1. Introduction

2. HBCD Distribution in Air

3. HBCD in Soil

3.1. Distribution and Levels of HBCD

Soil is a sink for HBCD, and there are many studies on HBCD in soil. Research from recent studies is collated in Fig. 1 to illustrate the distribution of HBCD in soil in China. Most studies on HBCD in soil in China have been concentrated in the northeast coastal areas, and after that, the Pearl River Delta region. This may be due to the fact that the HBCD-related products in China are mainly produced on the coastal areas. HBCD showed regional distribution in soil, with varying concentrations from 0.88 to 23,200 ng/g across different areas (Covaci et al., 2006; Zhu et al., 2017b; Cao et al., 2018). Spatially distributed, HBCD concentrations in soil decreased from southeast to north China, except for Yingkou. For instance, HBCD concentration is higher in Jiangmen (7.6 ng/g) (Lu et al., 2019) compared to Weihai (0.449 ng/g) (Zhang et al., 2016).

Figure 1. Distribution of HBCD concentration in soil in China.

Figure 1. Distribution of HBCD concentration in soil in China.

HBCD in soil mainly comes from the local discharge of industrial wastewater and waste and/or deposition of atmospheric pollution from HBCD-related industrial processes. Generally, high concentrations of HBCD were found in soils near HBCD manufacturing plants, followed by soils near plants producing HBCD-related products (Fig. 2) (Li et al., 2012; Zhu et al., 2017b; Cao et al., 2018; Zhang et al., 2018). For example, the highest HBCD concentration measured in China (11,700 ng/g) was found in soil near one of the biggest HBCD manufacturing facilities in China (Zhang et al., 2018). Similarly, HBCD at concentrations up to 6901 ng/g was detected in soil near HBCD manufacturing facilities in the Laizhou Bay, East China(Li et al., 2012). However, the concentrations of HBCD detected in soil near the HBCD-related production and processing plants in Tianjin (1730 ng/g) and Weifang (560.4 ng/g) were lower compared to the above-mentioned areas (Yi et al., 2016; Zhu et al., 2017b). Additionally, HBCD-related product wastes, including plastic waste, e-waste and textiles, are also sources of HBCD pollution in the environment. For example, high concentrations of HBCD in soil samples near plastic waste treatment site (11.0-624 ng/g) were detected in Dingzhou and Xinle (Huang et al., 2017a). Recycled plastic waste consisted mainly of EPX and XPS. E-waste and textile also contributed to increased HBCD concentrations in the environment, although the amount of HBCD used in textiles and electrical/electronic equipment was relatively small. For example, HBCD concentrations in soils near the e-waste recycling areas in Guangzhou were at moderate levels (0.38-284 ng/g), which was consistent with the level of HBCD detected in soil in Ningbo (below detection limits-102.6 ng/g), where the textile and electronic equipment industries are well developed. Above two areas had much higher HBCD concentrations in urban soils in Guangzhou (1.7-5.6 ng/g) and in soil in Chongming Island (the Yangtze River Delta) (not detected-93.8 pg/g) without industrial activities (Yu et al., 2008; Gao et al., 2011; Meng et al., 2011a; Tang et al., 2014).

Figure 2. Different sources of HBCD in the surface soil.

Figure 2. Different sources of HBCD in the surface soil.

Notable variations in HBCD concentrations exist across various soil types. For example, a study conducted in Ningbo analyzed six types of soil and revealed that significantly higher concentrations of HBCD were observed in waste dumping sites (mean: 67.4 ng/g) and industrial areas (mean: 37.9 ng/g), followed by traffic areas (mean: 31.8 ng/g), residential areas (mean: 14.1 ng/g), vegetable soils (mean: 11.0 ng/g) and farmland soils (mean: 7.75 ng/g) (Tang et al., 2014). Similarly, in Dingzhou and Xinle, the highest HBCD concentrations were observed in disposal/recycling sites (11–624 ng/g), followed by roadside soil (2.96–5.4 ng/g), and farmland soil (8.69–55.5 ng/g) (Huang et al., 2017a). However, only slight difference of HBCD concentration was observed in the different soil types in Chongming Island, where there is no industrial activity. The average concentration of HBCD in tideland soils (38.9 pg/g) was comparable to that in road soils (34.1 pg/g) and woodland soils (27.8 pg/g) and was only 2 times higher than in farmland soils (15.3 pg/g) and grassland soils (15.1 pg/g), respectively (Meng et al., 2011b). In addition, HBCD concentrations in soil clearly showed a decreasing trend with increasing distance (Remberger et al., 2004; Gao et al., 2011; Li et al., 2012; Yi et al., 2016; Zhang et al., 2016). It was also found that the concentration of HBCD decreased dramatically with distance up to a certain distance, and decreased more slowly beyond that distance. For example, at the biggest HBCD manufacturing plant in China, HBCD concentrations decreased sharply by 11,600 ng/g over a distance of 2 km, while for the next 4.1 km, the decrease was only 75.2 ng/g(Zhang et al., 2018). At another HBCD production plant in China, an approximate 466-fold decrease in HBCD concentrations in soil was observed within 7 km in two directions(Li et al., 2012). At an e-waste recycling site in Guangzhou. a dramatic decreasing trend of HBCD in the soil from 5.82 ng/g to 0.01 ng/g was observed in a range of 30 km (Gao et al., 2011).

3.2. Diastereoisomer Profiles in Surface Soils

γ-HBCD usually shared the major proportion to the total HBCD in the soil, while certain cases revealed a relatively high proportion of α-HBCD (Fig. 3). The composition of HBCD diastereoisomer in soils was strongly influenced by industrial emissions. For example, the diastereoisomer composition of HBCD in soils from the HBCD production facility area in China was found to be consistent with commercial HBCD products. Commercial HBCD consist mainly of 70–89% γ-HBCD and 10-13% α-HBCD and 1-12% β-HBCD. The proportions to the total HBCD of γ-HBCD, α-HBCD and β-HBCD were 76±10%, 16 ± 8% and 8 ± 6% in the soil near the largest HBCD manufacturer in China (Zhang et al., 2018). γ-HBCD was predominant (averaged 73.9%) in the brominated flame retardants production area in Shouguang, followed by α-HBCD (averaged 16.2%) and β-HBCD (averaged 9.9%) (Zhang et al., 2016). Similar patterns were also observed in soil samples collected from Laizhou Bay and Weifang (Li et al., 2012; Yi et al., 2016). And in Guiyu, α-HBCD was the dominant isomer in soils from e-waste recycling areas, accounting for 56%–57% (Gao et al., 2011), which is similar to the profiles reported for soils near plastics industrial parks in Cangzhou (α-HBCD: 42.8%-81.1%) (Zhang et al., 2016). The isomer pattern in HBCD-containing products and environment media can be changed by industrial processes, particularly extrusion molding and thermal cutting, before emission because of differences in the physical properties of the three isomers (Gao et al., 2011; Zhang et al., 2016; Zhang et al., 2018). Lower γ-HBCD proportion (55.5%) was found in polystyrene hard plastics consumer products (general purpose polystyrene, high impact polystyrene; etc.) with more industrial processing than EPS (Rani et al., 2014). And γ-HBCD was the predominant component of commercial technical HBCD products in EPS raw foam, but α-HBCD was found to be the main congener in particles emitted from thermal cutting of EPS. However, the diastereoisomer composition of HBCD in soils differed between point source and non-point source sites. For example, in Shouguang, the HBCD diastereoisomer composition in soils in point source sites (65.3% γ-HBCD, 23.2% β-HBCD and 11.5% α-HBCD) showed significant differences from that in non-point source sites (56.4% γ-HBCD, 23.8% α-HBCD and 19.7% β-HBCD) (p<0.028) (Zhu et al., 2014).

Figure 3. HBCD stereoisomer in soil in China (ASERS: Areas surrounding the e-waste recycling sites; EWRS: e-waste recycling sites).

Figure 3. HBCD stereoisomer in soil in China (ASERS: Areas surrounding the e-waste recycling sites; EWRS: e-waste recycling sites).

4. HBCD in Water, Sludge, and Sediment

4.2. HBCD in Sludge of Wastewater Treatment Plants

Due to its strong hydrophobicity (logK_ow of 5.4–5.8), HBCD has a propensity to adsorb onto suspended particulate matter and accumulate in sludge during wastewater treatment (Byrns, 2001; Marvin et al., 2011). To date, there have been few studies on HBCD in sludge of wastewater treatment plants in China. Two publications have reported on HBCD concentrations in sludge in Shanghai (0.1–37.2 ng/g) (Xiang et al., 2015) and the Pearl Delta River (112–136 ng/g) (Feng et al., 2012).

Figure 4. Concentration of HBCD (a) and corresponding isomers (b) in sludge from sewage treatment plants in Shanghai.

Figure 4. Concentration of HBCD (a) and corresponding isomers (b) in sludge from sewage treatment plants in Shanghai.

A comparison with the 2001 national sewage sludge survey conducted by the U.S. Environmental Protection Agency reveals that sludge from Shanghai exhibits lower concentrations of HBCD (mean: 4.7 ng/g) than the global average (mean: 19.8 ng/g) (Venkatesan and Halden, 2014). In terms of annual production in China, the proportion of HBCD that ends up in sewage sludge is extremely low (< 0.002%) (Xiang et al., 2015). α-HBCD (47.9%) and γ-HBCD (48.0%) are the two main components in sewage sludge in Shanghai (Fig. 4b), which is similar to the composition of sewage sludge in the United States (12.3 ng/g for γ-HBCD, 6.7 ng/g for α-HBCD) (Venkatesan and Halden, 2014), and Spain (375 ng/g for γ-HBCD, 175 ng/g for α-HBCD) (Guerra et al., 2010). Previous studies have demonstrated that γ-HBCD exhibits the highest hydrophobicity among the three HBCD isomers, leading to its accumulation in solid matrices such as soils and sediments (Wu et al., 2010; Gao et al., 2011; Meng et al., 2011b; Li et al., 2013). However, the isomer profiles of HBCD in Chinese sewage sludge have not been thoroughly investigated, necessitating further analysis.

In the Pearl River Delta region of China, HBCD concentrations (112–136 ng/g) in sewage treatment plant sludge are much higher than those in river sediment from the same region (not detected to 31.6 ng/g) (Feng et al., 2012). Similar results have been found in Spain (Gorga et al., 2013) and Turkey (Demirtepe and Imamoglu, 2019). This is mainly because the sludge from sewage treatment plants has a high organic matter content, and organic matter is a strong adsorbent of HBCD (Ganci et al., 2019). Consequently, HBCD remains in the sludge, resulting in a relatively high concentration. Therefore, the concentration of HBCD in sewage treatment plant sludge (13.1–616.2 ng/g) is generally 100–1000 times that in river sediment in the same area (Gorga et al., 2013; Fang et al., 2016; Demirtepe and Imamoglu, 2019).

4.3. HBCD in Sediment

Many studies have shown that because of its low water solubility, HBCD is readily adsorbed by particles in aquatic environments. It then enters sediments through sedimentation and other processes, and sediments are one of the main storage sites of HBCD in the natural environment.

4.3.1. Distribution and Levels of HBCD

Fig. 5 showed the geographical distributions and concentrations of total HBCD in sediments from the Yellow River, Yangtze River, Haihe River, Pearl River Delta, Liaohe River, Hunhe River, Ertix River, Tarim River and some tributaries (e.g., the Dongjiang River, Beijiang River, and Xijiang River).

Figure 5. Geographical distribution and level of HBCD in sediments from several river drainage basins in China.

Figure 5. Geographical distribution and level of HBCD in sediments from several river drainage basins in China.

The HBCD concentrations measured in sediment from the seven major rivers listed above range from not detected to 206 ng/g (Li et al., 2013). Generally, HBCD concentrations in sediment increased from the north to the southeast of China (Fig. 5). Compared with most other regions in the world, except for Europe and the coast of Korea (Ramu et al., 2010), the Yangtze River had very high HBCD concentrations. The average HBCD concentration in sediments from the Yangtze River drainage basin was as high as 206 ng/g, which was much higher than that from the second-most polluted area the Pearl River drainage basin (0.03–31.6 ng/g) (Feng et al., 2012). HBCD concentrations in sediments from the remaining five rivers (Haihe River, Ertix River, Yellow River, Tarim River and Liaohe River), which were all located in north China, were all lower than 0.5 ng/g. The HBCD concentrations decreased from the lower reaches to the upper reaches of the rivers. For example, the HBCD concentration in sediments from the upper reaches of the Hunhe River in northeast China (0.05–1.42 ng/g) is relatively low compared with that from the lower reaches (0.10–25.8 ng/g) (Su et al., 2015). Similar spatial distributions of HBCD have been observed in sediments from the Yangtze River and Pearl River (Li et al., 2013).

For tributaries of these rivers, the total HBCD was at a low level. The highest HBCD concentrations were detected in the Dongjiang River (mean: 6.89 ng/g) (Feng et al., 2012), followed by the Hunhe River (3.74 ng/g) (Su et al., 2015), Zhujiang River (2.85 ng/g) (Feng et al., 2012), Shunde tributaries (1.43 ng/g) (Feng et al., 2012), Beijiang River (0.69 ng/g), Xijiang River (0.44 ng/g), and Dayanhe River (0.21 ng/g). The concentration of HBCD in sediments from Taihu Lake was basically at the same level, ranging from 0.168 to 2.66 ng/g dw in 2009 and 0.046 ng/g dw to 2.56 ng/g dw in 2010 (Xu J, 2013; Wang et al., 2016).

Numerous industrial, commercial, and urban activities can affect the distribution of HBCD. The HBCD concentrations in sediments from industrial or urban areas were significantly higher (p < 0.023) than those from rural or less industrialized areas. For instance, higher HBCD concentrations were found in sediments from the downriver section of the Hunhe River, which was exposed to industrial effluent from Shenyang and Fushun. Similar results have been found in the Pearl River and Yangtze River Delta, which are on the southeast coast of China in regions with high industrialization and high population densities (Li et al., 2013); the Kuzuryu River in Japan, which is severely polluted with effluent from the textile industries containing HBCD (Oh et al., 2014); and Onsan Bay in Korea, which is a heavily industrialized and urbanized region (Ramu et al., 2010). The mean HBCD concentrations in the Beijiang River, Pearl River Delta, Xijiang River, and Dayanhe River were 0.69, 0.50, 0.44, and 0.21 ng/g, respectively (Feng et al., 2012). The HBCD concentration in Laizhou Bay near manufacturing facilities ranged from 2.93 to 1029 ng/g (Li et al., 2012). Higher concentrations of HBCD have been observed in certain regions of China, likely due to intensive industrial activity, relatively high population density, and market demand for HBCD in these areas. These findings indicate that industrial activities and urbanization serve as potential sources of HBCD.

4.3.2. Diastereoisomer Profiles in Sediment

Generally, γ-HBCD is the dominant isomer in sediment. This has been observed in sediment from the Hunhe River and coast of the northern Yellow Sea in China (Harrad et al., 2009a; Su et al., 2015), lakes in England (Harrad et al., 2009b), the coast of Korea (Ramu et al., 2010), and Tokyo Bay in Japan (Minh et al., 2007). The contribution of γ-HBCD to the total HBCD concentration was consistent with the relative abundance (70%–95%) of γ-HBCD in commercial technical products available in China, and with the concentrations measured in soils near manufacturing plants in North China (Huang et al., 2017a). For example, in Taihu Lake, γ-HBCD was the dominant isomer, accounting for 64.5% - 87.7% of total HBCD (Wang et al., 2016).

Numerous studies have reported variations in the isomer profiles of sediment samples. Specifically, certain studies have identified elevated concentrations of α-HBCD in sediment samples(Marvin et al., 2006). The proportions of HBCD isomers in different environmental matrices are potentially affected by thermal isomerization of HBCD-containing products (Heeb et al., 2010), photolysis under natural light (Harrad et al., 2009a), and different degradation kinetics under aerobic and anaerobic conditions (α-HBCD slower than β- and γ-HBCD) (Davis et al., 2006; Gerecke et al., 2006).

5. HBCD in Biota

5.1. HBCD in Animals

The overall distribution of HBCD in various types of aquatic biota in China is shown in Fig. 6. In many studies, fish and shellfish are the common target aquatic species of HBCD, and the concentration of HBCD varies greatly among different aquatic species and regions. Because of the lipophilicity of HBCD, the content of HBCD in organisms is positively correlated with its fat content. This conclusion is proved by the distribution in fish tissues, and studies have found that the concentrations of HBCD in fish tissues decreased in the order of eggs > liver > muscles (Zhang et al., 2013; Wang et al., 2018). In animals, the concentration distribution of HBCD among different tissues varies. HBCD concentrations in liver and eggs were higher than those in muscles of freshwater fish from the Yangtze River (Xian et al., 2008).

Figure 6. Geographical distribution and concentration of HBCD in various types of aquatic biota in China.

Figure 6. Geographical distribution and concentration of HBCD in various types of aquatic biota in China.

Overall, HBCD concentrations in aquatic biota in China are relatively low. HBCD concentrations in large yellow croaker and silver pomfrets, which are common consumer fish in coastal regions, from nine coastal cities (Shanghai, Zhoushan, Fuzhou, Dalian, Tianjin, Qingdao, Wenzhou, Quanzhou and Xiamen) ranged from 0.62 to 8.7 ng/g lw (lipid weight) and from 0.57 to 10.1 ng/g lw (lipid weight), respectively (Xia et al., 2011). The HBCD concentration of bluntsnout bream (Megalobrama amblycephala) and snubnose pompano (Trachinotus blochii) in Guangdong was 0.471–0.665 ng/g lw (Meng et al., 2012). Interestingly, the HBCD concentration increases from the south (average: 1.2 ng/g lw) to the north (average: 6.5 ng/g lw) in silver pomfrets (Fig. 6b) and other aquatic species, which may be due to the fact that the consumption of EPS/XPS containing HBCD is less in the south than in the north. HBCD concentrations (not detected–0.194 ng/g lw) have been detected in 12 consumer fish species in 11 coastal cities in South China (Fig. 6a) (Meng et al., 2012). Those in apple snails (Ampullaria gigas spix) and grass carp (Ctenopharyngodon idellus) from East China are 3.5–6.55 ng/g lw and 14.9–67.8 ng/g lw, respectively (Zhu et al., 2017a).

The obtained results demonstrate similarity to the concentrations found in benthic shellfish, including oysters, scallops, and mussels, in the northern region of the Yellow Sea, which ranged from 0.87 ng/g to 67.08 ng/g (Harrad et al., 2009a).However, they appear slightly lower in comparison to the concentrations detected in mollusks from the Bohai Sea, which ranged from 25.1 ng/g to 148.87 ng/g lw(Zhu et al., 2012).

The HBCD related industrial activities likely affect the concentration and geographical distribution of HBCD in biota. High concentration of HBCD is detected in rivers and lakes related to HBCD pollution in China. For instance, the high concentrations of HBCD in aquatic species (goby, silver carp, loach, and freshwater shrimp) (7.09 - 815 ng/g lw) was found near manufacturing facilities in Laizhou Bay which was the important HBCD production base in China (Li et al., 2012). In the e-waste recycling area of China, HBCD concentrations of up to 186, 377 and 1791 ng/g lw have been found in crucian, loach, carp and winkle, respectively (Zhang et al., 2009). HBCD concentrations in freshwater fish from the low reach of the Yangtze River ranged from 11 to 330 ng/g lw, where large amount of polymer raw materials, textiles, electronic appliances and fine petrochemicals plants located (Xian et al., 2008). HBCD concentrations were detected in rats (mean: 40.3 ng/g lw, range: 22.1-51.1 ng/g lw) from an e-waste dismantling region in East China (Zhu et al., 2017a). HBCD has been detected in samples collected near an EPS material manufacturing plant in Tianjin. In marine species such as shrimp, crab, and fish, the total concentration of HBCD ranged from 0.878 ng/g to 44.8 ng/g (Zhu et al., 2017b). Geographically, among the nine Chinese coastal cities, the highest HBCD concentrations was found in Dalian (large yellow croaker:5.2 ng/g lw; silver pomfrets:10.1 ng/g lw) which is located near Laizhou Bay in the Bohai Sea and has been seriously polluted by domestic and industrial effluent (Xia et al., 2011). HBCD concentrations were observed to be higher in terrestrial passerine birds inhabiting urban and e-waste sites in the Pearl River Delta, while rural sites exhibited the lowest concentrations (Sun et al., 2012). These results indicated that industrial activities related to HBCD affected the concentration level and distribution of HBCD

6. Conclusions

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