Chromate Adsorption on Amorphous Iron Oxyhydroxide in the Presence of Major Groundwater Ions

Authors

John M. Zachara, Pacific Northwest National LaboratoryFollow
Donald Girvln, Pacific Northwest National Laboratory
Ronald Schmidt, Pacific Northwest National Laboratory
C. Thomas Resch, Pacific Northwest National Laboratory

Date of this Version

1987

Citation

Environ. Sci. Technol. 1987, 21, 589-594

Abstract

Chromate adsorption on amorphous iron oxyhydroxide was investigated in dilute iron suspensions as a single solute and in solutions of increasing complexity containing CO₂ (g), SO₄^2-(aq), H₄Si0₄(aq), and cations [K⁺, Mg²⁺, Ca²⁺(aq)]. In paired-solute systems (e.g., CrO₄^2- =H₂CO₃*), anionic cosolutes markedly reduce CrO₄^2- adsorption through a combination of competitive and electrostatic effects, but cations exert no appreciable influence. Additionally, H4Si04 exhibits a strong time-dependent effect: CrO₄^2- adsorption is greatly decreased with increasing H₄SiO₄ contact time. In multiple-ion mixtures, each anion added to the mixture decreases CrO₄^2- adsorption further. Adsorption constants for the individual reactive solutes were used in the triple-layer model. The model calculations are in good agreement with the CrO₄^2- adsorption data for paired- and multiple-solute systems. However, the model calculations underestimate CrO₄^2- adsorption when surface site saturation is appr6ached. Questions remain regarding the surface interactions of both CO₂(aq) and H₄Si0₄. The results have major implications for the adsorption behavior of CrO₄^2- and other oxyanions in subsurface waters.