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Absorption of the selenite anion from aqueous solutions by thermally activated layered double hydroxide
water research 43 (2009) 1323–1329
Available at www.sciencedirect.com
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Absorption of the selenite anion from aqueous solutions by thermally activated layered double hydroxide Rui Liua,b, Ray L. Frosta,*, Wayde N. Martensa a
Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland University of Technology, 2 George Street, GPO Box 2434, Brisbane, Queensland 4001, Australia b Changchun Institute of Technology, Changchun Jilin130021, China
article info
abstract
Article history:
The presence of selenite or selenate in potable water is a health hazard especially when
Received 8 September 2008
consumed over a long period of time. Its removal from potable water is of importance. This
Received in revised form
paper reports technology for the removal of selenite from water through the use of ther-
2 December 2008
mally activated layered double hydroxides.
Accepted 2 December 2008
Mg/Al hydrotalcites with selenite in the interlayer were prepared at different times from 0.5 to
Published online 3 January 2009
20 h through ion exchange. X-ray diffraction of the MgAlSeO3 hydrotalcites indicates that the
Keywords:
was formed. Raman spectra proved the presence of selenite anion in the hydrotalcite inter-
Water purification
layer as the counter anion. The band intensity and width of MgAlSeO3 hydrotalcite in the
Potable water
region of 3800–3000 cm1 increase with the adsorption of selenite by the Mg/Al hydrotalcite.
selenite anion entered the interlayer spacing of Mg/Al hydrotalcite and MgAlSeO3 hydrotalcite
Thermally activated hydrotalcites
The characteristic bands of free selenite anions in the MgAlSeO3 hydrotalcites are located
Selenite adsorption
between the region between 850 and 800 cm1. The Raman spectra of the lower wave number region of 550–500 cm1 show a shift toward higher wave numbers with adsorption of the selenite. An estimation of the amount of selenite anion removed by the thermally activated layered double hydroxide was obtained through the measurement of the intensity of the selenite Raman bands at 814 and 835 cm1 resulting from the amount of selenite anion remaining in solution. Thermally activated LDHs provide a mechanism for removing selenite anions from aqueous solutions. ª 2008 Elsevier Ltd. All rights reserved.
1.
Introduction
Layered double hydroxides (LDH), also known as hydrotalcites or anionic clays have received increasing attention in recent years due to their wide range of applications as anion exchangers, adsorbents, ionic conductors, catalyst precursors and catalyst supports (Miyata, 1983; Mitchell and Wass, 2002; Scavetta et al., 2005). Hydrotalcites are naturally occurring minerals derived from a brucite structure (Mg(OH)2) in which e.g. Al3þ or Fe3þ substitutes for part of the Mg2þ. This substitution creates a positive layer charge on the hydroxyl layers,
which is compensated by interlayer anions or anionic complexes (Frost et al., 2007). These anions may be any anion with a suitable negative charge including the selenite anion (Hou and Kirkpatrick, 2000). Selenium occurs naturally in a number of inorganic forms, including selenide, selenate and selenite (Tsujia et al., 2000). For most aqueous medium, selenium exists in two primary oxidation states: Se(IV) usually as (selenite), and Se(VI) usually HSeO3 (biselenite) or SeO2 3 2 present as SeO2 4 (selenate) (Das et al., 2002). Selenite (SeO3 ), ubiquitous at various kinds of environment of soils, water and air fields (Peak et al., 1997), is an oxyanion of environmental
* Corresponding author. Tel.: þ61 7 3138 2407; fax: þ61 7 3138 1804. E-mail address: [email protected] (R.L. Frost). 0043-1354/$ – see front matter ª 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2008.12.030
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importance due to its toxicity to animals at higher concentrations, notably waterfowl and grazing animals. Selenite in potable water is toxic to humans. One means of studying the uptake of the selenite anion is to use Raman spectroscopy. Raman spectroscopy is especially sensitive to oxyanions. Raman spectroscopy for chemical analysis combines many of the advantages of FTIR and NIR absorption, having some added features based on resonance and/or surface enhancement, polarization measurements, compatibility and high spectral resolution (Montea et al., 2001). Raman spectroscopy, enables the identification of the interlayer anions and may be used to identify mixtures of anions in the interlayer of hydrotalcite (Kloprogge et al., 2004; Frost et al., 2005, 2006; Palmer et al., 2008). In this research, we present Raman spectroscopy on the selenite adsorption behavior upon hydrotalcite in the interlayer and explore the adsorption kinetics using a combination of X-ray diffraction and Raman spectroscopy. In this paper, we show that a thermally activated synthetic hydrotalcites can be used to remove selenite anions from aqueous solution.
2.
Experimental
2.1.
Synthesis of Mg/Al hydrotalcite
solution (0.1 M, 436 mL, pH w 12.5) and stirred under a nitrogen atmosphere. Aliquots (80 mL) were taken out at regular intervals. The mixture was stirred continuously and aliquots of solution were taken at intervals from 30 min to 20 h, filtered and dried before subsequent analysis. The resultant solution was vacuum filtered and rinsed two times with freshly decarbonated water. By using the hydrotalcite reformation effect, the selenite entered the interlayers of the thermally activated hydrotalcites and formed the MgAlSeO3 hydrotalcites. The hydrotalcites were left to dry in a vacuum desiccator to minimize exchange with carbon dioxide from the air.
2.3.
The samples of hydrotalcite were pressed in stainless steel sample holders. X-ray diffraction patterns were recorded ˚ ) on a Philips PANalytical X’ using CuKa radiation (n ¼ 1.5418 A Pert PRO diffractometer operating at 40 kV and 40 mA with 0.25 divergence slit, 0.5 anti-scatter slit, between 1.5 and 20 (2q) at a step size of 0.0167 . For XRD at low angle section, it was between 1 and 5 (2q) at a step size of 0.0167 with variable divergence slit and 0.125 anti-scatter slit.
2.4.
A mixed solution of aluminum and magnesium nitrates ([Al3þ] ¼ 0.25 M and [Mg2þ] ¼ 0.75 M) and a mixed solution of sodium hydroxide ([OH] ¼ 2 M) and the desired anion carbonate or selenite, at the appropriate concentration, normally 0.1 M, were placed in two separate vessels. In order to make the thermally activated hydrotalcite, it is easier and less expensive to make the hydrotalcite with carbonate in the interlayer. The carbonate anion can be readily removed by thermal treatment. Hydrotalcites were also synthesized with selenite anions in the interlayer as the only interlayer anion. All compounds were dissolved in freshly decarbonated water. The cationic solution was added to the anions via a peristaltic pump at 40 mL/min and the pH maintained above 9. The mixture was aged at 80 C for 18 h. The resulting precipitate was then washed thoroughly with room temperature decarbonated water to remove nitrates and left to dry in a vacuum desiccator for several days. In this way hydrotalcites with different anions in the interlayer were synthesized. The phase composition was checked by X-ray diffraction and the chemical composition by EDX analyses. In order to achieve the best result for adsorption of selenite from an aqueous medium, the thermally decomposed temperature was selected by studying the thermoanalytical patterns. The Mg6Al2(OH)16(CO3)4H2O hydrotalcite was thermally activated by rapid heating to 340 C. The dehydration, dehydroxylation and loss of carbonate have occurred by this temperature. The thermally activated hydrotalcite was kept in a sealed, dry container to minimize absorption of CO2 and water vapour preventing the reformation of the clay-like structure.
2.2.
Selenite adsorption
The thermally activated hydrotalcite powder (3.0 g) was then placed in a large excess of decarbonated sodium selenite
X-ray diffraction
Raman microprobe spectroscopy
Hydrotalcite crystals were placed and orientated on a polished metal surface on the stage of an Olympus BHSM microscope, which is equipped with 10 and 50 objective lenses. The microscope is part of a Renishaw 1000 Raman microscope system, which also includes a monochromator, a filter system and a Charge Coupled Device (CCD). Raman spectra were excited by a Spectra-Physics model 127 He–Ne laser (633 nm) at a resolution of 2 cm1 in the range between 100 and 4000 cm1. Repeated acquisition, using the highest magnification, was accumulated to improve the signal to noise ratio in the spectra. Spectra were calibrated using the 520.5 cm1 line of a silicon wafer. Powers of less than 5 mW at the sample were used to avoid laser induced degradation of the sample. Spectroscopic manipulation such as baseline adjustment, smoothing and normalisation were performed using the Spectracalc software package GRAMS (Galactic Industries Corporation, NH, USA). Band component analysis was undertaken using the Jandel ‘Peakfit’ software package, which enabled the type of fitting function to be selected and allows specific parameters to be fixed or varied accordingly. Band fitting was done using a Gauss–Lorentz cross-product function with the minimum number of component bands used for the fitting process. The Gauss–Lorentz ratio was maintained at values less than 0.7 and fitting was undertaken until reproducible results were obtained with squared correlations of r2 greater than 0.995.
3.
Results and discussion
3.1. X-ray diffraction of Mg/Al hydrotalcite and the MgAlSeO3 hydrotalcite Upon adding the thermally activated hydrotalcite to the sodium selenite solution, the selenite is absorbed into the
water research 43 (2009) 1323–1329
Fig. 1 – X-ray diffraction patterns of Mg/Al hydrotalcite and hydrotalcites exposed to selenite solutions from 0.5 to 20 h.
hydrotalcite interlayer through the reformation effect. In this way MgAlSeO3 hydrotalcites are formed. One method of following the uptake of the selenite anion is to expose the selenite solution to the thermally activated hydrotalcite for different lengths of time and to analyze the collected reformed hydrotalcite using powder X-ray diffraction. The XRD patterns for MgAlCO3 hydrotalcite and MgAlSeO3 hydrotalcite are shown in Fig. 1. The sharp, symmetric peaks of the basal (003), (006) planes are observed, three non-basal planes of (012) þ (009), (015) and (018) are also observed. All these features are characteristic of a typical hydrotalcite phase (Pe´rez-Ramı´rez et al., 2001, 2007; Wang et al., 2003, 2007). The original Mg/Al hydrotalcite has a d(003) and d(006) spacing of 0.773 and 0.386 nm, respectively. The MgAlSeO3 hydrotalcite has an almost identical d-spacing value after selenite-absorbed hydrotalcite after contact for 0.5 h. There is only a 0.003 nm increase in the d(003) spacing. The peak width increases with the formation of MgAlSeO3 hydrotalcite with increasing reaction times. The width of the XRD peak provides an indication of the MgAlSeO3 hydrotalcite crystallite size. The increase in peak width shows that the crystallite size of the selenite hydrotalcite decreases with exposure time. It is noted that the spacing value of d(003) increases as the reaction time of the thermally activated hydrotalcite and selenite increase. This is as expected. As more selenite anion is adsorbed into the reforming hydrotalcite, an increased d(003)
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Fig. 2 – Raman spectra of Mg/Al hydrotalcite and hydrotalcites exposed to selenite solutions from 0.5 to 20 h in the 3800–3000 cmL1 region.
spacing is expected. The d(003) values for 2, 4 and 20 h are 0.793, 0.801 and 0.806 nm respectively (Fig. 1). The distances between the hydroxyl layers increased with the reaction time providing an indication that the selenite anion was absorbed into the hydrotalcite interlayers. The d-spacing for the 2 h is 0.793 nm and 0.801 nm for 4 h exposure time, thus there is 0.008 nm increase. However, the measurement at 16 h for the d(003) value of MgAlSeO3 hydrotalcite increased from 0.801 to 0.806 nm, suggesting that the maximum expansion occurs during the period from 2 to 4 h. The value of the d(003) changes little after this period. Variation for three non-basal planes of (012) þ (009), (015) and (018) are observed although there is a certain increase in interlayer spacing for (012) þ (009), (015) and (018). These values provide evidence that the selenite anion entered the interlayer space of hydrotalcite. The XRD patterns for the uptake of the selenite anion by the thermally activated hydrotalcite show that the selenite anion is absorbed quite quickly and most of the selenite ion is adsorbed within the first 30 min.
3.2.
Raman spectroscopy
Raman spectroscopy is a tool for the study of the reformation of hydrotalcites and is most useful for the detection of oxyanions. Oxyanions such as carbonate and selenite will have
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Fig. 3 – Raman spectra of Mg/Al hydrotalcite and hydrotalcites exposed to selenite solutions from 0.5 to 20 h in the 1100–1000 cmL1 region.
bands in different parts of the spectrum and therefore can be distinguished. The uptake of the selenite anion may be followed by studying the increase in intensity of typical selenite bands in the 870–900 cm1 region. The Raman spectra in the 3800–3000, 1100–1000, 900–750, and 600–400 and 200–100 cm1 region of Mg/Al hydrotalcite and MgAlSeO3 hydrotalcite are shown in Figs. 2–6, respectively. The Raman spectra of the 3800–3000 cm1 region for Mg/Al hydrotalcite and MgAlSeO3 hydrotalcite are observed in Fig. 2. The Raman bands in the 3800–3000 cm1 region are attributed to the H-bonded stretching vibrations of the OH units of the brucite-like sheets, and stretching vibrations of water molecules. Raman spectroscopy is not as useful for the detection of water since the Raman scattering cross section is very low. Thus Raman spectroscopy is more useful for the detection of OH vibrations. The Raman spectroscopy of the OH stretching vibrations from the hydroxyl surface of the hydrotalcite is more readily observed compared with that of water. Two intense bands of 3595 and 3494 cm1 and the other less intense bands of 3680 and 3379 cm1 for Mg/Al hydrotalcite are observed (Fig. 2). The bands at around 3680, 3647 and 3595 cm1 for the 0.5 h MgAlSeO3 hydrotalcite are assigned to the OH stretching vibrations of OH units of –AlOH and –MgOH. There is no large
Fig. 4 – Raman spectra of Mg/Al hydrotalcite and hydrotalcites exposed to selenite solutions from 0.5 to 20 h in the 900–700 cmL1 region.
variation for the band of 3595 cm1 band in position and intensity comparing 0.5 h with 20 h MgAlSeO3 hydrotalcite, indicating the amount of selenite entering the interlayer spacing cannot influence the wave number and size of the peak. The band intensity of 3595 cm1 is constant for all samples of different exposure times. This indicates that the uptake of the selenite anion is immediate and occurs within the first 30 min. The Raman bands at 3504, 3401 and 3255 cm1 for 0.5 h MgAlSeO3 hydrotalcite are assigned to water OH stretching vibrations of the hydroxyl surface of the brucitelike layers. The band of 3504 cm1 shifts toward lower wave numbers and its intensity increases with prolonged exposure time, becoming the 3497 cm1 in wave number for 20 h, and its intensity also increased. This decrease in wave number is attributed to the increase in hydrogen-bond strength. In addition, the width of all bands of MgAlSeO3 hydrotalcite is larger than the hydrotalcite without adsorption of selenite (see bottom spectrum of Fig. 2). The spectra in the 1030–1080 cm1 region shows strong bands at 1061 cm1 attributed to the (CO3)2 symmetric stretching mode (Fig. 3). These bands show that carbon dioxide is adsorbed from the atmosphere during the experiment. An alternative explanation rests with the incomplete removal of the carbonate anion during the thermal activation of the hydrotalcite. The band at 1046 cm1 is also assigned to the (CO3)2 symmetric stretching mode. The position of the
water research 43 (2009) 1323–1329
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Fig. 5 – Raman spectra of Mg/Al hydrotalcite and hydrotalcites exposed to selenite solutions from 0.5 to 20 h in the 600–400 cmL1 region.
Fig. 6 – Raman spectra of Mg/Al hydrotalcite and hydrotalcites exposed to selenite solutions from 0.5 to 20 h in the 200–100 cmL1 region.
band suggests that the carbonate is non-bonded or is weakly bonded. The existence of 1061 and 1046 cm1 indicates the carbonate anion is absorbed into the structure of MgAlSeO3 hydrotalcite. The fact that carbonate anion is formed in the interlayer may assist in the absorption of selenite anion into the interlayer. The size of the selenite anion is larger than the carbonate anion and has a different structure compared with the carbonate anion. The carbonate anion inserts between the layers, thus enabling further expansion as the selenite anion enters the interlayer. After the thermally activated Mg/Al hydrotalcite is exposed to aqueous selenite solutions for 0.5 h, the main band at 1060 cm1 moves slightly toward higher wave numbers (Fig. 3). The shift may indicate an interaction between the carbonate anion and the selenite anion. Both anions are of a similar charge and are similar is size. The shoulder bands at around 1630 cm1 (not shown) also shifted from 3 to 12 cm1 toward higher wave numbers after absorption of selenite. It is suggested that the uptake of selenite changes the hydrogen bonding in the hydrotalcite interlayer. The shoulder bands at 1550–1400 cm1 did not change in position after absorption of selenite by the thermally activated hydrotalcite for 1 h, but wave number of the bands changed as the reaction time
prolonged, there is 1–8 cm1 increase for 20 and 4 h MgAlSeO3 hydrotalcite (Fig. 3). The selenite ion should show a maximum of six bands. The free ion will have C3v symmetry and four modes, 2A1 and 2E. Nakamoto (1970) gives these as 807, 432 cm1 (A1) and 737, 374 cm1 (E ). The bands from 900 to 750 cm1 for MgAlSeO3 hydrotalcite are shown in Fig. 4. In Fig. 4, no bands are observed for the Mg/Al carbonate hydrotalcite in this spectral region. There appear 2–3 peaks in the region of 900–750 cm1 for Mg/Al hydrotalcite with adsorption of selenite as the selenite intercalated hydrotalcite is formed. The bands between the region of 850 and 800 cm1 are attributed to bonded selenite anions in the hydrotalcite interlayer, suggesting that selenite anions are held in the spacing of hydrotalcite interlayer and the thermally activated Mg/Al hydrotalcite absorbs the selenite anion. An intense main band at 550 cm1 for the Mg/Al hydrotalcite and other three shoulder bands locating at 569, 558 and 540 cm1 are observed in Fig. 5. These intense bands are typical of Al(OH)6 unit due to the vibration of aluminum– oxygen bonds (Villegas et al., 2003; Wang et al., 2003). After the thermally activated Mg/Al hydrotalcite was exposed to aqueous selenite solutions for 0.5 and 1 h, no obvious
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water research 43 (2009) 1323–1329
Fig. 7 – Relative intensity of the Raman bands at 814 and 835 cmL1 as a function of time.
variation occurs in the spectra. This suggested that the reaction time was so short and no change in the spectra was observed. After 2 h, the major bands of 550 cm1 for Mg/Al hydrotalcite became 547 cm1 for MgAlSeO3 hydrotalcite, the wave number of the main peak decreased. In addition, the shoulder band of 540 cm1 for Mg/Al hydrotalcite also shifted toward the region of the lower wave number, becoming 533, 538, 538 and 533 cm1 for 0.5, 1, 2, 4 and 20 h MgAlSeO3 hydrotalcite, respectively. This confirms that the adsorption of the selenite anion into the interlayer of Mg/Al hydrotalcite. This proves that the thermally activated hydrotalcite adsorbed the selenite anion from an aqueous solution. Bands at 481 and 465 cm1 are observed for 0.5 h MgAlSeO3 hydrotalcite, there is a shift of 10 cm1 toward the region of higher wave number compared with the band of 475 cm1 of Mg/Al hydrotalcite (Fig. 5). With further increase in exposure time, the two bands for the 0.5 h MgAlSeO3 hydrotalcite divided into three bands for 1.0 h MgAlSeO3 hydrotalcite. The spectra in this region are identical to that of the hydrotalcite of 1 and 2 h exposure. However, the band intensity of 465 cm1 for 4 and 20 h increases, indicating that the hydrotalcite adsorbed more selenite and much more selenite anion entered the interlayer spacing of the thermally acted Mg/Al hydrotalcite. The Raman spectra of the 200–100 wave number region are shown in Fig. 6. A broad, asymmetric band for Mg/Al hydrotalcite is observed at around 145 cm1, which may be resolved into component bands. The bands extended from 120 to 180 cm1 are assigned to hydrogen-bond stretching vibrations involving the hydrotalcite OH units and the water in the interlayer (Palmer et al., 2008). The main band for MgAlSeO3 hydrotalcite shifts toward the region of the higher wave number and is at 148 cm1 for 1, 2 and 20 h MgAlSeO3 hydrotalcite. The band FWHM of MgAlSeO3 hydrotalcites is narrower comparing with that of Mg/Al hydrotalcite. This suggested that the amount of water decreases as more selenite entered the interlayer spacing of the Mg/Al hydrotalcite.
3.3. Removal of selenite by thermally activated hydrotalcite The amount of selenite anion that is adsorbed by the thermally activated hydrotalcite may be obtained by the
determination of the amount of selenite anion remaining in the solution. Such a measurement may be obtained by using the intensity of the selenite Raman bands as shown in Fig. 4. The bands in Fig. 4 have been scaled so that the intensity is similar in the figure. The variation of the amount of selenite anion remaining in solution as measured by the relative intensity of the selenite bands at 814 and 835 cm1 as a function of time is presented in Fig. 7. The figure demonstrates that 85% of the selenite is removed in the first 15 min. 95% is removed after 30 min. The graph also shows that some selenite anion is released with extended time. The concentration of the selenite anion increased to 0.15 after 10 h of contact. This is attributed to the displacement of the selenite anion by the carbonate anion in the hydrotalcite interlayer. The graph also suggests that there is an optimum time for the removal of selenite anion using thermally activated hydrotalcite.
4.
Conclusions
In this research we have shown that the thermally activated Mg/Al hydrotalcite can be used as an effective absorbent for removal of selenite from potable water. The selenite anion, as shown by the XRD and Raman spectroscopy, is incorporated into the hydrotalcite interlayer as the counter anion. The d(003), d(006) spacing between the hydroxyl layers were increased with increasing reaction time and the reaction is complete during the period from 2 to 4 h. The intercalation of selenite in the Mg/Al hydrotalcite occurs quickly and most is adsorbed in the first 30 min. Raman spectroscopy identified the nature of the anion in hydrotalcite. The band intensity and the FWHM of OH stretching vibrations of –AlOH and –MgOH units and water OH for MgAlSeO3 hydrotalcite increased with the exposure time in the region of 3800–3000 cm1. The bands at around 1800– 1040 cm1 are not influenced by the adsorption of selenite anion into the MgAlSeO3 hydrotalcite interlayer. The bands between the region of 850 and 800 cm1 are attributed to the selenite anions bonded to the brucite-like layer in the interlayer spacing of MgAlSeO3 hydrotalcite. The bands at 550 and 475 cm1 for the Mg/Al hydrotalcite shift toward lower wave number with intercalation of the selenite anion in the interlayer spacing of Mg/Al hydrotalcite. The water content appears to decrease with more selenite entering the interlayer spacing of the Mg/Al hydrotalcite. This research shows that a thermally activated hydrotalcite is very useful for the adsorption of the selenite anion from an aqueous solution. Such technology is most appropriate for the purification of potable water.
Acknowledgements The financial and infra-structure support of the Queensland University of Technology, Inorganic Materials Research Program of the School of Physical and Chemical Sciences is gratefully acknowledged. China Scholarship Council (CSC) is thanked for funding the scholarship of Rui Liu.
water research 43 (2009) 1323–1329
references
Das, J., Das, D., Dash, G.P., Parida, K.M., 2002. Studies on Mg/Fe hydrotalcite-like-compound. Removal of inorganic selenite from aqueous medium. Journal of Colloid and Interface Science 251, 26–32. Frost, R.L., Musumeci, A.W., Martens, W.N., Adebajo, M.O., Bouzaid, J., 2005. Raman spectroscopy of hydrotalcites with sulphate, molybdate and chromate in the interlayer. Journal of Raman Spectroscopy 36, 925–931. Frost, R.L., Musumeci, A.W., Kloprogge, J.T., Adebajo, M.O., Martens, W.N., 2006. Raman spectroscopy of hydrotalcites with phosphate in the interlayer: implications for the removal of phosphate from water. Journal of Raman Spectroscopy 37, 733–741. Frost, R.L., Musumeci, A.W., Adebajo, M.O., Martens, W., 2007. Using thermally activated hydrotalcite for the uptake of phosphate from aqueous media. Journal of Thermal Analysis and Calorimetry 89, 95–99. Hou, X., Kirkpatrick, R.J., 2000. Solid-state 77Se NMR and XRD study of the structure and dynamics of seleno-oxyanions in hydrotalcite-like compounds. Chemistry of Materials 12, 1890–1897. Kloprogge, J.T., Hickey, L., Frost, R.L., 2004. FT-Raman and FT-IR spectroscopic study of synthetic Mg/Zn/Al-hydrotalcites. Journal of Raman Spectroscopy 35, 967–974. Mitchell, P.C.H., Wass, S.A., 2002. Propane dehydrogenation over molybdenum hydrotalcite catalysts. Applied Catalysis A: General. 225, 153–165. Miyata, S., 1983. Anion-exchange properties of hydrotalcite-like compounds. Clays and Clay Minerals 31, 305–311. Montea, M.D., Ausset, P., Forti, P., Le`fevreb, R.A., Tolomelli, M., 2001. Air pollution records on selenite in the urban environment. Atmospheric Environment 35, 3885–3896. Nakamoto, K., 1970. Infrared Spectra of Inorganic and Coordination Compounds, second ed., 376 pp.
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Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/watres
Absorption of the selenite anion from aqueous solutions by thermally activated layered double hydroxide Rui Liua,b, Ray L. Frosta,*, Wayde N. Martensa a
Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland University of Technology, 2 George Street, GPO Box 2434, Brisbane, Queensland 4001, Australia b Changchun Institute of Technology, Changchun Jilin130021, China
article info
abstract
Article history:
The presence of selenite or selenate in potable water is a health hazard especially when
Received 8 September 2008
consumed over a long period of time. Its removal from potable water is of importance. This
Received in revised form
paper reports technology for the removal of selenite from water through the use of ther-
2 December 2008
mally activated layered double hydroxides.
Accepted 2 December 2008
Mg/Al hydrotalcites with selenite in the interlayer were prepared at different times from 0.5 to
Published online 3 January 2009
20 h through ion exchange. X-ray diffraction of the MgAlSeO3 hydrotalcites indicates that the
Keywords:
was formed. Raman spectra proved the presence of selenite anion in the hydrotalcite inter-
Water purification
layer as the counter anion. The band intensity and width of MgAlSeO3 hydrotalcite in the
Potable water
region of 3800–3000 cm1 increase with the adsorption of selenite by the Mg/Al hydrotalcite.
selenite anion entered the interlayer spacing of Mg/Al hydrotalcite and MgAlSeO3 hydrotalcite
Thermally activated hydrotalcites
The characteristic bands of free selenite anions in the MgAlSeO3 hydrotalcites are located
Selenite adsorption
between the region between 850 and 800 cm1. The Raman spectra of the lower wave number region of 550–500 cm1 show a shift toward higher wave numbers with adsorption of the selenite. An estimation of the amount of selenite anion removed by the thermally activated layered double hydroxide was obtained through the measurement of the intensity of the selenite Raman bands at 814 and 835 cm1 resulting from the amount of selenite anion remaining in solution. Thermally activated LDHs provide a mechanism for removing selenite anions from aqueous solutions. ª 2008 Elsevier Ltd. All rights reserved.
1.
Introduction
Layered double hydroxides (LDH), also known as hydrotalcites or anionic clays have received increasing attention in recent years due to their wide range of applications as anion exchangers, adsorbents, ionic conductors, catalyst precursors and catalyst supports (Miyata, 1983; Mitchell and Wass, 2002; Scavetta et al., 2005). Hydrotalcites are naturally occurring minerals derived from a brucite structure (Mg(OH)2) in which e.g. Al3þ or Fe3þ substitutes for part of the Mg2þ. This substitution creates a positive layer charge on the hydroxyl layers,
which is compensated by interlayer anions or anionic complexes (Frost et al., 2007). These anions may be any anion with a suitable negative charge including the selenite anion (Hou and Kirkpatrick, 2000). Selenium occurs naturally in a number of inorganic forms, including selenide, selenate and selenite (Tsujia et al., 2000). For most aqueous medium, selenium exists in two primary oxidation states: Se(IV) usually as (selenite), and Se(VI) usually HSeO3 (biselenite) or SeO2 3 2 present as SeO2 4 (selenate) (Das et al., 2002). Selenite (SeO3 ), ubiquitous at various kinds of environment of soils, water and air fields (Peak et al., 1997), is an oxyanion of environmental
* Corresponding author. Tel.: þ61 7 3138 2407; fax: þ61 7 3138 1804. E-mail address: [email protected] (R.L. Frost). 0043-1354/$ – see front matter ª 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2008.12.030
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importance due to its toxicity to animals at higher concentrations, notably waterfowl and grazing animals. Selenite in potable water is toxic to humans. One means of studying the uptake of the selenite anion is to use Raman spectroscopy. Raman spectroscopy is especially sensitive to oxyanions. Raman spectroscopy for chemical analysis combines many of the advantages of FTIR and NIR absorption, having some added features based on resonance and/or surface enhancement, polarization measurements, compatibility and high spectral resolution (Montea et al., 2001). Raman spectroscopy, enables the identification of the interlayer anions and may be used to identify mixtures of anions in the interlayer of hydrotalcite (Kloprogge et al., 2004; Frost et al., 2005, 2006; Palmer et al., 2008). In this research, we present Raman spectroscopy on the selenite adsorption behavior upon hydrotalcite in the interlayer and explore the adsorption kinetics using a combination of X-ray diffraction and Raman spectroscopy. In this paper, we show that a thermally activated synthetic hydrotalcites can be used to remove selenite anions from aqueous solution.
2.
Experimental
2.1.
Synthesis of Mg/Al hydrotalcite
solution (0.1 M, 436 mL, pH w 12.5) and stirred under a nitrogen atmosphere. Aliquots (80 mL) were taken out at regular intervals. The mixture was stirred continuously and aliquots of solution were taken at intervals from 30 min to 20 h, filtered and dried before subsequent analysis. The resultant solution was vacuum filtered and rinsed two times with freshly decarbonated water. By using the hydrotalcite reformation effect, the selenite entered the interlayers of the thermally activated hydrotalcites and formed the MgAlSeO3 hydrotalcites. The hydrotalcites were left to dry in a vacuum desiccator to minimize exchange with carbon dioxide from the air.
2.3.
The samples of hydrotalcite were pressed in stainless steel sample holders. X-ray diffraction patterns were recorded ˚ ) on a Philips PANalytical X’ using CuKa radiation (n ¼ 1.5418 A Pert PRO diffractometer operating at 40 kV and 40 mA with 0.25 divergence slit, 0.5 anti-scatter slit, between 1.5 and 20 (2q) at a step size of 0.0167 . For XRD at low angle section, it was between 1 and 5 (2q) at a step size of 0.0167 with variable divergence slit and 0.125 anti-scatter slit.
2.4.
A mixed solution of aluminum and magnesium nitrates ([Al3þ] ¼ 0.25 M and [Mg2þ] ¼ 0.75 M) and a mixed solution of sodium hydroxide ([OH] ¼ 2 M) and the desired anion carbonate or selenite, at the appropriate concentration, normally 0.1 M, were placed in two separate vessels. In order to make the thermally activated hydrotalcite, it is easier and less expensive to make the hydrotalcite with carbonate in the interlayer. The carbonate anion can be readily removed by thermal treatment. Hydrotalcites were also synthesized with selenite anions in the interlayer as the only interlayer anion. All compounds were dissolved in freshly decarbonated water. The cationic solution was added to the anions via a peristaltic pump at 40 mL/min and the pH maintained above 9. The mixture was aged at 80 C for 18 h. The resulting precipitate was then washed thoroughly with room temperature decarbonated water to remove nitrates and left to dry in a vacuum desiccator for several days. In this way hydrotalcites with different anions in the interlayer were synthesized. The phase composition was checked by X-ray diffraction and the chemical composition by EDX analyses. In order to achieve the best result for adsorption of selenite from an aqueous medium, the thermally decomposed temperature was selected by studying the thermoanalytical patterns. The Mg6Al2(OH)16(CO3)4H2O hydrotalcite was thermally activated by rapid heating to 340 C. The dehydration, dehydroxylation and loss of carbonate have occurred by this temperature. The thermally activated hydrotalcite was kept in a sealed, dry container to minimize absorption of CO2 and water vapour preventing the reformation of the clay-like structure.
2.2.
Selenite adsorption
The thermally activated hydrotalcite powder (3.0 g) was then placed in a large excess of decarbonated sodium selenite
X-ray diffraction
Raman microprobe spectroscopy
Hydrotalcite crystals were placed and orientated on a polished metal surface on the stage of an Olympus BHSM microscope, which is equipped with 10 and 50 objective lenses. The microscope is part of a Renishaw 1000 Raman microscope system, which also includes a monochromator, a filter system and a Charge Coupled Device (CCD). Raman spectra were excited by a Spectra-Physics model 127 He–Ne laser (633 nm) at a resolution of 2 cm1 in the range between 100 and 4000 cm1. Repeated acquisition, using the highest magnification, was accumulated to improve the signal to noise ratio in the spectra. Spectra were calibrated using the 520.5 cm1 line of a silicon wafer. Powers of less than 5 mW at the sample were used to avoid laser induced degradation of the sample. Spectroscopic manipulation such as baseline adjustment, smoothing and normalisation were performed using the Spectracalc software package GRAMS (Galactic Industries Corporation, NH, USA). Band component analysis was undertaken using the Jandel ‘Peakfit’ software package, which enabled the type of fitting function to be selected and allows specific parameters to be fixed or varied accordingly. Band fitting was done using a Gauss–Lorentz cross-product function with the minimum number of component bands used for the fitting process. The Gauss–Lorentz ratio was maintained at values less than 0.7 and fitting was undertaken until reproducible results were obtained with squared correlations of r2 greater than 0.995.
3.
Results and discussion
3.1. X-ray diffraction of Mg/Al hydrotalcite and the MgAlSeO3 hydrotalcite Upon adding the thermally activated hydrotalcite to the sodium selenite solution, the selenite is absorbed into the
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Fig. 1 – X-ray diffraction patterns of Mg/Al hydrotalcite and hydrotalcites exposed to selenite solutions from 0.5 to 20 h.
hydrotalcite interlayer through the reformation effect. In this way MgAlSeO3 hydrotalcites are formed. One method of following the uptake of the selenite anion is to expose the selenite solution to the thermally activated hydrotalcite for different lengths of time and to analyze the collected reformed hydrotalcite using powder X-ray diffraction. The XRD patterns for MgAlCO3 hydrotalcite and MgAlSeO3 hydrotalcite are shown in Fig. 1. The sharp, symmetric peaks of the basal (003), (006) planes are observed, three non-basal planes of (012) þ (009), (015) and (018) are also observed. All these features are characteristic of a typical hydrotalcite phase (Pe´rez-Ramı´rez et al., 2001, 2007; Wang et al., 2003, 2007). The original Mg/Al hydrotalcite has a d(003) and d(006) spacing of 0.773 and 0.386 nm, respectively. The MgAlSeO3 hydrotalcite has an almost identical d-spacing value after selenite-absorbed hydrotalcite after contact for 0.5 h. There is only a 0.003 nm increase in the d(003) spacing. The peak width increases with the formation of MgAlSeO3 hydrotalcite with increasing reaction times. The width of the XRD peak provides an indication of the MgAlSeO3 hydrotalcite crystallite size. The increase in peak width shows that the crystallite size of the selenite hydrotalcite decreases with exposure time. It is noted that the spacing value of d(003) increases as the reaction time of the thermally activated hydrotalcite and selenite increase. This is as expected. As more selenite anion is adsorbed into the reforming hydrotalcite, an increased d(003)
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Fig. 2 – Raman spectra of Mg/Al hydrotalcite and hydrotalcites exposed to selenite solutions from 0.5 to 20 h in the 3800–3000 cmL1 region.
spacing is expected. The d(003) values for 2, 4 and 20 h are 0.793, 0.801 and 0.806 nm respectively (Fig. 1). The distances between the hydroxyl layers increased with the reaction time providing an indication that the selenite anion was absorbed into the hydrotalcite interlayers. The d-spacing for the 2 h is 0.793 nm and 0.801 nm for 4 h exposure time, thus there is 0.008 nm increase. However, the measurement at 16 h for the d(003) value of MgAlSeO3 hydrotalcite increased from 0.801 to 0.806 nm, suggesting that the maximum expansion occurs during the period from 2 to 4 h. The value of the d(003) changes little after this period. Variation for three non-basal planes of (012) þ (009), (015) and (018) are observed although there is a certain increase in interlayer spacing for (012) þ (009), (015) and (018). These values provide evidence that the selenite anion entered the interlayer space of hydrotalcite. The XRD patterns for the uptake of the selenite anion by the thermally activated hydrotalcite show that the selenite anion is absorbed quite quickly and most of the selenite ion is adsorbed within the first 30 min.
3.2.
Raman spectroscopy
Raman spectroscopy is a tool for the study of the reformation of hydrotalcites and is most useful for the detection of oxyanions. Oxyanions such as carbonate and selenite will have
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Fig. 3 – Raman spectra of Mg/Al hydrotalcite and hydrotalcites exposed to selenite solutions from 0.5 to 20 h in the 1100–1000 cmL1 region.
bands in different parts of the spectrum and therefore can be distinguished. The uptake of the selenite anion may be followed by studying the increase in intensity of typical selenite bands in the 870–900 cm1 region. The Raman spectra in the 3800–3000, 1100–1000, 900–750, and 600–400 and 200–100 cm1 region of Mg/Al hydrotalcite and MgAlSeO3 hydrotalcite are shown in Figs. 2–6, respectively. The Raman spectra of the 3800–3000 cm1 region for Mg/Al hydrotalcite and MgAlSeO3 hydrotalcite are observed in Fig. 2. The Raman bands in the 3800–3000 cm1 region are attributed to the H-bonded stretching vibrations of the OH units of the brucite-like sheets, and stretching vibrations of water molecules. Raman spectroscopy is not as useful for the detection of water since the Raman scattering cross section is very low. Thus Raman spectroscopy is more useful for the detection of OH vibrations. The Raman spectroscopy of the OH stretching vibrations from the hydroxyl surface of the hydrotalcite is more readily observed compared with that of water. Two intense bands of 3595 and 3494 cm1 and the other less intense bands of 3680 and 3379 cm1 for Mg/Al hydrotalcite are observed (Fig. 2). The bands at around 3680, 3647 and 3595 cm1 for the 0.5 h MgAlSeO3 hydrotalcite are assigned to the OH stretching vibrations of OH units of –AlOH and –MgOH. There is no large
Fig. 4 – Raman spectra of Mg/Al hydrotalcite and hydrotalcites exposed to selenite solutions from 0.5 to 20 h in the 900–700 cmL1 region.
variation for the band of 3595 cm1 band in position and intensity comparing 0.5 h with 20 h MgAlSeO3 hydrotalcite, indicating the amount of selenite entering the interlayer spacing cannot influence the wave number and size of the peak. The band intensity of 3595 cm1 is constant for all samples of different exposure times. This indicates that the uptake of the selenite anion is immediate and occurs within the first 30 min. The Raman bands at 3504, 3401 and 3255 cm1 for 0.5 h MgAlSeO3 hydrotalcite are assigned to water OH stretching vibrations of the hydroxyl surface of the brucitelike layers. The band of 3504 cm1 shifts toward lower wave numbers and its intensity increases with prolonged exposure time, becoming the 3497 cm1 in wave number for 20 h, and its intensity also increased. This decrease in wave number is attributed to the increase in hydrogen-bond strength. In addition, the width of all bands of MgAlSeO3 hydrotalcite is larger than the hydrotalcite without adsorption of selenite (see bottom spectrum of Fig. 2). The spectra in the 1030–1080 cm1 region shows strong bands at 1061 cm1 attributed to the (CO3)2 symmetric stretching mode (Fig. 3). These bands show that carbon dioxide is adsorbed from the atmosphere during the experiment. An alternative explanation rests with the incomplete removal of the carbonate anion during the thermal activation of the hydrotalcite. The band at 1046 cm1 is also assigned to the (CO3)2 symmetric stretching mode. The position of the
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Fig. 5 – Raman spectra of Mg/Al hydrotalcite and hydrotalcites exposed to selenite solutions from 0.5 to 20 h in the 600–400 cmL1 region.
Fig. 6 – Raman spectra of Mg/Al hydrotalcite and hydrotalcites exposed to selenite solutions from 0.5 to 20 h in the 200–100 cmL1 region.
band suggests that the carbonate is non-bonded or is weakly bonded. The existence of 1061 and 1046 cm1 indicates the carbonate anion is absorbed into the structure of MgAlSeO3 hydrotalcite. The fact that carbonate anion is formed in the interlayer may assist in the absorption of selenite anion into the interlayer. The size of the selenite anion is larger than the carbonate anion and has a different structure compared with the carbonate anion. The carbonate anion inserts between the layers, thus enabling further expansion as the selenite anion enters the interlayer. After the thermally activated Mg/Al hydrotalcite is exposed to aqueous selenite solutions for 0.5 h, the main band at 1060 cm1 moves slightly toward higher wave numbers (Fig. 3). The shift may indicate an interaction between the carbonate anion and the selenite anion. Both anions are of a similar charge and are similar is size. The shoulder bands at around 1630 cm1 (not shown) also shifted from 3 to 12 cm1 toward higher wave numbers after absorption of selenite. It is suggested that the uptake of selenite changes the hydrogen bonding in the hydrotalcite interlayer. The shoulder bands at 1550–1400 cm1 did not change in position after absorption of selenite by the thermally activated hydrotalcite for 1 h, but wave number of the bands changed as the reaction time
prolonged, there is 1–8 cm1 increase for 20 and 4 h MgAlSeO3 hydrotalcite (Fig. 3). The selenite ion should show a maximum of six bands. The free ion will have C3v symmetry and four modes, 2A1 and 2E. Nakamoto (1970) gives these as 807, 432 cm1 (A1) and 737, 374 cm1 (E ). The bands from 900 to 750 cm1 for MgAlSeO3 hydrotalcite are shown in Fig. 4. In Fig. 4, no bands are observed for the Mg/Al carbonate hydrotalcite in this spectral region. There appear 2–3 peaks in the region of 900–750 cm1 for Mg/Al hydrotalcite with adsorption of selenite as the selenite intercalated hydrotalcite is formed. The bands between the region of 850 and 800 cm1 are attributed to bonded selenite anions in the hydrotalcite interlayer, suggesting that selenite anions are held in the spacing of hydrotalcite interlayer and the thermally activated Mg/Al hydrotalcite absorbs the selenite anion. An intense main band at 550 cm1 for the Mg/Al hydrotalcite and other three shoulder bands locating at 569, 558 and 540 cm1 are observed in Fig. 5. These intense bands are typical of Al(OH)6 unit due to the vibration of aluminum– oxygen bonds (Villegas et al., 2003; Wang et al., 2003). After the thermally activated Mg/Al hydrotalcite was exposed to aqueous selenite solutions for 0.5 and 1 h, no obvious
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Fig. 7 – Relative intensity of the Raman bands at 814 and 835 cmL1 as a function of time.
variation occurs in the spectra. This suggested that the reaction time was so short and no change in the spectra was observed. After 2 h, the major bands of 550 cm1 for Mg/Al hydrotalcite became 547 cm1 for MgAlSeO3 hydrotalcite, the wave number of the main peak decreased. In addition, the shoulder band of 540 cm1 for Mg/Al hydrotalcite also shifted toward the region of the lower wave number, becoming 533, 538, 538 and 533 cm1 for 0.5, 1, 2, 4 and 20 h MgAlSeO3 hydrotalcite, respectively. This confirms that the adsorption of the selenite anion into the interlayer of Mg/Al hydrotalcite. This proves that the thermally activated hydrotalcite adsorbed the selenite anion from an aqueous solution. Bands at 481 and 465 cm1 are observed for 0.5 h MgAlSeO3 hydrotalcite, there is a shift of 10 cm1 toward the region of higher wave number compared with the band of 475 cm1 of Mg/Al hydrotalcite (Fig. 5). With further increase in exposure time, the two bands for the 0.5 h MgAlSeO3 hydrotalcite divided into three bands for 1.0 h MgAlSeO3 hydrotalcite. The spectra in this region are identical to that of the hydrotalcite of 1 and 2 h exposure. However, the band intensity of 465 cm1 for 4 and 20 h increases, indicating that the hydrotalcite adsorbed more selenite and much more selenite anion entered the interlayer spacing of the thermally acted Mg/Al hydrotalcite. The Raman spectra of the 200–100 wave number region are shown in Fig. 6. A broad, asymmetric band for Mg/Al hydrotalcite is observed at around 145 cm1, which may be resolved into component bands. The bands extended from 120 to 180 cm1 are assigned to hydrogen-bond stretching vibrations involving the hydrotalcite OH units and the water in the interlayer (Palmer et al., 2008). The main band for MgAlSeO3 hydrotalcite shifts toward the region of the higher wave number and is at 148 cm1 for 1, 2 and 20 h MgAlSeO3 hydrotalcite. The band FWHM of MgAlSeO3 hydrotalcites is narrower comparing with that of Mg/Al hydrotalcite. This suggested that the amount of water decreases as more selenite entered the interlayer spacing of the Mg/Al hydrotalcite.
3.3. Removal of selenite by thermally activated hydrotalcite The amount of selenite anion that is adsorbed by the thermally activated hydrotalcite may be obtained by the
determination of the amount of selenite anion remaining in the solution. Such a measurement may be obtained by using the intensity of the selenite Raman bands as shown in Fig. 4. The bands in Fig. 4 have been scaled so that the intensity is similar in the figure. The variation of the amount of selenite anion remaining in solution as measured by the relative intensity of the selenite bands at 814 and 835 cm1 as a function of time is presented in Fig. 7. The figure demonstrates that 85% of the selenite is removed in the first 15 min. 95% is removed after 30 min. The graph also shows that some selenite anion is released with extended time. The concentration of the selenite anion increased to 0.15 after 10 h of contact. This is attributed to the displacement of the selenite anion by the carbonate anion in the hydrotalcite interlayer. The graph also suggests that there is an optimum time for the removal of selenite anion using thermally activated hydrotalcite.
4.
Conclusions
In this research we have shown that the thermally activated Mg/Al hydrotalcite can be used as an effective absorbent for removal of selenite from potable water. The selenite anion, as shown by the XRD and Raman spectroscopy, is incorporated into the hydrotalcite interlayer as the counter anion. The d(003), d(006) spacing between the hydroxyl layers were increased with increasing reaction time and the reaction is complete during the period from 2 to 4 h. The intercalation of selenite in the Mg/Al hydrotalcite occurs quickly and most is adsorbed in the first 30 min. Raman spectroscopy identified the nature of the anion in hydrotalcite. The band intensity and the FWHM of OH stretching vibrations of –AlOH and –MgOH units and water OH for MgAlSeO3 hydrotalcite increased with the exposure time in the region of 3800–3000 cm1. The bands at around 1800– 1040 cm1 are not influenced by the adsorption of selenite anion into the MgAlSeO3 hydrotalcite interlayer. The bands between the region of 850 and 800 cm1 are attributed to the selenite anions bonded to the brucite-like layer in the interlayer spacing of MgAlSeO3 hydrotalcite. The bands at 550 and 475 cm1 for the Mg/Al hydrotalcite shift toward lower wave number with intercalation of the selenite anion in the interlayer spacing of Mg/Al hydrotalcite. The water content appears to decrease with more selenite entering the interlayer spacing of the Mg/Al hydrotalcite. This research shows that a thermally activated hydrotalcite is very useful for the adsorption of the selenite anion from an aqueous solution. Such technology is most appropriate for the purification of potable water.
Acknowledgements The financial and infra-structure support of the Queensland University of Technology, Inorganic Materials Research Program of the School of Physical and Chemical Sciences is gratefully acknowledged. China Scholarship Council (CSC) is thanked for funding the scholarship of Rui Liu.
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