(1) Adsorption
The amorphous Al(OH)3(am) precursor with a large surface area generated during the aluminum salt flocculation defluorination process produces hydrogen bond adsorption on fluoride ions. Fluoride ions have a small radius and strong electronegativity, so this adsorption mode is easy to occur.
(2) Ion exchange
The radius and charge of fluoride ions are similar to those of hydroxide ions. During the aluminum salt flocculation defluorination process, polycations such as A113O4(0H)147+ added to the water and the amorphous Al(0H)3(am) precipitate formed after hydrolysis, the OH- in them exchanges with
F-. This exchange process is carried out under isoelectric conditions.
(3) Complex precipitation
F- can form six complexes with Al3+, such as AlF2+, AlF2+, AlF3, and AlF63-, and remove F- by complex precipitation. The complex ion equation is as follows: F-+Al3+→AlF2+↓+AlF2+↓+AlF3↓+AlF4-↓
+AlF52-↓+AlF63-↓The flocs produced by flocculation are effectively separated from the solid and liquid through the flotation device, and the effluent is discharged after sand filtration and activated carbon adsorption.
2. Operation effect
Since the completion and commissioning of this treatment facility, the environmental monitoring authority has sampled and monitored the F- concentration at the inlet and outlet of the facility many times.