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Mechanism of selenite removal by a mixed adsorbent based on FeMn hydrous oxides studied using X-ray absorption spectroscopy Natalia Chubar, Vasyl Gerda, and Ma#gorzata Szlachta Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 17 Oct 2014 Downloaded from http://pubs.acs.org on October 18, 2014
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Environmental Science & Technology
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Mechanism of selenite removal
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by a mixed adsorbent based on Fe-Mn hydrous oxides
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studied using X-ray absorption spectroscopy
5 Natalia Chubar12*, Vasyl Gerda23, Małgorzata Szlachta24
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*School of Engineering and Built Environment, Glasgow Caledonian University, Cowcaddens Road
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70, Glasgow, G40BA, United Kingdom 2
Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584CD,
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Utrecht, The Netherlands 3
Faculty of Chemistry, Kyiv Taras Shevchenko National University, Volodymyrska Street 60, 01601,
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Kyiv, Ukraine 4
Faculty of Environmental Engineering, Wrocław University of Technology, Wybrzeże S. Wyspiańskiego 27, 50-370 Wrocław, Poland
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KEYWORDS: Selenite, adsorption, mixed adsorbent, Fe-Mn hydrous oxide, XANES, EXAFS.
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ABSTRACT
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Selenium cycling in the environment is greatly controlled by various minerals, including Mn and
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Fe hydrous oxides. At the same time, such hydrous oxides are the main inorganic ion exchangers
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suitable (based on their chemical nature) to sorb (toxic) anions, separating them from water solutions.
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The mechanism of selenite adsorption by the new mixed adsorbent composed of a few (amorphous and
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crystalline) phases (maghemite, MnCO3 and X-ray amorphous Fe(III) and Mn(III) hydrous oxides) was
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studied by EXAFS spectroscopy (supported by FTIR and XRD data). The complexity of the porous
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adsorbent, especially the presence of the amorphous phases of Fe(III) and Mn(III) hydrous oxides, is
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the main reason for its high selenite removal performance demonstrated by batch and column
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adsorption studies shown in the previous work. Selenite was bound to the material via inner-sphere
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complexation (via oxygen) to the adsorption sites of the amorphous Fe(III) and Mn(III) oxides. This
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anion was attracted via bidentate binuclear corner-sharing coordination between SeO32- trigonal
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pyramids and both FeO6 and MnO6 octahedra; however, the adsorption sites of Fe(III) hydrous oxides
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played a leading role in selenite removal. The contribution of the adsorption sites of Mn(III) oxide
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increased as the pH decreased from 8 to 6. Since most minerals have complex structure (they are
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seldom based on individual substances) of various crystallinity this work is equally relevant to
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environmental science and environmental technology as it shows how various solid phases control
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cycling of chemical elements in the environment.
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INTRODUCTION
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Over the last several decades, selenium has been of great interest to both separation and environmental
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scientists. This chemical element is known for its dual properties. Selenium is an essential trace
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chemical element that plays an important role in the maintenance of human health, but it can be toxic in
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high doses.1-3 Environmental research experienced a turning point after the 1980s as a result of the
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environmental disaster in California (contamination of irrigation waters due to drainage activities).1
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New toxicological data resulted in a reduction of the maximum permissible concentration (MPC) of
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this chemical element in drinking water, which is as low as 10 µg/L in some countries.4 Nevertheless,
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at present, the main concern is selenium deficiency rather than selenium toxicity.
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The behavior and speciation of selenium in the environment is largely controlled by its
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adsorption by various minerals, especially metal hydrous oxides.1 Minerals based on hydrous oxides of
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iron and manganese play a major role in selenium adsorption in the geosphere. Synchrotron-based
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techniques allowed studies on selenium’s environmental behavior on the speciation level. EXAFS (X-
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ray absorption near-edge structure and extended X-ray absorption fine structure) was applied to study
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mechanisms of selenite adsorption by magnetite,5 maghemite,6 birnessite,7 montmorillonite,8 and γ-
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Al2O39 as well as the role of various solid phases in selenium redox transformations.10,11 However,
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selenium sorption by mixed materials of various crystalline states has yet to be investigated.
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Adsorption is one of the main approaches in separation science and technology and can be used
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for the recovery of valuable trace chemical elements, such as selenium. The removal capacities of the
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commercial adsorbents towards selenium species are usually low. Therefore, adsorption is not currently
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considered a suitable approach capable of recovering selenium at industrial scales due to the absence of
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commercial adsorptive materials with acceptable removal properties for selenate and selenite.12
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Improved understanding of the mechanism of selenium adsorption by various materials might be a
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turning point in the development of highly efficient adsorbents for selenium recovery and water
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treatment technology. It has been widely recognized that adsorbents (and catalysts) based on
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double/mixed/hybrid compounds have many advantages over individual substances (such as individual
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metal hydroxides), such as higher removal efficiency and a softer pH effect. However, all commercial
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inorganic ion exchange adsorbents are based on individual compounds, mainly AA (activated alumina)
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and GFH (granulated ferric hydroxide).
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The authors developed a new mixed adsorbent based on Fe(III) and Mn(III) hydrous oxides and
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Mn(II) carbonate (which includes crystalline and amorphous phases) and tested its performance
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towards several toxic anions (arsenate, arsenite, selenite).13,14 The new material demonstrated very
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good performance towards selenite in dynamic adsorption conditions.14 To explain its high efficiency,
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the mechanism of selenite removal by this mixed adsorbent was studied by XANES/EXAFS
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spectroscopy as well as FTIR (Fourier-Transform Infrared), XRD (X-ray Diffraction) and batch
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adsorption characterization. The results presented in this work are equally relevant to the fields of
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environmental science and environmental technology. Natural minerals are rarely individual
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substances. They are usually mixed/double/composites materials comprising few amorphous and
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crystalline phases. Thus, mixed materials, such as the investigated adsorbent, reflects the environmental
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processes better than individual compounds.
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Objectives of this work were using EXAFS spectroscopy (in combination with FTIR spectra and
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XRD patterns) (1) to study if high selenite adsorptive capacity of the mixed adsorbent was due to
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chemi-sorption mechanism, (2) to estimate contribution of each solid phase of the mixed materials in
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the removal efficiency, and (3) to investigate if decrease in pH results in the involvement of more solid
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phases of the mixed adsorbent in the removal process.
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EXPERIMENTAL METHODS
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Sample preparation. The mixed inorganic adsorbent based on Fe(III)/Mn(III) oxides and Mn(II)
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carbonate was synthesized by the hydrothermal precipitation approach in the presence of urea in an
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autoclave (Suurmond BV, Büchi Glas Uster AG, Switzerland).13 The product was filtered, washed and
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dried. The resulting powder was stored in closed flasks.
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The samples for EXAFS analysis were prepared in batch adsorption experiments at an adsorbent
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dose of 2 gdw/L. First, 0.15 g of the adsorbent was placed in contact with 75 ml of Na2SeO3·5H2O
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(Fluka) solution in 0.01 M NaNO3 background electrolyte and shaken in a temperature-controlled ACS Paragon Plus Environment
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orbital shaker at 22 °C at 200 rpm until adsorption equilibrium which was reached in 3-5 days
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depending on pH. Small samples were taken and Se concentration was measured. Constant pH of 6, 7
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or 8 was maintained throughout the experiment using 0.1 M HNO3 and/or NaOH. The initial selenium
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concentration was high enough (500 mg[Se]/L) to achieve the adsorptive capacity and full saturation of
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the sorption sites.1 The adsorption was calculated from the difference in the initial and final selenium
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concentrations in the solution: q =
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per gram of dry weight of the adsorbent, Co (mg L-1) is the initial concentration of Se, Ceq (mg L-1) is
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the final (or equilibrium) concentration of the anion in solution, V (L) is the volume of solution and m
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(Co − C eq. )V m
, where q (mg gdw-1) is the amount of selenium sorbed
(gdw) is the dry mass of the adsorbent.
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Sufficient amounts of the solids with sorbed selenite were prepared for EXAFS analysis in the
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preferred transmission mode. The solids were separated from the solution using a membrane filter with
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0.2-µm-diameter pores (Macherey-Nagel, filter type Porafil-PE), air-dried and used in the spectroscopy
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studies.
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The concentrations of selenium, iron and manganese were measured by inductively coupled
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plasma optical emission spectrometry (ICP-OES) with radial plasma (CIROSCCD, Spectro A.I.,
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Germany).
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XANES/EXAFS. The Se K-edge spectra (12658 eV) were recorded in transmission mode at
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ambient temperature on the Dutch-Belgian beamline (DUBBLE, BM26A) at the European Synchrotron
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Radiation Facility.15 The monochromator was calibrated by assigning an energy value of 12658 eV to
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the first inflection point in the absorption edge. The experimental samples (collected at the maintained
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pH of 6, 7 and 8) and references (Na2SeO3·5H2O, Na2SeO4·10H2O, Fluka) were ground into very fine
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powders, mixed with boron nitride and prepared as pellets. The IFFEFIT (Athena, Artemis, Atoms,
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FEFF9) software package was used for the data analysis, treatment and modeling.16,17 To model the
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first and second shells, feff.inp files were generated using the atomic coordinates of ACS Paragon Plus Environment
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Fe2(SeO3)3·3H2O,18 Mn(SeO3)219 as calculated by Giester, and MnSeO3 as calculated by Larranaga20
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using crystallography methods. Mn(SeO3)2 was used due to the absence of crystallography data for
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Mn2(SeO3)3. The applicability of Mn(SeO3)2 has been proved by the results for the second coordination
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shell of selenite obtained when Fe2(SeO3)3 and Mn(SeO3)2 were used for the computation (Table 2). All
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fits (based on both k2- and k3-weighted spectra and their Fourier Transforms) are shown in the SI
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(Supporting Information; Figures S1-S6).
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HyperChem8 software [http://www.hyper.com/] was used to demonstrate the results of EXAFS
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studies. Models of the characteristic clusters of the investigated mixed inorganic adsorbent and selenite
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surface complexes were built by Molecular Mechanics in gaseous environment.
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RESULTS AND DISCUSSION
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Brief characterization of the adsorbent and its adsorptive performance towards HSeO3-. The
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adsorbent comprises a few crystalline and amorphous phases: rhodochrosite (MnCO3), subordinate
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maghemite (Fe2O3, γ-Fe2O3), amorphous Mn(III) and Fe(III) (hydrous) oxides.13 The resulting material
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is porous (Ss = 124 m2/g) with smaller meso-sized pores (2.5 nm); based on this pore size, it is almost a
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nano-porous material (
