Environ Sci Pollut Res (2011) 18:1324–1332 DOI 10.1007/s11356-011-0486-4
RESEARCH ARTICLE
Mercury distribution in seawater discharged from a coal-fired power plant equipped with a seawater flue gas desulfurization system Xiyao Liu & Lumin Sun & Dongxing Yuan & Liqian Yin & Jinsheng Chen & Yaoxing Liu & Chengyu Liu & Ying Liang & Fangfang Lin
Received: 12 November 2010 / Accepted: 7 March 2011 / Published online: 29 March 2011 # Springer-Verlag 2011
Abstract Background and purpose More and more coal-fired power plants equipped with seawater flue gas desulfurization systems have been built in coastal areas. They release large amount of mercury (Hg)-containing waste seawater into the adjacent seas. However, very limited impact studies have been carried out. Our research targeted the distribution of Hg in the seawater, sediment, biota, and atmosphere, and its environmental transportation. Methods Seawater samples were collected from five sites: 1, sea areas adjacent to the power plant; 2, near discharge outlets; 3, the aeration pool of the power plant; and 4 and 5, two reference sites. The total gaseous Hg was determined in situ with a Tekran 2537B. Analyses of total Hg (TM) followed the USEPA methods. Results In most part of the study area, TM concentrations were close to the reference values and Hg transfer from the seawater into the sediment and biota was not obvious. However, in the aeration pool and near the waste discharge outlets, atmospheric and surface seawater concentrations of TM were much higher, Responsible editor: Vera Slaveykova X. Liu : L. Sun : D. Yuan (*) : L. Yin : Y. Liu : C. Liu : F. Lin State Key Laboratory of Marine Environmental Science, College of Oceanography and Environmental Science, Xiamen University, Xiamen 361005, China e-mail:
[email protected] L. Yin : J. Chen Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China Y. Liang The Eighth Department, Guilin University of Electronic Technology, Guilin 541004, China
compared with those at a reference site. The concentration ranges of total gaseous Hg and TM in seawater were 3.83– 8.60 ng/m3 and 79.0–198 ng/L near the discharge outlets, 7.23–13.5 ng/m3 and 186–616 ng/L in the aeration pool, and 2.98–4.06 ng/m3 and 0.47–1.87 ng/L at a reference point. Conclusions This study suggested that the Hg in the flue gas desulfurization waste seawater was not only transported and diluted with sea currents, but also could possibly be transferred into the atmosphere from the aeration pool and from the discharge outlets. Keywords Mercury . Coal-fired power plant . Seawater . Atmosphere . Distribution . Marine environment
1 Introduction Mercury (Hg) is a typical heavy metal, which has been known for several decades to be a toxic chemical with neurotoxic effects. Concerns for human and ecosystem health have attracted increasing attention to the study of Hg in the environment (Eric et al. 2010; Stephan et al. 2010). Natural sources include volcanic eruptions, geothermal activities, forest fires, and soil and water surface evaporation, whereas the numerous anthropogenic sources of inorganic Hg include coal combustion, mining, municipal waste incineration, and waste from industrial production (Nriagu 1989; Camargo 2002; Jaffe and Strode 2008). The largest share of Hg emission in Asia comes from coal combustion and smelting (Jaffe and Strode 2008). Chinese coal-fired power plants, metal smelters, and other industries release roughly a quarter of the world’s annual total Hg emissions, that is, about 700±300 tons out of an estimated global total of 1,930 tons (Wu et al. 2006; UNEP 2008).
Environ Sci Pollut Res (2011) 18:1324–1332
It is well recognized that burning coal introduces Hg pollution into the atmosphere (Yan et al. 2003; Goodarzi 2004), and for the last decade interest and concern have been paid to the waste seawater discharged from coal-fired power plants equipped with seawater flue gas desulfurization (SFGD) systems in coastal areas (Tokumura et al. 2006; Smitshuysen et al. 2009). Such power plants utilize fresh seawater to neutralize the acidic flue gas and thus produce a large amount of waste seawater containing various pollutants from the coal (Radojevic 1989; Vidal et al. 2007), including Hg, which originally exists as HgS in the coal. It is expected that a certain part of the Hg is removed by SFGD seawater (Chen 2007), and this part of the Hg would pollute the seawater and sediment as well as fisheries after being discharged to the sea area together with the waste seawater. To limit the impact of the SFGD waste seawater on coastal areas, the Ministry of Environmental Protection of China has strict regulations and long-term monitoring for seawater, biota, and sediments. However, there is a lack of impact assessment for the Hg from the SFGD waste seawater into the marine atmosphere. There have been many studies on Hg cycling and pollution in coastal water systems in the last decade, and San Francisco Bay (Conaway et al. 2003), Western Black Sea (Lamborg et al. 2008), Southern North Sea and Scheldt estuary (Leermakers et al. 2001), and California coastal areas (Black et al. 2009) are some typical examples. However, little study has been focused on the sea area adjacent to coal-fired power plants with SFGD systems, except for our previous study (Liang et al. 2010). Our present study targeted Hg in the waste seawater discharged from a coal-fired power plant (the SY Power Plant) with an SFGD system in Xiamen, China, in order to investigate Hg distribution in the marine environment. In the sea area near the waste seawater discharge outlet, the aeration pool of the power plant, and a nearby reference site, concentrations of total gaseous mercury (TGM) above the seawater surface were analyzed in situ, and total mercury (TM) in the surface seawater at these sites was analyzed in the laboratory, and the distributions of both were then plotted. The results should provide theoretical support for the risk assessment of Hg in the atmosphere around coal-fired power plants equipped with SFGD systems, as well as technical support for Hg control of coal-fired power plants in coastal areas.
2 Materials and methods 2.1 A brief description of the power plant and nearby environment The coal-fired power plant in Xiamen is the SY Power Plant, and it has a capacity of 1,200 MW and consumes
1325
10,000 tons of coal daily. Since the fall of 2006, the plant has been equipped with an SFGD system. Fresh seawater obtained from the nearby sea area is utilized to neutralize SO2 in the flue gas so as to reduce SO2 air pollution. Before being discharged back into the sea, the SFGD waste seawater is mixed with one third fresh seawater and aerated in two parallel aeration pools in the power plant in order to reduce CO2 and SO32−, as well as to increase pH and dissolved oxygen. After aeration and cooling, the waste seawater (as much as 0.112 Mm3/h) is directly discharged without further treatment through two pipes into the adjacent sea area at two underwater discharge outlets. Xiamen is a famous tourism city on the southeast coast of China, and there is no heavy industry in Xiamen and around the SY Power Plant and no other possible Hg discharging factories within a radius of at least 20 km. 2.2 Area investigated and sample collection Water, sediment, and biota samples were collected biannually from December 2007 to June 2010. As shown in Fig. 1a, there were five sampling sites, the first site was the sea area adjacent to the power plant, which included 23 sampling points (S series except S9 and S11); the second site was near the discharge outlets including two sampling points (S9 and S11); the third site was the aeration pools of the power plant, including only one sampling point (sampling point P); and the fourth and fifth were two reference sites (sampling points R1 and R2, respectively). The sampling points of S series were distributed around the discharge outlets (Fig. 1b) and S3 and S5 were located directly outside the discharge outlets. Based on local hydrological and meteorological conditions, reference point R1 was across the estuary in the upstream of the river, setting 2.5 km southwest from the power plant near a small islet with no residents and limited human impacts. And R2 was 8 km far away from the outlets as the reference point of less impact from the power plant. From the previous monitoring results it was found that the TM concentrations in R1 and R2 were significantly lower than those around the discharge outlets of the power plant. Seawater sampling was carried out during both high and low tide. Special care was taken in water sampling and all steps followed the US EPA (1996) Method 1669 sampling method. Surface seawater was from 0.2 m depth and samples for the profiles of Hg distribution were from depth of 0.2, 5, 10, and 15 m, respectively. The samples were collected into a clean borosilicate glass sample bottle using a peristaltic pump, then tightly sealed and double-bagged, and immediately cooled in an ice box. The samples were sent to the laboratory for pretreatment within 12 h. The samples were then divided into two subsamples, one of which was filtered through a 0.45-μm filter and used to
1326 Fig. 1 The study area and sampling sites. a Overall area. Site S (sampling points 1–25): sea area adjacent to the power plant; Site P: the aeration pool of the power plant; Site R1 and 2: the reference sites; and b details of sampling points S1–S23 of site S. S3 and S5 were at the discharge outlets
Environ Sci Pollut Res (2011) 18:1324–1332
a 24.45 N S24
P SY Power Plant
Xiamen
24.44 N R1
S25
24.43 N 24.42 N
R2
River estuary
Open sea
24.41 N
118.01 E
118.03 E
118.05 E
118.07 E
118.09 E
118.11 E
0 km 1 km 2 km 3 km 4 km
b 24.445 N SY Power Plant Drainage Outlets S1
S2 S7
S3 S9
S8
S14
S13
24.440 N S18
S19
S4 S5 S10 S11 S15 S16 S20 S21
S6 S12 S17 S22
S23
24.435 N 118.030 E 0m
300 m
determine dissolved Hg, and the other, an unfiltered one, was used for TM analysis. All the subsamples were preserved by adding bromine monochloride solution, and then kept at 4°C until analysis. Two biota samples and four sediment samples were collected during every sampling. Surface sediment samples were collected with a sediment collector. It has to be pointed out that the sediment in the study area had been dredged in the fall of 2008, which might provide a background value for studying the sedimentation of Hg from waste seawater since 2008. The biota samples were wild fish caught in the sea area adjacent to the power plant during seawater sampling, and were usually eels (Anguilla japonica), a typical local species, with normal weights of about 1.0 kg. Some oysters (Concha ostreae) on the reef near the discharge outlets were also collected. From December 2007 to June 2010, the TM in seawater collected at both high and low tide from six points (S9,
600 m
118.035 E 900 m
118.040 E
118.045 E
1200 m
S11, S15, S24, S25, and R1), in the surface sediment from two points (S15 and R1), and in the biota from the first site were monitored every half year. 2.3 Determination of TM The analysis of TM was carried out based on the US EPA (2002) method 1631, Revision E, the method of Gill and Fitzgerald (1987), and the US EPA (2001) appendix to method 1631, and is described briefly as follows: Each preserved seawater sample was reduced with hydroxylamine hydrochloride solution. An appropriate amount of the reduced seawater was weighed into a reaction vessel followed by the addition of SnCl2 solution. The reduced Hg was purged from the solution onto a gold trap. Subsequently, a double amalgamation in series was processed, followed by Hg detection with a cold vapor atomic fluorescence spectrophotometer (Beijing Rayleigh Analytical
Environ Sci Pollut Res (2011) 18:1324–1332
1327
Instrument Corporation). Particulate Hg was determined as the difference between TM and dissolved Hg. For sediment and biota samples, a known mass of sample (approximately 0.2 g) was weighed into a Teflon or PTFE vial, and digested with 5 mL 3:7 (v/v) nitric/sulfuric acid in an oven at 60°C overnight. The procedure which followed was the same as that for water samples. 2.4 Determination of TGM The TGM was determined in situ 1 m above the seawater surface with a Tekran vapor Hg analyzer 2537B (Tekran Instruments Corporation, USA), which is an elemental vapor analyzer designed especially for ultra-trace Hg vapor analysis. The instrumental detection limit was 0.1 ng/m3. 2.5 Determination of ancillary parameters Ancillary parameters, including seawater pH, temperature, salinity, wind speed, and sunlight intensity, were determined according to Chinese National Standards (State Environmental Protection Administration of China (SEPAC) 2007). The concentration of sulfite in seawater
was determined based on a method developed in our laboratory (Yin et al. 2009). 2.6 Quality assurance and quality control All vessels used were made of borosilicate glass or PTFE and cleaned thoroughly according to the US EPA (2002) method 1631, Revision E. The correlation coefficient, R2, of the calibration curve was higher than 0.997. Recoveries for matrix spiked seawater, sediment, and biota samples were within the acceptable range (71% to 125%). The relative percentage difference was less than 25% for matrix spiked duplicates and sample duplicates. The SD values of water, biota, and sediment samples were 1.6– 9.2 ng/L, 0.007–0.078 ng/g, and 0.007–0.057 ng/g. For sediment and biota samples, method performance was additionally checked using an Hg reference sediment material ERM-CC580 (ID 17486, Chinese CRM/RM Information Center, Beijing) and recoveries ranged 89– 115%. For TGM determination, the instrumental performance was checked using an internal automatic calibration unit, and a relative standard deviation lower than 10% was acceptable.
Table 1 TM in seawater, sediment, and biota, and the relevant criteria Sample type (unit)
Chinese criteria (State Environmental Protection Administration of China (SEPAC) 1997, 2002)
Sampling Distance to point the closest discharge outlet (km)
Seawatera 1st class 50, 2nd class 200 (ng/L)
Sedimentb 1st class 0.20, (mg/kg, 2nd class 0.50 dry) Biotac 1st class 0.5, (mg/kg, 2nd class 1.0 wet)
S9 S11 S15 S24 S25 R1 S15 R1 The wild eels caught in the study area during seawater sampling The oyster on the rock
0.15 0.10 0.25 2.0 1.6 2.5 0.20 8.0
0.20 km to S9 Near S24, 2.0 km to S9 Near R1, 3.0 km to S9
Sampling time November June 2007 2008
December June 2008 2009
42 71 32 31 32 28 0.14 0.090
98 81 7.5 8.0 5.5 6.0 0.17 0.21
42 43 7.0 16 3.5 12 0.075 0.099
15 23 2.5 4.0 3.5 5.5 0.13 0.052
81 42 8.0 7.5 5.5 7.5 0.082 0.081
37 84 10 10 7.5 15 0.021 0.031
0.070
0.14
0.065
0.014
0.052
0.032
2.1 0.045
0.42 0.059
0.10 0.040
0.038
0.025
0.029
a
Seawater samples at each sampling point were taken at both full tide and low tide, and the average concentrations are presented
b
One sediment sample was taken at each sampling point for each sampling time
c
December July 2009 2010
Biotic samples were collected in the study area. Two eel/fish samples collected in each sampling time were analyzed, and at least eight oysters were mixed and analyzed as one sample
1328 Fig. 2 The TM concentration distribution in the surface water of site S
Environ Sci Pollut Res (2011) 18:1324–1332 200 ng L-1
24.445 N SY Power Plant
160 ng L-1
Drainage Outlets S1
S2
S3
S7
S8 S13
24.440 N
S9 S14
S18 S19
S4 S5 S10 S11 S15 S16 S20 S21
S6 S12
120 ng L-1
S17 S22
80 ng L-1
S23
40 ng L-1
24.435 N 0 ng L-1
118.030 E 0m
300 m
600 m
2.7 Data analysis Statistical analysis was conducted using SPSS10.0 (SPSS Inc., Chicago). The TM concentrations in surface seawater among sampling times and sampling points were compared with one-way ANOVA at α=0.05.
3 Results and discussion 3.1 Long-term monitoring results at site S A long-term monitoring program was carried out over 3 years to investigate the effect of waste seawater from the SY Power Plant on the nearby sea. The TM values obtained for seawater, sediment, and biota are summarized in Table 1, together with the Chinese National Criteria (State Environmental Protection Administration of China (SEPAC) 1997, 2002; State Health Administration of China SHAC 2005) for such samples. Most of the results for seawater, sediment, and biota in the area studied were within the second class of Fig. 3 The TGM concentration distribution above the sea surface of site S
118.035 E 900 m
118.040 E
118.045 E
1200 m
the Chinese National Criteria, indicating a low pollution level. The concentration ranges of TM in the surface seawater, sediment, and biota were 2.5–98 ng/L, 0.021–0.21 mg/kg (dry wt), and 0.012–0.14 mg/kg (wet wt). Compared with the monitoring data obtained from other coastal areas such as the Gulf of Mexico, where the concentration ranges of TM in seawater and sediment are 0.05). This preliminary work indicates that the SFGD waste seawater did not seem to have much effect on the environmental quality of the waste seawater receiving sea area. The present study further showed that only a limited area (about
1329
0.1 km2) had TM level higher than 50 ng/L (Figs. 1 and 2). The fate of Hg in the waste seawater thus became very interesting and in need of further consideration, but with the study area focused on limited sites near the pollution
Fig. 4 The profiles of Hg distribution at sampling points S3 (a), S5 (b), and R1 (c). Solid lines TM. Dashed lines particulate Hg
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Environ Sci Pollut Res (2011) 18:1324–1332
source. Since Hg is a volatile element, there is a possibility that various Hg species in water will be transferred into the atmosphere if they are reduced to elemental Hg. Therefore, the studies described in the next section were carried out.
waste seawater transferred to the receiving sea area at the discharge outlets was calculated as 0.112 Mm3/h×24 h/ day×200 ng/L=0.54 kg/day. A part of this might be transferred into the air above the sea surface.
3.2 The distribution of Hg in the seawater and air at the first and second sites
3.3 The distribution of Hg in the seawater and air at the third site
Figures 2 and 3 show the concentration distribution of TM in the surface water and of TGM in the atmosphere in the sampling points of S series at the first and second sites. The TM concentrations near the waste seawater discharge outlets again had the highest level, ranging from 79 to 198 ng/L, and the TGM above the sea surface at these outlets also had the highest values, ranging from 3.83 to 8.60 ng/m3. A close correlation was observed between Hg in the surface water and the air (n=21, R2 =0.885, p
