Iron and manganese oxides in Finnish ground water treatment plants

Large amounts of ochreous precipitates are formed on aeration of Fe containing Finnish ground waters during purification for drinking purposes. Sixty-four precipitates were characterized chemically and mineralogically. X-ray diffraction (XRD) indicated that the Fe-rich precipitates consist mainly of a poorly ordered ferrihydrite (5 Fe₂O₃ · 9 H₂O) which only has 2–3 of the 6 XRD lines characteristic of better ordered ferrihydrite. The surface area ranges between 325 and 433 m² g⁻¹ corresponding to a particle size of 5 nm. The ferrihydrites contain 3–7% Si strongly associated with the ferrihydrite as indicated by an i.r. absorption band at 960–975 cm⁻¹ which is associated to Fe-O-Si bonds. Si-containing ferrihydrite typically forms by rapid oxidation of ground waters with 1–23 mg 1⁻¹ Fe and 7–12 mg 1⁻¹ Si at pH 6–7. Very similar products formed in a simulation experiment in which artificial ground water with 20 mg 1⁻¹ Fe was oxidized in the presence of 12 mg 1⁻¹ Si. A1 < 4 mg 1⁻¹ Si lepidocrocite (γ-FeOOH) was formed showing that Si in the system prevents the formation of the more stable and better crystallized FeOOH forms. A transformation of 2-line ferrihydrite to better ordered ferrihydrite or goethite with time is indicated. The Mn-oxide birnessite was identified in black precipitates formed in one plant.