Sorption of Se(IV) on Fe- and Al-modified bentonite
- 363 Downloads
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.
KeywordsModified bentonite Se(IV) Thermal treatment Sorption mechanism
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 . 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 . It exhibits high sorption capacities for anion such as arsenate  and phosphates . 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 . 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 .
79Se (half-life 3.27 × 105 a) is one of the most important nuclides considered in the safety assessment of HLW . 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−) . Up to now, several researchers have discussed abundant materials to remove selenite such as aluminum oxide [7, 8], iron-coated sand , 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.  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) . 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).
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) .
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
Batch sorption studies
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) . 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.
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. , 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 . 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 . 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 .
Langmuir and Freundlich parameters for the sorption Se(IV) on FeOH-B, AlOH-B and Fe-B at pH 3
q max (mg/g)
Maximum sorption capacities of Se(IV) on different sorbents
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 . 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 . 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) . 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.
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.
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.
- 21.Mosser-Ruck R, Cathelineau M, Guillaume D, Charpentier D, Rousset D, Barres O, Michau N (2010) Effects of temperature, pH, and iron/clay and liquid/clay ratios on experimental conversion of dioctahedral smectite to berthierine, chlorite, vermiculite, or saponite. Clay Clay Miner 58(2):280–291CrossRefGoogle Scholar