Zinc is a lustrous bluish-white metal. It is found in group IIb of the periodic table. It is brittle and crystalline at ordinary temperatures, but it becomes ductile and malleable when heated between 110°C and 150°C. It is a fairly reactive metal that will combine with oxygen and other non-metals, and will react with dilute acids to release hydrogen.

Applications

It is used principally for galvanizing iron, more than 50% of metallic zinc goes into galvanizing steel, but is also important in the preparation of certain alloys. It is used for the negative plates in some electric batteries and for roofing and gutters in building construction.
Zinc is the primary metal used in making American pennies, is used in die casting in the automobile industry. Zinc oxide is used as a white pigment in watercolors or paints, and as an activator in the rubber industry. As a pigment zinc is used in plastics, cosmetics, photocopier paper, wallpaper, printing inks etc, while in rubber production its role is to act as a catalyst during manufacture and as a heat disperser in the final product. Zinc metal is included in most single tablet, it is believed to possess anti-oxidant properties, which protect against premature aging of the skin and muscles of the body.

Zinc in the environment

Zinc is a very common substance that occurs naturally. Many foodstuffs contain certain concentrations of zinc. Drinking water also contains certain amounts of zinc, which may be higher when it is stored in metal tanks. Industrial sources or toxic waste sites may cause the zinc amounts in drinking water to reach levels that can cause health problems.

Zinc occurs naturally in air, water and soil, but zinc concentrations are rising unnaturally, due to addition of zinc through human activities. Most zinc is added during industrial activities, such as mining, coal and waste combustion and steel processing. Some soils are heavily contaminated with zinc, and these are to be found in areas where zinc has to be mined or refined, or were sewage sludge from industrial areas has been used as fertilizer.

Zinc is the 23rd most abundant element in the Earth's crust. The dominant ore is zinc blende, also known as sphalerite. Other important zinc ores are wurzite, smithsonite and hemimorphite. The main zinc mining areas are Canada, Russia, Australia, USA and Peru. World production exceeds 7 million tonnes a year and commercially exploitable reserves exceed 100 million tonnes. More than 30% of the world's need for zinc is met by recycling.

Health effects of zinc

Zinc is a trace element that is essential for human health. When people absorb too little zinc they can experience a loss of appetite, decreased sense of taste and smell, slow wound healing and skin sores. Zinc-shortages can even cause birth defects.

Although humans can handle proportionally large concentrations of zinc, too much zinc can still cause eminent health problems, such as stomach cramps, skin irritations, vomiting, nausea and anaemia. Very high levels of zinc can damage the pancreas and disturb the protein metabolism, and cause arteriosclerosis. Extensive exposure to zinc chloride can cause respiratory disorders.

In the work place environment zinc contagion can lead to a flu-like condition known as metal fever. This condition will pass after two days and is caused by over sensitivity.

Zinc can be a danger to unborn and newborn children. When their mothers have absorbed large concentrations of zinc the children may be exposed to it through blood or milk of their mothers.

Effects of zinc on the Environment

The world's zinc production is still rising. This basically means that more and more zinc ends up in the environment. 

Water is polluted with zinc, due to the presence of large quantities of zinc in the wastewater of industrial plants. This wastewater is not purified satisfactory. One of the consequences is that rivers are depositing zinc-polluted sludge on their banks. Zinc may also increase the acidity of waters.

Some fish can accumulate zinc in their bodies, when they live in zinc-contaminated waterways. When zinc enters the bodies of these fish it is able to bio magnify up the food chain.

Large quantities of zinc can be found in soils. When the soils of farmland are polluted with zinc, animals will absorb concentrations that are damaging to their health. Water-soluble zinc that is located in soils can contaminate groundwater. 

Zinc cannot only be a threat to cattle, but also to plant species. Plants often have a zinc uptake that their systems cannot handle, due to the accumulation of zinc in soils.

On zinc-rich soils only a limited number of plants has a chance of survival. That is why there is not much plant diversity near zinc-disposing factories. Due to the effects upon plants zinc is a serious threat to the productions of farmlands. Despite of this zinc-containing manures are still applied.

Finally, zinc can interrupt the activity in soils, as it negatively influences the activity of microrganisms and earthworms. The breakdown of organic matter may seriously slow down because of this.

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.