Comparative Study of the Defluorination of Hyperfluorinated Groundwater from the Diohine Senegal Borehole by Activated Carbon Based on Filao Litter and Zircon ()
1. Introduction
In Senegal, the natural presence of fluorides in groundwater is a well-documented problem, particularly in the peanut basin where the Maastrichtian aquifers are often affected by fluoride levels exceeding recommended standards [1]-[4]. According to the World Health Organization (WHO), the maximum permissible concentration of fluoride in drinking water is set at 1.5 mg/L [5] [6]. At low doses, fluoride has beneficial effects on oral health and the acidification of tooth enamel, particularly for young children under eight years of age [7]-[9]. However, at high concentrations and prolonged consumption, it becomes toxic and causes diseases such as dental fluorosis (staining and discoloration of teeth) and bone fluorosis (deformation and joint pain), with serious consequences on the quality of life of the affected populations [10] [11].
However, in some localities in Senegal such as Diouroup, a neighboring commune of Diohine, values exceeding 6 to 10 mg/L are regularly observed, thus posing a real public health problem [3] [12]. Faced with this situation, several methods of treating hyperfluorinated water such as reverse osmosis, ion exchange, chemical precipitation, nanofiltration and adsorption have been tested [2] [13] [14]. Among these approaches, adsorption on natural or modified materials is attracting growing interest, particularly in rural areas, due to its simplicity, low cost, local adaptability and availability [15]-[17].
Among the most promising materials for water defluoridation, zirconium-based adsorbents have attracted increasing interest due to their high affinity for fluoride ions, their chemical and thermal stability, as well as their non-toxic and economical nature. A comprehensive study conducted by Savari et al. (2023) provides a comprehensive review of zirconium-modified adsorbents, highlighting their notable effectiveness in removing fluoride from aqueous media [18]. The use of zircon in this study is part of an international dynamic of valorization of high-performance and available materials for defluoridation. This choice also aims to explore its potential in comparison with another local natural material; activated carbon based on filao litter, in order to propose solutions adapted to the environmental and economic realities in the locality of Diohine.
The general objective of this study is to evaluate and compare the effectiveness of two adsorbent materials, namely activated carbon based on filao litter and local zircon, in the defluoridation process of hyperfluorinated Diohine groundwater. To achieve this objective, it is first necessary to characterize the groundwater in the study area in order to determine its physicochemical parameters, including fluoride content. Then, activated carbon was prepared from filao litter and characterized according to usage standards. Laboratory adsorption tests were used to evaluate the respective performances of the two materials in terms of adsorption capacity and defluoridation efficiency. Finally, a comparative analysis is conducted to study the adsorption kinetics, maximum performance and optimal operating conditions of each adsorbent.
2. Methodology
In order to meet the objectives of this study and to rigorously compare the effectiveness of activated carbon based on filao litter and zircon in the defluorination of hyperfluorinated Diohine groundwater, a structured methodological approach was implemented. This section describes the different steps followed, from the location, collection and characterization of water samples to the preparation of adsorbent materials, including the implementation of adsorption tests.
2.1. Location of the Commune of Diohine
Diohine is a village in Senegal located in the west-central part of Senegal, in the region and department of Fatick, in the commune of Tattaguine. Figure 1 shows its location in the Fatick region. The Diohine borehole, which is the subject of this study, is located in the commune and constitutes the source of water supply for the population.
Figure 1. Location of the village of Diohine.
2.2. Characteristics of the Water from the Diohine Borehole
The assessment of the water characteristics of the Diohine borehole was carried out in two stages: an in situ analysis followed by a laboratory analysis.
The in situ analysis focused on the measurement of physical parameters: temperature, hydrogen potential (pH), electrical conductivity and total dissolved solids (TDS).
A water sample was taken and sent to the laboratory for the analysis of physicochemical parameters and the treatment test process.
2.3. Description of Zircon and Activated Carbon Used
The zircon used in this research was supplied by Grande Côte Opérations (GCO), a subsidiary of the Eramet group, specializing in mineral sand mining. GCO operates a deposit located in the dune formations of the Thiès and Louga regions of Senegal, and has two industrial processing units located in Diogo. The extracted ores consist mainly of zircon, ilmenite, rutile, and leucoxene.
As for activated carbon, it was synthesized from the litter of Casuarina equisetifolia (filao) collected on the coastal strip of Mboro. The manufacturing process includes controlled pyrolysis followed by chemical activation using calcium chloride (CaCl2), before grinding and sieving in the laboratory to obtain a particle size suitable for adsorption application. Figure 2 illustrates in A the image of zircon and in B that of carbon.
Figure 2. Materials used: (A) zircon; (B) active charcoal based on filao litter.
2.4. Process of Obtaining Activated Carbon
2.4.1. Carbonization of Filao Litter
Carbonization is a key step in the production of precursor charcoal for activation, aiming to enrich the biomass with fixed carbon and to initiate the formation of porosity. In this study, the previously collected Casuarina equisetifolia (filao) litter (Figure 3) was manually fragmented using metal scissors into small, homogeneous pieces to facilitate carbonization.
The process was carried out using a Nabertherm muffle furnace. The ground material was packaged in a metal pan with a perforated lid to allow controlled evacuation of volatile gases from pyrolysis. The whole was then introduced into the furnace to undergo dry pyrolysis in a controlled atmosphere at 600˚C for 45 minutes.
The muffle furnace, having a limited capacity, required several carbonization cycles in series to accumulate a sufficient quantity of carbon for the subsequent activation and characterization steps. Figure 4 illustrates the visual appearance of the carbon obtained after carbonization of the filao litter called carbonizate at 600˚C.
Figure 3. Filao litter.
Figure 4. The coal.
2.4.2. Drying, Grinding and Sieving of the Carbinisate
The carbonized material (Figure 5(A)) is washed with distilled water to remove impurities, then dried for 24 hours in an oven at 105˚C. It was then ground in a mortar to obtain two (2) types of particle size: one with a diameter of less than 0.5 mm (Figure 5(B)) and the other with a diameter of between 0.5 and 1 mm (Figure 5(C)). Figure 5 illustrates the stages of grinding and separation of the two aggregates of the carbonized material of the filao litter obtained in a muffle furnace at 600˚C.
Figure 5. Crushing and separation of aggregates.
2.4.3. Activation of the Carbonized Material
Activation was carried out with calcium chloride (CaCl2) prepared beforehand in the laboratory.
The charcoal powder, passing through a 0.5 sieve, is poured into a saucepan, then the calcium chloride solution is poured into it while stirring so that all the powder is in contact with the solution. This was done until a pasty mixture was obtained. The saucepan was closed and the mixture was left to stand for 24 hours. Then, the carbonized material was removed from the solution while pressing it to drain the water. The previously activated charcoal was dried in an oven for 24 hours. The activated charcoal was washed with distilled water to remove the activating agent and then dried in an oven for 24 hours at 105˚C. After activation of the carbonized material, the activated charcoal was obtained (Figure 6).
Figure 6. Activated carbon.
2.5. Determination of Iodine and Methyl Blue Values of Zircon
and Activated Carbon
2.5.1. Iodine Index
The iodine value is an indicator of the microporosity of an adsorbent; it measures its ability to retain small molecules in aqueous solution. It is expressed in milligrams of iodine adsorbed per gram of material. In this study, a 0.1 N iodine solution was prepared from potassium iodide (KI) and diiodine (I2), then standardized with a 0.1 N sodium thiosulfate solution. The zircon and activated carbon samples, dried at 105˚C for 24 h, were brought into contact with the iodine solution. After stirring for 5 to 6 min and filtration, a portion of the filtrate was titrated with thiosulfate until discoloration. The amount of iodine adsorbed, calculated by the difference in concentration before and after contact, gives the iodine value of the material. This index allows to compare the adsorption efficiency of zircon and activated carbon and to evaluate their ability to trap small molecules in aqueous phase. The iodine index expressed in (mg/g) was calculated according to Morvan et al. by Equation 1 [19].
Iodine index
(1)
with:
C0: the concentration of the initial iodine solution (mol∙L−1), Cn: the concentration of the sodium thiosulfate solution (mol∙L−1), Vn: the volume of the sodium thiosulfate solution at equivalence (mL), VI2: the volume of iodine solution dosed, M I2: the molecular molar mass of iodine, Vabs: the adsorption volume and mCA: the mass of activated carbon.
2.5.2. Methylene Blue Index
The methylene blue index (MBI) is a parameter used to evaluate the macroporosity of an adsorbent material such as activated carbon or zircon. It is defined as the volume of standard methylene blue solution decolorized by 0.1 g of adsorbent. In this work, a 1200 mg/L stock solution was prepared by dissolving 1200 mg of dye in one liter of distilled water. Adsorption is monitored by UV-Visible spectrophotometry, a quantitative technique based on the Beer-Lambert law that relates absorbance to solution concentration, extinction coefficient, and optical path length. Absorbance measurements are performed for different wavelengths (330 - 690 nm) to determine the maximum absorbance wavelength (λmax). The latter is used to establish a calibration line, an essential prerequisite for evaluating the adsorption capacity of the materials studied.
To measure the residual concentrations of methylene blue, 100 mg of adsorbent (charcoal or zircon) are brought into contact with 100 mL of dye solution for 20 minutes with stirring. The absorbance values obtained make it possible to determine the residual concentrations (Cmr), then to calculate the apparent adsorption capacity (QBM in mg/g) according to Equation 2 [20]:
(2)
Cmi: the initial mass concentration (mg/L) of the methylene blue (MB) solution. Cmr: the residual mass concentration (mg/L) of the MB solution. V: the volume of the MB solution (= 100 mL). mCA: the mass of the adsorbent used (g).
2.6. Absorption Device
The adsorption or filtration device is illustrated in Figure 7. In the context of the defluorination tests of hyperfluorinated brackish water, two experimental devices or single-column systems were designed; one filled with activated carbon based on filao bedding (Figure 7(A)), the other with zircon (Figure 7(B)). Each single-column device consists of a 5 - 10 L raw water reservoir, an inlet valve, a perforated connecting sleeve without a filter, an adsorbent column (zircon or activated carbon), a perforated connecting sleeve with a filter, and a filtration flow control valve as shown in Figure 7.
After characterizing the raw water, a volume of 500 mL was introduced into the reservoir of each device. The inlet valve was opened, and the filtration flow rate was adjusted to 0.83 mL/s using the shut-off valve. The contact time between the water and the adsorbent was set at 10 minutes. The filtrates obtained were collected in beakers and then stored in refrigerated bottles before being sent to the laboratories for physicochemical analyses.
Figure 7. Diagram of the measuring device.
3. Discussion Results
3.1. Characterization of Raw Water from Diohine Drilling
The analysis of the physicochemical parameters of raw water (Table 1) compared to the standards and limits recommended for drinking water, reveals that the majority of the measured values exceed the accepted thresholds, thus confirming the need for prior treatment before consumption. However, the present study does not aim to directly make the analyzed water potable, but rather to evaluate the relative effectiveness of adsorption on zircon and adsorption on activated carbon based on filao litter for the removal of fluorides.
Table 1. Characteristics of raw water.
Settings |
Concentration in mg/L |
Standards and limits |
Potassium K+ |
21.08 |
12 |
Sodium Na+ |
651.61 |
200 |
Magnesium Mg2+ |
4.63 |
30 |
Calcium Ca2+ |
9.44 |
200 |
Bicarbonate
|
579.5 |
250 |
Nitrate
|
6.59 |
50 |
Chloride Cl− |
614.5 |
250 |
Fluorine F− |
5.74 |
1.5 |
Sulfate
|
107.15 |
250 |
Total Dissolved Solids TDS |
2005.85 |
1000 |
pH |
7.92 |
6.5 – 8.5 |
3.2. Characteristics of Zircon and Activated Carbon
These results were obtained following analyses carried out with an X-ray fluorescence spectrometer, the Niton XLT900s, at the laboratory of the Applied Nuclear Technology Institute (ITNA) of the Cheikh Anta Diop University of Dakar (UCAD). Table 2 gives the composition of activated carbon and Table 3 that of zircon.
Table 2. Composition of coal.
Elements |
Nature of matter |
Symbols |
Concentration (g/kg) |
Percentage (%) |
Calcium |
Alkaline earth metal |
That |
113.14 |
66.50 |
Aluminum |
Alkaline earth metal |
Al |
20.85 |
12.25 |
Chlorine |
Halogen |
Cl |
12.15 |
7.14 |
Iron |
Transition metal |
Fe |
10.64 |
6.25 |
Potassium |
Alkali metal |
K |
4.74 |
2.79 |
zirconium |
Transition metal |
Zr |
3.88 |
2.28 |
Sulfur |
Non-metal |
S |
2.77 |
1.63 |
Titanium |
Transition metal |
You |
0.89 |
0.52 |
Manganese |
Transition metal |
Mn |
0.79 |
0.46 |
Copper |
Transition metal |
Cu |
0.15 |
0.09 |
Tungsten |
Transition metal |
W |
0.11 |
0.06 |
Arsenic |
Metalloid |
Ace |
0.01 |
0.01 |
Bismuth |
Metal |
Bi |
0.01 |
0.01 |
Total mass (g/kg) |
170.13 |
100.00 |
Density (g/L) |
0.25 |
- |
The results obtained highlight notable differences between activated carbon from filao litter and zircon in terms of fluoride removal. Chemical analysis showed that filao litter is particularly rich in calcium (66.5%), aluminum (12.25%) and iron (6.25%), elements known for their ability to interact with fluoride ions by precipitation or adsorption on metal hydroxides. In addition, its low density (0.25 g/L) and its porous structure after activation constitutes an advantage for the development of a large specific surface area, which favors adsorption. These characteristics explain the good performances observed during adsorption tests, consistent with the iodine indices close to those of commercial carbons (838 - 876 mg/g) [19].
In contrast, zircon has a composition dominated by zirconium (80.83%) and silica (15.32%), with a much higher density (3.16 g/L). Zirconium is known for its high affinity towards fluoride ion through the formation of stable complexes (Zr-F), giving the material remarkable selectivity and efficiency for defluorination [21]. Although its structure is denser and less porous than that of activated carbon, the high zirconium concentration compensates for this limitation by providing many specific active sites.
These results are consistent with those reported in the literature, particularly on activated carbons obtained from bitter almond shell and olive seeds, which also exhibited interesting fluoride adsorption capacities. Similarly, the performance observed with zircon confirms the conclusions of previous studies highlighting the effectiveness of zirconium-based materials in the defluoridation of groundwater [22] [21].
Table 3. Composition of zircon.
Elements |
Nature of matter |
Symbols |
Concentration (g/kg) |
Percentage (%) |
Zirconium |
Transition metal |
Zr |
708.73 |
80.83 |
Silica |
Alkaline earth metal |
If |
134.34 |
15.32 |
Sulfur |
Non-metal |
S |
22.31 |
2.54 |
Copper |
Transition metal |
Cu |
2.34 |
0.27 |
Molybdenum |
Transition metal |
Mo |
2.26 |
0.26 |
Calcium |
Alkaline earth metal |
That |
2.19 |
0.25 |
Barium |
Alkaline earth metal |
Ba |
1.51 |
0.17 |
Titanium |
Transition metal |
You |
1.13 |
0.13 |
Tungsten |
Transition metal |
w |
0.60 |
0.07 |
Niobium |
Transition metal |
Nb |
0.42 |
0.05 |
Nickel |
Transition metal |
Neither |
0.38 |
0.04 |
Money |
Metalloid |
Ag |
0.34 |
0.04 |
Arsenic |
Metalloid |
Ace |
0.29 |
0.03 |
Total mass (g/kg) |
876.85 |
100.00 |
Density (g/L) |
3.16 |
- |
It therefore appears that activated carbon from filao litter constitutes a versatile and accessible adsorbent, capable of treating several pollutants thanks to its porosity and the diversity of its constituent elements, while zircon, more specific, proves to be particularly effective in the targeted elimination of fluoride. The choice between these two materials therefore depends on the context of use on the one hand, the overall treatment of water quality for activated carbon and on the other hand, the selective and optimized treatment of fluoride for zircon.
3.3. Characterization of Activated Carbon
The characterization of activated carbon based on filao litter consisted of determining the moisture, volatile matter, ash, fixed carbon, iodine index and methylene blue index. The results obtained during this characterization are presented below. The characterization results are shown in Table 4.
The results obtained on filao litter (Table 4) are generally in agreement with those reported in the literature [23]. Indeed, the reference study on biomasses also showed low ash contents, low humidity levels and high proportions of volatile matter. This convergence confirms that filao litter, like other biomasses studied, constitutes a precursor suitable for the production of quality activated carbons, presenting both a good adsorption capacity and an interest for water treatment.
Table 4. Characteristics of activated carbon based on filao litter.
Biomass |
Humidity level (%) |
Dry matter content (%) |
Volatile matter content (%) |
Ash content (%) |
Fixed carbon rate (%) |
Filao litter |
33.12 |
66.88 |
88.57 |
6.38 |
3.30 |
3.4. Iodine and Methylene Blue Index of Zircon and Coal
The iodine and methylene blue indices of zircon and coal are grouped in Table 5.
Table 5. Iodine and methylene blue values of zircon and activated carbon.
Materials |
Iodine index |
Methylene blue index |
Coal |
1063.56 |
842.80 |
Zircon |
526.61 |
705.98 |
The results show that both zircon and activated carbon prepared from filao litter have iodine values comparable to those of commercial activated carbons [19] [23], indicating good efficiency in terms of adsorption capacity. However, activated carbon from filao litter develops a slightly higher performance than zircon, due to its porous structure more suited to the adsorption of methylene blue molecules. Zircon, although efficient, could release some mineral constituents into the treated water, which justifies the complementary use of activated carbon downstream.
3.5. Adsorption Spectrum of Methylene Blue
In Figure 8, we presented the results on the adsorption spectrum of methylene blue which allowed us to choose the best wavelength for the determinations of the methylene blue indices of coal and zircon.
Figure 8. Adsorption spectrum of methylene blue.
For different wavelengths, we find that in the adsorption spectrum of methylene blue, the absorbance reaches its maximum at the values of 580, 600, 610, 620, 630 nm. Thus, we chose the wavelength 620 nm for the rest of the work.
3.6. Methylene Blue Calibration Line
In the following Figure 9, we have illustrated the calibration line obtained during the determination of the methylene blue indices. The equation of the calibration line, representing the absorbance as a function of the concentration of Methylene Blue, with a regression coefficient R2 = 0.998, can be considered as a good linear fit. This equation is used, subsequently, for the calculation of the concentration of a given solution.
Figure 9. Methylene blue calibration line.
3.7. Treatment Results
The following device Figure 10 was adopted for the treatment as written in the methodology.
Figure 10. Device used for treatment.
The results of the analysis of raw water, treated water and standards are presented in Table 6.
Table 6. Comparison of fluoride contents of different waters and limits.
|
Concentration in (mg/L) of Fluorides |
Raw water |
5.74 |
Zircon treated water |
1.84 |
Water treated with activated carbon |
2.29 |
Limits and standards |
1.5 |
Analysis of the results obtained highlights an initial fluoride concentration in the raw water from the Diohine borehole of 5.74 mg/L, a value well above the standard recommended by the WHO set at 1.5 mg/L. This high content confirms the need for defluoridation treatment before any consumption [24]. After treatment, two distinct behaviors are observed depending on the adsorbent used. Zircon allows a significant reduction in the fluoride concentration, bringing it down to 1.84 mg/L, which corresponds to a reduction of approximately 67.9%. Although this value remains slightly above the limit set by the WHO, it demonstrates the high efficiency of this material and its strong affinity for fluoride ions [22] [25] [26]. In comparison, activated carbon made from filao litter reduced the concentration to 2.29 mg/L, a reduction of 60.1%. This significant result, however, remains lower than that obtained with zircon and thus reflects a more limited effectiveness with respect to fluoride.
These observations indicate that zircon performs better than filao activated carbon for fluoride removal. However, neither adsorbent can achieve the recommended potability standards. On a practical level, activated carbon has the advantage of being sourced locally and is very accessible, making it an attractive alternative for decentralized and sustainable treatment. Zircon, on the other hand, appears to perform better but could pose economic or supply constraints. In this sense, process optimization, particularly through combined or sequential treatments (zircon-carbon, or coupling with other adsorbents such as activated alumina), could make it possible to achieve fluoride concentrations that comply with international standards.
4. Conclusions
This present work has highlighted the persistence of the fluoride problem in Diohine groundwater, with concentrations well above WHO standards. In order to reduce the fluoride contents of these waters, defluoridation tests by adsorption on zircon columns and activated carbon based on filao litter have made it possible to obtain significant reductions.
The results obtained confirm that zircon has a higher efficiency (67.9% reduction) compared to activated carbon from filao litter (60.1%). However, neither adsorbent was able, under the experimental conditions applied, to reach the recommended potability threshold (1.5 mg/L). Activated carbon nevertheless retains the major advantage of being a local resource, inexpensive and easily mobilizable in a rural context, which makes it relevant from a decentralized treatment perspective.
In addition to the results obtained, this study offers new opportunities for process improvement. In particular, extending the retention time from 10 to 30 minutes, or even 120 minutes, could significantly improve the efficiency of activated carbon by promoting longer contact between the adsorbent and fluoride ions.
The study, therefore, highlights the importance of continuing studies on improving operational parameters and potential synergies between various adsorbents, in order to design viable, sustainable solutions adapted to the socio-economic and environmental realities of Senegal.