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Polychlorinated biphenyls (PCBs) have been detected in a variety of environmental media, and they have been continuously introduced into the environment for industrial reasons, despite their harmful impact upon ecological health. Considering the adverse effects of PCBs, many researchers have begun to analyze remediation technologies for PCB-contaminated soil. In this study, the rise and fall of PCBs, as well as their physical and chemical properties and environmental impact, were reviewed. The pollution status of PCBs in soil was summarized. Based on the analysis and comparison of various remediation technologies, the limitations of several phytoremediation technologies in PCB-contaminated soils were analyzed. The influencing factors and transformation mechanisms of the phytoremediation of PCB-contaminated soil were investigated, and the research direction of enhancing the degradation of PCBs through the use of plants was also discussed.
Polychlorinated biphenyls (PCBs) have 209 congeners depending on the location and number of chlorine atoms (Figure 1) [1]. PCBs were first produced in 1929 and are used on a large scale for industrial and commercial purposes due to their high chemical thermal stability and low electrical conductivity [2]. As studies found that PCBs have high toxicity, persistence, enrichment, migration, and other environmental hazard properties, as well as being involved in an increasing number of environmental pollution incidents, attention has gradually been drawn to the phenomenon of environmental pollution by PCBs [3]. The 1920s to the 1970s was the golden age of PCB production [4]. This continued until the 1980s, when countries signed the Stockholm Convention and agreed to stop the production of PCB products [5]. Although PCBs have been banned, they continue to be introduced into the environment in contexts such as oil field development, waste incineration, etc. In addition, commercial mixtures of PCBs still remain in large quantities in electrical systems, garbage dumps, and land [4]. This situation leads to the widespread detection of PCBs in the global marine environment, the terrestrial environment, animals, plants, and food [6, 7]. Therefore, in the coming decades or even centuries, we will continue to be exposed to the environment containing these PCB pollutants [8].
The harmful nature of PCBs to human beings and the environment cannot be ignored. Contact with PCB-contaminated soil threatens human health. In the past few decades, more and more studies have shown that people exposed to PCBs are more likely to suffer from chronic diseases (including endocrine, metabolic, cardiovascular, and cerebrovascular). Excessive biological contact with PCBs promotes the inflammation of inner epidermal cells and accelerates the development of arteriosclerosis [9, 10]. Recent studies have shown that the developing brains of humans and animals are particularly susceptible to the effects of PCBs, which destroy the normal metabolism of the human body [11]. In addition, PCB pollutants in the soil can further harm human health through transmission within the food chain. PCBs such as PCB-138, PCB-153, and PCB-180 have been found in large proportions in eggs and meat [12]. As PCBs are insoluble in water and highly lipid-soluble, they are also easily enriched in aquatic animals and can be toxic [13]. PCBs have been detected in fish in the Songhua River basin at varying levels, with a high percentage of PCBs detected in lower chlorinated biphenyls [14]. PCBs have long degradation half-life and strong long-distance transport capability, so they have been detected in some remote areas and marine environments. It was found that these pollutants were produced in the last century and were used as raw materials for the production of high voltage capacitors and capacitors [15, 16, 17]. In recent years, PCBs in the ocean have also attracted close attention from researchers. Although the concentrations of PCBs in seawater are low, they can bioaccumulate through the amplification of marine food chains and food webs, reaching high levels in higher-grade marine organisms [18]. The sampling and analysis of the Ross Sea and Drake Passage around the Antarctic revealed that over 51 PCBs were found in 23 samples [19].
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As the largest receptor of PCBs, it is difficult for soil to degrade them naturally. Therefore, large amounts of PCBs remaining in soil often cause serious soil pollution events. Research shows that a whole-body PCB concentration of about 150 ppm appears to be the threshold concentration above which acute mortality is observed [20]. In the study on PCB industrial contamination in a mixed urban–agricultural area, the dl-PCBs ranged from 1.4 to 3.5 pgTE(WHO)/g (all values higher than limits of regulation EC, 2001/102), and the determination of dl-PCBs indicates that the overall toxicity equivalent is significant [21]. The analysis showed that the biodegradation rate of PCBs decreased with the degree of chlorination, from 75% for PCB-3 to 22% for PCB-77 and Aroclor 1242 [22]. This potentially reflects the toxicity accumulation of high-concentration PCBs in organisms.
Soil around the world is threatened by PCB contamination, with about 20, 000 tons of PCBs in global soil and an uncountable range of site contamination [23]. In the United States, more than 300 sites have contained or still contain PCB pollution; in Europe, there are more than 242, 000 sites of PCB pollution; in Canada, there are more than 100 PCB pollution sites. The contamination concentrations of PCBs in the soil of developed countries such as the United Kingdom and the United States has reached 50 mg/kg [24, 25, 26]. Due to improper disposal of waste transformer facilities and equipment in Nigeria, PCBs have leaked, causing great damage to local soil resources and water resources, affecting people’s normal work and life [27]. The situation of PCB soil pollution in China is also not promising. It has been reported that the degree of pollution in the southeastern coastal areas is relatively high, and the concentration of PCBs in soil reaches 80 ng/g [26]. The content of PCBs detected in some surface soils in China is about 0.05–8.69 ng/kg, and the pollution level is relatively high [28]. Urban soil is dominated by low-chlorinated PCB compounds, and the content of PCBs in the surface soil around industrial areas is significantly higher than that of ordinary farmland [29, 30, 31]. The detection rate of PCBs in different soil types in southern Jiangsu was different, and the detection rate was the highest in vegetable fields, but the types of PCBs detected in soil were similar [32]. The PCB pollution of farmland soil in the Yangtze River Delta is relatively serious, and the detected concentration and average value are as high as 484.5 ng/g and 35.52 ng/g, respectively [33]. The PCB pollution of the irrigated areas around the Yellow River was studied and found to be similar to that of the Yangtze River Delta; the surrounding areas of the Yellow River also had higher pollution levels. In addition, there are spatial differences in the degree of PCB pollution in China, and the pollution situation in the southwest of China is severe due to industrial distribution and other reasons [34]. Overall, the PCB-based pollution of soil in China and around the world is not promising, with a high degree of pollution in a wide range of areas. Although PCBs have been banned in countries around the world at this stage, and the trend of PCB pollution has declined, large amounts of PCBs remain in the soil environment, and there is an urgent need to study how to effectively control PCB-contaminated soil [35].
Physical remediation techniques include adsorption, membrane filtration, and other methods [36]. Although landfill usually creates a relatively sealed space to isolate pollutants from the outside world, it does not fundamentally eliminate pollutant contamination, and the risk of leakage increases with time. The incineration method can efficiently remove the pretreated PCB pollutants under high temperature treatment, but this method has the problems of high cost and secondary pollution [37]. In addition, other methods such as microwave treatment technology show that the degradation degree of PCBs reaches 99.999% in 10 min at 50 °C, but the shortcomings are also obvious, as it cannot be processed continuously, but rather only by batch operation. Therefore, it has high
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