Status Distribution And Phytoavailability Of Heavy Metals And Metalloids In Soils Irrigated With Wastewater From Akaki River Ethiopia Implications For Environmental Management Of Heavy Metalmetalloid Affected Soils

Soils are indispensable resources that afford production of agricultural and industrial crops asrnwell as furnish manifold ecological services (as a filter, buffer, geochemical sink, andrntransformation system) and thus, safeguard the global ecosystem from the effect of pollution.rnHowever, being at the interface between the atmosphere and the earth’s crust, soils arernexposed to natural and anthropogenic inputs of heavy metals and metalloids which may entailrnpotential environmental ramifications.rnAkaki River, laden with untreated wastes from factories, commercial, public, and domesticrnutilities in Addis Ababa serves as irrigation water for vegetable farm on Eutric Fluvisol andrnPelli–Eutric Vertisol that stretch along its bank at Akaki. Besides, livestock forage on thernfallow when considerable part of the farm is inundated during high rains. Albeit the humanrnand environmental health risks associated with heavy metal/metalloid buildup in soils,rnpotential impact of wastewater irrigation has received little consideration. Consequently,rnsaving a few prefatory works, huge information gap exists regarding systematic andrncomparative study on behavior, availability, simultaneous uptake, and translocation of arrayrnof heavy metals/metalloids from Vertisol and Fluvisol irrigated with untreated wastewater.rnDespite the significance of phytoremediation techniques as low–tech and potentially cheap inrnsitu treatment alternatives to costly conventional remediation of contaminants, no prior studyrnhas been performed to screen candidate plants from mining landscape for phytoremediationrnof soils with polymetallic contamination.rnIn an attempt to address the above dearth of information, a series of studies involving greenrnhouse and field experiments on the study soils as well as complementary field surveys onrnuncontaminated soils and contaminated mine spoils were carried out in order: (1) to catch onrnthe levels of heavy metals/metalloids in uncontaminated soils; (2) to appraise the status ofrnheavy metals/metalloids and distinguish their forms of retention in contaminated soils; (3) tornassess the phytoavailability, uptake, within–plant distribution and potential risk of heavyrnmetals/metalloids in forage plants grown on contaminated Vertisol and Fluvisol underrncontrolled and actual field conditions and; (4) to assess the potential utility of plants collectedrnfrom mineral landscape for phytoremediation of heavy metal affected soils.rnThe study on the status of uncontaminated soils (Vertisol, Fluvisol, Solonetz, Andosol, andrnNitosol) established that all but Zn (>50 mg kg–1: Vertisol, Andosol, and Nitosol) had lowerrnmean heavy metal/metalloid levels than the corresponding common values for soils. In contrast, attributable largely to wastewater irrigation for the last few decades, the averagerncontents of Cr, Ni, Co, Cu, Zn, V, Hg and As of contaminated Vertisol and Fluvisol as wellrnas Pb and Se in contaminated Fluvisol surpassed the respective mean + 2sd of theirrnuncontaminated counterparts. Besides, sequential extraction of contaminated Vertisol andrnFluvisol demonstrated that considerable proportions of the total levels of most heavyrnmetals/metalloids in contaminated Vertisol and Fluvisol resided in non–residual fractions,rnand thus could be potentially mobile/phytoavailable. The green house and field studies onrnplant uptake and distribution of heavy metals/metalloids by forage plants grown onrncontaminated soils portrayed that the elements were highly phytoavailable and hencernaccumulated in the roots as well as remarkably translocated to the shoot. Thus, BCF > 1rn(bioconcentration factor: root to soil concentration ratio) BCF >1 was observed for Cu, Zn,rnCd, and Hg in grasses, while legumes had TF >1 for majority of heavy metals/metalloids.rnBesides, the mean levels of most heavy metals/metalloids of forage plants grown onrncontaminated soils were higher than the corresponding background levels for forage grassesrnand legumes. The study also demonstrated that soil type, kind of species, plant part, and theirrninteractions significantly influenced (p < 0.05) uptake, translocation, sequestration of heavyrnmetals/metalloids and thus, could govern their transfer through the food chain in the studyrnarea (and similar sites). Therefore, the data suggest that the potential environmental andrnhealth hazard could proceed from the use of fodder grasses/legumes, and cultivation ofrnvegetables in soils with polymetallic and metalloid contamination.rnConversely, native plants grown on mine spoils tolerated the edaphic stressors (pH 3.9–6.6;rnorganic carbon < 1.05%; total nitrogen < 0.12%, toxic metal levels reaching 307 mg kg–1),rnproduced considerable biomass (up to 228.4g, 178.3g, and 364.2g [dry weight] of shoot, root,rnand total, respectively) and sequestered appreciable levels of heavy metals (up to 740.45 mgrnkg–1). Overall, with sound agronomic practice, the naturally capacity of these plantsrn(extraction efficiency, EE = 176.90–25073μg, total plant levels = 211.37–28779.50μg plant–rn1) could be developed for phytoremediation of soils affected by polymetallic pollution.

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