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Journal of Radioanalytical and Nuclear Chemistry

, Volume 303, Issue 1, pp 107–113 | Cite as

Sorption of Se(IV) on Fe- and Al-modified bentonite

  • Hai Wang
  • Tao Wu
  • Jiang Chen
  • Qing Zheng
  • Chaohui He
  • Yaolin Zhao
Article

Abstract

The sorption behavior of Se(IV) on Fe- and Al-modified bentonite is studied through batch experiments. In order to introduce active centers for Se(IV) sorption, bentonite was modified with iron, iron oxocations and aluminum oxocations at 400, 600 and 800 °C. It was found that calcined temperature had great effect on the sorption with above 80 % sorption at 400 °C, whereas with only 5 % sorption at 800 °C. The sorption capacities for Se(IV) were 112.5 mg/g by FeOH-B, 60.1 mg/g by AlOH-B and 71.9 mg/g by Fe-B, respectively. The pH-dependent and ionic strength-independent Se(IV) sorption on these modified bentonites demonstrated that the sorption mechanism of Se(IV) was inner-sphere surface complexation at low pH values.

Keywords

Modified bentonite Se(IV) Thermal treatment Sorption mechanism 

Introduction

Bentonite has been selected as the candidate of backfill material for high level nuclear waste repository (HLW) because of its large specific area, low hydraulic conductivity, low cost and high cation exchange capacity for most of radionuclides. However, it had a low sorption capacity of anionic radionuclides like 79Se due to anion exclusion between negative charge surface of bentonite and anionic species. To overcome this difficulty, modified bentonites have been concerned by researchers. The surface and pore structure of montmorillonite is modified in the presence of Fe(III) species in the interlayer or by Fe(III) (hydr)oxide surface coatings. Such modifications may change sorption and desorption properties of the clay. Fe(III)-modified clay could be found in natural soils and sediments [1]. In HLW, bentonite could be altered by metallic iron and/or iron oxides because of the corrosion of the carbon steel (Fe) over pack container under anaerobic deep geological disposal conditions. Iron modified bentonite is considered as a good material to understand some basic properties such as swelling, hydraulic conductivity, tracer diffusion and sorption [2]. It exhibits high sorption capacities for anion such as arsenate [1] and phosphates [3]. For instance, Fe(III) species in Fe-montmorillite bind arsenate efficiently by forming small oligomeric species or surface clusters containing just a few iron atoms [1]. Phosphate has high sorption capacity on hydroxyl-aluminum, hydroxyl-iron and hydroxyl-iron-aluminum pillared bentonites. The sorption was highly pH-dependent, and decreased at high pH [4].

79Se (half-life 3.27 × 105 a) is one of the most important nuclides considered in the safety assessment of HLW [5]. Although selenium occurs in water in several forms depending on its oxidation state, the predominant water-soluble anionic species are selenite (HSeO3 or SeO3 2−) and selenate (SeO4 2−) [6]. Up to now, several researchers have discussed abundant materials to remove selenite such as aluminum oxide [7, 8], iron-coated sand [9], hematite [10, 11], layered double hydroxides [12, 13], NanoFe [14, 15] and so on [16, 17]. In Ref. [7, 18], it is found that Se(IV) formed inner-sphere complexes on the surface of Al oxide. Frost et al. [19] found that Se(IV) had higher sorption on montmorillonite than on kaoline because montmorillonite had greater particle edge surface area. However, materials with high sorption capacity such as layered double hydroxides and NanoFe are expensive. And some abundant geological materials like hematite have little sorption capacity (0.39 mg/g) [11]. It is necessary to investigate more useful and cost-effective sorbents to remove selenium contaminants from water.

The objectives of this article are: (a) to investigate the effect of different experimental conditions on Se(IV) sorption behavior on bentonite modified with iron, iron oxocations and aluminum oxocations; (b) to study the effect of calcined temperature on the microstructure and surface morphology of modified bentonites; (c) to understand the impact of microstructure and surface morphology of modified bentonites on the sorption property of Se(IV).

Experimental

Materials

A stock solution of Se(IV) (50 mg/L) was prepared in distilled water using SeO2(from Sinopharm Reagant). The concentration of Se(IV) was analyzed by ICP-OES (PerkinElmer Optima 2100DV).

Gaomiaozi Na-bentonite (GMZ) from the Beijing Research Institute of Uranium Geology was used. The cation exchange capacity (CEC) was 760 meq/kg. As the Na-bentonite was considered here as a precursor for new sorbent development, it was not purified. To activate bentonite, they were prepared by calcinating for 4 h at 400 °C after being soaked for 12 h in 20 % H2SO4.

Synthesis of sorbent

10 g activated bentonite were mixed with 500 mL water. After stirring for 24 h, a 0.1 mol/L FeCl3 solution was added to the clay suspension and made the Fe/clay ratio reach 10 mmol Fe3+ per gram clay. After shaking the above solution for another 24 h, it was washed until the supernatant without Cl (checked by AgNO3 solution). After drying at 50 °C, the clay was then calcined at 400, 600 and 800 °C for 4 h and filtered by 200 mesh sieve, respectively. Similar methods were used to make iron oxocations modified bentonite (FeOH-B) and Al oxocations modified bentonite (AlOH-B) [26].

Sorption experiments

The sorption of Se(IV) on the sorbents were investigated by batch technique in laboratory at ambinet condition in 0.1 mol/L NaClO4 solutions. All experiments were conducted in polyethylene tubes. The sorbents were first equilibrated with electrolyte NaClO4 for 2 days. The Se(IV) solution was added into suspension to start the sorption. 0.1 mol/L and 0.01 mol/L HCl or NaOH were used to achieve the desired pH of the aqueous suspensions. The test tubes were shaken for 3 days to achieve the sorption equilibration and then centrifuged at 5,000 rpm for 30 min to separate the solid phase from the liquid one. The concentration of Se(IV) sorbed on sorbents is expressed in Eq. 1.
$$ W = (C_{0} - C_{\rm{eq}} )/C_{0} \times 100\,\% . $$
(1)
where, C 0 and C eq are the initial and equlibirium concentration of Se(IV) in the solution (mg/L), respectively.

Characterization

Powder X-ray diffraction (XRD) patterns for modified bentonites were recorded on Beijing Purkinje General Instrument X-ray 6 with CuKα radiation (λ = 0.15406 nm). Samples were scanned for 2θ ranged from 3° to 80°. The surface morphology of sorbents was determined by scanning electron microscope (Hitachi S-3400N) coupled with energy dispersive spectrometer.

Results and discussion

Characterization of sorbents

The XRD diffraction patterns for sorbents are presented in Fig. 1. XRD patterns of activated and modified bentonites showed the peaks at 2θ diffraction angle of 6.11º and 19.98º, indicating the formation of pure phase of montmorillonite. The most pronounced modification occurred in the montmorillonite d(001) peak showed the reduction in intensity for activated bentonite comparing to original bentonite, and the peaks shift to lower diffraction angle with 6.01, 5.75 and 5.67ºfor AlOH-B, FeOH-B and Fe-B. The reduction in intensity and increase in width of peak indicates that the crystalline of montmorillonite is considerably affected by acid [20] and 400 °C activation temperature [21, 22]. The H2O contained between the 2:1 layers of montmorillonite would be lost irreversibly at 400 °C. The acid could clear impurities covering the active sorption sites and form active acid sites on montmorillonite. The acid can also decrease the crystallinity of montmorillonite. Comparison of Fe-B XRD diffraction pattern of the three different calcination temperatures (Fig. 1b) reveals no major differences between 400 and 600 °C except that the characteristic peaks of montmorillonite disappeared at 800 °C. It is in accordance with previously reported results [22, 23]. The SEM images Fig. 2(a–d) indicated that the surface of AlOH-B and FeOH-B were flocculent and Fe-B surface was platelike. Montmorillonite flakes were found in the surface of original bentonite (Fig. 2a) and Fe-B (Fig. 2c). One can also find some phase deposited on the flakes of the mineral from SEM of FeOH-B (Fig. 2d) and AlOH-B (Fig. 2b). These phases might be the agglomerate of crystals of iron oxides as found by Nguyen et al. [26] on the surface of their FeOH-B and Fe-B materials.
Fig. 1

XRD pattern of Fe-B, AlOH-B, FeOH-B, actived GMZ with 400 °C calcined temperature and GMZ (a) and Fe-B with 400, 600 and 800 °C calcined temperature (b). a′ is enlarged view on the small diffraction angle part of a

Fig. 2

SEM image of bentonite a AlOH-B, b Fe-B, c FeOH-B, d with 400 °C calcined temperature

Batch sorption studies

The sorption of Se(IV) was investigated on sorbents as a function of calcination temperature in the range of 400–800 °C (Fig. 3).
Fig. 3

Sorption of Se(IV) as a function of calcined temperature (initial concentration 50 mg/L, volume of solution 20 mL, sorbent 0.1 g, NaClO4 0.1 mol/L, pH 3)

It was observed that the sorption of Se(IV) on FeOH-B, AlOH-B and Fe-B decrease dramatically with increasing of calcination temperature. At 400 °C, the sorption on FeOH-B, AlOH-B and Fe-B were 98.1, 78.9 and 83.5 %, while the corresponding sorption on them at 800 °C were 2.7, 3.5 and 5.5 %, respectively. It can be explained as follows. Calcination could break the crystal structure and decrease the specific surface area and adsorbability. Destruction of the hydromica began in the vicinity of iron ions at a temperature of 500 °C, the layer structure of bentonite retained when it was calcined at 350–550 °C [24, 25]. The sorption capacity is relatively high at 400 °C. Therefore, sorbents under 400 °C condition were selected for follow-up experiments. When the temperature increased from 400 to 800 °C, the sorption on FeOH-B and Fe-B decrease faster than that on AlOH-B. It could be explained that FeOH-B and Fe-B structure is broken more quickly than AlOH-B with increasing of temperature. In addition, the sorption of Se(IV) on FeOH-B at 400 °C calcination temperature was higher than that on Fe-B. SEM images showed that the surface of FeOH-B was flocculence and Fe-B surface was platelike in Fig. 2a–d. The specific area value of FeOH-B (160 m2/g) was larger than that of Fe-B (63 m2/g) [26]. This may be the reason why FeOH-B has higher sorption ability.

The sorption of Se(IV) on sorbents are quick processes and 1 h is enough to achieve the equilibrium of Se(IV) sorption.

The effect of sorbent quantity on the sorption of Se(IV) was studied, as shown in Fig. 4. It was observed that the sorption increased from 33 to 100 % on FeOH-B, 19.3 to 85.2 % on Fe-B and 21 to 78.4 % on AlOH-B with increasing sorbent quantity from 0.004 to 0.1 g, respectively.
Fig. 4

Sorption of Se(IV) as a function of sorbent quantity (initial concentration 50 mg/L, volume of solution 20 mL, NaClO4 0.1 mol/L, pH 3)

The pH of electrolyte is one of the crucial parameters, which has significant influence on the sorption of Se(IV). The effect of pH was investigated in the range of pH 3–11 (Fig. 5). The sorption of Se(IV) on original montmorillonite is also studied as a function of pH under the initial concentration 3.5 mg/L, solid-to-liquid ratio 0.1 g/20 mL and 0.1 M NaClO4. The obtained sorption percentage is less than 20 % in the range of pH 3–10. It is shown that the sorption of Se(IV) on original montmorillonite is very weak. By the comparison of pH-Se(IV) sorption data of original montmorillonite with those of FeOH-B, AlOH-B and Fe-B modified materials in Fig. 5, one can find that modified materials has higher sorption rate than original montmorillonite in acid conditions.
Fig. 5

Sorption of Se(IV) as a function of pH (initial concentration 50 mg/L, volume of solution 20 mL, sorbent 0.1 g, NaClO4 0.1 mol/L)

For FeOH-B and AlOH-B, the pH in range of 3–6.5 had little effect on the sorption of Se(IV), whereas the sorption was dramatically decreased with increasing of the pH from 7 to 11.For Fe-B, the sorption was decreased from pH 4 to 11. The sorption of Se(IV) on FeOH-B and AlOH-B were decreased from 99.4 to 33.9 % and 63.9 to 5.9 % respectively, when the pH was increased from 6.5 to 11. While the sorption of Se(IV) on Fe-B was decreased from 65.4 to 2.7 % with increasing the pH from 4 to 11. It can be explained that the surface of the particle has more H+ at low pH and carried a positive charge. Consequently, it has higher sorption capacity. With increasing of the pH, the surface of the particle will become less positively charged resulting in a lower binding affinity for anion. These are consistent with the results by Ararem et al. [27], who showed that the points of zero charge (PZC) of iron pillared layered montmorillonite and α-FeOH are at pH 3.8 and 6.8. The PZC of Al-pillared montmorillonite is at pH 6.1 [28]. However, the mechanism of Se(IV) sorption on modified montmorillonites should be described in more details. The plausible interpretation of Se(IV) sorption on FeOH-B, Fe-B and AlOH-B may be obtained under further investigation at the molecular level using spectroscopic techniques such as EXAFS and XPS.

In order to investigate the influence of background electrolyte ions on Se(IV) sorption, the sorption of Se(IV) on sorbents was studied as a function of ion strength from 0.001 mol/L to 0.5 mol/L NaClO4 at pH 3. It showed little influence on the sorption of Se(IV) on FeOH-B, Fe-B and AlOH-B, indicating that Se(IV) forms inner-sphere complexations on these modified bentonites. It agrees with previously published results, Se(IV) formed bidentate inner-sphere surface complexes on Al-oxides, Fe-oxides and Fe-(hydr)oxides [18, 29]. And Se(IV) forms inner-sphere bidentate-binuclear (corner-sharing) and some outer-sphere surface complexes on hydrous aluminum oxide [8]. The sorption of Se(IV) on hydrous aluminum as well as clay minerals occur via ligand exchange, or the replacement of oxygen ions on hydrous oxide surfaces by anions [30].

Sorption modeling

The Langmuir and Freundlich models are generally used to describe the sorption characteristics and to establish the relationship between the amount of Se(IV) sorbed on sorbents and the concentration of Se(IV) remained in solution. The linear equation of Langmuir sorption isotherm supposes that sorption occurs in a monolayer with all sorption sites identical and energetically equivalent [30]. It is expressed as follows:
$$ q_{\rm{e}} = K{\rm{q}}_{\hbox{max} } C_{\rm{eq}} /(1 + KC_{\rm{eq}} ) . $$
(2)
where q e (mg/g) is the amount of Se sorbed after equilibrium; C eq (mg/L) is the equilibrium concentration of Se remained in solution; q max (mg/g), the maximum sorption capacity, is the amount of sorbate at complete monolayer coverage, and K (L/mg) is a constant that relates to the heat of sorption. The Freundlich sorption isotherm, which is a semi-empirical equation based on the sorption phenomenon occurring on heterogeneous surfaces, is expressed by following linearized equation:
$$ \log (q_{\rm{e}} ) = \log K_{\rm{F}} + (1/n)\log (C_{\rm{e}} ) . $$
(3)
where K F and 1/n are Freundlich constants related to sorption capacity and sorption intensity. The Langmuir and Freundlich isotherms for Se(IV) are shown in Fig. 6a, b.
Fig. 6

Langmuir (a) and Freundlich (b) isotherms for the sorption of Se(IV) on FeOH-B,AlOH-B and Fe-B at pH 3

The correlative coefficients R2 obtained from fitting the experimental isotherm data indicated that Langmuir model fit the data better than Freundlich model (see Table 1) because an exponential increase is supposed in the Freundlich model. A large value of K illustrates strong binding of Se(IV) ions on FeOH-B and we can get same answer from large KF coefficient of Freundlich model [31].
Table 1

Langmuir and Freundlich parameters for the sorption Se(IV) on FeOH-B, AlOH-B and Fe-B at pH 3

Sorbent

Langmuir

Freundlish

q max (mg/g)

K (L/mg)

R 2

K F

1/n

R 2

FeOH-B

112.5

0.086

0.952

19.34

0.431

0.954

AlOH-B

60.1

0.020

0.970

1.68

0.762

0.929

Fe-B

71.90

0.021

0.976

2.02

0.777

0.954

The monolayer capacity q max (mg/g) calculated from the Langmuir equation was higher for FeOH-B than those for AlOH-B and Fe-B. It demonstrated that FeOH-B (112.5 mg/g) had higher sorption capacity for Se(IV) than AlOH-B (60.1 mg/g) and Fe-B (71.9 mg/g) (Table 1).Table 2 shows a comparison of Sorption capacities from the present study with those from previous studies where different materials were used as sorbents.
Table 2

Maximum sorption capacities of Se(IV) on different sorbents

Material

pH

Maximum

sorption (mg/g)

Reference

Goethite

4

0.52

[11]

Hematite

4

0.39

[11]

Magnetite

4

15.06

[2]

Rice husk

1.5

26.46

[32]

Aluminum-oxide-coated sand

4.8

1.08

[7]

Fe3O4 nano-materials

 

2.38

[14]

FeOOH

5

26.3

[17]

FeOH-B

3

112.5

This study

AlOH-B

3

60.1

This study

Fe-B

3

71.9

This study

Some materials, such as FeOH and layered double hydroxides, have relatively large surface areas (0.02–0.12 km2/kg) and high anion exchange capacities (200–500 cmol/kg), however they are mostly synthesized under laboratory conditions and expensive [13]. Although goethite and hematite are present in many natural media and have been extensively researched as sorbent for a number of trace elements, their sorption capacity for Se(IV) are low [11]. Iron and aluminum oxide coated sand are made by mixing metal salt solution with sand and could effectively remove heavy metals but they also had low sorption capacity for Se(IV) [7]. Compared with the above materials, FeOH-B is cheap and has higher sorption capacity for Se(IV). Therefore, it has a potential application as the backfilling material in HLW to prevent 79Se(IV) migration to the geosphere.

Conclusion

Modified Bentonites were prepared, characterized and used for sorption of Se(IV) ions. It was demonstrated that calcinations temperature, pH and the sorbent dose played important roles in the sorption of Se(IV) on the modified materials. It was found that the sorption of Se(IV) on modified bentonites were decreased with the increasing of calcination temperature in the range of 400–800 °C, XRD results showed that the crystal structure of montmorillonite was destroyed at 800 °C. SEM results showed that the surface of AlOH-B and FeOH-B were flocculent and Fe-B surface was platelike. The sorption capacities for Se(IV) were 112.5 mg/g on FeOH-B, 60.1 mg/g on AlOH-B and 71.9 mg/g on Fe-B, respectively. FeOH-B has a potential application as the backfilling material in HLW to prevent 79Se(IV) migration to the geosphere. The sorption capacity could be improved by further investigations. Further work to assess the effect of surface structure on sorption capacity is ongoing in our laboratories and would be disseminated in subsequent publications. Since the surface charge of bentonites modified by iron oxocations become positive at low pH, it can also be used in the sorption of other anion, such as 129I, 99TcO4 , 37Cl, and so on.

Notes

Acknowledgments

The authors gratefully acknowledge the financially supported by National Natural Science Foundation of China (Grant No. 11275147 and Grant No. 21207035) and Project supported by the Scientific Research Starting Foundation for Returned Overseas Chinese Scholars, Ministry of Education, China.

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Copyright information

© Akadémiai Kiadó, Budapest, Hungary 2014

Authors and Affiliations

  • Hai Wang
    • 1
    • 2
  • Tao Wu
    • 2
  • Jiang Chen
    • 3
  • Qing Zheng
    • 2
  • Chaohui He
    • 1
  • Yaolin Zhao
    • 1
  1. 1.School of Nuclear Science and TechnologyXi’an Jiaotong UniversityXi’anPeople’s Republic of China
  2. 2.Department of ChemistryHuzhou Teachers CollegeHuzhouPeople’s Republic of China
  3. 3.Huzhou Environmental Protection Monitoring CenterHuzhouPeople’s Republic of China

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