Nova Biotechnologica et Chimica 13-1 (2014) 57 DOI 10.2478/nbec-2014-0007 © University of SS. Cyril and Methodius in Trnava RADIOCESIUM ADSORPTION BY ZEOLITIC MATERIALS SYNTHESIZED FROM COAL FLY ASH LUCIA REMENÁROVÁ, MARTIN PIPÍŠKA, EVA FLORKOVÁ, JOZEF AUGUSTÍN, MARIÁN ROZLOŽNÍK, STANISLAV HOSTIN, MIROSLAV HORNÍK Department of Ecochemistry and Radioecology, University of SS. Cyril and Methodius, J. Herdu 2, Trnava, SK-917 01, Slovak Republic (l.remenarova@ucm.sk) Abstract: Brown coal fly ash derived from the combustion of brown-coal in power plant ENO Nováky (Slovak Republic) was used as raw material for synthesis of zeolitic materials ZM1 and ZM3 by hydrothermal alternation with 1M NaOH and 3M NaOH, respectively. Fly ash and synthesized products were characterized using XRF and SEM-EDX analysis. Subsequently, zeolitic materials were tested as sorbents to remove Cs+ ions from aqueous solutions using radiotracer technique. Sorption of cesium by both types of zeolitic materials obeys Langmuir adsorption isotherm model. The maximum sorption capacities Qmax at pH 6.0 calculated from Langmuir isotherm were 1203 ± 65 μmol Cs+/ g for ZM1 and 1341 ± 66 μmol Cs+/ g for ZM3. The results showed that alkali treated fly ash can be used as effective sorbent for radiocesium removal from contaminated solutions. Key words: 137Cs, adsorption, fly ash, hydrothermal zeolitization, zeolitic materials 1. Introduction The development of nuclear science and technology, nuclear weapons testing, generation of electricity in nuclear power plants and the use of radionuclides in a research and medicine have led to the increase production of both liquid and solid wastes containing radionuclides and release of artificial radionuclides into the environment. Radiocesium 134Cs and 137Cs are one of the most abundant radionuclide in nuclear fission products which has a relatively long half-life and is considered as hazardous element (KHAN, 2003) due to its high solubility what enables easy migration into the environment. Effective separation technologies, e.g. fractional crystallization, solvent extraction, ion exchange using inorganic and organic ion exchangers have been developed for removal of radionuclides from liquid radioactive wastes. The most widely used methods for water treatment to remove radionuclides (Cs, Sr, U, Pu) are those based on co-precipitation and sorption (MILYUTIN et al., 2012). Many researchers have reported about utilization of zeolite and bentonite type minerals for removal of Cs+ ions (YILDIZ et al., 2011; ABD EL-RAHMAN et al., 2006; ATUN and BODUR, 2002). Ion exchange properties of zeolites have been received great attention, especially for application in radioactive liquid waste treatment (ABD EL-RAHMAN et al., 2006; EL-KAMASH et al., 2005). The cationic radioisotopes, present in the liquid effluents of low and intermediate level liquid waste, can be removed by the ion exchange with the Na+ ions of the zeolites. These inorganic materials possess high exchange capacity, selectivity and specificity, good resistant to radiation, and have proven advantages with respect to immobilization and Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 16.01.20 16:11 UTC 58 Remenárová, L. et al. final disposal when compared with organic ion exchangers (GONI et al., 2006; LI et al., 2005). While the particular zeolitic development may take thousands of years in order to form natural zeolites (KOUKOUZAS et al., 2010), formation of synthetic zeolitic materials can be speeded up in the laboratory. In that case the activation solution is an alkaline one, usually NaOH or KOH. The classical alkaline conversion of fly ash (FA) is based on the combination of different [activation solution]/ [fly ash] ratios. The methodologies developed on this field aim at the dissolution of Al-Si bearing phases of the FA and the subsequent precipitation of the zeolitic material (BLISSETT and ROWSON, 2012; AHMARUZZAMAN, 2010; HUI and CHAO, 2006; MOUTSATSOU and PROTONOTARIOS, 2006; QUEROL, 2002; IYER and SCOTT, 2001; HOLLMAN, 1999; AMRHEIN et al., 1996; SHIGEMOTO et al., 1993). In addition, the application of fly ash can contribute to the solution of another environmental problem, namely accumulation of fly ash produced by burning of coal in power stations. Fly ash is a massing waste material containing hazardous substances which may be released to the environment from dumping places which are still enlarged. In spite of the fact that there exist many ways of utilization of this toxic pollutant (e.g. civil engineering, metallurgy, mining, agriculture, etc.) it is necessary to increase the amount of fly ash being re-utilized. Single fly ash is known for low sorption capacities. However, after its alternation, products with better ion-exchange properties and several fold-enlarged surfaces comparable with natural zeolites can be obtained. Of the many applications of fly ash, zeolite synthesis has been reported to be of great interest due to the fact that zeolites have wide industrial application (MUSYOKA et al., 2013). In our work the brown coal fly ash was utilized for synthesis of two types of zeolitic materials. The main motivation for this work was inspired by the need for effective sorbents for radiocesium that are essential for purifying the radioactive contaminated aqueous solutions. The key objectives of our research are as follows: (a) to characterize fly ash and products of its hydrothermal alternation using scanning electron microscopy connected with EDX and X-ray fluorescence analysis; (b) to quantify sorption capacity of fly ash and zeolitic materials, and (c) to investigate the major mechanisms of cesium ions sorption. Equilibrium isotherm models according to Langmuir and Freundlich were used for mathematical description of sorption equilibrium in single systems. To obtain reliable experimental sorption data within a broad concentration range, radiotracer technique with 137CsCl was used. 2. Material and methods 2.1 Sorbent preparation Brown-coal fly ash (density ρ = 2 850 kg/m3) was obtained from power station Nováky (ENO) (Zemianske Kostoľany, Slovak Republic) and was stored in sealed containers before use. Fluid fly ash was washed in deionised water (0.054 μS/cm Simplicity 185, Millipore, USA) and oven-dried for 24 h at 60°C. Dried ash was used Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 16.01.20 16:11 UTC Nova Biotechnologica et Chimica 13-1 (2014) 59 as the raw material for zeolite synthesis according to the method of QUEROL et al. (2001). Fly ash (100 g) was resuspended in 300 ml of 1.0 M or 3.0 M NaOH for ZM1 and ZM3 preparation, and the mixture was boiled under reflux for 18 h. The products of hydrothermal zeolitization were thoroughly washed 3 times in surplus of deionised water, oven-dried for 24 h at 60°C and were stored in sealed containers before use in sorption experiments. 2.2 Sorbent characterization The surface structure analysis of sorbents (fly ash, ZM1 and ZM3) before and after Cs+ sorption and EDX microanalysis were performed by scanning electron microscope VEGA 2 SEM (TESCAN Inc., Czech Republic) coupled with QUANTAX QX2 detector (RONTEC, Germany) for electron dispersive X-ray analysis. Prior to the SEM and EDX analysis, the samples were dried for 24 h at 80°C and stuck to a sample holder using conductive adhesive (Ag). Samples were then coated with Au using BP 343.7 Evaporator (TESLA ELMI Inc., Czech Republic). The analyses were performed at voltage 30 kV and vacuum pressure 9.0×10-3 Pa. Elemental analysis of fly ash and zeolitic materials was performed by X-ray fluorescence analysis using high performance X-ray fluorescence spectrometer X-LAB 2000, SPECTRO, Germany. For determination of specific surface area (SSA) zeolitic material was subjected to N2 adsorption using BET method (BRUNAUER et al., 1938). 2.3 Adsorption experiments Batch adsorption experiments were carried out by suspending the sorbent (2.5 g/dm3, d.w. of fly ash, zeolitic material ZM1 and ZM3) in solutions with concentrations of 4000 μmol/dm3 of CsCl (analytical grade, Sigma Aldrich, Germany) spiked with 137Cs at pH adjusted to 6.0. The content was agitated on reciprocal shaker (120 rpm) at 22 °C. In time periods (5, 30, 60, 120, 240, 360 and 1440 min.) aliquot samples of solution were taken from individual flasks for radiometric determination of 137Cs. The Cs+ uptake was calculated as: M V CCQ eqeq )( 0 −= (1) where Q is the uptake (μmol/g), C0 and Ceq are the initial and the final Cs + concentrations in solution (μmol/dm3), V is volume (dm3) and M is the amount of dried sorbent (given in grams). For determination of the adsorption capacity of sorbents batch equilibrium experiments were carried out in solutions of CsCl in concentration range 100-6000 μmol/dm3, spiked with 137Cs. pH was adjusted to 6.0. The content was agitated on a reciprocal shaker (120 rpm) for 6 h at 22°C. To evaluate the effect of the pH on the uptake of Cs by studied sorbents, experiments were carried out in CsCl solutions (4000 μmol/dm3) spiked with 137Cs at pH ranging from 2.0 to 8.0. The content was agitated on reciprocal shaker (120 rpm) for 6 h at 22 °C. At the end of the experiments Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 16.01.20 16:11 UTC 60 Remenárová, L. et al. the sorbent was filtered out, washed in deionized water and the radioactivity of both the sorbent and the liquid phase was measured. 2.4 Radiometric analysis The gamma spectrometric assembly using the well type scintillation detector 54BP54/2-X, NaI(Tl) (Scionix, the Netherlands) and the data processing software Scintivision 32 (ORTEC, USA) were used for obtaining quantitative data of Cs+ sorption by sorbents at the energy of γ – photons: 137Cs – 661.64 keV. Standardized 137CsCl solution (5.4 MBq/cm 3, CsCl 20 mg/dm3 in 3 g/dm3 HCl) was obtained from the Czech Institute of Metrology, Prague (Czech Republic). 2.5 Data analysis The Langmuir (Eq. 2) and Freundlich (Eq. 3) adsorption models were used for describing equilibrium data. The integral equations are as follows: eq eq eq bC CbQ Q + = 1 max (2) )/1( neqeq KCQ = (3) where Qmax represents the maximum sorption capacity upon complete saturation of the sorbent, b is a constant related to the energy of adsorption. K and 1/n values are the Freundlich constants referring to adsorption capacity and intensity of adsorption, respectively. The Qmax values and the corresponding parameters of adsorption isotherms were calculated using the non-linear regression analysis performed by ORIGIN 7.0 Professional (OriginLab Corporation, Northampton, USA). The theoretical Cs speciation in solution was calculated using MINEQL+ Chemical Equilibrium Modeling System (version 4.6 for Windows). 3. Results and discussion 3.1 Synthesis of zeolitic materials and their characterization The chemical composition of zeolitic material ZM1 and ZM3 prepared from brown coal fly ash by hydrothermal activation is given in Table 1. The main components of fly ash are SiO2 (54.7%) and Al2O3 (19.2%) which present elementary building blocks for preparation of synthetized zeolites. Concentration of these components was further decreased in both zeolitic materials (SiO2 from 54.7 to 50.1% (ZM1) and 42.2% (ZM3)) as a result of dissolution within zeolitization in NaOH. Moreover, during the hydrothermal activation, both alumina and silicate ions are condensed and formed aluminum–silicate gel. Evident increase in Na2O amount from 1.0 (coal fly ash) to 6.2% (ZM1) and 9.8% (ZM3) is due to entrapment of sodium ions to neutralize the negative charge on aluminosilicate gel (Table 1). NaOH causes differentiation in the Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 16.01.20 16:11 UTC Nova Biotechnologica et Chimica 13-1 (2014) 61 distribution of the electric charge between the Al-O and Si-O bonds and the enhancement of the chemically active centres in the lattice. As was mentioned by other authors (KOUKOUZAS et al., 2010) generation of terminal groups such as ≡Si-OH, ≡Si-ONa, ≡Si-O-, (≡Si-O)3Al-O- in the presence of NaOH lead to the formation of zeolitic materials. Fig. 1. Scanning electron micrographs of zeolitic material ZM3 – magnification: (A) 100×; (B) 500×; (C) 1000× and (D) 2000×. The SEM microscopy was used for surface characterization of raw fly ash and synthesized ZM1 and ZM3. Although the synthesized zeolitic materials keep the original spherical shape of raw fly ash, pseudomorphs (zeolitic aggregates) on the surface of fly ash particles were created (Fig. 1A-D). Similar formation of pseudomorphs after hydrothermal reaction was observed by RÍOS et al. (2009). MURAYAMA et al. (2002) have synthetized zeolites in solutions of Na2CO3 and/or KOH. They observed that the crystal of zeolite P synthesized in Na2CO3 solution A B C D Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 16.01.20 16:11 UTC 62 Remenárová, L. et al. covers like a thin film on the fly ash surface. On the other hand, in the case of KOH solution, chabazite crystal grows as an ellipsoid on the fly ash surface. Table 1. The chemical composition of brown coal fly ash and synthesized zeolitic materials ZM1 and ZM3 (wt. %). Component (%) SiO2 Al2O3 Fe2O3 CaO MgO MnO TiO2 K2O Na2O P2O5 SO3 Raw fly ash 54.7 19.2 9.88 5.39 2.07 0.09 0.73 2.30 1.00 0.19 0.58 ZM1 50.1 18.8 8.38 4.16 2.10 0.09 0.61 2.06 6.20 0.13 0.09 ZM3 42.2 18.9 8.94 5.85 2.25 0.10 0.67 1.63 9.80 0.06 0.08 3.2 Influence of pH on cesium adsorption The pH value presents one of the crucial parameters in the sorption of radionuclides heavy metals by zeolites and zeolitic materials. Aqueous liquid radioactive waste is generated during nuclear reactor operations and during industrial and institutional application of radioisotopes. The chemical compositions and radioactivity levels of the generated wastes depend on the conducted operation (ABDEL-RAHMAN et al., 2011). Also the pH of aqueous radioactive waste depends on its origin. Therefore, to investigate the influence of pH on sorption of Cs+ ions, experiments were conducted in the pH range from 2.0 to 8.0. 2 3 4 5 6 7 8 0 20 40 60 80 100 Q eq ( % ) pH sorption of Cs+ onto ZM1 sorption of Cs+ onto ZM3 Fig. 2. Effect of initial pH on sorption of Cs+ (C0 = 4000 µmol/dm3 CsCl, 60 kBq/dm3 137Cs) by zeolitic material ZM1 and ZM3 (CS = 2.5 g/dm3, d.w.) from single metal solution. Error bars represent standard deviation (SD) of the mean (n = 2). The effect of initial pH on Cs sorption is depicted in Fig. 2. It was observed that cesium ions´ binding is pH dependent and that the amount of sorbed metal ions increased with increasing pH of the solution. Increasing pH value causes that the net- Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 16.01.20 16:11 UTC Nova Biotechnologica et Chimica 13-1 (2014) 63 electronegativity of zeolitic materials increases as a result of deprotonation of active sites present on surface. The highest uptake was observed at pH ranging from 5.0 to 6.0 and reached 57% (ZM1) and 61% (ZM3) respectively. Observed negligible sorption at pH 2.0 is closely related to competition between H+ and Cs+ ions for occupancy of the active sites. The slight decrease of Cs sorption at pH 8.0 is attributed to the abundance of OH- that results in an increased hindrance to the diffusion of cesium ions. RAO et al. (2006) pointed out that mineral acids affect the structure of zeolites In the zeolite framework, the Si-O-Al is weaker than Si-O-Si and can easily be attached by H+ ions affecting the zeolite structure. This defect is more pronounced in the case of zeolites with low Si/Al ratios such as zeolite A and X (RAO et al., 2006), as well as studied zeolitic materials ZM1 and ZM3. The extent of the damage to their structure depends on the pH of the solution. The structure of zeolites with low Si/Al ratios may collapse in the presence of acids with pH lower than 5.0, but the severity would be more below pH value of 3.0 (EL-KAMASH, 2008). Aluminosilicate structure is vulnerable also to strong basic environments since strong bases can lead to desilication (MISHRA and TIWARI, 2006). These morphological changes could influence the ZM sorption capacity. The pH of the solution also influences the ionization and speciation of metals in aqueous solutions. Metals can form various ionic forms depending on the pH of solution. It is generally known that zeolites have a very little affinity to anionic complexes due to their negative framework structure (ZIYATH et al., 2011). According to speciation calculation (Visual MINTEQ ver. 3.0) cesium, in pH from 2.0 to 8.0, exists exclusively in two forms Cs+ (>99.9%) and CsCl (0.1%) indicating that speciation has no significant influence on cesium uptake by zeolitic materials. Due mentioned facts, all further experiments in present study were carried out at initial pH value of 6.0. 3.3 Cs+ sorption kinetics by zeolitic materials and fly ash Zeolitic materials (ZM1 and ZM3) prepared from coal fly ash represent well sedimenting material with a large specific surface area, suitable for sorption of dissolved solutes primarily metal ions and radionuclides. The kinetic studies on the adsorption of cesium ions from single system by zeolitic materials and brown coal fly ash were performed to determine the minimum time needed to reach the sorption equilibrium. Results showed that the Cs sorption by zeolitic materials is a rapid process (Fig. 2). Maximum uptake, 933 µmol/g (59%) in case of ZM1 and 1015 µmol/g (63%) in case of ZM3 at sorbent dosage 2.5 g/dm3, initial cesium concentration C0 = 4000 µmol/dm 3 and 22 °C, was recorded after 30 min and equilibrium was established within 60 min. After this time there was no significant increase during next 23 hours, although a slight decrease of Cs uptake in case of ZM1 was observed. At the initial phase, rapid uptake of cesium ions onto surface of zeolitic materials occurs. Subsequently, the Cs+ ions diffused into the crystal structure through the micropores followed by chemical and physical adsorption at the active sites of the pores. Contrary, the equilibrium sorption of Cs+ ions by raw fly ash was obtained after 60 min and the cesium uptake reached only 4 µmol/g, indicating negligible affinity of Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 16.01.20 16:11 UTC 64 Remenárová, L. et al. fly ash towards cesium ions. EL-NAGGAR et al. (2008) also observed very fast sorption of cesium ions by NaA-X zeolite blend from fly ash. The largest amount of cesium ions was adsorbed within the first 90 min. Maximum sorption of Cs+ ions by Turkish clays (bentonite, zeolite, sepiolite and kaolinite) was reached within 4h (BAYÜLKEN et al., 2010). Similarly, the kinetic studies of FAGHIHIAN et al. (2013) showed that the process of Cs+ and Sr2+ sorption onto magnetic zeolite nanocomposite was quite rapid and 90% of equilibrium capacity was achieved within 30 min. On the contrary, YILDIZ et al. (2011), who studied the sorption behavior of Cs+ ions on clay minerals and zeolite from radioactive waste, found that in all cases, equilibrium was reached in about 2 days. 0 200 400 600 800 1000 1200 1400 1600 0 200 400 600 800 1000 1200 Q eq [μ m ol g -1 ] t [min] fly ash zeolitic material ZM1 zeolitic material ZM3 Fig. 2. Sorption kinetics of Cs+ ions by coal fly ash (C0 = 1000 µmol/dm3 CsCl, 60 kBq/dm3 137Cs, CS = 2.5 g/dm3, d.w.) and zeolitic materials ZM1, ZM3 (C0 = 4000 µmol/dm3 CsCl, 60 kBq/dm3 137Cs, CS = 2.5 g/dm3, d.w.) at 22°C and pH 6.0. 3.4 Sorption isotherm study The cesium sorption equilibrium is shown in Fig. 3. On the basis of obtained results we found that sorption capacity for cesium was 287 times higher (ZM1) and 316 times higher (ZM3) in comparison with raw coal fly ash. Due to the fact that we observed only negligible sorption capacity for cesium in case of fly ash, this sorbent was not used in further experiments. The Langmuir (Eq. 2) and Freundlich (Eq. 3) isotherms were fitted to the equilibrium data for Cs+ sorption on ZM1 and ZM3. Parameters of the models determined from the experimental data using non-linear regression analysis are reported in Table 2. In both cases the Langmuir isotherm fits experimental data better than the Freundlich isotherm, as is demonstrated by the more homogeneous standard deviation of each observed parameter obtained and by the Akaike´s information criterion values (Table 2).The maximum sorption capacity Qmax of zeolitic materials for cesium obtained from Langmuir isotherm was 1203 ± 65 µmol/g for ZM1 and 1341 ± Q eq [μ m ol /g ] Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 16.01.20 16:11 UTC Nova Biotechnologica et Chimica 13-1 (2014) 65 66 µmol/g for ZM3, indicating slightly higher sorption capacity of ZM3 for cesium cations. Affinity constant b indicate similar affinity of both materials for Cs+ ions at low concentrations (ZM1 b = 0.0042; ZM3 b = 0.0036). The difference in Qmax values is related to reaction conditions of zeolite preparation. For preparation of zeolitic material ZM3, by hydrothermal synthesis of fly ash, higher NaOH concentration (3 M) was used. This fact relates with higher molar content of exchangeable Na+ ions in the final product in comparison with ZM1 and raw coal fly ash (Table 1). The difference in maximum sorption capacities of studied materials was more visible in case of Cd2+ ions sorption. The Qmax were 696 ± 22 μmol/g for ZM1 and 1160 ± 44 μmol/g for ZM3 indicating higher capacity of ZM3 for cadmium (REMENÁROVÁ et al., 2014). 0 1000 2000 3000 4000 5000 6000 0 200 400 600 800 1000 1200 1400 Q eq [μ m ol /g ] Ceq [μmol/L] fly ash zeolitic material ZM1 zeolitic material ZM3 Fig. 3. The equilibrium sorption of Cs+ ions onto: (-■-) fly ash, (-●-) zeolitic material ZM1 and (-▲-) zeolitic material ZM3 (2.5 g/dm3, d.w.) after 6 h at 22°C, initial pH 6.0. Table 2. Langmuir and Freundlich isotherm model parameters (± SD) for sorption of Cs+ ions onto zeolitic material ZM1 and ZM3 at pH 6.0 (6 h, 22 °C) calculated using non-linear regression analysis. Isotherm model Qmax [μmol/g] b [dm3/μmol] K [dm3/g] 1/n R 2 Akaike´s weight Zeolitic material ZM1 Langmuir 1203 ± 65 0.0042 ± 0.0008 - - 0.989 97.58 Freundlich - - 76.4 ± 31.4 0.34 ± 0.06 0.951 2.42 Zeolitic material ZM3 Langmuir 1341 ± 66 0.0036 ± 0.0006 - - 0.992 97.93 Freundlich - - 62.9 ± 26.1 0.37 ± 0.06 0.960 2.07 It can be seen from Table 2 that the Freundlich intensity constant n are greater than unity for both studied zeolitic materials. This has physicochemical significance with Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 16.01.20 16:11 UTC 66 Remenárová, L. et al. reference to the qualitative characteristics of the isotherms, as well as to the interactions between metal ions species and zeolite (EL-KAMASH, 2008). Value n > 1 indicates that the zeolitic material has an increasing tendency for sorption with increasing sorbent concentration. This should be attributed to the fact that with progressive surface coverage of adsorbent, the attractive forces between the metal ion species such as van der Waals forces, increases more rapidly than the repulsive forces, exemplified by short-range electronic of long-range Coulombic dipole repulsion, and consequently, the metal ions manifest a stronger tendency to bind to the zeolitic materials (ABD EL-RAHMAN et al., 2006; MOHAN and SINGH, 2002). Table 3 compares the maximum sorption capacities Qmax of various inorganic sorbents for removing of cesium cations. Comparison of Qmax values obtained in our work with those of other authors indicates that studied synthetic zeolitic materials prepared from coal fly ash belong to the group of sorbents with significantly higher sorption capacity for cesium. Table 3. Comparison of Qmax values of different inorganic sorbents for Cs+ ions. Sorbent Qmax [µmol/g] References magnetic zeolite 1725 FAGHIHIAN et al. (2013) zeolite A 1561 EL-KAMASH (2008) bentonite J15 850 GALAMBOŠ et al. (2010) bentonite K15 950 GALAMBOŠ et al. (2010) bentonite L15 550 GALAMBOŠ et al. (2010) montmorillonite K10 280 GALAMBOŠ et al. (2010) zeolite (fine) 1610 BAYÜLKEN et al. (2010) zeolite (coarse) 671 BAYÜLKEN et al. (2010) coal fly ash 4* present study zeolite ZM1 1203 present study zeolite ZM3 1341 present study * maximum sorption capacity Qmax determined experimentally 3.5 Sorption mechanism Due to the fact that obtained results indicate higher capacity of ZM3 for cesium ions, sorption mechanism will be discussed only for ZM3. The specific surface area was determined from the N2 adsorption isotherm by applying the Brunauer-Emmett- Teller equation (BRUNAUER et al., 1938). The specific surface area (SSA) of ZM3 evaluated from BET isotherm equals to 9.24 m2/g. In the literature is often found an inconsistence in determination of specific surface area of zeolites and zeolitic materials. Significantly higher values of SSA were obtained in case of cancrinite-type zeolite synthetized from fly ash (278.9 m2/g) produced by QIU and ZHENG (2009), NaA-X zeolite blend from fly ash (593.6 m2/g) (EL-NAGGAR et al., 2008) and ZFA (278.9 m2/g) (QIU and ZHENG, 2009). This inconsistency could be associated with differences in the properties of zeolites, their origin and the method of study (CHOJNACKI et al., 2004). In case of synthetized zeolites, the differences in SSA are also related with the original fired coal, type of used combustion technology and also Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 16.01.20 16:11 UTC Nova Biotechnologica et Chimica 13-1 (2014) 67 the type of used additives (e.g., for stabilization of flame). WANG et al. (2009) reported that the removal of heavy metals increased with the increase in the specific surface area of zeolite. However, some studies have reported that the increase in specific surface area does not necessarily increase the sorption capacity (OREN & KAYA, 2006; LEYVA-RAMOS et al., 2004). SSA value of ZM3 was rather low indicating that the material either has low sorption capacity or the dominating mechanism of sorption is different than physical adsorption. Maximum sorption capacity acquired rather high values indicating that the physical adsorption is not the main binding mechanism. Table 4. Predicted values of pKa constants and site concentration of functional groups on zeolitic material ZM3. Potentiometric titration of ZM3 (3 g/dm3) was performed by 0.1 M NaOH in 0.1 M NaCl at 25 °C. Calculated by ProtoFit ver. 2.1 software. Functional group pKa cAn [mmol/g] pHzpc 1 0.00 – 1.46 0.020×10-3 2 5.76 – 5.95 0.013 3 9.55 – 10.60 1.050 4 11.10 – 11.20 1.910 10.9 ∑ 2.97302 On the basis of the chemical nature of zeolites, most studies (REMENÁROVÁ et al., 2014; MERRIKHPOUR and JALALI, 2013; QIU and ZHENG, 2009; SPRYNSKYY et al., 2006; ERDEM et al., 2004) expect that the dominating mechanism of metal cations sorption is ion-exchange in which cations in the zeolite framework are replaced by metal cations present in water. Fig. 4. Potentiometric titration of zeolitic material ZM3 prepared from coal fly ash (3.0 g/dm3) expressed in the form of first derivation according to Gran. Titration performed with 0.1 M NaOH at background electrolyte 0.1 M NaCl and 25 °C. 2 3 4 5 6 7 8 9 10 11 12 0 5 10 15 20 25 Δ pH Δ pr íd av ok N aO H pH Δ pH /Δ ad di tio n of N aO H pH Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 16.01.20 16:11 UTC 68 Remenárová, L. et al. Fig. 5. Scanning electron micrographs of zeolite ZM3: (A,B) before and (C-F) after cesium sorption (6000 µmol/dm3 CsCl) at pH 6.0; magnification 150. EDX elemental mapping of Na and K on zeolite ZM3: (A, B) before and (C, D) after cesium sorption; and (E, F) elemental mapping of Na, K and Cs after cesium sorption. A A B C D E F Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 16.01.20 16:11 UTC Nova Biotechnologica et Chimica 13-1 (2014) 69 To identify the types and the amount of functional groups present on the ZM3 surface, potentiometric titration of ZM3 was performed. For calculation of pKa values of functional groups and corresponding site concentrations cAn from titration curve behavior the modeling software ProtoFit using the non-electrostatic model (NEM) was used (Table 4). It was found that a four-site model provided a good fit of titration data and the results revealed the presence of four functional groups. Their detailed chemical identification was not possible. Fig. 4 illustrates potentiometric titration curve of ZM3. Total ion exchange capacity of zeolitic material ZM3 was 2.97 mmol/g, giving the theoretical maximum sorption capacity of ZM3 for Cs ions 2970 µmol/g. In previous experiments (REMENÁROVÁ et al., 2014) we confirmed by SEM EDX analysis and elemental mapping that the main mechanism of Cd2+ ions binding onto zeolitic materials was ion-exchange. Similar results were obtained also in case of Cs sorption. Evident decrease of sodium and also potassium and calcium peaks after Cs sorption was observed on the EDX spectra indicating the key role of ion-exchange mechanism on Cs+ ions sorption by zeolitic materials (data not shown). The expected replacement of sodium and potassium ions by cesium is evident from Fig. 5A-F. It has to be pointed out that beside ion-exchange also other mechanisms such as physisorption and chemisorption partly participate on Cs binding by zeolitic materials. This statement was also confirmed by slight decrease of sorption capacity at pH 8.0 which cannot be interpreted with a pure ion-exchange mechanism because pH- dependent surface sites would be expected to retain more cesium as the pH was elevated (BAYÜLKEN et al., 2010). 4. Conclusions Zeolitic materials based on brown coal fly ash produced by power plant ENO Nováky (Slovak Republic) were successfully synthesized by hydrothermal alternation. The results showed that zeolitic materials present excellent adsorbents for the removal of cesium ions from liquid radioactive wastes. Sorption capacities of studied zeolitic materials for Cs+ ions removal are more than 287 times higher in case of ZM1 and more than 316 times higher in case of ZM3 compared with the sorption capacity of the original coal fly ash. Zeolitic materials demonstrate high sorption capacities and rapid uptake of cesium ions from aqueous solutions. 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