fluorination and defluorinationReading time:
The term “fluorination” is preferred to “fluoration” because fluorine is added as fluoride (unlike chlorine). By analogy, we also speak of “defluorination”..
It is generally accepted that a low level of fluorine in mains water (0.4 to 1 mg · L–1 depending on the climate of the country concerned) promotes the formation of tooth enamel and protects teeth from decay. On the other hand, too much fluorine will destroy this enamel and cause a range of endemic type disorders that are generally called “fluoroses”: malformed teeth, staining of the enamel, decalcification, tendon mineralisation, digestive and nervous disorders, etc. These problems can appear in individuals for widely variable quantities of the product. Water must be discharged or treated as soon as it contains more than 1 to 1.5 mg · L–1 of F–.
Therefore, depending on the case, one needs to consider either an artificial injection of fluorine (when it is not provided in another form such as toothpaste), or remove this element.
Fluorination is mainly carried out in the USA. The following products can be used :
- sodium hexafluorosilicate: Na2SiF6, the most common;
- hexafluorosilicic acid: H2SiF6;
- sodium fluoride: NaF.
This type of treatment must be accompanied by all appropriate protection precautions concerning the plant’s personnel and the possibility of an accidental overdose.
Some natural waters contain more than 10 mg · L–1 of fluorine. This concentration has to be reduced to approximately 1 mg · L–1 (the acceptable concentration falling as the average annual temperature rises); the European standard has set 1.5 mg · L–1. The following processes are used :
filtration through activated alumina
This is the most widely used process. Fluoride ion fixing is reversible and regeneration is carried out with aluminium sulphate or, preferably, with sodium hydroxide and sulphuric acid. Retention capacity can vary considerably: depending on the water’s initial fluorine content, pH and overall salinity, the material’s particle size and operating conditions, 0.3 to 4.5 g of F– ion can be fixed per litre of product. It is recommended that preliminary tests are conducted in the laboratory in order to identify optimum conditions and the consequences for the composition of the treated water.
Aluminium sulphate treatment constitutes another means of using fluorine’s affinity for alumina but the coagulant demand will be very high (50 to 150 g per g of F– to be removed); therefore, it can only be used on raw water that requires sedimentation and that has a low fluoride content.
lime used to soften water
This method can be used but with a pH >10 and providing that the water has a sufficiently high magnesium level because it is the magnesium oxide that will adsorb the fluorine. It is felt that approximately 50 mg · L–1 of magnesium will be required to remove 1 mg · L–1 of fluorine.
using tricalcium phosphate
The affinity of fluorine for this compound has long been known because significant levels of fluorine have always been found in natural phosphates such as apatites, phosphorites (2 to 5%) and even in bones. Consideration has been given to using the following :
- either natural products: bone ash (animal charcoal) or powdered bone;
- or synthetic apatite that can be produced within the water through the controlled combination of lime and phosphoric acid.
When the fluoride ion has to be removed at the same time as excessive water mineralisation, then reverse osmosis may provide the answer.
Electrodialysis can also be considered.