Contaminant Mobility in Soil Columns Amended with Fly Ash and Flue Gas Desulfurization Gypsum

Abstract

The mobility of contaminants such as boron (B) and arsenic (As) present in coal combustion by-products, such as fly ash (FA) and flue-gas desulfurization gypsum (FDG), may limit the use of these materials as soil amendments. Column studies were conducted to determine the sorption and leaching characteristics of soluble constituents from an alkaline FA and 1:1 mixtures of FA + FDG that were surface applied to highly weathered, coarse-textured soils. Preliminary batch sorption studies indicated that As, as either As(III) or As(V), was sorbed to a greater degree than B. Both elements were sorbed to a greater degree by the finer-textured B_thorizon materials, presumably due to the higher Fe oxide and clay content. In column studies, FA or combinations of FA + FDG were surface applied at rates of 10 and 20 Mg ha^-1 to 10 cm diameter, 30 cm deep intact soil columns. Column leachates totaling 8 L (≈11 pore volumes) were monitored for levels of B, As, Ca, Mg, K and Na. Following leaching, columns were sectioned at 5cm intervals and the pH and EC of the soil, as well as the vertical distribution of As and B, were determined. Effluent B levels for the treated soils were greater than those of the control column, but remained below phytotoxic levels. Boron was readily mobile from the soil columns, a majority of which leached from the columns within the first 3 liters, suggesting that B toxicities observed in greenhouse pot studies may be less likely under field conditions with sufficient drainage. Column effluents for combined treatments (FA + FDG) were much higher in B, indicative of sulfate (SO ^2-₄ ) competition for sorption sites.The As levels present in the leachates from FA and FDG columns were generally low (<10µg L^-1) when compared to the control column and roughly correlated with effluent turbidity. Combined treatments (FA + FDG) enhanced Mg and K leaching due to the added competition of Ca for cation exchange sites. Levels of residual As were only slightly higher in the FA-amended columns, but no clear trend in residual B was observed due to its mobility.