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Adsorption of arsenite and selenite using an inorganic ion exchanger based on Fe–Mn hydrous oxide
Journal of Colloid and Interface Science 365 (2012) 213–221
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Adsorption of arsenite and selenite using an inorganic ion exchanger based on Fe–Mn hydrous oxide Małgorzata Szlachta ⇑, Vasyl Gerda, Natalia Chubar Department of Earth Sciences-Geochemistry, Faculty of Geosciences, Utrecht University, P.O. Box 80021, 3508 TA Utrecht, The Netherlands
a r t i c l e
i n f o
Article history: Received 13 July 2011 Accepted 9 September 2011 Available online 17 September 2011 Keywords: Adsorption Arsenite Fe–Mn hydrous oxide Inorganic ion exchanger Selenite
a b s t r a c t The adsorption behaviour and mechanism of As(III) and Se(IV) oxyanion uptake using a mixed inorganic adsorbent were studied. The novel adsorbent, based on Fe(III)–Mn(III) hydrous oxides and manganese(II) carbonate, was synthesised using a hydrothermal precipitation approach in the presence of urea. The inorganic ion exchanger exhibited a high selectivity and adsorptive capacity towards As(III) (up to 47.6 mg/g) and Se(IV) (up to 29.0 mg/g), even at low equilibrium concentration. Although pH effects were typical for anionic species (i.e., the adsorption decreased upon pH increase), Se(IV) was more sensitive to pH changes than As(III). The rates of adsorption of both oxyanions were high. Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) studies showed that the ion exchange adsorption of both anions took place via OH groups, mainly from Fe(III) but also Mn(III) hydrous oxides. MnCO3 did not contribute directly to As(III) and Se(IV) removal. A higher adsorptive capacity of the developed material towards As(III) was partly due to partial As(III) oxidation during adsorption. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Toxic levels of anionic species of arsenic and selenium can significantly affect drinking water quality. Arsenic contamination is a serious environmental problem, and its increasing concentration in natural water has been reported in many areas all over the world [1,2]. Inorganic arsenic is present in aquatic environments predominantly as arsenate and arsenite. Arsenite tends to be more toxic and mobile than arsenate and is more difficult to remove from water [3]. Selenium is a trace element of environmental significance and its oxyanionic species, i.e., selenate and selenite, are the predominant forms found in natural waters. Large quantities of selenium can be released into freshwater as a result of mining, agricultural or petrochemical activities, which lead to concentration levels that are hazardous for aquatic ecosystems [4]. To protect public health, environmental authorities have taken a more stringent attitude towards the presence of these toxic elements in drinking water. The World Health Organization (WHO) has recommended maximum contaminant levels of arsenic and selenium of 10 lg/L. On the basis of this recommendation, the concentration limit for arsenic has recently been reduced from 50 to 10 lg/L by both the US. Environmental Protection Agency (US EPA) and the European Commission (EC), whereas the maximum
⇑ Corresponding author. Fax: +31 302535302. E-mail address: [email protected] (M. Szlachta). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.09.023
concentration level for selenium has been set as 10 lg/L by the EC and 50 lg/L by the US EPA [5,6]. A number of adsorptive materials, such as impregnated activated carbon [7], activated alumina [8], exchange resin [9], low cost [10] and natural materials [11,12], have been more or less successfully tested for the removal of toxic anions. Recently, inorganic adsorbents that are based on metal oxides/hydroxides have been gaining popularity for ion exchange separation in water treatment where high removal efficiency is required. For example, double Mg–Al hydrous oxides [13], Mg–Fe-based hydrotalcite [14] and Fe–Mn oxide incorporated into diatomite [15] were found to be effective materials for the removal of different oxidation states of arsenic, whereas Al(III) oxides and Fe(III) oxides mixed with SiO2 [16] as well as c-Al2O3 [17] have proven to exhibit a high efficiency for the removal of selenium oxyanions from aqueous solutions. We have previously observed that adsorptive inorganic materials based on mixed and double metal hydrous oxides usually exhibit more-attractive adsorptive properties, compared with individual hydrous oxides, which include higher removal affinities and capacities, smoother pH effects and faster kinetics of adsorption [13]. Thus, the step-wise tasks of our present work were (1) to synthesise novel inorganic ion exchange adsorbents based on mixed or double hydrous oxides of Fe and Mn via a hydrothermal precipitation approach; (2) to test the new materials towards a list of target anionic species for drinking water treatment; (3) to select the ion exchanger that showed the best performance towards a few target anions and characterise its structure and surface
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chemical properties and (4) to run batch adsorption studies to investigate its mechanism of adsorption using Fourier transform infrared (FTIR) and X-ray photoelectron (XPS) spectroscopy. Nine adsorbents having three different ratios of Fe and Mn (i.e., 1:2, 1:1 and 2:1) that were dried/treated at three temperatures (i.e., 22 °C (ambient), 80 °C and 300 °C) were tested for their removal capacity towards arsenate, arsenite, phosphate, selenate, selenite, bromate, bromide, chromate, Cr(III) and borate. The hydrous oxide with an Fe–Mn ratio of 1:1 that was dried at 300 °C displayed the best performance towards the As(III) and Se(IV) oxyanions. Therefore, the structure, surface and adsorptive properties towards arsenite and selenite of the new material were studied and described in the present paper. 2. Materials and methods 2.1. Synthesis of inorganic ion exchanger based on Fe–Mn hydrous oxides The hydrothermal precipitation approach in the presence of urea was chosen for the material preparation. It was already proved by several researchers [18–21] that this synthesis method can produce good quality porous materials with a high specific surface area due to the unique properties of urea such as its weak Brønsted base characteristics, high solubility in water and the possibility to control the hydrolysis rate. Taking into account the chemical reactions occurring in the reactor, it was expected that having the salts of two or three various metals which are also in various valence states and urea in the reacting system could produce a variety of new materials in the form of mixed and double hydrous oxides depending on the selected metals. Double hydrous oxides would contain ionic carbonate in the interlayer space due to the presence of urea in the system. The possibility to produce layered double hydrous oxides by this synthesis approach has already been confirmed [22,23]. The texture of the final products can be regulated by changing the concentration of urea, temperature of synthesis and thermal treatment of the materials precipitated in the autoclave as well as by adjusting the heating regime of the autoclave. Regulating these parameters during and after synthesis can also change the ion selectivity and adsorptive capacity of the ion exchangers. New inorganic adsorbents were synthesised in versoclave of the type 3E/3.0 It (60 bar. N 5896) using an Autoclave Suurmond BV (Büchi Glas Uster AG, Switzerland). Mixed adsorbents based on Fe–Mn hydrous oxides and MnCO3 were precipitated together from aqueous solutions of iron(II) and manganese(II) sulphates under conditions of urea (CO(NH2)2) hydrothermal decomposition. Sigma–Aldrich chemicals of analytical grade were used for the synthesis. Three Fe:Mn ratios were prepared: 1:2, 1:1 and 2:1. Typically, a mixture of pre-calculated quantities of FeSO47H2O, MnSO4H2O and CO(NH2)2 (0.12, 0.12 and 0.64 mol, respectively) powders were dissolved in 2800 mL of distilled water. The solution was placed in a 3000 mL autoclave and maintained at 120 °C for 24 h. The autoclave was cooled down to room temperature. The agitation rate of 750 rpm was applied. The precipitate was filtered and thoroughly washed with distilled water and alcohol several times. Finally, the products were dried at 22 °C, 80 °C and 300 °C for 12 h. 2.2. Characterisation of the adsorbent structure Nitrogen physisorption isotherms were measured at 196 °C using a TriStar 3000 apparatus (Micromeritics, USA). The Brunauer–Emmet–Teller (BET) surface area was evaluated from the adsorption branch in the range of relative pressure from
0.009 to 0.996. For the calculation of the average pore diameter, the Barrett–Joyner–Halenda (BJH) method was applied to the desorption isotherm. Scanning electron microscopy (SEM) was performed using a high-resolution scanning electron microscope XL30 SFEG (Phillips, the Netherlands). X-ray diffraction (XRD) patterns were collected using a D2 Phaser X-ray powder diffractometer (Brucker AXS, Germany) with Co Ka radiation (k = 1.79026 Å). The apparatus was equipped with a LynxEye detector and operated at 30 kV and 10 mA, with a scanning speed of 2° min1. The Fourier transform infrared spectra were recorded using an FTIR spectrometer Nicolet 6700 (Thermo Scientific, USA) in the range of 4000–400 cm1 with 32 scans at a resolution of 2 cm1. KBr pellets were prepared using a Specac press. 2.3. Characterisation of the adsorbent surface Point of zero charge (PZC) was determined by measuring changes in zeta potential at 25 °C (Zetasizer nano S., Malvern, UK) as a function of pH, which ranged from about 2.0 to 12.5. The Fe–Mn hydrous oxide suspensions, which were prepared by adding 0.01 M NaNO3 solution and a desired amount of 0.1 M H2SO4 or NaOH, were shaken using a temperature-controlled orbital shaker (IKA, Germany) for 72 h. The equilibrium pH was recorded. XPS spectra were recorded with an Axis Ultra DLD electron spectrometer (Kratos Analytical, UK) using a monochromated Al Ka source that was operated at 150 W. The surface potential was stabilised by the spectrometer neutralisation system. Wide spectrum (pass energy of 160 eV) and high-resolution spectra of all detected elements (pass energy of 20 eV) were acquired. The binding energy (BE) scale was referenced to the C (1s) peak of aliphatic carbon, which was set to 285.0 eV. The analysis area was 0.3 0.7 mm2, and the depth of analysis was 6 nm. 2.4. Adsorption experiments Batch adsorption experiments were conducted using a bottlepoint method to obtain equilibrium data. A total of 20 mL of arsenite or selenite solution, ranging from 5 to 500 mg/L, was added to a capped 50 mL polypropylene tube containing 0.04 g of dry adsorbent. Stock solutions of arsenite and selenite (1000 mg/L) were prepared by dissolving sodium (meta) arsenite (NaAsO2, Sigma–Aldrich) and sodium selenite pentahydrate (Na2SeO35H2O, Fluka), respectively, in ultrapure water. As a background electrolyte, 0.01 M NaNO3 was used in all studies. The pH was maintained at either 6, 7 or 8 throughout the experiments by addition of a suitable volume of 0.1 M HNO3 or NaOH and measuring with a HQ40d multi-parameter meter (Hach, Germany). The tubes were shaken using a temperature-controlled orbital shaker at 200 rpm and 22 ± 1 °C until equilibrium was reached. The adsorption kinetics tests were performed using a magnetic stirrer (IKA, Germany) at 200 rpm. A total of 0.6 g of adsorbent was introduced into a glass beaker that contained 300 mL of As(III) or Se(IV) solution of the known concentration and at an initial pH of 6.5. Kinetic tests were run for 24 h, and approximately 8 mL of suspension was taken for ICP-OES measurements at specific time intervals. Kinetic studies were performed at three different anion concentrations, which were 5, 20 and 50 mg/L. Total As and Se concentrations were determined by Spectro CIROSCCD ICP-OES with radial plasma (Spectro A.I., Germany). Prior to analysis, samples were filtered through 0.2 lm membrane filters and acidified with concentrated HNO3.
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The amount of anion adsorbed (qe, mg/g) per unit mass of adsorbent was calculated by a difference of the initial (C0, mg/L) and equilibrium (Ce, mg/L) concentration of anion in the solution. 3. Results and discussion 3.1. Characterisation of the adsorbent structure 3.1.1. N2 adsorption and SEM studies The novel adsorbent, which was based on mixed Fe–Mn hydrous oxides, is a highly porous material with a specific surface area of 124 m2/g. Fig. 1 shows the N2 adsorption isotherm and pore diameter data along with SEM image for the Fe–Mn hydrous oxides. 3.1.2. XRD The phases and purity of the developed Fe–Mn hydrous oxidebased adsorbent were studied by XRD (Fig. 2). All main (the highest) diffraction peaks can be indexed to the rhombohedral rhodochrosite structure of MnCO3 [space group: R3c, (167)] with lattice constants a = 4.772 Å and c = 15.651 Å, which are consistent with the reported values (JCPDS Card 831763). There are also several low-intensity peaks (2H = 35.35, 37.54, 41.72, 45.36, 50.94, 63.49 and 74.37) that can be assigned to maghemite cubic c-Fe2O3 phase with lattice constants a = b = c = 8.35 Å (JCPDS: Card 24-81). Fig. 2 shows that the adsorbent is composed of rhodochrosite and subordinate maghemite as well as amorphous phases of Fe(III) and Mn(III). We can conclude that the new ion exchanger is a mixed material composed of Mn(II) carbonate and c-Fe2O3 crystalline phases as well as (taking into account data from XPS analysis (see Section 3.2.2)) of the amorphous phases of Fe(III) and Mn(III) hydrous oxides. 3.1.3. FTIR Further evidence for the presence of MnCO3 and c-Fe2O3 in the composition of the prepared adsorbent was obtained from FTIR investigations (Fig. 3). Bands between 400 cm1 and 2000 cm1 were mainly attributed to vibrations of the carbonate ðCO2 3 Þ of MnCO3 and vibrations of c-Fe2O3 (Fig. 3a). The FTIR spectrum of MnCO3 (rhodochrosite, the reference) (Fig. 3c) can be divided into three main regions due to different vibrations of the carbonate unit: the t4-associated band, which occurs at 728 cm1 in MnCO3 and is caused by the inplane bending vibration of the CO2 3 group; the t2-associated band,
Fig. 2. XRD patterns of the Fe–Mn hydrous oxides.
generated by the out-of-plane bending vibration of the CO2 3 group at 863 cm1 and a broad t3 band that is associated with the asymmetric stretching vibration of the CO2 group at 1405 cm1 [24]. 3 The other absorption bands that were assigned to carbonate ions were too weak or were located in the region of (much stronger) Fe–OH absorption bands. A very weak absorption band at 1797 cm1 represented the (t1 + t4) mode of CO2 3 , which demonstrates that carbonate is the bridge species. The medium intensity sharp band at 1630 cm1 is due to the deformation vibration of water [25]. The absorption bands at 1000–1200 cm1 and 400–700 cm1 can be assigned to M-O and M–OH stretching and bending vibrations modes, respectively. Further confirmation of c-Fe2O3 formation are the bands at 419, 444, 555, 634 and 690 cm1, which can be assigned to the characteristic vibrations of Fe–O in c-Fe2O3 [25–27]. It can also be concluded that there is a phase of poorly crystalline iron oxide in the adsorbent composition (Fig. 3a). The bands at 400–700 cm1 are not strong, but they were visible enough to be used for interpretation. If FTIR of c-Fe2O3 powder had weak peaks at 400–700 cm1, it would have demonstrated the presence of an amorphous iron oxide phase with a minimal long-range order. Enough strong (distinguishing) peaks at 550–580 cm1 and 440–460 cm1 could be assigned to the octahedral positions in the maghemite crystal structure [28]. Maghe-
Fig. 1. Isotherm of N2 adsorption, pore diameter data and SEM image for the Fe–Mn hydrous oxides.
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Fig. 3. FTIR spectra of (a) the Fe–Mn hydrous oxides before and after adsorption of As(III) at pH 7 and Se(IV) at pH 6, (b) the range of 400–1000 cm1, showing the influence of As(III) and Se(IV) adsorption and (c) the reference MnCO3.
mite (c-Fe2O3) has an inverse spinel structure; therefore, it can be understood as an iron-deficient form of magnetite. 3.2. Characterisation of the adsorbent surface 3.2.1. Zeta potential The point of zero charge (pHPZC) of the Fe–Mn hydrous oxide, defined from zeta potential measurements, was at a pH 4.5. Below this pH value, the adsorbent surface sites carried a positive charge, while above the pHPZC negatively charged sites dominated. The surface charge of the Fe–Mn hydrous oxide varied from +43.7 mV to 21.6 mV with an increasing pH value from 2.2 to 12.5. Within the tested pH range of 6–8, the surface of the mixed material was basic in character and the negative surface charge increased from approximately 9.5 mV to 15.5 mV, which is less favourable for the adsorption of oxyanions from solution due to electrostatic repulsion (Section 3.3). 3.2.2. XPS The surface of the developed adsorbent was investigated by XPS analysis (Fig. 4). XPS survey spectra (not shown) detected all the main chemical elements on the surface, which included manganese, iron, carbon and oxygen as well as arsenic and selenium on the surface of the ion exchanger following adsorption of anions. Table 1 summarises the data of the XPS survey spectra. The results
from the XPS analysis were in agreement with data obtained from XRD and FTIR studies. The carbon that was found on the surface of the mixed material indicated the presence of MnCO3 in the composition. It was also confirmed that the iron oxide in the composition of the adsorbent was indeed maghemite. The binding energy of Fe 2p(3/2) was 711.1 eV (Fig. 4a), which was assigned to Fe(III) ions [29]. Separation of the 2p(3/2) and 2p(1/2) spin–orbit level was approximately 13.6 eV, which is also consistent with the results presented by Armelao et al. for Fe(III) ions in solids [30]. The peaks for Fe 2p(3/2) and Fe 2p(1/2) were at 711.1 and 724.7 eV, respectively, which is in agreement with previous values reported for c-Fe2O3 [31,32]. For the interpretation of manganese oxidation states, XPS analyses are often focused on the 2p region, where Mn gives the most intense signal. However, Mn (2p) spectra are more complex due to the multiple splitting of Mn (2p) spectra from Mn(II), Mn(III) and Mn(IV) ions [33]. We think that the Mn (2p) region is not the most desirable way to determine Mn oxidation states due to the broadness of the peaks and the small shifts of the photopeaks, which are functions of the oxidation states [34]. Instead, we have given special attention to the Mn (3s) region to determine the Mn oxidation state(s). The energy difference between the multiplet splitting of 3s and unpaired 3d electrons [35] is a reliable method to distinguish between Mn(IV), Mn(III) and Mn(II). The splitting magnitudes of Mn (3s) XPS spectra strongly depend on the oxidation states of
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Fig. 4. XPS (a) Fe (2p) spectra, (b) Fe (3s) and Mn (3s) spectra of the ion exchanger surface before and after adsorption of arsenite and selenite.
Table 1 Data from XPS analysis of the Fe–Mn hydrous oxides before and after adsorption of arsenite and selenite. Line
Fe 2p3/2 Mn 2p3/2 C 1s, (CO2 3 ) O 1s, 1 (O2) O 1s, 2 O 1s, 3 (CO2 3 , OH ) As(III) 3d5/2 As(V) 3d5/2 Se(IV) 3d5/2 a b c
Adsorbent
Adsorbent + As(III)
Adsorbent + Se(IV)
BEa, eV
FWHMb, eV
ACc, at.%
BE, eV
FWHM, eV
AC, at.%
BE, eV
FWHM, eV
AC, at.%
711.1 642.5 290.2
2.05 2.15 1.20
22.21 13.23 6.44
710.8 642.5 289.9
2.05 2.30 1.10
15.86 11.89 6.96
710.8 642.4 290.0
2.20 2.35 1.10
16.00 13.45 7.05
530.3
1.20
33.01
532.2
1.55
22.44
530.0 530.9 531.9
1.20 1.00 1.70
21.78 2.88 29.15
530.0 531.0 532.0
1.15 1.05 1.55
26.34 4.63 23.29
44.0 45.2
1.05 1.05
0.59 1.75 58.8
1.35
1.47
BE – binding energy. FWHM – half-width of band. AC – atomic concentration.
Table 2 Mn (3s) multiplet splitting results. Sample
Mn5S, eV
Mn7S, eV
Multiplet splitting, eV
Average Mn oxidation state
2þ 3þ Fe3þ 1:68 Mn0:15 Mn0:85 ðCO3 Þ0:49 O2:5 ðOHÞ0:24
89.93
84.28
5.65
+2.85
2þ 3þ Fe3þ 1:33 Mn0:52 Mn0:48 ðCO3 Þ0:59 O1:83 ðOHÞ0:20 ðAsO3 Þ0:05 ðHAsO4 Þ0:15
89.75
83.73
6.02
+2.48
2þ 3þ Fe3þ 1:19 Mn0:22 Mn0:78 ðCO3 Þ0:52 O1:96 ðOHÞ0:16 ðSeO3 Þ0:11
89.63
83.91
5.72
+2.78
Mn ions. Mn (3s) multiplet splitting is smaller at higher oxidation state of Mn [36,37]. It is approximately 6.5, 5.5 and 4.5 eV for Mn(II), Mn(III) and Mn(IV), respectively [37]. These values were used for calibration in order to determine the oxidation state of the Mn ions (Fig. 4b) as is demonstrated in [38]. The binding energies of the Mn (3s) main peaks for all three samples (the pure adsorbent and the same material after adsorption of either arsenite and selenite) were around 83.91–84.28 eV, and the splitting energies were 5.65–6.02 eV (Fig. 4b). Because the splitting energies were within 6.5 eV (Mn(II)) and 5.5 eV (Mn(III)), the Mn oxidation state was between +2 and +3. Using a linear interpolate of the known splitting energies against the oxidation number [38], a formal oxidation state for the Mn ions was calculated (Table 2). Interestingly, the Mn (3s) main peak was asymmetric and consisted of two components. The weaker component at the lower binding energy was attributed to the Mn(II) ion, whereas the higher one was attributed to Mn(III). Furthermore, we estimated the ratio of these two ions and other species found on the surface of the adsorbent
before and after adsorption of arsenite and selenite, which are shown in Table 2. Additionally, O (1s) XPS spectra were used to study the oxygen speciation on the surface of the Fe–Mn hydrous oxide (Fig. 5). Two peaks from emission lines of O(1) and O(3) were detected, and their binding energies were 530.3 and 532.2 eV, respectively (Table 1). The first peak (530.3 eV) was assigned to the O2 ions of the metal oxides, whereas the second peak (532.2 eV) was ascribed to carbonate and hydroxide species. 3.3. Adsorption equilibrium The adsorption capacity of the inorganic ion exchanger for As(III) and Se(IV) was assessed on the basis of adsorption isotherms. Fig. 6 shows the results of adsorption equilibrium experiments of both oxyanions at three different pH values. Langmuir and Freundlich models were applied to fit the experimental data. The isotherm parameters for both models (with correlation coeffi-
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Fig. 5. XPS O (1s) spectra of the ion exchanger surface: (a) before, (b) after adsorption of arsenite and (c) selenite.
Fig. 6. Adsorption isotherms of As(III) and Se(IV) on the mixed adsorbent at different pH values fit by (a) the Langmuir and (b) Freundlich models (solid line – As(III), dash line – Se(IV)). Experimental conditions included an adsorbent content of 2 g/L, background electrolyte concentration of 0.01 M NaNO3, solution temperature of 22 °C and contact time of 14 d.
cients and sum of squares errors) are presented in Table 3. The data shown in Table 3 suggest that the applied models were suitable for describing the adsorption behaviour of As(III) and Se(IV) onto Fe– Mn hydrous oxides in the pH range from 6 to 8.
As can be seen from Fig. 6, all isotherms obtained were steep type, indicating the high affinity of the new mixed adsorbent towards both oxyanions. The pH effects of arsenite and selenite adsorption differ from each another. It was noted that adsorption
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M. Szlachta et al. / Journal of Colloid and Interface Science 365 (2012) 213–221 Table 3 Langmuir and Freundlich model parameters for As(III) and Se(IV) adsorption. Solution pH
Langmuir isotherm
Freundlich isotherm SSE
K (mg/g) (L/mg)
n
R2
SSE
0.922 0.933 0.945
143.09 114.94 87.71
17.87 16.27 13.94
0.117 0.181 0.202
0.949 0.945 0.958
96.14 93.12 64.74
0.916 0.972 0.982
56.41 8.30 4.49
15.49 10.43 8.77
0.115 0.129 0.142
0.972 0.848 0.848
18.05 46.32 36.67
qm (mg/g)
b (L/mg)
R
As(III) pH 6 pH 7 pH 8
42.94 42.13 42.18
0.456 0.292 0.125
Se(IV) pH 6 pH 7 pH 8
26.71 20.08 18.45
6.830 2.170 0.725
2
of Se(IV) was more sensitive to changes in pH, while the amount of As(III) adsorbed did not show a pronounced decrease within the pH range of 6–8. Additionally, a better performance in As(III) removal than Se(IV) at any given pH was also observed. Within the tested pH range, the surface of the ion exchanger was negatively charged (pHPZC 4.5), which was unfavourable for the adsorption of selenium anionic species. The increasing deprotonation of the adsorbent surface and the increase in repulsive forces between the selenium anions and the adsorbent surface decreased Se(IV) uptake from 28.98 mg/g at pH 6 to 18.14 mg/g at pH 8. This is in agreement with selenium speciation at the various pH values shown in Eh-pH diagrams [38,39]. At pH 6, HSeO 3 is the only species in the experimental solution, while at pH 7–8, SeO2 3 (which exhibits lower adsorptive ability) is present or dominant. Nevertheless, the Se(IV) removal still remained effective at higher solution pH (Fig. 6). Such pH dependency of selenium oxyanion adsorption might also suggest that the physical mechanism may occur alongside ligand exchange reactions, resulting in the formation of inner-sphere surface complexes. Similar observations have been reported by other researchers [40,41], who found that significant selenium species adsorption onto hydrous alumina and Mg/Fe hydrotalcite-like compounds occurred, even at pH values much higher than the pHPZC. Balistrieri and Chao [42] assumed that adsorption of selenite onto amorphous iron oxyhydroxide and manganese dioxide occurs by inner-sphere complexation, while Peak [43] suggested that inner-sphere complexation between selenite and hydrous aluminium oxide co-occurs with some outer-sphere selenite adsorption. As(III) exhibits different adsorption behaviour onto hydrous metal oxides than Se(IV). As can be seen from Fig. 6, there was
no significant enhancement of As(III) adsorption at lower pH values, and only a slight increase in the amount of oxyanion adsorbed was recorded (from 43.04 mg/g to 47.58 mg/g) as the pH decreased from 8 to 6. Throughout the range of pH values studied, the same, fully protonated, species of arsenic (H3AsO3 with pKa1 = 9.2) [44] dominate the solution, and therefore the pH effect of As(III) adsorption was negligible. It also means that uptake of the arsenic species is not controlled by an electrostatic interaction, but by a specific adsorption.
3.4. Adsorption kinetics Adsorption kinetics was used to determine the equilibrium time for oxyanions adsorption onto the developed ion exchanger. The obtained results are shown in Fig. 7. As expected, the uptake of arsenite and selenite increased with contact time, irrespective of their initial concentrations. However, the removal performance of the adsorbent was better for As(III) than to Se(IV). The amount of As(III) adsorbed increased from 2.43 to 22.42 mg/g with rising anion concentrations from 5 to 50 mg/L, while Se(IV) uptake changed from 2.65 to 15.99 mg/g during the same increase in concentration. A rapid initial uptake was observed for the adsorptions of both oxyanions. It was followed by a slower rise to reach adsorption equilibrium (the plateau), indicating the absence of easily accessible adsorption sites. After 1 h of the adsorption process, a complete depletion was reached for the lowest concentration of anions in solution. For higher concentrations (20–50 mg/L) the equilibrium adsorption capacity was achieved within 8 h, and no significant changes between 8 and 24 h were observed (i.e., adsorption in-
Fig. 7. Adsorption kinetics of (a) As(III) and (b) Se(IV) on the mixed adsorbent at different initial concentrations of anions. Experimental conditions included a solution pH 6.5, an adsorbent content of 2 g/L, background electrolyte concentration of 0.01 M NaNO3, solution temperature of 22 °C and a contact time of 24 h.
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creased no more than 5%). A similar phenomenon was reported in many time-dependent studies of As(III) and Se(IV) adsorption that have been presented in the literature. For example, Duc and coworkers [45] found that in the beginning of the process, the adsorption efficiency rapidly increased to a certain level, but between 4 and 65 h, no significant changes in the amount of Se(IV) adsorbed onto hematite was observed. Hao et al. [46] reported that a rapid initial uptake of As(III) took place followed by a slow adsorption when thiol-functionalized silica coated activated alumina was used as the adsorbent and about 65% removal of As(III) was achieved after 10 h.
3.5. Adsorption mechanism of As(III) and Se(IV) FTIR spectra of the new ion exchanger with adsorbed oxyanions did not show any bands that could be attributed to arsenite and selenite; however, the influences of As(III) and Se(IV) were found in the region of 400–700 cm1 (Fig. 3b), which are assigned to Fe–O and Fe–OH bands. These bands became smoother or disappeared completely, demonstrating that adsorption took place via the anion exchange of OH groups that belong to mainly Fe(III) hydrous oxides in addition to Mn(III) hydrous oxides. No changes in the main characteristic peaks of MnCO3 (863 and 728 cm1) in the FTIR spectra (Fig. 3b) were detected, indicating that MnCO3, which is one of the main components of the adsorbent, was not involved directly in the adsorption mechanism. XPS analysis of the C (1s) binding energy region also indicated that MnCO3 did not contribute directly to the adsorptive removal of Se(IV) and As(III). Table 1 shows that at.% of this chemical element and the related binding energies were almost the same for the pure adsorbent and for the samples with the adsorbed anions. The C (1s) line at 289.9–290.2 eV was due to the presence of CO2 3 . From O (1s) XPS data analysis, it is clear that the O (1s) spectra of the adsorbent before (Fig. 5a) and after arsenite (Fig. 5b) and selenite (Fig. 5c) adsorption are quite different, indicating the presence of As and Se on the adsorbent surface and shown as different ratios of oxygen species (Table 1). The O (1s) XPS spectra of the adsorbent after adsorption of arsenite and selenite had additional peaks at 530.9 and 531 eV, respectively. All spectra indicated the presence of the structural O2, OH and CO2 3 in the composition of the adsorbent before and after it was in contact with As(III) and Se(IV). Such data provide additional evidence that MnCO3 does not participate directly in the adsorption of both oxyanions. We suppose that the role of manganese carbonate with regard to the
adsorption of the investigated anions might be similar to the role of other inorganic catalysts’ supports, which are not involved in catalysis mechanisms, but increase the efficiency of the catalysed reactions. MnCO3 might behave differently in contact with other anions in upper oxidation states because it might reduce them. The As (3d) XPS spectrum of the adsorbent in contact with As(III) is shown in Fig. 8. The As 3d(5/2) peaks of As(III) and As(V) were located at 44.0 and 45.2 eV, respectively. Each of the As 3d(3/2) peaks was located at 0.7 eV greater binding energy, had the same FWHM, and had an intensity that was precisely two-thirds that of the 3d(5/2) peak. From XPS analysis, we concluded that the ratio of arsenite–arsenate on the adsorbent surface was 1:3, which indicated that most of the added As(III) was oxidised to As(V) by Mn(III). The binding energy of the As 3d(5/2) peak was 45.2 eV, which could be assigned to the HAsO2 4 species [30]. Surface redox reactions between As anions and metal oxides have been well documented, because it is known that manganese-containing materials (in higher oxidation states than Mn(II)) readily oxidise and adsorb many reduced species, such as As(III). For example, Deschamps and co-workers [47,48] found that natural Fe- and Mn-enriched materials (cFeMn) promoted the complete oxidation of As(III) to As(V), which was consistent with the results presented by Ghosh et al. [49] and Lakshmipathiraj et al. [50]. Experimental observations presented by other researchers [51,52] follow a similar adsorption mechanism of arsenite onto Mn-substituted goethite and untreated and treated (with Na2SO3) Fe–Mn binary oxide, respectively.
4. Conclusions Hydrothermal precipitation in the presence of urea has proven to be an efficient method for producing novel, efficient inorganic adsorbents based on mixed and double hydrous oxides of two metals. Using this synthesis approach, a new ion exchanger based on hydrous oxides of Fe and Mn and Mn(II) carbonate was synthesised. It was demonstrated that the developed material was a mixed adsorbent composed of MnCO3 and c-Fe2O3 crystalline phases as well as amorphous phases of Fe(III) and Mn(III) hydrous oxides. The adsorption data revealed that the ion exchanger exhibited a high selectivity and adsorptive capacity towards arsenite (up to 47.6 mg/g) and selenite (up to 29.0 mg/g), even at low equilibrium concentrations. This important advantage of the developed adsorbent makes it a promising material for treating As(III)- and Se(IV)contaminated waters. It was demonstrated that adsorption of Se(IV) was more sensitive to pH changes than that of As(III) due to the speciation of these anions at various pH values. The adsorption kinetics studies showed that, for both oxyanions, a rapid initial uptake occurred initially followed by a slower process to reach the plateau. FTIR and XPS studies showed that the ion exchange adsorption of arsenite and selenite occurred via the exchange of the OH of Fe(III) and Mn(III) hydrous oxides. MnCO3 did not contribute directly to As(III) and Se(IV) removal. The higher adsorptive capacity of this novel material towards As(III) was attributed to the oxidation of As(III) by Mn(III) hydrous oxides to the easily removable As(V).
Acknowledgments
Fig. 8. As(III)d XPS spectrum of the Fe–Mn hydrous oxides after adsorption of As(III).
This work was funded by the Center-in-Development Award to Utrecht University (No. KUK-C1-017-12) by King Abdullah University of Science and Technology.
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Dr. Andrey Shchukarev (Umeä University, Sweden) is gratefully acknowledged for XPS analysis. References [1] M. Bissen, F.H. Frimmel, Acta Hydrochim. Hydrobiol. 31 (2003) 9. [2] L.M. Camacho, W. Gutierrez, M.T. Alarcon-Herrera, M.D. Villalba, S.G. Deng, Chemosphere 83 (2011) 211. [3] D. Mohan, C.U. Pittman, J. Hazard. Mater. 142 (2007) 1. [4] J. Torres, V. Pintos, S. Dominguez, C. Kremer, E. Kremer, J. Solution Chem. 39 (2010) 1. [5] European Commission, Council Directive 98/83/EC, 1998. [6] US EPA, National Primary Drinking Water Regulations, 2009. [7] Q.G. Chang, W. Lin, W.C. Ying, J. Hazard. Mater. 184 (2010) 515. [8] T.Z. Su, X.H. Guan, G.W. Gu, J.M. Wang, J. Colloid Interface Sci. 326 (2008) 347. [9] N.B. Issa, V.N. Rajakovic´-Ognjanovic´, B.M. Jovanovic´, L.V. Rajakovic´, Anal. Chim. Acta 673 (2010) 85. [10] S. Chakravarty, V. Dureja, G. Bhattacharyya, S. Maity, S. Bhattacharjee, Water Res. 36 (2002) 625. [11] N. Jordan, C. Lomenech, N. Marmier, E. Giffaut, J.J. Ehrhardt, J. Colloid Interface Sci. 329 (2009) 17. [12] J. Gimenez, J. de Pablo, M. Martinez, M. Rovira, C. Valderrama, J. Colloid Interface Sci. 348 (2010) 293. [13] N. Chubar, J. Colloid Interface Sci. 357 (2011) 198. [14] T. Turk, I. Alp, H. Deveci, J. Hazard. Mater. 171 (2009) 665. [15] F.F. Chang, J.H. Qu, H.J. Liu, R.P. Liu, X. Zhao, J. Colloid Interface Sci. 338 (2009) 353. [16] Y.T. Chan, W.H. Kuan, T.Y. Chen, M.K. Wang, Water Res. 43 (2009) 4412. [17] E.J. Elzinga, Y.Z. Tang, J. McDonald, S. DeSisto, R.J. Reeder, J. Colloid Interface Sci. 340 (2009) 153. [18] A. Kajiyama, T. Nakamura, Colloids Surf., A 163 (2000) 301. [19] Y.W. Zhang, G. Xu, Z.G. Yan, Y. Yang, C.S. Liao, C.H. Yan, J. Mater. Chem. 12 (2002) 970. [20] M. Hirano, H. Morikawa, Chem. Mater. 15 (2003) 2561. [21] R. Si, Y.W. Zhang, S.J. Li, B.X. Lin, C.H. Yan, J. Phys. Chem. B 108 (2004) 12481. [22] U. Costantino, F. Marmottini, M. Nocchetti, R. Vivani, Eur. J. Inorg. Chem. (1998) 1439. [23] K.H. Goh, T.T. Lim, Z. Dong, Water Res. 42 (2008) 1343. [24] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds. Part A. Theory and Applications in Inorganic Chemistry, fifth ed., John Wiley & Sons, Inc., New York, 1997.
221
[25] M.P. Morales, S. Veintemillas-Verdaguer, M.I. Montero, C.J. Serna, A. Roig, L. Casas, B. Martinez, F. Sandiumenge, Chem. Mater. 11 (1999) 3058. [26] S.N. Inamdar, S.K. Haram, J. Nanosci. Nanotechnol. 6 (2006) 2155. [27] Z.H. Jing, Mater. Lett. 60 (2006) 2217. [28] A. Millan, F. Palacio, A. Falqui, E. Snoeck, V. Serin, A. Bhattacharjee, V. Ksenofontov, P. Gutlich, I. Gilbert, Acta Mater. 55 (2007) 2201. [29] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Mullenberg, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corp., Eden Prairie, MN, USA, 1979. [30] L. Armelao, R. Bertoncello, L. Crociani, G. Depaoli, G. Granozzi, E. Tondello, M. Bettinelli, J. Mater. Chem. 5 (1995) 79. [31] T. Hyeon, S.S. Lee, J. Park, Y. Chung, H. Bin Na, J. Am. Chem. Soc. 123 (2001) 12798. [32] Z.Y. Sun, H.Q. Yuan, Z.M. Liu, B.X. Han, X.R. Zhang, Adv. Mater. 17 (2005) 2993. [33] J.W. Murray, J.G. Dillard, R. Giovanoli, H. Moers, W. Stumm, Geochim. Cosmochim. Acta 49 (1985) 463. [34] J. Junta, M.F. Hochella, Geochim. Cosmochim. Acta 58 (1994) 4985. [35] A.J. Nelson, J.G. Reynolds, J.W. Roos, J. Vac. Sci. Technol., A 18 (2000) 1072. [36] V.A.M. Brabers, F.M. Vansetten, P.S.A. Knapen, J. Solid State Chem. 49 (1983) 93. [37] Q.H. Wu, J.M. Xu, Q.C. Zhuang, S.G. Sun, Solid State Ionics 177 (2006) 1483. [38] M.I. Leybourne, E.M. Cameron, Chem. Geol. 247 (2008) 208. [39] B. Sherwood Lollar, Environmental Geochemistry: Treatise on Geochemistry, first ed., vol. 9, Elsevier, 2005. [40] M.M. Ghosh, C.D. Cox, J.R. Yuan-Pan, Environ. Prog. 13 (1994) 79. [41] J. Das, D. Das, G.P. Dash, K.M. Parida, J. Colloid Interface Sci. 251 (2002) 26. [42] L.S. Balistrieri, T.T. Chao, Geochim. Cosmochim. Acta 54 (1990) 739. [43] D. Peak, J. Colloid Interface Sci. 303 (2006) 337. [44] P. Lu, C. Zhu, Environ. Earth Sci. 62 (2011) 1673. [45] M. Duc, G. Lefevre, M. Fedoroff, J. Colloid Interface Sci. 298 (2006) 556. [46] J.M. Hao, M.J. Han, C. Wang, X.G. Meng, Microporous Mesoporous Mater 124 (2009) 1. [47] E. Deschamps, V.S.T. Ciminelli, P.G. Weidler, A.Y. Ramos, Clays Clay Miner. 51 (2003) 197. [48] E. Deschamps, V.S.T. Ciminelli, W.H. Holl, Water Res. 39 (2005) 5212. [49] U.C. Ghosh, K. Gupta, A. Maity, J. Hazard. Mater. 184 (2010) 832. [50] P. Lakshmipathiraj, B.R.V. Narasimhan, S. Prabhakar, G.B. Raju, J. Colloid Interface Sci. 304 (2006) 317. [51] X.H. Sun, H.E. Doner, M. Zavarin, Clays Clay Miner. 47 (1999) 474. [52] G.S. Zhang, J.H. Qu, H.J. Liu, R.P. Liu, R.C. Wu, Water Res. 41 (2007) 1921.
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Adsorption of arsenite and selenite using an inorganic ion exchanger based on Fe–Mn hydrous oxide Małgorzata Szlachta ⇑, Vasyl Gerda, Natalia Chubar Department of Earth Sciences-Geochemistry, Faculty of Geosciences, Utrecht University, P.O. Box 80021, 3508 TA Utrecht, The Netherlands
a r t i c l e
i n f o
Article history: Received 13 July 2011 Accepted 9 September 2011 Available online 17 September 2011 Keywords: Adsorption Arsenite Fe–Mn hydrous oxide Inorganic ion exchanger Selenite
a b s t r a c t The adsorption behaviour and mechanism of As(III) and Se(IV) oxyanion uptake using a mixed inorganic adsorbent were studied. The novel adsorbent, based on Fe(III)–Mn(III) hydrous oxides and manganese(II) carbonate, was synthesised using a hydrothermal precipitation approach in the presence of urea. The inorganic ion exchanger exhibited a high selectivity and adsorptive capacity towards As(III) (up to 47.6 mg/g) and Se(IV) (up to 29.0 mg/g), even at low equilibrium concentration. Although pH effects were typical for anionic species (i.e., the adsorption decreased upon pH increase), Se(IV) was more sensitive to pH changes than As(III). The rates of adsorption of both oxyanions were high. Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) studies showed that the ion exchange adsorption of both anions took place via OH groups, mainly from Fe(III) but also Mn(III) hydrous oxides. MnCO3 did not contribute directly to As(III) and Se(IV) removal. A higher adsorptive capacity of the developed material towards As(III) was partly due to partial As(III) oxidation during adsorption. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Toxic levels of anionic species of arsenic and selenium can significantly affect drinking water quality. Arsenic contamination is a serious environmental problem, and its increasing concentration in natural water has been reported in many areas all over the world [1,2]. Inorganic arsenic is present in aquatic environments predominantly as arsenate and arsenite. Arsenite tends to be more toxic and mobile than arsenate and is more difficult to remove from water [3]. Selenium is a trace element of environmental significance and its oxyanionic species, i.e., selenate and selenite, are the predominant forms found in natural waters. Large quantities of selenium can be released into freshwater as a result of mining, agricultural or petrochemical activities, which lead to concentration levels that are hazardous for aquatic ecosystems [4]. To protect public health, environmental authorities have taken a more stringent attitude towards the presence of these toxic elements in drinking water. The World Health Organization (WHO) has recommended maximum contaminant levels of arsenic and selenium of 10 lg/L. On the basis of this recommendation, the concentration limit for arsenic has recently been reduced from 50 to 10 lg/L by both the US. Environmental Protection Agency (US EPA) and the European Commission (EC), whereas the maximum
⇑ Corresponding author. Fax: +31 302535302. E-mail address: [email protected] (M. Szlachta). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.09.023
concentration level for selenium has been set as 10 lg/L by the EC and 50 lg/L by the US EPA [5,6]. A number of adsorptive materials, such as impregnated activated carbon [7], activated alumina [8], exchange resin [9], low cost [10] and natural materials [11,12], have been more or less successfully tested for the removal of toxic anions. Recently, inorganic adsorbents that are based on metal oxides/hydroxides have been gaining popularity for ion exchange separation in water treatment where high removal efficiency is required. For example, double Mg–Al hydrous oxides [13], Mg–Fe-based hydrotalcite [14] and Fe–Mn oxide incorporated into diatomite [15] were found to be effective materials for the removal of different oxidation states of arsenic, whereas Al(III) oxides and Fe(III) oxides mixed with SiO2 [16] as well as c-Al2O3 [17] have proven to exhibit a high efficiency for the removal of selenium oxyanions from aqueous solutions. We have previously observed that adsorptive inorganic materials based on mixed and double metal hydrous oxides usually exhibit more-attractive adsorptive properties, compared with individual hydrous oxides, which include higher removal affinities and capacities, smoother pH effects and faster kinetics of adsorption [13]. Thus, the step-wise tasks of our present work were (1) to synthesise novel inorganic ion exchange adsorbents based on mixed or double hydrous oxides of Fe and Mn via a hydrothermal precipitation approach; (2) to test the new materials towards a list of target anionic species for drinking water treatment; (3) to select the ion exchanger that showed the best performance towards a few target anions and characterise its structure and surface
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chemical properties and (4) to run batch adsorption studies to investigate its mechanism of adsorption using Fourier transform infrared (FTIR) and X-ray photoelectron (XPS) spectroscopy. Nine adsorbents having three different ratios of Fe and Mn (i.e., 1:2, 1:1 and 2:1) that were dried/treated at three temperatures (i.e., 22 °C (ambient), 80 °C and 300 °C) were tested for their removal capacity towards arsenate, arsenite, phosphate, selenate, selenite, bromate, bromide, chromate, Cr(III) and borate. The hydrous oxide with an Fe–Mn ratio of 1:1 that was dried at 300 °C displayed the best performance towards the As(III) and Se(IV) oxyanions. Therefore, the structure, surface and adsorptive properties towards arsenite and selenite of the new material were studied and described in the present paper. 2. Materials and methods 2.1. Synthesis of inorganic ion exchanger based on Fe–Mn hydrous oxides The hydrothermal precipitation approach in the presence of urea was chosen for the material preparation. It was already proved by several researchers [18–21] that this synthesis method can produce good quality porous materials with a high specific surface area due to the unique properties of urea such as its weak Brønsted base characteristics, high solubility in water and the possibility to control the hydrolysis rate. Taking into account the chemical reactions occurring in the reactor, it was expected that having the salts of two or three various metals which are also in various valence states and urea in the reacting system could produce a variety of new materials in the form of mixed and double hydrous oxides depending on the selected metals. Double hydrous oxides would contain ionic carbonate in the interlayer space due to the presence of urea in the system. The possibility to produce layered double hydrous oxides by this synthesis approach has already been confirmed [22,23]. The texture of the final products can be regulated by changing the concentration of urea, temperature of synthesis and thermal treatment of the materials precipitated in the autoclave as well as by adjusting the heating regime of the autoclave. Regulating these parameters during and after synthesis can also change the ion selectivity and adsorptive capacity of the ion exchangers. New inorganic adsorbents were synthesised in versoclave of the type 3E/3.0 It (60 bar. N 5896) using an Autoclave Suurmond BV (Büchi Glas Uster AG, Switzerland). Mixed adsorbents based on Fe–Mn hydrous oxides and MnCO3 were precipitated together from aqueous solutions of iron(II) and manganese(II) sulphates under conditions of urea (CO(NH2)2) hydrothermal decomposition. Sigma–Aldrich chemicals of analytical grade were used for the synthesis. Three Fe:Mn ratios were prepared: 1:2, 1:1 and 2:1. Typically, a mixture of pre-calculated quantities of FeSO47H2O, MnSO4H2O and CO(NH2)2 (0.12, 0.12 and 0.64 mol, respectively) powders were dissolved in 2800 mL of distilled water. The solution was placed in a 3000 mL autoclave and maintained at 120 °C for 24 h. The autoclave was cooled down to room temperature. The agitation rate of 750 rpm was applied. The precipitate was filtered and thoroughly washed with distilled water and alcohol several times. Finally, the products were dried at 22 °C, 80 °C and 300 °C for 12 h. 2.2. Characterisation of the adsorbent structure Nitrogen physisorption isotherms were measured at 196 °C using a TriStar 3000 apparatus (Micromeritics, USA). The Brunauer–Emmet–Teller (BET) surface area was evaluated from the adsorption branch in the range of relative pressure from
0.009 to 0.996. For the calculation of the average pore diameter, the Barrett–Joyner–Halenda (BJH) method was applied to the desorption isotherm. Scanning electron microscopy (SEM) was performed using a high-resolution scanning electron microscope XL30 SFEG (Phillips, the Netherlands). X-ray diffraction (XRD) patterns were collected using a D2 Phaser X-ray powder diffractometer (Brucker AXS, Germany) with Co Ka radiation (k = 1.79026 Å). The apparatus was equipped with a LynxEye detector and operated at 30 kV and 10 mA, with a scanning speed of 2° min1. The Fourier transform infrared spectra were recorded using an FTIR spectrometer Nicolet 6700 (Thermo Scientific, USA) in the range of 4000–400 cm1 with 32 scans at a resolution of 2 cm1. KBr pellets were prepared using a Specac press. 2.3. Characterisation of the adsorbent surface Point of zero charge (PZC) was determined by measuring changes in zeta potential at 25 °C (Zetasizer nano S., Malvern, UK) as a function of pH, which ranged from about 2.0 to 12.5. The Fe–Mn hydrous oxide suspensions, which were prepared by adding 0.01 M NaNO3 solution and a desired amount of 0.1 M H2SO4 or NaOH, were shaken using a temperature-controlled orbital shaker (IKA, Germany) for 72 h. The equilibrium pH was recorded. XPS spectra were recorded with an Axis Ultra DLD electron spectrometer (Kratos Analytical, UK) using a monochromated Al Ka source that was operated at 150 W. The surface potential was stabilised by the spectrometer neutralisation system. Wide spectrum (pass energy of 160 eV) and high-resolution spectra of all detected elements (pass energy of 20 eV) were acquired. The binding energy (BE) scale was referenced to the C (1s) peak of aliphatic carbon, which was set to 285.0 eV. The analysis area was 0.3 0.7 mm2, and the depth of analysis was 6 nm. 2.4. Adsorption experiments Batch adsorption experiments were conducted using a bottlepoint method to obtain equilibrium data. A total of 20 mL of arsenite or selenite solution, ranging from 5 to 500 mg/L, was added to a capped 50 mL polypropylene tube containing 0.04 g of dry adsorbent. Stock solutions of arsenite and selenite (1000 mg/L) were prepared by dissolving sodium (meta) arsenite (NaAsO2, Sigma–Aldrich) and sodium selenite pentahydrate (Na2SeO35H2O, Fluka), respectively, in ultrapure water. As a background electrolyte, 0.01 M NaNO3 was used in all studies. The pH was maintained at either 6, 7 or 8 throughout the experiments by addition of a suitable volume of 0.1 M HNO3 or NaOH and measuring with a HQ40d multi-parameter meter (Hach, Germany). The tubes were shaken using a temperature-controlled orbital shaker at 200 rpm and 22 ± 1 °C until equilibrium was reached. The adsorption kinetics tests were performed using a magnetic stirrer (IKA, Germany) at 200 rpm. A total of 0.6 g of adsorbent was introduced into a glass beaker that contained 300 mL of As(III) or Se(IV) solution of the known concentration and at an initial pH of 6.5. Kinetic tests were run for 24 h, and approximately 8 mL of suspension was taken for ICP-OES measurements at specific time intervals. Kinetic studies were performed at three different anion concentrations, which were 5, 20 and 50 mg/L. Total As and Se concentrations were determined by Spectro CIROSCCD ICP-OES with radial plasma (Spectro A.I., Germany). Prior to analysis, samples were filtered through 0.2 lm membrane filters and acidified with concentrated HNO3.
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The amount of anion adsorbed (qe, mg/g) per unit mass of adsorbent was calculated by a difference of the initial (C0, mg/L) and equilibrium (Ce, mg/L) concentration of anion in the solution. 3. Results and discussion 3.1. Characterisation of the adsorbent structure 3.1.1. N2 adsorption and SEM studies The novel adsorbent, which was based on mixed Fe–Mn hydrous oxides, is a highly porous material with a specific surface area of 124 m2/g. Fig. 1 shows the N2 adsorption isotherm and pore diameter data along with SEM image for the Fe–Mn hydrous oxides. 3.1.2. XRD The phases and purity of the developed Fe–Mn hydrous oxidebased adsorbent were studied by XRD (Fig. 2). All main (the highest) diffraction peaks can be indexed to the rhombohedral rhodochrosite structure of MnCO3 [space group: R3c, (167)] with lattice constants a = 4.772 Å and c = 15.651 Å, which are consistent with the reported values (JCPDS Card 831763). There are also several low-intensity peaks (2H = 35.35, 37.54, 41.72, 45.36, 50.94, 63.49 and 74.37) that can be assigned to maghemite cubic c-Fe2O3 phase with lattice constants a = b = c = 8.35 Å (JCPDS: Card 24-81). Fig. 2 shows that the adsorbent is composed of rhodochrosite and subordinate maghemite as well as amorphous phases of Fe(III) and Mn(III). We can conclude that the new ion exchanger is a mixed material composed of Mn(II) carbonate and c-Fe2O3 crystalline phases as well as (taking into account data from XPS analysis (see Section 3.2.2)) of the amorphous phases of Fe(III) and Mn(III) hydrous oxides. 3.1.3. FTIR Further evidence for the presence of MnCO3 and c-Fe2O3 in the composition of the prepared adsorbent was obtained from FTIR investigations (Fig. 3). Bands between 400 cm1 and 2000 cm1 were mainly attributed to vibrations of the carbonate ðCO2 3 Þ of MnCO3 and vibrations of c-Fe2O3 (Fig. 3a). The FTIR spectrum of MnCO3 (rhodochrosite, the reference) (Fig. 3c) can be divided into three main regions due to different vibrations of the carbonate unit: the t4-associated band, which occurs at 728 cm1 in MnCO3 and is caused by the inplane bending vibration of the CO2 3 group; the t2-associated band,
Fig. 2. XRD patterns of the Fe–Mn hydrous oxides.
generated by the out-of-plane bending vibration of the CO2 3 group at 863 cm1 and a broad t3 band that is associated with the asymmetric stretching vibration of the CO2 group at 1405 cm1 [24]. 3 The other absorption bands that were assigned to carbonate ions were too weak or were located in the region of (much stronger) Fe–OH absorption bands. A very weak absorption band at 1797 cm1 represented the (t1 + t4) mode of CO2 3 , which demonstrates that carbonate is the bridge species. The medium intensity sharp band at 1630 cm1 is due to the deformation vibration of water [25]. The absorption bands at 1000–1200 cm1 and 400–700 cm1 can be assigned to M-O and M–OH stretching and bending vibrations modes, respectively. Further confirmation of c-Fe2O3 formation are the bands at 419, 444, 555, 634 and 690 cm1, which can be assigned to the characteristic vibrations of Fe–O in c-Fe2O3 [25–27]. It can also be concluded that there is a phase of poorly crystalline iron oxide in the adsorbent composition (Fig. 3a). The bands at 400–700 cm1 are not strong, but they were visible enough to be used for interpretation. If FTIR of c-Fe2O3 powder had weak peaks at 400–700 cm1, it would have demonstrated the presence of an amorphous iron oxide phase with a minimal long-range order. Enough strong (distinguishing) peaks at 550–580 cm1 and 440–460 cm1 could be assigned to the octahedral positions in the maghemite crystal structure [28]. Maghe-
Fig. 1. Isotherm of N2 adsorption, pore diameter data and SEM image for the Fe–Mn hydrous oxides.
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Fig. 3. FTIR spectra of (a) the Fe–Mn hydrous oxides before and after adsorption of As(III) at pH 7 and Se(IV) at pH 6, (b) the range of 400–1000 cm1, showing the influence of As(III) and Se(IV) adsorption and (c) the reference MnCO3.
mite (c-Fe2O3) has an inverse spinel structure; therefore, it can be understood as an iron-deficient form of magnetite. 3.2. Characterisation of the adsorbent surface 3.2.1. Zeta potential The point of zero charge (pHPZC) of the Fe–Mn hydrous oxide, defined from zeta potential measurements, was at a pH 4.5. Below this pH value, the adsorbent surface sites carried a positive charge, while above the pHPZC negatively charged sites dominated. The surface charge of the Fe–Mn hydrous oxide varied from +43.7 mV to 21.6 mV with an increasing pH value from 2.2 to 12.5. Within the tested pH range of 6–8, the surface of the mixed material was basic in character and the negative surface charge increased from approximately 9.5 mV to 15.5 mV, which is less favourable for the adsorption of oxyanions from solution due to electrostatic repulsion (Section 3.3). 3.2.2. XPS The surface of the developed adsorbent was investigated by XPS analysis (Fig. 4). XPS survey spectra (not shown) detected all the main chemical elements on the surface, which included manganese, iron, carbon and oxygen as well as arsenic and selenium on the surface of the ion exchanger following adsorption of anions. Table 1 summarises the data of the XPS survey spectra. The results
from the XPS analysis were in agreement with data obtained from XRD and FTIR studies. The carbon that was found on the surface of the mixed material indicated the presence of MnCO3 in the composition. It was also confirmed that the iron oxide in the composition of the adsorbent was indeed maghemite. The binding energy of Fe 2p(3/2) was 711.1 eV (Fig. 4a), which was assigned to Fe(III) ions [29]. Separation of the 2p(3/2) and 2p(1/2) spin–orbit level was approximately 13.6 eV, which is also consistent with the results presented by Armelao et al. for Fe(III) ions in solids [30]. The peaks for Fe 2p(3/2) and Fe 2p(1/2) were at 711.1 and 724.7 eV, respectively, which is in agreement with previous values reported for c-Fe2O3 [31,32]. For the interpretation of manganese oxidation states, XPS analyses are often focused on the 2p region, where Mn gives the most intense signal. However, Mn (2p) spectra are more complex due to the multiple splitting of Mn (2p) spectra from Mn(II), Mn(III) and Mn(IV) ions [33]. We think that the Mn (2p) region is not the most desirable way to determine Mn oxidation states due to the broadness of the peaks and the small shifts of the photopeaks, which are functions of the oxidation states [34]. Instead, we have given special attention to the Mn (3s) region to determine the Mn oxidation state(s). The energy difference between the multiplet splitting of 3s and unpaired 3d electrons [35] is a reliable method to distinguish between Mn(IV), Mn(III) and Mn(II). The splitting magnitudes of Mn (3s) XPS spectra strongly depend on the oxidation states of
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Fig. 4. XPS (a) Fe (2p) spectra, (b) Fe (3s) and Mn (3s) spectra of the ion exchanger surface before and after adsorption of arsenite and selenite.
Table 1 Data from XPS analysis of the Fe–Mn hydrous oxides before and after adsorption of arsenite and selenite. Line
Fe 2p3/2 Mn 2p3/2 C 1s, (CO2 3 ) O 1s, 1 (O2) O 1s, 2 O 1s, 3 (CO2 3 , OH ) As(III) 3d5/2 As(V) 3d5/2 Se(IV) 3d5/2 a b c
Adsorbent
Adsorbent + As(III)
Adsorbent + Se(IV)
BEa, eV
FWHMb, eV
ACc, at.%
BE, eV
FWHM, eV
AC, at.%
BE, eV
FWHM, eV
AC, at.%
711.1 642.5 290.2
2.05 2.15 1.20
22.21 13.23 6.44
710.8 642.5 289.9
2.05 2.30 1.10
15.86 11.89 6.96
710.8 642.4 290.0
2.20 2.35 1.10
16.00 13.45 7.05
530.3
1.20
33.01
532.2
1.55
22.44
530.0 530.9 531.9
1.20 1.00 1.70
21.78 2.88 29.15
530.0 531.0 532.0
1.15 1.05 1.55
26.34 4.63 23.29
44.0 45.2
1.05 1.05
0.59 1.75 58.8
1.35
1.47
BE – binding energy. FWHM – half-width of band. AC – atomic concentration.
Table 2 Mn (3s) multiplet splitting results. Sample
Mn5S, eV
Mn7S, eV
Multiplet splitting, eV
Average Mn oxidation state
2þ 3þ Fe3þ 1:68 Mn0:15 Mn0:85 ðCO3 Þ0:49 O2:5 ðOHÞ0:24
89.93
84.28
5.65
+2.85
2þ 3þ Fe3þ 1:33 Mn0:52 Mn0:48 ðCO3 Þ0:59 O1:83 ðOHÞ0:20 ðAsO3 Þ0:05 ðHAsO4 Þ0:15
89.75
83.73
6.02
+2.48
2þ 3þ Fe3þ 1:19 Mn0:22 Mn0:78 ðCO3 Þ0:52 O1:96 ðOHÞ0:16 ðSeO3 Þ0:11
89.63
83.91
5.72
+2.78
Mn ions. Mn (3s) multiplet splitting is smaller at higher oxidation state of Mn [36,37]. It is approximately 6.5, 5.5 and 4.5 eV for Mn(II), Mn(III) and Mn(IV), respectively [37]. These values were used for calibration in order to determine the oxidation state of the Mn ions (Fig. 4b) as is demonstrated in [38]. The binding energies of the Mn (3s) main peaks for all three samples (the pure adsorbent and the same material after adsorption of either arsenite and selenite) were around 83.91–84.28 eV, and the splitting energies were 5.65–6.02 eV (Fig. 4b). Because the splitting energies were within 6.5 eV (Mn(II)) and 5.5 eV (Mn(III)), the Mn oxidation state was between +2 and +3. Using a linear interpolate of the known splitting energies against the oxidation number [38], a formal oxidation state for the Mn ions was calculated (Table 2). Interestingly, the Mn (3s) main peak was asymmetric and consisted of two components. The weaker component at the lower binding energy was attributed to the Mn(II) ion, whereas the higher one was attributed to Mn(III). Furthermore, we estimated the ratio of these two ions and other species found on the surface of the adsorbent
before and after adsorption of arsenite and selenite, which are shown in Table 2. Additionally, O (1s) XPS spectra were used to study the oxygen speciation on the surface of the Fe–Mn hydrous oxide (Fig. 5). Two peaks from emission lines of O(1) and O(3) were detected, and their binding energies were 530.3 and 532.2 eV, respectively (Table 1). The first peak (530.3 eV) was assigned to the O2 ions of the metal oxides, whereas the second peak (532.2 eV) was ascribed to carbonate and hydroxide species. 3.3. Adsorption equilibrium The adsorption capacity of the inorganic ion exchanger for As(III) and Se(IV) was assessed on the basis of adsorption isotherms. Fig. 6 shows the results of adsorption equilibrium experiments of both oxyanions at three different pH values. Langmuir and Freundlich models were applied to fit the experimental data. The isotherm parameters for both models (with correlation coeffi-
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Fig. 5. XPS O (1s) spectra of the ion exchanger surface: (a) before, (b) after adsorption of arsenite and (c) selenite.
Fig. 6. Adsorption isotherms of As(III) and Se(IV) on the mixed adsorbent at different pH values fit by (a) the Langmuir and (b) Freundlich models (solid line – As(III), dash line – Se(IV)). Experimental conditions included an adsorbent content of 2 g/L, background electrolyte concentration of 0.01 M NaNO3, solution temperature of 22 °C and contact time of 14 d.
cients and sum of squares errors) are presented in Table 3. The data shown in Table 3 suggest that the applied models were suitable for describing the adsorption behaviour of As(III) and Se(IV) onto Fe– Mn hydrous oxides in the pH range from 6 to 8.
As can be seen from Fig. 6, all isotherms obtained were steep type, indicating the high affinity of the new mixed adsorbent towards both oxyanions. The pH effects of arsenite and selenite adsorption differ from each another. It was noted that adsorption
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M. Szlachta et al. / Journal of Colloid and Interface Science 365 (2012) 213–221 Table 3 Langmuir and Freundlich model parameters for As(III) and Se(IV) adsorption. Solution pH
Langmuir isotherm
Freundlich isotherm SSE
K (mg/g) (L/mg)
n
R2
SSE
0.922 0.933 0.945
143.09 114.94 87.71
17.87 16.27 13.94
0.117 0.181 0.202
0.949 0.945 0.958
96.14 93.12 64.74
0.916 0.972 0.982
56.41 8.30 4.49
15.49 10.43 8.77
0.115 0.129 0.142
0.972 0.848 0.848
18.05 46.32 36.67
qm (mg/g)
b (L/mg)
R
As(III) pH 6 pH 7 pH 8
42.94 42.13 42.18
0.456 0.292 0.125
Se(IV) pH 6 pH 7 pH 8
26.71 20.08 18.45
6.830 2.170 0.725
2
of Se(IV) was more sensitive to changes in pH, while the amount of As(III) adsorbed did not show a pronounced decrease within the pH range of 6–8. Additionally, a better performance in As(III) removal than Se(IV) at any given pH was also observed. Within the tested pH range, the surface of the ion exchanger was negatively charged (pHPZC 4.5), which was unfavourable for the adsorption of selenium anionic species. The increasing deprotonation of the adsorbent surface and the increase in repulsive forces between the selenium anions and the adsorbent surface decreased Se(IV) uptake from 28.98 mg/g at pH 6 to 18.14 mg/g at pH 8. This is in agreement with selenium speciation at the various pH values shown in Eh-pH diagrams [38,39]. At pH 6, HSeO 3 is the only species in the experimental solution, while at pH 7–8, SeO2 3 (which exhibits lower adsorptive ability) is present or dominant. Nevertheless, the Se(IV) removal still remained effective at higher solution pH (Fig. 6). Such pH dependency of selenium oxyanion adsorption might also suggest that the physical mechanism may occur alongside ligand exchange reactions, resulting in the formation of inner-sphere surface complexes. Similar observations have been reported by other researchers [40,41], who found that significant selenium species adsorption onto hydrous alumina and Mg/Fe hydrotalcite-like compounds occurred, even at pH values much higher than the pHPZC. Balistrieri and Chao [42] assumed that adsorption of selenite onto amorphous iron oxyhydroxide and manganese dioxide occurs by inner-sphere complexation, while Peak [43] suggested that inner-sphere complexation between selenite and hydrous aluminium oxide co-occurs with some outer-sphere selenite adsorption. As(III) exhibits different adsorption behaviour onto hydrous metal oxides than Se(IV). As can be seen from Fig. 6, there was
no significant enhancement of As(III) adsorption at lower pH values, and only a slight increase in the amount of oxyanion adsorbed was recorded (from 43.04 mg/g to 47.58 mg/g) as the pH decreased from 8 to 6. Throughout the range of pH values studied, the same, fully protonated, species of arsenic (H3AsO3 with pKa1 = 9.2) [44] dominate the solution, and therefore the pH effect of As(III) adsorption was negligible. It also means that uptake of the arsenic species is not controlled by an electrostatic interaction, but by a specific adsorption.
3.4. Adsorption kinetics Adsorption kinetics was used to determine the equilibrium time for oxyanions adsorption onto the developed ion exchanger. The obtained results are shown in Fig. 7. As expected, the uptake of arsenite and selenite increased with contact time, irrespective of their initial concentrations. However, the removal performance of the adsorbent was better for As(III) than to Se(IV). The amount of As(III) adsorbed increased from 2.43 to 22.42 mg/g with rising anion concentrations from 5 to 50 mg/L, while Se(IV) uptake changed from 2.65 to 15.99 mg/g during the same increase in concentration. A rapid initial uptake was observed for the adsorptions of both oxyanions. It was followed by a slower rise to reach adsorption equilibrium (the plateau), indicating the absence of easily accessible adsorption sites. After 1 h of the adsorption process, a complete depletion was reached for the lowest concentration of anions in solution. For higher concentrations (20–50 mg/L) the equilibrium adsorption capacity was achieved within 8 h, and no significant changes between 8 and 24 h were observed (i.e., adsorption in-
Fig. 7. Adsorption kinetics of (a) As(III) and (b) Se(IV) on the mixed adsorbent at different initial concentrations of anions. Experimental conditions included a solution pH 6.5, an adsorbent content of 2 g/L, background electrolyte concentration of 0.01 M NaNO3, solution temperature of 22 °C and a contact time of 24 h.
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creased no more than 5%). A similar phenomenon was reported in many time-dependent studies of As(III) and Se(IV) adsorption that have been presented in the literature. For example, Duc and coworkers [45] found that in the beginning of the process, the adsorption efficiency rapidly increased to a certain level, but between 4 and 65 h, no significant changes in the amount of Se(IV) adsorbed onto hematite was observed. Hao et al. [46] reported that a rapid initial uptake of As(III) took place followed by a slow adsorption when thiol-functionalized silica coated activated alumina was used as the adsorbent and about 65% removal of As(III) was achieved after 10 h.
3.5. Adsorption mechanism of As(III) and Se(IV) FTIR spectra of the new ion exchanger with adsorbed oxyanions did not show any bands that could be attributed to arsenite and selenite; however, the influences of As(III) and Se(IV) were found in the region of 400–700 cm1 (Fig. 3b), which are assigned to Fe–O and Fe–OH bands. These bands became smoother or disappeared completely, demonstrating that adsorption took place via the anion exchange of OH groups that belong to mainly Fe(III) hydrous oxides in addition to Mn(III) hydrous oxides. No changes in the main characteristic peaks of MnCO3 (863 and 728 cm1) in the FTIR spectra (Fig. 3b) were detected, indicating that MnCO3, which is one of the main components of the adsorbent, was not involved directly in the adsorption mechanism. XPS analysis of the C (1s) binding energy region also indicated that MnCO3 did not contribute directly to the adsorptive removal of Se(IV) and As(III). Table 1 shows that at.% of this chemical element and the related binding energies were almost the same for the pure adsorbent and for the samples with the adsorbed anions. The C (1s) line at 289.9–290.2 eV was due to the presence of CO2 3 . From O (1s) XPS data analysis, it is clear that the O (1s) spectra of the adsorbent before (Fig. 5a) and after arsenite (Fig. 5b) and selenite (Fig. 5c) adsorption are quite different, indicating the presence of As and Se on the adsorbent surface and shown as different ratios of oxygen species (Table 1). The O (1s) XPS spectra of the adsorbent after adsorption of arsenite and selenite had additional peaks at 530.9 and 531 eV, respectively. All spectra indicated the presence of the structural O2, OH and CO2 3 in the composition of the adsorbent before and after it was in contact with As(III) and Se(IV). Such data provide additional evidence that MnCO3 does not participate directly in the adsorption of both oxyanions. We suppose that the role of manganese carbonate with regard to the
adsorption of the investigated anions might be similar to the role of other inorganic catalysts’ supports, which are not involved in catalysis mechanisms, but increase the efficiency of the catalysed reactions. MnCO3 might behave differently in contact with other anions in upper oxidation states because it might reduce them. The As (3d) XPS spectrum of the adsorbent in contact with As(III) is shown in Fig. 8. The As 3d(5/2) peaks of As(III) and As(V) were located at 44.0 and 45.2 eV, respectively. Each of the As 3d(3/2) peaks was located at 0.7 eV greater binding energy, had the same FWHM, and had an intensity that was precisely two-thirds that of the 3d(5/2) peak. From XPS analysis, we concluded that the ratio of arsenite–arsenate on the adsorbent surface was 1:3, which indicated that most of the added As(III) was oxidised to As(V) by Mn(III). The binding energy of the As 3d(5/2) peak was 45.2 eV, which could be assigned to the HAsO2 4 species [30]. Surface redox reactions between As anions and metal oxides have been well documented, because it is known that manganese-containing materials (in higher oxidation states than Mn(II)) readily oxidise and adsorb many reduced species, such as As(III). For example, Deschamps and co-workers [47,48] found that natural Fe- and Mn-enriched materials (cFeMn) promoted the complete oxidation of As(III) to As(V), which was consistent with the results presented by Ghosh et al. [49] and Lakshmipathiraj et al. [50]. Experimental observations presented by other researchers [51,52] follow a similar adsorption mechanism of arsenite onto Mn-substituted goethite and untreated and treated (with Na2SO3) Fe–Mn binary oxide, respectively.
4. Conclusions Hydrothermal precipitation in the presence of urea has proven to be an efficient method for producing novel, efficient inorganic adsorbents based on mixed and double hydrous oxides of two metals. Using this synthesis approach, a new ion exchanger based on hydrous oxides of Fe and Mn and Mn(II) carbonate was synthesised. It was demonstrated that the developed material was a mixed adsorbent composed of MnCO3 and c-Fe2O3 crystalline phases as well as amorphous phases of Fe(III) and Mn(III) hydrous oxides. The adsorption data revealed that the ion exchanger exhibited a high selectivity and adsorptive capacity towards arsenite (up to 47.6 mg/g) and selenite (up to 29.0 mg/g), even at low equilibrium concentrations. This important advantage of the developed adsorbent makes it a promising material for treating As(III)- and Se(IV)contaminated waters. It was demonstrated that adsorption of Se(IV) was more sensitive to pH changes than that of As(III) due to the speciation of these anions at various pH values. The adsorption kinetics studies showed that, for both oxyanions, a rapid initial uptake occurred initially followed by a slower process to reach the plateau. FTIR and XPS studies showed that the ion exchange adsorption of arsenite and selenite occurred via the exchange of the OH of Fe(III) and Mn(III) hydrous oxides. MnCO3 did not contribute directly to As(III) and Se(IV) removal. The higher adsorptive capacity of this novel material towards As(III) was attributed to the oxidation of As(III) by Mn(III) hydrous oxides to the easily removable As(V).
Acknowledgments
Fig. 8. As(III)d XPS spectrum of the Fe–Mn hydrous oxides after adsorption of As(III).
This work was funded by the Center-in-Development Award to Utrecht University (No. KUK-C1-017-12) by King Abdullah University of Science and Technology.
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Dr. Andrey Shchukarev (Umeä University, Sweden) is gratefully acknowledged for XPS analysis. References [1] M. Bissen, F.H. Frimmel, Acta Hydrochim. Hydrobiol. 31 (2003) 9. [2] L.M. Camacho, W. Gutierrez, M.T. Alarcon-Herrera, M.D. Villalba, S.G. Deng, Chemosphere 83 (2011) 211. [3] D. Mohan, C.U. Pittman, J. Hazard. Mater. 142 (2007) 1. [4] J. Torres, V. Pintos, S. Dominguez, C. Kremer, E. Kremer, J. Solution Chem. 39 (2010) 1. [5] European Commission, Council Directive 98/83/EC, 1998. [6] US EPA, National Primary Drinking Water Regulations, 2009. [7] Q.G. Chang, W. Lin, W.C. Ying, J. Hazard. Mater. 184 (2010) 515. [8] T.Z. Su, X.H. Guan, G.W. Gu, J.M. Wang, J. Colloid Interface Sci. 326 (2008) 347. [9] N.B. Issa, V.N. Rajakovic´-Ognjanovic´, B.M. Jovanovic´, L.V. Rajakovic´, Anal. Chim. Acta 673 (2010) 85. [10] S. Chakravarty, V. Dureja, G. Bhattacharyya, S. Maity, S. Bhattacharjee, Water Res. 36 (2002) 625. [11] N. Jordan, C. Lomenech, N. Marmier, E. Giffaut, J.J. Ehrhardt, J. Colloid Interface Sci. 329 (2009) 17. [12] J. Gimenez, J. de Pablo, M. Martinez, M. Rovira, C. Valderrama, J. Colloid Interface Sci. 348 (2010) 293. [13] N. Chubar, J. Colloid Interface Sci. 357 (2011) 198. [14] T. Turk, I. Alp, H. Deveci, J. Hazard. Mater. 171 (2009) 665. [15] F.F. Chang, J.H. Qu, H.J. Liu, R.P. Liu, X. Zhao, J. Colloid Interface Sci. 338 (2009) 353. [16] Y.T. Chan, W.H. Kuan, T.Y. Chen, M.K. Wang, Water Res. 43 (2009) 4412. [17] E.J. Elzinga, Y.Z. Tang, J. McDonald, S. DeSisto, R.J. Reeder, J. Colloid Interface Sci. 340 (2009) 153. [18] A. Kajiyama, T. Nakamura, Colloids Surf., A 163 (2000) 301. [19] Y.W. Zhang, G. Xu, Z.G. Yan, Y. Yang, C.S. Liao, C.H. Yan, J. Mater. Chem. 12 (2002) 970. [20] M. Hirano, H. Morikawa, Chem. Mater. 15 (2003) 2561. [21] R. Si, Y.W. Zhang, S.J. Li, B.X. Lin, C.H. Yan, J. Phys. Chem. B 108 (2004) 12481. [22] U. Costantino, F. Marmottini, M. Nocchetti, R. Vivani, Eur. J. Inorg. Chem. (1998) 1439. [23] K.H. Goh, T.T. Lim, Z. Dong, Water Res. 42 (2008) 1343. [24] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds. Part A. Theory and Applications in Inorganic Chemistry, fifth ed., John Wiley & Sons, Inc., New York, 1997.
221
[25] M.P. Morales, S. Veintemillas-Verdaguer, M.I. Montero, C.J. Serna, A. Roig, L. Casas, B. Martinez, F. Sandiumenge, Chem. Mater. 11 (1999) 3058. [26] S.N. Inamdar, S.K. Haram, J. Nanosci. Nanotechnol. 6 (2006) 2155. [27] Z.H. Jing, Mater. Lett. 60 (2006) 2217. [28] A. Millan, F. Palacio, A. Falqui, E. Snoeck, V. Serin, A. Bhattacharjee, V. Ksenofontov, P. Gutlich, I. Gilbert, Acta Mater. 55 (2007) 2201. [29] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Mullenberg, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corp., Eden Prairie, MN, USA, 1979. [30] L. Armelao, R. Bertoncello, L. Crociani, G. Depaoli, G. Granozzi, E. Tondello, M. Bettinelli, J. Mater. Chem. 5 (1995) 79. [31] T. Hyeon, S.S. Lee, J. Park, Y. Chung, H. Bin Na, J. Am. Chem. Soc. 123 (2001) 12798. [32] Z.Y. Sun, H.Q. Yuan, Z.M. Liu, B.X. Han, X.R. Zhang, Adv. Mater. 17 (2005) 2993. [33] J.W. Murray, J.G. Dillard, R. Giovanoli, H. Moers, W. Stumm, Geochim. Cosmochim. Acta 49 (1985) 463. [34] J. Junta, M.F. Hochella, Geochim. Cosmochim. Acta 58 (1994) 4985. [35] A.J. Nelson, J.G. Reynolds, J.W. Roos, J. Vac. Sci. Technol., A 18 (2000) 1072. [36] V.A.M. Brabers, F.M. Vansetten, P.S.A. Knapen, J. Solid State Chem. 49 (1983) 93. [37] Q.H. Wu, J.M. Xu, Q.C. Zhuang, S.G. Sun, Solid State Ionics 177 (2006) 1483. [38] M.I. Leybourne, E.M. Cameron, Chem. Geol. 247 (2008) 208. [39] B. Sherwood Lollar, Environmental Geochemistry: Treatise on Geochemistry, first ed., vol. 9, Elsevier, 2005. [40] M.M. Ghosh, C.D. Cox, J.R. Yuan-Pan, Environ. Prog. 13 (1994) 79. [41] J. Das, D. Das, G.P. Dash, K.M. Parida, J. Colloid Interface Sci. 251 (2002) 26. [42] L.S. Balistrieri, T.T. Chao, Geochim. Cosmochim. Acta 54 (1990) 739. [43] D. Peak, J. Colloid Interface Sci. 303 (2006) 337. [44] P. Lu, C. Zhu, Environ. Earth Sci. 62 (2011) 1673. [45] M. Duc, G. Lefevre, M. Fedoroff, J. Colloid Interface Sci. 298 (2006) 556. [46] J.M. Hao, M.J. Han, C. Wang, X.G. Meng, Microporous Mesoporous Mater 124 (2009) 1. [47] E. Deschamps, V.S.T. Ciminelli, P.G. Weidler, A.Y. Ramos, Clays Clay Miner. 51 (2003) 197. [48] E. Deschamps, V.S.T. Ciminelli, W.H. Holl, Water Res. 39 (2005) 5212. [49] U.C. Ghosh, K. Gupta, A. Maity, J. Hazard. Mater. 184 (2010) 832. [50] P. Lakshmipathiraj, B.R.V. Narasimhan, S. Prabhakar, G.B. Raju, J. Colloid Interface Sci. 304 (2006) 317. [51] X.H. Sun, H.E. Doner, M. Zavarin, Clays Clay Miner. 47 (1999) 474. [52] G.S. Zhang, J.H. Qu, H.J. Liu, R.P. Liu, R.C. Wu, Water Res. 41 (2007) 1921.