In situ ATR-FTIR spectroscopic study of adsorption of perchlorate, sulfate, and thiosulfate ions onto chromium(III) oxide hydroxide thin

A thin chromium oxide hydroxide colloid film has been used as a model of th e passive stainless steel surface for studies of anion adsorption from aque ous solutions. The adsorption of perchlorate, sulfate, and thiosulfate ions has been investigated by in situ attenuated total reflection infrared (ATR -IR) spectroscopy. Surface charge was monitored from the surface excess con centrations of tetramethylammonium ions and of perchlorate ions using the i nfrared spectroscopic STIRS technique. The colloid film showed a high posit ive charge at low pH and a low negative charge at high pH. The adsorption o f sulfate was only observed for a positive surface charge. The infrared spe ctrum of adsorbed sulfate was significantly altered by the interfacial elec tric field, but there was no evidence of sulfate coordination to surface Cr (III) ions. Thiosulfate gave adsorption behavior analogous to sulfate. Adso rption isotherms for sulfate and for thiosulfate were determined from infra red spectral data, and Langmuir binding constants of (2.3 +/- 0.4) x 10(5) and (1.4 +/- 0.4) x 10(5) M-1 were obtained for the respective adsorbates. The lack of chemical binding of sulfate and of thiosulfate to the chromium oxide hydroxide surface may be part of the basis for the corrosion-promotin g properties of these ions at stainless steel surfaces.