Ash weathering controls on contaminant leachability.

摘要

Experimental and theoretical investigations were conducted to evaluate the extent to which fly ash leachability was influenced by different environmental conditions and flow regimes. Samples of fresh coal combustion fly ash as well as those obtained from an existing landfill were subjected to column leaching, batch leaching and batch extraction tests. Chemical transport models including the Van Genuchten's Analytical Leaching Model, Yalcin's Leaching Model, HYDRUS 2D and MINEQL+ were used to interpret the data and highlight the influence of flow rate and chemical reactions on the leachability signatures of calcium, chromium, iron and sodium that leached from the ash. For example, column testing at a commonly used, but relatively fast flow rate of 41.7 mL/hr resulted in a leachate concentration of 170 mg/L of calcium at one pore volume. By comparison, when tested at a slower, but more field-relevant flow rate of 4.2 mL/hr, the corresponding concentration was found to be 55 mg/L. Peclet numbers, dissolution rates and uniquely applied modified mass transfer coefficients were used to identify the processes which governed the leachability signature. A flow index was developed to describe the change in leachability signature as a function of flowrate and an analysis was conducted to relate short term batch data to column data. Electron microscopy, energy-dispersive x-ray spectrometry, x-ray diffraction and thermal gravimetric analysis were used to identify the presence of secondary minerals in ash samples obtained from both fresh and weathered ash. Results confirm the presence of secondary minerals, including kaolinite, that are typically not associated with fresh ash. Concatenating the obtained data and analysis, it appears that conventionally-applied leaching tests are not sufficient for characterizing the leaching potential of coal combustion fly ash.