Vibrational spectroscopy study of selenate and sulfate adsorption mechanisms on Fe and Al (hydr)oxide surfaces

The coordination and speciation of selenate (SeO4) and sulfate (SO4) on goethite and Al oxide were studied using Raman and ATR-FTIR spectroscopy. Raman spectra were collected from pastes of suspensions containing 4 mM SeO4 or SO4. For SO4, complementary data were collected by ATR-FTIR spectroscopy in goethite systems with 1 mM SO4 and in Al oxide systems with 4 mM SO4. The combined data set of Raman and ATR-FTIR spectra indicate that both inner- and outer-sphere surface complexes of SeO4 and SO4 occur on these metal (hydr)oxide surfaces. These spectral data show that SeO4 and SO4 have a similar complexation behavior on the same adsorbent. On goethite, these form predominantly monodentate inner-sphere surface complexes at pH <6, while at pH >6 these anions exist predominantly as outer-sphere surface complexes. On Al oxide, in contrast, these anions exist predominantly as outer-sphere surface complexes, but a small fraction is also present as an inner-sphere complex at pH <6. A comparison of the spectral intensities of these anions on goethite and Al oxide shows that complexation of these anions with Al oxide is weaker than with Fe oxide. (C) 2000 Academic Press.