Interaction of Fe(II) with phosphate and sulfate on iron oxide surfaces

Sulfate and phosphate, oxoanions common in natural systems, affect iron oxide growth and dissolution processes, the adsorption behavior of divalent cations, and iron oxide phase transformations. These oxoanions may thus influence Fe(II) adsorption behavior and subsequently alter the mechanisms and products of Fe(II)-catalyzed Fe(III) oxide recrystallization processes, such as trace metal repartitioning. In this study, the macroscopic and molecular-scale effects of the coadsorption of Fe(II) and sulfate or phosphate onto Fe(III) oxide surfaces were investigated. Macroscopic adsorption edges show that both sulfate and phosphate increase Fe(II) adsorption and that Fe(II) increases sulfate and phosphate adsorption. Attenuated total reflectance Fourier transform infrared spectroscopy shows that the cooperative adsorption behavior of oxoanions and aqueous Fe(II) likely results from a combination of ternary complexation and electrostatic interactions. Surface complexation modeling requires the inclusion of ternary complexes to simulate all conditions of the macroscopic data, further suggesting that these oxoanions and Fe(II) form ternary complexes on Fe(III) oxide surfaces. Despite clear evidence in previous research for Fe(II) oxidation upon adsorption on iron oxide surfaces, this work shows that Fe(II) also displays macroscopic and molecular-scale behaviors associated with divalent (i.e., non-oxidative) cation adsorption. Prior work has shown that metal release from iron oxides caused by ET-AE reactions is directly proportional to the macroscopically-determined Fe(II) surface coverage. Predicting the effects of sulfate and phosphate on processes controlled by ET-AE reactions at redox interfaces, such as mineral phase transformations and trace element repartitioning, may thus not require the explicit consideration of electron transfer processes. (C) 2015 Elsevier Ltd. All rights reserved.