Waterguide - Calcium
What is calcium?
Calcium is an alkaline earth metal (symbol Ca, atomic number 20, atomic mass 40 g/mol). It is one of the most common elements in the Earth's crust and is a major constituent of water in terms of quantity. Calcium is also an essential element for almost all living creatures. In water chemistry, calcium has a particular importance as a component in the lime-carbonic acid equilibrium.
In what form can calcium be found in the Earth’s crust?
Due to its high reactivity, calcium is only present as a chemical compound in minerals, and is almost always present in the oxidation state +2. In rocks, calcium is most commonly found as limestone (calcium carbonate CaCO3), dolomite (calcium magnesium carbonate CaMg(CO3)2) or gypsum (calcium sulphate CaSO4 · 2 H2O) or as an anhydrite (CaSO4). Just like clay, limestone can form as sedimentary rock in ocean basins (e.g. consisting of lime skeletons of corals or algae). Some of the most important calcium compounds for commerce include apatites (calcium phosphate minerals), used as a raw material in fertilizer production, but which also play an important role in the human body; for example, bones are 50 % hydroxyapatite.
How does calcium find its way into water and in what concentration can it be found there?
Calcium can dissolve in water in the following ways:
- Dissolution of calcium carbonate by water that contains CO2 (carbonic acid weathering)
- The dissolution of calcium sulphate from gypsiferous strata
- Neutralisation reactions with sulphuric acid or nitric acid (acid rain, fertiliser)
- Release of calcium chloride (waste product from chemical industry/mining) into waste water
Depending on the type of rock, the weathering of carbonic acid can cause the formation of striking terrain, such as underground caves. These types of terrain are known as karst. Around 25 % of the global population obtains drinking water from karst aquifers.
The type of rock from which an aquifer is made is the main factor that determines the calcium content of groundwater. Groundwater from a gypsum rock aquifer can contain a high concentration of calcium (> 200 mg/l) and sulphur. Groundwater from a limestone aquifer generally contains < 100 mg/l of calcium, as the lime-carbonic acid equilibrium limits how much can be dissolved.
The calcium content of surface water is generally quite low, but increases further downstream due to waste water discharge, and at the lower reaches of a river is approx. 50 – 100 mg/l.
The concentration of calcium in seawater is 410 mg/l, as evaporation causes the concentration of all minerals to increase.
Why does calcium have to be removed from water?
A lot of people prefer the taste of hard water due to the calcium and magnesium content. On the other hand, people generally think tea tastes more aromatic when it is made with soft drinking water. Calcium and magnesium form insoluble compounds with the tannins in the tea, which become visible as fine streaks (film on surface of tea) and can make the tea taste flat and bitter – although hard water is perfectly safe for human health.
However, water that contains too much calcium (and is therefore too hard) is not ideal for domestic purposes and for a number of commercial and industrial applications. This is because when the water is heated, calcium carbonate precipitates. This causes deposits to accumulate, which are sometimes merely a visual nuisance (e.g. lime spots on glasses and pans), but can also severely impair functionality (e.g. limescale can build-up on pipes and reduce heat transmission efficiency).
In the household, you can use citric acid – in the form of a descaling product – to remove calcium deposits. In an industrial context, it is common to remove calcium from the water through softening, decarbonisation or other methods.