Nickel is a naturally occurring element (the 24th most abundant element in the periodic table) and as a consequence of natural and man-made processes can be found ubiquitously in the air, soil, sediments and water. As with all metals and chemicals, excessive amounts of nickel in any of these environmental compartments can result in deleterious impacts on the quality of the environment for flora and fauna.
Natural sources of nickel to the atmosphere include soil dust, forest fires, particle releases from vegetation and sea salt. Anthropogenic sources of nickel released to the atmosphere are primarily from fossil fuel (oil and coal) combustion, high-temperature metallurgical operations, nickel primary production operations, and municipal waste incineration. Other sources include coke ovens, cement manufacturing, asbestos mining/milling, and cooling towers.
In major nickel producing countries such as Canada, the most important anthropogenic sources of nickel to air are the primary base metal smelter and refineries, followed by fossil fuel combustion. Nickel in air is mainly in the particulate form, with particle sizes ranging from 0.1 to 2 μm. Nickel associated with these particles is mainly in the form of nickel sulphate, nickel chloride, nickel nitrate, and nickel oxide, the sulphates resulting from the oxidation of nickel by sulphur dioxide. Airborne nickel particles eventually end up being precipitated with rainfall and therefore end up in water and soil.
Adverse health effects from the inhalation of nickel can occur and these are associated with inhalation of the inorganic form. The respiratory system is the prime target of nickel inhalation toxicity in both animals and humans. Further information on the health effects of nickel can be found here.
Nickel occurs naturally in soils as a result of the weathering of the parent rock (McGrath, 1995). The highest concentrations are found in ultramific igneous rocks with much lower levels found in sedimentary rocks including shales, clays, limestones, and sandstones. The underlying geology and soil-forming processes strongly influence the amount of nickel in soils with higher average concentrations reported in clays, silts, and fine grained loams relative to coarser grained loams, sandy and peaty soils. Soil nickel concentrations vary widely, ranging from 0.7 to 259 mg Ni/kg soil on a global basis, with an arithmetic mean of 23.9 mg Ni/kg. Nickel has been established as a micronutrient for plants, and some soils have been categorized as being deficient in nickel content. Excess nickel concentrations do, however, lead to adverse impacts to soil organisms. Further information on soil ecotoxicity of nickel can be found here.
Anthropogenic activity has resulted in the atmospheric deposition of nickel from the burning of oil and coal. Localised nickel contamination with high concentrations of nickel may also occur near to a nickel smelter, plating works or steel mill. Agricultural fertilisers, especially phosphates, are also a significant source of nickel in soil but it is unlikely to build-up in soil in the long term from their use. More important is the application of wastes to land including sewage sludge.
Nickel is generally present in water bodies at varying concentrations depending on the nature of the underlying geology. In river systems, the influence of natural and anthropogenic activities will also influence the ambient concentration of nickel. Where there are industrial activities, the levels of nickel may become more elevated. Sediments in freshwater and marine systems also contain nickel naturally, and this environmental compartment may act as a sink for nickel in systems that receive anthropogenic sources.
Nickel may end up in water from both point and non-point sources. Diffuse nickel emissions may stem from sewage treatment plants, power plants, waste incinerators and metal industries and application of sewage sludge to land. Nickel is directly emitted from a number of industries through direct discharge to surface waters, usually after waste treatment. The emission limits are strictly controlled in regions like Europe and North America.
Information on nickel aquatic toxicology is extensive, and general patterns have emerged. For example, the most sensitive aquatic organisms tend to be invertebrate zooplankton, whereas fish are generally more tolerant. Also, effects of nickel exposure vary greatly among different freshwater systems as a function of differences in water chemistry. The mechanisms behind these differences are well understood, and are beginning to be used in setting Environmental Quality Standards for nickel. Further information on water ecotoxicity of nickel can be found here for freshwater and here for the marine aquatic compartment.