Potential for Metal Leaching and Toxicity from Fly Ash Applied for Increasing Carbon Sequestration in Soil

Previous work with leaching of metals from several fly ashes (both class F and class C) indicated minimal potential of leaching of toxic levels of metals from most fly ashes tested. Also, mixing fly ash with soil and other amendments (phosphate fertilizer) significantly decreases Cr, Li, Pb, and Cd in the leachate. One concern is that the characteristics of fly ash may be changed by the addition of NOx removal equipment that potentially results in higher levels of ammonia in the fly ash. We are concerned with fly ash since our laboratory and field studies show that addition of fly ash may increase carbon sequestration in reclaimed mine soils. However, the use of such amendments must overcome public concerns about the possible release of toxic metals before the practice is generally accepted. We have recently been testing fly ash with a wide range of pH (3.7-12.4) originating from systems with SCR systems for NOx removal equipment. Three types of leaching experiments were done on these samples and soils from near a power plant. Batch and column leaching experiments were done using dilute CaCl solutions. In addition samples were leached using a nitric acid method. All three methods indicated that the potential for leaching heave metals was greatest in the highest ammonia sample tested. However, toxicity testing with the Microtox© system has indicated little potential toxicity in leachates from the fly ash sources except for the fly ash at the highest pH (12.4) and no measured toxicity in the highest ammonia sample. When the leachate from the high pH fly ash was neutralized, toxicity was eliminated. Additional data on ammonia and metal concentrations in high ammonia and high pH fly ash leachates should enable greater generalizeability of the results. 2005 World of Coal Ash (WOCA), April 11-15, 2005, Lexington, Kentucky, USA http://www.flyash.info