Introduction
Microplastics (MPs) have emerged as a growing global environmental concern, posing significant threats to marine ecosystems and human health. While the scientific community has made significant strides in understanding the prevalence and characteristics of MPs in various marine environments, limited research has focused on the quantitative and qualitative assessment of MPs in the Mekong Delta region of Vietnam.
The Mekong Delta, situated in the southernmost region of Vietnam, is a crucial freshwater ecosystem that supports the livelihoods of millions of people. As a major agricultural and industrial hub, the Mekong Delta is susceptible to various forms of pollution, including the accumulation of MPs. Understanding the extent and nature of MP contamination in this region is essential for developing effective mitigation strategies and safeguarding the delicate balance of the Mekong ecosystem.
This article presents a comprehensive evaluation of microplastic contamination in the surface water and sediments of the Mekong Delta, Vietnam. By combining quantitative and qualitative analyses, the study aims to provide a detailed assessment of the abundance, distribution, and physical/chemical characteristics of MPs in this vital freshwater system.
Methodology
Sample Collection and Preparation
The study was conducted across six strategic sampling sites within the Mekong Delta region. Surface water samples were collected using a Neuston net, while sediment samples were obtained using an Ekman Grab sampler at three selected sites.
Upon collection, the samples were carefully preserved and transported to the laboratory for further analysis. In the lab, the water samples underwent density separation using a saturated sodium chloride (NaCl) solution to isolate the microplastic particles. Sediment samples were subjected to an oxidative degradation process with 30% hydrogen peroxide (H2O2) to remove organic matter, followed by filtration through a Whatman Grade 42 filter paper.
Microplastic Identification and Characterization
The filtered microplastics were visually examined under a light microscope (Carl Zeiss Microscopy GmbH) at 40x magnification. The abundance of MPs was quantified and expressed as items per liter (L-1) for water samples and items per gram (g-1) for sediment samples.
Microplastics were categorized based on their size, shape, and color. Size ranges were determined, with particles classified into seven categories: 1-40 μm, 40-60 μm, 60-80 μm, 80-100 μm, 100-500 μm, 500-1000 μm, and 1000-5000 μm. Shapes were identified as filaments, fragments, films, and pellets.
To determine the polymer composition, approximately 30% of the microplastic particles were analyzed using Raman spectroscopy (Horiba Jobin Yvon-LabRam HR Evolution). The obtained Raman spectra were compared to reference databases to identify the polymer types present in the samples.
Quality Assurance and Quality Control
To ensure the accuracy and reliability of the results, the sampling equipment and analytical instruments were thoroughly calibrated prior to use. Additionally, strict protocols were followed to minimize potential contamination during sample processing and analysis, including the use of glass and metal labware, protective clothing, and thorough cleaning of the workspace.
Results and Discussion
Microplastic Abundance and Distribution
The abundance of microplastics in the surface water samples ranged from 0.75 ± 0.49 to 2.13 ± 0.25 items/L, with the highest concentration observed at Site 2, located in the central region of the Mekong Delta. In the sediment samples, the microplastic abundance varied from 0.84 items/g at Site 4 to 2.75 items/g at Site 3.
The spatial variation in microplastic abundance can be attributed to the complex hydrodynamic conditions of the Mekong Delta. The region is influenced by the Indonesian Throughflow (ITF), a significant ocean current system that connects the Pacific and Indian Oceans. This extensive circulation pattern plays a crucial role in the dispersal and deposition of marine debris, including microplastics, across the region.
Microplastic Characteristics
The analysis of microplastic morphology revealed the presence of four distinct shapes: filaments, fragments, films, and pellets. Filament-shaped microplastics were the most dominant, accounting for approximately 77% of the total particles observed. Fragments, films, and pellets were also present, but in lower proportions.
In terms of size distribution, the microplastics found in the surface water ranged from 4.70 μm to 3,799.25 μm, while those in the sediment samples were larger, ranging from 67.20 μm to 2,176.87 μm. The presence of larger microplastics in the seabed suggests that these particles may have undergone less fragmentation or were more prone to settling due to their higher density.
The microplastics observed in the Mekong Delta exhibited a diverse range of colors, including blue, brown, black, red, yellow, gray, green, and transparent. The prevalence of certain colors, such as blue, brown, and black, was consistent across multiple sampling sites, while other colors, like yellow, green, and transparent, were more site-specific.
Polymer Identification
The chemical analysis using Raman spectroscopy revealed that the predominant polymer types present in the Mekong Delta were polyethylene terephthalate (PET/PETE) and polyester (PE) film. These polymers are commonly used in various applications, including synthetic fibers, beverage bottles, and food packaging.
The identification of PET and PE as the primary polymer types aligns with the prevalent presence of filaments and fragments, which are often associated with the degradation of textile fibers and plastic packaging materials, respectively. The widespread use of these polymers in the region, coupled with improper waste management practices, likely contributes to the observed microplastic contamination in the Mekong Delta.
Implications and Recommendations
The findings of this study underscore the significant presence of microplastics in the surface water and sediments of the Mekong Delta, Vietnam. The high abundance of small-sized, filamentous, and colored microplastics is a matter of concern, as these characteristics can increase the bioavailability and potential ecological impacts of MPs in the aquatic ecosystem.
The prevalence of PET and PE polymers, which are widely used in various industries and consumer products, highlights the need for comprehensive waste management strategies and the implementation of extended producer responsibility policies. Addressing the sources of microplastic pollution, such as wastewater discharge and littering, will be crucial in mitigating the ongoing contamination of the Mekong Delta.
Furthermore, the complex hydrodynamics of the region, influenced by the Indonesian Throughflow, emphasizes the importance of regional and international collaboration in tackling the transboundary issue of marine plastic pollution. Coordinated efforts among neighboring countries and stakeholders will be essential to develop effective solutions and ensure the long-term sustainability of the Mekong Delta ecosystem.
Conclusion
This study provides a comprehensive assessment of microplastic contamination in the surface water and sediments of the Mekong Delta, Vietnam. The quantitative and qualitative analyses reveal the widespread presence of MPs, with significant spatial variations in abundance and diverse physical and chemical characteristics.
The findings underscore the urgent need for targeted interventions to address the sources and pathways of microplastic pollution in the Mekong Delta. Strengthening waste management infrastructure, promoting public awareness, and fostering international collaboration are crucial steps towards mitigating the environmental and public health risks associated with microplastic contamination in this vital freshwater system.
As the global scientific community continues to explore the complexities of microplastic pollution, studies like this one contribute to our understanding of the issue and inform the development of effective strategies to protect the Mekong Delta and other vulnerable ecosystems worldwide.
