FATE OF NI, CU, AND ZN DURING ATOM EXCHANGE BETWEEN AQUEOUS FE(II) AND METAL(II)-SUBSTITUTED GOETHITE AND HEMATITE

Abiotic interactions between aqueous Fe(II) and solid Fe(III) oxide minerals influence the fate and transport of nutrients and contaminants via surface mediated adsorption and redox reactions. Many studies have examined contaminant interactions with pure Fe(III) phases in the presence and absence of Fe(II). However, natural Fe(III) minerals, including goethite and hematite, often contain trace element impurities that substitute for Fe within the crystal structure. While many of these substituting elements are both micronutrients and water contaminants, little is known about their fate upon reaction of Fe(III) oxide minerals with aqueous Fe(II). Kinetic experiments show that aqueous Fe(II) catalyzes the release of Ni and Zn from both metal(II)-substituted hematite and goethite, whereas Cu is not released from the solid phase. Metal(II) release increases with pH and [Fe(II)]. Rapid adsorption of Fe(II) (t_1/2<1 hr) followed by a slower release of Ni and Zn (t_1/2>1 d) is consistent with a mechanism involving an Fe(II)-Fe(III) atom exchange redox reaction. Such processes are useful for understanding metal cycling and mobility in aquatic systems where aqueous Fe(II) and solid Fe(III) oxides coexist.