Short-term natural weathering of MSWI bottom ash

The release of heavy metals from MSWI bottom ash has been the key concern i n the management of this material. The leaching distribution values obtaine d from 100 freshly quenched bottom ash samples, according to the German DIN 38414-S4 procedure test, showed the release of lead, zinc and copper to be the main hazards associated with bottom ash utilisation as a secondary bui lding material. Currently, natural weathering of MSWI bottom ash, for an es timated period of 1-3 months, is the most economic treatment available to e nsure the eventual utilisation of this material. The leaching of natural we athered bottom ash in the short-term (up to 9 months) was studied. The most significant changes in the bottom ash were found to occur in the first 90 days. At pH values greater than 12, lead, zinc and copper were the main hea vy metals to be released from the MSWI freshly quenched bottom ash samples studied. Natural weathering for a period of about 90 days reduced the leach ing of heavy metals, stabilising the bottom ash pH to minimise the solubili ty of metal hydroxides, and enabled the residue to be used as secondary bui lding material. The profile of the pH neutralisation curve is similar to th at described by carbonates, which would suggest that the reaction is contro lled by CO2. The formation of insoluble oxides as well as carbonates contro l the immobilisation of certain heavy metals, e.g. lead and zinc. The leach ing of aluminium increases during this short natural weathering stage due t o elemental metal oxidation, Aluminium solubility is controlled by the prec ipitation of gibbsite or other aluminium-sulphate neoformations. The latter may contribute to the immobilisation of heavy metals. (C) 2000 Elsevier Sc ience B.V. All rights reserved.