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Influence of Flow Rate on Leachability

The leaching potential of coal fly ash is often approximated with laboratory-based methods that expose columns of compacted material to synthetic precipitation. While this procedure can simulate aspects of the field condition, it remains difficult to replicate site-specific thermodynamic and kinetic constraints on geochemical processes. This article explores one aspect that contributes to the field/laboratory disparity, namely, the influence of flow rate and intermittency. Seven column-based leaching experiments were conducted with the same ash but with different flow rates and intermittency (i.e., infiltration pulsing), and results were evaluated in terms of aqueous sodium, calcium, and chromium concentrations in the effluent. Flow rates ranged by three orders of magnitude, encompassing advectionand diffusion-dominated conditions as determined by Peclet number calculations. With few exceptions, the results revealed diminishing leachate concentrations with continued flushing, consistent with a declining source model. Notwithstanding differences in effluent concentration as a function of pore volume, general mass release followed similar patterns that likely reflected solubility control. Higher liquid-to-solid ratios revealed potentially nonequilibrium behavior at the highest flow rate (2400 mL/day) during intermittent flow conditions for chromium and calcium, and to a modest extent for sodium. The primary conclusion from this work is that for the constituents and ash tested, there was relatively little effect of flow rate or intermittency on leachability patterns. © 2018 The University of Kentucky Center for Applied Energy Research and the American Coal Ash Association All rights reserved. A R T I C L E I N F O Article history: Received 8 February 2018; Received in revised form 17 April 2018; Accepted 18 April 2018