{Adsorption of strontium on different sodium enriched bentonites} J. Serb. Chem. Soc. 82 (4) 449–463 (2017) UDC 546.42’33+666.32:544.723:628.316.12 JSCS–4980 Original scientific paper 449 Adsorption of strontium on different sodium-enriched bentonites SANJA R. MARINOVIĆ1*, MARIJA J. AJDUKOVIĆ1, NATAŠA P. JOVIĆ-JOVIČIĆ1**, TIHANA M. MUDRINIĆ1, BOJANA M. NEDIĆ-VASILJEVIĆ2, PREDRAG T. BANKOVIĆ1 and ALEKSANDRA D. MILUTINOVIĆ-NIKOLIĆ1# 1University of Belgrade, Institute of Chemistry, Technology and Metallurgy, Njegoševa 12, Belgrade, Serbia and 2University of Belgrade, Faculty of Physical Chemistry, Studentski trg 12–16, Belgrade, Serbia (Received 10 October, revised 19 December, accepted 21 December 2016) Abstract: Bentonites from three different deposits (Wyoming, TX, USA and Bogovina, Serbia) with similar cation exchange capacities were sodium enriched and tested as adsorbents for Sr2+ in aqueous solutions. X-Ray diffract- ion analysis confirmed successful Na-exchange. The textural properties of the bentonite samples were determined using low-temperature the nitrogen physi- sorption method. Significant differences in the textural properties between the different sodium enriched bentonites were found. Adsorption was investigated with respect to adsorbent dosage, pH, contact time and the initial concentration of Sr2+. The adsorption capacity increased with pH. In the pH range from 4.0– –8.5, the amount of adsorbed Sr2+ was almost constant but 2–3 times smaller than at pH ≈11. Further experiments were performed at the unadjusted pH since extreme alkaline conditions are environmentally hostile and inapplicable in real systems. The adsorption capacity of all the investigated adsorbents toward Sr2+ was similar under the investigated conditions, regardless of sig- nificant differences in the specific surface areas. It was shown and confirmed by the Dubinin–Radushkevich model that the cation exchange mechanism was the dominant mechanism of Sr2+ adsorption. Their developed microporous structures contributed to the Sr2+ adsorption process. The adsorption kinetics obeyed the pseudo-second-order model. The isotherm data were best fitted with the Langmuir isotherm model. Keywords: Sr2+; water purification; Na-enriched clays; cation exchange cap- acity; textural properties. *,** Corresponding authors. E-mail: (*)sanja@nanosys.ihtm.bg.ac.rs; (**)natasha@nanosys.ihtm.bg.ac.rs # Serbian Chemical Society member. doi: 10.2298/JSC161010008M 450 MARINOVIĆ et al. INTRODUCTION Strontium is abundantly present in the Earth’s crust in the form of minerals: SrCO3, SrSO4 and Al2O3∙SrO∙(SiO2)6∙4H2O. In surface waters, strontium origin- ates from the weathering of rocks or from the discharge of wastewater from industries that use strontium compounds. Ionic strontium is not toxic in small concentrations, and since its concentration in water is generally low, it could be regarded as a harmless substance. The environmental impact of Sr arises from its radioactive isotopes. Besides the four stable Sr isotopes naturally present in soil, there are also artificial, radioactive isotopes 89Sr and 90Sr.1,2 These radioactive isotopes occur as waste products in nuclear power plants and in the reprocessing of nuclear fuels.3 Strontium is an alkaline earth metal that resembles calcium and barium in chemical properties. Due to its similarity with calcium, strontium tends to accu- mulate in bone tissues in the same manner. In the case of radioactive strontium, accumulation in bone tissue causes radiation damage affecting human and/or ani- mal health.4 Therefore, the presence of radioactive Sr in the environment should be resolved. The disposal of radioactive wastewater from commercial nuclear plants is one of the major problems in nuclear waste management.5 The adsorption of strontium from wastewater prior to its discharge to water bodies in the environ- ment is among the most suitable solutions for pollution prevention. The adsorption of non-radioactive strontium has been investigated on differ- ent adsorbents, such as a PAN/zeolite composite,6 expanded perlite,7 activated carbon,2 etc. Some authors investigated the adsorption of radioactive strontium on kaolinite and montmorillonite,8 illite/smectite mixed clays9 and on magnetite and a magnetite silica composite.10 The adsorptive behavior of radioactive strontium isotopes is similar to that of non-radioactive strontium. This enables the potential adsorptive removal of radio- active strontium from aqueous solutions to be determined using experimental results and models obtained for systems containing the non-radioactive isotopes. Bentonites are widely used as adsorbents for the removal of heavy metals from wastewater.11–14 They are low cost, naturally occurring, non-toxic mat- erials, abundantly present in different parts of the world, including Serbia.12,14–16 Bentonites are clays rich in smectite minerals. Smectites are 2:1 phyllosilicates – layered minerals the layers of which are composed of an octahedral [AlO3(OH)3]6– sheet sandwiched between two opposing tetrahedral [SiO4]4– sheets.17 Smectites have a total (negative) layer charge between 0.2 and 0.6 per half unit cell and include the tri-octahedral smectites: hectorite, saponite, sauco- nite, stevensite and swinefordite, and di-octahedral smectites: beidellite, mont- morillonite, nontronite and volkonskoite. Beidellite and montmorillonite differ in the origin of the net structural charge. The net structural charge of beidellite is STRONTIUM ADSORPTION ON DIFFERENT BENTONITES 451 mainly located in smectite tetrahedral sheets, while that of montmorillonite is mainly located in smectite octahedral sheets.17 The present work is focused on the removal of Sr2+ from aqueous solutions using three different sodium-enriched bentonites with similar cation exchange capacities (CEC) but with different textural properties, particularly different spe- cific surface areas. The bentonites selected for this work included standard source clays from the Clay Minerals Society18 and a previously well-characterized clay from a Serbian deposit.19,20 The adsorption was studied as a function of ads- orbent dosage, suspension pH, adsorption time and the initial concentration of Sr2+. The goal of this investigation was to confirm the applicability of the inves- tigated bentonites as Sr2+ adsorbents and to determine the most significant ads- orption mechanism for Sr2+. EXPERIMENTAL Materials Two bentonites were supplied by the Clay Minerals Society: a Ca-montmorillonite dominant clay (STx-1) originating from the Manning Formation, County of Gonzales, Texas, USA and a Na-montmorillonite dominant clay (SWy-2) originating from the Newcastle Form- ation in the County of Crook, Wyoming, USA.18 The chemical composition (mass %) of STx-1 was: SiO2, 70.1; Al2O3, 16.0; TiO2, 0.22; Fe2O3, 0.65; FeO, 0.15; MnO, 0.009; MgO, 3.69; CaO, 1.59; Na2O, 0.27; K2O, 0.078; F, 0.084; P2O5, 0.026; S, 0.04 and CO2, 0.16 and ignition loss up to 1100 °C was 6.54 mass %. The composition of SWy-2 was: SiO2, 62.9; Al2O3, 19.6; TiO2, 0.090; Fe2O3, 3.35; FeO, 0.32; MnO, 0.006; MgO, 3.05; CaO, 1.68; Na2O, 1.53; K2O, 0.53; F, 0.111; P2O5, 0.049; S, 0.05 and CO2, 1.33, and the ignition loss up to 1100 °C was 6.06 mass %.18 The third bentonite material (the dominant smectite mineral phase of which consisted of montmorillonite and beidellite in the ratio of approx. 1:9)21 was obtained from the ‟Bogovina Coal and Bentonite Mine”, Bogovina, Serbia. It had been previously thoroughly charac- terized19,22 and tested as a Pb, Cd and Ni adsorbent.23 The chemical composition (mass %) of this clay was: SiO2, 57.51; Al2O3, 17.13; Fe2O3, 7.67; MgO, 2.35; CaO, 1.81; Na2O, 0.75; K2O, 1.18 and CO2, 0.5, and the ignition loss was 11.10 mass %.19 As previously reported, the cation exchange capacities (CEC) of the bentonites obtained by the ammonium acetate method24 were 84.4, 76.4 and 63.3 mmol of monovalent cation per 100 g of clay dried at 110 °C for the Texas, Wyoming and Bogovina originating bentonites, respectively.18,20 The samples were sieved through a 74-μm sieve. Sodium enrichment of the bentonite samples was performed using the previously reported procedure20 and the obtained materials based on Wyoming, Texas and Bogovina bentonites were denoted as Na-W, Na-T and Na-B, respectively. Sodium enrichment was performed with the goal of obtaining homoionic absorb- ents with Na+ as the exchangeable cation, which could be more easily replaced with Sr2+ in the adsorption process. SrCl2∙6H2O, with 98 % purity, was obtained from Carlo Erba and used as received. 452 MARINOVIĆ et al. Characterization methods The X-ray diffraction patterns of the powders of Na-enriched bentonites were obtained using a Philips PW 1710 X-ray powder diffractometer, equipped with a Cu anode (λ = = 0.154178 nm). The point of zero charge (pHPZC) was determined using a batch equilibration technique.25 Suspensions each containing 50 mg of one of the bentonite samples in 20 cm3 of a 10-2 mol dm-3 NaCl solution were prepared by shaking for 24 h at room temperature. The initial pH values (pHi) were adjusted in the pH range from 2 to 12 by the addition of small amounts of 0.1 mol dm-3 HCl or NaOH solution. The pHi and pHf (final pH measured after 24 h of shaking) were measured using a Meterlab pH/ION Meter PHM240 pH-meter. The point of zero charge was determined from the pHi vs. pHf diagram. Nitrogen physisorption isotherms were determined on a Thermo Finnigan Sorptomatic 1990 instrument at –196 °C and the values of the textural parameters were obtained using the ADP 2005 software. The samples were out-gassed at 160 °C for 20 h. Textural parameters’ values were calculated according to the usual methods.26-28 The specific surface area, SBET, was calculated from the adsorption data in the p/p0 range from 0.05 to 0.26 according to the Brunauer, Emmett, Teller method. The total pore volume, V0.98, was calculated according to the Gurvitsch method. The Dubinin–Radushkevich method was used for the calculation of the micropore volume, while specific surface area in the micropore region was obtained using the Horvath–Kawazoe method. Pore diameter distribution curves were obtained according to the Barrett, Joyner, Halenda method. To distinguish between micropores and the external surface area, the t-plot method was applied. The Harkins and Jura relation was used as the standard reference t-curve. Adsorption experiments Batch-type adsorption experiments were conducted in aqueous solutions in a tempe- rature-controlled water bath shaker (Memmert WNE 14 and SV 1422). Adsorption was car- ried out with respect to contact time and initial Sr2+ concentration. Aliquots were withdrawn from the shaker at regular time intervals and the solution was centrifuged at 17000 rpm for 10 min (Hettich EBA-21). The Sr2+ content in the supernatant solutions was determined by inductively coupled plasma optical emission spectrometry (ICP– –OES). The strontium concentrations in the supernatants were determined using an iCAP 6500 Duo ICP (Thermo Fisher Scientific, Cambridge, UK) inductively coupled plasma optical emission spectrometer (ICP-OES) with iTEVA operational software. The strontium calib- ration solutions were prepared using multi-element plasma standard solution 4, Specpure® (Alfa Aesar, Germany). Strontium concentration was measured at the emission wavelength Sr II 215.284 nm. For each sample, the measurement was performed in triplicate. The relative standard deviation was lower than 0.5 %. Analytical process quality control was performed using a certified reference material (EPA method 200.7 LPC solution; ULTRA Scientific, USA), which indicated a strontium recovery of 99.4 %. All experiments were performed in triplicate at 25 °C, using the same mass of adsorbent (mads = 20.0 mg) and volume of solution (V = 50.0 cm3). The amount of Sr2+ adsorbed qt (mg g-1) after time t was calculated from the following mass balance relationship: 0 ads t t c c q m − = (1) STRONTIUM ADSORPTION ON DIFFERENT BENTONITES 453 where c0 and ct are the concentration (mg dm-3) of Sr2+ in solution initially and after ads- orption time t, respectively. RESULTS AND DISCUSSION X-Ray powder diffraction The XRD patterns of the powders of the Na-enriched bentonites are pre- sented in Fig. 1. Fig. 1. The XRD patterns of the powders of the Na-enriched bento- nites (S – smectite, Q – quartz, F – feldspar and C – calcite). The obtained diffractograms confirmed the phase composition of bentonites with dominant smectite phase (either montmorillonite or a montmorillonite/bei- delite mixture) with different associated minerals in accordance with the different origins of the bentonites. As expected, Na-W contained the highest amount of feldspar among the investigated bentonites, while Na-B had the highest amount of quartz.22,29 Only Na-B contained a detectable amount of calcite as an imp- urity. The XRD pattern revealed that Na-exchange had been successfully per- formed since the position of the 001 smectite peak in all the investigated samples corresponded to the Na-forms of smectite with d001 at around 1.2–1.3 nm.30 The raw bentonites from Texas and Bogovina were dominantly smectites of the Ca- -type with characteristic basal spacing of d001 = 1.5 nm. Textural properties Low temperature N2 physisorption measurements resulted in the isotherms presented in Fig. 2. All the presented isotherms were reversible at lower equilibrium pressures, belonging to the type II isotherms according to IUPAC nomenclature. Type II isotherms are characteristic for materials consisting of aggregated planar particles that form slit shape pores. Such a result is in accordance with the literature.31 At equilibrium pressures above p/p0 = 0.4, a hysteresis loop of the H3 type appeared, 454 MARINOVIĆ et al. which indicates multilayer nitrogen adsorption and capillary condensation within the smectite mesopores of all the smectites.22,32 The adsorption isotherms of all the materials exhibited an inflection point at about p/p0 = 0.2. This indicates that no overlapping between monolayer and multilayer adsorption occurred, i.e., the formation of the second adsorption layer began only after monolayer adsorption had been completed. Selected textural data are presented in Table I. Fig. 2. Nitrogen adsorption–desorption isotherms of Na-T, Na-W and Na-B. TABLE I. Selected textural properties; SBET – specific surface area (Brunauer, Emmett, Teller – three parameter plot); V0.98 – total pore volume (Gurvitch); Vmes,BJH – mesopore volume (Barrett, Joyner, Hallenda); dmed – median pore diameter; dmax – the most abundant pore diameter; St – specific surface area (t-plot); and Smic – micropore specific surface area Sample SBET m2 g-1 V0.98 cm3 g-1 Vmes,BJH cm3 g-1 dmax nm dmed nm St m2 g-1 Smic=SBET–St m2 g-1 Na-T 113 0.2000 0.2101 3.83 9.17 83 30 Na-W 52 0.0759 0.0871 4.08 4.26 31 21 Na-B 98 0.0853 0.0788 3.93 3.99 43 55 All the Na-enriched materials exhibited meso- and microporosity developed to different extents. While the mesoporous surface area increased in the order Na-W < Na-B < Na-T, the increase of the surface area of micropores followed the sequence Na-W < Na-T < Na-B. The microporous surface area should be considered as more relevant in the investigated cation exchange process, since it occurs in the interlamellar smectite region. Therefore, if Sr2+ adsorption on the STRONTIUM ADSORPTION ON DIFFERENT BENTONITES 455 Na-enriched clays is dominantly dependent on the surface area, the efficiency of Sr2+ uptake is expected to follow the sequence Na-B > Na-T > Na-W. The effect of adsorbent dosage on the adsorption of Sr2+ In order for the influence of the adsorbent dosage on the adsorption of Sr2+ ions to be investigated, the mass of all investigated Na-enriched bentonites was varied from 10 to 100 mg. All the experiments were performed for three hours at 25 °C with an initial Sr2+ concentration of 50 mg dm–3. The influence of adsorbent dosage on the amount of Sr2+ ions adsorbed on the investigated Na-enriched bentonites after 180 min at 25 °C is presented in Fig. 3. Fig. 3. The influence of adsorbent dosage on the amount of adsorbed Sr2+. As expected, the quantity of adsorbed Sr2+ per unit mass of adsorbent dec- reased with increasing adsorbent dosage. It decreased steeply from a dosage of 10 mg to one of 20 mg, after which a plateau was attained. For dosages from 20 to 50 mg, the adsorption per unit mass did not change significantly. After the plateau, the drop in the adsorption per unit mass was more pronounced. For a dosage of 100 mg, almost complete Sr2+ removal was obtained. A dosage of 20 mg was chosen because it is in the plateau of the q–m diagram, while it is low enough to meet economic requirements. The effect of pH on the adsorption of Sr2+ The effect of the initial pH on the adsorption of Sr2+ was investigated within the pH range 2–11 and the results are presented in Fig. 4, together with the pH behavior of all the Na-enriched bentonites. The diagram shows the initial pH of the suspensions of Na-enriched ben- tonites in 0.01 M NaCl (pHi) vs. pH after 24 h of shaking (pHf). A plateau 456 MARINOVIĆ et al. between pH values of 4 and 8.5 was observed and the point of zero charge (pHPZC) was estimated to be 6.5 for Na-T, 6.3 for Na-W and 8.2 for Na-B. Fig. 4. The effect of pH on the adsorption of Sr2+ and pHi vs. pHf diagram. Lower than pHPZC, the surface charge of the samples is positive, while above these values, the surface charge is negative. On the other hand, strontium is in the cationic form within the entire investigated pH range. The quantity of adsorbed Sr2+ was very low in an extremely acidic environ- ment, which could be explained by the repulsion between positive surface charge of the adsorbents and Sr2+. In the 4–8.5 pH range, the amount of adsorbed Sr2+ was higher and almost constant due to a constant ratio between the positive and negative surface charges of the adsorbents. Further increases of the pH value (>8.5) led to very high amounts of adsorbed Sr2+ as a consequence of the electro- static interaction of the negative surface of adsorbents and the Sr2+. Further experiments were realized at an unadjusted pH (pH 6.8), which lies on the plateau in Fig. 4, where the adsorption efficiency is almost constant in the pH interval from 4 to 8.5.33 Unadjusted pH was also chosen because adjusting the pH would lead to an additional operation, which increases the procedure cost. Moreover, extreme pH values could cause ecological problems that should be avoided. The adsorption at unadjusted pH was satisfactory (around 30 % removal of Sr2+ from water) under the given experimental conditions.34 STRONTIUM ADSORPTION ON DIFFERENT BENTONITES 457 The effect of contact time on the adsorption of Sr2+ In order to determine the equilibrium time for the maximum uptake of Sr2+, the adsorption of Sr2+ was monitored with respect to contact time. The initial Sr2+ concentration was 50 mg dm–3. The effect of contact time on the amount of Sr2+ adsorbed on the bentonites is presented in Fig. 5. Fig. 5. The effect of contact time on the adsorption of Sr2+ on the three studied bentonites. For all adsorbents, adsorption rate was initially high and then, it gradually reached a plateau. All adsorbents exhibited similar t1/2 values (t1/2 – the time at which the amount of adsorbed Sr2+ is equal to half of the total amount adsorbed at equilibrium). The t1/2 values were 5.8, 6.9 and 5.5 min for Na-T, Na-W and Na-B, respectively. Both parameters that describe adsorbent quality, i.e., the amount of Sr2+ adsorbed per unit mass and t1/2, were similar for all the sodium- -enriched bentonites. The obtained results confirmed that the degree of the dev- elopment of a porous structure had no influence on the adsorption of Sr2+. Na-W, the material with the least developed micro and mesopore surface, even exhibited a slightly higher adsorption of Sr2+. This indicates that the ion exchange mech- anism was the dominant adsorption mechanism. The pseudo-first order and pseudo-second order kinetics models were tested for the adsorption of Sr2+ on the Na-enriched bentonites. The parameters calculated for the pseudo-first and pseudo-second order kin- etics models35,36 are presented in Table II. 458 MARINOVIĆ et al. TABLE II. Pseudo-first-order-kinetics and pseudo-second-order-kinetics for the adsorption of Sr2+ ions on Na-T, Na-W and Na-B; k1 is the pseudo-first order rate constant; k2 is the pseudo-second order rate constant; qeexp is the experimentally obtained value for the equi- librium adsorption capacity; qecalc is calculated value for the equilibrium adsorption cap- acity and R2 is the square of the correlation coefficients Sample Na-T Na-W Na-B qeexp / mg g-1 38.98 41.78 35.68 Pseudo-first order kinetics model qecalc / mg g-1 3.121 2.239 1.374 k1 / min-1 0.027 0.007 0.029 R2 0.9978 0.6747 0.6759 Pseudo-second order kinetics model qecalc / mg g-1 37.22 42.94 35.62 k2×102 / g mg-1 min-1 1.092 0.447 7.269 R2 0.9998 0.9998 0.9999 The square of the correlation coefficients (R2) for the pseudo-first order kinetics model were relatively low for Na-W and Na-B, indicating poor correl- ation of the data with the model. On the other hand, the R2 values for the pseudo- -second order kinetics model were > 0.999 for all bentonites. Although Na-T exhibited a rather high R2 value for the pseudo-first order kinetics, the values of the equilibrium adsorption capacity qecalc calculated from the equation for the pseudo-first order kinetics for all samples exhibited high discrepancy from the experimentally obtained adsorption capacity, qeexp. On the other hand, for the pseudo-second order kinetics model, qecalc and qeexp were in good agreement, confirming that the adsorption of Sr2+ on the studied bentonites obeyed pseudo- -second order kinetics. The pseudo-second-order kinetics model for the adsorption of Sr2+ on the investigated bentonites is shown in Fig. 6. The effect of the initial concentration of Sr2+ on their adsorption The effect of the initial concentration of Sr2+ on the adsorption capacity was also investigated for four different initial concentrations of Sr2+ in the concentra- tion range from 25 to 100 mg dm–3. The obtained data were fitted using the Langmuir, Freundlich and Dubinin–Radushkevich isotherm models.37–39 The value obtained for the adsorption energy could be used to predict the type of adsorption.40 If E is smaller than 8 kJ mol–1, the adsorption could be described as physisorption, while E in the range from 8−16 kJ mol–1 indicates chemisorption. Average values of repeated experiments for each adsorbent were fitted using the Langmuir and Freundlich models and the results are presented in Figs. 7a and b, respectively. The calculated parameters for the investigated models are pre- sented in Table III. STRONTIUM ADSORPTION ON DIFFERENT BENTONITES 459 Fig. 6. Plots of the pseudo-second- order kinetics model for the ads- orption of Sr2+ on the studied ben- tonites. Fig. 7. Adsorption isotherms: a) Langmuir and b) Freundlich isotherm model. The Langmuir model better fitted the experimental data, having R2 close to unity. It is appropriate to describe the adsorption of Sr2+ on all the studied ben- tonites. As previously stated, the ion exchange mechanism could be regarded as the dominant adsorption mechanism. In order to verify this statement, the qmax values calculated according to the Langmuir model were compared with the corresponding cation exchange capacity (CEC) values.18,20 Since strontium is divalent and the CEC values are given for monovalent cations, the qmax values should be compared with half of the CEC values. The corresponding values were as follows: for Na-T, qmax was 0.526 mmol g–1 and half the CEC value was 0.422 mmol g–1; for Na-W, qmax was 0.543 mmol g–1 and half of the CEC value was 0.382 mmol g–1 and for Na-B, qmax was 0.521 mmol g–1 and half of the CEC value was 0.316 mmol g–1. The obtained values of qmax exceeded the 460 MARINOVIĆ et al. amount of available exchangeable cations, indicating that although the ion exchange mechanism was dominant, there are other contributions to the adsorption process. Bearing in mind that the increase in the surface area of the micropores followed the sequence Na-W < Na-T < Na-B, it could be assumed that developed micro- porous structure contributed somewhat to the Sr2+ adsorption process. TABLE III. Parameters calculated for the Langmuir, Freundlich and Dubinin–Radushkevich isotherm models; qmax is the maximum adsorption capacity; KL is the Langmuir adsorption constant; KF and n are the Freundlich adsorption constants characteristic for the system; KDR is the adsorption energy constant and E is the energy of adsorption Sample Na-T Na-W Na-B Langmuir isotherm qmax / mg g-1 46.1 47.6 45.7 KL / dm3 mg-1 7.31 5.21 9.49 R2 0.968 0.998 0.973 Freundlich isotherm N 8.62 9.558 5.78 KF / dm3 mg-1 24.9 26.6 19.0 R2 0.689 0.901 0.880 Dubinin–Radushkevich KDR / mol2 kJ-2 6.81·10-3 6.99·10-3 7.43·10-3 E / kJ mol-1 8.57 8.46 8.20 R2 0.995 0.956 0.935 The squared correlation coefficients (R2) showed that the experimental data were in good agreement with the Dubinin–Radushkevich isotherm model (Table III). The calculated E values were similar for all investigated adsorbents and slightly higher than 8 kJ mol–1, indicating that the type of adsorption was chemi- sorption. According to literature data, the obtained E values were in the range of adsorption energy (8–16 KJ mol–1) characteristic for systems where ion exchange is the dominant mechanism.41 Since the experiments related to Sr2+ adsorption on various adsorbents reported in literature were performed under different experimental conditions, direct comparison of the obtained data is not possible. However, a few des- criptive data will be given. Chegrouche et. al.2 reported a maximum adsorption capacity of 0.507 mmol g–1 (44.4 mg g–1) on activated carbon, while Yusan and Erenturk6 obtained a maximum adsorption capacity of 0.011 mg g–1 on a PAN/ /zeolite composite adsorbent. Ahmadpour et al.5 investigated adsorption of Sr2+ on treated almond green hull and obtained a maximum adsorption capacity of 116.3 mg g–1. Considering the literature data, the adsorption of Sr2+ on the inves- tigated Na-enriched bentonites showed that they could be considered as possible adsorbents for the removal of Sr2+ from wastewater. STRONTIUM ADSORPTION ON DIFFERENT BENTONITES 461 CONCLUSIONS Three different bentonites were sodium-enriched in order to be tested as adsorbents for Sr2+. The characterization of these bentonites included XRD and detailed textural analysis. The XRD analysis revealed that Na-exchange had been successfully performed. The mesoporous surface area increased in the order Na-W < Na-B < Na-T, while the increase in the surface area of the micropores followed the sequence Na-W < Na-T < Na-B. The surface areas of the inves- tigated bentonites differed significantly. The adsorption of Sr2+ ions from aque- ous solutions was performed on all three Na-enriched bentonites. Adsorption was realized with respect to the adsorbent dosage, pH, contact time and initial con- centration of Sr2+. The adsorption capacity increased with pH. In the pH range from 4–8.5, the amount of adsorbed Sr2+ was almost constant but 2–3 times smaller than maximal obtained at pH ≈11. It is concluded that the adsorption was mainly governed by the ion exchange mechanism, since the bentonites with simi- lar CEC values and different specific surface areas had similar adsorption cap- acities. Bearing in mind that the increase of the surface area of micropores followed the sequence Na-W < Na-T < Na-B, it could be assumed that the dev- eloped microporous structure contributed somewhat to the Sr2+ adsorption process. The adsorption dynamics was described well by the pseudo-second-order kinetics model. The Langmuir isotherm model best fitted the experimental data. The Dubinin–Radushkevich isotherm was used to provide a better understanding of the adsorption mechanism and to distinguish physisorption from chemisorp- tion. The value obtained for the adsorption energy was slightly higher than 8 kJ mol–1. This indicated chemisorption and partially ion exchange as the dominant mechanisms. All the investigated adsorbents could be used for the removal of radioactive strontium, considering the fact that it has similar adsorptive behavior to its non-radioactive counterpart. Moreover, according to the findings in this study, bentonite from unexploited deposits could be used as a Sr2+ adsorbent. Knowing the cation exchange capacity of any clay, the prediction of its adsorp- tion potential is possible. Acknowledgement. This work was supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia (Project III 45001). И З В О Д АДСОРПЦИЈА СТРОНЦИЈУМА НА РАЗЛИЧИТИМ НАТРИЈУМОМ ИЗМЕЊЕНИМ БЕНТОНИТИМА САЊА Р. МАРИНОВИЋ1, МАРИЈА Ј. АЈДУКОВИЋ1, НАТАША П. ЈОВИЋ-ЈОВИЧИЋ1 , ПРЕДРАГ Т. БАНКОВИЋ1, ТИХАНА М. МУДРИНИЋ1, БОЈАНА М. НЕДИЋ ВАСИЉЕВИЋ2 и АЛЕКСАНДРА Д. МИЛУТИНОВИЋ-НИКОЛИЋ1 1Универзитет у Београду – Институт за хемију, технологију и металургију, Центар за катализу и хемијско инжењерство, Његошева 12, Београд и 2Универзитет у Београду – Факултет за физичку хемију, Студентски трг 12–16, Београд Бентонити из три различита налазишта (Вајоминг и Тексас, САД, и Боговина) са сличним капацитетима катјонске измене натријумски су измењени и испитани као 462 MARINOVIĆ et al. адсорбенси за уклањање Sr2+ из водених раствора. Успешност натријумске измене потврђена је рендгено-структурном анализом. Текстурална својства бентонитних узо- рака су одређена коришћењем методе нискотемпературне физисорпције азота. Добијене су значајне разлике у текстуралним својствима натријумски измењених бентонита раз- личитог порекла. Испитиван је утицај масе адсорбента, pH, времена контакта и почетне концентрације Sr2+ на адсорпцију Sr2+. Aдсорпциони капацитет расте са порастом pH. У опсегу pH од 4 до 8,5 количина адсорбованих Sr2+ је скоро константна, али 2 до 3 пута мања од максималне остварене при pH ≈11. У свим наредним експериментима кориш- ћен је неподешени pH раствора, јер је коришћење екстремно базних условa штетно за животну средину и сходно томе неприменљиво у реалним системима. Утврђено је да је адсорпциони капацитет свих адсорбенаса према Sr2+ сличан, без обзира на значајне разлике у специфичним површинама. Показано је да је механизам катјонске измене доминантан механизам при адсорпцији Sr2+ на натријумски измењеним бентонитима, што је потврђено и моделом Dubinin–Radushkevich. Развијена микропорозна структура такође у извесној мери доприноси процесу адсорпције Sr2+. Адсорпциона динамика прати кинетички модел псеудо-другог реда за све адсорбенсе, док Лангмирова изотерма најбоље описује адсорпцију у испитаним системима. (Примљено 10. октобра, ревидирано 19. децембра, прихваћено 21. децембра 2016) REFERENCES 1. J. R. Dojlido, G. A. Best, Chemistry of water and water pollution, Ellis Horwood, Chichester, 1993, p. 75 2. S. Chegrouche, A. Mellah, M. Barkat, Desalination 235 (2009) 306 3. S. Yusan, S. Erenturk, World J. Nucl. Sci. Technol. 1 (2011) 6 4. A. Horr, Chemistry of strontium in natural water, U.S. Government. Print. Off, Washington, DC, 1962, p. 4 5. A. Ahmadpour, M. Zabihi, M. Tahmasbi, T. Rohani Bastami, J. Hazard. Mater. 182 (2010) 552 6. S. Yusan, S. Erenturk, World J. Nucl. Sci. Technol. 1 (2011) 6 7. M. Torab-Mostaedi, A. Ghaemi, H. Ghassabzadeh, M. Ghannadi-Maragheh, Can. J. Chem. Eng. 89 (2011) 1247 8. H. N. Erten, S. Aksoyoglu, S. Hatipoglu, H. Göktürk, Radiochim. Acta 44/45 (1988) 147 9. T. Missana, M. Garcia-Gutierrez, U. Alonso, Phys. Chem. Earth 33 (2008) S156 10. A. Ebner, J. Ritter, J. Navratil, Ind. Eng. Chem. Res. 40 (2001) 1615 11. M. K. Uddin, Chem. Eng. J. 308 (2017) 438 12. Y. Bentahar, C. Hurel, K. Draoui, S. Khairoun, Appl. Clay Sci. 119 (2016) 385 13. M. Cruz-Guzman, R. Celis, M. C. Hermosin, W. C. Koskinen, E. A. Nater, J. Cornejo, Soil Sci. Soc. Am. J. 70 (2006) 215 14. R. Yua, S. Wanga, D. Wangb, J. Keb, X. Xinga, N. Kumadac, N. Kinomurac, Catal. Today 139 (2008) 135 15. L. Aloui, F. Ayari, A. Ben Othman, M. Trabelsi-Ayadi, Int. J. Eng. Appl. Sci. 2 (2015) 33 16. F. Ayari, E. Srasra, M. Trabelsi-Ayadi, Desalination 185 (2005) 391 17. F. Bergaya, G. Lagaly, Developments in Clay Science, in Handbook of Clay Science, Vol. 5A, 2nd ed., Elsevier, Amsterdam, 2013 18. Clay Minerals Society, Source Clay Physical/Chemical Data, http://www.clays.org/ /Sourceclays.html (August 1, 2016) 19. Z. Vuković, A. Milutinović-Nikolić, Lj. Rožić, A. Rosić, Z. Nedić, D. Jovanović, Clays Clay Miner. 54 (2006) 697 STRONTIUM ADSORPTION ON DIFFERENT BENTONITES 463 20. N. Jović-Jovičić, A. Milutinović-Nikolić, M. Žunić, Z. Mojović, P. Banković, I. Gržetić, D. Jovanović, J. Contam. Hydrol. 150 (2013) 1 21. T. Novaković, Lj. Rožić, S. Petrović, A. Rosić, Chem. Eng. J. 137 (2008) 436 22. Z. Vuković, A. Milutinović-Nikolić, J. Krstić, A. Abu-Rabi, T. Novaković, D. Jovanović, Mater. Sci. Forum 494 (2005) 339 23. N. Jović-Jovičić, A. Milutinović-Nikolić, M. Žunić, Z. Mojović, P. Banković, B. Dojči- nović, A. Ivanović-Šašić, D. Jovanović, J. Serb. Chem. Soc. 79 (2014) 253 24. US Environmental Protection Agency, Method 9080 - Cation exchange capacity of soils (ammonium acetate), USEPA, Washington, DC, 1986, https://www.epa.gov/sites/pro- duction/files/2015-12/documents/9080.pdf 25. Lj. Čerović, S. K. Milonjić, M. Todorović, M. Trtanj, Y. Pogozhev, Y. Blagoveschenskii, E. A. Levashov, Colloids Surfaces, A 297 (2007) 1 26. S. H. Gregg, K. S. Sing, Adsorption, Surface Area and Porosity, Academic Press, New York, 1982, p. 41 27. F. Rouquerol, J. Rouquerol, K. Sing, Adsorption by powders and porous solids, Acad- emic Press, London, 1999, p. 440 28. P. A. Webb, C. Orr, Analytical methods in fine particle technology, Micromeritics Instrument Corporation, Norcross, GA, 1997, p. 53 29. S. J. Chipera, D. L. Bish, Clay Clay Miner. 49 (2001) 398 30. T. Hayakawa, M. Minase, K. I. Ujita, M. Ogawa , Clay Clay Miner. 64 (2016) 275 31. P. Cañizares, J. L. Valverde, M. R. Sun Kou, C. B. Molina, Micropor. Mesopor. Mat. 29 (1999) 267 32. G. Leofanti, M. Padovan, G. Tozzola, B. Venturelli, Catal. Today 41 (1998) 207 33. N. Jović-Jovičić, A. Milutinović-Nikolić, M. Žunić, Z. Mojović, P. Bankovića, I. Gržetić, D. Jovanović, J. Contam. Hydrol. 150 (2013) 1 34. Z. Sandić, A. Nastasović, N. Jović-Jovičić, A. Milutinović-Nikolić, D. Jovanović, Appl. Polym. Sci. 121 (2011) 234 35. S. Lagergren, K. Vet. Akad. Handl. 24 (1898) 1 36. Y. S. Ho, G. McKay, Chem. Eng. J. 70 (2) (1998) 115 37. I. J. Langmuir, J. Am. Chem. Soc. 40 (1918) 1361 38. H. M. F. Freundlich, J. Phys. Chem. 57 (1906) 385 39. M. M. Dubinin, L. V. Radushkevich, Chem Zent. 1 (1947) 875. 40. M. Horsfall, A. I. Spiff, A. A. Abia, Bull. Kor. Chem. Soc. 25 (2004) 969 41. C. Y. Abasi, A. A. Abia, J. C. Igwe, Envron. Res. 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