Critical Review pubs.acs.org/est
Mercury in Municipal Solid Waste in China and Its Control: A Review Hefa Cheng* and Yuanan Hu State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China S Supporting Information *
ABSTRACT: Although a potentially significant and preventable source of environmental pollution, mercury in municipal solid waste (MSW) has not received adequate attention in China. Discarded mercury-containing products, if not recycled, ultimately release mercury to air, soil, and groundwater, even after being properly collected and disposed of in MSW management facilities. This review presents an overview on mercury in MSW and describes the emissions associated with landfilling, incineration, and composting in China. Besides endof-pipe technologies for controlling mercury emissions from MSW management, strategies for controlling mercury in MSW are also discussed, focusing on mercury source reduction and recycling. Batteries and fluorescent lamps contribute to approximately three-quarters of mercury in MSW, and are expected to remain as significant sources of mercury in the near future. Reducing or eliminating the mercury contents in household products, particularly batteries and fluorescent lamps, should be the top priority in controlling mercury in MSW, while it is also important to set mercury contents in consumer products at acceptable and achievable levels based on a life-cycle approach. Meanwhile, cost-effective recycling programs should be developed targeting products containing elemental mercury, such as medical thermometers and sphygmomanometers, and waste products with high mercury contents (e.g., button cells) as well. Elemental mercury, which accounts for ∼95% of atmospheric mercury in general, has a long atmospheric residence time of 0.5−2 years before undergoing wet deposition (after being photo-oxidized to water-soluble Hg2+ species) and dry deposition.2,3,6 As the distribution of elemental mercury is governed by local, regional, and global atmospheric circulation,3,4,6 excessive mercury emissions in any given region can influence the global mercury deposition.3,5,7−10 Mercury emissions from Asia account for over half of the global anthropogenic mercury releases,5,9,11 and a significant increase in anthropogenic mercury emissions is expected with the rapid economic and industrial development in Asia if no drastic control measures are taken.10 Despite the fact that mercury pollution is a problem with global reach, the technical capability, environmental awareness, and socioeconomic structures vary significantly among different countries, with most developing countries paying much less attention to the effective control of mercury emissions compared to the developed ones.10 Because elemental mercury can be longlived in the atmosphere and transported on intercontinental and hemispheric scales,4,8 a global approach is necessary to reduce mercury pollution in the long term. A legally binding
1. INTRODUCTION Mercury, in both inorganic and organic forms, is a significant public health and environmental concern because of its toxic, persistent, and bioaccumulative properties.1 It occurs in three valence states in nature: elemental (Hg0, boiling point: 356.7 °C), monovalent (Hg22+), and divalent (Hg2+), with divalent mercury being more stable and common than monovalent mercury in the environment. In particular, divalent mercury can be associated with organic molecules forming compounds such as methylmercury (CH3Hg+), the most toxic and bioaccumulative form of mercury.1 Mercury is introduced into the environment both from natural sources, including volcanoes, weathering of rocks, forest fires, and soils, and from human activities, with natural and anthropogenic sources contributing approximately equally to the total atmospheric budget of vapor-phase mercury.2−5 The total global mercury emissions into the atmosphere from anthropogenic sources, primarily coal and oil combustion, artisanal gold mining, and production of nonferrous metals and cement, are approximately 2000−2300 megagrams (Mg) annually (Figure S1, Supporting Information). The transport of different mercury species emitted into the atmosphere varies significantly. Oxidized mercury undergoes wet and dry deposition regional to the point of emission with a short atmospheric lifetime (∼1 week), while the particulate mercury bound to particles is locally and regionally distributed, with a somewhat longer atmospheric lifetime of 1−2 weeks.3,6,7 © 2011 American Chemical Society
Received: Revised: Accepted: Published: 593
July 31, 2011 December 1, 2011 December 2, 2011 December 2, 2011 dx.doi.org/10.1021/es2026517 | Environ. Sci. Technol. 2012, 46, 593−605
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Figure 1. Mercury uses in China and mercury contents in MSW. (a) Mercury consumption between 1995 and 2009, and the estimated mercury contents in MSW from discarded batteries, fluorescent lamps, medical thermometers, and sphygmomanometers. Artisanal gold mining process using gold−mercury amalgamation has been banned in China since 1996, but was still practiced by some private, illegal mines in remote areas.13−15 Only mercury use data from the state-owned gold mines are included here, while annual mercury use by small scale gold mines ranges from >500 to 264 Mg between 1995 and 2004.31 Mercury use in chemical reagents sharply declined after 2000 because of production bans and the lack of data on small-scale production. The mercury content in MSW was calculated by assuming 30% of the batteries and fluorescent lamps manufactured were consumed domestically, while 1/3 of the medical thermometers and sphygmomanometers were used domestically with 25% of them being disposed of in MSW. (b) Measured mercury contents in MSW from Chinese cities between 1991 and 2005 (data from 32−43). Geometric mean values are shown when averages are not available.
pogenic mercury emissions, with the rest contributed by cement production, mercury mining, household waste burning, and other less significant sources (Figure S2, Supporting Information). Monitoring data available indicate that atmospheric mercury pollution is widespread, while mercury pollution has also occurred in other environmental media in China (Supporting Information). Industrial sources, such as coal-fired power plants, cement plants, and MSW incinerators, which are typically located just beyond the fringe of large urban areas in China, vehicular emissions, and domestic stoves are responsible for the elevated mercury levels in urban air, while the atmosphere in remote areas is mainly impacted by mercury transported from the industrial sources.17 Overall, the major
global agreement to reduce the risks to human health and the environment from mercury, similar to the Stockholm Convention on Persistent Organic Pollutants, is being negotiated and is expected to be implemented in the near future.12 Unarguably, successful reduction in mercury releases from China, which is the world’s largest mercury consumer and contributes approximately a quarter of the global anthropogenic mercury emissions,8,13−16 will be a cornerstone of the global efforts on reducing mercury pollution.
2. MERCURY IN MUNICIPAL SOLID WASTE (MSW) Nonferrous metal smelting and coal combustion are the major mercury sources in China, accounting for >80% of anthro594
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Figure 2. Fate of mercury in typical MSW treatment processes: (a) transformation and cycling of mercury within landfills (anaerobic) and composts (typically aerobic); (b) speciation of mercury at different stages of MSW incineration. More than 80% of mercury in the MSW is released into the gas phase during incineration,47 and most of the vaporized elemental mercury reacts with HCl (released from chlorinated plastics and chloride salts in MSW) in the flue gas to form HgCl2.6 As the flue gas cools, some Hg0 and Hg2+ condense and adsorb onto the surface of fly ash particles, forming particulate mercury (Hgp). APCDs can capture some mercury from the flue gas, although their removal efficiencies vary significantly for different mercury species due to differences in their physical/chemical properties.6,48 Particulate control devices such as fabric bag filters can efficiently remove Hgp; Hg2+ is mostly removed by adsorption or chemical reaction, while Hg0, which is very volatile and not very reactive, is substantially more difficult to remove from the flue gas.
highly recycled by the informal recycling sector comprised of street pickers, dump pickers, and itinerant buyers.21,22 A variety of mercury-containing products, such as batteries, fluorescent lamps, and thermometers, are discarded as household waste, thus the collection, transport, and disposal of MSW can contribute significantly to mercury emissions. Globally, it is estimated that waste disposal accounts for approximately 8% of the total anthropogenic mercury emissions.23 Due to the high mercury contents in MSW and inefficient flue gas cleaning, MSW incineration used to be a major source of atmospheric mercury in developed countries relying heavily on incineration for MSW disposal.24,25 In China, MSW incineration was the
mercury exposure pathways are consumption of contaminated fish for residents living in coastal areas and inhalation in cities with severe atmospheric mercury pollution in China.13,14,18 For inhabitants of the mercury mining areas in inland China, intake of rice, rather than fish, has been shown as the major pathway for methylmercury exposure.19,20 Municipalities collect, transport, and dispose of discarded materials in urban areas that are generated by households and businesses, although small amounts of industrial waste and construction waste may also end up in the MSW in China.21 Only a few cities in China have introduced waste sorting, while waste items with resale values, such as metals and paper, are 595
dx.doi.org/10.1021/es2026517 | Environ. Sci. Technol. 2012, 46, 593−605
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in the MSW collected from the 661 middle-size and large cities in 2009 amounted to 78.5 Mg, more than 10% of the annual anthropogenic mercury emissions in China. Control of mercury emissions from MSW management can be more difficult compared to industrial point sources as the mercury-containing wastes are generated in individual households and are dispersed in large volumes of household and commercial waste items. Nonetheless, the mercury emissions from MSW management are not only potentially significant, but also largely preventable.
leading sector in mercury emission growth, increasing from 0.6 to 10.4 Mg·year−1 over the 1995−2003 period at an annual growth rate over 40%.15 With rapid economic growth and unprecedented urbanization (Figure S3, Supporting Information), MSW disposal has become a major challenge for many Chinese cities.21,22,26−28 A total of 157 teragrams (Tg) of MSW was collected from 661 middle-size and large cities in 2009.29 Landfilling, incineration, and composting accounted for approximately 80.2%, 18.2%, and 1.6% of the waste disposal capacity, respectively; together they disposed of only 70.6% of the collected MSW in 2009 (Table S1, Supporting Information). Due to limited space available for construction of new landfills, incineration is playing an increasingly important role in MSW management in China.21,22,26−28 By 2015, incineration is expected to account for over half of the MSW disposal capacity in the eastern provinces, economically more developed regions, and the densely populated regions, as well as over a quarter of the MSW disposal capacity in central and western China.22,30 Figure 1a shows the significant changes in mercury usage in China between 1995 and 2009. Production of caustic soda, polyvinyl chloride, and chemical reagents, and artisanal gold mining consumed large amounts of mercury during this period, depending on market demand and production bans. The amount of mercury used by the battery industry fell from 582.4 Mg in 1995 to 140 Mg in 2009, largely due to the commendable efforts by the manufacturers to eliminate or reduce mercury from retail batteries. Mercury use by the lighting industry did not follow the dramatic decrease observed for the battery industry. Instead, it increased slowly from 30.9 to 55 Mg over the same period. Medical devices, primarily medical thermometers and sphygmomanometers, also consumed large amounts of mercury, while their mercury use had decreased from the peak of approximately 270 Mg since 2004 due to global mercury reduction efforts. With very little collection and recycling, almost all the spent batteries and fluorescent lamps end up in the MSW stream, while significant fractions of medical thermometers and sphygmomanometers are also used in households and are discarded into MSW in China. The estimated concentrations of mercury in the MSW resulting from disposal of batteries, fluorescent lamps, medical thermometers, and sphygmomanometers are also shown in Figure 1a. It is estimated that mercury content in MSW contributed by these domestically used mercury-containing products decreased from 1.8 mg·kg−1 in 1995 to 0.5 mg·kg−1 in 2009, largely due to the significant reduction in mercury contents in batteries. Batteries comprised approximately 93% of the mercury in MSW in 1995, followed by fluorescent lamps (5%). By 2009, batteries still predominated as the major mercury source, contributing approximately 54% of the total mercury in MSW, with fluorescent lamps accounting for 21%, a considerably larger percentage than in 1995. Figure 1b shows the mercury concentrations measured in MSW from various cities in China during the period 1991−2005. Despite the significant variability, mercury contents were mostly lower than 3 mg·kg−1, which is comparable to those in the MSW from developed countries.44−46 Some MSW samples exhibited very high mercury levels, probably due to contamination by waste products that contained elemental mercury.39 Although the mercury contents of MSW are generally low in China, the cumulative amount contained in the large volume of MSW generated each year can be significant. Assuming an average mercury content of 0.5 mg·kg−1, the mercury contained
3. MERCURY EMISSIONS FROM MSW MANAGEMENT 3.1. Landfilling. After being buried in landfills, the mercury contained in a variety of discarded items can still impact the surrounding soil, groundwater, and even the air through leachate and gas emissions. Mercury transformation in landfills is mediated primarily by bacteria, but is also influenced by physical conditions, such as temperature, pH, and the presence of oxygen. As illustrated in Figure 2a, most of the mercury either precipitates as highly insoluble cinnabar (HgS) or binds to the abundant organic matter, while the rest can remain as elemental mercury in situ, vaporize into the gas phase, or leach out into groundwater.45,49 Because landfills reduce waste volume by generating methane with anaerobic bacteria, volatile methylated mercury compounds also form during MSW decomposition.50,51 Many studies have demonstrated that landfills could serve as a potential atmospheric mercury source (e.g., 40,52−54). Table 1 shows the concentrations of mercury species detected in leachate, landfill gas, and cover soils from various MSW landfills in China. Elevated levels of mercury were found in these media, clearly indicating the releases and emissions of mercury from mercury-containing waste products buried in landfills. Mercury concentration in leachate depends on many factors, such as the mercury content in the MSW, solution pH, presence of sulfide, and retention by organic and inorganic materials in MSW. Such leachate can cause groundwater contamination from unlined landfills and those with broken liners. Compared to the mercury in leachate, the mercury in landfill gas has received little attention in China until recently. Feng and co-workers measured the mercury emissions from the surface cover of several MSW landfills in China;40,59−62,81 they observed that mercury emissions from the landfills were 1−2 orders of magnitude greater than those from the background zones and highly correlated with the mercury contents in the upper substrate.62,81 Maximum mercury emissions occurred at the working face, which decreased significantly after being covered by soil or vegetation, while the vent pipes appeared to be the least significant mercury emission sources.40 Overall, methylated mercury species accounted for 250 mg·kg−1 mercury were banned. Alkaline batteries with >1 mg·kg−1 mercury were phased out. Maximum allowable mercury content limits were set at 1 mg·kg−1 for zinc−carbon batteries, and 20 mg·g−1 for zinc−air, alkaline, and silver oxide button cells. Zinc−air, alkaline, and silver oxide button cells containing ≤5 mg·kg−1 were classified as “mercuryfree” batteries, while those with 20 mg·g−1 mercury were classified as “mercury-containing” batteries. Zinc−mercury batteries and alkaline batteries containing >1 mg·kg−1 mercury were banned. 2001
1995
1999
Batteries with >1 mg·kg mercury were not eligible for the China Environmental Label.
mercury content requirement
−1
year of enactment
Table 2. Past and Current Regulations Restricting Mercury Use in Batteries in China
regulation/directive
based on the emission characteristics of the respective source. Leachate from modern sanitary landfills is often captured and treated, and the mercury in the leachate, which is primarily present in particulate form, can be easily removed by wastewater treatment systems.56,63 With only 35 landfill gas power plants in operation in 2010, landfill gas is not captured or simply flared in the vast majority of landfills (over 440) in China.90 Therefore, it is important to enhance the capture and treatment of landfill leachate, while improving landfill gas collection and installing mercury capture devices in landfill gas utilization facilities to minimize mercury emissions from landfills. Mercury pollution in composting can be reduced by requiring separate collection of compostable materials, and by employing centralized separation technologies to remove heavy metal sources, such as batteries and consumer electronics, at the composting facilities. MSW sorting has been shown to be effective at reducing the contamination of compost in developed countries.84 Although incinerators are required to control the emissions of mercury and other air pollutants, the current standard (0.2 mg·m−3) on mercury emissions from MSW incineration in China is much looser than those in Europe and the United States (0.05−0.08 mg·m−3).22,91 A more stringent mercury emission standard (0.05 mg·m−3) for newly constructed MSW incinerators in China has been proposed recently.89 Meanwhile, an effective administrative mechanism on MSW incineration and a strong monitoring capacity should also be developed for verification of the actual mercury emissions from MSW incineration facilities and their compliance with the relevant regulations.92 Although end-of-pipe technologies can effectively control mercury emissions from MSW management, minimization and elimination of mercury in the solid waste is far more desirable compared to the pollution control strategies.92 This can be achieved by preventing mercury-containing items from entering MSW and recycling them before sending the waste for disposal, and more preferably, by eliminating nonessential use of mercury in consumer products. 4.1. Batteries. The single most significant source of mercury in MSW in China is consumer batteries. Significant regulatory efforts have been made to reduce mercury contents in all types of batteries since the late 1990s, as summarized in Table 2. The then State Environmental Protection Administration issued a technical guideline on disposal and recycling of waste batteries, reiterating the limits on mercury contents in batteries in 2003.93 Besides restricting mercury contents in batteries, some regulations also outlined the obligations of manufacturers and importers on collecting and recycling highmercury-content batteries, and promoted mercury-free and low-mercury batteries.93,94 As a result of the joint efforts of government agencies and manufacturers, mercury contents in batteries and mercury consumption by the battery industry in China have been drastically reduced (Figure 1a). A new directive currently under review further aims to reduce the mercury consumption of battery industry by 80% (from 140 Mg in 2009 to 26 Mg in 2015) through implementing nonmercury production technologies.95 Today, batteries except button cells produced by the major manufacturers in China contain very low levels of mercury, although batteries made by some small producers may far exceed the mercury content limits (in violation of the relevant regulations). Despite the sharp decline in the use of mercury in consumer batteries, waste batteries remain a major source of mercury in MSW. The overall recycling rate of batteries in China is less
The certif iable technical requirement for environmental labeling products: Mercury-f ree dry cells and batteries (HJBZ 9-1995); State Environmental Protection Administration: Beijing, China, 1995. Catalogue of outdated production capacity, technologies and products to be phased out (batch 1); State Economic and Trade Commission: Beijing, China, 1999. Regulation on mercury content limitation for batteries; China Light Industry Association: Beijing, China, 1997.
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than 2%.96 Although environmental groups and activists have called for full recycling of batteries in China, the task is daunting and the cost is prohibitive given the large number of batteries discarded each year (approximately 8 billion). Meanwhile, mercury reduction in batteries began in the late 1990s and continues today. Most alkaline batteries are manufactured with “no mercury added”, whereas button cells (except the lithium ion type) still contain relatively high levels of mercury (up to 20 mg·g−1). Although recycling the batteries containing low levels of mercury reduces environmental impact relative to discarding them, such programs are not cost-effective at recovering mercury from the MSW stream.97 Because of the much smaller sizes and higher mercury contents of button cells, the costs associated with their collection and transport are much lower compared to those for other types of consumer batteries, thus they should be the primary target of battery recycling programs. Overall, reduction of mercury input to the MSW stream from batteries can be best achieved by continuing to reduce the mercury contents in all types of batteries through technological innovation, legislation, compliance and enforcement efforts, and by recycling the button cells with high mercury contents. 4.2. Fluorescent Lamps. As an energy-efficient alternative to incandescent light bulbs (ILBs), fluorescent lamps have become one of the main types of light source in China (Figure S4, Supporting Information). Fluorescent lamps contain varying amounts of mercury, depending on their type, size, and manufacturer. Although export-oriented manufacturers follow the strict standards set by the U.S. and E.U., compulsory regulations on mercury contents in fluorescent lamps are lacking in China and reduction of mercury use is largely based on voluntary efforts of the manufacturers. Fluorescent lamps containing
