Effect of Organic Carbon in MSWI Bottom Ash on the Mobilization of Heavy Metals

Bottom ash from municipal solid waste incineration (MSWI) is composed of inorganic and organic material. The organic part (TOC) may favor the mobility of pollutants after deposition, when organic carbon is mobilized as dissolved organic carbon (DOC). Therefore threshold values and methods for the determination for TOC are given in Switzerland (VVEA) and in the EU (EN 13137: CEN, 2001), respectively. Most standard methods for the Determination of TOC are based on the oxidation of carbon by heating, which includes organic (OC) and elemental carbon (EC). But only carbon that is assimilable (ROC) is transposed to its soluble form, as DOC. Dissolved organic compounds are known to form OC-metal complexes, which leads to mobilization of these metals in the leachates. Copper is known to build preferentially complexes with organic acids but the extent is pH-dependent, which is also a consequence of carbonation/aging of the BA. The presented study shows the differences in composition of bottom ash from 4 MSWI plants of different age (from 1 to 40 years) and their effect on the metal concentration in leachates from laboratory extraction experiments. As already shown in leachates from old deposits containing bottom ash compartments a general trend of DOC to heavy metal release is expected, where as the dependence on the ROC content in the bottom ash has not been investigated so far.