Beaker tests are carried out in order to study the oxidation capacities of iron in contact with different oxidants with variable pH. The oxidation kinetics of iron is carried out by oxygen, potassium permanganate and finally sodium hypochlorite. For this, a solution of water doped with 5 mg·L⁻¹ Fe²⁺ is prepared from iron sulphate (FeSO₄). The amount of oxidant (O₂, KMnO₄, NaClO) added is evaluated on the basis of the stoichiometric proportion of the reactions; in this case, 0.73 mg O₂/L should be bubbled, add 3.61 mg KMnO₄/L and 2.4 mg NaClO/L theoretical masses respectively in each beaker. As a result of these tests, we have moved to the application phase to remove iron from groundwater. The sampling was done in South Pout (Dakar/Senegal) precisely in the drilling PS2 (Figure 1).

The physic and chemical characterization study (Table 1) of the groundwater in South Pout (Senegal) is focused in situ measurements (pH, electrical conductivity, turbidity) and the laboratory determination of the major components and metallic trace elements (chloride Cl⁻, sulphates SO 4 2 − , nitrates NO 3 − , nitrites NO 2 − , calcium Ca²⁺, magnesium Mg²⁺, total iron (Fe²⁺ and Fe³⁺), manganese Mg²⁺, etc.).

In order to be in the calibration range, [Fe²⁺]₀ < 6 mg·L⁻¹, we prepared an initial solution of 4.10⁻⁵ mol·L⁻¹ iron sulfate (FeSO₄) dissolved in distilled water. In 100 mL volumetric flasks numbered 1 to 7, put the reagent volumes indicated in the following table, and adjust to the mark with distilled water (Table 2) and (Figure 2).

After adjusting to volume, mix thoroughly and let sit for 5 to 10 minutes. Meanwhile, prepare the reference, and then measure the optical density at 510 nm relative to the reference solution. Prepare a volumetric flask of 100 mL of the

reference solution by pouring 10 mL of methylene chloride of 10% hydroxylammonium, 5 mL of 2M sodium acetate, 8 mL of ortho-phenanthroline 0.25% and make up to gauge with distilled water (Figure 2).

The principle is to dissolve the theoretical mass of the oxidant (O₂, KMnO₄, NaClO) in the already known concentration of the iron (II) solution, while checking the pH and temperature, allowing the oxidation reaction to proceed with thorough agitation of the reaction medium (Figure 3). At the end of each operation, the precipitates deposited at the bottom of the beaker before proceeding to the spectrometric determination of the mixture are filtered. The oxidation reactions are summarized as following:

4 Fe 2 + + O 2 + 10 H 2 O → 4 Fe ( OH ) 3 ( s ) + 8 H + (4)

3 Fe 2 + + MnO 4 − + 2 H 2 O + 5 OH − → 3 Fe ( OH ) 3 ( s ) + MnO 2 ( s ) (5)

2 Fe 2 + + HClO + 5 H 2 O → 2 Fe ( OH ) 3 ( s ) + Cl − + 5 H + (6)

The calibration curve is then obtained from the measurement of the absorbance of each standard solution (Figure 4).

The concentration of the sample taken is calculated by multiplying by 10 the concentration obtained (to take into account the dilution factor). The corresponding iron concentration is calculated from the following relation:

[ Fe 2 + ] total = ABS − 0.0577 0.1418 (7)

A series of tests in beaker is carried out in order to verify the kinetics of oxidation of iron by different oxidants as a function of the pH. For this, the pH of the water is adjusted by concentrated hydrochloric acid (HCl) and sodium hydroxide (NaOH) solutions. The results obtained for each of the oxidants used are shown in Figures 5-7.

Fe²⁺ oxidation tests in beaker are carried out at three different pH ranges, pH = 3.4 - 3.6; pH = 7.3 - 7.5 and pH = 9.8 - 10. Indeed, for Figure 5, the results show that at pH = 9.8 in less than five minutes, 95% of ferrous iron (Fe²⁺) is removed whereas at pH = 7.3, only 60% of ferrous iron has disappeared. In the other hand, at acid pH, there is no removal of iron, and then iron is not oxidized by

oxygen at acidic pH. Depending on the pH of the water, the kinetics will be more or less rapid. It is possible to find the order of the kinetics by applying a trend line to the curves determined experimentally (here the exponential type allows to find an order line). This kinetics can therefore be modeled as follows:

− d [ Fe 2 + ] d t = K t (8)

These results are consistent with the results obtained by [11] [12] which showed that, in the presence of iron, a significant fraction of the iron can be oxidized by O₂ and at a relatively basic pH (Figure 5).

Beaker tests were also carried out with potassium permanganate (KMnO₄) as oxidant at different pH. The kinetics of oxidation of dissolved iron is reported in the graph of Figure 6. The results show that at pH = 9.85 in less than five minutes, 96.85% of ferrous iron (Fe²⁺) is removed while at pH = 7.5, only 63% of the ferrous iron is removed. In the same way, at acidic pH, the removal of iron is very weak or even non-existent. The kinetics of chemical oxidation of experimental values is written by the kinetic model of the equation above. These results are consistent with the work of some authors [11] [12] who showed that, in the presence of iron, a significant fraction of iron can be oxidized by KMnO₄ as well.

With regard to the removal of ferrous iron by sodium hypochlorite (NaClO) at different pH, the dissolved iron disappears after five minutes which is consistent with the work of KNOCKE et al., (1990a) who showed that the oxidation kinetics of Fe²⁺ by NaClO was very fast. The results (Figure 7) show that at pH = 9.8 in less than five minutes, 98.25% ferrous iron (Fe²⁺) is removed and has the most effective oxidant. The theoretical stoichiometric dose of NaClO is therefore too much to eliminate iron. Hence the possibility of the theoretical quantity being put in contact with the sample reduces by 1/3.

The oxidation capacity makes it possible to find the maximum amount of iron that the oxidant is capable of reducing Fe²⁺ ions to Fe³⁺. To find the oxidation capacities, groundwater samples of 5.403 mg Fe²⁺/L with an initial pH of 7.01 are regulated at pH = 9.8. In three 100 mL beakers, we put 30 mL of the sample plus the calculated theoretical oxidant mass. This suspension is placed in a rotary shaker for 5 minutes so that the maximum oxidation capacity of the iron is reached. The results of this experiment make it possible to calculate the percentage of iron eliminated by the various oxidants. They are presented in Table 3.

The results presented above make it possible to evaluate the treatment efficiency of the three oxidants used for the removal of iron (II) in groundwater of South Pout (PS2 site).

We recorded a percentage of iron removed of 93.74% for the use of oxygen as oxidant. Indeed, the iron is very unstable in the water in the presence of oxygen as seen on the diagram of Pourbaix of the iron [13], which is why the removal of

this element in presence only of aerated water (9.5 mg O₂/L) is very fast unless the pH of the water is basic (pH = 9.8). A second oxidant is then tested, potassium permanganate, oxidizing stronger than oxygen. For this experiment, the stoichiometric dose is respected, i.e. a dose of 3.61 mg KMnO₄/L starting from a theoretical initial concentration of Fe²⁺ of 5 mg/L. Potassium permanganate is a very strong oxidizer that achieves the almost complete oxidation of soluble iron almost instantaneously. Indeed, the results show that after one minute of experience, 85% of the soluble iron in groundwater is removed at pH = 9.8. This oxidant seems to be the most suitable for the removal of Mn²⁺, however, it should not be introduced in excess under penalty of a pink coloration of the samples. It is therefore necessary to dose it well so that it is present in order to only remove the soluble iron present in the water to be treated. The results show that the most efficient oxidant is sodium hypochlorite (NaClO), with a percentage of iron eliminated by 96% under the conditions of these tests. These results confirm the kinetics of iron oxidation by sodium hypochlorite (NaClO).

The advantage of these three systems is the need for a single filtration stage. By comparing them with each of the systems, it can be seen that the difference in the percentage of iron eliminated is relatively small. The difference is on average 1.58%, which could guide the choice of the type of oxidant to use. From the economic point of view, the use of oxygen as an oxidant would result in a lower investment cost compared to potassium permanganate (KMnO₄) or sodium hypochlorite (NaClO) because it is sufficient to air ventilate to reduce Fe²⁺.