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TECHNICAL REPORTS

Waste Management

Mechanisms Controlling the Leaching Kinetics of Fixated Flue Gas Desulfurization (FGD) Material under Neutral and Acidic Conditions

^a Dep. of Civil and Environ. Eng. and Geo. Sci., The Ohio State Univ., 470 Hitchcock Hall, 2070 Neil Ave., Columbus, OH 43210
^b School of Environ. and Nat. Res., The Ohio State Univ., 210 Kottman Hall, 2021 Coffey Rd., Columbus, OH 43210. C.-M. Cheng, current address, Institute for Combustion Science and Environmental Technology, Western Kentucky Univ., 2413 Nashville Rd., Bowling Green, KY 42101

^* Corresponding author (walker.455{at}osu.edu)

Received for publication August 21, 2006. A number of agricultural and engineering uses for fixated flue gas desulfurization (FGD) material exist; however, the potential for leaching of hazardous elements has limited widespread application and the processes controlling the leaching of this material are poorly understood. In this study, a flow-through rotating-disk system was applied to elucidate the relative importance of bulk diffusion, pore diffusion, and surface chemical reaction in controlling the leaching of fixated FGD material under pH conditions ranging from 2.2 to 6.8. Changing the hydrodynamics in the rotating disk system did not affect the leaching kinetics at both pH 2.2 and 6.8, indicating that bulk diffusion was not the kinetic-limiting step. Application of the shrinking core model (SCM) to the data suggested a surface reaction-controlled mechanism, rather than a pore diffusion mechanism. The leaching of fixated FGD material increased with decreasing pH, suggesting it can be described by a combination of an intrinsic hydration reaction and a proton-promoted dissolution reaction. X-ray diffraction (XRD) and elemental composition analyses before and after leaching suggests that for most elements a number of solid phases controlled the leaching process.

Abbreviations: BET, Brunauer, Emmett, Teller • FA, fly ash • FC, filter cake • FGD, flue gas desulfurization • GFAAS, graphite furnace atomic absorption spectrometry • ICP–AES, inductively coupled plasma atomic emission spectrometry • ICP–MS, inductively coupled plasma mass spectrometry • SCM, shrinking core model • SEM, scanning electron microscopy • XRD, X-ray diffraction