In the study, experiments were conducted to ascertain the feasibility of electrolytic defluoridation process in a continuous mode and the effects of operational parameters on electrolytic defluoridation process are discussed here.
Effect of Initial pH on Fluoride Concentration
The experiment was conducted to study the effect of initial pH on the efficiency of electrolytic defluoridation process. The influent pH was found to be one of the important factors affecting the performance. In the electrolytic defluoridation system, the fluoride on electrodes and flocs solely determines the fluoride removal efficiency. The adsorption on flocs and defluoridation by electrodes is of primary importance to understand fluoride distribution.
The optimal pH range found was 6-7, at which higher fluoride removal efficiency could be reached, but the fluoride removal efficiency varies with variation in pH. Lower pH is favorable for early fluoride removal in the EC process. The pH could be increased not only because of the hydrogen generation at the EC cathodes, but also the liberation of hydroxide ions from aluminum hydroxide due to the substitution of fluoride ions. Therefore, further increase of the influent pH would decrease the defluoridation capability. Since aluminum hydroxide is an amphoteric hydroxide, high pH will lead to the formation of Al (OH)- 4 which is soluble and does not take part in defluoridation. Effect of pH on fluoride reduction for batch and continuous processes are shown in Fig. 1 and 2 respectively.
The experiments were conducted to determine the effect of pH variation on fluoride removal efficiency, keeping other parameters constant and the optimum fluoride reduction was observed between pH 6 and 7 for influent fluoride concentration 5 mg/l. With this in view, the working pH in the optimum range of 6-7 was adopted as 6.5, which yielded the best results.
Effect of Current Intensity on Fluoride Concentration
It was found that current intensity has little effect on defluoridation because the principal factor influencing adsorption of fluoride by flocs is the pH of the solution. Consequently, an increase in current intensity leads to an increase in power requirement. Infact, there should be an optimal current applied for the electrolytic defluoridation system. Increasing current intensity would accelerate the liberation rate of Al3+ and OH- ions. The rapid liberation of Al3+ and OH-ions from the surface of the electrode would make the solution around the anode and cathode extremely acidic or basic respectively, which would decrease the removal efficiency of electrodes as depicted in Fig. 3, 4 and 5.
Both rapid liberation and short run time would lead to incomplete reaction around the electrodes due to insufficient time to reach dynamic equilibrium. Therefore, optimal current density should be maintained.
Effect of Influent Fluoride Concentration
The experiments were conducted by varying initial fluoride concentrations 5, 6, 8, 10 mg/L, keeping all other experimental conditions identical. Figures 6, 7, 8 and 9 illustrate fluoride reduction below 1 mg/l for pH 6.5 and current intensity 2, 2.5 and 3 amperes.
These figures reflect that the retention time required for an acceptable residual fluoride concentration, which is <1mg/l of fluoride, increases when the initial concentration is increased. Maximum fluoride removal efficiency was observed at current intensity of 3 A. The defluoridation efficiency of electrodes tended to decrease since they required longer time with the initial F- concentration rising.
Effect of Flow Rate on Defluoridation Efficiency
In the study, a continuous stirred tank reactor was utilized and a peristaltic pump was used to maintain the desired flow rate. To study the effect of flow rate on defluoridation efficiency, operating experimental parameters such as fluoride concentration 5 mg/l, 3 ampere current and pH 6.5 were kept constant for various flow rates. The study of the effect of flow rates on the removal efficiency of a continuous type of electrolytic process, which is given in Fig. 10, reveals that the flow rate of 550 ml/min gives the optimum results with residual fluoride concentration within the desirable limit.
Residual Aluminium in Treated Water
The residual aluminium found in treated water is one of the important factors, because leaching of the same was demonstrated in the experiments conducted. Aluminium found above 0.2 mg/L in drinking water, is a risk cause of Alzheimer disease (AD) in humans (WHO, 1997). Effect of different current intensities on the concentration of residual aluminium is shown in fig. 11. Higher residual aluminium was found at optimum removal efficiency with current intensity 3 ampere and pH 6.5, but it is not beyond permissible limit in drinking water for the current intensities 2 and 2.5 ampere for lower fluoride concentrations.
The optimum current intensity of 2 ampere with optimum pH of 6.5 gives better results in comparison to higher current intensities. Therefore, it can be inferred that the variation in current intensity with optimum pH of 6.5 for electrolytic defluoridation process is not effective.