Cadmium and lead hazards as occurring with their speciations in periurbain agroecosystems of Abidjan in Côte d’Ivoire

Ecotoxicity and vulnerability of Trace Metals in low industrialized environment

IJTEH

Cadmium and lead hazards as occurring with their speciations in periurbain agroecosystems of Abidjan in Côte d’Ivoire

Thierry Philippe Guety1, Brahima Kone2*, Yao Kouman Nestor Kouakou1, Yves Krogba Nangah3, Derving Baka4, Nantarie Toure5, Albert Yao-Kouame6

1, 2, 5, 6

Department of Soil Science, Unit Training and Research of Earth Sciences, University FELIX Houphouet Boigny, 22 BP 582 Abidjan 22, Ivory Coast. 3 University PELEFORO Gon Coulibaly. BP 1328 Korhogo, Ivory Coast.

4 Department of Hydrogeology and Environment, Unit Training and Research of Earth Sciences, University FELIX

Houphouet Boigny, 22 BP 582 Abidjan 22, Ivory Coast.

Environment pollution hazard awareness is required for less industrialized countries which are faced with increasing periurban agriculture practice however. Lead (Pb) and Cadmium (Cd) were characterized around Abidjan city (Bingerville, Port-Bouët and Yopougon) in soil, perched ground water and vegetable crops (Hibiscus and sweet potato). Total amounts and speciations of metals were determined respectively. The sites were mainly differing with pH observed at Yopougon characterized by highest soil content of Pb (40 mg kg

-1). In contrast with the low soil

contents of metals, plant contaminations were observed in the root for Cd and Pb at Yopougon and Port-Bouët sites respectively with variance involving above and below ground organs as specific contamination of Hibiscus or sweet potato. Skeleton fractions as exchangeable (F1) and carbonate bound (F2) were characterizing these contaminations although additional fraction as oxide bound (F3) Cd and organic (F4) Pb were required respectively for effectiveness. The non-polluted perched groundwater pH, Eh, temperature and O2 concentration were likely concerned by these fractions availability beside that of residual fraction (F5) of Cd. Enhance isomorphic substitution of anionic Pb forms transforming F2 into F5 and the cationic substitutions between Cd and Pb were suggested for pollution management.

Keywords: Isomorphic substitution, pollutant skeleton fractions, pollution awareness, peri-urban agriculture, Côte d’Ivoire INTRODUCTION Environment pollution as water, soil and plant contaminations is a threat mainly depending on man’s impact including urbanization and associated agriculture (peri-urban agriculture) as well as industrialization so far, has been developed the concept of anthroposphere or technosphere (Kabata-Pendias, 2011). Among pollutants (e.g. H2SO3, H2SO4, HF, HNO, HNO3, Zn, Cu, Pb, Fe, Co, Cd and Ni), metallic trace elements accumulations in plant, soil and water have been reported (Kurnia, 1999; Douay et al., 2001; Coulibaly et al., 2008; Soro et al., 2009; Godin, 2010) and Pb and Cd were likely the most

potential harmfulness contaminant in peri-urban agricultural production (Sposito, 2010; Guety et al., 2015). *Corresponding author: Brahima Kone, Department of Soil Science, Unit Training and Research of Earth Sciences, University FELIX Houphouet Boigny, 22 BP 582 Abidjan 22, Ivory Coast. Email: [email protected], phone: +225 06546189; fax: +225 22314567

International Journal of Toxicology and Environmental Health Vol. 1(1), pp. 002-014, September, 2015. © www.premierpublishers.org. ISSN: 2167-0449

Research Article


Guety et al. 002 In fact, Cd and Pb are considered as most ecotoxic metals that exhibit adverse effects on all biological processes of humans, animals, and plants characterized by a great adverse potential to affect the environment and the quality of food (Kabata-Pendias, 2011). Results of several studies (Gadde and Laitien, 1974; Forbes et al., 1976; Lamyet al., 1993) on Cd fixation by soil organic matter (SOM) and Fe/Mn hydroxides lead to some generalizations: (1) in all soil, Cd activity is strongly affected by pH, (2) in acid soils, the SOM and sesquioxides may largely control labile pool of Cd, (3) in alkaline soil, precipitation of Cd compounds is likely accounting for Cd equilibrium. Similarly, the stability of Pb (PbOH

+ ;Pb(OH)2;Pb(OH)3

-, PbNO3

+, PbCl

+, PbCl2 ,

PbCl3- and organic forms) is relevant to soil pH among

other physicochemical properties (Nriaguet al, 1978; Hem, 1985). Of course, the harmfulness characterizing both metals is depending on their mobility which is varying with their speciations in a given ecological conditions (Kabata-Pendias and Sadurski, 2004). There are many investigations about metal speciations in developed countries (Shoberet al., 2007; Medved et al., 2008; Mpunduet al., 2013) in contrast with pollutants characterization studies elsewhere, especially in Africa whereas increasing urbanization and industrialization coupled with environment pollution are observed. Hence, such knowledge is required for raising public awareness of pollution and saving environment quality as well as public health in urban zones. Therefore, the actual study was initiated in peri-urban agriculture areas of Abidjan (economic capital) in Côte d’Ivoire to explore soil, water and plant contaminations by Cd and Pb in relation with the characterization of the speciations respectively. MATERIAL AND METHODS Description of studied sites Abidjan is the economic capital of Côte d’Ivoire (West Africa), on the shoreline of the Guinea Golf within altitudes 5˚00 - 5˚30 N and longitudes 3˚50 - 4˚10 W. It accounts for 10 districts and the major industrial activities of the country. It is characterized by subequatorial climate with annual average rainfall amount fluctuating between 1637 mm and 2048 mm irregularly distributed in time and space scales as bimodal rainfall in pattern (Two rainy seasons alternating with two dry seasons). Annual averages air temperature and hygroscopic measurement are recorded between 24˚C - 30˚C and 75% - 88% respectively according to Guety et al. (2015). The soil is sand clayed Ferralsols somewhere Acrisols developed on tertiary and quaternary sand deposits. Actual survey was conducted in vegetable cropping areas of Port-Bouët (5˚25N - 3˚94W) and Yopougon (5˚35N - 4˚04 W)

characterized by higher industrial and commercial activities contrasting with Bingerville (5˚31 N - 3˚87 W) as control site with lower activities (figure 1).

Figure 1. Studied zones and sampling site localizations

Plants studied 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). Soil, plant and water sampling Multi-sites (Bingerville, Yopougon and Port-Bouët) survey was conducted in 2013 in the district of Abidjan (5°18 N; 4°00 W; 10 asl) around the localities of Port-Bouët and Yopougon characterized by higher industrial and commercial activity intensities while prevailing agricultural activity accounts for Bingerville. In 600 m

2 of vegetables cultivated area in each of these

locations, 12 soil composite samples were randomly taken in 0 – 20 cm and 20 – 40 cm depth using hand augur respectively. Furthermore, a soil profile (120 cm depth) was also sampled for each of the studied area. Soil sampling was coupled with that of plant selecting one plant between 4 soil samples’ positions. About 5 g of matured plant organs as fresh leaf, stem and root were taken from hibiscus and sweet potato respectively. Soil and plant samples were kept in plastic package and transported in icebox for laboratory analysis.


Int. J. Toxicol. Environ. Health 003 Groundwater was also collected from well for this purpose. Laboratory analysis Soil samples were dried in room condition before being ground and sieved (2mm). Soil pH were determined using glass electrode in 1/2.5 ratio of soil/solution (water) respectively. Soil electric conductivity (EC) measurement was coupled with that of soil pH.Soil chemical extraction of metal (Cd and Pb) was also done using a ground soil sample (0.3 g) of 150 μm in size and aqua regia [3 mL de HNO3 (65%; v/v) + 1 mL de HCl (37%; v/v)] as described by Baize et al. (2005) and Laurent (2003). Three measurements (Atomic Absorption Spectrometry) were done for each analysis and the average was reported and compared to the standard values (Table 1) defined by Barneaud (2006) and Godin (2010). Table 1. Maximum standard concentration of Cd and Pb in soil, plant and water

Metal maximum standard concentration

Cd Pb

Sol (mg kg-1

) 0.7 60

Plant (mg kg-1

) 1 8

Water (µm L-1

) 5 10

The chemical fractionations of metals were assessed following sequential extraction procedure based on coupled methods of Tessier et al. (1979) and Ure (1990). It was carried out progressively on initial weight of 1.0 g of homogenized material using the following extractions steps: The step 1 was processed with 0.5 M magnesium chloride adjusted to pH 7.0 with 10% ammonia solution in room temperature of about 30˚C for determination of metal exchangeable fraction (F1) from floated solution. Then, the step 2 was applied to the residue adding 1 M Sodium acetate adjusted to pH 5.0 with Acetic acid for the determination of carbonate bounded fraction (F2) concentration in subsequent floated solution. Similarly, bounded fractions to Fe-Mn oxide (step 3) and organic matter (step 4) were determined using 0.04 M Hydroxylamine hydrochloride in 25% Acetic acid (F3) and 30% Hydrogen peroxide in 0.02 M Nitric acid (F4) respectively. Then, residual/lithogenic fraction (F5) was determined in step 5 using Acidic reaction (HNO3). Plant material was washed in tap water to remove adhered soil particles and subsequently shredded, oven dried (60˚C), ground (1mm) for the use of 0.5 g of sample. Plant samples were digested in 6 ml of each of H202 and HNO3 during 3 hours at constant temperature of

95˚C in a Digi PREP. Flame atomic absorption spectrometry was use for the determination of plant concentrations of Cd and Pb. Pre-cleaned polyethylene sampling bottles were immersed about 10cm below the water surface as encountered in the well used for crop irrigation. About 0.5L of the water samples were taken at each sampling site. Samples were acidified with 10% HNO3, placed in an ice bath and brought to the laboratory. The samples were filtered through a 0.45μm micropore membrane filter and kept at 4°C until analysis. The samples were analyzed directly using inductively coupled plasma optical emission spectrometry (ICP-OES) fixing 228.8 nm and 220.4 nm as wave lengths for Cd and Pb respectively. Statistical analysis By descriptive analysis, the mean, maximum and minimum values of soil of Cd and Pb as well as pH were determined for a given site in 0 – 20 cm soil depth. Similarly, mean values of Cd and Pb concentrations in leave, stem and root were demined according to studied sites. By analyze of variance (ANOVA), mean values of perched ground water properties (T˚C, EC, Eh, pH, O2, Pb and Cd) were also determined running the test of Student-Newman-Keul. ANOVA was also done for determination of soil contents of different fractions in Cd and Pb in topsoil (0 – 40 cm) and subsoil (40 – 120 cm) for a given site. Pearson correlation was done to establish the relation soil total content of Cd and Pb considering their concentrations in plant root, stem and leaf. Similarly, both metal concentrations of different speciations (F1, F2, F3, F4 and F5) were used for Pearson correlation with plant organs equivalent concentrations likewise for perched groundwater. Statistical package of SAS (version 9) was used for these analysis considering α = 0.05. Studied sites (Bingerville (1), Port-Bouët (2) and Yopougon (3)) were discriminated by Cd and Pbspeciations (F1, F2, F3, F4 and F5) using SPSS 16 package. RESULTS Soil and groundwater characteristics in studied sites There is clearness of variation in soil acidity across the studied sites (Table 2) of which, Yopougon site is outstanding with highest mean value of pH (8.6) while, the others (Bingerville and Port-Bouët) are characterized by values ranging in acidic to neutral pH. However, soil content of Cd is often missing (0 mg kg

-1)

indifferently to sites and highest mean values account for Port-Bouët (1.5 mg kg

-1) and Yopougon (1.4 mg kg

-1)

sites contrasting with the lowest value recorded in the soil of Bingerville (0.56 mg kg

-1) as the control site. In turn,


Guety et al. 004

Table 2. Ranges of soil (0 – 20 cm) contents of totals Cd and Pb as well as the pH according to studied sites

Site

Bingerville Port-Bouët Yopougon

Cd (mg kg-1

) Maximum 2.0 4.9 5.7 Mean 0.56 1.5 1.4 Minimum 0 0 0

Pb (mg kg-1

) Maximum 62.7 56.1 57.2 Mean 21.0 33.6 40.0 Minimum 11.6 12.6 20.7

pH Maximum 5.8 7.1 8.7 Mean 5.6 7.0 8.6 Minimum 5.1 5.6 7.6

P (mg kg-1


CEC (cmol kg-1


Ca (cmol kg-1


Clay (%) Maximum 5 2 12 Mean 3 2 10.80 Minimum 2 1.5 10.65

MO (g kg-1


Number of sample = 24

there is always Pb content in soil wherever with minimum values ranging between 11.6 mg kg

-1 and 20.7 mg kg

-1

for a maximum reaching 62.7 mg kg-1

especially at Bingerville site which is also characterized by lowest mean value however. Mean value of soil content of Pb is almost twice higher for Yopougon site as compared to that of Bingerville which is 1/3 lower than the value observed for Port-Bouët site. No significant difference accounts for sites referring to ground water concentration of Pb and EC, however. In turn, significant lowest values of temperature (27˚3C), oxygen concentration (2.03 mg L

-1), redox potential (-7.03

mV) are determined for the ground water at Yopougon entirely contrasting with Bingerville site (control site). The ground water at Port-Bouët site is particularly characterized by highest value of temperature (30˚C). Except for Yopougon, no concentration of Cd is determined in groundwater. Site specific metal fractions in soils Figures 2 and 3 are showing the discrimination of studied sites in the basis of soil profile (0 – 120 cm) contents of Cd (variance: F1= 99, 6 %; F2= 0, 4 %) and Pb (variance: F1= 87, 7 %; F2= 12, 3 %) speciations as site characteristics respectively.

There is no influence of lithogenic fraction of Cd as site characteristic while Bingerville and Yopougon sites are diametrically opposed according to Cd speciations with exchangeable (F1), Ca-bound (F2) and oxide bound (F3) fractions positively characterizing Yopougon site (Figure 2). Furthermore, there is no influence of oxide-bound fraction in Port-Bouët site characteristics as observed for Cd while organic fraction accounts for negatively although with limited influence for all the studied sites. Roughly, similar picture is observed for sites characterization by Pb speciations in soils (Figure 3) with no influence of oxide bound fraction (F3). In addition, no significant influence of lithogenic fraction (F5) is observed like for Ca-bounded form (F2) of Pb. These bounded fractions were positively characterizing Bingerville site in opposition with Port-Bouët site more relative to the exchangeable and organic fractions of Pb as characteristic. By analysis of soil total contentment of the most significant Pb and Cd speciations (F1, F2, F4 and F5) according to the functions 1 of Figures 2 and 3 respectively, soil contents of Cd and Pb as total of F1 and F2 fractions are showing opposite scenarios in site difference according to soil depths (Table 5): significant difference is observed for Cd (F1+F2) in topsoil while no significant difference accounts for Pb (F1+F2) entirely


Int. J. Toxicol. Environ. Health 005

Figure 2. Sites (1: Port-Bouët; 2: Bingerville; 3: Yopougon) discrimination according to cadmium speciations

Figure 3. Sites (1: Port-Bouët; 2: Bingerville; 3: Yopougon) discrimination according to Lead speciations

contrasting with the measurement in the subsoil. Moreover, highest content of a given metal is associated to lowest content of the second (e.g. highest topsoil content of Cd is associated to the lowest amount of Pb in subsoil at Yopougon site). Highest content of Pb (8.42 mg kg

-1) is observed for Port-Bouët as total of F1+F4

fractions also outstanding with 1.85 mg kg-1

as F1+F2 fractions of Pb. No significant difference between sites is observed for total content of F2 + F5 fractions indifferently to studied metal. Topsoil contents of Pb and Cd as total of F1+F2 fractions (Figures 4 and 5) are often greater than that of the subsoil and the amounts of both metals are contrasting according to the sites. Highest amount of Pb accounts for Bingerville as control site, however (Figure 5).

However, no significant difference is observed between the mean values of topsoil contents of Pb speciations according sites respectively (Table 6) but, soil content of Pb-F2 (CaO-bound) at Bingerville site is twice greater (2.10 mg kg

-1) than that of Port-Bouët site (1.42 mg kg

-1)

which is also threes greater compared with that of Pb-F2 (0.4 mg kg

-1) at Yopougon. In turn, topsoil content of Pb-

F5 (residual fraction) at Bingerville site is twice and threes greater than that of Port-Bouët (4.53 mg kg

-1) and

Bingerville (3.66 mg kg-1

). In contrast, significant differences are observed between the mean values of subsoil contents of Pb-F1 and Pb-F2 according to sites. Highest values of both Pbs peciations are observed at Port-Bouët while lowest accounts for Yopougon site studiously. This trend is contrasting with


Guety et al. 006

Figure 4: Mean value of Cd as total of F1+F2 fraction in soil depths according to sites

Figure 5. Mean value of Cd as total of F1+F2 fraction in soil depths according to sites

that of Cd speciation’s, which are significantly highest for F1 and F5 in both top and subsoil at Yopougon beside similar observation in the topsoil for F2. Of these results (Table 6), no significant difference accounts for Pb speciation contents in topsoil between the sites studied as much as for Cd-F1, Cd-F2 and Cd-F5 while such of difference is characterizing the subsoil content of F1 for Pb and Cd respectively. Metal concentrations in plant and groundwater Figures 6 and 7 are showing the concentrations of Cd (Figure 6) and Pb (Figure 7) in plant organs (leaf, stem and root) according to studied sites respectively. Less than 6 mg kg

-1 as Cd concentrations account for

above ground organs (leaf and stem) indifferently to sites

except for Port-Bouët which is characterized by leaf concentration of Cd recorded over 0.7 mg kg

-1 and about

0.65 mg kg-1

for the stem (Figure 6). In turn, greater amount of Cd concentrations are observed in the root especially at Yopougon site where the concentration is brushing 0.85 mg kg

-1. However, Cd concentration is

likely decreasing when from the leaf to the root throughout stem at Port-Bouët site while studious increasing is observed at Bingerville site. Lowest concentration of Cd is noticed for the stem collected at Yopougon. The concentration of Pb is also increasing from above ground biomass (leaf and tem) to root at Yopougon site (Figure 7) remaining lower than 6 mg kg

-1 and contrasting

with the irregular trend observed at Port-Bouët in highest range of 8.5 mg kg

-1 to 10 mg kg

-1.


Int. J. Toxicol. Environ. Health 007

Figure 6: Cadmium (Cd) concentrations in crop leaf, stem and root in studied areas (Barres are representing standard deviation)

Figure 7. Lead (Pd) concentrations in crop leaf, stem and root in studied areas (Barres are representing standard deviation)

Moderate range (6 – 8 mg kg

-1) of fluctuation is

characterizing Pb concentrations in plant organs at Bingerville. Roughly, Port-Bouët site is outstanding with highest concentration of Pb indifferently to the plant organs. This trend is consistent with that of Pb concentrations in sweet potato and Hibiscus across sites (Table 7). But, highest accumulation of Pb (10.5 mg kg

-1) accounts for

stem of sweet potato at Port-Bouët as observed for stem and/or root of Hibiscus as genotype difference of Pb mobilization in plant according to environments (sites). Lowest values of Pb concentrations are characterizing

Yopougon site indifferently to plant and their organs respectively. In turn, highest concentrations of Cd are observed in there for roots of sweet potato and Hibiscus. Moreover, there is higher concentration of Cd in the root of sweet potato at Bingerville site (control site) when compared with that of Port-Bouët while reverse scenario accounts for Hibiscus. Overall, there is significant negative correlation between Pb concentration in the root and total soil contents of Cd (-0.47) and Pb (-0.48) respectively, all top medium (Table 8). But, these correlations are contrasting with the positive correlations observed for Cd concentration in the


Guety et al. 008

Table 3. mean values of soil contents of totals Cd and Pb as well as the pH according to studied sites

a and b are indicating mean values with significant difference in column for α=0.05

Table 4. Characteristics of perched groundwater as determined in the studied sites

T˚C EC (ms cm

-1)

O2 (mg L

-1)

Eh (mV)

pH Pb (mg L

-1)

Cd (mg L

-1)

Bingerville 28.7ab 0.08a 5.90a 72.90a 5.7c 0.12a ---- Port-Bouët 30.8a 0.65a 4.90ab 23.52b 6.5b 0.44a ---- Yopougon 27.3b 0.45a 2.03b -7.03c 7.2a 0.31a 0.03

GM P> F

29.4 0.01

0.46 0.101

4.44 0.06

28.22 0.001

6.5 0.0001

0.33 0.44

---- ----

a and b are indicating mean values with significant difference; ----: Missing data

Table 5. Mean values of topsoil and subsoil contents of combined F1+F2, F1+F4 as Cd and F1+F2,

F1+F4 and F2+F5 as Pb in Topsoil and Subsoil according to studied sites

Topsoil

Site Cd-F1+F2 (mg kg

-1)

Pb-F1+F2 (mg kg

-1)

Cd-F1+F4 (mg kg

-1)

Pb-F1+F4 (mg kg

-1)

Pb-F2+F5 (mg kg

-1)

Bingerville

0,06a 1,11b 0,09ab 3,90a 4,32a

Port-Bouët

0,21a 1,85a 0,03b 7,50a 11,67a

Yopougon

0,58a 0,29c 0,24a 4,62a 23,36a

P> F Mean (mg kg

-1)

0.04 0.30

0,02 0,93

0,04 0,14

0,32 4,90

0,47 13,40

Subsoil

Site Cd-F1+F2 (mg kg

-1)

Pb-F1+F2 (mg kg

-1)

Cd-F1+F4 (mg kg

-1)

Pb-F1+F4 (mg kg

-1)

Pb-F2+F5 (mg kg

-1)

Bingerville

0,11c 2,70a 0,12b 4,73b 5,77a

Port-Bouët

0,28b 2,83a 0,16b 8,42a 5,95a

Yopougon

1,90a 0,58a 1,01a 6,96ab 9,82a

P> F Mean (mg kg

-1)

0,001 0,54

0,39 2,33

0,01 0,31

0,03 6,65

0,10 6,65

a, b and c are indicating mean values with significant difference in column for α=0.05

root. Meanwhile, consistent negative correlations are observed for Cd and Pb concentrations in the stem with limited significant for Pb concentration however, especially for soil content of Pb. No significant correlation is observed for Cd concentration in leaf as referring to soil contents of Cd and Pb while leaf accumulation of Pb can decrease with the increasing of both metal contents

in soil according to the negative correlation significantly observed respectively. However, soil content of Cd can be released as F2 and F5 fractions with increasing of groundwater pH according to the significant positive correlations observed respectively while, negative correlations with these Cd fractions account for groundwater redox potential (Eh).

Cd (mg kg

-1) Pb (mg kg

-1) pH

Bingerville 0.65b 15.25c 5.6c Port-Bouët 0.5b 29.97b 6.8b Yopougon 2.33a 49.15a 8.6a

P> F 0.0001 0.0001 0.0001


Int. J. Toxicol. Environ. Health 009 Table 6. Mean values of Pb and Cd speciations (F1, F2, F3, F4 and F5) in soils according to studied sites (Bingerville, Port-Bouët and Yopougon)

Content of Pb

Topsoil Subsoil

F1 F2 F3 F4 F5 F1 F2 F3 F4 F5

Bingerville

0.60a 2.10a 4.18a 4.13a 3.66a Bingerville 0.55a 0.56b 4.34a 3.34a 3.76a

Port-Bouët

1.41a 1.42a 6.69a 7.01a 4.53a Port-Bouët 0.65a 1.20a 6.91a 6.85a 10.47a

Yopougon

0.18a 0.40a 3.89a 6.78a 9.42a Yopougon 0.08b 0.21b 12.77a 4.54a 23.15a

P> F 0.46 0.14 0.07 0.08 0.07 0.02 0.02 0.26 0.34 0.46

Content of Cd

Topsoil Subsoil

F1 F2 F3 F4 F5 F1 F2 F3 F4 F5

Bingerville

0.02c 0.09b 0.20a 0.10a 0.12b Bingerville 0.03b 0.03a 0.22a 0.06a 0.14b

Port-Bouët

0.12b 0.15b 0.29a 0.03a 0.26b Port-Bouët 0.01b 0.20a 0.79a 0.02a 0.73ab

Yopougon

0.89a 1.01a 0.21a 0.12a 1.27a Yopougon 0.18a 0.40a 2.09a 0.05a 1.88a

P> F 0.001 0.001 0.15 0.43 0.03 0.03 0.05 0.54 0.06 0.03

Table 7: Mean values of Pb and Cd concentrations in leaf, stem and root of sweet potato and Hibiscus according to sites respectively

Pb concentration (mg kg-1

)

Sweet potato Hibiscus

Bingerville Port-Bouët Yopougon Bingerville Port-Bouët Yopougon

Leaf 6.17cB 9.72bA 2.92bC 6.91bB 8.81bA 3.10cC Stem 6.41bB 10.50aA 5.01aC 6.17bB 9.42aA 5.52bC Root 7.80aB 9.60bA 5.81aC 8.42aB 9.71aA 6.12aC

P> F <0.001 <0.001 <0.001 <0.001 <0.001 <0.001

Cd concentration (mg kg-1

)

Leaf 0.52cB 0.81aA 0.57bC 0.46cC 0.64aA 0.48Bb Stem 0.64bB 0.72bA 0.41cC 0.53bB 0.60aA 0.47bC Root 0.72aB 0.65cC 0.94aA 0.61aC 0.66aB 0.77aA

P> F <0.001 <0.001 <0.001 <0.001 <0.001 <0.001

a, b and c are indicating mean values with significant difference in column for α=0.05; A, B and C are indicating mean values with significant difference in line for α=0.05


Guety et al. 010

Table 8. Pearson correlation coefficient (R) and probability (P) observed for soil total contents of Cd and Pb according to corresponding concentrations in crop leave, stem and root indifferently to sites

Soil [Cd] Soil [Pb]

Leaf [Pb] R -0.47 -0.48 P 0.003 0.002 Stem [Pb] R -0.29 -0.15 P 0.076 0.366 Root [Pb] R -0.34 -0.37 P 0.041 0.023 Leaf [Cd] R -0.18 0.08 P 0.272 0.623 Stem [Cd] R -0.41 -0.44 P 0.011 0.006 Root [Cd] R 0.41 0.56 P 0.013 0.0003

Table 9. Pearson correlation coefficient (R) and probability (P) of Cd and Pb fractions (F1, F2, F3, F4 and F5)

according to perched ground water properties

Cd fraction

F1 F2 F3 F4 F5

pH R 0.44 0.64 0.52 -0.18 0.82 P 0.200 0.044 0.112 0.609 0.003 T˚C R -0.54 -0.50 -0.23 -0.56 -0.45 P 0.101 0.139 0.508 0.086 0.186 Eh R -0.39 -0.59 -0.49 0.22 -0.77 P 0.257 0.069 0.143 0.530 0.008 O2 R -0.37 -0.53 -0.34 0.29 -0.68 P 0.284 0.107 0.324 0.413 0.028 EC R 0.104 0.10 -0.13 -0.34 -0.11 P 0.772 0.760 0.716 0.329 0.759

Pb fraction

F1 F2 F3 F4 F5

pH R -0.37 -0.31 0.54 0.34 0.63 P 0.341 0.382 0.101 0.326 0.047 T˚C R 0.35 0.54 -0.168 0.27 -0.29 P 0.307 0.105 0.641 0.450 0.405 Eh R 0.29 0.25 -0.53 -0.37 -0.60 P 0.419 0.490 0.114 0.287 0.062 O2 R 0.53 0.73 -0.48 -0.31 -0.44 P 0.104 0.013 0.153 0.377 0.196 EC R -0.14 0.21 -0.12 0.51 -0.10 P 0.697 0.552 0.726 0.127 0.771

The increase of oxygen amount in groundwater can induce the reduction of soil content of Cd-F5 likewise for Cd-F4 as illustrated by the correlation values observed for groundwater temperature (Table 9). Except for residual fraction of Pb (F5) which has contrasting correlations with groundwater pH (0.63) and Eh (-0.60) respectively beside the high positive correlation (0.73) between Ca-bound fraction (F2) and groundwater concentration of O2, there is limited relations between soil content of Pb and the studied characteristics of groundwater (Table 9). Furthermore, no influence of these characteristics are noticed for F1 (exchangeable) and F3 (oxide-bound) fractions indifferently to studied metals and no relationship is observed for EC whatever the metal and relevant fractions.

DISCUSSION Cadmium pollution hazards According to Heinrichs et al. (1980),worldwide average concentration of cadmium in the lithosphere is 0.098 mg kg

-1 and mean values in soils of most industrialized

countries (e.g. United States of America) are depending to soil types and the sites ranging below 1.5 mg kg

-1

however (Burau et al., 1973; Lund et aI., 1981; Logan and Miller, 1983 ; Holmgren et al., 1993). In contrast with the industrialization level of Côte d’Ivoire (Cherniwchan, 2012), this value is close to the mean values of Cd observed in the top soils at Port-Bouët (1.5 mg kg

-1) and

Yopougon (1.4 mg kg-1

) sites and maximum values


Int. J. Toxicol. Environ. Health 011 recorded across sites were ranging between 2 mg kg

-1

and 5.7 mg kg-1

(Table 2) over the threshold level of 0.7 mg Cd kg

-1 (Alloway, 1990) This is a major output of the

current study as an awareness pointing out the poor control of urbanization source of pollution early gusted by Innes and Haron (2000) as a guilty behavior in rapid industrialization regions. Beside the microbial transformation of soil total Cd (Czaban and Wróblewska, 2005), soil solution characteristics as pH, Eh, temperature and concentration of O2, may have significantly contributed to Cd speciations occurrence in soil: Residual fraction of Cd (F5) in soil was roughly predominant in both topsoil (0.40 mg kg

-1) and subsoil (0.95 mg kg

-1) across the studied

sites while CaO3-bound (0.30 mg kg-1

) and oxide-bound (1.08 mg kg

-1) fractions did so in topsoil and subsoil

respectively. Yopougon site was outstanding with highest residual and exchangeable fractions of Cd (Table 5). Likewise, for Cd concentration in root indifferently to studied plants (Figure 6, Table 6). Caution should be paid to sweet potato, which was characterized by 0.94 mg kg

-1

of Cd in tuber closely to the critical level of 1 mg kg-1

(Godin, 2010). Difference in soil pH and crop ability of low weight organic acid secretion in soil (Krishnamuri et al., 1997; Mann and Ritchie, 1993) may have contributed to this when referring to Cd speciations in soils as the combination of F1, F2 and F3 characterizing Yopougon site contrasting with Port-Bouët site in spite of almost similar total amount of Cd. Moreover, the concentration of Cd in the plant root was likely increasing with that of soil content of total Cd (Table 7). Therefore, harmfulness of Cd in Yopougon site for sweet potato was due to the total proportion of F1, F2 and F3 while F4 accounted for Port-Bouët site instead of F3 with limited potential of contamination. These analyze reveal potential interactions between Cd speciations as synergism or antagonism mechanisms as topic for further study in order to deepen knowledge. Contrast in lead pollution hazards Yet the peri-urban agriculture production of Abidjan was deemed contaminated by Pb (Guety et al., 2015), the soil contents of total Pb were heterogeneous across sites and highest amount was observed at Yopougon (40 mg kg

-1)

followed by that of Port-Bouët (33.6 mg kg-1

) though below the threshold level of 60 mg kg

-1 (Alloway, 1990).

Moreover, similar low concentrations of Pb (< 10 mg L-1

) (Fageria et al, 2010) were characterizing the groundwater which was likely concerned by carbonate and residual bound fractions enrichment in soil when increasing its pH and O2 concentration (Table 9). Indeed, trace metals do not exist in soluble forms for a long time in waters (Dossis and Warren, 1980): they are present mainly as suspended colloids or fixed by organic and mineral substances. Consequently, the contamination of plant as observed according to the high concentrations (> 8 mg

kg-1

) of Pb in leaf, stem and root respectively at Yopougon site (Figure 7) was likely relevant to the total amount of F1 and F4 in topsoil as well as that of F1+F2 in subsoil (Table 5) instead of single effects apart, especially in topsoil (Table 5). Base on the negative correlations observed between Pb concentrations in plant organs (leaf, stem and root) and that of soil respectively (Table 9), we assume the reduction of the concentrations of these harmful fractions of Pb, probably by transformation to residual fraction (F5) when soil total content of Pb increased. This assertion is supported by Figure 2 pointing out the most important fractions of Pb as F1, F2, F4 and F5 across sites and carbonate bound fraction (F2) coupled with the residual form (F5) were opposed to Port-Bouët site characteristics of Pb (F1 and F4) illustrating most sensitivity of F2 to be transform into F5 hence, reducing Pb harmfulness. Such transformation was reported as buffer mechanism controlling the migration and fixation of Pb as well as the bioavailability (Bolan et al., 2003) involving phosphorus availability in the soil, and resulting in pyromorphite formation (Cao et al., 2002; Scheckel and Ryan, 2004). Of course, available soil P may be higher in Port-Bouët compared to that of the other sites because of the prevailing neutral pH (Koné et al., 2011; 2014). Thought agricultural practice involving the application of chicken manure may be concerned by increasing of both P and Pb amounts in soil as specially in yopougon (Amadji et al., 2013). Hence, there is opportunity of Pb self-control of harmfulness in soil neutral pH condition somewhat differing with the case described by Cotter-Howells and Caporn, (1996) relative to mineral phosphate effect in Pb polluted soil. Endeavors of Pb and Cd pollution managements The self-control of Pb harmfulness in agricultural system may be relevant to anionic forms of Pb (e.g. Pb(OH)3

-,

PbCl3-, Pb(CO3)2

2-) because of P forms (H2PO4

– and

HPO42 –

) in soil solution hence, excluding the other forms of Pb (e.g. PbOH

+ ; Pb(OH)2;, PbNO3

+, PbCl

+, PbCl2).

Therefore, this strategy of Pb pollution management advocated by some experts including Melamed et al. (2003) may have partial efficacy. In turn, exchangeable form and carbonate bound fraction are constituting the skeleton forms required for Cd and Pb harmfulness in the studied ecology even though their effective pollution ability further involve oxide bound fraction for Cd and organic form for Pb respectively with contrast across sites (Figures 2, 3, 4 and 5): Increasing of skeleton fractions of Cd (F1+F2) amount in soil was associated with the reduction of that of Pb across sites (Figures 4 and 5) likely consecutive to isomorphic substitution between Cd

2+ and Pb

2+. In the light of this

analysis, polluted soil amendment with inorganic or synthetic material enriched in bivalent cations (e.g. Ca

2+,

Mg2+

even Al3+

, Fe3+

, Si4+

) may have potential to fixed (immobilize) both Cd

2+ and Pb

2+, hence reducing their


Guety et al. 012 bioavailability as F1 and F2 when, the residual form amounts will increase. Bentonite properties can be explored for this purpose referring to the successful test conducted by Schütz et al. (2013) for trace metals immobilization. Furthermore, the nitrification of bentonite has improved this potential by dividing the basic montmorillonite layers, providing more space for adsorption of cadmium cations and leads (Galamboš et al., 2010). Success of such attempt strategy can be expected in the studied ecology as spontaneously occurring according to figures 2 and 3: Negative influence of Cd oxide bound fraction was observed for the characteristics of Port-Bouët site saved from Cd pollution in spite of the amount of soil total Cd (1.5 mg kg

-1) somewhat greater than

elsewhere. Almost similar scenario was observed for Yopougon site with the turnover of Pb residual fraction consecutively to carbonate bound fraction transformation resulting limited pollution of soil Pb in spite of the greatest amount of soil Pb (40 mg kg

-1).

However, change in trace metal forms can be observed over time, including the release of more bioavailable fractions as observed for soil Cd according to soil pH (Mann and Ritchie, 1993). Because this eventuality, one might explore phytoremediation options which may be effective for the studied crops when considering Cd and Pb concentrations in non edible organs: Highest concentrations of Cd were observed for the stem and leaf of sweet potato attesting safety of the tuber for consumption and the removal of 1.58 mg Cd kg

-1 from the

soil at Port-Bouët. In the same manner, higher concentrations Pb were observed in the root and stem of Hibiscus at Yopougon and Port-Bouët as remediation aptitude of this species characterized by edible leaf. CONCLUSION There is stray impulse of Cd pollution at Yopougon site, especially for below ground organ of tuber crop as sweet potato and the harmfulness was most related to combined effect of exchangeable, carbonate bound and oxide bound fractions. In turn, Pb pollution was observed at Port-Bouët in leaf, stem and root indifferently to crop and the most harmful fractions included also the exchangeable and organic forms beside carbonate bound fraction which may be transform into residual bound fraction as buffer mechanism of soil Pb decontamination as self-control which may be limited to anionic forms however. Specific remediation potential of studied crops was recommended for sustaining peri-urban agro systems around Abidjan city. ACKNOWLEDGEMENTS We are particularly grateful to Dr Brahima Koné as

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Accepted 10 September, 2015. Citation: Guety TP, Kone B, Kouakou YKN, Nangah YK, Baka D, Toure N, Yao-Kouame A (2015). Cadmium and lead hazards as occurring with their speciations in periurbain agroecosystems of Abidjan in Côte d’Ivoire. International Journal of Toxicology and Environmental Health, 1(1): 002-014.

Copyright: © 2015 Guety et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are cited.

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Cadmium and lead hazards as occurring with their speciations in periurbain agroecosystems of Abidjan in Côte d’Ivoire