Resources, Conservation and Recycling 58 (2012) 8–17 Contents lists available at SciVerse ScienceDirect Resources, Conservation and Recycling journa l h o me pag e: www.elsevier.com/locate/resconrec Leaching of trace metals from high carbon fly ash stabilized highway base layers Bora Cetin a , Ahmet H. Aydilek a,∗ , Yucel Guney b a Department of Civil and Environmental Engineering, University of Maryland, College Park, MD 20742, USA b Department of Civil Engineering, Anadolu University, Eskisehir 26480, Turkey a r t i c l e i n f o Article history: Received 1 June 2011 Received in revised form 7 October 2011 Accepted 11 October 2011 Keywords: Fly ash Stabilization Heavy metals Leaching Coal combustion byproducts Highways a b s t r a c t 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. © 2011 Elsevier B.V. All rights reserved. 1. Introduction According to American Coal Ash Association (ACAA), 45% of the electricity consumed in the United States in 2009 was supplied from coal burning power plants. As a result, over 134 million tons of coal combustion byproducts (CCBs) were produced in the United States, and nearly 47% of these CCBs are fly ashes (ACAA, 2009). As of 2009, ACAA estimates that over 38 million tons of fly ash was landfilled, and the trend is expected to increase in future years. The fly ashes produced in Eastern U.S. are generally reused in concrete production. However, the fly ashes produced by several power plants in Maryland and elsewhere occasionally contains significant amounts of unburned carbon (i.e., high loss on igni- tion) due to the increasingly common use of low nitrogen oxide (NO x ) burners in recent years. This ash has a carbon content of 12–30%, cannot be efficiently re-burnt by using current technology, and has no value as a concrete additive as the unburned carbon tends to adsorb the air entrainment admixtures that are added to the cement to prevent crack formation and propagation. These ashes are typically classified as off-spec fly ashes meaning that they do not meet the physical and chemical requirements criteria out- lined in ASTM C618. Recent data indicate that approximately 68% of this high-carbon fly ash (HCFA) is placed in landfills, thereby ∗ Corresponding author at: University of Maryland, Department of Civil and Envi- ronmental Engineering, 1163 Martin Hall, College Park, MD 20742, USA. E-mail address: aydilek@umd.edu (A.H. Aydilek). consuming valuable land space and creating the potential to impact aquatic resources in Maryland. Reuse of fly ash in highway applications is gaining importance due to its potential to solve the landfilling problems and provide high strength material. The most important problem in highway constructions is building the suitable base layer that can provide enough support to the asphalt layer. The two conventional methods to stabilize the base layer are removing the soft soil and replac- ing it with a stronger material, such as granular materials (gravel), or in situ stabilization of the soil via physical and chemical tech- niques. However, these conventional methods can be costly and time consuming, and an alternative approach such as fly ash stabi- lization of unpaved road material (URM, road surface material from an unpaved road or a road undergoing rehabilitation) and their use as a base layer for newly paved roads is gaining acceptance (Hatipoglu et al., 2008; Camargo, 2008; Cetin et al., 2010). However, one key issue that precludes URM stabilization with fly ash, including HCFA, is the potential for groundwater and other environmental impacts caused by metals in the fly ash. Fly ash con- tains trace metals that can have environmental consequences when fly ash is used in geotechnical applications. The amount and distri- bution of metals in fly ash depend mainly on the type of coal and the burning process, and the leaching potential of different metals is difficult to know a priori and leaching tests are typically neces- sary to determine this potential (McKinley et al., 2001; Bin-Shafique et al., 2006; Sauer et al., 2005; Goswami and Mahanta, 2007; Chand and Subbarao, 2007; O’Donnell, 2009). Limited information exists on the reuse of high carbon off- spec fly ash in construction of highway pavements. This is 0921-3449/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.resconrec.2011.10.004