Removal of Fluoride from Water Using an Activated Alumina Modified with Iron Compounds

1. Access to safe drinking water is an important factor for human life. Presence of both chemical and microbial contaminants in water can lead to different types of disease. In areas which groundwater is contaminated to fluoride via dissolution from geological formations and earth crust, drinking water can be considered as a major source of exposure to fluoride. Dental fluorosis and skeletal fluorosis are health outcomes of exposure to high concentration of fluoride via drinking water. World Health Organization (WHO) guidelines [1] suggested that in areas with warm climate, the optimal fluoride concentration in drinking water should remain below 1 mg/L (1 ppm or part per million), while in cooler climates it could go up to 1.2 mg/L. The differentiation derives from this fact that we perspire more in hot weather and consequently drink more water. The guideline value (permissible upper limit) for fluoride in drinking water was set at 1.5 mg/L, considered a threshold where the benefit of resistance to tooth decay did not yet shade into a significant risk of dental fluorosis. It has long been known that excessive fluoride intake carries serious toxic effects. But scientists are now debating whether fluoride confers any benefit at all or no. There are different options for overcoming to fluoride problem in drinking water including replacement of contaminated water resources with safer one or removal of excessive fluoride from water through application of different treatment methods. Those methods are generally based on chemical absorption, chemical precipitation, ion exchange and physical removal [reverse osmosis (RO) and electrodialysis process (ED)] [2]. Present study aimed to investigate the efficiency of a modified activated alumina for removal of fluoride from aqueous solution. 2. Methodology2.1. Batch experiments:Modified Activated Alumina (MAA) by iron compounds was used in current study which was a commercial product of Alcan Company, Canada [3]. During batch absorption experiments, effects of solution pH (5-9), initial fluoride concentration (1-10 mg/L), adsorbent dose (1-50g/L), contact time (15-180min) and shaking rate (100-400rpm) on fluoride removal was investigated in the laboratory temperature (20±1ºC). 2.2. Fluoride measurement:SPADNS method was used for the measurement of fluoride based on standard methods for the examination of water and wastewater. The SPADNS colorimetric method is based on the reaction between fluoride and a zirconium-dye lake. Fluoride reacts with the dye lake, dissociating a portion of it into colorless complex anion (ZrF62-) and the dye. As the amount of fluoride increases, the color produced becomes progressively lighter. Spectrophotometer (Model: Jenway 6105 UV/Vis) was used at 570 nm for measurements [4]. 3. 3.1. Effect of absorbent dose, contact time, pH and shaking rate on fluoride removalAbsorbent dose of 1, 5, 10, 25 and 50 g/L removed respectively 70, 91, 95, 97 and 98 of fluoride with initial concentration of 10 mg/L during 120 minutes. Optimum dose of absorbent was determined 5 g/L in 120 minutes. Fluoride level was decreased from 10 mg/L to 0.8 mg/L by 5g/L MAA (as optimum dose). This concentration is in the range of national guideline for fluoride in drinking water. Results of the study indicated that fluoride removal efficiency increased by increasing the contact time. Removal efficiency in 15, 30, 60, 120, and 180 minutes were 70, 80, 88, 92 and 93 percent using optimum dose, respectively. The best removal efficiency was observed at neutral pH in which removal efficiency was 90-92%. The best removal performance was observed in both shaking rate of 300 and 400 rpm (93%). 3.2. Absorption isotherm:Langmuir and Freundlich isotherms were investigated for absorption model. Fig. 1 and Table 1 show achieved results of batch experiments for absorption models. As can be seen in the figure, the equilibrium isotherm for fluoride uptake corresponds closely to the Langmuir model.Table 1. Absorption isotherm parameters for removal of fluoride by MAAModel qm (mg/g) B (L/mg) Kf N R2Langmuir 8.23 0.147 - - 0.85Freundlich - - 5.16 5.22 0.434. According to observed removal efficiencies during the study, MAA used in the study can be considered as an efficient absorbent for the removal of fluoride from drinking water. However, conducting more research using real water samples and column experiments is highly recommended before filed application of the method.