Leaching of Metals from High Carbon Fly Ash Mixed Soils

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Fly ash produced by power plants in the United States occasionally contains significant amounts of unburned carbon due to the use of low nitrogen-oxide and sulphur-oxide burners in recent years. This ash cannot be reused in concrete production due to its reactivity with air entrainment admixtures and is largely placed in landfills. Highway structures have high potential for large volume use of high carbon fly ashes (HCFAs). However, in such applications, even though mechanical properties of the fly ash-amended highway base layers and embankments are deemed satisfactory, one key issue that precludes highway embankments built with fly ash is the potential for groundwater impacts caused by metals in the fly ash.

This study was conducted to evaluate the leaching potential of metals from high carbon fly ash stabilized highway base layers and high carbon fly ash amended highway embankment structures. Three different laboratory tests: (1) batch water leach tests, (2) toxicity characteristics leaching procedure tests, (3) column leach test and two different numerical modeling analyses: (1) WiscLEACH, (2) MINTEQA2, were carried out. Analysis were conducted on eight fly ashes and two locally available sandy soil materials that are mainly used in highway base layer and highway embankment structures.

Laboratory test results indicated that an increase in fly ash content in the soil fly ash mixtures yielded an increase in leached metal concentrations except Zn metal. The pHs had significant and different impacts on the leaching of metals. The leaching of Cr, Zn, Al, As and Se were increasing with pH while leaching of Ba, B, Cu, Fe, Mn, Sb, V were decreasing.

Numerical model WiscLEACH was used to simulate the leaching behavior of leached metals from HCFA stabilized highway base layers and amended highway embankment structures. WiscLEACH predicted field metal concentrations were significantly lower than the metal concentrations obtained in laboratory leaching tests, and field concentrations decreased with time and distance due to dispersion in soil vadose zone. Numerical model MINTEQA2 predicted that leaching of metals were solubility controlled except As, Se and Sb metals. Speciation analyses indicated that leached metals were present at their less or non-toxic forms.