Clean up of acidic leachates using fly ash barriers: Laboratory column studies

Laboratory column tests were conducted to study the efficiency of lignite fly ash barriers in removing
heavy metal ions, such as Fe, Zn, Mn, Ni, Cd, Co, Al and Cu, present in high concentrations in acidic
leachates produced in mining and waste disposal sites. The experimental configuration comprised two
40 cm long plexiglas continuous flow columns installed in series. An upward flow of synthetic acidic
solutions spiked with high concentrations of contaminants was maintained in order to simulate field
flow rates and residence times. Sampling ports fitted along the column walls allowed sampling and provided
information on geochemical mechanisms within the barrier and contaminant degradation rates
in the presence of the reactive material.
The geochemical speciation/mass transfer computer code PHREEQC and the MINTEQ database
were used for geochemical modeling of the process.
The experimental results underline the potential of lignite fly ash permeable reactive barriers to
remove almost completely, over a long period, high loads of inorganic contaminants from very acidic
leachates. The main mechanisms involved in contaminants removal are adsorption at the surface of fly
ash and the produced hydrous iron oxides, precipitation and co-precipitation. The toxicity of the reactive
material at the end of the treatment period, regarding all heavy metals studied, is limited.