Distribution of Fluoride in the Phosphorite Mining Area of Hahotoe–Kpogame (Togo)

content ranged between 0.15 and 0.63 mg/l (average = 0.41 mg/l; n = 22). The fluoride content in the water of the phosphorite mining area was higher than in Gbodjomé (reference area). Meanwhile, assessment of the effluents discharged into the ocean had a fluoride content ranging from 12 to 20 mg/l. In dusts, the P 2 O 5 and fluoride contents were 36.02% and 1.85%, respectively. Vegetables from the local market garden produce showed levels of fluoride up to 2.06%. The average contents of P 2 O 5 and fluoride in one of the phosphorite profiles were 32.38% and 3.00%, respectively. A significant correlation was observed between P 2 O 5 and fluoride. Conclusions. The correlation between P 2 O 5 and fluoride in phosphorites shows that phosphorite mining is the main source of fluoride pollution in this area. Competing Interests. The authors declare no competing financial interests. 10: 84–94 (2016)


Introduction
Numerous countries around the world such as India and Bangladesh experience fluoride concentrations in groundwater greater than 1.5 mg/l (World Health Organization (WHO) guideline). 1 In the 1980s, it was estimated that nearly 260 million people worldwide consumed water containing more than 1 mg/l of fluoride. More than 70 million people have fluorosis globally. 2 The impact of high concentrations of fluoride in the environment is well known. 3-9 Excessive fluoride ions in drinking water is a cause of serious poisoning. Fluoride is helpful to human health in low concentrations, but toxic at higher doses. Around 0.5 mg/l fluoride ions, fluoride in water plays a preventive role, but the risk of fluorosis begins at 0.8 mg/l and increases as fluoride concentration increases. The accepted standard concentrations range from 0.7 to 1.5 mg/l for temperatures between 12 and 25°C. 10 Interactions between water and rocks have been widely studied and recognized as the main causes of fluoride in groundwater. 11 In 2011, only 39% of the population of Togo had access to clean drinking water. This supply rate was lower than the Millennium Development Goals target, which was 49% in 2011. 12 In addition, it is estimated that 20,500 people die each year in Togo due to environmental risks; 4,800 due to water pollution, lack of sanitation and hygiene, and 3,400 due to air pollution (indoor and outdoor). World Health Organization enriched with numerous trace metals such as cadmium, chromium, copper, nickel, vanadium, zinc, lead, uranium, thorium, molybdenum, silver, fluorine, yttrium and major elements such as fluoride, aluminum, iron etc. 14- 16 The main phosphorite mineral in Togo's phosphorite is a carbonate fluorapatite, also called francolite, which allows numerous ionic substitutions. [17][18][19] The processing of the phosphorite ore to commercial grade is done mechanically by wet sieving using sieves and hydrocyclones. Seawater is pumped in a factory at Kpémé close to the beach situated 25 km from the mining sites. Three types of mine wastes are produced during this processing: fine-grained clayeymuddy tailings, coarse-grained waste and phosphorite dust rejected by the factory chimney. The muddy tailings are dumped directly into the coastal waters of Togo without any pre-treatment, and the coarser waste is disposed of on soils and areas around the factory. Mining activities in the area are a source of metal contamination of soils, air, water and biota. [20][21][22][23] Dental fluorosis, which is endemic in the local population, especially among children in the Hahotoé-Kpogamé and Kpémé mining areas, calls for more attention and study. Until now, no specific study has been conducted to examine the distribution of fluoride in the different environmental compartments of the mining area, as well as in the phosphorites and wastes, and to identify the source of pollution and provide solutions for the exposed population. Therefore, the present study is aimed at assessing the distribution of fluoride in this phosphorite mining environment.

Study Area
The industrial activities related to phosphorite mining are mainly located at two sites; the phosphorite mining area at Hahotoé and Kpogamé, and the processing plant at Kpémé.
Both are located in the southeastern region of Togo ( Figure 1). This is the area most affected by the industrial activity of ore mining and processing. This delimitation of the study area takes into account wind transport of dust particles and emitted gases and transport of mine tailings by marine currents.

Water
Groundwater was taken directly from boreholes and wells (Figure 2) in the mining area of Hahotoé and Kpogamé, and also from boreholes and wells in the processing area in the surrounding villages of Kpémé, Aglomé II, Sewatsri-Copé and Goumou-Copé. In those villages, people use water from boreholes and wells for drinking. Surface water samples were also taken from abandoned mining quarry ponds and from the muddy tailings at the outfall after decantation (Figure 3). Tanouayi

Research
Groundwater samples were also taken at Gbodjomé, the reference area very far away from phosphorite mining activities. For the analysis, water samples were then filtrated using Whatman filter paper and kept at room temperature before running the analysis.

Phosphorite Waste
Phosphorite waste (coarse waste and fine muddy tailings) samples (10) were collected from the processing plant and kept in clean plastic bags. Raw phosphorite materials were sampled from the phosphorite layer mined at Hahotoé-Kpogamé in three profiles (vertical samples). The distances between the samples in the profiles were irregular. 24 Dusts from the plant were collected from deposits on house roofs and on tree foliage ( Figure 5).
At each site, sampling of boreholes and wells was spatially equidistant.
The wells are open and there are no barriers to prevent their contamination with phosphorite dust. Four effluent samples from phosphorite processing were collected from pipes discharging tailing outputs into the sea (Figure 4). A total of 56 water samples were taken according to Rodier protocol. Samples were collected in sterile plastic bottles and kept cool at -4°C until analysis. Research reference. Soil samples were conserved in plastic bags (polyvinyl chloride) designed for this use. In the laboratory, soil samples were dried in an electric oven at a temperature below 40°C, sieved through a 2-mm sieve and milled (reference method NF X 31-101) in order to obtain a powder with a particle size smaller than 250 μm to improve the efficiency of acid attack during mineralization. 25

Vegetables
Vegetables growing around the treatment plant were sampled and packed in plastic bags at Goumou-Copé. These samples consisted of leaves (3

Quality Control and Precision Analysis
The validity of the analytical methods was verified by internal control. A procedural blank was prepared simultaneously using the same acids (nitric acid) and hydrogen chloride, for the samples in the same experimental conditions and measured for each sample series. This allows for the identification of sample contamination and elimination of errors. The standard solution for the measured elements (fluoride and P 2 O 5 ) was prepared and incorporated into the normal series of interval analyses to check the internal precision of the method. In addition, in order to verify the repeatability of the results, multiple trials on one sample (duplicates) were randomly incorporated into the analytical batch through a blind analytical procedure (without the knowledge of the analyst).

Phosphorus Pentoxide and Fluoride Content in Phosphorites
The P 2 O 5 and fluoride content of the crude and commercial phosphorite processing waste can be found in Table  1. The total average P 2 O 5 and fluoride content in the three profiles were 29.35% and 2.24%, respectively. Profile 1 and 2 presented high concentrations of clays, calcites and oxide of iron and manganese, which explains the low content of P 2 O 5 in both profiles. There was also a very significant correlation between the P 2 O 5 concentration and fluoride in Profile 3 (r = 0.90; P <0.0003), followed by Profile 2 (r = 0.76; p <0.0094) and Profile 1 (r = 0.54; P <0.10). This shows that fluoride is incorporated into the apatite structure. Table 1 indicates that the commercial phosphorite has been processed (enriched) from crude phosphorite, as the P 2 O 5 content in commercial phosphorite (37.51%) is higher than in crude phosphorite (29.35% total average for the three profiles).

Phosphorus Pentoxide and Fluoride Content in Phosphorite Processing Waste
The P 2 O 5 and fluoride content of the various types of processing waste is summarized in Table 2. In dusts, the P 2 O 5 content was 36.02% and 1.85% for fluoride. For iron oxide, the contents were 19.05% for P 2 O 5 and 1.43% for fluoride. Fluoride and P 2 O 5 levels were the highest in dust, followed by coarse mud, iron oxide, fine wet mud, and fine dry mud. It was also noted that there was a significant correlation between P 2 O 5 and fluoride in the waste (r = 0.58; p <0.0455).

Fluoride Content in Surface Water and Groundwater at the Mining area of Hahotoé-Kpogamé
The results of the analyses of surface and ground water in the mining site of Hahotoé Kpogamé are shown in Table  3 and compared to the reference area (Gbodjomé).
It appears from these results that the average fluoride content in surface groundwater was below the WHO guideline (1.5 mg/l), although 30% of the surface water was above this value. 9,30

Fluoride Content in Water, Soil and Vegetables Around the Phosphorite Processing Plant
The results of the analyses of ground water in villages around the phosphorite processing plant are shown in Table 4 and compared to the reference area (Gbodjomé). In all cases, the fluoride content in the water in these villages was below the WHO drinking water guideline of 1.5 mg/l. 9,30 The high fluoride content in the waters of Goumou-Copé may be due to a slightly basic pH. This pH level was the most basic of all of the water samples. A basic pH with moderate conductivity favors the dissolution of fluoride in water. Goumou-Copé is also next to the outfall of phosphorite tailings. Fluoride and conductivity in these waters is highly variable and this may be due to the intrusion of seawater into groundwater and the influence of surface water by rainfall.
The fluoride content in mining effluents is shown in Table 5. Overall fluoride contents in water and effluents showed a negative correlation with pH (r = -0.04; P <7760; n = 51) and a positive correlation with electrical conductivity (r = 0.97, P <0.0001, n = 51).
The results of the analyses of fluoride in soils of villages around the phosphorite processing plant compared to those of Gbodjomé (reference area) are reported in Table  6. There was a higher concentration of fluoride in the soil of Aglomé II, followed by soils in Kpémé, Goumou-Copé and Séwatsri-Copé. This can be explained by wind direction, which causes dust distribution from the Research processing site towards these villages, often in a northeast direction. As for village distribution, the village of Aglomé II is located northeast of the plant, Goumou-Copé is located east of the plant, Séwatsri-Copé is located north of the plant, and Kpémé is located to the northwest. The phosphorite treatment plant is located 3 km from Goumou-Copé, 1 km from Aglomé II, 2 km from Séwatsri-Copé and 1.6 km from Kpémé.
Because it had the highest fluoride content of the tested soils of the study villages, fluoride in market produce in Goumou-Copé was analyzed in order to estimate the accumulation in foodstuffs consumed by the local population. Figure 7 shows the fluoride content of different local produce. Daucus carota had a higher fluoride concentration (2.06%) compared to Allium cepa leaves (0.16%). It appears from these results that the reserve organs of these plants (tubers, bulbs) accumulate more fluoride than leafy vegetables.

Discussion
Most sedimentary phosphorite deposits contain a variety of carbonate-fluorapatite, grouped under the collective name of francolite. 31 By establishing a series of systematic relationships between francolites, some authors have used X-ray diffraction, chemical analysis and statistical methods to show that calcium, sodium, magnesium, phosphorus, carbon dioxide and fluorine contents can adequately describe most francolites. 17- 19 The incorporation of carbon dioxide in the structure of francolite is accompanied by increased and excess content of fluoride. 32-34 Substitution of the phosphate by the carbonate is coupled with that of fluoride, which explains the high fluoride content in francolite. 35 This is consistent with the results of our analysis, which showed (Table 1) an average content of fluoride and P 2 O 5 in one of the study profiles of 3.00% and 32.38%, respectively, suggesting that mining activities are the primary source of the fluoride contamination in other environmental compartments.
Coarse wastes have a higher fluoride content than fine mud (1.78% vs 1.23%), because the coarse wastes consist of apatite and coprolites (fossilized fish dung) accompanied by teeth, vertebrae and other pieces of fossilized animals, while fine mud is mostly rich in clay. Meanwhile, dusts have almost the same P 2 O 5 and fluoride content as phosphorites, because they mainly consist of fine apatite particles released into the Research daily), and this is not negligible. The inhabitants consume water from wells without filtration. The water contains phosphorite particles caused by dust deposit in the wells. The study by Gnandi in 2007 showed that once those particles enter the body through consumption, they can release fluoride in the stomach after digestion due to the presence of gastric acid. The study also found that up to 6.05 mg/l fluoride was found in well water particles that were separated by filtration and acid digestion. 38 The negative correlation found between fluoride content and pH (r = -0.04; P <7760, n = 51) and positive correlation with conductivity (r = 0.97; P <0.0001, n = 51) in the water was confirmed by Saxena in 2001. 39 Experimental results indicated that alkaline pH (7.6 to 8.6), high concentration of bicarbonate (ranging from 350 to 450 mg/l), and moderate specific conductivity (ranging from 750-1750 µS/cm) favor fluoride dissolution.
Like any solid particle, dust grains, even if sent by wind several kilometers away, end up falling naturally onto open surfaces such as soil, water (sea, lakes, rivers, wells etc.) and vegetation. These deposits cause water pollution. Phosphorite dust falling in the villages surrounding the plant and on soils is contaminated by fluoride with a content ranging from 0.05% to 1.54%, as the results have shown. Produce grown on these soils also accumulate fluoride. However, the fluoride accumulated in these products is not necessarily from the soil. Chemical elements can penetrate directly into the leaves after dust deposition. 40, 41 Thus, phosphorite dust with a fluoride content of 1.85% could be one of the eventual sources of contamination. In addition, garden crops could also accumulate fluoride from water.
Fluoride is not considered to be an essential element in mammals, but may be considered as a beneficial element, forming fluorapatite in teeth and bones. 42 The example of fluoride is revealing of this prophylactic role. Indeed, while the ingestion of a small amount (≤ 2.5 mg/day) can prevent or reduce tooth decay and strengthen bones, excessive ingestion of this element (> 7.75 mg/day) usually induces the development of conditions such as dental or skeletal fluorosis. 1 A mottled discoloration of teeth among many children and some adults was observed at the phosphorite mining area. This is due to accumulation of fluoride possibly caused by the consumption of fluoride from water, agricultural products or by the ingestion and inhalation of phosphorite dust. A previous study by found high fluoride levels in blood (0.52 mg/l) and breast milk (0.47 mg/l) of lactating women in this area. 43 In phosphate-rich regions of Morocco, high levels of fluoride in water, soil and plants were found, and dental and bone abnormalities have also been reported. 44-46 Haikel et al. (1986) demonstrated that dental fluorosis encountered in the Khouribga region of Morocco was mainly due to inhalation of phosphorite dust, not only fluoride content in drinking water and incidental accumulation of fluoride due to consumption of agricultural products. 47

Conclusion
We examined the distribution of fluoride in different environmental compartments of a phosphorite mining area in Togo. The high content of fluoride in raw phosphorite shows that in these phosphorites, the main apatite mineral is francolite. Waste from phosphorite processing also contains high levels of fluoride.

Research
The level of fluoride enrichment in groundwater was not negligible. The soils around the treatment plant were also found to be contaminated by fluoride. Produce from these soils accumulates fluoride. Consumption of water, agricultural products and dust cause dental fluorosis observed in the target populations. The significant correlation between P 2 O 5 and fluoride in phosphorites indicate that mining is the main source of fluoride pollution in this area.