Pollution Characteristics and Health Risk Assessment of Halogenated Organic Pollutants (HOPs) in Fish from the Laibin Section of Hongshui River, China

,


Introduction
Halogenated organic pollutants are a class of persistent organic pollutants containing halogenated elements, which are characterized by long-term residual, bioaccumulation, long-range migration, and high biotoxicity. Tey can be amplifed along the food chain step by step, which can cause great harm to biological and human health [1]. Te low cost and widespread use of these chemicals in the industrial, agricultural and health sectors have led to their continued use in many developing countries [2]. Studies have shown that residues of HOPs are still detectable worldwide [3][4][5][6]. Even today, there are still some HOPs being produced and used in China, which makes HOPs pollution a constant problem [7]. Terefore, it is necessary to test the Laibin section of the Hongshui River for HOPs levels to see if any pollutants remain.
In recent years, river ecosystems around the world have been polluted to varying degrees due to rapid population growth and economic development. Human activities change the hydromorphological, physicochemical, and biological parameters of surface water bodies, afecting their biodiversity and ecological functioning [8]. To monitor the pollution status of rivers, many researchers consider aquatic organisms as an important object of study. Aquatic organisms are capable of absorbing and enriching bioaccumulative pollutants through the food chain or directly from water and sediment [9]. Fish are almost ubiquitous in aquatic environment and play an important ecological role in aquatic food chain because they are energy carriers from low to high trophic levels. Terefore, understanding uptake, behavior and response of toxicant in fsh may have a high ecological signifcance [10]. At present, using fsh as a bioindicator of chemical pollutants has been a common procedure in several countries worldwide like China, Brazil, Argentina, the Atlantic Ocean, and other regions [11][12][13][14].
Te Hongshui River is the main stream of the Xijiang River system in the Pearl River basin. It has an excellent ecological environment for the growth and reproduction of aquatic organisms, high fshery yield, and wide variety of fsh species (Huang and Zeng, 2008). However, in recent years, there are fewer studies related to HOPs in fsh from the Hongshui River, and there is a lack of powerful data to explain whether fsh from the Hongshui River are contaminated with HOPs. Terefore, in this study, nine common food fsh species from the Laibin section of the Hongshui River were selected to determine the content of HOPs in their edible muscle, calculate the ecological contamination risk level, and assess the potential health risk of human consumption of HOPs.

Sample Collection and Preparation.
In July 2021 and December 2021, nine common edible fsh (Cirrhinus molitorella, Ctenopharyngodon idella, Cyprinus carpio, Squaliobarbus curriculus, Ptychidio jordani, Carassius auratus, Silurus asotus, Tachysurus fulvidraco, and Channa argus) were collected from the Laibin River section in the Hongshui River ( Figure 1). Five mature and well-formed individuals of each species were collected as experimental samples. Te basic characteristics of the fsh are shown in Table 1. All samples were sealed in plastic bags and stored at −20°C until chemical analysis.
Te samples thawed at room temperature were cleaned with deionized water, scaled, and peeled. Te edible muscles of their backs were dissected with a stainless-steel scalpel and weighed wet. Ten, put the sample into a tin foil paper bag, freeze-dried for 48 h, and weighed for dry weight. We grounded the freezedried samples into fne powder. Te operation is carried out on aluminum foil paper to avoid pollution.

Extraction and Analysis.
Te extraction and analysis of HOPs referred to a previous study [16] with minor modifcations according to laboratory conditions. Approximately 0.5 g of freeze-dried muscle sample was grounded into fne powder and then mixed with dichloromethane. 100 ng of PCB 24,82,and 198; 20 ng of PBDE 118 and 128; and 50 ng of 13 C 12 -BDE 209 were added as an internal standard before extraction. Te sample was ultrasonically extracted with 4 ml of dichloromethane three times. Te lipids were removed with 5 ml of concentrated sulfuric acid and then centrifuged at 3000 rpm for 5 minutes. Te supernatant was transferred to a new tube and concentrated to 1 ml under gentle nitrogen fow. Te extracts were purifed using a multilayer silica column packed with neutral silica gel (8 cm, 3% deactivated, activated at 180°C), 50% (on a weight basis) sulfuric acid silica (8 cm), anhydrous sodium sulfate, and 1 g anhydrous sodium sulfate (activated at 450°C). We used dichloromethane to elute HOPs. Te fnal eluate was concentrated to near dryness under a gentle stream of pure nitrogen. Known amounts of recoveries of surrogate standards (100 ng of PCB 30, 65, and 204; 20 ng of PBDE 77, 181, and 205) were added to the extract and make up to 100 μl with isooctane prior to instrumental analysis.
PBDEs were analyzed by a 7890 Agilent gas chromatograph (GC) coupled with a 5975 mass spectrometer (MS) operated in an electron capture negative ionization mode. Te concentrations of PCBs and OCPs were determined by a 7890 Agilent GC coupled with a 5975 MS in an electron ionization mode.

Quality Control and Quality
Assurance. Set the program blank in the chemical analysis process of the sample to check the sample pollution caused during the experiment. Te results showed that no target compound was detected in the blank sample of the program or the content was very low, indicated that there was no sample contamination caused by human or other environmental factors during the experiment. In this study, the standard internal method was used to detect the target compound, and the multipoint calibration curve was used for quantifcation. Te correlation coefcients of the standard curves were greater than 0.99. Te instrument limit of detection (LOD) and quantitative limit of quantitation (LOQ) was defned as the concentrations corresponding to 3 and 10 times the signal-to-noise ratio, respectively. Te LODs and LOQs for the fsh samples were 0.0001-0.0514 ng/g ww and 0.0002-0.3931 ng/g ww, respectively.

Evaluation Method.
Te assessment of human exposure to HOPs is usually expressed in terms of estimated daily intake (EDI), expressed as follows [17]: where PR is the daily per capita intake of fsh. In this paper, the per capita fsh consumption in southern China is 59.3 g per person per day [18,19]. C is the wet weight concentration of the target compound in fsh muscle tissue (ng/g ww), and BW is the average body weight of local residents 60 kg (National Bureau of Statistics, 2019). Te hazard quotient (HQ) for human consumption of fsh can be estimated from the acceptable daily intake (ADI) or reference intake (RfD) of PCBs, DDTs, HCHs, and BDE 99. HQ is usually determined by the ratio of EDI to ADI or RfD. If HQ is greater than 1, that is, EDI > ADI or RfD, there is a health risk, otherwise, there is no health risk [20] (Naqvi et al., 2020).

Statistical Analysis.
Te results were analyzed using descriptive statistical parameters such as mean value, standard deviation, and concentration range [21]. We used Origin 2021 to make the composition characteristic map of three kinds of diferent homologs in fsh and the composition characteristic map of homologs in fsh of diferent feeding habits and habitats.

Levels of HOPs in Fish Species.
Te detection rate of OCPs was 93.10-100%, and all selected OCP homologs were detectable in fsh samples. Table 2 shows the measured concentrations for all samples. Te levels of Σ 18 OCPs in fsh muscle tissue ranged from 8.94 to 43.32 ng/g ww, with a mean of 18.61 ng/g ww. Among them, C. idella had the highest average concentration, the concentration range was 13.94-43.32 ng/g ww and the average value was 23.81 ng/g ww. C. molitorella have the lowest average concentration, with the concentration range of 8.94-11.38 ng/g ww and the average value of 10.23 ng/g ww. Compared with those freshwater fsh concentrations of Σ 19 OCPs in Lake St Lucia, KwaZuluNatal (KZN), ranged from 74 to 510 ng/g lw [22], Σ 6 OCPs in Oguta Lake, Nigeria, mean concentration was 236.9 ng/g (2021), Σ 6 OCPs in Lake Tana, Ethiopia, ranged from ND to 5142.15 ng/g [23], concentrations of OCPs in the present study were in relatively low levels. Although the use of most pesticides has been restricted, their long half-life and long-distance mobility make them still detectable worldwide.
As can be seen from Table 3, the detection rate of PCBs ranged from 79.31 to 100% for all 18 PCB monomers selected for this study. Σ 18 PCBs in fsh muscle tissue ranged from 1.22 to 4.09 ng/g ww, with an average value of 1.94 ng/g ww. It was noteworthy that T. fulvidraco had the highest PCBs content in the total PCBs, which was 2.80 ng/g ww. Te lowest concentration was found in C. molitorella, with a concentration range of 1.22-1.61 ng/g ww and an average value of 1.45 ng/g ww. Te highest and lowest average contents of the 18 monomers analyzed were PCB 169 and PCB 18, with 0.51 ng/g ww and 0.00 ng/g ww, respectively. Te concentration of Σ 18 PCBs in the present study was moderate compared to the concentrations reported in other studies, in Lake Chapala, Mexico (Σ 39 PCBs, 1.06-6.07 ng/g) [24] and Cook County IL, Chicago (11.8-505 ng/g ww) [25].
Te detection rate of PBDEs ranged from 34.48% to 86.21% and the concentration ranged from ND to 0.13 ng/g ww, with an average value of 0.05 ng/g ww. Te average concentrations of omnivorous C. auratus and carnivorous T. fulvidraco were the highest and lowest, respectively, with average values of 0.09 ng/g ww and 0.01 ng/g ww, respectively. Te average content of BDE 154 and BDE 99 was the highest and lowest among the fve PBDE monomers, with 0.02 ng/g ww and ND, respectively. Please see Table 4 for details. ΣPBDE content in this study is lower than Σ 6 PBDEs in the Yellow River Delta, China (5.3-149 ng/g lw) [26], Σ 6 PBDEs in Lake Geneva, France and Switzerland (0.54-2.60 ng/g ww) [27], and Σ 5 PBDEs in Newfoundland, Canada (ND-368.27 ng/g) [28].       Journal of Applied Ichthyology

Journal of Applied Ichthyology
Currently, China attaches great importance to the emission situation of HOPs and has adopted emission reduction, waste incineration shutdown, and treatment process improvement to reduce the emission of HOPs. However, the late start and the imperfect monitoring system have resulted in residues of HOPs still being detected in various locations. Ten kilometers upstream from the sampling site, there is a large area of industrial and agricultural land, which is fat and far from living areas and suitable for industrial and agricultural industries. OCPs are likely to originate mainly from residues of pesticide use on agricultural land. PCBs accumulated in freshwater originate mainly from their industrial application in paints, insulation additives, and dielectric fuids (2012). Tere are metal, paper, and timber mills upstream of the Hongshui River, and the wastewater discharge may lead to residual PCBs in the Hongshui River. In addition, historical residues may also be a contributing factor. Te generally low concentration may be related to the limited use and short consumption history of PCBs in China [29]. PBDEs, an organic bromine compound, are widely used in the electronics, clothing, and furniture industries and help to reduce the fammability of products containing bromine [30]. Te increasing number of fame retardants being put on the market to sustain people's productive lives may explain the growing concern over PBDEs pollution in recent years [31]. Figure 2(a). Te composition patterns of OCPs in the nine fsh species were similar, with HCHs and endrin being the main contributing contaminants, followed by DDTs, heptachlor, endrin-aldehyde, MXC, and aldrin, with other OCPs contributing less signifcantly. Te total contribution of HCHs and endrin ranged from 44.04% to 75.48%. Te similar composition pattern shows that OCPs contained in nine fsh species have certain similarity or similar origin. Te main components of HCHs are α-HCH, β-HCH, c-HCH, and δ-HCH. Te highest percentage of HCHs in this study was β-HCH ( Figure 3). Tere are the following two main sources of HCHs in the environment: industrial HCH and Lindane, which is still used as a broad-spectrum pesticide in some areas [32]. Biodegradation of β-HCH in environmental media is challenging; in contrast, α-HCH and c-HCH are highly volatile and can be transported over long distances in the environment. Endrin is a highly toxic pesticide. Although endrin has never been produced on a large scale in China, its residues have been detected throughout the country [33]. Terefore, from the analysis of the compositional characteristics of OCPs, it could still be concluded that the main sources of OCPs in the region are historical residues and emissions as by-products from the manufacture and use of various industrial and agricultural chemicals [34].

PCBs.
In this study, 18 kinds of PCB monomers were classifed according to the number of chlorine atoms, mainly including tetra-CBs, penta-CBs, hexa-CBs, hepta-CBs, and deca-CBs. From Figure 2(b), it could be seen that hexa-CBs made the most signifcant contribution, accounting for 64.89% of the total. Te second was penta-CBs, accounting for 10.78% of the total. Te percentages of deca-CBs, tetra-CBs, and hepta-CBs in the total were similar at 8.23%, 8.15%, and 7.95%, respectively.
It is generally accepted that higher chlorinated congeners of PCBs accumulate more in organisms than lower chlorinated congeners due to their greater lipophilicity. In contrast, the content of higher chlorinated PCB congeners, such as deca-CBs, are very low, probably because these chlorinated compounds are too large to pass through the cell membranes of the organisms, resulting in low levels of accumulation [35]. Tis is consistent with the results of studies on PCBs in food fsh from the Negro River Basin, Patagonia, Argentina [36], where Ondarzav noted that PCB 138 and PCB 153 are more resistant to metabolism and slower to be eliminated, and that the inherent high lipophilicity, stability, and persistence of hexa-CBs lead to their easier accumulation in fsh. PCBs are industrial products and no natural sources, so it is inferred that the PCBs detected in the area are mainly from upstream industrial activities and historical residues.

PBDEs.
Te distribution of PBDEs in nine species of fsh was diferent (Figure 2(c)). Tere was almost no BDE 99 in the P. jordani, and BDE 154 had the highest contribution rate; there were fve kinds of PBDEs in C. idella, BDE 154 had the highest contribution rate and BDE 99 had the lowest contribution rate; there were fve kinds of PBDEs in S. asotus, BDE 154 had the highest contribution rate and BDE 100 had the lowest contribution rate; there were almost no BDE 99, BDE 154 and BDE 208 in T. fulvidraco, and the contribution rate of BDE 28 was higher than that of BDE 100; there were fve kinds of PBDEs in C. molitorella, BDE 154 had the highest contribution rate, followed by BDE 208 and had little difference with BDE 154, BDE 99 and BDE 100 had little difference; there was almost no BDE 28 in C. carpio, BDE 154 had the highest contribution rate, and BDE 99 and BDE 100 had a small contribution rate and little diference; there was almost no BDE 99 in S. curriculus, and BDE 28 had the highest contribution rate; there were fve kinds of PBDEs in C. argus, BDE 154 had the highest contribution rate and BDE 99 had the lowest contribution rate; there was almost no BDE 99 in C. auratus, and BDE 208 had the highest contribution rate.
It could be seen from Figure 2(c) that the composition characteristics of PBDEs homologs in the nine fsh species are diferent. Tis is similar to the research conclusion of Zhou et al. in Dianshan Lake, Shanghai, China [37]. Tis may be due to the diferent habitats of the fsh and the diferent metabolic and biotransformation mechanisms of each fsh [38]. Te main sources of PBDEs are local wastewater discharges and wastewater from plastic and textile product factories [39]. Terefore, it was tentatively determined that the residues of PBDEs in the water  Journal of Applied Ichthyology environment of the area are mainly of industrial origin. However, the distribution characteristics of the various PBDEs homologs are diferent due to the fsh themselves.

Compositional Characteristics of Pollutants in Fish of Diferent Feeding Habits and Habitats.
In this study, fsh of three diferent feeding habits and two diferent habitats were analyzed separately. In order to ensure the rationality of the data, C. molitorella and C. idella were selected for the analysis of herbivorous fsh. Te omnivorous fsh that were selected for the analysis were C. carpio and S. curriculus,. Te carnivorous fsh that were selected for the analysis were T. fulvidraco and C. argus. Selection was based on good recovery rates and preference for a high number of strips. As only one midwater fsh species was collected, midwater fsh were not involved in the analysis of the diferent habitat. Te remaining eight species were divided into four lower midwater fsh and four demersal fsh. As can be seen in Figures 4(a) and 4(b), the three HOPs (OCPs, PCBs, and PBDEs) have diferent compositional characteristics in fsh of diferent diets and inhabiting water layers. According to the analysis of three diferent feeding habits (carnivorous, omnivorous, and herbivorous), the order of OCPs and PCBs content was carnivorous > omnivorous > herbivorous; the order of PBDEs content was omnivorous > herbivorous > carnivorous. Te composition of the three contaminants in fsh from the two diferent habitats was characterized by demersal > lower midwater.
In a study of OCPs in fsh from Lake Koka, Ethiopia [40], it was noted that OCPs in the water bodies are more efciently transferred to phytoplankton, which herbivorous fsh    can take in. In contrast, carnivorous fsh that feed on these fsh accumulate more degraded forms of these contaminants, which are more persistent, resulting in higher levels of OCPs accumulated in carnivorous fsh. OCPs and PCBs have been found in many studies to have characteristics that are transmitted in the food chain species because of their high octanol-water partition coefcient (K ow ) [18,41,42]. It makes them more concentrated in carnivorous fsh with higher trophic level. Tis is consistent with the fndings in this study. However, in some studies, it is found that there is no biomagnifcation feature of PBDEs, so the enrichment degree of PBDEs is more dependent on the physicochemical properties and habitat [43,44].
In aquatic ecosystem, halogenated persistent organic pollutants are usually attached to sediments, making it easier for benthic fsh to absorb them into their bodies from their skin, gills, and feeding. As a result, demersal fsh typically have higher levels of contaminants than other water-layer fsh. However, many factors still infuence the concentration of HOPs in fsh, such as the age of the fsh, habitat environment, and the stability of pollutants. Terefore, more detailed studies are needed to analyze the characteristics of the composition of diferent pollutants in fsh in diferent environments. 50 and EDI 90 were calculated based on the 50th and 90th concentrations of pollutants in fsh, respectively, representing the average and maximum level of HOPs intake by Chinese residents through fsh consumption. Te results of the calculations are shown in Table 5. Te maximum daily intake of the fve compounds (except MXC) calculated in this study was far lower than the allowable daily intake (ADIs) recommended by the International Food and Health Organization (FAO/ WHO) and the reference dose (RfD, USEPA 2000a) of PCBs set by the United States Environmental Protection Agency [26,[45][46][47]. EDI of each compound was lower than the corresponding allowable daily intake (ADIs), that is, HQ > 1. At present, there are no national regulations or standards for MXC. Tis means that the estimated daily intakes in the nine fsh samples in this experiment were much smaller than the specifed reference measurements. To sum up, the daily consumption of fsh in the Laibin section of Hongshui River may not pose a health risk to local residents for PCBs, DDTs, HCHs, and BDE 99 compounds. However, there are still many persistent halogenated organic pollutants not involved in this study, so the risk of fsh consumption in this area still needs to be further studied.

Conclusion
In this study, three types of persistent halogenated organic pollutants (OCPs, PCBs, and PBDEs) could be detected. Endrin dominated in OCPs; hexachlorobiphenyl dominated in PCBs; BDE 154 dominated in PBDEs, but its composition characteristics were diferent in nine species of fsh. From the perspective of diferent feeding habits, OCPs and PCBs had the largest residues in carnivorous fsh, and PBDEs had the largest residues in herbivorous fsh. From the perspective of diferent habitats, the residues of OCPs, PCBs, and PBDEs in the demersal fsh were the largest. Tere were many factors that afect the enrichment of HOPs. Based on the test data and analysis in this paper, the following suggestions can be made: (1) Build a comprehensive environmental monitoring system, increase the number of environmental monitoring points, conduct longterm monitoring of pollutants and take corresponding treatment measures immediately if the standards are found to be exceeded. (2) Upgrade environmental monitoring technology, step up talent building and keep up with changes in technology. (3) Vigorously carry out pollution prevention and control publicity to enhance public awareness of pollutant emissions and control, so that the public can properly understand the hazards of pollutant emissions.

Data Availability
Data will be made available on request.

Conflicts of Interest
Te authors declare that there are no conficts of interest.