Geochemical Modeling of Weathering Reactions in Municipal Solid Waste Incinerator Bottom Ash

Abstract

The leaching of municipal solid waste incinerator (MSWI) bottom ash has been studied at different stages of natural weathering. Bottom ash samples, originating from a single incinerator, included grate siftings, unquenched, quenched, 6-week-old, 1.5-year-old and 12-year-old bottom ash. Leaching experiments were performed at various pH levels and liquid/solid ratios. The speciation code MINTEQA2 was used to evaluate whether the leachates are in equilibrium with minerals that are expected to form in MSWI bottom ash environments. Three major stages in weathering are identified, each stage having a characteristic pH that is controlled largely by Ca minerals and pCO 2, but also by soluble Al and SO 4: (1) unweathered bottom ash, with pH > 12 (grate siftings and unquenched samples); (2) quenched/non-carbonated bottom ash, with pH 10−10.5 (freshly quenched and 6-week-old samples); and (3) carbonated bottom ash with pH 8−8.5 (1.5- and 12-year-old samples). A comparison of the leaching mechanisms identified for these stages of weathering reveals major differences for Ca, Al, Fe, Si, and SO 4; for Na, K, and Cl, on the other hand, leaching mechanisms appear largely similar. The effect of weathering on the leaching of Pb, Zn, Cu, and Mo is discussed.