Conversion of microplastics to ocs during estuarine mixing the Aras River with the Caspian Sea


 Microplastics originated from various sources are carried by rivers into oceans, seas and lakes. In the last few years, the accumulation of microplastic particles in marine environments has been on the increase which causes irreversible damages to flora, fauna and human health. One of the most considerable processes in an estuary is the flocculation process. The flocculation process converts pollutants to flocs or greater particles. In the present study, the conversion of microplastics to flocs during estuarine mixing of the Aras River water and the Caspian Sea water is investigated for the first time. The results clearly show that a huge percentage of microplastics (99.95%) are converted to greater particles (> 5mm) due to the flocculation process. The maximum flocculation rate of microplastics (47.37%) is observed at the salinity of 0.25 ppt. Moreover, 35.71% of microplastics are flocculated at the salinity of 29 ppt. Salinity enhances the flocculation of microplastics.


Introduction
Plastics are synthetic organic polymers which can be utilized to manufacture various products (Ferreira et al. 2019; N V Lakshmi Kavya et al. 2020;Mason et al. 2018). Plastic particles smaller than 5 mm can be classi ed as primary and secondary microplastics (Klemeš and Jiang 2020). Primary microplastics are manufactured at a microscopic size for consumer care products or industrial uses, while secondary microplastics are degraded from larger-sized pieces (Cole et al. 2011;Frias et al. 2020;Han et al. 2020).
Microplastics derived from various point sources and non-point sources are eventually carried by rivers into water bodies including oceans, seas and lakes (Bergmann et al. 2015;El Hadri et al. 2020;Thushari and Senevirathna 2020). In the last few years, the accumulation of microplastics in marine ecosystems has been on the increase (Bonanno and Orlando-Bonaca 2020;Fadare et al. 2020). Such pollutants in marine and aquatic environments have long-lasting detrimental effects on marine organisms (Eerkes-Medrano 2015; Jaikumar 2019; Ozturk and Altinok 2020). A wide variety of deadly diseases can be observed in marine and aquatic organisms because of exposure to microplastic particles (Anbumani and Kakkar 2018;Prokić et al. 2019). Microplastics are ingested by marine organisms and transported through the water bodies via the food chain, eventually causing irreparable damages to human health (McGoran et al. 2017;Santana et al. 2017;Susanti et al. 2020). Estuaries are semi-enclosed coastal areas where the mixture of freshwater from the river and saline water from the ocean occur (Heidari 2019). One of the most signi cant processes in an estuary is the occulation process (Heidari 2019). A large percentage of pollutants is converted to ocs or greater particles due to the natural and estuarine occulation process (Karbassi and Heidari 2015). The vital role of the occulation process in the selfpuri cation of pollutants gives aid to the pollution load of water bodies to be on the decline. In other words, the conditions of marine ecosystems are enriched due to the natural and estuarine occulation process. In the present investigation, the effect of the occulation process on converting microplastic particles to greater particles during estuarine mixing of freshwater from the Aras River and saline water from the Caspian Sea is studied for the rst time.

Materials And Methods
Aras River with 5323 × 10 6 m 3 /year mean annual discharge nds its way into the Caspian Sea. A wide range of marine organisms dwell in the Caspian Sea that is the world's largest lake. The location of sampling is indicated in Fig. 1.
Four Niskin bottles (30 L) were used to sample freshwater and saline water simultaneously (Bagaev et al. 2018). River water samples were collected from the Aras River upstream (Fig. 1). For preventing from the mixture of river water and seawater, saline water samples were collected approximately 20 km away from the Caspian Sea coast (Fig. 1). The HNO 3 and HCl were mixed to acid wash all equipment. Moreover, rinsing was carried out with running Milli-Q water. Collected samples were then passed through a 50 μm stainless steel sieve, stored in glass bottles and packed with aluminum foils. 30 L of fresh and saline water samples were ltered through MF-Millipore™ Membrane Filter, 0.45 µm pore size. For the occulation experiments, 30 L of saline water sample was passed through 0.45 µm MF-Millipore™ Membrane Filter to ensure that the sample is free of microplastics. Different salinity regimes from 0.5 to 29 ppt were formed by mixing ltered saline water with freshwater at room temperature. Mixed samples were kept for 24 h with occasional stirring. The lters containing the microplastics and occulated particles were left to dry and transferred to Petri dishes for further experiments. The visual identi cation technique was used to identify microplastics and occulated particles preserved on the lters.

Results And Discussion
The concentration of occulants at various values of salinity, dissolved organic carbon (DOC), pH, dissolved oxygen (DO), electroconductivity (EC) and temperature (T) are shown in Table 1. The microplastic abundance in freshwater and saline water is also presented in Table 1. According to Table 1, the abundance of microplastics in the Aras River and the Caspian Sea is respectively 3800 and 1800 items/m 3 . Natural and estuarine occulation processes may not occur as indicated in Table 1. In other words, at the very rst stages of estuarine mixing, some of the microplastic particles are removed from the freshwater in the form of occulants (Table 2). of total microplastic particles (3800 items/m 3 ) are converted to greater particles (>5mm) due to the occulation process during estuarine mixing of the Aras River water and the Caspian Sea water. As a result, the occulation process gives aid to the self-puri cation of microplastic particles (Fig. 2).
According to the conceptual model (Fig. 2), microplastic particles derived from point sources and nonpoint sources nd their ways into the Aras River. In the estuary, the formation of occulation process eliminates 99.95% of microplastics which plays a vital role in enriching the condition of the Caspian Sea in an appropriate manner (Fig. 2). In other words, the mass balance between the Aras River and the Caspian Sea is in uenced by natural and estuarine occulation process. Heidari (2019) clearly showed that trace metals are occulated during estuarine mixing freshwater from the river with saline water from the sea. The occulation rate of microplastics at the salinity of 0.25 ppt is 1800 items/m 3 (47.37 %) ( Table 2). A huge percentage of occulants are formed at the earlier stages of mixing freshwater from the river with saline water from the sea (Farajnejad et al. 2017). Table 2 shows that 35.71% (1357 items/m 3 ) of microplastics are occulated at the highest studied salinity (29 ppt). The occulation rate of microplastic particles at different salinities could be arranged in the following order (Table 2): 47.37% (salinity of 0.25 ppt) > 35.71% (salinity of 29 ppt) > 5.26% (salinity of 6 ppt) > lower than 3% (other studied salinities ppt). The occulation of microplastic particles is varied during estuarine mixing due to forming various stages of mixing freshwater from the river with saline water from the sea. Such a condition gives aid to the negative net charge of microplastics to be on the decrease. A wide variety of salinity regimes including oligohaline (0.5-5.0 ppt), mesohaline (5.0-18.0 ppt) and polyhaline (18.0 30.0 ppt) can be seen in an estuary (Chang, 2012). The effect of salinity, DOC, pH, DO, EC and T on the occulation of microplastic particles is illustrated in Fig. 3.
According to Fig. 3, the conversion of microplastics to greater particles (> 5mm) can be controlled by salinity, DOC and EC due to the high similarity coe cient (0.93). Moreover, occulation of microplastic particles is governed by T, pH and DO with a similarity coe cient of around 0.86 (Fig. 3). Consequently, salinity, DOC, pH, DO, EC and T as effective factors enrich the occulation of microplastic particles.
Heidari (2019) showed that the natural and estuarine occulation process is in uenced by a wide variety of factors including salinity, pH and dissolved organic carbon (DOC). The occulation process was governed by dissolved oxygen (DO) (Karbassi and Heidari 2015). Considering the abundance of microplastics in the Aras River water (3800 items/m 3 ) and the mean discharge of the river (5323 × 10 6 m 3 /year), the annual load of microplastics carried by the Aras River into the Caspian Sea would be 2.02 × 10 13 items/year. Nevertheless, the results of such a study clearly show that 99.95% of microplastic particles are occulated during estuarine mixing of the Aras River water and the Caspian Sea water. As a result, the mean annual discharge of microplastics from the Aras River into the Caspian Sea would reduce from 2.02 × 10 13 to 1.01 × 10 10 items/year.

Conclusion
The results obtained in this study represent that 99.95% of microplastics are occulated at salinities of 0.5-29 ppt due to the natural and estuarine occulation process. At the salinities of 0.5 and 29 ppt, 47.37% and 35.71% of microplastics were respectively converted to greater particles (>5mm) during estuarine mixing freshwater from the Aras River with saline water from the Caspian Sea. Flocculation of microplastics was governed by salinity. Natural and estuarine occulation process remarkably reduced the microplastic load of the Caspian Sea which plays a vital role in enriching the conditions of marine ecosystems. As a result, the natural and estuarine occulation process should be applied to eliminate microplastic particles from water bodies (oceans, seas, lakes and rivers) and different types of wastewaters.

Declarations
Funding (University of Tehran) Con icts of interest/Competing interests (The authors declare that they have no known competing nancial interests or personal relationships that could have appeared to in uence the work reported in this paper) Availability of data and material (Not applicable) Code availability (Not applicable) Ethics approval (Not applicable) Consent to participate (Not applicable) Consent for publication (Not applicable) Figure 1 The location of collecting freshwater and saline water samples Cluster analysis of microplastics during estuarine mixing