A study of Cr(VI) in ashes from fluidized bed combustion of municipal solid waste: leaching, secondary reactions and the applicability of some speciation methods

The use of the fluidized bed technique for the combustion of municipal soli d waste is a rather new concept. This type of combustor produces ash residu es with somewhat different properties than the residues generated from the traditional mass burn techniques. Therefore, chemical characterization and the investigation of toxic metals behavior during ash water reactions are n ecessary for the safe disposal of these residues. In the present work, the total elemental composition, mineralogy and leaching behavior of ashes from the combustion of municipal solid waste in a fluidized bed combustion boil er have been investigated. The cyclone ash and, in particular, the filter a sh contained considerable amounts of soluble substances, thus giving leacha tes with high levels of Cl-, Na+, K+, Ca2+ and Al(III). On the other hand, the two ash fractions taken in the boiler, the bottom and hopper ashes, wer e much more stable with respect to the release of salts and heavy metals. S ince Cr(VI) is mobile and toxic its release from combustion residues can po se environmental problem. Even though the total Cr contents were similar in all ashes studied, the bottom ash gave about a thousand times higher level s of Cr(VI) in test leachates than the hopper, cyclone and filter ashes. Ho wever, it was found that the leached amount of Cr(VI) from the bottom ash d ecreased significantly when bottom ash was mixed with the hopper ash. The m ost probable cause for this decrease is the coupled oxidation of AI(0) to A l(III) and reduction of dissolved Cr(VI) to Cr(III). This finding that the mixing of two ash streams from the same boiler could result in the immobili zation of Cr may point at a simple stabilization method. Selective extracti on of water soluble, exchangeable and sparingly soluble forms of Cr(VI) was also investigated. Extraction methods were evaluated for their suitability for ash matrixes. It was found that interferences due to the presence of r educing substances in some ash materials may occur. (C) 2001 Elsevier Scien ce Ltd. All rights reserved.