Fluxes From Society To The Environment Some Case Studies

In the research conducted over the past few years with the double aim of closing cycles of trace metals in society and understanding the relative importance of different sources and fluxes from the anthroposphere to the environment, a certain number of "key questions" or "critical steps" have been identified. It was found that more comprehensive investigations were needed to further elaborate on these problems, the resolution of which would considerably improve our overall understanding of the processes involved. Among these "key questions", which have been the focus of considerable research efforts and - consequently - on which substantial new information has been produced since the compilation of the three original monographs on Metals in Society and in the Environment, the following ones appear to be particularly important and will be treated in this chapter:

• the flux of copper, zinc, chromium and nickel from buildings and constructions in the urban environment as a result of corrosion and runoff;

• the flux of trace metals from the traffic sector in built-up city areas to the aquatic environment, and to aquatic organisms in the near-field recipients;

• the flux of trace metals from households, including the plumbing systems in buildings, to sewage treatment plants (STPs), sewage sludge and agricultural soils;

• the flux of copper and zinc from mine wastes and long-term contaminated soils to surface waters (lakes, rivers, the sea), as exemplified by the Falu Copper Mine;

• the flux of metals from major point sources of metal emissions through the atmosphere and soils in the surroundings of the emission sources.

4.1 Case Study 1: Corrosion and runoff of metals from roofing materials made of copper, galvanized steel or stainless steel (Cu, Zn, Cr and Ni)

A team of researchers at the Division of Corrosion Science, KTH, conducted systematic studies of corrosion and metal runoff from sheets (with different age) made of copper, galvanized steel or stainless steel, exposed on roofs in the urban atmosphere in Stockholm. The main aim was to determine the rates of both corrosion and metal runoff under various environmental conditions. Also the speciation and fate of released metals (Cu and Zn) were studied during their transport from the roofs to the natural receiving bodies of water.

The atmospheric corrosion rate for both Cu and Zn typically changes with time. In contrast, the rate of metal runoff from the surfaces is much more constant in time. These basic findings were shown to be valid under different atmospheric conditions (air qualities, rain pH, etc.). In the initial phase of panel exposure, the runoff rate of both metals was lower than the corrosion rate, leading to continuous accumulation of metal in the corrosion products layer, remaining on the panel. After some time, the runoff rate will equal the corrosion rate, which happens on Zn after a few years exposure in Stockholm and probably after a few decades for Cu. For Cu sheet aged from 0 to 130 years, the runoff rate was confirmed to be in the range 1.0 - 2.0 g Cu/m2, y, and the Zn runoff rate was found to be about 3.1 g Zn/m2, y. Both rates are specific for the Stockholm urban atmosphere with low concentrations of sulphur dioxide (about 3 jug/m3) and annual rainfall of about 540 mm. The corresponding release rates of Cr and Ni from stainless steel were found to be 0.2 - 0.7 mg/m2 and 0.1 - 0.8 mg/m2, respectively, i.e. some 10,000 times lower than for Cu and Zn.

By using sophisticated metal speciation approaches, the Cu and Zn species in the runoff water were described. At the edge of the investigated roof, 60-100% of the released Cu was present as free hydrated cupric ions, while 95-99.9% of all Zn in the runoff was present as free, hydrated Zn(II) ions. Depending on the pH and sulphate content, Zn(OH)+ and ZnSO4(aq) could also be present. Measurements with Cu- or Zn-specific biosensors and tests for growth inhibition with green algae confirmed that almost the total amount of the metals occurred in a bioavailable form in the runoffjust at the edge of the roof. However, in tests where the Cu- or Zn-containing runoff percolated through a soil column, the retention of total Cu and Zn was found to be 96-99.8% and the fraction of bioavailable metal in the percolate was significantly reduced. Other tests with Cu-containing runoff in contact with concrete or limestone indicated that also these materials partly retained the metal.

It was concluded that Cu and Zn, occurring in the runoff from roofing materials and other structures exposed in the urban atmosphere, are initially highly bioavailable, but during transport from the site of origin towards the final recipient, the runoff will be in close contact with soil and other reactants that considerably reduce the amount of bioavailable metals. It is pointed out that the bioavailability of metals has a real meaning only in the receiving environment.

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