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Aging of air pollution control residues from municipal solid waste

incinerator: Role of water content on metal carbonation

Pin-Jing He

a,

*, Qun-Ke Cao

a

, Li-Ming Shao

a

, Duu-Jong Lee

b

aState Key Laboratory of Pollution Control and Resources Reuse, Tongji University, Shanghai 200092, China bChemical Engineering Department, National Taiwan University, Taipei, Taiwan, 10617

Received 4 March 2005; accepted 6 June 2005 Available online 5 July 2005

Abstract

This work studies the effect of water content on the aging of APC residues, with a liquid to solid ratio (L / S) of 0.25 or 10, aged with or without exposure to ambient air. After the residue was mixed with water, CaSO4and (Na,K)Al3(SO4)2(OH)6in the

raw sample yielded ettringite. When CO2were available, this ettringite was further transformed to gypsum, calcite and possibly

gibbsite. Experimental data revealed that the concentrations of Pb, Zn, Cd, Hg, and Cu fell with age, whereas that of Cr increased. Given L / S = 10, excess Ca2+ions were present in the suspension, so a precipitate of primarily calcite crystals of sizes under 5 Am formed on the air–water surface. This layer significantly reduced the rates of decline of Pb, Zn, Cd and Hg contents, and also reduced the increasing rate of Cr content in the suspension. This result follows from the mass transfer barrier of CO2

added at the air–water surface and the occurrence of subsequent chemical reactions in the suspension. An estimate of the mass transfer rate revealed that the rate-controlling step with L / S = 10 was the dissolution and diffusion of CO2in the bulk solution.

However, at L / S = 0.25, the rate-limiting step was the dissolution of metals from ash particles. Water content is a very important process factor, whose distribution in the sample, and the resulting competition between carbonate ion flux and heavy metal flux, govern the reaction time required during the natural aging process.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Municipal solid waste incinerator; Air pollution control residues; Aging; Carbonation

1. Introduction

Air pollution control (APC) residues from munic-ipal solid waste (MSW) incinerators are typically

classified as hazardous waste because they contain large amounts of leachable heavy metals, soluble salts and trace organic pollutants, which must be disposed of with care (Reijnders, 2005). He et al. (2005) reported for the first time the characteristics and pollution potentials of APC residues generated in an incinerator in Shanghai. When the APC residues were wet and exposed to air, aging (or natural weath-ering) occurred. Carbonation is regarded as the main 0048-9697/$ - see front matterD 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.scitotenv.2005.06.004

* Corresponding author. Tel.: +86 21 65986104; fax: +86 21 65986104.

E-mail addresses: solidwaste@mail.tongji.edu.cn, xhpjk@mail.tongji.edu.cn (P.-J. He).

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mechanism of the aging-induced reduction of the solubility of metals, owing to the low solubility of heavy metal carbonates. Accordingly, aging has been proposed to be a means of immobilizing residues that are contaminated with heavy metals (Macias et al., 1997; Walton et al., 1997; Bin-Shafique et al., 1998; Valls and Va`zquez, 2001; Venhuis and Reardon, 2001; Ecke et al., 2002).

Aging is a complex process of numerous interrelat-ed chemical reactions, including carbonation, hydroly-sis, hydration, dissolution/precipitation, complexation with organic and inorganic ligands, surface complex-ation, surface precipitcomplex-ation, sorption, formation of solid solutions and oxidation/reduction (Sabbas et al., 2003).Table 1lists the literature examines the effect of APC residues aging on metal immobilization. The partial pressure of CO2 and the reaction time were

proposed more important parameters than the amount of water added or the temperature of fly ash carbon-ation (Ecke et al., 2003). Water is a critical ingredient in carbonation, but an excess of water suppresses CO2

diffusion into the suspension (Ferna´ndez Bertos et al., 2004). By establishing that carbonation is much faster under dynamic conditions than under static conditions, Ferna´ndez Bertos et al. (2004) established that the diffusion of CO2into ash particles is the

rate-control-ling step. The optimal liquid to solid ratio (L / S) was 0.2–0.3 for APC residues, using pure CO2at pressure

of three bars.

If the transfer rate of CO2were the rate-controlling

step of the aging of APC residues, then the way in which water is added and distributed within samples should significantly affect the rate of carbonation. Table 1 demonstrates that the L / S ratios considered in the literature are under 0.6, and claims are made of the effect of CO2 on aging without experimental

support. In this work, APC residues with L / S = 0.25 and 10 were aged with or without exposure to ambient

air. This study shows that the distribution of the added water considerably influences the aging process, via mechanisms that are discussed in certain detail.

2. Materials and Methods

The APC residues employed in this study were sampled from the Yuqiao MSW Incineration Plant (mass burn) in Shanghai, which treats approximately 1000 tons of MSW daily. The plant removes acid gas from the flue gas stream using lime slurry (10% w/w); heavy metals and dioxins using activated carbon (50 mg m3), and particulates using bag filters. The APC residues were collected from the semidry reactors and the fabric filters, containing particles from the incin-eration chamber, reaction products and some excess reactants, primarily Ca(OH)2. The raw residues had a

low water content of 0.37% w/w.

Ferna´ndez Bertos et al. (2004) posited that the optimal L / S is 0.2–0.3 for APC residues. According-ly, in this study, raw ash was mixed with distilled water to a moisture content of 20% w/w (L / S = 0.25 w/w) to form a sample with a low to moderate mois-ture content. Then, some of the wet ash was sealed in a 500 ml PE bottle that contained no air; the rest of the sample was spread in enamel trays to a thickness of 20 mm and exposed to ambient air. Mineralogical anal-yses of both samples using a D/max 2550 X-ray diffractometer (XRD, Rigaku Corporation, Japan), were conducted before and after 30-day test. Compar-ing the data for the sealed and the spread samples elucidated the possible effect of exposure to CO2on

ash characteristics.

In each of the sixteen cylindrical chambers, 100 g raw ash was thoroughly mixed with 1 l distilled water (L / S = 10 w/w) and then kept still with its upper surface exposed to air, to determine whether the

dif-Table 1

Literature works considering ash aging process

Works L / S (w/w) CO2 Metals studied Remarks

Ecke et al. (2003) 0, 0.5 0.03% and 50% v/v Pb, Zn, Cd, Cr Flow chamber

Shimaoka et al. (2002) 0–0.6 0–30% v/v Pb Flow chamber

Kim et al. (2003) 0.05–0.2 10% v/v Pb, Ca, Cd Aged with bottom ash

Ferna´ndez Bertos et al. (2004) 0–0.6 (opt. 0.2–0.3) Pure CO2 (3 bars) Pb, Zn Shaken or still chambers This work 0.25, 10 0.03% v/v Ca, Cd, Cr, Cu, Ni, Pb, Zn, As, Hg Still chamber

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fusional resistance of CO2 limits the rate of

carbon-ation with a large amount of water. The amount L / S = 10 was adopted since it is also the recommended ratio in numerous leaching standard tests, including EN 12457 (EU), EPTSW (China), JLT 13 (Japan), and DIN38414 S4 (Germany). The ash quickly settled to a bottom layer with a thickness of 20 mm. CO2could be

supplied only from the top surface of the water. Such a test involves a sample with a high moisture content. In the sixteen chambers that contained APC residues at L / S = 10, a thin layer of precipitate would rapidly form on the top surface of the water. In eight cham-bers, this thin layer was not touched during the test. In the other eight chambers, this layer was carefully removed after it had formed. Ecke et al. (2002) revealed the rather high solubility (N5–10%) of fly ash at L / S = 10 and 25 8C. Suspension samples were extracted from the supernatant layer at 0, 8, 24, 48, 96, 144, 360 and 720 h, and were filtered using a 0.45 Am membrane before their pH and metal concentrations were measured. An AA-6501F FAAS (Shimadzu Cor-poration, Japan) was used to analyze the levels of Ca, Cd, Cr, Cu, Ni, Pb and Zn, and a XGY1012 AFS (atomic fluorescence spectrometer, Institute of Geo-physical and Geochemical Exploration of the Chinese Academy of Sciences, China) was used to measure the concentrations of As and Hg. The variability of pH measurement was within 0.08, and heavy metals con-centration within 2%. The cross-sectional composi-tions of the heavy metals on the particle surface were scanned using a scanning electron microscope (Philips XL30) equipped with an energy dispersive X-ray spectrometer (EDAX DX4).

3. Results and discussion 3.1. APC characteristics

Table 2 lists the leaching characteristics of the APC residues through the Extraction Procedure for Toxicity of Solid Waste (EPTSW). Independent tests revealed that the surface area of this ash sample was about 5 m2g1, so the material was not very porous. Notably, the leachate pH of ash, and the concentra-tions of Hg and Pb in leachate all violate the current national standards of China, so the material is clas-sified as hazardous. Additionally, the calcium con-centration in the leachate is high, at approximately 3000 mg l1, indicating that the carbonic acid is easily precipitated.

3.2. Effects of carbonation on mineralogy of APC residues (L / S = 0.25)

Comparing the XRD spectra of the raw ash sample and the carbonated ash (L / S = 0.25), the latter contained calcite and gypsum (Fig. 1). No calcite appeared in the sealed sample without air available, and the gypsum content was not substantially in-creased. Rather, ettringite [Ca6Al2(SO4)3(OH)12d 26H2O] had formed in the wet and sealed residues.

Shimaoka et al. (2002) also observed similar com-pounds in their scrubber residues, and found that aging almost completely eliminated them. According-ly, CaSO4 and (Na,K)Al3(SO4)2(OH)6 in raw ash

might react initially with water to produce ettringite, which was transformed to more stable forms of

sul-Table 2

Leaching characteristics of the APC residues samples based on Extraction Procedure for Toxicity of Solid Waste (GB5086.1-1997, 1997) Item pH As/(mg l1) Cd/(mg l1) Cr/(mg l1) Cu/(mg l1) Hg/(mg l1) Ni/(mg l1) Pb/(mg l1) Zn/(mg l1)

This study* 13.0 0.001 0.04 0.36 0.13 0.164 0.18 63.6 2.26

Average** 12.6 (0.3) 0.001 (0.001) 0.12 (0.07) 0.23 (0.17) 0.25 (0.17) 0.14 (0.16) 0.38 (0.20) 88 (40) 4 (2)

Limited value*** 12.5 1.5 0.3 10 50 0.05 10 3 50

Item Electric conductivity/(ms cm1) Cl/(mg l1) SO42/(mg l1) Ca/(mg l1)

This study* 27.1 8600 1900 2950

Average** 31.6 (3.4) 10,000 (3190) 1300 (600) 3410 (670)

* Referring to the leaching characteristics of the APC residues used in this study.

** The data cover leaching tests for ten APC residues sampled from the Yuqiao MSW Incineration Plant at different seasons in 2 years (He et al., 2004). The values in the bracket mean standard deviation.

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fate, carbonate and hydroxide, gypsum, calcite and possibly gibbsite, when CO2was present.

Fig. 2 displays typical SEM/EDS images of the raw and aged samples. The elemental compositions of the sample surfaces were estimated by averaging more than 40 images taken of the samples at random. The contents of the main elements in the raw residues followed the sequence O N C, Ca, Si, Cl N K, Na, Al, S (Fig. 3). In the aged residues, the abundance of C and O increased significantly from 12.4% to 24.9%, and from 33.1% to 47.0%, respectively, supporting to the conclusion that carbonation occurred in the ash samples by natural aging.

3.3. Aging of APC residues (L / S = 10)

The SEM micrographs reveal that the layer of precipitate noted in the L / S = 10 tests contains much calcite (Fig. 4(a)) with scattered spots of gypsum (Fig. 4(b)). Most of the precipitate was fine crystals of calcite that were smaller than 5 Am, packed

closely into a compact skin layer on the surface of the water. This occurrence was unsurprising be-cause the concentration of calcium ions was approx-imately 3000 mg l1 and the top water surface represented the only route along which CO2 could

enter. Fig. 5 presents the pH evolutions of the suspension of APC residues (L / S = 10) with and without the surface precipitate layer. Initially, the fly ash suspension was at pH 12.4. During the 720 h test, the pH of the suspension with the precipitate layer remained almost unchanged. In extended tests, the pH of the suspension remained above 12 for a month (data not shown). In the test with precipitate layer removed, the pH fell slowly from 12.4 to about 10.6 over 50 to 360 h. Some earlier studies also established that the pH of the fly ash suspension declined to around 10 (Hjelmar and Birch, 1997; Shimaoka et al., 2002; Bone et al., 2003; Ecke et al., 2003). After about 8 h, three distinct behaviors were observed. The concentra-tions of Pb, Zn, Cd, Hg and Cu began to fall

3 13 23 33 43 53 63 Intensity 2 (c) (b) SiO2 KCl NaCl

CaSO4 Ca6Al2(SO4)3(OH)12•26 H2O

___ ___ ___ ___ __ __ __ __

CaSO4.2H2O (Na,K)Al3(SO4)2(OH)6

CaCO3

(a)

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(Fig. 6(a)–(e)). Those of Ca and Ni remained un-changed (Fig. 6(f)–(g)), and that of Cr increased (Fig. 6(h)).

When many metal ions are present in a solution, the one that must bond with the least CO32to form

the carbonate precipitate settles first. The reaction equilibrium between the divalent metal ion and the carbonate ion is:

M2þ þ CO2

3 XMCO3A

The minimum concentration of CO32required to

form MCO3precipitate at a given metal ion

concen-tration is½CO23 0¼ Ksp

M2þ.Table 3presents the

calcu-lated [CO32]0with the metal ion concentrations in the

initial ash suspension (L / S = 10).

The data in Table 3 demonstrate that when CO2

was dissolved and transferred from the top water surface, the order of precipitation was Pb2+, Ca2+, Zn2+, (Cu2+, Hg+), Cd2+. With the exception of Ca2+, which was abundant in the fly ash and was supplied by continuous dissolution with aging (shown later), the concentrations of the other five metal ions decreased with aging (Fig. 6(a)–(e)). The very high concentration of Ca2+ easily precipitated when CO32 was available, promoting the formation

of the noted surface precipitate layer in the test. The CO32 concentration was as high as around

0.03 mol l 1 to precipitate Ni2+, which was not achievable in the test herein. This observation reveals why the Ni2+ concentration remained unchanged throughout the test.

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Unlike for the other metals examined, the concen-tration of Cr increased with aging, probably because Cr3+was oxidized by dissolved oxygen to the more soluble form, Cr6+(Ecke et al., 2003).

The decline in the concentrations of Pb, Zn, Cd and Hg, but not Cu, was faster in the test without a surface precipitate layer than in the test with a surface layer. This observation is attributable to the provision of a mass transfer barrier by the compact surface layer, through which CO2had to pass. For example, the drop

in the concentration of Pb from 105 to 25 mg l1took 150 h in the test without a surface precipitate layer. When the surface layer was present, the time required exceeded 720 h. The corresponding times required for the drop in the concentration of Zn from 15–20 mg

l1 to around 3 mg l1 were 100 and 400 h, respec-tively. Accordingly, when APC residues were mixed with water, the rate of carbonation varied greatly with the manner in which the precipitate was formed and where it was precipitated. When a compact precipitate was formed on the pathway of the CO2, although the

chemical equilibrium calculation revealed the same results, the corresponding carbonation reaction was substantially slower.

Fig. 6(h) unsurprisingly reveals that more Cr was present in soluble form in the test without a precipitate surface layer than in the test with a surface layer, because the compact surface layer also slowed the dissolution of oxygen into water. However, the in-crease in the dissolution of Cr became marked only after 100–150 h of testing. An optimal period of aging should be used to immobilize heavy metal. In this particular test, the optimal time was 100 h, without the release of Cr, which detrimentally affects the manage-ment of residues.

3.4. Rate-controlling step

Insights into carbonation can be gained by com-paring the incorporated mass transfer rate with the chemical reaction rate. For the system adopted for L / S = 10, CO2 dissolves at the top water surface,

according to Henry’s Law: CO2 mol fraction x0ð Þ ¼pe

E ¼

30 1:66 108

¼ 1:8  107 mol mol1 ð1Þ

where,

pe CO2partial pressure, 30 Pa in air;

E Henry coefficient, Pa, for CO2 in water,

which is 1.66  108Pa at 25 8C.

If, as posited byFerna´ndez Bertos et al. (2004), the diffusion of CO2into ash particles is the

rate-control-ling step in ash carbonation, then an order-of-magni-tude estimate of the rate at which dissolved CO2is

transferred from the top surface to the bottom layer of residues is as follows: NA;CO2 ¼ Dd x0 L ð2Þ C O Na Mg Al Si S Cl K Ca Fe Zr 0 10 20 30 40 50 weight % Element (a) C O Na Mg Al Si S Cl K Ca Fe Zr Pt P Br Ti 0 10 20 30 40 50 weight % Element (b)

Fig. 3. Mean weight percents of main elements of APC residues samples. (a) raw residues, (b) aged residues.

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where,

NA,CO2 Rate of diffusion of dissolved CO2from the

top surface to the bottom layer of residues; L thickness of water above the ash layer, 0.08

m herein;

D diffusion coefficient of CO2 in water at 25

8C, 1.96  10 9m2s1.

The CO2concentration on top surface of the water

(x0) = 1.8  107 5.56  10 4

= 1 102 mol m3. Accordingly, the rate of CO2diffusion is

approxima-ted as NA;CO2¼ 1:96  10

9 m2

s 

0:01 molm3

0:08m = 2.45 

1010 mol (m2 s)1. The cross-sectional area of the top water surface is (p / 4)  (0.12 m)2= 0.0113 m2, yielding a diffusional flow of 9.97  10 9mol h 1. If, however, all CO2is assumed to convert to CO32after

it is dissolved in water, given that the diffusion coef-ficient of the latter in water is only 0.92  10 9 m2s1, the corresponding diffusional flow would be even lower. Hence, the above estimate is an upper limit on the diffusional flow of carbonate ions.

Consider the consumption of Pb2+as an example of the calculation. The lead concentration fell from 104 mg l1 to almost zero over 100–700 h (Fig. 6(a)). The minimum quantity of Pb2+ consumed in the chamber was estimated as (1 l 104 mg l1 / (207 g mol1) = ) 5  104 mol. Therefore, the time taken for CO2to transfer from top surface to bottom

residues layer to precipitate all lead is around (5  10 4 mol / 9.97  10 9 mol h1= ) 50,000 h, two to three orders of magnitudes above the exper-imental values (order of (102h)). Accounting for this huge time span difference could be difficult, even when the effects of enhanced gas adsorption on the alkaline solution or those of transient mass transfer caused by local depletion of carbonate ions are considered. Accordingly, the chemical reaction could proceed effectively in the ash particles that settled in the bottom layer. Rather, the chemical reaction preferentially proceeds in the bulk solution, beginning after the CO2 has dissolved and

hydro-lyzed at the top surface of the water. The dissolution and diffusion of CO2in the bulk solution is the

rate-8 9 10 11 12 13 0 100 200 300 400 500 600 700 800 time (h) pH

pH, with precipitate layer pH, without precipitate layer

Fig. 5. pH evolutions of suspensions of fly ashes with or without surface precipitate layer. Fig. 4. The SEM micrographs of the surface precipitate layer with

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limiting step of the carbonation of ash suspension (L / S = 10).

Most heavy metal ions in water have a diffusion coefficient of the order of 10 9m2s1. For example,

Ca2+has a diffusion coefficient of 0.79  109m2s1 in water at 25 8C. If calcium ions were released from the bottom ash layer to form a highly saturated solu-tion with an ion concentrasolu-tion of 10 mol m3, then the

0 20 40 60 80 100 120 1 10 100 1000 time (h) time (h) time (h) without precipitate layer with precipitate layer (a) 0 5 10 15 20 25 1 10 100 1000

without precipitate layer with precipitate layer (b) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 1 10 100 1000 Hg (mg l -1) Zn (mg l -1) Pb (mg l -1)

without precipitate layer with precipitate layer (c) 0.00 0.02 0.04 0.06 0.08 1 10 100 1000 time (h) Cd (mg l -1)

without precipitate layer with precipitate layer (d)

Fig. 6. Concentrations of heavy metals in supernatant with or without surface precipitate layers. (a) Pb, (b) Zn, (c) Hg, (d) Cd, (e) Cu, (f) Ca, (g) Ni, (h) Cr.

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corresponding diffusional flux from the bottom to top surface is estimated as follows; NCa2þ ¼ 0:79

109 ms2 10 mol m3

0:08m  0:0113m

2 = 4.02 10 6

mol h1. This value greatly exceeds that of the

corresponding CO2 flow (9.97  10 9 mol h1),

yielding an almost constant Ca concentration through-out the aging test, as presented in Fig. 6(f). Mean-while, the decrease in metal concentrations displayed

1 10 100 1000 time (h) 1 10 100 1000 time (h) Cu (mg l -1) Ca (mg l -1) 0.0 0.5 1.0 1.5

without precipitate layer with precipitate layer (e) 0 1000 2000 3000 4000

without precipitate layer with precipitate layer (f) 0.0 0.1 0.2 0.3 1 10 100 1000 time ( h) 1 10 100 1000 time ( h) Ni (mg l -1) Cr (mg l -1)

without precipitate layer with precipitate layer (g)

0.0 0.5 1.0 1.5

without precipitate layer with precipitate layer (h)

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inFig. 6(a)–(e) during aging revealed that most dis-solvable Pb, Zn, Cd, Cu and Hg were released to the bulk suspension and completely consumed by carbon-ation in 150–700 h.

Therefore, given a water thickness of 8 cm, the rate-controlling step of aging was the dissolution of CO2 (or CO32) at the top water surface and its

transport through and reaction in the bulk solution. The particle size or the surface area of the ash particles does not affect natural aging when the moisture con-tent is high. Meanwhile, agitation should be able to promote the transfer rate of CO32, whence its reaction

rate in the bulk, as noted by Ferna´ndez Bertos et al. (2004).

In the tests with L / S = 0.25, the 100 g sample was mixed with 25 g of water. Under extreme conditions in which all particle surfaces were coated uniformly with a liquid film, the mean thickness of the liquid film was 50 nm (25 cm3/ (100 g  5 m2/g)). Given such a thin film, the corresponding NA,CO2 was

1.6  106times greater than that obtained in the L / S = 10 tests, yielding a CO2flux of 700 mol h1. The

ambient air cannot supply as much CO2. Even at 1%

of this value, the corresponding flux more than suf-fices to react with all metal ions in the ash samples within half an hour. Meanwhile, the dissolution of metals ions in the attached liquid film is far from complete. Accordingly, under this condition, the rate-limiting step is how fast the heavy metals could be transformed into ionic forms. Restated, the rate of dissolution of metals from ash particles controls the reaction rate. The particle size or surface area of the ash particles governs the rate of aging. Under this extreme condition, the water content is secondary,

and the conclusions drawn by Ecke et al. (2003) are unsurprising: the added water is not significant in relation to the reaction time and the partial pressure of CO2.

Water content is a very important process factor, whose distribution in the sample, along with the resulting competition between the carbonate ion flux and the heavy metal flux, determines the reaction time required in this natural aging process. Accordingly, the amount of added water and the reaction time are dependent variables and should not be used as inde-pendent variables in statistical analysis to interpret experimental data.

4. Conclusions

This study examines the natural aging of APC residues from an MSW incinerator in Shanghai, con-sidering in particular the effect of the amount of added water on the immobilization of the heavy metals. The liquid to solid ratio was fixed at 0.25 or 10 with or without CO2supplied from ambient air. With a L / S of

0.25, the CaSO4 and (Na,K)Al3(SO4)2(OH)6 in the

raw sample would initially react with water to yield ettringite. Then, if external CO2 were available, the

ettringite would be transformed to gypsum and calcite, along with a decline in the pH of the suspension. The C and O contents on ash particle surface also consid-erably increased following natural aging at ambient air exposure.

With a sufficiently high L / S (10 in this study), excess Ca2+ions were present in the suspension, so a precipitate of mostly crystals of calcite that were smaller than 5 Am form on the air–water surface. With or without this surface precipitate layer, the concentrations of Pb, Zn, Cd, Hg and Cu fell with time, because of carbonation, following closely the precipitation sequence determined by chemical equi-librium calculations. The levels of Ca and Ni in suspensions were almost constant throughout the test, because the Ca2+was too abundant to be com-pletely converted and the concentration of carbonate ions required to precipitate Ni was too high to achieve. Additionally, the declining rates of Pb, Zn, Cd and Hg contents when a surface precipitate layer presented were much lower than those without a surface layer. The amount of Cr increased with Table 3

Precipitation analysis of metal carbonate Compounds Ksp(25 8C)

(Cai et al., 1998; Zheng and Zhu, 1999) [M2+]/(mol l1) [CO3 2-]0/(mol l1) PbCO3 1.46  10 13 5.07  10 4 2.88  10 10 CaCO3 4.96  10 9 6.64  10 2 7.47  10 8 ZnCO3 1.19  10 10 1.89  10 4 6.30  10 7 CuCO3 1.4  10 10 1.62  10 5 8.65  10 6 Hg2CO3* 3.67  10 17 2.40  10 6 6.40  10 6 CdCO3 6.18  10 12 3.99  10 7 1.55  10 5 NiCO3 1.42  10 7 4.11 10 6 3.46  10 2 *For monovalent metal ion, [CO32-]0= K

sp

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aging, whose rate was also lower when the surface precipitate layer was present. This observation follows from the fact that the added mass transfer barrier of CO2 or O2 at the air–water surface influences the

occurrence of subsequent chemical reactions. A simple estimate of the rate of mass transfer revealed that the rate-controlling step at L / S = 10 was not the diffusion of CO2 into ash particles, as

proposed by Ferna´ndez Bertos et al. (2004). The most important chemical reactions occur in the bulk suspension rather than on the surface of the ash particles or inside the ash particles. The dissolution and diffusion of CO2in the bulk solution are the

rate-limiting steps in the carbonation of ash suspension (L / S = 10). However, in L / S = 0.25 tests, the rate-limiting step was the rate of dissolution of metals from ash particles. The water content is a very im-portant process factor, whose distribution in the sam-ple, along with the resulting competition between the carbonate ion flux and the heavy metal flux, deter-mines the reaction time required during natural aging process.

Acknowledgments

We thank Shanghai Council of Science and Tech-nology for the financial support through the project bResearch on beneficial use of MSW incineration residues and its demonstration projectQ (032312043).

Appendix A

D the diffusion coefficient of CO2in water at

25 8C, 1.96  10 9m2s 1;

E Henry coefficient, Pa, for CO2in water it is

1.66  108Pa at 25 8C; Ksp the solubility–product constant;

NA,CO2 the diffusion rate of dissolved CO2

trans-ferred from top surface to the bottom resi-dues layer;

NCa2+ the diffusional flux of Ca2+from the bottom

to top surface;

L thickness of water above the ash layer, 0.08 m here;

pe CO2partial pressure, 30 Pa in air;

x0 CO2mol fraction.

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數據

Table 1 demonstrates that the L / S ratios considered in the literature are under 0.6, and claims are made of the effect of CO 2 on aging without experimental support
Table 2 lists the leaching characteristics of the APC residues through the Extraction Procedure for Toxicity of Solid Waste (EPTSW)
Fig. 2 displays typical SEM/EDS images of the raw and aged samples. The elemental compositions of the sample surfaces were estimated by averaging more than 40 images taken of the samples at random
Fig. 2. The SEM/EDS spectra of APC residues samples. (a) raw residues, (b) aged residues.
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