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Article

Metal Content and Enrichment in Bivalves within the Drainage Area of Seawater Used for a Desulfurization Process in Zhanjiang Bay, China

by
Jibiao Zhang
1,
Demeng Peng
1,
Peng Zhang
1,*,
Yumei Rong
2,
Lifang Hu
2,
Lirong Zhao
1 and
Chunliang Chen
3
1
College of Chemistry and Environmental Science, Guangdong Ocean University, Zhanjiang 524088, China
2
Neusoft Institute Guangdong, Foshan 528225, China
3
Analytial and Testing Centre, Guangdong Ocean University, Zhanjiang 524088, China
*
Author to whom correspondence should be addressed.
Water 2022, 14(16), 2532; https://doi.org/10.3390/w14162532
Submission received: 10 July 2022 / Revised: 2 August 2022 / Accepted: 13 August 2022 / Published: 17 August 2022
(This article belongs to the Section Water Quality and Contamination)
Browse Figures
Versions Notes

Abstract

:
As heavy metals are easy to accumulate and have strong biological toxicity, they pose a potential threat to human health by entering the human body through the cumulative effect of marine life. Land-based input is an important source of heavy metals in the ocean, which has a great influence on coastal water quality. In this study, the spatial distribution characteristics of heavy metals (Zn, Cu, Cd, Pb, Cr, As) in the coastal waters of the desulfurization process outlet of a power plant in Zhanjiang Bay were investigated, and the enrichment behavior of heavy metals by organisms (oysters and barnacles) were also studied. The results showed that, before the seawater desulfurization system was closed, there were high concentrations of heavy metals (Cu, Zn, Cd, Pb and Cr) in the surface seawater near the drainage outlet. The concentrations of these heavy metals in the surface seawater were higher than those in the bottom seawater within 100 m of the drainage outlet. After the seawater desulfurization system was closed, the average concentrations of Cu, Cr and As in seawater at each station decreased by 17.04%, 37.52% and 29.53%, respectively, while the average concentrations of Zn, Cd and Pb increased by 17.05%, 32.87% and 48.77%, respectively. Single factor pollution index (SFI) and bio-concentration factor (BCF) showed that there was a potential high accumulation risk of Zn in oysters and barnacles near the drainage outlet of desulfurization wastewater (0.5 < SFI < 1 and BCF > 1000). The SFI and BCF of each metal in oysters and barnacles decreased with the increase in distance from the drainage outlet. Generally, the coastal water quality of desulfurization process drainage area in Zhanjiang Bay were below the class Ⅱof the “Seawater quality standard” (GB 3097-1997) of China. However, the heavy metals content in seawater and organisms near the drainage outlet is slightly higher. This suggested that if the seawater desulfurization process runs for a long time, it will have a negative impact on the coastal water and organisms.

1. Introduction

In recent years, seawater desulfurization technology has been rapidly applied in coastal thermal power plants. By using the alkaline characteristics of seawater to scrub and absorb SO2 in flue gas, seawater desulfurization technology is designed to achieve flue gas purification. Theoretically, there are no other by-products in the seawater desulfurization process [1,2]. However, in the process of flue gas washing, seawater in the absorption tower will absorb a large amount of SO2, and the SO2 gas in flue gas will generate H2SO3 after dissolving in water, making the pH of desulfurization wastewater generally appear acidic [3]. While under acidic conditions, the high concentration of F− and Cl− in desulfurization wastewater will generate HF and HCl [4], and the discharge of these wastewater into the ocean will have a great impact on the pH of seawater. Moreover, some heavy metals and other pollutants in the fly ash are also dissolved during flue gas washing [5], resulted in considerable amounts of suspended particulate matter and heavy metals such as mercury (Hg), arsenic (As), selenium (Se), and cadmium (Cd) in desulfurization wastewater [6,7]. These contaminants may accumulate in the aquatic food chain and pose a significant threat to the entire ecosystem [7]. In addition, SO2 in flue gas is washed and absorbed by seawater and further converted into large amounts of SO32−, an unstable oxygen-depleting substance. Therefore, only after sufficient aeration and oxidation can the flue gas desulfurization seawater be discharged into the sea area as SO42− combined with heavy metal substances [8,9]. Meanwhile, SO42− in the water body is easily reduced to S2− in an anaerobic environment, and these S2− combine with H+ in the water column to generate toxic and harmful H2S that escapes into the air causing pollution [10].
Heavy metal is a typical persistent inorganic pollutant widely existing in marine environments, which has the characteristics of strong toxicity, reduction resistance, degradation resistance, concealment, long-term and irreversible [11,12]. It can be directly transferred from seawater to marine organisms, or through contaminated sediments to marine benthic organisms. Through the accumulation and amplification mechanism of food chain, it poses a potential threat to human health [13,14]. Metals such as Cu and Zn are essential biological micronutrient elements required for the growth of many aquatic organisms, but these micronutrients may be toxic to aquatic organisms at high concentrations [15]. Others that are not essential for the growth of aquatic organisms, such as Cr, Pb and Cd, are all highly toxic to marine organisms in just trace amounts [16]. With the rapid development of coastal industry, heavy metal pollution has increasingly become a potential ecological risk in coastal and estuarine environments. Monitoring and control of marine heavy metal pollution has been a hot topic.
Physical and chemical analysis of seawater quality is an important and effective means to monitor marine heavy metal pollution. However, it cannot reflect the long-term actual damage of heavy metals to organisms and marine ecosystems only by a limited number of physical and chemical analysis of seawater. Therefore, marine organisms are taken as the biological indicators to monitor the heavy metal pollution in seawater and judge the toxicity of heavy metal pollution to marine organisms and its potential threat on marine ecosystems [17]. Marine shellfish have a small range of activity and are filter-feeding organisms that enrich pollutants in the water body while filtering biological bait [18] and have a strong bioconcentration capacity for pollutants such as heavy metals, and because of this habitat and physiological characteristics of shellfish, researchers often use it to indicate biological monitoring of environmental pollution [18,19,20,21,22,23]. Meanwhile Reinfelder [24] pointed out that oysters have a high assimilation rate and high uptake rate of heavy metals relative to other shellfish, and the rate of metal excretion from the body of oysters is relatively low. As a highly sensitive and low-cost indicator organism, oysters play an important role in carrying out marine environmental pollution indicators.
Zhanjiang Bay, a typical natural semi-enclosed bay, is located in the southernmost Leizhou Peninsula, Guangdong Province, China [25]. In Zhanjiang Bay, it is an irregular semidiurnal tide with an average tidal range of about 2.17 m and a maximum of 5.45 m. Moreover, the hydrodynamic dilution and diffusion effect is relatively intense in the bay due to the strong tidal current [26], which is an excellent choice for the seawater discharge of flue gas desulfurization in coal-fired power plants. With the rapid development of social economy in Zhanjiang, a large number of coastal industries are located along the coast of Zhanjiang Bay, anthropogenic impact on coastal environments is increasing by land pollutant emissions, marine aquaculture, ports, dock shipping and other human activities [27]. Zhanjiang Bay is a geographically and hydrodynamically complex area with many sources of metals, and coastal industries affect the dynamics and biogeochemical processes of heavy metals in Zhanjiang Bay, which in turn have an important impact on the environmental quality of the bay. In order to comprehensively understand the distribution of heavy metals in coastal power plant drainage area and its impact on the marine environment, three aspects of work in this study were carried out as following: (1) Monitoring the spatial distribution characteristics of heavy metals Cu, Zn, As, Cr, Cd and Pb in coastal seawater of drainage area; (2) The indicative oysters and barnacles were selected for the enrichment study of Cu, Zn, As, Cr, Cd and Pb, and the content level and enrichment behavior of heavy metals in organisms were monitored; (3) Combined with the analysis of the content level and spatial distribution characteristics of heavy metals in seawater and polluted organisms, it provides a reference for the monitoring of heavy metals in the drainage area of coastal power plants.

2. Materials and Methods

2.1. Study Area and Sampling Stations

As the developing area of Zhanjiang city, the Donghai Island has a population of 202,000 inhabitants and covers an area of 401 km2 [26]. Baosteel’s steel base of Zhanjiang is located in the northeast corner of this island. In order to ensure the electrical safety of 10 million tons of steelmaking system and reduce the gas emission of iron and steel plants, make full use of the by-product gas of steel production and protect the environment, Baosteel’s steel base had built a self-used power plant. The power plant has four 350 MW heating units mixed with gas and two 160 MW Combined Cycle Power Plant (CCPP) gas units. The flue gas desulfurization system of the power plant unit firstly adopts the seawater desulfurization process. The outlet of Seawater Desulfurization System is located inside Zhanjiang Bay, as illustrated in Figure 1b. In order to understand the influence of water diffusion discharged into Zhanjiang Bay by the desulfurization process on the sur-rounding water environment, 18 stations were arranged around the vicinity of the drainage outlet by taking full account of the tidal current and hydrodynamic conditions near the drainage outlet. So, a fan-shaped survey area with the radius of 10 m, 100 m, 500 m, 1000 m, 2000 m and 3000 m away from the drainage outlet was set up as shown in Figure 1c. Therein, S14 station is 3000 m away from the drainage outlet as the control site of upstream water body.
In order to further understand the toxicity of heavy metal content to marine organisms and its potential impact on marine ecosystems, we selected oysters and barnacles to study the enrichment of heavy metals in their soft tissues near the drainage area of seawater desulfurization system. Seven stations (A1, A2, A3, A4, A5, A6 and A0) were set up and the station A0 was used as the reference station. The distribution diagram of stations is shown in Figure 1, and the distance from each station to the drainage outlet is shown in Table 1.

2.2. Sample Collection, Preservation and Determination

2.2.1. Collection and Determination of Seawater Samples

In this study, the heavy metal content of the surface and bottom seawater near the outlet of a power plant in Zhanjiang Bay was monitored monthly from March 2015 to June 2016. The period from March 2015 to January 2016 was the monitoring stage when the seawater desulfurization system was in operation while from February to June 2016, the seawater desulfurization system was in a closed stage. Water samples collected were immediately transported back to the experimental laboratory and pre-treated within 24 h according to “The specification for marine monitoring—Part 3: Sample collection, storage and transportation” (GB 17378.3—2007) [28]. The water samples were filtered with 0.45μm acetate membrane (Shanghai Xingya Purification Material Factory, Shanghai, China), and then acidified to pH≈2 with HNO3 (Fluka, Sigma-Aldrich, Selzer, Germany). The contents of Cu, Zn, As, Cr, Cd and Pb in acidified samples were determined by an inductively coupled plasma mass spectrometer (ICP-MS) (Agilent 7500Cx, NYSE: A, Palo Alto, CA, USA), the experimental procedure is shown in Figure S1 in the supplementary material.

2.2.2. Collection, Preservation and Determination of Biological Samples

In March 2016, barnacles and oysters in an area of 30 × 30 cm2 were sampled on the surface and middle layers of the dock columns at each station where barnacles and oysters adhered uniformly and in a good growth. The surface layer was about 0.5 m below the high-water line, and the middle layer was about 1.5 m below the high-water line. The collected biological samples were put into a clean polyethylene plastic bag and brought back to the laboratory. Fifteen adult barnacles and oysters with the similar size were selected from each station and randomly divided into three groups, with five in each group. Collection and storage of biological samples were performed according to “The specification for marine monitoring—Part 6: Organism analysis” (GB 17378.6—2007) [29]. Soft tissues of barnacles and oysters were digested by microwave digestion and extraction system (MARS, CEM corporation, Matthews, NC, USA). Firstly, the soft tissue was removed from the barnacles and oysters by a clean hammer, and the feces and other residues in the viscera were removed. After weighing the wet weight, the soft tissue was washed by ultrapure water and dried, and the moisture content was calculated. Soft tissues were cut open, washed with ultrapure water, then ground and placed in a microwave digestion tank. Then add 8 mL of HNO3 (Fluka, Sigma-Aldrich, Selzer, Germany), 2 mL of 30% H2O2 (GR, Guangzhou Chemical Reagent Factory, Guangzhou, China) and 0.5 mL of HF (GR, Guangzhou Chemical Reagent Factory, Guangzhou, China). The digestion program was set to 1600 W 180 °C for 20 min. After digestion and cooling, 25 mL of digestion solution was made with ultrapure water, and the digestion solution was diluted 10 times and measured. ICP-MS was used to determine the contents of heavy metals in diluted digestion samples. During digestion and determination, a blank sample was prepared for each ten samples and the national standard reference material of oyster (GBW 10068, Beijing, China Institute of Metrology) was prepared for quality control, the experimental procedure is shown in Figure S1 in the supplementary material.

2.3. Data Statistics and Analysis Methods

The station map of seawater survey and fouling organisms survey and the regional distribution map of heavy metal content in seawater were drawn by Surfer software. The distribution characteristics of heavy metal content in organisms were plotted with Origin software. Single factor index (SFI) was used to assess the pollution degree of heavy metal content in oyster and barnacle organisms at each station. The bio-concentration factor (BCF) and bio-sediment accumulation factor (BSAF) were also used to evaluate the bio-accumulation ability of heavy metals in seawater.
The SFI was used to evaluate the status of heavy metal pollution in organisms at survey stations [30]. The calculation formula was as follows:
P i = C i / S i
In the formula, Pi represents the pollution index of pollution factor i, Ci represents the content of heavy metal i (wet weight) in organisms, and Si represents the evaluation standard value of the pollution factor i. The second standard value of “Marine biological quality” (GB18421-2001) [31] was used as the reference value of Si. The Si of Cu, Zn, As, Cr, Cd and Pb in oyster and barnacle were 25, 50, 5, 2, 2 mg/kg, respectively.
Bio-concentration factors (BCF) were used to evaluate the enrichment of heavy metals by organisms from the surrounding water [32]. The formula is:
B C F = C A / C W
Therein, CA is a heavy metal content in organisms; CW is the heavy metal content in water. Both are expressed as dry weight (dw).
The biota sediment accumulation factors (BSAF) of heavy metals can be used to determine whether the bioaccumulation of heavy metals in the study area can occur [33]. The calculation formula is as follows:
B S A F = C O / C S
Therein, CO is the content of some heavy metals in organisms; CS is the corresponding heavy metal content in sediments, the heavy metal origins in mean sediment concentrations were consistent with our earlier research [34]. Both are expressed as dry weight (dw).

3. Results

3.1. Content and Distribution Characteristics of Dissolved Heavy Metals in Seawater

The regional distributions of heavy metals Cu, Zn, Cd, Pb, Cr and As in seawater surface under normal operation (March 2015–January 2016) and after the closure of seawater desulfurization system (February 2016–June 2016) are shown in Figure 2. The high concentrations of Cu, Zn, Cd and Pb in surface seawater were found in the range of 100 m from the outlet (S4, S8, S9 and S10) under normal operation of the desulfurization process (Figure 2A–F). The high concentrations of Cu, Zn, Cd, Pb, Cr and As in the surface seawater were generally located in the area 500–3000 m away from the drainage outlet after the seawater desulfurization system was closed (Figure 2a–f). The concentration differences of heavy metals in surface seawater before and after the closure of seawater desulfurization system were as follows: at Station S9 (within 10 m from the drainage outlet), the concentrations of Cu, Zn, Pb, Cr and As were 1.49 times, 1.56 times, 1.89 times, 1.15 times, 1.89 time and 1.89 times higher than those after the closure, respectively; at stations S4, S8 and S10 (within 100 m from the outlet), the concentrations of Cu, Zn, Cd, Pb, Cr and As were 1.31–1.59, 0.74–1.08, 0.60–1.21, 0.5–1.24, 1.53–1.72 and 1.40–1.44 times higher, respectively, compared with those after the closure. Before and after the seawater desulfurization system was closed, the concentration of heavy metals in the bottom seawater was not significantly different, and there was no obvious law of gradual increase or decrease.
In the vertical direction, the concentrations of Cu, Zn, Cd, Pb and Cr in the surface layer were higher than those in the bottom layer in the range of 100 m from the outlet under the condition of normal operation of the desulfurization system, and there was no obvious variation of As concentration in the vertical direction. After the desulfurization system was closed, the Zn concentration in the bottom layer was higher than that in the surface layer, while Cu, Cd, Pb, Cr and As had no obvious change rule in the surface layer and bottom layer.
Under normal operation conditions, the average concentrations of heavy metals in the surface and bottom layers of seawater were Zn > Cu > As > Pb > Cr > Cd. After the seawater desulfurization process was stopped, the average concentrations of heavy metals in the surface and bottom layers of seawater were Zn > Cu > Pb > As > Cr > Cd (Table 2). After the closure of the seawater desulfurization system, the average concentrations of heavy metals Cu, Cr and As in seawater decreased and the average concentrations of Zn, Cd and Pb increased at each station. However, the average concentrations of all of the heavy metals in the surface layer of seawater decreased to different degrees at the stations near the outfall.
During normal operation of the seawater desulfurization system, the concentrations of heavy metals Zn, Cu, As, Pb, Cr and Cd in the surface layer of seawater near the drainage outlet were slightly higher than those in other regions. However, compared with “Seawater quality standard” (GB 3097-1997) [35], during the desulfurization process and after the seawater desulfurization system was closed, Cu, Zn, Cd, Cr and As in the seawater were all lower than the limits of class I of seawater quality standard, and Pb was lower than the limits of class II seawater quality standard in the area near the drainage outlet.

3.2. Content and Distribution Characteristics of Heavy Metals in Marine Fouling Organisms

As shown in Table 3, except for As, the contents of heavy metals (wet weight) in the soft tissues of oysters and barnacles in the surface and middle layers were in the order of Zn > Cu > Cr > Cd > Pb. In the regional distribution, the contents of Zn, Cu, Cr, Cd, Pb and As in the surface and middle layers of oysters and barnacles decreased as the distance from the drainage outlet increased (Figure 3, Figure 4, Figure 5 and Figure 6). The contents of Zn, Cu, Cr, Cd, Pb and As in the soft tissues of oysters and barnacles at 50 m from the drainage outlet (station A1) were much higher than at 2000 m from the drainage outlet (station A0), and the contents of Zn, Cu, Cr, Cd, Pb and As at station A1 were about 1.36 times, 3.14 times, 6.34 times, 3.13 times, 2.45 times and 1.76 times of those at station A0, respectively.
In the vertical distribution, in the range of 500 m (stations A1–A3), the contents of Cr, Cd, and As in the soft tissues of surface oysters and barnacles were 2.18, 1.72, and 1.19 times that in the soft tissues of middle oysters and barnacles, while the contents of Zn, Cu, and Pb were not significantly different between surface oysters and middle oysters and barnacles. On the whole, the contents of Zn, Cu and Pb in the surface and middle oysters and barnacles had no obvious change in the range of 500 m from the drainage outlet. The contents of Cr, Cd, Pb and As in the different levels of biological tissues were surface layer > middle layer (Figure 3, Figure 4, Figure 5 and Figure 6).

3.3. Pollution Degree and Enrichment Behavior of Heavy Metals in Marine Organisms

The spatial distribution characteristics of heavy metals in the soft tissues of and barnacles were used to assess the heavy metal pollution near the drainage area of Zhanjiang Bay. The single factor index (SFI) method was used to evaluate the pollution degree of heavy metals in the surface and middle layers of each sampling station. Comparison with the classification standard in “Technical specification of marine biological quality monitoring” (HY/T 078-2005) [36], when the pollution index is less than 0.5, the organism is not polluted by this factor; when the pollution index is between 0.5 and 1.0, the organism begins to be polluted by this factor; and the pollution index is greater than 1.0, indicating that the organism has been polluted by this factor. The evaluation results are shown in Table 4 and Table 5. As shown in Table 4, the SFI of Cu, Cd, Pb, Cr and As in oyster soft tissue of each survey station was less than 0.5, which was in the normal range, and Zn (except the surface layer of A5 station) was in the range of 0.5 < SFI < 1, indicated that oysters began to be polluted by Zn. As shown in Table 5, the SFI of Cu, As, Cd and Cr in the soft tissues of barnacles at each survey station were all less than 0.5, which were within the normal range, while the Zn and Pb SFI at those stations close to the drainage outlet was greater than 0.5. The pollution indexes of Zn in the surface and middle barnacles at stations A1 and A2 were between 0.5 and 1 (SFI = 0.570, 0.500, 0.532, 0.535), the pollution indexes of Pb in the surface of station A1 were greater than 1 (SFI = 1.001), showed that the potential pollution risk of Pb in surface layer of A1 station was high. In the vertical direction, the single factor pollution index of heavy metals Cu, Zn, Cd, Pb, Cr and As in the soft tissues of surface oysters and barnacles was higher than that in the middle layer within the range of 50 m (station A1). On the whole, the SFI of biological heavy metals in surface layer was higher than that in middle layer, and the closer the oysters and barnacles were to the drainage outlet, the greater the single-factor pollution index of each metal.
Bio-concentration factor (BCF) and biota sediment accumulation factor (BSAF) were used to evaluate the enrichment of heavy metals in oysters and barnacles, as shown in Tables S1 and S2 in the supplementary material. When BCF < 100, it indicated low accumulation, 100 < BCF < 1000, indicated medium accumulation, BCF > 1000, indicated high accumulation. When BSAF > 100%, it indicates that heavy metals have biological accumulation effect. The higher BSAF value, the stronger bioaccumulation. The BCF of heavy metals in soft tissues of oysters and barnacles was Zn > Cu > Cd > Cr (the BCF of Pb and As was less than Cr). Among them, the BCF of Zn and Cu in oyster and barnacle soft tissue was greater than 1000, which was at a high ac-cumulation level. The BCF of As, Cd, Pb and Cr was in the range of 100–1000, which was at a medium accumulation level. The BSAF of Zn, Cu and Cd in oyster soft tissue was more than 100%, and the BSAF of Zn, Cu and Cd in barnacle soft tissue was more than 100%, suggested that Zn, Cu and Cd have the potential to be accumulated in the organism. BSAF of heavy metals in soft tissues of oysters and barnacles showed Cd > Zn > Cu > Pb > Cr. Overall, the BCF and BSAF of heavy metals in surface layer were higher than those in middle layer, and the BCF and BSAF of each heavy metal in oyster and barnacle soft tissue showed that the closer to the drain outlet, the higher the BCF and BSAF.

4. Discussion

4.1. Effect of Seawater Desulfurization Drainage on Coastal Water Quality

For heavy metal water quality, there are three main sources, namely natural sources, land-based inputs [37,38,39,40,41,42,43] and atmospheric deposition [44,45]. Many human activities may contribute to the input of heavy metals in the coastal area, including the discharge of coastal industrial waste gas and wastewater, dredging projects, marine transportation, and urban sewage discharge [46,47]. According to the analysis of the regional distribution of heavy metals in desulfurization wastewater discharge area of Zhanjiang Bay, the high content areas of heavy metals Cu, Zn, Cd, Pb and Cr were located near the drainage outlet and in its downstream sea area, which may be related to the heavy metals contained in the desulfurization wastewater discharged from the outlet. The results of previous studies also indicated that the operation of the seawater desulfurization system will cause the change in water quality in the nearby sea area [40], especially the contents of Cr and Zn will increase [48]. Meanwhile, an obvious horizontal distribution feature appeared from the coast to the outer sea because of the diffusion of heavy metals under the action of tidal current dynamics [49]. After the seawater desulfurization process stopped operating, the average concentrations of Cu, Cr and As in seawater at each station decreased by 17.04%, 37.52% and 29.53%, respectively. Among them, the Cu content decreased the most at station S4 (100 m from the outfall) by 37.04%, Cr decreasing the most at station S9 (10 m from the outfall) by 47.14%, and As decreased at station S3 (500 m from the outfall) by 35.91%. The stations with the largest decrease in Cu, Cr and As concentrations were closer to the outfall because a large amount of desulfurization wastewater was discharged into the sea during the normal operation of the seawater desulfurization process, which brought a large amount of heavy metals and increased the heavy metal content in the area near the outfall. It indicated that desulfurization wastewater was an important source of Cu, Cr and As in the sea near the drainage area. The differences of Cu, Cr and As in the surface layer of seawater at the outfall and its downstream coastal waters (S5~S9) before and after the discontinuation of the seawater desulfurization process were smaller the further away from the outfall, but there was no such trend in the upstream and other areas because the water body flowed from the west side of the outfall to the east side, and the upstream and other areas were more influenced by the disturbance of the water body, resulting in a less obvious trend. Instead, they increased by 17.05%, 32.87% and 48.77%, respectively, after the seawater desulfurization process stopped operating, indicating that the desulfurization wastewater was not the main source of Zn, Cd and Pb. During the desulfurization process and after the seawater desulfurization system was closed, Cu, Zn, Cd, Cr and As in the seawater were all lower than the limits of class I of seawater quality standard, and Pb was lower than the limits of class II seawater quality standard in the area near the drainage outlet, this is similar to previous studies [34,50]. The average concentrations of Cu, Zn, Cd and Pb in seawater in Zhanjiang Bay were generally lower compared to those reported for Bohai Bay [51]. The average concentrations of Cu and Cd in seawater were similar to those in Jiaozhou Bay [52]. The average concentrations of Cu, Zn and Cd in seawater were higher than those in the Yangtze River Estuary [53,54]. The monitoring results of all heavy metal concentrations did not differ significantly in the vertical direction, probably due to the shallow water depth, strong tides, and large tidal differences [55], the large hydrodynamic changes caused by tidal currents, and the fast-circulating exchange of seawater, where heavy metal concentrations are usually evenly distributed in the vertical direction [56].

4.2. Enrichment Level of Heavy Metals in Oyster and Barnacle Soft Tissue

The contents of heavy metals in oyster soft tissues and barnacle soft tissues in Zhanjiang Bay were Zn > Cu > Cr > Cd > Pb, which were basically consistent with the concentrations of heavy metals in seawater of Zhanjiang Bay [34]. In the regional distribution, the concentrations of heavy metals in seawater and the contents of heavy metals in fouling organisms (oysters and barnacles) decreased with the increase in distance from the drainage outlet, which had good consistency. The concentrations of Zn, Cu, Cr and Pb in the surface seawater at the station closest to the outfall (S9) increased by 34.19%, 2.81%, 15.35% and 6.20% compared to the station farthest from the outfall (S14) during normal operation of the seawater desulfurization process. The levels of Zn, Cu, Cr, Cd, Pb and As in the soft tissues of the surface barnacles at the station closest to the outfall (A1) increased by 6.11%, 380.76%, 1043.77%, 327.55%, 47.33% and 65.28% compared to the station farthest from the outfall (A0), and the levels of Zn, Cu, Cr, Cd, Pb and As content increased by 4.52%, 70.70%, 127.82%, 97.45%, 161.61% and 50.49% compared to A0. This may be due to the existence of dissolved or complex heavy metals in flue gas desulfurization wastewater [4], and the slight increase in heavy metals in water. Oysters and barnacles were filter food organisms. They could absorb heavy metals through aqueous phase absorption and particulate phase absorption and accumulate heavy metals in their tissues [57]. Before the desulphurization system was closed, the content of heavy metals in the surface fouling organisms was higher than that in the middle seawater. This may be due to the shallow discharge of wastewater and the high temperature of wastewater discharged, which led to the retention of wastewater in the surface water and the greater impact on the surface water. The sampling stations in the nearshore surface were relatively close to the pollution source, and the exchange of water quality was slow. The heavy metals in the discharged wastewater were enriched into the organism through the food chain [58]. The higher concentrations of Zn and Cu in oysters and barnacles may be related to the higher concentrations of Zn and Cu in water. Studies have shown that there was a significant linear positive correlation between Zn and Cu in oysters and barnacles and Zn and Cu in water [59]. On the other hand, it may be because oysters and barnacles are net accumulation of Zn and Cu, and the biological half-life of removing heavy metals from the body was long [23,24]. The accumulation of heavy metals by marine organisms actually depended on the rate of metal entering and leaving organisms, and the relative rate change determined the accumulation of specific metals by organisms [8].
The bioconcentration factor (BCF) of oysters and barnacles in the coastal waters of Zhanjiang Bay was higher than that of biota sediment accumulation factor (BSAF), which may be related to the fact that oysters and barnacles are living in camps and feed on single-cell phytoplankton and organic debris [17,18,19]. During feeding, oysters and barnacles need to continuously filter seawater containing high concentrations of heavy metals such as suspended solids, organic debris and phytoplankton [19,20,21,22]. It may affect the amount of heavy metals in the sediment. Moreover, the content of heavy metals in general suspended solids was higher than that in sediments [60]. It was concluded that the content of heavy metals in marine organisms could reflect the level of heavy metals in corresponding sea areas to some extent.

4.3. Comparison of Heavy Metal Contents in Soft Tissues of Oysters and Barnacles in Other Estuaries and Sea Areas

Bivalves are filter-feeding organisms, which could absorb heavy metals through water phase absorption and particle phase absorption and accumulate heavy metals in their tissues [57]. In addition, bivalves were widely distributed, easy to collect and had strong resistance to the physical and chemical changes in the marine environment. They were therefore used as biological indicators to monitor heavy metal pollution in many aquatic environments [61,62].
According to the results of Table 6, compared with coastal water, Hong Kong, the content of Cu in oyster soft tissue was about 14–19 times lower, the content of Zn was about 6–26 times lower, and the difference of Cr content was small. Compared with Maowei estuary, China, the difference of Cu content was small, Cd content was 1–3 times higher, Pb content was 16–116 times lower, As content is 17–23 times lower. Compared with Hab River Delta, Balochistan, Pakistan, Cu content was about 18 times higher, Zn content was about 2–4 times lower, Cd content was about 17 times higher, Pb content was about 5–7 times lower, Cr content difference was small, and As content was about 1–3 times lower.
Compared with West coast of India, the Cu content in the soft tissue of oyster was about 128–310 times lower, and the Cu content was about 3.2–3360 times lower than that of Mazatlan piers and Mexico. Zn was about 23–89 times lower than West coast of India, about 0.5–2 times higher than Mazatlan pier, Mexico and Guaymas Harbour, Mexico, while Cd was much lower than Puerto Vallarta, Mexico and Mazatlan Harbours, Mexico. Compared with Hebe Haven pier in Hong Kong, Tso Wo Hang pier in Hong Kong and Sai Kung pier in Hong Kong, Cu content was about 60–352 times and Zn content was about 120–270 times lower. Compared with the west coast of Portugal, the content of Cu was 1.5–2.5 times higher, the content of Zn was 2–5 times lower, and the difference of Cd content was small.
In general, the contents of heavy metals Zn, Cu, Cr, As, Cd and Pb in oyster and barnacle soft tissues in the coastal waters of Zhanjiang Bay were at a low level compared with those in the coastal waters around the world, indicated that the input of heavy metals from land sources in Zhanjiang Bay was less, and the content of heavy metals in wastewater discharged during the operation of desulfurization system was low. However, the soft tissues of oyster and barnacle in the coastal waters of Zhanjiang Bay had accumulated higher heavy metals. If the desulfurization process runs for a long time, the heavy metals in desulfurization wastewater may have been accumulated and enlarged in marine organisms through the food chain, which was a threat to human safety.

5. Conclusions

This study investigated the effects of desulfurization process discharge on seawater quality and heavy metal content in marine organisms in coastal waters of Zhanjiang Bay. The conclusions are as follows:
(1)
When the seawater desulfurization system of Zhanjiang Bay coastal power plant was in normal operation, the heavy metals Cu, Zn, Cd, Pb and Cr in the seawater near the coastal power plant had obvious spatial distribution characteristics. In terms of regional distribution, the contents of Cu, Zn, Cd, Pb and Cr were higher near the drainage outlet and in the downstream waters. In the vertical direction, the concentrations of Cu, Zn, Cd, Pb, Cr and As in the surface layer were higher than those in the bottom layer within 100 m of the drainage outlet. The average concentrations of heavy metals Cu, Cr and As in the normal operation of seawater desulfurization process in Zhanjiang Bay were higher than those after the seawater desulfurization process stopped operating;
(2)
The content of heavy metals in the soft tissues of oysters and barnacles was Zn > Cu > Cr > Cd > Pb, which had obvious spatial distribution characteristics. In terms of regional distribution, the contents of heavy metals in the soft tissues of oysters and barnacles decreased with the increase in the distance from the outlet. In the vertical direction, the contents of heavy metals in the soft tissues of oysters and barnacles were in the order of surface layer > middle layer;
(3)
Single factor index (SFI) showed that the SFI of heavy metals Cu, Cr, As and Cd in the soft tissues of oysters and barnacles were in the normal range. Zn and Pb had potential pollution risks in surface water near the drainage outlet. The BCF of Zn in the soft tissues of oysters and barnacles was greater than 1000, and the BSAF was greater than 100%. The oysters and barnacles had high accumulation of Zn, and it decreased with the increase in the distance from the drainage outlet.
When the seawater desulfurization project of Zhanjiang Bay Coastal Power Plant was in normal operation, the water quality of the sea area near the coastal power plant was below class II of the Seawater quality standard (GB 3097-1997) of China. However, the heavy metal content in the seawater area near the coastal power plant was slightly higher than that in the offshore water. Thus, if marine organisms are in this environment for a long time, there is a potential high enrichment risk. Based on the purpose of environmental protection, the seawater desulfurization process should be replaced by other environmentally friendly technologies for a coastal power plant.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/w14162532/s1, Figure S1: Schematic diagram of the sample handling process; Table S1: Bio-concentration factors (BCF) of oysters and barnacles to heavy metals in coastal waters of Zhanjiang Bay; Table S2: Biota sediment accumulation factors (BSAF) of oysters and barnacles to heavy metals in coastal waters of Zhanjiang Bay (%).

Author Contributions

Conceptualization, J.Z. and P.Z., Methodology, J.Z. and P.Z., Software, D.P., Validation, D.P., Formal analysis, D.P., Writing-original draft preparation, P.Z. and D.P., Writing-review and editing, P.Z., J.Z. and D.P., Visualization, Y.R., L.H., L.Z. and C.C., Supervision, J.Z., Project management, J.Z., Funding acquisition, P.Z. and J.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Research and Development Projects in Key Areas of Guangdong Province (2020B1111020004), the Science and Technology Program Project of Guangdong Province (2016A020225004), the First-class Special Fund (231419018) and Innovation Strong School Project (230420021) of Guangdong Ocean University.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data sharing is not applicable to this article.

Acknowledgments

The authors are grateful for the anonymous reviewers’ careful review and constructive suggestions to improve the manuscript. Thanks are given to the Research and Development Projects in Key Areas of Guangdong Province (2020B1111020004), the Science and Technology Program Project of Guangdong Province (2016A020225004), the First-class Special Fund (231419018) and Innovation Strong School Project (230420021) of Guangdong Ocean University.

Conflicts of Interest

The authors declare no conflict of interest.

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Water 14 02532 g001 550
Figure 1. Survey station map. (a) Location of the survey area in mainland China. (b) Location of the survey area in Zhanjiang. (c) Seawater quality survey station. (d) Sampling stations of oysters and barnacles.
Figure 1. Survey station map. (a) Location of the survey area in mainland China. (b) Location of the survey area in Zhanjiang. (c) Seawater quality survey station. (d) Sampling stations of oysters and barnacles.
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Figure 2. The distribution characteristics of the concentrations of heavy metals Cu, Zn Cd, Pb, Cr and As in the surface seawater. (A–F) shows the distribution characteristics of the concentrations of heavy metals during the normal operation of seawater desulfurization process. (a–f) shows the distribution characteristics of heavy metals after seawater desulfurization process closed.
Figure 2. The distribution characteristics of the concentrations of heavy metals Cu, Zn Cd, Pb, Cr and As in the surface seawater. (A–F) shows the distribution characteristics of the concentrations of heavy metals during the normal operation of seawater desulfurization process. (a–f) shows the distribution characteristics of heavy metals after seawater desulfurization process closed.
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Figure 3. Changes in heavy metal contents in surface oyster soft tissues at different monitoring stations (wet weight).
Figure 3. Changes in heavy metal contents in surface oyster soft tissues at different monitoring stations (wet weight).
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Figure 4. Changes in heavy metal content in middle oyster soft tissue at different monitoring stations (wet weight).
Figure 4. Changes in heavy metal content in middle oyster soft tissue at different monitoring stations (wet weight).
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Figure 5. Changes in heavy metal content in soft tissue of surface barnacle at different monitoring stations (wet weight).
Figure 5. Changes in heavy metal content in soft tissue of surface barnacle at different monitoring stations (wet weight).
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Figure 6. Changes in heavy metal content in soft tissue of middle barnacle at different monitoring stations (wet weight).
Figure 6. Changes in heavy metal content in soft tissue of middle barnacle at different monitoring stations (wet weight).
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Table
Table 1. Distance from biofouling sampling stations to drainage outlet (m).
Table 1. Distance from biofouling sampling stations to drainage outlet (m).
Sampling StationA1A2A3A4A5A6A0
Distance~50~100~500~1000~800~900~2000
Table
Table 2. The average concentration of heavy metals Zn, Cu, Cd, Pb, Cr and As in seawater (μg/L).
Table 2. The average concentration of heavy metals Zn, Cu, Cd, Pb, Cr and As in seawater (μg/L).
Sampling TimeLayerCuZnCdPbCrAs
March 2015—January 2016 (Normal operation of desulfurization process)Surface layer2.94 ± 0.216.42 ± 1.230.13 ± 0.021.00 ± 0.220.63 ± 0.042.51 ± 0.14
Bottom layer2.95 ± 0.225.97 ± 1.230.12 ± 0.020.99 ± 0.230.68 ± 0.062.44 ± 0.18
February 2016—June 2016 (The desulfurization process was stopped)Surface layer2.43 ± 0.427.26 ± 0.960.16 ± 0.031.42 ± 0.310.39 ± 0.031.77 ± 0.11
Bottom layer2.57 ± 0.487.71 ± 1.830.17 ± 0.031.42 ± 0.310.41 ± 0.041.76 ± 0.09
Table
Table 3. Content changes in heavy metals in soft tissues of oyster and barnacle (wet weight).
Table 3. Content changes in heavy metals in soft tissues of oyster and barnacle (wet weight).
Fouling OrganismLayerEigenvalueCuZnAsCdPbCr
OysterSurface
layer		Min3.4121.880.080.060.030.08
Max8.3742.080.200.160.120.35
M ± SD4.48 ± 1.6133.1 ± 6.790.11 ± 0.040.1 ± 0.030.05 ± 0.030.15 ± 0.09
Middle
layer		Min2.3127.570.070.060.030.06
Max9.1838.760.150.130.060.34
M ± SD5.06 ± 2.0433.63 ± 3.850.11 ± 0.030.09 ± 0.020.04 ± 0.010.16 ± 0.09
BarnacleSurface
layer		Min0.528.010.070.130.020.14
Max3.5428.490.230.840.082.00
M ± SD1.61 ± 1.0918.87 ± 6.700.15 ± 0.050.42 ± 0.260.04 ± 0.021.02 ± 0.63
Middle
layer		Min0.4713.670.100.090.020.23
Max3.1631.110.250.530.100.95
M ± SD1.63 ± 0.8222.34 ± 5.690.14 ± 0.050.33 ± 0.150.06 ± 0.030.58 ± 0.24
Table
Table 4. Single factor pollution index (SFI) of heavy metals in oysters.
Table 4. Single factor pollution index (SFI) of heavy metals in oysters.
LayersStationsPi
CuZnAsCrCdPb
Surface layerA10.3350.8420.0390.1740.0790.059
A20.1410.6270.0240.0420.0560.016
A30.1530.7010.0210.0930.0550.036
A40.1750.6260.0190.0530.0430.025
A50.1660.4380.0150.0520.0310.019
A60.1490.8410.0170.0590.0410.022
A00.1360.5600.0180.0530.0280.016
Middle layerA10.3670.7750.0300.1710.0650.027
A20.2100.7250.0160.0680.0500.015
A30.2210.6940.0280.1080.0470.017
A40.2180.6750.0230.0540.0420.028
A50.1850.5510.0200.0790.0310.018
A60.0930.7200.0150.0410.0290.025
A00.1230.5690.0190.0310.0350.025
Table
Table 5. Single factor pollution index (SFI) of heavy metals in barnacles.
Table 5. Single factor pollution index (SFI) of heavy metals in barnacles.
LayersStationsPi
CuZnAsCrCdPb
Surface layerA10.1420.5700.0460.3600.0391.001
A20.1140.5320.0360.2860.0160.570
A30.0710.2790.0270.4210.0120.913
A40.0210.1600.0140.1010.0140.277
A50.0360.3070.0350.1490.0090.305
A60.0430.3640.0330.0990.0240.441
A00.0240.4300.0220.0670.0210.070
Middle layerA10.0780.5000.0240.3040.1620.501
A20.1260.5350.0270.3580.1930.285
A30.0640.4770.0380.2700.2150.456
A40.0730.6220.0490.4770.2630.138
A50.0280.2730.0250.3680.1980.152
A60.0190.3220.0190.1160.0470.221
A00.0690.3960.0200.1320.0650.035
Table
Table 6. Comparison of heavy metals Cu, Zn, Cd, Pb, Cr and As in oysters and barnacles worldwide (dry weight).
Table 6. Comparison of heavy metals Cu, Zn, Cd, Pb, Cr and As in oysters and barnacles worldwide (dry weight).
Fouling OrganismTypes of Heavy MetalsStudy AreaContent (μg/g)References
OysterCuCoastal water, Hong Kong211–825[63]
Maowei Estuary, China17.15 ± 8.61[64]
Hab River Delta, Balochistan, Pakistan0.791 ± 0.023[65]
Hab River Delta, Balochistan, Pakistan0.731 ± 0.045[65]
Zhanjing Bay, China14.46–39.91This study
ZnCoastal water, Hong Kong1049–7351[63]
Hab River Delta, Balochistan, Pakistan774.4 ± 9.723[65]
Hab River Delta, Balochistan, Pakistan737.8 ± 12.2[65]
Zhanjing Bay, China149.3–262.72This study
CdCoastal water, Hong Kong0.43–5.88[63]
Maowei Estuary, China0.16 ± 0.14[64]
Hab River Delta, Balochistan, Pakistan0.027 ± 0.004[65]
Hab River Delta, Balochistan, Pakistan0.019 ± 0.003[65]
Zhanjing Bay, China0.35–0.68This study
PbMaowei Estuary, China17.58 ± 10.82[64]
Hab River Delta, Balochistan, Pakistan1.322 ± 0.148[65]
Hab River Delta, Balochistan, Pakistan1.070 ± 0.092[65]
Zhanjing Bay, China0.15–0.51This study
CrHab River Delta, Balochistan, Pakistan0.471 ± 0.039[65]
Hab River Delta, Balochistan, Pakistan0.381 ± 0.030[65]
Zhanjing Bay, China0.40–1.51This study
AsMaowei Estuary, China10.27 ± 5.24[64]
Hab River Delta, Balochistan, Pakistan2.059 ± 0.107[65]
Hab River Delta, Balochistan, Pakistan1.696 ± 0.031[65]
Zhanjing Bay, China0.42–0.85This study
BarnacleCuWest coast of India805[66]
Indian River Lagoon (Florida Coast)1937[66]
Mazatlan piers, Mexico11–64[67]
Topolobampo piers, Mexico10083[68]
Hebe Haven piers in Hong Kong915.1 ± 104.5[69]
northwest coast of Portugal0.93–5.70[70]
northwest coast of Portugal0.76–6.09[70]
Zhanjing Bay, China2.59–15.02This study
ZnWest coast of India3618[68]
Mazatlan piers, Mexico29[68]
Guaymas Harbour, Mexico21.8[68]
Hebe Haven piers in Hong Kong18,440.4 ± 3089.4[69]
Tso Wo Hang piers in Hong Kong12,929.4 ± 1372.8[69]
Sai Kung piers in Hong Kong10,855.2 ± 913.7[69]
northwest coast of Portugal119–782[70]
northwest coast of Portugal413–976[70]
Zhanjing Bay, China40.07–155.54This study
CdPuerto Vallarta, Mexico63.1[68]
Mazatlan Harbours, Mexico9.2–50.2[68]
northwest coast of Portugal0.39–1.98[70]
northwest coast of Portugal0.35–3.75[70]
Zhanjing Bay, China0.67–3.51This study
PbPuerto Vallarta, Mexico29.2[68]
Topolobampo, Mexico25[68]
Guaymas Harbour, Mexico15.9[68]
Zhanjing Bay, China0.09–0.40This study
CrCoast of Mexico<3.8[68]
northwest coast of Portugal0.25–1.79[70]
northwest coast of Portugal0.45–3.13[70]
Zhanjing Bay, China0.70–8.01This study
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Zhang, J.; Peng, D.; Zhang, P.; Rong, Y.; Hu, L.; Zhao, L.; Chen, C. Metal Content and Enrichment in Bivalves within the Drainage Area of Seawater Used for a Desulfurization Process in Zhanjiang Bay, China. Water 2022, 14, 2532. https://doi.org/10.3390/w14162532

AMA Style

Zhang J, Peng D, Zhang P, Rong Y, Hu L, Zhao L, Chen C. Metal Content and Enrichment in Bivalves within the Drainage Area of Seawater Used for a Desulfurization Process in Zhanjiang Bay, China. Water. 2022; 14(16):2532. https://doi.org/10.3390/w14162532

Chicago/Turabian Style

Zhang, Jibiao, Demeng Peng, Peng Zhang, Yumei Rong, Lifang Hu, Lirong Zhao, and Chunliang Chen. 2022. "Metal Content and Enrichment in Bivalves within the Drainage Area of Seawater Used for a Desulfurization Process in Zhanjiang Bay, China" Water 14, no. 16: 2532. https://doi.org/10.3390/w14162532

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