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The example of cadmium

Step 1: Identification of materials containing cadmium and their pathways through the industrial economy

As an example of the approach taken in our study, the discussion focuses on the industrial metabolism of cadmium (Cd). Primary materials containing cadmium and its range of concentrations are shown in table 1.

Table 1 Natural occurrence of cadmium

Source: Boehm and Schaefers, 1990.

Cadmium enters the industrial economy inadvertently as a trace impurity of high-volume raw materials. The most important of these are phosphate ores, coal, oil, and iron ore. The production of phosphate fertilizer is a major source of aqueous cadmium pollution in the Rhine Basin (Elgersma et al., 1991), and fertilizer application is now the major source of cadmium pollution in agricultural soils. Combustion of coal and oil are major sources of atmospheric cadmium pollution. Iron and steel production results in the generation of large volumes of solid wastes contaminated with cadmium, as well as atmospheric and aqueous cadmium emissions. An added source of cadmium pollution in steel production is the input of steel scrap treated with cadmium as a surface coating.

The cadmium contained in zinc ores is generally of sufficiently high concentration that separating and refining it as a by-product of zinc refining is economically feasible. In fact, it is via this route that all primary cadmium is produced. There is some mining of Zn/Cd ores within the basin, but most of the ores, in the form of zinc concentrates, are imported. Inputs of cadmium to the Zn/Cd refinery are transformed into three outputs: refined cadmium metal; refinery emissions (to air, water, and soil); and a trace component in refined zinc metal (0.15-0.50 per cent in zinc produced at thermal refineries, and 0.02 per cent or less in zinc produced at electrolytic refineries). Cadmium is also present in lead and copper ores but, as shown in table 1, the concentrations are much lower than for zinc, and the production and use of lead and copper in the Rhine Basin is not an important source of cadmium pollution.

As shown in figure 3, cadmium and zinc pollution are linked for at least two zinc-containing products, automobile tyres and galvanized metals, which are significant sources of emissions, particularly in urban areas. Tyre wear and corrosion of galvanized zinc cause the release of zinc and associated cadmium. The deposited metals accumulate in street dust and may be transported as aqueous emissions during storm run-off or dispersed as windblown dust. The amount of cadmium contained as an impurity in finished zinc metal has been decreasing since the 1960s, as electrolytic refineries have accounted for an increasingly greater share of zinc production. Nevertheless, even in recent decades significant quantities of cadmium have entered the economy by this route.

For the Federal Republic of Germany in the period 1973-1986, it is estimated that 40 tons of cadmium per year on average (560 tons over the entire period) were contained in zinc products for domestic consumption (Rauhut and Balzer, 1976; Rauhut, 1978a, 1981, 1983, 1990; Balzer and Rauhut, 1987). Since the population of the West German part of the Rhine Basin comprises about one-half of the total West German population, and assuming an equal per capita distribution of zinc products, it is estimated that about 280 tons of cadmium entered the basin by this route from the Federal Republic of Germany alone. When account is taken of the rest of the population in the basin, a total of about 467 tons of cadmium is estimated to have been associated with zinc products over the 14-year period.

Fig. 3 The coupling of zinc and cadmium pollution caused by the presence of cadmium as a trace impurity in zinc-containing products

The largest inputs of cadmium to the Rhine Basin are not from inadvertent trace impurities, but rather through the refining of cadmium metal, and the production, use, and disposal of cadmium products. As shown in figure 4, cadmium metal, some of which is produced at the basin's zinc refineries and some of which is imported, is the input to plants that manufacture cadmium-containing products. The four major products are pigments (mostly for plastics), nickel-cadmium (Ni-Cd) batteries, plate (for surface protection of steel and other metals), and stabilizers (in PVC plastic). Emissions of cadmium occur for each of these manufacturing sectors.

Fig. 4 The flow of cadmium-containing products through the industrial economy

The next stage in the material flow is the use of cadmium in the basin. Ayres et al. (1988) have noted the importance of dissipative emissions, referring to emissions that may occur during the normal use of products. Such emissions are important, however, only when the chemical is easily mobilized during use. Of the four major cadmiumcontaining products, only cadmium plate might be expected to generate emissions to the environment via corrosion when exposed to polluted atmospheres during use (Carter, 1977). In the other three products, the cadmium is tightly embedded in the matrix of the product, and emissions during normal usage would be expected to be low or even negligible.

As shown in figure 4, recycling so far is important for only two cadmium-containing products, large industrial batteries and cadmiumplated steel. Recycling of batteries back to the battery manufacturers by the industrial users, mainly the railroad industry and operations requiring emergency power supplies, is a long-established practice. This is not the case, however, for small, sealed batteries, used by consumers in personal computers and other light electronic equipment. Most of these batteries still end up in municipal landfills or incinerators.

An estimated 30 to 40 per cent of cadmium-coated steel is recycled back to iron and steel producers as scrap. As noted earlier, this practice has been a major source of cadmium emissions in steel production. The remainder of the cadmium-plated metals ends up in landfills where most of the cadmium is corroded over time. Surface coatings for steel and other metallic substances were once the dominant use of cadmium in the basin. Currently, however, it is the smallest use, accounting for only about 20 tons in 1988, compared to about 250 tons in 1970.

Cadmium as a pigment in plastics is a major source of cadmium in the consumer waste stream (Schulte-Schrepping, 1981). Since the cadmium is tightly bound in the plastic matrix, however, it is expected that very little of it would be mobilized if the wastes were directly landfilled (Raede and Dornemann, 1981). When the plastics are incinerated, however, they constitute a major source of atmospheric cadmium emissions, as well as cadmium concentrated in the residue incinerator ashes.

Cadmium as a stabilizer in PVCs is used particularly in outdoor window frames. AS with cadmium-pigmented plastics, cadmium in this form is not likely to be appreciably mobilized, either during use or after disposal to landfills. Very little of the disposed PVC is incinerated, since most of it ends up in landfills with other debris from demolition of old buildings.

Step 2: Estimation of cadmium emissions and deposition to air, water, and solid wastes

Atmospheric emissions and deposition

ATMOSPHERIC EMISSIONS WITHIN THE RHINE BASIN. Because atmospheric pollutants may be transported over long distances, a substantial fraction of emissions generated in the basin are transported and deposited out of the basin and, conversely, some fraction of emissions generated out of the basin are transported and deposited in the basin. Therefore, the calculation of cadmium deposited in the basin from the atmosphere requires the incorporation of emission sources both inside and outside the basin.

Fig. 5 In-basin atmospheric emissions of cadmium

Such a European-wide database was provided by Pacyna (1988) and Pacyna and Munch (1988) for the early 1980s. Historical emissions were calculated using available production statistics for the relevant sectors generating the emissions, and estimations of the evolution of emission factors per sector since the 1950s (Pacyna, 1991). Deposition was calculated using the TRACE 2 model developed at IIASA and described in detail in Alcamo et al. (1992). The model employs "transfer matrices" which convert emission inputs into deposition outputs.

Figure 5 shows the in-basin atmospheric emissions of cadmium for selected years in the 1970s and 1980s. The Federal Republic of Germany has been the predominant source of emissions, accounting for between 75 and 80 per cent of total emissions over the entire period. Table 2 shows the distribution of emissions by sector for 1970 and 1988. One may observe that over the 18year period there was an overall reduction in air emissions of 87 per cent. This decrease occurred mostly from the implementation of emission-control technologies.

Additional factors, however, were also important. Emissions from coal and oil combustion declined because of the adoption of energy conservation measures, and the increased use of nuclear power. Emissions from iron and steel production declined in part because of the stagnation of production in the basin. The very large reductions in non-ferrous metal production (zinc, copper, and others) were in part the result of the closing down of large pyrometallurgical smelters. Another significant trend was the increase in the relative share of emissions from incinerator wastes. In 1970 these emissions only accounted for about 5 per cent of the total; by 1988 they already accounted for 14 per cent of total emissions.

Table 2 Atmospheric emissions of cadmium in the Rhine Basin by industrial sector, in tons per year and percentages

ATMOSPHERIC DEPOSITTON IN THE RHINE BASIN. Table 3 lists total atmospheric deposition in the basin, and the calculated contributions to the deposition by countries inside and outside the basin. One may observe that there was a 79 per cent decline in deposition over the 18year period, which is obviously strongly related to the decline in emissions within the Rhine Basin, as well as in the Western European nations in close proximity to it. Deposition in the basin contributed by the formerly socialist countries of Eastern Europe also declined during this period, although the decrease was not nearly as large as in the Western European countries.

Table 3 Atmospheric deposition of cadmium in the Rhine Basin - contribution by country, in (tons per year and percentages)^a

a. Percentages in parentheses.

The share of total deposition attributed to the five Rhine Basin countries showed a slight but continuous decline, beginning in 1975, when they accounted for about 58 per cent of the total deposition, until 1988, when they comprised about 50 per cent. The share of total deposition from the three Western European nations Belgium, the United Kingdom, and Italy dropped by about 37 per cent between 1970 and 1988. In fact this trend was dominated by large decreases in Belgium's contribution, which was reduced from 25 per cent in 1970 to about 12 per cent in 1988. Belgium is one of the leading producers of zinc/cadmium in the world. Until the early 1970s, cadmium was produced solely at thermal zinc refineries with large atmospheric emissions of cadmium. During the 1970s, the thermal smelters were phased out, in some cases by closures and in others by a switch to electrolytic zinc/cadmium production with greatly reduced air emissions.

The opposite trend is shown for the Eastern European countries. Emissions from the German Democratic Republic, Poland, Czechoslovakia, and the Soviet Union contributed only about 9 per cent of the total deposition in 1970, but the share increased to about 27 per cent in 1988. This occurred not because emissions from Eastern Europe increased so much, but rather because total deposition in the basin decreased so rapidly, thus increasing the shares of contributions from Eastern Europe. These shifts in percentage shares reflect regional differences in efforts to limit air pollution. While Western European nations, beginning in the 1970s, were able to reduce their emissions mainly through the extensive application of air pollution control equipment, emissions in Eastern Europe remained largely unchanged during this time.

The estimated distribution of cadmium deposition among the three major land uses (agriculture, forests, and urban areas) in the basin is given in table 4. About 44 per cent of the total deposition goes to agricultural lands, 31 per cent to forests, and about 25 per cent to urban areas. Relative to their spatial coverage in the basin (about 15 per cent of the total land), urban areas receive more cadmium than forests or agricultural lands. This is particularly so because most point sources are located in urban areas, and about 10 to 15 per cent of atmospheric emissions are deposited locally within a radius of up to 20 km from the point source. (As will be discussed below, urban atmospheric deposition constitutes a major source of diffuse cadmium loading to surface waters in the basin.)

Table 4 Distribution of atmospheric deposition of cadmium in the Rhine Basin according to land use, in tons per year

Figure 6 shows the estimated annual loads of cadmium for the time periods 1973-1977, 1978-1982, and 1983-1987 at various monitoring stations on the Rhine and some of its tributaries. The station at Lobith is on the GermanDutch border. Since there is no net sedimentation of cadmium on an annual basis in the River Rhine before the Netherlands border, the load represents the total inputs to the river from all upstream sources. The analysis was conducted by Behrendt and Boehme (1992) and is based on monitoring data provided by the International Commission for the Protection of the Rhine in Koblenz, Germany. The analysis also included a disaggregation of the total load into point and non-point (diffuse) loads by a methodology developed by Behrendt (1992).

One may observe the emergence of two major trends during this 14-year time period. Firstly, the cadmium load at Lobith decreased significantly over time, from 145 tons per year during the first period, to 96 tons per year in the second period, to 26 tons per year in the third period. Secondly, the relative contribution to the total load from point sources decreased from 82 per cent in the first period, to 79 per cent in the second period, to 42 per cent in the third period, while the contribution from diffuse sources increased from 18 to 21 to 58 per cent.

Aqueous emissions

INDUSTRIAL SOURCES. Table 5 lists estimated aqueous emissions of cadmium in the Rhine Basin during the 1970s and 1980s according to industrial sector (Elgersma et al., 1991). The estimates for a given point source were calculated by multiplying plant production by an assumed emission factor. Historical emissions were derived from production statistics for the various industrial sectors, together with an appraisal of changes in emission factors over time owing to the implementation of regulatory standards and the adoption of waterpollution control technologies. The assumptions applied in these estimations were calibrated by comparison with the estimates shown in figure 6.

Table 5 Aqueous emissions from industrial point sources in the Rhine Basin^a.

Source: Elgersma et al., 1991.

a. The emission values listed here are somewhat higher than the values for point sources shown in figure 6 because not all the aqueous emissions within the basin end up in the River Rhine. Some are trapped in sediments of tributaries. Also, this table includes point source emissions in the Netherlands, while figure 6 includes emissions only up to the German-Netherlands border.)

One may observe that during the 1970s primary and secondary production of zinc was by far the most important source of aqueous cadmium pollution, accounting for more than 45 per cent of all emissions. Most of the pollution from this sector can be attributed to two thermal zinc smelters that did not refine cadmium, but, rather, treated it as a disposable waste. Reductions in emissions from these plants in the late 1970s and 1980s were achieved in part by recycling the wastes to an electrolytic zinc/cadmium refinery located in the basin. Further reductions were achieved when one of the smelters ceased refining zinc ore in the early 1980s.

Fig. 6 Estimated annual loads of cadmium at various stations on the River Rhine and its tributaries (Source: Behrendt and Boehme, 1992)

The second-largest polluting sector was the iron and steel industry, including coke production, which accounted for between 20 and 33 per cent of the emissions during the 1970s and 1980s. As in the case of the thermal zinc-refining sector, reductions in aqueous cadmium emissions were in part achieved by recycling the cadmium-containing wastes to electrolytic zinc/cadmium refineries. In fact, recycling of industrial sludges and solid wastes increased significantly during the 1970s and 1980s for most industrial sectors in the basin. The wastes were either recycled as a feed stock to other industrial sectors, or used as a filler in construction materials such as cement or asphalt. Another significant factor in reducing aqueous cadmium emissions in steel production was the reduction in cadmium-plated steel as a component of recycled steel scrap. Annual use of cadmium plate in the basin averaged about 300 tons per year throughout the 1960s; by the early 1980s use had dropped to around 80 tons per year, and by the late 1980s it was less than 30 tons per year.

Historically, among the industries manufacturing cadmiumcontaining products, the production of cadmium pigments was a major source of water pollution. In the early 1970s the pollution load from this source comprised about 8 per cent of the total from all point sources. By the early 1980s, however' pigment manufacturers accounted for less than 2 per cent, and in the late 1980s less than 1 per cent of the total load. These decreases occurred in part because of the implementation of more efficient pollution-control technologies (thus lowering emission factors), and in part because of the decrease in production of cadmium-containing pigments. (Nearly 900 tons per year of cadmium were used in pigment manufacturing in the early 1970s; by the late 1980s cadmium use had been reduced to under 400 tons per year.)

Another major source of industrial aqueous cadmium emissions is the phosphate fertilizer manufacturing industry. This accounted for about 17-19 per cent of total industrial pollution in the 1970s, and in the 1980s was the dominant source of pollution, responsible for about 50 per cent in the mid 1980s and 75 per cent in the late 1980s. During the conversion of phosphate ore to phosphoric acid, gypsum, containing about 30 per cent of the cadmium contained in the ore, is produced as a waste product. The largest polluters have been two fertilizer companies located in the Netherlands, both of which discharged the gypsum directly to the Rhine. It is the intention of these companies, however, to limit their combined emissions to less than one ton by 1993 (Elgersma et al., 1991).

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