final approval of proofs. We confirm that we have provided a current, correct email address which is accessible by the Corresponding Author and which has been configured to accept email from “[email protected]”.
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Tayebe Abdollahi December 17, 2019.
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Jafar Towfighi December 17, 2019.
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Hadi Rezaei-Vahidian December 17, 2019.
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Declaration of interests
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☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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