Chromate adsorption on amorphous iron oxyhydroxide in the presence of major groundwater ions

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), H4Si04(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, H4Si04 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 decreases 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 appr6ached. Questions remain regarding the surface interactions of both CO2(aq) and H4Si04. The results have major implications for the adsorption behavior of CrO42- and other oxyanions in subsurface waters.