Long-Term Leaching Prediction of Constituents in Coal Bottom Ash Used as a Structural Fill Material

Abstract

Coal ash is generated during a coal combustion process. In general, ash collected at the bottom of the furnace is called bottom ash whereas fly ash is dispersed ash collected by air collector. Bottom ash can be used as a public work material, and fly ash can be used as a construction material. In Korea, an increasing amount of coal ash has been used as structural fill materials. Since physical properties of bottom ash are suitable for uses in land reclamation, bottom ash is likely to be treated as an alternate material for soil. When bottom ash is used as a structural fill material, the long-term leaching potential of the Chemicals Of Potential Concerns (COPCs) may need to be evaluated, because the bottom ash used will remain in the structural fill for a long time and there is no clear correlation between the total COPCs content and the short-term leaching concentration. Therefore, this study aims to predict the long-term leaching potential of coal bottom ash used as a structural fill material. Two coal bottom ash samples collected in the current study, YD and YH-coal bottom ash had similar physical and chemical properties. Antimony, Arsenic, Copper, and Zinc were selected as the COPCs. Percolation control scenario which assumed that uncovered structural fill receives only precipitation that percolates through the filled coal bottom ash was applied to predict long-term leaching of the COPCs from the coal bottom ashes applied as structural fills. Leaching was assumed to follow a first-order kinetics and it was assumed that the material with unknown degree of saturation had the same leaching patterns as the materials saturated with water. A long-term leaching prediction was carried out applying the percolation control scenario using the input parameters determined either from an up-flow percolation test or site-specific data available for study sites. The mass of COPCs leached and accumulated on a square meter of subsoil directly below the bottom of the filled bottom ash and the COPCs concentration in the leachate from the filled bottom ash were predicted as a function of time. The coal bottom ash-specific parameters such as leaching rate (kappa), total COPCs mass leached (EL/Slab), and liquid to solid ratio (L/S) were obtained from the up-flow percolation test (i.e., column test). The total masses of cations (e.g., Cu and Zn) leached from the YD-coal bottom ash were higher than that from the YH-coal bottom ash. However, the total masses of anions (e.g., As and Sb) leached from the YD-coal bottom ash were less than or similar to that from the YH-coal bottom ash. The Kappa values were also different between the two samples. The anions leached from the YD-coal bottom ash had higher kappa values than that from the YH-coal bottom ash anions, but the kappa values for the cations leached from the YD-coal bottom ash were about two times less than that from the YH-coal bottom ash cations. For site-specific long-term leaching prediction, sites where coal ash was or will be used as filling material in Korea were selected. The study, sites include Yeosu, Namhae, Mockpo, Busan, and Gunsan. The site-specific parameters such as infiltration rate and depth of application were obtained by literature review for each selected site. The accumulation of mass leached for each site was predicted for 100-year time frame. Liquid to solid ratio of the field (i.e., L/Sfield), which can be determined by using the infiltration rate and the depth of bottom ash applied, resulted in the slower leaching rate into the subsoil. Namhae had the lowest L/Sfield

(4.76 L/kg), which resulted in the most prolonged leaching of COPCs at the site. The Namhae site also had much higher accumulated mass than other sites due to deeper application depth. When YD-coal bottom ash and YH-coal bottom ash were assumed to be filled at Namhae site, kappa value and EL/Slab affected leaching time and amount of COPCs mass accumulation, respectively. The long-term leaching concentration of COPCs was used to estimate the time to reduce the initial leaching concentration (i.e., the maximum leaching concentration

Ct,max) by 50%. Since Namhae and arsenic of YH-coal bottom ash has the lowest L/Sfield and kappa values, respectively, the longest time to reach 50% of initial leaching concentration (C(t)max) was predicted. The relative sensitivity of bottom ash samples and selected sites was examined to find significant factors influencing the long-term accumulation of the COPCs mass. Significant factor of long-term leaching was leaching kinetics for anions and amount of coal bottom ash used as structural fil for cations when sufficiently long duration (e.g. 100 years) was considered. Additionally, environmental effect of coal bottom ash used as structural fill was estimated. By comparing 1% increase of background COPCs concentration, Hazardous Concentration 5, and pH of leachate with regulating level. I100yr of YD- and YH-coal bottom ash increased COPCs native concentration. Cations leaching concentration would not much affects to aquatic organisms if considering long-term period. Consideration of environmental effect, therefore, YD- and YH-coal bottom ash sufficient to reuse as structural fill materials. This study shows that the long-term leaching prediction of coal bottom ash used as a structural fill material needs to be carried out using the bottom ash- and application site-specific variables prior to application.