Occurrence, Abundance and Characteristics of Microplastics in the Sediments of the Zayandeh- Rud River

Abstract

Introduction: One of the most important emerging pollutants in aquatic environments that has recently attracted the attention of global researchers in recent years is plastic waste, especially microplastics. Aquatic fauna can ingest microplastics as they confuse it for their prey. Aquatic organisms can easily ingest microplastics because their size is similar to that of the larvae of several organisms, including plankton. Consequently, the animal may develop a sense of satiety after ingesting microplastics, causing reduced food uptake and decreased energy intake, which in turn may affect growth, survival, fecundity and reproduction rate, impacting general fitness of the organisms. Once ingested, these particles can be transferred to higher trophic levels. Despite the fact that freshwater is the only source of drinking water, knowledge about the effects of microplastic pollution in freshwater environments compared to marine environments is still in its infancy. Therefore, the aim of the present study is too comprehensive investigate the presence and distribution of microplastics in the sediments of Zayandeh-Rud River.Materials and Methods: From upstream to downstream of Zayandeh-Rud River, 21 sampling stations were selected and in each station, sampling was done in an area of ​​30 x 30 cm to a depth of approximately 10 cm above the sediment in three repetitions. Sample processing was done during two stages of organic material digestion using hydrogen peroxide and density separation using sodium chloride salt. Microplastics were identified using a standard light microscope with magnifications of 40x, 100x and 400x. Different standard protocols such as pressure, tension, additional light or hot needle tests were used to detect microplastics. After identifying the microplastic particles, their type and shape, color and size were determined and recorded. Raman spectrometer was used to determine the polymer composition of microplastics.Results: The average abundance of microplastics was 70.481±29.39 particles in 50 gr/dry sediment. Station 17 with an average number of 363.50±14.849 particles and stations 1, 6 and 7 with an average number of 1.33 ±1.528 particles in 50 gr/dry sediment had the highest and lowest levels of pollution, respectively. The size of all microplastics was in the range of 50-5000 micrometers. Microplastics less than 500 microns were the most abundant with 64.5%. The dominant form of microplastics with the frequency of 34.9%, 33% and 28.8% belonged to line, fragment and fiber, respectively. In total, 4 polymers, polypropylene, polyamide, polyurethane and polyethylene, were identified. The results were compared with the reference spectra provided in the Openspecy database. In total, 3 different polymers including polypropylene (PP), polyethylene (PE) and polyamide (PA) were found in sediments.Discussion: The result of this research indicates microplastics dispersion in the sediments of Zayandeh-Rud River. Polypropylene (PP), polyamide (PA) and polyethylene (PE) polymers found in Zayandeh-Rud River sediments are often used in textiles. The composition of the polymer in the sediments clearly indicates the discharge of wastewater from the washing of synthetic textile into the river. Polypropylene and polyethylene are used to make ropes and fishing nets. In addition, polypropylene fibers are widely used for the production of floor coverings, carpets and rugs, sports clothes, making agricultural mulch, fishing nets, packaging bags and ropes. Due to its high strength and hardness, polyamide is used to produce items such as ropes, safety belts, parachutes, thread and fishing nets, as well as in the production of synthetic fibers such as clothes, socks and carpet fibers.Therefore, it can be stated that human activities, including the discharge of urban and industrial wastewater, plastic waste by tourists on the river and also the use of fishing nets by fishermen, play an important role in microplastic pollution in this area. Therefore, developing a program to monitoring microplastics in the water and sediment of Zayandeh-Rud River will be of great help for management measures to control this pollution.

Keywords

• pollution
• sediment
• zayandeh- rud river
• microplastic