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Calcium (Ca) and water

Calcium and water: reaction mechanisms, environmental impact and health effects

Calcium occurs in water naturally. Seawater contains approximately 400 ppm calcium. One of the main reasons for the abundance of calcium in water is its natural occurrence in the earth's crust. Calcium is also a constituent of coral. Rivers generally contain 1-2 ppm calcium, but in lime areas rivers may contains calcium concentrations as high as 100 ppm.
Examples of calcium concentrations in water organisms: seaweed luctuca 800-6500 ppm (moist mass), oysters approximately 1500 ppm (dry mass).
In a watery solution calcium is mainly present as Ca2+ (aq), but it may also occur as CaOH+ (aq) or Ca(OH)2 (aq), or as CaSO4 in seawater.
Calcium is an important determinant of water harness, and it also functions as a pH stabilizer, because of its buffering qualities. Calcium also gives water a better taste.

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

Contrary to magnesium placed directly above calcium in the periodic chart, elementary calcium reacts with water at room temperature, according to the following reaction mechanism:

Ca (s) + 2H2O (g) -> Ca(OH)2 (aq) + H2 (g)

This reaction forms calcium hydroxide that dissolves in water as a soda, and hydrogen gas.

Other important calcium reaction mechanisms are erosion reactions. These usually occur when carbon dioxide is present. Under normal conditions calcium carbonate is water insoluble. When carbon dioxide is present carbonic acid is formed, affecting calcium compounds.

The reaction mechanism for carbon weathering is:

H2O + CO2 -> H2CO3 and CaCO3 + H2CO3 -> Ca(HCO3)2

And the total reaction mechanism:

CaCO3 (s) + CO2 (g) + 2H2O (l) -> Ca2+ (aq) + 2 HCO3- (aq)

The product is calcium hydrogen carbonate.

Solubility of calcium and calcium compounds

Elementary calcium reacts with water. Calcium compounds are more or less water soluble. Calcium carbonate has a solubility of 14 mg/L, which is multiplied by a factor five in presence of carbon dioxide. Calcium phosphate solubility is 20 mg/L, and that of calcium fluoride is 16 mg/L. Calcium chromate solubility is 170 g/L, and at 0oC calcium hypo chlorate solubility is 218 g/L. Solubility of other calcium compounds lies between the levels of these examples, for example calcium arsenate 140 mg/L, calcium hydroxide 1.3 g/L and calcium sulphate 2.7-8.8 g/L.

Why is calcium present in water?

Calcium is naturally present in water. It may dissolve from rocks such as limestone, marble, calcite, dolomite, gypsum, fluorite and apatite. Calcium is a determinant of water hardness, because it can be found in water as Ca2+ ions. Magnesium is the other hardness determinant.
Calcium is present in various construction materials, such as cement, brick lime and concrete. It is present in batteries, and is applied in plaster as calcium sulphate. The metal is applies for zirconium and thorium production. In steal industries calcium is applied as a blotter, and is added to aluminium, copper and lead alloys. Calcium can extract sulphur dioxide from industrial exhaust, and neutralize sulphuric acids before discharge. Other examples of calcium applications are calcium hypo chloride as bleach and for disinfection, calcium phosphate in glass and porcelain industries, calcium polysulphide and hydroxide as flocculants in wastewater treatment, and calcium fluoride as turbidity agent in enamel industries, in UV-spectroscopy, and as a raw material for fluid acid production. Calcium may also be applied for removal of carbon and sulphur from iron and iron alloys, and for dewatering oil. Limestone is applied as a paper filler, causing paper to colour whiter, and in plastics to improve stability.
Calcium often positively affects soil quality and various compounds are applied as a fertilizer. For example, CaCl2- or Ca(NO)3 solutions are applied in horticulture. Calcium oxide is a dehydrating molluscicide.

What are the environmental effects of calcium in water?

Calcium is a dietary requirement for all organisms apart from some insects and bacteria. Calcium carbonate is a building stone of skeletons of most marine organisms, and eye lenses. Calcium phosphate is required for bone structure and teeth structure of terrestrial organisms. Plants mainly contain calcium oxalate. Calcium storage in plants is about 1% of dry mass.
Calcium is largely responsible for water hardness, and may negatively influence toxicity of other compounds. Elements such as copper, lead and zinc are much more toxic in soft water.
In limed soils calcium may immobilize iron. This may cause iron shortages, even when plenty of iron is present in the soil.
Water hardness influences aquatic organisms concerning metal toxicity. In softer water membrane permeability in the gills is increased. Calcium also competes with other ions for binding spots in the gills. Consequently, hard water better protects fishes from direct metal uptake. pH values of 4.5-4.9 may harm salmon eggs and grown salmons, when the calcium, sodium and chlorine content is low.
Various calcium compounds may be toxic. The LD50 of rats for calcium arsenite is 20 mg/ kg body weight. Calcium carbide forms flammable ethyn when it comes in contact with water and is therefore considered hazardous.
Some environmental effects of water hardness include hardening of domestic equipment, because high temperatures cause carbonate hardness. This may dramatically decrease the lifespan of equipment, and causes an increase of domestic waste. Calcium carbonate interacts with detergents and cleansing agents. Complex formation causes a decrease in detergent efficiency, resulting in requirement for increased detergent application and softener purchases (see also magnesium and water).
Softening is often carried out by means of ion exchangers. These must be regenerates with kitchen salt, and therefore burden wastewater.
There are six stable calcium isotopes. Today, we know of eight instable calcium isotopes. 45Ca is highly radioactive and toxic.

What are the health effects of calcium in water?

Calcium is a dietary mineral that is present in the human body in amounts of about 1.2 kg. No other element is more abundant in the body. Calcium phosphate is a supporting substance, and it causes bone and tooth growth, together with vitamin D. Calcium is also present in muscle tissue and in the blood. It is required for cell membrane development and cell division, and it is partially responsible for muscle contractions and blood clotting. Calcium regulates membrane activity, it assists nerve impulse transfer and hormone release, stabilizes the pH of the body, and is an essential part of conception. In order to stimulate these body functions a daily intake of about 1000 mg of calcium is recommended for adults. This may be achieved by consuming dairy, grains and green vegetables.
Calcium carbonate works as a stomach acid remedy and may be applied to resolve digestive failure. Calcium lactate may aid the body during periods of calcium deficiency, and calcium chloride is a diuretic.
Hard water may assist in strengthening bones and teeth because of its high calcium concentration. It may also decrease the risk of heart conditions. Drinking water hardness must be above 8.4 odH. Calcium carbonate has a positive effect on lead water pipes, because it forms a protective lead(II)carbonate coating. This prevents lead from dissolving in drinking water, and thereby prevents it from entering the human body.
When one takes up large amounts of calcium this may negatively influence human health. The lethal dose of oral uptake is about 5-50 mg/ kg body weight. Metallic calcium corrodes the skin when it comes in contact with skin, eyes and mucous membranes.

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

Removing calcium and magnesium ions from water is carried out by water softeners. These are ion exchangers that usually contain Na+ ions, which are released and substituted by Ca2+ and Mg2+ ions.
Calcium compounds may be applied for wastewater treatment. Drinking water pH and hardness may be altered by means of calcium carbonate and calcium hydroxide.

Literature and the other elements and their interaction with water

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