Effects of Thermal Treatment on Mineralogy and Heavy Metal

cipitation with ferrous iron and subsequent thermal treatment. (at 600 and 900 °C) ... quantities of air pollution control (APC) residues containing ...
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Environ. Sci. Technol. 2000, 34, 4620-4627

Effects of Thermal Treatment on Mineralogy and Heavy Metal Behavior in Iron Oxide Stabilized Air Pollution Control Residues M E T T E A . S Ø R E N S E N , * ,† CHRISTIAN BENDER KOCH,‡ MAIREAD M. STACKPOOLE,§ RAJENDRA K. BORDIA,§ MARK M. BENJAMIN,# AND THOMAS H. CHRISTENSEN† Department of Environmental Science and Engineering, Technical University of Denmark (DTU), Building 115, DK-2800 Kgs. Lyngby, Denmark, Chemistry Department, The Royal Veterinary and Agricultural University (KVL), DK-1871 Frederiksberg C, Denmark, and Department of Materials Science and Engineering and Department of Civil and Environmental Engineering, University of Washington (UW), Seattle, Washington 98195

Stabilization of air pollution control residues by coprecipitation with ferrous iron and subsequent thermal treatment (at 600 and 900 °C) has been examined as a means to reduce heavy metal leaching and to improve product stability. Changes in mineralogy and metal binding were analyzed using various analytical and environmental techniques. Ferrihydrite was formed initially but transformed upon thermal treatment to more stable and crystalline iron oxides (maghemite and hematite). For some metals leaching studies showed more substantial binding after thermal treatment, while other metals either volatilized or destabilized with respect to leaching. Pb, in particular, exhibited increased reactivity following the formation of an ordered iron oxide structure at 900 °C. The thermal treatment had a positive effect on Cr release, which was reduced significantly at 900 °C in the presence of organic matter. Thermal treatment of the stabilized residues produced structures with an inherently better iron oxide stability. However, the concentration of metals in the leachate generally increased as a consequence of the decreased solubility of metals in the more stable iron oxide structure.

Introduction Municipal solid waste incineration (MSWI) produces large quantities of air pollution control (APC) residues containing various heavy metals in high concentrations. A new approach has recently been developed for treating these residues prior to landfilling. This process involves suspending the APC residue in a ferrous-containing solution and subsequent aeration of the slurry allowing oxidation of the precipitated iron hydroxides (1). The process, here referred to as the Ferrox * Corresponding author phone: +45 4525 1600; fax: +45 4593 2850; e-mail: [email protected]. † Department of Environmental Science and Engineering, DTU. ‡ Chemistry Department, KVL. § Department of Materials Science and Engineering, UW. # Department of Civil and Environmental Engineering, UW. 4620

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process, dissolves a significant part of the salts in the APC residues and exploits the ability of iron oxides to incorporate and adsorb simple heavy metal cations and complex metalloid oxyanions. Sorption and incorporation of heavy metals in iron oxides is well-known from geochemistry and has been used previously in treatment processes for removal of heavy metals from wastewater and liquid hazardous waste (e.g. refs 2-4). For simplicity, in this paper, we use the term iron oxide for both iron oxides, hydroxides, and oxyhydroxides. Landfilling of stabilized APC residues requires a product of high stability to limit heavy metal leaching in the long term. Physical and chemical changes in the stabilized product over time should not result in an unacceptable increase in leaching of heavy metals. In the Ferrox process an amorphous iron(III) oxide, identified as ferrihydrite, is formed in a slurry of the solid residues, by precipitation of heavy-metalcontaining ferrous hydroxides at high pH and by subsequent oxidation of the Fe(II). Ferrihydrite has a large binding capacity toward heavy metals, in part because of its extremely large surface area and in part because it readily integrates other metals in its structure. However, ferrihydrite is not stable from a thermodynamic point of view and may transform into more crystalline phases with different binding capacity for the heavy metals, depending on the structural constraints of the ordered phases. The change in metal binding over time in aquatic environments and the retarding effect that substituted metals induce on the transformation have both been described by several authors (e.g. refs 5-11). Thermal treatment of iron oxides under dry conditions at high temperature may force the transformation of amorphous iron oxides into more crystalline forms (12-15). Heating of APC residues stabilized by iron oxides would have the benefit of removing water from the stabilized product, thereby decreasing its volume and improving its physical properties in the context of landfilling, and would also lead to a thermodynamically more stable product. However, like long-term aging, dry heating of ferrihydrite might also change the heavy metal binding and properties (16). In this paper we investigate the effects of heating two different Ferrox treated APC residues in terms of changes in iron oxide characteristics and in heavy metal availability. Thermal treatment is considered as an optional and additional treatment method that can be used prior to landfilling, and the combination of time and temperature is selected with this in mind. The restructuring of the iron oxides formed in the Ferrox treatment and the changes induced by thermal treatment were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), Mo¨ssbauer spectroscopy, determination of specific surface area, and oxalate extraction. The properties of the heavy metals were characterized in terms of mass balances during thermal treatment, pH-static leaching tests, and kinetic extraction by HCl.

Materials and Methods Stabilization of APC Residues by the Ferrox Process. Treatment of APC residues from two Danish MSWI plants was examined. One residue was the product of a semidry APC system (SD) and the other a fly ash (FA) from an electrostatic precipitator. The SD or the FA residue was added to an FeSO4 solution, containing 5 g Fe/100 g SD or 2.5 g Fe/100 g FA, at a liquid-to-solid ratio (L/S) of 5 L/kg. After addition of residue to the FeSO4 solution, the slurry was stirred for 24 h at its natural pH (>10) and subsequently separated from the aqueous suspension by vacuum filtering. Both products were dried at 50 °C for 2 days in a conventional drying oven. The content of selected elements in the two 10.1021/es0009830 CCC: $19.00

 2000 American Chemical Society Published on Web 09/29/2000

TABLE 1. Metal Content of Selected Elements in the Two Ferrox Products after Drying at 50 °C (mg/kg Dry Product) producta element

SD

FA

Al Ba Ca Fe K Mg Mn Na Na S As Cd Co Cr Cu Hg Mo Ni Pb Zn

31,000 553 206,000 53,000 6600 9100 625 8800 8800 73,000 89 182 14 151 1162 27 11 45 6330 19,000

48,000 721 176,000 32,000 12,000 13,000 986 13,000 13,000 39,000 137 246 17 262 1649 3 19 66 6575 17,000

a SD: ferrox treated residue from a semidry APC system. FA: ferrox treated fly ash from an electrostatic precipitator.

Ferrox products is listed in Table 1, and a general description of the products and the phases formed is given in Table 2. Thermal Treatment. Initially Differential Thermal Analysis (DTA) and Thermo Gravimetric Analysis (TGA) studies were completed in order to determine relevant phase transition temperatures. DTA/TGA traces were obtained using a Netzsch thermal analysis instrument model STA409. Samples of 100-200 mg were heated at 5 °C/min to 1300 °C. The analysis was conducted in a flowing air atmosphere. DTA curves were not calibrated, and data, therefore, are only indicative of thermal transformations, temperatures at which they occur, and the endothermic or exothermic nature of the transformations. Samples of the two Ferrox treated products were thermally treated in a static air atmosphere in a Lindberg box furnace. Treatment temperature and time were chosen so that a significant structural change in the iron oxide structure was obtained. Samples were, thus, kept at either 600 °C for 3 h or at 900 °C for 45 min. Temperatures were reached within 15-20 min, and cooling to room temperature after treatment lasted 30-60 min. Thermal treatments of the samples were performed in conical Al2O3 crucibles of 200 mL covered partly by an Al2O3 lid. All samples were weighed before and after heating. All thermal treatments and analyses were performed in air, but traces of organic matter in the APC residues may have caused oxygen depletion and consequently the presence of reducing conditions locally in the samples. Characterization of the Ferrox Products. The Ferrox products were characterized before and after thermal treatment using the following methods and techniques: Powder X-ray diffraction (XRD) was performed on a Siemens D 5000 diffractometer using Co KR radiation. 57Fe Mo ¨ ssbauer spectra were obtained in transmission geometry using a conventional constant acceleration spectrometer and a source of 57Co in Rh at temperatures between 298 and 5 K and in an external magnetic field up to 4 T. The spectrometer was calibrated using a foil of natural iron at room temperature, and isomer shifts are given with respect to the centroid of this spectrum. Specific surface area was determined from the single point BET method using a Quantasorb surface area analyzer

(Quantachrome Corp.). Helium was used as carrier gas and N2 as the adsorptive. All samples were outgassed at low temperature (50 °C) in order to minimize structural changes prior to surface area measurement. The surface texture was examined using scanning electron microscopy (SEM). A highly reactive iron oxide fraction was quantified by extraction 2 h in the dark in 0.2 M ammonium-oxalate at pH 3 (17). All samples were digested by autoclaving in 7 M HNO3 to determine the chemical composition of the products before and after treatment. Some uncertainty was introduced due to an incomplete dissolution of the samples. Metal analyses were performed on the extract and mass balances were calculated. Metal Availability. Leaching tests were performed at constant pH values (pH 6, 7, 8, 9, and 10). All tests were performed at L/S 10 L/kg for 24 h. A computer controlled titrator was maintained at constant pH by the addition of either HNO3 or KOH. Leachate samples were filtered through a 0.45 µm polypropylene filter and acidified with acetic acid prior to analysis. Extraction with 1 M HCl (0.1 g solid + 30 mL 1 M HCl in single batches shaken between 15 min and 195 h) was used to relate metal availability of Pb, Cd, and Cr to the iron oxide solubility (modified after ref 18). Samples were filtered through a 0.45 µm polypropylene filter prior to analysis. Metal Analysis. Metal concentrations were measured using inductively coupled plasma techniques (atomic emission spectroscopy (ICP-AES) and mass spectroscopy (ICPMS)), atomic absorption spectroscopy (AAS) techniques (graphite furnace and flame), and atomic fluorescence spectroscopy (AFS), depending on the metal and the actual concentration level.

Results and Discussion Effects of Thermal Treatment on Weight and Metal Loss. TGA analyses (curves not shown) on both products showed a distinct mass loss at temperatures ranging from 100 to 200 °C. This is interpreted as loss of water from hydrous phases and from removal of adsorbed surface water. In the 200 to 900 °C temperature range, a continuous but less evident mass loss was observed with no distinct plateaus. The loss in this temperature range was interpreted as a combination of various reactions (dehydration, dehydroxylation, decomposition, transformation and vaporization). At temperatures in excess of 1000 °C a considerable mass loss was observed and this is attributed to the evaporation of salts and further decomposition reactions, e.g. reactions caused by the chemical reduction of metals present in oxide phases (particularly Fe(III)). DTA analyses (curves not shown) on both products showed two endothermic regions (25-200 °C and above 900 °C) associated with marked weight losses and a very broad exothermic region in between. The analyses showed no sharp peak in the 25-1250 °C temperature range. Consequently, no characteristic temperature for the transformation of ferrihydrite was identified. Assuming that the ash formation during waste incineration resulted in a high-temperature assemblage of phases in the residue, the information in the DTA/TGA measurements can be considered to be dominated by the properties of phases formed in the cold flue gas or subsequently in the Ferrox process, i.e., mainly gypsum and ferrihydrite (see below). In the cold flue gas, salts precipitate on particle surfaces. These salts are all volatile within the temperature range examined but are not expected to contribute significantly in the diffractograms, since they, to a great extent, have been washed off in the Ferrox stabilization process. VOL. 34, NO. 21, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 2. General Description of the Residues and the Phases Formed residue

semidry (SD)

fly ash (FA)

characteristics

Contains fly ash, various calcium compounds, and activated carbon. Ca(OH)2 and activated carbon is added in the SD APC system in order to reduce the content of acid gases, Hg, and dioxins in the flue gas. origin Sampled from the ash silo at Amagerforbrænding (Copenhagen, Denmark) Fe added (g/100 g residue) 5.0 a theoretical ferrihydrite supply 8.6 (g/100 g residue)

50 °C 600 °C 900 °C a

Contains particles withdrawn from the boiler and compounds, primarily salts, condensed in the cold flue gas. Additionally, traces of unburnt organic material are present. Sampled from an electrostatic precipitator at Vestforbrænding (Copenhagen, Denmark) 2.5 4.3

New Phases Identified after Treatment of SD and FA at ferrihydrite, gypsum anhydrous analogue of ferrihydrite, anhydrite maghemite and hematite, anhydrite

Structural formula based on ref 28.

Due to the complex composition of the Ferrox products all DTA/TGA traces reflected a combination of simultaneously occurring processes and reactions. Ferrihydrite represents a minor fraction of the products only, and changes related to its structural transformation are, therefore, difficult to separate. A characteristic transformation peak for pure ferrihydrite has been reported between 300 and 350 °C (19) but has also been proven highly dependent on the composition, e.g. the silicate content and the pH conditions in the precipitate (19, 20). The high pH in the Ferrox process and the content of many interfering components may, based on these observations, be expected to increase the transformation temperature and to broaden the peak, making it even harder to identify a characteristic transformation temperature. Silicate has, indeed, been found associated with the ferrihydrite phase in unheated Ferrox products using timeof-flight-secondary ion mass spectroscopy and analytical transmission electron microscopy (not published). The dehydration of gypsum (CaSO4‚2H2O) will contribute significantly to the mass losses observed at low temperatures. The decomposition occurs via different intermediate hydrates, eventually forming anhydrite (CaSO4) at temperatures between 120 and 450 °C (e.g. refs 21 and 22). The thermal stability of anhydrite is strongly dictated by redox conditions and by the presence of contaminating components, e.g. silicate. Therefore, under oxidizing conditions the decomposition occurs at higher temperatures than under reducing conditions, and the presence of some impurities has been shown to accelerate the decomposition and reduce the decomposition temperature (21, 23, 24). A study on sulfur rich coal ash showed the persistent presence of anhydrite in air at temperatures up to 1060 ( 10 °C (25) and the decomposition of anhydrite in the Ferrox products is, therefore, not anticipated when heated to 900 °C in air. Another contribution to the mass loss may come from the decomposition of calcite (CaCO3) formed in the APC system or, subsequently, when calcareous compounds react with atmospheric CO2. The decomposition of calcite to solid CaO and gaseous CO2 has been reported at various temperatures ranging between 650 °C and 900 °C (e.g. refs 22, 26, 27), depending on the origin and precipitation. Therefore, this reaction may also contribute to the mass losses observed. The dehydroxylation of ferrihydrite will similarly cause a reduction in mass due to the low iron content, but this loss will be less significant. The structural formula for ferrihydrite is not finally agreed upon but has been suggested as FeOOH‚ 0.4H2O (e.g. ref 28). If all Fe added in the Ferrox stabilization is present as ferrihydrite, dehydroxylation and restructuring to maghemite or hematite, Fe2O3, gives a theoretical weight loss of only 1.7 g/100 g treated SD residue and 0.9 g/100 g treated FA residue. 4622

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Treatment temperatures for the Ferrox products were chosen so that a change in crystal structure was obtained without an extensive loss of sample to the gas phase, particularly heavy metals. For the high-temperature treatment, heating time was adjusted to the typical residence time in a sintering oven at the two plants (45 min), and at the lower temperature, a treatment time of 3 h was chosen to induce some transformation in the iron oxide phase. The SD Ferrox product appeared inhomogeneous after thermal treatment at both 600 °C and 900 °C. A distinct color difference was observed between exposed sample surface and sample interior, particularly after treatment at 900 °C for 45 min. The sample immediately at the surface had a bright red color, whereas the remaining part was darker and brownish in color. The SD residue, in particular, initially contains a small amount of organic carbon, which may have caused reducing conditions in the lower layers of the sample while heating. The same inhomogeneity was observed, to some degree, after thermal treatment at 600 °C for 3 h. The longer treatment time (3 h) may have allowed sufficient time for oxidation of the lower layers as well. Due to the small quantity of the red layer observed at the sample exterior it was possible to collect only a minor sample of this fraction. The remaining product was mixed and used for all analyses, and XRD showed no notable differences between the red and the brownish fraction. Drying at 110 °C caused the SD and the FA Ferrox products to lose 36% and 35% of the total mass as water, respectively. Another 7% was lost from each product after treatment at 600 °C, whereas treatment at 900 °C resulted in an additional loss of 9% for the SD product and 8% for the FA product, for total mass losses of up to 45% compared to the initial product mass. All results shown in this work are based on the 110 °C mass. Acid digestions of the Ferrox products showed (Table 3), not surprisingly, a major loss of Hg during heating. Heating led to minor losses of Cd, Pb, and possibly other elements, e.g. As, Co, Ni, and Zn. The estimates for recoveries presented in Table 3 are all based on the assumption that no iron was lost while heating the samples. The numbers are subject to large uncertainties and mass losses less than 10% are not considered significant. When thermally treating a fly ash based product, significant evaporation can be expected due to the high content of volatile components. However, a substantial amount of chloride was washed out in the stabilization stage, meaning that less metal can evaporate as chloride salts when the Ferrox treated residues are heated. Chloride has been shown by several authors to control the evaporation of heavy metals from thermally treated fly ashes (29, 30). The evaporation of As, however, has been reported

TABLE 3. Relative Metal Content of Selected Metals after Thermal Treatment at 600 °C and 900 °C in Air (% Remaining of the content at 50 °C ()100%)) product SD Ferrox

FA Ferrox

element As Cd Co Cr Cu Hg Mo Ni Pb Zn Fe As Cd Co Cr Cu Hg Mo Ni Pb Zn Fe

600 °C

900 °C

96 100 90 123 104 1 108 86 101 101 100a 98 105 110 109 104 9 97 95 99 103 100

97 86 100 110 104 2 112 93 94 104 100a 86 66 85 89 94 4 91 88 79 86 100

a The mass balances are based on the assumption that the Fe content does not change.

independent, without the presence of chloride (31). As a result, a greater loss of As could be expected. Effects of Thermal Treatment on Iron Oxide Phases. XRD analyses of the APC residues revealed a mixture of both crystalline and noncrystalline compounds. The patterns (not shown) were highly complex and were not identified in detail. A prominent broad background in the pattern of the SD residue is consistent with the presence of activated carbon in the residue. The XRD patterns (not shown) of the Ferrox treated samples also revealed a mixture of crystalline and noncrystalline compounds. The crystalline phases were dominated by the presence of gypsum. No indications of crystalline iron oxides were observed in the X-ray diffractograms of the Ferrox products, indicating the absence of crystallinity in this phase. Heating induced major changes in the XRD patterns (not shown) showing anhydrite as the predominant crystalline compound in products treated at 600 and 900 °C. Due to the complexity of the diffractograms of the heated samples no clear identification of the iron oxide phases in these samples was possible, but a few peaks are identified as the iron oxides maghemite and hematite. Maghemite was primarily indicated in the SD Ferrox product and hematite primarily in the FA product. Products treated at 900 °C showed slightly sharper iron oxide peaks than products treated at 600 °C, suggesting the presence of larger or more perfect crystals, but the greatest differences were generally observed between heated and unheated products. The low temperature Mo¨ssbauer spectrum obtained for the SD residue prior to the Ferrox stabilization is shown in Figure 1a. At higher temperatures the magnetically ordered sextet collapsed into one paramagnetic doublet (not shown). The hyperfine parameters of the sextets observed at 14 K (magnetic hyperfine fields of approximately 53 T ( 1.0 T and quadrupole shifts of -0.1 mm s-1) were consistent with poorly crystalline hematite. The paramagnetic components were caused by Fe(III), quite possibly in a glasslike phase, but no definitive assignment of the phases was possible. The sample is very low in iron, and the statistics of the spectrum is, accordingly, weak. The spectrum of the FA sample (not shown) revealed a more crystalline hematite, an Fe(III) doublet, and in addition a minor Fe(II) doublet. Following

the Ferrox treatment, where Fe was added to the residues, these inherent components were significantly suppressed in their relative amount (completely in the spectra of the SD product, but only partly in the spectra of the FA product). The temperature dependence of the spectra (not shown) changed dramatically following the addition of iron. The new dominant phases exhibited magnetic order below 30 K only, and exhibited broadlined sextets at 5 K with negligible quadrupole shifts and magnetic hyperfine fields of approximately 48.5 T ( 0.3 T (Figure 1b). These hyperfine parameters and the low blocking temperature identified the new phases as ferrihydrite in both samples (32). Heating of the Ferrox samples to 600 °C induced only very minor structural changes in the iron oxides. At 80 K a relatively larger contribution of magnetically ordered components was found in the spectra (not shown), whereas the spectrum at low temperature (Figure 1c) exhibited hyperfine parameters similar to those of the untreated product (hyperfine field of approximately 49.0 T ( 0.3 T). The absence of polarization effects in the spectrum demonstrates the dominance of an antiferromagnetically ordered magnetic structure as in ferrihydrite. It is not expected that the water and hydroxyl groups associated with the ferrihydrite structure remain in the structure after treatment at such high temperature. Rather, it is suggested that an anhydrous analogue of ferrihydrite is present which exhibits a similar magnetic structure. A similar iron oxide phase has been described recently (15) after exposure of a Si-rich ferrihydrite to temperatures between 400 and 850 °C. Due to the relatively higher content of iron in the FA residue, both ferrihydrite and inherent hematite showed clearly in the spectrum of the FA product treated at 600 °C (not shown). After heating of the samples to 900 °C the spectra revealed significant changes. The broadlined sextets observed in the spectra at 298 and 80 K (not shown) transformed into a nonresolved, asymmetric sextet at 14 K (not shown) showing a magnetic hyperfine field of approximately 52 T. Application of an external magnetic field to the sample at 5 K (Figure 1d) induced splitting of the sextet into two sublattices due to the ferrimagnetic spin structure. The hyperfine fields of the components are 47.7 T ( 0.2 T (major component) and 53.8 T ( 0.2 T. This indicates the dominance of maghemite in the sample. The presence of nonvanishing intensity of the 2nd and the 5th line in the sextets was taken as an indication of the occurrence of spin canting in the oxide. The relative intensity of the sextets differs from ideal maghemite, suggesting that a substitution of heavy metals has occurred in the structure. However, the presence of vacancies may also add to this deviation. Campbell et al. (13) recently studied the transformation of ferrihydrite to maghemite by heating and observed that the presence of organic matter at elevated temperatures could induce the formation of maghemite. Thus, for the present samples it appears highly important for their ultimate structure that the APC products contain small amounts of charcoal and, additionally for the SD residue, activated carbon. This explains why the SD Ferrox product in particular was dominated by maghemite after thermal treatment. For the 900 °C treated FA sample a substantial overlap of hematite and maghemite components were found (not shown). Specific surface area analyses showed a decrease in accessible surface area after thermal treatment at 600 °C and further at 900 °C (Table 4). This is consistent with the increased crystallinity of the iron oxide found by Mo¨ssbauer spectroscopy and the sintering of the samples observed by SEM (micrographs not shown). The specific surface area of ferrihydrite is >200 m2/g (e.g. ref 28). Hence, the theoretical surface area supplied by ferrihydrite in the SD and the FA Ferrox products is approximately 17 and 9 m2/g, respectively (if all Fe added in the stabilization is present as ferrihydrite). VOL. 34, NO. 21, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Low-temperature Mo1 ssbauer spectra. SD residue prior to Ferrox stabilization (a). SD Ferrox product as prepared (b), and after heating to 600 °C and 900 °C (c and d). Temperature and external magnetic fields are indicated in the figure.

TABLE 4. Surface Area (m2/g) of the Treated APC Residues SD FA

50 °C

600 °C

900 °C

16.5 18.2

10.7 5.7

1.6 1.6

TABLE 5. Oxalate-Extractable Iron in the Treated APC Residues (% of the Total Fe Content) SD FA

50 °C

600 °C

900 °C

68 76

53 61

34 15

Other phases, of course, contribute to the total surface area too, and the surface area of the SD product is, therefore, lower than expected. For this reason, a part of the iron added in the stabilization process may not be present in a free ferrihydrite phase but may be captured by simultaneously precipitated phases, e.g. the gypsum phase. The surface area of anhydrite particles was shown to decrease dramatically after treatment at 900 °C (from 19.7 m2/g to less than 0.5 m2/g) (23), and these structural changes in the anhydrite phase will consequently add to the changes observed. The amount of oxalate extractable Fe (Table 5) indicates the amount of highly reactive or poorly crystalline iron oxide. Heating to 600 °C reduced this fraction by about 20%, whereas heating to 900 °C decreased the extractable fraction more significantly (between 50 and 80%). An extractable iron oxide fraction, thus, remained even after heating to 900 °C. The numbers in Table 5 should not be taken as exact percentages but should be seen as indicators for the relative iron oxide solubility only. Effect of Thermal Treatment on Metal Availability. Thermal treatment had an evident impact on the leaching of metals, and all metals showed a marked and characteristic 4624

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pH dependency (Figure 2). Oxyanion-forming metals (As, Cr, and Mo), thus, showed high leachate concentrations within the neutral to alkaline pH range, whereas other metals (Cd, Cu, Ni, Pb, and Zn) showed high concentrations in the low pH range. In most cases, the changes induced upon heating lead to an increase in metal release relative to the initial dry mass. Clearly, the oxyanion-forming metals As and Mo leached more easily after treatment at both 600 and 900 °C. Previous studies on changes in As substitution after aging of a hydrous and amorphous iron oxide coprecipitate (33) support these observations. Mo in particular showed a high release (more than 50% leached out of the products treated at 900 °C), whereas no more than 3% of the As content was found in the leachates. The leachate concentration of Cr is known to be fully controlled by chromate, and the leachate concentration is, therefore, greatly affected by changes in redox state upon heating. Thermal treatment at 600 °C resulted in a greater release for both products, giving a maximum leachable fraction of 20% for the FA product and 4% for the SD product. However, treatment at 900 °C reduced the Cr concentration significantly, particularly for the SD product. This decrease in concentration is very likely caused by the reducing environment formed locally while heating, thus causing a reduction of Cr(VI) to Cr(III). The SD product initially holds more organic matter than the FA product, and it is, therefore, not surprising that this product in particular show markedly lower Cr concentrations after treatment at 900 °C. Once Cr(VI) is reduced the leaching behavior changes dramatically. Cr(III) may stabilize by substitution in a spinel phase (34, 35) or it may precipitate as a highly stable Cr2O3 compound (36), either in a free phase or in solid solution with Fe2O3 (37). Ni also showed a lower release after exposure to high temperatures, thus suggesting incorporation of the metal in the crystalline structure. During the thermal treatment Ni may integrate structurally in the maghemite phase (34, 35)

FIGURE 2. Leaching of various metals at pH 6-10 (µg leached per kg dry product). but may also integrate in a Ni-ferrite phase or in a solid solution NiO-Fe2O3 (38). These possibilities all provide for excellent Ni binding. The leaching of Hg (not shown) decreased significantly after thermal treatment. This is solely ascribed to the fact that most Hg vaporized and, thus, was lost from the thermally treated products. Both Ferrox products showed elevated leachate concentrations of Cu after treatment at 600 °C. Treatment of the SD product at 900 °C, on the other hand, decreased the Cu concentration significantly, whereas a similar treatment for the FA product resulted in a further increase in concentration. However, the products showed a low Cu release, and less than 0.2% of the total content was in all cases found in the leachate. The structural incorporation of Cu in spinel phases has been described previously (34, 39, 40). Cd and Zn both showed the highest leachate concentrations after treatment at 600 °C; however, treatments at 900

°C did not increase the leachate concentrations as significantly. The same relative amounts of Cd and Zn were released from the two stabilized products, and a maximum of approximately 10% of the Zn and 30% of the Cd leached at pH 6 from the 600 °C treated products. Metals released at pH 6 have most likely been associated with particle surfaces in the products, whereas the remaining part may be integrated in the solid structure, either physically or chemically. The leaching tests, thus, suggest that the binding of both Cd and Zn was better in products treated at 900 °C than in products treated at 600 °C since more metal was released from the latter. Other investigators have shown that Cd has been integrated into spinel phases (35, 39, 41), and also the binding of Zn in spinel phases has been addressed previously (7, 34, 35, 39). Pb showed, like the metalloid oxyanions, significantly higher leachate concentrations after thermal treatment. This behavior suggests that Pb was ejected from the bulk structure VOL. 34, NO. 21, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. 1 M HCl extraction on Ferrox products. Extracted metal (% of total metal content) as a function of time. during its restructuring and, thus, was enriched at the surface of the particles. Pb sorbs readily to iron oxide surfaces (42), but the surface area decreased simultaneously and lowered, consequently, the number of sites available for sorption. The structural changes induced by heating will therefore lead to higher leachate concentrations of Pb. These changes are expected to affect the FA product in particular since it has the highest Pb/Fe ratio (the Pb/Fe ratio is 3 mol % in the SD product and 5 mol % in the FA product). Thermal treatment at 900 °C did, accordingly, show the most significant change in Pb concentration in leachate from the FA product. Several authors have noted that Pb2+ cannot be incorporated in crystalline iron oxides (e.g. refs 10, 11, 43-45). Despite the ejection of Pb, only a minor fraction was soluble. Less than 0.4% were in all cases found in the leachate. To further examine the heat induced changes in metal binding, the release of Pb, Cd, and Cr was examined relative to the Fe release by extraction in 1 M HCl. Pb, Cd, and Cr were chosen for further study since they are often present in high concentrations in ash leachates and since they all display different leaching behaviors. Figure 3 shows the extracted metal fractions. Numbers above 100% are merely reflections of the uncertainties associated with the experiments. Dissolution of the iron oxides in 0.5 M HCl reveals significant differences in kinetics depending on their stability and crystallinity (46, 47). In our study similar differences in dissolution rates were observed between heated and unheated samples. In accordance with the changes observed in the iron oxide phases by oxalate extractions and Mo ¨ ssbauer spectroscopy, extractions showed only minor changes in iron oxide stability after heating to 600 °C and a major improvement in stability was noted after heating to 900 °C. The extractions indicated that a major part of the heavy metals was associated with particle surfaces and, thus, released readily in the strongly acidic environment (pH 0.40.5). However, Cr and possibly Cd integrated partly in the solid product after thermal treatment, whereas Pb remained associated with the surface. Pb, Cd, and Cr, like Fe, followed similar extraction patterns in both products treated at 50 and 600 °C. At 900 °C, the SD Ferrox product showed much 4626

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slower and fairly parallel dissolution of Fe, Cd, and Cr, suggesting the presence of a more stable iron oxide phase containing Cd as well as Cr. Similar patterns were observed for the FA Ferrox product at 900 °C, except that the association of Cd with the crystalline iron structure was less substantial, allowing extraction of close to 100% of the Cd after 195 h. Also the binding of Cr seemed slightly less prominent in the FA product and 20% remained readily extractable after treatment at 900 °C. These differences may be explained by the higher content of iron in the SD product and additionally for Cr by the possible differences in redox state. Pb was found relocated to sites or separate phases outside the crystalline iron structure, easily accessible for dissolution by HCl, whereas Cd and Cr clearly stabilized in the solid structure after treatment at 900 °C. These results, thus, strongly support our leaching test data. The iron oxide phase is expected to control the release of heavy metals. However, gypsum and other calcium based compounds may also contribute to the metal binding properties (48-50). As a result, metals may substitute for calcium (as cations) or for sulfate (as oxyanions). Gypsum decomposes thermally after heating to 900 °C and a dense structure of anhydrite remains (23). As with iron oxides, this nonporous anhydrite compound may be capable of holding back metal ions. Other calcium based minerals are also capable of taking up substantial amounts of heavy metals, e.g. ettringite (3CaO‚Al2O3.3CaSO4‚32H2O) (49, 50). The Al content of the two APC residues is stoichiometrically high enough to form an ettringite based sink for all Pb and Zn present. Ettringite, however, was not identified by XRD, and it is, therefore, not expected to contribute to the heavy metal binding properties of the Ferrox products. Environmental and Technological Implications. Thermal treatment of iron oxide stabilized APC residues provides a stable product with reduced water content, a dense anhydrite structure, and a crystalline maghemite/hematite phase which supply a good sink for parts of the heavy metals present. However, the binding of trace metals changes significantly as the stabilized products are heated, meaning that most metals become more readily available for leaching. The initially amorphous iron oxides provide a significantly

large surface area, possibly controlling the release of toxic metals. Silicate and other contaminating ions help in stabilizing this meta-stable product, suggesting that at ambient temperatures the phase transformation will be very slow if at all significant. This, on the other hand, suggests that the amorphous product will remain vulnerable over a very long time toward reductive dissolution or metal desorption caused by acidic conditions. These conditions should, therefore, be avoided if unheated Ferrox products are landfilled. Hence, the overall effect of treating a Ferrox stabilized APC residue thermally is an initially more contaminated leachate but also a more stable product in the long term.

Acknowledgments The technical assistance by Carina Aistrup, Pernille Du¨hring, Christel Mortensen, and Bent Skov is gratefully acknowledged. Amagerforbrænding, Vestforbrænding, and AV Miljø are thanked for their economical support and for providing the APC residues. Susan Stipp and Michael Hochella are acknowledged for their structural and analytical analyses of the Ferrox products. Rajendra K. Bordia acknowledges support for this research from the United States National Science Foundation (Grant numbers DMR 9410981 and DMR 9257024). Thomas H. Christensen acknowledges support for his visit to University of Washington from the Danish Research Council (STVF: 9800462).

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Received for review February 9, 2000. Revised manuscript received May 3, 2000. Accepted August 10, 2000. ES0009830

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