FLUORIDE REMOVAL FROM HOT SPRING WASTE WATER BY AN ELECTROLYSIS METHOD AND ITS MECHANISM

Abstract

 A high concentration of fluoride in the wastewater from hot springs is an environmental issue in Japan, since some of the wastewater exceeds the national minimum effluent standards of 8 mg/L. However, an effective treatment for fluoride removal has not yet been developed. Accordingly, the temporal effluent standards of 30 mg/L or 50 mg/L have, so far, been applied to the wastewater from hot springs. In this study, an electrolysis system consisting of an anode bath and a cathode bath separated by a diaphragm made of a clay panel was tested for the fluoride removal. In an electrolysis system, fluoride is removed by co-precipitation with magnesium hydroxide formed in a cathode bath under a high pH condition. When 100 mg/L of magnesium was added to water from Gero hot spring, Gifu, Japan, the electrolysis system reduced the fluoride concentration from 17 mg/L to less than 8 mg/L when the flow ratio a:b was 0.6:9.4. The production of Cl₂ accompanied by the reduction of Cl^- reduced the H⁺ production in the anode, leading to high pH level in the cathode bath which enhanced the co-precipitation of fluoride with magnesium hydroxide.

 The mechanisms of the fluoride removal was elucidated by using a model. The fluoride removal was expressed by the following formula.

 -dF=k^*(-dMg)^*F

 F and Mg are fluoride and magnesium concentration, respectively. k is a constant.