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

Abstract

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 circumneutral reducing (bicarbonate-buffered ascorbic acid, pH 7.8, Eh ~-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.

Coprecipitation of Phosphate and Silicate Affects Environmental Iron (Oxyhydr)Oxide Transformations: A Gel-Based Diffusive Sampler Approach

“…By binding to FeOx surfaces, P and other strongly sorbing inorganic and organic species can passivate the particle surface and decrease the rate of FeOx transformation. 4,6,43,44 However, we note that adsorbed species (P and organic carbon) were not determined in the current study.…”

Thermal Stability and Decomposition Products of P-Doped Ferrihydrite

“…Although pure ferrihydrite is unstable and directly transforms to more stable phases such as goethite or hematite [2], many recent studies have proved that the admixtures present in ferrihydrite significantly inhibit its transformation [23,24]. In the presence of phosphate, the solubility of ferrihydrite decreases [25,26].…”

“…Although pure ferrihydrite is unstable and directly transforms to more stable phases such as goethite or hematite [ 2 ], many recent studies have proved that the admixtures present in ferrihydrite significantly inhibit its transformation [ 23 , 24 ]. In the presence of phosphate, the solubility of ferrihydrite decreases [ 25 , 26 ]. Consequently, the rate of ferrihydrite conversion is reduced [ 27 ] and the aggregation of colloidal iron oxide particles is observed [ 28 , 29 ] which leads to the preferred formation of hematite [ 27 , 30 ].…”

Effects of biotite syenite on the nutrient levels and electrical charges in a Brazilian Savanna Ferralsol

“…Thus, it is possible that the soil acidity may have contributed Effects of biotite syenite on the nutrient levels and electrical charges in a Brazilian Savanna Ferralsol to release P from the rock powder. A possible explanation for the slight decrease in the P levels at the highest dose of the biotite syenite might be the immobilization of P in iron oxihydroxides caused by the weathering of biotite syenite, resulting in iron oxidation (Kraal et al 2019). Additionally, the 1,200; 1,800; and 2,400 kg ha -1 doses presented a slightly lower Al content than that of the 600 kg ha -1 dose.…”