Environ. Sci. Techno/. 1995, 29, 629-646
Comarehensive Aaeroach toward Understanding Element Speciation and Leachino Behavior in Municipal Solid Waste Incineration Electrostatic Precipitator Ash T . TAYLOR E I G H M Y , L , t J . DYKSTRA E U S D E N , J R . , * J A M E S E . KRZANOWSK1,s DAVID S . D O M I N G O , t DOMINIQUE STAMPFLI,+ J O H N R . M A R T I N , " s L AND PATRICIA M . ERICKSONA Environmental Research Group, A1 15 Kingsbury Hall, University of New Hampshire, Durham, New Hampshire 03824, Geology Department, Carnegie Hall, Bates College, Lewiston, Maine 04240, Mechanical Engineering Department, 134 Kingsbury Hall, University of New Hampshire, Durham, New Hampshire 03824, Analytical Shared Experimental Facility, Center for Material Science and Engineering, Room 13-4137, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, and US.Environmental Protection Agency, 26 West Martin Luther King Drive, Cincinnati, Ohio 45268
A comprehensive approach was used to characterize speciation and leaching behavior of major, minor, and trace elements in electrostatic precipitator (ESP) ash from a Canadian M S W incinerator. Neutron activation analysis (NAA), X-ray powder diffraction (XRPD 1, s c a nning e Ie ctr on mic r os c o pyR-r ay microa naIysis (SEMRRM), Auger electron spectroscopy (AES), secondary ion mass spectrometry (SIMS), and X-ray photoelectron spectroscopy (XPS) were used to quantify elements, describe particles and phase associations, identify bulk and surface mineral phases, and identify the speciation of elements. SEM/XRM showed a complex polycrystalline material covering aluminosilicate spheres. XPS, as a surface technique, provided information on speciation at the particle surface where leaching first occurs. SIMS showed molecular fragments indicative of speciation and enrichment of volatile species (K, Zn, CI, S, Pb) in the fine polycrystalline material. Many of these phases readily dissolve during leaching. Dissolution behavior and pH-dependent leaching could be modeled with the geochemical thermodynamic equilibrium model MINTEQAZ. The abilityto model leaching behavior provides an opportunity to examine possible disposal or treatment behavior.
0013-936X/95/0929-0629$09.00/0
0 1995 American Chemical Society
Introduction Municipal solid waste (MSW) incineration is a viable management strategy for treating combustible municipal solid waste that cannot be recycled. Organic material is oxidized. The volume of material is reduced while exothermic energy is recovered. Less volatile inorganic contaminants in the waste feed remain in the bottom ash while more volatile inorganic contaminants are captured as residues in air pollution control devices. Air pollution control residues such as electrostatic precipitator (ESP) ash from municipal solid waste incineration have been generally characterized with respect to composition of some elements (1-11). Some speciation work has been attempted. Methods frequently employed involve X-ray powder diffraction (XRPD) (5, 7, 81, Fouriertransform infrared spectroscopy (41,and scanning electron microscopylx-ray microanalysis (SEMIXRM) (8). However, little research has been conducted on the speciation of elements in the ash using a comprehensive approach that relies on multiple methods to confirm the speciation of elements, particularly trace elements, in the residue as it relates to leaching behavior. The speciation of both major and trace elements in residues impacts leaching behavior. Frequently, the major mineral phases dictate leachate chemistry (pH, Eh, ionic strength) via dissolution and reprecipitation processes. The predominant phases can also control trace element leaching behavior via sorption processes. Combustion residue aging reactions (elemental metal oxidation, heat of hydration, carbon dioxide uptake, crystallization) involve changes in speciation of major and trace element solid phases. The selection of proper residue treatment technologies, management scenarios, or utilization strategies is also dependent upon speciation. Finally, the ability to understand, model, and predict longer-term leaching behavior requires fundamental knowledge about speciation. The approach taken by our group to characterize speciation of elements in municipal solid waste incineration ESP ash is shown in Figure 1. Elemental composition is determined by combinations of neutron activation analysis (NM) and HCUHNOJHF total digestion coupled with inductively-coupled argon plasma-atomic emission spectroscopy (ICAP-AES). Bulk sample mineralogy is determined by XRPD and SEM/XRM. Characterization of the surface environment is accomplished with secondary ion mass spectrometry (SIMS), Auger electron spectroscopy (AES),andX-ray photoelectron spectroscopy (XPS). These methods can provide semiquantitative compositional information and element associations by spatial mapping. SIMS can also provide information on surface speciation when molecular fragments are identified under various ion * Author to whom correspondence should be addressed; e-mail address:
[email protected];FAX: (603) 862-2364. Environmental Research Group, University of New Hampshire. Geology Department, Bates College. 5 Mechanical Engineering Department, University of New Hampshire. II Posthumously. Analytical Shared Fxperimental Facility, Massachusetts Institute of Technology. A U.S. Environmental Protection Agency.
*
VOL. 29, NO. 3, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
629
SEMRRM
Total Composition
SIMS
J. XRPD
I SPECIATION I
x's
AES
Leachability
Geochemical Modeling of Leaching Behavior
the methods shown in Figure 1. Additionally, we wished to evaluate the speciation data provided by each method. ESP ash from a Canadian MSW combustor was analyzed because of high concentrations of trace elements, including volatile elements; its relative crystallinity;and its potential for use in particle depth profilingstudies to examine particle surface enrichment of volatile elements. ESP ashes from MSW incineration have been characterized for composition and some speciation (1-8, 10, 12). We conclude that a number of the methods, particularlyXRPD,X P S , SIMS, and geochemical modeling are useful when these methods are integrated and speciation is related to leaching behavior.
Methods
Incinerator Description. The residues collected in this FIGURE 1. Approach used to characterize element speciation in study were from a Canadian mass burn MSW waste-toESP ash. energy facility. The facility has a rating of 900 t/day. The combustor is comprised of four parallel units consisting of sputtering schemes. Element valence states, bonding a vibratory feed hopper, vibratory grates, waterwall boiler/ environments, and speciation are also provided from X P S , economizer, two field ESP, dry lime scrubber, and fabric which is particularly valuable as a surface analytical filters. ESP ash was collected on April 30,1991, from screw technique because its information can be coupled to conveyors exiting the ESP ash hoppers. Samples were leaching behavior (13-151. All of these methods are used collected every 0.5 h over a 4-h period. The samples were to provide an integrated synopsis about speciation; parcomposited, mixed, and subsampled using a 5-cm i.d. ticularly as it relates to leaching behavior. We employ two stainless steel pipe. The pipe was used to core the leaching procedures and the geochemical thermodynamic composite to produce a 3-kg working sample. model MINTEQA2 to model solid-phase dissolution and to Processing. A l-kg subsample was created from the compare the controlling solids to the ones observed by the 3-kgworking sample using a cone and quartering procedure. other analytical methods. Powders were stored under vacuum desiccation. A fractionation scheme has been developed to separate Fractionation. Magnetic separation and density sepawastes as a function of magnetic properties and particle ration (1,1,2,2-tetrabromoethane, TBE, 2.95 g/cm3at 20 "C) density (Figure2). This scheme is necessary to concentrate were attempted. The residues were not amenable to either elements present at low concentration in the residue, separation process. Isodynamic separation using a Frantz thereby increasing the chance of detection with the aboveisodynamic separator (S. G. Frantz Co., Trenton, NJ) with mentioned methods. Particles (
