Study of the Sediments of the Dam of Okpara (Benin): Physico-Chemical Characterization and Speciation of Iron and Manganese

doi:10.4236/jwarp.2013.57071

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Journal of Water Resource and Protection, 2013, 5, 709-714

http://dx.doi.org/10.4236/jwarp.2013.57071 Published Online July 2013 (http://www.scirp.org/journal/jwarp)

Study of the Sediments of the Dam of Okpara (Benin):

Physico-Chemical Characterization and

Speciation of Iron and Manganese

Fidele Suanon1,2*, Biaou Dimon1,3, Daouda Mama1,4, A. Lyde Tominti1,4

1Laboratoire de Chimie-Physique (LCP), Département de Chimie, Faculté des Sciences et Techniques, Cotonou, Bénin

2Laboratoire d’Hydrologie Appliquée (LHA), Département de Chimie, Faculté des Sciences et Techniques, Cotonou, Bénin

3Centre Béninois de Recherches Scientifiques et Technologiques (CBRST), Cotonou, Bénin

4Laboratoire cd chimie Inorganique et de l’Environnement (LACIE), Cotonou, Bénin

Email: *[email protected]

Received April 30, 2013; revised May 31, 2013; accepted June 24, 2013

which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ABSTRACT

Iron and Manganese contents and parameters including pH, conductivity, and organic matter contents were determined

in the sediments of the Okpara dam in Northern Benin. Fifteen samples were collected during a one-month period and

analysed in laboratory using the method of sequential extraction of Tessier. The analyses indicated that sediments con-

tained high concentrations in reducible fraction of Iron and relatively high contents of exchangeable fraction, acidhy-

drolysable fraction and residual fraction of Manganese. The findings of this study confirmed the hypotheses that the

metal contents of the sediments were relatively high and varied according to the geochemical phases.

Keywords: Sediments; Iron, Manganese; Geochemical Phases; Pollution

1. Introduction

The diagnostic study of the situation of the Okpara dam,

led by the Coordination Team of the Local Water Part-

nership of Borgou-Alibori in 2007 revealed the existence

of serious threats to the safeguarding of this river and the

dam, as well as to secure drinking water supplied to the

population of Parakou. The major threats included con-

tinuous stranding of the river, pollution by municipal

solid waste, wastewater, chemical fertilizers, erosion, in-

vasion by plants, land conflicts and deforestation.

This situation is alarming that Okpara constitutes the

single source of drinking water for the population in the

region especially for the town of Parakou.

Several studies [1-3] showed that the human and in-

dustrial activities added to the increase of populations in

African countries and particularly in Benin, generated

many environmental problems; they involved a rapid

increase of various pollutants such as heavy metals in

lagoons, rivers, lakes which also received urban waste-

water. This also applied to the river of Kpara.

Indeed, the Beninese Society of Electricity and Water

(SBEE), company in charge of the production and the

distribution of water and electricity in Benin reported

that in the early 90s, consumers of the drinking water

coming from the treatment station provided by the Ok-

para dam, started complaining about the organoleptic

quality of the water in particular a change in color: some-

times reddish, sometimes brownish. The multiple phys-

icochemical analyses carried out revealed that the phe-

nomenon was due to the presence of Manganese in the

treated water. To look deeper into the phenomenon, the

central laboratory of this company conducted several

analyses at various levels: from the surface of water to-

wards the bottom. The studies undertaken showed that

the Iron and Manganese ions were highly concentrated

down at the bottom. Elsewhere, the results of this dam’s

water characterization work carried out over four years

by Zogo D., Soclo H., Bawa M. and Gbaguidi M [4] be-

tween the time period (2006-2010) revealed that between

June and January of each year, the raw water became

particularly rich in Iron and Manganese. The enrichment

of the water in these metals was due to the depletion of

oxygen in the water since at the closure of cofferdams,

the water-atmosphere exchange surface became almost

motionless. The author, in addition, noted that, the works

of treatment of water currently in place at the station of

*Corresponding author.

F. SUANON ET AL.

710

Parakou were not designed to eliminate the high concen-

trations of Iron and Manganese in Okpara during low

water period. The metals strongly contributed to the dete-

rioration of the organoleptic quality of the water; hence

the need to get rid of them before the drinking water is

supplied for consumption.

The speciation of a chemical element corresponded to

its distribution between the various physicochemical

species presented in the environment to assess their rela-

tive importance.

Whether in soil, sediment or aquatic systems, the fate

and the bioavailability of Metal Elements Traces (ETM)

are related to their speciation. The risk posed by the ETM

in soils depends on their ability to migrate from the solid

phase to the soil solution where they can be available for

crops. The speciation of metals related to the various

fixing phases of soil thus was most widely studied.

Various approaches were tested in order to carry out this

speciation. The most usual have long been the methods

of operational and functional fractionation even if in re-

cent years the physical speciation methods have rapidly

developed.

Operational fractionation consists of the successive

use of various reagents in order to specifically extract the

metals fixed to a given compartment of soil (organic

matter, Iron oxides, manganese or aluminum, carbonates,

sulphides etc.) and to assess its stock. However, these

extractants lack of specificity [5,6] and the ETM thus are

not really characterized by the elements of the soil to

which they are linked but rather by the reagent used to

extract them. That is why we talk of operational frac-

tionation. The procedure of operational fractionation

most known was developed by Tessier, A., Campbell,

P.G.C., Bisson, M. [7]. Five phases are highlighted: ex-

changeable, oxydable, acid-soluble, reducible and resid-

ual. Since, so many other protocols of operational frac-

tionation have been developed [8].

Functional fractionation consists in characterizing the

trace metallic element (ETM) of the soil according to

their “function” in the soil. They are made with a single

extractant (dilute acids, organic complexants, salt works

solutions etc.) supposed to simulate the physicochemical

conditions of the soil. The most current example is the

determination of the availability of the ETM for crops

(bioavailability). This type of fractionation is also used to

evaluate the variation of mobility and bioavailability of

metals in a soil after a treatment such as, a contribution

of station of purification mud. But it can also be used to

assess the effectiveness of a rehabilitation of polluted

soils by traces of metals [9,10]. Recently, in a harmoni-

zation care, a diagram of extraction was proposed by the

Community Office of Reference Community Office of

Reference (BCR) [11], aiming to appreciate the fraction

of bioavailable metal. The quantity of metal extracted is

supposed to be representative of the quantities likely to

pass in the solution of the soil and thus to be potentially

bioavailable. Both types of fractionation involve reaching

thermodynamic balance what limits their effectiveness.

Indeed, it is generally accepted that balance is not often

reached under natural conditions and that the kinetics of

dissociation of complexes can be a significant parameter

[12,13]. The concentration of free metal (good indicator

of bioavailability according to the FIAM [14] in the soil

solution, as in aquatic systems, results from a dynamic

equilibrium in which the formation and dissociation of

complexes in solution take place continuously. To prop-

erly estimate the amount of bioavailable metals in solu-

tion, it is necessary to obtain information on the kinetics

of dissociation of the complex, i.e. to consider their labil-

ity/unstability. But in the case of soils especially, given

the stocks of metals in presence, it is important to take

into account the contribution of metals fixed to the solid

fixing soil compartments and transfer speed of metal

fixed to the soil solution.

Finally, from a kinetic point of view, there are at least

two stages limiting potentially the transfer of metals from

soils to crops: 1) the dissociation of the labile complexes

in the solution of the soil at the solution/roots interface

and 2) the extraction of metals from the solid phase of

the soil towards the solution of the soil.

The objective of the study is to determine within the

sediments the various possible combinations of Iron and

Manganese which cause the enrichment of the water in

these pollutants. The study aims at testing the hypotheses

that the metal contents of the sediments are relatively

high and vary according to the geochemical phases.

2. Material and Methods

2.1. Study Area

The town of Parakou is located between 09˚21'N and

02˚36'E (Figure 1) in the North-East of Benin, at 450 km

from Cotonou, the economic capital of Benin, at an av-

erage altitude of 350 m and covering an area of 441 km2

(www.villeparakou.bj). It is limited in the North by the

commune of N’Dali, in the South, the East and the West

by the commune of Tchaourou. It runs along a ridge that

rises up to 390 m altitude and separates the Ouémé basin

in the West from the one of Okpara in the East. The

study area is based on a Precambrian crystalline pene-

plain and cristallophylienne made of granite and gneiss

that can store water reserves only after deterioration [15].

The Dam of Okpara is built on the river of the same

name, which represents one of the two principal tributar-

ies of the river Ouémé of Benin. It is established in the

district of Kika at Tchaourou and is at a distance of 12.3

km to the water treatment company of SONEB in Para-

kou. The catchment area of Okpara is composed of a

F. SUANON ET AL. 711

Figure 1. Location of the studrea: Parakou, the Okpara

rystalline peneplain comprising with hard rock hills. Its

2.2. Sampling Techniques

lected based on their ac-

s (pH, conductivity)

2.3. Sequential Extraction Protocol

ummarised in

3. Results and Discussion

w the curve of evolu-

y a

dam in northern Benin in West Africa (source: CENATEL,

2003).

area is of 2070 km² and covers completely or partially

five communes of the department of Borgou namely

Tchaourou, N’Dali, Pèrèrè, Nikki and Parakou. The cli-

mate is soudanian with an alternation of rainy season

(May-October) and a dry season (November-April). The

average annual rainfall is about 1200 mm. Temperatures

vary between 18˚C (December-January) with 38˚C (March-

April). The main soil types found in the catchment area

are mainly tropical ferruginous soils, lateritic soils, sandy

clay soils and granite-gneiss soils. The dam was initiated

by the Dahoméenne Society of Kénafe (SODAK) in

1969 to meet the water needs related to the production of

Kénafe, especially for washing the fibers. It was assigned

to the Benin Electric Power Corporation (SBEE) in 1975

to feed the people of the city of Parakou in drinking wa-

ter (PNE-Benin, DG-Eau, SONEB, in December 2008),

(www.villeparakou.bj).

Five sampling stations were se

cessibility and proximity of emissions. Fifteen sediment

samples were collected in the dam Okpara on board of a

boat and with a Heckman bucket at the following depths

5cm, 10, 20, 30, 40 ... 140, 150 cm from the water sur-

face. The samples were stored, since the boat to the

laboratory in a portable cooler at 4˚C. In the laboratory

we made the removal of stones and plant debris using a

mesh of 2 mm diameter size. Wet sediments are sieved

through a sieve of 63 μm in diameter and dried in an

oven at 90˚C. The sediments have been subsequently

treated through several analyzes including determination

of heavy metals and chemical parameters measuring (pH,

conductivity, organic matter etc.).

The physico-chemical parameter

ere measured using a multi-parameter in situ of the type

Combo by HANNA. Chemical analyses were performed

in the laboratory. The analytical method for the speci-

ation of Iron and Manganese is that of Tessier et al., [7]

based on the sequential extraction of different splits (ex-

changeable, acid-soluble, reducible, oxidizable and re-

sidual). In addition, digestion of sediments was per-

formed, using a microwave oven, by mixing strong acids:

HF-HNO3-HClO4 in (4-5-1) ml [16] proportions. The

evaluation of the content of Iron and Manganese in each

split has been performed by molecular absorption spec-

troscopy.

The protocol used for the extraction is s

Figure 2.

For each studied parameter, we dra

tion of the calculated contents’ average.

Figure 2. Diagramm of sequential extraction of tessier (1979).

F. SUANON ET AL.

712

3.1. Physico-Chemical Characteristics of

Sediments

The pH of tdiments in the dam is at an average

ediments of the dam are

The conductivity on an average value of about 126.15

s at points S

The organic matter contents observed are decreasing

of depth showing that

3.1.1. The pH of the Sediments

he se

value of 6.49 indicating that the s

slightly acid (Figure 3). This characteristic reflects the

tropical ferralitic and sandy-clay nature of the dam.

3.1.2. The Conductivity of the Sediments

µS/cm (Figure 4), it rises to significant peak3

(228 µS/cm) and S10 (298 µS/cm). This increase could be

due to the fact that the dam sediments are rich in mono-

valent and divalent ions, which come from various do-

mestic and industrial wastes.

3.1.3. Or ganic Mat te r (OM)

from surface sediments to those

the studied sediments are fairly loaded with OM (Figure

5). This result can result from the influence of waste wa-

ter loaded with organic matter. These contents are proba-

bly due to significant leaching by rainwater and the re-

turn of irrigation water from agricultural land rich in or-

ganic substances that are in the vicinity of the dam.

nt pH.

Figure 3. Spatial variation of sedime

ctivity of the sedi-

ments.

kpara Dam

Irondant metal in the sediments of the

evels can reach 14365.026

Figure 4. Spatial variation of the condu

3.2. Analysis of Metals in the Sediments of the

3.2.1. Iron

is the most abun

dam of Okpara where average l

µg/g (Table 1). The enrichment in Iron is due to the re-

gional geological context of the dam. Indeed, during his

studies in 1993 the BARBE [17] showed that the geology

of the soils of the dam gives it the nature of a soil rich in

the metal Iron. The same study showed that the soil of

this region is more or less hydromorphic, the bedrock is

formed by the red sandy clay, and it is topped by a hori-

zon of clay and ferric accumulation leaching out of the

upper horizon.

Moreover, the presence of Iron in the sediments of the

station is usual, since the latter is essentially due partly to

the structure of silicates which are of the main compo-

nents of the sediments [18].

Table 1 shows the distribution of Iron in the different

geochemical phases, of the extracts of sediment of the

dam of Okpara. It shows that in the sediments of the Ok-

para, Iron is unevenly distributed in the different geo-

chemical phases of the sediments. It is linked at 65.20%,

to metal oxides or reducible fraction, what confirms that

the environment is anoxic or reducing. It is also linked to

the residual fraction at a proportion of 19.90%, this, be-

Figure 5. Spatial Variation of the Organic matter.

Table 1. Results of sequential extractions, mass and per

centag -

e of Iron.

Iron chemical forms Iron Concentration (µg/g) Percentage (%)

Exchangeable Iron (F1)198.600 1.38

Acid-soluble Iron (F2)510.874 3.55

Reducible Iron (F3) 9368.700 65.2

Oxydizable Iron (F4) 1430.201 9.95

Residual Iron (F5) 2859.65 19.90

Total Fe 14365.026 100

F. SUANON ET AL. 713

damhe physiical analysis of the

sediments showed that the daxperiencing

cant orlution vividld in sedim

[1] V. Salvad, et al., “Surveillance des Eléments Nutritifs,

Les Pesticideses Eaux, les Sédi-

ments et les Pmide,” Archives of

u (Bénin),” Review Annal des Sci-

D’eutrophi-

istry,

cauthe dam posed of crystalline

clay pae mind stable nwork

and thanese is relaresistanticles

ese content of station 1 studied is

213 mg/g (Tab le 2). This is probably due to the leaching

the dam that are rich of manganese.

Indeed, Manganese is presented in

al contamination of sediments i

percentage (%)

se the soils of are com

rticles, crystallin

t manga

erals an

ted to the

part

during oxidation. The proportion 9.95% observed about

the Iron linked to the organic matter is due to anthropo-

genic pollution originating from sewage, and municipal

waste discharge. Moreover, the Iron has a low affinity for

carbonates (3.55%).

3.2.2. Mangan es e

The average mangan

of the soils around

The average variation of manganese in the sediments is

very irregular and does not seem to be related only to

discharges from the agglomeration of Parakou. This re-

sult would be due to domestic waste, agricultural leach-

ing and other activities (mechanical garages, industry of

vehicles surface treatment with paint, oil distribution

stations, one textile industry and one concrete pipes ma-

nufacture industry). The same observations were made

by Halima B. Bouih et al. [18], during their studies on

“trace metal contamination in the sediments of Lake

Fouarat” in Morocco.

Table 2 also indicates the unequal distribution of man-

ganese in geochemical phases, of the extracts of sediment

of the dam of Okpara.

changeable form with a proportion of 32.12%. We also

observed that this paradoxically Manganese, Iron, detain

high affinity to carbonates. By cons, it has an affinity for

oxides (13.61%) comparable to that of Iron. It is also

presented in the residual fraction with a high proportion

(25.77%) and this is due to the nature of the soil of the

dam that is composed of crystalline clay particles, stable

network crystalline minerals and to the fact that Manga-

nese is related to these particles resistant to oxidation.

4. Conclusion

The results obtained in this work allowed us to make

evaluation of met

n the

Table 2. Results of sequential extractions; Mn mass and %.

Chimicals Forms Concentration

of Manganese de Mn (µg/g)

Exchangeable Mn (F) 68.601 32.121

Ac 2)

id-soluble Mn (F52.647 24.650

Reducible Mn (F3) 29.062 13.607

Oxydizable Mn (F4) 8.210 3.844

Residual Mn (F5) 55.052 25.775

Total Mn 13.573 100

of Okpara. Tco-chem

m was e

y notice

signifi-

ents. Theganic pol

concentrations of metals found in the sediments were

very high, and one could say that the retention of Okpara

was heavily polluted by trace metal elements from dif-

ferent origins. In fact, human activities, wastewater, storm

water and those leaching from agricultural lands were

among others, responsible for the heavy pollution. The

color of the water often observed at the dam could be

justified by the high concentrations of the Metal Trace

Elements studied. Besides, the fragmentation related to

land use in the catchment area of the dam for the inten-

sive cultivation of cotton and other food product with the

spraying of large quantity of chemical fertilizers were

probably the causes of the lack of balance notice in this

aquatic ecosystem.

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