title: Potential for leaching of heavy metals in open-burning bottom ash and soil from a non-engineered solid waste landfill.
abstract: Bottom ash from open-burning of municipal waste practised in developing countries poses a risk of heavy metal leaching into groundwater. Compared to incineration ash, there is limited information on heavy metal leaching from open-burning ash and soil from non-engineered landfills. Batch and column experiments were conducted to address three specific objectives; (1) to determine aqua regia extractable concentrations of heavy metals in fresh ash, old ash and soil from beneath the landfill, (2) to determine the relationship between heavy metal leaching, initial and final pH of leaching solution, and aqua regia extractable concentrations, and (3) to determine the breakthrough curves of heavy metals in ashes and soil. Aqua regia extractable concentrations of Cd, Zn, Mn, Cu, Ni and Pb were significantly higher (p < 0.05) in fresh and old ashes than soil beneath landfill and uncontaminated soil (control). Increasing initial solution pH from 5 and 7 to 9 significantly reduced the mobility of Pb, Zn and Cu but not Cd whose mobility peaked at pH 7 and 9. Concentrations of desorbed heavy metals were not correlated with aqua regia extractable concentrations. Final pH of leachate rebounded to close to original pH of the material, suggesting a putative high buffering capacity for all materials. Both batch and column leaching showed that concentrations of leached heavy metals were disproportionately lower (<5%) than aqua regia extractable concentrations in most cases. The retardation of heavy metals was further evidenced by sigmoidal breakthrough curves. Heavy metal retention was attributed to precipitation, pH-dependent adsorption and formation of insoluble organo-metallic complexes at near-neutral to alkaline pH. Overall, the risk of heavy metal leaching from ash and soil from the waste dump into groundwater was low. The high pH and the presence of Zn, Fe, Mn and Cu make ash an ideal low-cost liming material and source of micronutrients particularly on acidic soils prevalent in sub-Saharan Africa.
authors.affiliation: Biosystems and Environmental Engineering Research Group, Department of Soil Science and Agricultural Engineering, University of Zimbabwe, P.O. Box MP167, Mount Pleasant, Harare, Zimbabwe. Electronic address: wgwenzi@yahoo.co.uk.
authors.forename: Willis
authors.initials: W
authors.lastname: Gwenzi
authors.affiliation: Biosystems and Environmental Engineering Research Group, Department of Soil Science and Agricultural Engineering, University of Zimbabwe, P.O. Box MP167, Mount Pleasant, Harare, Zimbabwe.
authors.forename: Dorcas
authors.initials: D
authors.lastname: Gora
authors.affiliation: Department of Polymer Science & Engineering, Harare Institute of Technology, PO Box BE 277, Belvedere, Harare, Zimbabwe.
authors.forename: Nhamo
authors.initials: N
authors.lastname: Chaukura
authors.affiliation: Marondera College of Agricultural Science and Technology, University of Zimbabwe, Private Bag 35, Marondera, Zimbabwe.
authors.forename: Tonny
authors.initials: T
authors.lastname: Tauro
mesh_terms: D006851:Hydrochloric Acid / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D017745:Incineration; D019216:Metals, Heavy / Q000032:analysis* / Q000737:chemistry; D017942:Nitric Acid / Q000737:chemistry; D012989:Soil Pollutants / Q000032:analysis* / Q000737:chemistry; D062611:Solid Waste; D064091:Waste Disposal Facilities
chemical_list: D019216:Metals, Heavy; D012989:Soil Pollutants; D062611:Solid Waste; D017942:Nitric Acid; D006851:Hydrochloric Acid; C022102:aqua regia
keywords: Breakthrough curves; Cd; Cu; Mn; Ni; Pb; Zn; pH