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1. College of Biological and Environmental Engineering, Binzhou University, Binzhou 256600, China; 2. Binzhou Bohai Middle School, Binzhou 256600, China

Abstract To evaluate the levels of heavy metal pollution and risks in consumption of aquatic products from a market in Binzhou, the contents of Cu, Zn, Cd, Cr and Pb in the edible part of eight aquatic species such as Ctenopharyngodon idellus were determined by inductively coupled plasma mass spectrometry. The results show that the levels of the five elements differed among different species of aquatic products. The ability of bivalves to accumulate heavy metals was significantly higher than that of fish. Compared with relevant food hygiene standards, there was a certain excess of Cr in Ruditapes philippinarum and Penaeus vannamei. The results of human exposure risk assessment show that the THQ values of the heavy metals in the eight aquatic species were all less than 1, indicating that the consumption of these aquatic products had no potential non-carcinogenic risks.

Key words Heavy metal, Aquatic product, Binzhou, Ingestion risk

1 Introduction

In recent years, with the development of social economy, the level of industrialization and urbanization has been continuously improved, and the pollutants entering the environment are also increasing. A variety of pollutants can be enriched in aquatic organisms and even biomagnified with the food chain, making aquatic product quality issues increasingly prominent. Heavy metal pollution has also become one of the hot issues of public concern[1-2]. Cu, Zn,etc. are essential elements of living things and participate in a variety of physiological functions. However, ingestion above a certain limit will produce a negative effect. Some elements such as Cd and Pb have strong toxic effects and even cause mutagenic, carcinogenic and teratogenic effects at very low doses, leading to physiological and metabolic disorders[3-4]. The living standards of people have been improved constantly, and their demand for aquatic products is also growing. Heavy metals are a class of pollutants that are ubiquitous and highly toxic in the environment, and their residues in aquatic products are related to human health, and therefore, they have become important indicators for evaluating food safety[5-7]. Relevant departments in China have promulgated relevant food safety quality standards, which specify the limits for heavy metals such as Cu, Zn, Cd, Cr and Pb[8-9]. Strengthening the monitoring of heavy metal pollution in aquatic products is of great significance for evaluating risks in the ingestion of these aquatic products and safeguarding the health of people[10-14].

This paper studied the pollution status of heavy metals including Cu, Zn, Cd, Cr and Pb in common aquatic products from Liujie Market of Binzhou, so as to understand the levels of heavy metals in the aquatic products sold in Binzhou and evaluate the hygiene quality of the aquatic products.

2 Materials and methods

2.1 Sample collectionFrom May to July, 2017, a certain number of common aquatic products were randomly purchased from the Liujie Market in Binzhou, covering three fishes [Ctenopharyngodonidellus(n=5),Carassiusauratus(n=5) andCyprinuscarpio(n=5)], two bivalves [Ruditapesphilippinarum(n=10)andSinonovaculaconstricta(n=10)] and three crustaceans [Penaeusvannamei(n=8),Oratosquillaoratoria(n=6)andPortunustrituberculatus(n=6)].

2.2 Sample treatmentThe body weight and body length of the aquatic product samples were measured in the laboratory. After dissection with stainless steel scissors or a scalpel, the muscle of the fishes and the edible soft tissue of the bivalves and crustaceans were sampled. Each tissue sample was homogenized, freeze-dried, ground and sieved through a 100-mesh sieve. A certain amount (0.15 g) of each dry sample was placed in a digestion tube, added with 5 mL of nitric acid, 3 mL of hydrochloric acid and 2 mL of hydrogen peroxide solution, and microwaved in a microwave digestion instrument at 140℃ for 30 min. The digested solution was transferred to a PTFE crucible, heated on a hot plate at 110℃ to remove the acids and diluted with deionized water to a certain volume.

The concentrations of Cu, Zn, Cd, Cr and Pb in the samples were determined by inductively coupled plasma mass spectrometry. Reagent blanks, standard material (TORT-2) and parallel samples were used for quality control. The recoveries of the standard reference substance were above 78%. The relative standard deviation of the parallel samples was below 10%.

2.3 Evaluation methodThe ingestion risks of the aquatic products polluted by heavy metals were evaluated through calculation of estimated daily intake (EDI) and target hazard quotient (THQ)[12]. The calculation formula forEDIis as follows:

EDI=FIR×C/BW

(1)

whereEDIis estimated daily intake [μg/(kg·d)];FIRis food intake (g/d), referring to the annual consumption of fish in urban residents of Shandong Province in 2013 (22.3 g/d)[6];Cis the concentration of heavy metal in aquatic product (mg/kg, wet weight);BWis average body weight (kg), and this paper used the average body weight of adults of 55.9 kg[10].

THQis established by the US Environmental Protection Agency, and can be used to assess the health risk of a single heavy metal and to assess the risk of multiple-heavy metal exposure (total target hazard quotients,TTHQ). The method uses the ratio of ingested dose of certain pollutant to the standard limit as a risk evaluation criterion. IfTHQ<1, it indicates that there are no obvious non-carcinogenic risks to exposed population; and ifTHQ>1, it indicates that there are potential non-carcinogenic risks to exposed population. The greater the value ofTHQ, the more serious the non-carcinogenic risk of harmful heavy metal to human health[10]. The calculation formula ofTHQis as follows:

THQ=EF×ED×FIR×C/(RfDo×BW×AT)

(2)

(3)

whereEFis the exposure frequency of population (365 d/y);EDis exposure duration and was assigned as 70 years for adults;RfDo(Oral Reference Dose) is reference dose, and is used to evaluate the toxicity threshold for non-carcinogenic health effect; AT is exposure time, and the exposure time of non-carcinogenic pollutants was 365 d/y×exposure duration (70 years for adults);iis the number of evaluation elements; andFIR,CandBWare the same as Formula (1). TheRfDovalues of Cu, Zn, Cd, Cr and Pb were 40, 300, 0.1, 3 and 4 μg/kg·bw/d, respectively[6].

3 Results and discussion

3.1 Contents of heavy metals in edible part of the aquatic productsThe contents of heavy metals in the edible part of the eight aquatic species from Liujie Market in Binzhou are shown in Table 1. The contents of Cu, Zn, Cd, Cr and Pb were 4.6-34, 40-113, 0.02-1.3, 1.7-16 and 0.51-2.8 mg/kg·dw, respectively. There were some differences in the contents of the heavy metals between different species. Overall, the contents of the heavy metals in all the aquatic products exceptRuditapesphilippinarumranked as Zn > Cu > Cr > Pb > Cd. Zn and Cu are essential elements of living things and participate in the metabolism process in the body, and aquatic organisms may ingest them more actively and selectively[7]. Great levels of residual amounts of Zn and Cu are also reported in aquatic products such as fish and shellfish in many places[7, 12]. Cr is also one of the essential elements for humans, but its demand is not high. The non-essential elements Cd and Pb are more toxic and they do not participate in the physiological activities of humans. Compared with the essential elements, their contents were low in the aquatic products[5].

The enrichment of heavy metals in aquatic products shows obvious inter-species differences.

The ability of bivalves to accumulate heavy metals was significantly higher than that of fish (P<0.01). The highest content of Cu was observed inS.constricta. The contents of Cu inS.constrictaandP.vannameiwere significantly higher than those in the other aquatic products (P<0.01). The three fish species tended to accumulate less Cu than the other aquatic species. This might be related to habits of different species. Bivalves and crustaceans are benthos, and they are exposed to more sediments, which are often considered to be important sinks of heavy metals[12]. Greatest level of Zn was found inP.trituberculatus. The contents of Zn in the fishes were lower than those in bivalves and crustaceans (P<0.05). Cu and Zn are essential elements of living organisms, and their contents in the muscles of various species were significantly higher than those of the other three metals. This is similar to the results of many studies on the characteristics of heavy metal pollution in aquatic products[11-13]. Cd is a non-essential element of organisms that is highly toxic and harmful to aquatic organisms. In the present study, the contents of Cd in the aquatic products were in the range of 0.02-1.3 mg/kg, and higher contents of Cd were found inR.philippinarumandP.trituberculatus(P<0.01). Higher content of Pb was found inR.philippinarum. The contents of Pb inR.philippinarum,S.constricta,P.vannameiandP.trituberculatuswere significantly higher than those in the fishes (P<0.05). The contents of Cr ranked as bivalves > crustaceans > fishes (P<0.01).

Table 1 Contents of heavy metals in edible part of the aquatic productsmg/kg

SpeciesCuZnCdCrPbC.idellus4.6±1.6040.0±16.00.02±0.022.00±1.500.94±0.58C.auratus7.8±1.5066.0±9.500.03±0.031.70±0.420.83±0.26C.carpio6.7±1.9076.0±15.00.02±0.012.30±0.790.75±0.56R.philippinarum8.2±1.4077.0±11.01.30±0.0816.00±3.402.80±0.10S.constricta34.0±4.2089.0±13.00.09±0.0214.00±2.001.50±0.05P.vannamei33.0±0.9286.0±3.200.08±0.0215.00±1.601.50±0.18O.oratoria10.0±0.72102.0±6.100.07±0.018.30±1.500.51±0.04P.trituberculatus12.0±0.74113.0±10.00.86±0.1411.24±0.201.37±0.02

3.2 Quality evaluation of aquatic products from perspective of heavy metal contentsReferring to the limits of pollutants in aquatic products specified by relevant national ministries and commissions, the quality of the aquatic products was evaluated according to NY5073-2006[15]and GB2762-2017[16]. Compared with the limit on Cu (50 mg/kg) specified in NY5073-2006, the contents of Cu in the aquatic products of this study (0.86-7.6 mg/kg, wet weight) did not exceed the standard. Compared with the limit on Zn (50 mg/kg), the contents of Zn in the aquatic products did not exceed the standard. The contents of Cd in the aquatic products were 0.004-0.20 mg/kg (wet weight), much lower than the limits specified in GB2762-2017 (fishes, 0.1 mg/kg; crustaceans, 0.5 mg/kg; bivalves, 2.0 mg/kg). The contents of Pb in the aquatic products were 0.08-0.43 mg/kg, lower than the limits specified in GB2762-2017 (fishes, 0.5 mg/kg; crustaceans, 0.5 mg/kg; bivalves, 1.0 mg/kg). The contents of Cr in the aquatic products ranged from 0.34 to 3.6 mg/kg. Compared with the limit on Cr in GB2762-2017 (2.0 mg/kg), this element had a certain excess in the samples ofP.vannameiandR.philippinarum. For Cr, if valence is different, the toxicity is also very different. The toxicity of hexavalent Cr is about 100 times that of trivalent Cr, so in the risk evaluation of Cd for human health, the chemical form of Cr must be taken into account. In addition, whether the Cr exceeds the standard or not may be affected by the selected batch. Therefore, the problem of excessive Cr in aquatic products needs further study. The results of this study are similar to those of the bivalves and crustaceans reported by Liu Yangetal.[17], which may be related to the quality of the culture environment of the two organisms. Therefore, residents should be advised to control the intake ofR.philippinarumandP.vannamei.

Table 2 Contents of heavy metals ingested from aquatic productsμg/kg·bw/d

SpeciesCuZnCdCrPbC.idellus0.3453.0640.0620.1760.079C.auratus0.6024.9810.0030.1340.067C.carpio0.4584.9960.0020.1610.061R.philippinarum0.6184.9360.0801.0590.170S.constricta1.4263.7720.0040.5920.060P.vannamei3.0307.9330.0081.4310.144O.oratoria0.6496.6080.0050.5670.033P.trituberculatus0.5925.6490.0450.5430.066Acceptabledailyintake(ADI)50-500300-10001.0000.83-33.33.570

Table 3 THQ of heavy metals in edible part of the aquatic products

SpeciesCuZnCdCrPbC.idellus4.6±1.6040.0±16.00.02±0.022.00±1.500.94±0.58C.auratus7.8±1.5066.0±9.500.03±0.031.70±0.420.83±0.26C.carpio6.7±1.9076.0±15.00.02±0.012.30±0.790.75±0.56R.philippinarum8.2±1.4077.0±11.01.30±0.0816.00±3.402.80±0.10S.constricta34.0±4.2089.0±13.00.09±0.0214.00±2.001.50±0.05P.vannamei33.0±0.9286.0±3.200.08±0.0215.00±1.601.50±0.18O.oratoria10.0±0.72102.0±6.100.07±0.018.30±1.500.51±0.04P.trituberculatus12.0±0.74113.0±10.00.86±0.1411.24±0.201.37±0.02

3.3 Health risk evaluationTheEDIcalculation results of the aquatic products from Liujie Market in Binzhou are shown in Table 2. Compared with the acceptable daily intake (ADI) of Cu (50-500 μg/kg·bw/d) and Zn (300-1 000 μg/kg·bw/d) specified by the Food and Agriculture Organization of the United Nations (FAO) and the World Health Organization (WHO), there was no health risk in Cu and Zn exposure in the aquatic products. Compared with the provisional tolerable weekly intake (PTWI) of Cd (7 μg/kg·bw/w) and Pb (25 μg/kg·bw/w) of FAO/WHO, the Cd and Pb in the aquatic products would not produce any negative effects on human health. Compared with the maximum tolerable daily intake (MTDI) of Cr (0.83-33.3 μg/kg·bw/w) recommended by the National Research Council of the United States (NRC, 1989), the Cr in the aquatic products was also barely produce negative effect on human health. In short, it is relatively safe for people to ingest these aquatic products at low frequency.

The calculation results ofTHQare shown in Table 3. TheTHQvalues of Cu, Zn, Cd and Pb in the eight kinds of aquatic products were all less than 0.1. TheTHQvalues of Cr inP.vannameiandR.philippinarumwere greater, but less than 1. It indicates within the recommended ranges, single one of the five heavy metals had no significant non-carcinogenic exposure risk to the population. Considering the combined pollution of the heavy metals, theTTHQwas calculated. TheTTHQvalue was also lower than 1, indicating that it was safe to intake the above aquatic products, and the health risk of heavy metal exposure was low.

In overall, the consumption risk of the aquatic products purchased from the Liujie Market of Binzhou was relatively low. However, the risk of Cr exposure was high, so residents should control the intake ofR.philippinarumandP.vannamei.

4 Conclusions

Five heavy metals including Cu, Zn, Cd, Cr and Pb were commonly detected in the aquatic products purchased from the Liujie Market in Binzhou. There were significant differences in the heavy metal residues among different species. In comparison, bivalves accumulated more heavy metals than fishes. Compared with the limits on heavy metals in aquatic products specified by the relevant ministries and commissions of China, there was a certain excess of Cr inR.philippinarumandP.vannamei. The calculation results of EDI and THQ show that it was relatively safe for Binzhou citizens to eat the above aquatic products, and the health risk of heavy metal exposure was relatively low, however, the intake ofR.philippinarumandP.vannameishould be controlled.