# Spatial and seasonal abundance and characteristics of microplastics along the Red River to the Gulf of Tonkin, Vietnam

Thi Thao Nguyen, van Hoi Bui, Stéphanie Lebarillier, Toan Khanh Vu, Pascal Wong-Wah-Chung, Vincent Fauvelle, Laure Malleret

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Thi Thao Nguyen, van Hoi Bui, Stéphanie Lebarillier, Toan Khanh Vu, Pascal Wong-Wah-Chung, et al. Spatial and seasonal abundance and characteristics of microplastics along the Red River to the Gulf of Tonkin, Vietnam. Science of the Total Environment, 2024, 957, 10.1016/j.scitotenv.2024.177778. insu-04835173

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# Spatial and seasonal abundance and characteristics of microplastics along the Red River to the Gulf of Tonkin, Vietnam

Thi Thao Nguyen<sup>a,b</sup>, Van Hoi Bui<sup>b</sup>, Stéphanie Lebarillier<sup>a</sup>, Toan Khanh Vu<sup>a,b,c</sup>, Pascal Wong-Wah-Chung<sup>a</sup>, Vincent Fauvelle<sup>c</sup>, Laure Malleret<sup>a,*</sup>

<figure><img src="image_3.png" /></figure>

## HIGHLIGHTS

* Microplastics were found in all water and sediment samples along the Red River.

* The most abundant polymers were PE, PP, PET; fiber being the predominant shape.

* Seasonal variability was noted, with higher plastic deposition during the dry season.

* The particle of 13–200 µm was the major size classification.

* The risk was high to danger for the hazard index.

## GRAPHICAL ABSTRACT

<figure><img src="image_4.png" /></figure>

## ARTICLE INFO

Editor: Damià Barceló

**Keywords:**

Microplastics

µ-FTIR

Surface water

Surface sediment

Risk assessment

Red River

Vietnam

## ABSTRACT

This study aimed to examine the occurrence of microplastics in surface water and sediment samples collected from Hanoi to the Ba Lat estuary along the Red River, the second-largest river in Vietnam (surface area: 156,451 km²). 21 stations were sampled during the dry (March 2023) and rainy (September 2023) seasons. The analytical procedure involved: digestion with hydrogen peroxide, flotation with potassium carbonate, and overflow filtration. The filters were analyzed by microscopy (Nikon SMZ645) to describe shapes and colors and by µ-FTIR (PerkinElmer Spotlight 400) to determine polymer types and abundances. Results showed that microplastic quantities throughout the river ranged from 10 to 203 items.m⁻³ in surface water and from 653 to 8069 and 990 to 21,610 items.kg⁻¹ dried weight (d.w.) in sediment during the rainy and dry seasons, respectively. MPs were classified into two main shape groups: fiber and fragment, with fibers being predominant, representing 82.0 % and 75.5 % of microplastics in water and sediment, respectively. The primary colors identified were white/ transparent, black, and blue. Particles between 13 and 200 µm were the predominant size class, accounting for 64.1 % and 72.4 % of the microplastics in water and sediment, respectively. Polyethylene, polypropylene, and polyethylene terephthalate were the main polymers, accounting for 71.5 % and 72.2 % of the microplastics in water and sediment, as revealed by µ-FTIR analyses. Overall, in the Red River, the MP pollution load is moderate,

<footnote><sup>a</sup> Aix Marseille Univ, CNRS, LCE, Laboratoire Chimie Environnement, FR ECCOREV, ITEM, OCEAN, Aix-en-Provence, France</footnote>

<footnote><sup>b</sup> Department of Water-Environment-Oceanography, University of Science and Technology of Hanoi (USTH), Vietnam Academy of Science and Technology (VAST), 18 Hoang Quoc Viet, Cau Giay, Hanoi 100000, Viet Nam</footnote>

<footnote><sup>c</sup> Université de Toulouse, LEGOS (CNES/CNRS/IRD/UPS), Toulouse, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: laure.malleret@univ-amu.fr (L. Malleret).</footnote>
but the type of particles detected represents a high to dangerous polymer risk, resulting in a very high potential ecological risk on the river.

# 1. Introduction

From 1.5 million tons (Mt) produced in the 1950s to >348 Mt. in 2017, the amount of plastics used has increased dramatically over the last 70 years (Li et al., 2021b) and has become a global problem (Ismanto et al., 2023).

Solid plastic particles between 1 µm and 5 mm in size are referred to as microplastics (MPs), a term first used in 2004 (Li et al., 2021b; Loganathan and Jacob, 2023; Thompson et al., 2004). MPs can be further classified according to their shapes (spheres, beads, pellets, foams, fibers, fragments, films, and flakes), colors, polymer types, and size distribution. Depending on their origin, primary MPs are intentionally manufactured for specific purposes, such as microbeads used in cosmetics. However, the more prevalent type in the environment is secondary MPs, which result from the fragmentation or degradation of larger plastics through mechanical, natural, or biological processes (Li et al., 2020, 2018; Waldschläger et al., 2020).

MPs are regarded as novel, ubiquitous, and hazardous entities that reach all the environmental compartments and have been found in oceans (Andreas et al., 2021; Zheng et al., 2023), rivers (Ta and Babel, 2020; Zhao et al., 2023a, 2023b), mangroves (Cordova et al., 2019; Morgado et al., 2022), and even within aquatic organisms (Cabansag et al., 2021; Espiritu et al., 2023; Karbalaei et al., 2020). Indeed, the bioavailability of particles and the release of polymers or additives are higher in MPs due to their small size and large surface-area-to-volume ratio (Alimi et al., 2021). This increases the sorption of toxic chemicals and pathogens, the leaching of potentially toxic polymers and additives, and the likelihood of erroneous ingestion and transfer across the food chain (Borges Ramirez et al., 2019; Mansuri et al., 2023; Wei et al., 2015). Furthermore, compared to macro-debris, MPs are challenging to remove from natural environments once released, and developing remediation solutions may take a long time, given the current state of technology. Their harmful effects and the limited possibilities for remediation make MPs a cause for concern, the consequences of which will persist for many years.

Globally, most MP studies have been conducted in high-income countries (31 % in China, 11 % in the United States, 8 % in Germany, and 8 % in the United Kingdom), while few evaluations of microplastics have been carried out in the lower-middle-income countries (Gabisa and Gheewala, 2022). In particular, the Association of Southeast Asian Nations (ASEAN) is one of the least studied region regarding microplastic contamination. Only 5 % of all scientific publications on microplastics were retrieved from Scopus in March 2022, according to Gabisa and Gheewala (2022), which is contradictory to recent estimates showing that ASEAN rivers contribute most of the world's continental plastic to the oceans (Curren et al., 2021; Jambeck et al., 2015; Walther et al., 2020). Of the 1.15 to 2.41 Mt. of plastic that flows from the world's rivers into the oceans annually, 34 to 71 % is carried by Asian rivers (Lebreton et al., 2017). This disproportionate contribution of Asian rivers to global plastic emissions was also confirmed by Schimdt et al. (2017). Of the top 10 countries ranked by mass of mismanaged plastics waste in 2010, eight are in ASEAN, and five are considered lower-middle-income countries by the World Bank: Indonesia, the Philippines, Vietnam, Sri Lanka, and Bangladesh (Jambeck et al., 2015).

Vietnam is a developing nation in ASEAN with an economic growth of 8 % and a population of nearly 100 million in 2022 (World Bank). Vietnam ranks 4th among the top 20 nations globally for the amount of plastic waste entering the ocean, with approximately 1.8 Mt. of mismanaged plastic waste discharged in 2010 and an estimated 4.17 Mt. in 2025, which is 10 % higher than the global average (Giles et al., 2021; Hai and Vu, 2019; Jambeck et al., 2015; Le Dai, 2019). The World Bank

estimated 2022 that 3.1 Mt. of plastic waste is discharged on land in Vietnam, and at least 10 % of this waste reaches the ocean. The amount of plastic in waterways could more than double by 2030 if the country's current waste collection, recycling, and treatment processes are not improved. Some recent studies on MP contamination were conducted in Vietnam (27 articles from 2018 to March 2022, Gabisa and Gheewala, 2022). Most of these studies focused on southern Vietnam, whereas the capital of Vietnam, Hanoi, is located in northern Vietnam. Hanoi hosted >5.2 million people in 2023, with an annual population growth rate of 3.4 % (World Population Review). Its economic and industrial development (6.27 % in 2023) may negatively impact its aquatic environment, especially on the Red River, which flows through Hanoi and is the largest river in Northern Vietnam. However, little data on microplastics in the water and sediment of the Red River has been reported. Specifically, one sampling point was investigated for microplastic contamination in the river water (Strady et al., 2021), sediment (Le et al., 2022b), and the mangroves (Dung et al., 2021).

As few studies in the region have attempted to evaluate the effect of temporal and spatial variations on MP concentration (Gabisa and Gheewala, 2022), this paper aims to address this data gap by documenting the occurrence of microplastics at 21 sampling points along the Red River to the Gulf of Tonkin during the dry season (March 2023) and the rainy season (September 2023). The abundance and seasonality of MPs were investigated in surface water and sediment to assess the main inputs arriving in the surface water layer and the proportion of high-density MPs dragged down to the underlying sediments, contributing to long-term pollution. Moreover, collecting surface samples allows researchers to establish consistent monitoring practices across different regions and times. In addition, the characteristics of MPs (shape, color, size, and polymer type) were identified, and the ecotoxicological risk was assessed based on the polymer hazard and pollution load indexes.

# 2. Materials and methods

## 2.1. Study area and sample collection

The present study was conducted on the Red River, the second-largest river in Vietnam and one of the five largest rivers along the East Asian coast. The Red River basin has a surface area of 156,451 km² (Le et al., 2018). Originating in the Yunnan province, China, the Red River runs through several provinces of Vietnam before flowing into Tonkin Bay through four estuaries: Ba Lat, Tra Ly, Lach Giang, and Day. The Ba Lat estuary is the main estuary of the Red River and belongs to two provinces, Thai Binh and Nam Dinh. The discharge at the Hanoi station reaches a maximum during the rainy season (July–August: approximately 23,000 m³.s⁻¹) and a minimum during the dry season (January–May: typically 700 m³.s⁻¹) (Vinh et al., 2014). The Red River Delta is the most densely populated region in the country, with >23 million inhabitants and a population density of 1.450 people.km⁻². The Red River basin is characterized by a tropical monsoonal climate with a wet season from May to October and a dry season from November to April (Dang et al., 2010). Economic activities such as industry, rice cultivation, other agriculture, fisheries, domestic waste, and population growth are increasingly becoming the main causes of environmental issues in this area.

Sediment samples were collected during the dry and rainy seasons in March 2023 and September 2023, respectively. Water samples were taken once in September 2023 (rainy season), at the time of maximum run-off. The first station was located upstream of Hanoi, and the last was at the Ba Lat estuary. Each sampling point was settled approximately
10–15 km from the next, and a total of 21 stations were sampled (Fig. 1). The physicochemical parameters measured in the water and sediment samples from the Red River for both campaigns are presented in Table S1.

Water samples were collected from the middle of the river at each station using a plankton net with an 80 µm mesh size and a 50 cm diameter for 4 min. The net was placed perpendicular to the current and towed by a local boat to ensure a flow speed of approximately 50 cm.s⁻¹ entering the net. The volume of water passing through the net (ranging from 3.77 to 46.74 m³, depending on the station) was measured using a mechanical flowmeter (General Oceanics, Model 2030 series, with a speed range of approximately 10 cm.s⁻¹ or 1/5 knot to 7.9 m.s⁻¹ or 15 knots) attached to the mouth of the plankton net, which was kept submerged during sampling. Items retained in the cod-end of the net were rinsed three times with filtered seawater (0.7 µm of porosity) in the field. The residue was transferred into a clean 500 mL glass bottle. Before the next sampling stage, the net was rinsed externally.

Bulk surface sediment samples (approximately 1 kg of wet sediment) were collected at each station in the intertidal zone using a Van Veen grab sampler and stored in a covered aluminum box. After transport to the laboratory, the samples were stored at 4 °C. Sediment samples were dried in a freeze-dryer for five days using a Buchi Lyovapor L-200.

## 2.2. Extraction

A sample volume of 20 mL of water was filtered using GF/A filter paper, and the filter was placed in a 150 mL glass beaker. Organic matter was removed by adding 20–30 mL of 20 % (v/v) H₂O₂. Saturated potassium carbonate (K₂CO₃) was used to separate small MPs from the sample matrix due to its density, low cost, and non-toxicity (Gohla et al., 2021). After 3–4 days, 100 mL of the filtered K₂CO₃ solution (density 1,53 g.cm⁻³) was added and stirred for 30 min, following (Gohla et al., 2021). After 1–2 days of sedimentation, the suspension overflowed into a 200 mL glass beaker. The supernatant was filtered under vacuum through six mesh filters with pore sizes of 500 µm, 200 µm, 100 µm, 58 µm, 25 µm, and 13 µm. The filters were stored in a glass Petri dish for subsequent analysis.

The sediment samples were freeze-dried, and a 1 mm mesh sieve was used to separate small MPs (<1 mm) from large MPs (>1 mm). Large-sized plastics were removed with forceps. A 10 g portion of sediment was accurately weighed and transferred into a 150 mL glass beaker. The subsequent digestion, separation, and filtration steps were the same as those for the water samples.

<figure><img src="image_5.png" /><figcaption>Fig. 1. The 21 surface water and sediment sampling sites collected in Red River, Vietnam.</figcaption></figure>
## 2.3. Determination of abundance, type of polymer, color, and shape

The types of polymers were determined using attenuated total reflection (ATR) for large sizes (>1 mm) and micro-Fourier transform infrared (μ-FTIR) for small sizes (<1 mm). The spectroscopic analysis was carried out using a PerkinElmer Spotlight™ 400 μ-FTIR imaging system in reflectance mode, with a liquid nitrogen-cooled linear array detector covering the IR spectral range from 4000 to 650 cm⁻¹. The spectral resolution was 8 cm⁻¹, using fifteen scans per pixel, and the pixel resolution was 25 µm, representing a good compromise between mapping time and signal-to-noise quality of the spectra. The analysis of one filter on the μ-FTIR took around 30 min. Once the spectral map images were acquired and the spectra of the particles generated, they were analyzed using siMPle software to obtain information about MP abundance and polymer types. An image with a map of the pixels representing the identified MPs was also obtained, and the spectrum for each MP was visualized and compared to a spectral reference database.

Filters with pore sizes over 100 µm were observed using a Nikon SMZ645 optical microscope at 50× magnification to record the color and shape.

## 2.4. Quality assurance and quality control

All sampling equipment was pre-cleaned with deionized water before each field trip. Water samples were stored in glass bottles, and sediment samples were covered in aluminum boxes. Lab coats and nitrile gloves were worn during microplastic extraction and analysis to avoid microplastic contamination (false positive). The workplace was cleaned with methanol, and all glassware was rinsed with deionized water and covered with aluminum foil to prevent airborne contamination. All water, H₂O₂, and K₂CO₃ solutions used in this study were filtered before use with a vacuum pump and a mesh filter (pore size: 13 µm). Preparations were done under a laminar flow hood. Blank samples were placed in the workplace to check for blank contamination. Of the ten blank samples analyzed, none showed the presence of microplastic, except for one in which a single MP item was detected.

## 2.5. Risk assessment

### 2.5.1. Pollution load index (PLI)

The pollution load index (PLI) has been extensively used to study the content of estuarine contaminants (Tomlinson et al., 1980; Yin et al., 2021). It has recently been extended to examine the status of MP pollution (Li et al., 2020; Ranjani et al., 2021). This approach is based on calculating a regional geographical index by combining the index for each sampling station, which illustrates the pollution level at individual sampling sites and allows for a detailed examination of pollution in a given area (Pan et al., 2021).

It is related to MP contamination factors (CFᵢ) as given in Eq. (1), calculated at each sampling station in comparison to C₀, which is the minimum baseline concentration taken from the available literature. However, the lowest MP abundance measured in this study was taken as the baseline concentration, due to a lack of available background data in similar environment. An integrated PLI zone can be calculated for the studied geographical area based on Eq. (2) (Hakanson, 1980; Rakib et al., 2022; Tomlinson et al., 1980).

$$CF_i = C_i/C_o \qquad (1)$$

$$PLI_{zone} = \sqrt[n]{CF_1 \times CF_2 \times \dots \times CF_n} \qquad (2)$$

where:

Cᵢ is the MP number abundance in site i,

C₀ is the lowest MP concentration detected,

n is the number of sampling stations.

### 2.5.2. Polymer hazard index (PHI)

The polymeric hazard due to MP contamination was assessed by considering MP chemical composition and hazard scores defined by Lithner et al. (2011) as the chemical toxicity coefficient for the identified MP polymers (as reported in Table S2). The polymer hazard index (PHI) in Eq. (3) was employed to estimate the ecological hazard due to MP contamination (Canesi et al., 2015; Yin et al., 2021).

$$PHI_i = \sum_{x=1}^{m} P_{x,i} \times S_x \qquad (3)$$

where:

x corresponds to a polymer type (e.g., PE, PP...),

m is the number of the identified polymer types,

Pₓᵢ is the percentage of polymer x identified at sampling station i,

Sₓ is the hazard score of each single polymer x (as given in Table S2.).

### 2.5.3. Potential ecological risk index (PERI)

The potential ecological risk index (PERI, also known as RI) of microplastics considers the effects of both MP abundance and polymer types on the ecological risk caused by microplastics. This index fully reflects the comprehensive effects of multiple pollutants in the environment and quantitatively classifies potential ecological risk levels (Al Nahian et al., 2023; Gurumoorthi and Luis, 2023; Peng et al., 2018; Praved et al., 2024; Rakib et al., 2022; Ranjani et al., 2021; Suyamud et al., 2024). PERIᵢ at a sampling station i was calculated following Eq. (4), and PERI zone for a geographical area can be calculated from Eq. (5).

$$PERI_i = CF_i \times PHI_i \qquad (4)$$

$$PERI_{zone} = \sqrt[n]{PERI_1 \times PERI_2 \times \dots \times PERI_n} \qquad (5)$$

# 3. Results and discussions

## 3.1. Abundances of microplastics in water and sediment samples

Microplastics were detected in all water and sediment samples from the 21 sampling sites. Some examples of the detected items are shown in Fig. 2, with their descriptions categorized by shape (fiber or fragment) and color.

The total observed MPs in water samples ranged from 10 to 203 items.m⁻³, averaging 65 ± 49 items.m⁻³ (n = 21 stations), as shown in Fig. 3a. The highest concentration (203 items.m⁻³) was recorded at station RR12, corresponding to the point where the Red River branches into the Tra Ly River, potentially carrying MPs. The second highest concentration of MPs was found at station RR14 (141 items.m⁻³), which is near Nam Dinh (1641 km²; 2,487,000 inhabitants in 2023) and Thai Binh (1584 km²; 2,483,200 inhabitants in 2023) districts, ranking in the top 13 and 11 population in Vietnam, respectively (Worldwide Population Review). This finding aligns with the previously observed link between MP input and proximity to urban and/or densely populated areas (Castro-Castellon et al., 2022).

In comparison with other studies conducted in Vietnam, the concentrations of MPs found in the water samples herein were higher than those in the previous studies on the Red River (2.3 items.m⁻³, n = 1 sampling point), Han River (2.8 items.m⁻³, n = 1), Dong Nai River (3.9 items.m⁻³, n = 1) reported in Strady et al. (2021), Sai Gon River (0.16–1.59 items.m⁻³, n = 15) stated in Nguyen et al. (2022), and Ma River (57.3 ± 55.8 items.m⁻³, n = 10) indicated in Nguyen et al. (2023a). However, the concentrations were lower than those found in the Tien River (140.7 ± 53.8 items.m⁻³, n = 6) according to Kieu-Le et al. (2023) and Nhue-Day River (49–2827 items.m⁻³, n = 9 in the dry season, and 400–1133 items.m⁻³, n = 9 in the rainy season) mentioned in Nguyen et al. (2023b).

We studied microplastics ranging from 13 to 5000 µm, while the
<figure><img src="image_6.png" /><figcaption>**Fig. 2.** Microscope images of microplastics on stainless mesh filter (sizes 0.1, 0.5, 1 mm) found in Red River using a Nikon SMZ645 optical microscope with 50× magnification.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 3.** Microplastics abundance in water (a) and sediment (b) samples collected in the Red River, upstream to downstream, during the dry (blue) and the rainy season (orange).</figcaption></figure>

studies cited earlier on Vietnamese rivers focused on MPs larger than 300 µm, which is certainly one of the main reasons for the higher abundance measured in the present work.

The total observed MPs in surface sediment are reported in Fig. 3b. In the dry season, they ranged from 990 to 21,610, with a mean and a median value of 5599 items.kg⁻¹ d.w. (n = 21) and 4071 items.kg⁻¹ (n = 21), respectively. The concentration of MPs was significantly higher in the dry season (990 to 21,610 items.kg⁻¹ d.w., n = 21) than in the rainy season (653 to 8069 items.kg⁻¹ d.w., n = 21) (t-test, *p* < 0.05). This seasonal trend was also previously observed in the Ba Lat estuary (Le et al., 2022b) and along the coast of the Eastern Gulf of Thailand

(Jualaong et al., 2023), Phuket Province, Thailand (Jiwarungrueangkul et al., 2021). RR05 had the highest abundance of MPs in both the dry and rainy seasons, at 21,610 items.kg⁻¹ d.w. and 8069 items.kg⁻¹ d.w., respectively. Specifically, RR04b and RR05 are located near the Yen So wastewater treatment plant (WWTPs), the largest WWTPs in Northern Vietnam, where almost all domestic, industrial, and hospital wastewater from the capital, is discharged.

The abundance and characteristics of MPs were compared with other published data on sediment from the Red River and river sediments in Asian countries and around the world (Table S3). In this study, the average concentration of microplastics was 5599 ± 4797 items.kg⁻¹ d.
w. (n = 21) in the dry season and 2761 ± 2164 items.kg⁻¹ d.w. (n = 21) in the rainy seasons, which is in a similar range to that of the Tien River (Kieu-Le et al., 2023), Phu Loc Channel (Tran-Nguyen et al., 2022), and higher than that of the Ma River (Nguyen et al., 2023a), Skudai River, and Tebrau River (200 ± 80 items.kg⁻¹ d.w. and 680 ± 140 items.kg⁻¹ d.w., n = 3, Sarijan et al., 2018), Tapi-Phumduang River (5–160 items.kg⁻¹ d.w., average 79 ± 18.3 items.kg⁻¹ d.w., n = 10, Chinfak et al., 2021), and Huangpu River branch (723 ± 306 items.kg⁻¹ d.w., n = 7, Peng et al., 2018).

## 3.2. Characteristics of microplastics in water and sediment samples

### 3.2.1. Shape and color

The microplastic features of the Red River are presented in Fig. 4a and Fig. 4b for water and sediment samples, respectively. MPs were classified into two main shape groups: fiber and fragment, with fibers being predominant, representing 82.0 % in water and 75.5 % in sediment. Fragmented microplastics are commonly found in the river due to the deterioration and degradation of larger plastic products (Omeyer et al., 2022; Zhao et al., 2023a, 2023b). Fiber microplastics are produced during the manufacturing of synthetic clothing, ropes, fishing lines, and other fabric products (Suardy et al., 2020; Zhang et al., 2020). The proportion of fragments in the river sediment was higher than that in the river water samples. The fragments have a higher density, lower specific surface area, and unique surface structure, which results in a greater accumulation of these microplastic items in sediments than in surface waters (Choong et al., 2021). These findings are consistent with those reported in the Vietnamese aquatic environment: aquaculture ponds (62.0–81.0 %, Le et al., 2022a), urban drainage channels (77.2–86.1 %, Tran-Nguyen et al., 2022), shrimp farmers (42.0–69.0 %, My et al., 2023), the Nhue-Day River basin (93.3 %, Nguyen et al., 2023b), and some publications from Southeast Asian countries (Han et al., 2020, 93.1 %; Mazlan et al., 2023, 99.4 %; Wang et al., 2020, 87.9 %).

In terms of color, transparent and white microplastics were the most common in the Red River, representing 60.4 % and 53.4 % in water and sediment, respectively. They were followed by black and blue, as shown in Fig. 4c and d for water and sediment, respectively. Microplastics can inherit their colors from their parent plastic products, but their colors

can change as a result of photodegradation and residence time in the water (Campanale et al., 2020).

### 3.2.2. Type of polymers

Regarding the identification of polymers, the results of the analysis via µ-FTIR revealed the presence of four main polymer types: polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), and polyvinyl chloride (PVC), accounting for 19.7 %, 26.7 %, 25.8 %, and 9.3 % in water, and 17.6 %, 39.7 %, 14.2 %, and 10.4 % in sediment, respectively. The densities and common uses of the polymers identified in this study are shown in Table S2. The high level of industrial relevance of PE and PP, which covers 58.0 % of the Vietnamese plastic demand (Ta et al., 2021), can explain their large abundance. Conversely, the low specific densities of PE and PP can result in their widespread distribution in aqueous systems. They can be easily transported by rainwater or rivers and accumulated in sediment. As expected, the total proportions of lower-density microplastics (PE and PP) were higher in water (57.3 %) than in sediment (46.4 %), while the proportion of PET (with density higher than water, 1.34–1.39 g.cm⁻³) was much lower in water (14.2 %) than in sediment (25.8 %). In addition to density, the vertical distribution of MPs is also impacted by complex phenomena, such as riverine inflow, wind (Shiye et al., 2015), and turbulence (Ballent et al., 2012). In comparison to published data, the percentages of PET and PVC found in this study were higher than in other studies conducted in Vietnam (Duong et al., 2022; Do et al., 2024; Kieu-Le et al., 2023; Nguyen et al., 2022). The main reason for this discrepancy may be that almost all studies conducted in Vietnam used NaCl solution to separate MPs. However, NaCl density (1.2 g.cm⁻³) is lower than PET and PVC (shown in Table S2), so these polymers cannot be separated during the flotation step when using NaCl. In Vietnam, PET and PVC account for 10 % and 29 % of plastic consumption, respectively (Ta et al., 2021). Because of their unique features, these thermoplastics can be used to manufacture a wide range of plastic products for industry and domestic consumption. PET is widely used in packaging (especially bottles), while PVC is commonly used in construction, piping, and cable insulation. In Vietnam, the rapid industrialization and growth of the packaging and construction sectors could lead to an increase in the discharge of these polymers into the environment. Their density suggests a greater risk of

<figure><img src="image_8.png" /><figcaption>**Fig. 4.** Distribution of microplastics in the Red River according to the shape a, b; the color c, d; the type of polymers e, f; in water (on top) and sediment (at bottom), respectively (% of the total number abundance of items).</figcaption></figure>

PE: Polyethylene, PP: Polypropylene, PET: polyethylene terephthalate, PVC: Polyvinyl chloride, EVA: Ethylene vinyl acetate, Others: Polystyrene, Polyvinyl alcohol.
accumulation in sediments, which could contribute to long-term
pollution.

3.2.3. Size distribution

The particle size of MPs was studied in the range of 13–5000 µm and divided into three classes according to size: 13–200 µm, 200–1000 µm, and 1000–5000 µm (Fig. 5). The highest proportion of MPs fell in the range of 13–200 µm (64.1 % and 72.4 %), followed by 200–1000 µm (32.9 % and 26.5 %) and 1000–5000 µm (3.0 % and 1.1 %) in water and sediment, respectively. Small MPs with a particle size of <1000 µm are most abundant in all sampling stations, accounting for 97 % and 98.9 % in water and sediment, respectively. There are two main sources of plastics with a particle size of <1 mm (Bian et al., 2022). The first is the discharge of factory sewage, sludge, domestic sewage, the use of plastic in cosmetic products, the discharge of pesticides, and agricultural wastewater. The second is linked to the process of plastic aging after use. Most plastics are discharged into the environment as large particles or items. But these plastics are continuously fragmented by environmental factors, such as sunlight, heat, wind, and waves, causing them to become brittle and break into smaller and smaller pieces, leading to a high abundance of smaller-sized particles. Yan et al. (2019) mentioned that over 90.0 % of the microplastics were <1 mm in all detected samples in the Pearl River estuary, while Li et al. (2021a, 2021b) found that 94.1 % of microplastics found in the Chishui River were smaller than 1 mm. In Thailand, Wang et al. (2020) found that small microplastics accounted for over 70.0 % of total microplastic numbers. Han et al. (2020) found that microplastics <200 µm were most abundant, representing a percentage of 87.9 %. As presented in Fig. 6, microplastics sized 13–200 µm in the river water and sediment were related to the total abundance (R² = 0.9601 and 0.9325, respectively). Similarly, a moderate correlation between total abundance and the size class of 200–1000 µm microplastics can be observed (R² = 0.5886 and 0.4836, respectively). In contrast, the larger microplastics showed a weak correlation with the total abundance. These findings are consistent with previous MP studies in water and sediment (Tsering et al., 2021; Zhao et al., 2024).

3.3. Risk assessment of microplastics

The degree of pollution in the environmental compartments (i.e.,
water and sediment) in the sampling area (PLI), the hazard due to the

type of polymer incorporated in the recovered particles (PHI), and the
potential ecological risk (PERI) due to MP pollution were estimated to
quantify the state of the Red River and allow comparisons with other
environments or in the future.

3.3.1. Pollution loading index (PLI)

The contamination factors (CF) results related to the presence of MPs for each studied station along the Red River are shown in Figs. 7a, S6a, and S6c for water and sediment samples from both seasons. The PLI<sub>zone</sub> for water and sediment in the Red River was 2.25 and 1.97, respectively, which falls within the same range as in Quang Ngai with a slight pollution PLI<sub>zone</sub> of 3.15 (Le et al., 2023) but is relatively lower than those in the Nhue, Day, and To Lich rivers, with PLI<sub>zone</sub> of 6.52, 10.53, and 17.3, respectively (Duong et al., 2023). In line with the abundance of MPs, the highest contamination factor (3.51 in the rainy and 4.67 in the dry seasons) was observed at station RR05 for sediment samples, while station RR12 had the highest CF for water samples (4.53). According to the CF and PLI scale shown in Table S4, river sediments and water along the Red River are moderately contaminated and polluted with MPs. The chemical composition of MPs has a minor effect on the PLI values because the PLI is based on the abundance of MPs compared to the background value.

3.3.2. Polymer hazard index (PHI)

The average polymer composition and their respective hazard score (Table S2) are considered for calculating the polymer hazard index. The polymers that ranked as most hazardous (e.g., polyurethanes, polyacrylonitriles, polyvinyl chloride, epoxy resins, and styrene copolymers) are made of monomers classified as mutagenic and/or carcinogenic (category 1A or 1B), and all have a significant global annual production (1–37 Mt) (Lithner et al., 2011). The PHI is presented as a boxplot in Fig. 7b and for each station in Fig. S6b and S6d. The PHI for the 21 sampling stations ranged from 4.5 to 1671.3 (average 792.4, median 707.3) in water and from 2.6 to 3096.5 (average 1207.2, median 1185.8) in sediment, respectively. These values qualify as minor (level I) to high (level III) or even dangerous (level IV) hazard indices, as shown in Fig. 7b according to the ranking provided in Table S5. Indeed, the highest hazard score reached up to 10,000 due to the high risk posed by PVC (the only monomer, vinyl chloride, classified as carcinogenic 1A; V), with the average percentage of this polymer being as high as 10.4 %

<figure><img src="image_9.png" /><figcaption>Fig. 5. Size distribution of microplastics (% of the total number abundance of items) in water samples (n = 21) (a) and in sediment samples (n = 42) (b) in Red River.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Fig. 6. Linear regression analysis testing the correlation between the total abundance of microplastics and the size population of microplastics in the surface water (a) and in the sediment (b) samples.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 7. Contamination factors (a), polymer hazard index (b), and potential ecological risk index (c) related to the presence of microplastics in water and sediment samples at the sampling stations in the Red River.</figcaption></figure>

in water and 9.3 % in sediment. The PHI values are significantly higher than those reported in research conducted in the center of Vietnam at Quang Ngai (Le et al., 2023), where a PHI of 256.1 was estimated. In the study by Le et al. (2023), polyurethane presented the highest hazard score of 13,844, but it accounts for only 0.8 % of the MP composition. The primary polymer components detected were PET and PE, which have very low hazard scores of 4 and 11, respectively, thus yielding a weak PHI compared to our study. On the other hand, Kabir et al. (2021) determined that polymeric risk varied from low to high in four rivers in Japan and also showed that the presence of more toxic polymers, such as polyurethane or PVC, resulted in a higher hazard score.

### 3.3.3. Potential ecological risk index (PERI)

The PERI values ranged from 2.6 to 9140 for sediment and from 9.5 to 5125 for water, as shown in the boxplot of Fig. 7c and for each sampling station in Fig. S6e and S6f.

The PERI<sub>zone</sub> due to MPs in surface sediments was 1560 in the rainy season and 1391 in the dry season, showing alarming levels (level V > 1200) for the geographical area. The moderate PLI but the high to dangerous PHI associated with the polymer types (see Fig. S6a and S6b) indicates that the risk is mainly caused by the prevalence of polymers

with a high hazard score. Therefore, even if the MP abundance is low, the PERI value can be strongly influenced by some polymers' high toxicity hazard score. It is advisable to focus attention on these highly toxic polymers and reduce their use or explore effective recycling methods. The highest PERI values were measured at station RR05 close to WWTP, where both abundance and polymer hazard were high during both seasons. Urban domestic waste appeared as a significant source of ecotoxicological risk and highlighted the need to improve collection and treatment in Vietnam. The PERI in Red River sediments was significantly lower than in sediments at To Lich River (4771) and Nhue River (1829) but higher than in Day River (866), according to Duong et al. (2023).

The PERI trend for water and sediment samples followed that of the PHI. The PERI<sub>zone</sub> was 754 (level IV), and the highest value was recorded at sampling station RR12, congruent with the highest levels of the PLI and PHI at this station.

All the stations in the Red River that posed a high polymeric risk also posed a high potential ecotoxicological risk. However, a station containing a large number of MPs does not necessarily present the highest potential ecological risk. This must be carefully taken into account in future regulations.
## 4. Conclusion

MP concentrations in surface water ranged from 10 to 203 items. m⁻³. In sediments, the MP load was higher in the dry season, with an abundance of 990 to 21,610 items.kg⁻¹ d.w., compared to the rainy season, ranged from 653 to 8069 items.kg⁻¹ d.w. Analysis of the size distribution of MPs revealed a predominant abundance of particles between 13 and 200 µm in size. Polyethylene, polypropylene, and polyethylene terephthalate were identified as the predominant polymer types in both water and sediment samples. Overall, in the Red River, the MP pollution load is moderate, but the type of particles detected represents a high to dangerous polymer risk, resulting in a very high potential ecological risk on the river. Thus, the type of polymer and its hazard score should be carefully considered by stakeholders in regulations for plastic reduction and replacement products.

In addition, the predominance of fibers of small size class (<200 µm) is a cause for concern, as this type of particle is more likely to be absorbed by aquatic life through oral ingestion or dermal exposure. These findings underscore the urgent need for more comprehensive studies on the extent of microplastic pollution in river systems, particularly in developing countries such as Vietnam, where plastic production, consumption, and recycling activities are rapidly increasing. To the best of our knowledge, this is the first study to assess MP contamination in the Red River from upstream of Hanoi to the Gulf of Tonkin, Vietnam. The present results provide a critical baseline for documenting microplastic pollution in Southeast Asia and may serve as a basis for further surveys to monitor the temporal evolution of contamination and assess the impact of implementing sustainable policies in the future.

### CRediT authorship contribution statement

**Thi Thao Nguyen:** Writing – original draft, Visualization, Methodology, Investigation, Formal analysis, Data curation. **Van Hoi Bui:** Writing – review & editing, Supervision, Resources, Project administration, Methodology, Funding acquisition, Conceptualization. **Stéphanie Lebarillier:** Validation, Methodology, Formal analysis. **Toan Khanh Vu:** Visualization, Resources, Methodology. **Pascal Wong-Wah-Chung:** Writing – review & editing, Validation, Resources, Methodology. **Vincent Fauvelle:** Writing – review & editing, Supervision, Project administration, Funding acquisition, Conceptualization. **Laure Maleret:** Writing – review & editing, Validation, Supervision, Resources, Project administration, Methodology, Funding acquisition, Conceptualization.

### 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

We are grateful to the A*Midex Fundation and Institute of Ocean Sciences (OCEAN) and the French National Research Institute for Sustainable Development (IRD) for their support towards the PhD thesis of Thi Thao NGUYEN. This paper contributes to the LOTUS International Joint Laboratory (http://lotus.usth.edu.vn) and the IRN SOOT-SEA project managed by the IRD. We would like to thank the Region Sud for its financial support for the purchase of part of the µ-FTIR equipment. We would like to express our sincere gratitude to Marine HERMANN from IRD/LEGOS and LMI Lotus, Marc TEDETTI from IRD/MIO, and Sylvain OUILLON from IRD/LEGOS and USTH for their unfailing support for this project. We would also like to extend our special thanks to Dr. DUONG Thi Thuy (Department of Environmental Biotechnology, Institute of Science and Technology for Energy and Environment, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet Road,

Cau Giay, Hanoi, Vietnam) for sharing the plankton net; to Thi Quynh Mai DUONG (School of Chemical Engineering, Hanoi University of Science and Technology, Hanoi, Vietnam) for her valuable contributions to the sampling campaigns and the sample pretreatments; and to Dr. Frédéric GUITER (Mediterranean Institute of Biodiversity and Marine and Continental Ecology) for providing access to the optical Microscope.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.scitotenv.2024.177778.

### Data availability

Data will be made available on request.

### References

Al Nahian, Sultan, Rakib, Md. Refat Jahan, Kumar, Rakesh, Haider, Sayeed Mahmood Balal, Sharma, Prabhakar, Idris, Abubakr M., 2023. Distribution, characteristics, and risk assessments analysis of microplastic in shore sediments and surface water of Moheshkhali channel of Bay of Bengal, Bangladesh. Sci. Total Environ. 855 (158892). https://doi.org/10.1016/j.scitotenv.2022.158892.

Alimi, O.S., Fadare, O.O., Okoffo, E.D., 2021. Microplastics in African Ecosystems: Current Knowledge, Abundance, Associated Contaminants, Techniques, and Research Needs. Sci. Total Environ. 755, 142422. https://doi.org/10.1016/j.scitotenv.2020.142422.

Andreas, Hadibarata, Tony, Sathishkumar, Palanivel, Prasetia, Hafizh, Hikmat, Eka Dian Pusfitasari, Tasfiyatı, Aprilia Nur, Muzdalifah, Dian, Waluyo, Joko, Randy, Ahmad, Ramadhaningtyas, Dillani Putri, Zuas, Oman, Sari, Ajeng Arum, 2021. Microplastic contamination in the skipjack tuna (Euthynnus affinis) collected from southern coast of Java, Indonesia. Chemosphere 276. https://doi.org/10.1016/j.chemosphere.2021.130185.

Ballent, Anika, Purser, Autun, Mendes, Pedro, Pando, Sibila, Thomsen, Laurenz, 2012. Physical transport properties of marine microplastic pollution. Biogeosci. Discuss. 9, 18755–18798. https://doi.org/10.5194/bgd-9-18755-2012.

Bian, Pengyang, Liu, Yixuan, Zhao, Kaihui, Yue, Hu, Zhang, Jie, Kang, Le, Shen, Weibo, 2022. Spatial variability of microplastic pollution on surface of rivers in a mountain-plain transitional area: a case study in the Chin Ling-Wei River Plain, China. Ecotoxicol. Environ. Saf. 232, 113298. https://doi.org/10.1016/j.ecoenv.2022.113298.

Borges Ramirez, Merle M., Caamal, Ricardo Dzul, von Osten, Jaime Rendón, 2019. Occurrence and seasonal distribution of microplastics and phthalates in sediments from the Urban Channel of the Ria and Coast of Campeche, Mexico. Sci. Total Environ. 672, 97–105. https://doi.org/10.1016/j.scitotenv.2019.03.472.

Cabansag, Jerome Benedict P., Olimberio, Roselle B., Villanobos, Zaivy Mae T., 2021. Microplastics in some fish species and their environs in eastern Visayas, Philippines. Mar. Pollut. Bull. 167 (September 2019), 112312. https://doi.org/10.1016/j.marpolbul.2021.112312.

Campanale, Claudia, Friederike, Stock, Carmine, Massarelli, Christian, Kochleus, Giuseppe, Bagnuolo, Georg, Reifferscheid, Felice, Uricchio Vito, 2020. Microplastics and their possible sources: the example of Ofanto River in Southeast Italy. Environ. Pollut. 258, 113284. https://doi.org/10.1016/j.envpol.2019.113284.

Canesi, Laura, Caterina, Ciacci, Elisa, Bergami, Monopoli, Marco P., Dawson, Keneth A., Stefano, Papa, Barbara, Canonico, Ilaria, Corsi, 2015. Evidence for immunomodulation and apoptotic processes induced by cationic polystyrene nanoparticles in the hemocytes of the marine bivalve Mytilus. Mar. Environ. Res. 111, 34–40. https://doi.org/10.1016/j.marenvres.2015.06.008.

Castro-Castellon, Ana T., Horton, Alice A., Hughes Jocelyne, M.R., Cordelia, Rampley, Jeffers, Elizabeth S., Gianbattista, Bussi, Paul, Whitehead, 2022. Ecotoxicity of microplastics to freshwater biota: considering exposure and hazard across trophic levels. Sci. Total Environ. 816, 151638. https://doi.org/10.1016/j.scitotenv.2021.151638.

Chinfak, Narainrit, Sompongchaiyakul, Penjai, Charoenpong, Chawalit, Shi, Huahong, Yeemin, Thamasak, Zhang, Jing, 2021. Abundance, composition, and fate of microplastics in water, sediment, and shellfish in the Tapi-Phumduang River System and Bandon Bay, Thailand. Sci. Total Environ. 781, 146700. https://doi.org/10.1016/j.scitotenv.2021.146700.

Choong, Wei Sheng, Hadibarata, Tony, Yuniarto, Adhi, Tang, Kuok Ho Daniel, Abdullah, Faizuan, Syafrudin, Muhammad, Al, Dunia A., Farraj, and Amal M. Al-Mohtaimeed., 2021. Characterization of microplastics in the water and sediment of Baram River Estuary, Borneo Island. Mar. Pollut. Bull. 172 (August), 112880. https://doi.org/10.1016/j.marpolbul.2021.112880.

Cordova, Muhammad Reza, Purwiyanto, Anna Ida Sunaryo, Suteja, Yulianto, 2019. Abundance and characteristics of microplastics in the northern coastal waters of Surabaya, Indonesia. Mar. Pollut. Bull. 142 (March), 183–188. https://doi.org/10.1016/j.marpolbul.2019.03.040.

Curren, Emily, Kuwahara, Victor S., Yoshida, Teruaki, Leong, Sandric Chee Yew, 2021. Marine microplastics in the ASEAN region: a review of the current state of knowledge. Environ. Pollut. 288 (April), 117776. https://doi.org/10.1016/j.envpol.2021.117776.
Dang, Thi Ha, Coynel, Alexandra, Orange, Didier, Blanc, Gérard, Etcheber, Henri, Le, Lan Anh, 2010. Long-term monitoring (1960-2008) of the river-sediment transport in the Red River watershed (Vietnam): temporal variability and dam-reservoir impact. *Sci. Total Environ.* 408 (20), 4654–4664. https://doi.org/10.1016/j.scitotenv.2010.07.007.

Do, Van Manh, Trinh, Van Tuyen, Le, Xuan Thanh Tao, Nguyen, 2024. Evaluation of microplastic bioacquisition capacity of mussel (Perna viridis) and surrounding environment in the north coast of Vietnam. *Mar. Pollut. Bull.* 199 (January 2024), 115987. https://doi.org/10.1016/j.marpolbul.2023.115987.

Dung, Luu Viet, Duc, Truong Huu, Linh, Le Thi Khanh, Ly, Trinh Thi Dieu, Hoang Anh, Hao, Nguyen Thi My, 2021. Depth profiles of microplastics in sediment cores from two mangrove forests in northern Vietnam. *Journal of Marine Science and Engineering* 9 (12). https://doi.org/10.3390/jmse9121381.

Duong, Thi Thuy, Le, Phuong Thu, Nguyen, Thi Nhu Huong, Hoang, Thi Quynh, Ngo, Ha My, Doan, Thi Oanh, Le, Thi Phuong Quynh, Bui, Huyen Thuong, Bui, Manh Ha, Trinh, Van Tuyen, Nguyen, Thuy Lien, Da Le, Nhu, Thanh Mai, Vu, Tran, Thi Kim Chi, Ho, Tu Cuong, Phuong, Ngoc Nam, Strady, Emilie, 2022. Selection of a density separation solution to study microplastics in tropical riverine sediment. *Environ. Monit. Assess.* 194 (2), 1–17. https://doi.org/10.1007/s10661-021-09664-0.

Duong, Thi Thuy, Nguyen-Thuy, Duong, Phuong, Ngoc Nam, Ngo, Ha My, Doan, Thi Oanh, Le, Thi Phuong Quynh, Bui, Ha Manh, Nguyen-Van, Huong, Nguyen-Dinh, Thai, Nguyen, Thi Anh Nguyet, Cao, Thi Thanh Nga, Pham, Thi Minh Hanh, Hoang, Thu Huong Thi, Gasperi, Johnny, Strady, Emilie, 2023. Microplastics in sediments from urban and suburban rivers: influence of sediment properties. *Sci. Total Environ.* 904. https://doi.org/10.1016/j.scitotenv.2023.166330.

Espiritu, Emilyn Q., Rodolfo, Raymond S., Sofia, Maria J., Evangelista, Jillian Jay, Feliciano, G., Alexandra, Marie N., Sumaway, Jiena Lynne, Pauco, R., Karl, Vincent N., Alvarez, and Erwin P. Enriquez, 2023. Microplastics contamination in the fishes of selected sites in Pasig River and Marikina River in the Philippines. *Mar. Pollut. Bull.* 187 (October 2022), 114573. https://doi.org/10.1016/j.marpolbul.2023.114573.

Gabisa, Elias W., Gheewala, Shabbir H., 2022. Microplastics in ASEAN region countries: a review on current status and perspectives. *Mar. Pollut. Bull.* 184 (September), 114118. https://doi.org/10.1016/j.marpolbul.2022.114118.

Giles, Rachel K., Nguyen, Cindy Anh Thu, Ho, Thu Thi Yen, Van Nguyen, Cong, Ngo, Ngoc Thi, Rochman, Chelsea M., 2021. Source-specific patterns of marine debris and associated ecological impacts in the Red Estuary of Xuan Thuy National Park, Vietnam. *Front. Environ. Sci.* 9 (May), 1–11. https://doi.org/10.3389/fenvs.2021.679530.

Gohla, Jan, Bračun, Sandra, Gretschel, Gerwin, Koblmüller, Stephan, Wagner, Maximilian, Pacher, Christian, 2021. Potassium carbonate (K2CO3) – a cheap, non-toxic and high-density floating solution for microplastic isolation from beach sediments. *Mar. Pollut. Bull.* 170, 0–4. https://doi.org/10.1016/j.marpolbul.2021.112618.

Gurumorthi, Kaliyamoorthi, Luis, Alvarino J., 2023. Recent trends on microplastics abundance and risk assessment in coastal Antarctica: regional meta-analysis. *Environ. Pollut.* 324, 121385.

Hai, Thanh Truong, Vu, Hai Nam, 2019. The “crisis” of plastic waste in Vitenam is real. *European Journal of Engineering Research and Science* 4 (9), 107–111. https://doi.org/10.2401/ejeng.2019.4.9.1523.

Hakanson, Lars, 1980. An ecological risk index for aquatic pollution control. A sedimentological approach. *Water Res.* 14 (8), 975–1001. https://doi.org/10.1016/0043-1354(80)90143-8.

Han, Mei, Niu, Xuerui, Tang, Man, Zhang, Bo Tao, Wang, Guoqiang, Yue, Weifeng, Kong, Xianglun, Zhu, Jiqian, 2020. Distribution of microplastics in surface water of the Lower Yellow River near estuary. *Sci. Total Environ.* 707, 135601. https://doi.org/10.1016/j.scitotenv.2019.135601.

Ismanto, Aris, Hadibarata, Tony, Sugianto, Denny Nugroho, Zainuri, Muhammad, Kristanti, Risky Aaya, Wisha, Ullung Jantana, Hernawan, Undang, Indanita, Asoka, Gonsilou, Audrey Primus, Elshikh, Mohamed Soliman, Al-Mohaimed, Amal M., Abbasi, Arshad Mehmed, 2023. First evidence of microplastics in the water and sediment of Surakarta City River Basin, Indonesia. *Mar. Pollut. Bull.* 196 (September), 115677. https://doi.org/10.1016/j.marpolbul.2023.115677.

Jamebeck, Jenna R., Geyer, Roland, Wilcox, Chris, Siegler, Theodore R., Perryman, Miriam, Andradry, Anthony, Narayan, Ramani, Law, Kara Lavender, 2015. Plastic waste inputs from land into the ocean. *Science* 347 (6223), 768–771. https://doi.org/10.1126/science.1260352.

Jiwarungreueangkul, Thanakorn, Phaksopa, Jitraporn, Sompongchaiyakul, Penjai, Tipmanee, Daiai, 2021. Seasonal microplastic variations in estuarine sediments from urban canal on the west coast of Thailand: a case study in Phuket Province. *Mar. Pollut. Bull.* 168 (May), 112452. https://doi.org/10.1016/j.marpolbul.2021.112452.

Jualaong, Suthep, Towatana, Prawit, Pradit, Siriporn, Pransilpauyapreecha, Mitila, 2023. Type and distribution of microplastic contamination in beach sediment along the coast of the lower gulf of Thailand. *Appl. Ecol. Environ. Res.* 21 (3), 2671–2682. https://doi.org/10.15666/aeer/2103_26712682.

Kabir, A.H.M., Enamul, Masahiko Sekine, Imai, Tsuyoshi, Yamamoto, Koichi, Kanno, Ariyo, Higuchi, Takaya, 2021. Assessing small-scale freshwater microplastics pollution, land-use, source-to-sink conduits, and pollution risks: perspectives from Japanese rivers polluted with microplastics. *Sci. Total Environ.* 144655. https://doi.org/10.1016/j.scitotenv.2020.144655.

Karbalaei, Samaneh, Golieskardi, Abolfazl, Watt, Dorothy Uning, Boiret, Mathieu, Hanachi, Parichehr, Walker, Tony R., Karami, Ali, 2020. Analysis and inorganic composition of microplastics in commercial Malaysian fish meals. *Mar. Pollut. Bull.* 150, 110687. https://doi.org/10.1016/j.marpolbul.2019.110687.

Kieu-Le, Thuy Chung, Thuong, Quoc Thinh, Truong, Tran Nguyen Sang, Le, Thi Minh Tam, Tran, Quoc Viet, Strady, Emilie, 2023. Baseline concentration of microplastics in surface water and sediment of the northern branches of the Mekong River Delta, Vietnam. *Mar. Pollut. Bull.* 187. https://doi.org/10.1016/j.marpolbul.2023.114605.

Le Dai, Thang, “Overview of marine plastic debris in Vietnam in relation to international context.” *FIG Working Week 2019*. In Proceedings of the Geospatial information for a smarter life and environmental resilience, Hanoi, Vietnam April 22-26, 2019.

Le, Thi Phuong, Quynh, Nhu Da, Le, Viet Nga, Dao, Emma Rochelle-Newall, Nguyen, Thi Mai Huong, Marchand, Cyril, Duong, Thi Thuy, Phung, Thi Xuan Binh, 2018. Change in carbon flux (1960-2015) of the Red River (Vietnam). *Environ. Earth Sci.* https://doi.org/10.1007/s12665-018-7851-2.

Le, Nhu Da, Hoang, Thi Thu Ha, Duong, Thi Thuy, Lu, XiXi, Pham, Thi Mai Huong, Phung, Thi Xuan Binh, Le, Thi My Hanh, Duong, Thi Huyen, Nguyen, Tien Dau, Le, Thi Phuong Quynh, 2022a. First observation of microplastics in surface sediment of some aquaculture ponds in Hanoi City, Vietnam. *Journal of Hazardous Materials Advances* 6 (March), 100061. https://doi.org/10.1016/j.hazadv.2022.100061.

Le, Nhu Da, Hoang, Thi Thu Ha, Duong, Thi Thuy, Phuong, Ngoc Nam, Le, Phuong Thu, Nguyen, Tien Dau, Phat, Thi Xuan Binh, Le, Thi My Hanh, Le, Thi Lien, Thi Huong, Vu, Le, Thi Phuong Quynh, 2022b. Microplastics in the surface sediment of the Main Red River Estuary. *Vietnam Journal of Earth Sciences* 1–14. https://doi.org/10.15625/2615-9783/17486.

Le, Xuan Thanh, Thao, Duy Thanh, Nguyen, Minh Tuan, Pham, Minh Viet, Trinh, Phuc Quang Le, Do, Van Manh, 2023. Risk assessment of microplastic exposure: a case study near a refinery factory at the central coast of Vietnam. *Mar. Pollut. Bull.* 196 (September), 115636. https://doi.org/10.1016/j.marpolbul.2023.115636.

Lebreton, Laurent C.M., van der Z wet, Joost, Damsteeg, Jan-Willem, Slat, Boyan, Andradry, Anthony, Reisser, Julia, 2017. River plastic emissions to the world's oceans. *Nat. Commun.* 8, 15611. https://doi.org/10.1038/ncomms15611.

Li, Jingyi, Huihui, Liu, Paul Chen, J., 2018. Microplastics in freshwater systems: a review on occurrence, environmental effects, and methods for microplastics detection. *Water Res.* 137, 362–374. https://doi.org/10.1016/j.watres.2017.12.056.

Li, Chaoran, Rosa, Busquets, Campos Luiza, C., 2020. Assessment of microplastics in freshwater systems: a review. *Sci. Total Environ.* 707, 135578. https://doi.org/10.1016/j.scitotenv.2019.135578.

Li, Jianlong, Ouyang, Zhuozhi, Liu, Peng, Zhao, Xiaonan, Renren, Wu, Zhang, Chutian, Lin, Chong, Li, Yiyong, Guo, Xuetao, 2021a. Distribution and characteristics of microplastics in the basin of Chishui River in Renhui, China. *Sci. Total Environ.* 773, 145591. https://doi.org/10.1016/j.scitotenv.2021.145591.

Li, Penghui, Wang, Xiaodan, Min, Su, Zou, Xiaoyan, Duan, Linlin, Zhang, Hongwu, 2021b. Characteristics of plastic pollution in the environment: a review. *Bull. Environ. Contam. Toxicol.* 107 (4), 577–584. https://doi.org/10.1007/s00128-020-02820-1.

Lithner, Delilah, Larsson, Ake, Dave, Göran, 2011. Environmental and health hazard ranking and assessment of plastic polymers based on chemical composition. *Sci. Total Environ.* 409 (18), 3309–3324. https://doi.org/10.1016/j.scitotenv.2011.04.038.

Loganathan, Yukeswaran, Jacob, Kizhakedathil Moni Philip, 2023. A review on microplastics - an indelible ubiquitous pollutant. *Biointerface Research in Applied Chemistry* 13 (2), 126. https://doi.org/10.32362/briac132.126.

Mansuri, Abdulkhalik, Kansara, Krupa, Raiyani, Dixit, Mazmudar, Dhairyap, Kumar, Ashutosh, 2023. New insight into long-term effects of phthalates microplastics in developing zebrafish: evidence from genomic alteration and organ development. *Environ. Toxicol. Pharmacol.* 99 (January), 104087. https://doi.org/10.1016/j.etap.2023.104087.

Mazlan, Zurzafirah, Shukhairi, Sarah Syazwani, Husin, Miftahul Jannah Muhammad, Shalom, Jemimah, Saud, Safaa Najah, Sani, Muhammad Shirwan Abdullah, Ong, Meng Chuan, Mohan, Naveen Kumar Naidu Chandra, Sopian, Nor Asyikin, 2023. Evaluation of microplastics isolated from sea cucumber Acaudina molpoides in Pulau Langkawi, Malaysia. *Heliyon* 9 (6), e16822. https://doi.org/10.1016/j.heliyon.2023.e16822.

Morgado, Vanessa, Gomes, Luis, Ricardo, J.N., da Silva, Bettencourt, Palma, Carla, 2022. Microplastics contamination in sediments from Portuguese inland waters: physical-chemical characterisation and distribution. *Sci. Total Environ.* 832 (April). https://doi.org/10.1016/j.scitotenv.2022.155053.

My, Tran Thi, Ai, Nguyen Duy, Dat, and Nguyen Quoc Hung, 2023. Occurrence and characteristics of microplastics in wild and farmed shrimps collected from Cau Hai Lagoon, Central Vietnam. *Molecules* 28 (12), 4634. https://doi.org/10.3390/molecules28124634.

Nguyen, Nguyen Thao, Nhon, Nguyen Thi Thanh, Hai, Ho Truong Nam, Chi, Nguyen Doan Thien, To Thi Hien, 2022. Characteristics of microplastics and their affiliated PAHs in surface water in Ho Chi Minh City, Vietnam. *Polymers* 14 (12), 1–20. https://doi.org/10.3390/polym14122450.

Nguyen, Thi Lan, Huong, Thi Lim, Duong, Thi Huong, Nguyen, Thuy, Dang, Tran Quan, Nguyen, Thi Hue, Dao, Ngoc Nhiem, Nguyen, Kien Trung, Duong, Cong Dien, Pham, Ngo Nghia, Nguyen, Bac Quang, 2023a. Microplastics and trace metals in river sediment: prevalence and correlation with multiple factors. *Sci. Total Environ.* 895 (June), 165145. https://doi.org/10.1016/j.scitotenv.2023.165145.

Nguyen, Huu Thang, Nguyen, Thi Ha, To Thi Hien, Hoang, Minh Trinh, 2023b. Distribution and characteristics of microplastics in Nhuè-Day River Basin, Vietnam. *Environment and Natural Resources Journal* 21 (3), 245–255. https://doi.org/10.32526/ennrj/21/202200244.

Omeyer, Lucy C.M., Duncan, Emily M., Aiemsomboon, Kornrawee, Beaumont, Nicola, Bureekul, Sujaree, Cao, Bin, Carrasco, Luis R., Chavanich, Suchana, Clark, James R., Cordova, Muhammad R., Couceiro, Fay, Cragg, Simon M., Dickson, Neil, Failler, Pierre, Ferraro, Gianluca, Fletcher, Stephen, Fong, Jenny, Ford, Alex T., Gutierrez, Tony, Hamid, Fauziah Shahul, Hiddink, Jan G., Hoa, Pham T.,
Holland, Sophie I., Jones, Lowenna, Jones, Nia H., Koldewey, Heather, Lauro, Federico M., Lee, Charlotte, Lewis, Matt, Marks, Danny, Matallana-Surget, Sabine, Mayorga-Adame, Claudia G., McGeehan, John, Messer, Lauren F., Michie, Laura, Miller, Michelle A., Mohamad, Zeeda F., Nor, Nur Hazimah Mohamed, Müller, Moritz, Neill, Simon P., Nelms, Sarah E., Onda, Deo Florence L., Ong, Joyce J.L., Pariatamby, Agamuthu, Phang, Sui C., Quilliam, Richard, Robins, Peter E., Salta, Maria, Sartimbul, Aida, Shakuto, Shiori, Skov, Martin W., Taboada, Evelyn B., Todd, Peter A., Toh, Tai Chong, Valiyaveettil, Suresh, Viyakarn, Voranop, Wonnapinij, Passorn, Wood, Louisa E., Yong, Clara L.X., Godley, Brendan J., 2022. Priorities to inform research on marine plastic pollution in Southeast Asia. Sci. Total Environ. 841 (June). https://doi.org/10.1016/j.scitotenv.2022.156704.

Pan, Zhong, Liu, Qianlong, Jiang, Ronggen, Li, Weiwen, Sun, Xiuwu, Lin, Hui, Jiang, Shuangcheng, Haining, Huang, 2021. Microplastic Pollution and Ecological Risk Assessment in an Estuarine Environment: The Dongshan Bay of China. Chemosphere 262, 127876. https://doi.org/10.1016/j.chemosphere.2020.127876.

Peng, Guyu, Pei, Xu, Zhu, Bangshang, Bai, Mengyu, Li, Daoji, 2018. Microplastics in freshwater river sediments in Shanghai, China: a case study of risk assessment in mega-cities. Environ. Pollut. 234, 448–456. https://doi.org/10.1016/j.envpol.2017.11.034.

Praved, P., Hari, K.V., Neethu, S. Bijoy, Nandan, N. Deepak, Sankar, E.H., Aravind, Sruthy Sebastian, Mirigoudar, S.R., Sharma, K.V., 2024. Evaluation of microplastic pollution and risk assessment in a tropical monsoonal estuary, with special emphasis on contamination in jellyfish. Environ. Pollut. 343 (July 2023), 123158. https://doi.org/10.1016/j.envpol.2023.123158.

Rakib, Md Refat Jahan, Hossain, M. Belal, Kumar, Rakesh, Ullah, Md Akram, Al Nahian, Sultan, Rima, Nazmun Naher, Choudhury, Tasrina Rabia, Liba, Samia Islam, Jimmy, Yu, Khandaker, Mayeen Uddin, Sulieman, Abdelmoneim, Sayed, Mohamed Mahmoud, 2022. Spatial distribution and risk assessments due to the microplastics pollution in sediments of Karnaphuli River Estuary, Bangladesh. Sci. Rep. 12 (1), 1–15. https://doi.org/10.1038/s41598-022-12296-0.

Ranjani, M., Veerasingam, S., Venkatachalapathy, R., Mugarilasan, M., Bagaev, Andrei, Mukhanov, Vladimir, Vethamony, P., 2021. Assessment of potential ecological risk of microplastics in the coastal sediment of India: a meta-analysis. Mar. Pollut. Bull. 163 (January), 111969. https://doi.org/10.1016/j.marpolbul.2021.111969.

Sarijan, Shazani, Azman, Shamila, Said, Mohd Ismid Mohd, Andu, Yusrina, Zon, Nur Farhan, 2018. Microplastics in sediment from Skudra and Tebrau River, Malaysia: a preliminary study. MATEC Web of Conferences 250. https://doi.org/10.1051/mateconf/201825006012.

Schimdt, Christian, Krauth, Tobias, Wangner, Stephan, 2017. Export of plastic debris by rivers into the sea. Environ. Sci. Technol. 51, 12246–12253. https://doi.org/10.1021/acs.est.7b02368.

Shiye, Zhu, Lixin, Li, Daoji, 2015. Microplastic in three urban estuaries, China. Environ. Pollut. 206, 597–604. https://doi.org/10.1016/j.envpol.2015.08.027.

Strady, Emilie, Dang, Thi Ha, Dao, Thanh Duong, Dinh, Hai Ngoc, Do, Thi Thanh Dung, Duong, Thanh Nghi, Duong, Thi Thuy, Hoang, Duc An, Kieu-Le, Thuy Chung, Le, Thi Phuong Quynh, Mai, Huong, Trinh, Dang Mau, Nguyen, Quoc Hung, Tran-Nguyen, Quynh Anh, Tran, Quoc Viet, Truong, Tran Nguyen Sang, Chu, Van Hai, Vo, Van Qi, 2021. Baseline assessment of microplastic concentrations in marine and freshwater environments of a developing Southeast Asian country, Viet Nam. Mar. Pollut. Bull. 162, 111870. https://doi.org/10.1016/j.marpolbul.2020.111870.

Suardy, Nur Hapizah, Tahrim, Nurfaizah Abu, Ramli, Suria, 2020. Analysis and characterization of microplastic from personal care products and surface water in Bangi, Selangor. Sains Malaysia 49 (9), 2237–2249. https://doi.org/10.17576/jsm-2020-4909-21.

Suyamud, Bongkotrat, Pan, Xiong, Yongxiang, Yu, Yuan, Wenke, Liu, Yi, Yang, Yuyi, 2024. First-of-its-kind: nationwide meta-analysis of microplastic pollution and risk assessment in Thailand. Chemosphere 364 (July), 143041. https://doi.org/10.1016/j.chemosphere.2024.143041.

Ta, Anh Tuan, Babel, Sandhya, 2020. Microplastics pollution with heavy metals in the aquaculture zone of the Chao Phraya River Estuary, Thailand. Mar. Pollut. Bull. 161 (October), 111747. https://doi.org/10.1016/j.marpolbul.2020.111747.

Ta, Thi Yen, Nguyen, Thi Anh Tuyet, Van, Hoang Thi Hong, 2021. Analysis of production, consumption and environmental burden of plastic industry in Vietnam by input-output table. Ministry of Science and Technology, Vietnam 63 (2), 89–96. https://doi.org/10.31276/vjste.63(2).89-96.

Thompson, Richard C., Olson, Ylva, Mitchell, Richard P., Davis, Anthony, Rowland, Steven J., John, Anthony W.G., McGonigle, Daniel, Russell, Andrea E.,

2004. Lost at sea: where is all the plastic? Science 304 (5672), 838. https://doi.org/10.1126/science.1094559.

Tomlinson, Derek L., Wilson, James G., Harris, C.R., Jeffrey, D.W., 1980. Problems in the assessment of heavy-metal levels in estuaries and the formation of a pollution index. Helgoländer Meeresun. 33 (1–4), 566–575. https://doi.org/10.1007/BF02414780.

Tran-Nguyen, Quynh Anh, Thi Bich Hau, Vu, Nguyen, Quy Tuan, Hoai, Nhu Y., Nguyen, Thi Mai, Le, Van Minh Vo, Trinh-Dang, Mau, 2022. Urban drainage channels as microplastics pollution hotspots in developing areas: a case study in Da Nang, Vietnam. Mar. Pollut. Bull. 175 (May 2021), 113323. https://doi.org/10.1016/j.marpolbul.2022.113323.

Tsering, Tenzin, Sillanpää, Mika, Sillanpää, Markus, Viitala, Mirka, Reinikainen, Satu Pia, 2021. Microplastics pollution in the Brahmaputra River and the Indus River of the Indian Himalaya. Sci. Total Environ. 789. https://doi.org/10.1016/j.scitotenv.2021.147968.

Vinh, Vu Duy, Guillon, Sylvain, Thanh, Tran Duc, Van Chu, La, 2014. Impact of the Hoa Binh Dam (Vietnam) on water and sediment budgets in the Red River Basin and Delta. Hydrol. Earth Syst. Sci. 3987-4005. https://doi.org/10.5194/hess-18-3987-2014.

Waldschläger, Kryss, Lechthaler, Simone, Stauch, Georg, Schüttrumpf, Holger, 2020. The way of microplastic through the environment-application of the source-pathway-receptor model (review). Sci. Total Environ. 713, 136584. https://doi.org/10.1016/j.scitotenv.2020.136584.

Walther, Bruno Andreas, Takashi, Kusui, Ning, Yen, Hu, Chieh-Shen, Lee, Hyemi, 2020. Plastic pollution in East Asia: macroplastics and microplastics in the aquatic environment and mitigation efforts by various actors. In: Plastics in the Aquatic Environment – Part I. The Handbook of Environmental Chemistry, vol. 111. https://doi.org/10.1007/6982020508.

Wang, Ying, Zou, Xinqing, Peng, Cong, Qiao, Shuqing, Wang, Teng, Wenwen, Yu, Khokiatiwong, Somkiat, Kornkanitnan, Narumol, 2020. Occurrence and distribution of microplastics in surface sediments from the Gulf of Thailand. Mar. Pollut. Bull. 152 (January), 110916. https://doi.org/10.1016/j.marpolbul.2020.110916.

Wei, Gao Ling, Li, Ding Qiang, Zhu, Mu Ning, Liao, Yi Shan, Xie, Zhen Yue, Guo, Tai Long, Li, Jun Jie, Zhang, Si Yi, Liang, Zhi Quan, 2015. Organophosphorus flame retardants and plasticizers: sources, occurrence, toxicity and human exposure. Environ. Pollut. 196, 29–46. https://doi.org/10.1016/j.envpol.2014.09.012.

Yan, Muting, Nie, Huayue, Kaihang, Xu, He, Yuhui, Yingtong, Hu, Huang, Yumei, Wang, Jun, 2019. Microplastic abundance, distribution and composition in the Pearl River along Guangzhou City and Pearl River Estuary, China. Chemosphere 217, 879–886. https://doi.org/10.1016/j.chemosphere.2018.11.093.

Yin, Kai, Wang, Dongxu, Zhao, Hongjing, Wang, Yu, Guo, Menghao, Liu, Yachen, Li, Baoying, Xing, Mingwei, 2021. Microplastics pollution and risk assessment in water bodies of two nature reserves in Jilin Province: correlation analysis with the degree of human activity. Sci. Total Environ. 799, 149390. https://doi.org/10.1016/j.scitotenv.2021.149390.

Zhang, Zhaoyong, Mamat, Zulpiya, Chen, Yinguang, 2020. Current research and perspective of microplastics (MPs) in soils (dusts), rivers (lakes), and marine environments in China. Ecotoxicol. Environ. Saf. 202 (February), 110976. https://doi.org/10.1016/j.ecoenv.2020.110976.

Zhao, Xiaoli, Qiang, Minmink, Yuan, Yuan, Zhang, Man, Wu, Wenjing, Zhang, Jiaocheng, Gao, Zesen, Gu, Xinmei, Ma, Sitian, Liu, Zihan, Cai, Lu, Han, Jianqiao, 2023a. Distribution of microplastic contamination in the major tributaries of the Yellow River on the Loess Plateau. Sci. Total Environ. 905 (3), 167431. https://doi.org/10.1016/j.scitotenv.2023.167431.

Zhao, Xiaoli, Liu, Zihan, Cai, Lu, Han, Jianqiao, 2023b. Occurrence and distribution of microplastics in surface sediments of a typical river with a highly eroded catchment, a case of the Yan River, a tributary of the Yellow River. Sci. Total Environ. 863 (3), 160932. https://doi.org/10.1016/j.scitotenv.2022.160932.

Zhao, Wenyu, Li, Jing, Liu, Mengyue, Wang, Rui, Zhang, Boxuan, Meng, Xiang Zhou, Zhang, Shengwei, 2024. Seasonal variations of microplastics in surface water and sediment in an inland river drinking water source in southern China. Sci. Total Environ. 908 (June 2023), 168241. https://doi.org/10.1016/j.scitotenv.2023.168241.

Zheng, Xuanjing, Sun, Ruikun, Dai, Zhenqing, He, Lei, Li, Chengyong, 2023. Distribution and risk assessment of microplastics in typical ecosystems in the South China Sea. Sci. Total Environ. 883 (February), 163678. https://doi.org/10.1016/j.scitotenv.2023.163678.