Levels of Potentially Toxic Elements and Polycyclic Aromatic Hydrocarbons in Sediment, Swiss Chard and Water of Akaki River, Lake Awassa and Lake Ziway, Ethiopia
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Date
2014-04-04
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Addis Ababa University
Abstract
In this work, the levels of potentially toxic elements and polycyclic aromatic hydrocarbons in
sediment, Swiss chard and water of Akaki River, Lake Awassa and Lake Ziway, Ethiopia, were
determined, employing comprehensive sample volume, effective sample pretreatment and various
state-of-the-art instrumentations. The implications of the levels of potentially toxic elements and
polycyclic aromatic hydrocarbons were also evaluated based on national and/or international
guidelines.
A total of thirty nine sediment and water samples were collected from Akaki River, Lake Awassa,
and Lake Ziway, Ethiopia while Swiss chard samples were collected from five farmlands
irrigated with Akaki River for determination of potentially toxic elements (V, Cr, Mn, Fe, Co, Ni,
Cu, Zn, As, Se, Ag, Cd, Sn, Hg and Pb) and polycyclic aromatic hydrocarbons. The potentially
toxic elements were determined using laser-induced breakdown spectrometry (LIBS), flameatomic
absorption spectrometry (F-AAS), inductively coupled plasma-optical emission
spectrometry (ICP-OES), inductively coupled plasma-mass spectrometry (ICP-MS) and
elemental mercury analyzer. The polycyclic aromatic hydrocarbons in sediment samples were
determined using gas chromatography-mass spectrometry (GC-MS) after microwave-assisted
extraction (MAE) with acetone/n-hexane (1:1, v/v).
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External calibrations were used for quantifications of potentially toxic elements and polycyclic
aromatic hydrocarbons. The precision and accuracy of the methods were verified using certified
reference materials. The results obtained were accurate and precise at the 95% confidence level
with recoveries >90% and the average relative standard deviations < 10% for most of the
quantified species.
In the present study, LIBS has been applied for the determination of levels of Cr, Mn and Fe in
sediment samples and the results were compared with those obtained using F-AAS. Fourteen
sediment samples were collected from Akaki River, Addis Ababa, Ethiopia. The LIBS results for
the elements varied from 0.060–0.707 mg/g for Cr, 0.895–3.64 mg/g for Mn and 26.9–71.8 mg/g
for Fe. The F-AAS results varied from 0.061–1.37 mg/g for Cr, 1.24–4.46 mg/g for Mn and 49.0–
83.3 mg/g for Fe. With reference to sediment quality guidelines, Akaki River is heavily polluted
by Cr.
Using elemental mercury analyzer, the concentration of mercury in sediment samples ranges
between 3.9 to 110 μg/kg for Akaki River, 12 to 67 μg/kg for Lake Awassa, and 17 to 110 μg/kg
for Lake Ziway. When compared to Lake Awassa, Lake Ziway had a higher concentration of
mercury. The average values for the areas are 24 μg/kg for Lake Awassa, 32 μg/kg for Akaki
River, and 44 μg/kg for Lake Ziway. The sources of mercury in the present study areas can be
related to natural than anthropogenic origin.
The concentration (in μg/L) of the potentially toxic elements in water were in the range of 0.992–
18.8, <0.416–83.0, 1.05–1580, <45–1760, <0.109–1.99, 0.432–12.8, 0.765–12.5, 6.00–300,
<0.937–1.96, <0.294–1.50, <0.012–0.633, 0.132–4.76, <0.289 and 0.198–7.18 for V, Cr, Mn,
Fe, Co, Ni, Cu, Zn, Se, Ag, Cd, Sn, Hg and Pb, respectively. The concentrations of all the
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quantified elements in the water, with the exception of Mn are in the range of the permissible
limits of the European Community and World Health Organization guidelines for both drinking
and irrigation water. In general, lower concentrations were observed for the known toxic
elements (such as Hg and Cd) while higher concentrations were observed for Zn, Mn and Fe.
The average concentrations of the potentially toxic elements in sediment of Akaki River follow
the pattern: As << Hg < Se < Cd < Ag < Sn < Co < Cu < Ni < V < Cr < Pb < Zn < Mn < Fe.
For Lake Awassa, the average concentrations of the potentially toxic elements in sediment
follow the pattern: As << Hg < Se < Cd < Co < Sn < Ag < Ni < V < Cu < Cr < Pb < Zn < Fe
< Mn. For Lake Ziway, the average concentrations of the potentially toxic elements in sediment
in sediment follow the pattern: As << Hg < Se < Cd < Sn < Pb < Co < Ag < Ni < Cr < V < Cu
< Zn < Fe < Mn. For both lakes similar pattern in elemental concentration is observed. For the
entire sampling areas, the minimum concentration is observed for the known toxic elements
(such as As, Hg and Cd) while the highest concentration is observed for Zn, Fe and Mn.
For the vegetable samples based on the elemental concentrations the pattern in leaf is: Ag ≈ Cd
<< Hg < Se < As < Co < Pb < V < Ni < Cr < Cu < Sn < Zn < Mn < Fe, whereas for stem: Ag
≈ Cd << Hg < Co < As < Se < Pb < V < Ni < Cr < Cu < Sn < Mn < Zn < Fe. For specific sites
the potentially toxic elements Cr, Fe, Pb, Zn and As in Swiss chard surpassed the maximum
permitted guideline values.
The assessment of sediment pollution on effect based sediment quality guidelines indicate that
some of the Akaki River sites are heavily polluted with Cu, Zn and Pb. Moreover, both lakes are
heavily polluted with Zn and even some of the sites are heavily polluted with Cu, Ni and Pb. With
respect to the ecotoxicological aspect of potentially toxic element contamination, at least
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possible contamination for biota could be observed in some of the Akaki River sites with Zn, Ag,
Cd, Hg and Pb, which could be an alarm for the responsible personnel. Both lakes have higher
concentration of Ag which suggests that the areas could be toxic to aquatic organisms with
probable effects, while for Cr, Cu, As and Hg are less than the minimum range. Based on the
bivariate correlation procedure, there is significant correlation between the two lakes at the 0.01
level (2-tailed) for the fifteen potentially toxic elements, which shows similar sources for both
lakes. In general, when looking the area wise average concentrations of potentially toxic
elements in the water and sediment samples, Lake Ziway > Akaki River > Lake Awassa.
The contents of the fourteen polycyclic aromatic hydrocarbons (dry mass) in sediments were
determined. The results obtained ranged from 0 to 3,070 ng/g, 24.9 to 410 ng/g and 15.0 to 310
ng/g for Akaki River, Lake Awassa and Lake Ziway, respectively. Source ratios indicated that the
polycyclic aromatic hydrocarbons are mainly from petrogenic sources. Sediment quality
guidelines showed that all the sediment sampling sites are of at lesser sediment toxicity risk and
should not exert adverse biological effects relative to the quantified polycyclic aromatic
hydrocarbons. Though the concentrations of some of the potentially toxic elements as well as
PAHs are within the limit proposed by national and/or international guidelines, the history of
their level and therefore the anthropogenic effects could not be determined, as there were no
reliable data in the literature for the specified sampling areas. Therefore this study can be used
as a baseline data for further studies. The qualities of water and sediments of the investigated
aquatic system are degrading. Therefore, remediation mechanisms should be implemented and
properly practiced around the rivers and lakes catchment areas.
Description
Keywords
Sediment, Swiss Chard, Water, Potentially Toxic Elements, Polycyclic Aromatic Hydrocarbons, Microwave Acid Digestion, Microwave-Assisted Extraction, Laser-Induced Breakdown, Spectrometry, Flame-Atomic Absorption Spectrometry, Inductively Coupled Plasma-Optical Emission Spectrometry, Inductively Coupled Plasma-Mass Spectrometry, RA-915+ Mercury Analyzer, Gas Chromatography-Mass Spectrometry, Akaki River, Lake Ziway, Lake Awassa, Ethiopia