Temporary stabilization of air pollution control residues
using carbonation
Hua Zhang
a, Pin-Jing He
a,*, Li-Ming Shao
a, Duu-Jong Lee
baState Key Laboratory of Pollution Control and Resources Reuse, Tongji University, Shanghai 200092, China bChemical Engineering Department, National Taiwan University, Taipei 10617, Taiwan, ROC
Accepted 5 February 2007 Available online 3 April 2007
Abstract
Carbonation presents a good prospect for stabilizing alkaline waste materials. The risk of metal leaching from carbonated waste was investigated in the present study; in particular, the effect of the carbonation process and leachate pH on the leaching toxicity of the alka-line air pollution control (APC) residues from municipal solid waste incinerator was evaluated. The pH varying test was conducted to characterize the leaching characteristics of the raw and carbonated residue over a broad range of pH. Partial least square modeling and thermodynamic modeling using Visual MINTEQ were applied to highlight the significant process parameters that controlled metal leach-ing from the carbonated residue. By lowerleach-ing the pH to 8–11, the carbonation process reduced markedly the leachleach-ing toxicity of the alkaline APC residue; however, the treated APC residue showed similar potential risk of heavy metal release as the raw ash when sub-jected to an acid shock. The carbonated waste could, thereby, not be disposed of safely. Nonetheless, carbonation could be applied as a temporary stabilization process for heavy metals in APC residues in order to reduce the leaching risk during its transportation and stor-age before final disposal.
2007 Elsevier Ltd. All rights reserved.
1. Introduction
Carbonation has been recognized to be an important process affecting alkaline materials (waste) such as bottom ash from municipal solid waste incinerators (MSWI) and cement-stabilized waste (Chimenos et al., 2000; Garra-brants et al., 2004; Gervais et al., 2004; Meima et al., 2002; Van Gerven et al., 2005; Walton et al., 1997). It has also been proposed as a means to stabilize Pb and Zn in air pollution control (APC) residues from MSWI (Bone et al., 2003; Ecke et al., 2003a; Kim et al., 2003). The effect of carbonation on the leaching toxicity of these samples was intensively investigated (Freyssinet et al., 2002; Meima and Comans, 1998; Polettini and Pomi, 2004; Van Gerven et al., 2004, 2005; Yu et al., 2005), which could be summarized as: (1) lowering pH of the leachate;
(2) changing the metal solubility due to precipitation of metal carbonates or formation of oxyanions (Cr and Mo); (3) reducing release of certain metals (Cu and Mo) by sorption to the neoformed minerals (in 1.5-year weath-ered bottom ash); and (4) decreasing the matrix porosity because of the formation of calcite.
However, certain leaching results have been interpreted based on single batch tests, which may obtain discrepant results (Astrup et al., 2006; Li et al., 2001). Detailed leach-ing behavior of uncarbonated and carbonated ash has been investigated using pH varying test or semi-dynamic leach-ing tests (Garrabrants et al., 2004; Meima et al., 2002; Van Gerven et al., 2005, 2006); most of these tested sam-ples were bottom ash or cement-stabilized ash. Leaching behavior of carbonated APC residues without cement sta-bilization is rarely discussed. Furthermore, it is inconclu-sive whether the change in metals retention properties are due to the precipitation of metal-carbonates as some researchers have suggested (Bone et al., 2003; Freyssinet et al., 2002) or to a shift in pore water pH as a result of 0956-053X/$ - see front matter 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.wasman.2007.02.005 *
Corresponding author. Tel./fax: +86 21 6598 6104. E-mail address:solidwaste@mail.tongji.edu.cn(P.-J. He).
www.elsevier.com/locate/wasman Waste Management 28 (2008) 509–517
carbonation which changed the solubility of metals (San-chez et al., 2002; Van Gerven et al., 2004). Therefore, the effects of environmental change on the leaching potential of carbonated APC residues have not been satisfactorily explored.
To address these problems, a previous study (He et al., 2006a) examined the leaching behaviors of the APC residue during the natural aging process evaluated by three kinds of regulatory leaching tests. It was demonstrated that both leaching tests and carbonation substantially affected the leaching results. Model calculations based on the geochem-ical thermodynamic equilibrium model MINTEQA2 sug-gested that the formation of metal carbonates did not correspond to the noted change in the leaching behaviors. Rather, the partial neutralization of alkaline ash by
dis-solved CO2, lowering the final pH of the leachate,
domi-nated the leaching characteristics. Metals immobilized by carbonation will be released again under an acidic environ-ment. However, the experimental proof to demonstrate this proposal remained preliminary.
This work is different fromHe et al. (2006a)in the
fol-lowing three aspects. (1) The pH varying test was con-ducted over pH 0.4–12.9 to demonstrate the leaching characteristics of raw and carbonated samples. (2) Effects of carbonation were analyzed using partial least square (PLS) modeling (Umetrics AB, 2005) of experimental data and using thermodynamic modeling, Visual MINTEQ (Allison et al., 1991; Gustafsson, 2005), considering redox reactions for illustrating the impact of carbonation on leaching characteristics. (3) Based on the experimental find-ings, the use of waste carbonation as a temporary pretreat-ment process for heavy metal stabilization was proposed. 2. Methods and materials
2.1. Residues
The APC residue samples were collected from the flue gas treatment units of a MSW incineration plant (mass burn) with a treatment capacity of approximately 1200 t/ d in Shanghai City, China, equipped with a semi-dry reac-tor with lime slurry injection, activated carbon adsorption, and bag filters to remove acid gas, heavy metals, dioxins,
and particulate matter, respectively.Table 1lists the
chem-ical compositions of the raw residue. Due to the presence of
a semi-dry process unit, calcium was the most abundant element in the APC residue. Moreover, the residue was
composed of Cl, K, Na, Fe, Mg, SO24 , and CO23 . The
concentrations of trace elements followed the order: Zn > Pb > Cu > Cr > Ni > Cd > As > Hg.
The raw APC residue was mixed with distilled water to make a sample of 20 wt.% moisture content, which was fed
into two columns (U 5 cm· 25 cm), with a 500-g residue
sample in each. In column 1, the residue was carbonated
at accelerated rates by flowing through pure CO2at a rate
of 0.4 m3/h for 10 h (rapidly carbonated). In column 2, air
(0.03% v/v CO2) was flowing through at 0.4 m3/h for 7
days to simulate a normal aging process (slowly
carbonated).
The amounts of carbonate (CO23 ) in raw, rapidly and
slowly carbonated residues were measured by mixing 1 g
of residue with 10 ml of 1 mol/l HNO3, and then analyzing
the CO2 gas release using a gas chromatograph (GC102,
Shanghai Analytical Instrument Overall Factory, China). 2.2. Leaching test
Leaching tests were conducted based on a regulatory leaching test (State Environmental Protection Administra-tion of China, 1997); 100 g of each residue sample (raw, rapidly and slowly carbonated) was mixed with 1 l of dis-tilled water (liquid to solid ratio, L/S = 10 l/kg) and tum-bled at 30 ± 2 rpm for 18 h. The mixture was vacuum-filtered through a 0.45-lm membrane. The leachate pH and ORP were recorded and the concentrations of Cd, Cr, Cu, Ni, Pb and Zn in the filtrate were then analyzed using an atomic absorption spectrophotometer (AAS), while those of As and Hg were analyzed using an atomic fluorescence spectrometer (AFS).
The pH varying test was carried out to determine the pH-dependent leaching behavior of the residues (van der Sloot et al., 1997). The raw residue sample was equally divided into 15 parts, each of 30-g by weight, which were extracted individually for 48 h by using 300-ml of NaOH solutions of 0.5, 0.2, 0.1, 0.05 mol/l, distilled water, and
HNO3 solutions of concentrations of 0.1, 0.2, 0.4, 0.6,
0.8, 1.0, 1.2, 1.5, 1.8, and 2.0 mol/l, respectively. A similar extraction procedure was adopted to the rapidly and slowly carbonated residues; however, they were each divided into only 10 parts, which were individually extracted by using NaOH solutions of concentrations of 0.5, 0.2, 0.05 mol/l,
distilled water, and HNO3 solutions of 0.2, 0.4, 0.5, 0.6,
0.8, 1.0 mol/l, respectively. The final pH and the metal con-centrations in the filtrates were measured.
The variability of Hg and As concentration in duplicates was within 10%, and other heavy metals concentration within 3%.
2.3. Leaching modeling
Solubility of carbonates is greatly dependent on leachate pH. Therefore, Visual MINTEQ was applied herein to Table 1
Raw APC residue composition
Major components Content (wt.%) Trace elements Content (mg/kg)
Ca 30 ± 1.4 As 71 ± 1.5 K 3.2 ± 0.04 Cd 57 ± 0.2 Na 2.9 ± 0.04 Cr 450 ± 21 Fe 3.0 ± 0.07 Cu 980 ± 19 Mg 1.4 ± 0.02 Hg 39 ± 4.2 Cl 9.4 ± 0.10 Ni 130 ± 2.0 SO2 4 1.5 ± 0.03 Pb 2620 ± 60 CO23 1.4 ± 0.14 Zn 5400 ± 140 PO34 <0.001 Mn 1000 ± 5
determine the chemical stability of pure metal carbonates as a function of pH, by solely inputting the metal carbon-ates as finite solid.
The equilibrium species of metals and As leached from the real carbonated samples were also modeled, by inputting in Visual MINTEQ the aqueous ion concentra-tions, and the pH and Eh values (18 mV for slowly car-bonated ash and 70 mV for rapidly carcar-bonated ash) measured in the regulatory leaching test, allowing precipi-tation/dissolution, redox, aqueous complexation reactions.
The input molar concentrations for each component (Al3+,
Ca2+, Cd2+, CrðOHÞþ2, Cu2+, Fe3+, H3AsO4, Hg2þ2 ,
H4SiO4, Mg2+, Mn2+, Ni2+, Pb2+, Zn2+, Cl, CO23 ,
SO24 , PO34 ) were based on the determined total amount
(Table 1) divided by the L/S ratio 10 and the molar mass of the component. The input redox couples included
H3AsO3=AsO34 , CrðOHÞ
þ 2=CrO 2 4 , Cr 2þ=CrðOHÞþ 2, Cu + / Cu2+, Fe2+/Fe3+, Hg2þ2 =HgðOHÞ2, HS /SO2 4 . The log K
value of Pb(OH)2concentration was taken as10.15, from
van der Bruggen et al. (1998). The other log K values referred to the default values in Visual MINTEQ. This work disregards the role of gas phase and surface complex-ation/precipitation (Eighmy et al., 1995; van der Bruggen et al., 1998; van Herck et al., 2000). The temperature in
cal-culations was set at 25C.
2.4. Multivariate data analysis
PLS modeling was used to highlight the inter-relation-ships between three process parameters (factors), being extractant concentrations (negative for alkaline and posi-tive for acid solution), extent of carbonation (rapid or slow, denoted by carbonate content), and leachate pH, on the leaching behavior (response variables).
3. Results
3.1. Stabilization of heavy metals and As in the APC residue using carbonation
During carbonation, CO2 was absorbed and reacted
with alkaline compounds in the residue. After 10 h of rapid
carbonation, the CO23 content in the residue increased
from 1.43 wt.% to 8.85 wt.%, comparable to the result of
7.2 wt.% in Van Gerven et al. (2005), and the leachate
pH (extracted by distilled water at L/S = 10 l/kg) decreased from 12.0 to 8.7 (Fig. 1), which was comparable to the car-bonation level of natural aging for more than 1 month (He et al., 2006a). Slow carbonation for 7 days, however, obtained a lower carbonation level, revealed by the low
CO23 content of 4.08 wt.% and relatively high leachate
pH of 10.1.
After carbonation, the acid neutralization capacity of
the ash decreased from 4.2 mmol-H+/g-ash to 3.9 and
3.3 mmol-H+/g-ash for slowly carbonated and rapidly
car-bonated ash, respectively (data not shown), taken pH 7 as endpoint.
Fig. 1presents the leached concentrations of metals and As from the raw and carbonated APC residues. The con-centrations of Hg and Pb leached from the raw APC resi-due, particularly the latter, were higher than the limit values (0.05 and 3 mg/l, respectively) set by the Chinese identification standard for hazardous waste (State Environ-mental Protection Administration of China, 1996). Hence, the raw ash was classified as hazardous and might pose risk to environmental safety and public health following dis-posal (Chandler et al., 1997).
The leaching behavior of metals and As following car-bonation could be categorized into three types: (raw
resi-due carbonated residue) As (around the limit value),
Cd, and Cu; (raw residue > carbonated residue) Hg, Ni, Pb and Zn; and (raw residue < carbonated residue) Cr only. After carbonation, the leachability of Hg and Pb had become much lower than the corresponding limit val-ues, and Zn was not even detected in the leachate of the carbonated residues. The carbonated residues would be then classified as non-hazardous for disposal. Based on
similar observations, Ecke (2003b), Kim et al. (2003), and
Meima and Comans (1999) proposed that carbonation could be used as a stabilization technique for Pb and Zn in APC residues or bottom ash. As reported in the follow-ing section, usfollow-ing Visual MINTEQ modelfollow-ing, we examined whether the heavy metals in the APC residue were trans-formed into more stable carbonate or oxide forms follow-ing carbonation, or simply some hydroxides after pH changes.
3.2. Change of metal speciation during the carbonation process
Based on X-ray diffraction (XRD) analysis, He et al.
(2006b) observed the increase of calcite and gypsum in the carbonated residues; however, heavy metal carbonates were not detected by XRD because of their trace amount. As such, it is also difficult to determine directly the changes of heavy metal species in the carbonated ash by other non-destructive analytical methods. Visual MINTEQ is an equilibrium speciation model used to calculate the equilib-rium composition of aqueous solutions. Thus, it can be used to simulate aqueous speciation of the carbonated product subjected to a leaching system, although it is inca-pable of indicating reactions occurred in the carbonation process.
Fig. 2 illustrates the modeled pH-dependent leaching behavior of pure metal carbonates. Calculations for As and Cr are lacking since equilibrium data for their carbon-ates were not available. In a fully carbonated system, leach-ate pH of the APC residue could reach a value as low as 8.3 (Bone et al., 2003). Within this pH range 8–13, carbonates
of all heavy metals are relatively stable except CdCO3,
which becomes significantly soluble at pH < 10. When pH further decreases to less than 6, carbonates turn into
soluble species following the order CdCO3> NiCO3>
0 2 4 6 8 10 12 14
Raw ash SC ash RC as h Raw ash SC ash RC ash
Raw ash SC ash RC as h Raw ash SC ash RC ash
Raw ash SC ash RC as h Raw ash SC ash RC ash
Raw ash SC ash RC as h Raw ash SC ash RC ash
Raw ash SC ash RC as h Raw ash SC ash RC ash
pH 0 2 4 6 8 10 C o nc ent ra ti on of C O3 2- , % 0. 0 0. 4 0. 8 1. 2 1. 6 0. 0 0. 1 0. 2 0. 3 0. 4 0 2 4 6 8 10 12 0 20 40 60 0 0.02 0.04 0.06 0.08 0 2 4 6 8 10 12 0 10 20 30 40 50 0 20 40 60 Concentration of Zn, mg/l Concentration of Pb, mg/l Concentration of Cu, mg/l Concentration of Cr , mg/l Concentration of As, mg/l Concentration of Cd, mg/l
Concentration of Hg, mg/l Concentration of Ni, mg/l
Fig. 1. Leaching characteristics of the raw, slowly carbonated (SC) and rapidly carbonated (RC) ash. The broad-brush solid line is the limit value set by the identification standard for hazardous waste (State Environmental Protection Administration of China, 1996).
products will dissolve to a large extent when they encounter an acid shock.
In the leaching system of APC residue, much more com-plex results, as the combined effect of oxidation/reduction, hydration, competitive precipitation/dissolution and
com-plexation, would be observed.Tables 2 and 3 summarize
the modeling results of the carbonated residues under the extraction conditions of the regulatory leaching test. The release ratios of metals and As obtained by direct measure-ment and modeling were compared.
The major precipitates in the raw APC residue included
CaO, CaCO3, SiO2, CaSO4Æ2H2O, Mg3Si2O7Æ2H2O, and
Fe2O3, among which the former three species and
anhy-drite (CaSO4) had been detected by XRD (He et al.,
2004). After carbonation, the content of CaCO3and
gyp-sum (CaSO4Æ2H2O) significantly increased, consistent
with the XRD analytical results (He et al., 2006b). Most of the modeling results, except for Cd, were in agreement with the experimental results that the carbon-ated ash was of less leaching toxicity. Predicted Cd concen-tration was significantly discrepant with the observed value. According to Visual MINTEQ, Cd could completely release from the rapidly carbonated ash at pH 8.7; how-ever, only 1.14% of Cd was leached from the ash in the real
extraction system. Apul et al. (2005) and Meima and
Comans (1998)had also found that the predicted Cd con-centrations could be one order of magnitude higher than the measured concentrations even when a surface complex-ation model by hydrous ferric oxide (HFO) and aluminum (hydr)oxides was included. The use of a surface precipita-tion model did not significantly improve the match between observed and predicted concentrations either. The model-ing results for Cd are still inconclusive.
As the model predicted, heavy metals of the carbonated residues precipitated in the leaching system mainly as
hydroxide or oxide, Cd4(OH)6SO4, Cr2O3, Ni(OH)2,
Pb2(OH)3Cl, ZnO, respectively. As, Cu, Hg formed insoluble
Ca3(AsO4)2Æ4H2O (slowly carbonated ash) or Mn3
-(AsO4)2Æ8H2O (rapidly carbonated ash), CuFeO2 and Hg
in the elute. No heavy metal carbonates precipitated, since
CO23 preferentially reacted with Ca2+ and Mg2+ to form
CaCO3and MgCO3and precipitated from the leachate.
0 20 40 60 80 100 0 2 4 6 8 10 12 14 pH Dissovled content, % CdCO3 CuCO3 HgCO3 NiCO3 PbCO3 ZnCO3 CdCO3 CuCO3 Hg2CO3 NiCO3 PbCO3 ZnCO3
Fig. 2. Modeled pH-dependent leaching behavior of pure metal carbonates.
Table 2
Species of major precipitates in the raw and carbonated APC residues based on modeling results
Raw ash (pH = 12.0) Slowly carbonated ash (pH = 10.1) Rapidly carbonated ash (pH = 8.7)
Major precipitates Concentration (mol/l) Major precipitates Concentration (mol/l) Major precipitates Concentration (mol/l)
CaO 4.91E 01
Fe2O3 2.60E 02
SiO2 2.41E 01 SiO2 2.62E 01 SiO2 2.63E 01
Mg3Si2O7Æ2H2O 1.86E 02 Mg3Si2O7Æ2H2O 1.86E 02 Mg3Si2O7Æ2H2O 1.85E 02 CaSO4Æ2H2O 8.70E 03 CaSO4Æ2H2O 1.10E 02 CaSO4Æ2H2O 1.07E 02
CaCO3 2.38E 02 CaCO3 6.79E 02 CaCO3 1.48E 01
MgCO3 1.73E 02 MgCO3 1.73E 02
Table 3
Heavy metals and As released from the carbonated APC residues based on experimental and modeling results
Slowly carbonated ash Rapidly carbonated ash
Measured release (%) Modeled releasea(%) Precipitates Measured release (%) Modeled releasea(%) Precipitates
As 0.00 0.29 Ca3(AsO4)2Æ4H2O 0.14 1.01 Mn3(AsO4)2Æ8H2O Cd 0.91 4.45 Cd4(OH)6SO4 1.14 100 – Cr 1.60 0.01 Cr2O3 1.43 0.01 Cr2O3 Cu 0.58 0.00 CuFeO2 0.54 0.00 CuFeO2 Hg 0.30 0.25 Hg 0.03 0.26 Hg Ni 2.98 0.00 Ni(OH)2 2.92 0.28 Ni(OH)2 Pb 0.31 0.01 Pb2(OH)3Cl 0.31 0.01 Pb2(OH)3Cl Zn 0.00 0.02 ZnO 0.00 0.04 ZnO
Future experiments to define the effect of surface com-plexation and precipitation on the release rates of heavy metals are desirable to improve the match between observed and predicted concentrations.
3.3. Effect of carbonation and pH on the leaching behavior of the APC residues
Fig. 3 demonstrates the pH varying test results of the raw, slowly carbonated and rapidly carbonated residue samples. Regardless of carbonation, all residue samples
possessed similar pH-dependent leaching behavior of indi-vidual metals and As.
Although complex reactions might occur in carbonation and leaching processes, the final pH of leachate, a result of combined effects of all reactions/processes, presents the major index influencing leaching equilibrium. As shown in Fig. 3, the heavy metals and As release curves can be divided into two main groups in the range pH 0–13: V shaped (Cd, Cr, Hg, Ni, Pb and Zn) and L shaped (As and Cu). With final suspension pH decreasing from 12.0 to 8.7, the concentration ranges of As and Cu are located
0 200 400 600 0 2 4 6 8 10 12 14 pH C onc ent ra ti on of Z n , m g l -1 raw carbonated aged 0 1 2 3 4 0 2 4 6 8 10 12 14 pH C o n c e n tr a ti o n o f As, m g l -1 raw carbonated aged 0 2 4 6 0 2 4 6 8 10 12 14 pH C onc ent ra ti on of C d , m g l raw carbonated aged 0 30 60 90 120 0 2 4 6 8 10 12 14 pH C onc ent ra ti on of C u , m g l -1 raw carbonated aged 0 2 4 6 8 10 0 2 4 6 8 10 12 14 pH C onc ent ra ti on of N i, m g l -1 raw carbonated aged 0 100 200 300 0 2 4 6 8 10 12 14 pH C onc ent ra ti on of P b , m g l -1 raw carbonated aged 0 5 10 15 20 0 2 4 6 8 10 12 14 pH C o nc ent ra ti on of C r, m g l -1 raw carbonated aged 0 1 2 3 0 2 4 6 8 10 12 14 pH C onc ent ra ti on of H g , m g l -1 raw carbonated aged Raw ash RC ash SC ash Raw ash RC ash SC ash Raw ash RC ash SC ash Raw ash RC ash SC ash Raw ash RC ash SC ash Raw ash RC ash SC ash Raw ash RC ash SC ash Raw ash RC ash SC ash Concentration of Pb, mg/l Concentration of Zn, mg/l Concentration of Hg, mg/l Concentration of Ni, mg/l
Concentration of Cr , mg/l Concentration of Cu, mg/l Concentration of Cd, mg/l Concentration of As, mg/l
at the alkaline side of the L curves, showing only weak fluc-tuation during carbonation. In the pH range of 8–13, Cd, Ni, Hg, Pb, and Zn leaching is situated on the right leg of the V curves, so their leaching concentrations decreased as pH lowers (or with carbonation). On the contrary, the Cr leaching is located in the left part of the V curve, where leaching concentration increases when pH approaches 8.7 when coming from more alkaline conditions.
Similar to the modeled pH-dependent result of pure metal carbonates (Fig. 2), within the pH range 8–11, As and heavy metals were relatively stable. When pH decreased to less than 7, however, a significant increase of Cd, Ni, and Zn leaching was observed. At pH 6, Cu and Pb became more leachable. As, Cr, and Hg were rela-tively stable under weak acidic conditions. From the regu-latory viewpoint, the carbonated APC residue was not really ‘‘stabilized’’ and could not be disposed of directly, with regard to its release potential under acid shock (pH < 6–7). On the other hand, rapid carbonation (10 h) or slow carbonation (7 d) might be favorably used as a sim-ple pretreatment process to control pollution before the final treatment and disposal of ash, by decreasing pH of the carbonated residue to 8–11.
3.4. PLS modeling
The PLS analysis is a regression algorithm relating input
and output samples (xi, yi) by a linear multivariate model
(Nadler and Coifman, 2005). It obtains PLS-weights for the variables. The weights for the X-variables indicate the importance of these variables, how much they ‘‘in a relative sense’’ participate in the modeling of Y, while the weights for the Y-variables indicate which Y-variables are modeled in the respective PLS model dimension. When these weights are plotted in a PLS loading plot, we obtain a pic-ture showing the relationships between X and Y-variables, which X-variables are important, and which Y-variables are related to which X, etc. (Umetrics AB, 2005).
In this study, the PLS modeling taking into account extractant concentration, carbonation level, and leachate pH, resulted in two principal components comprising 71% and 2.5% of the data variation. The first principal component dominantly explained data variation.
The loading plot (Fig. 4) illustrates the impact of various factors (extractant concentration, leachate pH, and car-bonation level) on the response variables (concentration of metals and As). The metals and As release depended sig-nificantly on the extractant (acid or alkaline) concentration and leachate pH. Compared to the positive correlation between extractant concentration and heavy metals release, leachate pH (which is the combining result of the extract-ant concentration, carbonation level, and leaching
pro-cess), was negatively correlated to metals leaching
concentration. The carbonation level dominating the sec-ond principal component, which covered only 2.5% of the data variation, was inconclusive.
All metals and As showed similar variation. If further subdivided according to their X-weight, Cu, Cd, Ni, and Zn were more significantly influenced by extractant and leachate pH. This observation corresponds to the results
shown in Fig. 3, that these metals were more
acid-leach-able, with rising release when pH fell to less than 7. As, Hg, Cr, and Pb were relatively resistant to pH decrease, whose leachability increased at pH less than 4–6.
4. Conclusions
After rapid and slow carbonation, the CO23 content in
the APC residue increased from 1.43 wt.% to 8.85 wt.% and 4.08 wt.%, respectively, and the leachate pH of the rap-idly and slowly carbonated ash decreased to 8.7 and 10.1, respectively. The leaching toxicity test showed that carbon-ation could significantly immobilize heavy metals (Hg, Pb and Zn) and turned the hazardous materials into ‘‘non-haz-ardous’’ waste. A slight decrease of Ni leaching and increase of Cr was found, while Cd and Cu leaching pH carbonation extractant As Cd Cr Cu Hg Ni Pb Zn -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 w*c[1] w* c[ 2]
concentration remained stable, all of them were far below the limit values. However, the so-called stabilization was mainly due to the decrease of leachate pH controlled by the carbonation in the short-term, which finally influenced the solubility of metals, according to the results of pH vary-ing test, PLS analysis, and Visual MINTEQ modelvary-ing. Although the long-term leaching potential of the carbon-ated APC residue was of concern when the residue was under an acid shock, carbonation in air via water spraying could be applied as a temporary stabilization stage during transportation and storage before final disposal.
Acknowledgments
We thank Shanghai Council of Science and Technology for the financial support through the project ‘‘Research on beneficial use of MSW incineration residues and its demon-stration project’’ (032312043).
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