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Sorption behavior of U(VI) onto Chinese bentonite: Effect of pH, ionic strength, temperature and humic acid
Journal of Molecular Liquids 188 (2013) 178–185
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Sorption behavior of U(VI) onto Chinese bentonite: Effect of pH, ionic strength, temperature and humic acid Jiang Xiao, Yuantao Chen ⁎, Wenhua Zhao, Jiangbo Xu Department of Chemistry, Qinghai Normal University, 810008, Xining, Qinghai, PR China
a r t i c l e
i n f o
Article history: Received 12 June 2013 Received in revised form 6 October 2013 Accepted 9 October 2013 Available online 22 October 2013 Keywords: U(VI) Sorption Bentonite pH Thermodynamic data Mechanism
a b s t r a c t Herein, a local bentonite from Huangshan county (Anhui province, China) was purified and the purge bentonite sample was characterized by using FTIR and XRD to determine its chemical constituents and microstructures. The sample was chosen as an adsorbent to remove uranium ions from aqueous solutions as a function of various environmental parameters such as contact time, pH, ionic strength, foreign ions, humic acid and temperature under ambient conditions. The results indicated that the sorption of U(VI) on the bentonite was strongly dependent on pH and ionic strength. At pH b 6.5, the sorption of U(VI) on the bentonite increased with increasing pH, whereas the sorption of U(VI) on bentonite decreased with increasing pH at pH N 6.5. The presence of humic acid (HA) enhanced the sorption of U(VI) on bentonite obviously at low pH while it reduced the U(VI) sorption on bentonite at high pH values. The enhancement of U(VI) sorption on HA-bentonite hybrids at low pH was attributed to the strong surface complexation of surface adsorbed HA with U(VI) on bentonite surface, whereas the decrease of U(VI) sorption on bentonite at high pH was attributed to the formation of free HA-U complexes in solution. The Langmuir, Freundlich and D–R models were applied to simulate the sorption isotherms of U(VI) at three different temperatures of 298, 318 and 338 K, respectively. The thermodynamic parameters (i.e., ΔH0, ΔS0 and ΔG0) calculated from the temperature dependent sorption isotherms indicated that the sorption process of U(VI) on bentonite was an endothermic and spontaneous process. At low pH, the sorption of U(VI) was mainly dominated by outer-sphere surface complexation and ion exchange with H+/Na+ on bentonite surfaces, whereas inner-sphere surface complexation was the main sorption mechanism at high pH values. Based on the experimental results, bentonite is a very suitable material for the preconcentration and solidification of U(VI) ions from large volumes of aqueous solutions in U(VI) pollution cleanup. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The physicochemical behavior of long-lived lanthanides and actinides has aroused great interest in terms of the radioactive waste disposal [1–4]. Uranium (U(VI)), a radioactive and hazardous heavy metal, originates from nuclear industry and other anthropogenic activities such as lignite burning in power stations, ore processing, and the use of fertilizers [5,6]. Uranium released into the environment is predominantly in the hexavalent form as mobile, aqueous uranyl ion (U(VI)) under normal environmental conditions. Owing to the long half-life (t1/2(235U)= 7.04 × 108 a; t1/2(238U)= 4.47 × 109 a), the release of uranium into the environment can cause significant geochemical and public health problems. Thereby, it is crucial to eliminate U(VI) from aqueous solutions before it is released into the environment. Exposure to uranium can result in serious biochemical and radioactive harms to biological organization such as toxic hepatitis, skin corrosion, kidney damage, histopathological system damage and even cancers. In view of this point, strict environmental protection legislation on the ⁎ Corresponding author. Tel.: +86 971 6303374. E-mail address: [email protected] (Y. Chen). 0167-7322/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molliq.2013.10.008
disposal of hazardous and radioactive metal ions causes increasing demands for the removal of those ions with cost-effective techniques. In the last decade, various technologies such as metal ion extraction [7], ion exchange [8], chemical precipitation [9,10], filtration [11], surface complexation [12,13], thermal treatment [14], and sorption [15–18] have been developed for the removal of radionuclides and heavy metal ions from aqueous solutions. Among these methods, sorption is an effective technique with the advantages of high treatment efficiency and easy operation. The sorption of U(VI) on different minerals and oxides has been studied extensively [18,19]. The results showed that the sorption of U(VI) was mainly dominated by outer-sphere surface complexation at low pH and by inner-sphere surface complexation or precipitation at high pH values. Bentonite is a 2:1 type of clay, and the main clay component is montmorillonite, which is composed of units of two silica tetrahedral sheets with a central alumina octahedral sheet. All tips of tetrahedrons point in the same direction and toward the center unit. Compared with other clay minerals, bentonite has excellent sorption sites and functional groups which are on the outer surface and edges for the formation of complexes with radionuclides. Bentonite has a series of outstanding physicochemical properties such as large specific area,
J. Xiao et al. / Journal of Molecular Liquids 188 (2013) 178–185
high cation exchange capacity, strong adsorptive affinity for organic and inorganic pollutants, low permeability, low cost, accessibility and ubiquitous presence in most soils. Based on these advantages, bentonite is considered one of the most promising candidates as geochemical barriers in nuclear waste repositories and hazardous chemical landfills [20–22]. In China, the bentonite from Huangshan county has been selected as a potential candidate as backfill material for nuclear waste disposal management. However, to the best of our knowledge, no available study was focused on the sorption behaviors of uranium on this kind of bentonite. In this study, a local bentonite from Huangshan county (Anhui province, China) was used as a sorbent for the removal of uranium from wastewater. The purposes of this work are: (1) to characterize the purified bentonite sample by using X-ray diffraction (XRD) and Fourier Transformed Infrared spectroscopy (FTIR) in detail; (2) to study the effect of different parameters, such as contact time, pH, ionic strength, foreign ions, humic acid (HA) and temperature on U(VI) sorption by using batch technique; (3) to calculate the thermodynamic data (ΔG0, ΔS0 and ΔH0) from the temperaturedependent sorption isotherms; and (4) to discuss the sorption mechanism of U(VI) on bentonite. 2. Experimental section 2.1. Materials The bentonite sample was obtained from Huangshan county (Anhui province, China). The sample was treated with 5% hydrochloric acid for 24 h in order to improve the specific surface area, and washed with double distilled water until no chloride was detected in supernatant with 0.01 mol/L AgNO3. The resulted bentonite sample was dried at 105 °C for 2 h to eliminate the free water. Finally, the sample was milled and passed through a 200-mesh screen for further usage in the following experiments. U(VI) stock solution was prepared by dissolving uranylnitrate hexahydrate (UO2(NO3)2 6H2O) in Milli-Q water. The stock solution was kept at pH 3 and used in the following sorption experiments. HA was extracted from the soil samples of Hua-jia county of Gansu province (China) and characterized in detail [23,24]. The main elements of HA are: H (3.53%), C (60.44%), N (4.22%), O (31.31%) and S (0.50%). All chemicals used in the experiments were purchased as analytical purity and used without any further purification. Milli-Q water was used in the experiments.
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for 24 h, the solid and liquid phases were separated by centrifugation at 9000 rpm for 30 min at the same temperature as in the sorption experiments. It was necessary to emphasize that the sorption of U(VI) on the tube wall was negligible according to the test of U(VI) sorption in the absence of bentonite. The concentration of U(VI) was analyzed by chlorophosphonazo III spectrophotometric method at the wavelength of 669 nm. The amount of U(VI) adsorbed on bentonite was calculated from the difference between the initial concentration and the equilibrium one. The sorption percentage (Sorption% = (C0 − Ce)/C0 × 100%) and distribution coefficient (Kd = (C0 − Ce)/Ce · V/m) of U(VI) on bentonite were calculated from the initial concentration of U(VI) (C0), the equilibrium concentration of U(VI) (Ce) in supernatant, the mass of bentonite (m) and the volume of the suspension (V). 3. Results and discussion 3.1. Characterization of bentonite Fig. 1 shows the FTIR spectrum of the purified bentonite sample. The broad bands at 3436 and 3625 cm−1 are due to the H\OH vibration of the free water molecules adsorbed on the solid surface and the O\H stretching vibration of the silanol (Si\OH) groups from the solid. The peaks at 2862 and 2933 cm−1 are assigned to the aliphatic C\H stretching vibration [23]. The spectral band at 1640 cm reflects the bending of H\OH bond of water molecules, which is retained in the matrix. The strong band at 1044 cm−1 represents the Si\O\Si groups of the tetrahedral sheet. The band at 798 cm−1 confirms the presence of quartz in the sample. The spectral band at 930 cm−1 reflects the stretching vibration of Al\O\(OH)\Al. The band at 624 cm−1 is assigned to the out-of-plane vibrations of coupled Al\O and Si\O. The bands at 468 and 519 cm−1 are corresponded to Si\O\Si and Al\O\Si bending vibrations [24]. The XRD pattern of the bentonite sample is shown in Fig. 2. The diffraction peaks of the planes at 2θ = 6.04°, 19.86° and 35.15° (marked by M) are the reflections indicative of 2:1 swelling clays. The other peaks are the impurities corresponded to quartz (marked by Q) and Cal–Fe(Ca)CO3 (marked by C). Crystallographic parameters are evaluated by measuring (001) and (060) peaks. The bentonite sample exhibits a diffraction peak of the (001) plane at 2θ = 6.039°, and its basal spacing is calculated to be 14.6 Å. The (060) reflection at 2θ = 62.00° implies the dioctahedral structure of bentonite. 3.2. Time-dependent sorption
2.2. Characterization Infrared spectrum of the purified bentonite sample ranging from 400 to 4000 cm−1 was recorded using FTIR spectrophotometer (Bruker EQUINOX55 Nexus) in KBr pellets. The spectral resolution was set to 1 cm−1, and 150 scans were collected for each spectrum. The XRD pattern was obtained from a D/Max-rB equipped with a rotation anode using Cu Kα radiation (λ = 0.15406 nm). The XRD device was operated at 40 kV and 80 mA. The measurements were carried out in the range of 5°–65°.
The contact time is an important parameter that can reflect the sorption kinetics of an adsorbent for a certain adsorbate concentration.
2.3. Experimental procedures The sorption of U(VI) on bentonite was investigated by using batch technique in polyethylene centrifuge tubes under ambient conditions. The stock suspension of bentonite and NaClO4 was pre-equilibrated for 24 h, then U(VI) stock solution and HA were added in bentonite suspension to achieve the desired concentrations of different components. The volume of the solution was 10 mL and the amount of the solid was 1.2 g/L for each experimental data. The pH values of the solution were adjusted by adding negligible volumes of 0.1 or 0.01 mol/L HClO4 or NaOH solutions. After the suspensions were stirred
Fig. 1. FTIR spectrum of purified bentonite sample.
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J. Xiao et al. / Journal of Molecular Liquids 188 (2013) 178–185
3.3. Influence of solid content Fig. 4 shows the sorption of U(VI) on bentonite as a function of solid content. The distribution of the coefficient, Kd, values as a function of the bentonite content is also plotted in Fig. 4. One can see that the sorption percentage of U(VI) increases from ~35% at m/V = 0.2 mg/L to ~51% at m/V = 1.2 mg/L. It is well known that the amount of functional groups at bentonite surfaces increases with increasing solid content. Thereby, more surface sites are available for the binding of U(VI) at higher solid contents. As can be seen from Fig. 4, the Kd values are constant with increasing solid content. This phenomenon is consistent with the physicochemical properties of Kd values, i.e., the Kd value is independent of solid content and solution concentration when both of them are low. The result resembles the sorption of U(VI) on carbon nanotubes [6]. Fig. 2. XRD pattern of purified bentonite sample.
3.4. Influence of pH and ionic strength
It was necessary to emphasize that the centrifugation time was also considered in the time-dependent sorption experiments for the reason that the sorption of U(VI) still occurred at low centrifugation rate. The sorption percentage of U(VI) on bentonite as a function of contact time is shown in Fig. 3. It is observed that the sorption percentage increases rapidly in the first contact time of 4 h, and then maintains high level with increasing contact time. The fast sorption kinetics suggests that the U(VI) sorption on bentonite is mainly dominated by chemisorption rather than physicosorption [25]. The result also suggests that 4 h is enough to achieve the sorption equilibrium of U(VI) on bentonite. In the following experiments, the shaking time is fixed at 24 h to guarantee that the sorption reaction can achieve complete equilibrium. A preudo-second-order rate equation is used to simulate the kinetic of sorption [26]: t 1 1 ¼ þ t qt 2k0 qe 2 qe
ð1Þ
where k′ (g/(mg·h)) is the pseudo-second-order rate constant of sorption; qt (mg/g) is the amount of U(VI) adsorbed on bentonite at time t (h), and qe (mg/g) is the equilibrium sorption capacity. The plot of t/qt versus t is shown in the inserted figure of Fig. 3. The values of k′ and qe calculated from the intercept and slope are 0.84 g/(mg·h) and 10.98 mg/g, respectively. The correlation coefficient of the pseudosecond-order rate equation for the linear plot is 0.999, which suggests that the kinetic sorption can be described by the pseudo-second-order rate equation well [27].
Fig. 3. Effect of contact time on U(VI) sorption onto bentonite. pH = 5.6 ± 0.1, T = 298 K, m/V = 1.2 g/L, C[U(VI)]initial = 8.00 × 10−5 mol/L, I = 0.01 mol/L NaClO4.
Solution pH has a significant influence on the migration and transformation behaviors of environmental contaminants at solid/water interfaces. In view of this point, the pH-dependent sorption of U(VI) on bentonite was investigated in the pH range of 2–11 at various electrolyte concentrations (i.e., 0.001, 0.01 and 0.1 mol/L NaClO4 solutions). As shown in Fig. 5, the sorption of U(VI) is strongly dependent on pH and ionic strength. The sorption percentage increases quickly at pH 3–7, and then decreases with increasing pH at pH N 7. The sorption edges spread over three pH units, which suggests the formation of multifarious surface complexes and manifests various sorption mechanisms [28,29]. The strong pH-dependent sorption can be interpreted in terms of zero point of charge (pHzpc) of bentonite and the species of U(VI) in solution. The relative species of U(VI) as a function of pH values in the presence of carbonate are shown in Fig. 6. At pH b 4, U(VI) is the predominant species (N 96%); at pH 4–7, the main species are UO2+ 2 , 0 UO2(OH)+, (UO2)3(OH)+ 5 , UO2(OH)2 and UO2CO3, and the prominent species are UO2CO3, UO2(CO3)2− and UO2(CO3)4– 2 3 at pH N 7. The surface charge of bentonite is negative at pH N pHzpc. The 4− electrostatic repulsion between UO2(CO3)2− 2 , UO2(CO3)3 and negative surface of bentonite becomes strong with increasing pH at pH N pHzpc, and thereby results in the decrease of U(VI) sorption on bentonite at high pH values. Similar results were also found in the sorption of U(VI) on phyllite [30]. From Fig. 5, one can see that the sorption of U(VI) is the highest in 0.001 mol/L NaClO4 solution, and is the lowest in 0.1 mol/L NaClO4 solution at pH b 7. This phenomenon may be ascribed to: (1) the decrease of competing salt concentration leads to the formation of
Fig. 4. Effect of sorbent content on U(VI) sorption onto bentonite. pH = 5.6 ± 0.1, T = 298 K, m/V = 1.2 g/L, C[U(VI)]initial = 8.00 × 10−5 mol/L, I = 0.01 mol/L NaClO4.
J. Xiao et al. / Journal of Molecular Liquids 188 (2013) 178–185
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(2) Exchange with Na+ ions: ≡ SONa þ UO2
2þ
→ ≡ SO ¼ UO2
2þ
þ
þ Na :
ð6Þ
(3) The hydrolysis of U(VI) in solution: UO2
2þ
2−m þ þ n H2 O→ UO2 ðH2 OÞn−m þmH :
ð7Þ
Being n N m, and exchange with hydrolyzed species: ≡ SOH þ UO2 ðOHÞm ðH2 OÞn−m þ ðn−mÞ H2 O:
Fig. 5. Effect of pH and ionic strength on U(VI) sorption onto bentonite. T = 298, m/V = 1.2 g/L, C[U(VI)]initial = 8.00 × 10−5 mol/L.
electrical double layer complexes, which favors the sorption of U(VI) on bentonite. This phenomenon is indicative of an ion exchange mechanism; (2) the ionic strength of solution influences the activity coefficient of U(VI) ions, which limits their transfer to bentonite surfaces [31,32]. In contrast, the sorption of U(VI) is the highest in 0.1 M NaClO4 solution and the lowest in 0.001 M NaClO4 solution at pH N 7. This phenomenon could be attributed to the reason that when the electrostatic attraction is repulsive, an increase in ionic strength will increase adsorption theoretically, which is beneficial to innersphere surface complexation and thereby increases the U(VI) sorption on bentonite [33]. The sorption of U(VI) is mainly via ion exchange with sodium and hydrogen ions that saturate the exchange sites [34,35]. The exchange can be expressed by the following reactions: (1) Exchange with hydronium ions: ≡ SOH þ UO2
2þ
2 ≡ SOH þ UO2 þ
→ ≡ SOUO2
2þ
≡ SOH2 þ UO2 þ
2þ
→ð ≡ SOÞ2 UO2
2þ
2 ≡ SOH2 þ UO2
→ ≡ SOHUO2
2þ
þ
þH
ð2Þ þ
2þ
þ 2H
ð3Þ
2þ
þH
þ
ð4Þ
→ð ≡ SOHÞ2 UO2
2þ
þ
þ 2H :
ð5Þ
Fig. 6. Relative proportion of U(VI) species in solution as a function of pH values in the presence of CO2. PCO2 = 3.8 × 10−4 atm, T = 298 K, C[U(VI)]initial = 8.00 × 10−5 mol/L, I = 0.01 mol/L NaClO4.
2−m
→ ≡ SOUO2 ðOHÞm
1−m
þH
þ
ð8Þ
Herein, the sorption of U(VI) on bentonite is complicated. The strong pH and ionic strength-dependent sorption of U(VI) on bentonite suggests that ion exchange is the dominant mechanism at pH b 7, while the pH-dependent and ionic strength-independent sorption at pH N 7 suggests an inner-sphere surface complexation mechanism [35]. 3.5. Influence of foreign ions Considering the influence of foreign cations, the sorption of U(VI) on bentonite as a function of pH was investigated in 0.01 mol/L LiClO4, NaClO4 and KClO4 solutions, respectively. As can be seen from Fig. 7, the sorption percentage of U(VI) is the highest in LiClO4 solution and is the lowest in KClO4 solution under the same pH values. The competitive cations hinder the exchange of U(VI) with pH changing on bentonite, which is a well-known phenomenon in ion exchange system [36]. The influence of foreign cations on U(VI) sorption can also be interpreted by the hydrated radius of the three cations (K+ = 2.32 Å, Na+ = 2.76 Å and Li+ = 3.40 Å) [37]. The hydration radius of K+ is smaller than those of the other two cations and therefore the influence of K+ on U(VI) sorption is larger than those of Na+ and Li+. Tan et al. [37] investigated the effect of Li+, Na+ and K+ on the sorption of Th(IV) on TiO2 and also found similar results. The concentration of the monovalent alkali ions (0.01 mol/L) is much higher than that of U(VI) (8.00 × 10−5 mol/L). Before the addition of U(VI), the bentonite has been pre-equilibrated with alkali ions. Thereby, the sorption of U(VI) on bentonite can be considered as the exchange of U(VI) with alkali ions and other reactions, and thereby the sorption of U(VI) on bentonite is influenced by Li+, Na+ and K+ ions. Fig. 8 illustrates the sorption of U(VI) on bentonite as a function of pH in 0.01 mol/L NaCl, NaNO3 and NaClO4, respectively. The results demonstrate that the sorption of U(VI) on bentonite is not influenced by the coexisting electrolyte anions. The radius order of inorganic acid
Fig. 7. Effect of foreign cations and pH on U(VI) sorption onto bentonite. T = 298 K, m/V = 1.2 g/L, C[U(VI)]initial = 8.00 × 10−5 mol/L.
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− − radicals is ClO− 4 N NO3 N Cl . Such negatively charged anions may form complexes with the oxygen-containing functional groups on the − surfaces of bentonite. However, the effects of Cl−, NO− 3 and ClO4 on U(VI) sorption to bentonite are still very weak, which suggests that surface complexes are formed on bentonite surfaces. The effect of foreign anions on U(VI) removal from solution to bentonite can be negligible. The result is similar to the sorption of U(VI) on attapulgite [38], different to the sorption of Ni(II) on hematite [39]. The results indicate that the influence of foreign ions on metal ion sorption is dominated by various factors, such as the properties of metal ions, the properties of adsorbent, and other environmental parameters like ionic strength and pH. It was also necessary to note that the mechanism of specific electrolytes anions on U(VI) sorption is difficult to be discriminated solely from the macroscopic experiment data and further investigation is needed to obtain in-depth microstructure information.
3.6. Effect of humic acid Fig. 9 shows the pH dependency of U(VI) sorption on bentonite in the absence and presence of HA. One can see that the presence of HA enhances U(VI) sorption on HA-bentonite hybrids at pH b 7, whereas reduces U(VI) sorption at pH N 7. The increase of U(VI) sorption on HA-bentonite hybrids in the low pH region may be attributed to a reduction in positive surface charge caused by the sorption of negatively charged HA at bentonite surfaces, which results in a more favorable electrostatic environment for U(VI) sorption and enhances the formation of ternary U(VI)–HA-bentonite surface complexes. The surface adsorbed HA also provides more available groups to form complexes of U(VI)–HA, and thereby enhances U(VI) sorption on HA-bentonite hybrids. At pH N 7, the surface of bentonite becomes negatively charged due to deprotonation reaction. The sorption of negatively charged HA on the negatively charged bentonite surface decreases with increasing pH due to electrostatic repulsion. Hence, more free HA molecules remained in solution and these HA molecules form soluble HA-U(VI) complexes. This process competitively diminishes the sorption extent of U(VI) on bentonite [40]. 3.7. Sorption isotherms and thermodynamic data The sorption isotherms of U(VI) on bentonite at 298, 318 and 338 K are illustrated in Fig. 10. The sorption of U(VI) increases with increasing temperature, indicating that the sorption of U(VI) on bentonite is favored at high temperature and blocked at low temperature. In order to gain a better understanding of U(VI) sorption mechanism and to quantify the sorption data, Langmuir, Freundlich and D–R models are
Fig. 8. Effect of foreign anions and pH on U(VI) sorption onto bentonite. T = 298 K, m/V = 1.2 g/L, C[U(VI)]initial = 8.00 × 10−5 mol/L.
Fig. 9. U(VI) sorption onto bentonite in the presence and absence of HA as a function of pH. T = 298 K, m/V = 1.2 g/L, C[U(VI)]initial = 8.00 × 10−5 mol/L, I = 0.01 mol/L NaClO4.
adopted to fit the sorption isotherms and to simulate the experimental data. The Langmuir isotherm model is a theoretical model for monolayer sorption, which can be expressed by the following equation [40]: Cs ¼
bC s max C e : 1 þ bC e
ð9Þ
Eq. (9) can be expressed in the linear form: Ce 1 Ce ¼ þ C s bC s max C s max
ð10Þ
where Ce is the equilibrium concentration of U(VI) remained in solution (mol/L); Cs is the amount of U(VI) adsorbed on per weight unit of bentonite after equilibrium (mol/g); Csmax, the maximum sorption capacity, is the amount of U(VI) at complete monolayer coverage (mol/g), and b (L/mol) is a binding constant that relates the heat of sorption. The Freundlich isotherm model is a semi-empirical equation describing the sorption onto heterogeneity surface and it is usually expressed as follows [40]: n
Cs ¼ K F Ce :
ð11Þ
Fig. 10. Sorption isotherms of U(VI) to bentonite at three different temperatures. pH = 5.8 ± 0.1, m/V = 1.2 g/L, I = 0.01 mol/L NaClO4.
J. Xiao et al. / Journal of Molecular Liquids 188 (2013) 178–185
183
The D–R isotherm is more general than the Langmuir isotherm, because it does not assume a homogeneous surface or constant sorption potential. It is valid at low concentration ranges and can be used to describe sorption on both homogeneous and heterogeneous surfaces. The D–R equation has the general expression [40]: 2 C s ¼ C s max exp −βε :
ð13Þ
Eq. (13) can be expressed in linear form as: ln C s ¼ ln C s max −βε
2
ð14Þ
where Cs and Csmax are defined above, β is the activity coefficient related to the mean sorption energy (mol2/kJ2), and ε is the Polanyi potential, which is equal to: 1 ε ¼ RT ln 1 þ Ce
ð15Þ
where R is ideal gas constant (8.314 J/(mol·K)), and T is the absolute temperature in Kelvin (K). E (kJ/mol) is defined as the free energy change that is required to transfer 1 mol of ions from solution to the solid surfaces. The relation can be described as follows:
1 E ¼ pffiffiffiffiffiffi : 2β
Fig. 11. Langmuir (A), Freundlich (B) and D–R (C) isotherms of U(VI) sorption on bentonite at three different temperatures, pH = 5.8 ± 0.1, m/V = 1.2 g/L, I = 0.01 mol/L NaClO4.
ð16Þ
The experimental data of U(VI) sorption (Fig. 10) are regressively simulated with the three models and the results are shown in Fig. 11. Table 1 shows the relative values calculated from the three models. It can be concluded from the correlation coefficients that Langmuir model simulates the experimental data better than Freundlich and D– R models. This phenomenon indicates that almost complete monolayer coverage of the bentonite particles. What's more, bentonite has a limited sorption capacity, thus the sorption could be better described by Langmuir model rather than by Freundlich model, since an exponentially increasing sorption was assumed in the Freundlich model. The values of Csmax acquired from the Langmuir model for U(VI) sorption on bentonite are the highest at T = 338 K and the lowest at T=298K, which indicates that the sorption is enhanced with increasing temperature. In the Freundlich model, the value of n is lower than 1, which indicates that a nonlinear sorption takes place on bentonite surfaces. The magnitude of E is of great importance for estimating the sorption mechanism. The E values calculated from Eq. (16) are 14.34 kJ/mol (298 K), 14.31 kJ/mol (318 K) and 14.84 kJ/mol (338 K). The E values are in the sorption energy range of chemical ion-exchange reaction [41], which indicates that U(VI) sorption on betonite should be attributed to chemical sorption rather than physical sorption. The Csmax values obtained from the D–R model are higher than those obtained from the Langmuir model. This may be attributed to the different assumptions considered in the formulation of the isotherms. The thermodynamic parameters (ΔH0, ΔS0 and ΔG0) for U(VI) sorption on bentonite are calculated from the temperature-dependent sorption isotherms. The Gibbs free energy change (ΔG0) is calculated by the following equation:
Eq. (11) can be expressed in linear form as: logC s ¼ logK F þ n logC e
ð12Þ
where kF (mol1 − nLn/g) represents the sorption capacity when metal ion equilibrium concentration equals to 1, and n represents the degree of dependence of sorption with equilibrium concentration.
0
ΔG ¼ −RT lnK
0
ð17Þ
where K0 is the sorption equilibrium constant. Values of lnK0 are obtained by plotting in Kd versus Ce (Fig. 12) and extrapolating Ce to zero. Constants of linear fit of lnKd versus Ce for sorption of U(VI) to bentonite are listed in
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Table 1 The parameters for Langmuir, Freundlich and D–R isotherms at different temperatures. T (K)
Langmuir Csmax (mol/g) −5
298.15 318.15 338.15
9.19 × 10 9.40 × 10−5 1.08 × 10−4
Freundlich b (L/mol) 3
4.66 × 10 2.05 × 103 1.24 × 103
R2 0.996 0.996 0.992
kF (mol1 − n∙Ln/g) −3
8.93 × 10 3.75 × 10−3 4.06 × 10−3
D–R n 0.531 0.414 0.400
R2 0.967 0.966 0.969
β (mol2/kJ2) −3
4.21 × 10 2.76 × 10−3 2.30 × 10−3
Csmax (mol/g) −4
5.98 × 10 4.23 × 10−4 4.66 × 10−4
R2 0.981 0.982 0.985
solutions and may suggest some structure changes of bentonite. The result of U(VI) sorption on bentonite is a spontaneous and endothermic process [42]. 4. Conclusion
Fig. 12. Linear plots of lnKd versus Ce. pH = 5.8 ± 0.1, m/V = 1.2 g/L, I = 0.01 mol/L NaClO4.
Table 2. Its intercept with vertical axis gives the value of lnK0. Standard entropy change (ΔS0) is calculated using the equation: 0
∂ΔG ΔS ¼ − ∂T 0
! ð18Þ p
The average standard enthalpy change (ΔH0) is then calculated from the relationship: 0
0
0
ΔS ¼ ΔG þ TΔS :
ð19Þ
The values obtained from Eqs. (18) and (19) are tabulated in Table 2. The positive value of ΔH0 indicates that the sorption is endothermic. One possible explanation for this positive enthalpy is that U(VI) is dissolved well in water, and the hydration sheath of U(VI) has to be destroyed before its sorption on bentonite. This dehydration process needs energy, and so it is favored at high temperature. This energy exceeds the exothermicity of cations to attach to the solid surface [41]. The assumption indicates that the endothermicity of the desolvation process is higher than the enthalpy of sorption to a considerable extent. The Gibbs free energy change (ΔG0) is negative, as expected for a spontaneous process under the conditions applied. The value of ΔG0 becomes more negative with the increase of temperature, which indicates more efficient sorption at high temperature. At high temperature, cations are readily desolvated and hence their sorption becomes more favorable. The positive value of entropy change (ΔS0) suggests the affinity of bentonite toward U(VI) ions in aqueous Table 2 Values of thermodynamic parameters for U(VI) sorption on bentonite. T(K)
ΔG0 (kJ/mol)
ΔS0 (J/(mol∙K))
ΔH0 (kJ/mol)
298.15 318.15 338.15
−18.07 −20.56 −22.90
120.8
17.94 17.88 17.95
In this study, a local bentonite sample was purified and characterized by using FTIR and XRD to determine its chemical constituents and microstructure. Batch technique was adopted to investigate the sorption of U(VI) from aqueous solutions onto bentonite as a function of various environmental factors such as contact time, pH, ionic strength, coexisting electrolyte ions, HA and temperature under ambient conditions. The sorption of U(VI) on bentonite from aqueous solution is strongly dependent on pH values and ionic strength. At low pH, outer-sphere surface complexation or ion exchange is the main sorption mechanism, whereas inner-sphere surface complexation is the predominant sorption mechanism at high pH values. The cations and anions influence the sorption of U(VI) on bentonite obviously at pH b 7.0, and no influence is found at pH N 7.0. The sorption isotherms of U(VI) on bentonite can be well described by the Langmuir model. The thermodynamic analysis derived from temperature dependent sorption isotherms suggests that the sorption process of U(VI) on bentonite is spontaneous and endothermic. Considering the wide raw material sources, simple disposal procedure, low cost, high removal efficiency and environmental friendliness, one can draw a conclusion that bentonite may be widely used for the cost-effective treatment of U(VI)-bearing wastewaters. Besides, one can also consider using the bentonite as a candidate backfilling material for the deep geological disposal of high-level radioactive waste. Acknowledgments The work was supported by the National Natural Science Foundation of China (21107115, 21225730), Financial Grant from the China Postdoctoral Science Foundation (2012M511432) and Special Foundation for High-level Waste Disposal of China (2007-840). References [1] X.L. Tan, Q.H. Fan, X.K. Wang, B. Grambow, Environ. Sci. Technol. 43 (2009) 3115. [2] C.L. Chen, X.K. Wang, M. Nagatsu, Environ. Sci. Technol. 43 (2009) 2362. [3] D.D. Shao, D. Xu, S.W. Wang, Q.H. Fan, W.S. Wu, Y.H. Dong, X.K. Wang, Sci. China B Chem. 52 (2009) 362. [4] X.L. Tan, X.K. Wang, H. Geckeis, T. Rabung, Environ. Sci. Technol. 42 (2008) 6532. [5] Y. Zhang, Y. Li, X. Zheng, Sci. Total Environ. 409 (2011) 625. [6] D.D. Shao, Z. Jiang, X. Wang, J. Li, Y. Meng, J. Phys. Chem. B113 (2009) 860. [7] S. Panja, P.K. Mohapatra, S.C. Tripathi, P.M. Gandhi, P. Janardan, Sep. Purif. Technol. 96 (2012) 289. [8] Y. Zhang, Y. Li, J. Li, L. Hu, X. Zheng, Chem. Eng. J. 171 (2011) 526. [9] G.N. Kim, D.B. Shon, H. Park, W.K. Choi, K.W. Lee, Sep. Purif. Technol. 79 (2011) 144. [10] T.K. Rout, D.K. Sengupta, L. Besra, Int. J. Miner. Process. 79 (2006) 225. [11] J. Li, Y. Li, Q. Meng, J. Hazard. Mater. 174 (2010) 188. [12] K. Trivunac, S. Stevanovic, Chemosphere 64 (2006) 486. [13] Y. Li, Y. Zhang, J. Li, X. Zheng, Environ. Pollut. 159 (2011) 3744. [14] T. Matsuo, T. Nishi, Carbon 38 (2000) 709. [15] X.K. Wang, C.L. Chen, J.Z. Du, X.L. Tan, D. Xu, S.M. Yu, Environ. Sci. Technol. 39 (2005) 7084. [16] Y. Zhang, Y. Li, J. Li, G. Sheng, Y. Zhang, X. Zheng, Chem. Eng. J. 185–186 (2012) 243. [17] J. Li, Y. Li, J. Lu, Appl. Clay Sci. 46 (2009) 314. [18] J. Li, J. Lu, Y. Li, J. Appl. Polym. Sci. 112 (2009) 26. [19] M. Kara, H. Yuzer, E. Sabah, M.S. Celik, Water Res. 37 (2003) 224.
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Sorption behavior of U(VI) onto Chinese bentonite: Effect of pH, ionic strength, temperature and humic acid Jiang Xiao, Yuantao Chen ⁎, Wenhua Zhao, Jiangbo Xu Department of Chemistry, Qinghai Normal University, 810008, Xining, Qinghai, PR China
a r t i c l e
i n f o
Article history: Received 12 June 2013 Received in revised form 6 October 2013 Accepted 9 October 2013 Available online 22 October 2013 Keywords: U(VI) Sorption Bentonite pH Thermodynamic data Mechanism
a b s t r a c t Herein, a local bentonite from Huangshan county (Anhui province, China) was purified and the purge bentonite sample was characterized by using FTIR and XRD to determine its chemical constituents and microstructures. The sample was chosen as an adsorbent to remove uranium ions from aqueous solutions as a function of various environmental parameters such as contact time, pH, ionic strength, foreign ions, humic acid and temperature under ambient conditions. The results indicated that the sorption of U(VI) on the bentonite was strongly dependent on pH and ionic strength. At pH b 6.5, the sorption of U(VI) on the bentonite increased with increasing pH, whereas the sorption of U(VI) on bentonite decreased with increasing pH at pH N 6.5. The presence of humic acid (HA) enhanced the sorption of U(VI) on bentonite obviously at low pH while it reduced the U(VI) sorption on bentonite at high pH values. The enhancement of U(VI) sorption on HA-bentonite hybrids at low pH was attributed to the strong surface complexation of surface adsorbed HA with U(VI) on bentonite surface, whereas the decrease of U(VI) sorption on bentonite at high pH was attributed to the formation of free HA-U complexes in solution. The Langmuir, Freundlich and D–R models were applied to simulate the sorption isotherms of U(VI) at three different temperatures of 298, 318 and 338 K, respectively. The thermodynamic parameters (i.e., ΔH0, ΔS0 and ΔG0) calculated from the temperature dependent sorption isotherms indicated that the sorption process of U(VI) on bentonite was an endothermic and spontaneous process. At low pH, the sorption of U(VI) was mainly dominated by outer-sphere surface complexation and ion exchange with H+/Na+ on bentonite surfaces, whereas inner-sphere surface complexation was the main sorption mechanism at high pH values. Based on the experimental results, bentonite is a very suitable material for the preconcentration and solidification of U(VI) ions from large volumes of aqueous solutions in U(VI) pollution cleanup. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The physicochemical behavior of long-lived lanthanides and actinides has aroused great interest in terms of the radioactive waste disposal [1–4]. Uranium (U(VI)), a radioactive and hazardous heavy metal, originates from nuclear industry and other anthropogenic activities such as lignite burning in power stations, ore processing, and the use of fertilizers [5,6]. Uranium released into the environment is predominantly in the hexavalent form as mobile, aqueous uranyl ion (U(VI)) under normal environmental conditions. Owing to the long half-life (t1/2(235U)= 7.04 × 108 a; t1/2(238U)= 4.47 × 109 a), the release of uranium into the environment can cause significant geochemical and public health problems. Thereby, it is crucial to eliminate U(VI) from aqueous solutions before it is released into the environment. Exposure to uranium can result in serious biochemical and radioactive harms to biological organization such as toxic hepatitis, skin corrosion, kidney damage, histopathological system damage and even cancers. In view of this point, strict environmental protection legislation on the ⁎ Corresponding author. Tel.: +86 971 6303374. E-mail address: [email protected] (Y. Chen). 0167-7322/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molliq.2013.10.008
disposal of hazardous and radioactive metal ions causes increasing demands for the removal of those ions with cost-effective techniques. In the last decade, various technologies such as metal ion extraction [7], ion exchange [8], chemical precipitation [9,10], filtration [11], surface complexation [12,13], thermal treatment [14], and sorption [15–18] have been developed for the removal of radionuclides and heavy metal ions from aqueous solutions. Among these methods, sorption is an effective technique with the advantages of high treatment efficiency and easy operation. The sorption of U(VI) on different minerals and oxides has been studied extensively [18,19]. The results showed that the sorption of U(VI) was mainly dominated by outer-sphere surface complexation at low pH and by inner-sphere surface complexation or precipitation at high pH values. Bentonite is a 2:1 type of clay, and the main clay component is montmorillonite, which is composed of units of two silica tetrahedral sheets with a central alumina octahedral sheet. All tips of tetrahedrons point in the same direction and toward the center unit. Compared with other clay minerals, bentonite has excellent sorption sites and functional groups which are on the outer surface and edges for the formation of complexes with radionuclides. Bentonite has a series of outstanding physicochemical properties such as large specific area,
J. Xiao et al. / Journal of Molecular Liquids 188 (2013) 178–185
high cation exchange capacity, strong adsorptive affinity for organic and inorganic pollutants, low permeability, low cost, accessibility and ubiquitous presence in most soils. Based on these advantages, bentonite is considered one of the most promising candidates as geochemical barriers in nuclear waste repositories and hazardous chemical landfills [20–22]. In China, the bentonite from Huangshan county has been selected as a potential candidate as backfill material for nuclear waste disposal management. However, to the best of our knowledge, no available study was focused on the sorption behaviors of uranium on this kind of bentonite. In this study, a local bentonite from Huangshan county (Anhui province, China) was used as a sorbent for the removal of uranium from wastewater. The purposes of this work are: (1) to characterize the purified bentonite sample by using X-ray diffraction (XRD) and Fourier Transformed Infrared spectroscopy (FTIR) in detail; (2) to study the effect of different parameters, such as contact time, pH, ionic strength, foreign ions, humic acid (HA) and temperature on U(VI) sorption by using batch technique; (3) to calculate the thermodynamic data (ΔG0, ΔS0 and ΔH0) from the temperaturedependent sorption isotherms; and (4) to discuss the sorption mechanism of U(VI) on bentonite. 2. Experimental section 2.1. Materials The bentonite sample was obtained from Huangshan county (Anhui province, China). The sample was treated with 5% hydrochloric acid for 24 h in order to improve the specific surface area, and washed with double distilled water until no chloride was detected in supernatant with 0.01 mol/L AgNO3. The resulted bentonite sample was dried at 105 °C for 2 h to eliminate the free water. Finally, the sample was milled and passed through a 200-mesh screen for further usage in the following experiments. U(VI) stock solution was prepared by dissolving uranylnitrate hexahydrate (UO2(NO3)2 6H2O) in Milli-Q water. The stock solution was kept at pH 3 and used in the following sorption experiments. HA was extracted from the soil samples of Hua-jia county of Gansu province (China) and characterized in detail [23,24]. The main elements of HA are: H (3.53%), C (60.44%), N (4.22%), O (31.31%) and S (0.50%). All chemicals used in the experiments were purchased as analytical purity and used without any further purification. Milli-Q water was used in the experiments.
179
for 24 h, the solid and liquid phases were separated by centrifugation at 9000 rpm for 30 min at the same temperature as in the sorption experiments. It was necessary to emphasize that the sorption of U(VI) on the tube wall was negligible according to the test of U(VI) sorption in the absence of bentonite. The concentration of U(VI) was analyzed by chlorophosphonazo III spectrophotometric method at the wavelength of 669 nm. The amount of U(VI) adsorbed on bentonite was calculated from the difference between the initial concentration and the equilibrium one. The sorption percentage (Sorption% = (C0 − Ce)/C0 × 100%) and distribution coefficient (Kd = (C0 − Ce)/Ce · V/m) of U(VI) on bentonite were calculated from the initial concentration of U(VI) (C0), the equilibrium concentration of U(VI) (Ce) in supernatant, the mass of bentonite (m) and the volume of the suspension (V). 3. Results and discussion 3.1. Characterization of bentonite Fig. 1 shows the FTIR spectrum of the purified bentonite sample. The broad bands at 3436 and 3625 cm−1 are due to the H\OH vibration of the free water molecules adsorbed on the solid surface and the O\H stretching vibration of the silanol (Si\OH) groups from the solid. The peaks at 2862 and 2933 cm−1 are assigned to the aliphatic C\H stretching vibration [23]. The spectral band at 1640 cm reflects the bending of H\OH bond of water molecules, which is retained in the matrix. The strong band at 1044 cm−1 represents the Si\O\Si groups of the tetrahedral sheet. The band at 798 cm−1 confirms the presence of quartz in the sample. The spectral band at 930 cm−1 reflects the stretching vibration of Al\O\(OH)\Al. The band at 624 cm−1 is assigned to the out-of-plane vibrations of coupled Al\O and Si\O. The bands at 468 and 519 cm−1 are corresponded to Si\O\Si and Al\O\Si bending vibrations [24]. The XRD pattern of the bentonite sample is shown in Fig. 2. The diffraction peaks of the planes at 2θ = 6.04°, 19.86° and 35.15° (marked by M) are the reflections indicative of 2:1 swelling clays. The other peaks are the impurities corresponded to quartz (marked by Q) and Cal–Fe(Ca)CO3 (marked by C). Crystallographic parameters are evaluated by measuring (001) and (060) peaks. The bentonite sample exhibits a diffraction peak of the (001) plane at 2θ = 6.039°, and its basal spacing is calculated to be 14.6 Å. The (060) reflection at 2θ = 62.00° implies the dioctahedral structure of bentonite. 3.2. Time-dependent sorption
2.2. Characterization Infrared spectrum of the purified bentonite sample ranging from 400 to 4000 cm−1 was recorded using FTIR spectrophotometer (Bruker EQUINOX55 Nexus) in KBr pellets. The spectral resolution was set to 1 cm−1, and 150 scans were collected for each spectrum. The XRD pattern was obtained from a D/Max-rB equipped with a rotation anode using Cu Kα radiation (λ = 0.15406 nm). The XRD device was operated at 40 kV and 80 mA. The measurements were carried out in the range of 5°–65°.
The contact time is an important parameter that can reflect the sorption kinetics of an adsorbent for a certain adsorbate concentration.
2.3. Experimental procedures The sorption of U(VI) on bentonite was investigated by using batch technique in polyethylene centrifuge tubes under ambient conditions. The stock suspension of bentonite and NaClO4 was pre-equilibrated for 24 h, then U(VI) stock solution and HA were added in bentonite suspension to achieve the desired concentrations of different components. The volume of the solution was 10 mL and the amount of the solid was 1.2 g/L for each experimental data. The pH values of the solution were adjusted by adding negligible volumes of 0.1 or 0.01 mol/L HClO4 or NaOH solutions. After the suspensions were stirred
Fig. 1. FTIR spectrum of purified bentonite sample.
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3.3. Influence of solid content Fig. 4 shows the sorption of U(VI) on bentonite as a function of solid content. The distribution of the coefficient, Kd, values as a function of the bentonite content is also plotted in Fig. 4. One can see that the sorption percentage of U(VI) increases from ~35% at m/V = 0.2 mg/L to ~51% at m/V = 1.2 mg/L. It is well known that the amount of functional groups at bentonite surfaces increases with increasing solid content. Thereby, more surface sites are available for the binding of U(VI) at higher solid contents. As can be seen from Fig. 4, the Kd values are constant with increasing solid content. This phenomenon is consistent with the physicochemical properties of Kd values, i.e., the Kd value is independent of solid content and solution concentration when both of them are low. The result resembles the sorption of U(VI) on carbon nanotubes [6]. Fig. 2. XRD pattern of purified bentonite sample.
3.4. Influence of pH and ionic strength
It was necessary to emphasize that the centrifugation time was also considered in the time-dependent sorption experiments for the reason that the sorption of U(VI) still occurred at low centrifugation rate. The sorption percentage of U(VI) on bentonite as a function of contact time is shown in Fig. 3. It is observed that the sorption percentage increases rapidly in the first contact time of 4 h, and then maintains high level with increasing contact time. The fast sorption kinetics suggests that the U(VI) sorption on bentonite is mainly dominated by chemisorption rather than physicosorption [25]. The result also suggests that 4 h is enough to achieve the sorption equilibrium of U(VI) on bentonite. In the following experiments, the shaking time is fixed at 24 h to guarantee that the sorption reaction can achieve complete equilibrium. A preudo-second-order rate equation is used to simulate the kinetic of sorption [26]: t 1 1 ¼ þ t qt 2k0 qe 2 qe
ð1Þ
where k′ (g/(mg·h)) is the pseudo-second-order rate constant of sorption; qt (mg/g) is the amount of U(VI) adsorbed on bentonite at time t (h), and qe (mg/g) is the equilibrium sorption capacity. The plot of t/qt versus t is shown in the inserted figure of Fig. 3. The values of k′ and qe calculated from the intercept and slope are 0.84 g/(mg·h) and 10.98 mg/g, respectively. The correlation coefficient of the pseudosecond-order rate equation for the linear plot is 0.999, which suggests that the kinetic sorption can be described by the pseudo-second-order rate equation well [27].
Fig. 3. Effect of contact time on U(VI) sorption onto bentonite. pH = 5.6 ± 0.1, T = 298 K, m/V = 1.2 g/L, C[U(VI)]initial = 8.00 × 10−5 mol/L, I = 0.01 mol/L NaClO4.
Solution pH has a significant influence on the migration and transformation behaviors of environmental contaminants at solid/water interfaces. In view of this point, the pH-dependent sorption of U(VI) on bentonite was investigated in the pH range of 2–11 at various electrolyte concentrations (i.e., 0.001, 0.01 and 0.1 mol/L NaClO4 solutions). As shown in Fig. 5, the sorption of U(VI) is strongly dependent on pH and ionic strength. The sorption percentage increases quickly at pH 3–7, and then decreases with increasing pH at pH N 7. The sorption edges spread over three pH units, which suggests the formation of multifarious surface complexes and manifests various sorption mechanisms [28,29]. The strong pH-dependent sorption can be interpreted in terms of zero point of charge (pHzpc) of bentonite and the species of U(VI) in solution. The relative species of U(VI) as a function of pH values in the presence of carbonate are shown in Fig. 6. At pH b 4, U(VI) is the predominant species (N 96%); at pH 4–7, the main species are UO2+ 2 , 0 UO2(OH)+, (UO2)3(OH)+ 5 , UO2(OH)2 and UO2CO3, and the prominent species are UO2CO3, UO2(CO3)2− and UO2(CO3)4– 2 3 at pH N 7. The surface charge of bentonite is negative at pH N pHzpc. The 4− electrostatic repulsion between UO2(CO3)2− 2 , UO2(CO3)3 and negative surface of bentonite becomes strong with increasing pH at pH N pHzpc, and thereby results in the decrease of U(VI) sorption on bentonite at high pH values. Similar results were also found in the sorption of U(VI) on phyllite [30]. From Fig. 5, one can see that the sorption of U(VI) is the highest in 0.001 mol/L NaClO4 solution, and is the lowest in 0.1 mol/L NaClO4 solution at pH b 7. This phenomenon may be ascribed to: (1) the decrease of competing salt concentration leads to the formation of
Fig. 4. Effect of sorbent content on U(VI) sorption onto bentonite. pH = 5.6 ± 0.1, T = 298 K, m/V = 1.2 g/L, C[U(VI)]initial = 8.00 × 10−5 mol/L, I = 0.01 mol/L NaClO4.
J. Xiao et al. / Journal of Molecular Liquids 188 (2013) 178–185
181
(2) Exchange with Na+ ions: ≡ SONa þ UO2
2þ
→ ≡ SO ¼ UO2
2þ
þ
þ Na :
ð6Þ
(3) The hydrolysis of U(VI) in solution: UO2
2þ
2−m þ þ n H2 O→ UO2 ðH2 OÞn−m þmH :
ð7Þ
Being n N m, and exchange with hydrolyzed species: ≡ SOH þ UO2 ðOHÞm ðH2 OÞn−m þ ðn−mÞ H2 O:
Fig. 5. Effect of pH and ionic strength on U(VI) sorption onto bentonite. T = 298, m/V = 1.2 g/L, C[U(VI)]initial = 8.00 × 10−5 mol/L.
electrical double layer complexes, which favors the sorption of U(VI) on bentonite. This phenomenon is indicative of an ion exchange mechanism; (2) the ionic strength of solution influences the activity coefficient of U(VI) ions, which limits their transfer to bentonite surfaces [31,32]. In contrast, the sorption of U(VI) is the highest in 0.1 M NaClO4 solution and the lowest in 0.001 M NaClO4 solution at pH N 7. This phenomenon could be attributed to the reason that when the electrostatic attraction is repulsive, an increase in ionic strength will increase adsorption theoretically, which is beneficial to innersphere surface complexation and thereby increases the U(VI) sorption on bentonite [33]. The sorption of U(VI) is mainly via ion exchange with sodium and hydrogen ions that saturate the exchange sites [34,35]. The exchange can be expressed by the following reactions: (1) Exchange with hydronium ions: ≡ SOH þ UO2
2þ
2 ≡ SOH þ UO2 þ
→ ≡ SOUO2
2þ
≡ SOH2 þ UO2 þ
2þ
→ð ≡ SOÞ2 UO2
2þ
2 ≡ SOH2 þ UO2
→ ≡ SOHUO2
2þ
þ
þH
ð2Þ þ
2þ
þ 2H
ð3Þ
2þ
þH
þ
ð4Þ
→ð ≡ SOHÞ2 UO2
2þ
þ
þ 2H :
ð5Þ
Fig. 6. Relative proportion of U(VI) species in solution as a function of pH values in the presence of CO2. PCO2 = 3.8 × 10−4 atm, T = 298 K, C[U(VI)]initial = 8.00 × 10−5 mol/L, I = 0.01 mol/L NaClO4.
2−m
→ ≡ SOUO2 ðOHÞm
1−m
þH
þ
ð8Þ
Herein, the sorption of U(VI) on bentonite is complicated. The strong pH and ionic strength-dependent sorption of U(VI) on bentonite suggests that ion exchange is the dominant mechanism at pH b 7, while the pH-dependent and ionic strength-independent sorption at pH N 7 suggests an inner-sphere surface complexation mechanism [35]. 3.5. Influence of foreign ions Considering the influence of foreign cations, the sorption of U(VI) on bentonite as a function of pH was investigated in 0.01 mol/L LiClO4, NaClO4 and KClO4 solutions, respectively. As can be seen from Fig. 7, the sorption percentage of U(VI) is the highest in LiClO4 solution and is the lowest in KClO4 solution under the same pH values. The competitive cations hinder the exchange of U(VI) with pH changing on bentonite, which is a well-known phenomenon in ion exchange system [36]. The influence of foreign cations on U(VI) sorption can also be interpreted by the hydrated radius of the three cations (K+ = 2.32 Å, Na+ = 2.76 Å and Li+ = 3.40 Å) [37]. The hydration radius of K+ is smaller than those of the other two cations and therefore the influence of K+ on U(VI) sorption is larger than those of Na+ and Li+. Tan et al. [37] investigated the effect of Li+, Na+ and K+ on the sorption of Th(IV) on TiO2 and also found similar results. The concentration of the monovalent alkali ions (0.01 mol/L) is much higher than that of U(VI) (8.00 × 10−5 mol/L). Before the addition of U(VI), the bentonite has been pre-equilibrated with alkali ions. Thereby, the sorption of U(VI) on bentonite can be considered as the exchange of U(VI) with alkali ions and other reactions, and thereby the sorption of U(VI) on bentonite is influenced by Li+, Na+ and K+ ions. Fig. 8 illustrates the sorption of U(VI) on bentonite as a function of pH in 0.01 mol/L NaCl, NaNO3 and NaClO4, respectively. The results demonstrate that the sorption of U(VI) on bentonite is not influenced by the coexisting electrolyte anions. The radius order of inorganic acid
Fig. 7. Effect of foreign cations and pH on U(VI) sorption onto bentonite. T = 298 K, m/V = 1.2 g/L, C[U(VI)]initial = 8.00 × 10−5 mol/L.
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− − radicals is ClO− 4 N NO3 N Cl . Such negatively charged anions may form complexes with the oxygen-containing functional groups on the − surfaces of bentonite. However, the effects of Cl−, NO− 3 and ClO4 on U(VI) sorption to bentonite are still very weak, which suggests that surface complexes are formed on bentonite surfaces. The effect of foreign anions on U(VI) removal from solution to bentonite can be negligible. The result is similar to the sorption of U(VI) on attapulgite [38], different to the sorption of Ni(II) on hematite [39]. The results indicate that the influence of foreign ions on metal ion sorption is dominated by various factors, such as the properties of metal ions, the properties of adsorbent, and other environmental parameters like ionic strength and pH. It was also necessary to note that the mechanism of specific electrolytes anions on U(VI) sorption is difficult to be discriminated solely from the macroscopic experiment data and further investigation is needed to obtain in-depth microstructure information.
3.6. Effect of humic acid Fig. 9 shows the pH dependency of U(VI) sorption on bentonite in the absence and presence of HA. One can see that the presence of HA enhances U(VI) sorption on HA-bentonite hybrids at pH b 7, whereas reduces U(VI) sorption at pH N 7. The increase of U(VI) sorption on HA-bentonite hybrids in the low pH region may be attributed to a reduction in positive surface charge caused by the sorption of negatively charged HA at bentonite surfaces, which results in a more favorable electrostatic environment for U(VI) sorption and enhances the formation of ternary U(VI)–HA-bentonite surface complexes. The surface adsorbed HA also provides more available groups to form complexes of U(VI)–HA, and thereby enhances U(VI) sorption on HA-bentonite hybrids. At pH N 7, the surface of bentonite becomes negatively charged due to deprotonation reaction. The sorption of negatively charged HA on the negatively charged bentonite surface decreases with increasing pH due to electrostatic repulsion. Hence, more free HA molecules remained in solution and these HA molecules form soluble HA-U(VI) complexes. This process competitively diminishes the sorption extent of U(VI) on bentonite [40]. 3.7. Sorption isotherms and thermodynamic data The sorption isotherms of U(VI) on bentonite at 298, 318 and 338 K are illustrated in Fig. 10. The sorption of U(VI) increases with increasing temperature, indicating that the sorption of U(VI) on bentonite is favored at high temperature and blocked at low temperature. In order to gain a better understanding of U(VI) sorption mechanism and to quantify the sorption data, Langmuir, Freundlich and D–R models are
Fig. 8. Effect of foreign anions and pH on U(VI) sorption onto bentonite. T = 298 K, m/V = 1.2 g/L, C[U(VI)]initial = 8.00 × 10−5 mol/L.
Fig. 9. U(VI) sorption onto bentonite in the presence and absence of HA as a function of pH. T = 298 K, m/V = 1.2 g/L, C[U(VI)]initial = 8.00 × 10−5 mol/L, I = 0.01 mol/L NaClO4.
adopted to fit the sorption isotherms and to simulate the experimental data. The Langmuir isotherm model is a theoretical model for monolayer sorption, which can be expressed by the following equation [40]: Cs ¼
bC s max C e : 1 þ bC e
ð9Þ
Eq. (9) can be expressed in the linear form: Ce 1 Ce ¼ þ C s bC s max C s max
ð10Þ
where Ce is the equilibrium concentration of U(VI) remained in solution (mol/L); Cs is the amount of U(VI) adsorbed on per weight unit of bentonite after equilibrium (mol/g); Csmax, the maximum sorption capacity, is the amount of U(VI) at complete monolayer coverage (mol/g), and b (L/mol) is a binding constant that relates the heat of sorption. The Freundlich isotherm model is a semi-empirical equation describing the sorption onto heterogeneity surface and it is usually expressed as follows [40]: n
Cs ¼ K F Ce :
ð11Þ
Fig. 10. Sorption isotherms of U(VI) to bentonite at three different temperatures. pH = 5.8 ± 0.1, m/V = 1.2 g/L, I = 0.01 mol/L NaClO4.
J. Xiao et al. / Journal of Molecular Liquids 188 (2013) 178–185
183
The D–R isotherm is more general than the Langmuir isotherm, because it does not assume a homogeneous surface or constant sorption potential. It is valid at low concentration ranges and can be used to describe sorption on both homogeneous and heterogeneous surfaces. The D–R equation has the general expression [40]: 2 C s ¼ C s max exp −βε :
ð13Þ
Eq. (13) can be expressed in linear form as: ln C s ¼ ln C s max −βε
2
ð14Þ
where Cs and Csmax are defined above, β is the activity coefficient related to the mean sorption energy (mol2/kJ2), and ε is the Polanyi potential, which is equal to: 1 ε ¼ RT ln 1 þ Ce
ð15Þ
where R is ideal gas constant (8.314 J/(mol·K)), and T is the absolute temperature in Kelvin (K). E (kJ/mol) is defined as the free energy change that is required to transfer 1 mol of ions from solution to the solid surfaces. The relation can be described as follows:
1 E ¼ pffiffiffiffiffiffi : 2β
Fig. 11. Langmuir (A), Freundlich (B) and D–R (C) isotherms of U(VI) sorption on bentonite at three different temperatures, pH = 5.8 ± 0.1, m/V = 1.2 g/L, I = 0.01 mol/L NaClO4.
ð16Þ
The experimental data of U(VI) sorption (Fig. 10) are regressively simulated with the three models and the results are shown in Fig. 11. Table 1 shows the relative values calculated from the three models. It can be concluded from the correlation coefficients that Langmuir model simulates the experimental data better than Freundlich and D– R models. This phenomenon indicates that almost complete monolayer coverage of the bentonite particles. What's more, bentonite has a limited sorption capacity, thus the sorption could be better described by Langmuir model rather than by Freundlich model, since an exponentially increasing sorption was assumed in the Freundlich model. The values of Csmax acquired from the Langmuir model for U(VI) sorption on bentonite are the highest at T = 338 K and the lowest at T=298K, which indicates that the sorption is enhanced with increasing temperature. In the Freundlich model, the value of n is lower than 1, which indicates that a nonlinear sorption takes place on bentonite surfaces. The magnitude of E is of great importance for estimating the sorption mechanism. The E values calculated from Eq. (16) are 14.34 kJ/mol (298 K), 14.31 kJ/mol (318 K) and 14.84 kJ/mol (338 K). The E values are in the sorption energy range of chemical ion-exchange reaction [41], which indicates that U(VI) sorption on betonite should be attributed to chemical sorption rather than physical sorption. The Csmax values obtained from the D–R model are higher than those obtained from the Langmuir model. This may be attributed to the different assumptions considered in the formulation of the isotherms. The thermodynamic parameters (ΔH0, ΔS0 and ΔG0) for U(VI) sorption on bentonite are calculated from the temperature-dependent sorption isotherms. The Gibbs free energy change (ΔG0) is calculated by the following equation:
Eq. (11) can be expressed in linear form as: logC s ¼ logK F þ n logC e
ð12Þ
where kF (mol1 − nLn/g) represents the sorption capacity when metal ion equilibrium concentration equals to 1, and n represents the degree of dependence of sorption with equilibrium concentration.
0
ΔG ¼ −RT lnK
0
ð17Þ
where K0 is the sorption equilibrium constant. Values of lnK0 are obtained by plotting in Kd versus Ce (Fig. 12) and extrapolating Ce to zero. Constants of linear fit of lnKd versus Ce for sorption of U(VI) to bentonite are listed in
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Table 1 The parameters for Langmuir, Freundlich and D–R isotherms at different temperatures. T (K)
Langmuir Csmax (mol/g) −5
298.15 318.15 338.15
9.19 × 10 9.40 × 10−5 1.08 × 10−4
Freundlich b (L/mol) 3
4.66 × 10 2.05 × 103 1.24 × 103
R2 0.996 0.996 0.992
kF (mol1 − n∙Ln/g) −3
8.93 × 10 3.75 × 10−3 4.06 × 10−3
D–R n 0.531 0.414 0.400
R2 0.967 0.966 0.969
β (mol2/kJ2) −3
4.21 × 10 2.76 × 10−3 2.30 × 10−3
Csmax (mol/g) −4
5.98 × 10 4.23 × 10−4 4.66 × 10−4
R2 0.981 0.982 0.985
solutions and may suggest some structure changes of bentonite. The result of U(VI) sorption on bentonite is a spontaneous and endothermic process [42]. 4. Conclusion
Fig. 12. Linear plots of lnKd versus Ce. pH = 5.8 ± 0.1, m/V = 1.2 g/L, I = 0.01 mol/L NaClO4.
Table 2. Its intercept with vertical axis gives the value of lnK0. Standard entropy change (ΔS0) is calculated using the equation: 0
∂ΔG ΔS ¼ − ∂T 0
! ð18Þ p
The average standard enthalpy change (ΔH0) is then calculated from the relationship: 0
0
0
ΔS ¼ ΔG þ TΔS :
ð19Þ
The values obtained from Eqs. (18) and (19) are tabulated in Table 2. The positive value of ΔH0 indicates that the sorption is endothermic. One possible explanation for this positive enthalpy is that U(VI) is dissolved well in water, and the hydration sheath of U(VI) has to be destroyed before its sorption on bentonite. This dehydration process needs energy, and so it is favored at high temperature. This energy exceeds the exothermicity of cations to attach to the solid surface [41]. The assumption indicates that the endothermicity of the desolvation process is higher than the enthalpy of sorption to a considerable extent. The Gibbs free energy change (ΔG0) is negative, as expected for a spontaneous process under the conditions applied. The value of ΔG0 becomes more negative with the increase of temperature, which indicates more efficient sorption at high temperature. At high temperature, cations are readily desolvated and hence their sorption becomes more favorable. The positive value of entropy change (ΔS0) suggests the affinity of bentonite toward U(VI) ions in aqueous Table 2 Values of thermodynamic parameters for U(VI) sorption on bentonite. T(K)
ΔG0 (kJ/mol)
ΔS0 (J/(mol∙K))
ΔH0 (kJ/mol)
298.15 318.15 338.15
−18.07 −20.56 −22.90
120.8
17.94 17.88 17.95
In this study, a local bentonite sample was purified and characterized by using FTIR and XRD to determine its chemical constituents and microstructure. Batch technique was adopted to investigate the sorption of U(VI) from aqueous solutions onto bentonite as a function of various environmental factors such as contact time, pH, ionic strength, coexisting electrolyte ions, HA and temperature under ambient conditions. The sorption of U(VI) on bentonite from aqueous solution is strongly dependent on pH values and ionic strength. At low pH, outer-sphere surface complexation or ion exchange is the main sorption mechanism, whereas inner-sphere surface complexation is the predominant sorption mechanism at high pH values. The cations and anions influence the sorption of U(VI) on bentonite obviously at pH b 7.0, and no influence is found at pH N 7.0. The sorption isotherms of U(VI) on bentonite can be well described by the Langmuir model. The thermodynamic analysis derived from temperature dependent sorption isotherms suggests that the sorption process of U(VI) on bentonite is spontaneous and endothermic. Considering the wide raw material sources, simple disposal procedure, low cost, high removal efficiency and environmental friendliness, one can draw a conclusion that bentonite may be widely used for the cost-effective treatment of U(VI)-bearing wastewaters. Besides, one can also consider using the bentonite as a candidate backfilling material for the deep geological disposal of high-level radioactive waste. Acknowledgments The work was supported by the National Natural Science Foundation of China (21107115, 21225730), Financial Grant from the China Postdoctoral Science Foundation (2012M511432) and Special Foundation for High-level Waste Disposal of China (2007-840). References [1] X.L. Tan, Q.H. Fan, X.K. Wang, B. Grambow, Environ. Sci. Technol. 43 (2009) 3115. [2] C.L. Chen, X.K. Wang, M. Nagatsu, Environ. Sci. Technol. 43 (2009) 2362. [3] D.D. Shao, D. Xu, S.W. Wang, Q.H. Fan, W.S. Wu, Y.H. Dong, X.K. Wang, Sci. China B Chem. 52 (2009) 362. [4] X.L. Tan, X.K. Wang, H. Geckeis, T. Rabung, Environ. Sci. Technol. 42 (2008) 6532. [5] Y. Zhang, Y. Li, X. Zheng, Sci. Total Environ. 409 (2011) 625. [6] D.D. Shao, Z. Jiang, X. Wang, J. Li, Y. Meng, J. Phys. Chem. B113 (2009) 860. [7] S. Panja, P.K. Mohapatra, S.C. Tripathi, P.M. Gandhi, P. Janardan, Sep. Purif. Technol. 96 (2012) 289. [8] Y. Zhang, Y. Li, J. Li, L. Hu, X. Zheng, Chem. Eng. J. 171 (2011) 526. [9] G.N. Kim, D.B. Shon, H. Park, W.K. Choi, K.W. Lee, Sep. Purif. Technol. 79 (2011) 144. [10] T.K. Rout, D.K. Sengupta, L. Besra, Int. J. Miner. Process. 79 (2006) 225. [11] J. Li, Y. Li, Q. Meng, J. Hazard. Mater. 174 (2010) 188. [12] K. Trivunac, S. Stevanovic, Chemosphere 64 (2006) 486. [13] Y. Li, Y. Zhang, J. Li, X. Zheng, Environ. Pollut. 159 (2011) 3744. [14] T. Matsuo, T. Nishi, Carbon 38 (2000) 709. [15] X.K. Wang, C.L. Chen, J.Z. Du, X.L. Tan, D. Xu, S.M. Yu, Environ. Sci. Technol. 39 (2005) 7084. [16] Y. Zhang, Y. Li, J. Li, G. Sheng, Y. Zhang, X. Zheng, Chem. Eng. J. 185–186 (2012) 243. [17] J. Li, Y. Li, J. Lu, Appl. Clay Sci. 46 (2009) 314. [18] J. Li, J. Lu, Y. Li, J. Appl. Polym. Sci. 112 (2009) 26. [19] M. Kara, H. Yuzer, E. Sabah, M.S. Celik, Water Res. 37 (2003) 224.
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