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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.


In what way and in what form does zinc react with water?

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)

Zinc reacts with H+ ions, according to the following reaction mechanism:

Zn(s) + 2H+ -> Zn2+(aq) + H2(g)

This reaction releases hydrogen, which reacts with oxygen explosively.

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.


Solubility of zinc and zinc compounds

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.
Examples of solubility of zinc compounds are: zinc chloride (ZnCl2) 4320 g/L, and zinc oxide (ZnO) or zinc vitriol (ZnSO4 . 7H2O) 580 g/L


Why is zinc present in water?

The most significant zinc ores include sphalerite (ZnS) and smithsonite (ZnCO3). These compounds end up in water on locations where zinc ores are found.
About three-quarters of the total zinc supply is used in metal form. The remainder is applied as various zinc compounds in various industries.
Industrial wastewaters containing zinc stem from galvanic industries, battery production, etc. Zinc compounds are applied for many different purposes. Zinc chloride is applied for parchment production, zinc oxide is a constituent of salves, paints and catalysers, zinc vitriol is applied as a fertilizer, and zinc bacitracine is applied as a growth stimulant in animal husbandry.
The larger part of zinc in wastewater does not stem from point sources. It stems from larger surface waters containing the element.
Zinc leaks from zinc pipes and rain pipes, consequential to circulation of carbon rich water. Car tires containing zinc and motor oil from zinc tanks release zinc compounds on roads. Zinc compounds are present in fungicides and insecticides, and consequently end up in water.
When inadequate safety measures are taken, zinc may be emitted from chemical waste dumps and landfills, or from dredge mortar.


What are the environmental effects of zinc in water?

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.
Zinc is a dietary mineral for humans and animals. Still, overdoses may negatively influence human and animal health and over a certain boundary concentration, zinc may even be toxic. Toxicity is low for humans and animals, but phytotoxicity may not be underestimated.
Sludge from wastewater treatment is applied in agriculture, horticulture and forestry, and zinc concentrations may therefore not exceed the 3 g/ kg boundary.
Ecotoxicological tests attributed a 50 μg/L PNEC value to dissolved zinc. This means a total concentrations of 150-200 μg/L of zinc in water. This PNEC value represents the maximum concentration where no environmental effect occurs (Predicted No Effect Concentration).
Industrial zinc emissions decreased strongly in the past decades. Current zinc values are not a very extensive environmental risk. Zinc concentrations in the River Rhine have reached optimal values. Unfortunately, locations of historical contamination still exist.
A total of five stable zinc isotopes occur naturally, among which are 64Zn, 66Zn en 68Zn. We now know of about fifteen instable zinc isotopes. 65Zn is present in nuclear reactor cooling water, and is applied in medicine.
Zinc appears to accumulates in some organisms.


What are the health effects of zinc in water?

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.
Higher zinc application appears to protect people from cadmium poisoning. Zinc may also decrease lead absorption. The relation copper : zinc in the human body is an important characteristic.
One may also absorb zinc overdoses. This does not occur very regularly. Symptoms include nausea, vomiting, dizziness, colics, fevers and diarrhoea and mostly occur after intake of 4-8 g of zinc. Intake of 2 g of zinc sulphate at once cause acute toxicity leading to stomach aches and vomiting.
Strikingly, zinc belongs to the same elemental group in the periodic chart as cadmium and mercury, which are both toxic.
Examples of zinc-related health effects also include mucous membrane infection from zinc chloride (lethal dose 3-5 g), and zinc vitriol poisoning (lethal dose 5 g).


Which water purification technologies can be applied to remove zinc from water?

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.

Literature and the other elements and their interaction with water







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