The addition of fluoride to water has caused much controversy and public debate. The problem seems to be that some see it as the addition of a poison, and others see it as the use of mass medication whether the individual wishes it or not.
Many waters do, however, have a natural fluoride content (Figure 33) and it has been suggested that the presence of fluoride in a concentration of 1.0 mg l−1 is beneficial in preventing dental decay. Above this concentration there is the likelihood of 'mottled teeth' occurring. The EU Drinking Water Directive specifies a maximum value of 1.5 mg 1−1.
Fluoride is added to the water as the last process in water treatment. There are three commonly used chemicals:
disodium hexafluorosilicate (Na2SiF6);
sodium fluoride (NaF);
hexafluorosilicic acid (H2SiF6).
In water, all these chemicals dissociate to give fluoride ions, e.g.
The three chemicals must be handled carefully during their addition to the water as they are harmful if they are inhaled, ingested or come into contact with the skin. It must, however, be remembered that the addition of chlorine to water is readily accepted and chlorine is a poisonous gas!
A natural water contains 0.55 mg l−1 of fluoride ion and is to be treated with sodium fluoride so that the final concentration will be 1.0 mg l−1 of fluoride ion. The flow of water to be treated is 1000 litres per second. Calculate the daily weight of sodium fluoride that will be required.
The fluoride ion to be added is the difference between the required concentration and the natural concentration, i.e.
The molecular mass of sodium fluoride = 23 + 19 = 42. Therefore, to add 0.45 mg of F− will require
This amount is required for 1 litre of water.
1000 kg of hexafluorosilicic acid (H2SiF6) is added per day to a flow of 15 000 l s−1 to achieve a concentration in the water of 1.0 mg l−1 fluoride ion. What was the original concentration of fluoride in the water?
Relative molecular mass of H2SiF6 = 2 + 28 + (6×19) = 144
As mentioned in Section 3.7, fluoride belongs to a group of chemicals called halogens. If compounds of bromine (another halogen) – called bromides – are present, and the water is treated with ozone (as might happen if pesticide residues are present), then there is a danger of bromates being formed. Bromates have been found to induce a high incidence of kidney tumours in male and female rats, and peritoneal mesotheliomas in male rats. Bromate is mutagenic in vitro and in vivo. There is therefore a limit of 10 mg l−1 imposed on levels of bromate in drinking water.