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TECHNICAL REPORTS

Heavy Metals in the Environment

Leaching of Carbonated Air Pollution Control Residues Using Compliance Leaching Tests

^a State Key Laboratory of Pollution Control and Resources Reuse, Tongji University, Shanghai 200092, China
^b Chemical Engineering Department, National Taiwan University, Taipei, Taiwan, 10617

^* Corresponding author (solidwaste{at}mail.tongji.edu.cn, xhpjk{at}mail.tongji.edu.cn)

Received for publication August 14, 2005.

	ABSTRACT

TOP ABSTRACT INTRODUCTION METHODS AND MATERIALS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
The leaching characteristics of air pollution control (APC) residues collected in Shanghai, China, were compared by performing three compliance leaching tests. These were the standard Chinese method for determining the leaching toxicity of solid waste (GB 5086.1-1997), the USEPA's Toxicity Characteristic Leaching Procedure (TCLP), and the new European shake test (EN 12457-3). In particular, behaviors of raw samples and samples that had been subjected to natural aging were compared. Both the leaching tests and natural aging substantially affected the leaching results concerning the APC residue samples. Most importantly, EN and GB tests classified the raw APC residues as hazardous, but the residues passed the TCLP test as nonhazardous. After it had been naturally aged for 720 h, however, the aged sample was classified as hazardous by the TCLP and EN tests, but as nonhazardous by the GB test. Metals that are thought to have been immobilized by carbonation were released at pH 6.3. Model calculations based on the geochemical thermodynamic equilibrium model MINTEQA2 revealed that the formation of metal carbonates did not correspond to the noted change in the leaching behaviors in the three leaching tests. Rather, the partial neutralization of alkaline ash by dissolved CO₂ changing the final pH of the leachate dominated the leaching characteristics. The leaching results showed a change in leachate pH.

Abbreviations: AAS, atomic absorption spectrophotometer • AFS, atomic fluorescence spectrometer • APC, air pollution control • EC, electrical conductivity • EN, European shake test (EN 12457-3) • GB, Chinese method for determining the leaching toxicity of solid waste (GB 5086.1-1997) • L/S, liquid to solid ratio • MSW, municipal solid waste • ORP, oxidation–reduction potential • TCLP, Toxicity Characteristic Leaching Procedure

	INTRODUCTION

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COMPLIANCE OR REGULATORY LEACHING TESTS have been performed to identify potential hazards of disposing wastes at land disposal sites (Van der Sloot et al., 1997; Halim et al., 2004). All allowed leaching tests have been conducted under specified conditions, so they may not be feasible when the characteristics of the samples and/or field conditions are not those that have been allowed for (USEPA, 1995; Van der Sloot, 2002; Jing et al., 2004).

The Toxicity Characteristic Leaching Procedure (TCLP) has been criticized recently because of the overly broad use of the test and some technical aspects of the method (Kosson et al., 2002; Townsend et al., 2003). The criticisms include the following. First, TCLP was developed to simulate the worst-case scenario of the co-disposal of waste and municipal solid waste (MSW), so an acidic leaching fluid cannot be used to classify nonputrescible wastes that have not been co-disposed with municipal wastes (Halim et al., 2005). Second, TCLP does not correctly evaluate the potential release of highly alkaline wastes, including cementitious wastes, whose alkalinity would rapidly neutralize the acidity of the leaching fluid (Poon and Lio, 1997; Halim et al., 2003, 2004). Finally, TCLP underestimates the leaching of heavy metals in a MSW landfill because these metals complex with organic materials in the MSW leachate and undergo other reactions (Hooper et al., 1998; Kendall, 2003; Ghosh et al., 2004). Other regulatory leaching tests may also encounter similar shortcomings as those faced by TCLP. This fact has not yet been considered in the literature.

Aging has been proposed as an approach to immobilizing heavy metals in alkaline wastes (bottom ash and air pollution control [APC] residues, for example) (Walton et al., 1997; Ecke et al., 2002; Shimaoka et al., 2002; Van Gerven et al., 2005). The long-term stability of carbonated waste, particularly when subject to acid shock, remains an open question. Meanwhile, as established herein, the regulatory leaching tests may easily neglect the potential hazards associated with these carbonated alkaline wastes.

The geochemical thermodynamic equilibrium model MINTEQA2 (Allison et al., 1991) has been used to provide useful insights into leaching characteristics of heavy metals (Chandler et al., 1997; Meima and Comans, 1998; Kosson et al., 2002). Modeling leaching behavior can improve our understanding of the leaching mechanisms and predictions of long-term leachability. Therefore, MINTEQA2 has been applied to describe the speciation and equilibrium concentration of heavy metals in a leaching system of APC residues (Eighmy et al., 1995; Van der Bruggen et al., 1998; Van Herck et al., 2000).

The levels of leached metals from a sample of APC residue from Shanghai were compared using the following three compliance leaching tests: the standard Chinese method for determining the leaching toxicity of solid waste (GB 5086.1-1997); TCLP, developed by the USEPA; and the new European shake test (EN 12457-3). pH-stat tests were also performed to characterize the leaching of APC residues over a wide range of pH. Samples were raw residues and naturally aged residues. Various leaching tests involved very different leaching reagents, liquid to solid ratios (L/S), one-step or multi-step extraction procedures, short or long leaching times, and other variables, so MINTEQA2 was used to identify the process parameters that dominate metal leaching and to elucidate the effect of carbonation on the residues.

	METHODS AND MATERIALS

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The Samples
The APC residue sample was obtained from the Yuqiao MSW Incineration Plant (mass burn) in Shanghai, which treats around 1000 Mg of MSW daily. The plant removes acid gas, heavy metals and dioxins, and particulates from the flue gas using lime slurry (10% w/w), activated carbon (50 mg m^–3), and bag filters, respectively. The resulting residues were collected from the semidry reactors and the bag filters, which contained particles from the incineration chamber, reaction products, and some excess reactants, mainly Ca(OH)₂. The raw residues had a low water content of 1.2% w/w.

The metal contents in the APC residues were measured. First, 0.5 to 0.7 g of each sample was mixed with 20 mL of concentrated nitric acid, it was then heated until only 2 to 3 mL of nitric acid remained. The concentrations of Ca, Cd, Cr, Cu, Fe, Mg, Ni, Pb, and Zn in the filtrate obtained from this suspension were analyzed using an atomic absorption spectrophotometer (AAS) (Model AA320N; Shanghai Analytical Instrument Overall Factory, China), and that of Hg was determined using an atomic fluorescence spectrometer (AFS) (Model XGY1012; Institute of Geophysical and Geochemical Exploration of the Chinese Academy of Sciences, China). The total Si and Al contents in the APC residues were estimated by averaging over 40 randomly selected images obtained by SEM-EDAX (Model XL30; Philips, Eindhoven, the Netherlands).

The total amounts of SO₄^2–, Cl^–, and PO₄^3– in the APC residues were determined by leaching the sample using 1 mol L^–1 HNO₃ solution (L/S = 10 L kg^–1) and then conducting BaSO₄ gravimetric analysis, AgNO₃ titration, and molybdene-blue spectrophotometry, respectively (State Environmental Protection Administration of China, 2002). The amounts of CO₃^2– in the raw or carbonated samples were measured by mixing 1 g of the sample with 10 mL of 1 mol L^–1 HNO₃ and then measuring the yield of CO₂ gas using a gas chromatograph (Model GC102; Shanghai Analytical Instrument Overall Factory, China).

Sample Aging and Leaching Tests
The raw residues were mixed with distilled water to a moisture content of 20% w/w, and then aged by direct contact with the ambient air, to a thickness of 20 mm. Room temperature and the humidity of the air during aging were recorded against time. Carbonated samples were collected at random from the trays after 0, 10, 25, 50, 96, 192, 360, and 720 h of aging. The collected samples were divided into four equal parts. One was used to measure the water content; the three leaching tests (GB, TCLP, and EN) were performed on the other three.

Figure 1 schematically depicts the procedures associated with these three tests. In the GB test (State Environmental Protection Administration of China, 1997), 100 g (dry weight) of raw or carbonated sample was mixed with 1 L of distilled water (L/S = 10 L kg^–1) and tumbled at 30 ± 2 rpm for 18 h. The mixture was then vacuum-filtered through a 0.45-µm membrane. In the TCLP test (USEPA, 1986), a 50-g sample was mixed in 1 L of acetic acid (0.1 mol L^–1 at pH 2.9) in a PE bottle, which was tumbled at 30 ± 2 rpm for 18 h; the extract was vacuum-filtered using a 0.8-µm membrane. In the EN test (Hage and Mulder, 2004), the simulated rainfall (diluted nitric acid at pH 4.0) was mixed with 100 g of ash sample at L/S = 2 L kg^–1 in a PE bottle. The bottle was then tumbled for 6 h and the suspension was filtered initially through a 0.45-µm membrane. The solid that remained on the membrane was resuspended in rainwater at L/S = 8 L kg^–1 and tumbled for another 18 h. The filtrate from this suspension was mixed with that collected from the first filtration, and measured. The filtrates of GB, TCLP, and EN were analyzed to determine their metal concentrations (Ca, Cd, Cr, Cu, Hg, Mg, Ni, Pb, and Zn) using an AAS and an AFS. The pH, electrical conductivity (EC), and oxidation–reduction potential (ORP) of the filtrates were also measured.

View larger version (19K): [in this window] [in a new window]   Fig. 1. The procedures adopted by the three mentioned leaching tests. EN, European shake test (EN 12457-3); GB, Chinese method for determining the leaching toxicity of solid waste (GB 5086.1-1997); TCLP, Toxicity Characteristic Leaching Procedure.

	RESULTS

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Characterization of the Air Pollution Control Residue
Table 1 presents the composition of the APC residue sample. The raw ash comprised excess calcium, mainly formed in the semidry APC process. The concentrations of heavy metals followed the order Zn > Pb > Cu > Cr > Ni > Cd > Hg. The concentrations of the metals varied mildly following carbonation (less than 7% except in Hg test), because of sampling errors and the absorption of CO₂. The CO₃^2– content rose markedly from 2.5 to 8.6% during the carbonation reaction.

Table 2 also lists the leaching levels at pH 6.3 and 3.1, obtained using pH-stat tests. These results are used as references to determine the amounts of metals that would be released from an APC residue under acid shock. The leaching levels at pH 3.1 normally exceeded those at pH 6.3, which is clearly expected. However, even under approximately neutral conditions (pH 6.3), which are easily achieved when the APC residue is co-disposed of with the typical Chinese municipal solid waste that includes a high proportion of food waste (He et al., 2005b), the amounts of Cd, Cu, Ni, and Zn released greatly exceeded the corresponding values obtained from regulatory leaching tests. The TCLP underestimated the leaching levels of all metals at pH 6.3, except for Cr and Hg.

Leaching Toxicity of the Carbonated Air Pollution Control Residues
Figure 2 presents the leaching levels of heavy metals from the carbonated APC residues, obtained using the three compliance leaching tests. The final leachate pH declined continuously with the period of carbonation, becoming constant after about 200 h. According to the EN and GB tests, the corresponding EC values also decreased, but that of the TCLP test remained unchanged. The ORP values in all three tests increased with carbonation. Carbonation would provide protons to neutralize alkalis in the residues, reducing pH, increasing ORP, and reducing EC in the leaching tests.

View larger version (24K): [in this window] [in a new window]   Fig. 2. Leachate characteristics of the air pollution control (APC) residues during the carbonation process using different compliance leaching tests. EN, European shake test (EN 12457-3); GB, Chinese method for determining the leaching toxicity of solid waste (GB 5086.1-1997); TCLP, Toxicity Characteristic Leaching Procedure.

The TCLP test revealed three different groups of metals. Of the seven metals reported herein, the level of only Cr was found to fall with carbonation, but not as much as the metals in the first group in the EN and GB tests. The leaching levels of Cd, Cu, Zn, and Ni increased with carbonation. In the third group, the low leaching levels of Hg and Pb remained unchanged during carbonation.

The leaching results obtained from carbonated APC residues also differed with the leaching test used. In particular, after 720 h of carbonation, the Cd level measured by TCLP and the Hg level measured by EN exceeded their corresponding limits, while the GB test would classify the same sample as nonhazardous. Ecke et al. (2003) also noted the increased mobility of Cd following carbonation.

Modeling Leaching
MINTEQA2 was applied to determine the equilibrium concentrations of metals and other ions, as functions of pH. The input molar concentrations for each component were based on the determined total concentrations in raw APC residues (Table 1) divided by the liquid to solid ratio and the molar mass of the component, but the CO₃^2– concentration was based on the total content of the APC residue carbonated for 720 h: 0.15 mol L^–1 of CO₃^2– in the GB and EN tests and 0.075 mol L^–1 in the TCLP test. Other input variables were the metal [Al³⁺, Ca²⁺, Cd²⁺, Cr(OH)₂⁺, Cu²⁺, Mg²⁺, Ni²⁺, Pb²⁺, Zn²⁺] concentrations, H₄SiO₄ concentration, and anion (Cl^–, SO₄^2–, PO₄^3–) concentrations. The calculated Hg concentrations did not converge to physically meaningful values, so were not considered herein. The log K value of Pb(OH)₂ concentration was taken as –10.15 from Van der Bruggen et al. (1998). Other log K values referred to the default values in MINTEQA2. The ionic strength was set to 0.3 mol L^–1, based on the concentrations of the metals in solution during the experiment. As in the work of Eighmy et al. (1995), Van der Bruggen et al. (1998), and Van Herck et al. (2000), neither the role of gas phase nor the metal sorption reactions were considered, and the pH was input as a fixed value, ranging between zero and 14. The temperature in the calculations was set to 25°C. In modeling, the case without carbonation, the initial CO₃^2– was set to zero.

Figure 3 summarizes the results of the calculations, along with the leaching data obtained from all three regulatory leaching tests. Results with or without carbonation (with or without initial input of CO₃^2–) were denoted as test-C or test-C-1, respectively. The plotted leaching curves based on the EN and GB tests were similar, so were shown as a single curve, such as EN/GB-C. For comparison, the calculated leaching behaviors of pure heavy metal carbonates were plotted as test-C-2: the temperature, the ionic strength, and the pH range were the same as in the above calculation and no gas phase or sorption was considered. For example, the curves EN/GB-C-2 and TCLP-C-2 in Fig. 3a plot the equilibrium concentrations of CdCO₃(s), in an environment with a constant pH. Calculations for Cr are lacking since equilibrium data for Cr₂(CO₃)₃ were not available.

View larger version (25K): [in this window] [in a new window]   Fig. 3. pH-dependent leaching profiles of heavy metals from the carbonated air pollution control (APC) residues based on experimental (, , x) and calculated results (GB/EN-C and TCLP-C are the results without input of CO32–; GB/EN-C-1 and TCLP-C-1 are the results with input of 0.15 and 0.075 mol L–1 CO32–, respectively; GB/EN-C-2 and TCLP-C-2 are the results of pure heavy metals carbonates). EN, European shake test (EN 12457-3); GB, Chinese method for determining the leaching toxicity of solid waste (GB 5086.1-1997); TCLP, Toxicity Characteristic Leaching Procedure.

	DISCUSSION

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Model calculations were consistent with present experimental results for Cr, Cu, Pb, and Zn, but overestimated the leaching levels of Cd and Ni at pH < 9.5 (Fig. 3). Precipitation–adsorption reactions, not considered in the calculations, may have been responsible for the noted discrepancies for Cd and Ni.

Figure 3 indicates that the heavy metals release curves can be divided into two main groups in the range pH 3 to 12.3: V-shaped (Hg, Pb, and Zn) and L-shaped (Cd, Cr, Cu, and Ni). According to the EN and GB tests, in which the final suspension pH ranged from 12.3 to 9.8, the Hg, Pb, and Zn concentrations were to the right of the V curves, so their leaching concentrations fell as pH decreased (or with carbonation, as in Fig. 2). Meanwhile, the concentrations of Cd, Cr, Cu, and Ni were located at the bottom of the L curves, indicating a weak dependence on carbonation. According to TCLP, however, all metals were initially at low concentrations, and the leaching levels of Cd, Cu, Pb, and Zn began to increase as the pH dropped (as carbonation proceeded). This result was particularly evident for both Cd and Zn, since their raw samples were originally located to the left (rising parts) of the curves.

According to model calculations, adding CO₃^2– reduced the equilibrium concentrations of all metals, but only at low pH. Over the pH ranges used in the three regulatory leaching tests, most metal carbonates, including CdCO₃(s), were unlikely to form, because most of the CO₃^2– was converted to CaCO₃ (Fig. 3a) in the leaching system. According to the model, the species that were likely to be precipitated were Cd(OH)₂ (pH 12–13.5), Cd₄(OH)₆SO₄ (pH 9.5–11.5), and otavite (CdCO₃) (pH 7–9). At pH > 5, Cr precipitates as Cr₂O₃. At pH 7 to 13, Cu was present mostly as tenorite (CuO). At pH > 9, NiO appeared. At pH 8 to 13.5, Pb(OH)₂ was the predominant precipitate. At pH > 7.5, Zn was present as ZnO. Regardless of the forms of the precipitates, they dissolved considerably in an acidic environment, showing the risk of mass release in response to acid shock.

The aforementioned results suggest the following regarding the carbonation and leaching of APC residues. During aging, the CO₂ gas was absorbed by the wet, alkali residues (20% w/w water) and most of it was transformed to metal carbonates, primarily CaCO₃ (He et al., 2005a). After the carbonated residue was suspended in leaching liquor in the regulatory leaching tests, the carbonates reacted with water to neutralize partially the ash alkaline, reducing the pH of the suspension (Fig. 2). The change in pH then influenced the speciation of heavy metals in the suspension, yielding very different leaching outcomes. As presented in Fig. 3, the metals exhibit low solubility, with or without carbonation, at pH 10.5 to 12.3 for Cd, 7 to 12.3 for Cr, 6.5 to 12.3 for Cu, 9 to 12.3 for Ni, 6 to 12 for Pb, and 8 to 12 for Zn. Following carbonation, the EN and GB tests demonstrated that pH had decreased from 12.3 to around 9.8. Hence, these leaching tests revealed that carbonation stabilized the metals that were relatively soluble at pH 12.3 but insoluble at pH 9.8 (Cu, Pb, and Zn). However, the TCLP test detected the bursting of metals from the APC residue that were relatively insoluble at pH 9.1 but easily soluble at pH 7. At a pH of 3.1, the amounts of metals released greatly exceeded the amounts indicated by the regulatory leaching tests.

The result herein, that pH dominates the characteristics of leaching from APC residues, is consistent with the conclusions of Brunori et al. (2001), Dijkstra et al. (2004), and Halim et al. (2005). Restated, although complex reactions occurred during aging (Sabbas et al., 2003), the final pH of the leachate, as the integrated effect of these physical and chemical influencing factors, became the major index that affected equilibrium leaching.

	CONCLUSIONS

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The leaching characteristics of an APC residue sample generated in Shanghai were compared, using the following three compliance leaching tests: the standard Chinese method for determining the leaching toxicity of solid waste (GB 5086.1-1997), the Toxicity Characteristic Leaching Procedure developed by the USEPA, and the new European shake test (EN 12457-3). In particular, the behavior of the raw sample was compared to those of samples subjected to natural aging. The three leaching tests yielded various levels of raw or aged APC residues. The final pH, EC, and ORP of the suspensions, according to EN and GB, were close, while the strong buffer capacity in the TCLP test led to the lower final pH of the samples determined by that method. The leaching characteristics of the seven tested metals fall into three groups: (EN GB < TCLP) Cd and Ni; (GB > EN > TCLP) Cu, Hg, Pb, and Zn; and (EN < GB < TCLP) Cr.

Natural aging (carbonation) yields a lower final leachate pH than that of the raw samples. The changes in the leached metals determined by EN and GB tests were similar. The leaching level decreased markedly with carbonation for Cu, Hg, Pb, and Zn; the level increased with carbonation for Ni and Cr; and the level was unaffected by carbonation for Cd. In the TCLP test, three patterns were also observed, but they were associated with different groups of metals: leaching level decreased with carbonation (Cr); leaching level increased with carbonation (Cd, Cu, Zn, and Ni); and leaching level was not significantly affected by carbonation (Hg and Pb).

Hence, both the leaching tests and natural aging substantially affected the leaching results obtained from the APC residue samples. In particular, the EN and GB tests classify the raw APC residue herein as hazardous, since the leached levels of Hg (0.134 and 0.43 mg L^–1) and Pb (32.3 and 62.6 mg L^–1) exceeded their corresponding limiting values, but the sample safely passed the TCLP test, as nonhazardous waste for safe disposal. After 720 h of natural aging, the Cd level determined by TCLP and the Hg level determined by EN exceeded their corresponding limiting values, although the GB test classified the sample as nonhazardous. The leaching levels at pH 6.3 and 3.1 obtained using pH-stat tests revealed that significant amounts of heavy metals were released from both raw and carbonated samples. Therefore, the potential risk of heavy metal release from APC residues in a weakly acidic environment, such as when the ash was co-disposed of with typical Chinese municipal solid waste that contains a large fraction of food waste, may be overlooked.

The geochemical thermodynamic equilibrium model MINTEQA2 was applied to elucidate the chemical speciation of metals in the three leaching systems of interest. The model calculations indicate that adding CO₃^2– reduced the equilibrium concentrations of all metals, but only at low pH. Over the pH ranges used in the three regulatory leaching tests, most metal carbonates, including CdCO₃(s), were very unlikely to form. Therefore, the change in the leaching characteristics of samples during natural aging did not contribute to the formation of metal carbonates, but to the partial neutralization of the alkaline suspension by dissolved CO₂. The natural aging of the APC residues exhibited two dependences on pH: V-shaped (Hg, Pb, and Zn) and L-shaped (Cd, Cr, Cu, and Ni), according to the buffering capacities in the three leaching tests, and the alkalinity of the tested APC residues that leached out during the leaching tests (pH from 12.3 to 9.8 for the EN and GB tests and from 9.0 to 6.9 for the TCLP test).

	ACKNOWLEDGMENTS

 
We thank the Shanghai Council of Science and Technology for the financial support through the project "Research on beneficial use of MSW incineration residues and its demonstration project" (032312043).

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