T. Taylor Eighmy

Comprehensive Approach toward Understanding Element Speciation and Leaching Behavior in Municipal Solid Waste Incineration Electrostatic Precipitator Ash

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 MSW incinerator. Neutron activation analysis (NAA), X-ray powder diffraction (XRPD), scanning electron microscopy/X-ray microanalysis (SEM/XRM), 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, Cl, 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 MINTEQA2. The abilityto model leaching behavior provides an opportunity to examine possible disposal or treatment behavior.

An approach for estimation of contaminant release during utilization and disposal of municipal waste combustion residues

The use of appropriate leaching tests and leaching models can help to predict how constituents in municipal waste combustion residues will leach during either utilization or disposal scenarios. This paper presents a general approach for estimating constituent release from MWC residues under a variety of management scenarios through use of fundamental leaching, site specific design and regional climatic parameters. Leaching behavior is categorized as being controlled by either (i) constituent availability or solubility for percolation-dominated scenarios with loose granular residues, or, (ii) controlled by diffusion for flow around scenarios with compacted granular residues or monolithic products containing residues. Three broad scenarios involving either disposal or utilization are used to illustrate the approach. The scenarios are applied to bottom ash, combined ash and APC residue. In two specific cases pertinent to bottom ash utilization, field data are used to verify the approach. Field data are also used to verify the approach for disposal of combined ash. These methodologies hold promise for serving as a basis for evaluating and comparing potential environmental impacts from different management scenarios for combustion residues and for other waste materials.

Carbonation processes in municipal solid waste incinerator bottom ash and their effect on the leaching of copper and molybdenum

The interaction of CO2 with municipal solid waste incinerator (MSWI) bottom ash was studied in order to investigate the resulting changes in pH and bottom ash mineralogy and the impact that these changes have on the mobility of Cu and Mo. Carefully controlled carbonation experiments were performed on bottom ash suspensions and on filtered bottom ash leachates. Changes in leachate composition were interpreted with the geochemical model MINTEQA2, and neoformed minerals were investigated by means of chemical and spectroscopic analysis. The leaching of Cu and Mo during artificial carbonation is compared to the leachability of Cu and Mo from a sample of naturally carbonated bottom ash from the same incinerator. During carbonation in the laboratory, a precipitate was formed that consisted mainly of Al-rich amorphous material, calcite, and possibly gibbsite. Carbonation to pH ≈8.3 resulted in a reduction of more than 50% in Cu leaching, and a reduction of less than 3% in Mo leaching. The reduction in Cu leaching is attributed to sorption to the neoformed amorphous Al-minerals. During natural weathering/carbonation of bottom ash, additional sorption sites are formed which further reduce the leaching of Cu and Mo on a time scale of months to years.

Heavy metal stabilization in municipal solid waste combustion bottom ash using soluble phosphate

Abstract Heavy metal chemical stabilization with soluble PO 4 3− was assessed for bottom ash from combustion of municipal solid waste. Bottom ash can contain heavy metals (e.g. Pb) that can leach. An experimental dose of 0.38 mols of soluble PO 4 3− per kg of residue was used without optimizing the formulation for any one heavy metal. The reduction in the fraction available for leaching according to the total availability leaching test was 52% for Ca, 14% for Cd, 98% for Cu, 99% for Pb, and 36% for Zn. pH-dependent leaching (pH 4, 6, 8) showed that the treatment was able to reduce equilibrium concentrations by 0.5 to 3 log units for these heavy metals. Bulk and surface spectroscopies showed that both crystalline and amorphous precipitates were present as insoluble metal phosphate reaction products. Dominant reaction products were calcium phosphates, tertiary metal phosphates, and apatite family minerals. Observed phases included, β-Ca 3 (PO 4 ) 2 (tertiary calcium phosphate); Ca 5 (PO 4 ) 3 OH (calcium hydroxyapatite); Pb 5 (PO 4 ) 3 Cl (lead chloropyromorphite); and Pb 5 (PO 4 ) 3 OH (lead hydroxypyromorphite). These are considered to be very geochemically stable mineral phases. The geochemical thermodynamic equilibrium model MINTEQA2 was modified to include both extensive phosphate minerals and simple ideal solid solutions in order to better model pH-dependent leaching. Both end members [e.g. Pb 5 (PO 4 ) 3 Cl, β-Ca 3 (PO 4 ) 2 ] and ideal solid solutions [e.g. (Pb 2 ,Ca)(PO 4 ) 2 ] were observed as controlling solids for Ca 2+ , Zn 2+ , Pb 2+ , and Cu 2+ . Controlling solids were not identified for Cd 2+ because pH dependent concentrations were generally below detection limits. The divalent metal cations in bottom ash were effectively stabilized by treatment with soluble PO 4 3− .

World terends in municipal solid waste management

The philosophy of the Waste Management Hierarchy (prevention/minimization, materials recovery, incineration and landfill) has been adopted by most industrialized nations as the menu for developing municipal solid waste (MSW) management strategies. The extent to which any one option is used within a given country (or region) varies depending on a large number of factors, including topography, population density, transportation infrastructures, socioeconomics and environmental regulations. Recognizing these differences, the International Ash Working Group (IAWG) compiled available waste data from Canada, Denmark, Germany, Japan, the Netherlands, Sweden and the United States of America, for presentation at the Seminar on Cycle and Stabilization Technologies of MSW Incineration Residues held in March 1996.

Petrogenesis of municipal solid waste combustion bottom ash

Abstract A petrographic study was conducted on a suite of bottom ash particles from 3 different modern municipal solid waste combustors. The object of the study was to evaluate the mineralogical characteristics and formation process of the bottom ash by using standard geological techniques of light microscopy, electron microscopy, and X-ray microanalysis. This information was subsequently used to model the bottom ash petrogenesis based upon an examination of the mineralogy, melt structure, and composition of the ash. Bottom ash can be divided into two major groups: 1) refractory waste products and 2) melt products. The refractory waste products consist largely of rock and mineral fragments, various waste metals, and unmelted glass shards. The melt products consist of two distinct glasses: 1) isotropic glass, and 2) opaque glass. Complex silicate minerals are precipitated from and are abundant in the isotropic glass whereas both metal oxide and silicate minerals are precipitated from the opaque glass. The isotropic and opaque glasses formed simultaneously in different locations on the combustor grate. The contrast in melting (liquidus) temperatures shown by these glasses suggests that the isotropic melts were produced at localized hot spots (1500°C to 1650°C) and the opaque melts formed at cold spots (1150°C to 1400°C) on the grate. This could be the result of heterogeneous distribution of combustible municipal solid waste on the grate or from localized hot spots where air is introduced through the grates. In some instances the two glasses then had the opportunity to variably mix with each other. Fe-oxides represent waste metal fragments that were assimilated by melting and later recrystallized. Bottom ash is produced via a co-mingled two melt system that forms melilite-bearing, alkaline, volcanic-like rocks. The great similarity of the bottom ash residues between these 3 different MSW combustors suggests that, despite variable combustor designs and heterogeneous waste feed, high temperature combustion of MSW produces bottom ash of fairly uniform composition and structure that formed via the petrogenetic process described above. Alterations to the combustion process or implementations of secondary treatment technologies may render the bottom ash residue into a more environmentally stable material better suited for aggregate or long term secure disposal in landfills.

Distribution and role of bacterial nitrifying populations in nitrogen removal in aquatic treatment systems

Abstract The distribution and role of autotrophic nitrifying bacterial populations in nitrogen removal in aquatic macrophyte-based aquatic treatment systems (ATSs) was assessed. Pilot-scale systems using the macrophytesElodea nuttallii, Myriophyllum heterophyllum andLemna minor were used to evaluate macrophyte type and seasonal effects on nitrifier distributions. Distributions of pelagic, epiphytic and sedimentary nitrifiers were determined using a most probable number (MPN) microtechnique. Nitrifying populations are present in the reactors year-round. Season has no apparent effect on distribution. Relative abundances between pelagic, epiphytic and sedimentary populations are similar. Ammonium oxidizers are more abundant than nitrite oxidizers. Seasonal net ammonium removal rates, net nitrification rates and nitrogen budgets were also conducted onElodea nuttallii reactors. Rates of net ammonium removal and net nitrate production are very much affected by macrophyte productivity. The data suggest that nitrification is relatively constant but that reactor nitrate levels are governed both by macrophyte utilization of ammonium or nitrate and by denitrification. Mass balances on total nitrogen (TN) support these observations. WhenElodea nuttallii productivity is low, detrital sedimentation is the predominant nitrogen removal mechanism. When productivity is high, macrophyte uptake of ammonium or nitrate and dentrification are the predominant nitrogen removal mechanisms; thus indicating that nitrifiers play a significant role in nitrogen removal in ATSs. In terms of using ATSs for nitrogen control for sewered small communities, consideration of macrophyte type, harvesting strategy and detrital sediment collection and removal should be given to control effluent TN, ammonium and nitrate.

Characterization and phosphate stabilization of dusts from the vitrification of MSW combustion residues

Abstract The use of soluble PO 4 3− as a heavy metal chemical stabilization agent was evaluated for a dust generated from melting or vitrification of municipal solid waste combustion residues. Vitrification dusts contain high concentrations of volatile elements such as Cl, Na, K, S, Pb, and Zn. These elements are present in the dusts largely as simple salts (e.g. PbCl 2 , ZnSO 4 ) which are highly leachable. At an experimental dose of 0.4 moles of soluble PO 4 3− per kg of residue, the pH-dependent leaching (pH 5,7,9) showed that the treatment was able to reduce equilibrium concentrations by factors of 3 to 100 for many metals; particularly Cd, Cu, Pb and Zn. Bulk and surface spectroscopies showed that the insoluble reaction products are tertiary metal phosphate [e.g. Zn 3 (PO 4 ) 2 ] and apatite [e.g. Pb 5 (PO 4 ) 3 Cl] family minerals. Geochemical thermodynamic equilibrium modeling showed that apatite family and tertiary metal phosphate phases act as controlling solids for the equilibrium concentrations of Ca 2+ , Zn 2+ , Pb 2+ , Cu 2+ , and Cd 2+ in the leachates during pH-dependent leaching. Both end members and ideal solid solutions were seen to be controlling solids. Soluble phosphate effectively converted soluble metal salts into insoluble metal phosphate phases despite the relatively low doses and dry mixing conditions that were used. Soluble phosphate is an effective stabilization agent for divalent heavy metals in melting dusts where leachable metals are present in high concentrations.

Particle Petrogenesis and Speciation of Elements in MSW incineration Bottom Ashes

Abstract The speciation of elements in municipal solids waste incineration bottom ash is important with respect to its impact on leaching behavior and to its treatment for utilization or disposal. We used a variety of techniques to identify the speciation of major, minor and trace elements in both intact bottom ash particles as well as bottom ash powders. Petrography and scanning electron microscopy/x-ray microanalysis (SEM/XRM) were used to classify intact particles and identify ash particle petrogenic sequences. Two distinct features were seen. Particles are comprised of about 15% of materials present in the MSW waste feed to the incinerator. The remaining portion of the particle (85%) is melt structure. A typical particle contains waste glass (10%) waste soil minerals such as pyroxenes, SiO 2 (quartz), and feldspars (2%), waste metals and metal alloys (2%), and waste organics (1%). Particles are also comprised of slag or melt products, derived from the MSW feed material, that include opaque glass (25%), isotropic glass (20%), schlieren (10%), and spinel-group minerals (magnetite, hercynite, chromite) (10%) and melilite group minerals such as CajAljSiO? (gehlenite) and MgCa 2 Si 2 6 7 (akermanite) (20%) which precipitated out of the melt as it cooled. The paragenic sequence is similar to that described for melelite-bearing, igneous rock systems. The system can best be described petrogenically using the CaO-MgO-AljOj-SiOj-NajO-FeO (CMASNF) system. The melt structure was formed at about 1,200°C. Thermodynamically incompatible phases are present in the ash, making it reactive to aging (oxidation, hydrolysis), weathering, and diagenesis. Increasing the silicon content of the ash could result in the formation of more geochemically stable phases. The residue was ground into powders less than 300 μm in size. Magnetic and density separations were performed to segregate powders for further analysis. The residue is comprised of approximate equal fractions of magnetic, high density; non-magnetic, low density; and non-magnetic, high density material. Isodynamic separation of the non-magnetic fraction was also effective in separating minerals. SEM/XRM of powders fractions in thin section was particularly effective in identifying major minerals in identifiable mineral structures as well as minerals associated with “hot spots” of minor and trace elements. These minerals include many pyroxenes, quartz, feldspars, and melilite-group minerals as well as many spinels. Lead appears to largely be incorporated in complex silicate melt structures. X-ray powder diffraction (XRPD) confirmed the presence of minerals seen by petrography and SEM/XRM. X-ray photoelectron spectroscopy (XPS) of powder surfaces also documented the presence of many of these minerals. A number of oxides and carbonates were also seen with XPS, reflecting the role of O 2 (g) and CO 2 (g) in altering the speciation of the particle exterior surface. XPS is particularly well suited for identifying phases associated with leaching at this surface. Solid phases controlling leaching, as determined with the geochemical thermodynamic code MINTEQA2, are not always the same as ones observed with the above mentioned methods. The role of mineral respeciation and diagenesis in controlling leaching is highlighted. The use of such models in predicting leaching behavior is discussed.

Approach Towards International Standardization: A Concise Scheme for Testing of Granular Waste Leachabnjty

Abstract A series of short and relatively simple leaching tests is proposed for rapid compliance testing of granular (waste) materials. The test conditions selected are based on results obtained from extensive testing programs that have identified several critical factors influencing leachability of a particular granular (waste) material. These factors include specific element solubility and availability or release potential. Solubility can be influenced by pH, complexation by inorganic species or dissolved organic matter, and reducing properties of the waste. The sum of all of these factors reflect the chemical speciation of constituents in the material. Most current regulatory protocols do not explicitly consider these fundamental waste properties during evaluation. The proposed testing protocol includes two serial batch extractions with deionized water, first at liquid to solid ratio of 2:1 followed by L/S 10; and two static pH extractions at L/S 50, first at pH = 8 and then at pH = 4. The entire procedure can be completed within 32 hours and is designed to be simple, concise and reliable. Some typical examples of test results are presented in comparison with more extensive test data.