Phosphate release induced by the reaction of synthetic lepidocrocite (γ-FeOOH) with dissolved sulfide

Abstract

Reductive dissolution of iron(III) oxyhydroxide minerals is an essential process for regulating the aqueous concentration and mobility of phosphate in natural systems. This is mainly a result of strong specific adsorption of phosphate species onto the particle surface of these minerals, which leads to a release of phosphate to the environment upon dissolution. In order to identify the exact mechanisms of phosphate release during the reductive dissolution reaction of iron(III) oxyhydroxide minerals, multiple flow-through experiments have been performed with synthetic lepidocrocite (γ-FeOOH) and dissolved sulfide. The experimental results are compared to the results from a modeling simulation, which leads to the recognition of two different mechanisms of phosphate release in this case. The process of competitive adsorption can be described as a ligand exchange reaction in which adsorbed phosphate on the lepidocrocite surface is replaced by dissolved sulfide. The kinetics of this process are almost instantaneous for mononuclear complexes, while binuclear complexes of phosphate are replaced much more slowly. Apart from this ligand exchange reaction, phosphate is simply released upon dissolution of iron atoms in its surroundings. This second mechanism accounts for the release of phosphate that is situated in the bulk phase, as well as for adsorbed phosphate release when the kinetics of competitive adsorption are slower than the reductive dissolution rate of lepidocrocite. The influence of pH on the release of phosphate is really prominent, which could be explained based on the speciation of adsorbed phosphate. The relative abundance of mononuclear complexes increases when the pH rises above the isoelectric point, which results in a more instantaneous type of pattern for the release of adsorbed phosphate in higher pH ranges.