RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

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), SO₄²⁻(aq), H₄SiO₄(aq), and cations [K⁺, Mg²⁺, Ca²⁺(aq)]. In paired-solute systems (e.g., CrO₄²⁻–H₂CO₃*), anionic cosolutes markedly reduce CrO₄²⁻ adsorption through a combination of competitive and electrostatic effects, but cations exert no appreciable influence. Additionally, H₄SiO₄ exhibits a strong time-dependent effect: CrO₄²⁻ adsorption is greatly decreased with increasing H₄SiO₄ contact time. In multiple-ion mixtures, each anion added to the mixture ecreases CrO₄²⁻ 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 CrO₄²⁻ adsorption data for paired- and multiple-solute systems. However, the model calculations underestimate CrO₄²⁻ adsorption when surface site saturation is approached. Questions remain regarding the surface interactions of both CO₂(aq) and H₄SiO₄. The results have major implications for the adsorption behavior of CrO₄²⁻ and other oxyanions in subsurface waters.