Leachability of trace elements from land-application of coal combustion by-products : intact vs. packed columns

Leaching experiments using both intact (10.2cm in diameter) packed (5 cm in diameter) soil columns were conducted to determine the ion release and weathering characteristics of a selected raw coal fly ash and also mixture of the ash and flue-gas desulfurization gypsum (FDG), both of which are coal combustion by-products (CCBP). The soil selected was the Appling clayey, kaolinitic thermic family of the Typic Hapludults from a cultivated plot. The treatments carried out were control, and surface application of 10 Mg/ha ash only and 20 Mg/ha 1:1 (wt.:wt.) ash:FDG mixture. FDG, which can act as a flocculant, was selected as a treatment to show the effect of turbidity on the movement of trace elements. Leachates totalling approximately 6 to 8 pore volumes were monitored for pH, electrical conductivity (EC) and levels of dissolved constituents. Results from both packed and intact columns for B and As leachability gave similar trends although the magnitude was much higher for the packed columns. The variability of replicates was surprisingly rather similar for the intact colimns which implied that not much spatial or structural variability existed among the columns tested. Flux or pore water velocity, which is the most important physical variable in subsurface solute transport, were different for the two column systems. The packed columns were run at constant flux but variable hydraulic head, while the intact columns were run at variable flux but constant hydraulic head. The choice was based on the manageability to conduct the experiment in the two systems. Boron levels in the leachates of treated soils were greater than those of control columns. The 20 Mg/ha ash:FDG treatment , which contain the same amount of ash as the 10 Mg/ha ash-only treatment, showed a much higher (2X) B concentration in the leachates. Sulfate from the gypsum may have enhanced B movement through the soil by inhibiting B sorption. The trend were similar for both packed and intact soil columns which indicated that flux did not have much of an influence on B movement or that the availability of B in the leachate is not kinetically dependent. The level of total As (dissolved and colloidal) present in the leachates present from the treated columns were insignificant when compared with the control. The movement of As seemed to be correlated to the turbidity of the leachates. The intact columns exhihited lower turbidity than intact columns of the same treatment due to the structural integrity of the soil cores. In summary, for the Appling soil, using small intact soil columns from cultivated plots (partially disturbed) gave rather similar results as those of packed columns.