XANES evidence of arsenate removal from water with magnetic ferrite
Yao-Jen Tu
a,*, Chen-Feng You
a,*, Chien-Kuei Chang
b, Shan-Li Wang
caEarth Dynamic System Research Center, National Cheng-Kung University, No 1, University Road, Tainan 701, Taiwan, ROC
bDepartment of Chemical and Materials Engineering, National Kaohsiung University of Applied Science, No 415, Chien Kung Road, Kaohsiung 807, Taiwan, ROC cDepartment of Agricultural Chemistry, National Taiwan University, Taipei 10617, Taiwan, ROC
a r t i c l e i n f o
Article history: Received 21 March 2012 Received in revised form 10 December 2012 Accepted 6 February 2013 Available online Keywords: Adsorption Arsenate Contaminated groundwater Magnetic ferrite As K-edge XANES
a b s t r a c t
Arsenic (As) in groundwater and surface water is a worldwide problem possessing a serious threat to public health. In this study, a magnetic ferrite, was synthesized and investigated for its As(V) removal efficiency. The adsorption of As(V) by magnetic ferrite exhibited an L-shaped nonlinear isotherm, suggesting limiting binding sites on the adsorbent surface. The As K-edge X-Ray Absorption Near-Edge Structure (XANES) revealed that the adsorbed As(V) on ferrite was not reduced to more toxic As(III) by Fe2þin the ferrite structure. The maximum As adsorption capacity of ferrite was 14 mg/g at pH 3 and decreased with increasing pH due to enhanced electrostatic repulsion between As(V) and the adsorbent surface. Desorption of As(V) using six different acid and salt solutions showed that the desorption rate decreased in an order of H3PO4> Na3PO4> H2SO4> Na2SO4> HCl > HNO3. These
results suggest that magnetic ferrite without surface modification is an effective adsorbent for removing As(V) from water, which was confirmed by the effective removal of As(V) from contami-nated groundwater using this material. The used material can then be recovered using a magnet because of its paramagnetism; the adsorbed As(V) on the material can be recovered using H3PO4or
Na3PO4solutions.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
Arsenic (As) contamination is threatening water supplies in many countries (Mohan and Pittman, 2007), especially in Southern Asia (Charlet and Polya, 2006;Polizzotto et al., 2008). The anthro-pogenic sources of As include the waste and wastewater of various industries utilizing As and the materials used in agricultural pro-duction, such as herbicide and wood preservative (ATSDR, 2000;
Mohan and Pittman, 2007). In addition to man-made pollutants, elevated levels of As in surface waters and groundwater also occur naturally in certain areas of the world as a result of leaching from As-bearing minerals (Charlet and Polya, 2006; Polizzotto et al., 2008). Arsenic is highly carcinogenic after long-term or high-dose exposure. The occurrence of As in water supplies can pose a dele-terious impact on public health. Thus, developing techniques to remove As from contaminated water is an important task for healthy living.
The common treatment techniques for removing As from contaminated waters include membrane filtration (Heimann and Jakobsen, 2007; Iqbal et al., 2007; Fogarassy et al., 2009),
precipitation (Jia et al., 2006; Janin et al., 2009;Xu et al., 2010), coagulation (Parga et al., 2005;Baskan and Pala, 2010;Ingallinella et al., 2011), and adsorption (Clifford, 1999; Opiso et al., 2009;
Mamindy-Pajany et al., 2011). The adsorption method is considered more advantageous over others because of its removal effective-ness, treatment cost, and ease in equipment handling. Because the oxyanions of As have relatively high affinity to Al and Fe oxides, there have been many studies proposing to use these oxide min-erals to remove As in water (Mohan and Pittman, 2007). According to Edwards (Edwards, 1994), adsorption of arsenate and arsenite onto Fe and Al oxides ranged from 0.04 to 0.7 mols per mol of oxide, depending on the pH in the solutions and surface area of oxides. Meanwhile, to increase the adsorption capacity, nano-sized oxides are often used. The major drawback of using nano-materials in water treatment is its difficulty to remove the used materials throughfiltration, centrifugation and sedimentation.
In this study, the As(V) removal was investigated by using magnetic ferrite. Ferrite is a magnetic Fe oxide with a spinel structure containing both Fe2þ and Fe3þ. The As(V) removal ca-pacity of a ferrite was reported to be 1.575 mg/g (Parsons et al., 2009) and 3.70 mg/g (Chowdhury and Yanful, 2011). To achieve a better removal efficiency of As from waters, previous studies also focused on different preparations or surface modification of ferrite. However, the surface modification or coating processes in
* Corresponding authors. Tel.: þ886 6 2757575x65438; fax: þ886 6 2758682. E-mail address:todojen@gmail.com(Y.-J. Tu).
Contents lists available atSciVerse ScienceDirect
Journal of Environmental Management
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j e n v m a n
0301-4797/$e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.jenvman.2013.02.006
preparing the materials imply an extra cost of applying them to treating As-containing water and wastewater. Thus, in this study, magnetic ferrite was synthesized without any modification and investigated for its As(V) adsorption capacity. The adsorption ca-pacity of magnetic ferrite is expected to be better than its coun-terpart with a large crystal size due to its increasing surface area available for As adsorption. After treating As-containing water, this magnetic material can be recovered using an external magnetic field to avoid the difficulty in separating the material using tradi-tional methods, such as centrifugation and filtration. Thus, the application of this magnetic material to treating As-containing water is expected to be more rapid and cost-effective. The objec-tives of this study were to investigate the effects of pH, contact time, and adsorbent dosage on the As(V) removal of magnetic ferrite and the possibility of regenerating the used material. The information provided by this study is essential for applying mag-netic ferrite to further development of effective and safe methods for scavenging and recovering As(V) in water.
2. Material and methods 2.1. Synthesis of magnetic ferrite
Magnetic ferrite was synthesized using a hydrothermal method. 27.8 g of reagent grade FeSO4$7H2O was dissolved into 1000 mL
water and the pH of the solution was subsequently adjusted to 8.0 by adding 0.1 M NaOH and 0.1 M HNO3. Under continuous air
purging (flow rate ¼ 3 L/min), this synthesis process proceeded at 80 C for 60 min while the pH of the solution was maintained constant at 8.0. The corresponding reaction is described as Eq.(1).
3Fe2þþ 6OHþ 1=2O2/Fe3O4þ 3H2O (1)
The synthesized product was collected using a magnetic sepa-ration method by taking advantage of its magnetism. The ferrite product was then washed with de-ionized water several times until the pH of the solution reached 7. The solid was then dried at 50C for 24 h in an oven and stored for further tests.
2.2. Characterization of ferrite
The synthesized ferrite sample was characterized using XRD (D8 Advance, Germany) with a graphite monochromatic copper radia-tion over the 2
q
range of 20e80. The BET surface area wasdeter-mined using an ASAP 2010 analyzer (Micromeritics, USA) and N2
adsorption at 77 K. The surface morphology and particle size were examined by scanning electron microscopy (JSM-6330, Japan). The saturation magnetization of the synthesized ferrite was measured using a Superconducting Quantum Interference Device (SQUID, MPMS-XL7, Quantum Design, USA) at 27C.
The point of zero charge (PZC) determination was carried out according to the procedures described bySmiciklas et al. (2000). Briefly, aliquots of 0.1 M KNO3were prepared in a series offlasks
and the pHs of the aliquots were adjusted to values ranging from 2 to 12 using 0.1 M KOH or HCl solution. The ferrite samples were then added into each of theflasks to have a solid-to-solution ratio of 1:200 (w/w). The suspensions were allowed to equilibrate for 24 h in a shaker thermostated at 27 1C. Then, the suspensions
were magnetically separated from the aqueous phase by using a magnet with 4000 Gauss and the pH values (pHf) of the residual
solutions were measured using a pH meter (WalkLAB TI 9000, TRANS instruments, Singapore). For each sample, the values of the final pH (pHf) were plotted against the values of the corresponding
initial pH (pHi). The experimental pHf at the stable values was
defined to be the PZC for the sample.
2.3. As(V) adsorption
As(V) adsorption experiments were conducted using the batch method. Ten mL of 10 mg/L As(V) solutions were added into 15 mL lid tubes containing 0.05 g magnetic ferrite. To investigate the effect of pH on the As(V) adsorption of ferrite, the pH of the As(V) solu-tions were controlled at 2.4 0.1, 3.0 0.1, 4.0 0.1, 5.0 0.1, 6.0 0.1, 7.0 0.1, 8.0 0.1, 9.0 0.1, 10.0 0.1, 11.0 0.1, 12.0 0.1 by adding 0.1 M NaOH or 0.1 M HNO3solution. The lid
tubes were then put on a rotary shaker with a rotating speed of 30 rpm and the temperature was maintained at 27 1C. The solid
and liquid phases were magnetically separated using a magnet with 4000 Gauss. The As concentrations in the supernatant were determined by ICP-MS (Element XR, Germany). The adsorbed amount of As(V) on the ferrite was determined using the differ-ences between the initial and equilibrium As concentrations. 2.4. As K-edge XANES analysis
As K-edge X-Ray Absorption Near-Edge Structure (XANES) analysis was conducted at the Beamline 17C in the National Syn-chrotron Radiation Research Center (NSRRC) in Hsin-Chu, Taiwan. Samples werefixed onto an aluminum holder sealed with Kapton tape. The As K-edge XANES spectra of the samples were obtained on fluorescent mode using a Lytle detector with a 6-m germanium filter and a set of Soller slits. All spectra were calibrated to the edge of metallic As at 11,867.0 eV. The scans for each sample were averaged, followed by background removal and normalization. The XANES spectra of NaAsO2and Na2HAsO4chemicals (J.T. Baker, Inc.)
were also obtained to serve as the reference standards for the As(III) and As(V) oxidation states.
2.5. As(V) desorption
Desorption experiments were conducted using six different acid and salt solutions (HNO3, HCl, H2SO4, Na2SO4, H3PO4, Na3PO4) with a
concentration of 0.1 M. Magnetic ferrite wasfirst reacted with 10 mg/ L As(V) solution at pH 3. Subsequently, the ferrite samples were washed with de-ionized water several times to remove excessive salts and the acid or salt solution was added into the sample to initiate the desorption process. The suspensions were shaken for 30 min, and the ferrite solids were then separated from the solutions using a magnet with 4000 Gauss. The desorption efficiency was calculated from the amount of As released into the solutions. 3. Results and discussion
3.1. Characterization of adsorbent
Fig. 1a displays the SEM of the synthesized ferrite, showing that the primary particle size ranged from 30 to 90 nm. This result confirms that the synthesized ferrite is a nano-scaled adsorbent. The XRD pattern of this material showed the diffraction peaks at the d-spacings of 2.966, 2.530, 2.422, 2.098, 1.713, 1.615, 1.483, and 1.280 A (Fig. 1b), which match well with those of magnetite (JCPDS file number 03-065-3107). No other crystalline phases were detected in the XRD pattern. The BET surface area, pore volume, and average pore diameter of the adsorbent were determined to be 40.3 m2/g, 0.07 cm3/g, and 15.41 A respectively. The point of zero charge (PZC) of the synthesized ferrite was around 7.1 (Fig. S1), determined based on the procedure described in
Smiciklas et al. (2000). This is consistent with the value measured by zeta sizer apparatus reported byZhang et al. (2010). The satu-ration magnetization of synthesized nano-ferrite was determined to be 82.53 emu/g (Fig. S2). No remanence was detected in
contamination level of drinking water regulated by WHO (i.e., 10
m
g/L). The As removal efficiency was tested by adding 0.05 g magnetic ferrite into 10 mL As contaminated groundwater. The results demonstrated that the As removal efficiency could reach more than 91.7% (Table 2). The highest residual As concentration was 7.0m
g/L, which was still lower than the maximum contamina-tion level of drinking water regulated by WHO. Although a small amount of ferrite was dissolved at pH< 3, magnetic ferrite generally exhibited a high efficiency of As removal in real groundwater. 4. ConclusionMagnetic nano-ferrite synthesized by a hydrothermal method without surface modification appeared to be an effective adsorbent for As(V). The results show that this adsorbent has a great potential for treating As-containing groundwater and can be recovered using an external magneticfield. Because the PZC of nano-ferrite was 7.1, it exhibited a better As(V) removal rate at low pH. It is thus rec-ommended that this nano-ferrite be used at low-to-neutral pH as an adsorbent for As(V). The removal of As(V) using this material from alkaline waters and wastewaters may be significantly enhanced via pre-acidification of the waters.
Acknowledgments
This work was financially supported by the Environmental Protection Administration (EPA-101-X007). The authors are grate-ful to Dr. Jyh-Fu Lee for his assistance in X-ray absorption mea-surements. This research was carried out (in part) at the National Synchrotron Radiation Research Center in Hsinchu, Taiwan. Appendix A. Supplementary data
Supplementary data related to this article can be found athttp:// dx.doi.org/10.1016/j.jenvman.2013.02.006.
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Chem. Eng. J. 158, 599e607. Table 2
Removal efficiency of As from six contaminated groundwaters using magnetic ferrite. Well station No. Adsorption pH As (mg/L) Fe (mg/L) Beforea Afterb Removal
(%) Beforea Afterb 1 6.97 21.6 b.d.c 100 0.75 0.75 3.12 21.6 b.d. 100 0.75 0.78 1.61 21.6 b.d. 100 0.75 26.80 2 6.93 82.8 b.d. 100 3.64 3.64 3.23 82.8 b.d. 100 3.64 3.88 2.15 82.8 5.6 93.2 3.64 8.03 3 7.65 84.3 4.6 94.5 0.15 0.15 3.18 84.3 b.d. 100 0.15 0.17 2.20 84.3 7.0 91.7 0.15 8.78 4 7.50 44.6 3.2 92.8 0.06 0.06 3.30 44.6 b.d. 100 0.06 0.06 2.01 44.6 b.d. 100 0.06 12.37 5 7.02 54.6 b.d. 100 1.45 1.45 3.02 54.6 b.d. 100 1.45 1.52 2.18 54.6 3.1 94.3 1.45 12.81 6 6.84 27.5 b.d. 100 0.43 0.43 3.11 27.5 b.d. 100 0.43 0.46 1.92 27.5 b.d. 100 0.43 29.06 Amount of adsorbent¼ 0.05 g Fe3O4, Volume¼ 10 mL, Temperature ¼ 27C,
Time¼ 4 h.
The adsorption pH were operated at neutral (pH 6.84e7.65) and acidic (pH 1.61e3.30) conditions to compare the As removal efficiency.
aConcentration before adsorption. b Concentration after adsorption.
c b.d.: below detection limit (For As: 2.3mg/L, Fe: 5.6mg/L).
