Abstract
Microplastic pollution and potentially toxic elements in aquatic environments pose significant environmental threats to marine ecosystems and human health. This study aimed to investigate the relationship between microplastic pollution and potentially toxic elements in the surface waters along the southwestern coasts of the Caspian Sea. In this study, the elements arsenic (As), cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), mercury (Hg), lead (Pb) and zinc (Zn) were selected due to their toxic potential, bioaccumulation ability and widespread presence in aquatic environments contaminated by industrial, agricultural and urban activities. To achieve this aim, sampling was conducted at three selected stations: Kiashahr, Anzali and Astara. These stations were chosen to assess the distribution and concentration of microplastics and potentially toxic elements in the region. For microplastic sampling in surface waters, a plankton net with a mesh size of 0.35 mm was used. The net was towed by a boat at a depth of 25 cm from the water surface at a constant speed of 5 km/h for 5 minutes. In the microplastic separation process, organic matter in the water samples was first digested using hydrogen peroxide (H₂O₂, 30%). Then, microplastics were separated from other suspended particles by density-based flotation in saturated NaCl solution. The concentrations of potentially toxic elements were measured using inductively coupled plasma mass spectrometry (ICP-MS). Data analysis was performed using SPSS version 27 at a 95% confidence level. The results showed that the highest mean concentrations of elements at the Kiashahr and Anzali stations were Cu > As > Hg and at the Astara station were As > Zn > Cu > Hg. The Anzali station showed the highest abundance of microplastics with 57.66 ± 5.50 particles per m³. The extracted microplastics were classified into two morphological types: fibres and fragments, appearing in six colors: blue, brown, red, transparent, black and green. Among them, blue was the dominant color with a frequency of 78%. The highest abundance of microplastics was observed in the size ranges of 4 to 5 mm and greater than 5 mm. FTIR-ATR spectroscopy identified four types of polymers in the microplastic samples: polyethylene, polypropylene, polyester and polystyrene, among which polyethylene was identified as the dominant polymer with the highest frequency. Comparison of the concentrations of potentially toxic elements against global standards revealed that the levels of the measured elements in the surface waters of the southwestern coasts of the Caspian Sea are generally within the established standard limits. However, the concentration of mercury in this region, considering the prohibition on discharging this element into marine ecosystems, exceeds the permissible environmental standards and pollution management principles. This result indicates the presence of mercury pollution in the coastal waters, which could have negative and long-term impacts on marine ecosystems and the health of aquatic organisms. Furthermore, correlation analysis revealed a significant positive correlation between the abundance of microplastics and mercury concentration at the Kiashahr station. These findings highlight the potential role of microplastics in the adsorption, transfer and accumulation of pollutants in aquatic environments. Overall, the results of this study emphasize the need for more precise monitoring and management of pollutant sources, as well as the evaluation of risks associated with the accumulation of microplastics in the coastal waters of the Caspian Sea. This is crucial for developing effective management strategies aimed at preserving ecosystem health and reducing pollution, particularly in coastal areas impacted by human activities.