Fluoride Removal from Wastewater by Electro-coagulation-flotation Process

• Main Authors: Ching-Yao Hu, 胡景堯
• Other Authors: 駱尚廉
• Format: Others
• Language: zh-TW
• Published: 2005
• Online Access: http://ndltd.ncl.edu.tw/handle/42449012701729556435

博士 === 國立臺灣大學 === 環境工程學研究所 === 93 === ABSTRACT Excessive fluoride in water is toxic for human and aquatics. Owing to the development of the semiconductor industry, how to treat fluoride-containing wastewater produced by the industry efficiently and economically has become an important issue for environmental engineers. Currently, the cheapest way to remove fluoride ions from high-fluoride containing wastewater is to form calcium fluoride (CaF2) by adding excess lime or other calcium salts. However, this process causes the problem of hardness of the effluent water due to over addition of calcium salts. This thesis used an electrochemical technique, electro-coagulation-flotation process to treat fluoride containing wastewater. The mechanism of fluoride removal by electro-coagulation, the effect of water quality parameters and charge loading on the defluoridation efficiency of the process and the kinetic of the defluoridation are investigated in this study. Finally, the calcium precipitation was combined with the electro-coagulation-flotation process to treat high fluoride containing wastewater with stoichiometric amount of calcium salt. The results shows coprecipitation is the major mechanism for fluoride removal for aluminum coagulation. The efficiency of defluoridation was approximately 100% when the sum (�脧H+F) of the molar ratio of hydroxide (�脧H) and fluoride ions (�脥) to Al(III) ions and was close to three. This finding reveals that the fluoride ions and the hydroxide ions can co-precipitate with Al(III) ions and the formula of the precipitate is AlnFm(OH)3n-m. However, when �脧H was less than 2.4, the co-precipitate is not stable. Therefore, the defluoridation efficiency, given �脧H+F =3, dropped as �脧H fell, as �脧H was less than 2.4. The type of the dominant anion directs the EC defluoridation reaction. The defluoridation efficiency was almost 100% and most of the fluoride removal reaction occurred on the surface of the anode in the solution without the co-existing anions, due to the electro-condensation effect. In the solutions with co-existing anions, the defluoridation efficiency was less than 100% and most of the defluoridation took place in bulk solution. The existence of sulfate ions inhibits the localized corrosion of aluminum electrodes, leading to lower defluoridation efficiency because of lower current efficiency. The presence of chloride or nitrate ions prevented the inhibition of sulfate ions, and the chloride ions were more efficient. Above phenomenon should be because the different corrosion types occurred in different anion-containing solution. The rates of fluoride removal follow zero order kinetics when the concentration of fluoride exceeds around 8mg/L in the solutions with co-existing anions. Otherwise, the rates follow first order kinetics. A variable order kinetics (VOK) model based on Langmuir equation was developed to simulate the kinetics of EC defluoridation. The result revealed VOK model is better than first order kinetic model used by previous researchers for simulation of EC defluoridation. However, the VOK model can only be employed in systems with very little initial acidity or with an initial acidity higher than that of the initial fluoride concentration. An anodic surfactant, sodium dodecyl sulfate (SDS), was employed to improve the flotation performance of the electro-coagulation-flotation (ECF) process to treated fluoride containing semiconductor wastewater following calcium precipitation. The dissolved fluoride ions and CaF2 particles in the wastewater after calcium precipitation were effectively removed in ECF process simultaneously. The dosage of SDS required for ECF was much less than those for dispersed air flotation (DiAF) or dissolved air flotation (DAF) processes because the CaF2 particles can be collected by hydro-fluoro-aluminum flocs in ECF. Thus, SDS is only served as a frother to make the bubble tiny and stable in ECF defluoridation process. The interference of co-existing anions can be overcome by increasing the dosage of calcium ions and SDS. The initial acidity of the wastewater after calcium precipitation can be modified by changing the [Ca(OH)2]/[Ca2+]T ratio and the appropriate ratio is approximately given by the acid dissociation constant of hydrofluoric acid and the initial pH of the wastewater before calcium precipitation.