Iron(III) Coordination and Phosphate Sorption in Peat Reacted with Ferric or Ferrous Iron

摘要

The molecular bonding configuration of Fe(III) associated with organic matter (OM) in soils potentially affects its binding affinity, reducibility, mobility, and interactions with other cations and anions. Given that Fe(III) can be incorporated into soil OM either directly as Fe(III) or through oxidation of Fe(II), the coordination and reactivity of OM-associated Fe(III) may depend on soil redox conditions. Our objective was to determine if the molecular coordination environment and phosphate sorption characteristics of OM-associated Fe(III) depend on the pathway of Fe(III) incorporation. Pahokee peat soil material was reacted at pH 2.5 with either Fe(III) or Fe(II) at 300, 900, or 1200 mmol Fe kg(-1) peat, equilibrated at pH 6.8 and oxidized. Extended x-ray absorption fine structure and wavelet transform analyses indicated the presence of mononuclear Fe(III) coordinated with organic ligands in all samples along with polynuclear Fe-hydroxide species at higher concentrations. The pathway of Fe incorporation did not significantly affect its local coordination environment or phosphate sorption isotherms. Our results suggest that OM-associated Fe(III) formed under oxidized or reduced soil conditions would have similar molecular-level coordination and be equally accessible for phosphate sorption.