Status, Distribution, and Phytoavailability of Heavy Metals and Metalloids in Soils Irrigated with Wastewater from Akaki River, Ethiopia: Implications for environmental management of heavy metal/metalloid affected soils
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Date
2009-07
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Addis Ababa Universty
Abstract
Soils are indispensable resources that afford production of agricultural and industrial crops as
well as furnish manifold ecological services (as a filter, buffer, geochemical sink, and
transformation system) and thus, safeguard the global ecosystem from the effect of pollution.
However, being at the interface between the atmosphere and the earth’s crust, soils are
exposed to natural and anthropogenic inputs of heavy metals and metalloids which may entail
potential environmental ramifications.
Akaki River, laden with untreated wastes from factories, commercial, public, and domestic
utilities in Addis Ababa serves as irrigation water for vegetable farm on Eutric Fluvisol and
Pelli–Eutric Vertisol that stretch along its bank at Akaki. Besides, livestock forage on the
fallow when considerable part of the farm is inundated during high rains. Albeit the human
and environmental health risks associated with heavy metal/metalloid buildup in soils,
potential impact of wastewater irrigation has received little consideration. Consequently,
saving a few prefatory works, huge information gap exists regarding systematic and
comparative study on behavior, availability, simultaneous uptake, and translocation of array
of heavy metals/metalloids from Vertisol and Fluvisol irrigated with untreated wastewater.
Despite the significance of phytoremediation techniques as low–tech and potentially cheap in
situ treatment alternatives to costly conventional remediation of contaminants, no prior study
has been performed to screen candidate plants from mining landscape for phytoremediation
of soils with polymetallic contamination.
In an attempt to address the above dearth of information, a series of studies involving green
house and field experiments on the study soils as well as complementary field surveys on
uncontaminated soils and contaminated mine spoils were carried out in order: (1) to catch on
the levels of heavy metals/metalloids in uncontaminated soils; (2) to appraise the status of
heavy metals/metalloids and distinguish their forms of retention in contaminated soils; (3) to
assess the phytoavailability, uptake, within–plant distribution and potential risk of heavy
metals/metalloids in forage plants grown on contaminated Vertisol and Fluvisol under
controlled and actual field conditions and; (4) to assess the potential utility of plants collected
from mineral landscape for phytoremediation of heavy metal affected soils.
The study on the status of uncontaminated soils (Vertisol, Fluvisol, Solonetz, Andosol, and
Nitosol) established that all but Zn (>50 mg kg–1: Vertisol, Andosol, and Nitosol) had lower
mean heavy metal/metalloid levels than the corresponding common values for soils. In contrast, attributable largely to wastewater irrigation for the last few decades, the average
contents of Cr, Ni, Co, Cu, Zn, V, Hg and As of contaminated Vertisol and Fluvisol as well
as Pb and Se in contaminated Fluvisol surpassed the respective mean + 2sd of their
uncontaminated counterparts. Besides, sequential extraction of contaminated Vertisol and
Fluvisol demonstrated that considerable proportions of the total levels of most heavy
metals/metalloids in contaminated Vertisol and Fluvisol resided in non–residual fractions,
and thus could be potentially mobile/phytoavailable. The green house and field studies on
plant uptake and distribution of heavy metals/metalloids by forage plants grown on
contaminated soils portrayed that the elements were highly phytoavailable and hence
accumulated in the roots as well as remarkably translocated to the shoot. Thus, BCF > 1
(bioconcentration factor: root to soil concentration ratio) BCF >1 was observed for Cu, Zn,
Cd, and Hg in grasses, while legumes had TF >1 for majority of heavy metals/metalloids.
Besides, the mean levels of most heavy metals/metalloids of forage plants grown on
contaminated soils were higher than the corresponding background levels for forage grasses
and legumes. The study also demonstrated that soil type, kind of species, plant part, and their
interactions significantly influenced (p < 0.05) uptake, translocation, sequestration of heavy
metals/metalloids and thus, could govern their transfer through the food chain in the study
area (and similar sites). Therefore, the data suggest that the potential environmental and
health hazard could proceed from the use of fodder grasses/legumes, and cultivation of
vegetables in soils with polymetallic and metalloid contamination.
Conversely, native plants grown on mine spoils tolerated the edaphic stressors (pH 3.9–6.6;
organic carbon < 1.05%; total nitrogen < 0.12%, toxic metal levels reaching 307 mg kg–1),
produced considerable biomass (up to 228.4g, 178.3g, and 364.2g [dry weight] of shoot, root,
and total, respectively) and sequestered appreciable levels of heavy metals (up to 740.45 mg
kg–1). Overall, with sound agronomic practice, the naturally capacity of these plants
(extraction efficiency, EE = 176.90–25073μg, total plant levels = 211.37–28779.50μg plant–
1) could be developed for phytoremediation of soils affected by polymetallic pollution.
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Keywords
Metal/metalloid affected soils