Source and risk assessment of heavy metals and microplastics in bivalves and coastal sediments of the Northern Persian Gulf, Hormogzan Province
Introduction
Coastal environments faced rapid urbanization and industrialization over the past century. Marine sediment is a valuable indicator for monitoring pollution since it acts both as a sink and as a secondary source of pollutants in the aquatic environment (Crawford and Quinn, 2016; Rocha-Santos and Duarte, 2017). Biota can also be used to biomonitor pollution, and itself can be a source of exposure to pollutants such as microplastics (fragments or original plastics < 5 mm) and heavy metals (de Sá et al., 2015; Maulvault et al., 2015; Arulkumar et al., 2017; Ward et al., 2019). The status of the coastal environment can be assessed through heavy metals contamination in the marine organisms and coastal sediments (Frias et al., 2016; Pellini et al., 2018a) and microplastics pollution. Microplastics have not been regularly monitored in recent years, however, they were first reported in coastal sites in the early 1970s (Di Cesare et al., 2020; Tien et al., 2020).
Heavy metals tend to bioaccumulate in marine organisms, therefore this contamination is recognized as a threat to ecosystems and may also cause a health risk to humans through the intake of contaminated seafood or fish (Uysal et al., 2009; Soltani et al., 2019). Heavy metals’ uptake by aquatic organisms occurs mostly through water and contact with superficial sediment via benthic and pelagic biota (Soltani et al., 2019). In contrast, the effect of microplastics on ecosystems is still unclear. Numerous marine organisms, including bivalves, fish and oysters pick up microplastics from the water column or sediments as they can be mistaken for food (Bessa et al., 2018; Pellini et al, 2018a, 2018b). When ingested, depending on their size and shape, microplastics can cause blockage of the digestive track of organism, (Akhbarizadeh et al., 2019a, Akhbarizadeh et al., 2019b) and they could exert toxicity when entered the bloodstream (microplastics <20 μm) (Rothen-Rutishauser et al., 2006). Microplastics can also act as carriers of organic and inorganic contaminants present in water (Zhou et al., 2020; Abbasi et al., 2020, 2021) and could cause enhanced exposure to these. Microplastics, depending on their size are also excreted (Xu et al., 2020) and then they do not exert apparent impact on amphipods or plants (Fang et al., 2021). The microplastics accumulated in organs of aquatic species (i.e. muscle, gut, gills, and liver) often tend to be very variable in terms of size, roughness, and shape (Abbasi et al., 2018) and finally can reach humans via indirect and direct pathways (Abbasi and Turner, 2021).
Heavy metals presence in coastal sediments can be related to atmospheric deposition, geological weathering, soil erosion, airborne dust, and human activities, including waste disposal. Recently, anthropogenic sources have dominated heavy metals contamination in sediments (Qiao et al., 2020). The most important anthropogenic sources of heavy metals include: fertilizers, pesticides, wastes from smelting ores, leachates from mining sites, leakage of brake fluids, vehicular traffic, sewage sludge, industrial wastes, and partially treated or untreated industrial wastewater (Wang et al., 2020). The total heavy metals concentration is not sufficient to evaluate the adverse effects of contaminated sediment, because heavy metals exist in various chemical species in sediment, which have different mobility, bioavailability and potential toxicity (Delshab et al., 2017). With reference to microplastics, their known main sources are the degradation of plastics released from industry, uncontrolled litter disposal, and inputs from wastewater, gray water and urban run-off (Li et al., 2020). It is important to identify heavy metals and microplastics in environmental studies because, although these are very different type of contaminants, they could have a synergistic effect.
Mobility/bioavailability monitoring studies of pollutants are often restricted by the numerous steps needed for sample treatment. The harmonized and most common procedure used for heavy metals fractionation is the European Community Bureau of Reference sequential extraction, BCR (Davidson et al., 1998; Quevauviller et al., 1994; Rauret et al., 2000). There are no harmonized methods for the monitoring of microplastics and current methods involve their separation from the environmental matrix by filtration or flotation; the characterization of their sizes and shapes with microscopy, and composition with IR or Raman spectroscopy (Li et al., 2020).
The Hormozgan Province coast is an important Iranian port and hosts a wide variety of marine organisms. In recent decades, the fishery industry has co-existed with the oil industry and the area has seen an expansion or new installation of power plants, zinc and steel processing plants, desalination plants, food-processing units, and cement factories. These developments and activities can result in the release of relevant pollutants in the marine ecosystem (Nozar et al., 2014). Since 2010, the presence and impact of heavy metals and microplastics in sediments and selected aquatic organisms from the Persian Gulf has been investigated (Abdolahpur Monikh et al., 2013; Abbasi et al., 2018, 2019; Delshab et al., 2017). However, there is no comprehensive study on the levels and distribution of microplastics and heavy metals in the Hormozgan Province coast. The objectives of the current study were: i) to investigate the concentration and associated biological risks of heavy metals in the sediments of Persian Gulf, ii) to identify chemical speciation and mobility potential of heavy metals in sediment, iii) to assess health risks of heavy metals via bivalve consumption, iv) to determine the spatial distribution and abundance of microplastics in coastal sediments and selected bivalves in the Persian Gulf. The possible sources of heavy metals were investigated using multiple linear regression with principal component analysis (MLR/PCA).
Section snippets
Study area
Hormozgan Province is located south of Iran facing the United Arab Emirates and Oman. Bandar Abbas City with an area of 70,697 km2 is the provincial capital. The province has fourteen islands in the Persian Gulf and 1000 km of coastline covering an area of 71,193 km2 with a population of 1,500,000 people (Hatam et al., 2015). Being located in an optimal geographic situation and having numerous natural, economic, fisheries, maritime, agricultural as well as livestock resources, Hormozgan is a
Heavy metals in sediments
Heavy metals in sediments and bivalves were instigated by their elevated levels reported previously in fish, prawn, and crab (Soltani et al., 2019; Keshavarzi et al., 2018). The heavy metals concentration in sediments and bivalves are summarized in Table 1. The highest concentrations of Mo, Pb, Zn, Cr, Cu, and Mn, Hg, and Sb were detected in areas with frequent human activities including Shahid Rajaee Port (S3), Shahid Bahonar Port (S1), and Tavanir station (S4), respectively. Shahid Rajaee
Conclusion
This study has evaluated the concentrations of heavy metals and microplastics in the edible tissues of bivalves and coastal sediments in the Northern Persian Gulf. The mean EF values are more than 5 for As, Mn, Cr, and Ni, which indicates the anthropogenic contribution to pollution in the sediment samples. The MPI revealed that sediments are moderate to severely polluted, mostly from anthropogenic sources and this would justify policies to regenerate the area and reduce anthropogenic pollution.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors wish to express their gratitude to the Research Committee and Medical Geology Center of Shiraz University for logistical and technical assistance.
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