Nickel (Ni) and water
Nickel and water: reaction mechanisms, environmental impact and health effects
|Seawater contains approximately 0.5-2 ppb of nickel, and rivers contain approximately 0.3 ppb. Phytoplankton contains 1-10 ppm nickel (dry mass), resulting in a 103-104 bioconcentration factor compared to seawater. Bentic algae can be found both in freshwater and salt water, and may contain between 0.2 and 84 ppm nickel. Lobsters contain 0.14-60 ppm nickel, molluscs 0.1-850 ppm, and fishes between 0.1 and 11 ppm (all values based on a dry mass). Nickel occurs in water as Ni2+ (aq) and sometimes as NiCO3. It may be either dissolved, or complexed with inorganic ligands. Nickel may also be bound to particles. |
Under normal conditions nickel does not react with water.
Elementary nickel is water insoluble at T=20oC pressure = 1 bar. However, nickel compounds may be water soluble. Nickel chloride is most water soluble; 553 g/L at 20oC, to 880 g/L at 99.9oC. Nickel carbonate has a water solubility of 90 mg/L, whereas other nickel compounds, such as nickel oxide, nickel sulphide and nickel tetra carbonyl are water insoluble.
Nickel may be found in slate, sandstone, clay minerals and basalt. The main nickel source is pentlandite. The element accumulates in sediments and is a part of various biological cycles.
Nickel is a dietary requirement for many organisms, but may be toxic in larger doses. Metallic nickel and some other nickel compounds are teratogenic and carcinogenic to mammals. Nickel concentrations in plants are usually 1 μg/g, and concentrations above 50 μg/g are toxic. Tea has an extraordinary nickel content of 7.6 mg/kg dried leaves. Nickel causes growth restraints in algae at concentrations of between 0.5 and 10 ppm. Fishes apparently are less susceptible to nickel, but this differs between species. For Daphnia hyaline the LD50 for 48 hours is 1.9 ppm. Chronic nickel toxicity for Daphnia magna lies between 30-150 ppb. The LD50 for marine lobsters lies between 150 and 300 ppm. In the organs of birds mainly living off water organisms nickel concentrations of 0.6-36 ppm (dry mass) were found. Nickel accumulation in rats mainly occurs in lungs, where concentrations exceed those in other organs by 4-40 times.
The human body contains approximately 10 mg nickel. Nickel is a dietary requirement for a number of organisms, therefore it might be of significance to humans. The human dietary need is estimated at only 5 μg, which is the result of a 150 μg intake. Nickel probably has a function in urea to ammonia conversion by the urease enzyme. Nickel cannot be resorbed in the digestive gland, unless it is complexed.
Nickel may be removed from water by means of active carbon adsorption. Coagulation is another feasible option. Nickel only fully precipitates under certain conditions, namely a pH value of at least 9.5, under which it is fully converted to nickel hydroxide.