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DOI: 10.14738/aivp.92.9434
Publication Date: 25th March, 2021
URL: http://dx.doi.org/10.14738/aivp.92.9434
Assessment of Contamination Marine Sediments From
Dakar Coast (Senegal) By Chlorine And Metallic Elements
(Cu, Zn and Cr)
Dame Cisse1, Birame Ndiaye1, Momar Ndiaye1*, Ibrahima Diagne1, Cheikh Tidiane
Dione1, Maoudo Hane1, Abdoulaye Diop1
1Faculty of Science and Technology, Laboratory of Organic Physical Chemistry and
Environmental Analysis (LCPOAE)-UCAD, Dakar, Senegal.
E-mail: momndiaye@hotmail.com
ABSTRACT
Coastal marine ecosystems are increasingly affected by human activity due to urban
and industrial discharges. The objective of this study is to assess the state of
pollution in chlorine and metals (copper, zinc and chromium) of the marine
sediments in the Dakar coast. The samples were taken, using a hand grab at depths
of 5 cm, at the three sites studied (Ngor, Hann and Soumbedioune). Chlorine, copper,
zinc and chromium were measured by photometric method. The results of the
analyses showed that the sediments of Soumbedioune and Hann are more
contaminated than those coming from Ngor area. The geo-accumulation index of
metals showed contamination in the order Cu>Cr>Zn and the comparative study
shows that the sediments of Soumbedioune are more loaded with chlorine 2 than
those of Hann and Ngor areas. These results constitute basic data for
environmentalists but also for those working in the protection of coastal areas and
marine organisms.
Key words: contamination, chlorine, heavy metals, sediments, Dakar coast.
1. INTRODUCTION
The contamination of aquatic ecosystems by chlorine and metals (copper, zinc,
chromium) remains a serious environmental problem of increasing concern [1].
This contamination of coastal ecosystems can be caused by the wastewater
discharged into these areas. These releases cause the displacement of suspended
matter, plant and animal debris. They travel and settle downstream of the canals to
form a large mass of sediment, a source of pollution. Among the chemicals released
into the environment, some such as metals accumulate in sediments [2]. Metal
contamination can be caused by diffusion into sediments. Accelerated urbanization,
industrialization and infrequent cleaning of canals are promoting the increase in
residual sludge and sediments are formed downstream of the discharge points
located along the coast of Dakar.
Coastal sediments are of diverse origins and are often reservoirs of many chemical
pollutants, particularly metals. Metal contamination can cause harm to the aquatic
ecosystem [3, 4]. Thus, it becomes important to study the impact of these pollutants
in this matrix in order to avoid ecological disasters and to contribute to an effective
and durable protection of these environments. The present study aims to assess the
level of contamination of marine sediments on the Dakar coast by chlorine and
heavy metals (copper, zinc and chromium).
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Cisse, D., Ndiaye, B., Ndiaye, M., Diagne, I., Dione, C. T., Hane, M., & Diop, A. (2021). Assessment of Contamination
Marine Sediments From Dakar Coast (Senegal) By Chlorine And Metallic Elements (Cu, Zn and Cr). European Journal
of Applied Sciences, 9(2). 107-114.
URL: http://dx.doi.org/10.14738/aivp.92.9434
2. MATERIALS AND METHODS
2.1. Sampling points
Ngor, Hann and Soumbedioune areas were chosen for the activities taking place
there (Figure 1).
Figure 1. Map of location of sediment sampling areas.
The bay of Hann (SD1) is home to fishing activities, textile factories, processing of
fishery products and receives wastewater from the East Canal. Soumbedioune beach
(SD2) is a place where fish products are unloaded and receives wastewater from the
West channel (open channel IV) which crosses a good part of the municipalities of
Dakar. The site of Ngor (SD3) is a beach which receives many visitors. It houses an
open-air canal transformed into a landfill, which crosses the entire district.
2.2. Sediment sampling and processing
Sampling of surface sediments was carried out using a small Van Veen type grab at
depths of 5 cm, using reference methods for studies of marine pollution [5]. The
sediments collected were stored in polyethylene bottles and then placed in the dark
in coolers at 4 °C [6].
In the laboratory, the sediments were placed in a freezer (-20 °C). After drying in an
oven (40 °C) for 3 hours, the sample was pre-screened for 2 mm in order to remove
the pieces of shells, branches and leaves before being sieved [7, 8]. Sediments
smaller than 63 μm were kept in plastic bottles, with a plastic tape around the
closure device to protect them from humidity, then stored in a cabinet in the dark
and cool (20 °C) for analysis [9-11].
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2.3. Sample preparation
For each sample, a quantity of dry and homogenized sediment of 0.20g is placed in
Teflon tubes and mineralized with 4 mL of concentrated nitric acid [12]. Heating is
carried out at 80 °C in the oven for one hour. After cooling to ambient air, the
solution obtained is quantitatively transferred to a 25 ml volumetric flask and the
mixture is made up with ultra-pure water up to the mark. The solution thus obtained
is used for reading with a PF-11 round cell photometer.
2.4. Analysis of samples by Photometry
The blank consists of the sample to be analysed without the standard solutions. For
each analysis, a 5 mL sample is taken, to which the reagents are added. It is very
important to respect the order and the time prescribed in the analysis protocol to
obtain a homogeneous solution.
2.5. Principle of the analysis
The Visocolor, Visocolor Eco or Nanocolor mode is chosen, as indicated in the
protocol, as is the filter number depending on the element to be dosed. These
numbers are between 1 and 6, and each corresponds to a wavelength. The
concentration zero serves as a reference for each measurement. It must be settled
before each determination. First, place the tube containing the blank in the
measuring well and press the button "zero". The photometer displays zero and then
indicates that the sample is ready for analysis. Then place the prepared sample in
the measuring well and press the M key to directly obtain its concentration in mg/L.
The relationship below allows to express the content of chlorine and metallic
elements in mg/ Kg.
C= [(A ×V) ÷ m] × F, where C represents the concentration of the substance in
mg/kg in the sample; A, that of the metal to be dosed in mg/L; V, the volume of the
solution in mL; m, the sample size in g and F, the dilution factor.
3. RESULTS AND DISCUSSION
Analysis of the fine fraction of the sediments collected between June and September
2018, reveals the presence of varying concentrations of chlorine, copper, zinc and
chromium at the different study sites. Table 1 shows the minimum, maximum and
average contents (mg/Kg) of dry weight, obtained for a series of four measurements
and their standard deviation.
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Cisse, D., Ndiaye, B., Ndiaye, M., Diagne, I., Dione, C. T., Hane, M., & Diop, A. (2021). Assessment of Contamination
Marine Sediments From Dakar Coast (Senegal) By Chlorine And Metallic Elements (Cu, Zn and Cr). European Journal
of Applied Sciences, 9(2). 107-114.
URL: http://dx.doi.org/10.14738/aivp.92.9434
Table 1. Minimum, maximum and average levels of chlorine 2 and metal elements
found in the sediments studied.
Content
of
elements
Hann (SD1) Soumbedioune
(SD2)
Ngor (SD3)
Min- Max
Mean ±
DS
Min-Max Mean ±
DS
Min-Max Mean ± DS
Cu
(mg/Kg)
0.2-
87.5
75 ± 2.5 50-112.5 90.6 ±
4.7
0.1-87.5 70.8 ± 3.6
Zn
(mg/Kg)
0.1-
37.5
37.5 ± 1.2 62.5-150 109.4 ±
6.2
37.5-75 68.8 ± 0.3
Cr
(mg/Kg)
16.2-
86.3
37.2 ± 2.8 36.3-55 45.4 ±
2.6
23.8-125 64.4 ± 2.8
Cl2
(mg/Kg)
12.5-
250
103.1 ±
4.8
100-
237.5
181.3 ±
6.7
0.1-100 75 ± 3.3
Analysis of Table 1 shows that the chlorine and metal contents vary from one site
to another. The chlorine contents, of the fine sediment fraction (< 63μm) from the
Dakar coast, range between 0.1 and 237.5 mg/Kg. With regard to metals, the zinc
contents vary between 0.1 and 150 mg/Kg while those of chromium and copper vary
between 16.2-125 and 0.1-112.5 mg/Kg, respectively. However, some levels exceed
the minimum effect threshold (28 mg/Kg) and the toxic effect threshold (86 mg/Kg)
established by Canadian legislation (Table 2).
Table 2. Canadian Standards for the Biological Effects of Metals in Benthic
Organisms: Screening Level Concentration Approach (SLCA) [13].
Metallic elements Minimal effet
(mg/kg)
Effet threshold
(mg/kg)
Copper 28 86
Chrome IV 55 100
Zinc 150 540
Figure 2 shows the chlorine and metal contents for the three sites studied on the
Dakar coast (Hann, Soumbedioune and Ngor). The analysis results show that the
content of chlorine is higher than that of metals such as copper, zinc and chromium.
Thus, the Soumbedioune’s range registers on average the highest chlorine content 2
with 181.3 mg/Kg and the Ngor’s range has the lowest value (75 mg/Kg). These high
chlorine contents can be attributed to its massive abundance in the earth's crust and
atmospheric fallout [14]. The presence of this element, in large quantities, can
constitute a danger for living beings. Thus, under a very long exposure time, chlorine
can affect the immune system, the blood, the heart and the respiratory system of
animals.
For metals, copper has the highest contents (70.8; 75 and 90.6 mg/Kg) followed by
zinc (37.5; 68.8 and 109.4 mg/Kg) and chromium records the lowest contents (37.2;
45.4; 64.4 mg/Kg). The high copper contents may indicate contamination of the
geochemical background in comparison with the content of the earth's crust
(approximately 70 mg/Kg) and that contained in the continental crust (14 mg/Kg)
[11, 15]. The average copper contents found are higher than the Threshold Effect
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Concentration (TEC) (31.6 mg/Kg) for freshwater sediments [16]. However, they
are lower than the concentration found by Cheikh Diop et al in 2012 (9.978 mg/Kg)
[17] and that found by Kikouama et al in 2013 (24.840 mg/Kg) [11].
Chromium has the lowest levels with average values between 37.2 and 64.4 mg/Kg.
The chromium contents recorded at Ngor are higher than the values found by
Cheikh Diop et al in 2012 (27.972 mg/Kg) [17]. However, the chromium contents
obtained in Soumbedioune and Hann are lower than the value found by Kikouama
et al in 2013 (51.4 mg/Kg) and the threshold value with minimal effect (55 mg/Kg)
[11].
Hann Soumbedioune Ngor
Figure 2. Levels of chlorine and metals in sediment from three areas of Dakar coast.
The copper and chromium contents found are higher than the threshold effect
producing concentrations (SEC) for marine sediments (19 mg/Kg and 52 mg/Kg,
respectively) [3]. The contents of the metals found are greater than those of the
reference values.
The sampling points for these sediments are discharge sites for the various
channels, the anthropogenic source of which can have a strong influence on the
sediment overload. Indeed, this wastewater, often loaded with organic matter, the
preferred carriers of metals, can justify this pollution [18]. These levels can
contaminate or alter the cellular functioning of benthic organisms which are often
in contact with sediments. Thus, the probability of observing harmful effects on
benthic organisms increases with the concentration of metallic elements. As the
fixation of metals is not final, the redistribution of these metals in surface waters
also disturbs the aquatic ecosystem.
To better assess the contamination of sediments, the geo-accumulation index (Igeo)
of metals is calculated. The calculation was made according to the following formula:
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Cisse, D., Ndiaye, B., Ndiaye, M., Diagne, I., Dione, C. T., Hane, M., & Diop, A. (2021). Assessment of Contamination
Marine Sediments From Dakar Coast (Senegal) By Chlorine And Metallic Elements (Cu, Zn and Cr). European Journal
of Applied Sciences, 9(2). 107-114.
URL: http://dx.doi.org/10.14738/aivp.92.9434
Igeo= log2( ��
�,����) where Cn represents the concentration measured for an element
and Bgx, the geochemical background for an element n. The results obtained are
collated in Table 3.
Table 3. Index of geoaccumulation (Igeo) of metals in sediment from the three
sites.
Igeo Hann Soumbedioune Ngor Moyenne
Zinc 0.049 0.72 0.42 0.396
Copper 1.305 1.052 1.27 1.209
Chrome 0.29 0.42 0.63 0.447
In the absence of an environmental standard, the metal contamination index was
used to compare the enrichment of a locality with regional values. Table 3 shows
that the geo-accumulation index of copper is higher than that of chromium and zinc.
According to the Muller scale (1981), sediments are valued from unpolluted to
slightly polluted (0≤Igeo <1) in chromium and zinc. These sediments are slightly
polluted to moderately polluted in copper (1≤Igeo <2) [19]. The comparative study
shows that the Soumbedioune sediments are more loaded with chlorine 2 than the
Hann and Ngor sediments (Figure 3).
Cr Cu Zn Cl2
Figure 3. Comparative study of sediments from three studied areas of the Dakar
coast
4. CONCLUSION
In this work, we evaluated the contamination of sediments from the beaches of
Hann, Soumbedioune and Ngor located at the coast of Dakar by the determination
of chlorine and metals such as copper, zinc and chromium. The analyses were
carried out by photometry.
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The measurement results show the presence of chlorine and metals (copper, zinc
and chromium). Thus, the content of chlorine is higher than that of metals such as
copper, zinc and chromium. The sediments of Soumbedioune and Hann are more
contaminated than the sediments of Ngor.
To estimate sediment pollution and make comparisons, the metal geo-accumulation
index was calculated. The results show that the sediments are more contaminated
with copper than the other metals (zinc and chromium).
The comparative study shows that the Soumbedioune sediments are more loaded
with chlorine 2 than the Hann and Ngor sediments.
The level of contamination of chlorine and heavy metal sediments in the coastal area
of Dakar highlights the urgency of reducing the discharge of pollutants into these
environments, which could avoid certain public health problems and the scarcity of
fishery resources.
The Authors Thank:
- Macherey Nagel who offered us the spectrophotometer and the standards kits.
- Mr Paul NKENG from University of Strasbourg.
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of Applied Sciences, 9(2). 107-114.
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