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January 15, 2003 Mechanisms of Selenate Adsorption on Iron Oxides and HydroxidesD. Peak1 and D.L. Sparks2 Selenium is a natural element that can be found in rocks and soil and can also be released by manufacturing processes. In the environment, selenium combines with oxygen to form many different molecules, such as selenate (SeO42-), which is known to bind to ferric oxides and hydroxides. Because of selenate’s toxicity to animals and its mobility in the soil, the mechanism of selenate adsorption on iron oxides has been the subject of intense debate. Researchers from the University of Delaware have determined selenate bonding mechanisms on hematite (α-Fe2O3), goethite (α-FeOOH), and amorphous iron hydroxide (Fe(OH)3).
In soils and sediments, selenate preferentially reacts with ferric [Fe(III)] oxides and hydroxides. However, the mechanism of selenate binding on iron oxides has been the subject of intense debate. Some researchers have proposed that the bonding is electrostatic and weak, while others have proposed that covalent chemical bonds form between selenate and iron oxides. The nature of bonding can have a large effect on the transport and availability of selenate in the environment, so we decided to investigate this adsorption mechanism when selenate binds to hematite (α-Fe2O3), goethite (α-FeOOH), and amorphous iron hydroxide (Fe(OH)3).
Extended x-ray absorption fine structure (EXAFS) was the primary spectroscopic tool chosen due to its suitability to determine local bonding environments of selenate on all three sorbents. EXAFS spectra were collected at the selenium K-edge at beam line X11A of the National Synchrotron Light Source at Brookhaven National Laboratory. Additional information about selenate adsorption mechanisms on hematite was obtained using attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. Selenate forms different surface complexes on iron oxides, depending upon pH, ionic strength, and iron oxide structure. On hematite, we noticed that selenate ions directly interact with iron, with no intermediate water molecules present, a structure called an inner-sphere complex. Also, the adsorbed selenate ions form single covalent bonds (formed by sharing electrons) with the surface, a process known as monodentate. These inner-sphere monodentate complexes were observed at all pH and ionic strength values considered in the study.
The EXAFS spectra are shown for aqueous selenate (pH 4) and selenate adsorbed on goethite at pH values of 6 and 3.5 in figure 1. A schematic representation of the outer- and inner-sphere monodentate complexes is provided in figure 2. This study and others have shown that outer-sphere and inner-sphere complexation often occur simultaneously. We noticed that changes in the relative importance of different surface complexes depend on the pH and ionic strength of the solution containing the sorbent, as well as on the nature of the sorbent. So, to adequately characterize selenate reactivity at the molecular level, we consider it is necessary to conduct EXAFS studies over a wide range of reaction conditions and with a range of minerals. Such studies are expected to produce a far clearer picture of how selenate, and oxyanions in general, reacts under constantly changing conditions in the natural world. BEAMLINE FUNDING PUBLICATION FOR MORE INFORMATION |
The National Synchrotron Light Source at Brookhaven National Laboratory in New York, is a national user research
facility funded by the U.S. Department of Energy's Office of Basic Energy Science. The NSLS operates two electron
storage rings: an X-Ray ring and a Vacuum UltraViolet (VUV) ring which provide intense light spanning the electromagnetic
spectrum from the infrared through x-rays. Each year over 2300 scientists from universities, industries and government
labs perform research at the NSLS.
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Selenate (SeO42-) is the fully oxidized form of selenium, an
essential micronutrient for plant growth. When soil selenate levels
are high, it often accumulates in plants and can then prove toxic to
animals that ingest this vegetation. Alternatively, deficiency
symptoms are commonly seen in animals when selenium levels in plants
are low. So, understanding the chemistry of selenate in soils is
important for minimizing potentially hazardous environmental effects
on animal populations. 
The objectives of our work were to determine the effects of pH
(the concentration of protons in solution), surface loading, and
ionic strength (an average of the concentrations of the ions
present) on selenate adsorption mechanisms. Hematite, goethite, and
amorphous iron hydroxide were chosen due to their important
differences in structure and their common occurance in soils. 
On goethite and amorphous iron hydroxide, we noticed that
selenate forms outer-sphere complexes, in which a water molecule is
positioned between each selenate and iron atom, at pH values of 6
(slightly acidic) and above (alkaline). For pH values between 3.5
and 6 (acidic), we noticed a mixture of outer- and inner-sphere
monodentate complexes. ATR-FTIR spectroscopy also suggests that some
hydrogen bonding is present in this inner-sphere complex.