Cr(VI) Leached from Lignite Fly Ash—Assessment of Groundwater Contamination Risk

The leaching of Cr(VI) from lignite fly ash was investigated by means of experimental standards and theoretical simulations. Two lignite fly ash samples with different calcium oxide concentration were examined. Acid extraction of fly ash and batch leaching tests were performed at various liquid (water) to solid (fly ash) ratios (L/S) for the estimation of Cr(VI) leaching rate. The mobility of Cr(VI) in soil and soil’s hydraulic conductivity at different temperatures was also examined. The results indicate that chromium in fly ash occurs mainly in its trivalent form and that Cr(VI) can be leached, but at lower degree. Along with Cr(VI), the strong reducing agent Fe(II) is also leached. A portion of the Cr(VI) is readily leached, while several hours are needed for reaching Cr(VI) equilibrium concentration in leachate. The fly ash hydraulic conductivity is greatly affected by its pozzolanic characteristics, decreasing from around 10 mm/h (2.78 × 10⁻⁶ m/s) to values lower than 1 mm/h (2.78 × 10⁻⁷ m/s) in few hours. In continuous leaching tests, relatively low Cr(VI) concentrations (< 20 μg/L) were detected, while in batch leaching tests with higher water/fly ash contact time, leachate Cr(VI) concentrations up to 700 μg/L were detected depending on the applied L/S ratio. However, for low L/S (0.5–1) ratios, no leachate was obtained. Theoretical calculations were performed in order to estimate the L/S ratios at real conditions (by taking into account the volume and the surface of an ash disposal, the density of ash, the hydraulic conductivity of ash, rainfall rate, and rainfall duration). It was found that in real conditions, high L/S can be obtained only when various specific conditions are fulfilled. Experiments with soil showed that only a small portion (approximately 4%) of the leached Cr(VI) can pass through a relatively thin (12–15 cm) layer of soil, even at low temperatures. Based on the experimental results and the groundwater and surface water quality data of a studied basin, a low contamination risk due to Cr(VI) leaching from lignite fly ash is assessed.