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Experimental study on the effects of H2O on PCDD/Fs formation by de novo

synthesis in carbon/CuCl2 model system

Ke Shao, Jianhua Yan *, Xiaodong Li, Shengyong Lu, Yinglei Wei, Muxing Fu

State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China

a r t i c l e i n f o

 Article history:Received 31 August 2009

Received in revised form 15 November 2009

Accepted 7 December 2009

Keywords:

Cuprous chloride

Model ash

Organic chlorine

Catalytic oxidation

De novo synthesis

a b s t r a c t

The effects of H2O vapor on de novo synthesis of polychlorinated dibenzo-p-dioxins and dibenzofurans(PCDD/Fs) and biphenyls (PCB) were investigated at two levels (none and 10 vol.%) in various model sys-

tems containing five different carbons, CuCl2, and quartz, exposed to a flow of 10% O2/N2 at 300 °C. The

influence of H2O was studied on (1) speciation and behavior of copper compounds, (2) catalytic oxidation

of carbons of distinct reactivity, and (3) formation of organic chlorine compounds, with the aim to inves-

tigate any effects on de novo synthesis. It is found that H2O converts CuCl2 to CuCl2ÁCuO, and finally to

CuO in a flow of 10% O2/N2 at 300 °C and that it decreases of organic chlorine (C–Cl) formation. When

CuCl2 is supported on carbon, the addition of H2O promotes the catalytic oxidation of this carbon. When

CuCl2 is supported on quartz, however, H2O inhibits carbon oxidation. A decrease in chlorination level of 

PCDD/Fs and PCBs with water addition is observed for all (six) model ashes; yet this addition affects the

yields of PCDD/Fs and PCBs differently. Under the experimental conditions tested H2O does not react with

Cu2Cl2, which is the catalyst of carbon oxidation. On the basis of the experimental results, the following

mechanism is proposed: conversion of CuCl2 into CuO which is less reactive in de novo synthesis and pro-

motion of catalytic oxidation of carbon by Cu2Cl2.

Ó 2009 Published by Elsevier Ltd.

1. Introduction

Incineration of municipal solid waste (MSW) allows a consider-

able volume reduction, as well as safe disposal and energy recov-

ery. Unfortunately, toxic compounds, such as polychlorinated

dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) are generated

(Olie et al., 1977). Precursor as well as de novo synthesis have both

been proposed as a crucial mechanism in the formation of dioxins

in the post-combustion zone of a MSW incinerator (Stieglitz and

Vogg, 1987). Numerous researches have been carried out to inves-

tigate the influence of individual process parameters, such as tem-

perature (Addink et al., 1991), carbon structure (Wikstrom et al.,

2004), chlorine origin (Wikstrom et al., 2003a), oxygen (Wilhelmet al., 1999) and types of catalyst (Stieglitz et al., 1989), on de novo

synthesis and its reaction mechanism.

The basic mechanistic steps for de novo formation have been

proposed by Stieglitz (1998): (a) transfer of chlorine to carbona-

ceous matter with formation of carbon–chlorine bonds, and (b)

oxidation of the (partly chlorinated) macromolecular structure to

carbon dioxide, at the same time releasing PCDD/Fs, PCBs and

numerous other compounds as reaction by-products. In both steps,

copper is thought to be a key catalyst. For the first step, it has been

demonstrated that CuCl2 can chlorinate activated carbon according

to the following reaction (Weber et al., 2001):

2CuCl2 þ R-H! 2CuCl þ R-ClþHCl: ðR1Þ

The Cu2Cl2 is then re-chlorinated by gas-phase chlorine species

(Takaoka et al., 2005). For the second step, Takaoka et al. (2005)

identify different copper species, and conclude that Cu2Cl2 plays

an important role in the catalytic oxidation of carbon and in the for-

mation of PCDD/Fs, PCBs, and CBzs in fly ash within the de novo

temperature window.

It should be noted that water vapor is always amply present in

the flue gas from MSW incineration: it is present in MSW, in com-

bustion air, and it is formed by combustion. In small plants, flue

gases are quenched with water before their cleaning. Therefore,

the influence of water on formation of PCDD/Fs must be clarified.

The research of  Ross et al. (1990) and Sakurai et al. (1996) indicate

that water enhances PCDD/Fs formation. On the other hand Briois

et al. (2007) found that the yield of PCDD/F was considerably re-

duced. However, both studied the heterogeneous precursor route,

using chlorophenols to produce PCDD/F on fly ash or model fly

ash. Contradictory results were also reported on the effect of water

on de novo synthesis of PCDD/F. Stieglitz et al. (1990) found that it

enhances the yield of PCDD/F. However, Jay and Stieglitz (1991)

observed considerably less PCDD and PCDF in the presence of 

water vapor. Meanwhile, Suzuki et al. (2004) reported that water

0045-6535/$ - see front matter Ó 2009 Published by Elsevier Ltd.doi:10.1016/j.chemosphere.2009.12.011

* Corresponding author. Tel.: +86 571 87952629; fax: +86 571 87952438.

E-mail address: yanjh@zju.edu.cn (J. Yan).

Chemosphere 78 (2010) 672–679

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vapor has little effect on dioxin formation in the dry zone of an iron

ore sintering bed.

In the presence of water, Wikstrom et al. (2003b) reported a

slightly enhanced carbon oxidation and a lower amount of total

Cl and organic chlorine in the ash. Water also slightly reduced

the yields of PCDD/Fs by 17–30% and significantly shifted the PCDF

homologue profile towards the lower chlorinated homologues.

Thus, the presence of water appears to play multiple roles in

PCDD/F formation; viz., enhancing the oxidation of active carbon

to CO and CO2, reducing Cl2 activity by converting it into HCl,

and dechlorinating the PCDD/Fs already-formed. Li et al. (2006)

indicate that water reduces the catalysis of CuCl2 and prevents di-

rect chlorination.

The present work tries to confirm these effects separately bystudying the effect of water on each mechanistic step of  de novo

synthesis. Considering the relationship between catalytic oxida-

tion of carbon and PCDD/Fs formation and the important role of 

copper chlorides in de novo synthesis, experiments on the effect

of H2O on the chemical speciation of copper, the formation of or-

ganic chlorine, and the catalytic oxidation of carbon were

conducted.

2. Experimental (Shao et al., 2009)

 2.1. Chemicals

Reagent-grade quartz (100–120 mesh) was first rinsed twice bydistilled water, and then completely dried at 120 °C. Glass wool

was cleaned by immersion in a diluted HNO3 solution for 12 h,

rinsed by distilled water, and then dried at 500 °C to remove all or-

ganic compounds. Five kinds of carbon were used. Their analysis

and characteristics are given in Table 1. The three kinds of partic-

ulate carbon blacks were obtained from Jinderui Corp. (labeled

CBA and CBB) and Cabot Corp. (labeled CBC). CBA and CBC are fur-

nace blacks, and CBB is a channel black. Three carbon blacks, acti-

vated carbon and graphite were first rinsed twice with distilled

water and acetone, respectively, and then completely dried at

100 °C.

Reagent-grade CuCl2Á2H2O, Cu(NO3)2, KNO3, HNO3, activated

carbon particles, pesticide-grade toluene, methanol, methylene

chloride and acetone, HPLC-grade n-hexane were used. N2 of 99.999% and O2 of 99.995% were supplied by Jingong Corp.

 2.2. Sample preparation

The model mixtures used in de novo synthesis were prepared as

follows: acetone and aqueous solution of CuCl2 was added to the

activated carbon and continuously hand-mixed. It was then dried

at 100 °C. The Cu content in this mixture was 2.7%. Then, 3.3 g of 

mixture was added to 96.7 g quartz and rotary mixed continuously

for a week to produce model ash [AC]. The model ashes [CBA],

[CBB], [CBC] and [Graphite] were prepared similarly as model ash

[AC].

An aqueous solution of CuCl2 was added to quartz and dried in a

rotary evaporator. It was then dried at 120 °C for 2 h. Its Cucontent

in model ash [CuCl2] was also 0.09 wt.%. Finally, 3 g of activated

carbon was added to 97 g [CuCl2] and continuously hand-mixedto produce model ash [ACÃ].

 2.3. Experimental procedure

 2.3.1. De novo experiment 

Tests were run in a tubular quartz reactor, inserted into a hori-

zontally mounted, three-stage electric furnace. The temperature at

each stage was controlled by a single S type thermocouple. Then

2 g model ash was held in the reactor with a plug of glass wool

and treated at 300 °C in a flow of either 10% O2/N2 or 10% H2O/

10% O2/N2. Total gas flow rate was 360 mL minÀ1. The total reac-

tion time was 30 min, unless otherwise noted.

 2.3.2. Sampling and analysis of PCDD/Fs, PCBs

For sampling gas-phase products, the outlet gas from the reac-

tion tube passed through ice-cooled traps consisting of a XAD-2

resinous column and an impinger absorber containing toluene.

After the desired reaction time, acetone, methylene chloride and

toluene were used for rinsing the inside of the reactor, gas tubes

and connectors. The rinses were included in the analysis of the

gas traps. Products remaining in the solid sample residue after

the experiment were also analyzed. The EPA 1613 isotope dilution

method was used for tetra–octa PCDD/Fs determination, and the

EPA 1618A isotope dilution method was used for PCBs determina-

tion. Samples were analyzed by means of high-resolution gas chro-

matography/high-resolution mass spectrometry (HRGC/HRMS

  JEOL, JMS-800D) with a DB-5MS (60 m length 0.25 mmID 0.25lm film) column.

 Table 1

Analysis of carbons used: short analysis, elementary analysis, metal analysis, BET specific surface and pore volume.

Activated carbon CBA CBB CBC Graphite

Moisture (%) 1.73 1.88 1.09 2.74 0.36

Ash (%) 2.09 0.27 0.01 0 0.13

Volatiles (%) 7.38 4.71 11.21 2.92 2.95

Fixed carbon (%) 88.80 93.14 87.69 94.34 96.56

Total short analysis (%) 100 100 100 100 100

C (%) 92.52 95.41 93.21 93.21 98.52H (%) 1.50 1.30 1.18 1.18 0.09

O (%) 1.67 0.61 3.76 0.64 0.82

N (%) 0.17 0.23 0.22 0.22 0.02

S (%) 0.19 0.30 0.54 0.06 0.06

Cl (%) 0.14 0 0 0 0

Total elementary analysis (%) 96.19 97.85 98.91 95.31 99.51

Al (ppm) 654.7 75.1 66.5 4.1 63.5

Ca (ppm) 591.9 776.1 407.0 471.5 728.2

Cu (ppm) 275.9 0.1 14.7 0.1 1.7

Fe (ppm) 506.9 252.8 642.9 1.8 78.1

Ni (ppm) 81.6 4.9 11.9 0.5 25.4

Zn (ppm) 54.0 114.2 6.9 1.6 71.0

Total metal (ppm) 2165 1223 1150 479.6 967.9

BET-SA (m2 gÀ1) 933.93 71.47 114.85 107.35 6.68

Pore volume (cm3 gÀ1) 0.63 0.54 0.69 0.74 0.03

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 2.3.3. Measurement of organic chlorine and C–ClFor sampling gas phase organic chlorine, the outlet gas was sent

through ice-cooled sampling columns of activated carbon. After the

reaction was finished, the activated carbon was washed by distilled

water and further by an aqueous solution of potassium nitrate (0.8

mass% KNO3) to remove inorganic chlorides. Then, the activated

carbon bulk was burnt at 1000 °C in 80% O2/20% H2O mixture. Dis-

tilled water was used to absorb the HCl in the outlet gas. Chlorine

ion concentrations were measured in the resulting solutions by ion

chromatography (Metrohm, 729 Basic IC). Also the organic chlo-

rides in the model ash were analyzed. Briefly, the residual ash

was mixed with 50 mg activated carbon, and extracted with a di-

luted nitric acid solution (pH = 2) at 70 °C for 30 min followed by

a rapid quench. After 60 min of mixing via a mechanical shaker,

the sample was filtered and washed with a nitrate solution to dis-place inorganic chlorides. Chlorine ion and sulfate ion concentra-

tions in the eluate were measured by ion chromatography. The

ash plus 50 mg of activated carbon was dried at 100 °C. The total

extractable organic chlorine (EOCl) remaining in the solid was ex-

tracted by toluene for 24 h. Then the amounts of EOCl and non-

extractable organic chlorine (NEOCl) in the model ash were mea-

sured similarly as for outlet gas analysis.

 2.3.4. Measurement of CO and CO 2

Through the experiments the CO2, CO, HCl concentrations in the

flue gas were measured continuously using an infrared gas ana-

lyzer (FTIR). An infrared spectrum was collected automatically

every 20 s. Each experimental condition was tested two times, to

check the reproducibility of the experiments.

3. Results and discussion

 3.1. Influence of H  2O on organic chlorine formation

A summary of the experiments performed and of the results ob-

tained is presented in Table 2; the values are averages of more than

two experiments. After 30 min reaction in a flow of 10% O2/N2, half 

of the original chlorine content in model ash [CuCl 2] was reduced

(Test 1). However, further de-chlorination of CuCl2 in this model

ash was much slower (Test 2). 46% of the original chlorine was left

after 60 min reaction. This suggests that CuCl2ÁCuO is formed. Then

CuCl2ÁCuO decomposes to form CuO and starting material, CuCl2(Takaoka et al., 2005).

4CuCl2 þ O2 ! 2CuCl2 Á CuOþ 2Cl2 ðR2ÞCuCl2 Á CuO$ CuCl2 þ CuO ðR3Þ

The addition of 10% H2O to the gas phase resulted in an appar-

ent reduction of the inorganic chlorine in the model ashes. In the

model ash [CuCl2], only 10% of the original chlorine was left after

30 min reaction (Test 9). It has been reported that CuCl2 can react

with H2O to form CuCl2ÁCuO by reaction (R4) (Lewis et al., 2009).

This result also suggests that the rate of R4 is much faster than that

of R2.

H2Oþ 2CuCl2 ! CuCl2 Á CuOþ 2HCl ðR4Þ

The inorganic chlorine in model ashes also decreased, compared

with the results without H2O. With the exception of [Graphite]

and [ACÃ

], the extent of decrease was much less than the resultsof [CuCl2]. This distinction may be attributed to the different chem-

ical speciation of copper between the model ashes.

The addition of H2O also decreased organic chlorine formation,

in line with the results of  Wikstrom et al. (2003b). However, Cl2

was not used in our experiments. Therefore, the decrease of organ-

ic chlorine was ascribed to the conversion of CuCl2 to CuCl2ÁCuO by

R4. For model ash [AC], the addition of H2O has little influence on

organic chlorine formation, suggesting that R1 is very fast in this

model ash. However, for model ash [ACÃ], a remarkable decrease

(60%) in C–Cl is observed when 10% H2O is introduced. In model

ash [ACÃ], the support of CuCl2 is quartz. H2O converted the CuCl2

to CuCl2ÁCuO. This inhibited the CuCl2 transfer to carbon and its

chlorination. For model ashes [CBA], [CBB], [CBC] and [Graphite],

addition of H2O decreases the formation of C–Cl, by 27%, 22%,40% and 38%, respectively. For all six model ashes, the inhibiting ef-

fect of H2O is substantially more pronounced for EOCl than for

NEOCl, suggesting that the release of organic chlorine from carbon

matrix is also inhibited.

 3.2. Effect of H  2O on catalytic oxidation of carbon

The effect of H2O on catalytic oxidation of carbon is illustrated

in Fig. 1, shown for CuCl2 with AC, CBC and graphite; the values

are averages of two experiments. The results of model ash [CBA]

and [CBB] (not shown) are similar to model ash [CBC]. The red lines

represent the CO2 produced from model ashes in 10% O2/N2 at

300 °C. The black lines represent the CO2 produced from model

ashes without CuCl2. The green and blue lines represent the CO2produced when water is added from 0 to 90 min (0–30 min for

 Table 2

Summary of the experimental conditions and results.

Test Experiment duration (min) Model ash H2O (%) Inorganic chlorine content (lg gÀ1) EOCl content (lg gÀ1) NEOCl content (lg gÀ1)

Gas-phase Solid-phase

1 30 [CuCl2] None 518 ± 6 N/A N/A N/A

2 60 [CuCl2] None 457 ± 17 N/A N/A N/A

3 30 [AC] None 349 ± 12 8 ± 3 6 ± 2 272 ± 34

4 30 [CBA] None 241 ± 8 62 ± 8 2 311 ± 21

5 30 [CBB] None 196 ± 3 49 ± 4 2 372 ± 23

6 30 [CBC] None 305 ± 8 74 ± 17 7 ± 3 326 ± 23

7 30 [Graphite] None 268 ± 40 48 ± 11 3 14 ± 5

8 30 [ACÃ] None 431 ± 12 4 ± 2 4 ± 2 156 ± 17

9 30 [CuCl2] 10 92 N/A N/A N/A

10 30 [AC] 10 248 ± 12 7 ± 3 6 ± 3 268 ± 7

11 30 [CBA] 10 178 ± 3 30 ± 8 3 ± 1 242 ± 11

12 30 [CBB] 10 150 ± 6 21 ± 5 2 307 ± 28

13 30 [CBC] 10 230 ± 25 29 ± 4 3 ± 2 213 ± 18

14 30 [Graphite] 10 89 ± 25 18 ± 3 n.d. 22 ± 5

15 30 [ACÃ] 10 95 ± 13 2 n.d. 64 ± 17

All experiments: 2.0 g model ash; Tests 1–7 in triplicate, Tests 8–15 in duplicate. Mean value ± standard deviation is shown, no standard deviation for Cl is given if negligible

(<1lg gÀ1); N/A = not analyzed; n.d. = non-detected, i.e. below detection limits (=1lg gÀ1).

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model ash [ACÃ]) and 30–60 min, respectively. The results show

that CuCl2 dramatically catalyzes the oxidation of carbon. Takaoka

et al. (2005) have reported that only low-valence copper com-

pounds were observed in model fly ash containing activated car-

bon at 300 °C under 10% O2/90% N2. This suggests that Cu2Cl2 isthe catalyst in oxidation of carbon.

For [AC], when 10% H2O was added during a time period from

30 to 60 min, the concentration of CO2 increased sharply at the

time of addition, and then dropped slowly with time (Fig. 1). After

stopping the addition, the CO2 concentration rapidly returned to

the same level as in the case without H2O. Similar results were alsoobserved when carbon blacks were used as carbon sources. These

0

50

100

150

200

250Graphite

Model ash [Graphite]

Model ash [Graphite] + H2O (0-90min)

Time (min)

0

500

1000

1500

2000

2500

3000

3500CBC

Model ash [CBC]

Model ash [CBC] + H2O (0-90min)

Model ash [CBC] + H 2O (30-60min)

0

100

200

300

400

500

600 AC

Model ash [AC*]

Model ash [AC*] + H2O (0-90min)

Model ash [AC*] + H2O (0-30min)

AC

Model ash [AC]

Model ash [AC] + H2O (0-90min)

Model ash [AC] + H2O (30-60min)

0 906030 754515

0 906030 754515

0

300

600

900

1200

1500

   C   O

   2   (  p  p  m   )

   C   O

   2   (  p  p  m   )

   C   O

   2   (  p  p  m   )

   C   O

   2   (  p  p  m   )

0 906030 754515

0 906030 754515

Fig. 1. Effect of H2O on catalytic oxidation of carbon (2 g of model ash, 300 °C, 10% O2, 10%H2O, balance N2).

K. Shao et al./ Chemosphere 78 (2010) 672–679 675

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results suggest that H2O can promote the catalytic oxidation of car-

bon and does not react with Cu2Cl2. This result confirms that de-

chlorination by H2O has less influence on copper chloride sup-ported on activated carbon or carbon blacks. Wikstrom et al.

(2003b) proposed that chemisorption of water to free carbon sites

or to oxygen in the macromolecular carbon structure in the fly ash

change the reactivity of the carbon and enhance carbon oxidation.

In another case, 10% H2O was added from the beginning and un-

til the end at 90 min (Fig. 1). For model ashes [AC], [CBC] and

[Graphite], the addition of H2O promoted the oxidation of carbons.

However, for model ash [ACÃ], the concentration of CO2 was much

lower, compared with results without H2O. This distinction may be

attributed to the difference of support of copper catalyst. In model

ash [AC], the support of CuCl2 is activated carbon. CuCl2 is reduced

by carbon to form Cu2Cl2. According to the results above, H2O does

not react with Cu2Cl2 but it promotes the catalytic oxidation of car-

bon. These results suggest that the chlorination of carbon by CuCl 2

is very fast, compared with the reaction between CuCl2 and H2O.

For [ACÃ], the support of CuCl2 is quartz. H2O converts the CuCl2

to CuCl2ÁCuO and finally to CuO which is less reactive in de novo

synthesis (Lewis et al., 2009).

 3.3. Effect of H  2O on PCDD/Fs, PCB formation

The influence of H2O on PCDD/Fs, PCBs formation by de novo

synthesis is illustrated in Figs. 2–5. The composition of model

ashes [AC] and [ACÃ] is the same. Surprisingly, the amounts of 

PCDD/Fs and PCBs produced in model ash [ACÃ] are 2.8 and 2.2

times more than those in model ash [AC], respectively. The support

of CuCl2 also influences the congener distribution. For model ash

[ACÃ

], OCDF and OCDD both accounted for approximately 70% of the total Te-OCDF and Te-OCDD formed, respectively. However,

the formation of moderately chlorinated PCDD/F (Te-Hx) was fa-

vored in the model ash [AC]. The chlorination level of PCBs formed

in [ACÃ] was also higher than that in [AC]. The reason for this dis-tinction is that the nature of the support of CuCl2 changes the

chemical form of the copper catalyst in model ash. In [ACÃ], only

part of CuCl2 was transferred to carbon. The copper chloride left

on the quartz remained at the bivalent state and could chlorinate

the PCDD/Fs and PCBs formed from de novo synthesis. The

[PCDD]:[PCDF] ratios for model ashes [AC], [CBA], [CBB], [Graph-

ite], and [ACÃ] were comparable, at 0.08, 0.08, 0.12, 0.09, 0.19,

and 0.21, respectively.

The addition of 10% H2O exhibits multiple effects on the

yields of PCDD/Fs when different model ashes are used. For mod-

el ashes [AC], [CBB] and [CBC], H2O promoted the PCDD/Fs for-

mation. For model ashes [ACÃ], [CBA] and [Graphite], H2O

exhibited lower yields of PCDD/Fs. The inhibiting effects in

[CBA] and [Graphite] were both slight (a decrease by 8% and11%, respectively). In [ACÃ], a remarkable decrease in PCDD/Fs

was observed when 10% H2O was introduced. This result is in

agreement with the results of influence of H2O on catalytic oxi-

dation of carbon in model ashes [ACÃ]. These results were not

unexpected, considering that H2O has two aspect influences on

de novo synthesis: Converting CuCl2 to CuCl2ÁCuO to decrease

the C–Cl and Cu2Cl2 formation and promoting catalytic oxidation

of carbon. It is also found that for all six model ashes, H2O in-

creases the [PCDD]:[PCDF] ratios, to 0.09, 0.10, 0.15, 0.10, 0.29

and 0.41, respectively.

While the effect of H2O on the amounts of PCDD/F produced ap-

peared to depend on carbon source and mixing manner of model

ashes, the effect on congener distribution was not. For all kinds

of model ashes, the introduction of 10% H2O decreased the chlori-nation level of PCDD/F congeners. This result is in agreement with

0

400

800

1200

1600

0

2

4

6

60

90

120

150

180

0

30

60

90

120

150

180

   A  m  o  u  n   t  o   f   P   C   D   D  s   i  n   t   h  e  g  a  s  s  a  m  p   l  e

   (  n  g

  g  -   1    s  a  m  p   l  e   )

   A  m  o  u  n   t  o   f   P   C   D

   D  s   i  n   t   h  e  s  o   l   i   d  s  a  m  p   l  e

   (  n  g  g

  -   1    s  a  m  p   l  e   )

PCDDs in the solid phase

PCDFs in the solid phase

   A  m  o  u  n   t  o   f   P   C   D

   F  s   i  n   t   h  e  s  o   l   i   d  s  a  m  p   l  e

   (  n  g  g

  -   1    s  a  m  p   l  e   )

0

3

6

9

12

15

18PCDDs in the gas phase

[Graphite]+H2O

[Graphite][CBC]+H

2O

[CBC][CBB]+H

2O

[CBB][CBA]+H

2O

[CBA][AC]+H

2O

[AC][AC*]+H

2O

[AC*]

[Graphite]+H2O

[Graphite][CBC]+H

2O

[CBC][CBB]+H

2O

[CBB][CBA]+H

2O

[CBA][AC]+H

2O

[AC][AC*]+H

2O

[AC*]

PCDFs in the gas phase

   A  m  o  u  n   t  o   f   P

   C   D   F  s   i  n   t   h  e  g  a  s  s  a  m  p   l  e

   (  n

  g  g

  -   1    s  a  m  p   l  e   )

Fig. 2. Effect of H2O on PCDD/Fs formation by de novo synthesis. All tests with two or three repeats, mean value with standard deviation is shown (2 g of model ash with

0.19%CuCl2, 3%C, bulk quartz; 300 °C, 10% O2, 10%H2O, balance N2, 30 min).

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previous findings. Two reasons can be used to explain this result:(1) decreasing the C–Cl formation to lower the chlorination level

of PCDD/F; (2) Converting copper (II) chloride to CuCl2ÁCuO to inhi-bit the further chlorination of PCDD/Fs.

0

20

40

60

80

100

OCDF

HpCDFHxCDF

PeCDF

TeCDF

0

20

40

60

80

100

OCDD

HpCDD

HxCDD

PeCDD

TeCDD

   P   C   D   F   h  o  m  o   l  o  g  u  e   d   i  s   t  r   i   b  u   t   i  o  n   %

   P   C   D   D   h  o  m  o   l  o  g  u  e

   d   i  s   t  r   i   b  u   t   i  o  n   %

[graphite]+H2O[CBC]+H

2O

[CBC][CBB]+H

2O

[CBB] [graphite][CBA]+H

2O

[CBA][AC]+H

2O

[AC][AC*]+H

2O

[AC*]

[graphite]+H2O[CBC]+H

2O

[CBC][CBB]+H

2O

[CBB] [graphite][CBA]+H

2O

[CBA][AC]+H

2O

[AC][AC*]+H

2O

[AC*]

Fig. 3. Percentage of PCDD and PCDF homologue mass distribution of the samples with and without H 2O during thermal treatment of model ashes in laboratory scale

experiments. All tests with two or three repeats, mean value is shown (2 g of model ash with 0.19%CuCl2, 3%C, bulk quartz; 300 °C, 10% O2, 10% H2O, balance N2, 30 min).

0

100

200

300

400

PCBs in the gas phase

PCBs in the solid phase

   A  m  o  u  n

   t  o

   f   P   C   B  s

   i  n   t   h  e  g  a

  s  p

   h  a  s  e  a  n

   d  s  o

   l   i   d  s  a  m  p

   l  e

   (  n  g  g

  -   1    s

  a  m  p

   l  e   )

[Graphite]+H2O

[Graphite]

[CBC]+H2O

[CBC]

[CBB]+H2O

[CBB]

[CBA]+H2O

[CBA][AC]+H

2O

[AC]

[AC*]+H2O

[AC*]

Fig. 4. Effect of H2O on PCBs formation by de novo synthesis. All tests with two or three repeats, mean value with standard deviation is shown (2 g of model ash with

0.19%CuCl2, 3%C, bulk quartz; 300 °C, 10% O2, 10%H2O, balance N2, 30 min).

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The influence of H2O on PCB formation by de novo synthesis is

illustrated in Figs. 4 and 5. With the exception of [ACÃ], the addition

of H2O increases the yields of PCBs formation by de novo synthesis,

by 3.3, 1.5, 2.1, 1.6 and 1.6 times, respectively. The PCBs increase

more than that PCDD/Fs, yet only Te-OCDD/Fs were measured in

our experiments. For model ash [ACÃ], a decrease of 9.5% in Di-

DeCB can be seen in Fig. 5. Addition of H2O also decreases the chlo-

rination level of PCBs.

In de novo synthesis of PCDD/Fs, copper is a key catalyst. Copper

catalyst oxychlorinates the carbon matrix to form dioxin-like

structures. CuCl2 also chlorinates carbon, forming C–Cl bonds,

while Cu2Cl2 transmits the gaseous oxygen to solids forming C–O; Cu2Cl2 also catalyzes the oxidation of carbon decomposing the

macromolecular carbon and releasing PCDD/Fs. Then PCDD/Fs are

further chlorinated by copper (II) chloride.

According to these experimental results, the mechanism by

which H2O influences upon PCDD/Fs de novo synthesis is as fol-

lows: (1) Promoting the conversion of CuCl2 into CuO inhibits the

C–Cl bond formation as well as further chlorination of PCDD/Fs;

(2) Promoting carbon oxidation catalysis by Cu2Cl2, thus decom-

posing macromolecular carbon, while releasing PCDD/F, PCBs, etc.

4. Conclusion

Model mixtures containing cupric chloride and five different

carbon sources was used to study the effects of H2O on PCDD/Fsformation by de novo synthesis, i.e. those on chemical speciation

of copper, organic chlorine (C–Cl) formation, catalytic oxidation

of carbon, and on PCDD/Fs and PCBs formation. It was found that

H2O could convert CuCl2 to CuCl2ÁCuO and promote conversion of 

CuCl2 to CuO, as confirmed by the decrease of organic chlorine

(C–Cl) formation by H2O. For model ashes where CuCl2 was sup-

ported by carbon ([AC], [CBA], [CBB], [CBC] and [Graphite]), the

addition of H2O promoted catalytic oxidation of this carbon. The

oxidation rate of carbon increased by 14–110% when 10% H2O

was introduced. After stopping H2O addition, the oxidation rate

dropped sharply to the level as in the cases without H2O. This re-

sult suggests that H2O does not react with Cu2Cl2 that catalyzes

carbon oxidation. The addition of H2O decreased the chlorination

level of both PCDD/Fs and PCBs. For model ashes [ACÃ

], [CBA] and[Graphite], addition of H2O decreased the amounts of PCDD/Fs pro-

duced. For other ashes, addition of H2O exhibited the opposite

effect.

According to the experimental results, the mechanism by which

H2O influences upon PCDD/Fs de novo synthesis is as follow: (1)

Promoting the conversion of CuCl2 into CuO inhibits the C–Cl bond

formation as well as further chlorination of PCDD/Fs. (2) Promoting

carbon oxidation catalysis by Cu2Cl2, thus decomposing macromo-

lecular carbon, while releasing the PCDD/Fs, PCBs, etc.

 Acknowledgements

The authors acknowledge the support of the National HighTechnology Research and Development Key Program of China

(2007AA061302) and (2007AA06Z336), National Natural Science

Foundation of China (50576082), Important Project on Science

and Technology of Zhejiang Province of China (2007C13084), Zhe-

  jiang Provincial Natural Science Foundation of China (R107532)

and Project on Science and Technology of Zhejiang Province of Chi-

na (2008C23090). The authors also thank Ms. Mao Lijuan, the tech-

nician of 985-Institute of Agrobiology and Environmental Sciences

of Zhejiang University, for her contribution in carrying through the

analysis of carbon.

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