Hierarchical porous structured zeolite composite for removal of ionic contaminants from waste streams and effective encapsulation of hazardous waste.

Sama M. Al-Jubouria, Nicholas A. Curryb, Stuart M. Holmesa*

a. Chemical Engineering & Analytical Science, The University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom. stuart.holmes@manchester.ac.ukb. Materials Science, The University of Manchester, Oxford Road, Manchester M13 9PL, United KingdomAbstractA hierarchical structured composite made from clinoptilolite supported on date stones carbon is synthesized using two techniques. The composites are manufactured by fixing a natural zeolite (clinoptilolite) to the porous surface of date stones carbon or by direct hydrothermal synthesis on to the surface to provide a supported high surface area ion-exchange material for metal ion removal from aqueous streams. The fixing of the clinoptilolite is achieved using sucrose and citric acid as a binder. The composites and pure clinoptilolite were compared to test the efficacy for the removal of Sr2+ ions from an aqueous phase. The encapsulation of the Sr2+ using either vitrification or a geo-polymer addition was tested to ensure that the hazardous waste can be made safe for disposal. The hierarchical structured composites were shown to achieve a higher ion exchange capacity per gram of zeolite than the pure clinoptilolite (65mg/g for the pure natural clinoptilolite and 72mg/g for the pure synthesized clinoptilolite) with the synthesized composite (160mg/g) having higher capacity than the natural clinoptilolite composite (95mg/g). The rate at which the equilibria were established followed the same trend showing the composite structure facilitates diffusion to the ion-exchange sites in the zeolite.

1. IntroductionZeolites are microporous minerals used commonly for catalysis, ion-exchange, water purification and softening, with other developing applications, such as chemical sensors and electrochemistry, pharmaceutical engineering, optics and photochemistry. The applications stem from the uniform microporous structure which gives zeolites their unique shape selectivity and high surface area coupled with high thermal stability [1-3].Generally, the diffusion of molecules and ions into/from active sites inside the micropores is the main factor determining the performance of zeolites in applications. Several strategies have been used to reduce mass transfer problems: reducing zeolite particle size reduces path length but leads to dense packed beds and large back pressure issues coupled with more complex synthesis. Pelletizing zeolites with inorganic (inert) binders to avoid excessive pressure drop in fixed bed reactors is common but can lead to fines/dust problems and dilutes the zeolite adding mass and volume which is particularly detrimental to disposal routes. Recently, introducing extra levels of porosity within the structure of zeolite has been examined by many research groups, since macropores facilitate the transport and address mass transfer limitations [2,4,5].Achieving a layer of zeolite strongly attached to a meso/macroporous surface is an important priority in the preparation of hierarchically structured zeolites to facilitate access to the micropores of zeolites through the pores of the support [2].Many approaches have been reported for the preparation of hierarchical zeolites. Li et al., [2] reported assembling ZSM-5 hollow fibers with tri-modal porosity using coaxial

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electrospinning. Dong et al., [6] prepared hollow capsules with homogeneous and dense zeolitic shells via vapour phase transport treatment. Tong et al., [4] and Ryoo et al., [7] used carbon as a transitional template to transform amorphous walls of a silica monolith to zeolite beta and CMK-1 with a tri-modal pore structure. A layer by layer (lbl) method was used to gain uniformly attached zeolite on a diatom surface utilising the electrostatic interaction between substrate materials and zeolite particles [8] modified using poly diallyl dimethyl ammonium chloride (PDDA).Strontium, plutonium, technetium, cesium and americium are the most significant isotopes in radioactive wastewaters [9]. Strontium is present in seawater with a concentration of 8×10–4

M (therefore it is the 10th most abundant element) and in the earth’s crust with concentration of 0.04% [10]. As a result of past atmospheric nuclear weapons tests, a radio-isotope 90Sr is present in soil; it is comparatively mobile therefore it can percolate to the groundwater [10]. In addition, strontium has many different uses, such as elimination of oxygen in electron tubes, fireworks and signal flares, in certain optical materials and production of glass for colour television tubes [9,10]. Removal of Sr2+ ions from commercial waste water is important because it is bio-toxic and highly soluble [9]. Several methods have been used to remove Sr2+ ions, membrane processes, chemical precipitation, flocculation adsorption and ion exchange [9,10]. Ion-exchange is very effective for the removal of the harmful ions from contaminated industrial water and zeolites are one of the most widely used ion-exchange materials. For example, zeolite A [11] and natural clinoptilolite [12] have both been used to remove Sr2+ ions from waste water.Unfortunately, ion-exchange leaves behind a material which is heavily contaminated with metal ions, a waste with a significant mass and volume which requires safe disposal without releasing the harmful ions to the environment. Finding an effective technology to treat the spent ion-exchange media requires additional study. Vitrification is widely used to treat high level hazardous radioactive wastes, radioactive wastes are immobilized through melting the zeolite ion-exchanger to glass at elevated temperatures encapsulating the waste ions (approx. 1,250ºC). The temperature makes vitrification unsuitable to encapsulate waste ions with a low boiling point, in this case, a geo-polymer can be used to immobilize harmful heavy metals and radioactive elements. Geo-polymers have been used as barriers, capping material and sealants at containment sites [13], which makes geo-polymer technology attractive for containment [14]. Geo-polymers are inorganic polymers made from cheap and abundant sources of reactive alumino-silicates (kaolin or waste materials such as blast furnace slag, mine tailings and fly ash) bound with metal hydroxide solutions [13,14]. Generally, the alumino-silicate used in the geo-polymerisation reaction is in a solid powder form and is activated by the alkaline solution (NaOH or KOH solution) and silicate solution (sodium silicate or potassium silicate solutions) at ambient temperature or slightly above [15].This paper demonstrates the preparation of a hierarchical monolithic clinoptilolite using a layer by layer (lbl) adhesion technique and then compares its ion uptake performance with that of a hierarchically structured clinoptilolite prepared by a hydrothermal treatment, for efficacy in removing of Sr2+ ions. Clinoptilolite was chosen for preparation of the monolithic composite as the natural form is cheap and possesses relatively high thermal stability and good ion-exchange capacity. Moreover, this work dealt with containment of the spent materials used for removal of Sr2+ ions using of vitrification and solidification by geo-polymers.

2. Chemicals and experimental work Materials: Carbon was produced from carbonization of date stones, collected from date palms in Iraq, heated at 900°C for 3 h under nitrogen. The materials used for preparation of monolithic clinoptilolite composite (CNC) were; natural clinoptilolite (NC) (300 micron)

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provided from Holistic Valley, sucrose (Sigma-Aldrich, 99.5%) and citric acid (Sigma-Aldrich, 99%). While, the materials used for preparation of hierarchically structured clinoptilolite (CSC) by hydrothermal treatment were anhydrous sodium aluminate (Sigma-Aldrich, 10.6%wt Na2O and 26.5%wt SiO2), colloidal silica (Sigma-Aldrich, LUDOX HS-40), NaOH pellets (99.9%wt from Fisher Scientific) and deionized water. For the ion-exchange study, anhydrous strontium nitrate (BDH laboratory supplies, purity of 99%) was used, hydrochloric acid (BDH laboratory supplies, purity of 31.5-33%) was use to prepare a 1 M solution for adjusting the pH of the solution. For the solidification study, sodium chloride (BDH laboratory supplies, purity of 99.5%), kaolin (WBB minerals, UK), fly-ash (450-S, Cemex, UK) and sodium silicate solution were used. Table 1 shows The EDS characterisation of kaolin and fly-ash.

Preparation of clinoptilolite composite using a layer by layer (lbl) methodThis method involves using NC directly. Hence, no need to use a hydrothermal treatment to obtain clinoptilolite. The preparation process involves adding sucrose and citric acid in two stages. Initially, 0.5 g of sucrose, 0.1 g of citric acid and 10 g of water were mixed together, then 1 g carbon and 2 g clinoptilolite were successively added to the sucrose/citric acid solution with continuous mixing. The combination was sonicated for 30 min. After which, it was heated at 130°C for 1 h with continuous mixing. Another sucrose/citric acid solution was prepared by dissolving of 0.5 g of sucrose and 0.1 g citric acid in 10 g water. The dried composition was added to the sucrose/citric acid solution, mixed for 10 min and then dried at 130°C for 1 h before carbonization at 650°C for 3 h in flowing nitrogen. The additional sucrose treatment gives significantly better adhesion to the support, without this step, there is loss of clinoptilolite from the surface during ion exchange.

Preparation of hierarchical composite by a hydrothermal treatmentClinoptilolite was prepared with a gel composition formula 6.5 Na2O: 3 Al2O3: 30 SiO2: 340 H2O under hydrothermal conditions in order to compare it with the natural clinoptilolite composites. The synthesis procedures were based on literature [16-19]. 3.259 g of anhydrous sodium aluminate was added to 20 g of 6%wt NaOH solution with vigorous mixing until completely dissolved. 26.91 g of colloidal silica were added to the aluminate solution agitating for 1 h until the mixture turned to a creamy gel. The gel was mixed with 1 g of natural clinoptilolite (seeds) for 15 min. The produced gel was discharged into an autoclave, heated at 140°C for 4 days. The sample was then recovered, washed and dried.The hierarchical composite clinoptilolite (CSC) over carbon of date stones was prepared with the ratio 5 carbon: 1 silica. Prior to hydrothermal treatment, the carbon was prepared by exposure to ultrasonication for 6 h in the presence of a clinoptilolite nanoparticle suspension solution prepared using ball milling. The carbon support was then added to the clinoptilolite gel and mixed for 15 min, then heated to 140°C for 4 days and recovered. The samples were characterized using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), and Thermo Gravimetric Analysis (TGA).

Ion-ExchangeStirred batch reactors were used to test the removal of Sr2+ ions by the NC and synthesised clinoptilolite (SC) and their composites. To identify the equilibrium time, 0.2 g of each materials was submerged in 100 ml of 300 ppm Sr2+ solution and equilibrated for 24 h under continuous shaking. Sr2+ solutions with concentrations: 50, 100, 200, 300, 400, 500, 600 and 700 ppm were prepared to study the kinetics of ion-exchange. Different doses of the four ion-exchange materials were chosen to study the ion-exchange capacity (0.2, 0.5, 0.8 and 1 g). The volume of batches was fixed at 100 ml. The pH of Sr2+ solution was changed to 2, 4, 6, 8

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and 10 to study the performance of the prepared materials at different pH levels. All ion-exchange samples were taken at intervals 1, 2, 3, 4 and 24 h. Tests with the pure carbon showed negligible uptake of ions.

Analytical techniqueThe Sr2+ ion concentrations were measured by Inductively Coupled Plasma Atomic Absorption Spectroscopy (ICP). The samples were filtered to avoid damaging the instrument and diluted, where necessary, to the instrument detection range.

Encapsulation of strontium ions- VitrificationFor the encapsulation study, the spent materials placed in a ceramic boat and heated in a muffle furnace at 1250°C for 2 hours. Then, 0.15 g of each material was immersed in 30 ml of 0.1 M NaCl solution and 30 ml of deionized water for contact time 1, 7 and 30 days with continuous shaking. The solution was then analysed by ICP to test for leached strontium ions.

- Solidification by geo-polymersThe geo-polymers were prepared by the activation of geo-polymer raw material (kaolin and fly ash) with alkaline activator solution according to the literature [20-22], the activator solution was prepared by mixing 2.5 ml NaOH solution (normality of 10 M) with 0.5 ml sodium silicate solution for 15 min. 0.25 g of the spent ion-exchanger was dry mixed with 2.5 g of kaolin by hand for 10 min to obtain a homogeneous mixture.In case of fly ash (which acts as a cheap supplementary silica and alumina source), 5 g was used; and in case of 50% kaolin-50% fly ash, 2 g of both were used. Then, 2.5 ml of the activator solution was added to the mixture followed by hand mixing for 15 min at room temperature. The produced slurries were transferred into moulds. Curing was conducted at 70°C for 24 h followed by for 7 days at room temperature. After that, the leachability of Sr2+ from the samples was examined by immersing samples in both water and 0.1 M NaCl solution.

3. Results and discussionSEM and EDS results of NC are shown in Figure 1, where, morphology appears to be agglomerated slim platelets, with a Si/Al ratio of ~ 4.3 according to EDS results shown in Figure 1 (b). The surface morphology of Composite Natural Carbon (CNC) (shown in Figure 2 (a)) showed a coherent layer of clinoptilolite coating the surface of the carbon. The presence of citric acid with sucrose enhances formation of poly-consistency which works to attach clinoptilolite crystals onto the carbon surface. At high temperature, the organic monomers which were added in two steps converts to a thin carbon layer attaching the clinoptilolite to the surface. Based on evidence from the difficulty to grind the samples and the lack of attrition during ion-exchange, it appears that the samples are mechanically strong and therefore could be applicable as catalysts for reactions below 500°C in a packed bed reactor. This approach could be used for other zeolite phases which have high Si/Al ratio and high thermal stability such as, ZSM-5 and mordenite.The EDS result of CNC (Figure 2 (b)) showed that the Si/Al ratio was 4.2, and potassium and calcium are the cations that balance the negative charge of clinoptilolite framework.Figure 3 presents the XRD pattern of CNC; it shows reduced peak height in comparison with XRD pattern of NC. This decline is due to dilution of the clinoptilolite concentration in the sample by the carbon support. TGA results (Figure 4) for CNC showed that it contains 59wt.% of clinoptilolite.

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The literature has demonstrated that it is difficult to prepare a pure phase of clinoptilolite [16-19]. XRD patterns of the SC in Figure 5 showed some peaks not assigned to clinoptilolite, gismondine (G) appeared at 2 theta ~ 19.76o and 27.98o; and mordenite (M) appeared at 2 theta ~ 6.50o and 31.04o (see Figure 5).The SEM image shown in Figure 6, shows the clinoptilolite crystals in the form of packed needles with a Si/Al ratio of ~ 4.1, according to the EDS analysis shown in Figure 6 (b). Figures 7 (a and b) show the SEM and EDS for CSC demonstrating a good distribution of clinoptilolite crystals over the carbon surface obtained by hydrothermal treatment. The Si/Al ratio of the composite clinoptilolite over date stones carbon is 4.3 which confirms crystallization of clinoptilolite rather than heulandite (the same structure with a higher Si/Al ratio). The percentage of clinoptilolite present in the composite is 42.33wt.% according to TGA results, see Figure 8.Table 2 displays the characteristic properties such as BET surface area, pore volume and mesopore volume of NC, CNC, SC and CSC. As expected, the composites showed lower surface area and pore volume (34.935 m2/g and 0.0543 cm3/g for CNC, and 89.1708 m2/g and 0.0649 cm3/g for CSC) than the NC and SC (40.184 m2/g and 0.1142 cm3/g for NC, and 236.092 m2/g and 0.1631 cm3/g for SC) due to the lower concentration of clinoptilolite coating on the carbon surface. The mesopore volume was 0.0412 cm3/g for CNC and 0.0343 cm3/g for CSC.Figures 9-13 show ion-exchange results of NC, CNC, SC and CSC. Generally, their ion-exchange ability showed the following sequence in the removal of Sr2+ ions: SC > CSC > NC > CNC. This can be attributed to the presence of Na+ in the structure of SC and CSC to balance the negative charge of their structures, Na+ is easier to ion-exchange than Ca2+ and K+

which are present in the structure of NC. However, this order became: CSC > SC > CNC > NC when it considers only the actual weight of clinoptilolite in the composite according to TGA results. A study of the equilibrium time for all four media shown in Figure 9 revealed that Sr2+concentration in the solution decreases until 3 h, after which ion concentration became stable which indicates equilibrium state. The results of ion-exchange were displayed in form of ion-exchange capacity mg ion/g ion-exchanger (Q) against ion equilibrium concentration (Ce) according to the equation:

Q = (C0-Ce)*v/w Equation 1

Where: C0 is initial concentration (ppm), v is volume of solution (l) and w is weight of ion-exchange material (g). The results are also presented as equilibrium concentration Ce against parameter affecting ion-exchange such as the mass of ion-exchange material, initial ion concentration and pH of the solution.Figure 10 shows the performance of the four materials as a function of varying mass of clinoptilolite or composite at a solution concentration of 300 ppm at pH 5.7 and room temperature. The material dosage was varied from 0.2, 0.5, 0.8 and 1 g. It is evident that the ionic removal increases with increasing clinoptilolite dose, hence, the ion equilibrium concentration in the solution decreases with increasing clinoptilolite dose for a given strontium initial concentration. This trend can be explained by an increasing number of active sites which were provided by zeolites at a constant initial ion concentration.The influence of initial ion concentration on the ion-exchange capacity is shown in Figure 11. The pH of the solution for each concentration was measured, the pH of solutions was 5.5-5.9. Both pure and composite synthesized clinoptilolite showed high ion-exchange capacity. In comparison, the ion-exchange capacity of NC and its composite was lower. Figure 12 shows the influence of pH of Sr2+ solution on the ion-exchange performance. The equilibrium concentration in solution falls when the pH of the ion solution increases. This

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may be attributed to the fact that H+ available in the acidic medium competes with Sr2+ ions to exchange with the non-framework cations present in clinoptilolite structure. SEM images, in Figure 13, show that high stability of both composites in the acidic media, where clinoptilolite crystals remain bound to the support surface after ion-exchange.Figure 14 shows samples of clinoptilolite used to remove Sr2+ ions before and after vitrification at 1250°C, where glassy products resulted. Significant volume reduction can be seen in the images. The colour of samples of NC and CNC changed to green after vitrification due to the presence of calcium and potassium ions which reacted with alumina in the structure of clinoptilolite. The samples of the SC show white glassy product. The samples of the spent clinoptilolite solidified by geo-polymers are shown on Figure 15. Table 3 displays the concentration of Sr2+ ions leaching from the spent materials after the vitrification step and solidification with geo-polymers. Negligible concentration of Sr2+ were detected in all cases. Vitrification gives the encapsulation of the harmful ions within a hard glassy structure and reduces the size of final waste. At elevated temperature, the alumino-silicate structure converts into a hard glassy product. In the case of the composites the, the size of the glassy product is significantly smaller than the pure clinoptilolite due to the presence of carbon which is released as CO2. Therefore, clinoptilolite /carbon composites improve the ion-exchange performance through improving the mass transfer, reduce the amount of clinoptilolite required for a given process and minimize the volume of the waste if vitrified.

Ion-exchange isotherm studyAn equilibrium isotherm study is important for ion-exchange materials. The Langmuir isotherm model and the Freundlich isotherm model were used to fit the experimental data of Sr2+ ion-exchange process. The linearized form of Langmuir and Freundlich isotherm models are described in Equation 2 and Equation 3, respectively:

qeq/Ceq= qmKL - KLqeq Equation 2

Where qeq (mg/g) is the weight of Sr2+ ion removed per weight of ion-exchanger, Ceq (mg/l) is the ion concentration at equilibrium, qm (mg/g) is Langmuir constant representing the maximum possible ion-exchange capacity and KL (l/mg) is Langmuir constant relating to the free energy of ion-exchange which corresponds to the affinity between the ion and the surface of the ion-exchange material [23-25]. The values of qm and KL can be obtained by evaluating the intercept and slope of the linear relation of qeq/Ceq with qeq.

ln qeq = ln Kf + (1/n)ln Ceq Equation 3

Where Kf (mg/g) is the Freundlich constant associated with ion-exchange capacity, and 1/n (unitless) is the Freundlich constant associated with energy heterogeneity and intensity of the reaction; it also indicates the size of the exchanged molecule and favorability of the process [23,26,27]. The values of Kf and 1/n can be obtained by estimating the intercept and slope of the linear relation between ln qeq and ln Ceq. The correlation factor (R2) was used to assess the agreement of experimental data with model.The Langmuir isotherm model assumes monolayer coverage, i.e. no transmigration of the molecules adsorbed in the plane of the surface, and maximum uptake happens at energetically identical sites when the adsorbate molecules form a saturated layer on the surface of the adsorbent [28-31]. The Freundlich isotherm model postulates that uptake can occur at a multilayer and heterogeneous active sites with non-identical energies [29,31-33].The calculated values of qm, KL and the correlation factor (R2) are displayed in Table 4. The values of Freundlich model constants (Kf and 1/n) and R2 are also presented in Table 4.

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According to the values of R2, the Freundlich isotherm model fitted the experimental equilibrium data with highest values of R2. As can be seen the values of 1/n obtained were < 1 referring to the heterogeneity of the ion-exchanger surface therefore the ion-exchange happened on heterogeneous multilayers and the ion-exchange process is favorable at high concentrations.

Kinetics study A kinetic study is essential in term of understanding the behavior of ion-exchange

process and providing information required for modeling a process. The kinetics of Sr2+ ion-exchange was investigated using pseudo first order and pseudo second order rate models [34,35].The integrated form of pseudo first order kinetic model is written as:

log (qeq - qt) = log qeq – (K1/2.303)t Equation 4

Where qt (mg/g) is the ion-exchange capacity at time = t and K1 (min-1) is the pseudo first order rate constant of the ion-exchange reaction. The values of K1 and qeq can be obtained from the slope and intercept of linear plots of log (qeq − qt) with t [34,36-38].The integrated form of pseudo second order kinetic rate equation is expressed as:

qt/t = K2(qeq)2 + K2qeqqt Equation 5

Where, K2 (g/mg.min) is the pseudo second order rate constant. The quantity K2(qeq)2 is the initial ion-exchange rate. The values of K2 and qeq can be calculated from evaluating the intercept and slope of the linear plots of qt/t versus qt [38,39].Figure 16 shows the plots of log (qeq − qt) versus t. Figure 17 shows the plots of t/qt versus t. The calculated values of K1, K2, qeq(Theoretical) and R2 are presented in Table 5. The obtained results show that Sr2+ ion-exchange reaction follows the pseudo second order kinetic model as the highest values of R2 and closest qeq(Theoretical) to the qeq(Experimental) values were obtained by pseudo second order kinetic model. This good match with the kinetic model indicates that the rate determining step of ion exchange process is the a chemical reaction related to valence forces [24,29,33,36].

ConclusionsA layer by layer assembly technique was used to fabricate a new hierarchically structured clinoptilolite/carbon composite. In addition, hydrothermal treatment was used to prepare clinoptilolite and hierarchically structured clinoptilolite/carbon. The lbl technique shows a simple procedure to prepare monolithic clinoptilolite on carbon surface from abundant and cheap precursors. Citric acid/sucrose combination gives the advantage of formulating the material working as a cheap adhesive to bind the clinoptilolite to the carbon. This paper shows results of ion-exchange of Sr2+ ions from an aqueous solution by ion-exchange with NC, CNC, SC and CSC.The equilibration time was achieved about between 3-4 h and the ability of materials to ion-exchange with Sr2+ ions showed the following sequence: CSC > SC > CNC > NC. The hierarchical structured composite was shown to have higher ion exchange capacity than the pure clinoptilolite (when nomalised for mass of clinoptilolite) with the synthesized composite having higher capacity than the natural clinoptilolite composite. However, the low cost of the components for the natural composite make it an attractive material for ion-exchange followed by disposal. The results of leaching tests showed negligible (sub ppm) concentration leached to water or NaCl solution after vitrification or solidification with geo-polymers. The Freundlich isotherm model successfully fitted the

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equilibrium data with high correlation coefficient (R2). The pseudo second order model effectively described kinetics of the ion-exchange process.

AcknowledgementsThe authors would like to thank Patrick Hill for his support. The Higher Committee for Development of Education in Iraq and the University of Baghdad are acknowledged for supporting Sama M. Al-jubouri.

Table 1: EDS quantification of kaolin and fly-ash.

Element Kaolin Fly ashWeight% Weight%

C 32.57 30.78O 27.46 22.80Na 0.07 0.89Mg 0.28 0.75Al 16.96 13.07Si 20.01 17.13S 0.17 0.35K 1.33 2.21Ca 0.19 3.46Ti 0.15 0.73Fe 0.80 7.84

Figure 1: a) SEM image of natural clinoptilolite (NC) and b) EDAX result of natural clinoptilolite.

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Figure 2: a) SEM image of monolithic clinoptilolite/carbon composite (CNC) and b) EDS result of the composite.

Figure 3: XRD patterns of natural clinoptilolite (NC) and monolithic clinoptilolite/carbon composite (CNC).

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Figure 4: TGA results of monolithic clinoptilolite/carbon composite (CNC).

Figure 5: XRD patterns of synthetic clinoptilolite (SC) and synthetic clinoptilolite/carbon composite (CSC).

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Figure 6: a) SEM image of synthetic clinoptilolite (SC) and b) EDS result of synthetic clinoptilolite.

Figure 7: a) SEM image of synthetic clinoptilolite/carbon composite (CNC) and b) EDS result of synthetic clinoptilolite/carbon composite.

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Figure 8: TGA results of synthetic clinoptilolite/carbon composite (CSC).

Table 2: Characteristic properties of ion-exchange materials obtained from characterization by N2-

adsorption.

Ion-exchange

materialsSBET (m2/g) VTotal (cm3/g) VMesopores (cm3/g)

NC 40.184±0.211 0.1142 -

CNC 34.935±0.078 0.0548 0.0412SC 236.092±5.667 0.1631 -

CSC 89.1708±0 0.0649 0.0343

SBET: BET surface area.VTotal: Total pore volume.VMesopores: Volume of mesopores.

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Figure 9: The study of equilibration time on the ion-exchange of Sr2+ ions by natural clinoptilolite, monolithic clinoptilolite/carbon composite, synthetic clinoptilolite and synthetic clinoptilolite/carbon composite (Initial Sr2+ ions concentration 300 ppm, ion-exchanger weight 2 g, solution pH 5.7 and at room temperature).

Figure 10: Effect of ion-exchanger dose on the ion-exchange of Sr2+ ions by natural clinoptilolite, monolithic clinoptilolite/carbon composite, synthetic clinoptilolite and synthetic clinoptilolite/carbon composite (Initial Sr2+ ions concentration 300 ppm, solution pH 5.7 and at room temperature).

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Figure 11: Effect of initial Sr2+ ions concentration on the ion-exchange of Sr2+ ions by natural clinoptilolite, monolithic clinoptilolite/carbon composite, synthetic clinoptilolite and synthetic clinoptilolite/carbon composite (Ion-exchanger weight 2 g, solution pH 5.7 and at room temperature).

Figure 12: Effect of Sr2+ solution pH on the ion-exchange of Sr2+ ions by natural clinoptilolite, monolithic clinoptilolite/carbon composite, synthetic clinoptilolite and synthetic clinoptilolite/carbon composite (Ion-exchanger weight 2 g, initial Sr2+ ions concentration 300 ppm and at room temperature).

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Figure 13: SEM images of a) synthetic clinoptilolite, b) monolithic clinoptilolite/carbon composite, c) synthetic clinoptilolite and d) synthetic clinoptilolite/carbon composite after ion-exchange with Sr2+ ions.

Figure 14: Samples of clinoptilolite after used to remove strontium ions a) before vitrification, b) after vitrification (at 1250°C).

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Figure 15: Samples of clinoptilolite after used to remove strontium ions after solidification by a) kaolin, b) fly ash, c) 50%kaolin-50%fly ash.

Table 3: Concentration of Sr2+ ions leaching from the spent materials in the solutions before/after vitrification and solidification with geo-polymers.

MaterialsConcentration of Sr2+ ions leached in the solutions

(ppm) after one month

H2O 0.1 M NaCl solutionSpent ion-exchangers before any treatment 0.661-0.985 8.626-121

Vitrified samples 0-0.036 0-0.176Samples solidified with kaolingeopolymer 0.11-0.72 0.07-0.24Samples solidified with fly ash geopolymer 0.11-0.58 0.1-0.91Samples solidified with 50%kaolin-50%fly

ash geopolymer0.05-0.1 0.05-0.19

Table 4: Langmuir and Freundlich constants for ion-exchange of Sr2+ ion over different ion-exchangers.Ion-exchange material Langmuir model Freundlich model

qm (mg/g) KL (l/mg) R2 Kf (mg/g) 1/n R2

NC 99.55 0.0202 0.7656 8.81 0.392 0.9795

CNC 95.62 0.013 0.7469 6.21 0.422 0.9786

SC 97.63 0.919 0.4176 31.85 0.189 0.9205

CSC 91.22 0.411 0.4103 26.55 0.206 0.9226

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Figure 16: Pseudo first order kinetics plots for Sr2+ ion-exchange over NC, CNC, SC and CSC.

Figure 17: Pseudo second order kinetics plots for Sr2+ ion-exchange over NC, CNC, SC and CSC.

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Table 5: Pseudo first order and pseudo second order parameters for ion-exchange of Sr2+ ion over different ion-exchangers.

Ion-exchange material

qeq(Experimental)

(mg/g)Pseudo first order Pseudo second order

qeq(Theoretical)

(mg/g)K1

(min-1)R2 qeq(Theoretical)

(mg/g)K2

(g/mg.min)R2

NC 63.64 7.85 9.8×10-3 0.8963 64.1 19×10-4 0.9998CNC 55.52 4.406 9.5×10-3 0.8920 55.556 54×10-4 1SC 73 45.29 11×10-3 0.9926 79.37 8×10-4 0.9999

CSC 69.64 1.29 2.65×10-

30.0710 69.93 42×10-4 1

Theoretical: means the ion-exchange capacity evaluated from experimental data.Experimental: means the ion-exchange capacity evaluated from kinetic model.

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