3.1. Combined qualification and quantification of microplastics by Raman microscopy
Visual identification: The viewing mode includes two objectives: 10x and 50x. The larger objective allowed to zoom powdered particles in the case of PA and PMMA. However, the images were not always clear. Thus, the shape, color and size (µm) were observed at 10x. The final size was calculated depending on the shape. It was expressed as length for microfibers, and the sum of width and length for non-fibrous forms.
The morphology of debris under the microscopic mode could be used to quantify the microplastics. Some criteria described by Zhao et al. 2016 were applied to distinguish microplastic fibers (clothing, fishing nets, for instance) from non-plastic fibers (organic, or cellular structures, Fig. 2a). Microplastic fibers were (1) clear and homogeneously colored, (2) soft and three-dimensional bendable Fig. 2c; (3) equally thick, not segmented and not tapered towards the ends. Fibers especially dark-colored ones meeting above requirements but having complicated spectra were called “unidentified group”. Moreover, the lengths of some fibers were close to the widths, and these cases were recorded in the shape of “bars or sticks” instead of particles as suggested by Enders et al. 2015. Non-fibrous shapes included particles with either spherical or aggregate of spheres, fragments (particles with jagged edges as a signal of fragmentation), and films with square or rectangle images. However, the film category always accounted for a small percentage, mainly because they were broken down into threads and filaments, thereby classified into fiber categories (Chubarenko et al. 2016). The other groups are foams, pellets (granules with the shape of a cylinder or a disk manufactured as a raw material of plastic goods).
In addition to shapes, colors can be a preliminary tool to guess the plastic type, behavior and origin of microplastics. Clear and transparent particles have been ascribed to polypropylene (PP) (Ismail et al. 2009), while black represented the sorption of PAHs and PCBs on PP and polystyrene (PS) (Frias et al. 2010). White pieces were assigned to polyethylene (PE) (Ismail et al. 2009), and their film or sheet shapes suggest they come from shopping bags, agricultural and food packaging films (Zhu et al. 2018). Colors can also indicate the weathering and decomposition degree, and residence time at the seawater surface. The discoloration can be caused by long exposure of UV-light (translucent color) (Kunz et al. 2016, Crawford and Quinn 2017). In other words, the discoloration (yellowing) is a result of oxidized PCBs adsorbed on plastic resins in the environment (Endo et al. 2005).
Spectroscopic identification: The Raman spectrometer with a 532 nm excitation laser and a charge-coupled device (CDC) detector was used to measure spectra of each item at x50 objective in an integration time of 15s and a grating of 900 gr/mm in a range wavelength from 50 to 3600 cm-1. In practice, plastic additives and organic pollutants such as PCBs, PAHs adsorbed on microplastics’ surface made the spectra more complex. The sample treatment was performed on all polymer standards. The results showed their morphology and Raman spectra were not affected by peroxide oxidation. Thus, the peak shifts of sampled microplastics were mainly caused by the degradation in the environment. Nevertheless, as can be seen in Table 1, vibration wavenumbers of functional groups in pure plastic molecules could be recognized in the sampled microplastics’ spectra, which pointed out the high specificity of Raman spectroscopy in identifying chemical composition of most common plastics.
Table 1. Raman band assignments of standard plastics and sampled microplastics
(e) Gündoğdu 2018, (f) Milani 2015, (g) Käppler et al. 2015, (h) Alexiou et al. 2020, (i) Solodovnichenko et al. 2016
3.2. Microplastic pollution in the freshwater at Saigon urban canals
Figure 3 showed the uneven distribution of microplastics in locations of Ho Chi Minh City. Microplastic amount was highest in residential areas, 45.89 ± 24.04 MPs/L in Bridge No. 1 (SG1 on the map, Fig. 1) where floating rubbish from NL-TN canal was gathered, varied from 44.33 to 58.57 MPs/L in Tau Hu Canal (SG8, SG9, SG1, SG11) belonging to Districts 6, 7, 8 and Tan Binh. Microplastics were found with a noticeably high abundance in some samples at the locations where the water environment was concentrated (dark colour) with a higher number of visible floating garbage such as food boxes, clothing pieces and nylon bags. Among the repetition times, there were replicates obtaining very high abundance of plastic debris, for instance, 160 MPs/L in Bridge No. 1 (SG1, Fig. S1a), 110 MPs/L in Letter Y Bridge (SG10), 125 MPs/L in Lo Gom canal (SG10), and 220 MPs/L in Kenh Te Bridge (SG12). There was a dramatic decrease in the microplastic amount from the pollution sources to Saigon River estuaries, for example 23.92 ± 19.23 MPs/L in Bach Đang Wharf (SG7), and 27.33 ± 6.23 MPs/L in Khanh Hoi Bridge (SG15). Indeed, Turkey’s multiple tests showed significant statistical differences between Kenh Te Bridge and Khanh Hoi (Sig.=0.005, Table S4), and Bach Dang Wharf (Sig.=0.021, Table S4). There were no statistical differences between other locations.
These locations are grouped into four canals according to geographical division of the Government. Nhieu Loc-Thi Nghe is a separate system inside HCMC flowing to Saigon River, while other canal systems connect HCMC with neighbour cities. Ben Nghe Canal (3.1 km) originates from Lo Gom-Tau Hu Canal system (9 km), Te Canal (KT, 4.5 km) is the continuation of Doi Canal system (8.5 km). Đoi-Te, Ben Nghe-Tau Hu canal basins join at the position of Nguyen Van Cu Bridge before flowing to Saigon River. Regarding the composition (Table 2), the percentage of plastic types were similar at close locations belonging to each canal system. In general, nylon was the most abundant component, followed by polyester fibers. PE was also the second most frequently sorted in Ben Nghe and Te basins. In Nhieu Loc and Lo Gom basins, PE and PP were identified at the same proportions. Polystyrene was found with a higher percent (around 12%) in the study of Emmerik et al. (2019) but made up small percentages in our study. In fact, many fragments of PS food boxes were seen floating on surface waters while sampling, but they were regarded as macroplastics. Small-size particles in black colour had only Raman band at 1000 cm-1 of PS, which were not enough to assign them as PS microplastics. Physical processes, particularly wind blowing and water mixing influenced sinking behavior of plastic debris, strongly affecting the presence of PS micro-size classes (< 2 mm) (Frère et al. 2016).
Table 2
Compositions of microplastics extracted from main Saigon urban canals
Plastic composition
|
Nhieu Loc –
Thi Nghe (NL)
|
Tau Hu
(TH)
|
Ben Nghe (BN)
|
Te Canal
(KT)
|
Total microplastics (MPs/L)
|
35.58 ± 4.75
|
45.19 ± 10.30
|
33.83 ± 10.54
|
43.41 ± 7.93
|
PE (%)
|
14.35
|
12.23
|
14.68
|
17.41
|
PP (%)
|
13.48
|
12.66
|
11.51
|
11.34
|
PET (%)
|
21.74
|
18.34
|
8.73
|
7.283
|
PVC (%)
|
5.65
|
6.11
|
11.51
|
4.86
|
PS (%)
|
3.04
|
2.18
|
4.76
|
1.21
|
PA (%)
|
22.17
|
23.58
|
14.68
|
17.41
|
PMMA (%)
|
1.31
|
0.87
|
0.79
|
1.21
|
Unidentifed (%)
|
18.26
|
24.02
|
33.33
|
39.27
|
Nhieu Loc-Thi Nghe canal system plays an important role in the city’s activities during the foundation and development. This 8.7-km long canal flowing through seven districts was the cleanest canal at the beginning of the reclamation of land in Southern Vietnam. It became the dirtiest canal from the 1970s mainly due to the discharge of solid waste and wastewater from the slums on both sides of the banks. The water quality in this canal has been significantly improved since 1993. It is now the cleanest canal in the city, thanks to the largest canal clean-up programs (HCMC Environmental Sanitation Project) funded by the World Bank and the City Government (Kieu-Le et al. 2016). Nonetheless, plastic pollution is still a concerning problem because plastic wastes can come from different sources such as direct dropping litter on land or at sea, blowing, leaching from landfills, and losses during the transport. According to a statistical data of Barnes et al. 2009, a large proportion of 40–80% of plastic garbage is from carrier bags, packaging, footwear, cigarette lighters and other domestic items. Indeed, the fact in all urban canals is floating debris which comes from discarding wastes of tourists and residents, and natural vegetal, mainly water hyacinths drifted from Sai Gon River during high tides. The floating debris life cycle is specific to each canal and not involved in solid waste management. The floating debris collection is very active in Nhieu Loc – Thi Nghe canal, conducted every day from 6 am to 6 pm. Indeed, the average values showed the lowest abundance of microplastics in this canal (Table 2), which is similar to the study of Lahens et al. (2018). The water quality of KT and TH-BN canal basins has been improved to some extent thanks to HCMC Water Environment Improvement Project funded JICA. Nonetheless, TH and LG canals flow along with the highest populated districts with many slum areas along the canals. The mismanaged wastewater management and ineffective floating debris collections in these districts led to higher pollution compared to Nhieu Loc canal (Kieu-Le et al. 2016).
The average of MPs in our study (38.45 ± 24.93) was two orders of magnitude lower than the range of total microplastics reported by Lahens et al. 2018 (172–519 fibers/L). The noticeable differences can be explained by different time periods, sampling techniques and analysis methods rather than the levels of plastic pollution in the studied regions as proposed by Enders et al. 2015. A few bundles of green fibers (Fig. 2a) were detected in the state of twisted together, thereby considered as “non-microplastics” due to unclear shapes. Moreover, we found a large number of black particles whose images and spectra were difficult to identify. Indeed, Raman analysis of black particles resulted in a considerably higher rejection rate, as suggested by Lenz et al. 2015. Hence, only black items with clearly homogenous shape and identifiable structural characteristics were included in microplastic data, consequently small percentages (1.58–6.7%) of black color.
3.3. Spatial distribution of marine microplastic from Can Gio Biosphere Reserve to the East Sea
The downstream of Saigon River merges with Dong Nai River to form Nha Be River and divides into two branches: Long Tau and Soai Rap Rivers, flowing to the East Sea (South China Sea) through Can Gio Mangrove. Plastic pollution sources, including direct dropping litter on land or at sea, blowing, leaching from landfills, and losses during the transport, are mainly in the transition zone (from CT1 to CT8 on the map). Nevertheless, its total microplastics (10.45 ± 3.67 MPs/L) are similar to the buffer zone (from CB1 to CB5 on the map) (10.75 ± 3.13 MPs/L), where human activities are more restricted. The geographical position is proposed to be the main reason. The transition zone consists of outer areas that are strongly affected by coastal features such as winds, waves, tides, river runoff which makes microplastics dispersed and diluted unevenly (Lusher et al. 2014). Moreover, there are no clear geographical maritime boundaries between the zones because adjacent areas share the local canals and rivers.
Figure 4 indicates the distribution of plastic types in the zones of Can Gio reserve. PE was the most abundant type in the transition and core zones, and almost all of them were fragments and particles. PET was the largest group in the form of fibers (35%) and fragments (35%) in the buffer zone but was the second dominant, mainly in fibrous form (53.57%) in the transition zone and in both fibers (31.25%) and particles (31.25%) in the core zone. Fibers were found to be abundant in the transition zone, especially coastal locations. Can Gio 30 April Beach is the most attractive recreational beach, which is considered as a vital pollution source, especially polyamide fibers (Khuyen et al. 2021a). Fishing is the main income source of the local people, especially in Can Thanh. Zhu et al. (2018) proposed that the origin of blue nylon fibers is cables used in fishery activities. Moreover, fibers could originate from shipping (Lusher et al. 2015), especially at the intersection of main rivers – Đong Tranh and Long Tau, where large ships circulate every day. In addition to plastics’ density at this location, buoyancy effects of marine plastics in the water column were mediated by a high level of surface mixing caused by boat movement and seawater turbidity (Lusher et al. 2014). As a result, PET and PA dominated (26.53%), following by PP (20.41%), and PE (10.2%).
In other words, microplastics were observed but with less number (7.48 ± 1.28 MPs/L) in the core zone (from CC1 to CC3 on the map) although human’s domestic activities are strictly prohibited. This proves microplastics can be detected in areas far from pollution sources, and their abundance would depend on the proximity to the urban sources. Microplastics, therefore, could be released from outer regions, floated under the natural forces of water flows and accumulated in convergent zones (Lusher et al. 2014). The sediment of mangrove forest can be a suitable environment for microplastic storage because we found some white fragments and blue fibers similar to ones in the seawater. The debris is difficult to detach from the sediment due to sticky characteristics of the sediment. Furthermore, pollutants retained in the core zone would be difficult to return to the marine environment because the waves are attenuated, and the water surface here remains calm (Mazda 2009). Some studies reported the presence of microplastics in the mangrove sediment and roots (Li et al. 2019), but the role of mangroves as a filter of plastic debris for the seawater needs more investigations.
Figure 5 demonstrates remarkable changes in the relative abundance of microplastics from Ho-Chi-Minh City (HCMC, Saigon urban canals/rivers) through Can Gio Mangrove (Can Gio seawater) to the East Sea (Soai Rap Estuary, from Long Tau Estuary to Ganh Rai Gulf, and White Tiger Oilfield). Mangrove is a special soil accumulating carbon, nutrients and sediments as “enhancer of sedimentation” (Valiela and Cole 2002). Sediments deposited into mangroves come from allochthonous sediments (external sources like terrestrial or oceanic sources), and autochthonous sources (re-suspended sediments) (Adame et al. 2010). Microlastics can be transported from the oceanic environment to the mangrove and accumulated in the sediment layers in a similar way as in the aquatic environment (Auta et al. 2017). This is considered one of the main reason for a low number of microplastics within Can Gio reserve. Indeed, the decrease in number of plastic debris highlights the feasible role of the mangrove in retaining contaminants including microplastics.
The Turkey’s test showed a statistical difference in total MPs between Binh Khanh ferry (CT1) with Ganh Rai Gulf (ES3) (Sig.=0.03, Table S3), and with Soai Rap estuary (Sig.=0.03, Table S3); Turkey’s test showed a statistical difference in total MPs between Binh Khanh ferry with Ganh Rai Gulf (Sig.=0.03, Table S3), and with Soai Rap estuary (Sig.=0.03, Table S3); between Rach Đon Bridge (Can Gio buffer zone, CB1) and Ganh Rai Gulf (Sig.=0.014, Table S3). The comparison tests indeed showed some statistical difference between Saigon urban canals with Can Gio Mangrove and the East Sea, for example, Bui Huu Nghia Bridge (SG5) with transition zone (C1-Sig.=0.01, C2-Sig.=0.038, C3-Sig.=0.018, Table S4), with downstream of Soai Rap River (ES2) (Sig.=0.004, Table S4), downstream of Long Tau River (ES1) (Sig.=0.036, Table S4), Ganh Rai Gulf (ES3) (Sig.=0.001, Table S4), and White Tiger Oilfield (ES4) (Sig.=0.011, Table S4).
The pollution sources concentrate in HCMC, and therefore the MPs amount was highest (nearly 40 items/L) and decreased gradually to the estuary areas. The number of microplastics in the Saigon river-estuarine system of our study falls into the range of concentrations reported by Strady et al. (2020), which was 22 to 251 MPs/L. At the intersection of HCMC and Can Gio (Binh Khanh Ferry), the microplastic concentration decreased to below 20 MPs/L. The average abundance of MPs was around 10 MPs/L within Can Gio reserve, which was in the range of microplastics in Three Gorges Reservoir in China (1.597–12.611 MPs/L) (Di and Wang 2018), Taihu Lake in China (3.4–25.8 MPs/L) (Su et al. 2016). This comparison could be made owing to the similarity of the position of Can Gio and Taihu in terms of geographical location and human activities. Nevertheless, Taihu Lake has become one of the most severely polluted lakes in China due to the development of the local economy and industry, presence of three wastewater treatment plants, but, in contrast, Can Gio is strictly protected by the government and UNESCO. Microplastics have eventually reached the East Sea, but with a smaller amount, from 3.1 ± 1.6 to 4.7 ± 3.2 MPs/L.
The total microplastics in Ganh Rai Gulf was much smaller than Jinhae Bay, South Korea (88 ± 68 MPs/L) (Song et al. 2015b), but equivalent to Hudson River (USA) to some extent (0.625–2.45 fibers/L) (Miller et al. 2017). Microplastics in Ganh Rai (ES3) are predicted to come from terrestrial and sea-based sources. The later source is supposed to maritime transport (Sheavly et al. 2015, Hasnat and Rahman 2018), because the sampling sites are around the intersection of cargo ships, vessels carrying imported-exported fuels and gases. However, it is still well understood which is the main reason for the presence of plastic debris in this area. According to Browne (2015), the source of maritime shipping is much smaller than land-based sources. In fact, microfibers accounted for larger percentage than micro-parctilces (Table 3), which is attributed to textile sources from the mainland. All of investigated plastic types were encountered from the ocean surface to the depths of 3 meters in the sampling locations of Ganh Rai Bay. PE dominated on the sea surface, whilst PA, PET, and PVC were predominant in the sub-surface samples, 2 meters and 3 meters, respectively. They are subjected to distribute more in the deeper seawater levels since their densities (up to 1.14, 1.3–1.5 and 1.15–1.70 g/cm3, respectively) were higher than seawater (1.023 g/cm3 in measured average) (Crawford and Quinn 2017; Weber et al. 2018; Issa and Kandasubramanian, 2021). PS was found in the sub-surface seawater with a higher abundance despite its similar density to the seawater. Hence, it can be seen that the vertical distribution of microplastics depends on not only the density of virgin polymer. Their distribution is also affected by the disturbance caused by the actions of internal wave and oceanic circulation modes in the seawater (Wang et al. 2016) since any abrasion, cracking and pitting of the surface can increase the density (Crawford and Quinn 2017).
As seen in Table 3, the majority of microplastics were fibers with various size fractions. There was a surprising similarity in their colours in most environments. White was always most predominant in all shapes of plastic types, particularly 18.1% for fibers. Pink (red), blue, purple, gray were popular colors of fibers (Fig. 2), 15.7%, 14.4%, 12.4% and 10.2%, respectively. Black accounted for 7.6% of fibers. However, the black was seen with purple or blue instead of completely black as activated carbon. These colours were also commonly found in fibers in the Northeast Atlantic Ocean (blue, black, red) (Lusher et al. 2014), Jinhae Bay, South Korea (green, blue, red) (Song et al. 2015b), Qatar (blue, white) (Castillo et al. 2016), Hudson River-USA (blue, black, red) (Miller et al. 2017). The results showed the distribution of plastic types was not much different by the depths in the water column. PE dominated in the surface samples, PET and PA were predominant in the sub-surface samples. PE, PP and many PET fragments were attributed to breakdown products of plastic items, especially single-use water bottles and tableware. In Ganh Rai, Soai Rap and White Tiger seawater, pink fibers were dominated by PET (Fig. 2e), while blue was more frequently observed in PA fibers (Fig. 2d). In the water of Saigon canals, some parts of several coloured fibers appeared in brown, which showed the contamination of coloured substances on the surface of microplastics in the environment.
Table 3. Comparisons of parameters of microplastics in Saigon urban canals, Can Gio Reserve and East Sea
Parameters
|
Saigon urban canals
(Saigon River)
|
Can Gio Reserve
|
Downstream of
Soai Rap River
|
Ganh Rai Gulf
(East Sea)
|
White Tiger oilfield (East Sea)
|
Plastic compositions
|
PA > PET > PE >
PP > PVC > PS > PMMA
|
PET > PE > PA > PP > PVC > PS > PMMA
|
PE > PP > PET >
PA > PVC = PS > PMMA
|
PET > PE > PA >
PP > PVC ≥ PS > PMMA
|
PET > PA > PP > PE > PVC > PS = PMMA
|
Common shapes (%)
|
Fiber: 35.4
Fragment: 20.6
|
Fiber: 27.7
Fragment: 34.2
|
Fiber: 31.3
Particle: 21.8
|
Fiber: 42.4
Particle: 20.14
|
Fiber: 44.1
Particle: 26.5
|
Common colors
|
white > gray > brown > green > pink
|
white > gray > blue > pink > brown
|
white > pink > blue > purple > yellow
|
white > blue > pink > gray > green
|
white > gray > pink > blue > purple
|
Size range (µm)
|
Fiber: 14.1–561.2
Non-fibers: 15.7–188.4
|
Fiber: 25–60
Non-fibers: 14–53.4
|
Fiber: 19.7–58.3
Non-fibers: 21.9–94.5
|
Fiber: 25–436.9
Non-fibers: 23.5–265
|
Fiber: 36.1–184.7
Non-fibers: 22–92
|
Fibers were also commonly encountered in Can Gio reserve. However, their percentage (21–22%) was still lower than that of fragments (33–36%) (Table 3). The size of plastic debris in this mangrove varied from 15 µm to 53.38 µm, so smaller than those in Saigon urban canals (from 15 µm to 197.5 µm). Similarly, microfibers were only 60 µm maximum (53.43 µm in average) in the mangrove but were up to 561.23 µm (122.76 µm in average) in Saigon canal systems. The shape of fibers in locations of Can Gio and East Sea (Fig. 2b, c, d and e) looked similar which is single colored fibers, which suggests that microfibers have dispersed in the marine environment. Differently, bundles of microfibers (Fig. 2a) were quite often found in only Saigon urban canals. The small size fraction (25–50 µm) is quite common in the mangrove systems as also reported in mangrove sediment of Maowei Sea (Li et al. 2019) and in Singapore’s coastal mangrove ecosystem (Nor and Obbard 2014). The high percentages of small plastic fragments revealed that mangrove systems might accelerate plastic decomposition and accumulation. This, therefore, can promote harmful effects of plastic debris on the mangrove benthos to some extent.