Studies of Cd(II)-sulfate interactions at the goethite-water interface by ATR-FTIR spectroscopy

A combination of macroscopic experiments and in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy was used to study Cd(II)-sulfate interactions on the goethite-water interface. The presence of SO₄ dramatically promoted Cd adsorption at lower pH (pH 5.5-6.5) and had a smaller effect at higher pH. ATR-FTIR studies indicated sulfate adsorption on goethite occurred via both outer- and inner-sphere complexation. The relative importance of both complexes was a function of pH and sulfate concentration. ATR-FTIR spectra provided direct evidence of the formation of Cd-SO₄ ternary surface complexes on goethite. In addition to ternary complexes, Cd specifically sorbed on goethite promoted SO₄ adsorption via changing the surface charge, and caused additional SO₄ adsorption as both inner- and outer-sphere complexes. The relative importance of ternary complexes versus electrostatic effects depended upon pH values and Cd concentration. Ternary complex formation was promoted by low pH and high Cd levels, whereas electrostatic effects were more pronounced at high pH and low Cd levels. A portion of SO₄ initially sorbed in inner-sphere complexes in the absence of Cd was transformed into Cd-SO₄ ternary complexes with increased Cd concentration.