Contamination of Selected Persistent Organic Pollutants (POPs) in Sediment of Some Areas in Vietnam

2.1. General characteristics of PCBs

PCBs are industrial products, which constitute a global environmental health hazard of solely anthropogenic origin. Theoretically, there are 209 PCB isomers and congeners with 1–10 chlorine atoms attached to the biphenyl molecule.

The term “PCBs homolog” is used to refer to all PCBs with the same number of chlorines. Homolog with different substitution patterns is referred to as isomer. The numbering system for the PCBs is shown above. Positions 2, 2′, 6, and 6′ are called ortho positions, positions 3, 3′, 5, and 5′ are called meta positions, and positions 4 and 4′ are called para positions. The benzene rings can rotate around the bond connecting them. The two extreme configurations are planar and the nonplanar in which the benzene rings are at a 90° angle to each other [1]. The benzene rings of non-ortho substituted PCBs, as well as monoortho substituted PCBs, may assume a planar configuration and are referred to as planar or coplanar congeners. The PCBs congeners are arranged in ascending numerical order using a numbering system that follow the IUPAC rules.

2.2. Contamination status of PCBs

Monitoring surveys of PCBs residue in sediment have been conducted during the early 1994s. In the northern Vietnam, PCBs were found in environmental sediment of Thaibinh province (Ba Lat Estuary, coast lines of Thai Binh province), Quangninh province (Halong Bay), and Hanoi city (Set, Kim Nguu, CauBay River, Yen So Lake). PCBs penetrated into the estuaries, urban rivers, lakes, and coastal areas. High PCB concentrations were found in sediment of Kim Nguu River (328 ng/g) and Yen So Lake of Hanoi city (384 ng/g) in 2006 [2]. Hoai et al. [2] reported that the sediment levels of PCBs measured in their study in 2006 revealed a clear increase compared to 0.79–40 ng/g (mean 13 ng/g) in 1997 and 15–120 ng/g (mean 45 ng/g) in 1999 [3, 4]. Until 2015, sediment levels of PCBs decreased compared to 31.72–167.32 ng/g [5]. Toan et al. reported that the main source of contamination in Hanoi city could originate from the dielectric oil used in old hanging transformers and capacitors [6]. From such equipment, PCBs could penetrate into the environment by re-filling of dielectric oil, mechanical damage, electrical accident, and fire. Statistics until 2006 show that the total amount of dielectric oil containing PCBs in the entire country is approximately 19,000 tons [7]. This clearly indicates a huge contamination source of PCBs in the environment in Vietnam. In central Vietnam, PCBs were found in the environmental sediment of Hue city (Phu Da, A Luoi, and Tam Giang-Cau Hai Lagoon), Quy Nhon city (Thi Nai Lagoon). PCBs penetrated into the lagoons and canals near paddy fields or municipal sewage at low levels (<25 ng/g). In southern Vietnam, PCBs were also found in Mekong River Delta (Tra Vinh), Can Tho city, and Hochiminh city. PCBs were distributed in wide spaces such as drainage from rice fields, rivers near ferry harbors, river near the mouth of Mekong, shrimp farming areas, and canals in the densely populated areas. Highest PCBs concentrations were found in sediment of Saigon River, Hochiminh city (590.5 ng/g) [8]. According to typical data about PCB residues in sediment in Vietnam in Table 1, we can draw a number of general conclusions about studies of PCB residues in sediment in Vietnam (Figures 1 and 2):

• Within 23 years (1994–2016), about 143 typical sediment samples in several areas of Vietnam were analyzed. Obtained results had a great effort to show the PCB residues level in a number of studied areas. However, the database of PCBs is still limited and further assessment is required in the future.

• Studies about total PCBs are quantified according to various PCBs standards. Several studies do not report the depth of sediment sampling, and the component of total PCBs is not the same (total PCBs can be the sum of 6, 7, 22, 53, 93, or even more than 100 PCBs isomers and congeners). Therefore, the comparisons of total PCBs of different studies are relative and are not precise. This calls for a unified standard about PCBs research for application and further study in the future. We recommend that the researchers can analyze only six indicator congeners (PCB 28, 52, 101, 138, 180). After, the sum of six PCBs can be multiplied by five relatively to get the value of total PCBs. This recommendation is in good agreement with Kohler et al. [16].

• Published studies about PCBs in sediment only provide an initial evaluation about the residue in a point in time without assessments about the time trend variation or in-depth studies about the consequences of PCBs residue in the studied areas. These problems can be additional research directions for PCBs in Vietnam in the future.

2.3. Composition analyses of PCBs

Concerning the PCB congeners, PCBs could be detected from tri-CB to octa-CB in the collected sediment samples. The mean percentages of six selected PCB indicators in the collected sediment samples from several studies (Table 1) followed the order PCB138 > PCB153 > PCB101 > PCB52 > PCB180 > PCB28. This order can be explained by physical and chemical properties of PCBs. According to Toan et al., lightly chlorinated PCBs are less persistent, have lower log K_ow, and are more volatile than heavily chlorinated PCBs. Therefore, heavily chlorinated PCBs are more accumulative in the sediment, whereas lightly chlorinated PCBs are degraded and volatilized faster [6].

Another explanation could be related to the compositions of PCB mixtures that probably escaped from the dielectric oil. Up to April 1998, 48.3% of the total quantity of dielectric oils in Vietnam was imported from the Soviet Union. Japan and China contributed with smaller percentages of 7.5 and 3.6%, respectively [6, 17]. According to Falandysz et al., the percentages of PCB138, PCB153, PCB101 (along with PCB90), PCB52, PCB180, and PCB28 (along with PCB31) in Sovol (trade name of Soviet Union dielectric oil) were 11.4, 7.0, 6.5, 3.6, 0.4, and 0.8%, respectively [18]. It seems that the predominance of heavily chlorinated PCBs, PCB138, and PCB153, still remained when they penetrated the sediment. In general, low percentages of lightly chlorinated PCBs and a high percentage of heavily chlorinated PCBs in the analyzed sediment samples reflect their long-time release Figure 3 [6].

2.4. Ecological risk assessment of PCBs

To evaluate the ecotoxicological significance of PCBs contamination, total PCBs in collected sediments were compared with the NOAA sediment quality guideline (SQG) [19]. This guideline specifies the “effects range low” (ERL) and the “effects range median” (ERM). The ERL represents the chemical concentration below which an adverse effect would rarely be observed. The ERM represents the concentration above which adverse effect would frequently occur [20]. Only total PCBs of sediment samples in two big cities (Hanoi and Ho Chi Minh cities) exceeded ERM levels (ERM of total PCBs is 180 ng/g). The other sediment samples listed in Table 1 that were collected from the estuaries, coastal areas, lagoons, canals near paddy field or municipal sewage, drainage from rice fields, rivers near ferry harbors, river near the mouth of Mekong, shrimp-farming areas, were lower than ERL levels (ERL of total PCBs is 22.7 ng/g). This finding raises the concern on PCBs impact in the two big cities of Vietnam. Thus, further investigation is required in Hanoi and Hochiminh cities to assess possible toxic effects on human health and ecological system.

3.1. General characteristics of PBDEs

Polybrominated diphenyl ethers (PBDEs) are used commercially as additives in plastics and textiles, building materials, carpets, and vehicles and aircraft with half-lives in the order of 2–10 years. In computers, these compounds are commonly used in printed circuit boards, components such as connectors, cables, plastic covers, and parts of keyboards and monitors. Theoretically, there are 209 PBDEs isomers and congeners with 1–10 bromine atoms attached to the biphenyl molecule.

PBDEs are highly resistant to heat, light, oxidizing, and reducing compounds. Thus, PBDEs are extremely persistent when released into the environment. The use of PBDEs has increased over the last 30 years with production estimated to be about 3000–5000 tons in Europe. Deca-BDE is the largest mix on the market and makes up over 80% of the total PBDE production, whereas penta-BDE and octa-BDE products constitute about 12 and 6%, respectively, of the total PBDE production [21]. The presence of high levels of these compounds in samples from remote areas suggests that they may now have been distributed worldwide as a result of long-range atmospheric transport. PBDEs have been associated with endocrine disruption, neurotoxicity, and cancer. Sediments are major sinks for these contaminants in aquatic environments, and their study is an important step in mapping possible pollution sources and exposure pathways that facilitate PBDE bioavailability to sediment-dwelling organisms [21].

3.2. Contamination status of PBDEs

From the north to the south of Vietnam, PBDEs was found in environmental sediment of Hanoi city (CauBay river), Quy Nhon city (Thi Nai Lagoon), Hochiminh city (canals), and Saigon-Dongnai River. PBDEs penetrated in the environmental sediment of rivers, lagoon urban canals, urban sewer systems, and estuary. Data about PBDE residue in sediment in these areas of Vietnam are presented in Table 2.

A number of general conclusions can be drawn from studies about PBDE in Vietnam:

• At present, there is a lack of studies about PBDE in Vietnam. In three representative studies presented in Table 2, the numbers of samples and research areas are not enough to represent PBDE residue in sediment in Vietnam. Published studies do not report the depth of sampling. There are also different viewpoints about the total PBDE value. Components of total PBDE can be the sum of 7, 9, or 11 PBDEs. Therefore, it is difficult to compare research results.

• Published studies about PBDE in sediment are mainly about initial evaluation of residue in a point in time. There is no assessment about the change in trend or in-depth research about the consequences of PBDE residue in the studied areas. Further studies about PBDE residue and its impact on the environment are necessary.

When compared with other regions in the world, the residues of total PBDEs in Hanoi and Hochiminh are lower than those in Dianchi lake, China and and higher than residues found in Hongfeng and Chenghai lake, China [23].

3.3. Composition analyses of PBDEs

Concerning the composition analyses, PBDEs congeners could be detected from tri-BDE to deca-BDE in the collected sediment samples (Table 2). BDE-209 was a predominant congener in sediment samples. In the past, BDE-209 is the largest mix on the market and makes up over 8% of the total PBDE production, whereas penta-BDE and octa-BDE products constitute about 12 and 6%, respectively, of the total PBDE production [24]. This is one of important factors to explain the predominance of BDE-209.

The mean percentages of six PBDEs indicators in the collected sediment samples followed the order: BDE-47 > BDE-99 > BDE-100 > BDE-154 > BDE-153 > BDE-27. This order is in agreement with chemical properties of PBDEs as well as the percentages of PBDEs in the commercial mixtures.

3.4. Ecological risk assessment of PBDEs

It has been suggested that PBDEs biomagnified as they move along a food web. In addition, PBDEs can inhibit growth in colonies of algae as well as depress the reproduction of zooplankton. Based on the toxicity data of benthic organisms, the multiple species have no observed effect on the concentrations of ΣPBDEs which is 3.1 mg/kg of sediment [21, 25]. Most of the collected sediment samples in Table 2 had residues of PBDEs lower than 3.1 mg/kg. However, the values of PBDEs in the urban canals and urban sewer system of Hochiminh city are very high (maximal 119 ng/g). Due to the propensity of PBDEs to highly accumulate in various compartments of wildlife and human food webs, further evaluation of ecological risk assessment in Hochiminh city should be undertaken as a high priority.

4.1. General characteristics of DDTs and HCHs

4.2. Contamination status of DDT and HCH

Data about DDT and HCH residues in sediment in Vietnam are relatively adequate, including sediment in freshwater, brackish water, and seawater. There are research results about DDT and HCH in sediment from 1994 up to now (Table 3). According to data in Table 3, DDT and HCH have deposited in the sediment in Vietnam in a wide range and for a long time with considerable extent. DDT and HCH residue in sediment in Vietnam is on the decreasing trend.

Monitoring surveys of DDTs and HCHs residue in sediment have been conducted during the early 1994s. In northern Vietnam, DDTs and HCHs were found in environmental sediment of Thaibinh province (Diem Dien Estuary, coast lines of Thai Binh province), Quangninh province (Halong Bay) and Hanoi city (Set, Kim Nguu, CauBay River; Yen So Lake). DDTs and HCHs have penetrated into the estuaries, urban rivers, lakes, and coastal areas. HCHs had low residues in most of the sediment samples. Besides, high DDTs concentrations were found in the sediment of Kim Nguu River (1100 ng/g) and Set River of Hanoi city (680 ng/g) in 2006 [2]. This can be explained by the usage of DDTs in the big cities of Vietnam in the past. Both DDTs and HCHs have been used in Vietnam in considerable amounts as pesticides for crop protection and as vector control for public health purposes. DDT had been imported and used in Vietnam from 1957 up to 1994.

In Central Vietnam, DDTs and HCHs were found in the environmental sediment of Hue city (Phu Da, A Luoi). DDTs and HCHs penetrated into the canals near paddy fields or municipal sewage at medium levels. In southern Vietnam, DDTs and HCHs also found in Mekong River Delta (Duyen Hai and Tra Vinh), Can Tho city, and Hochiminh city. DDTs and HCHs are distributed in wide spaces such as drainage from rice fields, river near ferry harbor, river near the mouth of Mekong, shrimp-farming areas, and canals in the densely populated areas. Highest DDTs concentrations were found in the sediment of Saigon River, Hochiminh city (253.6 ng/g) [8].

4.3. Composition analyses of DDTs and HCHs

Composition differences of HCHs isomers or DDTs metabolites in the environment could indicate different contamination sources. DDT can be biodegraded in the environment to DDD under anaerobic conditions and DDE under aerobic conditions. Thus, p,p’-DDD was a major breakdown product of DDT in sediment from different places in Vietnam. With regards to DDT metabolites, the ratio of (p,p’-DDE + p,p’-DDD)/ΣDDT in the sediment samples collected in 2012 from CauBay River ranged between 0.77 and 0.89 (mean 0.80). The obtained ratios indicated that the degradation of DDT occurred significantly, and there is no recent input of DDT in the study areas [28]. This conclusion was suitable with the fact of usage of DDTs and other studies since 2006 in Vietnam. Among the isomers, β-HCH has the lowest water solubility and vapor pressure, which is the most stable and relatively resistant to microbial degradation. Besides, there is the isomerization of α- to β-HCH and of γ- via α- to the more stable β-HCH, which is energetically more favorable in the environment [28]. Therefore, the β-HCH predominance reflects an old source of input of HCH in the environment. Low ratios of α-HCH/γ-HCH may represent the use of lindane, whereas high ratios of these isomers may depict the use of technical HCH [28]. An α-HCH/γ-HCH ratio in areas where lindane has been typically used ranges from 0.2 to 1 compared to a range from 4 to 15 for technical HCH. According to Toan et al., the mean percentages of HCH isomers analyzed in sediment samples from CauBay River followed the order of β-HCH (43.2%) > α-HCH (39.3%) > γ-HCH (10.7%) > δ-HCH (6.9%) [28, 29]. Therefore, the predominance of β-HCH reflects an old source of input of HCH in the environment. Besides, the ratio of α-HCH/γ-HCH in the analyzed sediment samples of CauBay River ranged between 2.68 and 4.09 (mean 3.73). It means that the use of technical HCH is the major source and lindane is the minor source in the study areas.

4.4. Ecological risk assessment of DDTs and HCHs

To evaluate the ecotoxicological significance of DDTs and HCHs contamination in collected sediments, our data in Table 3 were compared with the interim sediment quality guideline (ISQG) and the probable effective level (PEL), issued by the Canadian Council of Ministers of Environment [30]. Hoai et al. [2] reported that the concentrations of DDE, DDD, and DDT in all the Hanoi sediment samples were higher than the ISQG values (1.42, 3.54, and 1.19 ng/g, respectively). The DDE, DDD, and DDT generally exceed the PEL values (6.75 ng/g for DDE, 8.51 ng/g for DDD, and 4.77 ng/g for DDT) but vary among the sediment samples [2]. This conclusion is in agreement with DDTs residues in CauBay River and Hochiminh city [22, 28]. In general, most of the collected sediment samples in Table 3 had DDTs at low levels as well as lower than PEL values. With regards to HCHs, no values of ISQG and PEL were reported in the international guidelines. Due to the propensity of DDTs to highly accumulate in various compartments of wildlife and human food webs, further evaluation of ecological risk assessment in Hanoi and Hochiminh city should be undertaken Figures 4 and 5.