Metallic Trace Elements (MTE) in soils and plant organs of some crop in periurban of Abidjan (Ivory Coast)

Volume05-2017
Advances in Agricultural Science 05 (2017), 04: 13-31

Metallic Trace Elements (MTE) in soils and plant organs of some crop in periurban of Abidjan (Ivory Coast)

Thierry Guety 1*, Emile B. Bolou Bi 2, Bongoua-Devisme Affi Jeanne 2, Brahima Kone 1

University Felix Houphouet Boigny, UFR STRM, Department of Soil Science,  LIEVASOL, Côte d’Ivoire.
University Felix Houphouet Boigny, UFR STRM, Department of Soil Science,  LIFBIOS, Côte d’Ivoire.

ABSTRACT

The quality of the plant production in periurban agriculture is subjected to question given the potential contamination of soils that can affect the crops. The levels of contamination of soils and vegetables by Metallic trace elements (MTE) as copper (Cu), zinc (Zn), cadmium (Cd) and lead (Pb) in the district of Abidjan, have been evaluated. Multi-sites survey of cropping areas of sweet potato and Hibiscus was conducted in three municipalities in Abidjan (Port-Bouët, Yopougon and Bingerville) in relation to the intensity of industrial and commercial activities. The site of Bingerville has been used as the reference site referring to the low activities. Soil samples (in 0-20 cm and 20 – 40 cm), combined with that of plants (leaf, stem, and root), and water were collected, transported in laboratory for analysis. The total amounts of MTE in soil, as well as the different fractions extracted were determined in addition to the respective concentration in plants (Hibiscus and Sweet potato). Toxic level of Pb (< 8 mgkg-1) was observed in the plant organs collected at Port-Bouët site indifferently to crops while lowest content of Pb (35.5 mgkg-1) was accounting for the soil of Yopougon (39.8 mgkg-1). A neutral pH of the soil has been considered more favorable to the contamination of plants in Pb on the polluted sites somewhat differing for extractable fractions. The acidity and small width of leaf as observed for Hibiscus, were identified as the control factors of crop contamination in periurban agroecosystems prone to Pb pollution. To strengthen the consistency of the knowledge, studies of the interaction between Pb and Zn as well as the translocation of Pb in the plants to tubers are suggested in the tropical ecosystems.

Keywords: Trace metal, Mobility, Bioavailability, Remediation, Phytodisponibilité, Côte d’Ivoire


Introduction

In the area of wet drill bit of West Africa, the urban population is very important compared to that of the rural population (Drechsel et al, 1999). Approximately 2/3 of the population will be living representing a huge challenge for the security of the food supplies (Maxwell, 2000). Hence, there is emergence of new informal economies (United Nations, DESA, 2006) among which, periurban agriculture is major regarding to the additional income for populations. Indeed, the often precarious lifestyle of a fringe of the urban population imposes upon him the return to agriculture in the vicinities of the cities for its subsistence and to generate income (FAO, 2008). The urban agriculture or periurban areas, according to Moustier et al. ( 2004), refers to the forms of agricultures coproduced by the cities, that they are located on the inside of the city or in the urban periphery. This agriculture can supply approximately 30% of food requirement in the world (Smith et al, 1996; N’Dienor, 2006).

Ivory Coast, like many African countries, has experienced over the past few years, significant urbanization, with as a subsequent strong development of the periurban agriculture. In the district of Abidjan, vegetable production has taken an important part in this activity mainly characterized by production of leaf vegetables. It thus contributes to the supply of fresh products of the markets of the ten municipalities (Kouakou, 2009). The proximity of vegetable cropping site to industrial zone and traffic road are promoting the risk of pollution including an accumulation of metallic trace elements (MTE) in soils and crops (Sposito, 2010). The MTE or micro-metal pollutants are chemical elements in the crust of the earth for less than 0.1 per cent and their accumulation over is a danger man, animal and/or plants. Several studies are state of the harmfulness of such accumulation for the human and animal health, as well as for the plants to a certain extent (baize, 2000). Among the pollutants of most encountered in the soils, cadmium (Cd) and lead (Pb) are toxic even for trace concentation (Godin, 1983). In constrast, certain pollutants such as zinc (Zn) and copper (Cu) are becoming toxic for high concentration (FAO, 2008).

The current work is dealing with the pollution of sweet potato and Hibiscus (Hibiscus sabdaroufa), as popular diet in urban and rural areas. These ETM particularly concerns the Pb and Cd whose phytodisponibilité would depend on the Cu and Zn. The characterization of such affinities would deepen the knowledge of the Ecotoxicity in periurban agriculture and to consider more effective remediation strategies.

Therefore an exploration has been carried out in the periurban agroecosystems of the District of Abidjan to assess levels potential pollution of soils and contamination of a plant at tuber compared with a plant has not tuberous root with regard to Cu, Zn, Pb and Cd. This study aims to: (i) determine the levels of these chemicals in plants in relationship with the characteristics of the soil, (ii) identify relationships soil-plant promoting plant contamination, (iii) identify the edaphic conditions of potential contamination of Pb and Cd respectively, and (iv) characterize the potential for import of cultures studied. In the long term, this study will need to suggest methods of effective and sustainable management of the pollution in the periurban agroecosystems to arouse the public conscience on the dangers of pollution and save the quality of the environment as well as the public health in urban areas.

 

Materials and Methods

The study site

The city of Abidjan is the economic capital of Ivory coast (West Africa), at the edge of the Gulf of Guinea between latitudes 5°00 and 5°30 North and longitudes 3°50 and 4°10 west. It is composed of ten (10) commons and contains the main industrial activities and administrative provisions of the country (Ahoussi, 2013). The climate is of type equatorial, with a strong annual rainfall between 1637 and 2048 mm, unevenly distributed in time and space. This area characterized by a precipitation regime bimodal with four seasons including, two rainy seasons (April – July and October – November). Temperatures average between 24°C to 30°C with a relative humidity between 75 and 88% (81% in spatial average). The vegetation varies from the clear forest on the south coast to the forest dense evergreen rain and toward the north of the region. This forest is largely degraded following the thrust of the anthropogenic activities intense and urbanization. The geology of the area of study is composed of sedimentary rocks mainly located in the sedimentary basin Coastal and the Ivory Coast. This sedimentary basin extends over 400 km long and 40 km wide and represents only 2.5% of the area of the country (Ahoussi, 2013). The soil coverage is essentially composed of Ferralsols sandy-clayey, to Acrisol facies or pseudogleyic developed on sand tertiary or quaternary. In the framework of this study, the work has been carried out on sites to maraicher in the communes of Port-Bouet (5°25 N – 3°94 W), Bingerville (5°31 N- 3°87 W) and Yopougon (5°35 N- 4°04 W) (Figure1).

 

Figure 1. Location Map of the studied sites.

Plants

Two vegetable species characterized by edible leaf as local dietary habits (Kouakou, 2009) were concerned: Hibiscus (Hibiscus sabdaroufa) locally named “Dah” and sweet potato (Ipomoea batatas). While hibiscus is matured about 2 -3 months of cropping duration sweet potato does so in a longer period of 3 – 6 months depending to the cultivars and the roots are also edible as tubers. Both are characterized by shallow rhizosphere (0 – 30 cm) receiving manual daily irrigation using perched ground water of well (2 – 3 m in depth).

 

Sampling Soil, plants and water

Multi-sites (Bingerville, Yopougon and Port-Bouët) survey was conducted in 2013 in the district of Abidjan around the localities of (i) Port-Bouet and (ii) Yopougon characterized by higher industrial and commercial activity intensities while prevailing agricultural activity accounts for (iii) Bingerville, used as a control site in the framework of our study.

In 600 m2 of vegetables cultivated area in each of these locations, 12 soil composite samples resulting from 5 soil samples were randomly taken in 0 – 20 cm and 20 – 40 cm depth using hand augur respectively. Between four (4) points of soil sample l, each type of plants was also sampled. Approximately 5 g of plant organs (fresh leaves, stems and roots) were collected respectively for each crop and by site during the maturity period. The samples of soil and plants were keept in plastic bags for laboratory analysis.

A sampling of water has been collected in the well used to the watering of plants on each site. These samples of water collected in Plastic Bottles HDPE, were also transported to the laboratory to undergo treatment before analysis of the levels of MTE.

 

Laboratory analyzes

Processing and Analysis of the soil

The soil samples collected were dried in air temperature until constant weight before slightly prounding and sieved by 2mm. The resulting fine earth was splited in two (2) aliquots. The first was used for the determination of particle size (sand, silt, clay) according to the method of the eyedropper using Robinson pipette (Gee and Boder, 1986). The second aliquot for the determination of the pH-water, using a pH meter VWR in a soil/solution ratio of 1/2.5. The electrical conductivity (EC) of the ground water was measured with a multi-meter as the pH.. Finally, the last aliquot was finely crushed to 150 µm and homogenized. Approximately 300 mg of this powder were digested and mineralized in the aqua regia [3 ml of HNO3 (65% v/v) + 1 ml of HCl (37%; v/v)] in vial of 50 ml. The solution of metals by the aqua regia is qualified of pseudo-total (Baize et al., 2005, Laurent 2003). The solution obtained after mineralization has been analyzed by an atomic absorption spectrometer of Mark PerkinElmer., for the determination of concentrations of metals (Pb, Cd, Cu, and Zn). For each sample, 3 measures were done and the average was repported. This value is compared to that of the existing standard (Table 1) (Barneaud, 2006; Godin, 2010).

The Fractionation operational is carried out by the method of sequential extractions according to the Protocol of Tessier et al. (1979). This extraction was done for Cd and Pb (Figure 2).

 

Figure 2. Protocol of sequential extraction (Tessier et al., 1979).

Processing and Analysis of plants

The samples of plants collected are washed with distilled water, and then weighed and dried in an oven at 60°C up to the obtaining of a constant weight. After drying, the crop samples were crushed

Table 1. Maximum Concentration standard of the Cu, Zn, Cd and Pb in the soil, the plant and water

Cd Pb Cu Zn
Soil (mg/kg) 0.7 60 65 260
Plant (mg/kg) 1 8 10 50
Water (μm/L) 5 10 ND >15

 

and sieved (50 Micron). Approximately 0.5 g of each sample was mixed in 6 ml of hydrogen peroxide (H2O2) and 6 ml of nitric acid (HNO3) in a DigiPrep, at a 95°C during 3 hours.

Mineralized were analyzed for the determination of metal concentrations by mass spectrometer with inductively coupled plasma (ICP/MS). The digestion of the samples as well as the treatment of mineralized were made in the same manner for soil and plant as described previously.

Treatment and analysis of the water

The samples of water collected wells was been filtered to 0.45 μm by using filters to acetate and a vacuum filtration system of brand.

The resulting filtrate solution was splited into two aliquots. One was used for physic and chemical mesurements (temperature, conductivity, the content of dissolved oxygen, the potential for oxidation and the potential of hydrogen). The second was used for the measurement of MTE concentrations (Zn, Cu, Cd, and Pb) using an atomic absorption spectrometer of Mark PerkinElmer.

 

Table 2. The characteristics of the soil of the sites of studies simply put a medium-sized and écartypes as well as data from treatments stat (significant difference or not)

Site
Bingerville Port-Bouët Yopougon
Cd (mg kg-1) Maximum 1.50 4.90 5.17
Average 0.65 1.5 1.45
Pb (mg kg-1) Minimum 0.00 0.00 0.00
Maximum 28,07 43.65 57,21
Average 15.25 30 49.15
Minimum 11.65 14.72 20.67
Cu (mg kg-1) Maximum 32,04 42.05 25.04
Average 18,50 35,06 18.28
Minimum 5.96 0.00 8.67
Zn (mg kg-1) Maximum 62.75 400.50 418,00
Average 35,64 320,71 69,61
Minimum 11.45 12.25 25.05
pH Maximum 5.8 7.1 8.7
Average 5.6 7.0 8.6
P (mg kg-1) Minimum 5.1 5.6 7.6
Maximum 21.70 28.42 44.16
Average 12.72 18.79 19.76
CEC (cmol kg-1) Minimum 4.06 4.48 6.67
Maximum 2.01 3.33 28.21
Average 1.59 2.02 13.28
Ca (cmol kg-1) Minimum 0.88 1.53 3.94
Maximum 0.33 0.89 11.96
Average 0.26 0.39 4.68
Minimum 0.14 0.12 0.89
Clay (%) Maximum 5 2 12
Average 3 2 10.80
OM (g kg-1) Minimum 2 1.5 10.65
Maximum 22.5 21.25 15.51
Average 20.35 17.05 12.22
Minimum 19.32 14.07 7.3

 

 

Statistical analyzes

For the statistical analysis, we used two (02) Software: SAS (version 9) and SPSS 16.

A descriptive analysis of the data was realized by using SAS fixing α = 0.05. The average, the minimum and maximum values of Cu, Zn, Cd and Pb as well as the pH were determined for soil sample (0 – 20 cm depth) of each site. Similarly the average values of the concentrations of Cu, Zn, Cd and Pb in the different organs of the plants were determined for each site. By analysis of variance (ANOVA), the average values of the properties of the irrigation water (T˚C, EC, Eh, pH, O2, Cu, Zn, Pb and Cd) were determined according to the test of Student-Newman-Keul likewise for testing difference between mean values of fractions of Cd and Pb in the soil depths of 0 – 20 cm and 20 – 40 cm for each site. A Pearson correlation was processed evaluate the relationship between the total levels of Cu, Zn, Cd and Pb in the soil and their concentrations in the different organs of the plants. The different speciations of metals (F1, F2, F3, F4 and F5) were used for the Pearson correlation with equivalent concentrations in plant organs. The sites studied (Bingerville (1), Port-Bouët (2) and Yopougon (3)) were futher discriminated according to the speciations of Cd and Pb (F1, F2, F3, F4 and F5) using SPSS 16.

 

Results

Soils and waters Characteristics in studied sites 

Table 2 reports the results of the characteristics of different samples of subsoils collected in respective studied sites. The pH reveales acidic soil in Bingerville (pH = 5.6), neutral for Port-Bouet (pH = 7) and alkalinic for Yopougon (pH = 8.7). The soils of Bingerville and Yopougon have strong contents in organic matter (OM). Sites have a strong content in sand.

As for pseudototale contents of metal in soil, the sites of Port-Bouët (1.5 mgkg-1) and Yopougon (1.45 mgkg-1) record of strong content in Cd, contrasting with the low contents recorded in soil of

witness site of Bingerville (0. 65 mgkg-1). Moreover, Pb concentration is regularly observed in the soils of the studied sites in the range of 11.65 mgkg-1 and 20.67 mgkg-1 out standing about 57.21 mgkg-1. The average value of Pb in the soil is almost twice greater for the site of Yopougon in comparison to that of Bingerville which is 1/3 lower than the value observed for the site of Port-Bouët. No significant difference was observed between the sites in relation to the concentration in Pb of groundwater and the electrical conductivity (EC) (Table 3).

Significant low values of temperature (27°3c), oxygen concentration (2.03 mgL-1) and redox potential (-7.03 mV) are determined for the groundwater collected in Yopougon contrasting with Bingerville site. The perched ground water collected at Port-Bouët site is particularly characterized by high value of temperature (30°C) while no concentration of Cd was determined in the ground water collected at Yopougon site.

 

Specific metals fractions in studied soils

Figure 3 shows the ratios of Cd speciation in 0 – 20 cm and 20 – 40 cm horizons for each studied sites: The total Cd is abundant in the fractions F3 and F5 regardless of the horizon of soil, except for horizon 0 – 20 cm at Yopougon site.

 

Figure 3. Speciation of Cd in the horizons of the soil on the various study sites (fractions: Redeemable (F1); the carbonates (F2); oxides (F3); organic matter (F4); residual (F5)).

The Figure 4 shows the ratio of speciation for Pb. We are noting abundance in the fractions F4, F3 and F5.

 

Figure 4. Speciation of Pb in the horizons of the soil on the various study sites (fractions: Redeemable (F1); the carbonates (F2); oxides (F3); organic matter (F4); residual (F5)).

The Figure 5 and 6 present the discrimination of the sites studied respectively of the Cd and Pb. There was no influence of the fraction F5 with regard to the Cd. Bingerville (2) and Yopougon (3) are diametrically opposed according to the fraction F1, F2 and F3 which are characterizing Yopougon site. The F4 fraction has a negative impact despite a limited influence for all the sites studied. For what is the problem, there is no influence of the fraction F3. There is a positive influence of F5 and F2 characterized by the site of Bingerville (2) in opposition to the site of Port-Bouet (1) linked withF1 and F4 concentrations.

 

Figure 5. Discrimination of sites according to the speciations of Cd (1: Port-Bouët; 2: Bingerville; 3: Yopougon). Fractions: Redeemable (F1); the carbonates (F2); oxides (F3); organic matter (F4); residual (F5).

 

Figure 6. Discrimination of sites according to the speciations of Pb (1: Port-Bouët; 2: Bingerville; 3: Yopougon) fractions: Redeemable (F1); the carbonates (F2); oxides (F3); organic matter (F4); residual (F5).

Table 4 indicates the factors of mobility of Cd and Pb. We note a greater mobility of Cd in the Horizon 0 – 20 cm of the soil to Yopougon. No significant difference is observed between the sites compared to the mobility of metals in the Horizon 20 – 40 cm.

The Table 5 shows the relationship between the characteristics of the water and the fractions of Cd and Pb. There is a positive correlation between the pH and the fractions F2 and F5, while negative correlations are observed with the redox potential for Cd. As concern Pb, apart from the residual fraction F5 which has a contrasting correlations with the pH and Eh, and a correlation between F2 and the O2, there are limited relations between the content of the soil in Pb and the characteristics of the groundwater.

 

Metals Concentrations in plants 

Figures 7 and 8 present the average concentrations of metals in sweet potato and Hibiscus. The Cu and Zn are more concentrated in the leaves of sweet potato and Hibiscus in Bingerville, while the site of Yopougon shows low levels of Zn and Cu regardless of the type of organs.

 

Figure 7. Concentrations of Cu and Zn in the leaves, stems and roots of sweet potato and Hibiscus in Bingerville, Port-Bouët and Yopougon

 

Figure 8. Levels of Pb and Cd in the leaves, stems and roots of sweet potato and Hibiscus in Bingerville, Port-Bouët and Yopougon.

 

The site of Port-Bouet is remarkable with high concentration of Pb interchangeably of the organs of the plant. High concentrations of Cd are observed for the Roots interchangeably of plants.

Table 6 shows the relationship between the metals in the soil and those in the leaves of Hibiscus. We note a correlation (r = 0.75) between Zn and Cu, and between Cu and Pb (r = 0.61) as well as for Zn and Pb (r = 0.71). For the sweet potato, we are noticing correlations between the Pb (r = 0.76) and Cd (r = 0.91) of leaves and the Zn of soil, even between the Cu of the soil and the Pb (r = 0.63) and Cd (r = 0.77) of the leaves.

Table 7 shows the Pearson correlation coefficients (R), and the probability (P) observed in the leaf, stem, and root of the cultures studied (Potato and Hibiscus) as a function of different fractions of Pb and Cd in the ground, regardless of the sites studied. We note significantly (p< 0.05) of positive correlations between the fraction F2 (0.69; 0.43; 0.51) of Pb and concentrations of Pb in the leaves, stems and roots of the cultures studied. However, those of fractions F3 Pb (-0.48; -0.34) and F5 (-0.52; -0.38) are negative with the concentrations in the leaves and roots. These latest show no significant correlation with the rod. The fraction F1 (0.51), and the F3 fraction (0.38) of the Cd are significantly and positively correlated with the concentration of Cd of leaves, while the fractions F2 (0.46) and F5 (0.41) are, for this which is the concentration in Cd of the roots. Only the fraction F5 (-0.31) of the Cd is negatively correlated to concentrations in the rod of cultures.

 

Discussion

Potential contaminant in soil and plants

Soils have a relative richness in organic matter (7 to 22%), and this organic matter is more abundant in the surface horizons, on the three sites studied. It could be due to the contribution repeated of animal waste (droppings of chicken, feces of pork and other), but its presence may also be explained by the refund of crop residues and by the inputs of nutrients (Naman et al. 2002; Touré and al. 2010). According to these results, the availability of metals is important in the soils analyzed, in particular thanks to the anthropogenic activities (Alloway, 1990 and Adriano, 2001), which could facilitate the mobility of these elements.

The levels of soil Cd and Pb have been strong in Port-Bouët Yopougon and according to the norms of the Alloway (1990), contrasting sharply with the site witness of Bingerville. There is therefore an obvious pollution of these soils, probably under the action of the man (agriculture and industry), helped by the waters of runoff and those of the lagoon that affect the quality of the groundwater (Coulibaly et al. 2008; Soro et al. 2009) used for the watering of cultures.

There is a potential contamination of crops in Cd and Pb of the soil which could reduce, depending on the remoteness from the Ebrié lagoon already stigmatized by Koné et al. (2010) as a catalyst of the pollution in the coastal zone of Côte d’Ivoire.

This finding is reinforced by the low content of the soil in Zn (35.6 mgkg-1) in Bingerville, whereas the highest concentrations (> 50 mgkg-1) of this metal (Zn) is y observe, regardless of vegetative organs (root, stem and leaf) and of cultures (potato and sorrel). Also, the concentrations of Pb are-they high in the vegetative organs harvested at Port-Bouët (33.6 mgPbkg-1 soil) and Yopougon (39.8 mgPbkg-1soil). The taking into account of the respective speciations of these metals, remains very variables (Mäkäla-kurto, 2000), and the known interactions (Foy et al. 1978) would explain this: Pb2+, and Pb+ are organic forms of Pb assimilated by plants according to the conditions of the middle, then some forms of Cd (polysaccharide) are not extractable. In these conditions, the total levels of these metals are not enough to apprehend the contaminating character of cultures by the soil. On the other hand, it is established a synergism between Cd and Pb and, the ratios of Cu/Cd and Cd/Zn which would be characteristic of the phytodisponibilité Cd of soil according to the criteria of De Vries and Bakker (1998). Therefore, there would be a synergy between the nutrition of the plants in Cu and Pb as shown by the results of correlated during the present study.

In the light of the results obtained and in the light of this analysis, problem would be the metal pollutant of cultures in agroecosystems in periurban areas of Abidjan, with concentrations greater than the standard (> 8 mgkg-1) defined by Godin (2010). The conditions of Port-Bouët (pH-neutral, sandy, rich in organic matter, high conductivity, zinc content in high and content in Cu moderate) would be more favorable to the availability of Pb, causing the contamination of plants.

According Heinrichs et al. (1980), the concentration world average cadmium in the lithosphere reached 0.098 mgkg-1 and the average values in the soils of most of the industrialized countries (for example the United States of America), function of soil types and sites, and are below 1.5 mgkg-1 (Burau et al., 1973; Lund et al., 1981; Logan and Miller, 1983; Holmgren et al., 1993).

 

Table 3. Characteristics of the water on the studied sites

  T˚C EC

(Ms cm-1)

O2

(MgL-1)

Eh

(MV)

PH Pb

(MgL-1)

Cu

(MgL-1)

Zn

(MgL-1)

Cd

(MgL-1)

Bingerville 28,7ab 0.08a 5,90a 72,90a 5,7c 0.04a <LD <LD <LD
Port-Bouët 30.8a 0.65a 4,90ab 23,52b 6,5b 0,44a <LD <LD <LD
Yopougon 27,3b 0.45a 2.03b -7,03c 7,2a 0,31a 0.3 0.5 0.03
MG

P> F

29.4

0.01

0.46

0.10

4.44

0.06

Obtained 28.22

0.001

6.5

0.0001

0.33

0.44

—-

—-

—-

—-

—-

—-

<LD : less than the detection limit; Cu=200µg/kg; Zn =100 µg/kg; Cd=15 µg/kg

 

Table 4. Factor of mobility of Cd and Pb in the Horizon 0 – 20 cm and 20 – 40 cm on the various study sites

 0 – 20 cm 20 – 40 cm
FM Cd FM Pb FM Cd FM Pb
 Bingerville 21,53b 18,53a 12,71a 9,00a
 Port-Bouët 32,50b 13,29a 12,14a 6.58a
 Yopougon 54,28a 2.80a 15,09a 0,96a
P> F 0.039 0.33 0.96 0.08

A, B, C indicates the average values significantly different in the column.

 

Table 5. The Pearson correlation coefficient (r) and probability (P) observed for the properties of the water as a function of different fractions of Cd and Pb in the soil (whatever the sites)

Cd   Pb
F1 F2 F3 F4 F5   F1 F2 F3 F4 F5
PH R 0.44 0.64 0.52 -0.18 0.82 -0.37 -0,31 0.54 0.34 0.63
P 0.200 0.044 0.112 0,609 0.003 0.341 0.382 0.101 0,326 0.047
T˚C R -0,54 -0.50 -0.23 -0.56 -0.45 0.35 0.54 -0,168 0.27 -0.29
P 0.101 0.139 0.508 0.086 0.186 0.307 0.105 0.641 0.450 0.405
Eh R -0.39 -0,59 -0.49 0.22 -0.77 0.29 0.25 -0,53 -0.37 -0.60
P 0.257 0.069 0.143 0.530 0.008 0.419 0,490 0,114 0.287 0.062
O2 R -0.37 -0,53 -0.34 0.29 -0,68 0.53 0.73 -0.48 -0,31 -0.44
P 0.284 0.107 0.324 0.413 0.028 0.104 0.013 0.153 0.377 0.196
EC R 0.104 0.10 -0.13 -0.34 -0.11 -0.14 0.21 -0.12 0.51 -0.10
P 0.772 0.760 0.716 0.329 0.759 0.697 0.552 0.726 0.127 0,771

 

In contrasting with the level of industrialization of the Ivory Coast (Cherniwchan, 2012), this value is close to the average values of the Cd observed in the superficial layers of the site of Port-Bouët (1.5 mgkg-1) and of Yopougon (1.4 mgkg-1) and the maximum values recorded through the sites are included between 2 mgkg-1 and 5.7 mgkg-1 (Table 2), above the critical threshold of 0.7 mgkg-1.This is a major result of this study for a consciousness denouncing a low control of sources of urban pollution as indicated earlier by Innes and Haron (2000), as a culpable behavior in countries with rapid urbanization.

In addition to the microbial conversion of the total Cd of the soil (Czaban and WróBlewska, 2005), the characteristics of the soil solution such as the pH, Eh, and the temperature and the oxygen content, may have contributed significantly to the occurrence of speciations of Cd in the soil. The residual fraction of the Cd (F5) in the soil was quasi-predominant in the Horizons, 0 – 20 cm (0.40 mgkg-1) and 20 – 40 cm (0.95 mgkg-1), on the sites studied, while the fractions related to carbonates (0.30 mgkg-1) and oxides (1.08 mgkg-1) are respectively in the Horizon 0 – 20 cm and 20 – 40 cm. The site of Yopougon offers residual fractions and exchangeable into high Cd, as well as the concentration of Cd in the root, regardless of the plants studied. A great attention should be given on the sweet potato, which is characterized by a concentration in Cd 0.94 mgkg-1 in the tuber, closely near the threshold value of 1 mgkg-1.

The difference in pH of the soil and the low capacity of secretion of organic acid of the plant in the soil (Krishnamuri et al., 1997; Mann and Ritchie, 1993) may have contributed to this increase in the concentration of the Cd, with a combination of F1, F2 and F3 characterizing the site of Yopougon, which contrast with the site of Port-Bouët, despite the total content of Cd almost similar.

In addition, the content of Cd in the root probably increases with the total content of the Cd in the soil (Table 7). Therefore, the harmfulness of the Cd on the site of Yopougon for sweet potato would be due to the total proportion of F1, F2 and F3, while F4 is representative of the site of Port-Bouët, instead of F3, with a limited potential for contamination.

 

Contrast in the pollution linked to Pb

The total content of Pb of the soil is heterogeneous on the sites, and the higher content was observed in Yopougon (40 mgkg-1), followed by that of Port-Bouët (33.6 mgkg-1), although below the threshold of 60 mgkg-1. In addition, the low concentrations similar to the Pb (10 mgL-1) are characterized by water from the water which seems to be concerned by the enrichment of the fractions related to carbonates and the fraction lithogénique when the pH and the concentration of oxygen increase. In fact, trace metals do not exist under soluble forms for a long time in the waters (Dossis and Warren, 1980): They are mainly present in the form of colloids in suspension or fixed on the organic substances and minerals.

Therefore, the contamination of the plant such as observed by the elevated (> 8 mgkg-1) of Pb, respectively, in the sheet, rod and the root on the site of Yopougon (Figure 6) was related to the sum of F1 and F4 in the topsoil layer, as well as to that of F1+F2 in the deep layer of the soil, unlike the only effects of each of the fractions taken separately, especially in the topsoil.

On the basis of the negative correlations found between the concentrations of Pb in the organs of the plant (leaf, stem, and root), and of the soil (Table 7), we attribute the reduction of concentrations of these harmful fractions of Pb, probably to the transformation of the residual fraction (F5), when the total content of Pb in the soil increases. This assertion is supported by the Figure 4, which indicates the fractions The most important of the prob as F1, F2, F4 and F5 on the sites, and the

Table 6. The Pearson correlation coefficient (R) between the concentrations of the metals from the soil and those in the leaves of Hibiscus sorrel and Sweet Potato

Guinea sorrel
  Cu (Ground) Zn (Ground) Pb (Ground) Cd (Ground)
Cu (Ground) 1.00
Zn (Ground) 0.75 1.00
Pb (Ground) -0.01 0.01 1.00
Cd (Ground) 0.07 0.05 0.27 1.00
Cu (sheet) 0.07 0.09 -0.48 -0.03
Zn (sheet) -0.36 -0.41 -0.39 -0.25
Pb (sheet) 0.61*** 0.71*** -0.29 -0.11
Cd (sheet) 0.48 0.58 0.10 -0.06
Sweet potato
  Cu (Ground) Zn (Ground) Pb (Ground) Cd (Ground)
Cu (Ground) 1.00
Zn (Ground) 0.75 1.00
Pb (Ground) -0.01 0.01 1.00
Cd (Ground) 0.07 0.05 0.27 1.00
Cu (sheet) 0.16 0.19 -0.47 -0.09
Zn (sheet) -0.41 -0.50 -0.35 -0,19
Prob (sheet) 0.63*** 0.76*** -0.17 0.06
Cd (sheet) 0.77*** 0.91*** 0.15 0.16

*** Refers to significant levels at P < 0.001, ( ): indicates the origin

 

Table 7The Pearson correlation coefficient (r) and probability (P) observed for concentrations of Pb in the leaf, stem, and root in function of the different fractions of Pb in the soil (whatever the sites and the plant)

    Pb Concentration   Cd concentrations
    Sheet Rod Root   Sheet Rod Root
F1  R 0.21 0.18 0.22 0.51 0.16 0.15
P 0.21 0.30 0.20 0.0015 0.35 0.37
F2  R 0.69 0.43 0.51 0.26 -0.21 0.46
P <0.001 .0079 0.0013 0.12 0.20 0.0044
F3  R -0.48 -0.18 -0.34 0.38 0.0004 0.23
P 0.0028 0.30 0.039 0.02 0.99 0.16
F4  R -0.28 0.045 -0,163 -0.15 -0.10 0.03
P 0.10 0.79 0.34 0.37 0.55 0.87
F5  R -0.52 -0.21 -0.38 0.08 -0,31 0.41
P 0.0012 0.22 0.02 0.63 0.05 0.01

 

carbonate fraction (F2) coupled to the residual form (F5), opposite to the characteristics of the Pb (F1 and F4) on the site of Port-Bouët, thus illustrating the sensitivity of F2 to be transformed into F5 in particular, reducing the harmfulness of Pb.

Such a transformation has been reported as the mechanism governing the migration and the fixing of the Pb, as well as its bioavailability (Bolan et al., 2003) involving the availability of phosphorus in the soil and leading to the formation of the pyromorphite (Cao et al., 2002; Scheckel and Ryan, 2004).

The content of available P in the soil can be high at Port-Bouët, compared to that of other sites, to cause the neutral pH predominant (Koné et al., 2011; 2014) and the agricultural practice requiring the contribution of the dung of poultry, may be at the origin of the increase in the content of the Pb in the soil (Amadji et al., 2013).

There is therefore need to check the harmfulness of Pb of the soil pH neutral; this differs with the case described by Cotter-Howells and Caporn, (1996) relative to the effect of phosphate mineral in the polluted soil by the Pb.

 

Attempts of the management of the pollution of the Pb and Cd

Although the content of the soil in Pb (21 mgkg-1) is moderate in Bingerville, it can be assumed a synergy of low level with the other metals (Cd, Cu and Zn) that had low levels in the soil. Similarly, the low levels of Cu (18.2 mgkg-1) and Zn (69.6 mgkg-1) in the ground, of Yopougon have probably inhibited the availability of the Pb (39.8 mgkg-1), in spite of a higher content. These findings, coupled with the contamination of crops observed at Port-Bouët, allow to outlaw the application of fertilizers sources of Zn and Cu in agroecosystems likely to induce a pollution in Pb, in periurban agriculture. Similar recommendations have already been made for Ca and S whose presence may reduce the bioavailability of Pb (Jones et al. 1973; Kabata-Pendias, 2011). In any case, these practices do not improve the quality of the soil, which will remain inadequate for the microbial activity (Brookes and McGrath, 1984), indispensable for ensuring a sustainable service of the agricultural ecosystem, in addition to the cost factor (McGrath et al. 1996). This is why, the phytoremediation of Polluted Soils is considered to be a more effective strategy for the restoration of the soil: the bioextraction of metals is estimated by the capacity to accumulate a high concentration in the plant (Adriano and al. 1999), reducing the level of pollution of the soil.

Concentrations of Pb in the plant organs of the sweet potato are generally more low in the sheet that in the stem and the root. However, at Port-Bouët where the toxicity threshold has been crossed (> 8 mgkg-1), it is the rod which has displayed the concentration (10.5 mgkg-1) the more strong, contrasting with the idea of a low mobility (translocation) of this metal in the plant. An average harvest of 10 kg/ha of the rod would result in a reduction of 105 mg Pb/ha, a significant magnitude compared to the standard of 100 mg Pb/kg of land indicated by Godin (2010), given the actual density of the soil in Africa (Barrios and al. 1996). This denotes a potential of remediation of soil by the sweet potato vis-a-vis the Pb, with a reduced risk of toxicity in the man because, what are the leaves and roots which are consumed.

This ability is strengthened among the Hibiscus, whose roots and stems, non-consumables, concentrate approximately 20 mgPbkg-1, either double the concentration in the rod of sweet potato. In addition, the leaves the supplies for the sorrel displayed a concentration in Pb to the critical limit (8 mgPbkg-1), Port-Bouët, with an overall average of 6.3 mgPbkg-1, therefore, less toxic. All this contributes to argue the use of the culture of the Hibiscus for human consumption and the Remediation of Polluted Soils in Pb by report to the sweet potato. The consumption of the root of this last being a drawback in spite of a higher total concentration in the root and the rod, then Port-Bouët, that the tubers are assumed to have a moderate concentration compared to other vegetables (Alexander et al. 2006); the genotypic differences and ecological that can contribute to this, it is appropriate to explore these aspects, subsequently, in tropical ecosystems.

The follow-up to the harmfulness of Pb in the agricultural system can be relevant for the anionic forms of Pb (Pb(OH) 3, PbCl3, Pb(CO3)22-) because of the different forms of the P (H2PO4and HPO42-) in the soil solution, excluding other forms of Pb (PbOH+ ; Pb(OH) 2, PbNO3+, PbCl+, PbCl2). Therefore, this management strategy of the pollution of the Pb, advocated by a few experts whose Melamed et al. (2003), may have a partial effectiveness.

On the other hand, the form exchangeable and bound fraction carbonate constitute the skeletal forms required for the Cd and the harmfulness of Pb in the ecology studied, although their ability to effective pollution implies more far the fraction related to oxides, for the Cd, and the organic form, for the problem with a contrast on the sites (Figures 5 and 6):

In the light of this analysis, a polluted soil, amended with the Synthetic inorganic or enriched in divalent cations (Ca2+, Mg2+, , or even Al3+, Fe3+, Si4+), may be able to attach (immobilize) both Cd2+ that Pb2+, and reduce their bioavailability, such F1 and F2, when the levels of the residual form will increase.

The properties of the bentonite may be explored at this end, referring to the successful test led by Schütz et al. (2013) for the immobilization of trace metals. In addition, the solidification of the bentonite has improved this potential, by dividing the base layers of the montmorillonite, providing more space for the adsorption of cations of cadmium and lead (Galambos et al., 2010).

 

Conclusion

The periurban agrosystems of Abidjan present real risks of pollution in Pb and Cd, particularly for the underground body, the tuber of the sweet potato and the proximity of the Continental Waters seems more favorable to this that the properties of the soil in the same conditions geo-climatic. In contrast, at least moderate levels of the soil in Cu and Zn are necessary for a high accumulation of Pb in the plant tissues regardless of the total content in the soil in which the effect would be attributable to speciations. The harmfulness was more related to the combined effect of exchangeable fractions carbonate, and those related to the oxides that can be mitigated by the increase of the organic form of the Cd as observed at Port-Bouët. Similarly, the pollution of the Pb was observed at Port-Bouët in the sheet, rod and the Root Regardless of the plant and the fractions the more harmful included also the exchangeable forms and organic matter apart from the carbonate fraction which can be transformed into the residual fraction as buffer mechanism governing the decontamination of the soil in Pb which can be limited to the anionic forms however. Therefore, the amendment with the absorbent material such as the bentonite has been suggested for the cleaning of the soil reducing the harmful effects of Cd and Pb. However, the potential of specific remediation of the plants examined was recommended to clean the periurban agrosystems around Abidjan. However, this study raises issues of ecological order genotypic and with regard to the nutrition of the plants in Pb, in particular, the interaction with the Zn and the ability of tubers in the accumulation of Pb.

 

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