Environ. Sci. Technol. 2006, 40, 4481-4487
Effect of Accelerated Aging of MSWI Bottom Ash on the Leaching Mechanisms of Copper and Molybdenum JORIS J. DIJKSTRA,† A N D R EÄ V A N Z O M E R E N , † JOHANNES C. L. MEEUSSEN,† AND R O B N . J . C O M A N S * ,†,‡ Energy Research Centre of The Netherlands (ECN), P.O. Box 1, 1755 ZG Petten, The Netherlands, and Department of Soil Quality, Wageningen University, P.O. Box 8005, 6700 EC Wageningen, The Netherlands
The effect of accelerated aging of Municipal Solid Waste Incinerator (MSWI) bottom ash on the leaching of Cu and Mo was studied using a “multisurface” modeling approach, based on surface complexation to iron/aluminum (hydr)oxides, mineral dissolution/precipitation, and metal complexation by humic substances. A novel experimental method allowed us to identify that the solid/liquid partitioning of fulvic acids (FA) quantitatively explains the observed beneficial effect of accelerated aging on the leaching of Cu. Our results suggest that iron/aluminum (hydr)oxides are the major reactive surfaces that retain fulvic acid in the bottom ash matrix, of which the aluminum (hydr)oxides were found to increase after aging. A new modeling approach, based on the surface complexation of FA on iron/aluminum (hydr)oxides is developed to describe the pH-dependent leaching of FA from MSWI bottom ash. Accelerated aging results in enhanced adsorption of FA to (neoformed) iron/aluminum (hydr)oxides, leading to a significant decrease in the leaching of FA and associated Cu. Accelerated aging was also found to reduce the leaching of Mo, which is also attributed to enhanced adsorption to (neoformed) iron/aluminum (hydr)oxides. These findings provide important new insights that may help to improve accelerated aging technology.
Introduction Municipal Solid Waste Incinerator (MSWI) bottom ash, the major waste stream originating from the incineration of municipal solid waste, is considerably enriched in potentially toxic trace elements compared to its parent material (1). The material, when either reused or disposed, can be qualified as a potential risk to the environment, depending on the availability of the contaminants for leaching. MSWI bottom ash is reused as a construction material in many countries, but its application is often restricted by regulatory limits based on its leaching potential. In The Netherlands, the leaching of particularly Cu and Mo are critical with respect to environmental regulations (e.g., Dutch Building Materials decree (2)) * Corresponding author: phone: +31-224-564218; fax: +31-224568163; e-mail:
[email protected]. † Energy Research Centre of The Netherlands (ECN). ‡ Department of Soil Quality, Wageningen University. 10.1021/es052214s CCC: $33.50 Published on Web 06/10/2006
2006 American Chemical Society
MSWI bottom ash, being a high temperature product, is thermodynamically unstable under atmospheric conditions and is, therefore, subject to similar weathering processes as observed in natural equivalents, such as volcanic ashes (e.g., ref 3). The uptake of atmospheric CO2 is the primary weathering reaction of MSWI bottom ash and generally leads to a decreased leachability of a number of critical metal contaminants (e.g, refs 4-6). Accelerating the natural weathering process of MSWI bottom ash by optimizing parameters such as CO2 pressure and humidity have been demonstrated to be a potentially promising technology, both with respect to improved leaching properties as well as to reduce CO2 emissions from industrial sources (e.g., refs 6, 7, and references therein). Therefore, an improved understanding of the processes and parameters that control the leaching of contaminants before and after accelerated aging may contribute to further development of this technology. Recently, natural humic substances (humic (HA) and fulvic acid (FA)) have been identified in MSWI bottom ash and their leaching has been shown to constitute the key process responsible for the facilitated leaching of copper and possibly other metals (8). At present, no knowledge exists on the processes controlling the leaching of humic substances from MSWI bottom ash, and their relation with contaminant leaching. The objective of this paper is to provide a mechanistic insight into the effects of accelerated aging on the leaching of Cu and Mo as typical and important metal and oxyanionic contaminants in MSWI bottom ash. We focus on the potentially important factors that influence the leaching of these contaminants, particularly pH-control, the contents of reactive surfaces in the bottom ash matrix and the quantities and composition of dissolved organic carbon (DOC). Given the key role of DOC in facilitating the leaching of Cu and possibly other metals, DOC is characterized in terms of humic, fulvic, and hydrophilic acids over a wide pH-range, to identify the processes controlling the solid/liquid partitioning of these reactive organic ligands. With an independently determined set of analytical parameters, we attempt to predict the leaching of Cu and Mo over a wide pH-range, both before and after accelerated aging, using a “multisurface” modeling approach (i.e., adsorption processes to (hydr)oxide surfaces and humic substances). Model results are used to gain further insight into the major chemical factors that control the leaching of contaminants such as Cu and Mo in MSWI bottom ash treated by accelerated aging. A novel approach is developed to model the pH-dependent leaching of fulvic acid from MSWI bottom ash, being the major process controlling the leaching of Cu and possibly other metals.
Experimental Section MSWI Bottom Ash Samples. A batch of about 900 tons (by weight) of freshly quenched MSWI bottom ash was treated by accelerated aging early 2004. Before the treatment, the ashes were sieved to a particle size of
