CHEMICAL FIXATION OF METALS IN FLY ASH USING FERROUS SULFATE TREATMENT

Large volumes of fly ash and coal combustion by-products (CCBs) produced from coal-powered electric power plants located throughout the US are disposed in surface impoundments and landfills. Fly ash contains hazardous leachable trace metals such as As, B, Cr, Mo, Ni, Se, Sr and V. These impurities may have a negative impact on the environment due to potential leaching with time. For example, acidic fly ashes have been shown to have substantial amounts of leachable As and Se. It has also been observed that chromium concentrations in fly ash leachate exceed drinking water limits and is mostly in the hexavalent form (carcinogenic). Due to the fact that large amounts of fly ash are produced and stored as waste materials in landfills each year, there exists a continuing concern about the potential release of toxic elements to the environment due to leaching by acid rain, groundwater or acid mine drainage. Thus, simple and effective treatment processes are needed to stabilize metals in coal combustion by-products.

This paper reports the preliminary results of experiments designed to chemically treat fly ash to immobilize the toxic metals after disposal. Ferrous sulfate solution has been shown to successfully treat arsenic trioxide contaminated soil to promote the formation of insoluble metal-bearing phases. The current study is focused on the experimental treatment of fresh fly ash samples with ferrous sulfate solution. Fly ash samples were collected from four electric power plants: Plant Hammond, Plant Branch, Plant McDonough-Addison and Plant Daniel. These fly ash samples were treated with a ferrous sulfate solution (322 mg/L Fe) at solid:liquid ratios of 1:3 and 1:30. The effectiveness of the treatment was evaluated by batch sequential leaching experiments of treated and untreated ash using synthetic precipitation leaching procedure (SPLP) as the leachate (USEPA Method 1312). Essentially none of the fly ash trace metals were leached during the treatment process. The ferrous sulfate treatment reduced the mobility of many trace elements, especially at the 1:30 treatment ratio. The success of the treatment depends on the type of fly ash and the trace metals of interest. The higher solid:liquid treatment ratio produced a proportionately greater amount of Fe(OH)3 and decreased the mobility of all detectable trace metals relative to the untreated ash, particularly the oxyanions.