Comparison of Trace Element Emissions from Thermal Treatments of

Mar 29, 2012 - promising remediation techniques for heavy metal (HM) con- taminated soils. .... China. The hyperaccumulators were split from the fresh...
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Comparison of Trace Element Emissions from Thermal Treatments of Heavy Metal Hyperaccumulators Shengyong Lu,†,* Yingzhe Du,† Daoxu Zhong,‡ Bing Zhao,‡ Xiaodong Li,† Mengxia Xu,§ Zhu Li,‡ Yongming Luo,‡ Jianhua Yan,†,* and Longhua Wu‡,** †

State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering of Zhejiang University, Hangzhou 310027, People's Republic of China ‡ Key Lab Soil Environment & Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, People's Republic of China § Ningbo Key Laboratory of Persistent Organic Pollutants Detection & Control, Ningbo Environment Monitoring Center, Ningbo 315012, People's Republic of China ABSTRACT: Phytoextraction has become one of the most promising remediation techniques for heavy metal (HM) contaminated soils. However, the technique invariably produces large amounts of HM-enriched hyperaccumulators, which need further safe disposal. In this study, two different thermal treatment methods are investigated as potential options for evaporative separation of HMs from the residues. A horizontal tube furnace and a vertical entrained flow tube furnace were used for testing the disposal of grounded hyperaccumulators. The release characteristics of HMs (Cd, Cu, Pb, and Zn) into flue gas and residues were investigated for thermal treatment of the Cd and Zn hyperaccumulators Sedum plumbizincicola and Sedum alf redii. In a horizontal tube furnace, incineration favors the volatilization of Cu and Cd in contrast to pyrolysis. The percentages of HMs in residues after incineration are lower than those after pyrolysis, especially for Cd, Pb, and Zn. However, in an entrained flow tube furnace, Zn content in flue gas increases with increasing temperature, but Cu and Cd contents are fluctuated. In addition, a higher incineration temperature enhances the Cu content in residues.

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INTRODUCTION With increasing industrialization and economic development, the environmental and health issues resulting from HM-contaminated soil have attracted great attention in China. The anthropogenic sources of HMs mainly consist of wastes generated from metalliferous mining and smelting industries, overuse of fertilizers and pesticides, motor vehicle emissions, as well as sewage sludge.1,2 The area of HM contaminated agricultural soil in China is up to 200 000 km2. This accounts for 1/5 of the total agricultural soil in China. In particular, Cd- or Hg-contaminated soil causes the most concern.1 Current remedial technologies for HM-contaminated soil involve ex-situ physical and chemical methods such as solidification, electrokinetics, soil washing, or excavation.3,4 The state-of-the-art phytoextraction technique has been proposed as a promising, cost-effective, and environmentally sound option for remediating shallow and moderate contaminated soils.3−7 Phytoextraction is based on the utilization of metal hyperaccumulators that have the capacity to accumulate and tolerate high amounts of metals. Certain species of plants that have extremely high tolerances to specific elements are termed hyperaccumulators.8,9 Brooks et al.10 first used “hyperaccumulator” to describe plants that contain greater than 1000 μg/g of Ni in dry matter. This is an order of magnitude higher than that in “normal” plants. A major concern is that this technique will © 2012 American Chemical Society

invariably generate large quantities of highly contaminated secondary waste. Incineration is presently deemed as the most feasible, economically acceptable, and environmentally sound approach4,11 for disposal of hyperaccumulator. Xing and Pan used Cd and Pb hyperaccumulator Dicranopteris pedata, and concluded that incineration would cause a considerable loss of Cd and Pb and therefore was not feasible to recycle HMs from bottom ash (BA).12 However, Keller et al. used Cd and Zn hyperaccumulator Thlaspi caerulescens and confirmed that incineration was an acceptable technique to dispose of the plant. Additionally, they demonstrated that pyrolysis was a better method than incineration to increase volatilization and would allow the recycling of BA as fertilizer.11 The incineration experiments conducted on As hyperaccumulator Pteris vittata showed that mass percentages of As in residues were 13.7∼17.5% with no additives, and 12∼20% with additives (such as CaO, K2HPO4, and Fe2(SO4)3); the evaporation rate of As in flue gas increased with increasing temperature and a peak appeared near the As melting point of 814 °C.13 In general, elements with high boiling points tend to remain in Received: Revised: Accepted: Published: 5025

July 28, 2011 February 23, 2012 March 29, 2012 March 29, 2012 dx.doi.org/10.1021/es202616v | Environ. Sci. Technol. 2012, 46, 5025−5031

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Table 1. Elemental Analysis Results of the Hyperaccumulators Sedum plumbizincicola and Sedum alf redii horizontal tube furnace tests Cda (mg/kg)

Zna (mg/kg)

Cua (mg/kg)

Pba (mg/kg)

Cb (ad%)

Hb (ad%)

Nb (ad%)

Sb (t,ad%)

Ob (ad%)

ashc (ad%)

moisturec (ad%)

281

34 700

244

3930

37.8

5.31

1.08

0.20

36.9

14.3

4.41

Sedum plumbizincicola

entrained flow tube furnace tests a

Sedum plumbizincicola Sedum alfredii

a

a

a

Cd (mg/kg)

Zn (mg/kg)

Cu (mg/kg)

Pb (mg/kg)

Cb (ad%)

Hb (Cad%)

Nb (ad%)

Sb (t,ad%)

Ob (ad%)

ashc (ad%)

moisturec (ad%)

Cl (mg/kg)

152

7480

26.3

6.13

39.4

3.20

2.20

0.12

31.9

14.2

8.98

0.61

16.9

869

20.2

3.52

36.8

3.65

2.02

0.25

35.0

13.3

9.00

0.80

a

Analysis performed by Flame Atomic Absorption Spectrophotometry (Varian SpectrAA 220FS). bUltimate analysis data were expressed on the airdry (ad) basis. cProximate analysis data were expressed on the air-dry (ad) basis, and the total mass percentages of C, H, O, N, S, ash, and moisture equal 100%.

sufficiently cooled, the BA was taken out carefully, allowed to cool to room temperature, weighed, and sealed in a sampling bag. The fly ash retained in the glass wool and quartz tube surface was not sampled and analyzed. Vertical Entrained Flow Tube Furnace Experiments. Figure 1(b) shows the schematic diagram of the entrained flow tube furnace setup. The combustion reactor is 1100 mm in height, and 60 mm and 300 mm in inner and outer diameter, respectively. The reactor is electrically heated by a three-stage alloy resistance wire. The incineration temperature can be measured and controlled separately by thermocouples and a temperature controller. A water cooled screw feeder is feeding the dried and crushed biomass into the furnace. The feed rate was automatically controlled by a speed-adjustable electromagnetic motor. A stainless steel mesh was installed at the bottom of the furnace to collect BA. Test conditions are shown in Table 2. All samples were acquired isokinetically, and the sampling flow rate ranged from 0.8 to 1 L/min. The fly ash and bottom ash samples were collected from the cyclone separator and the bottom of furnace after each experiment. HMs in the flue gas was sampled by the aforementioned method. Analysis of Heavy Metals. Solid samples such as fly ash and bottom ash were digested with a solution of HNO3 and HClO4 (3:2 HNO3: HClO4 by volume) for determination of Cd, Zn, Pb, and Cu. Then the concentrations of Zn, Pb, and Cu were determined by Flame Atomic Absorption Spectrophotometry (Varian SpectrAA 220FS). Moreover, Cd concentration in the solution was determined by a Varian SpectrAA 220Z spectrophotometer using a graphite furnace.

bottom ash, whereas volatile elements are predominantly transferred into the gas phase.14 Hg and Cd are considered volatile, while Zn and Pb belong to the intermediate group, and last Ni, Cr, and Cu are nonvolatile metals.15 In this study, incineration and pyrolysis methods were used to treat the Cd and Zn hyperaccumulators Sedum plumbizincicola and Sedum alfredii, to develop a proper thermal disposal method. Incineration and pyrolysis tests were conducted in a horizontal tube furnace, and the vaporization behaviors of Cd, Cu, Pb, and Zn were investigated and compared at two corresponding temperature series: 450 °C, 550 °C, 650 °C and 550 °C, 750 °C, 950 °C. Incineration tests were also performed in an entrained flow tube furnace between 650 and 950 °C.

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MATERIALS AND METHODS Sample Preparation. Sedum pumbizincicola and Sedum alfredii were cultivated in Fuyang of Zhejiang Province, East China. The hyperaccumulators were split from the fresh plant, then cleaned up by washing with distilled water and kept in a dark and ventilated place to near dryness. Thereafter, they were crushed and dried at 85 °C to a constant weight, then mixed thoroughly and sealed in a plastic bag for future use. The elemental compositions of samples are listed in Table 1. Horizontal Tube Furnace Experiments. The experimental facility includes a high pressure gas source, flow control valve and meter, a horizontal quartz tube with a surrounding electrically heated furnace, and flue gas absorption device [Figure 1(a)]. The horizontal quartz tube is 25 mm in inner diameter, 1000 mm in length, of which 400 mm is in the temperature controlled reaction zone heated and controlled by the electrical furnace at 100−1300 °C with a precision of ±5 °C. For each experiment, all flue gas was sampled. The absorption device was based on US EPA Method 29.16 Glass wool was plugged in the joint between quartz tube and absorption bottle to avoid blocking the flue gas flow in the exhausting tube. Test conditions for the thermal treatment experiments include the type of reacting atmosphere, reaction temperature, and reaction time, and are listed in Table 2. About 0.5 g of Sedum plumbizincicola powder was placed in a dried and weighted ceramic boat, and transferred into the center of the quartz tube, which was already heated to the desired temperature and was purged with N2 in advance for about 10 min. The gas flow was introduced into the quartz tube at the start of the experiments. After each experiment, the absorption bottles with the 5% HNO3 + 10% H2O2 solution were removed and replaced by another set of bottles to dispose of the exhaust gas. At the same time, the heating source was cut off. After the tube furnace had

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RESULTS AND DISCUSSION Elemental Compositions in Sedum plumbizincicola and Sedum alf redii. The hyperaccumulator compositions are presented in Table 1. For the horizontal tube furnace experiments, Zn is at the top level, in the order of 10 000 mg/kg, followed by Pb, Cu, and Cd, which are on the orders of 100 mg/kg. However, the factors affecting metal accumulation and storage are still unclear and further study is needed. For the HMs in Sedum plumbizincicola used for the entrained flow tube furnace experiments, Zn and Cd are the most abundant HMs with concentrations 7480 mg/kg and 152 mg/kg, respectively. Moreover, the HMs concentrations in Sedum plumbizincicola are higher than those in Sedum alfredii due to different HM hyperaccumulation propensities. The criterion for the Zn hyperaccumulator suggested by Baker and Brookes is 10 000 mg/kg in shoot17 and was later reduced to 3000 mg/kg.18 5026

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Figure 1. Schematic diagram of the furnaces setup (a) horizontal tube furnace (b) entrained flow tube furnace. (a) 1, high-pressure gas source; 2, flow control valve; 3, flow meter; 4, horizontal quartz tube; 5, ceramic boat; 6, electrically heated furnace; 7, thermocouple; 8, temperature controller; 9, glass wool; 10, iced-water bath; 11, absorption solution 1 (5% HNO3 + 10% H2O2); and 12, absorption solution 2 (4% KMnO4 + 10% H2SO4). (b) 1, entrained flow tube furnace with electrically heating; 2, screw feeder with water cooling and a speed-adjustable electromagnetic motor; 3, air supply fan; 4, thermocouples; 5, cyclone separator; 6, ash box; 7, induced draft fan; 8, filter; 9, absorption solution (5% HNO3 + 10% H2O2); 10, iced-water bath; 11, gas flow meter; 12, pump; and 13, temperature controller and power supply.

flue gas were below 10% for most tests. Previous studies have shown that thermal treatments under reducing conditions increased the volatilization of mineral matters in coal.25 This is consistent with the emissions of Cu and Cd, as significantly higher Cu and Cd emissions were measured from pyrolysis than those measured from incineration. However, the opposite trend was observed for Pb emissions. Part of PbO with comparatively lower melting point vaporized in combustion zone may lead to the higher emission of Pb under incineration. The recovery rate of HMs in bottom ash produced from pyrolysis and incineration are displayed in Figure 3. Incineration causes a greater loss of residual HMs in comparison to pyrolysis. For instance, the recoveries for Cd, Pb, and Zn under pyrolysis conditions at 550 °C are 3.5, 2.7, and 2.3 times higher, respectively, than those under incineration conditions. This indicates that the oxidizing atmosphere favors the transfer of HMs such as Cd, Pb, and Zn to the gaseous phase, the same as it is observed in municipal solid waste incineration.26 Furthermore, the reaction time of pyrolysis was two times longer than

A cadmium hyperaccumulator is defined as a plant species capable of accumulating more than 100 mg/kg (dry wt.) in the shoot.19 Thus, the data presented here show that Sedum plumbizincicola is a Zn and Cd hyperaccumulator, and Sedum alfredii is a Zn and Cd accumulator, which confirms results reported in other studies.20−24 Incineration and Pyrolysis Tests for Sedum plumbizincicola in Horizontal Tube Furnace. The mass percentages of BA to original biomass after pyrolysis and incineration are generally decreasing with increasing temperatures. However, there is a significant difference in BA mass percentages between incineration and pyrolysis. Incineration reduced the biomass matrix to 13.2−17.5% of the original mass, whereas pyrolysis produced ash about three times higher (31.1−44.3% ash) than in the case of incineration. This observation agrees with Xing and Keller’s experiments and more carbon remained in ash under reducing atmosphere than under oxidizing atmosphere.11,12 The HM evaporation rates for pyrolysis and incineration experiments are depicted in Figure 2. The amounts of HMs in 5027

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Table 2. Key Parameters used for the Simulated Pyrolysis and Incineration Experiments conducted in Horizontal Tube Furnace and for the Incineration Experiments in Entrained Flow Tube Furnace horizontal tube furnace no.

atmosphere

1 2 3 4 5 6

N2 N2 N2 air air air

no.

biomass

1 2 3 4 5

Sedum plumbizincicola Sedum alfrediib

flow rate (ml/min)

T (°C)

350 450 350 550 350 650 300 550 300 750 300 950 entrained flow tube furnacea primary air flow rate (m3/h)

T (°C)

1.4 1.4 1.4 1.4 1.4

650 750 850 950 850

reaction time (min) 30 30 30 15 15 15

sample type

Figure 3. Mass percentages of HMs in bottom ash under different thermal conditions (“P” represents pyrolysis, and “I” represents incineration in the legend).

flue gas + fly ash + bottom ash

a

The average oxygen contents under different combustion conditions according to the table above are 7.7%, 9.7%, 10.7%, 10.4%, and 13%, respectively. The concentrations of heavy metals in flue gas were corrected with the oxygen content (11%). bThe combustion experiment of Sedum alfredii is a comparative one.

Figure 4. Relationship between mass percentage of HMs in bottom ash and melting point of HMs and their chlorides.

0.99 and 0.40 respectively. Therefore, it might be postulated that the HMs are accumulated in bottom ash in primarily elemental form rather than as chlorides during incineration. However, the behavior of HMs during pyrolysis seems complicated, and shows poor linear relationship with both the melting points of HMs and their chlorides (the correlation coefficients are 0.73 and 0, respectively). This might be due to the fact that part of the HMs has reacted into stable species, such as metal silicates or spinels, and thus preventing their volatilization.26 Overall, mass balances of HMs (estimated by emissions measured in flue gas plus retention measured in bottom ash) under pyrolysis conditions are better than those under incineration conditions. This might be partly due to the higher reaction temperatures of incineration than those of pyrolysis. Studies on the volatility and chemistry of HMs in a coal combustor suggests that HMs like Cd, Cu, Pb, and Zn are vaporized at intermediate temperatures (330−930 °C) and are emitted mostly as fly ash.27 The mass percentages of BA after incineration were only less than 15% of the initial biomass samples. Therefore, it might be inferred that a large amount of HMs during incineration are adsorbed onto the fly ash as well. Actually, in our experiments, part of the fly ash retained in the glass wool and quartz tube surface was not included in the mass balance calculation, which may contribute to the relatively low mass balances as compared to previous study.28

Figure 2. Mass percentages of HMs in flue gas under different thermal conditions (“P” represents pyrolysis, and “I” represents incineration in the legend).

that of incineration. This may cause a greater difference in HM release behavior between two thermal treatment methods. Unlike Al, Cr, Fe, and Mg, Cd, Cu, Pb, and Zn are mainly transferred into flue gas by evaporation rather than by entrainment during incineration.15,26 Therefore, the melting points or boiling points of HMs or their chlorides may be one of the factors affecting HM retention behavior in BA under different thermal treatment conditions. The average residual percentages at three reaction temperatures under different thermal conditions are shown in Figure 4 along with the corresponding melting points of the HMs and their chlorides. The residual percentages after incineration exhibit a better linear relationship with the melting points of HMs than with their chlorides, and the correlation coefficients between them are 5028

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Figure 5, the concentrations of Cd and Cu are 0.09−0.10 and 0.11−0.28 mg/Nm3, respectively, and Pb concentrations are below detection limit. All HM emissions are below their regulatory limits. The mass percentages of HMs in fly ash and bottom ash for the incineration tests at 650−950 °C are presented in Figure 6.

By comparison of the mass percentage of HMs in bottom ash from incineration and pyrolysis of hyperaccumulators, the reducing condition favors the immobilization of HMs in the bottom ash, especially the volatile heavy metals such as Cd, Pb, and Zn. More HMs retained in bottom ash makes recycling of these HMs more feasible. Moreover, the pyrolysis products like tar, heavy oil, and combustible gas can be further incinerated for energy recovery.4,28 From the view of environmental protection, pyrolysis would be better as a potential choice for thermal treatment of hyperaccumulators.28 Incineration of Sedum plumbizincicola in Vertical Entrained Flow Tube Furnace. The concentrations of HMs in flue gas generated from incineration of Sedum plumbizincicola in entrained flow tube furnace are shown in Figure 5. Notably,

Figure 6. Mass percentage of HMs in fly ash and bottom ash under different incineration temperatures by entrained flow tube furnace (FA represents for fly ash, and BA represents for bottom ash).

Figure 5. Concentrations of HMs in flue gas under different incineration temperatures by entrained flow tube furnace.

the concentration of Pb is below the detection limit and is not shown in the figure. In general, Zn levels in flue gas are approximately 2 orders of magnitude higher than those of Cd and Cu. Differences between Zn and Cd and Cu on this scale were also found in Sedum plumbizincicola. Previous studies indicated that the volatilization rate of HMs from waste incinerators is a complex function of many factors including metal species and concentrations in feed materials, combustion temperature, residence time, composition of flue gas, and performance of the air pollution control devices,26,29−31 which could also influence the distribution of HMs in flue gas from Sedum plumbizincicola incineration. Temperature has a significant impact on the HM volatilization process. The higher the temperature, the higher the HM vaporization rate, although vaporization rate is not linearly proportional to incineration temperature. Increasing the temperature could enhance vaporization by raising the vapor pressure of HM chlorides and enhancing the rates of diffusion.31,32 But the influence of temperature on volatilization of HMs is complex without a clear understanding. Linak et al. concluded that Pb and Cd are transferred to fine particles with increasing incineration temperature, but Ni does not obey the rule.33 Morf and Wey believed that the influence of temperature on the evaporation of HMs was not obvious.34,35 The level of Cd in flue gas remained constant with increasing temperature. However, the level of Cu fluctuates with increasing temperature and no obvious trend was found. In P.R. China, national emission limits of Cd, Cu, and Pb for hazardous waste incineration are 0.1, 4.0, and 1.0 mg/Nm3, respectively, and Zn emissions are not regulated.36 As shown in

Figure 7. Relationship between mass percentages in fly ash and bottom ash and melting points of HMs and their chlorides (FA represents for fly ash, and BA represents for bottom ash).

The mass percentages are mainly decreasing with the elevated temperatures, except for Cu in bottom ash, which generally follows the volatilization behavior of HMs during incineration. HMs initially vaporize in the flame, the resultant metallic vapors then undergo homogeneous nucleation to form an ultra fine aerosol.37,38 In post incineration region, flue gas cools rapidly, and the condensed aerosol grows continuously by heterogeneous coagulation. The partitioning of HMs during combustion process varies significantly with the element type. Metals such as Ba, Ce, Mg, Mn, Cr, and Cu concentrate in bottom ash, because they never vaporize38,39 and rapidly undergo a transition to various solid species. Metals such as Pb, Se, Zn, and As concentrate in particulate matter, whereas the others, such as Br, Hg, I, and Cd, concentrate in the gas phase.40,41 In light of the experimental results, the mass percentage of Cu in bottom ash is increasing and the vaporization percentage in fly ash decreasing with the elevated temperature. It can be postulated that high temperature tends to concentrate Cu in bottom ash, 5029

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Table 3. Heavy Metals in Solid Phase and Mass Balance under Different Combustion Temperature in Entrained Flow Tube Furnace Experiments (%) solid phasea

a

mass balance

T(°C)

Cu

Cd

Pb

Zn

Cu

Cd

Pb

Zn

650 750 850 950

76.3 68.4 90.4 88.1

9.83 9.62 7.83 2.63

21.1 17.8 11.4 2.07

29.5 21.1 17.5 9.56

79.9 70.5 96.1 92.0

10.1 10.0 8.1 2.9

21.1 17.8 11.4 2.1

31.2 23.1 20.3 12.3

Solid phase means fly ash and bottom ash.

Table 4. Concentrations of Heavy Metals in Flue Gas, Fly Ash, and Bottom Ash from Sedum plumbizincicola and Sedum alfredii Incineration by Entrained Flow Tube Furnace flue gas (mg/Nm3)

fly ash (mg/kg)

bottom ash (mg/kg)

plant

Cu

Cd

Zn

Cu

Cd

Pb

Zn

Cu

Cd

Pb

Zn

Sedum plumbizincicola Sedum alf redii

0.28 0.13

0.09 0.08

39.8 2.05

429 271

590 330

40.7 24.9

19 101 6827

299 268

57.8 47.1

1.95 11.6

17 483 3475

but not in volatile phases (flue gas and fly ash). Moreover, the mass percentages of HMs in solid phase are very low, in the range of 2.1∼29.5%, except that the mass percentages of Cu are normal and high enough, which are in the range from 68.4% to 90.4% (Table 3). Generally speaking, the mass percentages of Zn are approximately in accordance with those in previous studies, which are about 25−28 wt.%; whereas, the mass percentages of Cd and Pb are low (2.07−21.1%), in contrary with the more than 50 wt.% reported previously.41,42 The mass balance of HMs in flue gas, fly ash, and bottom ash at different incineration temperatures are shown in Table 3. Overall, the mass balance of Cu is much better than those of Cd, Pb, and Zn, due to Cu belongs to nonvolatile HMs. Moreover, mass percentage of Cu in bottom ash increases with the increasing temperature. The higher the burnout ratio with increasing temperature may enrich the HM content in bottom ash. Also, the trend of mass percentage of Cu can be bilaterally affected by the Cu concentration and the mass of bottom ash. In order to concentrate more HMs in bottom ash or fly ash rather than in flue gas, a preferable temperature 850 °C is chosen. In Figure 7, the mass percentages in bottom ash exhibit better linear relationship with HM boiling points than with their chlorides; but the mass percentage in fly ash seems to be opposite. This might be due to the fact that the boiling points of chlorides are much lower than those of their corresponding HMs. The metal chlorides are more volatile during incineration, and they are indeed the major evaporated compounds of HMs at 300−1000 °C.31,42,43 Because the boiling points of metals are high, they are different to evaporate during incineration, and possibly form the matrix of ash, then prefer to accumulate in bottom ash, rather than deposited on fly ash surfaces. Comparison of Incineration of Sedum plumbizincicola and Sedum alf redii in Entrained Flow Tube Furnace at 850 °C. The concentrations of HMs in different phases from incineration of Sedum plumbizincicola and Sedum alf redii in entrained flow tube furnace at 850 °C are shown in Table 4. Notably, the concentrations of Cu, Cd, and Pb in all phases from incineration of these two hyperaccumulators are in the same order of magnitude. But the Pb content in bottom ash from incineration test of Sedum alfredii is 1 order of magnitude higher than that from Sedum plumbizincicola. However, the concentrations of Zn in all phases from Sedum alf redii test are approximately an order of magnitude lower than those from

Sedum plumbizincicola test. Table 1 shows the similar trends of HMs contained in hyperaccumulators except for Cd. It is speculated that higher chlorine content of Sedum alfredii may cause the formation of the volatile HM compounds, thus increase metal partition to the fly ash and the flue gas.31,44,45

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AUTHOR INFORMATION

Corresponding Authors

*Tel: +86-571-87952629; fax: +86-571-87952438; e-mail: [email protected] (J.Y.); [email protected] (S.L.). **Tel: +86-25-86881126; fax: +86-25-86881128; e-mail: [email protected] (L.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Project is supported by Major State Basic Research Development Program of China (No. 2011CB201500), the National Key Technology R&D Program of China (2012AA06A204), the Australia-China Special Fund for S&T cooperation-International Science Linkages Program (2010DFA92360) and Program of Introducing Talents of Discipline to University (No. B08026). We thank Chun-wai Lee (senior scientist of U.S. EPA and visiting Professor at Zhejiang University), and William R. Stevens (ORISE Postdoctoral Researcher of U.S. EPA) for their constructive comments and editing on this manuscript.

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REFERENCES

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dx.doi.org/10.1021/es202616v | Environ. Sci. Technol. 2012, 46, 5025−5031