Zinc (Zn) and water
Zinc and water: reaction mechanisms, environmental impact and health effects
|Zinc is naturally present in water. The average zinc concentration in seawater is 0.6-5 ppb. Rivers generally contain between 5 and 10 ppb zinc. Algae contain 20-700 ppm, sea fish and shells contain 3-25 ppm, oysters contain 100-900 ppm and lobsters contain 7-50 ppm. |
The World Health Organization stated a legal limit of 5 mg Zn2+/L.
Elementary zinc does not react with water molecules. The ion does form a protective, water insoluble zinc hydroxide (Zn(OH)2) layer with dissolved hydroxide ions, according to the following reaction mechanism:
Zn2+ + 2OH- -> Zn(OH)2(s)
Zn(s) + 2H+ -> Zn2+(aq) + H2(g)
Zinc salts cause a milky turbidity in water in higher concentrations. Additionally, zinc may add an unwanted flavour to water. This occurs at concentrations of about 2 mg Zn2+/ L.
The solubility of zinc depends on temperature and pH of the water in question. When the pH is fairly neutral, zinc in water insoluble. Solubility increases with increasing acidity. Above pH 11, solubility also increases. Zinc dissolves in water as ZnOH+ (aq) or Zn2+ (aq). Anionic ZnCO3 has a solubility of 0.21 g/L.
The most significant zinc ores include sphalerite (ZnS) and smithsonite (ZnCO3). These compounds end up in water on locations where zinc ores are found.
Zinc was not attributed a water hazard class, because it is not considered a hazard. This however only concerns elementary zinc. Some zinc compounds, such as zinc arsenate and zinc cyanide, may be extremely hazardous.
The human body contains approximately 2.3 g zinc, and zinc has a dietary value as a trace element. Its functions involve mainly enzymatic processes and DNA replication. The human hormone insulin contains zinc, and it plays an important role in sexual development. Minimum daily intake is 2-3 g, this prevents deficiencies. The human body only absorbs 20-40% of zinc present in food, consequently many people drink mineral water rich in zinc. Symptoms of zinc deficiencies are tastelessness and loss of appetite. Children's immune systems and enzyme systems may be affected.
Zinc may be removed from water by different methods. To achieve a level that meets legal standards, one may apply such techniques as coagulation, ion exchange and active carbon. Sand filtration is perceived and excellent solution.