Cadmium natural cycle

Nitrile rubber can be cured by sulphur, sulphur donor systems and peroxides. However, the solubility of sulphur in nitrile rubber is much lower than in NR, and a magnesium carbonate coated grade (sulphur MC) is normally used this is added as early in the mixing cycle as possible. Less sulphur and more accelerator than is commonly used for curing natural rubber is required. A cadmium oxide/magnesium oxide cure system gives improved heat resistance, but the use of cadmium, a heavy metal, will increasingly be restricted. 

The data on annual wood biomass production in the main forest types have been taken from (Bazilevich, 1993). We used the value of maximum permissible level of lead in the wood equal to 0.(X)5 g/kg as an accepted in Russia (Moiseev, 1994). To estimate cadmium accumulation in the biomass, the background values equal to 0.25 mg/kg for coniferous species, 0.5 mg/kg for small leaf species and 0.6 mg/kg for broad leaf species (Uchvatov, 1995), were used. The metal accumulation in the wood biomass was not calculated for the areas without forest ecosystems, for example, tundra and steppe ecosystems. We supposed that metal conservation in the biomass of these natural ecosystems was only temporal with subsequent mineralization of organic matter and entering biogeochemical cycling. 

Cadmium is widely distributed in the earth s crust at an average concentration of about 0.1-0.2 mg/kg and is commonly found in association with zinc. Volcanic activity is major natural source of cadmium release to the atmosphere. The annual global flux from this source has been estimated to be 820 tons. Deep sea volcanism is also a source of environmental cadmium release, but the role of this process in the global cadmium cycle remains to be quantified (OECD, 1994). About 10-15% of total airborne cadmium emissions arise from natural processes (WHO, 1992a). 

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