Release of heavy metals under pre-set redox potentials in Musa estuary sediments, northwestern of Persian Gulf
Graphical abstract
Introduction
Potentially toxic elements (PTEs) find their way into aquatic systems through natural and anthropogenic sources. In marine ecosystems, studying heavy metals (HMs) is important due to their bioaccumulation, environmental persistence, and toxicity (Gopinath et al., 2010; Stankovic et al., 2014). Sediments act as a potential sink for various pollutants such as HMs (Schroeder et al., 2019). HMs in sediments exist in various geochemical phases, in which their mobility and bioavailability depend on sediment-bound fractions (Antić Mladenović et al., 2017b). The presence of Fe, Mn, and Al oxides, organic matter, and clay minerals can favor the adsorption of metals on sediments (Buyang et al., 2019; Huang et al., 2020).
However, the adsorbed HMs could re-enter the water column under biological activities or changes in aquatic conditions such as pH, redox potentials (Eh), salinity, and temperature (Frémion et al., 2017; Liu et al., 2019; Pejman et al., 2017; Tsai et al., 2003; Zang et al., 2017). Thus, these factors affect HMs mobility and bioavailability and their associated environmental risks.
The redox reaction is an important process in soils/sediments that can change the mobility of HMs. For instance, a study on Woolston Canal sediment showed that the dissolved concentrations of HMs (Cr, Cu, Ni, Pb, and Zn) increased under oxidizing conditions and decreased under reduced conditions in leachates (Hartley and Dickinson, 2010). Miao et al. (2006) reported Pb and Zn were positively correlated with Eh changes in sediments from Mississippi River deltaic plain freshwater lake. However, Antić Mladenović et al. (2017b) indicated a negative correlation between Eh and soluble Ni and Pb in a periodically flooded arable soil. Rinklebe et al., 2016a, Rinklebe et al., 2016b, Rinklebe et al., 2016c observed in a serpentine-enriched fluvisol that Ni is mobilized at low Eh but it is immobilized in oxidizing conditions. Therefore, there are no general rules for predicting the mobility and release of metals and they vary case by case. This highlights the need for studying Eh changes in different environments.
Musa estuary, located in the northwestern Persian Gulf receives a tremendous amount of organic and inorganic contaminants from industrial wastewater, urban, and agricultural effluents (Jaafarzadeh Haghighi Fard et al., 2017; Keshavarzi et al., 2018; Pejman et al., 2017). Nevertheless, due to the gentle topography and favorable environment for the fauna and flora, it is of significant ecological importance. Musa estuary hosts a biodiverse habitat of various kinds of fish, shrimp, and water bird species (Saadati et al., 2020). The surface sediments of this area are rich in benthic organisms as well (Pejman et al., 2015). It is also an economically important place for fisheries and aquaculture activities (Ravanbakhsh et al., 2020).
Several factors, such as limited circulation of water, long flushing time, high temperature, high salinity, and rapid evaporation may cause a long persistence of contaminants in Musa estuary (Abdolahpur Monikh et al., 2015; Jaafarzadeh Haghighi Fard et al., 2017). Thus, it is likely that a load of contamination in this estuary may reach a crucial level for the biota living in the area.
The continual discharge of the organic waste to Musa estuary and the accumulation of biodegradable organic pollutants expose this water body to a possible reduction of redox potential as a result of depletion of dissolved oxygen and the growth of micro-organisms. The reducing condition may lead to a release of large amounts of HMs trapped in sediments. The release of HMs from sediments to water leaves adverse effects on aquatic ecosystems. Excess of HMs may pose serious threats to fauna and flora. Additionally, metals in the soluble phase are easily available to plants and aquatic organisms and subsequently can be transferred to humans through the food chain. It should be noted that Musa estuary roles as a great source of fishery for the people living in the coastal cities of Mahshahr (population 150,000), Sarbandar (75,000) and Hendijan (50,000) (Soltani et al., 2019). This highlights the importance of predicting HMs mobility and bioavailability for an appropriate ecological risk evaluation and management strategy in Musa estuary.
To our best knowledge, few studies have been conducted on the redox behavior of HMs in sediments and no research has been reported about natural sediments rich in organic matter in anoxic conditions. Besides, the mobility and bioavailability of HMs in Musa estuary sediments under dynamic redox conditions has not been systematically studied to date. According to all above mentioned, the objectives of this study were:
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to assess the impact of pre-set Eh-conditions and Eh-dependent factors on the release dynamics of HMs (Co, Cr, Cu, Ni, Pb, V, and Zn) using an automated biogeochemical microcosm system.
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to identify the geochemical fractionations of selected HMs in sediments using the Tessier sequential extraction procedure.
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to assess the contamination and risk status using some credible indexes.
Section snippets
Sample collection
The main fishery port is located near the industrial complex and the people mostly consume sea food (Keshavarzi et al., 2018). The sampling station (latitude of 30° 26′ 46.31″ and longitude of 49° 6′ 47.86″) was located at the downstream discharge of the industrial complex main outlet (Fig. 1). This sampling station was selected as this area contains higher target HMs. Moreover, in more polluted zones, the number of resistant benthos (Bivalves) are abundant (Haghshenas, 2020). Therefore, this
Sediment characterization
Table 4 lists the basic properties and chemical constituents of the studied sediment. XRF and XRD analyses showed that calcite and quartz were the abundant minerals and the major compounds in the sediment sample were CaO (30.9%), SiO2 (22%), and Al2O3 (5.2%). Particle size distribution indicated the sediment is in the category of silt with a mean size of 12.32 μm. The sediment was slightly alkaline (pH of 8.14) which might be due to the presence of carbonate shells and the discharge of
Conclusion
Studying the behavior of HMs under dynamic redox conditions could serve as a precondition for evaluating the potential risk, the fate of HMs, and creating proper environmental management strategies on contaminated sites to avoid pollutant exposure to aquatic organisms. The redox-induced changes in the release dynamics of HMs (Co, Cr, Cu, Ni, Pb, V, and Zn) in the Musa estuary sediment were studied. The high concentration of Co, Ni, Pb, and Zn in the potentially mobile fractions was reflected in
CRediT authorship contribution statement
Reyhane Madadi: Conceptualization, Methodology, Formal analysis, Investigation, Writing – original draft, Visualization. Abdolreza Karbassi: Conceptualization, Methodology, Writing – review & editing, Supervision. Mohsen Saeedi: Conceptualization, Resources, Writing – review & editing, Supervision.
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
This work was supported by environmental research laboratory, Iran University of Science and Technology. We thank Sedigheh Mohamadi for her assistance in manuscript writing.
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