Aluminum and iron based coagulants are strongly acidic except sodium aluminate and polyaluminum chloride (PAC). Metal ions exist mainly as bare metal ions in the concentrated stock solution due to the extremely low hydroxyl ion (OH–) concentration at the low pH. However, as soon as coagulants are introduced to mixed liquor, metal ions rapidly form insoluble amorphous metal hydroxides, which are a mixture of metal ions with varying number of hydroxyl ligands. The formation of amorphous metal hydroxides occurs instantaneously as soon as metal ions contact with water since hydroxide ions can be produced very rapidly from the abundant water molecules around the metal ions. Under this condition, little mass transfer barrier exists due to the abundance of water molecules nearby.
The freshly formed metal hydroxides form insoluble flocs with open pore structure that can be easily accessed by phosphate ions. Moreover, the fresh hydroxide flocs have reactive surface properties that can exchange hydroxyl group with phosphate ions relatively quickly to form metal-phosphate complexes such as ≡MeH2PO4, ≡MeHPO4–, and ≡MePO42-, where ≡Me indicates the metal ions on the surface of the amorphous metal hydroxide. The mechanisms of chemical phosphorus removal is illustrated in Fig. 1. As the metal hydroxide floc aged, pores on metal hydroxide floc are closed, which results in slowing phosphate ion exchange.
When ferric ions are added to mixed liquor, hydrous ferric oxides (HFO) that consist of amorphous ferric hydroxide (am-Fe(OH)3(s)), ferrihydrite (FepOr(OH)s·nH2O), goethite (α-FeOOH), lepidocrocite (γ-FeOOH), and hematite (α-Fe2O3) are formed (Neethling, 2008). Initially reactive ferric hydroxides undergo aging processes and gradually lose their reactivity perhaps by the saturation of phosphate ions on easily accessible areas. A large difference was observed regarding phosphate absorption rate between 3 minutes old and 20 minutes old metal hydroxides (Smith, 2008).
In summary, formation of amorphous metal hydroxides is kinetically favored since hydroxide ions are abundant at the typical pH of mixed liquor. The metal hydroxides undergo ion exchange reaction with orthophosphate and gradually turn to metal phosphates. Although the ion exchange reaction is thermodynamically favored under the typical mixed liquor pH and temperature, it is a very slow reaction that may take days or weeks to reach a thermodynamic equilibrium condition. Therefore allowing enough contact time with enough mixing energy is crucial to fully utilize added metal ions and to obtain low Me/P ratio.
© Seong Hoon Yoon