Synthesis of mesoporous magnetic γ-Fe2O3 and its application to Cr(VI) removal from contaminated water

water research 43 (2009) 3727–3734

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Synthesis of mesoporous magnetic g-Fe2O3 and its application to Cr(VI) removal from contaminated water Peng Wang, Irene M.C. Lo* Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Hong Kong, China

article info

abstract

Article history:

In this study, mesoporous magnetic iron-oxide (g-Fe2O3) was synthesized as an adsorbent

Received 13 February 2009

for Cr(VI) removal. For material synthesis, mesoporous silica (KIT-6) was used as a hard

Received in revised form

template and to drive iron precursor into KIT-6, a ‘greener’, affinity based impregnation

24 May 2009

method was employed, which involved using a nonpolar solvent (xylene) and led to

Accepted 27 May 2009

recycling of the solvent. The results of Cr(VI) removal experiments showed that the

Published online 6 June 2009

synthesized mesoporous g-Fe2O3 has a Cr(VI) adsorption capacity comparable with 10 nm nonporous g-Fe2O3 but simultaneously has a much faster separation than 10 nm nonpo-

Keywords:

rous g-Fe2O3 in the presence of an external magnetic field under the same experimental

Adsorption

conditions. Cr(VI) adsorption capacity onto the mesoporous g-Fe2O3 increased with

Chromium

decreasing solution pH and could be readily regenerated. Therefore, mesoporous g-Fe2O3

Magnetic separation

presents a reusable adsorbent for a fast, convenient, and highly efficient removal of Cr(VI)

Mesoporous iron-oxide

from contaminated water. ª 2009 Elsevier Ltd. All rights reserved.

1.

Introduction

Hexavalent chromium, Cr(VI), is a hard oxidant and a group A carcinogen (Katz and Salem, 1993) and the industrial sources of Cr(VI) mainly include: alloys and steel manufacturing, metal finishing, electroplating, leather tanning, and pigments synthesis and dyeing (Shevchenko et al., 2008). During the past decade, considerable research attention has been diverted to selective removal of Cr(VI) from contaminated water via adsorption. In principle, adsorption not only can remove contaminants but also can recover and recycle them back to industrial processes (Singh and Tiwari, 1997). Various types of adsorbents have been studied for their effectiveness in this regard, including activated carbons (Aggarwal et al., 1999), polymeric adsorbents (Dabrowski et al., 2004), metal oxides (Ai et al., 2008), and even certain types of biosorbents (Volesky, 2007). It is worth mentioning that biosorption using biosorbents, including bacteria, fungi, yeast, algae, industrial

wastes, agricultural wastes and other polysaccharide materials, etc. has been a research hotspot in the field of heavy metal removal lately (Vijayaraghavan and Yun, 2008; Volesky, 2007). However, given the fact that almost all of the biosorbents require drying and chemical pretreatment, at least with acid or alkali pretreatment, for their effective performance, and also the concerns of safe disposal of spent biosorbents, large-scale application is still not foreseeable in the near future. Iron-oxide based materials, on the other hand, have distinguished themselves due to their effectiveness and selectiveness in Cr(VI) removal, quantitative recovery of Cr(VI), and their innocuousness and chemical stability over a wide pH range. Goethite (a-FeOOH) and hydrous ferric oxide (HFO) have been among the most studied iron-oxide based materials for Cr(VI) adsorption because of their natural abundance (Fendorf et al., 1997). In the past few years, another special property of some iron-oxide based materials (Fe3O4, g-Fe2O3), magnetism, came

* Corresponding author. Tel.: þ86 852 2358 7157; fax: þ86 852 2358 1534. E-mail address: [email protected] (I.M.C. Lo). 0043-1354/$ – see front matter ª 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2009.05.041

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to be realized and utilized in the context of environmental remediation. Utilizing the magnetic properties of these adsorbents, magnetic separation has been combined with adsorption for heavy metal removal from contaminated water at laboratory scales (Yavuz et al., 2006; Mayo et al., 2007; Hu et al., 2004, 2005a, b). In industry, magnetic separation is especially desirable because it overcomes many of the issues present in filtration, centrifugation or gravitational separation, and requires much less energy to achieve a given level of separation. It is believed that the contaminant adsorption capacity of an adsorbent is largely determined by the surface area available for adsorption, and an increase in the surface area is generally obtained by decreasing the particle size of the adsorbent. Not surprisingly, with the introduction of nanoscaled iron-oxide based materials, the contaminant removal efficiency can be increased dramatically. For example, Yavuz et al. (2006), Mayo et al. (2007), and Hu et al. (2005a) reported a significant increase in heavy metal removal capacities using 10 nm nonporous iron-oxide nanoparticles (Fe3O4, g-Fe2O3) as compared with big sized iron-oxide particles or bulk materials. However, as the size of a magnetic adsorbent decreases, it has also been noticed that its response to an external magnetic field decreases undesirably, making it increasingly difficult to retract the adsorbent after treatment is completed (Yavuz et al., 2006). Although an even higher magnetic field can still be applied to achieve a complete separation of the adsorbent, the cost of applying such a field might be so high that magnetic separation loses ground against other conventional separation methods. Thus, the ultimate objective of this study is to remove Cr(VI) from contaminated water using a newly synthesized magnetic iron-oxide based adsorbent that simultaneously possesses a high surface area and a high response to external magnetic fields. Recently, mesoporous materials have attracted a lot of attention in both scientific and industrial communities since the introduction of well-ordered mesoporous silicas in the 1990s because of their large surface areas and uniform and tunable pore sizes (2–50 nm) (Kresge et al., 1992; Zhao et al., 1998). The characteristics of mesoporous materials are also attractive to researchers seeking adsorbents for environmental remediation, not only because of their high surface area but also of their fast contaminant adsorption kinetics (Wang et al., 2009). Although the synthesis of mesoporous silicas and related materials has been well documented, wellordered mesoporous magnetic Fe2O3 (i.e., g-Fe2O3) was not synthesized until recently (Jiao et al., 2006a, b). In addition to a high surface area, synthesized mesoporous g-Fe2O3 usually has a much bigger particle size (>200 nm) compared with 10 nm nonporous g-Fe2O3 and presumably its response to an external magnetic field would be stronger than that of 10 nm nonporous g-Fe2O3, resulting in a faster separation of mesoporous g-Fe2O3. Therefore, mesoporous g-Fe2O3 has the potential of being a more efficient and cost-effective adsorbent for removal of Cr(VI) from wastewater because a shortened separation time of an adsorbent reduces the operation cost. The specific objectives of the study are (1) to synthesize mesoporous g-Fe2O3 using a modified, ‘greener’ method, which involves recycling and reusing the solvent; and (2) to

evaluate the effectiveness of the synthesized mesoporous g-Fe2O3 as a potential adsorbent for Cr(VI) removal from wastewater as compared with 10 nm nonporous g-Fe2O3 in terms of Cr(VI) adsorption capacities and magnetic separation.

2.

Materials and methods

2.1. Synthesis and characterization of mesoporous g-Fe2O3 The synthesis of mesoporous g-Fe2O3 involved three steps: (1) synthesis of mesoporous a-Fe2O3 using a hard-templating method; (2) reduction of a-Fe2O3 to Fe3O4 by H2 treatment; (3) oxidation of Fe3O4 to g-Fe2O3. In a typical synthesis of mesoporous a-Fe2O3, 3.0 g of Fe(NO3)3$9H2O was mixed with 80 mL of xylene at 60  C, followed by the addition of 2.0 g of mesoporous silica template, KIT-6. The preparation of KIT-6 was conducted following the procedure described by Kleitz et al. (2003). After stirring at 60  C for 4 h, the mixture was quickly filtered. The filtrate (xylene) was collected and stored for reuse again, whereas the filtered powder was dried overnight at 50  C. The dry powder was then slowly heated to 600  C and calcined at that temperature for 6 h. The resulting samples were treated with hot 2.0 M NaOH for three times to remove the silica template, followed by drying overnight at 50  C. This procedure leads to mesoporous a-Fe2O3, which is nonmagnetic. Reduction of a-Fe2O3 to Fe3O4 was achieved by heating the previously prepared mesoporous a-Fe2O3 at 300  C for 3 h under a 5% H2 atmosphere, and a mild oxidation of Fe3O4 to g-Fe2O3 was obtained by heating the as-prepared mesoporous Fe3O4 at 150  C in air for 2 h. A transmission electron microscope (TEM) (JEOL JEM2010, Japan), equipped with an energy dispersive X-ray analyzer (EDX), was used to characterize the structure properties of the synthesized materials. Scanning electron microscopy (SEM) studies were performed using an FEI XL40 Sirion FEG microscope. The composition of the materials was identified by powder X-ray diffraction (XRD) (Philips PW-1830, Netherlands). Magnetization measurement was performed using a vibrating sample magnetometer (VSM) (LakeShore EM7037/9509-P, USA) at room temperature. Surface area measurements were taken with a Brunauer, Emmett, Teller (BET) (Coulter SA-3100, USA) analyzer at liquid nitrogen temperature using conventional gas adsorption apparatus. Fourier transform infrared spectroscopy (FTIR) measurement was also conducted, and infrared spectra of diluted samples in KBr were recorded between 4000 and 400 cm1 in a Bruker IFS-66 V FTIR.

2.2.

Synthesis of 10 nm nonporous g-Fe2O3

In the literature, 10 nm nonporous magnetic iron-oxide nanoparticles (Fe3O4, g-Fe2O3), due to their high surface areas, have been reported as effective magnetic adsorbents for heavy metal removal. In this study, to compare the effectiveness of mesoporous g-Fe2O3 in terms of Cr(VI) removal and magnetic separation, 10 nm g-Fe2O3 nanoparticles were synthesized and used for Cr(VI) removal as well. Single crystal

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2.3.

Batch experiments

A chromium stock solution was prepared by dissolving a known quantity of potassium chromate (K2CrO4) in ultrapure water. Batch Cr(VI) adsorption studies were performed by mixing 0.05 g of mesoporous g-Fe2O3 with 40 mL of solution of varying Cr(VI) concentrations (5–100 mg/L) in a 40 mL glass vial end-over-end to reach Cr(VI) adsorption equilibrium. After adsorption reached its equilibrium, the adsorbent was separated by using a hand-held permanent magnet and the supernatant was collected for Cr(VI) concentration measurements. The Cr(VI) adsorption on the mesoporous g-Fe2O3 was first studied at three different pH values (2.5, 5.0, 7.0) to investigate the dependence of Cr(VI) adsorption on solution pH. Standard acid of 0.1 M HCl and a base of 0.1 M NaOH solution were used for pH adjustment. Solution pH was stable over the course of the experiments. All the adsorption experiments were carried out at a room temperature of 22  2  C and were performed in duplicate. Based on the results of pH dependence of Cr(VI) adsorption, further Cr(VI) adsorption studies were conducted only at a pH of 2.5 unless otherwise specified. The total aqueous concentrations of chromium were measured using a flame atomic absorption spectrometer (AAS, Varian 220FS). Sample dilution was conducted before AAS measurement where necessary. In some cases, adsorption experiments were immediately followed by desorption experiments. Briefly, after adsorption was completed and the supernatant was decanted, the chromium-loaded mesoporous g-Fe2O3 (0.05 g) was then mixed with 40 mL of 0.01 M NaOH to reach Cr(VI) desorption equilibrium. To test whether any chemical redox reaction occurring during the adsorption process, the concentration of Cr(VI) in the desorption solution was measured by 1,5-diphenylcarbazide colorimetric method, using a UV/visible spectrophotometer (Ultrospec 4300 Pro) at wavelengths of 540 nm, to check both the concentration of Cr(VI) and the speciation of chromium during the adsorption/desorption process. The kinetics of both Cr(VI) adsorption and desorption was investigated as well. The Cr(VI) adsorption capacity onto the 10 nm nonporous g-Fe2O3 nanoparticles was also measured at pH ¼ 2.5 and compared with that onto the mesoporous g-Fe2O3 under the same conditions.

2.4.

Regeneration and reuse of mesoporous g-Fe2O3

To study the regeneration and reusability of the mesoporous g-Fe2O3 as an adsorbent for Cr(VI) removal, experiments

pertaining to mesoporous g-Fe2O3 regeneration and Cr(VI) re-adsorption were carried out in 5 consecutive adsorption/ desorption cycles. For each cycle, 40 mL of 50 mg/L Cr(VI) solution was adsorbed first by 0.05 g of mesoporous g-Fe2O3 for 120 min to reach adsorption equilibrium. The supernatant was then decanted with an assistance of the permanent magnet and the adsorbed Cr(VI) on mesoporous g-Fe2O3 was then desorbed with 40 mL of 0.01 M NaOH for 120 min. After each cycle of adsorption/desorption, the mesoporous g-Fe2O3 was washed thoroughly with ultrapure water to neutrality and then reconditioned for adsorption in the succeeding cycle.

3.

Results and discussions

3.1. Fe2O3

Synthesis and characterization of mesoporous

Unlike the synthesis method reported by Jiao et al. (2006a, b), xylene was used in place of ethanol as the solvent. Ethanol is a polar solvent, in which Fe(NO3)3$9H2O is soluble. Therefore, in Jiao’s method, evaporation induced impregnation was employed to get the iron precursor into the mesoporous silica template, which inevitably led to the loss of the solvent. In the adapted synthesis, a nonpolar solvent (xylene) was used, within which the iron precursor entered the mesopores of the templating silica via an entropy driven mechanism. Since the internal surfaces of the silica mesopores are largely hydrophilic (SiOH), Fe3þ, in the form of hydrated Fe3þ, much prefers to enter these mesopores rather than staying in the nonpolar bulk solution. Thus, xylene serves as a medium which drives the iron precursor into the mesopores and it itself is not consumed at all. Based on this, the advantage of the use of a nonpolar solvent in the synthesis is that the solvent can be recycled and reused, leading to a ‘greener’ synthesis. Fig. 1 presents wide-angle XRD patterns of the synthesized a-Fe2O3 and g-Fe2O3. As can be seen, well-defined peaks corresponding to the crystal structure of a-Fe2O3 and g-Fe2O3 are clearly evident, in agreement with the previous studies (Jiao et al., 2006a, b), suggesting that the walls of both the mesoporous a-Fe2O3 and g-Fe2O3 are crystalline. It should be noted that because Fe3O4 gradually converts to g-Fe2O3 phase on

550

Intensity

10 nm Fe3O4 nanoparticles were first synthesized using a method modified from Kang et al. (1996). Briefly, 6.1 g of FeCl3$6H2O and 4.2 g FeSO4.7H2O were dissolved in 100 mL ultrapure water. A total of 25 mL 6.5 M NaOH was then slowly added and mixed with the above solution. The system was mixed for a further hour after the addition of NaOH was completed. The formed black precipitates were washed with ultrapure water several times with an assistance of an external magnetic field. This procedure leads to Fe3O4 nanoparticles with a size of around 10 nm. The 10 nm Fe3O4 nanoparticles were then oxidized in air at 150  C for 2 h to obtain 10 nm nonporous g-Fe2O3.

(b)

(a) 250 20

30

40

50

60

2θ (degrees) Fig. 1 – Wide-angle XRD for mesoporous (a) a-Fe2O3; (b) g-Fe2O3.

70

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Fig. 2 – TEM images of mesoporous (a) a-Fe2O3; (b) g-Fe2O3; SEM image of mesoporous g-Fe2O3 (c) and TEM image of 10 nm g-Fe2O3 (d). Note: the scale bar for (b) is 20 nm; that for (c) is 500 nm.

Fig. 4(a) presents the VSM measurement of 10 nm nonporous g-Fe2O3 and mesoporous g-Fe2O3. Although Fig. 4(a) is not intended to show the difference between the two types of materials in terms of their saturation magnetization, the magnitude of the saturation magnetization indicates that both materials are highly magnetic (Lu et al., 2007). Also, as indicated by a zero magnetic moment when the external magnetic field is absent, 10 nm g-Fe2O3 is superparamagnetic, while mesoporous g-Fe2O3 is not. More importantly, the ease with which the two types of materials can be separated

900

Intensity

exposure to air, no attempts were made to characterize mesoporous Fe3O4 in this study. Fig. 2(a) and (b) present TEM images of the mesoporous a-Fe2O3 and g-Fe2O3. As seen, the hard-templating method produces a well-ordered mesoporous a-Fe2O3 structure (a) and converting it to Fe3O4 by reduction and then to g-Fe2O3 (b) by oxidation retains largely the same ordered mesostructure throughout. The ability to carry out solid/solid transformations in mesoporous solids, with retention of the mesostructure, has been reported previously for metal-oxide based materials (Brezesinski et al., 2006; Shi et al., 2008). Fig. 2(c) presents an SEM image of the synthesized mesoporous g-Fe2O3. As shown, although the mesoporous g-Fe2O3 particles are not monodispersed in size, they have a size greater than 200 nm. The results of low-angle XRD for mesoporous a-Fe2O3 and g-Fe2O3 are shown in Fig. 3. Both materials exhibit a 2q peak around 0.9 , reflecting an ordered pore structure. N2 sorption analysis showed that the surface areas of the mesoporous a-Fe2O3 and g-Fe2O3 were 108 and 88 m2/g respectively and their mean pore sizes were both about 4 nm. A TEM image of the synthesized 10 nm nonporous g-Fe2O3 is presented in Fig. 2(d). The surface area of the 10 nm nonporous g-Fe2O3 was measured to be 95 m2/g, which is close to the value reported in the literature (Tuutijarvi et al., 2009) and to that of the mesoporous g-Fe2O3 synthesized in this study.

600

(b)

300

(a) 0 0.5

1

1.5

2

2.5

2θ (degrees) Fig. 3 – Low-angle XRD for (a) a-Fe2O3; (b) g-Fe2O3.

3

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a Moment (emu/g)

50

25

0 -6000

-4000

-2000

0

2000

-25

4000

6000

mesoporous γ-Fe2O3 10nm γ-Fe2O3

-50

Field (Oe)

b

c Mesoporous γ-Fe2O3

10nm γ-Fe2O3

10nm γ-Fe2O3

Mesoporous γ-Fe2O3

Fig. 4 – (a) VSM measurements for 10 nm g-Fe2O3 and mesoporous g-Fe2O3; demonstration of magnetic separation at (b) 5 min; and (c) 6 h.

aqueous medium on each unit weight of the larger particles than of the smaller ones.

3.2.

Cr(VI) adsorption onto mesoporous g-Fe2O3

The experimental data on Cr(VI) adsorption onto mesoporous g-Fe2O3 at various pH values are presented in Fig. 5. The Cr(VI)

pH=2.5

18

Cr(VI) adsorbed concentration (mg/g)

by a magnetic field significantly differentiates each type. A qualitative ranking of the magnetic susceptibility of the two materials was demonstrated in a simple laboratory setup with a hand-held magnet as presented in Fig. 4(b) and (c). The mesoporous g-Fe2O3 could be completely separated from the aqueous solution within 5 min, while it took hours for 10 nm g-Fe2O3 nanoparticles. Presumably, the reasons for the differentiated magnetic separation behaviors are twofold: first, since mesoporous g-Fe2O3 is not superparamagnetic while 10 nm nonporous g-Fe2O3 is, mesoporous g-Fe2O3 responds to the same external magnetic field more strongly than 10 nm nonporous g-Fe2O3; secondly, because 10 nm nonporous g-Fe2O3 nanoparticles are virtually individual single iron-oxide crystals with a size around 10 nm (Fig. 2(d)), while mesoporous g-Fe2O3 is actually an cluster of single iron-oxide crystals with crystal size comparable with 10 nm g-Fe2O3. However, the overall particle size of mesoporous g-Fe2O3 is greater than 200 nm (Fig. 2(c)). With a lot of single crystals being assembled together, the average resistance to each single crystal of mesoporous g-Fe2O3, exerted by the medium, against its directed movement under an applied magnetic field is much reduced, leading to a much easier separation for mesoporous g-Fe2O3. This result is similar to that of settling separation in an aqueous phase: although the gravitational acceleration constant (g) is the same all the time, larger particles settle faster than smaller ones due to less resistance exerted by the

pH=5.0

pH=7.0

16 14 12 10 8 6 4 2 0

0

20

40

60

80

Equilibrium Cr(VI) concentration (mg/L) Fig. 5 – Cr(VI) adsorption onto mesoporous g-Fe2O3 at various pH values.

100

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b Relative % Cr(VI) recovered

Relative % Cr(VI) removed

a 100% 80% 60% 40% 20% 0%

0

20

40

60

80

100

100% 80% 60% 40% 20% 0%

120

0

20

Time (minutes)

40

60

80

100

Time (minutes)

Fig. 6 – Cr(VI) (a) adsorption and (b) desorption kinetics from mesoporous g-Fe2O3.

The effect of common ions in chrome-plating wastewater (Naþ, Ca2þ, Mg2þ, Cu2þ, Ni2þ, NO3 and Cl) on Cr(VI) adsorption onto g-Fe2O3 has been thoroughly studied in our previous work and the competition of these ions on Cr(VI) adsorption was found negligible (Hu et al., 2005a). These results indicate a high selectivity of g-Fe2O3 for Cr(VI), implying a potential of recycling Cr(VI) back to industries by using mesoporous g-Fe2O3. In this study, for the purpose of comparison, the same adsorption experiments were conducted with 10 nm g-Fe2O3. The saturation adsorption capacity of Cr(VI) onto 10 nm g-Fe2O3 was measured as 14.6 mg/g at pH 2.5, which is close to that of mesoporous g-Fe2O3 (15.6 mg/g). Given the fact that mesoporous g-Fe2O3 can be separated much more easily than 10 nm g-Fe2O3, the operation cost for Cr(VI) removal can be much reduced using mesoporous g-Fe2O3. FTIR technique provides information on the state of adsorbed molecules (particularly anions), thereby shedding light on adsorption mechanisms. The FTIR spectra of the sample collected after Cr(VI) adsorption onto the mesoporous g-Fe2O3 at pH 2.5 is shown in Fig. 7. A new peak at 948 cm1 in the FTIR spectrum occurred, which belongs to CrO2 4 (Hu et al., 2007). This serves as another evidence of physical adsorption because in a physical adsorption at the mineral–water interface, an oxyanion will retain its hydration shell and will not form a direct chemical bond with the oxide surface (Petit et al., 1995).

Absorbance

adsorption onto mesoporous g-Fe2O3 showed a saturation adsorption behavior at high equilibrium aqueous Cr(VI) concentrations. Clearly, the Cr(VI) saturation adsorption capacities increased sharply from around 4.5–15.6 mg/g (calculated as the average of the top three points) when the pH was decreased from 7 to 2.5. It has been proposed that, at low pH values, iron-oxide surfaces are protonated so that the net surface charge is positive, which enhances the adsorption of the negatively charged oxyanionic Cr(VI) (CrO2 4 ) species. As the pH increases, the iron-oxide surfaces are increasingly deprotonated so that the net surface positive charges are decreasing, leading to a reduction in Cr(VI) adsorption (Hu et al., 2005a). Thus, the pH-dependent behavior of Cr(VI) adsorption onto the mesoporous g-Fe2O3 suggests that Cr(VI) adsorption is via physical adsorption, i.e., electrostatic attraction, at low pH. The surface charge is neutral at the point of zero charge (PZC), which is 6.5 for the mesoporous g-Fe2O3. At pH higher than PZC, the Cr(VI) adsorption can be explained by anion exchange on the metal-oxide surfaces (Hu et al., 2005b). Although a direct comparison of the mesoporous g-Fe2O3 with other adsorbents is difficult, due to the different experimental conditions, it was found, in general, that the adsorption capacity of the mesoporous g-Fe2O3 for Cr(VI) at pH of 2.5 (15.6 mg/g) is higher than or comparable with those of diatomite (11.55 mg/g), anatase (14.56 mg/g), commercial activated carbon (15.47 mg/g), and beech sawdust (16.13 mg/g) (Sandhya and Tonnin, 2004; Hu et al., 2005a). The adsorption kinetics of Cr(VI) onto mesoporous g-Fe2O3 at a pH of 2.5 is shown in Fig. 6. The rate of Cr(VI) adsorption (Fig. 6(a)) was quite fast. About 90% of the total Cr(VI) adsorption occurred during the first 5 min of the reaction, while only a very small part of the additional adsorption appeared during the following 15 min of contact. The rapid adsorption of Cr(VI) by mesoporous g-Fe2O3 further suggests the adsorption mechanism is mainly due to electrostatic attraction and it also implies that the mesopores are not a limiting factor for Cr(VI) diffusion into the interior of the mesoporous g-Fe2O3, which is consistent with other studies using mesoporous materials (Wang et al., 2009). In comparison, the equilibrium time for adsorption of Cr(VI) by some other adsorbents is much longer. For instance, adsorption of Cr(VI) onto activated carbon is around 10–50 h (Aggarwal et al., 1999).

1400

mesoporous γ -Fe2O3 with adsorbed Cr(VI) mesoporous γ -Fe2O3 only

CrO42-

1200

1000

800

600

0 400

Wave number (cm-1) Fig. 7 – FTIR spectra of chromium adsorbed mesoporous g-Fe2O3.

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Fig. 8 – (a) TEM image of Cr(VI)-loaded mesoporous g-Fe2O3 and (b) the relevant EDX spectra of the circled area (A) in (a).

TEM image of the mesoporous g-Fe2O3 sample after Cr(VI) adsorption at pH of 2.5 with 40 mL of 100 mg/L Cr(VI) and the relevant EDX spectra of the marked area of the sample is shown in Fig. 8. EDX analysis is an analytical technique used for the elemental analysis or chemical characterization of a sample. The percentage of chromium within the circled area (A as in Fig. 8(a)) is around 1.8%, which corresponds well with the measured amount of Cr(VI) adsorbed onto the sample (15.6 mg/g), implying a largely uniform Cr(VI) adsorption within the mesoporous structure.

3.3.

Regeneration and reuse of mesoporous g-Fe2O3

As shown earlier, since the adsorption of Cr(VI) onto the mesoporous g-Fe2O3 was highly dependent on the solution pH, the desorption of Cr(VI) can be achieved by increasing the solution pH. In this study, the Cr(VI)-loaded mesoporous g-Fe2O3 was desorbed by using 0.01 M NaOH. Fig. 6(b) presents the desorption kinetics of the adsorbed Cr(VI). It should be noted that the amount of Cr(VI) desorbed was measured with 1,5-diphenylcarbazide colorimetric method using a UV/visible spectrophotometer to check the speciation of chromium as this method only responds to Cr(VI). An almost full recovery of Cr(VI) supports the hypothesis that there is no redox reaction, confirming that there is no chemical adsorption taking place. This is one advantage of using iron-oxide based materials, with 100%

100%

4.

80%

Removed

% Recovered

% Removed

90%

90%

70%

Recovered

80%

60%

1

2

3

4

iron oxidation state being þ3, because the highest iron valency (þ3) ensures that there is no redox taking place to reduce Cr(VI) to Cr(III), and hence permits the regeneration of the adsorbent. In the case with Fe3O4, where part of the iron is in an oxidation state of þ2, chemical adsorption of Cr(VI) has been reported and the formation of precipitated Cr(III) on the adsorbent surfaces led to irreversible adsorption (Hu et al., 2004). In addition, the rapid desorption kinetics of the adsorbed Cr(VI) also serves as a proof of electrostatic interaction involved in the Cr(VI) adsorption onto the mesoporous g-Fe2O3. In a wastewater treatment process that uses adsorption, regeneration of the adsorbent is crucially important. Nowadays, in many applications, reuse of the adsorbent through regeneration of its adsorption properties is an economic necessity. As mentioned previously, a complete desorption of Cr(VI) can be achieved by using 0.01 M NaOH solution as an extraction medium. To test the regeneration and reusability of the mesoporous g-Fe2O3, the mesoporous g-Fe2O3 was used in five consecutive adsorption/desorption cycles, with 40 mL of 50 mg/L Cr(VI) solution being used in the adsorption step while 40 mL of 0.01 M NaOH in the desorption cycle as desorbing agent. As shown in Fig. 9, at the end of the fifth cycle, the mesoporous g-Fe2O3 retained more than 90% of its original Cr(VI) adsorption capacity, and >90% of the total adsorbed Cr(VI) can be recovered during the desorption steps.

5

Cycles Fig. 9 – Percentage Cr(VI) removed and recovered during five adsorption/desorption cycles.

Conclusions

In this study, mesoporous magnetic Fe2O3 (g-Fe2O3) was synthesized using a ‘greener’ synthesis method. The synthesized mesoporous g-Fe2O3 simultaneously has a surface area comparable with and a higher susceptibility to magnetic separation than 10 nm nonporous g-Fe2O3. Cr(VI) adsorption onto mesoporous g-Fe2O3 exhibited a highly pH-dependent behavior, with the Cr(VI) adsorption capacity increasing with decreasing pH. Cr(VI) adsorption capacity of mesoporous g-Fe2O3 is comparable with that of 10 nm nonporous g-Fe2O3, but a much faster magnetic separation of mesoporous g-Fe2O3 makes it a better adsorbent for Cr(VI) than 10 nm nonporous g-Fe2O3. The results showed that mesoporous g-Fe2O3 can be regenerated, maintaining almost the same Cr(VI) adsorption capacity. Therefore, mesoporous g-Fe2O3 is a promising adsorbent for Cr(VI) removal from contaminated water.

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Acknowledgements This research was supported by the Hong Kong University of Science and Technology Research Project Competition Program under grant RPC07/08.EG03.

references

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