Abstract: Chronic exposure to heavy metals through air, water, food and other media and product has serious negative health impact. Concentrations of heavy metals (Cd, Pb and Zn) in soil, water, Manihot esculenta tuber and Oreochromis niloticus around phosphates exploitation area in Togo were determined during dry and wet seasons using digestion and Atomic Absorption Spectrophotometer methods. In soil, Cd and Zn contents significantly decreased during dry season compared to wet season (p<0.001). During wet season, Akoumapé 2 and Akoumapé 3, Asso Apégan, had Cd concentrations in water lower than 0.003 mg L-1, however in dry season these concentrations were higher than the recommended maximum concentration. During wet season, all the sites had Pb concentrations (0.204 to 0.313 mg L-1) higher than values recommended by WHO. In cassava tubers, all Pb concentrations were below critical value of 2 mg kg-1; the highest concentrations of Cd (0.673 mg kg-1) and Pb (1.868 mg kg-1) were observed at Asso Apégan. Zn concentrations were below 60 mg kg-1 except at Agbozo Kpedzi where Zn level reached 104.660 mg kg-1. The values of Cd and Pb in Oreochromis niloticus at different sites were less than respectively 1 and 2 mg kg-1 except at Asso Apégan where Cd concentration during wet season reached 3.014 mg kg-1. The same site showed a maximum concentration of Zn (113.370 mg kg-1) during wet season higher than 100 mg kg-1. This study reveals that some sites of the exploitation zone of phosphate are polluted by the mining activities and constitute a risk to the population around through the food chain.
INTRODUCTION
Heavy metals also named metallic trace elements are element which occurred naturally in the environment but can be emitted by anthropogenic activities such as industrial and mining transformations. These metals are very toxic even at low concentrations (Mckenzie, 1997). The accumulation of heavy metals in soil, water and sediment contaminates crops and fishes (Kabata-Pendias and Pendias, 1991; Etesin and Benson, 2007; Iwegbue et al., 2008). A large number of factors influence metal transfer to different parts of food chain. Metal types and edaphic factors are important variables in all transfer routes (Tremel-Schaub and Feix, 2005). Transfer of non-essential metals, for instance Cd and Pb can be widely different from essential metal transfer. This is due to efficient mechanisms in place to regulate cellular uptake and accumulation of essential metals (Sheffield et al., 2001). Argillaceous layers, pH and total organic content are factors known to influence metal transfer from soil to plant (Obasi et al., 2012).
Air pollutants inhalation and food chain transfer are the major pathways for human exposure to heavy metals contamination (INERIS, 2003). Continuous exposure to heavy metals has many adverse health effects (cancer, organ damage, reduced growth and development). Lead exposure results in physiological, biochemical and behavioral abnormalities in laboratory animals. Cadmium causes different type of cellular damage including apoptosis (Hsu and Guo, 2002). Lead and cadmium are highly toxic elements to human beings. Children, infants and fetus are at particularly high risk for neurotoxic and development effects (Lauwerys, 1990; Lanphear et al., 2005).
Togo, one of the countries of the world exploiting natural phosphate deposits is confronted to the problem of pollution. Due to their chemical composition, phosphate deposits contain traces of many metallic elements such as cadmium, lead and mercury. According to the estimation of IFG (International Fertilizer Group), the treatment of phosphate at Kpémé (southern Togo) emits about 3.5 million tonnes of phosphate mining waste (containing Cd and Pb) in the coastal waters of Togo (MERF, 2005). These tailings show an average content of 14 mg Cd kg-1 and 69 mg Pb kg-1. The toxicity of lead and cadmium makes these two chemical elements a permanent risk to the environment and human health. Previous studies in this area by Gnandi et al. (2006) showed a contamination of marine waters (Gnandi, 2003a) and sediment of river Haho and Lake Togo by heavy metals (Gnandi, 2003b). Other studies have also shown high concentrations of Cd and Pb in aquatic species (fish and shellfish) in coastal waters (Gnandi et al., 2006).
The main objective of this study was to evaluate cadmium, lead and zinc concentration in soils, water, corps (Manihot esculenta) and fish (Oreochromis niloticus) in six phosphate mining sites of Togo.
MATERIALS AND METHODS
Study area: Phosphate area is located at 20 km of the coast between 1°24’ and 1°30 East and latitude 6°22’ and 6°28. Altitude of the study zone varies between 51 and 75 m (GPS) according to the position of landscape. The zone has subequatorial climate characterized by two rainy seasons (from April to July and from October to November) and two dry seasons (from December to March and from August to September). Annual average of precipitation is 1 321 mm.
Six sites (Fig. 1) were chosen according to phosphate mining and soil characteristic. A site was not affected by the mining (Agbozo-Kpédji, GPS 631). The remaining five sites were affected and were called: Akoumapé 1 (soil filled and restored by plant species such as Eucalyptus sp., Cacia oriculiformis, GPS 626), Akoumapé 2 and 3 (soil not filled and restored by the same species respectively GPS 627/635 and GPS 628), Hahotoé (soil filled and not restored, GPS 625) and Asso Apegan (soil not filled and not restored GPS 630/632).
Collection of samples: At the sites, soil, water, plant (cassava tuber) and fish were sampled. Sampling was carried out from October to November 2011 during the wet season and from February to March 2012 during the dry season. Samples of ground and cassava were collected from agricultural areas belonging to families living at the study zone.
| Fig. 1: | Sites location |
Soil samples were carried out with drill on a layer of 0-30 cm. Composite samples were obtained from three subsamples taken in three points corresponding to the angles of an unspecified triangle. Subsamples of soil (2-3 kg) taken from each site, were mixed and conditioned in polyethylene bags at ambient temperature prior to chemical analysis.
Cassava roots (5-10 kg) were also collected from the corresponding sites where soil was obtained.
Water and fish samples were collected at Akoumapé 2 and 3, Asso Apégan and Agbozo Kpedzi with water sampler (bucket) and fishing net. Tilapia fish (Oreochromis niloticus) samples were collected randomly from each location. Fishes were placed in clean polyethylene bags with ice and immediately taken to laboratory where the samples were deep frozen at -20°C until prepared for analysis.
Sample preparation: Soils were air-dried in a clean room and crushed to obtain fine fractions ranging between 0.1 μm to 2 mm. Soil pH was determined in a 1: 2.5 soil to water suspension with a glass electrode using a Hanna pH meter (Hanna Instruments, Kehl, Germany). Carbon of organic matter was oxidized in a mixture of 5 mL of dichromate of potassium and 7.5 mL of sulfuric acid concentrated.
NFU 44-041 standard refering to AFNOR X31-151 Standard for total digestion by fluorhydric and perchloric acid was used for metal elements determination. About 1.0 g of dried soil was digested with a mixture of HClO4 and HF (1:1, v/v). The solution was boiled and evaporated nearly dryness on hot plate at 160°C. The residue was diluted in HCl 2% and filtered through fast paper into a 100 mL volumetric flask. The filtrate was then analyzed for determination of Pb, Cd, Zn content using an atomic absorption spectrophotometer (AAS 110, VARIAN).
Cassava tubers were subjected to dried digestion according to the standard of NF EN 14082 of June 2003. Five grams of sample was placed in an oven at 450°C for 7 h. Ashes were recovered with 2 mL of HNO3 in a flask of 50 mL and demineralized water was added. The concentration of Pb, Cd and Zn in cassava was determined with atomic absorption spectrophotometer (AAS 110, VARIAN). Total metals were determined by a graphite furnace (GTE, AAS 110 VARIAN) when the flame (AAS) was insufficiently sensitive. A sample of certified reference material of food (DORM-3, Canada) and external reference material were carried through dry digestion and analyzed as part of the quality control protocol. Reagent blanks and standards were used wherever appropriate to ensure accuracy and precision in heavy metals analyses of variance. Determination of Pb, Cd and Zn of the various samples was carried out with calibration curves obtained with well-known concentration of standard solutions.
Water samples were subjected to wet digestion according to the standard of NF EN 14082 of June 2003. Nitric acid (2 mL) and sulfuric acid (1.5 mL) were added to water sample (5 g). The mixture in a hermetically closed bottle was placed in water bath at 80°C for 2 h. The resultant residue was recovered in a flask of 50 mL.
Statistical analyses and bioconcentration factors estimation: Statistical analysis was performed using two-way ANOVA, followed by Bonféroni test and the significance was reported at p<0.05. The bioconcentration factors (BCF) of Cd, Pb and Zn from the soil to plants is the ratio of the concentration of metals in the plant to the concentration of metals in the soil. The bioconcentration factors of Cd, Pb and Zn from the water to fish is the ratio of the concentration of metals in the fish to the concentration of metals in the water.
RESULTS
Content of soils in Cd, Pb and Zn: Levels of Cd, Pb and Zn in soils of different sites were reported in Table 1. Hahotoè and Agbozo Kpédzi had Cd values<LOD (limit of detection). At Asso Apegan, Cd concentrations (7.135 and 2.447 mg kg-1 in respectively wet and dry season) were higher than the value (0.43 mg kg-1) recommended for agricultural soils by NYS DEC (Grubinger and Ross, 2011).
Pb concentration at Hahotoè area (268.63 mg kg-1) was respectively three thousand fold and two hundred fold the value of Akoumapé 2 and Akoumapé 1 during the wet season. This Value is greater than the recommanded maximum cncentration (200 mg kg-1). Zn concentrations observed at all the sites were below the concentration recommended by NYS DEC (1100 mg kg-1) for agricultural soils.
In general, Cd and Zn contents significantly decreased during dry season compared to wet season (p<0.001). The pH at all sites was slightly acidic with values ranging from 6.28 and 6.98. Percentage of organic matter at each site was <3%.
Content of Cd, Pb and Zn in water: Maximum trace element concentrations in surface waters (Table 2) were low compared to those found in soil. During wet season, Akoumapé 2, Akoumapé 3 and Asso Apégan, had Cd concentrations lower than the RMC (0.003 mg L-1) whereas these concentrations were elevated than RMC on dry season. During wet season, all sites had Pb concentrations (0.204 to 0.313 mg L-1) higher than the value recommended by WHO.
| Table 1: | Content of soils in Cd, Pb and Zn |
| Results represent Mean±SEM (n = 4), ***p<0.001 dry season vs wet season, OM: Organic matter expressed in %, cadmium detection limit, DLCd: 6.10-6, RMC: Recommended maximum concentrations in agricultural soils by New York Department of environmental conservation (NYS DEC) Grubinger and Ross (2011) | |
| Table 2: | Content of waters in Cd, Pb and Zn |
| Results represent Mean±SEM (n = 4), **p<0.01, ***p<0.001 dry season vs wet season, RMC: Recommended maximum concentrations in water surface by WHO (2011) | |
Zinc values at all the sites were below 3 mg L-1 (WHO critical value). The results showed that values of Zn during the dry season were lower (p<0.01) compared to the values of the wet season except at Akoumapé 2.
| Table 3: | Content of cassava tuber (Manihot esculenta) in Cd, Pb and Zn |
| Results represent Mean±SEM (n = 4), *p<0.05, **p<0.01, ***p<0.001 dry season vs wet season, RMC: Recommended maximum concentrations in vegetable by FAO/WHO (2002) | |
Content of Cd, Pb and Zn in cassava tuber: Cadmium level in cassava at Hahotoé, Akoumapé 3, Asso Apégan and Agbozo Kpédzi were between 0.008 and 0.673 mg kg-1. At Akoumapé 1 and Akomapé 2, Cd levels were below LOD (6.10-6 mg kg-1) (Table 3). Lead concentration in dry season was higher (p<0.05) than that of wet season at all the sites. All values of Pb were below RMC (2 mg kg-1).The highest concentrations of Cd and Pb, respectively 0.673 and 1.868 mg kg-1 were observed at Asso Apégan.
Zn concentrations were below RMC (60 mg kg-1) except at Agbozo Kpedzi where Zn level reached 104.660 mg kg-1. A comparison between seasons showed that values of dry season were lower (p<0.01) compared to wet season.
Content of Cd, Pb and Zn in tilapia (Oreochromis niloticus): All values of Cd and Pb in tilapia at different sites were lower than the RMC (respectively 1 and 2 mg kg-1) except at Asso Apégan where Cd concentration during wet season reached 3.014 mg kg-1 (Table 4). The same site showed a maximum concentration of Zn (113.370 mg kg-1) during wet season higher than RMC (100 mg kg-1).
Bioconcentration factors soil-plant (BCFsoil-plant) of M. esculenta: BCFsoil-plant of M. esculenta were between 0.081 and 0.64 for Cd, 0.0002 and 1.875 for Pb and 0.422 and 1334.25 for Zn. FBCPb and FBCZn in dry season were higher than in wet season (Table 5).
Bioconcentration factors water-fish (BCFwater-fish) of O. niloticus: BCFwater-fish of O. niloticus was between 0.58 and 55166.6 for Cd, 0.1 and 932.64 for Pb and 48.76 to 593.115 for Zn. BCFCd in dry season was high compared to wet season (Table 6).
| Table 4: | Content of tilapia (Oreochromis niloticus) in Cd, Pb and Zn |
| Results represent Mean±SEM (n = 4), *p<0.05, **p<0.01, ***p<0.001 dry season vs wet season, RMC: Recommended maximum concentrations in fishes by FAO/WHO (2002) | |
| Table 5: | BCFsoil-plant of Manihot esculenta |
| The results represent the Mean±SEM (n = 4), CR*: Concentration range of FBCsol-plant by Kloke et al. (1984), ND: Not determined, The bioconcentration factors BCFsoil-plant of Manihot esculenta is the ratio of the concentration of metals in M. esculenta to the total concentration in the soil | |
DISCUSSION
Soil Organic Matter (SOM) content ranged from 0.72 to 1.82 in wet season and from 1.65 to 2.54 in dry season. Higher organic matter content observed in dry season is due to lower soil moisture contents during dry season which slow down microorganism activities involved in the organic matter decomposition thereby accumulating more organic matter in dry season (Oyedele et al., 2008).
| Table 6: | BCFwater-fish of Oreochromis niloticus |
| The results represent the Mean±SEM (n = 4), ND: Not determined, the bioconcentration factors, BCFwater-fish of Oreochromis niloticus is the ratio of the concentration of metals in O. niloticus to the total concentration in the water | |
Restored soils were from 25 to 35 years old. Previous study in our laboratory (ITRA, 2010), showed that restored soils were made up of clay and sand, not very compact and chemically poor. Their content in organic matter (0.91-2.54%) was fairly weak.
The soils non-affected by the mining (Agbozo Kpedzi) and of Hahotoé had Cd contents lower than the limit of detection. The filing materials of Hahotoé are made up of sterile sediments without economic value pickled before the exploitation of the phosphate layers. These filing materials had the same structure such as non-pickled soils of Agbozo Kpedzi. Higher level of Pb contents at Hahotoé may originate from extraction activities leading to spreading of fuel residues containing lead. Low content of organic matter were due to the unrestored soils used for filing.
Wet season mean values were significantly higher than the dry season values for Cd and Zn. This can be attributed to the effect of streaming water of rural routes. Generally in rainy season, eroded roads are frequently filled with solid waste from the phosphate treatment factory (Gnandi, 1998; Aduayi-Akue, 2010). This waste is very rich in metal elements traces such as Cd and Zn which strongly pollute the environment. Our results are in line with the work of Oluyemi et al. (2008). These authors showed that the runoff effect is capable of removing heavy metals from farmland during wet season. The highest rates of Cd were observed at Asso Apégan. The soil at this site was not filled and not restored with exploitation duration less than 10 years. This soil remained still contaminated by phosphate residues which were very rich in non assimilable phosphorus.
Usually conveyed runoff or atmosphere (dust, rain) metals gradually contaminate the 3 compartments of aquatic environment: water, sediments and organisms living in this environment (INERIS, 2004). Trace element concentrations in surface waters in this study were low compared to those found in soils because trace elements are fixed to the sediment and microorganism in water. Ponds of fish farming are filled during wet season by erosion water from roads and phosphate mining. These ponds also receive agriculture waste as source of food for fish. Water provision in wet season allows dilution of chemical elements in suspension, hence weak concentrations of Pb, Cd and Zn. In dry season, decomposition of agriculture waste dumped in water ponds and evaporation resulting in higher content of trace elements. The lowest concentration of pond water Pb detected in dry season would result from an accumulation of this element in the sediments and microorganism present in the water (Youssao et al., 2011).
All trace metals found in soil were present in cassava crops. The higher content of Pb and Cd observed in dry season confirmed the higher concentration of these elements in soil in wet season. Evaporation of water from soil and dehydration of plant leaves by transpiration increased metal concentration in the roots (Oyedele et al., 2008). During roots growth, accumulation of heavy metals concentration depends on the pH and the organic matter of the soil. The soil pH (unregistered) was slightly acidic with low organic matter content. At low pH, metals are more soluble in the soil solution, hence more bioavailable to plants. Toxicity problems are more severe in acidic soils than in alkaline soils (Oluyemi et al., 2008). Organic matter plays an important role in soil structure, water retention, cation exchange and in the formation of complexes. They contribute easily to the accumulation of trace element in clay layers of soils (Tremel-Schaub and Feix, 2005). Bioconcentration factor (BCF) which is a Transfer Factor (TF) of metal from soil to plant indicates the capacity of the plant to accumulate the metal. This factor is very variable and depends on the species (Kim et al., 1988; Edeogu et al., 2007).
In this study, it is shown that cassava tuber hyperaccumulate Zn than Cd and Pb. Hahotoé sampling presents high concentration of lead in soil but weak cassava bioconcentration factor. These observations are in line with the data of Liang et al. (2009) and Humbert (2010) showing a decrease of bioconcentration factors in individuals hyperaccumulators with increasing concentration of metals in the soil.
Fishes contain all the minerals present in water. The values of Cd and Pb in O. niloticus at different sites are less than the RMC (respectively 1 and 2 mg kg-1) except Pb concentration at Asso Apegan. The higher value of Cd (3.014 mg kg-1) and Zn (113.370 mg kg-1) in Tilapia observed at Asso Apegan may be attributed to higher concentrations of Cd and Zn in the soil during the wet season and in the water during dry season. Previous studies have shown high concentration of Cd, Pb and Zn in the liver of tilapia fish (O. niloticus) than the muscle which is the edible part of the fish (Taweel et al., 2012).
CONCLUSION
This study has revealed various concentrations of lead (Pb), cadmium (Cd) and zinc (Zn), in soils, water, cassava and fish harvested in wet and dry seasons. The values obtained at the study sites are within acceptable range except at Hahotoé and Asso Apégan. However, this finding suggested pollution due to mining activities of phosphate. Effort is needed to monitor closely the environment and to ensure appropriate mining technology to reduce the availability of these metals (Pb, Cd and Zn) in agricultural soils and also preserve the health of population of the exploitation area. Currently, studies are in progress in order to determine the exposition of this population to cadmium, lead and zinc.
ACKNOWLEDGMENT
The authors would like to thank laboratories of ITRA (Togolese Institute of Agronomic Research) technicians for their assistance. Gratitude to the population of phosphate mining area.