Nickel (atomic weight 58.69) is a ductile, malleable, silver-white metal (density 8.91 g/ mL). It presents the oxidation states -1, 0, +1, +2, +3, +4. More than 70% of nickel produced annually is devoted to the production of alloys; nickel is used in a variety of electrolytic procedures, in the manufacture of batteries and in welding procedures, as a catalyst in large scale processes, and in the glass and ceramics industry. In addition to 8.5 million tons per year of nickel in the atmosphere due to natural sources, 43 million tons are released by anthropogenic activities. Population exposed at soluble nickel concentration < 1 gg m-3 has no respiratory cancer risk, which is related to exposure to concentrations greater than 1 mg m-3 (workers in nickel industries). Dermal sensitivity to nickel is presented by 10-20 % of female and 1 % of male population. The nickel content in surface water ranges from 2 to 20 gg/L. The limit for nickel in drinking water is 0.07 mg/L (WHO, 2008).

1.1.3 Zinc

Zinc (atomic weight 65.41) is a soft, bluish-white metal (density 7.14 g/ mL). It presents the oxidation states 0, +2. Zinc and its products are widely used in alloy production, as anticorrosion coatings of steel and iron, in electrical devices, in rubber and tire industries, in paints, in pesticides and as chemical reagents in a number of applications. Zinc is the second most abundant trace metal in the human body: it appears in the active site of a variety of enzymes and many of the metabolic consequences of its deficiency are related to a diminished activity of zinc metallo-enzymes. Zinc is relatively nontoxic, even if daily doses greater than 100 mg during several months may lead to different disorders. Zinc imparts an undesirable astringent taste to water. Water containing zinc at concentrations in the range 3­5 mg/L also tends to appear opalescent and develops a greasy film when boiled. This feature allows the high zinc limit 3 mg/L in drinking water (WHO, 2008).