Releases and flows of chromium and nickel

Although extensive investigations have been conducted on the dispersion rates of copper and zinc from material surfaces on buildings and other constructions containing these metals (see sections 4.1.1-4.1.4), relatively few studies have been devoted to the release of metals from outdoor building elements made of stainless steel. Recently, however, release rates of chromium and nickel from pickled and skin passed 304 and 316 stainless steel were first determined and reported after one-year field exposures in the urban atmosphere of Stockholm (Odnevall Wallinder et al., 2002b). Later on, the exposures continued for a period of 4 years, and the results were reported in 2003 (Odnevall Wallinder et al., 2003).

Traditionally, 304 stainless steels have been used for roofs and exterior fa├žades on buildings in urban environments, and 316 stainless steels mainly for constructions in marine environments. However, the most common quality of stainless steel used today for roofing appications in both environments is 316 stainless steel. The average annual release rates of chromium and nickel from the two materials are as shown in Table 4.2.

Table 4.2. Annual release rates (mg/m2) of chromium and nickel from 304 and 316 stainless steels, exposed for four years to the urban atmosphere in Stockholm. After Odnevall Wallinder et al. (2003).

Type of stainless steel

Release of Cr

Release of Ni

304

0.2 - 0.6

0.1 - 0.5

316

0.2 - 0.7

0.3 - 0.8

The runoff rates of chromium and nickel from abraded metal surfaces increased significantly with decreasing pH of the rainwater, but were not directly influenced by rain intensity. It was also noted that surface conditions have a major influence on the release rate, which suggests a non-uniform corrosion attack (Odnevall Wallinder et al., 2002b). Immediately after release, chromium in the runoff is predominantly present as Cr(III) (>98.5%) and nickel as Ni(II) (>99%). The chemical forms change when runoff passes through different media, such as soil, where >94% of the two metals were found to be retained (Odnevall Wallinder et al., 2003).

The very minor releases of chromium and nickel from stainless steel used in the outdoor built-up environment in Stockholm for roofs and other building materials appears to be even smaller than was previously estimated, based on short-term atmospheric exposures of grade 304 stainless steel plates (Walterson, 1998). According to these earlier estimates, the runoff rate of chromium was 1.6 mg/m2, y and the nickel runoff rate 1.4 mg/m2, y. Based on these figures, Sorme et al. (2001b) calculated the release of chromium to be 0.6-0.7 kg/y and of nickel, 0.5-0.6 kg/y from a total surface of exposed stainless steel in Stockholm of 370,000-430,000 m2. With the new, more accurate release data, chromium and nickel losses from roofs and constructions (stainless grade 316) would be 0.25 kg/y and about 0.3 kg/y, respectively. This would represent less than 0.03% of the total chromium emissions from goods in Stockholm and about 0.05% of the total nickel emissions from goods.

4.2 Case Study 2: Relative importance of the traffic sector for metal fluxes from the urban environment to aquatic ecosystems

The fate and effects in the aquatic environment of trace metals emitted primarily from road and street traffic in Stockholm were investigated by a team of researchers at the University of Stockholm. The team selected a number of water bodies in the vicinity of central Stockholm, where major outfalls of urban storm-water (mainly street runoff) are located, and sampled water, sediment and several species of invertebrates (in addition, transplanted zebra mussels were used). The rationale was to find out to what extent the emitted, traffic-related metals would be bioavailable to benthic animals and thereby constitute an ecological risk.

It was found that the levels of tungsten (W), lead (Pb), Zn, antimony (Sb) and Cu showed strong co-variation in the storm-water receiving areas; the relationship with W and Pb, metals dominating the pollution picture, was considered to support the existence of a significant influence of road/street traffic (however, other plausible sources of W emissions were not discussed).

The body-burdens of Cr, Cu, Ni and Zn in transplanted zebra mussels were determined at 9 different sites after in situ exposure for 6 weeks. No enrichment of Ni and a slight enrichment of Cu was noted in the mussels along a gradient from unpolluted areas to central Stockholm, while Cr and Zn were enriched 4.3 times and 2.3 times, respectively. A comparison of metal body-burdens in mussels exposed at a site downstream of Greater Stockholm with those of mussels from reference sites upstream of Stockholm, showed that almost no enhancement of Cr, Cu or Ni could be detected, while Zn was slightly increased. Thus, the method used to assess the contamination with bioavailable metal species in road/street-related emissions from the city of Stockholm could not demonstrate any impact of Cr, Cu or Ni, but a slight increase in the body-burden of Zn in the mussels. However, it is not clear whether this enhancement of Zn in the mussel tissues was caused by current emissions from Stockholm or by historical contamination that influenced the mussels through resuspension of older bottom sediments.

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