Phosphorus Removal from Lake Water by Combined Dosage of Ferrous Iron and Diatomite

Effective technology and approaches for reducing phosphorus concentration in the lake water are essential to control cyanobacterial bloom and lake eutrophication. This study investigated the effectiveness of the combined usage of ferrous iron (FeCl₂) and diatomite for the phosphorus removal. Several combinations of FeCl₂ and diatomite in various ratios were tested. Higher amounts of phosphorus were removed from water by the combined usage of FeCl₂ and diatomite than that by FeCl₂ alone. The optimum diatomite to FeCl₂ ratio resulting in the highest efficiency was 0.8. Alkalinity and pH of lake water treated with the combined usage of FeCl₂ and diatomite decreased less than that treated by FeCl₂. The higher P removal efficiency achieved by the combined usage of FeCl₂ and diatomite can be explained as that dissolved silica provided by diatomite was incorporated into the crystalline structure of iron oxide during the ferrous oxidation and thus silica-stabilized ferrihydrite was formed. Silica-stabilized ferrihydrite was an effective phosphorus adsorbent and long-term phosphorus sink. However, ferric oxide formed without dissolved silica was transformed into more crystalline ferric oxide and then phosphorus would be released back to the lake water during this transformation. Diatom growth was stimulated by the increased dissolved Fe and dissolved Si which was provided by diatomite. The stimulated diatom growth can further uptake phosphorus following the phosphorus adsorption by the silicate stabilized ferrihydrite. It was concluded that the combined usage of FeCl₂ and diatomite is an effective approach for improving the chemical precipitation of phosphorus by FeCl₂.