**4. Discussion**

On 27 November 2020, the International Agency for Research on Cancer (IARC) updated its latest list of carcinogens, and both 2,3 ,4,4 ,5-PeCB (118) and 2,3,3 ,4,4 -PeCB (105) were classified as class I carcinogens. The findings of the current study revealed that the highest proportions of PCB-118 and PCB-105 were found in farm crabs (51.8% and 19.8%) and in market crabs (51.2% and 18.3%). Compared to PCDD/Fs, DL-PCBs were abundantly produced and used in the past, and they are relatively difficult to degrade under the influence of physical and chemical factors [28]. The most decent explanation for the above statement may be that the presence of residual PCB-118 and PCB-105 in crab may make DL-PCBs more stable. In one study, Grazia Barone et al. [1] found that PCB-118 was dominant in bluefin tuna samples. Wang X. et al. [29] reported that PCB-118 and PCB-105 were predominant in all of the fish samples they studied. Similarly, Danae Costopoulou et al. [30] also found that the main congener for monoorthmic PCBs produced in Greece was PCB-118 followed by PCB-105. These results align with our experiment's key finding, which state that PCB-118 and PCB-105 comprise the dominant concentrations and closely related to their ability to enrich organisms.

OCDD and 1,2,3,4,6,7,8-HpCDD are polychlorinated dibenzo-p-dioxins (PCDDs) as well as the dominant PCDD/F congener [31,32]. Battisti Sabrina et al. [33] found that OCDD was the most abundant PCDD/F congener in dairy products collected in the Latium region of Italy (2011–2017). L. P. Fang et al. [34] found that fish tissues contained a relatively large amount of OCDD, accounting for 54% of the total PCDD/F congeners. Our results showed that OCDD was dominant in both farm crabs (38.7%) and market crabs (12.7%), concordant with the above scholars' conclusions, our results suggest that OCDD has a strong enrichment capacity. Milena Dömötörová et al. [35] observed that 1,2,3,4,6,7,8- HpCDD was the second most abundant congener in most of the soil samples considered in their study. Kim K.S. et al. [36] found that 1,2,3,4,6,7,8-HpCDD was predominant in all of their soil samples. Regarding this substance, Loganathan B.G. et al. [37] observed the same pattern of HpCDD accumulation in sediment and mussel tissues. Our results showed that 1,2,3,4,6,7,8-HpCDD is the second highest PCDD/F congener in crabs (34.6% in farm crabs and 10.0% in market crabs) after OCDD.

We then conducted further research on content and analyzed the sources of PCDD/Fs and DL-PCBs in crabs to search for possible sources. Our results showed that the means of the TEQs of the PCDD/Fs and PCBs in crabs, sediments, aquacultural water, aquatic plants, shore plants, and feed were 9.37 pg TEQ/g, 12.92 pg TEQ/g, 0.53 pg TEQ/L, 0.28 pg TEQ/g, 0.17 pg TEQ/g, and 0.32 pg TEQ/g. We referred to Han Ying's [38] method to evaluate all of the substances that hairy crabs may ingest in a 1-hectare (ha) crab pond. In crab ponds, every hectare produces about 50 kg of Chinese mitten crab, and the edible portion accounts for about 26% of the crabs' weight. As a result, the production of crab meat and roe (crab paste) per hectare of crab pond is about 13 kg/ha, implying that the PCDD/Fs and DL-PCBs TEQ in crab meat and roe (crab paste) per hectare of crab pond is approximately 122 ng TEQ/ha. The recovery rate of hairy crabs was assumed to be 70% per hectare of crab pond, and a TEQ of 52 ng was found in dead crabs. A mean feed dose of 95 kg/ha resulted in a PCDD/Fs and DL-PCBs input of 30 ng TEQ/ha. The coverage rate of aquatic plants and shore plants in crab ponds was about 50% per hectare, or about 0.265 kg; the water (aquacultural water) content at a depth of 0.8 m was about 534 m3; the sediment weight of the 0.1 m disturbed layer was about 4269 kg. The total TEQ exposure of PCDD/Fs and DL-PCBs was 0.12 ng/ha, 283 ng/ha, and 55,155 ng/ha. We found that the amount of PCDD/Fs and PCBs input into crab ponds is much higher than that output to crabs. The TEQ of the PCDD/Fs and DL-PCB input to crabs in aquacultural water was 1.6 times higher than the TEQ in edible crab parts. Aquatic plants, shore plants, and feed contributed about 0.05% of the total TEQ inputs to crabs. Without considering biological processes, the TEQ contribution from sediment was 195 times that from aquacultural water. Therefore, sediment is considered the major contributor to PCDD/Fs and DL-PCBs in Chinese mitten crabs. All of the external substances added in the cultivation of crab culture can settle into

the sediment, affecting the historical concentration of pollutants in the sediment, increasing the exposure risk of hairy crab to pollutants.

The results showed that the total TEQ of PCDD/Fs and DL-PCBs in Shanghai's Qingpu district, China, were significantly higher than in Shanghai's Chongming district, China (Figure 5). This situation may be closely related to the surrounding industrial environment. The three aquaculture farms in Qingpu district were all raised by the Taipu River. Hong Yao et al. [39] found that a large amount of wastewater containing heavy metals was discharged into the Taihu Basin every year. The Taipu River, an important tributary of the Taihu Basin, is similarly plagued by wastewater contamination, which might be one of the explanations for the elevated levels of dioxins and dioxin-like PCBs in farms in the Qingpu district. To supplement the experimental results, we detected the level of pollutants in the soil around farms in the Qingpu and Chongming districts. The findings revealed that the level of pollutants in the soil near the farms in Qingpu district (4.89 pg TEQ/g dw in MH, 19.07 pg TEQ/g dw in RJ, 3.78 pg TEQ/g dw in NXC) was higher than that near the farms in Chongming district (1.00 pg TEQ/g dw in YF, 0.89 pg TEQ/g dw in HK, 0.86 pg TEQ/g dw in ZH). The pollutants migrated into crab farms over a long period of time, which further indicated that the crab farms in Qingpu district had high levels of pollutants. Detailed results are shown in Supplementary Tables S9 and S10.

Our above investigation on market crab revealed that the average residual value of dioxins and dioxin-like PCBs in crabs was 0.0027 pg TEQ/kg. Based on the average daily consumption of adults (60 kg) and children (13.1 kg) of 3 crabs and 1 crab (at an average weight of 100 g per crab), the TDIs of adults and children would be 0.0135 pg TEQ/g (weight)·d and 0.0206 pg TEQ/g (weight)·d, respectively, lower than the prescribed range, indicating no significant chronic ingestion risk for adults and children (WHO experts determined that the TDI range of total PCDD/Fs and PCBs was 1–4 pg TEQ/kg (weight)·d) [15]. The TDIs for adults and children were 0.054 pg TEQ/g (weight)·d and 0.0824 pg TEQ/g (weight)·d when calculated with the maximum residual value of 0.0108 pg TEQ/kg, respectively, both of which were below the prescribed range, indicating that there was no significant risk of acute ingestion in adults and children. Taking into account the above data analysis and the daily quota determined by the WHO, the consumption of Chinese mitten crabs does not seem to pose a threat to health. However, given the persistence and bioaccumulation of such pollutants, the excessive consumption of aquatic products may increase the burden of dioxin-type pollutants in the body. Therefore, it is recommended that the government increase its detection of pollutants, that individuals eat normal amounts of aquatic products, and that certain restrictions be imposed on the consumption of aquatic products. Although there may be some uncertainties and limitations, the study provides a valuable assessment of the health risks associated with PCDD/Fs and DL-PCBs exposure in Chinese river crabs.

### **5. Conclusions**

The PCDD/F and DL-PCB contents in crabs from the market and from farms in Shanghai were investigated. It was found that crabs bioaccumulate DL-PCBs more readily than PCDD/Fs. As the main source, the total TEQ exposure to PCDD/Fs and DL-PCBs in sediments was 55,155 ng/ha. The PCDD/F and DL-PCB contents in crabs was generally safe, and crabs had a strong enrichment ability for 2,3 ,4,4 ,5-PeCB (118), 2,3,3 ,4,4 -PeCB (105), OCDD, and 1,2,3,4,4,6,7,8-HpCDD. A health and safety assessment based on market crab samples showed no significant chronic or acute ingestion risk for adults and children, suggesting that eating crab several times a year may not cause PCDD/Fs and DL-PCBs to exceed safe limits. Finally, by further studying the PCDD/F and DL-PCB contents in the farming process, the PCDD/Fs and DL-PCBs in crabs mainly come from sediments. Therefore, regular sediment treatment can effectively reduce the exposure of crabs to pollutants.

**Supplementary Materials:** The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/foods11172556/s1, Table S1: Market crab sampling site information, Table S2: Summary of 29 compounds and WHO 2005 TEF Values, Table S3: Solutions of isotopically labelled quantitative internal standards for PCDD/Fs and DL-PCBs, Table S4: Solutions of isotopically labelled recovery internal standards for PCDD/Fs and DL-PCBs, Table S5: The concentrations of PCDD/Fs and DL-PCBs in Chinese mitten crabs and the potential sources, Table S6: PCDD/F and DL-PCB TEQs in Chinese mitten crabs from fresh markets, Table S7: The total PCDD/F and DL-PCB TEQs in Chinese mitten crabs and the potential sources, Table S8: The concentrations of PCDD/Fs and DL-PCBs in market crabs, Table S9: The total PCDD/F and DL-PCB TEQs in in the soil around Qingpu and Chongming district farms, Table S10: Informed detection limit (DL) of the method and the percentage of recovery of each PCDD/F and DL-PCB congener analyzed. Figure S1: Ratios of DL-PCBs/PCDD/Fs in farm Chinese mitten crabs and potential sources.

**Author Contributions:** Y.L.W. and K.H.: methodology and formal analysis; Y.L.W.: investigation; S.Y.F., T.W.W., H.T.W., X.T.L. and X.N.G.: data curation; Y.L.W. and S.Y.F.: writing—original draft; K.H.: writing—review and editing and funding acquisition. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work is funded by the National Key R&D Program of China (2019YFD0900102).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** All available data are presented in the article.

**Acknowledgments:** The authors thank the Shanghai Academy of Agricultural Sciences for its instrumental support.

**Conflicts of Interest:** The authors declare no conflict of interest.
