Environ. Sci. Technol. 2007, 41, 6283-6289
Silver Emissions and their Environmental Impacts: A Multilevel Assessment M A T T H E W J . E C K E L M A N * ,†,‡ A N D T . E . G R A E D E L ‡,§ Program in Environmental Engineering, Center for Industrial Ecology, and School of Forestry and Environmental Studies, Yale University, New Haven, Connecticut 06511
A detailed accounting of environmental releases of silver is presented for the year 1997, based on data from Yale University’s Stocks and Flows (STAF) project and other sources. The analysis is carried out for 64 countries, eight regions, and the world. From the chemical composition and receiving media of these different releases, each emission category is assigned an environmental impact score in accordance with the Indiana Relative Chemical Hazard (IRCH) ranking system. Flows are scaled by impact and land area to form an overall semiquantitative assessment of the environmental impact of silver. Of the 64 countries, the United States has the highest gross emissions for nearly all flows to the environment. On a regional basis, Asia is the largest emitter of silver directly to land and water. In major silver-producing countries, tailings tend to have the highest environmental impact of any emissions category; in nonproducing countries, it is dissipation to land (Hong Kong having the highest impact in this category). Globally, more than 13 Gg of silver are emitted annually to the environment, with that in tailings and landfills making up almost three-fourths of the total. The utility of this method for evaluating the environmental impact of other metals is explored.
1. Introduction The elemental quality of metals underscores their interest to economists, technologists, and environmentalists. Although metals are transformed in industrial systems and incorporated into products in a range of oxidation states, they can be neither created nor destroyed. Given enough energy and ingenuity it is theoretically possible to reprocess metals into pure form, regardless of their current state. Similarly, metals are transformed in environmental systems, where they can change valence state, adsorb to sediment, or be metabolized by organisms, among other fates, but they never cease to exist. For this reason, metals are excellent subjects for material flow analysis (MFA), which seeks to track materials as they flow through a given bounded system. One extensive MFA project, the Stocks and Flows project (STAF) at Yale University’s Center for Industrial Ecology, has conducted multilevel MFA studies for a variety of major metals, including copper, zinc, iron and steel, and silver (1-7). These studies * Corresponding author phone: 203-432-4985;
[email protected]. † Program in Environmental Engineering. ‡ Center for Industrial Ecology. § School of Forestry and Environmental Studies. 10.1021/es062970d CCC: $37.00 Published on Web 07/28/2007
2007 American Chemical Society
e-mail:
examine metals cycling on national, regional, and global scales. Silver is of particular interest because of its value, its relative scarcity, and its toxicity. Annual global demand for silver is currently 24 500 metric tons, and the metal is used in a vast array of industrial and consumer products (8). Approximately 57% of silver in discarded products worldwide is recycledsamong the highest rates for commonly used metalssbut much is lost in various emissions to the environment (4). Monovalent ionic silver (Ag+) is the form of most environmental concern, as it has antimicrobial properties. When ingested by humans, silver is metabolized and deposited in subcutaneous fat. Silver’s greyish-blue color gives rise to the mainly cosmetic disorder of argyria, resulting from excessive ingestion, in which the affected person’s skin is discolored. High concentrations of silver in water bodies in the United States and a growing concern about its potential environmental effects in the 1980s and 1990s led to a body of research on its effects on ecosystem health (9, 10), bioaccumulation (11), and environmental fate and transport (12). Silver in the surface waters and sediments of San Francisco Bay was extensively studied, leading to criticism of the photographic processors that were contributing much of the input (13, 14). It was found that although most of the rules and regulations regarding silver contamination are based upon the ionic form Ag+ (15, 16), ionic concentrations are usually extremely low due to complexation with sulfides and chlorides and adsorption to sediment (17, 18). Emissions of silver are still seen as a concern, however, and several studies have attempted to identify and quantify the sources of anthropogenic silver in the United States. The first comprehensive effort was probably that of GCA Corporation (19) for the U.S. Environmental Protection Agency, followed by a much more detailed assessment by Smith and Carson (20). Scow et al. built on this work to produce the first complete silver MFA study for the United States, using mass balances and life-cycle diagrams (21). After more information on the environmental and toxicological profile of silver became available, Purcell and Peters produced a study linking the masses and chemical forms of silver emitted, together with their environmental effects, albeit in a qualitative fashion (22). They conclude that silver does not pose a serious environmental threat but that it should be recovered because of its value as a precious metal. Similar efforts in Europe have concentrated on a small geographical area or a particular silver-containing product (23, 24). While much is known about how silver acts in the environment and in living organisms, few studies have attempted to link such information with anthropogenic emissions in a quantitative manner. Notable work in this area includes that of Guinee et al. on the flows of cadmium, copper, lead, and zinc in the Netherlands (25). In the present study, we develop a new and simple approach of semiquantitative impact assessment of silver emissions at the scales of nations, regions, and the globe for the year 1997.
2. Methodology 2.1. Silver Losses to the Environment. Johnson et al. (4) constructed a quantitative multilevel technological cycle for silver using available data for production, fabrication, and import/export for the year 1997. Anthropogenic emissions of silver to the environment were described in aggregate for many countries. However, the authors do not consider the quality of these emissions, such as their chemical form, emission route, or the receiving media (land, air, water). The VOL. 41, NO. 17, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Flow diagram for releases of silver to different environmental media. first task of the present study was to characterize the specific losses of silver to the environment from each of the metal life-cycle stages. Most releases occur during silver production and waste management (WM) but there are significant losses during fabrication and manufacture (F&M) and use as well. Figure 1 describes the different releases and their target media. The movement of silver from one medium to another (e.g., deposition of atmospheric silver) was not considered. 2.1.1. Production. The masses of silver mobilized during production were taken from Johnson et al. (4). Seventy five percent of silver is produced as a secondary metal in copper and lead/zinc mines (26). Following previous studies, an estimated 20% of the silver in extracted ore is returned to the lithosphere as tailings (4). This high percentage can be attributed to the fact that much of the silver is mined as a byproduct. During the mining and milling processes, a certain fraction of silver becomes airborne as dust particles. Vandegrift and Shannon use an emission factor of 1 g dust per kg ore mined (27), which was adopted here. The method of silver separation varies from region to region. Europe, for example, has prohibited the use of cyanide leaching, whereas in Asia cyanide leaching accounts for 16% of the total. The relative use of different separation technologies was assessed for each of the countries in question. The percentage loss of silver from each process was assigned as follows: 10% for tank leaching, 63% for heap leaching, 0.12% for smelting, and 11% for thiosulfate leaching (4). Silver is lost to slag during the refining process, though most of this is fed back into the furnace. Only an estimated 0.02% escapes during this process (4). 2.1.2. Fabrication and Manufacture. Again, the total mass of silver entering fabrication for each country was taken from Johnson et al. (4). Addicks estimates a 0.2% overall loss of silver during fabrication and manufacture of silver-bearing products (28). This occurs mainly in the production of jewelry, photographic chemicals, electroplating, and the spraying of silver coatings. Smith and Carson estimate that 80% of these losses are airborne and that the majority do not escape the factory (20). Silver that can be collected is usually recycled; 6284
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there are undocumented anecdotes of incinerating jeweler’s rugs and the roofing of some factories to recover the silver dust deposited there over time. The silver that escapes in wastewater is accounted for in the waste management section. 2.1.3. Use. Silver sees a wide variety of uses (4). The losses to the environment of silver from use are largely dissipative in nature and difficult to estimate. Dental and photographic uses of silver are accounted for in the waste management section. Silver used in electrical contacts is gradually aerosolized to form silver sulfide and silver oxide. However, this particulate matter generally does not escape the product and instead forms a film on the inside of the casing. There is some silver dissipated from jewelry, silverware, coins, and medals, but this amount is small and unquantified. In places such as India, where 60% of the total silver used is in jewelry, this dissipation will be relatively more significant (29). It is interesting to note that silver is increasingly being used in disposable products, such as antimicrobial coatings for bandages, clothing, or water treatment devices. These direct environmental losses of silver are difficult to quantify, but it is expected that this category of use will continue to grow (30). 2.1.4. Waste Management. The great majority of the silver that ends up the environment is processed through some sort of waste management system. Silver residues are deposited in a variety of forms and locations, depending on the waste stream and dominant waste management practices. Following Johnson et al. (4), the present study considers silver in municipal solid waste (MSW), waste electrical and electronic equipment (WEEE), hazardous waste (HW), industrial waste (IW), and sewage and sewage sludge (S/SS). Silver contained in end-of-life vehicles is generally reprocessed with the steel scrap; this silver is considered in the section on steel production. 2.1.4.1. MSW. For all countries, a per capita post-recycling MSW generation rate was used. This rate was scaled from a base of 0.00075% overall silver content in landfilled MSW (31), assumed to exclude silver in WEEE and HW. The scaling
was based on urban metal concentrations and the percentage of urban population for each country (32). Losses from recycling activity are assumed to be incorporated into refining and fabrication loss rates. MSW is landfilled, applied directly to land, or incinerated; Table S1 in the electronic supplement shows the fates of MSW of the countries and regions considered in the present study. Particulate silver emissions from incineration are highly dependent on the type of furnace and the pollution control technology being employed. An emission factor of 0.1% was found state-of-the-art pollution control (31), while a factor of 0.4% was found in the absence of controls (20). These factors were assigned to high- and low-income countries, respectively, while a factor of 0.25% was assigned to middleincome countries. Silver that does not escape as particulate matter is left in the fly ash or bottom ash and subsequently landfilled, applied to land, or incorporated into building materials. The incineration rates and emission factors for MSW were applied to all subsequent cases of waste incineration. 2.1.4.2. WEEE. As with MSW, a per capita generation rate was determined for each country where possible and on a regional basis otherwise. It has been estimated that 0.2% of circuit boards by weight is silver and that these make up 3% of WEEE (33). All of the silver in WEEE that was not recovered through recycling was assumed to be landfilled. 2.1.4.3. HW. Silver oxide in batteries and in dental amalgam was considered as the two constituents of HW. The annual per capita generation rate of silver in batteries was based on a German study that reported 0.08 g Ag/(cap-year) (34). This factor was scaled by per capita GNP for each country. Rates of HW generation from dental amalgam were similarly scaled from a detailed study from California (35). The silver in amalgam lost to sewers is considered in the discussion of S/SS, below. Amalgam in corpses is not considered here. After accounting for recycling (as in ref 4), the relative amount of HW incinerated or landfilled was assessed. For Africa, Asia, the former Soviet Union, Latin America, and the Middle East, it was assumed that HW was released to land, landfilled, or incinerated in the same proportion as MSW. Other world regions were assumed to landfill all HW. 2.1.4.4. IW. Historically, photographic uses have accounted for a large fraction of total silver use (20-23). In 1997, the photographic sector consumed 26% of the total silver demand (27). About 55% of photographic silver was found to be recycled on-site. The remainder is released to sewers (7%, included in per capita sewage generation rates below) or as IW (20%, considered as solid waste) (31). The fate of this solid IW was considered to be identical to HW. 2.1.4.5. S/SS- Following Johnson et al. (4), the per capita generation rate of silver in sewage of 0.33, 0.22, and 0.11 g Ag/(cap-year) was adopted for high-, middle-, and lowincome countries, respectively. This includes silver from photographic wastes, dental amalgam, hospital waste, electroplating, and human waste. Based on data on the percentage of sewage treated in any particular country, the amount of silver released directly to water as sewage was established. For sewage that undergoes treatment, 90% removal efficiency was assumed (22), which switched the silver containing medium to sewage sludge. The remaining 10% is released directly to water. Sewage sludge is either land applied, landfilled, incinerated, or dumped in the ocean. 2.1.5. Sources of Trace Silver. Due to its existence as a trace element in most metal ores and minerals, anthropogenic silver is also emitted in processes unrelated to the silver cycle. The three major sources are iron and steel production, cement production, and fossil fuel combustion (primarily coal). Silver emissions factors were derived from various sources (20, 21, 36, 37). Emissions from steel production depend on the type of smelting technology, primarily due to the different levels
of scrap metal used. The proportions of steel produced in basic oxygen, blast, and electric arc furnaces were found for all relevant countries. Data on international cement production was taken from the U.S. Bureau of Mines (37). The U.S. Energy Information Administration and the International Energy Agency provided information on global coal consumption (39, 40). Coal used in iron/steel and cement production was subtracted from coal consumption data to avoid double-counting. Petroleum products were not considered due to the low concentrations of silver present. The emission flows described above were then aggregated into seven categories, primarily based on their fate. The categories are tailings, slag, leachate, particulate silver, dissipation (or direct loss) to land, dissipation to water, and landfilled material. 2.2. Environmental Impact of Silver Emissions. To interpret the physical quantities of silver in the various emissions characterized above in terms of their environmental impact, information regarding the human and ecosystem health effects of silver was needed. Toffel and Marshall (41) critique a number of chemical hazard ranking systems for different applications. The most suitable ranking system for this study was found to be the Indiana Relative Chemical Hazard (IRCH) system, which gives a score (on a 0-100 basis) that incorporates health hazards to workers, toxicological data, and any measured effects on ecosystems (42). Additionally, the IRCH score takes into account the behavior of the compound in air, water, and land, and so is a mixed-media score. No chemicals meet all hazard criteria; the highest IRCH score listed is 61 for aziridine. Compared with other toxic materials, silver compounds are of little concern, and so are not included in most hazard ranking systems. The IRCH system has the most comprehensive coverage of silver compounds, though it still aggregates several compounds into one category. Chemical hazard scores for major silver compounds are shown in Table 1. The emission categories characterized above are generally composed of several different forms of silver whose relative proportion varies greatly with the receiving media and with local social and environmental conditions. To assign each emission category a unit environmental impact score, we multiplied the estimated relative proportion of each form of silver by its IRCH score and summed, as detailed in the electronic supplement. Silver in landfills and slag were not treated in this analysis, as the silver contained in these repositories has minimal exposure potential and poses little or no danger to human or ecosystem health. Tailings also offer little potential for exposure to humans but were retained to reflect the ecological damage that has historically resulted from mine waste (43). This information was gathered from the World Bank Indicators database (33). Finally, we transformed the mass results by scaling with the unit environmental impact score and land area, as shown in eq 1,
Ii,j ) (Ei,j Si)/Aj
(1)
where Ii,j is the environmental impact, Ei,j is the emission mass (in Mg Ag), Si is the unit environmental impact score, Aj is the land area (in 100 000 sq km), i is the emission category index, and j is the country index.
3. Results Environmental silver emissions for the various flows are shown in Figure 2. This plot simultaneously displays more than 400 individual data elements, allowing for examination of each element and of country-level and regional level patterns in the data. The color spectrum is scaled to the largest emission, which is the United States emission to VOL. 41, NO. 17, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Chemical Hazard Rankings for Various Silver Compounds from IRCH
a
chemical name
CAS no.
IRCH score
industrial uses
silver (metallic)
7440-22-4
17
lubricants, antimicrobials, textiles, catalysts, dental amalgam, electronics, bearings, brazing alloys, jewelry, foils
silver compounds silver bromide silver carbonate silver chloride silver iodide silver sulfadiazine silver sulfide silver cyanide
(SDO000) 7785-23-1 534-16-7 7783-90-6 7783-96-2 22199-08-2 21548-73-2 506-64-9
14a 14 14 14 14 14 14 26
silver nitrate
7761-88-8
16
silver oxide
155645-89-9
14a
photochemicals particulate emissions photochemicals photochemicals, cloud seeding medical devices, antimicrobials silver ore, particulate emissions electroplating, heap and tank leaching lab chemicals, films, medical devices, antimicrobials batteries, particulate emissions
Value calculated independently according to IRCH guidelines.
FIGURE 2. Comparisons of silver emissions to the environment for 64 countries, in alphabetical order by region. (FSU, Former Soviet Union; LAC, Latin America and the Caribbean; ME, Middle East; NA, North America; OC, Oceania) The units are logarithms of the silver flows in Mg (metric tons). landfill and appears in dark red. Areas in white represent those flows with a magnitude of less than 105 grams, or 100 kilograms. There are a number of features revealed by the plot: As a whole, landfills receive the largest amount of the silver emitted into or deposited in the environment, while slag is generally the smallest emission source; Regionally, Asia has consistently high amounts of silver being emitted directly to the land, which reflects the prevalence of informal waste management systems and land-applied sewage sludge; The United States is the single largest national source of silver emitted in nearly every category, due to its affluence, high per capita generation rates, relatively large population, and 6286
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active mining and processing industries; China, India, Indonesia, and the United States emit significant amounts of silver directly to water. For the Asian countries, this large flux stems from large populations and low rates of sewage treatment. In the U.S., this represents dissolved silver in the leaching fields of septic tanks and the silver that remains in treated wastewater; Particulate silver is emitted in large quantities in Chile, China, and the United States, the majority arising from coal combustion and cement production; Leachate emissions are not prevalent in Europe, the Middle East, or Africa. In general they are lower in mass than tailings emissions, except in Japan, Taiwan, and Brazil, where little mining but significant refining takes place; Silver in slag is
FIGURE 3. Silver emissions to the environment by world region.
FIGURE 4. Comparisons of environmental impacts of silver in different emission categories. lower than that in tailings, except in Germany, which imports significant quantities of silver ore and only uses smelting as a refining process; Though Chile and Canada emit roughly the same amount of silver in tailings, Canada’s emissions to landfill are almost 10 times that of Chile, due to domestic consumption. It is valuable to consider the results aggregated by region. As can be seen in Figure 3, North America dominates regional silver emissions to landfill and as tailings or leachate. This is not surprising, given the significant mining operations in
the United States, Canada, and Mexico, as well as the relative affluence of these countries. Asia as a whole emits the largest quantities of silver directly to land and water for any region. Africa has the highest ratio of silver emitted directly to land to that emitted to landfill, approximately 1.5:1, followed by Asia at 0.9:1. These high ratios reflect both prevalence of informal waste collection and processing systems in these regions as well as the direct application of much of the coal and incinerator ash directly to the land. VOL. 41, NO. 17, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Global Emissions of Silver to the Environment and Their Weighted Environmental Impact emission category
mass flow (Mg)
percent of total mass
tailings slag leachate particulate matter dissipation to water dissipation to land landfill
4041 13 1428 151 457 1379 5950
30.1% 0.1% 10.6% 1.1% 3.4% 10.3% 44.3%
total
13 420
100%a
a
Percentages do not sum to 100% due to rounding.
Looking at emissions on a global scale gives information relevant to biogeochemical cycles for silver but is not useful for evaluating environmental impact, which is local or at most regional in nature (for silver). Global emissions of silver are presented in Table 2. It is interesting to note that the total quantity of silver remaining in tailings is on the same order of magnitude to that in landfills. Both are geographically concentrated repositories of low concentrations of silver, although the concentration in tailings is significantly higher than that in landfilled material. The results for environmental impact are shown in Figure 4, which has units log[(Mg Ag)*impact/(100 000 km2)]. The scaling introduced multipliers along both axes, so that one can draw entirely different conclusions than for Figure 2: Globally, the silver emission category with the largest environmental impact is that of dissipation to land in Hong Kong. This is due to high discard rates, high population density (resulting in a high level of sewage generation), and an extremely small land area; In all major silver-producing countries, the silver emission with the highest environmental impact is tailings. The one exception is the United States, where cyanide leaching accounted for fully 31% of all silver separation in 1997; In Europe the most important emissions of silver in terms of environmental impact are in tailings in Poland and Sweden and in dissipation to land in Germany.
Discussion Using global metals cycles to evaluate environmental hazards is a fruitful area of research. Silver is an interesting case but is by no means the only possible example; this analysis could easily be extended to more toxic metals such as lead, chromium, and cadmium. While many rules and regulations specify maximum allowable concentrations (or stocks) of pollutants in air, water, and soils, consideration of pollutant flows can provide useful information on accumulation rates. One limitation to this analysis was the aggregation by the IRCH of many silver compounds into a single category. This resulted in a loss of resolution and detail in the hazard results. Compared to other pollutants and metals, silver has relatively low toxicity and is, therefore, of only modest concern for the toxicologists who provide data for the ranking system: whereas silver has five compounds that are individually ranked by IRCH, lead has more than 20. Future studies could also incorporate the flows of silver compounds from one medium to another. Much is known about the biogeochemical cycle of silver, and several studies have examined particular cases of anthropogenic silver moving between media (44, 45). It would also be useful to model silver emissions and sinks spatially, as has been done for mercury (Global Emissions Inventory Activity) or organic pollutants (GloboPOP). Such a spatial map could identify particular geographical areas of concern and thus help to allocate resources for monitoring and remediation. 6288
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As with the study from which many of our raw data are taken, there are significant uncertainties in this analysis. Trade data are only as good as the national bureaus that report them. Several assumptions have been made regarding the fate of silver in waste management, as outlined above. In general, we estimate that results for emissions of tailings, slag, leachate, and particulate matter have low uncertainty, while the emissions to land, water, and landfill have relatively high uncertainty. For some countries (generally those that collect significant data on waste management) the results will be robust for all emission categories. However, for many countries we have made calculations based on regional, and not national, data. Field analysis of the waste management systems of individual countries in Asia, Africa, and Latin America would greatly reduce the uncertainty of the results for those regions. Uses of silver have changed significantly in the past decade. The rise of digital photography has resulted in an enormous reduction of the amount of film manufactured and processed annually, accompanied by a decrease in attendant silver emissions. This reduction has been more than offset by the increased manufacture of electronic goods and the use of silver-containing conductive pastes and solders. Silver demand will likely continue to rise as silver finds new uses, particularly in the textiles, plastics, and medical industries, changing the pattern of silver emissions as these technologies and products diffuse through the global economy.
Acknowledgments We thank the Barnett F. Dodge Engineering Fellowship and the Yale University Graduate School of Arts and Sciences for financial support. Our STAF colleagues Jeremiah Johnson, Jing Cao, Daniel Mu ¨ ller, and Tao Wang provided crucial data and guidance. Thanks also to the anonymous reviewers for many helpful comments.
Supporting Information Available Detailed information about the assumed fate of MSW, IRCH scoring for environmental impact factors, and emission profiles for fourteen countries. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review December 14, 2006. Revised manuscript received May 30, 2007. Accepted May 31, 2007. ES062970D
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