Environ Sci Pollut Res (2015) 22:13077–13082 DOI 10.1007/s11356-015-4335-8
RESEARCH ARTICLE
Reducing dioxin formation by adding hydrogen in simulated fly ash J. Yang 1 & X. D. Li 1 & W. J. Meng 1 & S. Y. Lu 1 & T. Chen 1 & J. H. Yan 1 & A. Buekens 1 & K. Olie 1
Received: 13 October 2014 / Accepted: 6 March 2015 / Published online: 30 April 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract In this study, simulated fly ash containing CuO/ CuCl2 was heated at 350 °C in a flow of N2 and also in a nitrogen flow containing 10 vol% H2, to evaluate the influence of hydrogen adding on dioxin formation. The total polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) output derived from the CuO sample under N2 and 10 % H2 was 7.382 and 0.708 ng/g, respectively. As for CuCl2, it was 589 and 46.1 ng/g, respectively. The results show that the hydrogen adding has a good inhibition effect on PCDD/F formation; the inhibition rate was higher than 90 % for PCDD/Fs. HCl and NH3 were detected by Gasmet in the flue gas; the probable inhibition mechanism of hydrogen reaction was proposed, based on our measurements and others’ researches. Keywords Incineration . Fly ash . Formation . De novo synthesis . Nitrogen . Hydrogen . CuO/CuCl2 . PCDD/Fs . Inhibition
(1987), and Rappe et al. (1978). The presence of both catalytic metals and oxygen is essential for the de novo synthesis of PCDD/Fs (Gullett et al. 1990; Stieglitz et al. 1989; Vogg and Stieglitz 1986). The synthesis of PCDD/PCDF is catalyzed by Cu and Fe compounds, not by the chlorides of alkali, alkali earth, Zn, Mn, Hg, Cd, Sn, and Pb. The effect of an oxidizing atmosphere on the formation of PCDD/Fs was studied many times. Vogg et al. (1987) compared the annealing of fly ash in nitrogen and in its mixtures with 1, 4, and 10 vol% of oxygen. The formation of both PCDDs and PCDFs rose with oxygen content (Buekens et al. 2001; Vogg et al. 1987). Hagenmaier et al. discovered the destruction of PCDD/Fs under anoxic conditions (Hagenmaier et al. 1987). The [PCDD]:[PCDF] ratio rises with increasing [O2], but the average level of chlorination of PCDD and PCDF did not depend on O2 concentration (Addink and Olie 1995). The effect of oxygen was also studied for filter dust from the sintering belt. PCDD/Fs were quite low in pure N2 (Milligan and Altwicker 1995). But there was no comprehensive study about the effect of hydrogen adding on dioxin formation.
Introduction After the discovery of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in fly ash from municipal solid waste incineration (Olie et al. 1977), various pathways of formation were developed and a number of potential precursors were proposed, e.g., Buser (1979); Karasek and Dickson Responsible editor: Leif Kronberg * X. D. Li [email protected] 1
State Key Laboratory of Clean Energy Utilization, Zhejiang University, Zheda Road 38#, Hangzhou, China
Materials and methods Simulated fly ash was prepared from activated carbon (AC), admixed with different metal species by grinding in a mortar for about 5 min in the following proportions: AC (2.5 wt%, 200–500 mesh; the characteristics of the activated carbon are shown in Table 1), sodium chloride (NaCl; 10 wt% Cl), metal compound (0.1 wt% metal), and silicon dioxide (SiO2; remainder, 120–200 mesh).Metal compounds were bought from Aladdin Chemistry Co. Ltd. The metal species used were CuCl2 · 2H2O (99 % pure), CuO (99 % pure), and NaCl (99.99 % pure). The initial concentration of PCDD/F in simulated ash is shown in Table 2. The de novo experiments were
13078 Table 1
Environ Sci Pollut Res (2015) 22:13077–13082 Characteristics of the activated carbon (=air dry)
Proximate analysis (wt%)
Elemental analysis (wt%)
Trace metals (ppm)
Moisture
Ash
Volatile matter
Fixed carbon
C
H
N
S
O
Al
Cu
Fe
Ni
Zn
1.73
2.09
7.38
88.8
92.5
1.5
0.17
0.19
1.67
655
276
507
82
54
Table 2
Initial concentration of PCDD/F in simulated fly ash (ng/g)
T4CDD
P5CDD
H6CDD
H7CDD
O8CDD
T4CDF
P5CDF
H6CDF
H7CDF
O8CDF
I-TEQ
ND
0.005
0.007
0.007
0.036
0.027
0.026
0.042
0.057
0.078
0.006
carried out in a tubular reactor operating at atmospheric pressure and positioned in a tubular furnace (Fig. 1). An amount of 2 g of simulated fly ash was placed into the quartz tube and pushed in a furnace preheated at 350 °C. The de novo reaction lasted for 60 min and was conducted in a flow of N2 and 10 % H2 +90 % N2, supplied at 300 mL/min. The PCDD/Fs in the carrier gas were collected on XAD-2 (a resin that can adsorb dioxins in gas phase effectively, Sigma-Aldrich) and in two successive impingers containing 125 mL of toluene (HPLC, J.T. Baker). After the test, the PCDD/Fs in the simulated fly ash residue and in the gas were analyzed together. Every experiment was conducted twice and the average was recorded for further use. Each sample was spiked with a mixture of 13C12-labelled PCDD/F compound stock solution (5 μL) before Soxhlet extraction for 24 h. The extracts from the Soxhlet extraction were spiked with cleanup standard (5 μL) and subsequently followed by rotary evaporation and multilayer silica gel column cleanup procedure following the method of USEPA 1613. The extracts were blow-down to 20 μL under a gentle stream of nitrogen (N2), and 5 μL of 13C12-labelled PCDD/F Fig. 1 Experimental setup showing (from left to right): the gas cylinder supplying carrier gas, a mass flowmeter (CS200, Sevenstar), the ash sample mounted in a tubular reactor surrounded by the tubular furnace, and the XAD-2 and toluene bottles absorption train
internal standard solution was added before samples were subjected to analysis by high-resolution gas chromatography coupled with high-resolution mass spectrometry (HRGC/HRMS) (JEOL JMS-800D) with a DB-5MS column (60 m×0.25 mm×0.25 μm). The temperature program for the GC oven was set as follows: initial temperature was 150 °C, held for 1 min; increased at 25 °C/min to 190 °C; then increased at 3 °C/min to 280 °C; held for 20 min. The toxic 2,3,7,8-substituted PCDD/Fs (referred to as congeners) as well as tetra- to octa-chlorinated homologue were identified based on isotope ratios within ±15 % of the theoretical values, and signal to noise ratios of equal to or greater than 2.5 quantification of PCDD/Fs were determined by an isotope dilution method using relative response factors previously obtained from the five calibration standard solutions. Recoveries of internal standards, as determined against the external standard, generally varied between 70 and 110 %, and were all satisfied with the method of USEPA 1613. The limit of determination (LOD) of HRGC/HRMS is between 0.0005 and 0.001 ng/g based on the different isomers. The target compounds were tetra- to octa-CDD/Fs (136 isomers).
Environ Sci Pollut Res (2015) 22:13077–13082 Table 3 Dioxin concentrations under different atmosphere (ng/g)
13079
CuO N2
CuCl2 RSD (%)
10 % H2
RSD (%)
N2
RSD (%)
10 % H2
RSD (%)
T4CDD
1.635
22.4
0.081
19.7
4.25
18.2
1.7
19.9
P5CDD
0.866
26.1
0.043
16.3
7.53
17.8
1.34
14.6
H6CDD H7CDD
0.467 0.186
30.3 20.8
0.043 0.004
40.4 3.7
11.57 11.62
20.2 17.9
1.01 0.33
19.1 29.2
O8CDD
0.089
7.7
0.012
11.4
23.13
15.6
0.09
19.3
PCDD T4CDF
3.242 2.172
24.0 22.8
0.183 0.315
21.2 46.2
58.09 74.84
16.9 9.8
4.48 18.27
18.8 10.4
P5CDF H6CDF
1.104 0.616
27.9 27.0
0.131 0.063
42.4 54.0
106.5 146.33
11.7 10.5
13.13 7.12
13.2 21.7
H7CDF
0.229
23.9
0.011
8.1
149.76
12.1
2.43
22.0
O8CDF
0.018
11.5
0.004
33.3
53.43
19.0
0.62
20.0
PCDF PCDD/Fs
4.14 7.38
24.8 24.5
0.524 0.71
45.2 39.0
531 589
11.5 11.9
41.6 46.1
14.0 14.5
I-TEQ PCDD:PCDF
0.196 0.78
25.9
0.015 0.35
35.6
9.31 0.11
12.0
1.11 0.11
14.4
Cl average
4.79
All isotope standards were purchased from the Cambridge Isotope Laboratories, Inc.
Results and discussion The ten isomer groups of PCDDs and PCDFs, the corresponding international toxicity equivalent (I-TEQ) value, the ratio of PCDD to PCDF, and the weight average degree of chlorination of all PCDD/Fs are presented in Table 3. The total PCDD/F output derived from the CuO sample under N2 and 10 % H2 was 7.38 ng/g (0.196 ng I-TEQ/g) and 0.71 ng/g (0.015 ng I-TEQ/g), respectively. The ratios between PCDD/PCDF were 0.78 and 0.35. As for the CuCl2 sample, it was 589 ng/g (9.31 ng I-TEQ/g) and 46.1 ng/g (1.11 ng ITEQ/g), respectively, while the ratio kept constant at 0.11. The inhibition efficiency of each homologue is shown in Fig. 2. The results indicated that the addition of hydrogen had good inhibition ability on dioxin formation with the range of 60.0– 99.6 %. The homologues with a higher chlorinated level were more strongly suppressed. Considering that the level of chlorination decreases (4.79–4.70, 6.07–4.91), the chlorination reaction was supposedly prevented. As shown in Fig. 3, the inhibition rate of PCDD, PCDF, PCDD/Fs, and I-TEQ of the CuO sample was 94.4, 87.3, 90.4, and 92.4 %, respectively. Meanwhile, it was 92.3, 92.1, 92.2, and 88.1 % for the CuCl2 sample. Thus, the inhibition rate of hydrogen on PCDD formation is slightly better than that of
4.70
6.07
4.91
PCDF formation. One of the possible reasons is that DD can convert into DF under reducing conditions and the rate of disappearance of DD depends strongly on the concentration of hydrogen atoms (Altarawneh et al. 2009; Cieplik et al. 2002). But the inhibition of PCDD/Fs is mainly due to the reduction of PCDF formation. It accounted for 54.2 and 90.1 % of the total reduction of PCDD/Fs of the CuO and CuCl2 samples, respectively. The mass distribution of the PCDD and PCDF homologue of the samples under inert and hydrogen adding is shown in Figs. 4 and 5. There was little difference of PCDD/F
Fig. 2 Inhibition rate of each homologue comparing N2 +10 % H2 with N2
13080
Environ Sci Pollut Res (2015) 22:13077–13082
Fig. 3 Inhibition rate of PCDD/Fs and I-TEQ comparing N2 +10 % H2 with N2
distribution of the CuO sample with the addition of hydrogen. The percentage of the homologue decreased with the increasing of the chlorinated level, and the low-chlorinated dioxins (4, 5-Cl) dominated. On the contrary, the homologue distributions of the CuCl2 sample under these two atmospheres were very different. It seemed that the percentage of the homologue increased with the rise of the chlorinated number under an inert atmosphere. But it was completely reversed with the addition of hydrogen. In other words, hydrogen had better inhibition ability on high-chlorinated PCDD/Fs than the low-chlorinated ones. The concentration of O8CDD and O8CDF reduced from 23.13 and 53.43 ng/g to 0.09 and 0.62 ng/g, respectively. The inhibition rate was as high as 99.0 %. As for T4CDD and T4CDF, it was only 60.0 and 75.6 %, respectively. The chlorinated level dropped significantly; it reduced from 6.07 to 4.91. In order to study the probable inhibition mechanism of a reducing atmosphere, a Gasmet online monitor (Finland, FTIR DX-4000) was used to detect the flue gas at the same experimental conditions of CuO and CuCl2 samples under a reducing atmosphere; three detections were conducted during
Fig. 4 PCDD homologue distribution of CuO and CuCl2 samples
the experiment. As shown in Figs. 6 and 7, HCl and NH3 were found in the flue gas and there were three distinct cycles. The measurements vary from 0 to 20 ppm and 0 to 4 ppm for HCl and NH3, respectively. It indicated that NH3 played an important role during the reaction. The tests of NH3 effect on the formation of PCDD/Fs in both laboratory-scale (Vogg et al. 1987) and full-scale incinerators (Takacs et al. 1993; Takacs and Moilanen 1991) proved it to be an efficient inhibitor. The researches of Ruokojärvi et al. (1998) and Hajizadeh et al. (2012) also found the same effect of NH3 on dioxin formation. Ruokojärvi et al. (2004) found that the distinct effect was a decline in the degree of chlorination of PCDD/Fs, which could be seen in the tests with gaseous inhibitors. Addink and Olie (1993) also found that inhibitors reduced the degree of chlorination in their laboratory tests. Our results were consistent with their researches. Takacs et al. (1993) achieved PCDD/F reductions using NH3 and suggested that the mechanism of action was neutralization of HCl. Though we performed only a few measurements of HCl and NH3 concentration in our tests and the concentration was not so high, the neutralization of NH3
Environ Sci Pollut Res (2015) 22:13077–13082
13081
Fig. 5 PCDF homologue distribution of CuO and CuCl2 samples
Fig. 6 The concentration of HCl and NH3 in the flue gas of the CuO sample with hydrogen
cannot be excluded. Meanwhile, there might be another probable mechanism. Vogg et al. (1987) studied the effect of ammonia on MSWI fly ash and reported that NH 3 counteracted the catalytic action of CuCl2. Ruokojärvi et al. (2004) reported that one probable explanation for the inhibition of PCDD/Fs by molecules with lone pair electrons, e.g., those containing nitrogen or sulfur, may lie in their ability to Fig. 7 The concentration of HCl and NH3 in the flue gas of the CuCl2 sample with hydrogen
form stable complexes with metallic compounds (Tuppurainen et al. 1999), so that they eliminate or reduce the catalytic properties of the metals. Luna et al. (2000) used density functional theory (DFT) at the B3LYP level of theory and a valence triple-ζ to investigate the structures and bonding characteristics of the different complexes involved in the ureaCu+ potential energy surface (PES) and found the formation of
13082
CuNH3+. These could be the inhibition mechanism of hydrogen on dioxin formation, but more work should be done in the future to confirm.
Conclusion The simulated fly ash containing CuO/CuCl2 was studied under inert (N2) and reducing (N2 +10 % H2) atmospheres; the results showed that the reducing atmosphere had good inhibition ability on dioxin formation compared with the inert atmosphere, and the inhibition rate of I-TEQ was as high as 92.4 and 88.1 % for CuO and CuCl2 samples, respectively, under the experimental setup prescribed in this paper. Chlorination reaction also was prevented; the chlorination degree of PCDD/F formation via simulated fly ash (CuCl2) was reduced from 6.07 to 4.91. HCl and NH3 were detected by Gasmet in the flue gas. Based on the results in this paper and the research of Takacs et al. (1993) and Vogg et al. (1987), the inhibition mechanism of a reducing atmosphere was considered as NH3 forming in the reaction process and counteracting the catalytic action of metals. Acknowledgments This study was funded by the National Key Basic Research Development Program (No. 2011CB201500). Conflict of interest The authors declare that they have no conflict of interest.
References Addink R PR, Olie K. (1993): Inhibition of PCDD/F formation during de novo synthesis on fly ash using N- and S-compounds. Organohalogen Compd 12:27–30 Addink R, Olie K (1995) Role of oxygen in formation of polychlorinated dibenzo-p-dioxins/dibenzofurans from carbon on fly ash. Environ Sci Technol 29:1586–90 Altarawneh M, Dlugogorski BZ, Kennedy EM, Mackie JC (2009) Mechanisms for formation, chlorination, dechlorination and destruction of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). Prog Energ Combust 35:245–274 Buekens A, Stieglitz L, Hell K, Huang H, Segers P (2001) Dioxins from thermal and metallurgical processes: recent studies for the iron and steel industry. Chemosphere 42:729–735 Buser HR (1979) Formation of polychlorinated dibenzofurans (PCDFs) and dibenzo-para-dioxins (PCDDs) from the pyrolysis of chlorobenzenes. Chemosphere 8:415–424 Cieplik MK, Epema OJ, Louw R (2002): Thermal hydrogenolysis of dibenzo-p-dioxin and dibenzofuran. Eur J Org Chem 16:2792–2799
Environ Sci Pollut Res (2015) 22:13077–13082 Gullett BK, Bruce KR, Beach LO (1990) The effect of metal-catalysts on the formation of polychlorinated dibenzo-para-dioxin and polychlorinated dibenzofuran precursors. Chemosphere 20:1945– 1952 Hagenmaier H, Kraft M, Brunner H, Haag R (1987) Catalytic effects of fly-ash from waste incineration facilities on the formation and decomposition of polychlorinated dibenzo-para-dioxins and polychlorinated dibenzofurans. Environ Sci Technol 21:1080– 1084 Hajizadeh Y, Onwudili JA, Williams PT (2012) Effects of gaseous NH3 and SO2 on the concentration profiles of PCDD/F in flyash under post-combustion zone conditions. Waste Manag 32:1378–1386 Karasek FW, Dickson LC (1987) Model studies of polychlorinated dibenzo-para-dioxin formation during municipal refuse incineration. Science 237:754–756 Luna A, Amekraz B, Morizur JP, Tortajada J, Mo O, Yanez M (2000) Reactions of urea with Cu+in the gas phase: an experimental and theoretical study. J Phys Chem A 104:3132–3141 Milligan MS, Altwicker ER (1995) Mechanistic aspects of the de-novo synthesis of polychlorinated dibenzo-p-dioxins and furans in fly-ash from experiments using isotopically labeled reagents. Environ Sci Technol 29:1353–1358 Olie K, Vermeulen PL, Hutzinger O (1977) Chlorodibenzo-p-dioxins and chlorodibenzofurans are trace components of fly ash and flue gas of some municipal incinerators in The Netherlands. Chemosphere 6: 455–459 Rappe C, Marklund S, Buser HR, Bosshardt HP (1978) Formation of polychlorinated dibenzo-paea-dioxins (PCDDs) and dibenzofurans (PCDFs) by burning or heating chlorophenates. Chemosphere 7: 269–281 Ruokojärvi PH, Halonen IA, Tuppurainen KA, Tarhanen J, Ruuskanen J (1998) Effect of gaseous inhibitors on PCDD/F formation. Environ Sci Technol 32:3099–3103 Ruokojarvi PH, Asikainen AH, Tuppurainen KA, Ruuskanen J (2004) Chemical inhibition of PCDD/F formation in incineration processes. Sci Total Environ 325:83–94 Stieglitz L, Zwick G, Beck J, Roth W, Vogg H (1989) On the de-novo synthesis of PCDD/PCDF on fly-ash of municipal waste incinerators. Chemosphere 18:1219–1226 Takacs L, Moilanen GL (1991) Simultaneous control of PCDD/PCDF, HCl and NOx emissions from municipal solid-waste incinerators with ammonia injection. J Air Waste Manag 41:716–722 Takacs L, Mcqueen A, Moilanen GL (1993) Development of the ammonia injection technology (AIT) for the control of PCDD PCDF and acid gases from municipal solid-waste incinerators. J Air Waste Manag 43:889–897 Tuppurainen K, Aatamila M, Ruokojarvi P, Halonen I, Ruuskanen J (1999) Effect of liquid inhibitors on PCDD/F formation, prediction of particle-phase PCDD/F concentrations using PLS modelling with gas-phase chlorophenol concentrations as independent variables. Chemosphere 38:2205–2217 Vogg H, Stieglitz L (1986) Thermal-behavior of PCDD/PCDF in fly-ash from municipal incinerators. Chemosphere 15:1373– 1378 Vogg H, Metzger M, Stieglitz L (1987) Recent findings on the formation and decomposition of PCDD/PCDF in municipal solid-waste incineration. Waste Manage Res 5:285–294
RESEARCH ARTICLE
Reducing dioxin formation by adding hydrogen in simulated fly ash J. Yang 1 & X. D. Li 1 & W. J. Meng 1 & S. Y. Lu 1 & T. Chen 1 & J. H. Yan 1 & A. Buekens 1 & K. Olie 1
Received: 13 October 2014 / Accepted: 6 March 2015 / Published online: 30 April 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract In this study, simulated fly ash containing CuO/ CuCl2 was heated at 350 °C in a flow of N2 and also in a nitrogen flow containing 10 vol% H2, to evaluate the influence of hydrogen adding on dioxin formation. The total polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) output derived from the CuO sample under N2 and 10 % H2 was 7.382 and 0.708 ng/g, respectively. As for CuCl2, it was 589 and 46.1 ng/g, respectively. The results show that the hydrogen adding has a good inhibition effect on PCDD/F formation; the inhibition rate was higher than 90 % for PCDD/Fs. HCl and NH3 were detected by Gasmet in the flue gas; the probable inhibition mechanism of hydrogen reaction was proposed, based on our measurements and others’ researches. Keywords Incineration . Fly ash . Formation . De novo synthesis . Nitrogen . Hydrogen . CuO/CuCl2 . PCDD/Fs . Inhibition
(1987), and Rappe et al. (1978). The presence of both catalytic metals and oxygen is essential for the de novo synthesis of PCDD/Fs (Gullett et al. 1990; Stieglitz et al. 1989; Vogg and Stieglitz 1986). The synthesis of PCDD/PCDF is catalyzed by Cu and Fe compounds, not by the chlorides of alkali, alkali earth, Zn, Mn, Hg, Cd, Sn, and Pb. The effect of an oxidizing atmosphere on the formation of PCDD/Fs was studied many times. Vogg et al. (1987) compared the annealing of fly ash in nitrogen and in its mixtures with 1, 4, and 10 vol% of oxygen. The formation of both PCDDs and PCDFs rose with oxygen content (Buekens et al. 2001; Vogg et al. 1987). Hagenmaier et al. discovered the destruction of PCDD/Fs under anoxic conditions (Hagenmaier et al. 1987). The [PCDD]:[PCDF] ratio rises with increasing [O2], but the average level of chlorination of PCDD and PCDF did not depend on O2 concentration (Addink and Olie 1995). The effect of oxygen was also studied for filter dust from the sintering belt. PCDD/Fs were quite low in pure N2 (Milligan and Altwicker 1995). But there was no comprehensive study about the effect of hydrogen adding on dioxin formation.
Introduction After the discovery of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in fly ash from municipal solid waste incineration (Olie et al. 1977), various pathways of formation were developed and a number of potential precursors were proposed, e.g., Buser (1979); Karasek and Dickson Responsible editor: Leif Kronberg * X. D. Li [email protected] 1
State Key Laboratory of Clean Energy Utilization, Zhejiang University, Zheda Road 38#, Hangzhou, China
Materials and methods Simulated fly ash was prepared from activated carbon (AC), admixed with different metal species by grinding in a mortar for about 5 min in the following proportions: AC (2.5 wt%, 200–500 mesh; the characteristics of the activated carbon are shown in Table 1), sodium chloride (NaCl; 10 wt% Cl), metal compound (0.1 wt% metal), and silicon dioxide (SiO2; remainder, 120–200 mesh).Metal compounds were bought from Aladdin Chemistry Co. Ltd. The metal species used were CuCl2 · 2H2O (99 % pure), CuO (99 % pure), and NaCl (99.99 % pure). The initial concentration of PCDD/F in simulated ash is shown in Table 2. The de novo experiments were
13078 Table 1
Environ Sci Pollut Res (2015) 22:13077–13082 Characteristics of the activated carbon (=air dry)
Proximate analysis (wt%)
Elemental analysis (wt%)
Trace metals (ppm)
Moisture
Ash
Volatile matter
Fixed carbon
C
H
N
S
O
Al
Cu
Fe
Ni
Zn
1.73
2.09
7.38
88.8
92.5
1.5
0.17
0.19
1.67
655
276
507
82
54
Table 2
Initial concentration of PCDD/F in simulated fly ash (ng/g)
T4CDD
P5CDD
H6CDD
H7CDD
O8CDD
T4CDF
P5CDF
H6CDF
H7CDF
O8CDF
I-TEQ
ND
0.005
0.007
0.007
0.036
0.027
0.026
0.042
0.057
0.078
0.006
carried out in a tubular reactor operating at atmospheric pressure and positioned in a tubular furnace (Fig. 1). An amount of 2 g of simulated fly ash was placed into the quartz tube and pushed in a furnace preheated at 350 °C. The de novo reaction lasted for 60 min and was conducted in a flow of N2 and 10 % H2 +90 % N2, supplied at 300 mL/min. The PCDD/Fs in the carrier gas were collected on XAD-2 (a resin that can adsorb dioxins in gas phase effectively, Sigma-Aldrich) and in two successive impingers containing 125 mL of toluene (HPLC, J.T. Baker). After the test, the PCDD/Fs in the simulated fly ash residue and in the gas were analyzed together. Every experiment was conducted twice and the average was recorded for further use. Each sample was spiked with a mixture of 13C12-labelled PCDD/F compound stock solution (5 μL) before Soxhlet extraction for 24 h. The extracts from the Soxhlet extraction were spiked with cleanup standard (5 μL) and subsequently followed by rotary evaporation and multilayer silica gel column cleanup procedure following the method of USEPA 1613. The extracts were blow-down to 20 μL under a gentle stream of nitrogen (N2), and 5 μL of 13C12-labelled PCDD/F Fig. 1 Experimental setup showing (from left to right): the gas cylinder supplying carrier gas, a mass flowmeter (CS200, Sevenstar), the ash sample mounted in a tubular reactor surrounded by the tubular furnace, and the XAD-2 and toluene bottles absorption train
internal standard solution was added before samples were subjected to analysis by high-resolution gas chromatography coupled with high-resolution mass spectrometry (HRGC/HRMS) (JEOL JMS-800D) with a DB-5MS column (60 m×0.25 mm×0.25 μm). The temperature program for the GC oven was set as follows: initial temperature was 150 °C, held for 1 min; increased at 25 °C/min to 190 °C; then increased at 3 °C/min to 280 °C; held for 20 min. The toxic 2,3,7,8-substituted PCDD/Fs (referred to as congeners) as well as tetra- to octa-chlorinated homologue were identified based on isotope ratios within ±15 % of the theoretical values, and signal to noise ratios of equal to or greater than 2.5 quantification of PCDD/Fs were determined by an isotope dilution method using relative response factors previously obtained from the five calibration standard solutions. Recoveries of internal standards, as determined against the external standard, generally varied between 70 and 110 %, and were all satisfied with the method of USEPA 1613. The limit of determination (LOD) of HRGC/HRMS is between 0.0005 and 0.001 ng/g based on the different isomers. The target compounds were tetra- to octa-CDD/Fs (136 isomers).
Environ Sci Pollut Res (2015) 22:13077–13082 Table 3 Dioxin concentrations under different atmosphere (ng/g)
13079
CuO N2
CuCl2 RSD (%)
10 % H2
RSD (%)
N2
RSD (%)
10 % H2
RSD (%)
T4CDD
1.635
22.4
0.081
19.7
4.25
18.2
1.7
19.9
P5CDD
0.866
26.1
0.043
16.3
7.53
17.8
1.34
14.6
H6CDD H7CDD
0.467 0.186
30.3 20.8
0.043 0.004
40.4 3.7
11.57 11.62
20.2 17.9
1.01 0.33
19.1 29.2
O8CDD
0.089
7.7
0.012
11.4
23.13
15.6
0.09
19.3
PCDD T4CDF
3.242 2.172
24.0 22.8
0.183 0.315
21.2 46.2
58.09 74.84
16.9 9.8
4.48 18.27
18.8 10.4
P5CDF H6CDF
1.104 0.616
27.9 27.0
0.131 0.063
42.4 54.0
106.5 146.33
11.7 10.5
13.13 7.12
13.2 21.7
H7CDF
0.229
23.9
0.011
8.1
149.76
12.1
2.43
22.0
O8CDF
0.018
11.5
0.004
33.3
53.43
19.0
0.62
20.0
PCDF PCDD/Fs
4.14 7.38
24.8 24.5
0.524 0.71
45.2 39.0
531 589
11.5 11.9
41.6 46.1
14.0 14.5
I-TEQ PCDD:PCDF
0.196 0.78
25.9
0.015 0.35
35.6
9.31 0.11
12.0
1.11 0.11
14.4
Cl average
4.79
All isotope standards were purchased from the Cambridge Isotope Laboratories, Inc.
Results and discussion The ten isomer groups of PCDDs and PCDFs, the corresponding international toxicity equivalent (I-TEQ) value, the ratio of PCDD to PCDF, and the weight average degree of chlorination of all PCDD/Fs are presented in Table 3. The total PCDD/F output derived from the CuO sample under N2 and 10 % H2 was 7.38 ng/g (0.196 ng I-TEQ/g) and 0.71 ng/g (0.015 ng I-TEQ/g), respectively. The ratios between PCDD/PCDF were 0.78 and 0.35. As for the CuCl2 sample, it was 589 ng/g (9.31 ng I-TEQ/g) and 46.1 ng/g (1.11 ng ITEQ/g), respectively, while the ratio kept constant at 0.11. The inhibition efficiency of each homologue is shown in Fig. 2. The results indicated that the addition of hydrogen had good inhibition ability on dioxin formation with the range of 60.0– 99.6 %. The homologues with a higher chlorinated level were more strongly suppressed. Considering that the level of chlorination decreases (4.79–4.70, 6.07–4.91), the chlorination reaction was supposedly prevented. As shown in Fig. 3, the inhibition rate of PCDD, PCDF, PCDD/Fs, and I-TEQ of the CuO sample was 94.4, 87.3, 90.4, and 92.4 %, respectively. Meanwhile, it was 92.3, 92.1, 92.2, and 88.1 % for the CuCl2 sample. Thus, the inhibition rate of hydrogen on PCDD formation is slightly better than that of
4.70
6.07
4.91
PCDF formation. One of the possible reasons is that DD can convert into DF under reducing conditions and the rate of disappearance of DD depends strongly on the concentration of hydrogen atoms (Altarawneh et al. 2009; Cieplik et al. 2002). But the inhibition of PCDD/Fs is mainly due to the reduction of PCDF formation. It accounted for 54.2 and 90.1 % of the total reduction of PCDD/Fs of the CuO and CuCl2 samples, respectively. The mass distribution of the PCDD and PCDF homologue of the samples under inert and hydrogen adding is shown in Figs. 4 and 5. There was little difference of PCDD/F
Fig. 2 Inhibition rate of each homologue comparing N2 +10 % H2 with N2
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Fig. 3 Inhibition rate of PCDD/Fs and I-TEQ comparing N2 +10 % H2 with N2
distribution of the CuO sample with the addition of hydrogen. The percentage of the homologue decreased with the increasing of the chlorinated level, and the low-chlorinated dioxins (4, 5-Cl) dominated. On the contrary, the homologue distributions of the CuCl2 sample under these two atmospheres were very different. It seemed that the percentage of the homologue increased with the rise of the chlorinated number under an inert atmosphere. But it was completely reversed with the addition of hydrogen. In other words, hydrogen had better inhibition ability on high-chlorinated PCDD/Fs than the low-chlorinated ones. The concentration of O8CDD and O8CDF reduced from 23.13 and 53.43 ng/g to 0.09 and 0.62 ng/g, respectively. The inhibition rate was as high as 99.0 %. As for T4CDD and T4CDF, it was only 60.0 and 75.6 %, respectively. The chlorinated level dropped significantly; it reduced from 6.07 to 4.91. In order to study the probable inhibition mechanism of a reducing atmosphere, a Gasmet online monitor (Finland, FTIR DX-4000) was used to detect the flue gas at the same experimental conditions of CuO and CuCl2 samples under a reducing atmosphere; three detections were conducted during
Fig. 4 PCDD homologue distribution of CuO and CuCl2 samples
the experiment. As shown in Figs. 6 and 7, HCl and NH3 were found in the flue gas and there were three distinct cycles. The measurements vary from 0 to 20 ppm and 0 to 4 ppm for HCl and NH3, respectively. It indicated that NH3 played an important role during the reaction. The tests of NH3 effect on the formation of PCDD/Fs in both laboratory-scale (Vogg et al. 1987) and full-scale incinerators (Takacs et al. 1993; Takacs and Moilanen 1991) proved it to be an efficient inhibitor. The researches of Ruokojärvi et al. (1998) and Hajizadeh et al. (2012) also found the same effect of NH3 on dioxin formation. Ruokojärvi et al. (2004) found that the distinct effect was a decline in the degree of chlorination of PCDD/Fs, which could be seen in the tests with gaseous inhibitors. Addink and Olie (1993) also found that inhibitors reduced the degree of chlorination in their laboratory tests. Our results were consistent with their researches. Takacs et al. (1993) achieved PCDD/F reductions using NH3 and suggested that the mechanism of action was neutralization of HCl. Though we performed only a few measurements of HCl and NH3 concentration in our tests and the concentration was not so high, the neutralization of NH3
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Fig. 5 PCDF homologue distribution of CuO and CuCl2 samples
Fig. 6 The concentration of HCl and NH3 in the flue gas of the CuO sample with hydrogen
cannot be excluded. Meanwhile, there might be another probable mechanism. Vogg et al. (1987) studied the effect of ammonia on MSWI fly ash and reported that NH 3 counteracted the catalytic action of CuCl2. Ruokojärvi et al. (2004) reported that one probable explanation for the inhibition of PCDD/Fs by molecules with lone pair electrons, e.g., those containing nitrogen or sulfur, may lie in their ability to Fig. 7 The concentration of HCl and NH3 in the flue gas of the CuCl2 sample with hydrogen
form stable complexes with metallic compounds (Tuppurainen et al. 1999), so that they eliminate or reduce the catalytic properties of the metals. Luna et al. (2000) used density functional theory (DFT) at the B3LYP level of theory and a valence triple-ζ to investigate the structures and bonding characteristics of the different complexes involved in the ureaCu+ potential energy surface (PES) and found the formation of
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CuNH3+. These could be the inhibition mechanism of hydrogen on dioxin formation, but more work should be done in the future to confirm.
Conclusion The simulated fly ash containing CuO/CuCl2 was studied under inert (N2) and reducing (N2 +10 % H2) atmospheres; the results showed that the reducing atmosphere had good inhibition ability on dioxin formation compared with the inert atmosphere, and the inhibition rate of I-TEQ was as high as 92.4 and 88.1 % for CuO and CuCl2 samples, respectively, under the experimental setup prescribed in this paper. Chlorination reaction also was prevented; the chlorination degree of PCDD/F formation via simulated fly ash (CuCl2) was reduced from 6.07 to 4.91. HCl and NH3 were detected by Gasmet in the flue gas. Based on the results in this paper and the research of Takacs et al. (1993) and Vogg et al. (1987), the inhibition mechanism of a reducing atmosphere was considered as NH3 forming in the reaction process and counteracting the catalytic action of metals. Acknowledgments This study was funded by the National Key Basic Research Development Program (No. 2011CB201500). Conflict of interest The authors declare that they have no conflict of interest.
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