2.11.1 Metallic Forms of Heavy Metals in Bottom Ash

The bulk chemical composition of bottom ash revealed that the incineration products were silicate-based materials, with various amounts of other elements. SiO₂, CaO, Al₂O₃, FeO, MgO, Na₂O, K₂O, and TiO₂ were the dominant components, while minor and trace components, such as P, Cl, S, Pb, Zn, Cu, Mn, Ni, Cr, etc., also exist in different proportions in the bottom ash.

The glass phases are generally regarded as the principal incineration products of MSWI bottom ash. Based on microanalysis by optical and electronic microscopy, the glass phases in bottom ash can generally be divided into: (1) melt glass formed during combustion in the furnace and (2) waste stream glass. The latter is usually categorized as refractory (residual) components that have survived incineration as remnants of waste glass input. The melt glass phases are the main melting products of ash under high temperature and complex incineration conditions, which serve as a matrix for bottom ash particles. Optical and electron microscopy analysis reveals the following characteristics of the melt glass phases: (1) isotrope, transparent or translucent locally shifting to brown, light blue, green, and red colors as observed by plane polarized light (PPL) with respect to its chemical composition, (2) having peculiar banding due to the existence of minute metallic inclusions, (3) highly vesicular owing to the entrapment of air and gas bubbles, and (4) coexistence of various well-formed mineral species, quench crystallites, and refractory components. Waste stream glass is typically isotropic, presenting a more uniform appearance with few or no vesicles, without a banding feature, and usually with only the surface partially melted during the combustion process.

Opaque metallic phases (in the center and the top right corner) and fractured quartz fragments exist as remnants of waste input, and a great portion of these waste remnants are set into the glass matrix during combustion. Vesicles of various sizes are easily visible using the PPL mode of optical microscopy. The main compositions of melt glass phases from different incineration facilities suggest a very strong resemblance in compositional properties. It is understood that silicon, aluminum, calcium, and also some alkaline and alkaline-earth elements are the main components of the melt glass.

Melilites, with a general chemical formula of (CaNa)₂(AlMgFe²⁺)[(AlSi)SiO₇], are typical high temperature products of metamorphosed impure limestone and basic alkaline volcanic minerals. Moreover, melilites were detected as common constituents of MSW slag at melting temperatures of 1150–1400°C. Comprehensive microscopy observations of thin sections indicate that melilites are among the most abundant silicate-based melt minerals in bottom ash, consisting of a series of solid solutions of end-members, of which the most important are gehlenite and akermanite. The melilites found in bottom ash usually exist as crystal aggregates embedded in a glass matrix, characterized by subhedral to euhedral microphenocrysts. Optical observation and compositional analysis by SEM/EDX indicate that the melilites usually have a similar chemical composition to that of the melt glass matrix, and that the entrapping of melilites makes the melt glass easily distinguishable from waste stream glass.

Pseudowollastonite (CaSiO₃) is another silicate-based melt product of incineration, detected as radiating and fibrous aggregates. Quantitative analysis by electron microscope reveals that the pseudowollastonite in bottom ash contains a small amount of sodium, aluminum, and magnesium as additional substituents to calcium and silicon. A rough stoichiometric formula was estimated to be (Ca_0.48–0.69Na_0.06–0.21Mg_0–0.18)(Si_0.84–0.97Al_0.03–0.18)O₃, which implies undersaturation of cations, such as Ca, Na, and Mg. It should be noted that the pseudowollastonite shows a preference for embedding itself in high-silicon glass, and because the mandated temperature for MSWI furnaces in Japan is above 850°C, the pseudowollastonite might often not be detected in bottom ash.

The general chemical formula of spinel group minerals is A²⁺B₂³⁺O₃, where A²⁺ represents a metal having a valence of plus two (commonly Fe²⁺, Mg²⁺, Zn²⁺, Mn²⁺, Ni²⁺, and in rare cases Cu²⁺, Pb²⁺, Co²⁺, Cd²⁺), and B represent metals having a valence of plus three (Al³⁺, Fe³⁺, Cr³⁺, and sometimes Ti⁴⁺). Optical microscopy and SEM/EDX analysis disclose that MSWI bottom ash contains a considerable amount of spinels group minerals, which show a preference for embedding themselves in melt glass as aggregates.

In addition to spinels, metallic inclusions are another group of minerals encapsulated in the melt glass matrix. Most of the metallic inclusions have sizes ranging from one micron (or even less) to hundreds of microns with spherical or subspherical shapes, existing as single inclusion or clustered aggregates. These metallic inclusions are the most common heavy metal-bearing phases, primarily including Fe–P, Fe–S, Fe–Cu, Cu–Sn, Cu–Zn, Cu–S, and Cu–Pb couple-dominated combinations, of which Fe–P alloys account for more than 80% of the existing metallic inclusions. Moreover, complex associations of the above couple-dominated compounds are regular products of the solid waste incineration process. Typical images of the metallic inclusions were collected and shown in the following content, in order to exhibit their characteristic properties.

Combinations of Cu and Sn, as well as affiliated metallic inclusions, were frequently detected in bottom ash. It is presumed that it was possibly a Cu-Sn combination piece that was melted in the incineration furnace, on account of its melting point of 700–900°C. The Cu element congregated forming metallic inclusions, while the Sn was oxidized to SnO₂, owing to its low melting temperature. Occasionally, cuprous oxide (Cu₂O) was observed as coating of Cu. No cupric oxide (CuO) was found in this study.

In addition to the melt glass phase and the various encapsulated compounds (silicate-mineral phase and nonsilicate-mineral phase), other phases, such as calcium-rich mineral phases and refractory phases, account for a great proportion of the bottom ash. It is well known that in a MSWI furnace, carbonates are calcined to form lime (CaO).

The refractory materials are products from the waste stream that survive the solid waste incineration process, consisting of waste glass, minerals, ceramics, and unburned materials. Although frequently found in bottom ash, they are of less significance in terms of their influence on heavy metals. The refractory minerals, including quartz, K-feldspar, plagioclase, biotite, and in rare cases mafic minerals (pyroxene and amphibole) and cordierite, usually remain intact, or only the rim is partially altered. The group of refractory phases, owing to their high resistance to extreme conditions (high temperatures), basically keep their original chemical compositions (compositions formed in natural conditions) during the combustion process, and the impurity components cannot easily be chemically incorporated into these phases.

As a thermodynamically unstable multicomponent material, MSWI bottom ash should undergo evolution immediately after its production. Of all the weathering reactions undergone by the bottom ash, alteration of the calcium-rich phases (e.g., carbonation) should govern the initial stage of weathering. In an effort to ratify this hypothesis, an accelerated weathering experiment was conducted at elevated experimental conditions. Subsamples of the bottom ash were collected periodically for characterization of the secondary mineralization phenomenon and corresponding mobilization behavior of the heavy metals.

In an advanced weathering stage, evolution of other phases (e.g., glass), which are comparatively resistant to external environmental factors, should take place. Silicate-based glass has been determined to be the dominant phase in bottom ash, accounting for approximately 50 wt% of the ash material. Additionally, the glass phase has long been reported as being a metastable material. The specific geochemical conditions of landfill sites (moderately to highly alkaline) should enhance the evolution process of the glass as well of as the encapsulated compounds, resulting in textural and chemical evolutions. However, it should be emphasized that the evolution of the glass phase, unlike carbonation, appears to proceed in a slow manner, and the evolution products are usually not well-crystallized phases. These characteristics necessitate the utilization of long-term weathered ash samples and effective analytical methods for studying this weathering phenomenon. In this pursuit, a ten-year naturally weathered bottom ash sample was utilized in an effort to illustrate the alteration phenomenon of glass with respect to the microanalysis results. The primary glass presents more or less uniform morphology regardless of the perfect vertical banding texture (alignments of minute metallic inclusions). The fissures in the glass matrix disconnected two pieces of glass from the parent material, which provides more accessible channels for the percolated fluid.

It is revealed that glass and nonsilicate minerals (spinels and metallic inclusions) are the most important constituents in bottom ash in respect of its safe reutilization or disposal. The amount of heavy metals in bottom ash was relatively high, and the heavy metals were principally concentrated in the nonsilicate minerals that preferentially embedded in the glass matrix.

It has been outlined that the glass phases are the main constituents of bottom ash (approximately 50 wt%), entrapping assorted mineral phases—either refractory fragments (quartz and plagioclase) or melt clusters (melilites, spinels, and metallic inclusions), and having numerous discrete vesicles and fractures that resulted from rapid quenching. In particular, the existence of these vesicles and fractures may increase accessibility to water and gas, in view of the weathering process of glass. The heterogeneous chemical composition of the melt glass with respect to its major components SiO₂, CaO, Al₂O₃, Na₂O, and K₂O may suggest different weathering patterns (mechanism, weathering products, etc.).

The metals in bottom ash usually exist as various chemical combinations, e.g., several metals generally mixed together, or even with additional nonmetal elements. To gain a clear understanding of the behavior of bottom ash elements in the high temperature incineration process, particularly the behaviors of heavy metals, knowledge concerning the igneous process in geochemistry would be most helpful. According to Goldschmidt’s classification, the elements in the earth are categorized into four groups: lithophile (Na, K, Al, Si, Ti, P, O, Fe, Mn, Zn, etc.), siderophile (Fe, Ni, Cu, Zn, P, Mn etc.), chalcophile (Cu, Zn, Pb, S, Fe, Sn, etc.), and atmophile (H, N, and O). Only the elements detected in bottom ash are listed in the parentheses. Lithophile, siderophile, and chalcophile refer to the tendency of the element to partition into a silicate, metal, or sulfide liquid, respectively.

Magnetite, as well as a series of its Al-substituted and Ti-substituted varieties, is commonly identified in bottom ash. Heavy metals, such as Zn²⁺, Mn²⁺, and Cr³⁺, are frequently detected incorporated into these spinels in inconstant amounts. These findings are consistent with the fact that Zn²⁺, Mn²⁺, and Cr³⁺ have relatively high free energies (|ΔG_f°|) of oxidation, correspondingly 237.9, 281.1, and 362.9 kJ per oxygen atom respectively at 827°C, indicating their easily oxidable properties.

Fe–P-dominant alloys are the most abundant metallic inclusions in bottom ash, comprising 60–95 wt% of Fe and P, and usually associated with other minor elements (Si, Ca, Al, Cu, Zn, and Pb). Apart from the Fe–P alloys, combinations of Fe and S were frequently detected in bottom ash, which provided sound evidence of the siderophile property of Fe. Cu and its combinations or compounds are used for kitchen utensils, electrical wire, pigments, catalysts, stabilizers, and many other items. These Cu-rich combinations were melted or partially melted, then recomposed, and finally formed as numerous metallic grains and scattered in the bottom ash. In this study, Cu was constantly found as metallic inclusions in thin sections of bottom ash, including element Cu (usually more than 83 wt% of Cu), alloys (5–60 wt% of Cu, mainly Cu–Sn, Cu–Fe, Cu–Zn, Cu–Pb), sulfide, and in rare cases, lower oxides of Cu (Cu₂O). This is closely related to its strong resistance to oxidation, taking into account the low |ΔG_f°| of oxidation at high temperature. When bottom ash is exposed to the atmosphere (as in landfilling or recycling), further oxidation of Cu metals and their combinations might take place.

The metal Pb and its compounds are used mainly for the production of Pb batteries, solder for electronic devices, paint materials, crystal glass, ink, and a variety of other useful things. Pb is of great interest not only because of its economic importance, but also because of its pollution of surrounding environments. From the viewpoint of geochemistry, Pb is chalcophile, and perhaps slightly siderophile. Pb cannot easily be incorporated into mineral species in natural systems, which might be ascribed to its large ionic radius (1.26 Å) and charge (plus two valence). However, MSWI bottom ash was identified as having certain specific behaviors. The long-term stability of these specific products should be noted, as it might be thermodynamically unstable in natural environmental systems.

A clear understanding of the distribution of the heavy metals is essential for analyzing their fate in view of disposal or recycling. Of the entire range of weathering phenomena in MSWI bottom ash, carbonation, and glass alteration are emphasized in that the corresponding weathering products play an essential role in influencing the behavior of the heavy metals. Pb, Zn, and Cu become more stable as the weathering process proceeds, which should be primarily attributed to the declined pH. Furthermore, carbonation is generally regarded as a dominant process controlling pH variation, especially in the first few decades after disposal.

Evolution of the glass phase into a relatively stable phase, although not as rapid as carbonation in the bottom ash, has been identified in 10-year naturally weathered samples. The degree of mobility of the glass elements largely depends upon the pH conditions of the percolated fluid.

Bottom ash is derived from the thermal processing of MSW. Part of the inorganic fraction of the waste turns into melt components (glass and minerals), and some of them survive the thermal processing to form heat-resistant refractory phases. The glass phases are the main constituents, accounting for approximately 50 wt% of bottom ash, with various chemical compositions. Therefore, it is not easy to suggest a uniform alteration manner upon their exposure to the environment. However, compositional values from different incineration plants show a comparative resemblance in their properties, which suggests that high temperature processing (above 850°C) of solid waste produces more or less uniform outcomes in respect of both chemical and mineralogical characteristics. All the melt mineral phases (silicate minerals and nonsilicate minerals), as well as some of the refractory phases embed in a melt glass matrix.

It is understood that the incineration process of MSW results in a significant increase in the quantity of heavy metals. Some of them are elements of environmental concern, such as Pb, Zn, Cu, Mn, and Cr. Over all the various phases in bottom ash, the nonsilicate minerals were detected to be the most significant concentrators of heavy metals. Cr, Zn, and Mn are ubiquitously incorporated into spinels, while Cu and Pb show evidence of opposite behaviors, which are substantially associated with Fe, Sn, and Zn, present as metallic inclusions bound in the silicate glass matrix. The behavior of heavy metals during the incineration process depends on their own physical and chemical properties. The geochemical classification by Goldschmidt presents a good explanation for elements of concern, especially for P, Cu, and Pb. It should be noted that, despite the heterogeneous chemical compositions of the numerous metallic inclusions, most of them were in the metallic state (not oxidized).

Generally, it was concluded that the long-term behavior of the elevated quantity of heavy metals depends to a great extent upon the behavior of the host phases, especially the weathering rate and alteration products of the calcium-rich phases and the silicate-based glass phases. Of all the factors related to the mobilization behavior of the heavy metals, the carbonation process is of particular importance with respect to their dominant role in pH variation, especially in the first few decades, in that pH is a key factor that controls the leaching behavior of the heavy metals. In addition to the newly formed carbonates, the glass-derived secondary products also contributed to the immobilization of the heavy metals. It appeared that glass-derived products should play a long-lasting role, since evolution of the glass phase proceeds in a relatively slow manner. A clear understanding of the various phases in MSWI bottom ash, as well as the possible alternation phenomenon, when subjected to weathering, is essential for prediction and explanation of the behavior of heavy metals of environmental concern.

2.11.2 Evolution of Iron-Rich Constituents in Bottom Ash

The iron content in MSWI bottom ash normally accounts for 100–200 g FeO per kg bottom ash. The mineralogical analysis of fresh MSWI bottom ash by synchrotron powder X-ray diffraction, optical microscopy, and SEM/EDX disclosed that iron principally exists as components of spinels (including magnetite (Fe₃O₄) and a series of Al or Ti-substituted varieties), Fe-rich metallic inclusions (Fe–P, Fe–S, Fe–Cu, Fe–Cu–Pb, etc.), hematite (Fe₂O₃), and unburned iron pieces. Spinels and Fe-rich metallic inclusions show a preference for embedding themselves in the glass matrix in the course of incineration, thus remaining isolated from the percolated fluid before breakdown of the glass matrix. Detailed information concerning spinels and metallic inclusions has been published in a previous paper. It should be noted that, despite the heterogeneous chemical compositions of the numerous metallic inclusions, most of them are in the metallic state (not oxidized), and should undergo chemical and mineralogical changes when exposed to the environment.

From comprehensive observation of thin section specimens of weathered bottom ash, it was found that several forms of secondary Fe-rich products developed. These secondary products occurred either as simple or as complex combinations that infilled the pores, cavities, and channels. In a comparison of the fresh and weathered bottom ash, it was noticed that hydrous iron oxides (mostly goethite and lepidocrocite) are the major secondary products from the parent Fe-rich constituents. In addition, iron may also go to other amorphous phases in association with other major and minor elements; and these amorphous phases are “gel”-like, nonstandard materials.

Goethite is one of the most stable hydrous iron oxides at ambient temperature and is, therefore, the end-member of many transformations. Fe-rich metallic inclusions were ubiquitously detected as encapsulated compounds in almost all silicate-based glass phases of bottom ash. Owing to the metallic properties of these compounds, when exposed to ambient environment, their evolution should take place.

It should be noted that dissolution of the metallic inclusions usually takes place in the course of glass alteration. The metallic inclusions primarily embed themselves in the glass matrix, which is protected from contacted with the percolated fluid. Additionally, these metallic inclusions exist mainly in their metallic forms (not oxidized). Breakdown of the glass entity directly triggers exposure to the percolated fluid, finally leading to the dissolution of the metallic inclusions. It is clear that the dissolution of the metallic inclusions depends to a large extent upon the degree of alteration of the glass matrix. In return, the dissolving of metallic inclusions may also affect the alteration behavior of the glass matrix.

Regardless of the year of disposal of the weathered bottom ash, transformation of the primary iron species (hematite, magnetite, ferrous metal fragments, etc.) was commonly detected, e.g., evolution of primary magnetite to hydrous iron oxide and corrosion of ferrous fragments. These reactions may dominate in an oxidic condition.

Hydrous iron oxides have been recognized as being solid phases which exert a significant effect on the behavior of a large range of environmentally relevant substances, particularly heavy metals. Hydrous iron oxides indirectly affect the environment by influencing the mobility of these heavy metals via functions of surface adsorption or incorporation. Since the last half of the 20th century, numerous research projects have been undertaken on the adsorption behavior of these iron oxides or iron oxide-rich materials toward pollutants. In the research field of MSWI bottom ash, hydrous iron oxides are also regarded as an important sorbent that inhibits heavy metals from leaching out of the bottom ash deposit. Generally, the growth of hydrous iron oxide and its sorption capacity of certain heavy metals in landfill depend considerably upon the properties of the prevailing geochemical conditions, particularly the property of the percolated fluid (e.g., pH, ion varieties and concentrations). Hence, the microstructure and chemical compositions of these secondary products may disclose the evolution history of the Fe-rich constituents as a function of time.

From the microanalysis and bulk analysis of MSWI bottom ash, it was realized that the primary and secondary phases, including magnetite, metallic inclusions, hematite, iron fragments, and hydrous iron oxide, are the major Fe-rich constituents, and transformations frequently occur among these constituents. Such transformation reactions occur under the influence of the prevailing geochemical conditions during the weathering process of bottom ash. The metallic inclusions in bottom ash are primarily iron-rich compounds, and usually associated with different proportions of nonhazardous elements (P, S, Si, Ca, Al, etc.) as well as hazardous elements (Cu, Zn, Pb, Ni, etc.). In addition, these metallic inclusions in the bottom ash exist largely in the metallic state. It was understood that such metallic inclusions are important concentrators for heavy metals, and so evolution of these specific products should be taken into consideration, as they might be thermodynamically unstable once exposed to an oxidized condition.

Regardless of the physical and chemical reactions involved in the weathering process of bottom ash, electrochemical corrosion may also be held responsible for dissolution of the metal fragments and metallic inclusions under appropriate geochemical conditions. For instance, the abundance of various ions in the percolated fluid (e.g., Cl⁻, SO₄²⁻, Ca²⁺, Na⁺, etc.) could enhance the corrosion process of iron. It has been reported that the reactivity of zero-valent iron surfaces was maintained at a high level when corrosion proceeded in high ionic strength water in the presence of oxygen. Besides, the presence of chloride ions was reported to be a major factor in promoting further corrosion of iron. Although the exact roles of these geochemical parameters in accelerating the corrosion process of iron is still not clear, there is no doubt that they are essential factors for the long-term behavior of the Fe-rich constituents (especially metal fragments and metallic inclusions) of bottom ash.

Based on the above discussions, it was perceived that the transformation of the various Fe-rich constituents is controlled primarily by the percolated fluid. Conversely, the properties of the percolated fluid could also be affected by the evolution process of the Fe-rich constituents.

Despite the chemical forms of iron in MSWI bottom ash, formation of secondary Fe-rich products (including goethite, lepidocrocite, hematite, magnetite, wustite, Fe–Si-rich gel phase) invariably occurred in the vicinity of the primary Fe-rich constituents (magnetite, hematite, Fe-rich metallic inclusions, etc.). Of all these secondary products, goethite is thermodynamically the most stable one under oxidic conditions and therefore constitutes the end-members of many transformation routes in the weathered bottom ash. The ubiquitous development of the goethite could be ascribed to the prevailing geochemical environment (alkaline and oxidic) in landfill site. The degree of the transformation between the various Fe-rich materials, to a large extent, depends upon the timescale of the weathering of the ash. Hematite and ferric metal are usually the most accessible materials and their evolution occurs in the early stages of weathering. Alterations of magnetite and Fe-rich metallic inclusions, in that they preferentially embed themselves in the melt glass matrix, take place together with breakdown of the glass matrix; thus, their transformation should predominate at a later stage of ash weathering.

It should be noted that natural weathering of bottom ash is quite significant from the standpoint of environmental safety, as might be expected from the fact that all the reactions involved in such evolutions may locally change the properties of the percolated fluid by entrapping the dissolved heavy metals (Pb, Zn, Ni, Cu, etc.). Consequently, the quantity of such heavy metals leaching from the ash deposit is expected to diminish. In this process it can be seen that transformation of the various Fe-rich materials in the naturally weathered bottom ash plays an indispensable role, as it could enhance the stability of the ash materials and minimize the potential risks to surroundings. Furthermore, immobilization of heavy metals by these secondary phases also enhances the feasibility of excavation of the landfilled (naturally weathered) MSWI bottom ash as construction materials, which enables the disposal of more fresh MSWI residues into the landfill site.