Immobilization of Heavy Metals from the Structural Fill Using Fly Ash

Mitra, Manik Chandra
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According to ACAA, only 43 percent of the 130.1 million tons of coal ash produced in the US in 2011 was beneficially used (ACAA 2011). Since the fly ash contains hazardous material including heavy metals, the unused fly ashes are dumped in landfill which costs a huge amount of money every year. Although fly ash could be used in many geotechnical applications, such as, structural fill, soil stabilization, fillable fill etc., but the use of fly ash is restricted by environmental regulations due to leaching of hazardous materials possesses a huge threat to the environment. If the hazardous materials, especially the heavy metals could be stabilized, a huge amount of fly ash could be used for beneficial use.

This study is focused to immobilize the heavy metals from the fly ash structural fill using relatively low cost triple super phosphate (TSP) and the results are compared with widely used metal stabilization method using chelating agent (EDTA). One Class C and one Class F fly ash were mixed with different dosage of TSP (0%, 2%, 4%, 6%, and 8%) or EDTA (0.2%, 0.4%, 1%, and 2%), and then compacted after adding 15% water to achieve a compacted structural fill. The mixtures were allowed to cure for seven days and then subjected to standard Toxicity Characteristic Leaching Procedure (TCLP). Both TCLP extraction fluids with pH 4.93 and 2.88 were used for the TCLP tests. Additionally, a water leach test (extraction fluid pH = 7) was also conducted using the same solid-liquid ratio to understand the leaching characteristics at different pH of the extraction fluid. The leachates from the leaching tests were subjected to chemical analysis to determine the concentration of metals.

For both stabilizers, the leaching of metals showed three distinctive patterns as the stabilizer dosage was increased. Firstly, leaching of some of the metals remained unchanged as the stabilizer content was increased. Secondly, leaching of some of the metals decreased as the stabilizer content was increased, and finally leaching of some of the metals decreased dramatically and went below the detection limit even after using the lowest dosage of the stabilizers. However, the effect of TSP on metal leaching was significantly different than that of EDTA for different metals.

As the pH of the extraction fluid decreased, the leaching of metals increased for most of the metals. However, the effect of the stabilizers had the similar effect.

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Civil and Environmental Engineering