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Consumption-related heavy metal emissions

In the case of the heavy metals Ag, As, Cd, Cr. Cu. Hg, Pb, and Zn, we have found that there are ten categories of consumption that are readily distinguishable in terms of their different degrees of dissipation in use and different modes of release to the environment. These are as follows:

1. Metallic uses, e.g. in alloys. Environmental losses occur mainly in the production stage (discussed previously) and as a result of corrosion in use or discharge to landfills.

2. Plating and surface treatment (excluding paints and pigments) generate some losses in the platings or treatment process and some corrosion losses as above.

3. Paints and pigments generate losses at the point of application and from weathering and wear. Some are ultimately disposed of (e.g. in landfills) along with discarded objects or building materials.

4. Batteries and electronic devices have relatively short useful lives of 1-10 years. Production losses can be significant. Most are discarded to landfills.

5. Other electrical equipment as above, but may be longer-lived.

6. Industrial chemicals and reagents (e.g. catalysts, solvents, etc.) not embodied in products have short useful lives; catalysts and solvents are usually recycled, others are lost directly to air or water.

7. Chemical additives to consumer products include fuel additives, rubber vulcanizing agents and pigments, detergents, plasticizers, photographic film, etc. They are disposed of mainly to landfill or incinerators.

8. Agricultural pesticides, fungicides and herbicides are used dissipatively, on farms, nurseries, etc. Most are immobilized by soil or biologically degraded and volatilized. There is some uptake into the food chain and some amount of loss via run-off.

9. Non-agricultural biocides include the above, as used in homes and gardens, for termite control, etc. These uses are dissipative but most biocides are immobilized by soil, as above.

10. Pharmaceuticals, germicides, etc., are used in the home or in healthservice facilities and are largely discharged via sewage or to incinerators.

More detailed discussion of intermediate and final uses of each metal can be found in the Appendix that follows this chapter.

Table 8 Consumption-related emmissions factors (ppm)

a. As alloys or analgams (in the case of Hg) not used in plating, electrical equipment, catalysts or dental work. Losses can be assumed to be due primarily to wear and corrosion, except for mercury which volatilizes.

b. Protective surfaces deposited by dip coating (e.g. galvanizing, electroplating vacuum deposition, or chemical bath (e.g. chromic acid). The processes in question generally resulted in significant waterborne wastes until the 1970s. Cadmium-plating processes were particularly inefficient until recently (see discussion in Ayres et al., 1988, vol. II). Losses in use are mainly due to wear and abrasion (e.g. silverplate), or flaking (decorative chrome trim). In the case of mercury-tin "silver" for mirrors, losses were largely due to volatilization.

c. Paints and pigments are lost primarily by weathering (e.g. for metal-protecting paints), by wear, or by disposal of painted dyes or pigmented objects, such as magazines. Copper- and mercury-based paints slowly volatilize over time. A factor of 0.5 is rather arbitrarily assumed for all other paints and pigments.

d. Includes all metals and chemicals (e.g. phosphorus) in tubes and primary and secondary batteries, but excludes copper wire. Losses in manufacturing may be significant. Mercury in mercury vapour lamps can escape to the air when tubes are broken. In all other cases it is assumed that discarded equipment goes mainly to landfills. Minor amounts are volatilized in fires or incinerators or lost by corrosion; lead-acid batteries are recycled.

e. Includes solders, contacts, semiconductors and other special materials (but not copper wire) used in electrical equipment control devices. instruments, etc. Losses to the environment are primarily via discard of obsolete equipment to landfills. Mercury used in instruments is lost via breakage and volatilization or spillage.

f. Chemical uses not embodied in final products include catalysts. solvents, reagents, bleaches, etc. In some cases a chemical is basically embodied but there are some losses in processing. Losses in chemical manufacturing per se are included here. Major examples include copper and mercury catalysts (especially in chloride mfg); copper, zinc and chromium as mordants for dyes; mercury losses in felt manufacturing; chromium losses in tanning; lead in desulphurization of gasoline; zinc in rayon spinning, etc. In some cases virtually all of the material is actually dissipated. We include detonators such as mercury fulminate and lead azide (and explosives) in this category.

g. Chemical uses embodied in final products other than paints or batteries include fuel additives (e.g. TEL). anti-corrosion agents (e.g. zinc dithiophosphate), initiators and plasticizers for plastics (e.g. zinc oxide), etc. Also included are wood preservatives and chromium salts embodied in leather. Losses to the environment occur when the embodying productivity is utilized, for example gasoline containing TEL is burned and largely (0.75) dispersed into the atmosphere. However, copper, chromium, and arsenic are used as wood preservatives and and dispersed only if the wood is later burned or incinerated. In the case of silver (photographic film), we assume that 60 per cent is later recovered.

h. Agricultural pesticides, herbicides, and fungicides. Uses are dissipative but heavy metals are largely immobilized by soil. Arsenic and mercury are exceptions because of their volatility.

i. Non-agricultural biocides are the same compounds, used in industrial, commercial, or residential applications. Loss rates are high in some cases.

j. Medical/dental uses are primarily pharmaceutical (including cosmetics) germicides, also dental filling material. Most are dissipated to the environment via waste water. Silver and mercury dental fillings are likely to be buried with the dead body.

Emissions coefficient for consumption

The term "emission coefficient," as used in this context, means the fraction of the material in question that is released in mobile form (to the air or water) within a certain period (a decade, more or less). We exclude wastes that are recycled or disposed of in landfills or in sludge dumped offshore. We exclude toxic metals immobilized in clayey soil. In a few cases we also include production-related losses that were not included in the previous sections (e.g. process wastes in the plating, tanning, and chemical industries). These assumptions are obviously quite conservative, at least in the sense that a case could be made for significantly higher estimates of emissions.

It is unfortunate (and curious) that there are almost no published data on emissions coefficients consumption activities. Obviously, most analysts so far have not considered such activities to be "sources" of pollutants. In the absence of an existing body of literature (and of time to undertake more intensive research on this topic ourselves) we are led to a rather ad hoc choice of emissions coefficients. These are displayed in table 8. Each coefficient represents the fraction of total consumption in that category that is typically unrecoverable in principle.

It should be emphasized that these estimates are rather rough. In some cases, they are little better than "guestimates." The results presented here, therefore, are illustrative rather than authoritative. A task for the future is to improve the approach, and particularly to make it more relevant to major new policy initiatives.

In the case of tetra-ethyl lead (TEL) emissions from gasoline consumption, it is probably not necessary to compute emissions from an emission coefficient. Instead, on the assumption that all lead in gasoline is eventually emitted, input data on lead use as a gasoline additive should suffice. Such data are available from the Bureau of Mines. To compute strictly atmospheric emissions, however, the total lead used as a gasoline additive should be multiplied by a factor of 0.75 to reflect the fact that at least 25 per cent of the lead is trapped in the oil, oil filters, or exhaust system of the cars and not emitted directly to the atmosphere (Hirschler et al., 1957; Hirschler and Gilbert, 1964).

It must be pointed out that, while the numerical estimates in many cases are rather uncertain - sometimes even by a factor of two or three there are only a few important routes which clearly dominate the rest for each metal.

Historical usage patterns

The next and last step is to allocate total domestic usage of each of the eight metals among the ten categories over the past 100 years. The allocation among uses has been far from unchanging. Many formerly important uses have disappeared, while others have emerged as recently as the last decade. Consumption data by use are available, in general, only since the Second World War. For earlier periods one must rely on a scattering of real data supplemented by a variety of other clues.

Our composite picture of historical heavy metals usage patterns for the United States is summarized in tables A-H in the Appendix. Each table represents one metal, and is arranged as follows:

1. Percentage of metal use by consumptive category.

2. Consumption in metric tons (US).

3. Emissions due to consumptive use (US).

The Appendix also includes a final summary table (table I) of productionrelated and consumption-related emissions, and the consumption-related fraction, for seven of the eight metals (excluding silver, for which productionrelated emissions data are not available). The consumption fraction, expressed as a percentage, is plotted for two groups of metals in figure 1.

Conclusions

As noted already, the major results of our analysis are summarized in tabular form in the Appendix (see tables A-H).

The lower part of figure 1 displays, for chromium and copper, the ratio of consumption-related dissipative losses to production-related emissions (not including losses at the mine) in each decade. For these two metals, whose major uses are in metallic form or, in the case of chromite, as bricks for blastfurnace liners, production-related emissions are still dominant, but the consumption share is increasing steadily.

In the upper part of figure 1 the same data are shown for five other toxic heavy metals: arsenic, cadmium, lead, mercury, and zinc.

In two cases, arsenic and mercury, the consumption share has always been high. Arsenic has been used (until very recently) almost exclusively because of its biotoxic properties. Such uses are inherently dissipative. This is also partly true for mercury. For instance, mercury is the basis of a number of commercial fungicides, germicides, and preservatives. The major dissipative uses of cadmium, in the past, were in pigments and as a contaminant of zinc oxide used in tyres. The use of cadmium for red and orange pigments has declined sharply, while metallic usage (mainly in batteries) has increased even more sharply. This accounts for the inverted "U" shape of the cadmium curve. (As electronic uses of arsenic, in gallium arsenide, may grow in the future, a similar downturn may be expected in the future.)

Fig. 1 Consumptive emissions as a percentage of total emissions

The increasingly dissipative usage of lead is only partly due to its role as a gasoline additive (largely phased out since 1980, of course). In earlier decades lead was the basis of one of the most widely used agricultural insecticides (lead arsenate). In the nineteenth and early twentieth century, lead was also extensively used as a white pigment for oil-based paints. So-called white lead was later replaced by a zincbased white pigment (lithopone), which was subsequently replaced by the white pigment now used most widely, titanium dioxide. Red lead was the major metal-protective paint until the last decade or so. The yellow paints currently used on roadways and to protect heavy machinery - such as bulldozers - are largely chromium-based, which accounts in part for the rapid rise in dissipative uses of chromium. Zinc is also used in large quantities in tyres and paper.

As we indicated at the outset, for three of these five metals investigated the dissipative consumption-related emissions far outweigh the production-related emissions; in fact the consumption shares for arsenic, lead, and mercury are close to 100 per cent. In the case of zinc, that share is rising rapidly; for cadmium the consumptive share is still about 50 per cent of the total.

One of the eight metals included in the study was silver. Production-related emissions data are non-existent. However, since silver is a rather valuable metal, and since almost all of it is now obtained as a by-product of lead, zinc, or copper smelting and refining, one could probably argue that productionrelated emissions are essentially non-existent. On the other hand, one major consumptive use of silver is still in photography. While commercial photographic studios do recycle some silver, a significant fraction is lost. Thus, for silver, too, the consumption-related share of total emissions is probably close to 100 per cent.

The foregoing analysis was entirely historical. But one or two points worth considering for the future emerge clearly. One of them is the fact that several of these toxic heavy metals play a major and increasing role in electronics. These include lead (solder), arsenic (semi-conductors), cadmium (batteries), mercury (switches and batteries), and silver (batteries and connectors). Electronic wastes are accumulating in obsolete equipment at an enormous rate in the United States, and all around the world. Much of this electronic "junk" might be dumped in landfills in future years, and some will be inadvertently incinerated. Many states already classify such wastes as hazardous. Leaching - especially that due to increasingly acid rainfall and combustion will mobilize some of these toxic materials. There is, therefore, a strong need for more research on ways and means of closing the materials cycle.

References

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