Sorption of phosphate and silicate alters dissolution kinetics of poorly crystalline iron (oxyhydr)oxide

Iron (oxyhydr)oxides (FeOx) control retention of dissolved nutrients and contaminants in aquatic systems. However, FeOx structure and reactivity is dependent on adsorption and incorporation of such dissolved species, particularly oxyanions such as phosphate and silicate. These interactions affect the fate of nutrients and metal(loids), especially in perturbed aquatic environments such as eutrophic coastal systems and environments impacted by acid mine drainage. Altered FeOx reactivity impacts sedimentary nutrient retention capacity and, eventually, ecosystem trophic state. Here, we explore the influence of phosphate (P) and silicate (Si) on FeOx structure and reactivity. Synthetic, poorly crystalline FeOx with adsorbed and coprecipitated phosphate or silicate at low but environmentally relevant P/Fe or Si/Fe ratios (0.02-0.1 mol mol^-1) was prepared by base titration of Fe(III) solutions. Structural characteristics of FeOx were investigated by X-ray diffraction, synchrotron-based X-ray absorption spectroscopy and high-energy X-ray scattering. Reactivity of FeOx was assessed by kinetic dissolution experiments under acidic (dilute HCl, pH 2) and circum-neutral reducing (bicarbonate-buffered ascorbic acid, pH 7.8, E_h ∼ -300 mV) conditions. At these loadings, phosphate and silicate coprecipitation had only slight impact on local and intermediate-ranged FeOx structure, but significantly enhanced the dissolution rate of FeOx. Conversely, phosphate and silicate adsorption at similar loadings resulted in particle surface passivation and decreased FeOx dissolution rates. These findings indicate that varying nutrient loadings and different interaction mechanisms between anions and FeOx (adsorption versus coprecipitation) can influence the broader biogeochemical functioning of aquatic ecosystems by impacting the structure and reactivity of FeOx.