Fe²⁺ sorption at the Fe oxide-water interface: A revised conceptual framework

Sorption of aqueous Fe²⁺ at the Fe oxide-water interface has traditionally been viewed in the classic framework of sorption at static oxide surface sites as formulated in surface complexation models (SCMs). Significant experimental and theoretical evidence has accumulated, however, to indicate that the reaction of aqueous Fe²⁺ with Fe³⁺ oxides is much more complex and is comprised of sorption, electron transfer, conduction, dissolution, and, in some cases, atom exchange and/or transformation to secondary minerals. Here, we provide a brief historical review of Fe ²⁺ sorption on Fe oxides and present a revised conceptual model based on the semiconducting properties of Fe oxides that incorporates recent experimental evidence for Fe²⁺-Fe_oxide³⁺ electron transfer, bulk electron conduction, and Fe atom exchange. We also discuss the implications of this revised conceptual model for important environmental processes, such as trace metal cycling and contaminant fate.