Defluoridation by coagulation in drinkable water from the south east of Algeria

The drinkable water in the south east of Algeria contains high quantity of fluoride which causes dental fluorosis. To decrease fluoride, we have chosen a sample with the biggest content of fluoride among many sources in order to coagulate it with Al 2 (SO 4 ) 3 ·18H 2 O. After that the parameters influencing (concentration, pH, temperature) are studied to choose the best conditions for better reduction yield.


INTRODUCTION
The water of the south east of Algeria is characterized by high concentrations of fluoride, associated with severely high and excessive total mineralization. This water is the only source of drinking. The hot and dry climate has forced people to consume a lot of water which leads to raise the daily consumption rate of fluoride. In addition the eating a lot of dates and tea leads to the spread of fluorisis disease [1] which is characterized by the yellowish of tooth enamel according to the classification of the national program of school health [2,3]. To prevent these diseases from happening or reduce them, many techniques of defluoridation are used such as: membrane technologies, precipitation and adsorption. Our study is focused on adsorption with sulfate aluminium salt and determination of optimal conditions affecting the reduction of fluoride in drinking water.

1. Preparation of fluoride curve witness
To determine the concentration of fluoride inVarious samples, a potentiometer method was used (Rodier 2005) [4]. Different standard concentration solutions were prepared from NaF salt in cups of plastic. Then their potential is measured by using specific fluoride pole (ISE15381/1) and a pH-meter model (pH211), using a solution of TISAB (6) . The graph E = f(log C F ) is presented in Figure 1.

Determination of fluoride concentration in some samples of the study area
The concentration of fluoride has been determined in some water sources of the study area in order to determine and treat the largest content of fluoride. The results are presented in Table 1. The selected sample (cold water of Shuhada) has a concentration of fluoride 2.61 (mg / l).

3. Determination of the predominant concentration of ions in the studied water
The study was done according to (Rodier 2005) [4] on cold water of Shuhada as follows:

3. Total hardness
By complexity with EDTA (1) in the presence of Eriochrome Black T at buffer solution of pH = 10.

3. 3. Sodium and potassium
Using flame atomic absorption analysis. Results of Predominant ions are presented in Table 2.

4. Effect of pH
Based on adsorption of fluoride ion on Al(OH) 3 which is related to the pH of the studied water according to the equilibrium (1).
We repeat the same steps of the previous experiment as mentioned in (2.4.1) by fixing the temperature and the added optimal concentration of aluminium sulfate but changing the pH by buffer solutions. The results are presented in Table 4 and Figure 3.

4. 3. Effect of temperature T
The same steps of the experiment are repeated as mentioned in (2.4.1) by fixing the added optimal concentration of Al 2 (SO 4 ) 3 and the optimal pH ,but changing the temperature.
The results were presented in Table 5 and Figure 4 [5].

RESULTS AND DISCUSSION
 According to the results of Table 2 we observe that the high concentrations of (Ca 2+ , Mg 2+ , SO 4 2-, Clˉ) exceed the WHO standards of water. This related to the geological characteristics and the structure of rocks.  The results of Table 3 indicate that the concentrations of residual fluoride are decreased when the values of added aluminium concentration are increased. This can be explained by the formation of gelatinous aluminum hydroxide (equilibrum1) which adsorbs F - [5] . The optimal concentration of aluminium is 0.0024 g/l.  The results of table (4) show that:  In acidic medium: The residual Fis decreased when the pH is increased which is proved by equilibrium (2).  In basic medium: The residual Fis increased when the pH is increased which is proved by equilibrium (3). The optimal chosen pH = 7.6 leads to a residual fluoride concentration equals 1.5 mg/l which fits the OMS standard.  According to the results of Table 5 we observe that residual [F -] is decreased when T is increased, this can be proved by the following: Increasing T leads to the decrease of [equation of Guzman-andrad] (1), so V is increased [equation (2)]. As a result V mob is increased [equation (3)].

ILCPA Volume 10
V mob increases the probability of convergence Ffrom Al(OH) 3 particles which augment the adsorption of Fon the surface of the particles [6,7]. The optimal chosen T = 26.5 °C leads to a residual fluoride concentration equals 1.5 mg/l which fits the OMS standard.

CONCLUSION
 According to the quantity of fluoride in the water of Oasis region of Algeria ,it appear that most of them Contain surplus exceeds the standard value of (WHO) (9) with a total high hardness.  The present investigation indicates that reducing fluoride from water is economic method because using a small amount of Al 2 (SO 4 ) 3 is needed for this purpose.  Through the study of factors affecting (concentration, pH, temperature) it is possible to choose the best conditions for a reduction process with Al 2 (SO 4 ) 3 ·18H 2 O by adding an amount at a concentration of 0.131 g / l, pH = 7.6 and a temperature of 26.5 °C.