Minespoil amendment with dry flue gas desulfurization by-products: Element solubility and mobility

Greenhouse column studies of 8 mo duration investigated the solubility and mobility of salts and trace elements in dry flue gas desulfurization (FGD) by-products used for minespoil reclamation. Three mine-spoils were amended with two dry FGD by-products (lime injection multistage burners ash, LIMB; and, pressurized fluidized bed combustion ash, PFBC) using amounts from 0 to 320 g kg^-1. Two of the minespoils also received sewage sludge amendment of 60 g kg^-1. Columns were planted with 'Kentucky 31' tall rescue (Festuca arundinacea Schreber). Leachate analyses and pH determinations from column mixes were done at the beginning and the conclusion of the experiments. Both FGD by-products were effective in raising pH of the spoil materials. The largest LIMB amendment raised pH to near 12 and resulted in the formation of ettringite [(Ca₆Al₂(SO₄)₃(OH)₁₂ · 26 H₂O)]. Leachate pH, electrical conductivity, dissolved organic C, Ca, Mg, and S tended to increase with increased FGD amendment, while Al, Fe, Mn, and Zn decreased. Changes in leachate As, B, Cu, Ni, and Se depended on interactions among the type of FGD, the type of spoil, and the presence of sewage sludge, with pH being the most important variable. Overall, with FGD amendments of 120 g kg^-1 or less, leachate concentrations of most elements of environmental concern were less than drinking water standards. The amount of FGD by-product that can be applied to a minespoil is probably limited by soluble salts and initially high pH levels rather than by trace element loading of spoil or water.