Study On Fluoride Removal Using Unipolar Three-dimensional Electrodes

Fluoride is an essential element for human body. However, both deficiency andoverdosage of fluoride will be harmful to health. An acceptable range of fluorideconcentration in drinking water is 0.5~1.0mg/L. In China, endemic fluorosis is widelydistributed and affects large amounts of people severely. Therefore, the research offluoride removal techniques is necessary. In the paper, the removal of fluoride usingunipolar three-dimensional electrodes has been investigated on the basis of summaryof the existing techniques'deficiency. The particle electrodes had a large specific areafor adsorption of fluoride ions, which provided a new option for fluoride removal indrinking water.By means of static tests, the effect of three-dimensional electrodes and theoperation conditions were studied through the aspects of particle electrodes, spacebetween polar plates and voltage, and the thermomechanical analysis of fluorideadsorption was conducted. In addition, the efficiency and regeneration way ofcontinuous reactor was investigated.The results of static test showed that fluoride concentration could be decreasedfrom 4mg/L to below 1mg/L and reached the standard of drinking water of Chinawhen using activated carbon-1, activated carbon-2 and anthracite as the particleelectrodes. The minium voltage was 5V, 4V and 7V respectively when the spacebetween polar plates was 10mm. The fluoride removal efficiency increased withhigher specific area and porosity of the particle electrodes. However, the workingarea depended upon the pore-size distribution of the particles due to the overlappingof double electrode layer. The removal efficiency also increased with smaller spacebetween polar plates, for the space among particles led to voltage loss. When thevoltage increased, the removal efficiency increased, because the charging density onthe surface of electrodes was higher and more ions could be adsorbed. When thevoltage exceeded the electrolytic voltage of H2O, minus effect would occur. Theresult of thermomechanical analysis showed that the fluoride adsorption of three-dimensional electrodes agreed with Henry formula and Freundlich formula,whose adsorption coefficient grew exponentially with voltage, and proved thatadsorption capacity of fluoride increased with voltage.The result of continuous reactor showed that the fluoride concentration of mostpart of the reactor could be reduced to 0.5~0.8mg/L under the condition of 4.0L/h and3V, using activated carbon-2 as the particle electrodes. In the adsorption phase, thevoltage between polar plates increased gradually while the current decreased, and thechange rate of the two parameters decreased gradually, which showed that theadsorption process had characteristic of capacity charge. In the desorption phase,fluoride ions could not be desorbed effectively when raw water was used for flushing,leading to the performance reduction of three-dimensional electrodes. When 1% HClwas used, most adsorption capacity of fluoride could be regained, which concerned tothe force between fluoride ions and functional groups on the surface of activatedcarbon. Three-dimensional electrodes technique required low intake pressure. Thehead loss of continuous reactor remained 0.05m when the flow rate was 4.0L/h.