Environ. Sci. Techno/. 1995, 29, 142-147
Acid Neutralizing Capacity of Municipal Waste Incinerator C . ANNETTE J O H N S O N , * S A N D R O BRANDENBERGER, A N D PETER B A C C I N I Swiss Federal Institute for Environmental Science and Technology (EAWAG), CH-8600 Diibendorj Switzerland
One of the main concerns in relation to municipal solid waste incinerator (MSWI) bottom ash is the longterm mobility of heavy metals, which depends on leachate pH and thus on the acid neutralizing capacity of MSWI bottom ash. The acid neutralizing capacity is determined by titration. Independent of titration methodology, MSWI bottom ash appears to be predominantly buffered by Ca minerals. In alkaline c on d itio ns, c a Ic ium hy d r oxi d e s/si Iic at es a nd C a C 03 dominate the solution chemistry of the aqueous phase. They constitute the acid neutralizing capacity (ANC) equivalent to the titration end point at pH 7.5. Fresh samples have an ANC7.5 of 1.2-1.7 f 0.05 mequiv/ g. In neutral to acid conditions, silicate dissolution becomes increasingly important. Calcium carbonate is the predominant component of ANC7.5 in samples aged in landfills or foresttrack coffers. Determined ANC7.5 values of the aged samples range between 0.6 and 1.0 f 0.05 mequiv/g. Intrinsic neutralization reactions with COZ or silica are the most probable explanations for the absence of the basic calcium hydroxide/silicate components.
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ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29. NO. 1. 1995
Introduction The long-term leachabilty of solid waste materials is the most important factor in the assessment of the potential hazards associated with landfiling or use in construction materials (1,Z). The commonly used laboratory leaching tests provide data on the potential leaching characteristics under specific conditions. Samples are ground or sieved and generally equilibrated for up to 24 h under a variety of solid/liquid ratios (1-20) and pH values. Leach tests at low pH values (pH3-5) are employed to determine leaching potential ( 3 ) . Other tests are used to assess the influence of physical parameters such as permeability. However, the leaching of contaminants depends on a combination of physical and chemical parameters that cannot be discerned from a single leach test. The results are not necessarily applicable to field conditions and cannot be used for predicting the long-term leachability under differing conditions. For this, a knowledge of the geochemical properties and the reactivity of the waste material with the leachate as a function of time is essential. The matrix elements of municipal solidwaste incinerator (MSWI) bottom ash are comparable in concentration with igneous rocks, the main components being Si, Ca, Al,and Fe ( I , 4 ) . Mineralogical studies have found that MSWI bottom ash is composed of equal amounts of fine ash material and melted components, of which half have crystallized, small quantities of metallic components, ceramics, and stones (5).Minerals such as calcite,ettringite, haematite, quartz, gypsum, and silicates to name a few have been identified (5, 6). The trace metal and soluble salt concentrations are approximately 10- 100 times the average composition of the earth’s crust. In addition, residual organic components make up 1-2% of the MSWI bottom ash material ( I , 4 ) . Laboratory and field leachates contain high soluble salt concentrations. Hjelmar (7) observed that Na, K, C1, and SO4 concentrations were the major components (10-100 mmol/L) over a 15-year period in leachate emanating from a monofill containing MSWI bottom ash mixed with flue gas cleaning residues. In systems with high solid/water ratios, Ca concentrations are controlled by the formation of gypsum ( 4 ) . Laboratory leachate solutions are generally highly basic (pH 11-12.5) due to the presence of Ca(OHj2 as well as alkali metal hydroxides, whereas the pH values of field leachate samples are generally in the range of pH 8-11. Concentrations of trace metal cations in landfill leachates are typically in the low micromolar range. Average Pb concentrations ranging from 0.05 to 0.1pmol/L and Zn concentrations of 1.5-3pmoUL have been observed though the variability is very high (7, 8). One of the main concerns in relation to MSWI bottom ash is the long-term mobility ofheavymetals. Heavy metal solubility is primarily dependent on the chemical composition of the leachate, the phases in which the metals are bound, and the surfaces available for binding. Trace metal cation concentrations can only be expected to rise to substantial concentrations when leachate pH values decrease to the neutral and acidic pH range. The acid neutralizing capacity and factors affecting this parameter is thus one of the most important intrinsic properties of MSWI bottom ash with regard to trace metal mobility.
0013-936X/95/0929-0142$09.00/0
D 1994 American Chemical Society
Acid titrations are useful for the assessment of solid/ water interactions. They provide information on the dominant buffering components under differing solution conditions and together with a knowledge of MSWI bottom ash reactivity allow the interpretation of aging processes. The aim of this paper is to examine titration methodology, to determine the most important buffering components contributing to acid neutralizing capacity of MSWI bottom ash, and to use the methodology on samples of differing ages.
Experimental Section Sampling and Sample Preparation. The MSWI bottom ash used in these experiments originate from incinerators of the grate furnace construction type with excess air for combustion at an average temperature between 800 and 1000 “C. The municipal solid waste is not pretreated and is not mixed with other materials for the incineration process (9). The bottom ash material is quenched after leaving the furnace. The MSWI bottom ash utilized in road construction is crushed (‘5 cm) and sieved, and Fe, Cu, and alloys such as brass are removed. From the Hagenholz plant, samples were taken before the quench tank (HagenholzDry, HD1 and HD2), after the quench tank (HagenholzQuenched, HQ, 1month old),and after preparation for use in construction (Hagenholz Quenched and Prepared, HQP, 3 months old). A sample was also taken from the Buchs incinerator (Buchs Quenched, BQ, 3 weeks old). The Hagenholz MSWI bottom ashes were stored open to the atmosphere but protected from precipitation, whereas the Buchs MSWI bottom ash was not covered. Ten subsamples were taken at each site and combined to single samples weighing approximately 20 kg. Portions (5 kg) of the samples were dried for 2 h at 104 “C, ground in a ball mill for a further 2 h, and subsequently sieved to
