Identity and Character of Iron Precipitates

Engineers have commonly described the iron precipitate in water treatment plants as ferric hydroxide, Fe (OH)3, but have also considered the possibility that in certain cases it might be ferrous carbonate. These were reasonable assumptions based on equilibrium constant data; however, neither Fe(OH)3 nor FeCO3 were ever actually proven to exist in water treatment plants. However, Fe(OH)3 is not a recognized crystalline mineral and recent investigations at Iowa State University found that FeCO3 unlikely to form in most treatment plants. Rather, when oxygen is the oxidant, the precipitate is more likely to be very small crystals of lepidocrocite, γ-FeOOH, at low silica concentrations (LT7 mg/ at pH 7 with an iron concentration of 10 mg/1), but becomes amorphous at higher silica concentrations due to the adsorption of silica onto the growing crystals. When a strong oxidant such as KMnO4 is used the precipitate has a much smaller pore structure and is amorphous.