Nanoscale Formation, Structure, and Stability of Phosphate-Iron Colloids at Anoxic-Oxic Interfaces

The mobility of phosphorus (P) at anoxic-oxic interfaces is largely controlled by the formation of phosphate-iron (Fe) colloids, yet the nanoscale mechanisms underlying their formation, structure, and stability remain poorly described. Here, we systematically investigate these processes across a range of environmentally relevant conditions. Under anoxic conditions, phosphate-Fe(II) complexes exist predominantly as truly dissolved species. Upon transition into oxic conditions, phosphate-Fe(III) colloids form as a function of Fe/P molar ratio. At lower Fe/P ratios (e.g., ≤3), stable nanosized colloids (20-100 nm) form, whereas higher ratios (e.g., >3) promote aggregation into larger particles (>1000 nm) under the conditions tested. Structural analyses reveal that colloidal formation arises from a phosphate-stabilized, short-range ordered ferrihydrite phase, where phosphate inhibits Fe polymerization via corner-sharing Fe-O-P bonds and enhances electrostatic stabilization through surface enrichment. Natural organic matter (NOM, 2.5 and 10 mg C/L) promotes colloid generation under anoxic conditions and enhances stability at higher Fe/P ratios through electrostatic and steric interactions. Stability assessments in the tested water matrices show matrix-dependent persistence, with 0-41% of the colloids remaining suspended after 28 days without NOM, and substantially enhanced colloidal suspension (61-95%) in the NOM-amended system. This work provides mechanistic insight into one important pathway that may contribute to P mobility under redox-transition conditions.