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Title: 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.
Authors: Daniel, Fitamo
Advisors: Seyoum Leta(Dr.)
Mats Olsson(prof.)
Keywords: Phytoavailability
Heavy Metals
Akaki River
Copyright: Jul-2009
Date Added: 25-Jul-2012
Publisher: Addis Ababa University
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. Incontrast, 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.
Description: A Thesis Submitted to the School of Graduate Studies, Faculty of Science, Addis Ababa University in partial fulfillment of the requirements for the Degree of Doctor of Philosophy in Biology (Botanical Sciences)
URI: http://hdl.handle.net/123456789/3487
Appears in:Thesis - Biology

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