ABSTRACT
Nickel (Ni) is a transition metal with atomic number 28 in group VIII of the Periodic Table following iron and cobalt. Metallic nickel is a relatively hard and strong, lustrous, silver-white malleable metal that is moderately reactive. It has typical metallic properties, is magnetic, is a good conductor of electricity and heat, and unlike iron is resistant to corrosion. Nickel is the 24th element in order of natural abundance, making up approximately 0.008 percent of the Earth’s crust and 0.01 percent of igneous rocks. There are two generic types of naturally occurring nickel ores, the nickel-iron sulfides and the oxide or silicate laterites. Nickel is present in combination with iron in certain types of meteorites, and in manganese oxide nodules, which contain nickel, copper, and cobalt, and are found on the ocean bottom. Large amounts of nickel ores are also believed to exit in the Earth’s core (Cooley, 1987). Nickel is usually dipositive in compounds, but it can also exist in oxidation states of 0, +1, +2, +3, and +4, with +2 being the most common and important oxidation state under environmental conditions. In addition to simple compounds or salts, nickel also forms a variety of coordinate compounds or complexes (da Silva and Williams, 1991; ATSDR, 1993). The green or blue color of nickel compounds is due to its hydration or its binding to other ligands (Cooley, 1987).
