Leaching of trace metals from high carbon fly ash stabilized highway base layers

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. Roadways have high potential for large volume use of high carbon fly ash (HCFA). HCFA can be activated with lime kiln dust (LKD) and used as a base layer for newly paved roads. However, in such applications, the leaching of heavy metals from fly ash-stabilized base layers can be of concern. A series of batch water leach and column leach tests were conducted to investigate the leaching potential of six metals, Al, Cr, Fe, Mn, Sb and V, from the fly ash-lime kiln dust (LKD) stabilized soils. The results indicate that an increase in LKD amount, pH, and fly ash content have significant effects on leaching behavior of heavy metals from soil–fly ash–LKD mixtures. All six metals, except Al, exhibited a first flush pattern in column leach tests, and the measured concentrations quickly decreased to below EPA maximum concentration limits (MCLs) for drinking waters.

► The metal concentrations generally increase with increasing fly ash content in water leach tests (WLTs)
► The addition of lime had different effects on the column leach test (CLT) leaching of the six metals analyzed.
► The release of all metals from the soil mixtures in CLTs exhibited a first-flush pattern followed by a decrease in concentrations.
► The CLT concentrations of metals can be conservatively estimated from WLTs by using correlation factors.