University of Saskatchewan
Department of Soil Science
College of Agriculture and Bioresources

Environmental Soil Chemistry

Mechanisms of Selenate Adsorption on Iron Oxides

One oxyanion of interest to soil chemists is selenate, a weakly basic oxyanion with a +6 metal center. In aqueous solution, Se(VI) exists either as the fully-deprotonated selenate (SeO42-(aq)), or as the singly-protonated biselenate (HSeO4-aq). The pKa for the protonation reaction is ~1.9, making the fully-deprotonated form the dominant ion under normal soil conditions. Selenate chemistry is of interest to soil chemists for both environmental and agronomic reasons. Deficiency symptoms are commonly seen in grazing animals when selenium levels in plants are low, as selenium is an essential micronutrient for animals. However, when soil selenate levels are high, selenium often accumulates in plants and can prove toxic to animals that ingest the vegetation. Iron oxides are important geosorbents for selenate due to their high surface area, high points of zero charge, and common occurrence in soils. To study the effects of surface structure on adsorption, selenate bonding on hematite (alpha-Fe2O3), goethite (alpha-FeOOH), and amorphous iron hydroxide (Fe(OH)3) was studied with EXAFS spectroscopy.

Experimental: EXAFS spectra were collected at the Se K-edge (12.658 keV) at Beamline X11-A of the National Synchrotron Light Source at Brookhaven National Laboratory. The electron storage ring was operating at 2.8 GeV. All samples were scanned in fluorescence mode using a Lytle detector with a Krypton-filled ionization chamber. The sample chamber was designed so that samples were placed at a 45 degree angle to the incident beam, and a wide-angle collector was 90 degree to the incident beam. An Arsenic filter was placed between the sample compartment and the ionization chamber to eliminate elastically-scattered X-rays from entering the ionization chamber. WinXAS version 1.3 was used for all data analysis, along with ATOMS and FEFF7.

Results: Selenate forms different surface complexes on iron oxides, depending upon pH, ionic strength, and iron oxide structure.

Figure 1. Se K-edge EXAFS spectra of aqueous selenate, selenate adsorbed on goethite at pH 6.0 (outer-sphere), and selenate adsorbed on goethite at pH 3.5 (inner-sphere). Figure 1a presents k3 weighted chi data, while Figure 1b shows RSFs for the same samples. Figure 1c represents the bonding environment of selenate on goethite derived from EXAFS and ATR-FTIR spectroscopy.

On hematite, selenate reacts to form inner-sphere monodentate surface complexes all pH and ionic strength. On goethite and amorphous iron hydroxide, however, selenate forms outer-sphere surface complexes at pH 6.0 and above, and a mixture of outer- and inner-sphere monodentate surface complexes between pH 3.5 and 6. ATR-FTIR spectroscopy also suggests that some hydrogen bonding is present in this inner-sphere surface complex. The relative amount of outer-sphere to inner-sphere adsorption on goethite is affected by pH and ionic strength. Inner-sphere complexation is favored by lowering pH and increasing ionic strength. Adsorption on amorphous iron hydroxide was similar to goethite, but far more outer-sphere complexation was observed on amorphous iron hydroxides than on goethite.

Peak, J. D. and D.L. Sparks. 2002. "Mechanisms of selenate adsorption on iron oxides and hyroxides." Environ. Sci. Tech.36: 1460-1466