Atomic-Level Structural Differences between Fe(III) Coprecipitates Generated by the Addition of Fe(III) Coagulants and by the Oxidation of Fe(II) Coagulants Determine Their Coagulation Behavior in Phosphate and DOM Removal

Atomic-level structural changes of flocs reveal the advantagesof in situ Fe(III), formed in Fe(II)-KMnO4 processes, forDOM and phosphorus removal In situ Fe(III) coprecipitation fromFe(2+) oxidationis a widespread phenomenon in natural environments and water treatmentprocesses. Studies have shown the superiority of in situ Fe(III) (formedby in situ oxidation of a Fe(II) coagulant) over ex situ Fe(III) (usinga Fe(III) coagulant directly) in coagulation, but the reasons remainunclear due to the uncertain nature of amorphous structures. Here,we utilized an in situ Fe(III) coagulation process, oxidizing theFe(II) coagulant by potassium permanganate (KMnO4), totreat phosphate-containing surface water and analyzed differencesbetween in situ and ex situ Fe(III) coagulation in phosphate removal,dissolved organic matter (DOM) removal, and floc growth. Comparedto ex situ Fe(III), flocs formed by the natural oxidizing Fe2+ coagulant exhibited more effective phosphate removal. Furthermore,in situ Fe(III) formed through accelerated oxidation by KMnO4 demonstrated improved flocculation behavior and enhanced removalof specific types of DOM by forming a more stable structure whilestill maintaining effective phosphate removal. Fe K-edge extendedX-ray absorption fine structure spectra (EXAFS) of the flocs explainedtheir differences. A short-range ordered strengite-like structure(corner-linked PO4 tetrahedra to FeO6 octahedra)was the key to more effective phosphorus removal of in situ Fe(III)than ex situ Fe(III) and was well preserved when KMnO4 acceleratedin situ Fe(III) formation. Conversely, KMnO4 significantlyinhibited the edge and corner coordination between FeO6 octahedra and altered the floc-chain-forming behavior by acceleratinghydrolysis, resulting in a more dispersed monomeric structure thanex situ Fe(III). This research provides an explanation for the superiorityof in situ Fe(III) in phosphorus removal and highlights the importanceof atomic-level structural differences between ex situ and in situFe(III) coprecipitates in water treatment.