An in situ ATR-FTIR investigation of sulfate bonding mechanisms on goethite

The mechanism of sulfate adsorption on goethite was investigated in situ us ing attenuated total reflectance Fourier transform infrared (ATR-FTIR) spec troscopy. Sulfate adsorption was investigated at ionic strengths between 0. 005 and 0.1 M, reactant concentrations between 5 and 500 mu M, and pH value s between 3.5 and 9.0. It was determined that sulfate forms both outer-sphe re and inner-sphere surface complexes on goethite at pH less than 6. At pH values greater than 6, sulfate adsorbs on goethite only as an outer-sphere complex. The relative amount of outer-sphere sulfate surface complexation i ncreased with decreasing ionic strength. The spectrum of sulfate adsorbed o n goethite was also compared to the infrared spectrum of synthetic schwertm annite, an iron(III) oxy-hydroxy-sulfate. It was determined that in situ sp ectra of both schwertmannite and adsorbed sulfate are quite similar, sugges ting that a continuum of outer- and inner-sphere sulfate occurs in both cas es. (C) 1999 Academic Press.