Environ. Sci. Technol. 1998, 32, 1398-1403
Speciation of Cr in Leachates of a MSWI Bottom Ash Landfill M I C H A E L K E R S T E N , * ,† BURKHARD SCHULZ-DOBRICK,† THOMAS LICHTENSTEIGER,‡ AND C. ANNETTE JOHNSON‡ Geosciences Institute, Johannes Gutenberg-University, 55099 Mainz, Germany, and Swiss Federal Institute for Environmental Science and Technology (EAWAG S+E), 8600 Du ¨ bendorf, Switzerland
Cr concentrations and speciation were determined in leachate from a municipal solid waste incinerator bottom ash landfill both experimentally and by thermodynamic model calculations. Total dissolved Cr concentrations of 0.2 mmol L-1 were determined by GFAAS. Two orders of magnitude lower values were determined upon preconcentration by an in-situ solid-phase extraction technique based on the 8-HQ cation exchanger that is specific for Cr(III) but unspecific for Cr(VI). This suggests that chromate dominates the dissolved Cr concentrations in the leachates but was up to 5 orders of magnitude undersaturated with respect to the solubility of CaCrO4 or BaCrO4. Chromate adsorption by oxyhydroxides is less efficient in the highly alkaline environment, but coprecipitation and solid-solution formation with BaSO4 can explain the low chromate concentrations in the leachates. This model assumption was verified by EPMA/WDX measurement of Cr in secondary barite precipitates found in aged bottom ash. Scavenging by this secondary weathering product in landfilled MSWI ash can thus cause an efficient immobilization of the toxic chromate.
Introduction The environmental impact of municipal solid waste incineration (MSWI) has increasingly become the subject of public debate. The main goal of incineration is to develop a sustainable waste management by reducing the volume of nonavoidable and nonrecyclable municipal waste to be disposed and to reduce its postdepositional reactivity due to its organic matter inventory. While energy utilization is increasingly being discussed as merely a secondary effect, the extensive reduction and controllability of potential longterm emissions are the primary reason for the increasing role of MSWI in integrated waste management systems. A next generation of thermal treatment plants without relying on grate systems is currently being developed. These new systems are designed to separate more efficiently and thus to produce more inert ash qualities for construction-related applications (1). Even though the bottom ash can be utilized already with conventional incinerators based on the grate system, a major portion of these residues are still landfilled. A bottom ash landfill can be regarded as a “heterogeneous * To whom correspondence should be addressed. e-mail:
[email protected]. † Gutenberg-University. ‡ EAWAG. 1398
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 10, 1998
fixed bed reactor”, where fast and slow acid/base reactions occur and continue for a long term, with a yet unknown end point (2). Major cation and anion concentrations observed in aqueous extracts and leachates reflect the advance of those primarily inorganic reactions (3, 4). Priority pollutants are trace metals enriched in MSWI products (5, 6). Oxyanion-forming metals such as As, Se, Sb, Mo, and Cr deserve special attention due to their toxic behavior. While some information on the behavior of the first four metals are available from recently published work (7, 8), the behavior of Cr in MSWI bottom ash deposits is not yet well understood. In this paper, Cr measurements will be reported for MSWI landfill leachate. The resulting concentration vs pH relationships for the leachates will be evaluated by geochemical modeling to verify the dominant speciation and mechanism that control the dissolved Cr concentrations.
Experimental Section Landfill and Leachate Sampling Design. Leachate samples were taken from the bottom ash monofill Im Lostorf at the MSW incinerator in Buchs, Kt. Aargau, Switzerland, between December 1993 and July 1994. Bottom ash from this incinerator is collected in a water quench and is not mixed with electrostatic precipitator dust. Prior to deposition, the ash is screened for unburned bulky goods and treated magnetically to remove excess ferrous material. About 40 000 m3 has been deposited into an abandoned gravel pit since the landfill establishment in autumn 1991. The landfill is equipped with a clay bottom liner supporting a gravel drainage system between geotextile liners for leachate collection. The upper porous geotextile membrane prevents clogging of the installed HDPE pipe system. The final depth is 6 m to which the ash is being successively filled from east to west in discrete stages upon aging for several weeks on a separate open dump site. This pre-aging and the relatively low depth prevents a buildup of excessive heat production in the landfill due to the oxide hydratation reactions. The landfill is not covered, but the ash is being compacted by a roller truck. The leachate drains via the HDPE pipelines into a passable concrete outflow well situated at an edge of the landfill, from which it is occasionally pumped for discharge into the sewerage. The end of the drainage pipe was equipped with a PVC sampling water tap mounted before the flow meter, which allowed for fresh leachate sampling. Batch samples have been poured into acid-washed 100-mL HDPE bottles prefilled with argon gas and 1 mL of HNO3. The aerobic leachate (1-9 mg of O2 L-1) is characterized by its relatively high alkalinity and salinity. The pH varies between roughly 9 and 11 and is inversely proportional to the discharge: the higher the discharge, the lower is the pH, which is probably an effect of carbonation due to admixture of fresh precipitate (8-10). Sampling was performed occasionally at different discharge regimes to represent the full pH range. Temperature varied in a narrow range of 15 ( 2 °C. Major cations are the alkali and earth alkali elements (8-10). Care was taken to exclude CO2 contamination of the fresh leachate during sampling, otherwise calcite is rapidly precipitated from the alkaline water samples. Batch samples were not filtered prior to acidification, because suspended matter contents were usually very low (
