Environmental Toxicology and Chemistry, Vol. 31, No. 11, pp. 2431–2444, 2012 # 2012 SETAC Printed in the USA DOI: 10.1002/etc.1980
Critical Review ENVIRONMENTAL MERCURY IN CHINA: A REVIEW YAN LIN,*y ROLF VOGT,z and THORJORN LARSSENyz yNorwegian Institute for Water Research, Oslo, Norway zDepartment of Chemistry, University of Oslo, Oslo, Norway
(Submitted 19 April 2012; Returned for Revision 3 June 2012; Accepted 6 July 2012) Abstract— Mercury is a global pollutant that can be transported over long distances and can bioaccumulate. Currently, China is the
country that contributes most to atmospheric Hg emissions and has the greatest intentional (industrial) use of Hg. Mercury in the Chinese environment is generally elevated, particularly in air and water bodies. Remote areas in China also show elevated Hg levels in air and water bodies compared to other rural regions in the world. Large river estuaries are often heavily affected by upstream industrial sources. Mercury is also elevated in sediments, a direct result of contamination in river systems. Regardless of the few heavily polluted sites, the urban environment in Chinese cities is comparable to that of other megacities in terms of Hg pollution, considering the size and rapid development of Chinese cities. Studies on Hg in fish showed generally low levels of contamination resulting from low bioaccumulation of Hg in the mostly short food chains. Mercury in rice has recently received increased research interest; elevated concentrations have been reported from rice grown in contaminated areas and may pose a threat to people dependent on such locally grown food. For the general population, Hg exposure from rice is, however, small. In addition, Hg hair concentration in the Chinese population showed generally low levels of exposure to Hg, except for people with special occupational exposure. Environ. Toxicol. Chem. 2012;31:2431– 2444. # 2012 SETAC Keywords—Mercury
Chinese environment
Food sources
Exposure
emissions to the air to be 825 tons in 2005 [6], and Wu et al. [8] estimated the emissions to be 696 tons in 2003. Both studies found fossil fuel combustion to be the largest single source. There are, however, also important differences in source distribution between the two studies, illustrating the large uncertainties in the input data for the emission estimates. Wu et al. [8] report the nonferrous metals-smelting sector to be the second-largest contributor to Hg emissions, whereas the UNEP [6] reports artisanal gold production as the second-largest contributor. In the next few decades, anthropogenic Hg emissions in Asia are likely to increase significantly, especially in China [9]. China’s coal consumption is projected to rise from 2.6 billion tons in 2007 to 3.8 billion tons in 2015 [10]. Zinc production in China, the largest contributor to Hg emissions in the nonferrous metals-production sector, is projected to rise from 1 million tons in 2005 to 3.6 million tons in 2030 [11]; however, increasing production will not necessarily yield a corresponding increase in the release of Hg to the environment because larger and more modern plants typically have considerably lower Hg emissions per unit Zn produced [12]. The industrial (or intentional) use of Hg in China in 2005 was estimated to be 1,425 to 1,845 tons [11]. The most important sector is the use of Hg-containing catalysts in the production of vinyl chloride monomer, which consumed approximately 800 tons in 2005, a number that is projected to increase [13]. The other major Hg-consuming sectors are batteries, medical equipment, and fluorescent lamps [11]. The extent to which the Hg used in these industries is recycled versus released into the environment is uncertain. Mercury is a widespread pollutant and a threat to human health [2]. Mercury policy also has once again become a hot political topic; in February, 2009, the Governing Council of the UNEP agreed on the need to develop a global, legally binding agreement on Hg [14]. China, the largest contributor to Hg emission according to various studies [6,7,15], is therefore key
INTRODUCTION
Mercury is one of the most toxic heavy metals, especially for pregnant women and fetuses [1]. Mercury exposure can cause health problems not only at high, acute exposures, as in Minamata disease [2], but also at chronic low doses [3]. Its chemical properties and existence as a gas under standard pressure and room temperature allow Hg to be transported in the atmosphere over long distances. For these reasons, Hg has received considerable attention as a global pollutant. A series of complex chemical transformations allows the three oxidation states of Hg (0, I, and II) to cycle in the environment [4]. The majority of Hg encountered in most environmental media (i.e., water, soil, sediments, and biota, but not air) is in the form HgCl2, Hg(OH)2, HgS, or the organomercuric compound methylmercury (CH3Hgþ, MeHg), with other organomercurics (i.e., dimethylmercury and phenylmercury) existing in small fractions. In the atmosphere, the predominant form is elemental Hg (Hg0) [5]. The annual global anthropogenic Hg emission to the atmosphere was estimated to be 1,930 tons by the United Nations Environment Programme’s (UNEP) emission inventory [6] and 2,320 tons by Pirrone et al. [7]. The uncertainties in the estimates are great: UNEP reported a low-end estimate of 1,220 tons year1 and a high-end estimate of 2,900 tons year1 [6]. A major additional uncertainty in the global mass balance of Hg is the contribution from natural sources and the contribution from the re-emission of previously deposited Hg from oceans, lakes, and soils. China is currently the country with the greatest anthropogenic Hg emissions [6]. The UNEP estimated China’s Hg All Supplemental Data may be found in the online version of this article. * To whom correspondence may be addressed (
[email protected]). Published online xx Month 2012 in Wiley Online Library (wileyonlinelibrary.com). 2431
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to global Hg pollution control. Thus, an overview of Hg pollution in China is urgently needed. Although some efforts have been made to summarize the status of Hg pollution in China with various areas of emphasis [16–18], there has been too much focus on contaminated sites. No general comprehensive overview of the Hg levels in different environmental compartments has been made. The present study sought to separate the normal and contaminated environments and summarize the environmental status of Hg in China to improve understanding at the regional, national, and global levels. By separating the normal and heavily contaminated sites, the present study gives a more balanced summary of the environmental Hg status in China. MATERIALS AND METHODS
The present study provides an overview of the most recent studies on Hg in the Chinese environment. The literature cited here was extracted from the ISI Web of Knowledge citation index and search service (www.isiknowledge.com) using the following keywords: mercury (or Hg) and China. Thus, only publications in English were included in the present study. Because of the fast pace of development in China and the quality of the data, we considered only selected publications from 1999 until January, 2012, for inclusion in the present study. With these criteria, we reviewed 213 publications in total. Important water bodies and study sites are shown in Figure 1. During the review process, we found that heavily contaminated sites usually attracted more research and therefore generated more publications. For example, the Wanshan Hg mining area in Guizhou Province, which was the largest Hg mine in China, was examined in 21 publications from 1999 to 2012. Another site that was heavily contaminated by Zn smelting in Huludao City, Liaoning Province, was studied in five publica-
tions from 1999 to 2012. One-third (74 of 213) of the total number of articles that we reviewed focused on heavily contaminated sites (nonferrous metal smelting, Hg and gold mining, waste recycling, chemical plants, and power plants). This bias would hinder relatively comprehensive and balanced coverage of the overall status of Hg contamination in China. We therefore discuss the heavily contaminated sites in a separate section. The data presented here were used with the original significant digits; data collected in tables therefore vary in the number of significant digits. Many analysis methods for Hg and MeHg exist [19], and different studies use different qualitycontrol protocols. Among the reviewed articles, the method used most often was cold vapor atomic fluorescence spectroscopy (CVAFS); a few used atomic absorption spectroscopy (AAS). Generally, the CVAFS method has a lower detection limit and is therefore more widely used [20]. We reviewed the Hg distribution and occurrence in various environmental components in China: air, soil, water, sediment, and biota (Supplemental Data, Tables S1–5). The heavily contaminated sites are discussed in a separate section to avoid misunderstanding of the normal conditions in China. Mercury concentrations in human beings and exposure assessment are discussed based on Hg concentrations in human hair and blood samples as well. MERCURY IN THE CHINESE ENVIRONMENT
Air
Many major sources of Hg emission are located in or near urban areas and thus may lead to elevated Hg levels in the urban environment [21]. Urban districts, as densely populated areas, attract the most attention in studies of Hg contamination in
Fig. 1. Map of important water bodies and study sites in China.
Environmental Hg in China: A review
China. As rapid urbanization occurs in more areas of China, the number of people moving to cities is projected to increase [22]. This causes elevated Hg concentrations in both air and soil (due to deposition) in urban areas. The largest source of atmospheric Hg is fossil fuel combustion, especially coal [8]. Atmospheric Hg consists mainly of gaseous elemental Hg (GEM or Hg0), reactive gaseous divalent Hg (RGM), and particulate-bound divalent Hg (HgP) [5]. The major chemical form of Hg emitted to the atmosphere is Hg0, which constitutes approximately 53% of the total emissions, followed by RGM (37%) and HgP (10%) [23]. Unlike other heavy metals, which tend to exist in the atmosphere in the particulate phase, Hg exists mainly (>95%) in the gaseous phase. Reactive gaseous mercury and HgP are more reactive than Hg0 and are readily scavenged from the air via wet and dry deposition. Gaseous elemental Hg, which constitutes 90% of total gaseous Hg (TGM), is very stable in the atmosphere, allowing for a residence time between six months and two years [24]. This stability allows the long-range transport of Hg. Studies covered cities of different scales. Figure 2 shows the seasonal variation in gaseous Hg concentrations in Chinese urban and remote environments. Different studies used different parameters to report the atmospheric Hg concentration, but the most often used parameters are TGM and GEM. In the present study, both the TGM and GEM concentrations are included in the comparison, because the majority of TGM is GEM [24]. Generally, cities such as Beijing, Guiyang, and Changchun, in which coal is a major source of energy and heating, showed higher atmospheric Hg concentrations. Areas such as the Pearl River Delta and Yangtze River Delta (e.g., Shanghai) usually have shorter heating seasons and easier access to cleaner hydropower [25]; these factors resulted in lower atmospheric Hg concentrations in these areas. A study comparing atmospheric Hg concentration in Chinese cities with South Korean and Japanese cities showed that the Chinese cities usually have higher atmospheric Hg concentrations [26]. The regional background atmospheric Hg concentrations in China were measured at the China Global Atmosphere Watch Baseline Observatory at Mt. Waliguan in the Qinghai–Tibet Plateau; this station is an official World Meteorological Organization Global Atmospheric Watch Station. The regional background concentrations are estimated to be 1.7 1.1 ng m3 in
Fig. 2. Seasonal gaseous mercury concentrations in Chinese air. Bars show mean values with standard error marked if available (values from Supplemental Data, Table S1).
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summer and 0.6 0.08 ng m3 in winter [27]. These values are close to the background value in the Northern Hemisphere (1.5–1.7 ng m3) [28]. Studies have also documented the atmospheric Hg concentration in several remote areas, such as the natural reserve mountain areas and offshore areas. Three remote mountainous areas were studied, Mt. Gongga (Sichuan, southwest China) [29], Mt. Changbai (Jilin, northeast China) [30], and Mt. Leigong (Guizhou, southwest China [31]). The results showed similar levels of atmospheric Hg (1.9–5.7 ng m3) among the three areas [29–31]. The levels were lower than the findings in urban areas but were significantly elevated compared with other remote areas in Europe and North America (usually lower than 2.0 ng m3) [32–34]. The values were also higher than the background levels in China (0.6–1.7 ng m3) [27] and the Northern Hemisphere (1.5–1.7 ng m3) [28], indicating the extensive impact of large Hg emissions in China and the long-range transport of Hg. Soil
Mercury in soil usually comes from the deposition of atmospheric Hg, which causes an accumulation of Hg in the soil [5]; occasionally, the Hg also comes from irrigation water, which contains high levels of Hg in specific areas [35]. Methylmercury in soil usually accounts for less than 1% of total Hg (THg) [36], although the MeHg fraction in boreal forest peat may exceed 10% THg in some cases [37]. The background Hg value of soil in China is reported to be in the range of 0.02 to 0.2 mg kg1 [38]. The Chinese National Standard for Soil Environmental Quality [39] is divided into three classes: the Hg concentration in class I soils should be under 0.15 mg kg1, which is the natural background value; the Hg concentration in class II soils, which is the upper acceptable limit for agricultural soils, should be under 1 mg kg1; and the Hg concentration in class III soils should be under 1.5 mg kg1, which is the limit for the normal growth of plants. Figure 3 summarizes all studies of soil Hg concentration in China from the literature. We included only THg concentrations from topsoil in the figure because topsoil concentrations reflect more recent Hg contamination. There are many studies on Hg levels in soil in China with relatively complete coverage, probably as a result of the ease of sampling and analysis. In total, we reviewed articles on soil Hg concentrations at 48 locations, including four remote sites, 21 urban sites, and 23 rural sites. The average concentration was 0.24 mg kg1, and the median concentration was 0.17 mg kg1. Among the data points included in the present study, 46% are below 0.15 mg kg1 (class I soil environmental quality) [39], and 54% are below 1.0 mg kg1 (class II soil environmental quality) [39]. The elevated topsoil THg concentrations in many urban areas and agricultural areas were a direct impact from rapidly growing industries in recent decades as China has undergone massive industrialization [40]. However, these concentrations were still lower than the Chinese environmental soil standard class II limit for agricultural soils (Fig. 3) [39]. Cities with long histories, such as Beijing, show especially high Hg levels in topsoil, the legacy of a long history of human activities [41]. Generally, THg concentrations in soils in Chinese cities are comparable to those reported from North American and European cities, including Pittsburgh, Pennsylvania, USA (0.5 mg kg1) [42], Wexford, Ireland (0.1–3 mg kg1) [43], and Palermo, Italy (0.04–7.0 mg kg1) [44]. Atmospheric Hg speciation is also known to be affected by urban atmospheric chemistry and is associated with the cycles of some oxidants and particulates [45].
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Fig. 3. Mercury concentrations in Chinese soils: (A) all sites, (B) urban areas, and (C) rural areas (values from Supplemental Data, Table S2).
Few data, however, are available for Hg concentrations in soil at background sites in China. Data from Mt. Xiaoxing’an, a major natural forest reserve in northeastern China, show THg concentrations in the surface of peat bogs from 0.126 to 0.275 mg kg1 [46]. In the Fanjing Mountains, a natural preservation area near the Hg mining areas in Guizhou Province, similar concentrations have been reported (0.10–0.22 mg kg1) [47]. These concentrations are comparable to values reported for similar areas in Europe and North America: Langtjern, Norway (0.037–0.172 mg kg1) [48]; Florida Everglades National Park, USA (0.02–0.14 mg kg1) [49]; and northeastern Bavaria, Germany (0.120–0.30 mg kg1) [50]. Both urban and rural areas showed evidence of contamination. The level of atmospheric Hg in most Chinese cities is also elevated compared to that in large cities in Japan and South Korea [26]. Elevated Hg concentrations are due mainly to the large consumption of coal as an energy source and intense industrialization. Because the Hg in topsoil comes mainly from atmospheric deposition, the elevated atmospheric Hg concentrations in China will ultimately cause elevated Hg concentrations in topsoil. Generally, half of the reported Hg concentrations in urban and agricultural soils exceeded the background level of Hg in China (0.15 mg kg1) [38]. Most of the urban and agricultural topsoils are classified as class I or II by the Chinese environmental standard for soils [39]. Topsoil in
large cities with a long history usually showed significantly elevated Hg concentrations. Remote areas also showed elevated levels of Hg in both air and soil resulting from the long-range transport of Hg. Freshwater environments
Mercury in water exists mainly as divalent Hg(II) (Hg2þ) and a small but important fraction of MeHg. Divalent Hg has a strong affinity for sulfur-containing functional groups in organic matter and on minerals, which greatly limits the mobility of Hg [5]. Methylmercury typically accounts for less than 10% of the THg [5,51]; however, MeHg is of greater concern because it is readily assimilated and bioconcentrated by aquatic organisms and accumulated in fish, which are consumed by humans and may therefore pose health concerns [52]. The upper limit value for THg in drinking water set by the World Health Organization (WHO) [53] is 1,000 ngL1. In China, water quality is divided into five classes according to the water quality standard [54]. The class I and class II quality water bodies should not have a THg concentration greater than 50 ng L1. The limits for the class III and classes IV and V are 100 and 1,000 ng L1, respectively. Mercury in sediment usually reflects the direct impact from water bodies above the sediment, especially contaminated water at contaminated sites, such as chemical plants, mining areas, and smelters. The sediment core
Environmental Hg in China: A review
in remote clean areas is usually used as a good log of historic atmospheric Hg deposition. General background levels of THg were reported by Fuji [55] to be 50 mg kg1 in river sediments, 100 to 300 mg kg1 in lake sediments, and 50 to 80 mg kg1 in sea sediments. Methylmercury concentrations typically were found to be lower than 10 mg kg1 for estuarine sediments [56]. Craig [57] reported THg concentration ranges of 200 to 400 mg kg1 for uncontaminated river sediments, whereas sediments in urban, industrial, or mineralized areas can contain up to 100 mg kg1 THg. Especially high THg levels are found in sediments collected from recipients of large Hg pollution sources. Figure 4 shows the Hg concentrations in water of both freshwater and marine environments. Figure 5 shows the Hg concentrations in sediment of both freshwater and marine environments. Studies on Hg in Chinese waters have not been very systematic in general. China has three major river systems: the Yangtze River, the Yellow River, and the Pearl River. TheYangtze River Delta and the Pearl River Delta are also the most economically well-developed regions in China. However, none of the three rivers was studied systematically for the whole river basin with regard to Hg levels in water. Most studies have focused on rivers that had significant pollution occurrence, such as the Wuli River and the Songhua River. Rivers that originate from the Tibetan Plateau, such as the Yarlung Tsangpo River, the Salween River, the Mekong River, and the Yangtze River (upstream), were also studied because they provide a significant environmental baseline [58]. Reservoirs are also important water bodies, but most studies are now conducted in the reservoirs within Guizhou Province. Studies of first-order forest streams draining catchments with no direct industrial impact (Mt. Leigong, Tieshanping and Luchongguan) [59] inherently contain lower Hg concentrations than the large Chinese rivers receiving multiple anthropogenic discharge sources. The Hg concentrations in these small streams are comparable to the concentrations found in studies performed in Europe (0.84–38 ng L1) [48,50,60,61]. The annual total atmospheric input fluxes of Hg were 119.5 mg m2 a1 for Mt. Leigong and 291.2 mg m2 a1 for Tieshanping [59]. This
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Hg loading at subtropical, forested catchments in southwest China is much higher than that found for boreal forest catchments in Europe and North America (9.4–54 mg m2 a1) [48,50,62,63]. The high flux corresponds to the elevated atmospheric concentrations at remote sites discussed in the section titled Mercury in Chinese environment – air. Low Hg concentrations in runoff despite high loading indicate a strong retention of atmospheric Hg deposition in these catchments in southwest China. The reported THg in most large Chinese rivers varies greatly; the highest concentrations are usually an order of magnitude higher than those for large rivers elsewhere in the world, for example, the Amazon (4.2–50 ng L1) [64]. However, studies on rivers in the Tibetan Plateau by Huang et al. [58] and Zheng et al. [65] have yielded relatively low Hg concentrations (
