Defluorination in High Fluoride-Bearing Water with Electric Field and Limestone

High F
^-
in water can be removed by Ca
²⁺
in form of precipitation of CaF
₂
in water when their ion activity production(IAP) reaches up to equilibrium dissolution constant of fluorite based on the precipitation theory of insoluble minerals. Self-designed tank is used to generate a large number of Ca
²⁺
and CO
₃
^2-
from dissolution of limestone under the effect of electric field, then the Ca
²⁺
and F
^-
movement to the cathode and anode respectively leads to partial accumulation in certain area. When [Ca
²⁺
][ F
^-
]
²
>Ksp(CaF
₂
), CaF
₂
precipitates. The effect of voltage (U), distance of cathode and anode (d) and time (t) is discussed, the primary experiment results show that F
^-
concentration can be reduced to 1 mg/L(upper limit for Fluorine in drinking water in China) with initial F
^-
concentration of 10 mg/L, and the optimal conditions are as following: U = 8V, d = 2cm, t = 90min.