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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2023 , Vol. 35 >Issue 7: 1040 - 1052

DOI: https://doi.org/10.7536/PC221126

Review

Source and Environmental Characteristics of Hexachlorobutadiene

  • Chenyan Zhao 1, 2, 3 ,
  • Yuxiang Sun 1, 2, 3 ,
  • Lili Yang 1, 3 ,
  • Minghui Zheng 1, 2, 3 ,
  • Shuting Liu 1, 3 ,
  • Guorui Liu , 1, 2, 3, *
Expand
  • 1 State Key Laboratory of Environmental Chemistry and Ecotoxicology,Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences,Beijing 100085,China
  • 2 School of Environment,Hangzhou Institute for Advanced Study,UCAS,Hangzhou 310024,China
  • 3 College of Resources and Environment,University of Chinese Academy of Sciences,Beijing 100049,China
* Corresponding author e-mail:

Received date: 2022-11-28

  Revised date: 2023-03-17

  Online published: 2023-03-30

Supported by

Second Tibetan Plateau Scientific Expedition and Research Program(2019QZKK0605)

National Natural Science Foundation of China(92143201)

National Natural Science Foundation of China(22076201)

CAS Interdisciplinary Innovation Team(JCTD-2019-03)

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Abstract

Hexachlorobutadiene (HCBD) is a new persistent organic pollutant (POPs) added into the Stockholm Convention on POPs since 2015. HCBD has attracted worldwide attention due to its persistence, bioaccumulation, and potential for long-range transport, with potential adverse effects on humans and biota. However, the knowledge about the source, environmental characteristics, control techniques and strategies are still very lacking. The levels of HCBD in environmental and biological samples are summarized and analyzed in this review. The control process, potential emission sources and emission amount of HCBD are reviewed. The formation mechanism of hexachlorobutadiene, the degradation process in natural environment, and the related emission reduction strategies and control technologies are discussed. This paper can provide important reference for controlling the emission of HCBD, reducing their environmental level, and reducing human exposure.

Contents

1 Introduction

2 Environmental occurrence of HCBD

2.1 Atmosphere

2.2 Water

2.3 Soil

2.4 Organisms

3 Emission sources in China

3.1 Chemical production source

3.2 Waste incineration and landfill sources

4 Natural degradation

5 Control measures of HCBD at home and abroad

6 Corelation and synergistic emission reduction of HCBD and other POPs

6.1 Synergistic emission reduction

6.2 Emission reduction measures

7 Conclusion and outlook

Key words： hexachlorobutadiene; stockholm convention; persistent organic pollutants; pollution characteristics

Cite this article

Chenyan Zhao , Yuxiang Sun , Lili Yang , Minghui Zheng , Shuting Liu , Guorui Liu . Source and Environmental Characteristics of Hexachlorobutadiene[J]. Progress in Chemistry, 2023 , 35(7) : 1040 -1052 . DOI: 10.7536/PC221126

1 Introduction

Hexachlorobutadiene (HCBD) is an aliphatic halogenated hydrocarbon commonly used in the production of elastomers, rubber, heat transfer fluids, transformers, hydraulic fluids, pesticides, herbicides and fungicides. Its physicochemical properties are shown in Table 1. HCBD is chemically stable, bioaccumulative, persistent, and transported over long distances. Its molecular formula is C4Cl6, and its structure is shown in Fig. 1 (a), which is a butadiene with all six hydrogen atoms replaced by chlorine[1]. The substance has a low boiling point, is highly volatile, and is easily redistributed from water and soil to the atmosphere[2]. HCBD is not easy to degrade in the natural environment, and can remain in environmental media such as water, soil and sediment for years or even decades[3,4].
表1 HCBD的基本理化性质[1]

Table 1 The main physical and chemical properties of HCBD

English Name hexachlorobutadiene
CAS number 87-68-3
Physical state Liquid
Boiling Point 215℃
Melting Point -21℃
Density 1.68 g·cm-3 (20℃)
Vapor pressure 20 Pa (20℃)
Water solubility 3.2 mg·L-1 (25℃)
Henry's law constant 1044 Pa·m3·mol-1
(experimental)
log Kow 4.78
图1 六氯丁二烯的化学结构式 (a) 及价层电子密度 (b)

Fig.1 The chemical structure of HCBD (a) and the valence electron density (b)

The orbital dual descriptor is a real-space function proposed by Morell, Grand, and Toro-Labb Labbé in 2005 to predict nucleophilic and electrophilic reaction sites. The analysis of the valence shell electron density of HCBD by using the orbital weighted Fukui function and the orbital weighted dual descriptor shows that the nucleophilicity around the Cl atom is higher. Fig. 1 (B) shows the electron spin density distribution of the valence shell of the HCBD molecule. The orbital weight double descriptor in the blue part is negative, and the orbital weight double descriptor in the green part is positive. It can be seen from Fig. 1 (B) that a circle of isosurface around the Cl atom is blue, that is, the orbital weight double descriptor is negative, which reflects that this part of the region has high electrophilicity and shows the characteristics of a local Lewis base. The essence is that this region is the region where the rich lone pair electrons of Cl are mainly distributed. There is a blue isosurface in the vertical plane of the C = C double bond, that is, the π bond region, which is vulnerable to electrophilic attack in the same way. There is a green isosurface at the end of one of the Cl substituents, that is, if the orbital weight double descriptor is positive, it has a certain nucleophilicity, reflecting the characteristics of local Lewis acid. The green isosurface is surrounded by the C atom and the σ single bond connected to C, which is vulnerable to nucleophilic attack and nucleophilic reaction.
HCBD has high lipid solubility and low water solubility, and is easy to accumulate in organisms, with a bioconcentration factor (BCF) of up to 19 000 in fish[5]. Despite its high bioconcentration factor, some studies have pointed out that the biomagnification of HCBD is not obvious. Demers et al. Calculated the biomagnification factor of HCBD in invertebrates, fish, reptiles, amphibians, birds, mammals and humans, and the biomagnification factor values in several organisms are less than 1[6]. The calculation of BMF values in the herbivorous insect-toad-food chain in China also shows that its biomagnification is not obvious[7]. Therefore, compared with the transmission in the food chain, the environmental concentration effect is more worthy of attention.
HCBD has certain chronic toxicity and acute toxicity, and the median lethal concentration in freshwater fish is 0.09 mg/L. HCBD can be absorbed by organisms through respiratory exposure, dietary intake and skin contact, and has potential damage to kidney and liver[8,9][10,11]. As early as 1978, HCBD was considered to be the most nephrotoxic aliphatic halogenated hydrocarbon, which could trigger epithelial necrotizing nephritis[12]. At the same time, it can be further metabolized into nephrotoxic derivatives by binding with glutathione, which can specifically bind to surrounding tissues and damage renal proximal tubules. Studies have shown that there may be gender differences in the degree of damage[13][14]. In addition, potential respiratory damage, carcinogenicity, and genotoxicity are all of concern[8][15].

2 Hexachlorobutadiene in the environment

At present, HCBD has been widely detected in environmental media including water, soil, sediment and atmosphere, and even Antarctic ice cores[16]. In recent years, due to the control of international conventions, the content of environmental media in the world has decreased[17].

2.1 HCBD in the atmosphere

HCBD is hydrophobic and volatile, and HCBD in water environment is easy to partition into the atmosphere or adsorb on suspended particulate matter and stay for a long time[18,19]. At present, there are relatively few studies on the concentration of HCBD in the atmosphere, and the related studies mainly focus on the concentration level of HCBD in the atmospheric environment of industrial areas. HCBD with a concentration of 4.98~20.20μg/m3 was detected in the atmosphere of the new urban area of Zhengzhou, where petroleum enterprises are concentrated[20]. Higher concentrations of HCBD were detected in two studies on the vicinity of organochlorine pesticide plants in 2017 and 2018, with concentrations ranging from 0.03 to 0.33 and 0.01~2.23 ng/m3, respectively[21,22]. In 2018, HCBD levels of 0.21μg/m3 were detected near chlor-alkali plants in Catalonia, Spain[23]. However, a high concentration of HCBD (mean :225.8μg/m3,716.5μg/m3) was detected over a polluted river in Taiwan, China, which exceeded the maximum workplace limit of 210μg/m3 stipulated in the Occupational Safety and Health Regulations of Taiwan. Higher concentrations of HCBD with a maximum of 344 ng/PUF (PUF: polyurethane foam for passive sampling) were detected in Mongolia's environmental regional monitoring program in 2022[24]. The mean value was 75.7 ng/PUF, and the median value was 8.90 ng/PUF, which was much higher than that of other Asian countries (mean value was 3.7 ng/PUF, median = 2.6 ng/PUF). However, the content of HCBD is not high in most cities. For example, HCBD has not been detected in the rural areas of western Colorado in the United States, and the HCBD content in the air of Shanghai urban area is 0.06μg/m3[25][26]. Some studies have shown that under the influence of East Asian monsoon climate, its long-range transport will lead to the transport of the substance from pollution sources to the atmospheric environment of high-altitude cities. For example, the atmospheric HCBD level in North China may be affected by the emission sources in Central China and East China[27]. Table 2 is a summary of HCBD concentrations in the atmospheric environment in different years in different regions of the world.
表2 大气环境中HCBD的浓度

Table 2 Concentrations of hexachlorobutadiene in atmosphere in different areas

Time Location Country Concentration
(μg/m3)		 ref
2010 Fongshan Stream, Taiwan China Mean: 225.844
Max: 844		 28
2010 Fongshan Stream, Taiwan China Mean:334.472 28
2010 Fongshan Stream, Taiwan China n.d. - 716.517 28
2010 Chengde, Hebei China 4. 88 ± 4. 67 29
2010 Chongqing China < 0.05 29
2010 Qinghai Lake China < 0.05 29
2011 Qushui, Tibetan China 0. 89 ± 0. 39 29
2011 Changdao, Shandong China 0. 32 ± 0. 32 29
2011 Wuyi Mountain China 0. 11 ± 0. 11 29
2011 Shennongjia China 1. 23 ± 0. 80 29
2011 Qingyuan, Liaoning China < 0.05 29
2011 Greater Khingan Mountains China < 0.05 29
2016 Shanghai China 0.06 26
2017 Jiangsu China < 2.23 ng/m3 22
2018 Chongqing China < 2.23 ng/m3 22
2018 Barcelona Spain 0.21 23
2018 PCE plant
(Acetylene method)		 China 1170 30
2018 PCE plant (Carbon
tetrachloride method)		 China 5530 30
2018 PCE plant (Downwind) China 305 30
2017~2018 Mountain Tai China 0.33 27
2017~2018 Jinan, Shandong China 0.36 27
2017~2018 Taian, Shandong China 0.38 27

2.2 Hexachlorobutadiene in aquatic environment

Wastewater discharge from sewage treatment plants is one of the main sources of HCBD in the water environment. In addition, HCBD in sediments and soils pollutes the water body[31⇓~33]. At the same time, some sudden chemical leaks will also release a large amount of HCBD into the groundwater[34]. For example, in the late 1960s, chlorinated organic compounds leaked in France, and the by-product HCBD was released into the surrounding groundwater and soil, causing pollution[35]. The available data show that HCBD in water bodies mainly exists in surface water, but some groundwater is also polluted by HCBD. Table 3 summarizes the results of the investigation on the concentration of HCBD in the water environment in the past 15 years[36].
表3 HCBD在不同水体中的赋存水平

Table 3 Concentrations of hexachlorobutadiene in different water bodies

Time Location Type Concentration
(μg/L)		 ref
2002 Greece Industrial wastewater n.d. - 0.70 47
2010 Basel, Switzerland surface water < 0.05 48
2010 Spain Industrial wastewater 0.0083~0.11 49
2011 China surface water
and groundwater		 0.08~0.37 38
2011 Saudi Arabia surface water 0.46~0.81 39
2011 Southern Poland Landfill leachate 0.008~0.064 50
2012 Huai River, China surface water 0.93
RQa: 7.75		 37
2012 Dongxiang River
Basin of China		 groundwater n.d. - 349.02 47
2013 China’s five
major river basins		 surface water
and groundwater		 0.10~ 1.23
Mean:0.61		 37
2013 Korea surface water 0.029~0.067 40
2013 Yellow River,China surface water 1.23
RQmax: 10.3
RQmean: 5.29		 37
2013 Liaohe River, China surface water 0.76
RQmax: 6.33
RQmean: 5.33		 37
2016 Gran Canaria, Spain surface water 0.8 ×10-3 41
2016 Portugal sediment n.d. - 11.1 46
2016 Zhejiang,China surface water 0.4 51
2017 Dianshan Lake, Shanghai surface water 0.109 33

a RQ(risk quotients): 风险熵值

In 2013, 48 drinking water sources from the Yangtze River, Huaihe River, Yellow River, Haihe River and Liaohe River in five major river basins in China were studied, and HCBD was detected in the samples from the Huaihe River, Yellow River and Liaohe River, with concentrations ranging from 0. 10 to 1. 23 μg/L, exceeding the standard limit of 0. 6 μg/L, which may pose a potential ecological risk[37]. Compared with 2011, the concentration of HCBD in these waters is gradually increasing[38]. The concentration of HCBD in drinking water sources in China is similar to that in Saudi Arabia, but higher than that in South Korea and Spain[39][40][41]. The concentration of HCBD in surface water is often higher than that in groundwater.
Human activities may have a certain impact on the content of HCBD in natural water, and high concentrations of HCBD have been detected in wastewater from many sewage treatment plants at home and abroad. The concentration of HCBD in groundwater around an abandoned factory in Guangdong Province was as high as 91. 4 mg/L[42]. The average concentration of HCBD at the intakes of wastewater treatment plants in Jiangsu, Anhui, Shanghai and Zhejiang provinces in the Yangtze River Delta region ranged from 2. 27 to 4. 56 μg/L, which was higher than that in the Pearl River Delta region (0. 28 to 3. 42 μg/L) and the Beijing-Tianjin-Hebei region (1. 03 to 1. 4 μg/L), and much higher than the standard limit of 0. 6 μg/L for HCBD in natural water bodies. There are different degrees of HCBD pollution in the water bodies of Beijing-Tianjin-Hebei, Yangtze River Delta and Pearl River Delta, among which the average concentration of HCBD in the natural water bodies of the Pearl River Delta is the highest, which is 0. 64 μg/L, exceeding the standard concentration limit[43]. The concentration of HCBD in the Dongjiang River Basin of Guangdong Province was as high as 1. 04 μg/L, while HCBD was detected in the rest of the Yangtze River Delta and Beijing-Tianjin-Hebei, but did not exceed the limit.
Polar glaciers are located in remote areas, far away from human activities, and often represent the background areas for global POPs monitoring. Studies have shown that atmospheric deposition and snowfall may affect the concentration of HCBD in mountain lakes[44][45]. [33]. HCBDs may be imported into lake sediments through the watershed, and were detected in sediments from two tributaries of the Obidos Lagoon, Portugal[46]. HCBD was also detected in the Antarctic ice core with a variable flux of 0.45~2.81 pg·cm-2·yr-1[16]. This indicates that HCBD released unintentionally by human activities will accumulate in polar regions through long-distance migration.

2.3 HCBD in soil

Soil is an important sink of various organic pollutants. Atmospheric transport, atmospheric deposition and sewage irrigation may be the sources of HCBD in soil, and the concentration of HCBD in soil is related to economic activities[52,53]. There are also differences in the content of HCBD in soils with different uses. At present, most studies have focused on the soils around factories, while there is little information about the occurrence of HCBD in agricultural soils[54,55]. It has been found that organic pollutants, including those in agricultural soils, can be absorbed into crops by plants, posing a potential risk to human health[56,57]. The concentrations of HCBD in agricultural fields in Shaoxing of Zhejiang, Changzhou of Jiangsu, and the border of Shanghai and Zhejiang ranged from 0. 07 to 8. 47 ng/G, with an average of 0. 32 ng/G and a median of 0. 196 ng/G[58].
The pollution level of HCBD in most municipal sludges in China is generally not high. The concentration of HCBD in the sludge from a sewage treatment plant in Shanghai was the highest (74.3 ng/G), followed by two samples from Ningbo, Zhejiang Province, with concentrations of 7.41 and 10.7 ng/G dw, respectively[43]. This indicates that the discharge of wastewater may have little effect on the content of HCBD in the soil. The study also found that the content of HCBD in the industrial soil in the region with rapid economic development was significantly higher than that in the farmland soil, and the content was 9. 3 ~ 24.6 ng/G and 0. 13 ~ 2.67 ng/G, respectively[43]. This illustrates that agricultural activities and unintentional emissions from surrounding factories may be the main source of HCBD in soil. HCBD was detected in soils from an agricultural area in eastern China and a former pesticide factory in southwestern China, as well as in soils near a sewage treatment plant, with concentrations ranging from 0.02 to 5.59 ng/G dw[8,54][55]. HCBD is relatively volatile and easily enters the atmosphere from the soil over time[59].

2.4 HCBD levels in organisms

HCBD is bioaccumulative and has been detected in aquatic organisms, terrestrial organisms and plants[6]. In the study of wild fish, it was found that the content of HCBD in fish muscle, liver and brain was low, with the average concentration of 0.06 ng/G, 1.29 ng/G and 0.59 ng/G, respectively, which may be due to the fact that HCBD is easily biotransformed into polar metabolites[60]. HCBD can be detected in most fish, crustacean samples and all shellfish samples in the Pearl River Estuary, and the wet weight content of HCBD in fish samples (n. D. -0. 012 ng/G ww) is far below the environmental quality standard of HCBD in fish recommended by the European Union (55 ng/G ww). The existing data of Arctic biota show that the concentration of HCBD in terrestrial birds and mammals is relatively higher than that in fish and marine mammals, and the concentration in fish ranges from 0.17 to 0.64 ng/G lw, which also proves its long-distance migration. Table 4 shows the levels of HCBD in organisms in different regions.
表4 HCBD在不同地区生物体内的水平(μg·kg-1·lw)

Table 4 Concentrations of hexachlorobutadiene detected in organisms in different areas

Species Concentration Location Type Time ref
Herbivorous insects 1.3~8.2 Eastern China Terrestrial life 2014 15
Earthworm 1.3~8.2 Eastern China Terrestrial life 2014 8
Chinese toad 1.3~8.2 Eastern China Terrestrial life 2014 8
Insects and birds 1.65~3.80 Southwest China Terrestrial life 2015 8
Knotweed 0.03~24.6 Southwest China Plant 2015 54
Fish 0.6×10-4~
1.29×10-3		 South China Aquatic life 2021 60
Fish 0.17~0.64 ng·
g-1·lw		 North pole Aquatic life 2012 61
Fish n.d. - 0.012
ng·g-1·ww		 Pearl River Estuary, China Aquatic life 2012 2
Fish 112.8~827.3 Mississippi River Aquatic life 1976 62
Fish 2.7±0.59
ng/g ww		 Mexico Aquatic life 2018~2019 63

3 HCBD emission sources in China

Human production and industrial activities are the main sources of HCBD in the environment[64]. In the 1880s, HCBD was produced in large quantities as chlorinated solvents, herbicides, and pesticides[65][66][67]. However, with the awareness of its ecological hazards and the signing of the Stockholm Convention, most countries in the world have stopped the production and use of HCBD. Nevertheless, the emission of HCBD still exists, and the main emission sources of unintentionally produced HCBD include chemical production sources, such as chlorine-containing chemicals production and cement production; Metallurgical sources, such as the smelting of non-ferrous metals including magnesium[68]; There are also other emission sources, mainly including waste incineration sources[69]. In addition, HCBD emissions also exist in the landfill process[50].
The estimation of unintentional emission level of HCBD in China from 1992 to 2016 shows that the annual production of HCBD increased from 60.8 tons/year in 1992 to 2871.5 tons/year in 2016, with an average annual growth rate of 17.4%[70]. During the period from 1992 to 2003, the main emission source was the production process of chlorinated hydrocarbons. Since 2004, the main emission sources have been the reuse of HCBD and the production of chlorinated hydrocarbons. With the development of Trichloroethylene (TCE) and Perchloroethylene (PCE), the emission of HCBD may continue to increase.
Many evidences show that industrial production can also cause unintentional emission of HCBD. Industrial wastes, including waste water, waste gas and solid waste, may contain HCBD. HCBD is the highest VOC component in the chlorination residue and waste residue of trichloroethylene plants, accounting for 23% and 12% of the total, respectively[71]. HCBD was detected in different sections of trichloroethylene, tetrachloroethylene and chlorobenzene production processes, as well as in soils in and around chlorinated chemicals factories.The soil in the plant area contains a high concentration of HCBD (27.9 ng/G dw), and the concentration of pollutants in the soil decreases rapidly with the increase of the distance from the plant, so it can be inferred that the chemical plant is the main source of HCBD nearby[36]. Meanwhile, some studies have also detected HCBD in fly ash (0.25 ng/G dw) and flue gas samples (8.2 ng/m3N) from industrial waste incineration[72].

3.1 Unintentional production of chemical production sources

Unintentional emissions from chemical production processes are the main source of HCBD. Chemical plants producing trichloroethylene, tetrachloroethylene, and carbon tetrachloride (CTC) are typical sources of HCBD[36].
In the study of typical chemical plants, it was found that HCBD was more likely to be produced during the chlorination process, and the concentration of HCBD inadvertently produced in trichloroethylene and tetrachloroethylene plants was much higher than that in other plants[27]. C2/C4 olefins and ethylene may be important intermediates, and three possible paths for HCBD formation during chloride production are shown in Figure 2[36].
图2 HCBD的生成路径[36]

Fig.2 Three formation pathways of hexachlorobutadiene from several hydrocarbons synthesized from the results of researches by Heindl and Hutzinge[73], Sherry et al.[74], Tirey et al.[75], Wehrmeier et al.[76], summarized by Zhang[36].

In 1982, the United Nations Environment Programme reported that unintentional emissions from the production of chlorinated chemicals in the United States were 14,000 tons higher than artificially produced HCBD[67]. In 2008, 60% of HCBD discharged into water in Europe came from chlorinated hydrocarbon production[77,78]. In 2017, 99.8% of HCBD was unintentionally produced by organic chemicals manufacturing in the United States[79]. In recent years, with the awareness of its hazards, the unintentional emissions of HCBD in European and American countries have shown a downward trend, and the unintentional emissions of HCBD in the United States decreased from 50 tons to 1.064 tons from 1975 to 2011[80]. However, unintentional emissions from the production of chlorinated hydrocarbons still occur.
The amount of HCBD produced in the production process of chlorinated hydrocarbons in China can not be ignored. From 1992 to 2003, CTC production industry was the largest source of HCBD, accounting for 50.5% to 70.4% of the total production[70]. Due to the implementation of the Montreal Protocol, CTC has been produced as a by-product of methyl chloride in 2016, and the unintentional production of HCBD in this industry has declined rapidly, accounting for about 0.4% of the total unintentional production. With the rapid increase in the production of TCE and PCE, the two industries unintentionally produce more than 90% of HCBD. Studies on HCBD concentrations and emissions in the atmosphere of northern China also suggest that chloride production is an important source of HCBD emissions[27].
Before 2000, waste gas and wastewater containing HCBD were classified as ordinary waste, and it was difficult to remove HCBD by traditional waste disposal methods, and there was no relevant emission standard to restrict HCBD in China[67]. Therefore, a large amount of HCBD is discharged into the environment along with the waste. Tao Yuming and others studied the distribution characteristics of HCBD in the Yangtze River Delta, Beijing-Tianjin-Hebei and Pearl River Delta, and estimated the total amount of HCBD emitted in the industrial production process of the three regions in 2018[43]. The total emission of HCBD in the Yangtze River Delta region is 497.8 tons, which is much higher than that in the Beijing-Tianjin-Hebei region (0.370 tons) and the Pearl River Delta region (0.296 tons). This is mainly because the factories with an annual output of more than 10,000 tons of TCE and PCE in the three regions are mainly concentrated in the Yangtze River Delta region. TCE production is the industrial activity that emits the most HCBD, releasing 66.9% of HCBD. The industrial emission of HCBD in the Beijing-Tianjin-Hebei region and the Pearl River Delta region is almost entirely from the production of TCE. As a downstream product of the chlor-alkali industry, TCE has developed rapidly in China, and its output has increased year by year.
Magnesium production is also a source of HCBD emissions, but the level of unintentional production of HCBD varies greatly based on different production processes. According to the research of Van der Gon et al., a total of 2.53 tons of HCBD were emitted in 11 countries in Europe in 2000, of which the manufacture of magnesium accounted for 97%[68]. However, in China, the production of magnesium mainly uses electrolysis, and its proportion of unintentional emissions is small, and its HCBD emissions account for about 0.02% ~ 0.6% of the total emissions[70]. In 2001, Deutscher et al. Determined the organochlorine content in the process of magnesium production by electrolysis by gas chromatography. As the main aliphatic compound, HCBD accounts for about 0.0005% ~ 0.002% of the organochlorine waste gas[81]. Since 2002, the production of magnesium by Pijiang method has been popularized in China, and the utilization rate of magnesium by electrolysis method has gradually decreased, and its utilization rate has dropped to 5% in 2006[82][81].
Part of HCBD can be purified from by-products and used in the production of plastic materials, rubber and paint. HCBD emissions after reuse are relatively low, with emission factors of 16%, 18% and 59%, respectively. In a 2007 EPA report, only 0.2% of HCBD emissions came from plastic materials and rubber manufacturing[79]. HCBD emissions from these reuses increased from 29.5 tons/year in 2004 to 397.4 tons/year in 2012. However, after 2012, with the addition of HCBD to Annex C (unintentional production) of the Stockholm Convention, emissions continued to decline.
According to the information submitted to the Toxic Release Inventory (TRI) Program of the United States in 2011, cement manufacturing also emits a certain amount of HCBD into the atmosphere, accounting for 10.73% of the total emissions, but its mechanism and emission influencing factors need to be studied[80]. In New Zealand's Hazardous Substances Regulations, other sources of HCBD are proposed, such as cement kilns burning hazardous waste, secondary copper production, sintering plants in the iron and steel industry, secondary aluminum production, and secondary zinc production, but there is no relevant research at home and abroad.

3.2 Waste incineration and landfill sources

A certain amount of HCBD will also be produced in the process of waste disposal. Lenoir et al. Simulated the combustion process of acetylene and found that HCBD was more easily generated under the influence of CuCl2 catalyst, and the reaction mechanism is shown in Fig. 3[69]. Acetylene is present in a large number of incineration processes, which indicates that HCBD may be released during waste incineration[36]. In the United States, 42% of HCBD-containing waste is incinerated on site, and 5.3% is disposed of in landfills or closed dumps[79]. HCBD in China, as a by-product of its industrial production, can be divided into four categories: the first category is treated as a common industrial waste, the second category is purified from mixed by-products, the third category is recovered by fractionation, and the fourth category is incinerated at high temperature[70]. Therefore, in addition to direct release during disposal, HCBD may also be released from landfills and sewage treatment plants. At the same time, HCBD may also exist in other wastes. For example, in 2015, Lysychenko et al. Detected HCBD in Hexachlorobenzene (HCB) waste, accounting for about 20% ~ 27%[83].
图3 乙炔燃烧过程中HCBD的生成机理[69]

Fig.3 The formation mechanism of HCBD during acetylene combustion

Waste incineration also releases HCBD. The EPA reported that the amount of HCBD released from incineration in the United States in 2007 was 309 tons, accounting for 41.5% of the total[84]. The concentration of HCBD in the flue gas increases with the increase of polyvinyl chloride or polyester fiber in the waste[69]. Many experiments abroad have shown that HCBD will be released in the process of waste incineration. Lahaniatis et al. Detected HCBD in incineration gases of chlorinated organic solvents and PVC[85]. Matejczyk et al. Also detected HCBD in leachate from 22 landfills in Poland, with a detection rate of 45%, and suggested that landfills would pollute soil and surface water[50]. HCBD was detected in fly ash from five MSW incineration plants, with concentrations ranging from (1.39 ± 0.10) to (97.6 ± 7.51) ng/G, indicating that MSW incineration is one of the unintentional sources of HCBD emissions in China[86].

4 Degradation of hexachlorobutadiene

The effects of free radicals such as O3, Cl, NO3,HO2 and OH on the degradation of HCBD have been studied by density functional theory. The free radicals attack the C = C bond of HCBD and produce similar intermediates. The intermediates are further oxidized by O2 and NO2 to form the final products, which achieve the effect of decomposing HCBD[87]. In 2005, Environment Canada reported the reaction of HCBD with hydroxyl radicals in the atmospheric environment, and the estimated half-life of the degradation reaction ranged from 60 days to 3 years at hydroxyl radical concentrations of 7×105 molecules /cm3 and 17×105 molecules /cm3, respectively. At 298.15 K,The reaction rate constants of Cl, NO3, HO2, OH, and O3 with HCBD are 4.51×10-13,1.32×10-20, 4.33×10-29, 6.33×10-16 and 5.80×10-27cm3·mol-1·s-1. These reaction rates indicate that the oxidation of HCBD with OH and Cl radicals is more active than that with NO3, HO2 and O3, and the results of measured and simulated concentrations also support this theory[27].
There are few studies on the degradation of HCBD in water environment. HCBD in water can be treated by ultraviolet irradiation, oxidation and adsorption, and the most studied is the removal of HCBD by ozone. Ozonation was first proposed by Derco et al. In 2013, and the feasibility of ozonation to treat five organochlorine pesticides, including HCBD, pentachlorobenzene and hexachlorobenzene, was verified[88]. The results showed that 90.7% of HCBD could be removed within 60 s when the ozone dosage was 317 mg·L-1. The second-order rate constant for the reaction of ozone with HCBD is 6.2 mg·m-3·h-1. Ozone treatment in combination with UV irradiation, hydrogen peroxide, or with iron or copper salts as catalysts can greatly improve the performance of HCBD removal processes. In 2017, Simkovic et al. Verified the feasibility of removing HCBD by the combination of ozonation process and iron nanoparticles (nZVI-nanozero-valentiron), and the removal rate of HCBD by the H2O2/O3 advanced oxidation process was greater than 30% when the concentration of O3 was 0.5 mg·L-1 and the concentration of H2O2 was 0.25 mg·L-1[89]. However, some studies have reported that the removal rate of HCBD by ozone is not high, and the reason is not clear[88].

5 Control of HCBD at home and abroad

HCBD is regulated by a number of international treaties, dating back to 2009. HCBD is listed in the Convention on Long-range Transboundary Air Pollution, and its production and use are prohibited. In 2011, HCBD was proposed to be added to the Stockholm Convention's candidate list of persistent organic pollutants[5]. After reviewing the characteristics, risk characteristics and risk management assessment of POPs, the Stockholm Convention listed them in Annex A (elimination category) and Annex C (unintentional production category) in 2015 and 2017, respectively, to reduce their production and release[17]. Because of the potential harm of HCBD to human health and environmental organisms, many countries and organizations have formulated relevant laws and regulations. In 1993, the World Health Organization proposed to limit the content of HCBD in the Guidelines for Drinking Water Quality, and the WHO recommended value is 0.0006 mg/L, which is an important reference for water safety regulations and standards in 104 countries or regions. In 1999, the World Agency for Research on Cancer (IARC) classified HCBD as a Group 3 suspected carcinogen. Fig. 4 shows the time axis related to international control of HCBD.
图4 HCBD国际管控相关事件的时间轴

Fig.4 Timeline of international control process of hexachlorobutadiene

As one of the signatories of the Stockholm Convention, China has begun to control HCBD. Issue national standards at the end of some industrial activities to limit their HCBD emissions. In 2020, the List of Priority Controlled Chemicals was promulgated, in which HCBD was listed as a priority controlled chemical. The Action Plan for the Control of New Pollutants issued on May 24, 2022 proposes that: (1) the production, use, import and export of HCBD shall be prohibited; (2) Strictly implement the HCBD emission standards such as the Emission Standard of Pollutants for Petrochemical Industry (GB 31571) to meet the emission standards; (3) Waste HCBD shall be subject to environmental management as hazardous waste; (4) According to the Measures for Soil Environmental Management of Industrial and Mining Land (Trial), key facilities shall install anti-corrosion and anti-leakage facilities and leakage monitoring devices for facilities involving HCBD to prevent HCBD from polluting soil and groundwater. Table 5 shows the domestic control and determination standards of HCBD.
表5 HCBD的国内管控和测试标准

Table 5 Management and control of hexachlorobutadiene in China

Year Standard name Limits Of HCBD
1997 Water quality-Determination of volatile halogenated organic compounds-Headspace gas (GB/T 17130 - 1997) The minimum detection limit is 0.00002 mg/L[90]
2002 Environmental quality standards for surface water (GB 3838 - 2002) 0.0006 mg/L[91]
2007 Standard of Soil Quality Assessment for Exhibition Sites (HJ 350 - 2007) 1 μg/g
2007 Standards for drinking water quality (GB5749 - 2006) 0.0006 mg/L[92]
2014 Water quality-Determination of volatile organic compounds Purge and trap/gas chromatography The minimum detection limit is 0.1 mg/L[93]
2015 Emission standard of pollutants for petroleum chemistry industry (GB 31571 - 2015) 0.006 mg/L[94]
However, on the whole, the environmental management of HCBD and other chemicals in China started late, and the relevant measures still need to be improved. On the one hand, there is still much room for improvement in source management. HCBD may be unintentionally produced and released in thermal processes such as chloride production, waste incineration, non-ferrous metal smelting and coal-fired power generation, while China's industry emission standards are limited. On the other hand, the basic support for scientific research technology and management of HCBD is relatively weak, the occurrence of HCBD in different media is not clear, and the ability of environmental risk prevention and control needs to be further improved.

6 HCBD and other POPs interrelationship and co-reduction

HCBD may have similar sources or formation conditions with other persistent organic pollutants (POPs) in different industrial activities, which leads to significant correlations between HCBD and other POPs concentrations in some environmental media. However, due to the obvious differences in structure, their formation mechanisms must also have significant differences.

6.1 Coordinated Emission Reduction

It has been found that there is a correlation between the concentration of HCBD and pentachlorobenzene produced in the chemical synthesis process, and the Pearson correlation coefficient (R2) is 0.717, and the two-tailed test value is below 0.01[95]. In the preparation of tetrachloroethylene, the intermediate trichloroethylene can be converted into dichloroacetylene by chlorination or hydrogen chloride, and HCBD and hexachlorobenzene can be produced in the presence of metal catalysts such as CuCl2[73,74]. PCDD/Fs are finally formed by further chlorination of HCB. Relatively high concentrations of unintentionally produced POPs such as HCBD, PeCB, HCB and PCDD/Fs were detected in the CCl4 of by-products from the production of methyl chloride by methanol method[96]. In the study of the Yangtze River Delta region, there is a significant positive correlation between HCBD and the concentration of chlorine-containing pesticides in the environment, especially DDT, and there is a significant positive correlation between HCBD and the concentration of hexachlorobenzene, but there is no relevant literature to prove the correlation in the formation pathway[58].

6.2 Emission reduction measures

Current environmental HCBD sources include three parts: intentionally produced HCBD, unintentionally produced HCBD, and residues from historical stocks. There are different emission reduction or treatment measures for HCBD from different sources.
HCBD has been produced in large quantities in some countries in history. Since the 1980s, the residues produced by organochlorine solvents containing HCBD have usually been destroyed by high temperature incineration. However, before that, it has been reported that some chemical wastes containing HCBD may have been landfilled or stored and released unconsciously into the surrounding environment after a period of time[59,97,98]. Matejczyk et al. Found that HCBDs were detected in leachate from landfills in Poland, and these HCBDs may be released into the groundwater environment due to poor design or corrosion of the impermeable layer, and eventually enter organisms[50]. Measures to control HCBDs from historical stockpiles include establishing an inventory of relevant landfills and registering and investigating HCBDs and related pollutants at waste disposal sites.
At present, the emission reduction measures of HCBD are not mature, and the production mechanism of HCBD in the heat treatment process is different from that of dioxin-like substances, so it is difficult to directly learn from them. Emission reduction measures mainly start with reducing the amount of pollutants generated, and then take effective end emission reduction measures for the pollutants that have been generated.
Previous studies have shown that the main factors affecting the formation of persistent organic pollutants in industrial production processes are the composition of raw materials, the content of chlorine sources, reaction temperature, catalysts, precursors and so on[99⇓~101]. One of the possible synthesis paths of HCBD in industrial thermal process is through precursor synthesis, that is, chlorine-containing organic precursors are produced at 150-400 ℃, or chlorine-free organic precursors react with chlorine sources at 600-900 ℃ to produce HCBD. Another reference to the formation of dioxin may be through "de novo synthesis", that is, substances containing chlorine, hydrogen and oxygen are first adsorbed on the surface of fly ash particles and reacted under the action of catalysts[102]. Therefore, the reaction temperature should be strictly controlled in each section of industrial production to reduce the source of HCBD. In other thermal processes such as electric arc furnace steelmaking, reduce the input of chlorine sources, such as not using waste plastics and tires as carbon sources for steelmaking, and strictly limit the total amount of chlorine sources entering the incinerator. Replace the catalyst in the process to avoid the formation of organic pollutants accelerated by copper-containing catalyst. Do not use flue gas to preheat scrap steel, for example, China's "Technical Policy for Pollution Prevention and Control of Iron and Steel Industry" clearly States that "waste plastics and tires are not encouraged to be used as carbon sources for electric furnace steelmaking", all of which are to reduce the production of persistent organic pollutants such as HCBD from the source.
For the pollutants that have been generated, there are also many emission reduction technologies, such as efficient dust removal technology, physical adsorption technology and so on. The high efficiency dust removal technology can adsorb the persistent organic compounds discharged at the end, and the dust removal efficiency will increase with the decrease of the flue gas temperature at the inlet of the dust remover. Physical adsorption technology is to use injected adsorbent to physically adsorb organic pollutants. In addition, catalytic decomposition technology and Gore Remedia catalytic filtration technology can effectively reduce the emission of POPs at the end of industrial production[103]. On-line monitoring of sewage outlets can also better quantify and monitor organic pollutants such as HCBD, and reduce their unintentional emissions.

7 Conclusion and prospect

At present, through the study of the distribution of HCBD in different media and industrial emission sources in China, it is found that HCBD is widely detected in the environment, and its concentration is generally on the rise. The degree of its pollution is related to industrial activities, and the level of HCBD in industrial areas is higher. Even though some studies have pointed out that HCBD may have health risks in some sampling areas, there is no significant harm to human body and ecological environment in general. At present, there are few studies on the chemical mechanism of HCBD in the atmosphere, and there are few studies on its transport, transformation and degradation products in the atmosphere.
Although the production of HCBD has been discontinued in most countries, the release of unintentional HCBD cannot be ignored. At present, the research on the potential emission sources of HCBD mainly focuses on the production process of chlorinated hydrocarbons, while the research on the emission of other thermal processes is still rare, and the identification of the potential emission sources of HCBD still needs to be strengthened. The basic research on the production process and formation mechanism of HCBD unintentional emission industry in chemical industry is not in-depth.
In addition, we still need to focus on the health risks of people with occupational exposure. Strengthen the identification of human exposure risk of HCBD. At present, the health effects of HCBD in China are mainly concentrated in specific groups such as occupational workers, and it is suggested to strengthen the research on the exposure assessment model and risk assessment of HCBD. The population exposure and possible health effects were predicted to provide a reference for HCBD emission reduction.
The identification and quantification of different sources and environmental releases of HCBD are conducive to the formulation of effective control strategies, the assessment of possible environmental impacts, and the provision of a basis for the subsequent reduction of industrial emissions. At present, it is urgent to establish an emission inventory of HCBD-related industrial sources, explore HCBD control and disposal technologies, and improve the governance system.

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