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Destruction of decabromodiphenyl ether (BDE-209) in a ternary carbonate molten salt reactor
Journal of Environmental Management 127 (2013) 244e248
Contents lists available at SciVerse ScienceDirect
Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
Destruction of decabromodiphenyl ether (BDE-209) in a ternary carbonate molten salt reactor Zhi-tong Yao a, b, *, Jin-hui Li c, Xiang-yang Zhao c a
College of Materials Science and Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, China Key Laboratory for Solid Waste Management and Environment Safety, Ministry of Education of China, Tsinghua University, Beijing 100084, China c School of Environment, Tsinghua University, Beijing 100084, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 October 2012 Received in revised form 6 April 2013 Accepted 17 April 2013 Available online 10 June 2013
Soil contamination by PBDEs has become a significant environmental concern and requires appropriate remediation technologies. In this study, the destruction of decabromodiphenyl ether (BDE-209) in a ternary molten salt (Li, Na, K)2 CO3 reactor was evaluated. The effects of reaction temperature, additive amount of BDE-209 and salt mixture, on off-gas species, were investigated. The salt mixture after reaction was characterized by XRD analysis and a reaction pathway proposed. The results showed that the amounts of C2H6, C2H4, C4H8 and CH4 in the off-gas decreased with increases in temperature, while the CO2 level increased. When the reaction temperature reached 750 C, incomplete combustion products (PICs) were no longer detected. Increasing BDE-209 loading was not helpful for the reaction, as more PICs were produced. Larger amounts of salt mixture were helpful for the reaction and PICs were not observed with the mole ratio 1: 2000 of BDE-209 to carbonate melt. XRD analysis confirmed the capture of bromine in BDE-209 by the molten salt. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: PBDEs e-waste recycling Soil contamination Molten salt oxidation
1. Introduction Polybrominated diphenyl ethers (PBDEs) are brominated flame retardants used in polymeric materials to enhance the fire safety of electronics, plastics, textiles and furniture (WHO, 1994; Hites, 2004). There are three major commercial formulations on the global market: Pentabromodiphenyl ethers (Penta-BDEs), Octabromodiphenyl ethers (Octa-BDEs) and Decabromodiphenyl ether (Deca-BDEs) (Darnerud et al., 2001; Ma et al., 2012). PBDEs are a class of additive flame retardants not chemically bonded to their host, which therefore could be released into the environment during their lifetimes. The release pathways include mainly atmospheric emissions during manufacturing processes, volatilization from PBDE-treated consumer products, recycling of PBDEcontaining wastes and leaching from waste disposal sites (Streets et al., 2006; Watanabe and Sakai, 2003; Yu et al., 2011; Zhang et al., 2012). With the constant innovation in technologies and the upgrading of products, the quantities of e-waste generated are increasing
* Corresponding author. College of Materials Science and Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, China. Tel./fax: þ86 571 86919158. E-mail addresses: [email protected], [email protected] (Z.-T. Yao). 0301-4797/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jenvman.2013.04.040
rapidly throughout the world (Robinson, 2009). According to the report “Recyclingdfrom e-waste to resources” released by the UNEP, China produced about 2.3 million tons of e-waste in 2010, second only to the United States, which produced about 3 million tons. Because of its economic attractiveness, the e-waste recycling industry has boomed in China in the last few years. However, in addition to the formal treatment enterprises, informal sectors are also engaged in recycling operations in Guiyu and Qingyuan (Guangdong Province, South China), as well as in Taizhou (Zhejiang Province, Southeast China), etc. In the informal household workshops, e-waste is dismantled and recycled by simple and primitive methods, including heating printed circuit boards over coal-fired grills to remove electronic components, open burning to recover metals, open-pit acid baths to retrieve precious metals, chipping and melting plastics without proper ventilation, etc. (Ren et al., 2009; Chi et al., 2011). During these crude recycling processes, persistent toxic substances such as PBDEs are inevitably emitted into the environment. Soil is a major reservoir and sink for environmental pollutants (Darnerud et al., 2001; Dalla Valle et al., 2005; Ma et al., 2012). High levels of PBDEs have been reported in the soils at e-waste recycling sites in China(Table 1). Many experiments have been conducted on potential remediation technologies (Keum and Li, 2005; He et al., 2006; Li et al., 2010; Huang et al., 2011; Wu et al., 2012; Zhuang et al., 2012); however, remediation using molten salt oxidation (MSO) technology has not
Z.-t. Yao et al. / Journal of Environmental Management 127 (2013) 244e248
245
Table 1 PBDEs levels in soils at e-waste recycling sites in China. Location
Type
Qingyuan Farmland soils near recycling workshop Road soils near recycling workshop Farmland soils near open burning site Soil in plastic dumping site Soil in printer roller dumping site Guiyu Soils in acid leaching site Soils in printer roller dump site Taizhou Soils in e-waste disposal site
S PBDEs [ng/g dry weight (dw)]
References
10.0 (1.7e28.8)
(Luo et al., 2009)
1149.8 (121.7e3159.0) (Luo et al., 2009) 9.2 (1.6e25.6)
(Luo et al., 2009)
1140
(Leung et al., 2007)
1169
(Leung et al., 2007)
980.1
(Wang et al., 2005)
858.60
(Wang et al., 2005)
625
(Chen et al., 2011) Fig. 2. The TG-DTA curve of BDE-209.
yet been widely used or analyzed. MSO is a versatile and promising technology for the destruction of organics. It is capable of trapping halogen species during organic destruction, and has been tested for the destruction of PCBs, trichloro- and hexachlorobenzene, carbon tetrachloride and ion exchange resins, and has demonstrated high destruction efficiency (Navratil and Steward, 1996). Thus, it shows promise for use as a second reactor following the reaction that desorbs organic pollutants in soils. In this study, we attempt to use decabromodiphenyl ether (BDE-209) as a model contaminant to study its destruction by MSO technology. The effects of reaction temperature, additive amount of BDE-209 and salt mixture were investigated. The salt remaining after reaction was characterized by XRD analysis and a reaction pathway proposed.
particulates, and the transfer of the melt (Yao et al., 2011). Directly placing the soils in the MSO reactor, however, will influence the fluidity of the melt, and requires continuously draining off the salts. This problem led us develop an advanced reactor system (Fig. 1), consisting of a thermal desorption reactor with a capacity of 3 L and an MSO reactor with a 2-L capacity. The thermal desorption reactor could be used to model the desorption of BDE-209 from soils. The volatilized BDE-209 was then introduced into the MSO reactor and oxidized. The off-gas leaving the MSO reactor was cooled by a vertical air-to-gas heat exchanging condenser and subsequently absorbed by an alkaline solution and an organic solution, and vented.
2. Experimental material and methods
2.3. Procedure
2.1. Materials
Variable amounts of salt powders (12, 15, 18 and 20 mol) were measured and placed in the MSO reactor, then heated to different fixed temperatures (550, 650, 700 and 750 C) to achieve melting. Thereafter, different amounts of BDE-209 (0.005, 0.01, 0.0125 and 0.015 mol) were placed in the thermal desorption reactor and heated. Air was simultaneously introduced with a flow rate of 1.5 L/ min. When the temperature of the thermal desorption reactor reached 550 C, the off-gas was collected and its composition analyzed. It is worth mentioning that the selection of a temperature of 550 C was based on TG-FTIR analysis of BDE-209. From Fig. 2 it can
The BDE-209 with a chemical formula of C12Br10O in this study was obtained from Sino-Brom Compounds Co., Ltd. A ternary salt mixture (K2CO3: Li2CO3: Na2CO3 ¼ 1: 1.69: 1.14 mol) with eutectic temperature of 666 K was selected (Volkovich et al., 1998). Powdered anhydrous sodium carbonate and potassium carbonate were of analytical grade, from Sinopharm Chemical Reagent Co. Ltd. Lithium carbonate was also of analytical grade, from Beijing Modern Orient Fine Chemicals Co. Ltd. 2.2. Reactor system In the original one-step MSO reactor, the melt must remain fluid to enable the contact of waste with oxygen or air, the wetting of
Fig. 1. The schematic diagram of reactor system.
Fig. 3. The 3D FTIR spectrum of BDE-209.
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Fig. 4. The off-gas species at different temperatures (molten salt: 12 mol; BDE-209: 0.01 mol).
Fig. 6. The off-gas species with different amounts of salt mixture (BDE-209: 0.01 mol; temperature: 650 C).
be seen that the major mass loss occurred in the range of 300e 450 C. When the temperature was increased to 550 C, the mass loss reached as high as 99.7%. A weak peak at 306 C in the DTA curve was attributed to the melting of BDE-209, as that temperature corresponds to its melting point (Yang et al., 2002). Fig. 3 depicts the 3D FTIR spectrum of BDE-209. It shows that the absorption intensities were higher at temperatures of 382, 406 and 485 C. Except for the changes in absorption intensities, the locations of the functional groups were not significant at these three temperatures. By comparing the spectra with those of raw BDE209, it was found that the gas released was indeed BDE-209. The results confirmed the volatilization of BDE-209 at these temperatures, and that decomposition had not occurred.
using a Nicolet Nexus-670 spectrometer and a Mettler-Toledo TGA/ SDTA851e thermo analyzer at a heating rate of 10 C/min. Air was used as the carrier gas with a flow rate of 100 ml/min. Off-gas compositions were determined with a PerkineElmer Instruments Auto System XL gas chromatograph, equipped with FID and TCD.
2.4. Characterization and test XRD analysis was employed to determine the crystalline phases of salt after reaction. A Rigaku D/max-2550 X-ray power diffractometer was operated at 40 kV and 30 mA, with CuKa as the radiation source. The TG-FTIR analysis of BDE-209 was conducted
Fig. 5. The off-gas species with different amounts of BDE-209 (molten salt: 12 mol; temperature: 650 C).
3. Results and discussion 3.1. Effect of reaction temperature The off-gas species at different temperatures are depicted in Fig. 4, revealing that the gas by-products included mainly C2H6, C2H4, C4H8, CH4, CO2 and H2. The amounts of C2H6, C2H4, C4H8, CH4, and H2 decreased with the increase in temperature, while the CO2 level increased. This indicated that elevating the temperature was helpful for reaction and that PICs could be further oxidized and transformed into CO2. When the temperature was as low as 550 C, CO2 emission was also detected. When the temperature was further increased to 750 C, PICs were no longer detected. These results indicated the significant effect of reaction temperatures. By comparison, PBDEs produce toxic gases such as dioxins and furans during the combustion process in incinerators (Mousavi et al.,
Fig. 7. The XRD pattern of salt after reaction.
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247
Fig. 8. The reaction pathway of BDE-209.
2009), confirming the higher destruction efficiency of MSO technology. 3.2. Effect of additive amount of BDE-209 Fig. 5 illustrates the off-gas species with different additive amounts of BDE-209 added to 12 mol of the ternary carbonate melt. It can be seen that the amounts of CH4, C2H6, C2H4, C4H8, CO2 and H2 increased with the increasing addition of BDE-209. When the amount was increased to 0.0125 mol, C3H8 was also observed. This indicated that increasing BDE-209 loading was not helpful for the reaction, since more PICs were produced. 3.3. Effect of salt mixture amount Fig. 6 depicts the off-gas species with different amounts of salt mixture. It reveals that the amounts of C2H6, CH4, C2H4 and C4H8 decreased with the salt mixture amount increasing. Thus, increasing the amount of salt mixture could make the reaction complete and transform PICs into CO2. According to earlier studies (Yang et al., 2007), vaporized organic-air mixtures formed bubbles when entering the melt. These bubbles rose through the salt and disappeared at the airesalt interface. Increasing the mole ratio of molten salt to BDE-209 meant the increasing of melt depth (0.12 and 0.20 m for the salt amount of 12 and 20 mol), prolonging of gas retention time and thus promoted the reaction. When the amount was increased to 20 mol, BDE-209 was almost destroyed and PICs were no longer detected.
Gay, 1993), which was based on the reaction between graphite and sodium carbonate (Dunks et al., 1980; Dunks and Stelman, 1983; Dunks, 1984). At that time, molten salt chemistry was not completely understood, and it was assumed that the molten salt was acting as a catalyst. In further studies of developing catalystenhanced molten salt oxidation (CEMSO), (Griffiths and Volkovich, 2008 and Griffiths et al., 2005, 2010) explained that O2 was chemically dissolved in molten carbonates, thus forming oxidizing species, O2 2 and O2 . Based on this conclusion, the reaction pathway in this study was assumed to be as follows: First, the air was dissolved in the ternary molten salt (Li, Na, K)2CO3, forming an oxidizing species. Secondly, the strong oxidizing agents attacked BDE-209, and dissociation of the CeBr, CeC and CeO bonds occurred thereafter. In general, debromination follows a stepwise process, similar to the microbial and photochemical degradation of PBDEs (He et al., 2006; Fang et al., 2011; Young-Mo et al., 2012; Zhuang et al., 2012). In the first step, CeBr bonds were dissociated and bromines were removed from BDE-209 forming bromine radicals. Then phenyl rings were destroyed, forming PICs such as C2H6, CH4, C2H4 and C4H8. Under severe reaction conditions, these PICs could be further oxidized to CO2. The involvement of oxidizing species would also transform bromine radicals into bromide ions. The resulting bromide ions were retained in the molten salt and thus reduced the emission of bromide pollutants. In addition, other PICs could possibly appear at the end of the experiment, but they could not be accurately identified. A more comprehensive analytical approach is needed to understand the details of the pathway. A simple destruction pathway of BDE-209 is described in Fig. 8.
4. Mechanism and reaction pathway
5. Conclusions and future research
The XRD pattern of salt after reaction is shown in Fig. 7. It can be seen that the major crystalline phases were lithium sodium carbonate (a-LiNaCO3, JPCDS card no. 34-1193) and lithium potassium carbonate (LiK(CO3), JPCDS card no. 88-341) with little lithium bromide (LiBr, JPCDS card no. 74-1973) or sodium bromide (NaBr, JPCDS card no. 78-761) detected. In the earlier studies, the mechanisms of investigation were sparse. The only attempt was that by Stelman and Gay (Stelman and
Molten salt oxidation is a promising technology that can effectively treat BDE-209 without emission of brominated pollutants. The effects of reaction temperature, additive amount of BDE-209 and salt mixture on off-gas species were investigated. The results showed that the effect of reaction temperature was more significant compared with that of other two factors. The amounts of C2H6, C2H4, C4H8 and CH4 in off-gas decreased with an increase in temperature, while the CO2 level increased. When reaction temperature was
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increased to 750 C, PICs were no longer detected. Increasing BDE209 loading was not helpful for the reaction, as more high molecular hydrocarbons were produced. Larger amounts of salt mixture were helpful for the reaction and PICs were not observed with the mole ratio 1: 2000 of BDE-209 to carbonate melt. XRD analysis confirmed the capture of bromine by molten salt. However, it may not be necessary to use analytical grade carbonates in the treatment of practical contaminated soils, as the soils will soon contribute impurities into the melt, without significantly reducing the reaction efficiency. The air flow rate may be another factor, so in the further studies the effect of air flow rate at which the air is bubbled through the carbonate melt could be investigated for evaluating the optimum reaction conditions. To keep the system operating efficiently in a larger pilot or an industrial application of MSO technology, the air flow rate, the concentration of BDE-209 in the air, the depth of carbonate melt, and the melt temperature, have all to be kept in balance to guarantee no PICs and only CO2 and air in the off-gas. Since PBDEs are structurally similar to PCBs, this study will also provide references for the remediation of soils contaminated by PCBs and other volatile organics. Acknowledgments We gratefully acknowledge financial support from the Chinese National 863 High Technology (Grant no. 2009AA064001), the Scientific Research Foundation of Hangzhou Dianzi University (Grant no. KYS205612029), Zhejiang Provincial Natural Science Foundation of China (Grant no. LQ13B070005) and the Key Laboratory for Solid Waste Management and Environment Safety Open Fund (Grant no. SWMES 2011-07). We also would like to thank the anonymous referees for their helpful comments on this paper. References Chen, T., Zhou, C., Mou, Y.J., Yu, B.B., 2011. PBDEs pollution of soil in an e-waste disposal site and its surrounding area. J. Ecol. Rural Environ. 27, 20e24 (in Chinese). Chi, X.W., Streicher-Porte, M., Wang, M.Y.L., Reuter, M.A., 2011. Informal electronic waste recycling: a sector review with special focus on China. Waste Manag. 31, 731e742. Dalla Valle, M., Jurado, E., Dachs, J., Sweetman, A.J., Jones, K.C., 2005. The maximum reservoir capacity of soils for persistent organic pollutants: implications for global cycling. Environ. Pollut. 134, 153e164. Darnerud, P.O., Eriksen, G.S., Johannesson, T., Larsen, P.B., Viluksela, M., 2001. Polybrominated diphenyl ethers: occurrence, dietary exposure, and toxicology. Environ. Health Perspect. 109, 49e68. Dunks, G.B., 1984. Electrochemical studies of graphite oxidation in sodium carbonate melt. Inorg. Chem. 23, 828e837. Dunks, G.B., Stelman, D., 1983. Electrochemical studies of molten sodium carbonate. Inorg. Chem. 22, 2168e2177. Dunks, G.B., Stelman, D., Yosim, S.J., 1980. Graphite oxidation in molten sodium carbonate. Carbon 18, 365e370. Fang, Z.Q., Qiu, X.H., Chen, J.H., Qiu, X.Q., 2011. Debromination of polybrominated diphenyl ethers by Ni/Fe bimetallic nanoparticles: influencing factors, kinetics, and mechanism. J. Hazard. Mater. 185, 958e969. Griffiths, T.R., Volkovich, V.A., 2008. A new technology for the nuclear industry for the complete and continuous pyrochemical reprocessing of spent nuclear fuel: catalyst enhanced molten salt oxidation. Nucl. Techn. 163, 382e400. Griffiths, T.R., Volkovich, V.A., Anghel, E.M., Carper, W.R., 2005. Molten salt oxidation for the efficient destruction of radioactive, hazardous chemical, medical waste and munitions. In: 24th International Conference on Incineration and Thermal Treatment Technologies, Galveston, Texas. Griffiths, T.R., Volkovich, V.A., Carper, W.R., 2010. The structures of the active intermediates in catalyst-enhanced molten salt oxidation and a new method for
the complete destruction of chemical warfare arsenicals. Struct. Chem. 21, 291e297. He, J.Z., Robrock, K.R., Alvarez-Cohen, L., 2006. Microbial reductive debromination of polybrominated diphenyl ethers (PBDEs). Environ. Sci. Technol. 40, 4429e 4434. Hites, R.A., 2004. Polybrominated diphenyl ethers in the environment and in people: a meta-analysis of concentrations. Environ. Sci. Technol. 38, 945e956. Huang, H.L., Zhang, S.Z., Christie, P., 2011. Plant uptake and dissipation of PBDEs in the soils of electronic waste recycling sites. Environ. Pollut. 159, 238e243. Keum, Y.S., Li, Q.X., 2005. Reductive debromination of PBDEs by zero-valent iron. Environ. Sci. Technol. 39, 2280e2286. Leung, A.O.W., Luksemburg, W.J., Wong, A.S., Wong, M.H., 2007. Spatial distribution of polybrominated diphenyl ethers and polychlorinated dibenzo-pdioxins and dibenzofurans in soil and combusted residue at Guiyu, an electronic waste recycling site in southeast China. Environ. Sci. Technol. 41, 2730e2737. Li, X., Huang, J., Yu, G., Deng, S.B., 2010. Photodestruction of BDE-99 in micellar solutions of nonionic surfactants of Brij 35 and Brij 58. Chemosphere 78, 752e759. Luo, Y., Luo, X.J., Lin, Z., Chen, S.J., Liu, J., Mai, B.X., Yang, Z.Y., 2009. Polybrominateddiphenyl ethers in road and farmland soils from an e-waste recycling region in southern China: concentrations, source profiles, and potential dispersion and deposition. Sci. Total Environ. 407, 1105e1113. Ma, J., Qiu, X.H., Zhang, J.L., Duan, X.L., Zhu, T., 2012. State of polybrominated diphenyl ethers in China: an overview. Chemosphere 88, 769e778. Mousavi, M., Taymouri, A., Ghaffarian, V., 2009. Transient mass transfer modeling and simulation of polybrominated diphenyl ethers combustion in incinerators. Int. J. Enviorn. Sci. Technol. 6, 499e508. Navratil, J.D., Steward, A.E., 1996. Waste treatment using molten salt oxidation. Nukleonika 41, 57e71. Ren, G.F., Yu, Z.Q., Ma, S.T., Li, H.R., Peng, P.A., Sheng, G.Y., Fu, J.M., 2009. Determination of dechlorane plus in serum from electronics dismantling workers in South China. Environ. Sci. Technol. 43, 9453e9457. Robinson, B.H., 2009. E-waste: an assessment of global production and environmental impacts. Sci. Total Environ. 408, 183e191. Stelman, D., Gay, R.L., 1993. Fundamental Chemical and Process Differences between Molten Salt Oxidation and Incineration. Report for Rockwell International, Canoga Park, CA. Streets, S.S., Henderson, S.A., Stoner, A.D., Carlson, D.L., Simcik, M.F., Swackhamer, D.L., 2006. Partitioning and bioaccumulation of PBDEs and PCBs in Lake Michigan. Environ. Sci. Technol. 40, 7263e7269. Volkovich, V.A., Griffiths, T.R., Fray, D.J., Fields, M., 1998. Oxidation of ceramic uranium dioxide in alkali metal carbonate-based melts: a study using various oxidants and comparison with UO2 powder. J. Nucl. Mater. 256, 131e138. Wang, D., Cai, Z., Jiang, G., Leung, A., Wong, M.H., Wong, W.K., 2005. Determination of polybrominated diphenyl ethers in soil and sediment from an electronic waste recycling facility. Chemosphere 60, 810e816. Watanabe, I., Sakai, S., 2003. Environmental release and behavior of brominated flame retardants. Environ. Int. 29, 665e682. World Health Organization (WHO), 1994. Environmental Health Criteria 162: Brominated Diphenyl Ethers. International Program on Chemical Safety. Wu, C.D., Fan, C.P., Xie, Q.J., 2012. Study on electrokinetic remediation of PBDEs contaminated soil. Adv. Mater. Res. 518e523, 2829e2833. Yang, H.C., Cho, Y.J., Eun, H.C., Kim, E.H., 2007. Destruction of chlorinated organic solvents in a two-stage molten salt oxidation reactor system. Chem. Eng. Sci. 62, 5137e5143. Yang, Z.H., Chen, S.Z., Ye, J., Ou, C.Y., Zou, W.D., 2002. Catalytic synthesis of decabromodiphenyl ether by bromination and its purification. Chem. World 03, 150e152 (in Chinese). Yao, Z.T., Li, J.H., Zhao, X.Y., 2011. Molten salt oxidation: a versatile and promising technology for the destruction of organic-containing wastes. Chemosphere 84, 1167e1174. Young-Mo, K., Murugesan, K., Chang, Y.Y., Kim, E.J., Chang, Y.S., 2012. Degradation of polybrominated diphenyl ethers by a sequential treatment with nanoscale zero valent iron and aerobic biodegradation. J. Chem. Technol. Biotechnol. 87, 216e224. Yu, Z.Q., Zheng, K.W., Ren, G.F., Wang, D.C., Ma, S.T., Peng, P.G., Wu, M.H., Sheng, G.Y., Fu, J.M., 2011. Identification of monochloro-nanoabromodiphenyl ethers in the air and soil samples from South China. Environ. Sci. Technol. 45, 2619e2625. Zhang, W.H., Wu, Y.X., Simonnot, M.O., 2012. Soil contamination due to e-waste disposal and recycling activities: a review with special focus on China. Pedosphere 22, 434e455. Zhuang, Y., Jin, L.T., Luthy, R.G., 2012. Kinetics and pathways for the debromination of polybrominated diphenyl ethers by bimetallic and nanoscale zerovalent iron: effects of particle properties and catalyst. Chemosphere..
Contents lists available at SciVerse ScienceDirect
Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
Destruction of decabromodiphenyl ether (BDE-209) in a ternary carbonate molten salt reactor Zhi-tong Yao a, b, *, Jin-hui Li c, Xiang-yang Zhao c a
College of Materials Science and Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, China Key Laboratory for Solid Waste Management and Environment Safety, Ministry of Education of China, Tsinghua University, Beijing 100084, China c School of Environment, Tsinghua University, Beijing 100084, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 October 2012 Received in revised form 6 April 2013 Accepted 17 April 2013 Available online 10 June 2013
Soil contamination by PBDEs has become a significant environmental concern and requires appropriate remediation technologies. In this study, the destruction of decabromodiphenyl ether (BDE-209) in a ternary molten salt (Li, Na, K)2 CO3 reactor was evaluated. The effects of reaction temperature, additive amount of BDE-209 and salt mixture, on off-gas species, were investigated. The salt mixture after reaction was characterized by XRD analysis and a reaction pathway proposed. The results showed that the amounts of C2H6, C2H4, C4H8 and CH4 in the off-gas decreased with increases in temperature, while the CO2 level increased. When the reaction temperature reached 750 C, incomplete combustion products (PICs) were no longer detected. Increasing BDE-209 loading was not helpful for the reaction, as more PICs were produced. Larger amounts of salt mixture were helpful for the reaction and PICs were not observed with the mole ratio 1: 2000 of BDE-209 to carbonate melt. XRD analysis confirmed the capture of bromine in BDE-209 by the molten salt. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: PBDEs e-waste recycling Soil contamination Molten salt oxidation
1. Introduction Polybrominated diphenyl ethers (PBDEs) are brominated flame retardants used in polymeric materials to enhance the fire safety of electronics, plastics, textiles and furniture (WHO, 1994; Hites, 2004). There are three major commercial formulations on the global market: Pentabromodiphenyl ethers (Penta-BDEs), Octabromodiphenyl ethers (Octa-BDEs) and Decabromodiphenyl ether (Deca-BDEs) (Darnerud et al., 2001; Ma et al., 2012). PBDEs are a class of additive flame retardants not chemically bonded to their host, which therefore could be released into the environment during their lifetimes. The release pathways include mainly atmospheric emissions during manufacturing processes, volatilization from PBDE-treated consumer products, recycling of PBDEcontaining wastes and leaching from waste disposal sites (Streets et al., 2006; Watanabe and Sakai, 2003; Yu et al., 2011; Zhang et al., 2012). With the constant innovation in technologies and the upgrading of products, the quantities of e-waste generated are increasing
* Corresponding author. College of Materials Science and Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, China. Tel./fax: þ86 571 86919158. E-mail addresses: [email protected], [email protected] (Z.-T. Yao). 0301-4797/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jenvman.2013.04.040
rapidly throughout the world (Robinson, 2009). According to the report “Recyclingdfrom e-waste to resources” released by the UNEP, China produced about 2.3 million tons of e-waste in 2010, second only to the United States, which produced about 3 million tons. Because of its economic attractiveness, the e-waste recycling industry has boomed in China in the last few years. However, in addition to the formal treatment enterprises, informal sectors are also engaged in recycling operations in Guiyu and Qingyuan (Guangdong Province, South China), as well as in Taizhou (Zhejiang Province, Southeast China), etc. In the informal household workshops, e-waste is dismantled and recycled by simple and primitive methods, including heating printed circuit boards over coal-fired grills to remove electronic components, open burning to recover metals, open-pit acid baths to retrieve precious metals, chipping and melting plastics without proper ventilation, etc. (Ren et al., 2009; Chi et al., 2011). During these crude recycling processes, persistent toxic substances such as PBDEs are inevitably emitted into the environment. Soil is a major reservoir and sink for environmental pollutants (Darnerud et al., 2001; Dalla Valle et al., 2005; Ma et al., 2012). High levels of PBDEs have been reported in the soils at e-waste recycling sites in China(Table 1). Many experiments have been conducted on potential remediation technologies (Keum and Li, 2005; He et al., 2006; Li et al., 2010; Huang et al., 2011; Wu et al., 2012; Zhuang et al., 2012); however, remediation using molten salt oxidation (MSO) technology has not
Z.-t. Yao et al. / Journal of Environmental Management 127 (2013) 244e248
245
Table 1 PBDEs levels in soils at e-waste recycling sites in China. Location
Type
Qingyuan Farmland soils near recycling workshop Road soils near recycling workshop Farmland soils near open burning site Soil in plastic dumping site Soil in printer roller dumping site Guiyu Soils in acid leaching site Soils in printer roller dump site Taizhou Soils in e-waste disposal site
S PBDEs [ng/g dry weight (dw)]
References
10.0 (1.7e28.8)
(Luo et al., 2009)
1149.8 (121.7e3159.0) (Luo et al., 2009) 9.2 (1.6e25.6)
(Luo et al., 2009)
1140
(Leung et al., 2007)
1169
(Leung et al., 2007)
980.1
(Wang et al., 2005)
858.60
(Wang et al., 2005)
625
(Chen et al., 2011) Fig. 2. The TG-DTA curve of BDE-209.
yet been widely used or analyzed. MSO is a versatile and promising technology for the destruction of organics. It is capable of trapping halogen species during organic destruction, and has been tested for the destruction of PCBs, trichloro- and hexachlorobenzene, carbon tetrachloride and ion exchange resins, and has demonstrated high destruction efficiency (Navratil and Steward, 1996). Thus, it shows promise for use as a second reactor following the reaction that desorbs organic pollutants in soils. In this study, we attempt to use decabromodiphenyl ether (BDE-209) as a model contaminant to study its destruction by MSO technology. The effects of reaction temperature, additive amount of BDE-209 and salt mixture were investigated. The salt remaining after reaction was characterized by XRD analysis and a reaction pathway proposed.
particulates, and the transfer of the melt (Yao et al., 2011). Directly placing the soils in the MSO reactor, however, will influence the fluidity of the melt, and requires continuously draining off the salts. This problem led us develop an advanced reactor system (Fig. 1), consisting of a thermal desorption reactor with a capacity of 3 L and an MSO reactor with a 2-L capacity. The thermal desorption reactor could be used to model the desorption of BDE-209 from soils. The volatilized BDE-209 was then introduced into the MSO reactor and oxidized. The off-gas leaving the MSO reactor was cooled by a vertical air-to-gas heat exchanging condenser and subsequently absorbed by an alkaline solution and an organic solution, and vented.
2. Experimental material and methods
2.3. Procedure
2.1. Materials
Variable amounts of salt powders (12, 15, 18 and 20 mol) were measured and placed in the MSO reactor, then heated to different fixed temperatures (550, 650, 700 and 750 C) to achieve melting. Thereafter, different amounts of BDE-209 (0.005, 0.01, 0.0125 and 0.015 mol) were placed in the thermal desorption reactor and heated. Air was simultaneously introduced with a flow rate of 1.5 L/ min. When the temperature of the thermal desorption reactor reached 550 C, the off-gas was collected and its composition analyzed. It is worth mentioning that the selection of a temperature of 550 C was based on TG-FTIR analysis of BDE-209. From Fig. 2 it can
The BDE-209 with a chemical formula of C12Br10O in this study was obtained from Sino-Brom Compounds Co., Ltd. A ternary salt mixture (K2CO3: Li2CO3: Na2CO3 ¼ 1: 1.69: 1.14 mol) with eutectic temperature of 666 K was selected (Volkovich et al., 1998). Powdered anhydrous sodium carbonate and potassium carbonate were of analytical grade, from Sinopharm Chemical Reagent Co. Ltd. Lithium carbonate was also of analytical grade, from Beijing Modern Orient Fine Chemicals Co. Ltd. 2.2. Reactor system In the original one-step MSO reactor, the melt must remain fluid to enable the contact of waste with oxygen or air, the wetting of
Fig. 1. The schematic diagram of reactor system.
Fig. 3. The 3D FTIR spectrum of BDE-209.
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Fig. 4. The off-gas species at different temperatures (molten salt: 12 mol; BDE-209: 0.01 mol).
Fig. 6. The off-gas species with different amounts of salt mixture (BDE-209: 0.01 mol; temperature: 650 C).
be seen that the major mass loss occurred in the range of 300e 450 C. When the temperature was increased to 550 C, the mass loss reached as high as 99.7%. A weak peak at 306 C in the DTA curve was attributed to the melting of BDE-209, as that temperature corresponds to its melting point (Yang et al., 2002). Fig. 3 depicts the 3D FTIR spectrum of BDE-209. It shows that the absorption intensities were higher at temperatures of 382, 406 and 485 C. Except for the changes in absorption intensities, the locations of the functional groups were not significant at these three temperatures. By comparing the spectra with those of raw BDE209, it was found that the gas released was indeed BDE-209. The results confirmed the volatilization of BDE-209 at these temperatures, and that decomposition had not occurred.
using a Nicolet Nexus-670 spectrometer and a Mettler-Toledo TGA/ SDTA851e thermo analyzer at a heating rate of 10 C/min. Air was used as the carrier gas with a flow rate of 100 ml/min. Off-gas compositions were determined with a PerkineElmer Instruments Auto System XL gas chromatograph, equipped with FID and TCD.
2.4. Characterization and test XRD analysis was employed to determine the crystalline phases of salt after reaction. A Rigaku D/max-2550 X-ray power diffractometer was operated at 40 kV and 30 mA, with CuKa as the radiation source. The TG-FTIR analysis of BDE-209 was conducted
Fig. 5. The off-gas species with different amounts of BDE-209 (molten salt: 12 mol; temperature: 650 C).
3. Results and discussion 3.1. Effect of reaction temperature The off-gas species at different temperatures are depicted in Fig. 4, revealing that the gas by-products included mainly C2H6, C2H4, C4H8, CH4, CO2 and H2. The amounts of C2H6, C2H4, C4H8, CH4, and H2 decreased with the increase in temperature, while the CO2 level increased. This indicated that elevating the temperature was helpful for reaction and that PICs could be further oxidized and transformed into CO2. When the temperature was as low as 550 C, CO2 emission was also detected. When the temperature was further increased to 750 C, PICs were no longer detected. These results indicated the significant effect of reaction temperatures. By comparison, PBDEs produce toxic gases such as dioxins and furans during the combustion process in incinerators (Mousavi et al.,
Fig. 7. The XRD pattern of salt after reaction.
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Fig. 8. The reaction pathway of BDE-209.
2009), confirming the higher destruction efficiency of MSO technology. 3.2. Effect of additive amount of BDE-209 Fig. 5 illustrates the off-gas species with different additive amounts of BDE-209 added to 12 mol of the ternary carbonate melt. It can be seen that the amounts of CH4, C2H6, C2H4, C4H8, CO2 and H2 increased with the increasing addition of BDE-209. When the amount was increased to 0.0125 mol, C3H8 was also observed. This indicated that increasing BDE-209 loading was not helpful for the reaction, since more PICs were produced. 3.3. Effect of salt mixture amount Fig. 6 depicts the off-gas species with different amounts of salt mixture. It reveals that the amounts of C2H6, CH4, C2H4 and C4H8 decreased with the salt mixture amount increasing. Thus, increasing the amount of salt mixture could make the reaction complete and transform PICs into CO2. According to earlier studies (Yang et al., 2007), vaporized organic-air mixtures formed bubbles when entering the melt. These bubbles rose through the salt and disappeared at the airesalt interface. Increasing the mole ratio of molten salt to BDE-209 meant the increasing of melt depth (0.12 and 0.20 m for the salt amount of 12 and 20 mol), prolonging of gas retention time and thus promoted the reaction. When the amount was increased to 20 mol, BDE-209 was almost destroyed and PICs were no longer detected.
Gay, 1993), which was based on the reaction between graphite and sodium carbonate (Dunks et al., 1980; Dunks and Stelman, 1983; Dunks, 1984). At that time, molten salt chemistry was not completely understood, and it was assumed that the molten salt was acting as a catalyst. In further studies of developing catalystenhanced molten salt oxidation (CEMSO), (Griffiths and Volkovich, 2008 and Griffiths et al., 2005, 2010) explained that O2 was chemically dissolved in molten carbonates, thus forming oxidizing species, O2 2 and O2 . Based on this conclusion, the reaction pathway in this study was assumed to be as follows: First, the air was dissolved in the ternary molten salt (Li, Na, K)2CO3, forming an oxidizing species. Secondly, the strong oxidizing agents attacked BDE-209, and dissociation of the CeBr, CeC and CeO bonds occurred thereafter. In general, debromination follows a stepwise process, similar to the microbial and photochemical degradation of PBDEs (He et al., 2006; Fang et al., 2011; Young-Mo et al., 2012; Zhuang et al., 2012). In the first step, CeBr bonds were dissociated and bromines were removed from BDE-209 forming bromine radicals. Then phenyl rings were destroyed, forming PICs such as C2H6, CH4, C2H4 and C4H8. Under severe reaction conditions, these PICs could be further oxidized to CO2. The involvement of oxidizing species would also transform bromine radicals into bromide ions. The resulting bromide ions were retained in the molten salt and thus reduced the emission of bromide pollutants. In addition, other PICs could possibly appear at the end of the experiment, but they could not be accurately identified. A more comprehensive analytical approach is needed to understand the details of the pathway. A simple destruction pathway of BDE-209 is described in Fig. 8.
4. Mechanism and reaction pathway
5. Conclusions and future research
The XRD pattern of salt after reaction is shown in Fig. 7. It can be seen that the major crystalline phases were lithium sodium carbonate (a-LiNaCO3, JPCDS card no. 34-1193) and lithium potassium carbonate (LiK(CO3), JPCDS card no. 88-341) with little lithium bromide (LiBr, JPCDS card no. 74-1973) or sodium bromide (NaBr, JPCDS card no. 78-761) detected. In the earlier studies, the mechanisms of investigation were sparse. The only attempt was that by Stelman and Gay (Stelman and
Molten salt oxidation is a promising technology that can effectively treat BDE-209 without emission of brominated pollutants. The effects of reaction temperature, additive amount of BDE-209 and salt mixture on off-gas species were investigated. The results showed that the effect of reaction temperature was more significant compared with that of other two factors. The amounts of C2H6, C2H4, C4H8 and CH4 in off-gas decreased with an increase in temperature, while the CO2 level increased. When reaction temperature was
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increased to 750 C, PICs were no longer detected. Increasing BDE209 loading was not helpful for the reaction, as more high molecular hydrocarbons were produced. Larger amounts of salt mixture were helpful for the reaction and PICs were not observed with the mole ratio 1: 2000 of BDE-209 to carbonate melt. XRD analysis confirmed the capture of bromine by molten salt. However, it may not be necessary to use analytical grade carbonates in the treatment of practical contaminated soils, as the soils will soon contribute impurities into the melt, without significantly reducing the reaction efficiency. The air flow rate may be another factor, so in the further studies the effect of air flow rate at which the air is bubbled through the carbonate melt could be investigated for evaluating the optimum reaction conditions. To keep the system operating efficiently in a larger pilot or an industrial application of MSO technology, the air flow rate, the concentration of BDE-209 in the air, the depth of carbonate melt, and the melt temperature, have all to be kept in balance to guarantee no PICs and only CO2 and air in the off-gas. Since PBDEs are structurally similar to PCBs, this study will also provide references for the remediation of soils contaminated by PCBs and other volatile organics. Acknowledgments We gratefully acknowledge financial support from the Chinese National 863 High Technology (Grant no. 2009AA064001), the Scientific Research Foundation of Hangzhou Dianzi University (Grant no. KYS205612029), Zhejiang Provincial Natural Science Foundation of China (Grant no. LQ13B070005) and the Key Laboratory for Solid Waste Management and Environment Safety Open Fund (Grant no. SWMES 2011-07). We also would like to thank the anonymous referees for their helpful comments on this paper. References Chen, T., Zhou, C., Mou, Y.J., Yu, B.B., 2011. PBDEs pollution of soil in an e-waste disposal site and its surrounding area. J. Ecol. Rural Environ. 27, 20e24 (in Chinese). Chi, X.W., Streicher-Porte, M., Wang, M.Y.L., Reuter, M.A., 2011. Informal electronic waste recycling: a sector review with special focus on China. Waste Manag. 31, 731e742. Dalla Valle, M., Jurado, E., Dachs, J., Sweetman, A.J., Jones, K.C., 2005. The maximum reservoir capacity of soils for persistent organic pollutants: implications for global cycling. Environ. Pollut. 134, 153e164. Darnerud, P.O., Eriksen, G.S., Johannesson, T., Larsen, P.B., Viluksela, M., 2001. Polybrominated diphenyl ethers: occurrence, dietary exposure, and toxicology. Environ. Health Perspect. 109, 49e68. Dunks, G.B., 1984. Electrochemical studies of graphite oxidation in sodium carbonate melt. Inorg. Chem. 23, 828e837. Dunks, G.B., Stelman, D., 1983. Electrochemical studies of molten sodium carbonate. Inorg. Chem. 22, 2168e2177. Dunks, G.B., Stelman, D., Yosim, S.J., 1980. Graphite oxidation in molten sodium carbonate. Carbon 18, 365e370. Fang, Z.Q., Qiu, X.H., Chen, J.H., Qiu, X.Q., 2011. Debromination of polybrominated diphenyl ethers by Ni/Fe bimetallic nanoparticles: influencing factors, kinetics, and mechanism. J. Hazard. Mater. 185, 958e969. Griffiths, T.R., Volkovich, V.A., 2008. A new technology for the nuclear industry for the complete and continuous pyrochemical reprocessing of spent nuclear fuel: catalyst enhanced molten salt oxidation. Nucl. Techn. 163, 382e400. Griffiths, T.R., Volkovich, V.A., Anghel, E.M., Carper, W.R., 2005. Molten salt oxidation for the efficient destruction of radioactive, hazardous chemical, medical waste and munitions. In: 24th International Conference on Incineration and Thermal Treatment Technologies, Galveston, Texas. Griffiths, T.R., Volkovich, V.A., Carper, W.R., 2010. The structures of the active intermediates in catalyst-enhanced molten salt oxidation and a new method for
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