Investigation of the mechanisms of defluoridation of drinking water by using locally available magnesite.

The locally available magnesite at Chambogo, Same district was investigated as a media for fluoride removal from drinking water. The objective is to find a suitable mechanism of defluoridation using Magnesite. Media preparation involved crushing and calcination of magnesite at different temperatures and durations. Jar tests and pH chemostat experiments in batch and continuous flow on fluoride removal ability of the media were carried out. The results showed that, magnesite can be used in drinking water defluoridation when calcined at the right temperature and duration. High fluoride removal capacity was found to be 65 mg F/g in batch experiments. Adsorption of fluoride has a maximum value at optimum pH range of 10.5 to 11.0. The higher initial fluoride concentration reduced the dissolved magnesium and thus the total hardness residue. A fluoride removal capacity of 27 mg F/g was revealed in a bucket fill and draw method when using naturally fluoridated water with initial fluoride of 21 mg F/L. It was also found that fluoride removal capacity in a continuous flow is very low (at the optimum of 3.5 mg F/g) compared with a capacity of 26 mg F/g in a batch system. The process of fluoride uptake using calcined magnesite involves a combination of two or more reactions depending on the fluoride concentration in the solution, pH, alkalinity and dosage of magnesia. The process portrays a lag time at the end it is evident the process is not a simple sorption or chemisorption. Because of lack of direct correlation between changes in alkalinity and fluoride removal, it is concluded that the reaction is not a simple ion exchange. The main mechanism of removal is the uptake of fluoride by the first step of hydrolysis product of magnesia by water to form a complex component of magnesium mono-hydroxyl-fluoride.