- Email: [email protected]
Removal of manganese in batch and fluidized bed systems using beads of zeolite a as adsorbent
Accepted Manuscript Removal of manganese in batch and fluidized bed systems using beads of zeolite a as adsorbent Mina Jovanovic, Iztok Arcon, Janez Kovac, Natasa Novak Tusar, Bojana Obradovic, dr Nevenka Rajic, Prof. PII:
S1387-1811(16)30001-4
DOI:
10.1016/j.micromeso.2016.02.026
Reference:
MICMAT 7589
To appear in:
Microporous and Mesoporous Materials
Received Date: 9 November 2015 Revised Date:
28 January 2016
Accepted Date: 10 February 2016
Please cite this article as: M. Jovanovic, I. Arcon, J. Kovac, N.N. Tusar, B. Obradovic, N. Rajic, Removal of manganese in batch and fluidized bed systems using beads of zeolite a as adsorbent, Microporous and Mesoporous Materials (2016), doi: 10.1016/j.micromeso.2016.02.026. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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REMOVAL OF MANGANESE IN BATCH AND FLUIDIZED BED SYSTEMS
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USING BEADS OF ZEOLITE A AS ADSORBENT
3 Mina Jovanovica, Iztok Arconb, c, Janez Kovacc, Natasa Novak Tusard, b, Bojana
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Obradovice, Nevenka Rajice,*
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a
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Belgrade, Serbia
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b
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c
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d
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e
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Belgrade, Serbia
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Innovation Center of the Faculty of Technology and Metallurgy, Karnegijeva 4, 11000
University of Nova Gorica, Vipavska 13, 5000 Nova Gorica, Slovenia
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Institute Jozef Stefan, Jamova 39, 1000 Ljubljana, Slovenia
National Institute of Chemistry, Hajdrihova 19, 1000 Ljubljana, Slovenia
Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11000
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Mina Jovanovic: [email protected]
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Iztok Arcon: [email protected]
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Janez Kovac: [email protected]
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Natasa Novak Tusar: [email protected]
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Bojana Obradovic: [email protected]
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Corresponding author:
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Prof. dr Nevenka Rajic
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Faculty of Technology and Metallurgy
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University of Belgrade
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Karnegijeva 4
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11000 Belgrade
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Tel:+381113306623
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Fax:+381113370387
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e-mail: [email protected]
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ACCEPTED MANUSCRIPT Abstract
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In this study the uptake capacity of Mn(II) ions by zeolite A beads was investigated for
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different initial Mn concentration (100-400 mg Mn dm-3) in batch mode at 25-55 ºC.
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The obtained adsorption capacity varying from 30 to 50 mg Mn g-1 demonstrated a high
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affinity of zeolite A towards Mn(II) present in solutions. Kinetic studies indicated the
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intra-particle diffusion as the rate limiting step up to 45 °C with apparent diffusivities in
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the range (1.2 – 2.0)×10-13 m2 s-1 and the activation energy of 21.9 kJ mol-1, which
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implies strong interactions between the zeolite A and Mn ions. At 55 °C ion-exchange
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became the rate limiting step. The adsorption isotherms were studied at 25 oC showing
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that the Mn adsorption is the best described by the Langmuir model suggesting a
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homogenous zeolite surface. XPS analysis of the Mn-loaded beads showed that there is
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no surface accumulation of Mn but an almost uniform Mn distribution inside zeolite A,
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whereas XANES and EXAFS suggested that the adsorption of Mn(II) was followed by
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the Mn(II) oxidation and oxide formation. Regeneration of the spent zeolite was
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examined in 8 adsorption/desorption cycles by a chelating Na2EDTA in a fluidized
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column. It has been found that zeolite A beads could be reused for at least 4 cycles with
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satisfactory Mn(II) adsorption efficiencies of about 70 %.
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Keywords – zeolite A, manganese, adsorption kinetics, EXAFS/XANES, XPS
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1.
Introduction Manganese has been widely used in many industrial processes including
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production of stainless steel, ceramics, dry battery cells, or electrical coils. Wastewaters
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from these processes usually contain high manganese concentrations causing yellowish
54
water color and unpleasant taste and odor. Although Mn is an essential element for
55
humans and animals, necessary for normal bone formation, exposure to high Mn
56
concentrations causes nervous system damages and various disorders.
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Different methods have been used to reduce wastewater Mn concentration and to achieve the required water quality ranging including oxidation with potassium
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permanganate, precipitation, adsorption by magnetic nano- and microparticles [1], and
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various biological processes [2]. Most of these techniques are burdened with high
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investment and operational costs as well as with disposal of the spent materials. In order
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to overcome these limitations, current trends in wastewater treatments aim to devise
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effective, economic and easy to handle methods and materials [3].
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Adsorption is one of the approaches widely used due to high efficiency, simple operation and easy control of the process. Among different adsorbents, low cost zeolites
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attract attention due to good adsorption capacities. Zeolite A exhibits a high ion-
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exchange capacity of 5 – 6 mmol g-1 for different toxic metal ions [4-9]. However, to
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our knowledge, data considering removal of Mn(II) by zeolite A are lacking.
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In the present work we investigate Mn(II) adsorption from water solutions by
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commercial zeolite A beads. In specific, we focus on kinetics and mechanisms of
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Mn(II) adsorption including regeneration of the spent zeolite.
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The adsorption kinetics was experimentally investigated at different
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temperatures and initial Mn(II) concentrations followed by mathematical modeling.
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on the ion-exchange by natural clinoptilolite [10]. In order to reveal and quantify
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phenomena involved in the adsorption process, interactions of Mn with the surface of
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zeolite A were also investigated in detail using X-ray Photoelectron Spectroscopy (XPS
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or ESCA) and Mn K-edge XANES and EXAFS techniques.
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2.
Materials and methods
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2.1
Materials
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Beads of commercially available zeolite A (Asorbio ZAG1, Silkem d.o.o.,
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Slovenia) with an average diameter of 0.71 ± 0.07 mm and the density of 1466 ± 40 kg
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m-3 were used in this study. The beads contain zeolite 4A (86 ± 3 %), sodium silicate
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(13 ± 1 %) and water (1.5 ± 1 %) as specified by the producer.
Mn(II) solutions were prepared by dissolving the appropriate amounts of
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MnSO4×H2O (Sigma-Aldrich, Germany) in distilled water. NaCl (Carlo Erba, Italy) was
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used for the preparation of sodium chloride solution and C10H14N2Na2O8×2H2O (Lach-
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Ner, Czech Republic) for the preparation of Na2EDTA solutions. All chemicals used
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were of analytical grade.
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Adsorption experiments Mn(II) adsorption kinetics was studied in batch experiments at 25 and 45 °C
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using Mn(II) solutions with initial concentrations of C0 = 1.82, 3.64, 5.46 and 7.28
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mmol dm-3 (i.e. 100, 200, 300, 400 mg Mn(II) dm-3, respectively). Influence of
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temperature on the adsorption kinetics was studied at 25, 35, 45 and 55 °C at the initial
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Mn(II) concentration of 300 mg dm-3. In all experiments solid/liquid ratio was 3:1000,
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45 °C and 300 mg dm-3 at 55 °C in which the ratio of 1:500 was used in order to verify
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that the zeolite saturation was reached in all experiments. The Mn(II) adsorption
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isotherm was determined at 25 °C in the concentration range 100-600 mg Mn dm–3. The
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suspensions were shaken at about 100 rpm in a thermostated water bath (Memmert
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WPE 45, Germany). The Mn-loaded beads were recovered by filtration.
All experiments were carried out at the original pH (6.2) which slightly
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increased at the end of the experiments (6.5). Also, the experiments were carried out in
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duplicates under controlled conditions: temperature in the thermostated bath was
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maintained constant to within ±0.1 °C, the zeolite samples were weighted to four-digit
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accuracy.
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2.3
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Regeneration and reuse of the spent zeolite beads
Regeneration of the spent beads was studied by two experiments. In the first one,
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Mn-containing beads were suspended in NaCl (2 M) or Na2EDTA (0.01 M) solution
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followed by shaking in the water bath at ~100 rpm at 25 °C for 24 h. In the second
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experiment a fluidized bed system with recirculation described in details in our previous
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work [11] was used. The system consisted of a column with the inner and outer
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diameters of 2.4 and 3.0 cm, respectively, and the height of 16.7 cm connected to a
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glass reservoir and recirculated by a peristaltic pump (Masterflex L/S, Cole-Parmer,
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USA). Before the experiments, the beads were washed in distilled water under
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recirculation in the fluidized bed system for 24 h in order to release any exterior or
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loosely bound sodium silicate. Next, the column was filled with 15.2 g of washed
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zeolite beads (corresponding to 12.5 g of initial zeolite beads due to water absorption of
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cycles, the system was filled with 1.5 dm3 of the solution with the initial Mn(II)
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concentration of 300 mg dm-3, while in desorption cycles, 1.5 dm3 of 0.01 M Na2EDTA
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solution was used. Recirculation of solutions in all experiments was performed at the
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flowrate of 10.2 cm3s-1, while the fluidized bed height was determined as 9.3±0.4 cm.
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Adsorption cycles lasted for 24 h while desorption was performed during 2 h. At the end
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of each cycle liquid samples were analyzed for Mn(II) concentration. In total 8
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adsorption/desorption cycles were performed at room temperature.
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Mn(II) concentration was determined at four-digit accuracy by Atomic Absorption Spectroscopy (AAS) using Varian Spectra AA 55B (Varian Medical
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Systems, Australasia Pty Ltd., Australia); at least five measurements were done for each
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data point.
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The crystallinity of the beads was checked by powder X-ray diffraction analysis
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(PXRD) using a PANalytical X'Pert PRO diffractometer with a CuKα radiation (1.5406
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Å). The PXRD data were collected in the 2 theta range from 5 to 75° in steps of 0.02°
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with a scan step time 15 s.
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UV-visible absorption spectrum was recorded using a Shimadzu 3600 double
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beam UV-Vis-NIR Spectrophotometer. The X-ray Photoelectron Spectroscopy (XPS or
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ESCA) analysis was performed using a PHI-TFA XPS spectrometer, illuminating the
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sample surface by X-ray radiation from Al monochromatic source. The samples were in
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the form of 1 mm thick pressed pellets. The analyzed area was 0.4 mm in diameter and
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the analyzed depth was 2-5 nm. The XPS survey and high energy resolution scan
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order to follow the distribution of elements (in particular Mn) in the subsurface region,
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the XPS depth profiling was performed by alternating cycles of ion sputtering and
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acquisition of photoelectron spectra. Sputtering of solid surfaces by ion bombardment is
146
a common method to remove the surface material in order to reveal the subsurface
147
composition for characterization of surfaces and thin films. The ion bombardment
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process results in a desorption of the target (sample) atoms from the top-most surface
149
layer. By combining cycles of ion bombardment in the XPS spectrometer with
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subsequent XPS analyses of the newly formed surfaces one can obtain the depth
151
distribution of elements in the subsurface region. The result is a depth profile, which
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shows concentrations of elements in the direction from the surface to the bulk of the
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sample. In this way the subsurface region of up to several hundreds of nm can be
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investigated.
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Ion sputtering was performed with 1 keV Ar+ beam rastering over 3 x 3 mm2
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area. In this way a depth distribution of elements was obtained. Total sputtering time
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was 10 min with the sputtering rate of 1.0 nm min–1 measured on the Ni/Cr reference
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multilayer structure. The base pressure in the XPS analysis chamber was 2 x 10–9 mbar.
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During analysis the XPS spectra were slightly shifted for about 2 eV due to sample
160
charging. The binding energy of 284.8 eV for the C 1s peak (characteristic for C-C/C-H
161
bonds) was used as the reference energy for spectra alignment [12]. The spectra of the
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Si 2p, O 1s, Na 1s, Al 2p, Mn 2p and C 1s were acquired during analyses. The relative
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sensitivity factors provided by the instrument manufacturer were used to calculate
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concentrations in at.%. [12]. The composition was calculated in the model of
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homogenous matrix.
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X-ray absorption spectra XANES and EXAFS were measured in the energy region of the Mn K-edge in transmission mode at XAFS beamline of the ELETTRA
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synchrotron facility in Trieste, Italy. A Si (111) double crystal monochromator was used
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with about 0.7 eV resolution at Mn K-edge (6539 eV). The intensity of the
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monochromatic X-ray beam was measured by three 30 cm long consecutive ionization
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detectors respectively filled with the following gas mixtures: 580 mbar N2 and 1420
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mbar He; 90 mbar Ar, 910 mbar N2, and 900 mbar He; 35 mbar Ar , 52 mbar N2 and
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1000 mbar He. The absorption spectra were measured within the interval [-250 eV to
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1000 eV] relative to the Mn K-edge. In the XANES region equidistant energy steps of
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0.2 eV were used, while for the EXAFS region equidistant k steps of 0.03 Å-1 eV were
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adopted with an integration time of 2 s per step. In all experiments the exact energy
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calibration was established with simultaneous absorption measurement on 5-micron
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thick Mn metal foil placed between the second and the third ionization chamber.
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Absolute energy reproducibility of the measured spectra was ±0.01 eV.
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3.
Results and discussion
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3.1
Adsorption kinetics
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Kinetics of Mn(II) removal from aqueous solutions by zeolite A beads was
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studied in the temperature range from 25 to 55 °C using four initial Mn(II)
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concentrations, ranging from 100 to 400 mg Mn dm-3. For the experiments performed at
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25 and 45 °C it has been found that the adsorption rate and the amount of the adsorbed
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Mn per 1 g of zeolite (qt) do not depend on the initial Mn concentration (STD < 17 % ).
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Figure 1 shows the results obtained at 25 °C as typical ones.
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These results lead to conclusion that internal diffusion is involved in the adsorption and that it is the rate controlling step in the temperature range from 25 to
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45 °C. Accordingly, the adsorption kinetics can be described by the intra-particle
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diffusion model:
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qt = kd t 0.5
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where qt is the adsorbed concentration at time t and kd is the intra-particle
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diffusion rate constant. The rate constant is related to the intraparticle diffusivity, D,
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derived for a spherical particle as:
6qe R
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D
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kd =
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where R is the particle radius and qe is the adsorption capacity of the zeolite at equilibrium.
(2)
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π
For the intra-particle diffusion model, according to the Eq. (1), the plot qt vs. t0.5
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should be linear passing through the origin with the slope yielding kd. The plots of qt vs.
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t1/2 in the present adsorption experiments indeed yielded straight lines so that the model
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Eq. (1) predicts well the experimental data with a slight overestimate of the value of
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adsorbed Mn(II) at 45 °C after 24 h (Fig. 2).
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Figure 2
The intra-particle diffusion rate constants and experimentally determined zeolite
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adsorption capacities at equilibrium averaged for each temperature were applied in the
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Eq. (2) to calculate apparent Mn(II) diffusivities within the zeolite A beads at different
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temperatures (Table 1).
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Table 1. Intra-particle diffusion rate constants, kd, intraparticle diffusivities, D, and
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experimentally determined adsorbed concentrations of Mn(II) at equilibrium, qe, at
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different temperatures
-1 -0.5
kd (mg g h
)
D x1013 (m2 s-1)
35 °C
45 °C
28.5±1.1
34.1±2.5
44.8±1.0
5.5±0.4
7.7±1.6
1.2±0.2
1.6±0.7
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11.5±0.7
2.0±0.2
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qe (mg g-1)
25 °C
It could be noticed that the capacity of zeolite A for Mn(II) of about 30 mg g-1 at 25 ºC (Table 2) is significantly higher than that reported for natural zeolites (e.g. for
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natural clinoptilolite it has been reported to be in the range 4-15 mg g-1 [10, 13-15],
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about 16 mg g-1 for montmorillonite [16] and 4.3 mg g-1 for mordenite [17]). This
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suggests that zeolite A can be a promising adsorbent for Mn(II).
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Table 1 shows that both the internal diffusion rate constant and apparent
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diffusivity increase with the increase in temperature. Diffusivity dependence on
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temperature, T, is usually described by the Arrhenius’ equation: RT
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D = D0e Ea
(3)
where D0 is the pre-exponential constant, Ea is the activation energy and R is the
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universal gas constant (R = 8.314 J mol-1 K-1). Parameters in the Eq. (3) can be
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determined from the plot lnD vs. T based on the logarithm form of this equation:
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ln D = ln D0 − Ea RT
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The apparent diffusivity values determined in this study at three investigated
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(4)
temperature points yielded a linear trend (R2 = 0.998) in the Eq. (4) (data shown in
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Supplementary material) giving the value of 21.9 kJ mol-1 for the activation energy and
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8.0×10-10 m2 s-1 for D0.
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It should be noted that Mn(II) diffusion through the beads includes diffusion through macropores and the filler (i.e. sodium silicate) and diffusion through the zeolite
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A pore systems. Also, the apparent Mn(II) diffusivity values obtained in the present
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study are in the range of 10-12 - 10-17 m2 s-1 that were reported for various metal ions in
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different types of zeolites (e.g. diffusivities of 6.1 x 10-16 m2 s-1 and 1.3 x 10-17 m2 s-1
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were reported for Na+ and Ba(II), respectively, in chabazite [18] while diffusivities of
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Zn(II), Cd(II), Cs+ and Sr(II) in zeolite A at 25 ºC were reported in the range (1.8 – 6.5)
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x 10-12 m2 s-1 [4, 5]. Finally, the obtained value is quite similar to that previously found
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for Cu(II) (2 x 10-13 m2 s-1) [11]. Based on the reported data, it could be assumed that the
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apparent diffusivity values correspond to diffusion through the pore system of zeolite A
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as the rate limiting step.
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However, the intraparticle diffusion model does not describe the experimental data obtained at 55 °C, indicating that another process but not diffusion itself, is the rate
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limiting step. Accordingly, the adsorption kinetics at 55 °C was modeled by the
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previously developed kinetic model based on the ion exchange mechanism [10]. The
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model assumed adsorption of a divalent metal ion (M2+) through two reversible steps: 1)
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release of two Na+ from the zeolite lattice, and 2) M2+ binding to the remaining free site:
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+ → 2 Naaq 2 Naz P0.5 ← +P
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k3 → MzP M aq2+ + P ←
(6)
k1
k2
k4
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where k represents the kinetic rate constant of each of the reactions and P is the free cation site in the zeolite lattice for binding of one M2+ ion. Subscripts aq and z
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describe ions in the solution and in the zeolite, respectively, while the subscript 0.5 in
253
Eq. (5) indicates that 2 sites occupied by Na+ ions are close to each other in order to
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provide 1 site (P) for one M2+ ion. All details on the model derivation are provided in
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the Supplementary material.
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Taking the above into account and assuming that the concentration of free sites is small and constant over time, the final expression for the M2+ adsorption rate is
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derived as [10]:
dqt ,m dt
= k1
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(CMo − qt ,m )(CSzo − 2qt ,m )2 − 4qt3,m / K m 4k2' qt ,m 2 + (CMo − qt ,m )
(7)
where qt,m is the molar concentration of adsorbed M2+, CMo and CSzo are the initial
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concentrations of M2+ ions in the solution and Na+ ions in zeolite, respectively, k2’ is the
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constant defined as k2’ = k2/k3. Km is the equilibrium constant defined as:
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where qe,m is the equilibrium molar concentration of adsorbed M2+ on zeolite.
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4qe,m kk Km = 1 3 = k2 k 4 ( CMo − qe,m )( CSzo − 2qe, m )2
The model parameters qe,m, CSzo, Km, k1, and k2’ were determined as follows.
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Mn(II) capacity of the beads at 55 °C, qe,m, was experimentally determined as 0.958
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mmol g-1 (52.6±0.8 mg g-1) while the initial concentration of Na+ ions in zeolite A, CSzo,
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was calculated as 4.73 mmol g-1 based on the chemical formula of zeolite A. The
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equilibrium constant Km was then calculated using the Eq. (8). The rate constant of Na+
270
release from the zeolite lattice (k1) was shown previously to be approximately constant
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of 23.1 g mmol-1 h-1 [10]. Finally, the only independent parameter, the constant k2’ was
273
determined by the least-squares fit of experimental data by the model Eq. (7) using
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MATLAB software package. It should be noted that the M2+ desorption rate constant,
275
k4, is then resulting from the value of the equilibrium constant Km and Eq. (8).
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The model described accurately experimental results of adsorbed Mn(II)
277
concentration as a function of time (STD= 3.6 %, Fig. 3) with all constants summarized
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in Table 2 along with the previously reported constants determined for Mn(II)
279
adsorption in the natural clinoptilolite at 25 °C and at the same initial Mn(II)
280
concentration of 300 mg dm-3.
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Figure 3
Table 2. Kinetic parameters of the kinetic model based on the ion-exchange mechanism
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for Mn(II) adsorption by zeolite A and by natural clinoptilolite at the initial Mn(II)
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concentration of 300 mg dm-3 and at 55 and 25 °C, respectively
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zeolite A
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K (-)
k2’=k2/k3
(g mmol-1)
k4·102 (h-1) Ref.
natural clinoptilolite
55 °C
25 °C
0.25
0.039
8,000
13,976
1.2
4.2
This study
[10]
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The equilibrium constant for Mn(II) adsorption by zeolite A is significantly higher than that for the natural clinoptilolite (Table 2) reflecting the higher adsorbed
288
equilibrium concentration, qe,m, in the former case (52.6 and 8.2 mg g-1, respectively)
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due to the higher temperature. Higher temperature also led to slightly lower k2’ in
290
zeolite A indicating the increase in the overall exchange rate for Mn(II) adsorption with
291
the increase in temperature as also found previously for Cu(II) adsorption in the natural
292
clinoptilolite [10]. Moreover, the smaller value of desorption constant k4 implies
293
stronger interactions of Mn(II) with the lattice of zeolite A than with that of natural
294
clinoptilolite. Finally, excellent agreement of model predictions with experimental data
295
with the assumed rate constant k1 implies that the process of Na+ release is negligibly
296
affected by the zeolite type.
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The adsorption isotherms were studied at 25 °C (the results are presented in the
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Supplementary material). The equilibrium data have been analyzed by several empirical
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adsorption isotherm models [19]. The Langmuir model [20] gives excellent agremeent
300
with experimental results suggesting the Mn adsorption on a highly homogenous
301
surface. Moreover, the Langmuir constant of the saturated monolayer adsorption
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capacity predicted a value of 40.3 mg g-1 as the maximal concentration that could be
303
adsorbed on the zeolite beads.
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Regeneration and reuse of zeolite A beads
PXRD analysis confirmed that the Mn(II) adsorption does not affect the crystal
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structure of zeolite A (the results are presented in the Supplementary material)
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indicating that regeneration of zeolite A beads could be possible by a simple ion-
308
exchange reaction, i.e. by treatment of Mn-loaded beads in a NaCl solution. However,
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300 mg dm-3 at 25 ºC (Mn-300-25) and at 55 ºC (Mn-300-55) showed that the
311
regeneration in such a system does not occur readily. In specific, only 0.6 % of Mn was
312
desorbed from Mn-300-25 and about 8 % from Mn-300-55 in 2M NaCl solution. On the
313
other hand, the use of Na2EDTA solution (0.01 M) instead of NaCl provided better
314
results. For both samples about 80% of Mn was desorbed from the beads suggesting that
315
the desorption is not a simple process. It is well known that Na2EDTA exhibits a strong
316
affinity towards divalent metal cations due to its complexing ability. Namely, the
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hexadentate (chelate) ligand forms very stable soluble complexes.
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The regeneration of the spent beads using Na2EDTA solution was further
319
investigated in a fluidized bed system in 8 adsorption/desorption cycles. Figure 4 shows
320
Mn(II) adsorption efficiencies and adsorbed Mn(II) concentrations in the zeolite beads
321
as functions of the cycle number. Mn(II) adsorption efficiencies were calculated as the
322
ratio of the adsorbed Mn(II) amounts achieved in the column in each cycle and the
323
maximum possible amount that can be adsorbed when the equilibrium is accomplished
324
at 25 °C (Table 1). It should be mentioned that the first cycle should be regarded as a
325
preparatory phase since bead collisions induced mechanical abrasion of superficial
326
layers leading to reverting Mn to the solution and inducing yellowish color. After the
327
first cycle, both adsorption efficiency and zeolite capacity stayed approximately
328
constant during the next 4 cycles (68±7 % and 19.3±2.1 mg g-1, respectively) after
329
which they started to decrease (Fig. 4). This finding could be explained by incomplete
330
zeolite regeneration of about 80 %, as described above, which slowly led to manganese
331
accumulation within the beads, thus decreasing the zeolite capacity in repeated
332
adsorption cycles.
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3.3
Study of Mn interactions with the zeolite In order to improve regeneration efficacy, Mn-containing beads were
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Figure 4
characterized in detail. Firstly, the crystallinity of the beads recovered by Na2EDTA was
337
checked by PXRD. The pattern (given in the Supplementary material) clearly shows
338
that the treatment with Na2EDTA does not affect crystallinity of the zeolite A leading to
339
the conclusion that the recovery with Na2EDTA can include several simultaneous
340
processes: complexing of Mn species adsorbed on the zeolite, removal of the Mn
341
complex from the zeolite to aqueous solution, and an ion replacement including return
342
of Na+ ions into the zeolite lattice.
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UV-Vis spectrum of the Mn-300-25 was recorded in order to study oxidation and coordination states of manganese after adsorption. The spectrum (given in the
345
Supplementary material) displays an intense featureless absorption in the 250 to 800 nm
346
region. Similar results were reported for the spectra of both MnO2 and Mn2O3 [23].
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Moreover, the concentration profile of Mn inside the grains of Mn-300-25 was
348
studied by the XPS method to get a deeper insight into the interactions of Mn with the
349
zeolite. Figure 5a shows typical XPS survey spectra from the surface of the Mn-300-25
350
sample. The peaks of elements O, C, Si, Al, Mn and Na can be identified. Presence of
351
carbon is quite common originating from the contamination layer due to sample
352
handling in air [12]. Figure 5b shows that the C 1s signal drastically decreased after the
353
sputtering time of only 1 min (i.e. about 1 nm beneath the surface of the particles)
354
confirming that the carbon originates from the contamination layer. Moreover, the Mn
355
depth profile shows a nearly constant value of 2.0 at. % through the entire analyzed
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357
supports two facts: firstly, the Mn loading on the zeolite beads is governed by an ion
358
exchange process and secondly, the Mn species do not accumulate at the zeolite surface.
359
Figure 5c shows a high resolution Mn 2p spectrum. Two peaks Mn 2p3/2 at 641.5 eV
360
and Mn 2p1/2 at 653.4 eV are identified. The binding energies are similar to the values
361
reported for Mn2O3 on MCM-41 (640.9 and 652.5 eV, respectively) [24] but it should
362
be noted that the value for the peak Mn 2p3/2 is in the energy region (641.0-642.0)
363
corresponding to different valence states of manganese oxo-species (MnO, Mn2O3,
364
MnOOH, MnO2) [12, 25]. This indicates that the XPS results cannot identify precisely
365
the valence state of Mn at the surface of Mn-300-25.
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366 367
Figure 5
Finally, local environment and valence state of Mn were examined by Mn Kedge XANES and EXAFS analyses. Two zeolite samples with different amounts of Mn
369
(Mn-300-25 and Mn-300-55) were analyzed. The valence state of Mn in the sample is
370
deduced from the energy shift of the absorption edge [26-28]. The Mn XANES spectra
371
of Mn-300-25 and Mn-300-55 and reference compounds with known crystallographic
372
structures and Mn oxidation numbers are given in Fig. 6. The edge shifts of about 4 eV
373
per unit oxidation state are observed for the reference compounds. The edge shift can be
374
determined in a straightforward way only for similar edge profiles. Different
375
environments of the cation, most notably with different site symmetries, result in
376
different K-edge profiles. In such cases the comparison of the edge shifts is hindered
377
[29]. Among different approaches to precisely determine average valence state of the
378
Mn atom in the samples from the XANES spectrum, the best results are obtained by a
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380
valence states of the element, with similar symmetry, same type of neighbor atoms in
381
nearest coordination shells, arranged in a similar local structure [28, 30].
382 383
Figure 6
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The average Mn valence state in Mn-300-25 and Mn-300-55 was determined by the linear combination fit with the reference XANES spectra of four Mn oxides (MnO,
385
Mn3O4, Mn2O3, MnO2) as a reference for Mn2+, Mn2.67+, Mn3+ and Mn4+. The average
386
Mn valence 2.61+ is obtained for Mn-300-25 and 2.63+ for Mn-300-55, with a
387
precision of about 5%. The results indicate that in both the samples oxidation of Mn(II)
388
occurred.
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Further, for Mn-300-55, the Mn K-edge EXAFS spectra was recorded and
390
quantitatively analyzed for the coordination number, distance, and Debye-Waller factor
391
of the nearest coordination shells of neighbor atoms. The quantitative analysis of
392
EXAFS spectrum was performed with the IFEFFIT program package [30] using FEFF6
393
code [31] in which the photoelectron scattering paths were calculated ab-initio from a
394
presumed distribution of neighbor atoms. Fourier transforms of k3-weighted Mn
395
EXAFS spectra are shown in Figure 7.
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Structural parameters are quantitatively resolved by comparing the measured
397
signal with model signal. The EXAFS model of local environment of Mn atoms
398
comprised seven single scattering paths in the R range up to 4 Å: oxygen atoms in the
399
first coordination shell at three different distances, and Mn atoms in four more distant
400
coordination shells. The atomic species of neighbors are identified in the fit by their
401
specific scattering factor and phase shift.
19
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A very good agreement between the model and the experimental spectra is found in the R range from 1.0 to 3.8 Å and in a k range from 3.5 to 10 Å-1. The number of
404
variable parameters in the model was minimized to reduce correlations. For each
405
neighbor shell the neighbor distances and the coordination numbers were allowed to
406
vary, while a common Debye-Waller factor was used for each type of neighboring
407
atoms. A common shift of energy origin E0 was used for all scattering paths. The
408
EXAFS amplitude reduction factor S02 was kept fixed at the value of 0.8, determined on
409
MnO sample [26, 27]. The analysis yielded that the Mn cations are surrounded by about
410
six oxygen atoms at the distance between 1.87Å to 2.25Å. In more distant coordination
411
shells Mn neighbors are found at the distances from 2.7Å to 3.9Å. A complete list of
412
best-fit parameters is given in Table 3. The average local Mn neighborhood in the
413
sample clearly indicates presence of different Mn oxides.
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The XANES and EXAFS analyses clearly show that Mn(II) adsorption is accompanied by an oxidation process. The phenomenon could be explained by
416
readiness of Mn(II) present at the surface of different minerals including the
417
aluminosilicate ones to form oxide species in the presence of water and air oxygen [32,
418
33]. It is worth noticing that the heterogeneous reaction in which Mn(II) forms Mn(III)
419
oxide species when coupled with the reduction of O2 to H2O is thermodynamically
420
favorable under neutral to alkaline oxic conditions [32, 33]. The formed oxide film
421
inhibits regeneration of the spent beads by a simple ion exchange using NaCl.
422
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Figure 7
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ACCEPTED MANUSCRIPT Table 3. Structural parameters of the nearest coordination shells around Mn atom in
425
Mn300-55 samples: type of neighbor atom, average number N, distance R, and Debye-
426
Waller factor σ2. The amplitude reduction factor (S02 = 0.8) was determined on MnO
427
and kept fixed during the fit. Uncertainties in the last digit are given in the parentheses.
428
Measure for the quality of fit (r-factor) is 0.0034 as defined previously [30]. Neighbor
N
R (Å)
Mn300-55
(r-factor = 0.0034) 1.86(4)
O
1.2(9)
2.07(7)
0.002(1)
O
3(1)
2.25(4)
0.002(1)
Mn
3(1)
Mn
4(2)
Mn
2(1)
Mn
3(1)
Conclusion
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0.002(1)
2.73(4)
0.004(2)
2.95(6)
0. 004(2)
3.16(4)
0. 004(2)
3.94(4)
0. 004(2)
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4.
σ2 (Å2)
O
429 430
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Zeolite A beads show high capacity for Mn(II) varying from 30 mg g-1 at 25 °C to about 50 mg g-1 at 55 °C. From 25 to 45 °C adsorption rate does not depend on the
433
initial Mn(II) concentration (C0=100-400 mg dm-3, STD < 17 %) indicating diffusion
434
limitations. Apparent diffusivity values obtained by the intra-particle diffusion model
435
are in the order of magnitude of 10-13 m2 s-1 corresponding to those reported for
436
different metal ions in zeolites, and implying that the diffusion through the zeolite
437
lattice is the rate limiting step. At 55 °C, the process rate changes and the ion-exchange
438
becomes rate limiting step. This was confirmed by the previously developed kinetic
439
model based on the ion-exchange mechanism. Excellent agreement of the model
440
predictions with the experimental results was obtained.
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The adsorption isotherms were studied at 25 °C showing that the Mn removal is
442
the best described by the Langmuir model suggesting adsorption on highly homogenous
443
zeolite surface. XPS analysis of Mn-loaded zeolite A shows that there is no surface
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accumulation of Mn but an almost uniform Mn distribution inside zeolite A. EXAFS
446
and XANES analyses demonstrate that the adsorbed Mn(II) underwent an oxidation
447
followed by formation of different Mn oxides. The regeneration of the spent beads using
448
Na2EDTA was tested in a fluidized bed column. The treatment does not influence the
449
crystallinity of zeolite A and the beads can be reused for about 4 cycles with satisfactory
450
Mn(II) adsorption efficiency (~70 %).
451
Acknowledgments
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This work was supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia (grants III 45019 and 172018)
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and by Slovenia-Serbia bilateral project BI-RS/12-13-029. Access to synchrotron
455
radiation facilities of ELETTRA (beamline XAFS, project 20115112) is acknowledged.
456
Giuliana Aquilanti and Luca Olivi of ELETTRA are acknowledged for expert advice on
457
beamline operation. The authors are also thankful to Dr Aleksandra Samolov (Military
458
Technical Institute, Belgrade) for performing the UV-Vis analysis.
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Tables
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Table 1. Intra-particle diffusion rate constants, kd, intraparticle diffusivities, D, and
515
experimentally determined adsorbed concentrations of Mn(II) at equilibrium, qe, at
516
different temperatures
517
Table 2. Kinetic parameters of the kinetic model based on the ion-exchange mechanism
518
for Mn(II) adsorption by zeolite A and by natural clinoptilolite at the initial Mn(II)
519
concentration of 300 mg dm-3 and at 55 and 25 °C, respectively
520
Table 3. Structural parameters of the nearest coordination shells around Mn atom in
521
Mn300-55 samples: type of neighbor atom, average number N, distance R, and Debye-
522
Waller factor σ2. The amplitude reduction factor (S02 = 0.8) was determined on MnO
523
and kept fixed during the fit. Uncertainties in the last digit are given in the parentheses.
524
Measure for the quality of fit (r-factor) is 0.0034 as defined previously [30]
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Figure 1. Experimentally determined adsorbed Mn(II) per gram of zeolite (qt) for
528
different initial Mn(II) concentrations (100-400 mg dm-3) at 25 °C vs. time (data
529
represent average of at least two experiments)
530
Figure 2. Intra-particle diffusion model applied for Mn(II) adsorption at 25, 35 and 45
531
°C: experimental data of adsorbed Mn(II) concentrations (symbols) and model
532
predictions (lines) as functions of time; inset: best linear fits of the Eq. (1) (lines)
533
Figure 3. Adsorption of Mn(II) by zeolite A at 55 °C and at initial Mn(II) concentration
534
of 300 mg dm-3 as a function of time: experimental data (symbols) and predictions of
535
the kinetic model based on the ion-exchange mechanism (line). Data represent average
536
of n=2
537
Figure 4. Mn(II) adsorption efficiencies and adsorbed Mn(II) concentrations in zeolite A
538
beads, qt, as functions of the cycle number. Mn(II) adsorption efficiencies are calculated
539
with respect to the maximum possible amount that can be adsorbed when the
540
equilibrium is achieved
541
Figure 5. XPS survey spectrum from the surface of Mn-300-25 (a); XPS depth profile of
542
C and Mn (b); High-energy resolution Mn 2p spectrum (c)
543
Figure 6. Normalized Mn K-edge XANES spectra of samples Mn-300-25, Mn-300-55
544
and Mn reference samples MnO, Mn3O4, Mn2O3 and MnO2. The spectra are displaced
545
vertically for clarity
546
Figure 7. Fourier transformed magnitude of the k3-weighted Mn K-edge EXAFS spectra
547
of Mn-300-55 sample (solid line – experiment, dashed line - EXAFS model)
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Highlights High efficacy of zeolite A beads in Mn(II) removal
•
Diffusion or ion-exchange is rate controlling step depending on temperature
•
Interactions of Mn with zeolite A are elucidated by EXAFS/XANES
•
Capacity of reused fluidized beads stays at 20 mgMn g-1 in 4 cycles
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S1387-1811(16)30001-4
DOI:
10.1016/j.micromeso.2016.02.026
Reference:
MICMAT 7589
To appear in:
Microporous and Mesoporous Materials
Received Date: 9 November 2015 Revised Date:
28 January 2016
Accepted Date: 10 February 2016
Please cite this article as: M. Jovanovic, I. Arcon, J. Kovac, N.N. Tusar, B. Obradovic, N. Rajic, Removal of manganese in batch and fluidized bed systems using beads of zeolite a as adsorbent, Microporous and Mesoporous Materials (2016), doi: 10.1016/j.micromeso.2016.02.026. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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REMOVAL OF MANGANESE IN BATCH AND FLUIDIZED BED SYSTEMS
2
USING BEADS OF ZEOLITE A AS ADSORBENT
3 Mina Jovanovica, Iztok Arconb, c, Janez Kovacc, Natasa Novak Tusard, b, Bojana
5
Obradovice, Nevenka Rajice,*
6
a
7
Belgrade, Serbia
8
b
9
c
10
d
11
e
12
Belgrade, Serbia
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Innovation Center of the Faculty of Technology and Metallurgy, Karnegijeva 4, 11000
University of Nova Gorica, Vipavska 13, 5000 Nova Gorica, Slovenia
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Institute Jozef Stefan, Jamova 39, 1000 Ljubljana, Slovenia
National Institute of Chemistry, Hajdrihova 19, 1000 Ljubljana, Slovenia
Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11000
13
Mina Jovanovic: [email protected]
15
Iztok Arcon: [email protected]
16
Janez Kovac: [email protected]
17
Natasa Novak Tusar: [email protected]
18
Bojana Obradovic: [email protected]
19
Corresponding author:
20
Prof. dr Nevenka Rajic
21
Faculty of Technology and Metallurgy
22
University of Belgrade
23
Karnegijeva 4
24
11000 Belgrade
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1
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Tel:+381113306623
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Fax:+381113370387
28
e-mail: [email protected]
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32
In this study the uptake capacity of Mn(II) ions by zeolite A beads was investigated for
33
different initial Mn concentration (100-400 mg Mn dm-3) in batch mode at 25-55 ºC.
34
The obtained adsorption capacity varying from 30 to 50 mg Mn g-1 demonstrated a high
35
affinity of zeolite A towards Mn(II) present in solutions. Kinetic studies indicated the
36
intra-particle diffusion as the rate limiting step up to 45 °C with apparent diffusivities in
37
the range (1.2 – 2.0)×10-13 m2 s-1 and the activation energy of 21.9 kJ mol-1, which
38
implies strong interactions between the zeolite A and Mn ions. At 55 °C ion-exchange
39
became the rate limiting step. The adsorption isotherms were studied at 25 oC showing
40
that the Mn adsorption is the best described by the Langmuir model suggesting a
41
homogenous zeolite surface. XPS analysis of the Mn-loaded beads showed that there is
42
no surface accumulation of Mn but an almost uniform Mn distribution inside zeolite A,
43
whereas XANES and EXAFS suggested that the adsorption of Mn(II) was followed by
44
the Mn(II) oxidation and oxide formation. Regeneration of the spent zeolite was
45
examined in 8 adsorption/desorption cycles by a chelating Na2EDTA in a fluidized
46
column. It has been found that zeolite A beads could be reused for at least 4 cycles with
47
satisfactory Mn(II) adsorption efficiencies of about 70 %.
48
Keywords – zeolite A, manganese, adsorption kinetics, EXAFS/XANES, XPS
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1.
Introduction Manganese has been widely used in many industrial processes including
52
production of stainless steel, ceramics, dry battery cells, or electrical coils. Wastewaters
53
from these processes usually contain high manganese concentrations causing yellowish
54
water color and unpleasant taste and odor. Although Mn is an essential element for
55
humans and animals, necessary for normal bone formation, exposure to high Mn
56
concentrations causes nervous system damages and various disorders.
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Different methods have been used to reduce wastewater Mn concentration and to achieve the required water quality ranging including oxidation with potassium
59
permanganate, precipitation, adsorption by magnetic nano- and microparticles [1], and
60
various biological processes [2]. Most of these techniques are burdened with high
61
investment and operational costs as well as with disposal of the spent materials. In order
62
to overcome these limitations, current trends in wastewater treatments aim to devise
63
effective, economic and easy to handle methods and materials [3].
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Adsorption is one of the approaches widely used due to high efficiency, simple operation and easy control of the process. Among different adsorbents, low cost zeolites
66
attract attention due to good adsorption capacities. Zeolite A exhibits a high ion-
67
exchange capacity of 5 – 6 mmol g-1 for different toxic metal ions [4-9]. However, to
68
our knowledge, data considering removal of Mn(II) by zeolite A are lacking.
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In the present work we investigate Mn(II) adsorption from water solutions by
70
commercial zeolite A beads. In specific, we focus on kinetics and mechanisms of
71
Mn(II) adsorption including regeneration of the spent zeolite.
72
The adsorption kinetics was experimentally investigated at different
73
temperatures and initial Mn(II) concentrations followed by mathematical modeling.
4
ACCEPTED MANUSCRIPT Particularly, we have examined applicability of the previously developed model based
75
on the ion-exchange by natural clinoptilolite [10]. In order to reveal and quantify
76
phenomena involved in the adsorption process, interactions of Mn with the surface of
77
zeolite A were also investigated in detail using X-ray Photoelectron Spectroscopy (XPS
78
or ESCA) and Mn K-edge XANES and EXAFS techniques.
79
2.
Materials and methods
80
2.1
Materials
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81
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74
M AN U
Beads of commercially available zeolite A (Asorbio ZAG1, Silkem d.o.o.,
82
Slovenia) with an average diameter of 0.71 ± 0.07 mm and the density of 1466 ± 40 kg
83
m-3 were used in this study. The beads contain zeolite 4A (86 ± 3 %), sodium silicate
84
(13 ± 1 %) and water (1.5 ± 1 %) as specified by the producer.
Mn(II) solutions were prepared by dissolving the appropriate amounts of
86
MnSO4×H2O (Sigma-Aldrich, Germany) in distilled water. NaCl (Carlo Erba, Italy) was
87
used for the preparation of sodium chloride solution and C10H14N2Na2O8×2H2O (Lach-
88
Ner, Czech Republic) for the preparation of Na2EDTA solutions. All chemicals used
89
were of analytical grade.
90
2.2
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91
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Adsorption experiments Mn(II) adsorption kinetics was studied in batch experiments at 25 and 45 °C
92
using Mn(II) solutions with initial concentrations of C0 = 1.82, 3.64, 5.46 and 7.28
93
mmol dm-3 (i.e. 100, 200, 300, 400 mg Mn(II) dm-3, respectively). Influence of
94
temperature on the adsorption kinetics was studied at 25, 35, 45 and 55 °C at the initial
95
Mn(II) concentration of 300 mg dm-3. In all experiments solid/liquid ratio was 3:1000,
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ACCEPTED MANUSCRIPT with the exception of two experimental points: Mn(II) concentrations of 100 mg dm-3 at
97
45 °C and 300 mg dm-3 at 55 °C in which the ratio of 1:500 was used in order to verify
98
that the zeolite saturation was reached in all experiments. The Mn(II) adsorption
99
isotherm was determined at 25 °C in the concentration range 100-600 mg Mn dm–3. The
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96
100
suspensions were shaken at about 100 rpm in a thermostated water bath (Memmert
101
WPE 45, Germany). The Mn-loaded beads were recovered by filtration.
All experiments were carried out at the original pH (6.2) which slightly
103
increased at the end of the experiments (6.5). Also, the experiments were carried out in
104
duplicates under controlled conditions: temperature in the thermostated bath was
105
maintained constant to within ±0.1 °C, the zeolite samples were weighted to four-digit
106
accuracy.
107
2.3
M AN U
Regeneration and reuse of the spent zeolite beads
Regeneration of the spent beads was studied by two experiments. In the first one,
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108
SC
102
Mn-containing beads were suspended in NaCl (2 M) or Na2EDTA (0.01 M) solution
110
followed by shaking in the water bath at ~100 rpm at 25 °C for 24 h. In the second
111
experiment a fluidized bed system with recirculation described in details in our previous
112
work [11] was used. The system consisted of a column with the inner and outer
113
diameters of 2.4 and 3.0 cm, respectively, and the height of 16.7 cm connected to a
114
glass reservoir and recirculated by a peristaltic pump (Masterflex L/S, Cole-Parmer,
115
USA). Before the experiments, the beads were washed in distilled water under
116
recirculation in the fluidized bed system for 24 h in order to release any exterior or
117
loosely bound sodium silicate. Next, the column was filled with 15.2 g of washed
118
zeolite beads (corresponding to 12.5 g of initial zeolite beads due to water absorption of
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109
6
ACCEPTED MANUSCRIPT ~18 % during washing [11]) comprising the static bed height of 3.3 cm. In adsorption
120
cycles, the system was filled with 1.5 dm3 of the solution with the initial Mn(II)
121
concentration of 300 mg dm-3, while in desorption cycles, 1.5 dm3 of 0.01 M Na2EDTA
122
solution was used. Recirculation of solutions in all experiments was performed at the
123
flowrate of 10.2 cm3s-1, while the fluidized bed height was determined as 9.3±0.4 cm.
124
Adsorption cycles lasted for 24 h while desorption was performed during 2 h. At the end
125
of each cycle liquid samples were analyzed for Mn(II) concentration. In total 8
126
adsorption/desorption cycles were performed at room temperature.
127
2.4
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Analytical methods
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128
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119
Mn(II) concentration was determined at four-digit accuracy by Atomic Absorption Spectroscopy (AAS) using Varian Spectra AA 55B (Varian Medical
130
Systems, Australasia Pty Ltd., Australia); at least five measurements were done for each
131
data point.
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129
The crystallinity of the beads was checked by powder X-ray diffraction analysis
133
(PXRD) using a PANalytical X'Pert PRO diffractometer with a CuKα radiation (1.5406
134
Å). The PXRD data were collected in the 2 theta range from 5 to 75° in steps of 0.02°
135
with a scan step time 15 s.
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136
EP
132
UV-visible absorption spectrum was recorded using a Shimadzu 3600 double
137
beam UV-Vis-NIR Spectrophotometer. The X-ray Photoelectron Spectroscopy (XPS or
138
ESCA) analysis was performed using a PHI-TFA XPS spectrometer, illuminating the
139
sample surface by X-ray radiation from Al monochromatic source. The samples were in
140
the form of 1 mm thick pressed pellets. The analyzed area was 0.4 mm in diameter and
141
the analyzed depth was 2-5 nm. The XPS survey and high energy resolution scan
7
ACCEPTED MANUSCRIPT spectra of the emitted photoelectrons were taken at 187 eV and 29 eV, respectively. In
143
order to follow the distribution of elements (in particular Mn) in the subsurface region,
144
the XPS depth profiling was performed by alternating cycles of ion sputtering and
145
acquisition of photoelectron spectra. Sputtering of solid surfaces by ion bombardment is
146
a common method to remove the surface material in order to reveal the subsurface
147
composition for characterization of surfaces and thin films. The ion bombardment
148
process results in a desorption of the target (sample) atoms from the top-most surface
149
layer. By combining cycles of ion bombardment in the XPS spectrometer with
150
subsequent XPS analyses of the newly formed surfaces one can obtain the depth
151
distribution of elements in the subsurface region. The result is a depth profile, which
152
shows concentrations of elements in the direction from the surface to the bulk of the
153
sample. In this way the subsurface region of up to several hundreds of nm can be
154
investigated.
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142
Ion sputtering was performed with 1 keV Ar+ beam rastering over 3 x 3 mm2
156
area. In this way a depth distribution of elements was obtained. Total sputtering time
157
was 10 min with the sputtering rate of 1.0 nm min–1 measured on the Ni/Cr reference
158
multilayer structure. The base pressure in the XPS analysis chamber was 2 x 10–9 mbar.
159
During analysis the XPS spectra were slightly shifted for about 2 eV due to sample
160
charging. The binding energy of 284.8 eV for the C 1s peak (characteristic for C-C/C-H
161
bonds) was used as the reference energy for spectra alignment [12]. The spectra of the
162
Si 2p, O 1s, Na 1s, Al 2p, Mn 2p and C 1s were acquired during analyses. The relative
163
sensitivity factors provided by the instrument manufacturer were used to calculate
164
concentrations in at.%. [12]. The composition was calculated in the model of
165
homogenous matrix.
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ACCEPTED MANUSCRIPT 166
X-ray absorption spectra XANES and EXAFS were measured in the energy region of the Mn K-edge in transmission mode at XAFS beamline of the ELETTRA
168
synchrotron facility in Trieste, Italy. A Si (111) double crystal monochromator was used
169
with about 0.7 eV resolution at Mn K-edge (6539 eV). The intensity of the
170
monochromatic X-ray beam was measured by three 30 cm long consecutive ionization
171
detectors respectively filled with the following gas mixtures: 580 mbar N2 and 1420
172
mbar He; 90 mbar Ar, 910 mbar N2, and 900 mbar He; 35 mbar Ar , 52 mbar N2 and
173
1000 mbar He. The absorption spectra were measured within the interval [-250 eV to
174
1000 eV] relative to the Mn K-edge. In the XANES region equidistant energy steps of
175
0.2 eV were used, while for the EXAFS region equidistant k steps of 0.03 Å-1 eV were
176
adopted with an integration time of 2 s per step. In all experiments the exact energy
177
calibration was established with simultaneous absorption measurement on 5-micron
178
thick Mn metal foil placed between the second and the third ionization chamber.
179
Absolute energy reproducibility of the measured spectra was ±0.01 eV.
180
3.
Results and discussion
181
3.1
Adsorption kinetics
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Kinetics of Mn(II) removal from aqueous solutions by zeolite A beads was
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183
studied in the temperature range from 25 to 55 °C using four initial Mn(II)
184
concentrations, ranging from 100 to 400 mg Mn dm-3. For the experiments performed at
185
25 and 45 °C it has been found that the adsorption rate and the amount of the adsorbed
186
Mn per 1 g of zeolite (qt) do not depend on the initial Mn concentration (STD < 17 % ).
187
Figure 1 shows the results obtained at 25 °C as typical ones.
188
Figure 1 9
ACCEPTED MANUSCRIPT 189
These results lead to conclusion that internal diffusion is involved in the adsorption and that it is the rate controlling step in the temperature range from 25 to
191
45 °C. Accordingly, the adsorption kinetics can be described by the intra-particle
192
diffusion model:
193
qt = kd t 0.5
194
where qt is the adsorbed concentration at time t and kd is the intra-particle
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190
SC
(1)
diffusion rate constant. The rate constant is related to the intraparticle diffusivity, D,
196
derived for a spherical particle as:
6qe R
M AN U
195
D
197
kd =
198
where R is the particle radius and qe is the adsorption capacity of the zeolite at equilibrium.
(2)
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199
π
For the intra-particle diffusion model, according to the Eq. (1), the plot qt vs. t0.5
201
should be linear passing through the origin with the slope yielding kd. The plots of qt vs.
202
t1/2 in the present adsorption experiments indeed yielded straight lines so that the model
203
Eq. (1) predicts well the experimental data with a slight overestimate of the value of
204
adsorbed Mn(II) at 45 °C after 24 h (Fig. 2).
206
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205
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200
Figure 2
The intra-particle diffusion rate constants and experimentally determined zeolite
207
adsorption capacities at equilibrium averaged for each temperature were applied in the
208
Eq. (2) to calculate apparent Mn(II) diffusivities within the zeolite A beads at different
209
temperatures (Table 1).
10
ACCEPTED MANUSCRIPT 210
Table 1. Intra-particle diffusion rate constants, kd, intraparticle diffusivities, D, and
211
experimentally determined adsorbed concentrations of Mn(II) at equilibrium, qe, at
212
different temperatures
-1 -0.5
kd (mg g h
)
D x1013 (m2 s-1)
35 °C
45 °C
28.5±1.1
34.1±2.5
44.8±1.0
5.5±0.4
7.7±1.6
1.2±0.2
1.6±0.7
214
11.5±0.7
2.0±0.2
SC
213
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qe (mg g-1)
25 °C
It could be noticed that the capacity of zeolite A for Mn(II) of about 30 mg g-1 at 25 ºC (Table 2) is significantly higher than that reported for natural zeolites (e.g. for
216
natural clinoptilolite it has been reported to be in the range 4-15 mg g-1 [10, 13-15],
217
about 16 mg g-1 for montmorillonite [16] and 4.3 mg g-1 for mordenite [17]). This
218
suggests that zeolite A can be a promising adsorbent for Mn(II).
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215
Table 1 shows that both the internal diffusion rate constant and apparent
220
diffusivity increase with the increase in temperature. Diffusivity dependence on
221
temperature, T, is usually described by the Arrhenius’ equation: RT
EP
223
D = D0e Ea
(3)
where D0 is the pre-exponential constant, Ea is the activation energy and R is the
AC C
222
TE D
219
224
universal gas constant (R = 8.314 J mol-1 K-1). Parameters in the Eq. (3) can be
225
determined from the plot lnD vs. T based on the logarithm form of this equation:
226
ln D = ln D0 − Ea RT
227
The apparent diffusivity values determined in this study at three investigated
228
(4)
temperature points yielded a linear trend (R2 = 0.998) in the Eq. (4) (data shown in
11
ACCEPTED MANUSCRIPT 229
Supplementary material) giving the value of 21.9 kJ mol-1 for the activation energy and
230
8.0×10-10 m2 s-1 for D0.
231
It should be noted that Mn(II) diffusion through the beads includes diffusion through macropores and the filler (i.e. sodium silicate) and diffusion through the zeolite
233
A pore systems. Also, the apparent Mn(II) diffusivity values obtained in the present
234
study are in the range of 10-12 - 10-17 m2 s-1 that were reported for various metal ions in
235
different types of zeolites (e.g. diffusivities of 6.1 x 10-16 m2 s-1 and 1.3 x 10-17 m2 s-1
236
were reported for Na+ and Ba(II), respectively, in chabazite [18] while diffusivities of
237
Zn(II), Cd(II), Cs+ and Sr(II) in zeolite A at 25 ºC were reported in the range (1.8 – 6.5)
238
x 10-12 m2 s-1 [4, 5]. Finally, the obtained value is quite similar to that previously found
239
for Cu(II) (2 x 10-13 m2 s-1) [11]. Based on the reported data, it could be assumed that the
240
apparent diffusivity values correspond to diffusion through the pore system of zeolite A
241
as the rate limiting step.
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M AN U
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242
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232
However, the intraparticle diffusion model does not describe the experimental data obtained at 55 °C, indicating that another process but not diffusion itself, is the rate
244
limiting step. Accordingly, the adsorption kinetics at 55 °C was modeled by the
245
previously developed kinetic model based on the ion exchange mechanism [10]. The
246
model assumed adsorption of a divalent metal ion (M2+) through two reversible steps: 1)
247
release of two Na+ from the zeolite lattice, and 2) M2+ binding to the remaining free site:
249
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248
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243
+ → 2 Naaq 2 Naz P0.5 ← +P
(5)
k3 → MzP M aq2+ + P ←
(6)
k1
k2
k4
12
ACCEPTED MANUSCRIPT 250
where k represents the kinetic rate constant of each of the reactions and P is the free cation site in the zeolite lattice for binding of one M2+ ion. Subscripts aq and z
252
describe ions in the solution and in the zeolite, respectively, while the subscript 0.5 in
253
Eq. (5) indicates that 2 sites occupied by Na+ ions are close to each other in order to
254
provide 1 site (P) for one M2+ ion. All details on the model derivation are provided in
255
the Supplementary material.
256
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251
Taking the above into account and assuming that the concentration of free sites is small and constant over time, the final expression for the M2+ adsorption rate is
258
derived as [10]:
dqt ,m dt
= k1
M AN U
259
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257
(CMo − qt ,m )(CSzo − 2qt ,m )2 − 4qt3,m / K m 4k2' qt ,m 2 + (CMo − qt ,m )
(7)
where qt,m is the molar concentration of adsorbed M2+, CMo and CSzo are the initial
261
concentrations of M2+ ions in the solution and Na+ ions in zeolite, respectively, k2’ is the
262
constant defined as k2’ = k2/k3. Km is the equilibrium constant defined as:
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260
3
264 265
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(8)
where qe,m is the equilibrium molar concentration of adsorbed M2+ on zeolite.
AC C
263
4qe,m kk Km = 1 3 = k2 k 4 ( CMo − qe,m )( CSzo − 2qe, m )2
The model parameters qe,m, CSzo, Km, k1, and k2’ were determined as follows.
266
Mn(II) capacity of the beads at 55 °C, qe,m, was experimentally determined as 0.958
267
mmol g-1 (52.6±0.8 mg g-1) while the initial concentration of Na+ ions in zeolite A, CSzo,
268
was calculated as 4.73 mmol g-1 based on the chemical formula of zeolite A. The
269
equilibrium constant Km was then calculated using the Eq. (8). The rate constant of Na+
270
release from the zeolite lattice (k1) was shown previously to be approximately constant
13
ACCEPTED MANUSCRIPT irrespective of the initial M2+ concentration and temperature, yielding the average value
272
of 23.1 g mmol-1 h-1 [10]. Finally, the only independent parameter, the constant k2’ was
273
determined by the least-squares fit of experimental data by the model Eq. (7) using
274
MATLAB software package. It should be noted that the M2+ desorption rate constant,
275
k4, is then resulting from the value of the equilibrium constant Km and Eq. (8).
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271
The model described accurately experimental results of adsorbed Mn(II)
277
concentration as a function of time (STD= 3.6 %, Fig. 3) with all constants summarized
278
in Table 2 along with the previously reported constants determined for Mn(II)
279
adsorption in the natural clinoptilolite at 25 °C and at the same initial Mn(II)
280
concentration of 300 mg dm-3.
281
M AN U
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276
Figure 3
Table 2. Kinetic parameters of the kinetic model based on the ion-exchange mechanism
283
for Mn(II) adsorption by zeolite A and by natural clinoptilolite at the initial Mn(II)
284
concentration of 300 mg dm-3 and at 55 and 25 °C, respectively
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282
EP
zeolite A
AC C
K (-)
k2’=k2/k3
(g mmol-1)
k4·102 (h-1) Ref.
natural clinoptilolite
55 °C
25 °C
0.25
0.039
8,000
13,976
1.2
4.2
This study
[10]
285
14
ACCEPTED MANUSCRIPT 286
The equilibrium constant for Mn(II) adsorption by zeolite A is significantly higher than that for the natural clinoptilolite (Table 2) reflecting the higher adsorbed
288
equilibrium concentration, qe,m, in the former case (52.6 and 8.2 mg g-1, respectively)
289
due to the higher temperature. Higher temperature also led to slightly lower k2’ in
290
zeolite A indicating the increase in the overall exchange rate for Mn(II) adsorption with
291
the increase in temperature as also found previously for Cu(II) adsorption in the natural
292
clinoptilolite [10]. Moreover, the smaller value of desorption constant k4 implies
293
stronger interactions of Mn(II) with the lattice of zeolite A than with that of natural
294
clinoptilolite. Finally, excellent agreement of model predictions with experimental data
295
with the assumed rate constant k1 implies that the process of Na+ release is negligibly
296
affected by the zeolite type.
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287
The adsorption isotherms were studied at 25 °C (the results are presented in the
298
Supplementary material). The equilibrium data have been analyzed by several empirical
299
adsorption isotherm models [19]. The Langmuir model [20] gives excellent agremeent
300
with experimental results suggesting the Mn adsorption on a highly homogenous
301
surface. Moreover, the Langmuir constant of the saturated monolayer adsorption
302
capacity predicted a value of 40.3 mg g-1 as the maximal concentration that could be
303
adsorbed on the zeolite beads.
304
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305
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297
Regeneration and reuse of zeolite A beads
PXRD analysis confirmed that the Mn(II) adsorption does not affect the crystal
306
structure of zeolite A (the results are presented in the Supplementary material)
307
indicating that regeneration of zeolite A beads could be possible by a simple ion-
308
exchange reaction, i.e. by treatment of Mn-loaded beads in a NaCl solution. However,
15
ACCEPTED MANUSCRIPT the experiments with the spent beads obtained using the initial Mn(II) concentration of
310
300 mg dm-3 at 25 ºC (Mn-300-25) and at 55 ºC (Mn-300-55) showed that the
311
regeneration in such a system does not occur readily. In specific, only 0.6 % of Mn was
312
desorbed from Mn-300-25 and about 8 % from Mn-300-55 in 2M NaCl solution. On the
313
other hand, the use of Na2EDTA solution (0.01 M) instead of NaCl provided better
314
results. For both samples about 80% of Mn was desorbed from the beads suggesting that
315
the desorption is not a simple process. It is well known that Na2EDTA exhibits a strong
316
affinity towards divalent metal cations due to its complexing ability. Namely, the
317
hexadentate (chelate) ligand forms very stable soluble complexes.
M AN U
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309
The regeneration of the spent beads using Na2EDTA solution was further
319
investigated in a fluidized bed system in 8 adsorption/desorption cycles. Figure 4 shows
320
Mn(II) adsorption efficiencies and adsorbed Mn(II) concentrations in the zeolite beads
321
as functions of the cycle number. Mn(II) adsorption efficiencies were calculated as the
322
ratio of the adsorbed Mn(II) amounts achieved in the column in each cycle and the
323
maximum possible amount that can be adsorbed when the equilibrium is accomplished
324
at 25 °C (Table 1). It should be mentioned that the first cycle should be regarded as a
325
preparatory phase since bead collisions induced mechanical abrasion of superficial
326
layers leading to reverting Mn to the solution and inducing yellowish color. After the
327
first cycle, both adsorption efficiency and zeolite capacity stayed approximately
328
constant during the next 4 cycles (68±7 % and 19.3±2.1 mg g-1, respectively) after
329
which they started to decrease (Fig. 4). This finding could be explained by incomplete
330
zeolite regeneration of about 80 %, as described above, which slowly led to manganese
331
accumulation within the beads, thus decreasing the zeolite capacity in repeated
332
adsorption cycles.
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ACCEPTED MANUSCRIPT 333
335
3.3
Study of Mn interactions with the zeolite In order to improve regeneration efficacy, Mn-containing beads were
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334
Figure 4
characterized in detail. Firstly, the crystallinity of the beads recovered by Na2EDTA was
337
checked by PXRD. The pattern (given in the Supplementary material) clearly shows
338
that the treatment with Na2EDTA does not affect crystallinity of the zeolite A leading to
339
the conclusion that the recovery with Na2EDTA can include several simultaneous
340
processes: complexing of Mn species adsorbed on the zeolite, removal of the Mn
341
complex from the zeolite to aqueous solution, and an ion replacement including return
342
of Na+ ions into the zeolite lattice.
M AN U
343
SC
336
UV-Vis spectrum of the Mn-300-25 was recorded in order to study oxidation and coordination states of manganese after adsorption. The spectrum (given in the
345
Supplementary material) displays an intense featureless absorption in the 250 to 800 nm
346
region. Similar results were reported for the spectra of both MnO2 and Mn2O3 [23].
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344
Moreover, the concentration profile of Mn inside the grains of Mn-300-25 was
348
studied by the XPS method to get a deeper insight into the interactions of Mn with the
349
zeolite. Figure 5a shows typical XPS survey spectra from the surface of the Mn-300-25
350
sample. The peaks of elements O, C, Si, Al, Mn and Na can be identified. Presence of
351
carbon is quite common originating from the contamination layer due to sample
352
handling in air [12]. Figure 5b shows that the C 1s signal drastically decreased after the
353
sputtering time of only 1 min (i.e. about 1 nm beneath the surface of the particles)
354
confirming that the carbon originates from the contamination layer. Moreover, the Mn
355
depth profile shows a nearly constant value of 2.0 at. % through the entire analyzed
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17
ACCEPTED MANUSCRIPT depth indicating that the Mn ions are uniformly distributed (Figure 5b). This strongly
357
supports two facts: firstly, the Mn loading on the zeolite beads is governed by an ion
358
exchange process and secondly, the Mn species do not accumulate at the zeolite surface.
359
Figure 5c shows a high resolution Mn 2p spectrum. Two peaks Mn 2p3/2 at 641.5 eV
360
and Mn 2p1/2 at 653.4 eV are identified. The binding energies are similar to the values
361
reported for Mn2O3 on MCM-41 (640.9 and 652.5 eV, respectively) [24] but it should
362
be noted that the value for the peak Mn 2p3/2 is in the energy region (641.0-642.0)
363
corresponding to different valence states of manganese oxo-species (MnO, Mn2O3,
364
MnOOH, MnO2) [12, 25]. This indicates that the XPS results cannot identify precisely
365
the valence state of Mn at the surface of Mn-300-25.
M AN U
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356
366 367
Figure 5
Finally, local environment and valence state of Mn were examined by Mn Kedge XANES and EXAFS analyses. Two zeolite samples with different amounts of Mn
369
(Mn-300-25 and Mn-300-55) were analyzed. The valence state of Mn in the sample is
370
deduced from the energy shift of the absorption edge [26-28]. The Mn XANES spectra
371
of Mn-300-25 and Mn-300-55 and reference compounds with known crystallographic
372
structures and Mn oxidation numbers are given in Fig. 6. The edge shifts of about 4 eV
373
per unit oxidation state are observed for the reference compounds. The edge shift can be
374
determined in a straightforward way only for similar edge profiles. Different
375
environments of the cation, most notably with different site symmetries, result in
376
different K-edge profiles. In such cases the comparison of the edge shifts is hindered
377
[29]. Among different approaches to precisely determine average valence state of the
378
Mn atom in the samples from the XANES spectrum, the best results are obtained by a
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368
18
ACCEPTED MANUSCRIPT linear combination fit with XANES spectra of proper reference compounds with known
380
valence states of the element, with similar symmetry, same type of neighbor atoms in
381
nearest coordination shells, arranged in a similar local structure [28, 30].
382 383
Figure 6
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379
The average Mn valence state in Mn-300-25 and Mn-300-55 was determined by the linear combination fit with the reference XANES spectra of four Mn oxides (MnO,
385
Mn3O4, Mn2O3, MnO2) as a reference for Mn2+, Mn2.67+, Mn3+ and Mn4+. The average
386
Mn valence 2.61+ is obtained for Mn-300-25 and 2.63+ for Mn-300-55, with a
387
precision of about 5%. The results indicate that in both the samples oxidation of Mn(II)
388
occurred.
M AN U
SC
384
Further, for Mn-300-55, the Mn K-edge EXAFS spectra was recorded and
390
quantitatively analyzed for the coordination number, distance, and Debye-Waller factor
391
of the nearest coordination shells of neighbor atoms. The quantitative analysis of
392
EXAFS spectrum was performed with the IFEFFIT program package [30] using FEFF6
393
code [31] in which the photoelectron scattering paths were calculated ab-initio from a
394
presumed distribution of neighbor atoms. Fourier transforms of k3-weighted Mn
395
EXAFS spectra are shown in Figure 7.
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Structural parameters are quantitatively resolved by comparing the measured
397
signal with model signal. The EXAFS model of local environment of Mn atoms
398
comprised seven single scattering paths in the R range up to 4 Å: oxygen atoms in the
399
first coordination shell at three different distances, and Mn atoms in four more distant
400
coordination shells. The atomic species of neighbors are identified in the fit by their
401
specific scattering factor and phase shift.
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A very good agreement between the model and the experimental spectra is found in the R range from 1.0 to 3.8 Å and in a k range from 3.5 to 10 Å-1. The number of
404
variable parameters in the model was minimized to reduce correlations. For each
405
neighbor shell the neighbor distances and the coordination numbers were allowed to
406
vary, while a common Debye-Waller factor was used for each type of neighboring
407
atoms. A common shift of energy origin E0 was used for all scattering paths. The
408
EXAFS amplitude reduction factor S02 was kept fixed at the value of 0.8, determined on
409
MnO sample [26, 27]. The analysis yielded that the Mn cations are surrounded by about
410
six oxygen atoms at the distance between 1.87Å to 2.25Å. In more distant coordination
411
shells Mn neighbors are found at the distances from 2.7Å to 3.9Å. A complete list of
412
best-fit parameters is given in Table 3. The average local Mn neighborhood in the
413
sample clearly indicates presence of different Mn oxides.
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The XANES and EXAFS analyses clearly show that Mn(II) adsorption is accompanied by an oxidation process. The phenomenon could be explained by
416
readiness of Mn(II) present at the surface of different minerals including the
417
aluminosilicate ones to form oxide species in the presence of water and air oxygen [32,
418
33]. It is worth noticing that the heterogeneous reaction in which Mn(II) forms Mn(III)
419
oxide species when coupled with the reduction of O2 to H2O is thermodynamically
420
favorable under neutral to alkaline oxic conditions [32, 33]. The formed oxide film
421
inhibits regeneration of the spent beads by a simple ion exchange using NaCl.
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Figure 7
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ACCEPTED MANUSCRIPT Table 3. Structural parameters of the nearest coordination shells around Mn atom in
425
Mn300-55 samples: type of neighbor atom, average number N, distance R, and Debye-
426
Waller factor σ2. The amplitude reduction factor (S02 = 0.8) was determined on MnO
427
and kept fixed during the fit. Uncertainties in the last digit are given in the parentheses.
428
Measure for the quality of fit (r-factor) is 0.0034 as defined previously [30]. Neighbor
N
R (Å)
Mn300-55
(r-factor = 0.0034) 1.86(4)
O
1.2(9)
2.07(7)
0.002(1)
O
3(1)
2.25(4)
0.002(1)
Mn
3(1)
Mn
4(2)
Mn
2(1)
Mn
3(1)
Conclusion
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0.002(1)
2.73(4)
0.004(2)
2.95(6)
0. 004(2)
3.16(4)
0. 004(2)
3.94(4)
0. 004(2)
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σ2 (Å2)
O
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Zeolite A beads show high capacity for Mn(II) varying from 30 mg g-1 at 25 °C to about 50 mg g-1 at 55 °C. From 25 to 45 °C adsorption rate does not depend on the
433
initial Mn(II) concentration (C0=100-400 mg dm-3, STD < 17 %) indicating diffusion
434
limitations. Apparent diffusivity values obtained by the intra-particle diffusion model
435
are in the order of magnitude of 10-13 m2 s-1 corresponding to those reported for
436
different metal ions in zeolites, and implying that the diffusion through the zeolite
437
lattice is the rate limiting step. At 55 °C, the process rate changes and the ion-exchange
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becomes rate limiting step. This was confirmed by the previously developed kinetic
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model based on the ion-exchange mechanism. Excellent agreement of the model
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predictions with the experimental results was obtained.
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The adsorption isotherms were studied at 25 °C showing that the Mn removal is
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the best described by the Langmuir model suggesting adsorption on highly homogenous
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zeolite surface. XPS analysis of Mn-loaded zeolite A shows that there is no surface
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accumulation of Mn but an almost uniform Mn distribution inside zeolite A. EXAFS
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and XANES analyses demonstrate that the adsorbed Mn(II) underwent an oxidation
447
followed by formation of different Mn oxides. The regeneration of the spent beads using
448
Na2EDTA was tested in a fluidized bed column. The treatment does not influence the
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crystallinity of zeolite A and the beads can be reused for about 4 cycles with satisfactory
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Mn(II) adsorption efficiency (~70 %).
451
Acknowledgments
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This work was supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia (grants III 45019 and 172018)
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and by Slovenia-Serbia bilateral project BI-RS/12-13-029. Access to synchrotron
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radiation facilities of ELETTRA (beamline XAFS, project 20115112) is acknowledged.
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Giuliana Aquilanti and Luca Olivi of ELETTRA are acknowledged for expert advice on
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beamline operation. The authors are also thankful to Dr Aleksandra Samolov (Military
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Technical Institute, Belgrade) for performing the UV-Vis analysis.
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Tables
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Table 1. Intra-particle diffusion rate constants, kd, intraparticle diffusivities, D, and
515
experimentally determined adsorbed concentrations of Mn(II) at equilibrium, qe, at
516
different temperatures
517
Table 2. Kinetic parameters of the kinetic model based on the ion-exchange mechanism
518
for Mn(II) adsorption by zeolite A and by natural clinoptilolite at the initial Mn(II)
519
concentration of 300 mg dm-3 and at 55 and 25 °C, respectively
520
Table 3. Structural parameters of the nearest coordination shells around Mn atom in
521
Mn300-55 samples: type of neighbor atom, average number N, distance R, and Debye-
522
Waller factor σ2. The amplitude reduction factor (S02 = 0.8) was determined on MnO
523
and kept fixed during the fit. Uncertainties in the last digit are given in the parentheses.
524
Measure for the quality of fit (r-factor) is 0.0034 as defined previously [30]
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Figure 1. Experimentally determined adsorbed Mn(II) per gram of zeolite (qt) for
528
different initial Mn(II) concentrations (100-400 mg dm-3) at 25 °C vs. time (data
529
represent average of at least two experiments)
530
Figure 2. Intra-particle diffusion model applied for Mn(II) adsorption at 25, 35 and 45
531
°C: experimental data of adsorbed Mn(II) concentrations (symbols) and model
532
predictions (lines) as functions of time; inset: best linear fits of the Eq. (1) (lines)
533
Figure 3. Adsorption of Mn(II) by zeolite A at 55 °C and at initial Mn(II) concentration
534
of 300 mg dm-3 as a function of time: experimental data (symbols) and predictions of
535
the kinetic model based on the ion-exchange mechanism (line). Data represent average
536
of n=2
537
Figure 4. Mn(II) adsorption efficiencies and adsorbed Mn(II) concentrations in zeolite A
538
beads, qt, as functions of the cycle number. Mn(II) adsorption efficiencies are calculated
539
with respect to the maximum possible amount that can be adsorbed when the
540
equilibrium is achieved
541
Figure 5. XPS survey spectrum from the surface of Mn-300-25 (a); XPS depth profile of
542
C and Mn (b); High-energy resolution Mn 2p spectrum (c)
543
Figure 6. Normalized Mn K-edge XANES spectra of samples Mn-300-25, Mn-300-55
544
and Mn reference samples MnO, Mn3O4, Mn2O3 and MnO2. The spectra are displaced
545
vertically for clarity
546
Figure 7. Fourier transformed magnitude of the k3-weighted Mn K-edge EXAFS spectra
547
of Mn-300-55 sample (solid line – experiment, dashed line - EXAFS model)
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Highlights High efficacy of zeolite A beads in Mn(II) removal
•
Diffusion or ion-exchange is rate controlling step depending on temperature
•
Interactions of Mn with zeolite A are elucidated by EXAFS/XANES
•
Capacity of reused fluidized beads stays at 20 mgMn g-1 in 4 cycles
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