Metal fluxes from society to the environment and between environmental media

The use of 'materials (or substance) flow analyses' to establish metal mass balances in defined geographical areas has turned out to be an efficient approach to improving our understanding of resource availability and long-term environmental change. The construction of well-quantified metal cycles for a country - or a continent - may be helpful for designing sound policies for production, consumption and recycling of metals and can be used as a basis for decisions supporting an environmentally sustainable economic and social development.

A good example of such an exercise, where a team of researchers from Germany, Switzerland and the USA worked out a comprehensive copper cycle for an area covering the European Union and Poland, is presented and examined. The justification for this broad definition of the system boundaries is that both mining, primary and secondary production sites as well as a multitude of consumption sites are present within the boundaries. This exercise was made to incorporate all life stages and dissipative flows of copper in Europe, i.e. both recycling rates, various waste fluxes and dissipative flow rates have been estimated with greater accuracy than earlier. Consequently, the effort has been successful in providing a clearer picture of current copper fluxes and future needs to improve sustainability with regard to copper production and use on a continent-wide basis. Other recent attempts to develop practical tools for selecting the best options for nation-wide metals management, e.g. a research programme for dynamic mathematical modelling of metal flows in the Netherlands, turned out to be less successful.

In the effort to design sound management systems for metals on a continental scale, it may also be of great value to broaden our understanding of nature's tolerance to long-term metal exposure. Hence, we have included a brief summary of a recent book by L. Lindestrom, containing detailed descriptions of the variable environmental impacts caused by the mining and smelting activities that went on for one thousand years at the Falun Copper Mine.

On the other hand, studies of metal fluxes to and from systems covering a limited geographical area, e.g. a single city, may be used to focus on specific aspects, such as:

• the impacts of urban metal flows on society's metal recycling systems and on the local environment;

• the role and relative significance of diffuse sources of metal dispersion to the natural environment; and

• the transformation and speciation of metals during transport from sources in the anthroposphere to the final metal sinks.

A comprehensive analysis of the stores and fluxes of seven metals, including chromium, copper, nickel and zinc, within and out from the City of Stockholm was carried out, in 1995, as part of a broad Swedish research programme ("Metals in the Urban and Forest Environment"). This effort has generated invaluable information, that will be presented and critically discussed in Chapter 3 of this book.

The various attempts to establish metal balances for defined geographical areas have helped to identify gaps in the available knowledge and to initiate new research to fill in these gaps. In many cases, it was deemed necessary to quantify a specific metal flux more accurately, e.g. by using better experimental designs or improved measuring techniques. The new and more reliable data on metal fluxes that we will review in Chapter 4 relates to:

• fluxes resulting from corrosion of roofs and other metallic materials, followed by runoff from buildings and other urban constructions;

• fluxes from the traffic sector in built-up urban areas to biota in receiving waters; and

• metal releases from plumbing systems in houses followed by fluxes to sewage treatment plants, sewage sludge and agricultural soils.

0.2.1 The European copper cycle in the mid-1990s

Some of the major findings from the recently published copper cycle for Europe (EU + Poland), valid for the year 1994, may be summarized as follows:

• The European smelting industry was supplied with about 600 kt of copper from ores mined within the region and 280 kt in imported concentrates.

• However, most of the copper used in Europe, 1,900 kt, is mined, smelted and refined outside of the region.

• The industrial output of non-alloyed copper products amounts at 2,650 kt, while copper in alloys is about 780 kt (75% of copper in finished products is in pure form).

• Since about 920 kt of copper enters the waste management system, the yearly growth of the copper stock in Europe is about 2,500 kt or 6 kg per capita.

• The annual growth rate of copper in landfills and tailings ponds is 1.4 kg per capita.

• The fastest growing copper waste category is waste from electrical and electronic equipment (growth rate 5-10%), which requires more efficient recycling strategies.

• Since about 2 kg of copper is generated 'per capita' annually in consumer waste in the region, and only about half of this amount, on average, is recycled, the study gives valuable information as to what waste categories should be given priority in improving the recycling rate, e.g. by developing appropriate technologies to separate waste streams.

0.2.2 Metal fluxes from mining waste - Falun Copper Mine

During the lifetime of the Falun Copper Mine, where mixed sulphidic ores were mined for more than a millennium, huge amounts of mining and smelting waste were emitted to the surrounding environment. Current estimates have arrived at total emissions in the order of 6,000 kt of sulphur dioxide and 15 kt of copper released to the atmosphere, and 500 - 1,000 kt of copper, lead, zinc and cadmium discharged to forest soils and watercourses, at the time causing dramatic impacts on the environment as well as on human health. However, a detailed assessment of the present environmental situation has revealed that the soils and terrestrial ecosystems in the vicinity of Falun have recovered to a remarkable degree during the 20th century after a series of emission-reducing measures had been introduced .

After the late 1980s, when treatment of the mine-water started, also the effluent-receiving river and lakes, that used to be exposed to very high metal concentrations in the water (for total copper 140-fold, and for total zinc 1,000-fold increases above regional background levels), exhibited clear signs of recovery. Only based on a clear appreciation of the speciation and the low bioavailability of harmful metals and of possible antagonistic (protective) interactions between metals such as zinc and other, more toxic metals, e.g. cadmium, lead and copper, was it possible to explain this rapid return to functioning aquatic ecosystems.

0.2.3 Urban metal flows - the case of Stockholm

According to the final reports from the project "Metals in the Urban and Forest Environment", the stocks and fluxes of metals in the City of Stockholm, in 1995, were as follows:

Stock/flux

Unit

Copper

Zinc

Chromium

Nickel

Total stock in city

ktonnes

123

28

5.6

2.5

Net growth rate

% per year

1.6

4.3

4.6

6.4

do.

kg per capita

2.8

1.7

0.4

0.2

Loss (as solid waste)

% per year

0.24

2.5

1.8

1.2

Loss (to environment)

% per year

0.01

0.09

0.02

0.04

do.

tonnes/year

12

24

0.8

0.6

Emissions to water

tonnes/year

2.6

9.1

0.4

1.5

Increase over backgrd.

%

15-24

41-54

29-40

14-15

Flux to aquatic sedim.

tonnes/year

1.7

4.0

1.0

0

To sewage sludge

tonnes/year

9.0

12

0.9

0.8

Per capita growth of the copper stock (2.8 kg/year), thus, was about half of that in Europe.

The types of goods causing the greatest releases of metals in Stockholm were found to be:

• for copper: water pipes in buildings, followed by motor vehicle brakes;

• for zinc: motor vehicle tyres, followed by various galvanized materials;

• for chromium and nickel: road pavements, followed by tyres.

A substantial part of the released metals is channelled through the city sewerage system (including storm-water sewers) to sewage treatment plants, where metals are partitioned between sludge and effluents to the recipient, where a certain fraction is deposited in the bottom sediments and the remainder transported further out to the Baltic Sea. Some recent attempts to determine the degree of bioavailability - in the aquatic environment - of the fraction of metals that is emitted from the street traffic have not been entirely conclusive.

0.2.4 Critical steps in metal fluxes from cities to the environment

New and accurate information on critical metal fluxes has been produced in studies, e.g. by Swedish researchers. The annual runoff of metals from copper, galvanized steel or stainless steel sheets (of different age), after atmospheric corrosion in a relatively clean urban air (in Stockholm, with sulphur dioxide, SO2, levels of about 3 ^g/m3), has been measured during extended periods of exposure on roofs. While the corrosion rate shows a typical variation with time, the runoff rate remains relatively constant when atmospheric variables do not change. The runoff rate of copper from copper roofs is in the range 1.0-2.0 g Cu/m2 and year, depending on the age of the roof. Runoff rates are clearly a function of the atmospheric SO2 concentration, which has fallen dramatically in most major cities over the past 20-30 years, of rainwater pH and of the annual precipitation. Thus, in Singapore with up to 8 times higher air pollution and rainfall than in Stockholm, the copper runoff rate is about 5.7 g/m2 and year.

For zinc, the annual runoff rate in Stockholm was determined to be 3.1 g Zn/m2, while those of chromium and nickel from stainless steel were 0.20.7 mg/m2 and 0.1-0.8 mg/m2.

The speciation and fate of the released metals (copper, zinc) were studied in the runoff water on its transport from the roofs to the natural receiving water bodies. At the edge of a roof, copper and/or zinc in runoff largely occurs in the form of free, hydrated ions, but after the metal-laden water had percolated through soil or come in contact with concrete or limestone, 96-99.8 % of the total metal content in the runoff was retained and the remaining small residue of copper and zinc in the percolate had a very low bioavailability. These results indicate that environmental dispersal of bioavailable species of copper and zinc as a result of corrosion and runoff from roofs and galvanized structures can be effectively controlled by letting the runoff water percolate over concrete surfaces or through soil.

The extensive use of water pipes and heat exchangers made of copper may cause significant releases of copper, especially in areas with corrosive drinking water (rich in carbonate and/or organic ligands, forming soluble copper complexes). The released copper usually does not constitute any risk for human water consumers, but the copper in the sewage is finally incorporated into the sewage sludge. This is one reason why some concern has been expressed regarding the safety of sewage sludge in cases where such sludge is applied on arable land.

However, studies (over 18 growth seasons) of possible impacts of yearly application of copper-enriched sewage sludge to agricultural fields have come to the conclusion that an application rate in the range of 1.4 - 4.2 kg Cu per ha and year would not be harmful on soils used to raise cereals. The conclusion was drawn on the basis of no-observed-effects on soil fauna or soil microbes, on crop yields and copper content in crops even at the higher loading rate. Moreover, copper accumulation in the soil was marginal at the lower loading rate, with an insignificant risk of exceeding existing limit values for copper in agricultural soils to be fertilised with sludge, even in the long term. Thus, a provisional limit value for copper loading on most common types of agricultural soils would be 1.4 - 2.0 kg Cu per ha and year.

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