Investigation of natural weathering processes and artificial treatment techniques in the attenuation of toxic metals from coal fly ash
Coal fly ash contains high levels of hazardous trace elements such as As, B, Cr, Mo, Ni, Se, Sr and V, which may have a negative impact on the environment due to potential leaching by acid rain and groundwater. This study seeks to develop new fly ash management techniques by determining the effects of natural weathering on trace element mobility in fly ash and by evaluating the potential use of surfactant-modified zeolite and ferrous sulfate treatment for attenuating the mobility of trace elements associated with fly ash. The effects of weathering on trace element mobility in fly ash were studied using batch competitive adsorption experiments. Fresh fly ash shows high adsorption capacity for As, V and Mo, while weathered fly ash shows high affinity for Ni, Sr, As and V. Both fresh and weathered fly ash show low adsorption capacity for Se and B. Weathering reduced the adsorption capacities of fresh fly ash for As, B, Cr, Mo, Se and V, indicating increased mobility in ash disposal environments. The effectiveness of surfactant-modified zeolite (SMZ) as a PRB material was studied using batch experiments under competitive adsorption conditions. The results showed that SMZ preferably adsorbed V, Mo and Cr over As and Se. Unmodified zeolite (UMZ) showed high adsorption capacities for Ni and Sr. Both SMZ and UMZ failed to remove B from solution. The use of SMZ as a PRB material in coal fly ash management will be limited by its low affinity for B as well as its relatively low affinity for As, Se and cations. Ferrous sulfate treatment of coal fly ash successfully reduced the mobility of oxyanionic trace elements. The unbuffered 1:30 FS treatment was highly successful; oxyanion mobility reductions were: As (24-91%), Cr (82-97%), Mo (79-100%), Se (41-87%) and V (55-100%). On the other hand, the 1:30 FS fly ash treatment failed to reduce the mobility of B, Ni and Sr. Ferrous sulfate treatment is cost effective and can be applied directly to fresh fly ash produced in electric power plants, as well as to the fly ash already placed in the ash disposal facilities.