Materials Transactions, Vol. 46, No. 5 (2005) pp. 990 to 995 #2005 The Mining and Materials Processing Institute of Japan
Remediation of Contaminated Soil by Fly Ash Containing Dioxins from Incineration by using Flotation Atsushi Shibayama1; * , Younghun Kim2 , Sri Harjanto3 , Yuichi Sugai3 , Kimikazu Okada4 and Toyohisa Fujita5 1 Department of materials-process Engineering & Applied Chemistry for Environments Faculty of Engineering and Resource Science, Akita University, Akita 010-8502, Japan 2 Korea Resources Corporation (KORES), 646-48, Shindaebang-Dong, Dongjak-Gu, Seoul, 156-706, Korea 3 Venture Business Laboratory, Akita University, Akita 010-8502, Japan 4 Kubota Corporation, Hanshin Office, Amagasaki 661-8567, Japan 5 RACE & Department of Geosystem Engineering, Graduate School of Engineering, The University of Tokyo, Tokyo, 113-8656, Japan
Flotation was investigated to clean up the dioxin (Polychlorinated Dibenzo-p-Dioxins/Furans, PCDD/Fs and co-Planar Polychlorinated Biphenyls, co-PCBs)-contaminated soil, originated from fly ash contaminated by dioxins dispersed into soil. The primary purpose was to reduce the concentration of dioxins in soil by examining the conditions of flotation to remove selectively unburned carbons including a high dioxin concentration of fly ash generated from incineration processes. Three kind of materials were used, such as fly ash from an ESP (electrostatic precipitation) of an incinerator on vapor gas treatment (FA), artificial contaminated soil (mixture of soil and fly ash containing dioxins, ACS) and real dioxin-contaminated soil which excavated from a site of Japanese domestic incinerator area (CS). As a result, in the case of fly ash and artificial soil, fly ash containing dioxins would be enriched and then separated as float products by flotation under the conditions as follows, additional amount of kerosene and Dow 250, pulp density and pH were 40 kg/t, 7 kg/t, 5–30% and 2.8, respectively. In addition, it could be possible to recover approximately 80% of soil from dioxin-contaminated soil. The concentration of dioxins in the soil analyzed by AhImmunoassay after flotation was decreasing from 15 to 0.68 ng-DEQ/g and was satisfied with Japanese environmental regulation i.e. less than 10 ng-DEQ/g (analyzed by Ah-Immunoassay) or 1 ng-TEQ/g (analyzed by GC-MS methods). (Received November 10, 2004; Accepted March 3, 2005; Published May 15, 2005) Keywords: flotation, fly ash, dioxins, unburned carbons, contaminated soil, soil remediation
1.
Introduction
Contaminated soil is very serious issue for environmental protection and nature system to being a lifetime. In Japan, the dioxins (Polychlorinated Dibenzo-p-Dioxins/Furans and coPlanar Polychlorinated Biphenyls, PCDD/Fs and co-PCBs) law that is related with regulations for emission, disposal of ash and dust and measures against soil contamination by dioxins was promulgated in 1999.1) Since then, many developments for soil remediation technology such as washing, immobilization, incineration (thermal process), bioprocess (bacteria) and chemical treatment had been researched and operated in various fields.2–4) On the other hand, it is well known that the dioxins mostly generated by incineration process for MSW. According to an estimation of Japanese data, it is reported that 1.01 g-TEQ dioxins could be produced from 10,000 tones of incinerated municipal waste.5) Dioxins tend to enrich on fly ash by the presence of unburned carbon (C) which adsorbs dioxins (de-novo synthesis (Fig. 1)).6–9) The unburned carbon is a catalyst for dioxin formation in the incineration process and its surface easily adsorb the dioxins.6–9) Fly ash is accumulated and scattered at the surrounding of an incinerator. That is the reason for dioxin-contamination into the soil at the surrounding of an incinerator. For example, one estimated data, the fly ash is existed in the contaminated soil by weight ratio from 0.1% to 1.0%, and the unburned carbon is also existed in the fly ash by weight ratio from 3% to 10%. Therefore, the *Corresponding
author, E-mail:
[email protected]
O
O Clx
Cly
O Clx
Incinerator
Direct formation + Cl
Vapor gas (CO2, H2O, C nHn, etc.) and fly ash
Cly
De-novo synthesis and adsorption
Unburned carbon De-novo synthesis OH + Cl
Clx
Clx
Chlorination aromatic compounds
Fig. 1 Dioxin formation reaction in an incinerator. (modified from reference9))
unburned carbon adsorbing large amount of dioxins is calculated to exist in weight ratio from several ten to several hundred mg per kg in the soil. The environmental quality standard as basic standard for measures is shown in Table 1. That is to say, it is important to remove and recover the unburned carbon containing dioxins from contaminated soil to remedy the polluted and toxic site. In this research, flotation, one of the conventional mineral processing technologies was applied to clean up dioxincontaminated soil originated from fly ash containing dioxins dispersed into soil. Thus, the purpose of the study is to establish the conditions of flotation to remove unburned
Remediation of Contaminated Soil by Fly Ash containing Dioxins from Incineration by using Flotation Table 1
Law concerning special measures against dioxins in Japan.1Þ aÞ
Environmental standard 3
Air
0.6 pg-TEQ/m
Water Soil
bÞ
Conversion in Ah-I DEQ 6 pg-DEQ/m3
1 pg-TEQ/l
10 pg-DEQ/l
1000 pg-TEQ/g
10000 pg-DEQ/g
aÞ
TEQ stands for toxicity equivalency. As the toxicity of dioxins differs by isomer, it is used to evaluate the content and concentration of dioxins. ‘pg-TEQ/g’ shows the concentration of dioxins in soil; ‘pg-TEQ/l’ shows the concentration of dioxins in water. bÞ Only estimated (informal) data were using the factor of 10, our objective data were analyzed by Ah-Immunoassay.
carbons containing a high concentration of dioxins, selectively. Several conditions of flotation to decrease the dioxins concentration in the soil and fly ash had been examined such as pH, amount of kerosene as collector for carbon and Dow 250 as frother and pulp density. 2.
Materials and Methods
2.1 Materials The raw materials supplied for this study were fly ash from an ESP (electrostatic precipitator) of an incinerator vapor gas treatment, artificial contaminated soil (mixture of clean soil and fly ash containing dioxins) and real dioxin-contaminated soil which excavated from one site of Japanese municipal solid waste incinerator area. Thus, three kind of samples were prepared in the experiments, such as fly ash contaminated by dioxin (FA), artificial dioxin contaminated soil (ACS) and dioxin contaminated soil (CS). Some properties of the samples are listed in Table 2. The sample particles were kept at the size of less than 0.84 mm (20 mesh) for effectiveness of the flotation process. ACS were prepared by mixing clean soil (Akadama red soil, Japan) after sieving with 10 wt% of FA. CS was sieved before experiments to remove gravels (þ6 mesh or more than 4 mm). The CS particle with the size of þ20 mesh (more than 0.84 mm) was also used in the experiment after crushing by using mortar and sieving. 2.2 Flotation Prior to the flotation of CS, some preliminary experiments were performed. Firstly, the flotation conditions such as type and addition amount of a collector and frother, pulp density and pH were examined by using FA sample. Secondly, the
991
optimum condition of flotation obtained from the first preliminary experiment was investigated and evaluated for the ACS sample. Finally, based on the results of two preliminary experiments, the flotation process for CS were examined. Kerosene was selected as a collector of carbon from FA sample flotation. Several frothers used in the flotation process of carbon based materials, such as terpineol (terpene alcohol, C10 H18 O, M.W. ¼ 154), MIBC (methyl isobutyl carbinol, C6 H14 O, M.W. ¼ 102) and Dow 250 (polyoxypropylene glycol ether, CH3 -(O-C3 H6 )4 -OH), M.W. ¼ 250)10) were evaluated in the preliminary experiment. The condition of collector and frother addition, pulp density and pH in the flotation process were set at a range of 0.5–40 kg/t, 0.7– 70 kg/t, 10–30% and 2.8, respectively. Terpineol and MIBC belong to alcohol group frother. The first is a cyclic type of frother and being used so far in coal flotation. The second is an aliphatic and hydrophobic frother and the most commonly used frother in the mineral processing. On the other hand, Dow 250 is a brand name of polyoxypropylene glycol ether type frother manufactured by Dow Chemicals, Co. This frother is a combination of hydrophobic group represented by propylene and hydrophilic group represented by ether oxygen and hydroxyl. By changing the ratio of hydrophilic to hydrophobic groups and molecular weight during polimerization, its solubility in water can be changed. Flotation was conducted by using M.S. (Mineral Separation) type flotation machine under the following conditions, cell capacity and impeller speed were 250 ml and 2500 rpm respectively. Schematic of M.S. type flotation machine is shown in Fig. 2. During flotation, HCl and NaOH were used as pH regulator. Flotation process was undertaken in several steps, continuously. At first, conditioning agitation was conducted for pH adjustment for 3 minutes. Then, kerosene was added as collector in the flotation with conditioning agitation for 5 minutes. Another 5 minutes of conditioning agitation was done after frother addition. Then, flotation was continued for 10 minutes. After flotation, the floatability of the samples was calculated by the weight ratio of float products and feed. In the discussion, the term of a weight ratio of float products is often expressed to denote floatability. 2.3 Dioxin analysis Concentration of dioxins (PCDD/Fs and co-planar PCB)
Table 2 Some properties of the soils and fly ash.
aÞ
Properties
Fly ash (FA)
Artificial contaminated soil (ACS)
Contaminated soil (CS)
Mean particle size, d50 (mm)aÞ Moisture (%)
0.078 10.2
0.024 30.6
0.638 25.5
Carbon content (%)bÞ
7.3
n.a
n.a
Total-Cl (%)cÞ
6.8
n.a.
n.a.
Mean particle size distribution (d50 ) was determined from the particle size of 50% of cumulative fraction. The analysis was conducted according to JIS 8813 (Coal and coke—method for ultimate analysis). The carbon content in the soils (CS and ACS) were too low to be analyzed. cÞ The composition of alkali and earth-alkali elements of FA, such as Na, K Ca, Mg, is 3.18, 2.44, 19.12, 1.54%, respectively. bÞ
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A. Shibayama et al. 100
Impeller ( 2500rpm) Feed (Contaminated soil)
Dow 250 (pH = 10.6) Terpineol (pH = 10.6) MIBC (pH = 10.6)
Floatability (%)
80
60
40
20
0
Sink product
0
1
2
3
4
5
6
7
Frother amount (kg / t) Schematic representation of M.S. type flotation machine.
in soil was analyzed by Ah-Immunoassay (Ah-I) in term of DEQ unit (Dioxin Equivalency) and in some cases by gas chromatography-mass spectrograph (GC-MS) in term of TEQ (Toxicity Equivalents) for comparison, according to Japanese regulations. Ah-immunoassay is the analysis method which has strong correlationship with the measurement by using GC/MS.11–15) Ah-I was applied in this study because it is a relatively inexpensive way compared to conventional measurement (GC-MS) to provide significant reproducibility with good accuracy. Thus, the concentration of dioxins was presented in DEQ unit, with the conversion factor from TEQ to DEQ is in the factor of 10–13 (e.g.: [Ah-I DEQ] = 10 [TEQ]).12) Actually, carbon content in the samples were attempted to be analyzed according to JIS 8813 Coal and coke (Method for ultimate analysis). However, only carbon content data from FA was obtained. The carbon content in soils (Akadama soil and CS) was too low to be analyzed. Accordingly, the amount of carbon and dioxin concentration in the samples was not able to be compared. 3. 3.1
Results and Discussions
Flotation properties of fly ash contaminated by dioxin (FA) The effect of frother type and addition amount to the floatability of FA was investigated by using Dow 250, terpineol and MIBC at the concentration range of 0.7 to 7 kg/ t. The condition of kerosene as a collector, pulp density and pH were set at 40 kg/t, 5% and 10.6, respectively. The result is shown in Fig. 3. It was verified that over 65% of FA was floated at pH 10.6 by using Dow 250 (Fig. 3). In this case, weight loss due to dissolution of chloride compounds was predicted to be about 6–7%, according to the amount of soluble chloride in the raw materials of FA (see Table 2). The weight loss is still noteworthy for the FA flotation. From the above result, Dow 250 was selected as frother. Dow 250 is more effective compared with other frother such as terpineol and MIBC. It was confirmed that the separation of carbon containing dioxin to floating products was easier in
Fig. 3 Effect of frother amount on floatability of FA with various frothers. (kerosene: 40 kg/t, pulp density: 5%, pH 10.6)
100 Dow 250 (pH = 10.6) Dow 250 (pH = 6.6) Dow 250 (pH = 2.8)
80
Floatability (%)
Fig. 2
60
40
20
0 0
1
2
3
4
5
6
7
Frother amount (kg / t) Fig. 4 Effect of frother (Dow 250) amount on floatability of FA as a function of different pH. (kerosene: 40 kg/t, pulp density: 5%)
case of Dow 250. Afterwards, the floatability of FA in various addition amount of frother (0.7 to 7 kg/t) and pH (2.8, 6.6 and 10.6) were examined. Kerosene and pulp density were 40 kg/ t and 5%, respectively. Figure 4 shows the effect of pH to the weight ratio of float product of FA. At the pH 10.6, weight ratio of float products of FA is increased up to 65% with increasing amount of Dow 250. However, flotation of FA at pH 2.8 and 6.6, gave no effect to the floatability. Particularly, floatability was about 20%, regardless of an adding amount of frother. After the float product were filtered and dried, their colors were compared at each pH. It was estimated that unburned carbon of FA was removed effectively at pH 2.8 because the color was darker than that of at pH 10.6. To confirm these results, concentration of dioxin in the float and sink products was analyzed by Ah-Immunoassay. As listed in Table 3, concentration of dioxin was unchanged in case of pH 10.6, regardless of an adding amount of frother. Otherwise, dioxin in pH 2.8 was enriched to the float products with increasing in amount of frother up to 7 kg/t.
Remediation of Contaminated Soil by Fly Ash containing Dioxins from Incineration by using Flotation
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Table 3 Dioxin concentrations of float and sink products measured by the Ah-Immunoassay and GC-MS. (kerosene: 40 kg/t, pulp density: 5%)
pH ¼ 10:6
pH ¼ 2:8
Frother (Dow 250)
Float product
Ah-Immunoassay (ng-DEQ/g) Sink product
Float product
Sink product
0.7 kg/t
380
360
—
—
7 kg/t
330
270
—
—
0.7 kg/t
400
400
—
—
7 kg/t
620
400
75
24
100
100 Dow 250 (pH = 10.6) Dow 250 (pH = 6.6) Dow 250 (pH = 2.8)
90 80
Floatability (%)
80
Floatability (%)
GC-MS (ng-TEQ/g)
60 40 20
10% Fly ash ; kerosene (40kg/t), Dow 250 (7kg/t) 10% Fly ash ; kerosene ( 4kg/t), Dow 250 (7kg/t) 10% Fly ash ; kerosene ( 4kg/t), Dow 250 (0.7kg/t)
70 30 20 10 0
0 0
5
10 15 20 25 30 Kerosene amount (kg / t)
35
5
40
10
15
pulp density (%)
Fig. 5 Influence of the kerosene concentration on the floatability of FA as a function of different pH. (Dow 250: 7 kg/t, pulp density: 5%)
Fig. 6 Effect of kerosene and frother (Dow 250) concentration on floatability of ACS as a function of pulp density. (pH 2.8)
Next, floatability of FA was investigated by changing the amount of kerosene from 0.5 up to 40 kg/t at different pH (2.8, 6.6 and 10.6). Dow 250 addition dosage and pulp density were 7 kg/t and 5% respectively. As shown in Fig. 5, the weight ratio of float products of FA is increased up to 65% with the addition of kerosene up to 10 kg/t at pH 10.6. In case of pH 6.6 and 2.8, further addition of kerosene leads to no effect to the floatability, at approximately 20%. Concentration of dioxin of float and sink at the pH 2.8 was analyzed by Ah-Immunoassay. As shown in Table 4, removal efficiency of flotation for fly ash was higher in proportion to the addition of kerosene because dioxin was enriched in the side of float product. Particularly, in case of 40 kg/t adding amounts of kerosene, compared with 0.5 kg/t, a high concentration dioxin was found in the float products. As a result, the optimum conditions of flotation for fly ash were determined; pH, an addition amount of kerosene and Dow 250 were 2.8, 40 kg/t and 7 kg/t, respectively.
3.2
Table 4 Dioxin concentrations of float and sink products measured by the Ah-Immunoassay. (Dow 250: 7 kg/t, pulp density: 5%)
pH ¼ 2:8
Kerosene (kg/t)
Ah-Immunoassay (ng-DEQ/g) Float product
Sink product
0.5
1500
560
10
1500
460
40
1800
450
Flotation properties of artificial contaminated soil (ACS) Artificial dioxin contaminated soil (ACS) was prepared for flotation with the setting condition as follows. The pH, pulp density, addition amount of kerosene and Dow 250 were 2.8, 5–15%, 4–40 kg/t and 0.7–7 kg/t, respectively. In all conditions, a slightly increase of weight ratio of float product up to 20 through 30% is observed as shown in Fig. 6. Generally, there is an increase up to 30 mass% of floating products at 15% pulp density in case of the addition of kerosene and Dow 250 of 40 and 7 kg/t, respectively. Thus, the concentration of dioxin was analyzed by Ah-Immunoassay at 5 and 15% pulp density with a different addition amount of kerosene and Dow 250. As listed in Table 5, the addition of 40 kg/t kerosene and 7 kg/t Dow 250 gave higher dioxins concentration in the float products. Particularly, it should be recognized that dioxins concentration in the sink products was the lowest in case of 5% pulp density because the concentration in the float products was the highest. Based on this result, a removal efficiency of fly ash was examined in proportion to pulp density. Therefore, a weight ratio of float products was investigated by changing pulp density from 5 to 30% under the conditions as follows, pH 2.8, kerosene, 40 kg/t and Dow 250, 7 kg/t, respectively. As shown in Fig. 7, a weight ratio of float products increases in the higher pulp density. For example, a weight ratio of float products was increased from 15% at 5% pulp density up to 55% at 30% pulp density.
994
A. Shibayama et al. Table 5
Dioxin concentrations of float and sink products measured by the Ah-Immunoassay. (pH = 2.8) Ah-Immunoassay (ng-DEQ/g)
Kerosene, Dow 250 (kg/t)
Pulp density (5%)
Sink product
200
35
Kerosene 4, Dow 250 7
180
51
Kerosene 4, Dow 250 0.7
98
55
Kerosene 40, Dow 250 7
180
35
Kerosene 4, Dow 250 7
130
45
Kerosene 4, Dow 250 0.7
110
39
100
100
90
90
80
80
70
70
Floatability (%)
Floatability (%)
Pulp density (15%)
Float product Kerosene 40, Dow 250 7
60 50 40 30
60 50 40 30
20
20
10
10 0
0 5
10
15
20
25
10
30
Pulp density (%)
20
30
Pulp density (%)
Fig. 7 Influence of pulp density on floatability of ACS. (kerosene: 40 kg/t, Dow 250: 7 kg/t, pH: 2.8)
Fig. 8 Influence of pulp density on the floatability of CS. (kerosene: 40 kg/ t, Dow 250: 7 kg/t, pH: 2.8)
Table 6 Dioxin concentrations of sink products measured by the AhImmunoassay. (kerosene: 40 kg/t, Dow 250: 7 kg/t, pH: 2.8)
Table 7 Dioxin concentrations of float and sink products measured by the Ah-Immunoassay.
Pulp density (%)
Kerosene 40 kg/t Dow 250 7 kg/t pH ¼ 2:8
Ah-Immunoassay (ng-DEQ/g) Sink product
5%
35
10%
31
15%
35
20%
34
30%
30
Dioxin-concentration of sink products measured by AhImmunoassay is shown in Table 6. Other conditions were the same i.e. pH, addition amount of kerosene and Dow 250 were 2.8, 40 kg/t and 7 kg/t, respectively. As shown in Table 6, the best removal efficiency is presented at 30% pulp density. It can be also calculated that amount of dioxin in the sink product at the highest pulp density (30%) was the lowest. From an economic point of view, higher pulp density is more efficient than the lower one. However weight ratios of float products was higher up to 55% with increasing pulp density, which might decrease the recovery of sink products, or purified soil. 3.3
Flotation properties of real dioxin-contaminated soil (CS) Based on the result of preceding experiments, flotation for
Before flotation (ng-DEQ/g) 15
After flotation (ng-DEQ/g) Float product
Sink product
>6:7*
0.68
*maximum concentration by calculation: 58.0 ng-DEQ/g
real dioxin-contaminated soil (CS) was performed. Optimum conditions for this flotation with the reference to preliminary experiments were determined as follows: pH 2.8, kerosene as collector 40 kg/t and Dow 250 as frother 7 kg/t. In addition, weight ratio of float products was investigated by changing pulp density from 5 to 30%. The result is shown in Fig. 8. There was no change in weight ratio of float products, regardless of a increase in pulp density. Therefore, the problem, appointed in Fig. 7, that sink products decreased with increasing pulp density, seems to be solved. In addition, the average weight loss during flotation was observed to be 5.3%. It is still acceptable in the flotation process. Approximately, 80% purified soil (sink products) could be recovered at 30% pulp density. Thus, if a large treatment is performed, a good economical effect should have been established. The concentration of dioxin was analyzed by Ah-Immunoassay at 30% pulp density. As shown in Table 7, the dioxins concentrations of sink products purified by flotation are 0.68 ng-DEQ/g, which is satisfied with the environmental regulation (10 ng-DEQ/g). In addition, it is possible to
Remediation of Contaminated Soil by Fly Ash containing Dioxins from Incineration by using Flotation
4.
Sample (soil contaminated by fly ash containing dioxins) (Dry and crushing) Screening (20 mesh)
+20 mesh
Flotation on site Pulp density 30% Adjust pH (2.8) Collector (Kerosene) 40 kg/t Frother (Dow 250) 7kg/t
Sink products Cleaning (treatment) soil: (80%)
Float products Concentrated dioxins in soil: 20% (unburned carbon)
Fig. 9 Recommended remediation process of soil contaminated by fly ash containing dioxins using flotation.
recover approximately 80% of cleaned and treated soil or to decrease the concentration of dioxins from 15 to 0.68 ngDEQ/g after flotation. Moreover, the heavy contaminated soil with dioxins as float product by flotation could recover as much as 20% of soil. In addition, the average of concentration of dioxins in this float product was 6.7 ng-DEQ/g. However, it was achieved to the maximum of 58.0 ng-DEQ/g by calculated data according to dioxins distributions. As a result, it was confirmed that the flotation is possible to be a technology to purify dioxin-contaminated soil scattered at the surrounding of an incinerator. The remediation process flow of dioxin-contaminated soil scattered at the surrounding of a incinerator by using flotation is shown in Fig. 9. In the proposed process, first, dioxin-contaminated soil scattered at the surrounding of a incinerator is dried. Dried samples are ground in order to make flotation easier and then separated by means of a 20 mesh sieve. Oversize products from a 20 mesh sieve are returned to a grinding process and their undersize products were used as samples in a flotation experiments. The optimum conditions of pH, pulp density, amount of kerosene as a collector and Dow 250 as a frother are 2.8, 30%, 7 kg/t and 40 kg/t, respectively. Under the optimum conditions of flotation process, the purified soil could be recovered and satisfying environmental regulation value (10 ng-DEQ/g analyzed by Ah-Immunoassay) under the law concerning special against dioxins.
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Conclusions
In this study, flotation process for soil remediation to remove and reduce dioxins in a contaminated soil was investigated. The results are summarized as follows. (1) The dioxins was enriched into float products if flotation was performed for only fly ash contaminated by dioxins under the following conditions; pH 2.8; kerosene dosage (collector), 40 kg/t; Dow 250, 7 kg/t. (2) In artificial soil, a mixture of soil and 10 mass% fly ash contaminated by dioxin, was concentrated in float products by flotation under the conditions such as pH, 2.8; pulp density, 30%; an addition amount of kerosene as a collector, 40 kg/t and Dow 250 as a frother, 7 kg/t. (3) It was confirmed that the flotation process could be applied to purify real dioxin-contaminated soil scattered at the surrounding of an incinerator. The recovery achieved approximately 80% of clean soil having the concentration of dioxins being decreased after flotation from 15 to 0.68 ng-DEQ/g, which is less than the environmental regulation value (10 ng-DEQ/g analyzed by Ah-Immunoassay or 1 ng-TEQ/g analyzed by GC-MS methods).
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