Structural Variation of Precipitates Formed by Fe(II) Oxidation and Impact on the Retention of Phosphate

The oxidation-precipitation process of Fe(II) is ubiquitous in the environment and critically affects the fate of contaminants and nutrients in natural systems where Fe(II) is present. Here, we explored the effect of H2O2 concentration on the structure of precipitates formed by Fe(II) oxidation and compared the precipitates to those formed by Fe(III) hydrolysis. Additionally, the phosphate retention under different H2O2 concentrations was evaluated. XRD, TEM, PDA, XPS, and UV-visible Fe(II) retention absorbance spectroscopy were used to characterize the structure of the formed precipitates; UV-visible absorbance spectroscopy was also used to determine the residual phosphate and Fe(II) in solution. It was found that the predominant precipitates in Fe(II) solution changed from planar-shaped crystalline lepidocrocite (gamma-FeOOH) to poor short-range order (poorly crystalline) spherical-shaped hydrous ferric oxide (HFO) with increasing H2O2 concentrations. Although the HFO precipitates formed from Fe(II) resembled those formed from Fe(III) hydrolysis, the former was larger and had clearer lattice fringes. During the formation of gamma-FeOOH, both Fe(II)-Fe(III) complexes and ligand-tometal charge transfer processes were observed, and it was found that Fe(II) was present in the planar-shaped precipitates. Fe(II) might be present in the interior of precipitates as Fe(OH)(2), which could serve as a nucleus for the epitaxial growth of gamma-FeOOH. In addition, the extent of phosphate retention increased with the H2O2 concentration, indicating the increased reactivity of formed precipitates with H2O2 concentration. More phosphate was retained via coprecipitation with Fe than adsorption on the preformed Fe precipitates due to the incorporation of phosphate within the structure of the formed Fe hydroxyphosphate via coprecipitation.

Automated summary

The oxidation-precipitation process of Fe(II) plays a crucial role in the removal of contaminants and nutrients. This study investigated the effect of H2O2 concentration on the structure of Fe(II) precipitates and evaluated the retention of phosphate under different H2O2 concentrations.