The effect of the heavy metal immobilization agents on the strength of fly ash treated soils and the long-term stability of the agents

In United States, a large amount of electricity is produced by the burning of coal in power plants produce an immense amount of ash (bottom and fly ash) each year. Fly ash contains hazardous or toxic materials such as heavy metals. If without appropriate treatments, before dumping fly ash in land-fills, heavy metals may leach from the fly ash and contaminate the ground water. This study describes the laboratory evaluation of the effects of the heavy metal immobilization agent on the strength of fly ash treated soil and the long term stability of the immobilization agent. Ethylenediaminetetraacetic acid (EDTA) and Triple Super Phosphate (TSP) were used as immobilization agents and mixed at different ratios, ranging from 0 % to 6 %, with fly ash to check the effect on geotechnical properties of fly ash and long-term resistance to heavy metals found in fly ash. Two types of fly ash, Class C and Class F were mixed with different dosage of TSP (0 % to 6 %) and EDTA (0 % to 1 %) at optimum moisture contents (OMC) to achieve a compacted structural fills. The geotechnical properties of the samples were tested by means of unconfined compressive strength (ASTM-D2166/D2166M-13) at curing periods of 7, 14 and 28 days and for the long-term stability of the agents were tested by the means of standard test method for leaching solid material in a column apparatus (ASTM-D4874-95). For fly ash Class C, adding either EDTA or TSP decreased the fly ash's compressive strength. At 28 days, the average peak compressive strength of the cured fly ash specimens was 1549 kPa. EDTA lowered the compressive strength to 958 from 1549 kPa, while TSP lowered the compressive strength to 950 from 1549 kPa. Fly ash only gives a max compressive strength because of its cementious nature. For fly ash Class F, EDTA did not noticeably affect on the compressive strength of the fly ash, but TSP increased the strength. When TSP was added to fly ash, it showed increase from 58 (fly ash only) to 161 kPa. To immobilize the heavy metal from fly ash, it is recommended to use TSP as chelating agent to achieve the maximum compressive strength. TSP showing the same compressive strength compared to EDTA at 28 days curing time and TSP is very cheap compared to EDTA. For long-term stability of the immobilization agent used with fly ash Class C, it was founded that TSP worked well for almost all of the heavy metals except As. EDTA also showed good immobilization to heavy metals except Hg and Pb. For fly ash Class F, TSP showed better results for the stabilization of Hg and Pb and worst towards As. Addition of EDTA enhanced the stabilization of Cd, Cr, Zn, and As. EDTA gave good response to some metals and TSP gave well to others. The combination of EDTA and TSP showed excellent results towards immobilization of the metals.