Study On Mechanism And Technology Of Fluoride Removal From Drinking Water By Adsorption-ultrafiltration Process

Fluoride removal from drinking water has become an urgent task in China, owing to the wide distribution of high concentration fluoride groundwater, which could do serious harm to human health. So it is of great significance to explore a process which has high fluoride removal rate and maintain ability.In this study, La₂O₃·nH₂O and alumina were chosen as absorbent, and adsorption mechanism and adsorption - ultrafiltration combination process were researched. The main conclusions are shown as followed.Mechanism of fluoride removal was researched in static tests to provide a theoretical basis for the continuous experiments. Adsorption isotherms with the impact of pH were put forward, and the results calculated by related theory could perfectly inosculate with that coming from experiments, which validated that the effect of pH on fluoride removal by La₂O₃·nH₂O and activated alumina was resulted from ion exchange and competitive adsorption respectively. And the impact of pH on adsorption capacity could be predicted by the theory put award in this paper. Experimental results showed that La₂O₃·nH₂O could reach a high adsorption capacity when the pH was lower than 8 after reaction, and was hardly influenced by pH in that range. As well as the pH was in the range of 8 to 9, the adsorption capacity decreased slowly, and if the pH was higher than 9, the adsorption capacity decreased sharply. Activated alumina could reach the highest adsorption capacity when the pH was in the range of 4 to 6 after reaction.In order to determine the balance time for the adsorption capacity, a new dynamic model taking the initial concentration of fluoride and absorbent dosage into account was put forward. The model was very anastomosis with the results of 17 experiment sets. The model can be used to speculate the equilibrium time given known the adsorbent dosage and the initial fluoride concentration. 60 min was chosen in the adsorption capacity study according to the theoretical results. On this basis, the effect of particle size on the adsorption capacity of the two adsorbents was studied. Results showed that particle size had a significant impact on adsorption capability. The adsorption capacity increased by 5.5 times when the particle size of La₂O₃·nH₂O changed from 0.5-0.71 mm to 0.038-0.05 mm. The adsorption capacity of La₂O₃·nH₂O was higher than that of activated alumina. It was 6.4-22.2 times as much as that of activated alumina with the same particle size.The factor impacts on membrane fouling with the activated alumina adsorption– ultrafiltration and La₂O₃·nH₂O adsorption– ultrafiltration continuous process was studied. The optimization conditions for the activated alumina was determined as particle size 0.05-0.074 mm, adsorbent dosage 0.2 g/L, no aeration, back-washing cycle 6 h. From the analysis of membrane fouling mechanism the results as following could be gained. At the beginning, the membrane fouling was controlled by hole plugging, and later it was controlled by cake layer. And the whole stage would be controlled by cake layer if the adsorbent particle size was small.