Comparison of U(VI) removal from contaminated groundwater by nanoporous alumina and non-nanoporous alumina

Comparison of U(VI) removal from contaminated groundwater by nanoporous alumina and non-nanoporous alumina

Separation and Purification Technology 83 (2011) 196–203 Contents lists available at SciVerse ScienceDirect Separation and Purification Technology jou...

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Separation and Purification Technology 83 (2011) 196–203

Contents lists available at SciVerse ScienceDirect

Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur

Comparison of U(VI) removal from contaminated groundwater by nanoporous alumina and non-nanoporous alumina Yubing Sun a,⇑, Shitong Yang a, Guodong Sheng a, Zhiqiang Guo b, Xiaoli Tan a, Jinzhang Xu b, Xiangke Wang a a b

Key Laboratory of Novel Thin Film Solar Cells, Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei 230031, PR China School of Nuclear Science and Technology, Lanzhou University, 730000 Lanzhou, PR China

a r t i c l e

i n f o

Article history: Received 7 April 2011 Received in revised form 11 August 2011 Accepted 21 September 2011 Available online 5 October 2011 Keywords: Nanoporous alumina U(VI) Removal Groundwater

a b s t r a c t In this study, the sorption and desorption of U(VI) from contaminated groundwater by nanoporous and non-nanoporous alumina were investigated under ambient conditions. The nanoporous and non-nanoporous alumina were characterized by XRD, specific surface area analysis, TEM and potentiometric acid–base titration. The nanoporous alumina exhibited high sorption capacity, large specific surface area, high surface acidity constants, low difference of surface acidity constants and high pHPNZC (point of net zero charge) due to the nanoporous effect. The worm-like shape nanoporous alumina was transferred into the floc-like shape gibbsite after the sorption in terms of TEM images. Sorption kinetics and sorption isotherms of U(VI) on both nanoporous and non-nanoporous alumina can be interpreted by pseudosecond order kinetic model and the Langmuir model, respectively. The sorption of U(VI) on nanoporous alumina is strongly dependent on pH and independent of ionic strength, whereas U(VI) sorption on nonnanoporous alumina is dependent on pH and ionic strength. The sorption mechanism is assumed to be mainly inner-sphere surface complexation for nanoporous alumina and outer-sphere surface complexation for non-nanoporous alumina. Approximately 100% of U(VI) was desorbed from non-nanoporous alumina while only 5% of U(VI) was extracted from nanoporous alumina when the concentration of NaHCO3 was increased to 0.01 M in terms of sequential desorption experiments. The nanoporous alumina can be used as an efficient material for in situ immobilization of U(VI) from contaminated groundwater. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Uranium(VI), a chemical homolog of hexavalent actinides in subsurface environments, has attracted intense interest in multidisciplinary study because of its widespread distribution, long half-life radiation and carcinogenicity. Radioactive U(VI) can directly damage biological organization or produce reactive species (free radicals) that can subsequently react with bio-molecular when one inhales them from radiation sources, such as particles in soil, groundwater and air. For example, cancers, including lung cancer, female breast cancer, bone cancer, thyroid cancer and skin cancer, were observed in humans after exposure to radioactive contamination or ionizing radiation. Many countries, such as Sweden, Finland, Norway, USA, Canada, India, Iran and Brazil have problems with natural radioactivity in their groundwater [1]. According to the provisions of the WHO, it is recommended that the limit of uranium concentration in drinking water below 15 lg/L [2]. Total Indicative Dose (TID, <0.1 mSv/y) is also used as an additional ⇑ Corresponding author. Tel.: +86 551 5591368; fax: +86 551 5591310. E-mail address: [email protected] (Y. Sun). 1383-5866/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2011.09.050

guidance level for radionuclides in drinking water in many European Union countries [3,4]. For the sake of ecosystem stability and public health, it is necessary to decrease the concentration of U(VI) in contaminated groundwater to the permissible limits. Conventional methods such as sorption [5–7], chemical precipitation [8,9] and bio-reduction methods [3,10–15] have been widely used for the disposal of U(VI)-contaminated groundwater. However, these methods have some disadvantages such as low sorption capacity, weak chemical affinity, causing secondary contamination or requiring rigorous conditions (anaerobic, carbonate-free) [16–18]. Recent studies indicate that nanomaterials display novel behaviors not seen in analogous bulk systems [19–22], such as higher sorption capacity and stronger chemical affinity than non-nanomaterials. Um et al. [23] studied the removal of U(VI) onto nanoporous zirconium oxophosphate as a function of pH, ionic strength, U(VI) concentration and carbonate concentrations and indicated that the sorption capacity of nanoporous zirconium was higher than that of bulk zirconium. According to our literature survey, the study on the removal of U(VI) from contaminated groundwater by nanoporous alumina is still not available, especially the batch U(VI) desorption experiments.

Y. Sun et al. / Separation and Purification Technology 83 (2011) 196–203

To determine the environmental factors that affect of the sorption of U(VI) on nanoporous alumina and non-nanoporous alumina, the batch sorption and desorption experiments were conducted under various experimental conditions in this study. The objectives of this paper are: (1) to investigate the sorption kinetics and isotherms of U(VI) on nanoporous and non-nanoporous alumina; (2) to understand the nanoporous effect on the sorption–desorption behavior of U(VI) at nanoporous alumina-water interface; and (3) to discuss the sorption mechanism of U(VI) on nanoporous and non-nanoporous alumina and to estimate the possible application of these two materials in wastewater disposal.

2. Materials and methods 2.1. The composition of contaminated groundwater The U(VI)-containing groundwater was extracted from Area 2 of Oak Ridge Field Research Center (ORFRC) in Tennessee (Fig. 1), which is used as underground storage tanks for radioactive wastes generated from nuclear weapon production and nuclear power plant processing. The composition of contaminated groundwater is tabulated in Table 1. The predominant components are carbonate, including calcium carbonate, sodium carbonate and magnesium carbonate. The concentration of U(VI) in contaminated groundwater was measured to be 20 lg/L by kinetic phosphorescence analyzer (KPA-11, Chemchek instrument, Richland, WA).

2.2. Synthesis of nanoporous alumina The nanoporous alumina sample was synthesized by using selfassemble method: 100 mL aluminum Tri-sec-butoxide (ACROS Organic, US) and 20 mL pluronic P123 surfactant (BASF Corporation, NJ) as an organic precursor and a pore-structure-directing template, respectively, were completely dissolved in 95% ethanol and vigorously stirred for 24 h at room temperature. The mixture was heated at 75 °C for 6 h in polyethylene bottle and the final product was washed three times with deionized water. The organic surfactant was removed by calcinations at 450 °C in an air-flow furnace for 8 h [24]. Non-nanoporous alpha-alumina was purchased from Baikowski International Corporation (Baikowski, NJ) and used directly without any further purification.

197

2.3. Characteristics of nanoporous and non-nanoporous alumina The XRD patterns of nanoporous and non-nanoporous alumina were recorded on a Rapid-II powder diffractometer (D/RAPID II, Japan) using Co Ka radiation (k = 0.1059 nm) with a 0.02° step size and a 2 s step time over the range of 5° < 2h < 40°. The multi-point Brunauer–Emmett–Teller (BET) specific surface areas of nanoporous and non-nanoporous alumina were determined by N2 sorption–desorption isotherms at 77 K under atmospheric pressure by using the Quantachrome NOVA 4200e instruments (Quantachrome, Boynton Beach, FL). The samples were degassed for at least 16 h at 353 K prior to the measurements. The specific surface area of nanoporous alumina is measured to be 260.0 m2/g, which is 28 times higher than that of non-nanoporous alumina (9.0 m2/g). The TEM images of nanoporous and non-nanoporous alumina were investigated by the Philips CM 200UT transmission electron microscope (spherical aberration coefficient = 0.5 mm; point-topoint resolution = 0.19 nm) equipped with Noran Voyager X-ray energy-dispersive spectroscopy. The samples were mixed with methanol in an ultrasonic apparatus and superimposed on an appropriate grid of 3 mm in diameter for the observation. A drop of suspension was added on a carbon-coated copper grid (mesh size, 200 lm) and dried at room temperature. The potentiometric acid–base titration was carried by using a Mettler DL 50 programmable titrator under Ar atmosphere. 0.1 g nanoporous or non-nanoporous alumina was spiked into 50 mL 0.01 M or 0.1 M NaNO3 background electrolyte, respectively. The initial pH of the suspension was adjusted to pH  3.0 by adding 0.1 M HCl solutions, and then the suspension was titrated to pH  10.0 by 0.1 M NaOH. At last, the final suspension was titrated back to pH  3.0 by using 0.05 M HCl. 2.4. Batch sorption and desorption experiments The batch sorption of U(VI) on nanoporous and non-nanoporous alumina were investigated in 0.01 M NaNO3 solutions at a fixed pH (6.8 ± 0.1) by adding the PIPES buffer (1,4-piperazine-diethanesulfonic acid, 10 lM). It is noted that various concentrations of U(VI)bearing solutions were prepared by adding certain amount of analytical grade UO2(NO3)2 into original contaminated groundwater to achieve the desired concentrations. Sorption of U(VI) on nanoporous and non-nanoporous alumina as a function of pH

Fig. 1. The U(VI)-containing groundwater from the well of Area 2 of Oak Ridge Field Research Center (ORFRC, Oak Ridge, TN, US).

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Table 1 Components of contaminated groundwater from the Area 2 of ORFRC. Components

CaCO3

CaSO4

Ca(NO3)2

K2SO4

Na2CO3

UO2(NO3)2

MgCO3

pH

Conc. (lM)

2.75

0.9

0.25

0.1

2.0

0.05

1.1

6.8

and ionic strength in original contaminated groundwater were also investigated under batch techniques. The suspensions were agitated under gyratory shaker and then centrifuged at 8000 rpm for 30 min. The adsorbed amount of U(VI) on the nanoporous and non-nanoporous alumina were determined from the difference between the initial U(VI) concentration in solution and the final one in supernatant. The preliminary sorption experiments indicated that the sorption of U(VI) on the test tube wall was negligible under our experimental conditions.

nanoporous and non-nanoporous alumina in 0.1 M and 0.01 M NaNO3 solutions were tabulated in Table 2. The surface acidity constants of nanoporous alumina are higher than those of non-nanoporous alumina, but the difference of surface acidity constants (DpK = pK2  pK1) of nanoporous alumina is 10 times lower than that of non-nanoporous alumina, which indicates that there are more great nanoporous effect in nanoporous alumina [25]. The pHPNZC of nanoporous and non-nanoporous alumina can be defined by [26,27]:

2.5. Surface complexation model

pHPNZC ¼ 0:5  ðpK 1 þ pK 2 Þ

The diffuse double layer model (DDLM) with the aid of the software PHREEQC with LLNL database was employed to elucidate the sorption and desorption of U(VI) on nanoporous and non-nanoporous alumina. 3. Results and discussion 3.1. Characterization of nanoporous and non-nanoporous alumina The XRD patterns of nanoporous and non-nanoporous alumina before and after U(VI) sorption are shown in Fig. 2. The low intensity and widen peaks (2h = 21.8°, 29.4°) in the XRD pattern of nanoporous alumina indicate that the formation of amorphous nanoparticle with poor crystallinity. The peaks of non-nanoporous alumina at 2h = 11.8°, 16.1°, 17.2°, 19.7° and 25.7° are corresponded to the characteristics of corundum (a-Al2O3). Acid–base titration was conducted to determine the surface charge, the surface acidity constants and the pHPZC (point of zero charge) of alumina suspension in various background electrolyte solutions. Surface charge of nanoporous and non-nanoporous alumina was calculated by:

Q ðmol=kgÞ ¼

C A  C B  ½Hþ  þ ½OH  A

ð1Þ

D

(324)

(311)

C (002) (110)

The pHPNZC of nanoporous alumina is one unity higher than that of non-nanoporous alumina. The TEM and selected area electron diffraction (SEAD) images of nanoporous and non-nanoporous alumina before and after sorption are shown in Fig. 3. The two rings of SEAD images (Fig. 3A) before sorption clarified that the amorphous nanoparticle of nanoporous alumina was formed by using self-assemble method. By combining the TEM images with SEAD images, one can deduce that the worm-like shape nanoporous alumina (Fig. 3A) was transferred to the floc-like shape nanoporous gibbsite (Fig. 3B) after sorption. However, the TEM images of non-nanoporous alumina showed that there is little change of the non-nanoporous alumina morphology before and after sorption (Fig. 3C and D). 3.2. Sorption kinetics The results of U(VI) sorption kinetics on nanoporous and nonnanoporous alumina are showed in Fig. 4. One can see that the sorption of U(VI) on nanoporous alumina achieves the sorption equilibrium within 1 h, which is faster evidently than that of U(VI) on non-nanoporous alumina (about 20 h). According to sorption kinetics, one can see that the maximum amounts of nanoporous and non-nanoporous alumina are 28 lmol/g and 18 lmol/g after 48 h, respectively, under the experimental conditions. For model fitting, pseudo-second order kinetic model (Eq. (3)) were employed to compare the kinetic sorption behaviors [28]:

t 1 t ¼ þ qt K 2 q22 q2

ð3Þ

where qt, q2 refer to he sorption amount (mg/g) at time t and equilibrium sorption amount (mg/g) of pseudo-second order kinetic model respectively. K2 is sorption rate constants (g mg1 min1) of pseudo-second order kinetic model. The fitting results are showed in Fig. 4 and tabulated in Table 3. The results indicate that sorption of U(VI) on nanoporous and non-nanoporous alumina are specific suited to pseudo-second order kinetic model. The k2 values of nanoporous alumina (4.06) is more than two order of magnitudes than that of non-nanoporous alumina (0.0223), which further implies

(300)

(024)

(116)

(113) (110)

(012)

(104)

where CA and CB denote the acid concentration and base concentration respectively (mol/L), A refers to solid mass concentration (kg/L). The intrinsic surface acidity constants (pK1 and pK2 values) of

ð2Þ

B

A Table 2 The surface acidity constants of nanoporous and non-nanoporous alumina.

5

10

15

20

25

30

35

40

2θ Fig. 2. The XRD patterns of nanoporous and non-nanoporous alumina before and after U(VI) sorption. A and B refer to nanoporous alumina before and after U(VI) sorption respectively; C and D are non-nanoporous alumina before and after U(VI) sorption respectively.

Samples

I(NaNO3)

pK1

pK2

pHPNZC

DpK

Nanoporous Alumina Non-nanoporous Alumina

0.01 0.1 0.01 0.1

10.8 10.8 8.9 8.8

11 11 10.6 10.8

10.9 10.9 9.7 9.8

0.2 0.2 1.7 2.0

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Fig. 3. The TEM images of nanoporous and non-nanoporous alumina before and after U(VI) sorption. A and B are the nanoporous alumina before and after U(VI) sorption respectively, inserted is selected area electron diffraction (SAED) image; C and D are the non-nanoporous alumina before and after sorption respectively. The SAED images of non-nanoporous alumina no shown due to natural alpha alumina was used in this study and its morphology without any change.

28

Nanoporous alumina

24

Samples

Non-nanoporous alumina

20

Qe(umol/g)

Table 3 Model parameters of U(VI) sorption on nanoporous and non-nanoporous alumina in 0.01 M NaNO3 solutions.

16

Nanoporous alumina Non-nanoporous alumina

12

Pseudo-second order kinetic model

Langmuir model

k2 (g mg1 min1)

R2

ka (L/ mmol)

qmax (mg/g)

R2

4.06

1.0

2.14

11.6

0.966

0.0223

0.95

0.64

3.99

0.997

8 4 0 0

6

12

18

24

30

36

42

48

Time(h) Fig. 4. Sorption kinetics of U(VI) sorption on nanoporous alumina and non4 nanoporous alumina. CðUO2þ mol=L, pH = 6.8 ± 0.1, I = 0.01 M NaNO3, 2 Þ ¼ 1:0  10 m/V = 10.0 g/L and T = 25 °C.

that there are significant difference between nanoporous and nonnanoporous alumina because of the nanoporous effect. Nanoporous effect generally refers to occurrence of higher surface site concentration on nanoporous surface than that of an unconfined mineral–water interface [25] due to evident decrease of the difference between surface acidity constants (DpK = pK2  pK1).

The change in surface acidity constants results in a shift of ion sorption edges and also enhances ion sorption on nanopore surfaces, thus leading to the preferential enrichment of ions on nanopore surfaces [29]. The nanoporous effect modifies water properties, such as the viscosity [30,31], heat capacity [32], activation energy [33] and dielectric constants. The water activity (aw) and dielectric constants of solvent water were found to be higher than those of bulk water [32,34,35]. The molecular dynamics simulations showed that the dielectric constant of solvent water (affect Born solvation energy of both cations and anions) confined in a nano-dimensional spherical cavity was lower than that of bulk water [29,36]. On the other hand, the density of solvent water was found to be lower than that of bulk water. Therefore, the water activity in nanopores is much lower than that of bulk water. The reduction in water activity has two fundamental impacts on desorption behaviors. First, it reduces the hydration of aqueous species and therefore increases the possibility for inner-sphere surface complexation on nanopore surface.

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physical–chemical affinity and controlled the reactivity of a large fraction of solid-associated U(VI). Thereby nanoporous alumina can be used as effective material for retardation or immobilization of U(VI) migration.

12 Nanoporous alumina

Adsorbed U(VI)(umol/g)

10

8

3.4. Effect of pH

6 Non-nanoporous alumina

4

2

0 0

10

20

30

40

50

60

U(VI) in solution (umol/L) Fig. 5. Sorption isotherms of U(VI) on nanoporous and non-nanoporous alumina, 6 CðUO2þ —1:0  103 mol=L, pH = 6.8 ± 0.1, I = 0.01 M NaNO3, m/ 2 Þ ¼ 1:0  10 V = 10.0 g/L and T = 25 °C.

Second, the release of U(VI) from nanoporous alumina requires much more salvation energy than that of non-nanoporous alumina. A similar mechanism was also proposed for non-electrostatic exclusion of ions in thin water films [37]. Thereby, solute water in nanopores reduces uranyl and/or uranyl-carbonate solvation and therefore increases U(VI) sorption and chemical affinity, and thereby decreases U(VI) desorption from nanoporous alumina. 3.3. Sorption isotherms Fig. 5 shows the sorption isotherms of U(VI) on nanoporous alumina and non-nanoporous alumina. The sorption of U(VI) on nanoporous alumina increases evidently with increasing U(VI) concentration relative to non-nanoporous alumina. The steadystate of U(VI) sorption on nanoporous alumina was not found under our experimental conditions due to the formation of precipitates (such as schoepite, UO32H2O) at high initial U(VI) concentration (1.0  103 mol/L). Generally, sorption behaviors exhibited fast surface complexation and slow diffusion processes. Hsi and Langmuir [38] interpreted that the rapid uptake represented outer-sphere sorption by ion pair formation, whereas the slow step was the result from the diffusion into the matrix and alteration into inner-sphere. The U(VI) sorption on nanoporous and non-nanoporous alumina were regressively analyzed with the Langmuir model:

Langmuir model

Ce 1 Ce ¼ þ C s K a qmax qmax

ð4Þ

where Ka (L/mmol) and qmax (mg/g) are the sorption coefficient related to energy of sorption and the sorption capacity at saturation respectively. One can see that the sorption of U(VI) on nanoporous and nonnanoporous alumina follow Langmuir model in terms of their correlation coefficients (R2) (Fig. 5). The results strongly suggested that the sorption of U(VI) on nanoporous alumina and non-nanoporous alumina were assumed that there was no interaction between the adsorbate molecules and the adsorption was localized in a monolayer. The parameters of Langmuir models of U(VI) sorption on nanoporous and non-nanoporous alumina were also tabulated in Table 3. The results indicated that sorption capacity of nanoporous alumina was higher than that of non-nanoporous alumina, which was in good agreement with the results of maximum amount in terms of sorption kinetics. Occurrence of nanopores in nanoporous alumina surface exhibited very strong

The pH in aqueous solution is one of the important factors that influences the species and sorption of U(VI) in water–mineral interface. The predominant surface complexation reactions of U(VI) in aqueous solution are tabulated in Table 4. According to those reactions and log K values, the distribution of U(VI) species as a function of pH are illustrated in Fig. 6. The dominant species was found to be UO2þ at pH < 4, the UO2CO3 species was found 2 at pH  4.5–5.5 under our experimental conditions, and the aqueous U(VI)-carbonate complexes such as Ca2UO2(CO3)3 species was observed in the alkaline region (pH > 7.0). The sorption of U(VI) on nanoporous and non-nanoporous alumina as a function of pH are shown in Fig. 7. U(VI) sorption on nanoporous and non-nanoporous alumina increased with increasing pH from 1 to 6 due to the depleted electrostatic repulsion between UO2þ ions and alumina surface with positive charge 2 (pHPZC  9 from titration data). Interested enough, almost 100% of U(VI) was adsorbed by nanoporous alumina at pH  6.0 and maintained the steady-state at pH  6.0–10.0. About 90% of U(VI) was removed by non-nanoporous alumina at pH  7.5 and then remarkably decreased with increasing pH from 7.5 to 11.0. The pH-dependent sorption results determined that the extent of U(VI) sorption was particularly sensitive to carbonate because of the formation of aqueous U(VI)-carbonate complexes and hydrated U(VI) species at high pH values. For model fitting, the two sites DDLM with surface complexation constants listed in Table 5 was employed to simulate U(VI) sorption and the results were shown in Fig. 7. One can see that the modeled results of U(VI) sorption on non-nanoporous alumina and nanoporous alumina were consistent with experimental data very well by using two sites surface complexation. For non-nanoporous alumina, at pH < 5.0, the predominant surface species was þ found to be BAlsOUO2 species, which was in good agreement with 2þ the dominance UO2 species at pH < 5.0 (Fig. 6), and the þ BAlwOUO2 species was also observed at pH > 6.0. For nanoporous þ alumina, however, the BAlsOUO2 species was depleted thoroughly at pH > 7.0. 3.5. Effect of ionic strength The sorption of U(VI) on nanoporous alumina and non-nanoporous alumina as a function of pH under various ionic strength conditions are shown in Fig. 8. The sorption of U(VI) on nanoporous alumina is dependent on pH and independent of ionic strength, whereas the sorption of U(VI) on non-nanoporous alumina is dependent on both pH and ionic strength. The ionic strength dependent sorption suggests that the chemical affinity of nanoporous alumina for U(VI) is much higher than that of non-nanoporous alumina. The influence of ionic strength on U(VI) sorption is attributed to compete the reactive sites of adsorbent with adsorbate. Table 4 The predominant complexation reactions of U(VI) in 0.01 M NaNO3 solutions. Complexation reactions þ þ UO2þ 2 þ H2 O ¼ UO2 OH þ H

log K 5.2

Reference Davis et al. (2004)

15.55

Davis et al. (2004)

9.68

Davis et al. (2004)

2 2 UO2þ 2 þ 2CO3 ¼ UO2 ðCO3 Þ2

16.94

Davis et al. (2004)

2 2Ca2þ þ UO2þ 2 þ 3CO3 ¼ Ca2 UO2 ðCO3 Þ3

30.55

Dong et al. (2006)

þ þ 3UO2þ 2 þ 5H2 O ¼ ðUO2 Þ3 ðOHÞ5 þ 5H 2 UO2þ 2 þ CO3 ¼ UO2 CO3

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10 0

Percentage of U(VI) species

UO2

Table 5 Surface complexation constants (log K) for U(VI) sorption on non-nanoporous alumina and nanoporous alumina in 0.01 M NaNO3 solution.

Ca2UO2(CO3)3

2+

80

Complexation reactions

log K

þ

AlOH þ Hþ ¼ AlOH2

60

UO2CO3

+



AlOH = AlO + H

(UO2)3(OH)5

20

þ

þ AlsOH þ UO2þ 2 ¼ AlsOUO2 þ H

+

UO2(CO3)2

+

UO2(OH) H)

Huang (1973)

9.1 2.47 Turner (1996) 7.0 This study

þ

þ AlwOH þ UO2þ 2 ¼ AlwOUO2 þ H

40

Reference

7.9

þ 17.7 AlwOH þ Ca2 UO2 ðCOÞ3 ¼ AlwOUO2 þ 2CaCO3 þ HCO 3

2-

þ

AlsOH þ Ca2 UO2 ðCOÞ3 ¼ AlsOUO2 þ 2CaCO3 þ HCO 3

Turner (1996) This study

15.7

0 2

3

4

5

6

7

8

9

10

pH

Nanoporous alumina (I=0.01M NaNO3) Nanoporous alumina (I=0.1M NaNO3) Non-nanoporous alumina (I=0.01M NaNO3) Non-nanoporous alumina (I=0.1M NaNO3)

4.5 Fig. 6. The distribution of aqueous U(VI) species as a function pH in 0.01 M NaNO3 2þ solutions. CðUO2þ Þ ¼ 3:9 mmol=L, CðUO2 2 Þ ¼ 0:5 mmol=L, CðCa 3 Þ ¼ 5:85 mmol=L, and T = 25 °C.

4.0 3.5

Non-nanoporous alumina

90

Percentage of adsorbed U(VI)

log Kd (ml/g)

100

80 70

2.5 2.0

60

1.5

50

1.0

40

0.5

30

1

2

3

4

5

+

AlsOUO2

20

6

7

8

9

10

11

pH

+

AlwOUO2

10

Fig. 8. Sorption of U(VI) on nanoporous alumina and non-nanoporous alumina as a 4 function of pH at various ionic strengths. CðUO2þ mol=L, m/V = 10.0 g/L 2 Þ ¼ 1:0  10 and T = 25 °C.

0 1

2

3

4

5

6

7

8

9

10

11

pH

sorption complexation otherwise it belongs to inner-sphere sorption complexes. Combining the acid–base titration results that the surface acidity constants (pK1 and pK2 values) of nanoporous alumina do not change with increasing ionic strength (Table 2), one can deduce that the mechanism of U(VI) sorption on nanoporous alumina is predominately by inner-sphere surface complexation, while the sorption of U(VI) on non-nanoporous alumina is assumed to be dominated by outer-sphere surface complexation.

100 Nanoporous alumina

90

Percentage of adsorbed U(VI)

3.0

+

AlwOUO2

80 70 60 50

3.6. Batch desorption experiments

40 30 20 +

AlsOUO2

10 0 2

3

4

5

6

7

8

9

10

11

pH Fig. 7. The sorption of U(VI) on nanoporous and non-nanoporous alumina as a 4 function of pH, CðUO2þ mol=L, I = 0.01 M NaNO3, m/V = 10.0 g/L and 2 Þ ¼ 1:0  10 T = 25 °C, solid lines are DDLM model.

Thereby, the sorption of U(VI) onto non-nanoporous alumina decreases with increasing ionic strength. However, the effect of ionic strength on U(VI) sorption onto nanoporous alumina can be ignored due to the occurrence of substantial reactive sites on nanoporous alumina surface. It is reported that strongly ionic strengthdependent sorption behavior may be predominant by outer-sphere

The batch desorption experiments were conducted to determine the chemical affinity of nanoporous alumina with U(VI) by using sodium bicarbonate (NaHCO3) solution. NaHCO3, an amphoteric compound, are mildly alkaline in aqueous solution due to the formation of carbonic acid (H2CO3) and hydroxide ion (OH). Thereby, the H2CO3 species complexed easily with UO2þ 2 species that were adsorbed on alumina surface in presence of OH ion. The desorption of U(VI) from nanoporous and non-nanoporous alumina as a function of carbonate concentration was investigated by using sequential desorption experiments. Approximately 100% of U(VI) was extracted from non-nanoporous alumina, but only 5% of U(VI) was desorbed from nanoporous alumina when concentration of NaHCO3 solution was increased to 0.01 M (Fig. 9). The sequential desorption results indicate that nanoporous alumina exhibits strong chemical affinity relative to non-nanoporous alumina. The DDLM was also employed to model the desorption of U(VI) from nanoporous and non-nanoporous alumina. The modeling results are consistent with the experimental data very well (Fig. 9).

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100

Percentage of desorbed U(VI)

90 80 70 60 50

Non-nanoporous alumina Nanoporous alumina

40 30 20 10 0 0

1

2

3

4

5

6

7

8

9

10

[NaHCO3](mmol/L) Fig. 9. The desorption of U(VI) from nanoporous alumina and non-nanoporous alumina as a function of NaHCO3 concentrations, CðUO2þ 2 Þ ¼ 28:995 lmol=g, I = 0.01 M NaNO3, pH = 6.8 ± 0.1, m/V = 10.0 g/L and T = 25 °C. Solid lines are the DDLM simulation.

Table 6 Comparison of sorption capacities of U(VI) on various adsorbents. Adsorbents

Conditions

Hematite

pH = 4.0, I = 0.1 M NaNO3 pH = 7.0, I = 1.0 M NaNO3 pH = 4.5, I = 0.1 M NaNO3 pH = 4.5, I = 0.1 M NaNO3 pH = 5.0, I = 0.01 M NaClO4 pH = 4.5, I = 0.01 M NaNO3

Montmorillonite Goethite Ferrihydrite Carbon nanotube Nanoporous alumina

Cs max (lmol g1)

than that of non-nanoporous alumina. Sorption of U(VI) on nanoporous alumina was strongly dependent on pH and independent of ionic strength. The sorption of U(VI) on nanoporous alumina was predominant by inner-sphere surface complexation, whereas on non-nanoporous alumina was dominated by outer-sphere surface complexation. Almost 100% of U(VI) was extracted from nonnanoporous alumina, but only 5% of U(VI) was desorbed from nanoporous alumina when the concentration of NaHCO3 was increased to 0.01 M. The sorption of U(VI) on nanoporous alumina and nonnanoporous alumina previously exposed to significantly different sorption and desorption behaviors that were presented in this manuscript to determine nanoporous alumina exhibited higher sorption capacity and stronger chemical affinity than non-nanoporous alumina. Nanoporous alumina can be used as an efficient and promising candidate for backfill barrier material in the contaminated groundwater that proposed deep-geologic radioactive waste repositories. This research provides new insights into the development of appropriate engineering materials to sequestrate U(VI) from contaminated groundwater in subsurface environments. Acknowledgements We are indebted to Dr. Huifang Xu and Hiromi Konishi for providing TEM images acquisition. This work were supported financially by the National Natural Science Foundation of China (21007074; 20907055; 20971126; 21071147), and 973 Project of MOST (2011CB933700; 2007CB936602) are acknowledged.

Reference

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8.0

Hsi et al. (1985)

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The sorption capacity of U(VI) on different adsorbents were compared under various conditions (Table 6). One can see that the sorption capacity of U(VI) on nanoporous alumina is higher than the other adsorbents under the similar conditions except for carbon nanotubes, but carbon nanotubes are found to be more expensive relative to nanoporous alumina. This result indicates that nanoporous alumina can be used as an efficient and promising backfill material for immobilization of radionuclide from contaminated groundwater under ambient conditions due to the large sorption capacity and the strong chemical affinity. 4. Conclusions The batch sorption and desorption of U(VI) from contaminated groundwater by nanoporous and non-nanoporous alumina as a function of carbonate, pH and ionic strength were investigated under ambient conditions. The nanoporous alumina exhibited high surface acidity constants, low difference of intrinsic acidity constants, strong reactivity, high pHPNZC and high sorption capacity due to the nanoporous effect. The sorption kinetics of U(VI) on nanoporous alumina and non-nanoporous alumina followed pseudosecond order kinetic model and the sorption isotherms were simulated by Langmuir model very well. The sorption capacity of nanoporous alumina was found to be approximately three times higher

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